182
9 Hybridized Bonding
the unoccupied antibonding states by electrons of the surface dipoles above E F , by
integrating over morphological patches of a surface. Both STS and IPES measure
consistently the presence of surface polarization but STS probes the nonbonding
states that stem the surface dipoles.
Figure 3.2 compared the STS spectra collected from Cu(110) and O–Cu(110) surface at different atomic sites [1]. The clean surface shows an empty DOS at 0.8~1.8 eV
above E F with absence of any DOS features below E F . The “O
−2 :Cu
p :O
−2 ” chain
formation reduces the original empty-DOS above E F by occupation the states with
the polarized electrons upon chemisorption. Chemisorption creates additional DOS
features around −(1.4–2.1) eV below the E F with a sharp peak around −1.4 eV, as
indication of the nonbonding lone pairs. The sharp feature has also been detected
using the ARPES from O–Cu(110) and O–Cu(111) surfaces [4], the de-excitation
spectroscopy of metastable atoms [5], and VLEED spectrometrics from O–Cu(001)
surface [6, 7].
The STS profiles taken from O–Cu(110) surface between two Cu
p dipoles show
more pronounced nonbond features than that from atop one Cu
p . Taking the tip-size
effect of an STS (with ~2.5 Å lateral uncertainty) and the constant current mode into
consideration, the intensity difference between these two sites, atop and between
two Cu
p , results from that the STS collects more lone pairs information when its
tip stands between two Cu
p dipoles. The occupation of the above-E F feature results
from the two neighboring dipole protrusions while the below-E F information comes
from the lone pair of the O
−2 underneath. Therefore, the above-E F features of the
tip-between-Cu
p is stronger; the tip apt the Cu
p collects information of both the lone
pair and the dipole but the signal is weaker because of the positive curvature at the
dipole site.
The Cu 3d DOS are between −2 and −5 eV [4, 6–9], and the O–Cu bonding
derivatives are around the 2p-level of oxygen, −5.6 ~ − 7.8 eV below E F [10]. Both
the Cu-3d and the O–Cu bond DOS features are outside the scope of the STS (E F ±
2.5 eV).
Figure 9.1a shows the oxygen chemisorption induced STS spectra of the Nb(110)
surface [11] and the IPES spectra of O–Cu(110) surfaces [12]. The STS spectra
from the O–Nd(110) surface (Fig. 9.1a) show DOS features near the E F is not so
apparent as that from the O
−2 :Cu
p :O
−2 chain [11] (Fig. 9.10). The resonant peaks
at the sample positive bias (unoccupied states) arise from a tunneling via quantized
states in a potential well induced by the combination of the image states and the
applied electrical field [13, 14]. Since the lowest image state is energetically tied
to the vacuum level, the position of the first resonance can be used to estimate
the work function [14]. The successive higher resonance at 6.4 and 7.7 eV are in
accordance with resonances found for other transition metals such as Cu/Mo(110)
[13] and Ni(001) surfaces [14, 15]. These resonances shift to lower energies on the
O–Nb surfaces, independent of the tip positions, i.e. on or away from the O–Nb
chains. The intensity reduction of the IPES 2.0 eV peak indicates the occupancy of
the antibonding states by the surface dipole electrons induced by oxygenation. The
rest two peaks remain constant because of the resonant and bulk tunneling [12].
9 Hybridized Bonding
the unoccupied antibonding states by electrons of the surface dipoles above E F , by
integrating over morphological patches of a surface. Both STS and IPES measure
consistently the presence of surface polarization but STS probes the nonbonding
states that stem the surface dipoles.
Figure 3.2 compared the STS spectra collected from Cu(110) and O–Cu(110) surface at different atomic sites [1]. The clean surface shows an empty DOS at 0.8~1.8 eV
above E F with absence of any DOS features below E F . The “O
−2 :Cu
p :O
−2 ” chain
formation reduces the original empty-DOS above E F by occupation the states with
the polarized electrons upon chemisorption. Chemisorption creates additional DOS
features around −(1.4–2.1) eV below the E F with a sharp peak around −1.4 eV, as
indication of the nonbonding lone pairs. The sharp feature has also been detected
using the ARPES from O–Cu(110) and O–Cu(111) surfaces [4], the de-excitation
spectroscopy of metastable atoms [5], and VLEED spectrometrics from O–Cu(001)
surface [6, 7].
The STS profiles taken from O–Cu(110) surface between two Cu
p dipoles show
more pronounced nonbond features than that from atop one Cu
p . Taking the tip-size
effect of an STS (with ~2.5 Å lateral uncertainty) and the constant current mode into
consideration, the intensity difference between these two sites, atop and between
two Cu
p , results from that the STS collects more lone pairs information when its
tip stands between two Cu
p dipoles. The occupation of the above-E F feature results
from the two neighboring dipole protrusions while the below-E F information comes
from the lone pair of the O
−2 underneath. Therefore, the above-E F features of the
tip-between-Cu
p is stronger; the tip apt the Cu
p collects information of both the lone
pair and the dipole but the signal is weaker because of the positive curvature at the
dipole site.
The Cu 3d DOS are between −2 and −5 eV [4, 6–9], and the O–Cu bonding
derivatives are around the 2p-level of oxygen, −5.6 ~ − 7.8 eV below E F [10]. Both
the Cu-3d and the O–Cu bond DOS features are outside the scope of the STS (E F ±
2.5 eV).
Figure 9.1a shows the oxygen chemisorption induced STS spectra of the Nb(110)
surface [11] and the IPES spectra of O–Cu(110) surfaces [12]. The STS spectra
from the O–Nd(110) surface (Fig. 9.1a) show DOS features near the E F is not so
apparent as that from the O
−2 :Cu
p :O
−2 chain [11] (Fig. 9.10). The resonant peaks
at the sample positive bias (unoccupied states) arise from a tunneling via quantized
states in a potential well induced by the combination of the image states and the
applied electrical field [13, 14]. Since the lowest image state is energetically tied
to the vacuum level, the position of the first resonance can be used to estimate
the work function [14]. The successive higher resonance at 6.4 and 7.7 eV are in
accordance with resonances found for other transition metals such as Cu/Mo(110)
[13] and Ni(001) surfaces [14, 15]. These resonances shift to lower energies on the
O–Nb surfaces, independent of the tip positions, i.e. on or away from the O–Nb
chains. The intensity reduction of the IPES 2.0 eV peak indicates the occupancy of
the antibonding states by the surface dipole electrons induced by oxygenation. The
rest two peaks remain constant because of the resonant and bulk tunneling [12].