SUPPRESSED CARRIER SCATTERING IN CADMIUM SULFIDEENCAPSULATED LEAD SULFIDE NANOCRYSTAL FILMS
Upendra Rijal
A Thesis
Submitted to the Graduate College of Bowling Green
State University in partial fulfillment of
the requirements for the degree of
MASTER OF SCIENCE
August 2014
Committee:
Mikhail Zamkov, Advisor
Lewis P. Fulcher
Alexey Zayak
ii
ABSTRACT
Mikhail Zamkov, Advisor
One of the main problems on the development of efficient nanocrystal devices is the weak
strategy to deposit colloidal nanocrystals into films. The current work demonstrates the comparative
study of the several mechanisms to make nanocrystal films with the use of all-optical approach to
examine the dynamics of the electron transport process. The mechanism of carrier scattering on trap
states/defects is distinguished exclusively from other charge or energy transfer processes through
exciton dissociation by measuring the lifetimes for excited charges in matrix encapsulated or ligand
linked PbS nanoparticle solids, featuring a varying amount of ZnS nanocrystals. The carrier mobility and
the diffusion lengths for all types of solids within the hoping transport regime were then estimated using
the measured lifetimes. The matrix capped nanoparticle solids have shown a smaller probability for the
carrier scattering than that of nanoparticle films cross linked with 1,2-ethanedithiol molecule or 3mercaptopropionic acid. The density of the traps on the surface of nanoparticles/matrix interfaces are
comparatively lower due to the suppressed charge scattering in matrix capped nanoparticle films.
iii
ACKNOWLEDGMENTS
I would like to extend my sincere gratitude to my supervisor, Dr. Mikhail Zamkov for
his incredible support, guidance, and encouragement to enhance my graduate experience as
good researcher and graduate student. I am glad to a part of his research group, which has
opened a new way in my life.
I am also thankful to other committee members, Dr. Lewis Fulcher and Dr. Alexey
Zayak, for the time they spent to go through my manuscript.
Additionally, I am thankful to all the members of Dr. Zamkov research group, especially
Pavel, Geethika, and Prakash for their amiable collaboration and help. I would also like to thank
Matthew for his help during the work.
Lastly, I would also like to thank my family for the support and, in particular, my wife,
Padma, without whose love, encouragement, and editing assistance I would not have finished this
thesis.
v
TABLE OF CONTENTS
Page
CHAPTER 1. INTRODUCTION ..........................................................................................
1
CHAPTER 2. EXPERIMENTAL ........................................................................................
7
2.1 Materials ...........................................................................................................
7
2.2 Methods
...........................................................................................................
7
2.2.1 Synthesis of PbS quantum dots ............................................................
7
2.2.2 Preparation of CdCl2 precursor for the synthesis of hybride passivated
PbS quantum dots ..............................................................................
8
2.2.3
Synthesis of the hybride passivated PbS quantum dots .....................
8
2.2.4
Synthesis of PbSCdS coreshell quantum dots ...................................
9
2.2.5
Synthesis of ZnS quantum dots .........................................................
9
2.2.6
Preparation of the FTO/glass substrate ..............................................
10
2.2.7
Fabrication of the nanocrystal films ..................................................
10
2.2.8
In-filling of SMENA pores with ZnS ................................................
11
2.3 Characterization ...................................................................................................
12
CHAPTER 3. RESULTS AND DISCUSSION ..................................................................
13
CHAPTER 4. CONCLUSION ............................................................................................
33
REFERENCES
34
...........................................................................................................
vi
LIST OF FIGURES
Figure
1.1
Page
Band structure of type I and type II nanocrystal core/shell heterostructures. (a)
Type I Nanocrystal (b) Type II nanocrystal ..............................................................
1.2
The matrix encapsulated method to determine the carrier trapping rates in colloidal
semiconductor nanocrystal films nanocrystal film ....................................................
3.1
1
4
(a) Sketch of the general strategy for encapsulation of colloidal quantum dots into
matrices. (b-c) Typical TEM images of PbS/CdS core/shell and ZnS NCs used as
as nanoparticle precursors during film assembly. .....................................................
3.2
14
Absorbance spectra of the PbS cores (3.2-nm-black) and PbSCdS coreshell NCs
(3.2-nm-brown) featuring 3.0 nm core diameter .......................................................
15
3.3
Different stages of NC films seen on XPD spectrum ................................................
16
3.4
(a). Emission and FL intensity decay of PbSCdS core(shell) nanocrystals in
solution(ΔHCdS = 0.32 nm). (b). FL intensity decay of CdS embedded in PbS NCs
into matrices (Redge = 0.64 nm) with increasing amount of ZnS NCs in the film.
(c). Biexponential curve of the Fluorescence intensity decay indicating fast and
slow components .......................................................................................................
3.5
FL intensity decay of oleic acid-capped PbS nanocrystals in solution (black) and .
in a film ......................................................................................................................
3.6
17
The dynamics of the fluorescence decay intensity CdS embedded PbS nanocrystal
films. (a). Development of fast component of the fluorescence Intensity Decay
with growing amount of ZnS nanocrystals in the solid. The FL lifetime
(without-ZnS) of PbS films gives the exciton dissociation time. (b). Slow
20
vii
component of fluorescence intensity decay evolved with increasing amount of
ZnS nanocrystals in the solid. (c) Photoconductivity . measured on the same solids
as in (a) and (b) ..........................................................................................................
21
3.7
The positions of the excited energy levels of PbS QDs and bulk CdS ......................
22
3.8
(a). FL Intensity decay of weakly-coupled PbS NCs embedded into CdS matrices
(Redge = 2.7 nm) with increasing amount of ZnS NCs in solid. (b). Development
of the fast component of Flourescence decay with increasing amount of ZnS
nanoparticles in solid. (c) Development of the slow component of flourescence
decay with increasing amount of ZnS nanoparticles in solid ....................................
3.9
24
Flourescence decay of CdS encapsulated PbS nanoparticle solids (Redge = 0.64 nm)
before (black) and after (red) the in filling ....................................................................
25
3.10 (a). Sketch of SILAR method to fill the pore with ZnS. (b). The result of
implementation of the SILAR mechanism on Fl decay of CdS encapsulated
PbSZnS NC solids having lowered energy and charge transfer. The reduction of
rate of charge trapping resulted in the higher luorescence lifetime of the pore filled
NC film ......................................................................................................................
25
3.11 The FL intensity decay curve of PbS nanoparticles capped with MUA
(in methanol)..............................................................................................................
27
3.12 (a-c). Fluorescence intensity curve of EDT passivated PbS nanoparticles in
solution (a), MPA (b), MPA/Cl (c) different environment (d-f). Pl intensity decay
of PbS nanoparticle solids with EDT (d), MPA (e), and MPA/Cl (f) Functions
of raised amounts of ZnS NCs in the solid. (g-j) Fast and slow components Pl
intensity lifetime of PbS NCs ...................................................................................
28
viii
LIST OF TABLES
Table
T3.1
Page
FL Fluorescence intensity lifetime data and calculated transport properties of
matrix-encapsulated and ligand-linked PbS NC films .............................................
T3.2
32
Investigated drift scattering lengths for CdS matrix encapsulated and ligand linked
PbS NC films .............................................................................................................
32
1 CHAPTER 1. INTRODUCTION
A quantum dot is defined as a semiconductor material of nanoscale size, which confines
the motion of the electrons in the conduction band, of the holes in the valence band, or excitons
in all three spatial directions. Quantum dots demonstrate very interesting characteristics because
of their small size. These particles have recently proved themselves as promising materials with
applications such as solar cells, LEDs, LASERs, bio-imaging and other medical applications. A
hetero-structured semiconductor quantum dot is formed by developing one type of
semiconductor enclosed by different type of semiconductor. Such hetero-structure quantum dots
show unique optical and electronic properties which are not possible to achieve from either of
the semiconductors. A quantum dot core that is completely covered by the shell is called
coreshell quantum dot. The relative position of the energy band level of the coreshell determines
the optical and electronic properties. On the basis of the relative position of the conduction band
and valence band of semiconductor in the coreshell, the coreshells are classified into three types.
If the energy band gap of the shell is larger than that of core, and it lies completely outside, then
those coreshells are called type I coreshells (figure 1.1 a). If the bandgap of the core is wider than
Figure 1.1: Band structure of type I and type II nanocrystal core/shell heterostructures. (a) Type
I nanocrystal (b) Type II nanocrystal
2 that of the shell (exact opposite of type I), the coreshell is called a reverse type I coreshell.
Whereas if both valence and conduction band of shell lie above that of the core, then the
coreshell is known as type II (figure 1.1b).
The application of promising semiconductor nanocrystals has grown rapidly over the
past several years due to their properties such as quantum confinement and size tunability.1 This
process helps to optimize the film’s main characters, either increasing the quantum yield using
the core shell,2 managing to adjust the band gap,3 or adjusting the driving force at the junction
of the heterostrucutre.4 The thin film making process from the colloidal nanocrystal (quantum
dot) solution is cheaper than the regular process of making films, which requires extreme
conditions, namely high vacuum and high temperature. Due to these advantages over traditional
methods, different experimental works have been performed in the past to develop better
methods of making thin films from colloidal nanocrystals, and to improve the applications such
as LASERs,5-7 Photovoltaics (solar cells),8-27 LEDs,26-28 and FETs.29-35
The main reason behind low efficiency of semiconductor nanocrystal films is poor
conductivity, which is basically due to either energy disorder36 or trap states.37,38 It has already
been shown that the unpassivated anions form trap states above the highest level of valence
band and the cationic dangling bonds develop traps below the lowest level of the conduction
band.39 Due to significantly high surface-volume ratio of the semiconductor quantum dots, the
trap states can have substantial density. The charge transport in quantum dots is hence affected
by the recombination of electric charges on the surface of the dot or on the surface of other dots,
which is harmful to the efficiency of the device27.
The semiconductor quantum dots thin films are traditionally made by joining adjacent
quantum dots with inorganic or organic ligands. One of the major drawbacks of this method is
3 that the number of ligands must be equal to number of surface atoms. The inequality between
number of ligands and number of surface atoms results in the high density of trap states within
the nanocrystal band gap, which causes charge scattering to trap states. The density of trap
states on semiconductor nanocrystals for either organic or inorganic ligand nanocrystal films is
of the order of 1017 cm-3eV-1 as shown by the recent work of the Sargent group. 40 This result is
higher than the density of trap states of most crystalline semiconductors by at least three orders
of magnitude. The hybrid passivation method27, on the other hand, employs both organic and
inorganic ligands, which reduces the density of the trap states to about 2×1016 cm-3eV-1. Thin
film nanocrystal devices still need thorough treatment for the unpassivated surfaces to increase
the efficiency of the devices. It is therefore obvious to have trap states on the thin film if the
traditional ligand based strategy is followed to make the films.
The substitute for the traditional method is given by a ligand free film deposition
method of colloidal semiconductor nanocrystal.41-45 It is based on the inorganic ligands for the
surface passivation of NCs in a solid. Currently, we are particularly interested in a recently
developed matrix-encapsulation method, where CdS or ZnS quantum dot cores, which have
wide band gap are used to fill the gap between the PbS cores.43,44 The CdS or ZnS quantum dots
nanocrystals inserted between the PbS nanocrystals preserves the quantum confinement
effectively by protecting the quantum dot surface from interacting with neighboring quantum
dots. The main benefit of the matrix encapsulated method is that the ions (cations or anions) on
the surface of the quantum dots are forced to develop bonds with respective cations and anions
of the CdS or ZnS matrix. It stops the direct formation of trap states. The tunneling of the
charge can still occur on the matrix boundaries. However, the possibility of the charge trapping
is relatively low because of the wide band gap of CdS matrix. Hence, the formation of trap
4 states can be lowered by using the matrix encapsulated method over the traditional ligand based
methods.
Figure 1.2: The matrix encapsulated method to determine the carrier trapping rates in colloidal
semiconductor nanocrystal film.
In the current work, the electrical transport characteristics of matrix encapsulated and
ligand linked NC films will be compared by measuring the rate of charge trapping to trap states,
neighboring dots, and radiation recombination. In order to compare the dynamics of defects from
radiation recombination, insulating ZnS quantum dot cores are inserted in between NCs of a PbS
film in a controlled way, which helps to reduce the transfer of charges or energy to adjacent dots
5 (Fig. 1.2). The ligand-free film deposition method proves itself as a potential alternative to
the ligand based film deposition method, where a stable inorganic medium is used to
passivate the surface of the nanocrystal film. In this work, we are particularly interested in
the recently reported matrix encapsulated method, which uses materials having large band
gap such as CdS or ZnS quantum dots to separate the CdS quantum dots.45 The middling
matrix protects the quantum confinement of the surrounded quantum dots while preserving
the surfaces from possible interactions with surroundings. The main advantage of such
method is the idea of surface passivation, which forces all anions and cations on the outer
surface of the semiconductor quantum dots to stay heteroepitaxially bonded with their
respective counter ions of CdS matrix. This method prevents formation of trap states directly
on the surface of the quantum dots or reduces the trap states. Besides, the charge trapping can
still occur at the junction between the CdS matrix and quantum dots arrays. The possibility of
the charge trapping is comparatively low because of the quantum confinement produced by
the potential barrier developed by matrix. As a consequence, the development of trap states
are expected to be more suppressed in matrix encapsulated quantum dot films compared to
ligand-linked quantum dot films.
It can be proven that the rate of tunneling of the charge on the trap states is less for
solids made from the matrix encapsulated method than in the solids made from traditional
methods, where the QDs are connected by ligands, if the amount of ZnS QDs in the PbS QD
films is varied. Apart from the rate of charge scattering, the dynamics of charge decay can
also be used to investigate the exciton dissociation rate. The measured parameters are then
changed to diffusion length and charge mobilities. It has been found that the films of PbS
quantum dots covered by CdS shells produced the highest carrier diffusion length with 3-
6 mercaptopropionic acid (MPA)-linked PbS films containing the surfaces passivated by Cl. In
the meantime, the carrier diffusion with the shortest length was given by the 1,2ethanedithiol
(EDT)-linked QD films.
In general, a relative examination of charge dynamics in nanocrystal films involving
different types of matrices and ligands have revealed the fact that the matrix encapsulation
method might eventually offer better strategies to passivate the surface of quantum dots
leading to greater suppression of carrier scattering.
7 CHAPTER 2. EXPERIMENTAL
2.1 Materials
Chemicals. 1-octadecene (ODE, 90% Aldrich), oleic acid (OA, 90% Aldrich), cadmium
oxide (CdO, 99.99%, Aldrich), lead(II) oxide powder (PbO, 99.999% Aldrich), sodium sulfide
nonanhydrate (Na2S•9H2O, 98% Alfa Aesar), sulfur (S, 99.999% Acros), ethanol (anhydrous, 95%
Aldrich), hexane (anhydrous, 95% Aldrich), methanol (anhydrous, 99.8% Aldrich), toluene
(anhydrous, 99.8% Aldrich), isopropanol (anhydrous, 99.8% Acros), octane (anhydrous, 99%
Aldrich), 3-mercaptopropionic acid (MPA, 99% Alfa Aesar), 11-Mercaptoundecanoic acid (MUA,
95% Aldrich), Diethylzinc (Et2Zn, 15% wt., 1.1 M solution in toluene, Aldrich), bis(trimethylsilyl)
sulfide ((TMS)2S, Aldrich, synthetic grade), Tri-n-octylphosphine (TOP, 97% Strem), cadmium
chloride (CdCl2, 99.99% Aldrich), acetone (anhydrous, Amresco, ACS grade), zinc acetate (98+%
Acros), Tetradecylphosphonic Acid (TDPA, 97% Aldrich), 1,2-ethanedithiol (EDT, 98+% Fluka),
and triton X-100 (Alfa Aesar) were used as purchased. Argon atmosphere was provided using the
standard Schlenk method to all the reactions.
Fluorine-doped tin oxide (FTO) glass (TEC 15, 12-14 Ohm/sq) was received from Pilkington
Glass.
2.2 Methods
2.2.1 Synthesis of PbS quantum dots
For the synthesis of PbS NCs, the procedure developed by Hines, et al46 was adopted. First of
all, 0.49 gm of PbO, 18 ml of ODE, and 1.5 ml of OA were placed in a 100 ml three neck flask
containing a magnetic stirrer. The mixture was then degassed for two hours at 1200C. The
temperature of the mixture was further lowered to 1100C by setting up Argon flow. At the same
time, 10 ml of ODE was degassed for 2 hours at 1200C using a magnetic stirrer in another 50 ml 3
8 neck flask. After 2 hours, the mixture was pumped off, lowered to room temperature, and Argon gas
was pumped in. When the temperature of the second flask reached room temperature, 0.21 ml of
(TMS)2S (source of Sulfur) was added to the flask. It was stirred for five minutes and the mixture
from the second flask was added to the first flask. At that point, the mixture in first flask was cooked
for three minutes, and the temperature of the flask was reduced to room temperature using a water
bath.
When the mixture in the first flask was reduced to room temperature, the PbS quantum dot
solution was poured in equal amounts into 6 centrifuge tubes. 14 ml of Acetone were added in each
tube and centrifuged for 5 minutes. The PbS QDs were redispersed in hexane after separating the
QDS from solution containing from the tubes. Again, Acetone was added on each tubes up to 14 ml
and centrifuged for 5 minutes as before. The QDs are further separated from the solution containing
it and dispersed with hexane. Finally the PbS QDs were placed in a cleaned vial and labeled it.
2.2.2 Preparation of CdCl2 precursor for the synthesis of hybrid passivated PbS NCs.
For additional passivation of PbS quantum dots, the metal-halide precursor was prepared as
follows. 0.033gm (0.12 mmol) of TDPA and 0.30 gm (1.64 mmol) were placed in 5 ml Oleylamine
and degassed for 18 hours at 1000 C. Then the temperature was lowered to 800 C and the Argon gas
flow was set up to avoid solidification.
2.2.3 Synthesis of the hybrid passivated PbS nanocrystals.
Procedure reported by Ip et al.27 was used to prepare the hybrid passivaed PbS quantum dots.
PbS quantum dots with OA as ligands were synthesized by the regular procedure as explained above.
The synthesis process was further followed by reaction mixture while cooling. When the flask was
removed from the heating mantle, 1 ml solution from the CdCl2 solution was added in the reaction
mixture. The temperature was raised to 300C. As the temperature reached 300C, the quantum dots
9 were precipitated adding 50 ml Acetone. After that, the quantum dot solution was centrifuged and
nanocrystals were separated from other solvents and unwanted materials. The nanoparticles were
then redispersed in 15 ml of Hexane. This cleaning cycle was repeated two times to clean the
quantum dots better. Finally, the quantum dots were redispersed in hexane (5-6 ml).
2.2.4 Synthesis of PbSCdS coreshell quantum dots
PbSCdS nanoparticles were synthesized using the procedure reported in Ref. 47 by cation
exchange method. First of all, 0.30 gm of CdO, 10 ml of ODE, and 4 ml of OA were taken in a 100
ml three neck flask containing a magnetic stirrer and flow of Argon was set up. The mixture was
then heated for two and half hours at 2350C. When the solution in the flask became clear, the
temperature of the flask was reduced to 1350C. 4 ml of PbS NCs were then added to the flask and
cooked for 2 minutes. Samples of PbSCdS core shells from the flask were taken and the fluorescence
peak was checked constantly. Once the desired size of the quantum dots were formed, the reaction
was stopped using a water bath.
When the mixture in the first flask reached room temperature, the PbS quantum dot solution
was poured into 6 centrifuge tubes in equal amounts. 14 ml of Ethanol were added in each tube and
centrifuged for 5 minutes. The PbS QDs were redispersed in hexane after separating the QDs from
solution containing from the tubes. Again, Ethanol was added in each tube up to 14 ml and
centrifuged for 5 minutes as before. The QDs were further separated from the solution and dispersed
within hexane. Finally the PbS QDs were placed in a cleaned vial and labeled.
2.2.5 Synthesis of ZnS core quantum dots
ZnS quantum dots were synthesized using the typical procedure as follows. First of all, 2 ml
TOP were placed in a 3 neck flask and degassed at 1200C for 2 hours. After 2 hours, it was switched
with Argon flow. 0.2ml (TMS)2S and 1.0 ml diethyl zinc were separately injected into the flask
10 containing TOP. The temperature of the solution was reduced to 900C and stirred for 30 minutes. In
the meantime, 5 ml Butanol were injected into the flask to prevent the solidification of TOPO. The
solution containing ZnS quantum dots in the flask was cooled down to room temperature. The QD
solution was poured into 6 centrifuged tubes, and 14 ml of Acetone were added to each tubes. Then
the solution in the tubes was centrifuged for 5 minutes and the quantum dots were separated from the
solvent and other unwanted solutions. The quantum dot solution was centrifuged one more time to
make it cleaner. Finally, the quantum dots were redispersed in Hexane (5-6 ml) and kept in a clean
vial.
2.2.6 Preparation of the FTO/glass substrate.
For the preparation of the glass substrate, FTO coated glass was chosen and cut into 2.5 cm x
2.5 cm pieces. The glass pieces were then cleaned with detergent (Alconox) and rinsed with
deionized water. Soon after, the glass pieces were sonicated with Methanol, Acetone, and
Isopropanol each for five minutes.
2.2.7 Fabrication of nanocrystal films.
Standard Layer by Layer Techniques48 were used to deposit ligand linked Quantum dot films
inside the glove box with Argon flow. Using the procedure43, complete inorganic QD films
(SMENA) were deposited.
For the deposition of layer by layer complete inorganic PbSCdS coreshell QD films, 6-7
drops of PbSCdS QD solution made in Hexane (10 mg/ml) were injected dropwise on the surface of
FTO/glass substrate. The glass was then spun for 10 seconds at 3000 rpm. 8-10 drops of mixture of
MPA and Methanol (1:4) were placed at the center of the glass in order to exchange native OA
ligands with thermally degradable ligands MPA. The mixture on the glass was drenched for 10
seconds and spun for 10 seconds. After the replacement of OA ligands with MPA, 10 drops of
11 Methanol were placed on the glass and spun for 10 seconds to wash the film. The film was further
rinsed with Octane in the same fashion. Once two layers of film were deposited, the film was heated
at 1200C-1400C for 15 minutes. 6-7 layers of PbSCdS QD films in total were placed on the glass
substrate. The ratios of optical density of ZnS QDs (λ = 270 nm) and that of PbSCdS coreshells (λex
at ≈ 850 nm - 900 nm) were calculated. The originally prepared PbSCdS coreshell solution was
mixed with ZnS quantum dot solution in the different ratio as calculated before.
The Layer by Layer Spincoating Technique was used to deposit ligand (MPA)-linked PbS
quantum dot films in Argon flow. 6-8 drops of PbS quantum dots dispersed in Hexane were
deposited on the surface of FTO glass and spun for 10 seconds at 3000 rpm. Afterward, 8-10 drops
of mixture of Methanol and MPA (4:1) were placed on the glass, saturated for 10 seconds, and spun
for 10 seconds at 3000 rpm. Total 6-10 layers of NCs were made as per the procedure explained
before. ZnS cores were mixed with PbS NCs in different ratios, which were obtained by dividing the
optical density of the ZnS cores (λ = 270 nm) by that of PbS QDs. For the deposition of the
ethanedithiol (EDT)-linked QD films, 5-7 drops of PbS nanoparticles were placed on the FTO/glass,
spun for 10 seconds at 3000 rpm. The film was then adsorbed in 0.1 M EDT solution prepared in
Acetonitrile for one minute. After that, the film was dried and cleaned with 10 drops of Acetonitrile.
8-10 layers, in general, were needed for the fabrication of EDT-cross-linked PbS QDs and PbS QDs
with ZnS films.
2.2.8 In-filling of SMENA pores with ZnS.
The SILAR Method was employed49 for the pore filling process. An additional amount of
ZnS QDs, cores with a wide band gap, was injected in among NCs by soaking the heated NC films
containing thermally degradable ligands successively, completely inorganic QD film in solution of
Methanol with Zn and S precursors.
12 For this process, Sulfur and Zinc bath were used. 0.10 gm Zinc Acetate was dissolved in 20
ml of Methanol to produce a Zinc bath and 0.098 gm Na2S. 9 H2O was dissolved in 20 ml Methanol
to prepare the Sulfur bath. For a complete SILAR cycle, the film was soaked in the zinc bath for a
minute, washed with methanol for a minute, and soaked in a sulfur bath for a minute. Subsequently,
the film was rinsed with methanol. For the films, 2-10 SILAR cycles were completed for inorganic
films having longest PL-lifetime. Then, the films were heated at 1500 C for 15 minutes.
2.3 Characterization
Simadzu UV-3600 UV-vis-NIR and CARY 50 scan spectrophotometers were used to
investigate absorption spectra. To record the Photoluminescence spectra, Jobin Yvon Fluorolog FL311 fluorescence spectrophotometer was used. JEOL 3011UHR and 2010 transmission electron
microscopes were used for the measurements of High-resolution transmission electron microscopy
(HR-TEM), which were operated at 300 and 200 kV respectively. A small piece of nanocrystal film
was scratched for the preparation of the TEM sample. The scratched piece of film was dissolved in
Toulene and sonicated. The solution was then dropped on a carbon-coated copper grid and was
allowed to dry. Besides the instruments, a Scintag XDS-2000 X-ray powder diffractometer was used
to measure X-ray powder diffraction (XRD). To measure the FL lifetime, a time-correlated single
photon counting setup utilizing SPC-630 single-photon counting PCI card (Becker & Hickle GmbH)
was used, where a picosecond diode laser operating at 400 nm acted as an excitation source
(Picoquant), an id50 avalanche photodiode (Quantique), and long pass filters on 400nm, 532nm and
750nm.
13 CHAPTER 3.RESULTS AND DISCUSSION
The rate of charge trapping in quantum dot films is determined by uniquely distinguishing the
dynamics of this process from other processes of exciton decay such as variable range hopping (VRH),47
charge tunneling between neighboring quantum dots, and the resonant energy transfer to a dark state.48
These mechanisms have resulted in the dissociation of NC excitons, which cause a significant drop in
fluorescence life time. The binding energy of the exciton is remarkably smaller than thermal energy of
Photoinduced charges whereas the time of charge carrier hopping or tunneling is about the same as the
carrier ionization time. The measured FL lifetime of PbS quantum dots is, thus, useful to examine the
accumulative rate by which charge carriers are removed from the excited state. The rate of FL intensity
decay of solid is given as follows:
Γ
FL decay
=Γ +Γ
rad
non−rad
= Γrad + Γtrapping + Γtunneling + ΓVRH + Γenergy transfer
----------------------------------------(1)
The rate of radiative decay becomes negligible in comparison to the carrier removal rate
through the transfer processes if electrical coupling between adjacent quantum dots is strong, Γrad <<
Γnon-rad. Consequently, the band edge emission from each of the QDs in the film can be suppressed.
The FL lifetime (τFL = 1/ ΓFL decay) then is approximately the same as the nonradiative exciton decay
time as τFL ≈ τnon-rad = 1/(Γtrapping+ Γtunneling+ ΓVRH + Γenergy transfer). On the other hand, the
charge and energy transfer among PbS quantum dots in the film have comparatively lower probability
when the electrical coupling between adjacent QDs is weak. The role of radiative decay makes the rate
of total exciton decay significant. This decay is further demonstrated with the improvement of the FL
lifetime and the parallel growth in the emission quantum yield of nanocrystal films having large
interparticle distances.44
14 Figure 3.1: (a) Sketch of the general strategy for encapsulation of colloidal quantum dots into matrices.
(b-c) Typical TEM images of PbS/CdS core/shell and ZnS NCs used as nanoparticle precursors during
film assembly.
Equation 1 can be used to investigate average charge trapping time when the electrical coupling
between adjacent quantum dots is strong (Γrad << Γnon-rad), whereas the other mechanisms of exciton
dissociation are negligible, Γtunneling+ ΓVRH + Γenergy transfer → 0. By inserting wide band gap
materials such as ZnS semiconductor quantum dots in between the PbS NCs, this type of situation can
be created as shown in fig. 1(ii). It also increases the interparticle distance among the PbS quantum dots,
which basically lowers the rate of short-range hoping, energy transfer, and tunneling. The amplitude of
the energy transfer phenomenon is inversely proportional to the fourth power of interparticle separation,
which is reduced significantly when PbS QDs are separated by ZnS QDs. In the meantime, ZnS QDs
having wide band gap develop a considerable potential barrier for valence and conduction band charges
15 in the film as the PbS-PbS tunneling is also assumed to be insignificant. The VRH process, coupling the
resonance states of PbS QDs is lowered as well with the presence of insulating ZnS QDs. The short
range hopping rate, specifically, among the adjacent quantum dots can certainly be lessened by using a
higher amount/fraction of ZnS QDs in the film. In contrary, the rate of long range hopping among
resonant states with non-adjacent QDs is moderately suppressed since the hopping of excited charges in
resonance via longer distances can still exist.
Several mixed NC films with different ratios of ZnS to PbS QDs were fabricated to examine if
the ZnS NCs added to PbS NCs actually help to suppress the charge and energy transfer with
neighboring PbS QDs. Afterwards, the corresponding FL intensity lifetimes were measured. ZnS NCs to
PbS NCs were incorporated into CdS matrices (Fig. 3.1a) employing the semiconductor matrixencapsulated
Figure 3.2: Absorbance spectra of the PbS cores (3.2-nm-black) and PbSCdS coreshell NCs (3.2nm -brown) featuring 3.0 nm core diameter.
16 nanocrystal arrays (SMENA) technique43. Further, PbS QD cores covered with CdS semiconductor QD
shells (Fig. 3.2) are mixed with ZnS QDs. The mixture was then spincoated on an FTO/glass, native
ligands were exchanged with thermally degradable MPA ligands, and the adjacent QD Shells were
fused. It was finally heated at 1200C for 15 minutes. FTIR measurements were carried out to confirm the
removal of the native organic ligands.
Figure 3.3: Different stages of NC films seen on XPD spectrum.
(a) Bragg’s peaks displayed by PbS NCs (4.0-nm) for rock salt. (b) Fused PbSCdS coreshell (c)
PbSCdS coreshell and ZnS NCs before pore filling process. (d) Matching of the lattice structure at
the boundary between zinc blende CdS and rock-salt PbS crystals. (e) A small piece of PbSCdS
SMENA film.
Additional wide band gap materials such as CdS or ZnS NCs were used to fill the pores of the
matrices via ionic layer adsorption and reaction (SILAR) method. Fig. 3.3 clearly indicates the iffraction
spectra of PbS QDs, (Fig. 3.3a), PbS QDs in CdS (3.3b), and the mixture of PbS NCs and ZnS NCs in
17 CdS matrix (Fig. 3.3c). A remarkable band edge emission of PbS QDs obtained from the NC films of
PbS and Zns demonstrating the ZnS diffraction pattern, which indicates the successful encapsulation of
the both type of QDs into CdS NCs. Fig. 3.3e shows the TEM image of PbS QDs with embedded CdS.
The result of the increasing portion of ZnS NCs in CdS encapsulated PbSCdS NC films (without
pore filling) is demonstrated in Fig. 3.4b below. The biexponential decay (Fig. 3.4b black curve) having
slow and fast components almost equal to 50 ns and 1.6 ns respectively was shown by the FL lifetime of
the band edge excitons in PbS QDs before adding ZnS NCs. The higher rate of energy and charge
transfer to other particles resulted in a comparatively short lifetime of the fast component. It further
causes the detachment of excitons which suppresses the band gap emission. The dissociation process is
reserved due to the addition of increasing amount of ZnS QDs changing the increment of the fast
component from 1.6 ns to 8.9 ns (Fig. 3.4b and 3.6 a). In the meantime, the slow component of the PbS
exciton decay also grows with the addition of higher amount of ZnS QDs but at a lower rate ranging
from 50 ns to 87 ns. Eventually, both components saturate showing no further changes with the addition
of ZnS NCs.
Figure 3.4: (a). Emission and FL intensity decay of PbSCdS core(shell) nanocrystals in solution
(ΔHCdS = 0.32 nm). (b). FL intensity decay of CdS embedded in PbS NCs into matrices (Redge =
0.64 nm) with increasing amount of ZnS NCs in the film. (c). Biexponential curve of the
Fluorescence intensity decay indicating fast and slow components.
18 The ZnS nanocrystals that insulate the film are responsible for the origin of fast and slow
components of the fluorescence decay of the PbS excitons. In order to describe the significant rise in the
fast decay component of the NC films saturated with ZnS, it is important to remember that before the
addition of ZnS nanocrystals, the edge-to-edge distance between adjacent PbS nanocrystals, Redge= 2 x
0.32 nm = 0.64 nm, was short enough for the process of charge tunneling. Most photoconducting NC
devices43 use seperation distances similar to this because larger distances of the Redge make the material
exhibit insulating properties. As the ratio of ZnS to PbS increases, the PbS nanocrystals become more
covered by ZnS. As a result, the separation distance Redge reaches Redge = 2DH + dZnS , where ΔH is the
thickness of the CdS shell, and dZnS is the typical diameter of the ZnS nanocrystals (d=4.6nm). Redge
increases from 0.64nm to at least 5.2 nm, when the PbS(CdS) matrix is saturated with ZnS NCs. This
causes the PbS to PbS transport to be intensively suppressed. Since the PbS NCs are greatly separated by
the ZnS NCs insulator, all interparticle interactions such as tunneling, short-range hopping, and resonant
energy transfer will be suppressed at the saturation fraction of ZnS NCs. Phototcurrent measurements of
the same films backed up this hypothesis by showing that when ZnS concentration is increased, the
suppression of photoinduced current is increased as well (Fig. 3.6c). On the basis of the observations,
the exciton dissociation processes are therefore associated with the fast component of the FL decay as
demonstrated in Fig. 3.4c, this causes the charge and energy transfer between adjacent PbS NCs.
As mentioned before, the growth of the fast component of the FL intensity decay and the drop in
the photoconductivity of the films are caused by the destruction of exciton dissociation in the PbS
nanocrystal films saturated with ZnS NCs (Redge ≈ 0.64 nm). Despite the reduction of film
photoconductivity, the slow component only changes slightly. The slow component only increases from
50 to 87 ns (Fig. 3.6b). Concurrently, the lifetime of the slow component of FL intensity decay of PbS
NC films saturated with ZnS NCs is still much smaller (3.3 nm) than that of the PbSCdS coreshell NCs
in solution which have lifetimes greater than 1120 ns. This lowerbound estimate was found from the
19 emission lifetime of the NCs in chloroform (Fig. 3.4a). There was a difference between τrad and
τFL,slow of at least 20 times. This points to the another source of non radiative decay, which is
unaffected by seperation of nanoparticles. This phenomenon is being caused by charges being trapped
on the surface of the nanocrystals as per equation 1. As all the energy and charge transfer processes are
suppressed, the slow component of the Fluorescence lifetime of of PbS nanocrystal solids becomes
approximately equal to the timescale of the carrier trapping, τtrapping ≈ τFL, slow = 87 ns.
The dependence of the slow component of fluorescence decay on the ratio of ZnS NCs present in
the film further explains the trapping processes in PbS NC films. Figure 3.6b shows that τFL, slow is
increasing from 50 ns to 87 ns, showing that the some charge trapping actions are also being prevented
by the ZnS, whereas other mechanisms do not depend on the ZnS insulating effect. The concepts of
global and local traps can explain these phenomenon as demonstrated by Fig. 3.6. As the photoinduced
charges are trapped on the surface of the same PbS NCs (local trap state), the trapping rate is not
affected by the amount of ZnS, however if the charge is trapped on nearby dots and requires global trap
states, the amount of ZnS plays a significant role to reduce the trapping rate. Accordingly, both trap
states (Fig. 3.6b) are shown by the slow component of fluorescence intensity decay in PbS NC solids
(without ZnS NCs), whereas the local trap states in the NC films saturated with ZnS NCs primarily
determine the carrier decay.
In order to confirm the existence of global trap states in PbS NC solids, the fluorescence
intensity decay of PbS NC films with OA ligands was investigated. It is expected that new trap states are
not created when PbS NCs capped with OA ligands in solution are transferred into a solid. Therefore,
the local trap states stay constant for both solution and film if it solely determines the slow component of
the fluorescence decay. Measured data (Fig. 3.5), however, shows that the fluorescence lifetime of PbS
NCs drops at least by a factor of 30% with the deposition of film. The fluorescence intensity decay of a
PbS NC films picks up a rapid component (dissociation) over the slow one falling from 380 ns to 290 ns
20 unlike the PbS nanocrystal solution, where the fluorescence lifetime decay curve is single exponential.
Some global trap states, thus, take place between adjacent particles. This mechanism appears to be
viable for NC solid although the new trap states are not formed but clearly repressed for quantum dots in
solution.
Figure 3.5. FL intensity decay of oleic acid-capped PbS nanocrystals in solution (black) and
in a film.
The fast component and slow component of the fluorescence intensity decay of PbS(ZnS) films
evolves as being consistent with the dissociation and charge trapping process. Further explanation is
required for the lack of saturation of the fluorescence lifetime in NC films containing ZnS NCs.
Specifically, when the ratio of ZnS NCs volume with PbS(ZnS) solid approaches 10-15 range, all the
charge transfer and energy transfer mechanisms are expected to be prevented. Nonetheless, the fast
decay component can still be seen in the ZnS dominated PbS(ZnS) (Fig. 3.4b and 3.4c) solids.
Additionally, the photoconductivity of NC films cannot completely reach zero even if insulating ZnS
NCs cover the entire PbS NCs (Fig. 3.6c). The long range hopping process can explain this mechanism.
Higher amount of ZnS nanocrystals might not even be able to dissociate excitons with the long range
21 hopping process as in fig. 3.4b. In addition to this, the nonvanishing carrier conductivity of the solid can
also be explained by the long range hoping, which is resulted from the remaining photocurrent of the NC
films saturated with ZnS NCs.
Figure 3.6: The dynamics of the fluorescence decay intensity of CdS embedded PbS nanocrystal films.
(a). Development of fast component of the fluorescence Intensity Decay with growing amount of ZnS
nanocrystals in the solid. The FL lifetime (without-ZnS) of PbS films gives the exciton dissociation
time. (b). Slow component of fluorescence intensity decay evolved with increasing amount of ZnS
nanocrystals in the solid. (c) Photoconductivity measured on the same solids as in (a) and (b).
The dynamics of fluorescence intensity decay of the solids in weakly coupled structure of PbS
films were examined to check additional information that could strengthen the hypothesis presented for
slow and fast components to the trapping process and dissociation process. Soon after, the CdS thickness
22 was increased to 1.3 nm within the PbSCdS film-precursor and Redge distance of the PbSCdS matrix
was raised from 0.64 nm to 2.7 nm. The thickness of the shell is inversely proportional to the charge
trapping on the surface of the PbSCdS coreshell and the dissociation of the exciton (PbS-PbS) because
of the potential barrier offered by the CdS NCs to both carriers (Fig. 3.7). If charge carriers being
tunneled on the surface of the coreshell is considered as the main process of carrier trapping, then
Wentzel-Kramers-Brillouin (WKB) approximation49 can be used to calculate the probability of the
trapping process, by means of single exponential dependence on the thickness of the shell, Γthick/Γthin
= exp(-ΔHthick)/exp(-ΔHthin), where the charge trapping rate is given by Γ. The lattice defects which
are formed on the coreshell boundaries also contribute to the exciton dissociation. However, the
characteristic time of the dissociation process is bigger than other trapping processes. The fluorescence
lifetime measurement of the coreshell nanocrystals in solution (Fig. 4a, τ = 1120 µs) gives the lower
limit of the charge trapping time of the interface. The magnitude of this lifetime is higher than the
exciton lifetime of the PbS NCs bound in film by order of two. It is expected that the lattice structure of
Figure 3.7: The positions of the excited energy levels of PbS QDs and bulk CdS.
zinc blend CdS and rock salt PbS (Fig. 3.3d, strain ≈ 1.7%) is approximately matched and it has caused
a low probability of charge trapping on the interface of the PbSCdS coreshell.
23 The effects of the rising amount of ZnS NCs in the film of PbSCdS are demonstrated in figure
3.8 as the changes in the fluorescence intensity lifetime of PbSCdS NC film (Redge ≈ 2.7 nm). The fast
decay component limited within PbS-PbS exciton transfer (Eq. 1) is 11.6 ns before adding ZnS
nanoparticles. This value is remarkably higher than that of strongly coupled PbS nanoparticles in solids
having Redge= 0.64 nm. The fast component, according to WKB approximation, is exponential with
Redge, exp(-0.64)/exp(-2.7) = 7.8. This value is approximately equal to the observed Fl lifetime ratio,
τ2.7 nm/τ0.64 nm = 11.6/1.6 = 7.25. The charge transfer mechanism is limited even more with the
addition of the ZnS nanoparticles in the film, which caused the fast component to remain higher and
lowering the amplitude (Fig. 3.8b). The fast decay component of the fluorescence decay curve in the
PbS NC solid with the value of Redge equal to 2.7 nm is ultimately overcome by the slow decay
component like in case of Redge=0.64 nm. When the amount of ZnS nanoparticles within the NC films
are constantly increased, the slow decay component is finally saturated at τFL, slow = 370 ns. At this
point, trap states on the native surface are the only reason for exciton dissociation due to the strong
suppression of the energy transfer and charge transfer on these NC solids, τtrapping (local) ≈ τFL, slow
= 370 ns. The charge trapping time in case of strongly coupled PbS nanocrystal films (Redge=0.64 nm)
is nearly 87/370 = 1/(4.25) times that of weakly coupled PbS nanoparticle solids.
Charge transfer to the local trap states is further expected to be suppressed as a result of
implementing SILAR mechanism in matrix encapsulated NC solids. The boundaries of the PbSCdS
coreshells are in general defectless, (Γtrap(interfacial defects) < 1/450 ns), therefore the charge trapping
must have resulted from the tunneling of the charges onto the local surface of the CdS NC matrix. By
enhancing the size of the potential barrier that separates PbS localized charge carrier from trap states, the
probability of the carrier trapping on surface could be reduced, for eg., more layers of Zns or CdS NCs
could be deposited on the surface. Many layers of ZnS nanoparticles are deposited on the ZnS saturated
24 and ZnS non saturated PbS nanoparticle matrix through the SILAR mechanism to examine the result of
such a surface treatment. The fluorescence lifetime of the slow decay component is increased by 35% by
Figure 3.8: (a). FL Intensity decay of weakly-coupled PbS NCs embedded into CdS matrices (Redge = 2.7
nm) with increasing amount of ZnS NCs in solid. (b). Development of the fast component of
Flourescence decay with increasing amount of ZnS nanoparticles in solid. (c) Development of the slow
component of flourescence decay with increasing amount of ZnS nanoparticles in solid.
adding 6 monolayers of ZnS nanoparticles in case of no-ZnS NC films, where both global and local
charge trapping is present (Fig. 3.9). Such intermediate rise of time for the charge trapping is the
evidence for the existence of the global trap states. The SILAR mechanism on the other hand is more
effective with the ZnS saturated NC films that have only local trap states. The slow decay component
has three-fold rise, τFL, slow = 87 → 257 ns (Fig. 3.10b), which shows that the trap states at the local
25 surface are effectively passivated by the atomic layer deposition. But, the SILAR mechanism did not
fully inhibit the fast component of the fluorescence intensity decay in the ZnS-riched PbS NC solids.
The existence of this fast component is further validated by the long range hopping process, which
primarily depends upon interdot distance than passivation of the surface.
Figure 3.9: Flourescence decay of CdS encapsulated PbS nanoparticle solids (Redge = 0.64 nm) before
(black) and after (red) the in filling.
Figure 3.10: (a). Sketch of SILAR method to fill the pore with ZnS. (b). The result of implementation of
the SILAR mechanism on Fl decay of CdS encapsulated PbSZnS NC solids having lowered energy and
charge transfer. The reduction of rate of charge trapping resulted in the higher fluorescence lifetime of
the pore filled NC film.
The surface of the ligand-linked PbS nanocrystal films are passivated with the molecules having
short chain. By changing the amount of ZnS NCs used in the NC solid, we investigated whether or not
the both CdS encapsulated nanocrystal films and the NC film linked with ligands show the same exciton
dissociation and charge trapping rates. For this particular study, the choice of ligands have been
26 narrowed down to three types : MPA, EDT, and a combination of MPA/Cl molecules. The materials
have already been used as electric conducting surfactants in NC devices with excellent performance46.
First of all, the PbS NC (with MPA ligands) solids were fabricated using the spincoating
process46. When ZnS nanoparticles are added into PbS NC (with MPA ligands) solids (d=3.2 nm), the
charge transfer process is suppressed similar to the case of matrix encapsulated nanocrystal films. The
fast decay component of fluorescence intensity lifetime of all PbS NC films before adding ZnS NCs was
1.04 ns whereas FL intensity lifetime of PbS NC films (with OA ligands) having non-polar solvents was
480 ns (Figures 3.12e, and 3.12h). The lower fluorescence lifetime intensity of PbS NCs obtained while
transferring them from solution phase into a solid is further attributed to the resonant energy transfer,
resonant charge transfer, charge trapping, and hopping. In principle, the carrier ionization can also be
contributed by the photoinduced hole transfer of the MPA ligands. The rate of this process was not fast
enough to be competent with other processes like energy transfer and charge transfer because of the
fluorescence lifetime of PbS nanoparticles (with MUA ligands) in water/methanol solution, which is 220
ns (Fig. 3.11).
When the amount of ZnS nanoparticles in PbS dots linked with MPA ligands was increased, the
fast component of the fluorescence intensity decay was enhanced. This enhancement also shows the
reduction in the charge transfer between adjacent particles. The slow component of the fluorescence
intensity decay of the charges trapped on the local trap states increases from τ = 35 ns (no-ZnS) solids
(global + local traps) to a saturation level of τtrapMPA
= 60.5 ns (averaged over 3 films). This clearly
indicates charge carrier decay only in local trap states. This latest value is about four times shorter
than τtrap, local of matrix encapsulated PbS NC solids filled with ZnS NCs, which is τtrap
CdS
= 257
ns. The results for the EDT-linked solids were also similar (Table 3.1), where the time for the charge
trapping grew from 25 ns to 45 ns. The combination of MPA and halide anions recently was used as
ligands on NC solids to develop one of the best efficient solar cell/photovoltaic devices. The halide
27 elements such as chlorine are dense enough to penetrate the difficult-to-access spots of NCs, which
Figure 3.11: The FL intensity decay curve of PbS nanoparticles capped with MUA (in methanol).
provide improved surface passivation and lead towards the decreased density of mid gap trap
states.27 The exciton dissociation dynamics in the PbS nanoparticle films with MPA/Cl ligands (Fig.
3.12f, 3.12j) was measured to study the effects of halide passivation. The fluorescence lifetime
intensity and FL quantum yield of the PbS nanoparticles has enhanced by 10% (Fig. 3.12c) and 2025% respectively when chlorine ligands were added on the surface of the NCs during the solution
growth process. For no-ZnS NC solids, the fast component (3.1 ns) and slow components (30 ns) of
the fluorescence intensity decay were represented by the biexponential character shown by the
MPA/Cl-passivated PbS nanoparticles. By adding ZnS NCs further, the later lifetime was enhanced
to 193 ns, which is the carrier decay time in local trap states. As expected, the lower rate of charge
trapping on the trap states (τtrap = 193 ns) was resulted from the use of Cl-capped NCs in
comparison to the MPA linked PbS nanocrystal solids, agreeing well with the estimate of the newly
28 developed diffusion model.50
Figure 3.12: (a-c). Fluorescence intensity curve of EDT passivated PbS nanoparticles in solution (a),
MPA (b), MPA/Cl (c) different environment (d-f). Pl intensity decay of PbS nanoparticle solids with
EDT (d), MPA (e), and MPA/Cl (f) Functions of raised amounts of ZnS NCs in the solid. (g-j) Fast and
slow components Pl intensity lifetime of PbS NCs.
When the fluorescence lifetimes of the matrix encapsulated PbS nanoparticle films and ligand
crossed PbS nanoparticle films with same interparticle distance are compared, an important trend is
revealed. The rate of photoinduced charge trapping in CdS encapsulated PbS nanoparticles was found to
be reduced with respect to ligand linked PbS NC solids. The observed charge transport characteristics
are reviewed for a p-n junction for the better understanding of observed charge transport, where the
carrier transport completely depends upon the drift in the depletion region and diffusion elsewhere.
The characteristics length of the diffusion and drift can be calculated by the equation given below.
29 ldrift = µEτ ;
ldiffusion = √ Dτ
(2)
Where, charge carrier mobility is represented by µ, diffusion coefficient by D, and the electric field by
E. In both no-ZnS NC films and ZnS saturated NC films, charge trapping is the main mechanism of
charge scattering. The lifetime of the minority charge carrier is about the same as the trapping time τ ≈
τtrap. The carrier mobility (µ), and the diffusion coefficient (D) are required to be determined to find out
the values of ldrift and ldiff for tested NC solids.
Einstein’s relation of mobility and diffusion can be used for a NC solid in the regime of hoping
transport to determine the value of µ and D as follows:
µ = (ed2) ;
6kTτ
D = µkT
e
(3)
where, d: center-center distance between PbS nanoparticles in a solid.
1/τ :cumulative charge carrier diffusion rate having charge transfer and energy transfer
It is considered that the charge carrier diffusion occurs with tunneling of the excitation energy on
adjacent nanocrystals. This method, therefore, cannot be used to describe the band transport regime. The
diffusion-limited carrier mobility in closest adjacent hopping approximation is represented by the
resulting diffusion charge carrier mobility (µdiff). It should be noted that the µdiff is different than the
field effect transistor (FET) mobility. By determining the charge carrier diffusion rate (1/τdiff) first, the
µdiff for the tested solids is estimated. The fast component of the fluorescence intensity decay of the
PbS nanoparticle solids was measured to achieve 1/τdiff..In the absence of ZnS nanoparticles, all the
carrier transfer mechanisms occur. Hence the rate of the fluorescence intensity decay is given by:
Γ
FL decay
=Γ
trapping
+ Γ + Γ (Γ
rad
diff
tunneling
;Γ ;Γ
VRH
)
energy transfer
(4)
where, the τdiff is characterized by the fast decay component. τdiff ≈ τex.dissociation
30 τFL, fast for no ZnS NC solids. The equation 3 is therefore reduced to
µ
2
diff
ed
= 6kT
1
τ
no ZnS
FL , fast
2
d
µkT
; D = e = 6 no ZnS
τ
(5)
FL , fast
Hence, the carrier diffusion length becomes,
l
diffusion
= √ Dτ
trap
= d×√τ
trap
/6 τ
FL,fast
= d×√τ
FL,slow
/ 6τ
FL,fast
×τ
FL,slow
/ 6τ
FL, fast
(6)
The slow component of the fluorescence intensity decay in both ZnS saturated films (local trap
states only) and no ZnS (local and global trap states) in above equation is represented by τFL,slow.
The summary of the fluorescence intensity lifetime data for both type of films is presented in table
T3.1. As seen as in table T3.1, the suppressed charge trapping detected in case of matrix encapsulated
NC solids is the most important outcome of the all compared data. Specifically, the rates for the charge
carrier trapping in strongly coupled PbS(CdS) nanocrystal solids are 1.5 to 4 times shorter than that of
three different tested ligand linked NC films. PbS nanocrystal films passivated with Cl were also equally
efficient to discourage carrier tunneling on local traps with the characteristic scattering time (τtrap = 193
ns) above that for MPA linked NC solids by the factor of 3. Hence it is indirectly evidenced from the
above explanation that Cl passivated nanoparticle films and matrix encapsulated nanoparticle film reveal
relatively lower densities of traps on surface. Alternatively, µdiff for complete inorganic nanocrystal
solids was found to be 0.57*10-3 cm2/V/s, which is less than that for PbS nanocrystals with MPA ligands
(µdiff = 0.9*10-3 cm2/V/s). The electrical coupling of the CdS capped nanocrystal solids resulting from
PbS-PbS interdot distance has been found to be less than its maximum value stated previously
corresponding Redge= 0.5 nm. If we compare Redge = 0.64 nm and Redge = 2.7 nm PbS/CdS solids
(lines 1 and 2 in Table T3.1), it can be concluded that Redge has important role in the resulting value of
µdiff. To use the semiconductor nanocrystal quantum dots in photovoltaic/solar cell applications, charge
31 mobility over 10−2 cm2/V/s does not benefit the performance of the device because of the significant
exciton dissociation.46 It has been demonstrated from this study that the carrier trapping can be
significantly lowered by employing the matrix encapsulation mechanism instead of cross linking
method. It is certainly disclosed from the compared values of ldrift (Table T3.2) and ldiff (Table T3.1)
that CdS caped NC films show larger scatter free charge travel in comparison to other type of NC films.
32 Table T3.1: FL Fluorescence intensity lifetime data and calculated transport properties of matrixencapsulated and ligand-linked PbS NC films.
Num
ber of
the
films
tested
2
τtrap(ns)
τdiss(ns) global+loc
τfast, no al
ZnS
τslow; no
ZnS
1.6
65
(with
SILAR)
τtrap(ns)
local τslow;
ZnS Sat
257.0
(with
SILAR)
0.57*10-3
9.8
20.1
2
11.6
171
370
1.8*10-4
9.0
13.3
MPA-linked
3
PbS NCs
EDT-linked
1
PbS NCs
Hybrid (MPA/Cl)-linked 1
PbS NCs
1.04
35
60.5
0.9*10-3
9.0
11.8
3.4
25
45
0.2*10-3
4.2
5.7
3.1
33
193
0.3*10-3
5.1
12.3
Type of NC film
CdS-encapsulated
PbS NCs
(Redge = 0.64 nm)
CdS-encapsulated
PbS NCs
(Redge = 2.7 nm)
µ (cm2/V/s)
(Eq. 4)
ldiff (nm)
glob+loc local
Table T3.2: Investigated drift scattering lengths for CdS matrix encapsulated and ligand linked
PbS NC films
Type of NC film
CdS-encapsulated
PbS NCs
(Redge = 0.5 nm)
CdS-encapsulated
PbS NCs
(Redge = 1.5 nm)
MPA-linked
PbS NCs
EDT-linked
PbS NCs
Hybrid (MPA/Cl)linked PbS NCs
ldrift (local +global traps)
(nm*10-3*E (V/cm))
ldrift (local traps only)
(nm*10-3*E (V/cm))
19.0
75.04
9.4
20.3
15.0
25.9
1.9
3.5
6.6
38.7
33 CHAPTER 4. CONCLUSION
In conclusion, the characteristics of the exciton dissociation and carrier scattering mechanisms
were examined in many PbS nanoparticle films employing the fluorescence lifetime spectroscopy,
absorption spectroscopy, and TEM image techniques. Controlled amounts of ZnS NCs were inserted
into the films of PbS quantum dots to investigate the scattering time for the charge carrier for each type
of film. Both of the charge and energy transfer mechanisms between the PbS nanocrystals are
suppressed by the addition of the insulator ZnS into the nanocrystal films, resulting the charge carrier to
decay mostly on trap states. Afterward, the properties of the photoinduced charge carrier were measured
separately from that of other processes of charge transfer and energy transfer by using the fluorescence
lifetime technique. The exciton dissociation rate and charge trapping rate were investigated for many
types of nanocrystal solids on the basis of the observed relaxation times, which includes both matrix
encapsulation and cross linked methods. The scattering lengths and diffusion mobility for the different
types of films in the hoping regime were determined using the observed rates of carrier decay again. The
matrix encapsulate PbS nanocrystal solids, in general, demonstrated the smaller carrier trapping rate for
the trap states and larger diffusion lengths than the crossed linked PbS NC solids.
34 REFERENCES
1
Murray, C. B.; Nirmal, M.; Norris, D. J.; Bawendi, M. Synthesis and Structural Characterization
of II-VI Semiconductor Nanocrystallites (Quantum Dots). G. Z. Phys. D 1993, 26, S231–233.
2
Beard, M. C. Multiple Exciton Generation in Semiconductor Quantum Dots. J. Phys. Chem. Lett.
2011, 2, 1282–1288
3
Pal, B. N.; Ghosh, Y.; Brovelli, S.; Laocharoensuk, R.; Klimov, V. I.; Hollingsworth, J.; Htoon,
H. ‘Giant' CdSe/CdS core/shell nanocrystal quantum dots as efficient electroluminescent
materials: Strong influence of shell thickness on light-emitting diode performance. Nano Lett.,
2012, 12, 331–336.
4
Kramer, I. J.; Levina, L.; Debnath, R.; Zhitomirsky, D.; Sargent, E. H. Solar cells using
quantum funnels. Nano Lett. 2011, 11, 3701-3706.
5
Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.;
Eisler, H.; Bawendi, M. G. Optical gain and stimulated emission in nanocrystal quantum dots.
Science 2000, 290, 314-317.
6
Klimov, V. I. Mechanisms for Photogeneration and Recombination of Multiexcitons in
Semiconductor Nanocrystals: Implications for Lasing and Solar Energy Conversion J. Phys.
Chem. B 2006, 110, 16827- 16845.
7
Klimov, V. I.; Ivanov, S. A.; Nanda, J.; Achermann, M.; Bezel, I.; McGuire, J. A.; Piryatinski,
A. Single-Exciton Optical Gain in Semiconductor Nanocrystals. Nature 2007, 447, 441-446.
8
Hillhouse, H. W.; Beard, M. C. Solar Cells from Colloidal Nanocrystals: Fundamentals,
Materials, Devices, and Economics. Curr. Opin. Coll. Int. Sci. 2009, 14, 245-259.
9
Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Hybrid nanorod-polymer solar cells. Science
2002, 295, 2425-2427.
10
McDonald, S. A.; Konstantatos, G.; Zhang, S.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent,
E. H. Solution-processed PbS quantum dot infrared photodetectors and photovoltaics. Nat.
Mater. 2005, 4, 138-142.
11
Ellingson, R. J.; Beard, C. M.; Johnson, J. C.; Yu, P.; Micic, O. I.; Nozik, A. J.; Shabaev, A.;
Efros, A. L. Highly efficient multiple exciton generation in colloidal PbSe and PbS quantum
dots. Nano Lett. 2005, 5, 865-871.
12
Leschkies, K. S.; Divakar, R.; Basu, J.; Enache-Pommer, E.; Boercker, J. E.; Carter, C. B.;
Kortshagen, U. R.; Norris, D. J.; Aydil, E. S. Photosensitization of ZnO Nanowires with CdSe
Quantum Dots for Photovoltaic Devices. Nano Lett. 2007, 7, 1793.
13
Maria, A.; Cyr, P. W.; Klern, E. J. D.; Levina, L.; Sargent, E. H. Solution-processed infrared
photovoltaic devices with > 10% monochromatic internal quantum efficiency. Appl. Phys. Lett.
2005, 87, 213112.
14
Kim, S. J.; Kim, W. J.; Cartwright, A. N.; Prasad, P. N. Carrier multiplication in a PbSe
nanocrystal and P3HT/PCBM tandem cell. Appl. Phys. Lett. 2008, 92, 191107.
35 15
Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Air-stable all-inorganic nanocrystal solar
cells processed from solution. Science 2005, 310, 462-465.
16
Guo, Q.; Kim, S. J.; Kar, M.; Shafarman, W. N.; Birkmire, R. W.; Stach, E. A.; Agrawal, R.;
Hillhouse, H. W. Development of CuInSe2 Nanocrystal and Nanoring Inks for Low-Cost Solar
Cells. Nano Lett. 2008, 8, 2982–2987.
17
Leschkies, K. S.; Beatty, T. J.; Kang, M. S.; Norris, D. J.; Aydil, E. S. Solar Cells Based on
Junctions between Colloidal PbSe Nanocrystals and Thin ZnO Films. ACS Nano 2009, 3,
3638– 3648.
18
Ma, W.; Luther, J. M.; Zheng, H. M.; Wu, Y.; Alivisatos, A. P. Photovoltaic Devices
Employing Ternary PbSxSe1-x Nanocrystals. Nano Lett. 2009, 9, 1699–1703.
19
Riha, S. C.; Fredrick, S. J.; Sambur, J. B.; Liu, Y.; Prieto, A. L.; Parkinson, B. A.
Photoelectrochemical Characterization of Nanocrystalline Thin-Film Cu2ZnSnS4
Photocathodes. ACS Appl. Mater. Interfaces 2011, 3, 58-66.
20
Mora-Sero, I.; Bisquert, J.; Dittrich, T.; Belaidi, A.; Susha, A. S.; Rogach, A. L.
Photosensitization of TiO2 layers with CdSe quantum dots: Correlation between light
absorption and photoinjection. J. Phys. Chem. C 2007, 111, 14889-14892.
21
Yu, P. R.; Zhu, K.; Norman, A. G.; Ferrere, S.; Frank, A. J.; Nozik, A. J. Nanocrystalline TiO2
solar cells sensitized with InAs quantum dots. J. Phys. Chem. B 2006, 110, 25451-25454.
22
Luther, J. M.; Law, M.; Song, Q.; Reese, M. O.; Beard, M. C.; Ellingson, R. J.; Nozik, A. J.
Schottky Solar Cells Based on Colloidal Nanocrystal Films. Nano Lett. 2008, 8, 3488–3492.
23
Debnath, R.; Tang, J.; Barkhouse, D. A.; Wang, X.; Pattantyus-Abraham, A. G.; Brzozowski,
L.; Levina, L.; Sargent, E. H. Ambient-Processed Colloidal Quantum Dot Solar Cells via
Individual Pre-Encapsulation of Nanoparticles. J. Am. Chem. Soc. 2010, 132, 5952–5953.
24
Ma, W.; Swisher, S. L.; Ewers, T.; Engel, J.; Ferry, V. E.; Atwater, H. A.; Alivisatos, A. P.
Photovoltaic Performance of Ultrasmall PbSe Quantum Dots. ACS Nano 2011, 5, 8140–8147.
25
Acharya, K. P.; Khon, E.; O’Conner, T.; Nemitz, I.; Klinkova, A.; Khnayzer, R. S.;
Anzenbacher, P.; Zamkov, M. Heteroepitaxial Growth of Colloidal Nanocrystals onto Substrate
Films via Hot-Injection Routes. ACS nano 2011, 5, 4953-4964.
26
Wang, Q.; Zhu, K.; Neale, N. R.; Frank, A. J. Constructing Ordered Sensitized Heterojunctions:
Bottom-Up Electrochemical Synthesis of p-Type Semiconductors in Oriented n-TiO2 Nanotube
Arrays. Nano Lett. 2009, 9, 806-813.
27
Ip, A. H.; Thon, S. M.; Hoogland, S.; Voznyy, O.; Zhitomirsky, D.; Debnath, R.; Levina, L.;
Rollny, L. R.; Carey, G. H.; Fischer, A.; Kemp, K. W.; Kramer, I. J.; Ning, Z.; Labelle, A. J.;
Wei Chou, K.; Amassian, A.; Sargent, E. H. Hybrid passivated colloidal quantum dot solids.
Nature Nanotechnology 2012, 7, 577-582
28
Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic, V. Electroluminescence from single monolayers
of nanocrystals in molecular organic devices. Nature 2002, 420, 800-803.
29
Ridley, B. A.; Nivi, B.; Jacobson, J. M. All-inorganic field effect transistors fabricated by
printing. Science 1999, 286, 746-749.
36 30
Lee, S.; Jeong, S.; Kim, D.; Park, B. K.; Moon, J. Fabrication of a solution-processed thin-film
transistor using zinc oxide nanoparticles and zinc acetate J. Superlattices Microstruct. 2007, 42,
361-368.
31
Schneider, J. J.; Hoffmann, R. C.; Engstler, J.; Soffke, O.; Jaegermann, W.; Issanin, A.;
Klyszcz, A. A. A printed and flexible field-effect transistor device with nanoscale zinc oxide as
active semiconductor material. Adv. Mater. 2008, 20, 3383-3387.
32
Talapin, D. V.; Mekis, L.; Gotzinger, S.; Kornowski, A.; Benson, O.; Weller, H. CdSe/CdS/ZnS
and CdSe/ZnSe/ZnS core-shell-shell nanocrystals. J. Phys. Chem. B 2004, 108, 18826-18831.
33
Talapin, D. V.; Murray, C. B. PbSe Nanocrystal Solids for n- and p-Channel Thin Film FieldEffect Transistors. Science 2005, 310, 86-89.
34
Lee, J. S.; Shevchenko, E. V.; Talapin, D. V. Au-PbS Core-Shell Nanocrystals: Plasmonic
Absorption Enhancement and Electrical Doping via Interparticle Charge Transfer. J. Am.
Chem. Soc. 2008, 130, 9673-9675.
35
Lee, J-S; Kovalenko, M. V.; Huang, J.; Chung, D. S.; Talapin, D. V. Band-Like Transport, High
Electron Mobility and High Photoconductivity in All-Inorganic Nanocrystal Arrays. Nat.
Nanotech. 2011, 6, 348−352.
36
Guyot-Sionnest, P. Electrical Transport in Colloidal Quantum Dot Films. J. Phys. Chem. Lett.
2012, 3, 1169−1175.
37
Tang, J.; Sargent, E. H. Infrared Colloidal Quantum Dots for Photovoltaics: Fundamentals and
Recent Progress. Adv. Mater. 2011, 23, 12–29.
38
Talapin, D. V.; Lee, J. S.; Kovalenko, M. V.; Shevchenko, E. V. Prospects of colloidal
nanocrystals for electronic and optoelectronic applications. Chem. Rev. 2010, 110, 389-458.
39
Gai, I.; Peng, H.; Li, J. Electronic Properties of Nonstoichiometric PbSe Quantum Dots from
First Principles. J. Phys. Chem. C 2009, 113, 21506–21511.
40
Tang, J.; Kemp, K. W.; Hoogland, S.; Jeong, K. S.; Liu, H.; Levina, L.; Furukawa, M.; Wang,
X.; Debnath, R.; Cha, D.; Chou, K. W.; Fischer, A.; Amassian, A.; Asbury, J. B.; Sargent, E. H.
Colloidal-quantum-dot photovoltaics using atomic-ligand passivation. Nat. Mater. 2011, 10,
765– 771.
41
Liu, Y.; Gibbs, M.; Perkins, C. L.; Tolentino, J.; Zarghami, M. H.; Bustamante, J.; Law,
M.Robust, Functional Nanocrystal Solids by Infilling with Atomic Layer Deposition. Nano
Lett. 2011, 11, 5349– 5355.
42
Mashford, B.; Baldauf, J.; Nguyen, T. L.; Funston, A. M.; Mulvaney, P. Synthesis of quantum
dot doped chalcogenide glasses via sol-gel processing. J. Appl. Phys. 2011, 109, 094305.
43
Kinder, E.; Moroz, P.; Diederich, G.; Johnson, A.; Kirsanova, M.; Nemchinov, A.; O’Connor,
T.; Roth, D.; Zamkov, M. Fabrication of All-Inorganic Nanocrystal Solids through Matrix
Encapsulation of Nanocrystal Arrays. J. Am. Chem. Soc. 2011, 133, 20488-20499.
44
Khon, E.; Lambright, S.; Khon, D.; Smith, B.; O’Connor, T.; Moroz, P.; Imboden, M.;
Diederich,
37 G.; Perez-Bolivar, D.; Anzenbacher, P.; Zamkov, M. Inorganic solids of CdSe nanocrystals
exhibiting high emission quantum yield. Adv. Funct. Mater. 2012, 22, 3714-3722.
45
Choi, J.-H.; Fafarman, A. T.; Oh, S. J.; Ko, D.-K.; Kim, D. K.; Diroll, B. T.; Muramoto, S.;
Gillen, J. G.; Murray, C. B.; Kagan, C. R. Bandlike Transport in Strongly Coupled and Doped
Quantum Dot Solids: A Route to High-Performance Thin-Film Electronics. Nano Lett. 2012,
12, 2631– 2638.
46
Pattantyus-Abraham, A. G.; Kramer, I. J.; Barkhouse, A. R.; Wang, X.; Konstantatos, G.;
Debnath, R.; Levina, L.; Raabe, I.; Nazeeruddin, M. K.; Gratzel, M.; Sargent E. H. DepletedHeterojunction Colloidal Quantum Dot Solar Cells. ACS Nano 2010, 4, 3374-3380.
47
Chandler, R. E.; Houtepen, A. J.; Nelson, J.; Vanmaekelbergh, D.: Electron transport in
quantum dot solids: Monte Carlo simulations of the effects of shell filling, Coulomb repulsions,
and site disorder. Phys. Rev. B 2007, 75, 085325-10.
48
Zhitomirsky, D.; Voznyy, O.; Hoogland, S.; Sargent, E. H. Measuring Charge Carrier Diffusion
in Coupled Colloidal Quantum Dot Solids. ACS Nano, ACS Nano, 2013, 7 (6), pp 5282–5290.
49
Chandler, R. E.; Houtepen, A. J.; Nelson, J.; Vanmaekelbergh, D. Electron Transport in
Quantum Dot Solids: Monte Carlo Simulations of the Effects of Shell Filling, Coulomb
Repulsions, and Site Disorder. Phys. Rev. B 2007, 75, 085325-10.
50
Zhitomirsky, D.; Voznyy, O.; Hoogland, S.; Sargent, E. H. Measuring Charge Carrier Diffusion
in Coupled Colloidal Quantum Dot Solids. ACS