Introduction

Ultraviolet photoelectron spectroscopy (UPS) is an analytical method used to obtain data about the bonding and valence electron levels of atoms. UPS is a type of photoelectron spectroscopy (also known as photoemission spectroscopy).^[1]

Principle

A narrow beam of ultraviolet radiation is targeted towards a gas or vapour sample and the energy of the emitted photoelectrons are measured (Figure 1). The energy follows Einstein’s photoelectric law

E_k=hv-E_binding

where E_k is the kinetic energy, h Planck’s constant, v frequency of the ionizing beam and E_binding energy required to remove a certain electron from an orbital (≈orbital energy). From this equation, the valence electron structure and work function can be obtained.^[1]

Figure 1. Principle of ultraviolet photoelectron spectroscopy. Illustrated by Nina Lamminmäki.

Experimental methods

The instrumentation of UPS is very similar to other photoelectron spectroscopies. UPS typically uses He(I) or He(II) excitation source. With He(I) energies of about 21.2 eV or 23 eV can be achieved while with He(II) energies of about 41 eV are achieved. The electrons go through energy analyser, typically a hemispherical mirror analyser. The electrons arrive to the detector (electron multiplier). UPS usually uses discrete or continuous dynodes for this. The signal can be transferred into digital form for further analysis.^[1] The basic instrumentation is illustrated in Figure 1.

Advantages and disadvantages

The ultraviolet radiation used in UPS is typically obtained using a helium source which has a rather high wavelength (in comparison to X-ray radiation) and therefore a low photon energy. The ultraviolet radiation beam cannot get to the energy levels with high energy and causes only the valence electrons to be emitted. This makes UPS good for obtaining information about the bonding characteristics and the details of the valence electrons.^[1] With UPS it is possible to find out which orbitals are involved in the bonding between an absorbate and a substrate.^[2] UV radiation, like most radiation, can damage the sample, especially if it is organic.^[3]
Another common photoelectron spectroscopy technique, X-ray photoelectron spectroscopy (XPS), uses an X-ray beam with a much higher photon energy which causes the electrons with high energy to be emitted.^[1] Each element has their own characteristic core ionization energy which show up as lines in a XPS spectra. Therefore the technique is used to detect elements that are present on the sample surface. The core ionization are not affected by the chemical bonding as they are too tightly bound in the core levels.^[2] XPS is a more commonly used technique compared to UPS.^[4] UPS is limited due to the high wavelength of the UV radiation. However, the results obtained with it are rather accurate.^[1]

Example

Dharmadasa et al.^[5]studied the Fermi level position of CdTe thin films before and after CdCl₂ treatment. This treatment is used in the development of high efficiency CdTe solar cells, but the process step is not yet properly understood. Ultraviolet photoelectron spectroscopy was used to determine the position of Fermi level. Gold film was used to deal with charging effects, which would otherwise affect the UPS results ^[5]. The UPS results can be seen in the Figure 2.

Figure 2. The UPS spectra for gold films (a.-c.) and CdTe layers (d.-f.)^[5]. (License: CC BY 4.0)

References