Undergraduate Laboratory for Surface Science

Abstract

Surface science has developed into a multidisciplinary field of research with applications ranging from heterogeneous catalysis to semiconductor etching (1). Aspects of surface chemistry are now included in physical chemistry textbooks (2) and undergraduate curricula (3), but the perceived cost and complexity of equipment has deterred the introduction of surface science methods in undergraduate laboratories (4). Efforts to expose chemistry undergraduates to state-of-the-art surface instrumentation have just begun (5).To provide our undergraduates with hands-on experience in using standard techniques for characterizing surface morphology, adsorbates, kinetics, and reaction mechanisms, we have developed a set of surface science experiments for our physical chemistry laboratory sequence. The centerpiece of the laboratory is an ultrahigh vacuum (UHV) chamber for studies of single crystal surfaces. This instrument, shown in the figure, has surface analysis capabilities including low energy electron diffraction (LEED), Auger spectroscopy, and temperature-programmed desorption (TPD).The laboratory exercises involve experiments on the well-studied Pt(111) surface. Students prepare a previously mounted single crystal sample by sputtering it with an argon ion gun and heating it under O2. Electron diffraction patterns from the cleaned surface are then obtained with a reverse view LEED apparatus (Princeton Instruments). Images are captured by a charge-coupled device (CCD) camera interfaced to a personal computer for easy downloading and subsequent analysis. Although the LEED images from a Pt(111) surface can be readily interpreted using simple diffraction arguments, this lab provides an excellent context for introducing Miller indices and reciprocal lattices (6). The surface chemical composition can be investigated by Auger spectroscopy, using the LEED apparatus as a simple energy analyzer.The temperature programmed desorption experiment, which is nearly complete, will be be added to the curriculum this academic year. This experiment introduces students to the concepts of surface adsorption and desorption kinetics. The sample mount can be both heated to 900 oC and cooled by liquid nitrogen, allowing the study of the desorption of a variety of adsorbates, both chemisorbed and physisorbed. Adsorbed species evolving from the heated surface are detected with a quadrupole mass spectrometer. Initially, students will study the desorption kinetics of CO from a Pt(111) surface for a range of coverages and temperature programming rates in order to obtain rate parameters and to test the validity of Redhead's relationship between the activation energy of desorption and the peak desorption temperature. They then will be introduced to the mechanisms of surface reactions (Langmuir-Hinshelwood) in a study of CO oxidation on this surface.We have also set up a scanning tunneling microscopy (STM) laboratory using a commercially available instrument (Burleigh Instruments, Inc.), which complements the UHV surface structure experiments by introducing the topography of a real surface, for example, with steps and defect sites.With the apparatus now completed, we can explore other possible developments, for example, an applied physics track designed around experiments on semiconductor substrates. Future additions include X-ray photoelectron spectroscopy and completion of a separate surface infrared spectroscopy experiment on supported catalysts.We acknowledge support from the National Science Foundation, Division of Undergraduate Education Instrumentation and Laboratory Improvement (ILI) Program (Grant No. DUE-9352254) and an ATandT Special Purpose Grant.