Penn State Castleman Group

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Updated August 14, 2012

Research Overview

Some years ago, my students and I discovered a new class of molecular clusters termed Metallo-Carbohedrenes or Met-Cars for short. They are comprised of eight early transition metal atoms bound to twelve carbons. In view of their potential use as new electronic and optical materials, as well as predicted value as new catalysts, they have attracted wide interest in the chemistry community. Work is underway in our laboratory to investigate their molecular properties, reactivity, and routes for synthesis in the solid state. Excitation experiments using femtosecond lasers are providing new insights into the coupling of electronic and vibrational modes on the ultra short time scale, and are elucidating their photoinduced behavior.

Along the lines of exploring the physical basis for catalysis, my group is also engaged in a number of studies of the reactivities of transition metal compound clusters of widely varying composition and types, with particular attention to oxygen transfer reactions. Investigations are also under way to learn how the small cluster building blocks lead to different morphologies of growing particles that are of interest in wide-ranging areas from photocatalysis to developing new cluster assembled nanoscale materials.

Innovations in both the experimental techniques and the theoretical methods available for studying cluster behavior have been crucial in stimulating new activities emerging in this field. A particularly promising avenue that we are pursing involves materials design through cluster assembly often referred to as the "next frontier" in the field of nanoscience. Synergism arising from combined efforts in experiment and theory is serving to unravel new features specific to the reduced size dimensions and is opening new opportunities. We have a major collaborative effort underway with a theoretical group at the Virginia Commonwealth University, and another at the Humboldt University in Berlin, where my students and I spend a few weeks each year. The collaborative work with VCU has been particularly valuable, leading us to recent findings offering promise that it will be possible to construct a "new three dimensional table of the elements," based on clusters as superatoms. If these new ideas are borne out, there is the very promising and exciting prospect that new materials using atomic or compound clusters of selected electronic properties and geometries may be able to be formed through the self-assembly of superatoms that mimic selected elements of the periodic table. This would bring about an unprecedented level of atomic control that could be realized in using the properties of individual clusters in designing complex materials with desirable collective traits and could open up the possibility of assembling materials designed to display a pre-selected functional activity. The promises of developing new materials with tailored properties abound.

The vast majority of reactions of practical importance occur in liquids or on surfaces, yet from a molecular point of view they are far less well understood than reactions occurring in the gas phase. My group is also working to lay a foundation for connecting the gas and condensed phases using ultrafast lasers coupled to mass spectrometers. Work is in progress on studies of the spectroscopy and reactions of small solvated biological function groups, with the objective of learning more about the influence of hydrogen bonding on their properties and reactivity. Using femtosecond laser techniques we also have been able to answer age-old questions about the number of water molecules needed to dissolve an acid into ion-pairs, and the dynamical mechanisms of proton and related charge transfer.

Another major thrust of the group is learning more about atmospheric chemistry through cluster research. It is well recognized that small aerosol particles, as well as ice crystals and cloud droplets, play an important role in the conversion of many atmospheric molecules and employing water clusters as model systems, we are shedding light on the fundamentals of heterogeneous reactions occurring on ice and water cluster surfaces.

The breadth of activities and implications of findings in the cluster field are astounding, leading it to be a highly active subject at the cutting edge of science. As conveyed by a recent special section of the Proceedings of the National Academy of Sciences, activities in the field of cluster science do indeed provide a "Bridge across Disciplines."