358
18 Four-Stage Cu 3 O 2 Bonding Dynamics
18.6 Summary
The CuO 2 ⇒ Cu 3 O 2 transition bonding kinetics on Cu(001) surface is quantified as
follows (refer to Fig. 18.3 and Table 18.1) [4]:
(1) O ≤ 30 L: The dissociated oxygen atoms form one contracting (Q 1 = 12%)
ionic bond with a Cu atom (labeled 1) on the surface. The DCu x reaches 0.15
Å and ∠1O2 reaches 94.0°. O
−1 is located at a position above the surface and
forms an off-centered single or pairing-pyramid polarizing its rest neighbors.
(2) 30 L > O ≤ 35 L: O
−1 forms the second contracting (increase Q 2 from 0 to
4%) ionic bond with a Cu atom (labeled 2) in the substrate second layer, and as
a result, the O
−2 penetrates the bulk and evolves its position from the apical site
of the pyramid to the center of the tetrahedron upon the second bond formation.
Meanwhile, ∠1O2 increases from 94.0 to 98.0°, associated with the isolation
and evaporation of the missing-row atom.
(3) 35 L > O ≤ 200 L: The angle ∠1O2 increases resulting in the relaxation D 12 ,
and simultaneously, angle ∠102 increases from 98.0° to a saturation value of
102.0° while other parameters have little change.
(4) O > 200 L: The sp-orbital hybridization takes place with creation of the nonbonding lone pairs. The interaction between the O
−2 and the lone-pair-induced
Cu
p develops, which dominates the reaction at higher exposure and long-term
aging. Lone pairs push the dipoles outwardly, and as a result, pairing dipoles
form and bridge over the missing row. The DCu x increases from 0.15 up to 0.45
Å at about 800 L.
The variations of the structural-dependent z 0 (E) profiles (Fig. 18.2) agree with the
bonding kinetics. The features are summarized as follows:
(1) Features above 7.5 eV, particularly the small sharp peak at 7.1 eV, appearing in
the 30–600 L curves are derivatives of the nonbonding lone-pair states of the
O
−2 -hybrid [12].
(2) The insignificant outward-shift at higher exposures corresponds to the reduction of work function that is caused by the development of the oppositely
coupled metal dipoles.
(3) The absence of the nonbonding states at 25 L because of absenting the
O
−2 sp-orbital hybridization, while at higher exposures the 7.1 eV feature is
overtoned by the development of metal dipoles to be undetectable.
(4) Similarity of the exposure-resolved z 0 (E) profiles at energies greater than 7.5 eV
implies that chemisorption has little effect on the electrons in the bottom of
valence band or even deeper. The weak bonding states (~−5.0 eV) may go
beyond the resolution of VLEED due to the spatial convolution by saturated
Cu
p . Charge transferring occurs only to electrons in the upper valence band.
The effect of aging and annealing on the O-Cu(001) system is slightly complex and
therefore it is less explicit than the effect of increasing oxygen exposure. However,
it is certain that long-term aging effects the same as higher oxygen exposure on the
18 Four-Stage Cu 3 O 2 Bonding Dynamics
18.6 Summary
The CuO 2 ⇒ Cu 3 O 2 transition bonding kinetics on Cu(001) surface is quantified as
follows (refer to Fig. 18.3 and Table 18.1) [4]:
(1) O ≤ 30 L: The dissociated oxygen atoms form one contracting (Q 1 = 12%)
ionic bond with a Cu atom (labeled 1) on the surface. The DCu x reaches 0.15
Å and ∠1O2 reaches 94.0°. O
−1 is located at a position above the surface and
forms an off-centered single or pairing-pyramid polarizing its rest neighbors.
(2) 30 L > O ≤ 35 L: O
−1 forms the second contracting (increase Q 2 from 0 to
4%) ionic bond with a Cu atom (labeled 2) in the substrate second layer, and as
a result, the O
−2 penetrates the bulk and evolves its position from the apical site
of the pyramid to the center of the tetrahedron upon the second bond formation.
Meanwhile, ∠1O2 increases from 94.0 to 98.0°, associated with the isolation
and evaporation of the missing-row atom.
(3) 35 L > O ≤ 200 L: The angle ∠1O2 increases resulting in the relaxation D 12 ,
and simultaneously, angle ∠102 increases from 98.0° to a saturation value of
102.0° while other parameters have little change.
(4) O > 200 L: The sp-orbital hybridization takes place with creation of the nonbonding lone pairs. The interaction between the O
−2 and the lone-pair-induced
Cu
p develops, which dominates the reaction at higher exposure and long-term
aging. Lone pairs push the dipoles outwardly, and as a result, pairing dipoles
form and bridge over the missing row. The DCu x increases from 0.15 up to 0.45
Å at about 800 L.
The variations of the structural-dependent z 0 (E) profiles (Fig. 18.2) agree with the
bonding kinetics. The features are summarized as follows:
(1) Features above 7.5 eV, particularly the small sharp peak at 7.1 eV, appearing in
the 30–600 L curves are derivatives of the nonbonding lone-pair states of the
O
−2 -hybrid [12].
(2) The insignificant outward-shift at higher exposures corresponds to the reduction of work function that is caused by the development of the oppositely
coupled metal dipoles.
(3) The absence of the nonbonding states at 25 L because of absenting the
O
−2 sp-orbital hybridization, while at higher exposures the 7.1 eV feature is
overtoned by the development of metal dipoles to be undetectable.
(4) Similarity of the exposure-resolved z 0 (E) profiles at energies greater than 7.5 eV
implies that chemisorption has little effect on the electrons in the bottom of
valence band or even deeper. The weak bonding states (~−5.0 eV) may go
beyond the resolution of VLEED due to the spatial convolution by saturated
Cu
p . Charge transferring occurs only to electrons in the upper valence band.
The effect of aging and annealing on the O-Cu(001) system is slightly complex and
therefore it is less explicit than the effect of increasing oxygen exposure. However,
it is certain that long-term aging effects the same as higher oxygen exposure on the