John D. Baldeschwieler
J. Stanley Johnson Professor and Professor of Chemistry, Emeritus
Research InterestsMolecular structure and spectroscopy
Assistant: Elisha Okawa
Professor Baldeschwieler's research interests are in molecular structure and spectroscopy, including scanning probe microscopy, and the application of these techniques to the study of biological systems.
Scanning Tunneling Microscopy (STM) is based on positioning an extremely sharp metallic probe within a few angstroms of a molecule on a surface. When a small bias voltage is applied between the tip and the surface, electrons are transported from the tip to the surface (or vice versa) by quantum mechanical electron tunneling. Since the tunneling current is exquisitely sensitive to the distance between the tip and the molecule, when the tip is scanned, variations in the tunneling current can provide an image of the molecule and underlying surface to a resolution of 0.1Å normal to the surface, and less than 1Å in the plane of the surface. The best images to date of DNA on graphite have been obtained with the Caltech STM system. The images display the topography of the electronic state to which the tunneling occurs so that varying the bias voltage between the tip and the surface allows electronic spectroscopy with a field of view of <1Å. The interpretation of STM images is a challenging theoretical opportunity.
In Scanning Force or Atomic Force Microscopy (AFM) the deflection of a micromachined cantilever with a small tip on its end is measured when the tip is brought close to a surface. Attractive or repulsive forces between single atoms are sufficient to provide a deflection of the cantilever. By scanning such tips over a surface, the topography can be determined to atomic resolution. In addition, lateral forces cause torsion of the cantilever so that frictional forces at atomic scale can also be measured with the AFM. If the distance between the cantilever and surface is modulated, the derivative of force with respect to distance can be determined, thereby providing contrast based on the local elasticity of the sample.
Still another variation on these scanning methods is Near-Field Scanning Optical Microscopy (NSOM). Laser light is passed through an optical fiber drawn to an orifice that can be as small as 150Å. By scanning this spot of light over a sample, optical microscopy is possible with a resolution which is determined by the size of the orifice rather than usual diffraction limits. We have constructed a device in which the tip is scanned over a sample and images are obtained by recording time of flight mass spectra obtained as the spot of light interrogates each area of the sample.
The group's current research involves developing methods to bind reproducibly a single molecule at the end of STM or AFM tips. These techniques should provide an important window into new areas of nanotechnology.
In addition to these scanning probe techniques, the Baldeschwieler group has also developed a technique to create large combinatorial arrays of nucleic acids and other polymers based on technology similar to that used in ink-jet printers. These arrays show promise for enabling new approaches to sequencing, diagnostics, and many other applications.