Defect structure-electronic property correlations in transition metal dichalcogenide grain boundaries

Grain boundaries (Gb) in transition metal dichalcogenides are a rich source of interesting physics as well as a cause of concern because of its impact on electron transport across them in large area electronic device applications. Here, using first principles calculations, we show that beyond the conventional definition of grain boundaries based on misorientation, the defect structure present at the grain boundaries plays a significant role in defining the local electronic properties. We observed that even the standard 5-7 defect ring has differing electronic characteristics depending on its internal configuration. While the 5-7 ring presents shallow defect states, and induce long range strain fields with the local bandgap increasing up to 32.7%, the other commonly observed 4-8 defect rings introduce only mid-gap states, induce smaller strain fields with no observable bandgap change. The results show the seminal character of the individual defect structures at grain boundaries, and that their relative density can be used to determine the overall physico-chemical properties of the grain boundary.

Traction-separation laws of graphene grain boundaries

Molecular dynamics simulations are used to extract the traction-separation laws (TSLs) of symmetric grain boundaries of graphene. Grain boundaries with realistic atomic structures are constructed using different types of dislocations. The TSLs of grain boundaries are extracted by using cohesive zone volume elements (CZVEs) ahead of the crack tip. The traction and separation of each cohesive zone volume element are calculated during the crack growth. The traction and separation values obtained for the cohesive elements predict that the TSLs of grain boundaries have a bilinear form. The areas under the traction-separation curves are used to calculate the separation energy of the grain boundaries. The results show that as the grain boundary misorientation angle increases the separation energy of the grain boundaries decreases. The impact of temperature on the traction separation laws is studied. The results show that, with an increase of the temperature from 0.1 K to 300 K, the separation energy first increases to reach its peak at around 25 K and then slightly decreases.

Automatic software correction of residual aberrations in reconstructed HRTEM exit waves of crystalline samples

We develop an automatic and objective method to measure and correct residual aberrations in atomic-resolution HRTEM complex exit waves for crystalline samples aligned along a low-index zone axis. Our method uses the approximate rotational point symmetry of a column of atoms or single atom to iteratively calculate a best-fit numerical phase plate for this symmetry condition, and does not require information about the sample thickness or precise structure. We apply our method to two experimental focal series reconstructions, imaging a β-Si₃N₄ wedge with O and N doping, and a single-layer graphene grain boundary. We use peak and lattice fitting to evaluate the precision of the corrected exit waves. We also apply our method to the exit wave of a Si wedge retrieved by off-axis electron holography. In all cases, the software correction of the residual aberration function improves the accuracy of the measured exit waves.