Advancing Ultra-Pure TMDs Through Atomic-Scale Defect Characterization and Control

Over the past decade, semiconducting two-dimensional transition-metal dichalcogenides (TMDs) have become a leading platform for exploring quantum phenomena in reduced dimensions and advancing next-generation electronic and optoelectronic technologies. Their atomically thin structure and make TMDs uniquely sensitive to disorder, and device-scale behavior is often limited or even defined by atomic defects. Despite extensive research, the nature of the defects in even the highest-quality TMDs has remained uncertain, hindering further improvements in crystal purity and materials control. In this talk, I will describe our efforts to identify, control, and ultimately eliminate the dominant defects in high-purity self-flux-grown TMDs. Using scanning tunneling microscopy and spectroscopy (STM/STS), we directly compared monolayer and bulk WSe₂ and discovered that the most common defects are not chalcogen vacancies, as widely assumed, but oxygen substitutional impurities introduced during synthesis. This finding redefines how many optical and transport anomalies in TMDs are interpreted, as many observations have been attributed to vacancies. Building on this understanding, we developed a scalable method to exfoliate large-area monolayers from ultraclean bulk crystals and verified that no additional disorder is introduced during exfoliation. To accelerate materials optimization, we demonstrated that conductive atomic force microscopy (C-AFM) achieves comparable defect sensitivity to STM, dramatically improving throughput and closing the synthesis–characterization feedback loop. Leveraging this approach, we introduced dilute amounts of getter materials during growth to suppress oxygen incorporation, achieving record-purity TMD crystals. Finally, by comparing high purity TMDs as a benchmark to commercially available materials, we revealed how specific dopants drive exciton dissociation and modify optical conductivity. Together, these results establish a unified framework for understanding and controlling defects in 2D semiconductors, advancing scalability and materials quality while providing a foundation for deliberate defect engineering as a tool for quantum materials design.