Monolayer-like Exciton Recombination Dynamics of Multilayer MoSe₂ Observed by Pump-Probe Microscopy

Area of Science:

• Materials Science
• Condensed Matter Physics
• Semiconductor Physics

Background:

• Transition metal dichalcogenides (TMDCs) are layered semiconductors with significant interest in their electronic properties.
• Monolayer TMDCs are well-studied for their direct bandgap and exciton behavior, but multilayer dynamics remain less understood.
• Recent theories suggest excitons in bulk TMDCs are layer-confined, implying stable room-temperature populations.

Purpose of the Study:

• To investigate carrier dynamics in multilayer molybdenum diselenide (MoSe2) flakes.
• To compare exciton-exciton annihilation (EEA) and carrier recombination rates in MoSe2 flakes of varying thicknesses (16-125 layers).
• To understand the influence of layer thickness on carrier dynamics in MoSe2.

Main Methods:

• Experimental investigation of carrier dynamics in MoSe2 flakes using techniques sensitive to exciton behavior.
• Kinetic analysis to differentiate between exciton-exciton annihilation (EEA) and Shockley-Read-Hall recombination.
• Measurement of EEA rate constants across a range of multilayer MoSe2 thicknesses.

Main Results:

• The average exciton-exciton annihilation (EEA) rate constant in multilayer MoSe2 (0.003 cm²/s) was found to be nearly independent of flake thickness.
• This observed EEA rate is significantly lower (2 orders of magnitude) than that of unencapsulated monolayer MoSe2 but comparable to encapsulated monolayers.
• Both EEA and Shockley-Read-Hall recombination were identified as key kinetic processes.

Conclusions:

• Strong intralayer interactions in multilayer MoSe2 minimize the effect of layer thickness on recombination dynamics.
• Multilayer MoSe2 exhibits carrier dynamics analogous to encapsulated monolayers, with an apparent EEA rate intrinsic to the material.
• These findings provide new insights into carrier behavior in thicker TMDC flakes.