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

Unencapsulated, exfoliated black phosphorus (BP) flakes are found to chemically degrade upon exposure to ambient conditions. Atomic force microscopy, electrostatic force microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy are employed to characterize the structure and chemistry of the degradation process, suggesting that O2 saturated H2O irreversibly reacts with BP to form oxidized phosphorus species. This interpretation is further supported by the observation that BP degradation occurs more rapidly on hydrophobic octadecyltrichlorosilane self-assembled monolayers and on H-Si(111) versus hydrophilic SiO2. For unencapsulated BP field-effect transistors, the ambient degradation causes large increases in threshold voltage after 6 h in ambient, followed by a ∼103 decrease in FET current on/off ratio and mobility after 48 h. Atomic layer deposited AlOx overlayers effectively suppress ambient degradation, allowing encapsulated BP FETs to ma...

Effective Passivation of Exfoliated Black Phosphorus Transistors against Ambient Degradation

Sodium-ion batteries have recently attracted significant attention as an alternative to lithium-ion batteries because sodium sources do not present the geopolitical issues that lithium sources might. Although recent reports on cathode materials for sodium-ion batteries have demonstrated performances comparable to their lithium-ion counterparts, the major scientific challenge for a competitive sodium-ion battery technology is to develop viable anode materials. Here we show that a hybrid material made out of a few phosphorene layers sandwiched between graphene layers shows a specific capacity of 2,440 mA h g−1 (calculated using the mass of phosphorus only) at a current density of 0.05 A g−1 and an 83% capacity retention after 100 cycles while operating between 0 and 1.5 V. Using in situ transmission electron microscopy and ex situ X-ray diffraction techniques, we explain the large capacity of our anode through a dual mechanism of intercalation of sodium ions along the x axis of the phosphorene layers followed by the formation of a Na3P alloy. The presence of graphene layers in the hybrid material works as a mechanical backbone and an electrical highway, ensuring that a suitable elastic buffer space accommodates the anisotropic expansion of phosphorene layers along the y and z axial directions for stable cycling operation. The sodiation–desodiation properties of few-layer phosphorene are mostly preserved by sandwiching the material between graphene layers, a behaviour that makes phosphorene–graphene hybrids a potentially suitable anode material for sodium-ion batteries.

A phosphorene–graphene hybrid material as a high-capacity anode for sodium-ion batteries

Phosphorus is one of the most abundant elements preserved in earth, and it comprises a fraction of ∼0.1% of the earth crust. In general, phosphorus has several allotropes, and the two most commonly seen allotropes, i.e. white and red phosphorus, are widely used in explosives and safety matches. In addition, black phosphorus, though rarely mentioned, is a layered semiconductor and has great potential in optical and electronic applications. Remarkably, this layered material can be reduced to one single atomic layer in the vertical direction owing to the van der Waals structure, and is known as phosphorene, in which the physical properties can be tremendously different from its bulk counterpart. In this review article, we trace back to the research history on black phosphorus of over 100 years from the synthesis to material properties, and extend the topic from black phosphorus to phosphorene. The physical and transport properties are highlighted for further applications in electronic and optoelectronics devices.

Semiconducting black phosphorus: synthesis, transport properties and electronic applications

One hundred years after its first successful synthesis in the bulk form in 1914, black phosphorus (black P) was recently rediscovered from the perspective of a 2D layered material, attracting tremendous interest from condensed matter physicists, chemists, semiconductor device engineers, and material scientists. Similar to graphite and transition metal dichalcogenides (TMDs), black P has a layered structure but with a unique puckered single-layer geometry. Because the direct electronic band gap of thin film black P can be varied from 0.3 eV to around 2 eV, depending on its film thickness, and because of its high carrier mobility and anisotropic in-plane properties, black P is promising for novel applications in nanoelectronics and nanophotonics different from graphene and TMDs. Black P as a nanomaterial has already attracted much attention from researchers within the past year. Here, we offer our opinions on this emerging material with the goal of motivating and inspiring fellow researchers in the 2D materials community and the broad readership of PNAS to discuss and contribute to this exciting new field. We also give our perspectives on future 2D and thin film black P research directions, aiming to assist researchers coming from a variety of disciplines who are desirous of working in this exciting research field.

https://www.pnas.org/content/pnas/112/15/4523.full.pdf

The renaissance of black phosphorus

We report on a strong photoluminescence (PL) enhancement of monolayer MoS2 through defect engineering and oxygen bonding. Micro-PL and Raman images clearly reveal that the PL enhancement occurs at cracks/defects formed during high-temperature annealing. The PL enhancement at crack/defect sites could be as high as thousands of times after considering the laser spot size. The main reasons of such huge PL enhancement include the following: (1) the oxygen chemical adsorption induced heavy p doping and the conversion from trion to exciton; (2) the suppression of nonradiative recombination of excitons at defect sites, which was verified by low-temperature PL measurements. First-principle calculations reveal a strong binding energy of ∼2.395 eV for an oxygen molecule adsorbed on a S vacancy of MoS2. The chemically adsorbed oxygen also provides a much more effective charge transfer (0.997 electrons per O2) compared to physically adsorbed oxygen on an ideal MoS2 surface. We also demonstrate that the defect enginee...

Strong Photoluminescence Enhancement of MoS2 through Defect Engineering and Oxygen Bonding

There have been continuous efforts to seek novel functional two-dimensional semiconductors with high performance for future applications in nanoelectronics and optoelectronics. In this work, we introduce a successful experimental approach to fabricate monolayer phosphorene by mechanical cleavage and a subsequent Ar+ plasma thinning process. The thickness of phosphorene is unambiguously determined by optical contrast spectra combined with atomic force microscopy (AFM). Raman spectroscopy is used to characterize the pristine and plasma-treated samples. The Raman frequency of the A2g mode stiffens, and the intensity ratio of A2g to A1g modes shows a monotonic discrete increase with the decrease of phosphorene thickness down to a monolayer. All those phenomena can be used to identify the thickness of this novel two-dimensional semiconductor. This work on monolayer phosphorene fabrication and thickness determination will facilitate future research on phosphorene.

http://www.thenanoresearch.com/upload/justPDF/0446.pdf

Plasma-assisted fabrication of monolayer phosphorene and its Raman characterization

There have been continuous efforts to seek for novel functional two-dimensional semiconductors with high performance for future applications in nanoelectronics and optoelectronics. In this work, we introduce a successful experimental approach to fabricate monolayer phosphorene by mechanical cleavage and the following Ar+ plasma thinning process. The thickness of phosphorene is unambiguously determined by optical contrast combined with atomic force microscope (AFM). Raman spectroscopy is used to characterize the pristine and plasma-treated samples. The Raman frequency of A2g mode stiffens, and the intensity ratio of A2g to A1g modes shows monotonic discrete increase with the decrease of phosphorene thickness down to monolayer. All those phenomena can be used to identify the thickness of this novel two-dimensional semiconductor efficiently. This work for monolayer phosphorene fabrication and thickness determination will facilitates the research of phosphorene.

Low-dimensional materials such as graphene provide an ideal platform to probe the correlation between thermal transport and lattice defects, which could be engineered at the molecular level. In this work, we perform molecular dynamics simulations and non-contact optothermal Raman measurements to study this correlation. We find that oxygen plasma treatment could reduce the thermal conductivity of graphene significantly even at extremely low defect concentration (∼ 83% reduction for ∼ 0.1% defects), which could be attributed mainly to the creation of carbonyl pair defects. Other types of defects such as hydroxyl, epoxy groups and nano-holes demonstrate much weaker effects on the reduction where the sp(2) nature of graphene is better preserved. With the capability of selectively functionalizing graphene, we propose an asymmetric junction between graphene and defective graphene with a high thermal rectification ratio of ∼ 46%, as demonstrated by our molecular dynamics simulation results. Our findings provide fundamental insights into the physics of thermal transport in defective graphene, and two-dimensional materials in general, which could help on the future design of functional applications such as optothermal and electrothermal devices.

/pdf/defect-engineered-heat-transport-in-graphene-a-route-to-high-587otd79xv.pdf

Defect-Engineered Heat Transport in Graphene: A Route to High Efficient Thermal Rectification.

The ultra-clean and defect-free transfer of chemical vapor deposition (CVD) graphene is essential for its application in electronic devices. Here, we study the influence of commonly used etching solvents during the transfer process, i.e. ammonium persulfate, ferric chloride, and ferric nitrate, on the properties of CVD graphene by Raman spectroscopy. Obvious blue shift and broadening of Raman G and 2D peaks were observed for graphene transferred by ferric chloride and ferric nitrate, as compared to that transferred by ammonium persulfate. These changes are attributed to p-doping as well as reduction of phonon lifetime of graphene in the presence of residue iron compounds during the transfer process. The latter would also introduce a great reduction of thermal conductivity of graphene, e.g. with 76% reduction for graphene transferred by ferric nitrate as compared to that transferred by ammonium persulfate. This work would provide valuable information for the transfer of high-quality CVD graphene. Copyright © 2014 John Wiley & Sons, Ltd.

Zheng Liang

Papers

Strong Photoluminescence Enhancement of MoS2 through Defect Engineering and Oxygen Bonding

Plasma-assisted fabrication of monolayer phosphorene and its Raman characterization

Plasma-assisted fabrication of monolayer phosphorene and its Raman characterization

Defect-Engineered Heat Transport in Graphene: A Route to High Efficient Thermal Rectification.

The influence of chemical solvents on the properties of CVD graphene