Island Decay Research Articles

Linear and nonlinear dynamics of self-consistent collisionless tearing modes in toroidal gyrokinetic simulations

We investigate tearing modes (TM) driven by current density gradient in collisionless tokamak plasmas by using the electromagnetic gyrokinetic simulation code ORB5. We elucidate the TM width by simulations for flat profiles, as the absence of background diamagnetic flows implies a small rotation speed, while finite gradients are included to investigate the TM rotation. For flat profiles, the initial saturation width of nonlinearly driven magnetic islands is related to the TM linear growth rate; however, large islands in the initial saturation phase are prone to current density redistribution that reduces the island width in the following evolution. Island-induced E×B and diamagnetic sheared flows develop at the separatrix, able to destabilize the Kelvin–Helmholtz instability (KHI). The KHI turbulence enhances a strong quadrupole vortex flow that reinforces the island decay, resulting in a strong reduction of the island width in an eventual steady state. This process is enhanced by trapped electrons. For finite gradients profile, the TM usually rotates in the electron diamagnetic direction but can change direction when the ion temperature gradient dominates the other gradients. The reduced growth of the TM by diamagnetic effects results in a moderate island size, which remains almost unchanged after the initial saturation. At steady state, strong zonal flows are nonlinearly excited and dominate the island rotation, as expected from previous theoretical and numerical studies. When β is increased, the TM mode is suppressed and a mode with the same helicity but with twisting parity, coupled with the neighboring poloidal harmonics, is destabilized, similar to the kinetic ballooning mode.

Dynamical evolution of Ge quantum dots on Si(111): From island formation to high temperature decay

Heteroepitaxial growth is a process of profound fundamental importance as well as an avenue to realize nanostructures such as Ge/Si quantum dots (QDs), with appealing properties for applications in opto- and nanoelectronics. However, controlling the Ge/Si QD size, shape, and composition remains a major obstacle to their practical implementation. Here, Ge nanostructures on Si(111) were investigated in situ and in real-time by low energy electron microscopy (LEEM), enabling the observation of the transition from wetting layer formation to 3D island growth and decay. The island size, shape, and distribution depend strongly on the growth temperature. As the deposition temperature increases, the islands become larger and sparser, consistent with Brownian nucleation and capture dynamics. At 550°C, two distinct Ge/Si nanostructures are formed with bright and dark appearances that correspond to flat, atoll-like and tall, faceted islands, respectively. During annealing, the faceted islands increase in size at the expense of the flat ones, indicating that the faceted islands are thermodynamically more stable. In contrast, triangular islands with uniform morphology are obtained from deposition at 600°C, suggesting that the growth more closely follows the ideal shape. During annealing, the islands formed at 600°C initially show no change in morphology and size and then rupture simultaneously, signaling a homogeneous chemical potential of the islands. These observations reveal the role of dynamics and energetics in the evolution of Ge/Si QDs, which can serve as a step towards the precise control over the Ge nanostructure size, shape, composition, and distribution on Si(111).

Sulfur-enhanced dynamics of coinage metal(111) surfaces: Step edges versus terraces as locations for metal-sulfur complex formation

The propensity of trace amounts of sulfur adsorbed on coinage metal(111) surfaces to dramatically enhance surface dynamics has been demonstrated by STM observations of accelerated 2D island decay for Cu and Ag. It is generally accepted that this enhancement is due to the formation of adsorbed metal-sulfur complexes, which facilitate surface mass transport of the metal. These complexes were originally proposed to form on terraces following the extraction of metal atoms from step edges and subsequent combination with sulfur on the terraces. However, even when thermodynamically feasible, this mechanism may not be kinetically viable for some complexes due to limited coupling of the complex concentration to the surface diffusion flux of metal atoms. Focusing on the case of Cu, we assess various scenarios where complexes are formed either on terraces or instead directly at step edges, the latter being a new paradigm. A new pathway is proposed for the formation on terraces. A rich variety of structures incorporating S at step edges exist, which could provide a viable source for complexes, at least from a thermodynamic perspective. However, it is necessary to also assess the activation barrier for complex formation and detachment from step edges. This is facilitated by the nudged-elastic-band analysis of the minimum energy path for this process utilizing machine-learning derived potentials based on density functional theory energetics for the metal-sulfur system.

Enhanced Nanostructure Dynamics on Au(111) with Adsorbed Sulfur due to Au−S Complex Formation

AbstractThe front cover artwork is provided by Dr. Peter Spurgeon, Dr. Da‐Jiang Liu, and Prof. Jim Evans at Ames Laboratory USDOE and Iowa State University. The top images show the decay of a 2D Au island on Au(111) in the presence of 24 milli‐monolayers of S. The enhancement of island decay with increasing S coverage is quantified in the lower left image for ∼85 nm2 Au islands. The Au‐S complexes implicated as facile mass carriers enhancing decay are also illustrated. Read the full text of the Article at 10.1002/cphc.202000884.

Enhanced Nanostructure Dynamics on Au(111) with Adsorbed Sulfur due to Au-S Complex Formation.

Chemisorbed species can enhance the fluxional dynamics of nanostructured metal surfaces which has implications for applications such as catalysis. Scanning tunneling microscopy studies at room temperature reveal that the presence of adsorbed sulfur (S) greatly enhances the decay rate of 2D Au islands in the vicinity of extended step edges on Au(111). This enhancement is already significant at S coverages, θS , of a few hundredths of a monolayer (ML), and is most pronounced for 0.1-0.3 ML where the decay rate is increased by a factor of around 30. For θS close to saturation at about 0.6 ML, sulfur induces pitting and reconstruction of the entire surface, and Au islands are stabilized. Enhanced coarsening at lower θS is attributed to the formation and diffusion across terraces of Au-S complexes, particularly AuS2 and Au4 S4 , with some lesser contribution from Au3 S4 . This picture is supported by density functional theory analysis of complex formation energies and diffusion barriers.

Temporary late Holocene barrier-chain deterioration due to insufficient sediment availability, Wadden Sea, Denmark

Abstract Understanding the coupling between sediment availability and sea-level change is important for forecasting coastal-barrier (barrier islands and barrier spits) response to future sea-level rise (SLR). An extensive data set of sediment cores, seismic profiles, and a high-resolution chronology from the Wadden Sea (southeastern North Sea) documents that long-term barrier-chain progradation was interrupted by a period of widespread barrier deterioration between ca. 3.5 and 2.0 ka. The decay of the barrier islands resulted from a decrease in littoral drift triggered by regional-scale coastal reconfiguration. The formation of a large cuspate foreland updrift caused the depositional locus to shift away from the barrier coast. Our results demonstrate that the resulting reduction in marine sediment availability substantially decreased the stability of the barrier chain, causing the regional SLR thresholds to fall from between 2 and 9 mm yr−1 to ∼0.9 mm yr−1, and thus below contemporary rates of SLR. Hence, we argue that predicting the response of barrier coasts to ongoing SLR requires consideration of possible changes in sediment availability and the role of large-scale geomorphological feedbacks due to human and natural causes.

Non-equilibrium island size distribution in one dimension

In this paper, a novel analytical expression for the size distribution of one-dimensional structures at thermodynamic non-equilibrium is derived. The expression is obtained using one-dimensional lattice gas model. Ostwald ripening and decay of short one-dimensional islands is taken into account. The theoretical results are compared with kinetic Monte Carlo simulations and the experimental size distribution of one-dimensional wires. A correct method of extracting the binding energy from the experimental size distribution in one dimension is proposed.

Fundamentals of Au(111) Surface Dynamics: Coarsening of Two-Dimensional Au Islands

Au(111) surfaces play a central role in many applications, yet studies of fundamental aspects of their dynamics are limited. Thus, using Scanning Tunneling Microscopy (STM) at 300 K, we analyze the coarsening of first-layer 2D Au islands directly on the Au(111) substrate, and also of second-layer 2D Au islands. Specifically, we monitor the decay of Au first-layer islands with areas of about 100-500 nm^2 in the vicinity of larger islands or extended step edges over a period of approximately 40 hours - the relevant time scale for this process. Experimentally observed behavior is captured by analytic theory for terrace-diffusion-limited decay incorporating DFT results for the Au terrace diffusion barrier and the adatom formation energy. Experimental observations of second layer island decay were also compared with appropriate analytic theory and stochastic simulations, thereby determining the effective Ehrlich-Schwoebel barrier for Au on Au(111).

One-dimensional island size distribution: From non-equilibrium to equilibrium

We present a critical view of the analysis of experimental one-dimensional island size distribution as a function of time. We study the processes of island growth using large-scale kinetic Monte Carlo simulations with diffusion barriers calculated within the framework of the density functional theory. We have shown that one-dimensional island size distribution depends significantly on the time of the experiment. Our model predicts that during annealing or cooling, the transition from one state of thermodynamical equilibrium to another occurs through a non-equilibrium state. This transition consists of Ostwald ripening and decay of one-dimensional islands. The results of our work demonstrate that considering experimental one-dimensional island size distribution as an equilibrium is a big misconception.

Influence of misfit dislocations on nanoisland decay

We investigate the decay of Ag islands on Cu(111) by variable low temperature scanning tunneling microscopy between 195 and 250 K. Such islands exhibit a misfit dislocation pattern forming $(8\ifmmode\times\else\texttimes\fi{}8)$ to $(10\ifmmode\times\else\texttimes\fi{}10)$ superstructures because of a major lattice mismatch between silver and copper. The decay of islands smaller than $200{\mathrm{nm}}^{2}$ alternates between a slower and a faster decay. It is slower for specific island sizes, in particular those with magic numbers of superstructure unit cells. We relate these changes to the complexity of the heteroepitaxial decay, involving a deconstruction of the misfit dislocation pattern and a simultaneous diffusion of several adspecies during decay.

Tip-assisted directional growth of atomically thin Ag islands

The growth and decay of metal nanostructures are important in many areas of physical science and catalytic chemistry. Here, we show the directional growth of Ag island stripes at room temperature via tip-assisted coarsening of hexagonal islands on Ag(111) using scanning tunneling microscope (STM). The mechanism of this process is attributed to the alignment of the adatom and vacancy islands along the scanning direction via tip-sample interaction during the process of the intrinsic decay of the metastable Ag islands. We find that this tip-assisted island growth is not directly related to the tunneling current and the bias voltage. Experiments performed using Ag tips and W tips show similar coarsening effects. The chamber pressure and the O2 exposure also do not have systematic effects on the rate of island growth. The tip-assisted coarsening strongly depends on the tip morphology.

Dynamics of 2D adislands formed by sulfur adsorption on an O/Cu(110) nanotemplate: an STM study

We report on the stability of large sulfur islands, formed by exposing the nanostructured O/Cu(110) surface to H2S. We have found that these islands, composed of several hundreds of atoms, show a dynamic behavior at room temperature and undergo both Smoluchowski and Ostwald ripening. An additional decay mechanism has been observed at sub-saturation sulfur coverages. The island decay can be described by a classical power law. However, oscillatory deviations from the model have been observed and possible reasons for their occurrence are discussed. At saturation sulfur coverage, the islands are still mobile and undergo ripening, but no decay is observed. In these conditions, a new nanostructured S/Cu(110) surface is formed.

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 $\ensuremath{\langle}001\ensuremath{\rangle}$ 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 $\ensuremath{\langle}\overline{1}10\ensuremath{\rangle}$ 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 at $\ensuremath{\langle}\overline{1}10\ensuremath{\rangle}$ island edges, and the effect of rare kinks on $\ensuremath{\langle}\overline{1}10\ensuremath{\rangle}$ island edges. Significantly, it suggests facile two-way corner rounding at the island periphery during island decay, contrasting the previous picture.

Morphology of mesa surfaces on Si(111) prepared by molecular beam epitaxy at temperatures around the (7 × 7)-“1 × 1” surface phase transition

Development of surface morphology in Si molecular beam epitaxy on mesa-structured Si(111) at temperatures around the (7×7)-“1×1” surface phase transition was studied by atomic force microscopy. Significant changes in surface morphology were found for a small increase in temperature near the surface phase transition, accompanied by a strong increase in step-free area dimension. This behavior is reported for the first time for epitaxial silicon growth, but supports earlier in situ studies of the thermal decay of 2-dimensional islands and voids on Si(111) close to the surface phase transition using low energy electron microscopy by Hibino et al. (Phys. Rev. B 63, 245402, 2001). The observed changes in surface morphology clearly demonstrate the interplay of crystal growth and the surface phase transformation. In particular, the simultaneous appearance of two surface phases under certain conditions and their specific influence on the growth behavior are discussed with regard to this matter.

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 $\ensuremath{\langle}001\ensuremath{\rangle}$ direction. Adatoms mainly detach from the island ends with $\ensuremath{\langle}001\ensuremath{\rangle}$ step edges. 1D decay derives from the absence of corner rounding diffusion from $\ensuremath{\langle}001\ensuremath{\rangle}$ to $\ensuremath{\langle}\overline{1}10\ensuremath{\rangle}$ edges and from inhibited nucleation of new layers on $\ensuremath{\langle}\overline{1}10\ensuremath{\rangle}$ edges. In contrast, rounding from $\ensuremath{\langle}\overline{1}10\ensuremath{\rangle}$ to $\ensuremath{\langle}001\ensuremath{\rangle}$ 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.

Photography and the Representation of History in Georges Perec’s Ellis Island: Tales of Vagrancy and Hope

Photography and the Representation of History in Georges Perec’s Ellis Island: Tales of Vagrancy and Hope Peter Wagstaff (bio) Experimentation with form is a defining characteristic of the work of Georges Perec (1936–1982). His choices of subject matter and style reveal a similar eclecticism, frustrating any attempt at categorization by verbal or visual genre, by theme or mode of expression. It is possible, however, to identify a small number of deeply-rooted preoccupations that emerge from time to time, more or less hidden behind a veil of reticence, hesitation and, often, wilful obfuscation. If Perec has an overarching project—and his occasional commentaries on the progress of his work, in interviews and letters to acquaintances suggest that he does—it is an attempt to understand the functioning of memory as it is passed between individuals, families, generations, and cultures.1 Allied to this is an interest in the relationship between people and the spaces they inhabit, which are marked by their presence. The motivation for such an attempt lies in the traumatic loss of his immediate family and in his own experience of enforced isolation, then alienation, from a cultural and linguistic background of which he knows little. The shadow cast by Auschwitz is rarely visible in his work, but its existence is inescapably there, nevertheless. The experimental and eclectic nature of Perec’s work is a response to the need to discover a means of expression, a mode of representation for that which defies comprehension and rational explanation. In the spring of 1979 Perec crossed the Atlantic from Le Havre to New York (via Halifax) aboard the container ship S.S. Atlantic-cognac to make a documentary film for French television.2 The film, a collaboration with his friend and film director [End Page 33] Robert Bober, focused on Ellis Island and historic European migration to the United States. It seems reasonable to assume that he chose to travel in this way so that he could share, in however attenuated a manner, something of the experience of those who had made similar journeys in their millions in the late nineteenth and early twentieth centuries. In the course of the crossing Perec took several dozen Polaroid photographs of the ship and the surrounding mist-shrouded seascape. Verging on abstraction, these photographs reveal a monochrome blankness given form only by the geometric minimalism of the vessel’s rectilinear outline.3 It is a blankness that, seen in the context of Perec’s creative imagination and personal history, takes on a metaphorical significance out of all proportion to the banality of the images and finds an echo in other visual evocations of nothingness that recur throughout his work. The two-hour documentary film, Ellis Island: Tales of Vagrancy and Hope [Récits d’Ellis Island: histoires d’errance et d’espoir], which resulted from Perec’s time in New York, is broken into two parts. In the first, titled “Traces,” a commentary written and voiced by Perec accompanies a visual exploration of the Ellis Island site, in disrepair and decay following years of neglect and partial demolition. This is intercut with film of visitors exploring the site in the company of a guide whose anecdotes attempt to convey the experience of newly-arrived migrants. These men, women, and children would wait expectantly for assessment before their eventual transfer to New York and a new life. The second part of the film, “Mémoires” [“Remembrances”], consists of face-to-face interviews with elderly New Yorkers who had passed through Ellis Island many years previously. It is striking that these individuals are less moved by recollections of their Ellis Island experience (in general, they are more interested in recounting their subsequent experiences in America) than Perec is on his brief visit to Ellis Island, despite (or as a result of) his having no direct comparable experience of migration. The purpose of this article is to examine the reasons for Perec’s interest in and commitment to the project, to set it in the wider framework of his work as a whole, and to suggest that the film, through the choice of subject matter and formal construction, offers an exemplary and more widely...

Phase transformations of 4,4′-biphenyldicarboxylic acid on Cu(001)

The growth and structure of $4,{4}^{\ensuremath{'}}$-biphenyldicarboxylic-acid (BDA) on Cu(001) at temperatures between 300 and 400 K was studied by low energy electron microscopy and $\ensuremath{\mu}$-LEED. First, the adsorbed BDA molecules form a disordered dilute phase. Once this phase reaches a sufficiently high density, a crystalline phase nucleates, in which the molecules form a hydrogen-bonded two-dimensional (2D) supramolecular $c(8\ifmmode\times\else\texttimes\fi{}8)$ network. By a careful analysis of the bright-field image intensity, we can measure the density in the dilute phase, which is up to 30$%$ of that in the crystalline phase. From the respective equilibrium densities at different temperatures, we determine the 2D phase diagram and extract a cohesive energy of 0.35 eV. We also analyze the island decay behavior and estimate the BDA molecule diffusion constants. Steps are found to be highly transparent for diffusing BDA molecules. In the temperature range of 362--400 K, we find chemical diffusion constants between 850--1700$\phantom{\rule{0.28em}{0ex}}{\mathrm{nm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$.

Ehrlich-Schwoebel barrier and interface-limited decay in island kinetics on Ag(100)

We investigate diffusion and decay of adatom and vacancy islands on Ag(100) between $\ensuremath{\approx}10$ and $\ensuremath{\approx}1000$ nm${}^{2}$ in size at room temperature by fast scanning tunneling microscopy. Adatom and vacancy islands decay in the diffusion and in the attachment limit, respectively. This adatom-driven kinetics confirms the existence of an Ehrlich-Schwoebel barrier on Ag(100). The dependence of the diffusivity of vacancy islands on island size is consistent with a kink-dominated periphery diffusion mechanism. Quantitative differences to previously published work are discussed.

Off-lattice pattern recognition scheme for kinetic Monte Carlo simulations

We report the development of a pattern-recognition scheme for the off-lattice self-learning kinetic Monte Carlo (KMC) method, one that is simple and flexible enough that it can be applied to all types of surfaces. In this scheme, to uniquely identify the local environment and associated processes involving three-dimensional (3D) motion of an atom or atoms, space around a central atom is divided into 3D rectangular boxes. The dimensions and the number of 3D boxes are determined by the accuracy with which a process needs to be identified and a process is described as the central atom moving to a neighboring vacant box accompanied by the motion of any other atom or atoms in its surrounding boxes. As a test of this method to we apply it to examine the decay of 3D Cu islands on the Cu(100) and to the surface diffusion of a Cu monomer and a dimer on Cu(111) and compare the results and computational efficiency to those available in the literature.

Buried Pd slows self-diffusion on Cu(001)

Using low-energy electron microscopy, we determine that self-diffusion of the Cu(001) surface is slowed by the presence of a $c$($2\ifmmode\times\else\texttimes\fi{}2$)-Pd buried surface alloy. We probe surface diffusion using Cu-adatom island-ripening measurements. On alloyed surfaces, the island decay rate decreases monotonically as the Pd concentration is increased up to $\ensuremath{\sim}$0.5 monolayer (ML), where the $2\ifmmode\times\else\texttimes\fi{}2$ buried alloy is Pd saturated. We propose that the Pd slows island ripening by inhibiting the diffusion of surface vacancies across terraces. For dilute alloys ($\ensuremath{\lesssim}$0.2-ML Pd), this conclusion is supported by density-functional theory calculations, which show that surface vacancies migrate more slowly owing to an attraction to isolated buried Pd atoms. The results illustrate a fundamental mechanism by which even a dilute alloy thin-film coating may act to inhibit surface-diffusion-mediated processes, such as electromigration.