3.2 Characterization techniques 55
Figure 3.8: Left: Image of the glass cell used for in-situ XAS measurements. The
thin lms were deposited onto 200 m thick glassy carbon wafers, which are then
attached to a hole in the glass cell. A piece of copper tape is used as electrical
connection to the wafer. Right: A schematic of the signal pathways for the in-situ
XAS experimental set up from 198. The beam is coming from the lower right corner
and interacts with the sample as noted in the gure. Spherically bent analyzers send
the signal to an SDD detector underneath the sample.
3.2.4 Scanning Electron Microscopy
In Scanning Electron Microscopy, SEM, a focused electron beam is rastered over
a surface to generate a multitude of signals that can be used for analysis. The
energy of the incoming electrons is typically in the range of 1-30 kV. Among
the most used signals are secondary electrons, backscattered electrons, Auger
electrons and characteristic X-rays 199. An overall schematic of the signals
generated can be seen in gure 3.9.
Secondary electrons are emitted from the sample surface due to electrons excited
by the electron beam. The energy of these electrons varies from 10-200 eV which
means that approximately one nanometer thickness of the sample contributes
to the signal. The yield of these secondary electrons depends on the surface
topography, since local curvature changes the amount of electrons that can
escape. More specically, for protrusion edges a larger amount of secondary
electrons can escape, while the opposite holds for cavity edges. Other parameters
aecting the yield include the work function of surface, incident beam energy,
beam current and the density of the sample. Backscattered electrons are incident
electrons which undergo elastic scattering interactions with the surface atoms.
These electrons have energies close to the incident beam energy and therefore
a larger interaction volume is probed. The scattering process is sensitive to
the elements in the sample surface and for higher atomic numbers the back
scattering signal increases. It should be noted that backscattered electrons can