Epitaxy in Two Dimensions: Novel Structures and New Physics

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Non-technical Description: Nanoscience and nanotechnology often involve materials of reduced dimensionality, such as one-dimensional (1D) nanotubes and nanowires as well as 2D sheet materials. Reduced dimensionality frequently leads to new phenomena not observed in their 3D counterparts. This research project aims to synthesize two different 2D materials joined together to form a plane of single atomic-layer thickness. The constituent 2D materials in such an in-plane heterostructure are crystalline, and the atomic order is not disturbed crossing the sharp interfaces of the different materials. Due to these traits similar to those of epitaxial heterostructures in 3D, the synthesis of the in-plane heterostructures is called "epitaxy in 2D." Successful synthesis of such structures enables the investigation of the associated new physics, which is expected to lead to novel electronic, photonic, and magnetic devices. The new knowledge generated during the course of the research is passed directly to students by integrating research into teaching, not only to convey excitement of new discoveries but also to stimulate curiosity. This project also impacts K-12 STEM education through outreach activities. In addition, the team participates in a National Public Radio (NPR) program to educate the public on science and technology.Technical Description: This research project focuses on the synthesis and characterization of two-dimensional (2D) heterostructures. The long-term goals of the research are: 1) To establish the growth science of 2D heterostructures; 2) To reveal the very rich and often exotic physics of these structures; and 3) To develop novel electronic, photonic, and magnetic device concepts based on the novel physics. Specifically, the scope of this project includes: 1) Perfection of the zigzag boundary of the graphene-hexagonal boron nitride (graphene-hBN) in-plane heterostructure. This involves the elimination of antiphase disorders by growing the nonpolar graphene onto the edges of polar hBN in a well-controlled environment to overcome the chemical factors preventing this growth sequence. The interface is characterized by atomic resolution, element-contrast imaging to investigate the obtainability of a truly atomically sharp boundary. 2) Synthesis of complex in-plane heterostructures comprising multiple graphene-hBN junctions. Besides bottom-up growth, top-down control is incorporated when necessary. 3) Initial investigation of the novel physics, such as tunneling between localized boundary states and spin polarization of the boundaries, of these heterostructures. Dichroism in electron energy loss spectroscopy is the method of choice for the initial experimental probing of the spin polarization of the atomic-scale of the boundary states.