Study of Binary Physisorbed Films by Combined IRRAS and Ellipsometry

这个表面物理学项目将研究在单层到几层的范围内，小分子在石墨上的物理吸附，重点是二元混合物。 基底相互作用和维度效应，特别是在临界点附近，使薄膜不同于3-D流体。 此外，还存在一种组分从表面移位或层间分离的可能性。 广泛的计算机模拟已经做了简单的模型系统，但真实的二元系统的实验结果到目前为止是有限的，特别是对于解决方案的情况下，主要是由于难以分别测量的覆盖范围和每个组件的状态。 在这里，总覆盖率由椭圆偏振法确定，而合适分子的振动光谱同时由红外反射吸收光谱确定。 对于具有强IR吸收带的模型溶质分子，例如CF 4、CCl 4或SF6，以及可能具有IR活性或不具有IR活性的溶剂，这种技术的组合允许确定表面上每种组分的浓度。 此外，IR线位移区分溶质是相分离的还是分散的。 对于不太对称的分子，IR技术也提供了分子取向的信息。 石墨是从原子到微米尺度的理想均匀基底。 分子水平上的理解的行为，一个均匀的基板上的二元膜应提供一个基础，基本了解这种薄膜在不太理想的基板。 这可能与润滑和粘附问题有关，而与平版印刷、防腐蚀和多孔介质中的流动不太直接。 研究生，本科生和博士后学生在这个程序中工作获得各种物理原理和技术的应用经验，这为工业，政府实验室和教学的职业生涯提供了良好的准备。该研究项目将研究小分子二元混合物在石墨上的吸附，覆盖一到几个分子层。 结果可能是一种组分从表面取代另一种组分;或者一种组分吸附在另一种组分的层的顶部;或者它们在每层内形成溶液。 大量的计算机模拟已经做了简单的模型系统，但真实的二元系统的实验结果到目前为止是有限的，特别是对于案件以外的位移。 这主要是由于难以分别测量每个组件的覆盖范围和状态。 我们的设备允许通过光学技术测量总膜厚度，同时通过红外吸收光谱法测量合适的吸附分子的振动光谱。 这提供了足够的信息来确定表面上每种组分的浓度。 此外，IR吸收线位移区分溶质是溶解还是相分离。 对于不对称分子，IR技术还提供有关分子取向的信息。 石墨是从原子到微米尺度的理想均匀基底。 分子水平上的理解的行为，一个均匀的基板上的二元膜应提供一个基础，基本了解这种薄膜在不太理想的基板。 这可能与润滑和粘附问题有关，而与平版印刷、防腐蚀和多孔介质中的流动不太直接。 在这个程序中工作的学生获得的经验，涉及吸附的物理原理，以及红外光谱，真空，低温和光学技术，和计算机接口，这为在工业，政府实验室或教学的职业生涯提供了良好的准备。

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.