Selection rule ultrathin films

Studying the temperature evolution of UV Raman spectra was demonstrated to be an effective approach to determine the ferroelectric phase transition temperature in ferroelectric ultrathin films and superlattices, which is a critical but challenging step for understanding ferroelectricity in nanoscale systems. The T. determination from Raman data is based on the above mentioned fact that perovskite-type crystals have no first order Raman active modes in paraelectric phase. Therefore, Raman intensities of the ferroelectric superlattice or thin film phonons decrease as the temperature approaches Tc from below and disappear upon ti ansition into paraelectric phase. Above Tc, the spectra contain only the second-order features, as expected from the symmetry selection rules. This method was applied to study phase transitions in BaTiOs/SrTiOs superlattices. Figure 21.3 shows the temperature evolution of Raman spectra for two BaTiOs/SrTiOa superlattices. From the shapes and positions of the BaTiOs lines it follows that the BaTiOs layers remain in ferroelectric tetragonal... 

In this section, the physical mechanisms and selection rules of IR absorption by bulk material due to vibrations, electronic excitations, and free carriers (electrons and holes) are briefly discussed on the qualitative level from the viewpoint of quantum mechanics. In general, this problem is highly specialized, and for fuller details several standard textbooks [21, 34, 36-50] are reconunended. AU of these mechanisms also apply to ultrathin films, but their appearance, which will be discussed in Chapters 3 and 5-7, is quite specific. Note that although we will discuss absorption by solids, the mechanisms to be considered are also applicable to Uquids. 

From the general selection rule (1.27), it follows that, unlike the case of the transverse excitations = 0°), the longitudinal excitations (t = 90°) are nonradiative for any experimental geometry of experiment that is, they do not interact with the transverse electromagnetic wave. For ultrathin films, absorption of p-polarized radiation at the frequency close to surface mode produced by the so-called size effect (Section 3.2). 

The IRRAS method can be used to obtain information about ultrathin films not only at metals but also on semiconductor and dielectric (including liquid) substrates. This class of problem is applicable to many areas, including thin-fihn optics, electronic and electroluminescent devices [27] (Chapter 5), sensors and transducers [28], flotation technology [29] (Section 7.4.4), and biomedical problems [30, 31]. Although the sensitivity is much lower than when metallic substrates are used, the waiving of the metal selection rule allows both s- and /7-polarized spectra to be measured and thus a more thorough investigation of molecular orientation within the layer. 

The MO measurements provide information about the angular distribution of molecules in the x, y, and z film coordinates. To extract MO data from IR spectra, the general selection rule equation (1.27) is invoked, which states that the absorption of linearly polarized radiation depends upon the orientation of the TDM of the given mode relative to the local electric field vector. If the TDM vector is distributed anisotropically in the sample, the macroscopic result is selective absorption of linearly polarized radiation propagating in different directions, as described by an anisotropic permittivity tensor e. Thus, it is the anisotropic optical constants of the ultrathin film (or their ratios) that are measured and then correlated with the MO parameters. Unlike for thick samples, this problem is complicated by optical effects in the IR spectra of ultrathin films, so that optical theory (Sections 1.5-1.7) must be considered, in addition to the statistical formulas that establish the connection between the principal values of the permittivity tensor s and the MO parameters. In fact, a thorough study of the MO in ultrathin films requires judicious selection not only of the theoretical model for extracting MO data from the IR spectra (this section) but also of the optimum experimental technique and conditions [angle(s) of incidence] for these measurements (Section 3.11.5). 

The important information that can be provided by IR spectra is the molecular orientation in/on polymer films, which include SAMs as the specific case (Section 3.11). In the case of self-supporting anisotropic films, the linear dichro-ism is usually calculated from normal-incidence transmission spectra measured at two mutually perpendicular positions of the polarizer [684], Obviously, this approach is insensitive to the modes perpendicular to the film surface. This problem is circumvented by using a combination of the normal-incidence transmission with metallic IRRAS [727], since these methods have complimentary selection rules — the modes whose TDMs are parallel or perpendicular to the surface are active in transmission or IRRAS, respectively. This technique was used to study the MO in ultrathin n-aUcylacrylamide LB films [727, 728]. A strong biaxial distribution was found in these LB films in which the carbon-hydrogen chains are inclined in the dipping direction [727]. 

---