11745
J. Phys. Chem. 1994, 98, 11745-11750
Mechanism of Photosensitized Charge Injection from Organic Dyes Incorporated in Langmuir-Blodgett Films into SnOz Dimitri Noukakis, Mark Van der Auweraer,* and Frans C. De Schryver Luboratory for Molecular Dynamics, Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium Received: April 22, 1994; In Final Form: August 31, 1994@
The mechanism of spectral sensitization of SnOz electrodes by octadecyl-substituted cyanine dyes has been investigated. The Langmuir-Blodgett technique was used to manipulate the concentration of the adsorbed dyes and to vary the distance over which the sensitization occurs. When bis[ 1-octadecylbenzthiazo1-2-yl]monomethinecyanine perchlorate THIAM18 was excited at 435 nm, the distance dependence of the fluorescence quenching and of the anodic photocurrent could not be explained by the conventional mechanism of direct charge injection. In addition, a large cathodic photocurrent was obtained at electrode potentials cathodic of 0.15 f 0.02 V versus the SCE. When THIAM18 was replaced by the bis[l-octadecylbenzoxazol-2-yl]trimethinecyanine perchlorate OXAl8, the anodic photocurrent could still be obtained but the cathodic photocurrent was reduced by at least 1 order of magnitude. A detailed study of these systems showed that, under favorable circumstances, energy transfer can be the main path to photosensitization.
1. Introduction Spectral sensitization of semiconductors has been a wide and very active field of research the last three decades, driven mainly by its importance in photographic processes and solar energy conversion. In an early attempt' to elucidate the mechanism of sensitization of silver halides, the energy transfer mechanism was favored, while for some semiconductors2both energy and electron transfer were assumed to be possible. The mechanism of spectral sensitization is still not completely understood, although nowadays charge transfer is believed to be the predominant process3 In Scheme 1, the essential processes taking place during the spectral sensitization of a semiconductor by photoinduced electron transfer are depicted. It is striking that although kh, is often very high, typically on the order of 10l2 s-l, the overall photosensitization yield, remains low for several sensitizerlsemiconductorsystems: It is believed that deactivation through different channels, like electron-hole recombination at the surface states, accounts for such low yields. In fact, in systems where the surface properties of the semiconductor are o p t i m i ~ e d ,values ~ . ~ of close to 1 have been obtained. In the particular case of SnOa, @sens (defined as the number of electrons injected per number of incident photons) is extremely low and although it is a highly doped semiconductor (nD % lozo), there is no straightforward explanation for such a low value, as it persists at large anodic potentials and at high concentration of regenerator or supersensitizer. Spectral sensitization of large-band-gap semiconductors has already been extensively studied.6 Arden and F r ~ m h e r z have ~a~~ investigated the photosensitized injection of electrons into indium oxide by carbocyanines incorporated in fatty acid layers, studying the electrochemical properties of the electrode as well as the factors governing the generation of sensitized photocurrent. Using two different dyes as energy donor (D) and energy acceptor (A), they have performed a series of cosensitization and desensitization experiments, establishing a consecutive energy (from dye D to dye A) electron (from dye A to the @
Abstract published in Advance ACS Abstracts, October 15, 1994.
0022-365419412098- 11745$04.50/0
semiconductor) transfer mechanism. They have also reported that a small amount of sensitized photocurrent (SEC) persists even when the dye and the semiconductor are separated by two fatty acid layers, which they have assigned to breaks of the lamellar structure of these layers, leading to a close contact of the dye molecules with the semiconductor.
2. Choice of the System and Experimental Methods The central objective of this work was to obtain a better understanding of the reasons accounting for the low photosensitization yields obtained with SnOz. The dyes which led to the more interesting results were the bis[ l-octadecylbenzthiazol2-yl]monomethinecyanine perchlorate THIAMI 8 and the bis[ 1-octadecylbenzoxazol-2-yl] trimethinecyanine perchlorate
OXA18.
I
c1SH37
THlAMl8
OXAl8
In order to control both the concentration and the distance between the dye and the SnOz, we have used the LangmuirBlodgett (LB) technique for multilayer deposition. As shown in Figure 1, the dye was mixed with arachidic acid ( A M ) , in order to form stable monolayers and to control its concentration within each monolayer. The sensitization distance has been 0 1994 American Chemical Society
Noukakis et al.
11746 J. Phys. Chem., Vol. 98, No. 45, 1994
b Figure 1. Investigation of the distance dependence of the photosen-
sitized electron injection, using LB-filmsas spacers between the dye and the semiconductor.
SCHEME 1: General Model Describing the Main Features of Spectral Sensitization of a Semiconductor
Ecs
EF
I_
D*ID+
D :Sensitizingdye R : Regenerator
5-0
SS :Surface States
0
R+
varied by insertion of ARA layers between the SnO2 and the dye monolayer. Two different techniques have been used to investigate the efficiency of spectral sensitization. Fluorescence emission and excitation spectroscopy were used to determine the spectral features of the excited species involved in the process of photosensitization as well as in the quenching of the fluorescence of the dye due to energy or electron transfer to the semiconductor. These experiments were performed with a spectrofluorimeter SPEX-fluorolog, using front-face ill~mination.~ Another way to obtain information on the photosensitization processes is to record the sensitized photocurrent (see Figure 2). In this experiment, a three-electrode setup inside an electrolyte solution (NaC104 M) is converting the photoinjected electrons into a measurable current. The modulation frequency of the light is 10 Hz, and the sweep rate of the applied potential is 5 mV/s. The solution is buffered (pH = 3) and purged with N2 for 15 min, and in order to avoid rapid consumption of the dye, hydroquinone (-1 x M) is used as regenerator. A more detailed description of the experimental setup is given in ref 8b. Transient sensitized photocurrents were obtained using as excitation source a 250 W Xe lamp powered by a Muller SVX 1450 power supply and pulsed by a Muller MSP 05 pulse unit. The transients were recorded and averaged using a Gould 4072 digital oscilloscope.
-
3. Results and Data Analysis
3.1. Mixed Layers of THIAM18 and Arachidic Acid. The striking feature in Figure 3 is not only the presence of a considerable amount of anodic and cathodic photocurrent even at distances where any direct electron injection is impossible but also the fact that the ratio of anodic over cathodic photocurrent goes from 5-7 for the [dye/SnO2] case to -1.6 for the case of [dye110 ARA/Sn02]. The dark current (not shown here) remains always small, compared to the one obtained with uncovered Sn02,8bindicating that there is a good coverage of the SnO2 surface by the LB-films. Figure 4 points out some important features of this system. First of all, the difference between the cathodic and anodic action
spectra is too large to be explained by the normal electrochromic effect. In fact, the main contribution to the anodic photocurrent spectrum comes from the dimer band (centered at -405 nm), while in the cathodic one the contribution from the monomer band (centered at 435 nm) becomes predominant. Finally, the action spectrum obtained with a cathodically biased electrode matches well the fluorescence excitation spectrum except for the narrow band at 448 nm. This band is due to the absorption of J-aggregates, and for the system [THIAM18/Sn02] it has already been noticed that formation of these aggregates inhibits the photosensitization process? suggesting that they do not contribute to the action spectrum. In Figure 5 , the fluorescence emission and excitation spectra at two different sensitization distances are shown. A slight change in the relative contributions coming from the monomers, dimers, and J-aggregates is observed and is attributed to differences of the surface conditions. In fact, the sensitizing dye will experience different environments when it is in contact with the semiconductor or when it faces the polar heads of the arachidic acid monolayer. Fluorescence from the [THIAM 181 Sn021 system comes perhaps mainly from pathological sites where there is no contact between the monolayer and SnO2. Nevertheless, the number of A M layers has no significant influence on the shape of the fluorescence spectra. The relative intensities of fluorescence from the different excited species are not necessarily representative of the relative concentration of the ground-state species, because the fluorescence spectra can be biased either by energy transfer between monomers and aggregates or by the different absorption coefficients and emission yields of these species. As mentioned previously, it is not possible to explain the photosensitization observed over a range of ~ 1 4 A0 by solely invoking the direct electron injection mechanism from the dye to the semiconductor. The ensemble of results from the fluorescence and sensitized photocurrent measurements has been treated according to Forster's energy transfer model for fluorescence quenching.1° Equation 1 is a modification by H. Kuhn" of the original Forster equation.
In this model where the quencher is homogeneously distributed in a plane, energy transfer is treated as an electric dipole interaction and the rate of energy transfer from the sensitizer to the acceptor is proportional to d-4, d being the distance between the semiconductor and the plane where the sensitizing dye is located. Z and I, are the fluorescence intensities at a -18SnOz] distance d and at an infinite one. Figures 6 and 7 show the sensitized photocurrent and fluorescence quenching data of several experiments together with the fit resulting from the Forster's energy transfer model. In Figure 6, Za is the sensitized anodic photocurrent, Za.0 is the maximum photocurrent due to the energy transfer process, and do is the distance where Za.0 has dropped to half its initial value. Obviously, we have selected as la.0 the value from the system [Sn02/2 A M H I A M 1 8 1 (d = 54 8)and not [SnO2/ THIAM181, since in the latter system almost all of the anodic photocurrent is due to direct electron injection. It is also very reasonable to assume that, at d = 54 A, any direct electron injection from the dye to the semiconductor is very unlikely to happen. In fact this mechanism should become unimportant at distances larger than 30 A.12 In Figure 7, Z is the fluorescence intensity, l o is the fluorescence intensity at zero [Sn02-THIAM 181 distance, and Zm is the one at infinite [Sn02-THIAM 181 distance. The fact that one has to invoke lo (ideally, no fluorescence should be
Charge Injection from Organic Dyes into SnO2
DIGITIZER / COMPUTER
J. Phys. Chem., Vol. 98, No. 45, 1994 11747
-
DIVIDER
LOCK IN AMPLIFIER
1
POTEMIOSTAT
-
Xe Lamp 1500 W 1 1
-----_ _--CHOPPER
BEAM SPUTTER FOCUSING LENS Working electrode
Figure 2. Experimental setup for the investigation of the sensitized photocurrent. Broken lines depict the light’s path, and solid lines the electrical signals.
-2
I200
,
I
I
I”
I
1 ,,/ --
*
cathodic
D
anodic
0.8
0.6
-
of the photocurrent 0.4 -
6
v)
200
0.2 -0.3
-0.2 -0.1
0
0.1
0.3
0.2
0.4
0.5
0.6
0.7
Applied Electrode Potential [ V I
350
Figure 3. Sensitized photocurrent versus applied potential for the system THIAM18/AFL4 1 5 at two different separation distances: 0 8, for [dyelSnOz] and 270 8, for [dyellO ARA/Sn02]. The excitation wavelength was set to 435 nm, and the solutions were purged with NZ for at least 15 min. The phase difference of the photocurrentsat anodic and cathodic polarization is A@ 180.
I
,r,
I
cathodic SEC a1 -100 mV
-anodic SEC a t c 400 mV
400
450
500
550
5.0
I
z
126 f 7 0.9858
-
100
450
500
550
Figure 4. Action spectra (sensitized photocurrent against excitation wavelength) at two applied potentials. The spectra have been corrected for the spectrum of the excitation source and the response of the monochromator; the photocurrent due to the absorption of the SnO2 itself has been subtracted. The anodic photocurrent is several times higher than the cathodic one, but their relative intensities have been modified for comparison. left in this case) is due to the partial contact between the dye and the semiconductor. In fact, as it has already been found in a previous study,la the surface of SnO2 is not flat but covered by “valleys” and deep “craters” several hundred angstroms wide. The LB-films can easily bridge these structures, but a certain
300
200 Distance
Wavelength [nm]
= 1 / [l+(d/d ,)‘I
I, /
I
0 400
700
Figure 5. Fluorescence emission and excitation spectra for the systems [THIAM18/Sn02] and [THIAM18/8 ARA/Sn02] with front-face illumination and 02-free conditions.
0.5
350
650
600
Wavelengths [nm]
400
[A]
Figure 6. Sensitized anodic photocurrent versus the Sn02-THIAM18 distance. The experimental points are averages of many experiments, and they are corrected for a residual photocurrent of 10 nA. amount of dye is not in direct contact with the semiconductor surface,8b hence contributing to the overall fluorescence intensity. Mott-Schottky plots obtained for uncovered SnO2 and for SnO2 covered by LB-monolayerssa c o n f i i the above hypothesis. 3.2. Mixed Layers of OXA18 and Arachidic Acid. Photosensitization experiments using OXAl8 (which absorbs at longer wavelengths than THIAMl8) led to some interesting results, allowing a more refined interpretation of the results obtained with THIAM18.
11748 J. Phys. Chem., Vol. 98, No. 45, 1994
Noukakis et al.
I2W
(I-IJ / (I--$)
l/[l+(d Jd)‘]
0.2
0.9888 -0.3
0.0 0
100
200 Distance
300
-02
400
[A]
Figure 7. Fluorescence intensity versus the SnO2-THIAM18 distance. The experimental points are averages from different series of experiments in which the d = 0 system has always been measured.
-0.1
0
0.1
0.2
0.3
0.4
0.5
06
0.7
Applied Electrode Potential [ V I
9. Sensitized photocurrent versus applied potential for the system [THIAM18/Sn02]and [OXA18/SnOz]. The photocurrents were corrected for the wavelength dependence of the intensity of the excitation light source.
4. Discussion OXA18- SnO, OXAl a
I ~
- 2 AM - SnO,
I200
03
-02
-01
0
01
02
03
04
05
06
07
Applied Electrode Potential [ V I
Figure 8. Sensitized photocurrent versus applied potential for the system OXA18/ARA 1:5 at three different separation distances: 0 8, for [dye/SnO~],54 8, for [dye/2 W S n 0 2 1 , and 108 8, for [dye/4 ARA/Sn02]. The excitation wavelength was set to 490 nm, and the solutions were purged with N2 for at least 15 min. As can be seen from Figure 8, the anodic photocurrent decreases very quickly with the number of ARA layers, the ratio Za(0)/Za(2)being w15 and Ia(O)/Za(4) 50. On the other hand, the cathodic photocurrent stays always very small compared to the anodic one. In Figure 9, the SEC for the two dyes in contact with the semiconductor has been plotted; the ratio ZJZc goes from w5-6 ;or THIAM18 to w30 for OXA18. These results fully support the idea that energy transfer accounts for the long-range photosensitization observed with THIAM18. In such a case, the energy of the lower excited singlet state should determine whether a dye will transfer its energy of the semiconductor. The different behavior of the two dyes can thus be explained by the fact that the excited singlet state of OXA18 lies lower in energy than the one of THIAM18. It can not be excluded that some of the SEC observed in the systems where the dye is separated from the semiconductor by two and four layers of arachidic acid is due to imperfections of the spacer layers. In this case the SEC due to energy transfer will be even lower than those presented in Figure 8. It has to be noticed that the different behavior of the two dyes can not be attributed to their difference in excited state oxidation potentials (E’DC,~). In fact, the values obtained from the ground state oxidation potentials (EoD+ID)l3 and the zero phonon energies are -1.35 and -1.43 V (versus the SCE) for THIAM18 and OXA18, respectively. Therefore, on the basis of their redox properties, OXA18 should be a slightly better “long-range” sensitizer than THIAM18, which, as it has been shown, is not the case.
The sensitized photocurrent results together with the fluorescence quenching experiments for the two dyes strongly suggest that, in our system, energy transfer is the mechanism responsible for the “long-range” sensitization of anodic and cathodic photocurrents, while electron transfer accounts mainly for the generation of anodic photocurrent when the dye is in contact with the semiconductor. Similar results have been reported for spectral sensitization of AgBr evaporated layers,l4 and they have also been explained by the Forster’s energy transfer mechanism. However, if the sensitization of anodic photocurrents can still be understood in terms of a model using a large number of delocalized surface states, it is very difficult to understand the generation of the cathodic photocurrent. In order to explain both processes in a single model, one might consider that for the surface of SnO2 the conventional description of the semiconductor band-bending is not sufficient any more. As shown in Scheme 2, a model considering also the existence of noncommunicating localized states on the surface could be more appropriate, implying that electrons are trapped in these states. In the case of anodic photosensitization, if the dye is in contact with the semiconductor, this trapping will slow down significantly the electron injection rate kh,, since these localized states would only be weakly coupled to the bulk of the semiconductor. This would account for the low QSens observed with SnO2 spectral sensitization, since a low kinjSa will ‘allow different energy wasting recombination mechanisms to become dominant. In the case of long-range photosensitization, the excited dye will transfer its energy to localized surface states, where an electron-hole pair will be generated by exciting an electron from an occupied surface state. Provided that the electron can live long enough in these localized states, injection to the bulk of the semiconductor can take place. Generation of the cathodic photocurrent is more complex to explain. In an early study, Kuhn et al.15 studied the conductivity of LB-films and found that it rapidly decreases as the number of insulating fatty acid layers increases. In another study, Spider et al.16 reported generation of reduction photocurrents when Ti02 was sensitized by organic dyes solubilized in the electrolyte solution. This cathodic SEC was attributed to reactions involving triplet states of the dye molecules in solution and oxygen. More recently, in a photosensitization study of Sn02 covered by a squaraines layer,” a mechanism including reduction of 0 2 by the excited dye followed by electron transfer from the conduction band of the SnO2 to holes in the dye layer has been proposed. In our experiments, the electrolyte solutions have
Charge Injection from Organic Dyes into SnOz
J. Phys. Chem., Vol. 98, No. 45, 1994 11749
SCHEME 2: Mechanism of Generation of Sensitized Photocurrent for Anodically and Cathodically Biased Sn09 Anodicallv biased SnOz
I
EVE
k R+
A Cathodicallv
biased
SnOZ krad
D',D+
I A/A-
R
EVE L
4
Bulk of the semi-conduc tor
A -b Surface of the semi-conductor and deposited layers
Electrolyte solution
kred is the rate of electron transfer from the dye to the SnOz, k,, is the rate of electron transfer from the SnO2 surface states to an oxidizing species (A) in the electrolyte solution,kin, is the rate of electron injection from the surface to the bulk of the SnO2, kirecis the recombination rate of the electrons in the conduction band, and k,, and k,, are the rates of recombination from surface states and geminate recombination, re-
spectively. always been thoroughly purged by Nz or Ar (in fact, an increase of the cathodic photocurrent when 0 2 was present has also been observed) and generation of the cathodic photocurrent has been observed over distances where conduction through the layers is very unlikely to be effective. Although one can not exclude the possibility that electron transport through the layer" can add up to the energy transfer mechanism leading to a minor increase of the sensitized photocurrent, the efficiency of this process should not be large enough to account for our observations. This suggests that energy transfer is the main mechanism for generation of the cathodic photocurrent when highly doped SnOz is sensitized by THIAM18. In order to obtain some additional information on the direction and the time scale of the SEC generated under anodic and cathodic polarization, we have performed a series of experiments recording the transient photocurrent produced upon excitation with 3-10 ms light pulses. Under anodic (at +550 mV) and cathodic (at -50 to +50 mV) polarization, anodic and respectively cathodic photocurrents were obtained and their rise and decay followed that of the excitation pulse. Furthermore and in contrast to the experiments of Spitler et al., no transient currents of opposite sign could be detected within a time period up to 100 ms after the excitation pulse.
At the cathodic potentials used in our experiments, the band bending is very small. The electron from the photogenerated electron-hole pair will thus spend a large amount of time at the localized surface states, and in addition, because this electron will lie very high in energy, it will be a very strong reducing agent. By consequence, it will have enough time and energy to reduce some species in the electrolyte solution.18 Competition with the generation of cathodic photocurrent could occur by injection of electrons, generated by energy transfer at the surface states, into the conduction band. This latter process will become more important as the applied electrode potential becomes more anodic. This way an anodic photocurrent could be observed. In other terms, anodic photocurrents are generated by both mechanisms of electron and energy transfer while cathodic photocurrents are generated mainly by energy transfer. Therefore, within the conditions of our experiments, the photocurrents obtained at cathodic or slightly anodic potentials should remain small in comparison with those obtained at anodic potentials. When both mechanisms are available (contact case), the ratio Za/Zc will be larger than when only energy transfer is possible (distance case). This is confirmed by the experiments with THIAM18, the ratio laLC staying essentially constant during the long-range sensitization (Za/Zc FZ 1.6) while it is considerably higher in the contact case (la/&5-6). The features of the action spectra obtained at various applied potentials can be explained as follows: At anodic potentials and when electron transfer is more efficient than energy transfer, the action spectrum is mainly due to dimers. Formation of dimers is expected to decrease the efficiency of photosensitization when the energy transfer mechanism is the most important one, since the relaxed state of the excited sandwich dimers is characterized by a smaller transition moment. This would account for the increase of the contribution from the monomers in the action spectrum obtained at cathodically biased electrodes.
5. Conclusions In this work we have investigated the mechanism of photosensitization of highly doped SnOz by two cyanine dyes. The long-range photosensitization and the relatively high cathodic photocurrents obtained with THIAMl8 are explained by means of an energy transfer model, invoking the existence of localized states, weakly coupled with the bulk of the semiconductor."a These states would be associated with individual sites on the surface of SnOz. The roughness of the surface of SnOz, as characterized by SEM and STM microscopy,8a as well as the low sensitization quantum yields observed for this quality of SnO2, provides further support of this hypothesis. We therefore believe that when the energetics of the sensitizer and the surface characteristics of the semiconductor are appropriate, both electron and energy transfer can take place, their relative contribution to the overall photocurrent generation depending on the sensitization distance and the applied potential.
Acknowledgment. D.N. is a fellow under the EC's Human Capital and Mobility scheme. M.V.d.A is a Onderzoeksdirecteur of the Belgian Fonds voor Kollektief Fundamenteel Onderzoek. The continuing support of the Belgian Fonds voor Kollektief Fundamenteel Onderzoek and the Ministry of Science Programming through Projects IUAP 111-040 and IUAP 11-16 is gratefully acknowledged. References and Notes (1) Szentpaly, L. V.; Mobius, D.;Kuhn, H.J . Chem. Phys. 1969,5 (9),4618. (2) Memming, R.; Tributsch, H. J . Phys. Chem. 1971, 75 (4), 562.
11750 J. Phys. Chem., Vol. 98, No. 45, 1994 (3) (a) Arden, W.; Fromherz, P. J . Electrochem. Soc. 1980, 127 (2), 370. (b) Memming, R. Prog. Surf. Sci. 1984, 17, 7. (c) Willig, F.; Gerischer, H. Top. Curr. Chem. 1976, 61, 31. (4) (a) Bressel, B.; Gerischer, H. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 398. (b) Lazafame, J. M.; Miller, R. J. D.; Muenter, A. A,; Parkinson, B. A. J . Phys. Chem. 1992, 96, 2820. (5) Vlachopoulos, N.; Liska, P.; Augustynski, J.; Gratzel, M. J . Am. Chem. Soc. 1988, 110, 1216-1220. (6) (a) Nasielski, J.; Kirch-De Mesmaeker, A,; Leempoel, P. Nouv. J . Chim. 1978, 2, 497. (b) Arden, W.; Fromherz, P. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 868. (7) (a) Biesman, G.; Verbeek, G.; Verschuere, B.; Van der Auweraer, M.; De Schryver, F. C. Thin Solid Films 1989, 168, 127. (b) Biesmans, G.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 1990, 6, 227. (8) (a) Biesman, G.; Van der Auweraer, M.; Cathry, C.; De Schryver, F. C.; Yonezawa, Y.; Sato. T. Chem. Phys. 1992, 160, 97-121. (b) Biesmans, G.; Van der Auweraer, M.; Cathry, C.; Meerschaut, D.; De Schryver, F. C.; Storck, W.; Willig, F. J. Phys. Chem. 1991, 95 (9), 3771. (9) By changing the substrate surface characteristics (by first appropriate layers on the SnOz), one can readily enhance or inhibit the formation of J-aggregates.
Noukakis et al. (10) Forster, Th. Naturwissenschafen 1946, 33, 166. (11) Drexhage, K. H.; Zwick, M. M.; Kuhn, H. Ber. Bunsen-Ges. Phys. Chem. 1963,67,62. (12) (a) Kuhn, H. Modem Trends of Colloid Science, Proceedings of the International Symposium on Colloid and Surface Science, Interlaken, 1984; Eicke, H. F., Ed.; Birkhauser Verlag: Basel, 1985; p 97. (b) Miller, J. R.; Beitz, J. V. J . Chem. Phys. 1981, 74, 6746. (c) Miller, J. R.; Calcaterra, L.; Closs, G. L. J . Am. Chem. SOC.1984,106, 3047. (d) Hush, N.; Paddon-Row, M. N.; Cotsaris, E.; Oevenng, H.; Verhoeven, J. W. Chem. Phys. Lett. 1985, 117, 8. (13) Lenhard, J. J . Imaging Sci. 1986, 30, 27. (14) Steiger, R. Photogr. Sci. Eng. 1984, 28 (2), 35. (15) Kuhn, H.; Mann, B. J . Appl. Phys. 1971, 42, 4398. (16) Spitler, M. T.; Calvin, M. J . Chem. Phys. 1977, 66 (lo), 4294. (17) Kim, Y. S.; Liang, K.; Law, K. Y.; Whitten, D. G . J . Phys. Chem. 1994, 98, 984. (18) It is well established that even when many layers are deposited, small molecules will diffuse rather efficiently through the fatty acid layers to reach and react at the surface of SnOz.
