10
1 Introduction
for Cu atoms added on Cu(111) skin [117]. However, using STM/S alone, one can
hardly elucidate quantitatively how the local chemical bond relaxes and how to derive
the STM/S signatures.
In contrast, a photoelectron spectroscopy (PES, such as UPS and XPS) collects
the statistic and volumetric information of electron binding energy in the valence
band and below within a nanometer-thick skin [118–123]. The penetration depth of
the electron beam varies with its incident energy. If the beam energy is in the range
of 50–100 eV, the depth is 5–10 Å; if it is 500–880 eV, the depth is 10–20 Å; if
the beam energy reaches 3–6 keV, the penetration depth is 30–60 Å [124]. One can
hardly discriminate contribution of the monolayer skin from that of its bulk mixture.
The effect of atomic undercoordination and bond nature alteration in chemisorption
is dominant at most the outermost four atomic layers [3, 4, 45, 46, 125].
Experimental conditions create artifacts in measurements. Varying the incident
beam energy or the polar angle between the incident light or the emission electron
beam and the surface normal during PES experiment also modulates the spectral
peak energy and intensity. Increasing the incident beam energy or decreasing the
polar angle (close to the surface normal) could raise the intensity of the high-energy
component (close to E F ) [126–130]. For instance, at the same polar angle, increasing
slightly the beam energy from 370 to 380 eV increases the intensity of the Rh 3d 5/2
peak at 306.42 eV (bulk) more than that of the 307.18 eV (skin) component [131].
Likewise, at the same beam energy, decreasing the polar angle from 35° to 20° reduces
the intensity of the Rh(111) skin component at 307.18 eV [132]. Both beam energy
increase and polar angle reduction collect more information from the bulk than from
the skin. Experimental artifacts also include the charging effect and the ionization,
initial-final state relaxation that present throughout the course of measurements.
XPS profiles of Nb(100) [126], Tb(0001) [128], Al(100) [133, 134], Ta(100)
[130], and Be skins [135–138] follow the same polar angle and beam energy dependency. At higher beam energies or at smaller emission angles, the incident beam
excites more electrons from deeper layers compared with the otherwise [139]. Thus,
one can readily discriminate the contribution of the skin from that of the bulk by
varying the beam energy or the emission angle though the derived information was
qualitative.
Strikingly, the 2p energy of a Si-diode shifts significantly during its operation
under both forward and reverse bias [140, 141] or under high frequency charging
and discharging the specimen [142, 143]. Photon illumination also enhances the Si
2p energy shift [144]. This approach traces potential variation with the concentration
and location of dopants.
Since the advent of the third generation synchrotron light sources providing soft
X-rays up to 2 keV, XPS becomes a powerful tool for studying surface chemical
and physical properties at an unprecedented precision level [145]. The high resolution allows for identifying various surface species, small molecules, and even the
vibrational fine structures [146]. The near-ambient pressure XPS mimic the ambient
environment for examining the chemical effect on the core band shift of a substance
such as gold [147]. The liquid microjets PES and ultrafast liquid-jet UPS provide
1 Introduction
for Cu atoms added on Cu(111) skin [117]. However, using STM/S alone, one can
hardly elucidate quantitatively how the local chemical bond relaxes and how to derive
the STM/S signatures.
In contrast, a photoelectron spectroscopy (PES, such as UPS and XPS) collects
the statistic and volumetric information of electron binding energy in the valence
band and below within a nanometer-thick skin [118–123]. The penetration depth of
the electron beam varies with its incident energy. If the beam energy is in the range
of 50–100 eV, the depth is 5–10 Å; if it is 500–880 eV, the depth is 10–20 Å; if
the beam energy reaches 3–6 keV, the penetration depth is 30–60 Å [124]. One can
hardly discriminate contribution of the monolayer skin from that of its bulk mixture.
The effect of atomic undercoordination and bond nature alteration in chemisorption
is dominant at most the outermost four atomic layers [3, 4, 45, 46, 125].
Experimental conditions create artifacts in measurements. Varying the incident
beam energy or the polar angle between the incident light or the emission electron
beam and the surface normal during PES experiment also modulates the spectral
peak energy and intensity. Increasing the incident beam energy or decreasing the
polar angle (close to the surface normal) could raise the intensity of the high-energy
component (close to E F ) [126–130]. For instance, at the same polar angle, increasing
slightly the beam energy from 370 to 380 eV increases the intensity of the Rh 3d 5/2
peak at 306.42 eV (bulk) more than that of the 307.18 eV (skin) component [131].
Likewise, at the same beam energy, decreasing the polar angle from 35° to 20° reduces
the intensity of the Rh(111) skin component at 307.18 eV [132]. Both beam energy
increase and polar angle reduction collect more information from the bulk than from
the skin. Experimental artifacts also include the charging effect and the ionization,
initial-final state relaxation that present throughout the course of measurements.
XPS profiles of Nb(100) [126], Tb(0001) [128], Al(100) [133, 134], Ta(100)
[130], and Be skins [135–138] follow the same polar angle and beam energy dependency. At higher beam energies or at smaller emission angles, the incident beam
excites more electrons from deeper layers compared with the otherwise [139]. Thus,
one can readily discriminate the contribution of the skin from that of the bulk by
varying the beam energy or the emission angle though the derived information was
qualitative.
Strikingly, the 2p energy of a Si-diode shifts significantly during its operation
under both forward and reverse bias [140, 141] or under high frequency charging
and discharging the specimen [142, 143]. Photon illumination also enhances the Si
2p energy shift [144]. This approach traces potential variation with the concentration
and location of dopants.
Since the advent of the third generation synchrotron light sources providing soft
X-rays up to 2 keV, XPS becomes a powerful tool for studying surface chemical
and physical properties at an unprecedented precision level [145]. The high resolution allows for identifying various surface species, small molecules, and even the
vibrational fine structures [146]. The near-ambient pressure XPS mimic the ambient
environment for examining the chemical effect on the core band shift of a substance
such as gold [147]. The liquid microjets PES and ultrafast liquid-jet UPS provide