Full text of DOI:10.1557/JMR.2002.0253

Highly ordered and well-oriented single-crystal CdTe
nanowire arrays by direct-current electrodeposition
Dongsheng Xu,a) Yuguo Guo, Dapeng Yu, Guolin Guo, and Youqi Tang
State Key Laboratory for Structural Chemistry of Unstable and Stable Species,
Institute of Physical Chemistry, Peking University, Beijing 100871, People’s Republic of China
D.P. Yu
State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University,
Beijing 100871, People’s Republic of China
(Received 31 October 2001; accepted 15 April 2002)
Highly ordered CdTe nanowire arrays were prepared by electrochemical deposition
into the pores of anodic aluminum oxide templates from an ethylene glycol bath
containing CdCl2, TeCl4, and KI. Electron microscopy results showed that the length,
diameter, and direction of growth of the nanowires were quite uniform. X-ray
diffraction, transmission electron microscopy, and high-resolution electron microscopy
investigations showed that these nanowires had a crystalline structure of hexagonal
CdTe single crystal with a uniform [001] growth direction.
I. INTRODUCTION
nanowire arrays such as CdS, CdSe, Ag2Se, etc., have
been fabricated using direct or alternating current elec-
trodeposition processes,13–17 synthesis of aligned and
well-distributed single-crystal semiconductor nanowires
is still a challenging research area. Recently, CdS and
TiO2 single-crystal nanowires have been obtained by
electrochemically induced deposition.18,19 In this paper,
we report our work on synthesis of aligned single-crystal
CdTe nanowire arrays based on direct current (dc) elec-
trolysis into the pores of an anodic aluminum oxide
(AAO) template from an ethylene glycol bath containing
CdCl2, TeCl4, and KI.
Semiconductor nanowires have been attracting much
attention in recent years for their potential applications in
mesoscopic research and nanodevices.1–5 There are
mainly three experimental approaches to fabricating
semiconductor nanowires with uniform diameters. One
method is laser ablation of metal containing semiconduc-
tor targets through a vapor-liquid-solid growth mecha-
nism.6,7 A critical feature of this method is that the
catalyst used to define one-dimensional growth and thus
the metal always is observed at the nanowire end. The
second method is oxide-assisted semiconductor nanowire
growth by laser ablation, thermal evaporation, or chemi-
cal vapor deposition.8–11 In approach, each nanowire
consists of an amorphous oxide shell, and a high density
of defects has been observed in the crystalline semicon-
ductor core.9 The third is a template synthesis method,
the key components of which are template and electro-
deposition.12–19 In this method, thin fibrils of the desired
materials are electrochemically synthesized within the
voids of the template material. Due to cylindrical pore
geometry and monodisperse diameters, corresponding
cylindrical and oriented nanostructural materials with a
narrow diameter distribution are obtained. On the other
hand, because the growth is controllable almost exclu-
sively in the direction normal to the substrate surface,
electrochemical synthesis in a template is taken as one of
the most efficient methods in controlling the growth
of nanowires. Although crystallized semiconductor
II. EXPERIMENTAL PROCEDURE
The electrodeposition was carried out in a glass cell
fitted with a platinum counter electrode as the anode and
the AAO template with an Ag substrate as the cathode at
160–175 °C by immersing the cell in an oil bath. De-
tailed on the fabrication of the Ag/AAO electrode is
given in our previous reports.15–17 AAO templates with
about 10-, 20-, and 50-nm pore diameters were used. The
electrolyte solution consisted of 1.00 M CdCl2, 0.01 M
TeCl4, and 0.3 M KI in glycol. CdTe was cathodically
deposited in AAO template, as in the work by Gore
et al.20 After the deposition, the AAO templates with
CdTe nanowires were immediately removed from the
electrolyte and rinsed first with hot glycol followed by
ethanol, then washed with double distilled water, finally
dried in air at room temperature.
The AAO was completely removed from the
Ag/AAO/CdS samples by mounting the foil on a glass
using epoxy resin, and then dissolving the AAO template
a)Address all correspondence to this author.
e-mail: dsxu@chem.pku.edu.cn
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D. Xu et al.: Highly ordered and well-oriented single-crystal CdTe nanowire arrays by direct-current electrodeposition
in 1 M NaOH at 25 °C for 1 h and washing several times
with double-distilled water. For scanning electron micro-
scope (SEM) observation, the above CdTe nanowires
samples were directly mounted on Al stubs with conduc-
tive silver paint. For transmission electron microscope
(TEM) observation, the nanowires were detached from
the substrate by ultrasonic dispersion in water. SEM was
carried out using an AMARY 1910FE. A TEM (JEM-
200CX, Japan) operated at 200 keV and equipped with
x-ray energy dispersion analysis (EDAX 9100/6, Philips,
Holland) was employed for the study of the morphology
of the nanowires. For high-resolution electron micro-
scope (HREM) observations, a JEOL-2010 electron mi-
croscope (Japan) was employed at 300 kV at room
temperature. The electrochemical measurements and the
potentiostatic electrolysis were performed on a CHI
scanning poteniostat/galvanostat (Model 660A, China).
X-ray powder diffraction (XRD) was directly measured
on a bulk nanowire sample using Ni filtered Cu K␣ ra-
diation from a BD-86, PKU diffractometer (China).
AAO template was carried out by potentiostatic electro-
lyse in the potential range between −0.59 to −0.85 V
versus SCE.
Figure 2(a) shows a low-magnification SEM image of
CdTe nanowires electrodeposition in the AAO template
approximately 50 nm in diameter. This image indicates
that a thin layer of CdTe nanowires with approximately
4-␮m thickness was obtained in high densities. In the
higher-magnification SEM image [Fig. 2(b)], it can be
seen that straight nanowires with uniform diameter are
perpendicularly arraigned on the substrate.
The chemical composition and overall crystal structure
of the CdTe nanowires were characterized using EDAX
and x-ray diffraction analyses. The EDAX spectrum
shown in Fig. 3(a) reveals that the nanowires are com-
posed of Te and Cd, and quantitative analysis results
indicate that the atomic ratio of Cd to Te is very close to
1:1 stoichiometry. The XRD pattern of a bulk nanowire
sample 20 nm in diameter [Fig. 3(b)] can be indexed to a
hexagonal CdTe structure with lattice constants of
a ס 3.880 A and c ס 7.483 A [bulk CdTe: a ס 3.983 A
and c ס 7.506 A (ASTM Standard 8-45921)]. This crys-
tal structure of the nanowires is different from the cubic
crystalline structure of the CdTe films electrodeposited
on nickel plates under the same depositing conduction.20
The reflection peaks at 2␪ ס 22.92°, 23.77°, 25.36°,
39.37°, 46.00°, 62.19°, and 66.43° correspond to the
(100), (002), (101), (110), (200), (201), (210), and (105),
III. RESULTS AND DISCUSSION
The electrochemical processes during electrodeposi-
tion of CdTe were studied by means of cyclic voltam-
metry. The cyclic voltammograms on glass/indian-doped
oxide (ITO) working electrode as shown in Fig. 1 were
recorded in a stirred bath containing 1.0 M CdCl2 0.01 M
TeCl4, and 0.3 M KI solution. This voltammogram is
characterized by three cathodic peaks at 0.06, −0.23, and
−0.59 V versus saturated calomel reference electrode
(SCE), which are attributed to the formation of Te2+, Te,
and stochiometric CdTe, respectively.20 Beyond −0.85 V
versus SCE, the cathodic current exhibited an abrupt in-
crease due to the deposition of metallic Cd. In our ex-
periments, the deposition of the CdTe nanowire in the
FIG. 2. (a) Low-magnification and (b) higher-magnification SEM im-
ages of the CdSe nanowires electrodeposited in the AAO template
with a diameter of approximately 50 nm.
FIG. 1. Cyclic voltammogram on an ITO-coated glass cathode in
1.0 M CdCl2, 0.01 M TeCl4, and 0.3 M KI solution at 160 °C.
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D. Xu et al.: Highly ordered and well-oriented single-crystal CdTe nanowire arrays by direct-current electrodeposition
FIG. 3. (a) EDAX spectrum revealing that the nanowires are
composed of Te and Cd. (b) XRD spectrum recorded on bulk CdTe
nanowires in Ag/AAO membrane. The numbers above the peaks cor-
FIG. 4. (a) TEM image of the CdTe nanowires by electrodeposition in
respond to the (hkl) values of the hexagonal CdTe structure.
the AAO templates with a pore diameter of about 10 nm. (b) TEM
image of a bundle of the CdTe nanowires. (Inset) SAED pattern re-
corded the region indicated by the dashed circle.
respectively. The relative intensity of the 002 diffraction
peak, which corresponds to interplane distances
d ס 3.743 (A), is greater than that of the polycrystalline
CdTe powder. This fact indicates that the c axis of hex-
agonal crystals is preferentially aligned along the direc-
tion normal to the substrate rather than oriented
randomly.
Figure 4(a) shows a typical TEM image of nanowire
sample electrodeposited in the template. The nano-
wires have smooth surfaces and the diameters of the
nanowires are quite uniform. The average diameter of
these nanowires is about 10 nm, which corresponds to
the pore size of the template we used.
Figure 4(b) gives a TEM image of a bundle of 20-nm-
diameter CdTe nanowires deposited in the AAO
template. The nanowires have uniform lengths of approx-
imately 8 ␮m. The hooked shape of this bundle nano-
wires reveals the flexibility of the CdTe nanowires. The
shrinking at the top of the bundle nanowires may be due
to the aggregation of the individual nanowires after re-
moval of the template membrane. The selected-area elec-
tron diffraction (SAED) pattern taken from the region
indicated by dash circle with a diameter of 1 ␮m is
shown in the inset. The SAED pattern exhibits a sharp
regular pattern, which is consistent with hexagonal CdTe
single crystal. The diffraction pattern with somewhat dis-
persed and elongated spots implies that the orientation
within the selected area is limited; i.e., there is pro-
nounced texture. Indexing this pattern demonstrates that
the axes of the nanowires are along the [002] direction.
This SAED result strongly demonstrates that the CdTe
nanowires are perfectly single-crystal line in structure
with a uniform [002] growth direction. In addition, we
observed that the TEM images of many individual
nanowires have a uniform diffraction contrast along the
wire axis. This observation further confirms that
the nanowires are single crystals.
In agreement with the XRD, TEM, and SAED studies,
the HREM image of a single CdTe nanowire with a
diameter of 15 nm (Fig. 5) shows clearly a continuously
two-dimensional lattice resolved image of the single
crystalline structure of the nanowire. Unlike to the ob-
servation on CdS and CdSe nanowire by electrodeposit-
ing in dimethyl sulfoxide system,14,15 no thin amorphous
shell, which probably results from the oxidation of the
nanowires during the electrodeposit process, was found
on the CdTe nanowire.
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D. Xu et al.: Highly ordered and well-oriented single-crystal CdTe nanowire arrays by direct-current electrodeposition
REFERENCES
1. Y. Cui and C.M. Lieber, Science 291, 851 (2001).
2. Y. Huang, X.F. Duan, Q.Q. Wei, and C.M. Lieber, Science 291,
630 (2001).
3. X.F. Duan, Y. Huang, Y. Cui, J.F. Wang, and C.M. Lieber, Nature
409, 66 (2001).
4. M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind,
E. Weber, R. Russo, and P.D. Yang, Science 292, 1897 (2001).
5. Z.L. Wang, Adv. Mater. 12, 1295 (2000).
6. A.M. Morales and C.M. Lieber, Science 279, 208 (1998).
7. X.F. Duan and C.M. Lieber, Adv. Mater. 12, 298 (2000).
8. D.P. Yu, C.S. Lee, I. Bello, X.S. Sun, Y.H. Tang, G.W. Zhou,
Z.G. Bai, Z. Zhang, and S.Q. Feng, Solid State Commun. 105, 403
(1998).
9. S.T. Lee, N. Wang, Y.F. Zhang, and Y.H. Tang, MRS Bull. 24(8),
36 (1999).
10. Z.G. Bai, D.P. Yu, H.Z. Zhang, Y. Ding, Y.P. Wang, X.Z. Gai,
Q.L. Hang, G.C. Xiong, and S.Q. Feng. Chem. Phys. Lett. 303,
FIG. 5. HREM image of individual nanowires 15 nm in diameter.
311 (1999).
11. D.P. Yu, Q.L. Hang, Y. Ding, H.Z. Zhang, Z.G. Bai, J.J. Wang,
Y.H. Zou, W. Qian, G.C. Xiong, and S.Q. Feng, Appl. Phys. Lett.
73, 3076 (1998).
IV. CONCLUSION
In summary, we investigated the synthesis of CdTe
nanowires using dc electrodeposition in a non-aqueous
bath. SEM, TEM, EDAX, XRD, and HREM observa-
tions demonstrated that highly aligned single-crystal
CdTe nanowire arrays with hexagonal CdTe structure
and a uniform [002] growth direction were obtained. It is
expected that the electrodeposition in template approach,
which is based on choice of the appropriate electrodepo-
sition condition, can be extended to production of other
aligned single-crystal semiconductor nanowires arrays.
12. C.R. Martin, Science 266, 1961 (1994).
13. D. Routkevitch, T. Bigioni, M. Moskovits, and J.M. Xu, J. Phys.
Chem. 100, 14037 (1996).
14. S.A. Sapp, B.B. Lakshmi, and C.R. Martin, Adv. Mater. 11, 402
(1999).
15. D.S. Xu, Y.J. Xu, D.P. Chen, G.L. Guo, L.L. Gui, and Y.Q. Tang,
Chem. Phys. Lett. 325, 340 (2000).
16. D.S. Xu, X.S. Shi, G.L. Guo, L.L. Gui, and Y.Q. Tang, J. Phys.
Chem. B. 104, 5061 (2000).
17. R.Z. Chen, D.S. Xu, G.L. Guo, and Y.Q. Tang, J. Electrochem.
Soc. (2002, in press).
18. D.S. Xu, Y.J. Xu, D.P. Chen, G.L. Guo, L.L. Gui, and Y.Q. Tang,
Adv. Mater. 12, 520 (2000).
19. X.Y. Zhang, B.D. Yao, L.X. Zhao, C.H. Liang, L.D. Zhang, and
Y.Q. Mao, J. Electrochem. Soc. 148, G398 (2001).
20. R.B. Gore, R.K. Pandey, and S.K. Kulkarmi, J. Appl. Phys. 65,
2693 (1989).
21. Powder Diffraction File No. 19-192, International Centre for Dif-
fraction Data, Newton Square, PA.
ACKNOWLEDGMENT
This work was supported by the Major State Basic Re-
search Development Program, Peoples Republic of China
(Grant No. 2000077503).
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