Epitaxial Growth of Artificial Graphene on Conventional Semiconductor Surface towards Room Temperature Topological Quantum States

Graphene is a 2D hexagonal lattice made of \textit{sp}$^{2}$ hybridized carbon. Fundamental understanding of graphene has recently spurred a surge of searching for 2D topological quantum phases in solid-state materials. Here we demonstrate the epitaxial growth of artificial graphene, in which the carbon atoms are replaced by other elements, on conventional semiconductor surface to realize large-gap topological quantum phases. We show that Si(111) surface functionalized with 1/3 monolayer of halogen atoms [Si(111)-$\sqrt 3 \times \sqrt 3 $X (X$=$Cl, Br, I)] exhibiting a trigonal superstructure, provides an ideal template for epitaxial growth of heavy metals, such as Bi, which self-assemble into a hexagonal lattice with high kinetic and thermodynamic stability. Remarkably, the Bi overlayer show the feature of a ($p_{\mathrm{x}}$, $p_{\mathrm{y}})$ analogue of graphene that exhibits quantum spin Hall state with an energy gap as large as $\sim$ 0.8 eV. Growth of transition metals lead to the discovery of a new 2D material, \textit{sd}$^{2}$ graphene, characterized with bond-center electronic hopping, which surprisingly transforms the atomic hexagonal lattice into a hidden kagome lattice and exhibits a wide range of topological quantum phases. For example, quantum anomalous Hall states can be realized in W@Si(111)-$\sqrt 3 \times \sqrt 3 $-Cl, with an energy gap of $\sim$ 0.1 eV. These findings may pave the way for future exploration of Si-based topological quantum phases, by exploiting epitaxial growth and current available semiconductor technology. This research was supported by DOE (Grant No: DEFG02-04ER46148).