The scientists have been using benzene to study diffusion. Credit: Dreamstime
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Carbon plays a crucial role as a building block for nanotechnology. Carbon nanosystems have shown a number of commercially desirable properties such as super lubricity and super diffusion, as well as great strength and hardness. These properties give them numerous potential applications- such as in solar cells, hydrogen storage, environmental remediation and in biomedicine.
One of the most promising of these applications is in carbon based synthetic motors. These are tiny machines only measuring a few nanometres, inspired by protein motors found in the body that are used in intracellular cargo transport. “They could make dramatic changes to chemistry, chemical sensing and finally medicine: the idea is that these devices can move single molecules around and they are essential man made equivalents to bio-molecules,” says Dr Peter Fouquet from the Institut Laue-Langevin in Grenoble. At the moment however, these synthetic motors are restricted to the lab. There is still a lack of understanding of what’s happening within them at a nanometre scale, it is especially difficult to pinpoint the areas of friction within these systems.
"At present a lot of the research and development into these carbon systems is being done via experimental chemistry on a trial and error basis rather than by engineering these carbon systems from first principles,” Says Dr Fouquet.
Dr Fouquet’s team have been trying to gather a greater understanding of these carbon systems, using benzene – a simple carbon molecule - as a tool to investigate the dynamic friction on a surface at the microscale. The team published a paper in 2009, which revealed that Benzene’s movement can be described almost perfectly as Brownian diffusion - the random movement of particles in a fluid, a phenomenon first identified by Albert Einstein.
More recently, they used benzene and graphite to further investigate this diffusion, at various temperatures from 60K to 140K, as well as varying levels of benzene coverage. Using spectrometers at the ILL and the spectrometer OSIRIS at ISIS they were able to create a detailed 2D model of the system.
“It is amazing that such a "simple" model manages to accurately describe physics at the nanometre scale. The advantage of neutron spectroscopy is that it allows us to follow the motion of molecules with sub-nanometre resolution. Our measurements also tell us how much energy is needed to move a molecule a certain distance over a certain time. The spatial and time resolution combined in neutron spectroscopy is really unique.” Says Dr Fouquet.
The study revealed that - contrary to previous literature, the speed of diffusion is much slower when the density of particles is increased. The results were very similar to that for a “theoretically ideal liquid.” This is because slowing down occurs only upon collision between particles and as there are more particles to collide with, there is more slowing down and therefore less diffusion. They also observed that, at the lowest benzene coverages, the particles are moving faster than expected for a standard Brownian diffusion. This is known as super diffusion. The team are interested in this super diffusive behaviour because “it means the molecules are “flying” without any hindrance over a short distance, such a kind of super diffusion has never been measured for a molecular system before.” Said Dr Fouquet.
"This work has given us new insights into the nature of diffusion and the origins of friction", says Dr Fouquet. "The new, more accurate modelling of these processes will aid the search for low friction building blocks in nanotechnology, including those made of carbon. From a more fundamental physics viewpoint, what we have created here, an almost perfect Brownian gas 2D system, is also a brilliant test system for investigating the simple physics of colliding particles."
Research date: November 2014
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