Abstract

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Charge transport, trapping and decay in metal oxides

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Charge transport, trapping and decay in metal oxides


M. Chergui1

1Laboratoire de Spectroscopie Ultrarapide, Ecole Polytechnique Fédérale de Lausanne, Switzerland

Titanium dioxide (TiO2) and zinc oxide (ZnO) are among the most popular materials in solar energy conversion processes, either into electrical or chemical energy. These are entirely based on the generation of charge carriers (electrons and holes) by absorption of light, their transport and eventually, their localization due to the electron-phonon coupling and/or defects. In order to investigate these processes, element and geometry sensitive tools are required at high temporal resolution and at the working conditions of solar devices (i.e. room temperature). To this aim, we implemented picosecond (ps) and femtosecond (fs) X-ray absorption spectroscopy at the K-edges of Ti and Zn, on colloidal solutions of nanoparticles (NP) of typically ~20 nm diameter. In the anatase form of TiO2, the NPs are known to have an ordered core with a defect-rich surface shell containing a high degree of Ti under-coordination and since the spectral changes pointed to the reduced centres being in an amorphous-like environment, it was also concluded that these traps are mostly localized at pentacoordinated trapping site located in the shell region.[1] Using the slicing scheme at the SLS (Villigen), we find that electron trapping occurs within 200 fs, implying that the electron does not migrate prior to trapping.[2] ZnO NPs were studied by ps and sub-ps XAS, X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) at both synchrtrons and X-ray Free electron lasers. Time-resolved XAS results measured both 100 ps (Advanced Photon Source) and 2 ps (SACLA X-FEL) after photo excitation show clear signatures of structural changes due to the trapped electronic excitation in the nanoparticle. Our simulations of the XAS signals indicate that the primary contribution to our measurement is from a hole trapped at a native oxygen vacancy in the lattice, leading to an inward structural distortion of the neighbouring Zn atoms (VO++). This analysis is confirmed by the XES signal, which shows a slight charge density change on the Zn atoms, leading to a shift in the XES signal. Our transient XAS measurements from SACLA reveal that this charge trapping occurs on a sub-ps timescale, as in TiO2.

References:

[1] M. Hannelore Rittmann-Frank, C.J. Milne, J. Rittmann, M. Reinhard, T. J. Penfold and M. Chergui, Angewandte Chemie International Edition 53 5858 –5862(2014).

[2] F. G. Santomauro, A. Lübcke, J. Rittmann, E. Baldini, A. Ferrer, M. Silatani, P. Zimmermann, S. Grübel, J. A. Johnson, S. O. Mariager, P. Beaud, D. Grolimund, C. Borca, G. Ingold, S.L. Johnson, M. Chergui, Physical Review Letters (under review).