9. XPS, AES and ISS Flashcards
What are the basic principles of XPS? What are typical requirements to the equipment and how to perform the analysis of the XPS response?
X-ray Photoelectron Spectroscopy.
XPS uses the photoelectric effect. We irradiate a sample with x-rays, electrons will escape the solid, because it has gotten enough energy. We then detect these electrons as a function of their kinetic energy. Since we know the energy of the detector, we can calculate the binding energy of the electrons. These energies are characteristic for the elements.
The requirements for equipment:
• Ultra-high vacuum (the sample must withstand this)
• Need advanced detectors that can filter the electrons based on energy, which is the Hemi-Spherical analyser
- Analyze advantages and disadvantages of Auger and XPS methods regarding the surface analysis.
XPS:
+ Can perform depth profiles non-destructively
+ Surface sensitive (5-10nm)
+ Sensitive to chemical shifts
+ All elements are detectable, except H and He
+ Chemical/electronic state information is available
- Requires UHV, and the materials need to withstand high vacuum
Auger electron spectroscopy AES:
+ Surface sensitive (0.5-3nm), lower than XPS
+ Sensitive to chemical shift due to bonding
+ Straight-forward depth profiling is possible…
- … but destructive
- Poorer quantitative results
- Can get charging effects (in non-conducting samples), which can distort the AES peaks.
- High power electron beams can destroy sample (organic samples)
What are the principles of ARXPS?
X-ray beam to eject an electron
Non-destructive depth analysis of surface by measuring XPS at different angles.
The principle of Angle Resolved XPS is that we take consecutive XPS measurement at different angles. From this we can get information primarily of the surface (high angle) and more in the bulk (normal incidence). This takes advantage of the escape depth of 5-10nm, and the fact that the tilt will create an interaction volume closer to the surface. By using models and iterative algorithms, we can construct the depth profiles, but it is difficult.
Depth profile = elements measured as a function of depth
How the depth profile issues can be solved using AES (Auger electron spectroscopy)?
Electron beam to eject an electron
It is a non-destructive method. We can thus get information about the depth profiles of ultra-thin layers. Auger electrons are from the top atomic layer.
ISS: what are the basic principles and advantages?
Ion scattering spectroscopy. Here we get the information from the scattered ions.
The spectrum is obtained by recording the number of scattered primary ions collected per second as a function from zero to the energy of the primary beam. Higher energy scattering peaks are characterized of heavier elements, and low energy scattering peaks are characterized of lighter elements.
Analyze advantages, disadvantages and challenges of XPS, AES and ISS in characterization of energy materials.
XPS (X-ray photoelectron spectroscopy):
+ Surface sensitive
+ can get depth profiles up to 10nm without being destructive
+ nice for quantitative analysis
- must have elements that are high-vacuum compatible
- expensive equipment due to fancy detectors and UHV
- can´t visualize (depth profile modeling is hard)??
- Insensitive to H and He
AES (Auger electron spectroscopy):
+ surface sensitive (more than XPS)
+ can get depth profiles
- depth profiling is destructive.
- Insensitive to H and He
- Not as good for quantitative measurements
ISS (Ion scattering spectroscopy:
+ Sensitive to the first atomic layer
+ Sensitive to H and He
What can often determine the functionality of energy materials?
Surface and subsurface composition of the material.
Why is XPS and AES so surface sensitive?
Because of the low energies of the electrons. Only the electrons generated near the surface escape and are detected.
Draw schematics of the mechanisms of emission of photoelectron and emission of Auger electron.
Se bilde
What are the two most used x-ray lines? Why?
The Mg K alpha-line, and the Al K alpha-line. Because the have a narrow line, and have enough energy to reach many core levels.
What are the sample requirements for an XPS experiment?
- Has to withstand UHV (below 10^-7 torr)
- Has to withstand irradiation by X-rays
- Sample surface must be clean
- The sample should be «reasonable» sized.
Give examples of materials that can not be used in XPS chambers.
- Majority of organic compounds
- Some steels (only stainless is used)
- Lead (soldering is done with lead-free solder)??
- Indium (low melting point prevents it´s use in baked systems)
- Zinc, Casmium (high vapor pressure during system bakeout)
Why can we see F, N and C signals in an ITO-spectrum?
F because it is normally a dopant in ITO. N and C are contaminants.
How can one draw conclusions about chemical bonding states at the surface from an XPS spectrum?
Chemical bonds cause a small shift of the energy. This means that we can compare this to known data and draw conclutions of the nature of the chemical bonding state.
