NSF Award
2100248
This project is managed in the Division of Materials Research and funded by the Historically Black Colleges and Universities, Excellence in Research (HBCU-EiR) program and by the Established Program to Stimulate Competitive Research (EPSCoR).<br/><br/>NONTECHNICAL SUMMARY<br/>Quantum dots are tiny crystals, a few nanometers across, that display useful electronic and optical phenomena. This award supports theoretical research into such phenomena in dots made of lead sulfide, lead selenide, or lead telluride, the so-called lead chalcogenides. The distribution of possible electronic energies in a quantum dot depends on size, shape, and composition; tuning these parameters will permit applications in photovoltaics, optoelectronics, quantum information technologies, and medicine. Lead chalcogenide quantum dots are unique because they can be used in devices operating in the near-infrared and mid-infrared frequency ranges employed in telecommunications and medical imaging. Optical properties of these quantum dots are determined by particles called excitons, each consisting of a negatively charged electron and a positively charged empty state, or hole, where an electron should have been. At low resolution, many different exciton states share the same energy. However, competing interactions split the energies, so that different exciton states take different energy values. Being able to predict how these splittings depend on shape and size is key to engineering quantum dots for desired purposes. This research will be carried out at Jackson State University and will contribute to development of the science, technology, engineering, and math workforce at a historically Black university that primarily serves educational needs of local minority students.<br/><br/>TECHNICAL SUMMARY<br/>This award supports theoretical investigation of the exciton fine structure in lead chalcogenide quantum dots and its manifestation in optical phenomena. Description of band-edge exciton states in lead chalcogenide quantum dots comes up against fundamental complexity stemming from the fact that extrema of conduction and valence bands in bulk lead chalcogenides occur at four inequivalent L points of the Brillouin zone. In lead chalcogenide quantum dots, electron-hole exchange interaction and intervalley coupling split the otherwise 64-fold valley- and spin-degenerate ground exciton state into several groups of closely spaced states giving rise to a large intrinsic broadening of the photoluminescence spectral lines. Furthermore, an exciton effectively couples to confined optical and acoustic phonon modes. This results in complex photoluminescence spectral line-shapes which can be directly observed by means of single-dot spectroscopy. Additional information can be obtained from polarized photoluminescence measurements in external magnetic fields. This project aims to model lead chalcogenide quantum dots within the empirical tight-binding and effective-mass methods in order to provide theoretical description of physical processes underlying this kind of experiment. The effects of various perturbations and their interplay will be studied in order to gain control over the exciton fine structure through the control over the quantum dot shape.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
