J. K. Basu

Ordering and growth of Langmuir–Blodgett films: X-ray scattering studies

Abstract The interplay of ordering, confinement and growth in ultrathin films gives rise to various interesting phenomena not observed in bulk materials. The nature of ordering and interfacial morphology present in these films, in turn, depends on their growth mechanism. Well-ordered metal–organic films, deposited using an enigmatic Langmuir–Blodgett (LB) technique, are not only ideal systems for understanding the interplay between growth and structure of ultrathin films but also for studying chemical reactions and phase transitions in confined geometries. Studies on these LB films also enhance our understanding of the fundamental interactions of amphiphilic molecules important for biological systems. Advent of grazing incidence X-ray scattering techniques has enabled us to probe the interfacial structure of these multilayer films at very high resolution and as a result has improved our knowledge about the mechanism of growth processes and about physical/chemical properties of ultrathin films. In this review we will focus our attention on recent results obtained using these X-ray scattering techniques to understand the mechanism of growth leading to formation of remarkably well-ordered LB films after giving a brief outline of these scattering techniques. In addition, we also review recent results on growth and structure of nanoparticles formed by suitable chemical processes within the ordered matrix of LB films. Finally, we will discuss the work done on melting of LB films and its implications in our understanding of melting process in lower dimensions. In all these studies, especially those on as-deposited LB films results of atomic force microscopy measurements have provided important complementary morphological information.

Role of interfacial correlation in melting of Langmuir–Blodgett films

Abstract Cadmium arachidate Langmuir–Blodgett (LB) films, exhibiting two different in-plane correlation, namely self-affine and logarithmic, were studied to understand the role of interfacial correlation in the melting process. X-ray specular reflectivity and diffuse scattering measurements were performed on these films at different temperatures starting from room temperature (20°C) upto 110°C. Systematic analysis of specular and transverse diffuse scattering data showed that the average bilayer spacing and the roughness correlation at room temperature is retained for both the films even up to 100°C. Our results suggest that melting of the LB films occurs predominantly through the lateral motion of molecules, irrespective of the type of correlation. Between 100°C and 110°C the film disorders completely leading to the breakdown of the ordered bilayer structure existing up to 100°C.

Liquid-like interfacial correlation in LB films

We present here results of X-ray reflectivity and diffuse scattering study of three Langmuir-Blodgett films, all having nine monolayers. These systems are the cadmium arachidate film grown using the conventional method, the arachidic acid film with trapped CdS layers at the interfaces formed by exposing the cadmium arachidate film to hydrogen sulphide and ferric stearate film grown from a monolayer of the pre-formed salt on water. Self-consistent analysis of all the scattering data measured, viz., in specular, in transverse and in longitudinal directions, for each film shows that the interfaces are conformal in nature and the logarithmic in-plane correlation, characteristic of capillary waves on liquid surfaces, exists in all the three films.