Xray photoelectron spectroscopy XPS Bergds Bjrk Bringsdttir Danel

X-ray photoelectron spectroscopy (XPS) Bergdís Björk Bæringsdóttir Daníel Arnar Tómasson Kristján Einar Guðmundsson

Theory of xps • Based on the photoelectric effect • Emitted x-rays from the machine are focused towards the sample • The x-rays have sufficient energy (hv) to ionize core electrons (BE) and give them excess kinetic energy (KE): • Ke is the response with hv provided from the machine. BE can be determined • Identification of elements possible as binding energies for the core electrons are unique for every element

Internal factors affecting the xps spectral data Spin orbit splitting Chemical shifts Oxidation Noise signal from inner layer electrons losing energy in collisions with upper layer electrons (inelastic scattering) • Auger electrons peaks • And more • •

Spin orbit splitting of peaks • All core electrons with angular momentum (n=2, 3, 4. . ) experience spin orbit interactions • Coupling of the orbital magnetic moment and magnetic moment due to the intrinsic spin possession of the electron causes a change in energy for electrons which is observed as splitting of peaks in the xps

Chemical shifts and oxidation • Further induce peak splitting • Subtracted electron density due to oxidation means less efficient screening of the core electrons and causes a shift towards higher binding energy • Carbon chemical shift in a molecule • Different bond energies

XPS Instrument • XPS is also known as ESCA (Electron Spectroscopy for Chemical Analysis). • XPS works by irradiating atoms of a surface of any solid material with X-Ray photons, causing the ejection of electrons. • It is controlled by using a computer system which controls the X-Ray type and prepares the instrument for analysis.

XPS Instrument • XPS requires ultra high vacuum (UHV). • The Ultra High Vacuum environment will prevent contamination of the surface and aid an accurate analysis of the sample. • Ultra high vacuum environment to eliminate excessive surface contamination. Mostly water which covers all surfaces, sits in pours, until a very high vacuum is acquired.

XPS Technology • Consider as non-destructive • because it produces soft x-rays to induce photoelectron emission from the sample surface • Provide information about surface layers or thin film structures • Applications in the industry: • • • Polymer surface Catalyst Corrosion Adhesion Semiconductors Dielectric materials Electronics packaging Magnetic media Thin film coatings

XPS Instrument X-Ray Source Ion Source SIMS Analyzer Sample introduction Chamber

Diagram of the Side View of XPS System X-Ray source Ion source Detector SIMS Analyzer Axial Electron Gun Sample introduction Chamber Sample Holder sample Roughing Pump CMA Slits Ion Pump

Kratos Axis-165 XPS system in RRC-East

XPS Instrument Concentric Hemispherical Analyzer (CHA) or spherical sector analyzer The Atom and the X-Ray Free electron Valence electrons proton neutron electron Core electrons electron vacancy The core electrons respond very well to the X-Ray energy

Instrumentation Most common type of electrostatic deflectiontype analyzer: Concentric Hemispherical Analyzer (CHA) or spherical sector analyzer Energy resolution dependant on radius. Capable of collecting photoelectrons of larger angular distribution. Photoelectrons of a specific energy are focused by the lens at the slit of the spectrometer. Lens also controls sampling area. Photoelectrons travel through a circular path and exit into a series of channeltrons (electron multipliers).

FUN FACT Same kind of technology used to curve electron beam in Cern Supercollider and as in the CHA

Analytical Capabilities of XPS It can: • Identify elements or compounds (except H and He) • Determine oxidation states (for example Ti 3+ or Ti 4+) • Identify types of chemical bonds (like Si-O or Si-C) • Semi-quantitative analysis (with about 10 -15% error) • Determine film thickness Limitations: • Does not detect H or He • Radiation damage possible (worse for achromatic sources) • Charge neutralization needed for insulating material • Chemical analysis can be limited to functional groups and in some cases chemical shifts are not resolvable.

Analysis Capabilities continued Elemental vs. quantitative analysis: • Elemental Analysis: atoms have valence and core electrons so core -level binding energies provide a unique signature of elements. • Quantitative analysis: measures intensities, use standards or tables of sensitivity factor X-rays vs. e- Beam: • X-Rays: Hit all sample area simultaneously permitting data acquisition that will give an idea of the average composition of the whole surface. • Electron Beam: It can be focused on a particular area of the sample to determine the composition of selected areas of the sample surface.

XPS Spectrum The plot has characteristic peaks, which are sharp, for each element found in the surface of the sample In a XPS graph it is possible to see Auger electron peaks which are usually wider peaks in a XPS spectrum. There are tables with the KE and BE already assigned to each element. After the spectrum is plotted you can look for the designated value of the peak energy from the graph and find the element present on the surface.

Identification of XPS Peaks “Splitting” of 2 s, 2 p etc. due to spin-orbit splitting KINETIC ENERGY, e. V Electron shells