Coarsening Mechanisms in a Metal Film: From Cluster Diffusion to Vacancy Ripening

Coarsening of Ag films on Ag(100) at room temperature occurs primarily via diffusion-mediated coalescence of two-dimensional adatom clusters, rather than by Ostwald ripening, up to a coverage of 0.65 monolayer. Above 0.8 monolayer, vacancy clusters coarsen primarily via Ostwald ripening, due to their much lower diffusivity. An asymmetric transition region separates these two regimes, characterized by a near-percolating structure which undergoes self-similar coarsening

Diffusion of Large Two-Dimensional Ag Clusters on Ag(100)

Scanning tunneling microscopy shows that large two-dimensional Ag clusters on Ag(100) can diffuse. The value of the diffusion coefficient at room temperature is of order 10-17 cm2s-1 and varies little, if at all, with cluster size in the range studied, 100 to 720 atoms per cluster. This weak variation rules out periphery diffusion as the main mechanism of cluster diffusion, suggesting instead two-dimensional evaporation-condensation. This conclusion is compatible with the energetics of atomic-scale events within the cluster and with the dissolution of small clusters observed at low coverages

Surface Diffusion of Large Ag Clusters on Ag(100)

Scanning tunneling microscopy has shown that large, two-dimensional Ag clusters undergo observable diffusion on Ag(100). The variation of the diffusion coefficient with cluster size in the range studied, 100 to 540 atoms per cluster, indicates that two-dimensional evaporation-condensation is the main mechanism of cluster diffusion. The experimental data consistently show evidence for a backward-correlation in the cluster motion. The meaning and origin of this correlation is discussed

Ripening of monolayer vacancy pits on metal surfaces: Pathways, energetics, and size-scaling for Ag(111) versus Ag(100)

Scanning tunneling microscopy studies have revealed that monolayer-deep vacancy pits typically coarsen at 300 K via Smoluchowski ripening (SR) on Ag(111) surfaces and via Ostwald ripening (OR) on Ag(100) surfaces. We elucidate the underlying atomistic processes, the relevant energetics with some input from density functional theory analysis, and also the scaling of the ripening rate with mean pit size. Size scaling for SR reflects the size dependence of the pit diffusion coefficient, so we also discuss observed deviations from classical theories. SR dominates OR for pits on Ag(111) primarily due to its significantly lower effective energy barrier. However, the effective barrier for OR is not lower than that for SR for pits on Ag(100), and one must also account for the distinct size scaling of these pathways to explain the dominance of OR. We also briefly discuss the dependence on temperature of the dominant ripening pathway and the ripening behavior for adatom islands

Approaching the low-temperature limit in nucleation and two-dimensional growth of fcc (100) metal films Ag/Ag(100)

We analyze the formation of two-dimensional Ag islands following deposition of about 0.1 ML of Ag on Ag(100) over a temperature regime ranging from classical nucleation and growth behavior to almost immobile adatoms, from 300 to 125 K. Particular emphasis is placed on the post-deposition dynamics at the lower end of the temperature range, where the saturation island density is not reached at the end of the deposition, and nucleation and aggregation processes continue with adatoms from the remaining adatom gas. Our analysis combines VT scanning tunneling microscopy experiments with kinetic Monte Carlo simulation of appropriate atomistic models. The only adjustable parameters in the model are the terrace diffusion barrier and prefactor, which can be determined from island density behavior near room temperature. Other processes such as rapid edge diffusion, and “easy” nucleation and aggregation of diagonally adjacent adatoms, are treated as instantaneous. The model excellently reproduces all aspects of behavior at low temperatures, demonstrating that nucleation and growth processes can be described in one consistent scheme, down to the regime of almost immobile adatoms

Formation and Equilibration of Submonolayer Island Distributions in Ag/Ag(100) Homoepitaxy

We present an analysis both of the nucleation and growth of two-dimensional (2D) islands or clusters during deposition of Ag on Ag(100) at 295 K and of the subsequent postdeposition equilibration of such island distributions at coverages below about 0.25 monolayer. Island formation during deposition is shown to be effectively irreversible, and the island density and size and separation distributions are characterized using a combination of scanning tunneling microscopy (STM) and high-resolution low-energy electron diffraction. Postdeposition coarsening of the adlayer is monitored via STM and is shown to be dominated typically by diffusion and subsequent coalescence of large 2D clusters rather than by Ostwald ripening. Tailored studies of such coarsening elucidate both the kinetics and the underlying cluster diffusion process

Relationships between Social Spending and Childhood Obesity in OECD Countries:An Ecological Study

Objectives The burden of childhood obesity is clustered among children in low-socioeconomic groups. Social spending on children—public welfare expenditure on families and education—may curb childhood obesity by reducing socioeconomic disadvantages. The objective of this study was to examine the relationship between social spending on children and childhood obesity across the Organisation for Economic Cooperation and Development (OECD) countries. Design Ecological study. Setting Data on social spending on children were obtained from the OECD Social Expenditure Database and the OECD educational finance indicators dataset during 2000–2015. Data on childhood obesity were obtained from the NCD Risk Factor Collaboration database. Participants Aggregated statistics on obesity among children aged 5–19 years, estimated for OECD 35 countries based on the measured height and weight on 31.5 million children. Outcome measures Country-level prevalence of obesity among children aged 5–19 years. Results In cross-sectional analyses in 2015, social spending on children was inversely associated with the prevalence of childhood obesity after adjusting for potential confounders (the gross domestic product per capita, unemployment rate, poverty rate, percentage of children aged <20 years and prevalence of childhood obesity in 2000). In addition, when we focused on changes from 2000 to 2015, an average annual increase of US$100 in social spending per child was associated with a decrease in childhood obesity by 0.6 percentage points for girls (p=0.007) and 0.7 percentage points for boys (p=0.04) between 2000 and 2015, after adjusting for the potential confounders. The dimensions of social spending that contributed to these associations between the changes in social spending on children and childhood obesity were early childhood education and care (ECEC) and school education for girls and ECEC for boys. Conclusion Countries that increase social spending on children tend to experience smaller increases in childhood obesit

Nucleation and growth kinetics for intercalated islands during deposition on layered materials with isolated point-like surface defects

Theory and stochastic lattice-gas modeling is developed for the formation of intercalated metal islands in the gallery between the top layer and the underlying layer at the surface of layered materials. Our model for this process involves deposition of atoms, some fraction of which then enter the gallery through well-separated point-like defects in the top layer. Subsequently, these atoms diffuse within the subsurface gallery leading to nucleation and growth of intercalated islands nearby the defect point source. For the case of a single point defect, continuum diffusion equation analysis provides insight into the nucleation kinetics. However, complementary tailored lattice-gas modeling produces a more comprehensive and quantitative characterization. We analyze the large spread in nucleation times and positions relative to the defect for the first nucleated island. We also consider the formation of subsequent islands and the evolution of island growth shapes. The shapes reflect in part our natural adoption of a hexagonal close-packed island structure. Motivation and support for the model is provided by scanning tunneling microscopy observations of the formation of intercalated metal islands in highly-ordered pyrolytic graphite at higher temperatures

Youth with Disabilities Talk About Spirituality:A Qualitative Descriptive Study

There is little known about what spirituality means for youth with disability or about the potential relevance of youths’ spirituality in pediatric rehabilitation. This study explored perceptions of spirituality for youth with disabilities. Using a qualitative descriptive methodology, we examined the lived experiences of eighteen youth ages 11-20 years with disabilities including cerebral palsy, central nervous system disorder or autism spectrum disorder. In individual interviews, followed by a focus group, youth identified key spiritual themes – the importance of their beliefs, personal sources of comfort and strength, finding purpose in helping others, significance of personal connections, and strengths-based perspectives on disability. This study makes a unique contribution by informing health care professionals about the relevance of youths’ spirituality in service delivery