Three-dimensional microscopy for multi-scale imaging: from nano to macro

The structures studied in soft material science and biology are often hierarchical. One typical example of such hierarchical structures in polymer physics is a semi-crystalline region inside non-branched linear polymers, called spherulites [1]. Transmission electron microtomography (or simply electron tomography, ET) is known to be a very powerful tool in three-dimensional (3D) structural studies [2]. It has been extensively used to investigate polymer nano-morphologies [3,4], biological specimens [5], metallic alloys [6], etc. One of the biggest drawbacks of ET is that their observable thickness of specimens for 3D that is limited to less than ∼200 nm (in the case of accelerating voltage of 100–200 kV). In other words, structural elements with sizes on the order of several tens of nanometers are most suitable for ET. 3D observation of larger entities with sizes on the order of several hundreds of nanometers in hierarchical structures requires larger volume, i.e. a specimen with a greater thickness, i.e. the volume with a few micrometers. Recently, it has been shown that electron tomography with a scanning transmission mode, i.e. scanning transmission electron microscopy (STEM), is useful for the 3D observation of micrometer-thick specimens or ‘meso-scale’ volumes [7]. Hereafter, we refer to electron microtomography with STEM as scanning transmission electron microtomography (scanning electron tomography, SET). Although the resolution degradation due to the chromatic aberration is less pronounced because of the absence of an imaging system in STEM, however, multiple scattering of the electrons inside specimens commonly occurs both in STEM and TEM. The process of multiple scattering and accompanying beam broadening in STEM is rather complicated, particularly in thick specimens. Though the beam broadening could be another major source of the resolution degradation, this effect has not been discussed. As multiple scattering (and thus the beam broadening) strongly depends on the scattering angle, the detection angle (of the detector) should be a key parameter in SET. In the present talk, we discuss the detection-angle dependence of SET with particular emphasis on the beam broadening using a polymeric resin of thickness with ∼1 µm as a standard sample. Exactly the same volume of the polymeric specimen was observed by three different electron tomography modes. The difference between ET and SET together with some other 3D techniques has also been investigated [8].