Stochastic domain nucleation across the insulator-metal transition of isolated VO2 nanoparticles

Abstract

Vanadium dioxide (VO₂) has emerged as one of the most studied smart optical materials due to its reversible insulator-metal transition (IMT) that manifests morphology and size-dependent optical properties. Employing synchrotron-based nano-Fourier transform infrared spectroscopy (nano-FTIR) and scattering-type scanning near-field optical microscopy (s-SNOM), we report the IMT of isolated VO₂ nanoparticles (NPs). Thermal-induced random domain nucleation and metallization of V⁴⁺ (3d¹) correlated electrons in isolated VO₂ NPs are reported. Nano-FTIR as a function of temperature provides site-specific insights into VO₂ NPs by recording local amplitude and phase spectra that directly correlate the metallization of VO₂ NPs with their optical absorption characteristics. These spectra also indicate the phonon modes of the monoclinic phase of VO₂ (~680-750 cm⁻¹) that establish the distinct characteristics of electron-phonon interaction. Temperature-dependent infrared nano-imaging of VO₂ NPs (55-250 nm) reveals a different transition behaviour for individual NPs rather than an average transition temperature (T_c). The underlying microscopic mechanisms of the phase transition in VO₂ NPs and their size-dependent metallization are discussed within the framework of classical nucleation theory and a combined Peierls and Mott thermodynamic phenomenological model. The present observations offer profound implications for designing sensing devices based on these nanoscale elements.