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

Owing to their extraordinary electrical, chemical, optical, mechanical and structural properties, graphene and its derivatives have stimulated exploding interests in their sensor applications ever since the first isolation of free-standing graphene sheets in year 2004. This article critically and comprehensively reviews the emerging graphene-based electrochemical sensors, electronic sensors, optical sensors, and nanopore sensors for biological or chemical detection. We emphasize on the underlying detection (or signal transduction) mechanisms, the unique roles and advantages of the used graphene materials. Properties and preparations of different graphene materials, their functionalizations are also comparatively discussed in view of sensor development. Finally, the perspective and current challenges of graphene sensors are outlined (312 references).

/pdf/biological-and-chemical-sensors-based-on-graphene-materials-107h21x71b.pdf

Biological and chemical sensors based on graphene materials

The stability of two-dimensional (2D) layers and membranes is subject of a long standing theoretical debate. According to the so called Mermin-Wagner theorem, long wavelength fluctuations destroy the long-range order for 2D crystals. Similarly, 2D membranes embedded in a 3D space have a tendency to be crumpled. These dangerous fluctuations can, however, be suppressed by anharmonic coupling between bending and stretching modes making that a two-dimensional membrane can exist but should present strong height fluctuations. The discovery of graphene, the first truly 2D crystal and the recent experimental observation of ripples in freely hanging graphene makes these issues especially important. Beside the academic interest, understanding the mechanisms of stability of graphene is crucial for understanding electronic transport in this material that is attracting so much interest for its unusual Dirac spectrum and electronic properties. Here we address the nature of these height fluctuations by means of straightforward atomistic Monte Carlo simulations based on a very accurate many-body interatomic potential for carbon. We find that ripples spontaneously appear due to thermal fluctuations with a size distribution peaked around 70 \AA which is compatible with experimental findings (50-100 \AA) but not with the current understanding of stability of flexible membranes. This unexpected result seems to be due to the multiplicity of chemical bonding in carbon.

Intrinsic ripples in graphene

Nonlinear Oscillations: Exact Solutions and their Approximations

Graphene, a monolayer of graphite sheet consisting of sp2 hybridized carbon atoms covalently bonded to three other atoms (discovered in 2004), has recently attracted the attention of chemical sensor researchers owing to its unprecedented structural, mechanical and electrical properties. Excellent mechanical strength (Young modulus ∼0.05 TPa), potentiality of ultrafast electron transport (highest mobility ∼200,000 cm 2 /V s) along with the best surface to volume ratio has opened up the opportunity to use the material for future gas and vapor sensors with ultra fast speed and long-term durability. Since it is a two dimensional material, every atom of graphene may be considered a surface atom and as a result every atom site may be involved in the gas interactions. This feature of graphene can eventually be responsible for its ultra sensitive sensor response with the lowest detection capability approaching even a single molecule. Further, the ease of functionalization of the material either by chemical means (absorption of many molecules like oxygen or hydrogen) or by application of voltage or pressure, facilitates bandgap-engineering which in turn may lead to a possible solution to the selectivity issues, the perennial problems of chemical sensors. In this review, the latest advancement and new perspectives of graphene based gas and vapor sensors have been discussed critically.

Recent developments on graphene and graphene oxide based solid state gas sensors

A Review on the Application of Nonlocal Elastic Models in Modeling of Carbon Nanotubes and Graphenes

Nonlocal plate model for free vibrations of single-layered graphene sheets

The remarkable mechanical properties of carbon nanotubes, such as high elastic modulus and tensile strength, make them the most ideal and promising reinforcements in substantially enhancing the mechanical properties of resulting polymer/carbon nanotube composites. It is acknowledged that the mechanical properties of the composites are significantly influenced by interfacial interactions between nanotubes and polymer matrices. The current challenge of the application of nanotubes in the composites is hence to determine the mechanical properties of the interfacial region, which is critical for improving and manufacturing the nanocomposites. In this work, a new method for evaluating the elastic properties of the interfacial region is developed by examining the fracture behavior of carbon nanotube reinforced poly (methyl methacrylate) (PMMA) matrix composites under tension using molecular dynamics simulations. The effects of the aspect ratio of carbon nanotube reinforcements on the elastic properties, i.e. Young's modulus and yield strength, of the interfacial region and the nanotube/polymer composites are investigated. The feasibility of a three-phase micromechanical model in predicting the elastic properties of the nanocomposites is also developed based on the understanding of the interfacial region.

/pdf/mechanical-properties-of-carbon-nanotube-polymer-composites-4i08z8y6dp.pdf

Behrouz Arash

Papers

A Review on the Application of Nonlocal Elastic Models in Modeling of Carbon Nanotubes and Graphenes

Nonlocal plate model for free vibrations of single-layered graphene sheets

Mechanical properties of carbon nanotube/polymer composites

A review on applications of carbon nanotubes and graphenes as nano-resonator sensors

Nonlocal finite element model for vibrations of embedded multi-layered graphene sheets