Revealing exciton masses and dielectric properties of monolayer semiconductors with high magnetic fields (1904.03238v2)

Abstract: In semiconductor physics, many essential optoelectronic material parameters can be experimentally revealed via optical spectroscopy in sufficiently large magnetic fields. For monolayer transition-metal dichalcogenide semiconductors, this field scale is substantial --tens of teslas or more-- due to heavy carrier masses and huge exciton binding energies. Here we report absorption spectroscopy of monolayer MoS$_2$, MoSe$_2$, MoTe$_2$, and WS$_2$ in very high magnetic fields to 91~T. We follow the diamagnetic shifts and valley Zeeman splittings of not only the exciton's $1s$ ground state but also its excited $2s$, $3s$, ..., $ns$ Rydberg states. This provides a direct experimental measure of the effective (reduced) exciton masses and dielectric properties. Exciton binding energies, exciton radii, and free-particle bandgaps are also determined. The measured exciton masses are heavier than theoretically predicted, especially for Mo-based monolayers. These results provide essential and quantitative parameters for the rational design of opto-electronic van der Waals heterostructures incorporating 2D semiconductors.

• The paper demonstrates that high magnetic fields reveal multiple excitonic Rydberg states, enabling direct measurement of exciton effective masses.
• Analysis of magneto‐optical spectra in MoS₂, MoSe₂, MoTe₂, and WS₂ monolayers quantifies exciton binding energies and dielectric screening parameters.
• Findings indicate that heavier transition-metal monolayers have unexpectedly high exciton masses, prompting re-evaluation of conventional band structure models.

Insights into Exciton Masses and Dielectric Properties of Monolayer Semiconductors from High Magnetic Field Studies

The paper presented in this paper investigates the excitonic properties and dielectric screening in monolayer transition-metal dichalcogenide (TMD) semiconductors using high magnetic fields. This research focuses specifically on MoS$_2$, MoSe$_2$, MoTe$_2$, and WS$_2$ monolayers and scrutinizes their behavior under intense magnetic fields up to 91 Tesla. Such high-strength fields enable detailed optical spectroscopy to determine the fundamental optoelectronic properties of these materials, which are largely driven by excitonic effects due to the significant exciton binding energies and the heavy carrier masses involved.

Key Findings and Methodologies

1. Excitonic Rydberg States: The paper reveals the diamagnetic shifts and valley Zeeman splittings not just for the ground-state ($1s$) exciton, but also for higher Rydberg states ($2s$, $3s$, etc.). This comprehensive capture of multiple states underpins the direct experimental evaluation of crucial material parameters like effective exciton masses and dielectric constants.
2. Measurements and Models: By examining the magneto-optical spectra, distinct trends in exciton energy shifts are tracked. The high-field behavior predominantly provides insights into exciton mass, with the shifts in the higher Rydberg states offering effective separations giving a measure independent of any dielectric variation model.
3. Quantitative Determination: For WS$_2$, the effective reduced exciton mass is deduced to be significantly heavier than anticipated from theoretical models, demonstrating a possible need for adjustments in these theoretical approaches. For each material, key parameters such as the exciton binding energy and free-particle bandgaps are tabulated, providing valuable data for designing TMD-based electronic and photonic devices.
4. Comparisons and Trends: The paper compares the different TMD materials and identifies trends with respect to chalcogen atomic mass and transition metal types. Findings indicate that heavier transition-metal-based monolayers indeed exhibit higher exciton masses, aligning with broadened theoretical predictions.

Practical Implications

The experimentally determined exciton masses and dielectric parameters have substantial ramifications for the field of two-dimensional materials science. They provide the necessary input for theoretical models aiming to predict and tailor the electronic and optical behavior of TMDs in various environments, including dielectric-engineered heterostructures. The disclosed heavier exciton masses in Mo-based TMDs prompt a re-evaluation of electronic band structure models commonly applied to these materials.

Theoretical Implications

The deviation of experimental results, particularly exciton masses from theoretical predictions, suggests potential underlying mechanisms such as phonon-electron interactions or polaron formation in these systems. These elements require refined theoretical modeling to accurately capture and predict the behavior and properties of TMD monolayers.

Future Directions

Moving forward, this research lays the groundwork for more extensive studies into interaction-based effects that might arise at high charge densities in these materials, as well as expanded exploration into heterostructures involving multiple TMD layers or different encapsulating materials. Additionally, further research should be directed toward understanding how these fundamental excitonic parameters affect device performance, reliability, and scalability in real-world applications.

In conclusion, this work provides a detailed experimental framework for elucidating intrinsic optoelectronic properties of monolayer TMDs, empowering both theoretical predictions and practical advancements in the development of next-generation low-dimensional semiconductors.