The Analytical and Surface Chemistry Program of NSF Division of Chemistry is supporting the research program of Professor S. Alex Kandel in the Department of Chemistry and Biochemistry at the University of Notre Dame under a project titled, "Gas-Surface Chemistry of Self-Assembled Monolayers." Professor Kandel and his students are investigating how the exact arrangement of atoms and molecules that make up a surface influence the rate and type of chemical reactions that occur when the surface is exposed to reactive gases. The Kandel research group uses scanning tunneling microscopy to develop atomic-scale movies that map out the location and behavior of individual molecules during the course of a chemical reaction. They specifically investigate the reactions of thin organic films with hydrogen, oxygen, and chlorine atoms. The study could lead to development of new materials with improved stability in reactive chemical environments such as flames, plasmas, and the upper atmosphere. The project will provide excellent training opportunities to undergraduate and graduate students in the central discipline of physical chemistry and in the interdisciplinary fields of surface science and nanotechnology.
," focused on the direct observation of the chemical transformations that occur when the surfaces of materials react. Scanning tunneling microscopy (STM), an ultra-resolution technique capable of probing single atoms or molecules, was used to image surfaces at the nanometer scale. By alternately exposing a surface to a reactive gas and imaging the changes caused by reaction, this experimental approach provides a "stop motion movie" observation of chemical reactions as they occur. The particular emphasis of these experiments was to build a comprehensive understanding of how local structure -- defects, impurities, changes in composition, and disruption of order -- affects the mechanism and rate of chemical reactions. At the scale of individual atoms, even the most carefully prepared, crystalline material is loaded with defects: in terms of composition and structure, each location on the surface is different from every other. It has long been conventional wisdom in the field that defects and impurities can have a dramatic effect on the rates of chemical reactions or even the types of reactions that can occur. The funded research extended the fundamental basis of this wisdom, and provided some of the most direct evidence in its support. A molecular-scale view of reactivity is essential: except for the most carefully controlled model systems, real surfaces are highly heterogeneous, and in many cases, it is this heterogeneity that determines surface chemistry. Surface defects and impurities provide nucleation sites for oxidation, etching, and corrosion; steps and edges create low-coordination sites that are catalytically active; and domain sizes determine the length scales on which surfaces can be patterned. This project has resulted in 13 peer-reviewed publications, and an additional paper currently in press. The main experimental effort was in the study of the reactions of gas-phase radicals -- H and Cl atoms -- with organic monolayers adsorbed on gold surfaces. Alkanethiolate monolayers on gold are a well-characterized, experimentally tractable model system for polymers or coated surfaces; they additionally have potential direct applications as coatings or resists. Gas-phase radicals are important reagents in flames, plasmas, and the upper atmosphere. A series of STM experiments showed that disturbances in surface order present at the beginning of reaction have a substantial impact on how and where that surface reacts. Because reaction then creates further defects in the surface, even small initial defects can have an outsize effect on the surface’s overall reactivity. Reaction with hydrogen atoms shows a reaction rate that accelerates with time: reaction leads to removal of molecules from the surface, creating new defects that allow more reaction. The reaction rate near defect sites was found to be hundreds of times faster than in densely packed and ordered regions of the surface. In contrast, reaction with chlorine atoms starts quickly and then plateaus. While reaction at the defect sites likely occurs quickly, the products of this reaction are chlorinated hydrocarbon species, which are then less reactive than the original material. The fundamental goal of this research is an improved, fundamental understanding of how local surface structure influences reactivity. Potentially, this understanding could allow for the design and construction of more surfaces that are more stable and chemically resistant in these hostile environments. Alternately, an understanding of the surface chemistry involved could allow materials to be designed to react quickly via determined reaction pathways.
