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Ion beam lithographic fabrication of ordered VO2 nanoparticle arrays R. Lopez, J. Y. Suh, L. C. Feldman and R. F. Haglund, Jr. Department of Physics and Astronomy and Vanderbilt Institute of Nanoscale Science and Engineering Vanderbilt University, Nashville, TN, 37235 ABSTRACT Long-range ordered arrays of vanadium dioxide nanoparticles are fabricated by pulsed laser deposition in a patterned layer of poly(methyl methacrylate) resist. The twodimensional arbitrary pattern is created by focused ion beam exposure of the resist, followed by pulsed laser deposition and thermal annealing. Interaction of light with the nanoparticles is controlled by their geometrical arrangement as well as by the difference in optical properties displayed between the metallic and semiconducting phases of VO2. Arrays like this open opportunities to study optical resonances and interactions for nanoparticles in close proximity, in the framework of the metal-semiconductor phase transition in VO2. INTRODUCTION The control of light-matter interactions is one of the most interesting and active fields in the materials sciences, with the ultimate goal of optimizing the coupling between selected radiation modes and material excitations. Metal and semiconductor nanoparticles1,2 provide a unique suite of exceptional optical properties that, tailored via the proper design of periodic nanostructures, have many potential applications as photonic crystals,3 bio-chemical sensors4,5,6 and even sub-wavelength electromagnetic energy waveguides.7,8,9 The study of the particle-particle coupling is essential to the advance of these and related technologies. The geometrical disposition and the constrained electronic structure of the nanoparticles are the key to understanding these interesting effects. Vanadium dioxide nanocrystals can enhance the understanding of these phenomena since, in addition to the optical features derived from interacting metallic or semiconducting nanoparticles, VO2 can switch between these two general types of electronic behaviors. VO2 exhibits a semiconductor-metal phase transition10 (SMT) at a critical temperature Tc ~ 67 ºC that is a result of an atomic structural rearrangement. Above Tc, VO2 has a tetragonal rutile structure and exhibits metallic properties. Below Tc, VO2 is a narrow-gap semiconductor with monoclinic arrangement.11 The reversible VO2 SMT displays a 104 jump in conductivity and large changes in the optical properties, especially in the IR where VO2 tends to some degree of transparency when semiconducting and great opacity when metallic. The nanoarrays open the door to studying, for example, the excitation and relaxation of the surface plasmon resonance in a material that differs from a semiconductor solely in a minute alteration of its crystalline structure. In the present work, we describe the fabrication of ordered arrays of isolated vanadium-dioxide nanoparticles. The interaction of the light with the nanoparticles is controlled by the collective scattering properties of the geometrical arrangement as well