  Insight in the structure of nanoparticle assemblies up to a single particle
level is key to understand the collective properties of these assemblies, which
critically depend on the individual particle positions and orientations.
However, the characterization of large, micron sized assemblies containing
small, 10-500 nanometer, sized colloids is highly challenging and cannot easily
be done with the conventional light, electron or X-ray microscopy techniques.
Here, we demonstrate that focused ion beam-scanning electron microscopy
(FIB-SEM) tomography in combination with image processing enables quantitative
real-space studies of ordered and disordered particle assemblies too large for
conventional transmission electron tomography, containing particles too small
for confocal microscopy. First, we demonstrate the high resolution structural
analysis of spherical nanoparticle assemblies, containing small anisotropic
gold nanoparticles. Herein, FIB-SEM tomography allows the characterization of
assembly dimensions which are inaccessible to conventional transmission
electron microscopy. Next, we show that FIB-SEM tomography is capable of
characterizing much larger ordered and disordered assemblies containing silica
colloids with a diameter close to the resolution limit of confocal microscopes.
We determined both the position and the orientation of each individual
(nano)particle in the assemblies by using recently developed particle tracking
routines. Such high precision structural information is essential in the
understanding and design of the collective properties of new nanoparticle based
materials and processes.
