Surface Related Emission in CdS Quantum Dots. DFT Simulation Studies

In general, organic capping molecules are applied to passivate the surface of semiconductor nanomaterials to modulate the optical properties of these nanostructures. In this work, two alkylamines (n-butylamine (n-BA) and n-hexylamine (n-HA)) and oleic acid (OA) were used to modify the surface of moderately high luminescent CdS quantum dots (Qps). From the photoluminescence (PL) spectra and the quantum yield (QY) analyses, we observed that the PL QY of the CdS QDs decreased after introduction of the alkylamine and oleic acid molecules. The PL decay kinetics for these CdS-capping molecule systems were followed by time-resolved photoluminescence (TRPL), and the spectra were analyzed in terms of a biexponential model identifying two lifetime values, shorter lifetime (tau(S)) and longer lifetime (tau(L)). Compared to bare CdS QDs, for the CdS QDs surface modified by alkylamine or fatty acid, both the shorter and the longer excited state lifetimes were decreased; the fractional contribution by the longer-lifetime component became reduced and the shorter-lifetime component accounted for most of the total PL. Density function theory (DFT) simulation was employed using a Cd3S5 cluster to model the adsorption of organics to calculate the binding energy and the charge on Cd and S of CdS. By comparing the elemental charges of the bare CdS with those of the CdS modified by the organic molecules, it is suggested that n-BA, n-HA, and OA could decrease the surface related radiative charge-recombination process and the PL-QY of the CdS QDs.