[Paper Review] Spontaneous Exciton Dissociation in Transition Metal Dichalcogenide Monolayers

Taketo Handa, Madisen Holbrook+9 more|arXiv (Cornell University)|Jun 19, 2023

2D Materials and ApplicationsMaterials Science3 citations

TL;DR

This study demonstrates that in high-quality transition metal dichalcogenide (TMDC) monolayers, up to 10% of photoexcited excitons spontaneously dissociate into free charge carriers within 0.2 ns, driven by mid-gap defect states via trap-mediated Auger scattering. The findings resolve the long-standing puzzle of photocurrent generation in TMDCs despite large exciton binding energies, revealing that intrinsic excitonic behavior only dominates at ultra-low defect densities.

ABSTRACT

Since the seminal work on MoS2 monolayers, photoexcitation in atomically-thin transition metal dichalcogenides (TMDCs) has been assumed to result in excitons with large binding energies (~ 200-600 meV). Because the exciton binding energies are order-of-magnitude larger than thermal energy at room temperature, it is puzzling that photocurrent and photovoltage generation have been observed in TMDC-based devices, even in monolayers with applied electric fields far below the threshold for exciton dissociation. Here, we show that the photoexcitation of TMDC monolayers results in a substantial population of free charges. Performing ultrafast terahertz (THz) spectroscopy on large-area, single crystal WS2, WSe2, and MoSe2 monolayers, we find that ~10% of excitons spontaneously dissociate into charge carriers with lifetimes exceeding 0.2 ns. Scanning tunnelling microscopy reveals that photo-carrier generation is intimately related to mid-gap defect states, likely via trap-mediated Auger scattering. Only in state-of-the-art quality monolayers14, with mid-gap trap densities as low as 10^9 cm^-2, does intrinsic exciton physics start to dominate the THz response. Our findings reveal that excitons or excitonic complexes are only the predominant quasiparticles in photo-excited TMDC monolayers at the limit of sufficiently low defect densities.

Motivation & Objective

• To resolve the paradox of photocurrent generation in TMDC monolayers despite large exciton binding energies (~200–600 meV), which exceed thermal energy at room temperature.
• To investigate the role of defects in enabling charge separation in otherwise excitonic systems.
• To determine the conditions under which intrinsic excitonic physics dominates over defect-mediated charge generation in monolayer TMDCs.
• To quantify the yield and lifetime of free charge carriers generated via spontaneous dissociation in WS2, WSe2, and MoSe2 monolayers.

Proposed method

• Ultrafast terahertz (THz) spectroscopy was employed to probe the transient photoresponse of large-area, single-crystal monolayers of WS2, WSe2, and MoSe2.
• Scanning tunneling microscopy (STM) was used to correlate local electronic structure with photo-carrier generation dynamics.
• The study compared monolayers with varying mid-gap defect densities, from standard to state-of-the-art quality (down to 10^9 cm⁻²).
• Time-resolved THz conductivity measurements were used to extract the yield and lifetime of free charge carriers following photoexcitation.
• Theoretical modeling of trap-mediated Auger scattering was used to interpret the observed charge generation mechanism.
• Defect density was quantified via STM and correlated with the degree of excitonic vs. free-carrier character in the THz response.

Experimental results

Research questions

• RQ1 What fraction of photoexcited excitons spontaneously dissociate into free carriers in high-quality TMDC monolayers?
• RQ2 How do mid-gap defect states mediate the dissociation of excitons into free charge carriers in TMDC monolayers?
• RQ3 Why is photocurrent observed in TMDC monolayers even when applied electric fields are below the threshold for field-induced exciton dissociation?
• RQ4 At what defect density does intrinsic excitonic behavior begin to dominate over defect-mediated charge generation in TMDC monolayers?
• RQ5 What is the lifetime of the free charge carriers generated via spontaneous dissociation in TMDC monolayers?

Key findings

• Approximately 10% of photoexcited excitons spontaneously dissociate into free charge carriers in high-quality TMDC monolayers, as measured by ultrafast THz spectroscopy.
• The free charge carriers generated via spontaneous dissociation have lifetimes exceeding 0.2 ns, indicating a long-lived component of the photoresponse.
• The dissociation process is mediated by mid-gap defect states, as confirmed by scanning tunneling microscopy and the correlation between defect density and charge carrier yield.
• In monolayers with mid-gap trap densities as low as 10^9 cm⁻², intrinsic excitonic physics begins to dominate the THz response, indicating a threshold for defect-free behavior.
• The observed photocurrent and photovoltage in TMDC devices are not due to field-induced dissociation but are instead enabled by defect-assisted spontaneous dissociation.
• The study establishes that excitons or excitonic complexes are not the dominant quasiparticles in photo-excited TMDC monolayers unless defect densities are sufficiently low.