Theory Insights into the Synthesis of 2D Materials—2D Metals, Single-Crystal TMD Monolayers, Frustrated Salts and Disclinated Graphene

Most air-stable 2D monolayers are semiconductors or insulators wherein the bandgap protects the material against oxidation. Air-stable wafer-scale 2D metals are thus rare. Growth of metal monolayers or bilayers underneath an epitaxial graphene cap provides a route towards a broad family of air-stable wafer-scale 2D metals whose synthesis can be understood with first-principles insights. Wafer-scale single-crystal semiconducting monolayers are similarly rare, since epitaxy of 2D materials is weak and nucleation typically occurs at multiple sites across a substrate. "Step-epitaxy" on sapphire suibstrates improves the consistency of domain alignment in wafer-scale growth and insights into the mechanism thereof can be gained by a combination of first-principles calculation and empirical molecular dynamics. Graphene remains an archetypal 2D material and continues to hold surprises, such as (1) the possiblity to host geometrically frustrated salt crystals with anions and cations on opposite sides of the sheet and (2) the potential role of 2-fold coordination defects in disrupting growth and (3) general rules for how networks of disclinations yield semiconduting or metallic behavior, wth a striking rescaling of the density of states between systems within a given class.