Synthesis and characterization of single crystalline selenium nanowire arrays

Article abstract of DOI:10.1016/j.materresbull.2006.02.030

Ordered selenium nanowire arrays with diameters about 40 nm have been fabricated by electrodeposition using anodic porous alumina templates. As determined by X-ray diffraction, Raman spectra, electron diffraction and high-resolution transmission electron microscopy, selenium nanowires have uniform diameters, which are fully controllable. Single crystalline trigonal selenium nanowires have been obtained after postannealing at 180 °C. These nanowires are perfect with a c-axis growth orientation. The optical absorption spectra reveal two types of electron transition activity.

Full text of DOI:10.1016/j.materresbull.2006.02.030

Materials Research Bulletin 41 (2006) 1729–1734
www.elsevier.com/locate/matresbu
Synthesis and characterization of single crystalline selenium
nanowire arrays
a,b,*
a
, L.H. Xu , J.Y. Dai , Y. Cai , N. Wang
b
c
c
X.Y. Zhang
a
Department of Materials Science, The College of Anhui Architecture, Hefei, China
Applied Physics Department, The Hong Kong Polytechnic University, Hong Kong, China
b
c
Physics Department and Institute of Nano Science and Technology, The Hong Kong University
of Science and Technology, Hong Kong, China
Received 4 October 2005; received in revised form 4 January 2006; accepted 15 February 2006
Available online 15 March 2006
Abstract
Ordered selenium nanowire arrays with diameters about 40 nm have been fabricated by electrodeposition using anodic porous
alumina templates. As determined by X-ray diffraction, Raman spectra, electron diffraction and high-resolution transmission
electron microscopy, selenium nanowires have uniform diameters, which are fully controllable. Single crystalline trigonal selenium
nanowires have been obtained after postannealing at 180 8C. These nanowires are perfect with a c-axis growth orientation. The
optical absorption spectra reveal two types of electron transition activity.
#
2006 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures; A. Semiconductors; B. Crystal growth; C. Electron microscopy; D. Optical properties
1
. Introduction
In recent years, there has been increasing interest in the fabrication of one-dimensional nanostructures because of
their potential utilization in nanodevices [1–4]. Highly ordered nanowire arrays are particularly important from a
fundamental point of view as well as for future applications. The ability to precisely prepare such structures on a
large scale is an important goal of materials science. One strategy is to use porous nanochannel alumina (NCA)
template to fabricate such structures [4–6]. An important advantage of the template method is that the nanowires
prepared in this way can be diameter-controllable and well defined. Selenium is an important semiconductor
material, which has long been known to exhibit photoconductivity [7], low photomelting temperature [8] and
switching phenomenon [7]. Se has been widely used for xerographic photoreceptors. Se nanowires have been
synthesized previously by using solution-phase methods [9–11] and chemical vapor deposition method [12].
Ordered Se nanowire arrays are of considerable interest for obtaining scaled-up functional devices for use as
sensors, and in optoelectronic nanodevices. Here, we will report the fabrication and structure characterization of
ordered Se nanowire arrays by using electrodeposition within the NCA templates. The phase transition and the
optical properties of the Se nanowires have also been investigated.
*
Corresponding author. Tel.: +852 27665671; fax: +852 23337629.
E-mail address: apzhxy@polyu.edu.hk (X.Y. Zhang).
0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2006.02.030

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X.Y. Zhang et al. / Materials Research Bulletin 41 (2006) 1729–1734
2
. Experimental procedure
NCA templates were prepared by means of anodization [13,14]. Briefly, high purity (99.999%) aluminum foils
were used as the start material. Prior to anodizing, the aluminum was annealed at 500 8C in order to obtain
homogeneous conditions for pore growth over large areas. Subsequently, the foils were electropolished in a 1:9 by
volume mixture of HClO and C H OH. Anodization was carried under a constant cell voltage in an oxalic acid
4
2 5
solution. After the anodization, the remaining aluminum was removed in a saturated HgCl solution. Then, the pore
2
bottoms were opened and widened by chemical etching in an aqueous phosphoric acid. In order to fabricate the
nanowire arrays, a thin layer of Au about 100 nm was deposited as an electrode on one side of the anodic porous
alumina template using vacuum evaporation apparatus. Electrodeposition of Se nanowires was performed by using a
potentiostatic method from the following aqueous solution: 50 g/l SeO , 20 g/l H BO ; pH 2.0. The temperature of the
2
2
3
bath was maintained at 30 8C. A Pt film was used as a counterelectrode. The cathode potential was kept at À1.0 V
3
versus SCE and the cathode current density was about 0.2 A/dm . The length of the nanowires was controlled by the
deposition time. The deposition rate was about 200 nm/min. At the initial stage of deposition, the sample was orange in
color, which then turned grey. The as-prepared sample was then annealed at 180 8C for 1 h to obtain single trigonal
structure. The structure and morphology of the nanowire arrays were investigated using a X-ray diffractometer (D/
Max 2250V), a scanning electron microscope (SEM, JEOL JSM-6300), a transmission electron microscope (TEM,
JEOL-2010), and a laser Raman scattering spectrometer (Spex 1403) with a 514.5 nm incident wavelength and a
2
00 mW output power. The TG and DTA curves were taken using a thermal analysis apparatus.
3
. Results and discussion
Fig. 1(a) shows the SEM image of the empty NCA templates. The pore diameters and the thickness of the NCA
template are about 40 nm and 6 mm, respectively. Fig. 1(b) is the corresponding SEM image of the free standing
Fig. 1. (a) SEM image of the NCA template. (b) SEM image of the Se nanowire array. (c) XRD spectrum of the Se nanowire array. (d) Raman
spectrum of the Se nanowire array.

X.Y. Zhang et al. / Materials Research Bulletin 41 (2006) 1729–1734
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Fig. 2. (a) TEM image of the Se nanowires after completely dissolving the NCA template. (b) The corresponding selected electron diffraction
pattern of the nanowire marked by circle in (a). (c) TEM image of an isolated Se nanowire, and (d) the corresponding selected electron diffraction
pattern of the nanowire.
Se nanowires after etching away the alumina using a mixture of 6 wt.% phosphoric acid and 1.8 wt.% chromic acid
followed by cleaning in an ultrasonic bath. It can be seen that Se nanowires are of an equal height and have an
ordered tip array. X-ray diffraction (XRD) spectrum of the Se nanowires is shown in Fig. 1(c), where the refection
peaks are clearly distinguishable, and can be perfectly indexed as trigonal Se structure. Fig. 1(d) shows the
À1
corresponding Raman scattering spectrum of the Se nanowire arrays. The mode at 235.3 cm corresponds to the
bond-stretching mode of trigonal Se, which can be attributed to the vibration of Se helical chain. The structure of
the Se nanowires was further investigated by TEM. Fig. 2(a) shows the TEM image of the Se nanowires after
completely dissolving away the alumina. The Se nanowires are straight, the diameters and lengths are about 40 nm
and 2 mm, respectively. Fig. 2(b) shows the corresponding selected-area electron diffraction (SAED) pattern of one
Se nanowire in Fig. 2(a), which could be indexed as single crystal trigonal Se [1 1 0]. Fig. 2(c) shows an isolated Se
nanowire, Fig. 2(d) shows the corresponding selected-area electron diffraction (SAED) pattern, which could be
indexed as single crystal trigonal Se [0 1 0]. All the nanowires grow along the [0 0 1] direction, i.e., c-axis of the
trigonal phase. The single-crystal structure was further confirmed by high-resolution electron microscopy
(
HRTEM) images. Fig. 3 shows the HRTEM image of the Se nanowire in Fig. 2(c). This image shows the single-
crystal structure of the nanowire and the (0 0 1) and (1 0 0) lattice fringes with interplane spacings of around 0.5
and 0.378 nm, respectively.
In order to study the phase transformation of the Se nanowires in the NCA template, the TG and DSC curves were
measured simultaneously where heating the as-prepared samples from 25 to 500 8C. Fig. 4 shows the TG and DSC
curves, respectively. In the TG curve, the weight of the sample was significantly decreased in the temperature range
of 300–400 8C. The total mass loss is about 16 wt.%. The Se nanowires can be desorbed due to thermal motion
during the heating process, in fact, it can be seen that the Se nanowires do migrate out from the channels because the
sample color becomes lighter. The result of mass loss is reasonably comparable with the estimated Se nanowires
occupying weight ratio. There are two peaks located at 105 and 218 8C in the DSC curve, respectively. The
exothermic peak at 105 8C is associated with the phase transformation to trigonal Se. We have observed other

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Fig. 3. HRTEM image of the Se nanowire in Fig. 2c.
phases, such as monoclinic Se, in the as-deposited nanowires. Trigonal phase is the thermal equilibrium phase of Se,
so the peak at 105 8C shows an exothermic effect. This value agrees quite well with the values that have been
reported for ball-milled selenium [15]. The endothermic peak at 218 8C is clearly associated with the melting of
trigonal Se [16].
Fig. 5 shows the absorption spectrum of the trigonal Se nanowire array measured at room temperature for light
polarized perpendicular to the sample plane. The absorption spectrum can be decomposed into three bands with peak
Fig. 4. The TG and DSC curves of Se nanowire array measured as a function of temperature.

X.Y. Zhang et al. / Materials Research Bulletin 41 (2006) 1729–1734
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Fig. 5. The optical absorption spectrum of Se nanowire array measured at room temperature.
energy at 2.1, 3.34, and 4.1 eV. The bandgap and other transitions of trigonal Se nanowires were usually found to be
blue-shifted compared to bulk trigonal Se [9,11,12]. The lower energy at 2.1 eV corresponds to indirect interband
transition. This value is almost the same as that of trigonal Se nanowires with diameters of 32 nm [9]. So we assign the
band at 2.1 eV to the indirect transition of Se chain in trigonal-Se. The higher energy transitions were determined to
arise from covalent bonds within the Se chains [17]. The absorption bands in spectrum at 3.34 and 4.1 eV could be
attributed to direct transition, these values match well to the results reported for trigonal Se nanowires with diameters
about 32 nm (3.37 eV) [9] and diameters ranging from 20 to 40 nm (4.05 eV) [15].
4
. Conclusion
In conclusion, ordered single crystalline Se nanowire arrays with uniform diameters about 40 nm have been
fabricated by electrodeposition using anodic porous alumina templates. The trigonal Se nanowires are single
crystalline with growth direction along the [0 0 1] direction. These ordered Se nanowire arrays would be useful for
obtaining scaled-up functional devices for use as sensors, and in optoelectronic nanodevices.
Acknowledgments
This work was funded by the Natural Science Foundation of Anhui Province (99046345) and Postdoctoral Fellow
project in the Hong Kong Polytechnic University (G-YX17).
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