Non-ideal Surface Research Articles

CONVECTION CAUSES ENHANCED MAGNETIC TURBULENCE IN ACCRETION DISKS IN OUTBURST

We present the results of local, vertically stratified, radiation MHD shearing box simulations of MRI turbulence appropriate for the hydrogen ionizing regime of dwarf nova and soft X-ray transient outbursts. We incorporate the frequency-integrated opacities and equation of state for this regime, but neglect non-ideal MHD effects and surface irradiation, and do not impose net vertical magnetic flux. We find two stable thermal equilibrium tracks in the effective temperature versus surface mass density plane, in qualitative agreement with the S-curve picture of the standard disk instability model. We find that the large opacity at temperatures near $10^4$K, a corollary of the hydrogen ionization transition, triggers strong, intermittent thermal convection on the upper stable branch. This convection strengthens the magnetic turbulent dynamo and greatly enhances the time-averaged value of the stress to thermal pressure ratio $\alpha$, possibly by generating vertical magnetic field that may seed the axisymmetric magnetorotational instability, and by increasing cooling so that the pressure does not rise in proportion to the turbulent dissipation. These enhanced stress to pressure ratios may alleviate the order of magnitude discrepancy between the $\alpha$-values observationally inferred in the outburst state and those that have been measured from previous local numerical simulations of magnetorotational turbulence that lack net vertical magnetic flux.

A Markov chain approach to simulate Atomic Layer Deposition chemistry and transport inside nanostructured substrates

In this work, we present a new theoretical framework to model the transport and surface chemistry under molecular (Knudsen) flow. Our approach is based on casting the transport inside nanostructures as a single-particle discrete Markov chain process. One of the advantages of this approach is that it allows us to decouple the complexity of the surface chemistry from the transport model, thus allowing its application under general surface chemistry conditions, including atomic layer deposition (ALD) and chemical vapor deposition (CVD). Our model also allows us to determine statistical information of the trajectory of individual molecules, such as the average interaction time or the number of wall collisions for molecules entering the nanostructures as well as to track the relative contributions to thin-film growth of different independent reaction pathways at each point of the feature. This offers a straightforward way of incorporating into ALD simulations non-ideal surface processes, such as parasitic CVD or surface recombination. By studying the asymptotic behavior of the Markov chain process, we were also able to establish a direct link between ballistic models, kinetic Monte Carlo simulations, and continuous models based on the use of the diffusion equation under Knudsen conditions. Finally, we show that, under certain approximations, the coverage profile inside a nanostructure under ALD conditions is controlled by the total exposure, and not by the details of the surface flux dependence with time during the exposure, as long as the reaction probabilities are pressure independent.

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.

Searching for the thermodynamic limit – a DFT study of the step-wise water oxidation of the bipyramidal Cu7cluster

Oxidative degradation of copper in aqueous environments is a major concern in areas such as catalysis, electronics and construction engineering. A particular challenge is to systematically investigate the details of this process for non-ideal copper surfaces and particles under the conditions found in most real applications. To this end, we have used hybrid density functional theory to study the oxidation of a Cu7 cluster in water solution. Especially, the role of a large water coverage is explored. This has resulted in the conclusion that, under atmospheric H2 pressures, the thermodynamically most favored state of degradation is achieved upon the generation of four H2 molecules (i.e. Cu7 + 8H2O → Cu7(OH)8 + 4H2) in both condensed and gas phases. This state corresponds to an average oxidation state below Cu(I). The calculations suggest that the oxidation reaction is slow at ambient temperatures with the water dissociation as the rate-limiting step. Our findings are expected to have implication for, among other areas, the copper catalyzed water-gas shift reaction, and for the general understanding of copper corrosion in aqueous environments.

UV-Nanoimprint Lithography: Structure, Materials and Fabrication of Flexible Molds

Large-area nanopatterning technology has demonstrated high potential which can significantly enhance the performance of a variety of devices and products such as LEDs, solar cells, hard disk drives, laser diodes, wafer-level optics, etc. But various existing patterning technologies cannot well meet industrial-level application requirements in term of high resolution, high throughput, low cost, large patterned areas, and the ability to pattern on non-ideal surfaces or waters. Soft UV-nanoimprint lithography (UV-NIL) by using a flexible mold has been proven to be a cost-effective mass production method for patterning large-area structures up to wafer-level (300 mm) in the micrometer and nanometer scale, fabricating complex 3-D micro/nano structures, especially making large-area patterns on the non-planar surfaces even curved substrates at low-cost and with high throughput. In particular, it provides an ideal solution and a powerful tool for mass producing micro/nanostructures over large areas at low cost for the applications in compound semiconductor optoelectronics and nanophotonic devices, especially for LED patterning. That opens the way for many applications not previously conceptualized or economically feasible. The flexible mold is the most critical elements for soft UV-NIL. The performance of the flexible mold has a decisive effect on the soft UV-NIL in term of resolution, patterning area, throughput, uniformity of the imprinted patterns, and repeatability of multi-imprinting. The key enabler that can fulfill mass production of micro-and nanostructures over large areas by NIL is the continual advancement of mold techniques (structures, materials and fabrication processes) towards higher resolution over a larger area at a lower cost. This paper provides a comprehensive review on the structural types, materials used and fabrication methods of various flexible molds in soft UV-NIL, surveys major progress in various flexible molds, particularly highlights some concluding remarks and generalizations. Two key issues for flexible molds, deformation mechanism and controlling solution of soft molds as well as fabrication of large area (wafer level) master template, are described in detail. Furthermore, prospects, challenges and future directions for flexible molds are addressed. Finally, some potential or promising solutions for improving the performance of flexible molds and soft UV-NIL process, as well as some important conclusions are presented.

Structure, Chemistry and Energy of Carbon Surfaces

In this paper, the graphene layer is considered as the basic structural unit in a range of carbon materials. The effects of 2D and 3D order in defining the microstructures and macrostructures of carbons are reviewed along with the related concept of graphene layer orientation within the structure and the resulting ratio of basal plane to edge site as well as its effects on surface chemical reactivity and energy. Both non-specific and specific surface forces and the resulting potentials are considered. Theoretical physical models based on the Lennard-Jones potential applied to the basic structural units, either as an ideal, i.e. perfect, single graphene layer or as multiple layers of varied inter-layer spacing and size, are presented. Mention is then made of non-ideal surfaces (stepped or containing vacancies) and their resulting enhanced potential. The effects of incorporating chemical heterogeneity into the surfaces of these structures are also considered: carbon oxidation mechanisms are discussed in ter...

Compensating specular highlights for non-Lambertian projection surfaces

This paper concerns the compensation of specular highlight for handheld image projectors. By employing a projector-camera configuration, where the camera is aligned with the viewer, the distortion caused by nonideal (e.g., colored, reflective) projection surfaces can be estimated from the captured image and compensated for accordingly to improve the projection quality. This works fine when the viewing direction relative to the system is fixed. However, the compensation becomes inaccurate when this condition changes, because the position of the specular highlight changes as well. We propose a novel method that, without moving the camera, can estimate the specular highlight seen from any position and integrate it with Grossberg’s radiometric compensation framework to demonstrate how view-dependent compensation can be achieved. Extensive results, both objective and subjective, are provided to demonstrate the performance of the proposed algorithm.

Developing understanding of organic photovoltaic devices: kinetic Monte Carlo models of geminate and non-geminate recombination, charge transport and charge extraction

This Perspective discusses the physics, implementation and findings of Kinetic Monte Carlo (KMC) models applied to organic photovoltaic devices (OPVs). It is shown that KMC models can relate morphology and energy levels on the nano-scale to geminate and non-geminate recombination, charge transport, charge injection and charge extraction measured in macro-scale OPVs. In particular, KMC investigations probing the circumstances under which geminate recombination is, and is not, a significant loss mechanism in OPVs are reviewed. Furthermore, the mechanisms which yield non-geminate (bimolecular) recombination in OPVs that is both slower than the predictions of Langevin, and charge density dependent, are discussed. It is also shown how KMC models can predict average mobility, as well as spatial heterogeneity and temporal dispersion around this average value in disordered bulk heterojunctions. Finally, it is shown how KMC can be used to quantify the effect of non-ideal electrodes, interlayers and surface wetting layers on OPV performance.

Functionalised polycaprolactone films and 3D scaffolds via gamma irradiation-induced grafting.

Polycaprolactone (PCL) is frequently used as the base polymer in scaffolds targeted for tissue engineering applications. However, in the absence of further surface modification, the lack of functional moieties on the PCL chain results in non-ideal surface properties of such scaffolds, especially in terms of the inability to tailor the presentation of functional ligands for directed cell adhesion and growth. The current study investigates gamma irradiation-induced grafting as a means of improving the biofunctionality of the PCL surface. The surface presentation of carboxylic acid groups on 2D PCL films could be tailored by changing the acrylic acid (AAc) concentration and/or the solvent during grafting, as evaluated from X-ray photoelectron spectroscopy (XPS). From data obtained using Raman spectroscopy, it was concluded that the penetration depth of the grafted pAAc was affected by the solvent system with a mixed water-methanol system yielding high penetration. Grafted samples displayed a decreased elastic modulus of the surface correlating with pAAc penetration depth, as shown by nano-indentation using atomic force microscopy (AFM). The most promising grafting conditions found for the 2D PCL films were then applied to 3D thermally induced phase separation (TIPS) scaffolds and it was demonstrated using XPS that equivalent levels of grafting of pAAc could be achieved throughout the whole depth of the scaffold. The scaffolds maintained their overall integrity after grafting, even though we observed a decrease in the compressive modulus by 20% after surface modification. These combined studies confirm the utility of this surface modification methodology for scaffolds targeted at tissue engineering and cell culture applications.

On the characteristics of commonly nonideal rectangular ridge dielectric-loaded surface plasmon polariton waveguides in practical fabrication processing

In this paper, four commonly nonideal structures of the ideal rectangular dielectric ridge dielectric-loaded surface plasmon polariton waveguides (DLSPPWs) in actual fabrication processing were studied. The mode effective index, propagation length, field distributions, effects of geometric deviation, lateral field confinement and figures of merit of fundamental mode were calculated at the telecommunication wavelengths. It was found that, the propagation length, the lateral field confinement and figures of merit of four nonideal rectangle ridge DLSPPWs were changed compared with the ideal rectangular dielectric ridge DLSPPW. However, the field distributions were kept, and, the characteristics of nonideal rectangle ridge DLSPPWs were affected by the geometric deviation. This study presents the characteristics of commonly nonideal rectangle ridge DLSPPWs and provides substantial information for processing and optimizing the rectangular dielectric ridge DLSPPWs in practical fabrication processing.

Regulation of Cell-Surface Adhesion During Amoeboid Migration

During amoeboid migration, cell-substrate interactions play a critical role by supplying the traction allowing a cell to move itself forward. It has been shown that the ability of individual mammalian cells to move will depend on their adhesiveness to that surface, with moderately adhesive surfaces being ideal: moderate adhesion allows cells to gain traction to move forward while still permitting detachment of the cell rear. In addition to cell-substrate adhesion, migrating cells often experience cell-cell adhesion, and in many systems (such as wound healing or embryogenesis) such adhesion can dominate. The amoeba Dictyostelium discoideum naturally exhibits both cell-substrate and cell-cell adhesion during the aggregation process. We used both high- and low-magnification time-lapse microscopy to investigate the individual and collective migration of D. discoideum on substrates of varying adhesiveness, as well as on interfaces between substrates. We find that non-ideal adhesive surfaces can affect both individual cell migration as well as the behavior of cell groups. At the population scale, non-ideal surfaces slow down the initiation of aggregation and change how the process is performed: instead of forming large aggregation centers, the population forms small aggregation centers which coarsen over time until the standard aggregation center size is eventually reached. At the scale of single cells, we measure both adhesion ability as well as the area of contact between cells and surface for individual cells and cells that are part of groups. On all surfaces the contact area oscillates, reflecting the migration dynamics. Surprisingly, comparable forces are needed to rip cells off all surfaces, indicating that surface adhesion may be regulated by the cells.

Surface Wettability of (3-Aminopropyl)triethoxysilane Self-Assembled Monolayers

The (3-aminopropyl)triethoxysilane (APTES) self-assembled monolayer (SAM) has been widely used in fundamental research and engineering applications; however, characterization of its surface wetting properties remains problematic. Surface wetting properties of the APTES SAM were systematically investigated using different contact angle measurement techniques. The observed unique nonideal wetting was related to the APTES SAM structure, including surface hydrogen bond formation, the surface roughness, and the effect of water penetration. The contact angle decreased dramatically with the residence time on the APTES SAM surface, and a special contact angle hysteresis phenomenon was observed. The contact angle could be distorted by the calculation method used for the nonideal APTES SAM surface. Values calculated by the tangent-leaning method were thought to be more accurate and credible. Our findings demonstrated that static advancing contact angles were the most stable and credible for characterizing the APTES SAM surface wettability.

Electron Spectroscopy of Corrugated Solid Surfaces

This paper reviews work done of the influence of non-ideal surface topography on electron spectral intensities of surface-sensitive electron spectroscopic methods: primarily X-ray induced photoelectron spectroscopy (XPS) and concise Auger electron spectroscopy (AES), electron energy loss spectroscopy in the reflection mode (REELS), and elastic peak electron spectroscopy (EPES). Several attempts to solve the problem are mentioned, where (i) the effect of surface roughness is corrected using a single parameter, (ii) computer simulations based on a model of surface roughness composed of regular geometrical units are used for electron spectral intensity calculations, and finally, (iii) a semi-empirical method based on careful surface mapping of analyzed sample by atomic force microscopy (AFM) is discussed in greater detail. The first approach was found to be rather simple to properly include any complex surface topography. The second technique can help us to understand surface topography related phenomena. The latter method, suitable for arbitrary rough solid surfaces covered by conformal overlayer(s), can be incorporated in current quantitative procedures valid for ideally flat surfaces.

Kinetics of Catalytic Reactions on Nanoclusters

Kinetics of heterogeneous catalytic reactions on nanoclusters is considered. The size of organic molecules used in various catalytic transformations is often ca. 0.5-1 nm, whereas nanoparticles in the range 2-4 nm are used in supported heterogeneous catalysts. Proposed quantitative treatment of a heterogeneous catalytic mechanism with two kinetically significant steps assumes that a finite number of molecules per a single metal cluster can be adsorbed. A general equation with polynomial terms in the numerator and the denominator is derived for nonideal (nonuniform) surfaces when all kinetic and adsorption constants depend on the spatial arrangements of reacting molecules. A simplified case of a two-step catalytic cycle is discussed, with the first step being adsorption on nanoclusters and the second one being further transformation of absorbed species. The corresponding kinetic equations are derived and numerically analyzed for the case of ideal (uniform) surfaces as well as the presence of induced nonuniformity, when contrary to the ideal case there are lateral interactions between adsorbed species. The form of kinetic equations allows not only saturation, typical for Eley-Rideal kinetics, but also positive cooperativity, i.e., S-shaped behavior of the reaction rate as a function of substrate concentration. Comparison with experimental data on hydrodechlorination of dichlorobenzenes is given.

Size dependent interface energy and catalytic kinetics on non-ideal surfaces

Thermodynamic analysis is applied to describe the size dependent kinetics on surfaces with lateral interactions. A two-step sequence, often used in heterogeneous catalysis to account for catalytic kinetics, was modified to include the dependency of interfacial free energy on the particle size.

The influence of repellent coatings on surface free energy of glass plate and cotton fabric

The aim of this research was to determine the influence of chemical finishes on the surface properties of glass plate, considered as a model homogeneous smooth surface and cotton fabric as a non-ideal heterogeneous rough surface. Microscopic slides and 100% cotton fabric in plain weave were coated with fluorocarbon polymers (FCP), paraffin waxes with zirconium salts (PWZ), methylolmelamine derivatives (MMD), polysiloxanes with side alkyldimethylammonium groups (PSAAC) and aminofunctional polysiloxanes (AFPS). From the goniometer contact angle measurements of different liquids, the surface free energy of the coated glass plates was calculated according to approaches by Owens–Wendt–Kaelble, Wu, van Oss–Chaudhury–Good, and Li–Neumann–Kwok. The results showed that all the coatings decreased the surface free energy of the substrate, which was also influenced by the liquid combination and the theoretical approach used. In spite of the fact that the liquid contact angles were much higher on the coated fabric samples than on glass plates and resulted in the lower values of work of adhesion, a very good correlation between the coatings deposited on both substrates was obtained. The presence of repellent coatings FCP, PWZ and MMD converted the solid surface from polar to highly apolar by masking the functional groups of glass and cellulose. PSAAC and AFPS coatings did not decrease the solid surface free energy to such an extent as the former three coatings due to their monopolar character.

A proposal for the estimation of binary mixture activity coefficients from surface tension measurements throughout the entire concentration range

A method to estimate the concentration dependence of the bulk activity coefficients of both binary mixture components from experimental surface tension data covering the whole concentration range is presented. To this end, high-quality experimental surface tension data as a function of the concentration, paying special attention to the diluted regions, are needed. The method is based on the application of equilibrium conditions to the surface chemical potential arisen from the Volmer equation (the simplest non-ideal surface EOS) and to the bulk chemical potential coming from the three-suffix Margules equation for activity coefficients. It can be applied provided that positive aneotropy is not present and the second derivative of the surface pressure as a function of the mole fraction is not positive in any composition region, which means rather unrestrictive conditions. In order to test the method, a compilation of 25 systems for which surface tension and activity coefficient data are simultaneously available in the literature was performed, turning out that most of them involved water. It was found that surface tension data with enough concentration coverage and good quality are really scarce, which makes the development of this kind of methods difficult.

Impact of surface state modeling on the characteristics of InP/GaAsSb/InP DHBTs

We study the effect of non-ideal surface conditions on the characteristics of submicrometer type-II InP/GaAsSb double heterostructure bipolar transistors (DHBTs) by comparing two-dimensional numerical simulations with measured device characteristics. The inclusion of surface effects in simulations enables excellent agreement with measured Gummel characteristics and measured emitter size effects, and shows that recombination at the emitter mesa periphery on the extrinsic base surface is the major source on non-ideality in device characteristics. The simulations also demonstrate that AlInAs emitters should result in weaker surface recombination and emitter size effects.

A Comparison of Nanometer-Scale Growth and Dissolution Features on Natural and Synthetic Dolomite Crystals: Implications for the Origin of Dolomite

Abstract This paper describes experiments in which mechanisms of crystal growth are inferred from the surface nanotopography of natural and synthetic dolomite using atomic force and scanning electron microscopy. Synthetic dolomite was formed by replacement of calcite at 218°C in Mg–Ca–Cl solutions. The first products to form during dolomite synthesis experiments, as detected with X-ray diffraction (XRD), are invariably poorly ordered (nonideal) dolomite. As the reaction proceeds, the degree of cation order increases, yet the products remain relatively disordered. Upon examination, the surfaces of nonideal dolomite are covered with mounds 10–200 nm wide and 1–20 nm high. Following depletion of the calcite reactant, dolomite becomes relatively well ordered and stoichiometric (ideal). The surfaces of ideal dolomite are covered with broad, flat layers separated by steps that measure tenths to tens of nanometers high. Comparison of natural and synthetic dolomite surfaces after etching in 0.5% H2SO4 for 10–20 seconds indicates that high-temperature synthetic and low-temperature natural nonideal dolomite surfaces are covered with mounds identical to the growth features found on unetched nonideal synthetic dolomite. In contrast, synthetic and low-temperature ideal dolomite surfaces, when etched, are characterized by flat layers with deep, euhedral etch pits. These results suggest that despite the wide range in formation conditions, natural and synthetic dolomites form by the same growth mechanisms. Furthermore, the two different surface nanotopographies described here are consistent with a model in which nonideal dolomite forms by precipitation of amorphous mounds that quickly crystallize to nonideal dolomite while ideal dolomite later replaces nonideal dolomite and grows by spiral growth.

Development of electron reflection suppression materials for improved thermionic energy converter performance using thin film deposition techniques

Nonideal electrode surfaces cause significant degree of electron reflection from collector during thermionic converter operation. The effect of the collector surface structure on the converter performance was assessed through the development of several electron reflection suppression materials using various thin film deposition techniques. The double-diode probe method was used to compare the J-V characteristics of converters with polished and modified collector surfaces for emitter temperature and cesium vapor pressure in the ranges of 900–2000K and 0.02–1.5torr, respectively. The coadsorption of cesium and oxygen with respective partial vapor pressures of ∼1.27torr and a few microtorrs reduced the emitter work function to a minimum value of 0.99eV. It was found that the collector surfaces with matte black appearance such as platinum black, voided nickel from radio-frequency plasma sputtering, and etched electroless Ni–P with craterlike pore morphology exhibited much better performance compared with polished collector surface. For these thin films, the increase in the maximum output voltage was up to 2.0eV. For optimum performance with minimum work function and maximum saturation emission current density, the emitter temperature was in the range of 1100–1500K, depending on the collector surface structure. The use of these materials in cylindrical converter design and/or in combination with hybrid mode triode configuration holds great potential in low and medium scale power generators for commercial use.