Chemically Tunable Acoustic Phonons in 2D Materials

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NON-TECHNICAL SUMMARYAcoustic phonons are sound waves that move in materials and control important properties like stiffness, heat conduction, and how materials work in electronic devices. With this project, supported by the Solid State and Materials Chemistry program in the Division of Materials Research, Professor Kristie Koski at the University of California, Davis, will study acoustic phonons in new two-dimensional (2D) materials that are only a few atoms thick, where the behavior of phonons is very unusual and not well understood. 2D materials are exciting for a wide range of applications from electronics to energy to sensors. To fully realize these applications requires understanding and controlling material properties. This project uses precise laser-light spectroscopy measurements to map out the phonon behavior in detail, and it also explores how to modify and customize the phonon behavior using chemistry to produce 2D materials that have never before existed. This is the kind of knowledge needed to design 2D materials optimized for future devices, energy systems, and industries serving the national interest. Educational and broader outreach goals include an online Brillouin spectroscopy database for the scientific and broader public communities. US Military Veterans are recruited for participation in this research effort. Undergraduate students, with targeted recruitment of women and minorities, are trained for careers in science and engineering.TECHNICAL SUMMARYAs 2D materials become integrated into new technologies, and material dimensions reach quantum limits, acoustic phonon effects alter material mechanical properties, affect heat transport, and impact electronic transport. In this project, supported by the Solid State and Materials Chemistry program in the Division of Materials Research, aims to understand acoustic phonons in 2D materials, filling a major gap in fundamental knowledge while addressing the outstanding question: Where is the cross-over from bulk to quantum acoustic phonon behavior in a 2D material? This effort develops a transformative set of solid-state chemistry techniques to synthesize and chemically control the fundamental acoustic phonon behavior of 2D oxide and chalcogenide (S, Se, Te-based) materials. These materials are generated using new synthetic routes to heterostructures and new zero-valent intercalation chemistries. Brillouin laser light scattering spectroscopy is used to measure and understand acoustic phonons in 2D layered materials. Measurements give complete acoustic phonon dispersion relations, sound velocities, refractive indices, mechanical properties, and stiffness coefficients of 2D materials from the bulk to a monolayer. This effort lays critical foundation for the fundamental understanding and chemical control of phonons in 2D materials for future application. This work develops, enhances, and expands upon an online Brillouin spectroscopy database complementing similarly available databases in Raman and X-ray Diffraction. This work trains undergraduates in research with emphasis on recruitment of women and minorities. Military Veterans are recruited for participation in research through the UC Davis Veteran Success Center.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.