Alternate-layer Langmuir-Blodgett Films Research Articles

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.

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.

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.

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.

Study of horseradish peroxidase activity in alternate-layer Langmuir–Blodgett films

A previously proposed method of alternate-layer deposition, which enables the inclusion into the film of adsorbed proteins, was applied to the deposition of LB films with the structure (monolayer 1/adsorbed layer of horseradish peroxidase (HRP)/monolayer 2). Various combinations of the monolayers 1 and 2 were tested with regard to the following goals: to obtain a high efficiency of protein adsorption with retained enzymatic activity; to avoid detachment of protein molecules from the substrate in aqueous solutions; to increase duration of the activity preservation of HRP during sample storage; and to provide easy removal of protective monolayer 2 when the film is used for measurements. Creation of films with such characteristics is important for their successful bioanalytical applications. Spectrophotometric measurements of the HRP activity as well as quartz crystal microbalance studies reveal an interconnection between the properties of the deposited biologically active media, experimental conditions of their preparation, and film composition. To increase structural stability of films, electron beam sensitive polymer monolayers were used for the fabrication of some variants of the alternate-layer Langmuir–Blodgett (LB) films and enzymatic activity of irradiated samples was studied as well.

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.

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.

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.

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.

Some structural considerations for pyroelectric multilayer films

Some structural and pyroelectric properties of alternate-layer Langmuir-Blodgett films of 22-tricosenoic acid/1-docosylamine and 22-tricosenoic acid/4-octadecylaniline are reported. Reflection high energy electron diffraction experiments reveal that the acid/amine multilayer system possesses a structure similar to that of a fatty acid salt, with the long axes of the molecules oriented perpendicular to the substrate surface. In contrast, for acid/aniline films the molecules are tilted with respect to the substrate normal, resulting in a structure similar to that observed for a simple monocomponent fatty acid film. The pyroelectric coefficients of the acid/aniline multilayers are shown to depend upon the film thickness. It is suggested that this reflects some difference in structure for films possessing a small number of monolayers.