Using X-Rays to Investigate Order and Function in Nanotube Systems

In collaborative research, a team of scientists has developed a novel way of using x-rays at the NSLS to study arrays of nanotubes. In an ongoing series of research projects, they have determined the degree of order contained in certain nanotube systems - that is, to what extent they form organized patterns - and have investigated the structural and chemical properties of others.

A rendering of carbon nanotubes being studied using NEXAFS. Light comes in (left) and electrons are emitted (right).

The group is investigating assemblies containing two types of carbon nanotubes: single-walled nanotubes (SWNTs), consisting of a single-shelled cylinder, and multi-walled nanotubes (MWNTs), which resemble cylinders concentrically nested together. They also have completed a study of nanotubes composed of a different, yet equally intriguing material, boron nitride, which is composed of the elements boron and nitrogen.

The synchrotron-based technique is known as "near-edge x-ray absorption fine structure," or NEXAFS. In NEXAFS analysis, each nanotube sample is placed in the path of a beam of low-energy x-ray "photons," or particles of light. The x-ray photons are absorbed by each carbon atom's "core"

Scanning electron microscope images of (a) MWNT powder, (b) SWNT powder, (c) SWNT buckypaper, and (d) aligned MWNTs.

electrons - those closest to the nucleus -giving them an energy boost. As a result, they jump to an orbit further away from the nucleus. When this occurs across many, many atoms, scientists can record the sample's absorption "spectrum," which measures the absorption behavior of the sample based on specific energies of the x-ray photons.

There are two ways to measure the absorption spectrum: first, by measuring the photons emitted when the energized electrons de-energize and fall back to a lower orbit; or, in certain cases, by measuring electrons that are emitted.

At beamline U7A (owned by NIST and The Dow Chemical Company), the researchers aimed the x-rays at each sample from several angles, producing several spectra for each array. By analyzing the spectra, they uncovered information about the nanotubes' electronic and physical structures.

The researchers used NEXAFS to determine the degree of order in very thin films of single-walled nanotubes, known as "buckypaper," and a MWNT film grown on a surface of platinum metal. They compared their results to two "control" groups: graphite, a highly ordered form of carbon with well-understood electrical and structural properties, as well as SWNT and MWNT powder samples, which have essentially no order.

The group found that the buckypaper spectra are similar to graphite's in that, for both, the spectra change when the x-rays graze the sample rather than strike head-on. This property - the tendency of a material to react differently to outside fields depending on the direction the field is applied - is called "anisotropy." It is a very useful behavior to study, since high anisotropy often implies a high degree of order, and is often a desirable property for nanotube systems to have. Conversely, a material shows "isotropy" when its reaction to an outside field is the always the same, regardless of direction. Nanotube powders, containing nanotubes in all possible positions and orientations, display isotropy. Scientists often speak of anisotropy and isotropy when they are discussing order in a system.

In buckypaper, the group found, the anisotropy is not as pronounced as in graphite. They explored this result with further analysis, and calculated that approximately 87 percent of the nanotubes in the paper lie on their sides. Thus, the paper isn't as ordered on the nanoscale level as it could be. The analysis of the MWNT film is more preliminary, but shows that the film behaves as if most the nanotubes stand upright. Thus, the film also displays anisotropy.

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