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The dynamics of nanoscale clusters can be distinct from macroscale behavior described by continuum formalisms. For diffusion of 2D clusters of N atoms in homoepitaxial systems mediated by edge atom hopping, macroscale theory predicts simple monotonic size scaling of the diffusion coefficient, DN - Nβ , with β = 3/2. However, modeling for nanoclusters on metal(100) surfaces reveals that slow nucleation-mediated diffusion displaying weak size scaling β < 1 occurs for "perfect" sizes Np = L2 and L(L+1) for integer L = 3,4,. . . (with unique square or near-square ground state shapes) and also for Np+3, Np+4,. . . . In contrast, fast facile nucleation-free diffusion displaying strong size scaling β≈2.5 occurs for sizesNp+1 andNp+2.DN versusNoscillates strongly between the slowest branch (for Np+3) and the fastest branch (for Np+1). All branches merge for N = O(102), but macroscale behavior is only achieved for much larger N = O(103). This analysis reveals the unprecedented diversity of behavior on the nanoscale. [ABSTRACT FROM AUTHOR]

The work function (WF) of Al:LiF composite electrode is first reported to be tunable by electrostatic field effect via the polarization of LiF on Al atom layer. This observation was demonstrated in Kelvin probe force microscope measurement. The optimized Al:LiF composite electrode was employed as organic light emitting diode (OLED) cathode to show the electroluminescent brightness increased from 0 cd m-2 to >8000 cd m-2 and sub second temporal response in an electrolyte gated OLEDs, superior to LiF/Al and Al electrodes. These results are plausibly ascribed to the augmented electron injection with the concurrence of the upward shift of cathode WF induced by gating potential, leading to the improved electron/hole injection balance for efficient electroluminescence. [ABSTRACT FROM AUTHOR]

In order to gain insight into the nature of chemical bonding of sulfur atoms on coinage metal surfaces, we compare the adsorption energy and structural parameters for sulfur at four-fold hollow (4fh) sites on (100) facets and at three-fold hollow (3fh) sites on (111) facets of Cu nanoclusters. Consistent results are obtained from localized atomic orbital and plane-wave based density functional theory using the same functionals. PBE and its hybrid counterpart (PBE0 or HSE06) also give similar results. 4fh sites are preferred over 3fh sites with stronger bonding by ~0.6 eV for nanocluster sizes above ~280 atoms. However, for smaller sizes there are strong variations in the binding strength and the extent of the binding site preference. We show that suitable averaging over clusters of different sizes, or smearing the occupancy of orbitals, provide useful strategies to aid assessment of the behavior in extended surface systems. From site-projected density of states analysis using the smearing technique, we show that S adsorbed on a 4fh site has similar bonding interactions with the substrate as that on a 3fh site, but with much weaker antibonding interactions. [ABSTRACT FROM AUTHOR]

Using scanning tunneling microscopy, we observe an adlayer structure that is dominated by short rows of S atoms, on unreconstructed regions of a Au(111) surface. This structure forms upon adsorption of low S coverage (less than 0.1 monolayer) on a fully reconstructed clean surface at 300 K, then cooling to 5 K for observation. The rows adopt one of three orientations that are rotated by 30 from the close-packed directions of the Au(111) substrate, and adjacent S atoms in the rows are separated by √ 3 times the surface lattice constant, a. Monte Carlo simulations are performed on lattice-gas models, derived using a limited cluster expansion based on density functional theory energetics. Models which include long-range pairwise interactions (extending to 5a), plus selected trio interactions, successfully reproduce the linear rows of S atoms at reasonable temperatures. [ABSTRACT FROM AUTHOR]

A rich menagerie of structures is identified at 5 K following adsorption of low coverages (≤0.05 monolayers) of S on Cu(111) at room temperature. This paper emphasizes the reconstructions at the steps. The A-type close-packed step has 1 row of S atoms along its lower edge, where S atoms occupy alternating pseudo-fourfold-hollow (p4fh) sites. Additionally, there are 2 rows of S atoms of equal density on the upper edge, bridging a row of extra Cu atoms, together creating an extended chain. The B-type close-packed step exhibits an even more complex reconstruction, in which triangle-shaped groups of Cu atoms shift out of their original sites and form a base for S adsorption at (mostly) 4fh sites. We propose a mechanism by which these triangles could generate Cu-S complexes and short chains like those observed on the terraces. [ABSTRACT FROM AUTHOR]

Threshold versions of Schloegl's model on a lattice, which involve autocatalytic creation and spontaneous annihilation of particles, can provide a simple prototype for discontinuous non-equilibrium phase transitions. These models are equivalent to so-called threshold contact processes. A discontinuous transition between populated and vacuum states can occur selecting a threshold of N ≥ 2 for the minimum number, N, of neighboring particles enabling autocatalytic creation at an empty site. Fundamental open questions remain given the lack of a thermodynamic framework for analysis. For a square lattice with N = 2, we show that phase coexistence occurs not at a unique value but for a finite range of particle annihilation rate (the natural control parameter). This generic two-phase coexistence also persists when perturbing the model to allow spontaneous particle creation. Such behavior contrasts both the Gibbs phase rule for thermodynamic systems and also previous analysis for this model. We find metastability near the transition corresponding to a non-zero effective line tension, also contrasting previously suggested critical behavior. Mean-field type analysis, extended to treat spatially heterogeneous states, further elucidates model behavior. [ABSTRACT FROM AUTHOR]

We explore simple lattice-gas reaction models for CO-oxidation on 1D and 2D periodic arrays of surface adsorption sites with CO adsorption and desorption, dissociative O2 adsorption and recombinative desorption (at low rate), and CO + O reaction to form CO2. Adspecies interactions are neglected, and adspecies diffusion is effectively absent. The models are motivated by studies of CO-oxidation on RuO2(110) at high-pressures. Despite the lack of adspecies interactions, negligible adspecies diffusion results in kinetically induced spatial correlations. A transition occurs from a random primarily CO-populated steady-state at high CO-partial pressure, pCO, to a strongly correlated near-O-covered steady-state for low pCO as noted by Matera et al. [J. Chem. Phys. 134, 064713 (2011)]. In addition, we identify a second transition to a random near-O-covered steady-state at very low pCO. Furthermore, we identify and analyze the slow "diffusive dynamics" for very low pCO and provide a detailed characterization of the crossover to the strongly correlated O-covered steady-state as well as of the spatial correlations in that state. [ABSTRACT FROM AUTHOR]

We develop statistical mechanical models amenable to analytic treatment for the dissociative adsorption of O2 at hollow sites on fcc(100) metal surfaces. The models incorporate exclusion of nearest-neighbor pairs of adsorbed O. However, corresponding simple site-blocking models, where adsorption requires a large ensemble of available sites, exhibit an anomalously fast initial decrease in sticking. Thus, in addition to blocking, our models also incorporate more facile adsorption via orientational steering and funneling dynamics (features supported by ab initio Molecular Dynamics studies). Behavior for equilibrated adlayers is distinct from those with finite adspecies mobility. We focus on the low-temperature limited-mobility regime where analysis of the associated master equations readily produces exact results for both short- and long-time behavior. Kinetic Monte Carlo simulation is also utilized to provide a more complete picture of behavior. These models capture both the initial decrease and the saturation of the experimentally observed sticking versus coverage, as well as features of non-equilibrium adlayer ordering as assessed by surface-sensitive diffraction. [ABSTRACT FROM AUTHOR]

Scanning tunneling microscopy studies reveal that trace amounts of adsorbed S below a critical coverage on the order of 10 mML have little effect on the coarsening and decay of monolayer Ag adatom islands on Ag(111) at 300 K. In contrast, above this critical coverage, decay is greatly accelerated. This critical value appears to be determined by whether all S can be accommodated at step edges. Accelerated coarsening derives from the feature that the excess S (above that incorporated at steps) produces significant populations on the terraces of metal-sulfur complexes, which are stabilized by strong Ag–S bonding. These include AgS2, Ag2S2, Ag2S3, and Ag3S3. Such complexes are sufficiently populous and mobile that they can potentially lead to greatly enhanced metal mass transport across the surface. This picture is supported by density functional theory analysis of the relevant energetics, as well as by reaction-diffusion equation modeling to assess the mechanism and degree of enhanced coarsening. [ABSTRACT FROM AUTHOR]

We analyze a discontinuous nonequilibrium phase transition between an active (or reactive) state and a poisoned (or extinguished) state occurring in a stochastic lattice-gas realization of Schloegl’s second model for autocatalysis. This realization, also known as the quadratic contact process, involves spontaneous annihilation, autocatalytic creation, and diffusion of particles on a square lattice, where creation at empty sites requires a suitable nearby pair of particles. The poisoned state exists for all annihilation rates p>0 and is an absorbing particle-free “vacuum” state. The populated active steady state exists only for p below a critical value, pe. If pf denotes the critical value below which a finite population can survive, then we show that pf

Within the framework of a realistic atomistic lattice-gas model, we present the theoretical formulation and simulation procedures for precise analysis of the chemical diffusion flux of highly mobile CO within a nonuniform interacting mixed CO+O adlayer on a Pd(100) surface. The approach applies in both regimes of relatively immobile unequilibrated and fairly mobile near-equilibrated O adlayer distributions. Spatiotemporal behavior in surface reactions is controlled by chemical diffusion in mixed adlayers. Thus, we naturally integrate the above analysis with a previously developed multiscale modeling strategy to describe mesoscale reaction front propagation in CO oxidation on Pd(100). This treatment avoids using a simplified prescription of chemical diffusion and reaction kinetics as in traditional mean-field reaction-diffusion equation approaches. [ABSTRACT FROM AUTHOR]

We have developed an atomistic lattice-gas model for the catalytic oxidation of CO on single-crystal Pd(100) surfaces under ultrahigh vacuum conditions. This model necessarily incorporates an detailed description of adlayer ordering and adsorption-desorption kinetics both for CO on Pd(100), and for oxygen on Pd(100). Relevant energetic parameters are determined by comparing model predictions with experiment, together with some guidance from density functional theory calculations. The latter also facilitates description of the interaction and reaction of adsorbed CO and oxygen. Kinetic Monte Carlo simulations of this reaction model are performed to predict temperature-programed reaction spectra, as well as steady-state bifurcation behavior. [ABSTRACT FROM AUTHOR]

Using a lattice-gas model with pairwise interactions, we study the ordered structures, coverage dependence of the heat of adsorption, and other experimentally observable behavior of adsorbed CO overlayers on Pd(100) single crystal surfaces. Transfer matrix and Monte Carlo methods give accurate information regarding the lattice-gas model that often contradicts simple mean-field-like analysis. We demonstrate the usefulness of the model by reproducing experimental results over a large range of pressures and temperatures. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The development and synthesis of novel dye sensitizers are important for improving the power conversion efficiency of dye-sensitized solar cells (DSSCs) in terms of the role of dye sensitizers in photon to electricity energy conversion processes. How the different moieties tune the electronic structures and related properties is the fundamental issue in designing dye sensitizers. Here, the geometries, electronic structures, excitation properties, and free energy variations for electron injection (EI) and dye regeneration (DR) of porphyrin dye sensitizers SM315, GY50, FA and KS, containing bulky bis(2',4'-bis(hexyloxy)-[1,1'-biphenyl]-4- yl)amine, diarylamino group with two hexyl chains, quinolizinoacridine and triazatruxene as electron donors, respectively, were investigated. The Q bands absorption spectra of FA and KS exhibit a blue-shift relative to those of SM315 and GY50, resulting from weak conjugation effects. The transition configurations and molecular orbital analysis suggest that the electron donors in these dyes are effective chromophores for photon-induced EI in DSSCs. The torsion angle between the electron-donor and the conjugation-bridge has significant effects on electronic structures, excited states, charge transfer (CT) properties, and free energy variations for EI and DR. The transferred charges and CT distances demonstrate that quinolizinoacridine in FA is the most prominent electron donor moiety among these porphyrin dyes. [ABSTRACT FROM AUTHOR]

Applying a bidirectional sweeping gate voltage, the white-light illuminated copper phthalocyanine thin-film transistors (CuPc-TFTs) show obvious hysteresis effect and the hysteresis window is 32 V. This effect is dominated by the additional carriers generated by photoirradiation. Although small hysteresis exists in CuPc-TFTs without illumination, light makes the hysteresis more obvious. Thus, CuPc-TFTs are promising to realize light detection and/or light memory in a single organic device for future low-cost, ultrahigh-density organic photoelectric integration. [ABSTRACT FROM AUTHOR]

To elucidate surface diffusion in the presence of a coadsorbate with superlattice ordering, we consider particle hopping on a square lattice with some fraction, θ[sub B], of quenched blocking sites arranged with checkerboard or c(2x2) ordering. Behavior for low θ[sub B] corresponds to diffusion around isolated obstacles, and can be described by exact density expansions. Behavior for high θ[sub B] corresponds to percolative diffusion along (or sometimes away from) domain boundaries. The connectivity of these domain boundaries is closely related to the existence of symmetry breaking [i.e., long-range c(2x2) order] in the distribution of blocking sites. In some cases, symmetry breaking induces critical behavior for diffusive transport which is fundamentally different from that for the conventional "ant in the labyrinth" problem. Our results apply to recently developed models for CO oxidation, where CO(ads) diffuses rapidly through coadsorbed relatively immobile c(2x2)-O(ads). The characterization of CO diffusion in these systems is key to describing spatial pattern formation. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A theoretical analysis of light trapping properties of Gallium arsenide (GaAs) nanoneedle arrays (NNAs) solar cells is presented. The effect of geometric parameters on the optical absorption of NNAs has been analyzed by using rigorous coupled wave analysis algorithm and finite element method. Compared with nanowire arrays and thin-film layer counterpart, higher light absorption efficiency can be achieved in GaAs NNAs, due to the graded refractive index of NNAs that incident light can be coupled into the NNAs efficiently. The absorption profiles at different wavelength and angle-dependant optical properties of NNAs are also evaluated. Meanwhile, the short-circuit current of GaAs NNAs for various lengths at fixed filling factor is obtained. [ABSTRACT FROM AUTHOR]