Concepts for chemical state analysis at constant probing depth by lab‐based XPS/HAXPES combining soft and hard X‐ray sources

Abstract

The greater information depth provided in Hard X-ray Photoelectron Spectroscopy (HAXPES) enables non-destructive analyses of the chemistry and electronic structure of buried interfaces. Moreover, for industrially relevant elements like Al, Si and Ti, the combined access to the Al 1s, Si 1s or Ti 1s photoelectron line and its associated Al KLL, Si KLL or Ti KLL Auger transition, as required for local chemical state analysis on the basis of the Auger parameter, is only possible with hard X-rays. Until now, such photoemission studies were only possible at synchrotron facilities. Recently however, the first commercial XPS/HAXPES systems, equipped with both soft and hard X-ray sources, have entered the market, providing unique opportunities for monitoring the local chemical state of all constituent ions in functional oxides at different probing depths, in a routine laboratory environment. Bulksensitive shallow core-levels can be excited using either the hard or soft X-ray source, whereas more surface-sensitive deep core-level photoelectron lines and associated Auger transitions can be measured using the hard X-ray source. As demonstrated for thin Al2O3, SiO2 and TiO2 films, the local chemical state of the constituting ions in the oxide may even be probed at near constant probing depth by careful selection of sets of photoelectron and Auger lines, as excited with the combined soft and hard X-ray sources. We highlight the potential of lab-based HAXPES for the research on functional oxides and also discuss relevant technical details regarding the calibration of the kinetic binding energy scale. Supporting information:Advanced chemical state studies of oxide films by lab-based HAXPES combining soft and hard X-ray sources

Hard x-ray photoelectron spectroscopy: a snapshot of the state-of-the-art in 2020

“…The largest information depth occurs at the highest kinetic energy and therefore in the valence region, whereas deep core levels occur at lower kinetic energy and thus have smaller information depth. For TiO 2 , measured with Cr kα radiation the difference in information depth (3λ) can reach up 20 nm, [90] and may be even more pronounced for higher excitation energies. One strategy to obtain near-constant probing depth at one excitation energy it to use the angular dependence of the escape depth of the detected photoelectrons (see detailed discussion in section 3.1.5).…”

“…Laboratory-based HAXPES systems have exploited the accessibility of additional, deeper core levels to study depth-dependent phenomena in both bulk as well as multilayer systems. [85][86][87][88][89][90][91] A general advantage not just of laboratory systems, but any HAXPES experiment, is the ability to access deeper core levels opening up new experimental and analytical strategies. [43,[92][93][94][95] Figure 4 gives a schematic overview of core levels of transition metals and lanthanides that become available when moving A c c e p t e d M a n u s c r i p t…”

“…The ability to excite deep core levels consequently also enables the measurement of the associated Auger transmissions. [90,100] For many industrially relevant materials such as Al, Si, Ti and their corresponding oxides and nitrides, the 1s core level is not accessible using monochromated Al Kα radiation (see Figure 4). However, access to these lines enables chemical state analysis using the Auger parameter concept, which is very sensitive to changes in the local chemical environment and often more robust than the observation of core level binding energy shifts alone.…”

“…In singlesource HAXPES systems comparatively sharp Auger lines, such as the Cu L 2,3 M 4,5 M 4,5 , can provide alternative reference lines at the lower limit of the kinetic energy scale. [90] 2.2.1. PHI Quantes The first beta HAXPES system from Physical Electronics (PHI Quantes) was shipped in February 2016.…”

X-ray Photoelectron Spectroscopy in Mineral Processing Studies

“…The high-energy photoemission spectroscopy, or hard XPS (HAXPES) [128,153], most often utilizes the photon energies tunable from 2 keV to 10 keV at many synchrotron facilities and characterizes the layers extending up to 20-30 nm in depth. Commercial instruments employing hard X-ray sources, particularly with Cr (5.417 keV) and liquid Ga (9.243 keV) anodes [154,155], are developed now too. The HAXPES is an effective tool for destruction-free examination of multilayered structures, and surfaces and interfaces buried under reaction products, which provides information about both the concentration as a function of the excitation photon energy (depth profiles) and the depth-resolved chemical form of elements [130].…”