Structure Of Langmuir-Blodgett Films Research Articles

Dispersion control by using a bulky surfactant medium in the LB films for the enhancement of linearly polarized luminescence of Eu complexes

Abstract The ff transition of lanthanide ions such as Eu in the complex appears as unique energy relaxation process as luminescence after the intramolecular energy transfer from the photo-excitation of the ligand with π-electronic system. The Langmuir-Blodgett (LB) film consisting of Eu and amphiphilic naphtoic acids (NaphC15) induced the linearly polarized luminescence (LPL) with a medium, stearic acid (SA), to isolate the luminescence component by the layer structure and to keep the softness at the monolayer depositions on the quartz substrate as reported previously. Here, we used a bulky medium, 3-methyl-4-penthadecyl benzoic acid (MPBA), instead of SA, and found the Eu-NaphC15 in the LB film places more independently resulting in the 4-fold high quantum yields compared with previous system. The structures of LB films of Eu-NaphC15 with MPBA were elucidated by the measurement of synchrotron XRD and XPS. Various electronic spectra were quantitatively observed to discuss the effect molecular aggregation through much/less ππ-interaction of NaphC15 for the intensified LPL in the films.

Modulated Fluorescence in LB Films Based on DADQs—A Potential Sensing Surface?

Novel fluorescent Langmuir-Blodgett (LB) films have been constructed from three different amphiphilic dicynaoquinodimethanes (DADQs). The DADQs varied in functional group structure, which had an impact on the LB film structure and the fluorescence properties. As the fluorescence of DADQs competes with non-radiative decay (conformational change), the packing and/or free volume in the LB film will influence the average fluorescence lifetime and integrated intensity. The pristine (blank) LB films were then exposed to a selection of non-fluorescent target analytes (some with environmental relevance) and the fluorescence was measured and analyzed relative to the pristine LB film. Exposure of the LB films to selected target analytes results in a modulation of the fluorescence, both with respect to average fluorescence lifetime and integrated intensity. The modulation of the fluorescence is different for different DADQ LB films and can be attributed to restricted non-radiative decays or charge transfer reactions between target analyte and DADQ LB film. The response from the DADQ LB films shows that these systems can be developed into sensing surfaces based on fluorescence measurements.

Characterization of modified solid electrodes with organized thin films of a tetra-substituted zinc phthalocyanine

Organized thin films of a zinc phthalocyanine have been deposited on solid electrodes using several techniques, such as drop casting, immersion and Langmuir-Blodgett (LB), at several conditions, and their electrochemical response has been studied by cyclic voltammetry and differential pulse voltammetry. Topographical images by Atomic Force Microscopy reveal that the structure of immersion and LB films is uniform, showing granular aggregates and continuous layers, respectively. The electrochemical response of both kind of films is similar and shows peaks that can be correlated with redox processes occurring in the phthalocyanine ring. Drop casting films present a different topography, with rod-like structures, and the voltammograms show an enhancement in the peaks at positive potentials, which has been attributed to the influence of the atmospheric oxygen adsorbed during the film deposition. Immersion and LB films have also shown an increase in the oxidation current intensity of the voltammetric response after exposition to molecular oxygen. These results point to a possible application of these organized t-BuZnPc thin films as electrochemical oxygen sensors.

Axial ligands as interplanar spacers in macrocyclic solids

2,9,16,23-Tetra(neopentoxy)phthakwyaninato of cobalt ligand coordinated compounds of cyanide, pyridine, 4-cyanopyridine, pyrazine and 4,4'-bipyridyl, are isolated as solid powders and correlated to Langmuir-Blodgett film structure and compared to thermally induced film-surface ligand coordination processes.

Angular birefringence phase matching behaviour of Lithium Triborate (LBO) crystal for second harmonic generation in broad wavelength range

Second Harmonic Generation (SHG) is a nonlinear optical process in which the frequency of the incident radiation gets doubled. There are various types of applications of SHG such as ultra-short pulse measurement, high resolution optical microscopy, characterization of the structure of 2D materials and its physical properties like the thickness of interlayer stacking, stacking angle between different layers, the grain boundary etc. It has also been used to determine the structure of Langmuir-Blodgett films of low symmetry. For SHG, various commercial nonlinear optical (NLO) crystals are generally used. Lithium Triborate (LBO) is one of the best NLO material among them due to its wide transparency range (160–2600 nm), high damage threshold (19 GW/cm2 @1064 nm, 1 ns) and high second order nonlinear coefficient. It has also the ability to establish birefringence phase matching condition for efficient SHG for different wavelengths. The crystal can be used for generation for SHG in broad wavelength range through angular phase matching method. For this, in this work detail investigation has been made to find out the optimum wavelength range that can be used for efficient SHG. This has been done by finding the variation of phase matching angle, second order nonlinearity etc. with respect to wavelength using the SNLO software. Both Type I and Type II phase matching has been considered for various planes of the crystal.

Langmuir monolayers and Langmuir-Blodgett films of pH-sensitive lipid

Colloidal membranous vesicles based on the functionalized liposomes are promising for development of novel efficient systems of controlled and stimuli-triggered drug delivery. In particular, the specific metabolism of cancer cells results in significant acidification in tumors, and thus the corresponding pH value changes can be used as a stimulus for drug release specifically in the tumor areas. pH-sensitive liposomes can be formed via incorporation of special pH-sensitive amphiphilic compounds into the liposomal membrane. Molecules of such compounds undergo a conformational transition caused by certain changes in the pH value of aqueous environment and resulting in corresponding membrane perturbations with rapid leakage of drug content from those liposomes into the areas with corresponding acidity. In the present work the pH-induced changes of Langmuir monolayer compression isotherm and of the structure of corresponding Langmuir-Blodgett films of pH-sensitive synthetic lipid trans-4,5,didodecyloxycarbonyl-trans-2-morpholinocyclohexanol have been observed and studied. The substantial expansion of Langmuir monolayer of that lipid caused by change of the aqueous subphase pH value from 7 to 5 was observed. Corresponding monolayer Langmuir-Blodgett films were deposited on mica substrate surface and studied using AFM technique. The substantial differences in the structure of Langmuir-Blodgett films deposited at pH values 7 and 5 were found. The observed effects directly demonstrate the pH-induced structural transformations in organized condensed planar molecular systems formed by pH-sensitive lipid molecules which underlie the pH-sensitivity of corresponding liposomal bilayer membranes containing such lipids.

Preparation of Langmuir–Blodgett thin films of calix[6]arenes and p-tert butyl group effect on their gas sensing properties

Organic vapor sensing properties of Langmuir–Blodgett (LB) thin films of p-tert-butyl calix[6]arene and calix[6]arene, and their certain characterization are reported in this work. LB films of these calixarenes have been characterized by contact angle measurement, quartz crystal microbalance (QCM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). QCM system was used for the measurement of sensor response against chloroform, benzene, toluene and ethanol vapors. Forming of stable monolayers was observed at the water surface using surface pressure–area isotherm graph. The results indicate that good quality, uniform LB films can be prepared with a transfer ratio of over 0.95. Due to the adsorption of vapors into the LB film structures; they yield a response to all vapors as of large, fast, and reproducible.

Partially induced transition from horizontal to vertical orientation of helical peptides at the air–water interface and the structure of their monolayers transferred on the solid substrates

To apply the Langmuir–Blodgett (LB) technique as a platform for investigating the fundamental properties of amphiphilic peptides (APs), we have investigated the structure of LB films using the APs. To vertically orient the helical APs like transmembrane proteins in the membrane, the primary structure of the APs was designed to have two domains: a hydrophilic domain (three amino acids) and a hydrophobic domain (ca. 20 amino acids). However, we are still far from having full control of their orientation. This study reports the contribution of the subphase temperature to the change in the orientation of helical APs. When the surface pressure–area isotherm of AP was observed at the subphase temperature at 41.5°C, the isotherm exhibited a plateau, implying that a phase transition of the monolayer at the air–water interface occurred. Circular dichroism (CD) spectra of the monolayers transferred on the solid substrates revealed that the orientation of the helices changed at the pressure, where the plateau of the isotherm was observed. This change was not observed at 21.5°C, i.e., the horizontal alignment of helixes was maintained. Atomic force microscopy (AFM) was used to systematically investigate the surface structure of the monolayers transferred at different surface pressures. A structural model of the monolayer that did not contradict with the results obtained by the three different techniques (the isotherm, CD spectroscopy, and AFM) was derived, and it was concluded that the horizontally oriented helices partially changed their orientation to vertical upon compression in the plateau region of the isotherm.

Preparation and gas sensing properties of Langmuir–Blodgett thin films of calix[n]arenes: Investigation of cavity effect

Characterization and organic vapor sensing properties of Langmuir–Blodgett (LB) thin films of calix[n]arene (n=4, 6, 8) derivatives are reported in this work. Surface pressure–area isotherm graph shows that very stable monolayers are formed at the water surface. The results indicate that good quality, uniform LB films can be prepared with a transfer ratio of over 0.95. Calix[n]arene LB films have been characterized by contact angle measurements, quartz crystal microbalance (QCM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). LB film of calix[8]arene which has the largest cavity yields a gradient with a mass value of 773ng per layer according to the QCM results. AFM and SEM images showed a dense surface morphology obtained for all samples. QCM system was used for the measurement of sensor response against chloroform, benzene, toluene and ethanol vapors. These LB film samples yield a response to all vapors with a large, fast, and reproducible due to the adsorption of vapors into the LB film structures. Among them, calix[8]arene LB film has higher sensitivity toward the organic vapors because of a large cavity size. This study can be concluded that the cavity size of calix[n]arene molecule could have an important role in the research area of room temperature vapor sensing devices.

Electrical characterisation of stearic acid/calix[4]amine Langmuir–Blodgett thin film

Within this work we deposited 16 monolayers of stearic acid alternated with 15 monolayers of calix[4]amine to form a non-centrosymmetric Langmuir–Blodgett (LB) thin film onto an aluminized (50 nm coated) glass microscopic slide. Dielectric constant and dielectric loss for the film were determined using C-f and tan (δ-f) measurements. The value of the pyroelectric figure of merit was determined as 1.73 μC m−2 K−1. To elucidate the conduction mechanism of stearic acid/calix[4]amine LB film, DC current–voltage measurements between −4 and +4 V were carried out. The I(V) behaviour shows a symmetrical and highly non-linear behaviour. Analysis of this behaviour of the stearic acid/calix[4]amine LB film showed a conductivity value of 1.12 × 10−13 S m−1 for ohmic region. The exponential part of I(V) dependence obeyed the Schottky conduction mechanism with a barrier height of 1.67 eV. This LB film structure shows a typical insulating behaviour for low voltage values and the Schottky effect becomes dominant when the voltage increases. The frequency dependence of conductivity shows a power law relationship between conductance and frequency.

Study of an ethylene oxide-terminated bent–core compound: Synthesis and Langmuir–Blodgett film structure

Bent–core compounds have attracted interest due to their unusual supramolecular structures, uncommon physical properties such as ferro- and antiferroelectricity and potential applications in fields such as nonlinear optics. Their incorporation into nanostructured materials, however, needs to be improved in terms of accurate control of the packing and orientation of the molecules in practical structures. Here, we have synthesized a novel bent–core compound bearing a tetraethylene glycol (TEG) chain as the hydrophilic head group and studied its capacity to obtain monomolecular films by means of the Langmuir–Blodgett technique. We developed a synthetic route to the material and studied its behavior in Langmuir and Langmuir–Blodgett films by means of a variety of characterization techniques, including a new model for the interpretation of UV–Vis reflection spectra of bent–core compounds. We found that the new head group, while destroying the formation of bulk mesophases, stabilizes the formation of the monolayer at the air–water interface and allows core–core interactions to dominate film dynamics, thus providing a promising alternative to carboxylic acid head groups.

Supramolecular structure formation of Langmuir-Blodgett films of comblike precursor and polyimide

The surface structure of Langmuir-Blodgett films of a comblike polyimide precursor—a rigid-chain polyamic acid alkylamine salt bearing multichains of tertiary amine—and films of the corresponding polyimide were studied by atomic force microscopy (AFM). An analysis of the images of the surface of three-layer films revealed a domain structure. It was found that the Langmuir-Blodgett film formation of the precursor occurs as a result of the layer-by-layer deposition of two-dimensional domains (composed of polyamic acid salt molecules on the water surface) onto a substrate. The formation of domains in a monolayer is associated with the chemical structure of the precursor, to be more precise, with the rigidity of the main chain and the presence of closely spaced aliphatic side chains in the polymer chain, whose total cross-section area is close to the surface area of the projection onto the plane of the repeating unit of the main chain. Polyimide films inherit the domain structure of the precursor films; the inhomogeneity of the film thickness substantially decreases, whereas the domain size and character of their distribution in the film remain unchanged.

Interfacial assembly and characterization of hybrid ultrathin films of manganese 5,10,15,20-tetra (4-pyridyl) porphine chloride tetrakis (methochloride) and its polymeric derivative with poly (4-vinylpyridine)

Monolayers of a water-soluble metalloporphyrin, manganese tetra-(4-pyridyl) porphine chloride tetrakis (methochloride) (MnTMPyP), and its polymeric derivative with poly (4-vinylpyridine) (MnTMPyP-PVP) have been investigated at the air–water interface. These monolayers were transferred on the substrate surfaces to form hybrid ultrathin films by the Langmuir-Blodgett (LB) method. It was revealed that, although the water-soluble MnTMPyP could not form insoluble monomolecular layer on the pure water surface, it could be stabilized on the NaB(C6H5)4 subphase surface due to an electrostatic interaction between MnTMPyP and B(C6H5)4−, resulting in the formation of MnTMPyP-B(C6H5)4 hybrids. On the other hand, its polymeric derivative of MnTMPyP-PVP could form stable insoluble monolayers on the pure water, NaB(C6H5)4 and Na2PdCl4 subphase surfaces. Composition, structure and microscopic morphology of the as-prepared LB films were characterized by using UV–vis absorption, infrared and X-ray photoelectron spectroscope, as well as scanning electron microscope. The results suggested formation of the hybrid MnTMPyP-B(C6H5)4 and MnTMPyP-(PVP-)B(C6H5)4 and Pd-MnTMPyP-PVP coordination polymer ultrathin films at the interfaces. Cyclic voltammograms of the LB films revealed two couples of reversible redox waves centered at 0.2–0.3 and −0.1 to −0.2V (vs Ag/AgCl), corresponding to the Mn(II)/Mn(III)TMPyP-(PVP-)B(C6H5)4 and Mn(III)/Mn(IV)TMPyP-(PVP-)B(C6H5)4 electron transfer processes of the manganese porphyrins in the LB films.

Graphene Oxide as a Monoatomic Blocking Layer

Monolayer graphene oxide (mGO) is shown to effectively protect molecular thin films from reorganization and function as an atomically thin barrier for vapor-deposited Ti/Al metal top electrodes. Fragile organic Langmuir-Blodgett (LB) films of C(22) fatty acid cadmium salts (cadmium(II) behenate) were covered by a compressed mosaic LB film of mGO flakes. These hybrid LB films were examined with atomic force microscopy (AFM) and X-ray reflectivity, both with and without the metal top electrodes. While the AFM enabled surface and morphology analysis, the X-ray reflectivity allowed for a detailed structural depth profiling of the organic film and mGO layer below the metal top layers. The structure of the mGO-protected LB films was found to be perfectly preserved; in contrast, it has previously been shown that metal deposition completely destroys the first two LB layers of unprotected films. This study provides clear evidence of the efficient protection offered by a single atomic layer of GO.

Role of metal ions of Langmuir–Blodgett film in hydrophobic to hydrophilic transition of HF-treated Si surface

Hydrophobic to hydrophilic transition of HF-treated Si surface strongly depends upon the metal ions, which are present in the headgroups of the deposited Langmuir–Blodgett (LB) film. Structure of LB films studied by X-ray reflectivity technique and chemical analysis of LB film–substrate interfaces studied by X-ray photoelectron spectroscopy combinedly suggest that the partial transition or partial oxidation of the HF-treated Si surface takes place under the subphase water but further transition or oxidation is possible only in the presence of metal ions. Electrovalent and covalent natures of the metal ions tune this transition or oxidation. Ni ions, for which bonding with headgroups are electrovalent in nature, are favorable for such transition/oxidation and as a result, complete transition/oxidation takes place when nickel arachidate LB film is deposited. On the other hand, Cd ions, for which bonding with headgroups show covalent nature, is not favorable for such transition and can not oxidize the underlying H-passivated Si substrate totally when cadmium arachidate LB film is deposited on such HF-treated Si surface. This ion-specific hydrophobic to hydrophilic transition is visualized by X-ray reflectivity, contact angle and X-ray photoelectron spectroscopy measurements.

Effect of subphase temperature on the phase-separated structures of mixed Langmuir and Langmuir–Blodgett films of fatty acids and hybrid carboxylic acids

We examined the phase-separated structures of the mixed Langmuir–Blodgett (LB) films of C k H 2 k +1COOH (H kA: k = 17, 21) and C m F 2 m +1C 10H 20COOH (F mH10A: m = 6, 8) fabricated isothermally or after isobaric thermal treatments. Under isothermal fabrication conditions, disks and wire-type domains formed in the H17A/F6H10A LB films at high and low fabrication temperatures, respectively, because the line tension and dipole–dipole interaction were comparable with each other. The thermodynamically stable phases of H21A/F6H10A LB films at high and low fabrication temperatures were disks and polygonal domains, respectively. Isobaric thermal treatments of the Langmuir films affected the domain size and not the domain shape when the transfer temperature was the same. Isobaric thermal treatments were effective in controlling the domain size. The thermodynamically stable phases of both H17A/F8H10A LB films and H21A/F8H10A LB films were nanowires in the range of the fabrication temperatures studied. Isobaric thermal treatments of the Langmuir films did not affect the domain shape significantly and affected the domain size in both the LB films studied. The change in the value m of F mH10A was more effective in controlling the phase-separated structures of the mixed LB films than the value of k of H kA.

Growth and stability of Langmuir-Blodgett films on OH-, H-, or Br-terminated Si(001)

Growth of Langmuir-Blodgett (LB) films of nickel arachidate (NiA) on differently terminated (OH-, H-, or Br-terminated) Si(001) substrates and their structural evolution with time have been investigated by x-ray reflectivity technique and complemented by atomic force microscopy. Stable and strongly attached asymmetric monolayer (AML) of NiA is found to grow on freshly prepared oxide-covered Si substrate while unstable and weakly attached symmetric monolayer (SML) of NiA grows on H-terminated Si substrate, corresponding to stable hydrophilic and unstable hydrophobic natures of the substrates, respectively. The structure of LB film on Br-terminated Si substrate, however, shows intermediate behavior, namely, both AML and SML are present on the substrate, indicative of coexisting (hydrophilic and hydrophobic) nature of this terminated surface. Such coexisting nature of the substrate shows unusual growth behavior of LB films: (i) hydrophilic and hydrophobic attachments of NiA molecules in single up stroke of deposition and (ii) growth of few ring-shaped large-heights islands in subsequent deposition. These probably occur due to the presence of substrate-induced perturbation in the Langmuir monolayer and release of initially accumulated strain in the film structures near hydrophilic/hydrophobic interface, respectively, and provide the possibility to grow desired structures (AML or SML) of LB films by passivation-selective surface engineering.

Surface structure of Langmuir–Blodgett films of lipophilic guanosine derivatives

We studied surface organization of lipophilic guanosine derivatives with one, two and three alkanoyl tails in thin films on water surface and in Langmuir–Blodgett (LB) films transferred onto freshly cleaved mica substrate. The derivative with one alkanoyl group exhibits irreversible pressure–area isotherms and ribbon-like LB film structures. The derivatives with two and three side groups show reversible isotherms with similar critical surface pressures, while their LB film structures are quite different. The derivative with two tails forms films with flat terraces of micrometer size, while the derivative with three tails exhibits irregular thread-shaped assemblies. These observations demonstrate that molecular assembly of LB films of guanosine derivatives can be effectively manipulated by altering the number of attached lipophilic groups.

Self-assembled arrays of silica particles on the patterns reflecting the phase-separated structures of mixed Langmuir–Blodgett films

In this study, we constructed self-assembled arrays of silica particles on the micro- and nanopatterns of functionalized templates fabricated from phase-separated mixed Langmuir–Blodgett (LB) films. Electrostatic interaction between the carboxylic groups of silica particles and amino groups on the functionalized templates results in the formation of the self-assembled arrays of silica particles. The patterns of the silica particle arrays can be designed by controlling the phase-separated structures of the original mixed LB films.

Molecular dynamics simulations of sputtering of Langmuir-Blodgett multilayers by keV C(60) projectiles.

Coarse-grained molecular dynamics computer simulations are applied to investigate fundamental processes induced by an impact of keV C(60) projectile at an organic overlayer composed of long, well-organized linear molecules. The energy transfer pathways, sputtering yields, and the damage induced in the irradiated system, represented by a Langmuir-Blodgett (LB) multilayers composed from molecules of bariated arachidic acid, are investigated as a function of the kinetic energy and impact angle of the projectile and the thickness of the organic system. In particular, the unique challenges of depth profiling through a LB film vs. a more isotropic solid are discussed.The results indicate that the trajectories of projectile fragments and, consequently, the primary energy can be channeled by the geometrical structure of the overlayer. Although, a similar process is known from sputtering of single crystals by atomic projectiles, it has not been anticipated to occur during C(60) bombardment due to the large size of the projectile. An open and ordered molecular structure of LB films is responsible for such behavior. Both the extent of damage and the efficiency of sputtering depend on the kinetic energy, the impact angle, and the layer thickness. The results indicate that the best depth profiling conditions can be achieved with low-energy cluster projectiles irradiating the organic overlayer at large off-normal angles.