CAREER: Point Defects in Two-dimensional Material Systems: Fundamentals and New Perspectives

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Nontechnical Description: It is known that introducing defects or imperfections into a crystalline material, either by removing atoms or adding foreign atoms, can change material's properties, and this approach has led to many ground breaking applications such as light emitting diodes, transistors and sensors. Today, there is a good understanding of how to manipulate these imperfections to achieve the desired properties of traditional semiconductors such as silicon. However, researchers are just starting to understand how to use defects to manipulate a whole new class of materials: two-dimensional (2D) semiconductors that are atomically thin, which have an unusual set of properties and are expected to perform far better than traditional semiconductors in some regards. The research component of this CAREER award is to explore innovative ways to introduce defects in 2D materials and to study how defects change the 2D material's properties. One way this can be done is to remove atoms by striking the material with energetic particles. Alternatively, foreign atoms with different electrical charges can be anchored in their place. The research team uses state-of-the-art techniques to visualize how these imperfections form, how they react and how they impact the material properties. This project also brings nanoscience and 2D material concepts to the public through open house events and summer camps for high-school students, as well as participation of graduate and undergraduate students in active research. Findings from this project are disseminated through journal publications, conference presentations, and a publicly-accessible 2D materials database, which the team is developing in collaboration with a non-profit organization, MaterialsProject.org.Technical Description: This research project aims to uncover the physics of point defects, in particular those in emerging 2D semiconducting materials such as WS2, GeSe, GaSe, and InTe. The research team uses advanced high-resolution spectroscopy and microscopy techniques to investigate chalcogen vacancies, anti-site defects, and substitutionals. Defects are introduced at desired defect concentrations during growth or after growth by alpha-particle irradiation to achieve desired material properties. Nano-photoluminescence, ultra-fast spectroscopy, and electron energy loss spectroscopy measurements allow determination of energy states of defects and effects of defects on the excitonic complexes, optical absorption and emission characteristics, as well as electronic structure of 2D materials. Transmission electron microscopy (TEM) studies help to determine structural characteristics, relaxation dynamics, and diffusion speed of defects. The overarching goal is to measure, analyze, and establish the roles of point defects in determining the optical and electronic properties of select 2D semiconducting materials and explore new routes to control material properties on demand.