May 20-22, 2009

Workshop Highlights New Opportunities in Hard X-ray Photoelectron Spectroscopy

The third international workshop on hard x-ray photoelectron spectroscopy (HAXPES), held at Brookhaven Lab from May 20–22, 2009, brought together about 100 scientists from 14 countries to discuss progress and future prospects for this rapidly developing field of research. The three-day workshop was sponsored by the NSLS, the National Synchrotron Light Source-II Project, Brookhaven’s Condensed Matter Physics and Materials Science Department, and the National Institute of Standards and Technology.

The primary theme of the meeting was the newly emerging use of higher energy multi-keV x-ray photons for photoemission experiments that have historically used only the vacuum ultra-violet/soft x-ray region of the electromagnetic spectrum that is limited to ca. 1 keV. The talks considered both the opportunities and the challenges that utilizing this portion of the electromagnetic spectrum brings. The principal driver for such experiments is the longer inelastic electron mean-free paths that higher energy photons permit. Short inelastic electron mean-free paths have historically limited photoelectron spectroscopy to a surface science tool, probing only the first 1–2 nm of a sample. As was clear from the large number of presentations given at the meeting, both in situ and ex situ studies of real bulk systems and their chemical and electronic structures are now routinely being performed world-wide using HAXPES.

Participants in HAXPES 2009

The specific sessions of the conference included: "New Sources," "Electronic and Atomic Structure," "Ambient Pressure, Gas Phase, and Atomic Properties," "Microscopy and Analyzers," "Theory," "Industrial Applications I," "Industrial Applications II," "Photoemission with other Techniques," and "Correlated Materials." A special aspect of the meeting, triggered by restrictions on travel from Japan due to the HINI virus, was several remote talks by Japanese colleagues via a SPring8/NSLS internet connection.

The meeting began with a plenary lecture given by George Sawatzky from the University of British Columbia. George discussed new opportunities for HAXPES, drawing on his significant contributions to the field of electronic structure. The second plenary lecture was given by Hiroshi Yanagi from the Tokyo Institute of Technology on the topic of new transparent oxide semiconductors. The third lecture was given by Shigemasa Suga from Osaka University on the topic of Kondo resonances in rare earth compounds and metal insulator transitions. All plenary lectures succeeded in setting a high bar for the remaining talks of the day. The dinner lecture, held at Danfords Hotel and Marina, was given by Dennis Martinez-Galarce from Lockheed Martin Advanced Technology Center with the title, "From Nano to Galacto." Dennis presented the history of EUV/x-ray astronomy and showed exciting images obtained from several NASA (and ESA) orbiting astro- and solar-physics satellites. Concluding remarks and a general overview of the present status and future direction of HAXPES was given by Charles Fadley from the University of California at Davis and Lawrence Berkeley National Laboratory (LBNL).

The new sources session was opened by Steven Dierker, director of the NSLS-II Project, which, when completed, will be the brightest synchrotron in the world and an excellent opportunity for developing insertion device HAXPES beamlines. The audience then heard from Yong Cai, also of NSLS-II, about obtaining millivolt resolution at NSLS-II, and from Tien-Lin Lee of the Diamond Light Source and Yongfeng Hu of the Canadian Light Source about their facility plans for HAXPES. With the exception of the National Institute of Standards and Technology’s beamline X24A at the NSLS, HAXPES development has been centered at facilities outside of the United States. Hopefully, the NSLS-II will be a future site of HAXPES development.

The two industrial sessions featured talks given by Simon Bare of UOP, who spoke about the utilization of variable kinetic energy HAXPES for depth resolution in catalyst characterization; Shigeaki Zaima of Nagoya University and Giuseppina Conti of Applied Materials, who demonstrated the utility of HAXPES in probing the entire film stack of layered semiconductor dielectric structures; Patrick Lysaght of SEMATECH, who spoke about chemical interactions between thin buried layers in advanced metal oxide semiconductor structures; and Gary Mitchell of Dow Chemical, who gave an overview of industrial problem solving using HAXPES.

A variety of topics were presented in the session on ambient pressure, gas phase, and atomic properties session. Hendrik Bluhm, of LBNL, discussed the potential opportunities for the use of hard x-rays in photoemission at "ambient pressures" of several Torr due to the increased path length of the photoelectrons through solids, liquids, and gases. In addition, Marc Simon, of the SOLEIL synchrotron facility, and Dennis Lindle, of the University of Nevada, discussed the physics of the interactions of high-energy photons with atoms and molecules, in particular, non-dipole effects in photoionization cross sections.

In the correlated materials and electronic and atomic structure sessions, the audience heard from Wolfgang Drube, of HASYLAB, who gave an overview of work currently being performed at DESY and a detailed look at future HAXPES plans at PETRA III. Hao Tjeng, of the University of Cologne, spoke on the use of the energy dependence of orbital photoelectron cross sections to untangle density of states in strongly correlated transition metal oxides. Giancarlo Pianaccione, of TASC – INFM CNR Trieste, spoke of bulk measurements of core-level and the near Fermi-level spectra for correlated materials and the additional information provided by HAXPES both as a complement and an improvement to lower-energy photoemission measurements. The audience also heard from Ashish Chainani, of RIKEN/SPring-8, who spoke of HAXPES studies on metal-insulator transitions; Claudia Felser, of the University of Mainz, who discussed HAXPES applications to Huesler alloys; and Mihaela Gorgoi of the Helmholtz Center who reviewed a variety of activities on the HIKE facility at BESSY.

In the microscopy and analyzers session, Raymond Browning of R. Browning Consultants opened with a discussion of his new approach to improve performance and spatial resolution of a HAXPES microscope to be used at the new NSLS-II source. Eiji Ikenaga discussed new developments of HAXPES at Spring-8. Several vendors, including FOCUS GmbH, MB Scientific AB, SPECS GmbH, and VG Scienta Inc. also gave presentations on their latest designs and the state-of-the-art for high-energy x-ray photoelectron spectrometers. These talks, together with others in the workshop (see below), provided a complete overview of the current status in electron spectrometer design and instrumentation, as well as new beamlines and facilities that are being planned and constructed.

Joerg Zegenhagen from the ESRF opened the photoemission with other techniques session about his work combining HAXPES and x-ray standing waves to determine position sensitive atomic and electronic structure information. Munetaka Taguchi from RIKEN/SPring-8 then spoke of satellite features in core-and valence-level HAXPES from strongly correlated materials, and Shigenori Ueda from the National Institute for Materials Sciences spoke about the advantages of using magnetic circular dichroism HAXPES to obtain magnetic information.

The theory session opened with a talk by Hubert Ebert, from the University of Munich, on angle-resolved photoemission in the x-ray regime as calculated in the one-step model. The audience also heard from Yosuke Kayanuma, of Osaka Prefecture University, on the recoil effect, learning that the momentum of the photoelectron becomes important at the higher photon energies. John Rehr, from the University of Washington, then discussed a new many-pole description of photoemission. There were also talks focusing on the inelastic electron mean-free path due to its importance in dictating the sampling depth of any photoemission experiment, including talks by Cedric Powell, of the National Institute of Standards and Technology, and by Shigeo Tanuma, of the National Institute for Materials Science. The audience also heard about angle-resolved data at HAXPES energies by Christian Papp of LBNL. This data was coupled with theoretical support that demonstrated the feasibility of mapping bulk band structure from a three-dimensional metal with an unprepared surface.

The workshop clearly demonstrated that hard x-ray photoemission is rapidly developing as a powerful new technique that is complementary to photoemission at lower energies. Of paramount benefit is the ability to probe non-destructively more deeply into any material, thus deriving more truly bulk-related properties. From an industrial point of view, this means being able to take real samples from air, introduce them into a measurement system, and study them without any special surface treatment. HAXPES experiments are also ideally suited to exploring the buried layers and interfaces in multilayer structures that are ubiquitous in nanoscience and nanotechnology; this capability is enhanced by exploiting the energy tenability of the inelastic mean-free path to match the length scales of the technological system of interest. Further benefits lie in the generally reduced inelastic scattering backgrounds under spectra and the simpler interpretation of core-level intensities for quantitative analysis. Other HAXPES advances that will be critical for fundamental science and technology include angle-resolved HAXPES and bulk-sensitive band mapping, the interpretation of which should be simpler than at lower energies, core-level magnetic circular dichroism, the detection of satellite features that are unique to highly delocalized bulk screening effects in core- and valence-level HAXPES spectra from highly correlated materials, and photoelectron diffraction at HAXPES energies that promises to solve local atomic structures in complex materials.

The workshop was also balanced in discussing the challenges of going to higher energies. For example, one needs to compensate for the inherently lower photoelectron cross sections at higher energies with special high-brightness sources, which in turn leads to the need for high-resolution, high-energy, and high-intensity x-ray beamlines and spectrometers, which have now been achieved in several laboratories with resolutions in the 50 – 200 meV range and likely improvements in the future. It was also noted that non-dipole effects need to be considered in modeling intensities, but these effects are already included in many existing theoretical codes. The presence of recoil shifts due to the momentum of the high-energy photoelectrons must be considered for studies of light elements, but these will often be negligible for heavier atoms. The effect of phonon creation and annihilation on momentum conservation when studying valence levels is also an important effect which requires cryogenic cooling and limits how high in energy bands can be mapped. However, none of these complications are a barrier to a vastly expanded research role for HAXPES. Indeed, with the sources being developed it is clear that HAXPES has a bright future for the characterization and study of a broad variety of materials and devices. In view of the rapid development in the field, it was decided to hold the next HAXPES workshop in 2011 at DESY in Hamburg, with Wolfgang Drube as chair.

For more information and the complete list of workshop presentations with downloadable links, please visit the conference website.