The Supramolecular Nano‐Materials Group
Metal Nanoparticles and SelfAssembled Monolayer
Francesco Stellacci
Department of Materials Science
and Engineering
frstella@mit.edu
Outline
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• History
• Metal Nanoparticles Synthesis
• Electrical Double Layer
– Solubility
• Self-Assembly and Self-Assembled
Monolayer
• Ligand Coated Metal Nanoparticles
– Solubility
– Mixed Ligands
Nanoscale Materials
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1nm<d<100nm
d
Increasing size
molecule
Milan - Duomo
nanoparticle
Single Electron Transistor
Andres et al., Science, 1323, 1996
bulk material
Florence - S. Croce
What are they?
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0 dimensional nanomaterials:
unique properties due to
quantum confinement
and very high surface/volume ratio
Nanoparticles
Nanorods
1 dimensional nanomaterials:
extremely efficient
classical properties
Nanowires
Nanotubes
More Specifically
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0 dimensional nanomaterials:
unique properties due to
quantum confinement
and very high surface/volume ratio
Nanoparticles
Nanorods
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1 dimensional nanomaterials:
extremely efficient
classical properties
Nanowires
Nanotubes
Properties of Metal Nanoparticles
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Optical Properties
Electronic Properties
Nanoscale Materials Have Different
Properties when compared to their bulk
counterparts!
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A brief historical Background
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• the extraction of gold started in the 5th millennium B.C.
near Varna (Bulgaria) and reached10 tons per year in Egypt
around 1200-1300 B.C.when the marvelous statue of
Touthankamon was constructed.
• it is probable that “soluble” gold appeared around the 5th
or 4th century B.C. in Egypt and China.
• the Lycurgus Cup that was manufactured in the 5th to 4th
century B.C. It is ruby red in transmitted light and green in
reflected light, due to the presence of gold colloids.
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• The reputation of soluble gold until the Middle
Ages was to disclose fabulous curative powers
for various diseases, such as heart and venereal
problems, dysentery, epilepsy, and tumors, and
for diagnosis of syphilis.
• the first book on colloidal gold, published by the
philosopher and medical doctor Francisci Antonii
in 1618. This book includes considerable
information on the formationof colloidal gold sols
and their medical uses, including successful
practical cases.
• In 1676, the German chemist Johann Kunckels
published another book, whose chapter 7
concerned “drinkable gold that contains metallic
gold in a neutral, slightly pink solution that exert
curative properties for several diseases”. He
concluded that “gold must be present in such a
degree of communition that it is not visible to the
human eye”.
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A colorant in glasses, “Purple of Cassius”, is a colloid
resulting from the heterocoagulation of gold particles and
tin dioxide, and it was popular in the 17th century.
A complete treatise on colloidal gold was published in 1718
by Hans Heinrich Helcher. In this treatise, this philosopher
and doctor stated that the use of boiled starch in its
drinkable gold preparation noticeably enhanced its
stability.
These ideas were common in the 18th century, as indicated
in a French dictionary, dated 1769, under the heading “or
potable”, where it was said that “drinkable gold contained
gold in its elementary form but under extreme sub-division
suspended in a liquid”.
In 1794, Mrs. Fuhlame reported in a book that she had dyed
silk with colloidal gold.
In 1818, Jeremias Benjamin Richters suggested an
explanation for the differences in color shown by various
preparation of drinkable gold: pink or purple solutions
contain gold in the finest degree of subdivision, whereas
yellow solutions are found when the fine particles have
aggregated.
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• In 1857, Faraday reported the formation of deep
red solutions of colloidal gold by reduction of an
aqueous solution of chloroaurate (AuCl4-) using
phosphorus in CS2 (a two-phase system) in a well
known work.
• He investigated the optical properties of thin films
prepared from dried colloidal solutions and
observed reversible color changes of the films
upon mechanical compression (from bluish-purple
to green upon pressurizing).
• The term “colloid” (from the French, colle) was
coined shortly thereafter by Graham.
Marie-Christine Daniel and Didier Astruc, Chem. Rev. 2004, 104, 293-346
Synthesis of Metal Nanoparticles
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The citrate method
•It is easy
•It requires only water
•It requires skills
•Has reproducibility issues
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What is on the surface?
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The electrical double layer
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What are the limitations?
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Plasmons
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Optical Properties
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Mie Theory(1908)
9ωV0 ε m3 2
ε2
σ abs (ω ) =
2
2
c
ε
+
2
ε
+
ε
(1
m)
2
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Drude free electron model
ε (ω ) = 1−
ε (ω ) = ε 1 (ω ) + iε 2 (ω )
ω 2p
ω 2 − i γω
Empirically
a
γ (r ) = γ 0 +
r
Surface Plasmon Resonance is invariant with
respect to the size on the nanoparticle.
The FWHM scales with the radius of the
particles.
Assumes spherical particle
Particle diameter << λ/10

J.H. Hodak, et al. ; J. Phys Chem. B, 104(43), 9954, 2000.
Plasmon on nanoparticles
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Funny shapes
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Surface Enhanced Raman Scattering
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Local Field Enhancement
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Concentration Dependence
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Resonances
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Hemoglobin
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Metal Nanoparticles Fractal Clusters
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Metal nanoparticle fractal clusters
Theoretical simulation by
Shalaev et al., PRB 1997
→ Collective surface plasmon modes
“ strongly localized
“ resonate at VIS/NIR frequencies
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I/I0
• Enhancement of optical processes:
Raman scattering, Lasing, DFWM, TPA
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Sample Preparation
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AgNO3 +
Na - O
O
O
Na
O+
O Na
HO
O
HO
Ag0
O
+
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-
+
+
N
Δ
H2O
+
Ag0
-
Sedimentation
O
OH
Metal clusters
0.6
0.5
nanoparticles
O.D.
0.4
0.3
Fractal
clusters
0.2
0.1
0
200
400
600
800
Wavelength (nm)
1000
Glass slide
+ +
+ + +
-
-
-
-
+
-
+ +
+ + +
-
-
-
TPA Enhancement
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10
7000
TM70 only
Ag only
Ag + TM70
8
-1
)
6000
TM70 only
Ag only
Ag + TM70
Ag + TM70 (closer to edge)
5000
6
4000
4
3000
2
2000
0
1000
0
0
0.05
0.1
0.15
0.2
0.25
0.3
-2
-4.5
0.35
-4
-3.5
Power (W)
-3
-2.5
Ti:Sapphire
laser
He x
NC
He x
LWP
n
Ag
~800 nm
~100 fs
N
N
Glass substrate
-1
Hex
CN
Hex
Bu
-1.5
ln(Power/W)
Bu
TPA polymer
-2
Monochromator
sample
SWP
filters
PM
Amplifier
Counter
Spatial Inhomogeneity
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• TPF vs. position: very inhomogeneous as expected
Average enhancement factor ~ 235==> Peak enhancement factor >2000
(lower limit)
10
60
I/<I>
0
y (μm)
x (μm)
60
0
Frequency Dependence
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• Excitation
wavelength
dependence:
λexc = 720 nm
820 nm
890 nm
False Color Overlay
10 μm
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Sub-monolayers on MNFC
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Average enhancement factor
1200
Thickness dependence
1000
800
600
400
O
N
200
N
0
0
50
100
150
200
O
250
Thickness (nm)
HS
•Spatially averaged TPF enhancement factor ~ 20000
(referenced against one-photon fluorescence)
•Peak TPF enhancement factor > 160000
Morphology Effect
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O
N
N
O
Enhancement factor: 1
HS
Enhancement factor: 104
Enhancement factor: 5
Metal Nanoparticles Synthesis
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Metal Salt (AuHCl4) +
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HS
+
Reducing Agent (NaBH4)
HS
Direct mixed
ligands reaction**
F. Stellacci, et al. Adv. Mat. 2002, 14, 194
Ligand exchange
reaction*
A. C. Templeton, M. P. Wuelfing and R. W. Mu rray, Accounts Chem. Res. 2000, 33, 27
Synthesis Mechanism
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Synthesis Procedure
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Place Exchange Reactions
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Galvanic Exchange
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Solid State Photo-Patterning
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Photography
Dye
Ag+
LUMO
hν
-1eV
+0.25eV
+1eV
HOMO
Vs. Fc/Fc+
Photo-reduction
In solution: Huang et al., Langmuir, 12 (4),909,1996
Nanoparticle patterning
Nanoparticle Generation
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Dye and AgBF4 in a 1:1 molar ratio irradiated @ 488nm
2
2
1.5
abs.
abs
1.5
1
1
0.5
0.5
0
0
400
600
800
1000
400
1200
nm
500
600
nm
In PVK
In Acetonitrile
33nm
700
800
900
Applications for Nanoparticles
Patterns
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3D Optical Memories with
fluorescence-based and
refractive index-based
readout
10 μm
3 μm
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Holographic Data Storage
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All the images were saved on the same spot spaced of 70
7.6
7.5
diffraction efficiency (%)
Mask
Read
Ar+ laser
Sample
Write
Ar+ laser
CCD
camera
7.4
7.3
7.2
7.1
7
6.9
0
2
4
6
8
time (min)
10
12
14
General Concept
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Energy (eV)
ET
0.25
+ Ag
hν
Ag
Ag2
CR
CR
0.43
ET
+ Agn
0.71
Polymer
hν
dye
Agn+1
Ag+
e-
Dye
Ag+
Ag0
Free Energy (arb. units)
-1.85
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S Ag S
S
SS
S
SS S
1. Photo-reduction
Ag0
Ag0
Ag0
2. Growth
Ag0
Ag0
Ag0
S SS
3. Coalescence
Silver Nanoparticles Synthesis
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Ag+ + RSH
a
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(RSAg)n
NaBH4
(RSAg)n +Ag+
(RS)n
(RS)n Agm’
+Ag+
Dye
hν
Agm’
m’>n
(RS)n Agm’’
1
130
0.8
125
0.6
120
fwhm (nm)
absorbance
m’’>m’
0.4
0.2
115
110
105
0
100
-0.2
300 400 500 600 700 800 900 10001100
95
wavelength (nm)
Brust,et. al. Chem Commun., 1655, 1995.
0
5
10
15
20
time (min)
Kim et al., Langmuir 14,226, 1998
25
30
35
TPA induced Microfabrication
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Insoluble cross-linked polymer
O
2hν
O
O
O
D-π-D TPA dye
O
O
100 fs pulses 700 - 800
nm
1.4 NA , 0.35 μm spot
size
+z
Maruo, Nakamura, Kawata, Opt.Lett. 1997
Wu, Webb et al., Proc. SPIE 1992
+y
+x
Silver Patterns
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30 μm
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Wavelength: 488 nm
Average Power: 3 mW
Writing Speed: 25 μm/s
Transmission Image
30 μm
100 μm
Reflection Image
Two-Photon Absorption (TPA)
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σ one-photon cross section --> cm2 molecule-1
a
n-Bu
n-Bu
δ two-photon cross section --> cm4 s photon-1 molecule-1 n-Bu
S0
N
ω2
ω2
ω2
n-Bu
S1JS0
S0JS2
1.4
300
1.2
250
1.0
δ(ωδ(ω
2) 2) φf
φf
ω2
200
0.8
150
ωf
ωf
0.6
100
0.4
50
0.2
0.0
300
400
500
600
700
0
800
Wavelength (nm)
Albota et al. Science 281, 1653, 1998
Rumi et al., JACS 122, 9500,2000
δ (GM)
σ(ω
σ(ω
1) 1)
ω1
N
S0JS1
S3
S2
S1
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TPA Properties
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• 3D confinement
• Depth penetration
Twophoton
excitation
TPA ∝ δ I
1
I ~ 2
z
1
⇒ TPA ~ 4
z
2
One-photon
excitation
Excited volume as small as ~ 0.05 μm3
3D Metal Structures
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Schematic Drawing
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Two-Photon Microscopy
200 μm
200 μm
65 μm
Wavelength: 730 nm
Pulse duration: 120 fs
Average Power: 15 mW
Writing Speed: 25 μm/s
Transmission Optical Microscopy Scanning Electron Microscopy
20 μm
Solubility Issue
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liquid state
Solid interdigitated state
Solid de-interdigitated state
ΔHde-int
DSC-
ΔHsol
4
Differential
CAB1-cycle 1
CAB1-cycle 2
2
Scanning
< ENDO
0
Calorimetry
-2
liquid
decomposition
-4
ΔH
deinterdigitation
-6
-8
0
50
100
150
temperature C
200
250
30
Pradeep et al, Phys. Rev. B, 2(62), R739,2000.
Badia et. al. Chem. Europ. J., 2(3), 359,1996.
Ligand Length Effect
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Sample
d = 5 nm
d = 5 nm
d = 5 nm
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ligand
octadecanethiol
dodecanethiol
octanethiol
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ΔH(kJ/mol organic) Temp(K)
-42.3
402
-35.5
402
-20.7
401
increasing
length
ResultThe shorter the ligand the smaller the interdigitation enthalpy
Nanoparticle Size Effect
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Sample
d = 5 nm
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ligand
octanethiol
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relative ligand
ΔH(kJ/mol org) Temp(K)
amount
1
-20.7
401
1/3
-13.5
401
1
-20.7
382
1/2
-5.7
377
larger
d = 7 nm
d = 5 nm
3:1 oct/dod
larger
d = 8 nm
ResultThe larger the nanoparticle the smaller the interdigitation enthalpy, probably
because of surface curvature effect
Mixed Ligands Effect
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Sample
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3:1 oct/dod
N
a
ΔH(kJ/mol org)
ligand
d = 7 nm
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Temp(K)
-5.7
377
-6.0
384
-9.5
380
-20.6
420
14.1
384
SH
d = 5 nm
1:3 carbazolethiol/oct
d = 5 nm
1:1:1
oct/hep/dod
1:1 octadecane/dodecane
d = 5 nm
3:1 hep/dod
d = 5 nm
ResultMixture of ligands lower the interdigitation enthalpy
Order/Disorder Transition
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@1220C
Deinterdigitated
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@200C
10nm
Interdigitated
Thermal Annealing Evidences
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SEM
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TEM
Thick annealed film
50 nm
Pristine Film with a
thickness of ~ 20nm
Submonolayer
10 nm
After 1 thermal cycle
After 4 thermal cycles
10 nm
After 5 thermal cycles
Ligands on Nanoparticles
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Fluorescent Nanoparticles
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Nanoparticles synthesized by place exchange reactions
0.35
0.3
O
O
0.25
0.2
O
O
0.15
HS
0.1
0.05
0
~ 10 dyes per nanoparticles
300
400
500
600
700
[nm]
1
NO2
O
HS
~ 60 dyes per nanoparticles
absorbance [a.u]
O
O2N
0.8
0.6
0.4
0.2
0
300
400
500
600
[nm]
700
800
All Dye Coated Nanoparticles
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NaBH4
O2N
NO2
O
HS
M
+ AgNO3
EtOH/Acetone
COOH
DCM
~ 2500 dyes per nanoparticles
SH
Water Soluble
Fluorescence Quantum Yield (η) of the:
Free Dye
48% (2.2 ns)
Dye on the particle
33% (1.8 ns)
5 nm
Optical Spectroscopy
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One Photon
Fluorescence
Absorption
Two Photon
Absorption
nanoparticles
Free ligands
3 10 5
180
1
160
0.8
2.5 10 5
δ (GM)
0.6
120
2 10 5
δ (GM)
arb. units
140
100
0.4
1.5 10
80
5
60
0.2
1 10 5
40
0
300
400
500
600
nm
700
800
20
700
720
740
760
780
800
820
840
5 10 4
860
wavelength (nm)
•Two-Photon Cross Section per Nanoparticle δ: 3 105 GM