Surface atomic density

It is well known that the 0 of a metal depends on the surface crystallographic orientation.6,65,66 In particular, it is well established that 0 increases with the surface atomic density as a consequence of an increase in the surface potential M. More specifically, for metals crystallizing in the face-centered cubic (fee) system, 0 increases in the sequence (110) <(100) <(111) for those crystallizing in the body-centered cubic (bcc) system, in the sequence (111) < (100) <(110) and for the hexagonal close-packed (hep) system, (1120) < (1010) < (0001). 

Nm surface atom density of the catalyst surface atom/m2... 

Density of surface atoms at steps and kinks relative to surface atomic density... 

The growth of Ni layers on W(llO) and (100) subsequent to the first monolayer, determined by the breaks in the AES vs. TPD area curves shown in Figs. 26 and 27, does not yield TPD areas that correspond to simple multiple of the TPD area found for the monolayer TPD feature. This result indicate that the second and successive Ni layers have significantly altered Ni atomic densities compared with the first Ni layer. On W(110) the ratio of the first to second monolayer TPD areas, 0.78, compares favorably with the ratio of the surface atomic densities of Ni(lll) and W(110X 0.79, using the value 1.81 X 10 and 1.43 x 10 for the atomic densitie of Ni(lll) and W(llO), respectively. On W(IOO) the ratio of Ni atoms in the first to the second layers. 

Simple calculation gives a comparable distribution of the electrode potential in the two layers, (64< >h/64( sc) = 1 at the surface state density of about 10cm" that is about one percent of the smface atoms of semiconductors. Figure 5—40 shows the distribution of the electrode potential in the two layers as a function of the surface state density. At a surface state density greater than one percent of the surface atom density, almost all the change of electrode potential occurs in the compact layer, (6A /5d )>l, in the same way as occurs with metal electrodes. Such a state of the semiconductor electrode is called the quasi-metallic state or quasi-metallization of the interface of semiconductor electrodes, which is described in Sec. 5.9 as Fermi level pinning at the surface state of semiconductor electrodes. 

This surface is therefore the (111) surface. This surface is an important one because it has the highest possible density of atoms in the surface layer of any possible Miller index surface of an fee material. Surfaces with the highest surface atom densities for a particular crystal structure are typically the most stable, and thus they play an important role in real crystals at equilibrium. This qualitative argument indicates that on a real polycrystal of Cu, the Cu(l 11) surface should represent a significant fraction of the crystal s surface total area. 

Gas exposures are reported in units of Langmuir (1 Langmuir = 1 L = 1x10 torr-sec), uncorrected for ion gauge sensitivity and doser enhancement. Coverages, 0, are reported relative to the unreconstructed Au(lll) surface atom density (0 = 1 corresponds to 1.39x10 5 atoms/cm ). 

V at 40°. This AV. /pCl" value for the (100) orientation n-GaAs is approximately one-half that obtained with (111) n-GaAs crystals (2), indicating that the crystal surface atom density and type can be a significant factor in the interactions between substrate and electrolyte. Flat-band potential values for (100) and (111) n-GaAs/molten salt interphases and for the (111) n-GaAs/aqueous electrolyte interphase are compared in Table I. 

Regardless of bulk, epi, or SOI substrates, a key consideration that is paramount toward subsequent processing steps is the crystal orientation employed. Three important crystal planes for silicon include Si(lOO), Si(llO), and Si(lll) - Figure4.24. The surface atomic densities increase in the order Si(lOO) < Si(llO) < Si(lll). Empirically, this translates to available Si-Si bond densities of 6.77 x lO, 9.59 x 10, and 11.76 x 10 cm , respectively.Hence, the rates required to remove (etch), or react with, surface atoms (e.g., thin-film deposition) should follow the reverse order as above. However, the Si(l 10) orientation etches fastest due to its more... 

Determination of surface atom density on nanocrystals can be difficult, and imprecise, especially for very small particles that cannot be easily characterized microscopically. Nevertheless, reasonable accuracy can be obtained by using theoretical calculations informed by empirical data. In this work, the CdTe nanocrystals that were prepared (2.5-6 nm diameter) were found to be in the zinc blende crystal structure, allowing the use of the bulk density and interplanar distances of zinc blende CdTe in these calculations. It is likely that a variety of crystalline facets are exposed on individual nanocrystals, each with a range of planar densities of atoms. It is also likely that there is a distribution of different facets exposed across an assembly of nanocrystals. Therefore, one may obtain an effective average number of surface atoms per nanocrystal by averaging the surface densities of commonly exposed facets in zinc blende nanocrystals over the calculated surface area of the nanocrystal. In this work we chose to use the commonly observed (Iff), (100), and (110) zinc blende planes, which are representative of the lattice structure, with both polar and nonpolar surfaces. For this calculation, we defined a surface atom as an atom (either Cd or Te ) located on a nanocrystal facet with one or more unpassivated orbitals. Some facets, such as Cd -terminated 111 faces, have closely underlying Te atoms that are less than 1 A beneath the surface plane. These atoms reside in the voids between Cd atoms, and thus are likely to be sterically accessible from the surface, but because they are completely passivated, they were not included in this definition. 

For the same arrangement of the topmost layer, the same surface atomic density, and the same surface composition (100% Pt), but a difference in electronic structure = 0.33 eV), the ORR is enhanced by factor of 10 on Pt(lll)-skin relative to Pt(l 11). The rationale for this unprecedented increase in catalytic activity can be found in Fig. 3.4a, which shows that at the same potential attenuated sub-... 

For all three surfaces 1 ML is defined as one CO molecule per (1 x 1) unit cell. The surface atomic densities - or number of unit cells per cm - for Pt 211), Pt 311, andPt 411 are0.53x 10, 0.78x 10 and0.31 x 10 atomscm", respectively. Adsorption of CO on Pt 211 appears to proceed in three distinct stages, with an initial heat of adsorption and sticking probability of 185 kJ/mol and 0.76, respectively [4]. 

Si/SiOi Interface. There is little information on the interface of silicon and an anodic oxide film. For thermally grown oxides, a transition region exists at the Si/Si02 interface where there is an excess of unoxidized Si bonds with a density on the order of the surface atom density. The interface structurally consists of two distinct regions. A few atomic layers near the interface contain Si atoms in intermediate oxidation states, i.e., Sf (Si20), Si (SiO), and Si (Si203). The S atoms are located farther out than... 

Atomic scale roughness in terms of the density of kinks sites and steps relative to surface atomic density determines effect of surface lattice structure on the rate of reactions when the roughness is low relative to atomic density the reactions show a high degree of anisotropicity and when it is high the reactions tend to be isotropic. 

In the first part of this section we described how the density of surface atoms in growing DlC models was lowered due to increasing stresses in the IC structure. The MIC structure makes it possible to maximize the surface atom density and thus, for a well-defined size, clusters undergo a structural transition. In a cluster containing a few hundred atoms, the MIC structure is... 

Surface reconstruction usually leads to the formation of stable overlayers. This cannot occur without some mobility of catalyst substrate atoms. The reconstructed phase often has a different surface atom density from the non-reconstructed surface. For this reason the macroscopic consequence may be facetting of the catalyst along particular preferred crystallographic directions. This explains the often observed phenomenon that a heterogeneous catalyst often shows only stable performance after some initiation period. In a reactive system the surface composition of the adlayer may strongly vary with conditions and hence the details of surface facetting and surface reactivity. 

Up to 100°C under Ha flow for 30 minutes, then up to 400°C during 3 hours. The solids were flushed for 1 hour at 400°C under He flow and then cooled down at room temperature. CO pulses (71.74 pi) were performed. Based on these dispersion (D) data, the crystallite size can be estimated from the expression d (nm) = 112/D (%), assuming spherical particles and a Pd surface atom density of 1.27 x lO atom m [16]. 

It can be seen that only metals with close packed structures, that is the highest surface-atom density, catalyze this reaction ... 

Being semimetals, Bi and Sb show anomalies in the correlation of Ea=o with the surface atomic density [15, 16, 22], explained in terms of face-specific space charge effects. However, the definite dependence of Ea=o on the reticular density of plane has been established in a good agreement with general tendency. 

Fig. 15.10 (a) CV curves in Ar-purged 0.1 M KOH on Ag (hkl) sifffaces. All CVs were obtained at room temperatine with a sweep rate of 50 mV s . (b) FractiOTial charge per atom, obtained by integrating the anodic sweep direction of CV (fiorn a) after accounting for the different surface atom density of different Ag(hkl) orientations, (c) Fractional charge per atom expressed rm the rational potential scale (RPS) [47]... 

According to van Beurden and Kramerl l, the clean surfaces of these metals reconstruct because of the low values for vacancy formation as compared with the respective cohesion energies. The surface atom density changes upon reconstruction, therefore the heat of reconstruction, AHj., is given byl ... 

---