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Field effect transistors have been made, the channels of which consist of conducting, iodine-doped Langmuir-Blodgett (LB) films. In the chosen configuration, the modulation effect is shown to arise mostly from the source-channel contact, which behaves as a reverse-biased Schottky diode. The gate voltage modulates the height of the diode barrier by controlling the surface charge density of the oxide-LB film interface states. This model explains qualitatively and quantitatively the behaviour of the transistor and gives rise to remarkably good fits with the experimental transistor characteristics. The model is also supported by a counterexperiment in which the source and drain contacts are ohmic, as expected this leads to almost no drain current modulation.

Conducting Langmuir-Blodgett (LB) films based on tetracyanoquinodimethane (TCNQ) and ethylenedithio-tetrathiafulvalene (EDT-TTF) appear to be very promising candidates for phosphine sensors. Gas diffusion through the LB structure is an important factor which must be controlled. The study of a molecular assembly of silver behenate underlayers (the reaction of which towards PH3 is followed by infrared spectroscopy) on top of which the conducting film is transferred shows that phosphine penetration in these conducting films limits the response time of the sensor. The two types of conducting films lead to two different sensors.

This work presents two new amphiphilic molecules derived from the tetrathiafulvalene (TTF) cycle. The purpose of this work is to realize a controlled in-plane molecular architecture. In order to obtain well organized macrocycles at the air-water interface we have added to the TTF cycle four long alkyl chains, each with an acid (or ester) group. Compound I has been designed to form a regular paving in the polar plane whereas for compound II an identical arrangement in the hydrophobic plane is predicted. The synthesis and thermodynamic stability of the monolayers are investigated for both molecules. The results show that these macrocycles are lying flat on the water surface: this is the first step in two-dimensional molecular engineering.

We report recent work developed in Saclay. BEDT derivatives possessing two aliphatic chains both grafted at the same end of the electroactive nucleus have been used. Hence in the resulting films, the conjugated systems stand on edge with their long axis perpendicular to the support. After a controlled oxidation by iodine vapour, films are obtained as conducting sheets, the DC conductivity of which is close to 0.2 S cm−1 at room temperature. Such a structure is especially designed to mimic the sheet-like molecular array of classical (BEDT)2I3 highly conducting or even superconducting bulk crystals. In addition, the stoichoimetries are found to be basically the same. The DC conductivity abruptly decreases as the number of superimposed monolayers decreases and a bilayer thick film is found to be an insulator. Such a behaviour is understood in terms of superimposed conducting domains separated by more or less insulating boundaries. Basically different properties are currently obtained by the use of homodoped TCNQ containing films. The homodoping strategy consists of mixing in the same monolayer neutral amphiphilic octadecyl TCNQ (OD-TCNQ) together with the anion radical semi-amphiphilic TCNQ salts, for example octadecylsulfonium+ TCNQ−. Hybrid stacks are obtained in that way. The everage electronic charge of TCNQ (and hence the conductivity) can be accurately adjusted by properly choosing the molecular ratio of the individual components.

BEDT-TFF derivatives, labeled 1a and 1b are used to build Langmuir-Blodgett films. — In the first step, oxidation by iodine vapor leads, in both cases, to the formation of radical cation dimers: (1a·+)2 and (1b·+)2. However, the subsequent behavior of films strongly depends on the way the aliphatic substituents are grafted onto the electroactive nucleus. Built-up films of 1b go back reversibly to their initial insulating state, and those of 1a undergo a solid state reorganization leading to a permanent conducting phase. These differences are explained in terms of steric hindrance. Spectroscopic arguments are used to derive the stoichiometry of the conducting films of 1a, which is likely to be that of conducting bulk crystals of (BEDT-TFF)2I3.

The homodoping strategy, which consists of mixing within the same layer two derivatives of the same electroactive nucleus (TCNQ here), the first one being neutral and amphiphilic, the second one being charged and semi-amphiphilic, is used to build up conducting LB films. The results obtained when varying the cation of the semi-amphiphilic compound are reported, and some general features of these films are presented.

Preliminary resonance Raman spectroscopy experiments on Langmuir-Blodgett (LB) films made of a small number of porphyrin layers are reported. It is shown that the angular molecular distributions inside LB films can be obtained from these data.

We describe the properties of pristine and iodine-doped Langmuir-Blodgett films built up from a new semi-amphiphilic tetracyanoquinodimethane (TCNQ) salt, namely trimethyloctadecylphosphonium TCNQ. The films are transferred from a water subphase to a solid substrate. After exposure to diluted iodine vapour, the films become conducting. This behaviour is checked by the IR absorption spectrum. The conductivity estimated from the energy of the maximum of the charge transfer absorption band is about 20–50 Ω-1 cm-1.

A new method for building up conducting Langmuir-Blodgett (LB) films is described. The film is initially formed and compressed on a glycerol subphase from a semi-amphiphilic mixed-valence conducting tetracyanoquinodimethane (TCNQ) salt, namely octadecylpyridinium+ ∮(TCNQ1,45)−. We show that the conducting properties are retained on this subphase for surface pressures up to 18 mN m−1, which corresponds to the elasticity limit of the film. The film is then shifted onto a water subphase, contained in a dipping compartment, which has been previously incorporated in the main Langmuir trough, the water level being set identical to that of the glycerol. The shift onto the water allows the transfer onto a solid substrate with the classical dipping-withdrawing method. The propertie films are investigated by analysing their UV-visible and IR spectra and their linear dichroism. The IR and UV-visible spectra are reminiscent of those of mixed-valence TCNQ complexes with only a poor conductivity. This behaviour is verified bya d.c. conductivity measurement at room temperature, which provides a stable value of 10−4μ−1 cm−1. Furthermore, the high degree of order in the film is evidenced by its strong dichroic behaviour. It turns out that TCNQ molecules stand edge on with their molecular planes roughly perpendicular to the plane of the substrate and their long axis close to being perpendicular to it. In addition, the influence of the subphase in the fibrication of LB conducting films is pointed out.

Scanning electron microscopy (SEM) performed on ultrathin in-plane conducting Langmuir-Blodgett (LB) films with a transverse electric field applied across the sample (voltage-induced contrast) shows electrically active defects selectively. Scratches or cracks which cut off conduction can be sorted out from others which do not, whereas standard SEM or optical microscopy does not distinguish between the two sorts of defects. Grain boundaries in LB conducting films of several semiamphiphilic compounds of tetracyanoquinodimethane (TCNQ) were easily observed, but were completely invisible by other methods. Grain boundaries are shown to be more resistive than the insides of grains, but the situation is shown to be less drastic than in thin films of conventional molecular conductors, owing to the alignment in molecular conducting planes in LB films.

We report progress in the field of characterization of Langmuir-Blodgett films. Special attention is paid to intermolecular distance determination, film texture and visualization of defects, surface investigation and orientational order of molecules. Some aspects of the characterization of conducting films are also presented.

Resume Un modele d'ions rigides a 11 parametres comprenant les interactions generales entre premiers voisins, seconds voisins et les forces de Coulomb, est utilise pour expliquer a la fois les resultats experimentaux de diffusion de neutrons et les resultats d'absorption IR dans InP. Un modele mathematique qui utilise la technique des fonctions de Green est ensuite employe pour prevoir les frequences de vibration des impuretes substitutionnelles pour ce materiau dans l'approximation du defaut de masse et du defaut de constante de force entre premiers voisins. La frequence des modes localises, de gap et de bande est calculee pour diverses impuretes substitutionnelles en fonction du defaut de constante de force pris comme parametre. Le meme calcul est donne pour deux autres composes ayant egalement la structure de la blende et un grand gap de phonons: GaP et ZnS. Une comparaison est donnee avec les resultats experimentaux disponibles.

For the first time, conducting Langmuir-Blodgett films with reasonably low resistivity (10–50 Ω cm) have been obtained (N-docosylpyridinium-TCNQ-iodine ternary salts). These two-dimensional polycrystalline films behave as low-resistivity organic semiconductors. The aim of this paper is to determine the intrinsic nature of these films. As far as the electrical characterization is concerned, the Hall effect can hardly be used because of the very high internal resistance of the films. Furthermore, because of the polycrystalline structure, d.c. resistivity measurements are questionable. Optical measurements are especially suitable for an electrodeless evaluation of the resistivity; the IR optical band, located near 3 μm is discussed. More classical methods, including linear IR, dichroism and ESR measurements, are also presented and discussed.

An absorption spectrum with a resolved fine structure starting at 130 meV is measured on p -type CdTe. With the help of photoluminescence data, this spectrum is associated to a complex acceptor involving at least one group III donor and one cadmium vacancy.

Raman spectroscopy is an attractive method for the characterization of Langmuir-Blodgett (LB) films which has been shown to be workable. This communication reports on theoretical calculations as well as experimental studies developed with the aim of determining the orientations of the macrocycles of porphyrins in LB films. An experimental method, together with the procedure for the analysis of the results, is proposed through an example. After characterization of the molecule and its vibration modes from spectra of solutions we show that the porphyrin rings in the layers are oriented parallel to the film plane.

In monolayers the planar array of highly polarizable and adjoining dye molecules is favourable to singlet energy transfer. We give some evidences of energy transfer between sensitizers and acceptors in the plane of the layers. The radius of the excitonic cross section of the acceptor is about 60 A.

The infrared absorption of lithium doped CdTe shows sharp lines in the energy range 30–80 meV which are attributed to electronic transitions from the ground state to the excited states in the lithium acceptor and LO phonon assisted transitions. This excitation spectrum is analyzed using the theory of Baldereschi and Lipari what leads to a determination of the Luttinger valence band parameters.

We have obtained very stable Langmuir films from a (1:1) mixture of a tetrathiafulvalene (TTF) derivative (ethylenedithiodioctadecylthio TTF) with ω-tricosenoic acid, compressed on a pure water subphase at surface pressures up to about 35 mN/m. These films can be transferred by the classical Langmuir-Blodgett method with a constant transfer efficiency close to unity, and films more than 100 layers thick can be built up in this way. After exposure to iodine vapor and subsequent moderate heating at about 40°C, these films become conducting, and their in-plane direct current conductivity reaches a value of about 2 × 10−1 Ω −1 · cm−1 at room temperature. A preliminary interpretation of this behavior is proposed and strong arguments are brought out in favor of an electronic conduction mechanism arising from a mixed valence state in the TTF derivative array.

The Langmuir-Blodgett technique enables the production of ultrathin organic films. These layers are known for their high quality and well-organized structure. However, defects are present in them. In the present paper a review is given of the different types of defects together with their electrical characterization and their visualization by optical or electron microscopy. Several examples of decoration techniques and the importance of the defects for various applications are also presented.

Composite layers of behenic acid and tetraphenyl porphyrins made amphiphilic by aliphatic chain grafting were transferred onto solid substrates by the Langmuir method. The orientation of the macrocycles was studied by three independent methods: UV-visible linear dichroism, infrared linear dichroism, and electron spin resonance with paramagnetic markers. The first part of this paper is devoted to the theory relating to each of those methods, with the conditions of validity of formulas. The second part of the paper exposes the experimental results obtained not only by these three methods, but also by specific molecular area measurements and by observations on film crystallinity and stiffness. Results are discussed. All the observations converge to show that the macrocycles are excluded from the behenic film and that they organize aside this film according to order-generating ring-ring, and disorder-generating porphyrin chains-behenic chains, interactions.

For industrial work, an automatic, fast and above all continuous method of film production is needed. This can be achieved with a new generation of troughts, the operation of which is based on the following property; a rotating roller partially immersed in water transfers molecules from one side of the roller to the other. The trough is therefore divided into compartments separated by rotating rollers (patent). In the first compartment the film is continuously spread from the solution and the solvent is allowed to evaporate; a first “hydrophilic” roller provides a pressure of a few dynes per centimetre in the second compartment which supplies molecules to a “hydrophobic” roller feeding the last compartment (where the film is used). The servomechanisms and feedbacks necessary for operating all these cascaded stages are described. The trough is fully automatic and permits the covering of discrete classical samples as well as that of continuous strips such as those used in capacitors. The film fabrication is 10 times faster than with classical Langmuir troughs.

Earlier work by Ruaudel-Teixier et al. has shown that it is possible to build conducting Langmuir-Blodgett (LB) layers. These conductors are obtained in two successive steps: (i) transfer of a precursor N -docosylpyridinium-tetracyanoquinodimethane (TCNQ) salt onto a solid substrate; (ii) subsequent treatment of this precursor by iodine. Results concerning the structure of the precursor are presented in this paper. Electron spin resonance (ESR) measurement of the powder used to make the spreading solution showed the following features: (i) an asymmetric line at g = 2; (ii) a triplet signal. The latter is identical in all the samples and arises from (TCNQ - ) 2 dimers in the powder. The experimental spectra were correctly fitted by a spin hamiltonian H = gμ B · S +D(S z 2 − 1 3 S 2 )+E(S x 2 -S y 2 ) with D = 1.37 x 10 −2 cm −1 and E = 2.2 x 10 −3 cm −1 and a triplet activation energy J ≈ 0.14 eV. The same features are also observed in the ESR spectra of the LB films, but they now depend strongly on the orientation of the film in the magnetic field, eence allowing us to study molecular order. This study was complemented by linear dichroism measurements in the visible and the IR regions. In the visible region all the samples showed an intense band between 600 and 700 nm which was found to be polarized parallel to the substrate plane. Linear dichroism measurements in the IR region showed that the film is highly organized.

Using a very sensitive technique for linear dichroism measurement, the orientation of pigments in monolayers is studied according to the drawing conditions. We define some features of the molecular associations.