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Selective metal deposition on metal-semiconductor nanostructures
Vacuum/volume
50/number
I-2/pages 69 to 71/1998 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0042-207X/98 $19.00+.00
0
Pergamon PII
: SOO42-207X(98)00019-0
Selective metal deposition on metal-semiconductor nanostructures G A Ragoisha, Institute
of Physicochemical
Problems,
Belarusian
State University,
Minsk 220080, Belarus
Photostimulated selective electroless Ag and 0.1 deposition was investigated on thin TiO, films doped with small Ag particles and Pbl, films doped with Cu particles. The activation that is characteristic of the thin film structures at the electroless deposition was compared to the opposite effect of illumination in Zn vapour condensation. 0 1998 Elsevier Science Ltd. All rights reserved
Introduction
Selective electroless metal deposition is a powerful tool for amplification and detection of latent photostimulated changes in microsystems. This amplification technique is known in silver-halide’ and non-silver2 photography as a “physical development”. Actually, the “physical development” is a complex chemical reaction that comprises the autocatalytic reduction of a metal ion (CL?+, Ag+, Ni2+: etc.) coupled with a series of chemical and electrochemical transformations of a reducer (formaldehyde, hypophosphite, etc.). “Physical developer” solutions are the analogue of metal plating baths used in microelectronics, with chemical composition optimised for efficient detection of metal nucleL2 In the simplest case the development detects tiny metal particles in the initial stage of a photochemical reaction, but it can also give information on the effects of electric charging in microsystems3,4 and electron injection into semiconductor particles during oxidation decomposition of energy rich compounds.’ Due to low redox potential of metal nanophases, metal particles oxidise and dissolve in a “physical developer” solution below a critical size that usually varies from 2 to 10 nm for different metals and developers. Discrimination of metal nuclei in size is a common cause of selectivity in metal deposition on relatively large particles. Selective metal deposition onto certain particles below the critical size and onto non-metallic centres is based on kinetic rather then thermodynamic distinctions between the centres of different origin. In this case, metal particles start to grow in the developer solution as thermodynamically unstable dissipative structures, that comply with the conditions of nonlinear dynamics. Experimental investigation and computer simulation of the autocatalytic metal ion reduction coupled with the oxidation of formaldehyde and hypophosphite have revealed electrochemical oscillations and other nonlinear phenomena that result from peculiar catalytic properties of very small metal particles and the nonequilibrium character of the autocatalytic reaction.6.7 The initiation of metal ion autocatalytic reduction in nonlinear systems investigated in6.’ appeared to be highly sen-
sitive to electric control. Though the mechanisms of this control are not always clear, they give the possibility of monitoring the electric heterogeneity in microsystems by the autocatalytic metal ion reduction. In order to estimate the capacity of “physical development” to reveal the effect of a semiconductor support on tiny metal particles we have investigated the photoselective deposition of silver and copper onto thin TiO, and PbI, films doped with very small (less then 1 nm) Ag and Cu particles. Due to formation of traps for photoelectrons by metal particles, metal-semiconductor film structures retain electrically polarised state after illumination,3 which affects their capacity to initiate metal deposition from a developer solution. We have found that the effect of photoselective metal deposition was intrinsic to metal-semiconductor film structures of a certain low thickness. Unlike thin films, bulk materials do not exhibit the capacity to photostimulated activation in metal-semiconductor systems.
Experimental
Amorphous hydrated titanium dioxide nanoporous 100 nm films were obtained on glass supports by hydrolytic decomposition of polymeric butyl titanate using a technique that was developed for preparation of titanium dioxide transparent photographic layers.’ The usual temperature of hydrolysis was 100°C. In some cases additional heating in air at 500°C for 1 h was used to frit the nanoporous film and start crystallisation of Ti02. The amount of 4 x lo-‘” mol cm-’ Ag was introduced into the film by vacuum deposition. Ag-TiO, film structures were activated to metal deposition by integral radiation of Hg-lamp. A monochromator was used in some experiments to test spectral distribution of the photoactivation effect. PbI, films were deposited in vacuum onto BaSO, bonded paper preceded by 3 x lo-” mol cmm2 Cu deposition. The inverse order of deposition was used in this case to prevent the films from activation by light that was irradiated during Cu evaporation. 69
GA Ragoisha: Selective metal deposition on metal-semiconductor
Unlike Ag-TiO,, Cu-PbI, films were highly sensitive to light in the visible range of spectrum. We used a radiation of a filament lamp for their activation. Silver with metol (CH,NHC,H, “physical developer” OH * l/2H,S04) as a reducer of Ag+ and copper ‘*physical developer” with formaldehyde and ascorbic acid as reducers of CL?+’ were used to reveal the effect of irradiation in the exposed film structures. Optical density of deposited metal layers was measured in the transmitted light in the case of TiOz films and in the reflected light in the case of PbI, films. Irradiated film structures were treated also with Zn vapour in a vacuum chamber. It was interesting to compare the effects of controlled metal deposition using the deposition processes of different origin, because Zn vapour condensation is known to slow down on negatively charged sites of TiO, films.4 Results and discussion The amount of 4 x lo-‘” mol cme2 Ag on Ti02 and on glass supports was found to be inactive for Ag and Cu development. However, a short illumination of Ag-TiO, film structures with UV light activates them to subsequent metal deposition from the developer solutions. Figure 1 shows the dependence of the optical density of Ag deposit at 5 min development time on the exposure (H) of Ag-TiO> films. The maximum in the optical density and the corresponding exposure depend on the film structure. Amorphous hydrated TiOz films provide high rate of metal deposition that results in a high optical density of the deposited metal layer on the exposed Ag-TiO? structures. Annealed TiO, films show much higher sensitivity to light but the autocatalytic reaction in this case slows down resulting in a lower optical density of Ag deposit. We observed a decrease in the rate of the photostimulated metal deposition also with the increase in TiO, film thickness. The film thickness effect was especially strong in the case of Cu-Pbl, photoactivation. Figure 2 shows optical density of Cu deposit on exposed and unexposed Cu-PbI, layers after 5 min treatment in copper “physical developer” solution as a function of an average thickness (d) of Pb12 film. Maximum photoactivation effect was achieved on 7 nm film. We did not observe photoactivation of thick PbI, films doped with Cu, irrespective of the order of Cu and PbI, evaporation. The opposite effect at metal-semiconductor structures illumination was observed in the case of Zn particles growth from gaseous phase. Zn vapour condensation was slowed down on the illuminated areas of Cu-PbI, and Ag-TiOz films at the exposures that enhanced Ag and Cu deposition. This effect decreased at a
nanostructures
-unexposed
Figure 2. The dependence troless deposition
of Cu-Pbl, photoactivation on PbIz film thickness.
high exposure, when the inversion of a photostimulated effect was observed in the electroless metal deposition. We applied the discrepancy between these two amplification techniques to determine the contribution of a photoelectric polarisation into photochemical activation of TiO, films modified by Ag+ adsorption from AgNO, solution. Figure 3 shows the effect of irradiation of Ti02 films with adsorbed Ag+ on Ag electroless deposition (Fig. 3a) and Zn vapour condensation (Fig. 3b). Irradiation of TiO,-Ag+ system produces photolytic Ag nuclei that initiate Ag electroless deposition. From the comparison of Fig. 3a with Fig. 3b it is evident that higher activity of Ag nucleation centres in the film with lower Ag+ concentration is due to the negative charging of photolytic Ag particles. The negative charge attributed to Ag particles in the illuminated AgTiO, nanostructures is neutralised more easily with the increase of [Ag+]. It results in the decrease in the nucleating capacity of Ag particles in Ag electroless deposition. The same increase in [Ag+] results in the inversion of a photostimulated effect in Zn
”O.$% 0
” -5
-4 log H [J &]
Figure 1. The effect of Ag-TiOZ film structures exposure JO
H on the subsequent
electroless
-2.5
-2.0
IogH [J cm-‘]
-2
illumination Ag deposition.
effect at
Figure 3. Optical at various
density of (a) Ag and (b) Zn deposits modified with Ag’ as a function of exposure; numbers indicate [Ag+], mol cm-‘.
on TiO, films at the curves
GA Ragoisha:
Selective metal deposition on metal-semiconductor
vapour condensation, because the negatively charged Ag particles formed at low [Ag+] do not initiate Zn vapour condensation, unlike Ag electroless deposition. As the negative charge of Ag nuclei is neutralised by the excess Ag+, both the Ag electroless deposition and Zn vapour condensation become possible at higher [Ag+]. The residual photopolarisation that affects metal deposition on the exposed metal-semiconductor nanostructures is probably due to specific electronic properties of both their components. Small metal particles are characterised by a low redox potential compared to the bulk metal.” This deviation in the redox potential corresponds to a high position of electron energy levels formed by metal particles in the semiconductor-metalelectrolyte system. The variation of electronic properties of the semiconductor component with a film thickness tunes up the whole system for the interactions that were detected by selective metal deposition. Conclusions Selective deposition of metals is a highly sensitive technique for investigation of electronic processes in thin film metal-sem-
nanostructures
iconductor nanostructures. A combination of the autocatalytic reaction that produces particles of Ag or Cu phase with selective condensation of Zn vapour gives additional benefit to this technique, due to different effect of the electric charge on the initiation of metal particle growth in metal ion reduction and vapour condensation. References 1. James, T. H. ed., The Theory ofthe Photographic Process, Macmillan Publising Co., New York, Collier Macmillan Publishers, London, 1977. Sviridov, V. V., in Nonsiher Photogruphic Processes, ed. A. L. Kartuzhanski, Khimia, Leningrad, 1984, p. 242 (in Russian). Rakhmanov, S. K., Ragoisha, G. A.,-Branitski, G. A. and Sviridov, V. V.. Zh. Fiz. Khim. (J. Phvs. Chem.. USSR). 1980.54. 2565. Ragoisha, G. A. and &id&, V. V., Zh. Fiz: Khim. (J. Phys. Chem. USSR), 1987,61, 1433. Sokolov, V. G., Ragoisha, G. A., Nechepurenko. Y. V., Rakhmanov, A. K., Branitski, G. A. and Sviridov, V. V., Radiation-Stimulated Phenomena in Solids, 1988,8, 8 (in Russian). 6. Ragoisha. G. A., Surf. Sci., 1995,331/333. 300. I. Ragoisha, G. A., Vacuum, 1997,48,317 8. Henglein, A., Ber. Bunsenges. Phys. Chem.. 1990, 94, 600.
71
50/number
I-2/pages 69 to 71/1998 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0042-207X/98 $19.00+.00
0
Pergamon PII
: SOO42-207X(98)00019-0
Selective metal deposition on metal-semiconductor nanostructures G A Ragoisha, Institute
of Physicochemical
Problems,
Belarusian
State University,
Minsk 220080, Belarus
Photostimulated selective electroless Ag and 0.1 deposition was investigated on thin TiO, films doped with small Ag particles and Pbl, films doped with Cu particles. The activation that is characteristic of the thin film structures at the electroless deposition was compared to the opposite effect of illumination in Zn vapour condensation. 0 1998 Elsevier Science Ltd. All rights reserved
Introduction
Selective electroless metal deposition is a powerful tool for amplification and detection of latent photostimulated changes in microsystems. This amplification technique is known in silver-halide’ and non-silver2 photography as a “physical development”. Actually, the “physical development” is a complex chemical reaction that comprises the autocatalytic reduction of a metal ion (CL?+, Ag+, Ni2+: etc.) coupled with a series of chemical and electrochemical transformations of a reducer (formaldehyde, hypophosphite, etc.). “Physical developer” solutions are the analogue of metal plating baths used in microelectronics, with chemical composition optimised for efficient detection of metal nucleL2 In the simplest case the development detects tiny metal particles in the initial stage of a photochemical reaction, but it can also give information on the effects of electric charging in microsystems3,4 and electron injection into semiconductor particles during oxidation decomposition of energy rich compounds.’ Due to low redox potential of metal nanophases, metal particles oxidise and dissolve in a “physical developer” solution below a critical size that usually varies from 2 to 10 nm for different metals and developers. Discrimination of metal nuclei in size is a common cause of selectivity in metal deposition on relatively large particles. Selective metal deposition onto certain particles below the critical size and onto non-metallic centres is based on kinetic rather then thermodynamic distinctions between the centres of different origin. In this case, metal particles start to grow in the developer solution as thermodynamically unstable dissipative structures, that comply with the conditions of nonlinear dynamics. Experimental investigation and computer simulation of the autocatalytic metal ion reduction coupled with the oxidation of formaldehyde and hypophosphite have revealed electrochemical oscillations and other nonlinear phenomena that result from peculiar catalytic properties of very small metal particles and the nonequilibrium character of the autocatalytic reaction.6.7 The initiation of metal ion autocatalytic reduction in nonlinear systems investigated in6.’ appeared to be highly sen-
sitive to electric control. Though the mechanisms of this control are not always clear, they give the possibility of monitoring the electric heterogeneity in microsystems by the autocatalytic metal ion reduction. In order to estimate the capacity of “physical development” to reveal the effect of a semiconductor support on tiny metal particles we have investigated the photoselective deposition of silver and copper onto thin TiO, and PbI, films doped with very small (less then 1 nm) Ag and Cu particles. Due to formation of traps for photoelectrons by metal particles, metal-semiconductor film structures retain electrically polarised state after illumination,3 which affects their capacity to initiate metal deposition from a developer solution. We have found that the effect of photoselective metal deposition was intrinsic to metal-semiconductor film structures of a certain low thickness. Unlike thin films, bulk materials do not exhibit the capacity to photostimulated activation in metal-semiconductor systems.
Experimental
Amorphous hydrated titanium dioxide nanoporous 100 nm films were obtained on glass supports by hydrolytic decomposition of polymeric butyl titanate using a technique that was developed for preparation of titanium dioxide transparent photographic layers.’ The usual temperature of hydrolysis was 100°C. In some cases additional heating in air at 500°C for 1 h was used to frit the nanoporous film and start crystallisation of Ti02. The amount of 4 x lo-‘” mol cm-’ Ag was introduced into the film by vacuum deposition. Ag-TiO, film structures were activated to metal deposition by integral radiation of Hg-lamp. A monochromator was used in some experiments to test spectral distribution of the photoactivation effect. PbI, films were deposited in vacuum onto BaSO, bonded paper preceded by 3 x lo-” mol cmm2 Cu deposition. The inverse order of deposition was used in this case to prevent the films from activation by light that was irradiated during Cu evaporation. 69
GA Ragoisha: Selective metal deposition on metal-semiconductor
Unlike Ag-TiO,, Cu-PbI, films were highly sensitive to light in the visible range of spectrum. We used a radiation of a filament lamp for their activation. Silver with metol (CH,NHC,H, “physical developer” OH * l/2H,S04) as a reducer of Ag+ and copper ‘*physical developer” with formaldehyde and ascorbic acid as reducers of CL?+’ were used to reveal the effect of irradiation in the exposed film structures. Optical density of deposited metal layers was measured in the transmitted light in the case of TiOz films and in the reflected light in the case of PbI, films. Irradiated film structures were treated also with Zn vapour in a vacuum chamber. It was interesting to compare the effects of controlled metal deposition using the deposition processes of different origin, because Zn vapour condensation is known to slow down on negatively charged sites of TiO, films.4 Results and discussion The amount of 4 x lo-‘” mol cme2 Ag on Ti02 and on glass supports was found to be inactive for Ag and Cu development. However, a short illumination of Ag-TiO, film structures with UV light activates them to subsequent metal deposition from the developer solutions. Figure 1 shows the dependence of the optical density of Ag deposit at 5 min development time on the exposure (H) of Ag-TiO> films. The maximum in the optical density and the corresponding exposure depend on the film structure. Amorphous hydrated TiOz films provide high rate of metal deposition that results in a high optical density of the deposited metal layer on the exposed Ag-TiO? structures. Annealed TiO, films show much higher sensitivity to light but the autocatalytic reaction in this case slows down resulting in a lower optical density of Ag deposit. We observed a decrease in the rate of the photostimulated metal deposition also with the increase in TiO, film thickness. The film thickness effect was especially strong in the case of Cu-Pbl, photoactivation. Figure 2 shows optical density of Cu deposit on exposed and unexposed Cu-PbI, layers after 5 min treatment in copper “physical developer” solution as a function of an average thickness (d) of Pb12 film. Maximum photoactivation effect was achieved on 7 nm film. We did not observe photoactivation of thick PbI, films doped with Cu, irrespective of the order of Cu and PbI, evaporation. The opposite effect at metal-semiconductor structures illumination was observed in the case of Zn particles growth from gaseous phase. Zn vapour condensation was slowed down on the illuminated areas of Cu-PbI, and Ag-TiOz films at the exposures that enhanced Ag and Cu deposition. This effect decreased at a
nanostructures
-unexposed
Figure 2. The dependence troless deposition
of Cu-Pbl, photoactivation on PbIz film thickness.
high exposure, when the inversion of a photostimulated effect was observed in the electroless metal deposition. We applied the discrepancy between these two amplification techniques to determine the contribution of a photoelectric polarisation into photochemical activation of TiO, films modified by Ag+ adsorption from AgNO, solution. Figure 3 shows the effect of irradiation of Ti02 films with adsorbed Ag+ on Ag electroless deposition (Fig. 3a) and Zn vapour condensation (Fig. 3b). Irradiation of TiO,-Ag+ system produces photolytic Ag nuclei that initiate Ag electroless deposition. From the comparison of Fig. 3a with Fig. 3b it is evident that higher activity of Ag nucleation centres in the film with lower Ag+ concentration is due to the negative charging of photolytic Ag particles. The negative charge attributed to Ag particles in the illuminated AgTiO, nanostructures is neutralised more easily with the increase of [Ag+]. It results in the decrease in the nucleating capacity of Ag particles in Ag electroless deposition. The same increase in [Ag+] results in the inversion of a photostimulated effect in Zn
”O.$% 0
” -5
-4 log H [J &]
Figure 1. The effect of Ag-TiOZ film structures exposure JO
H on the subsequent
electroless
-2.5
-2.0
IogH [J cm-‘]
-2
illumination Ag deposition.
effect at
Figure 3. Optical at various
density of (a) Ag and (b) Zn deposits modified with Ag’ as a function of exposure; numbers indicate [Ag+], mol cm-‘.
on TiO, films at the curves
GA Ragoisha:
Selective metal deposition on metal-semiconductor
vapour condensation, because the negatively charged Ag particles formed at low [Ag+] do not initiate Zn vapour condensation, unlike Ag electroless deposition. As the negative charge of Ag nuclei is neutralised by the excess Ag+, both the Ag electroless deposition and Zn vapour condensation become possible at higher [Ag+]. The residual photopolarisation that affects metal deposition on the exposed metal-semiconductor nanostructures is probably due to specific electronic properties of both their components. Small metal particles are characterised by a low redox potential compared to the bulk metal.” This deviation in the redox potential corresponds to a high position of electron energy levels formed by metal particles in the semiconductor-metalelectrolyte system. The variation of electronic properties of the semiconductor component with a film thickness tunes up the whole system for the interactions that were detected by selective metal deposition. Conclusions Selective deposition of metals is a highly sensitive technique for investigation of electronic processes in thin film metal-sem-
nanostructures
iconductor nanostructures. A combination of the autocatalytic reaction that produces particles of Ag or Cu phase with selective condensation of Zn vapour gives additional benefit to this technique, due to different effect of the electric charge on the initiation of metal particle growth in metal ion reduction and vapour condensation. References 1. James, T. H. ed., The Theory ofthe Photographic Process, Macmillan Publising Co., New York, Collier Macmillan Publishers, London, 1977. Sviridov, V. V., in Nonsiher Photogruphic Processes, ed. A. L. Kartuzhanski, Khimia, Leningrad, 1984, p. 242 (in Russian). Rakhmanov, S. K., Ragoisha, G. A.,-Branitski, G. A. and Sviridov, V. V.. Zh. Fiz. Khim. (J. Phvs. Chem.. USSR). 1980.54. 2565. Ragoisha, G. A. and &id&, V. V., Zh. Fiz: Khim. (J. Phys. Chem. USSR), 1987,61, 1433. Sokolov, V. G., Ragoisha, G. A., Nechepurenko. Y. V., Rakhmanov, A. K., Branitski, G. A. and Sviridov, V. V., Radiation-Stimulated Phenomena in Solids, 1988,8, 8 (in Russian). 6. Ragoisha. G. A., Surf. Sci., 1995,331/333. 300. I. Ragoisha, G. A., Vacuum, 1997,48,317 8. Henglein, A., Ber. Bunsenges. Phys. Chem.. 1990, 94, 600.
71