Novel nano-tomography methods

Materials Today 10, 12, 18–25 (2007)

Handbook of Nanophysics, chapter “Tomography of Nanostructures”, Taylor and Francis (2010).

(i) Electron tomography for nanocrystals using incoherent electrons
In a nutshell: For computed tomography, the recorded image intensity must be proportional to the projection of object-density for the reconstructed quantity. Crystalline nanoparticles in bright field TEM often fail this requirement due to coherent Bragg scattering, which renders projections very dark if the tilt angle suddenly hits a Bragg angle condition. Incoherent imaging such as energy filtered TEM tuned to a single inelastic energy loss can overcome this problem.

See APL 79, 1369 (2001).

(ii) Spectroscopic electron tomography (4D ET) using EELS/EFTEM
In a nutshell: Incoherent electron tomography as of (i) can be extended to become a 3D chemical mapping technique, by acquiring several tomograms at various spectroscopic energy values. Distribution of chemical elements in 3D and exclusive reconstruction of particles from one chemical identity only are the main advantages. This is a first step to four-dimensional tomography with x,y,z, and E (energy, spectral coordinate) as dimensions. Both high-energy core-loss and low-loss plasmon spectral regions are usable.

See: APL 79, 1369 (2001), Ultramicroscopy, 96 , 433 - 451 (2003);

(iii) Electron induced x-ray emission tomography
In a nutshell: As an alternative to EFTEM-based spectroscopic tomography, energy dispersive X-ray mapping can be used for projection imaging. This technique allows for larger thicknesses of nano-objects than most other tomographies before losing its linearity. In analogy to PIXE-T the technique can be labelled EIXE-T. The main advantage is the simultaneous acquisition of many spectroscopic energies (4D tomography) without extra time or sample damage.

See Ultramicroscopy, 96, 433–451 (2003), Applied Physics Letters 91 , 251906 (2007).

(iv) Focused ion beam tomography
In a nutshell: Surfaces of samples are imaged by secondary electrons in a focused ion beam instrument. To obtain a 3D view, successive layers are milled away slice-by-slice and reimaged. The multiple slice-images are interpolated to reconstruct a 3D volume. Operating on micron-sized samples with resolution of ~ 10nm, the method fills the important gap in resolution and field-of-view between electron tomography in TEM on one hand and x-ray computed tomography on the other hand.

See Scripta Mater, 45, 7, 753-758 (2001); J. Microscopy 201, 212-220 (2001); Mat.Res.Soc.Symp.Proc., 649, (2000).

(v) Low-depth of focus 3D imaging and stereo-imaging in 3D
In a nutshell: Highly converging STEM nanobeams are explored, mainly by simulation, for their suitability of generating depth-sensitivity within a sample, which allows to map the z-coordinates of object details.

See Ultramicroscopy, 96 , 285 - 298 (2003); Proc. 16th Int.Microsc.Congr., Sapporo, Japan, 2, 949, (2006)

(vi) Tomographic nanofabrication
In a nutshell: 3D top-down nanostructuring by electron beam ablation is combined with 3D electron tomography reconstruction using the same specimen holder and tilt geometry. Fabrication and characterisation thus become part of the same experiment. As example, a metal wire tip is sharpened in 3D to unprecedented levels of radius of curvature.

See Appl.Phys.Lett 93 153102 (2008)

(vii) Quantitative electron tomography Reconstructed 3D volumes of a nanocomposite material (glass-oxide precipitate system) are post-processed to reveal stereological data and composite parameters such as fill factor, surface-volume ratio, surface shape morphology, etc… Tomographic results are compared to estimates that can be extrapolated approximately from single projections.

See Mater.Res.Soc.Symp.Proc., 1107, 239-245 (2008); PhysRev B 78 205428 (2008)

(viii) Geometric nanotomography, atomic ET, and hybrid tomographies
The method of Geometric Tomography, well established in macroscale CT, is introduced to nanoparticle matter and applicability evaluated. This method ignores projected intensities and only uses particle silhouettes at various tilt angles for reconstruction. In comparison with quantitative ET, a continuous transition from geometric CT to quantitative CT is derived by tuning the non-linear intensity-transfer characteristics.
In Atomic ET, the method of standard ET is explored at atomic resolution using HRTEM image simulations for very thin model particles, and reconstructions are evaluated depending on acceleration voltage and level of aberration correction.
In Hybrid ET, two complementary, originally separate, modes of electron tomography are superimposed to generate one single composite reconstruction result, such as to combine advantages of the two techniques (one technique might have superior resolution, another one superior chemical sensitivity). Examples demonstrated are the combination of atomic ET and geometric ET, or the overlay of plasmon-EFTEM ET with geometric ET;

See Journal of Microscopy 232, 186–195 (2008); Journal of Applied Physics, 106, 024304 (2009); Mater. Res. Soc. Proc. 1184 1184-HH02-03, 139–144 (2009)