Electronic properties of semiconducting graphitic microtubules

Abstract

The electronic structure of graphitic tubules with general chirality is thoroughly studied by means of a recursion tight-binding method. A detailed investigation of densities of states around the Fermi level for various types of tubules confirms the previous theoretical prediction that tubules become metals (M), narrow-gap semiconductors (NS's), or moderate-gap semiconductors (MS's), depending on an integer index k specifying the geometrical structure of the tubules. We point out that the MS's are systematically classified into two types, MS1 and MS2, according to this index. Namely, tubules correspond to M for k=0; otherwise they belong to semiconductors, being NS's, MS1, or MS2, respectively, for k modulo 3=0, 1, or 2. Our result is compared with an approximate expression for energy gaps of MS's, derived by White et al. and we discuss the deviation of our results from this expression. An empirical expression for the energy gap, which elucidates such a deviation, is suggested.