Size-dependent exciton recombination dynamics in single CdS nanowires beyond the quantum confinement regime

A deep understanding of the size, surface trapping, and scattering effects on the recombination dynamics of CdS nanowires (NWs) is a key step for the design of on-demand CdS-based nanodevices. However, it is often very difficult to differentiate these intertwined effects in the NW system. In this article, we present a comprehensive study on the size-dependent exciton recombination dynamics of high-quality CdS NWs (with diameters from 80 to 315 nm) using temperature-dependent and time-resolved photoluminescence (TRPL) spectroscopy in a bid to distinguish the contributions of size and surface effects. TRPL measurements revealed two distinct processes that dominate the band edge recombination dynamics - a fast decay process (τ₁) originating from the near-surface recombination and a slower decay process (τ₂) arising from the intrinsic free exciton A decay. With increasing NW diameters, τ₁ increases from ∼0.10 to ∼0.42 ns due to the decreasing surface-to-volume ratio of the NWs, whereas τ₂ increases from ∼0.36 to ∼1.21 ns due to decreased surface scattering in the thicker NWs - as validated by the surface passivation and TRPL studies. Our findings have discerned the interplay between size and surface effects and advanced the understanding of size-dependent optoelectronic properties of one-dimensional semiconductor nanostructures for applications in surface- and size-related nanoscale devices.