Physics
Permanent URI for this communityhttp://hdl.handle.net/1903/2269
Browse
272 results
Search Results
Item INFLUENCE OF THE ELECTROCHEMICAL ENVIRONMENT ON DIFFUSION PROCESSES NEAR STEP AND ISLAND EDGES: Ag(111) and Ag(100)(2000) Haftel, Michael I.; Einstein, Theodore L.The electrochemical cell provides a potentially powerful means of altering morphology and islanding phenomena on metallic surfaces. Diffusion and attachment processes on terraces and near step and island edges are known to profoundly affect island sizes, shapes and coarsening kinetics. Using the surface-embedded-atom-model (SEAM) for describing metallic surfaces in the electrolytic environment, we calculate the dependence of the activation energies for the aforementioned diffusion processes on the deposited surface charge for the Ag(111) and Ag(100) surfaces in an electrolytic environment. While all these processes show some degree of dependence on the potential, the step-edge barrier and the edge diffusion processes are the most sensitive. Step-edge barriers increase (to over 1 eV) with large positive potential (0.85 V), while edge diffusion barriers monotonically decrease with positive surface charge on Ag(100) and Ag(111). We assess the effect these diffusion barriers have on island size/shapes and coarsening dynamics and discuss the implications on electrochemical tuning of islanding phenomena.Item Terrace-width distributions of touching steps: Modification of the fermion analogy with implications for measuring step-step interactions(American Physical Society, 2009) Sathiyanarayanan, Rajesh; Hamouda, Ajmi BH.; Einstein, Theodore L.Using Monte Carlo simulations, we compute the terrace-width distributions (TWDs) of surfaces in which steps can touch each other, forming multiple-atomic height steps, but cannot cross (no overhangs), and so inconsistent with the standard mapping to spinless fermions. Our results show that the generalized Wigner distribution with minor modifications at small step separations, gives a very good fit for TWDs of touching steps. The interaction strength derived from the fit parameter indicates an effective attraction between steps. The strength of this effective attraction decreases for larger mean-step separations and decreasing step-touching energies; describable via finite-size scaling. Hence, accurate extraction of the true repulsion strength requires multiple vicinalities.Item Rings sliding on a honeycomb network: Adsorption contours, interactions, and assembly of benzene on Cu(111)(American Physical Society, 2009) Berland, K.; Einstein, Theodore L.; Hyldgaard, P.Using a van der Waals density functional (vdW-DF) (Phys. Rev. Lett. 92, 246401 (2004)), we perform ab initio calculations for the adsorption energy of benzene (Bz) on Cu(111) as a function of lateral position and height. We find that the vdW-DF inclusion of nonlocal correlations (responsible for dispersive interactions) changes the relative stability of eight binding-position options and increases the binding energy by over an order of magnitude, achieving good agreement with experiment. The admolecules can move almost freely along a honeycomb web of “corridors” passing between fcc and hcp hollow sites via bridge sites.Item Impurity Decoration for Crystal Shape Control: C60 on Ag(111)(American Physical Society, 2009) Stasevich, T. J.; Tao, Chenggang; Cullen, William G.; Williams, Ellen D.; Einstein, Theodore L.The decoration of hexagonal Ag/Ag(111) monolayer islands by chains of C60, observed via STM at 300 K, dramatically changes the nanocrystalline shape and fluctuations of the islands. We tune coverage so that a single chain of C60 fully decorates each Ag island boundary, forming a closed circular “necklace.” We model the C60-induced rounding in terms of competing energetic and entropic effects. We thereby characterize the decorated-step fluctuations and estimate the C60-Ag and C60?C60 attractions to be ?0.13 and ?0.03??eV, respectively. Generalizations of our model show that decorating molecules of both circular and rectangular surface-projected symmetry will similarly lower the energy of fully kinked boundaries, leading to corner rounding and reorientations by 30° on (111) surfaces and 45° on (100) surfaces.Item Effect of impurities on pentacene island nucleation(American Physical Society, 2008) Conrad, B. R.; Gomar-Nadal, Elba; Cullen, W. G.; Pimpinelli, A.; Einstein, Theodore L.; Williams, Ellen D.Pentacenequinone (PnQ) impurities have been introduced into a pentacene source material in a controlled manner to quantify the relative effects of the impurity content on the grain boundary structure and thin film nucleation. Atomic force microscopy has been employed to directly characterize by weight films grown by using 0.0%–7.5% PnQ in the source material. Analysis of the distribution of capture zone areas of submonolayer islands as a function of impurity content shows that for a large PnQ content, the critical nucleus size for forming a Pn island is smaller than for a low PnQ content. This result indicates a favorable energy for the formation of Pn-PnQ complexes, which, in turn, suggests that the primary effect of PnQ on Pn mobility may arise from the homogeneous distribution of PnQ defects.Item Step line tension and step morphological evolution on the Si(111) (1x1) surface(American Physical Society, 2008) Pang, A. B.; Man, K. L.; Altman, M. S.; Stasevich, T. J.; Szalma, F.; Einstein, T. L.The temperature dependence of the step line tension on the Si(111) (1×1) surface is determined from a capillary wave analysis of two-dimensional island edge fluctuations and straight step fluctuations that are observed with low energy electron microscopy. The line tension decreases by nearly 20% with a linear temperature coefficient of ?0.14meV/ÅK between 1145 and 1233K. Temporal correlations of step fluctuations exhibit the distinctive signature in the wavelength dependence of the relaxation time of a terrace diffusion-limited mechanism for step motion. We also find that the role of desorption in island decay increases dramatically in the temperature range (1145–1380K) that island decay is studied. Consequently, we generalize the current quasistatic model of island decay to take account of desorption. The evaluation of the island decay time with this model referenced to the temperature-dependent line tension accurately determines activation energies that are relevant to mass transport and sublimation.Item Capture-Zone Scaling in Island Nucleation: Universal Fluctuation Behavior(American Physical Society, 2007) Pimpinelli, Alberto; Einstein, Theodore L.In island nucleation and growth, the distribution of capture zones (in essence proximity cells) can be described by a simple expression generalizing the Wigner surmise (power-law rise, Gaussian decay) from random matrix theory that accounts for spacing distributions in a host of fluctuation phenomena. Its single adjustable parameter, the power-law exponent, can be simply related to the critical nucleus of growth models and the substrate dimensionality. We compare with extensive published kinetic Monte Carlo data and limited experimental data. A phenomenological theory elucidates the result.Item Using the Wigner-Ibach surmise to analyze terrace-width distributions: history, user's guide, and advances(Springer, 2007) Einstein, Theodore L.A history is given of the applications of the simple expression generalized from the surmise by Wigner and also by Ibach to extract the strength of the interaction between steps on a vicinal surface, via the terrace width distribution (TWD). A concise guide for use with experiments and a summary of some recent extensions are provided.Item Step-position distributions and the pairwise Einstein model for steps on crystal surfaces(American Physical Society, 2006) Benson, Amber N.; Richards, Howard L.; Einstein, Theodore L.The pairwise Einstein model of steps not only justifies the use of the generalized Wigner distribution (GWD) for terrace width distributions (TWDs), it also predicts a specific form for the step position distribution (SPD), i.e., the probability density function for the fluctuations of a step about its average position. The predicted form of the SPD is well approximated by a Gaussian with a finite variance.Item Analytic Formulas for the Orientation Dependence of Step Stiffness and Line Tension: Key Ingredients for Numerical Modeling(SIAM, 2006) Stasevich, T. J.; Einstein, Theodore L.We present explicit analytic, twice-differentiable expressions for the temperature-dependent anisotropic step line tension and step stiffness for the two principal surfaces of face-centered-cubic crystals, the square (001) and the hexagonal (111). These expressions improve on simple expressions that are valid only for low temperatures and away from singular orientations. They are well suited for implementation into numerical methods such as finite-element simulation of step evolution.