3D morphology and mechanical properties of nanoparticles

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 French National Institute for Research and Safety (INRS) distinguishes nanoparticles from nanomaterials. A nanoparticle is a particle whose three dimensions are between 1 and 100 nm.

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.

Even if the definition of nanoparticles has not yet been completely settled, all the players in this field agree that the "size" component is the most relevant factor for defining these elements.

 

Until today, several characterisation methods have been tried and tested on this subject: laser granulometry, PTA, DLS... These are to be forgotten as a tool for characterising the size as well as the number distribution. It is indeed the last information that interests us and not the distribution in mass or volume.

The only possible analyses are those related to microscopy: SEM, TEM, AFM should be considered.

Atomic Force Microscopy (AFM) is a technique that allows the three-dimensional morphology of a material's surface to be visualised with nanometric resolution, and to map certain of its properties (adhesive, mechanical, magnetic, electrical, etc.). The principle of AFM is based on the measurement of the various interaction forces (ionic repulsion forces, Van-der-Waals forces, electrostatic forces, etc.) between the atoms of the sample surface and the atoms of a nanometric probe tip, fixed under a flexible microlever.

TESCAN ANALYTICS has nearly 30 years of expertise in the use of AFM and its different modes on all types of materials. With the latest generation of instruments, our team of experts works with all industrial sectors.

Figure 1: 2D, 3D image - Number distribution - Identification of NPS

Figure 2: 2D, 3D image - Young's modulus per particle
 

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.

In this example, it was demonstrated that AFM is a suitable microscopy tool for measuring the dimensions of interest of spherical nanoparticles.

AFM allows the determination of the number distribution of the height of spherical nanoparticles.

AFM also allows the measurement of the mechanical properties of nanoscale objects, as shown above with nanoparticles.

In other work, it has been shown that AFM is an optimal technique for defining the 3D morphology of nanomaterials of all shapes. 

For more applications of AFM analysis or our other techniques, please ask us for information.

Other complementary techniques can be used to study the size and shape of nanoparticles (TEM, SEM) and to analyse their chemical composition such as EDX, XPS and ToF-SIMS.