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

Microstructured surfaces with continuous solid topography have many potential applications in biology and industry. To understand the liquid transport property of microstructured surfaces with continuous solid topography, we studied the sliding behavior of a droplet on microhole-structured surfaces. We found that the sliding friction of the droplet increased with increasing solid area fraction due to enlarged apparent contact area and enhanced contact angle hysteresis. By introducing a correction factor to the modified Cassie-Baxter relation, we proposed an improved theoretical model to better predict the apparent receding contact angle. Our experimental data also revealed that the geometric topology of surface microstructures could affect the sliding friction with microhole-decorated surfaces, exhibiting a larger resistance than that for micropillar-decorated surfaces. Assisted by optical microscopy, we attributed this topology effect to the continuity and the true total length of the three-phase contact line at the receding edge during the sliding. Our study provides new insights into the liquid sliding behavior on microstructured surfaces with different topologies, which may help better design functional surfaces with special liquid transport properties.

Sliding friction and contact angle hysteresis of droplets on microhole-structured surfaces

Energy‐absorbing materials with both high absorption capacity and high reusability are ideal candidates for impact protection. Despite great demands, the current designs either exhibit limited energy‐absorption capacities or perform well only for one‐time usage. Here a new kind of energy‐absorbing architected materials is created with both high absorption capacity and superior reusability, reaching 10 kJ kg−1 per cycle for more than 200 cycles, that is, unprecedentedly 2000 kJ kg−1 per lifetime. The extraordinary performance is achieved by exploiting the rate‐dependent frictional dissipation between prestressed stiff cores and a porous soft elastomer, which is reinforced by an intertwined stiff porous frame. The vast interfaces between the cores and elastomer enable high energy dissipation, while the magnitude of the friction force can adapt passively with the loading rate. The intertwined structure prevents stress concentration and ensures no damage and reusability of the constituents after hundreds of loading cycles. The behaviors of the architected materials, such as self‐recoverability, force magnitude, and working stroke, are further tailored by tuning their structure and geometry. This design strategy opens an avenue for developing high‐performance reusable energy‐absorbing materials that enable novel designs of machines or structures.

Harnessing Friction in Intertwined Structures for High‐Capacity Reusable Energy‐Absorbing Architected Materials

Bis(2-hydroxyethyl) terephthalate (BHET), the alcoholysis monomer of waste polyethylene terephthalate, can form a cross-linked network structure with the polyurethane main chain because of the unique structure of active hydroxyl groups. In this work, organic small molecular BHET was applied as a new chain extender in polyurethane synthesis. A green and facile method was proposed to fabricate waterborne polyurethane (WPU) by recycling waste polyester. It was found that WPU was successfully prepared by using BHET as chain extender. Compared with polyurethane prepared by small molecular chain extender 1,4-butanediol (BDO), mechanical strength and thermal stability of polyurethane were remarkably enhanced by the incorporation of BHET. Meanwhile, the water absorption rate of the polyurethane is reduced. The tensile strength of WPU/BDO materials with 3.77 wt% BDO was 5.3 MPa, while WPU/BHET materials with 3.77 wt% BHET was 9.4 MPa. Furthermore, the elongation at break of WPU/BHET materials reached the maximum with 5.66 wt% BHET. Moreover, WPU/BHET materials prepared by using BHET as a chain extender have better thermal stability in high temperature environments. BHET with distinctive structure and the uniform dispersion will have more potentials for the preparation of high-performance polyurethane composites, especially for green recycling of polyester materials. 

Research on synthesis of polyurethane based on a new chain extender obtained from waste polyethylene terephthalate

Supreme mechanical performance and tribological properties render graphene a promising candidate as a surface friction modifier. Recently, it has been demonstrated that applying in-plane strain can effectively tune friction of suspended graphene in a reversible manner. However, since graphene is deposited on solid surfaces in most tribological applications, whether such operation will result in a similar modulation effect becomes a critical question to be answered. Herein, by depositing graphene onto a stretchable substrate, the frictional characteristics of supported graphene under a wide range of strain are examined with an in situ tensile loading platform. The experimental results show that friction of supported graphene decreases with increasing graphene strain, similar to the suspended system. However, depending on the adherence state of the graphene/substrate interface, the system exhibits two distinct friction regimes with significantly different strain dependences. Assisted by detailed atomic force microscopy imaging, we attribute the unique behavior to the transition between two friction modulation modes, i.e., contact-quality-dominated friction and puckering-dominated friction. This work provides a more comprehensive view of the influence of strain on surface friction of graphene, which is beneficial for active modulation of graphene friction through strain engineering.

Revisiting Frictional Characteristics of Graphene: Effect of In-Plane Straining.

Monte Carlo simulations using a dielectrophoresis model were performed to investigate the microstructural evolution and the conductance change of an Ag30Ge17Se53 electrolyte film clapped by a Ag active electrode, at which a positive bias was applied, and a Pt inert electrode. It has been revealed that driven by the electrical field Ag ions were injected into the electrolyte from the Ag electrode to form conductive particles, thereafter, the particles align themselves in nanowires connecting Ag electrode and Pt electrode and leading to an electrical percolation. It is responsible for the resistive switching of the system. The transmission electron microscopic observations and resistive measurement results are in good agreement with the results of simulations.

Qunyang Li

Papers

Sliding friction and contact angle hysteresis of droplets on microhole-structured surfaces

Harnessing Friction in Intertwined Structures for High‐Capacity Reusable Energy‐Absorbing Architected Materials

Research on synthesis of polyurethane based on a new chain extender obtained from waste polyethylene terephthalate

Revisiting Frictional Characteristics of Graphene: Effect of In-Plane Straining.

Monte Carlo simulation of the percolation in Ag30Ge17Se53 amorphous electrolyte films