The Electronic and Superconducting Properties of Oxygen-Ordered MgB2 compounds of the form Mg2B3Ox

Possible candidates for the Mg2B3Ox nanostructures observed in bulk of polycrystalline MgB2 (Ref.1) have been studied using a combination of Z-contrast imaging, electron energy loss spectroscopy (EELS) and first-principles calculations. The electronic structures, phonon modes, and electron phonon coupling parameters are calculated for two oxygen-ordered MgB2 compounds of composition Mg2B3O and Mg2B3O2, and compared with those of MgB2. We find that the density of states for both Mg2B3Ox structures show very good agreement with EELS, indicating that they are excellent candidates to explain the observed coherent oxygen precipitates. Incorporation of oxygen reduces the transition temperature and gives calculated TC values of 18.3 K and 1.6 K for Mg2B3O and Mg2B3O2, respectively.Comment: Submitted to PR

Nanofiltration across Defect-Sealed Nanoporous Monolayer Graphene

Monolayer nanoporous graphene represents an ideal membrane for molecular separations, but its practical realization is impeded by leakage through defects in the ultrathin graphene. Here, we report a multiscale leakage–sealing process that exploits the nonpolar nature and impermeability of pristine graphene to selectively block defects, resulting in a centimeter-scale membrane that can separate two fluid reservoirs by an atomically thin layer of graphene. After introducing subnanometer pores in graphene, the membrane exhibited rejection of multivalent ions and small molecules and water flux consistent with prior molecular dynamics simulations. The results indicate the feasibility of constructing defect-tolerant monolayer graphene membranes for nanofiltration, desalination, and other separation processes.Samsung (Firm) (Fellowship)United States. Dept. of Energy. Office of Basic Energy Sciences (Award number DE-SC0008059)King Fahd University of Petroleum and Minerals (Center for Clean Water and Clean Energy at MIT and KFUPM, project number R10-CW-09

High-temperature phonons in h-BN: momentum-resolved vibrational spectroscopy and theory

Vibrations in materials and nanostructures at sufficiently high temperatures result in anharmonic atomic displacements, which leads to new phenomena such as thermal expansion and multiphonon scattering processes, with a profound impact on temperature-dependent material properties including thermal conductivity, phonon lifetimes, nonradiative electronic transitions, and phase transitions. Nanoscale momentum-resolved vibrational spectroscopy, which has recently become possible on monochromated scanning-transmission-electron microscopes, is a unique method to probe the underpinnings of these phenomena. Here we report momentum-resolved vibrational spectroscopy in hexagonal boron nitride at temperatures of 300, 800, and 1300 K across three Brillouin zones (BZs) that reveals temperature-dependent phonon energy shifts and demonstrates the presence of strong Umklapp processes. Density-functional-theory calculations of temperature-dependent phonon self-energies reproduce the observed energy shifts and identify the contributing mechanisms.Comment: 21 pages, 4 figures, 2 tables, 3 supplemental figures, 3 supplemental table

Selective Ionic Transport through Tunable Subnanometer Pores in Single-Layer Graphene Membranes

We report selective ionic transport through controlled, high-density, subnanometer diameter pores in macroscopic single-layer graphene membranes. Isolated, reactive defects were first introduced into the graphene lattice through ion bombardment and subsequently enlarged by oxidative etching into permeable pores with diameters of 0.40 ± 0.24 nm and densities exceeding 10[superscript 12] cm[superscript –2], while retaining structural integrity of the graphene. Transport measurements across ion-irradiated graphene membranes subjected to in situ etching revealed that the created pores were cation-selective at short oxidation times, consistent with electrostatic repulsion from negatively charged functional groups terminating the pore edges. At longer oxidation times, the pores allowed transport of salt but prevented the transport of a larger organic molecule, indicative of steric size exclusion. The ability to tune the selectivity of graphene through controlled generation of subnanometer pores addresses a significant challenge in the development of advanced nanoporous graphene membranes for nanofiltration, desalination, gas separation, and other applications.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R10-CW-09)United States. Dept. of Energy. Office of Basic Energy Sciences (Award DE-SC0008059)United States. Dept. of Energy. Office of Basic Energy Sciences (Oak Ridge National Laboratory. Center for Nanophase Materials Sciences

Ab Initio Structural Energetics of Beta-Si3N4 Surfaces

Motivated by recent electron microscopy studies on the Si3N4/rare-earth oxide interfaces, the atomic and electronic structures of bare beta-Si3N4 surfaces are investigated from first principles. The equilibrium shape of a Si3N4 crystal is found to have a hexagonal cross section and a faceted dome-like base in agreement with experimental observations. The large atomic relaxations on the prismatic planes are driven by the tendency of Si to saturate its dangling bonds, which gives rise to resonant-bond configurations or planar sp^2-type bonding. We predict three bare surfaces with lower energies than the open-ring (10-10) surface observed at the interface, which indicate that non-stoichiometry and the presence of the rare-earth oxide play crucial roles in determining the termination of the Si3N4 matrix grains.Comment: 4 Pages, 4 Figures, 1 tabl

Ultra-fast Vacancy Migration: A Novel Approach for Synthesizing Sub-10 nm Crystalline Transition Metal Dichalcogenide Nanocrystals

Two-dimensional materials, such as transition metal dichalcogenides (TMDCs), have the potential to revolutionize the field of electronics and photonics due to their unique physical and structural properties. This research presents a novel method for synthesizing crystalline TMDCs crystals with < 10 nm size using ultra-fast migration of vacancies at elevated temperatures. Through in-situ and ex-situ processing and using atomic-level characterization techniques, we analyze the shape, size, crystallinity, composition, and strain distribution of these nanocrystals. These nanocrystals exhibit electronic structure signatures that differ from the 2D bulk i.e., uniform mono and multilayers. Further, our in-situ, vacuum-based synthesis technique allows observation and comparison of defect and phase evolution in these crystals formed under van der Waals heterostructure confinement versus unconfined conditions. Overall, this research demonstrates a solid-state route to synthesizing uniform nanocrystals of TMDCs and lays the foundation for materials science in confined 2D spaces under extreme conditions.Comment: MS+S

High-K dielectric sulfur-selenium alloys.

Upcoming advancements in flexible technology require mechanically compliant dielectric materials. Current dielectrics have either high dielectric constant, K (e.g., metal oxides) or good flexibility (e.g., polymers). Here, we achieve a golden mean of these properties and obtain a lightweight, viscoelastic, high-K dielectric material by combining two nonpolar, brittle constituents, namely, sulfur (S) and selenium (Se). This S-Se alloy retains polymer-like mechanical flexibility along with a dielectric strength (40 kV/mm) and a high dielectric constant (K = 74 at 1 MHz) similar to those of established metal oxides. Our theoretical model suggests that the principal reason is the strong dipole moment generated due to the unique structural orientation between S and Se atoms. The S-Se alloys can bridge the chasm between mechanically soft and high-K dielectric materials toward several flexible device applications