In this work, we use optical spectroscopic techniques including micro-photoluminescence (PL), spatially-resolved PL imaging, time-resolved PL, PL polarization measurements to investigate the optical and electronic properties of single CdS and core-shell GaAs/AlGaAs nanowires (NWs) at both room and low temperatures.
CdS nanowires grown by the Vapor-Liquid-Solid (VLS) method show variation in morphology ranging from straight and uniform, to extremely irregular with bends, kinks, and irregular shaped clusters of material aggregating locally on the nanowires. At room temperature we observe that all nanowires, regardless of size and shape, emit PL with a single broad peak with Full Width at Half Maximum (FWHM) ~ 70 meV at energy below the band gap energy of bulk CdS (2.51 eV). The emission is often referred as the Near Band Edge (NBE) emission. At low temperature, PL emission from these nanowires are significantly different: PL from uniform wires still display a broad peak near 2.54 eV (FWHM ~32 meV) but irregular wires show a broad band with a high energy shoulder near ~2.54 eV and a number of sharp emission lines are observed on the lower energy side of the broad band. Spatially resolved PL imaging and temperature dependent PL measurements reveal that these sharp emission lines originate from specific positions along the
nanowire and are completely quenched by 80-90K. Time-resolved measurements show that the decay times of the emission
from the high energy shoulder (or near band edge) is very short (< 50 ps) while it is significantly longer for the sharp lines (500-1000 ps). We conclude that the sharp emission lines result from defect or surface related electron states.
For single core-shell GaAs/AlGaAs nanowires, we observe an order-of-magnitude higher quantum efficiency for emission than for bare GaAs nanowires. This results from passivation of the GaAs surface with AlGaAs, reducing non-radiative surface recombination. Non-equilibrium spin distributions in single GaAs/AlGaAs core-shell nanowires are excited using resonant polarized excitation at low temperature (6 K). The large dielectric mismatch between the semiconductor nanowire (epsilon = 12 for GaAs and AlGaAs in this case) and the surrounding vacuum (epsilon = 1) results in significant polarization anisotropy of both excitation and emission at all excitation energies. We observe a significant increase in PL intensity (by a factor of ~ 70) occurs when the exciting laser is polarized along the nanowire versus perpendicular to it. The suppression of the electromagnetic (photon) field in these nanowires results directly in a corresponding suppression of the
radiative rate for excitonic dipoles aligned perpendicularly to the nanowire. We observe two strong resonances at 1-LO and
2-LO phonons of GaAs and a third resonance likely from electronic states of the AlGaAs shell. More interestingly, we observe
that the polarization of the PL emission is strongly enhanced as the excitation energy comes closer to resonance with the exciton emission. This strong polarization enhancement indicates that resonant excitation creates non-equilibrium exciton spin distributions near resonance. Rate equation modeling allows us to estimate the spin relaxation times which range from ~5 ps at high energies to ~50ps at energies close to resonance.
Our results suggest the use of polarization measurements, spatially- and temporally-resolved optical spectroscopies are
important tools for rapidly characterizing nanowire growth for the optimization of future applications in nano devices.