Unraveling the dislocation core structure at a van der Waals gap in bismuth telluride.

Tetradymite-structured chalcogenides such as bismuth telluride (Bi2Te3) are of significant interest for thermoelectric energy conversion and as topological insulators. Dislocations play a critical role during synthesis and processing of such materials and can strongly affect their functional properties. The dislocations between quintuple layers present special interest since their core structure is controlled by the van der Waals interactions between the layers. In this work, using atomic-resolution electron microscopy, we resolve the basal dislocation core structure in Bi2Te3, quantifying the disregistry of the atomic planes across the core. We show that, despite the existence of a stable stacking fault in the basal plane gamma surface, the dislocation core spreading is mainly due to the weak bonding between the layers, which leads to a small energy penalty for layer sliding parallel to the van der Waals gap. Calculations within a semidiscrete variational Peierls-Nabarro model informed by first-principles calculations support our experimental findings.

π and 4π Josephson Effects Mediated by a Dirac Semimetal.

Cd_{3}As_{2} is a three-dimensional topological Dirac semimetal with connected Fermi-arc surface states. It has been suggested that topological superconductivity can be achieved in the nontrivial surface states of topological materials by utilizing the superconductor proximity effect. Here we report observations of both π and 4π periodic supercurrents in aluminum-Cd_{3}As_{2}-aluminum Josephson junctions. The π period is manifested by both the magnetic-field dependence of the critical supercurrent and the appearance of half-integer Shapiro steps in the ac Josephson effect. Our macroscopic theory suggests that the π period arises from interference between the induced bulk superconductivity and the induced Fermi-arc surface superconductivity. The 4π period is manifested by the missing first Shapiro steps and is expected for topological superconductivity.

Wet-chemical synthesis and consolidation of stoichiometric bismuth telluride nanoparticles for improving the thermoelectric figure-of-merit.

Bismuth telluride nanoparticles (NPs) have been synthesized using a low-temperature wet-chemical approach from bismuth(III) oleate and tri-n-octylphosphine telluride. The size and shape of the NPs can be controlled by adjusting the temperature, reaction time, and nature of the surfactants and solvents. Aromatic hydrocarbons (toluene, xylenes) and ethers (phenyl- and benzyl-ether) favor the formation of stoichiometric Bi2Te3 NPs of platelike morphology, whereas the presence of oleylamine and 1-dodecanethiol yields Bi-rich Bi2Te3 spherical NPs. XRD, IR, SEM, TEM, and SAED techniques have been used to characterize the obtained products. We show that the surfactants can be efficiently removed from the surface of the NPs using a two-step process employing nitrosonium tetrafluoroborate and hydrazine hydrate. The surfactant-free NPs were further consolidated into high density pellets using cold-pressing and field-assisted sintering techniques. The sintered surfactant-free Bi2Te3 showed electrical and thermal properties comparable to Bi2Te3 materials processed through conventional solid state techniques, and greatly improved over other nanostructured Bi2Te3 materials synthesized by wet-chemical approaches.

Finite-size effects on the structure of grain boundaries.

We present a combined experimental and theoretical analysis of the structure of finite-sized Sigma 3 [112] grain boundaries in Au. High-resolution electron microscopy shows lattice translations at the grain boundary, with the magnitude of the translation varying along the finite-sized grain boundaries. The presence of this structural profile is explained using continuum elasticity theory and first-principles calculations as originating from a competition between elastic energy and the energy cost of forming continuous [111] planes across the boundary. This competition leads to a structural transition between offset-free and nontrivial grain boundary structures at a critical grain boundary size, in agreement with the experiments. We also provide a method to estimate the energy barrier of the gamma surface.

Geometric origin of hexagonal close packing at a grain boundary in gold.

Using electron microscopy, we identify local, intergranular regions of hexagonal close-packing at a grain boundary in gold. By analyzing the topological defects that connect this layer to the adjacent face-centered cubic grains, we explain the geometric origin of this interfacial reconstruction. We extend this analysis to predict the stacking arrangements found over a range of intergranular misorientations. These results help to unify our understanding of the defects that control the behavior of polycrystalline materials by showing how line defects that are already well understood in the bulk also can determine the atomic arrangements at grain boundaries.