Phospholipid-based potassium-selective Langmuir-Blodgett films

The preparation and properties of mixed Langmuir-Blodgett films of the phospholipid L--phosphatidic acid dipalmitoyl and the ionophore valinomycin are described. IR spectroscopy shows clear evidence for the formation of the valinomycin-potassium complex when these layers are immersed in KCl solutions. However, and in contrast to previous studies with fatty acid-valinomycin mixtures, dissociation of the potassium complex is also observed.     

Langmuir-Blodgett films as chemical sensors

This review covers the fundamental scheme of chemical sensors, fields of major interest in chemical sensors with Langmuir-Blodgett (LB) films, and new and future trends in LB sensor studies. The topics discussed are the relation between sensor characteristics and LB film structure, the biomimetic approach to sensor developments, proposals for transducer improvements, and the possibility of fabricating molecular filters by LB techniques.     

Langmuir-Blodgett films from dioctadecylamine picrate

Langmuir-Blodgett (LB) films prepared from picrate of dioctadecylamine(PDODA) and dioctadecylamine (DODA) were studied for the high picrate dipole moment, potentially effective in pyroelectricity. Stable condensed monolayers were obtained on pure water at pH from 5 to 8. About 90% of the PDODA in the films was hydrolysed to DODA. Moderate repulsion between ionized polar heads in the films promoted the desirable X- or Z-type deposition on solid supports, but it prevented a successive deposition of more than five monolayers. At pH below 5 the content of CdCl₂ or picric acid in the subphase decreased the PDODA hydrolysis. Strong repulsion under these conditions caused an expansion of the films and prevented the deposition of more than a single monolayer.     

Langmuir-Blodgett films for nonlinear optical applications

Azo-benzene chromophores with an oxygen donor and five different end-groups have been attached to octadecyl chains or to polysiloxane backbones via hexadecyl spacer groups. These were deposited by the Langmuir-Blodgett (LB) technique using our own design of diamond-barrier trough. The LB deposition properties of the polymers were superior to those of the low molecular mass materials, which did not show second harmonic generation (SHG) as deposited monolayers. The strength of SHG from the polymers correlated with acceptor strength, but the maximum signal was only about 10% of that of hemicyanine. Only one of the polymers gave thick films of very high quality; this had a hydroxyl end-group and showed no SHG except when deposited from an acid subphase. This was presumed to result from protonation of the azo group to create an acceptor. The quality of the deposited films may correlate with their phase behaviour; the best material showed only a presumed soft crystalline phase. The film-forming properties of that film were remarkable; loss figures of around 2.5 dB cm^?1 have been recorded at 633 nm in films of 1.3 Μm thickness. Future work will seek to isolate and independently control the factors involved in good film formation and in high SHG.     

Ultrathin polymer Langmuir-Blodgett films for microlithography

We describe photolithographic properties of poly(N-dodecylmethacrylamide-co-t-butyl 4-vinylphenyl carbonate) [p(DDMA-tBVPC)], which has a structure being subject to main chain scission and deprotection of t-butoxycarbonyloxy group by deep UV irradiation. Positive-tone patterns of the p(DDMA-tBVPC) Langmuir-Blodgett (LB) film with 60 layers were obtained by deep UV light irradiation followed by development with alkaline aqueous solution. The resolution of the pattern was 0.75 μm, which is the resolution limit of the photomask employed. The etching resistance of p(DDMA-tBVPC) LB film deposited for the pattern of the gold film is also investigated.     

Optical applications of IR-absorbing Langmuir-Blodgett films

Langmuir-Blodgett (LB) films have been produced using a novel near-IR- absorbing dye based on 1,4-naphthoquinone. Wavelength compatibility with semiconductor diode lasers makes it particularly suitable for optical storage applications. Ablative data marks have been recorded using 830 nm light with a pulse energy of 3.6 nJ. The sensitivity is dependent on heat flow within the film and substrate, and can be increased using LB film buffer layers.     

Surface modification using polymer Langmuir-Blodgett films

We describe a simple and effective approach to introduce a functional group into polymer film on a solid surface using reactive polymer LB films. N-dodecylacrylamide copolymers containing terminal amino groups in the side chains as the reactive moiety form a stable monolayer, and the monolayer was transferred onto a solid support to modify the solid surface using the Langmuir-Blodgett method. The transferred coatings were characterized with fluorescence, IR spectroscopies, and X-ray diffraction. The reactivity of the terminal amino group incorporated in the LB films was investigated in detail using fluorescein isothiocyanate (FITC) as a fluorescent probe. The chemical reaction between amino groups in the LB films and FITC in the bulk solution was completed within approximately 30 minutes and the chemical bond formation was confirmed by infrared spectroscopy. Furthermore, the fluorescent image of the multilayers reacted with FITC were observed with fluorescent microscopy. This method is effective for tailoring functional organic ultrathin films on solid substrates.     

Conducting Langmuir-Blodgett films as electrode materials

Photoelectric cells based on Langmuir-Blodgett (LB) films were fabricated using dye monolayers as photoconductors and N-docosylpyridinium-di(tetracyanoquinodimethane) (PT) monolayers as one of the electrode materials. It is found that PT films can be used as a wiring material instead of large work function metals such as silver or Ni-Cr. However, in comparison with the photoelectric cells using the conventional metals, a few different properties were observed for those wired with PT. These differences are assignable to the difference between the LB film-metal and the LB film-LB film contacts.     

Semifluorinated TTF derivatives in Langmuir-Blodgett films

New semifluorinated tetrathiafulvalene (TTF) derivatives were prepared and used in Langmuir and Langmuir-Blodgett (LB) films. Transfer onto a solid substrate of such pure monolayers can be easily achieved leading to organized LB films in which the fluorinated chains are tilted with respect to the normal of the substrate as demonstrated by infrared and X-ray experiments. Doping of such films by iodine does not induce change in the layer spacing on the contrary to already published studies concerning alkyl derivatives of TTF. Such structural stability and values of the layer thickness suggest some interdigitation between TTF groups within the LB film. Depending on the exact chemical structure of the TTF moiety, the iodination is stabilized or not. When the doped film is stabilized in a mixed valency state, conductivity of approx. 10⁻² S cm⁻¹ was measured at room temperature.     

Photopatterning of heterostructured polymer Langmuir-Blodgett films

Heterostructured polymer Langmuir-Blodgett (LB) film prepared by using poly(N-dodecylacrylamide-co-t-butyl 4-vinylphenyl carbonate) (p(DDA-tBVPC53)) and poly(N-neopentyl methacrylamide-co-9-anthrylmethyl methacrylate) (p(nPMA-AMMA10)) polymer LB films which can act as photogenerator layers were investigated. Patterns with a resolution of 0.75 μm were obtained on heterostructured polymer LB films composed of 4 layers of p(nPMA-AMMA10) LB film (top layers) and 40 layers of p(DDA-tBVPC53) LB film (under layers) on a silicon wafer by deep UV irradiation followed by development with 1% tetramethylammonium hydroxide aqueous solution. The sensitivity of the heterostructured polymer LB films was improved without loss of the resolution compared with p(DDA-tBVPC53) LB film. The etch resistance of the heterostructured polymer LB films was sufficiently good to allow patterning of a copper film suitable for photomask fabrication.     

Polarization processes in pyroelectric Langmuir-Blodgett films

We report on an investigation into the origins of the pyroelectric effect in some alternate layer Langmuir-Blodgett (LB) films. The pyroelectric coefficient is measured as a function of temperature and of the thermal expansion coefficient of the substrate on which the film is deposited; these experiments enable a partial separation of the primary and secondary pyroelectric effects. The results indicate that thermal expansion of the substrate tends to reduce the total pyroelectric coefficient through a piezoelectrically induced effect. At low temperatures, the film behaves like a “free crystal”, and the pyroelectric coefficient shows a power law dependence on temperature. However, at temperatures above 240 K, the substrate- induced effect becomes dominant. The results are used to deduce methods by which the pyroelectric response of the LB film devices might be improved.     

Langmuir-Blodgett films for nonlinear optical applications

Azo-benzene chromophores with an oxygen donor and five different end-groups have been attached to octadecyl chains or to polysiloxane backbones via hexadecyl spacer groups. These were deposited by the Langmuir-Blodgett (LB) technique using our own design of diamond-barrier trough. The LB deposition properties of the polymers were superior to those of the low molecular mass materials, which did not show second harmonic generation (SHG) as deposited monolayers. The strength of SHG from the polymers correlated with acceptor strength, but the maximum signal was only about 10% of that of hemicyanine. Only one of the polymers gave thick films of very high quality; this had a hydroxyl end-group and showed no SHG except when deposited from an acid subphase. This was presumed to result from protonation of the azo group to create an acceptor. The quality of the deposited films may correlate with their phase behaviour; the best material showed only a presumed soft crystalline phase. The film-forming properties of that film were remarkable; loss figures of around 2.5 dB cm^–1 have been recorded at 633 nm in films of 1.3 m thickness. Future work will seek to isolate and independently control the factors involved in good film formation and in high SHG.     

Characterization of copper phthalocyanine Langmuir-Blodgett films

Asymetrically substituted copper phthalocyanine (as-CuPc) Langmuir-Blodgett (LB) films are prepared on metal-coated quartz substrates and the electronic behaviour of sandwich structures of metal/CuPc LB film/metal are studied under normal laboratory ambients. The results show that CuPc films are very stable and reproducible. D.c. measurements produce ohmicI(V) characteristics while the a.c. conductance shows a power-law dependence on the frequency.     

Organization of copper nanoclusters in Langmuir-Blodgett films

Stable nanoclusters of Cu were synthesized using Langmuir-Blodgett films of octadecylsuccinic acid (ODSA) as template. The Langmuir-Blodgett films of ODSA formed from subphase containing copper ions were first subjected to sulphidation (S) using sodium sulphide and then hydrogenated (H) using hydrogen gas. Diffuse reflectance UV-visible spectroscopy (DIR-UV-vis), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) used to characterize these films indicated the formation of Cu(0) metallic clusters ranging in size from 3 ∼ 10 nm.     

Friction force microscopy of mixed Langmuir-Blodgett films

Thin films of mixtures of fluorocarbons and hydrocarbons are investigated by friction force microscopy. For a mixture of equal molar parts of arachidic acid and partially fluorinated carboxylic acid, phase separation manifests itself as circular domains. By shortening the length of the hydrocarbon chain, the disorder of the system is increased and phase separation takes on a fractal geometry. The frictional forces are found to be material specific and distinguish between domains of fluorocarbons and hydrocarbons. With higher normal forces hydrocarbon particles are easily sheared away while the fluorocarbon region remains unbroken. The results are discussed in relation to the phenomenon of boundary lubrication.     

Langmuir-Blodgett films in amorphous silicon MIS structures

In this paper we describe the properties of MIS structures based on hydrogenated amorphous silicon (a-Si:H) and organic films deposited using the Langmuir-Blodgett technique. Results are reported for undoped a-Si:H passivated with an insulating film of cadmium stearate 80 nm thick. The deposition of the monolayers was found to be critically dependent on the surface condition of the semiconductor.The capacitance data display well-defined accumulation and depletion regions and suggest that inversion is obtained when the device is reverse biased. The conductance data are similar in shape to those observed for conventional MOS structures on crystalline silicon. However, hysteresis and frequency dispersion effects complicate their interpretation in terms of surface state densities.From this preliminary investigation we conclude that useful MOS devices incorporating both thick and thin insulating layers can be based on the a-Si:H/Langmuir-Blodgett film system.     

Preparation and characterization of protein Langmuir-Blodgett films

A protein monolayer of bovin serum albumin (BSA) was stabilized at the air- water interface using glutaraldehyde (GA). The GA-treated BSA film was transferred onto the glass surface, followed by immunochemical lamination of anti-BSA antibody. Protein A-β-galactosidase (pro A-β-gal) conjugate was then reacted with the antibody molecular layer. Biofunctional protein films can be prepared on the BSA base Langmuir-Blodgett film by intermolecular complementary affinity between the proteins concerned.     

On the degradation of conducting Langmuir-Blodgett films

Direct current through conducting Langmuir-Blodgett films was observed as a function of the applied voltage and alternating current as a function of frequency. In some cases voltage-current characteristics appeared to be quasi-ohmic but in other cases they exhibited tunnelling-like behaviour. This phenomenon was interpreted as being due to the progressive detachment of the film from the micro-electrodes, as confirmed by measurements of the conductivity as a function of time. This interpretation was also confirmed by estimating the tunnelling barrier parameters from voltage-current characteristics, by conductivity measurements before and after thermal treatment of the samples and by conductivity measurements of samples stored in air and under vacuum.