Introduction

The bombardment of a growing film with energetic particles has been observed to change for the better a number of characteristics and properties, critical to the performance of thin films and coatings, such as adhesion, densification of films grown at low substrate temperatures, modification of residual stresses, control of texture (orientation), modification of grain size and morphology, modification of optical properties, and modification of hardness and ductility.

The process of simultaneous thin-film deposition and directed ion bombardment from an ion source has been labeled by a variety of terms including: ion assisted coating (IAC); ion assisted deposition (IAD); ion vapor deposition (IVD); ion beam enhanced deposition (IBED); dynamic recoil mixing (DRM) at high energies; and ion beam assisted deposition (IBAD). This term, ion beam assisted deposition, or IBAD, will be used here in favor of its growing acceptance by the energetic-particle–solid interaction research community.

The important role of ions in thin-film deposition techniques has long been realized by the coating community. It is difficult, however, in many of the plasma based coating techniques, to separate out the degree to which ion and neutral particle fluxes as well as ion energies affect resultant coating properties. Mattox (1982) showed as early as 1963 that energetic ions within plamsa had an important influence on coating properties in his early development of ion plating. In addition, other plasma-based deposition processes, such as activated reactive evaporation (ARE), developed by R. F. Bunshah and co-workers (Bunshah, 1982), employ ionization to promote film properties.

Gases, liquids and solids

The three most common states, or phases, of matter, gases, liquids and solids are very familiar (Walton, 1976). Phases that are not so well known are plasmas and liquid crystals (although these are both found in electrical and electronic devices in everyday use). All these states are generally distinguished by the degree of translational and orientational order of the constituent molecules. On this basis some phases may be further subdivided. For example, solids, consisting of a rigid arrangement of molecules, can be crystalline or amorphous. In an amorphous solid (a good example is a glass), the molecules are fixed in place, but with no pattern in their arrangement. As shown in figure 1.1, the crystalline solid state is characterized by long-range translational order of the constituent molecules (the molecules are constrained to occupy specific positions in space) and long-range orientational order (the molecules orient themselves with respect to each other). The molecules are, of course, in a constant state of thermal agitation, with a mean translational kinetic energy of 3kT/2 (k is Boltzmann's constant, T is temperature; kT/2 for each component of their velocity). However, this energy is considerably less than that associated with the chemical bonds in the material and the motion does not disrupt the highly ordered molecular arrangement.

Deposition principles

The Langmuir—Blodgett (LB) technique, first introduced by Irving Langmuir and applied extensively by Katharine Blodgett, involves the vertical movement of a solid substrate through the monolayer/air interface (Blodgett, 1934; Blodgett, 1935; Langmuir, 1920). Blodgett's and Langmuir's original papers contain a wealth of useful experimental advice and are still excellent starting points for anyone considering working in the area today (Blodgett, 1935; Blodgett and Langmuir, 1937).

The surface pressure and temperature of the monolayer are first controlled so that the organic film is in a condensed and stable state. For fatty acid type materials, deposition generally proceeds from either the L2′, LS or S phase (with surface pressures in the range 20–40 mN m-1 and temperatures 15–20°C). However, it is also possible to start from one of the other monolayer states. The molecular organization in the resulting LB film will depend on these initial conditions.

Figure 3.1 shows the commonest form of LB film deposition. The substrate is hydrophilic and the first monolayer is transferred, like a carpet, as the substrate is raised through the water. The substrate may therefore be placed in the subphase before the monolayer is spread. Subsequently, a monolayer is deposited on each traversal of the monolayer/air interface.

Refractive index

The refractive index of materials is determined by the interaction of electromagnetic (EM) radiation with the molecules which they comprise (appendix B). This depends not only on the orientation of the electric field vector of the incident EM wave, but also on that of the electric dipoles produced in neighbouring molecules. Figure 7.1 shows an ideal arrangement of molecules in an LB monolayer. The sample coordinate system is (x, y, z) while that of the principal axes of the molecules is (x′, y′, z′). Careful measurements on fatty acid LB layers show that the films possess a biaxial symmetry with three independent permittivity values (Barnes and Sambles, 1987). However, two of the indices are very close in value and LB films are often approximated as uniaxial.

Several approaches can be used to measure the refractive indices of thin organic films (Petty, 1990). In some techniques, the film thickness is also obtained (section 5.8). The more popular methods, based on ellipsometry, surface plasmon resonance and waveguiding are discussed below. Figure 7.2 summarizes the results of such experiments by various workers using cadmium eicosanoate LB films (Swalen et al., 1978). It is evident that the refractive index for the extraordinary ray ne (p-polarization) is greater than that for the ordinary ray n∘ (s-polarization) by 0.04.

Bonding in organics

Carbon has an atomic number of six and a valency of four. Its electron configuration is 1s2, 2s2, 2p2, i.e., the inner s shell is filled and the four electrons available for bonding are distributed two in s orbitals and two in p orbitals. The s orbital is spherically symmetrical, as shown in figure A.1(a), and can form a bond in any direction. In contrast, the p orbitals, figure A.1(b), are directed along mutually orthogonal axes and will tend to form bonds in these directions. When two or more of the valence electrons of carbon are involved in bonding with other atoms, the bonding can be explained by the construction of hybrid orbitals by mathematically combining the 2s and 2p orbitals. In the simplest case, the carbon 2s orbital hybridizes with a single p orbital. Two sp hybrids result by taking the sum and difference of the two orbitals, as shown in figure A.2, and two p orbitals remain. The sp orbitals are constructed from equal amounts of s and p orbitals; they are linear and 180° apart.

Other combinations of orbitals lead to different hybrids. For example, consider three groups bonded to a central carbon atom. From the 2s orbital and two p orbitals (e.g., a px and a py), three equivalent sp2hybrids may be constructed.

Electromagnetic radiation

Besides what is commonly called light, electromagnetic radiation includes radiation of longer (infrared, microwave) and shorter (ultraviolet, X-ray) wavelengths (see p. xvii). As the name implies electromagnetic (EM) radiation contains both electric field E and magnetic field B components. The use of the bold typeface indicates that these are vector quantities. The relationship between the electric and magnetic fields is best illustrated by considering plane-polarized radiation. Here the electric vector is confined to a single plane. Figure B.1 depicts such radiation of wavelength λ travelling with phase velocity c (the velocity at which the crests of the wave travel) in a vacuum (c = 2.998 × 108 m s-1) along the x-axis. The electric component of the radiation is in the form of an oscillating electric field and the magnetic component is an oscillating magnetic field. These fields are orthogonal and are also at right angles to the direction of propagation of the radiation. The plane of polarization is conventionally taken to be the plane containing the direction of the electric field. Unpolarized radiation, or radiation of an arbitrary polarization, can always be resolved into two orthogonally polarized waves. If the two electric field components possess a constant phase difference and equal amplitudes, the resultant EM wave is said to be circularly polarized.

The gas—liquid interface

Certain organic molecules will orient themselves at the interface between a gaseous and a liquid phase (or between two liquid phases) to minimize their free energy. The resulting surface film is one molecule in thickness and is commonly called a monomolecular layer or simply a monolayer. In the previous chapter the individual properties of bulk phases were outlined. The interface region will now be examined.

The boundary between a liquid and a gas (e.g., the air/water interface) marks a transition between the composition and properties of the two bulk phases. A surface layer will exist with different properties from those of either bulk phase (Adamson, 1982; Gaines, 1966). The thickness of this region is very important. If the molecules are electrically neutral, then the forces between them will be short-range and the surface layer will be no more than one or two molecular diameters. In contrast, the Coulombic forces associated with charged species can extend the transition region over considerable distances.

The microscopic model of a real interface is one of dynamic molecular motion as molecules move in and out of it. However, for the interface to be in equilibrium, as many molecules must diffuse from the bulk of the liquid to its surface per unit time as leave the surface for the bulk.

Organization in long-chain materials

Long-chain aliphatic materials pack together with their hydrocarbon chains parallel. The simplest scheme is an hexagonal array, with the molecules freely rotating as rigid rods about their long axes. The diameter of the cylinder into which one molecule fits is about 0.48 nm (Kitaigorodskii, 1961). Such a plastic crystalline state (section 1.2), originally known as a rotator phase (Ungar, 1983), may be exhibited by straight chain alkanes and some monolayers just below their melting point. There may even be some similarity with the LS phase in floating monolayer films (section 2.4.3). However, there is still considerable debate about this (Ulman, 1991).

For infinite aliphatic molecules, the hydrocarbon chain takes the form of a zig-zag, repeating at 0.254 nm intervals along the chain axis. In the most stable state, all the CH2 group carbon atoms lie in a plane to give a flat zigzag (appendix A). Close-packed structures result from the hydrogen atoms in a CH2 group on one molecule fitting into depressions between hydrogen atoms on adjacent molecules. Different packing arrangements of the C2H4 repeat units define the crystallographic nature of the subcell or sublattice. There are three possible close-packed structures with similar packing densities: orthorhombic (R), monoclinic (M) and triclinic (T) (appendix C) (Kitaigorodskii, 1961).

The crystal lattice

An ideal crystal contains atoms arranged in a repetitive three-dimensional pattern. If each repeat unit of this pattern, which may be an atom or group of atoms, is taken as a point then a three-dimensional point lattice is created. A space lattice, such as that shown in figure C.1, is obtained when lines are drawn connecting the points of the point lattice. The space lattice is composed of box-like units, the dimensions of which are fixed by the distances between the points in the three noncoplanar directions x, y and z. These are known as unit cells and the crystal structure has a periodicity (based on the contents of these cells) represented by the translation of the original unit of pattern along the three directions x, y and z. These directions are called the crystallographic axes. Any directions may, in principle, be chosen as the crystallographic axes. However, it is useful to select a set of axes which bears a close resemblance to the symmetry of the crystal. This can result in x, y and z directions that are not at right angles to one another. In figure C.1, the angle between the y and z axes is designated α, between the z and x axes, β, and between the x and y axes, γ.

Fatty acids and related compounds

A simple long-chain fatty acid such as n-octadecanoic acid (stearic acid) consists of a linear chain (CnH2n+1) — an alkyl chain — terminating in a carboxylic acid group (COOH). The polar acid head confers water solubility while the hydrocarbon chain prevents it (section 2.2). It is the balance between these two opposing forces that results in the formation of an insoluble monolayer at the air/water interface. Any change in the nature of either the alkyl chain or the polar end group will affect the monolayer properties.

The solubility of fatty acids in water decreases as the length of the alkyl chain is increased. To obtain an insoluble monolayer of a nonionized fatty acid (i.e., the situation at sufficiently low pH values), the molecule must contain at least 12 carbon atoms. For example, n-dodecanoic acid (lauric acid — C11H23COOH) forms a slightly soluble gaseous monolayer at low temperatures. The addition of two more carbon atoms, to form n-tetradecanoic acid (myristic acid), causes the gas phase to condense at low surface pressures and an expanded monolayer phase to be formed (Stenhagen, 1955). If this monolayer is held at a surface pressure of 10 mM m-1 and a temperature of 20°C, then the loss in monolayer area due to solubility in the water subphase is 0.1% min-1.

Langmuir—Blodgett (LB) films have been the subject of scientific curiosity for most of the twentieth century. However, interest has grown significantly since the 1970s — a direct result of the work of Hans Kuhn and colleagues on energy transfer in multilayer systems. This introduced the idea of molecular engineering, i.e., using the LB technique to position certain molecular groups at precise distances to others. In this way new thin film materials could be built up at the molecular level and the relationship between these artificial structures and the natural world explored.

There are already several books that cover LB and related thin films. So why another? My own background is in electronics. While I have been involved in LB film research I have spent many hours pondering on chemical formulae, struggling with biological nomenclature and trying to understand the finer points of thermodynamics. The scope of the subject is continuing to grow and anyone now starting work in the area must assimilate an enormous amount of information. My intention therefore has been to provide a gentle introduction to newcomers with an emphasis on the multidisciplinary and interdisciplinary nature of the field.

Each chapter addresses a different issue. Chapter 1 describes the various bulk phases of matter and outlines physical principles that can be used to model these. Monolayer phases are introduced in chapter 2.