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

Incorporation of functional nanofillers can unlock the potential of polymers as advanced materials. Herein, single-layered and three-dimensional reduced graphene oxide (rGO)/Ti3C2Tx (B-rGO@Ti3C2Tx) nanohybrids were constructed using bis(2-hydroxyethyl) terephthalate (BHET) as a coupling agent between rGO and Ti3C2Tx through covalent and hydrogen bonds. It is found that BHET can not only resist the weak oxidization of Ti3C2Tx to some degree but also prevent the self-stacking of Ti3C2Tx and rGO sheets. Then, B-rGO@Ti3C2Tx was used as a functional nanofiller and three-dimensional chain extender for preparing the waterborne polyurethane (WPU) nanocomposite through in situ polymerization. Compared with WPU nanocomposites with an equivalent amount of Ti3C2Tx/rGO@Ti3C2Tx, although containing an equivalent amount of BHET, WPU/B-rGO@Ti3C2Tx nanocomposites show significantly improved performance. For example, 5.66 wt % of B-rGO@Ti3C2Tx endows WPU with a high tensile strength of 36.0 MPa (improved by 380%), thermal conductivity of 0.697 W·m-1·K-1, electrical conductivity of 1.69 × 10-2 S/m (enhanced by 39 times), good strain-sensing behavior, electromagnetic interference (EMI)-shielding performance of 49.5 dB in the X-band, and excellent thermal stability. Therefore, the construction of rGO@Ti3C2Tx nanohybrids with the aid of chain extenders may unlock new possibilities of polyurethane as smart materials.

Bis(2-hydroxyethyl) Terephthalate-Modified Ti3C2Tx/Graphene Nanohybrids as Three-Dimensional Functional Chain Extenders for Polyurethane Composite Films with Strain-Sensing and Conductive Properties.

Abstract Composite structures consisting of two-dimensional (2D) materials deposited on elastic substrates have a wide range of potential applications in flexible electronics. For such devices, robust 2D film/substrate interfacial adhesion is essential for their reliable performance when subjected to external thermal and mechanical loads. To better understand the strength and failure behavior of the 2D film/substrate interfaces, two types of graphene/polymer samples with distinct interfacial adhesion properties are fabricated and tested by uniaxially stretching the substrates. Depending on the interfacial adhesion, two drastically different debonding rates are observed, i.e., rapid snap-through debonding and more progressive crack propagation. Motivated by the experimental observation, we propose an improved shear-lag model with a trapezoidal-shaped cohesive zone to derive an analytical solution for the decohesion behavior. The theoretical model reveals that the decohesion behavior of the frictional adhesive interface is governed by three dimensionless parameters. Particularly, the dimensionless length of the film essentially determines the decohesion rate; while the other two parameters affect the critical substrate strain to initiate debonding. By fitting the experimental data with the theoretical model, the intrinsic adhesion properties of the two samples are obtained with physically meaningful values. This work offers an analytical solution to describing the decohesion behavior of general thin film/substrate systems with a frictional adhesive interface, which is beneficial for characterizing and optimizing the mechanical properties of various thin film/polymer devices. 

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Qunyang Li

Papers

Bis(2-hydroxyethyl) Terephthalate-Modified Ti3C2Tx/Graphene Nanohybrids as Three-Dimensional Functional Chain Extenders for Polyurethane Composite Films with Strain-Sensing and Conductive Properties.

Decohesion of graphene from a uniaxially-stretched substrate: Failure analysis of a frictional adhesive interface

Sono-electro-mechanical therapy for peripheral nerve regeneration through piezoelectric nanotracts

A parameter identification scheme of the visco-hyperelastic constitutive model of rubber-like materials based on general regression neural network

Α Model of Computation of Elastic and Plastic Contact Considering Adhesion Effect