Chapter 3
Modification of the Work Function
In Sect. 2.3, we examined the factors that determine the work function. In this chapter,
modification of the work function is described on the basis of the discussion in
Sect. 2.3. There are three main strategies for modifying the work function (Fig. 3.1).
Because the work function is primarily an atomic quantity, the first strategy is to mix
an element with the mother element, which is often carried out by device material
scientists. Mixing A (whose work function in the polycrystalline form is φ(A)) and
B (whose work function in the polycrystalline form is φ(B)) at a 1:1 ratio does
not necessarily result in an alloy with the work function of
1
2 (φ(A) + φ(B)). The
effect of mixing elements on the work function is discussed in detail in Sect. 3.1.
While the aim of mixing another element is to modify the bulk term of the work
function (though the surface term is inevitably modified at the same time), there is a
method of modifying only the surface term without changing the bulk. When another
element exists only at the surface, the surface electrostatic potential φ S is modified.
This situation is experimentally realized by adsorbing another element, letting the
impurity element in the bulk segregate on the surface, or making an underlayer
element diffuse and segregate on the surface, as schematically shown in Fig. 3.1b.
The mechanism and examples are given in Sect. 3.2. Because the work function
modified by adsorption is dependent on not only the adsorbed species but also the
number of adsorbed atoms, it is natural to consider multilayer adsorption as a work
function modification method, which is called “deposition” (Fig. 3.1c). In Sect. 3.3,
aspects of work function modification via the deposition method are discussed.
3.1 Mixing Elements
It has been experimentally shown that the work function of a binary compound
tends to be closer to that of the element with the lower work function of the two
constituent elements [1]. There have been attempts to estimate the work functions
© National Institute for Materials Science, Japan 2021
M. Yoshitake, Work Function and Band Alignment of Electrode Materials,
NIMS Monographs, https://doi.org/10.1007/978-4-431-56898-8_3
35
Modification of the Work Function
In Sect. 2.3, we examined the factors that determine the work function. In this chapter,
modification of the work function is described on the basis of the discussion in
Sect. 2.3. There are three main strategies for modifying the work function (Fig. 3.1).
Because the work function is primarily an atomic quantity, the first strategy is to mix
an element with the mother element, which is often carried out by device material
scientists. Mixing A (whose work function in the polycrystalline form is φ(A)) and
B (whose work function in the polycrystalline form is φ(B)) at a 1:1 ratio does
not necessarily result in an alloy with the work function of
1
2 (φ(A) + φ(B)). The
effect of mixing elements on the work function is discussed in detail in Sect. 3.1.
While the aim of mixing another element is to modify the bulk term of the work
function (though the surface term is inevitably modified at the same time), there is a
method of modifying only the surface term without changing the bulk. When another
element exists only at the surface, the surface electrostatic potential φ S is modified.
This situation is experimentally realized by adsorbing another element, letting the
impurity element in the bulk segregate on the surface, or making an underlayer
element diffuse and segregate on the surface, as schematically shown in Fig. 3.1b.
The mechanism and examples are given in Sect. 3.2. Because the work function
modified by adsorption is dependent on not only the adsorbed species but also the
number of adsorbed atoms, it is natural to consider multilayer adsorption as a work
function modification method, which is called “deposition” (Fig. 3.1c). In Sect. 3.3,
aspects of work function modification via the deposition method are discussed.
3.1 Mixing Elements
It has been experimentally shown that the work function of a binary compound
tends to be closer to that of the element with the lower work function of the two
constituent elements [1]. There have been attempts to estimate the work functions
© National Institute for Materials Science, Japan 2021
M. Yoshitake, Work Function and Band Alignment of Electrode Materials,
NIMS Monographs, https://doi.org/10.1007/978-4-431-56898-8_3
35