- Metallic nanoparticles
Since the end of the 80s, the organometallic chemistry has proven to be well adapted to controlling the synthesis of metallic nanoparticles. This approach enables metallic atoms to be released into a solution under soft conditions (ambient temperature or lower), thus allowing these atoms to condense and be protected by the molecules that are present (solvents, polymers, ligands, etc.). The following preparation conditions prevent the oxidation of formed particles and present several advantages :
- The particles have significant chemical reactivity,
- The particle composition is controlled by the concentration of precursors,
- These particles may easily coalesce, or change shape and size,
- The particles’ surface may be complexed by ligands that favour their organisation into two or three dimensions.
Applications of this chemistry involve inasmuch the physics (nanoelectronic, magnetism, optics, etc.), as the chemistry (chemical sensors, catalysis, etc.). For informational purposes, mono or bimetallic cobalt-based nanoparticles (ex.: Co-Rh, Co-Pt) less than 2 nm, exhibiting remarkable magnetic properties (significant enhancement of the moment and the magnetic anisotropy by comparison to the bulk) were evidenced.
The determining factors for obtaining ferromagnetic materials at ambient temperature are: i) the particle volume, ii) the nature of the nanomaterials (alloys such as FePt, CoPt, etc., present an anisotropy highly superior to cobalt), iii) the shape of the particles.
In this way, it was also shown that in nano-rods, the shape anisotropy contribution attained the same order of magnitude as the magnetocristalline anisotropy one.
The reaction of Co(C8H13)(C8H12) under 3 bars of H2 in the presence of oleic acid and oleyl amine, leads to the formation of spherical nanoparticles 3 nm in size, then after heating at 150°C for 48 hours, nano-rods of 9 nm in diameter and 40 nm in length. These rods are ferromagnetic at ambient temperature, and possess a significant coercitive field of 8900 Oe (1100 Oe for the nanoparticles), which favours magnetic recording.
The synthesis techniques of aggregates in gaseous phase, which brings into play the very high vapour quenching rates, exhibits remarkable functionalisation potential and attributes a distinct specificity to the “aggregate nanotechnological way”. We could cite the example of cage-like silicon or silicon-carbon (fullerenes and heterofullerenes), of very different structures from diamond structure for which the electronic and opto-electronic properties are unreported (nearly direct large gap, photoluminescence in visible). In the field of magnetic components, the major technological breach for producing very high-density systems from nano-objects is directly related to their superparamagnetic behaviour at ambient temperature. Through aggregates and composite systems such as Co-Sm, Fe-Pt, etc., nanostructures with very strong magnetic anisotropy features are able to be developed, and densities in the order of Tbits/in2 that are necessary for the coming generation of components are targeted. Other examples of optical applications also deserve to be highlighted, such as the plasmon resonance of composite metallic aggregates, or the photoluminescence of oxide aggregates (Eu3+ doped Gd2O3).
- Oxide-based nanoparticles
Numerous oxides possess interesting properties in fields such as electron conduction (ITO), gaseous detection (SnO2, In2O3, ZnO), luminescence (ZnO), or magnetic recording (ferrites). Since their properties are strictly related to the size of the particles, it is therefore also crucial to use reliable, high-performing synthesis methods. Among the methods used, certain consist of decorrelating the kinetic steps of nucleation and growth from the precipitation phenomenon. Others are aimed at limiting the growth of the object, with the help of complexing agents that play the role of inhibitors and stabilisers, and enable the surface/volume relationship to be controlled.
The idea of limiting the space offered to the growth of particles leads to very good results. Synthesis is therefore the most often drivren in multi-phased environments: the reactor is composed of micelles or vesicles formed in emulsions, microemulsions or in ion-exchange resins. The synthesis can also be carried out in homogenous but viscous environments (gels or polymer solutions): the reduction of the precursors' diffusion speed thus limits the size of the objects.
This type of approach is used for the synthesis of SnO2 nanoparticles, deposited on a silicon chip, a system that is currently used as sensitive layer for gas sensor applications.
We should also emphasize the synthesis of the 15 nm monodispersed Sn/SnOx core/shell nanoparticle composites, which is carried out using an organometallic/sol-gel composite mechanism. In the same way, ZnO nano-rods and nanowires for electronic or optic properties (varistors, nano-lasers) were prepared, starting with the thermal decomposition of the ZnCy2 precursor, followed by oxidation in the presence of a retarder (the HDA ligand).
Over and above their intrinsic properties, the importance of such materials resides in their extreme stability in an oxidizing atmosphere, unlike metallic nanoparticles. To these nanoparticles they confer their wide range of application possibilities and a easy implementation.