Langmuir-Blodgett Multilayers Research Articles

Reversible Thermochromic Transition between Different J-Aggregate States of Amphiphilic Merocyanine Dye Crystallites

ABSTRACTOn the subphase, which contains Cd2+ and Mg2+ ions, two-dimensional (2D) J-aggregate crystallites of merocyanine dye molecules exhibit a thermochromic transition. The J-aggregate absorption band of the low (LT) and high temperature (HT) phases are located at 620 and 595 nm, respectively. In order to clarify the in-plane structure of the 2D J-aggregate crystallites, grazing incident X-ray diffraction measurements were performed on the Langmuir-Blodgett multi-layers which consisted of the monolayers of LT and HT phases. It was revealed that the in-plane arrangement of the dye molecules is slightly different in the J-aggregate crystallites of LT and HT phases.

Photoluminescence Characteristics of Hemicyanine Derivatives in Langmuir-Blodgett Multilayers

Photoluminescence Characteristics of Hemicyanine Derivatives in Langmuir-Blodgett Multilayers

Biologically inspired polyoxometalate–surfactant composite materials. Investigations on the structures of discrete, surfactant-encapsulated clusters, monolayers, and Langmuir–Blodgett films of (DODA)40(NH4)2[(H2O)n ⊂ Mo132O372(CH3CO2)30(H2O)72] †

A detailed analysis of the supramolecular architecture of the nanoporous surfactant-encapsulated cluster (SEC) with the empirical formula (DODA)40(NH4)2[(H2O)n ⊂ Mo132O372(CH3CO2)30(H2O)72] 1 (n ≈ 50) is presented. The open framework architecture of the Keplerate cluster is investigated by means of small angle neutron scattering (SANS) in CDCl3 solutions containing discrete SECs. A simplifying core–shell model of 1 is developed, which describes the SEC as a solvent-filled nanocavity, surrounded by two concentric shells (a first polyoxometalate shell of 2.96 nm outer diameter, and a consecutive surfactant shell of 6.18 nm outer diameter, respectively). The model is successfully applied to probe the content of H2O/D2O guest molecules in the Keplerate host. Different surface analytical techniques are applied to characterize the hierarchical structures of monolayers and thin films of 1. Monolayers at the air–water interface are investigated by means of optical ellipsometry and Brewster angle microscopy. Electron density profiles of the monolayers of 1 are gained from synchrotron X-ray reflectance (XRR) measurements that provide further evidence for the supramolecular core-shell architecture of the SEC. Within the spatial resolution limits of these analytical methods, the current data support a monolayer model consisting of hexagonal close-packed arrays of discrete SECs, floating at the air–water interface. Langmuir–Blodgett (LB) transfer of compressed monolayers on to a solid substrate leads to homogeneous multilayers. In the XRR spectra of LB multilayers of 1 multiple Bragg reflections appear, thus indicating an intrinsic tendency of the SECs to adapt a 3-dimensional, highly ordered solid state structure. Considering the huge variety of structurally different polyoxometalates and the possibility to tailor the surfactant shell by means of classic organic synthesis, the self-organization of hierarchically structured thin films and solids based on SECs bears promising perspectives towards the engineering of functional materials.

Experimental and molecular modelling studies of overbased detergent particles

Langmuir trough, Langmuir–Blodgett (LB) film and ellipsometry experiments, and molecular modelling, were carried out to characterise a series of overbased detergent (OD) systems made with several surfactant types, including the first application of these approaches to a new class based on calixarates. Systems were synthesised with low, mid and high levels of basicity, which on the TBN (total base number) scale were 150, 250 and 350, respectively. The diameters determined from the Langmuir trough measurements were 3.4, 3.5 and 3.6 nm, respectively. These are consistently higher than those of phenate particles with the same basicities. The phenate and calixarate particles self-assembled into multilayer LB films on a stearic acid-coated silicon wafer. Film thicknesses were determined by ellipsometry. Differences in the estimated particle diameter for the phenates and calixarates by these two methods suggest that the OD particles are, in general, and to varying extents, non-spherical. In the phenate case the deposited LB film thickness was a strong function of the Langmuir film deposition surface pressure in the region where the monolayer is essentially close-packed, supporting the discoid shape for these particles, which was first predicted by our previous molecular modelling studies. The molecular modelling results suggest that the sulfonate systems are the most spherical, followed probably by the salicylates. Preliminary results on the calix[6]arates suggest that they form prolate ellipsoidal particles, capped at both ends with the surfactant molecule. The experiments and simulations are consistent in indicating that the particle shape and internal atomistic structure inside the inorganic core region of the particle are quite sensitive to the surfactant type.

Light-Induced Enhancement of Rectification by Monolayer of Hexadecylquinolinium Tricyanoquinodimethanide Between Al Electrodes

ABSTRACTPhotoinduced changes in electrical rectification were measured for a 5-layer Langmuir-Blodgett multilayer of hexadecylquinolinium tricyanoquinodimethanide, sandwiched between opaque and semitransparent Al electrodes, as a function of bias and wavelength. The current increases with the light intensity, but this may be simply due to heating of the sample, rather than to a photon-induced enhancement of the rectification.

Ultra-thin films of polymerised smectic liquid crystals. A study using AFM and ellipsometry

Langmuir–Blodgett multilayers very often consist of 2D crystallites which scatter light. One way of avoiding this problem is to use preformed polymers but even here there is a difficulty as multilayers formed from them have a poor degree of order in the direction normal to the plane of the film. This defect is exemplified by the fact that a low-angle X-ray diffractogram obtained from such films consists, usually, of 3 or less Bragg peaks and, at the most, 5 observable peaks. In an effort to obtain better order in multilayers both within the plane and normal to it, we have examined the properties of multilayers of a compound which can exist in the smectic A phase and can be polymerised with the aid of UV light while they are in this phase. We have studied the thickness of a number of multilayers of this compound as they go through the phase change using ellipsometric techniques and indeed find a thickness change which corresponds to the change in monolayer thickness found by X-ray diffraction and reported in our earlier studies. The ellipsometer that we have used gives the average thickness of a circular region having a diameter of about 1 mm. In order to obtain a better understanding of the behaviour of films on a much smaller horizontal scale, we have used atomic force microscopy. We display a number of typical cross sections of AFM scans and show that, on a molecular scale, the films are smoother in the smectic phase than they are in the crystalline phase and that polymerisation tends to maintain a film in a smooth condition.

Investigation of molecular diffusion across organic multilayers using neutron specular reflectivity

Langmuir–Blodgett (LB) multilayers of fatty acid salt molecules have been investigated by neutron specular reflectivity (NSR) at the reflectometer ADAM at the Institute Laue-Langevin in Grenoble, France. Due to the high flux at the sample site, the NSR curve could be recorded over more than six orders of magnitude, corresponding with q z ≤0.3 Å −1. Because of the incoherent hydrogen scattering, the detectable coherent NSR curve is limited to 10 −5. The incoherent background of the multilayer systems could be determined for the first time by means of a spin-polarized NSR experiment. The observed intermixing at freshly prepared LB multilayers increases during thermal annealing below the phase transition temperature, whereas the vertical structure remains. This is explained by thermally induced vertical reorganisation of molecules across the multilayer system. The molecular exchange can be described as an analogue to an order–disorder transition. The evaluated activation energy of about 1.7 eV is related to the breaking of van der Waals bonds between one particular molecule and its hexagonal neighbourhood. Time-resolved measurements at particular temperatures allow us to estimate a vertical “diffusion constant” in the range of 10 −23 m 2/s.

Linear and nonlinear optical properties of Langmuir–Blodgett multilayers from chromophore-containing maleic acid anhydride polymers

Organized polymer films prepared from amphiphilic side-chain polymers were investigated with respect to their application in passive or active photonic devices. The polymers were synthesized by means of polymer analogous reaction of azo chromophores with a maleic acid anhydride polymer which bears saturated hydrophobic side chains. The resulting polymer contains, besides hydrophilic groups and hydrophobic side chains, azo chromophores with high second-order hyperpolarizability which are attached to the main chain via a spacer. In order to increase the hydrophobic character of the chromophore, a fluorinated moiety was chosen as acceptor of the chromophore. Langmuir–Blodgett (LB) technique was applied for preparation of multilayer structures with distinct supramolecular architecture from these polymers. They are characterized by high anisotropy and a special combination of linear and nonlinear optical (NLO) properties. The polymers were deposited both in Y-type and Z-type LB layers. The Y-type layers of the azopolymer show high birefringence and dichroism but small nonlinear response which originates from the first deposited layer only. Multilayers of the azopolymer deposited in Y-type configuration are characterized by high internal order which can be concluded from a pronounced Bragg-peak and Kiessig-fringe patterns as obtained from gracing incidence X-ray scattering. Experiments to deposit the active polymer directly in Z-type configuration by passing a blank water compartment before layer deposition in a double compartment LB-trough led to the result that well-ordered, anisotropic films are obtained only if the number of deposited monolayers is limited to a value of about 30. The layers deposited in Z-type manner show low order. This can be concluded from the fact that only a weak Bragg-peak is observed in the X-ray scattering curves, and Kiessig-fringes are very weak or missing. The same conclusion can be drawn from ellipsometric and second-harmonic generation (SHG) experiments. A second kind of Z-type multilayers was prepared by alternate deposition of the active azopolymer and of another maleic acid anhydride polymer without chromophores. They show similar linear optical properties like Y-type layers but due to the noncentrosymmetric internal structure, a NLO second-order response grows proportional to the square of the sample thickness. Again, thick layers of the azopolymer prepared by alternate deposition LB-technique were prepared, which can be used as active waveguide for second-order NLO application.

Unimolecular Electrical Rectification by Hexadecylquinolinium Tricyanoquinodimethanide

The asymmetrical DC electrical conductivity (macroscopic and also nanoscopic) through Langmuir-Blodgett multilayers, and even a monolayer, of γ-(n-hexadecyl)quinolinum tricyanoquinodimethanide, C16H33Q-3CNQ (1) is due to rectification of electrical current by a molecule.

LB Multilayers and Superlattice Films of Erbium Fatty Acid Salts

Multilayers of Y-type bilayers of pure and mixed erbium palmitate(EP), nonadecanate(EN) and behenate(EB) on CaF2 substrates were prepared by conventional Langmuir-Blodgett (LB) method. It is demonstrated that two systems composed of alternating bilayer of different fatty acid salts are unidimensional superlattices. These LB films were characterized by means of x-ray photoelectronic Spectrometry (XPS), FTIR and x-ray diffraction (XRD) measurements.

Preparation and Characterization of DBSA Doped Polyaniline Thin Films

Stable monolayer of the polyaniline(PAn) doped with dodecyl benzenesulfonic acid(DBSA) can form on the pure water surface. The multilayer ultrathin film can be successfully deposited by Langmuir-Blodgett(LB) technique onto CaF2 substrate. The limiting mean molecular area and collapse pressure observed are 0.066 nm2 and 35 mN m−1, respectively. The multilayer LB film and casting film were all characterized by IR and UV-Vis-NIR spectroscopies.

Organized Langmuir-Blodgett Monolayers and Multilayers Based on Semi-Amphiphilic Binuclear Phthalocyanine: AFM Investigation and Photovoltaic Characteristics

A water-soluble binuclear cobalt phthalocyanine-sulphonate was used to prepare LB films by semi-amphiphilic technique. Atomic force microscopy (AFM) was applied to investigate the surface morphology of C18NH2-OME/bi-CoPc LB multilayer on silicon substrate and surface photovoltage spectra (SPS) was used to study the interfacial charge transfer properties.

Interlayer Energy Transfer between Carbazole and Two 9-Anthroyloxy Derivatives in Langmuir−Blodgett Films

The interlayer excitation energy transfer between 11-(9-carbazole)undecanoic acid (II-CU) and two 9-anthroyloxy derivatives, 9-(9-anthroyloxy)stearic acid (9-AS) and 2-(9-anthroyloxy)stearic acid (2-AS), in alternating multilayer Langmuir-Blodgett films has been studied. The 11-CU fluorescence is quenched by energy transfer to 9-AS or 2-AS as judged by steady-state and picosecond time-resolved fluorescence measurements. The fluorescence decay curves of 11-CU in the films were analyzed in the framework of several models: (1) a general model for interlayer energy transfer, (2) a two-exponential decay, (3) a Forster model for energy transfer in a two-dimensional system, and (4) a stretched-exponential decay, characteristic of Forster energy transfer in self-similar fractal-like structures. The recovered decay parameters suggest an inhomogeneous mixing of the acceptor molecules in LB films leading to a two-phase system. The phase separation during compression of the acceptor monolayers forms regions of acceptor concentration about 3 times that of the intended and regions with very low acceptor concentration.

Monolayer and Spectroscopic Studies of an Amphiphilic (Phenylethynyl)anthracene Probe in Pure and Mixed Films with Charged and Neutral Lipids

The amphiphilic and morphological properties of a novel, amino-subsituted bis(phenylethynyl)anthracene derivative (APA) are investigated at the air/water interface using a Langmuir film balance and fluorescence microscopy. The interfacial miscibility of APA and positively charged, negatively charged, and zwitterionic lipids in two-dimensional monolayer phases is characterized on the basis of surface pressure/area isotherms of binary monolayers recorded at different molar ratios of the lipid and dye components. “Excess” Gibbs free energies of mixing are calculated for the APA/lipid monolayers at different surface pressures. Organized supramolecular film structures of the (phenylethynyl)anthracene amphiphile and its mixtures with octadecylamine are deposited onto solid supports by using the Langmuir−Blodgett (LB) technique. Mono- and LB multilayers are characterized by means of UV−visible spectroscopy. Evidence is obtained for a J-type aggregation of APA in LB monolayers. The aggregation is likely favored b...

The Directional Dependence of Water Penetration into Langmuir–Blodgett Multilayers

The relationship between the mass uptake and the swelling of arachidic acid Langmuir–Blodgett (LB) films exposed to water vapor was investigated. Combining sorption data from the quartz crystal microbalance with swelling data from the surface forces apparatus (SFA), it was found that films exposed to water vapor (≈75% RH) absorb 0.22 moles water/mole LB film with an associated swelling of 0.2 Å/film layer. This degree of film swelling is less than that predicted if the water taken up exhibits the molar volume of bulk liquid water. The configuration of the films in the SFA, where they are trapped between impermeable surfaces, makes possible the measurement of the lateral diffusion coefficient of water in these layered materials. This was found to be 1.5 × 10−10 cm2/s, which is at least 100 times faster than diffusion normal to the layers as measured with the microbalance.

Langmuir–Blodgett multilayers of imidazoles

Langmuir–Blodgett (LB) multilayers have been formed from a wide variety of compounds but the basic studies from which others have developed have involved a long hydrocarbon chain terminated at one end by a hydrophilic group. The quaternary ammonium ion has a greater attraction for water than do the other groups which have been used and is so hydrophilic as to preclude the formation of LB layers of straight chain molecules terminated by this group. Certainly all such molecules up to and including octadecyl trimethyl ammonium bromide form micelles in preference to monolayers at the air/water interface though metastable monolayers of this compound have been reported. Quaternized imidazoles have, like quaternary ammonium ions, a positive charge but seem more likely to form LB films. We have thus explored the properties of several of these compounds with a view to producing one which has the desired properties. In order to combat the highly hydrophilic nature of the imidazole group it is necessary to employ a highly hydrophobic ‘tail group’. Thus our most successful compound contains two octadecyl alkyl chains. We have studied the behaviour of this compound at the air/water interface and have found that the nature of the subphase has a very marked influence on the shape of the isotherms. With nitric acid added to the subphase to bring about a pH of 3.5 it is possible to obtain good Y deposition with deposition ratios equal to one in both the upward and downward directions. However we have not been able to achieve LB deposition using any other acid. On heating, the compound in question it changes from a crystalline structure to a smectic structure at 55°C.

LB films of parent polypyrrole

Langmuir films of parent polypyrrole were investigated using surface pressure and surface potential isotherms, which showed that the films are not true monomolecular layers but multilayer stacks instead. The mean molecular area increased with decreasing subphase temperatures, but it did not change significantly when the compression speed was varied. These Langmuir films could be transferred onto solid substrates in the form of Langmuir-Blodgett (LB) films, with no need to mix the parent polypyrrole with builder materials such as cadmium stearate. Multilayer LB films were characterized by UV-vis, FTIR, surface potential and conductivity measurements.

Conformations of poly(dihexylsilane) embedded in Langmuir-Blodgett multilayers of arachidic acid

Poly(dihexylsilane) (PDHS) and arachidic acid (AA) were co-spread onto a cadmium-containing water subphase and transferred onto a solid substrate as multilayers by the Langmuir-Blodgett method. The structure of these films was evaluated by means of UV-visible absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray diffractometry, and film thickness measurements. The results indicated that the PDHS ultrathin film is incorporated into a well-structured AA multilayer matrix without appreciable destruction of the layer structure. The conformational behavior of PDHS embedded within this film was evaluated by UV-visible absorption spectroscopy. The phase transition temperature of the ordered (all-trans zigzag) to the disordered (helical gauche) state on the AA monolayer at the air-water interface is lowered and the conformational state is virtually maintained in the as-deposited multilayers during storage at room temperature at least for a month. PDHS in the AA multilayer on heating showed a substantially broadened transition around 40°C, which clearly reflects a geometrical restriction effected by the AA multilayer matrix.

Conformations of poly(dihexylsilane) embedded in Langmuir-Blodgett multilayers of arachidic acid

Poly(dihexylsilane) (PDHS) and arachidic acid (AA) were co-spread onto a cadmium-containing water subphase and transferred onto a solid substrate as multilayers by the Langmuir-Blodgett method. The structure of these films was evaluated by means of UV-visible absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray diffractometry, and film thickness measurements. The results indicated that the PDHS ultrathin film is incorporated into a well-structured AA multilayer matrix without appreciable destruction of the layer structure. The conformational behavior of PDHS embedded within this film was evaluated by UV-visible absorption spectroscopy. The phase transition temperature of the ordered (all-trans zigzag) to the disordered (helical gauche) state on the AA monolayer at the air-water interface is lowered and the conformational state is virtually maintained in the as-deposited multilayers during storage at room temperature at least for a month. PDHS in the AA multilayer on heating showed a substantially broadened transition around 40°C, which clearly reflects a geometrical restriction effected by the AA multilayer matrix.

Investigations of pH-dependent domain structure of lead arachidate Langmuir-Blodgett films by means of x-ray specular and diffuse scattering and atomic force microscopy

Langmuir-Blodgett (LB) multilayer systems are found to be inhomogeneous in morphology and structure. The most characteristic feature of these multilayer films is the appearance of three-dimensional domains in which the molecules arrange themselves as a result of the film transfer onto the substrate. A series of lead arachidate (PbA2) LB multilayers have been prepared as model systems to study the dependence of film morphology on the pH value in the subphase. Atomic force microscopy (AFM) and two different x-ray scattering methods [specular (XSR) and diffuse (XDS) x-ray reflectometry] have been used to investigate the intrinsic interface properties, such as the vertical electron density profile and lateral distribution of head groups and chains, as well as the microscopic description of the interface structure, thus providing an overall picture of the investigated multilayers. With AFM, discrete height variations of domains with minimum step widths of one double layer independent of the salt concentration in the films were observed. The lateral domain size shows a dependence on pH. It was found to be maximum at pH=4.2 (pure acid) but minimum at pH 7.0 (maximum salt content). The AFM pictures were treated by a statistical analysis to extract quantities that can be compared with the x-ray results. A considerable number of Bragg maxima were observed in XSR. The vertical correlation length LZ was calculated from the angular width of Bragg maxima along 2θ and was found to vary with pH value. It follows, in general, the tendency of the domain sizes, being maximum at pH=4.8 and minimum at pH=7.0, respectively. The lateral correlation length LX has been evaluated via XDS from the half widths Δω measured by rocking the sample across a fixed 2θ. It decreases for increasing pH. LX was compared with the respective quantity of the AFM analysis. Estimated by XDS, the correlation lengths for inner interfaces and domains complement one another with the lateral length scales resulting from AFM analysis of the surface.