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Spatially selective electrochemical deposition of composite films of metal and luminescent Si nanoparticles
Chemical Physics Letters 372 (2003) 415–418 www.elsevier.com/locate/cplett
Spatially selective electrochemical deposition of composite films of metal and luminescent Si nanoparticles A. Smith a, G. Belomoin a, M.H. Nayfeh a
a,*
, Taysir Nayfeh
b
Departments of Physics, University of Illinois at Urbana-Champaign, 1110 W. Green Street Urbana, IL 61801, USA b Industrial Engineering, Cleveland State University, Cleveland, OH, USA Received 5 December 2002; in final form 5 March 2003
Abstract We report on a procedure for selective deposition of Si nanoparticles using an electrochemical process. A conducting substrate is immersed in alcohol in which the particles are suspended. Biasing the substrate positively relative to a platinum electrode draws the Si particles to the substrate. Thin particle coatings on metal, foil, or silicon substrates are demonstrated. Fluorescent spectroscopy shows that the deposited particles retain the high luminescence efficiency and spectral distribution characteristic of the dispersed state. The process is used to deposit composite thin films of metal and Si nanoparticles. Dielectric masking allowed selective area deposition. These processes have implications for flat panel or flexible particle-based displays. Ó 2003 Elsevier Science B.V. All rights reserved.
Recently, we have introduced a method [1–11] which disperses bulk Si into luminescent ultrasmall nanoparticles of 1, 1.67, 2.15, and 2.9 nm in diameter fluorescing in the blue, green, yellow, and red, respectively [10,11], with high throughput and excellent size definition and control. Unlike bulk Si, a dull poorly fluorescent indirect gap material, the particles are highly fluorescent, two to threefold brighter than fluorescein or coumarine, such that emission from single particles is readily detected under two-photon near-infrared femto second excitation [1]. In addition to being ultrabright, the particles exhibit stimulated emission [2], di-
*
Corresponding author. Fax: 1-217-333-9819. E-mail address: [email protected] (M.H. Nayfeh).
rected beam emission [3,11], and harmonic generation [4]. The particle capacitance is ultrasmall such that the single electron charging and the confinement energy are significantly larger than the thermal energy, allowing room temperature single electronics [5]. The particles have been synthesized with H-, O-termination [6–8], or functionalized with N- [9] or C-linkages. Density functional methods with generalized gradient exchange-correlation potentials, configuration interaction and Monte Carlo approaches show that the structure is filled fullerene [12,13]. Particles must be delivered in controlled manner to meet practical requirements. Applications envisioned for the particles include synthesis of two-dimensional and three-dimensional films with unique optical and electrical properties. Other
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00421-4
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A. Smith et al. / Chemical Physics Letters 372 (2003) 415–418
applications include insertion in matrices to synthesize photonic materials, incorporation into the gate stacks of nanotransistors to construct nanoflash-memory cells, flat panel RGB applications, assays for biomedical applications, etc. Precipitating the particles from a colloid on high-quality Si, Si oxide, and in freestanding mode yields closepacked, optically clear microcrystallites of several hundred microns across [3,4]. However, particle diffusion limits the crystallization to a thin ribbon at the edge of the expanding liquid. In this Letter, we report on a procedure for the deposition of the nanoparticles using an electrochemical process. A silicon substrate is immersed in an alcohol solution in which the particles are suspended. Biasing the substrate positively with respect to a platinum electrode draws the Si particles to the substrate. Thin particle coatings on metals, foils, or silicon substrates are demonstrated. We deposited composite thin films of metal and Si nanoparticles. By modulating the conductivity of the Si substrate using oxide masking, we achieved selective area deposition. These processes have implications for flat panel or flexible particle-based displays. The synthesis and characterizations of the particles were described elsewhere [1–12,14–16]. We disperse crystalline Si using a highly catalyzed electrochemical etching process in a mixture of HF and H2 O2 . Incorporation of the highly oxidative H2 O2 catalyzes the etching, producing smaller nanoparticles, with high chemical and electronic quality. We use ð1 0 0Þ oriented, 1–10-X cm resistivity, p-type boron doped Si by lateral anodization while advancing the wafer in the etchent at a reduced speed. Subsequent immersion in an ultrasound bath crumbles the top film into ultrasmall particles. We prepared colloids of blue luminescent 1 nm particles and red luminescent 3 nm particles. The size of the constituent particles was determined by imaging a thin graphite grid coated with the particles by transmission electron microscopy (TEM) [9,10,12]. Electron photo spectroscopy and infrared Fourier spectroscopy show that the particles are composed of Si, dominated by H-termination with less than 10% of terminating oxygen. When the particle colloids are excited at 365 nm radiation, blue or red emission respectively is
observed with the naked eye as shown in Fig. 1a. We suspend the particles in alcohol. The surface to be coated is biased positively with respect to an immersed platinum wire. A current flow in the range 50–200 lA is established. Fig. 1b shows a post-deposition fluorescent image of a section of stainless steel plate under UV illumination at 365 nm, showing a red luminescent particle film. We have also used the same procedure to deposit 1 nm blue luminescent particles. In this case, the rate of deposition is found to be an order of magnitude slower than that for the red particles. Fig. 1c shows a fluorescent image of a p-type silicon wafer coated under similar conditions used for metal surface. In this case, we find additional particle deposits along the air–solution interface (the meniscus). This is likely due to the fact that the conductivity of the substrate is lower than the conductivity of the liquid, resulting in the
Fig. 1. Fluorescent images, under UV illumination at 365 nm, (a) of a fluorescent red silicon particles colloid, (b) of a coated stainless steel flat, (c) of a coated p-type silicon wafer, (d) of a coated p-type substrate with an oxide overlayer that has been etched away in a selected area (error bar), and (e) of an coated aluminum alligator clip.
A. Smith et al. / Chemical Physics Letters 372 (2003) 415–418
417
concentration of the current at the meniscus. Gradually advancing the substrate into the liquid sweeps the meniscus uniformly over a large area of the substrate, resulting in a more uniform coating. A variety of patterns on Si and several metallic objects, such as an aluminum alligator clip (shown in Fig. 1e), were successfully coated, demonstrating the versatility of the method. The process is self-limiting because the particles are essentially nonconducting; under doping of 1015 /cc boron, less than one particle in a million contains a boron atom. Once the coating is complete, the deposition current begins to drop. Additional material gets deposited but it may not adhere well. We believe the mechanism of the deposition of the particles is that of the well-known electrophoresis. In the process, the particles get attached to negative al cohol ions ðROHSiÞ . Alcohol is known to ionize to produce the negative ion in the presence of even trace amount of water [14–21]. The slow down of the deposition of the smallest particles may be a result of a smaller surface area. XPS spectra of the plated metal as well as the silicon substrate (Fig. 2) were taken. The spectra show that there is no carbon constituent above what is expected in background measurements of control samples. The spectra show also a silicon state indicating the presence of the silicon particles. The oxygen peak is consistent with an SiO2 state, indicating partial oxidation may have taken place. However, the oxide did not permeate the
particles. Fluorescent spectroscopy shows that the deposited particles retain the high luminescence efficiency and spectral distribution characteristic of the dispersed state. We added metal Al salt (AlCl3 ) to the particle colloid. The particle density is a 3–5% of the ion density. In this case the particles get deposited on the negatively biased electrode. The reversed direction indicates that the process proceeds largely in terms of the attachment of the silicon particles to metal ions, rather than by attachment to alcohol ions. The deposition results in a composite thin film of metal and partially oxidized (capped) nanoparticles. Fig. 3 gives material depth analysis profiling of the film using Auger electron spectroscopy. It confirms that the film is a composite of aluminum and silicon nanoparticles, with a concentration ration similar to that in the dispersion. Photoluminescence spectroscopy shows that the film is highly luminescent. Controlling the duration of the deposition, concentration of the material, and the current/voltage used controls the thickness and composition of the film. Some oxidation of metal may be a result of the presence of traces of water in the solution. Aluminum oxide is a very useful matrix. It is a high hardness, high temperature material. A form of Al2 O3 (corundum) is nearly as hard as diamond. The procedure can be extended to other metals. We successfully deposited thin film composites on metal plates, foils, and silicon substrates.
Fig. 2. XPS of coated metal substrates. Elemental silicon peaks are seen. The level of the carbon constituent is below the normal background of control samples. There is an oxygen peak.
Fig. 3. Auger electron spectroscopy of the composite film showing that the film is a uniform composite of silicon nanoparticles and aluminum (oxide).
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A. Smith et al. / Chemical Physics Letters 372 (2003) 415–418
Finally, we masked the substrate to spatially control the deposition process. A thermal oxide layer of 300 nm was grown on the p-type ð1 0 0Þ Si substrate. Patterns in the oxide were etched away to provide current paths. The substrate was then coated. Fig. 1d shows that the particles are selectively deposited within the etched pattern area (error bar). In conclusion, we reported on a procedure for deposition of ultrasmall highly fluorescent silicon nanoparticles on metals or semiconductors. The particles are delivered from alcohol colloids using electrochemical processes. Selective area deposition is demonstrated which is useful for flat panel display applications. By mixing with metal salts, thin film composites of metals and particles on metal, silicon substrates, and foils are demonstrated. The composite process is discussed in terms of the attachment of particles to metal ions. These results have implications to flexible particlebased displays.
Acknowledgements The authors acknowledge the State of Illinois Grant IDCCA No. 00-49106, US NSF Grant BES-0118053, the US DOE Grant DEFG02ER9645439, NIH Grant RR03155, Motorola, NSF, and the University of Illinois at UrbanaChampaign.
References [1] O. Akcakir, J. Therrien, G. Belomoin, N. Barry, E. Gratton, M. Nayfeh, Appl. Phys. Lett. 76 (2000) 1857.
[2] M. Nayfeh, O. Akcakir, J. Therrien, Z. Yamani, N. Barry, W. Yu, E. Gratton, Appl. Phys. Lett. 75 (1999) 4112. [3] M.H. Nayfeh, N. Barry, J. Therrien, O. Akcakir, E. Gratton, G. Belomoin, Appl. Phys. Lett. 78 (2001) 1131. [4] M.H. Nayfeh, O. Akcakir, G. Belomoin, N. Barry, J. Therrien, E. Gratton, Appl. Phys. Lett. 77 (2000) 4086. [5] J. Therrien, G. Belomoin, M. Nayfeh, Appl. Phys. Lett. 77 (2000) 1668. [6] G. Belomoin, J. Therrien, M. Nayfeh, Appl. Phys. Lett. 77 (2000) 779. [7] H. Thompson, Z. Yamani, L. AbuHassan, O. Gurdal, M.H. Nayfeh, Appl. Phys. Lett. 73 (1998) 841. [8] W.H. Thompson, Z. Yamani, L.H. Abuhassan, J.E. Greene, M. Nayfeh, J. Appl. Phys. 80 (1996) 5415. [9] E. Rogozhina, G. Belomoin, J. Therrien, P. Braun, M.H. Nayfeh, Appl. Phys. Lett. 78 (2001) 3711. [10] G. Belomoin, J. Therrien, A. Smith, S. Rao, R. Twesten, S. Chaieb, M.H. Nayfeh, Appl. Phys. Lett. 80 (2002) 841. [11] M.H. Nayfeh, S. Rao, N. Barry, J. Therrien, G. Belomoin, A. Smith, S. Chaieb, Appl. Phys. Lett. 80 (2002) 121. [12] L. Mitas, J. Therrien, G. Belomoin, M.H. Nayfeh, Appl. Phys. Lett. 78 (2001) 1918. [13] G. Belomoin, E. Rogozhina, J. Therrien, P.V. Braun, L. Abuhassan, M.H. Nayfeh, Phys. Rev. B 65 (2002) 193406. [14] Z. Yamani, H. Thompson, L. AbuHassan, M.H. Nayfeh, Appl. Phys. Lett. 70 (1997) 3404. [15] D. Andsager, J. Hilliard, J.M. Hetrick, L.H. AbuHassan, M. Plisch, M.H. Nayfeh, J. Appl. Phys. 74 (1993) 4783. [16] Z. Yamani, S. Ashhab, A. Nayfeh, M.H. Nayfeh, J. Appl. Phys. 83 (1998) 3929. [17] E. Lee, T. Bitner, J. Ha, M. Shane, M. Sailor, J. Am. Chem. Soc. 118 (1996) 5375. [18] E. Lee, J. Ha, M. Sailor, J. Am. Chem. Soc. 117 (1995) 8295. [19] V. Doan, M. Sailor, J. Appl. Phys. Lett. 60 (1992) 619. [20] Y. Kato, T. Ito, A. Hiraki, Appl. Surf. Sci. 41/42 (1989) 614. [21] M. Warntjes, C. Vieillard, F. Ozanam, J.-N. Chazalviel, J. Electrochem. Soc. 42 (1995) 4138.
Spatially selective electrochemical deposition of composite films of metal and luminescent Si nanoparticles A. Smith a, G. Belomoin a, M.H. Nayfeh a
a,*
, Taysir Nayfeh
b
Departments of Physics, University of Illinois at Urbana-Champaign, 1110 W. Green Street Urbana, IL 61801, USA b Industrial Engineering, Cleveland State University, Cleveland, OH, USA Received 5 December 2002; in final form 5 March 2003
Abstract We report on a procedure for selective deposition of Si nanoparticles using an electrochemical process. A conducting substrate is immersed in alcohol in which the particles are suspended. Biasing the substrate positively relative to a platinum electrode draws the Si particles to the substrate. Thin particle coatings on metal, foil, or silicon substrates are demonstrated. Fluorescent spectroscopy shows that the deposited particles retain the high luminescence efficiency and spectral distribution characteristic of the dispersed state. The process is used to deposit composite thin films of metal and Si nanoparticles. Dielectric masking allowed selective area deposition. These processes have implications for flat panel or flexible particle-based displays. Ó 2003 Elsevier Science B.V. All rights reserved.
Recently, we have introduced a method [1–11] which disperses bulk Si into luminescent ultrasmall nanoparticles of 1, 1.67, 2.15, and 2.9 nm in diameter fluorescing in the blue, green, yellow, and red, respectively [10,11], with high throughput and excellent size definition and control. Unlike bulk Si, a dull poorly fluorescent indirect gap material, the particles are highly fluorescent, two to threefold brighter than fluorescein or coumarine, such that emission from single particles is readily detected under two-photon near-infrared femto second excitation [1]. In addition to being ultrabright, the particles exhibit stimulated emission [2], di-
*
Corresponding author. Fax: 1-217-333-9819. E-mail address: [email protected] (M.H. Nayfeh).
rected beam emission [3,11], and harmonic generation [4]. The particle capacitance is ultrasmall such that the single electron charging and the confinement energy are significantly larger than the thermal energy, allowing room temperature single electronics [5]. The particles have been synthesized with H-, O-termination [6–8], or functionalized with N- [9] or C-linkages. Density functional methods with generalized gradient exchange-correlation potentials, configuration interaction and Monte Carlo approaches show that the structure is filled fullerene [12,13]. Particles must be delivered in controlled manner to meet practical requirements. Applications envisioned for the particles include synthesis of two-dimensional and three-dimensional films with unique optical and electrical properties. Other
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00421-4
416
A. Smith et al. / Chemical Physics Letters 372 (2003) 415–418
applications include insertion in matrices to synthesize photonic materials, incorporation into the gate stacks of nanotransistors to construct nanoflash-memory cells, flat panel RGB applications, assays for biomedical applications, etc. Precipitating the particles from a colloid on high-quality Si, Si oxide, and in freestanding mode yields closepacked, optically clear microcrystallites of several hundred microns across [3,4]. However, particle diffusion limits the crystallization to a thin ribbon at the edge of the expanding liquid. In this Letter, we report on a procedure for the deposition of the nanoparticles using an electrochemical process. A silicon substrate is immersed in an alcohol solution in which the particles are suspended. Biasing the substrate positively with respect to a platinum electrode draws the Si particles to the substrate. Thin particle coatings on metals, foils, or silicon substrates are demonstrated. We deposited composite thin films of metal and Si nanoparticles. By modulating the conductivity of the Si substrate using oxide masking, we achieved selective area deposition. These processes have implications for flat panel or flexible particle-based displays. The synthesis and characterizations of the particles were described elsewhere [1–12,14–16]. We disperse crystalline Si using a highly catalyzed electrochemical etching process in a mixture of HF and H2 O2 . Incorporation of the highly oxidative H2 O2 catalyzes the etching, producing smaller nanoparticles, with high chemical and electronic quality. We use ð1 0 0Þ oriented, 1–10-X cm resistivity, p-type boron doped Si by lateral anodization while advancing the wafer in the etchent at a reduced speed. Subsequent immersion in an ultrasound bath crumbles the top film into ultrasmall particles. We prepared colloids of blue luminescent 1 nm particles and red luminescent 3 nm particles. The size of the constituent particles was determined by imaging a thin graphite grid coated with the particles by transmission electron microscopy (TEM) [9,10,12]. Electron photo spectroscopy and infrared Fourier spectroscopy show that the particles are composed of Si, dominated by H-termination with less than 10% of terminating oxygen. When the particle colloids are excited at 365 nm radiation, blue or red emission respectively is
observed with the naked eye as shown in Fig. 1a. We suspend the particles in alcohol. The surface to be coated is biased positively with respect to an immersed platinum wire. A current flow in the range 50–200 lA is established. Fig. 1b shows a post-deposition fluorescent image of a section of stainless steel plate under UV illumination at 365 nm, showing a red luminescent particle film. We have also used the same procedure to deposit 1 nm blue luminescent particles. In this case, the rate of deposition is found to be an order of magnitude slower than that for the red particles. Fig. 1c shows a fluorescent image of a p-type silicon wafer coated under similar conditions used for metal surface. In this case, we find additional particle deposits along the air–solution interface (the meniscus). This is likely due to the fact that the conductivity of the substrate is lower than the conductivity of the liquid, resulting in the
Fig. 1. Fluorescent images, under UV illumination at 365 nm, (a) of a fluorescent red silicon particles colloid, (b) of a coated stainless steel flat, (c) of a coated p-type silicon wafer, (d) of a coated p-type substrate with an oxide overlayer that has been etched away in a selected area (error bar), and (e) of an coated aluminum alligator clip.
A. Smith et al. / Chemical Physics Letters 372 (2003) 415–418
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concentration of the current at the meniscus. Gradually advancing the substrate into the liquid sweeps the meniscus uniformly over a large area of the substrate, resulting in a more uniform coating. A variety of patterns on Si and several metallic objects, such as an aluminum alligator clip (shown in Fig. 1e), were successfully coated, demonstrating the versatility of the method. The process is self-limiting because the particles are essentially nonconducting; under doping of 1015 /cc boron, less than one particle in a million contains a boron atom. Once the coating is complete, the deposition current begins to drop. Additional material gets deposited but it may not adhere well. We believe the mechanism of the deposition of the particles is that of the well-known electrophoresis. In the process, the particles get attached to negative al cohol ions ðROHSiÞ . Alcohol is known to ionize to produce the negative ion in the presence of even trace amount of water [14–21]. The slow down of the deposition of the smallest particles may be a result of a smaller surface area. XPS spectra of the plated metal as well as the silicon substrate (Fig. 2) were taken. The spectra show that there is no carbon constituent above what is expected in background measurements of control samples. The spectra show also a silicon state indicating the presence of the silicon particles. The oxygen peak is consistent with an SiO2 state, indicating partial oxidation may have taken place. However, the oxide did not permeate the
particles. Fluorescent spectroscopy shows that the deposited particles retain the high luminescence efficiency and spectral distribution characteristic of the dispersed state. We added metal Al salt (AlCl3 ) to the particle colloid. The particle density is a 3–5% of the ion density. In this case the particles get deposited on the negatively biased electrode. The reversed direction indicates that the process proceeds largely in terms of the attachment of the silicon particles to metal ions, rather than by attachment to alcohol ions. The deposition results in a composite thin film of metal and partially oxidized (capped) nanoparticles. Fig. 3 gives material depth analysis profiling of the film using Auger electron spectroscopy. It confirms that the film is a composite of aluminum and silicon nanoparticles, with a concentration ration similar to that in the dispersion. Photoluminescence spectroscopy shows that the film is highly luminescent. Controlling the duration of the deposition, concentration of the material, and the current/voltage used controls the thickness and composition of the film. Some oxidation of metal may be a result of the presence of traces of water in the solution. Aluminum oxide is a very useful matrix. It is a high hardness, high temperature material. A form of Al2 O3 (corundum) is nearly as hard as diamond. The procedure can be extended to other metals. We successfully deposited thin film composites on metal plates, foils, and silicon substrates.
Fig. 2. XPS of coated metal substrates. Elemental silicon peaks are seen. The level of the carbon constituent is below the normal background of control samples. There is an oxygen peak.
Fig. 3. Auger electron spectroscopy of the composite film showing that the film is a uniform composite of silicon nanoparticles and aluminum (oxide).
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A. Smith et al. / Chemical Physics Letters 372 (2003) 415–418
Finally, we masked the substrate to spatially control the deposition process. A thermal oxide layer of 300 nm was grown on the p-type ð1 0 0Þ Si substrate. Patterns in the oxide were etched away to provide current paths. The substrate was then coated. Fig. 1d shows that the particles are selectively deposited within the etched pattern area (error bar). In conclusion, we reported on a procedure for deposition of ultrasmall highly fluorescent silicon nanoparticles on metals or semiconductors. The particles are delivered from alcohol colloids using electrochemical processes. Selective area deposition is demonstrated which is useful for flat panel display applications. By mixing with metal salts, thin film composites of metals and particles on metal, silicon substrates, and foils are demonstrated. The composite process is discussed in terms of the attachment of particles to metal ions. These results have implications to flexible particlebased displays.
Acknowledgements The authors acknowledge the State of Illinois Grant IDCCA No. 00-49106, US NSF Grant BES-0118053, the US DOE Grant DEFG02ER9645439, NIH Grant RR03155, Motorola, NSF, and the University of Illinois at UrbanaChampaign.
References [1] O. Akcakir, J. Therrien, G. Belomoin, N. Barry, E. Gratton, M. Nayfeh, Appl. Phys. Lett. 76 (2000) 1857.
[2] M. Nayfeh, O. Akcakir, J. Therrien, Z. Yamani, N. Barry, W. Yu, E. Gratton, Appl. Phys. Lett. 75 (1999) 4112. [3] M.H. Nayfeh, N. Barry, J. Therrien, O. Akcakir, E. Gratton, G. Belomoin, Appl. Phys. Lett. 78 (2001) 1131. [4] M.H. Nayfeh, O. Akcakir, G. Belomoin, N. Barry, J. Therrien, E. Gratton, Appl. Phys. Lett. 77 (2000) 4086. [5] J. Therrien, G. Belomoin, M. Nayfeh, Appl. Phys. Lett. 77 (2000) 1668. [6] G. Belomoin, J. Therrien, M. Nayfeh, Appl. Phys. Lett. 77 (2000) 779. [7] H. Thompson, Z. Yamani, L. AbuHassan, O. Gurdal, M.H. Nayfeh, Appl. Phys. Lett. 73 (1998) 841. [8] W.H. Thompson, Z. Yamani, L.H. Abuhassan, J.E. Greene, M. Nayfeh, J. Appl. Phys. 80 (1996) 5415. [9] E. Rogozhina, G. Belomoin, J. Therrien, P. Braun, M.H. Nayfeh, Appl. Phys. Lett. 78 (2001) 3711. [10] G. Belomoin, J. Therrien, A. Smith, S. Rao, R. Twesten, S. Chaieb, M.H. Nayfeh, Appl. Phys. Lett. 80 (2002) 841. [11] M.H. Nayfeh, S. Rao, N. Barry, J. Therrien, G. Belomoin, A. Smith, S. Chaieb, Appl. Phys. Lett. 80 (2002) 121. [12] L. Mitas, J. Therrien, G. Belomoin, M.H. Nayfeh, Appl. Phys. Lett. 78 (2001) 1918. [13] G. Belomoin, E. Rogozhina, J. Therrien, P.V. Braun, L. Abuhassan, M.H. Nayfeh, Phys. Rev. B 65 (2002) 193406. [14] Z. Yamani, H. Thompson, L. AbuHassan, M.H. Nayfeh, Appl. Phys. Lett. 70 (1997) 3404. [15] D. Andsager, J. Hilliard, J.M. Hetrick, L.H. AbuHassan, M. Plisch, M.H. Nayfeh, J. Appl. Phys. 74 (1993) 4783. [16] Z. Yamani, S. Ashhab, A. Nayfeh, M.H. Nayfeh, J. Appl. Phys. 83 (1998) 3929. [17] E. Lee, T. Bitner, J. Ha, M. Shane, M. Sailor, J. Am. Chem. Soc. 118 (1996) 5375. [18] E. Lee, J. Ha, M. Sailor, J. Am. Chem. Soc. 117 (1995) 8295. [19] V. Doan, M. Sailor, J. Appl. Phys. Lett. 60 (1992) 619. [20] Y. Kato, T. Ito, A. Hiraki, Appl. Surf. Sci. 41/42 (1989) 614. [21] M. Warntjes, C. Vieillard, F. Ozanam, J.-N. Chazalviel, J. Electrochem. Soc. 42 (1995) 4138.