Graphene And Its Derivatives: A Theoretical Study Of Doping, Strain And Interface Effects

Using first-principles calculations, we have studied the efects of doping, strain andinterface on graphene and its derivatives, and explored the fascinating quantum phenom-ena induced by these efects.Controlling the type and the concentration of charge carriers is the key step for de-veloping graphene electronics. We find that a change of carrier type from electron tohole can be achieved in monolayer epitaxial graphene on SiC(0001) by fluorine (F) inter-calation. F intercalation induces the interface charge redistribution, introducing unusualphysical phenomena: unexpected non-monotonous dependency of hole density on the Fcoverage and the presence of interface magnetism. More importantly, contrasting to theconventional doping method, the “interface modulation doping” method realizes the spa-tial separation between dopants and the conducting channel and thus produces minimalefects on the mobility of the graphene.Graphene is known as the strongest2D material in nature, yet we show that chargedoping of either electrons or holes can further enhance its ideal strength. This unusualelectronic enhancement, versus conventional structural enhancement, of the material’sstrength is achieved by an intriguing physical mechanism of charge doping changing theelectronic structure and resulting in phonon renormalization of graphene. The failuremechanism of graphene under tension lies in the softening of K1mode, which is closelyassociated with the strain enhancement of the Kohn anomaly in graphene. Charge dopinginduces suppression of the Kohn anomaly, which counteracts the strain efect, and hencehardens the K1mode. Electrons and holes work in the same way due to the high electron-hole symmetry around the Dirac point of graphene.Graphene has exhibited a wealth of fascinating properties, but is also known notto be a superconductor. If it were possible to find a way to induce superconductivityof graphene, it could improve the performance and enable more efcient integration ofa variety of promising device concepts. Remarkably, we show that graphene can bemade a conventional Bardeen-Cooper-Schriefer superconductor by the combined efectof charge doping and tensile strain. While the efect of doping obviously enlarges theFermi surface, the efect of strain profoundly increases the electron-phonon coupling. Atthe experimental accessible doping (～4×1014cm2) and strain (～16%) levels, the superconducting critical temperature Tcis estimated to be as high as～30K, the highestfor a single-element material above the liquid hydrogen temperature.Finally, we investigate the applications of graphene and its derivatives in the field oftopological insulators (TIs). We propose two-dimensional (2D) TIs in functionalized ger-manenes (GeX, X=H, F, Cl, Br, or I). We find GeI is a2D TI with a bulk gap of about0.3eV, while GeH, GeF, GeCl, and GeBr can be transformed into TIs with sizable gaps underachievable tensile strains. A unique mechanism is revealed to be responsible for the largetopologically nontrivial gap obtained: due to the functionalization, the σ orbitals withstronger spin-orbit coupling (SOC) dominate the states around the Fermi level, insteadof original π orbitals with weaker SOC. Our results suggest a realistic possibility for theutilization of topological efects at room temperature.