Research: Scanning Probe Microscopy
Although we understand well what physical properties control the contrast of an optical photograph or micrograph, relatively little is understood about the factors that produce the observed contrast in scanning tunneling microscopy images of molecules on surfaces. The goal of this work is to elucidate the link between the structure of molecules and their STM images in well-defined monomolecular layers on surfaces.
To study such systems, a well-packed structure is required so that the tip does not push the molecules of interest out of the way during the imaging process (at room temperature and ambient pressure). We utilize overlayers of simple alkanes, alkanols, and functionalized alkanes on atomically smooth terraces of surfaces such as graphite and MoS2. In such systems, one can obtain sub-molecular resolution, and often atomic resolution, of the electron tunneling probability contours of the overlayer through use of a scanning tunneling microscope.
Images of alkanols have revealed that the overlayer is oriented with the carbon-carbon skeleton parallel to the surface, and that the bright spots in such STM images are associated with the positions of the methylene hydrogens that point toward the STM tip. This has been confirmed by STM images of an allene, which shows the expected change in spot pattern as the orientation of the carbon-carbon skeleton changes by 90 degrees around the C=C=C linkage.
In addition, it is possible to assign the location and image contrast associated with various functional groups in the molecular overlayer. For instance, the alcohol functionality appears dark relative to the methylene chain. Strong evidence for this conclusion has been obtained by comparing images of 1-alkanols to images of alkane diols. The latter molecules have two alcohol functionalities, one at each end of the molecule, and consistently, twice as many dark trough regions are observed in the STM images of these systems relative to the parent 1-alkanols.
Related Publications:
Christopher L. Claypool, Francesco Faglioni, Adam J. Matzger, William A. Goddard III, and Nathan S. Lewis, Effects of Molecular Geometry on the STM Image Contrast of Methyl- and Bromo-Substituted Alkanes and Alkanols on Graphite, J. Phys. Chem. B, 1999, 103, 9690-9699.
Christopher L. Claypool, Francesco Faglioni, William A. Goddard III, and Nathan S. Lewis, Tunneling Mechanism Implications from an STM Study of H3C(CH2)15HC=C=CH(CH2)15CH3 on Graphite and C14H29OH on MoS2, J. Phys. Chem., 1999, 103, 7077-7080.
Christopher L. Claypool, Francesco Faglioni, William A. Goddard III, Harry B. Gray, Nathan S. Lewis, and Rudolph A. Marcus, Source of Image Contrast in STM Images of Functionalized Alkanes on Graphite: A Systematic Functional Group Approach, J. Phys. Chem., 1997, 101, 5978-5995.
Francesco Faglioni, Christopher L. Claypool, Nathan S. Lewis, and William A. Goddard III, Theoretical Description of the STM Images of Alkanes and Substituted Alkanes Adsorbed on Graphite, J. Phys. Chem., 1997, 101, 5996-6020.