auger electron spectroscopy - ferrothinfilms labphotoelectron spectroscopy • xps, also known as...
TRANSCRIPT
Photoelectron Spectroscopy
Xiaozhe Zhang
10/03/2014
A conception last time remain
• Secondary electrons are electrons generated as ionization
products. They are called 'secondary' because they are generated
by other radiation (the primary radiation). This radiation can be in
the form of ions, electrons, or photons with sufficiently high
energy, i.e. exceeding the ionization potential. Photoelectrons can
be considered an example of secondary electrons where the
primary radiation are photons.
What is photoelectron spectroscopy?
• Photoelectron spectroscopy utilizes photo-ionization
and analysis of the kinetic energy distribution of the
emitted photoelectrons to study the composition
and electronic state of the surface region of a
sample
Auger Electron
Free e-
e- Vacancy
e- of high energy that will occupy the vacancy of the core level
e- released to analyze
1
1, 2, 3 and 4 are the order of steps in which the e-s will move in the atom when hit by the e- gun.
e- gun
2
3
4
Auger electron and photoelectron
Auger electron and photoelectron
Photoelectron spectroscopy
• XPS, also known as ESCA, is the most widely used surface analysis technique because of its relative simplicity in use and data interpretation.
XPS X-ray Photoelectron SpectroscopyESCA Electron Spectroscopy for Chemical AnalysisUPS Ultraviolet Photoelectron SpectroscopyPES Photoemission Spectroscopy
Analytical Methods
Equation
KE=hν-EB-Ø
KE Kinetic Energy (measure in the XPS spectrometer)
Hν photon energy from the X-Ray source (controlled)
Ø spectrometer work function. It is a few eV, it gets morecomplicated because the materials in the instrument will affect it. Foundby calibration.
EB Binding energy(BE), is the unknown variable
Equation
• The equation will calculate the energy needed to get an e-
out from the surface of the solid.
• Knowing KE, hv and Ø the (BE)EB can be calculated.
KE=hv-EB-Ø
KE versus BE(EB)
E E E
KE can be plotted depending on BE
Each peak represents the amountof e-s at a certain energy that ischaracteristic of some element.
1000 eV 0 eV
BE increase from right to left
KE increase from left to rightBinding energy
# o
f e
lect
ron
s
(eV)
Interpreting XPS Spectrum: Background
• The X-Ray will hit the e-s in the bulk (inner
e- layers) of the sample
• e- will collide with other e- from top layers,
decreasing its energy to contribute to the
noise, at lower kinetic energy than the
peak .
• The background noise increases with BE
because the SUM of all noise is taken from
the beginning of the analysis.
Binding energy
# o
f e
lect
ron
s
N1
N2
N3
N4
Ntot= N1 + N2 + N3 + N4
N = noise
Analytical Methods
Orbital splitting
Orbital splitting
XPS Sampling Depth
XPS Sampling Depth
• Sampling Depth is defined as the depth from which 95% of
all photoelectrons are scattered by the time they reach the
surface ( 3λ)
• Most λ‘s are in the range of 1 ~ 3.5 nm for Al Kα radiation
• So the sampling depth (3λ) for XPS under these conditions is
3 ~ 10 nm
XPS spectrum example
• The XPS peaks are sharp.
• In a XPS graph it is possible to see Auger electron peaks.
• The Auger peaks are usually wider peaks in a XPS
spectrum.
• Aluminum foil is used as an example on the next slide.
XPS Spectrum
O 1s
O becauseof Mg source
C
AlAl
O 2s
O Auger
Sample and graphic provided by William Durrer, Ph.D.Department of Physics at the Univertsity of Texas at El Paso
Auger Spectrum
Characteristic of Auger graphsThe graph goes up as KE increases.
Sample and graphic provided by William Durrer, Ph.D.Department of Physics at the Univertsity of Texas at El Paso
Identification of XPS Peaks
• The plot has characteristic peaks for each element
found in the surface of the sample.
• There are tables with the KE and BE already assigned
to each element.
• After the spectrum is plotted you can look for the
designated value of the peak energy from the graph
and find the element present on the surface.
XPS Imaging
Thank you for your time!