Physics
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Item Summary Abstract: Relationship between many-parameter lattice gas systems and simpler models: Easy approximations for Tc(AIP, 1984) Bartelt, N. C.; Einstein, Theodore L.; Williams, Ellen D.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 Fluctuations on Ag(111) Surfaces with C60(American Physical Society, 2006) Tao, C.; Stasevich, T. J.; Einstein, Theodore L.; Williams, Ellen D.STM has been used to characterize fluctuation properties of segments of step edges partly covered by C60 on Ag(111) at room temperature. The distribution of C60 at step edges exhibits a step orientation dependence: Low-symmetry step edges are more favorable for C60 binding. The temporal correlation functions of step segments between C60-covered step regions scale as a power law, with an average exponent of 0.23+-0.02, indicating that fluctuations of these “confined” steps are consistent with step-edge diffusion limited fluctuations. Parameters extracted from temporal correlation and autocorrelation analysis consistently indicate that close-packed steps have smaller fluctuation magnitude and higher step mobility than low-symmetry steps. The measured effective system sizes of step segments with different lengths show at most a weak step-length dependence. Fluctuation features thus yield the surprising conclusion that C60 molecules are not acting as pinning points that constrain mass transport along the step edges.Item Distinctive Fluctuations in a Confined Geometry(American Physical Society, 2006) Degawa, M.; Stasevich, T. J.; Cullen, W. G.; Pimpinelli, Alberto; Einstein, Theodore L.; Williams, Ellen D.Spurred by recent theoretical predictions (Phys. Rev. E 69, 035102(R) (2004); Surf. Sci. Lett. 598, L355 (2005)), we find experimentally using STM line scans that the fluctuations of the step bounding a facet exhibit scaling properties distinct from those of isolated steps or steps on vicinal surfaces.Item A facet is not an island: step-step interactions and the fluctuations of the boundary of a crystal facet(Elsevier, 2005) Pimpinelli, Alberto; Degawa, M.; Einstein, Theodore L.; Williams, Ellen D.Item Correlations in nanoscale step fluctuations: Comparison of simulation and experiments(American Physical Society, 2006) Szalma, F.; Dougherty, D. B.; Degawa, M.; Williams, Ellen D.; Haftel, Michael I. Haftel; Einstein, Theodore L.We analyze correlations in step-edge fluctuations using the Bortz-Kalos-Lebowitz kinetic Monte Carlo (MC) algorithm, with a two-parameter expression for energy barriers, and compare with our variable-temperature scanning tunneling microscopy line-scan experiments on spiral steps on Pb(111). The scaling of the correlation times gives a dynamic exponent confirming the expected step-edge-diffusion rate-limiting kinetics both in the MC simulations and in the experiments. We both calculate and measure the temperature dependence of (mass) transport properties via the characteristic hopping times and deduce therefrom the notoriously elusive effective energy barrier for the edge fluctuations. With a careful analysis we point out the necessity of a more complex model to mimic the kinetics of a Pb(111) surface for certain parameter ranges.Item Distinguishing step relaxation mechanisms via pair correlation functions(American Physical Society, 2004) Dougherty, D. B.; Lyubinetsky, I.; Einstein, Theodore L.; Williams, Ellen D.Item Si(111) step fluctuations at high temperature: Anomalous step-step repulsion(American Physical Society, 2002) Cohen, Saul D.; Schroll, Robert D.; Einstein, Theodore L.; Metois, J.-J.; Gebremariam, Hailu; Richards, Howard L.; Williams, Ellen D.Using reflection electron microscopy we examine the step fluctuations of Si(111) at 1100°C. Evaporation is compensated by a replenishing flux. The step fluctuation behavior is qualitatively similar to that at 900°C (where sublimation is negligible), with unexplained quantitative differences. We focus on the three parameters of the step continuum model of vicinals. The step stiffness scales with an increase in T from 900°C as predicted by an appropriate lattice model. The kinetic coefficient is larger than scaling of the parameters from 900°C would predict. The step-step correlations are assessed in traditional and novel ways; step repulsions are at least 6 times as strong as predicted from lower temperatures, suggesting nonequilibrium effects probably due to electromigration.Item Dynamics of step fluctuations on a chemically heterogeneous surface of Al/Si(American Physical Society, 2002) Lyubinetsky, I; Dougherty, D. B.; Einstein, Theodore L.; Williams, Ellen D.