Traceable measurement of nanoparticle size using transmission scanning electron microscopy (TSEM)

Nanoparticles are increasingly applied in science and technology. Possible associated risks and regulatory demands emphasize the need for versatile and traceable methods for nanoparticle characterization. We developed a method which uses a scanning electron microscope operated in transmission mode (TSEM) to enable nanoparticle size measurements which are traceable to the SI unit ‘metre’. Traceable TSEM measurements rely on a calibration of the instrument and an appropriate physical model of the image formation process. To determine the particle boundary in TSEM images we use a threshold approach. The correct threshold level is calculated using a Monte Carlo simulation of the image generation taking into account the 3D shape and composition of the nanoparticles as well as relevant properties of the instrument. An automated image analysis tool enables the analysis of a series of TSEM images containing thousands of particles to attain particle size distributions. As examples, nanoparticles of three different material classes with sizes ranging from about 7 to 200 nm have been measured. The expanded uncertainty of the mean diameter is between about 1.5 to 5 nm.