Irreversible adsorption of colloid particles on heterogeneous surfaces

Irreversible adsorption of polystyrene latex particles of micrometer size range at heterogeneous surfaces was studied experimentally. Model substrate surfaces of controlled site coverage (heterogeneity degree) used in these studies were produced by preadsorption of positively charged latex particles on mica sheets. Deposition kinetics of latex was studied as a function of the site coverage, particle to site size ratio λ and ionic strength of the colloid suspension. Particle distributions over surfaces and coverage were quantitatively evaluated by the direct microscope observation techniques using the diffusion cell. In this way, pair correlation function for various coverage degree and particle size ratio was evaluated. It also was determined the dependence of the jamming coverage of colloid particles on site coverage and ionic strength of the suspension. It was demonstrated that the decrease in the ionic strength of the suspension resulted in a significant decrease in the jamming coverage. This was attributed to the effect of the electrostatic field generated by the interface whose range was increased for low ionic strength. These experimental data revealed, in accordance with theoretical predictions derived from numerical simulations, that the multiple site coordination exerted a pronounced effect on the jamming coverage and the structure of adsorbed layers. It also was shown that this effect can be regulated by changes in the ionic strength of particle suspensions. This could allow one to produce particle clusters at the surface of targeted composition.     

Effect of the amorphous thin layer on the surface growth of amorphous/crystalline binary multilayer films

The effect of the amorphous thin layer on the surface growth of amorphous/crystalline binary multilayer films has been studied by using a continuum model. It is shown that both the surface roughness and the growth exponent of amorphous/crystalline binary multilayer films decrease with increasing thickness ratio between amorphous and crystalline layers. Our simulations have also revealed, in contrast to the monotonous rise in surface roughness observed in single-layer films grown on flat substrates, the surface growth of a multilayer film consists of two processes: interface smoothing and roughening, namely the film roughness decreases during the growth of amorphous thin layers but increases monotonously during the growth of crystalline thin layers. The observed interface smoothing and roughening can be obviously influenced by the change in the thickness ratio between amorphous and crystalline layers. The rise in thickness ratio between amorphous and crystalline layers enhances the interface smoothing effect but lowers the interface roughening effect and consequently shows a marked smoothing effect on the surface roughness.     

Simulation of diffusio-phoretic motion of colloidal particle suppressed by bound solutes within adsorption shell

Colloidal particle submerged in a non-equilibrium fluid with a concentration gradient of solutes experiences diffusio-phoresis. Such directional transport originates from the driving forces that exert on the fluid in a microscopic boundary layer surrounding the colloid. Based on a simple model of spherical colloid fixed in a concentration gradient of solutes, molecular dynamics simulations are performed to determine the interaction parameters that maximise the diffusio-phoretic mobility, which cannot be properly measured by conventional continuum theory. The diffusio-phoretic mobility is found to depend non-monotonically on the strength of the interaction between the colloid and solutes, due to the presence of bound solutes within adsorption shell that cannot contribute to diffusio-phoresis. The results also show that the phoretic mobility depends sensitively on the density of solutes in bulk, due to the uneven distribution of excess particles surrounding the colloid at a microscopic level. The simulations suggest that diffusio-phoresis may in principle be applied to the selective transport, separation and purification for colloidal systems. By substituting the spherical colloid with other realistic macromolecules, the model could provide results that are quantitatively comparable with experiments.     

Fabrication of two-dimensional colloidal arrays

We report the effective fabrication of two-dimensional (2D) arrays of submicron colloidal particles. These colloidal arrays are produced in thin layers of monodisperse colloid suspensions on flat surfaces of solids such as clean glass or cleaved mica. The process of colloid assembling includes two steps, nucleation and growth, similar to those found in crystallization in solution but each of the steps in detail progresses with different mechanisms. The nucleation process is initiated by a special kind of capillary force acting parallel to the surface. The growth is guided by a laminar flow of water to the crystals, which is driven by water evaporation from 2D arrays. What is distinguishable in the 2D assembling is its active nature governed by the forces and flows, making a contrast to the diffusive mechanism in ordinary crystallization. With this two-step mechanism, a domain of hexagonally packed colloidal array can grow with time. A large and uniform film of particle monolayer is, thus, formed in a short period, from several seconds up to several minutes depending on the conditions controlled.     

Adsorption compression in surface layers

A recently elucidated aspect of adsorption, compression in confined phases, is discussed. Grand Canonical Monte Carlo simulations were performed for the adsorption of Lennard–Jones molecules and new details of intermolecular interactions in adsorbed layers are analysed. It is shown that a strong attraction to a surface can cause adsorption compression not only in the first layer, but also in higher layers. Compression of the first layer creates a pattern of active sites; the second layer tends to be commensurate with this pattern and has density higher than that of a ‘free’ layer. This pattern propagates to higher layers. However, there is a wide range of chemical potentials where the first layer is compressed and the second layer is not yet formed. It was found that transition to adsorption compression results in oscillations of the isosteric heat of adsorption. These oscillations are determined by a combination of (a) changes in adsorbed layers’ structure and (b) exchange of molecules between layers. In particular, at high affinity to adsorbent, the adsorption isotherm for the first layer has a slight maximum because an increase of the chemical potential causes molecules to leave the compressed first layer and move to the second layer. For this reason, the isosteric heat of adsorption decreases and can become negative. Analysis of adsorption compression mechanisms in the context of theory and emerging experimental results indicates that the significance of this phenomenon is not limited to fundamental aspects of adsorption and capillarity. These mechanisms play a crucial role in various applications, such as heterogeneous catalysis, membrane separations, and self-assembly on surfaces. Results are discussed in a broader context of theory, experiments and previous simulations.     

Hetero-Colloidal Metal Particle Multilayer Films Grown Using Electrostatic Interactions at the Air–water Interface

The formation of nanoparticle multilayer films by electrostatic immobilization of surface-modified colloidal particles at the air–water interface has been recently demonstrated by us. In this paper, we extend our study to show that multilayer assemblies consisting of metal particles of different chemical nature (hetero-colloidal particle superlattices) and size can be deposited by the versatile Langmuir–Blodgett technique. Multilayer films consisting of a different number of bilayers of gold and silver colloidal particles have been deposited and characterized using quartz crystal microgravimetry and UV–visible spectroscopy measurements. It is observed that while layer-by-layer deposition of the different colloidal particle assemblies is possible by this technique without a detectable variation in the cluster density in the different layers, a degree of post-deposition reorganization of the clusters occurs in the film. In addition to this aging behavior, the effect of different organic solvents on the reorganization process has also been studied.     

Self-assembled film thickness determination by focused ion beam

The thickness evolution of multilayer film is investigated by focused ion beam (FIB) in the domain of polymer multilayers. This method, currently used in the modification and the characterization of integrated circuits, proves it is possible to determine the polymer film thickness. Sample cutting and its observation of the cross-section are performed in the FIB without leaving the vacuum chamber. Two main conclusions can be drawn: (1) the roughness of the film increases with the number of layer deposit, (2) the film growth changes from nonlinear (called exponential) to linear beyond 300 nm (70 layers).     

Optical, structural and adsorption properties of zinc peroxide/hydrogel nanohybrid films

Hybrid nanofilms from zinc-peroxide/poly(acrylamide) (ZnO₂/PAAm) and zinc-peroxide/poly(N-isopropyl-acrylamide) (ZnO₂/PNIPAAm) were prepared using the photopolymerization procedure. The thin layers were prepared by the combination of the Layer-by-Layer (LbL) self-assembly method and photopolymerization using UV light in every step of the procedure. The hybrid multilayer films consisting of layers of zinc peroxide nanoparticles and hydrogel alternating in a sandwich-like fashion with thicknesses of 65-246 nm. The chemical structures of the hybrid films were investigated by FTIR spectroscopy, their morphology was studied by atomic force microscopy (AFM). The build up of the films was studied by measuring the optical reflection spectrum, and we have calculated the refractive index and layer thickness of the hybrid layers using simulating software. The adsorption properties of the ZnO₂/hydrogel nanohybrid composite networks were investigated by measuring water and ethanol vapour adsorption by a quartz crystal microbalance (QCM). It was established that on partially hydrophobic ZnO₂/PNIPAAm hybrids the adsorbed amounts were lower, against the hydrophilic ZnO₂/PAAm film the vapour amount was higher. These results correspond to those of the bulk gel swelling results.     

超亲水性SiO2-TiO2纳米颗粒阵列结构的制备与性能研究

用分子自组装的方法在玻璃衬底上分别制备了TiO₂纳米颗粒层和SiO₂-TiO₂复合纳米颗粒阵列结构. 其中,SiO₂ 纳米颗粒层用旋涂法制备,得到密排阵列结构,而TiO₂纳米颗粒层则用浸渍提拉法制备. 文章分析了TiO₂纳米颗粒层和SiO₂-TiO₂复合纳米颗粒阵列结构的理论粗糙度,并通过扫描电子显微镜研究了它们的微观结构,用接触角 关键词： 自清洁 表面粗糙度 光催化 分子自组装     

The Effect of an Absorbed Polyelectrolyte Layer on the Dynamic Mobility of a Colloidal Particle

Electroacoustic measurements can be used for determining the characteristics of polymer layers on colloidal particles. In this paper a theoretical formula is derived for calculating the effect of a polyelectrolyte layer on the colloid electroacoustic signal. Representative calculations are presented, and the application of these results for polyelectrolyte characterisation are discussed.     

Methane adsorption on graphite（0001） films： A first-principles study

Using first-principles methods, we have systematically investigated the electronic density of states, work function, and adsorption energy of the methane molecule adsorbed on graphite(0001) films. The surface energy and the interlayer relaxation of the clean graphite(0001) as a function of the thickness of the film were also studied. The results show that the interlayer relaxation is small due to the weak interaction between the neighboring layers. The one-fold top site is found most favourable on substrate for methane with the adsorption energy of 133 meV. For the adsorption with different adsorption heights above the graphite film with four layers, the methane is found to prefer to appear at about 3.21 A above the graphite. We also noted that the adsorption energy does not dependent much on the thickness of the graphite films. The work function is enhanced slightly by adsorption of methane due to the slight charge transfer from the graphite surface to the methane molecule.     

Integrated characterization of multilayer periodic systems with nanosized layers as applied to Mo/Si structures

The potential inherent in integrated characterization of multilayer periodic systems employed in development of extreme-ultraviolet mirrors was demonstrated using the example of Mo/Si structures grown by magnetron sputtering in different technological regimes. An integrated study provided mutually consistent data on the thicknesses and crystal structure of the layers, as well as on the quality of the interfaces. Measurements by atomic force microscopy permitted a comparison of surface roughness of the substrates and the multilayer systems grown on them. An analysis of the power spectral density functions revealed that low-frequency roughness is replicated from the substrate, whereas the high-frequency one can become smoothed out in the course of growth. X-ray diffractometry performed in the thin film mode showed that the Mo layers in the samples studied have different crystal structures, from the amorphous and polycrystalline to the [110]-textured one. An analysis of the transmission electron microscopy data confirmed that there is a difference in the degrees of crystallinity of Mo layers. The thicknesses of individual layers, the period, and the irreproducibility of the thicknesses and the period were determined using X-ray reflectometry. The root-mean-square roughness amplitude of the interfaces was estimated, and the existence of transition layers originating primarily from the Si layer was demonstrated. The study was used to formulate a proper strategy for the analysis of multilayer periodic systems with nanosized layers.     

Dislocation motion in anisotropic multilayer materials

Line integral forms for the elastic field of dislocations in anisotropic, multilayer materials are developed and utilized in Parametric Dislocation Dynamics (PDD) computer simulations. Developed equations account for interface image forces on dislocations as a result of elastic modulus mismatch between adjacent layers. The method is applied to study dislocation motion in multilayer thin films. The operation of dislocation sources, dislocation pileups, confined layer slip (CLS), and the loss of layer confinement are demonstrated for a duplex Cu/Ni system. The strength of a thin film of alternating nanolayers is shown to increase with decreasing layer thickness, and that the maximum strength is determined by the Koehler barrier in the absence of coherency strains. For alternating Cu/Ni nanolayers, the dependence of the strength on the duplex layer thickness is found to be consistent with experimental results, down to a layer thickness of ≈10nm.     

The effect of nature of polyions and treatment after deposition on wetting characteristics of polyelectrolyte multilayers

The sequential adsorption of oppositely charged polyelectrolytes (PE) occurs to be a powerful tool for obtaining various materials of precisely defined properties. The interfacial features of PE multilayer films are governed by the choice of polycation/polyanion pairs and the conditions of film formation. Additionally, the long time exposure to the conditions different than that encountered during formation usually affects polyelectrolyte multilayer structure.The wettability of heterogeneous surfaces produced by ‘layer-by-layer’ (LbL) adsorption of polyelectrolytes was investigated in this work. We focused on the influence of film treatment after deposition on wetting properties of obtained multilayers. The effect of the nature of the first layer was also studied. Apart from simple arrangements: (polyallylamine hydrochloride)/(polysodium 4-styrenesulfonate) (PAH/PSS) and (poly-l-lysine hydrobromide)/(poly-l-glutamic acid sodium salt) (PLL/PGA) more complicated structures were considered having as a first layer two types of polyethylene imines (PEI) of different molecular weight.Wetting properties of such polyelectrolyte films were determined experimentally by contact angle measurements using technique of direct image analysis of the shape of sessile drop.     

Surface-enhanced Raman spectroscopy based on ordered nanocap arrays

We fabricated the Ag cap array for surface-enhanced Raman scattering (SERS) by Ag deposition onto two dimensional polystyrene colloid sphere templates, and 4-mercaptopyridine (4-MPy) was used as the probing molecule. When the colloids with different size were chosen as the substrate for 20 nm Ag deposition, the film on 100 nm colloids gave the significant enhancement. SERS intensity increased with the increase of Ag thickness. When 20 nm Ag film was coated by Ta, the SERS signals decreased with the increase of Ta thickness, indicating the main effect from the top of cap structure. When Co layer was added under the Ag film, the SERS intensity decreased with the increase of Co thickness because the Co layer affects electromagnetic and plasmon resonance.     

Homogeneous model for surface roughness in identical double layer system

In this paper a homogeneous model for surface roughness in the identical double layer system has been presented. It has been shown that the reflectance change in non-absorbing layers is directly proportional to the square of the total thickness of the layers. In an absorbing layer, it has been shown that the thickness of each layer is equal to the surface roughness of the identical double layer system. The extinction coefficients of both layers are directly proportional to the thickness of that layer.     

Nonpolar a-plane GaN grown on r-plane sapphire using multilayer AlN buffer by metalorganic chemical vapor deposition

Mirror-like and pit-free non-polar a-plane (1 1 −2 0) GaN films are grown on r-plane (1 −1 0 2) sapphire substrates using metalorganic chemical vapor deposition (MOCVD) with multilayer high-low-high temperature AlN buffer layers. The buffer layer structure and film quality are essential to the growth of a flat, crack-free and pit-free a-plane GaN film. The multilayer AlN buffer structure includes a thin low-temperature-deposited AlN (LT-AlN) layer inserted into the high-temperature-deposited AlN (HT-AlN) layer. The results demonstrate that the multilayer AlN buffer structure can improve the surface morphology of the upper a-plane GaN film. The grown multilayer AlN buffer structure reduced the tensile stress on the AlN buffer layers and increased the compressive stress on the a-plane GaN film. The multilayer AlN buffer structure markedly improves the surface morphology of the a-plane GaN film, as revealed by scanning electron microscopy. The effects of various growth V/III ratios was investigated to obtain a-plane GaN films with better surface morphology. The mean roughness of the surface was 1.02 nm, as revealed by atomic force microscopy. Accordingly, the multilayer AlN buffer structure improves the surface morphology and facilitates the complete coalescence of the a-plane GaN layer.     

Self-assembled monolayer and multilayer formation using redox-active Ru complex with phosphonic acids on silicon oxide surface

The formation of self-assembled monolayer and multilayer using redox-active Ru complex molecules with phosphonic acids on SiO₂ surface has been examined using X-ray photoelectron spectroscopy (XPS), ellipsometry, and time of flight secondary mass-ion spectroscopy (TOF-SIMS). We found that an introduction of a Zr adlayer leads to higher surface molecular density of Ru complex SAMs on the SiO₂ surface, compared to that of obtained from the direct adsorption of Ru complex monolayer on the SiO₂ surface. We further tried to fabricate a multilayer film using this molecule with Zr(IV) ion acting as a chemical glue by a successive immersion process. The XPS data revealed that the molecular densities of the multilayers were also higher for the immobilization with Zr adlayer between Ru complex and SiO₂ surface than those without the Zr adlayer, suggesting that Zr adlayer is effective in forming highly packed molecular layer of phosphonic acids on SiO₂ surface. We found the film growth reached a saturation point after 6 layers on the SiO₂ surface. The film growth saturation can be explained by a molecular domain boundary effect encountered due to the large tilt angle of the molecular layer.     

Temperature profile of the multilayer structure irradiated by pulsed laser

Two programs are developed to calculate the temperature profile, as well as the reflectance, transmittance and absorption of a given multilayer film structure, in order to better understand the laser energy distribution between the reflectance, transmittance and absorption in each film layer. An inorganic Blu-ray recordable disc (BD-R) structure is used as a practical demonstration of the multilayer structure. The reflectance and absorption of the BD-R structure exhibit opposite trends and oscillate repeatedly with varying lower or upper dielectric layer thickness while the rest of the film thickness remains unchanged. The energy absorption in an absorbed layer depends on the thickness of the dielectric layers, its relative position in the structure and the extinction coefficient of its optical constant. The total absorption ratio of its maximum to minimum can be over 3 when changing the lower dielectric layer thickness of the studied structure. The layer thickness acts as an energy valve to control the energy flow into the multilayer structure. The thermal profile of the multilayer film structure irradiated by a pulsed laser is calculated at different positions in the film layers with time. The calculated temperatures in the recording alloy layer exhibit linear relationship with the applied power level. The effect of the laser duration time on the temperature increase in the recording layer is significant in the first few nanoseconds and becomes saturated if the heat balance is established in the structure. The calculated temperature is consistent with the experimental recording result when the structure is recorded at 4-time BD-R recording speed.     

The film thickness dependent thermal stability of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>:Ag thin films as high-temperature solar selective absorbers

The monolayer Al₂O₃:Ag thin films were prepared by magnetron sputtering. The microstructure and optical properties of thin film after annealing at 700 °C in air were characterized by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and spectrophotometer. It revealed that the particle shape, size, and distribution across the film were greatly changed before and after annealing. The surface plasmon resonance absorption and thermal stability of the film were found to be strongly dependent on the film thickness, which was believed to be associated with the evolution process of particle diffusion, agglomeration, and evaporation during annealing at high temperature. When the film thickness was smaller than 90 nm, the film SPR absorption can be attenuated until extinct with increasing annealing time due to the evaporation of Ag particles. While the film thickness was larger than 120 nm, the absorption can keep constant even after annealing for 64 h due to the agglomeration of Ag particles. On the base of film thickness results, the multilayer Al₂O₃:Ag solar selective thin films were prepared and the thermal stability test illustrated that the solar selectivity of multilayer films with absorbing layer thickness larger than 120 nm did not degrade after annealing at 500 °C for 70 h in air. It can be concluded that film thickness is an important factor to control the thermal stability of Al₂O₃:Ag thin films as high-temperature solar selective absorbers.