There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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

The reduction of graphene oxide

Graphene based materials: Past, present and future

Chemically derived graphene oxide (GO) is an atomically thin sheet of graphite that has traditionally served as a precursor for graphene, but is increasingly attracting chemists for its own characteristics. It is covalently decorated with oxygen-containing functional groups - either on the basal plane or at the edges - so that it contains a mixture of sp(2)- and sp(3)-hybridized carbon atoms. In particular, manipulation of the size, shape and relative fraction of the sp(2)-hybridized domains of GO by reduction chemistry provides opportunities for tailoring its optoelectronic properties. For example, as-synthesized GO is insulating but controlled deoxidation leads to an electrically and optically active material that is transparent and conducting. Furthermore, in contrast to pure graphene, GO is fluorescent over a broad range of wavelengths, owing to its heterogeneous electronic structure. In this Review, we highlight the recent advances in optical properties of chemically derived GO, as well as new physical and biological applications.

/pdf/graphene-oxide-as-a-chemically-tunable-platform-for-optical-2c6i758087.pdf

Graphene oxide as a chemically tunable platform for optical applications

The excellent electrical, optical and mechanical properties of graphene have driven the search to find methods for its large-scale production, but established procedures (such as mechanical exfoliation or chemical vapour deposition) are not ideal for the manufacture of processable graphene sheets. An alternative method is the reduction of graphene oxide, a material that shares the same atomically thin structural framework as graphene, but bears oxygen-containing functional groups. Here we use molecular dynamics simulations to study the atomistic structure of progressively reduced graphene oxide. The chemical changes of oxygen-containing functional groups on the annealing of graphene oxide are elucidated and the simulations reveal the formation of highly stable carbonyl and ether groups that hinder its complete reduction to graphene. The calculations are supported by infrared and X-ray photoelectron spectroscopy measurements. Finally, more effective reduction treatments to improve the reduction of graphene oxide are proposed.

Structural evolution during the reduction of chemically derived graphene oxide

We have studied the thermal conductance of tilt grain boundaries in graphene using nonequilibrium molecular dynamics simulations. When a constant heat flux is allowed to flow, we observe sharp jumps in temperature at the boundaries, characteristic of interfaces between materials of differing thermal properties. On the basis of the magnitude of these jumps, we have computed the boundary conductance of twin grain boundaries as a function of their misorientation angles. We find the boundary conductance to be in the range 1.5 × 1010 to 4.5 × 1010 W/(m2 K), which is significantly higher than that of any other thermoelectric interfaces reported in the literature. Using the computed values of boundary conductances, we have identified a critical grain size of 0.1 μm below which the contribution of the tilt boundaries to the conductivity becomes comparable to that of the contribution from the grains themselves. Experiments to test the predictions of our simulations are proposed.

http://utw10193.utweb.utexas.edu/Archive/RuoffsPDFs/265.pdf

Thermal transport across twin grain boundaries in polycrystalline graphene from nonequilibrium molecular dynamics simulations.

By use of molecular dynamics simulations, we have studied the evolution of epoxy and hydroxyl functional groups on graphene oxide (GO) during high-temperature thermal reduction. We find that the reduced GO sheets are characterized by a large number of stable holelike defects formed by breaking of C−C bonds in the basal plane. These defects are always decorated by the carbonyl (C═O) groups and are formed due to the strain in the basal plane created by epoxy and hydroxyl functional groups that are located close to each other. With very few exceptions, the carbonyl groups that are observed in Raman spectroscopy and other experimental studies are generally attributed to the C═O terminations of the edges. However, our study using first-principles calculations and a reactive force field approach clearly shows that the formation of carbonyl groups within the graphene basal plane is energetically favorable compared to other well-known functional groups such as epoxies and ethers. We have identified the specific r...

Stability and formation mechanisms of carbonyl- and hydroxyl-decorated holes in graphene oxide

A one-dimensional generalized thermoelasticity model of a disk based on the Lord–Shulman theory is presented. The dynamic thermoelastic response of the disk under axisymmetric thermal shock loading is studied. The effects of the relaxation time and coupling coefficient are studied. The Laplace transform method is used to transform the coupled governing equations into the space domain, where the Galerkin finite element method is employed to solve the resulting equations in the transformed domain. The dimensionless temperature, displacement, and stresses in the transformed domain are inverted to obtain the actual physical quantities using the numerical inversion of the Laplace transform method.

Akbar Bagri

Papers

Structural evolution during the reduction of chemically derived graphene oxide

Thermal transport across twin grain boundaries in polycrystalline graphene from nonequilibrium molecular dynamics simulations.

Stability and formation mechanisms of carbonyl- and hydroxyl-decorated holes in graphene oxide

Generalized coupled thermoelasticity of disks based on the lord–shulman model

Generalized coupled thermoelasticity of functionally graded annular disk considering the Lord–Shulman theory