Germanether: a two-dimensional auxetic semiconductor with tunable direct-band-gap and high electron mobility.

Pristine germanene is a zero-gap semi-metal, which may hinder its practical application in semiconducting devices. Here, on the basis of the structural characteristics of digermyl ether, we theoretically design a two-dimensional crystal, namely germanether. Germanether exhibits excellent dynamical and thermal stability. It possesses an indirect band gap of 1.37 eV and a high electron mobility of 2.32 × 103 cm2 V-1 s-1. The uniaxial strain and layer stacking order can trigger an indirect-to-direct band gap transition. More interestingly, germanether has remarkable in-plane negative Poisson's ratios with the largest one (∼-0.2) five times of borophenes and three times of penta-graphene. The negative Poisson's ratio arises from the interplay of Ge-O tetrahedron symmetry and Ge-4d orbitals involvement, which is different from previously reported auxetic materials. All these findings render germanether is a competitive material for the future application in nanomechanics and nanoelectronics.