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 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

Most of recent research on layered chalcogenides is understandably focused on single atomic layers. However, it is unclear if single-layer units are the most ideal structures for enhanced gas–solid interactions. To probe this issue further, we have prepared large-area MoS2 sheets ranging from single to multiple layers on 300 nm SiO2/Si substrates using the micromechanical exfoliation method. The thickness and layering of the sheets were identified by optical microscope, invoking recently reported specific optical color contrast, and further confirmed by AFM and Raman spectroscopy. The MoS2 transistors with different thicknesses were assessed for gas-sensing performances with exposure to NO2, NH3, and humidity in different conditions such as gate bias and light irradiation. The results show that, compared to the single-layer counterpart, transistors of few MoS2 layers exhibit excellent sensitivity, recovery, and ability to be manipulated by gate bias and green light. Further, our ab initio DFT calculations...

Sensing behavior of atomically thin-layered MoS2 transistors

Wrinkling is a ubiquitous phenomenon in two-dimensional membranes. In particular, in the large-scale growth of graphene on metallic substrates, high densities of wrinkles are commonly observed. Despite their prevalence and potential impact on large-scale graphene electronics, relatively little is known about their structural morphology and electronic properties. Surveying the graphene landscape using atomic force microscopy, we found that wrinkles reach a certain maximum height before folding over. Calculations of the energetics explain the morphological transition and indicate that the tall ripples are collapsed into narrow standing wrinkles by van der Waals forces, analogous to large-diameter nanotubes. Quantum transport calculations show that conductance through these “collapsedwrinkle” structures is limited mainly by a density-of-states bottleneck and by interlayer tunneling across the collapsed bilayer region. Also through systematic measurements across large numbers of devices with wide “foldedwrink...

https://researcher.watson.ibm.com/researcher/files/us-vperebe/graphene_wrinkle.pdf

Structure and Electronic Transport in Graphene Wrinkles

Carbon nanomaterials have been attracting a great deal of research interest in the last few years. Their unique electrical, optical and mechanical properties make them very interesting for developing the new generation of miniaturised, low-power, ubiquitous sensors. In the particular case of gas sensing, some carbon nanomaterials such as nanofibres, nanotubes and graphene are threatening the dominance position of other well established (nano)materials, yet the commercial exploitation of carbon nanomaterials is still a way off. This paper reviews the state of the art for electrical gas sensors employing carbon nanomaterials, identifies the bottlenecks that impair their commercialisation and also some recent breakthroughs. Finally an outlook in which challenges and opportunities are identified is given.

Gas sensors using carbon nanomaterials: A review

The advance of nanomaterials has opened new opportunities to develop ever more sensitive sensors owing to their high surface-to-volume ratio. However, it is challenging to achieve intrinsic sensitivities of nanomaterials for ultra-low level detections due to their vulnerability against contaminations. Here we show that despite considerable achievements in the last decade, continuous in situ cleaning of carbon nanotubes with ultraviolet light during gas sensing can still dramatically enhance their performance. For instance in nitric oxide detection, while sensitivity in air is improved two orders of magnitude, under controlled environment it reaches a detection limit of 590 parts-per-quadrillion (ppq) at room temperature. Furthermore, aiming for practical applications we illustrate how to address gas selectivity by introducing a gate bias. The concept of continuous in situ cleaning not only reveals the tremendous sensing potential of pristine carbon nanotubes but also more importantly it can be applied to other nanostructures.

/pdf/enhanced-gas-sensing-in-pristine-carbon-nanotubes-under-2mpmq4iqaa.pdf

Enhanced gas sensing in pristine carbon nanotubes under continuous ultraviolet light illumination

Formation of ripples on a supported graphene sheet involves interfacial interaction with the substrate. In this work, graphene was grown on a copper foil by chemical vapor deposition from methane. On thermal quenching from elevated temperatures, we observed the formation of ripples in grown graphene, developing a peculiar topographic pattern in the form of wavy grooves and single/double rolls, roughly honeycomb cells, or their combinations. Studies on pure copper foil under corresponding conditions but without the presence of hydrocarbon revealed the appearance of peculiar patterns on the foil surface, such as dendritic structures that are distinctive not of equilibrium solidified phases but arise from planar and/or convective instabilities driven by solutal and thermal capillary forces. We propose a new origin for the formation of ripples in the course of graphene growth at elevated temperatures, where the topographic pattern formation is governed by dynamic instabilities on the interface of a carbon–cat...

Formation of Ripples in Graphene as a Result of Interfacial Instabilities

In situ observation of the carbon nanotube nucleation process accompanied by dynamic reconstruction of the catalyst particle morphology is considered within a thermodynamic approach. It reveals the driving force for the detachment of the sp2-carbon cap, so-called lift-off—a crucial event in nanotube growth. A continuum model and detailed atomistic calculations identify the critical factors in the lift-off process: (i) catalyst surface energy, affected by the chemisorbed carbon atoms at the exterior surface of the catalyst, exposed to the carbon feedstock; and (ii) the emergence of a pristine, high-energy facet under the sp2-carbon dome. This further allows one to evaluate the range of carbon feedstock chemical potential, where the lift-off process occurs, to be followed by emergence of single-walled nanotube, and provides insights into observed catalyst morphology oscillations leading to formation of multiwalled carbon nanotubes.

Carbon Nanotube Nucleation Driven by Catalyst Morphology Dynamics

Understanding the performance volatility of carbon nanotube-based devices will expedite their applications. We performed in situ electrical and Raman scattering studies on an individual semiconducting single-walled carbon nanotube in the field-effect transistor geometry under different ambient and temperatures. The Raman G+ mode frequency responds in synchronization with changes in the charge density induced by an external gate voltage. Ambient caused a blueshift in the G+ mode and a reversible transformation of the device performance from n-type in vacuum to p-type in air, owing to the charge transfer-induced phonon renormalization by oxygen.

The performance volatility of carbon nanotube-based devices: Impact of ambient oxygen

In situ observation of the carbon nanotube nucleation process accompanied by dynamic reconstruction of the catalyst particle morphology is considered within a thermodynamic approach. It reveals the driving force for the detachment of the sp(2)-carbon cap, so-called lift-off-a crucial event in nanotube growth. A continuum model and detailed atomistic calculations identify the critical factors in the lift-off process: (i) catalyst surface energy, affected by the chemisorbed carbon atoms at the exterior surface of the catalyst, exposed to the carbon feedstock; and (ii) the emergence of a pristine, high-energy facet under the sp(2)-carbon dome. This further allows one to evaluate the range of carbon feedstock chemical potential, where the lift-off process occurs, to be followed by emergence of single-walled nanotube, and provides insights into observed catalyst morphology oscillations leading to formation of multiwalled carbon nanotubes.

/pdf/carbon-nanotube-nucleation-driven-by-catalyst-morphology-6ai9l1sw78.pdf

Elena Mora Pigos

Papers

Enhanced gas sensing in pristine carbon nanotubes under continuous ultraviolet light illumination

Formation of Ripples in Graphene as a Result of Interfacial Instabilities

Carbon Nanotube Nucleation Driven by Catalyst Morphology Dynamics

The performance volatility of carbon nanotube-based devices: Impact of ambient oxygen

Carbon Nanotube Nucleation Driven by Catalyst Morphology Dynamics | NIST