Blodgett Layers Research Articles

Frictional characteristics of molecular length ultra-thin boundary adsorbed films

The paper presents measurements of friction of any ultra-thin film entrained into the contact of a pair of very smooth specimen subjected to entrainment in a converging micro-wedge of a special-purpose micro-tribometer. An ultra-thin film is expected to form at the boundary solids through adsorption of boundary active molecules. Fluids with linear and branched molecules are used in the investigation. It is found that the frictional characteristics of these films can be adequately described through use of Eyring thermal activation energy and a potential energy barrier to sustain conjunctional sliding motion. The combined experimental measurement and the simple activation energy approach shows that the thin molecular adsorbed films act like hydro Langmuir–Blodgett layers, the formation and frictional characteristics of which are affected by the competing mechanisms of adsorption, forced molecular re-ordering and discrete-fashion drainage through the contact by the solvation effect. This process is a complex function of the contact sliding velocity as well as a defined Eyring activation density (packing density of the molecules within the conjunction). It is shown that the contribution of solvation to friction is in the form of energy expended to eject layers of lubricant out of the contact, which unlike the case of micro-scale hydrodynamic films, is not a function of the sliding velocity.

Tuning of electronic properties of fullerene-oligothiophene layers

Electronic properties of fullerene derivatives containing oligothiophene pendant chain (1–3 thiophene moieties) were investigated using the Kelvin probe technique and quantum chemistry methods. For electrochemical examination of these systems, Langmuir–Blodgett (LB) layers were prepared by the deposition on a gold substrate. The analysis of the experimental data shows that the value of the work function depends strongly on the length of oligothiophene chain. Similar dependence was also found for the surface photovoltage measurements conducted for the layers consisting of multiple LB films of the examined compounds deposited on gold surfaces. The assumption has been made that these changes are associated with the influence of oligothiophene chain on the electrostatic potential distribution near the surface of the sample. The hypothesis was confirmed by the results of DFT calculations, which revealed that the value of Fermi level energy shifts in the opposite direction to the determined work function. The key highlights of this study are as follows: electronic structure tuning by oligothiophene side chain; DFT calculation on fullerene-thiophene system; work function measurements of thin molecular layers.

Indium–chlorine and gallium–chlorine tetrasubstituted phthalocyanines in a bulk system, Langmuir monolayers and Langmuir–Blodgett nanolayers – Spectroscopic investigations

The paper deals with spectroscopic characterization of metallic phthalocyanines (Pc’s) (indium and gallium) complexed with chlorine and substituted with four benzyloxyphenoxy peripheral groups in bulk systems, 2D Langmuir monolayers and Langmuir–Blodgett nanolayers. An influence of the molecular structure of dyes (the presence of metal and of substitutes attached to the phthalocyanine macroring) on the in situ measurements of light absorption is reported. Molecular arrangement of the phthalocyanine molecular skeleton in the Langmuir monolayers on water substrate and in the Langmuir–Blodgett nanolayers is evaluated. A comparison of the light absorption spectra of the phthalocyanine monolayers with the spectra of the dyes in solution supports the existence of dye aggregates in the monolayer. It was shown that the type of dye aggregates (oblique and H types) depends markedly on the dye molecular structures. The NIR-IR, IR reflection–absorption and Raman spectra are also monitored for Langmuir–Blodgett nanolayers in non-polarized and polarized light. It was shown that the dye molecules in the Langmuir–Blodgett layers are oriented nearly vertically with respect to a gold substrate.

Spectroscopic properties and orientation of molecules in Langmuir–Blodgett layers of selected functionalized fullerenes

Vibrational properties of two fullerene derivatives: C60TZ-OT-5 (1) and C60TH-3HX (2) have been studied using infrared absorption and Raman scattering spectroscopies. Additionally, quantum chemical calculations of the equilibrium geometry and normal mode vibrations of these functionalized fullerenes were performed. It was stated that despite of distinct structural differences between the investigated molecules, their experimental spectra are quite similar and correspond well with the calculated ones. The orientation of the molecules in the Langmuir–Blodgett films was evaluated.

Estimation of contact angle for hydrophobic silica nanoparticles in their hexagonally ordered layer

Wetting properties of bilayered nanostructured coatings were studied in this work. Coatings were prepared by stratifying a compact silica sol–gel (SG) film and a Langmuir–Blodgett (LB) layer of differently sized (84, 131 and 227 nm) Stöber silica particles onto glass substrates. Joint high temperature annealing of the layers increased the mechanical stability of the particulate film facilitating its further investigations.Regular structure of LB layers allowed us to carry out wetting model investigations. Close-packed arrangement of particles was reinforced by optical measurements. Transmittance spectra of LB films were taken and evaluated by a theoretical model. Resulting effective refractive index and film thickness values indicated a good agreement between the real and the supposed (hexagonally close packed, monodisperse spherical particles) layer structure. It was confirmed by field emission scanning electron microscopy (FESEM) images, too.Advancing and receding water contact angles (CA) on differently (with mono- and bifunctional chlorosilanes) hydrophobized SG and combined films were measured applying the sessile drop method. SG films showed approximately the same CA-s after different silylation processes without considerable contact angle hysteresis (CAH). In case of combined films ca. 30° higher advancing CA-s were measured at stronger silylation and only the milder silylation resulted in significant CAH. It was explained by the surface heterogeneity of constituent particles. Layers of differently sized particles showed the same wetting properties in all cases in agreement to the Cassie–Baxter equation which was used for the estimation of CA values for the individual silica particles (88–90° and 105–106° depending on the silylation conditions).

Molecular orientation in self-assembled layers of two functionalized fullerenes—Role of bromine atom at the end of alkyl chain

The IR reflection–absorption spectra of Langmuir–Blodgett (LB) layers of the functionalized fullerenes: C60-fused N-methylpyrrolidine-5-(dodecyl)thiophene (1) and C60-fused N-methylpyrrolidine-5-(12-bromododecyl)thiophene (2) were investigated. It was shown that thin films of functionalized fullerenes are ordered and characteristic parts of the molecules are oriented. Their orientation was evaluated with a simple spectroscopic method. It was stated that bromine atom at the end of aliphatic chain of the sample 2 strongly influences the chain orientation. Similarly, the sulphur atom in the thiophene ring causes some bend of its plane.

Enhanced Photoluminescence of ZnO Langmuir–Blodgett Films on Gold-Coated Substrates by Plasmonic Coupling

The interaction between a nanostructured metal and a semiconductor can lead to advantageous enhancement of the emission properties of the semiconductor, which is important for light-emitting diode applications. Herein, we present a study of the interaction between ZnO nanoparticles (3 nm) and two types of gold nanostructures: 10-nm particles and 50-nm-thick films. Photoluminescence emission measurements were performed to characterize the effects of the gold on the emission properties of ZnO and to study the dependence of the interaction on the structure of Au. Stearic acid Langmuir–Blodgett layers were introduced onto the film to control the distance between the noble metal and the semiconductor. The largest (12-fold) enhancement in the emission intensity due to plasmonic coupling was observed for a continuous Au film with one spacer layer, that is, a 2.2-nm distance between the metal and the ZnO.

Spectroscopic characterization of selected fullerene–organic chromophore Langmuir–Blodgett films

Electronic absorption in polarized and unpolarized light and steady-state fluorescence emission and excitation spectra of zinc porphyrins, their dimers and dyads with a fullerene as well as perylene-derived molecular systems containing a fullerene were investigated. In particular, dyads of the fullerene covalently bonded to perylene- and porphyrin-derived dyes were comprehensively studied in chloroform solution and in a form of Langmuir–Blodgett layers on solid inorganic substrates. The spectroscopic examination of the samples allowed us to analysis charge redistribution after the chromophore–fullerene dyads formation – this effect was detected as changes of the band wavelengths and shape of the lines. The layer organization, in particular the dye molecule orientation, was estimated from. Usefulness of the investigated dyads in photovoltaic devices was signaled.

Reorganization of Langmuir–Blodgett layers of silica nanoparticles induced by the low energy, high fluence ion irradiation

The Langmuir–Blodgett (LB) layers with different selforganization of small, medium, and large size silica nanoparticles, when exposed to the low energy, high fluence ion beam, show reorganization. The reorganization appears as the net effect between the forces caused by charging and by heating of particles, whereas the Coulomb repulsive force between the particles overcomes their adhesion to the Si substrate. The threshold force was estimated for the case of small thermal effects (solid particle on the solid substrate), of higher thermal effects (soft particle on the solid substrate), and of the strong thermal effects (soft particle on the soft substrate), as function of the ion fluence. The increase of fluence causes simultaneous decrease of the Coulomb repulsion and increase of adhesion, so that the mobility of particles decreases affecting the reorganization process. The reorganization depends on the ion fluence, but also on the initial selforganization of the LB layer, being different for the small, medium, and the large size particles. The hydroclusters of small particles become reorganized into “infinite” complex irregular chain-cluster; the small crystal-grains of medium size particles become rhombic and hexagonal clusters, while the 2D hexagonal crystals of large size particles become only slightly disturbed. The reorganization occurs until the increasing ion fluence reaches the reorganization-breakdown-threshold, and overwhelming thermal effects turn the process into particle destruction.

Photoelectrochemical cells based on LB films of fullerene–thiophene derived dyads

Photoelectrochemical properties of fullerene with three oligothiophenes (1T-F, 2T-F, 3T-F), ethylenedioxythiophenyl (EDOT-F) and hexylthiophenyl (HXT-F) groups were investigated with the photocurrent measurements. Electron paramagnetic resonance (EPR) spectroscopy has been used for study the fullerene–thiophene derived systems in wide temperature range. For electrochemical examination of these systems Langmuir–Blodgett (LB) layers deposited on a gold substrate were prepared. The light-induced photocurrent study shows that the structure of the dyad has an influence on both photovoltage and photocurrent, although these two variables are not correlated with each other. The role of the structure of the dyads is also seen in electron paramagnetic resonance investigations. All these results supported by previous IR and UV–vis spectral studies suggest, that the photoinduced electron transfer from oligothiophenes or EDOT and HXT to C 60 is a primary process of the photocurrent generation in the fullerene–thiophene-derived dyads.

Covalent dyads of porphyrin–fullerene and perylene–fullerene for organic photovoltaics: Spectroscopic and photocurrent studies

Supermolecular complexes of zinc porphyrin or perylenediimide as covalent dyads with fullerene (C 60) in chloroform and as Langmuir–Blodgett layers on an Au substrate were studied. In our studies we have used following spectroscopic methods: electronic absorption, fluorescence and electron spin resonance in solution. Also infrared absorption spectra in a KBr pellet and reflectance–absorption in Langmuir–Blodgett layers were monitored. Photocurrent generation in a photoelectrochemical cell was also studied. The redistribution of charge both upon porphyrin linkage to C 60 and when the systems are deposited on the Au substrate was shown. Photocurrent examinations show a great influence of the fullerene presence on photoresponse of the systems.

Raman and infrared studies of molecular orientation in fullerene–thiophene films

The paper deals with spectral studies (IR absorption, IR reflection–absorption, Raman scattering, and electronic absorption) of Langmuir–Blodgett layers of selected fullerene–thiophene dyads; some information on preparation and properties of the layers are also given. The spectra were recorded with unpolarised and polarised light (p-polarisation and s-polarisation) to receive data on orientation of the dyads with respect to the solid substrate and on interaction of the dyads with gold substrates. The comparison of the dyad spectra suggests redistribution of charges in both moieties of the dyads after films deposition on the substrate. We have shown that the polarised reflection–absorption spectra of the thin films are strongly dependent on the light polarisation. From the analysis of the data on the basis of the results obtained from various spectral methods it was concluded that the Langmuir–Blodgett layers are ordered and some parts of the dyads are oriented.

Molecular orientation and spectral investigations of Langmuir–Blodgett films of selected copper phthalocyanines

Various spectral methods (IR absorption and reflectance-absorption, Raman scattering and UV–Vis absorption) were used for investigations of a family of copper phthalocyanines in non-polarized and polarized light. The spectra of bulk and thin film samples were compared and discussed; an orientation of selected copper phthalocyanines in Langmuir–Blogdett layers and interaction between the phthalocyanine dyes with solid substrates were studied and discussed. Molecular arrangement of the molecular skeleton on the solid substrates was also evaluated. The presence of dye aggregates in the Langmuir–Blodgett layers was shown confirming the data obtained in concentrated solutions and in Langmuir layers.

ChemInform Abstract: Synthesis of Amphiphilic Crown Compounds as a Basis for Langmuir- Blodgett Layers.

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γ-Fe2O3 nanoparticle adsorption at an OTS Langmuir monolayer

The assembling of magnetic nanoparticles in ordered structures as well as the preparation of very thin magnetic switchable polymer membranes is an important aim in many technical fields. We studied the influence of γ-Fe2O3 nanoparticles on the polymerization process and on the properties of the poly(organosiloxane)/nanoparticle-composite layer by surface rheological measurements, surface pressure/area (π/A) isotherm measurements, and Brewster angle microscopy. The adsorption process dynamics were studied by X-ray reflectivity and surface potential measurements. The results confirm the presence of attractive electrostatic interactions between the partial negatively charged monolayer and the positively charged nanoparticles. For further investigations, we prepared Langmuir–Blodgett layers of these polymer-nanoparticle composite and investigated them by atomic force microscopy and UV-Vis spectroscopy. We found that the concentration of nanoparticles was very low and the particles were mainly arranged below the polymer layer.

Sensing properties of two-component Langmuir–Blodgett layer and its porous derivative in SAW sensor for vapors of methanol and ethanol

Two-component Langmuir–Blodgett film has been fabricated from equimolar mixture of 5-[[1,3-dioxo-3-[4-(1-oxooctadecyl)phenyl]propyl]amino]-1,3-benzenedicarboxylic acid with cetylamine. Porous one-component derivative of this film has been obtained by removing of cetylamine. Both films have been investigated as sensing layers of the surface acoustic wave sensors for vapors of methanol and ethanol in air. These films react on the alcohol vapors. Time of the sensor reaction is less than 2 s and this reaction is reversible. The response parameters of the sensor depend on the kind of the alcohol and the structure of the layer. An explanation of the observed differences in the behavior of both layers in contact with vapors of the alcohols has been proposed.

Polyaniline Langmuir–Blodgett films: formation and properties

The deposition and characterisation of Langmuir-Blodgett (LB) layers of polyaniline (PAN) on solid supports is described. Langmuir films were spread as a mixture of PAN and dodecylbenzenesulfonic acid (DBSA) at the water/air interface and deposited on either glass or indium tin oxide (ITO). Mono- and multi-layer films of PAN/DBSA were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), absorption spectroscopy and cyclic voltammetry (CV). The ultrathin films produced were found to be highly uniform and very stable. Further characterisation of the films was accomplished by scanning electrochemical microscopy (SECM) in the feedback mode. It was found that the conductivity depended strongly on the pH of the solution and the number of layers deposited. Values for the pH-dependent lateral conductivity of PAN LB films are reported.

Nanolayers of selected porphyrin and phthalocyanine dyes on solid substrates studied by electronic absorption and IR reflection–absorption spectroscopy

Electronic absorption and IR reflection–absorption spectra in non-polarized and in polarized light for lead porphyrin as well as magnesium and lead phthalocyanine dyes when deposited in the form of Langmuir–Blodgett (LB) nanolayers on solid inorganic surfaces (quartz, semiconductor and metal) were measured. Some IR bands of the Langmuir–Blodgett dye layers’ spectra show frequency shifts and changes in the relative intensities as well as in half widths when compared with the vibrational features of powdered dyes dispersed in KBr pellets, which were used as references. The FT-IR spectroscopic examination of the Langmuir–Blodgett layers allowed to estimate electron redistribution at the interface between dye layer and solid substrates. The Langmuir–Blodgett films of different thicknesses (3, 5, 10 layers) were studied at various angles of IR beam incidence and different light polarizations. The most spectacular results were obtained for the grazing incidence (80°) and films of 5 layers for dyes on the Au substrate. The IR spectroscopy was supported with electronic absorption studies (UV–vis) to follow interaction at interface between the dye layers and the substrates as well as to evaluate linear dichroism and to determine arrangement of molecules in the Langmuir–Blodgett films. Molecular arrangement in the Langmuir–Blodgett layers was discussed. It was shown that the dye molecular planes are rather randomly oriented in the Langmuir–Blodgett films with a tendency that the Q y and Q x transition moments in the phthalocyanine macroring are slightly directed along the y-axis (Langmuir–Blodgett dipping direction) and the x-axis direction, respectively.

Vibrational properties of thin films and solid state of perylenediimide–fullerene dyads

Analysis of the vibrational properties of Langmuir–Blodgett multilayers of the perylenediimide–fullerene (PDI–C 60), perylenediimide–tetrathiafulvalene (PDI–TTF) dyads, and reference molecules adsorbed on various surfaces was performed. The comparison of their absorption and reflection–absorption spectra suggests small redistribution of the charges in both moieties of the dyads after formation of the Langmuir–Blodgett layers. This effect is observed as shifts of selected vibrational bands of the dyad layers in comparison with respective bands of the dyads in the bulk. Orientations of organic chain molecules grafted to a metal surface were suggested on the ground of IR reflection–absorption investigations.

Spectral and thermodynamic characterization of bacteriochlorophyll c and dipalmitoylphosphatidylcholine in the binary mixed monolayers

The spectral and thermodynamic properties of bacteriochlorophyll c and dipalmitoylphosphatidylcholine in binary mixed monolayers have been investigated. The model system of biological membrane was built up of a mixture of phospholipids and pigments dissolved in chloroform, at several mole ratios. The isotherms of these samples were recorded and then the monomolecular layers on quartz plate were produced. For the Langmuir layer obtained on the air–water interface the thermodynamic parameters were calculated. Additionally, by means of the absorption spectroscopy methods in natural and polarized light, pigment–pigment and pigment–lipid interactions were studied. The information about the average surface per molecule, aggregation properties and the molecular orientation of pigments in Langmuir–Blodgett layers, was obtained.