单晶外延VO2薄膜可控制备、MIT相变机理与尺寸效应的原子尺度探究

深入理解VO2金属-绝缘体相变（MIT）的微观机理是实现对VO2薄膜MIT相变特性进行有效调控的基础。虽已研究多年，VO2 MIT相变机理至今没有定论。不同于通常在大气环境中对VO2进行的非原位实验研究，本项目提出利用集薄膜生长（MBE）和原位表面分析表征手段于一体的超高真空系统开展VO2薄膜生长及MIT特性的研究。通过精细调节MBE生长参数、调控VO2生长模式，获得具有原子级光滑表面、厚度从亚原子层（纳米小岛）到数十个原子层精确可控的单晶VO2（超）薄膜，采用原子分辨的扫描隧道技术（STM/STS）原位观测表征VO2薄膜MIT相变过程，在原子尺度分析揭示VO2相变机理，探索VO2薄膜MIT量子尺寸效应。通过本项目研究，预期最终解决VO2 MIT微观机理问题，并定量建立薄膜厚度、晶粒尺寸、应力与VO2 MIT相变温度的变化关系，为有目的地调控VO2 MIT相变特性开发其实际应用提供指导。

A deep understanding of VO2 metal-insulator transition (MIT) mechanism is the basis for an effective regulation of the MIT characteristics of VO2 films. Despite many years of study, the VO2 MIT mechanism has not been resolved. Differing from the usual experiments conducted ex-situ in the atmosphere for VO2 study, in this project we carry out the growth of VO2 thin films and the study of their MIT characteristics in an ultrahigh vacuum system that combines molecular beam epitaxy (MBE) for thin film growth and in-situ surface characterization methods. Through fine-tuning of the MBE growth parameters and control of the growth mode, VO2 (Super-) thin films that have an atomically smooth surface with defined thickness from sub-monolayer (nano-islands) to dozens of monolayer are obtained. Then the MIT evolution of the VO2 samples is followed in-situ with atomically resolved STM and STS to reveal the MIT mechanism on an atomic level, and to explore the quantum size effect of the MIT. Through this project, it is expected that the VO2 MIT mechanism can be finally resolved, and a quantitative relationship between the film thickness, grain size and stress and the VO2 MIT phase transition temperature can be established, which will provide guidance for purposeful regulation of VO2 MIT characteristics towards achieving practical applications.