Doping experiments with

A complication in doping experiments with silica is that the state of the impurity (A13+) may depend on the presence of other impurities. In quartz and fused silica, aluminum can be put in a substitutional position only if a charge-compensating cation such as Na+ is also present (86). If this is also true for silica gel, then the concentration of color that can be produced in a given sample will depend on the concentrations of both aluminum and some monovalent cation. 

An interesting question arises for spiro-oligophenyls with two identical, perpendicular chromophores whether the double-charged species have their charges on the same branch or on different branches. Crispin et al. [94] performed doping experiments with Li and Na in sohd films and concluded that if Spiro-4 21 or Spiro-6 22 is doped with Li, dianions (bipolarons) are formed on the same spiro branch, whereas doping with Na leads to the formation of radical anions (polarons) on each branch. These results are based on photo electron and optical spectra and indicate that the size and interactions of the dopant play an important role in the formation of the charged species. 

Fig. 3.18. Kinetics of conductivity of ZnO film during adsorption of methyl radicals CH3 at room temperature depending on the degree of preliminary alloying of the surface by titanium atoms. 1 - Blank experiment with a clean (Ti-atom free) film (O - before doping - after heating of alloyed film at 350 C, i. e. after the film has been regenerated) 2-5 - Experiments with doped films. Doping degree increases in the following row 2<3<4<5.

The gas-phase decomposition of beryllocenes has been examined for the doping of semiconductor materials and in the preparation of thin metal films. Molecular-beam expitaxy has been used with (MeC5H4)2Be to dope InP semiconductors only a small amount of carbon is found in the doped films.54 Beryllocene 9 has been explored as a precursor for coating capsules with beryllium metal for use as targets in experiments with inertial confinement fusion.55... 

Two further related comments. Firstly, since N2 is a good ligand (Xma, 364 nm), with N2 doped Xe there is no trace even of Cr(C0)5...Xe since Cr/C0/N2 species predominate. Secondly, during experiments with Ni(CO)4/N2/liquid Kr (see above), photolysis in the complete absence of dissolved N2 led to the appearance of a transient carbonyl species with IR bands similar to those assigned to Ni2(CO)7 (J.E. Hulse and M. Moskovits unpublished data) -presumably the interaction of Ni(C0)3 with Ni(C0)4 is considerably stronger than with Kr. 

The third member, trimethylenemethane (3), had some relevance to our studies on carbenes, since besides methylene and its simply substituted derivatives trimethylenemethane 3 is one of the few molecules having a triplet ground state.22 Also the experience with 3 could be of help in order to deal with the singlet/triplet differentiation in matrix-isolated carbenes. We learned that, if the calculated IR spectra of the singlet and triplet molecule are sufficiently different, it might be possible to determine the multiplicity of the matrix-isolated species by comparison with the experimental IR spectrum. In this context it is also worth mentioning that we were able to measure the matrix IR spectrum of 3, but a special technique (irradiation in halogen-doped xenon matrices) had to be developed in order to achieve a concentration of 3 sufficient for its IR detection.23... 

According to the results obtained from rotating ring disk experiments and the determination of Tafel slopes, it was concluded that the mechanism of oxygen electroreduction on an Ag/C catalyst is similar to that on a Pt/C catalyst at high potentials, that is, interesting potentials for fuel cell applications. Lee et al. also showed that a 30 wt% Ag electrode displayed the same electroactivity towards the ORR as a 10 wt% electrode and that the activity could be enhanced by doping Ag with Mg [115],... 

Fulleride anions are often more soluble, especially in more polar solvents, than the parent fullerenes. For example, in bulk electrolysis experiments with tetra-n-butylammonium perchlorate (TBACIO4) as supporting electrolyte, carried out in acetonitrile where Cjq is completely insoluble, fairly concentrated, dark red-brown solutions of 50 can be obtained [81]. Upon reoxidation, a quantitative deposition of a neutral Cjq film on the surface of a gold/quartz crystal working electrode takes place. This Cjq film can be stepwise reductively doped with TBA, leading to (Cjo )... 

However, there are limitations to the use of superconductivity as a screen for new superconductors. Experience with various of the known high Tc phases has taught us the sensitivity of superconductivity to doping. For, example Tc depends critically on oxygen stoichiometry in Ba2YCus07. Thus,interesting phases might easily be overlooked. 

If the experiment is conducted properly, this method has the advantage that the reactive intermediate to be studied is truly isolated in the sense that it is (ideally) surrounded by nothing but the inert host material (doped perhaps with some deliberately added reagent). This feature can be very important, say, in the case of radicals which usually arise in parrs that have a propensity to recombine if trapped in the same matrix cavity, which often excludes method (3) above for the generation of such species. 

Deep state experiments measure carrier capture or emission rates, processes that are not sensitive to the microscopic structure (such as chemical composition, symmetry, or spin) of the defect. Therefore, the various techniques for analysis of deep states can at best only show a correlation with a particular impurity when used in conjunction with doping experiments. A definitive, unambiguous assignment is impossible without the aid of other experiments, such as high-resolution absorption or luminescence spectroscopy, or electron paramagnetic resonance (EPR). Unfortunately, these techniques are usually inapplicable to most deep levels. However, when absorption or luminescence lines are detectable and sharp, the symmetry of a defect can be deduced from Zeeman or stress experiments (see, for example, Ozeki et al. 1979b). In certain cases the energy of a transition is sensitive to the isotopic mass of an impurity, and use of isotopically enriched dopants can yield a positive chemical identification of a level. 

The identification of an impurity, defect, or impurity-defect complex by some particular technique must nearly always be accomplished in conjunction with doping experiments. Thus, the well-known, sharp, zero-phonon photoluminescence lines at 0.84 eV in GaAs are almost certainly associated with Cr, as established by Cr-doping experiments (Koschei et al., 1976). However, some care must be taken here. For example, a dominant electron trap (EL2) in -doped GaAs is probably not associated with O, according to recent experiments (Huber et al., 1979). Thus, the doping must be accompanied by a positive identification of the relevant impurity concentration, say by SSMS, or SIMS. These general considerations apply to all the techniques discussed below. 

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