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Structure dependent luminescence evolution of c-axis-oriented ZnO nanofilms embedded with silver nanoparticles and clusters prepared by sputtering
Journal of Alloys and Compounds 569 (2013) 144–149
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Structure dependent luminescence evolution of c-axis-oriented ZnO nanofilms embedded with silver nanoparticles and clusters prepared by sputtering Yuan-Chang Liang ⇑, Xian-Shi Deng Institute of Materials Engineering, National Taiwan Ocean University, Keelung 20224, Taiwan
a r t i c l e
i n f o
Article history: Received 7 February 2013 Received in revised form 12 March 2013 Accepted 19 March 2013 Available online 27 March 2013 Keywords: Ceramics Thin film Sputtering Microstructure Characterization
a b s t r a c t ZnO–Ag composite thin films with various degrees of Ag addition were grown on c-axis-oriented sapphire substrates by sputtering. By varying the Ag sputtering power level from 3 to 13 W, the effects of Ag addition in 80-nm-thick ZnO thin films could be determined. The X-ray diffraction patterns revealed that ZnO thin films prepared with and without Ag co-sputtering exhibited highly c-axis-oriented crystallographic features. Scanning electron microscopy and transmission electron microscopy images show that a high Ag content caused the ZnO crystallites and Ag nanoparticles (or Ag clusters) to group and form clusters of grains during thin-film growth. The X-ray photoelectron spectrometer measurements demonstrate a decrease of oxygen-related point defects in ZnO thin films with proper Ag additions. The blue emission band dominated the photoluminescence characteristics of pure ZnO and 3 W Ag co-sputtered ZnO thin films. The blue emission band was markedly quenched, and the UV emission band was significantly enhanced when the Ag-sputtering power level was raised to above 9 W. These are attributed to a reduction of zinc vacancies and an improvement of the exciton transition process by using proper Ag additions. The experimental results in this work show that the photoluminescence characteristics of ZnO thin films could be adjusted by varying the content of the Ag in ZnO thin films. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a direct band gap semiconductor. The characteristics of its wide band gap and large exciton binding energy at room temperature make it a promising candidate for various optoelectronic applications such as UV light emitting diodes, blue luminescent devices and UV photodetectors [1,2]. In order to change electric and optical properties of intrinsic ZnO to meet various applications in devices, several elements have been used as dopants in ZnO [3,4]. For practical device applications, the thin-film structure of ZnO is highly required. Many techniques including sputtering, chemical vapor deposition, and chemical solution methods are adopted to synthesize ZnO thin films with various degrees of crystalline quality [5–7]. Among the various thin-film preparation methodologies, the sputtering is a promising method to prepare ZnO-based composite thin films with easy process control and large area thin-film deposition. Noble-metal-doped ZnO thin-films and nanostructures or ZnO– noble metal composites have recently raised interest because of their promising applications in various sensor devices, surface ⇑ Corresponding author. E-mail address: [email protected] (Y.-C. Liang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.03.169
plasmon-related devices, and catalysts [8–10]. Among the various noble metals, Ag is an important material with optical properties in the visible spectrum. Several works have synthesized ZnO–Ag composites and investigated their physical properties [11,12]. Generally, the photoluminescence (PL) characteristics of ZnO are associated with a near-band-edge excitonic UV emission and a deeplevel emission in the visible range [13]. The microstructure is posited to greatly affect the PL properties of ZnO thin films. Several works on the PL properties of ZnO demonstrate the mechanisms for the defect-related deep-level emission [13,14]; however, the effects on the PL properties of ZnO when Ag is added are still undetermined. This is because both the preparation methodology and the Ag contents significantly influence the microstructure of ZnO–Ag composites. The complex interactions between the defects and the Ag particles in the ZnO matrix affect optical properties [15–18]. To expand the application of ZnO–Ag composite thin films in devices, understanding the microstructure-dependent optical properties is critical. However, a systematic investigation of the correlation between the microstructure evolution and optical properties of the ZnO–Ag thin films prepared by sputtering are limited. In this work, ZnO thin films were co-sputtered with various Ag contents, and the structure-dependent optical properties that resulted from the Ag additions are discussed.
Y.-C. Liang, X.-S. Deng / Journal of Alloys and Compounds 569 (2013) 144–149 2. Experimental ZnO–Ag composite thin films were sputter deposited on c-axis-oriented sapphire substrates using ZnO and Ag targets. The substrates were supersonic clean out in acetone, rinsed in alcohol and subsequently dried in flowing air gas before deposition. The ZnO–Ag thin film deposition was performed with a dual-gun sputtering system. During the deposition, the substrate temperature was maintained at 300 °C and the working pressure is 30 mTorr with a pure Ar atmosphere. The duration for thin-film deposition was fixed at 25 min and the Ag sputtering power was varied from 3 to 13 W during the cosputtering deposition. X-ray diffraction (XRD) with Cu Ka radiation was used to investigate the crystalline quality of the prepared thin films. The surface morphology and composition of the films are investigated with a field emission scanning electron microscopy (FESEM) equipped with energy dispersive X-ray spectrometer (EDS). The binding states of the constituent elements for the sputtering deposited ZnO–Ag thin films are examined by X-ray photoelectron spectrometer (XPS). The atomic structure of the ZnO–Ag thin films is further characterized by high-resolution transmittance electron microscopy (HRTEM). The composition analysis is performed using EDS attached to the TEM. The room temperature dependent photoluminescence (PL) spectra are obtained using the 325 nm line of a He–Cd laser.
3. Results and discussion Fig. 1 shows the XRD patterns of the ZnO thin films prepared with and without Ag co-sputtering. Fig. 1 shows clear Bragg reflections from ZnO (0 0 2); no additional crystallographic planes were observed that revealed that the ZnO thin films prepared with and without Ag co-sputtering were highly c-axis oriented. By increasing the sputtering power level of Ag from 3 to 13 W, the Ag (1 1 1) Bragg reflection became stronger, and that of the Ag (2 0 0) became apparent. This indicated that the content of Ag in the ZnO thin film increased with the sputtering power level. The fullwidth at half maximum of the (0 0 2) Bragg reflection for the ZnO thin film with the Ag co-sputtering broadened compared to that without Ag co-sputtering. Moreover, the intensity of ZnO (0 0 2) Bragg reflections decreased as the Ag-sputtering power level increased, possibly because large Ag clusters in the ZnO matrix deteriorated the continuous-film feature of the ZnO–Ag thin film at the high Ag-sputtering power level. Fig. 2 shows the SEM images of the ZnO thin films with and without Ag co-sputtering. The surface of pure ZnO thin film is flat, and consists of tiny surface grains of approximately 50–80 nm. The surface of the ZnO–Ag thin films became rough in comparison. In addition to the Ag particles (or clusters) randomly dispersed on the film surface, the surface grains of the ZnO–Ag thin films featured clusters that consisted of many ZnO crystallites and Ag particles. The size of the clustered ZnO–Ag composite crystallites
Fig. 1. XRD patterns of the ZnO thin films with and without Ag co-sputtering: (a) without Ag addition, (b) 3 W, (c) 9 W, and (d) 13 W Ag sputtering power levels.
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increased with the Ag-sputtering power level, resulting in a corrugated film surface. Fig. 3a and b shows the HRTEM image and EDS spectrum, respectively, of ZnO thin films prepared using Ag co-sputtering at 3 W of power. The EDS spectrum in Fig. 3b shows the Ag element signals emitted from the prepared thin film. Small Ag nanoparticles with a typical diameter of approximately 20 nm were observed in the sample. As shown in Fig. 3a, an HRTEM observation of one particle indicated that the estimated interplanar distance of approximately 0.23 nm was a close fit with the {1 1 1} plane of a metallic Ag nanoparticle, in agreement with the XRD results. The existence of (1 1 1)-oriented Ag nanoparticles in a ZnO matrix has been observed in Ag nanoparticle-stabilized ZnO nanosheets [19]. A further increase of the Ag content (to the 9 W sputtering power level) caused the ZnO–Ag thin film to feature clusters of ZnO crystallites and Ag (Fig. 3c). The EDS spectrum (Fig. 3d) shows the composition of the Zn, Ag, and O in the film. The aggregation of the Ag particles is attributed to the reduction of the metallic particle surface energy during the film growth. The HRTEM lattice fringes for the Ag clusters can be observed directly in Fig. 3e, with a spacing of approximately 0.23 and 0.20 nm corresponding to the interplanar distances of Ag (1 1 1) and Ag (2 0 0), respectively. Fig. 4 shows the XPS survey scan of the ZnO thin film with 13 W Ag co-sputtering. The primary features include the Ag 3d, O 1s, and Zn 2p peaks at approximately 367, 529, and 1022 eV, respectively. No indications of impurity atoms were observed. The ZnO thin films were grown in a pure Ar ambient at 300 °C under such an oxygen-deficient and low-temperature process, that the Zn interstitials might exist in ZnO lattices. The XPS Zn 2p signals cannot provide subtle information on the formation of Zn interstitials in ZnO lattices. The Zn-LMM Auger spectra were further recorded to evaluate the existence of Zn interstitials in ZnO lattices. Fig. 5a–d shows that the Zn-LMM Auger signals are in an asymmetric curve for the samples; two Gaussian curves fit the Zn-LMM Auger spectra. The relatively strong peak centered at 498.5 eV is ascribed to the bonding with the oxygen in the ZnO lattice. The relatively weak peak centered at a lower bonding energy of 495.1 eV is attributed to the existence of Zn interstitials in the ZnO lattice. The Zn interstitials may exist herein because of the formation of nonstoichiometric ZnO thin films with oxygen vacancies; the metallic Zn interstitial atoms in ZnO lattices are believed to be associated with the reduction of free energy caused by the formation of the oxygen vacancies in ZnO thin films under certain preparation conditions [20,21]. The relative content of the interstitial Zn atoms in ZnO thin films with and without Ag co-sputtering is shown in Fig. 5e. The content of the interstitial Zn atoms in the ZnO thin films with or without various Ag co-sputtering power levels was similar at the extent of approximately 20%; this indicated that the Ag co-sputtering had little influence on the content of the interstitial Zn atoms. Fig. 6a–c shows the core-level lines of the Ag 3d in ZnO thin films with various Ag co-sputtering power levels. The reported binding energy of Ag 3d5/2 for metallic Ag ranges from 367.9 to 368.3 eV [22,23]. For Ag+ ions, the reported values range from 367.7 to 368.4 eV [24,25]. Therefore, it is difficult to accurately separate the effects of the metallic Ag and Ag+ ion states on the ZnO–Ag thin films from the XPS Ag 3d spectra. For the samples in this study, the splitting of the Ag 3d doublet was reported to be 6.0 eV, indicating the metallic nature of Ag in the ZnO–Ag thin films [26]. This can be further supported by the SEM–EDS and XRD analyses. The SEM– EDS composition analysis indicates that the Ag content in the ZnO thin films is high, and is approximately 5.0, 22.1, and 33.5 at.% for the 3, 9, and 13 W Ag co-sputtered thin films, respectively. The Ag+ ion state is ascribed to Ag–O binding that could result from an effective Ag doping in ZnO lattices or exist in the forms of AgO or Ag2O oxidation states [17]. No XRD Bragg reflections from the Ag-based oxides were detected from the
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Fig. 2. SEM images of the ZnO thin films with and without Ag co-sputtering: (a) without Ag addition, (b) 3 W, (c) 9 W, (d) 13 W Ag sputtering power levels, and (e) a highmagnification SEM image of (d).
XRD measurements; therefore, the formation of the AgO or Ag2O phase should be excluded herein. Although some degree of Ag doping in the ZnO thin films may occur during thin-film deposition with various Ag co-sputtering power levels, it is reasonable to assume that the major Ag atoms in the ZnO thin films are of a metallic nature instead of in an ion state. Moreover, the binding energies of Ag 3d5/2 for the ZnO thin films with the 3, 9, and 13 W Ag-sputtering power levels are 367.3, 368.0, and 368.0 eV, respectively. The Ag 3d5/2 peak shifted slightly to a higher binding energy with an increase of the Ag-sputtering power level and attained the constant value when the Ag-sputtering power level was higher than 9 W. This may reveal that the chemical environment of Ag in the 3 W Ag co-sputtered thin film is slightly different from the 9 and 13 W Ag co-sputtered thin films. A similar Ag 3d peak shift to a higher binding energy has been observed in the Ag-doped ZnO thin films after thermal annealing because of the slight change in the Ag chemical environment based on the improved doping effect [27]. Fig. 7a–d shows that the XPS spectra of O 1s for the ZnO thin films with or without Ag co-sputtering have a symmetric curve feature. Two Gaussian curves fit the O1s spectra, and are centered at approximately 530.33 and 531.75 eV, which were respectively designated as OI and OII. The relatively low binding-energy component OI is associated with the oxygen ions in the wurtzite structure
of the ZnO. The high binding-energy component OII represents an oxygen vacancy within the oxide lattice [28]. The OII/(OI + OII) ratio indicates the content of the oxygen vacancy in the crystalline ZnO– Ag thin films. Fig. 7e shows the change of the oxygen vacancy density in the ZnO crystallites with various Ag-sputtering power levels. Fig. 7e shows that the as-deposited ZnO thin-film has a defective nature because the thin-film deposition was conducted in an oxygen-deficient ambient. The oxygen vacancy is easily formed in oxide thin films during the sputtering process in an oxygen-deficient ambient; this has been widely reported in various oxide thin-film systems [29–31]. Comparatively, an increase of the Ag-sputtering power level from 3 to 9 W reduced the content of the oxygen vacancy in the ZnO–Ag thin films, and the ZnO–Ag thin films with the 9 W Ag-sputtering power level had the lowest oxygen vacancy density among the samples. The mechanism that triggers a reduction in oxygen vacancy defects with proper Agsputtering power levels in the ZnO thin film remains unclear. The oxygen vacancy is a donor defect in ZnO, and the Ag substitution atoms in ZnO lattices act as acceptors [32]. The reduction in the oxygen vacancy density in this study may be associated with the increase of the Ag-sputtering power level from 3 to 9 W maybe having increased the efficiency of Ag doping in the ZnO lattice at the low deposition temperature of 300 °C, reducing the oxygen
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Fig. 3. (a) HRTEM image of the ZnO–Ag thin film with 3 W Ag sputtering power level. (b) EDS spectrum of the ZnO–Ag thin film with 3 W Ag sputtering power level. (c) Lowmagnification TEM image of the ZnO thin film with 9 W Ag sputtering power level. The image clearly shows the congregate feature of ZnO crystallites and Ag particles (or clusters). (d) EDS spectrum of the ZnO thin film with 9 W Ag sputtering power level. (e) HRTEM image of the ZnO thin film with 9 W Ag sputtering power level.
vacancies during thin-film growth. This is similar to a recent study on ZnMgO thin films, whereby suppression of oxygen-related defects during thin-film growth was achieved through Ag addition at a suitable substrate temperature [32]. In contrast, the oxygen vacancy density in ZnO thin films in this study increased when the Ag-sputtering power level reached 13 W. The seriously aggregation of Ag particles has been shown to cause bad contact ability with ZnO crystallites in hydrothermally synthesized Ag/ZnO composites [11]. This reduced the accommodation ability between the continuous oxide film and Ag clusters, further causing an increase of the oxygen vacancy density during thin-film growth. Many
congregate features were formed to constitute the relatively rugged morphology of the ZnO–Ag thin film with 13 W Ag-sputtering. Such a rugged surface morphology may indicate that the oxygen vacancies are more easily formed compared with those having Ag-sputtering power levels of 3 and 9 W. Fig. 8a–d shows the PL spectra of ZnO thin films with and without Ag co-sputtering. Broadened and asymmetric PL spectra were observed for the pure ZnO and 3 W Ag co-sputtered thin films (Fig. 8a and b). The PL spectra were further deconvoluted into three peaks; they are ascribed to the UV-, blue-, and green-emission bands. The blue emission band dominated the PL characteristics
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Fig. 4. XPS survey scan of the ZnO–Ag thin film with 13 W Ag sputtering power level.
Fig. 5. Zn-LMM Auger signals of the ZnO thin films with and without Ag cosputtering: (a) without Ag addition, (b) 3 W, (c) 9 W, and (d) 13 W Ag sputtering power levels. The content of Zn interstitial atoms in ZnO lattices was shown in (e).
of the pure ZnO and 3 W Ag co-sputtering thin films. The blue emission band was centered at 2.69 eV in this study. The cause of the blue emission band in sputtering deposited-ZnO thin films has been attributed to zinc interstitials and zinc vacancy-point defects in the ZnO lattices [33]. Moreover, the theoretical work by Kohan et al. revealed that the energy interval between the defect center of the zinc vacancy and the bottom of the conduction band
Fig. 6. XPS narrow scans of Ag 3d core-level doublet of ZnO–Ag thin films: (a) 3 W, (b) 9 W, and (c) 13 W Ag sputtering power levels.
Fig. 7. XPS narrow scans of O 1s of ZnO–Ag thin films: (a) without Ag addition, (b) 3 W, (c) 9 W, and (d) 13 W Ag sputtering power levels. The content of oxygen vacancies in ZnO lattices was shown in (e).
of ZnO is approximately 2.6 eV [34]. Comparatively, the blue emission band was quenched for the ZnO–Ag thin films with Ag-sputtering power levels of 9 and 13 W. From the analysis of the ZnLMM Auger signals, the content of the Zn interstitials in the oxides was not markedly changed among the various ZnO and ZnO–Ag
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4. Conclusions
Fig. 8. PL spectra of the ZnO thin films with and without Ag co-sputtering: (a) without Ag addition, (b) 3 W, (c) 9 W, and (d) 13 W Ag sputtering power levels.
thin films. The blue emission band in this study may have resulted from the existence of zinc vacancies. It has been shown that the zinc vacancy sites can be compensated for by effectively Ag doping the ZnO [35]. An increase of the Ag-sputtering power level above 9 W may have validated the Ag+ ions substituting for zinc vacancies under the low deposition temperature of 300 °C in this study. This accounts for the quench of the blue emission bands from the ZnO thin films with higher Ag-sputtering power levels of 9 and 13 W. The UV emission band from the ZnO has been attributed to the near-band-edge emission, which originated from the direct recombination of the free excitons [3,5,36]. Because Ag could behave as an acceptor in the ZnO, the UV emission may be enhanced because of Ag addition in the ZnO matrix. The intensity ratio of the UV to the visible emission band of the ZnO was enhanced from 0.08 to 0.25 with the 3 W Ag co-sputtering. Further increasing the Agsputtering power level above 9 W, a clear UV emission band feature was observed, as shown in Fig. 8c–d. The intensity ratio of the UV to visible emission band reached a maximum of 2.7 for the ZnO thin film with 9 W Ag co-sputtering and decreased to 0.78 when the Ag-sputtering power level was further increased to 13 W. The 9 W Ag sputtering was an optimal sputtering power level for obtaining the relatively strong UV emission peak in this study. The holes are believed to exist in the interface between the ZnO crystallites and Ag particles (or clusters). The electrons in the n-type ZnO matrix can arrive at the hetero-interfaces of ZnO crystallites and Ag particles (clusters) to validate the exciton transition; thus, UV emission is generated [36]. Comparatively, the effective enhancement of the UV emission band for the ZnO thin films with the 9 W Ag co-sputtering power level is attributed to more Ag nanoparticles and clusters maybe existing in the ZnO thin film, resulting in a relatively large portion of hetero-contact with ZnO crystallites. However, for the 13 W Ag co-sputtering of ZnO thin film, more crystal defects may have formed during the co-sputtering process because of excessive and large Ag clusters in the ZnO. This structural deterioration of ZnO–Ag thin film with the 13 W Ag-sputtering power level has been shown from these structural measurements. Therefore, a marked increase of the green emission band was observed for the ZnO–Ag thin film when the Ag-sputtering power was increased from 9 to 13 W.
Highly c-axis-textured ZnO–Ag composite thin films with various degrees of Ag addition were grown on c-axis-oriented sapphire substrates by sputtering at 300 °C. Structural analyses show that Ag nanoparticles seriously aggregated to form clusters with a high Ag content in the ZnO thin film. The ZnO thin-film surface was further roughened with Ag addition; a markedly rugged surface morphology of the ZnO–Ag thin film was observed for the 13 W Ag cosputtered thin film. The Ag addition in the ZnO thin film changed the density of native point defects in the ZnO prepared in an oxygen-deficient ambient by sputtering. This caused different PL characteristics of the ZnO thin films with and without the Ag addition. The blue emission band was found to dominate the pure and 3 W Ag co-sputtered thin films. The UV emission band dominated the 9 W Ag co-sputtered thin film because effective Ag doping reduced the density of zinc and oxygen vacancies in ZnO lattices. Many ZnO–Ag composite grains clustered on the thin-film structure when the Ag-sputtering power level was increased to 13 W. This incurred more oxygen-related point defects in ZnO lattices; thus, a green emission band was further enhanced. The correlation between the structure and PL characteristics of ZnO–Ag composite thin films in this study is a good reference for designing ZnO-based devices with various luminescence properties. References [1] D. Bagnall, Y. Chen, M. Shen, Z. Zhu, T. Goto, T. Yao, Appl. Phys. Lett. 70 (1997) 2230. [2] S. Liang, H. Sheng, Y. Liu, Z. Huo, Y. Lu, H. Shen, J. Cryst. Growth 225 (2001) 110. [3] Y.C. Liang, Ceram. Int. 38 (2012) 119. [4] M. Miyazaki, M. Sato, E. Budianu, E. Rusu, M. Danila, R. Gavrila, Thin Solid Films 403–404 (2002) 485. [5] Y.C. Liang, J. Alloys Comp. 508 (2010) 158. [6] K. Haga, M. Kamidaira, Y. Kashiwaba, T. Sekiguchi, H. Watanabe, J. Cryst. Growth 214–215 (2000) 77. [7] Y.C. Liang, M.Y. Tsai, C.L. Huang, C.Y. Hu, C.S. Hwang, J. Alloys Comp. 509 (2011) 3559. [8] S.W. Choi, S.S. Kim, Sensors Actuat. B 168 (2012) 8. [9] C.A. Lin, D.S. Tsai, C.Y. Chen, J.H. He, Nanoscale 3 (2011) 1195. [10] Q. Simon, D. Barreca, D. Bekermann, A. Gasparotto, C. Maccato, E. Comini, V. Gombac, P. Fornaiero, O.I. Lebedev, S. Turner, A. Devi, R.A. Fischer, G. Tendeloo, Int. J. Hydrogen Engergy 36 (2011) 15527. [11] Y. Zhang, J. Mu, J. Colloid Interface Sci. 309 (2007) 478. [12] I.S. Kim, E.K. Jeong, D.Y. Kim, M. Kumar, S.Y. Choi, Appl. Surf. Sci. 255 (2009) 4011. [13] Y.C. Liang, Ceram. Int. 38 (2012) 1697. [14] S. Pati, S.B. Majumder, P. Banerji, J. Alloys Comp. 541 (2012) 376. [15] S.K. Panda, C. Jacob, Physica E 41 (2009) 792. [16] X. Li, Y. Wang, J. Alloys Comp. 509 (2011) 5765. [17] R. Chen, C. Zou, J. Bian, A. Sandhu, W. Gao, Nanotechnology 22 (2011) 105706. [18] M.A. Thomas, J.B. Cui, J. Appl. Phys. 105 (2009) 093533. [19] D. Zhang, X. Liu, X. Wang, J. Alloys Comp. 509 (2011) 4972. [20] M. Chen, X. Wang, Y.H. Yu, Z.L. Pei, X.D. Bai, C. Sun, R.F. Huang, L.S. Wen, Appl. Surf. Sci. 158 (2000) 134. [21] J. Aranovich, A. Oritz, R.H. Bube, J. Vac. Sci. Technol. 16 (1979) 994. [22] J.C. Fuggle, E. Kallne, L.M. Watson, D.J. Fabian, Phys. Rev. B 16 (1997) 750. [23] G. Schoen, J. Electron Spectrosc. Relat. Phenom. 1 (1972) 377. [24] V.K. Kaushik, J. Electron Spectrosc. Relat. Phenom. 56 (1991) 273. [25] R. Romand, M. Roubin, J.P. Deloume, J. Electron Spectrosc. Relat. Phenom. 13 (1978) 229. [26] J. Chen, X. Yan, W. Liu, Q. Xue, Sensors Actuat. B 160 (2011) 1499. [27] K. Liu, B. Yang, H. Yan, Z. Fu, M. Wen, Y. Chen, J. Zuo, J. Luminescence 129 (2009) 969. [28] Y.C. Liang, C.Y. Hu, Y.C. Liang, CrystEngComm 14 (2012) 5579. [29] Y.C. Liang, H.Y. Lee, CrystEngComm 12 (2010) 3172. [30] Y.C. Liang, X.S. Deng, H. Zhong, Ceram. Int. 38 (2012) 2261. [31] Y.C. Liang, J.P. Chu, Jpn. J. Appl. Phys. 47 (2008) 257. [32] L. Cao, L. Zhu, J. Jiang, Y. Li, Y. Zhang, Z. Ye, J. Alloys Comp. 516 (2012) 157. [33] Z. Liang, X. Yu, B. Lei, P. Liu, W. Mai, J. Alloys Comp. 509 (2011) 5437. [34] A.F. Kohan, G. Ceder, D. Morgan, Phys. Rev. D 61 (2000) 15019. [35] H. Xue, X.L. Xu, Y. Chen, G.H. Zhang, S.Y. Na, Appl. Surf. Sci. 255 (2008) 1806. [36] Y.C. Liang, C.Y. Hu, H. Zhong, Appl. Surf. Sci. 261 (2012) 633.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Structure dependent luminescence evolution of c-axis-oriented ZnO nanofilms embedded with silver nanoparticles and clusters prepared by sputtering Yuan-Chang Liang ⇑, Xian-Shi Deng Institute of Materials Engineering, National Taiwan Ocean University, Keelung 20224, Taiwan
a r t i c l e
i n f o
Article history: Received 7 February 2013 Received in revised form 12 March 2013 Accepted 19 March 2013 Available online 27 March 2013 Keywords: Ceramics Thin film Sputtering Microstructure Characterization
a b s t r a c t ZnO–Ag composite thin films with various degrees of Ag addition were grown on c-axis-oriented sapphire substrates by sputtering. By varying the Ag sputtering power level from 3 to 13 W, the effects of Ag addition in 80-nm-thick ZnO thin films could be determined. The X-ray diffraction patterns revealed that ZnO thin films prepared with and without Ag co-sputtering exhibited highly c-axis-oriented crystallographic features. Scanning electron microscopy and transmission electron microscopy images show that a high Ag content caused the ZnO crystallites and Ag nanoparticles (or Ag clusters) to group and form clusters of grains during thin-film growth. The X-ray photoelectron spectrometer measurements demonstrate a decrease of oxygen-related point defects in ZnO thin films with proper Ag additions. The blue emission band dominated the photoluminescence characteristics of pure ZnO and 3 W Ag co-sputtered ZnO thin films. The blue emission band was markedly quenched, and the UV emission band was significantly enhanced when the Ag-sputtering power level was raised to above 9 W. These are attributed to a reduction of zinc vacancies and an improvement of the exciton transition process by using proper Ag additions. The experimental results in this work show that the photoluminescence characteristics of ZnO thin films could be adjusted by varying the content of the Ag in ZnO thin films. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a direct band gap semiconductor. The characteristics of its wide band gap and large exciton binding energy at room temperature make it a promising candidate for various optoelectronic applications such as UV light emitting diodes, blue luminescent devices and UV photodetectors [1,2]. In order to change electric and optical properties of intrinsic ZnO to meet various applications in devices, several elements have been used as dopants in ZnO [3,4]. For practical device applications, the thin-film structure of ZnO is highly required. Many techniques including sputtering, chemical vapor deposition, and chemical solution methods are adopted to synthesize ZnO thin films with various degrees of crystalline quality [5–7]. Among the various thin-film preparation methodologies, the sputtering is a promising method to prepare ZnO-based composite thin films with easy process control and large area thin-film deposition. Noble-metal-doped ZnO thin-films and nanostructures or ZnO– noble metal composites have recently raised interest because of their promising applications in various sensor devices, surface ⇑ Corresponding author. E-mail address: [email protected] (Y.-C. Liang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.03.169
plasmon-related devices, and catalysts [8–10]. Among the various noble metals, Ag is an important material with optical properties in the visible spectrum. Several works have synthesized ZnO–Ag composites and investigated their physical properties [11,12]. Generally, the photoluminescence (PL) characteristics of ZnO are associated with a near-band-edge excitonic UV emission and a deeplevel emission in the visible range [13]. The microstructure is posited to greatly affect the PL properties of ZnO thin films. Several works on the PL properties of ZnO demonstrate the mechanisms for the defect-related deep-level emission [13,14]; however, the effects on the PL properties of ZnO when Ag is added are still undetermined. This is because both the preparation methodology and the Ag contents significantly influence the microstructure of ZnO–Ag composites. The complex interactions between the defects and the Ag particles in the ZnO matrix affect optical properties [15–18]. To expand the application of ZnO–Ag composite thin films in devices, understanding the microstructure-dependent optical properties is critical. However, a systematic investigation of the correlation between the microstructure evolution and optical properties of the ZnO–Ag thin films prepared by sputtering are limited. In this work, ZnO thin films were co-sputtered with various Ag contents, and the structure-dependent optical properties that resulted from the Ag additions are discussed.
Y.-C. Liang, X.-S. Deng / Journal of Alloys and Compounds 569 (2013) 144–149 2. Experimental ZnO–Ag composite thin films were sputter deposited on c-axis-oriented sapphire substrates using ZnO and Ag targets. The substrates were supersonic clean out in acetone, rinsed in alcohol and subsequently dried in flowing air gas before deposition. The ZnO–Ag thin film deposition was performed with a dual-gun sputtering system. During the deposition, the substrate temperature was maintained at 300 °C and the working pressure is 30 mTorr with a pure Ar atmosphere. The duration for thin-film deposition was fixed at 25 min and the Ag sputtering power was varied from 3 to 13 W during the cosputtering deposition. X-ray diffraction (XRD) with Cu Ka radiation was used to investigate the crystalline quality of the prepared thin films. The surface morphology and composition of the films are investigated with a field emission scanning electron microscopy (FESEM) equipped with energy dispersive X-ray spectrometer (EDS). The binding states of the constituent elements for the sputtering deposited ZnO–Ag thin films are examined by X-ray photoelectron spectrometer (XPS). The atomic structure of the ZnO–Ag thin films is further characterized by high-resolution transmittance electron microscopy (HRTEM). The composition analysis is performed using EDS attached to the TEM. The room temperature dependent photoluminescence (PL) spectra are obtained using the 325 nm line of a He–Cd laser.
3. Results and discussion Fig. 1 shows the XRD patterns of the ZnO thin films prepared with and without Ag co-sputtering. Fig. 1 shows clear Bragg reflections from ZnO (0 0 2); no additional crystallographic planes were observed that revealed that the ZnO thin films prepared with and without Ag co-sputtering were highly c-axis oriented. By increasing the sputtering power level of Ag from 3 to 13 W, the Ag (1 1 1) Bragg reflection became stronger, and that of the Ag (2 0 0) became apparent. This indicated that the content of Ag in the ZnO thin film increased with the sputtering power level. The fullwidth at half maximum of the (0 0 2) Bragg reflection for the ZnO thin film with the Ag co-sputtering broadened compared to that without Ag co-sputtering. Moreover, the intensity of ZnO (0 0 2) Bragg reflections decreased as the Ag-sputtering power level increased, possibly because large Ag clusters in the ZnO matrix deteriorated the continuous-film feature of the ZnO–Ag thin film at the high Ag-sputtering power level. Fig. 2 shows the SEM images of the ZnO thin films with and without Ag co-sputtering. The surface of pure ZnO thin film is flat, and consists of tiny surface grains of approximately 50–80 nm. The surface of the ZnO–Ag thin films became rough in comparison. In addition to the Ag particles (or clusters) randomly dispersed on the film surface, the surface grains of the ZnO–Ag thin films featured clusters that consisted of many ZnO crystallites and Ag particles. The size of the clustered ZnO–Ag composite crystallites
Fig. 1. XRD patterns of the ZnO thin films with and without Ag co-sputtering: (a) without Ag addition, (b) 3 W, (c) 9 W, and (d) 13 W Ag sputtering power levels.
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increased with the Ag-sputtering power level, resulting in a corrugated film surface. Fig. 3a and b shows the HRTEM image and EDS spectrum, respectively, of ZnO thin films prepared using Ag co-sputtering at 3 W of power. The EDS spectrum in Fig. 3b shows the Ag element signals emitted from the prepared thin film. Small Ag nanoparticles with a typical diameter of approximately 20 nm were observed in the sample. As shown in Fig. 3a, an HRTEM observation of one particle indicated that the estimated interplanar distance of approximately 0.23 nm was a close fit with the {1 1 1} plane of a metallic Ag nanoparticle, in agreement with the XRD results. The existence of (1 1 1)-oriented Ag nanoparticles in a ZnO matrix has been observed in Ag nanoparticle-stabilized ZnO nanosheets [19]. A further increase of the Ag content (to the 9 W sputtering power level) caused the ZnO–Ag thin film to feature clusters of ZnO crystallites and Ag (Fig. 3c). The EDS spectrum (Fig. 3d) shows the composition of the Zn, Ag, and O in the film. The aggregation of the Ag particles is attributed to the reduction of the metallic particle surface energy during the film growth. The HRTEM lattice fringes for the Ag clusters can be observed directly in Fig. 3e, with a spacing of approximately 0.23 and 0.20 nm corresponding to the interplanar distances of Ag (1 1 1) and Ag (2 0 0), respectively. Fig. 4 shows the XPS survey scan of the ZnO thin film with 13 W Ag co-sputtering. The primary features include the Ag 3d, O 1s, and Zn 2p peaks at approximately 367, 529, and 1022 eV, respectively. No indications of impurity atoms were observed. The ZnO thin films were grown in a pure Ar ambient at 300 °C under such an oxygen-deficient and low-temperature process, that the Zn interstitials might exist in ZnO lattices. The XPS Zn 2p signals cannot provide subtle information on the formation of Zn interstitials in ZnO lattices. The Zn-LMM Auger spectra were further recorded to evaluate the existence of Zn interstitials in ZnO lattices. Fig. 5a–d shows that the Zn-LMM Auger signals are in an asymmetric curve for the samples; two Gaussian curves fit the Zn-LMM Auger spectra. The relatively strong peak centered at 498.5 eV is ascribed to the bonding with the oxygen in the ZnO lattice. The relatively weak peak centered at a lower bonding energy of 495.1 eV is attributed to the existence of Zn interstitials in the ZnO lattice. The Zn interstitials may exist herein because of the formation of nonstoichiometric ZnO thin films with oxygen vacancies; the metallic Zn interstitial atoms in ZnO lattices are believed to be associated with the reduction of free energy caused by the formation of the oxygen vacancies in ZnO thin films under certain preparation conditions [20,21]. The relative content of the interstitial Zn atoms in ZnO thin films with and without Ag co-sputtering is shown in Fig. 5e. The content of the interstitial Zn atoms in the ZnO thin films with or without various Ag co-sputtering power levels was similar at the extent of approximately 20%; this indicated that the Ag co-sputtering had little influence on the content of the interstitial Zn atoms. Fig. 6a–c shows the core-level lines of the Ag 3d in ZnO thin films with various Ag co-sputtering power levels. The reported binding energy of Ag 3d5/2 for metallic Ag ranges from 367.9 to 368.3 eV [22,23]. For Ag+ ions, the reported values range from 367.7 to 368.4 eV [24,25]. Therefore, it is difficult to accurately separate the effects of the metallic Ag and Ag+ ion states on the ZnO–Ag thin films from the XPS Ag 3d spectra. For the samples in this study, the splitting of the Ag 3d doublet was reported to be 6.0 eV, indicating the metallic nature of Ag in the ZnO–Ag thin films [26]. This can be further supported by the SEM–EDS and XRD analyses. The SEM– EDS composition analysis indicates that the Ag content in the ZnO thin films is high, and is approximately 5.0, 22.1, and 33.5 at.% for the 3, 9, and 13 W Ag co-sputtered thin films, respectively. The Ag+ ion state is ascribed to Ag–O binding that could result from an effective Ag doping in ZnO lattices or exist in the forms of AgO or Ag2O oxidation states [17]. No XRD Bragg reflections from the Ag-based oxides were detected from the
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Fig. 2. SEM images of the ZnO thin films with and without Ag co-sputtering: (a) without Ag addition, (b) 3 W, (c) 9 W, (d) 13 W Ag sputtering power levels, and (e) a highmagnification SEM image of (d).
XRD measurements; therefore, the formation of the AgO or Ag2O phase should be excluded herein. Although some degree of Ag doping in the ZnO thin films may occur during thin-film deposition with various Ag co-sputtering power levels, it is reasonable to assume that the major Ag atoms in the ZnO thin films are of a metallic nature instead of in an ion state. Moreover, the binding energies of Ag 3d5/2 for the ZnO thin films with the 3, 9, and 13 W Ag-sputtering power levels are 367.3, 368.0, and 368.0 eV, respectively. The Ag 3d5/2 peak shifted slightly to a higher binding energy with an increase of the Ag-sputtering power level and attained the constant value when the Ag-sputtering power level was higher than 9 W. This may reveal that the chemical environment of Ag in the 3 W Ag co-sputtered thin film is slightly different from the 9 and 13 W Ag co-sputtered thin films. A similar Ag 3d peak shift to a higher binding energy has been observed in the Ag-doped ZnO thin films after thermal annealing because of the slight change in the Ag chemical environment based on the improved doping effect [27]. Fig. 7a–d shows that the XPS spectra of O 1s for the ZnO thin films with or without Ag co-sputtering have a symmetric curve feature. Two Gaussian curves fit the O1s spectra, and are centered at approximately 530.33 and 531.75 eV, which were respectively designated as OI and OII. The relatively low binding-energy component OI is associated with the oxygen ions in the wurtzite structure
of the ZnO. The high binding-energy component OII represents an oxygen vacancy within the oxide lattice [28]. The OII/(OI + OII) ratio indicates the content of the oxygen vacancy in the crystalline ZnO– Ag thin films. Fig. 7e shows the change of the oxygen vacancy density in the ZnO crystallites with various Ag-sputtering power levels. Fig. 7e shows that the as-deposited ZnO thin-film has a defective nature because the thin-film deposition was conducted in an oxygen-deficient ambient. The oxygen vacancy is easily formed in oxide thin films during the sputtering process in an oxygen-deficient ambient; this has been widely reported in various oxide thin-film systems [29–31]. Comparatively, an increase of the Ag-sputtering power level from 3 to 9 W reduced the content of the oxygen vacancy in the ZnO–Ag thin films, and the ZnO–Ag thin films with the 9 W Ag-sputtering power level had the lowest oxygen vacancy density among the samples. The mechanism that triggers a reduction in oxygen vacancy defects with proper Agsputtering power levels in the ZnO thin film remains unclear. The oxygen vacancy is a donor defect in ZnO, and the Ag substitution atoms in ZnO lattices act as acceptors [32]. The reduction in the oxygen vacancy density in this study may be associated with the increase of the Ag-sputtering power level from 3 to 9 W maybe having increased the efficiency of Ag doping in the ZnO lattice at the low deposition temperature of 300 °C, reducing the oxygen
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Fig. 3. (a) HRTEM image of the ZnO–Ag thin film with 3 W Ag sputtering power level. (b) EDS spectrum of the ZnO–Ag thin film with 3 W Ag sputtering power level. (c) Lowmagnification TEM image of the ZnO thin film with 9 W Ag sputtering power level. The image clearly shows the congregate feature of ZnO crystallites and Ag particles (or clusters). (d) EDS spectrum of the ZnO thin film with 9 W Ag sputtering power level. (e) HRTEM image of the ZnO thin film with 9 W Ag sputtering power level.
vacancies during thin-film growth. This is similar to a recent study on ZnMgO thin films, whereby suppression of oxygen-related defects during thin-film growth was achieved through Ag addition at a suitable substrate temperature [32]. In contrast, the oxygen vacancy density in ZnO thin films in this study increased when the Ag-sputtering power level reached 13 W. The seriously aggregation of Ag particles has been shown to cause bad contact ability with ZnO crystallites in hydrothermally synthesized Ag/ZnO composites [11]. This reduced the accommodation ability between the continuous oxide film and Ag clusters, further causing an increase of the oxygen vacancy density during thin-film growth. Many
congregate features were formed to constitute the relatively rugged morphology of the ZnO–Ag thin film with 13 W Ag-sputtering. Such a rugged surface morphology may indicate that the oxygen vacancies are more easily formed compared with those having Ag-sputtering power levels of 3 and 9 W. Fig. 8a–d shows the PL spectra of ZnO thin films with and without Ag co-sputtering. Broadened and asymmetric PL spectra were observed for the pure ZnO and 3 W Ag co-sputtered thin films (Fig. 8a and b). The PL spectra were further deconvoluted into three peaks; they are ascribed to the UV-, blue-, and green-emission bands. The blue emission band dominated the PL characteristics
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Fig. 4. XPS survey scan of the ZnO–Ag thin film with 13 W Ag sputtering power level.
Fig. 5. Zn-LMM Auger signals of the ZnO thin films with and without Ag cosputtering: (a) without Ag addition, (b) 3 W, (c) 9 W, and (d) 13 W Ag sputtering power levels. The content of Zn interstitial atoms in ZnO lattices was shown in (e).
of the pure ZnO and 3 W Ag co-sputtering thin films. The blue emission band was centered at 2.69 eV in this study. The cause of the blue emission band in sputtering deposited-ZnO thin films has been attributed to zinc interstitials and zinc vacancy-point defects in the ZnO lattices [33]. Moreover, the theoretical work by Kohan et al. revealed that the energy interval between the defect center of the zinc vacancy and the bottom of the conduction band
Fig. 6. XPS narrow scans of Ag 3d core-level doublet of ZnO–Ag thin films: (a) 3 W, (b) 9 W, and (c) 13 W Ag sputtering power levels.
Fig. 7. XPS narrow scans of O 1s of ZnO–Ag thin films: (a) without Ag addition, (b) 3 W, (c) 9 W, and (d) 13 W Ag sputtering power levels. The content of oxygen vacancies in ZnO lattices was shown in (e).
of ZnO is approximately 2.6 eV [34]. Comparatively, the blue emission band was quenched for the ZnO–Ag thin films with Ag-sputtering power levels of 9 and 13 W. From the analysis of the ZnLMM Auger signals, the content of the Zn interstitials in the oxides was not markedly changed among the various ZnO and ZnO–Ag
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4. Conclusions
Fig. 8. PL spectra of the ZnO thin films with and without Ag co-sputtering: (a) without Ag addition, (b) 3 W, (c) 9 W, and (d) 13 W Ag sputtering power levels.
thin films. The blue emission band in this study may have resulted from the existence of zinc vacancies. It has been shown that the zinc vacancy sites can be compensated for by effectively Ag doping the ZnO [35]. An increase of the Ag-sputtering power level above 9 W may have validated the Ag+ ions substituting for zinc vacancies under the low deposition temperature of 300 °C in this study. This accounts for the quench of the blue emission bands from the ZnO thin films with higher Ag-sputtering power levels of 9 and 13 W. The UV emission band from the ZnO has been attributed to the near-band-edge emission, which originated from the direct recombination of the free excitons [3,5,36]. Because Ag could behave as an acceptor in the ZnO, the UV emission may be enhanced because of Ag addition in the ZnO matrix. The intensity ratio of the UV to the visible emission band of the ZnO was enhanced from 0.08 to 0.25 with the 3 W Ag co-sputtering. Further increasing the Agsputtering power level above 9 W, a clear UV emission band feature was observed, as shown in Fig. 8c–d. The intensity ratio of the UV to visible emission band reached a maximum of 2.7 for the ZnO thin film with 9 W Ag co-sputtering and decreased to 0.78 when the Ag-sputtering power level was further increased to 13 W. The 9 W Ag sputtering was an optimal sputtering power level for obtaining the relatively strong UV emission peak in this study. The holes are believed to exist in the interface between the ZnO crystallites and Ag particles (or clusters). The electrons in the n-type ZnO matrix can arrive at the hetero-interfaces of ZnO crystallites and Ag particles (clusters) to validate the exciton transition; thus, UV emission is generated [36]. Comparatively, the effective enhancement of the UV emission band for the ZnO thin films with the 9 W Ag co-sputtering power level is attributed to more Ag nanoparticles and clusters maybe existing in the ZnO thin film, resulting in a relatively large portion of hetero-contact with ZnO crystallites. However, for the 13 W Ag co-sputtering of ZnO thin film, more crystal defects may have formed during the co-sputtering process because of excessive and large Ag clusters in the ZnO. This structural deterioration of ZnO–Ag thin film with the 13 W Ag-sputtering power level has been shown from these structural measurements. Therefore, a marked increase of the green emission band was observed for the ZnO–Ag thin film when the Ag-sputtering power was increased from 9 to 13 W.
Highly c-axis-textured ZnO–Ag composite thin films with various degrees of Ag addition were grown on c-axis-oriented sapphire substrates by sputtering at 300 °C. Structural analyses show that Ag nanoparticles seriously aggregated to form clusters with a high Ag content in the ZnO thin film. The ZnO thin-film surface was further roughened with Ag addition; a markedly rugged surface morphology of the ZnO–Ag thin film was observed for the 13 W Ag cosputtered thin film. The Ag addition in the ZnO thin film changed the density of native point defects in the ZnO prepared in an oxygen-deficient ambient by sputtering. This caused different PL characteristics of the ZnO thin films with and without the Ag addition. The blue emission band was found to dominate the pure and 3 W Ag co-sputtered thin films. The UV emission band dominated the 9 W Ag co-sputtered thin film because effective Ag doping reduced the density of zinc and oxygen vacancies in ZnO lattices. Many ZnO–Ag composite grains clustered on the thin-film structure when the Ag-sputtering power level was increased to 13 W. This incurred more oxygen-related point defects in ZnO lattices; thus, a green emission band was further enhanced. The correlation between the structure and PL characteristics of ZnO–Ag composite thin films in this study is a good reference for designing ZnO-based devices with various luminescence properties. References [1] D. Bagnall, Y. Chen, M. Shen, Z. Zhu, T. Goto, T. Yao, Appl. Phys. Lett. 70 (1997) 2230. [2] S. Liang, H. Sheng, Y. Liu, Z. Huo, Y. Lu, H. Shen, J. Cryst. Growth 225 (2001) 110. [3] Y.C. Liang, Ceram. Int. 38 (2012) 119. [4] M. Miyazaki, M. Sato, E. Budianu, E. Rusu, M. Danila, R. Gavrila, Thin Solid Films 403–404 (2002) 485. [5] Y.C. Liang, J. Alloys Comp. 508 (2010) 158. [6] K. Haga, M. Kamidaira, Y. Kashiwaba, T. Sekiguchi, H. Watanabe, J. Cryst. Growth 214–215 (2000) 77. [7] Y.C. Liang, M.Y. Tsai, C.L. Huang, C.Y. Hu, C.S. Hwang, J. Alloys Comp. 509 (2011) 3559. [8] S.W. Choi, S.S. Kim, Sensors Actuat. B 168 (2012) 8. [9] C.A. Lin, D.S. Tsai, C.Y. Chen, J.H. He, Nanoscale 3 (2011) 1195. [10] Q. Simon, D. Barreca, D. Bekermann, A. Gasparotto, C. Maccato, E. Comini, V. Gombac, P. Fornaiero, O.I. Lebedev, S. Turner, A. Devi, R.A. Fischer, G. Tendeloo, Int. J. Hydrogen Engergy 36 (2011) 15527. [11] Y. Zhang, J. Mu, J. Colloid Interface Sci. 309 (2007) 478. [12] I.S. Kim, E.K. Jeong, D.Y. Kim, M. Kumar, S.Y. Choi, Appl. Surf. Sci. 255 (2009) 4011. [13] Y.C. Liang, Ceram. Int. 38 (2012) 1697. [14] S. Pati, S.B. Majumder, P. Banerji, J. Alloys Comp. 541 (2012) 376. [15] S.K. Panda, C. Jacob, Physica E 41 (2009) 792. [16] X. Li, Y. Wang, J. Alloys Comp. 509 (2011) 5765. [17] R. Chen, C. Zou, J. Bian, A. Sandhu, W. Gao, Nanotechnology 22 (2011) 105706. [18] M.A. Thomas, J.B. Cui, J. Appl. Phys. 105 (2009) 093533. [19] D. Zhang, X. Liu, X. Wang, J. Alloys Comp. 509 (2011) 4972. [20] M. Chen, X. Wang, Y.H. Yu, Z.L. Pei, X.D. Bai, C. Sun, R.F. Huang, L.S. Wen, Appl. Surf. Sci. 158 (2000) 134. [21] J. Aranovich, A. Oritz, R.H. Bube, J. Vac. Sci. Technol. 16 (1979) 994. [22] J.C. Fuggle, E. Kallne, L.M. Watson, D.J. Fabian, Phys. Rev. B 16 (1997) 750. [23] G. Schoen, J. Electron Spectrosc. Relat. Phenom. 1 (1972) 377. [24] V.K. Kaushik, J. Electron Spectrosc. Relat. Phenom. 56 (1991) 273. [25] R. Romand, M. Roubin, J.P. Deloume, J. Electron Spectrosc. Relat. Phenom. 13 (1978) 229. [26] J. Chen, X. Yan, W. Liu, Q. Xue, Sensors Actuat. B 160 (2011) 1499. [27] K. Liu, B. Yang, H. Yan, Z. Fu, M. Wen, Y. Chen, J. Zuo, J. Luminescence 129 (2009) 969. [28] Y.C. Liang, C.Y. Hu, Y.C. Liang, CrystEngComm 14 (2012) 5579. [29] Y.C. Liang, H.Y. Lee, CrystEngComm 12 (2010) 3172. [30] Y.C. Liang, X.S. Deng, H. Zhong, Ceram. Int. 38 (2012) 2261. [31] Y.C. Liang, J.P. Chu, Jpn. J. Appl. Phys. 47 (2008) 257. [32] L. Cao, L. Zhu, J. Jiang, Y. Li, Y. Zhang, Z. Ye, J. Alloys Comp. 516 (2012) 157. [33] Z. Liang, X. Yu, B. Lei, P. Liu, W. Mai, J. Alloys Comp. 509 (2011) 5437. [34] A.F. Kohan, G. Ceder, D. Morgan, Phys. Rev. D 61 (2000) 15019. [35] H. Xue, X.L. Xu, Y. Chen, G.H. Zhang, S.Y. Na, Appl. Surf. Sci. 255 (2008) 1806. [36] Y.C. Liang, C.Y. Hu, H. Zhong, Appl. Surf. Sci. 261 (2012) 633.