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，推动了对半导体材料更高效率和更好性能的迫切需求。通过有目的地在其他有序结构中引入缺陷来控制能量运动已经显示出希望，而对这一过程的彻底理解将使合理的、特定用途的材料设计成为可能。然而，由于能量发生的尺度很小，速度很快(分别是十亿分之一米和十亿分之一秒)，直接实时观察能量在材料中的移动是非常具有挑战性的。首席研究人员通过直接成像研究激光激发的能量在缺陷半导体材料中的运动和转换来解决这一挑战。调查是在超小型和超快尺度上进行的，以便对个别材料缺陷的影响产生详细的洞察。与研究部分密切结合的是针对研究生和本科生的各种教育和外联活动，包括来自代表人数不足的群体的学生，使他们认识到半导体材料和表征方法的重要性。这是通过产业界和学术界的合作和建立一个电子显微镜暑期班计划来实现的。技术描述：过渡金属二卤化物(TMD)的电子、光子和机械性能已被证明对静态和动态结构和形态特性高度敏感，TMDs是一类很有前途的半导体材料。然而，由于探测纳米-飞秒组合尺度而不必在相对较大的样品区域平均信号的挑战，对单个原子尺度的缺陷在这些材料中对能量传输和转换的作用缺乏详细的了解。该项目的主要目标是阐明在原子和飞秒相结合的水平上少数层和单层TMD中缺陷调制能量传输和转换的机制。一个具体的目标是确定相对于单个缺陷的高速纳米尺度相干应变波的成核位置和择优波矢，并在时空上绘制从最初的电子-声子耦合到声学声子发射和衰变的整个光致晶格响应。为了实现这一点，我们用超快电子显微镜的成像和衍射方式(如飞秒电子成像和超快会聚束衍射)研究了纳米级感兴趣区域内的结构动力学。该项目将通过补充形态和结晶学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.