Expert panel to discuss applications of nanotechnology at Congress start

Emily M. Smith
ASME NEWS

When ASME's International Mechanical Engineering Congress and Exposition gets underway in November, it will be a showcase for the field of nanotechnology.

The nanotechnology track will kick off Monday evening, Nov. 12, with a keynote panel of nanotech experts discussing their work in the field.

One of those experts will be Michael L. Simpson, who is the founder and principal investigator of the Molecular-Scale Electronics and Nanoscale Technologies Group at Oak Ridge National Lab.

Simpson's group has been active in the controlled synthesis and directed assembly of carbon nanofibers into devices and systems.

He spoke with ASME NEWS about the applications of carbon nanostructures and their implications for mechanical engineering.

ASME NEWS: In what industries are/will carbon nanotubes be most applicable?

Simpson: Since carbon nanostructures, which include nanotubes and nanofibers, have extraordinary charge transport properties, the electronics industry is often mentioned when applications are contemplated.

However, carbon nanotubes also have extraordinary mechanical properties, and there is much research interest in carbon nanotube composites for structural materials. The result would be light but extremely strong materials of great interest to industries such as aerospace. Furthermore, the electrical and mechanical properties of carbon nanotubes are coupled, and so-called "smart" materials can be contemplated that don't just bear forces, but can also transduce and react to these forces in an engineered manner.

We now know how to integrate carbon nanofibers with micro or nanostructured substrates. These extremely high-aspect-ratio nanoscale elements can form components in nanoelectromechanical systems (NEMS), or in devices for nanobiotechnology. One could envision an impact in fields ranging from sensors and actuators to biomedicine.

ASME NEWS: Carbon nanotubes, buckeyballs and quantum dots are typical examples of nanoengineered structures. But, you've said that the self-assembly of those products into higher-order structures will require improved technical advances. What does that mean in terms of a time frame for the deployment of an application?

Simpson: There are two fundamental approaches to the assembly of nanoscale systems: top-down and bottom-up.

Top-down assembly imposes a structure on the system through the definition of patterns by lithography or other means. Top-down assembly is widely and successfully applied at the microscale in the manufacture of integrated circuits, for example.

However, at smaller scales, optical lithography doesn't have the required resolution. And other lithography techniques, such as electron beam, X-rays or extreme UV, are either slow, expensive or filled with as yet unmet technical challenges.

Bottom-up assembly proceeds from the atomic or molecular scale and is driven by natural processes; therefore, the assembly of lipids into a cell membrane, for example, is thought of as self-assembly. To make practical devices on the nanoscale, we need directed self-assembly techniques that allow the assembly of complex systems composed of large numbers of nanoscale components.

We don't have complete self-assembly strategies that are widely applicable to useful devices and systems. These may indeed be many years in the future. However, hybrid strategies that involve the lithographic definition of a structure that serves as a template for self-assembly are being developed.

Some hybrid strategies are ready for deployment in applications now. This is true for carbon nanofibers. In our process, an array of catalyst nanoparticles are lithographically defined, but the growth of the nanofibers proceeds through a physical self-assembly process that determines the diameter, length and orientation of the nanofibers.

ASME NEWS: How would the potential application of carbon nanoengineered structures be an improvement over what currently exists to get a job done?

Simpson: Carbon nanostructures may have exceptional electrical or mechanical properties that can be exploited in a variety of devices. These structures lend themselves to the directed self-assembly methods I just described, resulting in nanoscale components that can't be fabricated with conventional top-down assembly. Because of their size and/or aspect ratio, these devices display particularly useful properties, such as low-voltage field emission, the ability to perform electronic functions using very low current and the ability to interface directly with single biomolecules.

ASME NEWS: Do carbon nano-structures have broad applications? Or is their function so specialized as to be nontransferable?

Simpson: Carbon nanostructures are likely to find applications as varied as structural materials, flat panel displays, electronic devices, biomedical devices, fluidic devices, sensors and actuators. The range of applications is extremely broad.

ASME NEWS: Carbon nanostructures have been described as having a number of applications, such as vacuum nanoelectronics, nanoelectromechanical systems (NEMS), bio-NEMS, and biomimetic devices. How do any of those applications translate in terms of a mechanical engineering function?

Simpson: I see the impact on mechanical engineering in two ways. First, the macroscopic properties of materials can be enhanced through the engineering of nanoscale composites. For example, single-wall carbon nanotubes have exceptional mechanical strength. Many groups are now working to make very strong polymer/carbon nano-tube composites.

Carbon nanostructures can have broad applications.

However, you also have to consider nanomechanical devices — mechanical sensors or actuators that depend upon the enhanced properties of nanoscale components for their functionality. Just as work at the interface between microscale technology and mechanical engineering created useful MEMS devices in the last decade, work at the interface between nanoscale technology and mechanical engineering will produce NEMS devices in the future.

This is not an exercise in making MEMS devices smaller and calling them NEMS. Rather, it is taking advantage of the unique properties that emerge at the nanoscale and developing the directed self-assembly strategies that allow for the fabrication of devices with entirely new functionality.

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