Formation of Metallic Nanoparticles in Silicate Glass through Ion Implantation Stepanov, Andrei; Popok, Vladimir; Hole, D.E.

Transcription

1 Aalborg Universitet Formation of Metallic Nanoparticles in Silicate Glass through Ion Implantation Stepanov, Andrei; Popok, Vladimir; Hole, D.E. Published in: Glass Physics and Chemistry Publication date: 2002 Document Version Publisher's PDF, also known as Version of record Link to publication from Aalborg University Citation for published version (APA): Stepanov, A., Popok, V., & Hole, D. E. (2002). Formation of Metallic Nanoparticles in Silicate Glass through Ion Implantation. Glass Physics and Chemistry, 28(2), General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.? Users may download and print one copy of any publication from the public portal for the purpose of private study or research.? You may not further distribute the material or use it for any profit-making activity or commercial gain? You may freely distribute the URL identifying the publication in the public portal? Take down policy If you believe that this document breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from vbn.aau.dk on: januar 08, 209

2 Glass Physics and Chemistry, Vol. 28, No. 2, 2002, pp Original Russian Text Copyright 2002 by Fizika i Khimiya Stekla, Stepanov, Popok, Hole. Formation of Metallic Nanoparticles in Silicate Glass through Ion Implantation A. L. Stepanov*, V. N. Popok**, and D. E. Hole*** * I Physikalisches Institut, Aachen Technical University, Aachen, Germany ** Göteborg University and Chalmers University of Technology, Göteborg, 4296 Sweden *** University of Sussex, Brighton BN 9QH, United Kingdom Received September 25, 2000; in final form, November 9, 200 Abstract The composite layers based on sodium calcium silicate glass that contains metallic nanoparticles synthesized by implantation with Ag + ions are studied. The ion implantation conditions are as follows: the energy is 60 kev, the doses are ( ) 0 6 ions/cm 2, and the temperatures of the irradiated glass are 20, 35, 50, and 60 C. The data on the distribution of implanted silver and the nucleation and growth of metallic nanoparticles over the depth as functions of the temperature and dose are obtained using the Rutherford backscattering technique and the reflectance optical spectra. It is demonstrated that small changes in the temperature of the irradiated glass substrate lead to considerable differences in the specific features of the nanoparticle formation in the bulk of samples. At implantation temperatures of 50 and 60 C, the reflectance spectrum contains overlapping bands with two selective maxima. This suggests the possible formation of two layers that are located at different depths and involve nanoparticles differing in size, whereas one layer composed of nanoparticles is formed at 20 C. INTRODUCTION It is well known that ion implantation is an efficient and powerful technique for incorporating impurities into a solid, specifically for producing composite materials based on dielectrics containing metallic nanoparticles []. These metal dielectric composite materials are of considerable practical importance for fabricating nonlinear optical combined optoelectronic devices and magnetic storage elements [2 5]. The main advantages of the ion implantation in the synthesis of new materials over sol gel technology, ion exchange, fusing of a glass and a metal, and other techniques are as follows: the ion implantation makes it possible to fill an implanted layer with atoms of virtually any metal in excess of the equilibrium solubility limit in an irradiated matrix and to exert strict control over both the concentration of the introduced impurity and the spatial location of a doping ion beam on the sample surface. However, despite obvious prospects, the synthesis of metallic nanoparticles in a glass matrix by ion implantation is a complex process and depends on a large number of controlling factors. The influence of the ion implantation on the structure and composition of glasses manifests itself in a number of physical and chemical processes and phenomena [6]. The occurrence of two radically different mechanisms of energy transfer from implanted ions to the glass through the excitation of electron shells (ionization) and nuclear collisions plays an essential role. One of the main features of ion implantation is a statistically nonuniform penetration depth of incorporated ions into a material and, as a consequence, different concentrations of impurity atoms from layer to layer in the surface region of the sample. This leads not only to the size distribution of synthesized nanoparticles in the plane parallel to the irradiated surface but also to a substantial size distribution of particles over the target depth, which has a decisive effect on the optical and magnetic characteristics of metal glass composite materials produced by implantation [, 4, 5, 7, 8]. The conditions of preparing metallic nanoparticles can be varied depending on the ion implantation parameters such as the ion energy, dose, ion current, and temperature of the irradiated target. In our earlier work [9], we theoretically proved that the irradiation temperature is a governing factor that is responsible for the distribution of implanted metal over the depth. Unfortunately, this factor is often ignored in performing experiments. As a result, the properties of materials synthesized are characterized by a low reproducibility. It was shown that an increase in the substrate temperature by only a few tens of degrees leads to the thermostimulated diffusion spread of impurities over the sample depth. This complicates the synthesis of metallic nanoparticles uniform in size due to a decrease in the concentration of metal atoms in local glass regions. The present paper reports the results of our experimental investigations carried out with the aim of verifying the model concepts and revealing the actual behavior of implanted impurities with a variation of the temperature in a narrow range /02/ $ MAIK Nauka /Interperiodica

3 FORMATION OF METALLIC NANOPARTICLES 9 EXPERIMENTAL TECHNIQUE Sodium calcium silicate glass (Societa Italiana Vetro) of chemical composition (mol %) 70SiO 2 20Na 2 O 0CaO with an optical transparency of ~90% in the spectral range nm was used as a substrate for the composite material. Samples were prepared in the form of 3.-mm-thick glass plates 3 3 cm 2 in size. The implantation was performed on a Whickham implanter with 07 Ag + ions at an energy of 60 kev with doses of 2 0 6, 3 0 6, and ions/cm 2 and an ion current density of 0 µa/cm 2 under vacuum (the residual pressure was 0 5 Torr). In order to achieve a uniform irradiation, the samples were scanned at a mean ion current no higher than 3 µa/cm 2. The glass substrates were cemented using a heat-conducting paint to a massive metallic target whose temperature was monitored and regulated with a system consisting of a resistance heater and a gas cooler. The substrate temperature upon implantation varied from experiment to experiment and was equal to 20, 35, 50, and 60 C. The distribution of silver atoms over the depth in the glass bulk was determined using the Rutherford backscattering technique with 4 He + ions (.89 MeV). The 4 He + ions were generated with the use of a van de Graaf electrostatic accelerator according to the procedure described earlier in [0]. The reflectance optical spectra were recorded at room temperature with a Monolight single-beam fiber waveguide instrument in the range nm at normal light incidence to the sample surface. RESULTS AND DISCUSSION As was noted above, an excess in the concentration of silver atoms in the glass above the equilibrium solubility limit leads to the nucleation and growth of metallic nanoparticles. Nuclear collisions that are accompanied by the displacement of target atoms from their positions and the breaking of chemical bonds in the structure of sodium calcium silicate glass are predominant in our case of ion implantation with heavy Ag + ions at low energies. The implantation is also attended by the effective loss of electrons by target atoms, with a consequent deionization of the Ag + ions implanted in the glass matrix and the formation of silver atoms Ag 0 with the neutral charge. Silver can either form chemical bonds with arising radicals and ions of the glass or participate in oxidation processes. However, owing to the difference in the Gibbs free energies, the Ag Ag bonds are predominantly formed. This results in the formation of clusters that consist of several Ag atoms and subsequently serve as nuclei of metallic nanoparticles. According to Gonella and Mazzoldi [], although the free energy of silver oxide ( 2.68 kcal/mol at 25 C) is somewhat lower than that of metallic silver (~0 kcal/mol), the free energy of formation of SiO 2 (~ 200 kcal/mol) appears to be even lower and the bonding of oxygen by silicon prevails. Even if shortterm Ag O bonds are formed, they tend to dissociate with the formation of the Si O and Ag Ag bonds in order to decrease the total energy of the system. At the same time, the formed silver particles, in principle, can be separated from the surrounding glass by a thin layer of silver oxide or have Ag O bonds at the surface [7]. Let us assume that the nucleation and growth of nanoparticles in the course of the ion implantation result from the sequential addition of implanted silver atoms Ag 0. In this case, the growth of metallic nanoparticles is determined by both the diffusion coefficient and the local concentration of silver atoms. At a relatively low mobility of Ag 0 silver atoms in the glass matrix, nanoparticles predominantly grow at the expense of newly implanted Ag + ions (the so-called particle growth under conditions of limited diffusion [2]). Since the absolute concentration of metal ions in the implanted layer increases according to the distribution of impurities over the depth as a function of ion implantation time (or the dose accumulation), the nucleation and growth of nanoparticles also depend on the time. Moreover, it is evident that the sizes of metallic particles formed at different depths turn out to be proportional to the metal filling factor at the same depth; i.e., they are determined by the concentration profile of implanted ions. Therefore, according to the calculated Gaussian distributions of silver ions in the glass upon ion implantation at doses of ~0 6 ions/cm 2 [3], it can be assumed that, at higher ion implantation doses, the large-sized nanoparticles are formed at depths corresponding to the maxima of the calculated distributions, whereas the formation of small-sized nanoparticles occurs deep into the sample and toward the surface in accordance with the descending tails of the Gaussian curve. Upon high-dose implantation with heavy ions, the sputtering of atoms at the surface of dielectric materials can lead to the fact that the maximum in the distribution of impurities over the depth and, hence, the depth of the formation of large-sized nanoparticles shift toward the sample surface [4]. This size distribution of metallic nanoparticles over the sample depth has been confirmed by electron microscopic observations of cross sections of implanted glasses [5, 6]. On the other hand, in the case of surface sputtering, the projective range of implanted ions at each subsequent implantation stage appears to be somewhat larger than that at the previous stage. Consequently, the nucleation of nanoparticles at a certain depth, i.e., the accumulation of the necessary concentration of metal atoms, depends on the irradiation time and the sputtering efficiency. In practice, the optical properties of glasses with implanted metallic particles are usually characterized by the optical absorbance or reflectance measured in the visible range [, 8]. The intensity and location of optical selective bands at a maximum for noble-metal

4 92 STEPANOV et al. Reflectance, % C 28 (c) C 35 C 20 C C 60 C (b) C 35 C C 50 C (a) C, 35 C Wavelength, nm Fig.. Reflectance optical spectra of sodium calcium silicate glass () prior to and (2) after implantation with Ag + ions at different substrate temperatures. Implantation dose (ions/cm 2 ): (a) 2 0 6, (b) 3 0 6, and (c) nanoparticles are determined by the plasma resonance effects and depend on the concentration and the size of particles. In the case of spherical particles and their low concentration, the spectral location of bands can be successfully predicted in terms of the Mie electromagnetic theory [7]. In particular, this theory makes it possible to determine quantitatively the mean size of particles when they are uniformly distributed over the sample bulk and have a narrow size distribution. However, a sufficiently broad size distribution of nanoparticles both in the sample plane and over the depth is typical of ion-implanted materials. This results in a superposition of the spectra of particles differing in size and, as a consequence, complicates the estimation of the mean size in the framework of the Mie theory [8]. On the other hand, the spectra of metallic nanoparticles can be analyzed in terms of the effective-medium theory [8], which takes into account the shift in the plasma resonance spectrum toward the long-wavelength range with an increase in the metal content in the sample. However, all these approaches are valid only when nanoparticles are uniformly distributed over the sample bulk. As was shown earlier in [8], the spectra can exhibit an opposite behavior (shift) in the case of a nonuniform distribution of nanoparticles over the size and depth in the sample. For implanted materials, it is necessary to take into consideration that the size distribution of particles as a function of the depth correlates with the con-

5 FORMATION OF METALLIC NANOPARTICLES 93 centration profile of the impurity; hence, the change in the optical spectra should be compared with the change in the concentration profiles. The reflectance spectrum of sodium calcium silicate glass prior to implantation is depicted in Fig.. This figure also shows the experimental optical spectra at different doses and, hence, at different stages of the nanoparticle formation upon Ag + ion implantation at different temperatures of the irradiated substrate (20, 35, 50, and 60 C). As is known, the broad selective reflection bands observed in the visible range are due to the formation of silver nanoparticles in the glass and are associated with the plasma resonance [8, 8]. The nucleation of small-sized silver particles begins at a dose of ions/cm 2, as is evidenced by less intense reflection bands with a maxima near 450 nm (Fig. a). It should be noted that the concentration of silver implanted at the given dose only slightly exceeds the solubility limit in sodium calcium silicate glass, and, hence, this dose corresponds to the initial stage of the nanoparticle formation. As follows from our earlier works [3, 4], the sputtering of the glass surface is insignificant at the given dose. Therefore, the size distribution of nanoparticles over the depth is represented by a typical Gaussian profile of the concentration of implanted ions (Fig. 2). In this case, the large-sized particles grow at the depth corresponding to the maximum of the profile, i.e., to the mean projective range of Ag + ions, whereas smaller-sized metallic fractions are formed at depths corresponding to the Gaussian distribution tails. It is also seen from Fig. a that the reflection bands in the spectra of sodium calcium silicate glasses implanted at different temperatures virtually coincide in location. This indicates that an increase in the diffusion mobility of silver atoms due to an increase in the temperature does not considerably affect the nucleation and growth of nanoparticles at the given implantation dose; i.e., silver ions implanted in excess of the solubility limit participate in the growth of nucleated nanoparticles without noticeable diffusion in the sample bulk. An increase in the dose to ions/cm 2 (Fig. b) leads to an increase in the intensity of the reflection bands and to their spectral shift toward the long-wavelength range as compared to those of the bands observed at a dose of ions/cm 2 (Fig. a). Since the depth profiles of impurities are similar for both doses (Fig. 2), the spectral shift of the bands at a dose of ions/cm 2, according to the Mie theory, suggests an increase in the particle size. Note that this increase is maximum at a temperature of 20 C and minimum at 60 C, which indicates an increase in the fraction of small-sized nanoparticles formed at a higher substrate temperature. An increase in the intensity of the bands in the reflectance spectra is explained by the fact that an increase in the temperature leads to an increase in the mobility of silver atoms and, hence, in the probability of their incorporation into Yield, arb. units Si Ca Channel no. quasi-nuclei that coexist with nanoparticles but are not activated at low temperatures. The most considerable differences in the optical spectra of sodium calcium silicate glasses implanted at different temperatures are observed at the largest used dose equal to ions/cm 2 (Fig. c). An increase in the reflection intensity with an increase in the temperature is even more pronounced than that in the previous case. The reflectance spectra of the samples implanted at temperatures of 50 and 60 C contain at least two overlapping bands with maxima in the vicinity of 470 and 50 nm. This suggests the possibility of forming two layers that are located at different depths and involve ensembles of metallic nanoparticles differing in size [8]. As was shown earlier in [8, 9], in the material that is prepared under conditions similar to those used in the present work and contains nanoparticles with a nonuniform distribution over the depth, the layer involving larger-sized silver nanoparticles is located close to the glass surface, whereas the layer composed of smaller-sized particles is situated at a large depth. The observed spectral differences and the aforementioned nonuniform spatial distribution of silver nanoparticles cannot be explained only by an increase in the thermostimulated diffusion mobility of silver atoms in the temperature range C. It seems likely that the appearance of two maxima of two overlapping bands in the reflectance spectra of the composite materials produced by implantation of sodium calcium silicate glass at elevated temperatures should be associated with a nonuniform distribution of silver particles over the depth within a thin surface layer. The thickness of this layer can be determined by the Rutherford backscattering technique. The results obtained by this technique (Fig. 2) permit us to make the inference that the distributions of implanted impurities over the depth (thick- 3 2 Ag Fig. 2. Rutherford backscattering spectra of sodium calcium silicate glass implanted with Ag + ions at a substrate temperature of 60 C. Implantation dose (ions/cm 2 ): () 2 0 6, (2) 3 0 6, and (3)

6 94 STEPANOV et al. Vacancy concentration, arb. units Depth, nm Fig. 3. Depth profiles of Ag + ions implanted at an energy of 60 kev and vacancies formed upon implantation in sodium calcium silicate glass (calculated according to the SRIM program). ness of the impurity-containing layer) at different implantation doses are similar in shape. Unfortunately, these data do not provide information on the clustering of silver ions or a nonuniform size distribution of nanoparticles. However, the previously obtained results on the implantation of Ag + ions into SiO 2 at energies higher than 50 kev indicate a bimodal impurity distribution over the depth [20]. The first, more intense maximum in the profile corresponds to the projective range of ions in the case of a Gaussian distribution. The second maximum is located at the depth that coincides with the depth of the maximum concentration of radiation-induced defects arising in the glass matrix upon ion implantation. This suggests the radiation-induced diffusion of silver toward the surface and the nucleation of nanoparticles in this region. Therefore, the described bimodal distribution qualitatively confirms the assumption that two local layers involving ensembles of silver nanoparticles with different sizes are formed upon implantation at a dose of ions/ cm 2. In our case, two peaks in the silver distribution could not be recorded because of the depth resolution limit of the Rutherford backscattering technique. By using the SRIM-2000 program, we calculated the profile of Ag + ions implanted at an energy of 60 kev and the profile of defects (vacancies) generated upon implantation (Fig. 3). As can be seen from Fig. 3, the spacing between the maxima does not exceed 0 nm, which is less than the depth resolution limit of the Rutherford backscattering instrument used in our work. CONCLUSION The influence of the implantation temperature on the formation of silver nanoparticles in sodium calcium silicate glass was experimentally investigated at different implantation doses, i.e., at different stages of the nanoparticle growth. It was shown that, at a dose of 2 5 Ag concentration, arb. units ions/cm 2 that corresponds to the metal concentration in the implanted layer only slightly above the solubility limit of silver in the glass, the temperature does not substantially affect the optical properties of the produced composite material and, hence, the nanoparticle nucleation. An increase in the implantation dose leads to a spectral shift in the reflection bands toward the long-wavelength range, which indicates an increase in the nanoparticle size. It was found that the spectral shift is minimum at the elevated substrate temperatures. This was explained by the increase in the diffusion mobility of Ag atoms and the formation of the larger number of nuclei. It was assumed that, at implantation doses of an order of ions/cm 2 and elevated temperatures, two layers that are located at different depths and contain silver nanoparticles differing in size are formed through the radiation-induced and thermoinduced silver diffusion from the region of ion retardation to the region with a maximum concentration of radiationinduced defects. Thus, the results obtained demonstrated that even an insignificant change (by several tens of degrees) in the temperature of the implanted glass substrate considerably affects the formation of metallic nanoparticles. This fact is unexpected and was disregarded earlier in the description of the implantation processes. We believe that the temperature of local heating of the glass in a track of an implanted ion is the decisive factor in the formation of metallic nanoparticles. This temperature was estimated at several thousands of degrees. ACKNOWLEDGMENTS We would like to thank Prof. P. Townsend (University of Sussex, UK) for helpful advice regarding the performance of the experiments and suggestions in discussions of the results. A.L. Stepanov acknowledges the support of Alexander von Humboldt-Stiftung (Germany), which made possible his visit to the I Physikalisches Institut (Aachen Technical University, Germany), and the Russian Foundation for Basic Research (project nos and ). V.N. Popok acknowledges the support of the Swedish Natural Science Research Council (SNFR), Wallenberg Foundation, and the Belarussian Foundation for Basic Research. REFERENCES. Townsend, P.D., Chandler, P.J., and Zhang, L., Optical Effects of Ion Implantation, Cambridge: Cambridge Univ. Press, Stepanov, A.L., Khaibullin, I.B., Townsend, P., Hole, D., and Bukharaev, A.A, Technique of Fabricating a Nonlinear Optical Material, RF Patent /28 (006367), Maruyama, O., Senda, Y., and Omi, S., Non-Linear Optical Properties of Titanium Dioxide Films Containing

7 FORMATION OF METALLIC NANOPARTICLES 95 Dispersed Gold Particles, J. Non-Cryst. Solids, 999, vol. 259, pp Bukharaev, A.A., Kazakov, A.V., Manapov, R.A., Khaibullin, I.B., and Yafaev, N.R., Structural Features of Iron-Implanted Glass, Fiz. Khim. Stekla, 986, vol. 2, no. 3, pp Nakajima, A., Nakao, H., Futatsugi, T., and Yokoyama, N., Microstructure and Electrical Properties of Sb Nanocrystals Formed in Thin, Thermally Grown SiO 2 Layers by Low-Energy Ion Implantation, J. Vac. Sci. Technol., B, 999, vol. 4, no. 4, pp Battaglin, G., Arnold, G.W., Mattei, G., Mazzoldi, P., and Dran, J.-C., Structural Modification in Ion- Implanted Silicate Glasses, J. Appl. Phys., 999, vol. 85, no. 2, pp Borsella, E., Cattaruzza, E., De Marchi, G., Gonella, F., Mattei, G., Mazzoldi, P., Quaranta, A., Battaglin, G., and Polloni, R., Synthesis of Silver Clusters in Silica-Based Glasses for Optoelectronics Applications, Non-Cryst. Solids, 999, vol. 245, pp Stepanov, A.L., Optical Reflection from Dielectric Layers Containing Metallic Nanoparticles Formed by Ion Implantation, Opt. Spektrosk., 2000, vol. 89, no. 3, pp Stepanov, A.L., Hole, D.E., and Popok, V.N., Effect of the Temperature of Irradiated Target Surface on the Distribution of Nanoparticles Formed by Ion Implantation, Pis ma Zh. Tekh. Fiz., 200, vol. 27, no. 3, pp Stepanov, A.L., Hole, D.E., Bukharaev, A.A., Townsend, P.D., and Nurgazizov, N.I., Reduction of the Size of the Implanted Silver Nanoparticles in Float Glass during Excimer Laser Annealing, Appl. Surf. Sci., 998, vol. 36, pp Gonella, F. and Mazzoldi, P., Metal Nanocluster Composite Glasses, in Handbook of Nanostructured Materials and Nanotechnology, Nalwa, H.S., Ed., San Diego: Academic, Zettlemoyer, A.C., Nucleation, New York: Marcel Dekker, Stepanov, A.L., Zhikharev, V.A., Hole, D.E., Townsend, P.D., and Khaibullin, I.B., Depth Distribution of Cu, Ag and Au Ions Implanted at Low Energy into Insulators, Nucl. Instrum. Methods Phys. Res., 2000, vols , pp Stepanov, A.L., Zhikharev, V.A., and Khaibullin, I.B., Features in the Depth Profiles of Metal Ions Implanted into Dielectrics at Low Energies, Fiz. Tverd. Tela (St. Petersburg), 200, vol. 43, no. 4, pp Nistor, L.C., van Landuyt, J., Barton, J.B., Hole, D.E., Skelland, N.D., and Townsend, P.D., Colloid Size Distributions in Ion Implanted Glass, J. Non-Cryst. Solids, 993, vol. 62, pp Hosono, H., Importance of Implantation Sequence in the Formation of Nanometer Size Colloid Particles Embedded in Amorphous SiO 2, Phys. Rev. Lett., 995, vol. 74, pp Bohren, C.F. and Huffman, D.R., Absorption and Scattering of Light by Small Particles, New York: Wiley, Kreibig, U. and Vollmer, M., Optical Properties of Metal Clusters, Berlin: Springer-Verlag, Stepanov, A.L., Optical Transmission of Dielectric Layers Containing Metallic Nanoparticles Nonuniformly Distributed over the Sample Thickness, Opt. Spektrosk., 200, vol. 9, no. 4, pp Matsunami, N. and Hosono, H., Colloid Formation Effects on Depth Profile of Implanted Ag in SiO 2 Glass, Appl. Phys. Lett., 993, vol. 63, no. 5, pp