Optical Properties of CdSe Nanowires — Experiment and Theory

CdSe nanowires are one-dimensional semiconductor nanostructures with unique properties. The wires studied in this work possess diameters on the nanometer scale while diameters are on the order of tens of microns. For CdSe structures this has the consequence that size quantization effects are at play along the radial dimension while longitudinal dimensions are outside the confinement regime. Physical and electrical properties are therefore expected to be diameter dependent, but not very sensitive to variations in length. Another important aspect to consider is that the diameters studied span a range from small (i.e. d~3-5 nm) where wires show discrete transitions akin to quantum dots all the way up to large (i.e. d~25 nm). At the largest size studied, the material does not show size quantization effects and its properties are similar to bulk CdSe. One of the key questions I would like to answer is what characterizes the transition from a regime controlled by quantum effects (i.e. small diameter nanowires) to bulk-like semiconductors (i.e. large diameter wires).

The tools used to carry out the characterization include a number of steady-state and time-resolved spectroscopic techniques that are used to probe semiconductor nanowires both on the ensemble and single nanowire level. For single wire level measurements, the tool of choice is optical microscopy. Using microscopy, individual nanowires can be identified and studied using various excitation and emission wavelengths to characterize their properties. Using emission and absorption spectroscopies, the electronic structure of the nanowire as well as how charges interact is revealed. It, in turn, allows identifying signatures that belong to photogenerated charges that exist as coulombically bound electron-hole pairs (excitons) and distinguish them from free charges.