Metal oxide semiconductor surface states

A wide variety of solid-state sensors based on hydrogen-specific palladium, metal oxide semiconductor (MOS), CB, electrochemical, and surface acoustic wave (SAW) technology are used in the industry for several years. Microelectromechanical systems (MEMS), and nanotechnology-based devices for the measurement of hydrogen are the recent developments. These developments are mainly driven by the demands of the fuel cell industry. Solid-state approaches are gaining rapid popularity within the industry due to their low cost, low maintenance, replacements, and flexibility of multiple installations with minimal labor. 

On semiconductors that are partially ionic and partially covalent, such as transition metal oxides, the surface ion-induced and the surface dangling states may coexist together. 

In related studies, Hoffmann and co-workers [142,143,150-153] examined the rates of photooxidation of selected organic compounds and the production of hydrogen peroxide on a variety of metal oxide semiconductors. In general, electron transfer occurs from the conduction band to dioxygen adsorbed on the surface of the excited state metal oxide as follows ... 

The second chemical contribution to the total interaction energy is present if an ionic or a covalent chemical bond between the adsorbed molecule and the surface can be formed. Since covalent bonds also depend on the overlap between the wave functions of the subsystems, their distance dependence is exponential, see Table 1, as is that of the Pauli repulsion. In general, covalent bonds are only possible if at least one of the two partners possesses partially occupied valence orbitals. In contrast to the adsorption at metal or semiconductor surfaces, such a situation is rarely encountered at insulator and in particular at oxide surfaces. In most cases, the ions at the surface of an insulator try to adopt a closed shell electronic structure as they do in the bulk, as for instance the Na+ and Ck ions in NaCl or the Mg + and ions in MgO. Counterexamples are transition metal oxides in which the metal cations possess partially occupied d-shells which might form chemical bonds with the adsorbed molecule. One famous example is the interaction between NO and the NiO(lOO) surface where both the Ni + cations (d configuration with a A2g ground state) and the NO radical ( 11 ground state) have partially filled valence shells (see below). 

Properties of a nanocrystal can be influenced markedly by encasing it in a sheath of another material [531]. The material of the shell in such a core-shell structure can be a metal, semiconductor, or an oxide. The shell material helps to impart novel, desired properties on the nanocrystals. For example, defects prevalent in the surface states of semiconductor nanocrystals can be transferred to a buffer layer of the shell material to obtain better emission from the nanocrystals. We use the notation, core-shell to denote core-shell structures. We employ the following classification to describe the nature of the core and the shell semiconductor-semiconductor, metal-metal and metal-oxide, semiconductor-oxide and oxide-oxide. The classification is artificial in that the nanocrystals result from similar synthetic strategies. The motivation for carrying out the modification of the shell material, however, differs in each case. 

Kisailus, D., Truong, Q., Amemiya, Y., Weaver, J.C. and Morse, D.E. (2006) Self-assembled bifunctional surface mimics an enzymatic and templating protein for synthesis of a metal oxide semiconductor. Proceedings of the National Academy of Sciences of the United States of America,... 

Figure Bl.22.4. Differential IR absorption spectra from a metal-oxide silicon field-effect transistor (MOSFET) as a fiinction of gate voltage (or inversion layer density, n, which is the parameter reported in the figure). Clear peaks are seen in these spectra for the 0-1, 0-2 and 0-3 inter-electric-field subband transitions that develop for charge carriers when confined to a narrow (<100 A) region near the oxide-semiconductor interface. The inset shows a schematic representation of the attenuated total reflection (ATR) arrangement used in these experiments. These data provide an example of the use of ATR IR spectroscopy for the probing of electronic states in semiconductor surfaces [44]-...

Although the role of rare earth ions on the surface of TiC>2 or close to them is important from the point of electron exchange, still more important is the number of f-electrons present in the valence shell of a particular rare earth. As in case of transition metal doped semiconductor catalysts, which produce n-type WO3 semiconductor [133] or p-type NiO semiconductor [134] catalysts and affect the overall kinetics of the reaction, the rare earth ions with just less than half filled (f5 6) shell produce p-type semiconductor catalysts and with slightly more than half filled electronic configuration (f8 10) would act as n-type of semiconductor catalyst. Since the half filled (f7) state is most stable, ions with f5 6 electrons would accept electrons from the surface of TiC>2 and get reduced and rare earth ions with f8-9 electrons would tend to lose electrons to go to stabler electronic configuration of f7. The tendency of rare earths with f1 3 electrons would be to lose electrons and thus behave as n-type of semiconductor catalyst to attain completely vacant f°- shell state [135]. The valence electrons of rare earths are rather embedded deep into their inner shells (n-2), hence not available easily for chemical reactions, but the cavitational energy of ultrasound activates them to participate in the chemical reactions, therefore some of the unknown oxidation states (as Dy+4) may also be seen [136,137]. 

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