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CAMP:Toward tunable 2D crystal properties with covalent surface termination

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Jay Gupta, Ohio State University
17 March 2015 from 3:30 PM to 4:30 PM
339 Davey Laboratory
Contact Name
Jie Shan
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The toolbox of 2D crystals has expanded in recent years from the semi-metal graphene, to wide gap insulators like boron nitride and direct/indirect gap semiconductors such as MoS2. This range of properties has inspired many ideas for applications, some of which are being commercialized already. Surprisingly, the inherent sensitivity of 2D crystals to their surfaces has received relatively little attention, but presents exciting new opportunities to tune properties. For example, graphene can be converted into the wide-gap insulator graphane, by covalent termination with hydrogen. More exotic changes in electronic structure such as a crossover between trivial and topological insulating states have been predicted for 2D crystals of heavier group IV elements: Si, Ge, Sn with specific surface terminations. While most of these crystals have yet to be realized experimentally, recent progress in preparing a graphite analogue with germanium, terminated by hydrogen or organic groups, has inspired my groups' efforts to grow new 2D crystals with controlled surface termination. I will discuss scanning tunneling microscope studies that probe how the electronic structure of 2D crystals changes as the surface termination is varied. Our initial efforts have focused on graphene, which we grow under ultrahigh vacuum conditions on Cu(111) to achieve pristine surfaces. The electronic structure of the gr/Cu system is studied through tunneling spectroscopy, which reveals changes in the surface work function and surface-confined Shockley and Image Potential states. Then, I will discuss how we may have prepared the first crystalline, hydrogenated graphene surface, using a field dissociation technique. This termination, which is localized to a micron-scale region near the STM tip, can be reversed by STM imaging above a threshold bias voltage. These results represent an initial step toward the realization of lateral heterostructures which can be defined within a single 2D crystal.