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Influence of annealing temperature on the photoluminescence properties of ZnO quantum dots
Applied Surface Science 256 (2010) 3862–3865
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Influence of annealing temperature on the photoluminescence properties of ZnO quantum dots Xiangqiang Zhang, Shili Hou, Huibing Mao *, Jiqing Wang, Ziqiang Zhu Key Laboratory of Polar Materials and Devices, East China Normal University, Shanghai 200241, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 November 2009 Received in revised form 8 January 2010 Accepted 13 January 2010 Available online 25 January 2010
The properties of ZnO quantum dots (QDs) synthesized by the sol–gel process are reported. The primary focus is on investigating the origin of the visible emission from ZnO QDs by the annealing process. The Xray diffraction results show that ZnO QDs have hexagonal wurtzite structure and the QD diameter estimated from Debye–Scherrer formula is 8.9 nm, which has a good agreement with the results from transmission electron microscopy images and the theoretical calculation based on the Potential Morphing Method. The room-temperature photoluminescence spectra reveal that the ultraviolet excitation band has a red shift. Meanwhile, the main band of the visible emission shifts to the green luminescence band from the yellow luminescence one with the increase of the annealing temperature. A lot of oxygen atoms enter into Zn vacancies and form oxygen antisites with increasing temperature. That is probably the reason for the change of the visible emission band. ß 2010 Elsevier B.V. All rights reserved.
PACS: 73.21.La 78.55.Et 78.67.Hc Keywords: Quantum dots Annealing Point defects Photoluminescence
1. Introduction In recent years, semiconductor quantum dots (QDs) have attracted significant attention in the nanomaterials. Semiconductor QDs with the quantum confinement effect exhibit unique optical and electrical properties. As an environmentally friendly material, ZnO has a wide bandgap of 3.37 eV and rather large exciton binding energy of 60 meV. Therefore, ZnO QDs naturally become considerably promising applications in many fields, such as optoelectronic light emitting diodes (LED), ultraviolet (UV) laser diodes, solar cells and bio-sensors. So far, various physical or chemical synthetic methods of ZnO QDs have been developed, such as thermal evaporation [1], pulsed laser deposition (PLD) [2], ion implantation [3], reactive electron beam evaporation [4], thermal decomposition [5] and sol–gel [6– 10]. For these approaches, we choose sol–gel method to obtain ZnO QDs because this technique is known to have many fascinating advantages such as process simplicity and high homogeneity, which offer the possibility of large-area yield at low cost and low temperature synthesis for technological application. At room temperature, ZnO QDs typically exhibit dual emission bands in the near UV and visible range. The band-edge UV emission
* Corresponding author. Tel.: +86 21 54345152; fax: +86 21 54345152. E-mail address: [email protected] (H. Mao). 0169-4332/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.01.041
is commonly sharp whereas the visible emission is broad. Generally, the UV emission is attributed to the exciton radiative recombination [11] while that of the visible emission remains a matter of controversy [12–16]. Although several research groups have focused on the effect of the annealing temperature on the structural and optical properties of ZnO QDs [17–19], there is no detailed investigation on the origin of the visible emission by the annealing process. For the future development of ZnO QDs based nanodevices, in-depth understanding of the origin of the visible emission from ZnO QDs is very necessary. In this work, we report the synthesis of ZnO QDs capped with polyvinylpyrolidone (PVP) by the sol–gel process. The morphology and structure are investigated by the methods of X-ray diffraction (XRD) and transmission electron microscopy (TEM). The effect of annealing temperatures on the optical properties of ZnO QDs is characterized by the photoluminescence (PL) spectra. Especially, the mechanism of the visible emission from ZnO QDs is discussed in detail. 2. Experimental details ZnO QDs were prepared by the sol–gel technique from the zinc acetate dehydrate (Zn(CH3COO)22H2O) and NaOH. The synthesis was performed by the following method. 0.08 M zinc acetate solution by dissolving zinc acetate in anhydrous ethanol at 60 8C and 0.5 M NaOH solution by dissolving NaOH in anhydrous ethanol at room temperature were prepared separately. At the same time,
X. Zhang et al. / Applied Surface Science 256 (2010) 3862–3865
the appropriate weight of PVP was dissolved into the Zn2+ solution. Then 48 ml NaOH solution was added drop-wise with rigorous stirring into 10 ml Zn2+ solution. The resulting solution was homogenized by stirring continuously for 30 min with a magnetic stirrer. The precipitate of ZnO QDs from the transparent sol was carried out by addition of n-heptane with constant stirring. After centrifugation, the precipitate was collected and redispersed into de-ionized water and ethanol sequentially. The removal of excess acetate and sodium ions from the dispersed ZnO was carried out by de-ionized water and ethanol. Finally, a part of precipitate was recollected and then dried for 6 h in the air for the powder XRD analysis. Five samples named as A, B, C, D and E were fabricated by dropping the other part of precipitate in ethanol onto the square quartz. Sample A was the unannealed ZnO QDs. Sample B, C, D and E were annealed for 5 min by the rapid thermal processing at 200, 300, 400 and 600 8C, respectively. X-ray diffraction (XRD) pattern was obtained on powder samples using a Bruker D8 Advance diffractometer with a Cu Ka line of 0.1541 nm. Transmission electron microscopy (TEM) studies for the microstructure and estimation of crystal size of ZnO QDs were carried out using a JEM-2100 TEM at an accelerating voltage of 200 keV. The PL spectra were measured at room temperature with a Jobin-Yvon HR800 spectrometer using the 325 nm line of a He–Cd laser as the excitation source. 3. Results and discussion XRD is used to identify the crystal phase of ZnO QDs. Fig. 1 is the powder XRD pattern of sample A. The appearance of diffraction peaks corresponding to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) planes is in good agreement with the standard XRD peaks of bulk ZnO with hexagonal wurtzite structure, demonstrating the formation of wurtzite ZnO. According to Debye–Scherrer formula [20], the (1 0 1) peak at 36.288 in Fig. 1 gives the ZnO QD diameter of 8.9 nm. The corresponding TEM images are shown in Fig. 2. It can be seen that the average size of ZnO QD is approximately 8.7 nm, which is consistent with the estimated results from the XRD pattern. Fig. 3 displays the UV emission spectra of sample A–E. The UV emission peak position of sample A is at 376 nm. With the increase of the annealing temperature, the UV emission peak at 300 8C shifts to 378 nm while that of annealed at 600 8C sample E is at 381 nm. There is a red shift for the UV peak when the annealing temperature increases. For the UV emission, it is commonly attributed to the recombination of the free excitons. As shown in Fig. 3, the intensity of the UV peak decreases after annealing at 600 8C. Normally, the quality of QDs will increase after annealing and the UV emission should be enhanced. However, the capping
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Fig. 1. X-ray diffraction pattern of ZnO QDs powders (Upper) and the bars at bottom show the standard XRD peaks for the wurtzite crystalline ZnO powders.
agent PVP may be destroyed at the higher temperature 600 8C, which will lead to the increase of the non-radiation centers and the decrease of the UV peak intensity. There are two theoretical models to calculate the nanoparticle size based on the UV peak position. One is the Brus formula [21] and the other is the Potential Morphing Method (PMM) [22,23]. However, it is well known that the PMM model is more accurate for small radii than the Brus formula. For the PMM, we use the following material parameters for ZnO: me ¼ 0:24 m0 , mh ¼ 0:45 m0 , e = 3.7, V0e ¼ V0h ¼ 0:08 Eg (matrix) = 400 meV [22,23] (assuming that the PVP matrix has the bandgap energy equal to 5 eV). From these data we have calculated the ZnO QD size, which are listed in Table 1. It is showed that the radii of ZnO QDs are between 4.5 and 5.7 nm. For sample A, the diameter of QD is 9.0 nm, which quite coincides with the results of 8.9 and 8.7 nm from the XRD pattern and TEM images, respectively. At the same time, it can be seen that ZnO QDs gradually grow bigger with the annealing temperature increasing. This is the reason for the red shift of the UV peak. Fig. 4 presents the visible emission spectra of sample A–E. For the unannealed ZnO QDs sample A, there are 3 luminescence bands, a yellow luminescence (YL) band with a peak near 564 nm, a green luminescence (GL) one with a peak at 523 nm, and an orange one (OL) located at 615 nm. Sample B, compared with sample A, has a shift of 17 nm for the yellow luminescence band with a peak at 547 nm. However, the green and orange luminescence bands do not shift. Meanwhile, the intensity of yellow luminescence band is
Fig. 2. Lower magnification (a) and one quantum dot (b) TEM images of ZnO QDs.
X. Zhang et al. / Applied Surface Science 256 (2010) 3862–3865
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Fig. 3. Room temperature PL UV emission spectra of ZnO QDs: (A) unannealed, (B) annealed at 200 8C, (C) annealed at 300 8C, (D) annealed at 400 8C and (E) annealed at 600 8C.
decreasing while that of the green luminescence one is increasing. However, the yellow luminescence band is still the main one of the visible emission. At the annealing temperature of 300 8C, the yellow luminescence band shifts to 545 nm and the intensity of the green luminescence band exceeds that of the yellow luminescence, becoming the main band. At this time, the band positions of the yellow and orange luminescence still have no change. As the annealing temperatures increase to 400 and 600 8C, the positions of the three bands are the same as that of 300 8C. Generally, different mechanisms for the visible emission PL in ZnO have been proposed, and the exact one remains uncertain. Partially, the reason is that the visible luminescence depends on the preparation processes. There are different optical properties for ZnO under different preparation conditions. Many researches have been reported on the connection of the point defects with the visible emission. Theoretical and experimental studies have shown that the point defects in undoped ZnO can be of several types: oxygen vacancies (VO), Zn vacancies (VZn), interstitial O (Oi), interstitial Zn (Zni) and oxygen antisite (OZn). Lin et al. [24] calculated the defect levels in undoped ZnO films using the method of full-potential linear muffin-tin orbital and concluded that the green emission corresponded to the local level composed by the OZn defect rather than others. Tuomisto et al. [25] verified by positron annihilation spectroscopy that the concentration of VZn or VZn-related complexes was sufficiently high (2 1015 cm3) in high-quality ZnO. Reshchikov et al. [26] studied the yellow luminescence band in undoped and N-doped ZnO layers grown on sapphire by MBE. The result showed that the yellow luminescence band, with a peak at 2.19 eV, was assigned to transitions from a shallow donor to a deep acceptor, namely a VZn-related complex. Wang et al. [27] fabricated a Table 1 The impact of annealing temperatures on the PL UV peak position, bandgap and radius of ZnO QDs. Sample number
Temperature (8C)
UV peak position (nm)
Bandgap (eV)
Radius (nm)
A B C D E
Unannealed 200 300 400 600
376.7 377.2 378.4 380.4 381.3
3.29 3.29 3.28 3.26 3.25
4.5 4.5 4.7 5.5 5.7
Fig. 4. Room temperature PL visible emission spectra of ZnO QDs: (A) unannealed; (B) annealed at 200 8C, (C) annealed at 300 8C, (D) annealed at 400 8C and (E) annealed at 600 8C.
hexagonal two-dimensional ZnO nanosheet thin film on an aluminum substrate by a wet-chemical approach, which exhibited very strong orange emission with a peak at 615 nm derived from Oi inside the grain. For the visible emission PL spectra of the annealed samples, the yellow luminescence band is the main band of the visible emission from sample A and B. In the annealing temperature range of 300– 600 8C, the green luminescence band becomes the strongest one. As for the salient change in PL spectra, we assume that VZn and OZn defects play an important role. The yellow, green and orange luminescence bands are mainly related with VZn, OZn and Oi defects [24–27], respectively. The unannealed sample A has many VZn. Thus, the main emission band is the yellow luminescence one. As the annealing temperature rises, the intensity of the green luminescence band increases while that of the yellow luminescence one decreases, which indicates that the amount of OZn increases while that of VZn decreases. The change of the defects can be explained as follows. When ZnO QDs are annealed in the air, many oxygen atoms enter into the Zn vacancy and form OZn. With the annealing temperature increasing, more and more oxygen atoms form OZn and the amount of VZn gradually decreases in ZnO QDs. Therefore, above 300 8C the green luminescence band has become the main one instead of the yellow one. In addition, the amount of the Oi defects has no evident change compared with that of OZn and VZn, so the intensity of orange luminescence band does not have changes. From the impact of the annealing process on the visible emission in ZnO QDs, it is well concluded that the main native defects are VZn at lower annealing temperatures, but the main defects become OZn with the increase of the annealing temperature. This probably results from that many oxygen atoms enter into VZn and form OZn. Hence, the results of the annealing process well demonstrate that the yellow and green luminescence bands are mainly related with VZn and OZn defects, respectively. 4. Conclusions In conclusion, PVP-capped ZnO QDs are prepared by the sol–gel process. The XRD results show the ZnO QDs have hexagonal wurtzite structure and the average QD size estimated from Debye– Scherrer formula is 8.9 nm, which has a good agreement with the results of 8.7 and 9.0 nm from the TEM images and the theoretical
X. Zhang et al. / Applied Surface Science 256 (2010) 3862–3865
calculation based on the PMM model, respectively. ZnO QDs are annealed in air at the temperatures from 200 to 600 8C, and the effect of annealing temperatures on the optical properties is characterized by the PL spectra. The room-temperature PL spectra exhibit a relatively narrow UV emission band and an intense broad visible emission band containing the yellow, green and orange luminescence bands. With the increase of the annealing temperature, the UV peak position has a red shift. Meanwhile, the main peak of the visible emission band becomes the green luminescence band instead of the yellow luminescence one. With the annealing temperature increasing, ZnO QDs continually cluster and grow bigger so that it results in the red shift of UV emission peak position. Additionally, the change of visible emission band is attributed to the change in the main native defects. After annealing, lots of oxygen atoms enter into Zn vacancies and form OZn with increasing temperature. As a result, the number of OZn increases more than that of VZn in ZnO QDs. Thus, the green luminescence band becomes the main one from the yellow luminescence. Acknowledgments This investigation was supported by Natural Science Foundation of China (60576059) and Chinese National Key Basic Research Special Found (2006CB921700). References [1] A. Chrissanthopoulos, S. Baskoutas, N. Bouropoulos, V. Dracopoulos, D. Tasis, S.N. Yannopoulos, Thin Solid Films 515 (2007) 8524.
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[2] L. Chen, Z.Q. Chen, X.Z. Shang, C. Liu, S. Xu, Q. Fu, Solid State Commun. 137 (2006) 561. [3] F. Ren, L.Y. Zhang, X.H. Xiao, G.X. Cai, L.X. Fan, L. Liao, C.Z. Jiang, Nanotechnology 19 (2008) 155610. [4] H.Z. Wu, D.J. Qiu, Y.J. Cai, X.L. Xu, N.B. Chen, J. Cryst. Growth 245 (2002) 50. [5] L.L. Yang, J.H. Yang, X.Y. Liu, Y.J. Zhang, Y.X. Wang, H.G. Fan, D.D. Wang, J.H. Lang, J. Alloys Compd. 463 (2008) 92. [6] L. Spanhel, M.A. Anderson, J. Am. Chem. 113 (1991) 2826. [7] E.A. Meulenkamp, J. Phys. Chem. B 102 (1998) 5566. [8] Y.L. Wu, A.I.Y. Tok, F.Y.C. Boey, X.T. Zeng, X.H. Zhang, Appl. Surf. Sci. 253 (2007) 5473. [9] L. Guo, S.H. Yang, C.L. Yang, P. Yu, J.N. Wang, W.K. Ge, G.K.L. Wong, Appl. Phys. Lett. 76 (2000) 2901. [10] Y.H. Tong, Y.C. Liu, S.X. Lu, L. Dong, S.J. Chen, Z.Y. Xiao, J. Sol–Gel Sci. Technol. 30 (2004) 157. [11] Y.C. Kong, D.P. Yu, B. Zhang, W. Fang, S.Q. Feng, Appl. Phys. Lett. 78 (2004) 407. [12] A.V. Dijken, E.A. Meulenkamp, D. Vanmaeklbergh, A. Meijerink, J. Phys. Chem. B 104 (2000) 1715. [13] Y.W. Heo, P.P. Norton, S.J. Pearton, J. Appl. Phys. 98 (2005) 073502. [14] X. Zhou, Q. Kuang, Z.Y. Jiang, Z.X. Xie, T. Xu, R.B. Huang, L.S. Zheng, J. Phys. Chem. C 111 (2007) 12091. [15] J.S. Kang, H.S. Kang, S.S. Pang, E.S. Shim, S.Y. Lee, Thin Solid Films 443 (2003) 5. [16] A.B. Djurisic, Y.H. Leung, K.H. Tam, L. Ding, W.K. Ge, H.Y. Chen, S. Guo, Appl. Phys. Lett. 88 (2006) 103107. [17] J.H. Yang, X.Y. Liu, L.L. Yang, Y.X. Wang, Y.J. Zhang, J.H. Lang, M. Gao, B. Feng, J. Alloys Compd. 477 (2009) 632. [18] H.W. Kim, M.A. Kebede, H.S. Kim, M.H. Kong, C. Lee, J. Lumin. 129 (2009) 1619. [19] L.C. Chao, J.W. Huang, C.W. Chang, Physica B 404 (2009) 1301. [20] H.P. Klug, L.E. Alexaander, X-ray Diffraction Procedure for Crystalline and Amorphous Materials, Wiley, New York, 1974, p. 662. [21] L.E. Brus, J. Chem. Phys. 80 (1984) 4403. [22] S. Baskoutas, A.F. Terzis, J. Appl. Phys. 99 (2006) 013708. [23] S. Baskoutas, A.F. Terzis, Mater. Sci. Eng. B 147 (2008) 280. [24] B.X. Lin, Z.X. Fu, Y.B. Jia, Appl. Phys. Lett. 79 (2001) 943. [25] F. Tuomisto, V. Ranki, K. Saarinen, D.C. Look, Phys. Rev. Lett. 91 (2003) 205502. [26] M.A. Reshchikov, J.Q. Xie, B. Hertog, A. Osinsky, J. Appl. Phys. 103 (2008) 103514. [27] N. Wang, H. Lin, J.B. Li, L.Z. Zhang, X. Li, J. Wu, C.F. Lin, J. Am. Ceram. Soc. 90 (2007) 635.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Influence of annealing temperature on the photoluminescence properties of ZnO quantum dots Xiangqiang Zhang, Shili Hou, Huibing Mao *, Jiqing Wang, Ziqiang Zhu Key Laboratory of Polar Materials and Devices, East China Normal University, Shanghai 200241, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 November 2009 Received in revised form 8 January 2010 Accepted 13 January 2010 Available online 25 January 2010
The properties of ZnO quantum dots (QDs) synthesized by the sol–gel process are reported. The primary focus is on investigating the origin of the visible emission from ZnO QDs by the annealing process. The Xray diffraction results show that ZnO QDs have hexagonal wurtzite structure and the QD diameter estimated from Debye–Scherrer formula is 8.9 nm, which has a good agreement with the results from transmission electron microscopy images and the theoretical calculation based on the Potential Morphing Method. The room-temperature photoluminescence spectra reveal that the ultraviolet excitation band has a red shift. Meanwhile, the main band of the visible emission shifts to the green luminescence band from the yellow luminescence one with the increase of the annealing temperature. A lot of oxygen atoms enter into Zn vacancies and form oxygen antisites with increasing temperature. That is probably the reason for the change of the visible emission band. ß 2010 Elsevier B.V. All rights reserved.
PACS: 73.21.La 78.55.Et 78.67.Hc Keywords: Quantum dots Annealing Point defects Photoluminescence
1. Introduction In recent years, semiconductor quantum dots (QDs) have attracted significant attention in the nanomaterials. Semiconductor QDs with the quantum confinement effect exhibit unique optical and electrical properties. As an environmentally friendly material, ZnO has a wide bandgap of 3.37 eV and rather large exciton binding energy of 60 meV. Therefore, ZnO QDs naturally become considerably promising applications in many fields, such as optoelectronic light emitting diodes (LED), ultraviolet (UV) laser diodes, solar cells and bio-sensors. So far, various physical or chemical synthetic methods of ZnO QDs have been developed, such as thermal evaporation [1], pulsed laser deposition (PLD) [2], ion implantation [3], reactive electron beam evaporation [4], thermal decomposition [5] and sol–gel [6– 10]. For these approaches, we choose sol–gel method to obtain ZnO QDs because this technique is known to have many fascinating advantages such as process simplicity and high homogeneity, which offer the possibility of large-area yield at low cost and low temperature synthesis for technological application. At room temperature, ZnO QDs typically exhibit dual emission bands in the near UV and visible range. The band-edge UV emission
* Corresponding author. Tel.: +86 21 54345152; fax: +86 21 54345152. E-mail address: [email protected] (H. Mao). 0169-4332/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.01.041
is commonly sharp whereas the visible emission is broad. Generally, the UV emission is attributed to the exciton radiative recombination [11] while that of the visible emission remains a matter of controversy [12–16]. Although several research groups have focused on the effect of the annealing temperature on the structural and optical properties of ZnO QDs [17–19], there is no detailed investigation on the origin of the visible emission by the annealing process. For the future development of ZnO QDs based nanodevices, in-depth understanding of the origin of the visible emission from ZnO QDs is very necessary. In this work, we report the synthesis of ZnO QDs capped with polyvinylpyrolidone (PVP) by the sol–gel process. The morphology and structure are investigated by the methods of X-ray diffraction (XRD) and transmission electron microscopy (TEM). The effect of annealing temperatures on the optical properties of ZnO QDs is characterized by the photoluminescence (PL) spectra. Especially, the mechanism of the visible emission from ZnO QDs is discussed in detail. 2. Experimental details ZnO QDs were prepared by the sol–gel technique from the zinc acetate dehydrate (Zn(CH3COO)22H2O) and NaOH. The synthesis was performed by the following method. 0.08 M zinc acetate solution by dissolving zinc acetate in anhydrous ethanol at 60 8C and 0.5 M NaOH solution by dissolving NaOH in anhydrous ethanol at room temperature were prepared separately. At the same time,
X. Zhang et al. / Applied Surface Science 256 (2010) 3862–3865
the appropriate weight of PVP was dissolved into the Zn2+ solution. Then 48 ml NaOH solution was added drop-wise with rigorous stirring into 10 ml Zn2+ solution. The resulting solution was homogenized by stirring continuously for 30 min with a magnetic stirrer. The precipitate of ZnO QDs from the transparent sol was carried out by addition of n-heptane with constant stirring. After centrifugation, the precipitate was collected and redispersed into de-ionized water and ethanol sequentially. The removal of excess acetate and sodium ions from the dispersed ZnO was carried out by de-ionized water and ethanol. Finally, a part of precipitate was recollected and then dried for 6 h in the air for the powder XRD analysis. Five samples named as A, B, C, D and E were fabricated by dropping the other part of precipitate in ethanol onto the square quartz. Sample A was the unannealed ZnO QDs. Sample B, C, D and E were annealed for 5 min by the rapid thermal processing at 200, 300, 400 and 600 8C, respectively. X-ray diffraction (XRD) pattern was obtained on powder samples using a Bruker D8 Advance diffractometer with a Cu Ka line of 0.1541 nm. Transmission electron microscopy (TEM) studies for the microstructure and estimation of crystal size of ZnO QDs were carried out using a JEM-2100 TEM at an accelerating voltage of 200 keV. The PL spectra were measured at room temperature with a Jobin-Yvon HR800 spectrometer using the 325 nm line of a He–Cd laser as the excitation source. 3. Results and discussion XRD is used to identify the crystal phase of ZnO QDs. Fig. 1 is the powder XRD pattern of sample A. The appearance of diffraction peaks corresponding to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) planes is in good agreement with the standard XRD peaks of bulk ZnO with hexagonal wurtzite structure, demonstrating the formation of wurtzite ZnO. According to Debye–Scherrer formula [20], the (1 0 1) peak at 36.288 in Fig. 1 gives the ZnO QD diameter of 8.9 nm. The corresponding TEM images are shown in Fig. 2. It can be seen that the average size of ZnO QD is approximately 8.7 nm, which is consistent with the estimated results from the XRD pattern. Fig. 3 displays the UV emission spectra of sample A–E. The UV emission peak position of sample A is at 376 nm. With the increase of the annealing temperature, the UV emission peak at 300 8C shifts to 378 nm while that of annealed at 600 8C sample E is at 381 nm. There is a red shift for the UV peak when the annealing temperature increases. For the UV emission, it is commonly attributed to the recombination of the free excitons. As shown in Fig. 3, the intensity of the UV peak decreases after annealing at 600 8C. Normally, the quality of QDs will increase after annealing and the UV emission should be enhanced. However, the capping
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Fig. 1. X-ray diffraction pattern of ZnO QDs powders (Upper) and the bars at bottom show the standard XRD peaks for the wurtzite crystalline ZnO powders.
agent PVP may be destroyed at the higher temperature 600 8C, which will lead to the increase of the non-radiation centers and the decrease of the UV peak intensity. There are two theoretical models to calculate the nanoparticle size based on the UV peak position. One is the Brus formula [21] and the other is the Potential Morphing Method (PMM) [22,23]. However, it is well known that the PMM model is more accurate for small radii than the Brus formula. For the PMM, we use the following material parameters for ZnO: me ¼ 0:24 m0 , mh ¼ 0:45 m0 , e = 3.7, V0e ¼ V0h ¼ 0:08 Eg (matrix) = 400 meV [22,23] (assuming that the PVP matrix has the bandgap energy equal to 5 eV). From these data we have calculated the ZnO QD size, which are listed in Table 1. It is showed that the radii of ZnO QDs are between 4.5 and 5.7 nm. For sample A, the diameter of QD is 9.0 nm, which quite coincides with the results of 8.9 and 8.7 nm from the XRD pattern and TEM images, respectively. At the same time, it can be seen that ZnO QDs gradually grow bigger with the annealing temperature increasing. This is the reason for the red shift of the UV peak. Fig. 4 presents the visible emission spectra of sample A–E. For the unannealed ZnO QDs sample A, there are 3 luminescence bands, a yellow luminescence (YL) band with a peak near 564 nm, a green luminescence (GL) one with a peak at 523 nm, and an orange one (OL) located at 615 nm. Sample B, compared with sample A, has a shift of 17 nm for the yellow luminescence band with a peak at 547 nm. However, the green and orange luminescence bands do not shift. Meanwhile, the intensity of yellow luminescence band is
Fig. 2. Lower magnification (a) and one quantum dot (b) TEM images of ZnO QDs.
X. Zhang et al. / Applied Surface Science 256 (2010) 3862–3865
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Fig. 3. Room temperature PL UV emission spectra of ZnO QDs: (A) unannealed, (B) annealed at 200 8C, (C) annealed at 300 8C, (D) annealed at 400 8C and (E) annealed at 600 8C.
decreasing while that of the green luminescence one is increasing. However, the yellow luminescence band is still the main one of the visible emission. At the annealing temperature of 300 8C, the yellow luminescence band shifts to 545 nm and the intensity of the green luminescence band exceeds that of the yellow luminescence, becoming the main band. At this time, the band positions of the yellow and orange luminescence still have no change. As the annealing temperatures increase to 400 and 600 8C, the positions of the three bands are the same as that of 300 8C. Generally, different mechanisms for the visible emission PL in ZnO have been proposed, and the exact one remains uncertain. Partially, the reason is that the visible luminescence depends on the preparation processes. There are different optical properties for ZnO under different preparation conditions. Many researches have been reported on the connection of the point defects with the visible emission. Theoretical and experimental studies have shown that the point defects in undoped ZnO can be of several types: oxygen vacancies (VO), Zn vacancies (VZn), interstitial O (Oi), interstitial Zn (Zni) and oxygen antisite (OZn). Lin et al. [24] calculated the defect levels in undoped ZnO films using the method of full-potential linear muffin-tin orbital and concluded that the green emission corresponded to the local level composed by the OZn defect rather than others. Tuomisto et al. [25] verified by positron annihilation spectroscopy that the concentration of VZn or VZn-related complexes was sufficiently high (2 1015 cm3) in high-quality ZnO. Reshchikov et al. [26] studied the yellow luminescence band in undoped and N-doped ZnO layers grown on sapphire by MBE. The result showed that the yellow luminescence band, with a peak at 2.19 eV, was assigned to transitions from a shallow donor to a deep acceptor, namely a VZn-related complex. Wang et al. [27] fabricated a Table 1 The impact of annealing temperatures on the PL UV peak position, bandgap and radius of ZnO QDs. Sample number
Temperature (8C)
UV peak position (nm)
Bandgap (eV)
Radius (nm)
A B C D E
Unannealed 200 300 400 600
376.7 377.2 378.4 380.4 381.3
3.29 3.29 3.28 3.26 3.25
4.5 4.5 4.7 5.5 5.7
Fig. 4. Room temperature PL visible emission spectra of ZnO QDs: (A) unannealed; (B) annealed at 200 8C, (C) annealed at 300 8C, (D) annealed at 400 8C and (E) annealed at 600 8C.
hexagonal two-dimensional ZnO nanosheet thin film on an aluminum substrate by a wet-chemical approach, which exhibited very strong orange emission with a peak at 615 nm derived from Oi inside the grain. For the visible emission PL spectra of the annealed samples, the yellow luminescence band is the main band of the visible emission from sample A and B. In the annealing temperature range of 300– 600 8C, the green luminescence band becomes the strongest one. As for the salient change in PL spectra, we assume that VZn and OZn defects play an important role. The yellow, green and orange luminescence bands are mainly related with VZn, OZn and Oi defects [24–27], respectively. The unannealed sample A has many VZn. Thus, the main emission band is the yellow luminescence one. As the annealing temperature rises, the intensity of the green luminescence band increases while that of the yellow luminescence one decreases, which indicates that the amount of OZn increases while that of VZn decreases. The change of the defects can be explained as follows. When ZnO QDs are annealed in the air, many oxygen atoms enter into the Zn vacancy and form OZn. With the annealing temperature increasing, more and more oxygen atoms form OZn and the amount of VZn gradually decreases in ZnO QDs. Therefore, above 300 8C the green luminescence band has become the main one instead of the yellow one. In addition, the amount of the Oi defects has no evident change compared with that of OZn and VZn, so the intensity of orange luminescence band does not have changes. From the impact of the annealing process on the visible emission in ZnO QDs, it is well concluded that the main native defects are VZn at lower annealing temperatures, but the main defects become OZn with the increase of the annealing temperature. This probably results from that many oxygen atoms enter into VZn and form OZn. Hence, the results of the annealing process well demonstrate that the yellow and green luminescence bands are mainly related with VZn and OZn defects, respectively. 4. Conclusions In conclusion, PVP-capped ZnO QDs are prepared by the sol–gel process. The XRD results show the ZnO QDs have hexagonal wurtzite structure and the average QD size estimated from Debye– Scherrer formula is 8.9 nm, which has a good agreement with the results of 8.7 and 9.0 nm from the TEM images and the theoretical
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calculation based on the PMM model, respectively. ZnO QDs are annealed in air at the temperatures from 200 to 600 8C, and the effect of annealing temperatures on the optical properties is characterized by the PL spectra. The room-temperature PL spectra exhibit a relatively narrow UV emission band and an intense broad visible emission band containing the yellow, green and orange luminescence bands. With the increase of the annealing temperature, the UV peak position has a red shift. Meanwhile, the main peak of the visible emission band becomes the green luminescence band instead of the yellow luminescence one. With the annealing temperature increasing, ZnO QDs continually cluster and grow bigger so that it results in the red shift of UV emission peak position. Additionally, the change of visible emission band is attributed to the change in the main native defects. After annealing, lots of oxygen atoms enter into Zn vacancies and form OZn with increasing temperature. As a result, the number of OZn increases more than that of VZn in ZnO QDs. Thus, the green luminescence band becomes the main one from the yellow luminescence. Acknowledgments This investigation was supported by Natural Science Foundation of China (60576059) and Chinese National Key Basic Research Special Found (2006CB921700). References [1] A. Chrissanthopoulos, S. Baskoutas, N. Bouropoulos, V. Dracopoulos, D. Tasis, S.N. Yannopoulos, Thin Solid Films 515 (2007) 8524.
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