12
1 Introduction
1.1.3 Known Mechanisms for Binding Energy Shift
Understanding the effect of irregular coordination on the CLS and valence band
evolution has been an issue of long debating from various hypothetic mechanisms
through the formulation and quantification of the CLS remains unrealistic in most
cases:
(1) The “initial-final state” relaxation, core-hole screening, and atomic covalency
effect has been a long dominant mechanism for the CLS of both solid skins [127,
128, 155, 156] and nanoparticles [157]. The initial-final state notion suggests that
the irradiation of the incident energetic beam ionizes the initially neutral surface
atom into the finally ionized one. The ionized atom becomes a ‘Z + 1 impurity’
sitting on the neutral substrate of Z metal atoms. The difference in cohesive
energy between the Z atom and the Z + 1 impurity amounts to the CLS by BE
= E z+1 (final) − E z (initial). The CLS may be negative, positive, or their mixture,
depending on the Z value that is tunable in computation. However, correlation
between the effective Z and the coordination and chemical environment remains
yet unknown.
(2) Another opinion is the skin interlayer contraction that creates the low-energy
CLS component [126, 130, 139]. A 12% contraction of the first layer spacing
deepens the Nb(100) 3d 3/2 level by 0.50 eV [126] and a (10 ± 3)% contraction of
the Ta(100) first layer spacing deepens the 4f 5/2(7/2) level by 0.75 eV [130]. Interlayer charge densification might enhance the incident light resonance, which
results in the positive CLS, according to this mechanism.
(3) Electronic configuration variation (the distribution of electrons among s, p, and d
orbits) mechanism [158–161] suggests that the chemical shifts and the structural
relaxations contribute to the atom-bulk CLS. The narrowing and shifting of the
surface DOS is responsible for the CLS [156, 162].
(4) A metal-to-nonmetal transition mechanism explains that this transition shifts
positively the CLS for nanocrystals [161, 163, 164]. However, the metalnonmetal transition takes place only in the size range of 1–2 nm diameter
consisting of 300 ± 100 atoms [165, 166] but the CLS extends continually
when a solid grows from an isolated atom into the bulk [167].
(5) Mechanisms of charge transfer between dissimilar metals of an alloy [160], sizeenhanced ionicity of copper and oxygen [168], and dipole formation between the
substrate and the particles [169] explain the hetero-coordination-induced CLS.
The density and the momentum of the dipoles might increase with reduction
of particle size. The formation of surface oxygen vacancy provides possible
mechanism for the O-coverage dependent CLS of Au on the MgO(100) and on
the TiO 2 (110) support [170].
(6) Surface electron structure and the increase of the effective size of surface O
anions [171] are suggested to be responsible for the 0.75 eV positive (deeper)
shift of the Mg 2p energy and the constant O 1s level for the MgO(100) skin.
(7) The latest development [2, 8, 139, 172, 173] suggests that atomic irregular
coordination perturbs the Hamiltonian that determines intrinsically the CLS and
1 Introduction
1.1.3 Known Mechanisms for Binding Energy Shift
Understanding the effect of irregular coordination on the CLS and valence band
evolution has been an issue of long debating from various hypothetic mechanisms
through the formulation and quantification of the CLS remains unrealistic in most
cases:
(1) The “initial-final state” relaxation, core-hole screening, and atomic covalency
effect has been a long dominant mechanism for the CLS of both solid skins [127,
128, 155, 156] and nanoparticles [157]. The initial-final state notion suggests that
the irradiation of the incident energetic beam ionizes the initially neutral surface
atom into the finally ionized one. The ionized atom becomes a ‘Z + 1 impurity’
sitting on the neutral substrate of Z metal atoms. The difference in cohesive
energy between the Z atom and the Z + 1 impurity amounts to the CLS by BE
= E z+1 (final) − E z (initial). The CLS may be negative, positive, or their mixture,
depending on the Z value that is tunable in computation. However, correlation
between the effective Z and the coordination and chemical environment remains
yet unknown.
(2) Another opinion is the skin interlayer contraction that creates the low-energy
CLS component [126, 130, 139]. A 12% contraction of the first layer spacing
deepens the Nb(100) 3d 3/2 level by 0.50 eV [126] and a (10 ± 3)% contraction of
the Ta(100) first layer spacing deepens the 4f 5/2(7/2) level by 0.75 eV [130]. Interlayer charge densification might enhance the incident light resonance, which
results in the positive CLS, according to this mechanism.
(3) Electronic configuration variation (the distribution of electrons among s, p, and d
orbits) mechanism [158–161] suggests that the chemical shifts and the structural
relaxations contribute to the atom-bulk CLS. The narrowing and shifting of the
surface DOS is responsible for the CLS [156, 162].
(4) A metal-to-nonmetal transition mechanism explains that this transition shifts
positively the CLS for nanocrystals [161, 163, 164]. However, the metalnonmetal transition takes place only in the size range of 1–2 nm diameter
consisting of 300 ± 100 atoms [165, 166] but the CLS extends continually
when a solid grows from an isolated atom into the bulk [167].
(5) Mechanisms of charge transfer between dissimilar metals of an alloy [160], sizeenhanced ionicity of copper and oxygen [168], and dipole formation between the
substrate and the particles [169] explain the hetero-coordination-induced CLS.
The density and the momentum of the dipoles might increase with reduction
of particle size. The formation of surface oxygen vacancy provides possible
mechanism for the O-coverage dependent CLS of Au on the MgO(100) and on
the TiO 2 (110) support [170].
(6) Surface electron structure and the increase of the effective size of surface O
anions [171] are suggested to be responsible for the 0.75 eV positive (deeper)
shift of the Mg 2p energy and the constant O 1s level for the MgO(100) skin.
(7) The latest development [2, 8, 139, 172, 173] suggests that atomic irregular
coordination perturbs the Hamiltonian that determines intrinsically the CLS and