Phonon-Mediated Interlayer Charge Separation and Recombination in a MoSe₂/WSe₂ Heterostructure

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Monolayer transition metal dichalcogenides (TMDs) show promise for photodetection and light harvesting due to high absorption.
• Efficient optoelectronic applications necessitate the dissociation of tightly bound photogenerated excitons.
• Vertical stacking of TMD monolayers creates heterostructures with favorable band alignment for interlayer charge transfer.

Purpose of the Study:

• To elucidate the mechanisms and temperature dependence of charge separation and recombination in MoSe2/WSe2 heterostructures.
• To investigate the role of vibrational modes in ultrafast charge dynamics within TMD heterostructures.

Main Methods:

• Two-color pump-probe microscopy was employed to experimentally probe charge carrier dynamics.
• Ab initio quantum dynamics simulations were utilized to model and rationalize the experimental observations.

Main Results:

• Charge separation in MoSe2/WSe2 heterostructures was found to be ultrafast (approximately 200 fs) and largely independent of temperature.
• Photogenerated exciton recombination exhibited a strong temperature dependence, accelerating significantly with increasing temperature.
• Simulations revealed barrierless, temperature-independent charge separation driven by dense acceptor states and high-frequency vibrations.
• Temperature-dependent recombination was attributed to transient bandgap modulation by low-frequency shear and breathing motions.

Conclusions:

• The study provides a detailed understanding of the fundamental mechanisms governing charge carrier dynamics in TMD heterostructures.
• Ultrafast, temperature-independent charge separation is facilitated by specific vibrational modes, enabling efficient charge transfer.
• Temperature-dependent recombination, influenced by lattice vibrations, highlights a critical factor for device performance and stability.