Surface Heterogeneity and Defects in Gas-Surface Reactions

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S. Alex Kandel, Surface Heterogeneity and Defects in Gas-Surface ReactionsChemists use formulas containing reactant and product molecule symbols to describe a chemical reaction. However the formula is only a very basic starting point for understanding how the reactions occur and, therefore, how to control them. Dr. Kandel is investigating the molecular changes, or mechanisms, of chemical reactions taking place at solid surfaces to shead light on how these reactions actually take place at the atomic scale. He is particularly interested in determining the effect of compositional impurities and atomic-level structure on the rate and the products of a reaction. In some cases, these changes are desired, such as the enhanced catalytic activity demonstrated by specific defects in many processes of industrial importance. In others, such as the preparation techniques used to create computer chips and other semiconductor devices, even a small number of imperfections in a surface can result in an unusable product. Dr. Kandel employs an imaging technique known as scanning tunneling microscopy (STM) to "see" individual atoms and molecules in order to determine how the solid surfaces change at the atomic level during the course of a chemical reaction. These experiments produce time-lapsed sequences that show how the particular local composition and geometry influence and control both the rate and the outcome of a chemical reaction. The fundamental understanding gained in this project helps the future design of new catalysts, surface coatings, and materials processing methods. Dr. Kandel's research is attractive to students at many levels, and he involves several high school students and their teachers as research assistants every year. He also maintains a data base of STM images that is accessible to the general public through his web page at the Univeristy of Notre Dame.In this research program, Dr. S. Alex Kandel is supported by the Macromolecular, Supramolecular and Nanochemistry (MSN) Program to study the reactions between gas-phase radical species and solid surfaces, with a particular emphasis on how the local molecular structure of a surface influences chemical reactivity. Hydrogen, chlorine, and methane radicals are used to chemically modify well-characterized organic monolayers, and the progression of atomic-scale changes caused by their interaction is monitored using scanning tunneling microscopy (STM). Radical-surface reactions are relevant to a variety of chemical processes in which a surface is exposed to a reactive environment, including combustion (hydrogen atoms), upper atmosphere chemistry (chlorine atoms), chemical vapor deposition (hydrogen, methyl), and plasma processing (hydrogen, chlorine). In addition, the systematic investigation of how various aspects of the surface monolayer structure influences reactivity helps build fundamental knowledge of surface-radical interactions and new mechanistic understandings of gas-surface chemistry. Dr. Kandel involves high school, undergraduate and graduate students in his research, and serves as a mentor for high school students in local and international science competitions. He also maintains a data base of STM images that is accessible to the general public through his web page at the Univeristy of Notre Dame.