Optical and transport studies of Ni(dmi-based organic conductors

Single-crystal Ni(dmit(Formula presented) salts, ((Formula presented)P)[Ni(dmit(Formula presented)(Formula presented), ((Formula presented)N(Formula presented)[Ni(dmit(Formula presented)(Formula presented)⋅(Formula presented)CN, and ((Formula presented)S)[Ni(dmit(Formula presented)(Formula presented), have been synthesized. All show semiconducting behavior in their temperature-dependent dc conductivities. Room-temperature polarized reflectance measurements have been made over the range between 100 and 32 000 (Formula presented) (12 meV-4 eV). For light polarized along the Ni(dmit(Formula presented) stacking axis, all spectra show an energy gap, with superimposed vibrational fine structure at low frequencies and charge-transfer bands at high frequencies. Band gaps determined from the optical conductivities are consistent with thermal activation energies from dc transport measurements. The stacking-axis conductivity shows the effect of electron-molecular vibration interaction; analysis for ((Formula presented)S)[Ni(dmit(Formula presented)(Formula presented) yields a dimensionless electron-phonon coupling constant λ∼0.27. For light polarized perpendicular to the stacking axis, only weak vibrational features are observed.