Exploring the Growth and Characterization of Synthetic Elemental Two-Dimensional MaterialsPublic Deposited
Two-dimensional (2D) materials such as graphene exhibit unique, superlative electronic, optical, and mechanical properties that are dictated by the precise atomic structure within each layer. Consequently, control of this atomic-scale configuration is critical to engineering desirable characteristics. To date, however, most 2D materials have been discovered by isolating mono- or few-layer flakes from bulk material consisting of loosely bound layers (e.g., graphene from graphite). Although effective thus far, this methodology is ultimately limited to the study of structures which have been derived from a bulk layered solid. In this thesis, I will discuss how the growth of entirely synthetic 2D materials—those without bulk analogues—dramatically expands our capability to develop materials with novel properties. In particular, I will focus on the growth of 2D silicon (i.e., silicon nanosheets) and boron (i.e., borophene). I show that the study of these materials is enabled through growth and characterization under pristine ultra-high vacuum conditions, which preserves the intrinsic properties of the as-grown material. Moreover, I will demonstrate how multiple independent characterization techniques can be used to determine whether a new 2D material is atomically thin and chemically distinct from the growth substrate. I will then apply these methodologies to multilayer silicon nanosheets, which I have discovered retain a bulk-like sp3 structure—in contrast to previous literature reports—and exhibit semiconducting properties as well as unique electronic edge states. I will also demonstrate the first growth of borophene, accompanied by comprehensive structural and chemical characterization. Early characterization suggests that borophene is metallic, in contrast to semiconducting bulk boron. Whereas multilayer silicon nanosheets constitute a nanostructured form of the bulk silicon structure, borophene is a unique 2D boron allotrope with no naturally occurring analogue, thus satisfying the criteria for a synthetic elemental 2D material. These results lay the foundation for further exploration of boron nano-allotropes, and demonstrate the potential to engineer new synthetic 2D materials through the appropriate choice of composition and substrate.