Image quality assessment and image fusion for electron tomography

Aberration‐corrected scanning transmission electron microscopy (STEM) is providing previously unattainable views of materials at the atomic scale. The quality of STEM data is now often limited by environmental and experimental factors instead of instrument factors (e.g. electron optics). Some of these environmental limitations can be overcome by collecting and processing STEM data using new data science techniques. These techniques expose new atomic‐scale materials information by improving our ability to measure atomic column positions, 3D structure, single point defects, and atomic‐scale composition. The precision in locating atomic column positions in STEM images is fundamentally limited by the image signal to noise ratio (SNR), but typically practical limits including sample and microscope instabilities that produce distortions in STEM images are encountered before reaching the SNR limit. We have developed a non‐rigid registration (NRR) technique that corrects image distortion of all length scales and enables averaging to enhance the SNR.[1–2] Sub‐pm precision images of single crystal materials have been achieved by NRR and averaging high angle annular dark field (HAADF) STEM images. NRR has allowed measurements of pm‐scale bond length variations of Pt nanocatalyst atoms that may help explain their catalytic activity. Figure 1 shows NRR and averaged HAADF STEM data of a Pt nanocatalyst on an alumina support from a side‐view that exhibits ˜2 pm precision. Displacement measurements of each atomic column position reveal moderate Pt surface bond length contraction and strong but localized strain of Pt atoms near nanoparticle‐support interface. Some of the interface strain is transferred up the twin boundary. Determining the three dimensional atomic structure of materials from two dimensional S/TEM images is a major hurdle. The standardless atom counting technique is one promising route to measure local sample thickness by quantitatively comparing experimental and simulated HAADF STEM images and can be used to deduce 3D structure.[3] Unlike previous examples, NRR and averaging STEM images have allowed standardless atom counting with the uncertainty no longer dominated by Poisson noise [1]. This should allow the unique determination of the number of atoms in atomic columns, although this has not yet been demonstrated due to other sample limitations. Point defects are critical to the properties of a wide range of materials, but imaging single defects is challenging. Quantitative STEM has allowed imaging single substitutional and interstitial dopant impurity atoms[4], but experimentally imaging single vacancies has remained elusive. We have used HAADF STEM frozen phonon multislice simulations to predict the detectability of La vacancies in LaMnO 3 by the reduced atomic column intensity and atomic column distortions around the vacancy. NRR and averaging HAADF STEM images of LaMnO 3 improves the SNR and the image precision sufficiently to potentially detect single La vacancies. Experimental images contain candidate single La vacancies that have local atomic column distortions and intensity variations which match simulated predictions. Atomic‐resolution composition maps can be created using STEM energy dispersive x‐ray spectroscopy (EDS) spectrum imaging (SI). However, long total dwell times that may introduce spatial distortions are required because of low x‐ray production and collection efficiency. The most common approach to minimize distortions is to sum multiple SIs using online drift‐correction software that discards the individual SIs and HAADF images. The quality of EDS SIs of a Nd 2/3 TiO 3 sample was improved by saving the simultaneously acquired raw HAADF and EDS SI series, and applying post acquisition NRR and averaging. The resulting elemental maps show less spatial distortions and more atomic localization of x‐rays. In addition, a novel non‐local principle component analysis further enhances the quality of EDS SIs compared to conventional denoising methods.[5]

Gold coverage and faceting of MBE grown silicon nanowires

An Al3Mn-type Al3(Mn, Pd) crystal and an Al–Mn–Pd decagonal quasicrystal (DQC) in an Al70Mn20Pd10 alloy are studied using a spherical aberration (Cs)-corrected scanning transmission electron microscope (STEM) with high-angle annular dark-field (HAADF) and annular bright-field (ABF) techniques, together with atomic-resolution energy dispersive X-ray spectroscopy (EDS). Mn and Pd atomic positions in the Al3(Mn, Pd) structure projected along the b-axis (pseudo-tenfold rotational axis) are represented by separate bright dots in observed HAADF-STEM images. Besides, Al as well as Mn and Pd atomic positions are represented as dark dots in ABF-STEM images. Most Mn and Pd atomic positions in the Al3(Mn, Pd) structure can be observed on atomic-resolution EDS maps. On the basis of the good correlation between the STEM images and the EDS maps, and also considering the structure of the Al3(Mn, Pd) crystal, which was determined by X-ray diffraction using a single crystal, observed HAADF and ABF-STEM images of the Al–Mn–Pd DQC have been interpreted. Pd and Mn atomic positions in the Al–Mn–Pd DQC can be detected on the observed EDS maps. It can be seen that Pd is enriched around the centre of the columnar clusters, having a decagonal section with 2 nm in diameter. It can therefore be concluded that Pd plays an important role in the stabilization of the decagonal clusters, which form the Al–Mn–Pd DQC structure.

Carbon-based nanofiber and microfiber cathodes exhibit very low voltages for the onset of electron emission, and thus provide exciting opportunities for applications ranging from high power microwave sources to field emission displays. CsI coatings have been experimentally shown to lower the work function for emission from the fiber tips, although little is known about the microstructure of the fibers themselves in their as-received state, after coating with CsI, or after being subjected to high voltage cycling. Longitudinal cross sections of the original, unused CsI-coated fibers produced by focused ion beam lift-out revealed a nanostructured graphitic core surrounded by an amorphous carbon shell with submicron sized islands of crystalline CsI on the outer surface. Aberration-corrected high resolution electron microscopy (HREM) of the fiber core achieved 0.10 nm resolution, with the graphite (200) clearly visible in digital fast Fourier transformations of the 2–4 nm highly ordered graphitic domains. As the cathode fibers are cycled at high voltage, HREM demonstrates that the graphitic ordering of the core increases with the number of cycles, however the structure and thickness of the amorphous carbon layer remains unchanged. These results are consistent with micro-Raman measurements of the fiber disordered/graphitic (D/G) band ratios. After high voltage cycling, a uniform ∼100 nm film at the fiber tip was evident in both bright field transmission electron microscopy (TEM) and high angle annular dark field scanning TEM (STEM). Low-dose electron diffraction techniques confirmed the amorphous nature of this film, and STEM with elemental mapping via x-ray energy dispersive spectroscopy indicates this layer is composed of CsIO. The oxidative evolution of tip composition and morphology due to impurities in the chamber, along with increased graphitization of the fiber core, contributes to changes in emission behavior with cycling.

Atomic layer deposited Al2O3 as a protective overlayer for focused ion beam preparation of plan-view STEM samples

New Approach To Quantitative Subcellular Imaging Of Phosphorus And Calcium Using Energy-filtered Transmission Electron Microscopy And Tomography

Recent technical advances in scanning transmission electron microscopy (STEM) and xray detector technology have made it possible to perform atomic-resolution chemical mapping using energy-dispersive x-ray spectroscopy (EDS) [1]. This ability allows for establishing a direct correlation between atomic-scale STEM images, such as high-angle annular dark-field (HAADF) images, and atomic-scale chemical EDS images, making chemical quantification of interfaces, defects and crystalline structure at the atomic-scale possible. Using these capabilities, we have lately quantified the chemical composition of an epitaxial (La0.7Sr0.3)MnO3 (LSMO)/BiFeO3 (BFO) quantum structure [2], cation occupancy in a Sm-doped SrTiO3 (STO) thin film and antiphase boundaries present within STO films [3], as well as determined structures of several intermetallic alloys [4]. In this study, we describe further use of these capabilities to quantify atomic-scale interdiffusion in self-assembled, vertically aligned nanocomposite (VAN) thin films. Self-assembled VAN thin films, consisting of two immiscible components heteroepitaxially grown on single crystals, have experienced an increase in research activity in recent years [5]. These structures offer the advantage of utilizing the functionalities of both components with the possibility of tuning the material’s properties by tailoring the volume ratio of the two components, the interface-to-volume ratio and hetero-epitaxial strain. In addition, modification of interface properties such as structural and elemental distribution across the interface offers an additional dimension to generate new properties and functionalities. The atomic-scale characterization of the interface plays an essential role in understanding the structure-property relationships. Here we describe the characterization of NiO:LSMO and ZnO:LSMO VAN thin

Epitaxial TiN formation on rutile titanium dioxide (0 0 1) single crystal by nitridation

Janus nanoparticles are nanoparticles (NPs) displaying two sides of different chemical nature which makes them of high interest. The formation of the Janus shell provides the NPs an amphiphilic character, which can therefore self‐assemble in a tunable fashion by varying different experimental parameters such as size, polymer ratio and temperature. Here, Au NPs coated by polyethylene glycol (PEG) and both polystyrene (PS) and poly(N‐isopropylacrylamide) (PNIPAM) were investigated. [1] The true Janus character of the NPs was determined by electron tomography. Characterization of polymer coated particles by electron microscopy is challenging due to the low contrast of the organic materials. The goal was to distinguish the two different polymers in two types of particles (PEG+PNIPAM and PEG+PS coated Au NPs). Different approaches were used to improve the contrast between the two polymers. The PEG+PNIPAM particles were characterized by specific staining of the hemispheres while the character of the PEG+PS particles was investigated by the growth of silica. First, the PEG+PNIPAM coated NPs were stained with 3 mM CuSO 4 .5H 2 O solution to generate sufficient contrast in the Transmission Electron Microscope (TEM). PNIPAM‐coated NPs showed damage of the polymer upon electron beam irradiation. For PEG‐coated NPs a stable shell was observed. In an attempt to distinguish PNIPAM and PEG, PEG+PNIPAM coated particles were stained and investigated with TEM (Fig. 1). Since conventional TEM images are 2D projections from a 3D object, electron tomography was performed. It must be noted that the conventional technique for tomography in materials science, high angle annular dark field scanning TEM (HAADF‐STEM), could not be applied due to the large difference in atomic number Z for Au and the elements in the polymer shell. Instead, Bright‐Field TEM tomography was applied. Images were acquired over the largest possible range (‐76° to +76°) every 2°, aligned and reconstructed by the SIRT (Simultaneous Iterative Reconstruction Technique) algorithm in the Astra Toolbox. [2] The obtained 3D reconstruction displayed rather low contrast between the shell and the background. Manual segmentation based on the orthoslices (xy‐, yx‐ and zx‐direction) could be carried out. This method provided a 3D reconstruction that clearly confirms the formation of stained half‐shells (Fig. 3). In the second case of PEG+PS coated Au NPs, an attempt of staining with calcium phosphate was carried out. In TEM images hemispheres could be observed, but selective staining could not be confirmed. Due to the high affinity of PEG by silica, PEG is able to act as a primer to promote silica condensation. Since PS is hydrophobic and does not allow silica nucleation, the silica shell was expected to occur only at the parts of the NPs that are coated by PEG (Fig. 2). Either staining or silica shell growth over PEG‐coated areas resulted in the observation of semi‐shells (Janus character) in Au NPs by TEM and electron tomography.

Silver–titania (Ag–TiO2) nanoparticles with smaller Ag nanoparticles attached to larger TiO2 nanoparticles were generated by hybrid ultrasonic vibration and picosecond laser ablation of Ag and Ti bulk targets in deionised water, for the first time. The laser has a wavelength of 1064 nm and a pulse duration of 10 ps. It was observed that without the ultrasonic vibration, Ag and TiO2 nanoparticles did not combine, thus the role of ultrasonic vibration is essential. In addition, colloidal TiO2 and Ag nanoparticles were generated separately for comparison under the same laser beam characteristics and process conditions. The absorption spectra of colloidal Ag–TiO2 cluster nanoparticles were examined by UV–Vis spectroscopy, and size distribution was characterised using transmission electron microscopy. The morphology and composition of Ag–TiO2 nanoparticles were examined using scanning transmission electron microscopy in high-angle annular dark field, and energy-dispersive X-ray spectroscopy. The crystalline structures were investigated by X-ray diffraction. The size of larger TiO2 particles was in the range 30–150 nm, and the smaller-sized Ag nanoparticles attached to the TiO2 was mainly in the range of 10–15 nm. The yield is more than 50 % with the remaining nanoparticles in the form of uncombined Ag and TiO2. The nanoparticles generated had strong antibacterial effects as tested against E. coli. A discussion is given on the role of ultrasonic vibration in the formation of Ag–TiO2 hybrid nanoparticles by picosecond laser ablation.

Nanofabrication is continuously searching for new methodologies to fabricate 3D nanostructures with 3D control over their chemical composition. A new approach for heterostructure nanorod array fabrication through spatially controlled-growth of multiple metal oxides within block copolymer (BCP) templates is presented. Selective growth of metal oxides within the cylindrical polymer domains of polystyrene-block-poly methyl methacrylate is performed using sequential infiltration synthesis (SIS). Tuning the diffusion of trimethyl aluminum and diethyl zinc organometallic precursors in the BCP film directs the growth of AlOx and ZnO to different locations within the cylindrical BCP domains, in a single SIS process. BCP removal yields an AlOx -ZnO heterostructure nanorods array, as corroborated by 3D characterization with scanning transmission electron microscopy (STEM) tomography and a combination of STEM and energy-dispersive X-ray spectroscopy tomography. The strategy presented here will open up new routes for complex 3D nanostructure fabrication.

The titanium oxide nanotubes have become a very attractive material with potential applications in biomedicine, photocatalysis, energy etc. Their properties depend mostly on morphology which is relatively easy to change by controlling the conditions of the anodization like the type of electrolyte used, the voltage and the time of anodization. There is a direct linear relation between the anodization voltage and the average diameter of the formed nanotubes, as the voltage increases the diameter of nanotubes also increases [1,2]. The possibility of the preparation of nanotubes with different size, shape and wall thickness leads to the control of their geometric surface area and specific surface area, which is an important parameter in the development of new substrates for example in heterogeneous catalysis. Nanotube surface functionalization with platinum nanoparticles is a way to fabricate active material for catalysis of oxidation reaction of methanol. Such nanostructured complex materials demand advanced methods for characterization and visualization of real structure, therefore application of TEM and electron tomography techniques is desired. In this work titania nanotubes were obtained by electrochemical oxidation of pure titanium at voltage of 10 and 25V, that resulted in creation of TiO 2 nanotubes with 40 and 110 nm in diameter, respectively. After anodization the heat treatment was performed at 450 o C for 1h to change amorphous structure of TiO 2 nanotubes into crystalline anatase structure. A suitable amount of Pt – 0.2 mg/cm 2 on the surface of the nanotubes was deposited using magnetron sputtering. FIB prepared specimens were analyzed in a Hitachi HD‐2700 dedicated STEM (Scanning Transmission Electron Microscopy). Fig. 1 shows cross‐section SEM images of TiO 2 nanotubes with 0.2 mg/cm 2 Pt deposit on the top. The platinum nanoparticles tend to choose the edges of TiO 2 nanotubes and side walls. A high amount of Pt fills the interior of the nanotubes as shown in Fig. 1b. As nanotubes diameter increases from 40 to 110 nm the depth of deposition into nanotubes also increases (Fig. 1c). Morphology of prepared structures was characterized by scanning transmission electron microscopy tomography. This method provides three‐dimensional structural information at nanoscale based on two dimensional projections acquired at different tilt angles. High angle annular dark field (HAADF) imaging was used for 2D projections. The results have shown distribution of platinum nanoparticles inside the nanotubes. The variations in platinum content introduced into different diameter nanotubes were also examined. Segmented volume of nanotubes was analyzed in terms of specific surface area and volume fraction.

Nature of the interfaces between the constituent phases in the high entropy alloy CoCrCuFeNiAl

High density InGaN quantum dots (QDs) grown on an underlying GaN layer that was partially masked with SiNx nanocrystals were investigated by cross-sectional high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM), high-resolution transmission electron microscopy, and energy dispersive x-ray spectroscopy. The layer of SiNx masks appeared as a dark line in the HAADF-STEM images, and from the thickness of that dark line, the height of the masks was roughly estimated to be less than 2nm. The InGaN QDs appeared as bright triangles in the HAADF-STEM images. The QDs can be regarded as nanosized island crystals consisting of {101¯1} sidewalls, with a height of several nanometers. The lattices in the InGaN crystals were strained as compared to the underlying and the capping GaN lattices and contacted them coherently.

Effect of Ag addition on the age hardening and precipitation behavior in an Al-Cu-Li-Mg-Zn-Mn-Zr alloy