Spontaneous Exciton Dissociation in Transition Metal Dichalcogenide Monolayers

• Main Authors: Handa, Taketo, Holbrook, Madisen A, Olsen, Nicholas, Holtzman, Luke N, Huber, Lucas, Wang, Hai I, Bonn, Mischa, Barmak, Katayun, Hone, James C, Pasupathy, Abhay N, Zhu, X. -Y
• Format: Journal Article
• Language: English
• Published: 19.06.2023
• Subjects: Physics - Mesoscale and Nanoscale Physics
• DOI: 10.48550/arxiv.2306.10814

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.