Layer Langmuir–Blodgett Film Research Articles

Acetylene Used as a New Linker for Molecular Junctions in Phenylene–Ethynylene Oligomer Langmuir–Blodgett Films

Langmuir and Langmuir–Blodgett films have been fabricated from an acetylene-terminated phenylene–ethynylene oligomer, namely 4-((4-((4-ethynylphenyl)ethynyl)phenyl)ethynyl)benzoic acid (HOPEA). Characterization of the Langmuir film by surface pressure vs area per molecule isotherms and Brewster angle microscopy reveals the formation of a high quality monolayer at the air–water interface. One layer Langmuir–Blodgett (LB) films were readily fabricated by the transfer of HOPEA Langmuir films onto solid substrates by the withdrawal of the substrate. The deposition mode was Z-type. Quartz crystal microbalance (QCM) experiments confirm the formation of directionally oriented, monolayer LB films, in which the HOPEA molecules are linked to the gold substrate by attachment through the acid group. The morphology of these films was analyzed by atomic force microscopy (AFM), which revealed an optimum transference surface pressure of 18 mN m–1 for the formation of homogeneous films. Cyclic voltammetry also showed a si...

Directionally Oriented LB Films of an OPE Derivative: Assembly, Characterization, and Electrical Properties

Langmuir films have been fabricated from 4-[4'-(4''-thioacetyl-phenyleneethynylene)-phenyleneethynylene]-aniline (NOPES) after cleavage of the thioacetyl protecting group. Characterization by surface pressure vs area per molecule isotherms and Brewster angle microscopy reveal the formation of a high quality monolayer at the air-water interface. One layer Langmuir-Blodgett (LB) films were readily fabricated by the transfer of the NOPES Langmuir film onto solid substrates. X-ray photoelectron spectroscopy (XPS), surface polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS), and quartz crystal microbalance (QCM) experiments conclusively demonstrate the formation of one layer LB films in which the functional group associated with binding to the substrate can be tailored by the film transfer conditions. Using LB methods this molecule could be transferred to gold samples with either the amine or thiol group attached to the gold surface. The amine group is directly attached to the gold substrate (Au-NH(2)-OPE-SH) when the substrate is initially immersed in the subphase and withdrawn during the transfer process; in contrast, monomolecular films in which the thiolate group is attached to the gold substrate (Au-S-OPE-NH(2)) are obtained when the substrate is initially out of the subphase and immersed during the transfer process. The morphology of these films was analyzed by atomic force microscopy (AFM), showing the formation of homogeneous layers. Film homogeneity was confirmed by cyclic voltammetry, which revealed a large passivation of gold electrodes covered by NOPES monolayers. Electrical properties for both polar orientated junctions have been investigated by scanning tunnelling microscopy (STM), with both orientations featuring a nonrectifying behavior.

Langmuir–Blodgett films of ethylenedithiotetrathiafulvalene derivative containing hydroxyl groups

Langmuir–Blodgett (LB) films of an ethylenedithiotetrathiafulvalene (EDT-TTF) derivative containing hydroxyl groups, 4, 5-bis(11-hydroxyundecylthio)-4′, 5′-ethylenedithiotetrathiafulvalene were deposited without using stabilizer molecules. Doping of the film with iodine leads to oxidation of EDT-TTF moiety as evidenced by UV–vis and IR spectroscopy. X-ray diffraction studies reveal the presence of layered arrangement of the EDT-TTF derivative molecules in the LB film. Cyclic voltammetry studies indicate that the electrochemical oxidation of the EDT-TTF derivative in its LB film is irreversible. Due to the insulating nature of the alkyl chains, only the first layer of the LB film was found to be redox-active. The electrical conductivity of the 25 layer LB film was found to increase by two orders of magnitude on doping with iodine. On equilibrating the doped film in air, the conductivity gradually reduced with time and finally reached the conductivity of undoped film. This reversibility could be because of the steric hindrance induced by the two long alkyl groups. The alkyl chains help in rendering the EDT-TTF molecule amiable to LB film formation but are found to reduce the electrical conductivity of the films and also cause instability of the doped state. The hydroxyl groups at the end of the alky chain impart amphiphilic nature to the molecules and help in stabilizing the film at the air–water interface.

Electrical characterisation of poly(methyl methacrylate)/arachidic acid Langmuir–Blodgett films

A series of poly(methyl methacrylate) (PMMA) molecules with different molecular weights and arachidic acid has been used to fabricate alternate layer Langmuir–Blodgett (LB) films onto aluminum coated glass substrates and electrical properties of these LB films have been investigated. The area per molecule for PMMA molecules was found to be approximately 2000 nm 2. I–V measurements showed that there is a polarity in the alternate layer LB films. Schottky barrier height values have been calculated using alternate layer type (AL-type) LB films.

Pure and mixed films of a nitrostilbene derivative at the air–water interface, Langmuir–Blodgett multilayer fabrication, and optical characterization

This paper reports the preparation and characterization of pure Langmuir and Langmuir–Blodgett (LB) films of a stilbene derivative containing two alkyl chains, namely 4-dioctadecylamino-4′-nitrostilbene. Mixed films incorporating docosanoic acid and the stilbene derivative are also studied. Brewster angle microscopy (BAM) analysis has revealed the existence of randomly oriented three-dimensional (3D) aggregates, spontaneously formed immediately after the spreading process of the stilbene derivative onto the water surface. These 3D aggregates coexist with a Langmuir film that shows the typical gas, liquid, and solid-like phases in the surface pressure and surface potential vs area per molecule isotherms, indicative of an average preferential orientation of the stilbene compound at the air–water interface, and a gradual molecular arrangement into a defined structure upon compression. A blue shift of 55 nm of the reflection spectrum of the Langmuir film with respect to the spectrum of a chloroform solution of the nitrostilbene indicates that two-dimensional (2D) H-aggregates are formed at the air–water interface. The monolayers are transferred undisturbed onto solid substrates with atomic force microscopy (AFM) revealing that the one layer LB films are constituted by a monolayer of the stilbene derivative together with some 3D aggregates. When the nitrostilbene compound is blended with docosanoic acid, the 3D aggregation is avoided in the Langmuir and Langmuir–Blodgett films, but does not limit the formation of 2D H-aggregates, desirable for second-order nonlinear optical response in the blue domain. The AFM images of the mixed LB films show that they are formed by a docosanoic acid monolayer and, on the top of it, a bilayer of the stilbene derivative.

Characterization of Langmuir–Blodgett films of arachidic acid and 1,2-bis(dodecyloxy)-4,5-diaminobenzene in pyroelectric devices

Non-centrosymmetric materials exhibiting a temperature-dependent electric polarisation are known as pyroelectrics. It is well known that the tilt mechanism and the proton transfer between acid–amine interactions play an important role for the pyroelectric effect in alternate layer Langmuir–Blodgett (LB) films. In this work arachidic acid (AA) has been deposited alternately with 1,2-bis(dodecyloxy)-4,5-diaminobenzene (DADB) using the LB film deposition technique to form multilayered LB films. Pyroelectric and dielectric measurements have been performed for different layered AA/DADB films. Pyroelectric coefficients for this LB film system have been obtained using a quasi-static measurement system. Dielectric measurements have been used to determine the relative permittivity and dielectric loss tangent. The data obtained from pyroelectric and dielectric measurements have been used to calculate the figure of merit for these LB films which has the average value of 6.34 μC m −2 K −1.

Electrical characterisation of stearic acid/eicosylamine alternate layer Langmuir–Blodgett films incorporating CdS nanoparticles

Electrical properties of stearic acid/eicosylamine alternate layer Langmuir–Blodgett (LB) film structure incorporating cadmium sulphide (CdS) nanoparticles are studied in the temperature range 300–400 K. The room temperature ohmic conductivity is found to be 3.87 × 10 − 13 Sm − 1 for the as deposited LB film and 7.75 × 10 − 12 Sm − 1 for the LB film containing CdS nanoparticles. At a relatively high field the electrode-limited Schottky effect is found to be responsible for conduction mechanism. The potential barrier height is calculated at several temperatures for the metal/LB film/metal system and the average temperature coefficient is found to be 0.0048 eV K − 1 for the untreated LB film and 0.005 eV K − 1 for the CdS containing LB film. Activation energies are found to be 0.46 eV and 0.19 eV for the untreated LB film and the LB film with CdS nanoparticles, respectively. The alternating current conductance for both samples shows typical power law dependence with a value of ∼ 0.9 for the frequency dependence exponent.

Pyroelectric and dielectric characterisation of alternate layer Langmuir–Blodgett films incorporating ions

Non-centrosymmetric Langmuir–Blodgett (LB) thin films exhibit a temperature-dependent electric polarisation, which is known as the pyroelectric effect. Alternate layer LB films were prepared by transferring polysiloxane alternately with eicosylamine from a subphase of Millipore water (18 MΩ cm − 1 ) incorporating different metallic ions onto a 50 nm thick thermally evaporated aluminium substrate. Pyroelectric and dielectric measurements were performed on LB films with a sandwich structure comprising film. Pyroelectric coefficients of these LB films are measured at several temperatures. The dielectric constant and dielectric losses were obtained to determine the pyroelectric figure of merit values of these LB films, which were calculated in the range of 24.9 to 56.7 μC m − 2 K − 1 .

Langmuir–Blodgett films of hydrophobins HFBI and HFBII

Hydrophobins are small fungal proteins, which have remarkable surface–chemical properties. They self-assemble at hydrophobic/hydrophilic interfaces and work as adhesive agents and coatings. Sixteen layer Langmuir–Blodgett films of hydrophobins HFBI and HFBII from the fungus Trichoderma reesei were prepared and studied using grazing-incidence X-ray diffraction and reflectivity techniques. Both kind of films contain hexagonally ordered crystallites on the substrate with unit cell parameters of a = b = 54 Å (HFBI) and a = b = 55 Å (HFBII). The structure is similar to the structure of monolayer Langmuir–Blodgett films.

Pyroelectric figure of merit for a calix [4] acid/amine alternate layer Langmuir–Blodgett film

In this work calix [4] acid (A) has been deposited alternately with calix [4] amine (B) using the LB film deposition technique to form a non-centrosymmetric ABABA… molecular architecture. Pyroelectric and dielectric electric measurements have been performed for a 31 layer calix [4] acid/amine alternate layer LB film. Pyroelectric coefficients for this LB film system have been obtained using a quasi-static measurement system. Dielectric measurements have been used to determine the dielectric constant and the dielectric loss tangent. The data obtained from pyroelectric and dielectric measurements have been used to calculate the figure of merit for these LB film systems which has a value of 2.56 μC m −2 K −1.

Pyroelectric effect in Langmuir-Blodgett films incorporating ions

Non-centrosymmetric materials exhibiting a temperature-dependent electric polarisation are known as pyroelectrics. It is well known that the tilting mechanism and proton transfer between acid/amine interactions play an important role for the pyroelectric effect in alternate layer Langmuir-Blodgett (LB) films. In this work, the ionic mechanism is investigated using copolysiloxane/eicosylamine alternate layer LB films incorporating different metallic cations such as cadmium, barium and manganese. Pyroelectric coefficients of these LB films are measured at several temperatures and the dielectric constant and dielectric losses are obtained using a dielectric measurement system. The figure of merit of these LB film assemblies shows the highest figure of merit for alternate layer LB films.

A theoretical model for the pyroelectric response in Langmuir–Blodgett films

Understanding the structure of pyroelectric materials is important to elucidate the nature of the temperature-dependent microscopic and macroscopic electric polarisation. Pyroelectric materials must have a non-centrosymmetric structure. Previously, researchers [C.A. Jones, PhD thesis, University of Durham, (1987); J. Mater. Chem. 1 (1991) 819; Langmuir 11 (1995) 4623] have reported the pyroelectric effect of non-centrosymmetric ultrathin LB films prepared using the alternate layer Langmuir–Blodgett (LB) deposition technique. They have proposed three main mechanisms to explain the origin of the pyroelectric activity, namely, tilting, proton transfer, and ionic processes. Linear and cyclic polysiloxane materials with aliphatic and aromatic side groups have been studied in this work. These materials have been alternately deposited with eicosylamine to form the active pyroelectric material in metal–LB film–metal (MIM) devices, whose pyroelectric coefficients have been measured using a quasi-static measurement technique [W.H. Majid, Abd., PhD thesis, Univ. of Sheffield (1994); Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 3 (1995) 197; Thin Solid Films 327–329 (1998) 369]. The relation between pyroelectric mechanism(s) and microscopic and macroscopic pyroelectric response for polysiloxane/eicosylamine alternate layer LB films will be explained in this work. Results indicate that the physical mechanism by which the pyroelectric activity arises in the LB films is critically dependent upon their structural quality, the ions within the multilayer arrangement and the molecular dipole moments of the molecules.

Pyroelectric figure of merit for a heterogeneous alternate layer Langmuir–Blodgett film system

The pyroelectric effect arises as a result of the change of the polarisation with temperature. Pyroelectric materials, which must have a non-centrosymmetric structure, exhibit an inherent electrical polarisation that changes when their temperature is changed. A heterogeneous monolayer system containing linear copolysiloxanes has been shown to produce a non-centrosymmetric structure in this work. Alternate layer Langmuir–Blodgett (LB) films have been fabricated using LB film deposition technique. The pyroelectric and dielectric properties of these alternate layer LB film assemblies have been measured and used to determine the pyroelectric figure of merit in the context of thermal imaging devices.

Qualitative evaluation of pyroelectric mechanisms in Langmuir–Blodgett films containing a cyclic polysiloxane substituted with aliphatic side chains using Fourier transform infrared (FTIR) spectroscopy

Alternate layer Langmuir–Blodgett (LB) films containing cyclic polysiloxane substituted with aliphatic side chains alternated with monomeric eicosylamine have been shown to exhibit pyroelectric activity. The pyroelectric activity for 5, 11, 15, 21, 25 and 31 monolayer LB films, is recorded at 21°C and shows highest activity for 5 monolayers with a pyroelectric coefficient of 0.5 μC m −2 K −1. The coefficient decreases as the thickness of the film increases from 5 to 15 monolayers, increases in the thickness range of 15–25 monolayers and decreases again for 31 monolayers film. Fourier transform infrared (FTIR) spectroscopy shows that proton transfer between the acid headgroups at the terminal end of the cyclic polysiloxane and the headgroups of the eicosylamine increases as the number of monolayer increases. However, the angle at which the molecular axis of the LB film tilts from the surface normal, α (as indicated by the overall ratio of symmetric stretching of CH 2 to CH 3, or asymmetric stretching of CH 2 to CH 3) increases as the number of monolayer increases from 5 to 15 layers and remains stable for 21–31 layers.

Photoinduced Electrochemical Properties of Organized Assembly of Amphiphilic Redox Polymers by Langmuir-Blodgett Technique

Spatial arrangement of the ruthenium dipyridyl complex and ferrocene derivative in hetero-deposited redox polymer Langmuir-Blodgett (LB) films was carried out for direction control of photocurrent flow produced by photoinduced electron transfer reaction and for high quantum efficiency. Two hetero-deposited structures were obtained by varying the deposition order of the redox polymer monolayers by the LB technique. Cyclic voltammograms of hetero-deposited redox polymer LB films exhibited current rectifying and charge storage properties. Light irradiation led to anodic photocurrent of hetero-deposited redox polymer LB films consisting of Ru copolymer LB film as an inner layer and Fc copolymer LB film as an outer layer on the ITO (Fc/Ru/ITO) electrodes. On the other hand, cathodic current was observed at the reverse layered structure (Ru/Fc/ITO) electrodes. The direction of photocurrent flow depended on the deposition order of redox polymer LB films on the ITO electrode. Photocurrent quantum efficiency of 5.9% of the hetero-deposited redox polymer LB film was achieved. High photocurrent conversion efficiency is explained by inhibition of recombination of photoinduced charge separation due to hetero-deposited LB film structures.

A structural effect on electrochemical behavior of hetero-deposited redox polymer Langmuir–Blodgett films containing ferrocene and tris(bipyridine)ruthenium derivatives

Abstract A structural effect on the electrochemical behavior of hetero-deposited redox polymer Langmuir–Blodgett (LB) films containing ferrocene (Fc copolymer) and tris(bipyridine)ruthenium derivatives (Ru copolymer) on electrodes was investigated by cyclic voltammetry. The hetero-deposited LB film structure consists of Ru copolymer LB film (three layers) as an inner layer and Fc copolymer LB film (two layers) as an outer layer on ITO electrodes. The hetero-deposited redox polymer LB films which are prepared by the deposition at a surface pressure higher than 28 mN m−1 show that the redox behavior of the ferrocene moiety in the outer Fc copolymer LB film is controlled by the redox condition of the ruthenium complex in the inner Ru copolymer LB film and not directly influenced by the electrode potential. On the other hand, in the LB films prepared at a pressure less than 25 mN m−1, such behavior was not observed because the inner Ru copolymer LB films cannot work as an insulator between the outer Fc copolymer LB films and the ITO electrode. The voltammetric shapes of the Ru copolymer LB films were distorted with increasing deposition surface pressure. These results indicate that the insulating property of the inner layer is improved whereas the electron transfer rate in the interlayer becomes slower with increasing deposition surface pressure because of the formation of the high packing density of alkyl side chains of the LB films.

Fabrication of Photoresponsive Nano-Organized Polymer Assemblies

We describe here fabrication of organized polymer assemblies where the vectrial photoinduced electron transfer can be controlled by the spatial molecular arrangement by LB technique. The hetero-deposited redox polymer Langmuir-Blodgett (LB) films consisting of poly(N-dodecylacrylamide-co-ferrocenylmethylacrylate)(Fc copolymer) and poly(N-dodecy-lacrylamide-co-(4-(acryloylmethyl)-4′-methyl-2,2′-bipyridine)-bis(2,2′-bipyridine) ruthenium diperchlorate)) (Ru copolymer) were investigated by cyclic voltammetry and the steady state photocurrent measurement. Two kinds of the hetero-deposited structures are constructed by varying with the deposition order of these redox polymer monolayers using the LB technique. The cyclic voltammograms of the hetero-deposited redox polymer LB films show current rectifying and charge storage properties. On light irradiation, the anodic photocurrent is observed at the hetero-deposited redox polymer LB films consisting of Ru copolymer LB film as an inner layer and Fc copolymer LB film as an outer layer on the ITO (Fc/Ru/ITO) electrodes, whereas cathodic one is observed at the reverse layered structure (Ru/Fc/ITO) electrodes. The direction of photocurrent flow is controlled by the deposition order of the redox polymer LB films on ITO electrode. The magnitude of photocurrent depends on electrode potential, light intensity, sacrifier concentration, and irradiation wavelength. The mechanism for photocurrent generation is discussed.

Effective Photoinduced Electron Transfer in Hetero-Deposited Redox Polymer LB Films

The spatial arrangement of the ruthenium dipyridyl complex and ferrocene derivative in heterodeposited redox polymer Langmuir−Blodgett (LB) films is used for the direction control of the photocurrent flow resulting from the photoinduced electron-transfer reaction and the high quantum efficiency. Two kinds of heterodeposited structures are constructed by varying the deposition order of these redox polymer monolayers by the LB technique. The cyclic voltammograms of the heterodeposited redox polymer LB films show current rectifying and charge storage properties. On light irradiation, the anodic photocurrent is observed at the heterodeposited redox polymer LB films consisting of Ru copolymer LB film as an inner layer and Fc copolymer LB film as an outer layer on the ITO (Fc/Ru/ITO) electrodes, whereas a cathodic photocurrent is observed at the reverse layered structure (Ru/Fc/ITO) electrodes. The direction of photocurrent flow depends on the deposition order of the redox polymer LB films on the ITO electrode. A photocurrent quantum efficiency of 5.9% of the heterodeposited redox polymer LB film is achieved. The high photocurrent conversion efficiency is explained by the inhibition of the recombination of the photoinduced charge separation due to the presence of the heterodeposited LB film structures.

Cd 2+ and Mn 2+ incorporation in copolysiloxane/eicosylamine alternate layer LB films: influence on pyroelectric activity

The pyroelectrical activity in copolysiloxane/eicosylamine alternate layer Langmuir–Blodgett (LB) films incorporating Cd 2+ and Mn 2+ ions is reported. The pyroelectric coefficients and dielectric constants have been measured in order to determine the figure of merit in the context of thermal imaging materials. The pyroelectric results indicate that cadmium ion incorporation leads to a significantly improved performance compared to the system without metal ions. Additionally, preparation of LB films using a metal-doped subphase leads to an extended operating temperature range for the fabricated pyroelectric devices.

Controlling the structure of transparent Langmuir–Blodgett films for nonlinear optical applications

Langmuir–Blodgett (LB) multilayers of (E)-N-alkyl-4-[2-(4-docosyloxyphenyl)ethenyl]pyridinium bromide, CnH2n+ 1—Py+—CHCH—C6H4—O—C22H45Br–(dye I), are centrosymmetric (Y-type) for n⩽ 6 and non-centrosymmetric (Z-type) for 8 ⩽n⩽ 20. The areas obtained from grazing incidence synchrotron X-ray diffraction indicate that the molecules adopt a ‘stretched’ rather than a ‘U-shaped’ configuration and, unlike conventional amphiphilic materials, the long chain homologues do not invert during deposition. The second-harmonic intensity from films of the higher homologues (n 10) increases quadratically with the number of LB layers (I2ω(N)=I2ω(1)N2) whereas for the lower homologues (n⩽ 6) the intensity is negligible for even numbers of layers. The nonlinear optical properties of the sulfur (dye II), selenium (dye III) and quinolinium (dye IV) analogues are also reported and, in each case, quadratic enhancement of the second-harmonic intensity has been observed for suitable combinations of alkyl chain lengths. The second-order susceptibility, χ(2)zzz, of a 100 layer LB film of (E)-N-dodecyl-4-[2-(4-docosyloxyphenyl)ethenyl]quinolinium bromide (dye IV) is 20 pm V–1.