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Morphology and crystalline nature of a Langmuir—Blodgett film of a TCNQ charge transfer salt

1988; Elsevier BV; Volume: 121; Issue: 2 Linguagem: Inglês

10.1016/0021-9797(88)90451-1

1095-7103

A. Barraud, M. Flörsheimer, Helmuth Möhwald, J. Richard, A. Ruaudel‐Teixier, M. Vandevyver,

Molecular Junctions and Nanostructures

In order to obtain an ultrathin conductor as a two-dimensional (2D) layered arrangement of organic molecules, we have built up Langmuir—Blodgett (LB) films from N-docosylpyridinium+-TCNQ charge transfer salt. The morphology of these films is closey investigated by means of three interrelated methods. (a) High-resolution optical microscopy, performed with light reflected under crossed polarizers from the air-water interface, enables us to observe the film directly during the compression, before its transfer onto a solid substrate. (b) Optical microscopy, using the Nomarsky interference contract method, allow us to observe the film which has been transferred onto a solid substrate even at low surface pressures through a special method, the so-called “touching” method. (c) Transmission electron microscopy together with electron diffraction provides information about the crystallinity of the transferred films. We observe a three-dimensional (3D) crystallization, which takes place in the film compressed at the air-water interface. This is shown by the birefringent behavior of some parts of the film, and by the different charges acquired by crystalline and amorphous areas in SEM, and also by electron diffraction patterns. This crystallization can arise even at surface pressures as low as 10 mN/m, if the time spent by the film at the air-water interface is long enough. It causes a plateau in the surface-pressure-area per molecule isotherm. The resulting 3D flat crystals exhibit a trapezoidal shape with well-defined angles and very sharp edges. Their surface appears rather rough, which is typical for amphiphilic molecules with loosely packed chains. From the thickness measurements performed on filmes transferred at a surface pressure of about 15 mN/m, after platinum shadowing, the crystals appear to be made of three super-imposed monolayers. The crystallization phenomenon is tentatively interpreted on the basis of an instability arising from the molecular arrangement in the film compressed at the air-water interface. The investigation of this system provides information about the molecular geometries required to build up genuine two-dimensional monomolecular layers, which could lead to ultrathin 2D LB conducting films.