Electron tomography

In the field of biology, bright-field transmission electron microscopy (BF-TEM) and high-resolution TEM (HRTEM) are the primary imaging methods for tomography tilt series acquisition. However, there are two issues associated with BF-TEM and HRTEM. First, acquiring an interpretable 3-D tomogram requires that the projected image intensities vary monotonically with material thickness. This condition is difficult to guarantee in BF/HRTEM, where image intensities are dominated by phase-contrast with the potential for multiple contrast reversals with thickness, making it difficult to distinguish voids from high-density inclusions. Second, the contrast transfer function of BF-TEM is essentially a high-pass filter – information at low spatial frequencies is significantly suppressed – resulting in an exaggeration of sharp features. However, the technique of annular dark-field scanning transmission electron microscopy (ADF-STEM), which is typically used on material specimens,{{cite journal|author=B.D.A. Levin|display-authors=etal|title=Nanomaterial datasets to advance tomography in scanning transmission electron microscopy|journal=Scientific Data |year=2016|volume=3|issue=160041|article-number=160041 ==Atomic Electron Tomography (AET)==

Atomic level resolution in 3D electron tomography reconstructions has been demonstrated. With the aid of computational ptychography, identification and precise 3D coordinates of every single atom in tiny objects have been imaged, clearly depicting molecular structures at large and small scales. Reconstructions of crystal defects such as stacking faults, grain boundaries, dislocations, and twinning in structures have been achieved. This method is relevant to the physical sciences, where cryo-EM techniques cannot always be used to locate the coordinates of individual atoms in disordered materials. AET reconstructions are achieved using the combination of an ADF-STEM tomographic tilt series and iterative algorithms for reconstruction. Currently, algorithms such as the real-space algebraic reconstruction technique (ART) and the fast Fourier transform equal slope tomography (EST) are used to address issues such as image noise, sample drift, and limited data. ADF-STEM tomography has recently been used to directly visualize the atomic structure of screw dislocations in nanoparticles. and 20,000 atoms in a multiply twinned palladium nanoparticle. The combination of AET with electron energy loss spectroscopy (EELS) allows for investigation of electronic states in addition to 3D reconstruction. ==See also==