Nanoparticle characterisation instrumentation. Nanometrology Section Physical Metrology Branch National Measurement Institute Australia

Transcription

1 Nanoparticle characterisation instrumentation Nanometrology Section Physical Metrology Branch National Measurement Institute Australia November 2012

2 Characterisation techniques Choice of measurement technique and knowledge of its limitations:

3 Dynamic light scattering (DLS) magipics.com.au Size range Measurand Advantages Limitations ~0.1 nm to ~ 6 µm. Autocorrelation function of the scattered light intensity; average hydrodynamic diameter. Fast and accurate for monomodal suspensions. An ensemble measurement technique, providing a good statistical representation of the sample. Particles must be in suspension and undergoing Brownian motion. Large particles scatter much more light (I x 6 ); even a small number of large particles can obscure the contribution from smaller particles. DLS results for suspensions containing different fractions of 20 nm and 100 nm PSL particles.

4 Differential centrifugal sedimentation (DCS) magipics.com.au DCS measurement of a suspension of PSL particles showing signs of aggregation. The inserts show scanning electron micrographs of different clusters of n individual particles with n=1...6 observed in this sample. Size range Measurand Advantages Limitations < 5 nm to ~30 µm. Light extinction as a function of sedimentation time, from which the intensity weighted distribution of the Stokes diameter can be derived. Very high resolving power because the particles are separated by their sedimentation rate before detection. An ensemble technique, providing a good statistical representation of the sample. Small and/or low density particles sediment over long time scales. The particles are detected optically, thus their optical properties must be known to derive volume or number based size distributions.

5 Transmission electron microscopy (TEM) Size range ~ 0.1 nm to ~ 2 µm. Measurand Equivalent diameter, expressed as the diameter of a circle having the same area as the projected area of the particle, or Feret s diameter, the mean value of the distance of parallel tangents of the projected outline of the particle position. Advantages Excellent for visualizing the sample and to produce representative EM images. Provides information about particle shape, size and size distribution. Enables visualisation of the degree of aggregation/agglomeration. Possibility to investigate crystalline phase and perform chemical analysis. Single particle technique. Limitations Poor statistical representation of a sample. Imaging and analysis can be time consuming. Sample preparation can be challenging. Au nanorods 30 nm Au 60 nm Au and 20 nm SiO2 Nominally 20 nm SiO2 particles analysed for particle size distribution using ImageJ. ZnO

6 Asymmetric flow field flow fractionation (AFFFF) magipics.com.au Size range Measurand Advantages Limitations ~ 0.1 nm to ~ 2 µm. Detector dependent, often equipped with light scattering detectors, e.g., for static and dynamic light scattering. Elution time can also be used to provide a measure of particle size. A separation measurement technique. Very high resolution for both high and low molecular weight particles. Provides sequential separation of particles based on a size dependent interaction of the particles with an applied force field (flow). Fractions can be collected for off line processing. It is also possible to hyphenate to, for example, ICP MS. Complex method development required to optimise particle separation.

7 Particle tracking analysis (PTA) Particles scatter in the laser beam Liquid Particles to be viewed are suspended in liquid NanoSight Ltd. Laser beam (approx 50 µm wide) Glass Metallised surface Size range Measurand Advantages Limitations ~20 nm to ~ 1 µm. Diffusion length based on 2 D tracking of particles moving under Brownian motion. Qualitative differentiation between particles of different composition based on scattering intensity. Allows measurements of particle number concentration (particles/ml). Single particle measurement technique. Strong dependence on operator through choice of settings for imaging and analysis. Limited statistical relevance due to limited number of particles analysed. Mix of 100 nm PS and 100 nm Au

8 Examples of Reference Materials NIST Au PSL AFM DLS TEM SEM SAXS DMA EC JRC IRMM SiO2 DLS DCS TEM/SEM SAXS Zeta potential* * indicative value only Thermo Scientific PSL DLS (20 50 nm) TEM (> 50 nm) See refmat.bam.de/en/ for a (non exhaustive) list of currently available nanoscale reference materials.

9 Take home messages: Nanoparticle characterisation is important. Understand your instrumentation. Use well characterised particles to check instrument performance. Bi modal or multi modal mixes of particles may give a better understanding of the instrument limitations.

10 Take home messages: Characterise your particle suspension before, during and after other experiments. Important to pose the right question, e.g.: What is the primary particle size? What is the size distribution of the agglomerates/aggregates? Use more than one technique. You can never know too much about a sample!

11 NMIA Nanometrology team: Bakir Babic, Heather Catchpoole, Victoria Coleman, Chris Freund, Jan Herrmann, Åsa Jämting, Malcolm Lawn, Maitreyee Roy and John Miles. Dr Jan Herrmann Nanometrology Section National Measurement Institute Bradfield Road West Lindfield NSW 2070 Australia