Valence Band

Conduction Band

Discovery of the

Photoelectric Effect

In 1887, Henrich Hertz (Germany;1857),

was studying electromagnetic waves

propagation at the University of

Karlsruhe. He observed for the first time

1845, discovered X-ray in 1895 while

performing his research on electrical

rays at the University of Würzburg.

This discovery earned him the Nobel

Prize of Physics in 1901.

Photoelectric

Effect Theory

It is not until 1905 that the

theory behind Hertz’s

observation would be

Following almost 20 years of research at the

University of Uppsala, Kai Siegbahn, born in

Sweden in 1918, published a very detailed paper

on XPS also known as “electron spectroscopy for

chemical analysis” (ESCA). This work would be

recognized by the Nobel Prize in 1981.

Following a collaboration with

Siegbahn Hewlett-Packard

The basic XPS setup is as follows: X-ray (high energy light source) is directed to the sample

surface and cause photoelectrons to be emitted. Photoelectrons are energized electrons that

are emitted from the surface when it is exposed to X-ray whose energy is above the threshold

energy. Those photoelectrons are then captured, and their energies (the difference between

the X-ray energy and the binding energy) are measured to determine which type of elements

Reading XPS Spectra

Typical XPS spectra have counts on the y-axis which represent the number of electrons

impacting the detector. The relative height differences that trace out peaks is important, not

the absolute counts. From a software side, the measured peak height and position are

compared with a library of reference spectra to identify the material present. Most elements

can be detected down to 0.1 atm % with a strong X-ray source or repeated sweeping counts.

Chemical Bonding Information by Peak Deconvolution

Sometimes XPS spectra contains overlapping signals, these are evident with asymmetry in

peak shape, double peaks or long shelves. To extract the most meaningful data from these

states. With a good library of peak positions and a high intensity X-ray source, XPS can give

powerful insights into materials.

Depth Profiling by XPS

XPS is typically performed in a vacuum environment to minimize the chance of the

photoelectrons colliding with anything other than the detector. Most systems are also outfitted

with a sputtering beam, typically Ar, to clean the surface and perform XPS as a function of

depth. This is useful if the layer of interest is not directly at the surface. X-rays penetrate deep

into materials ~1 µm but only electrons emitted near the top 10 nm surface make it to the

detector. By coupling XPS with the sputtering Ar ion beam, XPS can generates depth profiles

of chemical composition and bonding states which is a very powerful information to

D-parameter of the sample material the ratio of sp3 and sp2 bonding states can be estimated.