Abstract for the Fourth Foresight Conference on Molecular Nanotechnology.

Tip-Surface Interactions

Richard J. Colton
Chemistry Division, Code 6177
Naval Research Laboratory, Washington, DC 20375-5342 USA
Tel: 202-767-0801; Fax: 202-767-3321; e-mail: rcolton@stm2.nrl.navy.mil

The invention of the Scanning Tunneling Microscope (STM), Atomic Force Microscope (AFM) and other proximal probes has not only revolutionized the way we study the structure and properties of surfaces but has also enabled the pursuit of more ambitious goals in nanotechnology, namely, creating unique structures or devices by manipulating individual atoms or molecules, and developing new concepts in electronics, information storage, and sensor technology. The topic of "tip-surface interactions" is perhaps mundane compared to other titles at this conference; however, it is currently the basis of our link between the macroscopic and nanoscopic worlds. Tip-surface interactions, for example, permit atoms and molecules to be manipulated, provide contrast during imaging, allow many different forces or properties to be measured/imaged, and in short, enable nanotechnology.

Our work at NRL has been focussed on transitioning the tip-based proximal probes (particularly the AFM) from a qualitative imaging tool to a quantitative probe of the surface forces and properties of materials. Today, we are able to design experiments to elucidate the atomic or molecular-scale mechanisms of phenomena such as adhesion, friction/wear, and deformation/fracture. In some instances, we must use new theories and computational methods such as molecular dynamics simulations to visualize the processes in order to understand the observed behavior. Adhesive interactions between the tip and surface can have a profound effect on the imaging mechanism including image contrast, frictional force and compliance measurements. Using two or more component Langmuir-Blodgett (L-B) films, we measure discrete adhesion levels related to the thermodynamic phase and mechanical property differences over phase segregated areas of the mixed films. Frictional force measurements on L-B films are also highly dependent on adhesion. As a nanoindenter, the AFM has unprecedented force and penetration depth resolution. Using glass spheres as the indenter, we have measured the absolute modulus of materials.

The use of chemically modified tips extends the capability of the technique to include chemical imaging and measurement of specific molecular interactions. We have used the extreme force and displacement sensitivity of the AFM to measure directly the binding interaction between individual molecules placed between two surfaces. The specific interaction forces measured between complementary strands of DNA and other ligand-receptor systems explore the basis of molecular recognition. This work has also led to new chemical and biological sensing concepts with the potential to detect single molecules.

Work sponsored by the Office of Naval Research and done in collaboration with W. Barger, D. Baselt, S. Corcoran, C. Draper, K. Feldman, S. Hues, D. Koleske, G. Lee, D. Schaefer, and S. Sinnott.