Vernon L. Chi

Bending and buckling of carbon nanotubes under large strain

The curling of a graphitic sheet to form carbon nanotubes produces a class of materials that seem to have extraordinary electrical and mechanical properties. In particular, the high elastic modulus of the graphite sheets means that the nanotubes might be stiffer and stronger than any other known material,,, with beneficial consequences for their application in composite bulk materials and as individual elements of nanometre-scale devices and sensors. The mechanical properties are predicted to be sensitive to details of their structure and to the presence of defects, which means that measurements on individual nanotubes are essential to establish these properties. Here we show that multiwalled carbon nanotubes can be bent repeatedly through large angles using the tip of an atomic force microscope, without undergoing catastrophic failure. We observe a range of responses to this high-strain deformation, which together suggest that nanotubes are remarkably flexible and resilient.

Nanometre-scale rolling and sliding of carbon nanotubes

Understanding the relative motion of objects in contact is essential for controlling macroscopic lubrication and adhesion, for comprehending biological macromolecular interfaces, and for developing submicrometre-scale electromechanical devices,. An object undergoing lateral motion while in contact with a second object can either roll or slide. The resulting energy loss and mechanical wear depend largely on which mode of motion occurs. At the macroscopic scale, rolling is preferred over sliding, and it is expected to have an equally important role in the microscopic domain. Although progress has been made in our understanding of the dynamics of sliding at the atomic level, we have no comparable insight into rolling owing to a lack of experimental data on microscopic length scales. Here we produce controlled rolling of carbon nanotubes on graphite surfaces using an atomic force microscope. We measure the accompanying energy loss and compare this with sliding. Moreover, by reproducibly rolling a nanotube to expose different faces to the substrate and to an external probe, we are able to study the object over its complete surface.

Salphasic distribution of clock signals for synchronous systems

The design of a synchronous system having a global clock must account for propagation-delay-induced phase shifts experienced by the clock signal (clock skew) in its distribution network. As clock speeds and system diameters increase, this requirement becomes increasingly constraining on system designs. The paper describes a method that exploits properties of standing waves to reduce substantially clock skews due to unequal path lengths, for distribution network diameters up to several meters. The basic principles are developed for a loaded transmission line, and then applied to an arbitrary branching tree of such lines to implement a clock distribution network. The extension of this method to two- and three-dimensional distribution media is also presented, suggesting the feasibility of implementing printed circuit board clock planes exhibiting negligible phase shift over their extents. >

Manipulation of individual viruses: friction and mechanical properties

We present our results on the manipulation of individual viruses using an advanced interface for atomic force microscopes (AFMs). We show that the viruses can be dissected, rotated, and translated with great facility. We interpret the behavior of tobacco mosaic virus with a mechanical model that makes explicit the competition between sample-substrate lateral friction and the flexural rigidity of the manipulated object. The manipulation behavior of tobacco mosaic virus on graphite is shown to be consistent with values of lateral friction observed on similar interfaces and the flexural rigidity expected for macromolecular assemblies. The ability to manipulate individual samples broadens the scope of possible studies by providing a means for positioning samples at specific binding sites or predefined measuring devices. The mechanical model provides a framework for interpreting quantitative measurements of virus binding and mechanical properties and for understanding the constraints on the successful, nondestructive AFM manipulation of delicate samples.

A programmable HIPPI interface for a graphics supercomputer

As networks approach gigabit performance, supercomputer host interfaces are becoming the communication bottleneck. The Network Interface Unit (NIU) is a high-performance host interface for Pixel Planes 5, a custom graphics supercomputer. The design offers both performance and programmability through a balance of data-marshaling hardware and an embedded processor. The authors describe the NIU hardware and firmware architecture. They present some preliminary performance measurements and assess the applicability of this design for the targeted environment. They also comment on its suitability for other environments and outline plans for the future work.

Tracking a head-mounted display in a room-sized environment with head-mounted cameras

This paper presents our efforts to accurately track a Head-Mounted Display (HMD) in a large environment. We review our current benchtop prototype (introduced in {WCF9O]), then describe our plans for building the full-scale system. Both systems use an inside-oui optical tracking scheme, where lateraleffect photodiodes mounted on the users helmet view flashing infrared beacons placed in the environment. Churchs method uses the measured 2D image positions and the known 3D beacon locations to recover the 3D position and orientation of the helmet in real-time. We discuss the implementation and performance of the benchtop prototype. The full-scale system design includes ceiling panels that hold the infrared beacons and a new sensor arrangement of two photodiodes with holographic lenses. In the full-scale system, the user can walk almost anywhere under the grid of ceiling panels, making the working volume nearly as large as the room.

Surface modification tools in a virtual environment interface to a scanning probe microscope

The NanoManipulator system has been expanded from a virtual-reality interface for a specific scanning tunneling microscope to include control of atomic force microscopes. The current state of the system is reviewed, and new tools extending the users feel and control in manipulation and fabrication in the mesoscopic regime are detailed. Manipulations that could not be performed using the techniques available from commercial SPM systems are demonstrated, and the direction of ongoing research is outlined.

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.

VISTAnet: interactive real-time calculation and display of 3-dimensional radiation dose: An application of gigabit networking

Three-dimensional treatment planning can allow the clinician to create plans that are highly individualized for each patient. However, in lifting the constraints traditionally imposed by 2-dimensional planning, the clinician is faced with the need to compare a much larger number of plans. Although methods to automate that process are being developed, it is not yet clear how well they will perform. VISTAnet is a 3 year collaborative effort between the Departments of Radiation Oncology and Computer Science at the University of North Carolina, the North Carolina Supercomputing Center, BellSouth, and GTE with the medical goal of providing real-time 3-dimensional radiation dose calculation and display. With VISTAnet technology and resources, the user can inspect 3-dimensional treatment plans in real-time along with the associated dose volume histograms and can fine tune these plans in real-time with regard to beam position, weighting, wedging, and shape. Thus VISTAnet provides an alternate and, possibly, complementary approach to computerized searches for optimal radiation treatment plans. Building this system has required the development of very fast radiation dose code, methods for simultaneously manipulating and modifying multiple radiation beams, and new visualizations of 3-dimensional dose distributions.

Nanowelding: tip response during STM modification of Au surfaces

Abstract We have studied the response of the tunnel current and the feedback circuit during surface modification by an STM tip. Our studies show that several classes of events occur when “writing” by applying voltage pulses. We have studied W and Au tips on Au surfaces and PtRh tips on Pt surfaces. Strong correlation is found between the tunnel current response and the kind of surface change that occurs. One dramatic discovery is the formation of a wire between the tip and surface, which is characterized by an electrical shorting of the tip to the surface until the feedback circuit of the tip retracts enough to break the wire. Using a sophisticated graphics display system and allowing pulse placement by pointing at the desired location, we are able to distinguish easily between changes in surface morphology and changes in tip shape and length.