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Charge carrier dynamics in metal oxides

Periodic Reporting for period 3 - DYNAMOX (Charge carrier dynamics in metal oxides)

Reporting period: 2019-10-01 to 2021-03-31

DYNAMOX deals with the charge carrier dynamics of transition metal oxides (TMO's). These materials are of crucial importance in a wide range of light-related applications, in particular in solar energy conversion. The latter relies on the absorption of light and the creation of electrons and holes. To describe the way they are created, their evolution over time and their ultimate fate, one needs to first correctly describe the optical absorption spectra of the materials, which paradoxically has not been the case so far. Further, one needs material- and element-specific probes that in addition can reach temporal resolutions on the order of the femtosecond since the carrier dynamics occur on ultrashort time scales.
As mentioned above,TMOs such as Titanium dioxide (TiO2), zinc oxide (ZnO), Nickel oxide (NiO), Cobalt Oxide (CoO or Co3O4), etc. are already used in a number of applications, in particular in photocatalysis and photovoltaics, as transparent conductive oxides and they also hold promise of applications as mechanical sensors with an optical read-out.
Our objective is to characterize the optical spectra and the charge carrier dynamics using a wide range of cutting-edge experimental tools: a) because these materials have large gaps in the near to deep-ultraviolet, it is necessary to implement novel spectroscopic tools, which we have developed in our labs; b) these also offer a materials-specific probe of the interfacial charge dynamics involving TMOs; c) an element-specific probe is provided by ultrafast X-ray spectroscopy, which we pioneered and that is finding widespread use in TMOs; c) the ability to capture simultaneously both the electron and hole dynamics is a challenge that we are overcoming using time- and angle-resolved photoemission spectroscopy. The entire set of results from these very different approaches is pooled together to provide a comprehensive and definitive vision of the charge carrier dynamics in these materials as well as opening avenues for possible applications.
The project has been managed on different fronts, which represent the various experimental tools:
a) One pre-requisite is the description of the optical absorption spectrum, which calls for the use of steady-state and time-resolved near-to far-UV spectroscopic tools. These have been used to characterise the optical absorption spectrum of anatase and rutile TiO2, which has been analysed and published. In particular, we have unravelled the existence for the first time of 2-dimensional excitons in the 3-dimensional lattice of a solid, namely anatase TiO2. We also carried out the characterization of ZnO and spinel cobalt oxide, and the results are being analysed at present.
b) As an extension of the above studies, we have characterised the charge carrier dynamics and phonon dynamics in anatase TiO2. Several new results have been obtained, namely the electron cooling time has been determined for the first time, giant couplings between excitons and acoustic phonons have been discovered and hold promise of using anatase TiO2 as a mechanical sensor with all-optical read-out. We have also demonstrated how the excitonic resonance of the material can be used as a probe of electron injection and its time scale in photovoltaics and results were obtained for anatase TiO2 and for ZnO.
c) We have implemented a time- and angle-resolved photoemission spectroscopy (TR-ARPES) experiment and have carried out test studies on inorganic perovskite crystals. The results have been written up and will soon be submitted for publication. We are now pursuing studies on ZnO at present.
d) Optical ultrafast spectroscopic measurements have been carried out on spinel cobalt oxide and the results are being analysed. These studies are a pre-requisite for ultrafast X-ray emission and transient grating experiments for which beamtime is granted at both the European X-ray Free electron laser (XFEL, Hamburg) and the FERMI XFEL in Trieste (Italy).
e) By a combination of picosecond X-ray absorption and resonant inelastic X-ray scattering (RIXS), we have observed for the first time the trapping of holes in a TMO. The results have been published and another X-ray spectroscopy study with femtosecond resolution is being written up at present.
f) The implementation of the femtosecond RIXS experiment is proceeding despite delays due to the development of a new conceptual design and the delivery of equipment and parts. First experiments are planned this autumn.
the project pivots around three aspects which are beyond the state of the art:
a) the use of deep-ultraviolet spectroscopy, for which we are in unique position thanks to the performances of our set-up, has delivered several new results as it is the first time it is implemented on TMOs.
b) the TR-ARPES experiment, which has already delivered new results.
c) the implementation of the fs-XAS, XES and RIXS experiments, which has been achieved. the RIXS part is now being completed by new developments.
Cartoon of the 2-dimensional exciton in the 3-dimensional crystal lattice of anatase TiO2