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IrO2 is one of the most efficient electrocatalysts for the oxygen evolution reaction (OER), and also has other applications such as in pH sensors. Atomistic modeling of IrO2 is critical for understanding the structure, chemistry, and nanoscale dynamics of IrO2 in these applications. Such modeling has remained elusive due to the lack of an empirical force field (EFF) for IrO2. We introduce a first-principles-based EFF that couples the Morse (MS) potential with a variable charge equilibration method, QEq. The EFF parameters are optimized using a genetic algorithm (GA) on a density functional theory (DFT)-based training set. The resultant Morse plus QEq EFF, “MS-Q” in short, successfully reproduces the lattice parameters, elastic constants, binding energies, and internal coordinates of various polymorphs of IrO2 from DFT calculations. More importantly, MS-Q accurately captures key metrics for evaluating structural and chemical properties of catalysts such as surface energetics, equilibrium shape, electrostatic charges, oxygen vacancy formation energies, relative stability of low index rutile IrO2 surfaces, and pressure-induced phase transformations. The MS-Q EFF is used to predict the oxygen binding energy (Ead), a well-known descriptor for OER activity, on various sites of a nanocatalyst. We find Ead to be more favorable at low coordination sites, i.e. edges and corners, compared to planar facets; Ead is also correlated with charge transfer between the adsorbed O and nanocrystal, highlighting the importance of variable charge electrostatics in modeling catalysis on metal oxide surfaces. Our variable charge force field offers encouraging prospects for carrying out large-scale reactive simulations to evaluate catalytic performance of IrO2 surfaces and nanostructures. [ABSTRACT FROM AUTHOR]

Metallic nanoslit waveguides are promising candidates for ultrahigh-density optical interconnections. A variety of devices based on metallic nanoslit waveguides have already been proposed that show a great superiority over conventional photonic devices for compactness. However very few two-dimensional devices have been experimentally demonstrated with in-plane geometries due to fabrication difficulties. In this article, a feasible process is presented using traditional semiconductor fabrication technologies such as mix-and-match lithography and electroplating, which is cable of fabricating complicated 100 nm wide, 800 nm deep gold slit waveguides with multiple sharp right-angle corners. The process can be extended to volume production manufacturing with minor modifications, thus enabling the fabrication of nanoslit photonic circuits and networks. [ABSTRACT FROM AUTHOR]

We use a stochastic model to study photoinduced surface relief grating (SRG) formation due to molecular transport in azobenzene polymer films. The model is shown to reproduce the essential experimental features of SRG formation. In particular, it predicts SRG formation under both p and s polarizations, and the double peaked topographies that can occur at early times of the process. The evolving molecular positions and orientations during exposure are also followed, providing a useful mechanistic picture of SRG dynamics. [ABSTRACT FROM AUTHOR]

Apertureless scanning near-field optical microscopy is a method for obtaining subwavelength optical images of nanostructures. However, great care must be taken to avoid artifactual images. We report on one artifact related to the error signal in cantilever vibration amplitude when operating in tapping mode atomic force microscopy. The artifact is described experimentally and modeled by electromagnetic calculations based on the finite element method. We report specific steps to identify and avoid this artifact with experimental results on gold nanostructures. It is suggested that future apertureless scanning near-field optical microscopy studies verify that optical image does not correlate with error signal. [ABSTRACT FROM AUTHOR]

A correction to the article "Error Signal Artifact in Apertureless Scanning Near-Field Optical Microscopy," by L. Billot and colleagues in a 2006 issue.

We demonstrate irreversible continuous filament formation when a weak laser focus is positioned near the edge of an evaporating colloidal droplet containing carbon and gold nanoparticles. Optical trapping, hydrothermal, and chemical interactions lead to controlled colloidal synthesis of stable, irreversible mesoscale filaments of arbitrary shape and size. Mechanisms for this optically directed assembly are discussed with fluid dynamics, molecular dynamics, and lattice kinetic Monte Carlo calculations. [ABSTRACT FROM AUTHOR]

We report on a nano-optical imaging study of WSe2 thin flakes with scanning near-field optical microscopy (NSOM). The NSOM technique allows us to visualize in real space various waveguide photon modes inside WSe2. By tuning the excitation laser energy, we are able to map the entire dispersion of these waveguide modes both above and below the A exciton energy of WSe2. We found that all the modes interact strongly with WSe2 excitons. The outcome of the interaction is that the observed waveguide modes shift to higher momenta right below the A exciton energy. At higher energies, on the other hand, these modes are strongly damped due to adjacent B excitons or band-edge absorptions. The mode-shifting phenomena are consistent with polariton formation in WSe2. [ABSTRACT FROM AUTHOR]

Laparoscopes can suffer from fogging and contamination difficulties, resulting in a reduced field of view during surgery. A series of diamond-like carbon films, doped with SiO, were produced by pulsed laser deposition for evaluation as biocompatible, antifogging coatings. DLC films doped with SiO demonstrated hydrophilic properties with water contact angles under 40°. Samples subjected to plasma cleaning had improved contact angle results, with values under 5°. Doping the DLC films with SiO led to an average 40% decrease in modulus and 60% decrease in hardness. Hardness of the doped films, 12.0 - 13.2 GPa, was greater than that of the uncoated fused silica substrate, 9.2 GPa. The biocompatibility was assessed through CellTiter-Glo assays, with the films demonstrating statistically similar levels of cell viability when compared to the control media. The absence of ATP released by blood platelets in contact with the DLC coatings suggests in vivo hemocompatibility. The SiO doped films displayed improved transparency levels in comparison to undoped films, achieving up to an average of 80% transmission over the visible spectrum and an attenuation coefficient of 1.1 × 10 4 cm -1 at the 450 nm wavelength. The SiO doped DLC films show promise as a method of fog prevention for laparoscopes.

Understanding the formation of electrodynamically interacting assemblies of metal nanoparticles requires accurate computational methods for determining the forces and propagating trajectories. However, since computation of electromagnetic forces occurs on attosecond to femtosecond timescales, simulating the motion of colloidal nanoparticles on milliseconds to seconds timescales is a challenging multi-scale computational problem. Here, we present a computational technique for performing accurate simulations of laser-illuminated metal nanoparticles. In the simulation, we self-consistently combine the finite-difference time-domain method for electrodynamics (ED) with Langevin dynamics (LD) for the particle motions. We demonstrate the ED-LD method by calculating the 3D trajectories of a single 100-nm-diameter Ag nanoparticle and optical trapping and optical binding of two and three 150-nm-diameter Ag nanoparticles in simulated optical tweezers. We show that surface charge on the colloidal metal nanoparticles plays an important role in their optically driven self-organization. In fact, these simulations provide a more complete understanding of the assembly of different structures of two and three Ag nanoparticles that have been observed experimentally, demonstrating that the ED-LD method will be a very useful tool for understanding the self-organization of optical matter.

We describe a class of plasmonic crystal that consists of square arrays of nanoposts formed by soft nanoimprint lithography. As sensors, these structure show somewhat higher bulk refractive index sensitivity for aqueous solutions in the visible wavelength range as compared to plasmonic crystals consisting of square arrays of nanowells with similar dimensions, with opposite trends for the case of surface bound layers in air. Three-dimensional finite-difference time-domain simulations quantitatively capture the key features and assist in the interpretation of these and related results.

Surface plasmon polaritons (SPPs) and Rayleigh anomalies (RAs) are two characteristic phenomena exhibited by periodic grating structures made of plasmonic materials. For Au subwavelength hole arrays, SPPs and RAs from opposite sides of the film can interact under certain conditions to produce highly intense, narrow spectral features called RA-SPP resonances. This paper reports how RA-SPP effects can be achieved in subwavelength hole arrays of Pd, a weak plasmonic material. Well-defined resonances are observed in measured and simulated optical transmission spectra with RASPP peaks as narrow as 45 nm (FWHM). Dispersion diagrams compiled from angle-resolved spectra show that RA-SPP resonances in Pd hole arrays shift in wavelength but do not decrease significantly in amplitude as the excitation angle is increased, in contrast with RA-SPP peaks in Au hole arrays. The apparent generality of the RA-SPP effect enables a novel route to optimize resonances in non-traditional plasmonic media.