Controlling band concept

To this point, we have seen how the structural, electronic, optical, and electrical properties of semiconductors can be treated within a common framework. The bonding in the lattice determines the structure of the solid, and the structure of the lattice in turn affects the band structure. This band structure then can be used to describe the chemical, optical, and electrical properties of the semiconducting solid. Thus, chemical control over the electronic properties of semiconductors is an important component of modem research in solid-state chemistry and solid-state physics. The concepts described above enable this process to be understood from a relatively qualitative, chemically based viewpoint. Further... 

Double Layer Interactions and Interfacial Charge. Schulman et al (42) have proposed that the phase continuity can be controlled readily by interfacial charge. If the concentration of the counterions for the ionic surfactant is higher and the diffuse electrical double layer at the interface is compressed, water-in-oil microemulsions are formed. If the concentration of the counterions is sufficiently decreased to produce a charge at the oil-water interface, the system presumably inverts to an oil-in-water type microemulsion. It was also proposed that for the droplets of spherical shape, the resulting microemulsions are isotropic and exhibit Newtonian flow behavior with one diffused band in X-ray diffraction pattern. Moreover, for droplets of cylindrical shape, the resulting microemulsions are optically anisotropic and non-Newtonian flow behavior with two di-fused bands in X-ray diffraction (9). The concept of molecular interactions at the oil-water interface for the formation of microemulsions was further extended by Prince (49). Prince (50) also discussed the differences in solubilization in micellar and microemulsion systems. 

This chapter reviews important aspects of inorganic LED structures. Section 1.2 introduces the basic concepts of optical emission. Band diagrams of direct and indirect semiconductors and the spectral shape of spontaneous emission will be discussed along with radiative and nonradiative recombination processes. Spontaneous emission can be controlled by placing the active region in an optical... 

The previous section discussed several factors that cause band broadening. Those major factors that control how the bands can be resolved from each other are the capacity factor, selectivity, and the number of plates. Together, these are called resolution. Figure 19-9 will be used to illustrate these concepts. 

Identification of organic compounds by their absorption spectra has become a routine procedure for the past several years. It is a standard practice now , to record either the infra-red or the ultra-violet spectrum while proposing a structure for a new compound or while reporting its physical properties. Electronic absorption spectroscopy has been used as confirmatory evidence for the identity of a previously known substance, just as any other physical property (e.g., melting point, refractive index). Many examples may be cited where a particular structure of a compound was selected from several possibilities on the basis of its ultra-violet or visible spectrum. The high intensity of many of the absorption bands in the near ultra-violet and visible regions not only permits the identification with minute quantities of material, but also serves as an aid in the control of purification of substances. In this book, an attempt has been made to present the basic concepts of electronic spectroscopy and to survey its analytical and structural applications in the different branches of chemistry. 

The development of a theory of retention and band broadening for macro-molecular HPLC is intended primarily for improving such separations. We need to relate various separation goals to (1) experimental conditions, (2) the choice of column type and HPLC method (RPLC, SEC, etc.), and (3) the nature of the sample. The present model allows os to do this 1 prediction, rathor than by experiment Our main requirement is usuaOy to achieve adequate separation, or some minimum resolution R, betw adjacent bands of interest. In this section we will show that the separation of macro-molecular samples by either isocratic or gradient elution is understandable and controllable, using the same concepts that we use for optimizing the isocratic separation of small molecules. 

---