What to expect when searching for quantitative information using XPS?

In the particular moment of surface investigation when quantitative analysis of the material is required, one usually chooses the XPS technique. There are many advantages and straightforward approaches to that path but only when: the sample is uniform in depth (monophase), the sample is laterally uniform, the sample has appropriate dimensions and corrugation. XPS line intensities of all the elements present in the surface region probed with incidence X–rays obtained during the measurement of XPS spectra, in combination with atomic sensitivity factors (ASFs), serve for quantitative characterization using a standard approach. The obtained set of values corresponds to relative elemental concentrations and this is acceptable information but only when you have a monophase sample.

In the majority of real situations, you need to deal with multiphase or multilayer structures. Why is that? In the contact with air, bulk material becomes covered with a thin oxide layer due to air exposure. Besides, any air-exposed sample will be also covered with a thin layer of organic impurities on top of the oxide layer. While properties of the contamination layer usually are of no interest, the composition and thickness of the oxide layer are highly relevant in numerous applications. So, if you want to really understand and know the surface structure you often need to search a little bit more. Two of the possible commonly used approaches for in-depth analysis is the application of AR–XPS technique, or successive cycles of sputtering and XPS measurement. Instead, without dimension and corrugation limitations, and surface damage due to sputtering, we suggest you try our proposed method with some calculations using a fundamental equation for the XPS line intensity.

"The beginning is the most important part of the work."

Plato in The Republic

This model is developed based on first principles for the investigation in–depth of non–uniform surface structure and provides you with plenty of new information. The model is greatly useful in order to determine the specific surface structure of samples, where using atomic sensitivity factors is not possible. Application of this model, followed by additional results submitted by XPS and optionally Auger spectrum analysis, enables the determination of exact in-depth sample structures. For example, if you have the common case of double-layer structure, with layers of hydrocarbons and oxides on the surface of the material, especially when the sample is air exposed for a long time, you need to deal with three different regions. The first region is the bulk of the material, the second one is the oxide layer with bulk constitutive elements oxides, and as the third region, we have a contamination layer which is made of hydrocarbons. Every one of these layers has its own thicknesses and transport characteristics with respect to photoemitted electrons traveling through it. Having that in mind for every single element of every region you have the equation for the intensity of the XPS line. Using these equations and iterative processes, chemical bonds present in each of the three surface areas – bulk, oxide, and contamination layer, as well as samples surface layer thicknesses, can be determined.