Successful PhD defense of Jingrong Ji
Congratulations to Jingrong Ji!
Title: “Molecular beam epitaxial growth and scanning tunneling microscopy characterization and manipulation of monolayers and heterostructures of two-dimensional ferroelectrics"
Abstract
Since the experimental realization of monolayer graphene by mechanical exfoliation, the family of 2D materials has rapidly grown. Among them, 2D ferroelectric materials have become an important subset due to their potential for applications in electronic devices. The combination of molecular beam epitaxy (MBE) and scanning tunneling microscopy (STM) techniques has provided a new way explore ferroelectricity in high-quality ultrathin materials in an ultra-high vacuum (UHV) environment. In this thesis, I report work on the realization of high-quality, molecular beam epitaxial synthesis of group-IV monochalcogenide monolayers, and on their lateral heterostructures (LHSs) and lateral superlattices (LSLs), and the characterization and manipulation by scanning tunneling microscopy (STM). First, I will present the MBE growth of monolayer PbTe/SnTe lateral superlattices on graphene substrates. During the growth of these superlattices, we discovered an asymmetric diffusion at the PbTe/SnTe interfaces. Several factors influence the interdiffusion at such interfaces, including the thickness of the graphene substrate, and the growth sequence of these two materials. By combining experimental data and first principles calculations, we developed a model of the diffusion mechanism that exists in two-dimensional lateral monolayer superlattices. As a second topic, I introduce the discovery of vortex-oriented ferroelectric domains in PbTe /SnTe monolayer lateral heterostructures. The ferroelectric domains in SnTe regions that surrounding PbTe core form either clockwise or counterclockwise vortex-oriented quadrant configurations are a result of ferroelectricity, charge transfer, and interfacial strain. Lastly, I present my work on the defect-free nano-welding of semiconducting SnSe nanoplates. Combining scanning tunneling microscope (STM) manipulation and first-principles calculations, an anisotropic friction is found in SnSe monolayer nanoplates because of the commensuration (or lack thereof) between SnSe and graphene. Taking advantage of in-situ annealing in the VT-STM, we observed that nanoplates can merge without any grain boundaries after physically moving the nanoplates together. Our results are a major step towards the controllable manipulation and construction of nanostructures from diverse 2D materials.
