DOI: https://doi.org/10.29363/nanoge.incnc.2021.047
Publication date: 8th June 2021
We have studied bismuth oxyiodide (BiOI) nanoplatelets, a material which combines intriguing optical and structural properties for photocatalysis. In a pump-probe experiment, we show that excitation by a femtosecond laser pulse creates coherent phonons which leads to an oscillating modulation of the differential optical density. We found that the two underlying frequencies originate from lattice vibrations along the [001] crystallographic axis, the stacking direction of oppositely charged layers in BiOI. This is consistent with a sub-picosecond charge separation in the built-in dipolar field, which screens the electric field partially and thus creates coherent phonons. Furthermore, we determine the two major dephasing mechanisms that lead to the loss of vibronic coherence: (i) the anharmonic decay of an optical phonon into two acoustic phonons and (ii) phonon-carrier scattering.
Our results provide the first direct demonstration of the presence of an electric field in BiOI along the [001] axis and show its role in efficient charge separation that is crucial for photocatalytic applications of BiOI.
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