Molecular multilayers: structure and templating effects
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Molecular multilayers: structure and templating effects Sandrine Heutz, Sallie M. Bayliss, Rudi Cloots 1, Ruth L. Middleton, Garry Rumbles and Tim S. Jones Department of Chemistry and Centre for Electronic Materials and Devices, Imperial College, London SW7 2AY, U. K. 1 Laboratoire de Chimie Inorganique Structurale, Departement de Chimie B6, Universite de Liege, B-4000 Liege, Belgium ABSTRACT Powder X-Ray diffraction (XRD) has been used to study multilayered structures grown by organic molecular beam deposition based on the molecular materials PTCDA and metal-free phthalocyanine (H2Pc). Double layers of different polymorphic forms (α, β 1 and β 2) of H2Pc indicate that the structure of the second layer is determined by the properties of the first layer. It is also shown that the first layer completely disrupts the crystallinity of the second layer in heterostructures containing PTCDA and H2Pc. The implication is that a strong templating effect occurs during the growth of multilayer molecular thin film structures. INTRODUCTION The study of molecular materials has undergone a rapid development, since structures based on thin organic films offer significant potential for a variety of electronic and optoelectronic applications [1]. Archetypal molecular materials that have been intensively studied include the two conjugated planar molecules PTCDA and H2Pc. PTCDA crystallises in the P21/c space group and the molecules lie parallel to the (102) plane. In this plane, the PTCDA molecules adopt a herringbone structure with an angle of about 45 ° between their long axes and the molecules arrange in columns [2]. Two polymorphs with slightly different lattice parameters have been identified in thin films formed by vapour deposition depending on the growth conditions and the type of substrate [3], but growth on glass and amorphous substrates only produces the α-polymorph. Free-base phtahlocyanine (H2Pc) can also be grown as two different polymorphs [4]. The α phase is obtained by growth at room temperature, while annealing an α film, or high substrate temperature growth, produces the β phase. Both phases are monoclinic and characterised by a herringbone structure in which molecules stack along the b-axis. The α phase crystallises in the C2/c space group [5], while the β form belongs to the P21/a space group [6]. More recent studies have shown structural and morphological evidence that the β phase can be further differentiated as β 1 and β 2, for the annealed and the high substrate temperature growth modes respectively [7]. Electronic and optoelectronic device structures are usually based on two or more layers of molecular materials. It is thus important to know how the structure of the first layer will affect subsequent growth of different materials, or of an identical material deposited under different conditions. A number of studies have been made on periodic ultrathin multilayers, with particular emphasis on the electrical and optical properties of the heterojunction [8], or its structure [9], but the behaviour of thicker film
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