Two-dimensional (2D) materials have been the subject of intense scientific investigation as their atomically thin nature leads to a wide variety of novel optical and electronic properties. While numerous fundamental physical property investigations and device demonstrations have been performed, there is still a significant knowledge gap in understanding how relevant processing and operation conditions such as structural confinement (encapsulation), high temperature and large current flows affect their structure and performance. Such conditions are commonly encountered during high performance device operation in modern microelectronics. Understanding these properties is therefore critical for understanding the alloying and doping, heterojunction and contact formation, as well as electrical and thermal failure. This project helps understand the fundamental thermodynamic processes and transformations that these materials undergo under extreme physical confinement and temperatures by direct visualization and spectroscopic analysis with an electron beam inside an electron microscope. These experiments are then coupled with nanoscale optical imaging techniques to correlate the structural transformations with optical and electronic property changes. These investigations advance our fundamental knowledge, and help frame design rules for synthesis, processing and device operation considerations. This project also includes next generation workforce preparation by training graduate students in forefront materials synthesis, state of the art microscopy and optical spectroscopy. It also entails workforce development for undergraduate students via the Materials Research Science and Engineering Center's research experience for undergraduate student program. Finally, the research outcomes, and specifically interactive models, are disseminated to the broader public via public lectures, science outreach activities and undergraduate as well as graduate class room education.
research goals of this project is to investigate structural phase transformations and diffusion phenomena in van der Waals layered two-dimensional chalcogenide systems. A combination of near-field optical spectroscopy and in-situ transmission electron microscopy methods, combined with unique sample preparation on pre-patterned substrates are used to investigate the impact of structural confinement, current and temperature. The samples are prepared via mechanical exfoliaton and stacking, as well as via chemical vapor deposition growth. The work exploits the ability to layer atomically-thin layers of these chalcogenides and their vertical heterostructures and then encapsulate them in inert and refractory layers such as graphite or boron nitride. In addition, the ability to laterally stitch and grow in-plane heterostructures via chemical vapor deposition further permits investigation of in-plane diffusion and segregation upon passage of current or upon heating to elevated temperatures both with and without encapsulation. To investigate the structural evolution, high-frame rate image acquisition is used in-situ to produce quantitative data from electron micrograph images. Since the structural evolutions and in-homogeneities are expected to be on a sub-visible light wavelength scale, nanoscale, tip-enhanced optical photoluminescence and Raman spectroscopy is used to correlate them with in-situ electron microscopy data. These experiments allow quantitative understanding of how 2D chalcogenides and their heterostructures evolve under physical confinement, high current density and high temperatures, all relevant for high-performance device operation. These experiments allow further assessment of the mechanisms of diffusion and phase transformations processes at these extreme conditions and their impact on optoelectronic performance which provides insight into strategies to engineer both materials and devices for robust and high performance operation.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
