Optical absorption spectra of Xene and Xane (X=silic, german, stan).

We study optical absorption spectra of Xene and Xane (X = silic, german, stan). The results show that the optical absorption spectra of Xenes are dominated by a sharp peak near the origin due to direct interband transitions near theKpoint of the Brillouin zone. Meanwhile, the optical absorption spectra of Xanes are characterized by an excitonic peak. The Xenes are zero-gap materials with a Dirac cone at theKpoint, whereas Xanes are semiconductors with sizable band gaps. The quasiparticle band gaps of silicane, germanane, and stanane are 3.60, 2.21, and 1.35 eV, respectively; their exciton binding energies are 0.40, 0.33, and 0.20 eV, respectively.