Nanotechnology

On the Forefront of Nanotechnology

Whether demonstrating that miniature electric circuits, unlike those of normal size, adhere to the laws of quantum mechanics, or elaborating a nanogrid to sort and regulate the movements of molecules, researchers at CNRS are at the cutting-edge of nanotechnologies.

Researchers from the Pierre Aigrain laboratory¹ recently confirmed experimentally the hypothesis that microscopic-size electric circuits, unlike those of standard size, obey the rules of quantum mechanics. Predicted more than ten years ago by Markus Büttiker, of the University of Geneva, these results imply that the fundamental laws governing electric conduct,² set out in 1845 by Gustav Kirchoff, do not apply at the nanometric scale.

The experiment involved using nanolithography to build a constriction or extremely fine wire, and a condenser, both measuring only a few nanometers. Combined, these two components, which act as quantum resistance R and quantum capacity C respectively, resulted in a nano-size circuit. This type of circuit would be expected to function according to Kirchoff's circuit laws. However, the researchers observed that at this microscopic scale, a new set of laws applied.

As they reported recently in Science,³ at the nano-level, the strength of impedance (opposition to the flow of electric current) is 50 percent lower than in a classical circuit, so that electricity can circulate twice as fast. In this nano-circuit, impedance, which normally varies depending on the diameter of the wire, remains stable regardless of the wire's diameter and is consistently weaker than it would be in a regular circuit. Thus energy moves faster in miniature networks than in standard ones, paving the way for an era of upgraded technological applications using high-frequency electronics. “Our study showed that certain quantum effects must be taken into account in the use of mini-circuits at high frequency,” says Bernard Plaçais of ENS. “Our goal is now to conduct basic experiments with the small number of electrons that pass through these conductors, to ultimately control their start and stop times as well as their trajectories.”

Another significant breakthrough in nanotechnology was carried out by researchers from CEA/Saclay and CNRS, who focused on controlling movement on a miniature scale. They created a grid of molecules that can sort and regulate the movements of a second set of molecules, uncovering a new way to control the flow of nano-objects.⁴ Depending on the shape and size of molecules chosen to constitute the grid, a set of molecules heterogenous in size and shape can be filtered.⁵

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© CEA labo Nanophotonique

3-D representation of the scanning tunneling microscopy of hexabenzocoronene molecules (green) retained on a molecular matrix (red and yellow) made of a star-shaped stilbenoid compound.

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A scanning tunneling microscope (STM) was used to identify molecules derived from tristilbene capable of organizing themselves into a regular pattern, thus creating a nanostructured surface. A solution composed of a different set of molecules was then poured over this surface, filling its cavities. The larger molecules were caught in the pores while the smaller ones diffused more rapidly across the grid, spurred by the impulse of thermal agitation. “The surface is made of a material that is analogous to biological membranes, in that it ensures the structure's mechanic cohesion while letting certain chemical substances pass through,” explain André-Jean Attias of CNRS and Fabrice Charra of CEA, who worked on the project.

The study overcame a significant hurdle in scientific research, by observing several properties of molecular movement simultaneously. Researchers watched molecules in real time, at speeds close to that of video imaging; They tracked movement in situ in both liquid and solid material, at a variety of temperatures, some below O°C. According to Attias and Charra, “this combination of conditions is required for understanding the molecular diffusion mechanism at this scale, a mechanism which makes it possible to sort and regulate the molecules.”

This discovery clears the way for industrial applications such as ultraselective catalysis. After extending the implications to biological molecules, the findings could also improve drug-delivery strategies. Over the long term, the mechanism could be used to produce, replicate, and repair molecular nano-objects.

Melisande Middleton