Exciton Dynamics in Colloidal Lead Sulfide (PbS) Nanosheets

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Semiconductors play a central role in many modern technologies, from computer chips to light emitting diodes (LEDs). A key property of a semiconductor is its band gap, which is the smallest energy an electron in the crystal can absorb. While the band gap depends on the type of semiconductor, it also depends on the size of the crystal. As the crystal shrinks, the electrons become confined in smaller and smaller volumes. When it reaches only a few nanometers in size, or about 100,000 times thinner than a sheet of paper, its band gap can be very different. One can use this confinement effect to make LEDs of different colors simply by changing the size of the particle, rather than the material itself. However, while semiconductor nanostructures that emit visible light are common, the advancement of new materials for infrared light has remained a challenge. With support from the Macromolecular, Supramolecular and Nanochemistry Program in the Division of Chemistry, Professor Liangfeng Sun at Bowling Green State University is studying electrons confined in lead sulfide (PbS) nanosheets. Working with his students, Professor Sun is developing methods to grow PbS nanosheets with precise control over their dimensions. They use sophisticated experimental methods to study their confined electrons. Their discoveries could lead to more efficient solar cells and better semiconductor lasers. The project also provides training opportunities for future scientists in advanced experimental techniques. In addition, the project is engaging undergraduates from Central State University, an historically black university in Wilberforce, Ohio, as well as introducing high-school students to nanomaterial research.The research team is studying the radiative recombination rates, charge transfer, exciton-exciton interactions, and resonance energy transfer processes as a function of the nanosheet structure (sheet area, thickness, co-facial contact, and comparison to quantum dots). Steady-state spectroscopy and electrochemical methods are used to elucidate static properties such as energy levels, transition line widths of emission and absorption spectra as a function of size and temperature. Time-resolved spectroscopic techniques (fluorescence and transient absorption on the picosecond and femtosecond time scales, respectively) are used to investigate exciton dynamics in these novel structures. The research activities include: synthesizing high-quality PbS nanosheets with tunable thickness and lateral size, studying the dynamics of a single exciton in isolated nanosheets, studying the dynamics of the interacting multiple excitons in a nanosheet, and investigating the exciton dissociation and charge transfer from nanosheets to charge acceptors. The research group also investigates the exciton resonance energy transfer between the nanosheets.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.