Local phenomena at grain boundaries: An alternative approach to grasp the role of oxygen vacancies in metallization of VO2

Abstract

Vanadium dioxide (VO2) undergoes an insulator to metal transition (MIT) and an accompanied phase transition from a monoclinic (M) structure to rutile (R) structure near room temperature, forming the basis for many VO2-based functional devices. The MIT transition of VO2 and the functionality of VO2-based devices can be controlled by a variety of chemical and physical stimuli. With these external stimuli, defects, such as oxygen vacancies, are often inevitably introduced. However, due to the VO system-induced challenge to synthesize stable VO2 with different oxygen vacancy concentrations, the impact of oxygen vacancies on the resistance and transition of the VO2 is not fully understood. Oxygen vacancy, as one of the typical defects in VO2, is expected to concentrate at grain boundaries, and hence a concentration gradient of oxygen vacancies may exist between the grains interior and the boundaries, and this suggests a possibility to study the effects of oxygen vacancies on the transition of VO2 by probing local phenomena at the grain boundaries. For investigating local phenomena at the grain boundaries, Scanning Probe Microscopy (SPM) techniques are effective, which allows probing the structure and various properties at the nanoscale. In this work, a series of SPM techniques, including Atomic Force Microscopy (AFM), conductive-AFM (c-AFM), Electrochemical Strain Microscopy (ESM), and Kelvin Probe Force Microscopy (KPFM), are employed to measure variations of the surface structure, the resistance, the oxygen vacancy concentration, and the work function between the grain interior and the grain boundary. It has been demonstrated that, for most cases, both the resistance and the work function are lower at the grain boundaries as a result of the accumulation of oxygen vacancies at those positions. In addition, the resistance change induced by the electric field has been observed in the deposited VO2 thin films, which may be associated with the generation/annihilation of the oxygen vacancies, rather than charge injection. This work has demonstrated the effects of oxygen vacancies in the transition of VO2 by probing the local phenomena at grain boundaries, also provided a new insight into the resistance change of VO2 under an electric field.