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Size variation and optical absorption of sol-gel Ag nanoparticles doped SiO2 thin film
Thin Solid Films 515 (2006) 771 – 774 www.elsevier.com/locate/tsf
Size variation and optical absorption of sol-gel Ag nanoparticles doped SiO2 thin film A. Babapour a,b, O. Akhavan a, A.Z. Moshfegh a,*, A.A. Hosseini b a
Department of Physics, Sharif University of Technology, P.O. Box 11365-9161, Tehran, Iran b Department of Physics, Mazandaran University, Babolsar, Iran Available online 2 February 2006
Abstract In this research, we have focused on the formation of Ag nanoparticles dispersed in SiO2 matrix using sol-gel method. The influences of the metal concentration on the size variation of Ag nanoparticles and the size effect on the surface plasmon absorption have been studied. Sol-gel silica thin films containing Ag particles were synthesized by dip-coating on soda-lime glasses. The molar ratio of Ag / Si was chosen from 0.2% to 8%. All films were dried in air at 100 -C for 1 h. Using X-ray photoelectron spectroscopy, the Ag / Si ratios in the prepared films have been measured. In addition, it was shown that the prepared matrix was a stoichiometric composition as SiO2, and the synthesized nanoparticles were mainly in the metallic state. Size and distribution of the nanoparticles were measured by high resolution scanning as well as transmission electron microscopy and also atomic force microscopy analyses for low and high Ag concentrations, respectively. We have found that by decreasing the Ag / Si ratio from 8 to 0.2 mol%, the particle size reduces from 95 to 4 nm with a nearly spherical shape. UV-visible spectrophotometry showed that the size reduction of the Ag nanoparticles for the Ag / Si molar ratios ranging from 8 to 0.2 mol% leads to an intensity reduction of the absorption peak and a blue shift from 460 to 410 nm. D 2005 Elsevier B.V. All rights reserved. PACS: 78.67.Bf Keywords: Nanoparticles; Silver; Sol-gel; Silica matrix
1. Introduction Formation of metal nanoparticles dispersed in solid dielectric materials, which can result in novel optical properties, has been of increasing interest due to their potential applications in nonlinear optics [1,2]. Optical properties of the nanoparticles depending on the surrounding medium have special worth [3]. Recently, various techniques such as laser ablation, sputtering, and sol-gel methods have been used to fabricate such composite thin films (especially transparent matrix thin films containing nanoparticles). Among them, the sol-gel synthesis is one of the most useful methods for preparation of metallic nanoparticles in an oxide transparent matrix such as SiO2 or TiO2. Meantime, different metal particles such as gold [4 –7], copper [8,9], platinum [10,11],
* Corresponding author. Fax: +98 21 6601 2983. E-mail address: [email protected] (A.Z. Moshfegh). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.191
and silver [12 – 16] were introduced in glassy matrices by the sol-gel method. It is well known that the properties of such composite films are dependent on their microstructure, e.g. size, shape, composition, and also spatial distribution of the particles in the film [17,18]. Therefore, in order to describe the relationship between the optical properties and microstructure of the composite thin films, it is necessary to obtain the composite films with well-defined microstructures. Recently, the effect of some parameters controlling nano-silver growth in silica sol-gel films has been investigated [16]. In this investigation, we report a facile and fast route for synthesis of SiO2 thin films containing Ag nanoparticles by using the sol-gel technique. The influences of the metal concentration on the size variation of Ag nanoparticles, and so, on the surface plasmon absorption of the films have been studied. Moreover, the surface concentration of nano-silver and the chemical state of the grown matrix as well as the nanoparticles located on its surface have been determined.
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A. Babapour et al. / Thin Solid Films 515 (2006) 771 – 774
2. Experimental Sol-gel silica thin films containing silver nanoparticles were synthesized by dip-coating on soda-lime glass microscope slides. The SiO2 – Ag sol was prepared using tetraethyl orthosilicate (TEOS), ethanol, distilled water, nitric acid, and silver nitrate. The solution was prepared by mixing TEOS and ethanol in equal volume. After stirring for 30 min, we added distilled water to the solution drop by drop while stirring at room temperature. Then, different amounts of AgNO3 and HNO3 (to adjust pH of the solution about 2) were added to the solution. The assumed molar ratios of TEOS / C2H5OH / H2O / AgNO3 are 1, 3.8, 4, and n, respectively. The n is nominal molar ratio of Ag / Si which was chosen as 0%, 0.2%, 0.4%, 0.6%, 0.8%, 1.2%, 1.6%, and 8%. Following preparation of the solutions containing Ag colloids, the sols were left for aging until the viscosity reached approximately the range of 2 –4 cP. The coatings were performed by 60 s dipping of the glass slides in the solution with a pulling rate of 1 mm/s. All films were dried in air at 100 -C for 1 h which lead to obtaining transparent light brown color films. Meantime, for silica films containing 0.2 and 8 mol% Ag, we have considered an additional heat-treatment at 200 -C in air for 2 h. The surface roughness and topography of the films as well as grain size and size distribution of the Ag nanoparticles on the films were studied by atomic force microscopy (AFM) in air with a silicon tip of 10 nm radius in contact method. A Philips-CM200 high resolution transmission electron microscopy (HRTEM) was used at 200 kV to determine size of the nano-silver particles and also particle size distribution for the film containing 0.2 mol% Ag which was annealed at 200 -C in air for 2 h. The surface morphology of the films was examined
by a Philips-XL30 High resolution scanning electron microscopy (HRSEM). A Jascow-V530 UV-visible spectrophotometer was used to determine the optical absorption of the films in the wavelength range of 300– 1100 nm. Moreover, using the optical method, the film thickness was measured in the range 140– 190 nm, as a typical thickness of the dried films for all the silver concentrations. X-ray photoelectron spectroscopy (XPS) equipped with an Al – Ka x-ray source was employed to study the surface atomic concentration and chemical state of the silica films doped by silver nanoparticles. All binding energy values were determined by calibration the C(1s) line to 284.6 eV. Both survey scans and individual high-resolution scans for Ag(3d), Si(2p), C(1s) and O(1s) peaks were recorded. 3. Results and discussion AFM images of the silica films containing 0, 0.8, 1.6 and 8 mol% Ag have been shown in Fig. 1. For the silica film with no Ag concentration, as our reference morphology, a uniform surface (with no special feature on it) is observed (Fig. 1a). By increasing the Ag concentration to 0.4 mol%, some particlelike features with 18 nm average size were observed on the surface (not shown here). At 0.8 mol%, AFM images show a considerable amount of spherical particles on the silica surface with diameters ranging from 10 to 50 nm and average size of about 28 nm, as can be seen in Fig. 1b. AFM images of the dried films containing higher Ag concentrations showed that surface concentration and also average size of the particles are increased by raising the Ag concentration. The average size of the particles was measured about 48 and 65 nm for 1.2 and 1.6 mol% Ag concentrations, respectively. By increasing the Ag concentration to 8 mol%, AFM images showed that the thin
Fig. 1. AFM images (1 1 Am2) of the silica sol-gel films containing different Ag concentrations: a) 0, b) 0.8, c) 1.6 and d) 8 mol%.
A. Babapour et al. / Thin Solid Films 515 (2006) 771 – 774
film contains micrometer particles on its surface. However, after heat-treatment of the thin films at 200 -C, a particle size reduction into the nanometric scale was observed. Fig. 1d shows AFM image of the SiO2 films containing 8 mol% Ag heated at 200 -C in which the average size of the Ag particles is about 95 nm. The presence of silver on the film surface and its variation by changing the Ag concentration in the sol have been also confirmed by XPS analysis (see Fig. 4). Concerning the size reduction of the Ag particles due to the heat-treatment, two main mechanisms can be considered. In fact, by increasing the temperature, the silver particles concentrated on the surface diffuse into the substrate while they get oxidized. The diffusion of the surface particles results in the average size reduction of the remained particles on the surface which was observed by AFM analysis. Moreover, the oxidation process of the silver particles reduces the size of metallic Ag particles which is observable by the plasmon absorption peak of nanoscale Ag particles (see inset of Fig. 3). These results are also consistent with the results reported in Ref. [15]. Due to the well-known tip convolution effect in AFM analysis, the silver particle sizes and distributions at the lower silver concentrations (less than 0.8 mol%) have been observed by high resolution SEM and TEM methods. Based on SEM analysis of the films containing 0.8 mol% Ag (not shown here), the average particle size of nano-silvers was the same as its value measured by AFM technique (¨30 nm). For the silica films containing 0.4 mol% Ag, Fig. 2 shows SEM image of the Ag nanoparticles located on the surface. In this case, HRSEM gave a better image to distinguish the particles concentrated on the surface, as compared with AFM images. Anyway, this confirms that the average grain size measured by AFM analysis can be reasonably attributed to the average size of the Ag particles. For the dried silica films containing 0.2 mol% Ag, SEM images showed that the shape of the Ag particles is a nanostrip form, not a semispherical form on the surface (not shown here). However, the heat-treatment of the films at 200 -C transforms the form of the particles to a semispherical one. HRTEM image of this silica film (not presented here) showed that the average size of the obtained Ag particles was about 4 nm. This once again indicates that decreasing the Ag concentration in the sol results in reduction of particle size of nano-silvers, in a suitable condition.
Fig. 2. HRSEM image of the silica film containing 0.4 mol% Ag concentration.
773
Fig. 3. Optical absorption spectra for the silica films containing different silver concentrations from zero to 8 mol% Ag.
The optical absorption spectra of the sol-gel silica films with the different silver concentrations have been shown in Fig. 3. It is seen that the absorption curve with no silver or even a low silver concentration (0.2 mol%) shows a typical fringe pattern observed in well transparent films. In fact, for the thin films with a thickness around 200 nm, there are only a couple of interference fringes. Nevertheless, the absorption curves of the films containing higher silver concentrations show a red shift for the absorption peak from 408 to 456 nm wavelength as the Ag concentration increases from 0.4 to 1.6 mol%. In addition, the films with higher silver concentrations show increased absorption intensities. To study the surface chemical state of the Ag nanoparticles, the obtained Ag / Si molar ratio on the surface, and also stoichiometry of the silica matrix, XPS analytical technique were utilized. Fig. 4 shows the Ag / Si ratio that was measured based on the area under the Ag(3d5 / 2) and Si(2p) XPS peaks as a function of the Ag concentration in the sol. We have found that for our dried silica films, the concentration of Ag on the surface is much higher than its initial value in the sol. This means that during the drying process, the Ag particles formed in the gel have a more concentration on the surface, which is
Fig. 4. The measured Ag / Si ratio on the surface and the average size of Ag particles as a function of the initial Ag concentration in the sol.
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A. Babapour et al. / Thin Solid Films 515 (2006) 771 – 774
Fig. 5. Deconvoluted XPS spectra of the silica films containing 0.8 mol% Ag for a) Ag(3d) and b) Si(2p) peaks.
also consistent with our AFM observations. Hence, it is reasonable to perform a reliable measurement on the particle size by the AFM method. Fig. 4 also shows the average size of Ag particles as a function of the initial Ag concentration in the sol. It is seen that by increasing the Ag concentration in the sol from 0.2 to 1.6 mol% the average particle size increases from 4 to 65 nm in a nearly linear behavior. To determine the chemical state of the silver nanoparticles present on the surface, XPS spectra of the 0.8 mol% Ag silica films in the range covering the Ag(3d) photoemission peaks have been studied (Fig. 5a). According to the literature [19,20], the binding energy of Ag(3d5 / 2) for Ag, Ag2O, and AgO are 368.2, 367.8, and 367.4 eV, respectively. Detailed deconvolution of Ag(3d) shows that about 93% of the Ag nanoparticles formed on the surface were in metallic Ag0 state and the other particles oxidized in the form of Ag2O(6%) and AgO(1%). Fig. 5b shows Si(2p) peak of XPS spectrum for the silica film containing 0.8 mol% Ag. This peak consists of both Si(2p3 / 2) and Si(2p1 / 2) peaks due to the spin-orbit coupling. To determine binding energy of the peaks, the Si(2p) peak was deconvoluted to two 2p3 / 2 –2p1 / 2 peaks with fixed separation energy 0.6 eV. Moreover, the ratio of the peaks area is about 2, which is consistent with the theory for splitting of 2p levels. The binding energies of the Si(2p3 / 2) and Si(2p1 / 2) of the silica film were measured as 102.7 and 103.3 eV, respectively. It is reported that the Si(2p3 / 2) core level binding energy of elemental Si is at 99.8 eV, while the binding energies of SiO2 and SiO films are at 102.7 and 102.3 eV, respectively [21]. Therefore, the deposited silica films possess the stoichiometric SiO2 structure in our study. Similar results were also obtained for the fabricated pure silica films. 4. Conclusions Silica thin films containing silver nanoparticles have been prepared by the sol-gel method. The effects of the Ag concentration on the particle size of the synthesized nanosilvers, and also, on the surface plasmon absorption peak of the films have been studied. Using AFM images, it was found that as the Ag concentration decreased from 8 to 0.4 mol%, the average particle size of nano-silvers reduced from 95 to 18 nm. The study of this trend at the lower Ag concentration was completed by using high resolution SEM and TEM which showed that the particle size reduction reaches to less than 4 nm at 0.2 mol% Ag concentration. UV-visible spectrophotometry
indicated that the observed particle size reduction led to a blue shift from 460 to 410 nm for the optical absorption peak. XPS analysis determined that the silver nanoparticles are mainly concentrated on the film surface. These results demonstrate that particle size tailoring of the nano-silver dispersed in SiO2 matrix is feasible by adjusting Ag concentration in the sol and applying a low temperature heat-treatment. Acknowledgments The authors would like to thank Research Council of Sharif University of Technology and Ministry of Science, Research, and Technology of Iran as well as Nanotechnology Cooperation Office for financial support of the project. Partial support of the Third World Academy of Science (TWAS) – Iran chapter is also appreciated. Useful discussions with R. Azimirad and assistance of L. Samie are greatly acknowledged. References [1] G. Compagnini, L. D’Urso, O. Puglisi, Sci. Eng. C 19 (2002) 295. [2] K. Naoi, Y. Ohko, T. Tatsuma, J. Am. Chem. Soc. 126 (2004) 3664. [3] G. Yang, W. Wang, Y. Zhou, H. Lu, G. Yang, Z. Chen, Appl. Phys. Lett. 81 (21) (2002) 3969. [4] Y. Hosoya, T. Suga, T. Yanagawa, Y. Kurokawa, J. Appl. Phys. 81 (1997) 1475. [5] R. Trbojevich, N. Pellegri, A. Frattini, O. de Sanctis, P.J. Morais, R.M. Almeida, J. Mater. Res. 17 (2002) 1973. [6] Y. Katayama, M. Sasaki, E. Ando, J. Non-Cryst. Solids 178 (1994) 227. [7] B. Kutsch, O. Lyon, M. Schmitt, M. Mennig, H. Schmidt, J. Non-Cryst. Solids 217 (1997) 143. [8] S. Szu, C.-Y. Lin, C.-H. Lin, J. Sol-Gel Sci. Technol. 2 (1994) 881. [9] M. Mennig, M. Schmitt, B. Kutsch, H. Schmidt, Proc. SPIE 2288 (1994) 120. [10] S. Sakka, H. Kozuka, G. Zhao, Proc. SPIE 2288 (1994) 108. [11] H. Kozuka, G. Zhao, S. Sakka, J. Sol-Gel Sci. Technol. 2 (1994) 741. [12] G. Mitrikas, C.C. Trapalis, G. Kordas, J. Non-Cryst. Solids 286 (2001) 41. [13] M. Zayat, D. Einot, R. Reisfeid, J. Sol-Gel Sci. Technol. 10 (1997) 67. [14] Z.-J. Jiang, C.-Y. Liu, Y. Liu, Appl. Surf. Sci. 233 (2004) 135. [15] W. Li, S. Seal, E. Megan, J. Ramsdell, K. Scammon, G. Lelong, L. Lachal, K.A. Richardson, J. Appl. Phys. 93 (2003) 9553. [16] M.A. Villegas, M.A. Garcı´a, S.E. Paje, J. LLopis, Mater. Res. Bull. 40 (2005) 1210. [17] Y. Wang, N. Herron, J. Phys. Chem. 95 (1991) 525. [18] M. Xu, M.J. Dignam, J. Chem. Phys. 96 (1992) 3370. [19] J.S. Hammond, S.W. Gaarenstroom, N. Winograd, Anal. Chem. 47 (1975) 2194. [20] S.W. Gaarenstroom, N. Winograd, J. Chem. Phys. 67 (1977) 3500. [21] C.D. Wagner, D.E. Passoja, H.F. Hilery, T.G. Kinisky, H.A. Six, W.T. Jansen, J.A. Taylor, J. Vac. Sci. Technol. 21 (1982) 933.
Size variation and optical absorption of sol-gel Ag nanoparticles doped SiO2 thin film A. Babapour a,b, O. Akhavan a, A.Z. Moshfegh a,*, A.A. Hosseini b a
Department of Physics, Sharif University of Technology, P.O. Box 11365-9161, Tehran, Iran b Department of Physics, Mazandaran University, Babolsar, Iran Available online 2 February 2006
Abstract In this research, we have focused on the formation of Ag nanoparticles dispersed in SiO2 matrix using sol-gel method. The influences of the metal concentration on the size variation of Ag nanoparticles and the size effect on the surface plasmon absorption have been studied. Sol-gel silica thin films containing Ag particles were synthesized by dip-coating on soda-lime glasses. The molar ratio of Ag / Si was chosen from 0.2% to 8%. All films were dried in air at 100 -C for 1 h. Using X-ray photoelectron spectroscopy, the Ag / Si ratios in the prepared films have been measured. In addition, it was shown that the prepared matrix was a stoichiometric composition as SiO2, and the synthesized nanoparticles were mainly in the metallic state. Size and distribution of the nanoparticles were measured by high resolution scanning as well as transmission electron microscopy and also atomic force microscopy analyses for low and high Ag concentrations, respectively. We have found that by decreasing the Ag / Si ratio from 8 to 0.2 mol%, the particle size reduces from 95 to 4 nm with a nearly spherical shape. UV-visible spectrophotometry showed that the size reduction of the Ag nanoparticles for the Ag / Si molar ratios ranging from 8 to 0.2 mol% leads to an intensity reduction of the absorption peak and a blue shift from 460 to 410 nm. D 2005 Elsevier B.V. All rights reserved. PACS: 78.67.Bf Keywords: Nanoparticles; Silver; Sol-gel; Silica matrix
1. Introduction Formation of metal nanoparticles dispersed in solid dielectric materials, which can result in novel optical properties, has been of increasing interest due to their potential applications in nonlinear optics [1,2]. Optical properties of the nanoparticles depending on the surrounding medium have special worth [3]. Recently, various techniques such as laser ablation, sputtering, and sol-gel methods have been used to fabricate such composite thin films (especially transparent matrix thin films containing nanoparticles). Among them, the sol-gel synthesis is one of the most useful methods for preparation of metallic nanoparticles in an oxide transparent matrix such as SiO2 or TiO2. Meantime, different metal particles such as gold [4 –7], copper [8,9], platinum [10,11],
* Corresponding author. Fax: +98 21 6601 2983. E-mail address: [email protected] (A.Z. Moshfegh). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.191
and silver [12 – 16] were introduced in glassy matrices by the sol-gel method. It is well known that the properties of such composite films are dependent on their microstructure, e.g. size, shape, composition, and also spatial distribution of the particles in the film [17,18]. Therefore, in order to describe the relationship between the optical properties and microstructure of the composite thin films, it is necessary to obtain the composite films with well-defined microstructures. Recently, the effect of some parameters controlling nano-silver growth in silica sol-gel films has been investigated [16]. In this investigation, we report a facile and fast route for synthesis of SiO2 thin films containing Ag nanoparticles by using the sol-gel technique. The influences of the metal concentration on the size variation of Ag nanoparticles, and so, on the surface plasmon absorption of the films have been studied. Moreover, the surface concentration of nano-silver and the chemical state of the grown matrix as well as the nanoparticles located on its surface have been determined.
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A. Babapour et al. / Thin Solid Films 515 (2006) 771 – 774
2. Experimental Sol-gel silica thin films containing silver nanoparticles were synthesized by dip-coating on soda-lime glass microscope slides. The SiO2 – Ag sol was prepared using tetraethyl orthosilicate (TEOS), ethanol, distilled water, nitric acid, and silver nitrate. The solution was prepared by mixing TEOS and ethanol in equal volume. After stirring for 30 min, we added distilled water to the solution drop by drop while stirring at room temperature. Then, different amounts of AgNO3 and HNO3 (to adjust pH of the solution about 2) were added to the solution. The assumed molar ratios of TEOS / C2H5OH / H2O / AgNO3 are 1, 3.8, 4, and n, respectively. The n is nominal molar ratio of Ag / Si which was chosen as 0%, 0.2%, 0.4%, 0.6%, 0.8%, 1.2%, 1.6%, and 8%. Following preparation of the solutions containing Ag colloids, the sols were left for aging until the viscosity reached approximately the range of 2 –4 cP. The coatings were performed by 60 s dipping of the glass slides in the solution with a pulling rate of 1 mm/s. All films were dried in air at 100 -C for 1 h which lead to obtaining transparent light brown color films. Meantime, for silica films containing 0.2 and 8 mol% Ag, we have considered an additional heat-treatment at 200 -C in air for 2 h. The surface roughness and topography of the films as well as grain size and size distribution of the Ag nanoparticles on the films were studied by atomic force microscopy (AFM) in air with a silicon tip of 10 nm radius in contact method. A Philips-CM200 high resolution transmission electron microscopy (HRTEM) was used at 200 kV to determine size of the nano-silver particles and also particle size distribution for the film containing 0.2 mol% Ag which was annealed at 200 -C in air for 2 h. The surface morphology of the films was examined
by a Philips-XL30 High resolution scanning electron microscopy (HRSEM). A Jascow-V530 UV-visible spectrophotometer was used to determine the optical absorption of the films in the wavelength range of 300– 1100 nm. Moreover, using the optical method, the film thickness was measured in the range 140– 190 nm, as a typical thickness of the dried films for all the silver concentrations. X-ray photoelectron spectroscopy (XPS) equipped with an Al – Ka x-ray source was employed to study the surface atomic concentration and chemical state of the silica films doped by silver nanoparticles. All binding energy values were determined by calibration the C(1s) line to 284.6 eV. Both survey scans and individual high-resolution scans for Ag(3d), Si(2p), C(1s) and O(1s) peaks were recorded. 3. Results and discussion AFM images of the silica films containing 0, 0.8, 1.6 and 8 mol% Ag have been shown in Fig. 1. For the silica film with no Ag concentration, as our reference morphology, a uniform surface (with no special feature on it) is observed (Fig. 1a). By increasing the Ag concentration to 0.4 mol%, some particlelike features with 18 nm average size were observed on the surface (not shown here). At 0.8 mol%, AFM images show a considerable amount of spherical particles on the silica surface with diameters ranging from 10 to 50 nm and average size of about 28 nm, as can be seen in Fig. 1b. AFM images of the dried films containing higher Ag concentrations showed that surface concentration and also average size of the particles are increased by raising the Ag concentration. The average size of the particles was measured about 48 and 65 nm for 1.2 and 1.6 mol% Ag concentrations, respectively. By increasing the Ag concentration to 8 mol%, AFM images showed that the thin
Fig. 1. AFM images (1 1 Am2) of the silica sol-gel films containing different Ag concentrations: a) 0, b) 0.8, c) 1.6 and d) 8 mol%.
A. Babapour et al. / Thin Solid Films 515 (2006) 771 – 774
film contains micrometer particles on its surface. However, after heat-treatment of the thin films at 200 -C, a particle size reduction into the nanometric scale was observed. Fig. 1d shows AFM image of the SiO2 films containing 8 mol% Ag heated at 200 -C in which the average size of the Ag particles is about 95 nm. The presence of silver on the film surface and its variation by changing the Ag concentration in the sol have been also confirmed by XPS analysis (see Fig. 4). Concerning the size reduction of the Ag particles due to the heat-treatment, two main mechanisms can be considered. In fact, by increasing the temperature, the silver particles concentrated on the surface diffuse into the substrate while they get oxidized. The diffusion of the surface particles results in the average size reduction of the remained particles on the surface which was observed by AFM analysis. Moreover, the oxidation process of the silver particles reduces the size of metallic Ag particles which is observable by the plasmon absorption peak of nanoscale Ag particles (see inset of Fig. 3). These results are also consistent with the results reported in Ref. [15]. Due to the well-known tip convolution effect in AFM analysis, the silver particle sizes and distributions at the lower silver concentrations (less than 0.8 mol%) have been observed by high resolution SEM and TEM methods. Based on SEM analysis of the films containing 0.8 mol% Ag (not shown here), the average particle size of nano-silvers was the same as its value measured by AFM technique (¨30 nm). For the silica films containing 0.4 mol% Ag, Fig. 2 shows SEM image of the Ag nanoparticles located on the surface. In this case, HRSEM gave a better image to distinguish the particles concentrated on the surface, as compared with AFM images. Anyway, this confirms that the average grain size measured by AFM analysis can be reasonably attributed to the average size of the Ag particles. For the dried silica films containing 0.2 mol% Ag, SEM images showed that the shape of the Ag particles is a nanostrip form, not a semispherical form on the surface (not shown here). However, the heat-treatment of the films at 200 -C transforms the form of the particles to a semispherical one. HRTEM image of this silica film (not presented here) showed that the average size of the obtained Ag particles was about 4 nm. This once again indicates that decreasing the Ag concentration in the sol results in reduction of particle size of nano-silvers, in a suitable condition.
Fig. 2. HRSEM image of the silica film containing 0.4 mol% Ag concentration.
773
Fig. 3. Optical absorption spectra for the silica films containing different silver concentrations from zero to 8 mol% Ag.
The optical absorption spectra of the sol-gel silica films with the different silver concentrations have been shown in Fig. 3. It is seen that the absorption curve with no silver or even a low silver concentration (0.2 mol%) shows a typical fringe pattern observed in well transparent films. In fact, for the thin films with a thickness around 200 nm, there are only a couple of interference fringes. Nevertheless, the absorption curves of the films containing higher silver concentrations show a red shift for the absorption peak from 408 to 456 nm wavelength as the Ag concentration increases from 0.4 to 1.6 mol%. In addition, the films with higher silver concentrations show increased absorption intensities. To study the surface chemical state of the Ag nanoparticles, the obtained Ag / Si molar ratio on the surface, and also stoichiometry of the silica matrix, XPS analytical technique were utilized. Fig. 4 shows the Ag / Si ratio that was measured based on the area under the Ag(3d5 / 2) and Si(2p) XPS peaks as a function of the Ag concentration in the sol. We have found that for our dried silica films, the concentration of Ag on the surface is much higher than its initial value in the sol. This means that during the drying process, the Ag particles formed in the gel have a more concentration on the surface, which is
Fig. 4. The measured Ag / Si ratio on the surface and the average size of Ag particles as a function of the initial Ag concentration in the sol.
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A. Babapour et al. / Thin Solid Films 515 (2006) 771 – 774
Fig. 5. Deconvoluted XPS spectra of the silica films containing 0.8 mol% Ag for a) Ag(3d) and b) Si(2p) peaks.
also consistent with our AFM observations. Hence, it is reasonable to perform a reliable measurement on the particle size by the AFM method. Fig. 4 also shows the average size of Ag particles as a function of the initial Ag concentration in the sol. It is seen that by increasing the Ag concentration in the sol from 0.2 to 1.6 mol% the average particle size increases from 4 to 65 nm in a nearly linear behavior. To determine the chemical state of the silver nanoparticles present on the surface, XPS spectra of the 0.8 mol% Ag silica films in the range covering the Ag(3d) photoemission peaks have been studied (Fig. 5a). According to the literature [19,20], the binding energy of Ag(3d5 / 2) for Ag, Ag2O, and AgO are 368.2, 367.8, and 367.4 eV, respectively. Detailed deconvolution of Ag(3d) shows that about 93% of the Ag nanoparticles formed on the surface were in metallic Ag0 state and the other particles oxidized in the form of Ag2O(6%) and AgO(1%). Fig. 5b shows Si(2p) peak of XPS spectrum for the silica film containing 0.8 mol% Ag. This peak consists of both Si(2p3 / 2) and Si(2p1 / 2) peaks due to the spin-orbit coupling. To determine binding energy of the peaks, the Si(2p) peak was deconvoluted to two 2p3 / 2 –2p1 / 2 peaks with fixed separation energy 0.6 eV. Moreover, the ratio of the peaks area is about 2, which is consistent with the theory for splitting of 2p levels. The binding energies of the Si(2p3 / 2) and Si(2p1 / 2) of the silica film were measured as 102.7 and 103.3 eV, respectively. It is reported that the Si(2p3 / 2) core level binding energy of elemental Si is at 99.8 eV, while the binding energies of SiO2 and SiO films are at 102.7 and 102.3 eV, respectively [21]. Therefore, the deposited silica films possess the stoichiometric SiO2 structure in our study. Similar results were also obtained for the fabricated pure silica films. 4. Conclusions Silica thin films containing silver nanoparticles have been prepared by the sol-gel method. The effects of the Ag concentration on the particle size of the synthesized nanosilvers, and also, on the surface plasmon absorption peak of the films have been studied. Using AFM images, it was found that as the Ag concentration decreased from 8 to 0.4 mol%, the average particle size of nano-silvers reduced from 95 to 18 nm. The study of this trend at the lower Ag concentration was completed by using high resolution SEM and TEM which showed that the particle size reduction reaches to less than 4 nm at 0.2 mol% Ag concentration. UV-visible spectrophotometry
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