Crystallography at the nanoscale: planar defects in ZnO nanospikes

The examination of anisotropic nanostructures, such as wires, platelets or spikes, inside a transmission electron microscope is normally performed only in plan view. However, intrinsic defects such as growth twin interfaces could occasionally be concealed from direct observation for geometric reasons, leading to superposition. This article presents the shadow-focused ion-beam technique to prepare multiple electron-beam-transparent cross-section specimens of ZnO nanospikes, via a procedure which could be readily extended to other anisotropic structures. In contrast with plan-view data of the same nanospikes, here the viewing direction allows the examination of defects without superposition. By this method, the coexistence of two twin configurations inside the wurtzite-type structure is observed, namely [2 {\overline 1} {\overline 1} 0]^{\rm W}/(0 1 {\overline 1} 1) and [2 {\overline 1} {\overline 1} 0]^{\rm W}/(0 1 {\overline 1} 3), which were not identified during the plan-view observations owing to superposition of the domains. The defect arrangement could be the result of coalescence twinning of crystalline nuclei formed on the partially molten Zn substrate during the flame-transport synthesis. Three-dimensional defect models of the twin interface structures have been derived and are correlated with the plan-view investigations by simulation.

Uniaxial Compression Behavior of Bulk Nano-twinned Gold from Molecular Dynamics Simulation

Parallel molecular dynamics simulations were used to study the influence of pre-existing growth twin boundaries on the slip activity of bulk gold under uniaxial compression. The simulations were performed on a 3D, fully periodic simulation box at 300 K with a constant strain rate of 4×107 s−1. Different twin boundary interspacings from 2 nm to 16 nm were investigated. The strength of bulk nano-twinned gold was found to increase as the twin interspacing was decreased. However, strengthening effects related to the twin size were less significant in bulk gold than in gold nanopillars. The atomic analysis of deformation modes at the twin boundary/slip intersection suggested that the mechanisms of interfacial plasticity in nano-twinned gold were different between bulk and nanopillar geometries.

Structure and complex twinning of dysprosium disilicate (Dy2Si2O7), Type B

Dysprosium disilicate (Dy2Si2O7) is triclinic with a = 6.6158 (2), b = 6.6604 (2), c = 12.0551 (4) Å, α = 94.373 (2), β = 90.836 (2), γ = 91.512 (2)°, V = 529.4 (1) Å3, space group P\overline 1, Z = 4 and D x = 6.156 g cm−3. The structure (single-crystal X-ray, R = 0.033, wR = 0.041) is built from a linear triple tetrahedral group [Si3O10] and isolated [SiO4] tetrahedron cross-linked by Dy3+ in one sixfold and three eightfold coordinated positions, and corresponds to the presently revised type B structure of Ho2Si2O7. The formation of the unusual linear triple tetrahedral group in the type B structure allows for a more continuous transition in the mean size of REE3+O n (REE = rare earth element) polyhedra in REE disilicates through the 4f transition metal series. The crystal of Dy2Si2O7 investigated was complexly twinned such that the diffraction pattern was also consistent with a larger dimensionally monoclinic unit cell (a = 22.5354, b = 14.2102, c = 6.6158 Å, β = 91.788°), which resulted in an apparent superstructure of the type B structure in space group C1¯. Lattice coincidence with the type B unit cell appears to have been maintained during crystal synthesis and quenching by the complex sector-zoned growth twin.

Structure Determination of Tilt Boundaries in Gold by High Resolution Electron Microscopy

ABSTRACTA number of tilt grain boundaries prepared from evaporated gold thin films have been investigated by high resolution transmission microscopy. When the grain boundary is parallel to the electron beam and the beam is parallel to a low index rotation axis such as [110] or [001]it is possible to identify atomic positions at the cores of these boundaries as demonstrated here by a Σ = 3 70.5°/[110], (112) growth twin. In many cases it is not possible to make a full atomistic structure determination because the specimen does not have translational periodicity in the beam direction. This may be because the boundary plane is not parallel to the beam or the specimen contains dislocations which have a component of Burgers vector parallel to the beam. Examples are given of various low angle boundary structures in goldwhere there are complications because of the three dimensional nature of their structure.