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Fabrication and photoluminescence properties of highly ordered ZnS nanowire arrays embedded in anodic alumina membrane
Physics Letters A 372 (2008) 273–276 www.elsevier.com/locate/pla
Fabrication and photoluminescence properties of highly ordered ZnS nanowire arrays embedded in anodic alumina membrane Ming Chang, Xue Li Cao, Xi-Jin Xu ∗ , Lide Zhang Key Laboratory of Materials Physics, Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, PR China Received 9 April 2007; received in revised form 25 June 2007; accepted 18 July 2007 Available online 21 July 2007 Communicated by R. Wu
Abstract ZnS nanowire arrays embedded in anodic alumina membranes (AAM) were fabricated from an electrolyte containing ZnCl2 and elemental S in dimethylsulfoxide. Photoluminescence (PL) measurements show a broadband with three peaks centered at about 353, 425, and 520 nm that are attributed to vacancies or interstitial, sulfur vacancy, and point defects respectively. © 2007 Elsevier B.V. All rights reserved.
1. Introduction With the development of electric and optoelectronic devices such as field emission displays and data storage, high density and well-ordered arrays of nanowires are needed. As we all known, AAM produced by electrochemical oxidization of aluminum contains hexagonally ordered porous structure with a uniform size [1]. The diameter of the pores can be changed from 15 to 250 nm, while channel density is in the range of 1010 –1012 cm−2 . The unique structure features and its thermal and chemical stability make AAM an ideal template for the fabrication of different kinds of nanowires [2–15]. Zinc Sulfide (ZnS) is an important semiconductor material with a wide band gap of 3.72 eV for the cubic phase [16] and 3.77 eV for the hexagonal-wurtzite phase [17] at room temperature, and has received much research interest [18–21]. Therefore, much attention has been focused on the synthesis of ZnS nanowires [22–24] and nanoribbons [25] or other novel morphologies [26–30]. However, those methods used to fabricate ZnS nanomaterials need either high temperature or complex process and the nanowires have poor regularity. Here in this Letter, we report a simple and high-yield way to fabricate the ordered ZnS nanowire arrays in AAM by the electrochemical * Corresponding author.
E-mail address: [email protected] (X.-J. Xu). 0375-9601/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2007.07.031
method. By comparison, great highly ordered ZnS nanowire arrays fabricated in this way do not need high temperature, which will be propitious to the practical use. 2. Experimental details High-purity (99.999%) aluminum foil was used to fabricate the ordered array of nanopores. Before anodization, the aluminum was degreased with acetone and then annealed at 450 ◦ C for 4 h in the vacuum of about 5 × 10−5 Torr. The AAM used in this work was fabricated by two-step anodization process proposed by Masuda et al. [31]. The first anodization process was conducted under constant voltage (40 V) in 0.3 M oxalic acid (H2 C2 O4 ) at 10 ◦ C. After 3 h anodization, the sample was immersed in a mixture of 6 wt% H3 PO4 and 1.8 wt% H2 CrO4 at 60 ◦ C for 6 h to remove the alumina layer formed in above step. Then the second anodization process was carried out under the same conditions as the first anodization process for 12 h. The remaining aluminum layer was removed in a saturated CuCl2 solution and the barrier of the AAM was etched with the solution of 5 wt% H3 PO4 at 30 ◦ C for 1 h. In the end, a layer of Au was sputtered onto one side of the membrane to serve as the working electrode in an electrochemical cell. ZnS nanowires were cathodically deposited into the nanopores of AAM at a direct current density of 1.5 mA cm−2 for 60 minutes at 120 ◦ C in a polycarbonate cell fitted with
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graphite counter electrode as the anode and the AAM template with Au substrate as the cathode by immersing the cell in an oil bath. The electrolyte contained 7.5 gL−1 ZnCl2 and 6.1 gL−1 S in DMSO. After the deposition, the AAM templates with ZnS nanowires were rinsed with DMSO at 80 ◦ C followed by ethanol, and then washed with deioned-water several times. The XRD pattern was recorded with a diffractometer (Philips X’pert PRO with CuKα1 radiation, λ = 1.54056 Å). X-ray tubes were operated with voltage of 40 kV and current of 40 mA. The surfaces of AAM containing the nanowires were polished with sandpaper before the XRD experiment in order to remove the surface contamination. The morphologies of the products were observed with fieldemission scanning electron microscopy (FE-SEM, JEOL JSM6700F) and high-resolution transmission electron microscopy (HRTEM, JEOL-2010). For the TEM observation, 5 wt% NaOH solution was used to remove the Al2 O3 template. After removing Al2 O3 template, the ZnS nanowires were dispersed in absolute ethyl alcohol by ultrasonic vibration, and then one droplet of ethanol with ZnS nanowires was dropped onto carbon films covered on the copper grid. The sample for SEM observation was obtained by dissolving the upper part of the Al2 O3 template of sample with 5 wt% NaOH solution. Photoluminescence (PL) spectrum was performed with a HITACHI 850-type visible-ultraviolet spectrophotometer with a Xe lamp as the excitation light source at room temperature. The excited wavelength was 266 nm and a 305 nm filter was used. 3. Results and discussion 3.1. Structural characterization The XRD pattern of the ZnS nanowires embedded in the pores of AAM is shown in Fig. 1. All the peaks match well with Bragg reflection of the standard wurtzite (JCPDS: 75-1534), indicating the generation of ZnS. The TEM image of the ZnS nanowires prepared by dc electrodeposition in the AAM template with a pore size about 50 nm is shown in Fig. 2A. Fig. 2A reveals the typical TEM images of ZnS nanowires with the diameter of 50 nm. The nanowires have high aspect ratio, and their lengths are about several to several tens of micrometers. Fig. 2B shows the SEM image of ZnS nanowires embedded in AAM. The white dots seen from the image are the tops of the nanowires. It can be seen that the filling ratio of AAM is perfect. The inset in the top right of Fig. 2B is the energy-dispersed X-ray spectrometry (EDS) spectrum taken from the nanowires embedded in AAM. Quantitative analysis reveals that the atomic ratio of Zn to S is very close to 1:1 stoichiometrically. The O peak and Al peak are originated from AAM and the atomic ratio of O to Al is close to 3:2 which is identical to that of AAM. Fig. 3A and B show the SEM image and the HRTEM image; the inset in Fig. 3A is the corresponding selected area electron diffraction (SAED). It can be seen from the SAED pattern that the diffraction spots correspond to (110) and (002) diffraction planes of a hexagonal ZnS crystal. The SAED pattern indicates
Fig. 1. XRD pattern of ZnS nanowires embedded in the pores of AAM with the diameter of 50 nm (JPCDS: 75-1534).
that the as-prepared nanowires were hexagonal-wurtzite phase ZnS nanowires. The HRTEM image was taken from the edge side of a ZnS nanowire. The lattice planes of (002) are clearly shown and the interplanar spacing is about 0.31 nm, which corresponds to the {002} plane of the hexagonal system of ZnS. And the growth direction of the ZnS nanowires is along [110]. 3.2. PL spectrum Fig. 4 shows the PL spectrum of ZnS nanowires. The peak shape “a” can be decomposed to three pronounced sub-peaks with maximum located at 353 nm (3.512 eV), 425 nm (2.92 eV) and 520 nm (2.38 eV), respectively. The emission spectrum peak shows that the green light emission band centered at 520 nm is wide, the blue emission centered at 425 nm is stronger. The UV emission with peak at 353 nm has a blue shift by 23.8 nm from the band gap emission of the bulk material. The PL properties of ZnS doped with Cu2+ , Mn2+ have been studied extensively. [32–35] The origin of different PL properties is attributed to the transition from conduction band of ZnS to the different energy level caused by Cu2+ , Mn2+ in the ZnS band gap. As for the ZnS nanowires in our experiment, the emissions centered at 353 nm may be caused by vacancies or interstitial states and the blue emission centered at 425 nm may be attributed to sulfur vacancy. Khosravi et al. [36] have reported blue emission also centered at 425 nm from ZnS quantum particles induced by defect levels due to anion vacancies. The green emission centered at 520 nm may have been related to structure defects such as point defects that could potentially induced deep-level emission. Mitsui et al. [37] reported green emission (527 nm) from ZnS films induced by point defects generated by Ar ion milling. The further work will be done to understand the origins of the emission bands. As for the formation mechanism of the ZnS nanowires from the DMSO solution containing ZnCl2 and elemental S, we have studied it in detail in our previous work [38]. The results indicate that the formation mechanism of ZnS nanowires is that metal Zn cations is reduced and then reacts with elemental S to form nanowires in the nanopores of AAM. The process can
M. Chang et al. / Physics Letters A 372 (2008) 273–276
275
Fig. 2. (A) TEM image of ZnS nanowires. (B) SEM image of ZnS nanowires embedded in AAM. The inset plot is the EDS spectrum of ZnS nanowires.
Fig. 3. (A) The TEM image of a single nanowire, and the inset is the diffraction pattern of the nanowire. (B) The HRTEM image of the nanowire.
tion, the confinement of the nanopores and the Au substrates probably play a role on the formation of the crystalline nucleus due to the chemical reaction coupled to charge transfer for Au. 4. Conclusion
Fig. 4. PL spectrum of ZnS nanowires taken at room temperature excited by a 266 nm UV laser. A strong emission band edge was observed. These three curves (c, d, e) are Gaussian line shape analyses after a Gaussian fit (b).
be expressed be through the following reaction process. Firstly, Zn cations are firstly reduced: Zn2+ + 2e = Zn. then followed by the reaction between S and Zn: Zn + S = ZnS. In addi-
In summary, highly ordered single-crystal ZnS nanowires have been synthesized in AAM by dc electrodeposition from DMSO solution containing ZnCl2 and elemental S. XRD, TEM, SEM, and HRTEM results show that the ZnS nanowires have diameters of 50 nm and several micrometers in length. The nanowires have a wurtzite structure. The mechanism for the preparation of ZnS nanowires is also discussed. PL measurements show the UV emission could be caused by vacancies or interstitial states; the blue emission is attributed to sulfur vacancy and the blue emission is related to structure defects such as point defects. The method applied in the work can be possibly used for the fabrication of the II–VI group semiconductor nanowires.
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M. Chang et al. / Physics Letters A 372 (2008) 273–276
References [1] H. Masuda, K. Fukuda, Science 268 (1995) 1466. [2] Y. Wang, C. Ye, G. Wang, L. Zhang, Y. Liu, Z. Zhao, Appl. Phys. Lett. 82 (2003) 4253. [3] H. Cao, Y. Xu, J. Hong, H. Liu, G. Yin, B. Li, C. Tie, Z. Xu, Adv. Mater. 131 (2001) 393. [4] D.J. Peña, J.K.N. Mbindyo, A.J. Carado, T.E. Mallouk, C.D. Keating, B. Razavi, T.S. Mayer, J. Phys. Chem. B 106 (2002) 7458. [5] Y. Li, G.W. Meng, L.D. Zhang, F. Phillipp, Appl. Phys. Lett. 76 (2000) 2011. [6] M. Zheng, G. Li, X. Zhang, S. Zhang, Y. Lei, L. Zhang, Chem. Mater. 13 (2001) 3859. [7] S.A. Sapp, B.B. Lakshmi, C.R. Martin, Adv. Mater. 11 (1999) 402. [8] K. Miyazaki, S. Kainuma, K. Hisatake, T. Watanabe, N. Fukumuro, Electrochim. Acta 44 (1999) 3713. [9] Y.T. Pang, G.W. Meng, L.D. Zhang, Y. Qin, X.Y. Gao, A.W. Zhao, Q. Fang, Adv. Funct. Mater. 12 (2002) 719. [10] J. Choi, G. Sauer, K. Nielsch, R.B. Wehrsphon, U. Gsöele, Chem. Mater. 15 (2003) 776. [11] M. Tian, J. Wang, J. Synder, J. Kurtz, Y. Liu, P. Schiffer, T.E. Mallouk, M.H.W. Chan, Appl. Phys. Lett. 83 (2003) 1620. [12] X.F. Wang, J. Zhang, H.Z. Shi, Y.W. Wang, G.W. Meng, X.S. Peng, L.D. Zhang, J. Fang, J. Appl. Phys. 89 (2001) 3847. [13] Y. Peng, H. Li Zhang, S.L. Pan, H.L. Li, J. Appl. Phys. 87 (2000) 7405. [14] V.F. Puntes, K.M. Krishnan, Appl. Phys. Lett. 78 (2001) 2187. [15] K. Nielsch, R.B. Wehrspohn, J. Barthel, J. Krischner, S.F. Fischer, H. Kronmüller, T. Schweinböck, D. Weiss, U. Gösele, J. Mag. Mag. Mater. 249 (2002) 234. [16] T.K. Tran, W. Park, W. Tong, M.M. Kyi, B.K. Wangner, C.J. Summers, J. Appl. Phys. 81 (1997) 2803. [17] H.C. Ong, R.P. Hchang, Appl. Phys. Lett. 79 (2001) 3612. [18] A. IshiZumi, C.W. White, Y. Kanemitsu, Appl. Phys. Lett. 84 (2004) 2397. [19] T.K. Tran, W. Park, W. Tong, M.M. Kyi, B.K. Wanger, C.J. Summers, J. Appl. Phys. 81 (1997) 2803.
[20] M. Tan, W. Cai , L. Zhang, Appl. Phys. Lett. 71 (1997) 3697. [21] W.H. Zhang, J.L. Shi, H.R. Chen, Z.L. Hua, D.S. Yan, Chem. Mater. 13 (2001) 648. [22] G.Z. Shen, Y. Bando, J.Q. Hu, D. Golberg, Appl. Phys. Lett. 90 (2007) 123101. [23] D.F. Moore, Y. Ding, Z.L. Wang, J. Am. Chem. Soc. 126 (2004) 14372. [24] G.Z. Shen, Y. Bando, D. Golberg, Appl. Phys. Lett. 88 (2006) 123107. [25] Y. Jiang, W.J. Zhang, J.S. Jie, X.M. Meng, J.A. Zapien, S.T. Lee, Adv. Mater. 18 (2006) 1527. [26] Q. Xiong, G. Chen, J.D. Acord, X. Liu, J.J. Zengel, H.R. Gutierrez, J.M. Redwing, L.C. Lew Yan Voon, B. Lassen, P.C. Eklund, Nano Lett. 4 (2004) 1663. [27] Y. Jiang, X.M. Meng, J. Liu, Z. Xie, C. Sing Lee, S.T. Lee, Adv. Mater. 15 (2002) 323. [28] J.X. Ding, J.A. Zapien, W.W. Chen, Y. Lifshitz, S.T. Lee, X.M. Meng, Appl. Phys. Lett. 85 (2004) 2361. [29] Q. Wu, N. Zheng, Y. Ding, Y. Li, Inorg. Chem. Commun. 5 (2002) 671. [30] Y. Wang, L. Zhang, C. Liang, G. Wang, X. Peng, Chem. Phys. Lett. 357 (2002) 314. [31] H. Masuda, M. Satoh, Jpn. J. Appl. Phys. 35 (1996) 1126. [32] W. Sang, Y. Qian, J. Min, D. Li, L. Wang, W. Shi, Y. Liu, Solid State Commun. 121 (2002) 475. [33] M.F. Bulanyi, B.A. Polezhaev, T.A. Prokofev, Semiconductors 32 (1998) 603. [34] J. Huang, Y. Yang, S. Xue, B. Yang, S. Liu, J. Shen, Appl. Phys. Lett. 70 (1997) 2335. [35] N. Murase, R. Jagannathan, Y. Kanematsu, M. Watanabe, A. Kurita, K. Hirata, T. Yazawa, T. Kushida, J. Phys. Chem. B 103 (1999) 754. [36] A.A. Khosravi, M. Kundu, L. Jatwa, S.K. Deshpande, U.A. Bhagwat, M. Sastry, S.K. Kulkarni, Appl. Phys. Lett. 67 (1995) 2702. [37] T. Mitsui, N. Yamamoto, T. Tadokoro, S. Ohta, J. Appl. Phys. 80 (1996) 6972; T. Mitsui, N. Yamamoto, Jpn. J. Appl. Phys. 37 (1998) 1390. [38] X.J. Xu, G.T. Fei, W.H. Yu, W.W. Wang, L. Chen, L.D. Zhang, Nanotechnology 17 (2006) 426.
Fabrication and photoluminescence properties of highly ordered ZnS nanowire arrays embedded in anodic alumina membrane Ming Chang, Xue Li Cao, Xi-Jin Xu ∗ , Lide Zhang Key Laboratory of Materials Physics, Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, PR China Received 9 April 2007; received in revised form 25 June 2007; accepted 18 July 2007 Available online 21 July 2007 Communicated by R. Wu
Abstract ZnS nanowire arrays embedded in anodic alumina membranes (AAM) were fabricated from an electrolyte containing ZnCl2 and elemental S in dimethylsulfoxide. Photoluminescence (PL) measurements show a broadband with three peaks centered at about 353, 425, and 520 nm that are attributed to vacancies or interstitial, sulfur vacancy, and point defects respectively. © 2007 Elsevier B.V. All rights reserved.
1. Introduction With the development of electric and optoelectronic devices such as field emission displays and data storage, high density and well-ordered arrays of nanowires are needed. As we all known, AAM produced by electrochemical oxidization of aluminum contains hexagonally ordered porous structure with a uniform size [1]. The diameter of the pores can be changed from 15 to 250 nm, while channel density is in the range of 1010 –1012 cm−2 . The unique structure features and its thermal and chemical stability make AAM an ideal template for the fabrication of different kinds of nanowires [2–15]. Zinc Sulfide (ZnS) is an important semiconductor material with a wide band gap of 3.72 eV for the cubic phase [16] and 3.77 eV for the hexagonal-wurtzite phase [17] at room temperature, and has received much research interest [18–21]. Therefore, much attention has been focused on the synthesis of ZnS nanowires [22–24] and nanoribbons [25] or other novel morphologies [26–30]. However, those methods used to fabricate ZnS nanomaterials need either high temperature or complex process and the nanowires have poor regularity. Here in this Letter, we report a simple and high-yield way to fabricate the ordered ZnS nanowire arrays in AAM by the electrochemical * Corresponding author.
E-mail address: [email protected] (X.-J. Xu). 0375-9601/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2007.07.031
method. By comparison, great highly ordered ZnS nanowire arrays fabricated in this way do not need high temperature, which will be propitious to the practical use. 2. Experimental details High-purity (99.999%) aluminum foil was used to fabricate the ordered array of nanopores. Before anodization, the aluminum was degreased with acetone and then annealed at 450 ◦ C for 4 h in the vacuum of about 5 × 10−5 Torr. The AAM used in this work was fabricated by two-step anodization process proposed by Masuda et al. [31]. The first anodization process was conducted under constant voltage (40 V) in 0.3 M oxalic acid (H2 C2 O4 ) at 10 ◦ C. After 3 h anodization, the sample was immersed in a mixture of 6 wt% H3 PO4 and 1.8 wt% H2 CrO4 at 60 ◦ C for 6 h to remove the alumina layer formed in above step. Then the second anodization process was carried out under the same conditions as the first anodization process for 12 h. The remaining aluminum layer was removed in a saturated CuCl2 solution and the barrier of the AAM was etched with the solution of 5 wt% H3 PO4 at 30 ◦ C for 1 h. In the end, a layer of Au was sputtered onto one side of the membrane to serve as the working electrode in an electrochemical cell. ZnS nanowires were cathodically deposited into the nanopores of AAM at a direct current density of 1.5 mA cm−2 for 60 minutes at 120 ◦ C in a polycarbonate cell fitted with
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graphite counter electrode as the anode and the AAM template with Au substrate as the cathode by immersing the cell in an oil bath. The electrolyte contained 7.5 gL−1 ZnCl2 and 6.1 gL−1 S in DMSO. After the deposition, the AAM templates with ZnS nanowires were rinsed with DMSO at 80 ◦ C followed by ethanol, and then washed with deioned-water several times. The XRD pattern was recorded with a diffractometer (Philips X’pert PRO with CuKα1 radiation, λ = 1.54056 Å). X-ray tubes were operated with voltage of 40 kV and current of 40 mA. The surfaces of AAM containing the nanowires were polished with sandpaper before the XRD experiment in order to remove the surface contamination. The morphologies of the products were observed with fieldemission scanning electron microscopy (FE-SEM, JEOL JSM6700F) and high-resolution transmission electron microscopy (HRTEM, JEOL-2010). For the TEM observation, 5 wt% NaOH solution was used to remove the Al2 O3 template. After removing Al2 O3 template, the ZnS nanowires were dispersed in absolute ethyl alcohol by ultrasonic vibration, and then one droplet of ethanol with ZnS nanowires was dropped onto carbon films covered on the copper grid. The sample for SEM observation was obtained by dissolving the upper part of the Al2 O3 template of sample with 5 wt% NaOH solution. Photoluminescence (PL) spectrum was performed with a HITACHI 850-type visible-ultraviolet spectrophotometer with a Xe lamp as the excitation light source at room temperature. The excited wavelength was 266 nm and a 305 nm filter was used. 3. Results and discussion 3.1. Structural characterization The XRD pattern of the ZnS nanowires embedded in the pores of AAM is shown in Fig. 1. All the peaks match well with Bragg reflection of the standard wurtzite (JCPDS: 75-1534), indicating the generation of ZnS. The TEM image of the ZnS nanowires prepared by dc electrodeposition in the AAM template with a pore size about 50 nm is shown in Fig. 2A. Fig. 2A reveals the typical TEM images of ZnS nanowires with the diameter of 50 nm. The nanowires have high aspect ratio, and their lengths are about several to several tens of micrometers. Fig. 2B shows the SEM image of ZnS nanowires embedded in AAM. The white dots seen from the image are the tops of the nanowires. It can be seen that the filling ratio of AAM is perfect. The inset in the top right of Fig. 2B is the energy-dispersed X-ray spectrometry (EDS) spectrum taken from the nanowires embedded in AAM. Quantitative analysis reveals that the atomic ratio of Zn to S is very close to 1:1 stoichiometrically. The O peak and Al peak are originated from AAM and the atomic ratio of O to Al is close to 3:2 which is identical to that of AAM. Fig. 3A and B show the SEM image and the HRTEM image; the inset in Fig. 3A is the corresponding selected area electron diffraction (SAED). It can be seen from the SAED pattern that the diffraction spots correspond to (110) and (002) diffraction planes of a hexagonal ZnS crystal. The SAED pattern indicates
Fig. 1. XRD pattern of ZnS nanowires embedded in the pores of AAM with the diameter of 50 nm (JPCDS: 75-1534).
that the as-prepared nanowires were hexagonal-wurtzite phase ZnS nanowires. The HRTEM image was taken from the edge side of a ZnS nanowire. The lattice planes of (002) are clearly shown and the interplanar spacing is about 0.31 nm, which corresponds to the {002} plane of the hexagonal system of ZnS. And the growth direction of the ZnS nanowires is along [110]. 3.2. PL spectrum Fig. 4 shows the PL spectrum of ZnS nanowires. The peak shape “a” can be decomposed to three pronounced sub-peaks with maximum located at 353 nm (3.512 eV), 425 nm (2.92 eV) and 520 nm (2.38 eV), respectively. The emission spectrum peak shows that the green light emission band centered at 520 nm is wide, the blue emission centered at 425 nm is stronger. The UV emission with peak at 353 nm has a blue shift by 23.8 nm from the band gap emission of the bulk material. The PL properties of ZnS doped with Cu2+ , Mn2+ have been studied extensively. [32–35] The origin of different PL properties is attributed to the transition from conduction band of ZnS to the different energy level caused by Cu2+ , Mn2+ in the ZnS band gap. As for the ZnS nanowires in our experiment, the emissions centered at 353 nm may be caused by vacancies or interstitial states and the blue emission centered at 425 nm may be attributed to sulfur vacancy. Khosravi et al. [36] have reported blue emission also centered at 425 nm from ZnS quantum particles induced by defect levels due to anion vacancies. The green emission centered at 520 nm may have been related to structure defects such as point defects that could potentially induced deep-level emission. Mitsui et al. [37] reported green emission (527 nm) from ZnS films induced by point defects generated by Ar ion milling. The further work will be done to understand the origins of the emission bands. As for the formation mechanism of the ZnS nanowires from the DMSO solution containing ZnCl2 and elemental S, we have studied it in detail in our previous work [38]. The results indicate that the formation mechanism of ZnS nanowires is that metal Zn cations is reduced and then reacts with elemental S to form nanowires in the nanopores of AAM. The process can
M. Chang et al. / Physics Letters A 372 (2008) 273–276
275
Fig. 2. (A) TEM image of ZnS nanowires. (B) SEM image of ZnS nanowires embedded in AAM. The inset plot is the EDS spectrum of ZnS nanowires.
Fig. 3. (A) The TEM image of a single nanowire, and the inset is the diffraction pattern of the nanowire. (B) The HRTEM image of the nanowire.
tion, the confinement of the nanopores and the Au substrates probably play a role on the formation of the crystalline nucleus due to the chemical reaction coupled to charge transfer for Au. 4. Conclusion
Fig. 4. PL spectrum of ZnS nanowires taken at room temperature excited by a 266 nm UV laser. A strong emission band edge was observed. These three curves (c, d, e) are Gaussian line shape analyses after a Gaussian fit (b).
be expressed be through the following reaction process. Firstly, Zn cations are firstly reduced: Zn2+ + 2e = Zn. then followed by the reaction between S and Zn: Zn + S = ZnS. In addi-
In summary, highly ordered single-crystal ZnS nanowires have been synthesized in AAM by dc electrodeposition from DMSO solution containing ZnCl2 and elemental S. XRD, TEM, SEM, and HRTEM results show that the ZnS nanowires have diameters of 50 nm and several micrometers in length. The nanowires have a wurtzite structure. The mechanism for the preparation of ZnS nanowires is also discussed. PL measurements show the UV emission could be caused by vacancies or interstitial states; the blue emission is attributed to sulfur vacancy and the blue emission is related to structure defects such as point defects. The method applied in the work can be possibly used for the fabrication of the II–VI group semiconductor nanowires.
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M. Chang et al. / Physics Letters A 372 (2008) 273–276
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