Study of Binary Physisorbed Films by Combined IRRAS and Ellipsometry

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This surface physics project will study physical adsorption of small molecules on graphite in the monolayer to few layer regime, with emphasis on binary mixtures. Substrate interactions and dimensionality effects, especially near critical points, make the films different from 3-D fluids. In addition there is the possibility of displacement of one component from the surface or segregation between layers. Extensive computer simulations have been done for simple model systems but experimental results for real binary systems are so far limited, particularly for cases of solution, due primarily to the difficulty of measuring the coverage and state of each component separately. Here, the total coverage is determined by ellipsometry, while the vibrational spectra of suitable molecules is simultaneously determined by infrared reflection absorption spectroscopy. For model solute molecules with strong IR absorption bands, such as CF4, CCl4, or SF6, together with a solvent that may be IR-active or not, this combination of techniques allows determination of the concentration of each component on the surface. In addition, IR line shifts distinguish whether the solute is phase-separated or dispersed. For less symmetric molecules the IR technique also provides information on molecular orientation. Graphite serves as an ideal uniform substrate from atomic to micrometer scale. Molecular level understanding of the behavior of binary films on a uniform substrate should provide a basis for fundamental understanding of such films on less ideal substrates. This is likely to be relevant to problems in lubrication and adhesion, and less directly to lithography, corrosion-protection, and flow in porous media. Graduate, undergraduate, and post-doctoral students working in this program gain experience in application of a variety of physical principles and techniques, which provides good preparation for careers in industry, government labs, and teaching.%%%This research project will investigate adsorption of binary mixtures of small molecules on graphite, at coverages of one to a few molecular layers. The result may be that one component displaces the other from the surface; or one component adsorbs on top of a layer of the other component; or they form a solution within each layer. Extensive computer simulations have been done for simple model systems but experimental results for real binary systems are so far limited, especial for cases other than displacement. This is due primarily to the difficulty of measuring the coverage and state of each component separately. Our apparatus allows measurement of total film thickness by an optical technique and simultaneously the vibrational spectra of suitable adsorbed molecules by infrared absorption spectroscopy. This gives sufficient information to determine the concentration of each component on the surface. In addition IR absorption line shifts distinguish whether the solute is dissolved or phase-separated. For asymmetric molecules the IR technique also provides information on molecular orientation. Graphite serves as an ideal uniform substrate from atomic to micrometer scale. Molecular level understanding of the behavior of binary films on a uniform substrate should provide a basis for fundamental understanding of such films on less ideal substrates. This is likely to be relevant to problems in lubrication and adhesion, and less directly to lithography, corrosion-protection, and flow in porous media. Students working in this program gain experience in physical principles involved in adsorption, as well as infrared spectroscopy, vacuum, low temperature, and optical techniques, and computer interfacing, which provides good preparation for careers in industry, government labs, or teaching.