The FEI Nova 400 nanoSEM has a combination of features which allows for superior imaging quality at nanoscales. In addition to the FEG source which provides a smaller and more stable emitter than a tungsten thermionic emitter, there are special detector configurations that are ideal for high resolution work at small working distances.
The ET-detector works optimally for larger working distances because of the larger solid angle captured by the biased cage on the end the detector. For small working distances, a through lens detector (TLD) is better, as a larger solid angle can be captured by the opening of the objective lens and directed into the secondary electron detector (SED)-- the scintillator and photo-multiplier shared with the ET-detector, or in FEI jargon, ETD. To increase secondary electron detection efficiency, field immersion can be applied. In this case the field of the objective lens is allowed to penetrate into the space around the sample. Electrons generated within this region are directed into the objective lens (OL) and into the TLD.
The result is extraordinary resolution. In this image of Au clusters on HOPG taken at 15 kV with TLD in field immersion, the facets on and grain boundaries between individual Au-clusters are visible, as are clusters as small as 10 nm.
Much higher resolutions are also possible. The bottom image is of NIST SRM 8012-- 30 nm Au particles. This SRM was diluted and drop cast right onto a standard Al stub and imaged directly. n-mers of spherical particles are readily discernible, remarkable as TEM is generally required for structures of this size.