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Optical properties of ion-beam-synthesized Au nanoparticles in SiO2 matrix
Nuclear Instruments and Methods in Physics Research B 375 (2016) 56–59
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Optical properties of ion-beam-synthesized Au nanoparticles in SiO2 matrix Chang-Lin Hsieh a, Keiji Oyoshi b, Der-Sheng Chao c, Hsu-Sheng Tsai a, Wei-Lun Hong b,d, Yoshihiko Takeda b, Jenq-Horng Liang a,d,⇑ a
Institute of Nuclear Engineering and Science, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC Quantum Beam Unit, National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan Nuclear Science and Technology Development Center, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC d Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC b c
a r t i c l e
i n f o
Article history: Received 2 February 2016 Accepted 7 March 2016
Keywords: Au nanoparticles Surface plasmon resonance Photoluminescence Ion beam synthesis
a b s t r a c t In recent years, gold (Au) nanoparticles have been synthesized via various methods and used in optical and biomedical detection. Au nanoparticles contain some remarkable dimension-dependent optical properties due to surface plasmon resonance (SPR) in Au nanoparticles which causes high absorption in visible light regions. Since SPR in well-crystallized Au nanoparticles can enhance the local electromagnetic field, it is thus expected that greater efficiency in the photoluminescence (PL) originating from oxygen deficiency centers (ODC) can be achieved in Au-implanted SiO2 matrix. In order to demonstrate the enhancement of PL, Au nanoparticles were formed in SiO2 film using ion beam synthesis and their optical and microstructural properties were also investigated in this study. The results revealed that a clear absorption peak at approximately 530 nm was identified in the UV-Vis spectra and was attributed to SPR induced by Au nanoparticles in SiO2. The SPR of Au nanoparticles is also dependent on thermal treatment conditions, such as post-annealing temperature and ambient. The Au nanoparticle-containing SiO2 film also displayed several distinctive peaks at approximately 320, 360, 460, and 600 nm in the PL spectra and were found to be associated with ODC-related defects and non-bridging oxygen hole centers (NBOHC) in SiO2. In addition, the PL peak intensities increased as post-annealing temperature increased, a finding contradictory to the defect recovery but highly consistent with the SPR tendency. A maximum PL emission was achieved when the Au-implanted SiO2 film was annealed at 1100 °C for 1 h under N2. Therefore, the existence of Au nanoparticles in SiO2 film can induce SPR effects as well as enhance PL emission resulting from defect-related luminescence centers. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction The fabrication of composite glass (i.e., dielectric matrix that contains metallic nanoparticles) is one of the key issues in applications in the optoelectronic, photonic, and plasmonic fields. Numerous methods are currently being adopted to fabricate such composite materials. In order to synthesize nanoparticles that are manageable in terms of their size, shape, and depth distribution, ion beam synthesis is a promising method of forming metallic nanoparticles in solids due to its excellent spatial controllability through its adjustable implantation energy and ion fluence [1–4].
⇑ Corresponding author at: Institute of Nuclear Engineering and Science, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC. E-mail address: [email protected] (J.-H. Liang). http://dx.doi.org/10.1016/j.nimb.2016.03.013 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.
Nanostructures have received increasing attention in the past decade due to their unique properties which differ significantly from those of the corresponding bulk material. In particular, gold (Au) nanoparticles embedded in dielectric materials have received great attention due to their superior optical features resulting from the local surface plasmon resonance (SPR) phenomenon which can’t be observed easily in the bulk phase. The related extinction band is predominantly caused by a collective oscillation of conduction electrons in Au nanoparticles experiencing optical excitation and thus can be characterized as a localized SPR band. Also, SPR energy levels can easily be adjusted by changing the size, shape, and environment of the Au nanoparticles [5,6]. SPR in Au nanoparticles presents promising applications in technologies such as molecular detectors, biosensors, and surface enhanced Raman scattering (SERS) [7,8]. Furthermore, SPR may enhance photoluminescence [9–14] or the band emission of the
C.-L. Hsieh et al. / Nuclear Instruments and Methods in Physics Research B 375 (2016) 56–59
nanorod structure [15–17] due to the existence of Au nanoparticles. Previous studies have reported that in ion-implanted SiO2 matrix, some defects evolve during the ion implantation process. Some of these defects, such as oxygen-deficiency centers (ODC) [18,19,22] and non-bridging oxygen hole centers (NBOHC) [20,21], are radiative, thus resulting in photoluminescence (PL) emission. Since the SPR of well-crystallized Au nanoparticles can enhance the local electromagnetic field, it is thus anticipated that a higher efficiency of PL originating from ODC can be achieved in Au-implanted SiO2 matrix. Therefore, the objective of this study is to form Au nanoparticles in SiO2 films by means of ion beam synthesis. This study also details the characteristics of the optical and microstructural properties in Au nanoparticles in order to clarify the SPR effects of Au nanoparticles and demonstrate the enhancement of SPR-induced PL in Au-implanted SiO2 films. 2. Experiment In this study, thermally-grown SiO2 films 100 nm in thickness deposited on (100)-oriented n-type Si wafers were adopted as the matrix materials. Au ions extracted from a Nisshin High Voltage accelerator with an acceleration voltage of 60 kV were roomtemperature implanted into the SiO2 films at a fluence of 5 1016 ions/cm2. The as-implanted specimens were each annealed at 650, 900, and 1100 °C for 1 h in both air and N2 ambients. The UV-Vis absorption spectra of the specimens were measured by a HITACHI U-4100 spectrophotometer and performed at room temperature in wavelengths ranging from 370 to 800 nm. The PL spectra were detected by a HITACHI F-7000 fluorescence spectrophotometer which was stimulated by a 248-nm line (5 eV) from a Xe lamp with a spectral filter. X-ray diffraction (XRD) measurements were conducted using a Shimadzu XRD-6000. Spherical-aberrationcorrected transmission electron microscopy (TEM) (JEOL, JEMARM200FTH) operating at 200 kV was used to carry out the micro-structural analysis and selected area electron diffraction (SAED) of the specimens. The TEM samples were prepared in a cross-section in order to observe depth distribution of the Au nanoparticles. The depth profiles of the Au ions were measured utilizing a TOF-SIMS IV secondary ion mass spectrometer (SIMS). Defects and binding energy levels were analyzed using a ULVACPHI high-resolution X-ray photoelectron spectrometer (XPS). 3. Results and discussion In order to characterize the SPR effects caused by Au nanoparticles, the UV-Vis absorption spectra of the Au-implanted specimens annealed at various temperatures in air and N2 ambients are shown in Fig. 1. As can be seen, a clear absorption peak around 530 nm is evident in both the air- and N2-annealed specimens,
57
which corresponds to the SPR peak. This is due to the presence of Au nanoparticles as predicted according to the Mie Theory [1]. The SPR peak also becomes more prominent as annealing temperature increases, implying that high-temperature annealing enhances the synthesis of Au nanoparticles as well as nanoparticle coalescence. In the specimen annealed at 1100 °C in air, a broad shoulder appears in the wavelength between 400 and 500 nm, which can be attributed to the thicker SiO2 layer due to oxidation reaction during high-temperature air annealing. This is evident from the cross-sectional TEM images and the depth profile analysis. Fig. 2 shows the cross-sectional TEM images and the corresponding SIMS-measured depth profiles of the as-implanted, air-annealed, and N2-annealed specimens. It can be seen that the maximum concentration of Au ions is located at approximately the same depth (i.e., 30 nm) in three of the specimens. Hightemperature annealing enhances the redistribution and clustering of excess Au atoms in SiO2, thus resulting in a remarkable increase in the size of the Au nanoparticles. The air-annealed specimens especially have larger particles and narrower depth distributions when compared to the N2-annealed specimens. It can be seen in the TEM images that air annealing at 1100 °C for 1 h leads to an Au particle size with a diameter of about 10–15 nm, while the specimens annealed in N2 at the same temperature and for the same duration have diameters of less than 10 nm. This variation in particle size in these two annealing ambients is caused by the differing diffusivity of Au atoms in the SiO2 matrix. This phenomenon was mentioned by De Marchi et al. [1,2], who claim that Au atoms prefer clustering when undergoing annealing in an aerobic environment. The incorporation of oxygen can promote the diffusion of Au atoms as well as enhance the growth of Au nanoparticles. Therefore, the annealing ambient is an important factor in the synthesis process of Au nanoparticles. Fig. 3 (a) and (b) show the XRD spectra of the Au-implanted specimens annealed in air and N2, respectively. Given the specific diffraction peaks shown in Fig. 3, the mean size of the Au nanoparticles annealed in different annealing atmospheres can be estimated according to the Scherrer equation given below,
s¼
Kk b cos h
ð1Þ
where, s is the mean size of the ordered (crystalline) domains, K is a dimensionless shape factor, k is the X-ray wavelength, b is the line which broadens at halfway point to the maximum intensity (FWHM), and h is the Bragg angle. The shape factor, incident wavelength, and Bragg angle are the constants in this experiment. In regard to the relationship between the mean size factor s and the FWHM b, the spectra clearly show that when the temperature is increased, larger Au nanoparticles are produced. A narrower FWHM indicates a larger average particle size in air annealing when
Fig. 1. Optical absorption spectra of Au-implanted specimens annealed in (a) air, and (b) N2 for 1 h.
58
C.-L. Hsieh et al. / Nuclear Instruments and Methods in Physics Research B 375 (2016) 56–59
Fig. 2. Cross-sectional TEM images and corresponding SIMS-measured Au-ion depth profiles of (a) as-implanted specimen, (b) air-annealed specimen (1100 °C, 1 h), and (c) N2-annealed specimen (1100 °C, 1 h).
Fig. 3. XRD spectra of (a) air-annealed specimen and (b) N2-annealed specimen.
Fig. 4. PL spectra of (a) air-annealed specimen and (b) N2-annealed specimen.
compared with the N2-annealed specimens, reconfirmed by the cross-sectional TEM images shown in Fig. 2. This is due to the fact that Au diffusivity in SiO2 matrix DAu is strongly influenced by the annealing ambient, inferring that the oxygen in air plays an important role in enhancing the diffusivity of Au atoms. Fig. 4(a) and (b) show the PL spectra of the Au-implanted specimens annealed in air and N2, respectively. The characteristic PL peaks induced by some specific defect states are also shown. As
can be seen, four PL peaks located at wavelengths of approximately 320, 360, 460, and 600 nm can be observed in both the air- and N2-annealed specimens. These peaks can be attributed to the defect-induced luminescence centers in SiO2 matrix. The peaks at 320, 360, 460 nm correspond to oxygen-deficiency-related defects, namely ODC-II, ODC, and ODC-I, respectively, while the 600 nm peak is associated with the NBOHC [20,21]. It has been reported that in ion irradiation processes, high-energy incident ions break
C.-L. Hsieh et al. / Nuclear Instruments and Methods in Physics Research B 375 (2016) 56–59
59
4. Conclusions
Fig. 5. Energy band of the Au-SiOx system showing electron transfer between defect-related bands and Au nanoparticles.
the bond of SiO2 matrix and create E’ centers [22]. After thermal annealing, this irradiation-induced damage transforms into ODC and NBOHC structures which can act as luminescence centers for PL emission. In our previous studies, most of these defect-related luminescence centers can be recovered when the specimens are annealed at temperatures above 600 °C, thus reducing PL intensity. In addition, a distinct difference in PL intensity can be identified in the air- and N2-annealed specimens. This could be due to the fact that the presence of oxygen in air ambient enhances recovery in the ODC structures. However, in this experiment, PL intensity increases even at high temperature annealing (i.e., 1100 °C). This contradictory finding can be ascribed to SPR effects due to the existence of Au nanoparticles in the SiO2 matrix. Some studies suggest that the emitted photons produce the SPR through energy transfer, which in turn stimulates the electrons in Au nanoparticles by means of SPR waves [15–17]. The stimulated electrons then transfer back to the defect-related bands in the SiO2 matrix. Thus, the electron density in the defect-related bands increases via the electron transfer process. This process is depicted in Fig. 5. As a result, the SPR effects enhance the local electromagnetic field around the surface of the Au nanoparticles, thus increasing PL intensity. Such enhancement of the electromagnetic field contributes to PL intensity even though the number of ODC structures decreases after annealing. It is worth noting that there is considerable variation in the enhancement ratios between these two annealing ambients. This difference can be explained by examining the results of the cross-sectional TEM and XRD spectra shown in Figs. 2 and 3. The average size of the particles is smaller and the depth distribution of the particles is broader in the N2-annealed specimens. This implies that the total interfacial area between the Au nanoparticles and the SiO2 matrix is larger in the N2-annealed specimens such that more defect-related structures can be formed near the interface. Another explanation is that the ODC and NBOHC are more easily recovered in air annealing, thus causing the luminescence centers to vanish more quickly. The larger particle size in air annealing leads to stronger SPR effects, but the rapid decrease in defect density in the air-annealed specimens results in weaker PL intensity than that is the N2-annealed specimens. Defect recovery can be also confirmed by the XPS fitting data (not shown here).
In conclusion, Au nanoparticles were successfully synthesized in SiO2 matrix via ion implantation followed by thermal annealing in air and N2 ambients. The UV-Vis spectra revealed that an absorption peak located at approximately 530 nm can be identified in the annealed specimens, implying that the formation of Au nanoparticles leads to SPR effects. Higher annealing temperatures enhanced the synthesis of Au nanoparticles, especially in the specimens annealed in air ambient. The cross-sectional TEM images also demonstrated that Au nanoparticles are larger when annealed in air ambient compared to those annealed in N2. In addition, Auion implantation created defects in the SiO2 matrix, which formed specific defect-related luminescence centers (i.e., ODC and NBOHC structures) after thermal annealing. These luminescence centers created PL emission peaks located at specific wavelengths. Furthermore, these defect-related structures were recovered through high-temperature annealing. However, the PL peaks caused by these defect-related structures still remained after annealing and their intensity levels were even enhanced in the specimens containing Au nanoparticles. This enhancement in PL intensity can be attributed to the SPR effects induced by Au nanoparticles in the SiO2 matrix. Acknowledgments The authors would like to thank Mr. C.C. Wang (National Tsing Hua University, Republic of China) for the SIMS measurements. Special thanks are also given to Professor Y.C. Hung at the Institute of Photonics Technologies in National Tsing Hua University for his kind assistance with the fluorescence spectrophotometer. References [1] G. De Marchi, G. Mattei, P. Mazzoldi, C. Sada, Phys. Rev. B 63 (2001) 075409. [2] G. De Marchi, G. Mattei, P. Mazzoldi, C. Sada, A. Miotello, J. Appl. Phys. 92 (2002) 4249. [3] N. Araia, H. Tsujia, K. Ueno, T. Matsumoto, Y. Gotoh, K. Adachi, H. Kotaki, J. Ishikawa, Surf. Coat. Technol. 196 (2005) 44. [4] M. Dubiel, H. Hofmeister, E. Wendler, J. Non-Cryst, Solids 354 (2008) 607. [5] S. Charnvanichborikarn, J. Wong-Leung, J.S. Williams, J. Appl. Phys. 106 (2009) 103526. [6] Y. Ramjauny, G. Rizza, S. Perruchas, T. Gacoin, R. Botha, J. Appl. Phys. 107 (2010) 104303. [7] Y.Y. Jiang, X.J. Wu, Q. Li, J.J. Li, D.S. Xu, Nanotechnology 22 (2011) 385601. [8] S. Hong, X. Li, J. Nanomater. 2013 (2013) 790323. [9] H. Liao, W.J. Wen, K.L. Wong, J. Opt. Soc. Am. B 23 (2006) 2518. [10] Y.H. Su, S.L. Tu, S.W. Tseng, Y.C. Chang, S.H. Chang, W.M. Zhang, Nanoscale 2 (2010) 2639. [11] P. Yang, K. Kawasaki, M. Ando, N. Murase, J. Nanopart. Res. 14 (2012) 1025. [12] H. Luo, R. Wang, Y. Chen, D. Fox, R. O’Connell, J.J. Wang, H.Z. Zhang, Cryst. Eng. Comm. 15 (2013) 10116. [13] T. Cesca, C. Maurizio, B. Kalinic, C. Scian, E. Trave, G. Battaglin, P. Mazzoldi, G. Mattei, Nucl. Instr. Meth. B 326 (2014) 7. [14] S. Majumder, S.K. Jana, K. Bagani, B. Satpati, S. Kumar, S. Banerjee, J. Optmat. 40 (2015) 97. [15] T. Singh, D.K. Pandya, R. Singh, Thin Solid Films 520 (2012) 4646. [16] N. Zhang, W. Tang, P. Wang, X.T. Zhang, Z.Y. Zhao, Cryst. Eng. Comm. 15 (2013) 3301. [17] S. Park, S. An, H. Ko, C. Lee, Mater. Chem. Phys. 143 (2014) 735. [18] R.J. Walters, G.I. Bourianoff, H.A. Atwater, Nat. Mater. 4 (2005) 143. [19] J.S. Biteen, N.S. Lewis, H.A. Atwater, H. Mertens, A. Polman, Appl. Phys. Lett. 88 (2006) 131109. [20] K. Kajihara, L. Skuja, M. Hirano, H. Hosono, Appl. Phys. Lett. 79 (2001) 1757. [21] A.F. Zatsepin, V.S. Kortov, D.Y. Biryukov, Phys. Status Solidi 4 (2007) 789. [22] R. Salh, Defect related luminescence in silicon dioxide network: a review, in: S. Basu (Ed.), Crystalline Silicon – Properties and Uses, InTech, 2011, p. 135.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Optical properties of ion-beam-synthesized Au nanoparticles in SiO2 matrix Chang-Lin Hsieh a, Keiji Oyoshi b, Der-Sheng Chao c, Hsu-Sheng Tsai a, Wei-Lun Hong b,d, Yoshihiko Takeda b, Jenq-Horng Liang a,d,⇑ a
Institute of Nuclear Engineering and Science, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC Quantum Beam Unit, National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan Nuclear Science and Technology Development Center, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC d Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC b c
a r t i c l e
i n f o
Article history: Received 2 February 2016 Accepted 7 March 2016
Keywords: Au nanoparticles Surface plasmon resonance Photoluminescence Ion beam synthesis
a b s t r a c t In recent years, gold (Au) nanoparticles have been synthesized via various methods and used in optical and biomedical detection. Au nanoparticles contain some remarkable dimension-dependent optical properties due to surface plasmon resonance (SPR) in Au nanoparticles which causes high absorption in visible light regions. Since SPR in well-crystallized Au nanoparticles can enhance the local electromagnetic field, it is thus expected that greater efficiency in the photoluminescence (PL) originating from oxygen deficiency centers (ODC) can be achieved in Au-implanted SiO2 matrix. In order to demonstrate the enhancement of PL, Au nanoparticles were formed in SiO2 film using ion beam synthesis and their optical and microstructural properties were also investigated in this study. The results revealed that a clear absorption peak at approximately 530 nm was identified in the UV-Vis spectra and was attributed to SPR induced by Au nanoparticles in SiO2. The SPR of Au nanoparticles is also dependent on thermal treatment conditions, such as post-annealing temperature and ambient. The Au nanoparticle-containing SiO2 film also displayed several distinctive peaks at approximately 320, 360, 460, and 600 nm in the PL spectra and were found to be associated with ODC-related defects and non-bridging oxygen hole centers (NBOHC) in SiO2. In addition, the PL peak intensities increased as post-annealing temperature increased, a finding contradictory to the defect recovery but highly consistent with the SPR tendency. A maximum PL emission was achieved when the Au-implanted SiO2 film was annealed at 1100 °C for 1 h under N2. Therefore, the existence of Au nanoparticles in SiO2 film can induce SPR effects as well as enhance PL emission resulting from defect-related luminescence centers. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction The fabrication of composite glass (i.e., dielectric matrix that contains metallic nanoparticles) is one of the key issues in applications in the optoelectronic, photonic, and plasmonic fields. Numerous methods are currently being adopted to fabricate such composite materials. In order to synthesize nanoparticles that are manageable in terms of their size, shape, and depth distribution, ion beam synthesis is a promising method of forming metallic nanoparticles in solids due to its excellent spatial controllability through its adjustable implantation energy and ion fluence [1–4].
⇑ Corresponding author at: Institute of Nuclear Engineering and Science, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC. E-mail address: [email protected] (J.-H. Liang). http://dx.doi.org/10.1016/j.nimb.2016.03.013 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.
Nanostructures have received increasing attention in the past decade due to their unique properties which differ significantly from those of the corresponding bulk material. In particular, gold (Au) nanoparticles embedded in dielectric materials have received great attention due to their superior optical features resulting from the local surface plasmon resonance (SPR) phenomenon which can’t be observed easily in the bulk phase. The related extinction band is predominantly caused by a collective oscillation of conduction electrons in Au nanoparticles experiencing optical excitation and thus can be characterized as a localized SPR band. Also, SPR energy levels can easily be adjusted by changing the size, shape, and environment of the Au nanoparticles [5,6]. SPR in Au nanoparticles presents promising applications in technologies such as molecular detectors, biosensors, and surface enhanced Raman scattering (SERS) [7,8]. Furthermore, SPR may enhance photoluminescence [9–14] or the band emission of the
C.-L. Hsieh et al. / Nuclear Instruments and Methods in Physics Research B 375 (2016) 56–59
nanorod structure [15–17] due to the existence of Au nanoparticles. Previous studies have reported that in ion-implanted SiO2 matrix, some defects evolve during the ion implantation process. Some of these defects, such as oxygen-deficiency centers (ODC) [18,19,22] and non-bridging oxygen hole centers (NBOHC) [20,21], are radiative, thus resulting in photoluminescence (PL) emission. Since the SPR of well-crystallized Au nanoparticles can enhance the local electromagnetic field, it is thus anticipated that a higher efficiency of PL originating from ODC can be achieved in Au-implanted SiO2 matrix. Therefore, the objective of this study is to form Au nanoparticles in SiO2 films by means of ion beam synthesis. This study also details the characteristics of the optical and microstructural properties in Au nanoparticles in order to clarify the SPR effects of Au nanoparticles and demonstrate the enhancement of SPR-induced PL in Au-implanted SiO2 films. 2. Experiment In this study, thermally-grown SiO2 films 100 nm in thickness deposited on (100)-oriented n-type Si wafers were adopted as the matrix materials. Au ions extracted from a Nisshin High Voltage accelerator with an acceleration voltage of 60 kV were roomtemperature implanted into the SiO2 films at a fluence of 5 1016 ions/cm2. The as-implanted specimens were each annealed at 650, 900, and 1100 °C for 1 h in both air and N2 ambients. The UV-Vis absorption spectra of the specimens were measured by a HITACHI U-4100 spectrophotometer and performed at room temperature in wavelengths ranging from 370 to 800 nm. The PL spectra were detected by a HITACHI F-7000 fluorescence spectrophotometer which was stimulated by a 248-nm line (5 eV) from a Xe lamp with a spectral filter. X-ray diffraction (XRD) measurements were conducted using a Shimadzu XRD-6000. Spherical-aberrationcorrected transmission electron microscopy (TEM) (JEOL, JEMARM200FTH) operating at 200 kV was used to carry out the micro-structural analysis and selected area electron diffraction (SAED) of the specimens. The TEM samples were prepared in a cross-section in order to observe depth distribution of the Au nanoparticles. The depth profiles of the Au ions were measured utilizing a TOF-SIMS IV secondary ion mass spectrometer (SIMS). Defects and binding energy levels were analyzed using a ULVACPHI high-resolution X-ray photoelectron spectrometer (XPS). 3. Results and discussion In order to characterize the SPR effects caused by Au nanoparticles, the UV-Vis absorption spectra of the Au-implanted specimens annealed at various temperatures in air and N2 ambients are shown in Fig. 1. As can be seen, a clear absorption peak around 530 nm is evident in both the air- and N2-annealed specimens,
57
which corresponds to the SPR peak. This is due to the presence of Au nanoparticles as predicted according to the Mie Theory [1]. The SPR peak also becomes more prominent as annealing temperature increases, implying that high-temperature annealing enhances the synthesis of Au nanoparticles as well as nanoparticle coalescence. In the specimen annealed at 1100 °C in air, a broad shoulder appears in the wavelength between 400 and 500 nm, which can be attributed to the thicker SiO2 layer due to oxidation reaction during high-temperature air annealing. This is evident from the cross-sectional TEM images and the depth profile analysis. Fig. 2 shows the cross-sectional TEM images and the corresponding SIMS-measured depth profiles of the as-implanted, air-annealed, and N2-annealed specimens. It can be seen that the maximum concentration of Au ions is located at approximately the same depth (i.e., 30 nm) in three of the specimens. Hightemperature annealing enhances the redistribution and clustering of excess Au atoms in SiO2, thus resulting in a remarkable increase in the size of the Au nanoparticles. The air-annealed specimens especially have larger particles and narrower depth distributions when compared to the N2-annealed specimens. It can be seen in the TEM images that air annealing at 1100 °C for 1 h leads to an Au particle size with a diameter of about 10–15 nm, while the specimens annealed in N2 at the same temperature and for the same duration have diameters of less than 10 nm. This variation in particle size in these two annealing ambients is caused by the differing diffusivity of Au atoms in the SiO2 matrix. This phenomenon was mentioned by De Marchi et al. [1,2], who claim that Au atoms prefer clustering when undergoing annealing in an aerobic environment. The incorporation of oxygen can promote the diffusion of Au atoms as well as enhance the growth of Au nanoparticles. Therefore, the annealing ambient is an important factor in the synthesis process of Au nanoparticles. Fig. 3 (a) and (b) show the XRD spectra of the Au-implanted specimens annealed in air and N2, respectively. Given the specific diffraction peaks shown in Fig. 3, the mean size of the Au nanoparticles annealed in different annealing atmospheres can be estimated according to the Scherrer equation given below,
s¼
Kk b cos h
ð1Þ
where, s is the mean size of the ordered (crystalline) domains, K is a dimensionless shape factor, k is the X-ray wavelength, b is the line which broadens at halfway point to the maximum intensity (FWHM), and h is the Bragg angle. The shape factor, incident wavelength, and Bragg angle are the constants in this experiment. In regard to the relationship between the mean size factor s and the FWHM b, the spectra clearly show that when the temperature is increased, larger Au nanoparticles are produced. A narrower FWHM indicates a larger average particle size in air annealing when
Fig. 1. Optical absorption spectra of Au-implanted specimens annealed in (a) air, and (b) N2 for 1 h.
58
C.-L. Hsieh et al. / Nuclear Instruments and Methods in Physics Research B 375 (2016) 56–59
Fig. 2. Cross-sectional TEM images and corresponding SIMS-measured Au-ion depth profiles of (a) as-implanted specimen, (b) air-annealed specimen (1100 °C, 1 h), and (c) N2-annealed specimen (1100 °C, 1 h).
Fig. 3. XRD spectra of (a) air-annealed specimen and (b) N2-annealed specimen.
Fig. 4. PL spectra of (a) air-annealed specimen and (b) N2-annealed specimen.
compared with the N2-annealed specimens, reconfirmed by the cross-sectional TEM images shown in Fig. 2. This is due to the fact that Au diffusivity in SiO2 matrix DAu is strongly influenced by the annealing ambient, inferring that the oxygen in air plays an important role in enhancing the diffusivity of Au atoms. Fig. 4(a) and (b) show the PL spectra of the Au-implanted specimens annealed in air and N2, respectively. The characteristic PL peaks induced by some specific defect states are also shown. As
can be seen, four PL peaks located at wavelengths of approximately 320, 360, 460, and 600 nm can be observed in both the air- and N2-annealed specimens. These peaks can be attributed to the defect-induced luminescence centers in SiO2 matrix. The peaks at 320, 360, 460 nm correspond to oxygen-deficiency-related defects, namely ODC-II, ODC, and ODC-I, respectively, while the 600 nm peak is associated with the NBOHC [20,21]. It has been reported that in ion irradiation processes, high-energy incident ions break
C.-L. Hsieh et al. / Nuclear Instruments and Methods in Physics Research B 375 (2016) 56–59
59
4. Conclusions
Fig. 5. Energy band of the Au-SiOx system showing electron transfer between defect-related bands and Au nanoparticles.
the bond of SiO2 matrix and create E’ centers [22]. After thermal annealing, this irradiation-induced damage transforms into ODC and NBOHC structures which can act as luminescence centers for PL emission. In our previous studies, most of these defect-related luminescence centers can be recovered when the specimens are annealed at temperatures above 600 °C, thus reducing PL intensity. In addition, a distinct difference in PL intensity can be identified in the air- and N2-annealed specimens. This could be due to the fact that the presence of oxygen in air ambient enhances recovery in the ODC structures. However, in this experiment, PL intensity increases even at high temperature annealing (i.e., 1100 °C). This contradictory finding can be ascribed to SPR effects due to the existence of Au nanoparticles in the SiO2 matrix. Some studies suggest that the emitted photons produce the SPR through energy transfer, which in turn stimulates the electrons in Au nanoparticles by means of SPR waves [15–17]. The stimulated electrons then transfer back to the defect-related bands in the SiO2 matrix. Thus, the electron density in the defect-related bands increases via the electron transfer process. This process is depicted in Fig. 5. As a result, the SPR effects enhance the local electromagnetic field around the surface of the Au nanoparticles, thus increasing PL intensity. Such enhancement of the electromagnetic field contributes to PL intensity even though the number of ODC structures decreases after annealing. It is worth noting that there is considerable variation in the enhancement ratios between these two annealing ambients. This difference can be explained by examining the results of the cross-sectional TEM and XRD spectra shown in Figs. 2 and 3. The average size of the particles is smaller and the depth distribution of the particles is broader in the N2-annealed specimens. This implies that the total interfacial area between the Au nanoparticles and the SiO2 matrix is larger in the N2-annealed specimens such that more defect-related structures can be formed near the interface. Another explanation is that the ODC and NBOHC are more easily recovered in air annealing, thus causing the luminescence centers to vanish more quickly. The larger particle size in air annealing leads to stronger SPR effects, but the rapid decrease in defect density in the air-annealed specimens results in weaker PL intensity than that is the N2-annealed specimens. Defect recovery can be also confirmed by the XPS fitting data (not shown here).
In conclusion, Au nanoparticles were successfully synthesized in SiO2 matrix via ion implantation followed by thermal annealing in air and N2 ambients. The UV-Vis spectra revealed that an absorption peak located at approximately 530 nm can be identified in the annealed specimens, implying that the formation of Au nanoparticles leads to SPR effects. Higher annealing temperatures enhanced the synthesis of Au nanoparticles, especially in the specimens annealed in air ambient. The cross-sectional TEM images also demonstrated that Au nanoparticles are larger when annealed in air ambient compared to those annealed in N2. In addition, Auion implantation created defects in the SiO2 matrix, which formed specific defect-related luminescence centers (i.e., ODC and NBOHC structures) after thermal annealing. These luminescence centers created PL emission peaks located at specific wavelengths. Furthermore, these defect-related structures were recovered through high-temperature annealing. However, the PL peaks caused by these defect-related structures still remained after annealing and their intensity levels were even enhanced in the specimens containing Au nanoparticles. This enhancement in PL intensity can be attributed to the SPR effects induced by Au nanoparticles in the SiO2 matrix. Acknowledgments The authors would like to thank Mr. C.C. Wang (National Tsing Hua University, Republic of China) for the SIMS measurements. Special thanks are also given to Professor Y.C. Hung at the Institute of Photonics Technologies in National Tsing Hua University for his kind assistance with the fluorescence spectrophotometer. References [1] G. De Marchi, G. Mattei, P. Mazzoldi, C. Sada, Phys. Rev. B 63 (2001) 075409. [2] G. De Marchi, G. Mattei, P. Mazzoldi, C. Sada, A. Miotello, J. Appl. Phys. 92 (2002) 4249. [3] N. Araia, H. Tsujia, K. Ueno, T. Matsumoto, Y. Gotoh, K. Adachi, H. Kotaki, J. Ishikawa, Surf. Coat. Technol. 196 (2005) 44. [4] M. Dubiel, H. Hofmeister, E. Wendler, J. Non-Cryst, Solids 354 (2008) 607. [5] S. Charnvanichborikarn, J. Wong-Leung, J.S. Williams, J. Appl. Phys. 106 (2009) 103526. [6] Y. Ramjauny, G. Rizza, S. Perruchas, T. Gacoin, R. Botha, J. Appl. Phys. 107 (2010) 104303. [7] Y.Y. Jiang, X.J. Wu, Q. Li, J.J. Li, D.S. Xu, Nanotechnology 22 (2011) 385601. [8] S. Hong, X. Li, J. Nanomater. 2013 (2013) 790323. [9] H. Liao, W.J. Wen, K.L. Wong, J. Opt. Soc. Am. B 23 (2006) 2518. [10] Y.H. Su, S.L. Tu, S.W. Tseng, Y.C. Chang, S.H. Chang, W.M. Zhang, Nanoscale 2 (2010) 2639. [11] P. Yang, K. Kawasaki, M. Ando, N. Murase, J. Nanopart. Res. 14 (2012) 1025. [12] H. Luo, R. Wang, Y. Chen, D. Fox, R. O’Connell, J.J. Wang, H.Z. Zhang, Cryst. Eng. Comm. 15 (2013) 10116. [13] T. Cesca, C. Maurizio, B. Kalinic, C. Scian, E. Trave, G. Battaglin, P. Mazzoldi, G. Mattei, Nucl. Instr. Meth. B 326 (2014) 7. [14] S. Majumder, S.K. Jana, K. Bagani, B. Satpati, S. Kumar, S. Banerjee, J. Optmat. 40 (2015) 97. [15] T. Singh, D.K. Pandya, R. Singh, Thin Solid Films 520 (2012) 4646. [16] N. Zhang, W. Tang, P. Wang, X.T. Zhang, Z.Y. Zhao, Cryst. Eng. Comm. 15 (2013) 3301. [17] S. Park, S. An, H. Ko, C. Lee, Mater. Chem. Phys. 143 (2014) 735. [18] R.J. Walters, G.I. Bourianoff, H.A. Atwater, Nat. Mater. 4 (2005) 143. [19] J.S. Biteen, N.S. Lewis, H.A. Atwater, H. Mertens, A. Polman, Appl. Phys. Lett. 88 (2006) 131109. [20] K. Kajihara, L. Skuja, M. Hirano, H. Hosono, Appl. Phys. Lett. 79 (2001) 1757. [21] A.F. Zatsepin, V.S. Kortov, D.Y. Biryukov, Phys. Status Solidi 4 (2007) 789. [22] R. Salh, Defect related luminescence in silicon dioxide network: a review, in: S. Basu (Ed.), Crystalline Silicon – Properties and Uses, InTech, 2011, p. 135.