Tuning the optical absorption and exciton bound states of germanene by chemical functionalization.

We present a comprehensive study of buckled honeycomb germanene functionalized with alternately bonded side groups hydroxyl (-H), methyl (-CH) and trifluoro methyl (-CF). By means of most modern theoretical and computational methods we determine the atomic geometries versus the functionalizing groups. The quasiparticle excitation effects on the electronic structure are taken into account by means of exchange-correlation treatment within the GW framework. The Bethe-Salpeter equation is solved ab initio to derive optical spectra including excitonic and quasiparticle effects. Band edge excitons are investigated in detail. The binding properties are compared with those resulting from model studies. The functionalization leads to significantly modified band structures compared with pristine germanene. The Dirac bands near the K point are destroyed and direct gaps appear at the Γ point. Together with the many-body effects, quasiparticle gaps of 2.3, 1.8 or 1.0 eV result for -H, -CH and -CF functionalization. Totally different absorption spectra are found for in-plane and out-of-plane light polarization. Strongly bound excitons are visible below the quasiparticle band edge with binding energies of about 0.5, 0.4 or 0.3 eV. The nature of these band-gap excitons is investigated via their wave function, the contribution of various interband combinations and the dipole selection rules.