Island Shape Research Articles

Resolving the Nanostructure of Self Organized Thin Films using Corrected Scanning Transmission Electron Microscopy

ABSTRACTCorrected scanning transmission electron microscopy (STEM) was used to characterise a novel thin film displaying a complex three dimensional nanostructure. The film was prepared by plasma deposition in such a way that it self-organises into layers of silver islands (each with typical dimensions of a few nanometres) within an aluminium nitride matrix. Successful application of STEM imaging and subsequent analysis was able to determine critical information about the material structure, namely island size, shape and crystalline orientation and the detection of island – matrix intermixing. Such information is essential in being able to predict the properties of this material and the approach adopted here is applicable to any similarly structured material.

Island shape and electronic structure in diindenoperylene thin films deposited on Au(110) single crystals.

We have investigated diindenoperylene (DIP) thin films deposited on Au(110) single crystals, by using a multi-technique approach based on X-ray photoemission spectroscopy (XPS), resonant photoemission spectroscopy (RPES), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, atomic force microscopy (AFM) and photoemission electron microscopy (PEEM). DIP molecules are physisorbed on gold, with image-charge screening playing the major role as an interface phenomenon. DIP thin films show Stranski-Krastanov growth mode and the structural herringbone arrangement mimics the arrangement found in DIP single crystals. These results are common with the (100) and (111) gold substrate geometries. On the contrary, the island aggregation is substrate geometry-dependent. This paves the way to exploit the degree of anisotropy in different lattice geometries as a tool for molecular patterning of inorganic surfaces, keeping the electronic structure preserved.

The Effect of the Substrate Characteristics on the Electrochemical Nucleation and Growth of Copper

We have deposited copper electrochemically, from acidic copper sulfate solutions with and without presence of different polyether molecules, on as-received RuTa alloy substrates, electrochemically-treated RuTa alloys and on Pt. We focused on the effect of substrate properties on the nucleation and growth processes during galvanostatic deposition, especially formation of Cu underpotential layer, and its influence on the island shape and final island density. Analysis of the galvanostatic transients showed that an exponential dependence could be used to describe the relationship between the island density and the actual deposition overpotential for all three substrates. For Pt and as-received RuTa alloy, a general equation is proposed for determining final island density irrespective of solution composition and deposition parameters, making it easier to predict the potential use of the given substrates in applications where direct plating is required. We also use various physical characterization techniques to determine the point of minimum coalescence thickness for Cu on Pt as a model substrate, and extrapolate the validity of the exponential formulas to island sizes beyond the resolution of our scanning electron microscopes.

In situ asymmetric island sidewall growth of high-quality semipolar (112̄2) GaN on m-plane sapphire

In situ asymmetric island sidewall growth (AISG) was developed to enhance Ga-face facet growth and improve the crystalline quality of (112) GaN epilayers on m-plane sapphire substrates. In the early growth stage island shaping and sidewall faceting were distinct and controlled by growth design. Using in situ AISG, {0002} instead of {103} sidewall facets were formed on the Ga-rich island surface, which eliminated formation of a {103} phase during subsequent layer growth of semipolar GaN. Enhanced Ga-face sidewall facet growth led to +c regions overlapping −c regions, which reduced defect density. Pure semipolar (112) epilayers with a reduced surface striation density and a basal-plane stacking fault density of 8 × 103 cm−1 were obtained. The observation of a narrow EH2 peak and an intense E1(LO) peak in Raman spectra indicates that almost strain-free high-quality semipolar (112) GaN films were achieved. The photoluminescence emission intensity from the (112) GaN film prepared by in situ AISG was dominated by band-edge emission and enhanced ∼4 times more than that from conventional (112) GaN.

Facet-dependent study of efficient growth of graphene on copper by ethanol-CVD

The growth of graphene by chemical vapour deposition (CVD) on copper is the most promising scalable method for high-quality graphene. The use of ethanol, an economic and safe precursor, for the fast growth of graphene on copper by a home-built CVD set-up was analysed. Full coverage of uniform single-layer graphene with high crystalline quality was found on 〈100〉 textured Cu foils in just 30 s. The nucleation density of graphene islands was found to be independent of facets but the island shape showed facet dependence. Diamond-like islands were observed on Cu(100) facets while random shaped islands were seen on other facets. The last observation is discussed in terms of a competition between graphene-island growth and its relaxation rate on different facets. On Cu(100) slower island growth as compared to its relaxation leads to equilibrium shapes as opposed to other facets. Further, an observed evolution in graphene contrast in electron micrographs with time on different facets was discussed in terms of oxygen diffusion between graphene and Cu.

Annealed silver-islands for enhanced optical absorption in organic solar cell

Silver nano-islands are explored for enhancing optical absorption and photo-conversion efficiency in organic solar cells (OSCs) based on the surface plasmon resonance effect under diverse annealing conditions. Ag nano-islands have been deposited by RF magnetron sputtering at 15W for 10s and subsequently annealed between 100°C–250°C in air and Argon ambient. The optical properties of the reconstructed Ag islands demonstrate an increase and a blue shift in the absorption bands with increasing annealing temperature. This is the localized surface plasmon effect due to the Ag islands of diverse sizes, shapes and coverages. The increase in optical absorption with temperature is attributed to changes in island shape and density as collaborated by atomic force microscopy and TEM.As a proof of concept, an organic solar cell was characterized for current–voltage (I–V) measurements under dark and under solar simulated white light. Incorporation of annealed Ag islands has yielded an efficiency increment of between 4–24%.

Ge deposition on Si(1 0 0) in the conditions close to dynamic equilibrium between islands growth and their decay

The formation of islands arrays during Ge deposition on Si(100) at high temperatures is studied using scanning tunneling and electron microscopies. It is found that the island size and shape distributions, which are known to be bimodal at growth temperatures below 700°C, become monomodal at temperatures above 800°C. The obtained data suggest that the processes such as island nucleation and Ostwald ripening become less significant in the surface morphology formation, giving the advantage to selective attachment of deposited Ge atoms to island sidewalls and spatially inhomogeneous Si-Ge intermixing, as the temperature increases. At 900°C, the islands exhibit a tendency to form laterally ordered arrays when the growth conditions approach the dynamic equilibrium between the growth of islands and their decay by means of Si-Ge intermixing. The islands ordering is accompanied by their shape transformation into the cone with shallow sidewalls inclined from (100) by angles of around 10°.

Topological Analysis of Rough Surfaces Using Persistent Homology

This letter investigates rough surfaces using a topological method. The horizontal cross section of a rough surface consists of “islands”, and we focus on the topological changes in the island shapes (generation and annihilation of islands and lakes) with changes in elevation. We apply persistent homology to track these topological changes. We numerically confirm that the life spans of the islands and lakes follow power-law distributions, whose scaling exponents vary according to the roughness of the surface. We also provide a theoretical explanation for the relation between these scaling exponents and the roughness exponent with a simple scaling argument. The proposed method successfully connects a topological property with the roughness of a surface.

Understanding the effects of strain on morphological instabilities of a nanoscale island during heteroepitaxial growth

A comprehensive morphological stability analysis of a nanoscale circular island during heteroepitaxial growth is presented based on continuum elasticity theory. The interplay between kinetic and thermodynamic mechanisms is revealed by including strain-related kinetic processes. In the kinetic regime, the Burton-Cabrera-Frank model is adopted to describe the growth front of the island. Together with kinetic boundary conditions, various kinetic processes including deposition flow, adatom diffusion, attachment-detachment kinetics, and the Ehrlich-Schwoebel barrier can be taken into account at the same time. In the thermodynamic regime, line tension, surface energy, and elastic energy are considered. As the strain relief in the early stages of heteroepitaxy is more complicated than commonly suggested by simple consideration of lattice mismatch, we also investigate the effects of external applied strain and elastic response due to perturbations on the island shape evolution. The analytical expressions for elastic fields induced by mismatch strain, external applied strain, and relaxation strain are presented. A systematic approach is developed to solve the system via a perturbation analysis which yields the conditions of film morphological instabilities. Consistent with previous experimental and theoretical work, parametric studies show the kinetic evolution of elastic relaxation, island morphology, and film composition under various conditions. Our present work offers an effective theoretical approach to get a comprehensive understanding of the interplay between different growth mechanisms and how to tailor the growth mode by controlling the nature of the crucial factors.

Mesoscale simulation of the early evolution of ductile fracture in dual-phase steels

In this study, finite element simulations synchronized with in-situ tensile tests have identified microstructural damage models in a 600 MPa grade dual-phase steel with 10% martensite volume fraction. Firstly, in-situ tensile tests have been carried out on a double-notched, flat-plate, tensile specimen. These experiments have shown that the damage in this steel is dominated, up to moderate tensile strains, by fracture of the martensite islands and decohesion at the martensite/ferrite interfaces. Secondly, FE simulations have been conducted, by using 2.5D meshes that were adjusted to the real shape of the martensite islands, as revealed by the micrographs. The ductile fracture of the martensite was described by a three-parameter fracture envelope defined by Bao and Wierzbicki, where the equivalent strain to fracture is expressed as function of the stress triaxiality. The stiffness of the ferrite/martensite interfaces has been described by a four-parameter cohesive model of Xu-Needleman type. The comparison of the experimentally determined sequence of damage events with the FE simulations has allowed adjusting the damage parameters of the model and validating them up to equivalent plastic strains exceeding locally 0.60. After determination of the damage parameters, the event sequence of martensite fracture and ferrite/martensite decohesion, as well as the pattern of the strain localization in the ferrite phase, were quite satisfactorily predicted by the FE simulations. Furthermore, the results obtained have shown that the martensite fracture and the interface decohesion have a significant influence on the stress and strain partitioning between the two phases, and hence also on the pattern and intensity of the strain localization and on the void growth and coalescence during the subsequent deformation stages.

Effects of Ga-induced reconstructed surfaces and atomic steps on the morphology of GaSb islands on Si(1 0 0)

The effects of the Ga-induced reconstructed surface and atomic step type on the shape and morphology of GaSb islands on a Si(100) surface were studied using ultrahigh-vacuum scanning tunneling microscopy and atomic force microscopy. Though both anisotropic elongated islands and isotropic islands were formed on clean Si and Ga/Si(100)-2×3 substrates, isotropic islands were dominantly formed on Ga/Si(100)-2×2 substrates at 300°C. The density and size of GaSb islands on 2×2-Ga at 300°C were estimated to be 4.9×1010cm−2 and 29.1nm, respectively. The difference in the GaSb island shapes could be caused by the terrace and step of the substrate surface being changed by step rearrangement due to the deposition of Ga atoms at high temperatures. Above 350°C, the density and size of islands decreased to 2.7×109cm−2 and increased to 62.0nm, respectively. In the initial growth stage, scanning tunneling microscopy results revealed that a Sb/Si(100)-2×1 reconstructed surface was formed above 350°C. The large islands were assumed to aggregate from the surface diffusion of each atom because the Sb-terminated Si surface is inactive against Ga and Sb atoms. The type of the reconstructed surface is also suggested to affect the density and size of the islands.

Numerical investigation of rotating stall in centrifugal compressor with vaned and vaneless diffuser

This study presents a numerical simulation of the stall and surge in a centrifugal compressor and presents a description of the stall development in two different cases. The first case is for a compressor with vaneless diffuser and the second is for a compressor with vaned diffuser of the vane island shape. The main aim of this study is to compare the flow characteristics and behavior for the two compressors near the surge operating condition and provide further understanding of the diffuser role when back flow occurs at surge. Results showed that for a location near the diffuser entrance, the amplitude of the static pressure fluctuations for the vaneless diffuser case is higher than that for the vaned diffuser case near surge condition. These pressure fluctuations in the case of the vaneless diffuser appear with a gradual decrease of the mean pressure value as a part of the surge cycle. While for the case of the vaned diffuser, the pressure drop during surge occurs faster than the case of the vaneless diffuser. Also, results indicated that during surge in the case of vaneless diffuser, there is a region with low velocity and back flow that appears as a layer connecting all impeller passages near shroud surface and this layer develops in size with time. On the other hand, for the case of vaned diffuser during surge, the low velocity regions appear in random locations in some passages and these regions expand with time towards the shroud surface. Results showed that during stall, the impeller passages are exposed to identical impact from stall cells in the case of vaneless diffuser while the stall effect varies from passage to another in the case of the vaned diffuser.

Diffusion Limited Aggregation in Low Temperature Growth of Sodium Chloride

Sodium chloride is adsorbed at low temperature on Cu(111) and investigated by scanning tunneling microscopy. The ramified shape of the low-temperature grown islands and early second layer nucleation suggest diffusion-limited aggregation of crystallites as corroborated by atomically resolved images of the highly disordered salt clusters. A comparison to single crystalline islands grown at room temperature on Ag(111) reveals differences in bond length and angle and thus crystal order for the low-temperature phase.

Step free energy of an arbitrarily oriented step on a rectangular lattice with nearest-neighbor interactions

We have derived within the framework of a solid-on-solid model with anisotropic nearest-neighbor interactions an exact expression for the free energy of an arbitrarily oriented step edge or boundary on a rectangular two-dimensional lattice. The full angular dependence of the step free energy allows the determination of the surface free energy of any vicinal surface as well as the temperature evolution of the equilibrium island shape. Our method can also be applied to derive an expression for the boundary tension of an arbitrarily oriented domain boundary of the anisotropic nearest neighbor two-dimensional Ising model

Crystallographic Features of Nanostructures SnTe on Polyimide

It is done the statistical analysis of crystallographic angles of surface elements of tin telluride films deposited on a substrate of polyimide by open evaporation in a vacuum. Analysis of images obtained by atomic force microscope revealed the influence of technological factors on the features of shape and spatial orientation of surface islands. It is shown that the islands are a dome-shaped with changing the ratio of the width to the lateral diameter. Weak dependence of symmetry islands on the deposition conditions is revealed.

Multiscale simulations of the early stages of the growth of graphene on copper

Abstract We have performed multiscale simulations of the growth of graphene on defect-free copper (111) in order to model the nucleation and growth of graphene flakes during chemical vapour deposition and potentially guide future experimental work. Basic activation energies for atomic surface diffusion were determined by ab initio calculations. Larger scale growth was obtained within a kinetic Monte Carlo approach (KMC) with parameters based on the ab initio results. The KMC approach counts the first and second neighbours to determine the probability of surface diffusion. We report qualitative results on the size and shape of the graphene islands as a function of deposition flux. The dominance of graphene zigzag edges for low deposition flux, also observed experimentally, is explained by its larger dynamical stability that the present model fully reproduced.

Crystallographic Characterization and Control of Domain Structure within Individual Graphene Islands

Chemical vapor deposition of graphene on Cu foil was performed to clarify the origin of the characteristic domain structure within individual graphene islands grown on variously oriented Cu grain surfaces. While the domain structure strongly affected island shapes on Cu(111) and (101), islands grown on Cu(001) showed that they always had a four-lobed shape regardless of whether they consisted of a single domain or multiple domains. The formation of the specific shape is related to the fact that there are edges in two fast growth directions that were found to have equivalent atomic configurations for the oriented domain. Furthermore, hydrogen etching was found to preferentially act to misoriented domains within the island on Cu(001), leading to single-crystalline four-lobed island formation. Finally, we found an interesting feature in that the ratio of single-crystalline islands increased on the vicinal Cu(001) surface compared with that on the Cu(001) basal plane. Cu(111) microfacets existing on the vicinal surface may play a significant role in the nucleation stage. These findings shed new light on the graphene growth mechanism on Cu surface and may lead the way to enlarging the domain size in the complete graphene layer by means of precise control of the “intra-island” domain structures.

Unraveling the edge structures of platinum(111)-supported ultrathin FeO islands: the influence of oxidation state.

We used high-resolution scanning tunneling microscopy to study the structure of ultrathin FeO islands grown on Pt(111). Our focus is particularly on the edges of the FeO islands that are important in heterogeneous catalysis, as they host the active sites on inversed catalysts. To imitate various reaction environments we studied pristine, oxidized, and reduced FeO islands. Oxidation of the FeO islands by O2 exposure led to the formation of two types of O adatom dislocations and to a restructuring of the FeO islands, creating long O-rich edges and few short Fe-terminated edges. In contrast, reducing the FeO islands led to a dominance of Fe-rich edges and the occurrence of few and short O-rich edges. In addition, for reducing conditions we observed the formation of O vacancy dislocations on the FeO islands. Through the identification of O adatom and O vacancy dislocations known from closed ultrathin FeO films and geometrical considerations we unraveled the atomic structure of the predominant FeO boundaries of pristine, oxidized, and reduced FeO islands. The results indicate an astonishing flexibility of the FeO islands on Pt(111), since the predominant edge termination and the island shape depend strongly on the preparation conditions.

Effect of island shape on dielectrophoretic assembly of metal nanoparticle chains in a conductive-island-based microelectrode system

The electrical conduction quality of an electric circuit connection formed by dielectrophoretic (DEP)-assembled metal nanoparticle wires between small conductive elements plays a significant role in electronic devices. One of the major challenges for improving the electrical conductance of nanowires is optimizing their geometric morphology. So far, the electrical conduction quality has been enhanced by optimizing the AC frequency and conductivity of nanoparticle suspensions. Herein, the effect of the conductive island shapes on the dynamic process occurring in a DEP assembly of 10nm gold nanoparticles was investigated in a conductive-island-based microelectrode system. The nanoparticle wires between the microelectrodes were assembled in situ from colloidal suspensions. The wires were grown in a much straighter route by increasing the geometric angle of the conductive-island tip. To validate the experiments, the effects of mutual DEP interactions and electrothermally induced fluid flow on the dynamic behavior of particle motion for different island geometric configurations in the conductive-island-based microelectrode system were determined by numerical simulations. The simulation results are consistent with those of experiments. This indicates that different conductive island shapes change the distribution of DEP force and increase the electrothermally induced fluid flow to different extents in the vicinity, leading to different morphologies of DEP-assembled nanoparticle wires.

Study on the Design of Wetland Based on Hydrodynamic Condition - A Case of Candle Lake Wetland at Qingxiu Mountain Scenic Spot in Nanning City

Candle Lake wetland at Qingxiu Mountain scenic spot is one of the most important parts of the 'China Water Town' program in Nanning city. In order to figure out the scientific rationality of the situation and design of wetland system, five pilot programs are set up based on flow velocity of three inlets, quantity and position of five outlets and the shape of center island to simulate the flow field and velocity of Candle Lake wetland by using two-dimensional numeral simulation. The results indicated that as follows: 1. The flow field and velocity of Candle Lake wetland can't be improved by increasing the velocity of inlets in the wetland system, so that the problem of large standing water can’t be solved. 2. The flow field and velocity of Candle Lake wetland are influenced by the position and quantity of outlets, especially the outlet 2, outlet 4 and outlet 5, which can improve the flow field and velocity effectively. 3. The flow field and velocity of Candle Lake wetland are also influenced by the shape of shoreline. Optimizing shoreline of Chengxiang Island can improve the flow field and velocity of Candle Lake wetland near the island very well. The flow field and slow velocity of Candle Lake wetland can be improved effectively by the optimizing shape of the Chengxiang Island and the position of outlet 4 and outlet 5, and the problems of large standing water and circulation can be solved very well.