- Email: [email protected]
Raman and excitonic photoluminescence characterizations of ZnO star-shaped nanocrystals
ARTICLE IN PRESS
Journal of Luminescence 122–123 (2007) 415–417 www.elsevier.com/locate/jlumin
Raman and excitonic photoluminescence characterizations of ZnO star-shaped nanocrystals Chunping Lia,, Yuzhen Lva, Lin Guoa,, Huibin Xua, Xicheng Aib, Jianping Zhangb a
School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, China b Institute of Chemistry, Chinese Academy of Science, Beijing 100080, China Available online 15 March 2006
Abstract Optical properties of ZnO star-shaped nanostructures grown by wet chemical solution method were investigated by using Raman scattering and temperature-dependent photoluminescence. High intensity of Raman mode E high with narrow FWHM of 9 cm 1 2 1 appearing at 436 cm indicates the good quality of ZnO wurtzitic structure. Temperature-dependent ultraviolet photoluminescence was studied over the temperature range from 78 to 293 K. At low-temperatures and low-excitation intensities, the dominant spontaneous emissions are due to radiative recombination of excitons bound to donors and one longitudinal optical phonon assistant free exciton A. Finally, only the first-order longitudinal–optical phonon replica of the A free exciton recombination was observed at room temperature. The exciton emission behaviour, including excitonic energy, intensity and line width dependents on the temperature and excitation intensities are discussed. The deep-level emission band is barely observable both at room temperature and at cryogenic temperature measurements. r 2006 Elsevier B.V. All rights reserved. PACS: 78.55 Keywords: ZnO; Temperature-dependent photoluminescence; Raman; Exciton; Phonon
1. Introduction Wurtzitic ZnO is a wide-band-gap and high excitonic bound energy semiconductor, which is a good candidate for optoelectronics in the UV and blue spectral ranges [1,2]. The quality of the ZnO crystal nanostructure is crucial for optoelectronic device fabrication and their applications. Raman and photoluminescence are sensitive and nondestructive techniques for the detection of impurities and defects, thus have been acted as suitable tools to determine the crystalline quality as well as the energy band fine structures of the materials. In the recent study, we synthesized high-quality singlecrystalline ZnO star-shaped nanostructures by a simple and soft solution route at low temperature and with a short reaction time. One remarkable feature of the products is Corresponding authors. Tel./fax: +86 10 82338162.
E-mail addresses: [email protected] (C. Li), [email protected] (L. Guo). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.173
the low optical pump stimulated emission threshold (318 kW/cm2) obtained at room temperature, and the narrowed stimulated emission FWHM is less than 3 nm. To clearly understand the photoluminescence characteristics of the nanomaterials, it is necessary to study the fine energy band structure and the excitonic emissions, the variations of which are dependent on the temperature and excitation intensities. 2. Experiments The morphology of the nanostars was characterized by TEM and the crystal structure was characterized by Raman spectrum excited by a He–Ne laser 632.8 nm line. The Raman signals were detected by a microlaser Raman spectrometer made in France (LabRam HR800). Temperature-dependent photoluminescence measurements were carried out by the tripli-frequency of Nd: yttrium– aluminum–garnet laser (YAG Tempest 300 laser: 355 nm, 5 ns pulse width, 10 Hz pulse repetition rate). The samples
ARTICLE IN PRESS 416
C. Li et al. / Journal of Luminescence 122–123 (2007) 415–417
were mounted in an optical cryostat where the temperature could be varied from 78 K to room temperature. 3000
436.126
3.1. Morphologies and structures A typical single star morphology of the samples is shown in Fig. 1. Inset shows the SAED pattern corresponding to one arm. The TEM image in Fig. 1 reveals that the ZnO entities have a micron size and a star shape with six symmetric arms extending radially from the center. Each arm of the ZnO microstars has a sharp tip of several nanometers with a length of around 0.52 mm tapers. The angle between two neighboring arms is about 601. Details of the synthesis of ZnO nanostar were described elsewhere [3]. Fig. 2 shows the room temperature Raman spectrum of the ZnO star-shaped nanostructures ranging from 200 to 1400 cm 1. ZnO is wurtzite structure that belongs to the space group C6 mm [4,5]. Five Raman shift modes belong to group C6 mm are observed in Fig. 2. Raman shift of 383 cm 1 were assigned to A1(one)(TO) mode. Peak located at 436 cm 1 is due to E2 (high frequency) mode. The broadened Raman shift from 1039 to 1184 cm 1 is generally assigned to be the second-order vibrational mode [5]. Peaks located at 330 cm 1 and 992 cm 1 are known to be the vibration modes due to the multiple-phonon scattering processes [6], and the strong multiple-phonon scattering indicated the quantum confinement effects in our samples due to the small tips. Among these Raman shift
Fig. 1. TEM of a typical single star morphology of the samples. Inset shows the SAED pattern corresponding to one arm of the star.
intensity (a.u.)
3. Results and discussions
2000
FWHM 9 cm-1 991.956
1000
1130.13
329.818
383.166 400
800 Raman shift
1200 (cm-1)
Fig. 2. Room temperature Raman spectrum of ZnO nanostars excited by 632.8 nm incident laser.
peaks, the Ehigh mode at 436 cm 1 has the strongest 2 intensity and narrower line width 9 cm 1. The Ehigh 2 mode corresponds to band characteristic of wurtzite phase. The obtained Raman spectra clearly demonstrate that the composed ZnO has hexagonal wurtzite structure and good crystal quality. 3.2. Photoluminescence spectroscopy The three valence bands of wurtzite semiconductors ZnO result in three types of excitons, labeled A, B and C. Lowtemperature and low-excitation intensity photoluminescence measurements can reveal detailed excitonic emissions near the optical band gap of ZnO [7]. To check the optical properties of the products, we made temperature-dependent photoluminescence measurements in the range 78–293 K, as shown in Fig. 3. The inset figure shows the wide range normalized photoluminescence spectrum measured at 78 and 293 K. The photoluminescence spectrum at 78 K exhibits five emission peaks distributing from 3.342 to 3.1 eV with invisible green band emission. Actually, all the obtained photoluminescence spectra exhibit predominant UV emission and rarely observe green band visible emission in the whole examined temperature range. The almost negligible green band photoluminescence spectra reveal that the concentration of oxygen vacancies or zinc interstices are very low in our sample. The dominant excitonic emission lines at 3.342 and 3.304 eV in the spectrum are labeled as I9 and FXA-1LO, and the line widths of them are as narrow as 10 and 20 meV, respectively. Generally, the exciton peak tends to be narrower due to the reduction of the impurity scattering in the crystal with decreasing temperature. The photoluminescence band at around 3.232, 3.161 and 3.1 eV denoted by FXA-2LO, FXA-3LO, FXA-4LO is a 2-LO phonon, 3-LO phonon and 4-LO phonon-assisted radiative
ARTICLE IN PRESS C. Li et al. / Journal of Luminescence 122–123 (2007) 415–417 FXA-1LO
3.34
78 K 293 K
I9 450
500
550
emission energy (eV)
400
intensity (a.u.)
I9
3.32 FXA-2LO 600
FXA-3LO
78 K 95 K 125 K 140 K 160 K 185 K 200 K
3.28 3.26 3.24 3.22 3.20
230 K
3.18 3.16
274 K
FXA-1LO FXA-2LO FXA-3LO
3.30
242 K
360
417
3.14 380
400 wavelength (nm)
420
Fig. 3. Temperature-dependent photoluminescence spectra of star-shaped ZnO nanostructures. The inset shows the wide range normalized photoluminescence spectra measured at 78 and 293 K.
recombination of the free exciton A, respectively, because the distance between these lines is the precise energy of the longitudinal optical phonon in ZnO (70–72 meV). Fig. 3 shows that the photoluminescence intensity of the near band emissions decreases rapidly with the increase of temperature. The luminescence line at 3.342 eV attributed to the bound exciton quenches more quickly and disappears above 140 K. This is induced by the bound-exciton thermally ionized due to the small binding energy of the excitons bound to neutral donors, so we can estimate the binding energy or thermal activation energy of the donor bound-exciton is approximately 19 meV, consistent with previous reported value [8]. The variations of exciton emission peak position with temperature can be seen in Figs. 3 and 4. All the positions of exciton emissions assigned above shift to lower energy with increasing temperature due to gap shrinkage effect. The room temperature (293 K) photoluminescence shows a dominant emission from 1-LO phonon-assisted free exciton transition at around 3.227 eV. The gap shrinkage effect in semiconductors is generally caused by the cumulative effects of thermal lattice expansion, electron–phonon interaction and localization of charge carriers due to defects [2]. 4. Conclusions The properties of the excitonic luminescence for starshaped ZnO nanocrystalline are investigated by using the dependence of photoluminescence spectra on temperature. It can be seen clearly that the evolution of emission
50
100
150 200 temperature (K)
250
300
Fig. 4. Peak position of exciton related emission as a function of temperature.
frequency shift, line width, intensity of the donor-bound exciton and multi-order phonon assistant FXA luminescence. All the obtained photoluminescence spectra showed a remarkable band-edge transition and rarely observed green band visible emission. The present results suggest that the wet chemical method is one effective way to fabricate high crystal quality and optical quality ZnO nanostructures. Acknowledgements L. Guo thanks the financial support of National Natural Science Foundation of China under Grant no. 20373004 and Program for New Century Excellent Talents in University (NCET) as well as by Engineering Research Institute, Peking University (ERIPKU). References [1] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [2] C.L. Yang, J.N. Wang, W.K. Ge, L. Guo, S.H. Yang, D.Z. Shen, J. Appl. Phys. 90 (2001) 4489. [3] Y.Z. Lv, C.P. Li, L. Guo, Q.X. Wang, R.M. Wang, H.B. Xu, S.H. Yang, X. AI, J.P. Zhang, Appl. Phys. Lett. 87 (2005) 163103. [4] J.M. Calleja, M. Cardona, Phys. Rev. B 16 (1977) 3753. [5] B.J. Chen, X.W. Sun, C.X. Xu, B.K. Tay, Physica E 21 (2004) 103. [6] V.V. Ursaki, I.M. Tiginyanu, V.V. Zalamai, E.V. Rusu, G.A. Emelchenko, V.M. Masalov, E.N. Samarov, Phys. Rev. B 70 (2004) 155204. [7] D.W. Hamby, D.A. Lucca, M.J. Klopfstein, G. Cantwell, J. Appl. Phys. 93 (2003) 3124. [8] X.T. Zhang, Y.C. Liu, Z.Z. Zhi, J.Y. Zhang, Y.M. Lu, D.Z. Shen, W. Xu, X.W. Fan, X.G. Kong, J. Lumin. 99 (2002) 149.
Journal of Luminescence 122–123 (2007) 415–417 www.elsevier.com/locate/jlumin
Raman and excitonic photoluminescence characterizations of ZnO star-shaped nanocrystals Chunping Lia,, Yuzhen Lva, Lin Guoa,, Huibin Xua, Xicheng Aib, Jianping Zhangb a
School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, China b Institute of Chemistry, Chinese Academy of Science, Beijing 100080, China Available online 15 March 2006
Abstract Optical properties of ZnO star-shaped nanostructures grown by wet chemical solution method were investigated by using Raman scattering and temperature-dependent photoluminescence. High intensity of Raman mode E high with narrow FWHM of 9 cm 1 2 1 appearing at 436 cm indicates the good quality of ZnO wurtzitic structure. Temperature-dependent ultraviolet photoluminescence was studied over the temperature range from 78 to 293 K. At low-temperatures and low-excitation intensities, the dominant spontaneous emissions are due to radiative recombination of excitons bound to donors and one longitudinal optical phonon assistant free exciton A. Finally, only the first-order longitudinal–optical phonon replica of the A free exciton recombination was observed at room temperature. The exciton emission behaviour, including excitonic energy, intensity and line width dependents on the temperature and excitation intensities are discussed. The deep-level emission band is barely observable both at room temperature and at cryogenic temperature measurements. r 2006 Elsevier B.V. All rights reserved. PACS: 78.55 Keywords: ZnO; Temperature-dependent photoluminescence; Raman; Exciton; Phonon
1. Introduction Wurtzitic ZnO is a wide-band-gap and high excitonic bound energy semiconductor, which is a good candidate for optoelectronics in the UV and blue spectral ranges [1,2]. The quality of the ZnO crystal nanostructure is crucial for optoelectronic device fabrication and their applications. Raman and photoluminescence are sensitive and nondestructive techniques for the detection of impurities and defects, thus have been acted as suitable tools to determine the crystalline quality as well as the energy band fine structures of the materials. In the recent study, we synthesized high-quality singlecrystalline ZnO star-shaped nanostructures by a simple and soft solution route at low temperature and with a short reaction time. One remarkable feature of the products is Corresponding authors. Tel./fax: +86 10 82338162.
E-mail addresses: [email protected] (C. Li), [email protected] (L. Guo). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.173
the low optical pump stimulated emission threshold (318 kW/cm2) obtained at room temperature, and the narrowed stimulated emission FWHM is less than 3 nm. To clearly understand the photoluminescence characteristics of the nanomaterials, it is necessary to study the fine energy band structure and the excitonic emissions, the variations of which are dependent on the temperature and excitation intensities. 2. Experiments The morphology of the nanostars was characterized by TEM and the crystal structure was characterized by Raman spectrum excited by a He–Ne laser 632.8 nm line. The Raman signals were detected by a microlaser Raman spectrometer made in France (LabRam HR800). Temperature-dependent photoluminescence measurements were carried out by the tripli-frequency of Nd: yttrium– aluminum–garnet laser (YAG Tempest 300 laser: 355 nm, 5 ns pulse width, 10 Hz pulse repetition rate). The samples
ARTICLE IN PRESS 416
C. Li et al. / Journal of Luminescence 122–123 (2007) 415–417
were mounted in an optical cryostat where the temperature could be varied from 78 K to room temperature. 3000
436.126
3.1. Morphologies and structures A typical single star morphology of the samples is shown in Fig. 1. Inset shows the SAED pattern corresponding to one arm. The TEM image in Fig. 1 reveals that the ZnO entities have a micron size and a star shape with six symmetric arms extending radially from the center. Each arm of the ZnO microstars has a sharp tip of several nanometers with a length of around 0.52 mm tapers. The angle between two neighboring arms is about 601. Details of the synthesis of ZnO nanostar were described elsewhere [3]. Fig. 2 shows the room temperature Raman spectrum of the ZnO star-shaped nanostructures ranging from 200 to 1400 cm 1. ZnO is wurtzite structure that belongs to the space group C6 mm [4,5]. Five Raman shift modes belong to group C6 mm are observed in Fig. 2. Raman shift of 383 cm 1 were assigned to A1(one)(TO) mode. Peak located at 436 cm 1 is due to E2 (high frequency) mode. The broadened Raman shift from 1039 to 1184 cm 1 is generally assigned to be the second-order vibrational mode [5]. Peaks located at 330 cm 1 and 992 cm 1 are known to be the vibration modes due to the multiple-phonon scattering processes [6], and the strong multiple-phonon scattering indicated the quantum confinement effects in our samples due to the small tips. Among these Raman shift
Fig. 1. TEM of a typical single star morphology of the samples. Inset shows the SAED pattern corresponding to one arm of the star.
intensity (a.u.)
3. Results and discussions
2000
FWHM 9 cm-1 991.956
1000
1130.13
329.818
383.166 400
800 Raman shift
1200 (cm-1)
Fig. 2. Room temperature Raman spectrum of ZnO nanostars excited by 632.8 nm incident laser.
peaks, the Ehigh mode at 436 cm 1 has the strongest 2 intensity and narrower line width 9 cm 1. The Ehigh 2 mode corresponds to band characteristic of wurtzite phase. The obtained Raman spectra clearly demonstrate that the composed ZnO has hexagonal wurtzite structure and good crystal quality. 3.2. Photoluminescence spectroscopy The three valence bands of wurtzite semiconductors ZnO result in three types of excitons, labeled A, B and C. Lowtemperature and low-excitation intensity photoluminescence measurements can reveal detailed excitonic emissions near the optical band gap of ZnO [7]. To check the optical properties of the products, we made temperature-dependent photoluminescence measurements in the range 78–293 K, as shown in Fig. 3. The inset figure shows the wide range normalized photoluminescence spectrum measured at 78 and 293 K. The photoluminescence spectrum at 78 K exhibits five emission peaks distributing from 3.342 to 3.1 eV with invisible green band emission. Actually, all the obtained photoluminescence spectra exhibit predominant UV emission and rarely observe green band visible emission in the whole examined temperature range. The almost negligible green band photoluminescence spectra reveal that the concentration of oxygen vacancies or zinc interstices are very low in our sample. The dominant excitonic emission lines at 3.342 and 3.304 eV in the spectrum are labeled as I9 and FXA-1LO, and the line widths of them are as narrow as 10 and 20 meV, respectively. Generally, the exciton peak tends to be narrower due to the reduction of the impurity scattering in the crystal with decreasing temperature. The photoluminescence band at around 3.232, 3.161 and 3.1 eV denoted by FXA-2LO, FXA-3LO, FXA-4LO is a 2-LO phonon, 3-LO phonon and 4-LO phonon-assisted radiative
ARTICLE IN PRESS C. Li et al. / Journal of Luminescence 122–123 (2007) 415–417 FXA-1LO
3.34
78 K 293 K
I9 450
500
550
emission energy (eV)
400
intensity (a.u.)
I9
3.32 FXA-2LO 600
FXA-3LO
78 K 95 K 125 K 140 K 160 K 185 K 200 K
3.28 3.26 3.24 3.22 3.20
230 K
3.18 3.16
274 K
FXA-1LO FXA-2LO FXA-3LO
3.30
242 K
360
417
3.14 380
400 wavelength (nm)
420
Fig. 3. Temperature-dependent photoluminescence spectra of star-shaped ZnO nanostructures. The inset shows the wide range normalized photoluminescence spectra measured at 78 and 293 K.
recombination of the free exciton A, respectively, because the distance between these lines is the precise energy of the longitudinal optical phonon in ZnO (70–72 meV). Fig. 3 shows that the photoluminescence intensity of the near band emissions decreases rapidly with the increase of temperature. The luminescence line at 3.342 eV attributed to the bound exciton quenches more quickly and disappears above 140 K. This is induced by the bound-exciton thermally ionized due to the small binding energy of the excitons bound to neutral donors, so we can estimate the binding energy or thermal activation energy of the donor bound-exciton is approximately 19 meV, consistent with previous reported value [8]. The variations of exciton emission peak position with temperature can be seen in Figs. 3 and 4. All the positions of exciton emissions assigned above shift to lower energy with increasing temperature due to gap shrinkage effect. The room temperature (293 K) photoluminescence shows a dominant emission from 1-LO phonon-assisted free exciton transition at around 3.227 eV. The gap shrinkage effect in semiconductors is generally caused by the cumulative effects of thermal lattice expansion, electron–phonon interaction and localization of charge carriers due to defects [2]. 4. Conclusions The properties of the excitonic luminescence for starshaped ZnO nanocrystalline are investigated by using the dependence of photoluminescence spectra on temperature. It can be seen clearly that the evolution of emission
50
100
150 200 temperature (K)
250
300
Fig. 4. Peak position of exciton related emission as a function of temperature.
frequency shift, line width, intensity of the donor-bound exciton and multi-order phonon assistant FXA luminescence. All the obtained photoluminescence spectra showed a remarkable band-edge transition and rarely observed green band visible emission. The present results suggest that the wet chemical method is one effective way to fabricate high crystal quality and optical quality ZnO nanostructures. Acknowledgements L. Guo thanks the financial support of National Natural Science Foundation of China under Grant no. 20373004 and Program for New Century Excellent Talents in University (NCET) as well as by Engineering Research Institute, Peking University (ERIPKU). References [1] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [2] C.L. Yang, J.N. Wang, W.K. Ge, L. Guo, S.H. Yang, D.Z. Shen, J. Appl. Phys. 90 (2001) 4489. [3] Y.Z. Lv, C.P. Li, L. Guo, Q.X. Wang, R.M. Wang, H.B. Xu, S.H. Yang, X. AI, J.P. Zhang, Appl. Phys. Lett. 87 (2005) 163103. [4] J.M. Calleja, M. Cardona, Phys. Rev. B 16 (1977) 3753. [5] B.J. Chen, X.W. Sun, C.X. Xu, B.K. Tay, Physica E 21 (2004) 103. [6] V.V. Ursaki, I.M. Tiginyanu, V.V. Zalamai, E.V. Rusu, G.A. Emelchenko, V.M. Masalov, E.N. Samarov, Phys. Rev. B 70 (2004) 155204. [7] D.W. Hamby, D.A. Lucca, M.J. Klopfstein, G. Cantwell, J. Appl. Phys. 93 (2003) 3124. [8] X.T. Zhang, Y.C. Liu, Z.Z. Zhi, J.Y. Zhang, Y.M. Lu, D.Z. Shen, W. Xu, X.W. Fan, X.G. Kong, J. Lumin. 99 (2002) 149.