Question

Describe the photoelectric effect and explain how XPS is used for chemical analysis. List the similarities and differences between XPS and AES.  

Answer 1

The photoelectric effect is a phenomenon in physics. The effect is based on the idea that electromagnetic radiation is made of a series of particles called photons. When a photon hits an electron on a metal surface, the electron can be emitted. The emitted electrons.

X-ray Photoelectron Spectroscopy (XPS) also known as Electron Spectroscopy for Chemical Analysis (ESCA) is the most widely used surface analysis technique because it can be applied to a broad range of materials and provides valuable quantitative and chemical state information from the surface of the material being studied. The average depth of analysis for an XPS measurement is approximately 5 nm. PHI XPS instruments provide the ability to obtain spectra with a lateral spatial resolution as small as 7.5 µm. Spatial distribution information can be obtained by scanning the micro focused x-ray beam across the sample surface. Depth distribution information can be obtained by combining XPS measurements with ion milling (sputtering) to characterize thin film structures. The information XPS provides about surface layers or thin film structures is important for many industrial and research applications where surface or thin film composition plays a critical role in performance including: nanomaterials, photovoltaics, catalysis, corrosion, adhesion, electronic devices and packaging, magnetic media, display technology, surface treatments, and thin film coatings used for numerous applications.

XPS is typically accomplished by exciting a samples surface with mono-energetic Al k? x-rays causing photoelectrons to be emitted from the sample surface. An electron energy analyzer is used to measure the energy of the emitted photoelectrons. From the binding energy and intensity of a photoelectron peak, the elemental identity, chemical state, and quantity of a detected element can be determined.

The fundamental principles of the two techniques are the same. One is a single photo-emission step (XPS), the other is a two-step emission process (AES). Indeed, we can see AES peaks in XPS but not XPS peaks in AES.

A host of reasons exist for why we use an electron gun rather than an x-ray source for laboratory-level AES. We should also remember some practical differences between conventional AES (with an electron gun) and conventional XPS (with an x-ray source).

* AES requires conductive substrates whereas XPS does not

* AES is more surface sensitive and has a higher spatial resolution

* AES is more sensitive to chemical state shifts of the parent element, but the interpretation of the spectra to resolve that information is more difficult in AES than in XPS. This is why the term electron spectroscopy for chemical analysis (ESCA) traditionally only means XPS and not also AES

* Damage to the sample (chemical reaction changes) is induced by an electron beam in AES and by x-rays in XPS.