CAREER: Mapping and Manipulating Lattice Relaxation in Moire Superlattices of Group VI Transition Metal Dichalcogenides

非技术描述：结构扭曲对原子薄材料的性能有重大影响。了解二维（2D）晶格如何在原子尺度上变形以及如何操纵这些变形过程对于在下一代设备技术中定制其行为和开发具有新功能的材料系统至关重要。由具有小旋转偏移或晶格不匹配的垂直堆叠的二维薄片组成的moir<s:1>超晶格是一类二维结构，其中自然晶格弛豫过程和由此产生的应变与观测到的光学、电子和光子性质的变化密切相关。本项目旨在阐明驱动moir<s:1>超晶格弛豫的结构机制，并研究如何通过外部刺激（如电场或机械力）精确地修改松弛的moir<s:1>结构及其紧急物理。这些研究工作与旨在扩大STEM教育和科学研究参与的教育和推广计划相结合，包括扩大加州大学伯克利分校本科转校生的资助研究机会，以及为圣昆廷州立监狱塔马尔佩斯山学院的在押学生开展科学讨论会议。技术描述：二维莫尔维尔超晶格独特的、可调谐的电子能带结构和由此产生的性质对内在结构弛豫过程和相应的层内应变积累高度敏感。因此，这些材料的精确结构表征和对其弛豫机制的透彻理解对于利用它们在新型（光电）电子设备平台中的潜力至关重要。由于器件制造所需要的复杂的多组分异质结构中通常埋藏着感兴趣的层，因此，以前探测红外材料结构的努力变得复杂。因此，现有的晶格弛豫描述在很大程度上是定性的，而机械图像则完全基于模拟。为了应对这一挑战，该CAREER项目的研究目标是利用干涉四维扫描透射电子显微镜（4D-STEM），这是PI研究小组开发的一种基于衍射的成像方法，专门用于测量波纹结构（包括典型设备架构）中的机械变形和应变。本工作的主要目标是：(1)定量绘制由半导体六族过渡金属二硫化物（TMDs）组成的莫尔莫尔双分子层中控制弛豫的机械变形；(2)对外部电场或单轴机械应变存在下的弛豫TMD莫尔莫尔双分子层及其本征应变场的扰动进行操作测量。光致发光光谱、电子输运测量和理论计算的结合补充了成像实验，将观察到的结构与涌现的光学和电子特性联系起来。这项工作加深了对TMD摩尔超晶格中基本结构-性质关系的理解，并为利用结构扭曲和应变作为调节旋钮来修改这些系统的（光电）电子行为提供了一个框架。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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:Structural distortions have a significant impact on the properties of atomically thin materials. Understanding how two-dimensional (2D) lattices deform on an atomic scale and how these deformation processes can be manipulated is essential for tailoring their behavior in next-generation device technologies and developing material systems with new functionalities. Moiré superlattices, comprised of vertically stacked 2D sheets with a small rotational offset or lattice mismatch, are a class of 2D structures in which natural lattice relaxation processes and resulting strain are closely linked to changes in observed optical, electronic, and photonic properties. This project aims to elucidate the structural mechanisms driving moiré superlattice relaxation and to investigate how relaxed moiré architectures and their emergent physics can be precisely modified by external stimuli, such as an electric field or mechanical force. These research efforts are integrated with education and outreach initiatives that seek to broaden participation in STEM education and scientific research, including the expansion of funded research opportunities for undergraduate transfer students at the University of California at Berkeley and the development of scientific discussion sessions for incarcerated students at Mount Tamalpais College at San Quentin State Prison.Technical description:The unique, tunable electronic band structures and resultant properties of two-dimensional moiré superlattices are highly sensitive to intrinsic structural relaxation processes and corresponding accumulation of intralayer strain. Precise structural characterization of these materials and thorough understanding of their relaxation mechanisms are therefore critical to harnessing their potential in novel (opto)electronic device platforms. Efforts to probe the structure of moiré materials have previously been complicated by the fact that the layers of interest are often buried within complex multi-component heterostructures, as required for device fabrication. As such, existing descriptions of lattice relaxation are largely qualitative and mechanistic pictures are based purely on simulations. To address this challenge, the research aims in this CAREER project utilize interferometric four-dimensional scanning transmission electron microscopy (4D-STEM), a diffraction-based imaging methodology developed by the PI’s research group specifically for measuring mechanical deformations and strain in moiré structures, including those in typical device architectures. The primary goals of this work are (1) to quantitatively map out mechanical deformations that govern relaxation in moiré bilayers composed of semiconducting group VI transition metal dichalcogenides (TMDs) and (2) to perform operando measurements on the perturbation of relaxed TMD moirés and their intrinsic strain fields in the presence of an external electric field or uniaxial mechanical strain. A combination of photoluminescence spectroscopy, electronic transport measurements, and theoretical calculations supplement the imaging experiments to correlate the observed structures with emergent optical and electronic properties. This work deepens the understanding of fundamental structure–property relationships in TMD moiré superlattices and provides a framework for leveraging structural distortions and strain as tuning knobs for modifying the (opto)electronic behavior of these systems.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.