Transformation of CaSi overgrowth domains to the CaSi2 crystal phase via vacuum annealing

The phase transformation of overgrown CaSi crystal on an (00l)-oriented epitaxial CaSi2 film was studied using high-angle annular dark-field scanning transmission electron microscopy. After annealing at 450°C under vacuum conditions, the CaSi domain transformed to the CaSi2 phase with thin Si layers. The transformed CaSi2 crystal formed epitaxially along the under-layer epitaxial CaSi2 film. The results suggest that Ca atoms in the overgrown CaSi domain diffused to the outermost passivated silicon oxide layer during the low-temperature vacuum anneal.

Molecular Beam Epitaxial Growth of Sb2Te3-Bi2Te3 Lateral Heterostructures

We report on heteroepitaxial growth of Sb2Te3-Bi2Te3 lateral heterostructures using molecular beam epitaxy. The lateral heterostructures were fabricated by growing Bi2Te3 islands of hexagonal or triangular nanostructures with a typical size of several hundred nm and thickness of ~ 15 nm on graphene substrates and Sb2Te3 laterally on the side facets of the nanostructures. Multiple-step processes with different growth temperatures were employed to grow the lateral heterostructures. Electron microscopy techniques indicate that the inner region is Bi2Te3 and the outer Sb2Te3 was formed laterally on the graphene in an epitaxial manner. The interface between Bi2Te3 and Sb2Te3 from planar and cross-sectional views was studied by the aberration-corrected (Cs-corrected) high-angle annular dark-field scanning TEM (HAADF-STEM) technique. The cross-sectional electron microscopy investigation shows no wetting layer of Sb2Te3 on Bi2Te3, corroborating perfect lateral heterostructure formation. In addition, we investigated the topological properties of Sb2Te3-Bi2Te3 lateral heterostructures using first-principles calculations.

Elucidation of PVD MoS2 film formation process and its structure focusing on sub-monolayer region

Sputtering enables uniform and clean deposition over a large area, which is an issue with exfoliation and chemi-cal vapor deposition methods. On the other hand, the process of physical vapor deposition (PVD) film formationhas not yet been clarified. We prepared several samples from the sub-monolayer region, and performed Ra-man spectroscopy, X-ray photon spectroscopy and high-angle annular dark-field scanning transmission electronmicroscopy. From these results, the internal stresses inherent to PVD films, the bonding states specific to sub-monolayers, and the unique film structure and the grain formation process of PVD films were discussed fromthe perspective of sub-monolayers. As a conclusion, we found that it is important to suppress the formation ofsub-monolayers on the substrate to completely form the first layer.

Low Angle Annular Dark Field Scanning Transmission Electron Microscopy Analysis of Phase Change Material

The continuously growing demands in high-density memories drive the rapid development of advanced memory technologies. In this work, we investigate the mushroom type PCM cells based on Ge2Sb2Te5 at nanoscale by low angle annular dark field (LAADF) STEM imaging technique as well as energy dispersive X-ray spectroscopy (EDX) to study the changes in microstructure and elemental distributions in PCM mushroom cells before and after SET and RESET conditions. We describe the microscope settings used for LAADF image formation to reveal the amorphous dome in RESET device and discuss the application example in failure analysis of PCM test device.

Bi8Te3, the 11-Atom Layer Member of the Tetradymite Homologous Series

Bi8Te3 is a member of the tetradymite homologous series, previously shown to be compositionally and structurally distinct from hedleyite, Bi7Te3, yet inadequately characterized structurally. The phase is identified in a sample from the Hedley district, British Columbia, Canada. Compositions are documented by electron probe microanalysis and structures are directly imaged using high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). Results confirm that Bi8Te3 has an 11-atom layer structure, in which three Bi-Bi pairs are placed adjacent to the five-atom sequence (Te-Bi-Te-Bi-Te). Bi8Te3 has trigonal symmetry (space group R3¯m) with unit cell dimensions of a = ~4.4 Å and c = ~63 Å calculated from measurements on representative electron diffraction patterns. The model is assessed by STEM simulations and EDS mapping, all displaying good agreement with the HAADF STEM imaging. Lattice-scale intergrowths are documented in phases replacing Bi8Te3, accounting for the rarity of this phase in nature. These results support prior predictions of crystal structures in the tetradymite homologous series from theoretical modeling and indicate that other phases are likely to exist for future discovery. Tetradymite homologues are mixed-layer compounds derived as one-dimensional superstructures of a basic rhombohedral sub-cell. Each member of the series has a discrete stoichiometric composition and unique crystal structure.

The Mixed-Layer Structures of Ikunolite, Laitakarite, Joséite-B and Joséite-A

We used high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) to image the crystal structures of four minerals in the Bi4X3 isoseries (X = Te, Se, S), a subgroup of the tetradymite homologous series: ikunolite (Bi4S3), laitakarite (Bi4Se2S), joséite-B (Bi4Te2S), and joséite-A (Bi4TeS2). The four minerals are isostructural and interpretable in terms of regular stacking of seven-atom packages: [Bi–S–Bi–S–Bi–S–Bi], [Bi–Se–Bi–S–Bi–Se–Bi], [Bi–Te–Bi–S–Bi–Te–Bi], and [Bi–S–Bi–Te–Bi–S–Bi], respectively. The four phases are mixed-layer structures representing the Bi2kTe3 (k = 2) module within the tetradymite series. Diffraction patterns confirm they are seven-fold superstructures of a rhombohedral subcell with c/3 = d~1.89–1.93 Å. Modulation along the d* interval matches calculations of reflection intensity using the fractional shift method for Bi4X3. Internal structures can be discerned by high-resolution HAADF STEM imaging and mapping. Paired bismuth atoms are positioned at the outside of each seven-atom layer, giving the minerals a modular structure that can also be considered as being composed of five-atom (X–Bi–X–Bi–X) and two-atom (Bi–Bi) sub-modules. The presence of mixed sites for substituting cations is shown, particularly for Pb. Moreover, Pb may be important in understanding the incorporation of Ag and Au in Bi–chalcogenides. Visualisation of crystal structures by HAADF STEM contributes to understanding relationships between phases in the tetradymite homologous series and will play an invaluable role in the characterization of potential additional members of the series.

High Resolution STEM Images of the Human Tooth Enamel Crystals

High-resolution scanning transmission electron microscopy (STEM) images of human tooth enamel crystals, mainly in the high-angle annular dark-field (STEM-HAADF) mode, are presented in this work along the [1000], [10-11]. and [1-210] directions. These images allow knowing some structural details at the nanometric level of the human tooth enamel crystals and of the central dark line (CDL) observed at their centers. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of the CDL showed the Fresnel contrast. In the STEM bright-field (STEM-BF) and annular-dark-field (STEM-ADF) images, the CDL was observed as an unstrain hydroxyapatite (HAP)-like zone but surrounded by a strained zone. In the STEM-HAADF images, the CDL appeared with a weak contrast, and its contrasts’ thickness was registered between 3 and 8 Å. The arrangement obtained in the STEM-HAADF images by identifying the bright points with the Ca atoms produced the superposition of the HAP atomic sites, mainly along the [0001] direction. The findings provide further information on the structure details at the center of enamel crystals, which favors the anisotropic carious dissolution at the CDL.

Ru Catalyst Encapsulated into the Pores of MIL-101 MOF: Direct Visualization by TEM

Ru catalyst nanoparticles were encapsulated into the pores of a Cr-based metal-organic framework (MOF)—MIL-101. The obtained material, as well as the non-loaded MIL-101, were investigated down to the atomic scale by annular dark-field scanning transmission electron microscopy using low dose conditions and fast image acquisition. The results directly show that the used wet chemistry loading approach is well-fitted for the accurate embedding of the individual catalyst nanoparticles into the cages of the MIL-101. The MIL-101 host material remains crystalline after the loading procedure, and the encapsulated Ru nanoparticles have a metallic nature. Annular dark field scanning transmission electron microscopy, combined with EDX mapping, is a perfect tool to directly characterize both the embedded nanoparticles and the loaded nanoscale MOFs. The resulting nanostructure of the material is promising because the Ru nanoparticles hosted in the MIL-101 pores are prevented from agglomeration—the stability and lifetime of the catalyst could be improved.