Optical and Electrical Properties of Three-Dimensional Interlinked Gold Nanoparticle Assemblies

Article abstract of DOI:10.1021/ja0377605

The optical and electrical properties of 11-20 nm thick films composed of ～4 nm gold nanoparticles (Au-NPs) interlinked by six organic dithiol or bis-dithiocarbamate derivatives were compared to investigate how these properties depend on the core of the l

Full text of DOI:10.1021/ja0377605

Published on Web 02/24/2004
Optical and Electrical Properties of Three-Dimensional
Interlinked Gold Nanoparticle Assemblies
Jurina M. Wessels,*,† Heinz-Georg Nothofer,† William E. Ford,†
Florian von Wrochem,† Frank Scholz,† Tobias Vossmeyer,† Andrea Schroedter,‡
Horst Weller,‡ and Akio Yasuda†
Contribution from Sony International (Europe) GmbH, Materials Science Laboratories,
Hedelfingerstrasse 61, 70327 Stuttgart, Germany, and Institute of Physical Chemistry,
UniVersity of Hamburg, Bundesstrasse 45, 20146 Hamburg, Germany
Received August 5, 2003; E-mail: wessels@sony.de
Abstract: The optical and electrical properties of 11-20 nm thick films composed of ∼4 nm gold
nanoparticles (Au-NPs) interlinked by six organic dithiol or bis-dithiocarbamate derivatives were compared
to investigate how these properties depend on the core of the linker molecule (benzene or cyclohexane)
and its metal-binding substituents (thiol or dithiocarbamate). Films prepared with the thiol-terminated linker
molecules, (1,4-bis(mercaptomethyl)benzene, 1,4-bis(mercaptomethyl)cyclohexane, 1,4-bis(mercaptoac-
etamido)benzene, and 1,4-bis(mercaptoacetamido)cyclohexane), exhibit thermally activated charge trans-
port. The activation energies lie between 59 and 71 meV. These films show distinct plasmon absorption
bands with maxima between 554 and 589 nm. In contrast, the film prepared with 1,4-cyclohexane-bis-
(dithiocarbamate) has a significantly red-shifted plasmon band (∼626 nm) and a pronounced absorbance
in the near infrared. The activation energy for charge transport is only 14 meV. These differences are
explained in terms of the formation of a resonant state at the interface due to overlap of the molecular
orbital and metal wave function, leading to an apparent increase in NP diameter. The film prepared with
1,4-phenylene-bis(dithiocarbamate) exhibits metallic properties, indicating the full extension of the electron
wave function between interlinked NPs. In all cases, the replacement of the benzene ring with a cyclohexane
ring in the center of the linker molecule leads to a 1 order of magnitude decrease in conductivity. A linear
relationship is obtained when the logarithm of conductivity is plotted as a function of the number of
nonconjugated bonds in the linker molecules. This suggests that nonresonant tunneling along the
nonconjugated parts of the molecule governs the electron tunneling decay constant (âN-CON), while the
contribution from the conjugated parts of the molecule is weak (corresponding to resonant tunneling). The
obtained value for âN-CON is ∼1.0 (per non-conjugated bond) and independent of the nanoparticle-binding
group. Hence, the molecules can be viewed as consisting of serial connections of electrically insulating
(nonconjugated) and conductive (conjugated) parts.
Introduction
optical properties of such assemblies toward their composition
hence provides a unique possibility for optimizing the properties
Two- and three-dimensional (3-d) assemblies of metal nano-
particles (NPs) exhibit interesting optical and electrical proper-
ties that are distinct from their respective bulk properties and
can be designed on a molecular level. The characteristics of
such semitransparent conductive films can be tuned by varying
the size or shape of the particles as well as the distance between
them. In addition, the electrical coupling between adjacent
particles is controlled by the dielectric nature of the embedding
organic matrix.1-5 The high sensitivity of the electrical and
of these films for applications such as thin transparent conduc-
tive coatings, sensors,6,7 or catalysis.8
The low-bias conductance of 3-d arrays of interlinked
spherical Au-nanoparticle assemblies2-5,7,9,10 and monolayer-
protected Au-nanoparticle assemblies1,6,8,11-13a has been studied
(4) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14,
5425.
(5) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta,
V. R.; Kubiak, C. P.; Mahony, W. J.; Osifchin, R. G. Science 1996, 273,
1690.
(6) Zamborini, F. P.; Leopold, M. C.; Hicks, J. F.; Kulesza, P. J.; Mali, M. A.;
Murray, R. W. J. Am. Chem. Soc. 2002, 124, 8958.
(7) Vossmeyer, T.; Guse, B.; Besnard, I.; Bauer, R. E.; Muellen, K.; Yasuda,
A. AdV. Mater. 2002, 14, 238.
† Sony International (Europe) GmbH.
‡ University of Hamburg.
(1) Terrill, R. H.; Potlethwaite, T. A.; Chen, C.-H.; Poon, C.-D.; Terzis, A.;
Chen, A.; Hutchinson, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.;
Superfine, R.; Falvo, M.; Johnson., C. S., Jr.; Samulski, E. T.; Murray, R.
W. J. Am. Chem. Soc. 1995, 117, 12537.
(8) Musick, M. D.; Keating, C. D.; Lyon, L. A.; Botsko, S. L.; Pena, D. J.;
Holliway, D.; McEvoy, T. M.; Richardson, J. N.; Natan, M. J. Chem. Mater.
2000, 12, 2869.
(2) Musick, M. D.; Keating, C. D.; Keefe, M. H.; Natan, M. J. Chem. Mater.
1997, 9, 1499.
(9) Brust, M.; Kiely, C. J. Colloids Surf. A 2002, 202, 175.
(10) Joseph, Y.; Besnard, I.; Rosenberger, M.; Guse, B.; Nothofer, H.-G.;
Wessels, J. M.; Wild, U.; Knop-Gericke, A.; Su, D.; Schlo¨gl, R.; Yasuda,
A.; Vossmeyer, T. J. Phys. Chem. 2003, 107, 7406.
(3) Brust, M.; Bethell, D.; Shiffrin, D. J.; Kiely, C. J. AdV. Mater. 1995, 7,
795.
9
10.1021/ja0377605 CCC: $27.50 © 2004 American Chemical Society
J. AM. CHEM. SOC. 2004, 126, 3349-3356
3349

A R T I C L E S
Wessels et al.
in detail. Murray et al. used an Arrhenius-type activated
tunneling model (eq 1) to describe the electron transport through
a 3-d network of monolayer-protected gold nanoparticles:13a
Au-NPs at sufficiently high surface coverage,2,8 while Wohltjen
and Snow showed that the transition in a film of monolayer-
protected Au-NPs could be induced by varying the Au/
alkanethiol capping molecule ratio due to alteration of the Au-
NP diameter.11 Brust et al. observed an insulator-metal
transition in 3-d assemblies of 6 nm Au-NPs when the distance
between the particles was reduced to e0.5 nm.9 Heath and co-
workers induced a reversible metal-insulator transition in a 2-d
assembly of alkylthiol-stabilized Ag-NPs by reducing the
interparticle spacing at the air/water interface on a Langmuir
trough to values below δ/2r ) 1.2.16
Changes in the charge transport properties of self-assembled
metal nanoparticle films are accompanied by changes in the
optical absorption characteristics. In the insulating regime of
3-d networks of nanoparticles, an increase in conductivity is
accompanied by a broadening and red-shift of the plasmon
absorption band, as well as an increase in the absorption in the
near infrared.2,4,8-10 The absorption characteristics of colloidal
Au-NPs, in solution and deposited on substrates, are determined
by the local dielectric environment and size of the particles,
the interparticle spacing,17-21 and the chemical nature of the
interface.22,23 For isolated gold particles smaller than 25 nm,
the extinction cross section of the collective electron resonance
induced by the incident electromagnetic field is given by the
dipole absorption:
σEL(δ,T) ) σ0(e-âδ)e-E /k T
(1)
A
B
where â is the electron tunneling coefficient in units of Å-1, δ
the average interparticle distance,13b EA the activation energy,
and σ0e-âδ the conductivity at kBT . EA. The first exponential
term describes the electron tunneling between the particles
through the organic matrix. Generally, the charge transport
through molecules is governed by the nature of the molecule.
Methylene (CH2) groups are considered to be insulators. The
Fermi level of the metal is located in the HOMO-LUMO gap
of the molecule, and the side orbitals provide a superexchange
pathway for the electrons to tunnel along the molecule. The
conductivity depends exponentially on the length of the
molecule. When the Fermi level of the metal becomes resonant
or near resonant with the molecular orbital energy levels, the
charge transport shows a weak length dependence.14
The second exponential term in eq 1 describes the activation
energy for charge transport. Different models have been
developed for theoretical description of the activation energy.
On one hand, Abeles et al. developed a simple electrostatic
model to describe the activation energy for the activated charge
transport in granular metals. In the low-field regime, σ ≈
exp[-2(C/RT)0.5], where C ) δâEA,GM and EA,GM corresponds
to15
ꢀ2(ω)
ω
c
3/2
σ
abs(ω) ) 9 V0ꢀm
(3)
[ꢀ1(ω) + 2ꢀm]2 + ꢀ2(ω)2
e2
1
(
1
where V0 is the particle volume, ꢀm is the dielectric constant of
the surrounding medium, and ꢀ(ω) ) ꢀ1(ω) + iꢀ2(ω) is the
dielectric constant of the particle. The peak position of the
plasmon band is determined by the condition for resonance
ꢀ1(ω) ) -2ꢀm and hence is governed by the dielectric function
of the surrounding medium.19 If the particles are closely packed,
the average polarization field from the surrounding particles
additionally influences the plasmon resonance.18
EA,GM
)
-
(2)
)
8πꢀꢀ0 r r + δ
where r is the radius of the particles, δ the interparticle distance,
and ꢀ the dielectric constant of the surrounding matrix. On the
other hand, according to the Marcus theory, the free energy for
activation (∆G*) of electron transfer can also be obtained from
the repolarization of the organic dielectric matrix:
According to the chemical interface damping model devel-
oped by Persson, the shape and position of the plasmon band
reflect not only the dielectric properties of the particles and the
embedding matrix but also the nature of the chemical inter-
face.22,23 Static and dynamic charge transfer between adsorbate
states and the particles can affect the phase coherence of the
collective resonance. While the static charge transfer influences
the electric double layer, the dynamic charge transfer influences
the phase coherence of the conduction electrons. Dynamic
charge transfer can take place when there is an overlap between
the wave function of the metal nanoparticle and the molecular
orbitals associated with the adsorbate, which leads to the
formation of a resonant state. Electrons with energy levels close
to the Fermi level or hot electrons can tunnel through the
interface barrier into the resonant state and cause a dephasing
of the plasmon oscillation.22,23
λ
4
e2
1
1
1
1
1
∆G* )
)
+
-
-
(3)
(
)(
)
16πꢀ0 2r1 2r2 r ꢀop ꢀs
where λ is the “outer sphere” reorganizational barrier energy,
r1 and r2 are the radii of neighboring NPs, and r is the center-
center distance between the NPs.13 Murray and co-workers
compared experimentally obtained activation energies from films
of noninterlinked Au-NPs with both EA,GM and ∆G*.12,13 This
comparison showed that neither of the models provides an
adequate description of the experimentally observed activation
energies.
It has been shown that an insulator-metal transition can be
induced in 2-d and 3-d assemblies of metal NPs by changing
the interparticle distance δ. Musick et al. observed an insulator-
metal transition in 3-d films of 2-mercaptoethylamine interlinked
(11) Snow, A. W.; Wohltjen, H. Chem. Mater. 1998, 10, 947.
(12) Wuelfing, W. P.; Green, S. J.; Pietron, J. J.; Cliffel, D. E.; Murray, R. W.
J. Am. Chem. Soc. 2000, 122, 11465.
(13) Wuelfing, W. P.; Murray, R. W. J. Phys. Chem. B 2002, 106, 3139. (b)
Wuelfing, W. P.; Murray, J. Phys. Chem. B 2003, 107, 6018, Addition
and Correction.
(16) Markovich, G.; Collier, C. P.; Heath, J. R. Phys. ReV. Lett. 1998, 80, 3807.
(17) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427.
(18) Schmitt, J.; Ma¨chtle, P.; Eck, D.; Mo¨hwald, H.; Helm, C. A. Langmuir
1999, 15, 3526.
(19) Link, S. Mohamed, M. B., El-Sayed, M. A. Int. ReV. Phys. Chem. 2000,
19, 409.
(14) Ratner, M. A.; Davis, B.; Kemp, M.; Mujica, V.; Roitberg, A.; Yaliraki,
S. In Molecular Electronics; Avriam, A., Ratner, M., Eds.; Annals of the
New York Academy of Science, Vol. 852; The New York Academy of
Sciences: New York, 1998; p 22.
(20) Mulvaney, P. Langmuir 1996, 12, 788.
(21) Ho¨vel, H.; Fritz, S.; Hilger, A.; Kreibig, U. Phys. ReV. B 1993, 48, 18178.
(22) Kreibig, U.; Gartz, M., Hilger, A. Ber. Bunsen-Ges. Phys. Chem. 1997,
101, 1593.
(15) Abeles, B.; Sheng, P.; Coutts, M. D.; Arie, Y. AdV. Phys. 1975, 24, 407.
(23) Persson, N. J. Surf. Sci. 1993, 172, 557.
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3350 J. AM. CHEM. SOC. VOL. 126, NO. 10, 2004

Optical and Electrical Properties of Au-NP Films
A R T I C L E S
(Tencor P10) along a scratch in the film. XRD measurements were
performed with a Philips X’Pert PRO X-ray diffraction system, using
the Cu KR line (λ ) 154.06 pm). TEM measurements were performed
using the Tecnai F20 from FEI.
This contribution for the first time introduces linker molecules
that enable tuning of the conductivity in 3-d assemblies of Au-
nanoparticles from the semiconducting to the metallic limit by
varying the degree of conjugation of the linker molecule and
the nature of its metal binding groups. We present results from
optical and electrical characterizations of three-dimensional
assemblies of Au-nanoparticles interlinked with novel dithiol-
and dithiocarbamate derivatives as well as supporting results
from TEM imaging and XRD measurements.
Results and Discussion
The molecules used for the assembly of the 3-d networks of
interlinked Au-NPs can be grouped into three classes of
molecules according to the substituents in the para positions of
the benzene and cyclohexane cores. The molecules have
either mercaptomethyl (BDMT, cHDMT), mercaptoacetamide
(DMAAB, DMAAcH), or dithiocarbamate (PBDT, cHBDT)
substituents for binding to gold. An overview of the linker
molecules and the distances between opposing S atoms and S-
atoms, respectively, as calculated by computational quantum
chemistry software, is given in Table 1. The calculations were
done with the program Dmol using density functional theory
(DFT) at the nonlocal level. The Perdew-Wang generalized
gradient-corrected functional (GGA)24 was utilized together with
a double numeric basis set with p- and d-polarized functions
(DND).
Film Growth and Film Morphology. The films were
prepared using the layer-by-layer assembly technique, and the
growth of the films was monitored with UV/vis spectroscopy
after the films reached a sufficient optical density, i.e., usually
after the fifth deposition cycle. For all linker molecules the
absorbance increases linearly with the number of deposition
cycles, indicating that in each deposition cycle the same amount
of material is deposited (Figure 1). However, the average slope
of the curves varies slightly for the different linker molecules.
The slope obtained from the films prepared with the bisdithio-
carbamate linkers is roughly a factor of 1.8 smaller than the
average slope obtained from the dithiol interlinked Au-NP films.
The observed differences could be due to differences in the
extinction coefficients of the NPs within the interlinked films,
differences in the efficiency of exchanging the dodecylamine
capping molecule with the linker molecule, or differences in
packing density (vide infra).
Experimental Section
Materials. All chemicals and solvents were reagent grade or higher
quality and were used as received. Deionized water was purified using
a Millipore Milli-Q system (18.2 MΩ cm). 1,4-Bis(mercaptomethyl)-
benzene (BDMT) was purchased from Aldrich. The syntheses of 1,4-
bis(mercaptoacetamido)cyclohexane (cHDMT), 1,4-bis(mercaptoace-
tamido)benzene (DMAAB), 1,4-bis(mercaptoacetamido)cyclohexane
(DMAAcH), disodium 1,4-cyclohexane-bis(dithiocarbamate) (cHBDT),
and disodium 1,4-phenylene-bis(dithiocarbamate) (PBDT) are described
in the Supporting Information. Dodecylamine-stabilized Au-NPs having
a diameter of d ) 4.0 ( 0.8 nm were synthesized as described by
Joseph et al.10 Briefly, 160 mg of AuCl3 was dissolved in 20 mL of
H2O. Tetraoctylammonium bromide (639 mg) in toluene (20 mL) was
added. The mixture was vigorously stirred until the aqueous phase was
colorless. Subsequently, dodecylamine (1.17 g) in toluene (20 mL) and
a fresh solution of NaBH4 (221 mg) in H2O (15 mL) were added under
vigorous stirring. The organic phase turned immediately from red-
orange to deep purple. After stirring the mixture overnight two size
selective precipitation steps were performed by adding each time 40
mL of ethanol to the organic phase and storing the solution overnight
at -18 °C. The precipitates were separated by filtration through a nylon
membrane (0.45 µm pore size) and redissolved in toluene. The second
fraction was used for the preparation of the films.
Film Preparation. The interlinked Au-NP films were assembled
onto BK7 glass substrates having lithographically defined interdigitated
electrode structures with 20 µm gaps using the layer-by-layer assembly
technique. The substrates were rinsed with acetone, hexane, and
2-propanol and then subjected to oxygen plasma treatment (Plasma
PREP5, Gala Instruments, Germany). They were silanized by immersion
into a solution of 50 µL of 3-aminopropyldimethylethoxysilane in 5
mL of toluene at 60 °C for 30 min. Subsequently, the silanized glass
substrate was immersed for 15 min into a toluene solution of
dodecylamine-stabilized Au-NPs whose concentration corresponded to
an absorbance of 0.4 at λmax ) 514 nm (0.1 cm path length). After
rinsing the substrate with toluene, it was exposed for 15 min to a 1
mM solution of the linker molecule and subsequently washed with the
corresponding solvent of the linker molecule (toluene, DMF, or
2-propanol). The film growth was monitored by UV/vis spectroscopy.
Successive exposures to the Au-NP and linker solutions were repeated
until the maximum of the plasmon band reached an absorbance of ca.
0.3. During the assembly process the film grew on both sides of the
glass substrate. Thus the actual film thickness corresponds to an
absorbance of 0.15.
Figure 2 shows TEM images of Au-NPs interlinked with
DMAAcH (a), DMAAB (b), cHBDT (c), and PBDT (d),
respectively. The networks of Au-NPs interlinked with DMAAcH
and DMAAB are stable during TEM imaging. Both images (a
and b in Figure 2) show well-isolated nanoparticles with particle
to particle distances of roughly 1 nm. These values are slightly
smaller than the S-S distance of the energy-minimized struc-
tures of the linker molecules (Table 1). The networks of Au-
NPs interlinked with cHBDT and PBDT are very sensitive to
(24) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M.
R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671.
(25) Link, S.; Mohamed, M. B.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103,
3073.
(26) Chang, W.; Dong, S.; Wand, E. Chem. Mater. 2003, 15, 2495.
(27) Humeres, E.; Debacher, N. A.; Franco, J. D.; Lee. B. S.; Martendal, A. J.
Org. Chem. 2000, 67, 3662.
(28) Humeres, E.; Debacher, N. A.; Sierra, M. M.de S. J. Org. Chem. 1999,
64, 1807.
(29) Wold, D. J.; Frisbie, C. D. J. Am. Chem. Soc. 2001, 123, 5549.
(30) The resistivity (F) of bulk gold at T ) 20 °C is F ) 2 × 10-8 Ω‚m.
(31) Holmlin, R. E.; Ismagilov, R. F.; Haag, R.; Mujica, V.; Ratner, M. A.;
Rampi, M. A.; Whitesides, G. M. Angw. Chem. Int. Ed. 2001, 40, 2316.
(32) Cui, X. D.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Primak, A.; Moore, A.
L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M. Nanotechnology
2002, 13, 5.
(33) Anariba, F.; McCreery, R. L. J. Phys. Chem. B 2002, 106, 10355.
(34) Kornilovitch, P. E.; Bratkovsky, A. M.; Williams, R. S. Phys. ReV. B 2002,
66, 165436.
For XRD and TEM measurements, the films were prepared from
nanoparticles interlinked in solution. The thoroughly washed precipitates
were deposited onto Si substrates for XRD measurements and onto
carbon grids (Plano) for TEM analysis. Drop casting was used for the
preparation of the control sample of noninterlinked films of dodecyl-
amine-stabilized Au-NPs.
Instruments. UV/vis absorption spectra were recorded using a
Varian Cary 50 Scan spectrometer. Electrical characterization was
performed using a home-built setup consisting of a liquid nitrogen
container, a temperature control unit (Lake Shore), and a HP4241 source
measure unit. The films were contacted using conductive Ag paint.
The thickness of the assemblies was determined with a surface profiler
9
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A R T I C L E S
Wessels et al.
Table 1. Overview of Linker Molecules
a S-S distance of energy-minimized structure. b S--S- distance of energy-minimized structure.
assume that the Au-NP packing density is comparable for the
films interlinked with DMAAB, DMAAcH, cHBDT, and PBDT.
Optical Properties. Figure 4 shows characteristic absorption
spectra of the three-dimensional assemblies of Au-NPs inter-
linked by the three classes of molecules. The Au-NP films
interlinked with the dithiols BDMT, cHDMT, DMAAB, and
DMAAcH exhibit a distinct plasmon band between 550 and
590 nm and have a blue-red appearance. In contrast the Au-NP
film interlinked with cHBDT has a blue-green appearance and
a significantly broadened plasmon band. The film prepared with
PBDT appears to be greener compared to the film prepared with
cHBDT. In addition it exhibits also some golden color in
reflection. The film displays absorption characteristics similar
to bulk gold.
The plasmon band maxima of the Au-NP films interlinked
with BDMT and DMAAB are red-shifted in comparison to the
maxima of the Au-NP films interlinked with cHDMT and
DMAAcH, respectively (Table 2). These values are reproducible
within an uncertainty of ca. (4 nm. In case of the mercapto-
methylene linker molecules, exchanging the cyclohexane core
with a benzene core causes a red shift in the plasmon band of
∼31 nm. The difference in the S-S distance is 0.5 Å. In the
case of the acetamidothiols the shift is only ∼19 nm. The
differences in the S-S length is 0.1 Å. These results indicate
that the observed shifts in the peak position of the plasmon band
are not solely determined by changes in the length of the linker
molecule but that the polarizability of the linker molecule largely
contributes to the observed shifts.17,18,20,21
Figure 1. Absorbance of three-dimensional assemblies of Au-NPs inter-
linked with the bis-mercaptophenyl derivatives BDMT (1a) (b) and cHDMT
(1b) (O), the bis-acetamidothiol derivatives DMAAB (2a) (2) and
DMAAcH (2b) (∆), and the bis-dithiocarbamate derivatives PBDT (3a)
(9) and cHBDT (3b) (0) plotted as a function of the deposition cycle after
the fifth deposition cycle.
e-beam irradiation, causing rapid fusing of the Au-NPs upon
irradiation. Results of such fusion processes are apparent in
Figure 2c,d in several places. However, XRD measurements of
Au-NPs interlinked with PBDT show that the half-width of the
reflection size of the Au-NPs remains unchanged upon inter-
linking them with PBDT (Figure 3). The half-width of the (111)
reflex corresponds to a particle diameter of 2.8 nm, which is
slightly smaller than the particle diameter obtained from TEM
(4.0 ( 0.8 nm). The slight difference between the average
particle size obtained from TEM and the calculated particle size
is probably due to lattice mismatches of the crystalline gold
nanoparticles, which cause an additional broadening of the
reflexes in the XRD measurements. The grain boundaries of
the particles, however, remain stable after cross-linking with
PBDT, indicating that the observed changes in particle diameter
during TEM imaging are solely induced by e-beam irradiation.
At locations where fusion processes did not take place, well-
separated NPs were observed in several TEM images (e.g.,
Figure 2c and d), showing interparticle distances of roughly 1
nm for cHBDT and PBDT. On the basis of these results, we
Within this context, it is further interesting to note that for
both types of linker molecules the films prepared with the
cyclohexane derivatives are ∼5 nm thicker (see Table 2) than
the films prepared with the benzene derivatives, although the
films have the same absorbance. This is due to differences in
the number of assembly cycles necessary to reach an optical
density of 0.3 in the plasmon band maximum (Figure 1). If the
number of assembly cycles is kept constant, the absorbance of
the film varies between the cyclohexane and benzene derivatives
accordingly. It has been shown, in agreement with the Mie
theory, that with increasing dielectric constant the plasmon
resonance shifts to the red and the intensity of the plasmon band
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Optical and Electrical Properties of Au-NP Films
A R T I C L E S
Figure 2. TEM images of Au-NPs interlinked with the acetamidothiol derivatives DMAAcH (a) and DMAAB (b) and with the bis-dithiocarbamate derivatives
cHBDT (c) and PBDT (d).
Figure 3. XRD spectra of a film of Au-NPs interlinked in solution with
the bis-dithiocarbamate PBDT (1) and of a film of noninterlinked dodec-
ylamine-stabilized Au-NPs (2) prepared by drop casting (control). The small
scattering signals are probably due to PBDT crystals that formed upon drying
of the film from excess of the linker molecule.
Figure 4. Absorption spectra of three-dimensional assemblies of Au-NPs
interlinked with the bis-mercaptomethylene derivatives BDMT (1a) and
cHDMT (1b), the bis-acetamidothiol derivatives DMAAB (2a) and
DMAAcH (2b), and the bis-dithiocarbamate derivatives PBDT (3a) and
cHBDT (3b).
increases.17,18,20,25,26 This indicates that the extinction cross
section is smaller for the films interlinked with the cyclohexane
derivatives than for the films interlinked with the benzene
derivatives.
More pronounced changes are apparent in the absorption
spectra of the bis-dithiocarbamate (PBDT, cHBDT) interlinked
Au-NP assemblies (Table 2). In the case of PBDT (3a), the
plasmon band is nearly absent and the film exhibits a strong
absorbance in the near infrared, similar to the absorption spectra
of gold films. A distinct broadening of the plasmon band and
increase in absorbance in the near-IR is also observed with
cHBDT (3b); however, the spectrum still exhibits a distinct
plasmon band maximum at 626 nm. The peak position is
reproducible with an uncertainty of ca. (20 nm. As noted
earlier, metallic absorption spectra have been observed previ-
ously for Au-NP assemblies containing short thiols or dithiols.
The absorption spectrum of PBDT (3a) (Figure 4) is comparable
to the absorption spectrum of a film of 6 nm sized Au-NPs
interlinked with propanedithiol, where the spectral changes were
attributed to the close proximity of the nanoparticles.9 For films
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A R T I C L E S
Wessels et al.
Table 2. Overview of the Film Characteristics
linker
d (nm)a
λ
max (nm)
EA (meV)
resistivity (Ωm)
(1a) BDMT
(1b) cHDMT
(2a) DMAAB
(2b) DMAAcH
(3a) PBDT
13.5
19.8
13.4
18.1
11.1
12.3
589
558
573
554
59.7
58.8
65
0.09
0.85
4.2
92.8
0.0008
0.0042
71
(3b) cHBDT
626
13.8
a Film thickness d.
of Au-NPs interlinked with 2-mercaptoethylamine, Natan and
co-workers observed with increasing surface coverage a transi-
tion from a spectrum exhibiting a plasmon band to a spectrum
displaying the characteristics of bulk gold.2,8 In the present case,
however, the distinct broadening of the plasmon band as well
as the metallic absorption characteristics seem to be specific to
the nature of the dithiocarbamate linker molecules, since they
are evident from the start of the assembly process (see
Supporting Information). In addition, the results obtained from
the TEM images provide support that the interparticle separation
is approximately 1 nm and determined by the length of the linker
molecule (vide supra).
Kreibig et al. observed for metal clusters embedded in various
dielectric metal oxides a distinct red-shift and broadening of
the plasmon band with increasing dielectric constant of the
embedding matrix.22 These plasmon band characteristics were
explained in terms of the chemical interface-damping model.23
Excited electrons that are transferred between adsorbate induced
resonant states and the metal NPs disturb the phase coherence
of the surface plasmon as a result of inelastic scattering processes
and hence lead to a broadening of the plasmon band. This
requires an overlap between molecular orbital and metal wave
functions,23 which creates a resonant state close to the Fermi
level of the metal-NPs. It is described in the literature that the
strong electron-withdrawing nature of the thiocarbonyl intro-
duces a pronounced double bond characteristic to the C-N bond
in the bis-dithiocarbamate moiety and inhibits the delocalization
of the nitrogen electron pair into the benzene ring.27,28 The
electron-withdrawing nature of the thiocarbonyl may also
influence the electron distribution in the Au-S bond. It is
anticipated that the conjugated nature of the dithiocarbamate
group, which is directly coupled to the Au-NPs, leads to the
formation of a resonant state, causing the pronounced changes
in the absorption characteristics of the films.
Electronic Transport Properties. The current-voltage
measurements show ohmic behavior for all films between (1
V in the temperature range between 100 and 300 K. The
resistivities of the films at 295 K are summarized in Table 2.
The experimental uncertainty obtained from three to five
different assemblies is approximately (10%. For all three
classes of molecules, the resistivity of the film increases by
roughly 1 order of magnitude when the benzene ring is
substituted with a cyclohexane ring. The same order of
magnitude difference in the resistivity was observed in metal-
molecule-metal tunnel junctions fabricated from benzyl thiol
SAMs and hexyl thiol SAMs (vide infra).29
The conductivity is plotted as a function of T-1 in Figure 5.
Except for the film of PBDT interlinked Au-NPs, which displays
metallic behavior, the films exhibit transport characteristics of
a percolated network of nanoparticles; that is, the conductivity
decreases with decreasing temperature. The electron transport
Figure 5. σ(T) characteristics of three-dimensional assemblies of Au-NPs
interlinked with the bis-mercaptomethyl derivatives BDMT (1a), cHDMT
(1b), the bis-acetamidothiol derivatives DMAAB (2a), DMAAcH (2b), and
the bis-dithiocarbamate derivatives PBDT (3a) and cHBDT (3b).
in such films can be described in terms of a thermally activated
tunneling process between particles.10,13 From the slope of the
curves we obtained the activation energy EA for charge transport
(Table 2). The activation energies of the BDMT and cHDMT
interlinked films (EA ≈ 58-59 meV) and DMAAB and
DMAAcH interlinked films (EA ≈ 65-71 meV) are, within the
experimental uncertainty of ca. (10%, similar in value and lie
in the range reported for self-assembled films of Au-NPs
interlinked by alkanedithiols using also dodecylamine-stabilized
Au-NPs prepared according to the same protocol (d ) 4.0 (
0.8 nm).10
The distinct differences in the absorption characteristics of
the dithiol and dithiocarbamate interlinked Au-NP films are also
apparent in the electrical characteristics of the assemblies. The
film of PBDT interlinked Au-NPs shows metallic behavior; that
is, the conductivity increases with decreasing temperature. This
is in agreement with the optical properties of this film displaying
absorption characteristics of bulk metal (vide supra). Metallic
charge transport characteristics have been previously observed
in propanedithiol interlinked Au-NP films.9 Natan and co-
workers observed insulator-metal transitions for films consist-
ing of ∼6 nm Au-NPs interlinked by 2-mercaptoethylamine at
sufficient high surface coverage, as mentioned above.2 This
transition was attributed to the close proximity of the nano-
particles and a resulting overlap of the wave function from the
nanoparticles. In the PBDT interlinked Au-NP film, however,
the distance between the nanoparticles is roughly twice as large
compared to the 2-mercaptoethylamine interlinked films. The
above-described results indicate that, due to the high degree of
conjugation of PBDT and the good Au-NP/molecule contact,
the electron wave function between neighboring NPs also
overlaps. Metallic properties were also observed for ∼150 nm
thick films of 11 nm Au-NPs capped with 2-mercaptoethanol.
The resistivity of these films was 5 × 10-6 Ω‚m, which is only
200 times that of bulk gold.2,8 The resistivity of the 13.8 ( 2.1
nm thick PBDT interlinked Au-NP film at 295 K is 8 × 10-4
Ω‚m, which is about 3-4 orders of magnitude above that of
bulk gold.30 Since the conductivity in such films is roughly
proportional to the third power of the NP diameter (for NPs
having diameters comparable or smaller than the de Broglie
9
3354 J. AM. CHEM. SOC. VOL. 126, NO. 10, 2004

Optical and Electrical Properties of Au-NP Films
A R T I C L E S
as obtained from three to five different films for each linker
molecule. There is no evidence for a clear linear relationship
between ln(σIntercept) and the distance between the nanoparticles,
as it has been previously observed for NP films comprised of
alkanethiol-stabilized Au-NPs1,13 and metal-molecule-metal
junctions.29,31,32 Similar scattering in plots of ln(σIntercept) vs δ
was also observed by Murray and co-workers for thin films of
arenethiol monolayer-protected Au clusters.13 They obtained a
clear linear relationship when ln(σIntercept) was plotted as a
function of the number of methylene units, however. Taking
into consideration the conjugated nature of the C-N bond in
the dithiocarbamate moiety (vide supra), we find a linear
relationship between ln(σIntercept) and the number of nonconju-
gated bonds (NN-CON) (Figure 6b), suggesting that σ ≈ σ0
exp(-âΝNN-CON). This indicates that electron tunneling through
the conjugated parts of the molecules does not contribute
significantly to the decay of the electron wave function. For
resonant tunneling processes, only a weak dependence of the
electron transmission on the number of conjugated bonds is
expected.33 Thus, these data suggest that the electron transport
through these molecules is partially nonresonant along the side
orbitals and partially resonant through the conjugated parts of
the molecule. In other words, the molecules can be viewed as
consisting of two electrically insulating (the nonconjugated) parts
in series with conductive (conjugated) parts. This is in agreement
with the theoretical description of rectifying asymmetric mol-
ecules by Williams and co-workers, where a conducting level
such as a benzene ring is placed asymmetrically between
insulating alkane chains.34 From Figure 6b it is evident that the
ln(σIntercept) values obtained for cHDMT and cHBDT deviate
from the linear fit. This may be due to the fact that all single
bonds were treated as being equivalent, with respect to their
electronic coupling.
From the slope of the linear fit (Figure 6b) we obtain â )
0.99 per NN-CON, which is comparable to the values reported
for noninterlinked alkanethiol-stabilized Au-NP films13 and
SAMs of alkanethiols29,31,32 but larger than the values obtained
from alkanedithiol interlinked Au-NP films (0.61 e â e 0.71)10
and alkanedithiol SAMs contacted at both ends (â ) 0.57).35
The smaller values of â in the latter two cases were attributed
to the fact that the molecules were contacted at both ends.
Recently, however, Xu and Tao reported a tunneling decay
constant of â ) 1.0 ( 0.1 for n-alkanedithiols.36 The results
described above suggest that the contact to the nanoparticles
through either thiol or thiocarbamate groups is not the determin-
ing factor for â, since the data obtained from the respective
linker molecules fall on the same line. Whether the lower
â-value obtained from films of alkanedithiol interlinked Au-
NPs is related to the flexibility of the linker molecules and
associated changes in the geometry of the films or to the fact
that our linker molecules contain different heteroatoms needs
to be resolved in further experiments.
Figure 6. (a) Plot of ln(σIntercept) as a function of the length of the linker
molecule. The length of the bis-mercaptomethylene (9) and bis-acetami-
dothiol (b) and bis-dithiocarbamate (2) linker molecules corresponds to
the S-S and S--S- distances of the energy-minimized structures. (b) Plot
of ln(σIntercept) as a function of the number of nonconjugated bonds in the
bis-mercaptomethylene (9) and bis-acetamidothiol (b), and bis-dithiocar-
bamate (2) linker molecules, where the dithiocarbamate group is considered
to be conjugated.
wavelength),11 these differences in conductivity can be mainly
attributed to differences in the diameter of the Au-NPs.
The activation energy for the cHBDT interlinked Au-NP film
(EA ) 13.8 meV) is ∼4-5 times smaller than the activation
energies obtained from the films assembled with the dithiol
derivatives. As described above, the optical characteristics of
the films indicate that the strong electron-withdrawing nature
of the thiocarbonyl group leads to an overlap of the molecular
orbital and metal wave function that could lead to an apparent
increase in the Au-NP diameter. On the basis of the granular
metal theory,15 an effective increase in particle radius of 2.4 Å,
which corresponds to the Au-S bond distance in a thiocar-
bamate/Au conjugate, should lead to a decrease in the activation
energy of a factor of ∼3.
In this context it is further pertinent to note that, within each
class of linker molecules, the Au-NP films interlinked with the
benzene derivatives are 1 order of magnitude more conductive
than the films interlinked with the cyclohexane derivatives (vide
supra). Wold and Frisbie observed this difference also between
Equation 1 predicts a linear relationship between ln(σ) and
the interparticle distance δ. By extrapolating the plots in Figure
5, we obtain the intercept ln(σIntercept) ) [ln(σ0) - âδ]. In Figure
6a, ln(σIntercept) is plotted as a function of the S-S distance in
case of the dithiols and the S--S- distance in case of the bis-
dithiocarbamates, respectively. The error bars reflect the
measurement uncertainty in the conductivity of the 3-d films
(35) Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Moore, A.
L.; Moore, T. A.; Gust, D.; Nagahara, L. A.; Lindsay, S. M. J. Phys. Chem.
B 2002, 106, 8609.
(36) Xu, B.; Tao, N. J. Science 2003, 301, 1221.
9
J. AM. CHEM. SOC. VOL. 126, NO. 10, 2004 3355

A R T I C L E S
Wessels et al.
benzyl thiol SAMs and to hexyl thiol SAMs and attributed this
change in the resistance to differences in the HOMO-LUMO
gap between phenyl rings (∆EHOMO-LUMO ≈ 4 eV) and alkyl
chains (∆EHOMO-LUMO ≈ 8 eV).29 When comparing the differ-
ence in the HOMO-LUMO gap of the bis-mercaptomethyl,
bis-mercaptoacetamido, and bis-dithiocarabamate derivatives,
there is no clear relationship between these and the electrical
properties of the films (Table 1). However, 1 order of magnitude
difference in the conductivity of alkanedithiol interlinked Au-
NP films is also observed upon increasing the number of
methylene units by three. This corresponds to the number of
additional nonconjugated bonds in the cyclohexane-based linker
molecules, if all single bonds can be treated as being equiva-
lent.9,10
The conductivity of the film prepared with cHBDT is 1 order
of magnitude larger than the film prepared with BDMT, despite
the fact that the interparticle distance is larger and the linker
molecule has five nonconjugated bonds (PBDT has only four
nonconjugated bonds). We note, however, that the activation
energy is roughly a factor of 4 smaller in the case of cHBDT.
According to eq 1, a factor of ∼4 difference in EA would lead
to 1 order of magnitude difference in conductivity if the number
of nonconjugated bonds is constant. Taking into consideration
the difference in the number of nonconjugated bonds between
cHDMT and BDMT, the observed difference in EA would lead
to roughly a factor of 4 difference in the conductivity.
In summary, these investigations on a variety of novel dithiol
and bis-dithiocarbamate linker molecules show that the metal/
molecule contact has a pronounced influence on the optical and
electrical properties of the films. The nonresonant tunneling
process along the nonconjugated parts of a molecule mainly
determines the electron tunneling decay constant, while the
contribution from the conjugated parts is negligible, correspond-
ing to a resonant tunneling through the conjugated parts of the
molecule. These effects make it possible to tune the conductivity
from the insulating to the metallic limit without altering
significantly the interparticle spacing.
Acknowledgment. Partial financial support from the Euro-
pean Commission through the BIOAND IST-FET “Nanotech-
nology Information Devices” initiative, BIOAND project IST-
1999-11974, is acknowledged. We also acknowledge Dr.
Yvonne Joseph for fruitful discussions.
Supporting Information Available: Synthesis and analytical
characterization of Au-NPs and linker molecules, additional
absorption spectra of Au-NP films interlinked with PBDT and
cHBDT. This material is available free of charge via the Internet
at http://pubs.acs.org.
JA0377605
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3356 J. AM. CHEM. SOC. VOL. 126, NO. 10, 2004