The electronic structures of Si-skeleton chainlike [one-dimensional (1D)] and planar [two-dimensional (2D)] materials have been calculated by the first-principles local-density-functional method. 1D Si-skeleton material (chain polysilane) has a directly-allowed-type band structure with a band gap of about 4 eV. Interchain interaction disappears, independently of the chain configuration, if each chain is located over 8 A from other chains. Therefore, the electronic structure of some polysilane-chain aggregations can be discussed in terms of the result for the corresponding isolated polysilane chain. 2D material (planar polysilane) has an indirect band gap of 2.48 eV as well as a direct band gap of 2.68 eV. This structure is an intermediary between direct (1D) and indirect (3D). A characteristic single, double, and treble degeneracy is found at the highest occupied valence-band state, depending on the structure dimension of 1D, 2D, or 3D Si skeleton, respectively.
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