Low-Dimensional Si-Sn and Si-Ge-Sn Nanoalloys as High-Efficiency, Direct-gap Nanostructures for Visible to Infrared Optoelectronics.

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Non-technical Description: Developing high-efficiency light emitters from earth abundant elements is imperative to replace rare and expensive materials currently used in optical and electronic technologies. The prospective low-cost and non-toxic candidates such as silicon, however, show extremely low electricity to light conversion efficiency in their conventional arrangement. This project integrates the unique quantum effects in nanoscale silicon and tin alloying to produce silicon-tin and silicon-germanium-tin nanoparticles with size and composition tunable superior light absorption and emission properties for visible to infrared applications. The research team combines the material synthesis efforts with computational calculations and advanced optical and structural characterizations to garner a comprehensive understanding of the physical and optical properties and stability of nanoscale alloys. The collaborative nature of this research provides multidisciplinary training and mentoring of graduate and undergraduate students, to develop skills in materials design and synthesis, computational chemistry, nanoscience, and advanced optical spectroscopy. The summer outreach to Richmond Public Schools exposes K-12 students to cutting-edge materials research projects and develops age-appropriate materials science curricular modules, impacting hundreds of underrepresented minority students. Technical Description: Group IV semiconductor alloys that show high efficiency direct-gap emission have gained exceptional interest for realizing Si-based optoelectronic technologies. However, the narrow energy gaps and the extremely low solubility of Sn in Si and Ge hindered their fabrication and widespread application in visible to infrared optoelectronic studies. This project exploits the concerted influences of quantum confinement effects, Sn nano-alloying, and solution-phase synthesis to produce metastable Si-Sn and Si-Ge-Sn alloys and quantum dots (QDs) with size and composition tunable direct energy gaps and superior absorption and emission properties across visible to infrared spectrum. A series of monodisperse alloys having various sizes and compositions are produced by innovative colloidal chemistry methods. The influences of Sn alloying and quantum confinement on optical properties are thoroughly and systematically probed via steady-state and time-resolved photoluminescence and pump/probe transient absorption spectroscopy, guided by first-principles electronic structure and thermodynamic stability calculations. Experiments are designed to probe confinement- and composition-induced direct-gaps of silicon, dark vs. bright excitonic states and their dependence on nanocrystal size and composition, and carrier relaxation mechanisms involving QD core, surface, and their hybrid states to optimize the radiative efficiency. These later efforts along with solution processing and high thermal and optical stability of nanocrystal alloys enable the future design of high-efficiency, silicon-based optoelectronics.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.