Altering quantum confinement in semiconductor nanocrystals using strongly interacting exciton-delocalizing ligands

Abstract: Semiconductor nanocrystals have long been studied as alternatives to traditional bulk semiconductor materials as active components in optoelectronic devices due to their size-tunable absorption and emission properties, as well as their ability to be processed into thin films from colloidal solutions. However, creating highly conductive nanocrystal solids remains a substantial challenge due to the presence of long (~2-3 nm), insulating “native ligands” that are used to terminate nanocrystal growth and provide colloidal stability following their synthesis. In this work we have used a variety of structural and spectroscopic characterization to investigate the chemical and physical changes associated with exchanging these native ligands for ligands that have the proper energetics and orbital symmetry conditions to interact strongly with nanocrystal electronic states, termed “exciton-delocalizing ligands”. This interaction is proposed to allow for delocalization of carriers beyond the nanocrystal core and into the ligand shell by reducing the potential energy barrier at the nanocrystal-ligand interface, which can be used to improve transport properties in nanocrystal solids. Colloidal nanocrystal samples were investigated to determine if this strong interaction impacts carrier cooling rates, as this would provide insight into the degree of mixed nanocrystal-ligand character of these states. Using transient absorption spectroscopy we measured the change in electron and hole cooling rates following ligand exchange with the exciton-delocalizing ligand phenyldithiocarbamate and found that when excited near the nanocrystal band edge, the valence band states of the nanocrystals interact more strongly with the ligand than those in the conduction band. Solid-state ligand exchange with phenyldithiocarbamate was then carried out on nanocrystal films to determine if the strong nanocrystal-ligand interaction of these exciton-delocalizing ligands impacts the exciton mobility in nanocrystal solids. Using a combination of transient absorption spectroscopy and kinetic Monte-Carlo simulations we have found that treatment with phenyldithiocarbamate yields a drastic increase in the diffusivity of excitons in nanocrystal films, and this improved transport occurs via a unique tunneling-type mechanism rather than more traditional Förster Resonance Energy Transfer. A combination of multidimensional spectroscopy and transient absorption were used to probe the electronic structure and dynamics of ligand-exchanged nanocrystals in both solution and films. Contributions to homogeneous broadening are assigned as primarily due to ligand fluctuations in solution and energy transfer between NCs in solids. Finally, by varying the initial excitation energy in the nanocrystal we find that more highly excited carriers exhibit a greater degree of delocalization resulting from a greater degree of mixed nanocrystal-ligand character of these states.