Langmuir-Blodgett Multilayers Research Articles

Excitation Energy Transfer between a First Generation Dendrimer and a Pyrene Derivative in Langmuir−Blodgett Multilayers

A first generation dendrimer D1 was placed onto defined surfaces to study the excitation energy transfer between D1 as donor molecule and a pyrene containing polymer 213 as acceptor molecule in Langmuir−Blodgett (LB) multilayers. Fluorescence experiments were carried out at several excitation wavelengths (220, 260, 312, and 341 nm). D1 formed a closed LB layer with a collapse area of 73 A2 on an aqueous subphase, and a successful incorporation of D1 in LB multilayers onto quartz plates was possible. At 220 nm excitation wavelength the adjacent donor and acceptor layers showed a dramatically increased fluorescence activity of the pyrene group in 213, which is about 20 times higher than that of the acceptor layer 213 without the present donor layer D1. 213 quenches the fluorescence activity of D1 more than 90% although only 5% of the maleic acid groups in 213 are labeled with pyrene. The excitation energy transfer between D1 and 213 may be assumed as a Forster energy transfer which was examined versus dista...

Amphiphilic diblock dendrimers: synthesis and incorporation in Langmuir and Langmuir-Blodgett films.

A new dendron with peripheral long alkyl chains and containing five C(60) units in the branching shell has been prepared and attached to a Fréchet-type dendron functionalized with ethylene glycol chains. The peripheral substitution of the resulting globular dendrimer with hydrophobic chains on one hemisphere and hydrophilic groups on the other provides the perfect hydrophobic/hydrophilic balance allowing the formation of stable Langmuir films. Furthermore, a perfect reversibility has been observed in successive compression/decompression cycles. The diblock structure of the dendrimer has been also crucial for the efficient transfer of the Langmuir films in order to obtain well-ordered multilayered Langmuir-Blodgett films. This approach appears particularly interesting since functional groups not well adapted for the preparation of Langmuir and Langmuir-Blodgett films such as fullerenes can be attached into the branching shell of the dendritic structure and, thus, efficiently incorporated in thin ordered films.

Langmuir–Blodgett films of BEDT-TTF derivatives containing hydroxyl groups with long alkyl chains

BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene) derivatives 1 and 2 containing hydroxyl groups with long alkyl chains were synthesized and characterized. Studies with Langmuir–Blodgett (LB) technique indicated that both compounds 1 and 2 were able to form stable monolayers at the air–water interface, but these monolayers were found difficult to be transferred onto solid substrates. Multilayer LB films based on the mixtures of 1 and 2 with arachidic acid (AA), however, could be prepared easily, and the ordered structures of these LB films were confirmed by small angle X-ray diffraction (SAXRD) analysis. The LB films of 1, 2, and their mixtures with AA were oxidized with iodide, and the conductivity of resulting thin films was measured. For comparison, the conductivity of these LB films was also measured before the oxidation with iodide.

Controlled orientation of Langmuir-Blodgett films of substituted phthalocyanines

Attempts have been made to induce perpendicular stacking in Langmuir-Blodgett multilayers of phthalocyanine derivatives. Normally the largely hydrophobic phthalocyanines will lie on their edges to minimise hydrophobic/hydrophilic repulsion with the aqueous subphase. This study has prepared hydrophilic phthalocyanine derivatives that lie flat on the aqueous subphase. Our aim is to maximise the electro-optical effects in the direction perpendicular to the substrate.

Synthesis and monolayer formation of liquid crystal polysiloxanes having a lateral side chain with a perfluoroalkyl chain

Liquid crystal polysiloxanes with a lateral 4-(4-cyanophenoxycarbonyl)phenyl 4-(2-perfluoroalkylethoxy)-2-propyleneoxybenzoate side group and a 4-decyloxy-2-propyleneoxy-benzoate hydrocarbon analogue were prepared, and the mesomorphic properties were determined by the microscopic observation and differential scanning calorimeter (DSC) thermogram, and the layer structure of the smectic A phase was characterized by an X-ray diffraction experiment. The introduction of the perfluoroalkyl chain to the polymer remarkably enhances the smectic properties due to the strong fluorophilic interactions around the perfluoroalkyl chain. The effect of the perfluoroalkyl chain on the monolayer formation was studied at the air–water interface. The surface pressure–area isotherms show that the perfluoroalkyl chain enhances the stability of the monolayer versus surface pressure. In the condensed state the cyano group in the side chain is assumed to be anchored on the water surface and the mean axis of the liquid crystal core is approximately normal to the water surface. The monolayers were easily deposited on a hydrophobic glass slide, giving uniform Langmuir–Blodgett (LB) films by the vertical deposition method. The X-ray diffraction results indicate that the perfluorooctyl chain effectively stabilizes the layer structure in the multilayer LB film at room temperature. The side chain monomers also show a similar behavior to the polymers and give uniform LB films on hydrophobic glass slides.

Microscopic treatment of substrate effects on linear and quadratic optical response of model Langmuir–Blodgett multilayers

Microscopic calculations are reported of linear and quadratic nonlinear optical response in model Langmuir–Blodgett films on a substrate, which is treated by the method of images. The effect of the substrate is significant in the first two layers, and is greatest for tilted molecules with their head groups adjacent to the substrate. The main qualitative effect is to lower the symmetry relative to a free-standing film. Calculations for stearic acid films show that the substrate effect is most important for molecules with nonuniform response on a metallic substrate.

Glass transition and relaxation following PhotoPerturbation in thin polymeric films

In this letter we employ null ellipsometry on Langmuir Blodgett multilayers of a glass forming photosensitive side chain polymer to investigate both the structural changes and the related time relaxation as a function of temperature and sample thickness following isothermal optical perturbation. Below a thickness of a few layers the glassy multilayer system collapses to a smecticlike crystal. This effect is discussed in the context of the glass transition in restricted dimensionality.

Preparation of titanium dioxide and barium titanate nanothick film by Langmuir–Blodgett technique

The TiO 2 and BaO(TiO 2) x thin film was prepared by the Langmuir–Blodgett (LB) technique. The mixture of octadecyl polymaleicate and tetrabutoxytitanium was spread on the surface of water subphase, then transferred onto substrates. The obtained LB multilayer was converted into a nano-thick TiO 2 film by calcination at 500°C for 30 min. When an appropriate amount of BaCl 2 was added into a water subphase, it was co-transferred onto a LB multilayer film. After calcination at the same condition, BaO(TiO 2) x thin films with different ratios of x could be obtained by adjusting the pH value of the water subphase. The Fourier transform infrared spectroscopy and X-ray photoelectron spectra measurements were used to confirm the formation of TiO 2 and BaO(TiO 2) x thin films. The integrated peak area in X-ray photoelectron spectra gave the atomic ratio of Ti/O and Ti/Ba. The surface morphology of the LB multilayer before and after calcination was investigated by atomic force microscopy. The image showed that the smooth and high cover surface of TiO 2 and BaO(TiO 2) x thin films were obtained after calcination. The rms roughness for a 21-layer film after calcination was approximately 25 Å, which was less than that of before calcination of approximately 45 Å. The resulting particle size was approximately 0.05 μm and almost uniform.

Interactions and reactions of monolayers and Langmuir–Blodgett multilayers with compounds in the bulk phase

Studies performed on the interactions and reactions of compounds in the bidimensional state, essentially in monolayers and Langmuir–Blodgett multilayers, with substances in the aqueous subphase are reported. More precisely, the following is illustrated: (i) interactions between acid amphiphiles and prevalently bivalent ions placed in the aqueous support and between compounds capable of functioning like ion carriers in monolayers and ions in the subphase, in order to build mimetic membranes capable of selective ion transport; and the complexation of amphiphiles in monolayer with ions in the bulk liquid phase, in order to build chemical sensors to ions; (ii) the reactions of photoinduced electron transfer between a partner in mono- or multimolecular films and a partner in the subphase, which may determine the fundamental parameters and the differences with the same reactions in the bulk phase; and (iii) the reactions of enzymatic hydrolysis between the monolayer of a glyceride, which constitutes the reaction support, and the enzyme in the liquid bulk phase, which constitutes the subphase. The mechanism of the reactions and its inhibition are clarified. To conclude, possible future developments connected with the areas studied are examined.

Photoluminescence Study of Langmuir-Blodgett Films of Blend Poly 3-(2-(5-Chlorobenzotriazole)Ethyl) Thiophene

The multilayer Langmuir-Blodgett (LB) films of poly 3-(2-(5-chlorobenzotriazole)ethyl) thiophene (PCBET) blended with amphibious arachidic acid (AA) were prepared and characterized. The photoluminescence intensity of the blend film was enhanced as the AA increased by a certain amount. The PCBET excimers were not formed in the blend LB films.

Characterization of orientation of perfluorostearic acid Langmuir–Blodgett multilayers by infrared spectroscopic methods

The molecular orientation of perfluorostearic acid [CF 3(CF 2) 16COOH] deposited by Langmuir–Blodgett technique has been investigated by grazing incidence reflection absorption and transmission infrared spectroscopy. In the former, the electric field vector is normal to the film surface whereas in the latter, it is parallel to the surface. The intensities of the infrared bands that tend to exhibit perpendicular polarization, e.g. symmetric stretching of COO −, are much higher in reflection absorption than in transmission for deposited perfluorostearic acid. Therefore, these Fourier transform infrared results strongly support that the perfluorostearic acid molecules are preferentially oriented with their chain axes perpendicular to the substrate surface.

The mechanical stability of Langmuir–Blodgett multilayers

The properties and stability of confined Langmuir–Blodgett multilayers made of arachidic acid were investigated using the surface forces apparatus. Two different types of confined films were studied: one formed by bringing two separate LB films into molecular contact, the other formed by bringing one LB film into direct contact with mica. The measured thickness of confined material depends solely on the total number of trapped molecular layers (28 Å/layer). The force required to separate the substrates, however, depends on the type of contact made. More force (equivalent to a work of adhesion of 36 mJ/m 2) is required to separate an LB film from mica than from another LB film (28 mJ/m 2). Applied stress impacts the integrity of the LB multilayers; the films are particularly vulnerable to the combination of shearing and tensile stresses. Microscopic changes in film structure are sufficient to generate anomalous behavior at the macroscopic level, namely pinning of the contact region despite changes in load exerted on the deformable interacting bodies.

Langmuir–Blodgett film properties of a donor–acceptor Schiff base

A low-molecular weight Schiff base with donor–acceptor substituent on the Schiff base has been synthesized in order to study the Langmuir–Blodgett (LB) film properties. This is the first report wherein we demonstrate a short chain, Schiff base forms stable monolayers which have been transferred onto solid substrates to prepare Langmuir–Blodgett multilayers. Spectroscopic measurements of this Schiff base in chloroform as well as its LB film show changes in the spectral features indicating the changes in the environment around the chromophore.

Formation Mechanism of Multilayer Polyamic Acid Alkylamine Salt Langmuir–Blodgett Films

In this paper we present atomic force microscopy (AFM) and friction force microscopy (FFM) analyses on the surface structure of mono- and multilayer films prepared from two types of polyamic acid with long alkylamine chains, which were utilized as the precursors of polyimide Langmuir–Blodgett (LB) films. The characteristic domain structures on the monolayer films, namely, the plateau type and the hollow type, were examined while changing the film preparation conditions and the chemical structures of the precursors. The surface structures of the multilayer LB films were also analyzed by AFM, FFM and FT-IR techniques. It was found that the domain structures that appeared on the monolayer film were transformed to a smooth domain-free structure by multilayer deposition of the LB film. The formation and smoothing mechanism of the domain structures was discussed taking into account the fluidity of the polymer groups controlled by the frictional force at the film interface.

Microscopic calculations of linear and quadratic optical response in model Langmuir–Blodgett multilayers

Microscopic calculations are reported of linear and quadratic nonlinear optical response in Langmuir–Blodgett films. The films are modeled as ordered layers of molecules that can tilt from the vertical and have a higher polarizability and first hyperpolarizability in the head than in the tail. The local electric field varies little after the first layer and negligibly after the second, which simplifies the analysis. Tilted two-layer Y-type films can have quadratic susceptibility components as large as X- or Z-type ones. Overall the results provide a systematic account of the linear and nonlinear optical properties as a function of molecular and film structure that should assist interpretation of experimental results.

Selection of a model in reflectometry: use of the linear statistical inference

The theory of linear models is suggested for quantification of parameters’ significance in application to reflectivity data. Determination of the ultimate complexity of the model for real neutron reflectivity data by Ti/TiO 2 film as well as influence of experimental limitations on the number of reliably restored parameters for Langmuir–Blodgett multilayer is considered.

Point defects in Langmuir–Blodgett films of Cd arachidate

True molecular resolution with the atomic force microscope (AFM) has been achieved, on Langmuir–Blodgett multilayers, using films of cadmium arachidate on mica. The images were obtained with 12 μm scanner head and commercially purchased cantilevers with spring constant k=0.06 N m −1 under ambient conditions. The presence of vacancies in the Langmuir–Blodgett film was shown for the first time and is the evidence that AFM is a true probe for imaging molecular structure of soft organic materials. Well-preserved raw image of an isolated dislocation was resolved in this film clearly. The probability for existence of vacancies and free dislocations is discussed in the light of KTHNY theory.

Effect of anion type on monolayers and Langmuir–Blodgett films of gadolinium stearate

This work is focused on the preparation and characterization of gadolinium-containing stearic acid (SA) Langmuir–Blodgett (LB) films. SA monolayer behavior on the gadolinium-containing aqueous subphase has been analyzed by thermodynamic measurements. The compression isotherm shape changed substantially under varying anion type at the constant rare-earth cation content and pH in aqueous subphase. Those changes were ascribed to different monolayer surface charging under gadolinium binding. Small angle X-ray diffraction was used to study the structure of formed multilayer rare-earth-containing LB films. Considerable structural defects in multilayer LB films were found when gadolinium chloride solution was exploited as aqueous subphase for LB film formation, caused presumably by electrostatic interactions under charged monolayer deposition. The use of monodentate complexions like acetic acid residue was proposed to form bulk phase low-charged complexes with trivalent cations. The electroneutral ligand exchange reactions of such complexes with fatty acid Langmuir monolayer results in the formation of quasi-neutral trivalent cation complexes bound to the monolayer surface. This approach allowed effective neutralization of the monolayer surface under gadolinium binding and formation of condensed monolayer below the triple-point temperature. The compression and subsequent deposition of such monolayers resulted in the formation of highly ordered LB films with high content of adsorbed gadolinium cations arranged in two-dimensional (2-D) monatomic layers.

Measurement of intermolecular electron current in supramolecular assemblies created by the Langmuir–Blodgett technique

Intermolecular photoelectron transfer in a vector ordered molecular assembly, produced as a Langmuir–Blodgett multilayer, causes currents and voltage changes at electrodes. A comprehensive account is given of the experimental determination of the voltage changes, which can be done, in principal, with a time resolution of 25 ps. The present experimental limitations to this time resolution are described. The experiments allow determination of the time evolution of the charge-separation product. The means of determining the intermolecular transfer time of the carriers in such multilayers is described.

Surfactant-encapsulated clusters (SECs): (DODA)20(NH4)[H3Mo57V6(NO)6O183(H2O)18], a case study.

We present a comprehensive study of the partially reduced polyoxomolybdate [H3-Mo57V6(NO)6O183(H2O)18]21-encapsulated in a shell of dimethyldioctadecylammonium (DODA) surfacmolecules. Treatment of an aqueous solution of (NH4)21[H3Mo57V6-(NO)6O183(H2O)18] . 65H2O (1a) with a trichloromethane solution of the surfactant leads to instant transfer of the encapsulated complex anion into the organic phase. Results from vibrational spectroscopy. analytical ultracentrifugation, small-angle X-ray scattering, transmission electron microscopy, elemental analysis, and Langmuir compression isotherms are consistent with a single polyoxometalate core encapsulated within a shell of 20 DODA molecules. The molar mass of the supramolecular assembly is 20249 gmol(-1) and the diameter is 3.5 nm. A material with the empirical formula (DODA)20(NH4)[H3-Mo57V6NO)6O183(H2O)18] (2) was isolated as a dark violet solid, which readily dissolves in organic solvents. Slow evaporation of solutions of 2 on solid substrates forces the hydrophobic particles to aggregate into a cubic lattice. Annealing these so-formed films at elevated temperature causes de-wetting with terrace formation similar to liquid crystals and block copolymers. Compound 2 forms a stable Langmuir monolayer at the air-water interface; Langmuir-Blodgett multilayers are readily prepared by repeated transfer of monolayers on solid substrates. The films were characterized by optical ellipsometry, Brewster angle microscopy, transmission electron microscopy, and X-ray reflectance.