Disentangling the structural and electronic phase transitions in ultrathin vanadium dioxide

非技术描述：该项目涉及支撑极薄二氧化钒从绝缘到金属行为的可逆开关的科学。这项研究的目标是确定在纳米尺度上热驱动转变过程中发生了什么，以及如何控制它。了解并控制二氧化钒不同可能相之间的转变，将使节能计算机和调节建筑物冷暖的智能窗户的发展取得进展。延长了对德国不来梅大学进行实验的国际访问，以补充宾厄姆顿和不来梅大学之间联合在线课程的发展。来自代表性不足的少数民族的本科研究人员参与了研究活动，包括专门的国家研究机构实地考察，如布鲁克海文国家实验室。一项旨在吸引年轻学生学习科学和工程的K-12教育推广活动采用了在线视频演示和信息图表。技术细节：二氧化钒的突然金属绝缘体转变是电子和晶格之间复杂相互作用的一个典型例子。伴随着电子转变的结构相变可以由室温附近的小热扰动触发。在过渡期间预计会存在额外的相，或者可能被应变薄膜稳定。然而，这些阶段的确切性质和存在是一个争论的热点话题。高质量超薄膜的出现，新开发的纳米级光谱显微镜技术，以及最近复杂的计算研究为探索这些相提供了理想的机会。纳米尺度分辨率光谱显微镜技术可以准确地同时测定二氧化钒相变中共存的相分离区域的电子和几何构型。研究目的是确定纳米级二氧化钒的结构和电子相变是否本质上解耦，正如中间态的存在所表明的那样，或者材料是否可以通过应变和掺杂来定制。测量参数，如晶格常数、态密度和钒配位被用作二氧化钒能带结构计算的输入和约束。研究生和本科生都在美国和国外的国家研究机构接受尖端低能/光电子电子显微镜和小点x射线光谱学的培训。

NON-TECHNICAL DESCRIPTION: The project addresses the science that underpins the reversible switch from insulating to metallic behavior in extremely thin vanadium dioxide. The goal of this research is to determine what happens at the nanometer scale during the thermally driven transition, and how to control it. Understanding and controlling transitions between the various possible phases of vanadium dioxide will enable advances to be made in the development of energy-efficient computers, and smart windows that regulate the heating and cooling of buildings. Extended international visits to Bremen University in Germany to perform experiments complement the development of joint on-line courses between universities in Binghamton and Bremen. Undergraduate researchers from underrepresented minorities are involved with the research activities, including dedicated field trips to national research facilities, such as Brookhaven National Laboratory. A K-12 educational outreach to attract younger students to science and engineering employs on-line video demonstrations and infographics. TECHNICAL DETAILS: The abrupt metal insulator transition of vanadium dioxide is an archetypal example of the complex interplay between the electrons and the lattice. A structural phase transition accompanies the electronic transition, which can be triggered by small thermal perturbations near room temperature. Additional phases are expected to exist during the transition or may be stabilized by strained thin films. However, the exact nature and presence of these phases is a hot topic of debate. The advent of high quality ultrathin films, newly developed nanoscale spectromicroscopy techniques, and recent sophisticated computational studies present an ideal opportunity to explore these phases. Nanoscale resolution spectromicroscopy techniques can determine exactly and simultaneously the electronic and geometric configurations in phase-separated regions that coexist through the phase transitions of vanadium dioxide. The research objective is to determine whether the structural and electronic phase transitions in nanoscale vanadium dioxide are intrinsically decoupled, as suggested by the presence of intermediate states, or whether the material can be tailored to be so by strain and doping. Measured parameters, such as lattice constants, density of states, and vanadium coordination are used as inputs and constraints for band structure calculations of vanadium dioxide. Students, at both the graduate and undergraduate level, are being trained in cutting-edge low-energy/photoelectron electron microscopy and small-spot X-ray spectroscopy at national research facilities in the US and abroad.