Sharper insights into thin-film systems

Interfaces in semiconductor components or solar cells play a crucial role for functionality. Nevertheless, until now it has often been difficult to investigate adjacent thin films separately using spectroscopic methods. An HZB team at BESSY II has combined two different spectroscopic methods and used a model system to demonstrate how well they can be distinguished.

Photoelectron spectroscopy (PES) enables the chemical analysis of surfaces and semiconductor layers. In this process, an X-ray pulse (photons) hits the sample and excites electrons to leave the sample. With special detectors, it is then possible to measure the direction and binding energy of these electrons and thus obtain information about electronic structures and the chemical environment of the atoms in the material. However, if the binding energies are close to each other in adjacent layers, then it is hardly possible to distinguish these layers from each other with PES.

 A team at HZB has now shown how precise assignments can nevertheless be achieved: they combined photoelectron spectroscopy with a second spectroscopic method: Auger electron spectroscopy. Here, photoelectrons and Auger electrons are measured simultaneously, which gives the resulting method its name: APECS for Auger electron photoelectron coincidence spectroscopy (APECS). 

Read more on the HZB website

Image: The illustration shows how the APECS measurement works on a nickel single crystal with an oxidised surface. An X-ray beam ionises atoms, either in the nickel crystal or on the surface. The excited photoelectrons from the surface and from the crystal have slightly different binding energies. The Auger electrons make it possible to determine the origin of the photoelectrons. 

Credit: © Martin Künsting /HZB

Expanding horizons with a new instrument

Work is in full swing to construct the new European XFEL instrument SXP. Manuel Izquierdo, who is the Group Leader for SXP since December 2020, gave insights into how the instrument will expand the European XFEL portfolio, when it is set to begin operations and what his vision is for the instrument at this stage.

How would you describe the SXP instrument?

SXP stands for “Soft X-ray Port”. This name was chosen in keeping with the core idea of the project, that is, to provide the users an FEL beamline where they can temporarily set up their own experiment stations. And, this is what makes the instrument unique: users can bring and operate their own experiment stations. This will allow many techniques and experiments to be implemented. The successful proposals would be those that cannot be performed at the two soft X-ray instruments SCS or SQS. So basically, the idea is that the SXP instrument will expand the portfolio of techniques available to users at European XFEL.

What kind of experiments will be performed at SXP? 

In principle it is up to the user community to suggest. So far, three communities have contributed to the project. One community aims to use European XFEL as a laboratory for astrophysics, atomic physics, and fundamental research investigating highly charged ions. A second community proposed studies on chemical bond activation in biological reactions and inorganic catalysts. The third and biggest community aims to perform time and angle-resolved photoelectron spectroscopy experiments in solids. This technique will allow understanding the atomic structure, chemical, electronic and magnetic properties of materials. The counter part for atoms, molecules and clusters can be done at the SQS instrument.

Read more on the European XFEL website

Image: Panorama view of the SASE3 beamline, which feeds SQS and SCS, and will now include SXP

Credit: Photograph by Dirk Nolle (Copyright: DESY)

Unusual reversibility of molecular break-up of PAHs

By combining the high-resolution x-ray photoelectron spectroscopy at the SuperESCA beamline of Elettra with density functional theory a group of scientists from Italy, UK, Denmark and Germany has shown that the process of hydrogen removal from pentacene molecules adsorbed on Ir(111) follows a reversible chemical route, which allows hydrogen re-attachment to the carbon nanoribbon formed after the thermally induced C–H bond break-up. The thermal dissociation taking place upon controlled annealing can be reversed by cooling the system at room temperature and in a hydrogen atmosphere.


Besides the novelty of the chemical process, this phenomenon could have interesting implications for molecular electronics and for the manipulation of graphene nanoribbons which are known to present higher electron/hole mobilities and better thermal transport when dehydrogenated. 

Read more on the Elettra website

A probe of light-harvesting efficiency at the nanoscale

SCIENTIFIC ACHIEVEMENT

Using time-resolved experiments at the Advanced Light Source (ALS), researchers found a way to count electrons moving back and forth across a model interface for photoelectrochemical cells.

SIGNIFICANCE AND IMPACT

The findings provide real-time, nanoscale insight into the efficiency of nanomaterial catalysts that help turn sunlight and water into fuel through artificial photosynthesis.

Solar-fuel tech goes for gold

In the search for clean-energy alternatives to fossil fuels, one promising solution relies on photoelectrochemical (PEC) cells: water-splitting, artificial-photosynthesis devices that turn sunlight and water into solar fuels such as hydrogen. In just a decade, researchers have achieved great progress in the development of PEC systems made of light-absorbing gold nanoparticles (NPs) attached to a semiconductor film of titanium dioxide (TiO2).

Read more on the Advanced Light Source website

Image: Laser pulses were used to excite electrons in gold nanoparticles (AuNPs) on a titanium dioxide (TiO2) substrate. X-ray pulses were used to count the electrons moving between the nanoparticles and the substrate. (Credit: Oliver Gessner/Berkeley Lab)

Laser, camera, action: Ultrafast ring opening of thiophenone tracked by time-resolved XUV photoelectron spectroscopy

Light-induced ring opening reactions form the basis of important biological processes such as vitamin D synthesis, and are also touted as promising candidates for the development of molecular switches. In recent years, new time-resolved techniques have emerged to investigate these processes with unprecedented temporal and spatial resolution.

An international research team from the USA, UK, Germany, Sweden, Australia, and the local team at the FERMI free-electron laser, combined time-resolved photoelectron spectroscopy with high-level electronic structure and molecular dynamics calculations to unravel the dynamics of a prototypical reaction along the full photochemical cycle of a ring molecule (thiophenone) – from photoexcitation, ring opening, all the way through to the subsequent ground state dynamics, and spanning a range of tens of femtoseconds  to hundreds of picoseconds. “These processes have intrigued the photochemistry community for decades” says Prof. Daniel Rolles from Kansas State University “and it is now routinely possible to visualize electronic changes and the movement of atoms in the molecule at each step of a chemical reaction”.

Read more on the ELETTRA website

Image: Artistic rendering of the photo-induced ring opening of thiophenone (left) into several open-ring products (right). The thin white lines show smoothed paths of actual trajectories. Illustration: KSU, Daniel Roles.

PHELIX beamline – delivery of analyzer and spin detector

On July 22, 2020, the last components of the PHELIX end station were delivered to SOLARIS. The delivery included a high-resolution hemispherical photoelectron energy analyzer and a VLEED spin detector.

The PHELIX end station will be exceptional: it will allow scientists to perform circular dichroism measurements (CD-ARPES) and provide direct insights into the spin texture of electron states (SP-ARPES) in the same UHV system and for the same sample. Both of these methods give information about the electron spin, but the interpretation of the CD-ARPES results alone can be challenging. However, the combination of these two methods has a number of advantages allowing for the better understanding of the systems, as it excludes differences in quality between samples and the risk of surface contamination when transferring the sample between experimental systems. Both of these factors significantly affect the obtained results, and the limited control over them reduces the reliability of the research. To our knowledge, the PHELIX beamline will be one of the very few facilities in the world where such combined measurements can be performed.

Read more on the SOLARIS website