Interactive Microscopy of Hybrid Scattering Structures

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Non-technical AbstractThe goal of this project is to understand how electrical current flows in small devices where it encounters obstacles whose size is on the order of the size of a single atom. This is important for miniaturizing electrical devices in order to make computers faster and more energy efficient, but it also creates new problems as the devices begin to reach sizes comparable to the distance between constituent atoms. In this size regime tiny defects such as a single misplaced atom - which might not have mattered in the past - suddenly can become very significant. The importance of this project is that it helps to clarify precisely how different atomic-scale objects affect electrical current in small devices, thus helping technology to be successfully miniaturized to the greatest extent possible. The difficulty here is the need to image atomic-scale structure inside actual operating devices to determine the cause-and-effect relationship between atomic-scale structure and device performance. This is accomplished using a scanning tunneling microscope that can see single atoms and also image electrons as they flow around the smallest possible obstacles, like water in a stream, according to the rules of quantum mechanics. Other types of microscopes are used that involve focused electron beams and helium ion beams to intentionally create atomic-scale structures that are not naturally occurring and thus intentionally manipulate the structure of electrical devices over the smallest distances possible. The broader impacts of this project lie in its strong education and outreach components and the fact that it provides high-level scientific training to graduate students, undergraduates, and high school students, preparing them for careers in STEM fields. Outreach efforts are performed at all levels by the investigators and team members and include creation of educational materials on nanoscience and technology for the Berkeley School/University Partnership Outreach Implementation Plan, as well as participation in the Bay Area Science in Schools program. Underrepresented minority students will be offered 5-week internships in the laboratories through the Summer Math and Science Honors Academy program at Berkeley, as well as mentoring opportunities through partnership with the UC Leadership Excellence through Advanced Degrees Program.Technical AbstractThe main goal of this project is to better understand how electrons in 2D materials interact with different structures at the atomic-scale under both equilibrium (i.e., zero transport current) and nonequilibrium (non-zero transport current) conditions. Conventional fabrication and imaging techniques are unable to access the atomic-scale for operational devices and so a gap has opened up in the understanding of how atomic-scale structures in 2D materials alter device functionality. This project will help to fill that gap by developing new techniques to synthesize atomically-precise structures in 2D devices and also to image them at the atomic-scale during device operation. A central question that is addressed is how the equilibrium electronic properties of microscopic scattering structures lead to their nonequilibrium response under high current density and differing device conditions. Specific objectives include the control and visualization of this behavior for point scatterers, quantum dots, and 1D superlattices at the surfaces of single-layer graphene field-effect transistor (FET) devices. Novel transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and focused ion beam based synthesis techniques are used to modify boron nitride substrates with unprecedented spatial precision to engineer the electronic properties of graphene FET capping layers. This project combines the principal investigators' TEM, device fabrication, and scanned probe microscopy expertise to explore a unique set of nanoscale experimental systems. The intellectual merit of this project lies in the fact that it allows access to physical regimes that have never been explored, including the nonequilibrium properties of different atomic scale scatterers such as Coulomb impurities, resonant scatterers, and sp3 defects that break sublattice symmetry. The smallest possible atomically-precise quantum dots are explored in graphene, creating opportunity to visualize new types of edge-state behavior as well as electron lensing. Quantum dot wavefunctions are imaged with unprecedented resolution, enabling long-standing theoretical predictions to be tested. Defect behavior is studied in systems with engineered electrical anisotropy, a new frontier in 2D materials research.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.