- Email: [email protected]
Excimer laser annealing of glasses containing implanted metal nanoparticles
Nuclear Instruments and Methods in Physics Research B 166±167 (2000) 882±886
www.elsevier.nl/locate/nimb
Excimer laser annealing of glasses containing implanted metal nanoparticles A.L. Stepanov b
a,b
, D.E. Hole
a,*
, P.D. Townsend
a
a School of Engineering, University of Sussex, Pevensey Building, Brighton, BN1 9QH, UK Laboratory of Radiation Physics, Kazan Physical-Technical Institute, Russian Academy of Sciences, Sibirsky Trakt 10/7, 420029, Kazan, Russia
Abstract Silver and copper nanoparticles have been synthesized by ion implantation in silica and soda-lime silicate glass at 60 keV to a dose of 4 1016 ion/cm2 and at 50 keV to a dose of 8 1016 ion/cm2 , respectively. The glasses were annealed using pulses of a high-power KrF excimer laser (248 nm) in ambient atmosphere. This employed a single (25 ns) pulse ¯uence of 0.25 J/cm2 for Ag-implanted samples or of 0.21 J/cm2 for Cu-implanted ones. Several pulses from 1 to 250 of the same energy density at a frequency of 1 Hz were accumulated in the same area on the surface. The formation and modi®cation of metal nanoparticle was assessed via optical re¯ectance, combined with Rutherford backscattering analysis. Generally, changes induced by laser pulses suggest there are both reductions of the nanoparticles and some longer-range diusion of metal atoms into the glass. However, before the total dissolution of metal nanoparticles is accrued, the substrate temperature, increasing during many-pulse treatments, initiates the regrowth of new metal nanoparticles that leads to a rise of the re¯ectance. These results are discussed on the basis of a surface substrate melting. This work is part of an attempt to gain control over the size and depth distribution of such metal nanoparticles. Ó 2000 Published by Elsevier Science B.V. All rights reserved. PACS: 78.66.V; 78.66.J; 61.80.B; 81.05.Y; 78.66 Keywords: Ion implantation; Nanoparticles; Laser annealing; Optical re¯ectance; Rutherford backscattering
1. Introduction Despite the fact that ion implantation, as an approach for synthesis of metal nanoparticles in insulators with high values of the ®lling factor, has a real perspective for fabrication of the opto-elec-
*
Corresponding author. Tel.: +44-0-1273-678193; fax: +440-1273-678193. E-mail address: [email protected] (D.E. Hole).
tronic devices [1,2], there is some inconvenience in this method. One of the problems is a non-uniform statistical penetration of accelerated ions into the substrate that leads to the growth of metal particles with a very wide size distribution in the plane and in the depth of the irradiated glass surface [1,3]. Many optical properties, such as linear absorption and re¯ectance [1] and/or non-linear response [2], are size dependent, consequently for any applications, variations in size degrade the practical performance. One interesting way to
0168-583X/00/$ - see front matter Ó 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 7 3 1 - 4
A.L. Stepanov et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 882±886
modify the size distribution is the laser annealing of the samples after ion implantation. As an example, it is possible to mention experiments with the Nd:YAG laser treatment on reduction of the size of silver clusters embedded into the soda-lime glass [4] or the changes of the internal crystallographic structure of implanted iron particles in silica after ruby laser irradiation [5]. The feature of these experiments is that the laser light was employed directly into the spectral region of the transparency of the dielectric matrix, and hence, the intense laser pulses were primarily absorbed by the metal particles. Contrary to that, some years ago [6] a new approach for annealing was demonstrated, when ¯oat glass with implanted silver particles was irradiated by excimer ArF (193 nm) laser light at wavelengths of glass absorption in the ultraviolet region. When applying high-power laser pulses, a decrease of the re¯ectance intensity of modi®ed silver nanoparticles within the glass was observed. This new technique gives a variety of possibilities for controlled change of the size and the size distribution of nanoparticles in dielectrics, as demonstrated for the case of the combined excimer KrF (248 nm) laser and thermal annealing of ¯oat glass implanted with silver [7,8]. It was also shown, recently [9], that the excimer laser pulses, applied to silica glass containing implanted bismuth nanoclusters, removes bismuth from the surface by evaporation, similar to the cases of silver implanted glass [8]. All previous anneals with excimer laser were only carried out with pulses of the energy density lower than 0.2 J/cm2 , though the excimer laser gives the possibility for use of more powerful treatment of materials. The present study concentrates on the experiments with KrF laser annealing employing with a high level of energy density per pulse to evaluate the modi®cation of the metal implanted glasses. 2. Experimental The implantation of soda-lime silicate glass (SLSG) and silica pieces has been performed with Ag ions at 60 keV and to a dose of 4 1016 ion/ cm2 or Cu ions at 50 keV and to a dose of 8 1016 ion/cm2 at a beam current density of 10
883
lA/cm2 using a Whickham ion implanter. The implantation was carried out in a controlled vacuum of 10ÿ5 Torr at room temperature. The approximate chemical composition of the glass (from Societa Italiana Vetro) is 70% SiO2 , 20% Na2 O and 10% CaO. Silica is produced by Heraeus QUARZGLAS Company. Pulsed laser treatment has been made by a KrF excimer laser (ATLEX 210) with a wavelength of 248 nm and pulse length of 25 ns full width at halfmaximum. A number of pulses from 1 to 250 of equal energy density of 0.25 J/cm2 for Ag-implanted samples or 0.21 J/cm2 for Cu-implanted samples were accumulated in the same area on the surface at a repetition rate of 1 Hz. The laser treatment has been done in ambient atmosphere at room temperature. Depth analyses for some samples were made by Rutherford backscattering spectrometry (RBS) with an analysing beam of 4 He with an energy of 2 MeV, as described elsewhere [8]. Optical re¯ectance spectra at normal incidence of the light were recorded with a Monolight ®bre-optical system in the range from 350 to 800 nm at room temperature. 3. Results and discussion 3.1. Laser annealing of Ag-implanted insulators The re¯ectance spectra for Ag-implanted silica and SLSG for dierent numbers of pulses are shown in Figs. 1 and 2, respectively. All spectra of implanted samples as well as annealed ones are characterized by selective plasmon bands determined by metal particles. As seen from the ®gures, the laser treatment initiates a decrease of re¯ectance intensity from 28% to 12%. However, for silica samples such a reduction in intensity is reached after 100 pulses. In SLSG this intensity level is reached after only 50 pulses. The fall in re¯ectance following laser treatment in SLSG is consistent with previous results for the case of the lower pulse ¯uence of Ag-implanted SLSG [6±8]. It was suggested there that either the metal nanoparticles were converted into atoms, or the large particles were separated into such small units that
884
A.L. Stepanov et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 882±886
they no longer interact as particles. The salient feature of the present data corresponding to Agimplanted SLSG (Fig. 2) is a remarkable increase of re¯ectance when more than 50 pulses are applied. After about 100 pulses, where the re¯ectance reached a maximum value, the intensities decrease again.
Fig. 1. Re¯ectance of Ag-implanted silica for a dose of 7 1016 ion/cm2 treated by various numbers of excimer laser pulses.
Fig. 2. Re¯ectance of Ag-implanted soda-lime silicate glass for a dose of 7 1016 ion/cm2 treated by various numbers of excimer laser pulses.
RBS data show that laser treatment modi®es the silver pro®le in the glass by lowering of the local concentration in the maximum and by inward diusion in proportion to the number of pulses. Measurable changes of pro®les appear only if more than 50 pulses are employed. For a fewer number of laser pulses the RBS curves are very similar and symmetrical. As an example, in Fig. 3 the RBS spectra for the cases of the Ag-implanted sample and ones corresponding to two anneals with 50 and 100 pulses are presented. Exciting silver migration inward in the sample during annealing shows that there is heating of the treated sample surface, and as a consequence, the diusional mobility of metal atoms in the glass is increased. This laser heating eect is more obvious for the lower melting point SLSG than for silica where for similar laser pulses there is a negligible change in the RBS depth pro®le. Whilst both materials have similar thermal conductivity and speci®c heats at room temperature there are signi®cant dierences once the SLSG reaches a temperature for softening or melting. Hence above about 700°C metallic ions will diuse more readily in SLSG than in silica. Dierences between the RBS pro®les for the pulsed laser heated metal in the two materials is expected since the laser light of 5 eV photons (248 nm) is totally absorbed in the surface layers of SLSG, but not the silica. SLSG therefore softens or melts [8]. As was estimated earlier [7], the
Fig. 3. The RBS data for the Ag-implanted soda-lime silicate glass for a dose of 7 1016 ion/cm2 and after laser treatment.
A.L. Stepanov et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 882±886
temperature rise of the surface-treated glass is enough for melting and dissociation of metal nanoparticles, and that leads to decreasing re¯ectance, corresponding to annealing with the ®rst 50 pulses as seen in Fig. 2. The use of nanosecond scale pulses was in principle to minimize the role of long-range diusion, because the heated material undergoes rapid cooling and freezes in a metastable phase. For frequent high energy density laser pulses, the average bulk temperature slowly rises, reducing the cooling eect. After some 50 pulses there is evidence for a diusion tail as seen from the RBS data (Fig. 3). Thus the continuing heating of the glass with high concentration of silver atoms may lead to a new nucleation stage, and this allows regrowth of metal particles, that immediately develops into the increased re¯ectance (Fig. 2). Note the size and depth distributions are altered. After many laser treatments (>150) the fast metal diusion has lowered the silver concentration below the critical nucleation concentration and so prevents further particle nucleation. On the other hand, for silica whose band gap is larger than the spectral energy of the light, there is only weak absorption of energy by impurities, or defect sites (except for the metal nanoparticles) or by multiphoton processes [10]. Continued heating of a Ag-implanted sample is related to the number of pulses, and might stimulate some migration of metal atoms towards the bulk. However, since the RBS data for silica is relatively insensitive to the reduction of optical re¯ectance (Fig. 1), then it is possible to conclude that excimer-laser pulse heating is eective at dissolving silver nanoparticles into atoms, but without melting the silica surface. 3.2. Laser annealing of Cu-implanted insulators Results on the laser annealing of insulators containing Cu-implanted nanoparticles has not been published so far, and so the present study gives the evaluation of optical re¯ectance of such composites treated with an energy density of 0.21 J/cm2 , which is close to the typical values used in research so far [4,6±8]. Re¯ectances of Cu-implanted silica and SLSG for various numbers of laser pulses are presented in Figs. 4 and 5, re-
885
Fig. 4. Re¯ectance of Cu-implanted silica for a dose of 4 1016 ion/cm2 treated by various numbers of excimer laser pulses.
Fig. 5. Re¯ectance of Cu-implanted soda-lime silica glass for a dose of 4 1016 ion/cm2 treated by various numbers of excimer laser pulses.
spectively. The main characteristic of these anneals is that the intensity of re¯ectance is increasing, when the number of pulses increases (from 1 until 5±10 in the present case), similar to the rise in re¯ectance in Ag-implanted SLSG (Fig. 2). Continued laser treatment with more pulses leads to a decreasing re¯ectance.
886
A.L. Stepanov et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 882±886
Again, as in Ag-implanted samples, there is a dierence in laser annealing for silica and SLSG substrates. The re¯ectance reduction accrues more quickly (for a smaller number of pulses) in the silica compared with the SLSG. In the silica glass there is a remarkable fall of intensity after only one laser pulse. To correctly describe the in¯uence of type of the substrate on re¯ectance changes during laser treatment requires answers to several questions. One needs to know the role of non-uniform absorption of laser light, the metal distribution after implantation, the diusion coecient of metal in various materials and so on. All these factors have to be taken into account, and cannot be evaluated in this article. However, note that the observed eect of increasing re¯ectance, via increases with the number of pulses, as a result of regrowth of nanoparticles, must not be considered as speci®c to copper. It is probably a typical behavior of metal nanoparticles in insulators under high-power laser treatment. Although this eect was demonstrated using only an excimer laser, the use of the transparent silica matrix in these experiments allows the possibility of annealing with Nd:YAG and others lasers. 4. Conclusion We have shown that there is a new possibility for modi®cation of the metal nanoparticles situated in the insulator matrix using frequent high power laser pulses. As was shown in the present paper, the implanted nanoparticles may be transformed by their dissolution to smaller ones and to separate metal atoms with subsequent regrowth into new particles. In principle they may have
dierent size distributions and modi®ed properties. The details of the processes are highly complex and involve many variables, hence the method requires further study and optimisation, particularly for applications. Acknowledgements We are grateful to the Royal Society Ex-Quota Visit Programme for ®nancial support of Dr. A. Stepanov at University of Sussex (UK), also to the Brite Euram contract BE 4427 (AMENIDAD) and the Russian Foundation for Basic Research 99-02-17767. References [1] P.D. Townsend, P.J. Chandler, L. Zhang, Optical Eects of Ion Implantation, Cambridge University Press, Cambridge, 1994. [2] R.F. Haglund Jr., Mater. Sci. Eng. A 253 (1998) 275. [3] H. Hosono, Y. Abe, N. Matsunami, Appl. Phys. Lett. 60 (1992) 2613. [4] F. Gonella, G. Mattei, P. Mazzoldi, E. Cattaruzza, G.W. Arnold, G. Bertoncello, R.F. Haglund Jr., Appl. Phys. Lett. 69 (1996) 3101. [5] A.A. Bukharaev, A.V. Kazakov, R.A. Manapov, I.B. Khaibullin, Sov. Phys. Solid State 33 (1991) 578. [6] R.A. Wood, P.D. Townsend, N.D. Skelland, D.E. Hole, J. Barton Afonso, C.N. Afonso, J. Appl. Phys. 74 (1993) 5754. [7] A.L. Stepanov, D.E. Hole, P.D. Townsend, Nucl. Instr. and Meth. B 149 (1999) 89. [8] A.L. Stepanov, D.E. Hole, A.A. Bukharaev, P.D. Townsend, N.I. Nurgazizov, Appl. Surf. Sci. 136 (1998) 298. [9] S.Y. Park, T. Isobe, M. Senna, R.A. Weeks, R.A. Zuhr, Appl. Phys. Lett. 73 (1998) 2687. [10] K. Arai, H. Imai, H. Hosono, Y. Abe, H. Imagawa, Appl. Phys. Lett. 53 (1988) 1891.
www.elsevier.nl/locate/nimb
Excimer laser annealing of glasses containing implanted metal nanoparticles A.L. Stepanov b
a,b
, D.E. Hole
a,*
, P.D. Townsend
a
a School of Engineering, University of Sussex, Pevensey Building, Brighton, BN1 9QH, UK Laboratory of Radiation Physics, Kazan Physical-Technical Institute, Russian Academy of Sciences, Sibirsky Trakt 10/7, 420029, Kazan, Russia
Abstract Silver and copper nanoparticles have been synthesized by ion implantation in silica and soda-lime silicate glass at 60 keV to a dose of 4 1016 ion/cm2 and at 50 keV to a dose of 8 1016 ion/cm2 , respectively. The glasses were annealed using pulses of a high-power KrF excimer laser (248 nm) in ambient atmosphere. This employed a single (25 ns) pulse ¯uence of 0.25 J/cm2 for Ag-implanted samples or of 0.21 J/cm2 for Cu-implanted ones. Several pulses from 1 to 250 of the same energy density at a frequency of 1 Hz were accumulated in the same area on the surface. The formation and modi®cation of metal nanoparticle was assessed via optical re¯ectance, combined with Rutherford backscattering analysis. Generally, changes induced by laser pulses suggest there are both reductions of the nanoparticles and some longer-range diusion of metal atoms into the glass. However, before the total dissolution of metal nanoparticles is accrued, the substrate temperature, increasing during many-pulse treatments, initiates the regrowth of new metal nanoparticles that leads to a rise of the re¯ectance. These results are discussed on the basis of a surface substrate melting. This work is part of an attempt to gain control over the size and depth distribution of such metal nanoparticles. Ó 2000 Published by Elsevier Science B.V. All rights reserved. PACS: 78.66.V; 78.66.J; 61.80.B; 81.05.Y; 78.66 Keywords: Ion implantation; Nanoparticles; Laser annealing; Optical re¯ectance; Rutherford backscattering
1. Introduction Despite the fact that ion implantation, as an approach for synthesis of metal nanoparticles in insulators with high values of the ®lling factor, has a real perspective for fabrication of the opto-elec-
*
Corresponding author. Tel.: +44-0-1273-678193; fax: +440-1273-678193. E-mail address: [email protected] (D.E. Hole).
tronic devices [1,2], there is some inconvenience in this method. One of the problems is a non-uniform statistical penetration of accelerated ions into the substrate that leads to the growth of metal particles with a very wide size distribution in the plane and in the depth of the irradiated glass surface [1,3]. Many optical properties, such as linear absorption and re¯ectance [1] and/or non-linear response [2], are size dependent, consequently for any applications, variations in size degrade the practical performance. One interesting way to
0168-583X/00/$ - see front matter Ó 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 7 3 1 - 4
A.L. Stepanov et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 882±886
modify the size distribution is the laser annealing of the samples after ion implantation. As an example, it is possible to mention experiments with the Nd:YAG laser treatment on reduction of the size of silver clusters embedded into the soda-lime glass [4] or the changes of the internal crystallographic structure of implanted iron particles in silica after ruby laser irradiation [5]. The feature of these experiments is that the laser light was employed directly into the spectral region of the transparency of the dielectric matrix, and hence, the intense laser pulses were primarily absorbed by the metal particles. Contrary to that, some years ago [6] a new approach for annealing was demonstrated, when ¯oat glass with implanted silver particles was irradiated by excimer ArF (193 nm) laser light at wavelengths of glass absorption in the ultraviolet region. When applying high-power laser pulses, a decrease of the re¯ectance intensity of modi®ed silver nanoparticles within the glass was observed. This new technique gives a variety of possibilities for controlled change of the size and the size distribution of nanoparticles in dielectrics, as demonstrated for the case of the combined excimer KrF (248 nm) laser and thermal annealing of ¯oat glass implanted with silver [7,8]. It was also shown, recently [9], that the excimer laser pulses, applied to silica glass containing implanted bismuth nanoclusters, removes bismuth from the surface by evaporation, similar to the cases of silver implanted glass [8]. All previous anneals with excimer laser were only carried out with pulses of the energy density lower than 0.2 J/cm2 , though the excimer laser gives the possibility for use of more powerful treatment of materials. The present study concentrates on the experiments with KrF laser annealing employing with a high level of energy density per pulse to evaluate the modi®cation of the metal implanted glasses. 2. Experimental The implantation of soda-lime silicate glass (SLSG) and silica pieces has been performed with Ag ions at 60 keV and to a dose of 4 1016 ion/ cm2 or Cu ions at 50 keV and to a dose of 8 1016 ion/cm2 at a beam current density of 10
883
lA/cm2 using a Whickham ion implanter. The implantation was carried out in a controlled vacuum of 10ÿ5 Torr at room temperature. The approximate chemical composition of the glass (from Societa Italiana Vetro) is 70% SiO2 , 20% Na2 O and 10% CaO. Silica is produced by Heraeus QUARZGLAS Company. Pulsed laser treatment has been made by a KrF excimer laser (ATLEX 210) with a wavelength of 248 nm and pulse length of 25 ns full width at halfmaximum. A number of pulses from 1 to 250 of equal energy density of 0.25 J/cm2 for Ag-implanted samples or 0.21 J/cm2 for Cu-implanted samples were accumulated in the same area on the surface at a repetition rate of 1 Hz. The laser treatment has been done in ambient atmosphere at room temperature. Depth analyses for some samples were made by Rutherford backscattering spectrometry (RBS) with an analysing beam of 4 He with an energy of 2 MeV, as described elsewhere [8]. Optical re¯ectance spectra at normal incidence of the light were recorded with a Monolight ®bre-optical system in the range from 350 to 800 nm at room temperature. 3. Results and discussion 3.1. Laser annealing of Ag-implanted insulators The re¯ectance spectra for Ag-implanted silica and SLSG for dierent numbers of pulses are shown in Figs. 1 and 2, respectively. All spectra of implanted samples as well as annealed ones are characterized by selective plasmon bands determined by metal particles. As seen from the ®gures, the laser treatment initiates a decrease of re¯ectance intensity from 28% to 12%. However, for silica samples such a reduction in intensity is reached after 100 pulses. In SLSG this intensity level is reached after only 50 pulses. The fall in re¯ectance following laser treatment in SLSG is consistent with previous results for the case of the lower pulse ¯uence of Ag-implanted SLSG [6±8]. It was suggested there that either the metal nanoparticles were converted into atoms, or the large particles were separated into such small units that
884
A.L. Stepanov et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 882±886
they no longer interact as particles. The salient feature of the present data corresponding to Agimplanted SLSG (Fig. 2) is a remarkable increase of re¯ectance when more than 50 pulses are applied. After about 100 pulses, where the re¯ectance reached a maximum value, the intensities decrease again.
Fig. 1. Re¯ectance of Ag-implanted silica for a dose of 7 1016 ion/cm2 treated by various numbers of excimer laser pulses.
Fig. 2. Re¯ectance of Ag-implanted soda-lime silicate glass for a dose of 7 1016 ion/cm2 treated by various numbers of excimer laser pulses.
RBS data show that laser treatment modi®es the silver pro®le in the glass by lowering of the local concentration in the maximum and by inward diusion in proportion to the number of pulses. Measurable changes of pro®les appear only if more than 50 pulses are employed. For a fewer number of laser pulses the RBS curves are very similar and symmetrical. As an example, in Fig. 3 the RBS spectra for the cases of the Ag-implanted sample and ones corresponding to two anneals with 50 and 100 pulses are presented. Exciting silver migration inward in the sample during annealing shows that there is heating of the treated sample surface, and as a consequence, the diusional mobility of metal atoms in the glass is increased. This laser heating eect is more obvious for the lower melting point SLSG than for silica where for similar laser pulses there is a negligible change in the RBS depth pro®le. Whilst both materials have similar thermal conductivity and speci®c heats at room temperature there are signi®cant dierences once the SLSG reaches a temperature for softening or melting. Hence above about 700°C metallic ions will diuse more readily in SLSG than in silica. Dierences between the RBS pro®les for the pulsed laser heated metal in the two materials is expected since the laser light of 5 eV photons (248 nm) is totally absorbed in the surface layers of SLSG, but not the silica. SLSG therefore softens or melts [8]. As was estimated earlier [7], the
Fig. 3. The RBS data for the Ag-implanted soda-lime silicate glass for a dose of 7 1016 ion/cm2 and after laser treatment.
A.L. Stepanov et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 882±886
temperature rise of the surface-treated glass is enough for melting and dissociation of metal nanoparticles, and that leads to decreasing re¯ectance, corresponding to annealing with the ®rst 50 pulses as seen in Fig. 2. The use of nanosecond scale pulses was in principle to minimize the role of long-range diusion, because the heated material undergoes rapid cooling and freezes in a metastable phase. For frequent high energy density laser pulses, the average bulk temperature slowly rises, reducing the cooling eect. After some 50 pulses there is evidence for a diusion tail as seen from the RBS data (Fig. 3). Thus the continuing heating of the glass with high concentration of silver atoms may lead to a new nucleation stage, and this allows regrowth of metal particles, that immediately develops into the increased re¯ectance (Fig. 2). Note the size and depth distributions are altered. After many laser treatments (>150) the fast metal diusion has lowered the silver concentration below the critical nucleation concentration and so prevents further particle nucleation. On the other hand, for silica whose band gap is larger than the spectral energy of the light, there is only weak absorption of energy by impurities, or defect sites (except for the metal nanoparticles) or by multiphoton processes [10]. Continued heating of a Ag-implanted sample is related to the number of pulses, and might stimulate some migration of metal atoms towards the bulk. However, since the RBS data for silica is relatively insensitive to the reduction of optical re¯ectance (Fig. 1), then it is possible to conclude that excimer-laser pulse heating is eective at dissolving silver nanoparticles into atoms, but without melting the silica surface. 3.2. Laser annealing of Cu-implanted insulators Results on the laser annealing of insulators containing Cu-implanted nanoparticles has not been published so far, and so the present study gives the evaluation of optical re¯ectance of such composites treated with an energy density of 0.21 J/cm2 , which is close to the typical values used in research so far [4,6±8]. Re¯ectances of Cu-implanted silica and SLSG for various numbers of laser pulses are presented in Figs. 4 and 5, re-
885
Fig. 4. Re¯ectance of Cu-implanted silica for a dose of 4 1016 ion/cm2 treated by various numbers of excimer laser pulses.
Fig. 5. Re¯ectance of Cu-implanted soda-lime silica glass for a dose of 4 1016 ion/cm2 treated by various numbers of excimer laser pulses.
spectively. The main characteristic of these anneals is that the intensity of re¯ectance is increasing, when the number of pulses increases (from 1 until 5±10 in the present case), similar to the rise in re¯ectance in Ag-implanted SLSG (Fig. 2). Continued laser treatment with more pulses leads to a decreasing re¯ectance.
886
A.L. Stepanov et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 882±886
Again, as in Ag-implanted samples, there is a dierence in laser annealing for silica and SLSG substrates. The re¯ectance reduction accrues more quickly (for a smaller number of pulses) in the silica compared with the SLSG. In the silica glass there is a remarkable fall of intensity after only one laser pulse. To correctly describe the in¯uence of type of the substrate on re¯ectance changes during laser treatment requires answers to several questions. One needs to know the role of non-uniform absorption of laser light, the metal distribution after implantation, the diusion coecient of metal in various materials and so on. All these factors have to be taken into account, and cannot be evaluated in this article. However, note that the observed eect of increasing re¯ectance, via increases with the number of pulses, as a result of regrowth of nanoparticles, must not be considered as speci®c to copper. It is probably a typical behavior of metal nanoparticles in insulators under high-power laser treatment. Although this eect was demonstrated using only an excimer laser, the use of the transparent silica matrix in these experiments allows the possibility of annealing with Nd:YAG and others lasers. 4. Conclusion We have shown that there is a new possibility for modi®cation of the metal nanoparticles situated in the insulator matrix using frequent high power laser pulses. As was shown in the present paper, the implanted nanoparticles may be transformed by their dissolution to smaller ones and to separate metal atoms with subsequent regrowth into new particles. In principle they may have
dierent size distributions and modi®ed properties. The details of the processes are highly complex and involve many variables, hence the method requires further study and optimisation, particularly for applications. Acknowledgements We are grateful to the Royal Society Ex-Quota Visit Programme for ®nancial support of Dr. A. Stepanov at University of Sussex (UK), also to the Brite Euram contract BE 4427 (AMENIDAD) and the Russian Foundation for Basic Research 99-02-17767. References [1] P.D. Townsend, P.J. Chandler, L. Zhang, Optical Eects of Ion Implantation, Cambridge University Press, Cambridge, 1994. [2] R.F. Haglund Jr., Mater. Sci. Eng. A 253 (1998) 275. [3] H. Hosono, Y. Abe, N. Matsunami, Appl. Phys. Lett. 60 (1992) 2613. [4] F. Gonella, G. Mattei, P. Mazzoldi, E. Cattaruzza, G.W. Arnold, G. Bertoncello, R.F. Haglund Jr., Appl. Phys. Lett. 69 (1996) 3101. [5] A.A. Bukharaev, A.V. Kazakov, R.A. Manapov, I.B. Khaibullin, Sov. Phys. Solid State 33 (1991) 578. [6] R.A. Wood, P.D. Townsend, N.D. Skelland, D.E. Hole, J. Barton Afonso, C.N. Afonso, J. Appl. Phys. 74 (1993) 5754. [7] A.L. Stepanov, D.E. Hole, P.D. Townsend, Nucl. Instr. and Meth. B 149 (1999) 89. [8] A.L. Stepanov, D.E. Hole, A.A. Bukharaev, P.D. Townsend, N.I. Nurgazizov, Appl. Surf. Sci. 136 (1998) 298. [9] S.Y. Park, T. Isobe, M. Senna, R.A. Weeks, R.A. Zuhr, Appl. Phys. Lett. 73 (1998) 2687. [10] K. Arai, H. Imai, H. Hosono, Y. Abe, H. Imagawa, Appl. Phys. Lett. 53 (1988) 1891.