Optical and transport studies of Ni(dmit)2-based organic conductors

Single-crystal Ni(dmit${)}_2$ salts, (${\mathrm{Ph}}_4$P)[Ni(dmit${)}_2$ ${]}_3$, (${\mathrm{Bu}}_4$N${)}_2$[Ni(dmit${)}_2$ ${]}_7$⋅${2\mathrm{C}\mathrm{H}}_3$CN, and (${\mathrm{Me}}_3$S)[Ni(dmit${)}_2$ ${]}_2$, 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 ${\mathrm{cm}}^{\mathrm{-}1}$ (12 meV–4 eV). For light polarized along the Ni(dmit${)}_2$ 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 (${\mathrm{Me}}_3$S)[Ni(dmit${)}_2$ ${]}_2$ yields a dimensionless electron-phonon coupling constant λ∼0.27. For light polarized perpendicular to the stacking axis, only weak vibrational features are observed. © 1996 The American Physical Society.