Film methods, diffraction

Most polymers do not form crystals suitable for single crystal X-ray diffraction, so powder or film methods are usually employed. X-ray and LJV data recorded at various temperatures provide the detailed information required to correlate conformational and electronic properties, since the former is sensitive to the inter- and intrachain packing, and the latter is sensitive to the conformation. DSC provides further evidence for any phase transitions. Detailed studies have been performed by Winokur and West,260 261 who reported a comparison of the polymorphism, structure, and chromism in poly(di- -octylsilylene), (Si- -Oct2), 89, and poly(di- -dccylsilylcnc)(Si- -Dcc2) , 90. These investigations will be described in detail for the useful insights into polysilane structures that they afford. 

When diffracted X-ray beams fall on a photographic film at different angles, as the different layer lines in a cylindrical-film rotation photograph do, it is necessary to correct for the absorption of X-rays in different thicknesses of film. (Since double-coated films are normally used, the effect on the back layer depends on the absorption in the film.) This was first considered by Cox and Shaw (1930) Whittaker (1953) gives a formula which is more accurate and deals with greater obliquity and a thicker film Grenville-Wells (1955) gives the corrections when the multiple film method is used. 

In the X-ray method, a beam of monochromatic X rays is passed through a single crystal of the sample. The incident beam is diffracted at various angles a photograph, for example, will show a pattern of spots. The intensity of this set of diffracted beams will depend on the nature and arrangement of the atoms in the unit cell. In short, the intensities carry the information about the locations of the atoms in the unit cell, while the relative positions of the spots on the film carry the information about the arrangement of the unit cells in space. The positions and intensities are seldom measured by film methods today, but by a computer-controlled device known as a diffractometer. 

Three types of detector are available for measuring the X rays diffracted by a crystal Point detectors, linear and area detectors. The film method involves a two-dimensional detector, which can provide simultaneous information about every point in the region of reciprocal space investigated. The measurements are not immediately available in digital form, but powerful computer-controlled photometers can digitize them. 

These film methods may appear old fashioned, but this is wrong. Any serious X-ray laboratory will have a few of these machines, which are always necessary to give a first impression of the diffraction problem on a relatively cheap apparatus. especially when the expensive computer-controlled diffractometers are not immediately available or when there are problems and a three-dimensional inspection of the reciprocal space is necessary to gain an impression of the difficulties. 

In summary, using a two-step exposure method with a line-and-space photomask we fabricated an LC grating inner coated with PLCP photoalignment films. The diffraction efficiencies and the unique polarization conversion properties of the LC grating were well explained based on the simple binary model involving planar and TN alignments, the Jones matrix method, and diffraction theory. 

The schlieren microscope is able to detect refractive index variations to six decimal places. Any small difference in optical path (index difference, film thickness, etc) is very precisely detected by the schlieren microscope, especially in the Dodd modification. It is, in effect, a darkfield method. The specimen is illuminated with light in a portion of the illuminating cone and that direct light is masked in the conjugate back focal plane of the objective (Fig. 3). The only light to pass through this plane is refracted, reflected, or diffracted by the specimen. 

This chapter contains articles on six techniques that provide structural information on surfaces, interfeces, and thin films. They use X rays (X-ray diffraction, XRD, and Extended X-ray Absorption Fine-Structure, EXAFS), electrons (Low-Energy Electron Diffraction, LEED, and Reflection High-Energy Electron Diffraction, RHEED), or X rays in and electrons out (Surfece Extended X-ray Absorption Fine Structure, SEXAFS, and X-ray Photoelectron Diffraction, XPD). In their usual form, XRD and EXAFS are bulk methods, since X rays probe many microns deep, whereas the other techniques are surfece sensitive. There are, however, ways to make XRD and EXAFS much more surfece sensitive. For EXAFS this converts the technique into SEXAFS, which can have submonolayer sensitivity. 

For ultrathin epitaxial films (less than "100 A), Grazingincidence X-ray Diffraction (GrXD) is the preferred method and has been used to characterize monolayer films. Here the incidence angle is small ("0.5°) and the X rays penetrate only "100-200 A into the specimen (see below). The exit angle of the diffracted X rays is also small and structural information is obtained about (hkl) planes perpendicular to the specimen sur e. Thus, GIXD complements those methods where structural information is obtained about planes parallel to the surface (e.g., Bra -Brentano and DCD). 

Other excellent methods of phase identification include TEM and electron diffraction. These may be more useful for low-Z materials, ultrathin films, and for characterizing small areas, including individual grains. For multiphase films with incomplete texture, these methods and XRD are complementary, since in commonly used geometries, they probe atomic planes perpendicular and parallel to the thin film surface, respectively. 

Interdiffusion of bilayered thin films also can be measured with XRD. The diffraction pattern initially consists of two peaks from the pure layers and after annealing, the diffracted intensity between these peaks grows because of interdiffusion of the layers. An analysis of this intensity yields the concentration profile, which enables a calculation of diffusion coefficients, and diffusion coefficients cm /s are readily measured. With the use of multilayered specimens, extremely small diffusion coefficients (-10 cm /s) can be measured with XRD. Alternative methods of measuring concentration profiles and diffusion coefficients include depth profiling (which suffers from artifacts), RBS (which can not resolve adjacent elements in the periodic table), and radiotracer methods (which are difficult). For XRD (except for multilayered specimens), there must be a unique relationship between composition and the d-spacings in the initial films and any solid solutions or compounds that form this permits calculation of the compo-... 

Some of the techniques included apply more broadly than just to surfaces, interfaces, or thin films for example X-Ray Diffraction and Infrared Spectroscopy, which have been used for half a century in bulk solid and liquid analysis, respectively. They are included here because they have by now been developed to also apply to surfaces. A few techniques that are applied almost entirely to bulk materials (e.g.. Neutron Diffraction) are included because they give complementary information to other methods or because they are referred to significantly in the 10 materials volumes in the Series. Some techniques were left out because they were considered to be too restricted to specific applications or materials. 

---