Elucidating Structural Transformations in MoTe2 for Efficient Optoelectronic Memory

非技术摘要：在信息时代，负担得起的高效光电存储器是保存和传播知识和思想的基础。经历相变的材料，如DVD中常用的硫化物，对于将存储器与高速计算相结合但需要相对较大的编程能量的新兴应用尤其有希望，编程能量与切换的材料的体积成比例。对二维(2D)材料(如碲化钼(MoTe2))中的数据进行编码提供了克服这一基本限制的直接途径。在现有的可以进行相变的2D材料中，MoTe2被预测是最节能的，但明显缺乏实验证据来支持相互矛盾的理论模型，这些模型描述了MoTe2中光诱导相变的机制、动力学和局限性。该团队建议使用动态光学测量技术与超高分辨率透射电子显微镜相结合来解决这一知识鸿沟。该项目克服了以往工作的实验局限性，为需要高速、可靠和高效的光电存储器的应用提供了相关2D材料的新线索。该团队寻求利用互动研讨会和虚拟现实工具相结合的方式，在STEM领域历史上代表性不足的地区对初中生和高中生进行与日常生活中纳米材料相关的主题的教育。该项目还为两名纳米制造和表征技术的研究生提供培训，并在夏季几个月接待来自未被充分代表的团体的本科生，以扩大对STEM相关领域的参与。技术摘要：相变材料使光电存储器能够以光速处理存储器中的信息，从而有可能改变低能量、非冯诺依曼计算架构。原子扁平的相变材料(例如MoTe2及其合金Mo1-xWxTe2)将通过显著减少经历相变的有效体积来进一步减少配置其状态所需的能量。虽然在MoT2和相关材料中已经观察到了光致相变，但这些相变是不可逆的，不同于使用电化学掺杂和机械应变的类似观察。有限的经验证据和理论模拟表明，Te空位在相变过程中起着核心作用，但到目前为止，对MoTe2中2H和1T‘相之间的光学开关的动力学和物理机制仍然缺乏清晰的理解。该团队提出，可以通过材料合成、封装和W合金化来控制MoTe2中的光学诱导结构转变，从而实现更高的操作速度、更高的可靠性和更低的开关能量。为了验证这一假设，该项目包含以下三个目标：(1)通过控制MoTe2生长过程中Te空位的浓度来确定Te空位对光开关功率的影响；(2)封装MoTe2以减少光激发期间的Te损失--这是防止可逆光开关的预期机制；以及(3)将MoTe2与W合金结合，以设计出一种用于高效和可重写的光电相变存储器的最佳2D材料。该方法通过探测光域中的相变过程，克服了现有实验技术的时间限制。这项拟议的研究有望通过利用MoTe2中的结构转换来编码信息，从而开发高速、非易失性和高效的数据存储。这项研究首次使用光学和电光技术相结合的方法来解决关于MoTe2及其合金Mo1-xWxTe2的相变机制、动力学和最佳化学计量比的相互冲突的理论模型。对MoTe2相变过程的新见解有望在数据存储以外的领域有广泛的应用，如神经形态计算、电光转换、灵活电子学以及可重构的拓扑和量子器件。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 abstract:In the information age, affordable and efficient optical and electrical memory is foundational to the preservation and dissemination of knowledge and ideas. Materials which undergo a phase transition, such as chalcogenides that are commonly used in DVDs, are especially promising for emerging applications which combine memory with high-speed computing but require relatively large programming energies which is proportional to the volume of material switched. Encoding data in two-dimensional (2D) materials such as molybdenum tellurides (MoTe2) provides a direct route to overcome this fundamental limitation. Among available 2D materials which can undergo a phase transition, MoTe2 is predicted to be the most energy efficient, but there is a distinct lack of experimental evidence to support conflicting theoretical models governing the mechanisms, dynamics, and limitations of optically-induced phase transformations in MoTe2. The team proposes to address this knowledge gap using dynamic optical measurement techniques in combination with ultrahigh-resolution transmission electron microscopy. The project overcomes the experimental limitations of prior works to shed new light on related 2D materials for applications requiring high-speed, reliable, and efficient optoelectronic memory. The team seeks to educate middle- and high-school students on topics related to nanomaterials in daily life from districts with historically under-represented minorities in STEM fields using a combination of interactive workshops and virtual reality tools. This project also provides training for two graduate students in nanofabrication and characterization techniques and hosts undergraduates from underrepresented groups during the summer months to broaden participation in STEM-related fields.Technical abstract:Phase-change materials that enable optoelectronic memory have the potential to transform low-energy, non-von Neumann computing architectures by processing information in memory at the speed of light. A phase-change material that is atomically flat (e.g. MoTe2 and its alloy Mo1-xWxTe2) would further reduce the energy required to configure its state by drastically reducing the active volume undergoing a phase transition. While optically induced phase transformations have been observed in MoTe¬2 and related materials, these transformations have been irreversible unlike similar observations employing electrochemical doping and mechanical strain. Limited empirical evidence and theoretical modeling indicates Te vacancies play a central role in the phase transition process, but a clear understanding of the dynamics and physical mechanism of optical switching between the 2H and 1T’ phases in MoTe2 remains elusive to date. The team proposes that optically induced structural transformations can be controlled in MoTe2 through material synthesis, encapsulation, and W-alloying, resulting in higher operating speeds, improved reliability, and lower switching energies. To test this hypothesis, the project contains the following three aims: (1) determine the influence of Te vacancies on the optical switching power by engineering the concentration of Te vacancies during the MoTe2 growth process; (2) encapsulate MoTe2 to reduce Te loss during optical excitation—the expected mechanism preventing reversible optical switching; and (3) alloy MoTe2 with W to engineer an optimal 2D material for efficient and rewriteable optoelectronic phase-change memory. The proposed approach overcomes the temporal limitations of prior experimental techniques by probing the phase-transition process in the optical domain. The proposed research is expected to enable the development of high-speed, non-volatile, and efficient data storage by exploiting structural transformations in MoTe2 to encode information. This study is the first to use a combination of optical and electro-optical techniques to resolve conflicting theoretical models regarding the phase transformation mechanisms, dynamics, and optimal stoichiometry of MoTe2 and its alloy Mo1-xWxTe2. New insights into phase-transformation process of MoTe2 are expected to have broad application to fields beyond data storage, such as neuromorphic computing, electro-optic conversion, flexible electronics, and reconfigurable topological and quantum devices.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.