The optical properties of bismuth nanowires

Abstract

The optical absorption of bismuth nanowires in the energy (wavenumber) range of 600 - 4000cm-1 is studied. Optical reflection and transmission spectra reveal that bismuth nanowires have a large and intense absorption peak as well as several smaller absorption peaks which are not measured in bulk bismuth. The smaller absorption peaks fit reasonably well to theoretical models for intersubband absorption in bismuth nanowires. The wire diameter, polarization, and doping dependencies as well as the spectral shape of the dominant peak agree with simulations of the optical absorption resulting from an L-point valence to T-point valence band electronic transition. The large absorption peak is present even for nanowires too large to exhibit quantum confinement, thus showing that the absorption results from a surface-induced effect and not from quantum confinement. The enhanced optical absorption in nanowires over bulk bismuth is attributed to a surface term in the matrix element which results from the spacial gradient of the dielectric function and the large dielectric mismatch between bismuth and the surrounding alumina or air. A comparison of the measured spectra with simulations of optical absorption resulting from direct L-point electronic transitions demonstrated that this absorption mechanism is not dominant in our materials. In order to explore the optical properties of bismuth nanowires, two methods were developed. First, effective medium theory applied in reverse was used to deduce the dielectric function of materials smaller than the wavelength of light. Second, a technique to fabricate nanowires with diameters above 200nm was transfered into our laboratory.

(cont.) The enhanced coupling between the L-T point valence bands in nanowires may lead to a very accurate measurement of the band gap and band overlap in bismuth as a function of doping and temperature. In addition, the discovery of the enhanced interband coupling resulting from the surface contribution to the matrix element has many implications, especially if this result is applicable to other systems.