M. K. Sanyal

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.

X-ray scattering studies of surface roughness of GaAs/A1As multilayers

We discuss the theory of X-ray scattering from multilayers with conformal roughness of the interfaces, and illustrate with an analysis of specular, diffuse and wide-angle scattering from a GaAs/A1As multilayer. This is a highly coherent multilayer structure deposited on a stepped, but otherwise smooth surface. The roughness due to the steps propagates through the layers and a distinct anisotropy is observed in the diffuse scattering. We discuss a method to treat diffuse scattering from such surfaces with slightly irregular steps.

Morphology of thin silver film grown by dc sputtering on Si(001)

The morphology and growth mechanism of silver films approximately 150 A in thickness on Si(001) substrates have been studied by atomic force microscopy and x-ray reflectivity. The thin films prepared by dc sputtering at room temperature are composed of islands of silver. The shape and size distribution of these islands are studied using these two complementary measurement techniques.

Fourier Reconstruction of Density Profiles of Thin Films Using Anomalous X-Ray Reflectivity

The use of X-ray and neutron reflectivity measurements to determine the density profile across and interface or across thin films has become increasingly popular over the last few years. However, in general convenient model-independent methods of inverting the reflectivity profiles to obtain the density profile have been missing. We present here one such approach using the method of anomalous reflectivity from the substrate and demonstrate its applicability in the case of an organic thin film.

Energy dependent wavelength of the ion induced nanoscale ripple

Formation of periodic undulations or ripple like features on various materials with typical wavelength ranging from about 10 nm to 1µm, obtained by obliquely incident ion bombardment, has become an active research subject due to its possible technological applications, as varied as optical devices, templates for liquid crystal orientation and strain-free patterned substrates for heteroepitaxial growth of quantum wires. It is also expected that systematic study of ion beam induced nano ripple formation will help us to understand the basic processes prevalent in formation of sand dune like structures in deserts. Although this ion induced phenomenon was reported first in 1960s [1] and then in 1970s [2, 3], the improvement in experimental conditions such as, better vacuum and ion beam parameters and improved surface characterizing tools, has enabled us to control the growth of these ripple like features [4, 5, 6, 7, 8, 9, 10, 11]. The first widely accepted theoretical approach describing the process of ripple formation due to ion bombardment was developed by Bradley and Harper (BH) [12]. This linear theory [12] predicts the ripple wavelength and orientation in agreement with numerous experimental studies. However, this theory cannot explain a number of experimental observations, such as the saturation of the ripple amplitude [9], the appearance of rotated ripples [11] and kinetic roughening [13]. Moreover, according to the BH theory ripple wavelength should decrease with ion energy but this prediction has not been confirmed experimentally so far [5, 6, 7]. Recently a formalism [14, 15, 16] based on nonlinear continuum theory has been developed to understand these experimental observations not predicted by linear theory. In this new formalism, not only nonlinear and noise terms were included in the equation of height profile for eroded surface but also existence of two different surface diffusion processes were recognised. Based on Sigmund’s theory of sputtering [17], the height evolution h(x,y,t) of an ion eroded surface according to this nonlinear theory [14, 15, 16] can be described by

Evolution of surface morphology of ion sputtered GaAs(1 0 0)

In order to explore possible route to fabricate nano-scale semiconductor dots, a series of ion bombardment experiment on GaAs(1 0 0) was undertaken using a high current isotope separator and ion implanter with 40 Ar þ ions of an energy of 60 keV incident at an angle of 60 with respect to surface normal. Detailed surface topographical features of the bombarded samples were characterised by atomic force microscopy. To observe the growth of topography with time, the samples were bombarded at a number of doses. At a dose of 1 � 10 17 ions/cm 2 , no observable topography was developed. At a dose of 2 � 10 17 ions/cm 2 , the topography started to develop in the form of roughness along with islands or dots formation on the crest of waves or hillocks. Similar kind of topography has been observed up to a dose of 1 � 10 18 ions/cm 2 , remarkable with the formation of nano-dots with the maximum dimension of a few hundred nanometer. At the dose of 3 � 10 18 ions/cm 2 the surface became populated with ripple morphology without formation of any island or dot, in contrast with lower doses. 2002 Elsevier Science B.V. All rights reserved.

Reversible buckling in monolayer of gold nanoparticles on water surface

Formation of condensed films of nanoparticles having small ratio of metal-core-diameter to organic-shell-thickness is desired for several applications in nanotechnology. We report here results of a X-ray scattering study carried out to understand structure and morphology of monolayer of such nanoparticles having gold-core and thiol-shell directly on the water surface before the monolayer undergoes a continuous transition to a bilayer. Our results demonstrate buckling of the monolayer over a large surface pressure range (1 to 15 mN/m). The buckled state exhibits reversibility on decompression and can be annealed with temperature. We also show that condensed monolayer films of nanoparticles can be formed by annealing the buckled monolayer before transferring to solid substrates.

Gold Nanoparticles at the Liquid-Liquid Interface: X-ray Study and Monte Carlo Simulation

The behavior of mixed-ligand-coated gold nanoparticles at a liquid-liquid interface during compression has been investigated. The system was characterized by measuring pressure-area isotherms and by simultaneously performing in situ X-ray studies. Additionally, Monte Carlo (MC) simulations were carried out in order to interpret the experimental findings. With this dual approach it was possible to characterize and identify the different stages of compression and understand what happens microscopically: first, a compression purely in-plane, and, second, the formation of a second layer when the in-plane pressure pushes the particles out of the plane. The first stage is accompanied by the emergence of an in-plane correlation peak in the scattering signal and a strong increase of the pressure in the isotherm. The second stage is characterized by the weakening of the correlation peak and a slower increase in pressure.

Glass transition in ultrathin polymer films: a thermal expansion study.

The glass transition process gets affected in ultrathin films having thickness comparable to the size of the molecules. We observe systematic broadening of the glass transition temperature (T(g)) as the thickness of an ultrathin polymer film reduces below the radius of gyration but the change in the average T(g) was found to be very small. The existence of reversible negative and positive thermal expansion below and above T(g) increased the sensitivity of our thickness measurements performed using energy-dispersive x-ray reflectivity. A simple model of the T(g) variation as a function of depth expected from sliding motion could explain the results.

Morphology of nanostructured materials

Here we shall discuss the importance of grazing incidence X-ray scattering techniques in studying morphology of nanostructured materials confined in thin films and multilayers. In these studies, the shapes, sizes, and structures of nanostructured materials and their distribution in composites are investigated. These studies are important for understanding properties that may deviate considerably from the known bulk properties. We shall first outline basics of three X-ray scattering techniques, namely X-ray reflectivity, grazing incidence small-angle X-ray scattering, and grazing incidence diffraction, used for these studies. We shall then demonstrate the utility of these techniques using some known results.