Stimulated by the race towards miniaturization in the microelectronics industry, research in Nanosciences is conducted at two departments within the Physical Sciences Division: Iramis in Saclay and Inac in Grenoble.
This involves disciplinary fields at the crossroads of Chemistry, Physics and Biology. Activities at Saclay in the spotlight.
Trying to manipulate nano-objects, understanding the behavior of finely divided matter, exploring quantum effects: these are some of the challenges facing fundamental research in nanosciences.
||At the level of atoms and molecules, there is a whole world to explore: the nanoworld, christened thus in reference to the nanometer, a billionth of a meter. Observing atoms and molecules individually became possible at the start of the 1980s thanks to two inventions: the scanning tunneling microscope for materials that conduct electricity and its derivative, the atomic force microscope for insulating materials. These instruments are used both to observe surfaces and to manipulate atoms or molecules. Pooling the talents of chemists, physicists and biologists has played a decisive role in the creation of electronic devices and innovative materials|
How do the properties of particles change when their dimensions go from the micrometric to the nanometric scale?
Understanding the effect of size on the physical or chemical properties of particles is essential in nanosciences. The particles can be separated out individually in a powder state or bonded to solid materials. At the frontier between science and technology, researchers are shuttling back and forth between synthesizing materials, conducting experiments and performing numerical simulations and interpretations.
A more radiation-resistant ceramic
A ceramic is a material obtained by heat treatment (sintering) from powders generally of micrometric size. A team specializing in laser pyrolysis has developed an original technique for producing chemical composites in powders of calibrated sizes. According to recent experiments, ceramics made from nanometric powders produced in a laboratory are more resistant to radiation than traditional ceramics. In both cases, it is possible to see the grains, separated by grain boundaries, at different scales. Under the effect of radiation, defects appear in the crystalline organization of the grains and tend to merge until they hit an obstacle: the boundary. It seems that, in nanostructured ceramics, the appearance of radiation damage is delayed because the web of particles is a thousand times finer. From the “materials” point of view, these ceramic nanopowders could be used in the composition of composite materials for the nuclear reactors of the future.
A larger active surface area
Another example of a divided (or nanostructured) material is the platinum in fuel cells. The chemical reactions that produce the current in the fuel cell are accelerated (or catalyzed) by this metal when the reagents “meet”, coming into contact with it. The use of fine platinum particles makes it possible to reduce the quantity of metal required. The size of these particles varies from a few nanometers to tens of nanometers.
Researchers are proposing to replace them with particles of a perfectly calibrated size. A “coating” of organic molecules prevents the particles from forming clusters and means that the distance between metal cores can be finely regulated by the choice of grafted molecules. From the perspective of application to fuel cells, the electrical conductivity of these objects can be optimized according to their size. The icing on the cake is that combining these particles with carbon nanotubes would make the catalysis sites more accessible to the reagents and improve efficiency even more.
A choice of colors
One particular property of semiconductors is photoluminescence, which provides a spectacular illustration of the size effect.
When they are lit, these materials give out some of the energy they receive by emitting light. The color (or energy) of this light is determined by the chemical nature and size of the semiconductor. If the specimen size is reduced to a few nanometers, there is constant variation in this color: the energy of the emitted light increases as the size of the object decreases. The behavior of the nanocrystal, also known as a quantum dot, seems to gradually approach that of an isolated atom. In particular, silicon nanocrystals, still produced by laser pyrolysis, could act as in vivo tracers for the diagnosis and treatment of diseases. How do the properties of particles change when their dimensions go from the micrometric to the nanometric scale ?
Microscope resolution is limited by the diffraction of light crossing the specimen. This becomes even more of a problem as the wavelength of the light increases. Hence the idea of replacing photons with electrons, which have a shorter wavelength. In transmission electron microscopy (TEM), a flow of electrons is passed through the specimen and detected to form the image. Resolution can go below a nanometer. Meanwhile, the scanning electron microscope (SEM) uses secondary electrons emitted by the specimen when it is bombarded with electrons, on the same side as the source. This time the resolution is of the order of a nanometer.
How do we “see” the atoms and molecules in a solid individually? The “eye” of these microscopes is a tip that scans the surface to be analyzed by gliding over it at a fixed height of the order of a few atom diameters (a few tenths of a nanometer). This distance is adjusted by very shortrange interactions between the last atom right on the tip and the surface.
|In the scanning tunneling microscope (STM) this interaction, quantum in nature, is manifested by a weak electric current that flows between the atom on the tip and the surface. This current rapidly increases as the tip gets closer to the surface. In the atomic force microscope (AFM), similar forces to those that make atoms bond in a molecule are at work between the atom on the tip and the atoms on the surface. At even shorter distances, forces of repulsion predominate between the atomic nuclei. Subject to these antagonistic forces, the atom spontaneously tries to remain at a fixed distance. In both cases, a computer records either the current or the force, and keeps the tip at a constant distance from the surface. The relief “felt” by the tip can be reconstituted in this way with resolution of less than a nanometer, giving the user an atom-byatom picture of the material being studied.|