984
J . Phys. Chem. 1994, 98, 984-988
An Investigation of Photocurrent Generation by Squaraine Aggregates in Monolayer-Modified SnOz Electrodes Young-Soon Kim,fv* Kangning Liang,f Kock-Yee Law,*9+and David G. Whittenff NSF STC for Photoinduced Charge Transfer, Department of Chemistry, University of Rochester, Rochester, New York 14627, and Xerox Webster Research Center, 800 Phillips Road, 1 1 4-390, Webster, New York 14580 Received: September 20, 1993; I n Final Form: November 9, 1993"
The photocurrent generations from modified electrodes, consisting of monolayers of a surfactant squaraine DSSQ (4-(distearylamino)phenyl-4'-(dimethylamino)phenylsquaraine) deposited on SnO2 substrates by the Langmuir-Blodgett (LB) technique, have been studied, in a conventional photoelectrochemical cell. Absorption studies show that the A,, of the D S S Q S n 0 2 electrode is at 530 nm and is significantly blue-shifted from the monomeric absorption of DSSQ (Amax 633 nm), indicating that the squaraine chromophores form aggregates on the electrode. Under ambient conditions, a cathodic photocurrent is observed when the DSSQ-Sn02 electrode is illuminated by visible light. The action spectrum of the photocurrent generation is coincident with the absorption of the electrode, indicating that the squaraine aggregate in the LB film is responsible for the photocurrent. The observation suggests that electrons flow from the electrode through the LB film to the electrolyte solution, which is an aqueous solution of 1 M NaNO3. We have demonstrated that 0 2 is vital in the photocurrent generation process. While a 2-fold increase in photocurrent is obtained when the 1 M NaNO3 solution is saturated with 02, a sharp decrease (>90%) is observed when the solution is degassed with N2. The results suggest that the generation of cathodic photocurrent involves an electron transfer from the excited squaraine aggregates to 0 2 to form the superoxide anion radical with a subsequent electron transfer from the conduction band of the electrode to the holes residing in the squaraine aggregate. This model is supported by studies of the effects of insulating layer, applied bias voltage, and added electron donors and acceptors on the photocurrent generation. The implication of this work to the photogeneration mechanism of squaraine photoconductors in xerographic devices is discussed.
Introduction The squaraines are a class of organic photoconductors that exhibit efficient photogeneration in xerographic photoreceptors. In bilayer devices that consist of a charge generation layer and a charge (hole) transport layer, they are usually imbedded in a polymer matrix as a microcrystalline aggregate in the charge generation layer. Key events subsequent to photoexcitation of the aggregate are postulated to be (1) photogeneration of electronhole pairs within a dye aggregate, (2) electron and/or hole migration within a dye aggregate, and (3) electron transfer between the excited dye aggregate and an electron donor (or acceptor) across the interface to the charge transport layer. Basic studies of the molecular details of the photoinduced electrontransfer process are of particular value, not only for xerographic photoreceptors but also for many other man-made molecular electronic devices.2 The knowledge obtained can lead to a rational design and synthesis of novel molecular and supramolecular structures with optimal photoelectrical properties. Research efforts directed to gain fundamental understanding of these processes in organic photoconductors have been documented. However, most of the studies are focused on single crystals for reason of purity, structure, and orientation.3 Single crystals of most organic photoconductors, such as squaraines, are unfortunately very difficult to prepare. To circumvent the experimental dilemma, we have used squaraine aggregates assembled by the Langmuir-Blodgett (LB) technique as models. Three surfactant squaraines DSSQ, MSSQ, and TSSQ, which were designed to orient the squaraine chro-
mophore in three different orientations, when organized in monolayers and LB films were synthesized. Initial results showed that there are significant orientation effects on the aggregation of the squaraine chromophores in LB filmse4 Subsequent study has focused on DSSQ because it forms stable monolayers (both chemically and structurally) on water as well as when transferred to glass and SnOz substrates. Although the photophysical behavior of the squaraine aggregate in the LB film of DSSQ is not assessable by fluorescence or fluorescence quenching techniques owing to the lack of luminescence (or extremely low yield), we have been able to gain information about its photochemical properties using solution photoelectrochemical techniques. In this paper, we report an investigation of the photocurrent generation by DSSQ LB film-modified SnOz electrodes. The action spectrum of the DSSQ-SnOz electrode coincides with the absorption spectrum of the aggregate, indicating that the aggregate is responsible for the photocurrent generation. A mechanistic model for the photocurrent generation is proposed, and its technological implication is discussed. 0
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of Chemistry, University of Rochester. Permanentaddress: Departmentof Chemistry,Dongguk University, Seoul, 100-715, Korea. Xerox Webster Research Center. * To whom correspondence should be addressed. 0 Abstract published in Advance ACS Abstracts, December 15, 1993. 1 Department
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0 1994 American Chemical Society
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Photocurrent Generation by Squaraine Aggregates
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Experimental Section Materials. DSSQ (4-(distearylamino)phenyl-4’-(dimethy1amino)phenylsquaraine) was synthesized by condensing 1-@(dimethylamino)phenyl)-2-hydroxycyclobutene-3,4-dione with 2 equiv of N,N-dioctadecylaniline (from Pfaltz & Bauer) in refluxing 2-propanol using tributyl orthoformate as a dehydrating reagent.5 The product was isolated by filtration and purified by solvent recrystallization from a mixture of methylene chloride and methanol. Elemental analysis and spectroscopic data for the compound were reported earliere4 The spreading solvent was chloroform (HPLC, pentene stabilized) and was purchased from Fisher. Distilled water was in-house deionized water purified by passing through a Millipore water purification system. The electrolyte for the electrochemical experiment was N a N 0 3 and was obtained from Fisher. Hydroquinone (HQ, 99+%) was from Aldrich and was recrystallized from water before use. Ethylenediaminetetraacetic acid (EDTA, 99%) was from Aldrich, and triethanolamine (TEA) was from Eastman Chemicals; these materials were used as received. Methyl viologen dichloride (MV2+) was a purified sample generously provided by Dr. J. S. Facci (Xerox Webster Research Center). Preparation and Characterization of LB Films. Monolayers of DSSQ were obtained by spreading a chloroform solution of DSSQ (- 10-3 M) onto an aqueous subphase, which contained cadmium chloride (3 X 10-4 M) and sodium bicarbonate (5 X 104 M) in a KSV 5000 film balance at a subphase temperature of -35 OC. Typical transfer ratios on hydrophilic glass and Sn02 substrates were 1.0 i 0.1. All the DSSQSnOz assemblies used in the photoelectrochemical experiments consisted of a single layer of DSSQ deposited on precleaned SnOz substrates. The aggregation of the squaraine chromophore in LB films was studied by absorption spectroscopy using a Hewlett-Packard 8452A diode array spectrophotometer. The uncoated side of the substrate was used as a reference. Photoelectrochemical Experiments. The apparatus for photoelectrochemical studies was built in-house from an electrochemical analyzer, Model BAS 1OOB from Bioanalytical Systems, and a xenon arc lamp system (Model A1010) from Photon Technology International Inc. (PTI). The light output was put through a f/4 monochromator (Model 001, also from PTI), and the intensity at each wavelength was calibrated with a standardized silicon diode using an electrometer, Model 617 from Keithley Instruments. Photocurrents were recorded in a time-base mode, and the shutter was controlled manually. The photoelectrochemical cell was made of Pyrex and consisted of a DSSQSnOz assembly as a working electrode, a polished platinum wire as a counter electrode, and a Ag/AgCl reference electrode. Unless specified, the supporting electrolyte was an aqueous solution of NaN03 (1 M). Results Fabrication and Characterizationof DSSQSnOz Electrodes. DSSQ forms stable monolayers on water. The limiting molecular area is -52 Az/molecule and is about twice that expect9.d from a squarainechromophore that isvertically oriented on water (-25 Az/molecule). The compression isotherm suggests that the monolayer of DSSQ is controlled by the two hydrocarbon chains rather than the squaraine chromophores, since they typically occupy -25 i 5 A2 per linear hydrocarbon chain.&9 At a subphase temperature of -20 OC,the monolayer is quite rigid. This rigidity has caused collapse of the monolayer and erratic film transfers. In this work, we increase the temperature of the subphase to 35 OC. The DSSQ monolayer becomes more fluid, and film transfers have been facilitated. The transfer ratio to hydrophilic substrates, glass, and SnO2 electrode is about unity (1.0 f 0.1). There is a tendency for obtaining lower transfer ratios on hydrophobic substrates. To minimize film variability,
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0.08
8 9)
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0.06-
- 0.90
- 0.60 - 0.30
400
600
500
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Wavelength (nm)
Figure 1. Absorption spectra of DSSQ
(e-)
on Sn02 electrode and (-)
in dilute CHCla solution.
800
3
1
5 600 f
i
E 400 -
P
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0
L
400
500
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Figure 2. Spectral response curves of a DSSQ-SnOz electrode ( E = bias voltage in volts, against Ag/AgCl).
all DSSQ-Sn02 electrodes were prepared by layering a single monolayer of DSSQ on precleaned, hydrophilic SnOzsubstrates. The aggregation of the squaraine chromophores in the DSSQSnOz electrode was studied by absorption spectroscopy (Figure 1). The ,A, of the LB film is a t -530 nm and is significantly blue-shifted from the monomeric absorption ,A, of DSSQ in chloroform (633 nm). Since DSSQ could be quantitatively recovered from the substrate (by absorption study and chromatographic analysis), our data suggest that squaraine chromophores form physical aggrregates in the LB film on the S n 0 2 substrate. A similar aggregational behavior was also observed for DSSQ on glass; we previously concluded that the aggregate is the result of an intermolecular interaction between the C-O dipoles in the four-membered ring of squaraine via a staircase arrangement based on the orientation of the chromophore and the solid-state absorption spectrum of microcrystals of bis(4methoxyphenyl)squaraine? Photocurrent Generation from DssQSa Electrodes. Spectral Response. A cathodic photocurrent is observed when the DSSQSnOz electrode is illuminated by visible light (-2 X 10’6 photons/(cm2 s), 1 cm2). Figure 2 shows the action spectra of the cathodic photocurrents at two different bias voltages.1° Both spectral responses coincide with the absorption spectrum in Figure 1, suggesting that the aggregate of DSSQ in the LB film is responsible for the photocurrent generation. Assuming that each DSSQ molecule occupies -52 A2 on the electrode, there will be -2 X 1014 molecules/cmz. From the optical density of the film and the light flux, we can estimate that the “turnovern ratio is 10 per second. This estimation coupled with the fact that the DSSQ-SnOz electrode shows no appreciable bleaching after 1-2 h of irradiation suggests that the observed photocurrent
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The Journal of Physical Chemistry, Vol. 98, No. 3, 1994
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Figure 3.
electrode.
a-
Effect of 02 on the cathodic photocurrent of a DSSQ-SnOz
600
_...._._.._ N2 degassed Inside Barrler Layer Thlckness (A)
Effect of inserted stearic acid layers on the photocurrent of DSSQSn02 electrodes.
Figure 5.
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Bias Voltage vs. AglAgCl (volts)
Effect of bias voltage on the photocurrent generation of a DSSQSnOZ electrode. Figure 4.
is a result of spectral sensitization of the Sn02 electrode by the dye aggregate in the LB film. To test the reproducibility of the photocurrent data, four DSSQ-Sn02 electrodes were fabricated and tested in parallel. The results show that there is about a factor of 2.5 variation in the data. For example, the cathodic photocurrents vary from 120 to 250 nA and 250 to 650 nA at 530 nm at bias voltage of E = 0 and E = -0.1 V vs Ag/AgCl, respectively. The attainment of -650 nA indicates that a quantum yield of -0.3% is obtained from the working electrode which consists of a single layer of DSSQ on the Sn02 substrate. Effect of Oxygen. The observation of cathodic photocurrents indicates that electrons flow from the electrode through the LB film to the electrolyte solution, which is an aqueous solution of 1 M NaN03. We hypothesize that the dissolved 0 2 (-2.7 X 1 W M)” in the aqueous solution is the electron acceptor. This is verified by studying the photocurrent in the presence and absence of 0 2 . The experimental data are plotted in Figure 3. The cathodic photocurrent increases (X2) as the electrolyte solution becomes saturated with 0 2 ([O,] 1.2 X 10-3 M). It decreases sharply, by L90%, when 0 2 is removed by N2 degassing. We attribute the small residual photocurrent to trace amount of residual 0 2 in the electrochemical cell, which is hard to remove totally under our experimental conditions. The cathodic photocurrent can be restored, but very slowly, when the degassed electrolyte solution is readmitted to ambient. Effect of Bias Voltage and Fatty Acid Barrier Layers. To probe the electron-transfer process between the SnO2 electrode and the LB film, the effect of bias voltage was investigated. Results (Figure 4) show that the cathodic photocurrent increases as the negative bias of the electrode increases, and vice versa, under
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both ambient and N2 degassed conditions. The most significant observed is the reversal of the photocurrent at bias voltage >+0.2 V vs Ag/AgCl. The mechanistic implication for the anodic photocurrent will be discussed below. In addition, we also attempted to examine the electron-transfer process by inserting insulating saturated fatty acid layers between the SnOzelectrode and the LB film. The experiments were carried out by transferring 2,4,6, and 8 layers of stearic acid to the SnO2 electrodes, followed by a monolayer of DSSQ on the top of the fatty acid layers. The transfer ratios of all layers are about unity. The photocurrent generations of these electrodes were measured in duplicate experiments, and the data are plotted in Figure 5. The cathodic photocurrent is shown to decrease as the number of insulating layers between the LB films and the electrode increases. Measurable photocurrents are not obtained when eight layers of stearic acid are inserted between the electrode and the LB film. Effect of Electron Donors and Acceptors. Several watersoluble electron donors and acceptors were added to the electrolyte solution (1 M NaNOs) to study their effects on the photocurrent generation of the DSSQ-Sn02 electrode. In each experiment, the cathodic photocurrent observed a t 530 nm under ambient condition was used as a control for a given electrode. The electrode was then placed into different photoelectrochemical cells, which consisted of an aqueous solution of 1 M N a N 0 3 and an added donor or acceptor. The photocurrent was then determined under ambient as well as N2 degassed conditions. The effect of these additives on the photocurrent usually increases as the concentration of the additive increases and levels off at a higher concentration. The data (Table 1) show that addition of MV2+ to the electrolyte solution provides increases which are relatively small as compared to saturation of the solution with 0 2 . When this particular electrolyte solution is degassed with N2, the cathodic photocurrent is suppressed, but it is significantly higher than that of the control. The observation of MV2+-sensitizedphotocurrent complements the conclusion that 0 2 is an electron acceptor for the cathodic photocurrent generation. This sensitized photocurrent is, however, lower than that obtained under ambient conditions, and weattributeit totheinability of MV2+toefficiently diffuse through the assembly to the squaraines aggregate.l3 An opposite effect is observed for electron donors. They generally attenuate the cathodic photocurrent. In the case of
Photocurrent Generation by Squaraine Aggregates
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TABLE 1: Effect of Donors and Acceptors on the Photocurrent Generations of DSSO-SnOl Electrodes' photocurrent* (nA) expt donor redox potential wncn no. or acceptor (V vs Ag/AgCl) (mM) ambients 1 none 292 MVZ+ -0.23 (red)d 1.9 432 2
none
3
TEA none
0.17 (oxd)'
22
426 226 295 91 340 -110
N2 degassed
32 103 33 27 30 26 32 -420 (anodic)
EDTA 0.21 (oxd)' 6.1 none HQ -0.13 (oxd)e 3.9 (anodic) The oxidation potential of the LB film on Sn02 is 1 V vs Ag/AgCl in 1 M NaNO3 solution. Unlessspecified,the photocurrents arecathodic and there is no biasvoltageon the working electrode. Theconcentration of 0 2 is estimated to be -2.7 X 10-4 M (ref 11). Reference 12. e This work, determined in 1 M NaNO3 solution. 4
(I
a. Cathodic Photocurrent
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b. Anodic Photocurrent
Figure 6. Schematics of photocurrent generations by DSSQ-SnOz
electrodes: (a) cathodicphotocurrentunder ambientcondition;(b) anodic photocurrent in degassed 1 M NaNO3 in the presence of HQ.
HQ, we not only observe decreases in the cathodic photocurrent but also find that it redirects the flow of electrons and makes the photocurrent anodic, especially in the absence of 02.
Discussion Photocurrent Generation Mechanism. The observation of cathodic photocurrents upon illumination of the squaraine aggregate in the DSSQ-Sn02 electrode indicates that electrons flow from the electrode through the LB film to the electrolyte. The results in Figure 3 further show that molecular 0 2 in the electrolyte is needed as the electron acceptor. The generation of cathodic photocurrent probably involves an electron transfer from the excited squaraine aggregate to 0 2 to form the superoxide anion radical with a subsequent electron transfer from the conduction band of the SnO2 electrode to the holes residing in the squaraine aggregate. A schematic for the cathodic photocurrent generation is shown in Figure 6a, based on the oxidation potential of the DSSQ LB film (- 1 .O V vs Ag/AgCl), the optical absorption of the aggregate, and the energy level of the S n 0 2 electrode.14 Complementary evidence for 0 2 serving as an electron acceptor comes from the MVZ+ experiment, which shows that addition of MVZ+ in the electrolyte enhances the cathodic photocurrent in the presence and absence of 0 2 . The proposed mechanism in this work is similar to those given by Haraguchi et al.15 and Hada and c o - w o r k e r ~ , ~who ~ J ~showed that 0 2 is also responsible for the cathodic photocurrents from a number of J-aggregates of cyanine dyes on ZnO and Sn02 electrodes. The proposed model is supported by a study of the bias voltage on thecathodic photocurrent (Figure 4). For instance, we observe an enhanced cathodic photocurrent when the D S S Q S n 0 2 electrode is negatively biased.18 It is important to point out that, when a positive bias is applied on the electrode, we not only observe a decrease in cathodic photocurrent generation but also observe
a switch in the direction of the electrons flow a t E > +0.2 V vs Ag/AgCl. The photocurrent becomes anodic. The photocurrent, however, is very weak (2.4 nA); we suggest that it can be attributed to an electron transfer from the excited squaraine aggregate to the conduction band of the Sn02 electrode. Injections of photogenerated electrons from squaraine to ZnO and Ti02 semiconductor have been reported.19-20 The absence of such process in this work is not due to the unfavorable energetic.l4 We suspect that the combination of the facile electron transfer from the excited squaraine aggregate to 0 2 and the absence of any hole scavengers in the electrolyte solution is contributing to the observation. The latter is supported by the HQ experiment discussed below. The subject of electric conduction through monolayers of fatty acids was studied by Mann and KuhnZ1and others.zz-2' These authors measured the conductance up to 6-8 layers of fatty acid and showed that the conductivity of the LB films of fatty acids decreases as the number of layers increases. In the inside barrier experiment in this work, we observe a gradual decrease in cathodic photocurrent as the number of stearic acid layers increases (Figure 5). The decrease is attributable to the decrease in electric conduction across the inside barrier which leads to enhanced charge recombination. The result is consistent with the mechanistic model given in Figure 6a. The data obtained from experiments involving added donors also shed some light on the mechanism of photocurrent generation. For instance, for relatively weak electron donors such as TEA and EDTA, the photogenerated holes in the squaraine aggregate are scavenged, and thus the cathodic photocurrent is short circuited. In the case of strong electron donors such as HQ, the quenching of the excited squaraine aggregate becomes energetically favorable. As a result of the electron-transfer quenching, an anion radical of the squaraine aggregate is formed. The generated anion radical can transfer an electron to 02and provides a competitive process to the scheme described in Figure 6a. In the absence of 02, the anion radical can inject the electron into the conduction band of the SnO2 substrate, resulting in an anodic photocurrent (Figure 6b). Technological Implication. The observed dominance of electron transfer from the excited squaraine aggregate on the SnOz electrode to 0 2 in the photocurrent generation process suggests that a similar photogeneration process may occur in xerographic photoreceptor devices that incorporate squaraines. For example, instead of photogenerating electron-hole pairs upon excitation of the squaraine aggregate in xerographic device, the excited squaraine aggregate may transfer an electron to 02 to form the superoxide anion radical. An interfacial electron transfer from the neighboring charge transport layer to the holes in thesquaraine aggregate coupled with charge injection from the superoxide anion radical at the conductive substrate completes the xerographic photodischarge process.28 In other words, 0 2 may play an active role in the photodischarge of squaraine xerographic devices. The active role of 0 2 in the photogeneration of organic photoconductors is not unprecedented. Ahuja and HauffeZ9demonstrated that 0 2 is responsible for the photogeneration of charges in x-HzPc (xform metal-free phthalocyanine) in alkane solvents. Mizuguchi30v3' reported that the adsorbed 0 2 on evaporated thin films of O-CuPc is directly related to the photocurrent generation in thin-film photoelectrical cells of 8-CuPc. A similar conclusion was also reached recently in the case of a-CuPc by Yamamoto and co-workers32 and in the case of GaClPc by Pankow et al.33 In addition to 02,other electron acceptors have also been found to play a similar role and result in improved photoconductivity in various phthal~yaninedevices.~~~~~ While the photogeneration mechanism of phthalocyanines might have been studied in some detail, the present work represents the first piece of evidence about the involvement of 0 2 in the photogeneration mechanism of squaraine photoconductors.
988 The Journal of Physical Chemistry, Vol. 98, No. 3, 1994
Concluding Remarks Our results suggest that the major mechanism of cathodic photocurrent generation from the DSSQ-Sn02 electrode is by transfer of an electron from the excited squaraine aggregate to 02, followed by an electron transfer from the conduction band of the Sn02 electrode to the holes in the aggregate. The mechanism is supported by studies of the effects of bias voltage, added barrier layers, and added donors and acceptors on the photocurrent generation. We have now initiated transient absorption experiments on the D S S Q S n 0 2 electrodes and hope that rate constants of charge generation, charge transfer, and charge recombination can be determined. In this work, we have obtained a quantum yield of -0.3% from a single layer of DSSQ on the SnO2 electrode. While this efficiency may not be high, we are encouraged by the finding. Previous photoconductivity measurements on a microcrystalline squaraine, which forms the 'blue-shifted" aggregate, suggested that the photoconductivity of the "blue-shifted" aggregateis 100 times less than that of the "red-shift" aggregate.36 We are currently directing our effort to synthesize such a molecular assembly. A successful effort in producing the red-shifted aggregate is expected to not only improve the quantum efficiency of the LB film-modified electrode but also provide insightful information on the molecular architecture that is required for efficient photogeneration. Finally, we have also shown that electron acceptors sensitize the cathodic photocurrent and electron donors quench it. We are now extending our investigation by incorporating surfactant donors and acceptors in the LB film in an intra- and interlayer fashion. Issues related to the effects of distance and orientation on electron transfer will be studied using the photoelectrochemical technique described herein.
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Acknowledgment. The authors thank Dr. John Facci for consultation of the electrochemistry and Dr. Meisheng Zhou for preliminary experiments. This work is supported by the National Science Foundation in the form of a grant from the Center for Photoinduced Charge Transfer (CHE-9120001). References and Notes (1) Law, K. Y. Chem. Rev. 1993,93,449 and references therein. (2) Hong, F. T. Molecular Electronics, Biosensors and Biocomputers; Plenum Press: New York, 1989.
Kim et al. (3) Gutman, F.;Lyon, L. E. OrganicSemiconductors;Krieger: Malabar, FL, 1981;p 376. (4) Law, K. Y.; Chen, C. C. J . Phys. Chem. 1989,93,2533. (5) Law, K. Y.; Bailey, F. C. Can. J . Chem. 1993,71,494. (6) Nutting, G. C.; Harkins, W. D. J. Am. Chem. Soc. 1939.61, 1180. (7) Harkins, W. D.;Copeland, L. E. J. Chem. Phys. 1942,10,272. ( 8 ) Nutting, G. C.; Harkins, W. D.J . Am. Chem. SOC.1939,61,1182. (9) Gaines, G. L. Insoluble Monolayers at Liquid Gas Interfaces; Interscience: New York, 1966;p 249. (10)The action spectra in Figure 2 are not corrected for any light flux variations because the intensites from 450 to 700 nm are found to be constant within *lo%. (1 1) Morov, S.L. Handbook of Photochemistry; Marcel Dekker: New York, 1973;p 89. (12) Gritzel, M.; Moser, J. Proc. Acad. Sci. 1983,80,3129. (13) The 02 effect on the cathodic photocurrent of a modified SnO2 electrode consisting of 1 layer of DSSQ and 2 layers of a surfactant MV2+ is identical to that shown in Figure 3. The observation supports that close proximity between the excited squaraine aggregate and the electron acceptor (02or MV2+) is essential for efficient charge generation. Liang, K.; Law, K. Y.; Whitten, D. G. Unpublished results. (14) Gritzel, M. In Phoroinduced Electron Transfer,Part D Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, New York, 1988;p 410. (15)Haraguchi, A.; Yonezawa, Y.; Hanawa, R. Photochem. Photobiol. 1990,52, 307. (16) Hada, H.;Yonezawa, Y.; Synrh. Met. 1987,18,791. (17) Hada, H.; Yonezawa, Y.; Inaba, H. Ber. Bunsen-Ges. Phys. Chem. 1981,85,425. (18) Bard, A. J.; Faulkner, L. R. EIectrochemicalMethods. Fundamentals and Applications; John Wiley & Son: New York, 1980. (1 9) Kamat, P. V.; Das, S.; Thomas, K. G.; George, M. V. J . Phys. Chem. 1992,96, 195. (20) Kamat, P. V.; Das, S.; Thomas, K. G.; George, M. V. Chem. Phys. Lett. 1991,178,75. (21) Kuhn, H.; Mann, B. J. Appl. Phys. 1971,42,4398. (22) Sugi, M.; Iizima, S. Appl. Phys. Lett. 1979,34,290. (23) Sugi, M.; Fukui, T.; Iizima, S. Phys. Rev. B 1978,18,725. (24) Polymeropulos, E. E.;Sagiv, J. J . Chem. Phys. 1978,69, 1836. (25) Polymeropulos, E. E. J . Appl. Phys. 1977,48, 2404. (26) Iizima, S.; Sugi, M. Appl. Phys. Leu. 1976,28, 548. (27) Sugi, M.; Fukui, T.; Iizima, S. Appl. Phys. Lett. 1975,27, 559. (28) Law, K. Y.; Kim, Y. S.; Whitten, D. G. Proceedings, ISdtTs Ninth International Congress on Advances in Non-Impact Printing Technologies; The Society for Imaging Science and Technology, 1993;p 600. (29)Ahuja, R.C.; Hauffe, K. Ber. Bunsen-Ges. Phys. Chem. 1980,84, 68. (30) Mizuguchi, J. Jpn. J . Appl. Phys. 1981,20, 1855, 2065,2073. (31) Mizuguchi, J. Jpn. J . Appl. Phys. 1982,21, 822. (32) Yamamoto, K.;Egusa, S.; Sugiuchi, M.;Miura, A. Solid Stare Commun. 1993,85,5. (33) Pankow, J. W.; Arbour, C.; Dodelet, J. P.; Collins, G. E.; Armstrong, N. R. J . Phys. Chem. 1993,97,8485. (34) Loutfy, R. 0.; Menzel, E. R. J . Am. Chem. SOC.1980,102,4967. (35) Nakatani, K.; Hanna, J.; Kokado, H. J . Electrophotogr. 1985,24, 2. (36) Law, K. Y. J. Phys. Chem. 1988,92,4226.
