Nondestructively seeing subsurface nanostructures

Nondestructively seeing subsurface nanostructures

Using an electrostatic variant of an atomic-force microscope, researchers view carbon nanotubes in a polymer matrix

Researchers from the National Institute of Standards and Technology (NIST), Gaithersburg, MD, working with colleagues from the National Aeronautics and Space Administration, the National Institute of Aerospace, the University of Virginia, and the University of Missouri announced in late June that, under the right circumstances, an instrument similar to an atomic-force microscope can nondestructively determine the subsurface stucture of nanostructured composite materials, which mix carbon nanotubes in a polymer matrix, or base.

The information could prove useful for the design and manufacture of such materials for high-performance applications that require superior strength and electrical conductance. For example, the material used by the research team in their test case is being considered by NASA for use in spacecraft actuators because it may outperform the heavier ceramics now used.

According to one of the researchers, NIST materials scientist Minhua Zhao, “One of the critical issues to study is how the carbon nanotubes are distributed within the composite without actually breaking the part. There are very few techniques available for this kind of non-destructive study.” So the team decided to try an unusual application of atomic-force microscopy: electrostatic-force microscopy, or EFM.

An atomic-force microscope (AFM) works by using a delicate needle-like point that is placed just above the surface to be profiled; the point responds to weak, atomic-level forces, typically the very short-range van der Waals forces exerted by molecules or atoms. This restricts the instrument to the surface of samples.

The team modified an AFM to respond to stronger, longer-range electrostatic force and thus measure the interaction between the probe tip and a charged plate beneath the composite sample. This EFM technigue works because the nanotubes have a high dielectric constant, while the polymer has a low dielectric constant. With properly chosen voltages, the nanotubes show up as finely detailed fibers dispersed below the composite's surface (see figure ).

A polyimide/carbon nanotube composite, whose surface is shown in the AFM height image at left, is imaged at right using an an EFM to reveal the curved lines of nanotubes that are well below the surface. (Photo: NIST.)

The goal is to control the process well enough to allow quantitative measurements. At present, the group can discriminate different concentrations of carbon nanotubes in the polymer, determine conductive networks of the nanotubes, and map electric potential distribution of the nanotubes below the surface. But the measurement is quite tricky since many factors, including probe shape and humidity, can affect the electrostatic force. The team used a specially designed probe tip and a patented, NIST-designed AFM humidity chamber.

An interesting, not-yet-fully-understood effect, according to Zhao, is that increasing the voltage between the probe and the sample at some point causes the image contrast to invert, so that dark regions becoming light and vice versa. The team is studying the mechanism of such contrast inversion. “We are still optimizing this EFM technique for subsurface imaging,” says Zhao. “If the depth of nanostructures [relative to] the film surface can be determined quantitatively, this technique will be a powerful tool….” For further information, call Minhua Zhao at 301-975-8923 or e-mail minhua.zhao@nist.gov.

Richard Comerford