Electronic and optical properties of graphane and related 2-D systems

With the aid of ab-initio calculations, we have studied the effect of hydrogen functionalization on graphene. This new material, theoretically predicted in 2007 [1] and synthesized in 2008 [2], is known with the name 'graphane'. We have investigated the effect of hydrogen also on silicon and germanium based counterparts of graphene, polysilane and polygermyne monolayers. Our main objective of this study is to understand the effect of hydrogen in the optical absorption spectra and in the electronic affinity of such systems, in view of their possible application in photovoltaic and optoelectronic devices. Being the optical absorption spectrum and the electron affinity excited state properties of materials, we go beyond standard density-functional theory (DFT) calculations by introducing quasi-particle and excitonic effects within GW and Bethe Salpeter approaches. We show that upon H functionalization, graphene undergoes a metal-insulator transition, with an opening of an electronic gap at Gamma of about 6 eV. The electron affinity stays positive, at odd with hydrogen covered diamond surfaces. A smaller opening of a gap is observed in silicon and germanium 2-D counterparts, making these materials eligible for optoelectronic applications. [1] J.O. Sofo, A.S. Chaudhari, and G.D. Barber, Phys. Rev. B 75, 153401 (2007) [2] D.C. Elias, R.R. Nair, T.M.G. Mohiuddin, S.V. Morozov, P.Blake, M.P., A.C. Ferrari, D.W. Boukhvalov, M.I. Katsnelson, A.K. Geim, and K.S. Novoselov, Science 323, 610 (2009); S.Ryu, M.Y. Han, J.Maultzsch, T.F. Heinz, P.Kim, M.L. Steigerwald, and L.E. Brus Nanoletters 8, 4597 (2008).