Surface Localized Sub-bandgap States in Lead Chalcogenide Nanocrystals

In our information technology-based world semiconductors are essential to our modern way of life, they are the basis upon which the vast majority of our modern optoelectronic and computing technologies work. As industry has sought to increase performance, decrease the size and cost, and expand the capabilities of these technologies, nano-materials such as semiconducting nanocrystals are playing an ever-larger role in achieving these objects. Indeed, semiconducting nanocrystals are quickly gaining industry acceptance in an array of technological applications including flat screen displays, photovoltaics, lasers, chemical sensing and medical imaging applications, among others.The versatility of semiconducting nanocrystals derives from their superlative opto-electronic and charge transport properties, the chief of which is their ability to efficiently absorb and emit light, across the visible and near-infrared regions of the electromagnetic spectrum. The tunability of photon absorption and emission in semiconducting nanocrystals is based on factors such as chemical composition and stoichiometry, size, and crystal morphology. This tunability allows for the production of nanocrystals with frequency-specific light emission and absorption characteristics, important for many evolving technologies. Semiconductor nanocrystal charge transport capabilities and the ability for charge carrier manipulation, such as multiple exciton generation (MEG), present other avenues for technological innovation and improved efficiency over current semiconductor-based technologies. Most importantly, as a result of their solution processability, as opposed to traditional semiconductor manufacturing processes, technologies based on semiconducting nanocrystal have the potential for vast improvements in production efficiency, waste reduction, and considerable savings with respect to energy consumption and cost. Semiconducting nanocrystal technologies present scientists and engineers an array of options for novel device functionalities and architectures. However, because of their size and surface to volume ratio, the optoelectronic properties of semiconducting nanocrystal QDs are susceptible to significant perturbations depending on the surface chemistry and crystal defects, as well as internal composition and structure. Chemical and structural irregularities at their surfaces can lead to substantial alteration of their optoelectronic properties, presenting headaches, but also avenues of exploitation for scientists and device engineers. The research presented in this dissertation explores the existence of sub-bandgap states in PbS nanocrystals’ electronic structure due to differing local surface stoichiometry and surface reconstruction resulting from removal, or lack of passivating ligands.2023-08-0