4.4 Other Methods
93
4.4 Other Methods
There are many other methods of measuring the work function, both old and new.
Here, a distinctive indirect method called the photoemission of adsorbed xenon (PAX)
is described because its measurement principle is considerably different from those
of the above methods and it is still utilized for some specific measurements.
4.4.1 Photoemission of Adsorbed Xenon (PAX)
The work function is measured indirectly in this method, where the inner-shell
binding energy of xenon adsorbed on the surface of a specimen is measured. Originally, the method was developed to measure the local work function on patched
surfaces (patches with different work functions exist on the surface) at the time when
there were no scanning probe microscopy techniques. The principle of the method
is that the inner-shell binding energy (usually the Xe 5p 1/2 peak is used) of xenon on
the surface having a work function of φ is constant when the vacuum level is used
as the reference but becomes variable when the Fermi level is used as the energy
reference, as in Eq. (4.16).
E
V
B
Xe 5p 1
2
= E
F
B
Xe 5p 1
2
+ φ
(4.16)
Here, E
V
B
Xe 5p 1
2
is the binding energy of Xe 5p 1/2 with respect to the vacuum
level, E
F
B
Xe 5p 1
2
is that with respect to the Fermi level, and φ is the work function
of the specimen. This method is based on the fact that Xe adsorbs on the specimen
without chemical interaction and only with van der Waals interaction, meaning that
there is no chemical shift in the Xe binding energy upon adsorption. A schematic
energy diagram of this relation for two patches with different work functions is
shown in Fig. 4.27. The relationship described by Eq. (4.16) has been verified by
measuring E
E
B
Xe 5p 1
2
on various samples whose work functions are already known
(Table 4.1), and the value of E
V
B
Xe 5p 1
2
is given as 12.3 ± 0.1 eV [13].
As an extension of this method, the use of Kr and Ar as adsorbates instead of Xe
has also been proposed [14]. The work function obtained with Kr and Ar adsorption
is in agreement with those obtained with Xe adsorption. However, Wandelt [13], who
examined the work function of NiAl(110) with various compositions, cautioned that
there could be preferential adsorption sites on the surface and that work functions
obtained by inert gas adsorption do not necessarily agree with those obtained by
secondary electron cutoff spectroscopy (compared using values measured by UPS).
93
4.4 Other Methods
There are many other methods of measuring the work function, both old and new.
Here, a distinctive indirect method called the photoemission of adsorbed xenon (PAX)
is described because its measurement principle is considerably different from those
of the above methods and it is still utilized for some specific measurements.
4.4.1 Photoemission of Adsorbed Xenon (PAX)
The work function is measured indirectly in this method, where the inner-shell
binding energy of xenon adsorbed on the surface of a specimen is measured. Originally, the method was developed to measure the local work function on patched
surfaces (patches with different work functions exist on the surface) at the time when
there were no scanning probe microscopy techniques. The principle of the method
is that the inner-shell binding energy (usually the Xe 5p 1/2 peak is used) of xenon on
the surface having a work function of φ is constant when the vacuum level is used
as the reference but becomes variable when the Fermi level is used as the energy
reference, as in Eq. (4.16).
E
V
B
Xe 5p 1
2
= E
F
B
Xe 5p 1
2
+ φ
(4.16)
Here, E
V
B
Xe 5p 1
2
is the binding energy of Xe 5p 1/2 with respect to the vacuum
level, E
F
B
Xe 5p 1
2
is that with respect to the Fermi level, and φ is the work function
of the specimen. This method is based on the fact that Xe adsorbs on the specimen
without chemical interaction and only with van der Waals interaction, meaning that
there is no chemical shift in the Xe binding energy upon adsorption. A schematic
energy diagram of this relation for two patches with different work functions is
shown in Fig. 4.27. The relationship described by Eq. (4.16) has been verified by
measuring E
E
B
Xe 5p 1
2
on various samples whose work functions are already known
(Table 4.1), and the value of E
V
B
Xe 5p 1
2
is given as 12.3 ± 0.1 eV [13].
As an extension of this method, the use of Kr and Ar as adsorbates instead of Xe
has also been proposed [14]. The work function obtained with Kr and Ar adsorption
is in agreement with those obtained with Xe adsorption. However, Wandelt [13], who
examined the work function of NiAl(110) with various compositions, cautioned that
there could be preferential adsorption sites on the surface and that work functions
obtained by inert gas adsorption do not necessarily agree with those obtained by
secondary electron cutoff spectroscopy (compared using values measured by UPS).