CAREER: Defect-Modulated Energy Transport in Semiconducting Materials

非技术描述：不断增长的能源需求和对数字电子技术的日益依赖，迫切需要提高半导体材料的效率和性能。通过有目的地在有序结构中引入缺陷来控制能量运动已经显示出前景，对这一过程的彻底理解将使合理的、特定应用的材料设计成为可能。然而，直接观察能量在材料中的实时移动是极具挑战性的，因为它发生的尺度非常小，非常快（分别是十亿分之一米和十亿分之一秒的百万分之一）。首席研究员通过直接成像研究激光激发的能量运动和缺陷半导体材料的转换来解决这一挑战。研究在超小和超快尺度上进行，以便详细了解单个材料缺陷的影响。与研究部分紧密结合的是研究生和本科生的各种教育和推广活动，包括来自代表性不足群体的学生，了解半导体材料和表征方法的重要性。这是通过产学研合作和电子显微镜暑期学校项目的建立来实现的。技术描述：过渡金属二硫族化合物（TMDs）是一类很有前途的半导体材料，其电子、光子和力学性能对静态和动态结构和形态特性高度敏感。然而，由于不需要在相对较大的样品区域上平均信号而探测纳米-飞秒尺度组合的挑战，缺乏对这些材料中单个原子尺度缺陷在能量传输和转换中的作用的详细了解。该项目的总体目标是阐明在原子和飞秒级结合的多层和单层tmd中缺陷调制的能量传输和转换机制。具体目标是确定高速、纳米级相干应变波相对于单个缺陷的成核位置和优先波向量，并绘制整个光致晶格响应的时空图，从初始的电子-声子耦合到声学声子发射和衰变。为了实现这一点，利用超快电子显微镜的成像和衍射模式（例如，飞秒电子成像和超快会聚束衍射）研究了感兴趣的纳米级区域内的结构动力学。该项目将通过互补形态和晶体时空测量产生新知识，深入了解半导体材料中能量传输和转换的基本过程。

Non-technical Description: Rising energy demands and an increasing reliance on digital electronic technologies are driving an urgent need for greater efficiencies and improved performance in semiconducting materials. Control of energy motion by purposeful introduction of imperfections in otherwise ordered structures has shown promise, and a thorough understanding of this process would enable rational, application-specific materials design. However, directly watching energy move through materials in real time is exceedingly challenging owing to the associated very small and very fast scales (a billionth of a meter, and a millionth of a billionth of a second, respectively) at which it occurs. The principal investigator addresses this challenge through direct-imaging studies of laser-excited energy motion and conversion in defect-laden semiconducting materials. Investigations are conducted on combined ultrasmall and ultrafast scales such that detailed insight into the effects of individual material imperfections is generated. Closely integrated with the research component are various education and outreach activities with graduate and undergraduate students, including students from underrepresented groups, to the importance of semiconducting materials and characterization methods. This is accomplished through an industry-academia collaboration and establishment of an electron microscopy summer school program.Technical Description: Electronic, photonic, and mechanical properties of transition metal dichalcogenides (TMDs), a promising class of semiconducting materials, have been shown to be highly-sensitive to static and dynamic structural and morphological properties. However, a detailed understanding of the role of individual, atomic-scale defects on energy transport and conversion in these materials is lacking, owing to challenges of probing the combined nanometer-femtosecond scales without having to average signal over relatively large specimen areas. The overarching goal of the project is to elucidate mechanisms of defect-modulated energy transport and conversion in few- and single-layer TMDs at the combined atomic and femtosecond levels. A specific aim is to determine nucleation sites and preferential wave vectors of high-velocity, nanoscale coherent strain waves with respect to individual defects and to spatiotemporally map the entire photoinduced lattice response, spanning from initial electron-phonon coupling to acoustic phonon launch and decay. To accomplish this, structural dynamics within nanoscale regions of interest are studied with imaging and diffraction modalities of ultrafast electron microscopy (e.g., femtosecond electron imaging and ultrafast convergent-beam diffraction). This project will provide insight into fundamental processes of energy transport and conversion in semiconducting materials through generation of new knowledge via complementary morphological and crystallographic spatiotemporal measurements.