Key terms associated with surface charging, charge neutralization, and energy referencing in XPS are briefly described for those who may be unfamiliar with them. It should be noted that this discussion is focused on XPS relevant context only. Parts of the definitions below are adaptions of terminology developed by the International Organization for Standardization Technical Committee 201 on Surface Chemical Analysis contained in ISO 18115 part 1 but the descriptions below are provided for information and are not formally accepted or approved definitions.

Sample charging: In the context of XPS measurements, this term refers to the buildup of net charge in a sample due to its exposure to the x-ray beam and, possibly, to other incident particles, e.g., like flood gun electrons or ion beams. Inherently, the ionizing x-ray radiation tends to induce net positive charge. Sample charging can also evolve via other effects, including desorption or adsorption of molecules, temperature changes, and more.

Charging potential: The change in surface potential due to the development of sample charging. The charging potential is directly expressed as a change in the measured binding energy of XPS-detected signals.

Differential charging: A situation commonly encountered, for reasons discussed in this guide, where the spatial distribution of charge is nonuniform and, therefore, different charging potentials affect the spectrum simultaneously. The application of a charge neutralization system does not necessarily eliminate all causes of differential charging and, sometimes, can even magnify them.

Charge neutralization: In response to the charging effect, neutralization of the surface is usually attempted, such as to achieve zero net charge. This process is termed “charge neutralization.” Practically, however, perfect neutralization is very difficult to stabilize and, even worse, very difficult to directly measure and hence be identified. Therefore, partial control over the magnitude of charging is a much more common situation.

Charge compensation: The use of various means to reduce the amount of net charge at the surface and to achieve partial control on its magnitude. Most common is the application of low-energy electrons (via an electron flood gun), with or without low-energy ions of a noble gas, in order to stabilize very low net surface charge.

Charge neutralization system: Components in an XPS spectrometer intended to minimize or control the buildup of charge during an XPS measurement. As described in Sec. II C, these systems usually involve an electron flood gun of some type and may also involve low-energy ions.

Electron flood gun (eFG): Frequently, neutralization is attempted by supplying a flood of low-energy electrons from an eFG. Essentially, these electron sources are designed to supply a broad, large diameter, beam spot, such as to verify a uniform flux of electrons across the (much smaller) analysis area.

Vacuum level: In its electrical context, the vacuum level is defined as the energy of a free stationary electron that is outside of any material (it is in a perfect vacuum).⁶⁰ 

Local vacuum level: This term is an extension of the formal concept of vacuum level. It applies to cases where electrostatic fields dictate different vacuum levels at different spatial locations.⁶¹ In particular, the local vacuum level at the sample surface is frequently different from the one at the detector. Importantly, abrupt changes in the local vacuum level are frequently realized across interfaces within samples, in particular, between compounds of different work function (see below) values; a feature of broad use in devices consisting of electronic materials.

Work function: In its present context, the work function of a given material is the minimal work needed to be done on an electron within that material in order to bring it to the local vacuum level. Alternatively, derived from the latter definition, the work function is equal to the energy difference between the Fermi level and the vacuum level next to the surface of that material.

Fermi level referencing: Setting the energy scale such that its origin coincides with the Fermi level of the sample. Fermi level referencing is the common convention for binding energies, because under contact with the spectrometer, the sample’s Fermi level equalizes with that of the instrument and, hence, with a very reliable and robust reference: the instrument’s electrical ground. Note that the measured photoelectron energies are normally independent of the sample work function, but instead, depend on the detector’s work function, which is in principle a known instrumental parameter.⁶² 

Vacuum level referencing: Setting the energy scale such that the local vacuum level at the sample surface equalizes with the one at an instrumental component (often the eFG). As discussed by many, including Lewis and Kelly,⁶² for insulating and electrically isolated samples, there is no Fermi level alignment and measured photoelectron energies are determined relative to a vacuum level at the spectrometer. This lack of Fermi level alignment can also occur in cases where the sample substrate is metallic, with a good back contact, but an insulating medium on top of the substrate prevents establishing thermodynamic equilibrium between the sample’s surface (where XPS signals are probed from) and the substrate. To maintain the convention of referencing peak energies to the Fermi level in XPS, a correction is needed for the energy scale.⁶² A procedure for extracting the instrumental parameter is described in Ref. 63.

Note that, by historic convention, the electron energies in Auger electron spectroscopy (AES) are referenced to the vacuum level of the spectrometer, because kinetic energies in this process are independent of the excitation source and, therefore, gain broader validity than Fermi-level-based scales for Auger electrons. Comparison of Auger peaks collected during an XPS measurement and observed on the BE scale (hence, Fermi level referencing) with those collected using a dedicated AES instrument requires conversion from the BE scale to a kinetic energy scale and appropriate accounting for the spectrometer work function.^64,65

Charge correction (Δ_corr): This term refers to the correction in energy required due to charging-induced energy shifts. Frequently, when differential charging is encountered, a single Δ_corr is insufficient to correct all observed photoelectron peak energies. Note that this energy scale adjustment is often, somewhat ambiguously, referred to as charge referencing, which is misleading, because referencing in XPS applies to energies, not charge. Practically, charge correction is often attempted using a known internal reference, or an external one like adventitious carbon or gold decoration, all subject to limitations described in this guide.

AdC referencing: Determining the charge correction (Δ_corr) for a sample by comparing the experimentally measured C 1s binding energy of hydrocarbons adsorbed on the sample surface, with a standard binding energy value associated with these molecules. As discussed in Sec. III A, the standard value is not necessarily known a priori, due to system specific variations in the hydrocarbon C 1s binding energy. Yet, values are limited to remain within a range of a few electron volts at the most, usually less than ±1 eV, such that a rough energy scale calibration can be verified, which is particularly helpful in extreme cases of charging.

Internal referencing: Determining the charge correction (Δ_corr) for a specific sample by comparing the experimentally determined binding energy of an element (or elements) in a known chemical state in that sample, to a standard binding energy value for that signal. Using the C 1s of a specific group within the sample may be such an example, but, in general, internal references are sample specific. There are several reasons and circumstances for which internal referencing has limited accuracy, often including multiphase and other complex samples. As with other energy referencing methods, it needs to be used with appropriate care and an evaluation of data consistency (Sec. III E).

Gold decoration: Use of a very small quantity of gold with an assumed binding energy of 84.0 eV, deposited as unconnected islands on an insulator, for establishing the charge correction (Δ_corr). Limitations of this method are discussed in Sec. III B.

Peak position: The peak position of a given spectral line refers to the energy at which the signal intensity is maximal. This term, which is common to spectroscopy, in general, and normally used for expressing the related measured core-level binding energy, should be considered carefully when differential charging is encountered, and line shape distortions are encountered.^22,29

Correct use of terminology is important for reliable and reproducible reporting of XPS measurements.²⁸ The full set of this terminology (ISO 18115 part 1) is available at no cost from several websites.^65,66 This terminology has been developed by many people over four decades and is now undergoing a systematic review. Several terms are being updated based on new or revised concepts and identification of inconsistencies that have evolved over time. Suggestions for improvements, clarification, or additional terms are most welcome and can be made to the chairman or secretaries of ISO TC201 or the terminology subcommittee ISO TC201 SC1.