14.4 Energy-Dependent 3D-SPB
283
14.4 Energy-Dependent 3D-SPB
14.4.1 Initiatives
The SPB describes the charge distribution both in real space and in energy space,
which links to the change of valence states and geometrical arrangement of atoms of
the surface [46]. The SPB is a complex function. The real part describes the image
elastic potential and the imaginary part describes the charge distribution in the real
and energy domain. The former charges the direction and phase shift of the reflected
electron beams and the latter causes inelastic damping of the incident beam energy.
The inelastic damping has been assumed as monotonically energy-dependent. For
clean metals, such as Cu(001) [47], W(001) [1, 48, 49], Ru(110) [9, 50], and Ni [51],
the SPB approaches a uniform layer of thin-film interference.
STM/S observations [41, 52] support the uniform-SPB approximation for clean
metals. For example, STM reveals that the ion cores with small protrusions (0.15–0.30
Å) arrange regularly in the homogeneous background or Fermi sea; STS studies of the
Cu(110) surface [15] confirmed the uniformity of the DOS below E F . Hence, clean
metal surfaces can be described as nearly-ideal Fermi systems and the uniform-SPB
approximation is acceptable within the error of detection.
Surfaces with chemisorbed oxygen differ much from those of the pure metals in
that the chemisorbed surfaces have many “rather local” features—varying site from
site around an atom. The chemisorption of oxygen results not only in the dislocation
of ion cores but also in the alternation of atomic sizes and atomic valence states
by charge transportation and polarization. More importantly, as consequences of
oxygen-metal bonding, the formation of the dipole layer and, in some cases, the
removal of atoms roughens the surface largely.
Even at very low exposure to oxygen, the Cu(001) surface is roughened by the
protruding domain boundaries, see Fig. 13.1a. The STM scale differences of the
systems with chemisorbed oxygen are much higher (0.45–1.1 Å) compared with
pure metals of 0.3 Å or lower. The STS profiles from the O–Cu chain region on
the O–Cu(110) surface show that there is a general elevation of energy states. The
above-E F empty surface state is occupied and a new DOS feature is generated below
E F .
Furthermore, in decoding VLEED data from the O–Cu(001) surface, Thurgate and
Sun [3] found that the spectrum collected at azimuth closing to the <11> direction
(perpendicular to the missing-row) could not be simulated with the uniform-SPB by
using either the Cu(001)-c(2 × 2) − 2O
−1 or the (
√
2 × 2
√
2) R45°− 2O
−2 structure,
or even their combination with various SPB parameters. Therefore, it is implausible
with VLEED to determine the SPB and the crystal structure without proper modeling
parameterization.
The atomic-scale localization and the on-site variation of energy states of the systems with chemisorbed oxygen suggest that it is necessary to consider the electron
distribution on the surface site by site. The 3D effect, the variation of energy states
283
14.4 Energy-Dependent 3D-SPB
14.4.1 Initiatives
The SPB describes the charge distribution both in real space and in energy space,
which links to the change of valence states and geometrical arrangement of atoms of
the surface [46]. The SPB is a complex function. The real part describes the image
elastic potential and the imaginary part describes the charge distribution in the real
and energy domain. The former charges the direction and phase shift of the reflected
electron beams and the latter causes inelastic damping of the incident beam energy.
The inelastic damping has been assumed as monotonically energy-dependent. For
clean metals, such as Cu(001) [47], W(001) [1, 48, 49], Ru(110) [9, 50], and Ni [51],
the SPB approaches a uniform layer of thin-film interference.
STM/S observations [41, 52] support the uniform-SPB approximation for clean
metals. For example, STM reveals that the ion cores with small protrusions (0.15–0.30
Å) arrange regularly in the homogeneous background or Fermi sea; STS studies of the
Cu(110) surface [15] confirmed the uniformity of the DOS below E F . Hence, clean
metal surfaces can be described as nearly-ideal Fermi systems and the uniform-SPB
approximation is acceptable within the error of detection.
Surfaces with chemisorbed oxygen differ much from those of the pure metals in
that the chemisorbed surfaces have many “rather local” features—varying site from
site around an atom. The chemisorption of oxygen results not only in the dislocation
of ion cores but also in the alternation of atomic sizes and atomic valence states
by charge transportation and polarization. More importantly, as consequences of
oxygen-metal bonding, the formation of the dipole layer and, in some cases, the
removal of atoms roughens the surface largely.
Even at very low exposure to oxygen, the Cu(001) surface is roughened by the
protruding domain boundaries, see Fig. 13.1a. The STM scale differences of the
systems with chemisorbed oxygen are much higher (0.45–1.1 Å) compared with
pure metals of 0.3 Å or lower. The STS profiles from the O–Cu chain region on
the O–Cu(110) surface show that there is a general elevation of energy states. The
above-E F empty surface state is occupied and a new DOS feature is generated below
E F .
Furthermore, in decoding VLEED data from the O–Cu(001) surface, Thurgate and
Sun [3] found that the spectrum collected at azimuth closing to the <11> direction
(perpendicular to the missing-row) could not be simulated with the uniform-SPB by
using either the Cu(001)-c(2 × 2) − 2O
−1 or the (
√
2 × 2
√
2) R45°− 2O
−2 structure,
or even their combination with various SPB parameters. Therefore, it is implausible
with VLEED to determine the SPB and the crystal structure without proper modeling
parameterization.
The atomic-scale localization and the on-site variation of energy states of the systems with chemisorbed oxygen suggest that it is necessary to consider the electron
distribution on the surface site by site. The 3D effect, the variation of energy states