Size-dependent optical properties of VO2 nanoparticle arrays.

TLDR: In this paper, the authors observed that the optical contrast between the semiconducting and metallic phases is dramatically enhanced in the visible region, presenting sizedependent optical resonances and size-dependent transition temperatures.

Abstract: The size effects on the optical properties of vanadium dioxide nanoparticles in ordered arrays have been studied. Contrary to previous VO2 studies, we observe that the optical contrast between the semiconducting and metallic phases is dramatically enhanced in the visible region, presenting sizedependent optical resonances and size-dependent transition temperatures. The collective optical response as a function of temperature presents an enhanced scattering state during the evolving phase transition. The effects appear to arise because of the underlying VO2 mesoscale optical properties, the heterogeneous nucleation behind the phase transition, and the incoherent coupling between the nanoparticles undergoing an order-disorder-order transition. Calculations that support these interpretations are presented. The study of photon-matter interactions at nanometer length scales has as its ultimate goal the optimization of the coupling between selected radiation modes and specific material excitations. Among the most exciting prospects are those that incorporate metal or semiconductor nanostructures into periodic arrays, with potential applications as photonic crystals [1], biochemical sensors [2], and near-field electromagnetic waveguides [3]. In periodic arrays of nanostructures, new properties arise from the combination of nanoscale material features and the periodicity of the arrays. These include the size-dependent and shape-dependent shift of optical resonances, and near-field or far-field coupling already observed in two-dimensional metal nanoparticle arrays [4 –7]. In this Letter, we describe first observations of the collective optical response of vanadium dioxide nanoparticle (NP) arrays during a reversible semiconductorto-metal phase transition (SMT). A previously unreported resonance in the visible region is found in the spectral signature of the SMT, and the resonance peak, apparently due to multipole effects, is blueshifted with decreasing NP size. Unlike the hysteretic response of VO2 NPs with inhomogeneous size and spatial distributions, the hysteresis associated with the NP arrays shows a distinctive three-state behavior resulting from the differential scattering efficiency between the metal and semiconducting phases of VO2. This unique optical response may be viewed as an order-disorder transition in the spectral response of the array, superimposed on the hysteretic response of individual NPs, and contains significant new insights about the stochastic nature of the mechanism that triggers the metal-insulator transition. VO2 exhibits a SMT at a critical temperature Tc 67 C that is a result of an atomic rearrangement. Above T c , VO 2 has a tetragonal rutile structure and exhibits metallic properties. Below Tc, VO2 is a narrow-gap semiconductor with a monoclinic unit cell [8]. The reversible VO2 SMT displays a 10 4 jump in conductivity and