Analytical methods
Photoelectron Spectroscopy
Photoelectron spectroscopy (PES) is an analytical tool for the characterization of the occupied electronic states of a sample. It is used in the analysis of elemental surface compositions and valence structures. In the PES process, one electron is excited from its ground state to a state above the vacuum level (EVac), where its kinetic energy (Ek) is measured by an electron analyzer with respect to the Fermi energy (EF). Depending on the source of excitation, PES is also called x-ray photoelectron spectroscopy (XPS) or ultraviolet photoelectron spectroscopy (UPS).
Inverse Photoemission Spectroscopy
Inverse photoemission spectroscopy (IPES) is a method for probing the unoccupied electronic states of a sample. It is the inverse process to PES, i.e. electrons from above the vacuum level couple into unoccupied states of the conduction band, subsequently decaying into lower unoccupied states emitting UV photons, which are detected.
UPS and IPES are used to determine the position of the valence band maximum (VBM) and the conduction band minimum (CBM), respectively. Hence, by combination of these techniques it is possible to directly measure the electronic surface band gap of the sample.
Because of the short inelastic mean free path of the electrons, PES and IPES are very surface-sensitive techniques (information depth: a few nm).
Scanning Probe Microscopy
Complementary to the laterally integrating spectroscopic methods (PES, IPES), it is intended to use the scanning probe microscope (SPM) to investigate the morphology and the electronic structure of the sample surface laterally resolved.
In addition to our lab-based techniques (PES, IPES), we also use synchrotron-based characterization metho
X-ray Emission Spectroscopy
As PES, the x-ray emission spectroscopy (XES) probes occupied states and hence can be used to investigate the chemical and electronic structure of samples. For XES a core-level electron is excited by (soft) x-ray photons. The created core hole is filled by an outer-level (most often a valence) electron, causing the emission of a photon, which is detected by an x-ray spectrometer. The cross section for this process (compared to that of, e.g., direct photoemission) is very small, demanding the experiments to be performed at high-flux synchrotron light sources. Because the radiative relaxation process takes place with respect to the created core hole, XES is an excellent tool to probe the local structure of a specific atomic center.
Since XES (contrary to PES) is a photon-in/photon-out technique, the information depth is roughly two orders of magnitude larger than that of PES.
When the core-level electron is excited to the lowest unoccupied state of the samples conduction band, it is referred to as resonant inelastic x-ray scattering (RIXS). RIXS is used to map the electronic sample structure and is therefore uniquely suited to “test” calculated band structures.
Schematic presentation of the processes involved in XPS, UPS, and IPES.
Picture of the ultra-high vacuum surface-analysis system of the Young Investigator Group (April 2009).
Schematic presentation of the processes involved in XES and XAS.
Picture of the SALSA (Solid and Liquid Spectroscopic Analysis) endstation at Beamline 8.0 (Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, USA). SALSA was set up by and is operated in close collaboration with the University of Nevada, Las Vegas (UNLV) and Würzburg University.
X-ray Absorption Spectroscopy
X-ray absorption spectroscopy (XAS) investigates unoccupied states. It is an element-specific technique, sensitive to bonding environment and geometry. XAS involves the excitation of core-level electrons into unoccupied states of the samples conduction band. For this, the excitation energy is tuned to the absorption edge of the investigated core level. Secondary processes, as e.g., radiative decay (fluorescence yield, FY) or the current drawn by the sample to maintain charge neutrality (electron yield, EY) are used as detection channels.Depending on what detection channel is used, the XAS information depth is similar to that of XES (® FY) or of PES (® EY).
Similar to PES and IPES, also XES and XAS can be combined to reveal the band gap* of the near-surface bulk of the sample.
The synchrotron-based characterization efforts are based on regular experimental campaigns at the synchrotron light source in Berlin (BESSY II) and in Berkeley (Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, USA).
* Note that this value is the band gap in the presence of a core hole, i.e. due to the potential existence of core excitons, it can be considered as a lower-bound value for the ground-state band gap.
