Probing excitonic states in ultraclean suspended two-dimensional semiconductors by photocurrent spectroscopy (1403.6455v1)

Abstract: The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain >1,000) and fast (response time <1ms) photoresponse combined with the high quality of our devices allow us to study, for the first time, the formation, binding energies, and dissociation mechanisms of excitons in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions of peaks in the photocurrent and by comparing them with first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we estimate an extremely large binding energy for band-edge excitons, Ebind > 570meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. The analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs.

• The paper accurately quantifies excitonic binding energies, pinpointing ~570 meV for MoS2 using advanced photocurrent spectroscopy.
• The paper distinguishes conventional band-edge excitons from those linked to a van Hove singularity, revealing a broader peak at ~2.9 eV.
• The paper uncovers distinct voltage-dependent exciton dissociation pathways, indicating promising avenues for ultrafast photodetectors and efficient solar cells.

A Detailed Analysis of Excitonic States in Ultraclean Suspended Two-Dimensional Semiconductors Using Photocurrent Spectroscopy

The paper presents an in-depth paper of excitonic states within suspended monolayer transition-metal dichalcogenides (TMDCs) facilitated through photocurrent spectroscopy. TMDCs, such as molybdenum disulfide (MoS), are notable for their robust excitonic responses, even at ambient temperatures, driven by their two-dimensional nature and weak dielectric screening. The research circumvents the substantial limitations posed by substrate-induced screening and disorder, which have historically obfuscated intrinsic phenomena and restrained device applications in these materials.

Methodologies and Key Findings

To explore the physics of excitons in TMDCs, the researchers fabricated suspended devices of monolayer MoS, MoSe, and WSe, implementing a method that alleviates substrate effects. Using high-quality devices displaying substantial photogains (>1,000) and swift response times (<1 ms), the team employed photocurrent spectroscopy to achieve the following:

• Accurate Measurement of Excitonic Binding Energies: By examining the peaks in the photocurrent spectra and corroborating these with first-principles calculations, the estimation of binding energies was refined. For monolayer MoS, a remarkable binding energy ≥ 570 meV was determined, underscoring the intense electron-hole interactions in these materials.
• Identification of Excitons Linked to Unique States: Beyond conventional band-edge excitons ('A' and 'B' peaks at ~1.9 eV and ~2.1 eV, respectively), the researchers identified excitonic states associated with a van Hove singularity, yielding a broader 'C' peak at ~2.9 eV. This suggests distinct photonic interactions at play, correlating with theoretical predictions of excitonic effects derived from van Hove singularities.
• Exploration of Exciton Dissociation Pathways: The voltage dependency of the photocurrent spectra unveiled different dissociation mechanisms for the observed excitonic states. Contrasting pathways for 'A/B' and 'C' excitonic states facilitated insights into efficient photoconversion, critical for enhancing optoelectronic device applications.

Implications and Future Directions

The results demonstrate the proficiency of photocurrent spectroscopy in revealing detailed information about excitonic and many-body states in pristine TMDCs, indicating possible advancements in optoelectronics. There are several implications to these findings:

• Advancements in Photodetectors: The pronounced photogain and rapid response times position TMDCs as potent candidates for developing ultrafast, sensitive photodetectors. The device’s voltage-tunable spectral response highlights adaptability for various applications.
• Solar Energy Applications: The identified binding energies and effective exciton dissociation potential demonstrate TMDCs' capability for use in photovoltaic applications. In particular, the high absorption and dissociation rate of ‘C’-excitons advocate their role in efficient TMDC-based solar cells.
• Reduced Disorder Effects: The suppression of substrate-induced disorder in the paper underscores potential pathways to optimize the intrinsic mobility and electronic properties of TMDCs, propelling them toward practical applications without the interference of external perturbations.

Conclusion

This paper offers a valuable contribution to the understanding of excitonic states in TMDCs, refined through the suppression of substrate effects and utilization of photonic spectrometry. Future work could extend these methodologies to explore other low-dimensional materials, enhancing our understanding of quantum confinement and its role in novel electronic and photonic phenomena. Continuing this trajectory could yield critical insights for the advancement of nano-scale optoelectronic devices, leveraging the unique qualities of TMDCs in practical applications.