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 (1 picosecond is a millionth of a millionth of a second).

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."

Research Report: Edoardo Baldini, Tania Palmieri, Thomas Rossi, Malte Oppermann, Enrico Pomarico, Gerald Aubock, Majed Chergui. Revealing hole trapping in zinc oxide nanoparticles by time-resolved X-ray spectroscopy. Nature Communications 02 February 2018. DOI: 10.1038/s41467-018-02870-4

Related Links
Ecole Polytechnique Federale de Lausanne
All About Solar Energy at SolarDaily.com

Tweet

Thanks for being here; We need your help. The SpaceDaily news network continues to grow but revenues have never been harder to maintain. With the rise of Ad Blockers, and Facebook - our traditional revenue sources via quality network advertising continues to decline. And unlike so many other news sites, we don't have a paywall - with those annoying usernames and passwords. Our news coverage takes time and effort to publish 365 days a year. If you find our news sites informative and useful then please consider becoming a regular supporter or for now make a one off contribution.
SpaceDaily Contributor $5 Billed Once credit card or paypal		SpaceDaily Monthly Supporter $5 Billed Monthly paypal only

Octopus completes refinancing of subsidy free Italian solar projects
London, UK (SPX) Feb 02, 2018
Octopus Investments ("Octopus"), part of the Octopus Group and the largest commercial solar investor in Europe, has completed the refinancing of 5 solar projects with 64MW of generation capacity near to Montalto di Castro in the Lazio region of Italy. The 23m euros financing package has been provided by MPS Capital Services Banca per le Imprese (MPSCS) and represents a new landmark deal for solar in Europe - securing project finance debt against solar projects without government subsidy. Ita ... read more