CdSe nanowires (NWs) are structures with high absorption cross sections and easy current flow along their long axis – desirable properties for applications, for example in photovoltaics. Additionally, easily affordable methods of chemical synthesis can be used to prepare nanowires with different structural properties. Specifically quantum nanowires (QNWs) – nanowires with radii below the exciton Bohr radius – are of interest, as the wires' effective band gaps depend on their respective radius.

Photoluminescence spectroscopy of single QNWs can give valuable information about the NWs' electronic structure and charge carrier dynamics. Confocal microscopy and spectroscopy of single QNWs at temperatures at about 5 K yield additional information. The single broad band which can be observed at room temperature splits into several narrow bands. The definition and narrowness of these bands is highly dependent on the individual wire samples but can be reproduced within each batch – i.e. synthesis – of wires.

We show for the first time that for QNWs the spectrum consists of two groups of several sharp peaks. The peaks of the high energy group are narrowly spaced. The spectrum's low energy side consists of one or more bands with pronounced LO phonon replicas. Both groups are subject to a spectral shift to higher energies with decreasing wire diameter.

In both spectral groups the bands are highly dynamic in terms of spectral position and intensity. Both groups show apparently independent spectral shifts, as well as particular intensity fluctuations on remarkably different time scales. The low energy bands with their respective LO phonon replicas show discrete On and Off states with the dark states of these bands lasting up to a minute. The high energy bands fluctuate on a much smaller time scale.

Our experimental findings can be explained by model calculations based on the effective mass approximation. By assuming the occurrence of positively and negatively charged defect sites as steep potential traps in the QNW the low energy side of the spectrum can be explained. These phonon-assisted peaks arise from recombinations of charge carriers, each trapped by such a site. This is analogous to the donor–acceptor pair recombinations commonly assigned in bulk CdSe fluorescence spectra. The higher energy bands accordingly result from recombinations of free or nearly free excitons bound to very shallow potential undulations.