Buildup of ultrathin multilayer films by a self-assembly process, 1 consecutive adsorption of anionic and cationic bipolar amphiphiles on charged surfaces

An anionic and a cationic bipolar amphiphile containing rigid biphenyl cores were synthesized. The compounds were dissolved in a mixture of dimethylsulfoxide (DMSO) and water and pure water, respectively. When a solid substrate with a positively charged planar surface is immersed in the solution containing the negatively charged bipolar amphiphile, a monolayer of the amphiphile is adsorbed and due to its bipolar structure the surface charge is reversed. After rinsing in pure water the substrate is immersed in the solution containing the positively charged bipolar amphiphile. Again a monolayer is adsorbed but now the original surface charge is restored. By repeating both steps in a cyclic fashion alternating multilayer assemblies of both compounds are obtained. It is demonstrated that multilayer films, composed of at least 35 consecutively alternating layers, which corresponds to a total film thickness of 170 nm can be assembled.     

Stable aqueous nanoparticle film assemblies with covalent and charged polymer linking networks

The construction of highly stable and efficiently assembled multilayer films of purely water soluble gold nanoparticles is reported. Citrate-stabilized nanoparticles (CS-NPs) of average core diameter of 10 nm are used as templates for stabilization-based exchange reactions with thioctic acid to form more robust aqueous NPs that can be assembled into multilayer films. The thioctic acid stabilized nanoparticles (TAS-NPs) are networked via covalent and electrostatic linking systems, employing dithiols and the cationic polymer poly(L-lysine), respectively. Multilayer films of up to 150 nm in thickness are successfully grown at biological pH with no observable degradation of the NPs within the film. The characteristic surface plasmon band, an optical feature of certain NP film assemblies that can be used to report the local environment and core spacing within the film, is preserved. Growth dynamics and film stability in solution and in the air are examined, with poly(L-lysine) linked films showing no evidence of aggregation for at least 50 days. We believe these films represent a pivotal step toward exploring the potential of aqueous NP film assemblies as a sensing apparatus.     

A multiscale structural study of nanoparticle films prepared by the Langmuir–Blodgett technique

Arrays of magnetic nanoparticles (NPs) represent a very interesting challenge toward the development of new devices for magnetic applications such as data storage and spintronic. The final properties of such assemblies depending essentially on the spatial arrangement of NPs, it is of first importance to investigate precisely their structure. Here, the structure of monolayer and multilayer films of magnetic iron oxide NPs assembled by the Langmuir–Blodgett (LB) technique has been studied by usual techniques such as SEM, AFM and ellipsometry and by a new and an easy to process enhanced optical technique: the Surface Enhancement Ellipsometry Contrast (SEEC) microscopy. This technique is based on the use of a new generation of microscope slides used as substrates which allow the strong enhancement of the sample contrast to a point where it becomes possible to visualize the structure of monolayer and multilayer films at the nanoscale with a conventional optical microscope. The SEEC microscopy is demonstrated to be complementary to usual characterization techniques to study the structure of NPs films, especially for films containing very small nanosized NPs which are more difficult to analyze by usual techniques. While the film structure is investigated with lateral resolution of microns, the layer thickness is analyzed at the nanoscale (with a precision of 0.3 nm) with a close fit to the experimental measurements on local (AFM) and on larger (ellipsometry) areas. This technique presents the advantage to visualize directly the topography of NPs assemblies on very large areas by extracting information such as the height profile, the film roughness and generating 3D images.     

Silk layering as studied with neutron reflectivity

Neutron reflectivity (NR) measurements of ultrathin surface films (below 30 nm) composed of Bombyx mori silk fibroin protein in combination with atomic force microscopy and ellipsometry were used to reveal the internal structural organization in both dry and swollen states. Reconstituted aqueous silk solution deposited on a silicon substrate using the spin-assisted layer-by-layer (SA-LbL) technique resulted in a monolayer silk film composed of random nanofibrils with constant scattering length density (SLD). However, a vertically segregated ordering with two different regions has been observed in dry, thicker, seven-layer SA-LbL silk films. The vertical segregation of silk multilayer films indicates the presence of a different secondary structure of silk in direct contact with the silicon oxide surface (first 6 nm). The layered structure can be attributed to interfacial β-sheet crystallization and the formation of well-developed nanofibrillar nanoporous morphology for the initially deposited silk surface layers with the preservation of less dense, random coil secondary structure for the layers that follow. This segregated structure of solid silk films defines their complex nonuniform behavior in the D(2)O environment with thicker silk films undergoing delamination during swelling. For a silk monolayer with an initial thickness of 6 nm, we observed the increase in the effective thickness by 60% combined with surprising decrease in density. Considering the nanoporous morphology of the hydrophobic silk layer, we suggested that the apparent increase in its thickness in liquid environment is caused by the air nanobubble trapping phenomenon at the liquid-solid interface.     

The effects of network structure on the resistance of silane coupling agent layers to water-assisted crack growth

Silane adhesion promoters are commonly used to improve the adhesion, durability, and corrosion resistance of polymer-oxide interfaces. The current study investigates a model interface consisting of the natural oxide of(100) Si and an epoxy cured from diglycidyl ether ofbisphenol A (DGEBA) and triethylenetetraamine (TETA). The thickness of (3-glycidoxypropyl)trimethoxysilane (GPS) films placed between the two materials provided the structural variable. Five surface treatments were investigated: a bare interface, a rough monolayer film, a smooth monolayer film, a 5 nm thick film, and a 10 nm thick film. Previous neutron reflection experiments revealed large extension ratios (>2) when the 5 and 10 nm thick GPS films were exposed to deuterated nitrobenzene vapor. Despite the larger extension ratio for the 5 nm thick film, the epoxy/Si fracture energy (Gc) was equal to that of the 10 nm thick film under ambient conditions. Even the smooth monolayer exhibited the same Gc. Only when the monolayer included a significant number of agglomerates did the Gc drop to levels closer to that of the bare interface. When immersed in water at room temperature for 1 week, the threshold energy release rate (Gth) was nearly equal to Gc for the smooth monolayer, 5 nm thick film, and 10 nm thick film. While the Gth for all three films decreased with increasing water temperature, the Gth of the smooth monolayer decreased more rapidly. The bare interface was similarly sensitive to temperature; however, the Gth of the rough monolayer did not change significantly as the temperature was raised. Despite the influence of pH on hydrolysis, the Gth was insensitive to the pH of the water for all surface treatments.     

Effect of thickness on the thermal properties of hydrogen-bonded LbL assemblies

Layer-by-layer (LbL) assemblies have attracted much attention for their functional versatility and ease of fabrication. However, characterizing their thermal properties in relation to the film thickness has remained a challenging topic. We have investigated the role of film thickness on the glass transition temperature (T(g)) and coeffecient of thermal expansion for poly(ethylene oxide)/poly(acrylic acid) (PEO/PAA) and PEO/poly(methacrylic acid) (PEO/PMAA) hydrogen-bonded LbL assemblies in both bulk and ultrathin films using modulated differential scanning calorimetry (modulated DSC) and temperature-controlled ellipsometry. In PEO/PAA LbL films, a single, well-defined T(g) was observed regardless of film thickness. The T(g) increased by 9 °C relative to the bulk T(g) as film thickness decreased to 30 nm because of interactions between the film and its substrate. In contrast, PEO/PMAA LbL films show a single glass transition only after a thermal cross-linking step, which results in anhydride bonds between PMAA groups. The T(g), within error, was unaffected by film thickness, but PEO/PMAA LbL films of thicknesses below ~2.7 μm exhibited a small amount of PEO crystallization and phase separation for the thermally cross-linked films. The coefficients of thermal expansion of both types of film increased with decreasing film thickness.     

Computer simulation studies of Newton black films

We study via molecular dynamics simulations thin films (Newton black films, NBF) consisting of water coated with sodium dodecyl sulfate (SDS) surfactants. We analyze in detail the film properties (distribution of particles, pair correlation functions, roughness of the film, tilt angle of the hydrocarbon chain, electron density profiles, and mobility of water molecules) as a function of water content in the film core (i.e., film thickness, H). Our simulations indicate that water is part of the bilayer structure as solvation water. We estimate that around 2.25 water molecules per surfactant are part of this solvation structure. The structural analysis of the NBF shows that the headgroups exhibit a high degree of in-plane ordering. We find evidence for the existence of cavities in the monolayer, where only water is present. The basic structure of the monolayer is conserved down to water contents of the order of 4 water molecules per surfactant (H approximately equal to 11 A). The computed monolayer roughness for the present model is 2.5 A, in good agreement with the experimental data. We find that the roughness is very sensitive to the details of the interatomic potentials. Water mobility calculations emphasize the sluggish dynamics of very thin NBF. Diffusion coefficients of water in the lateral direction strongly decrease with film thickness. We find that the typical mean squared displacement of water in the direction normal to the bilayer is between 9 and 80 A2. Overall, our results indicate that the equilibrium SDS Newton black films studied in the X-ray experiments contain from 2 to 4 water molecules per surfactant.     

Synthesis of nanostructured metal-organic films: surface X-ray radiolysis of silver ions using a langmuir monolayer as a template

An application of the radiolysis method using an X-ray synchrotron beam is developed as a novel approach to the synthesis of metal-organic films with controlled shapes and thickness. We demonstrate that a Langmuir monolayer deposited onto a silver ion containing subphase, irradiated by an incident beam impinging below the critical angle for total reflection, induces the synthesis of a stable nanostructured silver-organic ultrathin film at the air-water interface. The X-ray scattering is also used to monitor in situ the structure of the silver layer during the synthesis process. The layer is observed by atomic force microscopy after its transfer onto a silicon substrate. One observes a film thickness of 4.6 nm, in good agreement with the X-ray penetration depth, about 4.5 nm. The silver structure is oriented by the initial organic film phase. This experiment demonstrates the considerable potential of this approach to produce various controlled metal-organic films with a surfactant self-assembly as a template.     

Preparation, structure, and optical properties of nanoporous gold thin films

Thin nanoporous gold (np-Au) films, ranging in thickness from approximately 40 to 1600 nm, have been prepared by selective chemical etching of Ag from Ag/Au alloy films supported on planar substrates. A combination of scanning electron microscopy (SEM) imaging, synchrotron grazing incidence small angle X-ray scattering, and N2 adsorption surface area measurements shows the films to exhibit a porous structure with intertwined gold fibrils exhibiting a spectrum of feature sizes and spacings ranging from several to hundreds of nanometers. Spectroscopic ellipsometry measurements (300-800 nm) reveal the onset of surface plasmon types of features with increase of film thicknesses into the approximately 200 nm film thickness range. Raman scattering measurements for films functionalized with a self-assembled monolayer formed from 4-fluorobenzenethiol show significant enhancements which vary sharply with film thickness and etching times. The maximum enhancement factors reach approximately 10(4) for 632.8 nm excitation, peak sharply in the approximately 200 nm thickness range for films prepared at optimum etching times, and show high spot to spot reproducibility with approximately 1 microm laser spot sizes, an indication that these films could be useful as durable, highly reproducible surface-enhanced Raman substrates.     

Effect of substituent position in coumarin derivatives on the interfacial assembly: reversible photodimerization and supramolecular chirality

The Langmuir and Langmuir-Schaefer (LS) films of two coumarin derivatives, 4-octadecyloxylcoumarin (4-CUMC18) and 7-octadecyloxylcoumarin (7-CUMC18), were newly synthesized, and their interfacial assemblies were investigated. Owing to the different substituent position of the long octadecyloxy chain in the coumarin parent, the two compounds showed completely different behaviors in the interfacial assemblies. When they were spread at the air/water interface, 7-CUMC18 formed a monolayer while 4-CUMC18 formed a multilayer film on the water surface. The spreading films on the water surface were transferred onto solid substrates by a Langmuir-Schaefer method, and the transferred films were characterized by UV-vis, Fourier transfer infrared, X-ray diffraction, circular dichroism, and atomic force microscopy spectra. Different packing of the molecules in the multilayer films was observed. While coumarin groups stacked in a face-to-face way in 7-CUMC18 film, they stacked in a head-to-tail manner in 4-CUMC18 film. Furthermore, distinct properties of the multilayer films were observed. It is revealed that a reversible [2+2] photodimerization and photocleavage could be induced in the LS film of 7-CUMC18 under photoirradiation with UV light of 365 and 254 nm, respectively. No photodimerization occurred in the 4-CUMC18 film. However, the film of 4-CUMC18 showed a supramolecular chirality although the compound itself is achiral.     

Layer-by-layer assembly of thiol-capped au nanoparticles on a water surface and their deposition on H-terminated Si(001) by the Langmuir-Blodgett method

A monolayer of dodecanethiol-encapsulated Au nanoparticles when compressed laterally transforms into layer-by-layer assemblies on water surface. These layer-by-layer assemblies of Au nanoparticles have been deposited on H-terminated Si(001) substrates by using one down-up cycle (two strokes) in the Langmuir-Blodgett (LB) method. The transformation from monolayer to layer-by-layer assembly on a water surface is irreversible; i.e., if the compressed film is decompressed the layer-by-layer structure cannot regenerate the monolayer structure. Unlike layer-by-layer growth, only odd numbers of layers grow from the monolayer on the H-terminated Si(001) substrates by using different numbers of down-up cycles. Z-type LB deposition occurs only in the first down-up cycle of the hydrophobic substrate, whereas Y-type LB deposition takes place in the successive cycles. Such layer-by-layer assemblies of Au nanoparticles, which are made on bare silicon surfaces and where thickness can be controlled at the nanoscale level, are very promising for their novel applications in the field of nanoscience.     

Crystalline ice growth on Pt(111) and Pd(111): nonwetting growth on a hydrophobic water monolayer

The growth of crystalline ice films on Pt(111) and Pd(111) is investigated using temperature programed desorption of the water films and of rare gases adsorbed on the water films. The water monolayer wets both Pt(111) and Pd(111) at all temperatures investigated [e.g., 20-155 K for Pt(111)]. However, crystalline ice films grown at higher temperatures (e.g., T>135 K) do not wet the monolayer. Similar results are obtained for crystalline ice films of D2O and H2O. Amorphous water films, which initially wet the surface, crystallize and dewet, exposing the water monolayer when they are annealed at higher temperatures. Thinner films crystallize and dewet at lower temperatures than thicker films. For samples sputtered with energetic Xe atoms to prepare ice crystallites surrounded by bare Pt(111), subsequent annealing of the films causes water molecules to diffuse off the ice crystallites to reform the water monolayer. A simple model suggests that, for crystalline films grown at high temperatures, the ice crystallites are initially widely separated with typical distances between crystallites of approximately 14 nm or more. The experimental results are consistent with recent theory and experiments suggesting that the molecules in the water monolayer form a surface with no dangling OH bonds or lone pair electrons, giving rise to a hydrophobic water monolayer on both Pt(111) and Pd(111).     

Unusual crystallization behaviors of anatase nanocrystallites from a molecularly thin titania nanosheet and its stacked forms: increase in nucleation temperature and oriented growth

Crystallization behaviors of anatase nanocrystallites from an ultrathin two-dimensional reactant composed of exfoliated titania nanosheets have been studied by monitoring the heating process of their well-organized films, with which the film thickness can be controlled from a molecularly thin monolayer to a stacked multilayer structure with a stepwise increment of approximately 1 nm. The heated products were identified by means of total reflection fluorescence X-ray absorption near-edge structure analysis and in-plane X-ray diffraction measurements using a synchrotron radiation source. The films composed of five or more layers of stacked nanosheets were transformed into anatase at 400-500 degrees C, which is a normal crystallization temperature of anatase from bulk reactants. As the film became thinner by decreasing the number of nanosheet layers to five or less, the crystallization temperature was found to increase and finally reached 800 degrees C for the monolayer film. Interestingly, preferential growth of anatase along the c-axis was strongly promoted for these ultrathin films. These unusual behaviors may be understood in terms of crystallization from the two-dimensional system of scarcely distributed reactants. The titania nanosheet crystallite is much thinner than the unit cell dimensions of anatase, and therefore, extensive atomic diffusion is required for the transformation particularly for the ultrathin films with a critical number (2-3) of stacked nanosheet layers. There is some structural similarity between anatase and titania nanosheet, which may account for the oriented growth of anatase nanocrystallites.     

Coating process regimes in particulate film production by forced-convection-assisted drag-out

Operating conditions for the deposition of monolayer and bilayer particulate coatings from aqueous 20-nm-diameter silica dispersions are identified in the context of a drag-out operation assisted by forced convection. The dry film thickness, uniformity, and morphology are assessed within an operating window parametrized by the capillary number and silica dispersion weight fraction. Three film deposition regimes with respect to the capillary number are observed: convective film deposition at low process rates, film entrainment at moderate process rates, and a thin-film transition regime at intermediate process rates. Locally ordered particulate films of variable layering thickness, including (i) a discontinuous submonolayer or (ii) a mixed submonolayer and monolayer, (iii) a mixed monolayer and bilayer, and (iv) multilayers, are dominant under convective deposition conditions. A map of morphologies is presented within the capillary number-weight fraction operating window, where monolayer and mixed monolayer-bilayer films are demonstrated in the thin-film transition regime at an intermediate dispersion weight fraction. A complementary map of the morphologies formed by the drag-out of 110 nm silica dispersions reveals a broader applicability to this type of operability diagram. These operating maps are constructed using model silica dispersions and are therefore relevant to particulate coatings of other inorganic materials.     

On sub-T(g) dewetting of nanoconfined liquids and autophobic dewetting of crystallites

The glass transition temperature (T(g)) of thin films is reduced by nanoconfinement, but it is also influenced by the free surface and substrate interface. To gain more insights into their contributions, dewetting behaviors of n-pentane, 3-methylpentane, and toluene films are investigated on various substrates as functions of temperature and film thickness. It is found that monolayers of these molecules exhibit sub-T(g) dewetting on a perfluoro-alkyl modified Ni substrate, which is attributable to the evolution of a 2D liquid. The onset temperature of dewetting increases with film thickness because fluidity evolves via cooperative motion of many molecules; sub-T(g) dewetting is observed for films thinner than 5 monolayers. In contrast, monolayers wet substrates of graphite, silicon, and amorphous solid water until crystallization occurs. The crystallites exhibit autophobic dewetting on the substrate covered with a wetting monolayer. The presence of premelting layers is inferred from the fact that n-pentane crystallites disappear on amorphous solid water via intermixing. Thus, the properties of quasiliquid formed on the crystallite surface differ significantly from those of the 2D liquid formed before crystallization.     

Particle zips: vertical emulsion films with particle monolayers at their surfaces

Vertical emulsion films with particle monolayers at their surfaces have been studied by direct microscope observations. The effects of particle wettability and surface coverage on the structure and stability of water films in octane and octane films in water have been investigated. Monodisperse silica particles (3 microm in diameter) hydrophobized to different extents have been used. It is found that the structure and stability of emulsion films strongly depend on the film type (water-in-oil or oil-in-water), the particle contact angle, the interactions between particles from the same and the opposite monolayer, and the monolayer density. Stable films are observed only when the particle wettability fulfills the condition for stable particle bridges--in agreement with the concept that hydrophilic particles can give stable oil-in-water emulsions, whereas hydrophobic ones give water-in-oil emulsions. In the case of water films with dilute disordered monolayers at their surfaces, the hydrophilic particles are expelled from the film center toward its periphery, giving a dimple surrounded by a ring of particles bridging the film surfaces. In contrast, the thinning of octane films with dilute ordered monolayers at their surfaces finally leads to the spontaneous formation of a dense crystalline monolayer of hydrophobic particles bridging both surfaces at the center of the film. The behaviors of water and octane films with dense close-packed particle monolayers at their surfaces are very similar. In both cases, a transition from bilayer to bridging monolayer is observed at rather low capillary pressures. The implications of the above finding for particle stabilized emulsions are discussed.     

四(十六烷硫基)四硫富瓦烯/硬脂酸混合LB膜的结构研究

我们考察了四(十六烷硫基)四硫富瓦烯/硬脂酸(THT-TTF:SA=1:n)在气-液界面的聚集状态及在LB膜中的排列,当n=1时,THT-TTF分子在气-液界面形成了双分子膜;当n>6时,THT-TTF分子形成了单分子膜;1相似文献   

Mechanics of nanoindentation on a monolayer of colloidal hollow nanoparticles

We explore the collective mechanical behavior of monolayer assemblies composed of close-packed arrays of hollow silica nanoparticles using a spherical nanoindentor. Seven types of well-defined hollow nanoparticles are studied with their radii ranging from 100 to 300 nm and shell thickness ranging from 14 to 44 nm. Micromechanical models reveal the underlying deformation mechanisms during indentation, where the consecutive contacting of the indentor with an increasing number of nanoparticles results in a nonlinear increase in the indentation force with penetration depth. Each contacted hollow nanoparticle successively locally bends, flattens, and then locally buckles. The effective indentation modulus of the monolayer film, which is obtained by a Hertzian fit to the experimental data, is found to be proportional to the elastic modulus of the nanoparticle shell material and scales exponentially with the ratio of particle shell thickness t to radius R to the power of 2.3. Furthermore, we find that for a constant film density with the same (t)/(R) of the constituent nanoparticles, smaller particles with a thinner shell can provide a higher effective indentation modulus, compared to their larger diameter and thicker shell counterparts. This study provides useful insights and guidance for constructing high-performance lightweight nanoparticle films and coatings with potential applications in tailoring stiffness and mechanical energy absorption.     

Precursor method for polymeric LB films

First, the general concept of the “Precursor Method” for the preparation of polymeric Langmuir-Blodgett (LB) films that possess no long alkyl chain between film layers, and the preparation of polybenzothiazole 7 LB film are described. The preparation of 7 LB film was carried out by the same procedure as that used to make polyimide LB films via precursor LB films of polyamides that contain β-carboxyethylthio alkylamine salts 6. Precursor 6 LB film had a Y type structure with monolayer thickness of 2.8 nm, while polybenzothiazole 7 LB film had 0.34 nm. The nonlinear susceptibility χ⁽³⁾ of polybenzothiazole 7 LB film in parallel to the dipping direction was 3.8 × 10⁻¹¹ esu, whereas the susceptibility in the perpendicular direction was about one fifth of that of the parallel direction.     

Electron beam irradiation of poly(vinyl methyl ether) films: 1. Synthesis and film topography

Temperature-sensitive hydrogel layers on silicon (Si) substrates were synthesized by electron beam irradiation of spin-coated poly(vinyl methyl ether) (PVME) films. The influences of the used solvent, the polymer concentration, and the spinning velocity on the homogeneity and the thickness of the PVME film were investigated. In the range of concentration c(p) = 1-15 wt% PVME in ethanol solution, homogeneous films with a thickness between d = 50 nm and 1.7 mum were obtained. The films were cross-linked by electron beam irradiation under inert atmosphere and analyzed by sol-gel-analysis. The results were compared with bulkgels formed by electron beam irradiation of PVME in the dry state. The film topography was analyzed by high-resolution field emission scanning electron microscopy and atomic force microscopy. An islandlike structure in the dry, swollen, and shrunken state of the hydrogel films was observed.