Ultrafast non-radiative dynamics of atomically thin MoSe₂

Area of Science:

• Condensed Matter Physics
• Materials Science
• Ultrafast Spectroscopy

Background:

• Photo-induced non-radiative energy dissipation is key for structural-phase transitions in 2D materials.
• Understanding real-time atomic motion and lattice temperature is crucial but experimentally challenging.
• Previous techniques lacked the resolution to probe subpicosecond energy conversion dynamics.

Purpose of the Study:

• To quantitatively investigate the real-time dynamics of photoenergy conversion to lattice vibrations.
• To elucidate the role of charge carrier density in ultrafast energy dissipation.
• To explore the mechanisms governing structural-phase transitions in 2D semiconductors.

Main Methods:

• Ultrafast electron diffraction (UED) to directly probe atomic motion and lattice dynamics.
• Experimental investigation on a model bilayered semiconductor, molybdenum diselenide (MoSe2).
• First-principles nonadiabatic quantum molecular dynamics (NA-QMD) simulations for theoretical validation.

Main Results:

• Direct observation of subpicosecond photoenergy conversion to lattice vibrations in MoSe2.
• Demonstration of efficient energy transfer to the lattice within one picosecond at high charge carrier densities.
• NA-QMD simulations successfully reproduced the experimental findings of ultrafast lattice heating.

Conclusions:

• Ultrafast electron diffraction provides unprecedented insight into photoenergy dissipation pathways.
• Efficient and rapid energy transfer to the lattice is confirmed in MoSe2.
• Softening of vibrational modes in the excited state likely mediates the observed ultrafast energy transfer.