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

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

Graphene is one of the most promising materials in nanotechnology. The electronic and mechanical properties of graphene samples with high perfection of the atomic lattice are outstanding, but structural defects, which may appear during growth or processing, deteriorate the performance of graphene-based devices. However, deviations from perfection can be useful in some applications, as they make it possible to tailor the local properties of graphene and to achieve new functionalities. In this article, the present knowledge about point and line defects in graphene are reviewed. Particular emphasis is put on the unique ability of graphene to reconstruct its lattice around intrinsic defects, leading to interesting effects and potential applications. Extrinsic defects such as foreign atoms which are of equally high importance for designing graphene-based devices with dedicated properties are also discussed.

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Structural defects in graphene

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

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Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

Many proposed applications of graphene require the ability to tune its electronic structure at the nanoscale1,2. Although charge transfer3 and field-effect doping4 can be applied to manipulate charge carrier concentrations, using them to achieve nanoscale control remains a challenge. An alternative approach is ‘self-doping’5, in which extended defects are introduced into the graphene lattice. The controlled engineering of these defects represents a viable approach to creation and nanoscale control of one-dimensional charge distributions with widths of several atoms6. However, the only experimentally realized extended defects so far have been the edges of graphene nanoribbons7,8,9,10, which show dangling bonds that make them chemically unstable11,12,13. Here, we report the realization of a one-dimensional topological defect in graphene, containing octagonal and pentagonal sp2-hybridized carbon rings embedded in a perfect graphene sheet. By doping the surrounding graphene lattice, the defect acts as a quasi-one-dimensional metallic wire. Such wires may form building blocks for atomic-scale, all-carbon electronics. A stable extended defect in graphene consisting of octagonal and pentagonal rings produces one-dimensional charge localization, allowing it to act as a metallic wire embedded in an otherwise perfect graphene sheet.

An extended defect in graphene as a metallic wire

In the molecular electronics field it is highly desirable to engineer the structure of molecules to achieve specific functions. In particular, diode (or rectification) behaviour in single molecules is an attractive device function. Here we study charge transport through symmetric tetraphenyl and non-symmetric diblock dipyrimidinyldiphenyl molecules covalently bound to two electrodes. The orientation of the diblock is controlled through a selective deprotection strategy, and a method in which the electrode-electrode distance is modulated unambiguously determines the current-voltage characteristics of the single-molecule device. The diblock molecule exhibits pronounced rectification behaviour compared with its homologous symmetric block, with current flowing from the dipyrimidinyl to the diphenyl moieties. This behaviour is interpreted in terms of localization of the wave function of the hole ground state at one end of the diblock under the applied field. At large forward current, the molecular diode becomes unstable and quantum point contacts between the electrodes form.

Rectification and stability of a single molecular diode with controlled orientation

The layer-dependent structural, electronic, and vibrational properties of ${\mathrm{SnS}}_{2}$ and ${\mathrm{SnSe}}_{2}$ are investigated using first-principles density functional theory (DFT). The in-plane lattice constants, interlayer distances and binding energies are found to be layer-independent. Bulk ${\mathrm{SnS}}_{2}$ and ${\mathrm{SnSe}}_{2}$ are both indirect band gap semiconductors with ${E}_{g}=2.18$ and $1.07\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, respectively. Few-layer and monolayer 2D systems also possess an indirect band gap, which is increased to 2.41 and $1.69\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ for single layers of ${\mathrm{SnS}}_{2}$ and ${\mathrm{SnSe}}_{2}.$ The effective mass theory of 2D excitons, which takes into account the combined effect of the anisotropy, nonlocal 2D screening and layer-dependent 3D screening, predicts strong excitonic effects. The binding energy of indirect excitons in monolayer samples, ${E}_{x}\ensuremath{\sim}0.9\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, is substantially reduced to ${E}_{x}=0.14\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ in bulk ${\mathrm{SnS}}_{2}$ and ${E}_{x}=0.09\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ in bulk ${\mathrm{SnSe}}_{2}$. The layer-dependent Raman spectra display a strong decrease of intensities of the Raman active ${A}_{1g}$ mode upon decreasing the number of layers down to a monolayer, by a factor of 7 in the case of ${\mathrm{SnS}}_{2}$ and a factor of 20 in the case of ${\mathrm{SnSe}}_{2}$, which can be used to identify the number of layers in a 2D sample.

Layer-dependent properties of SnS 2 and SnSe 2 two-dimensional materials

A novel growth mechanism of graphene on Ni(111) has been discovered that occurs at temperatures below 460 °C. At these conditions, a surface-confined nickel−carbide phase coexists with single layer graphene. The graphene grows by in-plane transformation of the carbide along a one-dimensional phase-boundary, which is distinctively different from known growth processes on other transition metals and on Ni above 460 °C, where carbon atoms attach to “free” edges of graphene islands.

Graphene growth on Ni(111) by transformation of a surface carbide.

The pentazolates, the last all-nitrogen members of the azole series, have been notoriously elusive for the last hundred years despite enormous efforts to make these compounds in either gas or condensed phases. Here, we report a successful synthesis of a solid state compound consisting of isolated pentazolate anions N5–, which is achieved by compressing and laser heating cesium azide (CsN3) mixed with N2 cryogenic liquid in a diamond anvil cell. The experiment was guided by theory, which predicted the transformation of the mixture at high pressures to a new compound, cesium pentazolate salt (CsN5). Electron transfer from Cs atoms to N5 rings enables both aromaticity in the pentazolates as well as ionic bonding in the CsN5 crystal. This work provides critical insight into the role of extreme conditions in exploring unusual bonding routes that ultimately lead to the formation of novel high nitrogen content species.

Ivan Oleynik

Papers

An extended defect in graphene as a metallic wire

Rectification and stability of a single molecular diode with controlled orientation

Layer-dependent properties of SnS 2 and SnSe 2 two-dimensional materials

Graphene growth on Ni(111) by transformation of a surface carbide.

High-Pressure Synthesis of a Pentazolate Salt