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

The beam spot size used in microscale laser shock peening is of the same order as grain size in many materials. Therefore, the deformation is induced in only a few grains so that it is necessary to treat the material as being anisotropic and heterogeneous. In order to investigate the corresponding anisotropic features, different experimental techniques and three-dimensional finite element simulations are employed to characterize and analyze anisotropic responses for single crystal aluminum under single pulse shock peening at individual locations. X-ray microdiffraction techniques based on a synchrotron light source affords micron scale spatial resolution and is used to measure the residual stress spatial distribution along different crystalline directions on the shocked surface. Crystal lattice rotation due to plastic deformation is also measured with electron backscatter diffraction. The result is experimentally quantified and compared with the simulation result obtained from finite element analysis. The...

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Study of anisotropic character induced by microscale laser shock peening on a single crystal aluminum

Electrocodeposition of alumina nanoparticles and copper thin film on silicon wafers was performed. The volume fraction of the nanoparticle is about 5% and the size is about 50 nm. Comparison between the static tensile behaviors of specimens with and without nanoparticles reveals that the Young’s modulus is significantly increased by incorporating nanoparticles into the copper film. However, the ultimate tensile strength of the nanocomposite (235 MPa) is slightly lower than that of the pure copper reference specimen (250 MPa). For the nanocomposite, the strain at failure is 7.8%, which is lower than that of the pure copper film (10.5%). Distinct microscale deformation mechanisms are observed: the main deformation mechanism of the pure copper film is slip followed by strain hardening, whereas for the nanocomposite, multistage failure behaviors are found due to the debonding at the nanoparticle/copper interface. Notched specimens were also tested and compared with the unnotched specimens. In addition, cyclic loading tests on the nanocomposite were conducted to show its hardening behavior.

Deformation and fracture behavior of electrocodeposited alumina nanoparticle/copper composite films

Electron Backscatter Diffraction (EBSD) based Orientation Imaging Microscopy (OIM) is used routinely at ~500 materials laboratories worldwide for the characterization and development of diverse crystalline materials. Statistically significant data sets (~107 individual EBSD measurements) can be collected and analyzed within time periods of acceptable beam stability (~105s). However, limitations in angular and spatial resolution have motivated a continued search for more robust EBSD-based methods. Herein is a gathered presentation of advanced techniques in use, intended as a guide to researchers in selecting the most appropriate method for their work. Wilkinson’s method has been shown to increase angular resolution nearly two orders of magnitude to ±0.006°, facilitating measurement of elastic strain, lattice curvature, and dislocation density. A simulated pattern adaptation of Wilkinson’s method extends these measurement capabilities to polycrystalline materials, by avoiding the need for an experimental strain free reference pattern. The angular resolution limit obtained is ~0.04°. Accurate pattern center calibration, essential to the high resolution methods, is accomplished by parallelization of band edges projected onto a sphere centered at the interaction volume. FFT powered cross-correlation functions improve the spatial resolution near grain boundaries and correct for measurement inaccuracies induced by overlapping patterns. To corroborate these claims, exemplary results taken from a wedge-indented nickel single crystal, cold-worked copper polycrystal, and rolled nickel polycrystal are shown.

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Techniques and Applications of the Simulated Pattern Adaptation of Wilkinson's Method for Advanced Microstructure Analysis and Characterization of Plastic Deformation

Erratum: Nonlinear elastic behavior of two-dimensional molybdenum disulfide [Phys. Rev. B 87 , 035423 (2013)]

An ab initio analysis based on the density functional theory is carried out to investigate the effects of axial strain and radial pressure on the radial breathing mode (RBM) frequency of individual single-walled carbon nanotubes with armchair and zigzag chiralities. It is found that the RBM frequency is not sensitive to axial tensile strain or small axial compressive strain. When the axial compressive strain exceeds a critical value, the RBM frequency is significantly reduced. On the other hand, the RBM frequency is highly sensitive to the radial pressure; a pressure of 1GPa can increase the RBM frequency by about 8cm−1. Under a high radial pressure, the difference between the frequencies of the positive and negative half periods of the RBM vibration sharply increases, which may lead to the separation of the RBM peaks. Very good agreement is found between the present numerical analysis and previous experimental results.

Jeffrey W. Kysar

Papers

Study of anisotropic character induced by microscale laser shock peening on a single crystal aluminum

Deformation and fracture behavior of electrocodeposited alumina nanoparticle/copper composite films

Techniques and Applications of the Simulated Pattern Adaptation of Wilkinson's Method for Advanced Microstructure Analysis and Characterization of Plastic Deformation

Erratum: Nonlinear elastic behavior of two-dimensional molybdenum disulfide [Phys. Rev. B 87 , 035423 (2013)]

Numerical analysis of the radial breathing mode of armchair and zigzag single-walled carbon nanotubes under deformation