Tailoring Optical Absorption Properties of Carbon Nanothreads

Carbon nanothreads form a novel class of hydrogenated, diamond-like materials synthesized at high pressure from simple aromatic substances, theoretically predicted to exhibit unique mechanical properties, some of which may also exhibit optical and transport properties of potential technological interest, depending on the particular aromatic precursor. Our study focuses on cocrystals of two very similar aromatic molecules: diphenylacetylene and stilbene, both comprised of two rings connected by two-carbon units featuring triple and double bonds, respectively. We prepared the cocrystals by recrystallization from solution, producing six different compositions between the two end-member values of 100% diphenylacetylene and 100% stilbene. These samples were then compressed to final pressures of ∼30 GPa, in diamond anvil cells, at room temperature. The compression induced copolymerization results in the formation of double-core carbon nanothreads. These nanothreads are comprised of two one-dimensional diamond-like cores connected through cis-polyacetylene-like backbones of variable length, produced from the topochemical polymerization of the acetylene moieties of diphenylacetylene. The resulting materials were characterized via optical absorption spectroscopy and X-ray diffraction. Very interestingly, the recovered materials exhibited variable optical absorption in the visible and near-infrared spectral region, resembling the low-energy edges of HOMO-LUMO band gaps in dielectric materials. Particularly, the absorption edge of our materials shifts to lower energies with increasing the diphenylacetylene content within the cocrystal precursor and, consequently, with increasing the lengths of the conjugated carbon chains. The materials properties range from semiconductor behavior to wide band gap insulating behavior at the two extremes of 100% and null diphenylacetylene content. Pressure-induced copolymerization thus represents a methodology for synthesizing novel carbon nanothreads with finely variable optical properties.