Metal dopants, influence

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... 

Changes in properties and their corre-lation with composition and structure of semiconductors will be discussed later, but it should be mentioned that the influence of various metal dopants on semiconductor properties is rather well studied [2, 9, 12, 141 and Refs, therein], whereas the peculiarities of their structure are studied poorly, while... 

For doped metal oxides, the aliovalent metal ions influence the overall defect concentrations via what is sometimes called the First Law of Doping. This law simply states that adding an aliovalent dopant increases the concentration of defects with opposite charges, and decreases the concentration of defects with charges of the same sign. 

It is well established, that the poor selectivity of tin-oxide sensors can partly be overcome by adding catalysts to the sensitive layer. Most common additives are noble metals like gold (Au), platinum (Pt) or palladium (Pd). They can be mixed with the tin oxide during paste formation before deposition. The influence of dopants on the gas sensor response is still subject to debates. The two most established mechanisms are the spill-over and the Fermi-level mechanism [82]. 

Figure 4.4.1 Schematic representation of the model systems discussed within the chapter (A) nanoparticle growth influenced by dopants in the support, (B) nanoparticle deposition from solution, (C) strong metal support interaction, and (D) photochemistry at supported nanoparticles as a function of size.

Dopants also influence the emission processes from PPVs. Improved red dopants have been based on pyran dyes" while Qo doping appears to be variable" "". Doping with electron transport materials such as oxadiazoles give polymers with balanced properties for hole transport". The avoidance of low molecular weight material in the synthesis of cyano based PPVs is important" as are head to head and tail to tail chain sequences in thiophene based polymers. Head to tail tetramer sequences were the most fluorescent. Metal ion doped PPV s are claimed to be good chemosensors and broad emission is observed from titania doped PPV . Electron rich dopants enhance the emission in the red region while electro and photoinduced infrared bands from PPV are similar . ... 

In reduction of the doped oxide, part of the dopants is incorporated during the CVT growth of the metal particles in the form of silicates. Excess dopant, which remains on the tungsten crystal surfaces, is removed by subsequent leaching of the powder in HCl and HF acids, while the dopants which are internally trapped are retained. The amount of incorporated dopants, the size of the inclusions, and their chemical composition can be influenced by the reduction conditions within certain limits. About 100-150 ppm K, 60-100 ppm Al, and 200-300 ppm Si commonly remain in the acid-washed powder. 

Ramis et al. [44] studied the effect of dopants and additives on the state of surface vanadyl species of vanadia on titania catalysts by means of FTIR spectroscopy. Additives such as alkali and alkali-earth metal cations (typically Cs, K, Na, Li and Mg), oxoanions (such as sulphates and arsenates), and other species (such as AP+, MoO +, and WO ), influence the position of V=0 stretching frequencies. The position of vy=o for a 3 wt% V2O5 on titania was observed at 1035 Two percent W or Mo did not show any shift of the stretching frequency of V=0, whereas Cs lowered the band position by 45 cm . This was explained in terms of the formation of strong basic sites and the exchange of Ti in 0=V-0-Ti by 0=V-0-Cs. The elements Al, S, and As shift the position of vv=o to higher frequencies. Oxoanions are coordinatively bond to vanadyl centers [44],... 

Apart from those mentioned above, there are other contaminations known for diamond films, too. Oxygen and silicon are frequently observed with the latter being incorporated into the lattice in a substitutional way as well. Oxygen and further impurities like sulfur or metallic defects have not yet been studied in sufficient detail to give a final statement on their arrangement in the lattice and on their influence on the diamond properties. Calculations indicate that boron, nitrogen, phosphorous, and silicon are the only dopants suitable for a stable incorporation into the lattice. 

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