Optically Induced Coherent Phonons in Bismuth Oxyiodide (BiOI) Nanoplatelets

Area of Science:

• Materials Science
• Solid-State Physics
• Photocatalysis

Background:

• Bismuth oxyiodide (BiOI) possesses unique optical and structural properties, making it a material of interest for photocatalysis.
• Understanding the dynamics of charge carriers and lattice vibrations in BiOI is essential for optimizing its photocatalytic performance.

Purpose of the Study:

• To investigate the ultrafast dynamics of coherent phonons in Bismuth oxyiodide (BiOI) following femtosecond laser excitation.
• To elucidate the role of the internal electric field in BiOI's charge separation and photocatalytic mechanisms.

Main Methods:

• Femtosecond laser spectroscopy was employed to excite coherent phonons in BiOI.
• Time-resolved optical density measurements were used to probe lattice vibrations and vibronic coherence.
• Analysis of phonon frequencies and dephasing mechanisms provided insights into charge dynamics.

Main Results:

• Femtosecond laser pulses induced coherent phonons in BiOI, manifesting as oscillating optical density modulations.
• Two distinct phonon frequencies were identified, corresponding to lattice vibrations along the [001] crystallographic axis.
• Subpicosecond charge separation, driven by a built-in dipolar field, was observed, leading to phonon generation.

Conclusions:

• The study provides direct evidence for an electric field along the [001] axis in BiOI.
• This electric field plays a critical role in efficient charge separation, a key factor for BiOI's photocatalytic applications.
• Anharmonic phonon decay and phonon-carrier scattering were identified as the primary dephasing mechanisms limiting vibronic coherence.