How large is “small”?

Nanoparticles often exhibit other properties than "large" particles made of the same material. This exactly is what makes them so interesting for various applications – but it also bears risks for man and the environment. To estimate the risks, and to advance the technical development, procedures to determine the exact size of these particles are important. PTB has therefore developed a new measurement procedure for nanoparticles which is based on a scanning electron microscope (SEM) equipped with an additional transmission detector. Such equipment brings about a solution to a general problem which occurs when spherical nanoparticles are measured with high precision: the precise determination of the particle boundary, which – even in high-resolution electron microscopic
images – is somewhat “blurred”.

The question is: At which grey scale value does the particle begin, and which image pixel still belongs to the background? To determine this signal threshold level, a deeper understanding of how the image is created inside the electron microscope is necessary. The detector signal can be calculated by means of a Monte Carlo simulation which was developed at PTB and which takes into account the interactions of the electrons with the spherical particle and the properties of the detector. This has shown that
the signal threshold level at the edge of the particle depends on the properties (material and size) of the particle.

To be able to take into account these two properties, an automatic image evaluation algorithm has been developed. It computes iteratively for each single particle an individual signal threshold level for the particle's edge based on the simulation results. This enables an accurate determination of the size, adapted to the corresponding particle. Despite the complexity of the procedure, hundreds of images can be analysed in less than an hour. In addition, the total time needed to fully characterise a nanoparticle  sample is considerably reduced by the use of a newly developed routine which makes it possible to capture numerous images of nanoparticles.

Thanks to this new procedure, medium-sized particles of down to approx. 7 nm can be determined with low measurement uncertainties from 1 nm to 2 nm. The work was carried out within the framework of the European Metrology Research Project "Traceable characterization of nanoparticles" – in which also an extension of the procedure to non-spherical particles is aspired to. Within the scope of this project, also integral measurement procedures are developed by PTB in which lightand X-ray scattering at particle ensembles in an aqueous environment is measured in order to determine the size distribution of nanoparticles.