Name the characteristics of photoelectron spectroscopies.
surface technique with 2nm penetration depth
based on photoelectric effect
information about chem. composition of the surface (qualitative and quantitative)
information on chemical enviroment (nearest neighbors, bonds, oxidation)
depth concentration profile (with sputtering)
How does XPS work?
The Xray photons give the electrons the energy to leave the binding of the atom. These Electron are evaluated.
How can you calculate the binding energy of the ejected electrons?
What does XPS stand for?
X-ray photoelectron spectroscopy
What is the penetration depth of XPS?
2nm=1-2 atom layers
What quantity is measured in XPS?
The kinetic energy of the photoejected electrons.
What is the mechanism of XPS measurements and on which effect do they rely?
XPS measurements rely on the photoelectric effect.
For a measurement X-ray photons are shot at the surface, which leads to an ejection of electrons. By measuring the kinetic energy of those electrons, one can calculate the binding energy.
In which way is the result of the XPS analysis noted?
Describe the Auger electron analysis (AES).
The Energy difference of the electron moving to a lower energy level is applied to another outer electron, which leads to it ejecting. These electrons are measured.
What is meant be the chemical shift in XPS?
The chemical shift is a change in binding energy due to the chemical influence on the atoms.
What are factors that influence the chemical shift?
the binding energy increases with the oxidation state of the element
the binding energy increases with the electronegativity of its neares neighbour
What are typical XPS sources and what is their energy?
Mg K alpha with hv=1253,6 eV
Al K alpha with hv=1486,6 eV
Describe the rough structure of a XPS.
Name a practical way to enhance the surface sensitivity of the XPS.
Changing the angle between analyser and surface. (take off angle)
Which properties are required of the samples in XPS?
stability (due to ultra high vacuum conditions and X-ray influence)
Conductivity (if charge accumulation, results will be greatly distorted)
What are experimental problems in XPS?
X-rays may cause damage to the samples
Sensitive samples may decompose: damage to the instrument
charge up of electrically insulated materials due to x-rays
different charges in different regions: Differential charging leads to peak broadening
Name 2 applications of XPS.
Analysis of the purety of semicondutors
in combination with sputtering for determination of a oxide surface layer thickness
Explain the difference between XPS, AES and UPS.
XPS: Composition, oxidation state, dispersion
AES: Surface composition, depth profiles
UPS: Chemical bonding, work function
How can a depth profile be obtained from XPS?
By removing the surface via sputtering
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