Nanoparticles are everywhere. Many innovations in various fields are based on these very special compounds. For several years now, French regulations have been taking a close interest in the use of these nanoparticles in consumer products. The great difficulties of these control processes are due to the wide variety of existing nanoparticles, which may differ in shape, size, chemical composition and physicochemical properties. The first step of this work consists in the precise definition of these different factors, the most accepted today being their size.
The INRS defines nanomaterials as "a material of which at least one external dimension is on the nanometric scale, i.e. between 1 and 100 nm, or which has an internal or surface structure on the nanometric scale" (definition found in the ISO TS 80004-1 standard). The European Commission considers a threshold concentration of 50% of nanoparticles to define a nanomaterial.
Since nanoparticles have a very strong tendency to agglomerate, it is necessary to disperse them on a functionalized substrate in order to ensure that they remain on the sample during the passage of the tip. Once this step is completed, the solution containing the NPs is deposited on the substrate to obtain a monolayer of perfectly distributed nanoparticles.
The size of the nanoparticles is measured by measuring their height. Indeed, given the convolution between the tip of the AFM and the nanoparticles, direct measurement of the diameter would lead to an overestimation of the latter. However, the height of a nanoparticle placed on the surface, considering that it is perfectly spherical, is equal to its diameter.
In figure 1, the AFM allows the morphology of nanoparticles to be obtained in 2D and 3D. Using image processing software, individual nanoparticles are selected and their maximum height measured. In this example, the height distribution of the silicon nanoparticles is centred between 82 and 85 nm. In figure 2, the Peak Force QNM mode allows to measure the Young's modulus of Silicon nanoparticles with a diameter of about 80 nm, with the help of a diamond tip fixed on a microlever with a stiffness equal to 250 N/m. The modulus is measured at the top of the particles and in this example gives an average modulus of 16.2 Gpa. The observed shape of the nanoparticles illustrates the convolution between the tip and the particles. Indeed, the diamond tip is "bigger" than a conventional AFM tip, which implies that the convolution problems are more visible.