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

Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Shape‐Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?

Recent years have seen a renewed interest in the harvesting and conversion of solar energy. Among various technologies, the direct conversion of solar to chemical energy using photocatalysts has received significant attention. Although heterogeneous photocatalysts are almost exclusively semiconductors, it has been demonstrated recently that plasmonic nanostructures of noble metals (mainly silver and gold) also show significant promise. Here we review recent progress in using plasmonic metallic nanostructures in the field of photocatalysis. We focus on plasmon-enhanced water splitting on composite photocatalysts containing semiconductor and plasmonic-metal building blocks, and recently reported plasmon-mediated photocatalytic reactions on plasmonic nanostructures of noble metals. We also discuss the areas where major advancements are needed to move the field of plasmon-mediated photocatalysis forward.

Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy

Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2-3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? The quantum mechanical coupling * To whom correspondence should be addressed. E-mail: dvtalapin@uchicago.edu. † The University of Chicago. ‡ Argonne National Lab. Chem. Rev. 2010, 110, 389–458 389

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Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications

Localized Surface Plasmon Resonance Sensors

Galvanic replacement reaction provides a remarkably simple and versatile route to metal nanostructures with controllable hollow interiors and porous walls. The key step of this process involves the replacement reaction between a suspension of nanoscale metal templates and a salt precursor containing a relatively less active metal. This method has been successfully applied to prepare gold-based hollow nanostructures with a wide range of different morphologies, including cubic nanoboxes, cubic nanocages, triangular nanorings, prism-shaped nanoboxes, single-walled nanotubes, and multiple-walled nanoshells or nanotubes. In addition to gold, hollow platinum and palladium nanostructures have also been prepared by using appropriate salt precursors for the replacement reaction. These hollow and porous metal nanostructures show intriguing optical and mechanical properties, with their surface plasmon resonance peaks tunable from the visible to the near-infrared region. These materials are expected to find a...

Galvanic replacement reaction: A simple and powerful route to hollow and porous metal nanostructures

Branched nanocrystals display interesting optical and catalytic properties on account of their high surface areas and tips with small radii of curvatures. However, many synthetic routes toward branched nanocrystals result in inhomogeneous samples on account of asymmetric branching. Seed-mediated coreduction is a recently developed route to symmetrically branched nanocrystals where the symmetry of the seeds is transferred to the final stellated morphologies. Here, general guidelines to stellated nanocrystals are outlined by surveying coreduction of Au and Pd precursors in the presence of a variety of shape-controlled Au seeds to achieve Au/Pd nanostructures. Single-crystalline, twinned, and anisotropic seeds were analyzed to expand the classes of stellated nanostructures synthetically accessible. Significantly, single-crystalline Au seeds adopt {100}-terminated intermediates prior to branching, regardless of initial seed shape. We compared these results with those obtained with shape-controlled Pd seeds, a...

Diffusion and Seed Shape: Intertwined Parameters in the Synthesis of Branched Metal Nanostructures

The global economic, security, and health challenges presented by counterfeit goods require new approaches toward anti-counterfeit labels. This review describes recent advances in the use of metal nanomaterials for optical anti-counterfeit labels that may offer a multiplexed approach to security tags that can be easily fabricated, offer large coding capacity, and be interrogated throughout the supply chain and by the end user. This review also critically discusses the current approach to developing continuously more complex labels and offers awareness toward the need for simple, yet unclonable, taggants.

Metal nanomaterials for optical anti-counterfeit labels

Transient absorption experiments with diffraction-limited spatial resolution have been used to study the optical absorption properties and dynamics of isolated, single silver nanowires. The images and polarization analysis show that the near-IR pump and near-UV probe beams couple to fundamentally different electron motions. The near-IR pump laser excites the propagating surface plasmon polariton (SPP) modes of the wires when focused at the ends, and multipolar plasmon modes (antenna modes) for medial excitation. The images show that these two modes have comparable optical absorption cross-sections. In contrast, the near-UV probe couples to the transverse plasmon resonance of the wire independent of the spatial position. For either end-on or medial excitation, pump laser absorption causes lattice heating and coherently excites the breathing vibrational mode of the nanowires. The vibrational quality factors depend on the acoustic impedance mismatch between the nanowire and the environment, and are similar to those recently measured for silver nanocubes. Experiments performed with spatially separated pump and probe beams, with the pump beam focused at one end to excite the propagating SPP modes, show that the amplitudes of the initial transient absorption signal and the breathing motion decrease with distance along the wire. This arises because the propagating SPP mode decays as it moves down the wire, which reduces the number of electronic excitations and, therefore, the signal level in the experiments. The measured length scale for the SPP decay is similar to that obtained in previous light scattering experiments.

Coupling to light, and transport and dissipation of energy in silver nanowires

In this Perspective, the use of ultrasound in the synthesis and modification of carbon materials is surveyed. Ultrasound is a common laboratory tool used to nebulize solutions into fine mists, emulsify mixtures, and drive chemical reactions. Given society’s continued use of carbon materials (e.g., carbon black and activated carbon) as pigments, adsorbents, and composite components as well as the exciting new applications being explored for carbon nanotubes, graphene, and meso- and macroporous carbons, the use of ultrasound in the synthesis and modification of carbon materials is of general interest. Here, carbon materials prepared by both ultrasonic spray pyrolysis and high intensity ultrasound will be discussed, with the properties and applications enabled by their preparation highlighted within the individual examples. This article is concluded with some personal perspectives on the directions toward which future research may be aimed.

Sara E. Skrabalak

Papers

Galvanic replacement reaction: A simple and powerful route to hollow and porous metal nanostructures

Diffusion and Seed Shape: Intertwined Parameters in the Synthesis of Branched Metal Nanostructures

Metal nanomaterials for optical anti-counterfeit labels

Coupling to light, and transport and dissipation of energy in silver nanowires

Ultrasound-assisted synthesis of carbon materials