Low-Temperature Absorption Studies in Bismuth Nanowires

Bulk bismuth has a small band overlap between the conduction and valence bands and a highly anisotropic electron effective-mass tensor. Previously, we have shown evidence for strong quantum confinement in Bi nanorods with diameters $\sim $10 nm which undergo a transition from a semimetal with a small band overlap to a semiconductor with a small indirect band gap. These quantum confinement effects can be potentially useful in optical and electro-optical devices. Here, we report the low temperature (77 K) optical absorption properties of $\sim $10 nm diameter Bi nanorods using Fourier Transform Infrared spectroscopy. The Bi nanorods exhibit a strong absorption peak ($\sim $1000 -- 1400 cm$^{-1}$, depending on the diameter) in the mid-IR that is not present in bulk bismuth. The full width at half maximum intensity of the IR absorption peaks decrease from 26 cm$^{-1}$ at 300 K to 15 cm$^{-1}$ at 77 K. No significant blue-shift in energy was observed, and these changes will be discussed in terms of the temperature dependence of the L-point and T-point electron energies.