Exciton-Dominated Ultrafast Optical Response in Atomically Thin PtSe₂

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Strongly bound excitons are key in 2D semiconductors, driving unique light-matter interactions.
• Platinum diselenide (PtSe2) is an emerging 2D material with promising optical and electrical properties and air stability.
• While bulk PtSe2 is a semimetal, its thin forms exhibit semiconducting behavior, suggesting potential for strong excitons.

Purpose of the Study:

• To experimentally observe and theoretically confirm excitons and their ultrafast dynamics in monolayer, bilayer, and trilayer PtSe2.
• To investigate the thickness dependence of exciton absorption resonances and spectral shifts.
• To provide insights into the photophysics of excitons in platinum dichalcogenides.

Main Methods:

• Steady-state optical microscopy to identify exciton absorption resonances.
• First-principles calculations to confirm experimental observations.
• Ultrafast transient absorption microscopy to study exciton dynamics.

Main Results:

• Direct observation and theoretical confirmation of excitons in mono-, bi-, and tri-layer PtSe2.
• Exciton absorption resonances show a clear dependence on the film thickness.
• Ultrafast dynamics reveal exciton dominance, characterized by strong bleaching and band-gap renormalization.
• Transient spectra redshift with increasing thickness due to shrinking band gaps.

Conclusions:

• This study provides the first direct observation and theoretical validation of excitons in PtSe2 ultrathin films.
• The findings highlight the significant role of excitons and their thickness-dependent behavior in PtSe2 photophysics.
• The research offers novel insights into exciton dynamics in platinum dichalcogenides, paving the way for new optical applications.