Challenges in surface analysis

The most commonly used technique for surface analysis is XPS as it provides the simplest spectrum and is the easiest to quan fy. XPS instruments also have a much lower cost than AES, SIMS, etc., so when groups are financially constrained, they tend towards XPS. In most cases, XPS also provides excellent informa on on the chemical state of surface atoms. AES can some mes provide superior chemical informa on, such as the chemical state of carbon on metal surfaces (Haas et al., 1972;Hooker and Grant, 1977). While XPS and AES do not directly detect hydrogen and helium in materials, the effect of hydrogen on other elements in the surface can some mes be observed with XPS (Smentkowski et al., 1995) and AES (Bevolo, 1985). On the other hand, SIMS can detect all elements as well as dis nguish isotopes; spectra can be quite complex as large molecular fragments from the material are also formed. Isotope detec on can be very useful when an oxygen beam is used for analysis, where oxygen from the surface of the material can be dis nguished from the oxygen in the beam.The number of papers published each year in AES and SIMS has been fairly constant for the past 20 years, whereas those published in XPS con nue to increase. This is illustrated in Figure 1, which is plo ed on a logarithmic scale to be er show their growth since the early years. The publica ons using the terms ESCA, PES, HAXPES, NAP-XPS, ARXPS (angle resolved XPS), and ARPES (angle resolved PES) have been included along with those using XPS to compare with the other techniques. Figure 2 illustrates the use of the different terms for XPS and shows that ESCA was the preferred term in the 1960s and 1970s, but was overtaken by the term XPS in the 1980s and remains dominant. The term PES is o en used by those using X-rays from synchrotrons; the other terms are more specific and generally have lower numbers of publica ons.Surface analysis grew rapidly in the late 1960s and early 1970s with the introduc on of commercial instrumenta on that was largely spearheaded by industrial research laboratories (Smentkowski, 2014). Instruments for surface analysis improved in the decades that followed, and s ll con nue to improve, especially for XPS. Spa al resolu on in XPS instruments is now in the 1 to 10 μm region, and resolu ons as low as 150 nm are obtainable at synchrotrons. Spa al resolu ons are much higher for AES and SIMS instruments, as the electron and ion beams can be focused to small areas. Many instruments have a high degree of automa on and can be operated remotely.The number of manufacturers of commercial AES and SIMS instruments has decreased over the decades, although there is s ll compe on between manufacturers of these instruments. Some of the early manufacturers of XPS instruments stopped making them, par cularly in the 1970s and 1980s, but others remained and expanded their offerings. Today, there is good compe on between manufacturers of XPS instruments, with many of them offering improved and versa le instruments. For example, hard X-ray photoelectron spectroscopy (HAXPES, also called hard X-ray photoemission spectroscopy), which was usually done using hard X-rays produced at synchrotrons, can now be done in the laboratory using silver, chromium or gallium X-ray sources instead of the usual aluminum (and magnesium) X-ray sources. The number of publica ons on HAXPES has reached about 100 per year, and is expected to con nue to increase, par cularly since the hard X-rays allow greater analysis depths and studies of deeper interfaces. Besides being able to probe higher binding energy core levels, HAXPES also reduces the effects of surface contamina on, and preferen al spu ering effects from ion-beam spu ering.Some of the early XPS/ESCA instruments did not use ultra-high vacuum, and surfaces that had been cleaned in-situ by argon ion bombardment quickly adsorbed gases from the vacuum system and were contaminated. This precluded basic research studies of the adsorp on of pure gases on reac ve metals, such as tanium and tungsten, in such systems. For several decades, all XPS instruments relied on ultrahigh vacuum chambers. However, a great advance in XPS has been the introduc on of commercial, near ambient pressure, XPS instruments (NAP-XPS). These instruments allow the chemical analysis of surfaces in reac ve working environments and are well suited to studies of corrosion, microorganisms and bacteria. The number of publica ons on NAP-XPS has been about 50 per year for the past five years, and papers using NAP-XPS are especially welcome.Although acquisi on so ware is fairly mature, allowing experiments to be put in a queue with their own acquisi on parameters and incident beam parameters, data processing so ware has many shortcomings. Even for XPS which has the simplest spectrum, peak iden fica on using so ware is not always reliable with peaks some mes incorrectly iden fied, or unable to be iden fied at all. Although some peak iden fica on so ware claims to look for confirming peaks in a spectrum, this is not always successful. Another issue is that rela ve intensi es of peaks from the same element or species are not checked in the so ware. Some manufacturers of instruments claim that their automa on is so good that the specimen can be analyzed without user input (other than se ng prior loca ons for analysis), and a composi on report can be generated and printed automa cally. This is o en not possible and such reports o en contain errors. Organiza ons such as ISO con nue to look at methods for improving data analysis, and this is a major challenge for the manufacturers and independent so ware writers.Peak fi ng (or curve fi ng) is one of the most commonly used procedures in XPS, yet it is unfortunate that in about 40% of the papers where peak fi ng is used show incorrect fi ng of peaks (Major et al., 2020). This is one of the biggest challenges in the XPS field and is due to the lack of understanding about peak shapes. For example, in many metals where the photoelectron peaks are asymmetrical, users tend to add addi onal, unnecessary, symmetrical peaks to obtain be er peak fits instead of using an appropriate asymmetrical line shape. Another issue is that constraints in peak fi ng are o en not used, resul ng in incorrect rela ve intensi es for doublets with well-known rela ve intensi es, and known peak separa ons. On the other hand, constraints are some mes used incorrectly such as se ng the fullwidth at half the maximum peak height (FWHM) of doublet peaks to be iden cal to each other, which is o en not the case. For example, the Ti 2p 1/2 FWHM is about 20% larger than the FWHM of the Ti 2p 3/2 peak. The challenge here is to be er explain the parameters chosen in peak fi ng and the jus fica ons for them.There is a great need for open access to papers in surface analysis, and it is the aim of this journal to meet these needs. Since the papers are published online, there is no need to curtail the presenta on of the work and the discussion of the results, as is the case with most journals. This also eliminates the need for "Supplementary Material" which is accessed separately from a main paper. Research must be reproducible and details of the experiment are necessary to accomplish this goal.Another advantage for publishing in this journal is that the papers stay alive. The publica on pla orm allows post-publica on commen ng and discussion on ar cles even a er the peer-review process. The name of the Associate Editor that handled the review process and the names of the reviewers are published with the paper.The journal seeks high-quality papers on surface analysis including all aspects of:  XPS (including HAXPES, synchrotron-based work, NAP-XPS and ARXPS/ARPES)  AES  SIMS (including D-SIMS, G-SIMS, ToF-SIMS and SNMS)  electron energy loss spectroscopy (EELS, REELS)  surface structure and diffrac on  imaging of surface composi ons  ion bombardment and spu ering  thin film and interface studies (including destruc ve methods such as spu er depth profiling, and non-destruc ve methods such as HAXPES, ARXPS, and signal backgrounds)  instrumenta on developments  data processing  quan ta ve analysis  matrix effects, and  studies on metals, alloys, compounds, polymers, organic materials, semiconductors, ceramics, biomaterials, fibers, spherical par cles, and nanomaterials.Industrial applica ons are welcome, especially where surface analysis techniques have been used to improve products and solve manufacturing issues. Papers on surface analysis standards, and guides for using and interpre ng surface analysis methods also fall within the scope of the journal. Finally, special research topics, such as papers presented at workshops and conferences, and collec ons of papers honoring esteemed careers are welcome. Let us all work together to move the surface analysis field forward.Figure 1. The number of publica ons per year as a func on of calendar year for some common surface analysis methods. Note that "XPS etc" includes the acronyms ESCA, PES, HAXPES, NAP-XPS, ARXPS and ARPES, and the number of papers per year has been increasing rapidly compared to the other methods. Data are from the Web of Science TM .Figure 2. The number of publica ons per year as a func on of calendar year for X-ray photoelectron spectroscopy, showing the number of publica ons using the acronyms XPS, ESCA, PES, HAXPES, NAP-XPS, ARXPS and ARPES separately, based on the name chosen in the publica on. Data are from the Web of Science TM .