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Enhanced room temperature ferromagnetism in Co-doped ZnO mediated by interstitial H
Materials Letters 89 (2012) 209–211
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Enhanced room temperature ferromagnetism in Co-doped ZnO mediated by interstitial H Hua Zhang a, Yanqiang Cao b, Zaixing Yang c, Lifang Si a, Wei Zhong c, Di Wu b, Mingxiang Xu a, Qingyu Xu a,n a
Department of Physics, Southeast University, Nanjing 211189, China Department of Materials Science and Engineering, Nanjing University, Nanjing 210008, China c Department of Physics, Nanjing University, Nanjing 210008, China b
a r t i c l e i n f o
abstract
Article history: Received 29 June 2012 Accepted 24 August 2012 Available online 3 September 2012
Strongly enhanced room temperature ferromagnetism with saturate magnetization (Ms) of 0.98 emu/g (0.71 mB/Co) has been obtained in Zn0.98Co0.02O powders after annealing in H2 atmosphere at 500 1C for 2 h. Co3O4 has been clearly resolved in the as-prepared Zn0.98Co0.02O powders, and significantly suppressed by the hydrogenation. The structural characterizations have confirmed the incorporation of the interstitial H, and excluded the reduction of Co ions to the metallic state. Our results clearly demonstrate the ferromagnetic mediation between the neighboring Co ions by the interstitial H ions. & 2012 Elsevier B.V. All rights reserved.
Keywords: Magnetic materials Semiconductors Sol–gel preparation
1. Introduction Diluted magnetic semiconductors (DMS) have attracted much interest for the potential applications in semiconductor spintronics [1]. It is important to achieve the room temperature (RT) ferromagnetism in DMS for the practical applications. After Dietl’s theoretical prediction of RT ferromagnetic ZnO [2], intensive researches have been done on ZnO by 3d transition metal (TM) doping [3]. Generally, Zener– RKKY free-carrier exchange or bound magnetic polarons models are always used to explain the observed ferromagnetism in ZnO-based DMS [4]. However, lack of ferromagnetism has been reported in epitaxial Co-doped ZnO films with wide electron concentration range, indicating that the itinerant conduction band electrons alone are not sufficient to induce ferromagnetism [4,5]. Furthermore, only paramagnetic signal has been observed for the doped Co ions by X-ray magnetic circular dichroism in ferromagnetic Co-doped ZnO [6,7], excluding the possible ferromagnetic exchange interaction between the doped Co ions. The interaction between the neighboring 3d TM ions doped in ZnO through O2 is superexchange antiferromagnetic. To obtain ferromagnetism, the ferromagnetic exchange interaction between the neighboring Co ions is required. Park and Chadi has predicted that the interstitial H in ZnO may provide ferromagnetic mediation between the neighboring 3d TM ions doped in ZnO [8]. Several works in experiments have demonstrated the enhanced ferromagnetism in Co-doped ZnO by annealing in H2 atmosphere [9–11], but
the mechanism is still under debate, since the defects might be induced, and even metallic Co might precipitate during the hydrogenation [10,11]. In this work, we prepared Zn0.98Co0.02O powders by the sol–gel method. Strongly enhanced RT ferromagnetism has been observed after annealing in H2 atmosphere at 500 1C for 2 h. By careful structural characterization, we conclude the ferromagnetic mediation between neighboring Co ions by the interstitial H ions.
2. Experimental details Zn0.98Co0.02O powders were synthesized by the sol–gel method. Proper amount of (Zn(Ac)2) (AR) and Co(NO3)2 (AR) were dissolved in de-ionized water. Citric acid monohydrate (C6H8O7 H2O, AR) and ethylene glycol (HOCH2CH2OH, AR) were added to the solution. The obtained solution was dried at 80 oC, then calcined at 300 1C in air for 12 h. After that, the obtained powders were sintered in air at 500 1C for 24 h. The annealing of Zn0.98Co0.02O powders were performed in H2 (ZnCoO–H) flow at 500 1C for 2 h to prevent the formation of metallic Co [11]. The structure was studied by X-ray diffraction (XRD) measurements with y–2y scans using a Cu Ka source, Photoluminescence (PL), Raman and X-ray photoelectron spectroscopy (XPS) with Al Ka X-ray source (hn ¼ 1486.6 eV). The magnetization was measured by a physical property measurement system (PPMS-9, Quantum Design) at 300 K.
3. Results and discussion n
Corresponding author. Tel.: þ86 25 l52090600x8308; fax: þ 86 25 52090600x8203. E-mail address: [email protected] (Q. Xu). 0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.08.116
Fig. 1 shows the XRD patterns of ZnCoO and ZnCoO–H powders. All the peaks can be indexed to the pure wurtzite structure.
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H. Zhang et al. / Materials Letters 89 (2012) 209–211
Fig. 1. XRD patterns of (a) ZnCoO and (b) ZnCoO–H. The inset shows the enlarged view of (103) peaks.
Fig. 2. M–H curves of (a) ZnCoO and (b) ZnCoO–H. Insets of (a) shows ferromagnetic magnetization of ZnCoO (left), the M–H curves of ZnCoO–Ar and ZnCoO–Hair (right). Insets of (b) show the ferromagnetic magnetization of ZnCoO–H (left), ZnCoO–Ar and ZnCoO–H-air (right).
The diffraction peak became asymmetric after the hydrogenation, as shown by the (103) peaks. The hydrogenation might modify the structure of the outer regions of the crystallites, and the core–shell structure might be formed. The shift of diffraction peak to higher angles is consistent with our previous studies on ZnO and Zn0.98 Cu0.02O annealed in H2 [12,13] and can be attributed to the decrease of the concentration of O vacancies [14]. Fig. 2(a) shows the magnetic hysteresis (M–H) loop of ZnCoO and ZnCoO–H measured at 300 K. It is obvious that the ZnCoO powders are paramagnetic. The ferromagnetic contribution can be obtained by subtracting the paramagnetic contributions from the high field linear part. The saturate ferromagnetic magnetization (Ms) is about 1.0 10 4 emu/g, which is close to the senstivity limit of the PPMS-9. After the hydrogenation, clear magnetic hysteresis loop can be observed, as shown in Fig. 2(b). The Ms is strongly enhanced to 0.98 emu/g (0.71 mB/Co). Fig. 3(a) shows the PL spectra of ZnCoO and ZnCoO–H. Two dominant emission peaks are observed: the near-band-edge (NBE) emission centered at 375 nm and the broad yellow emission centered
Fig. 3. (a) PL and (b) Raman spectra of ZnCoO and ZnCoO–H.
at around 500 nm. The stronger NBE emission after the hydrogenation can be attributed to the excitions bound to the incorporated H [15]. The broad yellow emission has been attributed to the presence of OH groups [16]. The PL results clearly confirm the incorporation of H in ZnCoO–H by the hydrogenation. Fig. 3(b) shows the Raman spectra of ZnCoO and ZnCoO–H. The peaks at about 327 and 435 cm 1 can be observed in both spectra, corresponding to the phonon modes of the wurtzite ZnO structure [17]. Three strong peaks at 488, 523 and 622 cm 1 were observed in the spectroscopy of ZnCoO, but nearly absent in that of ZnCoO–H. The peak at 488 cm 1 corresponds to Eg and the peaks at 523 and 622 cm 1 correspond to F2 g phonon modes of the Co3O4 [17], which cannot be revealed by XRD due to the senstivity limit. The results indicate that the hydrogenation strongly suppresses the formation of Co3O4. Furthermore, there are two more peaks, one broad peak located at around 551 cm 1, and the other sharp peak located at 714 cm 1. These two peaks can be attributed to Co–O bonding due to the Co doping [17], which confirms the substitution of Co in ZnO. The peak at 713 cm 1 has been strongly suppressed after the dydrogenation, this can be understood by the incorporation of H in ZnCoO–H and Co–H bonds instead of Co–O bonds are formed. To check the possible metalic Co after the hydrogenation, Co 2P XPS spectra of ZnCoO and ZnCoO–H were taken, referenced to the surface impurity C 1 s binding energy (284.8 eV) [18], as shown in Fig. 4. The Co 2P3/2 in ZnCoO is 780.48 eV, and shifts slightly to the lower energy in ZnCoO–H (780.29 eV). The binding energies of Co 2P3/2 in both Co3O4 and CoO are at around 780 eV, and that of the metallic Co is at around 778 eV [19]. Thus the possible metallic Co formed during the hydrogenation can be excluded, which is further confirmed by the satallite peaks marked by arrows in Fig. 4. Since the energy position of Co 2P3/2 in Co3O4 is a little larger than that in CoO, the slight shift of the Co 2P3/2 to lower energy is consistent with the Raman results that the Co3O4 was significantly suppressed by hydrogenation. Based on the above structural results, the metallic Co impurity can be excluded, and the observed ferromagnetism can be considered to be intrinsic. To further confirm ferromagnetic mediation by the interstitial H, and not from other defects, we annealed ZnCoO powders in Ar atmosphere at 500 1C for 2 h (ZnCoO–Ar). ZnCoO–Ar exhibits clear paramagnetism, as shown in right inset of Fig. 2(a). And only weak ferromagnetic contribution (Ms 0.0025 emu/g) has been observed, which is much smaller than that of ZnCoO–H. Our results clearly demonstrate that the enhanced ferromagnetism can be attributed to the neighboring Co ions mediated by the interstitial
H. Zhang et al. / Materials Letters 89 (2012) 209–211
211
ions to the metallic Co. The ferromagnetic contribution from defects has been excluded. Our results clearly demonstrated that the enhanced ferromagnetism originates from ferromagnetic mediation between the neighboring Co ions by the interstitial H ions.
Acknowledgment This work is supported by the National Key Projects for Basic Researches of China (2010CB923404), the National Natural Science Foundation of China (51172044), the National Science Foundation of Jiangsu Province of China (BK2011617), by NCET-09-0296, the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and Southeast University (the Excellent Young Teachers Program and Seujq201106).
References
Fig. 4. Co 2P XPS spectra for ZnCoO and ZnCoO–H.
[1] [2] [3] [4] [5]
H ions. We further annealed the ZnCoO–H in air at 500 1C for 2 h to check the thermal stability. Drastic drop of ferromagnetism was observed (right inset of Fig. 2(a)). The Ms is only about 0.0075 emu/g, which is about 2 orders smaller. Hao et al. reported that the magnetization is nearly unchanged after the reannealing in O2 at 400 1C [9]. The interstitial H can only stay in ZnO stably up to 400 1C [16], and the strongly suppression of the RT ferromagnetism can be attributed to the removal of the interstitial H during the annealing in air.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
4. Conclusion The as-prepared Zn0.98Co0.02O powders are paramagnetic at RT, and strongly enhanced RT ferromagnetism with Ms of 0.98 emu/g (0.71 mB/Co) has been obtained after annealing in H2 atmosphere at 500 1C for 2 h. The structural characterizations have confirmed the incorporation of the interstitial H, and excluded the reduction of Co
[16] [17] [18] [19]
Ohno H. Science 1998;281:951. Dietl T, Ohno H, Matsukura F, Cibert J, Ferrand D. Science 2000;287:1019. Liu C, Yun F, Morkoc- HJ. Mater Sci 2005;16:555. Kaspar TC, Droubay T, Heald SM, Nachimuthu P, Wang CM, Shutthanandan V, et al. N J Phys 2008;10:055010. Xu Q, Zhou S, Marko´ D, Potzger K, Fassbender J, Vinnichenko M, et al. J Phys D 2009;42:085001. Barla A, Schmerber G, Beaurepaire E, Dinia A, Bieber H, Colis S, et al. Phys Rev B 2007;76:125201. Gacic M, Jakob G, Herbort C, Adrian H. Phys Rev B 2007;75:205206. Park CH, Chadi DJ. Phys Rev Lett 2005;94:127204. Hao W, Li J, Xu H, Wang J, Wang T. ACS Appl Mater Interfaces 2010;2:2053. Singhal RK, Samariya A, Kumar S, Xing YT, Saitovitch E. Mater Lett 2010;64:1846. Deka S, Joy PA. Appl Phys Lett 2006;89:032508. Xu Q, Wen Z, Zhang H, Qi X, Zhong W, Xu L, et al. AIP Adv 2011;1:032127. Zhang H, Wen Z, Qi X, Si LF, Xu L, Zhong W, et al. J Alloys Compd 2012;536:184. Panigraphy B, Aslam M, Bahadur D. Appl Phys Lett 2011;98:183109. Huang XH, Tay CB, Zhan ZY, Zhang C, Zheng LX, Venkatesan T, et al. CrystEngComm 2011;13:7032. Tam KH, Cheung CK, Leung YH, Djuriˇsic´ AB, Ling CC, Beling CD, et al. J Phys Chem B 2006;110:20865. Bhargava R, Sharma PK, Kumar S, Pandey AC, Kumar N. J Raman Spectrosc 2011;42:1802. Wei L, Li Z, Zhang WF. Appl Surf Sci 2009;255:4992. Wagner CD, Riggs WM, Davis LE, Moulder JF. Mullenberg GE. Handbook of X-ray photoelectron spectroscopy. Minnesota: Perkin–Elmer Corporation; 1979 p. 78.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Enhanced room temperature ferromagnetism in Co-doped ZnO mediated by interstitial H Hua Zhang a, Yanqiang Cao b, Zaixing Yang c, Lifang Si a, Wei Zhong c, Di Wu b, Mingxiang Xu a, Qingyu Xu a,n a
Department of Physics, Southeast University, Nanjing 211189, China Department of Materials Science and Engineering, Nanjing University, Nanjing 210008, China c Department of Physics, Nanjing University, Nanjing 210008, China b
a r t i c l e i n f o
abstract
Article history: Received 29 June 2012 Accepted 24 August 2012 Available online 3 September 2012
Strongly enhanced room temperature ferromagnetism with saturate magnetization (Ms) of 0.98 emu/g (0.71 mB/Co) has been obtained in Zn0.98Co0.02O powders after annealing in H2 atmosphere at 500 1C for 2 h. Co3O4 has been clearly resolved in the as-prepared Zn0.98Co0.02O powders, and significantly suppressed by the hydrogenation. The structural characterizations have confirmed the incorporation of the interstitial H, and excluded the reduction of Co ions to the metallic state. Our results clearly demonstrate the ferromagnetic mediation between the neighboring Co ions by the interstitial H ions. & 2012 Elsevier B.V. All rights reserved.
Keywords: Magnetic materials Semiconductors Sol–gel preparation
1. Introduction Diluted magnetic semiconductors (DMS) have attracted much interest for the potential applications in semiconductor spintronics [1]. It is important to achieve the room temperature (RT) ferromagnetism in DMS for the practical applications. After Dietl’s theoretical prediction of RT ferromagnetic ZnO [2], intensive researches have been done on ZnO by 3d transition metal (TM) doping [3]. Generally, Zener– RKKY free-carrier exchange or bound magnetic polarons models are always used to explain the observed ferromagnetism in ZnO-based DMS [4]. However, lack of ferromagnetism has been reported in epitaxial Co-doped ZnO films with wide electron concentration range, indicating that the itinerant conduction band electrons alone are not sufficient to induce ferromagnetism [4,5]. Furthermore, only paramagnetic signal has been observed for the doped Co ions by X-ray magnetic circular dichroism in ferromagnetic Co-doped ZnO [6,7], excluding the possible ferromagnetic exchange interaction between the doped Co ions. The interaction between the neighboring 3d TM ions doped in ZnO through O2 is superexchange antiferromagnetic. To obtain ferromagnetism, the ferromagnetic exchange interaction between the neighboring Co ions is required. Park and Chadi has predicted that the interstitial H in ZnO may provide ferromagnetic mediation between the neighboring 3d TM ions doped in ZnO [8]. Several works in experiments have demonstrated the enhanced ferromagnetism in Co-doped ZnO by annealing in H2 atmosphere [9–11], but
the mechanism is still under debate, since the defects might be induced, and even metallic Co might precipitate during the hydrogenation [10,11]. In this work, we prepared Zn0.98Co0.02O powders by the sol–gel method. Strongly enhanced RT ferromagnetism has been observed after annealing in H2 atmosphere at 500 1C for 2 h. By careful structural characterization, we conclude the ferromagnetic mediation between neighboring Co ions by the interstitial H ions.
2. Experimental details Zn0.98Co0.02O powders were synthesized by the sol–gel method. Proper amount of (Zn(Ac)2) (AR) and Co(NO3)2 (AR) were dissolved in de-ionized water. Citric acid monohydrate (C6H8O7 H2O, AR) and ethylene glycol (HOCH2CH2OH, AR) were added to the solution. The obtained solution was dried at 80 oC, then calcined at 300 1C in air for 12 h. After that, the obtained powders were sintered in air at 500 1C for 24 h. The annealing of Zn0.98Co0.02O powders were performed in H2 (ZnCoO–H) flow at 500 1C for 2 h to prevent the formation of metallic Co [11]. The structure was studied by X-ray diffraction (XRD) measurements with y–2y scans using a Cu Ka source, Photoluminescence (PL), Raman and X-ray photoelectron spectroscopy (XPS) with Al Ka X-ray source (hn ¼ 1486.6 eV). The magnetization was measured by a physical property measurement system (PPMS-9, Quantum Design) at 300 K.
3. Results and discussion n
Corresponding author. Tel.: þ86 25 l52090600x8308; fax: þ 86 25 52090600x8203. E-mail address: [email protected] (Q. Xu). 0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.08.116
Fig. 1 shows the XRD patterns of ZnCoO and ZnCoO–H powders. All the peaks can be indexed to the pure wurtzite structure.
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H. Zhang et al. / Materials Letters 89 (2012) 209–211
Fig. 1. XRD patterns of (a) ZnCoO and (b) ZnCoO–H. The inset shows the enlarged view of (103) peaks.
Fig. 2. M–H curves of (a) ZnCoO and (b) ZnCoO–H. Insets of (a) shows ferromagnetic magnetization of ZnCoO (left), the M–H curves of ZnCoO–Ar and ZnCoO–Hair (right). Insets of (b) show the ferromagnetic magnetization of ZnCoO–H (left), ZnCoO–Ar and ZnCoO–H-air (right).
The diffraction peak became asymmetric after the hydrogenation, as shown by the (103) peaks. The hydrogenation might modify the structure of the outer regions of the crystallites, and the core–shell structure might be formed. The shift of diffraction peak to higher angles is consistent with our previous studies on ZnO and Zn0.98 Cu0.02O annealed in H2 [12,13] and can be attributed to the decrease of the concentration of O vacancies [14]. Fig. 2(a) shows the magnetic hysteresis (M–H) loop of ZnCoO and ZnCoO–H measured at 300 K. It is obvious that the ZnCoO powders are paramagnetic. The ferromagnetic contribution can be obtained by subtracting the paramagnetic contributions from the high field linear part. The saturate ferromagnetic magnetization (Ms) is about 1.0 10 4 emu/g, which is close to the senstivity limit of the PPMS-9. After the hydrogenation, clear magnetic hysteresis loop can be observed, as shown in Fig. 2(b). The Ms is strongly enhanced to 0.98 emu/g (0.71 mB/Co). Fig. 3(a) shows the PL spectra of ZnCoO and ZnCoO–H. Two dominant emission peaks are observed: the near-band-edge (NBE) emission centered at 375 nm and the broad yellow emission centered
Fig. 3. (a) PL and (b) Raman spectra of ZnCoO and ZnCoO–H.
at around 500 nm. The stronger NBE emission after the hydrogenation can be attributed to the excitions bound to the incorporated H [15]. The broad yellow emission has been attributed to the presence of OH groups [16]. The PL results clearly confirm the incorporation of H in ZnCoO–H by the hydrogenation. Fig. 3(b) shows the Raman spectra of ZnCoO and ZnCoO–H. The peaks at about 327 and 435 cm 1 can be observed in both spectra, corresponding to the phonon modes of the wurtzite ZnO structure [17]. Three strong peaks at 488, 523 and 622 cm 1 were observed in the spectroscopy of ZnCoO, but nearly absent in that of ZnCoO–H. The peak at 488 cm 1 corresponds to Eg and the peaks at 523 and 622 cm 1 correspond to F2 g phonon modes of the Co3O4 [17], which cannot be revealed by XRD due to the senstivity limit. The results indicate that the hydrogenation strongly suppresses the formation of Co3O4. Furthermore, there are two more peaks, one broad peak located at around 551 cm 1, and the other sharp peak located at 714 cm 1. These two peaks can be attributed to Co–O bonding due to the Co doping [17], which confirms the substitution of Co in ZnO. The peak at 713 cm 1 has been strongly suppressed after the dydrogenation, this can be understood by the incorporation of H in ZnCoO–H and Co–H bonds instead of Co–O bonds are formed. To check the possible metalic Co after the hydrogenation, Co 2P XPS spectra of ZnCoO and ZnCoO–H were taken, referenced to the surface impurity C 1 s binding energy (284.8 eV) [18], as shown in Fig. 4. The Co 2P3/2 in ZnCoO is 780.48 eV, and shifts slightly to the lower energy in ZnCoO–H (780.29 eV). The binding energies of Co 2P3/2 in both Co3O4 and CoO are at around 780 eV, and that of the metallic Co is at around 778 eV [19]. Thus the possible metallic Co formed during the hydrogenation can be excluded, which is further confirmed by the satallite peaks marked by arrows in Fig. 4. Since the energy position of Co 2P3/2 in Co3O4 is a little larger than that in CoO, the slight shift of the Co 2P3/2 to lower energy is consistent with the Raman results that the Co3O4 was significantly suppressed by hydrogenation. Based on the above structural results, the metallic Co impurity can be excluded, and the observed ferromagnetism can be considered to be intrinsic. To further confirm ferromagnetic mediation by the interstitial H, and not from other defects, we annealed ZnCoO powders in Ar atmosphere at 500 1C for 2 h (ZnCoO–Ar). ZnCoO–Ar exhibits clear paramagnetism, as shown in right inset of Fig. 2(a). And only weak ferromagnetic contribution (Ms 0.0025 emu/g) has been observed, which is much smaller than that of ZnCoO–H. Our results clearly demonstrate that the enhanced ferromagnetism can be attributed to the neighboring Co ions mediated by the interstitial
H. Zhang et al. / Materials Letters 89 (2012) 209–211
211
ions to the metallic Co. The ferromagnetic contribution from defects has been excluded. Our results clearly demonstrated that the enhanced ferromagnetism originates from ferromagnetic mediation between the neighboring Co ions by the interstitial H ions.
Acknowledgment This work is supported by the National Key Projects for Basic Researches of China (2010CB923404), the National Natural Science Foundation of China (51172044), the National Science Foundation of Jiangsu Province of China (BK2011617), by NCET-09-0296, the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and Southeast University (the Excellent Young Teachers Program and Seujq201106).
References
Fig. 4. Co 2P XPS spectra for ZnCoO and ZnCoO–H.
[1] [2] [3] [4] [5]
H ions. We further annealed the ZnCoO–H in air at 500 1C for 2 h to check the thermal stability. Drastic drop of ferromagnetism was observed (right inset of Fig. 2(a)). The Ms is only about 0.0075 emu/g, which is about 2 orders smaller. Hao et al. reported that the magnetization is nearly unchanged after the reannealing in O2 at 400 1C [9]. The interstitial H can only stay in ZnO stably up to 400 1C [16], and the strongly suppression of the RT ferromagnetism can be attributed to the removal of the interstitial H during the annealing in air.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
4. Conclusion The as-prepared Zn0.98Co0.02O powders are paramagnetic at RT, and strongly enhanced RT ferromagnetism with Ms of 0.98 emu/g (0.71 mB/Co) has been obtained after annealing in H2 atmosphere at 500 1C for 2 h. The structural characterizations have confirmed the incorporation of the interstitial H, and excluded the reduction of Co
[16] [17] [18] [19]
Ohno H. Science 1998;281:951. Dietl T, Ohno H, Matsukura F, Cibert J, Ferrand D. Science 2000;287:1019. Liu C, Yun F, Morkoc- HJ. Mater Sci 2005;16:555. Kaspar TC, Droubay T, Heald SM, Nachimuthu P, Wang CM, Shutthanandan V, et al. N J Phys 2008;10:055010. Xu Q, Zhou S, Marko´ D, Potzger K, Fassbender J, Vinnichenko M, et al. J Phys D 2009;42:085001. Barla A, Schmerber G, Beaurepaire E, Dinia A, Bieber H, Colis S, et al. Phys Rev B 2007;76:125201. Gacic M, Jakob G, Herbort C, Adrian H. Phys Rev B 2007;75:205206. Park CH, Chadi DJ. Phys Rev Lett 2005;94:127204. Hao W, Li J, Xu H, Wang J, Wang T. ACS Appl Mater Interfaces 2010;2:2053. Singhal RK, Samariya A, Kumar S, Xing YT, Saitovitch E. Mater Lett 2010;64:1846. Deka S, Joy PA. Appl Phys Lett 2006;89:032508. Xu Q, Wen Z, Zhang H, Qi X, Zhong W, Xu L, et al. AIP Adv 2011;1:032127. Zhang H, Wen Z, Qi X, Si LF, Xu L, Zhong W, et al. J Alloys Compd 2012;536:184. Panigraphy B, Aslam M, Bahadur D. Appl Phys Lett 2011;98:183109. Huang XH, Tay CB, Zhan ZY, Zhang C, Zheng LX, Venkatesan T, et al. CrystEngComm 2011;13:7032. Tam KH, Cheung CK, Leung YH, Djuriˇsic´ AB, Ling CC, Beling CD, et al. J Phys Chem B 2006;110:20865. Bhargava R, Sharma PK, Kumar S, Pandey AC, Kumar N. J Raman Spectrosc 2011;42:1802. Wei L, Li Z, Zhang WF. Appl Surf Sci 2009;255:4992. Wagner CD, Riggs WM, Davis LE, Moulder JF. Mullenberg GE. Handbook of X-ray photoelectron spectroscopy. Minnesota: Perkin–Elmer Corporation; 1979 p. 78.