Atomic-Scale Design, Synthesis and Characterization of Two-Dimensional Material Interfaces

The reduction of material dimensions to near atomic-scales leads to changes in the properties of these materials. The most recent development in reduced dimensionality is the isolation of atomically thin materials with 2 "bulk" or large-scale dimensions. The isolation of a single plane of carbon atoms has thus paved the way for the study of material properties when one of three dimensions is confined. Early studies revealed a wealth of exotic physical phenomena in these two-dimensional (2D) layers due to the valence and crystalline symmetry of the materials, focusing primarily on understanding the intrinsic properties of the system. Recent studies have begun to investigate the influence that the surroundings have on the 2D material properties and how those effects may be used to tune the composite system properties. In this thesis, I will examine the synthesis and characterization of these 2D interfaces to understand how the constituents impact the overall observations and discuss how these interfaces might be used to deliberately manipulate 2D materials. I will begin by demonstrating how ultra-high vacuum (UHV) conditions enable the preparation and synthesis of 2D materials on air-unstable surfaces by utilizing a characteristic example of crystalline silver. The lack of catalytic activity of silver toward carbon-containing precursors is overcome by using atomic carbon to grow the graphene on the surface. The resulting system provides unique insight into graphene-metal interactions as it marks the lower boundary for graphene-metal interaction strength. I will then show how new 2D materials can be grown utilizing this growth motif, demonstrating the methodology with elemental silicon. The atomically thin 2D silicon grown on the silver surfaces clearly demonstrates a diamond-cubic crystal structure, including an electronic bandgap of ~1eV. This work marks the realization of both a new 2D semiconductor and the direct scaling limit for bulk sp3 silicon. The common growth technique is extended to integrate the two 2D materials onto the same silver surface under vacuum conditions; these new interfaces reveal characteristics of van der Waals interactions and electronic decoupling from the metallic substrate. The heterogeneous 2D system provides key insight into the competition between physical and chemical interactions in this novel material system. Finally, a larger scale graphene-semiconductor interface is examined between graphene and crystalline germanium. The covalent-bonding of the germanium crystal provides strong anisotropy at the surface, leading to symmetry-dependent growth and behavior. These systems show unique tunability afforded by strain at the interface, leading to the potential for wafer-scale manipulation. These results clearly call for the treatment of 2D material interfaces as composite material systems, with effective properties derived from each constituent material.