Structural order and stability of phospho(silico)molybdate and tungstophosphate Langmuir-Blodgett multilayers

In the present paper we report on a structural study of alkylic long-chain ammonium salt of 12-phosphomolybdate (PMo12O403-), 12-silicomolybdate (SiMo12O403-), and 12-tungstophosphate (PW12O403-) Langmuir-Blodgett (LB) multilayers by using X-ray specular, off-specular reflectivity and diffuse scattering measurements. The structural ordering of the multilayers is investigated as a function of the metal oxide anions used in the synthesis of the LB films. In particular, the specular and off-specular X-ray reflectivity measurements allow us to analyze the molecular packing along the growth direction, while the in-plane arrangement is obtained by the X-ray diffuse scattering analyses. All the samples were measured within a few days after the synthesis and again two months after growth in order to study the structural stability as a function of time. No significant differences are found by comparing PMo12O403- and SiMo12O403- films. Both compounds show highly correlated interfaces, uniform thickness and a pronounced ordering along the growth direction. However, the structural quality of both compounds deteriorate with time, demonstrating a quite unstable molecular structure. On the contrary, the PW12O403- LB multilayer does not show any structural transformation in time. Nevertheless, this LB film reveals a lower degree of vertical correlation among the interfaces and a less pronounced ordering along the growth direction. (C) 1998 Elsevier Science S.A.