Nanocrystal/J-aggregate Constructs: Chemistry,
Energy Transfer, and Applications
by
Mi~SSACHUSETTS INSTI1TJTE
ASSACHUSETS INSTirITE
OF TECHNOLOGY
Brian J. Walker
JUN 0 7 2011
Submitted to the Department of Chemistry
in partial fulfillment of the requirements for the degree of
LIBRARIES
ARCHIVES
Doctor of Philosophy
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 2011
@ Massachusetts Institute of Technology 2011. All rights reserved.
A uthor .......................
........
Department of Chemistry
May 2, 2011
Certified by.........
Moungi G. Bawendi
Lester Wolfe Professor in Chemistry
Thesis Supervisor
Accepted by .............
Robert W. Field
Chairman, Department Committee on Graduate Students
2
This doctoral thesis has been examined by a committee of the
Department of Chemistry as follows:
Professor Andrei T'iakoff
Thesis Committee Chairman
Professoi Moungi G. Bawendi
Thesis Adviser
Thesis Committee Member
Professdr Vladimir Bulovid
Thesis Committee Member
4
Nanocrystal/J-aggregate Constructs: Chemistry, Energy
Transfer, and Applications
by
Brian J. Walker
Submitted to the Department of Chemistry
on May 2, 2011, in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy
Abstract
The interaction of light with matter is one of the most central subjects to modern
chemistry. Two types of materials, semiconductor nanocrystals and J-aggregates of
cyanine dyes, have been developed chiefly due to their potential for interacting with
light in interesting and productive ways. At the same time, existing spectroscopy
and microscopy tools enable the study of these photonic materials, their dynamics, and their interactions. Although semiconductor nanocrystals and J-aggregates
have complementary physical properties, the coupling between them requires new
methods to control the interface between the organic (J-aggregate) and inorganic
(nanocrystal) material. This thesis is about the interfacial chemistry, photophysical
characterization, and selected applications of J-aggregated cyanine dyes conjugated
with semiconductor nanocrystals.
Chapter 1 begins with a brief review of J-aggregates and semiconductor nanocrystals together with referrals to other scources and motivation for the present work. Because the electronic excited states of both J-aggregates and semiconductor nanocrystals are characterized by a bound electron-hole pair, they can be grouped under the
class of excitonic materials, and the coupling of J-aggregates with excitonic inorganic
materials is reviewed. To control J-aggregate/nanocrystal interactions, it is important to preserve the aggregate structure while achieving favorable energy transfer.
This challenge is the subject of Chapters 2 and 3, in which new ligand chemistry
was developed to achieve near-unity energy transfer efficiency from the J-aggregates
to the nanocrystal quantum dots in solution (Ch. 2) and in a solid state thin film
(Ch. 3). These hybrid J-aggregate/nanocrystal constructs result in emission enhancement through energy transfer across the organic/inorganic interface, with the
strongly-coupled J-aggregates serving as optical antennae to the nanocrystals. In the
process, it was discovered that the ligand directs formation of J-aggregates onto the
nanocrystal surface.
In Chapter 4, the template-directing ligand is used on semiconductor nanowires
grown from solution to realize a new photodetector design. Here, the excitation energy transfers from J-aggregated dyes to the nanowires, enhancing the photocurrent
of the device and creating an artificial solid-state photodetector whose self assembly and aggregated antenna molecules are analogous to a photosynthetic light harvesting complex. Additionally, the nanowire/J-aggregate self-assembly generalizes to
J-aggregates of three different color dyes (red/green/blue), providing a wavelength
selectivity absent in biological light harvesting.
In Chapter 5, the kinetics of indium phosphide (InP) semiconductor nanocrystal
synthesis is discussed. InP is benficial for nanocrystal applications in biology or
display technologies, as it does not contain lead or cadmium. However, the molecular
mechanism of InP nanocrystal synthesis had been essentially unexplored. By studying
the reaction kinetics of InP synthesis, a mechanism is proposed for InP. As in the case
of the chemistry described in Chapters 2-4, it is clear that non-covalent interactions
are vital to achieving control during nanocrystal synthesis.
Thesis Supervisor: Moungi G. Bawendi
Title: Lester Wolfe Professor in Chemistry
0.1
Acknowledgments
My research and this thesis owe their existence to the influence of many colleagues,
friends, and loved ones. My first thanks go to my advisor, Moungi Bawendi. When I
was new at MIT and told Moungi about my background and interests, he suggested
that I look into J-aggregates. Even as he provided valuable feedback and suggestions
for my experiments, Moungi always expressed a willingness to allow me to follow
my own interests, and this kind of balance seems to characterize all of Moungi's
interactions with us. I am extremely fortunate to have been in his group and to
have worked with him. My collaborating investigator Vladimir Bulovid has been an
extremely valuable source of feedback and knowledge, and I am grateful for the time
he has taken to talk with me about device physics and engineering, to invite me to
meetings, and to brainstorm ideas. I am constantly impressed with the intuitive grasp
that Andrei Tokmakoff, my thesis committee chair, has for science. He shared with
me with the kind of thoughtful advice I hoped to receive as a physical chemist at
MIT.
The Institute's research support staff are vital to much of this work. Between
instrument training and assistance with techniques and data analysis, much of my research could not happen without them: Bill DiNatali (Institute for Soldier Nanotechnologies), 1 Jeffrey Simpson (Department of Chemistry Instrumentation Facility), 2
Anne Gorham (Department of Chemistry Instrumentation Facility),3 Yong Zhang
(Center for Materials Science and Engineering), 4 Elisabeth Shaw (Center for Materials Science and Engineering),' Gang Liu (Department of Chemistry),6 and Debby
Pheasant (Department of Chemistry Biophysical Instrumentation Facility). 7
My collaborators on projects, whether the results came or not, were really central
to my motivation and love of my work at MIT. I am indebted to Jonathan (Yaakov)
Tischler for instilling in me an irrational love of strongly coupled chromophores, for
his tremendous energy and for his drive to learn. I hope that I have retained a small
fraction of the enthusiasm that always filled a room when Yaakov walked into it.
Jonathan Halpert introduced me to the art of colloidal quantum dot synthesis, and
his mentorship led me to collaborate with him on our first quantum dot/J-aggregate
project. I will always appreciate the high premium Jon placed on collegiality and I
am glad our paths continue to cross. Gautham Nair has been a great colleague and
friend; apart from teaching me to set up transient photoluminescence experiments,
our conversations and debates brightened many days in the basement. I also enjoyed
working with Lisa Marshall, who offered key suggestions for my work in the laser
lab. She always has great ideas about doing creative science and communicating it
well. From the time Peter Allen, Hee-Sun Han and I joined the lab I've been glad
'Focused ion beam tomography
General analysis, kinetics, and heteronuclear multiple bond correlation spectroscopy
3
Diffusion-Ordered Spectroscopy and basic training
4
Basic electron microscopy training and scanning tunneling electron microscopy
5 Atomic force microscopy
6
Electronics and electron microscopy
7 Dynamic light scattering
2
to have them as classmates. Peter Allen was my collaborator for the InP synthetic
mechanism, which fulfilled our long-standing interests. It was so much fun to have
a collaborator who was as invested in the project as I was. Hee-Sun has become
the senior student on the biological projects, and as her background was the inverse
of my own (moving from physical chemistry to synthesis) we have a complimentary
approach to science. I also enjoyed the time I spent with Juan Guan, who entered
the lab with Hee-Sun, Peter and me and is now at the University of Illinois.
Scott Geyer developed most of the scale-up apparatus that we used to synthesize
large quantities of quantum dots, and he was as great a reource for working in the
electronics lab as he was for awesome parties. I also collaborated with Gleb Akselrod, Dylan Arias, Will Tisdale, Liz Young, Kathy Stone, Jenn Scherer, Dan Harris,
Russ Jensen, Geoffrey Supran, Tim Osedach, Johnny Lunt, Shane Yost, and Tamar
Mentzel. One of the best aspects of research is the excitement to try new things, and
I have been fortunate that my collaborators have been so generous with their ideas,
their time, and their passion.
There are also many people to thank who have contributed to my development
more generally as a scientist, friend, and human being. The importance of this last
category cannot be overemphasized at MIT-a place that does some things extremely
well in part because it does not profess to do everything. Adam Bezigian's Taco Friday
parties have been a major part of my weekends. Liang-Yi Chang's work ethic in the
lab and explanations of concepts in electronics have really been excellent additions
to the group, and it was great to sit across from him for two years. Jian Cui never
fails to ask an incisive question, and his late-night tendencies have been heartening
when I find myself in the same place. I always get the critical mix of optics expertise,
internet memes, and cutting-edge hiphop from Raoul Correa. Kit Werley and Tony
Colombo have been awesome training partners and relay teammates.
I enjoyed living with Jason Rich, in our first and second years, and chatting about
our lives as students and where they might go. During the year I lived with Leeping
Wang, I never lacked for a thought-provoking scientific discussion. Brandi Cossairt
and Jared Silvia, my housemates for two years, were also my muses of coordination
chemistry, my inspiration in the kitchen, and the people I looked foward to seeing at
the ends of our days together.
Dorothy Couper's arrival in Somerville brought back our shared lives in Severna
Park and made my life more musical; I am so lucky to have such a good friend
live around the corner from me. Andrew Greytak, Zoran Popovic, Euan Kay, and
Dave Strasfeld have all been postdocs in the group while I was there; they have
been great resources on syntheses, laser spectroscopy, and the day-to-day balance of
professionalia with research. I had a great time hiking and planning adventures with
Yogi Surendranath-these were among my happiest memories. Nate Silver was a
constant friend and problem set partner throughout our first year in graduate school.
I've also enjoyed my interactions with Wen Liu, Cliff Wong, Jing Zhao, and Ni Zhao;
interspersed in our casual lunch breaks or conversations in the lab there was usually
some substantial scientific (and in the case of Cliff, philosophical) insight for me to
find. JM Lee's great playlists have lifted the mood in the lab and make many of us
dance.' Tara Sarathi has brought cheer and a social instinct to a bunch of hardened
graduate students and postdocs; with her boldness and ebullience I know she'll do
well in graduate school. I've had great times with Johanna and Misha Wolfson:
between our side projects and problem sets, strategy games, and the odd dinner
party I couldn't spend time with more delightful people.
I have also been fortunate to be in several groups with many members, but which
I must at least acknowledge collectively. The Bawendi Group, as a whole, is a remarkable, vibrant place to work and to grow as a scientist; I could not have asked
for a better group of colleagues. Both the ONE Lab and the J-aggregate supergroup
have been additional intellectual homes for me. The Graduate Student Symposium
Planning Committee was a great side project whose content ("Chemistry and Policy:
Solving Problems at the Interface") has had a strong effect on me.
The Chemistry Resources for Easing Friction and Stress (REFS) have not left a
single aspect of my life unimproved (professional and otherwise), and my interactions
with Ruthy Rosenberg, Toni Robinson, and Mary Rowe were among the principle
benefits to me. Nobody understands humans like they do.
The Chemistry Department support staff keep our department running, our stipends
paid, and our equipment and facilities moving along. Chiefly I thank Li Miao, who
has the ability to anticipate and correct seemingly any administrative problem before
it happens. There is a reason that the Chemistry Education Office (Susan Brighton,
Melinda Cerny, and Mary Turner) receive awards from the institute: they work tirelessly, they care, and they never fail to brighten my day whenever I come by even
when they aren't giving away food. Anne Hudson has been a huge help with my
questions and with the work with the REFS.
Thanks go to Jimmy DeFrancesco, Matt Donaghey, and Kevin Pulli, the night
custodians who clean 18-067. Thanks also to Claudio Gomes, who delivers our packages.
I am grateful that both Dbrthe Eiesele and Darcy Wanger read substantial parts
of this thesis. D6rthe will be carrying on the work on J-aggregates; in many ways I
am learning as much from her as she is from me. Darcy's insights have contributed
to the thesis as well as to the work that preceded it, and though her work intersects
only tangentially with mine, she has always offered good suggestions to me. I have
made changes since Darcy, D6rthe, or my committee read it, so of course I am solely
responsible for any errors that remain.
By tradition, I thank my family last. To my parents Dan Walker and June Bronfenbrenner, and my wife Jenny Harrison: there is nothing more to say apart from
"thank you."
8
when we think nobody is watching.
10
To my parents and my other teachers,
who encouraged my love of learning at every stage.
"So halt' ich's endlich denn in meinen Hinden,
Und nenn' es in gewissem Sinne mein."
Goethe
12
Contents
0.1
Acknowledgments...... . . . . . . . . . . . . . . . . .
. . . . . .
1 Introduction
29
1.1
W hat is a J-aggregate? . . . . . . . . . . . . . . . . . . . . . . . . . .
29
1.2
Why study J-aggregates?.
34
1.3
How does J-aggregation occur?
1.4
Introduction to Quantum Dots (QDs)... . . . . . . . . .
1.5
Coupling between J-aggregates and excitonic inorganic materials.
1.5.1
1.5.2
2
7
. . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . .. .
.
. . . . . . ..
35
. . . . .
36
. .
37
Planar structures: the first coupling between organic and inorganic m aterials. . . . . . . . . . . . . . . . . . . . . . . . . . .
37
Nanocrystal/J-aggregate constructs in solution . . . . . . . . .
39
1.6
Challenges addressed in this thesis
1.7
Conclusions and outlook.. . . . . . . . . . . .
. . . . . . . . . . .
43
1.8
Appendix to Chapter 1 . . . . . . . . . . . . . . . . . . . . . . . . . .
44
1.8.1
Frenkel Exciton Hamiltonian..... .
44
1.8.2
Lineshape narrowing in linear J-aggregate chains
. . . . . . . . . . . . . . . . . . .
. . . . . . . . . . .
. . . . . . .
41
44
Narrowband absorption-enhanced Quantum Dot/J-aggregate Conjugates in Solution
47
2.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
47
2.2
D iscussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
49
2.3
C onclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
56
2.4
Appendices to Chapter 2 . . . . . . . . . . . . . . . . . . . . . . . . .
57
2.4.1
57
Synthetic Procedures . . . . . . . . . . . . . . . . . . . . . . .
3
2.4.2
Spectroscopy and Analysis . . . . . . .
2.4.3
Energy Transfer Calculations
2.4.4
Sizing Data . . . . . . . . . . . . . . .
. . . . .
Quantum Dot/J-aggregate Blended Films for Light Harvesting and
Energy Transfer
67
3.1
Introduction ......................
67
3.2
Results and Discussion . . . . . . . . . . . . .
69
3.3
Conclusion . . . . . . . . . . . . . . . . . . . .
76
3.4
Appendices to Chapter 3 . . . . . . . . . . . .
77
3.4.1
Synthesis and deposition procedures . .
77
3.4.2
Characterization Methods . . . . . . .
78
3.4.3
Determining the Attenuation Length of a TC J-aggregate film
79
Measurements of linear spectral parameters for TC J-aggregate films
80
3.5.1
81
3.5
On "brightness" in measurements of absorption enhancement .
4 Color-enhanced photocurrent by coupled J-aggregate/nanowires
5
4.1
Introduction...................
. .. . . . . . ..
4.2
Discussion . . . . . . .............
4.3
Conclusion . . . . . . . . . . . . . . . . . . .
4.4
Appendices to Chapter 4 . . . . . . . . . . .
. ..
4.4.1
Synthesis and deposition procedures .
4.4.2
Characterization Methods . . . . . .
4.4.3
Electrostatic self assembly . . . . . .
4.4.4
Excitation power dependence
4.4.5
Model of photocurrent modulation by templated J-aggregates
85
85
. . . .
Mechanistic Insights into the Formation of InP Quantum Dots
103
5.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
103
5.2
Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .
105
5.3
C onclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
111
5.4
Appendices to Chapter 5.. . .
. . . . . . . . . . . . . . . . . . ..
112
5.4.1
Experimental Details . . . . . . . . . . . . . . . . . . . . . . .
112
5.4.2
Synthetic details
. . . . . . . . . . . . . . . . . . . . . . . . .
112
5.4.3
Kinetic analysis: general rate determinations . . . . . . . . . .
113
5.4.4
Representative kinetics data . . . . . . . . . . . . . . . . . . . 117
16
List of Figures
1-1
Solution-phase absorption spectra of thiacyanine (TC) dye in methanol
(blue curve) and aqueous ammonia (red curve). The spectral signatures
of the monomer and the red-shifted J-aggregate forms are distinct. . .
1-2
30
Schematic of the process of aggregation. (a) Structure of the thiacyanine dye (TC) use extensively in Chapters 2-4. The monomer transition dipole is shown. (b) Illustration of selection rules for two limiting
cases of molecular aggregates.
For head-to-tail aggregates, only the
red-shifted, J-type state is allowed. For parallel, stacked aggregates,
only the parallel, blue-shifted, H-type state is allowed.
(c) Cartoon
of the induced polarization effects for a linear aggregate of three dye
molecules. The net dipole moment of the aggregate exceeds that of the
monomer due to the longer polarization length, although no charges
have transfered between constituent molecules. .
2-1
48
..................................
Representative transmission electron micrograph of CdSe/CdZnS QDs,
with an average radius of 3.6 nm. . . . . . . . . . . . . . . . . . . . .
2-3
31
The structures and abbreviations of organic dyes and ligands used in
Chapter 2........
2-2
. . . . . . ....
49
Absorption spectra in DI H2 0. The spectral features of both QDs and
TC monomers are discernable in the QD/J-aggregate conjugate sample, as is the new J-aggregate feature at 467 nm. .
2-4
. . . . . . ...
50
Excitation spectra taken at 635 nm emission. The J-aggregates contribute only when conjugated to QDs . . . . . . . . . . . . . . . . . .
50
2-5
Emission spectra taken at 460 nm excitation. The integrated emission
of QD/J-aggregate is 5x greater than for QDs alone. Inset: Emission
spectra, renormalized to J-aggregate emission. In the QD/J-aggregate
the 472 nm emission is indistinguishable from baseline, consistent with
near-unity energy transfer efficiency . . . . . . . . . . . . . . . . . . .
2-6
51
Photograph of QD/J-aggregate and QD alone at identical QD concentration, illuminated at 457 nm light using a band pass filter [630 ± 15
nm] to remove excitation light . . . . . . . . . . . . . . . . . . . . . .
2-7
52
Time-resolved emission decays taken at 472 nm. (a) QD/J-aggregates,
fit with a convolution of the instrument response (shown) with an exponential decay function. This fit accounts for 97% of the exciton depopulation. (b) Thiacyanine J-aggregates, fit using an exciton-exciton
annihilation model.. . . . . . .
2-8
. . . . . . . . . . . . . . . . . . .
53
Steady-state optical characterization of QD(-)/J-aggregates (+). (a)
Emission spectra with 570 nm excitation. Spectrally integrated emission of QD/J-aggregate is 4x greater than for QDs alone. (b) Emission
spectra, renormalized to J-aggregate emission. J-aggregate was taken
in electrolyte solution to maximize aggregation. In the QD/J-aggregate
the 595 nm emission is reduced by 82% relative to the emission of Jaggregates alone. (c) Absorption spectra. The spectral features of both
QDs
and J-aggregates are discernable. Note that residual monomer
is suppressed in the QD/J-aggregation construct relative to the Jaggregate in H2 0. (d) Excitation spectra taken at 635 nm emission.
The J-aggregates contribute little unless conjugated to QDs. . . . . .
2-9
54
Photograph of QD(-)/J-aggregate(+) and QD alone at identical QD
concentration, illuminated with 568 nm light. A band pass filter [630
± 15 nm] was used to remove excitation light. . . . . . . . . . . . . .
55
2-10 Control experiment of mps-QDs combined with BIC. There is no significant difference in the emission intensity at 635 nm for the combined
J-aggregate(-)/ QD(-) sample [black] relative to the algebraic sum
of the isolated sample spectra [red], indicating that efficient energy
transfer does not occur from J-aggregate to QD. This lack of emission
enhancement indicates an absence of energy transfer and therefore an
absence of close QD/J-aggregate assembly. . . . . . . . . . . . . . . .
56
2-11 Nuclear Magnetic Resonance spectra for the synthesized mta ligand.
(a) 'H NMR spectrum of mta in D20 (500 MHz). Product resonances
are observed at 3.50, 3.11, and 2.91 ppm. (b)
13
C NMR spectrum of
mta in D20 (500 MHz). Product resonances are observed at 17.10,
53.47, and 68.59 ppm .
. . . . . . . . . . . . . . . . . . . . . . . . . .
60
2-12 Demonstration of the left-right invariance of the photography setup.
Photographs of QD and QD/J-aggregate constructs were taken at constant QD concentration, and a band pass filter centered at 635 nm was
used to remove excitation light. Samples were illuminated using a diffuse, collimated 457 nm Ar+ laser beam, and sample vials were placed
at an identical distance from the beam center. . . . . . . . . . . . . .
61
2-13 Plot of optical density at 350 nm as a function QD concentration. The
mass extinction coefficient was determined from a linear fit of this data
using a 1 cm quartz cuvette. . . . . . . . . . . . . . . . . . . . . . . .
63
2-14 Dynamic light scattering data for mta-capped QDs after dialysis in DI
H2 0. The peak at 4.6 nm comprises 98% of the sample mass (r = 4.3
± 0.4 nm), and its breath spans two bars in the histogram. . . . . . .
64
2-15 Dynamic light scattering data for mta-QDs conjugated to TC J-aggregates.
(a) Titration curve of the dominant QD/J-aggregate hydrodynamic radius, as a function of the dye molecule/QD ratio. Discrete particles
with radii comparable to QDs are present for dye/QD ratios well below 0.5, but many QDs begin aggregating abruptly when the ratio
reaches 0.5. The sulfonate groups of the thiacyanine dye are likely
cross linking positively-charged QDs; such cross-linking does not occur
in the oppositely-charged construct (Figure 2-16). (b) Representative
dynamic light scattering data for 0.91 dyes:dot. Although the sample mass is predominately composed of species at 29 ± 3 nm, larger
aggregates are also present.
. . . . . . . . . . . . . . . . . . . . . . .
65
2-16 Dynamic Light Scattering data for QD(-)/J-aggregate(+) species. (a)
mps-capped QDs conjugated to TTBC J-aggregates. The peak at 4.2
nm comprises 89.2% of the sample mass (r
=
4.2 nm ± 0.4 nm). The
larger species with r = 27.0 nm may consist of QDs bridged by large Jaggregates. (b) mps-capped QDs alone after dialysis. The peak at 4.0
nm comprises 99.6% of the sample mass (r
=
4.0 ± 0.4 nm). TTBC
J-aggregates alone have much larger hydrodynamic radii in aqueous
solution, and it is concluded that QD(-)/J-aggregate(+) form distinct
species in solution and that the QD surface acts as a template for Jaggregation. Zeta potential measurements of mps-QDs (-20 ± 1 mV)
and of mps-QD/TTBC J-aggregate conjugates (-7 ± 2 mV) are consistent with a negatively charged QD surface that is partially screened
by the oppositely charged J-aggregates. . . . . . .
. . . . . . . .
66
3-1
(a) Schematic of electrostatic conjugation of thiacyanine J-aggregates
at the surface of a quantum dot after ligand exchange with mta. Na+
and Cl- ions are not drawn for clarity. (b) Transition electron micrograph of a QD/J-aggregate heterojunction in cross section. The
thin film is supported below by the fused silica substrate and encased
above by graphite/gold deposited via focused ion beam. (c) QD and
QD/J-aggregate thin film absorption spectra. (d) Photoluminescence
excitation spectra of QD and QD/J-aggregate films, collected at the
peak quantum dot emission (A= 620 nm). Excitation of the QD/Jaggregate film at A= 465 nm is enhanced by 2.5-times relative to a
film of QDs alone, and overlaps well with the emission of an InGaN
light em itting diode.
3-2
. . . . . . . . . . . . . . . . . . . . . . . . . . .
70
Photographs of QD and QD/J-aggregate thin films, taken with a A=
630 nm band pass filter to remove excitation light. (a) Films excited
using columated A= 457 nm illumination at the intersection of the two
films. The average, whole-film brightness enhancement from QD/Jaggregates is apparent. (b) Films excited using broadband UV source
centered at A= 360 nm. The near-identical brightness is consistent
with the spectral specificity of the J-aggregate. . . . . . . . . . . . . .
3-3
Fluorescence micrographs of thin films, excited at A=465 nm.
71
All
images depict emission that has been waveguided to the edge of the
film for (a) QD/J-aggregates; (b) Quantum Dots; (c) J-aggregates. The
enhanced quantum dot emission and quenched J-aggregate emission is
consistent with efficient energy transfer from J-aggregates to QDs. . .
3-4
72
Emission spectra taken at A= 455 nm excitation. The quenched Jaggregate emission and enhanced QD absorption in the QD/J-aggregate
indicates that efficient energy transfer occurs.
. . . . . . . . . . . . .
73
3-5
Tapping-mode atomic force micrographs for height (a, b, c) and phase
(d, e, f) of QD/J-aggregates, QDs, and J-aggregates. The measured
RMS roughness of these films are 6.43 nm, 1.46 nm, and 0.80 nm,
respectively, determined using WSxM.
3-6
. . . . . . . . . . . . . . . . .
74
Photographs of TC J-aggregates excited with A=360 nm broad band
illumination and a long pass filter (A=485 nm) to remove excitation
light. (a) Two TC J-aggregate films photographed immediately after
deposition in air, showing nearly identical brightness and hue. (b) Left
film stored in ambient air without light exposure; right film exposed
to ambient air and UV light for 1 h. The relative decrease in J-band
emission is apparent in the right film. (c) Left film stored in inert
air without light exposure, right film stored under N2 with UV light
exposure for 48 h. The brightness and color are largely preserved in
an inert environment. . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-7
74
Photographs of QD/J-aggregates using a long pass filter (A=600 nm) to
remove excitation light. (a) Two QD/J-aggregate films photographed
immediately after deposition in air, showing nearly identical brightness
when excited at A=360 nm. (b) Films excited at A=457 nm. Right
film stored in N2 without light exposure; left flim exposed to UV light
continuously for 48 h under N2. (c) Films excited at A=360 nm. Left
film was stored under N2 with continuous UV light exposure for 48 h;
right film was stored under N2 without light exposure.
3-8
. . . . . . . .
75
Summary of the spectral characteristics of TC J-aggregate thin films,
sampled at a range of concentrations and solvent compositions. Based
on both (a) maximizing ratio of J-aggregate to monomer absorption
intensity and (b) minimzing the the full-width at half maximum for
the J-aggregate peak, the most complete J-aggregate formation occurs when spin coating solutions with concentrations of 0.2 mg/mL
<[TC]<1.0 mg/mL from ~ pure TFE solvent. . . . . . . . . . . . . .
79
3-9
Determining the attenuation length of a thiacyanine J-aggregate film.
A linear fit is shown in both cases; see text for fit parameters.
(a)
Optical density at the A= 46 5 nm J-aggregate resonance, measured as
a function of [TC] in the TFE spin coating solution. (b) Average film
thickness measured as a function of [TC] in the TFE solution. ....
80
3-10 (a) Reflectance, (b) Transmittance, and (c) Absorption coefficients
measured at the TC J-aggregate maximum (A=465 nm) for deposition solutions with a range of [TC].
4-1
. . . . . . . . . . . . . . . . . . .
81
Overview of the J-aggregate/CdSe nanowire color-selective photodetector. (a) Schematic of J-aggregate-coated nanowires contacted between
electrodes. (b) Optical micrograph, showing interdigitated electrodes
used to grow and test devices. Faint brown shading between electrodes
is the sparse network of CdSe nanowires. (c) Detail of J-aggregatetemplated CdSe nanowires, with the structure of J-aggregating dyes 1,
2, 3, and 4. .........
4-2
.............................
86
Atomic force micrographs of a small area of a functional J-aggregate/
nanowire device. (a) CdSe nanowires before deposition of J-aggregates.
(b) Same area as in a, after deposition of J-aggregates.
4-3
. . . . . . . .
87
Current-voltage characteristic of J-aggregate 2/nanowire photodetector, in both the light and the dark. The photoresponse is recorded
near the J-aggregate absorption maximum (E=2.1 eV), and the device
shows Ohmic behavior both in the dark and light. . . . . . . . . . . .
4-4
88
Cross-sectional scanning tunnelling electron micrograph of a nanowire/Jaggregate device. The nanowire is lying on the Si0
2
substrate, with
graphite/Au deposited for focused ion beam processing. Energy dispersive x-ray spectroscopy indicates that the organic molecules template
onto the CdSe nanowire. . . . . . . . . . . . . . . . . . . . . . . . . .
88
4-5
Spectral characterization of J-aggregate/nanowire devices. (a) Absorption spectra of J-aggregates 1-4 and the respective monomers in dilute
methanol solutions. (b) Absorption spectra of J-aggregate/nanowire
devices templated with J-aggregates 1-4, shown with a bare CdSe
nanowire device. (c) Photoaction spectrum of a J-aggregate/nanowire
device, compared against the bare nanowire device.
The photocur-
rent is enhanced 2.8-fold. (d) Relative photocurrent enhancement as
a function of excitation energy due to J-aggregates 1-3. No photocurrent enhancement is observed for J-aggregate 4. Color-selectivity was
changed by removing one J-aggregate and depositing another. ....
4-6
90
Description of the processes governing J-aggregates/nanowire photodetection. (a) Schematic of J-aggregate/nanowire photodetector operation via energy transfer. (b) Schematic of J-aggregate/nanowire photodetector operation via electron transfer. (c) Relative photocurrent
enhancement as a fucntion of excitation energy due to varying thicknesses of J-aggregate 2 in a nanowire/J-aggregate photodetector. (d)
Relative change in photocurrent at the J-aggregate absorption maximum as a function of J-aggregate layer thickness, from (c). Solid curve
corresponds to the model in the text and fit with respect to a . . . . .
4-7
Demonstation of electrostatic self-asembly.
92
(a) Structures of com-
pounds 3 and 3c. The chromophores of the monomers are identical.
(b) Absorption spectra of 3 and 3c, as both J-aggregate films and as
monomers in dilute methanol solutions. Both compounds have similar
aggregation properties. (c) Relative change in the photocurrent density due to J-aggregates 3 and 3c, for J-aggregate/nanowire photodetectors. The negatively-charged 3 enhances photocurrent by 44%, and
the positively-charged 3c attenuates photocurrent by 70% at E=2.3
eV. The contrast between the two J-aggregate behaviors show the importance of electrostatic assembly for photocurrent enhancement by
energy transfer process. . . . . . . . . . . . . . . . . . . . . . . . . . .
97
4-8
Data demonstrating linear excitation power dependence. (a) Plot of
photoaction spectra at various light intensities. The similarity in the
spectra on the semilog plot demonstrates that the spectra are qualititavely similar across many excitation intensities. (b) Photocurrent
at the excitation maximum (E=2.1 eV) of the J-aggregate 2/nanowire
device. The photocurrent increases linearly with increasing intensity
of the incident light.
4-9
. . . . . . . . . . . . . . . . . . . . . . . . . . .
98
Plots of the relative change in photocurrent due to J-aggregates in the
J-aggregate/nanowire device. The enhancement is plotted as a function
of J-aggregate layer thickness, for several parameters. (a) Varying the
J-aggregate absorption length a from 0 nm-1 [blue] to 0.1 nm-
1
[red],
in 0.01 nm-1 steps. (b) Varying the F6rster radius, Ro, from 0 nm
[blue] to 20 nm [red] in 2 nm steps. (c) Varying nanowire absorption
fraction Aw, from 0.15 [red] to 0.95 [blue], in 0.1 increments . . . . .
5-1
101
Proposed mechanistic pathway for amine-inhibited InP synthesis. Both
the formation of outer sphere complex 1 and the irreversible formation
of intermediate 2 are inhibited by increased solvation. Due to the large
negative activation entropy and the large rate decrease with added
amine, a charge dispersion SN2 transition state is inferred for TMS-X
bond formation and P-TMS bond cleavage. . . . . . . . . . . . . . . . 104
5-2
'H NMR spectra of (a) (TMS) 3 P at 20 0 C before injection, (b) reaction mixture of In(MA) 3 and (TMS) 3 P in toluene-d8 three minutes after mixing at 25 C, and (c) reaction mixture of (TMS) 3 P and
6:1 OA:In(MA) 3 in 1,2-dichlorobenzene-d4 after heating at 178 C for
one minute. Both (b) and (c) demonstrate quantitative conversion of
(TMS) 3 P to TMS-MA shortly after the reactions are initiated. . . . .
105
5-3
(a) Time-resolved 1H NMR spectra at 40 C, showing evolution of
(TMS) 3 P and TMS-myristate (TMS-MA) resonances during standard
conditions for InP QD synthesis with amines.
Inset: Detail of 15-
minute 1 H NMR spectrum. The doublet at 0.26 ppm is assigned to 2.
(b) Concentration profiles of (TMS) 3P and TMS-MA protons, determined with 1 min resolution via the integration of spectra represented
in (a). (c) Concentration profiles of (TMS) 3 P for InP QD syntheses
with varying amine concentrations at 40 C, normalized at initial time.
The reaction rate decreases with increasing amine concentration; reactions without amines reached completion too rapidly to be resolved
with our method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-4
107
Integration of all 'H NMR spectral peaks of all identifiable TMScontaining species throughout the reaction. The overall concentration
of TMS protons does not change during the course of the reaction. . .
5-5
3 1P-iH
108
heteronuclear multiple bond coupling spectrum (HMBC) for the
InP reaction mixture at 25 C after 40 minutes. The horizontal axis
corresponds to the chemical shift for 31 P in ppm; the vertical axis is the
chemical shift of 1 H, also in ppm. The only cross-coupled resonances
correspond to (TMS) 3 P and intermediate 2.. . . . . . . .
5-6
'H-decoupled
31 P
. . . .
108
spectrum for the InP reaction mixture after 30 min
at 25 C. Only resonances for (TMS) 3 P (241.3 ppm) and intermediate
2 (256.8 ppm) are visible. Chemical shifts are referenced to 85% H3 PO4
(0 ppm)...... ........
5-7
110
..........................
Eyring plot for amine-based synthesis of colloidal InP QDs, with AHI =
51.9 t 1.3 kJ mol-1 and ASt = -126 ± 4 J mol
1
K- 1 . Each point
was taken in triplicate, and temperature error was negligible. . . . . .
5-8
1H
NMR spectrum of TMS-MA (0.303 ppm) in toluene-d8 referenced
to diphenylmethane (3.771 ppm). . . . . . . . . . . . . . . . . . . . .
5-9
1H
110
114
NMR spectrum of TMS-OA (0.132 ppm) in toluene-d8 referenced
to diphenylmethane (3.771 ppm). . . . . . . . . . . . . . . . . . . . .
114
5-10 Absorbance spectra of InP QDs grown in an NMR tube at 40 C for
60 minutes in the presence of octylamine. . . . . . . . . . . . . . . . .
5-11 Plot of (TMS) 3 P concentration vs.
115
rate (measured from the early
time approximation) with a fixed concentration of In(MA) 3 , 0.02 M at
25 C. The rate does not follow a clear order dependence in (TMS) 3 P
concentration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
5-12 Plot of In(MA) 3 concentration vs.
rate (measured from the early
time approximation) with a fixed concentration of (TMS) 3 P, 0.02 M at
25 0 C. The rate does not follow a clear order dependence in In(MA) 3
concentration...... . . . . . . . . . . . . . . . . . . . . . .
5-13 Evolution of TMS-MA concentration vs.
. . . 116
time measured for <15%
conversion as described above. This is the first replicate of 40 C data
for Eyring analysis. The slope, (5.12 mol L-1 min-) is the rate of
. . . . . 117
reaction........ . . . . . . . . . . . . . . . . . . . . .
5-14 Evolution of TMS-MA concentration vs. time measured for <15%
conversion as described above. This is the second replicate of 400 C
data for Eyring analysis. The slope, (5.46 mol L-1 min-') is the rate
of reaction. .........
. . . . . . . . . .
. . . . . . . . . . . . . 118
5-15 Evolution of TMS-MA concentration vs. time measured for <15%
conversion as described above. This is the second replicate of 40 0 C
data for Eyring analysis. The slope, (5.91 mol L-1 min-)
of reaction.. . . .
is the rate
. . . . . . . . . . . . . . . . . . . . . . . . . .
118
28
Chapter 1
Introduction
This chapter provides an introduction to J-aggregates, citations to reviews of semiconductor nanocrystals, and a summary of previous work to couple these two materials
together.
The chapter will conclude with the scope and outline of the remaining
chapters in the thesis.
1.1
What is a J-aggregate?
A J-aggregate is a specific supramolecular arrangement of molecular chromophores,
held together via non-covalent intermolecular forces, whose structure leads to the
strong coupling of the molecular transition dipole moments and a consequentially a
delocalization of the excited states.
The consequences of J-aggregation are best understood by considering the optical
properties of dyes. When preparing samples of organic chromophores, among the most
conspicuous observations is that there are clear differences between the absorption
spectrum of a dilute dye solution and the spectrum of an aggregate of the same dye
(e.g. in a thin film formed when the solvent evaporates). Compared to the dilute
solution, the aggregate often has a broader absorption feature and exhibits either a
red- or blue-shift. The broadening and peak shift are often attributable to vibronic
coupling or inhomogeneous broadening [1], and solvatochromatism
[2,
3], respectively.
However, Jelley [4] and Sheibe [5] independently reported in 1936 that specifically
Monomer
-. 8--J-aggregate
0.8
C
M
0
0.6-
~0.40.2-------
0.0-
200
400
300
500
600
Wavelength (nm)
Figure 1-1: Solution-phase absorption spectra of thiacyanine (TC) dye in methanol
(blue curve) and aqueous ammonia (red curve). The spectral signatures of the
monomer and the red-shifted J-aggregate forms are distinct.
prepared aggregates of pseudoisocyanine (PIC) [6] exhibit a large (>100 meV) redshift of the aggregate absorption band relative to a solution of the unaggregated dye
(the "monomer") absorption feature, and that the aggregate feature is significantly
narrower and more intense than that of the monomer. The resulting band exhibited
large deviations from the Lambert-Beer Law [7], and similar phenomena have been
observed for other organic chromophores'.
Generally the presence of an intense,
narrow, and red-shifted aggregate absorption feature is both necessary and sufficient
to demonstrate the presence of a Scheibe or Jelley aggregate, aggreviated as S- or
J-aggregates (Figure 1-1).2
Additionally, several other observations have been noted for J-aggregates of PIC,
either in 1936 or subsequently, and they are often significant properties of J-aggregates:
1) The red-shift of the J-aggregate fluorescence relative to the J-aggregate absorption is far smaller than for most molecular dyes. That is, J-aggregates often have
'For an excellent overall review of the supramolecular chemistry of J-aggregation, see [8]. Mishra
has also written an extensive, chemistry-centered review [7], and Kobayashi's book thoroughly reviews many aspects, especially the physics and applications of J-aggregates [9].
2
It is likely that events during the 1930s-40s contributed to the general adoption of "J-aggregate"
compared with "S-aggregate." This thesis will use the term J-aggregate throughout, but it is important to emphasize both the initial discovery [5] and subsequent studies [10, 8] by Scheibe.
a)
Monomer
transition dipole
b)
)
ci
/\
S
0N
-
S
Head-Tail
Parallel
.
Si
-
C1
N
03S
S03
so
J-band
H-band
c)
-5+
--+
6 -1 6+
*
Linear J-aggregate transition dipole
Figure 1-2: Schematic of the process of aggregation. (a) Structure of the thiacyanine
dye (TC) use extensively in Chapters 2-4. The monomer transition dipole is shown.
(b) Illustration of selection rules for two limiting cases of molecular aggregates. For
head-to-tail aggregates, only the red-shifted, J-type state is allowed. For parallel,
stacked aggregates, only the parallel, blue-shifted, H-type state is allowed. (c) Cartoon of the induced polarization effects for a linear aggregate of three dye molecules.
The net dipole moment of the aggregate exceeds that of the monomer due to the
longer polarization length, although no charges have transfered between constituent
molecules.
exremely small Stokes shifts. 3
2) The fluorescence quantum yield of a J-aggregate is larger than the fluorescence
quantum yield of the monomer [11].
3) The radiative lifetime of a J-aggregate is shorter than that of the monomer [12]
4) The electron transfer efficiency from a J-aggregate donor exceeds that from the
monomer [13].
5) Like other instances of self aggregating molecules, J-aggregate formation is
generally reversible [5, 4].
6) J-aggregation is often accompanied with the formation of an identifiable colloidal or crystalline condensed phase of the chromophore [5, 11, 4]
3
This small Stokes shift (or "resonance radiation") was the emphasis of Jelley's original 1936
communication.
A brief early history of the theory of molecular excitons can be found in a review
by Kasha et al. [14].
By 1958, contributions by Kautsky, Merkel, F6rster, Lewis,
Kasha, McRae, and others led to a theory based on a model of strongly-coupled
chromophores with weak electronic interactions, thus acting in the perturbative limit
for the monomer bond energies (i.e. coupling without the formation of chemical
bonds) [14]. In such a limit, the excitations are described as Frenkel excitons formed
through the coupling of transition dipoles on neighboring molecules [15, 16].
As shown in Figure 1-2a and supported by excited state density functional theory
calculations [17], the transition dipole of a cyanine dye lies along the conjugated
portion of the the molecule; thus, in the excited state, the difference between the
ground and excited state electron density is polarized along this direction. Depending
on the arrangement of the transition dipole vectors, there are two limiting cases in
which monomer states can strongly couple (Figure 1-2b, right side). If the molecules
align directly on top of one another, e.g. as dictated by r-stacking, then the transition
dipole vectors add in a parallel arrangement. Of the two new states that form from
such an interaction, only the state with a higher energy than the monomer has a
net non-zero transition dipole, 4 so this phenomenon is termed H-aggregation, as the
aggregate transition has a hypsochromic (blue) shift relative to that of the monomer.
J-aggregation arises from the other limiting case, which occurs due to head-to-tail
addition of transition dipoles (Figure 1-2b left side). This arrangement results in
the opposite selection rule from the parallel case: only the lower-energy state has a
non-zero vector sum, so the observed aggregate transition is red-shifted with respect
to that of the monomer.
A hypothetical material manifestation of a J-aggregate is shown in the cartoon
in Figure 1-2c. The close proximity of aggregated chromophores means that each
molecule can polarize and is polarized by its neighbors, which are aligned in compliance with the selection rules described above. Because the i-aggregate excited state
results from the mutual, directed excitation (and hence polarization) of the aggregated
molecules, the excited state now depends on the properties of the aggregate rather
4The net dipole is determined by taking the vector sum of the constituent states.
than the single monomers in isolation. In this limit, where the excited state spans
several molecular lengths without transfering electrons or forming covalent bonds,
the aggregate is described by a delocalized Frenkel exciton [16].' The delocalized polarization of N linearly coupled monomers effectively increases the transition dipole
moment of the J-aggregate relative to the transition dipole moment of the monomer
by a factor of N.
The increase in the J-aggregate dipole moment (relative to the monomer) has several consequences. First, the heterogeneous lineshape [19] should decrease by a factor
of Ni/ 2 due to exchange narrowing of the exciton on various different lattice sites.
For a transition dipole p, the radiative rate is proportional to the oscillator strength
p|2;
hence, the radiative rate should increase by N-fold relative to the monomer.
In practice, the orientation of J-aggregates is generally not linear, and molecules assume slip-stacked orientations that complicate the theoretical descriptions of spectra. 6
Hence, the aggregate transition dipole will be increased by somewhat less than a factor of N depending on the precise orientations of the transition dipoles. Although
coupling between molecules results in a band of many-body excited states (corresponding to different quantum numbers) the vast majority of the oscillator strength
from the ground state occurs to the lowest excited state in each band (81% to the
lowest exciton and 70% to the lowest biexciton, respectively) [12]. Because most of
the excitation fluxes used in this thesis are low, generally the contribution of the BX
will be ignored, and the J-aggregate will be considered as a two-level system.
In subsequent years, much work has gone into extending the theoretical understanding of J-aggregates. In particular, Knoester and colleagues have reported on
the role of energetic disorder [16, 20, 21], the coupling in various J-aggregate geometries [22], and other related theoretical questions that seek to relate the structure of
J-aggregating chromophores with their optical properties; these questions remain active areas of inquiry both for J-aggregates and for other coupled systems with similar
'Such an exciton is distinguished from disparite Frenkel excitons (e.g. in an inorganic crystal)
that propagate between lattice site by incoherent hopping [18].
6
For a more physical treatment of the supramolecular structure of J-aggregates, including examples for several families of dyes supported by crystallography, see Wiirthner's review [8].
photophysics [15, 23]. Furthermore, the ultrafast spectroscopy of J-aggregates and
related systems is an area with both considerable history and ongoing activity but
will not be discussed herein.
1.2
Why study J-aggregates?
Light harvesting processes have long captured the imaginations of scientists [24].
With the discovery of the photosynthetic light harvesting complex of the purple bacterium Rhodopseudommas viridis [25] and subsequent studies into light harvesting
by its bacteriochlorophyll (BChl) pigments [26, 27, 28, 29], it has become clear that
the earliest steps of photosynthesis depend on energy transfer involving molecular
aggregates that self-assemble and are extremely efficient at capturing photons [30].
Such light harvesting complexes are composed of extensive supramolecular structures
that include proteins and other attendent molecules. As a result, there is a strong
incentive to study and engineer model systems that display analogous photophysics
with the evolved light harvesting complexes, without the complexity inherent in the
full photosynthetic architecture.
In their central photophysics, J-aggregates have similar characteristics with the
light harvesting complex (LHC) of purple bacteria [31]. The spectra of both classes
of aggregates are well-described by a molecular (Frenkel) exciton Hamiltonian that
includes nearest neighbor coupling (see appendix) [15, 12, 29]. In both J-aggregates
[12] and LHCs [29], the spectral density is redistributed over the density of electronic states, indicating that coherent coupling between chromophores is an essential
feature of their physics [29]. The theoretical description for both J-aggregates and
LHCs shows that excitons are delocalized over several sites, which indicates that
the J-aggregated dyes are excited collectively [12, 29].7 However, J-aggregates can
self-assemble from constituent monomers using inexpensive chemical or physical treat7
For the J-aggregates used in most of this thesis, the features of interchromophoric coupling are
more pronounced than they are in the LHC. For example, the magnitude of the nearest-neighbor
coupling between J-aggregated dyes (-120 meV, [16]) is greater than that calculated for the chromophores in either the LHC B850 ring (~40 meV [29]) or the B800 ring (~3 meV [32, 33]), and the
delocalization length is longer in J-aggregates [12] than in the LHC [29].
ments, so it is more practical to study delocalized excitations in J-aggregates than in
photosynthetic light harvesting systems [34, 35].
Beyond light harvesting, systems with correlated optical and structural properties are plentiful; however J-aggregates are important model systems for additional
reasons.
J-aggregates have distinct, addressable states readily studied via optical
spectroscopic techniques, which also make them interesting for applications that require color sensitivity. Because a J-aggregate structure is closely correlated with its
optical spectra, and because optical characterization methods have high sensitivity
(achieving even single-molecule resolution), J-aggregates are an ideal system to study
the implications of non-covalent interactions, the emerging supramolecular structure,
and the resulting implications for excited states.
Finally, the short exciton lifetimes ([0]~10-" s) [12, 36] and long exciton diffusion
lengths in J-aggregates ([0]~10-7 m) [36] imply that excitons (and the attendant
energy and information content) can move through a coherently-coupled J-aggregate
with a speed that is comparable to the Fermi velocities of metals and far more rapidly
than drift velocities of electrons through a metal at room temperature [37].
1.3
How does J-aggregation occur?
Because the optical criteria for J-aggregation depends strongly on the intermolecular
forces between constituent dye molecules, much work has focused on understanding
and manipulating the assembly of dye materials such that J-type features result [38, 7].
Jelley noted in 1936 that rapid modulation of solvent polarity or temperature change
resulted in J-aggregates of PIC
[4],
and modification of this approach have been used
through the present [39]. For film photography, J-aggregates have been templated
onto silver halide (AgX) "grains" (or micro- or nanoparticles) [9, 13] and the coupling
of J-aggregate excitons to metallic plasmonic modes have been studied through other
such templating methods [40, 41, 42].
While the reversibility of the aggregation
process was useful in uncovering the physics of interaction, subsequent efforts have
focused on preserving the stabilities of J-agregates, e.g. either by incorporating them
within a host-guest type architecture [43, 44, 45, 46], an emulsion [47, 48] or other
colloidal dispersion [49, 50], or at a phase interface as a Langmuir-Blodgett film
[51, 52].
More recently, Yonezawa, Fukumoto, Bradley, Tichler, and Bulovid have
developed techniques for building well-defined layers of J-aggregate films via layer-bylayer self assembly based on electrostatic attraction [53, 54, 55]. In spite of substantial
work on studying the supramolecular ordering of chromophores [56, 48, 57], however,
the actual synthesis of J-aggregating dyes remains an empirical science [58, 59], and
the challenge of accurately predicting the appropriate conditions for J-aggregation
remains open.
1.4
Introduction to Quantum Dots (QDs)
As the physics of semiconductor nanocrystals, particularly colloidal quantum dots
(QDs), has been ably discussed in other theses [60, 61], the key points will be summarized briefly here. When the radius of a semiconductor nanocrystal is smaller
than the Bohr exciton radius for that material, the confinement potential of the
nanocrystal becomes the most significant effect on the electronic structure. Because
this confinement potential depends on the size of the nanocrystal, these nanocrystals
exhibit a quantum size effect in their energy level structure. Any detailed study of
the energy states of a nanocrystal quantum dot thus depends on the ability to sample a narrow range of nanocrystal sizes-either by investigating single nanocrystals
or by synthesizing samples with uniform size. The second of these two methods is
dependent on the analytical techniques associated with single particle studies; and
the first method is contingent on preparing a monodisperse sample of semiconductor
nanocrystals. Both of these strategies have been pursued succesfully by the Bawendi
Group in the past [62, 63], although there are still synthetic challenges associated
with the monodisperse synthesis of some semiconductor materials.
For more information on the synthesis of semiconductor nanocrystals, the reader is
directed to excellent recent reviews by Murray [64] and Rogach [65]. Single molecule
spectroscopy of nanocrystals has recently been reviewed in Marshall's thesis [66].
1.5
Coupling between J-aggregates and excitonic
inorganic materials.
J-aggregates, which were once widely used in film photography
[9],
have regained
interest due to their quantum mechanical coupling at ambient conditions, their nonlinear optical properties, and their delocalized excitations (described using a molecular
exciton model). Such Frenkel excitons [67] contrast with the Wannier-Mott excitons
characteristic of inorganic semiconductors, and J-aggregates have been combined with
various inorganic semiconductor materials to study the effects of coupling in these
hybrid systems [68].
This section reviews the progress to date on J-aggregates coupled with excitonic
inorganic materials. After summarizing the motivation and first proposals in the
use of these materials, the experimental realization of semiconductor quantum wells
and quantum dots coupled to J-aggregated dyes is discussed. An outlook on future
directions is then presented.
1.5.1
Planar structures: the first coupling between organic
and inorganic materials.
Organic dyes have been long-established in film photography to enhance the spectral
sensitivity [69] of silver halide (AgX) particles, both in their aggregated and unaggregated states. Understanding the basic physics associated with energy transfer
[70, 71] and electron transfer [19, 72, 13] thus constituted a major motivation for
research on J-aggregates.
Pioneering reports by Forrest [73] and Agranovich [74]
already noted the possibility of coupling between Frenkel and Wannier-Mott excitons,
but the delocalized excitation in a J-aggregate constitutes a further scientific and
engineering challenge.
In 2007, Zhang et al. reported the coupling of J-aggregates with semiconductor
nanocrystals, the first example of energy transfer between J-aggregates and an inorganic semiconductor [75]. The photonic structures used for the experiments were com-
posed of films made via layer-by-layer self-assembly [53, 54, 76]. The CdSe(ZnCdS)
nanocrystals in these experiments were coated with thioglycolic acid, which conferred
both water solubility and a negative charge to the nanocrystals. J-aggregates in this
study were composed of negatively-charged TDBC. To associate the two negativelycharged excitonic materials, a thin (1.7 nm) layer of transparent, positively-charged
polyelectrolyte PDDA was deposited in an alternating fashion before either the Jaggregates or the nanocrystals [75].
Initially, Zhang et al. discussed films in which the nanocrystals had a larger band
gap than the J-aggregate [75].
These nanocrystal/J-aggregate films demonstrated
>95% energy transfer efficiency from the QDs to the J-aggregate, and resulted in
a gain of the J-aggregate emission intensity by a factor of 10. Because the band
gap of the nanocrystals exhibits a quantum size effect, the photonic device could
change entirely by fabricating a different photonic structure that used larger quantum dots. In the new structure, the J-aggregate had a larger band gap energy than
the nanocrystal, thus reversing the direction of energy transfer. For the second structure, energy transfer from the J-aggregate donors resulted in a photoluminescence
enhancement in the nanocrystal acceptors. The energy transfer from the J-aggregate
films to the nanocrystals resulted in a 10-fold increase in the photoluminescence intensity compared to layer-by-layer films of quantum dots alone when illuminated at
the J-aggregate absorption maximum.
The ability to tune the spectral properties of J-aggregates was a major advance.
As semiconductor nanocrystals can be excited at any energy greater than their band
edge, the use of nanocrystal donors allows a J-aggregate/quantum dot system to be
pumped with a broad range of excitation wavelengths, while retaining the narrow,
ultrafast emission of the J-aggregate. In the opposite direction, the quantum dot could
be pumped in a narrow excitation band defined by the J-aggregate, while preserving
the size-tunable emission characteristic of nanocrystals.
Soon afterward, Chanyawadee et al. demonstrated that a two-dimensional semiconductor quantum well could be used to transfer energy to J-aggregates of the cyanine dye U3 [77]. The photonic structure consisted of GaAs/AlGaAs single quantum
wells fabricated by molecular beam epitaxy, and the J-aggregates were deposited via
spin-casting directly onto the top-most AlGaAs spacer layer directly from methanol.
While spin casting had been used earlier by Tani et al. to deposit a range of Jaggregating dyes [78, 79], the report was the first demonstration of a non-aqueous
method for the deposition of J-aggregates into a hybrid organic/inorganic structure.
Because the quantum well photoluminescence peak position depends on the thickness of the GaAs layer, the device architecture offers a convenient method to vary the
spectral overlap while largely maintaining the separation between the quantum well
donor and the J-aggregate acceptor. On the basis of transient reflectivity measurements and time correlated single photon counting, Chanyawadee et al. constructed
a kinetic model that fully accounted for the energy transfer from the QW to the
J-aggregates. As noted by Chanyawadee et al., the stacked device architectures have
potential in device applications, where both the benefits of J-aggregation and of inorganic semiconductor structures can be gained. Many other device architectures are
also possible, depending on the nature of the link that mediates that organic/inorganic
conjugation.
1.5.2
Nanocrystal/J-aggregate constructs in solution
Until this point, the two reported device structures for energy transfer from inorganic semiconductors to a J-aggregate layer have been arrayed in a parallel planar
geometry: either energy transfer from a quantum well or a layer of semiconductor
nanocrystals (quantum dots) arranged in a two-dimensional structure. However, the
three-dimensional quantum confinement of semiconductor nanocrystals suggested the
possibility that discrete quantum dots (QDs) could be coupled to J-aggregates, forming QD/J-aggregate constructs that could be made and studied in solution. Such
a solution-phase QD/J-aggregate construct would allow the direct examination of
the photophysical interactions at organic/inorganic interfaces, as the QD/J-aggregate
constructs in the dilute solution limit are too isolated from one another for long range
transport to occur. The relative isolation between QD/J-aggregate constructs is verifiable through dynamic light scattering experiments, which also provide information
about the arrangement of QDs and J-aggregates in solution.
QD/J-aggregate constructs in solution also have potential applications in biological labeling and fluorescence multiplexing applications [80]. The maximum spectral
density of color channels varies inversely with the emission linewidth, so the narrow
J-aggregate emission (about 50 meV full width at half maximum) allows an increased
number of different color channels to be used in a given spectral range. Practical
multiplexing depends on the use of a single excitation source for all of the different
color channels, so it is beneficial to impart a broad absorption spectrum to the Jaggregate in solution. By tuning the nanocrystal (donor) emission to overlap with
the J-aggregate (acceptor) absorption, efficient energy transfer from the nanocrystal to the J-aggregate thus results in nanocrystal/J-aggregate constructs that have a
broad nanocrystal absorption while retaining the lineshape of J-aggregate emission.
These QD/J-aggregate constructs in solution were first reported by Halpert et
al. [81]. Using an amphiphilic polymer synthesized from a poly(acrylic acid) backbone [82], CdSe(ZnCdS) QDs were rendered water soluble while preserving the native
quantum yield of the underlying core(shell) inorganic nanocrystal. The negatively
charged carboxylate groups on the polymer were used to associate the QD with the
J-aggregates of the positively-charged dye TTBC.
The resulting structures were composed of 100-200 dye molecules per nanocrystal,
and the amphiphilic polymer ligand also induced the formation of aggregates. Such an
effect differs from the methods of inducing J-aggregate formation using amphiphiles
above the critical micelle concentration, as the outward ligand functional groups also
biased the formation of J-aggregates. Although suspensions of TTBC J-aggregates
in aqueous solutions span several orders of magnitude in physical size [81], light scattering measurements on the nanocrystal/J-aggregate constructs demonstrated that
there was a uniform 70% increase in size compared to the water-soluble nanocrystals
alone.
The nanocrystal/J-aggregate constructs also demonstrated efficient energy transfer from the nanocrystals to the J-aggregates (>99%), as observed in J-aggregate
luminescence and in QD quenching. The measured energy transfer rate was con-
sistent with an energy transfer mechanism between two coupled transition dipoles.
Due to the nature of the distinct, addressable excited states in the QD [83] and the
J-aggregates [12] the energy transfer between them constitutes a model process for
coupling in hybrid systems.
1.6
Challenges addressed in this thesis
With the current prominence of digital photography, photovoltaics, and LEDs, the
properties of J-aggregates suggest their application to optoelectronic devices [84].
Such applications could utilize the J-aggregates' ultrafast radiative lifetime, their
narrow emission, their intense absorption, or their small Stokes shift. In spite of
our increasingly detailed understanding of biological systems noted above, and the
increasing sophistication with which devices can make use of supramolecular interactions [85] there have been few attempts toward designing synthetic light harvesting
schemes that utilize the physics of delocalized Frenkel excitons-most notably by Kim
et al. [86], Walker et al. [87], and Ma et al. [88].
In practice, several challenges have hindered the application of i-aggregates to
optoelectronics. First, they are current insulators. Several time of flight studies on
evaporated i-aggregate films have demonstrated that both electron and hole mobilities are low (10-6 to 10-4 cm 2 V-1 s-1) [89]. Hence, any device application requires
interfacing the i-aggregates with another material. Although the energy transfer from
inorganic species to i-aggregates is an efficient process [75, 77, 81], energy transfer
from organic dyes to nanocrystals is generally less efficient, [75, 90] and thus required
a redesign of the ligand used to mediate the conjugation.
Additionally, the aggregation process depends strongly on the macromolecular
structure of constituent chromophores, as described above, and precedent has dictated
the use of aqueous processing to induce i-aggregation. However, the incorporation
of J-aggregates into other electronic architectures is often incompatible with aqueous
processing, and hence there was a demand for non-aqueous deposition of J-aggregate
films. In particular, it is advantageous to have processing methods for i-aggregates
that are compatible with excitonic semiconductors, such as those described in the
preceding section.
These challenges are addressed during Chapters 2-4. To take advantage of the
intense absorption of the J-aggregate, a new ligand for quantum dot/J-aggregate
conjugation was developed. The resulting organic/inorganic materials were studied
in solution in Chapter 2, and demonstrate near-unity energy transfer efficiency from a
coherently coupled antenna system to a semiconductor nanocrystal. Additionally, the
selective formation of J-aggregates in the presence of ligand-substituted quantum dots
indicates that there is a strong templating effect of J-aggregates at the nanocrystal
surface.
Chapter 3 describes the studies associated with incorporating quantum dot/Jaggreate constructs into blended films. In this architecture, the J-aggregate structure
is preserved, and the energy transfer efficiency is maintained through the blended organic/inorganic film that is assembled using non-aqueous solutions. Thus, the quantum dot/J-aggregate blended film serves as a proof of concept for complete device
fabrication using nanocrystals and J-aggregates.
In Chapter 4, the same conjugation chemistry and non-aqueous processing was
used to template i-aggregates onto the surface of semiconductor nanowires, grown
electrocatalytically between electrodes. The resulting J-aggregate/nanowire photodetectors constitute a new device design, where energy transfer from the i-aggregates to
the nanowires leads to photocurrent enhancement in the i-aggregate absorption band.
The self-assembly of the i-aggregates is both reversible and general for i-aggregates
at three different wavelengths (red, green and blue), and the photodetectors demonstrate color-selectivity by removing one J-aggregate and depositing another on the
same underlying device. The versatility of the templating and energy transfer sensitization process suggests a potential application of these i-aggregate/nanowires for
sub-diffraction limit color imaging. By incorporating i-aggregates as interchangeable
antennae with delocalized excited states, capable of self-assembly and efficient energy
transfer, the device design thus replicates some of the characteristics of a biological
light harvesting complex.
For semiconductor nanocrystals, synthetic methods to produce uniform-sized particles are vital to both basic studies and applications (as discussed above). Although
optimized syntheses exist for CdSe and PbSe nanocrystals, the interest in eliminating lead- and cadmium-containing semiconductor materials has motivated the development of III-V nanocrystal syntheses (indium arsenide and indium phosphide)
[91, 92, 93, 62, 94]. However, the existing syntheses for III-V nanocrystals produce
highly polydisperse products, and the synthetic mechanism had not been explored on
a molecular level. The kinetic study of indium phosphide nanocrystals is the subject
of Chapter 5, and the study yields both a molecular mechanism and elucidates the
most significant barriers toward a controlled InP nanocrystal synthesis.
1.7
Conclusions and outlook
J-aggregate hybrid materials continue to have potential to yield new insights into
light/matter interactions, as well as interactions between disparate materials, and
there are several promising areas engendered by recent progress. As already observed
in the studies above and elsewhere, the extent of the J-aggregation process itself
serves as a probe of the dye environment. In an environment that promotes close,
ordered packing the i-aggregate spectral features become far more apparent, and the
intensity of the i-aggregate resonance thus provides an optical indicator of local order.
For example, such a localized probe was used with AgNO 3 solution [57] to oxidize
the outer wall of a double-walled i-aggregate, while retaining the resonance from the
inner wall
[95].
This selective transformation demonstrated that the inner and outer
walls were electronically isolated from one another, thus helping to resolve the nature
of electronic coupling in double-walled J-aggregates [96].
In recent years, new excitonic couplings with i-aggregates have been explored with
many inorganic materials. Because the delocalized Frenkel excitons in i-aggregates
have notable analogies with the excitations in light harvesting photosynthetic systems
[97],
i-aggregate/inorganic structures may serve as synthetic model systems in which
rapid energy transfer and charge carrier separation are distributed among different
materials. Having already achieved practical use in photographic technology, there
is promise for the use of J-aggregates and coupled inorganic excitonic materials in a
new generation of optoelectronic devices.
1.8
Appendix to Chapter 1
1.8.1
Frenkel Exciton Hamiltonian
The Frenkel exciton Hamiltonian, H, is defined for a linear aggregate using a tightlybound excitation characterized by both the site energy (hw) and an exchange term
(J) with the nearest neighbors:
h
wi
li)
(il +
J(i)
(i + 1| +
li
+ 1) (il)
The eigenvalue energies associated with this Hamiltonian are described in detail
elsewhere [15, 12]. Although these energies correspond to particle-in-a-box states, in
practice the lowest energy state dominates both the X and BX manifolds. Generalizing the X manifold from the J-dimer depicted in Figure 1-2, the overall X manifold
has an equal density of states above and below the monomer state. The net energetic
stabalization of the J-aggregate absorption is J = Emonomer - Ej-aggregate.
Thus,
the degree of spectral red shift of the J-band relative to the monomer indicates the
excitonic coupling energy of the aggregated dyes.
1.8.2
Lineshape narrowing in linear J-aggregate chains
The narrow lineshape of a J-aggregate is related to the delocalization of the exciton
over many different environments. If the inhomogeneous broadening of an absorption
lineshape generally occurs as a result of large number of linearly-distributed random
fluctuations, then this broadening can be approximated by a Gaussian, and the energy
of the Frenkel exciton at any particular lattice site (i.e. a monomer in the aggregate)
is an uncorrelated Gaussian variable [16].
Then the variance of the sum (i.e. the square of the broadening in the aggregate,
var[E]) is the sum of the variance in energy at each lattice site (E):
var
var[r[
ar[ N
11
Nc
NNN
1var
E]]=2
- Nvar[E1
2N ar[E]
[E]
e
(1.2)
cN
Hence, if the inhomogeneous broadening in the J-aggregate and monomer are proportional to
var[E] and
var[E], respectively, then the ratio of the inhomogeneous
linewidths is proportional to /N. In practice, both the line-shape narrowing and radiative lifetime shortening indicate that the J-aggregates relevant to this thesis have
delocalization lengths of 16 molecular units [12, 98]. A single cyanine molecule is
~ 0.8 nm in length, hence the effective dipole separation for a linear J-aggregate is
~ 12 nm.
Because this excitation dipole separation distance is comparable to the donoracceptor distance for the nanocrystals and J-aggregates, the consequences of this
short donor/acceptor separation may be interesting for further investigation.
46
Chapter 2
Narrowband absorption-enhanced
Quantum Dot/J-aggregate
Conjugates in Solution
2.1
Introduction
This chapter discusses the narrowband absorption enhancement of colloidal quantum
dots (QDs) through efficient F6rster Resonance Energy Transfer from J-aggregated
cyanine dyes. These J-aggregate donors are electrostatically associated to the QDs
in solution via surface ligands.
As mentioned in the preceding chapter, coherent coupling of molecules in a J
aggregate leads to narrow, intense, absorption feature [16], so that a thin film of
J-aggregates can attenuate the same fraction of incident light (at the J-aggregate
absorption maximum) than a far thicker film of unaggregated dye [99].
Efficient
energy transfer from a J-aggregate donor to a QD acceptor results in an enhanced
QD
absorption cross section-in the J-aggregate absorption feature-while retaining
tunable emission. Such narrowband-sensitized QDs may have applications in down'Reproduced with permission from Walker, B. J., Nair, G. P., Marshall, L. F., Bulovi6, V. and
Bawendi, M. G. Journal of the American Chemical Society, 2009, 131, 9624-9625 [98]. Copyright
2009 American Chemical Society.
Cl/Q
s
\/CI
S
I @
.N
C1
N
(CH2)3SO3' Na
0 3S(H 2C) 3
\/
N
Oci
Thiacyanine dye (TC)
TTBC
HS
CI
C
N
N(Me) 3CI
mta
C
N
D
(C H2)3S0 3
B
BIG
(CH2)3SO3 Na
HS,-
SO 3Na
mps
Figure 2-1: The structures and abbreviations of organic dyes and ligands used in
Chapter 2.
conversion LEDs or in engineering hybrid optical structures in thin films [100, 1011.
Others have studied energy transfer between films of J-aggregates and QDs
[75]
and between films of different J-aggregates [102, 103] Previously energy transfer was
reported from QDs to associated J-aggregates in solution [81].
The QD emission
spectrum had a large spectral overlap with the dye absorption, and the energy transfer
rate was increased through the presence of many acceptors. Thus efficient FRET
occurred even though the QDs and J-aggregates were separated by long ligands or
amphiphilic polymers. As in the case of the layer-by-layer films, the roles of donor and
acceptor materials can be reversed so that there is resonant energy transfer from a
higher energy J-aggregate to a lower-energy-emitting QD. The present report details
the fabrication of a QD/J-aggregate conjugate in solution that exhibits energy transfer
from J-aggregates to a QD with transfer efficiency approaching unity.
d~
Figure 2-2: Representative transmission electron micrograph of CdSe/CdZnS QDs,
with an average radius of 3.6 nm.
2.2
Discussion
In this work, a negatively charged, 472 nm-emitting thiacyanine J-aggregate (TCJ,
Figure 2-1) and CdSe/ZnCdS QDs (635 nm emission) are used.
Energy transfer
from organic dyes to nanocrystals is generally inefficient [90], and thus any efficient
process required a redesign of the ligand used to mediate the coupling. The large,
core/shell QDs in this study have a greater absorption cross section than smaller QDs,
but their size also leads to a larger donor/acceptor separation. To compensate for
this separation and to promote electrostatic attraction, the short ligand 2-mercaptoN,N,N-trimethylethaneaminium chloride (mta, Figure 2-1) was synthesized. 2 Ligand
exchange with mta confers a pH-insensitive positive charge and water solubility, while
avoiding spurious reactivity with the QD surface. Dynamic light scattering (DLS)
gave a hydrodynamic radius of 4.3 ± 0.4 nm, a small increase over the nanocrystal
radius (3.6 nm by TEM, Figure 2-2).
The absorption spectrum of the QD/dye sample showed that both constituents
2
1n practice, it is often easier to achieve a high energy transfer efficiency by decreasing the donoracceptor separation than through any other controllable parameter, such as the quantum yield of
the donor or the spectral overlap between the donor emission and acceptor absorption. Among the
ligand platforms used to date, several [104, 105, 106] have been designed such that they achieve
optimal energy transfer efficiencies.
0.4-
03s5045
550s
650
Wavelength (nm)
Figure 2-3: Absorption spectra in DI H2 0. The spectral features of both QDs and
TC monomers are discernable in the QD/J-aggregate conjugate sample, as is the new
J-aggregate feature at 467 nm.
Excitation Wavelength (nm)
Figure 2-4: Excitation spectra taken at 635 nm emission. The J-aggregates contribute
only when conjugated to QDs.
E
N
"0
E I
-
0
J-aggregate
1.0 -
~50
El
L
0
o
04
480
500
20
540
Wavelength (nm)
.2 -0 0.5
LU
a)
565
595
625
655
685
Emission Wavelength (nm)
Figure 2-5: Emission spectra taken at 460 nm excitation. The integrated emission
of QD/J-aggregate is 5x greater than for QDs alone. Inset: Emission spectra, renormalized to J-aggregate emission. In the QD/J-aggregate the 472 nm emission is
indistinguishable from baseline, consistent with near-unity energy transfer efficiency.
were present (Figure 2-3). Because the dye requires electrolytes to aggregate in aqueous solution and because unbound electrolytes have been removed, the J-aggregate
feature at 465 nm indicates that the mta-QD surface acts as a template for Jaggregation. The excitation spectrum collected while monitoring emission at 635
nm (Figure 2-4) has similar features to the absorption spectrum.
Both absorp-
tion and excitation of QD/J-aggregate conjugates are 5x greater at the J-band than
the corresponding QD spectra, demonstrating narrowband absorption enhancement
(FWHM= 13 nm).
TCJ emission is negligible at 635 nm, so excitation from J-
aggregate absorption enhances QD emission, as expected for F6rster Resonance Energy Transfer (FRET).
The integrated emission near 635 nm of QD/J-aggregate conjugates was 5x greater
than for QDs alone, consistent with the excitation spectrum and enhanced QD emission (Figure 2-5).
Furthermore, emission at 472 nm for the QD/J-aggregate was
attenuated by >99% relative to emission of J-aggregates alone (Figure 2-5, inset). A
photograph of QD and QD/J-aggregate samples illuminated near the J-band qualitatively confirms narrowband enhancement (Figure 2-6).
Absorption/re-emission cannot account for the increase in QD emission, as the
QD/J
OD
Figure 2-6: Photograph of QD/J-aggregate and QD alone at identical QD concentration, illuminated at 457 nm light using a band pass filter [630 ± 15 nm] to remove
excitation light.
same J-aggregate emission quenching and QD emission enhancement was observed
using a cell with a 90% shorter path length. In this short cell the QD optical density
is 0.02 at 465 nm, so the quenched donor emission and enhanced acceptor emission
point to near-field energy transfer and are consistent with FRET with >99% efficiency.
The transfer efficiency was verified via transient photoluminescence measurements
at the TCJ emission (472 nm). For the J-aggregate alone, a two-parameter fit [12]
indicates that the dominant decay pathway has a 308 t 13 ps lifetime (Figure 2-7).3
The 472 nm emission of the QD/J-aggregate had a lifetime of about 6 ps, (Figure
2-7a), a 48-fold increase in the exciton depopulation rate over TCJ alone. Efficient
energy transfer is more rapid than the donor's radiative lifetime, and this rate corresponds to a 98% energy transfer efficiency from J-aggregates to QDs. Thus, the
results from transient and steady-state emission spectra are consistent to within about
1%.
Our results were compared to the predictions of F6rster theory to assess its applicability [107]. For the measured TCJ quantum yield, QD absorption cross section
3
This is longer than excitons in TCJ films, and similar differences have been observed for other
J-aggregates, see Refs [7, 75].
E
C
e QD/J-aggregate
fit, T = 6 ps
Instrument
response
-
cU
50
100
Time (ps)
E
C
0*
0
C0
.oS
5
eJ-aggregate
-
'~ ,
200
500
600
fit, '= 308 ps
800
Time (ps)
Figure 2-7: Time-resolved emission decays taken at 472 nm. (a) QD/J-aggregates,
fit with a convolution of the instrument response (shown) with an exponential decay
function. This fit accounts for 97% of the exciton depopulation. (b) Thiacyanine
J-aggregates, fit using an exciton-exciton annihilation model.
a)
b)
E E
1.0
--
QD/J-aggregate '
0
0.8
-
J- aggregate
E
E 1.0
0.6
E
0.8
0
0.6
0.4
V)
/J-aggregate
~~
J-aggregate
0.4
0.2
0.2
600
620
00
640
660
680
580
Emission Wavelength (nm)
c)
600
620
640
660
680
Emission Wavelength (nm)
d)
0.4
1.0
- 0D/J-aggregate
-CD
agreat
-J
0.3
-
-
0.8
aggregate
I
0Irga
QD-Q
ag
aggregate
.e0
0.6
0
0.2-
0.4
0
0.1
rnQ/<rgr
0.2
E
0
300
400
500
600
700
Wavelength (nm)
800
950
400
450
500
550
600
Excitation Wavelength (nm)
Figure 2-8: Steady-state optical characterization of QD(-)/J-aggregates (+). (a)
Emission spectra with 570 nm excitation. Spectrally integrated emission of QD/Jaggregate is 4x greater than for QDs alone. (b) Emission spectra, renormalized to
J-aggregate emission. J-aggregate was taken in electrolyte solution to maximize aggregation. In the QD/J-aggregate the 595 nm emission is reduced by 82% relative
to the emission of J-aggregates alone. (c) Absorption spectra. The spectral features of both QDs and J-aggregates are discernable. Note that residual monomer
is suppressed in the QD/J-aggregation construct relative to the J-aggregate in H2 0.
(d) Excitation spectra taken at 635 nm emission. The J-aggregates contribute little
unless conjugated to QDs.
QD
QD/J
Figure 2-9: Photograph of QD(-)/J-aggregate(+) and QD alone at identical QD
concentration, illuminated with 568 nm light. A band pass filter [630 ± 15 nm] was
used to remove excitation light.
and spectral overlap, the Fbrster radius for this donor-acceptor pair is 8.8 nm (see
appendix). A 98% FRET efficiency would occur for donor-acceptor separations of 4.6
nm. This is identical to the hydrodynamic radius of mta-QDs within error (4.3 ±
0.4 nm) and is consistent with energy transfer predominantly occurs from the closest
possible point, adjacent to the quantum dot surface.
This short donor-acceptor distance is reasonable due to the delocalized nature of
J-aggregate excitation. J-aggregates consist of segments of coherently-coupled dye
molecules, and these segments transfer energy between one another [9, 108]. Even
if the J-aggregates extend far beyond the QD surface in a construct, an exciton can
transfer within the aggregate until it comes close to the quantum dot acceptor. The
short mta ligand length allows J-aggregates to approach the QD surface more closely
than in previous studies [75]. These aggregates act like narrow band optical antennae
through rapid, efficient FRET.
To demonstrate the generality of our method, negatively charged QDs were conjugated to positively-charged J-aggregates that emit at 594 nm, TTBC (Figure 2-1).
After ligand exchange with sodium 3-mercapto-1-propanesulfonate (mps, Figure 2-1),
the anionic QDs were associated with TTBC J-aggregates in solution. DLS data of
these QD/J-aggregate conjugates showed species with a well-defined hydrodynamic
radius (4.2 nm), and the TTBC J-aggregates add little to the radius of mps-QDs in
SE
5
.N
E
1.2-
Lfn
" II
0.8-
E
LU
c
0.4-
0'
605
625
645
665
685
Emission Wavelength (nm)
Figure 2-10: Control experiment of mps-QDs combined with BIC. There is no significant difference in the emission intensity at 635 nm for the combined J-aggregate(-)/
QD(-) sample [black] relative to the algebraic sum of the isolated sample spectra [red],
indicating that efficient energy transfer does not occur from J-aggregate to QD. This
lack of emission enhancement indicates an absence of energy transfer and therefore
an absence of close QD/J-aggregate assembly.
solution (4.0 nm). In this pair, QD emission was enhanced and J-aggregate emission
was quenched by 82% relative to J-aggregates alone. Incomplete energy transfer is attributed to decreased spectral overlap of TTBC emission with QD absorption relative
to TC emission with QD absorption (Figure 2-8, Figure 2-9).
In a control experiment, negatively charged mps-QDs were combined with a different negatively-charged J-aggregating dye, BIC (Figure 2-1) [109].
Excitation of
this sample near the J-band shows no significant enhancement of QD emission, and
it is evident that the QD and J-aggregate were not associated (Figure 2-10).
2.3
Conclusion
In conclusion, QDs were used as templates for J-aggregation in solution and demonstrated rapid FRET with near-unity efficiency from cyanine J-aggregates to associated
QDs.
The J-aggregates serve as narrowband optical antennae and enhance the QD
absorption by five-fold at the J-aggregate absorption band.
2.4
2.4.1
Appendices to Chapter 2
Synthetic Procedures
Chemicals. All reagents were used without additional purification unless noted. The
dye Benzothiazolium, 5-chloro-2-[[5-chloro-3-(3-sulfopropyl)-2(3H)-benzothiazolylidene]
methyl]-3-(3-sulfopropyl)-, inner salt, sodium salt (thiacyanine, or TC) was purchased
from Charkit Chemical Corporation; (catalogue no. NK 3989). 1,1',3,3'-tetraethyl5,5',6,6'-tetrachloro-benzimidazolocarbo-cyanine (TTBC, NK 2156) and 5,6-dichloro2-[3-[5,6-dichloro-1-ethyl-3-(3-sulfo-propyl)-2(3H)-benzimidazolidene]-1-propenyl]
-1-
ethyl-3-(3-sulfo-propyl) benzimidazolium hydroxide, inner salt, sodium salt (BIC, NK
2203) were purchased from Charkit Chemical Corp. The following were purchased
from Sigma-Aldrich: Trioctylphosphine oxide (TOPO, 99% and technical grade),
1-dodecanal (DDA), hexadecylamine (HDA), selenium shot, hexamethyldisilathiane
[(TMS) 2S], decylamine (96%), acetylthiocholine chloride (99%), 3-merpcapto- 1-propanesulfonate (90%), mps. Butanol, methanol, hexane, and chloroform (CHCl3 ) were purchased from VWR. Trioctylphosphine (TOP, 98%) and cadmium 2,4-pentanedionate
(99%) were purchased from Strem. Hexylphosphonic acid (HPA, 99%) and tetramethylammonium hydroxide pentahydrate were purchased from Alpha Aesar. Diethyl
zinc and dimethyl cadmium were purchased from Strem and filtered through a 200
nm pore syringe filter before use. Trioctylphosphine selenide (1.5M TOP-Se) was
prepared by stirring selenium shot in trioctylphosphine overnight.
Quantum Dot core synthesis. Colloidal CdSe quantum dots (QDs) were synthesized according to a 20-fold scale-up of a published procedure [110]. Briefly, a 1 L
roundbottom flask was charged with 62 g of TOPO (99%), 62 g of TOPO (90%),
115 g of HDA, and degassed at 160'C for over 12 h. The solution was transferred
to a dry flask fitted with a mechanical stirrer and a Teflon adapter for a pneumatic
caulking gun. This caulking gun permits the injection of large (>300 mL) fluid volumes within 2 s, and both the gas inlet and fluid outlet adapters were designed to
preserve an air-free environment. The injection flask was heated under Ar to 360 C.
In a 500 mL round bottom flask, a solution of 6.4 g of cadmium 2,4-pentadionate, 10
ml of DDA, and 160 ml of TOP was degassed at 110 C. After cooling, 40 ml of 1.5
M TOP-Se was added to the bright yellow solution in an inert air glove box and the
solution was transferred to the cartridge of a caulking gun. The gun was attached to
pressurized N2 , the precursor solution was injected into the flask at 360 C, and the
reaction was allowed to cool to room temperature.
CdZnS shell growth. A 50% alloyed CdZnS shell was grown according to known
procedure [111]. A 4-neck flask containing a solution of 10 g TOPO and 0.4g HPA
was degassed (160 C at 150 torr, 2 h), before cooling it to 50 0 C. A small fraction
(1/40) of the CdSe QDs from the previous step, or 200 nmol of QDs were precipitated
with butanol/methanol, redispersed in hexane, and filtered through a 200 nm filter.
The QD solution was injected into the 4-neck flask and the hexane was removed by
vacuum for 1.5 h. The flask was then heated under Ar to 100 0 C. Decylamine (0.3
mL) was added, and after stirring for 1 h the solution was heated to 160 'C. Two
solutions were added simulataneously at a 1.5 mL/h syringe pump. The first solution
consisted of 36 mg dimethyl cadmium, 36 mg diethyl zinc in 5 ml of TOP and the
other contained 90 mg (TMS) 2S in 5 ml of TOP. The reaction mixture was cooled
and QDs were precipitated by adding butanol and methanol. QDs were resuspended
in chloroform, and quantum yield of the completed QDs in chloroform was found to
be 60% compared to DCM 640 (QY= 44% in ethanol).
Preparation of 2-mercaptoethyl-NN,N-trimethylammonium chloride (mta). The
following procedure is adapted from Kamper et al. [112]. Acetylthiocholine chloride
(4 g, 5.4 mmol) was dissolved in 1 M HCl (20 mL) and refluxed at 110'C under inert
atmosphere. After 10 h, the reaction was suspended and acetic acid, HCl and H2 0
were removed via vacuum distillation, leaving a white solid. After recrystallizing from
methanol, the yield was 70%. TLC (2:1 CHCl3:MeOH): Rf = 0.23; 1H NMR (500
MHz): 3.50, 3.11, and 2.91 ppm. See Figure 2-11.
Ligand exchange with mta. The procedure is similar to that outlined by Liu et
al. [104]. Briefly, 50 mg of mta was dissolved in 1 mL of phosphate buffer (pH=6.0).
A solution of CdSe/CdZnS QDs (0.7 mL, 2.5 pmol/L) was precipitated once from
growth solution, resuspended in CHCl 3 , filtered through a 200 nm PTFE filter, and
added to the buffered mta solution. After stirring vigorously for 1.5 min at room
temperature, the QDs had transferred into the aqueous phase.
The phases were
separated and residual CHCl3 was removed from the aqueous phase with vacuum.
QDs were preciptitated by adding EtOH and resuspended in DI H2 0. Dialysis (30k
cutoff Millipore Amicon Ultra filter) and filtration (200 nm Acrodisc, Tuffryn filter)
removed excess mta and aggregated QDs, respectively. The quantum yield for these
water-soluble QDs was 10% compared to rhodamine 640 (QY
=
100% in ethanol).
Ligand exchange with 3-merpcapto-1-propanesulfonate (mps). The procedure is
identical to that described above, except that 50 mg of mps in 1 mL of phosphate
buffer (pH=6.0).
QD(+)/J-aggregate(-) formation. A sample of resuspended, dialyzed QDs in H2 0
(4.9 pmol/L, 50 pL) was added to a narrow 7 mL vial, and 2.2 mL H2 0 was added.
On addition of a TC solution (100 puL, 0.1 mg/mL) with a glass syringe, the sample
gave rise to a J-aggregate absorption feature.
QD(-)/J-aggregate(+) formation. To a suspension of resuspended, dialized QDs
(4.9 pmol/L, 50 pL) in a vial, 100 pL of methanol was added. A TTBC monomer
solution (0.2 mg/mL in methanol, 100 pL) was added, and then 2.5 mL H2 0 was
rapidly injected. After sonication and filtration through 0.2 Am pores, the suspension
had the characteristics described elsewhere.
J-aggregate controls. To induce aggregation in the absence of QDs, an appropriate
volume of cyanine monomer solution was added to DI water. Aqueous NaCl solution
was added dropwise to achieve the appropriate spectral features.
2.4.2
Spectroscopy and Analysis
Transient photoluminescence traces were taken using a Hamamatsu C5680 Streak
Camera, and the excitation source was a regeneratively-amplified, frequency doubled
Ti:sapphire laser (150 fs) operating at 1 kHz. The concentration of J-aggregate was
constant in both the QD/J-aggregate and J-aggregate control samples, and the excitation power was 8 pW in both cases. Time traces were recorded on a 1 ns time
domain, and samples were contained within 1 mm cuvettes with stirring. For the
HS
N
a
CI
3
~)Lj~i
-C
L~J L~J
El
2
HS
N
C
C1
I-,
Figure 2-11: Nuclear Magnetic Resonance spectra for the synthesized mta ligand. (a)
'H NMR spectrum of mta in D2 0 (500 MHz). Product resonances are observed at
3.50, 3.11, and 2.91 ppm. (b) 13 C NMR spectrum of mta in D2 0 (500 MHz). Product
resonances are observed at 17.10, 53.47, and 68.59 ppm.
60
QD
QD/J
QD/J
QD
Figure 2-12: Demonstration of the left-right invariance of the photography setup.
Photographs of QD and QD/J-aggregate constructs were taken at constant QD concentration, and a band pass filter centered at 635 nm was used to remove excitation
light. Samples were illuminated using a diffuse, collimated 457 nm Ar+ laser beam,
and sample vials were placed at an identical distance from the beam center.
QD/J-aggregate conjugate, our fitting function was a convolution of the instrument
response function with a single exponential decay. This fit is well correlated with
the data for 97% of the decay in emission intensity, and holds until it falls below
noise. The error from fitting data at 472 nm is 6.4 ± 0.2 ps, but additional error
is introduced because the convolution uses an instrument response function recorded
at 400 nm. As the IRF is wavelength dependent, this variable wavelength error is
likely more significant than the fitting error. Thus the lifetime extracted from our fit
is reported as "about 6 ps" to avoid overstating the confidence in our measurement.
The diffuse excitation beam has a symmetric power density, as demonstrated by
the left-right invariance of the samples shown in Figure 2-12.
2.4.3
Energy Transfer Calculations
The expected energy transfer rates and efficiencies were computed from a J-aggregate
donor to a quantum dot (QD) acceptor. The QD is treated as a point dipole located
at the center of the nanocrystal [113], with a thin layer of J-aggregates on the QD
surface. In spite of the extended nature of the J-aggregate transition moment, it was
initially assumed that energy transfer from these J-aggregates would be dominated by
the transition dipole, which implicates a F6rster resonance energy transfer (FRET)
mechanism. The efficiency IFRET for FRET from a donor luminophore to an acceptor
at a distance r is [107, 114]
T7ET
r6 +
Ro is the threshold value of r at which
6
9 C4
- n-4
Re - 87
2
K77
R6
f
(2.1)
6
Rf
77FRET
J
= 50%, and is defined such that
(W)
10
U QD
4
W
(w)
do
(2.2)
with normalized donor emission Sj, donor quantum yield qj, acceptor absorption
cross section
UQD,
dipole orientational factor K, index of refraction y, and speed of
light in medium c/n.
The FRET rate was estimated from J-aggregates to QDs as follows. Because the
QD
acceptors are nearly spherical, it was assumed that the angular dependence for
the J-aggregates emission was negligible, so ,2 =
2
corresponding to energy transfer
between isotropically arranged donors and acceptors [115]. The TC J-aggregate has
a 45% quantum yield in PBS solution (measured against Coumarin 480, 95% QY in
absolute ethanol), and the PBS solution of TC J-aggregates had a similar degree of
aggregation as with the mta QDs in the construct.
To determine the acceptor absorption cross section, the QD absorbance at 350
was related to the QD number density using a modification of an established method
[116]. A concentration series of diluted CdSe/CdZnS QD samples was prepared in
hexane, and their absorbance at 350 nm was measured. Then the remainder of the QD
sample was precipitated and resuspended 3x before drying under vacuum. The dry
mass was used to calculate an extinction coefficient on a sample mass basis (Figure
2-13).
The nanocrystalline mass fraction of the sample was determined via thermal gravimetric analysis (83.6%). Transmission electron micrographs of the core and shell and
0
Beer's law data points
Linear fit
0.4
E
C
0.3-
0
0.2-
O
0.01
0.02
0.03
0.04
0.05
0.06
0.07
QD Concentration (mg/mL)
Figure 2-13: Plot of optical density at 350 nm as a function QD concentration. The
mass extinction coefficient was determined from a linear fit of this data using a 1 cm
quartz cuvette.
Table 2.1:
Energy Transfer Efficiencies for QD/J-aggregates
Construct
QD(+)/J-aggregate(-)
QD(-)/J-aggregate(+)
r|FRET1 rFRET2
>99%
82%
98%
N/A
Predicted TIFRET 3
98%
85%
from donor photoluminescence quenching;
Determined from transient photoluminescence;
1Determined
2
3 Calculated for R = 4.6 nm;
the densities of wurtzite CdSe and ZnCdS were used to estimate the mass per overcoated QD (Figure 2-2). This particle mass and the measured dry sample mass gave
the number of particles in the sample and led to the absorption cross section per particle at 350 nm (8.3 x 10-15 cm 2 ). The absorption cross section was used to obtain
the number density of QDs from the measured absorbance at 350 nm. Our resulting
energy transfer efficiencies are summarized in Table 2.1.
Because the dipole-dipole energy transfer mechanism adequately describes the energy transfer in this case, the results support FRET as the mechanism for narrowband
absorption enhancement of nanocrystals by J-aggregates.
CD,
m40-0
20
' 0
104
-
1-
10
- -
102
Hydrodynamic radius (nm)
Figure 2-14: Dynamic light scattering data for mta-capped QDs after dialysis in DI
H2 0. The peak at 4.6 nm comprises 98% of the sample mass (r = 4.3 ± 0.4 nm), and
its breath spans two bars in the histogram.
2.4.4
Sizing Data
Dynamic Light Scattering. Hydrodynamic radii were measured using a DynaPro
NanoStar and Dynamics@ software. Each dynamic light scattering sample was prepared immediately before use and filtered with a hydrophilic 200 nm-pore filter. All
data consist of averages of multiple experiments.
a)
Dye molecule/QD Ratio
0.0
0.5
1.5
1.0
2.0
40
30-
20
.
E
0
Co
102
_0
0.0
0.2
0.4
0.6
0.8
1.0
Volume of dye added (uL)
Hydrodynamic radius (nm)
Figure 2-15: Dynamic light scattering data for mta-QDs conjugated to TC Jaggregates. (a) Titration curve of the dominant QD/J-aggregate hydrodynamic radius, as a function of the dye molecule/QD ratio. Discrete particles with radii comparable to QDs are present for dye/QD ratios well below 0.5, but many QDs begin
aggregating abruptly when the ratio reaches 0.5. The sulfonate groups of the thiacyanine dye are likely cross linking positively-charged QDs; such cross-linking does not
occur in the oppositely-charged construct (Figure 2-16). (b) Representative dynamic
light scattering data for 0.91 dyes:dot. Although the sample mass is predominately
composed of species at 29 ± 3 nm, larger aggregates are also present.
90
7560b)
453015
00
-1
-
10
- -
10
-
102
Hydrodynamic Radius (nm)
60
Cl)
Cn 40
S20
Hydrodynamic Radius (nm)
Figure 2-16: Dynamic Light Scattering data for QD(-)/J-aggregate(+) species. (a)
mps-capped QDs conjugated to TTBC J-aggregates. The peak at 4.2 nm comprises
89.2% of the sample mass (r = 4.2 nm ± 0.4 nm). The larger species with r =
27.0 nm may consist of QDs bridged by large J-aggregates. (b) mps-capped QDs
alone after dialysis. The peak at 4.0 nm comprises 99.6% of the sample mass (r =
4.0 ± 0.4 nm). TTBC J-aggregates alone have much larger hydrodynamic radii in
aqueous solution [81], and it is concluded that QD(-)/J-aggregate(+) form distinct
species in solution and that the QD surface acts as a template for J-aggregation. Zeta
potential measurements of mps-QDs (-20 ± 1 mV) and of mps-QD/TTBC J-aggregate
conjugates (-7 ± 2 mV) are consistent with a negatively charged QD surface that is
partially screened by the oppositely charged J-aggregates.
Chapter 3
Quantum Dot/J-aggregate Blended
Films for Light Harvesting and
Energy Transfer
3.1
Introduction
This chapter describes the solution preparation of thin films composed of quantum
dots and thiacyanine J-aggregates, making use of the size tunable emission of quantum dots and the narrow, intense absorption of J-aggregates in the solid state. These
blended films exhibit 90% energy transfer efficiency from J-aggregates to quantum
dots and can uniformly cover a large area. Because the presence of the J-aggregates
enhances the QD photoluminescence intensity by 2.5-fold over QDs alone, these solid
state materials may be useful in downconversion applications or in fundamental investigations of light harvesting.
Colloidal quantum dots (QDs) have become fixtures in many fields of science and
technology
[651,
and one common material design theme involves energy transfer be-
tween quantum dots and another physically or chemically conjugated species [117].
The set of these quantum dot conjugates has expanded to many optical materials,
'Reproduced with permission from Walker, B. J., Bulovi6, V. and Bawendi, M. G. Nano Letters,
2010, 10, 3995-3999 [99]. Copyright 2010 American Chemical Society.
including J-aggregates of cyanine dyes [75, 81, 98]. J-aggregates have long attracted
interest due to their large oscillator strength, spectral red shift, small Stokes shift, and
narrow absorption features at ambient conditions [84]. Collective excitation phenomena in J-aggregates have been well-described by a Frenkel exciton model, in which
the characteristics of J-aggregates are due to coupling between chromophore transition dipoles and the consequent delocalization of excitation energy in the aggregate
[15, 12, 16, 118, 119, 120]. The spectrally narrow, intense absorption of J-aggregates
led to their widespread use in photographic film, where light absoprtion by the dye
aggregates is followed by reduction of silver halide particles [9, 57].
J-aggregates can also act as photosensitizers via excitation energy transfer (EET)
[121, 10]. It was previously reported that QD/J-aggregates in solution demonstrate
efficient EET from J-aggregates to conjugated QDs, resulting in enhanced light absorption and increased photoluminescence at the quantum dot wavelength [98]. By
incorporating QDs within the J-aggregate matrix in thin film, the light harvesting
and light emitting processes are distributed between materials that are optimized for
each role. The close conjugation of QDs and coherently-coupled dyes may aid in photoluminescence downconversion [122] beyond the levels already achieved through field
enhancements [123, 124, 42, 125], changes in QD chemistry [126], device architecture
[127], or matrix dispersion conditions [128], and energy transfer has already been used
to enhance light harvesting in heterostructured devices [117, 129, 114, 130, 101]. As a
13 nm-thick film of J-aggregated thiacyanine dyes (TC J-aggregate) can absorb 40%
of the incident light at A= 465 nm wavelength (see SI), the TC J-aggregate optical
penetration length is 2.4-times shorter than a film of bulk CdS and 4-times shorter
than the TC dye monomer [131]. Thus, efficient energy transfer from J-aggregates to
associated QDs should result in increased light harvesting and enhanced photoluminescence relative to a film with the same quantity of QDs alone. Others have noted
that the molecular excitons in J-aggregates are analogous to the physics of biological
light harvesting systems [23, 97], and blended QD/J-aggregate films may be model
systems for fundamental studies of light-matter interactions [86, 45, 34, 22, 35].
3.2
Results and Discussion
Like most J-aggregates, aggregation of TC in films depends on dye concentration and
polarity of the deposition solvent(s) [7, 78]. Conditions that lead to a high degree
of J-aggregation, such as highly polar or electrolytic solutions [48, 133], are often
incompatible with native quantum dot ligands and the hydrophobic solvents commonly used for solution processing [134]. Although all conjugations were previously
conducted in aqueous solutions, the low volatility and poor wetting of these solutions
make them less desirable than organic solvents for QD/J-aggregate deposition, and
as a result the deposition of the QD/J-aggregates represents a material processing
challenge.
The QD/J-aggregate constructs (Figure 3-la) were deposited in blended films as
follows. After a ligand exchange using 2-mercaptoethyl-(N,N,N-trimethylammonium)
chloride (mta) [98], 3.8 nm ZnCdS(CdSe) [shell(core)] QDs were suspended in 2,2,2trifluoroethanol (TFE) and further purified. This QD suspension was combined with
a TFE solution of thiacyanine dye [79], and the resulting solution was spun cast onto
a cleaned 2.5 cm 2 fused silica slide in air (see experimental section for details). Prior
to spin coating, the TC dye is dissolved in TFE in its non-aggregated form, and
it is evident that the spin casting process aids in the formation of TC J-aggregates
from solution [79].
Additionally, the alcohol-based mta ligand exchange results in
a somewhat modest drop in photoluminescence quantum yield (a 15% decrease),
compared to a 30-50% decrease in photoluminescence quantum yield (QY) associated
with ligand exchange of QDs in aqueous conditions [105, 135]. Cross sectional TEM
demonstrates the formation of a uniform thin film (Figure 3-1b).
The QD/J-aggregate film has the absorption spectrum in Figure 3-1c, and the
QD
absorption spectrum is shown for comparison. As in solution [98], the absorption
spectrum of the QD/J-aggregate thin film has both a J-aggregate absorption feature
at A= 465 nm and the spectral characteristics of QDs. The absorbance of the two
films at the lowest excitonic QD feature are comparable, indicating that the quantity
of QD material is approximately equal in both films.
graphite/Au
QD/J-aggregate
fused silica
d)
10
0.3
-Q0
2
0
-00D/J-aggregate
-naN
LED
electroluminescence
-
0.2-
-QD/J-aggregate
0
0L
06
0.4
060
>
400
500
600
Wavelength (nm)
700
0
400
Soo
600
7C
Wavelength (nm)
Figure 3-1: (a) Schematic of the electrostatic conjugation of thiacyanine J-aggregates
at the surface of a quantum dot after ligand exchange with mta. Na+ and Cl- ions
are not drawn for clarity. (b) Transition electron micrograph of a QD/J-aggregate
heterojunction in cross section. The thin film is supported below by the fused silica
substrate and encased above by graphite/gold deposited via focused ion beam. (c) QD
and QD/J-aggregate thin film absorption spectra. (d) Photoluminescence excitation
spectra of QD and QD/J-aggregate films, collected at the peak quantum dot emission
(A= 620 nm). Excitation of the QD/J-aggregate film at A= 465 nm is enhanced by
2.5-times relative to a film of QDs alone, and overlaps well with the emission of an
InGaN light emitting diode [132].
a)
Excited at 457 nm
b)
Excited at 360 nm
Figure 3-2: Photographs of QD and QD/J-aggregate thin films, taken with a A= 630
nm band pass filter to remove excitation light. (a) Films excited using columated
A= 457 nm illumination at the intersection of the two films. The average, wholefilm brightness enhancement from QD/J-aggregates is apparent. (b) Films excited
using broadband UV source centered at A= 360 nm. The near-identical brightness is
consistent with the spectral specificity of the J-aggregate.
The excitation spectrum of the QD/J-aggregate film, monitored at A= 620 nm
emission, demonstrates a 2.5-fold enhancement over a film with the same effective
thickness of deposited QDs (Figure 3-1d). Thus, for a light source of a given intensity tuned near the J-aggregate absorption maximum at A= 465 nm, a thin film of
QD/J-aggregates
results in greater blue light attenuation and a luminescence inten-
sity equivalent to a film of QDs that has 2.5-times as much material. The particular
TC J-aggregates used in this study may be particularly suited for this downconversion application, as they overlap well with the emission of an InGaN light emitting
diode [132], as shown in the overlay of Figure 3-1d. It is also clear that the higher
fluorescence in the QD/J-aggregate film does not result solely from diluting the QDs
(i.e. from a decrease in self-quenching among QDs), as the QD and QD/J-aggregate
films have similar emission intensities far from the A= 465 nm feature.
The enhanced emission of QD/J-aggregates is qualitatively apparent using the
waveguiding characteristic of the SiO 2 substrate. 2 First, the QD/J-aggregate film
demonstrates a clear brightness enhancement relative to the QD film in a photograph
taken using A= 457 nm excitation (Figure 3-2a). No such enhancement was observed
for illumination far from the J-aggregate absorption maximum (Figure 3-2b). Fluorescence micrographs of the films from an inverted optical microscope showed similar
2
For a discussion of the use of brightness to quantify absorption enhancement, see below.
Figure 3-3: Fluorescence micrographs of thin films, excited at A=465 nm. All images
depict emission that has been waveguided to the edge of the film for (a) QD/Jaggregates; (b) Quantum Dots; (c) J-aggregates. The enhanced quantum dot emission
and quenched J-aggregate emission is consistent with efficient energy transfer from
J-aggregates to QDs.
energy transfer and enhancement of the QD emission without the use of a band pass
filter (Figure 3-3, supporting information).
An inverted fluorescence microscope was used as an alternative means to image
energy transfer and absorption enhancement in QD/J-aggregate blended films (Figure 3-3). The fluorescence signals were maximized by imaging the edge of the film,
thus taking advantage of waveguiding quartz substrate. The thin QD layer yields little
photoluminescence when excited near the J-band maximum, and the J-aggregate control film exhibits strong blue-green emission. This J-aggregate emission is quenched
in the QD/J-aggregate film, which also shows the enhanced QD emission indicative
of energy transfer. The qualitative results from both experiments are consistent with
the PLE spectra in Figure 3-1c and with narrowband absorption enhancement via
EET.
Photoluminescence spectra of the QD, J-aggregate, and QD/J-aggregate films are
also consistent with efficient excitation energy transfer (Figure 3-4). Upon illumination at A= 457 nm, the QD film emits at A= 620 nm and the J-aggregate film
emits at A= 472 nm. In the QD/J-aggregate film, where the J-aggregate donors are
blended with QD acceptors, the A= 620 nm emission is enhanced and the A= 472
nm emission is quenched. The intensities of the J-aggregate donor emission measured
alone (ID) and in the presence of QD acceptor (IDA) permitted the calculation of the
EET efficiency (rEET), as r/EET
1
-
In this case the extent of donor quenching corresponds to an EET efficiency
4.U
-- QD
-QD/J-aggregate
3.0
- - - - J-aggregate
-
2.0
02.
E
LU
1.0-
0.0
450
~
500
550
600
'~~~
650
700
Wavelength (nm)
Figure 3-4: Emission spectra taken at A= 455 nm excitation. The quenched Jaggregate emission and enhanced QD absorption in the QD/J-aggregate indicates
that efficient energy transfer occurs.
>90%.This EET efficiency exceeds that reported in a previous experiment (38%)
[75], likely because the donors and acceptors in the previous work were separated by
alternating layers of a polyelectrolyte. The high EET efficiency in the present work
approaches the EET efficiencies previously achieved in solution (>98%) [98] using
identical quantum dots, charged surface ligands, and J-aggregates.
Morphology of the QD/J-aggregate thin films was characterized using atomic force
microscopy (Figure 3-5a), with the QD and J-aggregate films shown for comparison
(Figure 3-5b,c). The QD/J-aggregate film has less spatial continuity than either of
the constitutent films, and it consists of mounded structures that may arise from
disordered QD intercalation into the long-range structure of J-aggregates. The phase
contrast image of the QD/J-aggregate film (Figure 3-5d) is substantially different
from the phase contrast of either the QD or the J-aggregate, (Figure 3-5e,f) indicating that the tip-to-surface interactions of the QD/J-aggregate also differ from
either constituent [137]. The spacing between adjacent QD/J-aggregate surface peaks
(~50 nm) is similar to the estimated length scales for exciton declocalization of
other J-aggregates in the solid state [138, 139], and the morphology of the QD/Jaggregate blended films is thus consistent with efficient excitation energy transfer
from J-aggregates to QDs.
QD film
QD/J-aggregate film
J-aggregate film
4.4 nm
38.3 nm1
Height
Onm
Onm
0 nm
37.5*
12.5*
11.6*
Phase
00
0
Figure 3-5: Tapping-mode atomic force micrographs for height (a, b, c) and phase (d,
e, f) of QD/J-aggregates, QDs, and J-aggregates. The measured RMS roughness of
these films are 6.43 nm, 1.46 nm, and 0.80 nm, respectively, determined using WSxM
[136].
Figure 3-6: Photographs of TC J-aggregates excited with A=360 nm broad band illumination and a long pass filter (A=485 nm) to remove excitation light. (a) Two TC
J-aggregate films photographed immediately after deposition in air, showing nearly
identical brightness and hue. (b) Left film stored in ambient air without light exposure; right film exposed to ambient air and UV light for 1 h. The relative decrease in
J-band emission is apparent in the right film. (c) Left film stored in inert air without
light exposure, right film stored under N2 with UV light exposure for 48 h. The
brightness and color are largely preserved in an inert environment.
Figure 3-7: Photographs of QD/J-aggregates using a long pass filter (A=600 nm) to
remove excitation light. (a) Two QD/J-aggregate films photographed immediately
after deposition in air, showing nearly identical brightness when excited at A=360
nm. (b) Films excited at A=457 nm. Right film stored in N2 without light exposure;
left flim exposed to UV light continuously for 48 h under N2 . (c) Films excited at
A=360 nm. Left film was stored under N2 with continuous UV light exposure for 48
h; right film was stored under N2 without light exposure.
Like many organic fluorophores, J-aggregates are susceptible to photo-oxidation
when exposed to light under ambient conditions. To assess the degradation of TC Jaggregate donors in the solid state, the brightness of J-aggregate films were evaluated
qualitatively before and after continuous excitation at A= 360 nm and a power of
~0.41 W/cm2
Immediately after spinning two TC J-aggregate films under identical conditions,
the two films have identical photoluminescence in air (Figure 3-6a). The two films
were exposed to UV light from a lamp, (A=360 nm), with approximately -2.6 W of
excitation flux on the film. One of the two films was left underneath the UV lamp
for 1 h in air, and it demonstrated both a spectral shift and a reduction in brightness
relative to the other film, which was stored in air without light exposure (Figure 3-6b).
Although a sample film grew dim after 1 h of excitation in air, an identical sample
that was excited in a pure nitrogen atmosphere (< 0.2 ppm of 02) largely retained its
quantum yield even after 48 h of continuous exposure. QD/J-aggregate films stored
in N2 were similarly robust: a film exposed to an identical excitation source for 48 h
had undiminished brightness compared to a film stored without light exposure. These
experiments indicate that although films of both i-aggregates and QD/J-aggregates
can degrade, the films maintain photostability for an extended period if protected
from an oxidizing environment.
Another pair of films was transferred immediately to an inert air glove box. One
film was exposed to continuously to the same UV lamp as above, and the other
was stored to avoid light. After 48 h, the UV lamp excitation itself had changed in
appearance (Figure 3-6c), but the emission wavelength and intensity were largely the
same as the unexposed control film.
To account for the possibility that an electron transfer process could interfere with
light sensitization via energy transfer, the inert air bleaching experiment was repeated
for the QD/J-aggregate film. After deposition of two identical QD/J-aggregate films
(Figure 3-7a), both were stored under N2 . One of the two films was protected from
light exposure; the other was continuously excited using the same excitation source at
A=360 nm. After 48 h, the luminescence of the two films was compared for excitation
at A=457 nm (Figure 3-7b) and at A=360 nm (Figure 3-7c). In both cases, there is no
evidence of diminished QD emission, indicating that the direct light absorption and
energy transfer pathways are equally robust to photobleaching, as long as adequate
environmental controls are in place.
3.3
Conclusion
In conclusion, the uniform deposition of QD/J-aggregate thin films has been demonstrated via spin casting. Designed to benefit from both the intense, narrow absorption
of J-aggregates and the spectral tunability of quantum dots, these blended films exhibit 2.5-fold excitation enhancement over a film with the same quantity of QDs
when excited at A= 465 nm. The QD/J-aggregate films also demonstrate high energy transfer efficiency from the J-aggregates to the quantum dots, suggesting the use
of these heterostructures as model systems for studies of light harvesting. Because
energy transfer effectively imparts a large, tunable Stokes shift to the quantum dots,
a QD/J-aggregate film will have a low probability of reabsorbing the emitted light.
Avoidance of re-absorption is a critical property for luminescent solar concentrators
[140], so these films may be useful in tandem with other material sets. Finally, the
solution-phase deposition of QD/J-aggregates and the overlap of the J-aggregate ab-
sorption with the emission of common InGaN LEDs may make these QD/J-aggregate
films useful for luminescence downconversion applications.
3.4
3.4.1
Appendices to Chapter 3
Synthesis and deposition procedures
Chemicals and Reagents. All reagents were used without additional purification unless
noted. Thiacyanine dye (TC, no. NK 3989) was obtained from Charkit Chemical
Corp.; 2,2,2-trifluoroethanol (TFE) and Girard's Reagent T [CAS 123-46-6] were
obtained from Sigma Aldrich. 1-butanol, methanol, acetone, and isopropanol were
all OmniSolv brand purchased from VWR. Micro-90 detergent was also purchased
from VWR. Quartz slides (2.5 cm x 2.5 cm) were purchased from Chemglass.
Confirmation of J-aggregationfor TC J-aggregates. The optical properties of the
relevant thiacyanine dye has been previously reported elsewhere [40, 41, 141, 133].
Briefly, the formation of TC J-aggregate films was confirmed by comparison with their
spectra in various solvents. A dilute, 4x10-4 mg/mL solution of TC in methanol gives
rise to the monomer spectrum depicted in Figure 1-1 in Chapter 1. At identical dye
concentration in 0.2 M aqueous ammonium hydroxide, the optical signature of the
J-aggregate is evident.
Ligand exchange of ZnCdS(CdSe) with mta. The shell(core) quantum dots and
ligand in this study were identical to those reported earlier [98]. A solution of QD
growth solution (8 mL, 2.5 pmol/L) was flocculated via addition of n-butanol and
methanol, followed by centrifugation at 3900 rpm. After evaporation of residual
solvents in air from the separated QD solids, 2 mL of a 2-mercaptoethyl-(N,N,Ntrimethylammonium) chloride (mta) solution (65 mg/mL) was added directly to the
centrifuge tube. The QDs were immediately resuspended in the solution at 23 C
ambient temperature, with minimum stirring required. Excess ligands were removed
as follows. The QDs were precipitated two additional times by adding H2 0 (~0.2
mL) until turbid, then centrifuged to separate the water soluble-mta ligands from
the floculated QDs. TFE (2 mL) was added to resuspend the QDs. After the final
resuspension, the QDs were centrifuged at 5000 rpm and filtered (200 nm Acrodisc
PTFE syringe filter) to remove large particulates. The photoluminescence quantum
yield of these QDs in TFE was 60%.
It has been noted previously [142] that ZnS(CdSe) QDs become dispersable in
alcohol via ligand exchange with amines that contain polar functional groups. For
additional material versatility, cationic surface charge can be imparted to ZnS(CdSe)
QDs via a ligand exchange similar to that described above, using Girard's Reagent T
rather than mta. The photoluminescence QY of these species in solution was 57%.
Thin film deposition. To 1 mL of the above QD solution, a solution of TC in TFE
(1 mL, 0.3 mg/mL in TC dye) was added. The quartz substrates used in this study
were cleaned via a sequential washing procedure as reported earlier [54]; the procedure
is briefly summarized here. After washing the slide with a detergent solution of Micro90 and rinsing with distilled water, the slide was rinsed sequentially using acetone and
isopropanol. The solvent was then evaporated in a vertical orientation and used for
further spin coating as follows. 20 pL of the combined QD/TC solution was dropped
onto the center of the cleaned slide, which was immediately accelerated to 2000 RPM
on a spin coater chuck. After 1 minute, the sample was removed, and the solvent was
allowed to evaporate for 1 h before further characterization.
Control films were prepared in an analogous manner, with identical concentrations in either J-aggregates or QDs as appropriate. The photoluminescence QY of
CdSe(ZnCdS) QDs after deposition in thin film was 40%.
3.4.2
Characterization Methods
A Cary 5000 Spectrophotometer was used in double-beam mode for all thin film transmittance measurements. For absolute reflectance measurements, the Cary was fitted
with a 450 specular reflectance attachment and both of the two incident beams had
matching plane polarizations. Photoluminescence and photoluminescence excitation
measurements were taken using a Fluoromax-3 Fluorimeter. All thin film photoluminescence quantum yield measurements were taken using an integrating sphere.
a)
b)
J-band :Monomer
Ratio of Absorption Intensities
FWHM of J-band (nm)
35
5
10
5
UU
6
U
E
i
2
4
1
2
U 8
E
20
2,
20
0
.1
0.S
1
5
[Thiacyanine], mg/mL
for deposition
10
1
0.5
0.1
0.1
15
0.5
1
5
10
[Thiacyanine], mg/mL
for deposition
Figure 3-8: Summary of the spectral characteristics of TC J-aggregate thin films,
sampled at a range of concentrations and solvent compositions. Based on both (a)
maximizing ratio of J-aggregate to monomer absorption intensity and (b) minimzing
the the full-width at half maximum for the J-aggregate peak, the most complete Jaggregate formation occurs when spin coating solutions with concentrations of 0.2
mg/mL <[TC]<1.0 mg/mL from ~ pure TFE solvent.
Fluorescence micrographs were obtained using a Nikon Eclipse ME600 epifluorescence
optical microscope fitted with a Nikon DXM1200 digital camera. All other qualitative
images were taken using a Canon Powershot G9 digital camera.
Surface characterization was performed using a Veeco/Digital Instruments Dimension D3100s-1 atomic force microscope (AFM). Transmission electron micrographs
were taken with a JEOL 200CX TEM operating at 120 kV. Focused ion beam tomography was carried out using a JEOL JEM-9310 Focused Ion Beam system.
All film thicknesses were determined by scratching the film or by lifting off a corner
of the film with tape, then analyzing the height difference via AFM. AFM data was
processed using WSxM [136].
3.4.3
Determining the Attenuation Length of a TC J-aggregate
film
The attenuation length of a TC J-aggregate film was determined from the transmittance and reflectance data. TC J-aggregates were spun from TFE solutions of
varying concentrations onto quartz substrates, which were then measured via optical
0
0-
0
0-
0-
0 Data
0
0
1
0
-_
Linear Fit
2
3
4
5
[Thiacyanine], mg/mL for deposition
0
102
Data
Linear Fit
3
[Thiacyanine], mg/mL for deposition
Figure 3-9: Determining the attenuation length of a thiacyanine J-aggregate film. A
linear fit is shown in both cases; see text for fit parameters. (a) Optical density at the
A=465 nm J-aggregate resonance, measured as a function of [TC] in the TFE spin
coating solution. (b) Average film thickness measured as a function of [TC] in the
TFE solution.
spectroscopy and AFM (Figure 3-9). The linear fits from the thickness vs. [TC] and
O.D. vs. [TC) are
(9.9 ±0.8)
[mLnmTC]+
(4.0 ± 1.5) nm
O.D.= (0.15 ± 0.01) "L
mg [TC]+ (0.08 ± 0.04).
From the slopes of these two curves the attenuation length (in O.D.) was estimated
for a given concentration of thiacyanine dye [TC].
3.5
Measurements of linear spectral parameters
for TC J-aggregate films
A detailed study of relative contributions of the reflectance and absorption coefficients
is outside the scope of this work.3 As in other J-aggregate thin films, however, the
TC J-aggregate films have a significant reflectance coefficient (Figure 3-10a), and it
increases with increasing quantity of the deposited dye. Together with the absorption
3
For an example of such a study on a J-aggregate film prepared via electrostatic self-assembly
with an oppositely charged polyelectrolyte, see reference [54].
b)
c)
60
40
100 .
30
80
20
60
0)
40.
0
20-
10
CL
0.
40
s
0
20
1
2
-
3
0
[Thiacyanine dye], mg/mL
1
2
3
[Thiacyanine dye], mg/mL
0
1
2
3
[Thiacyanine dye], mg/mL
Figure 3-10: (a) Reflectance, (b) Transmittance, and (c) Absorption coefficients measured at the TC J-aggregate maximum (/\=465 nm) for deposition solutions with a
range of [TC].
spectra that level off at 40% absorption (Figure 3-10b,c), these data indicate that
there is a J-aggregate optical density at which the absorption coefficient is far greater
than the reflectance coefficient, (approximately O.D.< 0.3). Our samples have O.D.
that are far from this threshold value for the entire spectrum, so the QD/J-aggregate
films reported in this study have a favorable optical density for J-aggregate light
absorption and subsequent energy transfer to quantum dots.
3.5.1
On "brightness" in measurements of absorption enhancement
The connection between brightness and absorption enhancement is not always intuitive, and absorption measurements in particular are more complicated in thin film
than in solution. Therefore it is beneficial to consider the meaning of "brightness" in
more detail.
Here, the term brightness is considered synonymous with "the number of photons
collected."
This has an inherent spectral dependence, as the number of photons
collected depends on the spectral range of the detector. Photoluminescence is also a
function of the excitation wavelength.
Brightness is quantified by
nPL
(A1 ,2 ), the number of photons detected at wave-
length A2 after excitation at a wavelength A,. Then
nPL
where
A2
TPL
(A1 ,2)
= r7PL
(3.1)
(A, 2 ) nA (A)
(A, 2 ) is the photoluminescence quantum yield for emission at wavelength
after excitation at a wavelength A,, and nA (A,) is the number of photons absorbed
by the measured film area at (A1 ).
nA (A) itself is a product of two factors: the incoming flux of photons <D (A,),
which is constant during all replicated experiments, and the absorption coefficent of
the film A (A). In summary, nA(A1) = @ (A) A (/).
Determining the absorption coefficient is more complicated in thin films than
for homogeneous solutions, due to significant film reflectance.
Based on relation-
ships between absorption, reflection and transmission, the absorption coefficient A at
wavelength A1 is
A (A,)
=
1
-
(3.2)
10-o(Ai)p1 - R (A,)
with absorption cross section o (A1 ), density of absorbers p, path length 1, and
reflectance coefficient R (A1). The density of absorbers is the number of absorbers XA
in the excitation region of interest divided by volume; this volume contains both an
area term A and a path length, which cancels with 1. Introducing this change, and
substituting eq. 3.2 into eq. 3.1,
nPL (A1,2 ) = TPL (A1,2 )
ID(A,) [1 -
-
f (A,)]
(3.3)
Now consider the application of this relationship to the current experiment. If
the quantum yield of the emitting QDs is reasonably constant before and after the
addition of the J-aggregates, and if the quantity of QDs and their area of coverage
is approximately the same, then the only parts of this expression that change significantly during the addition of J-aggregates to the QD film are a (A1 ) and R (A,).
As noted above, the reflectance coefficient R increases monotonically over the rele-
vant range of TC J-aggregate film thicknesses, resulting in an upper bound for the
absorbance of the thin film. The films discussed in this paper are far from this limit
however, and after subracting the contribution from reflectance it is evident that most
of the light attenuation is due to absorbance.
Thus, -(A,) is the chief experimental variable in the measurement of the number
of photons ("brightness") from a film of QDs energy transfer acceptors with and
without asociated J-aggregate donors.
84
Chapter 4
Color-enhanced photocurrent by
coupled J-aggregate/nanowires
4.1
Introduction
J-aggregates are ordered clusters of coherently-coupled molecular dyes
[9],
and they
have been used as light sensitizers in film photography due to their intense absorptions. Hybrid structures containing J-aggregates may also have applications in devices that require spectral specificity, such as color imaging or optical signaling [143].
However the use of J-aggregates in optoelectronic devices has posed a long-standing
challenge [84, 87], due to the difficulty of controlling aggregate formation and the
low charge carrier mobility of many J-aggregates in solid state. This chapter describes a modular method to assemble three different cyanine i-aggregates onto CdSe
nanowires, resulting in a photodetector that is color-sensitized in three specific, narrow absorption bands. Both the J-aggregate and nanowire device components are
fabricated from solution and the sensitizing wavelength is switched from blue, to
red, to green, using only solution-phase exchange of the J-aggregates on the same
underlying device.
In photodetection and photovoltaic devices, organic components are particularly
'Reproduced with permission from Walker, B. J., Dorn, A., Bulovid, V., Bawendi, M. G. Nano
Letters, submitted, Copyright 2011 American Chemical Society.
CdSe-
8)C)
thiocholine
J-aggregated Dye
A
Pt electrodes
ith Bi catalyst
Q
C.Qyfr
fused SiO 2
0o3S(H2 C)3
(H
2 )3S0 3
Na
(
1
b)
H
2)3SO3N(C
2)3O*
3
Cl
N(C2 N
C
C1
Cl
C)
2 )3
(2
(CH
S03 Na
(()
3
SO 3 H
4
Figure 4-1: Overview of the J-aggregate/CdSe nanowire color-selective photodetector.
(a) Schematic of J-aggregate-coated nanowires contacted between electrodes. (b)
Optical micrograph, showing interdigitated electrodes used to grow and test devices.
Faint brown shading between electrodes is the sparse network of CdSe nanowires. (c)
Detail of J-aggregate-templated CdSe nanowires, with the structure of 3-aggregating
dyes 1, 2, 3, and 4.
interesting due to their low cost and ease of processing. However organics are generally
poorer photoconductors than crystalline inorganic semiconductors, and many device
designs have improved performance by combining organic with inorganic materials
in hybrid devices [144, 145, 146, 147]. Because the nature of the organic/inorganic
interface determines device performance for a range of different applications, it is
crucial to develop modular assembly techniques that can be used to template materials
at interfaces [148]. This need for control is particularly vital for J-aggregates [149,
150, 86, 54, 42, 120], whose wavelength specificity, intense absorptions, and small
feature sizes make them promising materials for a range of applications, including
color-resolved sub-diffraction limit imaging.
Our strategy was to design a photodetector based on a combination of J-aggregates
and a crystalline inorganic nanowire, thus distributing the light harvesting and charge
transport functions between two materials optimized for each role (Figure 4-la). 3aggregates have a large cross section, and they can attenuate 40% of the incident
light at the maximum absorption wavelength in a 13 nm film. However, 3-aggregates
b)
a)
26.7nm
32.7nm
200nmonm
CdSe nanowires
20
monm
nanowire/J-aggregates
Figure 4-2: Atomic force micrographs of a small area of a functional J-aggregate/
nanowire device. (a) CdSe nanowires before deposition of J-aggregates. (b) Same
area as in a, after deposition of J-aggregates.
have low carrier mobilities (10-6-10-4 cm2 V-1 s-1) in solid state [89]. The CdSe
nanowires in this study have continuous absorption above the band edge (1.7 eV),
and their high crystallinity and Ohmic contacts (Figure 4-3) make them suited to
photocurrent transport. We demonstrate switchable color sensitization by depositing
one cyanine J-aggregate, removing it, and repeating this process for three different
color sensitizing species in the red, green, and blue.
4.2
Discussion
Electrostatic self-assembly was used to direct the formation of J-aggregates onto
nanowires [75, 54]. CdSe nanowires were grown by the electrically controlled solutionliquid-solid method (EC-SLS) [151], from bismuth-coated [152, 153] platinum electrodes (Figure 4-1a). A light micrograph (Figure 4-1b) shows the intedigitated electrode structures used to grow and test devices.
After growth, the CdSe surface was treated with thiocholine [98], followed by
deposition of one of the J-aggregating dyes via spin casting (compounds 1, 2, 3, and
4, Figure 4-1c). These J-aggregates were all selected due to their strongly negative
10
10
10
-14
10
10
Light (2.1 eV)
-5-Dark
107
-5
-4
J
-3
-2
-1
0
1
2
3
4
5
Source-Drain Voltage (V)
Figure 4-3: Current-voltage characteristic of J-aggregate 2/nanowire photodetector,
in both the light and the dark. The photoresponse is recorded near the J-aggregate
absorption maximum (E=2.1 eV), and the device shows Ohmic behavior both in the
dark and light.
--
graphite/Au
Cadrnium
Nanowire
Selenium J
Sulfur
J-aggregate
Chloine
Bismuth
CdSe/
J-aggregate
SiO 2
250 nm
Figure 4-4: Cross-sectional scanning tunnelling electron micrograph of a nanowire/Jaggregate device. The nanowire is lying on the SiO 2 substrate, with graphite/Au
deposited for focused ion beam processing. Energy dispersive x-ray spectroscopy
indicates that the organic molecules template onto the CdSe nanowire.
electrostatic charge, and the similarity among their chemical structures suggested
the possibility of a modular deposition process. The results from a control device
with positively-charged cyanine dyes and positively charged ligands are consistent
with electrostatic repulsion (see supporting information), indicating that electrostatic
attraction drives the association between nanowire surface ligands and negativelycharged J-aggregates 1-4.
Morphological features of J-aggregate/nanowire surfaces are visible via atomic
force microscopy (AFM). AFM images of a detailed area before (Figure 4-2a) and
after J-aggregate deposition (Figure 4-2b) show an increase in the nanowire diameter
upon J-aggregate deposition from 30 to 60 nm, indicating a radial coating of 15 nm as
well as the presence of fibrous, untemplated J-aggregates in background. J-aggregates
of similar sulfonated cyanine dyes are known to form linear or sheet-like extended
structures
[154].
The increase in wire diameter due to J-aggregates is comparable to
the radii of the J-aggregates in the background, which is consistent with a templated
coating of J-aggregates around the positively-charged nanowire surface.
Additional analysis of the J-aggregate-coated nanowires was done using scanning
tunneling electron microscopy. The image of a J-aggregate-coated nanowire in cross
section is shown in Figure 4-4. Elemental line scans were conducted via energy dispersive X-ray analysis and indicate the presence of Cd, Se, S, and Cl. Because the
only sources of S and Cl are the organic compounds noted above, this composition
analysis is consistent with J-aggregate templating onto the nanowire surface.
J-aggregate formation is characterized by light absorption that is red shifted and
narrowed with respect to the monomer spectrum. In the present system, thin films of
cyanine dyes 1-4 exhibit J-aggregate characteristics, as their resonances are narrow
(about 50 meV full width at half maximum) and red-shifted (>200 meV) relative
to corresponding monomer spectra in dilute methanol solutions (Figure 4-5a).
J-
aggregate absorption features of 1-4 have the same spectral features on the wires
(Figure 4-5b) as when deposited on planar substrates.
The sensitivity enhancement in J-aggregate/nanowire photodetectors was determined via photoaction spectroscopy, by recording the photocurrent as a function of
a)
-J-agg.
I
monomer I
C)
x 011
J-agg. 2
-- monomer 2
J-agg. 3
-.--.monomer 3
- J-agg. 4
-
J-agg. 1 + wires
4.0
wires
-
monomer 4
-wires,
post-washing
E
-3.0
CL
U,
1.0
....
-
4..
-
.
.........
b)
..
-..........--------
..:..
.. ..
0.0
0 0
__d)
------ J-agg. 1 + wires
60
-
200
J-agg. 3 + wires
-
J-agg. 4 + wires
J-agg. 2 + wires wires
------ J-agg. I + wires
--
J-agg. 3 + wires
40-
0
- J-agg. 2 + wires
\%
150
J-agg. 4 + wires
U)
-1
%o
%
50
-
20
0
00
1.5
2.0
2.5
Energy (eV)
3.0
1.5
2.0
2.5
3.0
Energy (eV)
Figure 4-5: Spectral characterization of J-aggregate/nanowire devices. (a) Absorption spectra of J-aggregates 1-4 and the respective monomers in dilute methanol
solutions. (b) Absorption spectra of J-aggregate/nanowire devices templated with
J-aggregates 1-4, shown with a bare CdSe nanowire device. (c) Photoaction spectrum of a i-aggregate/nanowire device, compared against the bare nanowire device.
The photocurrent is enhanced 2.8-fold. (d) Relative photocurrent enhancement as
a function of excitation energy due to i-aggregates 1-3. No photocurrent enhancement is observed for i-aggregate 4. Color-selectivity was changed by removing one
i-aggregate and depositing another.
excitation wavelength. A representative photoaction spectrum is shown for a device
templated with J-aggregate 1 in (Figure 4-5c). Prior to the deposition of J-aggregates
the device has the photoaction spectrum shown in black. Photocurrent onset occurs
at E=1.7 eV, the same as bulk CdSe. After deposition of J-aggregate 1, the photoaction spectrum retains nanowire features and shows a distinct peak that correlates
with the J-aggregate absorption in Figure 4-5a and Figure 4-5b. The overall enhancement of the photocurrent is 2.8-fold above bare CdSe nanowires at maximum absorption (E=2.1 eV). No photocurrent is observed for J-aggregates deposited between
electrodes in the absence of CdSe wires, indicating that the light absorbed by the
J-aggregates only enhances the photocurrent via conjugation to the CdSe nanowires.
The J-aggregate deposition process is reversible, as washing the devices with TFE
resulted in removal of the J-aggregate and recovery of the original CdSe photoaction
spectrum (Figure 4-5c). To demonstrate the versatility of our method, four different
J-aggregating dyes were successively deposited onto the same device one at a time;
the photocurrent enhancement for these dyes is shown in Figure 4-5d. Devices made
with J-aggregates 1, 2, and 3 all demonstrated photocurrent enhancement, while
devices made with J-aggregate 4 did not show any enhancement.
Because the excited state energies of J-aggreates 1, 2, and 3 are all above the CdSe
band gap (E=1.7 eV) but J-aggregate 4 (bandgap E=1.2 eV) is below the CdSe band
gap, the device performance is consistent with photocurrent enhancement via energy
transfer from the J-aggregates to the nanowires (Figure 4-6a). This interpretation is
supported by efficient energy transfer previously reported from J-aggregates to CdSe
quantum dots both in solution [98] and in thin film [99], which indicates that such
an electronic coupling is favored. An alternate model for photocurrent enhancement
based on charge transfer at the J-aggregate/nanowire interface is also possible (Figure
4-6b). However, no analogous electron transfer was observed from J-aggregates to
quantum dots in previous experiments [98].
For an increase in photocurrent to be observed at the absorption wavelength of
the J-aggregate, several conditions have to be met. First, absorption of the plain
CdSe nanowire photo detector at the maximum J-aggregate absorption energy has to
a)
S50.
hac)
J-aggregate
Nanowire
A
0 -
0
1.6
d)
Energy transfer
0
d
2.0
2.4
Energy (eV)
Enhancement
60
2.8
Light-blocking
CD 20
b)
-20
Absorption dihffusionl
= -60
-100
0
60
40
20
J-agg. layer thickness, dj(nm)
Figure 4-6: Description of the processes governing J-aggregates/nanowire photodetection. (a) Schematic of J-aggregate/nanowire photodetector operation via energy
transfer. (b) Schematic of J-aggregate/nanowire photodetector operation via electron
transfer. (c) Relative photocurrent enhancement as a fucntion of excitation energy
due to varying thicknesses of J-aggregate 2 in a nanowire/J-aggregate photodetector. (d) Relative change in photocurrent at the J-aggregate absorption maximum as
a function of J-aggregate layer thickness, from (c). Solid curve corresponds to the
model in the text and fit with respect to a.
be significantly smaller than unity. Secondly, excitons in the J-aggregate layer must
be tranferred efficiently to the nanowires. Finally, the effect of light absorption by the
J-aggregate coating and subsequent energy transfer to the nanowires has to outweigh
the reduced direct light absorption by the nanowires resulting from blocking by the
i-aggregates.
The last two conditions place thickness constraints on the J-aggregate layer to
observe wavelength-selective photocurrent enhancement. If the J-aggregate layer is
too thin, only a small amount of additional light is absorbed and hence only a small
increase in photocurrent at the sensitization wavelength is achievable.
However a
very thick J-aggregate layer will absorb a large amount of light at the expense of
decreasing energy transfer effciency, as the absorbing regions become farther displaced
from the surface of the nanowires. This tradeoff suggests that there is an optimal
J-aggregate layer thickness at which light absorption and subsequent energy transfer
are balanced so that the photocurrent is maximized. The characteristic parameters
governing the optimal J-aggregate layer thickness (dj) are the absorption length of
the J-aggregate film (a), the J-aggregate/nanowire F6rster energy transfer radius
(Ro), and the excition diffusion length within the J-aggregate layer (Figure 4-6a).
For a quantitative measure of the optimal J-aggregate layer thickness, our device
was modeled in the thin film approximation (see supporting information). Photocurrent is taken to be proportional to exciton creation rate in the CdSe nanowires, which
is the sum of three terms: direct light absorption by nanowires, blocking of direct
nanowire light absorption by the J-aggregate film, and light absorption by the Jaggregate film with subsequent energy transfer to nanowires. Light absorption in the
where Io is the
i-aggregate was taken to follow the Lambert-Beer law, I = Ioe-"
intensity of incident light. The rate of energy transfer
kET
from the site of absorp-
tion in the i-aggregate layer to the CdSe nanowire was modeled by a point-to-bulk
(
where kD is
)3
the radiative rate of an exciton in the J-aggregate layer and r is the distance from an
interaction of two transition dipoles in the near field:
kET=
kD
r),
exciton in the i-aggregate to the CdSe surface.
In figure 4-6c the model is compared against four J-aggregate/nanowire devices
with varying thickness of J-aggregate 2 determined by AFM
.
The F6rster radius for
J-aggregate 2 donor and nanowire acceptor, obtained from emission and absorption
spectra, is Ro=7.8 nm. The absorption coefficient (a=5.0 x 105 ± 3 x 10' cm- 1 ) was
determined in a one-parameter fit to our model.
Several features are apparent from the relationship between photocurrent enhancement and layer thickness of J-aggregate 2 (Figure 4-6d). First, the proposed
model is consistent with the observed J-aggregate-enhanced photocurrent. The Jaggregate/nanowire device performance spans both the enhancement and light-blocking
regimes, solely by depositing thinner or thicker coatings of J-aggregate. Thus for device operation with dj >20 nm, the transport of excitons from the J-aggregate to the
nanowire is ultimately limiting. Our current model, without including exciton diffusion, shows good agreement with the thickness-dependent data. Hence, our results
are consistent with device operation in which direct energy transfer dominates over
exciton diffusion. Because the exciton delocalization length in sulfonated cyanine Jaggregates is comparable to the thickness of the J-aggregate layer noted above [12],
a non-diffusive process is plausible in our devices.
The color-selective response in this paper suggests further applications in absorption multiplexing and imaging. For example, J-aggregate/nanowire photodetector arrays may be useful for color-sensitive sub-diffraction limit imaging. This J-aggregate
nanowire device could be a model system for studying the coupling between Frenkel
excitons and Wannier-Mott excitons [155, 73].
4.3
Conclusion
The solution-phase assembly of cyanine J-aggregates and CdSe nanowires was demonstrated to form color-selective photodetectors.
The sensitizing wavelength of the
photodetectors switched from blue to red to green using exclusively solution phase
processing, with a resulting photocurrent enchancement of up to 2.8-fold greater than
the bare CdSe nanowire device. These results indicate that an array of J-aggregating
dyes can be used in optoelectronic applications, potentially enabling new capabilities
and device designs.
4.4
4.4.1
Appendices to Chapter 4
Synthesis and deposition procedures
Chemicals and Reagents. All reagents were used without additional purification unless noted. J-aggregating cyanine dyes 1, 2, 3, and 3c were obtained from Charkit
Chemical Corp; the catalog numbers are NK-3989, NK-2203, NK-1952, and NK-1538,
respectively. Dye 4 was obtained from Spectrum Info (Kiev), catalog no. S1318.
2,2,2-trifluoroethanol (TFE), Girard's Reagent T [CAS 123-46-6], trioctylphosphine
oxide (99% ReagentPlus) were obtained from Sigma Aldrich. Tetradecylphosphonic
acid (99%) and cadmium(II) oxide (99.998% metals basis) were obtained from Alfa
Aesar. 1-butanol, methanol, acetone, and isopropanol were all OmniSolv brand purchased from VWR. Micro-90 detergent was also purchased from VWR. Quartz slides
(1 cm x 1 cm) were purchased from Chemglass.
Thiocholine. The thiocholine ligand was synthesized following a modificaiton of
an earlier procedure [98]. Acetylthiocholine chloride (5 g, 6.8 mmol) was dissolved
in 1 mol/L HCl (20 mL) and refiuxed at 120'C under continuously-circulating N2.
After 20 h, the reaction was suspended and acetic acid, HCl, H2 0 were removed
through repeated washing and solvent removal in vacuo, leaving a white crystalline
solid. Recrystallization of the solid from methanol gave a 60% synthetic yield.
Substrate cleaning. All device substrates were cleaned according to a modification
of the procedure in Bradley et al. [54], using aqueous solutions of Micro-90, distilled
water, acetone, and isopropanol. Substrates were allowed to air-dry after cleaning.
Nanowire growth. A 3-neck, 50 mL round bottom flask was charged with trioctylphosphine oxide (20 g), cadmium (II) oxide (19 g, 6.8 mmol), tetradecylphosphonic acid (126 mg, 0.45 mmol). One of the ports was fitted with a digital temperature
controller and the other with an interdigitated Pt electrode substrate coated with a
20 nm film of bismuth [151]. The reaction mixture was heated at 150'C at 100 mTorr
for 1.5 h, then heated to 300 C. Upon reaching 290 C the solution became clear,
with no reddish residue. Then 100 pL of trioctylphosphine selenide (1.5 mol/L) was
injected. The wires were grown at 285 C, and a 1 V source-drain potential between
the electrodes. After 10 min, the electrode substrate wires were removed from the
growth solution for further processing.
Ligand exchange of CdSe nanowires with thiocholine. Immediately after wire
growth, samples were rinsed with hexane (3x 200 mL), then immersed in clean hexane
(100 mL) for 20 h. The substrate was rinsed with TFE, then immersed in a solution
of thiocholine (10 mL, 0.55 mg/mL in TFE) for 1 h. After washing with TFE, the
sample was immersed in TFE (2x 100 mL, 1 h ea.) and allowed to air-dry.
J-aggregate deposition. A solution of TC in TFE (1 mL, 1.5 mg/mL in TC dye)
was prepared. 20 tL of the combined TC solution was dropped onto the center of
the cleaned slide, which was immediately accelerated to 2000 RPM on a spin coater
chuck. After 1 minute, the sample was removed, and the solvent was allowed to
evaporate for 1 h before further characterization.
4.4.2
Characterization Methods
A Cary 5000 Spectrophotometer was used in double-beam mode for all thin film
transmittance measurements. For reflectance measurements, the Cary was fitted with
a specular reflectance attachment and both of the two incident beams had matching
plane polarizations; the transmittance (T) and reflectance (R) were used to obtain
the absorbance (A), as A = 1 - T - R. Photoaction spectroscopy was performed
using a blackbody Xenon lamp with a 300 nm/cm blazed monochrometer, with a
bias voltage applied using a calibrated Yokogawa 7651 programmable DC source.
Photo- and dark currents were measured using an Agilent 34401A 6.5 digit multimeter
and a preamplifier (model no. 1211 from DL Instruments). All photocurrents were
calibrated using a standard photodetector (Newport Optics 818-UV).
Surface characterization was performed using a Veeco/Digital Instruments Dimension D3100s-1 atomic force microscope (AFM). AFM scans were all performed using
the same tip, to minimize uncertainty in tip-surface convolution. Scanning transmis-
C)
b)
a)
-
--
4-
N/\/40-
3
- --
40-
J-agg.
3 Monomer
R
ON
Q
3
--oR 30so03
S03H
'N(C2H,)
-
3C J-agg.
U
----- 3C Monomer
0
.
C
~20< 10-
-40-
10
i
-
--
CIO
3C
0
-
2.0
---.---.-
2.5
__
3.0
Energy (eV)
.
-
,
3.5
-80
1.5
2.5
3.5
Energy (eV)
Figure 4-7: Demonstation of electrostatic self-asembly. (a) Structures of compounds
3 and 3c. The chromophores of the monomers are identical. (b) Absorption spectra
of 3 and 3c, as both J-aggregate films and as monomers in dilute methanol solutions. Both compounds have similar aggregation properties. (c) Relative change in
the photocurrent density due to J-aggregates 3 and 3c, for J-aggregate/nanowire
photodetectors. The negatively-charged 3 enhances photocurrent by 44%, and the
positively-charged 3c attenuates photocurrent by 70% at E=2.3 eV. The contrast
between the two J-aggregate behaviors show the importance of electrostatic assembly
for photocurrent enhancement by energy transfer process.
sion electron micrographs were taken with a JEOL 2010 FEG ATEM operating at
250 kV, with an EDX spectrometer. Focused ion beam tomography was carried out
using a JEOL JEM-9310 Focused Ion Beam system. AFM data were processed using
WSxM [136].
4.4.3
Electrostatic self assembly
To demonstrate the importance of electrostatic self-assembly, we templated a device
with a J-aggregate identical to one that was analyzed previously, but having the
opposite static charge (Figure 4-7a). Compound 3c has an identical chromophore
structure to compound 3, except for the 2nd carbon on its N-substitutents. Because
this change is electronically removed from the molecule's conjugated center, such a
change is not expected to substantially affect the monomer electronic states. Absorption spectra of both the J-aggregate films and dilute monomer solutions indicate both
that the relative energy levels are similar, and also that sign and magnitude of the
a)
b)
110
3.0
10
c
-
2.0
10
Power ensi
2
C
)
(x10-5 W/cm0
10
0
X
-12
o
10
0~0
-12
-86.6
64.8
-14
0
1.0
.c
51.3
10
39.4
17.5
1.99
-15
0
0
10
Excitation Energy (eV)
50
100
150
Power density @ 2.1 eV, (x1 0~5 W/cm 2)
Figure 4-8: Data demonstrating linear excitation power dependence. (a) Plot of
photoaction spectra at various light intensities. The similarity in the spectra on
the semilog plot demonstrates that the spectra are qualititavely similar across many
excitation intensities. (b) Photocurrent at the excitation maximum (E=2.1 eV) of the
J-aggregate 2/nanowire device. The photocurrent increases linearly with increasing
intensity of the incident light.
J-couplings are similar for 3 and 3c (Figure 4-7b).
Upon templating the J-aggregates onto a CdSe nanowire device, however, the two
compounds have different effects on device performance (Figure 4-7c). J-aggregate 3
leads to a 44% enhancement in the colected photocurrent density, while J-aggregates
of 3c reduce photocurrent by 70%. This result indicates that the J-aggregates of 3c
are able to absorb incident light but are not sufficiently close to the nanowires to transfer energy. Thus we conclude that the positively charged J-aggregates are repelled
by the positively charged nanowire surface, and conversely that the J-aggregates 3
and the other sulfonated dyes 1, 2, and 4, are likewise templated by electrostatic
attraction.
4.4.4
Excitation power dependence
The full photoaction spectrum of the J-aggregate 2/nanowire device was measured as
a function of incident power density (Figure 4-8a). The similar shapes of the curves on
a semilog plot indicates the photophysical response of the J-aggregate/nanowires are
similar at the measured excitation fluxes. Taking a cross-section at the J-aggregate excitation maximum, it is evident that the photocurrent has a linear power-dependence
(Figure 4-8b). This linear power (or light intensity) dependence is consistent with
photocurrent enhancement by either electron or energy transfer from the J-aggregates.
4.4.5
Model of photocurrent modulation by templated Jaggregates
Photocurrent enhancement from J-aggregates is highly sensitive to device geometry.
After absorption, the excitation energy in the J-aggregate must transfer efficiently
to CdSe to avoid losses. The effect of J-aggregate energy transfer to the nanowire
also must outweigh a decrease in direct nanowire light absorption due to J-aggregate
absorption. To understand these physical constraints on energy transfer, the photocurrent enhancement in J-aggregate/nanowire photodetectors was modeled using
the thin film approximation, in which energy transfer from J-aggregates to nanowires
is modeled as the point-to-bulk interaction of two transition dipoles in the near field.
We use a simplified dipole-dipole model:
kET= kD
3
Ro
(4.1)
-)
where kET is the energy transfer rate from the J-aggregate to the CdSe nanowire,
kD is the radiative rate of an exciton in the J-aggregate layer, Ro is the characteristic
radius for energy transfer (F6rster radius), and r is the distance from exciton in the
J-aggregate to the CdSe surface. The decrease in light intensity due to absorption is
assumed to follow the standard Lambert-Beer law:
I = 1
je--d
(4.2)
where Io is the intensity of incedent light, c is the absorption coefficient of the
J-aggregate layer, and dj is the J-aggregate layer thickness (shown in Figure 5a of
the main paper). In addition, we assume that energy transfer occurs from the site of
absorption in the J-aggregate layer directly to CdSe nanowires.
To determine the change in photocurrent in the CdSe nanowires, it is also necessary to take into account the reduction in direct light absorption by the CdSe resulting
from the light absorption by J-aggregates. The net effect of the J-aggregate layer is
'net
and is the difference between the excitation enhancement and reduction due to
the presence of J-aggregates:
Inet =
Aw
(4.3)
(Ienhanced - Ioe~"dj)
where Aw is the absorption efficiency of the plain nanowire photodetector. In
the limit of low excitation the exciton creation rate in the nanowires is proportional
to the photocurrent. The change in photocurent as a function of J-aggregate layer
thickness (AZIre)
relative to bare CdSe nanowires is:
ae-ax
AIrei(dj) =
1
Aw
(djx)
dx - (1
-
e-adj)
(4.4)
3
In the final model plotted in the main text, experimental values from optical and
AFM measurements were used for the parameters in the model. The nanowire absorption without enhancement was asssumed to be in the single wire limit (A, = 0.15),
and the F6rster radius for J-aggregate 2 donors and nanowire acceptors (determined
from absorption and emission spectra) is Ro=7.8 nm. The four J-aggregate nanowire
devices were fit against the model for incident photon energies 1.8-2.3 eV at 137 data
points, and the fitted result for the J-aggregate absorption coefficient was a=5.0 x
105
±2x
104 cm- 1.
The physical parameter space is summarized by the plots shown in Figure 4-9.
In particular, for all other parameters held constant, a more strongly-absorbing Jaggregate (i.e. higher a) results in a substantially higher photocurrent enhancement.
Likewise, the photocurrent enhancement effects are most pronounced for thinner wires
(lower A,) so that the proportional photocurrent enhancement due to J-aggregates
is higher.
100
a)
0
0
1.0
0.
dj (nm)
0
20
< -1.01
b)
3.0
0
.
0.0
c)
0
1.0
d (nm)
2.
60
80
0
e
1.0
20.
d (nm)
-3 -1.0
Figure 4-9: Plots of the relative change in photocurrent due to J-aggregates in the Jaggregate/nanowire device. The enhancement is plotted as a function of J-aggregate
layer thickness, for several parameters. (a) Varying the 3-aggregate absorption length
a from 0 nm-- [blue] to 0.1 nm- 1 [red], in 0.01 nm- 1 steps. (b) Varying the F6rster
radius, Ro, from 0 nm [blue] to 20 nm [red] in 2 nm steps. (c) Varying nanowire
absorption fraction Aw, from 0.15 [red] to 0.95 [blue], in 0.1 increments
101
102
Chapter 5
Mechanistic Insights into the
Formation of InP Quantum Dots 1 ,2
5.1
Introduction
This chapter presents a mechanistic exploration of colloidal III-V indium phosphide
quantum dot (InP QD) syntheses [91, 92, 93]. InP QDs are of increasing technological
interest as a replacement for CdSe QDs in visible light applications. However, the
synthetic methodology for InP QDs has not produced QDs with narrow size distributions relative to CdSe and PbSe QDs [91, 92, 93, 62, 94]. Studies of the molecular
mechanisms involved in the formation of QDs have only recently been reported for IIVI CdSe and IV-VI PbSe QDs [157, 158] As for InP QDs, the molecular mechanisms
underlying QD formation are essentially unknown. The reactions involved in InP QD
formation were investigated in order to understand the broad size distributions in
current InP QD syntheses.
In a simplified view of the formation of monodisperse colloids two general events
'This is the pre-peer reviewed version of the following article: P. A. Allen, B. J. Walker, M. G.
Bawendi. "Mechanistic Insights into the Formation of InP Quantum Dots." Angewandte Chemie
InternationalEdition, 2010, 49, 760-762 [156]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA.
Reproduced with permission.
2
Brian J. Walker (B.J.W.) and Dr. Peter M. Allen (P.M.A.) contributed equally to this chapter.
The experiments were designed and conceived by P.M.A and B.J.W. P.M.A. is responsible for the
synthetic aspects of the work, and B.J.W. is responsible for NMR data collection and analysis.
103
[In(MA) 3].(NH 2R)n
+
(MA)2|n
[In(MA) 3 ]
overall
(TMS) 3 P
R= C8H17
-jP
-
+ 3 TMS-X
{ln(MA)2[P(TMS) 2]} + TMS-X
(NH
2R)n
MA =
[InP]
(TMS)2
-{[P(TMS)3] (NH2R)n}_$---(N2]
1
-
0 2C- C13H27
2
X = NH-R or MA
Figure 5-1: Proposed mechanistic pathway for amine-inhibited InP synthesis. Both
the formation of outer sphere complex 1 and the irreversible formation of intermediate
2 are inhibited by increased solvation. Due to the large negative activation entropy
and the large rate decrease with added amine, a charge dispersion SN2 transition
state is inferred for TMS-X bond formation and P-TMS bond cleavage.
should occur; (i) an initial nucleation of colloids followed by (ii) subsequent growth
of these nuclei from molecular precursors [159, 160] Studies on the growth of CdSe
and PbSe QDs have shown these systems fulfill (i) and (ii) [157, 158] For InP QDs,
the molecular phosphorus precursors are completely depleted following InP nucleation, indicating that subsequent QD growth is due exclusively to ripening from
non-molecular InP species. The inability of InP QD syntheses to satisfy (ii), due to
depletion of molecular precursors, may explain the broad size distributions of InP
QDs relative to CdSe or PbSe QDs.
Colloidal InP QDs are synthesized by the injection of precursors into a hot solution of surfactants, or by mixing precursors at room temperature (RT) followed
by heating [91, 92, 93]. In these reactions, Indium (III) myristate [In(MA) 3] reacts
with tris(trimethylsilyl) phosphine [(TMS) 3 P] at elevated temperatures to produce
TMS-myristate (TMS-MA) and InP QDs (Figure 5-1, overall). By operating at reduced temperatures, with amines, it is possible to monitor the evolution of molecular
species during InP formation. From these experiments it is evident that amines inhibit
precursor decomposition, contrary to previous claims that amines act as "activating
104
Pre-injection
(TMS) 3 P
(a)
(b
TMS-MA
25 OC
No Amines
(c)
TMS-MA
178 0C
w/ Amines
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
ppm
Figure 5-2: 1 H NMR spectra of (a) (TMS) 3 P at 20 0 C before injection, (b) reaction
mixture of In(MA) 3 and (TMS) 3 P in toluene-d8 three minutes after mixing at 250 C,
and (c) reaction mixture of (TMS) 3P and 6:1 OA:In(MA) 3 in 1,2-dichlorobenzene-d4
after heating at 178 0 C for one minute. Both (b) and (c) demonstrate quantitative
conversion of (TMS) 3 P to TMS-MA shortly after the reactions are initiated.
agents" in InP QD synthesis [92, 161, 162, 163, 164].
A mechanism for amine-inhibited InP QD synthesis is proposed in Figure 5-1. Initially In(MA) 3 is coordinated to Lewis base(s), such as octylamine (OA), in the outer
(solvation) sphere. In the reversible first step, one [(TMS) 3 P] becomes incorporated
into the solvation sphere (complex 1). 1 then loses a myristate ligand, a stable In-P
bond forms, and the coordinated phosphine loses a TMS group, irreversibly forming
a molecular intermediate (complex 2). 2 reacts further to form [InP] clusters and
nanocrystals. Evidence that supports this mechanism will be described below.
5.2
Results and Discussion
To probe the evolution of molecular species during InP QD synthesis,
1H
NMR
spectroscopy was used to investigate species with trimethylsilyl (TMS) substituents.
In this reaction the TMS group is the only likely ligand for molecular phosphines,
105
so the high 1 H NMR sensitivity of the TMS group permits the observation of any
P-containing molecules or decomposition products present at significant concentration. Reactions were performed in sealed NMR tubes with 0.02 M In(MA) 3 , 0.01 M
(TMS) 3 P, 0.0-1.44 M OA, and 0.03 M diphenylmethane as an internal standard, in
toluene-d8.
In the absence of OA, a 1H NMR spectrum taken within three minutes of mixing In(MA) 3 and (TMS) 3 P at RT showed quantitative conversion of (TMS) 3 P to
TMS-MA (Figure 5-2a-b). The rapid decomposition of (TMS) 3 P is due to the direct
approach of (TMS) 3 P to the In center, circumventing outer sphere equilibria en route
to the production of a stable Si-O bond in the TMS-MA product. The exceedingly fast
conversion of (TMS) 3 P to TMS-MA at RT occurs on a time-scale that is not practical
for monodisperse QD synthesis or kinetic analysis by NMR [157, 158, 159, 160].
For literature procedures at elevated temperatures containing OA, no molecular
precursors remain shortly after reaction initiation at high temperature.
A sealed
NMR tube with a composition similar to those previously reported was heated at the
published reaction temperature (178 0 C)[92] and showed no (TMS) 3 P after 1 minute
(Figure 5-2c).
Our findings, with and without OA, demonstrate that previously
reported reaction conditions for InP QDs are based on ripening of non-molecular InP
species, similar to those used for InAs [165], and do not fulfill the requirements for
the formation of monodisperse colloids.
However, when OA containing InP reaction mixtures were run at 40 0 C, the reaction rate slowed sufficiently to permit kinetic analysis. From time-resolved 1 H NMR
spectra (Figure 5-3), four species account for 100% of all TMS groups.
(TMS) 3 P
and TMS-MA together account for over 90% of all TMS groups; the remaining TMS
groups are either associated with the intermediate 2 (Figure 5-3a, inset) or the silated
octylamine (TMS-OA) that occurs with <5% yield under standard conditions (Figure
5-4). The relative rates of (TMS) 3 P depletion and of TMS-MA growth are consistent
with near quantitative conversion of (TMS) 3 P to TMS-MA (Figure 5-3b).
Linear
evolution at early times facilitated the analysis of initial rates.
The doublet at 0.26 ppm was attributed to 2 as noted above. The doublet is
106
Complex 2
(TMS),P
TMS-Myristate
5 min
15 min -
0.35
0.25
25 min
-
35 min
45 min
55 min
I I | | | | |
0.45
0.35
0
0
0
V
0> .8A
CO
1-
A
0
A
0
0
0
A0
o3:
0
Time (min)
(a)
0
V
CL
o
5-3:
ppm
0
o
r
T
Figure
0.15
1.00
0.3-,
-
0.25
Time-resolved
Time (min)
1H
NMR
spectra
at
400C,
showing
evolution
of
(TMS)3P and TMS-myristate (TMS-MA) resonances during standard conditions for
InP
QD synthesis
with
amines.
Inset:
doublet at 0.26 ppm is assigned to 2.
TMS-MA
represented
protons,
in
determined
(a).
(c)
with
Concentration
1
Detail
of 15-minute 'H
NMR spectrum.
The
(b) Concentration profiles of (TMS)3P and
min
profiles
resolution
via
of
(TMS)3P
the
integration
for
InP
of
spectra
QD syntheses
with varying amine concentrations at 40 C, normalized at initial time. The reaction
tmTe
reacnhed
rate decreases with increasing amine concentration;reatinital
completion too rapidly to be resolved with our method.
107
20
40
100
80
60
Time (min)
Figure 5-4: Integration of all 'H NMR spectral peaks of all identifiable TMScontaining species throughout the reaction. The overall concentration of TMS protons
does not change during the course of the reaction.
F2
(ppm)
0.20--
0.25-0.30-(0J
CO
i
0.35
0.40
270
260
250
240
230
220
F1 (ppm)
Figure 5-5: 3 1 P-'H heteronuclear multiple bond coupling spectrum (HMBC) for the
InP reaction mixture at 25 C after 40 minutes. The horizontal axis corresponds to
the chemical shift for 3 1 P in ppm; the vertical axis is the chemical shift of 'H, also in
ppm. The only cross-coupled resonances correspond to (TMS) 3 P and intermediate 2.
108
split by 4.4 Hz (consistent with
31
P- 1 H 3-bond coupling) [166], which suggests a
P-containing species that retains at least one TMS group. This assignment was corroborated with a heteronuclear multiple bond coherence (HMBC) experiment, 3 which
showed that 'H resonances of (TMS) 3 P and 2 were cross-coupled to respective
31 p
resonances (Figure 5-5). The 'H NMR resonance of 2 has a line-width comparable to
that of molecular (TMS) 3 P and TMS-MA noted above, indicating that the species has
a distinct molecular structure. Finally, the time evolution of the 2 resonance shows
the characteristic growth and depletion of a reaction intermediate. Upon increasing
the OA concentration from 6:1 to 18:1 the rate of (TMS) 3 P depletion continued to decrease (Figure 5-3c); thus amines inhibit, rather than activate [92, 161, 162, 163, 164]
InP synthesis. As a change in amine ratio influences the rate even at high concentrations, and as the rates do not have a clear power dependence on OA concentration in
either this or prior studies [163] the amine does not have a well-defined stoichiometry
during the rate-determining step. At OA:In ratios greater than 18:1 a diminishing
change is evident in the rate with added amine. The high OA content reactions also
yielded an increase in the TMS-OA product, complicating kinetic analysis.
There was no evidence for other molecular phosphorus compounds, such as PH containing species. The only resonances in the
31 P
NMR spectra correspond to
(TMS) 3 P and 2 (Figure 5-6). When monitoring the depletion of (TMS) 3 P at various
OA concentrations, no doublets were observed in the 'H NMR that would indicate
the formation of P-H species, so this reaction does not appear to be a significant
pathway in the decomposition of (TMS) 3 P.
To account for the mechanism of amine inhibition the initial reaction rate was
measured at various temperatures, and these data are summarized on an Eyring plot
(Figure 5-7).
The activation entropy indicates that the reaction proceeds via an
ordered transition state, likely from an arrangement of two or more species. The
activation enthalpy (AHI= 51.9 ± 1.3 kJ mol-') is about 10 kJ mol-' less than that
measured by Liu et al. for CdSe nanocrystal formation [158] which is plausible for
3
1n HMBC, the only resonances that appear are those in which the relevant cross-coupling is
observed; in this case, 3 1P and IH.
109
iTh;,i~iriirf.7I
i
l
If.; ~~12 I I i~IIU~iSiM~
I
'
300
I
'
I
'
I
'
220
240
260
280
200
ppm
Figure 5-6: 'H-decoupled 31 P spectrum for the InP reaction mixture after 30 min at
25 C. Only resonances for (TMS) 3 P (241.3 ppm) and intermediate 2 (256.8 ppm)
are visible. Chemical shifts are referenced to 85% H3 PO 4 (0 ppm).
65
*
0.0029
55
I l
I
'
45
I
I
'
35
'
25
I
I
T(C)
'
I
Data
Linear Fit
0.0030
0.0031
0.0632
0.0033
0.0034
1/T(K-')
Figure 5-7: Eyring plot for amine-based synthesis of colloidal InP QDs, with AHt =
51.9 ± 1.3 kJ mol-1 and ASI = -126 ± 4 J mol- 1 K-1. Each point was taken in
triplicate, and temperature error was negligible.
110
the highly reactive precursors used during InP QD synthesis. The addition of OA
may decrease the rate (and increase AGI) by stabilizing the enthalpy of precursors
(increasing AHI), but OA may also influence the activation entropy.
The amine inhibition of InP synthesis results from solvation effects during the
steps that lead to complex 1, 2, or both.
As the concentration of a competing
Lewis base increases, the sterically-hindered (TMS) 3 P is less likely to approach the
In center. Thus an increase in amine concentration decreases the formation of 1,
and the observed saturation behavior may arise from crowding of the In(MA) 3 ligand
sphere.
The large negative ASI= -126 ± 4 J mol-1 K-- and the > 103 -fold decrease in rate
upon addition of amines are also consistent with solvation changes during nucleophilic
substitution [167]. Strong solvation effects are rarely observed for isopolar transition
states (e.g. for pericyclic reactions), so it is unlikely that the route from 1 to 2 occurs
in a single step. A potential step for this reaction is via a SN2 transition state that
results in charge dispersion. During charge dispersive SN2, charged species react to
form uncharged products. Because octylamine can hydrogen bond more strongly with
the attacking nucleophiles than with 2, an increase in amine concentration hinders
the formation of the transition state and decreases the formation of 2.
5.3
Conclusion
The currently-reported InP QD syntheses are based on non-molecular ripening processes. This study demonstrates that amines inhibit precursor decomposition which
can be rationalized via one or more solvation effects. It is apparent that the challenges
in the synthesis of III-V QDs are not due to differences in the bonding between 1I-VI
or IV-VI and III-V semiconductors, but instead result from the depletion of molecular
precursors following QD nucleation in current III-V QD syntheses.
111
5.4
5.4.1
Appendices to Chapter 5
Experimental Details
Reactions were performed in 600 mhz J-Young NMR tubes. Toluene-d8, 1,2-dichlorobenzene-d2 (C.I.L.), diphenylmethane, octylamine (Fluka) and all reaction solutions
were stored over 4 Amolecular sieves prior to use. (TMS) 3 P (Strem) was used without
further purification and In(MA) 3 was prepared as previously reported [168] NMR samples were prepared in toluene-d8 by mixing 0.35 ml of [In(MA) 3 , 0.04 M)], diphenylmethane [0.06 M internal standard], and [OA , 0.0-1.44 M] with 0.35 ml of [(TMS) 3 P,
0.02 M] in a nitrogen filled glovebox and then immediately transferred into a variable
temperature 300 mhz or 500 mhz Varian NMR for 'H and
31 P
analysis. A detailed
explanation of the kinetic analysis can be found below.
5.4.2
Synthetic details
Reagents: Octylamine (Fluka, 99%, dried over calcium hydride), Toluene d-8 (Cambridge Isotope Labs), diphenylmethane (Sigma, >99%, degassed), and 1,2-dichloromethane
(Cambridge Isotope Labs) were placed into a nitrogen filled glovebox and stored over
4 Amolecular sieves prior to use. Myristic acid (Sigma, >99%), 1-octadecene (Tech
grade, Sigma), hexanes (anhydrous, Acros) and Indium acetate (99.99%, Alfa Aesar)
were used to synthesize Indium (III) myristate [In(MA) 3]. Trimethylsilyl chloride
(Fluka, 99%, re-distilled) was used to synthesize TMS-OA and TMS-MA NMR reference compounds. Tris(trimethylsilyl) phosphine [(TMS) 3 P, caution pyrophoric] was
obtained from Strem and used as received. J-young NMR tubes (Chemglass, 600
mhz) were dried at 120 C prior to use. All other glassware was dried at 180 C. All
samples were prepared under minimal lighting in a nitrogen filled glovebox (mBraun,
<0.1% oxygen).
Synthesis of In(MA) 3 : The synthesis of In(MA) 3 was performed as previously
reported [168] All solutions containing In(MA) 3 were additionally dried for two days
over 4 Amolecular sieves to remove residual water.
112
NMR Solutions: Example of a typical solution preparation for OA:In (6 :1): Solution (A) 0.24 mmol of In(MA) 3 , 1.44 mmol of OA, and 0.36 mmol of diphenylmethane
were mixed in 6 ml of toluene-d8. In(MA 3 dissolves readily in the presence of amines.
Without amines, the solution was heated to 60 C to produce a homogeneous solution. The solutions were stored over 4 Amolecular sieves for two days prior to use.
Solution (B) 0.12 mmol of (TMS) 3 P and 6 ml toluene-d8 were combined and stored
in the dark at -35
C prior to use.
NMR Reaction Tubes: In a typical reaction 0.35 ml of solution (A) and 0.35 ml of
solution (B) were injected into a J-young NMR tube in the glovebox. The NMR tube
was then immediately transferred for NMR measurements. All samples were handled
under minimal lighting to prevent decomposition of (TMS) 3 P.
NMR Spectroscopy: All spectra were taken on 300 MHz Mercury or 500 MHz Varian Inova NMR spectrometer using either a variable temperature broadband switchable probe or a variable temperature indirect detection probe. Synthesis of TMS-MA:
10 mg of trimethylsilyl chloride was added to a stirred solution of 60 mg of myristic
acid and 50 mg of tri-n-octylphosphine in 1 ml of toluene-d8 and heated to 60'C. No
further purification was performed prior to 1H NMR analysis (Figure 5-8).
Synthesis of TMS-OA: 10 mg of trimethylsilyl chloride was added to a stirred
solution of 75 mg of octylamine (OA) in 1 ml of toluene-d8. The solution was filtered
to remove the OA-HCl salt prior to 1 H NMR analysis (Figure 5-9).
5.4.3
Kinetic analysis: general rate determinations
All kinetic data was obtained via integrals of the 'H NMR spectra of the samples
described above. Each spectrum in the series retained the same integration interval
throughout for all times, and all integrals were referenced to an internal diphenylmethane standard. The colloidal reaction products had the optical absorption spectra
expected for nucleation and growth at that concentration and temperature condition
(Figure 5-10).
The only identifiable TMS-containing species are (TMS) 3 P, TMS-MA, the intermediate In(MA) 2 P(TMS) 2 (2) and the TMS-OA. The sum of these four species is
113
0.5
0.4
0.3
0.2
0.1
0.0
ppm
Figure 5-8: 'H NMR spectrum of TMS-MA (0.303 ppm) in toluene-d8 referenced to
diphenylmethane (3.771 ppm).
0.5
0.4
0.2
0.3
0.1
0.0
ppm
Figure 5-9: 'H NMR spectrum of TMS-OA (0.132 ppm) in toluene-d8 referenced to
diphenylmethane (3.771 ppm).
114
InP
0.8-
a) 0.6(U
-
0
0.4-
0.2 0.0 300
400
500
600
Wavelength (nm)
Figure 5-10: Absorbance spectra of InP QDs grown in an NMR tube at 40 0 C for 60
minutes in the presence of octylamine.
constant throughout (Figure 5-4), indicating that our analysis accounts for all TMS
groups introduced during the reaction.
The method of initial rates was used using early time reaction data at i 15%
conversion to fit a linear regression. The slope of the fit line gave the rate of the
reaction. This rate was then used to determine an experimental rate constant, using
relationship from the experimental rate law
rate
[In(MA) 3]x[(TMS) 3 P]Y
From reactions run with variable (TMS) 3 P, 0 < x < 1 (Figure 5-11). A noninteger value for the order in (TMS) 3 P is plausible, as the TMS groups are each
present with 1/3 the molar ratio of the (TMS) 3 P and as each TMS group on every
possible intermediate likely has a different relative reactivity. The heterogeneous
reactivity permits only the determination of the average order in (TMS) 3 P for the
ensemble. It is inferred that for the conditions studied, on average no more than 1
P-containing species enters the transition state of the rate limiting step.
115
0.00020 U
0.00015
U
U
E
0.00010 U
0.00005
.
0.005
0.010
0.015
0.020
[(TMS) 3P], (mol/L)
Figure 5-11: Plot of (TMS) 3 P concentration vs. rate (measured from the early time
approximation) with a fixed concentration of In(MA) 3 , 0.02 M at 25 C. The rate
does not follow a clear order dependence in (TMS) 3 P concentration.
-5
E
0.00028
-
0.00026
-
0.00024
-
U
U
U
0)
0.00022
-
0.00020
-
U
0.006
0.008
0.010
0.012
0.014
[In(MA)3], mol/L
Figure 5-12: Plot of In(MA) 3 concentration vs. rate (measured from the early time
approximation) with a fixed concentration of (TMS) 3 P, 0.02 M at 25 C. The rate
does not follow a clear order dependence in In(MA) 3 concentration.
116
0.006O
-
S
0.005-
[10/6/2009 01:21 "/1myris40" (2455110)]
Linear Regression for myris4OB:
Y=A+B*X
0.004-
Parameter
E
<
B
Linear Fit of myris4oB
Value
Error
A
B
0.0017 2.93814E-5
5.12143E-4
6.56988E-6
R
SD
0.003 -
0.002
0.99959
0
2
4
6
N
P
3.47645E-5
7
<0.0001
8
Time (min)
Figure 5-13: Evolution of TMS-MA concentration vs. time measured for <15% conversion as described above. This is the first replicate of 40 C data for Eyring analysis.
The slope, (5.12 mol L1 min-1) is the rate of reaction.
Similarly, there was no evidence that the average order in In(MA) 3 is greater than
1 (ie 0 < y < 1), and the bounds are also sensible for similar reasons as the order in
(TMS) 3 P (Figure 5-12).
For Eyring analysis, the order in (TMS) 3 P and in In(MA) 3 is applied uniformly to
each rate to estimate k, so any error in estimating x and y affects only the activation
entropy, not the activation enthalpy.
An upper bound was obtained on AS, by
computing k with x = y = 1. This upper bound on ASI (-126 ± 4 J mol-1 K-1) is
still a large negative value, as noted above.
5.4.4
Representative kinetics data
Time-resolved data points obtained from the relevant 1 H NMR spectra are shown in
Figures 5-13, 5-14, and 5-15 for three replicate InP syntheses, at 40 C.
117
0.006O C
Linear Fitof myris4OC
[10/6/2009 01:25 "2myris4O" (2455110)]
Linear Regression
for myrls4OC:
Y=A+B*X
-
0.005 -
0.004 Parameter
Value
Error
0.003 -
A
B
0.0013 6.62555E-5
5.46429E-4
1.48152E-5
0.002 -
R
SD
0.99817
N
P
7.83946E-5
7
<0.0001
0.001 1I
0
2
4
6
8
Time (min)
Figure 5-14: Evolution of TMS-MA concentration vs. time measured for <15% conversion as described above. This is the second replicate of 40 C data for Eyring
analysis. The slope, (5.46 mol L- 1 min-') is the rate of reaction.
0.007-
0
---
D
Linear Fit of myris40 D
0.006-
[10/6/2009 01:23
(2455110)]
Linear Regressionfor myris4OD:
Y=A+B*X
0.005-
Parameter
N/3myris4O"
Value
Error
A
B
0.00209
5.91429E-4
1.90552E-4
3.95897E-5
R
SD
P
0.004-
0.003-
0.99116
N
1.65616E-4
6
1.16952E-4
Time (min)
Figure 5-15: Evolution of TMS-MA concentration vs. time measured for <15% conversion as described above. This is the second replicate of 40 0 C data for Eyring
analysis. The slope, (5.91 mol L- min-1) is the rate of reaction.
118
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Brian J. Walker
77 Massachusetts Ave
Room 18-080
Cambridge, MA 02139 USA
bwalkr@mit.edu
EDUCATION
Massachusetts Institute of Technology
Cambridge, MA
PhD in physical chemistry, expected June 2011
Thesis: Nanocrystal/J-aggregate Constructs: Chemistry, Energy Transfer, and Applications
Advisor: Moungi G. Bawendi
Cornell University
Ithaca, NY
BA in chemistry, May 2006
4.03 GPA (Summa Cum Laude, with distinction in all subjects)
Thesis: Catalysts in Polymer Capsules: Progress toward Efficient Multistep Syntheses
Advisor: D. Tyler McQuade
HONORS
Herchel Smith Postdoctoral Fellow for Materials in Sustainable Energy, 2011
National Science Foundation Graduate Research Fellow, 2009-2010
National Defense Science and Engineering Graduate Fellow, 2006-2009
Merrill Presidential Scholar, 2006 (top 1% of University class for academics and leadership)
Leo and Berdie Mandelkern Prize, 2006 (top award from Cornell Chemistry Department)
Robert B. Holden Research Scholar, 2005 (1 award for undergraduate Cornell chemists)
Cornell Presidential Research Scholar, 2004-2006
Eagle Scout
RESEARCH FUNDING
Herchel Smith Postdoctoral Fellowship, 2010 (E38. 1k/year, 3 yr)
National Science Foundation Graduate Research Fellowship, 2007 ($40.5k/yr, 3 yr)
National Defense Science and Engineering Graduate Fellowship 2006 ($63k/yr, 3 yr)
Robert B. Holden Scholarship, 2005 ($4.5k)
Cornell Presidential Research Scholarship 2004-2006, ($5k)
129
PUBLICATIONS
8.
B. J. Walker, M. G. Bawendi. "Coupling Between J-aggregates and Excitonic Inorganic
Materials."
7.
J-aggregates,Volume 2. World Scientific, Singapore.
B.J. Walker, A. Dom, V. Bulovid, M.G. Bawendi. "Color-selective photocurrent
enhancement in coupled J-aggregate/nanowires formed in solution." Submitted.
6.
D. Harris, P.M. Allen, H.-S. Han, B.J. Walker, J.M. Lee, M.G. Bawendi. In Press
Journalof the American Chemical Society.
5.
B.J. Walker, V. Bulovid, M.G. Bawendi. "Quantum Dot/J-aggregate Blended Films for
Light Harvesting and Energy Transfer." Nano Letters. 2010, 10, 3995-3999.
4.
*P.M. Allen, *B.J. Walker, M.G. Bawendi. "Mechanistic Insights into the Formation of
InP Quantum Dots." Angewandte Chemie InternationalEdition. 2010, 49, 760-762.
3.
B.J. Walker, G.P. Nair, L.F. Marshall, V. Bulovid, M.G. Bawendi. "Narrowband
Absorption Enhanced Quantum Dot/J-aggregate Conjugates." Journalof the American
ChemicalSociety. 2009, 131, 9624-9625.
2.
J.E. Halpert, J.R. Tischler, G.P. Nair, B.J. Walker, W. Liu, V. Bulovid, M.G. Bawendi.
"Electrostatic Formation of Quantum Dot/J-aggregate FRET Pairs in Solution." Journal
of PhysicalChemistry C, 2009, 113, 9986-9992.
1.
K.E. Price, S.J. Broadwater, B.J. Walker, D.T. McQuade. "A New Interpretation of the
Baylis-Hillman Mechanism." Journalof Organic Chemistry, 2005; 70(10); 3980-3987.
PATENTS
3.
G. M. Akselrod, B.J. Walker, W. A. Tisdale, J.R. Tischler, M.G. Bawendi, V. Bulovid.
"Luminescence enhancement by resonant energy transfer from an absorptive thin film." Filed
October 2010, patent pending.
*
These authors contributed equally to this work.
130
2.
B.J. Walker, A. Dom, V. Bulovi6, M.G. Bawendi. "Color-selective photocurrent
enhancement in coupled J-aggregate/nanowires formed in solution." Filed December 2010,
patent pending
1. B.J. Walker, V. Bulovid, M.G. Bawendi. "Quantum Dot/J-aggregate Blended Films for Light
Harvesting and Energy Transfer." Filed May 2010, patent pending.
PRESENTATIONS (* denotes invited)
Materials Research Society Fall Meeting. "Assembly and Analysis of Semiconductor
Nanocrystal/J-aggregate Constructs and Applications to Light Harvesting and Energy Transfer."
Poster Presentation, Boston, December 2010.
*University of Cambridge, Department of Physics. "Surface chemistry and energy transfer
between semiconductor nanocrystals and cyanine J-aggregates." Nov. 12, 2010.
Gordon Research Conference, Electronic Processes in Organic Materials. "Nanocrystal/Jaggregate blended films for light harvesting and energy transfer." Poster presentation, Holyoke,
July 2010
Materials Research Society Fall Meeting. "Narrowband Absorption-Enhanced Quantum Dot/Jaggregate Conjugates and Device Applications." Oral presentation, Boston, November 2009.
MIT Department Seminar: Metals in Synthesis. "Mechanistic studies in the synthesis of
monodisperse semiconductor nanocrystals." Oral presentation, Boston, December 2009
Gordon Research Conference, Clusters, Nanocrystals, and Nanostructures. "Narrowband
Absorption-Enhanced Quantum Dot/J-aggregate Conjugates in Solution." Poster presentation,
Holyoke, July 2009
American Chemical Society National Meeting. "Narrowband Absorption-Enhanced Quantum
Dot/J-aggregate Conjugates in Solution." Oral presentation, Salt Lake City, March 2009.
PROFESSIONAL MEMBERSHIPS
Materials Research Society
November 2007-present
American Chemical Society
August 2006-present
MIT Energy Club
February 2008-present
131
PROFESSIONAL ACTIVITIES
Representative, MIT Committee on Toxic Chemicals
September 2008-present
Mediator, MIT Resources for Easing Friction and Stress
October 2007-present
Environmental Health and Safety Representative, Bawendi Group
June 2008-May 2009
Graduate Student Symposium Planning Committee
July 2008-August 2010
American Chemical Society
132