Abstract
We investigated the surface work function (WS) and its spatial distribution for epitaxial VO2/TiO2 thin films using Kelvin probe force microscopy (KPFM). Nearly grain-boundary-free samples allowed observation of metallic and insulating domains with distinct WS values, throughout the metal–insulator transition. The metallic fraction, estimated from WS maps, describes the evolution of the resistance based on a two-dimensional percolation model. The KPFM measurements also revealed the fractal nature of the domain configuration.
Highlights
Some metal oxides exhibit first-order phase transitions, in which different electronic and magnetic phases coexist during the transition[1]
The domain configurations significantly affect the phase transition behavior of VO2 thin films, as well as the performance of prototypical devices based on VO2 thin films, including a split-ring-resonator-based metamaterials[17], active terahertz nanoantennas[18], ultra-thin perfect absorbers[19], and phase-transition-driven memristive systems[20]
All of these considerations raise the following question: How can we directly observe the metallic and insulating domain configurations of VO2 thin films and their evolution throughout the transition? Conventional x-ray diffraction (XRD), photoemission spectroscopy, and optical spectroscopies can be used to investigate the physical properties of VO2 during the MIT21; these techniques provide only the physical quantities averaged at the macroscopic scale
Summary
Some metal oxides exhibit first-order phase transitions, in which different electronic and magnetic phases coexist during the transition[1]. All of these considerations raise the following question: How can we directly observe the metallic and insulating domain configurations of VO2 thin films and their evolution throughout the transition? We demonstrated that KPFM measurements of the local surface work function (WS) can be used to reveal the intriguing phase transition behaviors of the VO2/TiO2 thin films.
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