R.H. Tredgold

LANGMUIR–BLODGETT FILMS OF SIMPLE ESTERIFIED PORPHYRINS

Abstract Protoporphyrin IX dimethyl ester, cobalt mesoporphyrin IX dimethyl ester, and tetra(4-carbomethoxyphenyl)porphyrin form condensed Langmuir films which can be transferred at 20–30 mN m −1 to quartz or aluminized substrates by an unusual Z deposition process. Some preliminary electrical measurements for Al/porphyrin/Au in a conventional sandwich cell arrangement and for Au/porphyrin/Au on quartz in a surface cell arrangement are reported. The capacitance and dark d.c. conductance of monolayers and multilayers on aluminium suggest that films of uniform quality and high dielectric constant have been produced. Protoporphyrin IX dimethyl ester is highly photoconductive in both cell arrangements, with a conductivity anisotropy of 10 4 −10 5 . Its action spectrum corresponds closely to the absorption spectrum of the solid film. The behaviour is compared and contrasted with that of chlorophyll-a and of the cobalt complexes of chlorophyll-a and the mesoporphyrin and is discussed in terms of the film structure which allows close packing of the porphyrin rings.

Langmuir-Blodgett monolayers of preformed polymers

The authors have shown that it is possible to form monolayers of three distinct preformed polymers on Al-Al2O3 substrates. Using gold top electrodes and an annealing technique it is possible to obtain exceptionally stable and resistive films.

Langmuir-Blodgett films made from preformed polymers☆

Abstract A brief general review of work in this field is given. A particular family of copolymers derived from maleic anhydride is described and the way in which these copolymers behave at the air-water interface and as Langmuir-Blodgett multilayers is discussed with particular reference to the influence of the nature of the side-chains on isotherms and structure. The application of these materials to the formation of low loss optical waveguides, electron beam resists and photodiodes is reviewed.

Langmuir-Blodgett monolayers of preformed polymers on n-type GaP

We have shown that it is possible to vary the Schottky barrier height of GaP by varying the anhydride content of an insulating monolayer of poly(octadecene-1-maleic anhydride) deposited onto the semiconductor. This is attributed to negative charges on the anhydride groups, the surface state density being unchanged by the presence of a polymer monolayer.

X-ray analysis of some porphyrin and polymer Langmuir-Blodgett films

Derivatives of mesoporphyrin IX dimethylester, poly(octadec-1-ene-maleic anhydride) and poly(styrene-maleic anhydride) have been prepared as Langmuir-Blodgett (LB) films. Examining their layer structure using X-ray diffraction analysis has led to several interesting and important conclusions about their packing arrangement.

Langmuir-Blodgett films from preformed polymers: derivatives of octadec-1-ene-maleic anhydride copolymers

Abstract We show here that good quality Langmuir-Blodgett films of octadec-1-ene- maleic anhydride copolymer derivatives can be deposited onto a number of substrates. The effects on the isotherm, deposition and packing of a number of derivatives are studied and these copolymers compared with a number of possible alternative amphiphilic polymer systems.

Electrical conductivity in Langmuir-Blodgett films of porphyrins: in-plane and through-the-film studies

Abstract We have prepared Langmuir-Blodgett (LB) multilayers of several metal complexes of mesoporphyrin IX dimethyl ester and investigated their dark conductivity normal to the film plane (on aluminized glass substrates with a gold top electrode) and parallel to the film plane (on quartz substrates with two interdigitating gold top electrodes). The silver(II) complex is amongst the most conductive of LB film materials yet measured in the film plane (resistivity, 4 × 10 4 Ω m) and the conductance is anisotropic. The dependence of the current on film thickness, voltage, temperature and frequency has been extensively investigated in both directions and reveals that these multilayers have some unusual conductivity properties which are quite different from those normally reported for conventional insulating multilayers. Conduction through the film is barrier limited with some evidence that the barrier is in the first few monolayers of the film. Conduction along the film is bulk limited but does not scale with thickness, the thinnest films ( n ⩽ 5 layers) being anomalously resistive. The conduction in both directions is thermally activated, with the same activation energy (0.43–0.48 eV). The results are discussed in terms of two possible models: one model involves structural defects in the films and the other model assumes that a depletion layer occurs in the film near an interface as in a conventional semiconductor.

Electron-beam resists from Langmuir-Blodgett films of poly(styrene/maleic anhydride) derivatives

Abstract A range of styrene/maleic anhydride polymer derivatives and some related polymers have been synthesized and their performance as electron-beam resists assessed, initially as spin-coated films and then as Langmuir-Blodgett (LB) films on silicon. The effects of molecular structure, molecular weight and packing have been investigated in designing materials for optimum sensitivity and resolution. The most promising materials contain vinyl or ethynyl crosslinking groups and are negative resists, resolution ≈ 0.25 μm and gel dose in the range 1–40 μC cm −2 . Exposed and developed LB films only 50 layers (≈ 50 nm) thick have good plasma etch resistance, allowing the processing of tri-level resist structures (polyimide/Al/LB film) with little loss of resolution. This indicates that LB films based on poly(styrene/maleic anhydride) have the potential for the fabrication of submicrometre electronic devices.

Tunnelling currents in Langmuir-Blodgett monolayers of stearic acid

Tunnelling currents in stearic acid monolayers stored at 15°C in dry He have been monitored over an 8 week period. Measurements were made as a function of both temperature and also applied voltage. It is deduced that the observed currents are associated with defects which are gradually annealed out.

Optical and structural characterization of Langmuir-Blodgett multilayers of non-polymeric and polymeric hemicyanines

Abstract Non-polymeric and polymeric amphiphilic hemicyanine dyes have been synthesized and extensively characterized as Langmuir films by their isotherm properties, and as Langmuir-Blodgett films by their deposition behaviour, optical absorption spectra and X-ray diffraction. Multilayers of non-polymeric dyes showed optical evidence for both non-aggregated ( λ max = 492 nm and aggregated ( λ max ≈ 420 nm) chromophores. Halide salts in the subphase, and particularli KI, had a marked effect on the isotherms and iodide ions were incorporated into the multilayers where they favoured a non-aggregated structure, presumably by a spacing effect. In alternating multilayers, aggregate formation was avoided by using the cadmium salts of fatty acids, or a polymer, as the optically inactive material. By contrast, multilayers of a polymeric hemicyanine were always free from chromophore-chromophore interactions however deposited ( λ max = 463 nm) and were clear and homeotropic. X-ray diffraction showed good evidence for a superlattice in multilayers of non-polymeric hemicyanine alternating with cadmium arachidate. These findings will be useful in selecting systems for future non-linear optics applications.