Nanoparticle metrology with atomic force microscope (AFM) aims to determine an average particle size from measurements of individual nanoparticles derived by image analysis. This constrains the statistical relevance of the measurement due to the limited number of particles which can be practically imaged and analyzed. Consequently, the number density of particles on samples prepared for particle measurement is an important parameter of sample preparation. A number density that is too low makes it difficult to obtain sufficient measurement statistics, whereas a number density that is too high can result in particle agglomeration on the substrate and limit the area of uncovered substrate that is required to obtain a reliable reference for measuring the particle height.
We present imaging and measurement results of a particle number density gradient of 16 nm gold nanoparticles deposited using a gradual immersion process. Results demonstrate how samples with particle number density gradients can facilitate identification of an area on a sample with optimal particle number density for AFM particle metrology and thereby improve measurement efficiency and reliability.
Nanoparticles and products incorporating nanoparticles are a growing branch of nanotechnology industry. They have
found a broad market, including the cosmetic, health care and energy sectors. Accurate and representative determination
of particle size distributions in such products is critical at all stages of the product lifecycle, extending from quality
control at point of manufacture to environmental fate at the point of disposal. Determination of particle size distributions
is non-trivial, and is complicated by the fact that different techniques measure different quantities, leading to differences
in the measured size distributions.
In this study we use both mono- and multi-modal dispersions of nanoparticle reference materials to compare and contrast
traditional and novel methods for particle size distribution determination. The methods investigated include ensemble
techniques such as dynamic light scattering (DLS) and differential centrifugal sedimentation (DCS), as well as single
particle techniques such as transmission electron microscopy (TEM) and microchannel resonator (ultra high-resolution
Atomic force microscopy (AFM) can provide a link in the traceability chain between dimensional measurement
techniques for nanoparticles, such as dynamic light scattering and differential centrifugal sedimentation, and the
realization of the definition of the SI metre. Despite the size of nanoparticles being well within the resolution range of
typical AFMs, the accurate measurement of nanoparticles with AFM presents a number of challenges. One of these
challenges is the number density of particles deposited on substrates for AFM imaging and measurement. If the number
density is too low, it is difficult to obtain adequate measurement statistics, whereas a number density that is too high can
result in particle agglomeration on the substrate and make it difficult to image sufficient substrate area to obtain a
reliable reference for height measurements. We present imaging and measurement results of 16 nm gold nanoparticles
deposited on a substrate functionalized to produce a surface with a number density gradient across the sample. This
substrate functionalization shows great potential for achieving reliable and efficient nanoparticle metrology with AFM.
We give an overview of the design and planned operation of the metrological Scanning Probe Microscope (mSPM)
currently under development at the National Measurement Institute Australia (NMIA) and highlight the metrological
principles guiding the design of the instrument. The mSPM facility is being established as part of the nanometrology
program at NMIA and will provide the link in the traceability chain between dimensional measurements made at the
nanometer scale and the realization of the SI meter at NMIA. The instrument will provide a measurement volume of
100 μm × 100 μm × 25 μm with a target uncertainty of 1 nm for the position measurement.
Three different methods for extracting zinc oxide (ZnO) and titanium dioxide (TiO<sub>2</sub>) nanoparticles from commercially
available sunscreen were investigated to determine the most appropriate route for producing a sample suitable for
measuring the primary particle size. Direct dilution of the formulation, centrifugal methods and chemical washing were
trialed in combination with ultrasonic processing and surfactant addition to generate samples that are suitable for particle
size analysis. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) were used to monitor the
extraction and re-dispersion process. Washing with hexane, methanol and water to remove the formulation, in
combination with pulsed high-powered ultrasonication and the addition of a charge-stabilizing surfactant was found to be
the most efficient way of producing de-agglomerated samples. DLS measurements gave average hydrodynamic particle
diameters of 87 nm for ZnO and 76 nm for TiO<sub>2</sub>, compared to equivalent spherical particle diameters of 21 ± 12 nm for
ZnO (81 particles) and 19 ± 14 nm for TiO<sub>2</sub> (81 particles) obtained from TEM analysis.
ZnO nanoparticles are a challenging material to disperse and stabilize due to their high density, tendency to aggregate
and chemical properties. Manufactured ZnO nanoparticles often posses a high degree of size and shape dispersity, adding
additional complexity to both sample preparation and subsequent characterization. In this paper, procedures for
achieving stable and representative dispersions of ZnO nanoparticles from commercially available sources are discussed,
and the average particle size determined from dynamic light scattering measurements is qualitatively evaluated against
transmission electron microscopy images. The results highlight a number of important issues that need to be taken into
consideration when performing a metrological assessment of particle sizes and size distributions in such systems.
In this study, the non-linear errors in a commercial heterodyne interferometer are investigated. There are two types of cyclic nonlinearities present in heterodyne interferometers and it is desirable to be able to measure these nonlinearities in order to quantify the uncertainty of the interferometer setup. The current study investigates whether the nonlinearities can be detected by measuring the optical power of the interferometers output signal as a function of its phase. In theory, the optical power can be described as a perfect circle in polar coordinates in the absence of cyclic errors. The cyclic errors present, then manifest themselves as ellipticity of this circle and a translation of its centre. In this study large cyclic nonlinearities were deliberately introduced into a standard heterodyne interferometer setup, making them large enough to
measure directly from the displacement data. Comparison with predicted nonlinearities calculated from the optical power data showed a good fit, indicating that it is possible to predict cyclic nonlinearities by reading the optical power from the measurement board.