Exciton binding energy in semiconductor nanowires in the presence of dielectric de-confinement

Fluorescence spectra of 10-, 25- and 50-nm diameter CdS nanowires (relative dielectric constant = 5.4) self assembled in a porous alumina matrix (relative dielectric constant = 8-10) reveal peaks associated with free electron-hole recombination. The 10-nm wires also show an additional lower energy peak due to exciton recombination. In spite of dielectric de-confinement caused by the insulator having a higher dielectric constant than the semiconductor, the exciton binding energy increases almost 8-fold from its bulk value. This increase is most likely due to quantum confinement accruing from the fact that the exciton Bohr radius (~5 nm) is comparable to or larger than the wire radius, especially if side depletion is taken into account. Such an increase in the binding energy could be exploited to make efficient room temperature luminescent devices in the visible range.