Author: Eric Ronald Kennehan

Publisher:

Published: 2019

Total Pages:

ISBN-13:

Hot-carrier cooling, which has been predicted to be slowed in semiconductor quantum dots (QDs) due to the anticipated hot-phonon bottleneck, has not been readily observed, with carrier cooling rates of QDs approaching bulk cooling rates. PbS QDs are excellent potential candidates to exhibit extended hot-carrier lifetimes since confinement of both electrons and holes are nearly identical, thus circumventing an efficient Auger mechanism which has been found to increase cooling rates in other materials. Despite, the advantages of PbS QDs for enhanced hot-carrier lifetimes, rapid thermalization of hot-carriers persist in these materials and the mechanism which directs this fast cooling is not well understood. Using a combination of steady-state and time-resolve, electronic optical spectroscopy, this work has provided new insights into the behavior of excited electronic states in PbS QDs. This work identified the spectroscopic signature of the formally forbidden 1Se(h)-1De(h) intraband transition, allowing vibronic coupling to be investigated for this high energy state, along with the 1Pe(h) and band edge states. It was determined that significant homogenous broadening of these states occurs as a result of a highly size-dependent exciton-phonon coupling mechanism. It is this vibronic coupling that results in resonant energy relaxation pathways between the intraband states of the QDs that are amplified for smaller diameter nanocrystals, leading to rapid bulk-like cooling in these materials. Additionally, steady-state and time-resolved vibrational spectroscopy were used to identify possible sources of the vibronic coupling in PbS QDs. Through this work, vibrational coupling in PbS QDs, was found to be independent of the types of ligands at the crystalline surface and was in fact mediated by the surface phonons. Strong nonadiabatic coupling is occurs at the QD surface because of the broad phonon spectrum associated with the strained crystalline lattice, dangling bonds, and exposed crystal facets. This work suggests that surface modification techniques that reduce the coupling of electronic states to surface phonons may enable quantum confined structures with slower carrier cooling rates. Such surface modifications may include the use of thin inorganic shells or controlled composition gradients to reduce dangling bonds and tune the lattice strain at QD surfaces.