*4.3. X-Ray Techniques*

X-ray diffraction (XRD) is one of the most widely used techniques for the characterization of NCM. Ideally, crystalline structure, phase behaviour, lattice parameters, and crystallite grain size are derived from XRD [86]. Crystallite grain size is determined using the Scherrer equation [87]. It utilizes the diffractogram of the sample by analysing the broadening of the peaks of the highest intensity. XRD has the advantage of resulting in volume-averaged values while being statistically representative. Despite its applicability, XRD is often replaced in use by single-crystal X-ray diffraction (ScXRD) as the latter is a more definitive technique for determining three-dimensional crystal lattice structures. ScXRD provides an accurate representation of atomic coordinates and thermal parameters. These are obtained by using parameters such as molecular geometry and intermolecular distances [88]. ScXRD does have a major disadvantage in that it requires the accessibility of a single crystal. Unfortunately, in many instances, polycrystallites are formed, especially when using the top-down methods. In such cases, powder X-ray diffraction (PXRD) is the obvious alternative to ScXRD [88].

X-ray photon spectroscopy (XPS) capitalizes on the photoelectric effect [89]. In XPS, following adsorption of incident photons from an X-ray source, core electrons are emitted from a sample and the kinetic energy determined by conservation of energy. As a result of inelastic processes from scattering deep in the bulk, only surface electrons escape without energy loss. Identification of specific elements is accomplished on the basis that each element has a characteristic set of binding energies. The concentration of the element directly correlates to the number of photoelectrons and as such, after background removal, peak areas can be used as a means to quantify specific elements [89].
