[Paper Review] Non-Hydrogenic Exciton Rydberg Series in Monolayer WS2

Alexey Chernikov, Timothy C. Berkelbach+7 more|arXiv (Cornell University)|Mar 17, 2014

2D Materials and Applications7 citations

TL;DR

This study experimentally measures the ground and first four excited excitonic states in monolayer WS2, revealing a large 0.32 eV exciton binding energy and strong deviations from hydrogenic Rydberg behavior. Using a microscopic theory that accounts for non-local dielectric screening, the authors explain the unconventional electron-hole interactions, which are expected to be common in atomically thin 2D semiconductors.

ABSTRACT

We have determined experimentally the energies of the ground and first four excited excitonic states of the fundamental optical transition in monolayer WS2, a model system for the growing class of atomically thin two-dimensional semiconductor crystals. From the spectra, we establish a large exciton binding energy of 0.32 eV and a pronounced deviation from the usual hydrogenic Rydberg series of energy levels of the excitonic states. We explain both of these results using a microscopic theory in which the non-local nature of the effective dielectric screening modifies the functional form of the Coulomb interaction. These strong but unconventional electron-hole interactions are expected to be ubiquitous in atomically thin materials.

Motivation & Objective

• To experimentally determine the energies of the ground and first four excited excitonic states in monolayer WS2.
• To investigate the nature of excitonic binding and energy level structure in atomically thin 2D semiconductors.
• To explain the deviation from hydrogenic Rydberg series in WS2 using a microscopic theory of electron-hole interactions.
• To establish the role of non-local dielectric screening in modifying Coulomb interactions in 2D materials.

Proposed method

• High-resolution optical spectroscopy to measure the energies of excitonic states in monolayer WS2.
• Experimental extraction of the exciton binding energy from the measured energy level spacings.
• Development of a microscopic many-body theory incorporating non-local dielectric screening effects.
• Calculation of the effective Coulomb interaction using a non-local dielectric function to model screening in 2D systems.
• Comparison of theoretical predictions with experimental data to validate the model of modified Coulomb interaction.

Experimental results

Research questions

• RQ1 What are the experimentally measured energies of the ground and first four excited excitonic states in monolayer WS2?
• RQ2 Why does the excitonic energy level structure in monolayer WS2 deviate significantly from the hydrogenic Rydberg series?
• RQ3 How does non-local dielectric screening alter the effective electron-hole interaction in atomically thin 2D semiconductors?
• RQ4 What is the magnitude of the exciton binding energy in monolayer WS2, and how does it compare to bulk or other 2D materials?

Key findings

• The exciton binding energy in monolayer WS2 is experimentally determined to be 0.32 eV, significantly larger than in bulk semiconductors.
• The energy levels of the excitonic states show a pronounced deviation from the hydrogenic Rydberg series, indicating non-standard electron-hole interactions.
• The deviation is explained by a microscopic theory that accounts for the non-local nature of dielectric screening in 2D materials.
• The effective Coulomb interaction in WS2 is modified by non-local screening, leading to stronger and unconventional excitonic binding.
• These unconventional electron-hole interactions are expected to be widespread in atomically thin 2D semiconductors due to their unique dielectric response.