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

Evaluation local strain of twisted bilayer graphene via moiré pattern

With the advent of the 5G era, the research and development of efficient thermal management materials has become more and more urgent. Here, a novel reduced graphene oxide-boron nitride (rGO-BN) hybrid was first designed and fabricated by Michael addition and Schiff base reaction, and then its orientation in thermoplastic polyurethane (TPU) matrix was regulated by fused deposition modeling (FDM) 3D printing technology to prepare an efficient polymer-based thermal management material. The results indicated that the preparation of rGO-BN hybrids enabled the synergistic thermal conductivity effect of rGO and BN to be fully exerted. Moreover, the orientation of the filler could be effectively controlled by flexibly regulating the FDM printing methods, thereby promoting the construction of efficient heat conduction paths. Finally, the thermal conductivity of TPU composites reached 2.61 W m−1 K−1, which was 1022% higher than that of pure TPU. In addition, the composite also maintained excellent electrical insulation performance, which further guaranteed its thermal management applications in electronic devices. This work provides a new idea for the design of efficient thermal conductive composites, and further expands the application of 3D printing technology in the field of functional composites. 

Design of rGO-BN hybrids for enhanced thermal management properties of polyurethane composites fabricated by 3D printing

With ultra-high mechanical strength and inherent lubrication property, graphene exhibits great potential as an atomically thin solid lubricant to address wear problems that affect mechanical function and longevity. Despite the excellent wear resistance of graphene at the nanoscale, wear of graphene is difficult to avoid at micro- to macroscale friction tests in ambient conditions and sensitive to the environmental atmosphere and humidity. As an atomically thin material, contaminants from the environment have a non-ignorable effect on graphene surface properties. To study the influence of airborne contaminants on graphene macroscale wear behavior, we performed sliding tests on new (as-prepared) and aged monolayer chemical vapor deposition grown graphene using a 5 mm-radius lens under load down to 0.1mN. The atomic force microscope characterization demonstrated the existence of airborne contaminants on aged graphene. The tribological experiments showed the aged graphene exhibited better wear resistance than the new one due to that the airborne contaminants separated the counterpart from the graphene wrinkles, from where graphene wear was initiated.

Effect of airborne contaminants on the macroscopic anti-wear performance of chemical vapor deposition graphene

Reducing friction and wear is a long-standing concern for machinery. Bulk solid lubricants have been widely adopted to address the tribological challenges at the macroscale, yet their implementation is relatively limited for small-scale applications. Recently, the emergence of two-dimensional (2D) materials brings a new horizon of a range of ultrathin solid lubricants for micro- and nanoscale sliding interfaces. Using ball-on-disk friction tests, we found that monolayer MoS2 possessed an outstanding lubrication performance when the normal load was below a threshold, beyond which wear set in. Further scratch tests on monolayer MoS2 showed that this critical normal load was significantly affected by the residual strain in MoS2. As revealed by finite-element simulations, MoS2 failed primarily because of in-plane stretching during tribological sliding, and the tensile residual strain would add to the in-plane strain inside MoS2, resulting in a noticeably weaker antiwear performance. This work provides guideli...

Qunyang Li

Papers

Evaluation local strain of twisted bilayer graphene via moiré pattern

Design of rGO-BN hybrids for enhanced thermal management properties of polyurethane composites fabricated by 3D printing

Effect of airborne contaminants on the macroscopic anti-wear performance of chemical vapor deposition graphene

Antiwear Performance of Monolayer MoS2 Modulated by Residual Straining

Continuous epitaxy of single-crystal graphite films by isothermal carbon diffusion through nickel.