Exciton and charge carrier dynamics in few-layer WS2

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Semiconducting transition metal dichalcogenides (TMDs) are crucial for optoelectronic devices like photodetectors and solar cells.
• High exciton binding energy in TMDs, especially in thinner layers, indicates excitons are primary photoexcited states.
• Existing time-domain studies predominantly focus on MoS2, necessitating research on other TMDs like WS2.

Purpose of the Study:

• To investigate exciton and charge dynamics in few-layer tungsten disulfide (WS2) after photoexcitation.
• To elucidate the primary photoexcited species and their subsequent dissociation and transport mechanisms.
• To identify and characterize charge trapping processes at defects in WS2.

Main Methods:

• Femtosecond optical spectroscopy was employed to study ultrafast photoexcitation dynamics.
• Time-resolved transient absorption spectroscopy was used to monitor exciton and charge carrier evolution.
• Analysis focused on spectral features to differentiate between excitons, free charges, and trapped charges.

Main Results:

• Excitons were confirmed as the primary photoexcited species in few-layer WS2.
• Exciton dissociation into charge pairs occurred with a time constant of approximately 1.3 picoseconds.
• A novel process of charge diffusion and trapping at defects (e.g., flake edges, grain boundaries) was resolved, altering transient absorption spectra.

Conclusions:

• Few-layer WS2 exhibits excitons as the initial photoexcitations, which rapidly dissociate into charge carriers.
• Charge carriers in WS2 undergo diffusion and trapping at structural defects, influencing their optoelectronic behavior.
• This study highlights the importance of defect engineering for controlling charge dynamics in TMDs and opens avenues for defect characterization.