Rapid Evaluation of Time Dynamics and Excitation Spectra of Quantum Dots

QDs have attracted a lot of interest in recent years due to their potential use in solar cells, light-emitting diodes (LEDs), displays, photo-detectors and biological sensing and imaging. Due to their size (1-10 nm) their electronic structure resembles that of both a molecule and a solid. At the band edges QDs show a set of discrete states like those in a molecule, while the orbitals lying deeper in the bands form a continuum resembling the electronic structure of a solid. The size of the quantum dots makes their optical properties difficult to describe from a theoretical perspective. For example, a CdSe/ZnS core shell QD, with a diameter of around 2 nm, consists of more than 500 atoms, possible ligand atoms not included. This QD is on the upper end of what is possible to describe computationally, but is still rather small compared to the experimentally observed average size of 3.5 nm. Moreover, hybrid density functional theory (DFT) is required to obtain reasonable accuracy for semiconductors, which further increases the computational cost. Furthermore, in order to reveal the full optical properties of QDs one needs to apply time-dependent DFT (TDDFT). However, the high computational cost of TDDFT, on top of everything mentioned previously, usually allows the computation of only a few, low-lying excited states for rather small QDs.<br/>In this computational study, we present a way to obtain accurate excitation spectra for quantum dot sizes which are not feasible by current standard TDDFT approaches. By combining of short real-time propagation TDDFT (RT-TDDFT) and approximated linear-response TDDFT (LR-TDDFT), we are able to rapidly obtain excitation spectra for the entire optical range. We demonstrate the capabilities of our approach by studying the optical properties of CdSe/ZnS core shell QDs.