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

Nanostructures of transition metal oxides (TMO), such as ZnO, have attracted considerable interest for solar-energy conversion and photocatalysis. For the latter, trapping of charge carriers has an essential role. The probing of electron trapping in the conduction band of room temperature photoexcited TMOs has recently become possible owing to the emergence of time-resolved element-sensitive methods, such as X-ray spectroscopy. However, because the valence band of TMOs is dominated by the oxygen 2p orbitals,holes have so far escaped observation. Herein we use a novel dispersive X-ray emission spectrometer combined with X-ray absorption spectroscopy to directly probe the charge carrier relaxation and trapping pro-cesses in ZnO nanoparticles after above band-gap photoexcitation. Here, our results, supported by simulations, demonstrate that within our temporal resolution of 80 ps, photo-excited holes are trapped at singly charged oxygen vacancies, turning them into doubly charged vacancies, which causes an outward displacement by approximately 15% of the four surrounding Zn atoms away from the central vacancy. These traps recombine radiatively with the delocalised electrons of the conduction band yielding the commonly observed green luminescence. This identification of the hole traps and their evolution provides new insight for future developments of TMO-based nanodevices.
Authors:
 [1] ;  [2] ;  [3] ;  [4] ; ORCiD logo [5] ;  [6] ;  [6] ;  [6] ;  [3] ;  [7] ; ORCiD logo [3] ; ORCiD logo [7]
  1. Newcastle Univ., Newcastle upon Tyne (United Kingdom). Chemistry-School of Natural and Environmental Sciences
  2. Paul Scherrer Inst. (PSI), Villigen (Switzerland). SwissFEL; Polish Academy of Sciences (PAS), Krakow (Poland). Inst. of Nuclear Physics
  3. Ecole Polytechnique Federale Lausanne (Switzlerland)
  4. European X-ray Free-Electron Laser (XFEL), Schenefeld (Germany); Hamburg Centre for Ultrafast Imaging, Hamburg (Germany)
  5. European X-ray Free-Electron Laser (XFEL), Schenefeld (Germany); Adam Mickiewicz Univ., Poznan (Poland). Faculty of Physics
  6. Argonne National Lab. (ANL), Argonne, IL (United States)
  7. Paul Scherrer Inst. (PSI), Villigen (Switzerland). SwissFEL
Publication Date:
Grant/Contract Number:
AC02-06CH11357; 2015/18/E/ST3/00444; 2016/ 22/E/ST4/00543
Type:
Accepted Manuscript
Journal Name:
Nature Communications
Additional Journal Information:
Journal Volume: 9; Journal Issue: 1; Journal ID: ISSN 2041-1723
Publisher:
Nature Publishing Group
Research Org:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Chemical Sciences, Geosciences & Biosciences Division; Swiss National Science Foundation (SNSF); National Science Centre, Poland (NCN); German Research Foundation (DFG); European Research Council (ERC)
Country of Publication:
United States
Language:
English
Subject:
77 NANOSCIENCE AND NANOTECHNOLOGY; Nanoparticles; Optical materials; X-rays
OSTI Identifier:
1429363