Bad Peak-fits produce Bad Information
| Explanation Why Peak-fit is Bad ? |
Bad Peak-fit Example |
Good Peak-fit Example |
Explanation Why is it Good ? |
| Ni (2p3) – Ni metal foil – ion etched | Ni (2p3) – Ni metal foil – ion etched clean | Ni (2p3) – Ni metal foil – ion etched clean | |
| Because the sample is conductive, we should use a Shirley type background. Shirley is commonly used, but is not necessarily the optimum background for all peak-fits. Conductors normally require some peak asymmetry due to valence-core interactions. The sample was ion etched and has only a trace of “adsorbed” oxygen so it is impossible to have many metal-oxide peaks. It is very difficult to identify so many different metal oxide species even if the surface really has much more oxygen. Based on the survey spectrum the adsorbed oxygen for this sample is <5 atom%. |  |
 |
Shirley background was used because nickel is conductive. Asymmetry was applied to the main peak because sample is conductive. To avoid over-fitting the data, only two more peaks were added. If we know more about the material, then we might add another peak. But we must explain and identify each peak. Peak “C” has been attributed to a 2-electron process. |
| C (1s) – PET (Mylar) film – IPA cleaned | C (1s) – PET (Mylar) film – IPA cleaned | C (1s) – PET (Mylar) film – IPA cleaned | |
| This peak-fit uses the same FWHM for all peaks which is reasonable as a first try. FWHM are usually in the 1.0 to 1.6 eV range for chemical compounds. Chi-Square is a little high. Baseline region shows a good fit, but the tops of B and C show small but significant gaps. Peak B (30% L) was given a larger amount of Lorentzian tail than Peaks A and C (20% L) to try to make Chi-square smaller. A linear background was used.A clean surface of PET has 3 different chemical states (A,B,C) for Carbon. The empirical ratio of these 3 peak areas is 3: 1: 1. The peak-fit shown here has a 55: 23: 19.2 ratio that reduces to 2.9 : 1.2: 1.0. The difference in the ratios is small, but the question is: Why? |  |
 |
Polymers usually do not have any contamination. But, here is an exception. By adding and constraining a small new hydrocarbon peak, and a small alcohol peak, the peak-fit produced the expected empirical ratio 3: 1: 1 shown in the C (1s) spectrum at the right. Peaks D and E are due to a single pi->pi* shake-up.Be sure not to use too small or too large a FWHM for compounds, A small FWHM for conductors is OK. |
| C (1s) – PET (Mylar) film – IPA cleaned | C (1s) – PET (Mylar) film – cleaned with IPA | C (1s) – PET (Mylar) film – cleaned with IPA | |
| This peak-fit has a small Chi-square which indicates a good peak-fit. Baseline region and the tops of B and C peaks have good fits, no gaps. All peaks use a 80:20 Gaussian:Lorentzian Sum peak shape ratio which is normal for similar chemical states. Variation in the FWHM values is <20% which is a little higher than normal 10%. This indicates a difference in chemical states. The background is linear. This looks like a good peak-fit. However, we know that the material is PET (Mylar). For this reason the data-analyst must constrain the peak area ratios of peaks A, B, and C to produce the empirical 3:1:1 ratio of PET. |  |
 |
Polymers usually do not have any contamination. But, here is an exception. By adding and constraining a small new hydrocarbon peak, and a small alcohol peak, the peak-fit produced the expected empirical ratio 3: 1: 1 shown in the C (1s) spectrum at the right. Peaks D and E are due to a single pi->pi* shake-up. |
| O (1s) – Native Silicon Oxide – as recd | O (1s) – Native Silicon Oxide – as received | O (1s) – Native Silicon Oxide – as received | |
| This peak-fit has 5 synthetic peaks each having a FWHM of 1.00. The chemical shifts are all the same, ~0.7 eV. Chi-square is small, <4. The sample is a native oxide of silicon. In general, FWHM for compounds range from 1.0-1.6 eV. O (1s) FWHM are normally 1.5-1.8 eV. Because the FWHMs are 1.0 eV, this fit is not good. There is no chemical reason for Silicon to have 5 different types of SiOx. Native Oxides usually form mainly the most stable and most oxidized from of the element. If the sample was contaminated, you need to identify chemical state of all 5 peaks. |  |
 |
This peak-fit has 2 synthetic peaks each having a FWHM of 1.7 eV, typical of O (1s). Chi-Square is ~4. Because this is a native oxide, there can be an oxide and a hydroxide or a 2nd type of oxide. We need to think what chemical states are possible. Using many narrow peaks does not match reality, |
| O (1s) – Native Silicon Oxide – as recd | O (1s) – Native Silicon Oxide – as received | O (1s) – Native Silicon Oxide – as received | |
| This peak-fit has 3 peaks each having FWHM of 1.7 eV. Chi-Square is ~5. OK. Chemical shifts are ~0.3 eV. The presence of 3 peaks means there are 3 different chemical states for Oxygen attached to Silicon. This is possible. But chemical shifts are rarely only 0.3 eV apart. It is possible, but we need good reference BEs to use such small shifts. |  |
 |
This peak-fit has 3 peaks each having a FWHM of 1.6 eV, typical of O (1s). Chi-Square is ~2. Great! Because this is a native oxide, there can be an oxide and a hydroxide or a 2nd type of oxide. We need to be able to assign good states. |
| Ce (3d) – CeO2 dried in air from slurry | Ce (3d) – CeO2 dried in air from slurry | Ce (3d) – CeO2 dried in air from slurry | |
| Using basic peak-fitting logic, the data-analysis would count the number of peaks and shoulders and use that as a starting point. That approach produces peaks with large FWHM (see spectrum at right). If we think all FWHM should be similar, then we add more peaks to produce the spectrum at the left.The Ce (3d) signal, like many rare earth signals, has multiplet splittings due to interactions of unpaired electrons. The questions is: How many peaks and what FWHM values should be used? FWHM for Multiplet Splittings is not well documented. CeO2 is often contaminated with low levels of Ce2O3. Theoreticians and experts have produced the plots shown at the far right. This is just a fraction of the info available on Multiplet Splitting in Rare Earths. | . |
 |
Here are two peak-fits with assignments to guide your peak-fits of Ce (3d).  |
| . | . | . |