CAREER: Defect-Modulated Energy Transport in Semiconducting Materials

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.