Journal of Electron Spectroscopy and Related Phenomena 190 (2013) 278–288 Contents lists available at ScienceDirect Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec High energy photoelectron spectroscopy in basic and applied science: Bulk and interface electronic structure Ronny Knut a , Rebecka Lindblad a , Mihaela Gorgoi b , Håkan Rensmo a , Olof Karis a,∗ a Department of Physics and Astronomy, Uppsala University, SE-751 21 Uppsala, Sweden b Helmholtz Zentrum Berlin für Materialien und Energie GmbH, Albert-Einstein-Str. 15, 12489 Berlin, Germany article info Article history: Available online 8 September 2013 PACS: 79.60.Jv 71.20.Be 73.20.-r Keywords: Hard X-ray photoelectron spectroscopy Materials for energy Bulk properties Multilayers Interface properties Alloying Electronic structure abstract With the access of new high-performance electron spectrometers capable of analyzing electron energies up to the order of 10 keV, the interest for photoelectron spectroscopy has grown and many new applica- tions of the technique in areas where electron spectroscopies were considered to have limited use have been demonstrated over the last few decades. The technique, often denoted hard X-ray photoelectron spectroscopy (HX-PES or HAXPES), to distinguish the experiment from X-ray photoelectron spectroscopy performed at lower energies, has resulted in an increasing interest in photoelectron spectroscopy in many areas. The much increased mean free path at higher kinetic energies, in combination with the elemental selectivity of the core level spectroscopies in general has led to this fact. It is thus now possible to investi- gate the electronic structure of materials with a substantially enhanced bulk sensitivity. In this review we provide examples from our own research using HAXPES which to date has been performed mainly at the HIKE facility at the KMC-1 beamline at HZB, Berlin. The review exemplifies the new opportunities using HAXPES to address both bulk and interface electronic properties in systems relevant for applications in magnetic storage, energy related research, but also in purely curiosity driven problems. © 2013 Published by Elsevier B.V. 1. Introduction The surface sensitivity of X-ray photoelectron spectroscopy (XPS, PES) when performed with photoelectrons having kinetic energies in the range of 50–1000 eV has been key to many impor- tant contributions in surface science (for example, see Ref. [1]). On the other hand, the surface sensitivity can also be problematic when bulk properties are sought or when properties of buried structures and interfaces are investigated. Often samples needs to be prepared in situ to minimize contributions from surface contamination, as the probing depth of ‘conventional’ photoelectron spectroscopy is in the range of 1 nm. If the sample is sensitive to oxidation it needs a protective capping layer or the top layers will not be represen- tative for the bulk properties. In either case, the top layers needs to be removed which is difficult without affecting the sample. To study structures which are buried deeper than a few nanometers is not possible using traditional XPS [2–7]. However, as the kinetic energy of the emitted photoelectron is increased the inelastic mean free path is also increased and can approach tens of nanometers for kinetic energies in the range of 4 keV and higher. During the last decade there has been a large ∗ Corresponding author. Tel.: +46 704250329. E-mail addresses: olof.karis@physics.uu.se, olof.karis@gmail.com (O. Karis). increase in the use of hard X-ray photoelectron spectroscopy (HAX- PES; HX-PES, HIKE are common synonyms). With the availability of new bright synchrotron sources and monochromators with a resolving power in the range of 10 5 combined with electron ana- lyzers capable of analyzing electron energies in the range of 10 keV with meV resolution, technology has finally reached the neces- sary level of perfection to fully enable the potential pointed out by Lindau et al. already in the seventies [8]. Since the first report of HAXPES using contemporary instrumentation [9,10], the method has rapidly developed into a promising tool to address electronic properties of buried interfaces and bulk layers [3–5,11–35], as it is one of the few methods that enable non-destructive bulk sensitive studies. The great advantage of HAXPES is the accurate measure- ment of shifts in core-level binding energies of bulk atoms, which reflect changes in chemical environment and give us information about the intermixing of interface atoms and alloying of the multi- layers. In this article we review some of our work on analyzing bulk and interface properties in several systems using photoelectron spectroscopy at excitation energies in excess of 2 keV. Regarding interface studies, we will provide examples from metallic multilay- ers relevant for magnetic sensors, and from interfaces in molecular solar cells. To exemplify how HAXPES can be used to address bulk- like electronic properties we have again chosen examples from both spin-based electronics and energy relevant materials, but also 0368-2048/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.elspec.2013.08.007