Engineering The Electronic Anisotropy In VO₂ Flake Via Interfacial-Stress-Induced Oriented Metal-Insulator Transition

In recent years,the in-plane anisotropy of two-dimensional(2D)materials,which originates from the low plane-symmetry structure,has broad prospects in the application of new optoelectronic devices due to their unique angle-dependent physical properties.However,the anisotropy of 2D materials is limited by their inherent crystal structure,and the small intrinsic anisotropy ratio results in difficulty in distinguishing the orientation accurately,which hinders their practical application.Therefore,it is of great significance to explore new strategies that can effectively improve the anisotropy of materials.As we know,the different crystal structures of 2D materials with various properties,and the phase transition result in enormous property changes,hence,the modulation of the anisotropy at a large degree is expected to be achieved via phase transition.In this work,based on the transition metal oxide vanadium dioxide(VO₂)with a metal-insulator transition(MIT),we explored a new strategy to modulate the anisotropy of 2D material by phase transition.Firstly,the 2D VO₂ flakes were successfully synthesized by chemical vapor deposition(CVD)method,then its oriented phase transition was induced by interfacial-stress.Finally,the electronic anisotropy of VO₂ flakes modulated by oriented phase transition was investigated.The main work is listed as follows:(1)The oriented phase transition of 2D VO₂ flakes induced by interfacial-stress.High-quality 2D VO₂ flakes were successfully synthesized via the CVD method.The periodic distribution of different phases in 2D VO₂ flakes was induced by interfacial-stress,due to the different thermal expansion coefficient between mica substrate and 2D VO₂flakes.Both the in-situ Raman and optical microscope give evidence to confirm that the oriented phase transition of 2D VO₂ flakes related to the effects of interfacial-stress,which is important to modulate the anisotropy of 2D VO₂ flakes.Furthermore,the formation and distribution of interfacial-stress,as well as the morphology of surface wrinkles on 2D VO₂were simulated by Finite Element Analysis(FEA),which further elucidated the process of interface-stress induced oriented phase transition in VO₂ flakes.(2)Modulation of electronic anisotropy of 2D VO₂ flakes via oriented phase transition.In view of the interfacial-stress-induced oriented phase transition in VO₂ flakes,we explored the effect of oriented phase transition on modulating the electronic anisotropy of2D VO₂ devices by in-situ temperature-dependent electrical tests.The electronic anisotropy ratio in[011]_M1 and[100]_M1 crystal directions was successfully enhanced from 1.5 times to112.9 times.Meanwhile,the process of the oriented phase transition was observed by in-situ optical microscope.This work provides insights into the mechanism of electronic anisotropy modulation,verifying the feasibility of the modulation of the electronic anisotropy by oriented phase transition.