Island Shape Research Articles

Three-Dimensional Fourier Monolayer Stress Microscopy

Many important biological processes, such as endothelial mechanotransduction of hemodynamic forces and neutrophil extravasation, involve the transmission of stresses across a cell monolayer. During these processes, the monolayer undergoes both lateral distortion due to the in-plane traction forces generated by the cells, and bending due to the out-of-plane component of the traction forces. However, the contribution of this bending to the monolayer stresses has been neglected in the literature. Here, we present a novel technique to determine monolayer stresses that considers both lateral distortion and bending. To illustrate the method, and to quantify the relative importance of the lateral and bending stresses, we measure the monolayer stresses in micropatterned endothelial cell islands of varying sizes and shapes. The cell islands are cultured on flexible polyacrylamide gels embedded with fluorescent beads, which deform due to traction forces exerted by the cells. We measure the three-dimensional gel deformation using previously established 3D Traction Force Microscopy methods, and recover the monolayer stresses from the measured deformation using Kirchoff-Love thin plate theory. The equations are solved numerically in an efficient manner using a Fourier pseudo-spectral method. The boundary conditions corresponding to the geometry of the cell islands are enforced within the Fourier framework using a relaxation iterative method. Our results indicate that, regardless of island shape, the three-dimensional bending stresses are dominant at the center of the island while the lateral stresses are more important near the island edge. Also, comparing the results from islands of different sizes shows that the relative importance of the bending stresses decreases with island size. These results suggest that it is necessary to resolve bending stresses to accurately determine the monolayer stresses, and reveal that the transmission of forces across cell junctions is three-dimensional and more complex than previously believed.

덕적도 지형을 고려한 소형풍력발전기 발전량 평가

덕적도 지형을 고려한 소형풍력발전기 발전량 평가

Galvanic synthesis of three-dimensional and hollow metallic nanostructures.

We report a low-cost, facile, and template-free electrochemical method of synthesizing three-dimensional (3D) hollow metallic nanostructures. The 3D nanoporous gold (3D-NPG) nanostructures were synthesized by a galvanic replacement reaction (GRR) using the different reduction potentials of silver and gold; hemispherical silver nanoislands were electrochemically deposited on cathodic substrates by a reverse-pulse potentiodynamic method without templates and then nanoporous gold layer replicated the shape of silver islands during the GRR process in an ultra-dilute electrolyte of gold(III) chloride trihydrate. Finally, the wet etching process of remaining silver resulted in the formation of 3D-NPG. During the GRR process, the application of bias voltage to the cathode decreased the porosity of 3D-NPG in the voltage range of 0.2 to -0.62 V. And the GRR process of silver nanoislands was also applicable to fabrication of the 3D hollow nanostructures of platinum and palladium. The 3D-NPG nanostructures were found to effectively enhance the SERS sensitivity of rhodamine 6G (R6G) molecules with a concentration up to 10-8 M.Electronic supplementary materialThe online version of this article (doi:10.1186/1556-276X-9-679) contains supplementary material, which is available to authorized users.

Analysis of island shape evolution from diffuse x-ray scattering of organic thin films and implications for growth

Understanding the growth of organic semi-conducting molecules with shape anisotropy is of high relevance to the processing of optoelectronic devices. This work provides insight into the growth of thin films of the prototypical rodlike organic semiconductor diindenoperylene on a microscopic level, by analyzing in detail the film morphology. We model our data, which were obtained by high-resolution grazing incidence small angle x-ray scattering (GISAXS), using a theoretical description from small angle scattering theory derived for simple liquids. Based on form factor calculations for different object types we determine how the island shapes change in the respective layers. Atomic force microscopy measurements approve our findings.

Island shape anisotropy in organic thin film growth induced by ion-beam irradiated rippled surfaces.

Controlling thin film growth of conjugated molecules is a key factor in organic electronics. Here, we report on the growth of the organic semiconductor para-hexaphenyl (6P) on ion bombarded, rippled TiO2(110) surfaces. These surfaces represent a one-dimensionally patterned substrate with alternating descending and ascending step trains with typical step distances below 1 nm. A clear island shape anisotropy and a discretization of the island width according to the ripple wavelength have been observed which are addressed to anisotropic detachment of molecules differently bound to the island rim at ascending and descending steps. By changing the average ripple length from ∼11 nm to ∼60 nm, the islands' length-to-width ratio could be tuned between ∼1.5 : 1 and ∼4.5 : 1. Further, strong diffusion anisotropy along and perpendicular to the ripples was found to be responsible for an increasing island density with decreasing ripple length.

In-situ investigation of thermal instabilities and solid state dewetting in polycrystalline platinum thin films via confocal laser microscopy

Solid state dewetting and the subsequent morphological changes for platinum thin films grown on zinc oxide (ZnO) buffered (001) silicon substrates (Pt/ZnO/SiO2/(001)Si system) is investigated under vacuum conditions via a custom-designed confocal laser microscope coupled with a laser heating system. Live imaging of thin film dewetting under a range of heating and quenching vacuum ambients reveals events including hillock formation, hole formation, and hole growth that lead to formation of a network of Pt ligaments, break up of Pt ligaments to individual islands and subsequent Pt islands shape reformation, in chronological fashion. These findings are corroborated by ex-situ materials characterization and quantitative electron microscopy analysis. A secondary hole formation via blistering before film rupture is revealed to be the critical stage, after which a rapid dewetting catastrophe occurs. This process is instantaneous and cannot be captured by ex-situ methods. Finally, an intermetallic phase forms at 900 °C and alters the morphology of Pt islands, suggesting a practical limit to the thermal environments that may be used for these platinized silicon wafers in vacuum conditions.

Dissociation of OCS by high energy highly charged ion impact

Various dissociation channels of OCS q+ (where q = 2 to 4), formed in the interaction of 5 MeV u-1 Si12+ ion beam with neutral OCS, have been studied using recoil–ion momentum spectroscopy. The concerted and/or sequential nature of dissociation is inferred from the shape and slope of the coincidence islands in the 2D coincidence map. It is observed that the C+ + S+ + O channel results from concerted as well as sequential decay of OCS2+. However the other channels originate purely from the concerted process in which the two terminal fragments (oxygen and sulphur) fly back to back and the central carbon fragment is left with negligible momentum. The kinetic energy release (KER) distributions for all the fragmentation channels arising from the dissociation of OCS q+ (where q = 2 to 4) have been measured and compared with the available data in the literature. It is observed that the KER values for complete Coulomb fragmentation channels are much smaller than those of incomplete Coulomb fragmentation cases and the KER increases with the increasing charge states of the parent molecular ions. From the momentum correlation map, we estimated the geometry of the precursor molecular ion undergoing three–body dissociation and inferred that bent dissociative states are involved in most of the fragmentation channels of OCS q+.

Submonolayer growth study using a solid-on-solid model for 2 × 1 reconstructed surfaces of diamond-like lattices

We have developed a simple cubic solid-on-solid (SOS) model of growth that captures many of the features of 2×1 reconstructed surfaces. Energetics in the model is based on bond-counting and the tendency to form dimer rows is controlled by a reconstruction energy parameter δ. We examine the dependence of the shapes and sizes of islands formed at submonolayer coverage on the parameter δ. The aspect ratio distribution of islands indicates a transition from two-dimensional to one-dimensional growth as δ is increased. At coverages below 0.2ML, the islands are mostly made of dimer chains with aspect ratios as large as 14. On annealing the system, the islands coalesce into larger islands with a much smaller aspect ratio of approximately 3. Through extensive simulations, we have found a set of parameters that reproduces the known experimental results on submonolayer growth on Si(001) in Molecular Beam Epitaxy(MBE). These results include island size and shape distribution and their dependence on temperature. This model can be used to study heteroepitaxial thin film growth on reconstructed surfaces.

Evaluating ex vivo fluorescence confocal microscopy images of basal cell carcinomas in M ohs excised tissue

Fluorescence confocal microscopy (FCM) is an emerging technology for rapid imaging of excised tissue, without the need for frozen- or fixed-section processing. Basal cell carcinomas (BCCs) can be detected in Mohs excisions although few studies have described the major BCC findings as seen on FCM. To describe the major BCC findings of excised tissue during Mohs surgery and to correlate them with histopathology. Freshly excised tumours and frozen-thawed discarded tissue of BCC during Mohs surgery were analysed by means of FCM. A side-by-side correlation between FCM images and histological sections was performed. The FCM features of overlying skin and adnexal structures were also described. Sixty-four BCC cases were analysed. Distinct BCC types appeared unique in terms of shape and size of tumour islands [bigger in nodular (18/25), smaller and rounded in micronodular (7/7) and tiny cords for infiltrative ones (24/30)] and for the presence of clefting, palisading and increased nucleus/cytoplasm ratio. An excellent correlation was found between FCM and histological findings (Cohen's κ statistics = 0·9). In six cases, the presence of sebaceous glands and intense stroma reaction represented possible confounders. Fluorescence confocal microscopy is a fast and new imaging technique that allows an excellent visualization of skin structures and BCC findings during Mohs surgery.

Measurement of microscopic coupling constants between atoms on a surface: Combination of LEEM observation with lattice model analysis

We present a method combining low-energy electron microscopy (LEEM) and lattice model analysis for measuring the microscopic lateral coupling constants between atoms on a surface. The calculated step (interface) stiffness in a honeycomb lattice Ising model with the nearest neighbor and the second nearest neighbor interactions (J1=93.8meV and J2=9.38meV) matched the experimental step quantity values on an Si(111)(1×1) surface reported by Pang et al. and Bartelt et al. based on LEEM measurements. The experimental value of step tension obtained by Williams et al. lies on the calculated step tension curve. The polar graphs of the step tension and a two-dimensional island shape at the temperature T=1163K also agree well with the experimental graphs reported by Métois and Müller. The close agreement between the LEEM observations and the lattice model calculations on a Si(111) surface suggests that our method is also suitable for measuring microscopic lateral coupling constants on the surface of other materials that are less well-studied than Si.

Graphene growth process modeling: a physical–statistical approach

As a zero-band semiconductor, graphene is an attractive material for a wide variety of applications such as optoelectronics. Among various techniques developed for graphene synthesis, chemical vapor deposition on copper foils shows high potential for producing few-layer and large-area graphene. Since fabrication of high-quality graphene sheets requires the understanding of growth mechanisms, and methods of characterization and control of grain size of graphene flakes, analytical modeling of graphene growth process is therefore essential for controlled fabrication. The graphene growth process starts with randomly nucleated islands that gradually develop into complex shapes, grow in size, and eventually connect together to cover the copper foil. To model this complex process, we develop a physical–statistical approach under the assumption of self-similarity during graphene growth. The growth kinetics is uncovered by separating island shapes from area growth rate. We propose to characterize the area growth velocity using a confined exponential model, which not only has clear physical explanation, but also fits the real data well. For the shape modeling, we develop a parametric shape model which can be well explained by the angular-dependent growth rate. This work can provide useful information for the control and optimization of graphene growth process on Cu foil.

Morphology transition of Ag ultrathin films on Pt (1 1 1): Kinetic Monte Carlo simulation

Kinetic Monte Carlo simulations are carried out to explore the growth of the Ag ultrathin film on Pt (111) in the early growth stage. With increasing temperature, the island shapes are demonstrated to transform from fractal to compact and resemble the surface crystal structure at high temperature. The transition temperature of the island evolution is demonstrated to increase with the deposition rate. The competing mechanism between nucleation and growth is pursued. The transition temperature of island evolution is deduced by the average island density distributions and demonstrated by the island morphology evolutions. In addition, the extent of a non-ideal substrate surface is explored. As the interaction energy between adatom and defect point increases, the island shape becomes small and irregular. The effect of surface defect on the nucleation and growth is demonstrated.

Growth of Ge islands on SrTiO3 (001) 2 × 1 reconstructed surface: Epitaxial relationship and effect of the temperature

The structural properties of Ge islands grown by molecular beam epitaxy on SrTiO3 (001) substrates are investigated. We report on the effect of the temperature on the epitaxial relationship and morphology of the Ge islands. By combining X-ray diffraction measurements and atomic force microscopy we evidence a correlation between the island size and shape, the structural properties of the interface and the sample texture. In particular, we show that the growth temperature affects the nature of the interface between the semiconductor and the perovskite oxide and thus the island orientation, that evolves from fully (001) oriented to a mixture of (111) and (001) oriented islands.

An investigation into defect reduction techniques for growth of non‐polar GaN on sapphire

AbstractIn this paper we describe the implementation and the characterisation of five different in‐situ defect reduction techniques for non‐polar (a ‐plane) GaN growth on r ‐plane sapphire. Sample 1 (3D/2D) employs a methodology frequently applied on the c ‐plane, involving a low temperature nucleation layer (LTNL) followed by 3D GaN island formation and lateral coalescence. For Sample 2 (d3D) GaN islands are grown directly onto the sapphire with no LTNL, followed by lateral growth. Sample 3 (d3D Si) follows a similar procedure, but with high silicon doping in order to adjust the 3D GaN island shape. Sample 4 (SiNx) utilises a silicon nitride interlayer between a LTNL and subsequent growth of a GaN layer. Sample 5 is grown by epitaxial lateral overgrowth (ELOG) coupled with a SiNx interlayer. X‐ray diffraction, scanning electron microscopy and cathodoluminescence are used to identify defects, and determined the threading defect density to vary from 1 x 1010–1 x 109 cm–2 and basal‐plane stacking fault (BSF) density to vary from 5 x 105 – 5 x 103 cm‐1. The improvement in crystal quality is reflected in the photoluminescence spectra by a comparison of the ratio of the GaN near band edge (NBE) emission to the BSF associated emission. It was determined that the ELOG method was most successful in blocking BSFs, with a density reduction of 2 orders of magnitude resulting in a fifteen‐fold increase in the NBE:BSF emission ratio increase. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of chemical composition on the microstructure and mechanical properties of gravity cast Mg–xZn–yRE–Zr alloy

The microstructures and mechanical properties of Mg–xZn–1.25RE–Zr (x=3.5, 4.2, 5.0wt%) and Mg–4.2Zn–yRE–Zr (y=1.0, 1.25, 1.5wt%) alloys in as-cast and 325°C peak-aged condition were investigated in this study. The as-cast Mg–xZn–yRE–Zr alloys consist of α-Mg matrix, T-phase and Mg51Zn20 phase. For Mg–xZn–1.25RE–Zr alloys, 4.2wt% Zn addition led to smallest average grain size and better eutectics morphology of discontinuous network, short-bar and island shape. For Mg–4.2Zn–yRE–Zr alloy, increase in RE content gradually refined the microstructure and contributed to more triangular particles and networks. After peak-aged at 325°C, the strengthening of Mg–xZn–yRE–Zr alloys occurred through the precipitations of rod-like β′1 phases. With Zn content from 3.5 to 5.0wt%, the strengthening effect first increased and then decreased, with a turning point of 4.2wt% Zn. The descend of strengthening effects at 5.0wt% was mainly ascribed to the formation of plate-like β′2 phases and reduced number density of β′1 phases. With RE addition from 1.0 to 1.5wt%, the strengthening effect increased because of the denser and finer β′1 phases. When x=4.2 and y=1.25–1.5wt%, the Mg–xZn–yRE–Zr alloy reveals good combination of strength and elongation in 325°C peak-aged condition, and was chosen as optimal chemical composition.

In-situ observation of equilibrium transitions in Ni films; agglomeration and impurity effects

Dewetting of ultra-thin Ni films deposited on SiO2 layers was observed, in cross-section, by in situ scanning transmission electron microscopy. Holes were observed to nucleate by voids which formed at the Ni/SiO2 interface rather than at triple junctions at the free surface of the Ni film. Ni islands were observed to retract, in attempt to reach equilibrium on the SiO2 layer. SiO2 layers with 120 nm thickness were found to limit in situ heating experiments due to poor thermal conductivity of SiO2. The formation of graphite was observed during the agglomeration of ultra-thin Ni films. Graphite was observed to wet both the free surface and the Ni/SiO2 interface of the Ni islands. Cr forms surface oxide layers on the free surface of the SiO2 layer and the Ni islands. Cr does not prevent the dewetting of Ni, however it will likely alter the equilibrium shape of the Ni islands.

Energy of Step Defects on the TiO2 Rutile (110) Surface: An ab initio DFT Methodology

We present a novel methodology for dealing with quantum size effects (QSE) when calculating the energy per unit length and step–step interaction energy of atomic step defects on crystalline solid surfaces using atomistic slab models. We apply it to the TiO2 rutile (110) surface using density functional theory (DFT) for which it is well-known that surface energies converge in a slow and oscillatory manner with increasing slab size. This makes it difficult to reliably calculate step energies because they are very sensitive to supercell surface energies, and yet the surface energies depend sensitively on the choice of slab chemical formula due to the dominance of QSE at computationally practical slab sizes. The commonly used method of calculating surface energies by taking the intercept of a best fit line of total supercell energies against slab size breaks down and becomes highly unreliable for such systems. Our systematic approach, which can be applied to any crystalline surface, bypasses such statistical estimation techniques and cross checks and makes robust what is otherwise a very unreliable process of extracting the energies of steps. We use the calculated step energies to predict island shapes on rutile (110) which compare favorably with published scanning tunneling microscopy (STM) images.

Epitaxial Graphene Growth and Shape Dynamics on Copper: Phase-Field Modeling and Experiments

The epitaxial growth of graphene on copper foils is a complex process, influenced by thermodynamic, kinetic, and growth parameters, often leading to diverse island shapes including dendrites, squares, stars, hexagons, butterflies, and lobes. Here, we introduce a phase-field model that provides a unified description of these diverse growth morphologies and compare the model results with new experiments. Our model explicitly accounts for the anisotropies in the energies of growing graphene edges, kinetics of attachment of carbon at the edges, and the crystallinity of the underlying copper substrate (through anisotropy in surface diffusion). We show that anisotropic diffusion has a very important, counterintuitive role in the determination of the shape of islands, and we present a "phase diagram" of growth shapes as a function of growth rate for different copper facets. Our results are shown to be in excellent agreement with growth shapes observed for high symmetry facets such as (111) and (001) as well as for high-index surfaces such as (221) and (310).

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.

Tuning Ag/Si(100) island size, shape, and density

Ag islands grown by depositing Ag onto Si(100) heated to 300–500 °C populate two distinct families. “Big islands” have basal dimensions in the few hundred nm to μm range, while “small islands” have basal diameters in the 10s of nm range. Big islands are clearly faceted, their bases are crystallographically aligned with the substrate and their bases form in a variety of shapes. Small islands are mound-like and not clearly faceted. The dependence of big island density on deposition temperature reveals that they form via nucleation and growth with a nucleation energy of 0.73 eV. The small island size and density are insensitive to growth temperature, indicating that they form via precipitation of Ag contained in a planar layer between the big islands that is thicker than the room-temperature Stranski-Krastanov layer. The big islands exhibit peculiar anti-coarsening behavior during anneals at their growth temperature: their areal density increases and their size decreases. Moreover, the proportion of each distinct basal shape evolves during the anneal and the big islands generally trend toward a more elongated shape. Since the small islands do not exist at the elevated growth and annealing temperatures, their size and density are relatively unaffected by annealing parameters. The results presented here provide guidance for tuning Ag/Si(100) island size, shape, and areal density that could have import for integration of plasmonics with Si devices.