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Significance and Use

As AFM measurement technology has matured and proliferated, the technique has been widely adopted by the nanotechnology research and development community to the extent that it is now considered an indispensible tool for visualizing and quantifying structures on the nanoscale. Whether used as a stand-alone method or to complement other dimensional measurement methods, AFM is now a firmly established component of the nanoparticle measurement tool box. International standards for AFM-based determination of nanoparticle size are nonexistent as of the drafting of this guide. Therefore, this standard aims to provide practical and metrological guidance for the application of AFM to measure the size of substrate-supported nanoparticles based on maximum displacement as the probe is rastered across the particle surface to create a line profile.

Scope

The purpose of this document is to provide guidance on the quantitative application of atomic force microscopy (AFM) to determine the size of nanoparticles deposited in dry form on flat substrates using height (z-displacement) measurement. Unlike electron microscopy, which provides a two-dimensional projection or a two-dimensional image of a sample, AFM provides a three-dimensional surface profile. While the lateral dimensions are influenced by the shape of the probe, displacement measurements can provide the height of nanoparticles with a high degree of accuracy and precision. If the particles are assumed to be spherical, the height measurement corresponds to the diameter of the particle. In this guide, procedures are described for dispersing gold nanoparticles on various surfaces such that they are suitable for imaging and height measurement via intermittent contact mode AFM. Generic procedures for AFM calibration and operation to make such measurements are then discussed. Finally, procedures for data analysis and reporting are addressed. The nanoparticles used to exemplify these procedures are National Institute of Standards and Technology (NIST) reference materials containing citrate-stabilized negatively charged gold nanoparticles in an aqueous solution.