Temperature-Dependent Hole Transfer from Photoexcited Quantum Dots to Molecular Species: Evidence for Trap-Mediated Transfer

Abstract

The effect of temperature on the rate of hole transfer from photoexcited quantum dots (QDs) is investigated by measuring the driving force dependence of the charge transfer rate for different sized QDs across a range of temperatures from 78 to 300 K. Spherical CdSe/CdS core/shell QDs were used with a series of ferrocene-derived molecular hole acceptors with an 800 meV range in electrochemical potential. Time-resolved photoluminescence measurements and photoluminescence quantum yield measurements in an integrating sphere were both performed from 78 to 300 K to obtain temperature-dependent rates for a series of driving forces as dictated by the nature of the molecular acceptor. For both QD sizes studied and all ligands, the Arrhenius plot of hole transfer exhibited an activated (linear) regime at higher temperatures and a temperature-independent regime at low temperatures. The extracted activation energies in the high-temperature regime were consistent across all ligands for a given QD size. This observation is not consistent with direct charge transfer from the QD valence band to the ferrocene acceptor. Instead, a model in which charge transfer is mediated by a shallow and reversible trap more accurately fits the experimental results. Implications for this observed trap-mediated transfer are discussed including as a strategy to more efficiently extract charge from QDs.

Polariton-Mediated Electron Transfer via Cavity Quantum Electrodynamics

“…A particular system that we proposed here for H en is the colloidal nanocrystal (NC) with organic molecular acceptors. NCs are advantageous in this application because (i) they are well-established electron 47 and hole donors 48 with long excited state lifetimes; (ii) they are exceptionally photostable which is important given the large optical fields inside the cavity, and (iii) they have been strongly coupled to several types of optical cavity. [86][87][88][89] Further, polaritonic photochemistry in such system is advantageous since the transition dipole moment between the ground and the donor state is large and isotropic due to the spherical symmetry of the NC.…”

Influence of Surface Ligands on Charge-Carrier Trapping and Relaxation in Water-Soluble CdSe@CdS Nanorods

“…Hence, under certain circumstances, surface trapping of charges can also support the desired function and increase their availability to surface-proximal catalytic centers and/or hole scavengers. A similar concept has recently been introduced as trap-state mediated charge transfer [83,84]. Thus, deciding on an optimal surface ligand is a sophisticated task and affords detailed knowledge on the function determining mechanistic steps and the impact of the ligand on these.…”

Prospects of Coupled Organic–Inorganic Nanostructures for Charge and Energy Transfer Applications

“…This can be rationalized in terms of the well‐known dependence of Dexter energy transfer on large orbital overlap between the exciton donor and acceptor. Charge transfer from CdSe/CdS core–shell quantum dots to X‐type ferrocene derivatives proceeds in two short‐range charge transfer steps via a reversible, shallow trap state reservoir at the quantum dot surface [62] . It can be argued that the reversibility and short‐range transfer mechanism increases the overall transfer probability with respect to a direct, single transfer.…”