Probing the Structures and Chemical Bonding of Size-Selected Boron and Metal-Boride Nanoclusters

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In this project funded by the Chemical Structure, Dynamics and Mechanisms (CSDM-A) program of the Chemistry Division, Professor Lai-Sheng Wang of Brown University and his students are using advanced spectroscopic techniques to study tiny structures (nanoclusters) formed from boron atoms and metal borides, which combine boron with metallic elements. Boron is next to carbon on the periodic table, but having one-fewer electrons than carbon leads to boron being characterized as an electron-deficient element. The electron deficiency of boron results in distinctive chemical and physical properties for boron-containing molecules and materials, and boron and metal boride nanoclusters display a rich range of structural and chemical bonding characteristics. There are significant experimental challenges to study these clusters, which exist only briefly before reacting to form more stable species, and, thus, must be carefully prepared. Professor Wang and his students are overcoming these challenges by developing innovative experimental techniques to investigate the structures and bonding of boron and metal-boride nanoclusters. The research not only provides fundamental knowledge about the boron-boron and metal-boron chemical bonding, but also has the potential to discover new boron and metal-boride nanomaterials valuable for industrial applications. The students engaged in this research are gaining critical thinking skills and valuable experience in advanced physical chemistry techniques. The project focuses on 1) structural evolution of large boron clusters to lay the foundation for new boron nanostructures and 2) metal-boride clusters to probe the nature of the metal-boron chemical bonds. These clusters are produced using a laser-vaporization cluster source and investigated using two different types of home-built photoelectron spectroscopic apparatuses. One apparatus involves a magnetic-bottle photoelectron analyzer and is aimed to provide spectra with a wide range of photon energies. Well-resolved photoelectron spectra provide electronic fingerprints, which are crucial to be used to compare with theoretical calculations to elucidate the structures and bonding of size-selected clusters. A cryogenically-cooled ion trap is being developed to create cold boron clusters, which are essential to obtain well-resolved photoelectron spectra for large clusters. The second apparatus involves high-resolution photoelectron imaging, which is aimed to obtain vibrational information. The broader impact of this work includes the discovery of novel boron-based nanomaterials, which have potential technological applications and societal benefits. This project is integrated with the teaching of physical chemistry by Professor Wang, as well as providing training opportunities for students and postdoctoral associates in the design and construction of advanced experimental instrumentation and computational chemistry.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.