X-ray Photoelectron Spectroscopy

Overview

Source: Faisal Alamgir, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA

X-ray photoelectron spectroscopy (XPS) is a technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material. XPS spectra are obtained by irradiating a material with a beam of X-rays while simultaneously measuring the kinetic energy and number of electrons that escape from the top several nanometers of the material being analyzed (within ~ the top 10 nm, for typical kinetic energies of the electrons). Due to the fact that the signal electrons escape predominantly from within the first few nanometers of the material, XPS is considered a surface analytical technique.

The discovery and the application of the physical principles behind XPS or, as it was known earlier, electron spectroscopy for chemical analysis (ESCA), led to two Nobel prizes in physics. The first was awarded in 1921 to Albert Einstein for his explanation of the photoelectric effect in 1905. The photoelectric effect underpins the process by which signal is generated in XPS. Much later, Kai Siegbahn developed ESCA based on some of the early works by Innes, Moseley, Rawlinson and Robinson, and recorded, in 1954, the first high-energy-resolution XPS spectrum of NaCl. Further demonstration of the power of ESCA/XPS for chemical analysis, together with the development of the associated instrumentation for the technique, led to the first commercial monochromatic XPS instrument in 1969 and the Nobel Prize for Physics in 1981 to Siegbahn in acknowledgement of his extensive efforts to develop the technique as an analytical tool.

Procedure

The following procedure applies to a specific XPS instrument and its associated software, and there may be some variations when other instruments are used.

The sample is a thin film of Pt (3 atomic layers thick) grown on a single layer of graphene, which is supported on a commercial silica (SiO₂) glass slide. The graphene (which is a single layer of carbon) was grown on Cu and then transferred to the glass substrate. The Pt atomic layers were then deposited by electrodeposition methods.

Results

Figure 1 shows a survey spectrum from the sample, clearly showing the Pt, Si, C and O emissions. In Figure 2, we see the high resolution scan of the Pt 4f_7/2 and 4f_5/2 peaks from the sample. The binding energies of each of the core level peaks can be compared to those found in databases such as the one maintained by the National Institute of Standards and Technology (NIST) (at https://srdata.nist.gov/xp

Application and Summary

XPS is a surface chemical analysis technique that is versatile in the range of samples it can be used to investigate. The technique provides quantification of chemical composition, chemical state and the occupied electronic structure of the atoms within a material.

XPS provides elemental the composition of the surface (within 1-10 nm usually), and can be used to determine the empirical formula of the surface compounds, the identity of elements that contaminate a surface, the chemical or elect