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

Graphene is a wonder material with many superlatives to its name. It is the thinnest known material in the universe and the strongest ever measured. Its charge carriers exhibit giant intrinsic mobility, have zero effective mass, and can travel for micrometers without scattering at room temperature. Graphene can sustain current densities six orders of magnitude higher than that of copper, shows record thermal conductivity and stiffness, is impermeable to gases, and reconciles such conflicting qualities as brittleness and ductility. Electron transport in graphene is described by a Dirac-like equation, which allows the investigation of relativistic quantum phenomena in a benchtop experiment. This review analyzes recent trends in graphene research and applications, and attempts to identify future directions in which the field is likely to develop.

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Graphene: Status and Prospects

Graphene has been attracting great interest because of its distinctive band structure and physical properties. Today, graphene is limited to small sizes because it is produced mostly by exfoliating graphite. We grew large-area graphene films of the order of centimeters on copper substrates by chemical vapor deposition using methane. The films are predominantly single-layer graphene, with a small percentage (less than 5%) of the area having few layers, and are continuous across copper surface steps and grain boundaries. The low solubility of carbon in copper appears to help make this growth process self-limiting. We also developed graphene film transfer processes to arbitrary substrates, and dual-gated field-effect transistors fabricated on silicon/silicon dioxide substrates showed electron mobilities as high as 4050 square centimeters per volt per second at room temperature.

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Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils

There is intense interest in graphene in fields such as physics, chemistry, and materials science, among others. Interest in graphene's exceptional physical properties, chemical tunability, and potential for applications has generated thousands of publications and an accelerating pace of research, making review of such research timely. Here is an overview of the synthesis, properties, and applications of graphene and related materials (primarily, graphite oxide and its colloidal suspensions and materials made from them), from a materials science perspective.

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Graphene and Graphene Oxide: Synthesis, Properties, and Applications

The richness of optical and electronic properties of graphene attracts enormous interest. Graphene has high mobility and optical transparency, in addition to flexibility, robustness and environmental stability. So far, the main focus has been on fundamental physics and electronic devices. However, we believe its true potential lies in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, even in the absence of a bandgap, and the linear dispersion of the Dirac electrons enables ultrawideband tunability. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light-emitting devices to touch screens, photodetectors and ultrafast lasers. Here we review the state-of-the-art in this emerging field.

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Graphene photonics and optoelectronics

The impact of the thickness of an AlN spacer in AlGaN/AlN/GaN high electron mobility transistor (HEMT) structures on the Hall mobility was investigated in a range of 30 K - 340 K. The AlN spacer has a strong impact on the mobility at temperatures below 150 K. This effect is linked to a reduction of alloy scattering. Optical and scanning electron microscopy revealed hexagonal shaped defects which also have an effect on the mobility. These defects can be avoided by an appropriate adjustment of the AlN layer thickness.

Impact of AlN Spacer on Electron Mobility of AlGaN/AlN/GaN Structures on Silicon

Epitaxial Graphene on Silicon Carbide as a Tailorable Metal–Semiconductor Interface

The speed of an active electronic semiconductor device is limited by RC timescale, i.e., the time required for its charging and discharging. To circumvent this ubiquitous limitation of conventional electronics, we investigate diodes under intense mid-infrared light-field pulses. We choose epitaxial graphene on silicon carbide as a metal/semiconductor pair, acting as an ultrarobust and almost-transparent Schottky diode. The usually dominant forward direction is suppressed, but a characteristic signal occurs in reverse bias. For its theoretical description, we consider tunneling through the light-field–modulated Schottky barrier, complemented by a dynamical accumulation correction. On the basis only of the DC parametrization of the diode, the model provides a consistent and accurate description of the experimentally observed infrared phenomena. This allows the conclusion that cycle-by-cycle dynamics determines rectification. As the chosen materials have proven capabilities for transistors, circuits, and even a full logic, we see a way to establish light-field-driven electronics with rapidly increasing functionality.

Light-field-driven electronics electronics in the mid-infrared regime: Schottky rectification

Current-voltage characterization and thermal dielectric relaxation current (TDRC) measurements are carried out on 4H silicon carbide (SiC) n-channel MOSFETs processed with different post oxidation anneals (POAs) in O2, N2O, and NO atmospheres at high temperature. In all samples we observe a distinct peak at a temperature of 70 K in the TDRC spectra due to a defect close to the conduction band of 4H-SiC having a high density of states (>1013 cm-2eV-1). We show that this defect is related to the degradation of the device performance such as the MOSFET conductivity. Comparing the different POAs, NO strongly reduces the density of states close to the conduction band and thus increases the amount of free channel electrons. Based on TDRC measurements we want to suggest a method for more accurate estimation of the true channel mobility accounting for the reduced channel electron density due to trapping.

Determination of Performance-Relevant Trapped Charge in 4H Silicon Carbide MOSFETs

Abstract The sequential vertical polyfunctionalization of 2D addend‐patterned graphene is still elusive. Here, we report a practical realization of this goal via a “molecular building blocks” approach, which is based on a combination of a lithography‐assisted reductive functionalization approach and a post‐functionalization step to sequentially and controllably link the molecular building blocks ethylpyridine, cis‐dichlorobis(2,2′‐bipyridyl)ruthenium, and triphenylphosphine (4‐methylbenzenethiol, respectively) on selected lattice regions of a graphene matrix. The assembled 2D hetero‐architectures are unambiguously characterized by various spectroscopic and microscopic measurements, revealing the stepwise stacking of the molecular building blocks on the graphene surface. Our method overcomes the current limitation of a one‐layer‐only binding to the graphene surface and opens the door for a vertical growth in the z‐direction.

https://opus4.kobv.de/opus4-fau/files/20312/ANIE_ANIE202201169.pdf

Heiko B. Weber

Papers

Impact of AlN Spacer on Electron Mobility of AlGaN/AlN/GaN Structures on Silicon

Epitaxial Graphene on Silicon Carbide as a Tailorable Metal–Semiconductor Interface

Light-field-driven electronics electronics in the mid-infrared regime: Schottky rectification

Determination of Performance-Relevant Trapped Charge in 4H Silicon Carbide MOSFETs

Molecular Stacking on Graphene