Real Time Quantification of Diffusion and Alloying in Atomically Thin Capillaries

非技术描述：二维(2D)材料一直是密集的科学研究的主题，因为它们的原子薄性质导致了各种各样的新的光学和电学特性。虽然已经进行了大量的基本物理性能调查和器件演示，但在了解结构限制(封装)、高温和大电流等相关工艺和操作条件如何影响其结构和性能方面，仍存在显著的知识差距。在现代微电子学的高性能器件操作过程中，经常会遇到这样的情况。因此，了解这些性质对于了解合金化和掺杂、异质结和接触形成以及电和热故障至关重要。该项目通过在电子显微镜内使用电子束进行直接可视化和光谱分析，帮助了解这些材料在极端物理限制和温度下经历的基本热力学过程和转变。然后，将这些实验与纳米级光学成像技术相结合，将结构变化与光学和电学性质变化相关联。这些研究提高了我们的基础知识，并有助于为合成、加工和器件操作考虑因素制定设计规则。该项目还包括通过培训研究生在前沿材料合成、最先进的显微镜和光学光谱学方面为下一代劳动力做准备。它还包括通过材料研究科学与工程中心的本科生研究经验项目为本科生培养劳动力。最后，研究成果，特别是互动模式，通过公开讲座、科学推广活动以及本科生和研究生课堂教育向更广泛的公众传播。技术描述：该项目的研究目标是研究范德华层状二维硫化物体系中的结构相变和扩散现象。采用近场光学光谱和原位电子显微镜相结合的方法，结合在预图案化衬底上独特的样品制备，研究了结构限制、电流和温度的影响。样品是通过机械剥离和堆积以及化学气相沉积生长来制备的。这项工作利用了将这些硫化物及其垂直异质结构原子薄层覆盖的能力，然后将它们封装在惰性和耐火层中，如石墨或氮化硼。此外，通过化学气相沉积横向缝合和生长面内异质结构的能力进一步允许在电流通过时或在加热到高温时(无论有没有封装)研究面内扩散和分离。为了研究结构的演变，高帧频图像采集被用来从电子显微图像中产生定量数据。由于结构演化和不均匀被认为是在亚可见光波长尺度上的，纳米尺度的尖端增强的光致发光和拉曼光谱被用来将它们与原位电子显微镜数据相关联。这些实验可以定量地了解2D硫化物及其异质结构在物理限制、高电流密度和高温下的演变，所有这些都与高性能器件的运行有关。这些实验可以进一步评估在这些极端条件下的扩散和相变过程的机制及其对光电性能的影响，这为设计材料和器件以实现坚固和高性能操作的战略提供了洞察力。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical description: Two-dimensional (2D) materials have been the subject of intense scientific investigation as their atomically thin nature leads to a wide variety of novel optical and electronic properties. While numerous fundamental physical property investigations and device demonstrations have been performed, there is still a significant knowledge gap in understanding how relevant processing and operation conditions such as structural confinement (encapsulation), high temperature and large current flows affect their structure and performance. Such conditions are commonly encountered during high performance device operation in modern microelectronics. Understanding these properties is therefore critical for understanding the alloying and doping, heterojunction and contact formation, as well as electrical and thermal failure. This project helps understand the fundamental thermodynamic processes and transformations that these materials undergo under extreme physical confinement and temperatures by direct visualization and spectroscopic analysis with an electron beam inside an electron microscope. These experiments are then coupled with nanoscale optical imaging techniques to correlate the structural transformations with optical and electronic property changes. These investigations advance our fundamental knowledge, and help frame design rules for synthesis, processing and device operation considerations. This project also includes next generation workforce preparation by training graduate students in forefront materials synthesis, state of the art microscopy and optical spectroscopy. It also entails workforce development for undergraduate students via the Materials Research Science and Engineering Center's research experience for undergraduate student program. Finally, the research outcomes, and specifically interactive models, are disseminated to the broader public via public lectures, science outreach activities and undergraduate as well as graduate class room education. Technical description: The research goals of this project is to investigate structural phase transformations and diffusion phenomena in van der Waals layered two-dimensional chalcogenide systems. A combination of near-field optical spectroscopy and in-situ transmission electron microscopy methods, combined with unique sample preparation on pre-patterned substrates are used to investigate the impact of structural confinement, current and temperature. The samples are prepared via mechanical exfoliaton and stacking, as well as via chemical vapor deposition growth. The work exploits the ability to layer atomically-thin layers of these chalcogenides and their vertical heterostructures and then encapsulate them in inert and refractory layers such as graphite or boron nitride. In addition, the ability to laterally stitch and grow in-plane heterostructures via chemical vapor deposition further permits investigation of in-plane diffusion and segregation upon passage of current or upon heating to elevated temperatures both with and without encapsulation. To investigate the structural evolution, high-frame rate image acquisition is used in-situ to produce quantitative data from electron micrograph images. Since the structural evolutions and in-homogeneities are expected to be on a sub-visible light wavelength scale, nanoscale, tip-enhanced optical photoluminescence and Raman spectroscopy is used to correlate them with in-situ electron microscopy data. These experiments allow quantitative understanding of how 2D chalcogenides and their heterostructures evolve under physical confinement, high current density and high temperatures, all relevant for high-performance device operation. These experiments allow further assessment of the mechanisms of diffusion and phase transformations processes at these extreme conditions and their impact on optoelectronic performance which provides insight into strategies to engineer both materials and devices for robust and high performance operation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.