Subsurface impurities

The positions of the vague maxima were not reproducible. In our opinion this type of behaviour is due to small amounts of Si and/or B subsurface impurities, which were not detectable in our AES analysis. Small amounts of these elements are known to form stable oxides at the surface (15-19). Type I and type II, however, were fully reproducible. Type I (fig.3d) is a Pt-like behaviour comparable to those of pure Pt (fig.3a) and the Pt-rich alloy (fig.3b). Type II (fig.3e), which shows a maximum for the oxygen intensity likewise at 800 K, is a Rh-like behaviour (compare fig.3c). The maximum relative intensity is lower than that observed for pure Rh. The figures also show that for the Rh-rich alloy the dashed line is much lower relative to the solid line than for pure Rh. This might indicate that the surface oxygen is more easily removed by the residual gas on the alloy than on the pure Rh. It was shown earlier that on a Pt-Rh alloy surface oxygen preferentially occupies the Rh sites leaving the Pt sites initially free (10). If many free Pt sites are present at the surface... 

It is interesting to compare our results on single crystal surfaces with those of Turner and coworkers for Pt, Pd, and Ir. In this study wires of Pt formed less than one layer of oxide under CO oxidation conditions. Considering that the Pt wires were known to have substantial Si impurities, which form subsurface oxides , it is not surprising that some oxide was formed. The absence of impurities on the rigorously cleaned, Pt single crystal surface used in this study precluded the formation of any oxides during CO oxidation. 

Contaminant precipitation involves accumulation of a substance to form a new bulk solid phase. Sposito (1984) noted that both adsorption and precipitation imply a loss of material from the aqueous phase, but adsorption is inherently two-dimensional (occurring on the solid phase surface) while precipitation is inherently three-dimensional (occurring within pores and along solid phase boundaries). The chemical bonds that develop due to formation of the solid phase in both cases can be very similar. Moreover, mixtures of precipitates can result in heterogeneous solids with one component restricted to a thin outer layer, because of poor diffusion. Precipitate formation takes place when solubility limits are reached and occurs on a microscale between and within aggregates that constitute the subsurface solid phase. In the presence of lamellar charged particles with impurities, precipitation of cationic pollutants, for example, might occur even at concentrations below saturation (with respect to the theoretical solubility coefficient of the solvent). 

The impurities lead, cadmium, and tin, if present in castings in amounts greater than the established maximums (0.005% lead 0.004% cadmium 0.003% tin), cause subsurface network corrosion. These limits are close to critical values. Iron is held to 0.10% maximum to prevent excessive skimming losses and machining problems. 

The comparison of more complete kinetic equations (242) and (243) with experimentation is hampered by the instability of activity of silver catalysts (59). The effects arising from the penetration of oxygen into the subsurface silver layer (63) and the formation of a polymer film on the surface (70), an extremely high sensitivity of the catalyst to the traces of compounds of such elements as S and Cl that may be present in the reactants as impurities, can be the sources of this instability. 

Figure 15 Relaxed structure of the S133BPH36 co-doped nanocrystal (diameter = 1.10 nm). Gray balls represent Si atoms, while the light gray balls are the H used to saturate the dangling bonds. B (dark gray) and P (black) impurities have been located at subsurface positions in substitutional sites on opposite sides of the nanocrystals. The relaxed impurity distance is DBP = 3.64 A.

Figure 16 Formation energy for single-doped and co-doped Si-NCs. In the co-doped nanocrystals, the impurities are placed as second neighbors in the first subsurface shell. Squares are related to S135H35, diamonds to S187H76, and circles to Si Hioo based nanocrystals. The lines are a guide for the eyes.

Figure 28 Formation Energy for the co-doped Si-NW (shown in the inset) as function of the related position between the two dopants. The B (impurity) is frozen in a subsurface site, while the P atom occupies different substitutional sites labelled 1, 2, and 3. The lines are guides for the eyes.

Since SEEE are localized in one or two subsurface monolayers, the discussion is actually about the effects of electronic excitations in the transition layer on optical properties of a crystal. Note also that even in pure crystals without impurities could be localized adsorbed molecules on the surface that makes the value of D(n ) a random function. The process of attaching foreign molecules to the surface lead to an analogous effect. As SSSE can be experimentally observed,... 

Clearly, this looks like a less reliable doping mechanism, as it ultimately relies on the concentration of the preexisting doping impurities and on how many of them were passivated at subsurface locations, then amenable to reactivation. 

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