A three-step phase transition upon high charge injection in VO2 platelets

Abstract

The present study reports the dynamics of phase transitions at the high electron injection limit in two-dimensional layers of VO2, an archetypical strongly correlated oxide that undergoes an insulator to metal transition (IMT) at 67 °C. Prior studies to date have reported electron doping as high as ∼1021 cm−3 in epitaxial thin films and nanoparticles of VO2 through catalytic spillover and electrochemical gating in ionic liquid electrolytes, which has been shown to induce a sequential insulator-to-metal-to-insulator phase transition. With the use of two-dimensional crystalline platelets, which enable fast Li+ diffusion and out diffusion kinetics during electrochemical gating, we show that an electron density as high as 4 × 1023 cm−3 can be reversibly injected into VO2 without significant structural damage. This leads to a giant conduction modulation involving an unprecedented three-step insulator-to-metal-to-insulator-to-metal transition along with a switch in the electrical polarity from n-type to p-type due to electron doping. A unified “lattice redox model” to explain the origin of thermal-, electrochemical-, and compositional-induced IMT that involves vanadium redox-induced band filling, structural distortion, and electron correlative effects is proposed.