Concerted single-nanowire absorption and emission spectroscopy: Explaining the origin of the size-dependent Stokes shift in single cadmium selenide nanowires

Abstract

Concerted single-nanowire (NW) absorption and emission spectroscopies have been used to measure Stokes shifts in the optical response of individual CdSe NWs. Obtained spectra are free of inhomogeneous broadening inherent to ensemble measurements. They reveal apparent size-dependent NW Stokes shifts with magnitudes on the order of 30 meV. Given that an effective mass model previously used to explain CdSe NW excited state progressions predicts no sizable emission Stokes shift, we have investigated modifications to the theory to rationalize their existence. This has entailed better accounting for the effects of crystal field splitting on NW band edge states. What results are important changes to the spectroscopic assignment of NW band edge transitions that arise from the crossing of hole levels. Furthermore, these modifications simultaneously predict Stokes shifts with size-dependent magnitudes up to 20 meV. However, quantitative agreement with experiment is only achieved by accounting for the role of exciton trap states. Consequently, we conclude that CdSe NW Stokes shifts contain both intrinsic and extrinsic contributions-the latter arising from band edge exciton potential energy fluctuations. At a broader level, these concerted absorption and emission measurements have provided detailed insight into the electronic structure of CdSe NWs, beyond what could be obtained using either single-particle absorption or emission spectroscopies alone.

Dimensional crossover in semiconductor nanostructures

“…Three to four transitions (labelled α, β, γ, and δ) are apparent in each spectrum and are excitonic in nature as predicted by a model which explicitly accounts for both spatial confinement and enhanced electrostatic interactions in NRs 15 . These states can be explicitly linked to analogous α, β, γ, and δ transitions in individual CdSe NWs 12 14 16 . The data in Fig.…”

“…In particular, carriers experience additional confinement along the NR z -axis, adding to the radial ( ρ ) confinement present exclusively in NWs 12 15 16 . To assess such confinement effects, we have constructed a modified version of an effective mass model previously used to explain absorption spectra of individual CdSe NWs 12 14 16 .…”

“…This is accomplished using spatial modulation microscopy12141617, which entails modulating a NW/NR's position in and out of a focused laser beam. Subsequent lock-in detection measures the transferred transmitted laser power modulation, which relates to individual nanostructure extinction cross-sections (Supplementary Note 1 and Supplementary Fig.…”

“…Importantly, they rely on photoluminescence and tunnelling spectroscopies 7 9 . However, dark/bright exciton splitting 13 , trap-induced Stokes shifts 14 and enhanced exciton binding energies 15 can significantly alter perceived bandgaps, complicating accurate E g estimates.…”

“…Here we directly probe this 1D-to-0D transition using single nanowire (NW)/NR absorption spectroscopy to eliminate ambiguities as to the actual evolution of electronic structure across dimensionality. This is accomplished using spatial modulation microscopy 12 14 16 17 , which entails modulating a NW/NR's position in and out of a focused laser beam. Subsequent lock-in detection measures the transferred transmitted laser power modulation, which relates to individual nanostructure extinction cross-sections ( Supplementary Note 1 and Supplementary Fig.…”