Electron microscopy

Electron microscopy – TEM and SEM - is the only analytical technique, which allows for direct imaging and chemical analysis with nanometer resolution or better. The recent advances in the field of TEM have brought forth a new generation of instruments which enable direct sub-Å resolution and give access to information about, for example, physical size and shape of single-core crystals, total size of multi-core particles, thickness of surface coating as well as chemical composition and atomic bonding and coordination on the subnanometer scale. Furthermore, the integration of tomography in the TEM also enables the reconstruction of 3D structures of multi-core particles with a spatial resolution down to around 1 nm. Using electron holography in the TEM it is also possible to obtain information about magnetic domains on the nano scale. Since electron microscopy provides direct imaging of nanoparticles no further modelling is required to interpret size and shape data. The well-defined atomic distances in the materials also give access to an internal calibration scale with Ångström accuracy for precise measurements of physical parameters. Analysis of particles ensembles is usually limited to a few hundred or thousands of particles depending on the information of interest. The electron microscope operates under a high vacuum and particles that are suspended in a liquid must be dried or prepared by special means before analysis. Particles with water permeable coatings may therefore suffer from shrinkage or structure alterations. 

 

By embedding the sample in a plastic resin or using cryo-techniques where the suspensions are instantaneously frozen and fixed the original state of the whole nanoparticle suspension may be retained. Complementary information on size, shape and aggregation can be achieved by SEM that provides sample's surface topography and composition.

  

These analysis methods give information of the physical size and shape of single-core crystals, total size of multi-core particles, thickness of surface coating (using low energy electrons), 3D-pictures of particles and material analysis (phases). The minimum number of particles to be counted for determining mean size and size-distributions will be analysed as well as the type of sample holder and sample preparation that can alter the aggregation state of the particles in the TEM holder respect to the state in suspension.