An artist's representation of an amine functional group attached to an AFM tip approaching a carbon nanotube surface in toluene solution. Translucent blue shape on the nanotube represents the polarization charge forming on the nanotube as the result of the interaction with the approaching molecule. Chemical force microscopy measures the tiny forces generated by this single functional group interaction. (Illustration by Scott Dougherty, LLNLnanotube
Carbon nanotubes have been employed for a variety of uses including composite materials, biosensors, nano-electronic circuits and membranes.
While they have proven useful for these purposes, no one really knows much about what's going on at the molecular level. For example, how do nanotubes and chemical functional groups interact with each other on the atomic scale? Answering this question could lead to improvements in future nano devices.
In a quest to find the answer, researchers for the first time have been able to measure a specific interaction for a single functional group with carbon nanotubes using chemical force microscopy - a nanoscale technique that measures interaction forces using tiny spring-like sensors. Functional groups are the smallest specific group of atoms within a molecule that determine the characteristic chemical reactions of that molecule.
A recent report by a team of Lawrence Livermore National Laboratory researchers and colleagues found that the interaction strength does not follow conventional trends of increasing polarity or repelling water. Instead, it depends on the intricate electronic interactions between the nanotube and the functional group.
This work pushes chemical force microscopy into a new territory," said Aleksandr Noy, lead author of the paper that appears in the Oct. 14 online issue of the journal, Nature Nanotechnology.
Understanding the interactions between carbon nanotubes (CNTs) and individual chemical functional groups is necessary for the engineering of future generations of sensors and nano devices that will rely on single-molecule coupling between components. Carbon nanotubes are extremely small, which makes it particularly difficult to measure the adhesion force of an individual molecule at the carbon nanotube surface. In the past, researchers had to rely on modeling, indirect measurements and large microscale tests.
But the Livermore team went a step further and smaller to get a more exact measurement. The scientists were able to achieve a true single function group interaction by reducing the probe-nanotube contact area to about 1.3 nanometers (one million nanometers equals one millimeter).
Adhesion force graphs showed that the interaction forces vary significantly from one functionality to the next. To understand these measurements, researchers collaborated with a team of computational chemists who performed ab initio simulations of the interactions of functional groups with the sidewall of a zig-zag carbon nanotube. Calculations showed that there was a strong dependence of the interaction strength on the electronic structure of the interacting molecule/CNT system. To the researchers delight, the calculated interaction forces provided an exact match to the experimental results.
"This is the first time we were able to make a direct comparison between an experimental measurement of an interaction and an ab initio calculation for a real-world materials system," Noy said. "In the past, there has always been a gap between what we could measure in an experiment and what the computational methods could do. It is exciting to be able to bridge that gap."
This research opens up a new capability for nanoscale materials science. The ability to measure interactions on a single functional group level could eliminate much of the guess work that goes into the design of new nanocomposite materials, nanosensors, or molecular assemblies, which in turn could help in building better and stronger materials, and more sensitive devices and sensors in the future.
Thin films of silicon nanoparticles roll into flexible nanotubes
By depositing nanoparticles onto a charged surface, researchers at the University of Illinois at Urbana-Champaign have crafted nanotubes from silicon that are flexible and nearly as soft as rubber.
"Resembling miniature scrolls, the nanotubes could prove useful as catalysts, guided laser cavities and nanorobots," said Sahraoui Chaieb, a professor of mechanical and industrial engineering at Illinois and a researcher at the Beckman Institute for Advanced Science and Technology.
To create their flexible nanotubes, Chaieb and his colleagues - physics professor Munir Nayfeh and graduate research assistant Adam Smith - start with a colloidal suspension of silicon nanoparticles (each particle is about 1 nanometer in diameter) in alcohol. By applying an electric field, the researchers drive the nanoparticles to the surface of a positively charged substrate, where they form a thin film.
Upon drying, the film spontaneously detaches from the substrate and rolls into a nanotube. Nanotubes with diameters ranging from 0.2 to 5 microns and up to 100 microns long have been achieved.
Using an atomic force microscope, the researchers found that the Young's modulus (a measure of a material's elasticity) of the film was about 5,000 times smaller than that of bulk silicon, but just 30 times larger than that of rubber.
"We suspect that the nanotubes consist of silicon nanoparticles held together by oxygen atoms to form a three-dimensional network," Chaieb said. "The nanotubes are soft and flexible because of the presence of the oxygen atoms. This simple bottom-up approach will give other researchers ideas how to build inexpensive active structures for lab-on-chip applications."
"Because the silicon nanoparticles - which are made using a basic electrochemical procedure - have properties such as photoluminescence, photostability and stimulated emission, the resulting nanotubes might serve as nanodiodes and flexible lasers that could be controlled with an electric field," Nayfeh said.
The results will be reported in an upcoming issue of the journal Applied Physics Letters. The work was funded by the National Science Foundation and the state of Illinois.
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