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Synthesis of sulfur-doped ZnO nanowires by electrochemical deposition
ARTICLE IN PRESS
Materials Science in Semiconductor Processing 10 (2007) 241–245
Synthesis of sulfur-doped ZnO nanowires by electrochemical deposition X.H. Wang, S. Liu, P. Chang, Y. Tang School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, PR China Available online 9 May 2008
Abstract Sulfur-doped zinc oxide (ZnO) nanowires have been successfully synthesized by an electric field-assisted electrochemical deposition in porous anodized aluminum oxide template at room temperature. X-ray diffraction and the selected area electron diffraction results show that the as-synthesized nanowires are single crystalline and have a highly preferential orientation. Transmission electron microscopy observations indicate that the nanowires are uniform with an average diameter of 70 nm and length up to several tens of micrometers. X-ray photoelectron spectroscopy further reveals the presence of S in the ZnO nanowires. Room-temperature photoluminescence is observed in the doped ZnO nanowires, which exhibits a violet emission and blue emissions besides the typical photoluminescence spectrum of a single crystal ZnO. r 2008 Elsevier Ltd. All rights reserved. PACS: 61.46.Hk; 78.67. n; 81.16.Be Keyword: ZnO nanowires; Electrochemical deposition; Photoluminescence
1. Introduction In the late 1990s, interest in the II–VI semiconductor zinc oxide (ZnO) was renewed when roomtemperature, optically pumped lasing was demonstrated for ZnO thin films [1–5]. Since that time the optical properties of ZnO bulk single crystal, thin films, and nanostructures have been studied extensively. In order to obtain different ZnO nanostructured materials, varieties of methods have been developed. For example, by the high-temperature physical evaporation [6], the microemulsion hydrothermal process [7], the template-induced method [8–10], or the reduction and oxidation of ZnS [11]. Corresponding author.
E-mail address: [email protected] (S. Liu). 1369-8001/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2008.02.003
Although controlling the size of the nanowires can bring about changes in their optical and electronic properties, doping of the nanowires either through in-situ or postprocessing techniques will provide a far more favorable approach to modulate their properties. Efficient impurity incorporation including Mg and transition metals in ZnO has been carried out in order to obtain novel properties and broaden possible applications [12–15]. However, little attention has been paid to S doping in ZnO thin films and nanostructures because of the difference between the low stability of sulfur and the high growth temperature of ZnO. Sulfur has a much smaller electronegativity (2.58) than that of oxygen (3.44), and the atom radius of sulfur (1.09 A˚) is much larger than that of oxygen (0.65 A˚). The difference between S and O makes it possible to
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X.H. Wang et al. / Materials Science in Semiconductor Processing 10 (2007) 241–245
obtain some novel properties of ZnO via S doping. Yoo et al. [16] have reported that the band gap of ZnO could be tuned via S dopant. S impurities are also expected to increase the electrical conductivity of ZnO by supplying excess carriers [17]. In this article, we report the synthesis and luminescence of sulfur-doped ZnO nanowires. Herein, anodized aluminum oxide (AAO) template was used because AAO template synthesis (by electrochemical deposition) is a versatile and particularly simple approach, especially in the preparation of ionic doped compound nanowires. Generally, photoluminescence (PL) spectrum of a single crystal ZnO consists mainly of two bands [18]. The as-synthesized nanowires exhibit broad visible emission. It has been suggested that sulfur substitutes for oxygen within the ZnO lattice, and the enhanced visible emission is due to the commensurate increase in oxygen vacancies [19–21]. This special characteristic suggests that the S-doped ZnO nanomaterial may be used for visible light emitter. 2. Experiment The AAO templates with ordered nanopore were prepared by using a two-step anodization process [22,23]. ZnO:S nanowires were synthesized by an electric field-assisted electrochemical deposition in porous AAO template at room temperature. The device used in the experiment was reported recently by the group, i.e., G. H. Yue et al. [24]. Na2S aqueous solution prepared was 0.0125 mol/L. The consistency Zn(NO3)2 aqueous solution was 0.00125 mol/
L. The cathode connects the aqueous solution of Na2S, and the anode connects the aqueous solution of Zn(NO3)2. The transverse electric field was added with a voltage of 12.5 VAC and a frequency of 50 Hz. In the experiment, the doped ZnO nanowires were obtained at a constant voltage of 20 VDC for 20 min. The current density was about 10 mA/cm2 at first. It decreased gradually to about 10 3 mA/cm2 after 20 min, and then it nearly did not change. For a reference purpose, a sample of undoped ZnO nanowires was prepared under almost the same conditions but with 0.0125 mol/L NaNO3 aqueous solution substituting for Na2S. Structural characterization was performed by means of X-ray diffraction (XRD) using a Rigaku/Max-2400 diffractometer with Cu Ka radiation (wavelength 1.54056 A˚). The bright-field image of nanowires and selected area electron diffraction pattern (SAED) were taken by an EM-400T transmission electron microscope (TEM). The X-ray photoelectron spectrum (XPS) data were collected on a PHI-5702 X-ray photoelectron spectrometer using a monochromatic Al Ka X-ray sources. PL spectrum excited with 325 nm UV light at 295 K were collected with an FLS920T combined fluorescence lifetime and a steady-state spectrophotometer. 3. Results and discussion Fig. 1(a) and (b) show the TEM images of dispersed nanowires with an average diameter of 70 nm obtained by electrochemical deposition in the anodic aluminum oxide templates. The dispersed
Fig. 1. TEM image of the ZnO:S nanowires: (a) and (b) images of the ZnO:S nanowires. The inset is the SAED pattern of as-synthesized products.
ARTICLE IN PRESS X.H. Wang et al. / Materials Science in Semiconductor Processing 10 (2007) 241–245
nanowires were obtained by dissolving AAO templates in 1 mol/L H2SO4 solution for 24 h at 30 1C. The detailed information of the ZnO:S nanowires was studied by SAED. The SAED pattern shown in the inset of Fig. 1(b) suggests that the as-synthesized nanowires are single crystalline with a wurtzite structure. Fig. 2 illustrates the XRD patterns of doped and undoped ZnO nanowires, which revealed that the lower Bragg angle peaks are located at 36.091 and 35.851, respectively. Except for ZnO (1 0 1) orientations, no extra diffraction peaks from S-related secondary phases or impurities were observed. This indicates that both the as-grown products have single phase and (1 0 1) preferred orientation. It is
Fig. 2. XRD patterns of ZnO and ZnO:S nanowires.
243
known that the ionic radii of the substitute S2 (184 pm) is larger than that of O2 (124 pm). Thus, doping with S ions causes a slight shift of about 0.241 of XRD peaks toward the lower diffraction angle. The slight variation of the doped sample as compared to the undoped sample is an indication of incorporation of S ions in the ZnO lattice. The ultra strong intensities relative to the background signal indicate high purity of the resulting products. Fig. 3(a)–(c) show Zn 2p, O 1s, and S2p XPS spectra of ZnO:S nanowires, respectively. The binding energies in all the XPS spectra have been calibrated using C 1s at 284.6 eV as shown in Fig. 3(d). The binding energy of Zn 2p3/2 is located at 1022.2 and 1021.8 eV, showing that the Zn 2p3/2 peak consists of ZnO and ZnS, respectively [25,26]. Figs. 3(b) shows O 1s XPS spectrum, the stronger O 1s peak at 530.2 eV may be attributed to O2 ions in the Zn–O bond [27]. While another at 531.3 eV is usually associated with the loosely bound oxygen (e.g., adsorbed O2, –OH) [28]. Fig. 3(c) shows the S 2p core level peak. The 2p3/2 component is located at about 162.0 eV. This binding energy is smaller than those of sulfur and related compounds, namely, elemental sulfur (164.0 eV), chemisorbed SO2 (163–165.5 eV), sulfite (–SO3) (166.4 eV), and sulfate ( SO4) (168–170 eV) [29]. This implies that the peak can be assigned to that of ZnS [30]. The atomic concentrations of the chemical elements were calculated from the peak areas and indicated 8.4 atomic % S, 37.9 atomic % O. The EDX spectrum was shown in Fig. 4 to further reveal the presence of S in the ZnO nanowires. EDX analysis
Fig. 3. XPS spectra of (a) Zn 2p, (b) O 1 s, (c) S 2p, and (d) C 1s core levels for ZnO:S nanowires.
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band luminescence in the region of 350–650 nm is observed by PL spectra of the as-grown products. Visible emission in the sulfur doped ZnO nanowires can be attributed to the increase in oxygen vacancies caused by the sulfur substitutes for oxygen within the ZnO lattice. In the PL spectrum for the assynthesized ZnO:S nanowires, blue emissions at around 406, 420, and 434 nm were also observed. Acknowledgment This work was supported by the Nature Science Foundation of Gansu Province (Grant no. 3ZS051A25-034). Fig. 4. Room-temperature photoluminescence spectrum for the ZnO:S nanowires.
Reference confirms that these wire-like nanomaterials are indeed composed of Zn, S, and O. The S content of individual nanowire is identified to be in the range of 5–10% by EDX. These results indicate that pure ZnO:S nanocrystals formed. The optical properties of the synthesized samples have been studied by using PL measurements that were performed at room temperature. From Fig. 4, we observed that the near-band-edge (NBE) ultraviolet (UV) peaks at 378 nm (3.2 eV) and 392 nm (3.37 eV). From Fig. 4 we also observed the spectra of visible emissions, showing a violet emission at 456 nm (2.74 eV) and green emissions at 533 nm (2.35 eV) and 507 nm (2.43 eV). It is generally accepted that the visible emissions are attributed to the single ionized oxygen vacancy in the ZnO, and the emission results from the radiative recombination of a photogenerated hole with an electron occupying the oxygen vacancy, while the UV emission can be explained by the NBE emission of the wide band gap ZnO [31,32]. Blue emissions at around 406, 420, and 434 nm were also observed in the PL spectrum for the as-synthesized ZnO:S nanowires. 4. Conclusion To conclude, we have reported the synthesis of ZnO:S nanowires via electric field-assisted electrochemical approach in AAO template at room temperature. TEM, SAED, XRD, and XPS analyses show that the ZnO:S nanowires are single crystals with a wurtzite structure. The as-grown products have an average diameter of 70 nm. Broad-
[1] Bagnall DM, Chen YF, Zhu Z, Yao T, Shen MY, Goto T. Appl Phys Lett 1998;73:1038–40. [2] Tang ZK, Wong GKL, Yu P, Kawasaki M, Ohtomo A, Koinuma H, Segawa Y. Appl Phys Lett 1998;72:3270–2. [3] Zu P, Tang ZK, Wong GKL, Kawasaki M, Ohtomo A, Koinuma H, Segawa Y. Solid State Commun 1997;103:459. [4] Bagnall DM, Chen YF, Zhu Z, Yao T, Koyama S, Shen MY, Goto T. Appl Phys Lett 1997;70:2230–2. [5] Reynolds DC, Look DC, Jogai B. Solid State Commun 1996;99:873. [6] Wang YW, Zhang LD, Wang DZ, Peng XS, Chu ZQ, Liang CHJ. Cryst Growth 2002;234:171–5. [7] Zhang J, Sun LD, Pan HY, Liao CS, Yan CH. New J Chem 2002;26:33–4. [8] Park WI, Kim DH, Jung SW, Yi GC. Appl Phys Lett 2002;80:4232–4. [9] Vayssieres L, Keis K, Hagfeldt A. J Phys Chem B 2001;105:3350–2. [10] Li Y, Meng GW, Zhang LD, Philipp F. Appl Phys Lett 2000;76:2011–3. [11] Hu JQ, Li Q, Wong NB, Lee CS, Lee ST. Chem Mater 2002;14:1216–9. [12] Yoo YZ, Fukumura T, Jin Z, Hasegana K, Kawasaki M, Ahmet P, Chikyow T, Koinuma H. J Appl Phys 2001;90:4246. [13] Ohtomo A, Kawasaki M, Koida T, Masubuchi K, Koinuma H, Sakurai Y, Yoshida Y, Yasuda T, Segawa Y. Appl Phys Lett 1998;72:2466. [14] Choopun S, Vispute RD, Yang W, Sharma RP, Venkatesan T, Shen H. Appl Phys Lett 2002;80:1529. [15] Yoo YZ, Osaka Y, Fukumura T, Jin ZW, Kawasaki M, Koinuma H, Chikyow T, Ahmet P, Setoguchi A, Chichibu SF. Appl Phys Lett 2001;78:616. [16] Yoo YZ, Jin ZW, Chikyow T, Fukumura T, Kawasaki M, Koinuma H. Appl Phys Lett 2002;81:3798. [17] Geng BY, Wang GZ, Jiang Z, Xie T, Sun SH, Meng GW, Zhang LD. Appl Phys Lett 2003;82:4791. [18] Shi WS, Agyeman O, Xu CN. J Appl Phys 2002;91:5640. [19] Bae SY, Seo HW, Park J. J Phys Chem B 2004;108:5206–10. [20] Geng BY, Wang GZ, Jiang Z, Xie T, Sun SH, Meng GW, Zhang LD. Appl Phys Lett 2003;82:4791.
ARTICLE IN PRESS X.H. Wang et al. / Materials Science in Semiconductor Processing 10 (2007) 241–245 [21] Shen G, Cho JH, Yoo JK, Yi GC, Lee CJ. J Phys Chem B 2005;109:5491–6. [22] Liu CH, Zapien JA, Yao Y, Meng XM, Lee CS, Fan SS, Lifshitz Y, Lee ST. Adv Mater (Weinheim, Germany) 2003;15:838. [23] Masuda H, Satoh M. Jpn J Appl Phys, part 2 1996;35: L126. [24] Yue GH, Yan PX, Liu JZ, Fan XY, Zhuo RF. Appl Phys Lett 2005;87:262505. [25] Laajalehto K, Kartio I, Nowak P. Appl Surf Sci 1994;81:11. [26] Peled A, Dragnea B, Alexandrescu R, Andrei A. Appl Surf Sci 1995;86:538.
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[27] Peng WQ, Cong GW, Qu SC, Wang ZG. Nanotechnology 2005;16:1469. [28] Chen M, Wang X, Yu YH, Pei ZL, Bai XD, Sun C, Huang RF, Wen LS. Appl Surf Sci 2000;158:134. [29] Rodriguez JA, Jirsak T, Chaturvedi S, Kuhn M. Surf Sci 1999;442:400. [30] Vdovenkova T, Vdovenkov A, Tornqvist R. Thin Solid Films 1999;343-344:332. [31] Ryu YR, Zhu S, Budai JD, Chandrasekhar HR, Miceli PF, Miceli PF, White HW. J Appl Phys 2000;88:201. [32] Chen Y, Hong K, Ko HJ, Nakajima M, Yao T, Segawa Y. Appl Phys Lett 2000:76.
Materials Science in Semiconductor Processing 10 (2007) 241–245
Synthesis of sulfur-doped ZnO nanowires by electrochemical deposition X.H. Wang, S. Liu, P. Chang, Y. Tang School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, PR China Available online 9 May 2008
Abstract Sulfur-doped zinc oxide (ZnO) nanowires have been successfully synthesized by an electric field-assisted electrochemical deposition in porous anodized aluminum oxide template at room temperature. X-ray diffraction and the selected area electron diffraction results show that the as-synthesized nanowires are single crystalline and have a highly preferential orientation. Transmission electron microscopy observations indicate that the nanowires are uniform with an average diameter of 70 nm and length up to several tens of micrometers. X-ray photoelectron spectroscopy further reveals the presence of S in the ZnO nanowires. Room-temperature photoluminescence is observed in the doped ZnO nanowires, which exhibits a violet emission and blue emissions besides the typical photoluminescence spectrum of a single crystal ZnO. r 2008 Elsevier Ltd. All rights reserved. PACS: 61.46.Hk; 78.67. n; 81.16.Be Keyword: ZnO nanowires; Electrochemical deposition; Photoluminescence
1. Introduction In the late 1990s, interest in the II–VI semiconductor zinc oxide (ZnO) was renewed when roomtemperature, optically pumped lasing was demonstrated for ZnO thin films [1–5]. Since that time the optical properties of ZnO bulk single crystal, thin films, and nanostructures have been studied extensively. In order to obtain different ZnO nanostructured materials, varieties of methods have been developed. For example, by the high-temperature physical evaporation [6], the microemulsion hydrothermal process [7], the template-induced method [8–10], or the reduction and oxidation of ZnS [11]. Corresponding author.
E-mail address: [email protected] (S. Liu). 1369-8001/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2008.02.003
Although controlling the size of the nanowires can bring about changes in their optical and electronic properties, doping of the nanowires either through in-situ or postprocessing techniques will provide a far more favorable approach to modulate their properties. Efficient impurity incorporation including Mg and transition metals in ZnO has been carried out in order to obtain novel properties and broaden possible applications [12–15]. However, little attention has been paid to S doping in ZnO thin films and nanostructures because of the difference between the low stability of sulfur and the high growth temperature of ZnO. Sulfur has a much smaller electronegativity (2.58) than that of oxygen (3.44), and the atom radius of sulfur (1.09 A˚) is much larger than that of oxygen (0.65 A˚). The difference between S and O makes it possible to
ARTICLE IN PRESS 242
X.H. Wang et al. / Materials Science in Semiconductor Processing 10 (2007) 241–245
obtain some novel properties of ZnO via S doping. Yoo et al. [16] have reported that the band gap of ZnO could be tuned via S dopant. S impurities are also expected to increase the electrical conductivity of ZnO by supplying excess carriers [17]. In this article, we report the synthesis and luminescence of sulfur-doped ZnO nanowires. Herein, anodized aluminum oxide (AAO) template was used because AAO template synthesis (by electrochemical deposition) is a versatile and particularly simple approach, especially in the preparation of ionic doped compound nanowires. Generally, photoluminescence (PL) spectrum of a single crystal ZnO consists mainly of two bands [18]. The as-synthesized nanowires exhibit broad visible emission. It has been suggested that sulfur substitutes for oxygen within the ZnO lattice, and the enhanced visible emission is due to the commensurate increase in oxygen vacancies [19–21]. This special characteristic suggests that the S-doped ZnO nanomaterial may be used for visible light emitter. 2. Experiment The AAO templates with ordered nanopore were prepared by using a two-step anodization process [22,23]. ZnO:S nanowires were synthesized by an electric field-assisted electrochemical deposition in porous AAO template at room temperature. The device used in the experiment was reported recently by the group, i.e., G. H. Yue et al. [24]. Na2S aqueous solution prepared was 0.0125 mol/L. The consistency Zn(NO3)2 aqueous solution was 0.00125 mol/
L. The cathode connects the aqueous solution of Na2S, and the anode connects the aqueous solution of Zn(NO3)2. The transverse electric field was added with a voltage of 12.5 VAC and a frequency of 50 Hz. In the experiment, the doped ZnO nanowires were obtained at a constant voltage of 20 VDC for 20 min. The current density was about 10 mA/cm2 at first. It decreased gradually to about 10 3 mA/cm2 after 20 min, and then it nearly did not change. For a reference purpose, a sample of undoped ZnO nanowires was prepared under almost the same conditions but with 0.0125 mol/L NaNO3 aqueous solution substituting for Na2S. Structural characterization was performed by means of X-ray diffraction (XRD) using a Rigaku/Max-2400 diffractometer with Cu Ka radiation (wavelength 1.54056 A˚). The bright-field image of nanowires and selected area electron diffraction pattern (SAED) were taken by an EM-400T transmission electron microscope (TEM). The X-ray photoelectron spectrum (XPS) data were collected on a PHI-5702 X-ray photoelectron spectrometer using a monochromatic Al Ka X-ray sources. PL spectrum excited with 325 nm UV light at 295 K were collected with an FLS920T combined fluorescence lifetime and a steady-state spectrophotometer. 3. Results and discussion Fig. 1(a) and (b) show the TEM images of dispersed nanowires with an average diameter of 70 nm obtained by electrochemical deposition in the anodic aluminum oxide templates. The dispersed
Fig. 1. TEM image of the ZnO:S nanowires: (a) and (b) images of the ZnO:S nanowires. The inset is the SAED pattern of as-synthesized products.
ARTICLE IN PRESS X.H. Wang et al. / Materials Science in Semiconductor Processing 10 (2007) 241–245
nanowires were obtained by dissolving AAO templates in 1 mol/L H2SO4 solution for 24 h at 30 1C. The detailed information of the ZnO:S nanowires was studied by SAED. The SAED pattern shown in the inset of Fig. 1(b) suggests that the as-synthesized nanowires are single crystalline with a wurtzite structure. Fig. 2 illustrates the XRD patterns of doped and undoped ZnO nanowires, which revealed that the lower Bragg angle peaks are located at 36.091 and 35.851, respectively. Except for ZnO (1 0 1) orientations, no extra diffraction peaks from S-related secondary phases or impurities were observed. This indicates that both the as-grown products have single phase and (1 0 1) preferred orientation. It is
Fig. 2. XRD patterns of ZnO and ZnO:S nanowires.
243
known that the ionic radii of the substitute S2 (184 pm) is larger than that of O2 (124 pm). Thus, doping with S ions causes a slight shift of about 0.241 of XRD peaks toward the lower diffraction angle. The slight variation of the doped sample as compared to the undoped sample is an indication of incorporation of S ions in the ZnO lattice. The ultra strong intensities relative to the background signal indicate high purity of the resulting products. Fig. 3(a)–(c) show Zn 2p, O 1s, and S2p XPS spectra of ZnO:S nanowires, respectively. The binding energies in all the XPS spectra have been calibrated using C 1s at 284.6 eV as shown in Fig. 3(d). The binding energy of Zn 2p3/2 is located at 1022.2 and 1021.8 eV, showing that the Zn 2p3/2 peak consists of ZnO and ZnS, respectively [25,26]. Figs. 3(b) shows O 1s XPS spectrum, the stronger O 1s peak at 530.2 eV may be attributed to O2 ions in the Zn–O bond [27]. While another at 531.3 eV is usually associated with the loosely bound oxygen (e.g., adsorbed O2, –OH) [28]. Fig. 3(c) shows the S 2p core level peak. The 2p3/2 component is located at about 162.0 eV. This binding energy is smaller than those of sulfur and related compounds, namely, elemental sulfur (164.0 eV), chemisorbed SO2 (163–165.5 eV), sulfite (–SO3) (166.4 eV), and sulfate ( SO4) (168–170 eV) [29]. This implies that the peak can be assigned to that of ZnS [30]. The atomic concentrations of the chemical elements were calculated from the peak areas and indicated 8.4 atomic % S, 37.9 atomic % O. The EDX spectrum was shown in Fig. 4 to further reveal the presence of S in the ZnO nanowires. EDX analysis
Fig. 3. XPS spectra of (a) Zn 2p, (b) O 1 s, (c) S 2p, and (d) C 1s core levels for ZnO:S nanowires.
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band luminescence in the region of 350–650 nm is observed by PL spectra of the as-grown products. Visible emission in the sulfur doped ZnO nanowires can be attributed to the increase in oxygen vacancies caused by the sulfur substitutes for oxygen within the ZnO lattice. In the PL spectrum for the assynthesized ZnO:S nanowires, blue emissions at around 406, 420, and 434 nm were also observed. Acknowledgment This work was supported by the Nature Science Foundation of Gansu Province (Grant no. 3ZS051A25-034). Fig. 4. Room-temperature photoluminescence spectrum for the ZnO:S nanowires.
Reference confirms that these wire-like nanomaterials are indeed composed of Zn, S, and O. The S content of individual nanowire is identified to be in the range of 5–10% by EDX. These results indicate that pure ZnO:S nanocrystals formed. The optical properties of the synthesized samples have been studied by using PL measurements that were performed at room temperature. From Fig. 4, we observed that the near-band-edge (NBE) ultraviolet (UV) peaks at 378 nm (3.2 eV) and 392 nm (3.37 eV). From Fig. 4 we also observed the spectra of visible emissions, showing a violet emission at 456 nm (2.74 eV) and green emissions at 533 nm (2.35 eV) and 507 nm (2.43 eV). It is generally accepted that the visible emissions are attributed to the single ionized oxygen vacancy in the ZnO, and the emission results from the radiative recombination of a photogenerated hole with an electron occupying the oxygen vacancy, while the UV emission can be explained by the NBE emission of the wide band gap ZnO [31,32]. Blue emissions at around 406, 420, and 434 nm were also observed in the PL spectrum for the as-synthesized ZnO:S nanowires. 4. Conclusion To conclude, we have reported the synthesis of ZnO:S nanowires via electric field-assisted electrochemical approach in AAO template at room temperature. TEM, SAED, XRD, and XPS analyses show that the ZnO:S nanowires are single crystals with a wurtzite structure. The as-grown products have an average diameter of 70 nm. Broad-
[1] Bagnall DM, Chen YF, Zhu Z, Yao T, Shen MY, Goto T. Appl Phys Lett 1998;73:1038–40. [2] Tang ZK, Wong GKL, Yu P, Kawasaki M, Ohtomo A, Koinuma H, Segawa Y. Appl Phys Lett 1998;72:3270–2. [3] Zu P, Tang ZK, Wong GKL, Kawasaki M, Ohtomo A, Koinuma H, Segawa Y. Solid State Commun 1997;103:459. [4] Bagnall DM, Chen YF, Zhu Z, Yao T, Koyama S, Shen MY, Goto T. Appl Phys Lett 1997;70:2230–2. [5] Reynolds DC, Look DC, Jogai B. Solid State Commun 1996;99:873. [6] Wang YW, Zhang LD, Wang DZ, Peng XS, Chu ZQ, Liang CHJ. Cryst Growth 2002;234:171–5. [7] Zhang J, Sun LD, Pan HY, Liao CS, Yan CH. New J Chem 2002;26:33–4. [8] Park WI, Kim DH, Jung SW, Yi GC. Appl Phys Lett 2002;80:4232–4. [9] Vayssieres L, Keis K, Hagfeldt A. J Phys Chem B 2001;105:3350–2. [10] Li Y, Meng GW, Zhang LD, Philipp F. Appl Phys Lett 2000;76:2011–3. [11] Hu JQ, Li Q, Wong NB, Lee CS, Lee ST. Chem Mater 2002;14:1216–9. [12] Yoo YZ, Fukumura T, Jin Z, Hasegana K, Kawasaki M, Ahmet P, Chikyow T, Koinuma H. J Appl Phys 2001;90:4246. [13] Ohtomo A, Kawasaki M, Koida T, Masubuchi K, Koinuma H, Sakurai Y, Yoshida Y, Yasuda T, Segawa Y. Appl Phys Lett 1998;72:2466. [14] Choopun S, Vispute RD, Yang W, Sharma RP, Venkatesan T, Shen H. Appl Phys Lett 2002;80:1529. [15] Yoo YZ, Osaka Y, Fukumura T, Jin ZW, Kawasaki M, Koinuma H, Chikyow T, Ahmet P, Setoguchi A, Chichibu SF. Appl Phys Lett 2001;78:616. [16] Yoo YZ, Jin ZW, Chikyow T, Fukumura T, Kawasaki M, Koinuma H. Appl Phys Lett 2002;81:3798. [17] Geng BY, Wang GZ, Jiang Z, Xie T, Sun SH, Meng GW, Zhang LD. Appl Phys Lett 2003;82:4791. [18] Shi WS, Agyeman O, Xu CN. J Appl Phys 2002;91:5640. [19] Bae SY, Seo HW, Park J. J Phys Chem B 2004;108:5206–10. [20] Geng BY, Wang GZ, Jiang Z, Xie T, Sun SH, Meng GW, Zhang LD. Appl Phys Lett 2003;82:4791.
ARTICLE IN PRESS X.H. Wang et al. / Materials Science in Semiconductor Processing 10 (2007) 241–245 [21] Shen G, Cho JH, Yoo JK, Yi GC, Lee CJ. J Phys Chem B 2005;109:5491–6. [22] Liu CH, Zapien JA, Yao Y, Meng XM, Lee CS, Fan SS, Lifshitz Y, Lee ST. Adv Mater (Weinheim, Germany) 2003;15:838. [23] Masuda H, Satoh M. Jpn J Appl Phys, part 2 1996;35: L126. [24] Yue GH, Yan PX, Liu JZ, Fan XY, Zhuo RF. Appl Phys Lett 2005;87:262505. [25] Laajalehto K, Kartio I, Nowak P. Appl Surf Sci 1994;81:11. [26] Peled A, Dragnea B, Alexandrescu R, Andrei A. Appl Surf Sci 1995;86:538.
245
[27] Peng WQ, Cong GW, Qu SC, Wang ZG. Nanotechnology 2005;16:1469. [28] Chen M, Wang X, Yu YH, Pei ZL, Bai XD, Sun C, Huang RF, Wen LS. Appl Surf Sci 2000;158:134. [29] Rodriguez JA, Jirsak T, Chaturvedi S, Kuhn M. Surf Sci 1999;442:400. [30] Vdovenkova T, Vdovenkov A, Tornqvist R. Thin Solid Films 1999;343-344:332. [31] Ryu YR, Zhu S, Budai JD, Chandrasekhar HR, Miceli PF, Miceli PF, White HW. J Appl Phys 2000;88:201. [32] Chen Y, Hong K, Ko HJ, Nakajima M, Yao T, Segawa Y. Appl Phys Lett 2000:76.