Langmuir-Blodgett Multilayers Research Articles

X-ray specular reflectivity and grazing incidence X-ray diffraction of new Langmuir-Blodgett multilayers

In this work we report on a structural study performed by specular reflectivity, diffuse scattering and grazing angle X-ray diffraction measurements of new Langmuir-Blodgett multilayers of a perylene-3,4,9,10-tetracarboxyldiimide derivative (PTCDI-Opent) and of a long chain-ammonium salt of 12-phosphomolybdate (PM12O 40 3− ). The periodic arrangement of the multilayers along the growth direction as well as the lateral crystalline in-plane ordering is investigated. Our results indicate a good structural quality of these complex organic molecules. Precisely, the specular reflectivity spectra exhibit very well defined Kiessig fringes implying a uniform thickness of both the investigated structures. Moreover, both films show correlated interfaces and a lateral structural arrangement of the multilayers in domains (70–140 A). A fairly good structural ordering within these domains is found.

Preparation and characterization of mixed LB films of polyaniline and cadmium arachidate

Langmuir isotherms of mixed monolayers of polyaniline and cadmium arachidate have been studied, and mixed monolayers have been transferred uniformly onto substrates as Langmuir-Blodgett (LB) multilayers. The LB films were characterized by XRD, UV-Vis and FTIR spectroscopy, dc conductivity and cyclic voltammetric measurements. The FTIR results suggest that the films contain arachidate salt, and the XRD results indicate the presence of a layered structure in the mixed LB films.

Stabilisation of fatty acid mono- and multilayers by simultaneous polyelectrolyte complexation and salt formation with cadmium ions

The complexation of highly ordered fatty acid monolayers with polyelectrolytes is expected to yield well-ordered Langmuir films suitable for the formation of Langmuir-Blodgett multilayers with improved long-term stability. Studies of the surface pressure-area isotherms and of the surface potential kinetics yield detailed information regarding the influence of these polymeric counterions on the monolayer properties. The injection of bivalent metal salts into the subphase after the complexation was used to improve the order and stability of the mono- and multilayers. The corresponding Langmuir-Blodgett films were investigated by means of X-ray reflectivity measurements and scanning force microscopy. The polyion complex multilayers show a strongly increased mechanical stability compared with films of fatty acid salts formed with bivalent metal ions. These structures are expected to be suitable as ultrathin separation layer for gas separation or ultrafiltration membranes.

Supramolecular structures formed from heterocyclic aromatic molecules

This paper describes the formation and structure investigation of Langmuir monolayers and Langmuir-Blodgett multilayers formed from amphiphilic derivatives of 2,5-diphenyl-1,3,4-oxadiazole. The 2,5-diphenyl-1,3,4-oxadiazole group as a functional unit with interesting physical and chemical properties is maintained, while the head group, the length of the alkyl chain and the structure of the coupling unit between aromatic and aliphatic part of these linear short-chain amphiphiles is systematically varied in order to explore the influence of this change on the film forming properties and the stability of Langmuir and Langmuir-Blodgett films. Molecular mechanics simulations are shown by these systematic variations to be suitable for the prediction of optimal chemical structures allowing for a stable stratified molecular packing. The combination of a detailed structure investigation of the multilayers based on scanning force microscopy and X-ray data with molecular mechanics simulations yields an insight into the packing of the molecules and the intermolecular interactions.

Electrical Rectification by a Molecule of Hexadecylquinolinium Tricyanoquinodimethanide

AbstractThe asymmetrical forward versus reverse - bias DC electrical conductivity (macroscopic and also nanoscopic) through Langmuir-Blodgett multilayers and monolayers of γ-(nhexadecyl) quinolinum tricyanoquinodimethanide, C16H33Q-3CNQ (1) is attributable to rectification of electrical current by a single molecule.

Dynamics of deposition of Langmuir–Blodgett multilayers

The process of Langmuir–Blodgett deposition is considered from the viewpoint of (de)wetting dynamics. Two models for the description of the dependence of the dynamic contact angle during deposition on the three-phase contact-line velocity are considered: a purely hydrodynamic model and a combined molecular–hydrodynamic one. The latter takes into account both viscous dissipation and the ‘molecular-kinetic’ type of dissipation caused by adsorption in the immediate vicinity of the moving contact line. Comparison with the available experimental data shows that the combined model satisfactorily describes the dependence of the receding dynamic angle on the contact-line velocity, whereas the purely hydrodynamic model alone fails in this respect. The analysis of the energy dissipated during the contact-line motion shows that, in the systems under consideration, the motion of the contact line is hindered mainly by the ‘molecular-kinetic’ type of friction whereas viscous drag becomes important at small values of the dynamic contact angle, i.e. when the contact-line velocity is close to the critical velocity of liquid film entrainment.

Photoconductivity of Langmuir-Blodgett films of corbathiene

Conductivity and photoconductivity properties of multilayer Langmuir-Blodgett (LB) films of corbathiene ( ∼ 100 nm thick) have been investigated. The conductivity exhibits anisotropic behaviour, i.e. it is different for lateral and transverse directions. Measurements of lateral photoconductivity and its dependence on wavelength and intensity of exciting light have been performed. The time-resolved decay of the photoinduced conductivity is indicative of more than one recombination mechanism. The results are related to the optical absorption features and discussed in terms of semiconducting concepts. Lateral photoconductivity has been found to be essentially a bulk property of the films. All data obtained arc well reproducible which means that the LB corbathiene films arc stable at room temperature in the current density range investigated ( 10 −7 to 10 −6 A cm −2) when subjected to illumination in the wavelength range 300–900 nm (10 16–10 17 photons cm −2 s −1).

Langmuir-Blodgett monolayers and multilayers of gramicidin A and lipids as membrane-mimetic models

Monolayers and multilayers of Gramicidin A (channel-forming protein in biological membranes and its mixtures with methyl stearate and ethyl palmitate were studied in order to prepare mimetic membranes for the selective ion transport using Langmuir-Blodgett (LB) films. Monolayers of Gramicidin A were spread at different temperatures and on different subphases containing Na +, K + and Ca 2+ ions, and ions were investigated using FT-IR-ATR spectra of LB multilayers to detect conformational changes at the interface. The dependence of the values of molecular areas and the collapse pressure as a function of molar fractions of Gramicidin A in mixtures with methyl stearate and ethyl palmitate, proved that the components showed complete and ideal miscibility. The FT-IR-ATR spectra of LB monolayers and multilayers of these mixtures were also reported.

Etude de la conformation des acides n-nonanoique, Z et E-9 octadecenoiques à 90 K par spectrométries infrarouge et Raman II — Etude de la conformation des chaines hydrocarbonées des acides Z et E-9 octadecenoiques à 90 K par spectrométrie infrarouge et Raman

A vibrational and attendant conformational analysis, from Raman and infrared spectra measured in the ranges 0–4000 cm −1 and 200–4000 cm −1 respectively, of solid oleic acid in the γ form and elaidic acids, i.e. Z and E-9 configurations of octadecenoic acid, is presented at 90 K and for comparison at 300 K for elaidic acid. The methylenic and skeletal vibration bands of the alkyl chains of these acids are interpreted using the frequency phase difference relationships of polyethylene and n-paraffins. Study of the vibrations more sensitive to chain conformational changes (CC stretching (1160-950 cm −1), δCCC deformation (particularly LAM1-3; range below 600 cm −1) and rocking-twisting deformation (1050-720 cm −1)) leads to the following results. In the two solid compounds, the dimeric carboxylic group has C i local symmetry and couples with the methylenic vibrational modes of the two nine-carbon chains located at both sides. Moreover, the CH 2 modes of the two alkyl segments situated between the double bond and the methyl group are observed. For oleic acid, the skeleton vibrations of the overall C 18 central pseudo-paraffinic and of the C 9 methylated segments are observed. In elaïdic acid we found that the skeletal modes involve the overall C 36 dimeric segment, as for stearic acid in the C form. The observed intramolecular coupling between the vibrational skeletal modes of the two monomeric chains and the in-plane modes of the hydrogen bonded dimer δ(OH⋯O) and ν(O-O) is responsible for the behaviour of the acids. This fact explains why amphiphilic oleic or elaïdic molecules joined together in ordered packing with a sufficient degree of fluidity are suitable for forming myelinic tubes or Langmuir-Blodgett multilayers which are models of biological membranes. Care must be taken in the vibrational analysis used to determine the chain conformations of phospholipid bilayers involving the olefin group, the all-trans lengths being sensitive to the hydrogen bond and to the nature of the carboxylic group.

Molecular packing in new Langmuir-Blodgett systems investigated by X-ray specular reflectivity and grazing incidence X-ray diffraction

In this work we investigate the periodic arrangementand crystalline order of Langmuir-Blodgett multilayers of a perylene-3,4,9,10-tetracarboxyldimide derivative (PTCDI-Opent) and of a long chain ammonium salt of 12-phosphomolybdate (PMo 12O 40 3−) deposited on (111)-oriented Si substrates by X-ray specular reflectivity (XSR) and grazing incidence X-ray diffraction measurements (GIXD). This is the first experimental study of the structural order performed on these complex compounds. Our results show that both films are quite uniform in thickness as demonstrated by well defined Kiessig fringes collected in the XSR spectra. The presence of sharp specular Bragg peaks in the XSR measurements of the PMo 12O 40 3− film indicates a very good crystalline order in the growth direction. For the PTCDI-Opent structure, an unexpected molecular reorganization in the film along the growth direction is found if compared to the molecular conformation at the air-water interface and the transfer ratios. Both films can be identified as monophasic structures. Moreover, the in-plane structural arrangement derived by GIXD shows domains of 70–140 Å lateral size.

The domain structure of LB multilayers prepared from fatty acid salts

Langmuir-Blodgett (LB) multilayers from fatty acid salts can be characterized by finite sized scattering aggregates. High-resolution atomic force microscopy maps reveal a substructure of molecular ordering. These blocks are to be considered as 3D domains. Furthermore, macroscopic holes and scratches appear at the surface. These defect structures have to be taken into account when interpreting observed X-ray and neutron reflectivity curves. The density loss due to the holes can be detected from the angular shift of the critical angle of total external reflection. Offdiagonal scans demonstrate a slight inclination of domains relative to each other. Neutron reflectivity patterns of sequentially deuterated and hydrogenated carbon chains provide information about the disorder of the organic chains. In the second part of the paper a structure model of the molecular order within the domains is developed using 16 in-plane Bragg peaks and their truncation rods measured by X-ray grazing incidence diffraction. The refined 3D unit cell contains at least four monolayers in a herringbone arrangement of the hydrocarbon chains. Its stability is given by the inter-chain coupling between two neighbouring monolayers.

Interfacial behaviour and mesomorphic properties of triazine-modified triphenylene oligomers

The supramolecular structures of new functional discotic oligomers formed on the water surface, within Langmuir-Blodgett (LB) multilayers and in the mesomorphic bulk state are described. The novel discotic oligomers are composed of triphenylene side groups attached to amino-substituted 1,3,5-triazine units in the main chain. The oligomeric triphenylenes form stable monofilms at the air-water interface with an edge-on orientation of the hydrophobic triphenylene side groups. LB multilayers are characterized by an edge-on bilayer packing with interdigitation of discotic side groups of adjacent bilayers. An unusual lamellar layer arrangement due to hydrogen bonding between the backbones is found for the bulk phase.

Langmuir films and Langmuir-Blodgett multilayers incorporating mechanically-threaded molecules-pseudorotaxanes

This paper describes the synthesis of a π-electron rich aromatic lipid containing a polyether thread intercepted by a 1,5-dioxynaphthalene moiety. The terminus of one end of the polyether is a methoxyl group and the other a glycerol unit linked via the 2-oxo position, while the 1,3-oxo functions are covalently attached to octadecyl chains. This compound, when cospread at the air-water interface with the tetracationic cyclophane, cyclobis(paraquat- p-phenylene), and the phospholipid, dimyristoylphosphatidic acid, as its monoanion, forms self-assembled pseudorotaxanes which are self-organized into a two-dimensional supramolecular array at the interface. The electrostatic interactions between the tetracationic cyclophane and the monoanions of the phospholipid anchor the cyclophane to the interface. This anchoring allows the naphthalene moiety to remain threaded through the cavity of the cyclophane by a combination of charge-transfer and hydrogen-bonding interactions. These Langmuir films can, subsequently, be transferred to a quartz support to fabricate Langmuir-Blodgett multilayers. In characterising these self-organized systems, surface pressure-area isotherms and surface potential measurements have been used in conjunction with fluorescence and UV-Vis spectroscopies. This research may have implications in sensor technology and molecular information storage devices.

Preparation and characterization of 14-[3,6-(didodecyl)- N-carbazolyl]tetradeca-10,12-diynoic acid LB multilayers

An amphiphilic diacetylene monomer containing an alkyl-substituted carbazole side group has been synthesized with the purpose of combining the electronic properties of polydiacetylenes with the charge-transporting properties of carbazole arrays. Langmuir-Blodgett multilayers have been prepared and characterized. The monomer film was polymerized by irradiation at 254 nm. X-ray diffraction revealed the presence of ordered structures with an interlayer spacing of about 63 Å in both the monomer and polymer films. A preferred orientation of the polymeric backbone parallel to the dipping direction was inferred from the dichroic ratio observed in the UV-Vis spectra in polarized light. The relevant anisotropic effect measured in this work is peculiar of this polymer, hence it is probably related to the stiffness of the alkylsubstituted carbazole ring. Investigations with scanning electron microscopy revealed a uniform thickness of the sample with smooth surface and an interlayer spacing in agreement with X-ray results.

Characterization of monolayers and LB multilayers of dioctadecyldimethylammonium chloride

Pressure-area ( π- A) isotherms were obtained from dioctadecyldimethylammonium chloride (DODAC) monolayers at the air-water interface for a range of subphase salt concentrations. Simultaneous surface potential measurements confirmed the important role of electrostatic repulsion between ionized headgroups in controlling the liquid-expanded to liquid-condensed phase transition. The extrapolated area per molecule measured in the condensed phase, 0.58 nm 2, was in good agreement with previously reported X-ray diffraction measurements on hydrated crystals. The Langmuir-Blodgett (LB) deposition of uniform monolayers of DODAC onto aluminium and gold substrates was investigated by surface potential measurements, phase contrast microscopy and surface plasmon resonance microscopy. Atomic force microscopy confirmed the regular molecular order of the deposited monolayer, yielding a spacing of approximately 0.46 nm between rows of adjacent molecules.

Photo-orientation in LB multilayers of amphotropic polymers

Amphotropic polymers with photochromic azobenzene side groups like 2-hydroxyethyl polyacrylates and “hairy rod” polyglutamates combine amphiphilic and thermotropic properties. Upon UV irradiation the initial double layer structures were irreversibly destroyed by the E–Z photoisomerization. New ordered structures with modified spacing, in-plane anisotropy and surface topology were generated by a subsequent Vis irradiation step. Optical anisotropy was modified due to a photo-orientation process on irradiation with linearly polarized Vis light. Relations between photoinduced modification and thermotropic self-organization are demonstrated for Langmuir-Blodgett (LB) layers of both types of polymers. The dependence of the photoinduced reorientation process on the structure, supramolecular order and the aggregation of the azobenzene moieties is shown for the different ordered LB systems.

New ways for the preparation of LB multilayers of polymeric metal complexes

Two routes to obtain well-ordered Langmuir-Blodgett (LB) films from polymeric metal complexes were evaluated. Due to the chemical and structural diversity of metal complexes the methods have to work with different complexes. Two different ways have been investigated. Firstly, ligand functions were introduced in amphiphilic maleic acid copolymers. To these polymers a lot of coordinatively unsaturated precursor complexes may be attached. The complexes' geometries have a strong influence on their orientation in the multilayers. Secondly, bifunctional amphiphilic ligands on the basis of methylene-bis-salicylidene hexadecylamine are able to form linear coordination polymers with bivalent metal cations. As in the first case it has been proved to be the best way to form the polymeric metal complexes directly at the water surface in an interfacial reaction between the dissolved metal component and the spread amphiphilic ligand or polymer, respectively. The formation of the polymeric complexes was studied by appropriate monolayer experiments and LB layers were investigated by spectroscopic methods.

Charge carrier trapping states in thin evaporated and Langmuir-Blodgett organic films

Thermally modulated space-charge-limited current (TM-SCLC) and conventional SCLC methods have been used to determine the energy parameters of the local charge trapping states for three heterocyclic organic compounds in two different structural states — as thin vacuum-evaporated layers and Langmuir-Blodgett (LB) multilayer films. The TM-SCLC method provides higher resolution and accuracy of trap depth (Et) determination with error not greater than ± 0.02 eV. In sandwich-type samples Me/organic layer/Me of metal-free phthalocyanine (α-H2Pc), five sets of hole trapping states are determined in the energy range from 0.55 to 0.07 eV; for dimethylamino-benzylidene indandione-1,3 (DMABI) samples also, five sets of trapping states are detected in the 0.5–0.28 eV range; however, for indandione pyridinium betaine-1,3 (IPB) samples, only a very shallow set of traps at Et ≈︂ kT emerged. In the case of evaporated layers the conductivity activation energy (Ea) dependence on voltage energy (U), Ea = Ea(U), decreases in a step-like manner, every step giving the trap depth Et of the corresponding set of trapping states. In the case of LB multilayers of vanadyl phthalocyanine (VOPc) and IPB films the Ea = Ea(U) dependences are of complex, oscillatory-type character, possibly caused by a spatially non-uniform trap distribution in the bulk of the sample.

Structural characterization of Langmuir-Blodgett films of nickel complexes with amphiphilic compounds

Spreading monolayers and Langmuir-Blodgett (LB) multilayers are appropriate supramolecular structures for molecular recognition. Specifically oriented ligands in the head group of the amphiphiles in the monolayer can selectively interact with the ions to be detected in solution. We investigated spreading monolayers of dioctadecyldithiocarbamate (DODTC) and nonadecylpiridyne (NDP), on subphases containing transition metal ions, by means of π- and Δ V-area isotherms. UV-Vis spectroscopy absorption and electron spectroscopy for chemical analysis measurements performed on LB films of DODTC and NDP transferred from subphases containing nickel ions indicated that the Ni ions combine with the monolayer to form stable adducts. The structure of the resulting LB films were further characterized using X-ray diffraction and atomic force microscopy. Both experiments indicated that in the presence of the transition metal ion there is a drastic variation in the structural organization of the monolayers and in the morphology of the LB films.

Low-temperature plasma treatment of magnesium stearate Langmuir-Blodgett films

Ultrathin Langmuir-Blodgett (LB) multilayers have been the focus of interest for use as gas separation membranes because of their well-defined thickness and defect-free structure. A considerable drawback from a technological point of view is the low thermal, chemical and mechanical stability of these films. One solution to this problem is the conversion of an organic multilayer structure, containing inorganic ions, to an organo-ceramic ultrathin film by low-temperature plasma treatment. This article describes the interfacial behaviour and the deposition of magnesium stearate monolayers and their subsequent conversion to organo-ceramic structures. X-ray, FTIR and AFM measurements were used to characterize the Langmuir-Blodgett (LB) multilayer structures, chemical compositions and surface topographics respectively: The LB-films consist of highly ordered Y-type multilayer structures with a stepwise surface height profile and nearly all amphiphiles are complexed as the magnesium salt. The modification of these LB-films by both argon and oxygen plasma leads to a decrease in the content of organic material by physical ablation and chemical etching processes. Whilst an internal order is retained within the plasma-treated films, the surface roughness increases significantly. Two parameters of the plasma process, the radio frequency power and the exposure time, were varied in order to find suitable process conditions. It is shown that the non-uniform multilayer structure can be converted to a uniform grainy organo-ceramic structure by plasma treatment.