Strong interlayer coupling and unusual antisite defect-mediated p-type conductivity in GePx (x = 1, 2).

As an emerging candidate for anisotropic two-dimensional materials, the group IV-V family (e.g. GeP, GeP2) has appealing applications in photoelectronics. However, their intrinsic point defect properties, which largely determine the device performance and optimization, are still poorly explored. In our study, through density functional theory (DFT) calculations, antisite defects were affirmed to be dominant with the lowest formation energies in 2D GePx semiconductors because of the similar atomic size and electronegativity of elemental components, which is in contrast to previous calculations and experimental speculation. These antisite defects could introduce relatively shallow states within the bandgap in bulk cases. The transition energy levels and electronic structures of defects reveal that GeP and PGe antisites act as dominant acceptors and donors, respectively. Strong interlayer coupling between anions results in a significant upshift of the valence band maximum (VBM) and shallower acceptor behaviors of GePx. Together with the dominant GeP antisite defect, the large upshift of the VBM in GeP leads to a remarkable transition of conductivity from intrinsic in the monolayer to p-type in the bulk. Such a synergistic effect in GeP2 is rather weak due to the strong inherent intralayer coupling of anions. Our research provides deep insights into the strong anion coupling effects on the electronic structures and defect properties of GeP and GeP2, which sheds light on defect engineering and electronic applications of GePx based semiconductors.

Microstructure of long period stacking ordered phase in an as-cast Mg-Ni-Zn-Dy alloy.

An as-cast Mg-5Ni-4Zn-1Dy (wt.%) alloy was prepared by traditional ingot metallurgy. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were used to study the microstructural characteristics of 14H LPSO phase in the as-cast Mg-5Ni-4Zn-1Dy (wt.%) alloy. Selected-area electron diffraction, convergent-beam electron diffraction, energy dispersive spectrum and high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and atomic-resolution energy dispersive spectrum (EDS) mapping were used to study the microscopic characteristics of the 14H LPSO phase. The unit cell of 14H LPSO phase has two ABCA-type building blocks and the Zn, Ni and Dy elements were enriched on the ABCA-type stacking sequence, especially on the B and C layers. Space group of the 14H LPSO phase was determined to be P63mc and atomic structural model was constructed.

Transmission electron microscopy investigations of the microstructures in rapidly solidified Mg-Sn ribbons.

Ribbons of an Mg-9.76 wt.%Sn alloy have been fabricated using rapid solidification technology. X-ray diffraction and transmission electron microscopy confirm that the rapidly solidified ribbons consist of α-Mg, ß-Mg2Sn and ß″-Mg3Sn phases. The α-Mg grains are refined in the ribbons. The predominant fraction of the ß-Mg2Sn phase distributes at the grain boundaries of the α-Mg grains. The minor fraction of the ß-Mg2Sn phase reveals a spherical morphology with a typical grain size of 170 nm. The orientation relationship between the ß″-Mg3Sn particles and α -Mg matrix is identified in the ribbons and an atomic structure model of the ß″-Mg3Sn phase is proposed.

Atomistic Interface Dynamics in Sn-Catalyzed Growth of Wurtzite and Zinc-Blende ZnO Nanowires.

Unraveling the phase selection mechanisms of semiconductor nanowires (NWs) is critical for the applications in future advanced nanodevices. In this study, the atomistic vapor-solid-liquid growth processes of Sn-catalyzed wurtzite (WZ) and zinc blende (ZB) ZnO are directly revealed based on the in situ transmission electron microscopy. The growth kinetics of WZ and ZB crystal phases in ZnO appear markedly different in terms of the NW-droplet interface, whereas the nucleation site as determined by the contact angle ϕ between the seed particle and the NW is found to be crucial for tuning the NW structure through combined experimental and theoretical investigations. These results offer an atomic-scale view into the dynamic growth process of ZnO NW, which has implications for the phase-controllable synthesis of II-VI compounds and heterostructures with tunable band structures.

Three-dimensional structures of magnesium nanopores.

The optimization of nanopore-based devices is closely related to the nanopore three-dimensional (3D) structures. In this paper, faceted nanopores were fabricated in magnesium (Mg) by aligning the electron beam (e-beam) along the [0001] direction. Detailed structural characterization by transmission electron microscopy reveals the existence of two 3D structures: hexagonal prism-shaped and hourglass-shaped 3D morphologies. Moreover, the 3D structures of nanopores are also found to depend on the widest nanopore diameter-to-thickness ratio (D/t). A plausible formation mechanism for different 3D structures is discussed. Our results incorporate a critical piece of information regarding the nanopore 3D structures in Mg and may serve as an important design guidance for the size- and shape-controllable fabrication of solid-state nanopores applying the e-beam sculpting technique.

Non-centro-symmetric electron diffraction pattern of icosahedral quasicrystal induced by combination of linear phason strain and curvature of Ewald sphere.

Non-centro-symmetric characteristics are observed in the experimental electron diffraction patterns (EDPs) from the icosahedral quasicrystalline precipitates in ZrAlNiCuNb alloys. Different from the well-known breaking of the Friedel's law, where a strong dynamical effect will reveal in EDPs the concealed non-centro-symmetry originated from the crystal structures themselves, the current results can be interpreted in terms of changes in deviation parameters due to a delicate combination of the linear phason strain characteristic of quasicrystals and the curvature of Ewald sphere. After taking this effect into consideration, the corresponding simulated EDPs fit quite well to the experimental data.

Electron beam-assisted healing of nanopores in magnesium alloys.

Nanopore-based sensing has emerged as a promising candidate for affordable and powerful DNA sequencing technologies. Herein, we demonstrate that nanopores can be successfully fabricated in Mg alloys via focused electron beam (e-beam) technology. Employing in situ high-resolution transmission electron microscopy techniques, we obtained unambiguous evidence that layer-by-layer growth of atomic planes at the nanopore periphery occurs when the e-beam is spread out, leading to the shrinkage and eventual disappearance of nanopores. The proposed healing process was attributed to the e-beam-induced anisotropic diffusion of Mg atoms in the vicinity of nanopore edges. A plausible diffusion mechanism that describes the observed phenomena is discussed. Our results constitute the first experimental investigation of nanopores in Mg alloys. Direct evidence of the healing process has advanced our fundamental understanding of surface science, which is of great practical importance for many technological applications, including thin film deposition and surface nanopatterning.

High spatially resolved morphological, structural and spectroscopical studies on copper oxide nanocrystals.

Copper oxide nanocrystals decorated on multi-wall carbon nanotubes have been prepared. Comprehensive morphological, structural and spectroscopical studies have been carried out on the nanometre/atomic scale by the combination of high-resolution transmission electron microscopy and electron energy-loss near-edge structure in electron energy-loss spectroscopy, which has a high spatially resolved capacity advantage over the normally used analytical techniques such as x-ray powder diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). The result reveals that highly crystalline cubic Cu(2)O nanocrystals with highly uniform dispersion, homogeneous size of about 5.3 nm and nearly spherical morphology are synthesized as the predominant phase, while rare individual monoclinic CuO nanocrystals with irregular shape are still present as the minor phase. The analysis based on the survey result and the structural symmetry difference between Cu(2)O and CuO demonstrates that XRD underestimates the presence of the CuO phase with much lower structural symmetry while XPS overestimates the proportion of CuO phase.