Evaluation of nanoscale roughness measurements on a plasma treated SU-8 polymer surface by atomic force microscopy

A comparison between roughness data obtained with an atomic force microscope (AFM) on different surfaces requires reliable roughness parameters. In order to specify the appropriate parameters for nanoscale roughness measurements, we compared the root mean square (rms) roughness and the relative surface area (sdr) as function of varying scan size, speed and pixel size. By using oxygen plasma (24 kJ) treated SU-8 with an average rms roughness of 2.6 ± 0.5 nm as reference surface, the repeatability of the method was evaluated for dynamic (tapping) and contact mode. The evaluation of AFM images indicated a decrease of the effective tip radius after a few measurements. This degradation of the tip lowers the resolution of the image and can affect roughness measurements.     

Nonlinearities in sedimentation: a microscopic study

Effect of temperature and surface tension on surface during sedimentation is studied using atomic force microscopy. Effects of surface tension is incorporated by using ∼1 mm radius water droplets. Surface tension of water droplet directed the deposition of dissolved particles around the perimeter of the droplet; leaving almost hollow circle in the middle with light deposition due to the particles being close to the surface. Evaporating shallow water solutions left random salt structures on the surface. Growth exponents are calculated along with the fractal dimension. For sedimentation process in shallow water (depth of ∼2 mm), a transition is observed from continuum model (1.88±0.2) to KPZ (0.406±0.082) universality class around ∼1 μm at room temperature. For droplets evaporation, turbulent (multi-affine) to self-affine transition is observed. Fractal dimensions (FD) for the droplets are found to be between one and two. The FD values are consistent with the fact that competing nonlinear terms are present in the system.     

Effect of sputtering power on surface topography of dc magnetron sputtered Ti thin films observed by AFM

Titanium films were deposited on glass substrates at room temperature by direct current (dc) magnetron sputtering at fixed Ar pressure of 1.7 Pa and sputtering time of 4 min with different sputtering power ranging from 100 to 300 W. Atomic force microscopy (AFM) was used to study topographic characteristics of the films, including crystalline feature, grain size, clustering and roughening. The amorphous-like microstructure feature has been observed at 100-150 W and the transition of crystal microstructure from amorphous-like to crystalline state occurs at 200 W. The increase in grain size of Ti films with the sputtering power (from 200 to 300 W) has been confirmed by AFM characterization. In addition, higher sputtering power (300 W) leads to the transformation of crystal texture from globular-like to hexagonal type. The study has shown that higher sputtering power results in the non-linear increase in deposition rate of Ti films. Good correlativity between the surface roughness parameters including root mean square (RMS) roughness, surface mean height (Ra) and maximum peak to valley height (P-V) for evaluating the lateral feature of the films has been manifested. Surface roughness has an increasing trend at 100-250 W, and then drops up to 300 W.     

Thermally responsive wettability of self-assembled methylcellulose nanolayers

Thermo-responsive cellulosic nanolayers were prepared from methylcellulose (MC), which is known to have a unique lower critical solution temperature. Thiosemicarbazide (TSC) was selectively introduced into the MC reducing end groups, and the corresponding MC-TSC derivative was spontaneously chemisorbed on an Au substrate at 4 °C to give MC self-assembled monolayers (SAMs). Linear MC chains were stably fixed onto the Au substrate, yielding an MC-SAM of thickness ca. 15 nm with a root mean square value less than 1 nm. The MC-SAM surface exhibited thermally responsive wetting characteristics; the water contact angle was found to rise and fall around 70 °C, possibly due to the solid-state phase transition of the MC nanolayers resulting from the inherent gelation of MC molecules in water. Such wetting behavior was shown to be reversible following repeated heating and cooling. The MC-SAM immersed in salt solution revealed lower phase transition temperatures, and an increase in sodium chloride concentration ranging from 0.0 to 1.0 M brought about a dramatic decrease in the apparent phase transition temperature from ca. 70 to 30 °C. For the purposely designed MC nanolayers, such controllable wetting properties are expected to prompt growing interest in the applications of cellulosic biopolymer interfaces.     

微纳复合结构表面稳定润湿状态及转型过程的热力学分析

自然界中的微纳复合结构超疏水表面由于其独特的润湿性质引起了人们的广泛关注, 大量实验研究表明了仿生人工微纳复合结构表面润湿性能的优越性, 然而液滴在微纳复合结构表面的润湿状态和转型过程的理论研究还并不完善. 本文首先用热力学方法分析了液滴在微纳复合结构表面可能存在的所有状态(四种稳定润湿状态和五种亚稳态到稳定态转型中的过渡态), 推导出了相应的能量表达式及表观接触角方程; 基于最小能量原理, 确定液滴在微纳复合结构表面的稳定状态, 较以往模型相比, 能够更好的预测已有的实验结果; 其次研究了微纳结构尺寸对稳定润湿状态和亚稳态到稳定态转型过程的影响; 最后提出了微纳复合结构表面设计原则, 即确定“超疏水稳定区”尺寸范围, 为超疏水表面的制备提供理论依据.     

Preparation and characterization of indium tin oxide films formed by oxygen ion beam assisted deposition

Indium tin oxide (ITO) films were produced by low-energy oxygen ion beam assisted electron-beam evaporation. The dependence of surface morphology, electrical and optical properties on evaporation rate, oxygen ion beam energy and density, as well as substrate temperatures was characterized by atomic force microscopy, X-ray photoelectron spectroscopy, Hall-effect and optical transmittance measurements. The results show that high-quality ITO films (resistivity of 7.0×10⁻⁴ Ω cm, optical transmittance above 85% at wavelength 550 nm, surface roughness of 0.6 nm in root mean square) can be obtained at room temperature.     

Two-Dimensional Wetting Transition Modeling with the Potts Model

A droplet of a liquid deposited on a surface structured in pillars may have two states of wetting: (1) Cassie-Baxter (CB), the liquid remains on top of the pillars, also known as heterogeneous wetting, or (2) Wenzel, the liquid fills completely the cavities of the surface, also known as homogeneous wetting. Studies show that between these two states, there is an energy barrier that, when overcome, results in the transition of states. The transition can be achieved by changes in geometry parameters of the surface, by vibrations of the surface or by evaporation of the liquid. In this paper, we present a comparison of two-dimensional simulations of the Cassie-Wenzel transition on pillar-structured surfaces using the cellular Potts model (CPM) with studies performed by Shahraz et al. In our work, we determine a transition diagram by varying the surface parameters such as the interpillar distance (G) and the pillar height (H). Our results were compared to those obtained by Shahraz et al. obtaining good agreement.     

Nano-structures in YSZ(1 0 0) surfaces: Implications for metal deposition experiments

The surface morphology of yttria stabilized zirconia (YSZ)(1 0 0) single crystals are examined by AFM and XPS before and after thermal annealing in air to 1000 °C. The surfaces show a large variability in topography which can be categorized in three types: (1) surfaces with well defined terraces, (2) surfaces with etch pits and no visible terraces, (3) surfaces with large square or rectangular holes with flat bottoms. All three types of surfaces show a large number of defects (pits, adatoms, steps) originating from the manufacturing process, and certain samples show large nano-structured arrays of self-organized lines at step edges. The evolution of the surfaces with time at 1000 °C and with higher temperatures was studied. Terraces are ultimately obtained for all sample types at 1300 °C, but the terrace shape is affected by the original defect structure. This history dependence is attributed to defect interactions modifying the annealing process. This is true even for UHV samples prepared using sputter-anneal cycles. The surface type is found to affect the nucleation, growth and sintering behaviour of palladium deposited by electron beam evaporation. For type 3 samples the metal nucleates at step edges outside the holes to particles 6 Å in height, following heating to 135 °C the particles move inside the holes forming agglomerates up to 20 Å.     

亲水性微观粗糙表面润湿状态转变性能研究

以亲水性微观粗糙表面上不同几何形貌及分布的微柱阵列为对象, 讨论了液滴在亲水性粗糙表面上的润湿过程以及润湿状态的转变阶段. 从能量角度分别考察了微观粗糙结构几何形貌及分布、微柱几何参数、固体表面亲水性、接触角滞后作用等因素对液滴润湿状态转变的影响规律. 研究发现: 在亲水粗糙表面, 正方形微柱呈正六边形阵列分布时, 液滴更容易形成稳定的Cassie状态, 或者液滴仅发生Cassie状态向中间浸润状态的转变; 与此同时, 减小微柱间距、增大方柱宽度或圆柱直径、增大微柱高度、增强固体表面的亲水性将有利于液滴处于稳定的Cassie状态, 或阻止润湿状态向伪-Wenzel或Wenzel状态转变; 然而, 当液滴处于Cassie状态时, 较小的固-液界面面积分数或减弱固体表面亲水性能均有利于增大液滴的表观接触角, 因此在亲水表面设计粗糙结构时应综合考虑润湿状态稳定性和较大表观接触角两方面因素; 此外, 接触角滞后作用对于液滴状态的稳定性以及疏水性能的实现具有相反作用的影响. 研究结果为液滴在亲水表面获得稳定Cassie状态的粗糙结构设计方法提供了理论依据. 关键词： 亲水表面 微观粗糙结构 表面自由能 润湿状态转变     

Wetting and evaporation behaviors of molten Mg on partially oxidized SiC substrates

The wetting and evaporation behaviors of molten Mg drops on pressureless-sintered SiC surfaces were studied in a flowing Ar atmosphere at 973-1173 K by an improved sessile drop method. The initial contact angles are between 83° and 76°, only mildly depending on temperature. The formation of a ridge at the triple junction as a result of reaction between molten Mg and the SiO₂ film on the SiC surface pins the triple line and leads to a constant contact diameter mode during the entire evaporation process. Moreover, the diffusion coefficients of the Mg vapor at different temperatures were evaluated based on a simple model.     

Direct experimental confirmation of the critical behaviour of antiphase domain boundaries at the second order phase transition of Al-Fe binary alloys

The present work reports, for the first time, a direct experimental observation of the critical phenomenon associated with the B2-A2 order-disorder transition of Al-Fe binary alloys. Transmission electron microscopy and energy dispersion spectroscopy are employed to span the morphological changes through the transition line from the ordered B2 phase to the disordered A2 phase. Dark field images of the microstructure around the transition line for samples aged at 973 and 1073 K for various times show an interface roughening for the {100} antiphase domain boundaries in body-centered cubic binary alloys having the B2 structure. This observation confirms theory about the instability of the second-order transition in such alloys. This behaviour occurs for compositions with Al-content slightly higher (by ∼4 at.% Al) than that of the critical point of the equilibrium order-disorder transition. In addition, roughness-induced wetting transition is also observed for alloys having compositions ranging from 1.3 to 1.5 at.% Al above the transition line. The interface roughening transition is thought to be unstable second-order while the wetting transition is suggested to be a stable first-order one.     

液滴在梯度微结构表面上的铺展动力学分析

本文通过改变肋柱宽度和间距, 构造了二级和多级梯度微结构表面, 采用格子-Boltzmann方法对液滴在两种梯度表面上的铺展过程进行了研究, 探析液滴运动的机理和调控方法. 结果表明, 在改变肋柱间距的二级梯度表面上, 当液滴处于Cassie态时, 接触角滞后大小与粗糙度梯度成正比关系; 当液滴从Cassie态转换为Wenzel态或介于两者之间的不稳定态时, 这一正比关系不再遵循. 在改变肋柱宽度的二级梯度表面上, 接触角滞后大小与粗糙度梯度始终成正比关系. 在多级梯度表面上, 随液滴初始半径增大, 接触角滞后减小, 但液滴平衡位置相较于初始位置偏离增大. 对梯度微结构表面上液滴运动和接触角滞后的定量分析, 可为实现梯度微结构表面液滴运动调控提供理论依据.     

Scattering from unidirectional ground steel surfaces in the specular direction

Scattering in the specular direction from unidirectional ground steel surfaces having random roughness is studied theoretically and experimentally. The grooves were oriented perpendicular and parallel to the plane of incidence and were illuminated by a light beam that was smaller than the sample size. Expressions for scattering from a one-dimensional rough surface in the specular direction were derived for the both orientations of the grooves. For the same groove orientations, scattering (λ = 0.633 μm) from the ground surfaces was measured in the specular direction at angles of incidence from 6° to 82°. The measured rms roughnesses of the surfaces were 0.096 μm, 0.143 μm, 0.311 μm, and 0.501 μm, respectively. The measured scattering was independent of the orientation of the grooves for the two smoother surfaces and depended on the groove orientation for the other surfaces. Calculations using the derived expressions taking into account the experimental results show that the scattering is independent of the orientation of the grooves if the rms roughness is no larger than ∼0.16 μm.     

Surface morphology of metal films deposited on mica at various temperatures observed by atomic force microscopy

Thin films of eight metals with a thickness of 150 nm were deposited on mica substrates by thermal evaporation at various temperatures in a high vacuum. The surface morphology of the metal films was observed by atomic force microscopy (AFM) and the images were characterized quantitatively by a roughness analysis and a bearing analysis (surface height analysis). The films of Au, Ag, Cu, and Al with the high melting points were prepared at homologous temperatures T/T_m = 0.22-0.32, 0.40, and 0.56. The films of In, Sn, Bi, and Pb with the low melting points were prepared at T/T_m = 0.55-0.70, where T and T_m are the absolute temperatures of the mica substrate and the melting point of the metal, respectively. The surface morphology of these metal films was studied based on a structure zone model. The film surfaces of Au, Ag, and Cu prepared at the low temperatures (T/T_m = 0.22-0.24) consist of small round grains with diameters of 30-60 nm and heights of 2-7 nm. The surface heights of these metal films distribute randomly around the surface height at 0 nm and the morphology is caused by self-shadowing during the deposition. The grain size becomes large due to surface diffusion of adatoms and the film surfaces have individual characteristic morphology and roughnesses as T increases. The surface of the Al film becomes very smooth as T increases and the atomically smooth surface is obtained at T/T_m = 0.56-0.67 (250-350 °C). On the other hand, the atomically smooth surface of the Au film is obtained at T/T_m = 0.56 (473 ± 3 °C). The films of In, Sn, Bi, and Pb prepared at T/T_m = 0.55-0.70 also show the individual characteristic surface morphology.     

Dynamic modeling of manipulation of micro/nanoparticles on rough surfaces

In this paper, the dynamic behavior of spherical micro/nanoparticles, while being pushed on rough substrates, is studied by means of an Atomic Force Microscope (AFM). For this purpose, first, the contact adhesion force, and the areas and penetration depths of rough surfaces are derived based on the Johnson-Kendall-Roberts (JKR) theory, the Schwarz method, and the Rumpf/Rabinovich models. Then, the dynamic model of particle manipulation on rough substrates is revised using the specified contact theory for rough surfaces. And finally, the pushing of spherical particles with 50, 100, 200, 500, and 10000 nm radii is simulated. The results show that the critical force and the critical time of manipulation decrease when the particles are pushed on the rough surfaces as compared to the smooth ones. It is also observed that the critical force for a rough substrate containing asperities of low height and large radius approaches a comparable critical force magnitude to the smooth substrate, as is expected. Also, when the asperity radius in the substrate is within the range of 0.5 < r < 5 nm, the critical force of pushing decreases; however, as the asperity radius becomes larger than 5 nm, the critical force begins to increase again. Furthermore, the critical values are generally more sensitive to the changes of the asperity radius than the height. It is also found that the difference between the critical values based on the Rumpf and Rabinovich models is negligible. However, the estimation of particles’ dynamic behavior using the Rumpf model could be wrong for the rough substrates with small radius asperities, which is considerable in the manipulation and assembly practices. Moreover, the dynamic behavior of particles of small radius (r < 500 nm) change during the pushing process on rough surfaces, and the rolling behavior could be possible on the surfaces that have small radius asperities. The probability of this occurrence is increased in the pushing of larger particles on rougher substrates.     

Investigation on femtosecond laser irradiation energy in inducing hydrophobic polymer surfaces

This study investigates the use of ultrashort femtosecond laser pulses to induce hydrophobic properties on PMMA surfaces. The modification of surface wetting property exhibits a strong dependence on the amount of energy deposited on the PMMA surface. A simple equation has been deduced from the laser parameters to express the energy deposition. It was revealed that water contact angle (WCA) of more than 120°, with a maximum of around 125°, could be achieved when the total energy deposited per unit area on the PMMA surface ranged from 600 J/cm² to 900 J/cm² at an energy deposition rate of around 50 J/cm²/s. Beyond this range, WCA reduced with increasing amount of energy deposition. Furthermore, with higher energy deposition rate or higher laser fluence, total energy required to induce hydrophobic surfaces was reduced. Under different energy deposition, the quantity of polar groups or non-polar groups induced was responsible for the changes in WCA and thus the different surface hydrophobicity.     

Energetics of the growth mode transition in InAs/GaAs(0 0 1) small quantum dot formation: A first-principles study

Based on first-principles density-functional pseudopotential calculations, the growth of InAs on the GaAs(0 0 1) surface has been studied. By analyzing the free energies of the surfaces with different thicknesses of the InAs coverages, the critical thickness of the layer-by-layer (2D) to island (3D) growth mode transition is predicted to be around 1.5 ML. Comparing the total energy differences between layer-by-layer growth models and 3D island models, the mechanism of the 2D-3D growth mode transition near the critical thickness (θ_crit) is studied which indicates that at the initial stage of InAs quantum dots formation, small 3D islands are formed randomly.     

Quantification of the lift height for magnetic force microscopy using 3D surface parameters

In this work, the quantitative conditions for the lift height for imaging of the magnetic field using magnetic force microscopy (MFM) were optimized. A thin cobalt film deposited on a monocrystalline silicon (1 0 0) substrate with a thickness of 55 nm and a thin nickel film deposited on a glass with a thickness of 600 nm were used as samples. The topography of the surface was acquired by tapping mode atomic force microscopy (AFM), while MFM imaging was performed in the lift mode for various lift heights. It was determined that the sensitivity of the measurements was about 10% higher for images obtained at a scan angle of 90° compared to a scan angle of 0°. Therefore, the three-dimensional surface texture parameters, i.e., average roughness, skewness, kurtosis and the bearing ratio, were determined in dependence on the lift height for a scan angle of 90°. The results of the analyses of the surface parameters showed that the influence of the substrate and its texture on the magnetic force image could be neglected for lift heights above 40 nm and that the upper lift height limit is 100 nm. It was determined that the optimal values of the lift heights were in the range from 60 to 80 nm, depending on the nature of the sample and on the type of the tip used.     

Superhydrophobic and transparent ZnO thin films synthesized by spray pyrolysis technique

Superhydrophobic and transparent zinc oxide (ZnO) thin films were deposited by a simple and cost effective spray pyrolysis technique (SPT) onto the glass substrates at 723 K from an aqueous zinc acetate precursor solution. The solution concentration was varied from 0.1 to 0.4 M and its effect on structural, morphological, wetting and optical properties of ZnO thin films was studied. The synthesized films were found to be polycrystalline, with preferential growth along c-axis. A slight improvement in the crystallite size and texture coefficient is observed as the concentration of the solution is increased. SEM micrographs show the uniform distribution of spherical grains of about 60-80 nm grain size. The films were specular and highly transparent with average transmittance of about 85%. The spectrum shows sharp absorption band edge at 381 nm, corresponding to optical gap of 3.25 eV. The samples of texture coefficient less than 90% and roughness less than 75 nm are hydrophobic and above these values they become superhydrophobic in nature. The hydrophobicity coupled with high transmittance is of great importance in commercial application such as transparent self-cleaning surfaces, anti-fog, anti-snow, fluid microchips and microreactors.     

Effects of surface inactivation, high temperature drying and preservative treatment on surface roughness and colour of alder and beech wood

Although extensive research has been conducted in wood surface quality analysis, a unified approach to surface quality characterisation does not exist. Measurements of the variation in surface roughness and surface colour are used widely for the evaluation of wood surface quality. Colour is a basic visual feature for wood and wood-based products. Colour measurement is one of the quality control tests that should be carried out because the colour deviations are spotted easily by the consumers. On the other hand, a common problem faced by plywood manufacturers is panel delamination, for which a major cause is poor quality glue-bonds resulting from rough veneer. Rotary cut veneers with dimensions of 500 mm × 500 mm × 2 mm manufactured from alder (Alnus glutinosa subsp. barbata) and beech (Fagus orientalis Lipsky) logs were used as materials in this study. Veneer sheets were oven-dried in a veneer dryer at 110 °C (normal drying temperature) and 180 °C (high drying temperature) after peeling process. The surfaces of some veneers were then exposed at indoor laboratory conditions to obtain inactive wood surfaces for glue bonds, and some veneers were treated with borax, boric acid and ammonium acetate solutions. After these treatments, surface roughness and colour measurements were made on veneer surfaces. High temperature drying process caused a darkening on the surfaces of alder and beech veneers. Total colour change value (ΔE^*) increased linear with increasing exposure time. Among the treatment solutions, ammonium acetate caused the biggest colour change while treatment with borax caused the lowest changes in ΔE^* values. Considerable changes in surface roughness after preservative treatment did not occur on veneer surfaces. Generally, no clear changes were obtained or the values mean roughness profile (R_a) decreased slightly in R_a values after the natural inactivation process.