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

Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.

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Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

Ultrathin two-dimensional nanosheets of layered transition metal dichalcogenides (TMDs) are fundamentally and technologically intriguing. In contrast to the graphene sheet, they are chemically versatile. Mono- or few-layered TMDs - obtained either through exfoliation of bulk materials or bottom-up syntheses - are direct-gap semiconductors whose bandgap energy, as well as carrier type (n- or p-type), varies between compounds depending on their composition, structure and dimensionality. In this Review, we describe how the tunable electronic structure of TMDs makes them attractive for a variety of applications. They have been investigated as chemically active electrocatalysts for hydrogen evolution and hydrosulfurization, as well as electrically active materials in opto-electronics. Their morphologies and properties are also useful for energy storage applications such as electrodes for Li-ion batteries and supercapacitors.

http://nanotubes.rutgers.edu/PDFs/nchem_TMDs_2013.pdf

The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets

As a fascinating conjugated polymer, graphitic carbon nitride (g-C3N4) has become a new research hotspot and drawn broad interdisciplinary attention as a metal-free and visible-light-responsive photocatalyst in the arena of solar energy conversion and environmental remediation. This is due to its appealing electronic band structure, high physicochemical stability, and “earth-abundant” nature. This critical review summarizes a panorama of the latest progress related to the design and construction of pristine g-C3N4 and g-C3N4-based nanocomposites, including (1) nanoarchitecture design of bare g-C3N4, such as hard and soft templating approaches, supramolecular preorganization assembly, exfoliation, and template-free synthesis routes, (2) functionalization of g-C3N4 at an atomic level (elemental doping) and molecular level (copolymerization), and (3) modification of g-C3N4 with well-matched energy levels of another semiconductor or a metal as a cocatalyst to form heterojunction nanostructures. The constructi...

Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability?

Graphene-Like Two-Dimensional Materials

The interactions between four different graphenes (including pristine, B- or N-doped and defective graphenes) and small gas molecules (CO, NO, NO2 and NH3) were investigated by using density functional computations to exploit their potential applications as gas sensors. The structural and electronic properties of the graphene-molecule adsorption adducts are strongly dependent on the graphene structure and the molecular adsorption configuration. All four gas molecules show much stronger adsorption on the doped or defective graphenes than that on the pristine graphene. The defective graphene shows the highest adsorption energy with CO, NO and NO2 molecules, while the B- doped graphene gives the tightest binding with NH3. Meanwhile, the strong interactions between the adsorbed molecules and the modified graphenes induce dramatic changes to graphene's electronic properties. The transport behavior of a gas sensor using B- doped graphene shows a sensitivity two orders of magnitude higher than that of pristine graphene. This work reveals that the sensitivity of graphene-based chemical gas sensors could be drastically improved by introducing the appropriate dopant or defect.

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Improving gas sensing properties of graphene by introducing dopants and defects: a first-principles study.

Layered two-dimensional (2D) nanomaterials such as graphene are a conceptually new class of materials that offers new access to low-dimensional physics. Besides wellknown graphene, inorganic graphene analogues (IGAs) such as layered transition metal dichalcogenides (e.g., MoS2 and WS2) [5–7] and boron nitride (BN) have been attracting rapidly increasing attention in the past few years. These IGAs were expected to exhibit unique properties and have great potential in applications like transistors, energy storage, thermal conductors, and topological insulators. Moreover, IGAs like MoS2 and WS2 have intrinsic band gap and high mobility, and may even compete with graphene in certain fields. However, investigations on IGAs have been significantly hindered by the practical difficulties in the preparation and assembly of these 2D nanomaterials. Only a few approaches to obtain few-layered IGAs have been reported. Mechanical exfoliation was first used to obtain layered IGAs from their bulk materials. Other approaches that are being explored include chemical synthesis and liquid exfoliation. Coleman et al. recently reported a surfactant-free liquid-exfoliation method which can produce few-layered nanosheets of IGAs dispersed in various organic solvents. Thermodynamic analysis suggested that, because of the high surface energy of IGAs, the best solvents are likely to have high boiling points. Using nonvolatile solvents makes it difficult to process IGAs into devices, due to the difficulties in the removal of solvent and the occurrence of aggregation during the slow solvent evaporation. To date, liquid exfoliation of layered MoS2 and WS2 in volatile solvent has met with very limited success. Herein we demonstrate a versatile and scaleable mixedsolvent strategy for liquid exfoliation of IGAs, including WS2, MoS2, and BN, in volatile solvents. By choosing solvents with appropriate composition, highly stable IGA suspensions can be obtained in low-boiling solvent mixtures, which can then be easily used in further applications. The dispersion of nanomaterials in liquids can be partially predicted by the theory of Hansen solubility parameters (HSP), which is a semi-empirical correlation developed to explain dissolution behavior. Three HSP parameters are used to describe the character of a solvent or material: dD, dP, and dH, which are the dispersive, polar, and hydrogen-bonding solubility parameters, respectively. The dissolution process is one of adaptation between the HSP parameters of solvents and solutes. The HSP distance Ra is used to evaluate the level of adaptation [Eq. (1)].

A Mixed‐Solvent Strategy for Efficient Exfoliation of Inorganic Graphene Analogues

Developing 'smart' membranes that allow precise and reversible control of molecular permeation using external stimuli would be of intense interest for many areas of science: from physics and chemistry to life-sciences. In particular, electrical control of water permeation through membranes is a long-sought objective and is of crucial importance for healthcare and related areas. Currently, such adjustable membranes are limited to the modulation of wetting of the membranes and controlled ion transport, but not the controlled mass flow of water. Despite intensive theoretical work yielding conflicting results, the experimental realisation of electrically controlled water permeation has not yet been achieved. Here we report electrically controlled water permeation through micrometre-thick graphene oxide (GO) membranes. By controllable electric breakdown, conductive filaments are created in the GO membrane. The electric field concentrated around such current carrying filaments leads to controllable ionisation of water molecules in graphene capillaries, allowing precise control of water permeation: from ultrafast permeation to complete blocking. Our work opens up an avenue for developing smart membrane technologies and can revolutionize the field of artificial biological systems, tissue engineering and filtration.

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Electrically controlled water permeation through graphene oxide membranes.

We demonstrate a strategy for designing high-performance, ambipolar, acene-based field-effect transistor (FET) materials, which is based on the replacement of C−H moieties by nitrogen atoms in oligoacenes. By using this strategy, two organic semiconductors, 6,13-bis(triisopropylsilylethynyl)anthradipyridine (1) and 8,9,10,11-tetrafluoro-6,13-bis(triisopropylsilylethynyl)-1-azapentacene (3), were synthesized and their FET characteristics studied. Both materials exhibit high and balanced hole and electron mobilities, 1 having μh and μe of 0.11 and 0.15 cm2/V·s and 3 having μh and μe of 0.08 and 0.09 cm2/V·s, respectively. The successful demonstration of high and balanced ambipolar FET properties from nitrogen-containing oligoacenes opens up new opportunities for designing high-performance ambipolar organic semiconductors.

High and Balanced Hole and Electron Mobilities from Ambipolar Thin-Film Transistors Based on Nitrogen-Containing Oligoacences

In this work, we use Raman spectroscopy as a nondestructive and rapid technique for probing the van der Waals (vdW) forces acting between two atomically thin crystals, where one is a transition metal dichalcogenide (TMDC). In this work, MoS2 is used as a Raman probe: we show that its two Raman-active phonon modes can provide information on the interaction between the two crystals. In particular, the in-plane vibration (E2g1) provides information on the in-plane strain, while the out-of-plane mode (A1g) gives evidence for the quality of the interfacial contact. We show that a vdW contact with MoS2 is characterized by a blue shift of +2 cm–1 of the A1g peak. In the case of a MoS2/graphene heterostructure, the vdW contact is also characterized by a shift of +14 cm–1 of the 2D peak of graphene. Our approach offers a very simple, nondestructive, and fast method to characterize the quality of the interface of heterostructures containing atomically thick TMDC crystals.

Kai-Ge Zhou

Papers

Improving gas sensing properties of graphene by introducing dopants and defects: a first-principles study.

A Mixed‐Solvent Strategy for Efficient Exfoliation of Inorganic Graphene Analogues

Electrically controlled water permeation through graphene oxide membranes.

High and Balanced Hole and Electron Mobilities from Ambipolar Thin-Film Transistors Based on Nitrogen-Containing Oligoacences

Raman modes of MoS2 used as fingerprint of van der Waals interactions in 2-D crystal-based heterostructures.