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

Graphene devices on standard SiO(2) substrates are highly disordered, exhibiting characteristics that are far inferior to the expected intrinsic properties of graphene. Although suspending the graphene above the substrate leads to a substantial improvement in device quality, this geometry imposes severe limitations on device architecture and functionality. There is a growing need, therefore, to identify dielectrics that allow a substrate-supported geometry while retaining the quality achieved with a suspended sample. Hexagonal boron nitride (h-BN) is an appealing substrate, because it has an atomically smooth surface that is relatively free of dangling bonds and charge traps. It also has a lattice constant similar to that of graphite, and has large optical phonon modes and a large electrical bandgap. Here we report the fabrication and characterization of high-quality exfoliated mono- and bilayer graphene devices on single-crystal h-BN substrates, by using a mechanical transfer process. Graphene devices on h-BN substrates have mobilities and carrier inhomogeneities that are almost an order of magnitude better than devices on SiO(2). These devices also show reduced roughness, intrinsic doping and chemical reactivity. The ability to assemble crystalline layered materials in a controlled way permits the fabrication of graphene devices on other promising dielectrics and allows for the realization of more complex graphene heterostructures.

Boron nitride substrates for high-quality graphene electronics

Graphene-Like Two-Dimensional Materials

A two-dimensional crystal of molybdenum disulfide (MoS2) monolayer is a photoluminescent direct gap semiconductor in striking contrast to its bulk counterpart. Exfoliation of bulk MoS2 via Li intercalation is an attractive route to large-scale synthesis of monolayer crystals. However, this method results in loss of pristine semiconducting properties of MoS2 due to structural changes that occur during Li intercalation. Here, we report structural and electronic properties of chemically exfoliated MoS2. The metastable metallic phase that emerges from Li intercalation was found to dominate the properties of as-exfoliated material, but mild annealing leads to gradual restoration of the semiconducting phase. Above an annealing temperature of 300 °C, chemically exfoliated MoS2 exhibit prominent band gap photoluminescence, similar to mechanically exfoliated monolayers, indicating that their semiconducting properties are largely restored.

Photoluminescence from Chemically Exfoliated MoS2

Graphene based materials: Past, present and future

ABSTRACT We derive an extension of the Maugis-Dugdale-Johnson-Greenwood model for 2D adhesive Hertzian contact to viscoelastic materials. This results in extremely simple approximate results for the maximum amplification of the pull-off due to viscoelastic effects, for arbitrary form of the viscoelastic linear properties. In particular, we assume that the initial loading state is fully relaxed, and unloading occurs at very large pulling speeds. We show that the maximum amplification (of the order of , the ratio between instantaneous and relaxed modulus to the 2/3 power) is only reached for large and soft cylinders, namely, large enough (relaxed) Tabor-Maugis parameters of the order of the order of , and therefore typically much larger than the Tabor-Maugis parameters to have a “short-range” JKR adhesion regime in quasi-static conditions, which is . The results agree well with recent numerical ones by Muser-Persson, but there is an shift factor in the Tabor-Maugis parameter which requires further study and may depend on initial contact area.

Maugis-Tabor parameter dependence of pull-off in viscoelastic line Hertzian contacts

Super-high mechanical strength and excellent lubrication property at the nanoscale render graphene a promising ultra-thin solid lubricant. However, macroscale friction tests have shown that wear of graphene often occurs resulting in failure of interface lubrication. As graphene coatings are atomically thin, a sensitive and reliable detection method for monitoring wear has been a great challenge to date. In this work, we performed tribological experiments on monolayer graphene on SiO2/Si while simultaneously measuring the in-plane electric resistance of graphene during the sliding tests. Our experiments showed that the electric resistance exhibited a sudden increase signifying the onset of graphene wear well before the macroscale friction increased rapidly. The variation of in-plane electric resistance was found to depend on the wear track length and relative orientation. Longer wear track and wear track perpendicular to the current would typically lead to higher increase in graphene electric resistance. Our work demonstrates that monitoring in-plane electric resistance can be a simple, sensitive and predictive method for detecting graphene wear, which makes it a useful inspecting technique for a wide range of mechanical applications.

Electric resistance as a sensitive measure for detecting graphene wear during macroscale tribological tests

Geometrical effects in contact mechanics: From atomic membranes to evolving asperities

Solid propellant is a composite material exhibiting classic nonlinear viscoelastic mechanical characteristic, which is due in a large part to a cumulative damage process caused by the formation and growth of microflaws inside. The standard relaxation tests and uniaxial tension tests under different velocities of hydroxyl-terminated polybutadiene (HTPB) propellant are carried out in this paper, where Digital Image Correlation (DIC) technique is applied to record deformation. The experimental results show that the material mechanical behavior is rate-dependent. It is also observed that the yield stress and failure stress are significantly rate-dependent on the tensile velocity. Based on these experimental results, it can be inferred that the stiffness degradation and damage evolution of HTPB propellant are a rate-dependent processes. Therefore, the damage accumulation of HTPB propellant is considered rate-dependent in this research. In order to describe the mechanical characteristic precisely, a nonlinear viscoelastic constitutive model with rate-dependent cumulative damage is developed. The damage model is developed based on the concept of pseudo strain, in which a Prony series representation of viscoelastic material functions is applied. Besides, a rate-dependent damage variable is introduced into the model through considering the rate-dependent characteristics of cumulative damage process. In addition, a new normalized failure criterion is derived on the basis of the proposed damage model, which is independent of strain-rate after normalization. Finally, it is implemented in commercial finite element software for stress analysis to verify the predictive capacities of the damage model. The accuracy of the constitutive model and failure criterion is validated under uniaxial tensile tests of various strain rates.

/pdf/a-nonlinear-viscoelastic-constitutive-model-for-solid-13kgeno6.pdf

A Nonlinear Viscoelastic Constitutive Model for Solid Propellant with Rate-Dependent Cumulative Damage

By using Monte Carlo simulation on a ferromagnetic core/antiferromagnetic shell nanoparticle, we investigate in details the exchange bias of the magnetic hysteresis as a function of both core radius and shell thickness, at low temperature. It is found that the exchange bias is very sensitive to the core radius and a small variation of the radius may lead to a big fluctuation of the bias. In a general tendency the exchange bias is enhanced by increasing shell thickness and decreasing core radius. The intrinsic correlation between the exchange bias and the spin configuration on the core-shell interface is demonstrated. We further investigate the dependence of the exchange bias on temperature and random field inside the nanoparticle, indicating a monotonous decreasing of the bias with the magnitude of random field and temperature, respectively.

Qunyang Li

Papers

Maugis-Tabor parameter dependence of pull-off in viscoelastic line Hertzian contacts

Electric resistance as a sensitive measure for detecting graphene wear during macroscale tribological tests

Geometrical effects in contact mechanics: From atomic membranes to evolving asperities

A Nonlinear Viscoelastic Constitutive Model for Solid Propellant with Rate-Dependent Cumulative Damage

Exchange bias in a core/shell magnetic nanoparticle: Monte Carlo simulation