Materials Letters 63 (2009) 444–446

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Materials Letters

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Structure and photoluminescence of S-doped ZnO nanorod arrays

Xiaohui Zhang a, Xiaoqin Yan a, Jing Zhao a, Zi Qin a, Yue Zhang a,b,⁎a Department of Materials Physics and Chemistry, University of Science and Technology Beijing, 30 Xueyuan Road, Beijing 100083, Chinab State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, 30 Xueyuan Road, Beijing 100083, China

⁎ Corresponding author. Department of Materials Phyof Science and Technology Beijing, 30 Xueyuan Road, Bei62334725; fax: +86 10 62333113.

E-mail address: yuezhang@ustb.edu.cn (Y. Zhang).

0167-577X/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.matlet.2008.11.006

a b s t r a c t

a r t i c l e i n f o

Article history:

Vertically aligned S-doped Received 29 September 2008Accepted 5 November 2008Available online 12 November 2008

Keywords:ZnONanorod arraysNanomaterialsLuminescence

ZnO nanorod arrays have been successfully synthesized by hydrothermalmethod at 90 °C for 2 h. The obtained nanorod is ∼70 nm in diameter and 1.2 μm in length. The XRDpattern and the Raman spectra indicate that the S-doped nanorod arrays are orientated at [001] and aresingle crystals with hexagonal wurtzite structure. The photoluminescence (PL) spectra show that S-dopedZnO nanorod arrays exhibit a relative weak ultraviolet (UV) emission, a violet emission and a strong greenemission. The effects of S-doping on the structure and photoluminescence of ZnO nanorod arrays arediscussed in detail.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Since the discovery of carbon nanotubes (CNTs) in 1991[1], onedimensional (1D) nanomaterials have attracted a great deal ofattention because of their numerous potential applications [2–5]. Asan n-type semiconductor with the wide band gap (3.37 eV) and largeexciton binding energy (60 meV), ZnO plays an important role in the1D nanomaterials field. The recent researches of 1D ZnO have focusedon its applications on optical waveguides, piezoelectric transducers,surface acoustic wave devices, varistors, phosphors, transparentconductive oxides, and UV-light emitters [6–8]. Doping of ZnOnanostructures either through in-situ or post-processing techniquescan help modulate the electrical and optical properties of 1D ZnO.More and more attention has been paid to chalcogen element dopingin ZnO. S-doped ZnO is expected to modify the properties between Sand O. Currently, S-doping ZnO nanostructures such as nanowires,nanorods and nanoporous materials have been successfully synthe-sized by various methods [9–11]. However, the synthesis of verticallyaligned S-doped ZnO nanorod/nanowire arrays is still a challenge.

In this letter, we report the structures and photoluminescenceproperties of S-doped ZnO nanorod arrays. Generally, the PL spectrumof single crystal ZnO consists mainly of two bands [12], while the as-synthesized S-doped ZnO nanorod arrays exhibit three bands due to

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the increase in oxygen vacancy [13–15] and zinc vacancy [16]. Thisspecial characteristic suggests that the S-doped ZnO nanorod arrayscan be used for visible green and violet light emitters.

2. Experiments

Zinc acetate dehydrate (Zn (CH3COO)2·2H2O, 0.5 M) andmonoethanolamine (HO(CH2)2NH2, 0.5 M) were dissolved inethylene glycol monomethylether (CH3OCH2CH2OH) to composethe coating solution. The coating solution was added onto (001)silicon substrates drop by drop at room temperature. Subse-quently, the substrate was pre-coated by spin coating and thenannealed at 300 °C for 10 min. The above steps were repeatedthree times.

Zinc chloride (ZnCl2, 0.1 M) and thiocarbamide (NH2CSNH2,0.1 M) were dissolved into de-ionized water to form a hydro-thermal reaction solution and ammonia was added to adjust the PHvalue to 10. The hydrothermal reaction solution and the pre-coatedsubstrates were transferred into a Teflon-lined stainless autoclaveof 50 ml capacity. The tank was conducted in an electric oven at90 °C for 2 h. In addition, the sample without S doping was alsosynthesized in 0.1 M zinc chloride aqueous solution (pH=10) at90 °C for 2 h. The products were rinsed by de-ionized water anddried at 60 °C in the air.

The morphologies of the products were characterized by fieldemission scanning electric microscopy (FE-SEM) (Zeiss, SUPRA-55). Energy dispersive X-ray spectra (EDX), X-ray diffraction(XRD) (Rigaku, DMAX-RB), X-ray photoelectron spectrum (XPS)(Microlab MKII), photoluminescence spectra (SPEX 1403) andRaman spectra (SPEX 1403) were designed to examine the

445X. Zhang et al. / Materials Letters 63 (2009) 444–446

composition, structures and properties of the S-doped ZnOnanorod arrays.

3. Results and discussions

A typical FE-SEM image of S-doped ZnO nanorod arrays is shown in Fig. 1 (a).Vertically aligned hexagonal nanorods are oriented on the Si substrate. The

Fig. 1. (a) the FE-SEM morphologies of the S-doped ZnO nanorod arrays. (b) the EDXspectrum of the S-doped ZnO nanorod arrays. (c) the typical XRD pattern of the S-dopedZnO nanorod arrays.

inserts in Fig. 1 (a) present the morphologies of the top ends of the nanorods andthe cross section of the nanorod arrays. The diameter of the nanorods is about70 nm and the length of the nanorods is about 1.2 μm. Fig. 1 (b) reveals the EDXspectrum of the sample. A small amount S exists in the sample and the content ofS is determined to be 2.7 at.%. The XRD pattern taken from the S-doped ZnOnanorod arrays is shown in Fig. 1 (c). There is only one obvious Bragg angle peakat 34.257°, which is smaller than the pure ZnO (002) peak at 34.400° (PDF 80-0075). Except for ZnO (002) orientations, no extra diffraction peaks from S-relatedsecondary phases or impurities were observed. It demonstrates the synthesizednanorod arrays are wurtzite structures and S substitutes O successfully in ZnOlattice.

The S-doped ZnO nanorod arrays were also characterized by XPS. Fig. 2(a), (b),and (c) shows the spectra of Zn2p, O1s,and S2p of the sample, respectively. Thewhole scanning spectrum is shown in Fig. 2 (d), and the binding energy in all theXPS spectra was calibrated using C1s at 284.6 eV. There is one peak located at161.8 eV indicating the S2p region (Fig. 2 (c)). This binding energy is lower than thatof sulfur and related compounds, namely, elemental sulfur (164.0 eV), chemis-sorbed SO2 (163.0–165.5 eV), sulfite (–SO3) (166.4 eV), and sulfate (–SO4) (168.0–170.0 eV) [17]. These results imply that S atoms are bonded with Zn atoms.

Further, Raman scattering was designed to determine the structures of the S-doped ZnO nanorod arrays. Fig. 3 shows the Raman spectrum of the S-dopedZnO nanorod arrays. The Raman peaks at 97 cm−1 and 437 cm−1 denote the E2low andE2high mode of ZnO, respectively, which reveal the typical ZnO hexagonal structure.The peaks at 615 cm−1, 666 cm−1, and 812 cm−1 represent the classical second orderRaman modes of ZnO TA+TO, TA+LO, LA+LO [18]. Generally, these second orderRaman modes are not very obvious. However, in our study, the second ordersRaman modes are enhanced. Compared with those of pure ZnO, the typical Ramanpeaks of the S-doped ZnO nanorod arrays sample appear red-shifted, which isrelated to doping effects. Moreover, three additional peaks at 300 cm−1, 937 cm−1

and 974 cm−1 are observed which are from the Si substrate and the zinc acetatedehydrates residuum on the surface of the sample.

Room temperature PL spectra are shown in Fig. 4 to explore the opticalproperty of the S-doped and un-doped ZnO nanorod arrays. Two luminescencebands have been observed in the ZnO nanorod arrays without doping. One is anultraviolet emission at 376 nm, which corresponds to the near band edge (NBE)emission [19]. The other luminescence band is observed at 532 nm, a green lightemission. It is generally accepted that the visible emissions are attributed to thesingle oxygen vacancy in the ZnO, and it results from the radiative recombina-tion of a photogenerated hole with an electron occupying the oxygen vacancy[20].

With the doping of S element, the PL spectrum of the ZnO nanorods appearsnew features. Three peaks are observed in the S-doped ZnO nanorods, a UVemission at 380 nm, and two visible emissions at 401 nm and 509 nm. Comparedwith the un-doped ZnO nanorod, the UV emission of the S-doped ZnO nanorodarrays appear red-shift, which is probably caused by the band gap shifting to thelower energy [21]. The green light emission gets relatively stronger due to theincrease of the oxygen vacancy from S incorporation [22]. The violet emission at401 nm of the S-doped ZnO nanorod arrays is attributed to the electron transitionfrom the bottom of the conduction band to zinc vacancy defect energy level[16,22].

4. Conclusions

In summary, the vertically aligned S-doped ZnO nanorodarrays have been synthesized by a facile hydrothermal method.The as-obtained nanorods are ∼70 nm in diameter, and 1.2 μm inlength. XRD pattern and Raman spectra reveal that the S-dopedZnO nanorods are single crystals with wurtzite structure. Exceptfor the UV emission, the two visible emissions at 401 nm and509 nm of the S-doped ZnO nanorod arrays can be attributedto the zinc vacancy and the increasing of oxygen vacancy bythe sulfur substituting for oxygen in the ZnO lattice. Theseresearch results suggest that the vertically aligned S-doped ZnOnanorod arrays obtained may offer more opportunities for bothfundamental researches and technological applications in lightemitters.

Acknowledgement

This work was supported by the National Basic Research Programof China (No. 2007CB936201), the National High TechnologyResearch and Development Program of China (No. 2006AA03Z351),and the Major Project of International Cooperation and Exchanges(Nos. 50620120439, 2006DFB51000).

Fig. 4. The PL spectra of S-doped ZnO nanorod arrays and undoped ZnO nanorod arrays.

Fig. 3. Raman spectrum of the S-doped ZnO nanorod arrays.

Fig. 2. The XPS spectra of Zn2p (a), O1s (b), S2p (c), and the whole scanning spectrum (d) line of the S-doped ZnO nanorod arrays.

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