Zero-gap semiconductors

These surprising results can be understood on the basis of the electronic structure of a graphene sheet which is found to be a zero gap semiconductor [177] with bonding and antibonding tt bands that are degenerate at the TsT-point (zone corner) of the hexagonal 2D Brillouin zone. The periodic boundary... 

Lombos BA, Lee EYM, Kipling AL, Krawczyniuk RW (1975) Mercury Chalcogenides, Zero gap semiconductors. J Phys Chem Solids 36 1193-1198... 

Our results with PITN are very encouraging to the development of an organic semimetal (zero gap semiconductor) since a relatively minor modification allowed to reduce the energy gap within a family of poly(heterocycles) by 1 eV (23 Kcal). Further modifications by annulation and substitution (electron donor or acceptor) are expected to further reduce the energy gap of a PT-based polymeric conductor. We are currently actively engaged in the preparation of such neutral organic conductors. 

Let us recall that nanotubes can be considered as graphene sheets rolled up in different ways. If we consider the so-called chiral vectors c = nai + na2, in which a and a2 are the basis vectors of a 2D graphite lattice, depending on the value of the integers n and m, one can define three families of tubes armchair tubes (n = m), zig-zag tubes (n or m = 0), and chiral tubes (n m 0). Band structure calculations have demonstrated that tubes are either metallic compounds, or zero-gap semiconductors, or semiconductors [6,7]. More commonly, they are divided into metallic tubes (when n-m is a multiple of 3) or semiconducting ones. 

The density of states is a linear function with the energy as shown in Figure 6.5b, which is reflected by the linear energy dispersions. The density of states decreases linearly as the energy approaches the Fermi level for both the conduction and valence bands, resulting in the absence of the density of states just at the Fermi level. This feature in the electronic state makes graphene a zero-gap semiconductor. 

Two other questions should be addressed in this system. Why is BaPb03 a metal and why is the system semiconducting above x—0.3 We know that BaPb03 is best viewed as a zero gap semiconductor, i.e. the conduction and valence bands just slightly overlap. By this explanation we might expect that BaPb1.xBix°3 compositions should remain metallic even if BiIV disproportionates to remove Bi 6s states from Ef. 

As illustrated by this simple model, graphene is a semimetal or a zero-gap semiconductor. At low temperatures it does not possess superconducting properties however, as demonstrated by Pathak et al. (2008) using variational Monte Carlo, there is a possibility that doped graphene may superconduct. 

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