Mass Transport and Chalcogen-Silver Interactions on Silver Surfaces

Electronegative adsorbates such as sulfur, oxygen, and chlorine can strongly affect metal transport on surfaces of coinage metals. Hence, they can affect processes of self-assembly (including nucleation and growth) and coarsening of metal nanostructures. These processes are important to many applications that exploit nanoscale particles of these metals, such as surface enhanced Raman scattering and catalysis. To understand how and why the adsorbate affects metal transport, it is necessary to first understand the basic interaction of the adsorbate with the metal surface. Both adsorbed oxygen and sulfur reconstruct coinage metal surfaces and enhance metal island coarsening, under certain conditions. We have found that atomic S interacts strongly with Ag, inducing surface reconstruction and accelerating Ag island coarsening or sintering. In other words, S destabilizes the Ag surface and nanostructures. On the other hand, molecular H2S interacts weakly with the Ag surface at low temperature, forming only adsorbate structures. The relative effect of O or S depends on the geometry of the substrate, in terms of the structures that appear and the rate of metal island coarsening. Sulfur reconstructs both the Ag(111) and Ag(100) surfaces resulting in long-range ordered phases composed of both S and Ag. Sulfur accelerates Ag island coarsening by 1 order of magnitude on Ag(100) and by 3 or more orders of magnitude on Ag(111). Low coverages of oxygen enhance Ag island coarsening on Ag(100), but has no effect on Ag islands on Ag(110). In addition, the nature of the chalcogen (O vs S) seems to have larger influence on surface structures than does the nature of the metal (Cu vs Ag). In this thesis, we describe work in which we have expanded the understanding of fundamental processes that govern nanostructure formation and dynamics by employing single crystals in ultra-high vacuum (UHV) and surface analytical techniques, including variable and low temperature scanning tunneling microscopy (STM), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED), in addition to density functional theory (DFT) calculations. This research may identify a commonality in chalcogen induced mass transport on the coinage metal surfaces and ultimately lead to controlled production of nanoclusters

Destabilization of Ag nanoislands on Ag(100) by adsorbed sulfur

Sulfur accelerates coarsening of Ag nanoislands on Ag(100) at 300 K, and this effect is enhanced with increasing sulfur coverage over a range spanning a few hundredths of a monolayer, to nearly 0.25 monolayers. We propose that acceleration of coarsening in this system is tied to the formation of AgS2 clusters primarily at step edges. These clusters can transport Ag more efficiently than can Ag adatoms (due to a lower diffusion barrier and comparable formation energy). The mobility of isolated sulfur on Ag(100) is very low so that formation of the complex is kinetically limited at low sulfur coverages, and thus enhancement is minimal. However, higher sulfur coverages force the population of sites adjacent to step edges, so that formation of the cluster is no longer limited by diffusion of sulfur across terraces. Sulfur exerts a much weaker effect on the rate of coarsening on Ag(100) than it does on Ag(111). This is consistent with theory, which shows that the difference between the total energy barrier for coarsening with and without sulfur is also much smaller on Ag(100) than on Ag(111)

Communication: Structure, formation, and equilibration of ensembles of Ag-S complexes on an Ag surface

We have utilized conditions of very low temperature (4.7 K) and very low sulfur coverage to isolate and identify Ag-S complexes that exist on the Ag(111) surface. The experimental conditions are such that the complexes form at temperatures above the temperature of observation. These complexes can be regarded as polymeric chains of varying length, with an Ag4S pyramid at the core of each monomeric unit. Steps may catalyze the formation of the chains and this mechanism may be reflected in the chain length distribution

Analytic formulations for one-dimensional decay of rectangular homoepitaxial islands during coarsening on anisotropic fcc (110) surfaces

Submonolayer homoepitaxial fcc (110) systems display behavior reflecting strong anisotropy at lower temperatures, including one-dimensional decay during Ostwald ripening of rectangular islands maintaining constant width in the 〈001〉 direction. To appropriately describe this behavior, we first develop a refined continuum Burton-Cabrera-Frank formalism, which accounts for a lack of equilibration of island shape and importantly also for inhibited incorporation of adatoms at almost-faceted 〈1̄10〉 island edges through effective kinetic coefficients. This formalism is shown to describe accurately the adatom diffusion fluxes between islands and thus island evolution for a complex experimental island configuration, as confirmed by matching results from realistic atomistic simulations for this configuration. This approach also elucidates basic dependencies of flux on island geometry and temperature. Second, a further refinement is presented incorporating separate terrace and edge adatom density fields either in a continuum setting or alternatively in a spatially discrete diffusion equation setting. The second approach allows more flexibility and accuracy in accounting for edge-diffusion kinetics including corner rounding, a lack of equilibration of the edge adatom density atisland edges, and the effect of rare kinks onisland edges. Significantly, it suggests facile two-way corner rounding at the island periphery during island decay, contrasting the previous picture

Automated Radiosynthesis of [11C]UCB-J for Imaging Synpatic Density by PET

An automated radiosynthesis of carbon-11 PET radiotracer [11C]UCB-J for imaging the synaptic density biomarker synaptic vesicle glycoprotein SV2A was established using Synthra RNPlus synthesizer. Commercially available trifluoroborate UCB-J analogue was used as a radiolabelling precursor and the desired radiolabelled product was isolated in 11 ± 2% (n = 7) non-decay corrected radiochemical yield and formulated as a 10% EtOH solution in saline with molar activities of 20-100 GBq/μmol. The method was based upon the palladium(0)-mediated Suzuki cross-coupling reaction and [11C]CH3I as a radiolabelling synthon. The isolated product was cGMP compliant as demonstrated by the results of quality control analysis.National Institute for Health Research (NIHR) Cambridge Biomedical Research Centre, Medical Research Council grant MR/K02308X/1

Anisotropic coarsening: One-dimensional decay of Ag islands on Ag(110)

Scanning tunneling microscopy studies show that coarsening of arrays of rectangular single-layer Ag islands on Ag(110) at 220 K and below occurs by one-dimensional (1D) decay of narrower islands, which maintain roughly constant width in the 〈001〉 direction. Adatoms mainly detach from the island ends with 〈001〉 step edges. 1D decay derives from the absence of corner rounding diffusion from 〈001〉 to 〈1̅ 10〉 edges and from inhibited nucleation of new layers on 〈1̅ 10〉 edges. In contrast, rounding from 〈1̅ 10〉 to 〈001〉 edges is active. The island decay rate exhibits an unexpectedly low effective Arrhenius energy due to a combination of strong anisotropy in terrace diffusion and a decrease with temperature of typical island end-to-end separations. Behavior is described by atomistic modeling, which accurately captures both the thermodynamics and the edge diffusion kinetics of the system, in contrast to previous treatments. Kinetic Monte Carlo (KMC) simulations assess model behavior and clarify the driving force for coarsening, as well as various detailed features of the 1D decay process. Refined “atom-tracking” KMC simulations for island configurations matching the experiment recover the experimentally observed island decay times and further elucidate spatial aspects of the transfer of adatoms between islands

Low-temperature adsorption of H2S on Ag(111)

H2S forms a rich variety of structures on Ag(111) at low temperature and submonolayer coverage. The molecules decorate step edges, exist as isolated entities on terraces, and aggregate into clusters and islands, under various conditions. One type of island exhibits a (×)R25.3° unit cell. Typically, molecules in the clusters and islands are separated by about 0.4 nm, the same as the S–S separation in crystalline H2S. Density functional theory indicates that hydrogen-bonded clusters contain two types of molecules. One is very similar to an isolated adsorbed H2S molecule, with both S–H bonds nearly parallel to the surface. The other has a S–H bond pointed toward the surface. The potential energy surface for adsorption and diffusion is very smooth