Ion Doped Semiconductors

As shown in reactions (6.5.13) and (6.5.14), metal ion dopants influence the photoreactivity of metal oxides by acting as electron (or hole) traps thereby altering the e /h -pair recombination rate. The energy level of should be less negative than that of the 

Tnble 6.7 Visible light-driven (X, 420 nm) photocatalysts for Mi and/or O from 

Semiconductors doped Band gap (uV) Sacrificial reagent Rate of gas evolution (fimol/h) Ref 

Reactions were carried out in presence of sacrificial agents. O2 evolution aqueous silver nitrate solution. Hi evolution aqueous methanol solution, 

IrOs has also been used as a co-catalyst with NaTaOs La [146] it was found that IrOs is involved in the formation of sites active for O2 evolution. Other metal oxides such as y-BisOs and 

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Ion implantation is a method commonly used for doping semiconductors. Because the concentrations of the dopants (mostly B and P) are very low, a dynamic range of more than five orders of magnitude is often necessary. Measurement of is more difficult than that of B, because of the mass interference of °Si H. High mass resolution of m/Am = 5000, or an energy offset of 300 V, is necessary. 

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]. 

In the past several years noble metal loading, ion doping, composite metal-oxide semiconductors, and multi-component semiconductors have been meticulously designed, fabricated, and then investigated... 

Zou Z, Ye J, Sayama K, Arakawa H (2001) Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst. Nature 414 625-627 Konta R, Ishii T, Kato H, Kudo A (2004) Photocatalytic activities of noble metal ion doped SrTiOs under visible light irradiation. J Phys Chem 108 8992-8995 Kato H, Kudo A (2002) Photocatalytic activities of noble metal ion doped SrTiOs under visible light irradiation. J Phys Chem B 106 5029-5034... 

In doped semiconductors I is due to direct overlap in transitional-metal oxides the overlap between the d-orbitals is frequentiy via the oxygen ions and is then often called a superexchange interaction. Figure 3.4 shows the kinds of wave function expected. As regards magnitudes, if B 1 eV, 17 10 eV and z=4, kBTN should be 0.01 eV so that 100K, which shows why low Neel temperatures are common. 

As we shall see below, for dilute solutions the electron is not attached to the alkali ion but is trapped in a cavity, around which the ammonia is polarized. The problem of the metal-insulator transition, then, is one of a random array of one-electron centres, as in a doped single-valley semiconductor. On the other hand, the disorder is less because the strong overlap between the wave functions of some pairs of centres characteristic of doped semiconductors is absent. In doped semiconductors there is no discontinuity in s2 at the transition. As explained in Chapter 5, this may be because of the very strong disorder or, in many-valley systems, because of self-compensation. In metal-ammonia solutions, as in the fluid alkali metals discussed in Section 4, both are absent. 

In recent years, the electronics industry has made increasing use of ion imptamaiion as a method of doping semiconductors. Since rhe number of ions implanted is determined hy the charge transferred to the substrate and Iheir depth distribution hy the incident energy, ion implantation has improved the controllability and reproducibility of certain semiconductor device processing operations. Also, ion implantation processes do not... 

So-called wet solar cells show promise, particularly because of their relative ease of fabrication. In this type of photovoltaic cell, the junction is formed, between a semiconductor and a liquid electrolyte. No doping is required because a junction forms spontaneously when a suitable semiconductor, such as GaAs, is contacted with a suitable electrolyte, Three knotty problems (accelerated oxidation of surface of semiconductor exchange of ions between semiconductor and electrolyte forming a blocking layer and deposition of ions of impurities on the surface of the semiconductor) all have been solved and thus the concept now appears technically viable,... 

In the past decade, lanthanide ions doped in nanocrystalline semiconductors have been the subject of numerous investigations. Although quantum size effects are not expected on lanthanide energy level structures, influence of quantum confinement in semiconductor on the luminescence properties of the lanthanides is expected. One of the advantages of lanthanide-doped semiconductor nanocrystals is that the lanthanide luminescence can be efficiently sen-... 

Recently, many researchers have paid attention to the optical properties of lanthanide-doped III-V and II-VI semiconductor nanocrystals prepared by ion implantation, molecular-beam-epitaxy (MBE) or wet chemical syntheses. Although some controversies still exist, many important results have been achieved, which may be beneficial to the understanding of the basic physical or chemical properties of lanthanide-doped semiconductor nanocrystals. 

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