Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

The electronic properties of ultrathin crystals of molybdenum disulfide consisting of N=1,2,…,6 S-Mo-S monolayers have been investigated by optical spectroscopy Through characterization by absorption, photoluminescence, and photoconductivity spectroscopy, we trace the effect of quantum confinement on the material's electronic structure With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by more than 06 eV This leads to a crossover to a direct-gap material in the limit of the single monolayer Unlike the bulk material, the MoS₂ monolayer emits light strongly The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 10⁴ compared with the bulk material

Atomically thin MoS2: a new direct-gap semiconductor

Graphene is a wonder material with many superlatives to its name. It is the thinnest known material in the universe and the strongest ever measured. Its charge carriers exhibit giant intrinsic mobility, have zero effective mass, and can travel for micrometers without scattering at room temperature. Graphene can sustain current densities six orders of magnitude higher than that of copper, shows record thermal conductivity and stiffness, is impermeable to gases, and reconciles such conflicting qualities as brittleness and ductility. Electron transport in graphene is described by a Dirac-like equation, which allows the investigation of relativistic quantum phenomena in a benchtop experiment. This review analyzes recent trends in graphene research and applications, and attempts to identify future directions in which the field is likely to develop.

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Graphene: Status and Prospects

Problems associated with large-scale pattern growth of graphene constitute one of the main obstacles to using this material in device applications. Recently, macroscopic-scale graphene films were prepared by two-dimensional assembly of graphene sheets chemically derived from graphite crystals and graphene oxides. However, the sheet resistance of these films was found to be much larger than theoretically expected values. Here we report the direct synthesis of large-scale graphene films using chemical vapour deposition on thin nickel layers, and present two different methods of patterning the films and transferring them to arbitrary substrates. The transferred graphene films show very low sheet resistance of approximately 280 Omega per square, with approximately 80 per cent optical transparency. At low temperatures, the monolayers transferred to silicon dioxide substrates show electron mobility greater than 3,700 cm(2) V(-1) s(-1) and exhibit the half-integer quantum Hall effect, implying that the quality of graphene grown by chemical vapour deposition is as high as mechanically cleaved graphene. Employing the outstanding mechanical properties of graphene, we also demonstrate the macroscopic use of these highly conducting and transparent electrodes in flexible, stretchable, foldable electronics.

Large-scale pattern growth of graphene films for stretchable transparent electrodes

There is intense interest in graphene in fields such as physics, chemistry, and materials science, among others. Interest in graphene's exceptional physical properties, chemical tunability, and potential for applications has generated thousands of publications and an accelerating pace of research, making review of such research timely. Here is an overview of the synthesis, properties, and applications of graphene and related materials (primarily, graphite oxide and its colloidal suspensions and materials made from them), from a materials science perspective.

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Graphene and Graphene Oxide: Synthesis, Properties, and Applications

Grain boundary strengthening effect for polycrystalline copper is studied numerically using crystal plasticity in conjunction with cylindrical indentation simulations under the plane strain condition. In order to compare with an isotropic, heterogeneous continuum model a new constitutive relation is developed. This new nonlocal continuum model that encompasses the heterogeneity in yield strength based on the exponentiated Weibull function can predict the plastic properties of materials in the micron length scale. The spatial description of the deformation gradient two-point tensor is utilized to capture the intrinsic size effect in line with the subsequent deformation measures. Moreover, the total geometrically necessary dislocation density is obtained from the non-zero components of Nye dislocation density tensor. From the simulation, the relationship between the effective Green–Lagrange strain and effective stress measures is explained using the persistent long-range order and intermittent short-range order. The observation derived from the analogy between the cylindrical indentation and the progress in cylindrical voids describes how different average grain sizes of polycrystalline materials are compared with the behavior of isotropic materials. The trajectories of directions of both principal stretch and maximum shear strain explain that the internal stresses induced by cylindrical indentation either hinder or reinforce the dislocation flow, forming strain localization sporadically. The grain size dependence of polycrystalline modeling incorporates the Hall–Petch strengthening as well as the homogenization of anisotropic polycrystalline metal into the isotropic effective medium. This is a physically-based model that is used to model failure characterization in metals.

Grain size dependence of polycrystalline plasticity modeling in cylindrical indentation

Nanocrystalline metals exhibit a phenomenon called plastic strain recovery whereby plastic strain introduced through a load cycle is gradually recovered under no external loading over a time period of hours and days. In this study, we experimentally explore the diffusive mechanisms and the strain rates for nanocrystalline thin films of copper with an average grain size of about 35 nm during plastic strain recovery and creep. The experiments are performed via the plane strain bulge test and the thin film samples are deposited using thermal evaporation and sputtering. The specimens recover their residual strain in a period of time with two characteristic strain rates, a transient strain recovery rate of the order of 10−7/s and a steady-state strain recovery rate of the order of 10−9/s and there is a characteristic time at which the transition occurs between the two rates. The results suggest that a diffusive mechanism in conjunction with voids within the nanocrystalline material can explain the two plastic strain recovery rates and the transition between the two.

Experimental Investigation of Plastic Strain Recovery and Creep in Nanocrystalline Copper Thin Films

The disclosed subject matter relates to a system and method for delivery of therapeutic agent to the inner ear. The system includes a plurality of micro-needles which can be delivered to the round window membrane by a delivery device, e.g. catheter, and is capable of controlled penetration of the round window membrane to create temporary and self-closing perforations. In some embodiments the micro-needles are hollow with a lumen for local drug deliver}' into the perforations. In other embodiments the micro-needles are solid and include a coating of therapeutic agent on exterior surface for delivery into the perforations.

System and method to locally deliver therapeutic agent to inner ear

Force, pressure, or stiffness measurement or calibration can be provided, such as by using a graphene or other sheet membrane, which can provide a specified number of monolayers suspended over a substantially circular well. In an example, the apparatus can include a substrate, including a substantially circular well. A deformable sheet membrane can be suspended over the well. The membrane can be configured to include a specified integer number of one or more monolayers. A storage medium can comprise accompanying information about the suspended membrane or the substrate that, with a deflection displacement response of the suspended membrane to an applied force or pressure, provides a measurement of the applied force or pressure.

Force, pressure, or stiffness measurement or calibration using graphene or other sheet membrane

Single crystal Aluminum of (001) orientation were shock peened using laser beam of 12 micron diameter and observed with X-ray micro-diffraction techniques based on a synchrotron light source. The X-ray micro-diffraction affords micron level resolution as compared with conventional X-ray diffraction which has only mm level resolution. The asymmetric and broadened diffraction profiles registered at each location were analyzed by sub-profiling and explained in terms of the heterogeneous dislocation cell structure. For the first time, the spatial distribution of residual stress induced in micro-scale laser shock peening was experimentally quantified and compared with the simulation result obtained from FEM analysis. Difference in material response and microstructure evolution under shock peening were explained in terms of material property difference in stack fault energy and its relationship with cross slip under plastic deformation.Single crystal Aluminum of (001) orientation were shock peened using laser beam of 12 micron diameter and observed with X-ray micro-diffraction techniques based on a synchrotron light source. The X-ray micro-diffraction affords micron level resolution as compared with conventional X-ray diffraction which has only mm level resolution. The asymmetric and broadened diffraction profiles registered at each location were analyzed by sub-profiling and explained in terms of the heterogeneous dislocation cell structure. For the first time, the spatial distribution of residual stress induced in micro-scale laser shock peening was experimentally quantified and compared with the simulation result obtained from FEM analysis. Difference in material response and microstructure evolution under shock peening were explained in terms of material property difference in stack fault energy and its relationship with cross slip under plastic deformation.

Jeffrey W. Kysar

Papers

Grain size dependence of polycrystalline plasticity modeling in cylindrical indentation

Experimental Investigation of Plastic Strain Recovery and Creep in Nanocrystalline Copper Thin Films

System and method to locally deliver therapeutic agent to inner ear

Force, pressure, or stiffness measurement or calibration using graphene or other sheet membrane

Spatially resolved characterization of residual stress induced by micro scale laser shock Peening