X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (English: X- ray photoelectron spectroscopy, XPS, often electron spectroscopy for chemical analysis, ESCA) is an established method from the group of photoelectron spectroscopies (PES) to determine the chemical composition, especially of solids and their surface non-destructive. This gives a first answer to the question of the qualitative element analysis, and thus consists of these chemical elements in the solid state. Only hydrogen and helium can not be directly detected due to low cross sections. The depth of information corresponds to the Ausdringtiefe the unscattered or elastically scattered electrons and is usually up to three nanometers. By using angle -resolved photoelectron spectroscopy, the electronic structure of a solid is investigated.

Principle of measurement

Photoelectron spectroscopies (PES) based on the external photoelectric effect, in which photoelectrons by electromagnetic radiation ( X-rays in this case) to be released from a solid. Here, the removal of the electrons from the inner atomic orbitals takes place ( core electrons ). In a simplified model of the process of photoemission proceeds in three steps. First, the excitation of the electron is accomplished by the incident photon, and then the transportation of the excited electron from the surface and the third step of the exit of the photoelectron. The binding energy EB, which can be determined from the kinetic energy of the photoelectrons is characteristic of the atom (specifically, even for the orbital), from which the electron. The analyzer used for the measurement (generally a hemispherical analyzer ) is adjusted by means of electrostatic lenses, and reverse voltages that may he only electrons of a certain energy to pass. For the XPS measurement, the electrons arriving at the end of the analyzer still detected by a secondary electron multiplier, so that a spectrum is produced, which is usually on a graph by the plot of the intensity ( count rate ) is displayed on the kinetic energy of the photoelectrons.

In the picture, a typical spectrum is shown in which the kinetic energy of the electrons on the photoelectric equation has already been translated into the binding energy. The nomenclature of the labeling means: Fe2P for electrons, the more accurately are made ​​of an iron atom (Fe) in the p- orbital of the L shell. Analogously, the abbreviated notation O 1s electron from the s orbital of the K shell of oxygen.

The spectrum reveals the significant spin - orbit splitting between the 2p1/2- and the 2p3/2-Niveau in the iron atom on the basis of two Fe2P emission lines. The additional subscripts 1 /2 or 3/2 give it the total angular momentum of the electron in these orbitals. A similar decomposition can be found (except for the s orbitals ) of all elements with emissions from all orbitals.

Quantitative evaluation of the measurement

The intensity, so the counting of these measurements is proportional to the frequency of occurrence of the various elements in the sample. To determine the chemical composition of a solid, must be below the surface of the observed lines that are characteristic of the elements analyzed. In this case, however, some measurement-specific features are observed. For example, a photoelectron before it leaves the solid body, stimulate more electrons, thereby surrendering a part of its kinetic energy to them. These so-called secondary electrons have virtually no discrete energy distribution and thus contribute equally to the growth of the ground in an XP spectrum at. In figure alongside an XP spectrum of magnetite can be seen (toward larger negative binding energy ) of the count rate after each line of this behavior at the step-like increase. This background has to be subtracted before the evaluation of the areas through appropriate methods, for example by subtracting a linear substrate. More specifically, the underground deduction is for a method, which goes back to DA Shirley and Shirley -background correction is called. The most accurate ( and most expensive ) method is to determine the course of the substrate by means of electron energy loss spectroscopy, and then subtracting the accurate measurement results from the XP spectrum; this method is referred to as Tougaard -background correction.

It should also be observed when the XPS measurements that the probability of triggering a photoelectron is energy-dependent, element -specific and orbital dependent. To account for this fact, the values ​​for the areas that are determined by the respective lines to the so-called sensitivity factors or cross sections are corrected, which can be found in different table works must.

Furthermore, the probability depends that photoelectrons generated by the X-ray radiation can be detected and actually left the solid, depending on how often they are scattered or reabsorbed in the solid state. This loss rate depends on the kinetic energy of the photoelectrons and the composition of the solid. One can account for this effect by considering the mean free path of electrons in the solid. The corresponding data are tabulated at least for most of the elements and simple compounds.

Taking all these effects, the evaluation of the spectrum shown opposite results in an iron -to- oxygen ratio of 3:4, the molecular formula of the investigated material is so Fe3O4 is magnetite. The detection limits of the various chemical elements vary considerably because of the widely varying cross sections. A less strong influence of the different electron scattering losses. Poor sensitivity values ​​( in common Al- Ka - radiation ), the light elements lithium, beryllium, and boron (1s lines) from about 1 at%. Very good sensitivities has, for example, for the elements of copper, tin and gallium (2p - lines), tellurium and adjacent elements (3D - lines) as well as the heavy elements of gold to uranium (4f lines) with values ​​usually significantly less than 0.1 at%. For the limited sensitivity of the noise of the measured significant wave height at the scattered electrons subsoil contributes.

Advanced Analysis

A further essential information about the chemical bonding in the sample based on the discovery of Kai Siegbahn, that the spectra of core electrons of the chemical environment of the investigated system depend. In the XPS spectra of the same element show, depending on the chemical form it exists differences in the binding energy of a core electron of up to several electron volts, it is referred to as chemical shift (English chemical shift ). For example, can be determined using the determination of the charge distribution in a chemical compound which has an oxidation state of atoms or atomic group. In addition, the shape of the spectra can provide information on the valence state of an element in many cases.

The photoemission accompany other physical processes, such as photoluminescence, or the occurrence of Auger electrons and much more., Who in turn found its application as a separate measurement method.

A constitution elucidation of chemical compounds with the XPS method is not always possible because the selectivity is much lower than for example in nuclear magnetic resonance spectroscopy. In addition to other spectroscopic methods, however, the method can be helpful in certain cases.

Radiation sources

The most common X-ray sources, see the XPS their use, are Al- Ka - or Mg - Ka - sources, more exotic X-ray sources, however, also produce silicon, titanium or zirconium - ray lines. In the last 20 years, the use of synchrotron radiation, which is ideal as an excitation source due to their virtually unlimited tunability of the photon energy and the monochrome, enforced more and more. Thus, the range of available stimulating photon energy of a few discrete values ​​(e.g., Al K? Hv = 1486.6 eV and Mg K? Hv = 1253.6 eV) on a continuum, from several electron volts to 20 keV has been filed extended.

Technical details

Since there is a very high technical (and thus financial) effort to perform measurements using synchrotron radiation, see the aforementioned X-ray tubes used widely in standard XPS analyzes. The energy of the photoelectrons produced by these X-ray sources is in the range between 0 eV and 1500 eV, which for PES measurements means that the electrons emitted from a maximum depth of the investigated sample, which is between 0 and 100 Å, originate. The limiting factor here is the mean free path of electrons in the solid. Here lies the reason why the XPS is mainly used for the analysis of solid surfaces.

For such measurements, a base pressure of the analysis chamber in the range of ultra-high vacuum (UHV ) is usually necessary, of between 0.5 · 10-10 hPa and 5 · 10-10 hPa ( air pressure at sea level: about 1013 hPa). This requirement for extreme vacuum conditions resulting from the unavoidable contamination of the sample with adsorbates from the ambient air, such as water or carbon, which may be several micrometers, and thus would not allow for the measurement of interest to the solid surface. Samples are To work around this issue brought into the UHV chamber and prepared using suitable methods, such as sputtering, annealing, filing or columns of single crystals of high purity.

However, this inherent weakness in the measuring method can be transformed into a strong advantage. The information depth of the PES is limited by the mean free path of electrons in a solid. For a metal, for example, is this just 1 to 2 nm, which is three orders of magnitude smaller compared to the penetration depth of X-rays ( depending on the material 1 to 10 microns ). It is clear that only the photoelectrons are detected by the detector, which may also leave the solids. The intensity contribution to the spectrum thus decreases exponentially with increasing depth. In the angle-resolved X-ray photoelectron spectroscopy (English angle -resolved X - ray photoelectron spectroscopy, ARXPS ), an extreme sensitivity of the measurement surface can be achieved by changing the angle of the detector relative to the sample to be measured. In the field of surface science can be studied in this way almost exclusively to the first monolayer of a sample.