Nanoparticle size and 3D shape measurement by electron tomography: An Inter-Laboratory Comparison

Abstract

Electron tomography (ET) has been used for quantitative measurement of shape and size of objects in three dimensions (3D) for many years. However, systematic investigation of repeatability and reproducibility of ET has not been evaluated in detail. To assess the reproducibility and repeatability of a protocol for measuring size and three-dimensional (3D) shape parameters for nanoparticles (NPs) by ET, an inter-laboratory comparison (ILC) has been performed. The ILC included six laboratories and six instruments models from three instrument manufacturers following a standard measurement protocol. A technical specification describing the normative steps of the protocol is published by the International Standards Organization (ISO). Gold NPs with 30 nm nominal diameter contained within a rod-shaped carbon support were measured. The use of a rod-shaped sample support eliminated the missing wedge effect in the experimental tilt series of projected images for improved quantification. A total of 443 NPs were initially measured by NRC-NANO and then 115 out of the 443 NPs were measured by five other labs to compare measurands such as the Volume (V), maximum Feret diameter (F_max), minimum Feret diameter (F_min), volume-equivalent diameter (D_eq) and aspect ratio (F_rat) of the NPs. The results of the five labs were compared with the results obtained at NRC-NANO. The maximum disagreement in measurements of F_min and F_max obtained by the participating labs did not exceed 7 %. The measured D_eq was between 27.5 nm and 30.3 nm in agreement with the NP manufacturer's specification (28 nm-32 nm). In addition to the above, the influence of the missing wedge effect and beam-induced NP movement was quantified based on the differences of the results between labs.

Keywords: Beam-induced nanoparticle movement; Feret diameter; Image alignment; Inter-laboratory comparison (ILC); Missing wedge; Nanoparticles; Nanoscale 3D morphology; Quantitative electron tomography; Statistical analysis; Transmission electron microscope (TEM).