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Modifications in structure and optical property of Cu nanoparticles in SiO2 by post heavy ion irradiation
Nuclear Instruments and Methods in Physics Research B 326 (2014) 28–32
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Modifications in structure and optical property of Cu nanoparticles in SiO2 by post heavy ion irradiation Changlong Liu a,b,c,⇑, Nana Wang a, Jun Wang a, Huixian Liu a, Guangyi Jia a, Xiaoyu Mu a a
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 c Key Laboratory of Beam Technology and Material Modification of Ministry of Education, Beijing Normal University, Beijing 100875, PR China b
a r t i c l e
i n f o
Article history: Received 29 June 2013 Accepted 3 October 2013 Available online 20 January 2014 Keywords: Ion beam technology Nanoparticles Distribution control Optical properties
a b s t r a c t The implantation-synthesized Cu nanoparticles (NPs) in silica were irradiated with 500 keV Xe and Ar ions, respectively. After Xe ion irradiation at a fluence of 2 1016/cm2, the average diameter of Cu NPs was increased from 7.3 to 8.5 nm, and especially, Cu NPs with a diameter of 11–14 nm were formed beyond the projected range of Cu ions and nearly aligned at the same depth, which presented a higher volume fraction. As a result, the Cu surface plasmon resonance (SPR) absorption peak was enhanced. However, if Xe ion fluence was less than 1 1016/cm2, no clear variation of the Cu SPR absorption peak could be found. Further, it was also revealed that Xe ion irradiation caused the Cu SPR absorption peak to more drastically change than Ar ion irradiation at the same ion fluence. The underlying processes for the above findings were discussed and tentatively proposed. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Owing to unique surface plasmon resonance (SPR) property with a frequency in the visible region, glass-based nanocomposites containing noble metal nanoparticles (NPs) are considered as excellent candidates for the fabrication of ultrafast optical switches [1], optical sensors [2], and waveguides [3], etc. Among various methods to synthesize this kind of nanocomposites, ion implantation has been proved to be quite suitable since it allows reaching a high metal filling factor (i.e., volume fraction) in the substrate without constrain of solubility limit as well as high cluster free energy of formation suffered by other methods [4–6]. Nevertheless, it should be noted that the formed NPs in the implanted samples usually present broad size and spatial distributions due to the uncontrolled nucleation and growth processes, which inevitably deteriorate the SPR properties of nanocomposites [6–8]. Although the position of precipitates along with the size of NPs can be controlled to some extent via implantation parameter optimization and/or post annealing [9,10], it is still a challenging task to tailor the size and spatial distributions of the implantation-synthesized NPs [6]. Recently, heavy ion irradiation has been conceived as a promising method to modify the embedded NPs in dielectric matrices. In virtue of the swift heavy ion (SHI) irradiation, as summarized in ⇑ Corresponding author at: School of Science, Tianjin University, Tianjin 300072, PR China. Tel./fax: +86 22 27403425. E-mail address: [email protected] (C. Liu). 0168-583X/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.10.045
Ref. [11], spherical NPs could be transformed into anisotropic ones or even nanorods along the ion tracks. One of the most widely accepted mechanisms is the interplay of the ion-hammering effect of the host matrix and the melting of NPs by the thermal spike effect, in which the molten NPs are elongated by the in-plane stress of the ion-hammering effect [12]. That is to say, the electronic excitation induced effects on both NPs and substrate are superior to other factors in the case of SHI irradiation [13,14]. In nature, the heavy ion irradiation induced modifications of the embedded NPs can be attributed to the energy deposition from ions by means of nuclear collision and electronic excitation processes [15]. Roles of the above two processes are strongly dependent on ion energy and species. With decreasing the ion energy down to the keV range, the deposited energy via the nuclear collision process becomes more and more prominent whereas that via the ionization and/or electronic excitation process gradually decreases. Therefore, the heavy ion irradiation in the keV energy range can be adopted to effectively tailor the size and spatial distributions of the embedded NPs through the relatively dominant nuclear collision process, especially, when the atomic number of ions is much larger than that of target atoms. In fact, an early work by our group has already evidenced that the uniform ZnO NPs could be fabricated in the silica after annealing in oxygen ambient by combining Zn ion implantation with post 500 keV Xe ion irradiation [16]. Moreover, we also employed 500 keV Xe ion irradiation at a fluence of 1 1016/cm2 to modify the size and spatial distributions of Ag NPs embedded in the Ag ion implanted SiO2, leading to the elimination of small
C. Liu et al. / Nuclear Instruments and Methods in Physics Research B 326 (2014) 28–32
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Ag NPs along with the formation of large ones near the end of Ag ion range [17]. In this work, the Cu ion implanted silica glasses were subsequently irradiated with 500 keV Xe and Ar ions, respectively, and the size and spatial distributions of the embedded Cu NPs along with their optical absorption property were studied. The work was intended to serve three purposes, i.e., tentatively proposing the underlying processes for the size and spatial redistributions of Cu NPs, revealing the dependence of the Xe ion irradiation induced modifications on ion fluence, and demonstrating the difference of Xe and Ar ion irradiations. The obtained results may be potential for better understanding the interaction between the heavy ions in the keV energy range, the embedded NPs and the host matrix. 2. Experimental Optical-grade amorphous SiO2 (a-SiO2) slices of 1 mm in thickness were implanted with 45 keV Cu ions at a fluence of 1 1017/cm2 to fabricate the embedded Cu NPs. For easy reference, the Cu ion implanted a-SiO2 slice is called the Cu sample in the following text. The Cu implantation was performed on a metal vapor vacuum arc (MEVVA) implanter with a tilted ion beam by 45° from the sample surface. During the Cu ion implantation, the ion beam flux density was maintained at 4 lA/cm2 or below, and the target plate was set to rotate at a constant speed in order to assure the implantation uniformity. Subsequently, some of the Cu samples were irradiated with 500 keV Xe ions at normal incidence to three different fluences, i.e., 1 1015, 1 1016 and 2 1016/cm2. To reveal the effect of ion species on particle modification, one of the Cu samples was also irradiated with 500 keV Ar ions at normal incidence to a fluence of 1 1016/cm2. Irradiations of Xe and Ar ions were conducted on a LC-4 high energy ion implanter, and the ion beam flux density for both cases was kept below 1 lA/ cm2. Without regard to the surface sputtering, the projected ranges of Cu ions (oblique incidence with an angle of 45°), Xe and Ar ions (normal incidence) in SiO2 can be calculated by using SRIM 2010 code [18], which are about 27, 194 and 529 nm, respectively. After implantation and/or irradiation, the samples were subjected to furnace annealing at 500 °C for 1 h in flowing nitrogen ambient. Grazing incidence X-ray diffraction (GIXRD) measurements were performed to identify the formation of Cu NPs along with their structure on a Philips X’pert pro X-ray diffractometer using Cu Ka line (0.154 nm) at 0.3° incidence angle as the radiation source. GIXRD spectra were registered in an angle range from 30° to 50°, and the scanning speed was set as 1°/min. Crosssectional transmission electron microscope (XTEM) observations were made on a Tecnai G2 F20 S-Twin TEM operating at an acceleration voltage of 200 kV. Bright field imaging technique was used to evaluate the size and spatial distributions of Cu NPs. Moreoever, optical absorbance (OA) spectra were recorded in a wavelength range from 200 to 800 nm on a double-beam spectrophotometer (UV–3100PC). The scanning interval and the bandwidth were 0.5 and 2 nm, respectively. 3. Results and discussion Fig. 1 shows the GIXRD spectra of the Cu samples before and after irradiation of 500 keV Xe ions at different fluences. One can see that the unirradiated Cu sample presents a weak and broad diffraction peak at about 43.4°, which corresponds to Cu (1 1 1) plane of fcc-structured Cu, indicating the formation of Cu NPs in the substrate. After Xe ion irradiation, Cu (1 1 1) diffraction peak becomes stronger and narrower with increasing Xe ion fluence, indicating that Xe ion irradiation causes the embedded Cu NPs to grow in size.
Fig. 1. GIXRD spectra of the Cu samples before and after irradiation of 500 keV Xe ions at different fluences. All spectra were shifted vertically for clarity.
Fig. 2 presents the main XTEM results on the Cu samples before and after 500 keV Xe ion irradiation at a fluence of 2 1016/cm2. In the unirradiated Cu sample (Fig. 2(a)), one can see that Cu NPs are mainly distributed in a region from surface up to of about 60 nm in the bulk, which can be further divided into three layers, i.e., A, B and C. Large Cu NPs with a diameter of 7–12 nm are mainly embedded in layer B, while small ones are mainly formed in layers A and C. Layer B is centered at a depth of about 27 nm, which matches well with the projected range of Cu ions. The Cu NPs in the unirradiated Cu sample exhibit a broad size distribution ranging from 2 to 12 nm in diameter, and their average diameter is about 7.3 nm, as shown in the inset of Fig. 2(a). After Xe ion irradiation, the size and spatial distributions of Cu NPs are remarkably changed, as demonstrated in Fig. 2(b) and its inset. Firstly, the region distributed with Cu NPs is slightly narrowed to about 50 nm in thickness. Secondly, large Cu NPs with a size of 11–14 nm in diameter are nearly aligned at the same depth of about 42 nm, which greatly exceeds the projected range (27 nm) of Cu ions. Thirdly, lots of Cu NPs with relatively small size are formed in a region corresponding to layers A and B labeled in Fig. 2(a). Lastly, the percentage of large Cu NPs with a diameter from 11 to 14 nm dramatically increases while that of small ones slightly decreases (see the inset of Fig. 2(b)). As a result, the average diameter of Cu NPs in the irradiated Cu sample is increased to about 8.5 nm. The results shown in Fig. 2(b) can be explained as follows. Firstly, owing to the nuclear collisions between Xe ions and Cu atoms, large numbers of Cu atoms can be knocked out from NPs and move toward deep positions via forward recoil processes. As a result, on the one hand, the Cu NPs formed in the shallow region, i.e., those in layers A and B (see Fig. 2(a)), are partially dissolved and become small in size. On the other hand, the concentration of Cu atoms in a region beyond the projected range of Cu ions can be increased significantly. Secondly, the ion beam heating, which comes from the ion kinetic energy [19] and can be approximately described by the electronic energy loss of the incident ions [20], can give rise to the nucleation and growth of Cu NPs via the classical Ostwald ripening process [21]. It can be rationally speculated from Fig. 2(b) that in the neighborhood of a depth of about 42 nm, the nucleation and growth of Cu NPs should be more prominent due to a higher concentration of Cu atoms, and thus the formation of large Cu NPs, along with the elimination of small Cu clusters formed near the end of layer C in Fig. 2(a), can be expected. Thirdly, the irradiation induced defects in the substrate, which
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Fig. 2. XTEM images of the Cu samples (a) before and (b) after 500 keV Xe ion irradiation at a fluence of 2 1016/cm2. The inserted figures give the size distribution of Cu NPs in two samples, respectively.
mainly result from the nuclear collisions of the incident ions with the substrate atoms in the case of heavy ion irradiation in the keV energy range, can markedly enhance the diffusivity of Cu atoms [22]. Thereby the growth of Cu NPs is promoted to a great degree since it is a defect-controlled process [7,20]. Based on the above discussion, one can conclude that the size and spatial redistributions of Cu NPs together with their increased average size can be attributed to three synergic factors, i.e., the nuclear collisions of Xe ions with Cu atoms, the ion beam heating and the irradiation induced defects. Among them, the nuclear collisions between Xe ions and Cu atoms play a crucial role. The OA spectra of the unirradiated and Xe ion irradiated Cu samples at different fluences before and after 500 °C annealing for 1 h in flowing nitrogen ambient are given in Figs. 3(a) and (b), respectively. From Fig. 3(a), it is clear that the unirradiated Cu sample presents an absorption peak at about 563 nm, which can be ascribed to SPR of the formed Cu NPs in the substrate
Fig. 3. OA spectra of the Cu samples unirradiated and irradiated with 500 keV Xe ions at different fluences (a) before and (b) after 500 °C annealing for 1 h in flowing nitrogen ambient.
[23,24]. After 500 °C annealing, as shown in Fig. 3(b), the absorbance of the unirradiated Cu sample is unitarily increased, and the Cu SPR absorption peak becomes stronger and narrower together with a slight red shift of about 8 nm. In addition, a weak absorption peak also appears around 310 nm (4 eV). The unitarily increased absorbance can be attributed to the dispersed Cu atoms [23], resulting from the dissolution of small Cu NPs due to their lower melting point [25]. In the work by Amekura et al. [26], the absorption cross section spectra of Cu colloids with different diameters were calculated according to the effective medium theory, and the obtained results demonstrated that with increasing the particle diameter, the Cu SPR absorption peak became stronger together with position blue shift. However, it should be also noted that by increasing the volume fraction of metal NPs, the corresponding SPR absorption peak can be enhanced and shift to a longer wavelength [7]. Therefore, we correlate the enhanced Cu SPR absorption peak and its position red shift for the unirradiated Cu sample after annealing with the increased size and volume fraction of Cu NPs resulting from the aggregation and growth of Cu NPs driven by thermal treatment. As for the weak absorption peak at about 310 nm, it may be related to the direct excitation of conduction electrons to high levels, which requires energy of 4 eV in Cu [8]. Actually, it is very reasonable to assign this weak peak to the absorption band between 280 and 510 nm [27], which can be usually observed in the silica implanted with Cu ions and subsequently annealed at a higher temperature in protective or reducing atmosphere [23,24,27]. Although the intensity of the absorption band between 280 and 510 nm is dependent on the size of Cu NPs [26], a latest research has also revealed that this band is quite visible for the crystalline Cu NPs but nearly undetectable for the amorphous Cu NPs [23]. On this basis, one can presume that the appearance of either the weak peak at about 310 nm or the absorption band between 280 and 510 nm is an indication of the transformation of amorphous Cu NPs to crystalline ones induced by thermal treatment or other methods. Returning to Fig. 3(a), it clearly reveals the relation between spectral changes and Xe ion fluence. Firstly, the absorbance of the Cu sample is unitarily decreased after Xe ion irradiation at a fluence of 1 1015/cm2, whereas it is gradually increased with increasing Xe ion fluence from 1 1016 to 2 1016/cm2. The unitarily decreased absorbance for the lowest fluence is mainly due
C. Liu et al. / Nuclear Instruments and Methods in Physics Research B 326 (2014) 28–32
to the recovery of the Cu ion implantation produced defects in the host matrix, in which the ion beam heating plays a dominant role. In contrast, the unitarily increased absorbance for two other higher fluences can be attributed to the Xe ion irradiation induced defects in the substrate, whose amount is directly proportional to Xe ion fluence. Secondly, no clear change of the Cu SPR absorption peak can be found if the Xe ion fluence is less than 1 1016/cm2, indicating in principle a critical fluence for Xe ion irradiation. After increasing Xe ion fluence up to 2 1016/cm2, a significantly enhanced and narrowed SPR absorption peak appears at about 574 nm, which can be mainly assigned to those Cu NPs formed at a depth of about 42 nm due to their large size, high volume fraction and relatively narrow size distribution as revealed in Fig. 2(b). Thirdly, the weak absorption peak at about 310 nm cannot be found for the Cu sample irradiated at a fluence of 1 1015/cm2, whereas it is quite clear for the Cu samples irradiated at two other higher fluences, i.e., 1 1016 and 2 1016/cm2. Since the appearance of the absorption peak around 310 nm is an indication of the transformation of amorphous Cu NPs to crystalline ones as presumed above, an annealing crystallization process of Cu NPs along with their growth in size as revealed in Fig. 1 is believed to occur owing to the stronger ion beam heating induced by Xe ion irradiation at higher fluences. After 500 °C annealing, it can be observed from Fig. 3(b) that owing to the thermal growth of Cu NPs, the Cu SPR absorption peak accordingly becomes stronger and narrower together with a position red shift for each irradiated Cu sample. In addition, it can be also found from Fig. 3(b) that the absorbance of the Cu sample irradiated at a fluence of 1 1015/cm2 is unitarily increased, whereas those of the Cu samples irradiated at two other higher fluences are decreased to different degrees. The former, which is quite similar to that detected from the unirradiated Cu sample, can be attributed to the dispersed Cu atoms as a result of the dissolution of small Cu NPs. The latter is mainly due to the recovery of the Xe ion irradiation induced defects. Based on the above results and the spectral evolution of the unirradiated Cu sample, it seems to be reasonable to consider the small Cu clusters formed near the end of Cu ion range (refer to layer C in Fig. 2(a)) as main contributors to the unitarily increased absorbance for the Cu samples unirradiated and irradiated at a fluence of 1 1015/cm2 after thermal treatment. In the Cu sample irradiated at a fluence of 2 1016/cm2, small Cu clusters can be hardly found beyond the region distributed with large Cu NPs, as demonstrated in Fig. 2(b). Thereby, the unitarily decreased absorbance after annealing for this sample is basically determined by the recovery of the radiation damage. Fig. 4 shows the OA spectra of the Cu samples unirradiated and irradiated with 500 keV Xe and Ar ions at the same fluence of 1 1016/cm2 before and after 500 °C annealing for 1 h in flowing nitrogen ambient. From Figs. 4(a) and (b), it can be clearly found that both Xe ion irradiation and Ar ion irradiation can lead to enhancement of the Cu SPR absorption peak along with a position red shift. A further comparison between two Cu SPR absorption peaks detected from the Cu samples irradiated by Xe and Ar ions either before or after annealing unambiguously confirms that Xe ion irradiation can cause the Cu SPR absorption peak to more drastically change than Ar ion irradiation at the same energy and fluence. In addition, such a comparison also suggests that different physical processes should take place in the Cu samples irradiated with Xe and Ar ions. The ion irradiation induced modification in the Cu SPR absorption peak, more correctly, the ion irradiation induced modifications of the embedded Cu NPs in size and spatial distributions are actually related to the energy deposition from the incident ions. The deposited energy can be semi-quantitatively estimated with the nuclear energy loss (Sn) and electronic energy loss (Se) of ions in the substrate. To better reveal the underlying processes in the Xe
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Fig. 4. OA spectra of the Cu samples unirradiated and irradiated with 500 keV Xe and Ar ions at the same fluence of 1 1016/cm2 (a) before and (b) after 500 °C annealing for 1 h in flowing nitrogen ambient.
and Ar ion irradiated Cu samples, Fig. 5 gives the depth profiles of Sn and Se of 500 keV Xe and Ar ions in SiO2 as well as the concentration profile of Cu atoms in SiO2. The depth profiles of Sn and Se were calculated according to the algorithm proposed by Messenger et al. [28], and the concentration profile of Cu atoms was simulated by using SRIM 2010 code [18]. Especially, the contribution of the recoiled ions was also taken into consideration during the calculations of Se. From Fig. 5, one can extract that around the projected range of Cu ions, Sn and Se of Xe ions are 1.04 and 1.01 keV/nm, while Sn and Se of Ar ions are 0.18 and 0.69 keV/nm, respectively. Obviously, the deposited energy in the Cu ion implanted region via the electronic energy loss process during Xe ion irradiation is higher than that during Ar ion irradiation, and thus the ion beam heating induced by Xe ions is more intense than that induced by Ar ions. In particular, the nuclear energy loss of Xe ions is quite larger than that of Ar ions in the Cu ion implanted layer, indicating that the nuclear collisions of the incident ions with Cu atoms and the substrate atoms in the Xe ion irradiated Cu sample are much more prominent as compared with those in the Ar ion irradiated Cu sample. On these bases, again noting the atomic numbers of Xe and Ar elements, one can conclude that after Xe ion irradiation,
Fig. 5. The calculated concentration profile of Cu atoms in SiO2 and depth profiles of nuclear energy loss (Sn) and electronic energy loss (Se) for 500 keV Xe and Ar ions in SiO2.
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the size and spatial distributions of the embedded Cu NPs can be substantially modified, and large Cu NPs with a relatively high volume fraction can be formed, which may be similar to those shown in Fig. 2(b). Therefore, a clearly enhanced and narrowed Cu SPR absorption peak can be detected from the Xe ion irradiated Cu sample. In contrast, the embedded Cu NPs cannot be effectively broken up during Ar ion irradiation, and thereby the ion beam heating is the prevailing factor, which mainly leads to the growth of Cu NPs around the projected range of Cu ions (see Fig. 2(a)) at expense of smaller clusters in the neighborhood. Although an enhanced and narrowed Cu SPR absorption peak can also be observed after Ar ion irradiation, it is still not comparable with that induced by Xe ion irradiation. 4. Conclusions In summary, 500 keV Xe ion irradiation can be used to effectively modify the size and spatial distributions along with the optical absorption property of the implantation-synthesized Cu NPs in silica if the adopted fluence reaches and exceeds a critical value of about 1 1016/cm2. Three synergic factors, i.e., the nuclear collisions of Xe ions with Cu atoms, the ion beam heating and the irradiation induced defects, are responsible for such modifications. Among them, the nuclear collisions between Xe ions and Cu atoms play a crucial role in the size and spatial redistributions of NPs. Owing to the higher energy loss (especially, nuclear energy loss) in the Cu ion implanted region as well as the larger atomic number, Xe ions are more effective to modify the spatial distribution of Cu NPs than Ar ones at the same energy and fluence. The above findings may be potential not only for further understanding the interactions between the keV heavy ions and the embedded NPs, but also for practical applications, e.g., the fabrication of optoelectronics, optical sensors, waveguides and antibacterial materials. Acknowledgments Authors acknowledge the financial supports from Natural Science Foundation of China (NSFC) (Nos. 11175129 and 11175235) and Natural Science Foundation of Tianjin (No. 12JCZDJC 26900).
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Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Modifications in structure and optical property of Cu nanoparticles in SiO2 by post heavy ion irradiation Changlong Liu a,b,c,⇑, Nana Wang a, Jun Wang a, Huixian Liu a, Guangyi Jia a, Xiaoyu Mu a a
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 c Key Laboratory of Beam Technology and Material Modification of Ministry of Education, Beijing Normal University, Beijing 100875, PR China b
a r t i c l e
i n f o
Article history: Received 29 June 2013 Accepted 3 October 2013 Available online 20 January 2014 Keywords: Ion beam technology Nanoparticles Distribution control Optical properties
a b s t r a c t The implantation-synthesized Cu nanoparticles (NPs) in silica were irradiated with 500 keV Xe and Ar ions, respectively. After Xe ion irradiation at a fluence of 2 1016/cm2, the average diameter of Cu NPs was increased from 7.3 to 8.5 nm, and especially, Cu NPs with a diameter of 11–14 nm were formed beyond the projected range of Cu ions and nearly aligned at the same depth, which presented a higher volume fraction. As a result, the Cu surface plasmon resonance (SPR) absorption peak was enhanced. However, if Xe ion fluence was less than 1 1016/cm2, no clear variation of the Cu SPR absorption peak could be found. Further, it was also revealed that Xe ion irradiation caused the Cu SPR absorption peak to more drastically change than Ar ion irradiation at the same ion fluence. The underlying processes for the above findings were discussed and tentatively proposed. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Owing to unique surface plasmon resonance (SPR) property with a frequency in the visible region, glass-based nanocomposites containing noble metal nanoparticles (NPs) are considered as excellent candidates for the fabrication of ultrafast optical switches [1], optical sensors [2], and waveguides [3], etc. Among various methods to synthesize this kind of nanocomposites, ion implantation has been proved to be quite suitable since it allows reaching a high metal filling factor (i.e., volume fraction) in the substrate without constrain of solubility limit as well as high cluster free energy of formation suffered by other methods [4–6]. Nevertheless, it should be noted that the formed NPs in the implanted samples usually present broad size and spatial distributions due to the uncontrolled nucleation and growth processes, which inevitably deteriorate the SPR properties of nanocomposites [6–8]. Although the position of precipitates along with the size of NPs can be controlled to some extent via implantation parameter optimization and/or post annealing [9,10], it is still a challenging task to tailor the size and spatial distributions of the implantation-synthesized NPs [6]. Recently, heavy ion irradiation has been conceived as a promising method to modify the embedded NPs in dielectric matrices. In virtue of the swift heavy ion (SHI) irradiation, as summarized in ⇑ Corresponding author at: School of Science, Tianjin University, Tianjin 300072, PR China. Tel./fax: +86 22 27403425. E-mail address: [email protected] (C. Liu). 0168-583X/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.10.045
Ref. [11], spherical NPs could be transformed into anisotropic ones or even nanorods along the ion tracks. One of the most widely accepted mechanisms is the interplay of the ion-hammering effect of the host matrix and the melting of NPs by the thermal spike effect, in which the molten NPs are elongated by the in-plane stress of the ion-hammering effect [12]. That is to say, the electronic excitation induced effects on both NPs and substrate are superior to other factors in the case of SHI irradiation [13,14]. In nature, the heavy ion irradiation induced modifications of the embedded NPs can be attributed to the energy deposition from ions by means of nuclear collision and electronic excitation processes [15]. Roles of the above two processes are strongly dependent on ion energy and species. With decreasing the ion energy down to the keV range, the deposited energy via the nuclear collision process becomes more and more prominent whereas that via the ionization and/or electronic excitation process gradually decreases. Therefore, the heavy ion irradiation in the keV energy range can be adopted to effectively tailor the size and spatial distributions of the embedded NPs through the relatively dominant nuclear collision process, especially, when the atomic number of ions is much larger than that of target atoms. In fact, an early work by our group has already evidenced that the uniform ZnO NPs could be fabricated in the silica after annealing in oxygen ambient by combining Zn ion implantation with post 500 keV Xe ion irradiation [16]. Moreover, we also employed 500 keV Xe ion irradiation at a fluence of 1 1016/cm2 to modify the size and spatial distributions of Ag NPs embedded in the Ag ion implanted SiO2, leading to the elimination of small
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Ag NPs along with the formation of large ones near the end of Ag ion range [17]. In this work, the Cu ion implanted silica glasses were subsequently irradiated with 500 keV Xe and Ar ions, respectively, and the size and spatial distributions of the embedded Cu NPs along with their optical absorption property were studied. The work was intended to serve three purposes, i.e., tentatively proposing the underlying processes for the size and spatial redistributions of Cu NPs, revealing the dependence of the Xe ion irradiation induced modifications on ion fluence, and demonstrating the difference of Xe and Ar ion irradiations. The obtained results may be potential for better understanding the interaction between the heavy ions in the keV energy range, the embedded NPs and the host matrix. 2. Experimental Optical-grade amorphous SiO2 (a-SiO2) slices of 1 mm in thickness were implanted with 45 keV Cu ions at a fluence of 1 1017/cm2 to fabricate the embedded Cu NPs. For easy reference, the Cu ion implanted a-SiO2 slice is called the Cu sample in the following text. The Cu implantation was performed on a metal vapor vacuum arc (MEVVA) implanter with a tilted ion beam by 45° from the sample surface. During the Cu ion implantation, the ion beam flux density was maintained at 4 lA/cm2 or below, and the target plate was set to rotate at a constant speed in order to assure the implantation uniformity. Subsequently, some of the Cu samples were irradiated with 500 keV Xe ions at normal incidence to three different fluences, i.e., 1 1015, 1 1016 and 2 1016/cm2. To reveal the effect of ion species on particle modification, one of the Cu samples was also irradiated with 500 keV Ar ions at normal incidence to a fluence of 1 1016/cm2. Irradiations of Xe and Ar ions were conducted on a LC-4 high energy ion implanter, and the ion beam flux density for both cases was kept below 1 lA/ cm2. Without regard to the surface sputtering, the projected ranges of Cu ions (oblique incidence with an angle of 45°), Xe and Ar ions (normal incidence) in SiO2 can be calculated by using SRIM 2010 code [18], which are about 27, 194 and 529 nm, respectively. After implantation and/or irradiation, the samples were subjected to furnace annealing at 500 °C for 1 h in flowing nitrogen ambient. Grazing incidence X-ray diffraction (GIXRD) measurements were performed to identify the formation of Cu NPs along with their structure on a Philips X’pert pro X-ray diffractometer using Cu Ka line (0.154 nm) at 0.3° incidence angle as the radiation source. GIXRD spectra were registered in an angle range from 30° to 50°, and the scanning speed was set as 1°/min. Crosssectional transmission electron microscope (XTEM) observations were made on a Tecnai G2 F20 S-Twin TEM operating at an acceleration voltage of 200 kV. Bright field imaging technique was used to evaluate the size and spatial distributions of Cu NPs. Moreoever, optical absorbance (OA) spectra were recorded in a wavelength range from 200 to 800 nm on a double-beam spectrophotometer (UV–3100PC). The scanning interval and the bandwidth were 0.5 and 2 nm, respectively. 3. Results and discussion Fig. 1 shows the GIXRD spectra of the Cu samples before and after irradiation of 500 keV Xe ions at different fluences. One can see that the unirradiated Cu sample presents a weak and broad diffraction peak at about 43.4°, which corresponds to Cu (1 1 1) plane of fcc-structured Cu, indicating the formation of Cu NPs in the substrate. After Xe ion irradiation, Cu (1 1 1) diffraction peak becomes stronger and narrower with increasing Xe ion fluence, indicating that Xe ion irradiation causes the embedded Cu NPs to grow in size.
Fig. 1. GIXRD spectra of the Cu samples before and after irradiation of 500 keV Xe ions at different fluences. All spectra were shifted vertically for clarity.
Fig. 2 presents the main XTEM results on the Cu samples before and after 500 keV Xe ion irradiation at a fluence of 2 1016/cm2. In the unirradiated Cu sample (Fig. 2(a)), one can see that Cu NPs are mainly distributed in a region from surface up to of about 60 nm in the bulk, which can be further divided into three layers, i.e., A, B and C. Large Cu NPs with a diameter of 7–12 nm are mainly embedded in layer B, while small ones are mainly formed in layers A and C. Layer B is centered at a depth of about 27 nm, which matches well with the projected range of Cu ions. The Cu NPs in the unirradiated Cu sample exhibit a broad size distribution ranging from 2 to 12 nm in diameter, and their average diameter is about 7.3 nm, as shown in the inset of Fig. 2(a). After Xe ion irradiation, the size and spatial distributions of Cu NPs are remarkably changed, as demonstrated in Fig. 2(b) and its inset. Firstly, the region distributed with Cu NPs is slightly narrowed to about 50 nm in thickness. Secondly, large Cu NPs with a size of 11–14 nm in diameter are nearly aligned at the same depth of about 42 nm, which greatly exceeds the projected range (27 nm) of Cu ions. Thirdly, lots of Cu NPs with relatively small size are formed in a region corresponding to layers A and B labeled in Fig. 2(a). Lastly, the percentage of large Cu NPs with a diameter from 11 to 14 nm dramatically increases while that of small ones slightly decreases (see the inset of Fig. 2(b)). As a result, the average diameter of Cu NPs in the irradiated Cu sample is increased to about 8.5 nm. The results shown in Fig. 2(b) can be explained as follows. Firstly, owing to the nuclear collisions between Xe ions and Cu atoms, large numbers of Cu atoms can be knocked out from NPs and move toward deep positions via forward recoil processes. As a result, on the one hand, the Cu NPs formed in the shallow region, i.e., those in layers A and B (see Fig. 2(a)), are partially dissolved and become small in size. On the other hand, the concentration of Cu atoms in a region beyond the projected range of Cu ions can be increased significantly. Secondly, the ion beam heating, which comes from the ion kinetic energy [19] and can be approximately described by the electronic energy loss of the incident ions [20], can give rise to the nucleation and growth of Cu NPs via the classical Ostwald ripening process [21]. It can be rationally speculated from Fig. 2(b) that in the neighborhood of a depth of about 42 nm, the nucleation and growth of Cu NPs should be more prominent due to a higher concentration of Cu atoms, and thus the formation of large Cu NPs, along with the elimination of small Cu clusters formed near the end of layer C in Fig. 2(a), can be expected. Thirdly, the irradiation induced defects in the substrate, which
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Fig. 2. XTEM images of the Cu samples (a) before and (b) after 500 keV Xe ion irradiation at a fluence of 2 1016/cm2. The inserted figures give the size distribution of Cu NPs in two samples, respectively.
mainly result from the nuclear collisions of the incident ions with the substrate atoms in the case of heavy ion irradiation in the keV energy range, can markedly enhance the diffusivity of Cu atoms [22]. Thereby the growth of Cu NPs is promoted to a great degree since it is a defect-controlled process [7,20]. Based on the above discussion, one can conclude that the size and spatial redistributions of Cu NPs together with their increased average size can be attributed to three synergic factors, i.e., the nuclear collisions of Xe ions with Cu atoms, the ion beam heating and the irradiation induced defects. Among them, the nuclear collisions between Xe ions and Cu atoms play a crucial role. The OA spectra of the unirradiated and Xe ion irradiated Cu samples at different fluences before and after 500 °C annealing for 1 h in flowing nitrogen ambient are given in Figs. 3(a) and (b), respectively. From Fig. 3(a), it is clear that the unirradiated Cu sample presents an absorption peak at about 563 nm, which can be ascribed to SPR of the formed Cu NPs in the substrate
Fig. 3. OA spectra of the Cu samples unirradiated and irradiated with 500 keV Xe ions at different fluences (a) before and (b) after 500 °C annealing for 1 h in flowing nitrogen ambient.
[23,24]. After 500 °C annealing, as shown in Fig. 3(b), the absorbance of the unirradiated Cu sample is unitarily increased, and the Cu SPR absorption peak becomes stronger and narrower together with a slight red shift of about 8 nm. In addition, a weak absorption peak also appears around 310 nm (4 eV). The unitarily increased absorbance can be attributed to the dispersed Cu atoms [23], resulting from the dissolution of small Cu NPs due to their lower melting point [25]. In the work by Amekura et al. [26], the absorption cross section spectra of Cu colloids with different diameters were calculated according to the effective medium theory, and the obtained results demonstrated that with increasing the particle diameter, the Cu SPR absorption peak became stronger together with position blue shift. However, it should be also noted that by increasing the volume fraction of metal NPs, the corresponding SPR absorption peak can be enhanced and shift to a longer wavelength [7]. Therefore, we correlate the enhanced Cu SPR absorption peak and its position red shift for the unirradiated Cu sample after annealing with the increased size and volume fraction of Cu NPs resulting from the aggregation and growth of Cu NPs driven by thermal treatment. As for the weak absorption peak at about 310 nm, it may be related to the direct excitation of conduction electrons to high levels, which requires energy of 4 eV in Cu [8]. Actually, it is very reasonable to assign this weak peak to the absorption band between 280 and 510 nm [27], which can be usually observed in the silica implanted with Cu ions and subsequently annealed at a higher temperature in protective or reducing atmosphere [23,24,27]. Although the intensity of the absorption band between 280 and 510 nm is dependent on the size of Cu NPs [26], a latest research has also revealed that this band is quite visible for the crystalline Cu NPs but nearly undetectable for the amorphous Cu NPs [23]. On this basis, one can presume that the appearance of either the weak peak at about 310 nm or the absorption band between 280 and 510 nm is an indication of the transformation of amorphous Cu NPs to crystalline ones induced by thermal treatment or other methods. Returning to Fig. 3(a), it clearly reveals the relation between spectral changes and Xe ion fluence. Firstly, the absorbance of the Cu sample is unitarily decreased after Xe ion irradiation at a fluence of 1 1015/cm2, whereas it is gradually increased with increasing Xe ion fluence from 1 1016 to 2 1016/cm2. The unitarily decreased absorbance for the lowest fluence is mainly due
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to the recovery of the Cu ion implantation produced defects in the host matrix, in which the ion beam heating plays a dominant role. In contrast, the unitarily increased absorbance for two other higher fluences can be attributed to the Xe ion irradiation induced defects in the substrate, whose amount is directly proportional to Xe ion fluence. Secondly, no clear change of the Cu SPR absorption peak can be found if the Xe ion fluence is less than 1 1016/cm2, indicating in principle a critical fluence for Xe ion irradiation. After increasing Xe ion fluence up to 2 1016/cm2, a significantly enhanced and narrowed SPR absorption peak appears at about 574 nm, which can be mainly assigned to those Cu NPs formed at a depth of about 42 nm due to their large size, high volume fraction and relatively narrow size distribution as revealed in Fig. 2(b). Thirdly, the weak absorption peak at about 310 nm cannot be found for the Cu sample irradiated at a fluence of 1 1015/cm2, whereas it is quite clear for the Cu samples irradiated at two other higher fluences, i.e., 1 1016 and 2 1016/cm2. Since the appearance of the absorption peak around 310 nm is an indication of the transformation of amorphous Cu NPs to crystalline ones as presumed above, an annealing crystallization process of Cu NPs along with their growth in size as revealed in Fig. 1 is believed to occur owing to the stronger ion beam heating induced by Xe ion irradiation at higher fluences. After 500 °C annealing, it can be observed from Fig. 3(b) that owing to the thermal growth of Cu NPs, the Cu SPR absorption peak accordingly becomes stronger and narrower together with a position red shift for each irradiated Cu sample. In addition, it can be also found from Fig. 3(b) that the absorbance of the Cu sample irradiated at a fluence of 1 1015/cm2 is unitarily increased, whereas those of the Cu samples irradiated at two other higher fluences are decreased to different degrees. The former, which is quite similar to that detected from the unirradiated Cu sample, can be attributed to the dispersed Cu atoms as a result of the dissolution of small Cu NPs. The latter is mainly due to the recovery of the Xe ion irradiation induced defects. Based on the above results and the spectral evolution of the unirradiated Cu sample, it seems to be reasonable to consider the small Cu clusters formed near the end of Cu ion range (refer to layer C in Fig. 2(a)) as main contributors to the unitarily increased absorbance for the Cu samples unirradiated and irradiated at a fluence of 1 1015/cm2 after thermal treatment. In the Cu sample irradiated at a fluence of 2 1016/cm2, small Cu clusters can be hardly found beyond the region distributed with large Cu NPs, as demonstrated in Fig. 2(b). Thereby, the unitarily decreased absorbance after annealing for this sample is basically determined by the recovery of the radiation damage. Fig. 4 shows the OA spectra of the Cu samples unirradiated and irradiated with 500 keV Xe and Ar ions at the same fluence of 1 1016/cm2 before and after 500 °C annealing for 1 h in flowing nitrogen ambient. From Figs. 4(a) and (b), it can be clearly found that both Xe ion irradiation and Ar ion irradiation can lead to enhancement of the Cu SPR absorption peak along with a position red shift. A further comparison between two Cu SPR absorption peaks detected from the Cu samples irradiated by Xe and Ar ions either before or after annealing unambiguously confirms that Xe ion irradiation can cause the Cu SPR absorption peak to more drastically change than Ar ion irradiation at the same energy and fluence. In addition, such a comparison also suggests that different physical processes should take place in the Cu samples irradiated with Xe and Ar ions. The ion irradiation induced modification in the Cu SPR absorption peak, more correctly, the ion irradiation induced modifications of the embedded Cu NPs in size and spatial distributions are actually related to the energy deposition from the incident ions. The deposited energy can be semi-quantitatively estimated with the nuclear energy loss (Sn) and electronic energy loss (Se) of ions in the substrate. To better reveal the underlying processes in the Xe
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Fig. 4. OA spectra of the Cu samples unirradiated and irradiated with 500 keV Xe and Ar ions at the same fluence of 1 1016/cm2 (a) before and (b) after 500 °C annealing for 1 h in flowing nitrogen ambient.
and Ar ion irradiated Cu samples, Fig. 5 gives the depth profiles of Sn and Se of 500 keV Xe and Ar ions in SiO2 as well as the concentration profile of Cu atoms in SiO2. The depth profiles of Sn and Se were calculated according to the algorithm proposed by Messenger et al. [28], and the concentration profile of Cu atoms was simulated by using SRIM 2010 code [18]. Especially, the contribution of the recoiled ions was also taken into consideration during the calculations of Se. From Fig. 5, one can extract that around the projected range of Cu ions, Sn and Se of Xe ions are 1.04 and 1.01 keV/nm, while Sn and Se of Ar ions are 0.18 and 0.69 keV/nm, respectively. Obviously, the deposited energy in the Cu ion implanted region via the electronic energy loss process during Xe ion irradiation is higher than that during Ar ion irradiation, and thus the ion beam heating induced by Xe ions is more intense than that induced by Ar ions. In particular, the nuclear energy loss of Xe ions is quite larger than that of Ar ions in the Cu ion implanted layer, indicating that the nuclear collisions of the incident ions with Cu atoms and the substrate atoms in the Xe ion irradiated Cu sample are much more prominent as compared with those in the Ar ion irradiated Cu sample. On these bases, again noting the atomic numbers of Xe and Ar elements, one can conclude that after Xe ion irradiation,
Fig. 5. The calculated concentration profile of Cu atoms in SiO2 and depth profiles of nuclear energy loss (Sn) and electronic energy loss (Se) for 500 keV Xe and Ar ions in SiO2.
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the size and spatial distributions of the embedded Cu NPs can be substantially modified, and large Cu NPs with a relatively high volume fraction can be formed, which may be similar to those shown in Fig. 2(b). Therefore, a clearly enhanced and narrowed Cu SPR absorption peak can be detected from the Xe ion irradiated Cu sample. In contrast, the embedded Cu NPs cannot be effectively broken up during Ar ion irradiation, and thereby the ion beam heating is the prevailing factor, which mainly leads to the growth of Cu NPs around the projected range of Cu ions (see Fig. 2(a)) at expense of smaller clusters in the neighborhood. Although an enhanced and narrowed Cu SPR absorption peak can also be observed after Ar ion irradiation, it is still not comparable with that induced by Xe ion irradiation. 4. Conclusions In summary, 500 keV Xe ion irradiation can be used to effectively modify the size and spatial distributions along with the optical absorption property of the implantation-synthesized Cu NPs in silica if the adopted fluence reaches and exceeds a critical value of about 1 1016/cm2. Three synergic factors, i.e., the nuclear collisions of Xe ions with Cu atoms, the ion beam heating and the irradiation induced defects, are responsible for such modifications. Among them, the nuclear collisions between Xe ions and Cu atoms play a crucial role in the size and spatial redistributions of NPs. Owing to the higher energy loss (especially, nuclear energy loss) in the Cu ion implanted region as well as the larger atomic number, Xe ions are more effective to modify the spatial distribution of Cu NPs than Ar ones at the same energy and fluence. The above findings may be potential not only for further understanding the interactions between the keV heavy ions and the embedded NPs, but also for practical applications, e.g., the fabrication of optoelectronics, optical sensors, waveguides and antibacterial materials. Acknowledgments Authors acknowledge the financial supports from Natural Science Foundation of China (NSFC) (Nos. 11175129 and 11175235) and Natural Science Foundation of Tianjin (No. 12JCZDJC 26900).
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