Metrological Protocols for Reaching Reliable and SI-Traceable Size Results for Multi-Modal and Complexly Shaped Reference Nanoparticles.

The study described in this paper was conducted in the framework of the European nPSize project (EMPIR program) with the main objective of proposing new reference certified nanomaterials for the market in order to improve the reliability and traceability of nanoparticle size measurements. For this purpose, bimodal populations as well as complexly shaped nanoparticles (bipyramids, cubes, and rods) were synthesized. An inter-laboratory comparison was organized for comparing the size measurements of the selected nanoparticle samples performed with electron microscopy (TEM, SEM, and TSEM), scanning probe microscopy (AFM), or small-angle X-ray scattering (SAXS). The results demonstrate good consistency of the measured size by the different techniques in cases where special care was taken for sample preparation, instrument calibration, and the clear definition of the measurand. For each characterization method, the calibration process is described and a semi-quantitative table grouping the main error sources is proposed for estimating the uncertainties associated with the measurements. Regarding microscopy-based techniques applied to complexly shaped nanoparticles, data dispersion can be observed when the size measurements are affected by the orientation of the nanoparticles on the substrate. For the most complex materials, hybrid approaches combining several complementary techniques were tested, with the outcome being that the reliability of the size results was improved.

Influence of electron landing energy on the measurement of the dimensional properties of nanoparticle populations imaged by SEM.

Scanning Electron Microscopy (SEM) technique is widely used to characterize nanoparticle (NP) size. The landing energy (LE) of the primary electron beam is considered to be a key parameter related to the ability of electrons to penetrate the sample. However, few studies have been carried out so far on the influence of this parameter on the measurement of NP size by SEM. The increasing needs for reference materials consisting of size-controlled NP suspension for microscope calibration induce new issues. This paper focuses on the effect of electron landing energy on the measurement of the equivalent diameter of several NP populations by SEM. To evaluate the influence of LE, particles of different sizes and chemical compositions were analyzed. The results showed the variation of the measured diameter as a function of LE. SEM secondary electron (SE) yield modeling by the Monte Carlo method allowed us to relate this variation to the information volume in the material. Finally, the use of reference particles and transmission electron microscopy (TEM) allowed us to determine an optimal value of LE to be applied, depending on the chemical composition and particle size to limit the bias in the SEM measurement. We showed that this operating point can be simply determined without reference nanomaterials by scanning an LE range.