Ambipolar-transporting coaxial nanotubes with a tailored molecular graphene-fullerene heterojunction

Despite a large steric bulk of C-60, a molecular graphene with a covalently linked C-60 pendant [hexabenzocoronene (HBC)-C-60; 1] self-assembles into a coaxial nanotube whose wall consists of a graphite-like pi-stacked HBC array, whereas the nanotube surface is fully covered by a molecular layer of clustering C-60. Because of this explicit coaxial configuration, the nanotube exhibits an ambipolar character in the field-effect transistor output [ hole mobility (mu(h)) = 9.7 x 10(-7) cm(2) V-1 s(-1); electron mobility (mu(e)) = 1.1 x 10(-5) cm(2) V-1 s(-1)] and displays a photovoltaic response upon light illumination. Successful coassembly of 1 and an HBC derivative without C-60 (2) allows for tailoring the p/n heterojunction in the nanotube, so that its ambipolar carrier transport property can be optimized for enhancing the open-circuit voltage in the photovoltaic output. As evaluated by an electrodeless method called flash-photolysis time-resolved microwave conductivity technique, the intratubular hole mobility (2.0 cm(2) V-1 s(-1)) of a coassembled nanotube containing 10 mol % of HBC-C-60 (1) is as large as the intersheet mobility in graphite. The homotropic nanotube of 2 blended with a soluble C-60 derivative [(6,6)-phenyl C-61 butyric acid methyl ester] displayed a photovoltaic response with a much different composition dependency, where the largest open-circuit voltage attained was obviously lower than that realized by the coassembly of 1 and 2.