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

/pdf/prospects-of-colloidal-nanocrystals-for-electronic-and-3dxpyhl3r7.pdf

Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications

Localized Surface Plasmon Resonance Sensors

Chemically synthesized metallic nanostructures can exhibit a strong local optical field enhancement associated with their high degree of crystallinity and well-defined geometry-dependent surface plasmon resonances. The extension of the plasmon modes into the mid-IR spectral range (3−30 μm) is shown for micrometer-sized nanowires with high aspect ratios available in the form of pentagonally twinned Ag crystallites as grown by polyol synthesis. Using scattering-scanning near-field optical microscopy, the associated IR plasmon modes are identified, and their underlying spatial distribution and enhancement of the optical polarization density is measured via phase, amplitude, and polarization resolved optical vector-field mapping. The transition from dipolar to multipolar resonances is observed and described by modeling the Ag wires using a modified cylindrical waveguide theory. For 10.6 μm excitation, dipole antenna resonances are observed at a resonant length of L = λeff/2 with λeff ≈ 10.6 μm/(1.8 ± 0.5) ≈ 6...

Mid-IR plasmonics: near-field imaging of coherent plasmon modes of silver nanowires.

Bimetallic nanoparticles display unique optical and catalytic properties that depend on crystallite size and shape, composition, and overall architecture. They may serve as multifunctional platforms as well. Unfortunately, many routes toward shape and architecturally controlled bimetallic nanocrystals yield polydisperse samples on account of the challenges associated with homogeneously nucleating a defined bimetallic phase by co-reduction methods. Developed by the Skrabalak laboratory, seed-mediated co-reduction (SMCR) involves the simultaneous co-reduction of two metal precursors to deposit metal onto shape-controlled metal nanocrystalline seeds. The central premise is that seeds will serve as preferential and structurally defined platforms for bimetallic deposition, where the shape of the seeds can be transferred to the shells. With Au-Pd as a model system, a set of design principles has been established for the bottom-up synthesis of shape-controlled bimetallic nanocrystals by SMCR. This strategy is successful at synthesizing symmetrically stellated Au-Pd nanocrystals with a variety of symmetries and core@shell Au@Au-Pd nanocrystals. Achieving nanocrystals with high morphological control via SMCR is governed by the following parameters: seed size, shape, and composition as well as the kinetics of seeded growth (through manipulation of synthetic parameters such as pH and metal precursor ratios). For example, larger seeds yield larger nanocrystals as does increasing the amount of metal deposited relative to the number of seeds. This increase in nanocrystal size leads to red-shifts in their localized surface plasmon resonance. Additionally, seed shape directs the overgrowth process during SMCR so the resultant nanocrystals adopt related symmetries. The ability to tune structure is important due to the size-, shape- and composition-dependent optical properties of bimetallic nanocrystals. Using this toolkit, the light scattering and absorption properties of Au-Pd octopods, 8-branched nanocrystals, could be tuned and were shown to be highly sensitive to changes in refractive index. The refractive index sensitivity displayed a linear correlation to the localized surface plasmon resonance initial position, where the sensitivity is greater than that of monometallic Au structures. Due to their bimetallic composition and unique architecture enabled by SMCR, Au-Pd octopods are promising refractive index based sensors. This Account summarizes the underlying principles for synthesis of bimetallic nanocrystals by SMCR, which have been established by systematic manipulation of synthetic parameters in a model Au-Pd system. These principles are anticipated to be general to other bimetallic systems, allowing for the design and synthesis of new nanocrystals with fascinating optical and catalytic properties.

http://pubs.acs.org/doi/pdf/10.1021/acs.accounts.5b00300

Seeding a New Kind of Garden: Synthesis of Architecturally Defined Multimetallic Nanostructures by Seed-Mediated Co-Reduction.

Eloquent routes to colloidal metal nanostructures have emerged in recent years, and a central component to any successful nanosynthesis is the initial selection of metal complexes with an appropriate ligand environment. This local ligand environment may be predetermined by the coordination complex selected as the metal precursor; however, recent studies reveal that the ligand environment of coordination complexes can be modified through exchange with other components for the synthesis that include solvent molecules, capping agents, anions, and even reducing agents. Importantly, ligands can often play multiple roles in a synthesis and direct the outcome by manipulating the rates of precursor reduction and particle coalescence, providing colloidal and facet stabilization and even serving as reducing agents themselves. This Feature Article highlights examples in which the ligand environments of metal precursors and nanoparticles contribute to product formation in multiple ways. Acknowledgment of the dual roles of ligands in nanomaterial synthesis will enable new strategies for nanostructures by decoupling the often contradictory roles of ligands.

On the dual roles of ligands in the synthesis of colloidal metal nanostructures.

Nitrogen-doped titania (N-TiO2)/tin oxide (SnO2) composites were synthesized and evaluated as photocatalysts for the first time. These composites represent a potential new class of Z-scheme photocatalysts wherein the enhanced light absorption properties of N-TiO2 are anticipated to couple with the observed benefits of composite TiO2/SnO2 systems. The N-TiO2/SnO2 composites were characterized by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N2-adsorption, diffuse reflectance spectroscopy (DRS), electron paramagnetic resonance (EPR) spectroscopy, and scanning electron microscopy (SEM). It was found that composites with low SnO2 content display enhanced photocatalytic properties under visible light irradiation when compared to standards: TiO2, N-TiO2, SnO2, and TiO2/SnO2 composites. This enhancement is attributed to a synergistic effect between the two composite components and changes in the textural properties of the materials upon composite formation. Significantly, the enhanced p...

Achieving Synergy with a Potential Photocatalytic Z-Scheme: Synthesis and Evaluation of Nitrogen-Doped TiO2/SnO2 Composites

Carbon powders composed of porous micrometer-sized spheres were synthesized from simple organic salt precursors using ultrasonic spray pyrolysis (USP). These materials were tested as catalyst supports for a direct methanol fuel cell (DMFC) catalyst and as pore formers in a membrane electrode assembly (MEA). The effect of these materials on unit cell performance was evaluated and compared to traditional Vulcan XC-72 carbon nanoparticle powder. USP provides a simple and facile way to prepare porous carbons with various morphologies and pore sizes. In exploring these new morphologies of carbon, two types of micrometer-sized spherical porous carbons were tested as pore-forming additives for both the anode catalyst (i.e., PtRu/C for methanol oxidation) and the cathode catalyst (i.e., Pt/C for O2 reduction). The anode catalyst mixture of PtRu/Vulcan and PtRu/PC-I (weight ratio 2:1, 33 wt % PC-I) showed the highest performance improvement and is attributed to the synergic effect of two carbons (PC-I and Vulcan X...

Sara E. Skrabalak

Papers

Mid-IR plasmonics: near-field imaging of coherent plasmon modes of silver nanowires.

Seeding a New Kind of Garden: Synthesis of Architecturally Defined Multimetallic Nanostructures by Seed-Mediated Co-Reduction.

On the dual roles of ligands in the synthesis of colloidal metal nanostructures.

Achieving Synergy with a Potential Photocatalytic Z-Scheme: Synthesis and Evaluation of Nitrogen-Doped TiO2/SnO2 Composites

Porous Carbon Supports Prepared by Ultrasonic Spray Pyrolysis for Direct Methanol Fuel Cell Electrodes