Is precession electron diffraction kinematical? Part II: A practical method to determine the optimum precession angle

A series of experiments was undertaken to investigate the kinematical nature of precession electron diffraction data and to gauge the optimum precession angle for a particular system. Kinematically forbidden reflections in silicon were used to show how a large precession angle is needed to minimise multi-beam conditions for specific reflections and so reduce the contribution from dynamical diffraction. Small precession angles were shown to be detrimental to the kinematical nature of some low-order reflections. By varying precession angles, precession electron diffraction data for erbium pyrogermanate were used to investigate the effect of dynamical diffraction on the output from structure solution algorithms. A good correlation was noted between the precession angle at which the rate of change of relative intensities is small and the angle at which the recovered structure factor phases matched the theoretical kinematical structure factor phases.     

Analysis of structural distortion in Eshelby twisted InP nanowires by scanning precession electron diffraction

Transmission electron microscopes (TEM) are widely used in nanotechnology research. However, it is still challenging to characterize nanoscale objects; their small size coupled with dynamical diffraction makes interpreting real- or reciprocal-space data difficult. Scanning precession electron diffraction ((S)PED) represents an invaluable contribution, reducing the dynamical contributions to the diffraction pattern at high spatial resolution. Here a detailed analysis of wurtzite InP nanowires (30–40 nm in diameter) containing a screw dislocation and an associated wire lattice torsion is presented. It has been possible to characterize the dislocation with great detail (Burgers and line vector, handedness). Through careful measurement of the strain field and comparison with dynamical electron diffraction simulations, this was found to be compatible with a Burgers vector modulus equal to one hexagonal lattice cell parameter despite the observed crystal rotation rate being larger (ca. 20%) than that predicted by classical elastic theory for the nominal wire diameter. These findings corroborate the importance of the (S)PED technique for characterizing nanoscale materials.

     

Nanoscale Disorder and Deintercalation Evolution in K-Doped MoS2 Analysed Via In Situ TEM

Intercalation and deintercalation processes in van der Waals crystals underpin their use in nanoelectronics, energy storage, and catalysis but there remains significant uncertainty regarding these materials’ structural and chemical heterogeneity at the nanoscale. Deintercalation in particular often controls the robustness and cyclability of the involved processes. Here, a detailed analysis of potassium ordering and compositional variations in as-synthesised K intercalated MoS₂ as well an analysis of deintercalation induced changes in the structure and K/Mo elemental composition is presented. By combining 4D scanning transmission electron microscopy (4DSTEM), in situ atomic resolution STEM imaging, selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDS) the formation of previously unknown intermediate superstructures during deintercalation is revealed. The results provide evidence supporting a new deintercalation mechanism that favors formation of local regions with thermodynamically stable ordering rather than isotropic release of K. Systematic time-temperature measurements demonstrate the deintercalation behavior to follow first-order kinetics, allowing compositional and superstructural changes to be predicted. It is expected that the in situ correlative STEM-EDS/SAED methodology developed in this work has the potential to determine optimal synthesis, processing and working conditions for a variety of intercalated or pillared materials.     

Low Energy Twist Defects in Pentatwinned Silver Nanowires

Transmission electron microscopy investigation of networks of pentatwinned silver nanowires has identified the presence of nanoscale defects within individual wires, showing narrow regions of banded contrast normal to the nanowire axis (bamboo faults). Structural analysis using machine learning decomposition of scanning precession electron diffraction data identifies these bamboo faults as a pair of twist boundaries normal to the wire axis creating low energy coincident site lattices between the faulted region and the parent nanowire. This leads to a conservation of the relative misorientation of the twinned structure within the faulted region, leading to an apparent local rotation of the nanowire. Their presence after spraying and nonthermal processing suggests that they may form during nanowire synthesis. However, examination of the networks before and after cyclic straining finds an increase in the density of the defects, indicating that they may also form as a result of mechanical deformation.