Scott Andrew Paulson

Torsional electromechanical systems based on carbon nanotubes

Carbon nanotubes (CNTs) are among the most highly studied nanomaterials due to their unique (and intertwined) mechanical and electrical properties. Recent advances in fabrication have allowed devices to be fabricated that are capable of applying a twisting force to individual CNTs while measuring mechanical and electrical response. Here, we review major results from this emerging field of study, revealing new properties of the material itself and opening possibilities for advances in future devices.

Controlled manipulation of molecular samples with the nanoManipulator

The nanoManipulator system adds a virtual-reality interface to an atomic-force microscope (AFM), thus providing a tool that can be used by scientists to image and manipulate nanometer-sized molecular structures in a controlled manner. As the AFM tip scans the sample, the tip-sample interaction forces are monitored, which, in turn, can yield information about the frictional, mechanical, material, and topological properties of the sample. Computer graphics are used to reconstruct the surface for the user, with color or contours overlaid to indicate additional data sets. Moreover, a force feedback stylus, which is connected to the tip via software, allows the user to directly interact with the macromolecules. This system is being used to investigate carbon nanotubes, DNA, fibrin, adeno- and tobacco mosaic virus. It is now also possible to insert this system into a scanning electron microscope which provides the user with continuous images of the sample, even while the AFM tip is being used for manipulations.

In situ resistance measurements of strained carbon nanotubes

We investigate the response of multiwalled carbon nanotubes to mechanical strain applied with an atomic force microscope probe. We find in some samples, changes in the contact resistance dominate the measured resistance change. In others, strain large enough to fracture the tube can be applied without a significant change in the contact resistance. In this case, we observe that enough force is applied to break the tube without any change in resistance until the tube fails. We have also manipulated the ends of the broken tube back in contact with each other, re-establishing a finite resistance. We observe that, in this broken configuration, the resistance of the sample is tunable to values 15–350 kΩ greater than prior to breaking.

Investigation and modification of molecular structures with the nanoManipulator

The nanoManipulator system adds a virtual reality interface to an atomic force microscope (AFM), thus providing a tool that enables the user not only to image but also to manipulate nanometer-sized molecular structures. As the AFM tip scans the surface of these structures, the tip-sample interaction forces are monitored, which in turn provide information about the frictional, mechanical, and topological properties of the sample. Computer graphics are used to reconstruct the surface for the user, with color or contours overlaid to indicate additional data sets. Moreover, by means of a force-feedback pen, which is connected to the scanning tip via software, the user can touch the surface under investigation to feel it and to manipulate objects on it. This system has been used to investigate carbon nanotubes, fibrin, DNA, adenovirus, and tobacco mosaic virus. Nanotubes have been bent, translated, and rotated to understand their mechanical properties and to investigate friction on the molecular level. AFM lithography is being combined with the nanoManipulator to investigate the electromechanical properties of carbon nanotubes. The rupture forces of fibrin and DNA have been measured. This article discusses how some of the graphics and interface features of the nanoManipulator made these novel investigations possible. Visitors have used the system to examine chromosomes, bacterial pili fibers, and nanochain aggregates (NCAs). Investigators are invited to apply to use the system as described on the web at http:@www.cs.unc.edu/Research/nano/doc/biovis it.html.

Pearls found on the way to the ideal interface for scanned-probe microscopes

Since 1991, our team of computer scientists, chemists and physicists have worked together to develop an advanced, virtual-environment interface to scanned-probe microscopes. The interface has provided insights and useful capabilities well beyond those of the traditional interface. This paper lists the particular visualization and control techniques that have enabled actual scientific discovery, including specific examples of insight gained using each technique. This information can help scientists determine which features are likely to be useful in their particular application, and which would be just sugar coating. It can also guide computer scientists to suggest the appropriate type of interface to help solve a particular problem. We have found benefit in advanced rendering with natural viewpoint control (but not always), from semi-automatic control techniques, from force feedback during manipulation, and from storing/replaying data for an entire experiment. These benefits come when the system is well-integrated into the existing tool and allows export of the data to standard visualization packages.

Hands-on tools for nanotechnology

We describe some mechanical and electrical measurements on carbon nanotubes. We discuss electron beam lithography techniques to form metal wire contacts to the as-found nanometer structures. Starting from a unique collaborative perspective, we suggest some improved design and alignment methods.

Rolling nanotubes: Atomic lattices as gears and contacts

The commensurate contact between lattices has profound affects on friction and on the electrical transport across the interface. We report on experiments in which multiwall carbon nanotubes (CNTs) are manipulated with AFM on a graphite (HOPG) substrate. We find certain discrete orientations in which the lateral force of manipulation dramatically increases as we rotate the CNT in the plane of the HOPG surface with the AFM tip. The three-fold symmetry of these discrete orientations indicates commensurate contact of the hexagonal graphene surfaces of the HOPG and CNT. As the CNT moves into commensurate contact, we observe the motion change from sliding/rotating in-plane to stick-roll motion. We have begun the electrical characterization of the nanotube/HOPG interface and find that the junction resistance is significantly lower when the lattices are in registry.