We have investigated atomic arrangements and electronic properties of two-dimensional Si compounds having the Si(111) skeletons with H and/or OH-radicals terminators, using first-principles calculations within the density functional theory. A mechanism of the onset of a direct gap for the planar siloxene has been clarified from the chemical bonding points of view. The direct energy gap does not change with increasing thickness of the Siskeleton of the planar siloxene, but the indirect one

more »

... ses: the direct and indirect gaps of the planar siloxene are associated with the surface state and the Si-skeleton states, respectively. If both surfaces of the planar siloxene are terminated with the OH-radicals, the stacking of the Si-skeleton changes from the diamond structure to the Wurtzite one as the Si-skeleton thickness increases. In such artificial compounds, a direct gap appears only at the thinnest Si thickness as well as the case of the planar siloxene. faces of Si(111)-1BL and the other has a Si-skeleton with a larger Si BL thickness. We show an artificial siloxene-like new material has a direct gap.