Z-type Langmuir-Blodgett Films Research Articles

Growth of CdS nanoparticles in Y- and Z-type Langmuir–Blodgett thin film using 1,3-bis-(p-iminobenzoic acid)indane

Cadmium sulphide (CdS) nanoparticles were formed in 1,3-bis-(p-iminobenzoic acid)indane by exposing Cd2+ doped Y- and Z-type multilayered Langmuir–Blodgett (LB) films to H2S gas. The growth of CdS nanoparticles were monitored by UV–visible spectroscopy measurements. It was observed that CdS nanoparticles in both Y- and Z-type LB films cause a blue-shift in absorption spectra. The surface morphology of LB films were characterized with atomic force microscopy DC electrical measurements were carried out for these LB films grown in a metal/LB film/metal sandwich structures with and without CdS nanoparticles. By analyzing I–V curves and assuming Schottky conduction mechanism the barrier height was found to be as 1.25 and 1.17 eV for Y-type unexposed and exposed samples; 1.18 and 1.25 eV for Z-type unexposed and exposed samples, respectively.

Effects of Hydrogen Bonding on the Monolayer Properties of Amphiphilic Double-Decker-Shaped Polyhedral Silsesquioxanes

"Core-corona" type amphiphiles, which comprise double-decker-shaped POSSs (DDSQs) as the core and two or four di(ethylene glycol) (DEG) units as the coronae, have recently been reported to form a stable monolayer at the air-water interface. In this paper, another core-corona amphiphile, 2DEGNH-DDSQ, which has a urethane group at the end of the coronae, was synthesized to elucidate the effects of hydrogen bonding on monolayer properties. The surface pressure-area isotherm and Brewster angle microscopy revealed that 2DEGNH-DDSQ initially formed rodlike assemblies. They subsequently coalescence to form a uniform monolayer with compression. Actually, 2DEGNH-DDSQs are well ordered in the rodlike assembly because of the strong hydrogen bonds among the urethane groups, as confirmed by FT-IR spectra. Although the monolayer was not transferred onto a solid substrate, mixing of 2DEGNH-DDSQ with 2DEG-DDSQ, which has already been reported to form a liquidlike monolayer, overcame this problem. The 1:1 molar mixture of 2DEGNH-DDSQ and 2DEG-DDSQ forms a uniform liquidlike monolayer. The mixed monolayer was transferred onto a solid substrate as a Z-type Langmuir-Blodgett film. Atomic force microscopic (AFM) images of the mixed-bilayer film showed a uniform surface with root-mean-square surface roughness of 0.21 nm. The intermolecular hydrogen bonds between the urethane groups in 2DEGNH-DDSQ and the hydroxyl groups in 2DEG-DDSQ improve the monolayer properties, which enable successful transfer of the LB film.

Understanding the optical spectroscopy of amphiphilic molecular rectifiers: A density functional approach

We present results of first principles density functional theory calculations of the electronic and atomic structural properties of model Z-type Langmuir-Blodgett (LB) layers comprising amphiphilic quinolinium tricyanoquinodimethanide (Q3CNQ) chromophores. We find that the chromophore electronic ground state is not as clearly "zwitterionic" as required by models to explain electrical rectification purportedly seen in such systems. The computed visible region transitions are not what have been assumed to be the intervalence charge transfer bands seen in the visible region of molecules in Z-type LB films. Our own LB deposition and spectroscopic studies suggest that almost all visible region features previously seen may be ascribed to aggregates. The calculated lowest energy electronic excitation between HOMO and LUMO levels, which is located in the near infrared region, has a transition moment aligned approximately 9° off the molecular long axis, and has a normalized oscillator strength of 1 order of magnitude higher than those of the visible region transitions. This most dominant feature has been neglected from discussions of Langmuir-Blodgett layer rectification but our own deposition studies show no sign of this feature, indicating that the structure of the modeled system differs from that of typical experimental structures. The model indicates that such idealized LB layer structures cannot confidently be invoked to explain their experimental optical or electrical properties.

Linear and nonlinear optical properties in Langmuir–Blodgett multilayers of new cyanine dye

We investigated on cyanine dye (HQ) in Langmuir and Langmuir–Blodgett (LB) films by UV–vis spectra and second-harmonic generation (SHG) methods. UV–vis spectra indicated a uniform film transfer. The significant SHG fringe signal as a function of angle of incidence for Z-type LB films was observed respectively. The refractive indexes at 532 nm and average tilt angle ψ between the molecular (dipolar) axis and surface normal from fitted SHG data are 1.4841, 57° and 1.4775, 53° without and with Cd 2+ ions, respectively. The second-order susceptibility χ ZZZ ( 2 ) = 42 pm V − 1 and 32 pm V −1 for LB films with and without Cd 2+ ions are obtained by measuring SHG signals. These high nonlinearities are due to resonant enhancement resulting from the proximity of the second-harmonic wavelength to the absorption band of the films.

Theoretical study of molecular aggregation in Langmuir–Blodgett films using a dipolar interaction model

We present a dipole–dipole interaction model based on the classical electrostatic theory in the Langmuir–Blodgett (LB) films to calculate the state of aggregation of rod-like molecules with permanent electronic dipole moment. The dependence of the molecular aggregation spectral properties on the structure parameters in the Z-type LB films, such as the intra-layer molecular separation distance, the adjacent layer separation distance, the molecular orientation angle and the layer number of LB films have been studied. It is found that the H–J-type transition of the aggregation could be realized by modulating the structure parameters of the LB films. The theoretical results show a qualitative agreement with experimental data.

Preparation and Characterization of Monolayer and Multilayer Langmuir−Blodgett Films of a Series of 6-O-Alkylcelluloses

Monolayer and multilayer Langmuir-Blodgett (LB) films of 6-O-alkylcelluloses with various chain lengths were prepared and studied. The surface pressure (pi)-area (A) isotherms of 6-O-alkylcelluloses exhibited characteristic behaviors depending on the length of the alkyl chain and temperature. 6-O-Stearylcellulose on the subphase formed a homogeneous monolayer at 10 mN m(-1). By transfer ratio, FT-IR, and contact angle measurements, it was proved that the monolayer of 6-O-stearylcellulose on the water surface was transferred successfully onto a substrate by a vertical dipping method to form a Z-type LB film. The transmission and reflection absorption IR spectrum indicated that the hydrocarbon chains had all-trans rotamers and were oriented nearly perpendicular to the surface in the film. AFM section analysis revealed that the thickness per layer was calculated to be 2.35 nm. These results suggested that the hydrocarbon chains were inclined at an angle of about 25.3 degrees to have high packing density in the alkyl region.

Langmuir and Langmuir–Blodgett Films Containing a Porphyrin–Ruthenium Complex

The fabrication of supramolecular structures from the tetraruthenated porphyrin-containing phosphines, {TPyP[RuCl3(dppb)]4}, RuTPyP, is demonstrated with Langmuir and Langmuir-Blodgett films. The surface pressure-molecular area isotherms (pi-A) point to an edge-on arrangement for the RuTPyP molecules in the condensed state. Weak aggregation in the Langmuir films was indicated by non-zero surface potentials at large areas per molecule and a slight red shift in the ultraviolet-visible absorption spectrum in comparison to the spectrum in solution. Further aggregation occurs in the Z-type Langmuir-Blodgett films, which was confirmed with ultraviolet-visible spectroscopy of the deposited films. Fourier transform infrared and Raman spectroscopic data for powder and Langmuir-Blodgett films indicate that the RuTPyP molecules are chemically stable in Langmuir-Blodgett films regardless of the contact with water during film fabrication. The nanostructured nature of the Langmuir-Blodgett films was manifested in cyclic voltammetry due to the high sensitivity of the metallic centers in RuTPyP. Electrodes modified with Langmuir-Blodgett films exhibit an anodic peak at 100 mV and a cathodic peak at 7 mV, which is assigned to RuIII/RuII redox processes. Furthermore, Langmuir-Blodgett films from RuTPyP showed electrocatalytic activity for oxidation of benzyl alcohol, illustrated by a large shift of 100 mV in the anodic peak at 400 mV, while electropolymerized and cast films of the same compound displayed smaller and no activities, respectively.

Studies of Langmuir and Langmuir–Blodgett films of NLO-active amphiphilic 1,3-indanedione derivatives

We studied Langmuir and Langmuir–Blodgett (LB) films of several strong nonlinear optical (NLO) active amphiphilic derivatives of 1,3-indanedione-5,6-dicarboxylic acid. The surface pressure–area (π–A) isotherms at the air–water interface were investigated at different temperatures and pH values. Non-centrosymmetric Z-type LB films were deposited. UV–visible spectra indicated a uniform film transfer. The orientation of the molecular transition moments was calculated from the polarized UV–visible spectra. Packing within the LB films was observed by UV–visible absorption and fluorescence spectroscopy. The morphology of the LB films was examined by AFM and compared to ellipsometric measurements, and the nature of the films was studied by contact angle measurements. Significant second harmonic generation (SHG) by the LB films was observed. Alternate layer Y-type LB films of two different dyes were fabricated to achieve enhanced stability and SHG.

Mono- and multi-layer langmuir-blodgett films of maleimide polymers possessing nonlinear optical-active side chains

A copolymer P[OSA-MI] was synthesized by copolymerization of its corresponding monomers,N-phenyl maleimide (MI) and 2-octen-1-ylsuccinic anhydride (OSA). The polymer (poly[2-[1-(2,5-dioxo-1-phenylpyrrolidin-3-ylmethyl)heptyl]-succinic acid 4-(2-{ethyl-[4-(4-nitrophen-ylazo) phenyl]amino}ethyl) ester]) P[DR1MA-MI] was obtained from the reaction of P[OSA-MI] with 2-[4-(4-nitrophenylazo)-N-ethylphenylamino] ethanol (DR1). A stable monolayer of P[DR1MA-MI] was formed by spreading the solution of the polymer in chloroform. In Y-type Langmuir-Blodgett (LB) films prepared using this Langmuir-Blodgett method, the second harmonic waves generated from adjacent monolayers canceled each other out. In X- and Z-type LB films, the second harmonic intensity increased upon increasing the number of monolayers, but this increase was somewhat smaller than predicted by the square law. This phenomenon is due to defects or imperfect alignment of the dipoles in the LB film. The generation of second harmonic waves from Y-type LB films having an even number of monolayers supports this argument. The degree of imperfection seemed to increase as the number of layers increased. The second-order nonlinear optical properties of spin-cast films of these polymers were also measured. The largest second harmonic coefficient of the poled P[DR1MA-MI] film coated on a glass plate was 19 pm/V.

Characteristics of Organic Light-Emitting Diodes Using PECCP Langmuir-Blodgett Film as an Emissive Layer

We have synthesized green light-emitting material, poly(3,6-N−2-ethylhexyl carbazolyl cyanotere-phthalidene)(PECCP), which has an electron-donor and electron-acceptor moiety in a repeated unit of polymer. Organic light-emitting diodes were made with PECCP Langmuir-Blodgett(LB) films in a sandwich structure between indium-tin-oxide(ITO) and aluminum electrodes. Z-type LB films were manufactured using a Kuhn type to use for an emissive layer. We have observed green photoluminescent and electroluminescent spectra at λmax = 536 nm corresponding to 2.31 eV.

Electrical Rectification by a Monolayer of Hexadecylquinolinium Tricyanoquinodimethanide Measured between Macroscopic Gold Electrodes

Unimolecular rectification was detected between oxide-free Au electrodes for a Langmuir−Blodgett (LB) monolayer of the zwitterionic D+-π-A- molecule hexadecylquinolinium tricyanoquinodimethanide, C16H33Q−3CNQ. The top gold pad was deposited by a process that cools the metal vapor before deposition. The maximum rectification ratio is 27.5 at 2.2 V (the average rectification ratio is 7.55). The currents are as large as 9.04 × 104 electrons molecule-1 s-1. The result reinforces previous work with oxide-bearing Al electrodes, but the currents with Au electrodes are larger by 3−5 orders of magnitude. Rectification was also seen for a nine-monolayer Z-type LB film between similar Au electrodes but not for a monolayer of arachidic acid. The direction of enhanced electron current is from the negatively charged dicyanomethylene end of C16H33Q−3CNQ to the quinolinium ring, as predicted by the Aviram−Ratner analysis. However, there is a good fit to the behavior expected for quantum conduction dominated by a single molecular level. The best-fit energy for this level is 1.31 ± 0.25 eV above the Fermi level of the Au electrode.

FTIR studies on Langmuir–Blodgett (LB) films of amphiphile with Schiff base moiety as headgroup

In the present study, an amphiphile 2,4-dihydroxy- N-octadecylbenzylideneamine with novel structure was designed and synthesized. The aggregation structure of this amphiphile in LB films was investigated through FTIR spectroscopic techniques. The surface pressure-dependent infrared spectra of LB films have revealed some important insights on the bonded properties of the headgroup for this amphiphile. It was found that the types of H-bond formed among/between the Schiff base headgroups are determined by the deposition pressure. Intermolecular H-bond is prominent in Z-type LB film deposited at a surface pressure of ≤30 mN/m, while intramolecular H-bond formed between 2-hydroxyl group and the azomethine nitrogen is more favorable in Y-type LB film fabricated at a surface pressure of ≥40 mN/m. The variable-temperature infrared spectra of LB films further provide powerful evidence for the conversion between intermolecular and intramolecular H-bond, depending on the surface pressure.

Monolayers of a series of viologen derivatives and the electrochemical properties in Langmuir–Blodgett films

The effects of a hydrocarbon layer on the electrochemical property of viologens were studied by immobilizing a series of symmetrically bis-alkylated viologens (C n V, n=12–18) on an indium–tin oxide electrode surface by using the Langmuir–Blodgett (LB) method. The electrochemical properties of viologen LB films were studied for different parameters; length of alkyl chains, the nature of counterions in the subphase, and deposition model (X- and Z-type LB films). Cyclic voltammetric measurements indicated that π-complex dimers were easily formed for viologens that had a ‘thin’ hydrocarbon barrier (CH 2=11 and 13) in Z-type LB films, but formed with difficulty for viologens that had a ‘thick’ barrier (CH 2≥15) and for all viologens in X-type LB films. A weak peak current and a small surface coverage of ‘electroactive’ viologens were observed for X-type LB films. For every viologen-modified In and Sn oxide (ITO) electrode, the reduced potential was negatively shifted; the thicker the barrier, the larger the shift.

Influence of Poly(ethylenimine) on the Monolayer of Oleic Acid at the Air/Water Interface

The effect of poly(ethylenimine) (PEI) dissolved in water on the surface pressure–area isotherm of oleic acid (OA) at the air/water interface was investigated. On a concentrated PEI solution, the isotherm of the OA monolayers exhibited a noticeable difference as a function of subphase pH. PEI caused the collapse pressure of the OA monolayer to increase up to 45 mN/m, due to a stronger acid–base-type interaction occurring between the amine group of the PEI and the carboxyl group of OA; on a pure water subphase, the collapse pressure was 28 mN/m. On the other hand, owing to a stronger OA–PEI interaction, the OA monolayers favored a liquid-expanded state more on the PEI-containing water subphase than on the pure water. From the QCM measurement, each OA molecule appeared to interact, on average, with 4.3–5.8 ethylenimine repeating units at basic pHs. We also found that OA multilayers could be assembled on a hydrophilic substrate by a Z-type Langmuir–Blodgett (LB) deposition in a PEI-containing subphase at basic pHs. The ATR-IR spectral data revealed that, in a Z-type LB film, the headgroup of OA was mostly present as carboxylate, interacting in an ionic state with the protonated amine groups of PEI. In acidic conditions, neither a Y-type nor a Z-type deposition was really accomplished. Nonetheless, the ATR-IR spectral data suggested that OA molecules should exist in a monomeric state in a LB film assembled at acidic pHs without PEI while they would form intermolecular hydrogen bridges and/or dimers in the presence of PEI.

Second-harmonic generation from alternate-layer and Z-type Langmuir–Blodgett films: optimisation of the transparency/efficiency trade-off

Langmuir–Blodgett (LB) films of (E)-4-[(N-octadecyl-5,6,7,8-tetrahydro-5-isoquinolylidene)methyl]-N,N-dialkylaniline octadecyl sulfate, where the donor group is dimethylamino (1a), diethylamino (1b), dibutylamino (1c) and dihexylamino (1d), have optimum susceptibilities when the dyes are co-deposited in a 1∶1 mole ratio with octadecanoic acid. Alternate-layer structures of the mixed films, interleaved with poly(tert-butyl methacrylate), are non-centrosymmetric and the second-harmonic intensity increases with thickness as to N>100 bilayers. Furthermore, when the dialkylamino group is sufficiently hydrophobic, i.e. for 1c and 1d, the dyes form non-centrosymmetric Z-type structures without interleaving spacer layers. The optimum susceptibilities, chromophore tilt angles, thicknesses and refractive indices of the interleaved and non-interleaved mixed films are as follows: alternate-layer (1b), = 67 pm V−1 at 1.064 µm for φ = 37°, l = 4.08 nm bilayer−1, nω = 1.50 and n2ω = 1.58; Z-type (1c), = 76 pm V−1 for φ = 33°, l = 3.15 nm layer−1, nω = 1.52 and n2ω = 1.58. The moderately high susceptibilities arise from an optimised transparency/efficiency trade-off, the films in this series being transparent at the fundamental wavelength (1.064 µm) and having a very slight absorbance of 5 × 10−4 per active layer at 532 nm.

Z-type Langmuir–Blodgett films of a three-component optically nonlinear dye

Langmuir–Blodgett (LB) films of N-octadecyl-4-[(4-dimethylaminophenyl)azo]pyridinium iodide are Y-type and consequently the second-harmonic intensity is modest when the number of layers is odd and negligible when even. However, the required non-centrosymmetric structural arrangement may be obtained by combining the cationic dye in a 1:1:1 ratio with an amphiphilic ammonium cation and a complex dianion; the second-harmonic intensity from LB films of [ N-octadecyl-4-[(4-dimethylaminophenyl)azo]pyridinium] +[dioctadecyldimethylammonium] +[bis(2-thione-1,3-dithiol-4,5-dimercapto)zinc] 2− then increases as I 2 ω( N) = I 2 ω(1) N 2, where N is the number of layers. The films have an effective second-order susceptibility of χ (2) eff =10 pm V −1 at 1.064 μm.

Molecular rectification with M|(D-σ-A LB film)|M junctions

Molecular materials of the form electron donor-sigma-bridge-electron acceptor (D-σ-A) have been synthesized and incorporated into non-centrosymmetric Langmuir-Blodgett (LB) multilayer structures. Electrical characterization has been performed using a metal|(Z-type LB film)|metal (M|LB|M) junction construction. Current density-voltage data demonstrate striking rectification behaviour. Computational modelling of the electronic structure of the material has been carried out using a first principles, density functional approach. Possible conduction mechanisms are discussed with reference to the results of this modelling.

Langmuir−Blodgett Films of a New Pyridinium−Dicyanomethanide Dye and Their Potential Optical Applications

We report on Langmuir−Blodgett (LB) characterization of 4-[5-(dicyanomethanidyl)thien-2-yl]-N-cetylpyridinium (C16H33−PDCNT). Surface pressure−area isotherms of monolayers were investigated at T = 291 K. LB films of Y- and Z-type were characterized by means of UV−vis and FTIR spectroscopies. The compound is photobleachable and undergoes a change in molecular structure in monolayer and in solution. The molecule strongly absorbs in the visible region due to a charge transfer band both in homogeneous solution and LB films. Depending upon dipping conditions, a sharp absorption band appears at about 422 nm that could be attributed to molecular aggregates, probably of H-type. Both the 422 nm band and the broad charge transfer band at longer wavelength are photobleachable, and the former could have potential applications for optical data storage. The molecule seems to possess a large second-order hyperpolarizability, and non-centrosymmetric Z-type LB films could have promising nonlinear optical properties.

Synthesis of a series of chiral copolymers with azo groups and investigations of reversible liquid crystalline alignment induced by the LB films of these materials

A series of liquid crystalline copolymers, poly{2-hydroxyethyl methacrylate}-co-{6-[4-(S2-methyl-l-butyloxycarbonylphenylazo)phenoxy]hexyl methacrylate} with an azobenzene moiety as photoreactive mesogenic unit, was prepared and investigated by using DSC, polarized optical microscopy and X-ray diffraction. The results show that these polymers exhibit smectic phases. Z-type Langmuir-Blodgett films of these copolymers were successfully deposited onto calcium fluoride and quartz. Reversible homeotropic and planar liquid crystal alignments were induced by using the photochromism of the LB films of one of the copolymers containing 20.6 mol% of the azo unit.

Evaluation of C15TCNQ Langmuir-Blodgett Ultrathin Films on Aluminum Thin Films by Attenuated Total Reflection Measurements

The attenuated total reflection (ATR) properties of C15TCNQ Langmuir-Blodgett (LB) ultrathin films deposited on the Al thin films about 20 nm thick were measured in order to evaluate the actual structure. The C15TCNQ LB ultrathin films were prepared by the vertical dipping method, using two deposition processes. The dielectric constants and the thicknesses of these deposited LB films were theoretically calculated from the ATR curves, taking into account the presence of the natural oxide layer on Al. It was considered that Z-type LB films were obtained when the deposition of each monolayer was carried out only as the substrates were withdrawn. In contrast, when monolayers were deposited during both upward and downward movements of the substrates, the resulting LB films were considered to be not perfect Y-type, but partially Z-type.