Development of a new method for the traceability of the nanoparticle size measurements by AFM and SEM

N. Feltin, A. Delvallée, S. Ducourtieux, C. Ulysse
Laboratoire national de métrologie et d'essais, FR

Keywords: AFM, SEM, nanoscale materials characterization

Summary:

The commercial products containing nanomaterials are already a part of our everyday life and new products enter the market regularly. But the nanometrology, i.e., science of the measurement at the nanometer scale, is still in its early stages. However, all actors concerned by nanoscience and nanotechnologies in the industrial sector or academic world agree to say that the development of a metrology dedicated to nanomaterials would have a determining catalytic effect on the growing of nanotechnology market. Furthermore, in a near future, the metrology institutes will have to provide traceable measurement methods in order to enhance the reliability of results concerning the risk assessment of nanomaterials. Indeed, several review papers show the lack of reliable measurements prevents from drawing conclusions about the toxicity of nanoparticles. In this context, the French metrology institute, LNE (Laboratoire National de métrologie et d’Essais) has been developing a platform, called CARMEN, dedicated to the metrology of nano-objects for five years. This platform is capable of measuring all the parameters characterizing a nanoparticle such as defined by ISO/TC 229: size, distribution in size, shape, crystal structure, agglomeration/aggregation state, specific surface area, surface charge… This activity is currently focused on the development of the traceability of dimensional measurements. In particular, we propose a traceable method for measuring the dimensions of a nanoparticule by combining Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Indeed, AFM imaging makes it possible to measure the height of a nano-object with a sub-nanometric accuracy, but the lateral measurements (along X and Y axes) contain large errors due to the tip/sample interaction. Deconvolution algorithms are currently developed in some metrology institutes but they are relatively heavy to implement and the obtained measurement uncertainties are still large. We propose an alternative method by using a SEM for measuring the lateral dimensions of the nano-object. Thus, we take advantage of the complementary nature of both techniques. This method has been tested with the measurement of spherical nanoparticules. Firstly, the traceability of the measurements performed with AFM and SEM is ensured by a calibration process implementing the LNE’s metrological atomic force microscope (mAFM) [1]. The latter is the key instrument in the traceability route of the nanodimensional measurements at the national level. Then, the uncertainty budget has been established for each microscopy techniques. The comparison of both methods and their complementarity has been enabled by using reference nanoparticules and a location system. Indeed, after imaging a group of nanoparticles, it is impossible to find the same population with another instrument. That’s the reason why substrates have been specially marked with lithographied crosses. The nanoparticules are deposited on this substrate and can be easily localized by both instruments through these repositioning patterns. A method of sampling has also been developed in order to obtain highly dispersed nanoparticles on the substrate. This condition is required to minimize the measurement errors by microscopy. Finally, a semi-automatic software has been developed for counting only the isolated nanoparticles and measuring their size. This program makes it possible to exclude all particle agglomerates or artefacts, which may affect the measurements. An optimized histogram of size distribution is then obtained and these measurements are directly traceable to the International System of Units (SI). [1] S. Ducourtieux and B. Poyet, “Development of a metrological atomic force microscope with minimized Abbe error and differential interferometer-based real-time position control”, Measurement Science and Technology 22 (2011) 094010.