Surface segregation functionality

The principal applications of REELS are thin-film growth studies and gas-surface reactions in the few-monolayer regime when chemical state information is required. In its high spatial resolution mode it has been used to detect submicron metal hydride phases and to characterize surface segregation and difRision as a function of grain boundary orientation. REELS is not nearly as commonly used as AES orXPS. 

We will limit ourselves to the surface segregation energy of an impurity of atomic number Z + 1 in a BCC matrix of atomic number Z and study the variation of this energy as a function of the number Nj of d electrons per atom in the d band of e transition metal Z. 

Surface composition. The principle of surface segregation in ideal systems is easy to understand and to derive thermodynamically the equilibrium relations (surface concentration Xg as a function of the bulk concentration Xb at various temperatures) is also very easy (4,8). Even easier is a kinetic description which can also comprise some of the effects of the non-ideality (9). We consider an equilibrium between the surface(s) and the bulk(b) in the exchange like ... 

When a steady state condition has been achieved. Equation 21 implies that the relative surface concentrations are only functions of the bulk concentrations and the sputtering coefficients. This point cannot be overemphasized. Many authors have misinterpreted their data because they did not understand the consequences of this result. Once the sputtering coefficients are known, then thermodynamic properties, such as a tendency towards surface segregation, do not affect the surface concentration. However, the sputtering yields themselves are partially determined by binding energies and the type of compounds which are present in the surface region. These parameters are, of course, influenced by thermodynamic considerations. 

Table 5.1. Adsorption properties of metal monolayers on metal substrates. The clean substrate properties are also given for comparison. Substrates are ordered by lattice type (fee, bcc, hep, cubic, diamond and rhombic). The structures, nearest neighbor distances and heats of vaporization refer to the bulk material of the substrate or the adsorbate. VD, ID and S stand for vapor deposition, ion beam deposition and surface segregation, respectively. TD, WF and TED stand for thermal desorption, work function measurements and transmission electron diffraction, respectively...

For k>kf the adspecies mass transfer process is described by the diffusion Eq. (63). If the species migration in the subsurface region and the exchange with the gaseous phase occur fast, then k — l, therefore the boundary condition comprises the 3rd kind condition. Otherwise, it would be necessary to take into account the temporal evolution of the species in subsurface layers k , and the kinetic equations for these layers can contain the time derivatives. Most works devoted to mass transfer problems and also to the surface segregation of the alloy components [155,173]. The boundary conditions in the non-ideal systems are discussed in Ref. [174]. They require the use of equations for the pair functions of the type d(6,Jkq)/dx — 0. When describing the interphase boundary motion, the 3rd kind boundary conditions are also possible, although the 1st and the 2nd kind conditions are used more often. The latter are mainly applied to the description of many problems with species redistribution in the closed volume [175],... 

All direct depth profiling techniques used to study the surface segregation from binary polymer mixtures have a depth resolution [29] p limited to some 5-40 nm HWHM (half width at half maximum of the related Gaussian function). They cannot observe the real composition profile < )(z) (for the sake of comparison mimicked by mean field prediction (dashed line) in Fig. 16a) but rather its convolution (solid line in Fig. 16a) with an instrumental resolution function characterized by p. The total surface excess z however provides a good parameter, independent of resolution, as it has been concluded based on experimental data obtained using different direct techniques [170]. 

At least three of the studied blends [16,145] h86 (N86=1520)/d75 (N75=1625, deuteration extent e=0.4),d75/h66 (N66=2030) and h66/d52 (N52=1510 as well as e=0.34) may be described by a rather small Afs driving surface segregation. In an extreme case of the lowest Afs magnitude the enrichment-depletion effect is expected, as observed for the h66/d52 blend (see the next section). Here we characterize this class of mixtures with the results obtained for the h86/d75 blend. Surface excess z has been determined [145] as a function of bulk concentration at two different temperatures the corresponding (f) values are denoted as open circles (O) in the h86/d75 phase diagram (see inset to Fig. 21a). Surface segregation of the h86 component with a local concentration ( >(z) has been stud-... 

Figure 3.6. Surface segregation in the Au-Ag system, (a) Surface excess of Ag as a function of bulk composition at 300 K. (b) Surface excess of Ag as a function of bulk composition at 900 K.

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