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Research Horizon: when nanomaterials write our future

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Research horizon : when the nanomaterials write our future...

nanomaterials experience

In the "materials" family, I'd like the "nanos". Nanomaterials have made a spectacular breakthrough into the public square since the beginning of this century. In the United States, according to the "Emerging Nanotechnologies Project"1, more than five hundred everyday objects contain them: food packaging, cleaning products, computers and hard drives, phones, paints and varnishes, cosmetics, toys, cars, etc. This is the logical follow on of a long history. Because in fact people have been using them since ancient times – without knowing it – in their inks and dyes.

(1) http://www.nanotechproject.org/index.php?id=44&action=introNouvelle fenêtre

When nanomaterials write our future

What is a nanomaterial?

A nanomaterial is a material composed of "particles" measuring between 1 and 100 nanometres (1 nanometre or nm = one billionth of a metre, 10-9 m). These "nano-objects" can have different shapes (spheres, tetrahedral, fibres, tubes, thin layers...) have particular physico-chemical properties specific to these incredibly small dimensions. Small size is an advantage, but can also be a problem, because nanometric objects can penetrate and fix to organs of the human body more easily than bigger particles. They are more chemically reactive, and could cause inflammations and toxic effects in the short or long term, like asbestos fibres, which have caused many cancers of the pleura. Toxicology studies need to be done on a case-by-case basis to eliminate these risks.

Two production methods

How are such small materials manufactured? There are two methods (see the explanation below). The first, called "top down", is taken from microelectronics. This consists of miniaturizing material as much as possible using different physical or mechanical methods. The second, "bottom up" method, tries to build nanometric structures in a controlled way by assembling atoms into aggregates of molecules. This article illustrates the second method, with three examples of research carried out in the UMPC.

This is more modern and inventive than the conventional method, and is closer to what happens in living matter (a protein is a controlled assembly of amino acids, which are themselves composed of atoms of carbon, nitrogen, oxygen and hydrogen), and also in natural minerals. Opal, for example, gets its colours and pearly aspect from light diffracted by perfectly ordered silicon beads.
The combination of this "bottom-up" method with the microelectronic "top-down" method will no doubt produced the nano-objects of the future.

Top-down method
Massive material
Bottom-up method
Mechanical synthesis, consolidation
and densification, strong technique,

Laser pyrolysis,
thermal plasma, sol-gel technique,
Vapour phase reactions (CVD)

Bio-inspired nanomaterials

Bones, seashells, spider web silk, insect and crustacean carapaces, diatom shells... nature is full of examples of delicately worked material that is beautiful and very resistant. For decades now, scientists have been taking inspiration from them to create new materials. Kevlar, the most solid man-made fibre, was based on spider silk. Today, bio-inspired materials are also moving into the "nano" age.

Multi-purpose coatings

In particular, materials that make up new coatings are riding the crest of the wave. These surfaces are based on nanolayers, and are particularly resistant to abrasion and to chemical aggression. They can be multifunctional, with, for example, self-cleaning or optical (colour, antireflection, UV absorption, etc.) properties. Water plants of the lotus family (Nelumbo nucifera), whose leaves remain dry and clean thanks to their surface covered with bumps and dips ("lotus effect"), inspired "superhydrophobic" plastic materials that are waterproof and self-cleaning, and that will be marketed before long.
Illustration of the "lotus effect". On the left (a), a drop of water slides on the hydrophobic surface of a lotus leaf, carrying dirt with it. On the right (b), graphic reconstitution of the surface.

Source: a) Nees Institut und Botanischer Garten, Bonn Universität
http://www.lotus-effekt.de/en/lotus_effect_html.htmlNouvelle fenêtre
b) William Thielicke, Wikipedia

Sol-Gel chemistry

This nano-layer revolution is based on a new chemistry, called Sol-Gel, which became widespread in the 1980s, encouraged by Jacques Livage at the UMPC. Unlike conventional processes for producing materials, by processing mineral ores or petrol derivatives at high temperatures, this "soft" chemistry can combine organic and inorganic molecules at low temperatures, as in nature, to build structures with predefined properties.

Multi-purpose chemistry

"Today, the potential of this chemistry seems almost limitless", says an enthusiastic Clément Sanchez, head of the UMPC's Condensed Matter Chemistry laboratory in Paris (LCMCP). In theory, it becomes possible to associate all sorts of organic and inorganic molecules. These materials, whose structure can be analysed by nuclear magnetic resonance (NMR), could be used to design many hi-tech products, apart from the coatings already mentioned,  such as nanosensors, catalysts, nanofilters, microelectronic components, optics, energetics, vectors for carrying therapeutic drugs, medical imaging agents, etc.

The secrets of natural nanocomposites

But the real challenge will be to "tailor" new materials with properties and architectures comparable to diatom shells or mother-of-pearl. Mother-of-pearl is a composite material formed by a succession of calcium carbonate and protein layers. Its synthesis is controlled by proteins that interact chemically with the carbonate. Some of the active principles (proteins and oligoelements) are used in cosmetics, and an artificial nano-mother-of-pearl could become a perfect biomaterial for orthopaedic surgery or designing artificial body parts. However, in order to imitate these natural syntheses, scientists would have to control the interaction between proteins and minerals in time and space. For the moment, they are far from this goal. But this nanobiochemistry certainly has a very bright future...

Example of biocompatible nanoparticles that can be used for magnetic resonance imaging (MRI). These are magnetic nanocrystals (maghemite) encapsulated in a silicon dioxide sphere carrying organic groups.
© The Royal Society of Chemistry 2007 J. Mater. Chem., 17, 1563–1569.

Superordered nanocrystals

Although it is still at the research stage, scientists can already build some surprising nanomaterial "scaffolding" from atomic "bricks".  Using an "inverse micelle" process invented in the Mesoscopic and Nanometric Materials Laboratory (LM2N, UMR CNRS 7070), directed by Marie-Paule Pileni in the UMPC, it is possible to organise the spatial arrangement of atoms, in the same way as salt crystals are formed naturally by the regular assembly of chlorine and sodium atoms.

Atomic masonry

Using this method, LM2N researchers also know how to build "supracrystals", 3-D networks of gold, silver or cobalt nanocrystals. They do this by letting a concentrated solution of nanocrystals evaporate slowly on a graphite substrate.
The nanocrystals spontaneously organise themselves in a perfectly ordered and oriented manner (epitaxy). By varying the temperature, the size and shape of the crystals can be controlled. 

Formation of triangular silver crystals after 8 days (right) from silver nanocrystals (left) that organise themselves spontaneously at high temperature (50°C).
© A. Courty et al. Nature Materials
Vol 6 November 2007,
www.nature.com/naturematerialsNouvelle fenêtre

Lego that's full of surprises

These supracrystals have surprising properties. In April 2005; Marie-Paule Pileni and her coworkers showed that the nanocrystals of the same supracrystal vibrate together ("in coherence") when they are subjected to a laser beam.
"This vibrational behaviour, due to interactions between the nanoparticles of the supracrystal, is similar to the behaviour of the atoms of a nanocrystal bombarded with energy, although the scale is much bigger and the physical forces are different", says Marie-Paule Pileni. What physical mechanisms are hiding behind the collective behaviour of the nanoparticles of a supracrystal?  For the moment, the phenomenon is still being analysed. But supracrystals are already considered as the nanomaterials with a great future, because their mechanical, electrical, optical and magnetic properties seem to be unparalleled in other nanomaterials.

From spintronics to nanowhiskers

Obviously, the most desirable nanomaterials are those that have new properties. One spectacular example is that which won the 2007 Nobel Physics Prize for the Frenchman Albert Fert (Mixed Physics Unit, CNRS-Thales, Palaiseau, and Paris-Sud University, Orsay), and the German Peter Grunberg, (Julich Research Centre). In 1988, the two physicists independently discovered the "giant magnetoresistance" effect. It is based not on the flow and trapping of electric charges, like conventional electronics, but on electron spin, which is particles rotating in space. This discovery created a new area of research, spintronics, or the electronics of spin.

Spin, or how to store information

Electron spin can be manipulated by a magnetic field, just like a compass needle. When electrons circulate in a ferromagnetic material (iron, cobalt, nickel and alloys of these), that can become "spin polarized".
By controlling the polarization, it is possible to design new electronic devices such as magnetic memories. Fert and Grunberg showed that, in iron and chromium nanostructures, iron, a magnetic material, can inject a spin polarized current into the adjacent non-magnetic chromium layer.
By placing a second ferromagnetic electrode nearby, it is possible to determine the orientation of the magetisation of the two ferromagnetic layers, called parallel and antiparallel, simply by measuring their electrical resistance.

Elementary bricks

Starting from these, it is very tempting to broaden this concept to nanomaterials that would have new properties. At the Paris NanoSciences Institute (INSP), attached to the CNRS and the UPMC, Victor Etgens's team has developed a means of producing hybrid systems that associate semiconductors and magnetic material, using the "bottom-up" approach mentioned earlier. The synthesis process used is "molecular jet epitaxy". The equipment, housed in the new premises on the Jussieu site in Paris, is composed of three interconnected chambers in which nanolayers of various materials grow. At each stage, researchers can control the atomic structure of the sample surface.
This apparatus produces a mixture of "elementary bricks" that have very different properties and that can be associated to lead to new functions, as Victor Etgens explains. Putting these "bricks" together to obtain a given materials and doing the functional tests (a complex set of operations) are carried out in collaboration with Albert Fert's team. For the moment, however, the success in associating the magnetic components and the semiconductors is coming up against a technical difficulty: keeping the spin-polarized magnetization currents, which are produced in the magnetic materials, within the semiconductors. "We try, empirically and with the help of theorists, to find the right material coupling, but it's a long haul, because there are so many possibilities", admits Victor Etgens. "A manganese arsenide (a ferromagnetic material) associated with III-V semiconductors (gallium arsenide GaAs, aluminium arsenide AlAs) is the best performing coupling."

The cat's nanowhiskers

Recently, the UPMC team set out on a track that's the height of fashion internationally, making semiconducting "nanowhiskers", which may have applications ranging from nanoelectronics to biology and medicine. Mahmoud Eddrief, a UMPC research engineer, has succeeded in producing whiskers with a diameter of tens of nanometres and that can be over one thousand nanometres long. They grow using a process called "vapour-liquid-solid" (VLS) in the presence of a catalyst made of gold droplets (see below).
Today the team is thinking about associating spin electronics, with which it is familiar, with semiconducting nanowhiskers, by inserting atoms with magnetic properties into the whiskers. A technical challenge which, the researchers hope, could lead to producing, to order, "spintronic nanowhiskers" with brand new properties. Zinc selenide (ZnSe) nanowhiskers on gallium arsenide (GaAs). © INSP