Emeritus Professor of Chemistry
Adjunct Professor of Physics
Ph.D. University of Chicago, 1969
Chemical reactions involving condensed phases take place at interfaces. We are all familiar with billiard ball models which describe how gas phase species react with one another, but how can we visualize reactions at surfaces? This question is at the heart of understanding phenomena as diverse as corrosion, heterogeneous catalysis, fuel cells and electrochemistry. Although whole industries have been built up in these areas, working knowledge comes from a largely empirical rather than from an atomic level understanding of what is really going on. It is a goal of surface scientists to provide an atomic level description of structure, reaction kinetics and reaction dynamics at interfaces.
Professor Engel and his group specialize in the study of the interactions between gases and solid surfaces. When taking on such a task, physical chemists generally seek to find a model system which mimics reality as closely as possible, but which has few enough adjustable parameters that each can be varied to determine how they influence the process being studied. How does a surface scientist approximate the solid-gas interface? In the Engel laboratory, it is done by using single crystal surfaces in an ultravacuum (UHV) environment in which the partial pressure of individual gases can be controlled down to the 10-11 torr range. To understand how oxygen interacts with silicon, the Engel group can either allow a controlled pressure of molecular oxygen or a molecular beam of atomic or molecular oxygen to impinge on the surface. They ask questions such as: What is the probability that the incoming particle forms a bond with the surface? How strong is the chemical bond formed? Does the reaction proceed differently if atomic rather than molecular reactants are used? How do the chemisorbed oxygen atoms arrange themselves on the surface? If volatile gaseous products are formed, what is their identity and what is the activation energy associated with their formation?
These are familiar questions for the student of chemistry, but they require new techniques when applied to surfaces. They have at their disposal five UHV systems with an array of surface analysis methods such as low energy electron diffraction (LEED), Auger electron spectroscopy (AES), ion scattering spectroscopy (ISS), X-ray photoelectron spectroscopy (XPS or ESCA), modulated beam mass spectrometry, scanning tunneling microscopy (STM), and atomic force microscopy (AFM). It is the combination of these various methods which allows difficult problems in surface science to be studied deeply. It is also the breadth of these techniques coupled with a training in vacuum technology and instrument design which prepares graduate students in the group for a future professional activity in surface science.
Currently, the group is focused on two broad areas which come from requirements of the electronic materials and the environmental remediation communities. The next generation of electronic devices will use gate oxides whose thickness is on the order of 40 atomic layers. At present it is not known how to prepare silicon wafer substrates with the necessary uniformity and how to form an oxide layer with the required homogeneity. In Professor Engel's laboratory, they are using molecular beam and scanning tunneling microscopy experiments to help solve this problem on a laboratory scale and working with scientists in industry to test the protypical interfaces which they make. In the environmental area, they are studying the surfaces of oxides, which are among the most common materials in the real world. These experiments using beam techniques and STM are designed to probe how surface defects and chemical reactivity are related. These examples illustrate their approach to science which is to model problems of societal importance in a way that questions can be asked and answered on an atomic level. This approach leads to good science and a purpose centered education.
"Evidence for Structure Sensitivity in the Thermally Activated and Photocatalytic Dehydrogenation of 2-Propanol or TiO2.", D. Brinkley, T. Engel. Submitted for publication in J. Physical Chem. B.
"Photocatalytic Dehydrogenation of 2-Propanol on TiO2(110).", D. Brinkley, T. Engel, J. Phys, Chem. B, 102 7596 (1998).
"Active Site Density and Reactivity for the Photocatalytic Dehydrogenation of 2-Propanol on TiO2 (110).", D. Brinkley, T. Engel, Surf. Sci., 415 (1998).
"A Modulated Molecular Beam Study to the Extent of Dissociation of H2O on TiO2(110).", D. Brinkeley, M. Dietrich, T. Engel, P. Farrall, G. Gantner, A. Schaefer, A. Szuchmacher, Surf. Sci., 395 292 (1997).
"A Liquid Nitrogen Cooled Ionozer for a Quadrupole Mass Spectrometer.", P. D. Farrall, T. Engel, Rev. Sci. Instrum., 67 4027 (1996).
ACS Award in Surface Chemistry