Dynamics of phonons, charge-regulated itinerant VO2 states, and their impacts on the memristor properties of thin VO2 films

Abstract

A molecular VO2 semiconductor (SC) reveals exotic properties of electronic switches, memristors, neuromorphic computing, and other devices at a nonlinear charge order at coupled ‘e-e’ and ‘e-p’ interactions (e: electron and p: phonon). Temperature (and/or other means) induced electron-hole ‘e−−h+’ pairs order at successive VO2 polymorphic phases, with a congruent SC to metallic phase transition at Tc ∼ 345 K. To develop these strategies, here we study the phonons (Raman bands, 100–800 cm−1), minor I-V (current-voltage) loops at small ramps of 1 to 30 V fields, and impedance resistance (at 102 to 106 Hz frequencies) at thin VO2 films (≤ 100 nm thickness). The films contain epitaxial (011) VO2 nanocrystals (NCs). The temperature is varied from 295 K to 385 K in which the VO2 phase orders led over thermal-induced charge carriers. A model VO68− molecular VO2 unit is used to analyse the phonon bands that soften and lose intensities (vanish at the Tc) at the metallicity rises up towards the Tc point. It describes the total 18 phonons (with three ones at a shorter V4+−O − V4+ dimer bond) that are sorted out at varied temperatures in a thermal cycle. Due transitions M1 → M2/R1/R2/R-VO2 are resolved (at a monoclinic M1-VO2 phase) at the specific conditions. An applied frequency is shown to promote the distinct VO2 phases at a nonlinear charge order mediated with the phonons states. An inbuilt strain induces a phonon hardening what is it releases at softer metallic VO2 states. The minor I-V loops swipe oppositely over the applied fields at the two major types of VO2 electronic bands. The results are described in correlation to the VO2 network structure in small NCs in the films of a memristor. © 2024 Elsevier B.V.