CRYSTAL GROWTH & DESIGN
CdS Multipod-Based Structures through a Thermal Evaporation Process Guozhen Shen* and Cheol-Jin Lee Department of Nanotechnology, Hanyang University, 17-Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea Received October 23, 2004;
2005 VOL. 5, NO. 3 1085-1089
Revised Manuscript Received November 25, 2004
ABSTRACT: CdS multipod-based structures, such as flowerlike microstructures, tetrapod-like microrods, and long branched nanowires, were selectively prepared by atmospheric pressure thermal evaporation of CdS nanoparticles without the use of a catalyst. The morphologies could be well controlled by simply adjusting the deposition position. The phase structures, morphologies, and optical properties of the products were investigated by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, and photoluminescence spectroscopy. A vapor-liquid mechanism was proposed for the formation of CdS multipod-based structures. The present synthetic route is expected to be applied to the synthesis of other II-VI groups or other group’s semiconducting materials with controllable morphologies. Introduction As one of the most important group II-VI semiconductors, CdS has been extensively investigated during the past decades. It is a well-studied semiconductor with a direct band gap of 2.4 eV at room temperature, and it is now widely used for photoelectric conversion in solar cells, in light-emitting diodes for flat-panel displays, and other optical devices based on its nonlinear properties.1-4 A number of methods have been explored to grow CdS-based structures especially one-dimensional CdS nano-/microstructures. For instance, Lieber’s group has fabricated thin CdS nanowires by laser ablation of CdS and a metal catalyst mixed target.5 Solution-basedrouteswithorwithoutorganicsurfactants6-8 and gas-phase routes9-11 were also developed rapidly to synthesize one-dimensional CdS structures, such as nanorods, nanowires, nanotubes, and nanobelts. It is well-known that the realization of technologically useful micro-/nanocrystal-based materials depends not just on the quality of the crystal (e.g., size, shape) and their surface chemistry, but also on their special orientation and arrangement.12-14 Recently, much effort has been paid to the synthesis of complex structures based on micro-/nanorods, which may offer both opportunities to exploit the distinctive optical, electronic properties of the 1-D micro-/nanostructure itself and the possibilities to prove potentially new phenomena arising from their 3-D organization.14-25 Most of research concerning this field is on ZnO-based anisotropic micro-/nanorods, and many amazing 3-D organizations based on ZnO micro-/nanorods, such as multipods, tetrapods, nanobridges, nanonails, nanopropeller arrays, and so on, have been successfully obtained.14,19-25 However, as for the CdS micro-/nanorod-based 3-D organization, most of the reports are based on a solution process using organic surfactants,15,17 and there is only one short report about gas phase to dendritic CdS crystals on W substrate by Jiao et al.26 It is still a challenge for materials scientists to find straightforward and control* To whom correspondence should be addressed. Tel: +82-2-22934744. Fax: +82-2-2290-0768. E-mail:
[email protected].
Figure 1. Apparatus for the synthesis of CdS multipod-based structures.
lable methods to fabricate CdS micro-/nanorods into 3-D nanostructures, such as tetrapods, multipods, or other patterns. In this paper, we report a simple atmospheric pressure thermal evaporation method to synthesize selfassembled CdS multipod-based structures on silicon substrate without the use of a catalyst. Studies found that the morphologies of the products could be well controlled by adjusting the deposition position. Experimental Section Our method for synthesizing CdS multipod-based structures is based on atmospheric pressure thermal evaporation of CdS nanoparticles without the presence of an Au catalyst. The apparatus was illustrated in Figure 1. CdS nanoparticles (ca. 30 nm, 0.2 g, self-made by refluxing Cd(NO3)2 and thiourea in ethylene glycol) was placed in the heating center of the quartz tube. Five pieces of Si wafers were placed downstream in the quartz tube to act as the deposition substrates for the materials growth. The distance between the first silicon substrate (substrate 1) and the source material is 7 cm, and the distance between each substrate is 1 cm as shown in Figure 1. Highpurity argon was adopted as a protecting medium during the evaporation at a constant flow rate of 200 sccm. Prior to heating of the sample, the system was flushed with high-purity Ar for 10 min to eliminate O2. Then the furnace was rapidly heated to 900 °C, held at this temperature for 60 min, and subsequently cooled to room temperature. The synthesized products were characterized using X-ray diffraction (XRD, Rigaku DMAX 2500), scanning electron microscopy (SEM, Hitachi S-4700), high-resolution transmis-
10.1021/cg0496437 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/14/2005
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Figure 2. products.
Shen and Lee
Typical XRD pattern for the deposited CdS
sion electron microscopy (HRTEM, Philips Tecnai F20), and energy dispersed X-ray spectrometry (EDX). For HRTEM observations, the synthesized products were ultrasonically dispersed in ethanol, and a drop of the solution was then placed on a Cu grid coated with a holey carbon film. The Raman spectra were produced at room temperature with a LABRAM-HR Confocal Laser MicroRaman spectrometer. Photoluminescence (PL) measurements were carried out using 520 nm line of an Ar laser as the excitation source.
Results and Discussion Figure 2 shows a typical XRD pattern of the product deposited on the Si substrate. All the peaks in this pattern can be indexed to hexagonal CdS wurtzite structure with lattice constants a ) 0.4141 nm, and c ) 0.6716 nm (JCPDS Card, No. 77-2306). No peaks due to other phases, such as CdO, Cd, S, were detected, indicating the high purity of the products. SEM images shown in Figure 3a show that the synthesized products on substrate 1 are composed of CdS multipod structures, which look like flowers in which each pod radially grows from one center. A highmagnified SEM image (Figure 3b) shows that the diameter of each pod of these structures is quite uniform along its length, and the typical diameters of the pods range from 500 nm to 1 µm. The length of each pod is about 4 µm. From the SEM images, we also found that many small nanoparticles with a diameter of several tens of nanometers were decorated on the pods. Figure 4a shows the SEM image of the sample deposited on substrate 2, which was placed about 8 cm away from the source materials. It clearly shows that the products on substrate 2 are composed of large-scale tetra pod-based structures. This structure is quite different from the flowerlike structures on substrate 1. Figure 4b is the high-magnified SEM image of a single CdS tetrapod. It indicates that the diameter of each pod is also uniform along its length and the typical diameter is about 1-1.5 µm. Besides these CdS tetrapods, many CdS arrow arrays were also detected under SEM observation, and the image is shown in Figure 4c. In each array, many CdS arrows with diameter of about 1 µm grew shoulder to shoulder and finally they formed the compacted CdS arrow arrays. Figure 5 shows typical SEM image of the as-prepared CdS multipods deposited on substrate 3, which was placed about 10 cm from the source materials. Compared with those products on substrate 1 and 2, the CdS multipods deposited on substrate 3 are of quite different
Figure 3. (a, b) SEM images of CdS flowerlike structures on substrate 1.
Figure 4. (Left) SEM image of the CdS tetrapods deposited on substrate 2; (right, top) SEM image of a single CdS tetrapod; (right, bottom) SEM image of a single CdS arrow array.
structures. The diameters of the pods along the length are no longer uniform, and the diameter ranges from about 300 nm at the root to about 1 µm at the tip. Highmagnified SEM images shown in Figure 6 show the general structure of each CdS pod. Figure 6a is the SEM image of a single CdS micropod with diameter ranging from 250 nm at the root to 400 nm at the tip. The CdS micropod shows a clear hexagonal cross section as emphasized with a dashed line in the image. Besides the micropods with a hexagonal cross section, most of
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Figure 8. (a) TEM image and (b) HRTEM image of the branched CdS nanowires deposited on substrate 4.
Figure 5. SEM image of CdS multipod structures on substrate 3.
Figure 6. SEM images of CdS micropod with (a) hexagonal cross section; (b) irregular cross section.
obtained at the position, and they formed a network structure on the Si substrate. The branched nanowires were clearly indicated with dashed circles in Figure 7a. Figure 7b,c shows the magnified SEM images of the deposited branched CdS nanowires. The diameter of the CdS nanowires is about 70 nm, and their length is up to several tens of micrometers. The composition of the branched CdS nanowires was analyzed by EDX, and the pattern is shown in Figure 7d. The EDX pattern shows only peaks of Cd and S, confirming the formation of pure CdS. In addition, transmission electron microscopy (TEM) images of individual branched CdS nanowire provide further insight into the structure of these materials. Figure 8a is the low-magnified TEM image of a single branched CdS nanowire. It shows that the diameter of the nanowire is about 70 nm, in agreement with the results from SEM observations. The corresponding SAED pattern shown in Figure 8a inset shows the single-crystal nature of the nanowire. Figure 8b corresponds to the HRTEM image of the CdS nanowires. It also reveals that the nanowire has a single crystalline structure. The lattice fringe is about 0.336 nm, corresponding to the (0002) fringes, confirming [0001] as the preferred growth direction for the wurtzite CdS nanowires. Vapor-solid (VS) and vapor-liquid-solid (VLS) mechanisms have been widely used to explain the formation of one-dimensional structures.27,28 In our work, we considered that the formation of CdS multipod-based structures could be enucleated by the VS mechanism. According to the previous study, the hot CdS gas will decompose to generate Cd gas and S2 gas at high temperature.29 Therefore, the chemical reactions we employed in the synthesis of CdS multipod-based structures could be described as follows:
CdS (s) f CdS (g)
(1)
2CdS (g) f 2Cd (g) + S2 (g)
(2)
Figure 7. (a-c) SEM images of branched CdS nanowires deposited on substrate 4; (d) the corresponding EDX pattern.
2Cd (g) + S2 f CdS (s)
(3)
the CdS micropods with a larger diameter (> 500 nm) are of an irregular cross section as shown in Figure 6b. The SEM images of the product deposited on substrate 4 were shown in Figure 7a-c. The images indicated that long branched CdS nanowires were
During the reactions, Cd and S2 gases flow with the Ar carrier gas to the low temperature region and react with each other to form CdS again. When the supersaturation increases to a level at which nuclei form, the produced CdS nuclei grow to sizes larger than the critical size and the nuclei deposit on the substrate. For
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Figure 9. Illustration of {CdS4} tetrahedral in wurtzite CdS.
wurtzite CdS, it is intrinsically an anisotropic material with a unique c axis. It can be regarded as a stacking of {CdS4} tetrahedral by sharing their common corners (Figure 9). The growth direction of CdS crystal is determined by the relative stacking rate of the constituent tetrahedral in various crystal faces, and the stacking rate is strongly dependent on the bonding force of atoms in the tetrahedron at the interface. As for the interface of CdS, each tetrahedron has a corner in the [001] direction, which favors the growth of CdS nanorods along the [001] axis. So the CdS nuclei will grow into rod-shaped structures along c axis. During deposition, the CdS vapor pressure and concentration decrease with the increase in distance between the source materials and the substrates. Near the source materials, the CdS vapor pressure and concentration are very high and a nearly spherical, but still a faceted CdS nucleus must be formed to minimize the surface area, so the products deposited on substrate 1 are multipod microrods with spherical symmetry. With the increase in distance between the source materials and the substrate, the CdS vapor pressure and concentration decrease rapidly so only CdS tetrapod microrods form on substrate 2, where the CdS concentration is still high enough for the formation of high density of microrods. For substrate 3, only low-density CdS tetrapod-like nanorods formed for the lack of a high enough concentration of CdS gas. The decomposition of CdS to generate Cd and S2 gases was also confirmed by our results as well as the previous report. Figure 10a shows the XRD pattern of the product deposited on substrate 5, which was put at the downstream exit of the furnace. All the peaks in this pattern can be indexed to the hexagonal Cd phase with lattice constants comparable to the literature (JCPDS Card, No. 65-3363). No peaks of CdS or other impurities were detected. Figure 10b,c showed the SEM images of the deposited Cd products. They clearly revealed that the products are composed of large amounts of nanowires. Most of the nanowires were highly curved and about 100 nm in diameter and about tens of micrometers in length. The corresponding EDX pattern shown in Figure 10d showed the peak of only Cd, confirming the formation of pure Cd nanowires. The peak of Si in this pattern comes from the silicon substrate. The formation of Cd nanowires can be explained as follows. Cd gas has a much higher partial pressure than does S2 gas,29 so some Cd can be separated from S2 gas and deposited at
Figure 10. (a) XRD pattern, (b-c) SEM images, and (d) EDX pattern of Cd nanowires deposited on substrate 5.
Figure 11. Raman spectra of CdS flowerlike structures.
the low-temperature region and form the final Cd nanowires. The S2 gas may be transferred to the lowtemperature region and carried out of the quartz tube or deposited on the end part of the tube, and we did observe yellow S powders at the end of the quartz tube. The detailed growth mechanism for the formation of Cd nanowires undoubtedly needs further studies. Figure 11 displays the Raman spectra of the flowerlike CdS structures. Because of the strong fluorescence background, the Raman intensities of the flowerlike CdS structures are very weak. But from the spectra, it can be seen that there are two Raman peaks at 300 and 605 cm-1, which corresponds to the first- and second-order longitudinal optical phonon (LO) modes of CdS, respectively. It was in good agreement with previous report on CdS nanostructures.30 We also studied the PL properties of the deposited CdS multipod-based structures obtained in the present experiments. Figure 12 is the room-temperature PL spectra of flowerlike CdS multipods. The intense peak at 505 nm is due to the near-band-edge emission of CdS, in accordance with the previous reports. A very low intense broad emission centered at 670 nm can be detected, which is often connected with the structure defects, ionized vacancies, or impurities.10,17,31,32 To know more about PL of the CdS multipod-based structures, all the products with different morphologies have been investigated. It was found that all the products
CdS Multipod-Based Structures
Figure 12. Room-temperature PL spectra of CdS multipodbased structures.
had similar PL spectra at room temperature. No obvious shift was detected for all the products. Conclusion In conclusion, we successfully synthesized CdS multipod-based structures by atmospheric thermal evaporation of CdS nanoparticles without the use of catalyst. Studies found that the morphologies of the products could be well controlled by adjusting the deposition position. Vapor-solid growth mechanism is proposed for the formation of CdS multipod structures. Considering the simplicity of the procedure, the easily morphologycontrolled and high yield of products, the method described here is likely to be of interest to commercial production and to synthesize other II-VI or other group’s semiconductor micro-/nanostructures with controllable morphology. Acknowledgment. This work was supported by Center for Nanotubes and Nanostructured Composites at SKKU, by National R&D Project for Nano Science and Technology of MOST. References (1) Mcclean, I. P.; Thomas, C. B. Semicond. Sci. Technol. 1992, 7, 1394. (2) Deshmuhk, I. P.; Holikatti, S. G.; Hankare, P. P. J. Phys. D: Appl. Phys. 1996, 27, 1784. (3) Peng, X.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019.
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