Chemistry Reference
In-Depth Information
The main parts of an XPS device are: x-ray source, electron optic, energy analyzer and
detector. In modern spectrometers, the x-rays are energy filtered or monochromatized
using a quartz crystal to get x-rays with small energy width. The monochromatic x-
rays enable a higher energy resolution and prevent additional peaks (satellites). A
hemispherical analyser (HSA) is applied to measure the energy of the electrons
emitted. Small pass energies of the HSA reduce the signal peak width.
Each element has a unique set of electrons in orbitals with well-defined binding
energies. The kinetic energy of an emitted electron is characteristic for its source
orbital and hence for each element.
The number of electrons measured for a given transition is proportional to the
number of atoms at the surface. For that reason, XPS can be used to identify and
determine the concentration of the elements in the surface excluding hydrogen and
helium. The number of electrons counted in dependance of binding energy yields an
XPS spectrum, see Figure 7.11.
Quantitative data can be obtained from the peak areas divided by the atomic
sensitivity factors (ASF) which depend on the XPS instrument. Concentrations down
to 0.1 at% can be detected. Variations in the elemental binding energies arise from
differences in the chemical neighborhood and binding state. These chemical shifts
can be used to identify the chemical state of the materials being analyzed.
In the case of conducting samples, the detected electron energies can be ref-
erenced to the Fermi energy of the spectrometer. An absolute energy scale can be
established. Energy calibration is significant for nonconducting samples. Electrons
leaving the sample surface cause a potential difference between the sample and the
10,000
C 1s
8,000
6,000
4,000
F 1s
O 1s
N 1s
NH
3
-plasma treated PS
2,000
0
Untreated PS
1200
1000
800
600
400
200
0
Binding energy [eV]
FIGURE 7.11
Wide scan—comparison between untreated polystyrene (PS) and NH
3
-
plasma-treated PS after chemical derivatization of amino groups by trifluoromethylbenzalde-
hyde introducing a F 1s peak.
