G.G. Roberts

The preparation and properties of stable metal-free phthalocyanine Langmuir-Blodgett films☆

Abstract In this paper we show for the first time that it is possible to prepare stable Langmuir-Blodgett films of metal-free phthalocyanine. They are of reproducible quality and adhere tenaciously to various substrates and to each other. Preparation conditions and isotherms are described for both a tetra-tert-butyl-substituted phthalocyanine and the unsubstituted molecule. Preliminary results describing the structural, electrical and optical properties of the films are also presented.

Electrical conduction and low voltage blue electroluminescence in vacuum-deposited organic films

Abstract Bright blue (400–500 nm) recombination electroluminescence (EL) has often been observed from anthracene crystals with solid electrodes; external quantum efficiencies of 1%–8% have been seen and the theoretical efficiency is approximately 40%. The power efficiency has always been greatly degraded by the high voltages necessary to obtain reasonable currents through the comparatively thick crystals used. We report the preparation, and the structural, electrical and EL properties of approximately 0.6 μm vacuum-deposited anthracene films through which large steady state current densities can be passed at rather low applied voltages. The films showed the critical optimization effect, various structural and electrical properties exhibiting sharp singularities when the films were prepared at substrate temperatures within a narrow range near −60 °C. EL visible in normal room lighting was obtained at applied voltages of only about 30 V and up; in thinner films, EL could be seen in a darkened room at only about 12 V. This is apparently the first time that clearly visible EL has been reported from an organic material at voltages significantly less than 100 V. Broadly similar results were obtained for perylene and 1,12-benzperylene films; EL does not appear to have been observed before from these substances in any form. We discuss in some detail the prospects for practical EL devices based on organic films. In view of our results, and of the great improvements which are now probably feasible, we conclude that the prospects are promising; if these improvements can be realized, such devices could include high efficiency blue light-emitting diodes, and large-area broader wavelength systems for illumination applications.

ELECTRICAL AND PHOTOELECTRICAL TRANSPORT PROPERTIES OF LANGMUIR–BLODGETT FILMS AND A DISCUSSION OF POSSIBLE DEVICE APPLICATIONS

Abstract A rather comprehensive review is given of the electrical and photoelectrical transport properties of Langmuir-Blodgett (LB) built-up monolayers and multilayers. Both conventional (fatty acid) and unconventional materials are included. It is shown that this field has undergone enormous progress in the last few years; many promising scientific areas have opened up and a wide range of device applications can now be considered. The feasibility of several devices using insulating LB films has in fact been demonstrated, and it is now possible to design complex structures and supermolecular “organizates” to fulfil specific “active” electrical functions, and to fabricate them predictably. An extensive review is given of the possible device applications for LB films; these extend from a whole range of electronic devices utilizing the thinness and perfection of insulating LB films, to complex supermolecular structures designed for such purposes as solar energy conversion and high temperature superconductivity. The last possibility is discussed in some detail and is concluded to be a very promising area for future research.

An investigation by electron spectroscopy for chemical analysis of chemical treatments of the (100) surface of n-type InP epitaxial layers for Langmuir film deposition

Abstract The surface chemistry of InP substrates used for the fabrication of electronic devices (MIS structures) was examined by ESCA. Spectroscopic data on related materials such as indium metal, plasma-oxidized indium metal, In 2 O 3 , InPO 4 ( x H 2 O), P 4 O 10 , AlPO 4 , (C 6 H 5 O) 3 PO and (C 6 H 5 ) 3 P were collected so as to aid the assignment of components in the spectra of as-received and etched InP surfaces on the basis of the chemical shifts and the intensity ratios. Surface features of InP substrates from several batches were examined in the as-received and the chemically etched forms. The effectiveness of several types of etchants and the features thus produced on the substrate surface were evaluated and are discussed.

Electronic devices incorporating stable phthalocyanine Langmuir-Blodgett films☆

Abstract This paper describes the critical role of the solvent in the preparation of monolayers of phthalocyanine. Pressure-area isotherms demonstrate that realistic monomolecular areas can be achieved for a number of compounds provided that the central hydrogen atoms in the ring system are replaced by appropriate metal ions. Particularly good results have been obtained with an asymmetrically substituted copper phthalocyanine. Transmission electron diffraction data for copper phthalocyanine tris(CH2NHC3H7-iso) show a preferred orientation over a sample region of 3 mm. The excellent stability of the Langmuir-Blodgett films has enabled us to incorporate them in a number of electronic devices. These include a gas-sensitive structure based on silicon, a bistable switch using GaAs and an electroluminescent metal-insulator-semiconductor ZnSeS diode.

Second-harmonic generation in mixed hemicyanine: fatty-acid Langmuir–Blodgett monolayers

The second-order nonlinear optical properties of a hemicyanine-dye-containing monolayer are measured by using second-harmonic generation. An unexpected large enhancement of the second-harmonic signal from the dye is observed on dilution by optically passive fatty acid, with the maximum enhancement occurring at around 50% dilution. Possible mechanisms are discussed.

A new model for the deposition of ω-tricosenoic acid Langmuir-Blodgett film layers

Abstract Langmuir-Blodgett (LB) films can be assembled one monolayer at a time to produce a high quality planar sheet of accurately controlled thickness. A possible application is in the area of electron beam lithography where the best results to date have been obtained with ω-tricosenoic acid. Films of this material have acceptable sensitivity, contrast and resolution to produce very small feature sizes. However, no study has yet been reported of the time required for deposition of a film of reasonable thickness. To rectify this omission, in this paper the limitations on ω-tricosenoic acid deposition speed are investigated and a model is proposed to explain the observations. This model is based on molecular reorganization and is thought to have general applicability to LB film deposition. Transmission electron diffraction data are presented to support the theory and, for optimum dipping conditions, excellent orders is revealed even using fast dipping speeds. We therefore conclude that the time for deposition of an ω-tricosenoic acid resist film need not be significantly longer than that required for spin coating a layer of resin.

Electroluminescence, photoluminescence and electroabsorption of a lightly substituted anthracene langmuir film

Abstract Optical data are presented for a novel thin film structure based on anthracene. Thin films of the 9-butyl–10-anthryl propionic acid derivative were prepared using the Langmuir-Blodgett technique. By applying electric fields in excess of 10 6 V cm −1 both electroabsorption and electroluminescence results have been obtained. The electroluminescence spectrum exhibits a shoulder at the wavelength where the maximum in the fluorescence spectrum occurs.

A Langmuir trough for the production of organic superlattices

Abstract This paper describes the design and operation of a Langmuir trough used to produce high quality organic superlattices. Ease of fabrication of the films has been accomplished by adding a fixed beam and revolving central section to an automated constant-perimeter barrier system. The multilayer assemblies have been characterized using electron diffraction and optical absorption techniques.

A.C. and D.C. conduction in lightly substituted anthracene Langmuir films

Abstract The a.c. and d.c. properties of short-chain derivatives of anthracene prepared by using the Langmuir-Blodgett technique are reported; these thin films display interesting conduction effects on account of their unusual molecular structure. Emphasis is placed on the 9-alkyl-10-anthrylpropionic acid derivatives in which the alkyl chain is either C4H9 (C4 anthracene) or C6H13 (C6 anthracene). As expected, the dark conduction properties are markedly different from those of unsubstituted crystalline anthracene. For example, there is a marked anisotropy in the conductivity of C4 anthracene, the in-plane value exceeding that in a direction perpendicular to the layers by a factor of 108. By use of appropriate electrodes double injection effects accompanied by blue light emission were found. The admittance data for these novel films differ slightly from those of films of conventional fatty acid salts.