Bicrystalline CdS Nanoribbons - Crystal Growth & Design (ACS

Bicrystalline CdS Nanoribbons - Crystal Growth & Design (ACS...

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Bicrystalline CdS Nanoribbons Xia Fan,†,‡ Ming-Liang Zhang,† Ismathullakhan Shafiq,† Wen-Jun Zhang,† Chun-Sing Lee,† and Shuit-Tong Lee*,† Center of Super-diamond and AdVanced Films and Department of Physics and Materials Science, City UniVersity of Hong Kong, Hong Kong SAR, P. R. China, and Nano-organic Photoelectronic Laboratory and Laboratory of Organic Optoelectronic Functional Materials and Molecular Engineering, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, China

2009 VOL. 9, NO. 3 1375–1377

ReceiVed June 9, 2008; ReVised Manuscript ReceiVed NoVember 23, 2008

ABSTRACT: We report the synthesis and characterization of a unique CdS architecture made of two intersecting ribbons that form a ridge-shaped bicrystalline CdS nanoribbon via simple thermal evaporation of CdS powder. The CdS bicrystalline nanoribbons have a tapering width decreasing from 600 to 50 nm and lengths of several tens of micrometers. The nanostructures show strong band gap emission and weak defect-related emission. Zigzag CdS bicrystalline nanoribbons are occasionally formed at the tip region. Introduction Recently, a variety of single-crystal semiconductor nanostructures have been synthesized, motivated by their potential applications as building blocks for optical and electronic devices, and biosensors.1,2 As structural parameters such as size and shape have significant influences on materials properties, extensive efforts have been expended to achieve controlled synthesis of nanostructures.3,4 Because of their unique and interesting geometry, a variety of bicrystalline nanostructures have been reported, such as ZnS, ZnO, In2O3, Si, and GaN nanowires and nanoribbons5-11 with a planar twinning boundary parallel to the long axis of the nanostructures. While there are a large number of reports on planar bicrystal nanostructures, there are relatively few on nonplanar nanostructure with a twin-crystal boundary.4 CdS, as an important II-VI group semiconductor with a direct band gap of 2.45 eV at 300 K, is of particular interest owing to its wide-ranging optoelectronic applications. A variety of CdS nanostructures, including nanowires and nanoribbons,12-15 have been synthesized and fabricated into nano-optoelectronic devices,16-19 such as optical switches, fieldeffects transistors. Herein, we report the synthesis and characterization of a unique CdS architecture made of two intersecting ribbons forming a ridge-shaped bicrystalline CdS nanoribbons. Experimental Section The synthesis apparatus includes a high-temperature horizontal tube furnace, as described previously.4 A total of 2 g of CdS powder (Alfa Aldrich, 99.9%) was put in an alumina boat and placed at the highest temperature zone of the quartz tube. A piece of silicon wafer coated with a gold film was put downstream of the tube. After the system was evacuated to 4.5 × 10-4 Torr, high-purity argon was fed at a rate of 50 sccm. The temperature at the central part of the furnace was then ramped to 800 °C and held at this temperature for 2 h. During the whole heating process, the pressure inside the tube was maintained at 100 Torr. The deposits on the silicon wafer were analyzed with X-ray diffraction (XRD) with Cu KR radiation (Siemens D-500), fieldemission scanning-electron microscopy (FE-SEM, Philips, XL 30), transmission-electron microscopy (TEM, Philips, CM200 FEG and CM20 operated at 200 kV), and energy dispersive X-ray spectroscopy (EDS) in a CM20 TEM. Photoluminescence (PL) spectra of the samples were obtained at room temperature using a pulsed Nd:YAG laser (266

Figure 1. (a) Low-magnification SEM image; (b) and (c) enlarged SEM images; and (d) XRD spectrum of CdS bicrystalline nanoribbons. nm) as an excitation source. Cathodoluminescence (CL) spectra were obtained with a CL system attached to a Philips XL 30 FEG SEM.

Results and Discussion * To whom correspondence should be addressed. E-mail: [email protected] † City University of Hong Kong. ‡ Technical Institute of Physics and Chemistry, Chinese Academy of Sciences.

For SEM investigation, the CdS deposit on the Si substrate was directly loaded into the SEM, without disturbing the original nature of the yellowish product. Figure 1a shows that the deposit consists

10.1021/cg800599r CCC: $40.75  2009 American Chemical Society Published on Web 01/15/2009

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Fan et al.

Figure 2. (a) Bright-field TEM image of bicrystalline nanoribbons; (b) TEM image showing the catalyst particle; (c) EDS of the nanoribbon in (b); (d) EDS of the region marked with red rectangle in (b); (e) SAED pattern; and (f) HRTEM image of a bicrystalline nanoribbon shown in (a).

of nanoribbons with lengths up to several tens of micrometers. A representative high-magnification SEM image (Figure 1b) shows that most of the nanoribbons are composed of two distinctive parts fused together to form the ribbon (indicated by arrows). The thickness of the bicrystal nanoribbons is about 60 nm. In contrast to the previous reports, there is a small angle between two parts of the ribbon. Figure 1c is the SEM image of the product deposited at the lower temperature region. It clearly shows that the nanoribbon has two intersecting ribbons sharing a common “spine” over the entire length. Thus, the CdS nanoribbon is not planar. A powder XRD spectrum of the nanoribbons is shown in Figure 1d, and is in good agreement with the JCPDS card (41-1049) for the typical hexagonal wurtzite CdS crystals with lattice constants of a ) 4.141 Å and c ) 6.720 Å. Hence, the as-prepared nanoribbons are determined to be pure hexagonal CdS. Detailed structure and composition of the product were characterized using TEM, EDS, and selected-area electrondiffraction (SAED). Figure 2a shows a low-magnification TEM image of the as-grown CdS nanoribbons. It clearly shows that the nanoribbon is composed of two distinctive parts forming a bicrystal structure. The nanoribbon has a clear grain boundary at the center and along the length. Further, the width of bicrystalline nanoribbons typically tapers from 600 to 50 nm from one end to the other. TEM image in Figure 2b shows a metallic particle at the narrower end of the CdS bicrystalline nanoribbon. The EDS elemental analysis (Figure 2c) reveals the body of the nanoribbon contains only Cd and S (Cu signal comes from the Cu TEM sample grid) with a Cd/S atomic ratio of 52:48. At the narrow end of the nanoribbon marked with a red rectangle in Figure 2b, gold can be detected (Figure 2d), as Au was used as catalyst during growth process. Figure 2e shows a typical SAED pattern of the CdS bicrystalline nanoribbon. It can be indexed as the [2-1-10] zone axis diffraction pattern with two sets of spots. The two patterns have a common (0-113) plane, which agrees well the HRTEM results. HRTEM image Figure 2f clearly shows the growth direction of the nanoribbon to be [03-32]. The grain boundary is parallel to the (0-113) plane, which is a symmetric tilt boundary. The angle between two (0001) planes is 64.2°. These results further confirm that the deposit is bicrystal CdS nanoribbons.

Figure 3. Room-temperature PL (a) and CL spectra (b) of bicrystalline CdS nanoribbons.

The PL spectrum of the nanoribbons in Figure 3a shows only a strong peak at 513 nm due to band gap emission of CdS, and no other defect-related emission peaks. In the CL spectrum of nanoribbons (Figure 3b) the peak at 504.5 nm is due to band gap emission, whereas the broad emission at 600-800 nm centering around 675 nm is due to defect emission, such as S atoms vacancies.20 The appearance of defect emission only in CL spectrum suggests that defects primarily reside in the nearsurface region, since CL is more surface sensitive than PL due to shorter excitation depth of the electron beam. In our experiment, the growth of bicrystalline CdS nanoribbons with a tapering width may be understood with the combination of two growth processes, vapor-liquid-solid (VLS) and vapor-solid (VS) mechanism.15 During growth, evaporated CdS was carried by the carrier gas and traveled to the lower temperature region, where they would be absorbed and combined into the Au droplets on the silicon wafer. When CdS concentration in the droplet reached supersaturation, CdS bicrystalline nanoribbons would form to release the high surface/ interface energy between gold particle (l) and CdS (s). The growth in the length direction would follow Au-catalyzed VLS growth with [03-32] direction, while the surrounding Cd and S vapors would deposit on the sides of the nanoribbon following the VS process. Finally, nanoribbons with a tapering width would be formed with Au tips. In addition to the CdS bicrystalline nanoribbon, we observed two other kinds of nanoribbons of different morphologies. Figure 4a shows that the width of the bicrystalline nanoribbon tapers from 500 to 150 nm toward the tip. Figure 4c shows the TEM image of a nanoribbon with a sharp and narrow tip, but the

Bicrystalline CdS Nanoribbons

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bicrystalline nanoribbons have two intersecting parts with tapering widths decreasing from 600 to 50 nm, and lengths of several tens of micrometers. The growth of CdS bicrystalline nanoribbons is regarded to follow a combination of VLS and VS processes. Optical and CL characterization suggests the bicrystalline nanoribbons are high-quality crystal with low defect density in the near-surface region. Acknowledgment. The work is supported by a NSFC/RGC Joint Research Scheme (N_CityU125/05) from the Research Grants Council of Hong Kong SAR, the US Army International Technology Center - Pacific and the National Basic Research Program of China (973 Program) (Grant No. 2006CB933000 and 2007CB936000). Supporting Information Available: SEM images, EDS, and TEM images of bicrystalline CdS nanoribbons with gold particles as catalysts. This information is available free of charge via the Internet at http://

References Figure 4. (a) SEM image and (c) TEM image of CdS bicrystalline nanoribbons with sharply decreasing widths; (b) SEM image of zigzag bicrystalline nanoribbons; (d) a side-view TEM image of zigzag bicrystalline nanoribbon; (e) TEM image of a zigzag bicrystalline nanoribbon; (f), (g) SAED patterns of the respective rectangle region in panel e (scale bar is 200 nm).

ribbon still has a clear midgrain boundary along the whole length. Another kind of nanostructure, shown in Figure 4b, is a zigzag shaped CdS nanoribbon, which has a length of less than 3 µm with the zigzag structure at the tip region. The periodic length in the zigzag ranges from 200 nm to 1 µm, and the angle of the zigzag junctions is about 115.8°. Figure 4d is the sideview TEM image of the zigzag bicrystalline nanoribbon, revealing that the wave amplitude of the zigzag is in the range of 300-400 nm. Figure 4e shows the TEM image of the zigzag nanoribbon, while Figure 4f,g shows the respective SAED patterns of the rectangle regions in Figure 4e. The TEM results show that the zigzag shape is due to a change of the growth direction from [03-32] to [03-3-2]. While the zigzag-shaped nanostructure has been reported,21,22 its formation in CdS nanoribbons is observed here for the first time, and should offer an interesting system for study. At the initial stage, Cd and S vapor concentrations are expected to be in proper balance for growing a long and smooth bicrystalline nanoribbon. Toward the end of growth, the source material would become deficient. Since S vapor in the furnace tube would be more easily carried to a lower temperature region, a higher ratio of the Cd (g) to S (g) concentration would result. The lower S content is consistent with EDS analysis yielding Cd:S of 52:48, and with the CL results showing S atom vacancies. Changes of supersaturating concentration ratio, temperature, partial pressure, etc, in the reaction system would affect the chemical potential of different crystallographic planes and change the active growth interface (between Au(l) and CdS(s)). According to crystallography, all directions are equivalent in hexagonal CdS. We suggest that the changes in reaction conditions at the end of growth would induce a change in the growth direction from [03-32] to [03-3-2] of bicrystalline CdS nanoribbon or vice versa, leading to the formation of the zigzag structure (Figure 4b).23,24 Conclusions In summary, ridge-shaped bicrystalline CdS nanoribbons were fabricated by simple thermal evaporation of CdS powders. The

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