Nano-Optics of Bismuth Nanowires
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Nano-Optics of Bismuth Nanowires M. R. Blacka) , P. L. Hagelsteina) , M. S. Dresselhausa,b) a Department of Electrical Engineering and Computer Science b Department of Physics Massachusetts Institute of Technology, Cambridge, MA 02139-4307
Abstract The optical absorption from an indirect L-T point valence band transition in bismuth nanowires is simulated based on a surface sensitive term, which is very weak in bulk bismuth, but effectively couples the L and T point bands in bismuth nanowires. The experimental Fourier transform infrared absorption spectra in the energy range 0.08 to 0.5 eV are compared to theoretical simulations for absorption resulting from indirect interband transitions between subbands at the L point valence band to subbands at the T point valence band as the polarization, tellurium doping, and wire diameter are varied. The simulated absorption from this indirect transition reproduces many of the experimentally observed trends and allows us to conclude that the dominant feature in the experimental absorption spectra of Bi nanowires results from an indirect L-T point transition which is enhanced in Bi nanowires.
Introduction Nanowires possess optical properties that differ significantly from those of bulk materials. In this paper we focus on Bi nanowires within an alumina template, and explore one of the more interesting and potentially useful differences in the optical properties of bismuth nanowires and bulk bismuth. The Bi nanowires used in this study are fabricated by using a liquid pressure injection process to fill porous anodic alumina templates[1, 2] with bismuth. The alumina, which is a wide bandgap semiconductor, electrically isolates individual wires from each other, protects them from oxidation, and allows the researcher to control the nanowire diameter. Inside the porous anodic alumina template, the bismuth forms a self-assembled array of single crystalline, crystallographically oriented bismuth nanowires of small diameter and long length. Using Bismuth permits the observation of quantum confinement effects at relatively large diameters. If the diameter of the bismuth nanowires is small enough, subbands form in the electronic band structure, transforming the nanowires into one-dimensional conductors. For wire diameters as large as 49(17)nm at 77(293)K, quantum confinement effects are strong enough to cause the bismuth to transform from a semi-metal to a semi-conductor. In these wire diameter ranges, electronic transitions between the subbands give rise to significant optical absorption features. In previous work, we have measured the optical absorption in bismuth nanowire arrays, identifying a sharp and intense absorption peak in the mid-IR at ∼ 1000cm−1 (10µm, 124meV ), which is not observed in bulk bismuth.[3] Previous work also reported the shape of the absorption spectra [4] and its dependence on polarization[5], on wire diameter[5], and on doping concentration.[6] This paper builds on this work by
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Experimentally Measured
Simulation of the indirec
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