Three‐dimensional nanostructure determination from large diffraction dataset
Authors: Meng Yifei , Zuo Jian Min 
Published: December 2016

Electron tomography relies on mass‐thickness contrast. However, chemically homogeneous nanostructures can not be studied using this technique. Nanocrystalline materials in general are structurally featured by a large density of grain boundaries and large surface/volume ratio, which have attracted significant interest for their unique mechanical, chemical and electronic properties.
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Transmission electron diffraction (TED) is an appropriate technique for complex nanostructure analysis because it is highly sensitive to local structure and it can be obtained using a small electron beam.

Previously we have developed a TEM based scanning electron nanodiffraction (SEND) technique that uses the built‐in TEM deflection coils to shift the beam.
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Here, we report a new technique called 3D‐SEND by coupling the SEND with diffraction tomography. This technique aims at determining the 3D morphologies and orientations of grains in nanocrystalline materials. A special holder design is employed for a high‐angle (up to 87°) sample rotation.
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The design employs a needle‐shape specimen and a sample rotation driven by the goniometer itself. Diffraction pattern recording and beam scanning are automated using a DigitalMicrograph
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script to control the TEM deflection coils and camera readout.
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A stack of diffraction patterns are acquired for each sample rotation step.

The reconstruction starts with the identification of 2D grain morphologies, using dark‐field images generated with diffraction spots. Similar contrasts are grouped using the normalized cross‐correlation. Diffraction spots belong to one grain are sorted into one diffraction pattern. Electron diffraction pattern indexing is achieved by a combination of diffraction peak search and peak indexing using both length and angle information. The tomographic reconstruction of the grain is performed using the algebraic reconstruction technique. We apply prior conditions concerning the sample outline and the damping of scattering intensity. A smoothed isosurface is created to illustrate the grain's 3D morphology. The grain orientation is determined from the indexing results.

We demonstrate the performance of 3D‐SEND on a TiN thin‐film nanocrystalline sample prepared by unbalanced magnetron sputtering. The FIB cut and lift‐out was performed perpendicular to the growth direction. The sample was milled to a tip with a diameter of 200 nm. The beam was set to 7nm in FWHM. The scanning covered a 2626 pixels area and each step is 11nm. The sample was tilted over a range of ±85° with a step of 5°. In total 23660 DPs are recorded. Figure 1 shows the reconstruction results of seven major grains and their orientations. The reconstruction result is shown in Fig. 1.

The spatial resolution of this technique is ultimately limited by the electron probe size under the column approximation. For a JEOL 2100 TEM, we can form a probe with a FWHM of 2.3 nm using a 10 um condenser aperture in the CBD mode with alpha = 1. Compared with the alternative technique 3D‐OMiTEM,
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3D‐SEND has a better diffraction pattern resolution, a wider sample rotation range and a much lower electron dose. Our approach also grants a combinative study with other techniques such as atom‐probe tomography and in‐situ deformation. In the future, 3D‐SEND may potentially be improved for the five‐parameter characterization of grain boundaries.