Electronic and Chemical Properties of Germanene: The Crucial Role of Buckling

Abstract

The heavier analogues of graphene, namely silicene and germanene, are known to be buckled. Such buckling leads to interesting properties like direct band gap in hydrogenated germanene, known as germanane. This article shows that the sequential replacement of C by Ge in benzene leads to increasing buckling with the maximal buckling distance (d = 0.61 Å) in Ge6H6. The origin of such buckling induced lowering of symmetry (D6h → D3d) is traced to pseudo Jahn–Teller (PJT) distortion along the b2g normal mode arising out of mixing of the nondegenerate (A1g) ground state with low lying (Δ0 = 4.36 eV) excited state of B2g symmetry. Buckling also leads to enhanced chemical reactivity of germanene toward hydrogen to form germanane. The large affinity of germanene toward hydrogenation explains the experimental synthesis of exfoliated layers of germanane by Goldberger and co-workers [ACS Nano 2013, 7, 4414−4421]. Germanene → germanane formation leads to the opening up of a large band gap making hydrogenation a chemical route to control the electronic properties in these new 2D materials. The presence of buckling in germanene leads to higher hole reorganization energies than polyaromatic hydrocarbons (PAH) of the same nuclearity.