© Copyright 1996 by the American Chemical Society
VOLUME 100, NUMBER 23, JUNE 6, 1996
LETTERS Studies of Electron Tunneling at Semiconductor Electrodes Y. Gu and D. H. Waldeck* Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260 ReceiVed: December 19, 1995; In Final Form: March 7, 1996X
The recently developed ability to prepare self-assembled monolayers on n-InP material has been used to study the photooxidation of Fe(CN)64- to Fe(CN)63-. In particular, the dependence of the photocurrent on the thickness of the monolayer film has been investigated. The distance dependence of the electronic coupling through the organic layer is observed to be “softer” than corresponding reports for such layers on gold electrodes.
A major goal in the study of electron-transfer reactions is to obtain a better understanding of the electronic coupling between the donor and acceptor moieties and how it is mediated by the spacer between them. Much of the mechanistic understanding of electron-transfer reactions, in general, and the electronic coupling, in particular, results from studies in homogeneous systems. Recent work1,2 on intramolecular electron-transfer systems has found that through-bond coupling (or pathway models) provides an adequate picture of the electronic coupling, in the weak coupling limit. In pathway models the distance dependence of the coupling depends on the energetics of the bridging unit (or spacer unit), relative to the donor and acceptor. By comparison, little work has been performed for heterogeneous systems, and it has been limited to the study of tunneling through insulating barriers on gold electrodes,3-6 silver electrodes7 and on silicon electrodes.8 Gu et al.9 recently demonstrated the ability to create self-assembled monolayers (SAMs) of alkanethiols on the (100) face of the III-V semiconductor InP. This study exploits that chemistry to measure the distance dependence of the electron transfer between the InP electrode and a redox couple in solution, by changing the thickness of the SAM. Conclusive mechanistic studies of heterogeneous electron transfer require chemical control of the interface and the X
Abstract published in AdVance ACS Abstracts, May 15, 1996.
S0022-3654(95)03754-3 CCC: $12.00
interfacial properties.10 The only previous work which studies the distance dependence of the electron-transfer rate has been performed on gold electrodes.3-6 Most of the studies on metal electrodes involve the investigation of the electron-transfer rate constant as a function of the overpotential and the extraction of the rate constant through a Tafel analysis.3a,11 These studies have found that electron transfer occurs by tunneling through the layers, so that the rate is given by
ket ∝ exp(-βn) where 1/β is the characteristic decay length and n is the number of methylene units. The β parameter for the alkane layer is about 1 per CH2 (the reported values range from 0.8 to 1.2). Interestingly, Miller3c has demonstrated that the characteristic decay length depends on the overvoltage for some redox couples, but this dependence is quite shallow for Fe(CN)63-/ Fe(CN)64-. By contrast, the studies reported here involve the use of a semiconductor electrode, which is beneficial for a variety of reasons. For example, the distance dependence of the reorganization energy is expected to be weaker.12 Other interesting features of these systems are the ability to vary the exothermicity of the reaction and the ability to change the tunneling energy. In these studies the electron transfer, as a function of the film thickness, is monitored by measuring the photocurrent through the electrochemical cell.13 The charac© 1996 American Chemical Society
9574 J. Phys. Chem., Vol. 100, No. 23, 1996
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Figure 1. Dependence of the photocurrent on bias potential is shown for three electrodes (O is untreated; 4 is covered with octanethiol;+ is covered with hexadecanethiol).
Figure 2. This figure shows the measured photocurrent (j) as a function of the alkane chain length. A is the area of the electrode. See text for details.
teristic decay length reported here is “softer” than the length dependence found on gold electrodes (0.52 per methylene group as opposed to 1 per methylene on gold). The self-assembled monolayers were prepared in the manner described previously.9 The thickness of the film was controlled by variation of the methylene units in the alkyl chain from 7 to 15. The n-type InP had a dopant density of 3 × 1018 cm-3. An ohmic contact was prepared on the back of the electrode using Ga/In eutectic. The counter electrode in these studies was Pt gauze and the reference electrode was SCE. The redox couple was an equimolar (0.08 M), aqueous solution of Fe(CN)63-/ Fe(CN)64- with 0.10 M Na2SO4 as a supporting electrolyte. The photocurrent apparatus and other electrochemical equipment has been described previously.14 Studies were performed using both a tungsten halogen lamp (with a cutoff filter to block the IR) and a HeNe laser as light sources. The measured distance dependence was independent of the light source. The light intensities were of the order 0.4 mW/cm2 or less. Figure 1 shows the dependence of the photocurrent on the applied potential. The potential was swept in the positive direction. These data correspond to three different sample preparations. The circles show data for an InP electrode which is freshly etched with HF but not treated with any alkanethiol. The other two curves correspond to InP electrodes with monolayers formed from octanethiol (triangles) and hexadecanethiol (crosses). The magnitude of the current response changes dramatically, but the dependence of the photocurrent on the bias potential is similar for each system. The inset of the figure shows the same data normalized to the maximum current. The onset of the photocurrent occurs near -0.7 V versus SCE, and its value is independent of the alkane chain length. This value is similar to the flatband potential found from a Mott-Schottky analysis15 of the InP electrode. The photocurrent was measured at a bias potential of 0.0 V versus SCE for a number of InP electrodes with different film thicknesses. The film thickness was changed by varying the number of methylene units in the alkane chain. Figure 2 shows a plot of the natural logarithm of the measured photocurrent as a function of the film thickness. Each data point in this plot represents a measurement on a different electrode. The main source of scatter in the data points is likely to be the surface area of the electrode, which was determined using a calipers. The best-fit line has a slope of -0.53 ( 0.04, and the correlation coefficient for the data set is 0.97.16 Studies of this sort were performed at a number of different bias potentials. Although the magnitude of the photocurrent changed with bias potential, the slope of the line did not change significantly.
The slope reported here is a factor of 2 softer than that found for alkane chains on gold electrodes. The factor of 2 in the current (or rate) corresponds to a factor of 1.4 in the electronic coupling’s decay exponent, which is still a significant difference. Three possible explanations might be offered for this difference in the distance dependence. First is the possibility that the alkane layers have defects and pinholes which contribute to the photocurrent in parallel with the tunneling. Second is the possibility that parameters in the rate expression other than the electronic coupling have a distance dependence that needs to be incorporated. Third is the possibility that the effective tunneling barrier for the charge transfer is significantly different for InP than it is for Au. Each of these explanations is discussed, in turn, below. The presence of pinholes and defects would act to create a parallel mechanism for charge transfer that has a weaker distance dependence than does the nominally exponential distance dependence associated with the electronic coupling. The existence of these two parallel rate processes would produce an apparent shallower observed distance dependence. Because this experiment employs a freely diffusing redox couple, it is not possible to fully discount the possible influence of pinholes and defects on the measured distance dependence. However, these films are quite compact, as measured by the contact angle for water.17 The studies below suggest that pinholes are not likely to be significant for the system studied. The importance of the pinholes was investigated by observing the quenching of the bandgap emission of the InP by adsorbed species18 for the dodecanethiol monolayers. The dopant density of the InP in these studies is 1016 cm-3, rather than the 1018 cm-3 used in the photocurrent studies, because the time profile of the emission is more sensitive to the surface recombination velocity at lower dopant densities.19 Figure 3A shows the fluorescence decay data20 for a freshly etched InP electrode immersed in deionized water (top curve), an InP electrode with a dodecyl SAM overlayer immersed in deionized water (middle curve [dots]) and a freshly etched InP electrode immersed in a 0.04 M Fe(CN)63-/Fe(CN)64- solution. Clearly the SAM overlayer causes a faster decay of the fluorescence emission than the freshly etched surface, but it is still longer than that for the freshly etched electrode in the ferricyanide solution. The electrode is kept under open-circuit conditions which results in no net current flow. The solution with the redox couple shows a faster emission decay. The faster decay results from an increase in the nonradiative charge-carrier recombination rate at the surface.14,18 A study of three different ions in solution showed that the recombination rate was highest for Ag+, smaller for Fe(CN)63-/Fe(CN)64- and negligible for Ru(bpy)32+. This
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Figure 3. Fluorescence decay curves for the bandgap emission of n-InP (
