Effect of Confinement on Melting Behavior of Cadmium Arachidate Langmuir-Blodgett Multilayer

The effect of confinement between two metallic layers on the melting behavior of a 13 monolayer cadmium arachidate (CdA) Langmuir-Blodgett (LB) multilayer has been studied. Temperature dependent diffraction measurements provide information about structural changes occurring in the film plane as well as in the out-of-plane direction. X-ray standing waves have been used to achieve depth selectivity in diffraction measurements. It is found that the difference in melting behavior of the surface and the bulk, which is observed in the film with free surface, disappears in the case of confined films; while the free surface transforms to hexaticlike phase via an intermediate smectic phase, confinement results in disappearance of this phase, and the sequence of transformations in the bulk and the interfacial regions becomes identical. Some anisotropy between (01 + 1 (1) over bar) and (10) directions remains, with coherence along (10) direction decreasing at a faster rate. The confinement between metallic layers also significantly reduces the tilting of the chains observed at higher temperature. Further, both in the case of film with free surface and confined films, melting at the surface/interface occurs at a lower temperature as compared to the bulk.