X‐Ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) uses x rays of a characteristic energy (wavelength) to excite electrons from orbitals in atoms. The photoelectrons emitted from the material are collected as a function of their kinetic energy, and the number of photoelectrons collected in a defined time interval is plotted versus kinetic energy. This results in a spectrum of electron counts (number per second) versus electron kinetic energy (eV). Peaks appear in the spectrum at discrete energies due to emission of electrons from states of specific binding energies (orbitals) in the material. The positions of the peaks identify the chemical elements in the material. Peak areas are proportional to the number of orbitals in the analysis volume and are used to quantify elemental composition. The positions and shapes of the peaks in an XPS spectrum can also be analyzed in greater detail to determine the chemical state of the constituent elements in the material, including oxidation state, partial charge, and hybridization. This article details the overall protocal for XPS analysis. X-ray photoelectron spectroscopy is widely applied to all types of solids, including metals, ceramics, semiconductors, and polymers, in many forms, including foils, fibers, and powders. It has also been used to obtain spectra of gas-phase compounds. In general, it is a nondestructive method of analysis. When applied to solids, XPS is a surface-sensitive technique. The best sensitivity of XPS for quantifying elemental composition in solids is on the order of 0.1 at. %. All elements with atomic number greater than three can be detected. The detection sensitivity varies for each element, with some elements requiring greater concentrations to reach a nominal detection threshold in a reasonable analysis time. Thin film samples can be analyzed with XPS to determine film thickness. Typical methods of analysis involve measuring composition either as a function of sputter depth into the sample (sputter profiling) or as a function of electron emission angle (angle-resolved XPS). The chemical state information obtained with XPS includes oxidation states or hybridization states for chemical bonds of the elements. Assignment of chemical state using XPS is not without ambiguities, because other factors besides chemical environment shift the peaks in an XPS spectrum. The use of references with known composition and chemistry can usually help resolve these ambiguities. The primary limitation of XPS is the need for ultrahigh vacuum conditions during analysis. This generally limits the type of material to those with a low vapor pressure (<10−8 mbar) at room temperature and limits the sample size to that which will fit through the introduction ports on the vacuum chamber. Some compounds, such as polymers, can also degrade under the x-ray flux. Another limitation in interpretation of spectra from insulator and semiconductor samples arises from sample charging. This is an artifact that can be corrected either during or after data acquisition, although the extent of the correction is often not straightforward to determine. Keywords: X-ray photoelectron spectroscopy (XPS); high resolution spectrum; XPS survey spectrum; instrumentation; applications; method automation; principles; peak shapes; problems; specimen modification; sample preparation; data analysis