SusChEM: Synthesis and Structure-Property Elucidation of Direct-Bandgap Group IV Alloy Nanocrystals for Optoelectronic Applications

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Non-technical Description: The production of high-efficiency optoelectronic materials based solely on low-cost, non-toxic, and abundant Group IV elements such as silicon is challenging as Group IV elements are less efficient in electron-photon conversion process compared to the widely used optoelectronic materials. This project utilizes the unique nanoscale size confinement effects and alloying with tin to produce silicon-tin, germanium-tin, and silicon-germanium-tin nanocrystals that exhibit superior light absorption and emission properties. The collaborative team supports the synthetic efforts along with advanced optical characterization and theoretical calculations to garner a deep understanding of the emerging materials properties and enhance the optoelectronic performance. The interdisciplinary research provides valuable training to the graduate and undergraduate students in the areas from smart material design to device testing. Other education and outreach activities include K-12 nanoscience outreach efforts and involving the under-represented high school students and women in summer research activities. Technical Description: Group IV semiconductor-based alloys with a direct bandgap are a long-sought goal in the field of optoelectronic materials. However this effort has been hindered primarily by the challenges in material design and synthesis, as well as the limited solubility of Sn in Si and Ge. This project utilizes both the quantum confinement effect and the Sn nano-alloying approach to produce direct bandgap Group IV alloys in nanocrystal form. The nanocrystals exhibit size and composition tunability that leads to wider effective bandgaps and superior photophysical properties. For example, GeSn, SiSn, and GeSiSn alloy nanocrystals with well controlled morphology and composition, far beyond the miscibility limit of Sn in bulk Si or Ge, are achieved via an innovative high-temperature colloidal synthesis method. Sn composition and quantum confinement effects on the indirect-to-direct bandgap transition are systematically investigated using steady-state and ultrafast absorption and emission spectroscopy, guided by computational simulations. The electronic structure calculations are used to establish a fundamental understanding of confinement-induced quasi direct bandgap behavior, composition-induced transition to true direct-gap, and surface/defect related effects on photophysical properties. Selected alloy nanocrystals linked by molecular metal chalcogenides are deposited on various substrates using low-cost, solution-based processing methods to demonstrate their potential for optoelectronics.