Scaling behavior of surface irregularity in the molecular domain: From adsorption studies to fractal catalysts

For an unexpected variety of solids, the surface topography from a few up to as many as a thousand angstroms is very well described by fractal dimension,D. This follows from measurements of the number of molecules in surface monolayers, as function of adsorbate or adsorbent particle size. As an illustration, we present a first case, amorphous silica gel, whereD has been measured independently by each of the two methods. (The agreement, 3.02±0.06 and 3.04±0.05, is excellent, and the result is modeled by a heavy generalized Menger sponge.) The examples as a whole divide into amorphous and crystalline materials, but presumably all of them are to be modeled as random fractal surfaces. The observedD values exhaust the whole range between 2 and 3, suggesting that there are a number of different mechanisms by which such statistically self-similar surfaces form. We show that fractal surface dimension entails interfacial power laws much beyond what is the source of theseD values. Examples are reactive scattering events when neutrons of variable flux pass the surface (this is of interest for locating fractal substrates that may support adlayer phase transitions); the rate of diffusion-controlled chemical reactions at fractal surfaces; and the fractal implementation of the traditional idea that the active sites of a catalyst are edge and apex sites on the surface.     

PbS红外探测器低频噪声物理模型及缺陷表征研究

为了表征PbS薄膜光导红外探测器的材料缺陷,详细推导了1/f和产生-复合(g-r)噪声物理模型,并由实验数据验证了模型的准确性. 利用1/f噪声与表面缺陷关系,计算了不同偏压下表面陷阱密度. 得到该值随偏压升高而增加,由此得出1/f噪声与所加偏压成正比变化,与实验测试结果相一致. 在此模型基础上,研究了g-r噪声与深能级缺陷特征参量的关系,提出由低频噪声表征缺陷激活能、简并因子、俘获截面等缺陷参数的方法. 关键词： 红外探测器 1/f噪声')" href="#">1/f噪声 噪声')" href="#">g-r噪声 缺陷     

Electronic structure and topography of annealed SrTiO3(1 1 1) surfaces studied with MIES and STM

Perovskites of ABO₃ type like strontium titanate (SrTiO₃) are of great practical concern as materials for oxygen sensors operating at high temperatures. It is well known that the surface layer shows different properties compared to the bulk. Numerous studies exist for the SrTiO₃(1 0 0) and (1 1 0) surfaces which have investigated the changes in the electronic structure and topography as a function of the preparation conditions. They have indicated a rather complex behaviour of the surface and the near surface region of SrTiO₃ at elevated temperatures. Up to now, the behaviour of the SrTiO₃(1 1 1) surfaces under thermal treatment is not sufficiently known. This contribution is intended to work out the relation between alteration of the surface topography with respect to the preparation conditions and the simultaneous changes of the electronic structure. We applied scanning tunneling microscopy (STM) to investigate the surface topography and, additionally, metastable impact electron spectroscopy (MIES) to study the surface electronic structure of reconstructed SrTiO₃(1 1 1) surfaces. The crystals were heated up to 1000 °C under reducing and oxidizing conditions. Both preparation conditions cause strong changes of the surface topography and electronic structure. A microfaceting of the topmost layers is found.     

The influence of cone-covered surface structures on sputtering and secondary electron emission: Model

Abstract

The yield and the polar angular distributions of secondary particles emitted from cone-covered surfaces were studied. A model taking into account both the influence of the local incidence angle I and the blocking of the emitted particles by the surface relief was elaborated. For an emission angle a, the transport effect of the particles generated inside the solid was simulated by the standard relation F(a) = (cos a) ⁿ and the emission dependence on the local incidence angle was assumed to behave as E(I) = (cos I) ^-m . Calculations showed that for m= 1 the surface topography only slightly alters the polar angular distribution shape but the yield of the collected particles is always higher than that from a flat surface when m is more than unity. For n= 2, the shape of the distribution curve is significantly different from that of the corresponding flat surface whatever the m values. For n=0, when E(I) very rapidly increases with the incidence angle both the angular distributions shape and the yield are strongly modified by the surface topography.     

In situ characterization of MBE grown GaAs and Al x Ga1−x As films using RHEED,SIMS, and AES techniques

A combination of the surface diagnostic techniques Auger electron spectroscopy (AES), reflection high energy electron diffraction (RHEED), and secondary ion mass spectroscopy (SIMS) was used in order to get more detailed information on basic processes which lead to the formation of high quality monocrystalline GaAs and Al _x Ga_1−x As films by molecular beam epitaxy (MBE) under ultra-high vacuum conditions. The formation and changes of reconstructed surface structures on (100) GaAs as a function of growth parameters were observedduring growth by RHEED. AES was used to determine the relative ratio of Ga/As on the surface for different reconstructed structures, to investigate the impurity contamination on substrate surfaces and grown films, and to study the surface segregation of Sn in MBE GaAs during doping. Finally, intentional and unintentional impurities incorporated during the growth of GaAs and Al _x Ga_1−x As by MBE were detected by the SIMS technique immediately after growth within the reaction chamber.     

Dependence on substrate topography of growth of nanosized dendritic structures in an electron-beam-induced deposition process

Nanometer-sized W-dendrites are fabricated on Al₂O₃ substrates with an electron-beam-induced deposition process. Dependence of growth of nanodendrite on surface topography is investigated with transmission electron microscopy. It is confirmed that the nanodendrite grows on convex surfaces but not around a hole on a substrate. These are attributed to different distribution of charges on surfaces with different topographies during electron beam irradiation when charges are produced on the surface due to emission of second electrons. The charges accumulate on convex surface and do not distribute around a hole. Therefore, the nanodendrite grows on the former and not on the latter.     

粗糙表面形貌对微尺度流动换热的影响

基于分形几何定量描述了多尺度自仿射的粗糙表面形貌,建立了微通道内层流流动换热的理论模型并对表面形貌的影响进行了数值模拟.研究表明,自仿射分形维数直接反映了表面轮廓的不规则度,对于两个具有相同统计粗糙度的轮廓,可能存在不同的分形维数;与常规尺度通道不同,雷诺数、粗糙高度和粗糙表面分形维数都对微通道内层流流动换热有着重要影...     

Novel insights into controlled drug release from coated pellets by confocal Raman microscopy

The development of novel therapeutics with improved efficacy implies increasing complexity of drug delivery systems, which in turn require advanced methods for their analytical characterization. Among these systems, pellets represent upcoming carrier systems, which show several advantages like simplified dosing and improved compliance among children and the aged population. However, rational development of such systems is hampered by the lack of non‐destructive, chemically selective analytical insight into compound distribution and drug release mechanisms. The aim of this study was to evaluate confocal Raman microscopy (CRM) for investigation of coated drug‐loaded pellets based on visualization of compound distribution and elucidation of drug release mechanisms. Three complementary approaches were applied for pellet characterization: analysis of cross sections after bisectioning, non‐invasive visualization of the pellet surface, and virtual cross sectioning in x–z direction. As the surface of such pellets is structured, a complementary approach of optical topography and CRM was applied for three‐dimensional analysis. Based on the individual Raman peak patterns, the drug and excipients forming the matrix of the pellets and the film coating were successfully visualized with high spatial resolution, verifying homogeneous drug distribution and intact polymer coating of the pellet. Further, analysis of the pellets after certain time intervals during drug release testing revealed pore formation in the polymer coating facilitating drug release and preceding drug depletion in the pellets matrix. CRM represents an upcoming technique for analytical characterization of carrier systems and elucidation of their complex drug release mechanisms, thus supporting rational development of novel therapeutics. Copyright © 2016 John Wiley & Sons, Ltd.     

Perturbation theory results for the diffuse scattering of light from two-dimensional randomly rough metal surfaces

Abstract

Using the unitarity and reciprocity preserving formulation of Brown et al a perturbation treatment, correct to fourth order in the surface profile function, is given for the scattering of electromagnetic waves from a weakly rough, two-dimensional, random metal surface. In this formulation the boundary conditions on the electromagnetic fields are satisfied using the extinction theorem in conjunction with the Rayleigh hypothesis and the vector equivalent of the Kirchhoff integral. The theory is applied to, and results are presented for, several different types of rough surfaces which are characterized by power spectra that are extensions to two-dimensional random surfaces of the power spectrum of some one-dimensional random surfaces recently fabricated by West and O'Donnell. These surfaces, which can be realized experimentally, favor coherent, interferent, multiple scattering of electromagnetic waves via surface plasmon polaritons in intermediate states, and clearly exhibit enhanced backscattering caused by the surface plasmon polariton mechanism. Theoretical results are presented for silver surfaces at optical wavelengths.     

A Monte Carlo method for simulating fractal surfaces

A Monte Carlo method is presented for simulating rough surfaces with the fractal behavior. The simulation is based on power-law size distribution of asperity diameter and self-affine property of roughness on surfaces. A probability model based on random number for asperity sizes is developed to generate the surfaces. By iteration, this method can be used to simulate surfaces that exhibit the aforementioned properties. The results indicate that the variation of the surface topography is related to the effects of scaling constant G and the fractal dimension D of the profile of rough surface. The larger value of D or smaller value of G signifies the smoother surface topography. This method may have the potential in prediction of the transport properties (such as friction, wear, lubrication, permeability and thermal or electrical conductivity, etc.) on rough surfaces.     

Dentinal tubules driven wetting of dentin: Cassie-Baxter modelling

We investigate the wetting properties of dentin surfaces submitted to a phosphoric acid etching followed by an air drying procedure, as in clinical situations of adhesive dentistry. The surface topography of the etched surfaces was characterized by AFM, and the wetting properties of water on these rough and heterogeneous surfaces were studied, by contact angle measurements. We showed that the contact angle increases with the acid exposure time and consequently with both surface roughness and the organic-mineral ratio of the dentin components. From the whole results, obtained on dentin and also on synthesized hydroxyapatites samples, we inferred a water contact angle of ∼ 133^° on the dentinal tubule. These experimental results may be described by the Cassie-Baxter approach, and it is suggested that small air pockets could be formed inside the dentinal tubules.     

Synchrotron scanning photoemission microscopy of homogeneous and heterogeneous metal sulfide minerals

Scanning photoemission microscopy (SPEM) has been applied to the investigation of homogeneous and heterogeneous metal sulfide mineral surfaces. Three mineral samples were investigated: homogeneous chalcopyrite, heterogeneous chalcopyrite with bornite, and heterogeneous chalcopyrite with pyrite. Sulfur, copper and iron SPEM images, i.e. surface‐selective elemental maps with high spatial resolution acquired using the signal from the S 2p and Cu and Fe 3p photoemission peaks, were obtained for the surfaces after exposure to different oxidation conditions (either exposed to air or oxidized in pH 9 solution), in addition to high‐resolution photoemission spectra from individual pixel areas of the images. Investigation of the homogeneous chalcopyrite sample allowed for the identification of step edges using the topography SPEM image, and high‐resolution S 2p spectra acquired from the different parts of the sample image revealed a similar rate of surface oxidation from solution exposure for both step edge and a nearby terrace site. SPEM was able to successfully distinguish between chalcopyrite and bornite on the heterogeneous sample containing both minerals, based upon sulfur imaging. The high‐resolution S 2p spectra acquired from the two regions highlighted the faster air oxidation of the bornite relative to the chalcopyrite. Differentiation between chalcopyrite and pyrite based upon contrast in SPEM images was not successful, owing to either the poor photoionization cross section of the Cu and Fe 3p electrons or issues with rough fracture of the composite surface. In spite of this, high‐resolution S 2p spectra from each mineral phase were successfully obtained using a step‐scan approach.     

Stochastic analysis of different rough surfaces

This paper shows in detail the application of a new stochastic approach for the characterization of surface height profiles, which is based on the theory of Markov processes. With this analysis we achieve a characterization of the scale dependent complexity of surface roughness by means of a Fokker-Planck or Langevin equation, providing the complete stochastic information of multiscale joint probabilities. The method is applied to several surfaces with different properties, for the purpose of showing the utility of this method in more detail. In particular we show evidence of the Markov properties, and we estimate the parameters of the Fokker-Planck equation by pure, parameter-free data analysis. The resulting Fokker-Planck equations are verified by numerical reconstruction of the conditional probability density functions. The results are compared with those from the analysis of multi-affine and extended multi-affine scaling properties which is often used for surface topographies. The different surface structures analysed here show in detail the advantages and disadvantages of these methods.Received: 5 April 2004, Published online: 12 October 2004PACS: 02.50.-r Probability theory, stochastic processes, and statistics - 02.50.Ga Markov processes - 68.35.Bs Surface structure and topography of clean surfaces     

Surface engineering with ion beams: from self-organized nanostructures to ultra-smooth surfaces

Low-energy ion-beam sputtering, i.e. the removal of atoms from a surface due to the impact of energetic ions or atoms, is an inherent part of numerous surface processing techniques. Besides the actual removal of material, this surface erosion process often results in a pronounced alteration of the surface topography. Under certain conditions, sputtering results in the formation of well-ordered patterns. This self-organized pattern formation is related to a surface instability between curvature-dependent sputtering that roughens the surface and smoothing by different surface relaxation mechanisms. If the evolution of surface topography is dominated by relaxation mechanisms, surface smoothing can occur. In this presentation the current status of self-organized pattern formation and surface smoothing by low-energy ion-beam erosion of Si and Ge is summarized. In detail it will be shown that a multitude of patterns as well as ultra-smooth surfaces can develop, particularly on Si surfaces. Additionally, the most important experimental parameters that control these processes are discussed. Finally, examples are given for the application of low-energy ion beams as a novel approach for passive optical device engineering for many advanced optical applications. PACS 81.16.Dn; 81.16.Rf; 81.65.Cf; 81.65.Ps; 68.35.Ct     

Low-coherence interferometry based roughness measurement on turbine blade surfaces using wavelet analysis

In this paper, a non-contact optical system, a low-coherence interferometer (LCI), is introduced for the purpose of measuring the surface roughness of turbine blades. The designed system not only possesses a high vertical resolution and is able to acquire the roughness topography, but also it has a large vertical scanning range compared to other commonly used optical systems. The latter characteristic allows us to measure turbine blades surfaces with large curvature without collisions between the lens and the measurement object. After obtaining the surface topography, wavelet analysis is applied to decompose the original surface into multiple bandwidths to conduct a multiscale analysis. The results show that the developed LCI system proofs a good performance not only in obtaining the surface topography in the roughness scale but also in being able to measure surfaces of objects that possess a complex geometry in a large vertical range. Furthermore, the applied biorthogonal wavelet in this study has performed good amplitude and phase properties in extracting the roughness microstructures from the whole surface. Finally, the traditional roughness parameters, such as the mean surface roughness S_a and the Root Mean Square (RMS) roughness S_q, are evaluated in each decomposed subband and their correlations with the scale of each subband are analyzed.     

Ab-initio density functional theory study of a WO3 NH3-sensing mechanism

WO 3 bulk and various surfaces are studied by an ab-initio density functional theory technique.The band structures and electronic density states of WO 3 bulk are investigated.The surface energies of different WO 3 surfaces are compared and then the (002) surface with minimum energy is computed for its NH 3 sensing mechanism which explains the results in the experiments.Three adsorption sites are considered.According to the comparisons of the energy and the charge change between before and after adsorption in the optimal adsorption site O 1c,the NH 3 sensing mechanism is obtained.     

Study of the effect of initial conditions on the scaling properties of CaF2 thin films

The growth of thermally deposited CaF₂ films was studied using three different substrates for deposition: glass, gold and silicon. Each substrate was chosen because of its different topography and used to determine the effect of substrate roughness on the growth of CaF₂ films. After thermally depositing a range of CaF₂ film thicknesses on the substrates, the CaF₂ surfaces were imaged using atomic force microscopy. The images were then used to determine the characteristic exponents which described the surface. In each case the Hurst exponent, H was found to rapidly increase from the initial substrate condition to a constant value (H ≈ 0.85) with increasing CaF₂ film thickness. This rapid crossover is quite remarkable and occurs in films with nominal thicknesses less than ≈20 nm. These data indicate that the roughness of the substrate, or in other words the initial conditions, have little effect on the growth properties of CaF₂ films beyond the crossover at very small values of the film thickness. The scaling of the dynamic exponent, β, is also presented as are measurements of the CaF₂ film porosity.     

About possible mechanisms of influence of gas bubbles on characteristics of turbulent boundary layer

Two different mechanisms responsible for the were revealed impact of gas bubbl injected into a boundary layer on the shear stress on the wetted surfaces. Both mechanisms exist due to extremely high sensitivity of bubbles even to very low pressure gradients and due to a high value of the virtual mass and coefficient of viscous drag for bubbles. The first mechanism manifests itself at the interaction of vortex structures with bubbles in the near-wall layer y ⁺ < 250. The second mechanism is due to pressure gradient along the wetted surface. Ascertainment of these mechanisms explains the known discrepancies in the experimental results on gas saturation obtained on different experimental setups.     

Modeling of Surface Diffusion in hcp Zr and Ti

The statics and dynamics of vacancies and adatoms on different surface orientations in two hcp materials are studied by using static relaxation techniques and many-body potentials. Formation and migration energies and entropies as well as attempt frequencies are evaluated and used in the random walk approach to obtain correlation factors and diffusivities. It is found that the main features of surface diffusion are dominated by jumps on and between a few atomic layers, so that a consistent comparison between the two mechanisms is feasible. The activation energies and the diffusivities for different environments, namely, bulk Q _b, D _b, symmetric grain boundaries Q _gb, D _gb, and surfaces, Q _s, D _s, calculated using the same simulation technique and interatomic potentials, fulfil the expected relationships Q _s < Q _gb < Q _b and D _s > D _gb > D _b. It is also found that generally adatoms are faster surface diffusers than vacancies.