Non-ideal Surface Research Articles

Entropy flow and generation in radiative transfer between surfaces

Entropy of radiation has been used to derive the laws of blackbody radiation and determine the maximum efficiency of solar energy conversion. Along with the advancement in thermophotovoltaic technologies and nanoscale heat radiation, there is an urgent need to determine the entropy flow and generation in radiative transfer between nonideal surfaces when multiple reflections are significant. This paper investigates entropy flow and generation when incoherent multiple reflections are included, without considering the effects of interference and photon tunneling. The concept of partial equilibrium is applied to interpret the monochromatic radiation temperature of thermal radiation, T λ ( λ, Ω), which is dependent on both wavelength λ and direction Ω. The entropy flux and generation can thus be evaluated for nonideal surfaces. It is shown that several approximate expressions found in the literature can result in significant errors in entropy analysis even for diffuse-gray surfaces. The present study advances the thermodynamics of nonequilibrium thermal radiation and will have a significant impact on the future development of thermophotovoltaic and other radiative energy conversion devices.

Reflectivity and optical surface height requirements in a broadband coronagraph 1Contrast floor due to controllable spatial frequencies

We derive the broadband contrast floor in a coronagraphic telescope having nonideal optical surfaces. We consider only fundamental spatial frequencies within the control bandwidth of the coronagraph's deformable mirror. Cross terms arising from the beating of spatial frequencies beyond the deformable mirror control bandwidth will be considered in a second paper. Two wavefront control systems are analyzed:a zero-path difference Michelson interferometer with two deformable mirrors at a pupil image, and a sequential pair of deformable mirrors with one placed at a pupil image. We derive requirements on optical surface figure and reflectivity uniformity for both cases.

Determining the Association Constant and Adsorption Properties of Ion Pairs in Water by Fitting Surface Tension Data

Association constants and adsorption parameters of tetraalkylammoniumdodecyl sulfate (TAADS) ion pairs in water were determined. We have analyzed water/air surface tension measurements obtained for mixtures of sodium dodecyl sulfate (SDS) and tetraalkylammonium bromide of increasing chain lengths (TMAB, TEAB, TPAB, and TBAB). To reproduce the experimental isotherms, we coupled the association equilibrium of the ion pairs to the equations proposed by Fainerman and co-workers to model the adsorption of binary mixtures of surfactants (SDS and TAADS) with different molar areas at a nonideal surface layer. The parameters found showed that the model is not convenient to describe the effect of the addition of TMAB but a clear coherency was obtained for the three longer compounds. Ranging from TEADS to TBADS increasing hydrophobic interactions give rise to a higher associability but to a lower surface activity. Self-interactions coefficients extracted by the fitting procedure confirmed the importance of attractive interactions between the ion pairs. The calculated surface coverage showed that in every case the compound mainly adsorbed at the interface was the ion pair. For TBADS strong attractive interactions result in a phase transition at very low concentration.

A bulk-micromachined bistable relay with U-shaped thermal actuators

This paper reports a deep-reactive ion etching (DRIE)-through-etched laterally bistable MEMS relay for power applications, with a primary emphasis on the design and modeling of its U-shaped transient thermal actuators, and a secondary emphasis on the design and fabrication of its contact element. In this relay, a contact crossbar is carried by a curved-beam bistable mechanism , which is toggled by transient U-shaped thermal actuators with their hot beam adiabaticly heated by electrical pulses. Each U-shaped thermal actuator comprises uniform-thickness hot and cold beams with a gap between them so they bend differently. This paper develops both a basic model and a complete model for the actuator that are verified by Finite Element Analysis and serve as effective design tools. The DRIE process creates nonideal etched surfaces, which pose challenges for good relay contacts. Both contact design and process development are discussed to help alleviate this problem. The fabricated relay exhibits a minimum total on-state resistance of 60 m/spl Omega/, and a maximum current carrying capacity of 3 A. It switches with a 1 ms actuation pulse, and a maximum 5 Hz repetition rate.

Authors' Response

The authors are grateful to the Editor for the opportunity to reply to the letter by Lupini et al. The authors of the above letter comment on a set of recent articles in which the novel technique of imaging at a negative value of the spherical aberration coefficient of the objective lens in an aberration-corrected transmission electron microscope (NCSI technique) is methodically described and applied to the measurement of the occupancy of atomically resolved oxygen columns in perovskites. In particular, the authors raise doubts about the possibility of inferring quantitative data from measurements of the local image intensity at the position of the oxygen atom columns. With reference to the study by Jia et al. (2003a), the letter authors present an image simulation on the basis of which it is stated that the observed effect of a reduced intensity at the oxygen atomic columns should not be interpreted in terms of reduced oxygen occupancy but can, as the authors claim, be “better” explained on the basis of the effect of surface roughness on contrast. In addition, the authors emphasize the work of Kim et al. (2001) with respect to the nonstoichiometry of the oxygen occupancy in grain boundaries of SrTiO3and criticize our reference to the literature in which it is reported that oxygen cannot be observed by the scanning transmission electron microscopy (STEM) technique in Z-contrast. In the following, we shall demonstrate that in spite of the fact that a nonideal surface morphology can—as in the application of any (!) electron microscopic technique whether used in TEM or in STEM—have an effect on local image intensity, meaningful quantitative measurements of relative oxygen-atom site occupancies can be carried out employing the NCSI technique.

Transitions from static wetting to steady dewetting and deposition of liquid-film on partially wettable polymer surface

The transitions from static to steadily moving wetting perimeter and further to deposition of a liquid-film on partially wettable surface were studied with the same system under the same conditions. A polyethylene terephthalate (PET) tape was vertically withdrawn at constant velocity from glycerol–water mixture. Elevation L of the three-phase contact line above the liquid level was measured under static, steady, and dynamic wetting. The static receding Θ R and the apparent dynamic angles Θ app at different withdrawal velocities U were calculated from the static relationship Θ( L). It was found that the limiting static angle Θ R,min, at which the wetting perimeter starts moving, depends on withdrawal velocity. Extrapolation of the Θ R,min/ U dependence to U = 0 yields the quasi-static value of this parameter Θ R , min 0 , that coincides with the relaxation static angle Θ R,rlx achieved after meniscus motion ceases. This conclusion holds also for the wetting mode, where the limiting static advancing angle Θ A , max 0 = Θ A,rlx. Both the limiting and relaxation angles could be used for calculation of the effective Young's contact angle on non-ideal surface following Adam's suggestion [N.K. Adam, Adv. Chem. Ser. 43 (1964) 53.]. The critical velocity U cr anfd apparent dynamic angle Θ app,cr, at which transition between steady dewetting and dynamic wetting occurs, were determined. The value of Θ app,cr = 0° ± 5° agrees with our previous results [R.V. Sedev, J.G. Petrov, Colloids and Surfaces, 53 (1991) 147] implying a quasi-static shape of the moving meniscus up to U cr. At U > U cr, the speed V of the contact line relative to the solid wall is independent of withdrawal velocity and thickness of the deposited film. The present data confirm the earlier findings [J.G. Petrov, R.V. Sedev, Colloids and Surfaces, 13 (1985) 317, T.D. Blake, K.J. Ruschak, Nature, 282 (1979) 489] that at U = U cr, V reaches its maximum value V max, which is most important parameter of dewetting kinetics. Weak linear decrease of Θ app with U was found above Ca = 2.4 × 10 −5 up to the critical capillary number Ca cr = 4.1 × 10 −4. Below Ca = 10 −5 the apparent receding angle depends much stronger on withdrawal velocity. The hydrodynamic (HD), and the simple and more general versions of the molecular-kinetic (MK) and molecular-hydrodynamic (MHD) theories of the wetting dynamics were used for quantitative characterization of the system in the steady dewetting regime. The effective Young's angle was used in the MK and MHD treatment of the experimental data following our previous publication [J.G. Petrov, J. Ralston, M. Schneemilch, R. Hayes, J. Phys. Chem B, 107(7) (2003) 1637]. The HD theory only qualitatively satisfies our experimental data giving physically unreasonable value of the hydrodynamic cut-off (slip) length and too small static receding angle at U = 0. The MK theory gives acceptable values of the oscillation frequency K 0 of the molecules at the contact line. Its more general version, including the viscous dissipation in the contact line vicinity, yields higher oscillation frequency. Very large distance λ between adsorption centers on the solid substrate (about five times the diameter of a glycerol molecule) was obtained with both MK and MHD theories. The too small frequency K 0 obtained with the simple MHD theory is removed by the more general version, accounting for contact line and viscous friction in the inner and intermediate zone of the moving meniscus. All theories show discrepancies between theoretically expected and experimentally estimated values of some of the parameters of wetting dynamics.

Persistent photocurrent and surface trapping in GaN Schottky ultraviolet detectors

GaN-based Schottky detectors were implemented and their photoresponse as a function of the incident power and time was measured. The measured photoresponse shows gain saturation and persistent photoconductivity behavior. These effects are shown here to be related to each other, arising from a nonideal semiconductor surface. A microscopic model of the gain mechanism to explain these observations is presented. Trap density at the semiconductor metal interface, characteristic lifetime, and carrier capture coefficient are extracted from our measurements.

Sb and Bi surfactant effects on homo-epitaxy of GaAs on ( [formula omitted]) patterned substrates

Anisotropic lateral growth during GaAs ( 0 0 1 ) epitaxy can have dramatic effects on the evolution of patterned features and surface morphology. Many new opto-electronic devices require growth on patterned or non-ideal surfaces. Controlling lateral growth will be essential for the production of these devices. In this study, GaAs epilayers were grown by organometallic vapor-phase epitaxy on patterned GaAs ( 0 0 1 ) wafers. During these growth experiments, trimethylantimony and trimethylbismuth were used as surfactant precursors to investigate the effects of Sb and Bi on GaAs lateral growth rates. Both surfactants were found to enhance the [1 1 0] lateral growth rate by nearly 300 %, while having a negligible effect on the lateral growth rate in the orthogonal direction. Kinetic simulations assisted in determining a plausible surfactant mechanism: The enhanced [1 1 0] lateral growth rate is due to an increase in the frequency of [1 1 0] diffusion events (decreased hop barrier).

Diffusion of nanoclusters

The diffusion of nanoclusters on various surfaces is investigated. It is shown that the misfit between the cluster and substrate is the main parameter determining the magnitude of the cluster diffusion on perfect surfaces. The motion of the cluster on non-ideal surfaces is also explored, and shows surprising results.

Diffusion of nanoclusters on non-ideal surfaces

We use molecular-dynamics simulations to investigate the diffusion of nanoclusters on non-ideal surfaces, containing bond-length disorder, vacancy islands or monoatomic steps. We find that bond-length disorder does not affect diffusion significantly. When vacancy islands are present, it is found that the diffusion path avoids the neighborhood of these defects, which can be explained in terms of an effective repulsion between the defect and the cluster. Indeed, when a step is present, we find a barrier which opposes diffusion, analogous to the well-known Ehrlich–Schwoebel barrier for adatom diffusion.

Contact angle measurements on two (wood and stone) non-ideal surfaces

Surface properties of wood and stones are very important in influencing the bonding and finishing of wood and for road construction. Two important surface properties are wettability and surface free energy. Contact angle measurement is a simple, useful and sensitive tool for quantifying the wettability and the surface energy of different materials in contact with pure water and/or aqueous surfactant solutions. Nevertheless, a sessile drop on a real surface shows different contact angle values due to its lack of symmetry or to the loss of volume by capillary action. For this reason alternative techniques which supply an average contact angle value are required. In view of this fact, we applied axisymmetric drop shape analysis-diameter (ADSA-D) using a top view of a sessile drop and axisymmetric drop shape analysis-profile (ADSA-P) with a side view of a captive bubble. Both techniques calculate the contact angle by solving the Young–Laplace equation although with different algorithms, the former needs the maximum drop diameter while the last makes use of the complete bubble profile. ADSA-D was applied to study two wood species (eucalyptus and pine) and ADSA-P with polished rocks of two different compositions (silicate and calcite). The maximum contour of the drop was fitted to an ellipse after being detected and extracted by means of a semi-automatic procedure of image processing. Thus, two different contact angles were found out. The captive bubble method in conjunction with the current ADSA-P technique allows to obtain comfortable, automatic and reproducible measurements of contact angle on porous stones.

Fast Adsorption on Nonideal Surfaces

The initial adsorption kinetics in stagnant flow of both rigid spherical poly(propylene imine) dendrimers as well as of flexible branched poly(ethylene imine) with similar building-blocks on glass are quantitatively described as Smoluchowski−Levich convective-diffusion-controlled up to concentrations cb ≈ 10 mg/L using a sticking probability β, defined as the ratio between the experimental adsorption rate and the theoretical prediction. This parameter β accounts for the nonideality of the substrate: There is only a limited amount of surface sites in the glass gel-layer available for adsorption, and this affects the collision efficiency. β is on the order of 0.03 for the small spheres and 0.8 for the flexible polyelectrolyte, perfectly in line with the expectation that the flexible polyelectrolyte, able to conform along the surface, has a much higher chance of finding a site within its contact area than the small dendrimers. At higher concentrations, the experimental data no longer show the predicted line...

Core level photoemission studies of the sulphur terminated Ge(100) surface

A core level photoemission spectroscopy study of the deposition of sulphur on the germanium (Ge(100)) surface has been performed. The sulphur is deposited from an electrochemical cell onto the clean Ge surface at room temperature in ultra-high-vacuum (UHV). The Ge surface dimers are broken and the sulphur terminated surface displays a (1×1) reconstruction. Analysis of the core level spectra reveal that all four Ge oxidation states are present indicating that a non-ideal surface termination has been formed. Annealing of the sulphur terminated surface to 200°C further promotes the formation of a S–Ge reacted phase which desorbs at higher annealing temperatures, re-establishing the clean surface spectrum. The results are consistent with recent studies indicating that the thermal desorption of the sulphur layer causes a surface etching process to occur.

In-Flight Calibration of the Near Earth Asteroid Rendezvous Laser Rangefinder

The Near Earth Asteroid Rendezvous (NEAR) mission began a year-long rendezvous with 433 Eros on 14 February 2000. The NEAR Laser Rangefinder (NLR) will measure ranges from the spacecraft to the surface of Eros with a single shot accuracy of a few meters. The NLR topographic data, when combined with doppler tracking of the spacecraft, will enable determinations of the asteroid's shape, mass, and density and will contribute to understanding of its internal structure and collisional evolution. NLR is the first space-borne laser altimeter with an in-flight calibration capability, achieved by means of an optical fiber which directs a small portion of every outgoing laser pulse back to the receiver with a known, fixed time delay. Key results of groundbased and in-flight calibrations of NLR are presented for: in-flight measurement of receiver noise statistics, confirmation of instrument stability over the 4-year cruise to Eros, absolute calibration of range measurements for ideal targets (flat, uniform surfaces normal to the boresight), and a prediction of single-pulse detection probability and range errors in the presence of pulse dilation from nonideal target surfaces, based on a Webb's approximation model of receiver performance. We find that pulse dilation is the major source of uncertainty in the single-shot range measurements from NLR, and that this uncertainty is consistent with the 6-m range measurement requirement for NEAR.

Effect of Surfactant Aggregation in the Adsorption Layer at the Liquid/Fluid Interface on the Shape of the Surface Pressure Isotherm

The thermodynamic model of Fainerman and Miller (Langmuir 1996, 12, 6011) for the description of ideal adsorption layers of surfactants forming small surface aggregates is generalized for the case of very large aggregates in the surface layer, for nonideal surface layers, and ionic surfactants. The models are used to explain results in the literature. It is shown that the transition region observed in SDS adsorption layers on concentrated NaCl substrates as discussed for example by Nikolov, Martynov, and Exerowa (J. Colloid Interface Sci. 1981, 81, 116) is due to the formation of surface aggregates with an aggregation number of the order of 100.

Errors in the refractive indices of solids measured using a 45°angle-of-incidence technique

By measuring the ratio of p- to s-polarized light intensitiesreflected at an angle of incidence of 45° from a transparent solid, therefractive index of that solid can be determined. The critical factorsaffecting the accuracy of the refractive index measurement using thistechnique are discussed. The extension of this technique to materials whichare weakly absorbing, or that have non-ideal surfaces, is also discussed andthe errors that arise from the neglect of these effects are quantified.

Electrical Impact of SiC Structural Crystal Defects on High Electric Field Devices

Commercial epilayers are known to contain a variety of crystallographic imperfections. including micropipes, closed core screw dislocations. low-angle boundaries, basal plane dislocations, heteropolytypic inclusions, and non-ideal surface features like step bunching and pits. This paper reviews the limited present understanding of the operational impact of various crystal defects on SiC electrical devices. Aside from micropipes and triangular inclusions whose densities have been shrinking towards manageably small values in recent years, many of these defects appear to have little adverse operational and/or yield impact on SiC-based sensors, high-frequency RF, and signal conditioning electronics. However high-power switching devices used in power management and distribution circuits have historically (in silicon experience) demanded the highest material quality for prolonged safe operation, and are thus more susceptible to operational reliability problems that arise from electrical property nonuniformities likely to occur at extended crystal defects. A particular emphasis is placed on the impact of closed-core screw dislocations on high-power switching devices, because these difficult to observe defects are present in densities of thousands per cm,in commercial SiC epilayers. and their reduction to acceptable levels seems the most problematic at the present time.

Influence of microstructure of substrate surface on early stage of thin film growth

The early stage of thin film growing is studied by a Monte Carlo simulation (MCS). We focus on the effect of the microstructure of the substrate surface (lattice structure and surface defect) at different substrate temperature. The results illustrate that the surface microstructure affects strongly the film morphology with increasing substrate temperature. With increasing temperature, the growth of island film goes through fractal, dendritic and compact growth processes. At high temperature, the island geometry shape resembles the substrate lattice structure. There is a temperature region in which the island film transforms from fractal-to-compact growth, 310–400 K for triangular lattice substrate and about 250–390 K for square lattice substrate. A surface factor is introduced to represent the effect of a non-ideal surface, changing this factor, different island morphologies can be obtained at the same temperature.

Single and Mixed Gas Adsorption Equilibria of Carbon Dioxide/Methane on Activated Carbon

Single gas adsorption isotherms of methane and carbon dioxide on micro-porous Norit RB1 activated carbon were determined in a gravimetric analyser in the temperature range of 292 to 349 K and pressures to 0.8 Mpa. Furthermore binary isotherms of carbon dioxide and methane mixtures were determined at 292 K and pressures up to 0.65 MPa. Adsorbed phase compositions were determined from the gravimetric data by the rigorous thermodynamic method of Van Ness. These experimental binary equilibrium data were compared with equilibrium data calculated by the Ideal Adsorbed Solution (IAS) model. Only moderate agreement could be obtained. Finally, activity coefficients, accounting for the non-ideality of the adsorbate mixture, were calculated from the experimental data. The Wilson equation, derived for bulk solutions, was fitted on these activity data and the Wilson interaction parameters were determined. The Wilson equation proved to correlate the experimental data reasonably. However, the Wilson interaction parameters are not only completely different from those found for bulk solutions, but also the physical interpretation of these parameter values is completely lacking. It is concluded that new solution models should be developed encompassing both non-ideal solution behaviour and surface heterogeneity.

Quantification problems in light element determination by grazing emission X-ray fluorescence

Grazing emission X-ray fluorescence analysis (GEXRF) is a new approach of X-ray fluorescence analysis to determine light elements by using very small take-off angles for the fluorescence radiation. The technique is in high demand in quantification procedures, since the two known theoretical ways of deduction do not cover all possible cases met in the analyses. They do not match the situations, e.g., where the samples show non-ideal flat surfaces, but have irregular, non-periodic structures. Additionally, a number of fundamental data are often not known accurately enough, or are even unknown. We have tested two empirical methods of quantification, which are already well established in classical X-ray fluorescence analysis: the calibration and the standard additions method. In particular the influence of matrix elements on the determination of the main component has been tested. Finally, for samples with large and flat surfaces (e.g., wafers), the use of the scattered lines for quantification is proposed. Topographical restructurization of the sample surface during a spin coating process seems to be confirmed by the shifts in the critical angle.