16
2 What is the Work Function?: Definition and Factors …
Table 2.1 Magnitude of
image potential at different z
z (nm) Image potential (eV) z (nm) Image potential (eV)
0.1
14.420
6
0.240
0.2
7.210
7
0.206
0.3
4.807
8
0.180
0.4
3.605
10
0.144
0.5
2.884
12
0.120
0.6
2.403
14
0.103
0.8
1.803
16
0.090
1
1.442
20
0.072
1.2
1.202
25
0.058
1.4
1.030
30
0.048
1.6
0.901
35
0.041
2
0.721
40
0.036
2.5
0.577
45
0.032
3
0.481
50
0.029
4
0.361
60
0.024
5
0.288
75
0.019
100
0.014
2.3 Factors Determining the Work Function
The work function comprises a bulk term and surface term, as discussed in the
previous section. The factors that determine the bulk term and surface term are
separately discussed here. The bulk term represents how strongly valence electrons
are bound inside the solid, and thus it is reasonable to assume that it has a strong
correlation with the electronegativity since the electronegativity χ is “a chemical
property that describes the tendency of an atom or a functional group to attract
electrons (or electron density) towards itself” [6].
In Fig. 2.10, the correlation between Pauling’s electronegativity [7] and the work
function of an elemental (polycrystalline) metal [8] is plotted. Values plotted as
triangles are taken from Ref. [7] and those plotted as circles are re-examined values
[9, 10]. The values are scattered along the straight line expressed by the following
equation.
W F (eV) = 2.27 × EN + 0.34
(2.8)
Here, EN represents Pauling’s electronegativity. Because Pauling’s electronegativity is a quantity belonging to each element, the above correlation implies how the
work function is dependent on the element.
However, Pauling’s electronegativity is determined on the basis of the experimental results of formation enthalpy and is defined only for elements. Therefore,
2 What is the Work Function?: Definition and Factors …
Table 2.1 Magnitude of
image potential at different z
z (nm) Image potential (eV) z (nm) Image potential (eV)
0.1
14.420
6
0.240
0.2
7.210
7
0.206
0.3
4.807
8
0.180
0.4
3.605
10
0.144
0.5
2.884
12
0.120
0.6
2.403
14
0.103
0.8
1.803
16
0.090
1
1.442
20
0.072
1.2
1.202
25
0.058
1.4
1.030
30
0.048
1.6
0.901
35
0.041
2
0.721
40
0.036
2.5
0.577
45
0.032
3
0.481
50
0.029
4
0.361
60
0.024
5
0.288
75
0.019
100
0.014
2.3 Factors Determining the Work Function
The work function comprises a bulk term and surface term, as discussed in the
previous section. The factors that determine the bulk term and surface term are
separately discussed here. The bulk term represents how strongly valence electrons
are bound inside the solid, and thus it is reasonable to assume that it has a strong
correlation with the electronegativity since the electronegativity χ is “a chemical
property that describes the tendency of an atom or a functional group to attract
electrons (or electron density) towards itself” [6].
In Fig. 2.10, the correlation between Pauling’s electronegativity [7] and the work
function of an elemental (polycrystalline) metal [8] is plotted. Values plotted as
triangles are taken from Ref. [7] and those plotted as circles are re-examined values
[9, 10]. The values are scattered along the straight line expressed by the following
equation.
W F (eV) = 2.27 × EN + 0.34
(2.8)
Here, EN represents Pauling’s electronegativity. Because Pauling’s electronegativity is a quantity belonging to each element, the above correlation implies how the
work function is dependent on the element.
However, Pauling’s electronegativity is determined on the basis of the experimental results of formation enthalpy and is defined only for elements. Therefore,