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Enhancement of Ag nanoparticles concentration by prior ion implantation
Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Enhancement of Ag nanoparticles concentration by prior ion implantation Xiaoyu Mu a, Jun Wang a, Changlong Liu a,b,c,⇑ a
School of Science, Tianjin University, Tianjin 300072, PR China Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparation Technology, Faculty of Science, Tianjin 300072, PR China c Key Laboratory of Beam Technology and Material Modification of Ministry of Education, Beijing Normal University, Beijing 100875, China b
a r t i c l e
i n f o
Article history: Received 10 August 2016 Received in revised form 24 February 2017 Accepted 20 March 2017 Available online xxxx Keywords: Ag nanoparticles Suppressing effect of self-sputtering Prior implantation Enhancement of the deposition
a b s t r a c t Thermally grown SiO2 layer on Si substrates were singly or sequentially implanted with Zn or Cu and Ag ions at the same fluence of 2 1016/cm2. The profiles of implanted species, structure, and spatial distribution of the formed nanoparticles (NPs) have been characterized by the cross-sectional transmission electron microscope (XTEM) and Rutherford backscattering spectrometry (RBS). It is found that preimplantation of Zn or Cu ions could suppress the self sputtering of Ag atoms during post Ag ion implantation, which gives rise to fabrication of Ag NPs with a high density. Moreover, it has also been demonstrated that the suppressing effect strongly depends on the applied energy and mobility of pre-implanted ions. The possible mechanism for the enhanced Ag NPs concentration has been discussed in combination with SRIM simulations. Both vacancy-like defects acting as the increased nucleation sites for Ag NPs and a high diffusivity of prior implanted ions in SiO2 play key roles in enhancing the deposition of Ag implants. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Noble metal nanoparticles (NPs) embedded in a dielectric film deposited on monocrystal semiconductors have great potential applications in the novel electronic/plasmonic devices. For an example, the stable nanocomposites have been proposed to be used in the noninvasive and highly sensitive optical sensors based on localized surface plasmon resonance (LSPR) or surface enhanced Raman scattering (SERS) [1–3]. Among noble metal NPs, Ag NPs are superior to Au or Cu ones in the aspect of plasmonic enhancement by virtue of low interference between the intraband and interband electronic transitions [3–6]. A variety of ways were used to synthesize Ag-NPs in dielectrics, including magnetron co-sputtering [7], sequential evaporation [8], sol-gel deposition [9] and ion implantation [10–19]. Among these methods, ion implantation at low energy is a very promising technique for wafer-scale fabrication of Ag NPs embedded in the near-surface region of a silica layer on a silicon substrate (SiO2/Si) [14,15]. The kinetic energy of the implanted ions in keV scale can offer a way to control the position of the metal nanostructures in nanoscale from the near surface, which is crucial for optical applications based on LSPR and SERS
⇑ Corresponding author at: School of Science, Tianjin University, Tianjin 300072, PR China. E-mail address: [email protected] (C. Liu).
in the tunneling or ‘‘near-field” range [16–18]. However, the volume fraction of the fabricated nanoparticles is largely limited by the heavy sputtering of the substrate during the implantation due to low energy, large mass and high fluence of the applied ions. In order to overcome this weakness, we have employed Xe ion irradiation in prior to Ag ion implantation in silica, which can largely increase the concentration of Ag NPs in the substrate [19]. Recently, Shen et al. also found that Ag concentration was significantly enhanced by Ni pre-implantation in SiO2 [11]. However, the above phenomenon has been reported without further discussion on the underlying physical mechanisms in details. In this work, Zn or Cu and Ag ions were singly or sequentially implanted into SiO2/Si, which is aimed at the effect of Zn or Cu pre-implantation on the surface morphology, structure and spatial distribution of Ag NPs formed during the following Ag ion implantation. It has been demonstrated that the pre-implantation of Zn or Cu ions can largely suppress the self-sputtering effect during the following Ag ion implantation, which gives rise to significant increase in the concentration of Ag NPs. Moreover, the suppressing effect strongly depends on the energy and mobility of the preimplanted ions. In combination with SRIM simulations, we suggest that vacancy defects acting as the nucleation sites for Ag NPs, along with the intrinsic high diffusivity of prior implanted ions play an important role in suppressing self-sputtering of Ag ions. Our findings thus could have pointed out an effective route to fabricating
http://dx.doi.org/10.1016/j.nimb.2017.03.113 0168-583X/Ó 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: X. Mu et al., Enhancement of Ag nanoparticles concentration by prior ion implantation, Nucl. Instr. Meth. B (2017), http:// dx.doi.org/10.1016/j.nimb.2017.03.113
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metal NPs with a high volume fraction in the shallow near-surface region of dielectrics, which have potential applications based on LSPR and SERS. 2. Experimental On a metal vapor vacuum arc (MEVVA) implanter, Zn or Cu ions and Ag ions were singly or sequentially implanted into a 300 nm thick thermally grown SiO2 layer on a Si (100) substrates at the same fluence of 2 1016/cm2. The implanted energies of Zn ions were 15, 30 and 45 keV, and that of Cu and Ag ions were fixed at 30 and 21 keV, respectively. During implantation, the flux density was kept at about 4 mA/cm2 and the incident beam was tilted by 45° from the sample surface. In order to assure uniformity of implantation, the target plate was set to rotate at a constant speed. Nine sets of samples were prepared, whose names and implantation parameters are listed in Table 1. Moreover, the projected ranges of different ions were simulated by using SRIM 2010 code [20]. The results are also given in Table 1. To characterize the structure, shape and spatial distribution of the formed nanostructures, cross-sectional transmission electron microscope (XTEM) and high-resolution transmission electron microscope (HRTEM) measurements were performed with a Tecnai G2 F20 S-Twin microscope operating at an acceleration voltage of 200 kV. Fast Fourier transform (FFT) was performed using free software, DigitalMicrograph. The concentration profiles of the implanted elements were measured by Rutherford backscattering spectrometry (RBS) with 400 keV He ions at 107.5° scattering angle. 3. Results and discussion Fig. 1(a) shows the typical XTEM image of the Ag sample. It is clear that spherical NPs are formed in the shallow region from surface to a depth of 19.6 nm. The FFT pattern inserted in Fig. 1(a) indicates that the NPs are the face-centered cubic (FCC) Ag. Results from the statistical measurements demonstrate that the formed Ag NPs have an average diameter of 4.4 ± 1.4 nm, as shown in Fig. 1 (b). For the Zn15+Ag, Zn30+Ag and Zn45+Ag samples, the spherical NPs are also observed (Fig. 1(c), (e) and (g)), which are mainly distributed in a region with a thickness of 18.2, 28.7 and 28.2 nm, respectively. The size of the formed NPs ranges from 1 to 6 nm (Fig. 1(d), (f) and (h)). Beyond this region, only small and sparse NPs are formed. Besides, a depleted layer with few small NPs is found on each surface of the three Zn + Ag samples, and its thickness shows dependence on the implanted energy of Zn ions. Formation of the depleted layer is useful to enhance the stability against oxidation of the embedded NPs. FFT patterns inserted in Fig. 1(c), (e) and (g) clearly show that the formed NPs are FFC Ag in all three Zn + Ag samples. Nevertheless, appearance of both halo-ring and spots of FCC Ag in the patterns indicates that the
polycrystalline and monocrystal Ag NPs have coexisted with amorphous Ag NPs. Actually, the three categories have also been detected by HRTEM observations of the NPs distributed at different regions (not shown here). It should be pointed out that the polycrystalline Ag NPs are numerous. The formation of polycrystalline Ag NPs may be attributed to the rapid agglomeration of implants against the relatively weak ion beam heating. By comparison with the Ag sample, two aspects should be emphasized: (1) The volume fractions of Ag NPs increase significantly in the three Zn+Ag samples and the increased amounts show strong dependence on the Zn-implanted energies. (2) The depth corresponding to the maximum volume fraction becomes deeper as the kinetic energy of Zn ions increases from 15 to 45 keV. The close relations suggest that the pre-implantation of Zn ions can strongly affect the subsequent Ag nucleation, diffusion and spatial distribution. Fig. 1(i) presents the XTEM image obtained from the Cn30+Ag sample sequentially implanted with 30 keV Cu and 21 keV Ag ions at the same fluence of 2 1016/cm2. It is clear that spherical Ag NPs with a high density are mainly distributed in a narrower region from surface to a depth of 19.5 nm. The corresponding size distribution of Ag NPs is given in Fig. 1(j). One can see that the average diameter of Ag NPs in the Cn30 + Ag sample is 4.5 ± 1.8 nm, which is larger than that in the Zn30 + Ag sample (3.2 ± 1.1 nm). Moreover, the increased amounts and the spatial distributions of Ag NPs also have significant differences between the Cu30 + Ag sample (Fig. 1(i)) and Zn30 + Ag sample (Fig. 1(e)), which could be attributed to the differences of the intrinsic properties of the preimplanted ions. Fig. 2 shows the RBS spectra of all the prepared samples. The spectra were normalized by using the silicon profile for the purpose of comparisons. In Fig. 2(a), the Zn15 + Ag sample exhibits a weaker Zn peak than that in the Zn15 sample, and a stronger Ag peak than that in the Ag sample. The weaker Zn peak is mainly ascribed to sputtering loss of the Zn atoms near the surface induced by the post Ag implantation. The stronger Ag peak indicates an enhanced deposition process of Ag implants caused by the prior Zn implantation. The prior Zn implantation produces lots of defects in the substrate including vacancies, which can enhance the forward diffusion of Ag ions during post Ag implantation [21]. Therefore, Ag nucleation and particle growth occur at a deeper region away from the surface. As a result, the sputtering loss of Ag atoms can be suppressed effectively, and a high Ag concentration can be reached in the Zn15 + Ag sample. In the Zn30 + Ag, Zn45 + Ag and Cu30+Ag samples, the similar phenomenon, i.e. a high Ag concentration, also appears (see Fig. 2(b) –(d)). By simulations of the RBS spectra, Ag content in the Zn15 + Ag, Zn30 + Ag, Zn45 + Ag and Cu30+Ag samples is higher than that in the Ag sample by 124.7%, 136.9%, 124.5% and 87.2%, respectively. The enhanced deposition process of Ag implants provides a reasonable explanation for a high volume fraction of Ag NPs (see Fig. 1). In our previous work, Ag content in the 500 keV Xe pre-irradiated sample
Table 1 Summary of the sample names and implantation parameters. Sample name
Zn15 Zn30 Zn45 Cu30 Ag Zn15 + Ag Zn30 + Ag Zn45 + Ag Cu30 + Ag
Prior ion implantation
Ion implantation
SRIM
Energy (keV)
Ion species
Energy (keV)
Ion species
Rp (nm)
15 30 45 30 — 15 30 45 30
Zn Zn Zn Cu — Zn Zn Zn Cu
— — — — 21 21 21 21 21
— — — — Ag Ag Ag Ag Ag
11.5 18.9 26.0 19.7 12.9 — — — —
Please cite this article in press as: X. Mu et al., Enhancement of Ag nanoparticles concentration by prior ion implantation, Nucl. Instr. Meth. B (2017), http:// dx.doi.org/10.1016/j.nimb.2017.03.113
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Fig. 1. XTEM results of (a) Ag, (c) Zn15 + Ag, (e) Zn30 + Ag, (g) Zn45 + Ag and (i) Cu30 + Ag samples. The insets in (a), (c), (e), (g) and (i) are the corresponding FFT patterns. (b), (d), (f), (h) and (j) are the size distributions of nanoparticles.
Please cite this article in press as: X. Mu et al., Enhancement of Ag nanoparticles concentration by prior ion implantation, Nucl. Instr. Meth. B (2017), http:// dx.doi.org/10.1016/j.nimb.2017.03.113
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Fig. 2. RBS spectra of the (a) Zn15 and Zn15 + Ag, (b) Zn30 and Zn30 + Ag, (c) Zn45 and Zn45 + Ag, (d) Cu30 and Cu30 + Ag samples. All the spectra were normalized by using the silicon profile of Ag sample.
was higher by 67.4% than the Ag sample [19]. Thus it is clear that Ag content is highest in the Zn+Ag sample, especially the Zn30 + Ag sample, indicating that the deposition process of Ag implants is significantly enhanced by Zn pre-implantation and related to the implanted energy of Zn ions. Since sputtering loss of the implanted ions should be quite heavy [10,20] for the ultra-low-energy ion implantation with a relatively high ion fluence during the actual production, the enhanced deposition process of implants is of potential interest for practical applications. In addition, RBS spectrum of the Ag sample shows bimodal distribution of Ag depth profile, which can be attributed to the heat effect of a high flux density. In the present case, the flux density of Ag ion beam was maintained at about 4 mA/cm2 and samples were placed at the holder without cooling equipment. Ion beam heating induced by the Ag implantation can dramatically increase the mobility of Ag atoms, which greatly promote diffusion of Ag atoms to lower potential energy sites including the sample surface and the interface of SiO2/Si. Fig. 3(a) shows the depth of the Ag peak concentration versus the implanted energy obtained from the XTEM images, RBS simulations and SRIM simulations of all the samples. For the Ag sample, the depth of Ag maximum concentration evaluated from the XTEM observation is 13.0 nm, consistent with the projected range (12.9 nm) from SRIM simulations. Moreover, the depths of the Ag peak concentration measured from XTEM images are in agreement with ones simulated from RBS spectra of the Zn + Ag or Cu+Ag samples. Additionally, it can be also found that the depth corresponding to Ag peak concentration becomes deeper as the kinetic energy of Zn ions increases from 15 to 30 keV, and almost keeps unchanged with further increasing Zn energy to 45 keV. Fig. 3(b)
shows the depth of the Zn peak concentration versus the Znimplanted energy obtained from the RBS simulations. One can see that post Ag ion implantation could cause migration of Zn atoms to deeper region. The migration distances corresponding to the depth of Zn peak concentration are about 5.5, 5.0 and 2.0 nm at the Zn-applied energy of 15, 30 and 45 keV, respectively. Such Zn migration can be attributed to the ion beam heat induced by post Ag ion implantation. However, the depths of Cu peak concentration are almost fixed for the Cu30 and Cu30 + Ag samples, suggesting that the mobility and diffusivity of Cu atoms are lower than that of Zn atoms under the same condition. It may be one of the primary factors to enhance the deposition process of Ag implants. Fig. 4 gives the atomic and vacancy concentration profiles of implants in singly ion implanted samples, as simulated by SRIM code [20]. One can find that the overlap region of Zn and Ag atomic concentration profiles decreases as the kinetic energy of Zn ions varies from 15 to 45 keV (see Fig. 4(a)). In the Ag sample, the Ag atoms mainly distribute in the depth range of 25 nm from surface, marked by solid diamond symbol in Fig. 4(a). However, the amount of the vacancies in this region induced by Zn ion implantation increases with increasing the implanted energy (see Fig. 4(b)). By simulations of the RBS spectra in Fig. 2, the Ag contents in the Zn15 + Ag, Zn30 + Ag and Zn45 + Ag samples are higher than that in the Ag sample by 124.7%, 136.9% and 124.5%, respectively, which indicates that the synergistic effect of the Zn atoms and vacancies contributes to the enhancement of the Ag concentration. By comparing Zn ion implantation with Cu ion implantation at the same energy of 30 keV (Fig. 4), it is found that the atomic content and vacancy concentration profiles have no significant difference
Please cite this article in press as: X. Mu et al., Enhancement of Ag nanoparticles concentration by prior ion implantation, Nucl. Instr. Meth. B (2017), http:// dx.doi.org/10.1016/j.nimb.2017.03.113
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Fig. 4. Atomic (a) and vacancy (b) concentration profiles of implants in singly implanted samples with Zn, Cu or Ag ions, as calculated by SRIM code. The digits in each legend denote the kinetic energy of the corresponding implants. Solid symbol: Atom, hollow symbol: Vacancy.
Fig. 3. Depth of the Ag (a) and Zn (b) peak concentration versus ion implanted energy obtained from XTEM images, RBS simulation and SRIM calculation of all samples.
between Zn and Cu ion implantation. Nevertheless, RBS simulations show that the enhancement of Ag concentration is 87.2% in the Cn30 + Ag sample, which is lower than that (136.9%) in the Zn30 + Ag sample. It suggests that Ag precipitation process is affected by the intrinsic properties of the pre-implanted species as well. The above results clearly show that the pre-implantation of Zn or Cu ions could increase the concentration of Ag NPs by suppressing the self-sputtering effect during the following Ag ion implantation, which strongly depends on the pre-implanted energy and species. The main reasons for the enhancement of Ag NPs concentration could be attributed to the following two aspects. Firstly, vacancies, non-bridging oxygen (NBO) hole centers, threefoldcoordinated silicon centers and electron-hole pairs [22,23] could be created in the substrate after Zn or Cu ion implantation. These defects not only act as lower potential energy sites accommodating the subsequent Ag nucleation [24], but also greatly promote the diffusion of Ag atoms [21]. As we know, the effective diffusivity of implants is one of the key factors to determine the deposition and spatial distribution of NPs in the ion implanted matrix [25]. Synergy of the numerous nucleation sites and the promoted Ag diffusion provides a possibility to enhance the deposition of Ag implants and form Ag NPs in a high volume fraction. Actually,
the similar phenomenon was reported by Wang et al. in the Xe pre-irradiated SiO2 [19] and by Shen et al. in the Ni preimplanted SiO2 [11]. Secondly, the metallic Zn or Cu atoms/clusters could be introduced in the substrate after Zn or Cu ion implantation, which may also play an important role in promoting Ag precipitation in the substrate. On one hand, the ion beam heating induced by the following Ag ion implantation, can dramatically enhance the mobility of Ag atoms themselves [26], along with thermal diffusion coefficient of the pre-implanted atoms. On the other hand, the following Ag implantation could cause migration of the pre-implanted atoms to the deep region via forward recoil processes [27], which has been demonstrated by the RBS simulations in Fig. 3(b). The diffusion and migration of pre-implanted atoms may further carry the mobile Ag atoms initially near the surface into the deeper region, which has been confirmed by the RBS simulations in Fig. 3(a). Consequently, a depleted layer is formed near the surface, which could suppress the sputtering loss of Ag atoms in the near-surface and increase the Ag concentration. Since the intrinsic diffusivity of the metallic Zn atoms in SiO2 is higher than that of Cu atoms [28], Ag content in the Zn30 + Ag sample (136.9%) is more than that in the Cu30 + Ag sample (87.2%). It should be pointed out that the related compounds (e.g. Zn or Cu oxides and Zn silicate [29,30]) could also be formed in the substrate after pre-implantation of Cu or Zn. According to the work by Amekura et al. [30], Zn2SiO4 was formed in the deeper region of the SiO2 after Zn implantation, which is more stable and hardly diffuses. The formation of various compounds may change the properties of substrate, and thus affect sputtering processes in
Please cite this article in press as: X. Mu et al., Enhancement of Ag nanoparticles concentration by prior ion implantation, Nucl. Instr. Meth. B (2017), http:// dx.doi.org/10.1016/j.nimb.2017.03.113
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some extent. In a word, the diffusion and deposition processes of Ag implants are enhanced along the tracks of pre-implanted ions. 4. Conclusion In summary, Ag NPs with a high volume fraction in SiO2/Si substrate are synthesized by sequential implantation of Zn or Cu and Ag ions. The pre-implantation of Zn or Cu ions can largely suppress the self-sputtering effect during the following Ag ion implantation, which gives rise to increasing the Ag content in the substrate. Moreover, it has been also demonstrated that the suppressing effect strongly depends on the applied energy and species of preimplantation ions. It is suggested that the enhanced deposition of Ag implants can be attributed to vacancy defects, acting as trapping centers for silver and greatly promoting the diffusion of Ag atoms, as well as the intrinsic high diffusivity of prior implanted ions. Our findings may provide an effective route to fabricating metal NPs with a high volume fraction in the shallow nearsurface of dielectrics, which have potential applications based on LSPR and SERS. Acknowledgements This work was supported by the Natural Science Foundation of China (Nos. 11535008, 11675120). References [1] M.E. Stewart, C.R. Anderton, L.B. Thompson, J. Maria, S.K. Gray, J.A. Rogers, R.G. Nuzzo, Chem. Rev. 108 (2008) 494. [2] J.N. Anker, W.P. Hall, O. Lyandres, N.C. Shah, J. Zhao, R.P. Van Duyne, Nat. Mater. 7 (2008) 442. [3] W. Li, X. Xiao, Z. Dai, W. Wu, L. Cheng, F. Mei, X. Zhang, C. Jiang, J. Phys.: Condens. Matter 28 (2016) 254003. [4] U. Kreibig, M. Vollmer, Springer, Berlin, Germany (1995).
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Please cite this article in press as: X. Mu et al., Enhancement of Ag nanoparticles concentration by prior ion implantation, Nucl. Instr. Meth. B (2017), http:// dx.doi.org/10.1016/j.nimb.2017.03.113
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Enhancement of Ag nanoparticles concentration by prior ion implantation Xiaoyu Mu a, Jun Wang a, Changlong Liu a,b,c,⇑ a
School of Science, Tianjin University, Tianjin 300072, PR China Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparation Technology, Faculty of Science, Tianjin 300072, PR China c Key Laboratory of Beam Technology and Material Modification of Ministry of Education, Beijing Normal University, Beijing 100875, China b
a r t i c l e
i n f o
Article history: Received 10 August 2016 Received in revised form 24 February 2017 Accepted 20 March 2017 Available online xxxx Keywords: Ag nanoparticles Suppressing effect of self-sputtering Prior implantation Enhancement of the deposition
a b s t r a c t Thermally grown SiO2 layer on Si substrates were singly or sequentially implanted with Zn or Cu and Ag ions at the same fluence of 2 1016/cm2. The profiles of implanted species, structure, and spatial distribution of the formed nanoparticles (NPs) have been characterized by the cross-sectional transmission electron microscope (XTEM) and Rutherford backscattering spectrometry (RBS). It is found that preimplantation of Zn or Cu ions could suppress the self sputtering of Ag atoms during post Ag ion implantation, which gives rise to fabrication of Ag NPs with a high density. Moreover, it has also been demonstrated that the suppressing effect strongly depends on the applied energy and mobility of pre-implanted ions. The possible mechanism for the enhanced Ag NPs concentration has been discussed in combination with SRIM simulations. Both vacancy-like defects acting as the increased nucleation sites for Ag NPs and a high diffusivity of prior implanted ions in SiO2 play key roles in enhancing the deposition of Ag implants. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Noble metal nanoparticles (NPs) embedded in a dielectric film deposited on monocrystal semiconductors have great potential applications in the novel electronic/plasmonic devices. For an example, the stable nanocomposites have been proposed to be used in the noninvasive and highly sensitive optical sensors based on localized surface plasmon resonance (LSPR) or surface enhanced Raman scattering (SERS) [1–3]. Among noble metal NPs, Ag NPs are superior to Au or Cu ones in the aspect of plasmonic enhancement by virtue of low interference between the intraband and interband electronic transitions [3–6]. A variety of ways were used to synthesize Ag-NPs in dielectrics, including magnetron co-sputtering [7], sequential evaporation [8], sol-gel deposition [9] and ion implantation [10–19]. Among these methods, ion implantation at low energy is a very promising technique for wafer-scale fabrication of Ag NPs embedded in the near-surface region of a silica layer on a silicon substrate (SiO2/Si) [14,15]. The kinetic energy of the implanted ions in keV scale can offer a way to control the position of the metal nanostructures in nanoscale from the near surface, which is crucial for optical applications based on LSPR and SERS
⇑ Corresponding author at: School of Science, Tianjin University, Tianjin 300072, PR China. E-mail address: [email protected] (C. Liu).
in the tunneling or ‘‘near-field” range [16–18]. However, the volume fraction of the fabricated nanoparticles is largely limited by the heavy sputtering of the substrate during the implantation due to low energy, large mass and high fluence of the applied ions. In order to overcome this weakness, we have employed Xe ion irradiation in prior to Ag ion implantation in silica, which can largely increase the concentration of Ag NPs in the substrate [19]. Recently, Shen et al. also found that Ag concentration was significantly enhanced by Ni pre-implantation in SiO2 [11]. However, the above phenomenon has been reported without further discussion on the underlying physical mechanisms in details. In this work, Zn or Cu and Ag ions were singly or sequentially implanted into SiO2/Si, which is aimed at the effect of Zn or Cu pre-implantation on the surface morphology, structure and spatial distribution of Ag NPs formed during the following Ag ion implantation. It has been demonstrated that the pre-implantation of Zn or Cu ions can largely suppress the self-sputtering effect during the following Ag ion implantation, which gives rise to significant increase in the concentration of Ag NPs. Moreover, the suppressing effect strongly depends on the energy and mobility of the preimplanted ions. In combination with SRIM simulations, we suggest that vacancy defects acting as the nucleation sites for Ag NPs, along with the intrinsic high diffusivity of prior implanted ions play an important role in suppressing self-sputtering of Ag ions. Our findings thus could have pointed out an effective route to fabricating
http://dx.doi.org/10.1016/j.nimb.2017.03.113 0168-583X/Ó 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: X. Mu et al., Enhancement of Ag nanoparticles concentration by prior ion implantation, Nucl. Instr. Meth. B (2017), http:// dx.doi.org/10.1016/j.nimb.2017.03.113
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metal NPs with a high volume fraction in the shallow near-surface region of dielectrics, which have potential applications based on LSPR and SERS. 2. Experimental On a metal vapor vacuum arc (MEVVA) implanter, Zn or Cu ions and Ag ions were singly or sequentially implanted into a 300 nm thick thermally grown SiO2 layer on a Si (100) substrates at the same fluence of 2 1016/cm2. The implanted energies of Zn ions were 15, 30 and 45 keV, and that of Cu and Ag ions were fixed at 30 and 21 keV, respectively. During implantation, the flux density was kept at about 4 mA/cm2 and the incident beam was tilted by 45° from the sample surface. In order to assure uniformity of implantation, the target plate was set to rotate at a constant speed. Nine sets of samples were prepared, whose names and implantation parameters are listed in Table 1. Moreover, the projected ranges of different ions were simulated by using SRIM 2010 code [20]. The results are also given in Table 1. To characterize the structure, shape and spatial distribution of the formed nanostructures, cross-sectional transmission electron microscope (XTEM) and high-resolution transmission electron microscope (HRTEM) measurements were performed with a Tecnai G2 F20 S-Twin microscope operating at an acceleration voltage of 200 kV. Fast Fourier transform (FFT) was performed using free software, DigitalMicrograph. The concentration profiles of the implanted elements were measured by Rutherford backscattering spectrometry (RBS) with 400 keV He ions at 107.5° scattering angle. 3. Results and discussion Fig. 1(a) shows the typical XTEM image of the Ag sample. It is clear that spherical NPs are formed in the shallow region from surface to a depth of 19.6 nm. The FFT pattern inserted in Fig. 1(a) indicates that the NPs are the face-centered cubic (FCC) Ag. Results from the statistical measurements demonstrate that the formed Ag NPs have an average diameter of 4.4 ± 1.4 nm, as shown in Fig. 1 (b). For the Zn15+Ag, Zn30+Ag and Zn45+Ag samples, the spherical NPs are also observed (Fig. 1(c), (e) and (g)), which are mainly distributed in a region with a thickness of 18.2, 28.7 and 28.2 nm, respectively. The size of the formed NPs ranges from 1 to 6 nm (Fig. 1(d), (f) and (h)). Beyond this region, only small and sparse NPs are formed. Besides, a depleted layer with few small NPs is found on each surface of the three Zn + Ag samples, and its thickness shows dependence on the implanted energy of Zn ions. Formation of the depleted layer is useful to enhance the stability against oxidation of the embedded NPs. FFT patterns inserted in Fig. 1(c), (e) and (g) clearly show that the formed NPs are FFC Ag in all three Zn + Ag samples. Nevertheless, appearance of both halo-ring and spots of FCC Ag in the patterns indicates that the
polycrystalline and monocrystal Ag NPs have coexisted with amorphous Ag NPs. Actually, the three categories have also been detected by HRTEM observations of the NPs distributed at different regions (not shown here). It should be pointed out that the polycrystalline Ag NPs are numerous. The formation of polycrystalline Ag NPs may be attributed to the rapid agglomeration of implants against the relatively weak ion beam heating. By comparison with the Ag sample, two aspects should be emphasized: (1) The volume fractions of Ag NPs increase significantly in the three Zn+Ag samples and the increased amounts show strong dependence on the Zn-implanted energies. (2) The depth corresponding to the maximum volume fraction becomes deeper as the kinetic energy of Zn ions increases from 15 to 45 keV. The close relations suggest that the pre-implantation of Zn ions can strongly affect the subsequent Ag nucleation, diffusion and spatial distribution. Fig. 1(i) presents the XTEM image obtained from the Cn30+Ag sample sequentially implanted with 30 keV Cu and 21 keV Ag ions at the same fluence of 2 1016/cm2. It is clear that spherical Ag NPs with a high density are mainly distributed in a narrower region from surface to a depth of 19.5 nm. The corresponding size distribution of Ag NPs is given in Fig. 1(j). One can see that the average diameter of Ag NPs in the Cn30 + Ag sample is 4.5 ± 1.8 nm, which is larger than that in the Zn30 + Ag sample (3.2 ± 1.1 nm). Moreover, the increased amounts and the spatial distributions of Ag NPs also have significant differences between the Cu30 + Ag sample (Fig. 1(i)) and Zn30 + Ag sample (Fig. 1(e)), which could be attributed to the differences of the intrinsic properties of the preimplanted ions. Fig. 2 shows the RBS spectra of all the prepared samples. The spectra were normalized by using the silicon profile for the purpose of comparisons. In Fig. 2(a), the Zn15 + Ag sample exhibits a weaker Zn peak than that in the Zn15 sample, and a stronger Ag peak than that in the Ag sample. The weaker Zn peak is mainly ascribed to sputtering loss of the Zn atoms near the surface induced by the post Ag implantation. The stronger Ag peak indicates an enhanced deposition process of Ag implants caused by the prior Zn implantation. The prior Zn implantation produces lots of defects in the substrate including vacancies, which can enhance the forward diffusion of Ag ions during post Ag implantation [21]. Therefore, Ag nucleation and particle growth occur at a deeper region away from the surface. As a result, the sputtering loss of Ag atoms can be suppressed effectively, and a high Ag concentration can be reached in the Zn15 + Ag sample. In the Zn30 + Ag, Zn45 + Ag and Cu30+Ag samples, the similar phenomenon, i.e. a high Ag concentration, also appears (see Fig. 2(b) –(d)). By simulations of the RBS spectra, Ag content in the Zn15 + Ag, Zn30 + Ag, Zn45 + Ag and Cu30+Ag samples is higher than that in the Ag sample by 124.7%, 136.9%, 124.5% and 87.2%, respectively. The enhanced deposition process of Ag implants provides a reasonable explanation for a high volume fraction of Ag NPs (see Fig. 1). In our previous work, Ag content in the 500 keV Xe pre-irradiated sample
Table 1 Summary of the sample names and implantation parameters. Sample name
Zn15 Zn30 Zn45 Cu30 Ag Zn15 + Ag Zn30 + Ag Zn45 + Ag Cu30 + Ag
Prior ion implantation
Ion implantation
SRIM
Energy (keV)
Ion species
Energy (keV)
Ion species
Rp (nm)
15 30 45 30 — 15 30 45 30
Zn Zn Zn Cu — Zn Zn Zn Cu
— — — — 21 21 21 21 21
— — — — Ag Ag Ag Ag Ag
11.5 18.9 26.0 19.7 12.9 — — — —
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Fig. 1. XTEM results of (a) Ag, (c) Zn15 + Ag, (e) Zn30 + Ag, (g) Zn45 + Ag and (i) Cu30 + Ag samples. The insets in (a), (c), (e), (g) and (i) are the corresponding FFT patterns. (b), (d), (f), (h) and (j) are the size distributions of nanoparticles.
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Fig. 2. RBS spectra of the (a) Zn15 and Zn15 + Ag, (b) Zn30 and Zn30 + Ag, (c) Zn45 and Zn45 + Ag, (d) Cu30 and Cu30 + Ag samples. All the spectra were normalized by using the silicon profile of Ag sample.
was higher by 67.4% than the Ag sample [19]. Thus it is clear that Ag content is highest in the Zn+Ag sample, especially the Zn30 + Ag sample, indicating that the deposition process of Ag implants is significantly enhanced by Zn pre-implantation and related to the implanted energy of Zn ions. Since sputtering loss of the implanted ions should be quite heavy [10,20] for the ultra-low-energy ion implantation with a relatively high ion fluence during the actual production, the enhanced deposition process of implants is of potential interest for practical applications. In addition, RBS spectrum of the Ag sample shows bimodal distribution of Ag depth profile, which can be attributed to the heat effect of a high flux density. In the present case, the flux density of Ag ion beam was maintained at about 4 mA/cm2 and samples were placed at the holder without cooling equipment. Ion beam heating induced by the Ag implantation can dramatically increase the mobility of Ag atoms, which greatly promote diffusion of Ag atoms to lower potential energy sites including the sample surface and the interface of SiO2/Si. Fig. 3(a) shows the depth of the Ag peak concentration versus the implanted energy obtained from the XTEM images, RBS simulations and SRIM simulations of all the samples. For the Ag sample, the depth of Ag maximum concentration evaluated from the XTEM observation is 13.0 nm, consistent with the projected range (12.9 nm) from SRIM simulations. Moreover, the depths of the Ag peak concentration measured from XTEM images are in agreement with ones simulated from RBS spectra of the Zn + Ag or Cu+Ag samples. Additionally, it can be also found that the depth corresponding to Ag peak concentration becomes deeper as the kinetic energy of Zn ions increases from 15 to 30 keV, and almost keeps unchanged with further increasing Zn energy to 45 keV. Fig. 3(b)
shows the depth of the Zn peak concentration versus the Znimplanted energy obtained from the RBS simulations. One can see that post Ag ion implantation could cause migration of Zn atoms to deeper region. The migration distances corresponding to the depth of Zn peak concentration are about 5.5, 5.0 and 2.0 nm at the Zn-applied energy of 15, 30 and 45 keV, respectively. Such Zn migration can be attributed to the ion beam heat induced by post Ag ion implantation. However, the depths of Cu peak concentration are almost fixed for the Cu30 and Cu30 + Ag samples, suggesting that the mobility and diffusivity of Cu atoms are lower than that of Zn atoms under the same condition. It may be one of the primary factors to enhance the deposition process of Ag implants. Fig. 4 gives the atomic and vacancy concentration profiles of implants in singly ion implanted samples, as simulated by SRIM code [20]. One can find that the overlap region of Zn and Ag atomic concentration profiles decreases as the kinetic energy of Zn ions varies from 15 to 45 keV (see Fig. 4(a)). In the Ag sample, the Ag atoms mainly distribute in the depth range of 25 nm from surface, marked by solid diamond symbol in Fig. 4(a). However, the amount of the vacancies in this region induced by Zn ion implantation increases with increasing the implanted energy (see Fig. 4(b)). By simulations of the RBS spectra in Fig. 2, the Ag contents in the Zn15 + Ag, Zn30 + Ag and Zn45 + Ag samples are higher than that in the Ag sample by 124.7%, 136.9% and 124.5%, respectively, which indicates that the synergistic effect of the Zn atoms and vacancies contributes to the enhancement of the Ag concentration. By comparing Zn ion implantation with Cu ion implantation at the same energy of 30 keV (Fig. 4), it is found that the atomic content and vacancy concentration profiles have no significant difference
Please cite this article in press as: X. Mu et al., Enhancement of Ag nanoparticles concentration by prior ion implantation, Nucl. Instr. Meth. B (2017), http:// dx.doi.org/10.1016/j.nimb.2017.03.113
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Fig. 4. Atomic (a) and vacancy (b) concentration profiles of implants in singly implanted samples with Zn, Cu or Ag ions, as calculated by SRIM code. The digits in each legend denote the kinetic energy of the corresponding implants. Solid symbol: Atom, hollow symbol: Vacancy.
Fig. 3. Depth of the Ag (a) and Zn (b) peak concentration versus ion implanted energy obtained from XTEM images, RBS simulation and SRIM calculation of all samples.
between Zn and Cu ion implantation. Nevertheless, RBS simulations show that the enhancement of Ag concentration is 87.2% in the Cn30 + Ag sample, which is lower than that (136.9%) in the Zn30 + Ag sample. It suggests that Ag precipitation process is affected by the intrinsic properties of the pre-implanted species as well. The above results clearly show that the pre-implantation of Zn or Cu ions could increase the concentration of Ag NPs by suppressing the self-sputtering effect during the following Ag ion implantation, which strongly depends on the pre-implanted energy and species. The main reasons for the enhancement of Ag NPs concentration could be attributed to the following two aspects. Firstly, vacancies, non-bridging oxygen (NBO) hole centers, threefoldcoordinated silicon centers and electron-hole pairs [22,23] could be created in the substrate after Zn or Cu ion implantation. These defects not only act as lower potential energy sites accommodating the subsequent Ag nucleation [24], but also greatly promote the diffusion of Ag atoms [21]. As we know, the effective diffusivity of implants is one of the key factors to determine the deposition and spatial distribution of NPs in the ion implanted matrix [25]. Synergy of the numerous nucleation sites and the promoted Ag diffusion provides a possibility to enhance the deposition of Ag implants and form Ag NPs in a high volume fraction. Actually,
the similar phenomenon was reported by Wang et al. in the Xe pre-irradiated SiO2 [19] and by Shen et al. in the Ni preimplanted SiO2 [11]. Secondly, the metallic Zn or Cu atoms/clusters could be introduced in the substrate after Zn or Cu ion implantation, which may also play an important role in promoting Ag precipitation in the substrate. On one hand, the ion beam heating induced by the following Ag ion implantation, can dramatically enhance the mobility of Ag atoms themselves [26], along with thermal diffusion coefficient of the pre-implanted atoms. On the other hand, the following Ag implantation could cause migration of the pre-implanted atoms to the deep region via forward recoil processes [27], which has been demonstrated by the RBS simulations in Fig. 3(b). The diffusion and migration of pre-implanted atoms may further carry the mobile Ag atoms initially near the surface into the deeper region, which has been confirmed by the RBS simulations in Fig. 3(a). Consequently, a depleted layer is formed near the surface, which could suppress the sputtering loss of Ag atoms in the near-surface and increase the Ag concentration. Since the intrinsic diffusivity of the metallic Zn atoms in SiO2 is higher than that of Cu atoms [28], Ag content in the Zn30 + Ag sample (136.9%) is more than that in the Cu30 + Ag sample (87.2%). It should be pointed out that the related compounds (e.g. Zn or Cu oxides and Zn silicate [29,30]) could also be formed in the substrate after pre-implantation of Cu or Zn. According to the work by Amekura et al. [30], Zn2SiO4 was formed in the deeper region of the SiO2 after Zn implantation, which is more stable and hardly diffuses. The formation of various compounds may change the properties of substrate, and thus affect sputtering processes in
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some extent. In a word, the diffusion and deposition processes of Ag implants are enhanced along the tracks of pre-implanted ions. 4. Conclusion In summary, Ag NPs with a high volume fraction in SiO2/Si substrate are synthesized by sequential implantation of Zn or Cu and Ag ions. The pre-implantation of Zn or Cu ions can largely suppress the self-sputtering effect during the following Ag ion implantation, which gives rise to increasing the Ag content in the substrate. Moreover, it has been also demonstrated that the suppressing effect strongly depends on the applied energy and species of preimplantation ions. It is suggested that the enhanced deposition of Ag implants can be attributed to vacancy defects, acting as trapping centers for silver and greatly promoting the diffusion of Ag atoms, as well as the intrinsic high diffusivity of prior implanted ions. Our findings may provide an effective route to fabricating metal NPs with a high volume fraction in the shallow nearsurface of dielectrics, which have potential applications based on LSPR and SERS. Acknowledgements This work was supported by the Natural Science Foundation of China (Nos. 11535008, 11675120). References [1] M.E. Stewart, C.R. Anderton, L.B. Thompson, J. Maria, S.K. Gray, J.A. Rogers, R.G. Nuzzo, Chem. Rev. 108 (2008) 494. [2] J.N. Anker, W.P. Hall, O. Lyandres, N.C. Shah, J. Zhao, R.P. Van Duyne, Nat. Mater. 7 (2008) 442. [3] W. Li, X. Xiao, Z. Dai, W. Wu, L. Cheng, F. Mei, X. Zhang, C. Jiang, J. Phys.: Condens. Matter 28 (2016) 254003. [4] U. Kreibig, M. Vollmer, Springer, Berlin, Germany (1995).
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Please cite this article in press as: X. Mu et al., Enhancement of Ag nanoparticles concentration by prior ion implantation, Nucl. Instr. Meth. B (2017), http:// dx.doi.org/10.1016/j.nimb.2017.03.113