J. Phys. Chem. B 2000, 104, 5061-5063
5061
Electrochemical Preparation of CdSe Nanowire Arrays Dongsheng Xu, Xuesong Shi, Guolin Guo,* Linlin Gui, and Youqi Tang Institute of Physical Chemistry, Peking UniVersity, Beijing 100871, P. R. China ReceiVed: August 25, 1999; In Final Form: March 6, 2000
We have successfully prepared CdSe nanowire arrays by direct-current electrodeposition in porous anodic aluminum oxide templates from a dimethyl sulfoxide solution containing CdCl2 and elemental Se. Electron microscopy results show that the length, diameter, and direction of growth of the nanowires are quite uniform. X-ray energy dispersion analysis indicates that the atomic composition of Cd and Se is very close to a 1:1 stoichiometry. Furthermore, X-ray diffraction and high-resolution electron microscope investigations demonstrate that the CdSe nanowires are a uniform hexagonal CdSe crystal.
Introduction Since the successful growth of carbon nanotubes, onedimensional materials have been a focused research field both because of their fundamental importance and the wide-ranging potential applications in nano devices.1-6 There are many experimental approaches to fabricate nanowires, utilizing a variety of nanofabrication techniques7,8 and crystal growth methods.1,4,9-12 Because the growth is controllable almost exclusively 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 nanowires12,13 and has been used to produce a variety of metal nanowire arrays.12,13 However, studies on the preparation of semiconductor nanowires by electrodeposition using templates are still scarce. Only CdS nanowires have been produced in porous anodic aluminum oxide (AAO) templates by alternatingcurrent (ac) electrodeposition.14,15 Recently, we have obtained well-controlled CdS nanowire arrays by direct-current (dc) electrodeposition.16 In this paper, we report our work of fabricating CdSe nanowire arrays based on dc electrolysis into the pores of an AAO template. The nanostructure and morphology as well as the atomic composition characterization of the nanowires were carried out using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray energy dispersion analysis (EDAX). Experimental Section The AAO templates with pore diameters of about 20 nm and with a thickness of 28 µm were grown by potentiostatic anodization of aluminum plates (0.15 mm thick, 99.9+%) in an aqueous solution containing 14% H2SO4 and 1.5% H2C2O4 at 20 °C. The anodizing voltage was 20 V and the anodizing time was 90 min. After the anodization, the remaining aluminum was etched by a 20% HCl-0.1 mol L-1 CuCl2 mixed solution. Then, the barrier layer was dissolved using 20% H2SO4. Finally, a silver film was deposited by vacuum evaporation onto a surface of the template membrane to provide a conductive contact. The electrodeposition was carried out in a glass cell fitted with a platinum counterelectrode as the anode and the AAO template with Ag substrate as the cathode at 185 °C by * To whom all correspondence should be addressed. Fax: 86-1062751725; e-mail:
[email protected].
immersing the cell in an oil bath. The temperature of the electrolyte was maintained to within 0.5 °C with a heater controlled by a temperature controller. The electrolyte solution consisted of 0.05 mol L-1 CdCl2 and saturated elemental selenium in dimethyl sulfoxide (DMSO). CdSe was cathodically deposited at the constant dc density of 0.85 mA cm-2 between the Ag/AAO working electrode and a Pt counterelectrode for 30-60 min. After the deposition, the AAO templates with CdSe nanowires were immediately removed from the electrolyte and rinsed first with hot DMSO (about 160 °C) three to four times, then with ethanol, and then washed with double-distilled water. The templates were finally dried in air at room temperature. The AAO was removed by mounting the foil on an n-type Si wafer using epoxy resin, and then dissolving the AAO template in 1 M NaOH at 25 °C for 1 h and washing several times with double-distilled water. For the SEM image, the above CdSe nanowire samples were directly mounted on Al stubs with conductive silver paint. For TEM, the CdSe nanowires were detached from the substrate by ultrasonic dispersion in 1 cm3 water; then a small drop of the solution was placed on the Cu grids. SEM was carried out using an AMARY 1910FE. A transmission electron microscope, JEM-200CX, operated at 160 keV and equipped with EDAX 9100/6 (Philips), was used for the study of the morphology of the CdSe nanowires. For the high-resolution electron microscope (HREM) observations, a JEOL-2010 electron microscope was used at 300 kV at room temperature. XRD was carried out on a BD-86 PKU diffractometer using Ni-filtered Cu KR radiation. Results and Discussion A typical morphology of the CdSe nanowires prepared by electrodeposition in AAO templates is shown in the SEM image in Figure 1(a). Most CdSe nanowires are laid on the surface of the substrate. In Figure 1(a) and (b), it is shown that CdSe nanowires have uniform diameters of about 20 nm, which corresponds closely to the pore diameters. The nanowires are about 6 µm in length. After electrochemical deposition for 90 min, their lengths can be up to 15 µm. In our experiment, the nanowires can be found in the whole sample with an area of 2 cm2. This result indicates that the nanowires can be filled uniformly in the pores of the AAO template by dc electrodeposition. Figure 2 shows the TEM image of CdSe nanowires prepared in an AAO template with a diameter of 20 nm. These nano-
10.1021/jp9930402 CCC: $19.00 © 2000 American Chemical Society Published on Web 05/05/2000
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Figure 3. EDAX spectrum revealing that the nanowires are composed of Se and Cd. The X-ray excitation energy for Se and Cd are 1.38 and 3.13 keV, respectively. No correction of the sample thickness has been made.
Figure 1. (a) Low-magnification and (b) higher magnification SEM images revealing the general morphology of the CdSe nanowires.
Figure 4. X-ray diffractogram of CdSe nanowire arrays.
Figure 2. TEM image reveals that the CdSe nanowires are not randomly oriented. The inset plot is the electron diffraction pattern of the CdSe nanowires, which can be indexed as the hexagonal CdSe crystal structure.
wires are not randomly oriented and the length, diameter, and direction of growth of the nanowires are quite uniform. The nanowires arrange tightly, which corresponds to the high pore density of the AAO template. The electron diffraction pattern taken from the CdSe nanowires is shown in the inset on the upper right of the micrograph. The diffraction spots correspond to the (002), (101), and (100) diffraction planes of a hexagonal CdSe crystal. The diffraction pattern with somewhat dispersed and elongated spots implies that the CdSe nanowires grow with uniform crystal structure nearly parallel to each other.
The chemical composition of the CdSe nanowires was determined using EDAX. In the EDAX spectrum of CdSe nanowires (Figure 3), the peaks of Se and Cd are found. Quantitative analysis results indicate an atomic composition of 50.5% Cd and 49.5% Se, which is very close to a 1:1 stoichiometry. The X-ray diffractogram of the CdSe nanowire arrays is shown in Figure 4, where the diffraction peaks could be assigned to CdSe, Ag, and AAO without elemental Se and Cd. These XRD data indicate that the nanowires have diffraction patterns corresponding to the hexagonal phase of CdSe (American Society for Testing and Materials standard 8-459). The interplanar diffraction spacing (dhkl) of CdSe nanowires with diameters of 20 nm differs slightly from those reported for polycrystalline CdSe. The relative intensity of the 002 diffraction peak, which corresponds to interplane distances d ) 3.49-3.51(A), is greater than that of the polycrystalline CdSe powder. This fact indicates that the c-axis of hexagonal crystals is preferentially aligned along the direction normal to the substrate rather than oriented randomly. Furthermore, the dimensions of the crystallites of the nanowires were estimated from the widths of the major diffraction peaks observed in Figure 4 through the Scherrer formula
Dhkl ) kλ/(∆hkl cos θ) where Dhkl is the linear dimension of the coherent diffracting
Preparation of CdSe Nanowire Arrays
J. Phys. Chem. B, Vol. 104, No. 21, 2000 5063 TABLE 1: Mean Crystallite Sizes Dhkl (nm) along the [100], [002], [101], [102], and [110] Zone Axes diffraction plane (hkl)
peak position (d, A)
peak areas (I, %)
Dhkl (nm)
(100) (002) (101) (102) (110)
3.705 3.496 3.285 2.532 2.142
38 80 12 35 100
22 120 15 58 30
morphology and a uniform width of 20 nm. The diffraction pattern (the inset plot) confirms that the nanowire has a hexagonal CdSe crystal structure. Figure 5(b) shows a typical HREM image of a single CdSe nanowire with a diameter of 20 nm. The interplanar spacing is about 0.329 nm, which corresponds to the {101} plane of the hexagonal system of CdSe. This image furthermore reveals that the structure of the nanowire is a uniform hexagonal CdSe crystallite. In summary, we have prepared CdSe nanowire arrays by dc electrodeposition in an AAO template from a DMSO solution containing CdCl2 and elemental Se. SEM, TEM, HREM, and XRD results demonstrated that these CdSe nanowires have a uniform length, diameter, and direction of crystal growth. This work further demonstrates that dc electrodeposition directly into the AAO template is a simple and efficient method of producing semiconductor nanowire arrays with well-controlled crystalline structure. Acknowledgment. Project supported by the Major State Basic Research Development Program (Grant No. 2000077500). References and Notes Figure 5. (a) TEM image of a single CdSe nanowire. (b) HREM image of a CdSe nanowire. The inset plot in (a) is the electron diffraction pattern taken from this nanowire.
domain along a direction normal to the diffraction plane (hkl), λ is X-ray wavelength in angstroms (1.5405 Å in our case), k is a crystal shape constant (0.9), θ is the angle of reflection of the peak, and ∆hkl is the corrected full width at half-maximum (fwhm) of the peak in radians, which was calculated as the square root of the difference between the squares of the sample’s and the reference’s fwhm. The dimensions of the CdSe crystallites calculated from the widths of the XRD reflections in Figure 4 are listed in Table 1. These results show that the crystallite domains of the CdSe nanowires are c-axis oriented and the crystallite dimension in the radial direction is very close to the pore size of the AAO template used. Both the crystallite dimension along the [002] axis and the crystallite orientation with an aspect ratio of 5:1 calculated from the crystallite dimensions along the [002] and [100] axes are larger than in the CdS nanowires fabricated by ac electrolysis.14 Figure 5 (a) shows the TEM image of a single CdSe nanowire. It can be seen that the CdSe nanowire has a relatively straight
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