Chapter 4
Measurement of Work Function
The principle of work function measurement is primarily divided into two concepts
in accordance with the electron density of states (DOS) at the Fermi level. As we
saw in Sect. 2.1, if there are electrons at the Fermi level, it is possible to excite
these electrons and observe the minimum energy needed for excitation, which is
the work function as defined previously. In this case, the absolute values of the
work function are obtainable. There are different methods of electron excitation,
as shown in Fig. 4.1. Measurement techniques can also be divided in accordance
with the method of measuring emitted electrons, namely, current measurement or
spectroscopic methods. In the case of semiconductors or insulators, there are almost
no electrons at the Fermi level. In this case, the minimum energy necessary for the
emission of electrons is not the work function but the ionization energy. Therefore,
a less direct method, utilizing the contact potential difference, is applied for work
function measurement, wherein only relative values can be obtained. The method
utilizing the contact potential difference is also applicable when electrons exist at
the Fermi level, and is often used for measurements of, for example, the change in
the work function with time upon chemisorption on metals. In a special case, an
absolute work function can be measured for semiconductors or insulators, which
will be discussed in Sect. 4.2.2.
4.1 Utilizing Electron Emission Current Measurement
The measurement of the electric current of emitted electrons either by thermal excitation (heating a specimen) or by the application of a high electric field is rather
simple from an instrumental viewpoint. No sophisticated, expensive equipment is
necessary. Therefore, work function values were mostly obtained by these measurement methods until the 1970s. Thermal and field emissions are still widely used
© 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_4
71
Measurement of Work Function
The principle of work function measurement is primarily divided into two concepts
in accordance with the electron density of states (DOS) at the Fermi level. As we
saw in Sect. 2.1, if there are electrons at the Fermi level, it is possible to excite
these electrons and observe the minimum energy needed for excitation, which is
the work function as defined previously. In this case, the absolute values of the
work function are obtainable. There are different methods of electron excitation,
as shown in Fig. 4.1. Measurement techniques can also be divided in accordance
with the method of measuring emitted electrons, namely, current measurement or
spectroscopic methods. In the case of semiconductors or insulators, there are almost
no electrons at the Fermi level. In this case, the minimum energy necessary for the
emission of electrons is not the work function but the ionization energy. Therefore,
a less direct method, utilizing the contact potential difference, is applied for work
function measurement, wherein only relative values can be obtained. The method
utilizing the contact potential difference is also applicable when electrons exist at
the Fermi level, and is often used for measurements of, for example, the change in
the work function with time upon chemisorption on metals. In a special case, an
absolute work function can be measured for semiconductors or insulators, which
will be discussed in Sect. 4.2.2.
4.1 Utilizing Electron Emission Current Measurement
The measurement of the electric current of emitted electrons either by thermal excitation (heating a specimen) or by the application of a high electric field is rather
simple from an instrumental viewpoint. No sophisticated, expensive equipment is
necessary. Therefore, work function values were mostly obtained by these measurement methods until the 1970s. Thermal and field emissions are still widely used
© 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_4
71