Michel Vandevyver

Novel Conducting LB Films

A layered anisotropic conducting structure is described. The LB technique is used for an amphiphilic TCNQ salt.

Structure of porphyrin multilayers obtained by the Langmuir Blodgett technique

Abstract 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.

Structural properties of Langmuir–Blodgett films of charge transfer salts: Pristine and iodine doped conducting films of (N‐docosyl‐pyridinium)TCNQ

We have investigated the molecular constitution and ordering of Langmuir–Blodgett films prepared by deposition of N‐docosylpyridinium–TCNQ charge transfer salt onto CaF2 substrates and by their subsequent doping with iodine vapor to yield reasonably good (∼10−1 Ω−1 cm−1) conducting films. Optical and electron scanning microscopy of the precursor film show that it is made up of a mosaic of highly crystalline platelets lying flat on the substrate though with random orientation. Structural information at the molecular level is gained by measuring and analyzing the ESR spectra and their angular and temperature dependence as well as the UV‐visible and infrared spectra and their linear dichroism. A triplet fine structure is observed in the ESR spectra of the precursor films with zero field splitting parameters ‖D‖=7.48×10−3 cm−1 and ‖E‖=1.2×10−3 cm−1 and singlet–triplet activation energy J=0.20 eV, markedly different from those of the N‐docosylpyridinium–TCNQ powder salt. The UV‐visible and infrared spectra sho...

Vibrational properties of HgCdTe system

The results of reflection spectra at 300 °K in the far‐infrared region (100–400 cm−1) are reported due to low concentration of Hg atoms in the Cd‐rich CdHgTe mixed crystals. A gap mode is observed at 131 cm−1 due to isolated Hg occupying Cd in CdTe. The experimental results are analyzed theoretically using the well‐know Green’s function technique. The perturbation caused by low concentration of Cd(Hg) is described by the mass change at the impurity site as well as local variation of impurity‐host interactions. In the nearest neighbor approximation, all the involved Green’s function matrix elements have been computed numerically by incorporating the lattice phonons from an eleven parameter rigid‐ion model fitted to recent neutron data of HgTe and CdTe. We find that different substitutional defects in Hg‐Cd tellurides can give rise to well‐defined symmetry vibrations both within the gap and outside the maximum phonon frequency regions. Our calculations suggest that a gap mode of Hg substituted to Te is like...

Conducting Langmuir-Blodgett films☆

Abstract 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.

Structure and properties of an n-docosylpyridinium-tetracyanoquinodimethane salt in Langmuir-Blodgett multilayers

Abstract 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 10−2cm−1 and E = 2.2 x 10−3cm−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.

Energy transfer in dye monomolecular layers

Abstract 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.

Optical properties of a donor-cadmium vacancy complex in CdTe

Abstract 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.

A new highly conducting Langmuir-Blodgett film based on a radical ion salt of a tetrathiafulvalene derivative

Abstract 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.

Defect characterization and detection in Langmuir-Blodgett films

Abstract 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.