- Email: [email protected]
Ion beam assisted formation of Ag nanoparticles in SiO2 and their optical properties
Nuclear Instruments and Methods in Physics Research B 193 (2002) 867–870 www.elsevier.com/locate/nimb
Ion beam assisted formation of Ag nanoparticles in SiO2 and their optical properties M. Nikolaeva
a,* ,
M. Sendova-Vassileva a, D. Malinovska a, Y. Sarov b, J.C. Pivin
c
a
Central Laboratory for Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria b Central Laboratory of Optical Storage and Processing of Information, Bulgarian Academy of Sciences, Akad. G. Bonchev str. bl. 101, Sofia, Bulgaria c CSNSM, IN2P3-CNRS, Batiment 108, 91405 Orsay Campus, France
Abstract Ag nanoclusters in SiO2 matrix were formed by magnetron co-sputtering followed by ion irradiation. The refractive index and optical extinction spectra of the films were studied. The SiO2 :Ag thin films exhibit a plasmon resonance in the visible region. In the as-deposited films the Ag nanoclusters are less then 0.5 nm in size. The intensity of the plasmon peak is low. After irradiation with 4.5 MeV Au ions the intensity of the peak increases and it becomes narrower. The FWHM of the plasmon peak corresponds to a mean radius increasing with the ion fluence. The method of the disappearing diffraction pattern was used for the refractive index determination. It is a critical angle method and is applicable for measurements of layers with thickness comparable with the used wavelength. Moreover it is insensitive to surface gradients of the real part of the refractive index and roughness, which disturb elipsometric data. The refractive index enhances with the increasing Ag concentration and irradiation ion fluence, as is expected from the light scattering theory. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 6146; 6180.J; 7865; 7755; 7890 Keywords: Solid clusters and nanoparticles; Radiation damage and others irradiation effects; Optical properties of thin films; Dielectric thin films
1. Introduction The formation of metallic nanoclusters in a dielectric medium provides it with interesting nonlinear optical properties, due to an enhanced thirdorder susceptibility, with potential applications in optoelectronics, e.g. non-linear wave guide devices. These systems are used for passive optical
*
Corresponding author. E-mail address: [email protected] (M. Nikolaeva).
elements such filters as well [1]. Nano-meter-sized clusters of free electron metals in a dielectric medium exhibit a strong characteristic extinction peak, due to plasmon resonance. The shape, intensity and position of the peak depend on the size and shape of the nanoclusters [2] and the possible interaction between them. In this paper we study the optical extinction spectra and changes of the refractive index n of SiO2 thin films containing Ag nanoclusters prepared by rf magnetron co-sputtering and subsequent irradiation with heavy ions. We also
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 9 1 7 - 5
868
M. Nikolaeva et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 867–870
estimated the average radius of Ag clusters using simulations of resonances for isolated clusters in silica, based on the Mie theory presented in [3].
2. Experimental The SiO2 thin films were prepared by magnetron co-sputtering from a complex target in an Ar atmosphere. Ag chips were placed on the surface of the SiO2 target. The Ag concentration in the films was varied between 0.7 and 2 at.% by changing the Ag quantity on the target. The thickness of the films was 750 nm. Some of the samples were irradiated with incremented fluences of 4.5 MeV Au ions delivered by the ARAMIS accelerator (CNRS Orsay, France). The ion current was limited to 0.3 lA cm2 and the sample holder cooled with water in order to prevent a target heating over 20 °C. The range of Au ions was estimated from TRIM calculations at 900 nm. RBS spectra of 2.5 MeV He2þ ions were recorded at normal incidence and at a scattering angle of 165°. The optical transmission of the films was measured in the range 200–800 nm with a CARY UV-VIS-NIR spectrophotometer. The method of the disappearing diffraction pattern (MDDP) was used to measure the real part of the refractive index n of the films. It is a critical angle type method [4,5] and gives the volume n value, averaged over the penetration depth of the evanescent field. Moreover it is insensitive to surface gradients of n and roughness, which disturb elipsometric data. The experimental setup is shown in Fig. 1. The SiO2 :Ag layers – 6, are deposited on TF4 glass substrates, 5 (heavy crown). The substrate is fitted between a prism – 4, with a high refractive index (npr ¼ 1:6950) and the a diffraction grating (DG) – 7, by index matching liquid – 1-bromonaphthalene. The refraction angle of the prism is 70°. The prism is placed on a rotatory table – 3, to change and measure the angle of incidence with accuracy of 1’. A He–Ne laser – 1, is used as a light source (k ¼ 632:8 nm). A polarizer – 2, determines p polarization, where the n determination error is minimal [6]. When the light penetrates on the interface between the substrate and the layer at an angle smaller than the angle of total internal reflection
Fig. 1. Experimental set-up. 1: He–Ne laser, 2: polarizer, 3: rotatory table, 4: prism, 5: substrate, 6: investigated layer, 7: DG, 8: screen.
(TIR), part of the beam refracted into the layer reaches the DG and gives diffraction orders on the screen – 8. If the angle is equal or beyond the critical, ucr , there is no refracted light, all diffraction orders disappear and only totally reflected light remains. The n of the layers is calculated from the measured values of the critical angle, ucr .
3. Results and discussion Metal nuclei already exist in the as-deposited layers, but their size can hardly be determined by TEM because it is at the limit of the resolution of the used microscope. After irradiation with increasing Au ion fluence some of them grow and new ones are formed as shown in a TEM study [7]. A saturation of the irradiation effect at fluences P 51015 cm2 was observed in a previous study [8]. The fluences, which we use in this study in order to modify the size of the clusters, are between 1 1014 and 1 1015 ions cm2 . The optical extinction spectra of the plasmon resonance of samples with two different Ag concentration (0.7 and 2 at.%) are shown on Figs. 2 and 3 respectively. A constant background has been subtracted from these raw optical extinction spectra. The as-deposited SiO2 :Ag films exhibit a plasmon resonance, the intensity of which increases with the Ag concentration. The extinction peaks are broad as it can be seen from the figures. After irradiation the intensity of the peaks becomes higher than that for the as-deposited films.
M. Nikolaeva et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 867–870
869
our films is relatively low and their shape nearly spherical. For clusters whose size is much smaller than the wavelength of the external electric field Eext , the scattering and retardation effects inside the particles are negligible and the cross section calculated with the exact Mie formula reduces to rext ðR; xÞ ¼
9x 3=2 e V c I
Fig. 2. Extinction spectra of SiO2 thin films with 0.7 at.% Ag irradiated with increasing ion fluences. Lines are experimental spectra, symbols are simulated spectra.
ImðeM Þ 2
½ReðeM Þ þ 2ReðeI Þ þ ImðeM Þ
2
;
where eI , eM are the dielectric constants of the insulating matrix and of the metal, Re and Im their real and imaginary parts, and V is the volume of the particles. The effect of colloidal size on extinction spectra arises from the dependence of dielectric function on the colloidal radius eðx; RÞ. As it can be seen from the Figs. 2 and 3 there is good agreement between experimental (lines) and calculated (symbols) resonances in the region of the peaks and slight discrepancies in the region of the interband absorption edge, which are probably due to the modification of the band structure in very small clusters [3]. The results for the radii of the clusters calculated by this simulation are shown in Fig. 4. The size variations are between 0.4 and 1.6 nm. These small variations of the cluster size and the levelling off at high fluence seems to indicate that the mixing of Ag atoms by collision cascades is balanced by their segregation
Fig. 3. Extinction spectra of SiO2 thin films with 2 at.% Ag irradiated with increasing ion fluences. Lines are experimental spectra, symbols are simulation spectra.
The position of the plasmon peaks is at a constant mean energy 3.1 eV, whatever the Ag concentration and treatment. The changes are only in the FWHM of the plasmon band. The changes of the FWHM depend also on the Ag concentration. As it can be seen from the figures the peaks become narrower with increasing Ag content. These alterations can be related with the variation of the clusters size. To estimate the radius R of Ag nanoparticles we used the Mie formula [3] since the filling factor of
Fig. 4. Dependence of the average radius of the clusters on the ion fluence.
870
M. Nikolaeva et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 867–870
Table 1 Real refractive index n dependence of Ag concentration and ion fluence SiO2 with different Ag content (at%)
Fluence (ions cm2 )
Refractive index (n)
0.7 0.7 0.7 1.5 1.5 1.5 2.0 2.0 2.0
Non irradiated 1 1014 1 1015 Non irradiated 1 1014 1 1015 Non irradiated 1 1014 1 1015
1.539 1.543 1.555 1.582 1.584 1.595 1.585 1.587 1.600
or the cluster coalescing by radiation enhanced diffusion. We also measured the real part of the refractive index of SiO2 :Ag layers after irradiation with two different fluences using a MDDP. The first fluence is low – 1 1014 ions cm2 and the mean radius obtained from the simulations is 0.7–0.75 nm. The second fluence is higher – 1 1015 ions cm2 and the mean radius is between 1 and 1.5 nm. The results from the real part of the refractive index n measurements are presented in Table 1. The layer’s n is calculated from the following equation: n ¼ npr sin A arcsin
sin ucr npr
4. Conclusion A plasmon resonance extinction peak is observed in as-deposited and ion irradiated co-sputtered SiO2 films with homogeneously distributed Ag clusters. The metal clusters formed in as-deposited films are less than 0.5 nm in size and their resonance peak is broad. After irradiation the size of the metal clusters increases. The intensity of the plasmon resonance extinction peak grows with the irradiation ion fluence. The FWHM of the plasmon peak corresponds to an increasing mean radius with the ion fluence. There is a good agreement between experimental and simulated spectra using the Mie formula. The increase of the refractive index is related to formation of the new nanoclusters. These SiO2 :Ag thin films with Ag nanoclusters prepared by the proposed technology can be used for optical filters. Acknowledgements This work has been financially supported by the Bulgarian National Science Fund – project X-903 and partially supported by a cooperation project between the Bulgarian Academy of Sciences and CNRS, France. We acknowledge Dr. M.A. Garcia for providing us with the fitting program.
;
ð1Þ
where ucr is the critical angle at which there is no refracted light, npr is the refractive index of the heavy glass substrates. The error in the n value is smaller than 1 103 . The results show a small enhancement of n with increasing ion fluence. The measured index is that of the effective medium composed of silica and clusters. Since no change of optical absorption and of refractive index is observed for pure silica films submitted to similar irradiation the result is ascribed to the growth of the Ag clusters by radiation enhanced diffusion.
References [1] R.F. Haglund Jr., Li Yang, R.H. Magruder III, C.W. White, R.A. Zuhr, Lina Yang, R. Dorsinville, R.R. Alfano, Nucl. Instrum. and Meth. B 91 (1994) 493. [2] R.W. Cohen, G.D. Cody, M.D. Coutts, B. Abeles, Phys Rev B 8 (1973) 3689. [3] U. Kreibig, L. Genzel, Surf. Sci. 156 (1985) 678. [4] S. Sainov, N. Dushkina, Appl. Opt. 29 (1990) 1046. [5] S. Sainov, Rev. Sci. Instrum. 62 (1991) 3106. [6] G. Meeten, Meas. Sic. Technol. 8 (1997) 728. [7] J.C. Pivin, M.A. Garcia, H. Hoffmaster, A. Martucci, M. Sendova-Vassileva, M. Nikolaeva, O. Kaitasov, J. Llopis, Eur. Phys. J. D, submitted. [8] J.C. Pivin, M. Sendova-Vassileva, M. Nikolaeva, D. Dimova-Malinovska, A. Martucci, Appl. Phys. A (2002) in press.
Ion beam assisted formation of Ag nanoparticles in SiO2 and their optical properties M. Nikolaeva
a,* ,
M. Sendova-Vassileva a, D. Malinovska a, Y. Sarov b, J.C. Pivin
c
a
Central Laboratory for Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria b Central Laboratory of Optical Storage and Processing of Information, Bulgarian Academy of Sciences, Akad. G. Bonchev str. bl. 101, Sofia, Bulgaria c CSNSM, IN2P3-CNRS, Batiment 108, 91405 Orsay Campus, France
Abstract Ag nanoclusters in SiO2 matrix were formed by magnetron co-sputtering followed by ion irradiation. The refractive index and optical extinction spectra of the films were studied. The SiO2 :Ag thin films exhibit a plasmon resonance in the visible region. In the as-deposited films the Ag nanoclusters are less then 0.5 nm in size. The intensity of the plasmon peak is low. After irradiation with 4.5 MeV Au ions the intensity of the peak increases and it becomes narrower. The FWHM of the plasmon peak corresponds to a mean radius increasing with the ion fluence. The method of the disappearing diffraction pattern was used for the refractive index determination. It is a critical angle method and is applicable for measurements of layers with thickness comparable with the used wavelength. Moreover it is insensitive to surface gradients of the real part of the refractive index and roughness, which disturb elipsometric data. The refractive index enhances with the increasing Ag concentration and irradiation ion fluence, as is expected from the light scattering theory. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 6146; 6180.J; 7865; 7755; 7890 Keywords: Solid clusters and nanoparticles; Radiation damage and others irradiation effects; Optical properties of thin films; Dielectric thin films
1. Introduction The formation of metallic nanoclusters in a dielectric medium provides it with interesting nonlinear optical properties, due to an enhanced thirdorder susceptibility, with potential applications in optoelectronics, e.g. non-linear wave guide devices. These systems are used for passive optical
*
Corresponding author. E-mail address: [email protected] (M. Nikolaeva).
elements such filters as well [1]. Nano-meter-sized clusters of free electron metals in a dielectric medium exhibit a strong characteristic extinction peak, due to plasmon resonance. The shape, intensity and position of the peak depend on the size and shape of the nanoclusters [2] and the possible interaction between them. In this paper we study the optical extinction spectra and changes of the refractive index n of SiO2 thin films containing Ag nanoclusters prepared by rf magnetron co-sputtering and subsequent irradiation with heavy ions. We also
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 9 1 7 - 5
868
M. Nikolaeva et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 867–870
estimated the average radius of Ag clusters using simulations of resonances for isolated clusters in silica, based on the Mie theory presented in [3].
2. Experimental The SiO2 thin films were prepared by magnetron co-sputtering from a complex target in an Ar atmosphere. Ag chips were placed on the surface of the SiO2 target. The Ag concentration in the films was varied between 0.7 and 2 at.% by changing the Ag quantity on the target. The thickness of the films was 750 nm. Some of the samples were irradiated with incremented fluences of 4.5 MeV Au ions delivered by the ARAMIS accelerator (CNRS Orsay, France). The ion current was limited to 0.3 lA cm2 and the sample holder cooled with water in order to prevent a target heating over 20 °C. The range of Au ions was estimated from TRIM calculations at 900 nm. RBS spectra of 2.5 MeV He2þ ions were recorded at normal incidence and at a scattering angle of 165°. The optical transmission of the films was measured in the range 200–800 nm with a CARY UV-VIS-NIR spectrophotometer. The method of the disappearing diffraction pattern (MDDP) was used to measure the real part of the refractive index n of the films. It is a critical angle type method [4,5] and gives the volume n value, averaged over the penetration depth of the evanescent field. Moreover it is insensitive to surface gradients of n and roughness, which disturb elipsometric data. The experimental setup is shown in Fig. 1. The SiO2 :Ag layers – 6, are deposited on TF4 glass substrates, 5 (heavy crown). The substrate is fitted between a prism – 4, with a high refractive index (npr ¼ 1:6950) and the a diffraction grating (DG) – 7, by index matching liquid – 1-bromonaphthalene. The refraction angle of the prism is 70°. The prism is placed on a rotatory table – 3, to change and measure the angle of incidence with accuracy of 1’. A He–Ne laser – 1, is used as a light source (k ¼ 632:8 nm). A polarizer – 2, determines p polarization, where the n determination error is minimal [6]. When the light penetrates on the interface between the substrate and the layer at an angle smaller than the angle of total internal reflection
Fig. 1. Experimental set-up. 1: He–Ne laser, 2: polarizer, 3: rotatory table, 4: prism, 5: substrate, 6: investigated layer, 7: DG, 8: screen.
(TIR), part of the beam refracted into the layer reaches the DG and gives diffraction orders on the screen – 8. If the angle is equal or beyond the critical, ucr , there is no refracted light, all diffraction orders disappear and only totally reflected light remains. The n of the layers is calculated from the measured values of the critical angle, ucr .
3. Results and discussion Metal nuclei already exist in the as-deposited layers, but their size can hardly be determined by TEM because it is at the limit of the resolution of the used microscope. After irradiation with increasing Au ion fluence some of them grow and new ones are formed as shown in a TEM study [7]. A saturation of the irradiation effect at fluences P 51015 cm2 was observed in a previous study [8]. The fluences, which we use in this study in order to modify the size of the clusters, are between 1 1014 and 1 1015 ions cm2 . The optical extinction spectra of the plasmon resonance of samples with two different Ag concentration (0.7 and 2 at.%) are shown on Figs. 2 and 3 respectively. A constant background has been subtracted from these raw optical extinction spectra. The as-deposited SiO2 :Ag films exhibit a plasmon resonance, the intensity of which increases with the Ag concentration. The extinction peaks are broad as it can be seen from the figures. After irradiation the intensity of the peaks becomes higher than that for the as-deposited films.
M. Nikolaeva et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 867–870
869
our films is relatively low and their shape nearly spherical. For clusters whose size is much smaller than the wavelength of the external electric field Eext , the scattering and retardation effects inside the particles are negligible and the cross section calculated with the exact Mie formula reduces to rext ðR; xÞ ¼
9x 3=2 e V c I
Fig. 2. Extinction spectra of SiO2 thin films with 0.7 at.% Ag irradiated with increasing ion fluences. Lines are experimental spectra, symbols are simulated spectra.
ImðeM Þ 2
½ReðeM Þ þ 2ReðeI Þ þ ImðeM Þ
2
;
where eI , eM are the dielectric constants of the insulating matrix and of the metal, Re and Im their real and imaginary parts, and V is the volume of the particles. The effect of colloidal size on extinction spectra arises from the dependence of dielectric function on the colloidal radius eðx; RÞ. As it can be seen from the Figs. 2 and 3 there is good agreement between experimental (lines) and calculated (symbols) resonances in the region of the peaks and slight discrepancies in the region of the interband absorption edge, which are probably due to the modification of the band structure in very small clusters [3]. The results for the radii of the clusters calculated by this simulation are shown in Fig. 4. The size variations are between 0.4 and 1.6 nm. These small variations of the cluster size and the levelling off at high fluence seems to indicate that the mixing of Ag atoms by collision cascades is balanced by their segregation
Fig. 3. Extinction spectra of SiO2 thin films with 2 at.% Ag irradiated with increasing ion fluences. Lines are experimental spectra, symbols are simulation spectra.
The position of the plasmon peaks is at a constant mean energy 3.1 eV, whatever the Ag concentration and treatment. The changes are only in the FWHM of the plasmon band. The changes of the FWHM depend also on the Ag concentration. As it can be seen from the figures the peaks become narrower with increasing Ag content. These alterations can be related with the variation of the clusters size. To estimate the radius R of Ag nanoparticles we used the Mie formula [3] since the filling factor of
Fig. 4. Dependence of the average radius of the clusters on the ion fluence.
870
M. Nikolaeva et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 867–870
Table 1 Real refractive index n dependence of Ag concentration and ion fluence SiO2 with different Ag content (at%)
Fluence (ions cm2 )
Refractive index (n)
0.7 0.7 0.7 1.5 1.5 1.5 2.0 2.0 2.0
Non irradiated 1 1014 1 1015 Non irradiated 1 1014 1 1015 Non irradiated 1 1014 1 1015
1.539 1.543 1.555 1.582 1.584 1.595 1.585 1.587 1.600
or the cluster coalescing by radiation enhanced diffusion. We also measured the real part of the refractive index of SiO2 :Ag layers after irradiation with two different fluences using a MDDP. The first fluence is low – 1 1014 ions cm2 and the mean radius obtained from the simulations is 0.7–0.75 nm. The second fluence is higher – 1 1015 ions cm2 and the mean radius is between 1 and 1.5 nm. The results from the real part of the refractive index n measurements are presented in Table 1. The layer’s n is calculated from the following equation: n ¼ npr sin A arcsin
sin ucr npr
4. Conclusion A plasmon resonance extinction peak is observed in as-deposited and ion irradiated co-sputtered SiO2 films with homogeneously distributed Ag clusters. The metal clusters formed in as-deposited films are less than 0.5 nm in size and their resonance peak is broad. After irradiation the size of the metal clusters increases. The intensity of the plasmon resonance extinction peak grows with the irradiation ion fluence. The FWHM of the plasmon peak corresponds to an increasing mean radius with the ion fluence. There is a good agreement between experimental and simulated spectra using the Mie formula. The increase of the refractive index is related to formation of the new nanoclusters. These SiO2 :Ag thin films with Ag nanoclusters prepared by the proposed technology can be used for optical filters. Acknowledgements This work has been financially supported by the Bulgarian National Science Fund – project X-903 and partially supported by a cooperation project between the Bulgarian Academy of Sciences and CNRS, France. We acknowledge Dr. M.A. Garcia for providing us with the fitting program.
;
ð1Þ
where ucr is the critical angle at which there is no refracted light, npr is the refractive index of the heavy glass substrates. The error in the n value is smaller than 1 103 . The results show a small enhancement of n with increasing ion fluence. The measured index is that of the effective medium composed of silica and clusters. Since no change of optical absorption and of refractive index is observed for pure silica films submitted to similar irradiation the result is ascribed to the growth of the Ag clusters by radiation enhanced diffusion.
References [1] R.F. Haglund Jr., Li Yang, R.H. Magruder III, C.W. White, R.A. Zuhr, Lina Yang, R. Dorsinville, R.R. Alfano, Nucl. Instrum. and Meth. B 91 (1994) 493. [2] R.W. Cohen, G.D. Cody, M.D. Coutts, B. Abeles, Phys Rev B 8 (1973) 3689. [3] U. Kreibig, L. Genzel, Surf. Sci. 156 (1985) 678. [4] S. Sainov, N. Dushkina, Appl. Opt. 29 (1990) 1046. [5] S. Sainov, Rev. Sci. Instrum. 62 (1991) 3106. [6] G. Meeten, Meas. Sic. Technol. 8 (1997) 728. [7] J.C. Pivin, M.A. Garcia, H. Hoffmaster, A. Martucci, M. Sendova-Vassileva, M. Nikolaeva, O. Kaitasov, J. Llopis, Eur. Phys. J. D, submitted. [8] J.C. Pivin, M. Sendova-Vassileva, M. Nikolaeva, D. Dimova-Malinovska, A. Martucci, Appl. Phys. A (2002) in press.