Surface segregation energy

Here, we will first briefly recall the principles of this method in the case of transition metals. Then we will apply it to two illustrative examples the surface segregation energy of an impurity is a pure host and the growth of adislands on FCC(lll) surfaces of the same chemical species. 

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. 

This case is particularly interesting since the surface segregation energy can be directly compared to surface core level binding energy shifts (SCLS) measurements. Indeed, if we assume that the excited atom (i. e., with a core hole) is fully screened and can be considered as a (Z + 1) impurity (equivalent core approximation), then the SCLS is equal to the surface segregation energy of a (Z + 1) atom in a Z matrixi. in this approximation the SCLS is the same for all the core states of an atom. 

In the case of W(H0) (Nd=4.4eVatom), we have also calculated the modification of the surface segregation energy of a Re impurity when a p(2 x 1) overlayer of oxygen is present at the surface (Eig. 3). Then, there are two geometrically inequivalent atomic rows, labelled a and b, of W atoms on the surface (and in the sublayers). However, the modification of their effective atomic levels relative to the bulk is vanishingly small beyond the second... 

G. Abramovici, M. C. Desjonqu res and D. Spanjaard, W Surface Core Levels Shifts of O/W(110) Deduced from Surface Segregation Energies, /. de Physique 15 907 (1995)... 

Ruban AV, Skriver HE, Nprskov JK. 1999. Surface segregation energies in transition-metal alloys. Phys Rev B 59 15990-16000. 

GENERAL TRENDS FOR SURFACE SEGREGATION ENERGIES IN TRANSITION METAL ALLOYS... 

In most cases the experimental techniques used to study surface phenomena do not seem to yield consistent values for the surface segregation energies. One important exception is the special case of an atom of atomic number Z+1 in a host of atoms of atomic number Z, where the surface segregation energy may in fact be extracted with a high degree of accuracy from X-ray photoemission spectroscopy (XPS) measurements of surface core-level shifts (SCLS) [39]. In contrast they may be calculated quite accurately by modern first-principles methods [18,25,40]. 

Fig. 6. Surface segregation energies of transition metal impurities (solute) for the closed-packed surfaces of transition metal hosts.

The simplest surface specific feature of an ordered phase is the fact that there usually are different truncations of the bulk ordered alloy by the same surface orientation. In this case the problem is to find the stable truncation which, as we will show in this section, is usually directly related to the surface segregation energy of the deposited element to the corresponding surface of the substrate. 

A similar growth of the ordered NisAl alloy is observed experimentally during deposition of A1 on the (100) surface of Ni [46]. Here the formation of a stable c-(2x2) ordered NiAl alloy was found on the surface while the second layer was an almost entirely pure Ni layer and the third layer was enriched by Al. This type of structure corresponds to the NiAl termination of the NisAKlOO) surface, which also has an alternative truncation. The surface segregation energy of Al on the (100) surface of Ni is only about -0.1 eV, and as has been shown [24], the NiAl termination is more stable than Ni termination by approximately half of this value. 

General trends for the surface segregation energies in transition metal alloys... 

Nilekar AU, Ruban AV, Mavrikakis M (2009) Surface segregation energy in low-index open surfaces of bimetallic transition metal alloys. Surf Sci 603 91-96... 

Zhang Y, Duan Z, Xiao C, Wang G (2011) Density functional theory calculation of platinum surface segregation energy in PtsNi (111) surface doped with a third transition metal. Surf Sei 605 1577-1582... 

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