Nanostructure-Dependent Metal-Insulator Transitions in Vanadium-Oxide Nanowires Jeong Min Baik, † Myung Hwa Kim, † Christopher Larson, Alec M. Wodtke,* and Martin Moskovits* Department of Chemistry & Biochemistry, UniVersity of California, Santa Barbara, California 93106 ReceiVed: June 23, 2008; ReVised Manuscript ReceiVed: July 19, 2008 Single-crystal VO 2 nanowires were synthesized using atmospheric-pressure and physical vapor deposition and outfitted with electrodes for current-voltage measurements. The Mott insulator-to-metal transition temperatures of several nanowires with varying lateral dimensions were determined by measuring the voltage values at which the sharp current step, signaling that the occurrence of the insulator-to-metal or the reverse transitions, had taken place. The observed Mott transition temperatures, which ranged between 62 and 70 °C for the nanowires measured, trended downward with decreasing nanowire width. We ascribe this to strong interactions between the nanowire and the underlying silica substrate. However, the scatter in the Mott- temperature versus nanowire width exceeded the experimental uncertainty in the values of the Mott temperature, indicating that other parameters also contribute to the precise value of the Mott transition temperature of nanostructured VO 2 . Introduction Vanadium dioxide (VO 2 ) is the iconic material which undergoes a Mott metal-insulator transition at a temperature of 68 °C in the bulk. 1,2 As a result, vanadium oxide has been proposed as a suitable material for constructing thermochromic devices and Mott field-effect transistors. 3-6 In general, the Mott transition in VO 2 occurs alongside a structural phase transition from monoclinic (the insulator or semiconductor phase) at temperatures below the Mott transition temperature to tetragonal (the metallic phase) above that temperature, which is ac- companied by several orders of magnitude jump in conductivity. The connection between the structural phase transition and the Mott transition has been considered in a number of publications. 6-8 Because the Mott transition in these systems is brought on by a change in crystalline structure wherein the two phases differ in density, the application of hydrostatic pressure, or, more generally, of stress by whatever means, is expected to have a significant effect on the Mott transition temperature, the sharpness of the transition, and other physical and structural consequences. The Mott transition can also be observed as a result of resistive heating of a VO 2 film or wire by applying an appropriate DC bias across the device while keeping the system at an ambient temperature below the structural transition temperature. 9,10 Because in most structures the local resistance can vary from location to location, Mott transitions induced by the application of a DC bias might bring about local changes in structure accompanying the Mott transition rather than global structural changes that are induced by changes in ambient temperature and pressure. In this paper, we report the synthesis by atmospheric pressure, physical vapor deposition (APPVD) of single-crystal VO 2 nanowires with a narrow width distribution. The nanowires were configured as single-nanowire resistors, and their current-voltage characteristics were measured as a function of ambient tem- perature. The current passing through the nanowire, which is held at a temperature below the Mott transition, resistively heats the nanowire, causing the current passing through the nanowire to increase abruptly at the voltage value at which the nanowire’s temperature first reaches the Mott transition. This transition occurs at an applied voltage which depends on the ambient temperature. This fact, which can be used to improve the precision with which the Mott transition temperature is deter- mined, was used to probe the Mott transition temperature for nanowires with various dimensions. Experimental Section Vanadium oxide nanowires were grown by APPVD on single crystal (100) Si wafers covered with a 200nm thick, thermally grown SiO 2 layer. The wafers were first cleaned in the standard manner. 11 A total of 0.1 g of fine meshed VO 2 (99.9%, Aldrich) powder was placed at the center of a 10 cm long quartz boat without further purification. The SiO 2 /Si substrate was then located in the quartz boat approximately 5 mm from the VO 2 . The quartz boat was then located at the center of quartz tube furnace and high purity He carrier gas (99.999%) was a flowed through the furnace as 300 sccm. The furnace temperature was increased to a temperature in the range of 550-650 °C. After 10∼60 min of growth the samples were allowed to cool to room temperature in He and the nanowire-covered substrate removed from the furnace. High yields of good-quality VO 2 nanowires were only obtained when the procedure was conducted at atmospheric pressure. Field-emission scanning electron microscopy (FEI Sirion XL30-FEG) was used to observe the morphologies of the VO 2 nanowires. The structural properties were determined by the energy dispersive X-ray spectroscopy, X-ray diffraction, and single nanowire Raman spectroscopy. * To whom correspondences should be addressed. E-mail: mmoskovits@ itsc.ucsb.edu; wodtke@chem.ucsb.edu. † These authors contributed equally to this work. 13328 10.1021/jp805537r CCC: $40.75  2008 American Chemical Society Published on Web 08/07/2008 2008, 112, 13328–13331