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In little more than a decade, the scanning tunneling microscope (STM) has proved to be a powerful revolutionary research tool and the first generation of a family of scanning probe microscopes (SPMs) that can be used to study bare surfaces as well as atoms and molecules adsorbed on surfaces. With equal speed SPM principles and practices are finding a place in undergraduate and high school laboratory curricula as a demonstration of state-of-the-art surface analytical techniques. The introduction of SPM to students at this level derives from the relative ease of use of the instrumentation and the quick visual images obtained during the experiments. Several papers in this journal have demonstrated the types of laboratory experiments that can be conducted using these microscopes (1, 2). Generally, these experiments emphasize collection of bare surface topographies and recognition of morphological surface changes due to reaction.
The investigation described below adds to the ideas enunciated in these previous papers by stressing the role of SPM as an atomic or molecular camera. Here, however, the students are asked not only to take "snapshots" of the atoms of bare surfaces but also to identify chemically significant portions (functional groups) of molecular adsorbates placed upon those surfaces. Functional group characterization by SPM is more pictorial and appears more concrete than the traditional "fingerprint" methods of identification used in electronic and vibrational spectroscopies. This empowers students by allowing them to obtain visual evidence for concepts that to this point in their studies may have seemed abstract. For example, students can see the effects of hydrogen bonding in molecular thin film formation. In addition, theoretical methods are introduced here, underscoring the complementary relationship between theoretical predictions and experimentally measured results. Calculations using commercially available software routines enable students to reflect critically on the transfer to a graphical form of the ideas taught in lecture (specifically, ab initio and semiempirical methods in quantum mechanics).
We describe a scanning tunneling microscope investigation of bare graphite and 11-bromoundecanol adsorbed on graphite suitable for undergraduate physical chemistry laboratory students. In addition to collecting substrate (surface) and adsorbate (molecule) images, students use Spartan, a generally accessible computational chemistry software package, to calculate the shapes of the frontier molecular orbitals of a structurally similar but shorter (and thus computationally more...