Synthesis of CdO Nanoneedles for Photonic and Sensing Applications
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Synthesis of CdO Nanoneedles for Photonic and Sensing Applications Xiaolei Liu, Chao Li, Song Han, Chongwu Zhou Dept. of E.E.-Electrophysics, University of Southern California, Los Angeles, CA 90089, U.S.A. ABSTRACT Single-crystalline needle-shaped CdO nanostructures were synthesized using a chemical vapor deposition method and characterized using a variety of techniques. Devices consisting of individual CdO nanoneedles were fabricated and high conductance as well as high carrier concentrations was observed. The temperature dependence of the conductance revealed thermal excitation as the dominating transport mechanism. Our devices exhibited good sensitivity to both infrared light and diluted NO2 gas, indicating potential applications as infrared photo-detectors and toxic gas sensors. INTRODUCTION In the past few years, extensive efforts have focused on the synthesis and characterization of one-dimensional nanostructures such as semiconductive nanowires.1-7 However, there is a lack of report on the synthesis and electronic transport properties of one-dimensional transparent conductive oxide (TCO) nanostructures, despite their enormous potential to be used in front panel displays and solar batteries.8 Among the TCO family, nanostructures based on CdO are particularly interesting because of their potential to serve as electrodes for nanoscale light emitting diodes and lasers.9 In fact, bulk CdO shows a wide direct band gap of 2.27 eV and a narrow indirect band gap of 0.55 eV.10 Several techniques have been used to prepare polycrystalline CdO nanowires,11 single crystalline nanobelts12 and micro-whiskers;13 however, further investigations on the electrical, optical and chemical properties of high-quality, singlecrystalline CdO one-dimensional nanostructures are still lacking. Here we report the use of a chemical vapor deposition (CVD) method to grow single-crystalline CdO nanoneedles and our investigations on their electronic, opto-electronic and chemical sensing properties. EXPERIMENTAL DETAILS In our experiments, Si/SiO2 substrates were cleaned and then coated with 20~30Å gold, followed by baking the substrates under argon environment for half an hour. The cadmium source and the substrates were then loaded into the upper stream and the down-stream of a quartz tube furnace, respectively. The substrates zone was heated up to 850~900 °C followed by heating up the Cd source to 350 °C to generate Cd vapor, which was carried down-stream by a constant argon flow with about 0.02% oxygen for about 30 minutes. After cooling down, the sample surfaces looked rough and showed a dark brown or crimson color. Fig. 1 shows a scanning electron microscope (SEM) image of the final product. The synthesized nanostructures appeared needle-shaped with sharp tips about 40~100 nm in diameter and wide butts of several hundred nanometers to several micrometers at the other end, while the lengths of the nanoneedles
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