Mixed crystals

It is also interesting to mix carbon and BN polymers in the solid state, to explore properties with potential technological applications. The interactions between graphitic sheets are known to be comparable for both carbon and BN, giving rise to similar interlayer spacings in graphite, graphitic BN and their mixtures. This has been observed in nanoparticles and nanotubes made out of carbon and BN sheets [55]. There are indications [56] that similar interactions are also present in the solid phase of PBZ, consisting of a layered structure with spacings between 3.4-3.7 Å. We have calculated the structural and electronic properties of the crystal phases of a mixture of carbon and BN polymers, assuming that, since the carbon and BN monomers are very similar in length and the polymers probably have similar inter-chain interactions, they could be combined in the same crystal. The starting crystal structures, then relaxed, were those of the carbon phases. One of two polymer chains in the orthorhombic (space group $Pbam$) crystal cell of PPP was replaced by its BN equivalent, thus creating a mixed crystal. A similar procedure was also applied to PPV, using the monoclinic structure with $P2_1/n$ symmetry. Upon relaxation of the lattice parameters and atomic coordinates, using a $4 \times 4 \times 4$ $k$-point grid, we found no significant deviation from the initial configurations: the chains remained separated with small variations in the cell volume.

Figure 10: Electronic densities of states (DOS) for the mixed solids of PPP/PBZ (left) and PPV/PVB (right). The zero of the energy corresponds to the Fermi level.

In Fig. 10 the electronic densities, projected onto the carbon (bottom panels) and BN (top panels) chains, are shown for both crystals. As for the isolated polymer chains, the energy gap between occupied and unoccupied states is larger for the BN chain than for the carbon one. On both the occupied and unoccupied sides of the gap, the carbon states are closer to the Fermi level than the BN ones, suggesting that electrons and holes will reside predominantly on the carbon polymers in the mixed crystals. This would result in larger ionization energies and lower electron affinities for the BN chains than for the carbon ones. In a hypothetical mixed device of carbon and BN polymers, holes could be injected into the BN system and would then be transferred to the carbon system.