Both UPS and XPS are photoelectron spectroscopy techniques – UPS stands for ultraviolet photoelectron spectroscopy, where XPS stands for X-ray photoelectron spectroscopy. While they are both photoelectron spectroscopy techniques, they vary in both their principles and application. Here, we take a look at both to see how they work and what they are used for.
Different photoelectron spectroscopies are relatively similar, regardless of the type, and are used to determine the specific properties and composition of a surface material. The main variant is in the radiation source. While this is only one component, it is significant, as the analysis and effects on the material vary. This is mainly due to the penetration depth and ionization potential of the radiation source, which can affect the local atoms in different ways.
In addition to the radiation source, photoelectron spectroscopy instruments are composed of high vacuum pumps, an ultra-high vacuum (UHV) chamber, and an electron energy analyzer (usually a concentric hemispheric analyzer).
UPS is commonly used to excite the core levels of a material and to measure the kinetic energy of the molecules irradiated with ultraviolet photons. This energy emittance is used to determine the energies of molecular orbitals in the valence band. UPS produces a spectrum where the individual peaks are attributed to molecular orbitals, including the bonding, nonbonding and antibonding orbitals.
UPS commonly uses a gas discharge lamp to produce helium radiation that penetrates the sample at an average depth of 2.5 nm. Because of the low penetration depth, UPS can only ionize the electrons in the outermost electronic level – i.e., the valence electrons.
UPS has been widely used to determine the electronic structure of solids and the effectiveness of molecular adsorption on a surface. UPS is often used to measure the valence band hybridization, position of the valence band maximum, electron affinity, ionization energy, activation energy and the work function of a material.
However, UPS cannot be used to determine the true value of the band gap, as the process only gives the position of the Fermi level. It is also a susceptible technique, and the results can be easily affected by a small concentration of contaminants in the sample. The sensitivity of UPS is also a beneficial factor, as it allows for the resolution (0.01 eV) of components that can’t be probed using XPS. The sensitivity is attributed to the radiation possessing a very narrow line width, and there is a high flux of photons available from the radiation source.
XPS is more commonly used than UPS and is often be referred to as Electron Spectroscopy for Chemical Analysis (ECSA). In contrast to UPS, it is often used for composition analysis, to determine what elemental composition is present at the surface of the material (quantity and types of elements). It also examines whether any contamination exists at the surface of a sample, the identification of chemical states, the empirical formula of a material, the depth of the surface layer, the binding energy of electronic states and the density of electronic states in a material.
XPS commonly uses MgK or AlK radiation to create monochromatic X-rays that penetrate a sample up to 10 nm in depth. This extra depth allows many different features of the sample to be determined, more than is possible with UPS. However, while XPS can remove electrons beyond the valence band, and into the p and d orbitals of an atom, is not as sensitive as UPS and only possesses a resolution of 0.3 eV to 0.7 eV.
The XPS spectrum features a profile of the ejected electrons as a range of kinetic energies. The number of electrons is counted at characteristic energy values, and peaks are produced on the spectrum. The energy values and intensities of the peaks can then be used to determine the type, and number, of elements at the surface (except hydrogen).
XPS and UPS can be used as stand-alone techniques as they both determine different properties of a material. However, for many applications, they are often used in conjunction with each other to provide a comprehensive analysis of the sample’s structure and properties. Used together, they complement each other well and are a powerful combination of techniques for the analysis of surfaces.
Sources and Further Reading