Photon Science Roadmap- for Research Infrastructures 2025-2028 by the Swiss Photon Community
Majed Chergui - elected to the European Academy of Sciences
Ruth Signorell - elected to the European Academy of Sciences
Proof of concept ERC Grant for Ursula Keller Dual-comb laser driven terahertz spectrometer for industrial sensing (DC-THz)
Farewell and Welcome!Chris Milne leaves for the European XFEL, Camila Bacellar takes over
SY-GAIA expedition - measures aerosols in the North-Atlantic
Synergy grants for MUST-AssociatesSylvie Roke (EPFL) and Gebhard Schertler (PSI/ETH).
Promotion to Associate Professor of Photonicscongratulations to Rachel Grange!
First light in the SwissFEL Maloja endstation- on track for first experiments in 2021
New scientific highlights- by MUST PIs Chergui, Milne, Wörner, Vaníček and Röthlisberger

A novel way of detecting positive charges (holes) and their trapping in solar materials

February 2, 2018

Revealing hole trapping in zinc oxide nanoparticles by time-resolved X-ray spectroscopy

Transition metal oxides such as Zinc Oxide (ZnO) are at the center of the recent surge in research and development on solar energy conversion into electrical (photovoltaics) or chemical (photocatalytic) forms, but also of applications such as detectors of high-energy radiation. All of these applications rely on the generation of negative (electrons) and positive (holes) charges, and the understanding of their evolution as a function of time is crucial for these applications.

While electrons have been detected by various techniques, holes have so far escaped observation. Various reasons are behind this: the signal of holes is obscured by that of the electrons and/or element-selective strategies cannot be implemented because they require working under vacuum, i.e. conditions which remote from the practical ones, e.g. the solution phase.

The lab of Majed Chergui at EPFL, within the Lausanne Centre for Ultrafast Science, along with scientists from the Paul-Scherrer-Institut and the Argonne National Laboratory (Chicago) have now successfully detected holes and identified their trapping sites after above band-gap photoexcitation using time-resolved element-selective techniques. The researchers used a novel dispersive X-ray emission spectrometer, combined with X-ray absorption spectroscopy. The technique allowed them to directly detect the trapping of holes with a resolution of 80 picoseconds.

The data, supported by computer simulations, revealed that photo-excited holes become trapped in the substrate at singly charged oxygen vacancies. The hole trapping turns the latter into doubly charged vacancies, which causes four zinc atoms around them to move outwards by approximately 15%. The hole traps then recombine radiatively with the delocalized electrons of the conduction band, which generates the green luminescence that is commonly detected when ZnO is used as a detector of high-energy radiation. Identifying the hole traps and their evolution opens up new insights for the future development of devices and nanodevices based on transition metal oxides.

"This is only the beginning," says Majed Chergui. "With the launch of the new Swiss X-ray free electron laser, SwissFEL at the Paul-Scherrer-Institut, a new era is opening before us."


Figure 6: Structural changes. a Schematic of the ZnO structure without the oxygen vacancy, which is the dominant structure probed in the ground state, and (b) structural distortion around the V2+O vacancy, which occurs upon the trapping of a hole at a V+O vacancy and the corresponding outward displacement of the four nearest neighbour Zn atoms (Zn1–4) by ~15%

Reference:  Penfold, T. J., J. Szlachetko, F. G. Santomauro, A. Britz, W. Gawelda, G. Doumy, A. M. March, S. H. Southworth, J. Rittmann, R. Abela, M. Chergui and C. J. Milne (2018). Revealing hole trapping in zinc oxide nanoparticles by time-resolved X-ray spectroscopy. Nature Communications 9: 478. (10.1038/s41467-018-02870-4) Penfold-2018

See also: ScienceDaily, EPFL News, EurekAlert.

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