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Nano-wire pig-tailed ZnO nano-rods synthesized by laser ablation
Thin Solid Films 506 – 507 (2006) 274 – 277 www.elsevier.com/locate/tsf
Nano-wire pig-tailed ZnO nano-rods synthesized by laser ablation T. Okada *, K. Kawashima, Y. Nakata Graduate School of Information Science and Electrical Engineering, Kyushu University, Fukuoka 812-8581, Japan Available online 4 October 2005
Abstract This paper describes the synthesis and the characterization of ZnO nano-rods by laser ablation in a high temperature background gas. Two unique nano-structured ZnO crystals were obtained. One is a nano-wire pig-tailed ZnO nano-rod which has a nano-wire of less than 100 nm in diameter at the hexagonal top of nano-rods. Another is a ZnO nano-cone with a bottom diameter of 500 nm and a height of 2 Am. The unique geometrical shapes may have a great potential in the application to the field emission devices and nano-optics. D 2005 Elsevier B.V. All rights reserved. Keywords: ZnO; Nano-rods; Laser ablation; Nano-particle assisted PLD
1. Introduction Wide-gap semiconductor materials with nano-structures such as nano-particles, nano-rods, nano-wires and so on, have been paid a great interest due to their importance both in scientific researches and in potential technological applications. Zinc oxide (ZnO) is one of such a wide-gap compound II –VI semiconductor. ZnO has the direct band gap of about 3.37 eV at room temperature and its large exciton binding energy of about 60 meV, which is significantly larger than the thermal energy at room temperature (26 meV), can ensure an efficient exciton emission at room temperature under low excitation energy [1]. Thus, ZnO is one of the most promising materials suitable for generating ultraviolet (UV) light. So far, various types of nano-structured ZnO crystals have been synthesized by different approaches, such as chemical vapor deposition [2], physical vapor deposition [3], molecular beam epitaxy [4]. ZnO nano-wires have been synthesized simply by heating Zn powders containing catalyst nano-particles [5], where the vapor – liquid –solid (VLS) mechanism is responsible for the nano-wire growth,
* Corresponding author. E-mail address: [email protected] (T. Okada). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.340
in which a metal or an oxide catalyst is necessary to dissolve feeding source atoms in a molten state initiating the growth of nano-materials. UV stimulated emission at room temperature from optically pumped nano-wires has also been reported [6,7]. The pulsed-laser ablation technique, on the other hand, is also a powerful method and has been widely used for the synthesis of ZnO thin films. But there are only a few efforts to employ this method in synthesis of nano-structured ZnO crystals, such as nano-wires or nano-rods. In the previous letters, we have reported the growth of large quantities of ZnO nano-rods by a newly developed nano-particle-assisted pulsed-laser deposition (NAPLD) without using any catalyst [8,9]. We have demonstrated the stimulated emission in the ZnO nano-rods under an optical pumping and the size control of the nano-rods by controlling the size and the density of the nano-particles in the gas phase [10]. However, only c-axis oriented ZnO nano-rod crystals were obtained by the previous NAPLD. In this paper, we explored the further possibility in obtaining various types of nano-structured ZnO crystals for specific optoelectronic applications. For this purpose, the NAPLD method was extended into the wider region of the process parameters with respect to background gas pressure and temperature. We have successfully synthesized nanowire pig-tailed and cone-shaped ZnO nano-crystals by
T. Okada et al. / Thin Solid Films 506 – 507 (2006) 274 – 277 gas outlet oven
A
laser beam
target
substrate
B
lens
gas inlet
Fig. 1. Schematics of experimental apparatus. The capital letters A and B indicate the positions where the samples were collected or synthesized.
NAPLD in a high temperature and high pressure background gas.
2. Preparation of ZnO nano-crystals The experimental apparatus for the synthesis of ZnO nano-crystals is shown in Fig. 1. It is a laser-ablation system in a high-temperature background gas that has been used to synthesize various types of semiconductor nanowires [11 – 13]. A rotating ZnO sintered target was evaporated by laser-ablation plasma produced by a KrF excimer laser in a quartz furnace. The temperature of the furnace was varied in the range from 600 -C to 1100 -C. The ablation fluence was about 3 J/cm2 at a repetition rate of 20 Hz. Ar gas was slowly flown from the entrance port of the laser beam. After about 1 h of deposition, white deposits were observed on the inner wall of the quartz tube. Deposits were collected by several different positions, indicated in Fig. 1 by the capital alphabetical letters and are referred in the following section. The position A is on the wall of the quartz tube near the outlet of Ar gas and the temperature was almost room temperature there. The deposits at the position A were taken off mechanically from the wall on a proper substrate. The position B is in front of the ZnO target, where the temperature is estimated to be the same as that measured by a thermo-couple at the outer wall of the furnace. At the position B, crystals were grown on sapphire (0001) and (1120) substrates. The crystallinity of the deposits was analyzed by an X-ray diffraction (XRD) pattern and the surface morphology was observed by a scanning electron microscope (SEM). The optical properties were investigated by observing the photoluminescence under the excitation at 355 nm from a frequency tripled Q-switched Nd:YAG laser.
3. Results and discussion Fig. 2 shows a typical SEM image of nano-structured ZnO crystals collected at the position A. They were deposited at an Ar gas pressures of 270 Torr and a furnace temperature of 1000 -C. They have a web-like structure that consisted of ZnO nano-wires with a size of around 50 nm in diameter and several Am in length. Some large crystals in a size of around 200 nm were also observed in the web. They
275
are similar to the previous reports on the formation of nanowires [11 –13]. At the position B, on the other hand, a unique ZnO nanostructured crystals were synthesized on sapphire (0001) substrates. SEM images of the crystals are shown in Fig. 3(a) and (b) which were synthesized at an Ar gas pressure of 260 Torr and 500 Torr at 1000 -C, respectively. The XRD pattern for the crystals in Fig. 3(a) is shown in Fig. 4. In both SEM images, hexagonal nano-rods with a diameter of 500 nm were observed and they have nano-wires of less than 100 nm in diameter at the end of each hexagonal nanorod. The nano-rods grew directly from the substrate surface, but they were declined from the substrate normal. In the XRD pattern in Fig. 4, only the diffraction from the planes of the hexagonal ZnO crystal were observed. When the (1120) sapphire substrate was used instead of (0001) substrates, the ZnO cone-shaped crystals perpendicular to the substrate surface was obtained, as shown in Fig. 5. The similar effect of differently oriented substrates on the ZnO crystal growth has been reported [14]. The ZnO coneshaped and nano-wire pig-tailed crystals are very much interesting for the application to the field emission, because of their unique geometry for the field enhancement. The most interesting observation in Fig. 3 is that the nano-wires grew at the center of the hexagonal top of each nano-rod. The reason why the nano-wires grew on the top of the nano-rods has not been understood, but one possible reason may be as follows. In the present experiment, it took about 30 min for the furnace to be cooled down from 1000 -C to 600– 700 -C, after the laser ablation was stopped. During the cooling time, the species evaporated from the target by the laser ablation remained and the different synthesis conditions from the laser ablation period could be maintained. The detailed investigation is now being carried out to clarify the formation mechanism of the nano-wire pigtailed ZnO nano-rods. Finally the photoluminescence characteristics were examined and the results for the crystals in Fig. 3(a) are summarized in Fig. 6 under different excitation powers. In
500 nm
Fig. 2. SEM image of ZnO crystals collected at the position A.
276
T. Okada et al. / Thin Solid Films 506 – 507 (2006) 274 – 277
1 µm m Fig. 5. SEM image of cone-shaped ZnO crystals on sapphire (1100) substrate at the position A, synthesized at an Ar pressure of 260 Torr and a substrate temperature of 800 -C.
Fig. 3. SEM images of ZnO crystals on sapphire (0001) substrate at position B synthesized at 1000 -C and an Ar pressure of 260 Torr (a) and 500 Torr (b).
these experiments, many crystals were excited simultaneously and the photoluminescence from those crystals were observed. Therefore, it should be noted that the spectra in Fig. 6 are those averaged over many different crystals. The characteristic photoluminescence was observed near the band gap of ZnO crystals and a weak visible emission near 500 nm that attributes to the oxygen vacancy [15] was also observed. In pure Ar background gas, oxygen atoms generated by the laser ablation were not sufficient for full oxidation of Zn atoms. It was also observed that the peak wavelength was slightly shifted towards the longer wavelength, when the excitation power was increased from 0.1 mJ/cm2 to 0.8 mJ/cm2. The stimulated emission occurs at a longer wavelength side of the fluorescence spectrum [9], so it is expected to realize the stimulated emission from the pig-tailed ZnO nano-rods after further optimization of the process parameter and the excitation fluence.
5
4000
(002)
4
(101)
3
2
1
(100) (102)
0 20
Excitation fluence [ J/cm2 ] 0.8
3500 Intensity [arb. units]
Intensity (arb. Units)
(sapphire)
25
30
35
40
45
50
3000 2500
0.6
2000
0.4
1500 0.1 1000 500
(110) 55
2 θ [deg] Fig. 4. XRD pattern of ZnO crystals of Fig. 3(a).
60
0 360
380
400 420 Wavelength [nm]
440
460
Fig. 6. Photoluminescence spectra of pig-tailed ZnO crystals for the crystals in Fig. 3(a) under different excitation fluences.
T. Okada et al. / Thin Solid Films 506 – 507 (2006) 274 – 277
4. Conclusion We have successfully synthesized nano-structured ZnO crystals with unique geometrical shapes. One is a nano-wire pig-tailed ZnO nano-rod which has a nano-wire of less than 100 nm in diameter at the hexagonal top of nano-rods. The other is a ZnO nano-cone with a bottom diameter of 500 nm and a height of 2 Am. The unique geometrical shapes may have a great potential in the application to the field emission devices and nano-optics.
Acknowledgement This work was financially supported in part by Grants-inAid for Scientific Research of JSPF, Nippon Sheet Glass Foundation for Materials Science and Engineering, and Murata Science Foundation. The authors would like to thank Mr. Agung Budi Hartanto for his great contributions in the experiments.
References [1] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Appl. Phys. Lett. 70 (1997) 2230.
277
[2] J.J. Wu, S.C. Liu, Adv. Mater. 14 (2002) 215. [3] Y.C. Kong, D.P. Yu, B. Zhang, W. Fang, S.Q. Feng, Appl. Phys. Lett. 78 (2001) 407. [4] Y.W. Heo, V. Varadarajan, M. Kaufman, K. Kim, D.P. Norton, F. Ren, P.H. Fleming, Appl. Phys. Lett. 81 (2002) 3046. [5] Y.W. Wang, L.D. Zhang, G.Z. Wang, X.S. Peng, Z.Q. Chu, C.H. Liang, J. Cryst. Growth 234 (2002) 171. [6] P. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J. Pham, R. He, H.J. Choi, Adv. Funct. Mater. 12 (2002) 323. [7] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [8] M. Kawakami, B.H. Agung, Y. Nakata, T. Okada, Jpn. J. Appl. Phys. 42 (2003) 33. [9] B.H. Agung, X. Ning, Y. Nakata, T. Okada, Appl. Phys. A 78 (2004) 299. [10] T. Okada, B.H. Agung, Y. Nakata, Appl. Phys. A 78 (in press). [11] Y.F. Zhang, Y.H. Tang, N. Wang, D.P. Yu, C.S. Lee, I. Bello, S.T. Lee, Appl. Phys. Lett. 72 (1998) 1835. [12] Y. Wu, R. Fan, P. Yang, Nano Lett. 2 (2002) 83. [13] Y.H. Tang, T.K. Sham, A. Ju¨rgensen, Y.F. Hu, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 80 (2002) 3709. [14] J.C. Johnson, H. Yan, R.D. Schaller, L.H. Haber, R.J. Saykally, P. Yang, J. Phys. Chem., B 105 (2001) 11387. [15] B.J. Chen, X.W. Sun, C.X. Xu, B.K. Tay, Physica E 21 (2004) 103.
Nano-wire pig-tailed ZnO nano-rods synthesized by laser ablation T. Okada *, K. Kawashima, Y. Nakata Graduate School of Information Science and Electrical Engineering, Kyushu University, Fukuoka 812-8581, Japan Available online 4 October 2005
Abstract This paper describes the synthesis and the characterization of ZnO nano-rods by laser ablation in a high temperature background gas. Two unique nano-structured ZnO crystals were obtained. One is a nano-wire pig-tailed ZnO nano-rod which has a nano-wire of less than 100 nm in diameter at the hexagonal top of nano-rods. Another is a ZnO nano-cone with a bottom diameter of 500 nm and a height of 2 Am. The unique geometrical shapes may have a great potential in the application to the field emission devices and nano-optics. D 2005 Elsevier B.V. All rights reserved. Keywords: ZnO; Nano-rods; Laser ablation; Nano-particle assisted PLD
1. Introduction Wide-gap semiconductor materials with nano-structures such as nano-particles, nano-rods, nano-wires and so on, have been paid a great interest due to their importance both in scientific researches and in potential technological applications. Zinc oxide (ZnO) is one of such a wide-gap compound II –VI semiconductor. ZnO has the direct band gap of about 3.37 eV at room temperature and its large exciton binding energy of about 60 meV, which is significantly larger than the thermal energy at room temperature (26 meV), can ensure an efficient exciton emission at room temperature under low excitation energy [1]. Thus, ZnO is one of the most promising materials suitable for generating ultraviolet (UV) light. So far, various types of nano-structured ZnO crystals have been synthesized by different approaches, such as chemical vapor deposition [2], physical vapor deposition [3], molecular beam epitaxy [4]. ZnO nano-wires have been synthesized simply by heating Zn powders containing catalyst nano-particles [5], where the vapor – liquid –solid (VLS) mechanism is responsible for the nano-wire growth,
* Corresponding author. E-mail address: [email protected] (T. Okada). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.340
in which a metal or an oxide catalyst is necessary to dissolve feeding source atoms in a molten state initiating the growth of nano-materials. UV stimulated emission at room temperature from optically pumped nano-wires has also been reported [6,7]. The pulsed-laser ablation technique, on the other hand, is also a powerful method and has been widely used for the synthesis of ZnO thin films. But there are only a few efforts to employ this method in synthesis of nano-structured ZnO crystals, such as nano-wires or nano-rods. In the previous letters, we have reported the growth of large quantities of ZnO nano-rods by a newly developed nano-particle-assisted pulsed-laser deposition (NAPLD) without using any catalyst [8,9]. We have demonstrated the stimulated emission in the ZnO nano-rods under an optical pumping and the size control of the nano-rods by controlling the size and the density of the nano-particles in the gas phase [10]. However, only c-axis oriented ZnO nano-rod crystals were obtained by the previous NAPLD. In this paper, we explored the further possibility in obtaining various types of nano-structured ZnO crystals for specific optoelectronic applications. For this purpose, the NAPLD method was extended into the wider region of the process parameters with respect to background gas pressure and temperature. We have successfully synthesized nanowire pig-tailed and cone-shaped ZnO nano-crystals by
T. Okada et al. / Thin Solid Films 506 – 507 (2006) 274 – 277 gas outlet oven
A
laser beam
target
substrate
B
lens
gas inlet
Fig. 1. Schematics of experimental apparatus. The capital letters A and B indicate the positions where the samples were collected or synthesized.
NAPLD in a high temperature and high pressure background gas.
2. Preparation of ZnO nano-crystals The experimental apparatus for the synthesis of ZnO nano-crystals is shown in Fig. 1. It is a laser-ablation system in a high-temperature background gas that has been used to synthesize various types of semiconductor nanowires [11 – 13]. A rotating ZnO sintered target was evaporated by laser-ablation plasma produced by a KrF excimer laser in a quartz furnace. The temperature of the furnace was varied in the range from 600 -C to 1100 -C. The ablation fluence was about 3 J/cm2 at a repetition rate of 20 Hz. Ar gas was slowly flown from the entrance port of the laser beam. After about 1 h of deposition, white deposits were observed on the inner wall of the quartz tube. Deposits were collected by several different positions, indicated in Fig. 1 by the capital alphabetical letters and are referred in the following section. The position A is on the wall of the quartz tube near the outlet of Ar gas and the temperature was almost room temperature there. The deposits at the position A were taken off mechanically from the wall on a proper substrate. The position B is in front of the ZnO target, where the temperature is estimated to be the same as that measured by a thermo-couple at the outer wall of the furnace. At the position B, crystals were grown on sapphire (0001) and (1120) substrates. The crystallinity of the deposits was analyzed by an X-ray diffraction (XRD) pattern and the surface morphology was observed by a scanning electron microscope (SEM). The optical properties were investigated by observing the photoluminescence under the excitation at 355 nm from a frequency tripled Q-switched Nd:YAG laser.
3. Results and discussion Fig. 2 shows a typical SEM image of nano-structured ZnO crystals collected at the position A. They were deposited at an Ar gas pressures of 270 Torr and a furnace temperature of 1000 -C. They have a web-like structure that consisted of ZnO nano-wires with a size of around 50 nm in diameter and several Am in length. Some large crystals in a size of around 200 nm were also observed in the web. They
275
are similar to the previous reports on the formation of nanowires [11 –13]. At the position B, on the other hand, a unique ZnO nanostructured crystals were synthesized on sapphire (0001) substrates. SEM images of the crystals are shown in Fig. 3(a) and (b) which were synthesized at an Ar gas pressure of 260 Torr and 500 Torr at 1000 -C, respectively. The XRD pattern for the crystals in Fig. 3(a) is shown in Fig. 4. In both SEM images, hexagonal nano-rods with a diameter of 500 nm were observed and they have nano-wires of less than 100 nm in diameter at the end of each hexagonal nanorod. The nano-rods grew directly from the substrate surface, but they were declined from the substrate normal. In the XRD pattern in Fig. 4, only the diffraction from the planes of the hexagonal ZnO crystal were observed. When the (1120) sapphire substrate was used instead of (0001) substrates, the ZnO cone-shaped crystals perpendicular to the substrate surface was obtained, as shown in Fig. 5. The similar effect of differently oriented substrates on the ZnO crystal growth has been reported [14]. The ZnO coneshaped and nano-wire pig-tailed crystals are very much interesting for the application to the field emission, because of their unique geometry for the field enhancement. The most interesting observation in Fig. 3 is that the nano-wires grew at the center of the hexagonal top of each nano-rod. The reason why the nano-wires grew on the top of the nano-rods has not been understood, but one possible reason may be as follows. In the present experiment, it took about 30 min for the furnace to be cooled down from 1000 -C to 600– 700 -C, after the laser ablation was stopped. During the cooling time, the species evaporated from the target by the laser ablation remained and the different synthesis conditions from the laser ablation period could be maintained. The detailed investigation is now being carried out to clarify the formation mechanism of the nano-wire pigtailed ZnO nano-rods. Finally the photoluminescence characteristics were examined and the results for the crystals in Fig. 3(a) are summarized in Fig. 6 under different excitation powers. In
500 nm
Fig. 2. SEM image of ZnO crystals collected at the position A.
276
T. Okada et al. / Thin Solid Films 506 – 507 (2006) 274 – 277
1 µm m Fig. 5. SEM image of cone-shaped ZnO crystals on sapphire (1100) substrate at the position A, synthesized at an Ar pressure of 260 Torr and a substrate temperature of 800 -C.
Fig. 3. SEM images of ZnO crystals on sapphire (0001) substrate at position B synthesized at 1000 -C and an Ar pressure of 260 Torr (a) and 500 Torr (b).
these experiments, many crystals were excited simultaneously and the photoluminescence from those crystals were observed. Therefore, it should be noted that the spectra in Fig. 6 are those averaged over many different crystals. The characteristic photoluminescence was observed near the band gap of ZnO crystals and a weak visible emission near 500 nm that attributes to the oxygen vacancy [15] was also observed. In pure Ar background gas, oxygen atoms generated by the laser ablation were not sufficient for full oxidation of Zn atoms. It was also observed that the peak wavelength was slightly shifted towards the longer wavelength, when the excitation power was increased from 0.1 mJ/cm2 to 0.8 mJ/cm2. The stimulated emission occurs at a longer wavelength side of the fluorescence spectrum [9], so it is expected to realize the stimulated emission from the pig-tailed ZnO nano-rods after further optimization of the process parameter and the excitation fluence.
5
4000
(002)
4
(101)
3
2
1
(100) (102)
0 20
Excitation fluence [ J/cm2 ] 0.8
3500 Intensity [arb. units]
Intensity (arb. Units)
(sapphire)
25
30
35
40
45
50
3000 2500
0.6
2000
0.4
1500 0.1 1000 500
(110) 55
2 θ [deg] Fig. 4. XRD pattern of ZnO crystals of Fig. 3(a).
60
0 360
380
400 420 Wavelength [nm]
440
460
Fig. 6. Photoluminescence spectra of pig-tailed ZnO crystals for the crystals in Fig. 3(a) under different excitation fluences.
T. Okada et al. / Thin Solid Films 506 – 507 (2006) 274 – 277
4. Conclusion We have successfully synthesized nano-structured ZnO crystals with unique geometrical shapes. One is a nano-wire pig-tailed ZnO nano-rod which has a nano-wire of less than 100 nm in diameter at the hexagonal top of nano-rods. The other is a ZnO nano-cone with a bottom diameter of 500 nm and a height of 2 Am. The unique geometrical shapes may have a great potential in the application to the field emission devices and nano-optics.
Acknowledgement This work was financially supported in part by Grants-inAid for Scientific Research of JSPF, Nippon Sheet Glass Foundation for Materials Science and Engineering, and Murata Science Foundation. The authors would like to thank Mr. Agung Budi Hartanto for his great contributions in the experiments.
References [1] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Appl. Phys. Lett. 70 (1997) 2230.
277
[2] J.J. Wu, S.C. Liu, Adv. Mater. 14 (2002) 215. [3] Y.C. Kong, D.P. Yu, B. Zhang, W. Fang, S.Q. Feng, Appl. Phys. Lett. 78 (2001) 407. [4] Y.W. Heo, V. Varadarajan, M. Kaufman, K. Kim, D.P. Norton, F. Ren, P.H. Fleming, Appl. Phys. Lett. 81 (2002) 3046. [5] Y.W. Wang, L.D. Zhang, G.Z. Wang, X.S. Peng, Z.Q. Chu, C.H. Liang, J. Cryst. Growth 234 (2002) 171. [6] P. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J. Pham, R. He, H.J. Choi, Adv. Funct. Mater. 12 (2002) 323. [7] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [8] M. Kawakami, B.H. Agung, Y. Nakata, T. Okada, Jpn. J. Appl. Phys. 42 (2003) 33. [9] B.H. Agung, X. Ning, Y. Nakata, T. Okada, Appl. Phys. A 78 (2004) 299. [10] T. Okada, B.H. Agung, Y. Nakata, Appl. Phys. A 78 (in press). [11] Y.F. Zhang, Y.H. Tang, N. Wang, D.P. Yu, C.S. Lee, I. Bello, S.T. Lee, Appl. Phys. Lett. 72 (1998) 1835. [12] Y. Wu, R. Fan, P. Yang, Nano Lett. 2 (2002) 83. [13] Y.H. Tang, T.K. Sham, A. Ju¨rgensen, Y.F. Hu, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 80 (2002) 3709. [14] J.C. Johnson, H. Yan, R.D. Schaller, L.H. Haber, R.J. Saykally, P. Yang, J. Phys. Chem., B 105 (2001) 11387. [15] B.J. Chen, X.W. Sun, C.X. Xu, B.K. Tay, Physica E 21 (2004) 103.