Role of Surface States in the Exciton Dynamics in CdSe Core and Core/Shell Nanorods

Abstract

Static and time-resolved photoluminescence (PL) along with transient absorption (TA) measurements have been used to elucidate the relaxation and recombination dynamics of single excitons and multiexcitons in CdSe nanorods with organic ligands and CdSe/CdS core/shell nanorods. The core/shell particles are synthesized from the same 3.5 × 17 nm nanorods as used in the studies on ligated particles. The core/shell particles are studied with and without adsorbed hole acceptors, specifically, hexadecanethiol (HDT) and phenothiazine (PZT). The time-resolved PL and quantum yield results show that following single-exciton photoexcitation, the core nanorods undergo exciton recombination dynamics on several different time scales. The sample is inhomogeneous, and most of these particles are not observed at all. Of the particles that are observed, most undergo exciton quenching by hole transfer to surface states, leaving the electron in the conduction band. The extent of hole trapping depends on the adsorbed ligands, with alkyl amines partially passivating the hole traps. High fluence excitation produces a significant concentration of biexcitons, which undergo Auger recombination on the approximately 100 ps time scale. Very different dynamics are observed in the CdSe/CdS core/shell nanorods. These nanorods exhibit high PL quantum yields (>50%), indicating that most of the surface hole traps have been passivated. High-fluence TA measurements indicate extensive conduction band state filling by multiexcitons. The Auger rate depends on the number of electron−hole pairs and is therefore state dependent. Auger recombination times are approximately 700 ps for the lowest conduction band level, 100 ps for the next level at 780 cm−1, and 13 ps for the π conduction band level at 3650 cm−1. The presence of adsorbed hexadecanethiol or phenothiazine changes the single- and multiexciton dynamics. Hole transfer quenches excitons, resulting in much faster PL decays, with little change in the X0 TA kinetics. These dynamical changes are accompanied by the appearance of a broad absorption of the PTZ+• radical cation, which persists for >1 ns. In the case of adsorbed HDT, the loss of valence band holes results in slower Auger recombination of electrons in the π conduction band level.