X ray photoelectron spectroscopy

Editor's Notes

• #3: What information can you obtain from XPS? •Identification of elements near the surface and surface composition •Local chemical environments •Oxidation states of transition metals •Valence band electronic structure •Morphology of thin films XPS detects all elements with (Z) >3. It cannot detect H (Z = 1) or He (Z = 2) because the diameter of these orbitals is so small, reducing the catch probability to almost zero XPS is routinely used to analyze inorganic compounds,metals,semiconductors,polymers, ceramics,etc. Organic chemicals are not routinely analyzed by XPS because they are readily degraded by either the energy of the X-rays or the heat from non-monochromatic X-ray sources
• #4: X-ray photoelectron spectroscopy (XPS) is based on the photoelectric effect. Each atom in the surface has core electron with the characteristic binding energy that is conceptually, not strictly, equal to the ionization energy of that electron. When an Xray beam directs to the sample surface, the energy of the X-ray photon is adsorbed completely by the core electron of an atom. If the photon energy, hn, is large enough, the core electron will then escape from the atom and emit out of the surface. BE= Electron Binding Energy KE= Electron Kinetic Φ= Spectrometer Work Function hn is the X-ray photon energy
• #8: How Does XPS Technology Work? A monoenergetic x-ray beam emits photoelectrons from the from the surface of the sample. The X-Rays either of two energies: Al Ka (1486.6eV) Mg Ka (1253.6 eV) The x-ray photons The penetration about a micrometer of the sample The XPS spectrum contains information only about the top 10 - 100 Ǻ of the sample. Ultrahigh vacuum environment to eliminate excessive surface contamination. Cylindrical Mirror Analyzer (CMA) measures the KE of emitted e-s. The spectrum plotted by the computer from the analyzer signal. The binding energies can be determined from the peak positions and the elements present in the sample identified.
• #11: The sample will be introduced through a chamber that is in contact with the outside environment It will be closed and pumped to low vacuum. After the first chamber is at low vacuum the sample will be introduced into the second chamber in which a UHV environment exists
• #14: The electrons ejected will pass through a device called a CMA. The CMA has two concentric metal cylinders at different voltages. One of the metal cylinders will have a positive voltage and the other will have a 0 voltage. This will create an electric field between the two cylinders. The voltages on the CMA for XPS and Auger e-s are different. When the e-s pass through the metal cylinders, they will collide with one of the cylinders or they will just pass through. If the e-’s velocity is too high it will collide with the outer cylinder If is going too slow then will collide with the inner cylinder. Only the e- with the right velocity will go through the cylinders to reach the detector. With a change in cylinder voltage the acceptable kinetic energy will change and then you can count how many e-s have that KE to reach the detector.
• #15: KE Kinetic Energy (measure in the XPS spectrometer) hv photon energy from the X-Ray source (controlled) Ø spectrometer work function. It is a few eV, it gets more complicated because the materials in the instrument will affect it. Found by calibration. BE is the unknown variable The equation will calculate the energy needed to get an e- out from the surface of the solid. Knowing KE, hv and Ø the BE can be calculated.
• #17: Chemical shift: change in binding energy of a core electron of an element due to a change in the chemical bonding of that element. If a charge q is added to (or removed from) the valence shell due to chemical bond formation, the electrostatic potential felt by the electron inside the atom is changed. • When atom loses valence charge (Si0 --> Si4+ ) BE increases. • When atom gains valence charge (O --> O--) BE decreases. Photoelectron core level peaks in elemental samples occur at the same binding, for example, a gold surface. In compounds, where ionic or covalent bonding occurs, the peak position may shift. For example, Si in pure Si wafer, the binding energy of Si 2p is 99 eV, In contrast, Si in SiO2, the binding energy of Si 2p is 102.3 eV Chemical shift: change in binding energy of a core electron of an element due to a change in the chemical bonding of that element. Core binding energies are determined by electrostatic interaction between it and the nucleus, and change with: • the electrostatic shielding of the nuclear charge from all other electrons in the atom (including valence electrons) • removal or addition of electronic charge as a result of changes in bonding will alter the shielding Atoms of a higher positive oxidation state exhibit a higher binding energy due to the extra coulombic interaction between the photo-emitted electron and the ion core. This ability to discriminate between different oxidation states and chemical environments is one of the major strengths of the XPS technique Binding energy of a core electron depends on number and distribution of valence electrons. •Rule of thumb: higher valence electron density = lower binding energy •Final state effects can revert this trend. •Oxidation leads to higher BE of the cation (e.g. Mn+ in metal oxide) compared to reduced element (less valence electrons). •Typically: +1eV per oxidation state An increase in oxidation state causes the binding energy to increase due to a decrease in the screening of the bound electron from the ion core. The ability of XPS to determine oxidation states is used extensively in catalysis research n principle quantum number l orbital angular quantum number s spin angular quantum number j total angular quantum number (j=l+s)
• #19: Emission from non-monochromatic x-ray sources produces satellite peaks in XPS spectrum at lower BE.