In Situ TEM Study Of VO₂（A） First-order Phase Transition

First-order phase transition in solid materials has long time been intensely investigated due to its high academic value and wide applications in many fields.However,it remains challenging to exactly determine the nucleation sites at the very beginning of transformation,especially the direct structural evidence of defect-mediated phase transitions and their dynamics at the atomic scale is still missing.VO2 as a typical system of first-order phase transformation has been extensively investigated.Nevertheless,as a metastable phase,the studies of VO2(A)which exhibits a metal-insulator transition(MIT)from low temperature phase(LTP)to high temperature phase(HTP)at 162 ℃ are limited.There is no real time information of the phase transition of VO2(A)at atomic resolution,which makes the whole transition process of VO2(A)elusive till now.In our work,the in-situ heating experiments were carried out in a conventional TEM for HRTEM images and in a STEM for HAADF images.Firstly,our results reveal that the HTP of VO2 nucleated within the LTP and extended along the nano wires’ growth direction.The interface structure between the LTP and HTP is unveiled by Cs-corrected scanning transmission electron microscopy(STEM)high angle annular dark field(HAADF)images.Such information is crucial for understanding the phase transition mechanism of VO2(A).In addition,we report the atomic-scale observation of a unique defect-mediated reversible phase transition between LTP and HTP of VO2.In-situ Cs-corrected STEM images clearly indicate that both phase transitions(from HTP to LTP and from LTP to HTP)start at the defects sites in parent phases.Intriguingly,the structure of the defects within the LTP is demonstrated to be HTP,and the defect in the HTP of is determined to be LTP structure,which means a nucleation seed of a new phase already exist in original phase,so nucleation process is not necessary and the phase transition can start at the defect site.These findings are expected to broaden our current understanding of first-order phase transition and shed light on controlling materials’ structure-property phase transition by "engineering" defects in applications.