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Two different mechanisms on UV emission enhancement in Ag-doped ZnO thin films
Journal of Luminescence 158 (2015) 396–400
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Two different mechanisms on UV emission enhancement in Ag-doped ZnO thin films Linhua Xu a,b,n, Gaige Zheng a,b, Lilong Zhao a,b, Shixin Pei a,b a b
School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China Optics and Photonic Technology Laboratory, Nanjing University of Information Science & Technology, Nanjing 210044, China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 July 2014 Received in revised form 12 October 2014 Accepted 13 October 2014 Available online 22 October 2014
Ag-doped ZnO thin films were prepared by a sol–gel method. The structural, morphological and optical properties of the samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), UV–vis and photoluminescence spectra. The results show that the Ag in the ZnO thin films annealed at 500 1C for 1 h substitutes for Zn and exists in the form of Ag þ ion (AgZn) while the Ag in the ZnO thin films without a post-annealing mainly exists in the form of simple substance (Ag0). The incorporation of Ag indeed can improve the ultraviolet emission of ZnO thin films and suppress the visible emissions at the same time. However, the mechanisms on the ultraviolet emission enhancement in the annealed and unannealed Ag-doped ZnO thin films are very different. As for the post-annealed Ag-doped ZnO thin films, the UV emission enhancement maybe mainly results from more electron–hole pairs (excitons) due to Ag-doping while for the unannealed Ag-doped ZnO thin films; the UV emission enhancement is attributed to the resonant coupling between exciton emission in ZnO and localized surface plasmon in Ag nanoparticles. & 2014 Elsevier B.V. All rights reserved.
Keywords: ZnO thin films Ag-doping Sol–gel method Ultraviolet emission enhancement Surface plasmon resonance
1. Introduction ZnO is a II–VI group compound semiconductor with excellent thermal and chemical stability. It has a large excitonic binding energy ( 60 meV) and a wide direct band gap ( 3.37 eV) at room temperature. These characteristics make ZnO materials be suitable for short-wavelength optoelectronic devices such as blue lightemitting diodes [1], UV light-emitting diodes [2], UV lasers [3], UV photoconductive detectors [4], solar cells [5], and so on. At present, ZnO thin films, as a promising material, have attracted a lot of attention. The physical properties of ZnO thin films are closely connected with deposition methods, deposition parameters, annealing treatments and doping. Among these factors, the doping has been widely used to adjust the structural, electrical and optical properties of ZnO thin films [6–15]. Among the used doped-elements, Ag has drawn considerable attention [16–20]. For example, Gruzintsev et al. [16] prepared Ag-doped ZnO thin films by electron beam evaporation and they found that Ag incorporation led to a pronounced increase in the resistance and photosensitivity of ZnO thin films; Xu et al. [17] deposited Ag-doped ZnO n Corresponding author at: School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Ningliu Road 219#, Nanjing, 210044, China. Tel./fax: þ 86 25 58731174. E-mail address: [email protected] (L. Xu).
http://dx.doi.org/10.1016/j.jlumin.2014.10.028 0022-2313/& 2014 Elsevier B.V. All rights reserved.
thin films by a sol–gel method, and they found that the doping of Ag led to a large increase in the intensity of UV emission and a p-type conduction in ZnO; Kang et al. [18] fabricated Ag-doped ZnO thin films by pulsed laser deposition, and they found that the deposition temperature had an important influence on the conduction type of Ag-doped ZnO thin films; Liu et al. [19] prepared ZnO thin films embedded with Ag nanoparticles by magnetron sputtering, and they found that with increase of Ag contents the UV emission of ZnO was enhanced but the position of UV peak was not shifted. There is no doubt that these studies are important for us to deeply understand the effect of Ag-doping on the structural, electrical and optical properties of ZnO thin films. However, we can see that the reported results of Ag-doped ZnO thin films are very different. For instance, (1) the effect of Ag-doping on the luminescence of ZnO: Gruzintsev et al. [16] found that the UV emission of 3 at% Ag-doped ZnO thin films was largely enhanced compared to that of pure ZnO thin films while the UV emission in 1 and 5 at% Ag-doped ZnO thin films decreased; Xue et al. [20] prepared 1.8, 2.6 and 3.3 at% Ag-doped ZnO thin films, and they found that the UV emission in all Ag-doped ZnO thin films was enhanced compared to that in pure ZnO thin films (the strongest UV emission was found in the 2.6 at% Ag-doped ZnO thin films); Liu et al. [21] deposited 3 and 6 at% Ag-doped ZnO thin films by spray pyrolysis at 500 1C with a spray rate of 0.15 ml/min, and the films were annealed at 700 1C in air for 1 h. They found that the
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incorporation of Ag made UV emission decrease and visible emissions increase in ZnO. (2) The effect of Ag-doping on the optical band gap of ZnO thin films: Xu et al. [17] found that Ag-doping did not change the optical band gap of ZnO thin films; Xue et al. [20] found that Ag-doping increased the optical band gap of ZnO thin films; Chelouche et al. [22] found that only 2% Ag-doping could increase the optical band gap when ZnO thin films were doped with 1–3% Ag. In addition to the above-mentioned optical properties, the reported results about the Ag-doped ZnO thin films in structural and electrical properties are also mostly inconsistent. Therefore, the Ag-doped ZnO thin films still need to be more studied. There are two reasons for us to start the present study. On one hand, the interesting physical properties of ZnO thin films due to Ag-doping greatly attract us; on the other hand, the results of Agdoped ZnO thin films are still mostly inconsistent. In this work, we prepared Ag-doped ZnO thin films by the sol–gel method and mainly studied the optical properties of the films. It is found that the incorporation of Ag in ZnO can indeed improve the UV emission of ZnO thin films but there are two different mechanisms for the UV emission enhancement. Among them, the UV emission enhancement caused by surface plasmon resonance is more attractive.
2. Experiments Fig. 1. XRD patterns of the samples.
First, ZnO sols were prepared using zinc acetate, silver nitrate, ethanol and monoethanolamine (MEA) as the precursor, dopant, solvent and stabilizer, respectively. The molar ratio of zinc acetate to MEA was 1.0 and the Zn2 þ concentrations were 0.32 mol/L in the sols. The rate of Ag/Zn in the ZnO sols is 0, 1, 3 and 5 at%. Correspondingly, the resulted ZnO thin films were labeled as samples A, B, C, and D. Because silver nitrate is slightly soluble in ethanol, some undissolved silver nitrate is found in the 3 and 5 at% Ag-doped ZnO sols. The ZnO sols were aged for 24 h at room temperature and then ZnO thin films were deposited on Si and glass substrates by a spin-coating method. After a ZnO sol layer was finished, it was put into a furnace for preheating at 280 1C for 3 min; and then the next layer was deposited. The procedure from spin-coating to preheating was repeated for several times in order to obtain the desired film thickness. At last, the films were postannealed at 500 1C in air atmosphere for 1 h. Another 3 at% Agdoped ZnO thin film without post-annealing was also prepared at the same conditions as used for other films. This sample was labeled as sample E. The crystal structures of the films were analyzed by an X-ray diffractometer (Bruker D8). The surface morphology of the samples was observed by a field emission scanning electron microscope (S4800). The chemical binding of silver was investigated by X-ray photoelectron spectroscopy. The transmittance and absorbance were recorded by a spectrophotometer (TU-1901). The luminescence behavior was investigated by a spectrometer (LABRAM 800) with an excitation wavelength of 325 nm from a He–Cd laser.
3. Results and discussion 3.1. Structural and morphological properties of Ag-doped ZnO thin films Fig. 1 shows the XRD patterns of Ag-doped ZnO thin films. All the films show a strong (0 0 2) peak located at 34.51 except for sample E. There are no AgO and Ag2O related diffraction peaks to be found. This means that all the films have a wurtzite phase and are preferentially oriented along the c-axis direction. Although
sample E does not show obvious diffraction peak, there is still a contour of diffraction peak at 34.51 to be observed. This suggests that some tiny ZnO crystals may be formed in sample E and most of this film is amorphous. It can be seen from Fig. 1 that the (0 0 2) peak is gradually weakened with increase of Ag-doping content. The similar results are also reported by Xue and Zhang et al. [20,23]. This means that the Ag-doping decreases the degree of preferred orientation of ZnO thin films along the c-axis. In addition, the position of (0 0 2) peak slightly shifts toward a smaller angle direction with increase of Ag doping level. The similar results are also observed by others. For instance, Liu et al. [21] observed that the (0 0 2) peak shifted toward the smaller angle direction when Ag was doped into ZnO and attributed this to the increase of c-axis lattice constant caused by substitution of Zn2 þ ions by Ag þ ions. Kang et al. [18] observed that the (0 0 2) peak of Ag-doped ZnO thin film first shifted to a lower angle and then shifted back to a higher angle with increasing deposition temperature. Kang et al. deemed that the shift of the (0 0 2) peak was closely related to the lattice sites of Ag in ZnO. In fact, the variation of intrinsic point defects of ZnO also leads to a change of the (0 0 2) peak position. Fig. 2 displays the surface morphology images of the Ag-doped ZnO thin films. All the films show obvious round and oval ZnO grains in plane except for sample E. With the rise of Ag doping concentration, the ZnO grains gradually increase. This means that the Ag-doping does not influence ZnO crystal growth. It maybe results from the low Ag-doping concentration. At present, the reported results about the effect of Ag-doping on ZnO crystal growth are inconsistent. For example, Liu et al. [21] found that the Ag-doping led to a smaller ZnO grains; Zhang et al. [23] observed that the grain sizes in Ag-doped ZnO thin films were more nonuniform than those in pure ZnO thin films. However, Liu et al. [19] found that the Ag-doping resulted in smaller ZnO grains when the Ag-doping concentration was low while Ag nanoparticles were formed when the Ag-doping concentration was high. It is surprising that the crystallinity of ZnO does not decline with the precipitation of Ag nanoparticles. On the contrary, the ZnO grains grow up with increase of Ag nanoparticles. We think that these differences mainly result from the various lattice sites of Ag in ZnO
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Fig. 2. Surface morphology images of samples A (a), B (b), C (c), D (d) and E (e).
due to different film deposition methods and annealing treatments. As for our samples, it can be seen that the Ag-doping almost does not influence ZnO grain growth and lead to no larger nonuniformity on grain size. But for sample D, the pores on the surface of the film obviously increase. The increase of pores maybe originates from the rise of gasification substance in the film due to more dopants. The pores in the films deposited by the sol–gel method are often observed. No obvious ZnO grains can be observed in sample E, but many small bright particles are uniformly distributed on the film surface. From the SEM image, the diameter of the bright particles can be evaluated to be about 10 nm. These bright particles should be Ag nanoparticles. The similar results are also reported by Houng and Huang [24]. For the samples prepared by Houng and Huang, the Ag nanoparticle size is bigger than that in our samples, which results from the different
ZnO sol systems and deposition parameters. In order to further confirm that the bright particles are Ag nanoparticles, we performed XPS analyses on samples C and E. The obtained XPS spectra are shown in Fig. 3. As for samples C and E, the Ag 3d5/2 peaks of them are located at 367.3 and 368.0 eV, respectively. The variation of this peak position indicates the change of Ag valence [21]. The XPS spectra show that the main existing form of silver in sample C is Ag þ (AgZn) and Ag0 in sample E. Post-annealing treatments produce an important effect on the lattice sites of Ag in ZnO. The similar results have been reported by Liu et al. [21]. But Liu et al. found that the silver existed mainly in the form of Ag þ before Ag-doped ZnO thin films were post-annealed and Ag0 after post-annealing treatment. Their results are just opposite to ours. This shows that the status of Ag is often different in ZnO thin films deposited by different techniques. On the other hand, the
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Fig. 5. Absorbance spectra of the samples. Fig. 3. XPS spectra of Ag 3d of samples C and E.
2
Fig. 4. Transmittance spectra of the samples.
Fig. 6. The plots of ðαhvÞ hv for obtaining optical band gaps of the samples.
same result is also observed in our and Liu's experiments. That is to say, the intensity of Ag 3d5/2 peak for Ag0 is stronger than that for Ag þ .
(3.294 eV) are almost identical. However, the optical bandgap of sample D (3.290 eV) slightly decreased. Previously, Chelouche et al. [22] also prepared Ag-doped ZnO thin films by the sol–gel method. They found that the optical bandgap of 2% Ag-doped ZnO thin film was increased while the optical bandgaps of 1% and 3% Ag-doped ZnO thin films were decreased compared to those of pure ZnO thin film. Xue et al. [20] prepared Ag-doped ZnO thin films by magnetron sputtering and found that the optical bandgaps of the films all increased by Ag-doping. However, Xu et al. [17] found that the incorporation of Ag had almost no influence on the optical bandgaps of ZnO thin films. Therefore, it can be concluded that the optical bandgap of ZnO thin films is strongly dependent on the lattice sites of Ag in ZnO which is in turn dependent on film deposition methods and annealing treatments. Fig. 7 exhibits the photoluminescence spectra of Ag-doped ZnO thin films. All the samples show two luminescent regions, namely a strong, narrow UV emission peak and a wide, weak visible emission band (yellow–green band). The UV emission results from the recombination of free excitons while the yellow–green emission is deemed to be connected mainly with oxygen vacancy defects [26]. Apparently, the Ag-doping enhances the UV emission and depresses the visible emissions at the same time. However, the annealed and unannealed Ag-doped ZnO thin films also show different luminescence behavior. As for the post-annealed Agdoped ZnO thin films, with increase of Ag content, the intensity of UV emission increases, the full width at half-maximum of UV peak decreases, and the position of UV peak has a slight blue-shift compared with that of pure ZnO thin film. The details are shown in Table 1. Comparing sample E with A, it can be found that the UV emission of sample E was largely enhanced, while the position of UV peak has a slight red-shift. Although both the UV emissions of annealed and unannealed Ag-doped ZnO thin films are enhanced, the mechanisms on the enhancement are evidently different. From
3.2. Optical properties of Ag-doped ZnO thin films Fig. 4 presents the transmittance spectra of Ag-doped ZnO thin films. As for samples A, B, C and D, they all show high transmittance in the visible range and a sharp absorption edge in the near-UV region. The transmittance of sample D slightly decreased, which is because the increase of pores on the film surface leads more incident light to be scattered. Obviously, the transmittance spectrum of sample E is very different from others. The most obvious feature of sample E is that there is a strong absorption band centered at 440 nm, which is often attributed to surface plasmon resonance (SPR) absorption of Ag nanoparticles. In fact, this also indirectly confirms that the bright particles in the SEM image of sample E are just Ag nanoparticles. The SPR absorption peak of Ag particles usually occurs in the range of 400–500 nm, which is dependent on the shape and size of Ag particles [24,25]. Fig. 5 shows the absorbance spectra of Ag-doped ZnO thin films. Sample E shows a strong SPR absorption feature while others show a pronounced excitonic absorption peak located at 375 nm or so. For the direct band gap semiconductor, its absorption coefficient and bandgap obey the following relationship: 2 ðαhvÞ ¼ A hv Eg where α is the absorption coefficient, hv is the photon energy, A is a 2 constant and Eg is the optical bandgap. If one plots ðαhvÞ hv, the optical band gap Eg can be obtained by extrapolating the linear part 2 of the curve to ðαhvÞ ¼ 0. The related curves for our samples are shown in Fig. 6. As for samples A, B and C, their optical bandgaps
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4. Conclusion
Fig. 7. Photoluminescence spectra of the samples.
Table 1 Position and FWHM of the UV emission peaks. Sample
A
B
C
D
E
UV peak position (nm) FWHM of UV peak (nm)
383.5 44
383.2 29
383.1 32
379.6 22
388.0 35
Fig. 5, it can be seen that the post-annealed Ag-doped ZnO thin films all show strong excitonic absorption peaks while the unannealed Ag-doped ZnO thin films display a strong surface plasmon resonance absorption band instead of an excitonic absorption peak. Generally speaking, the stronger the excitonic absorption of ZnO, the stronger the excitonic emission of ZnO, and vice versa. However, in here, the UV emission of unannealed Ag-doped ZnO thin film is much stronger than that of post-annealed one. This means that the intensity of UV emission in Ag-doped ZnO thin films is not only connected with the free exciton density but also something else. It should have two different mechanisms for the enhancement of UV emission in Ag-doped ZnO thin films. We deem that the difference of the two mechanisms mainly originates from the different status of Ag in ZnO. As for the post-annealed Ag-doped ZnO thin films, from the XPS analysis, we can know that most of Ag has replaced Zn in the form of Ag þ ions. According to the suggestion of Zhang et al. [23], the photocarriers may escape more easily from Ag ions than from Zn ions, which leads to the quick diffusion of excitons in ZnO and more electron–hole pairs (excitons). This finally results in stronger UV emission. However, with respect to the unannealed Ag-doped ZnO thin film, its XPS spectrum shows that the doped Ag mainly exists in ZnO in the form of single substance (Ag0) and its absorption spectrum also exhibits strong surface plasmon resonance absorption due to Ag nanoparticles. Therefore, the large enhancement of the UV emission in sample E should result from the resonant coupling between exciton emissions and localized SPs in Ag nanoparticles [19,27–30]. Previously, some groups adopted the noble metal layers as either the capping layer or buffer layer for ZnO thin films to improve the UV emission of ZnO through surface plasmon resonance [28–30]. Unlike planar SPs in metal surfaces, SPs in metallic particles can be excited by incident light without any special conditions [19]. Compared with the post-annealed ZnO thin films, the unannealed one is found to have more efficient UV emission and its preparation is relatively simpler. Thus, as for improving UV emission of ZnO, the enhancement from SP resonance coupling is more advantageous. As a result, the ZnO thin films embedded with Ag nanoparticles should have a potential application in UV emitters.
In this work, Ag-doped ZnO thin films were prepared by the sol–gel method. The photoluminescence of the samples shows that the Ag-doping can indeed enhance UV emission performance of ZnO and depress the visible emissions at the same time. However, the characteristics of the UV emission in the post-annealed and unannealed Ag-doped ZnO thin films are different. So we lay a strong emphasis on the enhancement mechanism of the UV emission. The XPS spectra of the samples show that Ag in the post-annealed ZnO thin films exists mainly in the form of Ag þ ions while Ag0 in the unannealed one. The absorbance spectrum of the unannealed film displays a strong absorption band centered at 440 nm due to surface plasmon resonance of Ag nanoparticles. Therefore, it can be confirmed that the UV emission enhancement in the unannealed Ag-doped ZnO thin film results from the resonant coupling between excitonic emissions and surface plasmon of Ag nanoparticles. The UV emission enhancement in the post-annealed Ag-doped ZnO thin films maybe originate from the more electron–hole pairs (excitons) due to Ag-doping.
Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant no. 61203211). References [1] T. Wang, H. Wu, Z. Wang, C. Chen, C. Liu, Appl. Phys. Lett. 101 (2012) 161905. [2] H. Zhang, R. Shen, H. Liang, Y. Liu, Y. Liu, X. Xia, G. Du, J. Phys. D: Appl. Phys. 46 (2013) 065101. [3] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [4] H.I. Abdulgafour, Z. Hassan, N.M. Ahmed, F.K. Yam, J. Appl. Phys. 112 (2012) 074510. [5] R.J. Hong, K. Helming, X. Jiang, B. Szyszka, Appl. Surf. Sci. 226 (2004) 378. [6] Z.F. Wu, X.M. Wu, L.J. Zhuge, B. Hong, X.M. Yang, T. Yu, J.J. He, Q. Chen, Appl. Surf. Sci. 256 (2010) 2259. [7] N.H. Erdogan, K. Kara, H. Ozdamar, R. Esen, H. Kavak, Appl. Surf. Sci. 271 (2013) 70. [8] C. Lee, S.Y. Park, J. Lim, H.W. Kim, Mater. Lett. 61 (2007) 2495. [9] M. Wang, E.J. Kim, S.H. Hahn, J. Lumin. 131 (2011) 1428. [10] P. Chen, X. Ma, D. Yang, J. Alloys Compd. 431 (2007) 317. [11] H.T. Derraz, N. Benramdane, D. Nacer, A. Bouzidi, M. Medles, Sol. Energy Mater. Sol. Cells 73 (2002) 249. [12] A. Singh, D. Kumar, P.K. Khanna, B.C. Joshi, M. Kumar, Appl. Surf. Sci. 258 (2011) 1881. [13] C. Ge, C. Xie, S. Cai, Mater. Sci. Eng. B 137 (2007) 53. [14] R.J. Mendelsberg, J. Kennedy, S.M. Durbin, R.J. Reeves, Curr. Appl. Phys. 8 (2008) 283. [15] A. Bouaoud, A. Rmili, F. Ouachtari, A. Louardi, T. Chtouki, B. Elidrissi, H. Erguig, Mater. Chem. Phys. 137 (2013) 843. [16] A.N. Gruzintsev, V.T. Volkov, E.E. Yakimov, Semiconductors 37 (2003) 259. [17] J. Xu, Z.Y. Zhang, Y. Zhang, B.X. Lin, Z.X. Fu, Chin. Phys. Lett. 22 (2005) 2031. [18] H.S. Kang, B.D. Ahn, J.H. Kim, G.H. Kim, S.H. Lim, H.W. Chang, S.Y. Lee, Appl. Phys. Lett. 88 (2006) 202108. [19] M. Liu, S.W. Qu, W.W. Yu, S.Y. Bao, C.Y. Ma, Q.Y. Zhang, J. He, J.C. Jiang, E.I. Meletis, C.L. Chen, Appl. Phys. Lett. 97 (2010) 231906. [20] H. Xue, X.L. Xu, Y. Chen, G.H. Zhang, S.Y. Ma, Appl. Surf. Sci. 255 (2008) 1806. [21] K. Liu, B. Yang, H. Yan, Z. Fu, M. Wen, Y. Chen, J. Zuo, Appl. Surf. Sci. 255 (2008) 2052. [22] A. Chelouche, D. Djouadi, H. Merzouk, A. Aksas, Appl. Phys. A 115 (2014) 613. [23] Y. Zhang, Z. Zhang, B. Lin, Z. Fu, J. Xu, J. Phys. Chem. B 109 (2005) 19200. [24] B. Houng, C.J. Huang, Surf. Coat. Technol. 201 (2006) 3188. [25] L. Irimpan, V.P.N. Nampoori, P. Radhakrishnan, Chem. Phys. Lett. 455 (2008) 265. [26] A.E. Rakhshani, J. Kokaj, J. Mathew, B. Peradeep, Appl. Phys. A 86 (2007) 377. [27] A.P. Abiyasa, S.F. Yu, S.P. Lau, E.S.P. Leong, H.Y. Yang, Appl. Phys. Lett. 90 (2007) 231106. [28] J.B. You, X.W. Zhang, Y.M. Fan, S. Qu, N.F. Chen, Appl. Phys. Lett. 91 (2007) 231907. [29] Y.J. Jin, Q.X. Yu, S.J. Liu, Y. Liao, J. Chin., Quantum Electron. 25 (2008) 43. [30] D.Y. Lei, J. Li, H.C. Ong, Appl. Phys. Lett. 91 (2007) 021112.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Two different mechanisms on UV emission enhancement in Ag-doped ZnO thin films Linhua Xu a,b,n, Gaige Zheng a,b, Lilong Zhao a,b, Shixin Pei a,b a b
School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China Optics and Photonic Technology Laboratory, Nanjing University of Information Science & Technology, Nanjing 210044, China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 July 2014 Received in revised form 12 October 2014 Accepted 13 October 2014 Available online 22 October 2014
Ag-doped ZnO thin films were prepared by a sol–gel method. The structural, morphological and optical properties of the samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), UV–vis and photoluminescence spectra. The results show that the Ag in the ZnO thin films annealed at 500 1C for 1 h substitutes for Zn and exists in the form of Ag þ ion (AgZn) while the Ag in the ZnO thin films without a post-annealing mainly exists in the form of simple substance (Ag0). The incorporation of Ag indeed can improve the ultraviolet emission of ZnO thin films and suppress the visible emissions at the same time. However, the mechanisms on the ultraviolet emission enhancement in the annealed and unannealed Ag-doped ZnO thin films are very different. As for the post-annealed Ag-doped ZnO thin films, the UV emission enhancement maybe mainly results from more electron–hole pairs (excitons) due to Ag-doping while for the unannealed Ag-doped ZnO thin films; the UV emission enhancement is attributed to the resonant coupling between exciton emission in ZnO and localized surface plasmon in Ag nanoparticles. & 2014 Elsevier B.V. All rights reserved.
Keywords: ZnO thin films Ag-doping Sol–gel method Ultraviolet emission enhancement Surface plasmon resonance
1. Introduction ZnO is a II–VI group compound semiconductor with excellent thermal and chemical stability. It has a large excitonic binding energy ( 60 meV) and a wide direct band gap ( 3.37 eV) at room temperature. These characteristics make ZnO materials be suitable for short-wavelength optoelectronic devices such as blue lightemitting diodes [1], UV light-emitting diodes [2], UV lasers [3], UV photoconductive detectors [4], solar cells [5], and so on. At present, ZnO thin films, as a promising material, have attracted a lot of attention. The physical properties of ZnO thin films are closely connected with deposition methods, deposition parameters, annealing treatments and doping. Among these factors, the doping has been widely used to adjust the structural, electrical and optical properties of ZnO thin films [6–15]. Among the used doped-elements, Ag has drawn considerable attention [16–20]. For example, Gruzintsev et al. [16] prepared Ag-doped ZnO thin films by electron beam evaporation and they found that Ag incorporation led to a pronounced increase in the resistance and photosensitivity of ZnO thin films; Xu et al. [17] deposited Ag-doped ZnO n Corresponding author at: School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Ningliu Road 219#, Nanjing, 210044, China. Tel./fax: þ 86 25 58731174. E-mail address: [email protected] (L. Xu).
http://dx.doi.org/10.1016/j.jlumin.2014.10.028 0022-2313/& 2014 Elsevier B.V. All rights reserved.
thin films by a sol–gel method, and they found that the doping of Ag led to a large increase in the intensity of UV emission and a p-type conduction in ZnO; Kang et al. [18] fabricated Ag-doped ZnO thin films by pulsed laser deposition, and they found that the deposition temperature had an important influence on the conduction type of Ag-doped ZnO thin films; Liu et al. [19] prepared ZnO thin films embedded with Ag nanoparticles by magnetron sputtering, and they found that with increase of Ag contents the UV emission of ZnO was enhanced but the position of UV peak was not shifted. There is no doubt that these studies are important for us to deeply understand the effect of Ag-doping on the structural, electrical and optical properties of ZnO thin films. However, we can see that the reported results of Ag-doped ZnO thin films are very different. For instance, (1) the effect of Ag-doping on the luminescence of ZnO: Gruzintsev et al. [16] found that the UV emission of 3 at% Ag-doped ZnO thin films was largely enhanced compared to that of pure ZnO thin films while the UV emission in 1 and 5 at% Ag-doped ZnO thin films decreased; Xue et al. [20] prepared 1.8, 2.6 and 3.3 at% Ag-doped ZnO thin films, and they found that the UV emission in all Ag-doped ZnO thin films was enhanced compared to that in pure ZnO thin films (the strongest UV emission was found in the 2.6 at% Ag-doped ZnO thin films); Liu et al. [21] deposited 3 and 6 at% Ag-doped ZnO thin films by spray pyrolysis at 500 1C with a spray rate of 0.15 ml/min, and the films were annealed at 700 1C in air for 1 h. They found that the
L. Xu et al. / Journal of Luminescence 158 (2015) 396–400
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incorporation of Ag made UV emission decrease and visible emissions increase in ZnO. (2) The effect of Ag-doping on the optical band gap of ZnO thin films: Xu et al. [17] found that Ag-doping did not change the optical band gap of ZnO thin films; Xue et al. [20] found that Ag-doping increased the optical band gap of ZnO thin films; Chelouche et al. [22] found that only 2% Ag-doping could increase the optical band gap when ZnO thin films were doped with 1–3% Ag. In addition to the above-mentioned optical properties, the reported results about the Ag-doped ZnO thin films in structural and electrical properties are also mostly inconsistent. Therefore, the Ag-doped ZnO thin films still need to be more studied. There are two reasons for us to start the present study. On one hand, the interesting physical properties of ZnO thin films due to Ag-doping greatly attract us; on the other hand, the results of Agdoped ZnO thin films are still mostly inconsistent. In this work, we prepared Ag-doped ZnO thin films by the sol–gel method and mainly studied the optical properties of the films. It is found that the incorporation of Ag in ZnO can indeed improve the UV emission of ZnO thin films but there are two different mechanisms for the UV emission enhancement. Among them, the UV emission enhancement caused by surface plasmon resonance is more attractive.
2. Experiments Fig. 1. XRD patterns of the samples.
First, ZnO sols were prepared using zinc acetate, silver nitrate, ethanol and monoethanolamine (MEA) as the precursor, dopant, solvent and stabilizer, respectively. The molar ratio of zinc acetate to MEA was 1.0 and the Zn2 þ concentrations were 0.32 mol/L in the sols. The rate of Ag/Zn in the ZnO sols is 0, 1, 3 and 5 at%. Correspondingly, the resulted ZnO thin films were labeled as samples A, B, C, and D. Because silver nitrate is slightly soluble in ethanol, some undissolved silver nitrate is found in the 3 and 5 at% Ag-doped ZnO sols. The ZnO sols were aged for 24 h at room temperature and then ZnO thin films were deposited on Si and glass substrates by a spin-coating method. After a ZnO sol layer was finished, it was put into a furnace for preheating at 280 1C for 3 min; and then the next layer was deposited. The procedure from spin-coating to preheating was repeated for several times in order to obtain the desired film thickness. At last, the films were postannealed at 500 1C in air atmosphere for 1 h. Another 3 at% Agdoped ZnO thin film without post-annealing was also prepared at the same conditions as used for other films. This sample was labeled as sample E. The crystal structures of the films were analyzed by an X-ray diffractometer (Bruker D8). The surface morphology of the samples was observed by a field emission scanning electron microscope (S4800). The chemical binding of silver was investigated by X-ray photoelectron spectroscopy. The transmittance and absorbance were recorded by a spectrophotometer (TU-1901). The luminescence behavior was investigated by a spectrometer (LABRAM 800) with an excitation wavelength of 325 nm from a He–Cd laser.
3. Results and discussion 3.1. Structural and morphological properties of Ag-doped ZnO thin films Fig. 1 shows the XRD patterns of Ag-doped ZnO thin films. All the films show a strong (0 0 2) peak located at 34.51 except for sample E. There are no AgO and Ag2O related diffraction peaks to be found. This means that all the films have a wurtzite phase and are preferentially oriented along the c-axis direction. Although
sample E does not show obvious diffraction peak, there is still a contour of diffraction peak at 34.51 to be observed. This suggests that some tiny ZnO crystals may be formed in sample E and most of this film is amorphous. It can be seen from Fig. 1 that the (0 0 2) peak is gradually weakened with increase of Ag-doping content. The similar results are also reported by Xue and Zhang et al. [20,23]. This means that the Ag-doping decreases the degree of preferred orientation of ZnO thin films along the c-axis. In addition, the position of (0 0 2) peak slightly shifts toward a smaller angle direction with increase of Ag doping level. The similar results are also observed by others. For instance, Liu et al. [21] observed that the (0 0 2) peak shifted toward the smaller angle direction when Ag was doped into ZnO and attributed this to the increase of c-axis lattice constant caused by substitution of Zn2 þ ions by Ag þ ions. Kang et al. [18] observed that the (0 0 2) peak of Ag-doped ZnO thin film first shifted to a lower angle and then shifted back to a higher angle with increasing deposition temperature. Kang et al. deemed that the shift of the (0 0 2) peak was closely related to the lattice sites of Ag in ZnO. In fact, the variation of intrinsic point defects of ZnO also leads to a change of the (0 0 2) peak position. Fig. 2 displays the surface morphology images of the Ag-doped ZnO thin films. All the films show obvious round and oval ZnO grains in plane except for sample E. With the rise of Ag doping concentration, the ZnO grains gradually increase. This means that the Ag-doping does not influence ZnO crystal growth. It maybe results from the low Ag-doping concentration. At present, the reported results about the effect of Ag-doping on ZnO crystal growth are inconsistent. For example, Liu et al. [21] found that the Ag-doping led to a smaller ZnO grains; Zhang et al. [23] observed that the grain sizes in Ag-doped ZnO thin films were more nonuniform than those in pure ZnO thin films. However, Liu et al. [19] found that the Ag-doping resulted in smaller ZnO grains when the Ag-doping concentration was low while Ag nanoparticles were formed when the Ag-doping concentration was high. It is surprising that the crystallinity of ZnO does not decline with the precipitation of Ag nanoparticles. On the contrary, the ZnO grains grow up with increase of Ag nanoparticles. We think that these differences mainly result from the various lattice sites of Ag in ZnO
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Fig. 2. Surface morphology images of samples A (a), B (b), C (c), D (d) and E (e).
due to different film deposition methods and annealing treatments. As for our samples, it can be seen that the Ag-doping almost does not influence ZnO grain growth and lead to no larger nonuniformity on grain size. But for sample D, the pores on the surface of the film obviously increase. The increase of pores maybe originates from the rise of gasification substance in the film due to more dopants. The pores in the films deposited by the sol–gel method are often observed. No obvious ZnO grains can be observed in sample E, but many small bright particles are uniformly distributed on the film surface. From the SEM image, the diameter of the bright particles can be evaluated to be about 10 nm. These bright particles should be Ag nanoparticles. The similar results are also reported by Houng and Huang [24]. For the samples prepared by Houng and Huang, the Ag nanoparticle size is bigger than that in our samples, which results from the different
ZnO sol systems and deposition parameters. In order to further confirm that the bright particles are Ag nanoparticles, we performed XPS analyses on samples C and E. The obtained XPS spectra are shown in Fig. 3. As for samples C and E, the Ag 3d5/2 peaks of them are located at 367.3 and 368.0 eV, respectively. The variation of this peak position indicates the change of Ag valence [21]. The XPS spectra show that the main existing form of silver in sample C is Ag þ (AgZn) and Ag0 in sample E. Post-annealing treatments produce an important effect on the lattice sites of Ag in ZnO. The similar results have been reported by Liu et al. [21]. But Liu et al. found that the silver existed mainly in the form of Ag þ before Ag-doped ZnO thin films were post-annealed and Ag0 after post-annealing treatment. Their results are just opposite to ours. This shows that the status of Ag is often different in ZnO thin films deposited by different techniques. On the other hand, the
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Fig. 5. Absorbance spectra of the samples. Fig. 3. XPS spectra of Ag 3d of samples C and E.
2
Fig. 4. Transmittance spectra of the samples.
Fig. 6. The plots of ðαhvÞ hv for obtaining optical band gaps of the samples.
same result is also observed in our and Liu's experiments. That is to say, the intensity of Ag 3d5/2 peak for Ag0 is stronger than that for Ag þ .
(3.294 eV) are almost identical. However, the optical bandgap of sample D (3.290 eV) slightly decreased. Previously, Chelouche et al. [22] also prepared Ag-doped ZnO thin films by the sol–gel method. They found that the optical bandgap of 2% Ag-doped ZnO thin film was increased while the optical bandgaps of 1% and 3% Ag-doped ZnO thin films were decreased compared to those of pure ZnO thin film. Xue et al. [20] prepared Ag-doped ZnO thin films by magnetron sputtering and found that the optical bandgaps of the films all increased by Ag-doping. However, Xu et al. [17] found that the incorporation of Ag had almost no influence on the optical bandgaps of ZnO thin films. Therefore, it can be concluded that the optical bandgap of ZnO thin films is strongly dependent on the lattice sites of Ag in ZnO which is in turn dependent on film deposition methods and annealing treatments. Fig. 7 exhibits the photoluminescence spectra of Ag-doped ZnO thin films. All the samples show two luminescent regions, namely a strong, narrow UV emission peak and a wide, weak visible emission band (yellow–green band). The UV emission results from the recombination of free excitons while the yellow–green emission is deemed to be connected mainly with oxygen vacancy defects [26]. Apparently, the Ag-doping enhances the UV emission and depresses the visible emissions at the same time. However, the annealed and unannealed Ag-doped ZnO thin films also show different luminescence behavior. As for the post-annealed Agdoped ZnO thin films, with increase of Ag content, the intensity of UV emission increases, the full width at half-maximum of UV peak decreases, and the position of UV peak has a slight blue-shift compared with that of pure ZnO thin film. The details are shown in Table 1. Comparing sample E with A, it can be found that the UV emission of sample E was largely enhanced, while the position of UV peak has a slight red-shift. Although both the UV emissions of annealed and unannealed Ag-doped ZnO thin films are enhanced, the mechanisms on the enhancement are evidently different. From
3.2. Optical properties of Ag-doped ZnO thin films Fig. 4 presents the transmittance spectra of Ag-doped ZnO thin films. As for samples A, B, C and D, they all show high transmittance in the visible range and a sharp absorption edge in the near-UV region. The transmittance of sample D slightly decreased, which is because the increase of pores on the film surface leads more incident light to be scattered. Obviously, the transmittance spectrum of sample E is very different from others. The most obvious feature of sample E is that there is a strong absorption band centered at 440 nm, which is often attributed to surface plasmon resonance (SPR) absorption of Ag nanoparticles. In fact, this also indirectly confirms that the bright particles in the SEM image of sample E are just Ag nanoparticles. The SPR absorption peak of Ag particles usually occurs in the range of 400–500 nm, which is dependent on the shape and size of Ag particles [24,25]. Fig. 5 shows the absorbance spectra of Ag-doped ZnO thin films. Sample E shows a strong SPR absorption feature while others show a pronounced excitonic absorption peak located at 375 nm or so. For the direct band gap semiconductor, its absorption coefficient and bandgap obey the following relationship: 2 ðαhvÞ ¼ A hv Eg where α is the absorption coefficient, hv is the photon energy, A is a 2 constant and Eg is the optical bandgap. If one plots ðαhvÞ hv, the optical band gap Eg can be obtained by extrapolating the linear part 2 of the curve to ðαhvÞ ¼ 0. The related curves for our samples are shown in Fig. 6. As for samples A, B and C, their optical bandgaps
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4. Conclusion
Fig. 7. Photoluminescence spectra of the samples.
Table 1 Position and FWHM of the UV emission peaks. Sample
A
B
C
D
E
UV peak position (nm) FWHM of UV peak (nm)
383.5 44
383.2 29
383.1 32
379.6 22
388.0 35
Fig. 5, it can be seen that the post-annealed Ag-doped ZnO thin films all show strong excitonic absorption peaks while the unannealed Ag-doped ZnO thin films display a strong surface plasmon resonance absorption band instead of an excitonic absorption peak. Generally speaking, the stronger the excitonic absorption of ZnO, the stronger the excitonic emission of ZnO, and vice versa. However, in here, the UV emission of unannealed Ag-doped ZnO thin film is much stronger than that of post-annealed one. This means that the intensity of UV emission in Ag-doped ZnO thin films is not only connected with the free exciton density but also something else. It should have two different mechanisms for the enhancement of UV emission in Ag-doped ZnO thin films. We deem that the difference of the two mechanisms mainly originates from the different status of Ag in ZnO. As for the post-annealed Ag-doped ZnO thin films, from the XPS analysis, we can know that most of Ag has replaced Zn in the form of Ag þ ions. According to the suggestion of Zhang et al. [23], the photocarriers may escape more easily from Ag ions than from Zn ions, which leads to the quick diffusion of excitons in ZnO and more electron–hole pairs (excitons). This finally results in stronger UV emission. However, with respect to the unannealed Ag-doped ZnO thin film, its XPS spectrum shows that the doped Ag mainly exists in ZnO in the form of single substance (Ag0) and its absorption spectrum also exhibits strong surface plasmon resonance absorption due to Ag nanoparticles. Therefore, the large enhancement of the UV emission in sample E should result from the resonant coupling between exciton emissions and localized SPs in Ag nanoparticles [19,27–30]. Previously, some groups adopted the noble metal layers as either the capping layer or buffer layer for ZnO thin films to improve the UV emission of ZnO through surface plasmon resonance [28–30]. Unlike planar SPs in metal surfaces, SPs in metallic particles can be excited by incident light without any special conditions [19]. Compared with the post-annealed ZnO thin films, the unannealed one is found to have more efficient UV emission and its preparation is relatively simpler. Thus, as for improving UV emission of ZnO, the enhancement from SP resonance coupling is more advantageous. As a result, the ZnO thin films embedded with Ag nanoparticles should have a potential application in UV emitters.
In this work, Ag-doped ZnO thin films were prepared by the sol–gel method. The photoluminescence of the samples shows that the Ag-doping can indeed enhance UV emission performance of ZnO and depress the visible emissions at the same time. However, the characteristics of the UV emission in the post-annealed and unannealed Ag-doped ZnO thin films are different. So we lay a strong emphasis on the enhancement mechanism of the UV emission. The XPS spectra of the samples show that Ag in the post-annealed ZnO thin films exists mainly in the form of Ag þ ions while Ag0 in the unannealed one. The absorbance spectrum of the unannealed film displays a strong absorption band centered at 440 nm due to surface plasmon resonance of Ag nanoparticles. Therefore, it can be confirmed that the UV emission enhancement in the unannealed Ag-doped ZnO thin film results from the resonant coupling between excitonic emissions and surface plasmon of Ag nanoparticles. The UV emission enhancement in the post-annealed Ag-doped ZnO thin films maybe originate from the more electron–hole pairs (excitons) due to Ag-doping.
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