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

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.

/pdf/science-and-technology-roadmap-for-graphene-related-two-4q9m7czlkc.pdf

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

This article presents an overview of the essential aspects in the fabrication of silicon and some silicon/germanium nanostructures by metal-assisted chemical etching. First, the basic process and mechanism of metal-assisted chemical etching is introduced. Then, the various influences of the noble metal, the etchant, temperature, illumination, and intrinsic properties of the silicon substrate (e.g., orientation, doping type, doping level) are presented. The anisotropic and the isotropic etching behaviors of silicon under various conditions are presented. Template-based metal-assisted chemical etching methods are introduced, including templates based on nanosphere lithography, anodic aluminum oxide masks, interference lithography, and block-copolymer masks. The metal-assisted chemical etching of other semiconductors is also introduced. A brief introduction to the application of Si nanostructures obtained by metal-assisted chemical etching is given, demonstrating the promising potential applications of metal-assisted chemical etching. Finally, some open questions in the understanding of metal-assisted chemical etching are compiled.

/pdf/metal-assisted-chemical-etching-of-silicon-a-review-2djk1xh2iu.pdf

Metal-Assisted Chemical Etching of Silicon: A Review

Large-Scale Fabrication of Boron Nitride Nanosheets and Their Utilization in Polymeric Composites with Improved Thermal and Mechanical Properties

All highly-efficient organic–inorganic halide perovskite (OIHP) solar cells to date are made of polycrystalline perovskite films which contain a high density of defects, including point and extended imperfections. The imperfections in OIHP materials play an important role in the process of charge recombination and ion migration in perovskite solar cells (PSC), which heavily influences the resulting device energy conversion efficiency and stability. Here we review the recent advances in passivation of imperfections and suppressing ion migration to achieve improved efficiency and highly stable perovskite solar cells. Due to the ionic nature of OIHP materials, the defects in the photoactive films are inevitably electrically charged. The deep level traps induced by particular charged defects in OIHP films are major non-radiative recombination centers; passivation by coordinate bonding, ionic bonding, or chemical conversion have proven effective in mitigating the negative impacts of these deep traps. Shallow level charge traps themselves may contribute little to non-radiative recombination, but the migration of charged shallow level traps in OIHP films results in unfavorable band bending, interfacial reactions, and phase segregation, influencing the carrier extraction efficiency. Finally, the impact of defects and ion migration on the stability of perovskite solar cells is described.

/pdf/imperfections-and-their-passivation-in-halide-perovskite-mfrto61u3p.pdf

Imperfections and their passivation in halide perovskite solar cells

The impact of gate metals on the threshold voltage (VTH) and the gate current of p-GaN gate high-electron-mobility transistors (HEMTs) is investigated by fabricating p-GaN gate HEMTs with different work function gate metals-Ni and W. p-GaN gate HEMTs incorporate a p-GaN layer under the gate electrode as the gate stack on top of the AlGaN/GaN layer. In comparison to the Ni-gate p-GaN HEMTs, the W-gate p-GaN HEMTs showed a higher VTH of 3.0 V and a lower gate current of 0.02 mA/mm at a gate bias of 10 V. Based on TCAD device simulations, we revealed that these high VTH and low gate current are attributed to the low gate metal work function and the high Schottky barrier between the p-GaN and the W gate metal.

p-GaN Gate HEMTs With Tungsten Gate Metal for High Threshold Voltage and Low Gate Current

We report the presence of defects in CH3NH3PbI3, which is one of the main factors that deteriorates the performance of perovskite solar cells. Although the efficiency of the perovskite solar cells has been improved by curing defects using various methods, deeply trapped defects in the perovskite layer have not been systematically studied, and their function is still unclear. The comparison and analysis of defects in differently prepared perovskite solar cells reveals that both solar cells have two kinds of deep level defects (E1 and E2). In the one-pot solution processed solar cell, the defect state E1 is dominant, while E2 is the major defect in the solar cell prepared using the cuboid method. Since the energy level of E1 is higher than that of E2, the cuboid solar cell shows higher open-circuit voltage and efficiency.

Deep level trapped defect analysis in CH3NH3PbI3 perovskite solar cells by deep level transient spectroscopy

One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the electronic band structure of the Si nanowire to obtain a direct band gap. Here, we present a new strategy for controlling the electronic band structure of Si nanowires. Our method is attributed to the band structure modulation driven by uniaxial strain. We show that the band structure modulation with lattice strain is strongly dependent on the crystal orientation and diameter of SiNWs. In the case of [100] and [111] SiNWs, tensile strain enhances the direct band gap characteristic, whereas compressive strain attenuates it. [110] SiNWs have a different strain dependence in that both compressive and tensile strain make SiNWs exhibit an indirect band gap. We discuss the origin of this strain dependence based on the band features of bulk silicon and the wave functions of SiNWs. These results could be helpful for band structure engineering and analysis of SiNWs in nanoscale devices.

Strain-driven electronic band structure modulation of si nanowires.

Evolution of a single graphene layer with disorder generated by remote oxygen plasma irradiation is investigated using atomic force microscopy, Raman spectroscopy and electrical measurement. Gradual changes of surface morphology from planar graphene to isolated granular structure associated with a decrease of transconductance are accounted for by two-dimensional percolative conduction by disorder and the oxygen plasma-induced doping effect. The corresponding evolution of Raman spectra of graphene shows several peculiarities such as a sudden appearance of a saturated D peak followed by a linear decrease in its intensity, a relatively inert characteristic of a D' peak and a monotonic increase of a G peak position as the exposure time to oxygen plasma increases. These are discussed in terms of a disorder-induced change of Raman spectra in the graphite system.

http://iopscience.iop.org/article/10.1088/0957-4484/20/37/375703/pdf

The structural and electrical evolution of graphene by oxygen plasma-induced disorder.

Normally off Al2O3/GaN MOSFETs are fabricated by utilizing a simple tetramethylammonium hydroxide (TMAH) treatment as a postgate-recess process. The TMAH-treated device with a gate length of 2.5 μm exhibited excellent device performances, such as a threshold voltage of 3.5 V, a maximum drain current of 336 mA/mm, and a breakdown voltage of 725 V, along with extremely small gate leakage current of about 10-9 A/mm at Vgs = 15 V, which is approximately six orders lower in magnitude compared to that of the device without TMAH treatment.

Jai-Kwang Shin

Papers

p-GaN Gate HEMTs With Tungsten Gate Metal for High Threshold Voltage and Low Gate Current

Deep level trapped defect analysis in CH3NH3PbI3 perovskite solar cells by deep level transient spectroscopy

Strain-driven electronic band structure modulation of si nanowires.

The structural and electrical evolution of graphene by oxygen plasma-induced disorder.

Effects of TMAH Treatment on Device Performance of Normally Off $\hbox{Al}_{2}\hbox{O}_{3}/\hbox{GaN}$ MOSFET