Formation and Geometry of a High-Coverage Oxygen Adlayer on Ru(001), the p(2 × 2)-3O Phase

A detailed low-energy electron diffraction (LEED)-IV analysis, complemented by scanning tunneling microscopy (STM) observations, was carried out for the apparent (2 × 2) structure of the oxygen-covered Ru(001) surface at a coverage of 0.75 ML. We present STM images of incomplete layers which allow one to define the symmetry of the ordered layer, in particular of the novel high density p(2 × 2)-3O phase. In the LEED-IV analysis we have tested 28 model structures; the results can be used for conclusions about the discrimination of this type of geometry determination. Our quantitative LEED analysis in connection with the STM results corroborates the model proposed before and shows that all of the oxygen atoms sit in the hcp sites with an averaged vertical distance to the outermost Ru layer of d^⊥_Ru–O = 1.22 Å. This value falls into the general trend of increasing d^⊥_Ru–O with oxygen coverage observed for the other ordered structures of adsorbed oxygen on Ru and is also predicted by recent total energy calculations. The O–Ru bonding distance of about 2.0 Å is essentially unchanged compared to the other structures. Considerable lateral and vertical displacements of both the O and the Ru atoms are found, with the O atoms being slightly displaced towards the fcc hollow site located in the center of three oxygen atoms. The two uppermost substrate layers are buckled; in the first layer three out of four Ru atoms of the (2 × 2) unit cell are shifted away laterally from their bulk positions. These shifts, globally as well as locally, can be understood in terms of local electron density changes induced by the adsorbed oxygen atoms.     

Formation of Ge-Sn nanodots on Si(100) surfaces by molecular beam epitaxy

The surface morphology of Ge_0.96Sn_0.04/Si(100) heterostructures grown at temperatures from 250 to 450°C by atomic force microscopy (AFM) and scanning tunnel microscopy (STM) ex situ has been studied. The statistical data for the density of Ge_0.96Sn_0.04 nanodots (ND) depending on their lateral size have been obtained. Maximum density of ND (6 × 10¹¹ cm^-2) with the average lateral size of 7 nm can be obtained at 250°C. Relying on the reflection of high energy electron diffraction, AFM, and STM, it is concluded that molecular beam growth of Ge_1-x Sn _x heterostructures with the small concentrations of Sn in the range of substrate temperatures from 250 to 450°C follows the Stranski-Krastanow mechanism. Based on the technique of recording diffractometry of high energy electrons during the process of epitaxy, the wetting layer thickness of Ge_0.96Sn_0.04 films is found to depend on the temperature of the substrate.     

Bulk copper electrodeposition on gold imaged by in situ STM: morphology and influence of tip potential

Electrochemical measurements were carried out simultaneously with acquisition of in situ STM images of copper electrodeposition at low cathodic overpotentials and subsequent dissolution from the underlying polycrystalline gold surfaces. The morphologies of the copper deposits were examined for correlation with features of the current-voltage diagram. Copper growth is by nucleation and formation of 3D islands. During the initial stages of bulk copper growth the potentials were fixed at selected values and a balance observed between formation of polycrystalline copper nuclei and of copper crystals. After the first cycle of copper deposition and dissolution the morphology of the polycrystalline gold surface had apparently changed into a recrystallized phase of a copper-gold alloy. At a given stage of the cycle the potential of the electrode was found to depend linearly on the tip potential. In a wide range of tip potentials the onset of copper deposition and end of dissolution showed a potential separation of 59 ± 5 mV indicating a single electron process.     

Electrical Characteristics of 8-nm SOI n-FinFETs

This study is aimed at presenting the electrical characteristics of a nanoscale SOI n-channel fin field-effect transistor (FinFET) structure with 8 nm gate length using Al₂O₃ as the dielectric material and their sensitivity to the number of fins and fin-thickness with 3C-SiC material in the channel region. In this work, the numerical simulation tool Silvaco-Atlas is used to simulate the device in three-dimensions and to extract new results concerning the electrical characteristics of the device at room temperature (300 K) in comparison to earlier generations. The threshold voltage, subthreshold slope, transconductance, drain induced barrier lowering, leakage current, on-current, and On/Off current ratio are analyzed. Simulation results show that the higher drain current and transconductance are obtained by increasing the number of fins. The use of a higher value of gate dielectric constant can increase the drain current and improve the leakage current. It is found that reducing the fin-thickness is beneficial in reducing the subthreshold slope, drain induced barrier lowering, and leakage current. It should be highlighted that the achieved results can be useful for further manufacturing processes.     

Field emission devices for advanced electronics comprised of lateral nanodiamond or carbon nanotube emitters

Nanocarbon-derived electron emission devices, specifically, nanodiamond lateral field emission diodes and gated carbon nanotube triodes are new configurations for robust nanoelectronic devices. These novel micro/nanostructures provide an alternative and efficient means of accomplishing electronics that are impervious to temperature and radiation. Nitrogen-incorporated nanocrystalline diamond has been lithographically micropatterned to utilize the material as an electron field emitter. Arrays of laterally arranged “finger-like” nanodiamond emitters constitute the cathode in a versatile diode configuration with small interelectrode separation. Nanodiamond lateral tip conditioning techniques are employed to improve emission and the subsequent device performance discussed. A low diode turn-on voltage of 7 V and a high emission current of 90 μA at an anode voltage of 70 V (electric field of ∼ 7 V/μm) is reported for the nanodiamond lateral device. Also, the development of a field emission triode amplifier based on aligned carbon nanotubes (CNTs) with low turn-on voltage and small gate leakage current, utilizing a dual-mask microfabrication process is reported. The2 × 20 μm CNT triode array displays a gate turn-on voltage of ∼ 44 V, and low gate currents less than 3% of the anode currents. The low gate leakage currents observed confirmed the effectiveness of the convex-shaped gated CNT emitter in alleviating the cathode-gate leakage problem that compromises the operation of a field emission triode.     

Carbon nanotubes in electron donor-acceptor nanocomposites

This Account presents recent advances in the design, synthesis, characterization, and potential applications of new hybrid materials based on carbon nanotube and electron donors. Fast charge separation and slow charge recombination are consistently observed in a variety of composites that contain porphyrin derivatives. The ultimate goal of using these systems to prepare practical photoelectrochemical devices is discussed, and a cell with a monochromatic efficiency of 8.5% conversion of light into current is illustrated.     

Investigation of electronic properties of graphene/Si field-effect transistor

We report a high-performance graphene/Si field-effect transistor fabricated via rapid chemical vapor deposition. Oligolayered graphene with a large uniform surface acts as the local gate of the graphene transistors. The scaled transconductance, g_m, of the graphene transistors exceeds 3 mS/μm, and the ratio of the current switch, I_on/I_off, is up to 100. Moreover, the output properties of the graphene transistor show significant current saturation, and the graphene transistor can be modulated using the local graphene gate. These results clearly show that the device is well suited for analog applications.     

Industry-wide energy saving by complex separation networks

Different column configurations in a distillation network can achieve the same ultimate products, with different amounts of energy to do so. Only recently, methods have been discovered for the systematic generation and evaluation of all possible complex column configurations for the separation of multi-component mixtures. However, there is a lack of robust methods for the globally optimal design of complex column sequences. This presentation reviews recent advances to tackle the challenges of synthesizing complex column networks with computer-aided design algorithms. An automatic procedure to generate optimal column sequences based on a thermodynamic problem transformation called temperature collocation is discussed and its current capabilities and limitations illustrations with the help of two realistic case studies.     

STM tip-induced local electrochemical dissolution of silver

Local dissolution/deposition processes under in situ scanning tunneling microscopy (STM) imaging conditions are studied in the systems Ag(111)/Ag⁺, ClO₄⁻ and Ag(111)/Ag⁺, SO₄²⁻. The results show that in both systems the local kinetics of these processes strongly depend on the polarization conditions. At STM-tip potentials more positive than the Ag/Ag⁺ equilibrium potential, a local dissolution of the Ag(111) substrate is observed even at cathodic substrate overpotentials at which the overall substrate current density is cathodic. This tip-induced Ag dissolution is in agreement with results obtained recently in the system Cu(111)/Cu²⁺. The enhanced local Ag dissolution is explained by a reduced Ag⁺ concentration underneath the STM tip promoted by both an electrostatic repulsion of Ag⁺ and a reduction of the mass transport due to the shielding effect of the tip. The possibility for a preparation of negative Ag nanostructures by STM tip-induced electrochemical dissolution is demonstrated.     

Thin film composite membranes with desirable support layer for MeOH/MTBE pervaporation

A comprehensive study was performed on a new application of thin film composite membranes and selecting a stable sublayer for them as pervaporation membranes in organic solvent separation. For this purpose, four different polymeric sublayers of polyethersulfone (PES), cellulose acetate, polyacrylonitrile, and polyetherimide were prepared, and the interaction of methanol (MeOH) and methyl tert butyl ether (MTBE) with them was investigated. The contact angle results, scanning electron microscopy images, and swelling and mechanical strength measurements obviously displayed the effect of immersion in organic solvents on the sublayers. Finally, a polyamide active layer was subsequently deposited on the PES membrane surface as the stable sublayer via interfacial polymerization based on a multistep statistical optimization strategy involving fractional factorial design and a response surface method. The prepared TFC membranes were tested in the pervaporation of a MeOH/MTBE mixture and exhibited excellent performance compared with the current membranes in this context. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47519.     

In situ Control of Si/Ge Growth on Stripe-Patterned Substrates Using Reflection High-Energy Electron Diffraction and Scanning Tunneling Microscopy

Si and Ge growth on the stripe-patterned Si (001) substrates is studied using in situ reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). During Si buffer growth, the evolution of RHEED patterns reveals a rapid change of the stripe morphology from a multifaceted “U” to a single-faceted “V” geometry with {119} sidewall facets. This allows to control the pattern morphology and to stop Si buffer growth once a well-defined stripe geometry is formed. Subsequent Ge growth on “V”-shaped stripes was performed at two different temperatures of 520 and 600°C. At low temperature of 520°C, pronounced sidewall ripples are formed at a critical coverage of 4.1 monolayers as revealed by the appearance of splitted diffraction streaks in RHEED. At 600°C, the ripple onset is shifted toward higher coverages, and at 5.2 monolayers dome islands are formed at the bottom of the stripes. These observations are in excellent agreement with STM images recorded at different Ge coverages. Therefore, RHEED is an efficient tool for in situ control of the growth process on stripe-patterned substrate templates. The comparison of the results obtained at different temperature reveals the importance of kinetics on the island formation process on patterned substrates.     

Effect of wall velocities on the determination of optimal separation times in electrical field flow fractionation (EFFF)

Electrical field flow fractionation (EFFF) has two perpendicular driving forces that help to produce an optimal separation of solute in a mixture [Giddings, Science 1993; 260:1456–1465]. For Couette flow based devices, the ratio of the velocity of the capillary walls offers an extra parameter that can be exploited to enhance the efficiency of EFFF applications. The analysis of the effects of this parameter on optimal times of separation is the subject matter of this contribution. The use of this additional parameter increases flexibility in the design of new devices for the improvement of the separation of solutes, such as proteins, DNA, and pharmaceuticals, as it will be illustrated with the results of this analysis (Jaroszeski et al., 2000 ; Trinh et al., 1999 ). The analysis has been illustrated by selecting parameter values that represent a number of potential useful applications. A set of five parameters (i.e., z, the valence; µ, electrophoretic mobility; Pe, Peclet number; Ω, the orthogonal applied electrical field; and R, the ratio of channel wall velocities) has been combined to obtain the best operating conditions for optimal separation of solutes. Results indicate that R, the ratio of the channel wall velocities, is actually the most important driving parameter.     

Design and CFD studies of multiphase separators—a review

The multiphase separators are generally the first and largest process equipment in an oil production platform. This primary separation step is a key element in the oil and gas production facilities in that downstream equipment, such as compressors, are completely dependent on the efficient performance of these multiphase separators. The literature on this critical unit operation, multiphase separators, abounds with macro studies and design methodologies for two‐ and three‐phase vertical and horizontal separators. There are very few studies that provide the micro details of the actual separation process. In fact, the popular classic methods for separator design, mostly due to a lack of a usable mathematical model for estimation of droplet ‘separation velocities’, do result in a conservative design and would specify extremely oversized separators. In order to reflect the current situation and address recent findings, this study will review the important literature on design and CFD simulation of multiphase separators. This review will show the benefits that CFD analyses can provide in optimising the design of new separators and solving problems with existing designs. © 2011 Canadian Society for Chemical Engineering     

A Rule of Thumb for Binary Isotope Separations in a Gas Centrifuge

Abstract

A simple hypothetical model of the binary isotope separation process in a modern countercurrent Gas Centrifuge is proposed. Like the usual Cohen-Onsager separation theory, internal fluid dynamics are obviously involved. But unlike that theory it completely obviates the flow integrals for Cohen's E, thereby allowing an immediate estimate of the flow efficiency of a given design by visual inspection of the flow field. At times this should be checked later by the usual analyses. To shed some light on this idea, derivations for two simple assumed idealized hydrodynamics are given, but a rigorous proof remains an open question. Then our hypothesis is tested against a battery of about 10 new “exact” formulas for E based upon analytical solutions to several variants of Onsager's pancake equation and is found to be “reasonably” accurate and surprisingly robust. Finally, some limitations of our rule are explored.     

No-Mix and Ideal Separation Cascades

Abstract

Ideal countercurrent recyle cascades are characterized by two propertie: 1) The compositions of heads and tails streams forming the feed stream to individual stages are the same, and 2) the heads separation factor for each stage is constant. When these two criteria are met, the heads and tails separation factors are constant and equal to the square root of the stage separation factor. The mixing of streams with different compositions within a separation cascade obviously constitutes an inefficiency since it is precisely the reverse of this process that is desired, hence Condition 1, which is often referred to as the no-mix-criterion. Separation cascades for which both criteria are valid are termed ideal. However, the ramifications of Condition 2 are not obvious and it is possible to design no-mix cascades which do not meet the second condition. Mathematical relationships between ideal and no-mix separation cascades are derived to quantify these differences. It is shown that Condition 2 minimizes the total interstage flow required to make a given separation for any no-mix cascade design, i.e., it is an ideal cascade by definition. Finally, the required number of ideal stages necessary to perform a specific separation with no-mix cascades are evaluated and compared to those of the ideal cascade.     

A method for joining individual graphene sheets

A graphene-based device requires the graphene to have an ideal shape, structure, and orientation, and be large enough, to allow them to be formed into a new device. Here the joining of individual single-layer and multi-layer graphene is performed in a transmission electron microscope-scanning tunneling microscope (TEM–STM) holder inside a 200 kV field emission TEM. Attempts have been made to join individual graphene sheets (GSs) with the so-called “top-to-top” and “layer-to-layer” geometries by applying a voltage. In the two geometries, the “top-to-top” form has resulted in a seamless joining for both single-layer and multi-layer GSs. The as-joined GSs show the same excellent electrical and mechanical properties as those of the original GSs. Large Joule heating originating from the field emission current will cause atom diffusion and self-assembly and then rearrangement of carbon networks at the GS edge front. In this way individual GSs could be extended and mended with the so-called “top-to-top” geometries by applying a constant voltage, to meet the required and desired shape, size, configuration, and functions for a variety of the special micro/nano scaled devices.     

Wärme-, Stoff- und Impulsübertragung in abgelösten Strömungen

Heat, mass and momentum transfer in separated flows . The simple model of a backward-facing step in flat plate boundary layers provides basic information on heat, mass and momentum transfer in local separated regions. These new investigations cover nearly the whole range of existence of incompressible separated flows with laminar as well as turbulent boundary layers at separation, including the two main parameters Re_δ1 and Re_s: The Reynolds number Re_δ1 represents the boundary layer at separation, the Reynolds number Re_s the shape of the body. It is demonstrated that independently of the state of the boundary layer at separation, there exist three types of local separated regions. Therefore a new general valid classification of local separated flows is introduced based on the actual state of the separated boundary (shear) layer from separation to reattachement. The new results presented are limited to the case where no temperature or concentration boundary layers exist at separation, i.e. to the case of unheated or inert starting length before separation: Only then can the analogy of heat, mass and momentum transfer give useful results over the whole range of existence of local separated regions. A new shear layer model is introduced to facilitate transfer of the results obtained for backward-facing steps to arbitrary shaped bodies. A compilation and comparison of available literature values shows that this shear layer model for the step also permits the calculation of heat and mass transfer in separated areas on arbitrarily shaped bodies.     

The Effects of Power Law Fluid Rheology on Coil Heat Transfer for Agitated Vessel in the Transitional Flow Regime

A CFD model of heat transfer from power‐law fluids to helical cooling coils in the transitional flow regime of a baffled tank mixed with a pitched blade turbine was developed with Fluent^TM. The model captured local temperature and velocity gradients. Simulations were run, varying Re, Pr, K and n. The results indicate that a Sieder‐Tate type correlation, with the exponent on and the coefficient in front of the Reynolds number being a function of n, is recommended for estimating ho. Also, a new two coil bank design was found to be more efficient when 450 < Re < 650.     

Bioproduct adsorption in immobilized adsorbent: Local thermodynamic equilibrium model

A mathematical model using local thermodynamic equilibrium isotherms for adsorption on immobilized adsorbents is proposed in order to optimize the design parameters inin situ bioproduct separation process. The model accurately follows the experimental data on the adsorption of berberine, secondary metabolite produced in plant cell culture. The result shows that the lower loading capacity in immobilized adsorbents is due to the decrease in the maximum solid phase concentration and the isotherm equilibrium constant, not the effective diffusivity. Design parameters inin situ bioproduct separation process, such as the size of the beads, the ratio of beads to bulk volume and the adsorbent content of the bead, are evaluated by using the model. The decrease of bead size is the most effective parameter for adsorption of berberine in immobilized adsorbent due to a reduction in the overall diffusional resistance.     

Effect of Changes in Operating Variables on Stage-wise and Cumulative Separation in Binary Multistage Distillation

Abstract

The progressive separation that occurs stage-wise within a multistage distillation column is characterized by the cumulative extent of separation, ζ _N ; while the contribution of individual stages, δ _N , to the overall separation is given by the difference between ζ _N for successive stages. These indexes permit the “goodness” of separation for individual stages and for the entire column to be compared on an equivalent basis. This paper examines the effects of changing operating variables of reflux ratio, feed rate, feed composition, and feed stage location on the separation obtained in a distillation column containing a fixed number of ideal stages, and how the single-stage contribution changes when these variables are altered from the design value. The calculations show that the reflux ratio (R) is probably the most important variable in determining how well a column makes a separation. Separation declines rapidly as R is reduced below the design value, as the feed rate is increased at constant boil-up rate, and as the feed concentration drops below the design value. Changing the feed stage location of ±5 stages in a 50-stage column has a minimal effect on separation at all feed compositions. δ _N clearly shows how the contribution of individual stages changes when operating variables are varied from the design values.