Breakthrough in investigating mechanics and dynamics of the strain-induced M1-M2 structure phase transition in individual VO2 nanowires
Title: Mechanics and Dynamics of the Strain-Induced M1-M2 Structural Phase Transition in Individual VO2 Nanowire
Abstract: The elastic properties and structural phase transitions of individual VO2 nanowires were studied using an in situ push-to-pull microelectromechanical device to realize quantitative tensile analysis in a transmission electron microscope and a synchrotron X-ray microdiffraction beamline. A plateau was detected in the stressstrain curve, signifying superelasticity of the nanowire arising from the M1-M2 structural phase transition. The transition was induced and controlled by uniaxial tension. The transition dynamics were characterized by a one-dimensionally aligned domain structure with pinning and depinning of the domain walls along the nanowire. From the stressstrain dependence the Young’s moduli of the VO2 M1 and M2 phases were estimated to be 128+10 and 156+10 GPa, respectively. Single pinning and depinning events of M1M2 domain wall were observed in the superelastic regime, allowing for evaluation of the domain wall pinning potential energy. This study demonstrates a new way to investigate nanoscale mechanics and dynamics of structural phase transitions in general.
我校材料学院“微纳尺度材料行为研究中心（CAMP-Nano）”青年教师陈凯博士在美国劳伦斯伯克利国家实验室留学期间，多方合作，利用透射电子显微镜（TEM）和同步辐射微区劳埃衍射（Synchrotron X-ray Laue microdiffraction）技术，首次原位观察并研究了应变导致的单根VO2 纳米线的相变，并在美国化学协会出版的国际纳米科学技术领域权威刊物《纳米快报》（Nano Letters）上发表了题为“Mechanics and Dynamics of the Strain-Induced M1–M2 Structural Phase Transition in Individual VO2 Nanowires”的论文（Nano Lett., 2011, 11 (8), pp 3207–3213）。陈凯博士与美国劳伦斯伯克利国家实验室的郭华博士为本文并列第一作者。合作者中包括我校材料学院“微纳尺度材料行为研究中心（CAMP-Nano）”的单智伟教授、美国劳伦斯伯克利国家实验室的A.M. Minor教授、加州大学伯克利分校的J. Wu教授、C. Dejoie博士和K.Wang博士，美国Hysitron公司Y. Oh博士、O.L. Warren和S. A. S. Asif博士等。其中单智伟教授，J. Wu教授和A.M. Minor教授为本文的并列通讯作者。
近年来，VO2 因为其较低的相变温度以及伴随相变而发生的电学性能的变化而在材料学界引起了广泛的关注。陈凯博士等人工作的主要特异之处在于，首次利用TEM原位样品台实现了对单根VO2 单晶纳米线的定量拉伸实验，原位观察到了VO2 单晶纳米线在外加应变状态下的相变及相界面的成核、生长过程，获得了准确的VO2 单晶纳米线的力学性能数据，包括杨氏模量、相变应力等，同时该工作也是国际上首次在同步辐射线站上实现了对单根纳米线在动态拉伸条件下形变相变行为的原位观察和研究，借助同步辐射衍射技术针对晶相、晶体取向与晶格应变等演变行为在超高分辨率下研究了应变状态下VO2 单晶纳米线微结构的变化，对理解这种新型材料的结构提供了重要的科学数据，进一步促进了该材料的潜在应用，而在该工作中发展起来的原位TEM与原位同步辐射衍射相结合的方法也势必大大拓展并推动其在微纳米尺度材料结构及性能研究中的应用。
