X-Ray Photoelectron Spectroscopy
XPS measurements are available to the staff of the Department of Chemistry and of the Faculty of Mathematical, Physical and Natural Sciences of La Sapienza. The XPS lab carries out activities also for third parties.
How to apply
To fix an appointment, send an email to:
specifying all the following info:
- Name and title of the applicant;
- Organization and research team;
- E-mail address;
- Phone number;
- Type of analysis required;
- Number of samples to be analyzed;
- Always add an explicit statement that each sample consists of non-toxic, non-hazardous species; alternatively, list the properties of the sample that make them hazardous materials.
Measurements can only be made on solid samples which are good electrical conductors. In some cases XPS can successfully be applied also to samples which are not good conductors. The size of the exposed surface must be of ca. 1 cm x 1 cm; the thickness and the total weight of the sample must not be high. XPS measurements are carried out under ultra-high vacuum, pressure ≤ 10 to the -9 mbar. The sample is subjected to a bias of ca. 1.5 keV for the time of the measurements, typically one to two hours. The typical power dissipated over the exposed area is 164 W (16 mA x 14 kV).
A basic knowledge of photoemission spectroscopy can help understanding the advantages and limitations that this analysis can offer on samples.
Description of the technique
Photoelectron spectroscopies are a set of techniques operating in ultra-high vacuum, affording information related to the electronic (but also magnetic and, in special cases, geometric) structure of samples in different states of aggregation. The photoemission phenomenon consists of a stimulated emission of free electrons in vacuum. Photoemission (the ''photoelectrons'') is the result of exciting a solid sample with photons (but also electrons) of an energy at least higher than the work function of the solid examined (typically 4-5 eV). The process of interaction of radiation with matter generates photoelectrons with kinetic energy KE which can be converted into the corresponding binding energy, BE, if the energy of the exciting photon, hv, is known. This conversion is obtained through the Einstein relation for the photoelectric effect: hv - KE = BE. The photoelectrons carrying such primary information come from a very small volume of the solid sample which includes its surface, the volume within which the photoelectrons have not undergone energy loss by different phenomena, mainly electron-electron scattering. In fact, the thickness investigated by XPS corresponds to the first atomic layers, 2-5 nm (depending on the chemical nature of the sample). Photoemission requires use of a stable source of known energy for an experimental determination of the values of binding energy of the photoelectrons and to assign them to specific electronic transitions.
The shift in binding energy, measured with respect to an appropriate reference (usually the atomic element taken in its zerovalent state) is called "chemical shift" in XPS. This parameter is crucial to assign oxidation states to the various atomic components of a sample. For example, one can easily distinguish sulphates from sulphites or sulphides or elemental sulphur, as well as thiols from thiolates. In addition to providing information on the oxidation state of each element (only H and He have not core orbitals), XPS is also a surface analytical technique. The results of a quantitative XPS analysis are expressed as stoichiometric ratios, not as absolute amounts. The (semi)quantitative XPS analysis is usefully supplemented by independent information on the composition of the sample mass. Such analysis can highlight differences in the distribution between surface and bulk for all the characteristic elements. In simple terms, XPS provides an accurate image of the exposed surface of the sample, both in terms of elementary composition and of oxidation state, as well as in terms of relative quantities.
This technique is widely used both in pure and in applied scientific research (studies on metallic materials and semiconductors, on polymers, catalysts, particles of various sizes, etc).