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Silica-coated ZnO nanowires
Vacuum 86 (2012) 789e793
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Silica-coated ZnO nanowires Hyoun Woo Kim a, *, Hyo Sung Kim b, Han Gil Na a, Ju Chan Yang b a b
Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea Division of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea
a b s t r a c t Keywords: ZnO Nanowires Silica Sputtering Photoluminescence
ZnO-core/SiOx shell nanowires were successfully fabricated and their morphology, structure, Raman and photoluminescence properties were examined. Not only the sputter-coated product had an onedimensional morphology, but the tubular structure of SiOx shell was also continuous, smooth, and uniform, along the core nanowires. It was found that two fundamental modes (334, 437 cm1) and 2 s order modes (1106, 1156 cm1) of hexagonal ZnO appeared in the Raman spectrum of ZnO-core/SiOx shell nanowires. The photoluminescence (PL) spectrum of the coreeshell nanowires were deconvoluted into three Gassian functions, centered at 382, 500, and 758 nm, whether the subsequent thermal annealing was performed or not. The integrated intensities of UV (382 nm) and green (758 nm) emissions were changed by means of the shell-coating and thermal annealing. We have discussed the possible emission mechanisms. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction One-dimensional (1D) heterostructures with modulated composition and interfaces are known to have diverse functionalities, along with their potential applications in nanodevice fabrication [1,2]. Among them, radial heterostrucures, in which different materials of nanowires (as cores) and nanotubes (as sheaths) are assembled in the radial direction, have attracted great attention. They offer numerous peculiar characteristics and advantages, including enhanced emission efficiency along with the band-gap tailoring [3], moving surface states away from the core to improve the performance of the nanodevices [4,5], enhanced pseudoelastic behavior [6], and enhanced electrocatalytic activity [7], etc. Zinc oxide (ZnO) is a wide band-gap semiconductor oxide with a large excitation binding energy (60 meV). Since the first report of ultraviolet (UV) lasing from ZnO nanowires [8], ZnO has been one of the most important functional materials with unique properties of near-UV emission, optical transparency, electric conductivity [9], piezoelectricity [10], gas sensing [11], bio-sensing [12], and photocatalytic activity [13], etc. In addition, ZnO nanowires have recently been used in a variety of energy harvesting devices, including dye sensitized solar cells [14]. With its excellent properties such as high voltage breakdown potential, high dielectric constant, considerable mechanical strength,
* Corresponding author. Tel.: þ82 2 2220 0382; fax: þ82 2 2220 0389. E-mail address: [email protected] (H. W. Kim). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.07.024
perfect insulating performance, and exceptional resilience to environmental factors [15e17], the SiOx shell will offer several advantages, including chemical stability [18], thermal stability [19], insulating characteristics [17], and protection from contamination, mechanical and radial damages. Furthermore, SiOx-associated processes are compatible with the well-established Si integrated circuit (IC) scheme [20]. The SiOx surface is easily functionalized with coupling reagents, allowing for robust attachment of a variety of specific ligands [21,22]. Furthermore, the SiOx coating layer does not degrade the intrinsic optical properties of core materials [19]. In spite of several advantages, it is still challenging to directly form the uniform and sound SiOx shell on the semiconductor core nanowires. In regard to the ZnO/SiOx heterostructures, SiOx/ZnO coreeshell nanowires have been fabricated [23]. In the present work, for the first time, we report on the fabrication of coaxial nanowires with a ZnO core and a SiOx shell, via a simple and conventional sputtering technique. In addition, we have studied structural and photoluminescence (PL) properties. In order to understand the PL emission mechanism, we have investigated the effects of thermal annealing. It is inevitable that real device fabrication includes the thermal annealing process and the annealing may change the characteristics of the coreeshell nanowires. 2. Experimental First, we have prepared core ZnO nanowires on platinum (Pt: about 10 nm)-coated Si substrates, by heating pure Zn powders in a tube furnace. A mixture of Ar (flow rate: 100 sccm) and NH3
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H.W. Kim et al. / Vacuum 86 (2012) 789e793
(flow rate: 20 sccm) gases was flowed at a temperature of 950 C for 1 h. Secondly, the substrates were transferred to a turbo sputter coater (Emitech K575X, Emitech Ltd., Ashford, Kent, UK) [24]. Being similar to our previous work [25], a piece of p-type (100) Si wafer has been used as the sputter target. After the vacuum chamber was evacuated to a base pressure of 2 104 Pa, the sputter time was set to 1 min in high-purity (99.999%) argon (Ar) ambient. During the sputtering process, the DC current and power were maintained at 120 mA and 33 W, respectively. Subsequently, some of ZnO-core/ SiOxshell nanowires were annealed in a vertical quartz tube furnace for 10 min in air ambient. The annealing temperature was set to 500 C. This temperature not only is close to the heating environment in the real IC fabrication but also helps to understand the PL emission mechanism in the present study. The products were characterized and analyzed by powder X-ray diffraction (XRD, Philips X’pert MRD diffractometer), scanning electron microscopy (SEM, Hitachi, S-4200), transmission electron microscopy (TEM, Philips CM-200), selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDX) attached to the TEM instrument. The PL measurement was carried out at room temperature using a 325 nm HeeCd laser. A 55 mW Kimmon laser beam was focused on the nanowires. Raman spectra of the samples were taken in order to characterize the nanowires, by means of using a Renishaw Raman spectromicroscope scanning from 100 cm1e1200 cm1 at room temperature in an open air. For
Fig. 1. SEM images of (a) core ZnO nanowires and (b) as-fabricated ZnO-core/SiOxshell nanowires.
Raman excitation, a HeeNe laser beam with a wavelength of 633 nm was utilized.
3. Results and discussion Fig. 1a and b show typical SEM images of the uncoated and SiOxcoated ZnO nanowires, respectively. Both SEM image confirmed the fabrication of 1D nanowires. By comparing Fig. 1b with Fig. 1a, we reveal that the morphology of the nanowires was not significantly altered by the shell-coating. Fig. 2a and b show the room temperature Raman spectra of the core ZnO nanowires and SiOx-coated ZnO nanowires, respectively. Both spectra appeared to be comprised of similar peaks, in which the spectra were normalized with respect to the main peak at 520 cm1. The sharp peak at 520 cm1 was identified as the TO phonon mode in the silicon (Si) crystal structure [26], originating from the Si substrates. Also, the weak peak centered at 969 cm1 corresponds to the surface phonon mode of amorphous silica. Since this peak can also be observed from the core ZnO nanowires without SiOx coating (Fig. 2a), it should have originated from the surface of Si substrate [27]. It is noteworthy that the usual or fundamental modes in ZnO, such as 334 cm1 (E2(high)-E2(low)) [28e30]) and 437 cm1 (E2(high) [31e33]), are clearly observed. Besides, the relatively weak peaks of the second order modes in ZnO, i.e. 1106 cm1 (A1, E2 acoustic combination) and 1156 cm1 (A1 optical combination), can be found [34]. The peaks at 1106 cm1 and 1156 cm1 are related to the acoustic combinations and optical overtones, respectively [35]. Fig. 3a shows a TEM image of ZnO-core/SiOx-shell nanowires prior to thermal annealing. The image clearly exhibits a wire-like core and thin coating layers on both sides, revealing that the shell has been uniformly coated. It is estimated that the wire diameter is about 85 nm, whereas the shell thickness about 6e8 nm Fig. 3b and c are EDX spectra from the B and C region, respectively, indicated in
Fig. 2. Room temperature Raman spectra of (a) core ZnO nanowires and (b) asprepared ZnO-core/SiOx-shell nanowires.
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Fig. 3. (a) TEM image of an as-synthesized ZnO-core/SiOx-shell nanowire. (b,c) EDX spectra from the (b) B and (c) C region, respectively, in (a). (d) Associated SAED pattern image. (e) Lattice-resolved TEM image enlarging an area near the surface of the nanowires in (a).
Fig. 4. PL spectra of (a) core ZnO nanowires, (b) as-fabricated SiOx-coated ZnO nanowires, and (c) annealed SiOx-coated ZnO nanowires. (d) Integrated PL intensities for three Gaussian functions.
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Fig. 3a. From EDX analysis, the atomic ratios of Zn/Si in B and C regions are 6.8 and 1.4, respectively, indicating that the coreeshell nanowire is indeed a ZnO-core/SiOx shell structure. The corresponding SAED pattern (Fig. 3d) could be indexed as the <010> zone axis of hexagonal ZnO, revealing it single crystalline nature. The lattice fringes observed in the high-resolution TEM image (Fig. 3e) correspond to the (002) lattice plane of hexagonal ZnO (spacing: 0.26 nm). The PL spectra of ZnO nanowires without and with the SiOx coating are presented in Fig. 4a and b, respectively, for a comparison. In addition, Fig. 4c shows a PL spectrum of the ZnO/SiOx coreeshell nanowires, which have been subsequently annealed at 500 C. All PL spectra could be convoluted into the three Gaussian functions, exhibiting relatively sharp ultraviolet (UV) emission bands (382 nm) and broad green emission bands (500 nm), being analogous to typical PL spectra of ZnO nanowires reported previously [36e38], as well as a weak red emission peak at around 758 nm. Fig. 4d shows the integrated PL intensities for three Gaussian functions, which reveals the effect of SiOx coating and subsequent thermal annealing. The integrated PL intensities of UV emission have been slightly increased by the SiOx coating, whereas that of green emission was significantly reduced. UV emissions are related to the excitons in ZnO [39,40]. On the other hand, the broad green emission is known to be associated with surface defects including oxygen vacancy (VO) and/or Zn vacancy (VZn) [41,42]. It has been reported that coating the ZnO surface with a dielectric layer such as Al2O3 or polymer can lead to a reduction of the surface traps due to surface passivation effects. Accordingly, the separation of electrons and holes within the surface depletion region is reduced, which means that more electronehole pairs are generated. As a result, the UV emission can be enhanced, whereas the green emission is suppressed. By the way, Fig. 4d indicated that both UV and green emissions have been reduced by the subsequent thermal annealing at 500 C. During the annealing process in air ambient which comprises abundant O2 gases, the surface oxygen vacancy can be suppressed and this will deactivate the green emission. The other possibility is the oxygen atoms from SiOx layer can diffuse into the interface, annihilating the oxygen vacancies. Also, it is possible that excessive oxygen from air ambient and/or SiOx layer will form the excessive oxidized layer on the interface, which degrades the UV emission [42]. Fig. 5 shows an XRD spectrum of 500oC-annealed coreeshell nanowires. The diffraction peaks correspond to (100), (002), (101), (102), (110), (103), (200), (112), and (201) reflections of a hexagonal wurtzite ZnO structure (JCPDS Card No. 02-1078), can be seen. In
addition, there exist diffraction peaks, being related to the cubic Si (JCPDS Card No. 72-1426) and the cubic Pt (JCPDS Card No. 04-0802). Apart from the reflection peaks being related to the cubic Pt, cubic Si, and hexagonal ZnO, there exist very weal peaks corresponding to the rhombohedral Zn2SiO4, with lattice constants of a ¼ 13.938 Å and c ¼ 9.310 Å (JCPDS Card No. 37-1485). Since this phase has not been observed from the unannealed samples (Supplementary Materials S-1), a small amount of Zn2SiO4 phase has been generated by the thermal annealing at 500 C and it is known that the Zn2SiO4 phase emits green emission [43e45]. We therefore suppose that the annihilation of oxygen vacancies rather than the generation of Zn2SiO4 phase played a decisive role in suppressing green emission in the annealed sample. On the other hand, it has been reported that the Zn interstitials are more likely sources for the red and IR emissions from ZnO [46,47]. Since the intensity of red emission was not significantly changed, we surmise that SiOx coating and subsequent thermal annealing did not alter the concentration of Zn interstitials. Further detailed investigation will be carried out soon. 4. Conclusions In summary, we have fabricated ZnO-core/SiOx-shell nanowires, in which the tubular shell structure was sputtered by using a Si target. SEM images reveal that the shell-coating did not alter the morphology of 1D nanowires, suggesting the possibility of continuous coating. TEM investigation indicates that the SiOx shell has been uniformed coated on the core ZnO nanowires. Raman spectrum of core ZnO nanowires was not significantly changed by the SiOx coating and it is comprised of two fundamental modes (334, 437 cm1) and 2 s order modes (1106, 1156 cm1) of hexagonal ZnO, in addition to the Si-substrate-related peaks. Gaussian deconvolution study reveals that PL spectrum of core ZnO nanowires are comprised of three emission bands, being centered at 382 nm (UV), 500 nm (green), and 758 nm (red). While the three peak positions (382, 500, and 758 nm) were not altered by the SiOx coating and thermal annealing, the integrated intensities of UV and green emissions were changed. By the SiOx coating, the UV emission is enhanced, whereas the green emission is suppressed. The intensity of red emission was not significantly changed, suggesting that the Zn interstitials were not affected by the shell-coating and thermal annealing. Acknowledgments This work was supported by the research fund of Hanyang University (HY-2011-201100000000434). Appendix. Supplementary material Supplementary data related to this article can be found online at doi:10.1016/j.vacuum.2011.07.024. References [1] [2] [3] [4] [5] [6] [7] [8]
Fig. 5. XRD spectrum of annealed SiOx-coated ZnO nanowires.
[9]
Li Q, Wang C. J Am Chem Soc 2003;125:9892. Gudiksen MS, Lauhon LJ, Wang J, Smith DC, Lieber CM. Nature 2002;415:617. Goldthorpe IA, Marshall AF, McIntyre PC. Nano Lett 2009;9:3715. Shi W-S, Peng H-Y, Xu L, Wang N, Tang Y-HH, Lee S-T. Adv Mater 2000;12: 1927. Wagner JB, Sköld N, Wallenberg LR, Samuelson L. J Cryst Growth 2010;312: 1755. Mastorakos IN, Zbib HM, Bahr DF, Parsons J, Faisal M. Appl Phys Lett 2009;94: 043104. Drury A, Chaure S, Kröll M, Nicolosi V, Chaure N, Blau WJ. Chem Mater 2007; 19:4252. Huang MH, Mao S, Feick H, Yan H, Wu Y, Weber E, et al. Science 2001;292: 1897. Tsai LT, Chiu SP, Lu JG, Lin JJ. Nanotechnology 2010;21:145202.
H.W. Kim et al. / Vacuum 86 (2012) 789e793 [10] Lin SS, Song JH, Lu YF, Wang ZL. Nanotechnology 2009;20:365703. [11] Santra S, Guha PK, Ali SZ, Hiralal P, Unalan HE, Covington JA, et al. Sens Actuators B Chem 2010;146:559. [12] Yeh P-H, Li Z, Wang ZL. Adv Mater 2009;21:4975. [13] Fang CW, Wu JM, Lee LT, Yeh HH, Wu WT, Lin YH, et al. Electrochem Sol Stat Lett 2010;13:K63. [14] Law M, Greene LEW, Johnson JC, Saykally R, Yang P. Nature 2005;4:455. [15] Liang X, Tan S, Tang Z, Kotov NA. Langmuir 2004;20:1016. [16] Meng X-M, Hu J-Q, Jiang Y, Lee C-S, Lee S-T. Appl Phys Lett 2003;83:2241. [17] Kovtyukhova NI, Mallouk TE, Mayer TS. Adv Mater 2003;15:780. [18] Wang Y, Tang Z, Liang X, Liz-Marzan LM, Kotov NA. Nano Lett 2004;4:225. [19] Pan A, Wang S, Liu R, Li C, Zou B. Small 2005;1:1058. [20] Wang C-Y, Chan L-H, Xiao D-Q, Lin T-C, Shih HC. J Vac Sci Technol B 2006;24:613. [21] Bruchez Jr M, Moronne M, Gin P, Weiss S, Alivisatos AP. Science 1998;281:2013. [22] Schroedter A, Weller H, Eritja R, Ford WE, Wessels JM. Nano Lett 2002;2:1363. [23] Kim HW, Kebede MA, Kim HS, Kong MH, Lee C. J Lumin 2009;129:1619. [24] Kim HW, Shim SH, Lee JW. Carbon 2007;45:2695. [25] Kim HW, Lee JW, Kim HS, Kebede MA. Surf Coat Technol 2008;203:666. [26] Hirasawa M, Orii T, Seto T. Appl Phy Lett 2006;88:093119. [27] Glinka YD, Jaroniec M. J Appl Phys 1997;82:3499. [28] Manouni AE, Manjón FJ, Mollar M, Marí B, Gómez R, López MC, et al. Superlatice Microst 2006;39:185. [29] Mensah SL, Kayastha VK, Ivanov IN, Geohegan DB, Yap YK. Appl Phys Lett 2007;90:113108.
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[30] Umar A, Karunagaran B, Suh E-K, Hahn YB. Nanotechnology 2006;17:4072. [31] Tak Y, Park D, Yong K. J Vac Sci Technol B 2006;24:2047. [32] Tzolov M, Tzenov N, Dimova-Malinovska D, Kalitzova M, Pizzuto C, Vitali G, et al. Thin Solid Films 2000;379:28. [33] Alim KA, Fonoberov VA, Balandin AA. Appl Phys Lett 2005;86:053103. [34] Fauteux C, Longtin R, Pegna J, Therriault D. Inorg Chem 2007;46:11036. [35] Wang RP, Xu G, Jin P. Phys Rev B 2004;69:113303. [36] Yang P, Yan H, Mao S, Russo R, Johnson J, Saykally R, et al. Adv Funct Mater 2002;12:323. [37] Wan Q, Song ZT, Liu WL, Lin CL, Wang TH. Nanotechnology 2004;15:559. [38] Kim HW, Shim SH, Lee JW, Lee C, Jeoung SC. Opt Mater 2008;30:1221. [39] Reynolds DC, Look DC, Jogai B, Litton CW, Collins TC, Harsch W, et al. Phys Rev B 1998;57:12151. [40] Jung SW, Park WI, Cheong HD, Yi G-C, Jang HM, Hong SS, et al. Appl Phys Lett 2002;80:1924. [41] Cheng CW, Sie EJ, Liu B, Huan CHA, Sum TC, Sun HD, et al. Appl Phys Lett 2010; 96:071107. [42] Kang HS, Kang JS, Kim JW, Lee SY. J Appl Phys 2004;95:1246. [43] Wei Q, Meng G, Ye M, An X, Hao Y, Zhang L. J Phys Chem C 2007;111:1924. [44] Xu C, Chun J, Rho K, Kim DE. Nanotechnology 2005;16:2808. [45] Wang XD, Summers CJ, Wang ZL. Adv Mater 2004;16:1215. [46] Cross RBM, Souza MMD, Narayanan EMS. Nanotechnology 2005;16:2188. [47] Koyano M, Quocbao P, Thanhbinh LT, Hongha L, Ngoclong N, Katayama S. Phys Status Solidi (a) 2002;193:125.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Silica-coated ZnO nanowires Hyoun Woo Kim a, *, Hyo Sung Kim b, Han Gil Na a, Ju Chan Yang b a b
Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea Division of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea
a b s t r a c t Keywords: ZnO Nanowires Silica Sputtering Photoluminescence
ZnO-core/SiOx shell nanowires were successfully fabricated and their morphology, structure, Raman and photoluminescence properties were examined. Not only the sputter-coated product had an onedimensional morphology, but the tubular structure of SiOx shell was also continuous, smooth, and uniform, along the core nanowires. It was found that two fundamental modes (334, 437 cm1) and 2 s order modes (1106, 1156 cm1) of hexagonal ZnO appeared in the Raman spectrum of ZnO-core/SiOx shell nanowires. The photoluminescence (PL) spectrum of the coreeshell nanowires were deconvoluted into three Gassian functions, centered at 382, 500, and 758 nm, whether the subsequent thermal annealing was performed or not. The integrated intensities of UV (382 nm) and green (758 nm) emissions were changed by means of the shell-coating and thermal annealing. We have discussed the possible emission mechanisms. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction One-dimensional (1D) heterostructures with modulated composition and interfaces are known to have diverse functionalities, along with their potential applications in nanodevice fabrication [1,2]. Among them, radial heterostrucures, in which different materials of nanowires (as cores) and nanotubes (as sheaths) are assembled in the radial direction, have attracted great attention. They offer numerous peculiar characteristics and advantages, including enhanced emission efficiency along with the band-gap tailoring [3], moving surface states away from the core to improve the performance of the nanodevices [4,5], enhanced pseudoelastic behavior [6], and enhanced electrocatalytic activity [7], etc. Zinc oxide (ZnO) is a wide band-gap semiconductor oxide with a large excitation binding energy (60 meV). Since the first report of ultraviolet (UV) lasing from ZnO nanowires [8], ZnO has been one of the most important functional materials with unique properties of near-UV emission, optical transparency, electric conductivity [9], piezoelectricity [10], gas sensing [11], bio-sensing [12], and photocatalytic activity [13], etc. In addition, ZnO nanowires have recently been used in a variety of energy harvesting devices, including dye sensitized solar cells [14]. With its excellent properties such as high voltage breakdown potential, high dielectric constant, considerable mechanical strength,
* Corresponding author. Tel.: þ82 2 2220 0382; fax: þ82 2 2220 0389. E-mail address: [email protected] (H. W. Kim). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.07.024
perfect insulating performance, and exceptional resilience to environmental factors [15e17], the SiOx shell will offer several advantages, including chemical stability [18], thermal stability [19], insulating characteristics [17], and protection from contamination, mechanical and radial damages. Furthermore, SiOx-associated processes are compatible with the well-established Si integrated circuit (IC) scheme [20]. The SiOx surface is easily functionalized with coupling reagents, allowing for robust attachment of a variety of specific ligands [21,22]. Furthermore, the SiOx coating layer does not degrade the intrinsic optical properties of core materials [19]. In spite of several advantages, it is still challenging to directly form the uniform and sound SiOx shell on the semiconductor core nanowires. In regard to the ZnO/SiOx heterostructures, SiOx/ZnO coreeshell nanowires have been fabricated [23]. In the present work, for the first time, we report on the fabrication of coaxial nanowires with a ZnO core and a SiOx shell, via a simple and conventional sputtering technique. In addition, we have studied structural and photoluminescence (PL) properties. In order to understand the PL emission mechanism, we have investigated the effects of thermal annealing. It is inevitable that real device fabrication includes the thermal annealing process and the annealing may change the characteristics of the coreeshell nanowires. 2. Experimental First, we have prepared core ZnO nanowires on platinum (Pt: about 10 nm)-coated Si substrates, by heating pure Zn powders in a tube furnace. A mixture of Ar (flow rate: 100 sccm) and NH3
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H.W. Kim et al. / Vacuum 86 (2012) 789e793
(flow rate: 20 sccm) gases was flowed at a temperature of 950 C for 1 h. Secondly, the substrates were transferred to a turbo sputter coater (Emitech K575X, Emitech Ltd., Ashford, Kent, UK) [24]. Being similar to our previous work [25], a piece of p-type (100) Si wafer has been used as the sputter target. After the vacuum chamber was evacuated to a base pressure of 2 104 Pa, the sputter time was set to 1 min in high-purity (99.999%) argon (Ar) ambient. During the sputtering process, the DC current and power were maintained at 120 mA and 33 W, respectively. Subsequently, some of ZnO-core/ SiOxshell nanowires were annealed in a vertical quartz tube furnace for 10 min in air ambient. The annealing temperature was set to 500 C. This temperature not only is close to the heating environment in the real IC fabrication but also helps to understand the PL emission mechanism in the present study. The products were characterized and analyzed by powder X-ray diffraction (XRD, Philips X’pert MRD diffractometer), scanning electron microscopy (SEM, Hitachi, S-4200), transmission electron microscopy (TEM, Philips CM-200), selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDX) attached to the TEM instrument. The PL measurement was carried out at room temperature using a 325 nm HeeCd laser. A 55 mW Kimmon laser beam was focused on the nanowires. Raman spectra of the samples were taken in order to characterize the nanowires, by means of using a Renishaw Raman spectromicroscope scanning from 100 cm1e1200 cm1 at room temperature in an open air. For
Fig. 1. SEM images of (a) core ZnO nanowires and (b) as-fabricated ZnO-core/SiOxshell nanowires.
Raman excitation, a HeeNe laser beam with a wavelength of 633 nm was utilized.
3. Results and discussion Fig. 1a and b show typical SEM images of the uncoated and SiOxcoated ZnO nanowires, respectively. Both SEM image confirmed the fabrication of 1D nanowires. By comparing Fig. 1b with Fig. 1a, we reveal that the morphology of the nanowires was not significantly altered by the shell-coating. Fig. 2a and b show the room temperature Raman spectra of the core ZnO nanowires and SiOx-coated ZnO nanowires, respectively. Both spectra appeared to be comprised of similar peaks, in which the spectra were normalized with respect to the main peak at 520 cm1. The sharp peak at 520 cm1 was identified as the TO phonon mode in the silicon (Si) crystal structure [26], originating from the Si substrates. Also, the weak peak centered at 969 cm1 corresponds to the surface phonon mode of amorphous silica. Since this peak can also be observed from the core ZnO nanowires without SiOx coating (Fig. 2a), it should have originated from the surface of Si substrate [27]. It is noteworthy that the usual or fundamental modes in ZnO, such as 334 cm1 (E2(high)-E2(low)) [28e30]) and 437 cm1 (E2(high) [31e33]), are clearly observed. Besides, the relatively weak peaks of the second order modes in ZnO, i.e. 1106 cm1 (A1, E2 acoustic combination) and 1156 cm1 (A1 optical combination), can be found [34]. The peaks at 1106 cm1 and 1156 cm1 are related to the acoustic combinations and optical overtones, respectively [35]. Fig. 3a shows a TEM image of ZnO-core/SiOx-shell nanowires prior to thermal annealing. The image clearly exhibits a wire-like core and thin coating layers on both sides, revealing that the shell has been uniformly coated. It is estimated that the wire diameter is about 85 nm, whereas the shell thickness about 6e8 nm Fig. 3b and c are EDX spectra from the B and C region, respectively, indicated in
Fig. 2. Room temperature Raman spectra of (a) core ZnO nanowires and (b) asprepared ZnO-core/SiOx-shell nanowires.
H.W. Kim et al. / Vacuum 86 (2012) 789e793
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Fig. 3. (a) TEM image of an as-synthesized ZnO-core/SiOx-shell nanowire. (b,c) EDX spectra from the (b) B and (c) C region, respectively, in (a). (d) Associated SAED pattern image. (e) Lattice-resolved TEM image enlarging an area near the surface of the nanowires in (a).
Fig. 4. PL spectra of (a) core ZnO nanowires, (b) as-fabricated SiOx-coated ZnO nanowires, and (c) annealed SiOx-coated ZnO nanowires. (d) Integrated PL intensities for three Gaussian functions.
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H.W. Kim et al. / Vacuum 86 (2012) 789e793
Fig. 3a. From EDX analysis, the atomic ratios of Zn/Si in B and C regions are 6.8 and 1.4, respectively, indicating that the coreeshell nanowire is indeed a ZnO-core/SiOx shell structure. The corresponding SAED pattern (Fig. 3d) could be indexed as the <010> zone axis of hexagonal ZnO, revealing it single crystalline nature. The lattice fringes observed in the high-resolution TEM image (Fig. 3e) correspond to the (002) lattice plane of hexagonal ZnO (spacing: 0.26 nm). The PL spectra of ZnO nanowires without and with the SiOx coating are presented in Fig. 4a and b, respectively, for a comparison. In addition, Fig. 4c shows a PL spectrum of the ZnO/SiOx coreeshell nanowires, which have been subsequently annealed at 500 C. All PL spectra could be convoluted into the three Gaussian functions, exhibiting relatively sharp ultraviolet (UV) emission bands (382 nm) and broad green emission bands (500 nm), being analogous to typical PL spectra of ZnO nanowires reported previously [36e38], as well as a weak red emission peak at around 758 nm. Fig. 4d shows the integrated PL intensities for three Gaussian functions, which reveals the effect of SiOx coating and subsequent thermal annealing. The integrated PL intensities of UV emission have been slightly increased by the SiOx coating, whereas that of green emission was significantly reduced. UV emissions are related to the excitons in ZnO [39,40]. On the other hand, the broad green emission is known to be associated with surface defects including oxygen vacancy (VO) and/or Zn vacancy (VZn) [41,42]. It has been reported that coating the ZnO surface with a dielectric layer such as Al2O3 or polymer can lead to a reduction of the surface traps due to surface passivation effects. Accordingly, the separation of electrons and holes within the surface depletion region is reduced, which means that more electronehole pairs are generated. As a result, the UV emission can be enhanced, whereas the green emission is suppressed. By the way, Fig. 4d indicated that both UV and green emissions have been reduced by the subsequent thermal annealing at 500 C. During the annealing process in air ambient which comprises abundant O2 gases, the surface oxygen vacancy can be suppressed and this will deactivate the green emission. The other possibility is the oxygen atoms from SiOx layer can diffuse into the interface, annihilating the oxygen vacancies. Also, it is possible that excessive oxygen from air ambient and/or SiOx layer will form the excessive oxidized layer on the interface, which degrades the UV emission [42]. Fig. 5 shows an XRD spectrum of 500oC-annealed coreeshell nanowires. The diffraction peaks correspond to (100), (002), (101), (102), (110), (103), (200), (112), and (201) reflections of a hexagonal wurtzite ZnO structure (JCPDS Card No. 02-1078), can be seen. In
addition, there exist diffraction peaks, being related to the cubic Si (JCPDS Card No. 72-1426) and the cubic Pt (JCPDS Card No. 04-0802). Apart from the reflection peaks being related to the cubic Pt, cubic Si, and hexagonal ZnO, there exist very weal peaks corresponding to the rhombohedral Zn2SiO4, with lattice constants of a ¼ 13.938 Å and c ¼ 9.310 Å (JCPDS Card No. 37-1485). Since this phase has not been observed from the unannealed samples (Supplementary Materials S-1), a small amount of Zn2SiO4 phase has been generated by the thermal annealing at 500 C and it is known that the Zn2SiO4 phase emits green emission [43e45]. We therefore suppose that the annihilation of oxygen vacancies rather than the generation of Zn2SiO4 phase played a decisive role in suppressing green emission in the annealed sample. On the other hand, it has been reported that the Zn interstitials are more likely sources for the red and IR emissions from ZnO [46,47]. Since the intensity of red emission was not significantly changed, we surmise that SiOx coating and subsequent thermal annealing did not alter the concentration of Zn interstitials. Further detailed investigation will be carried out soon. 4. Conclusions In summary, we have fabricated ZnO-core/SiOx-shell nanowires, in which the tubular shell structure was sputtered by using a Si target. SEM images reveal that the shell-coating did not alter the morphology of 1D nanowires, suggesting the possibility of continuous coating. TEM investigation indicates that the SiOx shell has been uniformed coated on the core ZnO nanowires. Raman spectrum of core ZnO nanowires was not significantly changed by the SiOx coating and it is comprised of two fundamental modes (334, 437 cm1) and 2 s order modes (1106, 1156 cm1) of hexagonal ZnO, in addition to the Si-substrate-related peaks. Gaussian deconvolution study reveals that PL spectrum of core ZnO nanowires are comprised of three emission bands, being centered at 382 nm (UV), 500 nm (green), and 758 nm (red). While the three peak positions (382, 500, and 758 nm) were not altered by the SiOx coating and thermal annealing, the integrated intensities of UV and green emissions were changed. By the SiOx coating, the UV emission is enhanced, whereas the green emission is suppressed. The intensity of red emission was not significantly changed, suggesting that the Zn interstitials were not affected by the shell-coating and thermal annealing. Acknowledgments This work was supported by the research fund of Hanyang University (HY-2011-201100000000434). Appendix. Supplementary material Supplementary data related to this article can be found online at doi:10.1016/j.vacuum.2011.07.024. References [1] [2] [3] [4] [5] [6] [7] [8]
Fig. 5. XRD spectrum of annealed SiOx-coated ZnO nanowires.
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