- Email: [email protected]
Tailoring the size and distribution of Ag nanoparticles in silica glass by defects
Nuclear Instruments and Methods in Physics Research B 321 (2014) 14–18
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Tailoring the size and distribution of Ag nanoparticles in silica glass by defects Yitao Yang a,⇑, Chonghong Zhang a, Yin Song a, Jie Gou a, Liqing Zhang a, Hengqing Zhang a,b, Juan Liu a,b, Yongqiang Xian a,b, Yizhun Ma a a b
Materials Research Center, Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 19 November 2013 Received in revised form 16 December 2013
Keywords: Ag nanoparticle Defect Nanofabrication Silica glass
a b s t r a c t The composites embedded with metallic nanoparticles show large nonlinear optical susceptibility and strong surface plasmon resonance absorption, which enable potential application in opto-electronics. Ion implantation has been proven to be a powerful technique of synthesis of metallic nanoparticles due to its versatility and compatibility. However, the synthesis of nanoparticles by ion implantation inevitably leads to a broad size distribution due to Ostwald ripening process. The broad size distribution has a negative effect on improving the figure of merits for nonlinear optics. In this paper, we tried to introduce defects in silica glass to act as pre-nucleation centers to mediate the size and distribution of Ag nanoparticles. In experiment, the silica glass samples were pre-irradiated by 200 keV Ar ions to fluences of 0.8, 2.0 and 5.0 1016 ions/cm2, and then 200 keV Ag ions were implanted into the pre-irradiated samples to fluence of 2.0 1016 ions/cm2. UV–VIS results show that the absorbance intensity of Ag SPR peak initially increases and then decreases with pre-irradiation fluence, which implies the change in size and density of Ag nanoparticles in samples. TEM results verify that Ag nanoparticles in the sample pre-irradiated to the fluence of 0.8 1016 ions/cm2 grow bigger and distribute in a relatively narrow region comparing with that without pre-irradiation. With further increase of pre-irradiation fluence, the size of Ag nanoparticles shows a depth dependent distribution. A boundary can be clear seen at the depth of 110 nm, larger Ag nanoparticles disperse in region shallower than 110 nm, and smaller Ag nanoparticles disperse in the region deeper than 110 nm. The average size of Ag nanoparticles initially increases and then decreases with pre-irradiation fluence. Therefore, the introduction of defects by pre-irradiation could be an effective way to tailor the size and distribution of metallic nanoparticles in matrix. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Due to large surface to volume ratio and electron confinement, the composites embedded with metallic nanoparticles show large nonlinear optical susceptibility (v3) and strong surface plasmon resonance absorption [1–3], which enable promising application in opto-electronics, e.g. ultrafast optical switch, waveguides, alloptical memories etc. [4,5]. Flytzanis et al. define a figure of merit for strongly absorbing nonlinear optical materials to be v3/as, where s is relaxation time, a is the absorption coefficient [6]. However, both v3 and a have a peak near the same wavelength. For the insulating matrix, the relaxation time s is long, which decreases the figure of the merit for these materials. One way to maintain a high value of v3 while decrease the surface plasmon absorption is to have a narrow size distribution of extremely small particles, ⇑ Corresponding author. Address: No. 509 Nanchang Road, Lanzhou 730000, China. Tel.: +86 931 4969036; fax: +86 931 8272100. E-mail address: [email protected] (Y. Yang). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.12.015
since in the quantum confinement regime v3 varies as 1/r3, and a is decreased due to the weakening of the plasmon resonance [1]. Among various methods of synthesis of metallic nanoparticles in matrix, ion implantation has been proven to be a powerful technique due to its versatility and compatibility with CMOS (Complementary Metal Oxide Semiconductor) technique [4,7,8]. However, the synthesis of nanoparticles by ion implantation inevitably leads to a broad size distribution due to Ostwald ripening process [1,7]. The broad size distribution has a negative effect on improving the figure of merits for nonlinear optics. The contribution of tailoring of size and size distribution by controlling the parameters of ion implantation, e.g. energy, fluence, flux, temperature etc., is rather limited [7]. During the nucleation and growth process, two factors control the size and distribution of nanoparticles [9]. One is the number of nuclei of implanted atoms, and another is the local concentration of implanted atoms around the nuclei. Therefore, one promising way to mediate the size and distribution of nanoparticles is to mediate the nucleation process, e.g. the introduction of
Y. Yang et al. / Nuclear Instruments and Methods in Physics Research B 321 (2014) 14–18
pre-nucleation centers. Recently, some studies have tried to introduce defects in insulating matrix to act as pre-nucleation centers of metallic nanoparticles. Fedorov et al. investigated the effects of nanocavities formed by helium ion implantation and annealing on trapping of Au atoms in MgO substrate [10]. Patel et al. studied the influence of defects produced by ion pre-implantation on the size of Cu nanoparticles synthesized by ion beam mixing in silica glass [11]. It revealed that the average size of Cu nanoparticles can be controlled by the pre-implantation dose. Giulian et al. presented a typical result of almost mono-dispersed Pt nanoparticles synthesized by pre-irradiation and Pt ion implantation in silica glass [12]. In addition, our previous results indicated that the defects introduced by ion pre-irradiation can effectively decrease the threshold of nucleation atom concentration for metallic nanoparticles in matrix [13], and also have a positive effect on mediating size distribution along depth [14,15]. In the present paper, the nucleation and growth of Ag nanoparticles at different defect concentration in silica glass is investigated. The defects in silica glass are produced by ion pre-irradiation. 2. Experimental The silica glass samples were initially irradiated with 200 keV Ar ions to fluences of 0.8, 2.0 and 5.0 1016 ions/cm2, respectively, to produce defects with different concentration. Subsequently, a pristine sample and the irradiated samples were simultaneously
15
implanted with 200 keV Ag ions to a fluence of 2.0 1016 ions/ cm2. All the irradiation/implantation experiments were performed at room temperature. According to the SRIM 2012 simulation [16], the damage produced by Ar ion irradiation mainly locates in the depth ranging from surface to 350 nm with a damage peak around 160 nm. The peak values are 8.4, 20.7 and 51.8 dpa for the three increasing fluences, respectively. The implanted Ag atoms mainly locate in the depth ranging from 30 to 175 nm with a concentration peak around 100 nm, which overlaps with the damage region produced by Ar ion irradiation. After implantation samples were characterized by optical absorption spectrometry (UV–VIS, PerkinElmer lambda900) and transmission electron microscope (TEM, JEOL JEM-2010). 3. Results and discussion Fig. 1a shows the UV–VIS results for the samples with and without Ar ion pre-irradiation. It is seen that surface plasmon resonance (SPR) absorbance peak from Ag nanoparticles is observed at wavelength of 400 nm, which is consistent with the prediction from Mie theory [17]. For the samples pre-irradiated by Ar ions, the absorbance intensity of SPR peak varies with pre-irradiation fluence, which initially increases and then decreases, as shown in Fig. 1b. The variation in absorbance intensity of SPR peak implies the change in size and density of Ag nanoparticles in matrix with different defect concentration.
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Ar ion fluence ( 10 ions/cm ) Fig. 1. UV–VIS spectra for pristine sample, sample with Ag ion implantation and samples with Ar ion pre-irradiation implanted with silver ions (a) and the evolution of absorbance intensity of SPR peak with pre-irradiation fluence (b).
Fig. 2. Cross-section TEM graphs for the sample implanted with Ag ions (a), and the samples with pre-irradiation fluence of 0.8 1016 (b), 2.0 1016 (c), and 5.0 1016 ions/cm2 (d).
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TEM micrograph, as shown in Fig. 2a, also verifies the formation of Ag nanoparticles in matrix after Ag ion implantation. It is seen that there are two groups of Ag nanoparticles dispersing in matrix. One is in the depth region from 20 to 35 nm, and another is in the depth region from 50 to 220 nm. Only a few Ag nanoparticles disperse in the region from 35 to 50 nm. This kind of distribution of Ag nanoparticles can only be observed for the implantation with high fluence, which is considered to be related to these defects produced during Ag ion implantation [9,18]. The largest particles distribute at the depth around 100 nm, which is consistent with the SRIM simulation. Large numbers of Ag nanoparticles with smaller size disperse in the depth ranging from 140 to 220 nm, which is deeper than that predicted by SRIM simulation. This is probably associated with the fast diffusion of Ag atom during implantation. For the samples pre-irradiated with Ar ions, there are also two groups of nanoparticles dispersing in matrix, which is similar to the sample without pre-irradiation. However, the size and distribution of Ag nanoparticles in deeper region are quite different, as shown in Fig. 2b–d. For the sample pre-irradiated with lowest fluence, it is seen that Ag nanoparticles grow bigger, and mainly
distribute in a narrow region ranging from 50 to 180 nm. With further increase of pre-irradiation fluence, the size of Ag nanoparticles shows a depth dependent distribution. A boundary can be clearly seen at the depth of 110 nm, larger Ag nanoparticles disperse in shallower region (approximately from 50 to 110 nm), and smaller Ag nanoparticles disperse in deeper region (approximately from 110 to 180 nm). From the statistical results of Ag nanoparticles, shown in Fig. 3a–d, it is seen that the fraction of larger Ag nanoparticles initially increases and then decreases with pre-irradiation fluence, a reverse situation occurs for smaller Ag nanoparticles. The average size of Ag nanoparticles also increases and then decreases with pre-irradiation fluence, as shown in Fig. 3e. The density of Ag nanoparticles also seems to vary with pre-irradiation fluence. The implanted Ag atoms initially present at a super-saturation state in sample, and then phase separation results in the formation of Ag nanoparticles [7,19]. In the process of nucleation and growth, two factors control the size and distribution of Ag nanoparticles. One is the number of Ag nuclei, and another is the local concentration of Ag atoms around the nuclei [9]. When Ag nuclei surpass a
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Ar ion fluence ( 10 ions/cm ) Fig. 3. Statistical results of silver nanoparticles for the sample implanted with Ag ions (a) and the samples with pre-irradiation fluence of 0.8 1016 (b), 2.0 1016 (c) and 5.0 1016 ions/cm2 (d), the evolution of average size of Ag nanoparticles with pre-irradiation fluence is shown in Fig. 3e.
Y. Yang et al. / Nuclear Instruments and Methods in Physics Research B 321 (2014) 14–18
critical radius, they will continuously grow into nanoparticles. In the case with the same Ag atom concentration (in this study, Ag ion fluence is the same for all samples), the only factor influencing the size and distribution is the number of Ag nuclei. For the sample with pre-irradiation, large numbers of defects are produced in matrix. Due to strong trapping capacity of defects to impurity atoms, the subsequently implanted Ag atoms could be trapped preferentially by these defects. This makes Ag atoms easy to nucleate. In this case, defects in matrix produced by pre-irradiation actually act as nucleation centers for Ag nanoparticles. The number and distribution of defects have a direct influence on the number and distribution of Ag nuclei/nanoparticles in the sample with preirradiation. The study by Giulian et al. also revealed that defects produced in pre-irradiation process governed nanoparticle size [12]. In the case with low concentration of defects, only Ag atoms close to these defects can be easily trapped to form Ag nuclei or participate in the process of Ag nanoparticle growth, other Ag atoms far away from these defects cannot be effectively trapped, they could nucleate mainly by self aggregation. With increase of defect concentration, more Ag atoms involve in the nucleation and growth processes, which can effectively improve the efficiency of nucleation and growth of Ag nanoparticles, namely, increase the size and density of Ag nanoparticles. However, with further increase of defect concentration, the average number of Ag atoms trapped by defect probably decreases, some Ag atoms just present as Ag nuclei or smaller nanoparticles, which cannot continuously grow into larger particles due to the trapping of most Ag atoms by defects and difficult migration of these trapped Ag atoms. This could result in a decrease of density and size of Ag nanoparticles. In experiment, we observed that the average size of Ag nanoparticles increases, and Ag nanoparticles distribute in a narrow region for the sample pre-irradiated with fluence of 8.0 1015 ions/ cm2, the density of Ag nanoparticle seems to decrease. This could correspond to the case with low defect concentration. For the sample with pre-irradiation fluence of 2.0 1016 ions/cm2, the size and density of Ag nanoparticles obviously increase in the region from 50 to 110 nm, but large numbers of Ag nanoparticles with smaller size are formed in the region from 110 to 180 nm, which makes the average size of Ag nanoparticles slightly decreases. The significant difference of size distribution in the two regions is probably caused
17
by the variation of Ag atom concentration and damage level, as shown in Fig. 4. It is seen that the profile of Ag atoms along depth nearly shows Gaussian distribution, which gradually increases in the region from 30 to 100 nm and then decreases in the region from 100 to 180 nm. However, the damage produced by pre-irradiation in this region gradually increases. When Ag atom concentration starts to decrease with depth, the damage at the corresponding depth still increases, which could result in most Ag atoms involving in the nucleation process and only a few Ag atoms participating in the growth process. As a result, the size of Ag nanoparticles is much reduced in deeper region (from 110 to 180 nm). With further increase of pre-irradiation fluence, defect concentration also increases, the influence from defects on the nucleation and growth becomes more pronounced so that the size of Ag nanoparticles in shallower region (from 50 to 110 nm) is also reduced, which causes a further decrease of average size of Ag nanoparticles in the sample with pre-irradiation fluence of 5.0 1016 ions/cm2. From these results, it is seen that Ag nanoparticles show different size and distribution under different defect concentration. Therefore, the introduction of defects by pre-irradiation could be an effective means to mediate the size and distribution of metal nanoparticles in matrix. 4. Conclusion Defects with different concentration were produced in silica glass by Ar ion pre-irradiation with different fluence, and then Ag ions were implanted into the pre-irradiated silica glass to study the nucleation and growth of Ag nanoparticles at different defect concentration. It is found that average size of Ag nanoparticles initially increases, and then decreases with pre-irradiation fluence, and Ag nanoparticles distribute in a relatively narrow region comparing with that without pre-irradiation. In addition, for the samples with pre-irradiation fluences of 2.0 1016 and 5.0 1016 ions/ cm2, it is seen that larger Ag nanoparticles disperse in the region shallower than 110 nm and smaller Ag nanoparticles disperse in the region deeper than 110 nm. Therefore, the introduction of defects by pre-irradiation could be an effectively way to mediate the size and distribution of nanoparticles in matrix. Acknowledgements This work was sponsored by the National Natural Science Foundation of China (NSFC, Grant No. 11005132), the National Basic Research Program of China (Grant No. 2010CB832904) and the National Magnetic Confinement Fusion Program (Grant No. 2011GB108003). References
Fig. 4. The simulated damage (including the damage produced by Ar ion preirradiation (red dash line), the damage produced by Ag ion implantation (yellow dot line) and the total damage (blue dash line)) and Ag atom concentration (green dash line) distribution with depth are shown together with the corresponding TEM graph for the sample with pre-irradiation fluence of 2.0 1016 ions/cm2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
[1] A. Meldrum, R.F. Haglund Jr., L.A. Boatner, C.W. White, Adv. Mater. 13 (2001) 1431. [2] U. Kreibig, L. Genzel, Surf. Sci. 156 (1985) 678. [3] P. Mulvaney, MRS Bull. 26 (2001) 1009. [4] R.F. Haglund Jr., Mater. Sci. Eng. A 253 (1998) 275. [5] H. Inouye, K. Tanaka, I. Tanahashi, T. Hattori, H. Nakatsuka, Jpn. J. Appl. Phys. Part 1 39 (2000) 5132. [6] C. Flytzanis, F. Hache, M.C. Klein, D. Ricard, P. Roussignol, Prog. Opt. 29 (1991) 321. [7] K.H. Heinig, T. Muller, B. Schmidt, M. Strobel, W. Moller, Appl. Phys. A 77 (2003) 17. [8] A.L. Stepanov, Rev. Adv. Mater. Sci. 26 (2010) 1. [9] Z. Liu, H. Li, X. Feng, S. Ren, H. Wang, Z. Liu, B. Lu, J. Appl. Phys. 84 (1998) 1913. [10] A.V. Fedorov, M.A. van Huis, A. van Vee, H. Schut, Nucl. Instrum. Methods Phys. Res. B 166–167 (2000) 215. [11] M.K. Patel, B.J. Nagare, D.M. Bagul, S.K. Haram, D.C. Kothari, Surf. Coat. Technol. 196 (2005) 96. [12] R. Giulian, P. Kluth, L.L. Araujo, D.J. Llewellyn, M.C. Ridgway, Appl. Phys. Lett. 91 (2007) 093115.
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[13] Y. Yang, C. Zhang, B. Li, L. Zhang, C. Xu, X. Jia, Nucl. Instrum. Methods Phys. Res. B 268 (2010) 3529. [14] Y. Yang, C. Zhang, Y. Song, L. Zhang, J. Gou, Y. Meng, H. Zhang, Y. Ma, Vaccum 99 (2014) 115. [15] L. Zhou, C. Zhang, Y. Yang, B. Li, L. Zhang, Y. Fu, H. Zhang, Nucl. Instrum. Methods Phys. Res. B 267 (2009) 58.
[16] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. [17] G. Mie, Ann. Phys. 25 (1908) 377. [18] Z. Liu, H. Wang, Nucl. Instrum. Methods Phys. Res. B 122 (1997) 45. [19] M. Strobel, K.H. Heinig, W. Moller, A. Meldrum, D.S. Zhou, C.W. White, R.A. Zuhr, Nucl. Instrum. Methods Phys. Res. B 147 (1999) 343.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Tailoring the size and distribution of Ag nanoparticles in silica glass by defects Yitao Yang a,⇑, Chonghong Zhang a, Yin Song a, Jie Gou a, Liqing Zhang a, Hengqing Zhang a,b, Juan Liu a,b, Yongqiang Xian a,b, Yizhun Ma a a b
Materials Research Center, Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 19 November 2013 Received in revised form 16 December 2013
Keywords: Ag nanoparticle Defect Nanofabrication Silica glass
a b s t r a c t The composites embedded with metallic nanoparticles show large nonlinear optical susceptibility and strong surface plasmon resonance absorption, which enable potential application in opto-electronics. Ion implantation has been proven to be a powerful technique of synthesis of metallic nanoparticles due to its versatility and compatibility. However, the synthesis of nanoparticles by ion implantation inevitably leads to a broad size distribution due to Ostwald ripening process. The broad size distribution has a negative effect on improving the figure of merits for nonlinear optics. In this paper, we tried to introduce defects in silica glass to act as pre-nucleation centers to mediate the size and distribution of Ag nanoparticles. In experiment, the silica glass samples were pre-irradiated by 200 keV Ar ions to fluences of 0.8, 2.0 and 5.0 1016 ions/cm2, and then 200 keV Ag ions were implanted into the pre-irradiated samples to fluence of 2.0 1016 ions/cm2. UV–VIS results show that the absorbance intensity of Ag SPR peak initially increases and then decreases with pre-irradiation fluence, which implies the change in size and density of Ag nanoparticles in samples. TEM results verify that Ag nanoparticles in the sample pre-irradiated to the fluence of 0.8 1016 ions/cm2 grow bigger and distribute in a relatively narrow region comparing with that without pre-irradiation. With further increase of pre-irradiation fluence, the size of Ag nanoparticles shows a depth dependent distribution. A boundary can be clear seen at the depth of 110 nm, larger Ag nanoparticles disperse in region shallower than 110 nm, and smaller Ag nanoparticles disperse in the region deeper than 110 nm. The average size of Ag nanoparticles initially increases and then decreases with pre-irradiation fluence. Therefore, the introduction of defects by pre-irradiation could be an effective way to tailor the size and distribution of metallic nanoparticles in matrix. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Due to large surface to volume ratio and electron confinement, the composites embedded with metallic nanoparticles show large nonlinear optical susceptibility (v3) and strong surface plasmon resonance absorption [1–3], which enable promising application in opto-electronics, e.g. ultrafast optical switch, waveguides, alloptical memories etc. [4,5]. Flytzanis et al. define a figure of merit for strongly absorbing nonlinear optical materials to be v3/as, where s is relaxation time, a is the absorption coefficient [6]. However, both v3 and a have a peak near the same wavelength. For the insulating matrix, the relaxation time s is long, which decreases the figure of the merit for these materials. One way to maintain a high value of v3 while decrease the surface plasmon absorption is to have a narrow size distribution of extremely small particles, ⇑ Corresponding author. Address: No. 509 Nanchang Road, Lanzhou 730000, China. Tel.: +86 931 4969036; fax: +86 931 8272100. E-mail address: [email protected] (Y. Yang). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.12.015
since in the quantum confinement regime v3 varies as 1/r3, and a is decreased due to the weakening of the plasmon resonance [1]. Among various methods of synthesis of metallic nanoparticles in matrix, ion implantation has been proven to be a powerful technique due to its versatility and compatibility with CMOS (Complementary Metal Oxide Semiconductor) technique [4,7,8]. However, the synthesis of nanoparticles by ion implantation inevitably leads to a broad size distribution due to Ostwald ripening process [1,7]. The broad size distribution has a negative effect on improving the figure of merits for nonlinear optics. The contribution of tailoring of size and size distribution by controlling the parameters of ion implantation, e.g. energy, fluence, flux, temperature etc., is rather limited [7]. During the nucleation and growth process, two factors control the size and distribution of nanoparticles [9]. One is the number of nuclei of implanted atoms, and another is the local concentration of implanted atoms around the nuclei. Therefore, one promising way to mediate the size and distribution of nanoparticles is to mediate the nucleation process, e.g. the introduction of
Y. Yang et al. / Nuclear Instruments and Methods in Physics Research B 321 (2014) 14–18
pre-nucleation centers. Recently, some studies have tried to introduce defects in insulating matrix to act as pre-nucleation centers of metallic nanoparticles. Fedorov et al. investigated the effects of nanocavities formed by helium ion implantation and annealing on trapping of Au atoms in MgO substrate [10]. Patel et al. studied the influence of defects produced by ion pre-implantation on the size of Cu nanoparticles synthesized by ion beam mixing in silica glass [11]. It revealed that the average size of Cu nanoparticles can be controlled by the pre-implantation dose. Giulian et al. presented a typical result of almost mono-dispersed Pt nanoparticles synthesized by pre-irradiation and Pt ion implantation in silica glass [12]. In addition, our previous results indicated that the defects introduced by ion pre-irradiation can effectively decrease the threshold of nucleation atom concentration for metallic nanoparticles in matrix [13], and also have a positive effect on mediating size distribution along depth [14,15]. In the present paper, the nucleation and growth of Ag nanoparticles at different defect concentration in silica glass is investigated. The defects in silica glass are produced by ion pre-irradiation. 2. Experimental The silica glass samples were initially irradiated with 200 keV Ar ions to fluences of 0.8, 2.0 and 5.0 1016 ions/cm2, respectively, to produce defects with different concentration. Subsequently, a pristine sample and the irradiated samples were simultaneously
15
implanted with 200 keV Ag ions to a fluence of 2.0 1016 ions/ cm2. All the irradiation/implantation experiments were performed at room temperature. According to the SRIM 2012 simulation [16], the damage produced by Ar ion irradiation mainly locates in the depth ranging from surface to 350 nm with a damage peak around 160 nm. The peak values are 8.4, 20.7 and 51.8 dpa for the three increasing fluences, respectively. The implanted Ag atoms mainly locate in the depth ranging from 30 to 175 nm with a concentration peak around 100 nm, which overlaps with the damage region produced by Ar ion irradiation. After implantation samples were characterized by optical absorption spectrometry (UV–VIS, PerkinElmer lambda900) and transmission electron microscope (TEM, JEOL JEM-2010). 3. Results and discussion Fig. 1a shows the UV–VIS results for the samples with and without Ar ion pre-irradiation. It is seen that surface plasmon resonance (SPR) absorbance peak from Ag nanoparticles is observed at wavelength of 400 nm, which is consistent with the prediction from Mie theory [17]. For the samples pre-irradiated by Ar ions, the absorbance intensity of SPR peak varies with pre-irradiation fluence, which initially increases and then decreases, as shown in Fig. 1b. The variation in absorbance intensity of SPR peak implies the change in size and density of Ag nanoparticles in matrix with different defect concentration.
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Ar ion fluence ( 10 ions/cm ) Fig. 1. UV–VIS spectra for pristine sample, sample with Ag ion implantation and samples with Ar ion pre-irradiation implanted with silver ions (a) and the evolution of absorbance intensity of SPR peak with pre-irradiation fluence (b).
Fig. 2. Cross-section TEM graphs for the sample implanted with Ag ions (a), and the samples with pre-irradiation fluence of 0.8 1016 (b), 2.0 1016 (c), and 5.0 1016 ions/cm2 (d).
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Y. Yang et al. / Nuclear Instruments and Methods in Physics Research B 321 (2014) 14–18
TEM micrograph, as shown in Fig. 2a, also verifies the formation of Ag nanoparticles in matrix after Ag ion implantation. It is seen that there are two groups of Ag nanoparticles dispersing in matrix. One is in the depth region from 20 to 35 nm, and another is in the depth region from 50 to 220 nm. Only a few Ag nanoparticles disperse in the region from 35 to 50 nm. This kind of distribution of Ag nanoparticles can only be observed for the implantation with high fluence, which is considered to be related to these defects produced during Ag ion implantation [9,18]. The largest particles distribute at the depth around 100 nm, which is consistent with the SRIM simulation. Large numbers of Ag nanoparticles with smaller size disperse in the depth ranging from 140 to 220 nm, which is deeper than that predicted by SRIM simulation. This is probably associated with the fast diffusion of Ag atom during implantation. For the samples pre-irradiated with Ar ions, there are also two groups of nanoparticles dispersing in matrix, which is similar to the sample without pre-irradiation. However, the size and distribution of Ag nanoparticles in deeper region are quite different, as shown in Fig. 2b–d. For the sample pre-irradiated with lowest fluence, it is seen that Ag nanoparticles grow bigger, and mainly
distribute in a narrow region ranging from 50 to 180 nm. With further increase of pre-irradiation fluence, the size of Ag nanoparticles shows a depth dependent distribution. A boundary can be clearly seen at the depth of 110 nm, larger Ag nanoparticles disperse in shallower region (approximately from 50 to 110 nm), and smaller Ag nanoparticles disperse in deeper region (approximately from 110 to 180 nm). From the statistical results of Ag nanoparticles, shown in Fig. 3a–d, it is seen that the fraction of larger Ag nanoparticles initially increases and then decreases with pre-irradiation fluence, a reverse situation occurs for smaller Ag nanoparticles. The average size of Ag nanoparticles also increases and then decreases with pre-irradiation fluence, as shown in Fig. 3e. The density of Ag nanoparticles also seems to vary with pre-irradiation fluence. The implanted Ag atoms initially present at a super-saturation state in sample, and then phase separation results in the formation of Ag nanoparticles [7,19]. In the process of nucleation and growth, two factors control the size and distribution of Ag nanoparticles. One is the number of Ag nuclei, and another is the local concentration of Ag atoms around the nuclei [9]. When Ag nuclei surpass a
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Ar ion fluence ( 10 ions/cm ) Fig. 3. Statistical results of silver nanoparticles for the sample implanted with Ag ions (a) and the samples with pre-irradiation fluence of 0.8 1016 (b), 2.0 1016 (c) and 5.0 1016 ions/cm2 (d), the evolution of average size of Ag nanoparticles with pre-irradiation fluence is shown in Fig. 3e.
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critical radius, they will continuously grow into nanoparticles. In the case with the same Ag atom concentration (in this study, Ag ion fluence is the same for all samples), the only factor influencing the size and distribution is the number of Ag nuclei. For the sample with pre-irradiation, large numbers of defects are produced in matrix. Due to strong trapping capacity of defects to impurity atoms, the subsequently implanted Ag atoms could be trapped preferentially by these defects. This makes Ag atoms easy to nucleate. In this case, defects in matrix produced by pre-irradiation actually act as nucleation centers for Ag nanoparticles. The number and distribution of defects have a direct influence on the number and distribution of Ag nuclei/nanoparticles in the sample with preirradiation. The study by Giulian et al. also revealed that defects produced in pre-irradiation process governed nanoparticle size [12]. In the case with low concentration of defects, only Ag atoms close to these defects can be easily trapped to form Ag nuclei or participate in the process of Ag nanoparticle growth, other Ag atoms far away from these defects cannot be effectively trapped, they could nucleate mainly by self aggregation. With increase of defect concentration, more Ag atoms involve in the nucleation and growth processes, which can effectively improve the efficiency of nucleation and growth of Ag nanoparticles, namely, increase the size and density of Ag nanoparticles. However, with further increase of defect concentration, the average number of Ag atoms trapped by defect probably decreases, some Ag atoms just present as Ag nuclei or smaller nanoparticles, which cannot continuously grow into larger particles due to the trapping of most Ag atoms by defects and difficult migration of these trapped Ag atoms. This could result in a decrease of density and size of Ag nanoparticles. In experiment, we observed that the average size of Ag nanoparticles increases, and Ag nanoparticles distribute in a narrow region for the sample pre-irradiated with fluence of 8.0 1015 ions/ cm2, the density of Ag nanoparticle seems to decrease. This could correspond to the case with low defect concentration. For the sample with pre-irradiation fluence of 2.0 1016 ions/cm2, the size and density of Ag nanoparticles obviously increase in the region from 50 to 110 nm, but large numbers of Ag nanoparticles with smaller size are formed in the region from 110 to 180 nm, which makes the average size of Ag nanoparticles slightly decreases. The significant difference of size distribution in the two regions is probably caused
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by the variation of Ag atom concentration and damage level, as shown in Fig. 4. It is seen that the profile of Ag atoms along depth nearly shows Gaussian distribution, which gradually increases in the region from 30 to 100 nm and then decreases in the region from 100 to 180 nm. However, the damage produced by pre-irradiation in this region gradually increases. When Ag atom concentration starts to decrease with depth, the damage at the corresponding depth still increases, which could result in most Ag atoms involving in the nucleation process and only a few Ag atoms participating in the growth process. As a result, the size of Ag nanoparticles is much reduced in deeper region (from 110 to 180 nm). With further increase of pre-irradiation fluence, defect concentration also increases, the influence from defects on the nucleation and growth becomes more pronounced so that the size of Ag nanoparticles in shallower region (from 50 to 110 nm) is also reduced, which causes a further decrease of average size of Ag nanoparticles in the sample with pre-irradiation fluence of 5.0 1016 ions/cm2. From these results, it is seen that Ag nanoparticles show different size and distribution under different defect concentration. Therefore, the introduction of defects by pre-irradiation could be an effective means to mediate the size and distribution of metal nanoparticles in matrix. 4. Conclusion Defects with different concentration were produced in silica glass by Ar ion pre-irradiation with different fluence, and then Ag ions were implanted into the pre-irradiated silica glass to study the nucleation and growth of Ag nanoparticles at different defect concentration. It is found that average size of Ag nanoparticles initially increases, and then decreases with pre-irradiation fluence, and Ag nanoparticles distribute in a relatively narrow region comparing with that without pre-irradiation. In addition, for the samples with pre-irradiation fluences of 2.0 1016 and 5.0 1016 ions/ cm2, it is seen that larger Ag nanoparticles disperse in the region shallower than 110 nm and smaller Ag nanoparticles disperse in the region deeper than 110 nm. Therefore, the introduction of defects by pre-irradiation could be an effectively way to mediate the size and distribution of nanoparticles in matrix. Acknowledgements This work was sponsored by the National Natural Science Foundation of China (NSFC, Grant No. 11005132), the National Basic Research Program of China (Grant No. 2010CB832904) and the National Magnetic Confinement Fusion Program (Grant No. 2011GB108003). References
Fig. 4. The simulated damage (including the damage produced by Ar ion preirradiation (red dash line), the damage produced by Ag ion implantation (yellow dot line) and the total damage (blue dash line)) and Ag atom concentration (green dash line) distribution with depth are shown together with the corresponding TEM graph for the sample with pre-irradiation fluence of 2.0 1016 ions/cm2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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