Journal of Colloid and Interface Science 416 (2014) 289–293
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Photoluminescence investigation about zinc oxide with graphene oxide & reduced graphene oxide buffer layers Jijun Ding, Minqiang Wang ⇑, Xiangyu Zhang, Zhi Yang, Xiaohui Song, Chenxin Ran Electronic Materials Research Laboratory, Key Laboratory of Ministry of Education, School of Electronic and Information Engineering, International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China
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i n f o
Article history: Received 19 August 2013 Accepted 21 October 2013 Available online 8 November 2013 Keywords: ZnO films Graphene oxide Reduced graphene oxide Tunable photoluminescence
a b s t r a c t ZnO with graphene oxide (GO–ZnO) & reduced graphene oxide (rGO–ZnO) buffer layers were fabricated. Photoluminescence (PL) properties of GO–ZnO and rGO–ZnO compositions induced by oxygen vacancies defects were investigated using photoluminescence spectroscopy. The results showed that blue emission is quenched while yellow–orange emissions from GO–ZnO and rGO–ZnO compositions are significantly increased as compared to that of ZnO films. In stark contrast to enhanced yellow–orange emissions, PL spectra show three sharp, discrete emissions that characterize the dominant optical active defect, which is the oxygen vacancies and extended oxygen vacancies. Our results highlight the ability of GO & rGO buffer layers to modulate defect concentrations in ZnO and contribute to understanding the optical properties of deep-level defects, which is significant for development of long-wavelength photoelectric devices related with graphene materials. Crown Copyright Ó 2013 Published by Elsevier Inc. All rights reserved.
1. Introduction Graphene oxide (GO) is a graphene sheet modified with oxygen functional groups [1], and it exhibits interesting steady-state photoluminescence (PL) properties [2–4]. Such as, Eda et al. observed, in chemically derived GO, a relatively narrow PL peak centered at 390 nm and an apparently equivalent PL peak centered at 440 nm [5]. Luo et al. reported broadband visible PL from solid GO and discussed possible gapping mechanisms [6]. Mei and Zhang synthesized highly photoluminescent GO nanosheets and proposed a fluorescence ‘‘off-to-on’’ mechanism [7,8]. PL has been reported in chemically modified GO where the electronic structure has been modified [9]. Chien et al. reported tunable PL of GO with increased reduction [10]. Xin et al. also observed tunable PL of GO from near-ultraviolet to blue region through controlled hydrazine (N2H4) reduction [11]. In addition, reduced graphene oxide (rGO) also exhibits interesting steady-state PL properties. Cuong et al. demonstrated that optical emissions of rGO were ascribed to recombination of electron–hole pairs in localized electronic states [12]. Lui et al. observed significant light emission from graphene under excitation by ultrashort (30-fs) laser pulses [13]. Gokus et al. confirmed making graphene luminescent by oxygen plasma treatment [2]. ZnO is an II–VI semiconductor with a wide and direct band gap (Eg = 3.37 eV at 300 K), excellent chemical and thermal stability, ⇑ Corresponding author. Fax: +86 29 82668794. E-mail address: [email protected] (M. Wang).
and specific electrical and optoelectronic property of having a large exciton binding energy (60 meV) [14]. It possesses many important applications in electronic and optical devices [15,16]. Hybridization of carbon materials with ZnO offers a powerful way to obtain many interesting properties. Interestingly, optical properties of graphene & GO and ZnO composites have also been investigated. Such as, Yang et al. reported UV–vis absorption properties of functionalized graphene sheets (FGS)/ZnO nanocomposites exposed under UV light at regular time intervals [17]. Luo et al. presented UV–vis diffused reflectance spectra from RGO-hierarchical ZnO hollow sphere composites [18]. Kavitha et al. prepared graphene nanosheets decorated with ZnO nanoparticles which showed an emission peak at 400 nm associated with the recombination of excitons in ZnO [19]. Singh et al. investigated ZnO decorated with different molar ratios GO, and as the concentration of GO was increased, the PL quenching extent at 550 nm increased and observed large blue-shift of 0.15 eV in PL [20]. These ZnO decorated GO & rGO sheets display distinct optoelectronic characteristics, emitting near-ultraviolet to blue-green luminescence. However, other longer wavelength emission can also be expected for GO–ZnO and rGO–ZnO compositions, which is significant for development of low cost nanodevices. To our knowledge, there are no reports about the modulation of GO & rGO on larger wavelength range emission of ZnO. Also, PL mechanism in these compositions is still not clear. In this paper, GO & rGO sheets were deposited on indium tin oxide (ITO) substrates by electrophoretic deposition (EPD), and then ZnO films were deposited on down-layer GO & rGO sheets
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by magnetron sputtering technique to prepare GO–ZnO and rGO– ZnO compositions. The crystal structures, morphology, optical properties were analyzed by using X-ray diffraction (XRD) pattern, atomic force microscopy (AFM) and scanning electron microscopy (SEM) images, Raman spectra, PL and photoluminescence excitation (PLE) spectra. The effect of GO & rGO on PL spectra ranging from 300 to 800 nm of ZnO is investigated. PL mechanism in these compositions is proposed based on these experimental results. 2. Materials and experiments Natural graphite (99.9%) powders are commercially available from Wodetai Ltd., Co., Beijing, China. ZnO target (prior to 99.99%, 3 inch in diameter) is commercially available from General Research Institute for Nonferrous Metals, Beijing, China. GO–ZnO and rGO–ZnO compositions were fabricated by the following procedures. Firstly, GO was prepared from natural graphite by a classical Hummers method with some modification [21]. The exfoliated GO was obtained in the supernatant. The rGO used here was reduced from GO followed a convenient method under a lowtemperature and atmosphere pressure in our previous work [22]. Finally, GO and rGO were dispersed to form solutions with an ultrasonic, followed by centrifuging at 4800 rpm for 30 min. Then, GO & rGO sheets were deposited on indium tin oxide (ITO) substrates by electrophoretic deposition (EPD). ITO was acted as working electrode and platinum plate was cathode that was kept 15 mm away from the counter electrode. The evenly dispersed GO & rGO solutions were used directly as electrolyte, and the concentration of GO & rGO in the solution was assigned to 0.5 mg/ml. A constant current of 0.2 mA was applied for 20 min to coat GO & rGO on the ITO substrates. Finally, ZnO films were deposited on down-layer GO & rGO sheets by magnetron sputtering technique. The base pressure for the system was 2 104 Pa and film growth was carried out in the sputtering ambient with Ar:O2 = 4:1 (sccm) at a working pressure of 0.8 Pa. ZnO films were deposited on downlayer GO & rGO sheets with sputtering power of 100 W used during sputtering for 1.5 h. XRD patterns were studied by using a D/Max-2400 X-ray diffractometer. AFM and SEM images were characterized by a Dimension Icon3 atomic force microscope and a FEI Quanta 250 scanning electron microscope, respectively. Raman spectra were investigated using a JY LabRAM HR800 laser Raman spectrometer from 1000 to 4000 cm1 at room temperature. The 514.5 nm line of the laser was used as the excitation source. PL spectra within 350–650 nm measurements were carried out by a LS-55 fluorescence spectrometer under the 325 nm excitation wavelength. PL spectra within 450–800 nm were done using a SENS-9000 fluoroSENS fluorescence spectrometer under the 426 nm excitation wavelength. PLE spectra were measured using a SENS-9000 fluoroSENS fluorescence spectrometer monitored at 606 nm. All spectra were measured at room temperature in air.
Fig. 1. XRD pattern of as prepared GO and rGO.
Fig. 2. XRD pattern of ZnO films, GO–ZnO and rGO–ZnO compositions.
Fujimura et al. [24] suggested that the surface energy density of the (0 0 2) orientation is the lowest in the ZnO crystal. This means that the (0 0 2) texture of the film may easily form. This texture growth results in good crystalline quality. Some minor peaks could be ascribed to ITO substrates. Generally, the above mentioned GO (0 0 2) and rGO (0 0 2) are theoretically expected to be observed in the XRD pattern of GO–ZnO and rGO–ZnO compositions. However, they did not appear in the corresponding compositions, indicating that the surfaces of rGO are fully covered by ZnO films [25,26]. Therefore, it can be seen that the diffraction peaks are mainly from the up-layer ZnO phase in the GO–ZnO and rGO– ZnO compositions. Table 1 shows XRD parameters of ZnO films, GO–ZnO and rGO– ZnO compositions. The average crystallite size D was calculated by the following Scherrer formula [27]:
D¼ 3. Results and discussions Fig. 1 shows XRD patterns of as prepared GO and rGO. As prepared GO shows a sharp diffraction peak centered at 2h = 10.8° corresponding to GO (0 0 2). In XRD pattern of as prepared rGO, GO (0 0 2) diffraction peak completely disappears and there occurs a broad diffraction band ranging from 10° to 30°, which indicates that the GO is completely reduced and there presents few defects in rGO during chemical reduction process. Fig. 2 shows XRD pattern of ZnO films, GO–ZnO and rGO–ZnO compositions. In these XRD patterns, a major peak could be indexed to ZnO (0 0 2), indicating that all the compositions are single crystalline and oriented along c-axis of hexagonal structure [23].
0:9k b cos h
ð1Þ
where k, h and b are the X-ray wavelength (1.5406 Å), diffraction angle and the full width at half-maximum (FWHM) of the ZnO (0 0 2) peak. As can be seen from Table 1, GO–ZnO compositions have the minimum FWHM and the largest crystallite size.
Table 1 XRD parameters of ZnO films, GO–ZnO and rGO–ZnO compositions.
ZnO films GO–ZnO rGO–ZnO
FWHM (°)
D (nm)
r (MPa)
dhkl (nm)
0.216 0.200 0.204
38.1 41.1 40.3
1.0742 1.0742 1.7007
0.2614 0.2614 0.2621
J. Ding et al. / Journal of Colloid and Interface Science 416 (2014) 289–293
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In addition, according to the Bragg formula: k = 2 dhkl sin h, where dhkl denotes the crystalline plane distance for indices (hkl). It was found that all dhkl values are larger than that of standard ZnO powder d0, which is equal to 0.2603 nm. The lattice constants c of (0 0 2) peak can also be calculated by the following formula [28]:
"
2 dhkl
2
2
2
4ðh þ k þ hkÞ l ¼ þ 2 3a2 c
#1 ð2Þ
where a and c are the lattice constants, and dhkl is the crystalline plane distance for indices (hkl). According to Eq. (2), the lattice constant c is equal to 2d for the (0 0 2) diffraction peak. The calculation of the film stress is based on the biaxial strain model. To derive the film stress r parallel to the film surface, the following formula is used, which is valid for single crystal ZnO [29]:
r ¼ 233ðc c0 Þ=c0 MPa
ð3Þ
where c is the lattice constant of ZnO film, c0 = 0.5206 nm is the lattice constant of standard ZnO powder. Together the minimum film stress and the largest crystallite size, we concluded that GO–ZnO compositions have the best crystal quality. Fig. 3 shows AFM and cross-sectional SEM images of ZnO films (a and d), GO–ZnO (b and e) and rGO–ZnO (c and f) compositions. The surface morphology of ZnO films (a), GO–ZnO (b) and rGO–ZnO (c) compositions can be clearly observed. The insets are their corresponding 3-dimensional AFM surface morphology. All the samples show rods-like structures and rGO–ZnO compositions have the largest average surface roughness. The cross-sectional SEM image of ZnO films (d), GO–ZnO (e) and rGO–ZnO (f) compositions were shown. At the same deposition conditions, ZnO films grown on rGO sheets have the typically largest thickness of about 360 nm, which correspond with GO–ZnO compositions having the best crystal quality. Fig. 4 shows Raman spectra of GO and rGO. In the GO and rGO spectrum, three sharp peaks at 1350, 1583 and 2695 cm1 correspond to the D, G and 2D peaks, respectively. The D peak is due to the presence of structural disorders in graphene sheets. The G
Fig. 4. Raman spectra of GO and rGO.
peak attributes to optical E2g phonons at the Brillouin zone center, whereas the ratio of the intensity of the G-band to the D-band is related to the in-plane crystallite size [30]. Besides, the intensity ratio of IG/ID is widely used to characterize the defect quantity in graphene and a low ratio indicates a great disorder arising from structural defects [31]. As can be seen in Fig. 4, the integrated intensity ratios of IG/ID in rGO (IG/ID 1.17) are larger than in GO (IG/ID 1.00). These indicate that the structural defects in GO structures are partially renovated after being reduced. Fig. 5 shows PL spectra of ZnO films, GO–ZnO and rGO–ZnO compositions at a 325 nm excitation wavelength. The PL spectrum of ZnO films clearly shows a broad blue emission peak at 425 nm (2.9 eV), which originates from the electron transition from the interstitial Zn levels to the top of the valence band [32–34]. Interestingly PL from GO–ZnO and rGO–ZnO compositions is significantly quenched as compared to that of ZnO films, which is due to interfacial charge transfer from ZnO to graphene [35]. In addition, PL from rGO–ZnO red shifted to 441 nm, while PL from GO–ZnO blue shifted to 420 nm. Singh et al. [20] observed that PL from rGO–ZnO was blue shifted as compared to that of bare ZnO, which is ascribed to a depletion layer formation between
Fig. 3. AFM and cross-sectional SEM images of ZnO films (a and d), GO–ZnO (b and e) and rGO–ZnO (c and f) compositions.
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Fig. 7. Proposed mechanisms of blue and yellow–orange emissions.
[40]. Zeng et al. reported that the oxygen vacancies arise from the Schottky reaction and further ionization reactions [41]:
Fig. 5. PL spectra of ZnO films, GO–ZnO and rGO–ZnO compositions at 325 nm excitation wavelength.
0 () V XZn þ V XO
ð4Þ
V XO () V O þ e0
ð5Þ
0 V O () V O þe
ð6Þ
the p-type graphene and n-type ZnO interface. So depletion layer formation significantly affects charge transition from shallow levels to valence band. This results in blue peak quenches and shifts. Fig. 6 shows PL spectra of ZnO films, GO–ZnO and rGO–ZnO compositions at a 426 nm excitation wavelength. At a longer excitation wavelength, all samples show three sharp peaks at 570, 603 and 630 nm, respectively. More interestingly, the PL spectra of GO– ZnO and rGO–ZnO compositions are significantly increased, which is different from previous PL quenching phenomenon, while the peak position has no change as compared to that of ZnO films. Djurišic´ et al. reported yellow–orange emission (612–640 nm) at different excitation wavelength [36], but the origin of this emission was not fully clear. Ke et al. thought that yellow–orange emission in ZnO films was related to the presence of O interstitials [37]. Eda et al. reported that a broad and intense emission band at 450– 600 nm may arise from single ionized oxygen vacancy in ZnO and the recombination of electron–hole pairs localized within sp2 carbon clusters embedded within a sp3 matrix in the GO sheets [5]. Likovich et al. found that the observed deep-level emission peaks are unusually narrow and fingerprint the electronic structure of the predominant near-surface optically active defect, which they propose is an oxygen vacancy [38]. We propose another mechanism for the observed abnormal PL increase in GO–ZnO and rGO–ZnO compositions at a longer excitation wavelength. Vlasenko and Watkins showed experimentally that interstitial zinc is a very instable defect [39], but the oxygen vacancy is stable. Janotti and Van de Walle calculated that interstitial zinc are fast diffusers with a migration barrier as low as 0.57 eV, while that for oxygen vacancies is in the range 1.7–2.4 eV
According to previous calculation results, the formation energy of oxygen vacancies is about 1.7 eV [42] and 2.4 eV [43], and it can be significantly reduced after further ionization. Three sharp peaks at 570, 603 and 630 nm correspond to photon energies of 2.18, 2.06 and 1.97 eV, respectively. So, as can be seen in Fig.7, three sharp peaks are attributed to the transitions from oxygen vacancies and extended oxygen vacancies states to the valance band, respectively. Here the extended oxygen vacancies states could be ionized oxygen vacancies, complex defect or localized oxygen vacancies states. In addition, peak at 425 nm originates from the electron transition from the interstitial Zn levels to the top of the valence band. As above mentioned, interstitial zinc is a very instable defect. The interaction between GO & rGO and ZnO caused interstitial zinc diffusing with a migration barrier, so 425 nm emission quenching could be observed in GO–ZnO and rGO–ZnO compositions. However, three sharp peaks at 570, 603 and 630 nm are attributed to the oxygen vacancies and extended oxygen vacancies. As above AFM results mentioned, GO–ZnO and rGO–ZnO compositions contained more defects than in the ZnO films. The ZnO film is an n-type semiconductor and it means that most defects are interstitial Zn and oxygen vacancies. After depositing GO & rGO sheets, the interaction between GO & rGO and ZnO caused interstitial zinc decrease. So according to charge equilibrium, GO–ZnO and rGO–ZnO compositions contained more oxygen vacancies than in the ZnO films. This explains why blue emission related with interstitial zinc will decrease, while three emission peaks related with oxygen vacancies will increase in GO–ZnO and rGO–ZnO compositions. Fig. 8 shows PLE spectra of ZnO films, GO–ZnO and rGO–ZnO compositions monitored at a 606 nm wavelength. The PLE spectra
Fig. 6. PL spectra of ZnO films, GO–ZnO and rGO–ZnO compositions at 426 nm excitation wavelength.
Fig. 8. PLE spectra of ZnO films, GO–ZnO and rGO–ZnO compositions monitored at 606 nm.
J. Ding et al. / Journal of Colloid and Interface Science 416 (2014) 289–293
exhibit three major peaks at 290 (4.3 eV), 325 (3.8 eV) and 426 nm (2.9 eV), respectively and some minor peaks. For 606 nm monitored wavelength, the energy (2.9 eV) below the band gap (3.2– 3.4 eV) is the optimal excitation energy and other lower energy is still effective. Here the extended oxygen vacancies states could be ionized oxygen vacancies, complex defect or localized oxygen vacancies states. Some minor peaks in the lower energy in the PLE spectrum have verified their existence. 4. Conclusions In this paper, we reported photoluminescence from GO–ZnO and rGO–ZnO compositions. In detail, at a 325 nm excitation wavelength, blue emission peak at 425 nm from GO–ZnO and rGO–ZnO compositions is significantly quenched as compared to that of ZnO films. At a 426 nm excitation wavelength, all samples show three sharp peaks at 570, 603 and 630 nm, respectively. More interestingly, the PL spectra of GO–ZnO and rGO–ZnO compositions are significantly increased, which is different from previous PL quenching phenomenon. We proposed that blue emission peak at 425 nm originates from the electron transition from the interstitial Zn levels to the top of the valence band, while three sharp peaks at 570, 603 and 630 nm are attributed to the oxygen vacancies and extended oxygen vacancies. After depositing GO & rGO sheets, the interaction between GO & rGO and ZnO caused interstitial zinc decrease. So GO–ZnO and rGO–ZnO compositions contained more oxygen vacancies than in the ZnO films. Our results highlight the ability of GO and rGO buffer layers to modulate defect concentrations in ZnO and contribute to understanding the optical properties of deep-level defects, which is significant for development of longwavelength photoelectric devices related with graphene materials. Acknowledgments The authors gratefully acknowledge financial support from Natural Science Foundation of China (Grant Nos. 91123019 and 61176056). This work has been financially supported by the International Collaboration Program and the ‘‘13115’’ Innovation Engineering Project of Shaanxi Province (Grant Nos. 2013KW-1205 and 2010ZDKG-58) and by the open projects from Institute of Photonics and Photo-Technology, Provincial Key Laboratory of Photoelectronic Technology, Northwest University, China. References [1] W.W. Cai, R.D. Piner, F.J. Stadermann, S. Park, M.A. Shaibat, Y. Ishii, D.X. Yang, A. Velamakanni, S.J. An, M. Stoller, J.H. An, D.M. Chen, R.S. Ruoff, Science 321 (2008) 1815–1817. [2] T. Gokus, R.R. Nair, A. Bonetti, M. Bohmler, A. Lombardo, K.S. Novoselov, A.K. Geim, A.C. Ferrari, A. Hartschuh, ACS Nano 3 (2009) 3963–3968. [3] J. Lu, J. Yang, J. Wang, A. Lim, S. Wang, K.P. Loh, ACS Nano 3 (2009) 2367–2375.
293
[4] D.Y. Pan, J.C. Zhang, Z. Li, M.H. Wu, Adv. Mater. 22 (2010) 734–738. [5] G. Eda, Y.Y. Lin, C. Mattevi, H. Yamaguchi, H.A. Chen, I.S. Chen, C.W. Chen, M. Chhowalla, Adv. Mater. 22 (2010) 505–509. [6] Z.T. Luo, P.M. Vora, E.J. Mele, A.T. Charlie Johnson, J.M. Kikkawa, Appl. Phys. Lett. 94 (2009) 111909. [7] Q.S. Mei, K. Zhang, G.J. Guan, B.H. Liu, S.H. Wang, Z.P. Zhang, Chem. Commun. 46 (2010) 7319–7321. [8] Q.S. Mei, Z.P. Zhang, Angew. Chem. Int. Ed. 51 (2012) 5602–5606. [9] X.M. Sun, Z. Liu, K. Welsher, J.T. Robinson, A. Goodwin, S. Zaric, H.J. Dai, Nano Res. 1 (2008) 203–212. [10] C.T. Chien, S.S. Li, W.J. Lai, Y.C. Yeh, H.A. Chen, I.S. Chen, L.C. Chen, K.H. Chen, T. Nemoto, S. Isoda, M.W. Chen, T. Fujita, G. Eda, H. Yamaguchi, M. Chhowalla, C.W. Chen, Angew. Chem. Int. Ed. 51 (2012) 6662–6667. [11] G.Q. Xin, Y.N. Meng, Y.F. Ma, D. Ho, N. Kim, S.M. Cho, H. Chae, Mater. Lett. 74 (2012) 71–73. [12] T.V. Cuong, V.H. Pham, E.W. Shin, J.S. Chung, S.H. Hur, E.J. Kim, Q.T. Tran, H.H. Nguyen, P.A. Kohl, Appl. Phys. Lett. 99 (2011) 041905. [13] C.H. Lui, K.F. Mak, J. Shan, T.F. Heinz, Phys. Rev. Lett. 105 (2010) 127404. [14] R.E. Marotti, P. Giorgi, G. Machado, E.A. Dalchiele, Sol. Energy Mater. Sol. Cells 90 (2006) 2356–2361. [15] D.P. Norton, Y.W. Heo, M.P. Ivill, K. Ip, S.J. Pearton, M.F. Chisholm, T. Steiner, Mater. Today 7 (2004) 34–40. [16] Z.K. Tang, G.K.L. Wong, P. Yu, Appl. Phys. Lett. 72 (1998) 3270. [17] Y. Yang, L.L. Ren, C. Zhang, S. Huang, T.X. Liu, ACS Appl. Mater. Interfaces 3 (2011) 2779–2785. [18] Q.P. Luo, X.Y. Yu, B.X. Lei, H.Y. Chen, D.B. Kuang, C.Y. Su, J. Phys. Chem. C 116 (2012) 8111–81117. [19] T. Kavitha, A.I. Gopalan, K.P. Lee, S.Y. Park, Carbon 50 (2012) 2994–3000. [20] G. Singh, A. Choudhary, D. Haranath, A.G. Joshi, N. Singh, S. Singh, R. Pasricha, Carbon 50 (2012) 385–394. [21] W.S. Hummers Jr., R.E. Offema, J. Am. Chem. Soc. 80 (1958) 1339–1340. [22] C.X. Ran, M.Q. Wang, W.Y. Gao, J.J. Ding, Y.H. Shi, X.H. Song, H.W. Chen, Z.Y. Ren, J. Phys. Chem. C 116 (2012) 23053–23060. [23] S. Dhara, P.K. Giri, J. Appl. Phys. 110 (2011) 124317. [24] N. Fujimura, T. Nishihara, S. Goto, J. Xu, T. Ito, J. Cryst. Growth 130 (1993) 269– 279. [25] Y.L. Chen, Z.A. Hu, Y.Q. Chang, H.W. Wang, Z.Y. Zhang, Y.Y. Yang, H.Y. Wu, J. Phys. Chem. C 115 (2011) 2563–2571. [26] A.R. Marlinda, N.M. Huang, M.R. Muhamad, M.N. Anámt, B.Y.S. Chang, N. Yusoff, I. Harrison, H.N. Lim, C.H. Chia, S. Vijay Kumar, Mater. Lett. 80 (2012) 9– 12. [27] G. Sanon, R. Rup, A. Mansingh, Thin Solid Films 190 (1989) 287–301. [28] G.J. Fang, D.J. Li, B.L. Yao, Phys. Status Solidi A 193 (2002) 139–152. [29] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Prog. Mater. Sci. 50 (2005) 293–340. [30] K.S. Subrahmanyam, S.R.C. Vivekchand, A. Govindaraj, C.N.R. Rao, J. Mater. Chem. 18 (2008) 1517–1523. [31] M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, L.G. Cancado, A. Jorio, R. Saito, Phys. Chem. Chem. Phys. 9 (2007) 1276–1291. [32] J.J. Ding, X.B. Yan, Q.J. Xue, Mater. Chem. Phys. 133 (2012) 405–409. [33] H.X. Chen, J.J. Ding, F. Shi, Y. Li, W.G. Guo, J. Alloys Compd. 534 (2012) 59– 63. [34] J.J. Ding, H.X. Chen, S.Y. Ma, Physica E 42 (2010) 1861–1864. [35] G. Williams, P.V. Kamat, Langmuir 25 (2009) 13869–13873. [36] A.B. Djurišic´, Y.H. Leung, K.H. Tam, L. Ding, W.K. Ge, H.Y. Chen, S. Gwo, Appl. Phys. Lett. 88 (2006) 103107. [37] L. Ke, S.C. Lai, J.D. Ye, V.L. Kaixin, S.J. Chua, J. Appl. Phys. 108 (2010) 084502. [38] E.M. Likovich, R. Jaramillo, K.J. Russell, S. Ramanathan, V. Narayanamurti, Appl. Phys. Lett. 99 (2011) 151910. [39] L.S. Vlasenko, G.D. Watkins, Phys. Rev. B 72 (2005) 035203. [40] A. Janotti, C.G. Van de Walle, Phys. Rev. B 76 (2007) 165202. [41] H.B. Zeng, G.T. Duan, Y. Li, S.K. Yang, X.X. Xu, W.P. Cai, Adv. Funct. Mater. 20 (2010) 561–572. [42] P. Erhart, K. Albe, A. Klein, Phys. Rev. B 73 (2006) 205203. [43] S.B. Zhang, S.H. Wei, A. Zunger, Phys. Rev. B 63 (2001) 075205.
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Photoluminescence investigation about zinc oxide with graphene oxide & reduced graphene oxide buffer layers Jijun Ding, Minqiang Wang ⇑, Xiangyu Zhang, Zhi Yang, Xiaohui Song, Chenxin Ran Electronic Materials Research Laboratory, Key Laboratory of Ministry of Education, School of Electronic and Information Engineering, International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
i n f o
Article history: Received 19 August 2013 Accepted 21 October 2013 Available online 8 November 2013 Keywords: ZnO films Graphene oxide Reduced graphene oxide Tunable photoluminescence
a b s t r a c t ZnO with graphene oxide (GO–ZnO) & reduced graphene oxide (rGO–ZnO) buffer layers were fabricated. Photoluminescence (PL) properties of GO–ZnO and rGO–ZnO compositions induced by oxygen vacancies defects were investigated using photoluminescence spectroscopy. The results showed that blue emission is quenched while yellow–orange emissions from GO–ZnO and rGO–ZnO compositions are significantly increased as compared to that of ZnO films. In stark contrast to enhanced yellow–orange emissions, PL spectra show three sharp, discrete emissions that characterize the dominant optical active defect, which is the oxygen vacancies and extended oxygen vacancies. Our results highlight the ability of GO & rGO buffer layers to modulate defect concentrations in ZnO and contribute to understanding the optical properties of deep-level defects, which is significant for development of long-wavelength photoelectric devices related with graphene materials. Crown Copyright Ó 2013 Published by Elsevier Inc. All rights reserved.
1. Introduction Graphene oxide (GO) is a graphene sheet modified with oxygen functional groups [1], and it exhibits interesting steady-state photoluminescence (PL) properties [2–4]. Such as, Eda et al. observed, in chemically derived GO, a relatively narrow PL peak centered at 390 nm and an apparently equivalent PL peak centered at 440 nm [5]. Luo et al. reported broadband visible PL from solid GO and discussed possible gapping mechanisms [6]. Mei and Zhang synthesized highly photoluminescent GO nanosheets and proposed a fluorescence ‘‘off-to-on’’ mechanism [7,8]. PL has been reported in chemically modified GO where the electronic structure has been modified [9]. Chien et al. reported tunable PL of GO with increased reduction [10]. Xin et al. also observed tunable PL of GO from near-ultraviolet to blue region through controlled hydrazine (N2H4) reduction [11]. In addition, reduced graphene oxide (rGO) also exhibits interesting steady-state PL properties. Cuong et al. demonstrated that optical emissions of rGO were ascribed to recombination of electron–hole pairs in localized electronic states [12]. Lui et al. observed significant light emission from graphene under excitation by ultrashort (30-fs) laser pulses [13]. Gokus et al. confirmed making graphene luminescent by oxygen plasma treatment [2]. ZnO is an II–VI semiconductor with a wide and direct band gap (Eg = 3.37 eV at 300 K), excellent chemical and thermal stability, ⇑ Corresponding author. Fax: +86 29 82668794. E-mail address: [email protected] (M. Wang).
and specific electrical and optoelectronic property of having a large exciton binding energy (60 meV) [14]. It possesses many important applications in electronic and optical devices [15,16]. Hybridization of carbon materials with ZnO offers a powerful way to obtain many interesting properties. Interestingly, optical properties of graphene & GO and ZnO composites have also been investigated. Such as, Yang et al. reported UV–vis absorption properties of functionalized graphene sheets (FGS)/ZnO nanocomposites exposed under UV light at regular time intervals [17]. Luo et al. presented UV–vis diffused reflectance spectra from RGO-hierarchical ZnO hollow sphere composites [18]. Kavitha et al. prepared graphene nanosheets decorated with ZnO nanoparticles which showed an emission peak at 400 nm associated with the recombination of excitons in ZnO [19]. Singh et al. investigated ZnO decorated with different molar ratios GO, and as the concentration of GO was increased, the PL quenching extent at 550 nm increased and observed large blue-shift of 0.15 eV in PL [20]. These ZnO decorated GO & rGO sheets display distinct optoelectronic characteristics, emitting near-ultraviolet to blue-green luminescence. However, other longer wavelength emission can also be expected for GO–ZnO and rGO–ZnO compositions, which is significant for development of low cost nanodevices. To our knowledge, there are no reports about the modulation of GO & rGO on larger wavelength range emission of ZnO. Also, PL mechanism in these compositions is still not clear. In this paper, GO & rGO sheets were deposited on indium tin oxide (ITO) substrates by electrophoretic deposition (EPD), and then ZnO films were deposited on down-layer GO & rGO sheets
0021-9797/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.059
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by magnetron sputtering technique to prepare GO–ZnO and rGO– ZnO compositions. The crystal structures, morphology, optical properties were analyzed by using X-ray diffraction (XRD) pattern, atomic force microscopy (AFM) and scanning electron microscopy (SEM) images, Raman spectra, PL and photoluminescence excitation (PLE) spectra. The effect of GO & rGO on PL spectra ranging from 300 to 800 nm of ZnO is investigated. PL mechanism in these compositions is proposed based on these experimental results. 2. Materials and experiments Natural graphite (99.9%) powders are commercially available from Wodetai Ltd., Co., Beijing, China. ZnO target (prior to 99.99%, 3 inch in diameter) is commercially available from General Research Institute for Nonferrous Metals, Beijing, China. GO–ZnO and rGO–ZnO compositions were fabricated by the following procedures. Firstly, GO was prepared from natural graphite by a classical Hummers method with some modification [21]. The exfoliated GO was obtained in the supernatant. The rGO used here was reduced from GO followed a convenient method under a lowtemperature and atmosphere pressure in our previous work [22]. Finally, GO and rGO were dispersed to form solutions with an ultrasonic, followed by centrifuging at 4800 rpm for 30 min. Then, GO & rGO sheets were deposited on indium tin oxide (ITO) substrates by electrophoretic deposition (EPD). ITO was acted as working electrode and platinum plate was cathode that was kept 15 mm away from the counter electrode. The evenly dispersed GO & rGO solutions were used directly as electrolyte, and the concentration of GO & rGO in the solution was assigned to 0.5 mg/ml. A constant current of 0.2 mA was applied for 20 min to coat GO & rGO on the ITO substrates. Finally, ZnO films were deposited on down-layer GO & rGO sheets by magnetron sputtering technique. The base pressure for the system was 2 104 Pa and film growth was carried out in the sputtering ambient with Ar:O2 = 4:1 (sccm) at a working pressure of 0.8 Pa. ZnO films were deposited on downlayer GO & rGO sheets with sputtering power of 100 W used during sputtering for 1.5 h. XRD patterns were studied by using a D/Max-2400 X-ray diffractometer. AFM and SEM images were characterized by a Dimension Icon3 atomic force microscope and a FEI Quanta 250 scanning electron microscope, respectively. Raman spectra were investigated using a JY LabRAM HR800 laser Raman spectrometer from 1000 to 4000 cm1 at room temperature. The 514.5 nm line of the laser was used as the excitation source. PL spectra within 350–650 nm measurements were carried out by a LS-55 fluorescence spectrometer under the 325 nm excitation wavelength. PL spectra within 450–800 nm were done using a SENS-9000 fluoroSENS fluorescence spectrometer under the 426 nm excitation wavelength. PLE spectra were measured using a SENS-9000 fluoroSENS fluorescence spectrometer monitored at 606 nm. All spectra were measured at room temperature in air.
Fig. 1. XRD pattern of as prepared GO and rGO.
Fig. 2. XRD pattern of ZnO films, GO–ZnO and rGO–ZnO compositions.
Fujimura et al. [24] suggested that the surface energy density of the (0 0 2) orientation is the lowest in the ZnO crystal. This means that the (0 0 2) texture of the film may easily form. This texture growth results in good crystalline quality. Some minor peaks could be ascribed to ITO substrates. Generally, the above mentioned GO (0 0 2) and rGO (0 0 2) are theoretically expected to be observed in the XRD pattern of GO–ZnO and rGO–ZnO compositions. However, they did not appear in the corresponding compositions, indicating that the surfaces of rGO are fully covered by ZnO films [25,26]. Therefore, it can be seen that the diffraction peaks are mainly from the up-layer ZnO phase in the GO–ZnO and rGO– ZnO compositions. Table 1 shows XRD parameters of ZnO films, GO–ZnO and rGO– ZnO compositions. The average crystallite size D was calculated by the following Scherrer formula [27]:
D¼ 3. Results and discussions Fig. 1 shows XRD patterns of as prepared GO and rGO. As prepared GO shows a sharp diffraction peak centered at 2h = 10.8° corresponding to GO (0 0 2). In XRD pattern of as prepared rGO, GO (0 0 2) diffraction peak completely disappears and there occurs a broad diffraction band ranging from 10° to 30°, which indicates that the GO is completely reduced and there presents few defects in rGO during chemical reduction process. Fig. 2 shows XRD pattern of ZnO films, GO–ZnO and rGO–ZnO compositions. In these XRD patterns, a major peak could be indexed to ZnO (0 0 2), indicating that all the compositions are single crystalline and oriented along c-axis of hexagonal structure [23].
0:9k b cos h
ð1Þ
where k, h and b are the X-ray wavelength (1.5406 Å), diffraction angle and the full width at half-maximum (FWHM) of the ZnO (0 0 2) peak. As can be seen from Table 1, GO–ZnO compositions have the minimum FWHM and the largest crystallite size.
Table 1 XRD parameters of ZnO films, GO–ZnO and rGO–ZnO compositions.
ZnO films GO–ZnO rGO–ZnO
FWHM (°)
D (nm)
r (MPa)
dhkl (nm)
0.216 0.200 0.204
38.1 41.1 40.3
1.0742 1.0742 1.7007
0.2614 0.2614 0.2621
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In addition, according to the Bragg formula: k = 2 dhkl sin h, where dhkl denotes the crystalline plane distance for indices (hkl). It was found that all dhkl values are larger than that of standard ZnO powder d0, which is equal to 0.2603 nm. The lattice constants c of (0 0 2) peak can also be calculated by the following formula [28]:
"
2 dhkl
2
2
2
4ðh þ k þ hkÞ l ¼ þ 2 3a2 c
#1 ð2Þ
where a and c are the lattice constants, and dhkl is the crystalline plane distance for indices (hkl). According to Eq. (2), the lattice constant c is equal to 2d for the (0 0 2) diffraction peak. The calculation of the film stress is based on the biaxial strain model. To derive the film stress r parallel to the film surface, the following formula is used, which is valid for single crystal ZnO [29]:
r ¼ 233ðc c0 Þ=c0 MPa
ð3Þ
where c is the lattice constant of ZnO film, c0 = 0.5206 nm is the lattice constant of standard ZnO powder. Together the minimum film stress and the largest crystallite size, we concluded that GO–ZnO compositions have the best crystal quality. Fig. 3 shows AFM and cross-sectional SEM images of ZnO films (a and d), GO–ZnO (b and e) and rGO–ZnO (c and f) compositions. The surface morphology of ZnO films (a), GO–ZnO (b) and rGO–ZnO (c) compositions can be clearly observed. The insets are their corresponding 3-dimensional AFM surface morphology. All the samples show rods-like structures and rGO–ZnO compositions have the largest average surface roughness. The cross-sectional SEM image of ZnO films (d), GO–ZnO (e) and rGO–ZnO (f) compositions were shown. At the same deposition conditions, ZnO films grown on rGO sheets have the typically largest thickness of about 360 nm, which correspond with GO–ZnO compositions having the best crystal quality. Fig. 4 shows Raman spectra of GO and rGO. In the GO and rGO spectrum, three sharp peaks at 1350, 1583 and 2695 cm1 correspond to the D, G and 2D peaks, respectively. The D peak is due to the presence of structural disorders in graphene sheets. The G
Fig. 4. Raman spectra of GO and rGO.
peak attributes to optical E2g phonons at the Brillouin zone center, whereas the ratio of the intensity of the G-band to the D-band is related to the in-plane crystallite size [30]. Besides, the intensity ratio of IG/ID is widely used to characterize the defect quantity in graphene and a low ratio indicates a great disorder arising from structural defects [31]. As can be seen in Fig. 4, the integrated intensity ratios of IG/ID in rGO (IG/ID 1.17) are larger than in GO (IG/ID 1.00). These indicate that the structural defects in GO structures are partially renovated after being reduced. Fig. 5 shows PL spectra of ZnO films, GO–ZnO and rGO–ZnO compositions at a 325 nm excitation wavelength. The PL spectrum of ZnO films clearly shows a broad blue emission peak at 425 nm (2.9 eV), which originates from the electron transition from the interstitial Zn levels to the top of the valence band [32–34]. Interestingly PL from GO–ZnO and rGO–ZnO compositions is significantly quenched as compared to that of ZnO films, which is due to interfacial charge transfer from ZnO to graphene [35]. In addition, PL from rGO–ZnO red shifted to 441 nm, while PL from GO–ZnO blue shifted to 420 nm. Singh et al. [20] observed that PL from rGO–ZnO was blue shifted as compared to that of bare ZnO, which is ascribed to a depletion layer formation between
Fig. 3. AFM and cross-sectional SEM images of ZnO films (a and d), GO–ZnO (b and e) and rGO–ZnO (c and f) compositions.
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Fig. 7. Proposed mechanisms of blue and yellow–orange emissions.
[40]. Zeng et al. reported that the oxygen vacancies arise from the Schottky reaction and further ionization reactions [41]:
Fig. 5. PL spectra of ZnO films, GO–ZnO and rGO–ZnO compositions at 325 nm excitation wavelength.
0 () V XZn þ V XO
ð4Þ
V XO () V O þ e0
ð5Þ
0 V O () V O þe
ð6Þ
the p-type graphene and n-type ZnO interface. So depletion layer formation significantly affects charge transition from shallow levels to valence band. This results in blue peak quenches and shifts. Fig. 6 shows PL spectra of ZnO films, GO–ZnO and rGO–ZnO compositions at a 426 nm excitation wavelength. At a longer excitation wavelength, all samples show three sharp peaks at 570, 603 and 630 nm, respectively. More interestingly, the PL spectra of GO– ZnO and rGO–ZnO compositions are significantly increased, which is different from previous PL quenching phenomenon, while the peak position has no change as compared to that of ZnO films. Djurišic´ et al. reported yellow–orange emission (612–640 nm) at different excitation wavelength [36], but the origin of this emission was not fully clear. Ke et al. thought that yellow–orange emission in ZnO films was related to the presence of O interstitials [37]. Eda et al. reported that a broad and intense emission band at 450– 600 nm may arise from single ionized oxygen vacancy in ZnO and the recombination of electron–hole pairs localized within sp2 carbon clusters embedded within a sp3 matrix in the GO sheets [5]. Likovich et al. found that the observed deep-level emission peaks are unusually narrow and fingerprint the electronic structure of the predominant near-surface optically active defect, which they propose is an oxygen vacancy [38]. We propose another mechanism for the observed abnormal PL increase in GO–ZnO and rGO–ZnO compositions at a longer excitation wavelength. Vlasenko and Watkins showed experimentally that interstitial zinc is a very instable defect [39], but the oxygen vacancy is stable. Janotti and Van de Walle calculated that interstitial zinc are fast diffusers with a migration barrier as low as 0.57 eV, while that for oxygen vacancies is in the range 1.7–2.4 eV
According to previous calculation results, the formation energy of oxygen vacancies is about 1.7 eV [42] and 2.4 eV [43], and it can be significantly reduced after further ionization. Three sharp peaks at 570, 603 and 630 nm correspond to photon energies of 2.18, 2.06 and 1.97 eV, respectively. So, as can be seen in Fig.7, three sharp peaks are attributed to the transitions from oxygen vacancies and extended oxygen vacancies states to the valance band, respectively. Here the extended oxygen vacancies states could be ionized oxygen vacancies, complex defect or localized oxygen vacancies states. In addition, peak at 425 nm originates from the electron transition from the interstitial Zn levels to the top of the valence band. As above mentioned, interstitial zinc is a very instable defect. The interaction between GO & rGO and ZnO caused interstitial zinc diffusing with a migration barrier, so 425 nm emission quenching could be observed in GO–ZnO and rGO–ZnO compositions. However, three sharp peaks at 570, 603 and 630 nm are attributed to the oxygen vacancies and extended oxygen vacancies. As above AFM results mentioned, GO–ZnO and rGO–ZnO compositions contained more defects than in the ZnO films. The ZnO film is an n-type semiconductor and it means that most defects are interstitial Zn and oxygen vacancies. After depositing GO & rGO sheets, the interaction between GO & rGO and ZnO caused interstitial zinc decrease. So according to charge equilibrium, GO–ZnO and rGO–ZnO compositions contained more oxygen vacancies than in the ZnO films. This explains why blue emission related with interstitial zinc will decrease, while three emission peaks related with oxygen vacancies will increase in GO–ZnO and rGO–ZnO compositions. Fig. 8 shows PLE spectra of ZnO films, GO–ZnO and rGO–ZnO compositions monitored at a 606 nm wavelength. The PLE spectra
Fig. 6. PL spectra of ZnO films, GO–ZnO and rGO–ZnO compositions at 426 nm excitation wavelength.
Fig. 8. PLE spectra of ZnO films, GO–ZnO and rGO–ZnO compositions monitored at 606 nm.
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exhibit three major peaks at 290 (4.3 eV), 325 (3.8 eV) and 426 nm (2.9 eV), respectively and some minor peaks. For 606 nm monitored wavelength, the energy (2.9 eV) below the band gap (3.2– 3.4 eV) is the optimal excitation energy and other lower energy is still effective. Here the extended oxygen vacancies states could be ionized oxygen vacancies, complex defect or localized oxygen vacancies states. Some minor peaks in the lower energy in the PLE spectrum have verified their existence. 4. Conclusions In this paper, we reported photoluminescence from GO–ZnO and rGO–ZnO compositions. In detail, at a 325 nm excitation wavelength, blue emission peak at 425 nm from GO–ZnO and rGO–ZnO compositions is significantly quenched as compared to that of ZnO films. At a 426 nm excitation wavelength, all samples show three sharp peaks at 570, 603 and 630 nm, respectively. More interestingly, the PL spectra of GO–ZnO and rGO–ZnO compositions are significantly increased, which is different from previous PL quenching phenomenon. We proposed that blue emission peak at 425 nm originates from the electron transition from the interstitial Zn levels to the top of the valence band, while three sharp peaks at 570, 603 and 630 nm are attributed to the oxygen vacancies and extended oxygen vacancies. After depositing GO & rGO sheets, the interaction between GO & rGO and ZnO caused interstitial zinc decrease. So GO–ZnO and rGO–ZnO compositions contained more oxygen vacancies than in the ZnO films. Our results highlight the ability of GO and rGO buffer layers to modulate defect concentrations in ZnO and contribute to understanding the optical properties of deep-level defects, which is significant for development of longwavelength photoelectric devices related with graphene materials. Acknowledgments The authors gratefully acknowledge financial support from Natural Science Foundation of China (Grant Nos. 91123019 and 61176056). This work has been financially supported by the International Collaboration Program and the ‘‘13115’’ Innovation Engineering Project of Shaanxi Province (Grant Nos. 2013KW-1205 and 2010ZDKG-58) and by the open projects from Institute of Photonics and Photo-Technology, Provincial Key Laboratory of Photoelectronic Technology, Northwest University, China. References [1] W.W. Cai, R.D. Piner, F.J. Stadermann, S. Park, M.A. Shaibat, Y. Ishii, D.X. Yang, A. Velamakanni, S.J. An, M. Stoller, J.H. An, D.M. Chen, R.S. Ruoff, Science 321 (2008) 1815–1817. [2] T. Gokus, R.R. Nair, A. Bonetti, M. Bohmler, A. Lombardo, K.S. Novoselov, A.K. Geim, A.C. Ferrari, A. Hartschuh, ACS Nano 3 (2009) 3963–3968. [3] J. Lu, J. Yang, J. Wang, A. Lim, S. Wang, K.P. Loh, ACS Nano 3 (2009) 2367–2375.
293
[4] D.Y. Pan, J.C. Zhang, Z. Li, M.H. Wu, Adv. Mater. 22 (2010) 734–738. [5] G. Eda, Y.Y. Lin, C. Mattevi, H. Yamaguchi, H.A. Chen, I.S. Chen, C.W. Chen, M. Chhowalla, Adv. Mater. 22 (2010) 505–509. [6] Z.T. Luo, P.M. Vora, E.J. Mele, A.T. Charlie Johnson, J.M. Kikkawa, Appl. Phys. Lett. 94 (2009) 111909. [7] Q.S. Mei, K. Zhang, G.J. Guan, B.H. Liu, S.H. Wang, Z.P. Zhang, Chem. Commun. 46 (2010) 7319–7321. [8] Q.S. Mei, Z.P. Zhang, Angew. Chem. Int. Ed. 51 (2012) 5602–5606. [9] X.M. Sun, Z. Liu, K. Welsher, J.T. Robinson, A. Goodwin, S. Zaric, H.J. Dai, Nano Res. 1 (2008) 203–212. [10] C.T. Chien, S.S. Li, W.J. Lai, Y.C. Yeh, H.A. Chen, I.S. Chen, L.C. Chen, K.H. Chen, T. Nemoto, S. Isoda, M.W. Chen, T. Fujita, G. Eda, H. Yamaguchi, M. Chhowalla, C.W. Chen, Angew. Chem. Int. Ed. 51 (2012) 6662–6667. [11] G.Q. Xin, Y.N. Meng, Y.F. Ma, D. Ho, N. Kim, S.M. Cho, H. Chae, Mater. Lett. 74 (2012) 71–73. [12] T.V. Cuong, V.H. Pham, E.W. Shin, J.S. Chung, S.H. Hur, E.J. Kim, Q.T. Tran, H.H. Nguyen, P.A. Kohl, Appl. Phys. Lett. 99 (2011) 041905. [13] C.H. Lui, K.F. Mak, J. Shan, T.F. Heinz, Phys. Rev. Lett. 105 (2010) 127404. [14] R.E. Marotti, P. Giorgi, G. Machado, E.A. Dalchiele, Sol. Energy Mater. Sol. Cells 90 (2006) 2356–2361. [15] D.P. Norton, Y.W. Heo, M.P. Ivill, K. Ip, S.J. Pearton, M.F. Chisholm, T. Steiner, Mater. Today 7 (2004) 34–40. [16] Z.K. Tang, G.K.L. Wong, P. Yu, Appl. Phys. Lett. 72 (1998) 3270. [17] Y. Yang, L.L. Ren, C. Zhang, S. Huang, T.X. Liu, ACS Appl. Mater. Interfaces 3 (2011) 2779–2785. [18] Q.P. Luo, X.Y. Yu, B.X. Lei, H.Y. Chen, D.B. Kuang, C.Y. Su, J. Phys. Chem. C 116 (2012) 8111–81117. [19] T. Kavitha, A.I. Gopalan, K.P. Lee, S.Y. Park, Carbon 50 (2012) 2994–3000. [20] G. Singh, A. Choudhary, D. Haranath, A.G. Joshi, N. Singh, S. Singh, R. Pasricha, Carbon 50 (2012) 385–394. [21] W.S. Hummers Jr., R.E. Offema, J. Am. Chem. Soc. 80 (1958) 1339–1340. [22] C.X. Ran, M.Q. Wang, W.Y. Gao, J.J. Ding, Y.H. Shi, X.H. Song, H.W. Chen, Z.Y. Ren, J. Phys. Chem. C 116 (2012) 23053–23060. [23] S. Dhara, P.K. Giri, J. Appl. Phys. 110 (2011) 124317. [24] N. Fujimura, T. Nishihara, S. Goto, J. Xu, T. Ito, J. Cryst. Growth 130 (1993) 269– 279. [25] Y.L. Chen, Z.A. Hu, Y.Q. Chang, H.W. Wang, Z.Y. Zhang, Y.Y. Yang, H.Y. Wu, J. Phys. Chem. C 115 (2011) 2563–2571. [26] A.R. Marlinda, N.M. Huang, M.R. Muhamad, M.N. Anámt, B.Y.S. Chang, N. Yusoff, I. Harrison, H.N. Lim, C.H. Chia, S. Vijay Kumar, Mater. Lett. 80 (2012) 9– 12. [27] G. Sanon, R. Rup, A. Mansingh, Thin Solid Films 190 (1989) 287–301. [28] G.J. Fang, D.J. Li, B.L. Yao, Phys. Status Solidi A 193 (2002) 139–152. [29] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Prog. Mater. Sci. 50 (2005) 293–340. [30] K.S. Subrahmanyam, S.R.C. Vivekchand, A. Govindaraj, C.N.R. Rao, J. Mater. Chem. 18 (2008) 1517–1523. [31] M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, L.G. Cancado, A. Jorio, R. Saito, Phys. Chem. Chem. Phys. 9 (2007) 1276–1291. [32] J.J. Ding, X.B. Yan, Q.J. Xue, Mater. Chem. Phys. 133 (2012) 405–409. [33] H.X. Chen, J.J. Ding, F. Shi, Y. Li, W.G. Guo, J. Alloys Compd. 534 (2012) 59– 63. [34] J.J. Ding, H.X. Chen, S.Y. Ma, Physica E 42 (2010) 1861–1864. [35] G. Williams, P.V. Kamat, Langmuir 25 (2009) 13869–13873. [36] A.B. Djurišic´, Y.H. Leung, K.H. Tam, L. Ding, W.K. Ge, H.Y. Chen, S. Gwo, Appl. Phys. Lett. 88 (2006) 103107. [37] L. Ke, S.C. Lai, J.D. Ye, V.L. Kaixin, S.J. Chua, J. Appl. Phys. 108 (2010) 084502. [38] E.M. Likovich, R. Jaramillo, K.J. Russell, S. Ramanathan, V. Narayanamurti, Appl. Phys. Lett. 99 (2011) 151910. [39] L.S. Vlasenko, G.D. Watkins, Phys. Rev. B 72 (2005) 035203. [40] A. Janotti, C.G. Van de Walle, Phys. Rev. B 76 (2007) 165202. [41] H.B. Zeng, G.T. Duan, Y. Li, S.K. Yang, X.X. Xu, W.P. Cai, Adv. Funct. Mater. 20 (2010) 561–572. [42] P. Erhart, K. Albe, A. Klein, Phys. Rev. B 73 (2006) 205203. [43] S.B. Zhang, S.H. Wei, A. Zunger, Phys. Rev. B 63 (2001) 075205.
