Theoretical Chemistry Ph.D. project

PI: Eran Rabani

Quantum dots (QDs) are confined systems of semiconductor materials with nanometer-scale sizes. They exhibit ultranarrow linewidth in single particle photoluminescence (PL) spectroscopy at low temperature. This property makes QDs ideal candidates for applications in many technologies, such as solar cells, lasers, and bioimaging probes.

The nature of the PL linewidth has been attributed to exciton-phonon coupling in these QD systems. However, earlier theoretical (and experimental) studies gave divergent results on exciton-phonon coupling strength, zero-phonon line width, and phonon sidebands in CdSe QD systems.

In this work, a complete theoretical framework of describing exciton-phonon coupling and its effect in spectroscopy is given. A pure dephasing model is established, by describing the electronic degrees of freedom of the model system using the atomic semi-empirical pseudopotential, and the phonon modes of the model system using the classical Stillinger-Weber force field. The model, treating the phonons as harmonic quantum mechanical particles, connects the exciton fine structure with exciton-phonon coupling, and gives a precise analytical expression of the photoluminescence spectra.

Compared to previous theoretical investigations, our model has a few advantages. First, it is an atomistic model based on pseudopotentials that are parameterized from first-principles calculations. Exciton-phonon couplings are calculated for each excitonic state and phonon modes, and the form of the spectral density isn't fitted to any empirical parameters. Secondly, a unified treatment of normal modes in the QD system is used in our model. No separate treatments of different coupling mechanisms (deformation potential, Frohlich) or different phonon branches is needed. Lastly, our model gives good quantitative agreement to experimental PL measurements of said QD systems in both line shape and zero-phonon linewidth, especially in the low temperature regime.