Acquisition and evaluation procedure to improve the accuracy ofSAED

Abstract

The achievement of this work is that fine tuning of experimental and evaluation parameters can improve the absolute accuracy and reproducibility of selected area electron diffraction (SAED) to 0.1% without using internal standard. Due to the proposed procedure it was possible to reach a reproducibility better than 0.03% for camera length between sessions by careful control of specimen height and illumination conditions by monitoring lens currents. We applied a calibration specimen composed of nanocrystalline grains free of texture and providing narrow diffraction rings. Refinements of the centre of the diffraction pattern and corrections for elliptic ring distortions allowed for determining the ring diameters with an accuracy of 0.1%. We analyze the effect of different error sources and reason the achieved absolute accuracy of the measurement. Application of the proposed evaluation procedure is inevitable in case of multicomponent nanocomposites or textured materials and/or having close diffraction rings where application of automated procedures is limited. The achieved accuracy of 0.1% without internal standard is approaching that of routine laboratory XRD, and reduction of instrumental broadening due to the elaborated evaluation procedure allows for separation of close reflections, provides more reliable ring width and thus improved input parameters for further nanostructure analysis as demonstrated on dental enamel bioapatite.

Improved Method for Electron Powder Diffraction-Based Rietveld Analysis of Nanomaterials

“…During the analysis, our approach was to reduce the number of refined parameters as much as possible. By applying standard acquisition conditions, the variation in camera length was below 0.1% [ 3 ], so the correction for detector distance/camera length could be skipped. Data pre-processing before obtaining the integrated intensity profile allowed the diffraction pattern center to be found and corrected ellipticity with high accuracy [ 20 ]; thus, no further refinement of the center displacement or tilt error was needed.…”

“…The correct alignment of the TEM ensures not only calibration accuracy and the reproducibility of camera length but also the minimization and steadiness of instrumental peak broadening. We expect that the combination of the standard acquisition procedure [ 3 ] with the in-TEM determination of instrumental broadening will promote a more reliable and sophisticated analysis of nanostructure parameters of complex nanomaterials, like the preferred orientation of multicomponent thin films or anisotropic crystallite size in biological nanomaterials.…”

“…SAED patterns were recorded using the highest (4 k × 4 k) camera resolution with special care of avoiding saturation and keeping the intensity in the linear range of the detector. SAED patterns were taken as detailed in [ 3 ]. Measurements were carried out at a 650 mm nominal camera length, which provided scattered intensity at a large enough angular range for Rietveld refinement.…”

“…Selected area electron diffraction (SAED) measurements can be performed on nanocrystalline thin film areas even below 100 nm of the diameter or using extremely small quantities of nanopowder (electron powder diffraction, EPD) and provide sufficient signal and excellent signal-to-noise ratio for quantitative analysis on the second timescale. The reproducibility and accuracy of electron diffraction can be compatible with those of routine laboratory X-ray diffraction (XRD) measurements, provided that acquisition parameters are properly controlled and the resulting SAED patterns are calibrated [ 3 ]. Additionally, EPD or SAED is complementary to other local transmission electron microscopy (TEM)-based characterization techniques like high-resolution transmission electron microscopy (HRTEM) or nanobeam diffraction because they provide nanostructure information of statistical relevance.…”

“…As an alternative approach, in this work, we present a single step in TEM procedure, which allows the instrumental broadening function of TEM to be obtained using a multilayer graphene calibration sample by a single SAED measurement. This procedure is based on our previous work on SAED calibration [ 3 ], exploiting its reproducibility, the achievable ±0.1% absolute accuracy of SAED measurements, and the minimization of instrumental broadening. The potential of this procedure is demonstrated on nanopowder and nanocrystalline thin film specimens.…”