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

Vanadium dioxide (VO<inf>2</inf>) 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 VO<inf>2</inf> nanoparticles (NPs). Thermal-induced random domain nucleation and metallization of V⁴⁺(3d¹) correlated electrons in isolated VO<inf>2</inf> NPs are reported. Nano-FTIR as a function of temperature provides site-specific insights into VO<inf>2</inf> NPs by recording local amplitude and phase spectra that directly correlate the metallization of VO<inf>2</inf> NPs with their optical absorption characteristics. These spectra also indicate the phonon modes of the monoclinic phase of VO<inf>2</inf> (∼680–750 cm⁻¹) that establish the distinct characteristics of electron-phonon interaction. Temperature-dependent infrared nano-imaging of VO<inf>2</inf> NPs (55–250 nm) reveals a different transition behaviour for individual NPs rather than an average transition temperature (T<inf>c</inf>). The underlying microscopic mechanisms of the phase transition in VO<inf>2</inf> 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.