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Ultraviolet emission enhancement in ZnO thin films modified by nanocrystalline TiO2
Optical Materials 67 (2017) 139e144
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Ultraviolet emission enhancement in ZnO thin films modified by nanocrystalline TiO2 Gaige Zheng a, b, c, *, Xi Lu a, b, Liming Qian a, b, Fenglin Xian a, b a Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean, Nanjing University of Information Science & Technology, Nanjing 210044, China b School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China c Jiangsu Collaborative Innovation Center on Atmospheric Environment and Equipment Technology (CICAEET), Nanjing University of Information Science & Technology, Nanjing 210044, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 February 2017 Received in revised form 18 March 2017 Accepted 30 March 2017
In this study, nanocrystalline TiO2 modified ZnO thin films were prepared by electron beam evaporation. The structural, morphological and optical properties of the samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), UV-visible spectroscopy, fluorescence spectroscopy, respectively. The composition of the films was examined by energy dispersive X-ray spectroscopy (EDX). The photoluminescent spectrum shows that the pure ZnO thin film exhibits an ultraviolet (UV) emission peak and a strong green emission band. Surface analysis indicates that the ZnO thin film contains many oxygen vacancy defects on the surface. After the ZnO thin film is modified by the nanocrystalline TiO2 layer, the UV emission of ZnO is largely enhanced and the green emission is greatly suppressed, which suggests that the surface defects such as oxygen vacancies are passivated by the TiO2 capping layer. As for the UV emission enhancement of the ZnO thin film, the optimized thickness of the TiO2 capping layer is ~16 nm. When the thickness is larger than 16 nm, the UV emission of the ZnO thin film will decrease because the TiO2 capping layer absorbs most of the excitation energy. The UV emission enhancement in the nanocrystalline TiO2 modified ZnO thin film can be attributed to surface passivation and flat band effect. © 2017 Elsevier B.V. All rights reserved.
Keywords: ZnO thin films UV emission enhancement TiO2 surface modification Surface passivation Flat band effect
1. Introduction In recent years, Zinc oxide (ZnO), as a multifunctional material, has attracted wide attention in the world. At room temperature, ZnO has a wide direct bandgap of ~3.37 eV and a high excitonic binding energy of ~60 meV, which makes ZnO a suitable material for fabrication of UV light-emitting devices [1e3]. If one wants to prepare ZnO-based UV emitters, he should first get ZnO materials with high performance of UV emissions such as ZnO thin films. ZnO thin films have been deposited by many methods such as molecular beam epitaxy [4], atomic layer deposition [5], metal organic chemical vapor deposition [6], magnetron sputtering [7], electron beam evaporation [8], ultrasonic spray pyrolysis [9], sol-gel method [10], etc. Among these methods, the first three methods, namely molecular beam epitaxy, atomic layer deposition, metal organic * Corresponding author. School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Ningliu Road 219#, Nanjing 210044, China. E-mail address: [email protected] (G. Zheng). http://dx.doi.org/10.1016/j.optmat.2017.03.056 0925-3467/© 2017 Elsevier B.V. All rights reserved.
chemical vapor deposition, can deposit high-quality ZnO thin films, but the related equipments are expensive, leading to a high production cost. Although the production cost for ZnO thin films deposited by other methods is low, the ZnO thin films usually exhibit strong visible emissions caused by many defects in the films [9,11e20]. In order to improve the UV emission and suppress visible emissions of ZnO thin films, some strategies have been applied. These strategies include: (1) Surface plasmon resonance [21]; (2) fluorescence resonance energy transfer [8]; (3) surface passivation [22e24]. As for the first strategy, the Ag or Au nanoparticles are often used to modify ZnO surface. However, Ag nanoparticles are easy to be oxidized when they are exposed to air for a long time, leading to lose surface plasmon resonance effect. On the other hand, gold is a very precious metal. As for the second strategy, if fluorescence resonance energy transfer happens between two materials, three harsh conditions must be met [8,25]. Compared with the first two strategies, the last one “surface passivation” seems simple and easy to be performed. For example, Jayalakshmi et al. [22] adopted thiol and amine to modify ZnO surfaces and
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found that the UV emission of ZnO thin films was enhanced and the visible emissions were suppressed; the authors attributed this to the surface defects passivation. Vijayalakshmi et al. [24] used MgO to cap ZnO thin films; they found that the UV emission of ZnO was improved and the UV emission peak had a blue-shift with the increase of MgO layer thickness. In this work, we prepared ZnO thin films by electron beam evaporation and used nanocrystalline TiO2 to modify the ZnO surface to tailor the luminescence. That the nanocrystalline TiO2 is applied to modify the ZnO thin films is based on the following considerations: (1) TiO2 is non-toxic, cheap and stable; (2) the TiO2 has a larger bandgap, so it will not absorb the UV emission of ZnO; (3) if the TiO2 thickness is suitable, it will not absorb lots of the excitation energy (325 nm). To our knowledge, there are few reports on the photoluminescence behavior of ZnO thin films modified by nanocrystalline TiO2 nanolayers. The results show that the UV emission of ZnO thin films modified by the TiO2 is largely enhanced, but there exists an optimized value for the TiO2
thickness. The details will be depicted below. 2. Experiments The ZnO thin films and TiO2 modified ZnO thin films were deposited by electron beam evaporation on thoroughly washed and dried glass and Si substrates. The raw material for the preparation of the films is high purity ZnO and TiO2 particles. When the ZnO thin films and TiO2 capping layer were deposited, the oxygen was introduced into the deposition chamber and the working pressure was 2.0 10 2 Pa. The substrate temperature was 200 and 100 C for ZnO and TiO2, respectively. The thickness of ZnO and TiO2 was monitored by a quartz crystal oscillator. The thickness of ZnO thin films was set to 100 nm. The thickness of the TiO2 capping layer was set to 8, 12, 16 and 20 nm, respectively (accordingly, the resultantly samples were marked as B, C, D and E, respectively.). The ZnO thin film without a TiO2 capping layer was marked as sample A. All the samples were not annealed.
Fig. 1. Surface and sectional morphology images of sample A (a), B (b), C (c), D (d) and E (e).
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growth direction of ZnO and the crystallinity of ZnO and TiO2 were analyzed by cross-sectional transmission electron microscopy. The optical absorbance of the samples was measured by a spectrometer (TU-1900). The photoluminescence spectra were recorded by a spectrophotometer (LABRAM800) with an excitation wavelength of 325 nm. The surface composition and chemical state of the samples were analyzed by an X-ray photoelectron spectrometer (AXIS UltraDLD).
3. Results and discussion
Fig. 2. XRD patterns of the samples.
The crystal phase of the samples was determined by an X-ray diffractometer (Druker D8 Advance). The surface and section morphologies of the samples were observed by a scanning electron microscope (S4800) and the chemical composition of the samples was analyzed by an energy dispersive X-ray spectrometer. The
Fig. 1 shows the surface and section morphology images of ZnO thin film and TiO2 modified ZnO thin films. For all the samples, it can be seen that ZnO thin films are composed of columnar grains with a direction perpendicular to the substrate surface, also evidenced by the following HRTEM images. All the films are very compact. As for sample B, C, D and E, it can be observed from the section morphology images that they all contain two layers and the top layer is TiO2. The TiO2 thickness is in agreement with the designed thickness. In addition, it can be seen from the surface morphology images that the surface morphology is similar for the samples modified by TiO2 capping nanolayer and the TiO2 particles are very uniform. Fig. 2 displays the XRD patterns of the samples. All the films show a strong (002) peak, suggesting that the ZnO thin films have a wurtzite structure and are preferentially orientated along the c-axis direction. Except the (002) peak of ZnO, there is no other peaks related to TiO2, indicating the TiO2 nanolayer exhibits an amorphous-like nature. As a matter of fact, the definition of “amorphous” is difficult for TiO2, as there is always some degree of local ordering observed [26]. Therefore, the amorphous nature of the TiO2 nanolayer here can not be decided only by the XRD results. That no XRD peaks are observed for TiO2 maybe implies that “amorphous TiO2” is just the phase where no signal can be detected
Fig. 3. Surface image (a), typical mapping images (b, c, d) and a EDX spectrum (e) of sample E.
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Fig. 4. The cross-section TEM image of sample E (a), HRTEM images for the three selected regions (b, c, d) and the EDS spectra for TiO2 (e) and ZnO (f).
by XRD, which means the material has crystalline domains of a size below the detection limit of X-ray [26]. Previously, P. B. Nair et al. [26] prepared TiO2 thin films at various RF powers. They found that the samples deposited at various RF powers at 0.01 mbar and annealed at 873 K were amorphous-like nature proofed by XRD
Fig. 5. Photoluminescence spectra of the samples.
patterns, while the signals from micro Raman spectra indicated the samples were crystalline. As for our samples, the crystallinity of TiO2 also needs other techniques to decide. For learning the film composition and the distribution of different elements, the EDX spectra and mapping images were taken. The typical results of sample E are shown in Fig. 3. From Fig. 3, it can be seen that except the Si signal from the Si substrate, only Zn, O and Ti occurred in the spectrum. It means that these films have no impurities. In addition, it can be seen from mapping images that Ti is distributed very uniformly, suggesting that the TiO2 has been uniformly deposited on ZnO thin films. In order to determine the crystallinity of TiO2 and the growth direction of ZnO, the HRTEM images are taken for sample E deposited on a Si substrate, as shown in Fig. 4. Fig. 4 (a) shows the cross-section TEM image. It can be seen that the ZnO thin film is very compact and composed of columnar grains which is covered by a TiO2 nanolayer. Fig. 4 (b) is the HRTEM image of the region 1. It clearly shows the growth direction of ZnO grains along the c-axis which is perpendicular to the Si substrate surface. In addition, a SiO2 nanolayer is also observed between ZnO and the substrate, which is formed during the deposition of ZnO. Fig. 4 (c) gives the HRTEM image of region 2, which exhibits a high c-axis oriented nature of the ZnO layer. Fig. 4 (d) shows the HRTEM image of the region 3. The upper layer is TiO2 as evidenced by EDS spectrum in Fig. 4 (e), and the bottom layer is ZnO as evidenced by a EDS
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Fig. 6. XPS spectra of sample A.
spectrum in Fig. 4 (f). The TiO2 nanolayer contains some nanocrystalline TiO2 particles and amorphous boundaries. The ZnO layer and TiO2 layer is tightly glued together, which is favorable for passivating the surface defects (such as oxygen vacancy, dangling bond etc.) of the ZnO layer. Fig. 5 gives the photoluminescence spectra of the samples. As for sample A, it has a UV emission peak located at 385 nm which is the near-band-edge emission of ZnO, originating from the recombination of free excitons [5,8,9]. Except the UV emission peak, sample A still has a strong green emission band centered at 516 nm. This green emission has been observed in many ZnO materials and
many researchers deem that the green emission is mainly connected with the oxygen vacancy defects on the ZnO surface [8,11,12,27]. Xu and Shan et al. [27,28] studied the aging effect of photoluminescence of ZnO thin films; they found that the green emission of ZnO gradually decreased with the increase of aging time, suggesting that the oxygen vacancies on the ZnO surface were gradually filled up. It is well known to all that the XPS is an important surface analysis technique which not only provides the surface composition information but also gives the chemical state of each element. In order to further confirm the existence of massive oxygen vacancies on the surface of ZnO thin films, we
Fig. 7. Absorbance spectra (a) and bandgaps (b) of ZnO and TiO2.
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measured the XPS spectra of the pure ZnO thin film and the XPS spectra are shown in Fig. 6. It can be seen from Fig. 6 (b) that the O 1s contains two components. One component located at 530.1 eV is attributed to lattice oxygen bonded as O2 in the ZnO matrix [7]; the other component located at 531.9 eV is associated with nonstoichiometric oxygen in ZnO [7,20]. This evidences that the ZnO thin film has many oxygen vacancy defects on the surface. As for the ZnO thin films modified by the TiO2 nanolayer, the green emission is largely suppressed while the UV emission is greatly enhanced. With the increase of the TiO2 nanolayer thickness, the UV emission is also gradually intensified. However, the UV emission is reduced again when the thickness of the TiO2 nanolayer is above 16 nm. As for the UV emission enhancement in TiO2 modified ZnO materials, two proposals have been put forward. Lin and Xu et al. [8,25] think that the UV emission enhancement in TiO2 modified ZnO materials is mainly ascribed to the fluorescence resonance energy transfer (FRET) between ZnO and TiO2. It should be noted here that the TiO2 deposited by Lin and Xu is crystalline and has UV emissions. Bahadur et al. [29] deem that the surface passivation produced by TiO2 surface modification plays an important role for the UV emission enhancement of ZnO. As for our deposited TiO2, there are no UV emissions under excitation by the 325 nm UV light. Therefore, it does not satisfy the three conditions in which FRET occurs [25]. As a consequence, for our samples, the UV emission enhancement should be mainly attributed to surface modification effect caused by the TiO2 nanolayer. In addition, the flat-band effect originating from TiO2 modification may be favorable for the UV emission enhancement [30]. Fig. 7 (a) shows the absorption spectra and bandgaps of ZnO and TiO2. From the position of absorption edge of TiO2, it can be known that when the TiO2 nanolayer is very thin, its absorption for the UV light of 325 nm is little. However, when the TiO2 nanolayer is relatively thick, its absorption for the UV light of 325 nm is significant. As a result, when the TiO2 nanolayer is thinner than 16 nm, most of the UV light of 325 nm penetrates the TiO2 nanolayer and enter into ZnO. But when the TiO2 nanolayer is more than 16 nm, most of the UV light of 325 nm will be absorbed by the TiO2 nanolayer, which decreases the excitation energy for ZnO and leads to the decline of the UV emission of 385 nm. Furthermore, from Fig. 7(b), it can be known that the bandgap of the TiO2 is 3.90 eV which is larger than the excitation energy of 3.80 eV (325 nm). Therefore, the excitation energy will not lead to electron transition from valence band to conduction band in TiO2 (but probably lead to electron transition from valence band to some levels in the forbidden band). As a result, the TiO2 has no UV emissions here. 4. Conclusion In this investigation, ZnO thin films modified by the TiO2 nanolayer were deposited by electron beam evaporation. The influence of TiO2 modification and the thickness of TiO2 nanolayer on the luminescent behavior of ZnO thin films were studied. The
results showed that the surface modification of TiO2 passivated surface defects, suppressed the green emission and enhanced the UV emission of ZnO. As for our samples, the optimized thickness of the TiO2 nanolayer was ~16 nm, which could lead ZnO to obtain the best UV emission efficiency. The UV emission enhancement of ZnO thin films modified by TiO2 was attributed to the surface passivation effect and flat band effect. The TiO2/ZnO thin films have a potential application in UV light-emitting devices. Acknowledgements This work is partly supported by the Natural Science Foundation of Jiangsu Province (Grant no. BK20141483), the National Natural Science Foundation of China (Grant nos. 41675154, 61203211), Six Major Talent Peak expert of Jiangsu Province (2015-XXRJ-014) and Jiangsu 333 High-Level Talent Cultivation Program (BRA2016425). References [1] R. Deng, B. Yao, Y.F. Li, Y. Xu, J.C. Li, B.H. Li, Z.Z. Zhang, L.G. Zhang, H.F. Zhao, D.Z. Shen, J. Lumin. 134 (2013) 240. [2] S. Li, G. Fang, H. Huang, H. Long, H. Wang, X. Mo, B. Dong, X. Zhao, Appl. Phys. B 107 (2012) 497. [3] A. Baltakesmez, S. Tekmen, S. Tuzemen, J. Appl. Phys. 110 (2011) 054502. [4] M. Wei, R.C. Boutwell, W.V. Schoenfeld, Appl. Surf. Sci. 277 (2013) 263. [5] J.L. Tian, H.Y. Zhang, G.G. Wang, X.Z. Wang, R. Sun, L. Jin, J.C. Han, Superlattices Microstruct. 83 (2015) 719. [6] M.L. Addonizio, L. Fusco, J. Alloys Compd. 622 (2015) 851. [7] J.W. Zhang, G. He, T.S. Li, M. Liu, X.S. Chen, Y.M. Liu, Z.Q. Sun, Mater. Res. Bull. 65 (2015) 7. [8] L. Xu, H. Shen, X. Li, R. Zhu, J. Lumin. 130 (2010) 2123. [9] N. Zebbar, L. Chabane, N. Gabouze, M. Kechouane, M. Trari, M.S. Aida, S. Belhousse, F.H. Larbi, Thin Solid Films 605 (2016) 89. [10] L. Xu, X. Li, J. Yuan, Superlattices Microstruct. 44 (2008) 276. [11] X. Meng, Y. Zhou, X. Zeng, X. Chen, Ceram. Int. 42 (2016) 13819. [12] A. Mortezaali, Q. Taheri, Z.S. Hosseini, Microelectron. Eng. 151 (2016) 19. [13] Z.Z. Li, M. Bao, S.H. Chang, Z.Z. Chen, X.M. Ma, Vacuum 86 (2012) 1448. [14] S. Singh, M.S.R. Rao, Scr. Mater. 61 (2009) 169. [15] Y.M. Lu, X.P. Li, S.C. Su, P.J. Cao, F. Jia, S. Han, Y.X. Zeng, W.J. Liu, D.L. Zhu, J. Lumin. 152 (2014) 254. [16] S. Talu, M. Bramowicz, S. Kulesza, S. Solaymani, A. Ghaderi, L. Dejam, S.M. Elahi, A. Boochani, Superlattices Microstruct. 93 (2016) 109. [17] C. Manoharan, S. Dhanapandian, A. Arunachalam, M. Bououdina, J. Alloys Compd. 685 (2016) 395. [18] A. Screedhar, J.H. Kwon, J. Yi, J.S. Kim, J.S. Gwag, Mater. Sci. Semicond. Process 49 (2016) 8. [19] H. Chen, J. Ding, X. Wang, X. Wang, G. Chen, L. Ma, Opt. Mater. 62 (2016) 505. [20] U. Ilyas, P. Lee, J.L. Tan, R. Chen, A.W. Anwar, S. Zhang, H.D. Sun, R.S. Rawat, Appl. Surf. Sci. 387 (2016) 461. [21] Y. Zhang, X.H. Li, C.X. Peng, Chin. Phys. Lett. 29 (2012) 107803. [22] G. Jayalakshmi, K. Saravanan, T. Balasubramanian, J. Lumin. 140 (2013) 21. [23] R. Jetson, K. Yin, K. Donovan, Z. Zhu, Mater. Chem. Phys. 124 (2010) 417. [24] K. Vijayalakshmi, K. Karthick, P.D. Raj, M. Sridharan, Ceram. Int. 40 (2014) 827. [25] H.Y. Lin, Y.Y. Chou, C.L. Cheng, Y.F. Chen, Opt. Express 15 (2007) 13832. [26] P.B. Nair, V.B. Justinvictor, C.P. Daniel, K. Joy, V. Ramakrishnan, P.V. Thomas, Appl. Surf. Sci. 257 (2011) 10869. [27] L. Xu, L. Shi, X. Li, Appl. Surf. Sci. 255 (2009) 5957. [28] F.K. Shan, G.X. Liu, W.J. Lee, G.H. Lee, I.S. Kim, B.C. Shin, Appl. Phys. Lett. 86 (2005) 221910. [29] N.M. Bahadur, T. Furusawa, M. Sato, F. Kurayama, N. Suzuki, Mater. Res. Bull. 45 (2010) 1383. [30] W.C. Sun, Y.C. Yeh, C.T. Ko, J.H. He, M.J. Chen, Nanoscale Res. Lett. 6 (2011) 556.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Ultraviolet emission enhancement in ZnO thin films modified by nanocrystalline TiO2 Gaige Zheng a, b, c, *, Xi Lu a, b, Liming Qian a, b, Fenglin Xian a, b a Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean, Nanjing University of Information Science & Technology, Nanjing 210044, China b School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China c Jiangsu Collaborative Innovation Center on Atmospheric Environment and Equipment Technology (CICAEET), Nanjing University of Information Science & Technology, Nanjing 210044, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 February 2017 Received in revised form 18 March 2017 Accepted 30 March 2017
In this study, nanocrystalline TiO2 modified ZnO thin films were prepared by electron beam evaporation. The structural, morphological and optical properties of the samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), UV-visible spectroscopy, fluorescence spectroscopy, respectively. The composition of the films was examined by energy dispersive X-ray spectroscopy (EDX). The photoluminescent spectrum shows that the pure ZnO thin film exhibits an ultraviolet (UV) emission peak and a strong green emission band. Surface analysis indicates that the ZnO thin film contains many oxygen vacancy defects on the surface. After the ZnO thin film is modified by the nanocrystalline TiO2 layer, the UV emission of ZnO is largely enhanced and the green emission is greatly suppressed, which suggests that the surface defects such as oxygen vacancies are passivated by the TiO2 capping layer. As for the UV emission enhancement of the ZnO thin film, the optimized thickness of the TiO2 capping layer is ~16 nm. When the thickness is larger than 16 nm, the UV emission of the ZnO thin film will decrease because the TiO2 capping layer absorbs most of the excitation energy. The UV emission enhancement in the nanocrystalline TiO2 modified ZnO thin film can be attributed to surface passivation and flat band effect. © 2017 Elsevier B.V. All rights reserved.
Keywords: ZnO thin films UV emission enhancement TiO2 surface modification Surface passivation Flat band effect
1. Introduction In recent years, Zinc oxide (ZnO), as a multifunctional material, has attracted wide attention in the world. At room temperature, ZnO has a wide direct bandgap of ~3.37 eV and a high excitonic binding energy of ~60 meV, which makes ZnO a suitable material for fabrication of UV light-emitting devices [1e3]. If one wants to prepare ZnO-based UV emitters, he should first get ZnO materials with high performance of UV emissions such as ZnO thin films. ZnO thin films have been deposited by many methods such as molecular beam epitaxy [4], atomic layer deposition [5], metal organic chemical vapor deposition [6], magnetron sputtering [7], electron beam evaporation [8], ultrasonic spray pyrolysis [9], sol-gel method [10], etc. Among these methods, the first three methods, namely molecular beam epitaxy, atomic layer deposition, metal organic * Corresponding author. School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Ningliu Road 219#, Nanjing 210044, China. E-mail address: [email protected] (G. Zheng). http://dx.doi.org/10.1016/j.optmat.2017.03.056 0925-3467/© 2017 Elsevier B.V. All rights reserved.
chemical vapor deposition, can deposit high-quality ZnO thin films, but the related equipments are expensive, leading to a high production cost. Although the production cost for ZnO thin films deposited by other methods is low, the ZnO thin films usually exhibit strong visible emissions caused by many defects in the films [9,11e20]. In order to improve the UV emission and suppress visible emissions of ZnO thin films, some strategies have been applied. These strategies include: (1) Surface plasmon resonance [21]; (2) fluorescence resonance energy transfer [8]; (3) surface passivation [22e24]. As for the first strategy, the Ag or Au nanoparticles are often used to modify ZnO surface. However, Ag nanoparticles are easy to be oxidized when they are exposed to air for a long time, leading to lose surface plasmon resonance effect. On the other hand, gold is a very precious metal. As for the second strategy, if fluorescence resonance energy transfer happens between two materials, three harsh conditions must be met [8,25]. Compared with the first two strategies, the last one “surface passivation” seems simple and easy to be performed. For example, Jayalakshmi et al. [22] adopted thiol and amine to modify ZnO surfaces and
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found that the UV emission of ZnO thin films was enhanced and the visible emissions were suppressed; the authors attributed this to the surface defects passivation. Vijayalakshmi et al. [24] used MgO to cap ZnO thin films; they found that the UV emission of ZnO was improved and the UV emission peak had a blue-shift with the increase of MgO layer thickness. In this work, we prepared ZnO thin films by electron beam evaporation and used nanocrystalline TiO2 to modify the ZnO surface to tailor the luminescence. That the nanocrystalline TiO2 is applied to modify the ZnO thin films is based on the following considerations: (1) TiO2 is non-toxic, cheap and stable; (2) the TiO2 has a larger bandgap, so it will not absorb the UV emission of ZnO; (3) if the TiO2 thickness is suitable, it will not absorb lots of the excitation energy (325 nm). To our knowledge, there are few reports on the photoluminescence behavior of ZnO thin films modified by nanocrystalline TiO2 nanolayers. The results show that the UV emission of ZnO thin films modified by the TiO2 is largely enhanced, but there exists an optimized value for the TiO2
thickness. The details will be depicted below. 2. Experiments The ZnO thin films and TiO2 modified ZnO thin films were deposited by electron beam evaporation on thoroughly washed and dried glass and Si substrates. The raw material for the preparation of the films is high purity ZnO and TiO2 particles. When the ZnO thin films and TiO2 capping layer were deposited, the oxygen was introduced into the deposition chamber and the working pressure was 2.0 10 2 Pa. The substrate temperature was 200 and 100 C for ZnO and TiO2, respectively. The thickness of ZnO and TiO2 was monitored by a quartz crystal oscillator. The thickness of ZnO thin films was set to 100 nm. The thickness of the TiO2 capping layer was set to 8, 12, 16 and 20 nm, respectively (accordingly, the resultantly samples were marked as B, C, D and E, respectively.). The ZnO thin film without a TiO2 capping layer was marked as sample A. All the samples were not annealed.
Fig. 1. Surface and sectional morphology images of sample A (a), B (b), C (c), D (d) and E (e).
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growth direction of ZnO and the crystallinity of ZnO and TiO2 were analyzed by cross-sectional transmission electron microscopy. The optical absorbance of the samples was measured by a spectrometer (TU-1900). The photoluminescence spectra were recorded by a spectrophotometer (LABRAM800) with an excitation wavelength of 325 nm. The surface composition and chemical state of the samples were analyzed by an X-ray photoelectron spectrometer (AXIS UltraDLD).
3. Results and discussion
Fig. 2. XRD patterns of the samples.
The crystal phase of the samples was determined by an X-ray diffractometer (Druker D8 Advance). The surface and section morphologies of the samples were observed by a scanning electron microscope (S4800) and the chemical composition of the samples was analyzed by an energy dispersive X-ray spectrometer. The
Fig. 1 shows the surface and section morphology images of ZnO thin film and TiO2 modified ZnO thin films. For all the samples, it can be seen that ZnO thin films are composed of columnar grains with a direction perpendicular to the substrate surface, also evidenced by the following HRTEM images. All the films are very compact. As for sample B, C, D and E, it can be observed from the section morphology images that they all contain two layers and the top layer is TiO2. The TiO2 thickness is in agreement with the designed thickness. In addition, it can be seen from the surface morphology images that the surface morphology is similar for the samples modified by TiO2 capping nanolayer and the TiO2 particles are very uniform. Fig. 2 displays the XRD patterns of the samples. All the films show a strong (002) peak, suggesting that the ZnO thin films have a wurtzite structure and are preferentially orientated along the c-axis direction. Except the (002) peak of ZnO, there is no other peaks related to TiO2, indicating the TiO2 nanolayer exhibits an amorphous-like nature. As a matter of fact, the definition of “amorphous” is difficult for TiO2, as there is always some degree of local ordering observed [26]. Therefore, the amorphous nature of the TiO2 nanolayer here can not be decided only by the XRD results. That no XRD peaks are observed for TiO2 maybe implies that “amorphous TiO2” is just the phase where no signal can be detected
Fig. 3. Surface image (a), typical mapping images (b, c, d) and a EDX spectrum (e) of sample E.
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Fig. 4. The cross-section TEM image of sample E (a), HRTEM images for the three selected regions (b, c, d) and the EDS spectra for TiO2 (e) and ZnO (f).
by XRD, which means the material has crystalline domains of a size below the detection limit of X-ray [26]. Previously, P. B. Nair et al. [26] prepared TiO2 thin films at various RF powers. They found that the samples deposited at various RF powers at 0.01 mbar and annealed at 873 K were amorphous-like nature proofed by XRD
Fig. 5. Photoluminescence spectra of the samples.
patterns, while the signals from micro Raman spectra indicated the samples were crystalline. As for our samples, the crystallinity of TiO2 also needs other techniques to decide. For learning the film composition and the distribution of different elements, the EDX spectra and mapping images were taken. The typical results of sample E are shown in Fig. 3. From Fig. 3, it can be seen that except the Si signal from the Si substrate, only Zn, O and Ti occurred in the spectrum. It means that these films have no impurities. In addition, it can be seen from mapping images that Ti is distributed very uniformly, suggesting that the TiO2 has been uniformly deposited on ZnO thin films. In order to determine the crystallinity of TiO2 and the growth direction of ZnO, the HRTEM images are taken for sample E deposited on a Si substrate, as shown in Fig. 4. Fig. 4 (a) shows the cross-section TEM image. It can be seen that the ZnO thin film is very compact and composed of columnar grains which is covered by a TiO2 nanolayer. Fig. 4 (b) is the HRTEM image of the region 1. It clearly shows the growth direction of ZnO grains along the c-axis which is perpendicular to the Si substrate surface. In addition, a SiO2 nanolayer is also observed between ZnO and the substrate, which is formed during the deposition of ZnO. Fig. 4 (c) gives the HRTEM image of region 2, which exhibits a high c-axis oriented nature of the ZnO layer. Fig. 4 (d) shows the HRTEM image of the region 3. The upper layer is TiO2 as evidenced by EDS spectrum in Fig. 4 (e), and the bottom layer is ZnO as evidenced by a EDS
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Fig. 6. XPS spectra of sample A.
spectrum in Fig. 4 (f). The TiO2 nanolayer contains some nanocrystalline TiO2 particles and amorphous boundaries. The ZnO layer and TiO2 layer is tightly glued together, which is favorable for passivating the surface defects (such as oxygen vacancy, dangling bond etc.) of the ZnO layer. Fig. 5 gives the photoluminescence spectra of the samples. As for sample A, it has a UV emission peak located at 385 nm which is the near-band-edge emission of ZnO, originating from the recombination of free excitons [5,8,9]. Except the UV emission peak, sample A still has a strong green emission band centered at 516 nm. This green emission has been observed in many ZnO materials and
many researchers deem that the green emission is mainly connected with the oxygen vacancy defects on the ZnO surface [8,11,12,27]. Xu and Shan et al. [27,28] studied the aging effect of photoluminescence of ZnO thin films; they found that the green emission of ZnO gradually decreased with the increase of aging time, suggesting that the oxygen vacancies on the ZnO surface were gradually filled up. It is well known to all that the XPS is an important surface analysis technique which not only provides the surface composition information but also gives the chemical state of each element. In order to further confirm the existence of massive oxygen vacancies on the surface of ZnO thin films, we
Fig. 7. Absorbance spectra (a) and bandgaps (b) of ZnO and TiO2.
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measured the XPS spectra of the pure ZnO thin film and the XPS spectra are shown in Fig. 6. It can be seen from Fig. 6 (b) that the O 1s contains two components. One component located at 530.1 eV is attributed to lattice oxygen bonded as O2 in the ZnO matrix [7]; the other component located at 531.9 eV is associated with nonstoichiometric oxygen in ZnO [7,20]. This evidences that the ZnO thin film has many oxygen vacancy defects on the surface. As for the ZnO thin films modified by the TiO2 nanolayer, the green emission is largely suppressed while the UV emission is greatly enhanced. With the increase of the TiO2 nanolayer thickness, the UV emission is also gradually intensified. However, the UV emission is reduced again when the thickness of the TiO2 nanolayer is above 16 nm. As for the UV emission enhancement in TiO2 modified ZnO materials, two proposals have been put forward. Lin and Xu et al. [8,25] think that the UV emission enhancement in TiO2 modified ZnO materials is mainly ascribed to the fluorescence resonance energy transfer (FRET) between ZnO and TiO2. It should be noted here that the TiO2 deposited by Lin and Xu is crystalline and has UV emissions. Bahadur et al. [29] deem that the surface passivation produced by TiO2 surface modification plays an important role for the UV emission enhancement of ZnO. As for our deposited TiO2, there are no UV emissions under excitation by the 325 nm UV light. Therefore, it does not satisfy the three conditions in which FRET occurs [25]. As a consequence, for our samples, the UV emission enhancement should be mainly attributed to surface modification effect caused by the TiO2 nanolayer. In addition, the flat-band effect originating from TiO2 modification may be favorable for the UV emission enhancement [30]. Fig. 7 (a) shows the absorption spectra and bandgaps of ZnO and TiO2. From the position of absorption edge of TiO2, it can be known that when the TiO2 nanolayer is very thin, its absorption for the UV light of 325 nm is little. However, when the TiO2 nanolayer is relatively thick, its absorption for the UV light of 325 nm is significant. As a result, when the TiO2 nanolayer is thinner than 16 nm, most of the UV light of 325 nm penetrates the TiO2 nanolayer and enter into ZnO. But when the TiO2 nanolayer is more than 16 nm, most of the UV light of 325 nm will be absorbed by the TiO2 nanolayer, which decreases the excitation energy for ZnO and leads to the decline of the UV emission of 385 nm. Furthermore, from Fig. 7(b), it can be known that the bandgap of the TiO2 is 3.90 eV which is larger than the excitation energy of 3.80 eV (325 nm). Therefore, the excitation energy will not lead to electron transition from valence band to conduction band in TiO2 (but probably lead to electron transition from valence band to some levels in the forbidden band). As a result, the TiO2 has no UV emissions here. 4. Conclusion In this investigation, ZnO thin films modified by the TiO2 nanolayer were deposited by electron beam evaporation. The influence of TiO2 modification and the thickness of TiO2 nanolayer on the luminescent behavior of ZnO thin films were studied. The
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