Holes chemical species

As seen from Fig. 5, upon absoption of photons with the energy hv > Eg, an electron and hole centres are formed. They migrate to different sites on the PC surface, thus becoming spatially separated. Note, that what solid state physisists call surface electron and hole centers, in fact are some definite chemical species with strong reducing and oxidizing... 

The photoreactivity of the involved catalyst depends on many experimental factors such as the intensity of the absorbed light, electron-hole pair formation and recombination rates, charge transfer rate to chemical species, diffusion rate, adsorption and desorption rates of reagents and products, pH of the solution, photocatalyst and reactant concentrations, and partial pressure of oxygen [19,302,307], Most of these factors are strongly affected by the nature and structure of the catalyst, which is dependent on the preparation method. The presence of the impurities may also affect the photoreactivity. The presence of chloride was found to reduce the rate of oxidation by scavenging of oxidizing radicals [151,308] ... 

Fig. 1.4 Diagram showing the principle of operation of a time-of-flight atom-probe. The tip is mounted on either an internal or an external gimbal system. The tip orientation is adjusted so that atoms of one s choice for chemical analysis will have their images falling into the small probe-hole at the screen assembly. By pulse field evaporating surface atoms, these atoms, in the form of ions, will pass through the probe hole into the flight tube, and be detected by the ion detector. From their times of flight, their mass-to-charge ratios are calculated, and thus their chemical species identified.

The atom-probe field ion microscope is a device which combines an FIM, a probe-hole, and a mass spectrometer of single ion detection sensitivity. With this device, not only can the atomic structure of a surface be imaged with the same atomic resolution as with an FIM, but the chemical species of surface atoms of one s choice, chosen from the field ion image and the probe-hole, can also be identified one by one by mass spectrometry. In principle, any type of mass analyzer can be used as long as the overall detection efficiency of the mass analyzer, which includes the detection efficiency of the ion detector used and the transmission coefficient of the system, has to be close to unity. 

The reactivity of a surface depends on many factors. These include the adsorption energies of chemical species and their dissociation behavior, their diffusion on the surface, the adatom-adatom interactions, the active sites where a chemical reaction can occur, and the desorption behavior of a new chemical species formed on the surface. The site specificity depends on at least three factors the atomic configuration of the surface, the electronic structures of the surface, and the localized surface field. In atom-probe experiments, the desorption sites can be revealed by a timegated image of an imaging atom-probe as well as by an aiming study with a probe-hole atom-probe, the electronic structure effect of a chemical reaction can be investigated by the emitter material specificity, and the surface field can be modified by the applied field. 

Point defects, electrons, and holes as chemical species... 

Concentration equilibrium among A , A , A , and h is discussed on the assumption that these equations can be treated as chemical equilibrium ones. (Similarly, D", D, (donor levels), and e are regarded as chemical species, see Fig. 1.24(c).) We have a reasonable reason for regarding these species as chemical species. As is well known, the electrical properties of metals and alloys are independent of the concentration of point defects or imperfections existing in their crystals, because the number of electrons or holes in metals or alloys is roughly equal to that of the constituent atoms. For the case of semiconductors or insulators, however, the number of electrons or holes is much lower than that of the constituent atoms and is closely correlated to the concentration of defects. In the latter case, electrons and holes can be considered as kinds of chemical species, for a reason similar to that discussed above for the case of point defects. Let us consider the chemical potential, which is most characteristic of chemical species. Electrochemical potential of electrons is written as... 

Thus, lattice defects such as point defects and carriers (electrons and holes) in semiconductors and insulators can be treated as chemical species, and the mass action law can be applied to the concentration equilibrium among these species. Without detailed calculations based on statistical thermodynamics, the mass action law gives us an important result about the equilibrium concentration of lattice defects, electrons, and holes (see Section 1.4.5). 

It has been shown in Section 1.3.7 that in semiconductors or insulators the lattice defects and electronic defects (electrons and holes), derived from non-stoichiometry, can be regarded as chemical species, and that the creation of non-stoichiometry can be treated as a chemical reaction to which the law of mass action can be applied. This method was demonstrated for Nii O, Zr Cai Oiand Cuz- O in Sections 1.4.5, 1.4.6, and 1.4.9, as typical examples. We shall now introduce a general method based on the above-mentioned principle after Kroger, and then discuss the impurity effect on the electrical properties of PbS as an example. This method is very useful in investigating the relation between non-stoichiometry and electrical properties of semiconductive compounds. 

In the corrosion example, we defined a layer of material containing oxidation and corrosion products. In this domain named C, for example, two phases were specified to exist, according to some particular model of the process. One of the phases is denoted CuO, and represents an oxide coating that has formed. The second phase, is designated CuS2, and contains products of an atmospheric sulfidation process. Within each phase, different chemical species may reside, for example, Cu+ ions, elemental Cu, holes, and electrons. The list of species depends on the particular reaction mechanism. 

In addition to stress, the other important influence on solid state kinetics (again differing from fluids) stems from the periodicity found within crystals. Crystallography defines positions in a crystal, which may be occupied by atoms (molecules) or not. If they are not occupied, they are called vacancies. In this way, a new species is defined which has attributes of the other familiar chemical species of which the crystal is composed. In normal unoccupied sublattices (properly defined interstitial lattices), the fraction of vacant sites is close to one. The motion of the atomic structure elements and the vacant lattice sites of the crystal are complementary (as is the motion of electrons and electron holes in the valence band of a semiconducting crystal). 

Like the performance of chemical reactors, in which the transport and reactions of chemical species govern the outcome, the performance of electronic devices is determined by the transport, generation, and recombination of carriers. The main difference is that electronic devices involve charged species and electric fields, which are present only in specialized chemical reactors such as plasma reactors and electrochemical systems. Furthermore, electronic devices involve only two species, electrons and holes, whereas 10-100 species are encountered commonly in chemical reactors. In the same manner that species continuity balances are used to predict the performance of chemical reactors, continuity balances for electrons and holes may be used to simulate electronic devices. The basic continuity equation for electrons has the form... 

The proper theoretical study of core-hole excited states has been one of the most challenging problems in molecular quantum mechanics. The difficulty stems from the fact that these states lie above the continuum part of the spectra, and their attainment as high-energy roots in an ordinary configuration interaction calculation is impossible [46]. However, core-hole excited states play an important role in the identification of chemical species by X-ray based spectroscopic techniques. Additionally, there are many interesting effects that are particular to this region of the spectra and their study are the object of active research [47]. 

Figure 34 shows a summary of the photochemistry and dynamics in the polar stratosphere, illustrating the time profiles of the key chlorine species coupled to the temperature requirements in the vortex. Figure 35 shows measurements of a range of chemical species and temperature in the 2004 ozone hole. The data reflects the main chemical and physical features on the ozone hole. 

Figure 35 Chemical species and temperature as measured by satellite in the 2004 ozone hole (image courtesy of NASA). The white line on the ozone figure gives the extent of the vortex...

Fig. 3.17 Schematic representation of some photophysical and photochemical processes in and on a semiconductor (SC) particle (for example Ti02). bg- Band gap energy VB valence band CB conduction band h electron hole ( defect electron ) in the valence band e photoelectron in the conduction band LT lattice trap ST surface trap A ds, Dads chemical species adsorbed on the surface of the SC particle with A being an electron acceptor and D an electron donor. Formation of an electron-hole pair (exciton) by irradiation SC-i-hv ecb + hvb (modified according to Serpone, 1996 and Bottcher 1991).

The situation here is very unusual in that the breaking of a single chemical bond at a solid surface can be direcdy recognized. Fig. 3 shows the electrolytic dissolution of two n-type Ge electrodes, where the supply of holes is limited, which is to be contrasted to the dissolution of a p-type electrode where there is an unlimited supply of holes at the surface (7). As shown in Fig. 4, the dissolution rate can also be limited by the supply of chemical species needed for the formation of the final ions in solution (7). Fig. 4 also shows that hydroxyl ions lead to formation of metagermanate ions much faster than do water molecules. A similar behavior has also been found with fluoride ions. 

Regarding the second aspect, generally, as shown in Fig. 29, three species may be involved in the etching reaction charge carriers, that is, electrons and holes, at the semiconductor surface chemical species such as OH-, NO3- H2O, and so on near the surface in the solution and active surface silicon atoms which are favorable sites for reaction and removal. Unlike the other two species, charge carriers may or may not be involved depending on whether the reaction is of an electrochemical nature. The concentrations of each of these species are determined by different processes such as diffusion, migration,... 

To determine the thickness of the silicon film as a function of radius, both the Navier-Stokes equations and the chemical species balances are written and solved. Then, the volumetric flow rate of the feed stream, and its distribution through the showerhead holes, is adjusted to obtain a more uniform film. 

Chemical doping The doping charge (electron or hole) on the conjugated molecules or polymers comes from another chemical species (atom or molecule). The chemical species become the counterions of the polarons created on the conjugated materials. 

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