  Three-dimensional epitaxial heterostructures are based on covalently-bonded
interfaces, whereas those from 2-dimensional (2D) materials exhibit van der
Waals interactions. Under the right conditions, however, material structures
with mixed interfacial van der Waals and covalent bonding may be realized.
Atomically thin layers formed at the epitaxial graphene (EG)/silicon carbide
(SiC) interface indicate that EG/SiC interfaces provide this unique environment
and enable synthesis of a rich palette of 2D materials not accessible with
traditional techniques. Here, we demonstrate a method termed confinement
heteroepitaxy (CHet), to realize air-stable, structurally unique, crystalline
2D-Ga, In, and Sn at the EG/SiC interface. The first intercalant layer is
covalently-bonded to the SiC, and is accompanied by a vertical bonding gradient
that ends with van der Waals interactions. Such structures break out of plane
centrosymmetry, thereby introducing atomically thin, non-centrosymmetric 2D
allotropes of 3D materials as a foundation for tunable superconductivity,
topological states, and plasmonic properties.
