Dipti Jasrasaria, University of California, Berkeley
Despite the fundamental role that electron-phonon interactions play in quantum dot (QD) technologies, there is no unified picture, either via theory or experiment, for understanding these interactions across a variety of QD systems. We have constructed a framework to calculate the exciton-phonon coupling in QDs of experimentally relevant sizes and its dependence on QD size, composition, and the energies of associated excitons and phonons. We perform atomistic electronic structure calculations within the semiempirical pseudopotential approach and solve the Bethe-Salpeter equation to obtain correlated electron-hole pair (i.e. excitonic) states of CdSe QDs with about 10^3 to 10^4 atoms. Electron-hole correlations are necessary to accurately describe excitons in QDs. We use molecular dynamics simulations to obtain phonon frequencies and modes and, finally, we compute the exciton-phonon coupling within a harmonic approximation.
We have validated our model by computing the reorganization energy associated with the rearrangement of the QD lattice after exciton formation. For CdSe QDs ranging from 2 to 5 nm in diameter, we calculated reorganization energies of 80 to 25 meV, which compare favorably with previous calculations and experiments. Our calculations also show that while excitons interact with a range of acoustic and optical phonon modes in CdSe QDs, they are particularly strongly coupled to spheroidal (breathing) and torsional acoustic modes via deformation potential coupling. Because these modes involve large collective motions of all the atoms of the QD, exciton formation can induce relatively large atomic displacements of about 0.1 angstrom, especially for those atoms at the surface.
This framework can provide valuable insight into exciton-phonon coupling in QDs on the atomic scale and can be applied to study core-shell QDs, nanorods and nanoplatelets, and exciton coupling to localized trap states.
Abstract Author(s): Dipti Jasrasaria, Eran Rabani