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

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

Methods of identifying and recruiting older people at risk of social isolation and loneliness: A mixed methods review

BackgroundLoneliness and social isolation are major determinants of mental wellbeing, especially among older adults. The effectiveness of interventions to address loneliness and social isolation among older adults has been questioned due to the lack of transparency in identifying and recruiting populations at risk. This paper aims to systematically review methods used to identify and recruit older people at risk of loneliness and social isolation into research studies that seek to address loneliness and social isolation.MethodsIn total, 751 studies were identified from a structured search of eleven electronic databases combined with hand searching of reference bibliography from identified studies for grey literature. Studies conducted between January 1995 and December 2017 were eligible provided they recruited community living individuals aged 50 and above at risk of social isolation or loneliness into an intervention study.ResultA total of 22 studies were deemed eligible for inclusion. Findings from these studies showed that the most common strategy for inviting people to participate in intervention studies were public-facing methods including mass media and local newspaper advertisements. The majority of participants identified this way were self-referred, and in many cases self-identified as lonely. In most cases, there was no standardised tool for defining loneliness or social isolation. However, studies that recruited via referral by recognised agencies reported higher rates of eligibility and enrolment. Referrals from primary care were only used in a few studies. Studies that included agency referral either alone or in combination with multiple forms of recruitment showed more promising recruitment rates than those that relied on only public facing methods. Further research is needed to establish the cost-effectiveness of multiple forms of referral.ConclusionFindings from this study demonstrate the need for transparency in writing up the methods used to approach, assess and enrol older adults at risk of becoming socially isolated. None of the intervention studies included in this review justified their recruitment strategies. The ability of researchers to share best practice relies greatly on the transparency of research

A communal catalogue reveals Earth's multiscale microbial diversity

Our growing awareness of the microbial world's importance and diversity contrasts starkly with our limited understanding of its fundamental structure. Despite recent advances in DNA sequencing, a lack of standardized protocols and common analytical frameworks impedes comparisons among studies, hindering the development of global inferences about microbial life on Earth. Here we present a meta-analysis of microbial community samples collected by hundreds of researchers for the Earth Microbiome Project. Coordinated protocols and new analytical methods, particularly the use of exact sequences instead of clustered operational taxonomic units, enable bacterial and archaeal ribosomal RNA gene sequences to be followed across multiple studies and allow us to explore patterns of diversity at an unprecedented scale. The result is both a reference database giving global context to DNA sequence data and a framework for incorporating data from future studies, fostering increasingly complete characterization of Earth's microbial diversity.Peer reviewe