The Structural Phase Transition in Individual Vanadium Dioxide Nanoparticles
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The Structural Phase Transition in Individual Vanadium Dioxide Nanoparticles Felipe Rivera, Art Brown, Robert C. Davis, Richard R. Vanfleet Brigham Young University – Department of Physics and Astronomy, N283 ESC, Provo, UT 84602, U.S.A. ABSTRACT Vanadium dioxide (VO2) single crystals undergo a structural first-order metal to insulator phase transition at approximately 68°C. This phase transition exhibits a resistivity change of up to 5 orders of magnitude in bulk specimens. We observe a 2-3 order of magnitude change in thin films of VO2. Individual particles with sizes ranging from 50 to 250 nm were studied by means of Transmission Electron Microscopy (TEM). The structural transition for individual particles was observed as a function of temperature. Furthermore, the interface between grains was also studied. We present our current progress in understanding this phase transition for polycrystalline thin films of VO2 from the view of individual particles. INTRODUCTION Vanadium is a transition metal whose oxides undergo a metal-to-insulator transition (MIT) at some characteristic temperature.[1,2] Among vanadium’s oxides, vanadium dioxide (VO2) has been extensively studied because its transition temperature occurs near room temperature (340K 68◦ C).[2–4] In single crystals, vanadium dioxide undergoes significant, abrupt, and reversible changes in several of its properties during its phase transition. These changes to its properties include: 1) A structural change from a low-temperature semi-conducting monoclinic phase to a high-temperature tetragonal metallic phase; 2) a resistivity change of several orders of magnitude; and 3) a sharp change in optical transmittance in the infrared region. These optical and electronic properties that vanadium dioxide exhibits due to its phase transition hint at the use of this material for optical[5–8] and electronic[9,10] applications, such as thermochromic coatings for windows, thermal sensors, or fast optical and electronic switches. VO2 shifts between a low-temperature, low-symmetry monoclinic structure to a hightemperature, higher-symmetry tetragonal structure.[11–14] Table I shows the standard structures for VO2. Due to this structural change, it is possible to use scattering techniques to probe the transition of crystalline VO2.[11–13,16] In this present study, Transmission Electron Microscopy (TEM) was employed to study this phase transition by taking advantage of the structural change in VO2. Furthermore, Scanning Transmission Electron Microscopy (STEM) was used to probe the phase transition of individual grains as well as the interface between grains. Even though the phase transition for VO2 is cited to be 68°C (it was first induced by temperature [1]) there are several factors that will modify, tune, alter, or even induce the phase transition. Variations in stoichiometry, particle size, stress, misorientations between grains, morphological faults, dopants, and other “imperfections” have been used qualitatively to describe changes in the transition temperatur
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