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Effect of N ion irradiation combined with thermal annealing on the optical absorption properties of Cu nanoparticles embedded in SiO2
Nuclear Instruments and Methods in Physics Research B 270 (2012) 140–143
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Effect of N ion irradiation combined with thermal annealing on the optical absorption properties of Cu nanoparticles embedded in SiO2 L.H. Zhang a, X.D. Zhang a, Y.Y. Shen a, F. Zhu a, C.L. Liu a,b,⇑ a b
School of Science, Tianjin University, Tianjin 300072, PR China Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, Institute of Advanced Materials Physics Faculty of Science, Tianjin 300072, PR China
a r t i c l e
i n f o
Article history: Received 24 May 2011 Received in revised form 13 September 2011 Available online 29 September 2011 Keywords: Cu nanoparticles SiO2 substrate N ion irradiation Optical properties
a b s t r a c t Effect of the post N ion irradiation on Cu nanoparticles (NPs) has been investigated. The N irradiation at the fluences of 5 1015 and 1 1016 ions/cm2 give rise to different influences on surface plasmon resonance (SPR) of Cu NPs. Low fluence N ion irradiation can induce clear enhancement of the Cu SPR signal with a reduction of full width at half maximum (FWHM) after 600 °C annealing by tailoring the distribution of Cu NPs, while no obvious change of the Cu SPR signal is found in high fluence irradiated samples. The SPR peak becomes very weak after 800 °C annealing. The results from Rutherford back-scattering spectroscopy measurements give the evidence that with the increasing annealing temperature, the distribution of Cu content for all samples becomes narrower in SiO2 matrix. The Cu atoms can diffuse at high annealing temperature. The present study shows the interest of using N ion irradiation for modifying the optical properties of Cu NPs. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Metal nanoparticles (MNPs) embedded in dielectric matrices have been recently received much attention due to the optical nonlinearity associated with the surface plasmon resonance (SPR), which is attractive for applications in all-optical-switch devices [1–3]. Ion implantation is a versatile method to synthesize MNPs embedded in insulators because of the high control of the depth and concentration of implanted atoms, allowing the growth of NPs in a well-defined space region of the host matrix. Many MNPs, such as Ag, Cu, Zn NPs, have been fabricated in SiO2 and Al2O3 in this way [4–6]. However, owing to the uncontrolled nucleation and growth processes, NPs usually exhibit broad spatial and size distributions, which reduce the actual applications because their electronic and optical properties are usually size- and shapedependent [7,8], especially the surface plasmon resonance (SPR). Actually, many theories have been developed to explain the observed experimental behaviors [9]. Although nucleation and growth during implantation can be controlled to some extent by varying ion flux and substrate temperature, the control of the NPs size and shape distribution still remains a challenge. Therefore, alternative approaches should be explored to meet the practical demand.
⇑ Corresponding author at: School of Science, Tianjin University, Tianjin 300072, PR China. Tel./fax: +86 22 27403425. E-mail address: [email protected] (C.L. Liu). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.09.014
In order to tailor the SPR of metallic NPs embedded in matrices, various methods, such as, annealing in different atmospheres, laser annealing, post ion irradiation, etc. [10,11,13], have been employed. Among them, the post ion irradiation has been received much more attention. Up to now, lots of the interesting phenomena have been revealed concerning the effects on formation and evolution of NPs embedded in insulators. For example, a shape transformation from spherical to elongated rods has been observed for MeV ion irradiated Ge, Au, and Ag NPs [12–15]. Moreover, NPs with special structures, such as hollow and sandwiched NPs have also been reported by several groups [16–18]. However, the effects of ion irradiation in keV range on the formation and evolution of NPs embedded insulators as well as its effects on the material’s optical property have been neglected. In this work, we aim to study the effect of the N ion irradiation in keV range on the evolution of Cu NPs embedded in SiO2 matrix with emphasis on the observations of Cu SPR peak. Cu NPs were fabricated in SiO2 by implantation of 45 keV Cu ions, and were then subjected to irradiation of 190 keV N ions at different fluences. The enhanced Cu SPR has been observed upon annealing in nitrogen atmosphere. 2. Experimental Optical-grade silica glasses of 0.5 mm in thickness were used as samples, and were implanted with 45 keV Cu ions at a fluence of 1 1017 ions/cm2 by using a metal vapor vacuum arc (MEVVA) implanter. Some of the implanted samples were then irradiated
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at room temperature by 190 keV N ions at fluences of 5 1015 ions/cm2 (labeled as N1 sample) and 1 1016 ions/cm2 (labeled as N2 sample), respectively. According to the SRIM (Stopping Range of Ion in Matter) 2008 code simulations [19], the projected ranges (Rp) of Cu and N ions are about 38 nm and 480 nm, respectively. After implantation and/or irradiation, the samples were subjected to furnace annealing in the temperature range of 400–800 °C for 1 h with a flow of nitrogen gas. The ultraviolet–visible spectroscopy (UV–vis) measurements were carried out in a wavelength range of 400–700 nm to characterize the optical properties by using a Perkin–Elmer-Lambda 9 UV–vis–NIR spectrometer. Rutherford backscattering spectrometry (RBS) was used to determine the Cu content and depth profile before and after N ion irradiation using a 2.0 MeV He+ beam of 1 mm in diameter with a scattering angle of 170°.
3. Results and discussion Fig. 1 presents optical absorption spectra of the Cu implanted SiO2 samples before and after N ion irradiation and followed by annealing at different temperatures. As shown in Fig. 1(a), a broad absorption peak occurs at about 563 nm in the Cu implanted sample, which is attributed to the surface plasmon resonance (SPR) peak of metallic Cu NPs, indicating that the Cu NPs have been formed even in the as-implanted state. The SPR peak is mainly due to the free electron oscillations in metal particles when excited by electromagnetic radiation. Similar absorption peak has been reported in the Cu implanted SiO2 by several authors [20,21]. After annealing at 400 and 600 °C, a slight red-shift of 5 nm could be found. The spectra of plasmon excitations in small metal particles are well described by the electromagnetic Mie theory [22]. According to the previous calculations, the increasing size in NPs can induce the red-shift of SPR [23,24]. Therefore, the red-shift is mainly related to the nucleation and growth of Cu NPs upon annealing. With increasing annealing temperature to 800 °C, the Cu SPR peak almost disappears. The disappearance of Cu SPR peak at higher annealing temperature has been reported by a variety of researches, and it could be mainly attributed to the following two aspects. On one hand, the different diffusivity of Cu atoms in different atmospheres plays an important role. Comparing to annealing in vacuum, the Cu atoms diffuse a little faster in nitrogen ambient. As the annealing temperature increases, lots of Cu NPs diffuse to the surface and then evaporate there [25–27]. On the other hand, the phenomenon is recognized as a limit of thermal processing of Cu NPs or atoms in SiO2 [27,28]. Owing to size dependence of the melting point of the nanoparticles, lots of Cu NPs or atoms could be dissolved in the matrix as the annealing temperature increases. For example, the melting point of Au bulk is about 1064 °C, while that of Au NPs with diameters less than 6 nm decreases to 627 °C [29]. The UV–vis absorption spectra of the low fluence N ion irradiated sample (i.e. N1 sample) at different annealing temperatures are displayed in Fig. 1(b). At 400 °C annealing, the spectra of the irradiated sample is almost identical to that of the un-irradiated sample. However, when the annealing temperature increases to 600 °C, the intensity of Cu SPR peak (573 nm) is enhanced obviously, along with a reduction of FWHM. It is well known that heavy ion irradiation in the higher energy regime has interesting effects on metal nanoparticles. In this energy regime swift heavy ion irradiation has been found to result, in some cases, in inverse Ostwald ripening [30], that is to say the size of NPs become smaller after irradiation. However, with the lower energy irradiation combined with thermal annealing, Joseph et al. have found the increase of the nanoparticles size and a more uniform distribution of NPs [31]. Thus, the enhancement of SPR can be ascribed to the growth
Fig. 1. Optical absorption spectra of the Cu implanted SiO2 samples before and after N ion irradiation and followed by annealing at different temperatures for 1 h. (a) Cu implanted sample, (b) N1 sample, and (c) N2 sample.
of Cu NPs and a well-defined redistribution. After annealing at 800 °C, the SPR peak of Cu NPs still remains, which nearly disappears in the un-irradiated sample (Fig. 1(a)). Fig. 1(c) gives the optical absorption spectra of the high fluence N ion irradiated SiO2 (e.g. N2 sample) at different annealing temperatures. It is obvious that in the annealing temperature range up to 600 °C, only slight changes are obtained in the Cu SPR peak comparing with the un-irradiated sample, which include the peak position, intensity and FWHM. However, after 800 °C annealing, one can see that the SPR peak of Cu NPs completely disappears. With emphasizing on the effect of irradiation, the optical absorption spectra of the Cu-implanted, N1 and N2 samples after 600 °C annealing are compared and shown in Fig. 2. One can clearly
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Fig. 2. Optical absorption spectra of the Cu implanted, N1 and N2 samples after 600 °C annealing in nitrogen atmosphere for 1 h.
see that the Cu SPR is situated at 568 nm with its FWHM of about 55 nm for the Cu-implanted sample at 600 °C annealing. The post N ion irradiation obviously changes the Cu SPR peak upon annealing, which shows strong dependence on the N ion fluence. After N ion irradiation at low fluence of 5 1015/cm2, the annealing results in a sharp SPR peak at 573 nm with a narrow FWHM (49 nm). As for the high fluence N irradiated SiO2 sample, nevertheless, there is no obvious change of SPR peak upon 600 °C annealing in comparison with that in the un-irradiated sample. The SPR of metal nanoparticles is determined by many factors, such as the dielectric constant of the substrate, the size and shape of the metal NPs, etc. For keV range ion irradiation, the shape of the metal nanoparticles is approximately spherical, and different irradiation fluences give rise to different sizes of nanoparticles [31]. Thus, for our samples, the observed change in the absorption band after N ion irradiation could be mainly due to the change in size variation and redistribution of the Cu NPs. The above results indicate that with appropriate N ion irradiation conditions, it is possible to obtain tailored SPR band, which is very promising for technological applications. The RBS analyses were performed to determine the distribution of Cu atoms for all the samples. Fig. 3(a) presents the RBS spectra obtained in the Cu implanted samples after annealing at different temperatures. It is clear that the profile of Cu atoms in Cu as-implanted sample is nearly symmetrical along the depth. After annealing at 600 °C, most of Cu elements still retain in the matrix, and the peak of Cu atoms moves towards the surface side slightly (see the inseted figure). After 800 °C annealing, the content of Cu atoms sharply decreases, indicating that most of Cu atoms have been evaporated from the surface or dissolved in the matrix. The results are in good accordance with the absorption spectra in Fig. 1(a). Therefore, the nearly disappearance of the Cu SPR peak at 800 °C should be mainly ascribed to the lower content of Cu NPs in matrix. The RBS profiles of Cu concentration in the post N irradiated SiO2, i.e. N1 and N2 samples, are shown in Fig. 3(b) and (c). From the figures, one can find that a majority of Cu atoms retain in the substrate after annealing at 600 °C for both N irradiated samples, which is similar to that observed in the un-irradiated sample. However, no clear shift of the Cu profile towards the surface is found. The result may suggest that the N ion irradiation prevents migration of the Cu atoms towards the surface upon 600 °C annealing. With increasing the annealing temperature to 800 °C, the Cu atoms diffuse towards the sample surface and some fraction of Cu atoms still retain in the matrix for N1 sample. Nevertheless, the Cu RBS peak almost disappears after 800 °C annealing in the
Fig. 3. RBS spectra of the Cu implanted SiO2 samples before and after N ion irradiation and followed by annealing at different temperatures for 1 h. (a) Cu implanted sample, (b) N1 sample, and (c) N2 sample.
N2 sample, which is consistent with the disappearance of the Cu SPR peak (see Fig. 1(c)). In order to give more information in effect of N ion irradiation on evolution of Cu profile, the RBS spectra of the Cu implanted sample before and after N ion irradiation at 600 °C annealing are given in Fig. 4. It can be found the distribution of Cu atoms gets narrower and becomes better after irradiation. The similar phenomena are also confirmed by Joseph et al. [31]. Moreover, one can also see that the Cu profile of the N2 sample moves a bit deeper into the matrix. Actually, under irradiation, the N ions can interact with the Cu NPs, or even penetrate them. As a result, Cu atoms displaced from the NPs may enter into the SiO2 substrate partially owing to forward recoils during the collisions of ions with atoms. The amount of forward recoils obviously depends on ion fluence, i.e. the higher fluence, the more recoils.
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under N ion irradiation has been discussed in terms of irradiation damage, such as defect. However, further study should be carried out to optimize the irradiation parameters to improve the optical properties since the irradiation damage could significantly affect the optical properties of the formed NPs. Acknowledgement This work is supported by National Natural Science Foundation of China (Grant Nos. 10975107 and 11175129). References
Fig. 4. RBS spectra of the Cu implanted, N1 and N2 samples after 600 °C annealing in nitrogen atmosphere for 1 h.
Besides the atoms knocked out from the already formed Cu NPs by irradiation, the N ion irradiation could also introduce various defects in SiO2 matrix, mostly silicon and oxygen vacancies, by direct and/or indirect (recoil implantation) processes [32]. For low fluence irradiation, the small Cu NPs could be broken into atoms, which diffuses as a result of the heat from the irradiation and are absorbed by the large Cu NPs [33]. After 600 °C annealing, a conventional Ostwald ripening will work, resulting in a growth of Cu NPs to larger size, which induce the enhancement of Cu SPR peak in N1 sample. However, when the irradiation fluence increases, the energetic N ions could also interact with the big Cu NPs, the more Cu atoms would be knocked out and penetrated into SiO2 substrate. When the Cu atoms concentration exceeds the solubility of the substrate, a nucleation of Cu NPs occurs at the tail of project range. Upon 600 °C annealing, the Cu NPs grow. It is well known that the SPR is closely relate to the large NPs, the Cu NPs size of high fluence irradiation must be smaller than the low fluence irradiation, which directly result in the difference effects on the SPR peak. Moreover, as irradiation fluence increases, concentration of defects in the SiO2 substrate occurs, the possibility of forming vacancy-impurity (Cu atoms) complexes is more in the ion-irradiated SiO2 matrix. It is reported that defect complexes are more mobile than the lone defects [34]. In this situation, the defect-enhanced diffusion of Cu atoms will happen in the SiO2 matrix at high annealing temperature. The increase of ion irradiation fluence could create more vacancies. Thus, the defect-enhanced diffusion of Cu atoms in the SiO2 matrix would be stronger. That is to say, in the case of high fluence N ion irradiation, the Cu atoms are easier to diffuse to the surface and evaporate from the surface at high temperature than that of low fluence irradiation, which directly induces the disappearance of SPR peak. 4. Conclusion In conclusion, Cu NPs were formed in SiO2 by implantation of 45 keV Cu ions at a fluence of 1 1017 ions/cm2, and were then irradiated by 190 keV N ions at fluences of 5 1015 and 1 1016 ions/cm2, respectively. The enhanced Cu SPR signal has been observed in low fluence N ion irradiated sample upon 600 °C annealing, while no clear modification of the Cu SPR band has been found for the high fluence irradiated sample. The result suggests that the N ion irradiation in keV range could be used to modulate the Cu SPR band in SiO2 if the irradiation and annealing conditions could be adequately chosen. The evolution of the Cu NPs
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Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Effect of N ion irradiation combined with thermal annealing on the optical absorption properties of Cu nanoparticles embedded in SiO2 L.H. Zhang a, X.D. Zhang a, Y.Y. Shen a, F. Zhu a, C.L. Liu a,b,⇑ a b
School of Science, Tianjin University, Tianjin 300072, PR China Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, Institute of Advanced Materials Physics Faculty of Science, Tianjin 300072, PR China
a r t i c l e
i n f o
Article history: Received 24 May 2011 Received in revised form 13 September 2011 Available online 29 September 2011 Keywords: Cu nanoparticles SiO2 substrate N ion irradiation Optical properties
a b s t r a c t Effect of the post N ion irradiation on Cu nanoparticles (NPs) has been investigated. The N irradiation at the fluences of 5 1015 and 1 1016 ions/cm2 give rise to different influences on surface plasmon resonance (SPR) of Cu NPs. Low fluence N ion irradiation can induce clear enhancement of the Cu SPR signal with a reduction of full width at half maximum (FWHM) after 600 °C annealing by tailoring the distribution of Cu NPs, while no obvious change of the Cu SPR signal is found in high fluence irradiated samples. The SPR peak becomes very weak after 800 °C annealing. The results from Rutherford back-scattering spectroscopy measurements give the evidence that with the increasing annealing temperature, the distribution of Cu content for all samples becomes narrower in SiO2 matrix. The Cu atoms can diffuse at high annealing temperature. The present study shows the interest of using N ion irradiation for modifying the optical properties of Cu NPs. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Metal nanoparticles (MNPs) embedded in dielectric matrices have been recently received much attention due to the optical nonlinearity associated with the surface plasmon resonance (SPR), which is attractive for applications in all-optical-switch devices [1–3]. Ion implantation is a versatile method to synthesize MNPs embedded in insulators because of the high control of the depth and concentration of implanted atoms, allowing the growth of NPs in a well-defined space region of the host matrix. Many MNPs, such as Ag, Cu, Zn NPs, have been fabricated in SiO2 and Al2O3 in this way [4–6]. However, owing to the uncontrolled nucleation and growth processes, NPs usually exhibit broad spatial and size distributions, which reduce the actual applications because their electronic and optical properties are usually size- and shapedependent [7,8], especially the surface plasmon resonance (SPR). Actually, many theories have been developed to explain the observed experimental behaviors [9]. Although nucleation and growth during implantation can be controlled to some extent by varying ion flux and substrate temperature, the control of the NPs size and shape distribution still remains a challenge. Therefore, alternative approaches should be explored to meet the practical demand.
⇑ Corresponding author at: School of Science, Tianjin University, Tianjin 300072, PR China. Tel./fax: +86 22 27403425. E-mail address: [email protected] (C.L. Liu). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.09.014
In order to tailor the SPR of metallic NPs embedded in matrices, various methods, such as, annealing in different atmospheres, laser annealing, post ion irradiation, etc. [10,11,13], have been employed. Among them, the post ion irradiation has been received much more attention. Up to now, lots of the interesting phenomena have been revealed concerning the effects on formation and evolution of NPs embedded in insulators. For example, a shape transformation from spherical to elongated rods has been observed for MeV ion irradiated Ge, Au, and Ag NPs [12–15]. Moreover, NPs with special structures, such as hollow and sandwiched NPs have also been reported by several groups [16–18]. However, the effects of ion irradiation in keV range on the formation and evolution of NPs embedded insulators as well as its effects on the material’s optical property have been neglected. In this work, we aim to study the effect of the N ion irradiation in keV range on the evolution of Cu NPs embedded in SiO2 matrix with emphasis on the observations of Cu SPR peak. Cu NPs were fabricated in SiO2 by implantation of 45 keV Cu ions, and were then subjected to irradiation of 190 keV N ions at different fluences. The enhanced Cu SPR has been observed upon annealing in nitrogen atmosphere. 2. Experimental Optical-grade silica glasses of 0.5 mm in thickness were used as samples, and were implanted with 45 keV Cu ions at a fluence of 1 1017 ions/cm2 by using a metal vapor vacuum arc (MEVVA) implanter. Some of the implanted samples were then irradiated
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at room temperature by 190 keV N ions at fluences of 5 1015 ions/cm2 (labeled as N1 sample) and 1 1016 ions/cm2 (labeled as N2 sample), respectively. According to the SRIM (Stopping Range of Ion in Matter) 2008 code simulations [19], the projected ranges (Rp) of Cu and N ions are about 38 nm and 480 nm, respectively. After implantation and/or irradiation, the samples were subjected to furnace annealing in the temperature range of 400–800 °C for 1 h with a flow of nitrogen gas. The ultraviolet–visible spectroscopy (UV–vis) measurements were carried out in a wavelength range of 400–700 nm to characterize the optical properties by using a Perkin–Elmer-Lambda 9 UV–vis–NIR spectrometer. Rutherford backscattering spectrometry (RBS) was used to determine the Cu content and depth profile before and after N ion irradiation using a 2.0 MeV He+ beam of 1 mm in diameter with a scattering angle of 170°.
3. Results and discussion Fig. 1 presents optical absorption spectra of the Cu implanted SiO2 samples before and after N ion irradiation and followed by annealing at different temperatures. As shown in Fig. 1(a), a broad absorption peak occurs at about 563 nm in the Cu implanted sample, which is attributed to the surface plasmon resonance (SPR) peak of metallic Cu NPs, indicating that the Cu NPs have been formed even in the as-implanted state. The SPR peak is mainly due to the free electron oscillations in metal particles when excited by electromagnetic radiation. Similar absorption peak has been reported in the Cu implanted SiO2 by several authors [20,21]. After annealing at 400 and 600 °C, a slight red-shift of 5 nm could be found. The spectra of plasmon excitations in small metal particles are well described by the electromagnetic Mie theory [22]. According to the previous calculations, the increasing size in NPs can induce the red-shift of SPR [23,24]. Therefore, the red-shift is mainly related to the nucleation and growth of Cu NPs upon annealing. With increasing annealing temperature to 800 °C, the Cu SPR peak almost disappears. The disappearance of Cu SPR peak at higher annealing temperature has been reported by a variety of researches, and it could be mainly attributed to the following two aspects. On one hand, the different diffusivity of Cu atoms in different atmospheres plays an important role. Comparing to annealing in vacuum, the Cu atoms diffuse a little faster in nitrogen ambient. As the annealing temperature increases, lots of Cu NPs diffuse to the surface and then evaporate there [25–27]. On the other hand, the phenomenon is recognized as a limit of thermal processing of Cu NPs or atoms in SiO2 [27,28]. Owing to size dependence of the melting point of the nanoparticles, lots of Cu NPs or atoms could be dissolved in the matrix as the annealing temperature increases. For example, the melting point of Au bulk is about 1064 °C, while that of Au NPs with diameters less than 6 nm decreases to 627 °C [29]. The UV–vis absorption spectra of the low fluence N ion irradiated sample (i.e. N1 sample) at different annealing temperatures are displayed in Fig. 1(b). At 400 °C annealing, the spectra of the irradiated sample is almost identical to that of the un-irradiated sample. However, when the annealing temperature increases to 600 °C, the intensity of Cu SPR peak (573 nm) is enhanced obviously, along with a reduction of FWHM. It is well known that heavy ion irradiation in the higher energy regime has interesting effects on metal nanoparticles. In this energy regime swift heavy ion irradiation has been found to result, in some cases, in inverse Ostwald ripening [30], that is to say the size of NPs become smaller after irradiation. However, with the lower energy irradiation combined with thermal annealing, Joseph et al. have found the increase of the nanoparticles size and a more uniform distribution of NPs [31]. Thus, the enhancement of SPR can be ascribed to the growth
Fig. 1. Optical absorption spectra of the Cu implanted SiO2 samples before and after N ion irradiation and followed by annealing at different temperatures for 1 h. (a) Cu implanted sample, (b) N1 sample, and (c) N2 sample.
of Cu NPs and a well-defined redistribution. After annealing at 800 °C, the SPR peak of Cu NPs still remains, which nearly disappears in the un-irradiated sample (Fig. 1(a)). Fig. 1(c) gives the optical absorption spectra of the high fluence N ion irradiated SiO2 (e.g. N2 sample) at different annealing temperatures. It is obvious that in the annealing temperature range up to 600 °C, only slight changes are obtained in the Cu SPR peak comparing with the un-irradiated sample, which include the peak position, intensity and FWHM. However, after 800 °C annealing, one can see that the SPR peak of Cu NPs completely disappears. With emphasizing on the effect of irradiation, the optical absorption spectra of the Cu-implanted, N1 and N2 samples after 600 °C annealing are compared and shown in Fig. 2. One can clearly
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Fig. 2. Optical absorption spectra of the Cu implanted, N1 and N2 samples after 600 °C annealing in nitrogen atmosphere for 1 h.
see that the Cu SPR is situated at 568 nm with its FWHM of about 55 nm for the Cu-implanted sample at 600 °C annealing. The post N ion irradiation obviously changes the Cu SPR peak upon annealing, which shows strong dependence on the N ion fluence. After N ion irradiation at low fluence of 5 1015/cm2, the annealing results in a sharp SPR peak at 573 nm with a narrow FWHM (49 nm). As for the high fluence N irradiated SiO2 sample, nevertheless, there is no obvious change of SPR peak upon 600 °C annealing in comparison with that in the un-irradiated sample. The SPR of metal nanoparticles is determined by many factors, such as the dielectric constant of the substrate, the size and shape of the metal NPs, etc. For keV range ion irradiation, the shape of the metal nanoparticles is approximately spherical, and different irradiation fluences give rise to different sizes of nanoparticles [31]. Thus, for our samples, the observed change in the absorption band after N ion irradiation could be mainly due to the change in size variation and redistribution of the Cu NPs. The above results indicate that with appropriate N ion irradiation conditions, it is possible to obtain tailored SPR band, which is very promising for technological applications. The RBS analyses were performed to determine the distribution of Cu atoms for all the samples. Fig. 3(a) presents the RBS spectra obtained in the Cu implanted samples after annealing at different temperatures. It is clear that the profile of Cu atoms in Cu as-implanted sample is nearly symmetrical along the depth. After annealing at 600 °C, most of Cu elements still retain in the matrix, and the peak of Cu atoms moves towards the surface side slightly (see the inseted figure). After 800 °C annealing, the content of Cu atoms sharply decreases, indicating that most of Cu atoms have been evaporated from the surface or dissolved in the matrix. The results are in good accordance with the absorption spectra in Fig. 1(a). Therefore, the nearly disappearance of the Cu SPR peak at 800 °C should be mainly ascribed to the lower content of Cu NPs in matrix. The RBS profiles of Cu concentration in the post N irradiated SiO2, i.e. N1 and N2 samples, are shown in Fig. 3(b) and (c). From the figures, one can find that a majority of Cu atoms retain in the substrate after annealing at 600 °C for both N irradiated samples, which is similar to that observed in the un-irradiated sample. However, no clear shift of the Cu profile towards the surface is found. The result may suggest that the N ion irradiation prevents migration of the Cu atoms towards the surface upon 600 °C annealing. With increasing the annealing temperature to 800 °C, the Cu atoms diffuse towards the sample surface and some fraction of Cu atoms still retain in the matrix for N1 sample. Nevertheless, the Cu RBS peak almost disappears after 800 °C annealing in the
Fig. 3. RBS spectra of the Cu implanted SiO2 samples before and after N ion irradiation and followed by annealing at different temperatures for 1 h. (a) Cu implanted sample, (b) N1 sample, and (c) N2 sample.
N2 sample, which is consistent with the disappearance of the Cu SPR peak (see Fig. 1(c)). In order to give more information in effect of N ion irradiation on evolution of Cu profile, the RBS spectra of the Cu implanted sample before and after N ion irradiation at 600 °C annealing are given in Fig. 4. It can be found the distribution of Cu atoms gets narrower and becomes better after irradiation. The similar phenomena are also confirmed by Joseph et al. [31]. Moreover, one can also see that the Cu profile of the N2 sample moves a bit deeper into the matrix. Actually, under irradiation, the N ions can interact with the Cu NPs, or even penetrate them. As a result, Cu atoms displaced from the NPs may enter into the SiO2 substrate partially owing to forward recoils during the collisions of ions with atoms. The amount of forward recoils obviously depends on ion fluence, i.e. the higher fluence, the more recoils.
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under N ion irradiation has been discussed in terms of irradiation damage, such as defect. However, further study should be carried out to optimize the irradiation parameters to improve the optical properties since the irradiation damage could significantly affect the optical properties of the formed NPs. Acknowledgement This work is supported by National Natural Science Foundation of China (Grant Nos. 10975107 and 11175129). References
Fig. 4. RBS spectra of the Cu implanted, N1 and N2 samples after 600 °C annealing in nitrogen atmosphere for 1 h.
Besides the atoms knocked out from the already formed Cu NPs by irradiation, the N ion irradiation could also introduce various defects in SiO2 matrix, mostly silicon and oxygen vacancies, by direct and/or indirect (recoil implantation) processes [32]. For low fluence irradiation, the small Cu NPs could be broken into atoms, which diffuses as a result of the heat from the irradiation and are absorbed by the large Cu NPs [33]. After 600 °C annealing, a conventional Ostwald ripening will work, resulting in a growth of Cu NPs to larger size, which induce the enhancement of Cu SPR peak in N1 sample. However, when the irradiation fluence increases, the energetic N ions could also interact with the big Cu NPs, the more Cu atoms would be knocked out and penetrated into SiO2 substrate. When the Cu atoms concentration exceeds the solubility of the substrate, a nucleation of Cu NPs occurs at the tail of project range. Upon 600 °C annealing, the Cu NPs grow. It is well known that the SPR is closely relate to the large NPs, the Cu NPs size of high fluence irradiation must be smaller than the low fluence irradiation, which directly result in the difference effects on the SPR peak. Moreover, as irradiation fluence increases, concentration of defects in the SiO2 substrate occurs, the possibility of forming vacancy-impurity (Cu atoms) complexes is more in the ion-irradiated SiO2 matrix. It is reported that defect complexes are more mobile than the lone defects [34]. In this situation, the defect-enhanced diffusion of Cu atoms will happen in the SiO2 matrix at high annealing temperature. The increase of ion irradiation fluence could create more vacancies. Thus, the defect-enhanced diffusion of Cu atoms in the SiO2 matrix would be stronger. That is to say, in the case of high fluence N ion irradiation, the Cu atoms are easier to diffuse to the surface and evaporate from the surface at high temperature than that of low fluence irradiation, which directly induces the disappearance of SPR peak. 4. Conclusion In conclusion, Cu NPs were formed in SiO2 by implantation of 45 keV Cu ions at a fluence of 1 1017 ions/cm2, and were then irradiated by 190 keV N ions at fluences of 5 1015 and 1 1016 ions/cm2, respectively. The enhanced Cu SPR signal has been observed in low fluence N ion irradiated sample upon 600 °C annealing, while no clear modification of the Cu SPR band has been found for the high fluence irradiated sample. The result suggests that the N ion irradiation in keV range could be used to modulate the Cu SPR band in SiO2 if the irradiation and annealing conditions could be adequately chosen. The evolution of the Cu NPs
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