Alternate-layer Langmuir-Blodgett Research Articles

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 .

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.

Langmuir and Langmuir−Blodgett Films of NLO Active 2-(p-N-Alkyl-N-methylamino)benzylidene-1,3-indandioneπ/A Curves, UV−Vis Spectra, and SHG Behavior

We study 2-(p-N-hexadecyl-N-methylamino)benzylidene-1,3-indandione (1a) as a model compound for the preparation of nonlinear optically (NLO) active Langmuir−Blodgett (LB) layers. The pressure−area (π−A) isotherm of (1a) at the air−water interface is investigated. A limiting area of 0.60 ± 0.02 nm2 per molecule is found. The alternate-layer LB deposition of compound (1a) and an inert spacer, cadmium stearate, at different surface pressures is performed. The effects of temperature and delay time between spreading of the Langmuir film and the deposition of the LB films are studied. An increase of collapse pressure from 20 to 30 mN/m is observed as the delay time increases. UV−vis spectra indicate a uniform transfer of (1a) and show compression-induced changes as a function of the deposition conditions. A split of the absorption maximum is observed for LB films deposited at higher pressures and lower temperatures. The two bands can result from two different molecular electronic transitions, affected by the aligning influence of the surface, and the different environments in the film compared to solution. Alternatively, these two bands can be associated with (at least) two different conformers. The orientation of the transition moments is evaluated on the basis of polarized UV−vis spectra at different angles of incidence, and found to be 40−44°. Significant second harmonic (SH) generation by the LB films is observed. Analysis of the SH response gives a tilt angle of 44°, in agreement with the finding from UV absorbance.

Second-Harmonic Generation from Alternate-Layer Langmuir-Blodgett Films of Optically Non-Linear Materials in which the Interleaving Species is a Complementary Dye or Poly(t-butyl methacrylate)

Non-centrosymmetric Langmuir–Blodgett (LB) films of optically non-linear dyes have been fabricated by alternating layers of complementary molecules, where the attachment of the hydrophobic alkyl chain, relative to the donor–(π -bridge)–acceptor chromophore, is reversed. The second-harmonic intensity increases quadratically with the number of LB bilayers, but the normalized intensity, I 2ω(N) /N 2 , is suppressed because the intramolecular and interlayer dipoles (D–π –A....D–π –A) are opposed. The second-order susceptibility, thickness and chromophore tilt angle, relative to the substrate normal, are X(2)zzz = 110 pm V–1 at 1.064 µm, l= 3.4 nm and φ = 31° for E-N-octadecyl-4-{2-[4-(N,N-dimethylaminophenyl)]ethenyl}pyridinium iodide (D–π–A-C 18 H 37 ) and X(2)zzz = 170 pm V –1 ,l = 2.5 nm and φ = 26° for E-N-methyl-4-[2-(4-N-methyl-N-octadecylaminophenyl))ethenyl]pyridinium iodide (C 18 H 37–D–π–A), the data corresponding to monolayer films deposited at 35 mN m –1 . The susceptibility of the alternate-layer structure is only modest (X(2)zzz = 20 pm V -1) but may be increased to c.70 pm V –1 by interleaving the active layers with passive spacers. The harmonic wavelength overlaps the charge-transfer band, whereas films of the naphthyl analogue, E-N-octadecyl-4-{2-[4-(N,N-dimethylaminonaphthyl)]ethenyl}pyridinium octadecylsulfate, interleaved with poly(t-butyl methacrylate), have a very slight absorbance of c.4.7 × 10 –4bilayer–1 at 532 nm and are transparent at 1.064 µm. The second-order susceptibility is 36 pm V –1 .

Quadratic Enhancement of the Second-Harmonic Intensity from Alternate-Layer Langmuir-Blodgett Films of an Optically Nonlinear Dye and Poly(T-Butyl Methacrylate)

Langmuir-Blodgett films of the optically nonlinear dye are non-centrosymmetric when the LB layers are interleaved by poly(t-butyl methacrylate). The second-harmonic intensity increases as I2ω(N) = I2ω(1) N2 where N is the number of active layers, and the normalised intensity is independent of film thickness to more than 100 bilayers. The second-order susceptibility and chromophore tilt angle are as follows: χ(2) zzz = 60 pm V−1 at 1.064 μm and φ = 32° relative to the normal to the substrate. The susceptibility is resonantly enhanced but the absorbance at the harmonic wavelength is only 1.5 × 10−3 bilayer−1.

Surface roughness in Langmuir-Blodgett multilayer films studied by AFM and X-ray diffraction

Langmuir-Blodgett multilayer films consisting of up to 100 layers of cadmium-substituted arachidic acid on silicon wafers have been studied. The samples were deposited from a water subphase containing Cd 2+ ions by using an alternate-layer Langmuir-Blodgett (LB) trough under computer control. Long-range orientational order was observed by atomic force microscopy on the surface of films with a small number of layers. X-ray reflectivity curves showed regularly spaced Bragg peaks arising from multilayer structure. The periodicity of the LB film is in agreement with twice the estimated molecular length, which confirms that the multilayer structure has a bilayer (Y) configuration. We demonstrate that the growth process of the LB films must have been stochastic. Our results are in good agreement with the Eden model of growth.

Conductivity of novel, thin-film, heterogeneous polymer electolyte systems

Over the last few years polymer electrolytes have generated much interest as potential components in devices such as batteries and smart windows. Some new systems synthesised recently have been proved to be excellent candidates for the preparation of thin films, especially Langmuir-Blodgett (LB) films. This technique enables the production of single- or multi-molecular layers of precisely known thickness. The work in this paper concentrates on the investigation of a bilayer structure composed of two different types of polyether-alkyl polymer electrolyte utilising the alternate-layer LB technique. We have demonstrated the viability of producing bilayer structures 19 molecular layers (approximately 450 Å) thick. In addition, samples prepared by using ions in the subphase exhibit higher conductivities than those without added ions, demonstrating that ion uptake from the subphase is taking place. Also, conductivity normal to the layer planes was extremely low in contrast to the higher values obtained on bulk samples. This demonstrates that ions are unable to cross the alkyl layers in the highly ordered LB system.

Intermolecular Charge Transfer in Langmuir-Blodgett Films: Formation of Mixed and Alternate-Layer LB Films of Quinhydrone and TTF-TCNQ Derivatives

Intra- and interlayer charge transfer properties were examined in the mixed and the alternate-layer Langmuir-Blodgett (LB) films of 2-methyl-5-octadecyl-1,4-dihydroxybenzene (C18HQ) and 2-methyl-5-octadecyl-1,4-benzoquinone (C18Q), which is an example of the “neutral” charge transfer complex, and 2-hexadecyl-3-heptadecyl-6,7-dimethyltetrathiafulvalene (C16C17DMTTF) and 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (C18TCNQ) for the “ionic” complex. In the former case, intermolecular charge transfer took place in both types of LB films. The charge transfer band between C18HQ and C18Q appeared at 590 nm even for the bilayer film. The charge transferred state should be accompanied by hydrogen bonding, although the IR spectra revealed that a certain amount of non-hydrogen bonded C˭O group exists in the film. The 1:1 mixture of C16C17DMTTF and C18TCNQ gave an ionic charge transfer complex in the form of mixed LB films. UV-visible and IR spectra revealed the strong complex formation during the film-forming process. On the other hand, the charge transfer interaction between the adjacent layer of the interleaved LB films essentially did not take place.

Langmuir-Blodgett Films of Alkyl-Substituted Quinhydrone

Research Institute for Electronic Science and Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060, JAPAN The charge transfer (CT) properties were examined in the mixed and the alternate-layer Langmuir-Blodgett (LB) films of 2-methyl-5-octadecyl-1,4-dihydroxybenzene (C18HQ) and 2-methyl-5-octadecyl-1,4-benzoquinone (C18Q). Intermolecular CT took place in both types of LB films. The CT band between C18HQ and C18Q appeared at 590 nm suggested the neutral CT state as in the case of unsubstituted quinhydrone. The direction of CT was examined by the polarized spectra. The CT state was accompanied by hydrogen bonding, which was confirmed by the hydrogen bonded and non-hydrogen bonded C=O stretching bands appeared in the IR spectra.

Optimisation of the pyroelectric figure of merit of porysiloxane/amine superlattices

Alternate-layer Langmuir-Blodgett (LB) films containing pendant side-chain polysiloxanes and eicosylamine have been shown to possess an exceptionally high pyroelectric coefficient (~14 μCm −2K −1). This study details the optimisation procedure adopted in order to achieve such high thermal sensitivity. The dependence of the pyroelectric activity on the number of deposited monolayers and cadmium ion content are described. The figure of merit for the best LB pyroelectric sensors prepared in this work is high as 80% of the value of poly (vinylidenedifluoride).

High pyroelectric sensitivity in alternate layer Langmuir-Blodgett superlattices

A family of polysiloxanes which have been designed for the purpose of achieving high pyroelectric activity in Langmuir-Blodgett (LB) film form is reported. The alternate layer LB deposition process has been used in which each siloxane monolayer (S) is co-deposited with a monomeric eicosylamine monolayer (E) to form alternate layer multilayer assemblies of the form SESESE… This approach ensures a macroscopic polarization within the LB film, which is an essential criterion for observation of the pyroelectric effect. Pyroelectric devices have been fabricated using aluminium electrodes and their pyroelectric sensitivity has been measured using a quasi-static technique. This collaborative study has involved investigating the effect on the pyroelectric response of (i) varying the degree of substitution on the polysiloxane backbone with aliphatic pendant side-chains, (ii) varying the length of the aliphatic pendant side-chains and (iii) incorporating polar aromatic pendant side-chains. This iterative strategy has led to an improvement in the pyroelectric coefficient from around 0.8 μC m −2 K −1 for an aliphatic substituted homopolymer to 4.6 μC m −2 K −1 for a copolysiloxane substituted with an aromatic pendant side-chain. The effects of LB film thickness (number of deposited monolayers) and the electrode overlap area have also been investigated and have brought about a further dramatic increase in activity to around 13 μC m −2 K −1. This is the highest pyroelectric coefficient ever reported for an alternate layer LB film assembly. It is believed that the activity of these thin films arises predominantly from the primary pyroelectric effect since an investigation into the response from two substrates possessing very different thermal expansion coefficients yielded almost identical behaviour, suggesting that the secondary piezoelectric component is small.

Temperature-dependent polarization in LB films of a stilbazole complex of iridium(I)†

The pyroelectric effect occurs in non-centrosymmetric, polar crystals and thin films whose spontaneous polarization changes as a function of temperature. In this paper, we report on a new organo-iridium complex which exhibits the pyroelectric effect. The alternate layer Langmuir-Blodgett deposition technique has been utilized to fabricate molecular assemblies which are polar at the macroscopic level. This technique facilitates the co-deposition onto solid substrates of monolayer films of two different materials (A and B) floating at an air-water interface. These monolayers are deposited alternately such that the resulting layer structure is of the ABABA form. This fabrication technology enables polar molecules to be organized into thin multi-lammellae assemblies which possess a macroscopic polarization. The pyroelectric coefficients of these films have been measured over a range of temperature. Several LB samples containing different numbers of iridium stilbazole monolayers have been examined showing that...

Preparation and characterization of C 60 and C 60/phtalocyanine derivative alternate-layer Langmuir-Blodgett films

The monolayer of C 60 mixed with arachidic acid (AA) on a water subphase is deposited on different substrates such as Si, GaAs single crystals and CaF 2, SiO 2 plates by the Langmuir-Blodgett (LB) technique. The position of AA molecules is determined at the vacancy of C 60 molecules from measuring the π − A isotherms of C 60 and AA monolayers, separately. According to previous work, C 60 and copper tetra-4-(2,4-di-t-amylphenoxy) phthalocyanine (CuPc) alternate-layer LB film is deposited on different substrates. Obvious in the UV-Vis absorption spectra of C 60/CuPc alternate film is not observed, compared with C 60 and CuPc LB films.

Second harmonic generation from alternate-layer LB films: quadratic enhancement with film thickness

The second harmonic generation (SHG) from alternate-layer Langmuir-Blodgett (LB) films of stilbazolium dyes and 4, 4'-dioctadecyl-3, 5, 3', 5'-tetramethyldipyrryl-methene hydrobromide increases as I 2ω(N) = l 2ω(1) N 2 for N ≥ 2 bilayers, but the equivalent monolayer signal ( I 2ω(1) ) is significantly less than the intensity from LB monolayers of the corresponding dye on glass. This is attributed, in part, to the different molecular tilts and, in part, to the position of the anion which influences the charge distribution within the chromophore. For alternate-layer films of E-N-octadecyl-4-[2-(4-dimethylaminophenyl)ethenyl]pyridinium octadecylsulphate (dye I) the second-order susceptibility and chromophore tilt angle are χ (2) zzz = 45 pm V -1 and ϕ = 30° respectively, whereas the methoxy congener, E-N-octadecyl-4-[2-(4-methoxyphenyl)ethenyl]pyridinium octadecylsulphate (dye II), has χ (2) zzz = 11 Pm V -1 and ϕ = 35°. The nonlinear optical properties of such films are dependent upon the compatibility of the dye and spacer.

Cyclic polysiloxanes in polar LB assemblies: synthesis, evaluation and pyroelectric behaviour

A family of cyclic polysiloxanes, of general structure ((CH 3)Si(X)O) 4, substituted at position X with either aliphatic acid or aromatic acid side-chains has been synthesized and evaluated. These compounds form monolayer films when spread from solution onto a pure water subphase. They can be transferred using the Langmuir-Blodgett (LB) deposition technique onto glass and aluminum-coated glass substrates to form Y-type multilayer films, but they also form alternate layer assemblies when co-deposited with monomeric eicosylamine monolayers. Such acid/amine multilayers exhibit the pyroelectric effects since the interaction of carboxylic acid and amine groups yields a temperature-dependent spontaneous polarisation. The quasi-static pyroelectric coefficients of LB films containing 11 monolayers (six monolayers of cyclic polysiloxane/five monolayers of eicosylamine) have been measured over a range of temperatures for each cyclic polysiloxane/eicosylamine assembly. The coefficient is significantly larger for the cyclic polysiloxanes substituted with ω-(4-alkoxyphenyl)alkanoic acid moieties than for those substituted with entirely aliphatic side-chains. This suggests that the increased dipolar nature of the former compounds plays an important role in the pyroelectric behaviour of these alternate layer LB structures. Dramatic changes in the magnitude of the pyroelectric coefficients have been observed when relatively subtle variations to the structure of the aromatic side-chains have been made. In particular, the pyroelectric activity depends on the length of the spacer groups separating the oxyphenyl moiety from the siloxane ring and the carboxylic headgroup.

Pyroelectric activity of aromatic-substituted copolysiloxane/ eicosylamine alternate-layer Langmuir-Blodgett films

The pyroelectric behaviour of alternate-layer Langmuir-Blodgett (LB) films containing monomelic eicosylamine and a linear copolysiloxane substituted with ×omega;-(4-alkoxyphenyl)alkanoic acid side-groups has been investigated. A quasi-static detection method has been used to measure the rate of change of electric polarization with respect to temperature, i.e. the pyroelectric coefficient of these molecular thin films. The coefficient measured at 303 K is 6·4 μCm−2 K−2, which is one of the highest values ever reported for an LB film device.

Fourier transform infrared spectroscopic studies on alternate-layer Langmuir-Blodgett films with nonlinear optical properties

Fourier transform infrared spectroscopic studies on alternate-layer Langmuir-Blodgett films with nonlinear optical properties

Tricos-22-enoic acid/1-docosylamine alternate-layer Langmuir–Blodgett films: polymerisation, pyroelectric properties and infrared spectroscopic studies

Pyroelectric alternate-layer Langmuir-Blodgett films of tricos-22-enoic acid and 1-docosylamme have been polymerised by ultravoilet irradiation. Partially polymerised films retained a significant proportion of the initial Pyroelectric activity. However, irradiation for long periods of time resulted in decarboxylation and photodesorption, which eventually led to the loss of the pyroelectric response.

Alternate-layer Langmuir-Blodgett films of long-chain TCNQ and TTF derivatives

Langmuir-Blodgett films of a long-chain TCNQ derivative alternated with a long-chain TTF compound have been deposited onto glass substrates. These layers exhibit a bulk lateral conductivity of (5±1) × 10 −3 S cm −1 at room temperature. Over the temperature range 100 – 300 K, typical semiconducting behaviour is observed, with a thermal activation energy of 0.26 ± 0.01 eV.

Small angle X-ray analysis of alternate-layer Langmuir—Blodgett films

Abstract Using small-angle X-ray diffraction, we demonstrate in this Letter that alternate-layer Langmuir-Blodgett films of tricosanoic acid/1-docosylamine and docosanoic acid/1-docosylamine retain their ‘as-deposited’ structure. Both of these non-centrosymmetrical bilayer structures exhibit a smaller tilting of their hydrocarbon tails than that observed for the corresponding centrosymmetric monocomponent systems. The electron-density profiles obtained for the alternate-layer systems reveal that the points of contact for the ends of the hydrocarbon chains are shifted from the centre of the unit cell and that the density maxima, corresponding to the positions of the polar head groups, possess an asymmetrical shape.