HOMO-LUMO gaps of large polycyclic aromatic hydrocarbons and their implication on the quantum confinement behavior of flame-formed carbon nanoparticles

Flame-formed carbon nanoparticles exhibit apparent size-dependent optical and electronic band gaps, consistent with the quantum confinement effect found in semiconductor materials. Understanding the effect of chemical composition on the band gap variations in carbon nanoparticles requires the investigation of the HOMO-LUMO gap of large aromatics. Geometry optimization with empirical force fields, followed by a density functional theory calculation, is employed here to enable the calculation of HOMO-LUMO gaps of aromatic species of sizes substantially larger than those reported in the past. Large hexagonal polycyclic aromatic hydrocarbons (PAHs) are shown to have diminishing HOMO-LUMO gaps, consistent with the behavior of two-dimensional quantum dots and approaching the zero band gap of graphene, while the HOMO-LUMO gap of polyacenes levels off at a finite value. The HOMO-LUMO gap of an aromatic cluster is found to be determined mostly by the large PAH molecules with smaller HOMO-LUMO gaps; small PAH molecules in the cluster make small or negligible contributions to the gap size of the overall cluster. Thus, if the quantum confinement of flame-formed carbon nanoparticles must be understood within the context of particle composition, the dependency of the band gap on particle size may be explained simply by the fact that larger flame-formed carbon nanoparticles contain larger PAH molecules or perhaps more precisely, have higher probabilities of containing more light absorbing species, e.g., large PAH molecules.