Tailoring quantum dot assemblies to extend exciton coherence times and improve exciton transport

Abstract

Electron energy transfer (EET) through nanostructured assemblies plays a crucial role in a wide range of emerging technologies such as quantum dot solar cells, quantum computing, molecular electronics, excitonic transistors, and light emitting diodes. These technologies are very dependent on excitonic lifetimes which are short on the order of a nanosecond. In order to efficiently use this short time scale, EET needs to be as fast as possible. This leads to an interest in the application of coherent exciton transfer. To examine the possibility of coherent transfer, we ask a simple question: How rapidly do coherent superpositions of excitonic states dephase between quantum dots? We assume that the major source of decoherence at room temperature is from the internal phonon modes of silicon quantum dots. The question is then addressed using a combination of ab initio calculations and a master equation formulation for the evolution of the electronic density operator for a dimer of interacting two-level systems coupled to a shared bath of harmonic oscillators. A combination of density functional theory (DFT) and frozen phonon method (FPM) analysis was used to obtain exciton-phonon coupling in various sizes of silicon quantum dots. As expected, coherent EET is faster in comparison to incoherent EET in assemblies of identical nanostructures. In cases of non-identical assemblies of nanostructures, the low energy regions act as trap states for the exciton. In this case, a combination of coherent and incoherent transport leads to the fastest transport rate.