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

This paper discusses a set of recent experimental results in which the mechanical properties of monolayer graphene molecules were determined. The results included the second-order elastic modulus which determines the linear elastic behavior and an estimate of the third-order elastic modulus which determines the non-linear elastic behavior. In addition, the distribution of the breaking force strongly suggested the graphene to be free of defects, so the measured breaking strength of the films represented the intrinsic breaking strength of the underlying carbon covalent bonds. The results of recent simulation efforts to predict the mechanical properties of graphene are discussed in light of the experiments. Finally, this paper contains a discussion of some of the extra challenges associated with experimental validation of multi-scale models.

Direct comparison between experiments and computations at the atomic length scale: a case study of graphene

This letter shows the ability to perform characterization of the strain field in an aluminium bicrystal subject to plane strain condition induced by micro scale laser shock peening. Intensity contrast method, previously used in topographic measurements of strain fields in thin films is employed here. Our results show that this method is applicable for measurements of the plastically deformed bulk materials. Moreover, heterogeneous response of the grain boundary is observed as periodic nature of deformation.

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Characterization of Heterogeneous Response of Al Bicrystal Subject to Micro Scale Laser Shock Peening

Micro-scale laser shock peening ({mu}LSP) can potentially be applied to metallic structures in microdevices to improve fatigue and reliability performance. Copper thin films on a single-crystal silicon substrate are treated by using {mu}LSP and characterized using techniques of X-ray microdiffraction and electron backscatter diffraction (EBSD). Strain field, dislocation density, and microstructure changes including crystallographic texture, grain size and subgrain structure are determined and analyzed. Further, shock peened single crystal silicon was experimentally characterized to better understand its effects on thin films response to {mu}LSP. The experimental result is favorably compared with finite element method simulation based on single-crystal plasticity.

Response of thin films and substrate to micro-scale laser shock peening

Path of light in near crack tip region in anisotropic medium and under mixed-mode loading

Laser shock peening by a micron sized laser beam is a process in which compressive residual stresses are induced in order to improve material fatigue life of micro scale components. The size of the laser target interaction zone is of the same order of magnitude as the target material grains and thus the effects of anisotropic material response must be taken into account. Single crystals are therefore chosen to study such anisotropy. It is also of interest to investigate the response of symmetric and asymmetric slip systems with respect to the yield surface. In presented work, analytic, numerical and experimental investigations of two different orientations, (110) and (11¯4) of aluminum single crystals are studied. Anisotropic slip line theory is employed for the construction of slip line fields for both orientations and compared with numerical results. Theory is further used to explain the difference in plastic deformation for two different orientations. Lattice rotations on the top surface and cross section are also measured using Electron Backscatter Diffraction (EBSD), while residual stress is measured using X-ray microdiffraction. Both the analytical and numerical models are then validated via experimental results.Laser shock peening by a micron sized laser beam is a process in which compressive residual stresses are induced in order to improve material fatigue life of micro scale components. The size of the laser target interaction zone is of the same order of magnitude as the target material grains and thus the effects of anisotropic material response must be taken into account. Single crystals are therefore chosen to study such anisotropy. It is also of interest to investigate the response of symmetric and asymmetric slip systems with respect to the yield surface. In presented work, analytic, numerical and experimental investigations of two different orientations, (110) and (11¯4) of aluminum single crystals are studied. Anisotropic slip line theory is employed for the construction of slip line fields for both orientations and compared with numerical results. Theory is further used to explain the difference in plastic deformation for two different orientations. Lattice rotations on the top surface and cross sect...

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Jeffrey W. Kysar

Papers

Direct comparison between experiments and computations at the atomic length scale: a case study of graphene

Characterization of Heterogeneous Response of Al Bicrystal Subject to Micro Scale Laser Shock Peening

Response of thin films and substrate to micro-scale laser shock peening

Path of light in near crack tip region in anisotropic medium and under mixed-mode loading

Comparative study of symmetric and asymmetric deformation of Al single crystal under micro scale laser shock peening