How to analyse XPS spectra (Photoemission Spectroscopy)

As we have seen the instrument gives a plot of Kinetic energy Vs Number of electrons counted, So number of electrons counted is plotted in y axis and kinetic energy is plotted in x axis. So you can see that it starts from the lower KE in the left and goes to higher KE in right as normally a graph is plotted. 

But in most modern instruments KE is converted to BE with the formula

h𝑣 = BE + KE + 𝜙,

The lower KE becomes higher BE and higher KE becomes lower BE.

This means now x axis starts from higher BE and end at lower BE. In future we will always use Binding energy for the reference. So graph will be always starting from high BE to lower BE in x axis.

And the peaks coming at different BE can be compared to the actual electronic configuration of the element. e.g. here in the image you can see the xps spectra of Ni, and at higher BE of 1008 eV comes Ni2s peak, then at 852eV comes Ni2p followed by Ni3s peak at 110eV and finally at 66eV for Ni 3p peak.

This clearly indicates the order of peak appearing in the graph is the exact electronic levels or orbitals of the element. Also as 2s is inner shell it appears at higher BE and as we move to outer electrons BE goes on decreasing.

Another interesting application of XPS is we can make the energy level diagram of any element and find the position of electron in the orbital. Making of an energy level diagram is a tricky task which we will discuss in upcoming videos. 

For now let’s stick to the number of signals and peak positions. In some cases when we have to identify unknown peak we generally use the NIST xps database of the standard values of the BE for pure elements.

Here we have list of BE line position when Mg x ray is used and when Al X-ray is used. You can easily find the peak position from the graph and from this chart identify the elements present in your sample. I will give you the link in description for these charts which you can download for your reference.

Line Position from Mg X ray

Line Position from Al X ray

Now lets try to find the unknown peaks from a sample of Iron. Here is the spectra which shows large number of small peaks. So lets start.

The peaks at 720 and 707eV corresponds to Fe 2p peaks, they are split into two because of spin orbital coupling. Then there are Fe 3s and Fe 3p peaks at 91 and 53eV respectively. But still we have more peaks, lets find what are those peaks from our previous table by matching the binding energy. So the peak at 287eV exactly matches with the C 1s peak. And peak at 531eV matches with O 1s peak. So we can also say that the sample is contaminated with Carbon and oxygen. Carbon and Oxygen comes from the atmospheric exposure, this means sample was exposed to air. The broad peaks at 304 and 553eV are due to the losses of C1s and O 1s electrons. Losses we will discuss in details in upcoming videos. Also O2s peak is visible at 24eV. 

Also some of the peaks of very low intensities are visible like Ca 2p at 348eV, N 1s at 400eV and Mn 2p at 643eV. The signal for these peaks are very low but still they are present. Also small peaks at 781eV and 498eV represents auger peaks. Auger lines are appearing because of energy transfer from one electron to the other electrons during photoemission process. This also we will discuss in detail in next videos. 

For now we can easily find the number of elements present in our sample just by matching the binding energy of different peaks with the standard values as given in table. 

Once sample is clean and we measured the xps spectra we can calculate the area under each peak. As area is directly related to number of electron, which indirectly gives the elemental composition of sample. Please note that for calculating area under the peak we must subtract the background of the peak and also the sensitivity factor of each electron should be accounted for the calculation of concentration. 

The formula for the concentration of x component becomes

$$C_x = \frac {n_x}{\sum n_i}$$

where n is the number of electrons, which is given by Intensity I by sensitivity factor S for each electron. Because each electron is equally sensitive to the cross section of the instrument. 

In the graph peak area of important components are shown here so lets calculate the percentage concentration of each.

Another important application of XPS is to find the percentage concentration of different components present in the sample. Please note that the xps is only surface sensitive technology and it gives the percentage concentration of sample surface unto a depth of 10 to 12 nm only. So if surface is different than the bulk. It is better to clean the surface very well before analysis. Cleaning of surface is done by sputtering and annealing, which removes, few atomic layers off the surface.

Atomic Sensitivity Factor

So if we have intensity of C 1s peak 40396 and its sensitivity factor is 0.205 then the ratio I/s becomes 197054, in this way we can calculate the I/s for each elements, and now sum of I/s will be 249108.

And finally ration of I/S with sum I/S for Carbon will be 79.10%, for Oxygen it is 17.39%, N is 1.44% Fe is 0.52%, Ca is 1.07% and Mn is 0.49%.

That means with simple calculation we can find the elemental composition of our sample.

 

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