We are interested in the detailed dynamics of reactions at well-characterized surfaces and interfaces. These studies combine techniques such as electron diffraction, electron spectroscopy, and scanning probe microscopy for the characterization of the surface, with laser excitation sources, optical spectroscopy, mass spectroscopy and molecular beam methods for the state preparation and characterization of gas phase reactants and products. This combination of approaches provides a powerful tool for obtaining a detailed understanding of the dynamics of heterogeneous reactions. The following paragraphs describe these studies:
Chirality, or the “handedness” of molecules, is a well known property of organic molecules. The study of three dimensional chirality dates from the work of Pasteur to separate chiral crystals of sodium ammonium tartrate individually. Chirality is exhibited in two dimensions as well, in particular in the adsorption of chiral and achiral organic molecules on solid surfaces. Scanning probe microscopy has made the structural study of these two dimensional chiral layers possible with atomic and molecular resolution. We have recently studied the formation of chiral monolayers from the adsorption of long chain substituted hydrocarbons on highly oriented pyrolytic graphite (HOPG) surfaces. We have observed the formation of chiral structures by the asymmetric distortion of achiral molecules such as octadecanol. We have examined the chiral pairing of molecules adsorbed from enantiomeric mixtures of iodine substituted alkenoic acids. We have also observed chiral structures that exhibit chirality only under the scanning tunneling microscope that is sensitive to the polarity of double bonds in the adsorbed molecules. The goal of our work in this project is to understand the complex interactions that govern the structures that form when long chain organic molecules adsorb on HOPG, and to develop predictive capability for the formation of chiral monolayers of particular structure. This understanding will be useful for the design of chirally active sensors, chiral catalytic materials, and chiral separations media. A detailed understanding of two dimensional chiral structures will also be relevant in trying to unravel the mystery of homochirality in biological molecules. See, for example,
F. Tao and S.L. Bernasek, “Understanding the Odd-Even Effects in Organic Self Assembled Monolayers”, Chemical Reviews, 107, 1408 (2007). Full: HTML/PDF
F. Tao and S.L. Bernasek, “Self-assembly of 5-Octadecyloxyisophthalic Acid and Its Coadsorption with Terephthalic Acid”, Surface Sci., 601, 2284 (2007). Full: HTML/PDF
The bonding between an organometallic species and an oxide or nitride supporting substrate is important to the chemistry of supported catalysts, electronic device processing, and adhesion and lubrication phenomena. This bonding may be investigated with the aid of ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). When combined with other surface probes such as thermal desorption spectroscopy (TDS), AES, LEED, and vibrational spectroscopic characterization by high resolution electron energy loss spectroscopy (HREELS) or reflection-absorption infrared spectroscopy (RAIRS), a relatively complete picture of the surface properties of the adsorbed species becomes available. We are exploring, in collaboration with Professor Jeffrey Schwartz, the interaction of a number of transition metal and phosphonate complexes with well characterized oxide surfaces. In this work electron spectroscopic measurements are correlated with the chemical behavior of the supported complex, as studied by conventional heterogeneous catalysis methods and mass sensitive kinetic methods. This approach is also extended to the study of important oxide and nitride bound metallic layers, which are of interest in electronic materials chemistry, in bio-compatible materials chemistry, and in adhesion and corrosion inhibition applications. See, for example,M. Mc Dowell, I.G. Hill, J.E. McDermott, S.L. Bernasek and J. Schwartz, “Improved Organic Thin-Film Transistor Performance Using Novel Self-assembled Monolayers”, App. Phys. Lett. 88, 1 (2006). Full: HTML/PDF
HREELS and TDS in combination provide a very detailed view of the kinetics and mechanisms of small molecule reactions on well-characterized solid surfaces. We have applied this approach over the years to the study of a number of small molecule reactions on the clean and adsorbate modified Fe(100) surface. This surface exhibits a very rich and complex chemistry, and serves as a useful model for the investigation of structure-reactivity correlations in surface chemistry. Currently, we are using these methods to examine the interaction of a number of sulfur, nitrogen, and phosphorous containing molecules, which are promising candidates for corrosion inhibitors on the iron surface. The goal of this project is to understand in detail how these molecules bind to the iron surface, and to determine the mechanism of their thermal decomposition, so that more effective high temperature corrosion inhibitor molecules can be designed and synthesized. We have seen, for example, that long chain thiol molecules decompose on the reactive iron surface at relative low temperatures via ß-hydrogen elimination. This observation has led us to investigate the decomposition of molecules appropriately substituted at the ß-position to block this decomposition route. We have also examined substituted thiophenes, and are investigating alkylphosphonates in this application as well. See, for example,
G. Bhargava, T.A. Ramanarayanan, I. Gouzman, and S.L. Bernasek, “Imidazole as an Inhibitor for Fe Corrosion: a Study Combining Surface Science and Electrochemistry”, ECS Transactions, 1, 195 (2005). Abstract Full: PDF
The detection of small electromagnetic fields in the presence of much larger stray-field sources is a problem of great importance with application in vehicle detection, security sensors, and mineral and petrochemical prospecting. Laser addressed magnetometers detect magnetic fields by monitoring the spin precession of laser prepared spin polarized atoms in the gas phase. The cells that contain these spin polarized atoms (usually rubidium or cesium atoms) are often coated with paraffin wax on the interior surfaces in an attempt to prevent the wall-collision relaxation of the spin polarization. These coatings are very empirical, and little is known about how or why they work to improve the sensitivity of these laser addressed magnetometer devices. In a collaborative research project with the atomic physics group in the Department of Physics at Princeton University, and the atomic physics group in the Department of Physics at the University of California, Berkeley, we are working to develop an understanding of these anti-relaxation coatings, and to develop new “designer” coatings that will enhance the sensitivity of these devices. We are carrying out detailed studies of the physical and chemical properties of functional anti-relaxation coatings, using electron spectroscopy and scanning probe microscopy to understand the electronic structure and morphology of these layers. We are using our self-assembled monolayer methodology, and phosphonate coating technologies developed in our earlier work to develop “designer” coatings to improve the anti-relaxation nature of these surfaces. We are carrying out fundamental studies of the collisional relaxation of spin polarized atoms on well characterized surfaces to provide the basic underpinning for the rational use of surface chemistry to improve the sensitivity and lifetime of these micromagnetometer systems. See, for example,
S.J. Seltzer and M.V. Romalis, “Unshielded 3-axis vector operation of a spin-exchange relaxation-free atomic magnetometer”, Appl. Phys. Lett., 85, 4804 (2004). Abstract Full: HTML / PDF
L. Liew, S. Knappe, J. Moreland, H.G. Robinson, L. Hollberg, and J. Kitching, “Microfabricated alkali atom vapor cells”, Appl. Phys. Lett. 84, 2694 (2004). Abstract Full: HTML / PDF
E.L. Bruner, N. Koch, A.R. Span, S.L. Bernasek and J. Schwartz, “Controlling the Work Function of Indium Tin Oxide”, J. Am. Chem. Soc., 124, 3192 (2002). Abstract Full: HTML / PDF
The collision of a gas molecule with a surface, followed by trapping of the molecule on the surface, must precede any surface reactions which might take place. Some knowledge of this fundamental process, along with the accompanying energy transfer to the surface, is essential to a complete understanding of heterogeneous reaction dynamics.
We have used infrared spectroscopic methods to investigate the detailed dynamics of a very important prototype surface reaction, the catalytic oxidation of CO on platinum. This reaction, which is essential in the control of pollution from automobile exhaust, has generated an enormous number of studies. In spite of all this work, the detailed dynamics of the reaction are just starting to become clear. In work begun in our laboratory several years ago, we observed that the CO2 product of this reaction is very highly vibrationally excited. This information was obtained using infrared chemiluminescence methods: the reaction product CO2 is vibrationally hot enough to emit easily measurable amounts of radiation. Much of this emission from the product CO2 originates from high lying bend-stretch combination states, suggesting that these bending and asymmetric stretching motions contribute strongly to the reaction coordinate for the surface oxidation of CO. We have set up a diode laser absorption spectrometer, which has been used to probe this process in more detail. The absorption spectrometer provides much better resolution and is sensitive to the possible production of cold CO2 in the oxidation reaction. We have used this method to map out the detailed ro-vibrational populations of the product CO2 and changes in these populations with changing surface reaction conditions. These studies have been extended to the reaction of CO with NO and methanol with O2. See, for example,
D. Bald, R. Kunkel, and S.L. Bernasek, "Diode Laser Absorption Study of Internal Energies of CO2 Produced from Catalytic CO Oxidation", J. Chem. Phys., 104, 7719 (1996). Abstract Full: PDF
D.J. Bald and S.L. Bernasek, “The Internal Energy of CO2 Produced From Catalytic Oxidation of CO by NO”, J. Chem. Phys., 109, 746 (1998). Abstract Full: PDF
In summary, the work presently underway in our laboratory combines studies of the structure and composition of transition metal, transition metal compound and semiconductor surfaces with studies of the energy state of the reactants and products involved in heterogeneous reactions on these surfaces. These studies address the effect of surface structure, composition, and electronic properties on the detailed molecular dynamics of reactions at surfaces and interfaces.