SoftX‐Ray Photoelectron Spectroscopy in Analysis of Surfaces

Abstract

AbstractX‐ray photoelectron spectroscopy (XPS) is widely used to measure the composition and chemical states of species in the near‐surface region of a solid material in ultrahigh vacuum (UHV). In XPS, the core‐level, or inner‐shell, electrons that are emitted via the photoelectric effect are measured. The spectra provide information related to the electronic structure and chemical composition of the near‐surface region and to the bonding of adsorbates onto the surface. Because the mean free path for electrons traveling in solids is small, the spectra reflect the composition of the outermost few atomic layers. When synchrotron light is used as the excitation source, the technique is known as soft X‐ray photoelectron spectroscopy (SXPS). There are many advantages of synchrotron radiation over conventional XPS. For example, the photon energy is tunable over a wide range, which allows for the enhancement of particular spectral features and for unique types of spectroscopy in which the photon energy is continuously varied. The spectral resolution is also greatly enhanced, which allows for the observation of features not otherwise discernible. Also, the brightness of the beam is much greater than that of conventional X‐ray sources, thereby allowing for the detection of very small photoelectron signals. SXPS has been applied to a wide variety of materials, including metals, semiconductors, adsorbates on surfaces and nanoparticles. The information obtained from SXPS includes determining the core‐level binding energies of individual surface atoms, the position of the Fermi level within the gap of a semiconductor, and the surface work function. Shifts of the core‐level binding energy are observed for the outermost atoms on clean surfaces because they are in a different chemical environment than the atoms in the bulk. When an adsorbate attaches to a surface atom, there is a shift in binding energy that is related to the degree of charge transfer during bonding, i.e. the oxidation state. Analysis of the intensity of the shifted components provides the coverage and chemical state distribution of each adsorbate. Novel uses of SXPS include ultrahigh‐resolution spectroscopy, SXPS microscopy, photoelectron diffraction and spin‐polarized photoemission. The equipment utilized for SXPS includes an electron storage ring, an X‐ray monochromator, and an electron spectrometer.