Langmuir 2001, 17, 403-412
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Studies on Charge Transport in Self-Assembled Gold-Dithiol Films: Conductivity, Photoconductivity, and Photoelectrochemical Measurements N. Fishelson, I. Shkrob, O. Lev,* J. Gun, and A. D. Modestov Division of Environmental Sciences, Fredy and Nadine Herrmann School of Applied Science, The Hebrew University of Jerusalem, Jerusalem, 91904 Israel Received June 15, 2000. In Final Form: October 25, 2000 Electronic transport in gold-dithiol nanoparticle films was studied using conductivity, photoconductivity, and photoelectrochemical means. The films were characterized by SEM and optical spectroscopy. GC/MS was used for the analysis of the pyrolysis products during heat treatment. Films were assembled on glass substrates using gold sol and different alkanethiol spacers (1,2-ethanedithiol (C2), 1,5-pentanedithiol (C5), and 1,8-octanedithiol (C8)). Resistance-temperature measurements revealed that the effective activation energies for conduction were 0, 5, and 15 meV for films assembled using C2, C5, and C8 spacers, respectively. Light action spectra of photoconductivity of gold-dithiol nanoparticle films revealed 0.8-1.0 eV threshold photon energy. The difference between the observed threshold energies points to different mechanisms for conductivity and photoconductivity. The low effective activation energy for dark conduction is attributed to a mixed mechanism of conduction, tunneling between insulated particles, and metal conduction through defects which are ascribed to direct contact points between metal particles. The photoconductivity mechanism involves photoemission from metal particles into the insulator layer. Photoelectrochemical studies of gold nanoparticle electrodes in aqueous electrolyte revealed 3.5 eV photon energy threshold of the photocurrent at an electrode potential of E ) 0 V vs Ag/AgCl reference. The much higher photoelectrochemical threshold energies are ascribed to direct photoemission processes from the surface metal particles into the electrolyte. Heat treatment of the films decreased film resistance and increased the temperature coefficient of resistance to values approaching that of metal gold. These trends are attributed to pyrolysis of spacer molecules, which favor the metal conduction mechanism.
Introduction Self-assembled 2-D and 3-D nanoparticle films represent a rapidly developing field of nanostructured materials.1-11 3-D self-assembled metal nanoparticle films can be prepared by cross-linking the nanoparticles using bifunctional organic compounds that have high affinity to gold, such as organic dithiols10,11 and diamines.9 Properties of the nanoparticle films can be adjusted by changing gold particle size and the type and length of the bifunctional cross-linker. This technology provides a cheap wet chemistry means to construct thin semitransparent conductive coatings on insulators or semiconductors. The mechanism of electrical conductivity through nanoparticle films is of particular interest. Charge transport through metal island-structure films received scientific and technological attention with the development of the so-called Cermets (ceramic metal conductors).12 Electron transport through nanoparticle self-assembled *Corresponding author. E-mail:
[email protected]. (1) Wang, Z. L. Adv. Mater. 1998, 10, 13. (2) Simon, U. Adv. Mater. 1998, 10, 1487. (3) Feldheim, D. L.; Keating, C. D. Chem. Soc. Rev. 1998, 27, 1. (4) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P. Science 1997, 278, 252. (5) Hu, K.; Chai, Z.; Whitesell, J. K.; Bard, A. J. Langmuir 1999, 15, 3343. (6) Fink, J.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. Chem. Mater. 1998, 10, 922. (7) Bharathi, S.; Joseph, J.; Lev, O.; Lev, Z. Electrochem. Solid State Lett. 1999, 2, 284. (8) Bharathi, S.; Fishelson, N.; Lev, O. Langmuir 1999, 15, 1929. (9) Musick, M. D.; Peo`a, D. J.; Botsko, S. L.; McEvoy, T. M.; Richardson, J. N.; Natan, M. J. Langmuir 1999, 15, 844. (10) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Adv. Mater. 1995, 7, 795. (11) Musick, M. D.; Keating, C. D.; Keefe, M. H.; Nata, M. J. Chem. Mater. 1997, 9, 1499.
films involves motion of electrons through metal particles interrupted by transport over spacer molecules potential barriers. An electron energy diagram for gold nanoparticles embedded in an insulator matrix is shown in Figure 1. There are two basic mechanisms for electron transport between metal particles separated by thin insulating barriers: transport of electrons through the energy barrier by quantum mechanical tunneling;13,14,15 thermionic emission of electrons over the potential barrier.16 For low electric fields in the insulating gap between particles (i.e., eV < kT, where V is the potential drop between neighboring metal islands), the thermionic emission of electrons between metallic islands gives the sheet conductivity (C):16
C ) RT 2(e/kT) exp(-φ/kT)
(1)
In this case, the activation energy (Ea) for conductivity equals the barrier height φ. Here k is Boltzmann’s constant, e is the elemental charge, T is the absolute temperature, and R is a constant. The barrier height equals the metal-insulator work function (φmi) reduced by the energy associated with the image forces (δφ). The latter becomes important for small, nanometer scale interparticle gap.16 According to ref 17, the image forces lower the energy barrier (eV) by (12) Coutts, T. J. Electrical Conduction in Thin Metal Films; Elsevier: Amsterdam, 1974. (13) Hill, R. M. Proc. R. Soc. A 1969, 309, 377. (14) Sheng P.; Abeles, B. Phys. Rev. Lett. 1972, 28, 34. (15) Abeles, B.; Sheng, P.; Coutts, M. D.; Arie, Y. Adv. Phys. 1975, 24, 407. (16) Van Steensel, K. Philips Res. Rep. 1967, 22, 246. (17) Simons, J. D. J. Appl. Phys. 1966, 34, 1793.
10.1021/la000830q CCC: $20.00 © 2001 American Chemical Society Published on Web 12/29/2000
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tunneling barrier width, φx is the barrier height at distance x from a particle, φav is the averaged barrier height, and δEel is the electrostatic energy required to generate charge carriers in the initially neutral metal islands by transfer of an electron between two nearby islands. For low electric fields (eV < kT), eq 3 gives Ohm’s law. The conductivity activation energy in this case equals δEel, which is given by the following expression:14,15
δEel ) 0.5e2s/{(4π0r)(r + s)}
Figure 1. Energy diagram of a gold nanoparticle film. Electron energy levels in a gold particle and vacuum and the conductivity level in an insulator are shown. Symbols: φmv, gold-vacuum work function; φmI, gold-insulator work function; χ, electron affinity in the insulator; φ, energy barrier for thermionic emission; δEel, electrostatic energy required to transfer electron between two nearby islands, which were initially neutral.
δφ ≈ 2/(opts)
(2)
where opt is the high-frequency dielectric constant of the insulator16,18 and s is the distance between metal particles, expressed in nanometers. Letting n ) 1.5 for the organic spacer layer (corresponding to typical refractive index of liquid hydrocarbons19), then opt ) n0.5 ≈ 1.2. For barrier width s ) 1 nm, the barrier energy is lowered by δφ ≈2 eV. The gold-vacuum work function is φmv ) 5.1 eV.20 The metal-polar hydrocarbon work function, φmI, is lowered compared to the metal-vacuum value by the electron affinity of the polar hydrocarbon (χ), which is of the order of 0.4-1 eV (e.g., for methanol χ ) 0.39 eV; for water χ ) 1.12 eV).21 Thus, the sum of two components, the image forces effect and the electron affinity of polar hydrocarbons, lowers the energy barrier in respect to the gold-vacuum work function by ca. 2.5 eV, thus giving a barrier height of φ ≈ 2.5 eV. However, this is only a rough estimate since both contributions are calculated assuming insulator phase between the particles rather than bridges of organic molecules, which are chemically bonded to gold particles on both sides. Electron transfer between metal islands by quantum mechanical tunneling was discussed in refs 12-15 and 22. Equation 3 describes the current (J) passing between two metal islands:12
J ∼ (8πme/h3B2) sinh(eV/kT) exp(-Aφav0.5) exp{-(δΕel/kT) [1- exp(-BδEel)]} (3) A ) 4πs(2m)0.5/h B ) A/(2φav0.5) φav)s-1
∫Sφsx dx
where m is the mass of the tunneling electron, h is Planck’s constant, A and B are functions dependent on the (18) Henisch, H. K. Semiconductor Contacts. An Approach to Ideas and Models; Clarendon Press: Oxford, U.K., 1984; p 58.
(4)
where r is the particle radius, 0 is permittivity of free space, and stands for the static dielectric constant of the insulator. This energy is required mostly to overcome the electrostatic image forces. However, it should be noted that the contribution of orientation polarization of spacer molecules should be subtracted from , since adsorbed spacer molecules are not free to change orientation. The goal of the article is to study the mechanism of electric conductivity through the self-assembled golddithiol nanoparticle films as a first step toward their use as working electrodes in electrochemical cells and sensing devices. The goal is approached by (1) studies of the temperature dependence of conductivity at the subambient range, (2) studies of conductivity changes at elevated temperatures (up to 200 °C), (3) photoconductivity measurements, and (4) photoelectrochemical studies. Gas chromatography-mass spectrometry was used to detect gaseous emissions during the heat treatment. To study the dependence of conductivity on the presence of defects in the nanoparticle self-assembled structures, we increased the number of defects using a controlled heat treatment of the films in an inert atmosphere. The change in film conductivity with temperature was reversible in the subambient range and irreversible at higher temperatures, indicating irreversible structural transformations of the films at high temperatures. The increase of the observed conductivity and change of the temperature coefficient of resistance (TCR) after heat treatment indicate gradual increase of the contribution of metallic conductivity as compared to electron hopping. The conductivity of heat-treated films approached, within 1 order of magnitude, the conductivity of bulk gold. Experimental Section Reagents and Materials. All chemicals were of reagent grade and were used as received. 1,2-Ethanedithiol (C2), 1,5-pentanedithiol (C5), 1,8-octanedithiol (C8), (3-mercaptopropyl)trimethoxysilane (MPTMOS), 95%, and hydrogen tetrachloroaurate were received from Aldrich, toluene was from J. Baker, and sodium borohydride was from Merck. All other chemicals were of reagent grade. Water was deionized (conductivity
