Using optical measurements to improve electronic models of bismuth nanowires

The optical properties of nanowires are studied because the singularities in their electronic density of states enhance the Seebeck coefficient without much decrease in the electrical conductivity, thus relaxing the trade-off between a high electrical conductivity and a high Seebeck coefficient, which is usually found in bulk materials. Filling a porous anodic alumina template with bismuth forms aligned, self-assembled arrays of bismuth nanowires. The previously measured strong optical absorption feature is compared to the simulated absorption spectrum resulting from an indirect electronic transition. The simulated and measured absorption spectra are similar in spectral shape and in their dependence on wire diameter and polarization. However, the simulated absorption peak occurs at 420 cm^-1 higher than the measured absorption peak. This difference could result from inaccurate values of the L and T point masses or of the band overlap in bulk bismuth at room temperature. Since the large optical absorption at ∼1000 cm^-1 which is observed in bismuth nanowires is not observed in bulk bismuth, the optical properties of bismuth nanowires are also shown to provide an important tool for determining the band properties of bulk bismuth, which sensitively affect the thermoelectric properties of bulk bismuth and bismuth nanowires.