Controlling the Band Structure of 2D Semiconductors by their Dielectric Environment

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Non-technical description: The field of atomically thin two-dimensional (2D) materials started ten years ago with the discovery of graphene. The range of such materials that are only one or a few atoms thick has now expanded to encompass 2D semiconductors, metals, and insulators. The unique combination of electrical, optical, and mechanical properties of these materials has opened up many new scientific frontiers and led to the development of a variety of nanoscale devices - from transistors to light emitting diodes and sensors. Critical to the further development of the field is the ability to manipulate the way electrons behave in these materials, particularly by controlling their available energy levels. The electronic properties of a material are normally altered by modifying the material itself, for example, by changing the material's chemical composition. This project explores a new approach to controlling the behavior of electrons in 2D materials. Rather than changing the material itself, the energy levels in the 2D material are modified by changing the surrounding media. For 2D materials that are only one or a few atoms thick, the external environment actually modifies the forces acting between electrons within the layer and, hence, changes the available energy levels for electrons. The research examines both uniform and nanostructured environments as a means of developing building blocks for a new class of nanoscale electronic and optical devices. Graduate students and undergraduate interns involved in the project learn to prepare, characterize, and analyze 2D materials and devices. Important professional development activities include frequent presentations of their own research and other technical topics within the group and in departmental forums at Stanford University. Technical description: A critical factor for the development of the field of atomically thin two-dimensional 2D materials is the control of their band gap on the nanoscale. This is essential to explore the fundamental physical properties of the materials and to realize many technological applications, ranging from tuning the photon energy of light emission to the control of the transport characteristics by lateral junctions. Traditional approaches, like alloying, are, however, difficult to implement on the nanoscale. This project takes advantage of the pronounced influence of non-local dielectric screening in the limit of atomically thin materials to tune externally many-body Coulomb interactions in 2D semiconductors. This allows tuning of the band structure and band gap of the 2D material simply by tailoring its dielectric environment. The research involves three distinct components: (1) Systematic assembly of 2D heterostructures with varying band gaps and environmental screening; (2) real-space profiling of the band gap at abrupt discontinuities and study of localization for 0D and 1D structures; and (3) determination of conduction and valence band offsets across dielectrically engineered junctions. Characterization tools for these investigations include optical spectroscopy, electron energy-loss spectroscopy and cathodoluminescence, and scanning tunneling spectroscopy.