Graphene has a unique electronic band structure, called the Dirac cone, and is being studied as a next-generation semiconductor material owing to its high carrier mobility. [1] However, there is a major problem in using graphene as a device material because it has no bandgap. In the past decade, other monolayer sheets of group IV materials such as silicon (Si), germanium (Ge) and Tin (Sn) were also experimentally formed. Monolayer honeycomb sheet of Ge, called germanene, were predicted to have very curious physical properties superior to graphene. In this study, We first report on the atomic and electronic band structure of germanene formed on Al(111) substrate measured using low-energy ion scattering spectroscopy and angle-resolved photo emission spectroscopy, respectively. Although germanene [3] have been reported and attracted large interests of researchers, they can exist only on certain substrates and in a vacuum condition. On the other hands, Si and Ge sheets terminated by hydrogen or functional groups, called silicane and germanane, are stable even in air while the physical properties are quite different from those in bulk Si and Ge. As the second topic, we report on the electrical characteristics of hydrogenated germanane (GeH) and methylated germanane (GeCH₃) which are semiconductor of about 1.6 eV and 1.8 eV direct bandgaps, respectively, even in a multilayered structure. We also report the photocurrent characteristics of GeCH₃ thin film by illumination of monochromatic light of various wavelengths.