[Paper Review] Exciton-dominated Dielectric Function of Atomically Thin MoS2 Films

Yiling Yu, Yifei Yu+9 more|arXiv (Cornell University)|Oct 12, 2015

2D Materials and Applications4 references3 citations

TL;DR

This study reveals that excitonic effects dominate the dielectric function of atomically thin MoS2 films with fewer than 5–7 layers, causing a non-monotonic dependence on layer count—initially decreasing then increasing with thickness. The authors demonstrate that excitonic contributions exceed band-structure effects, extracting layer-dependent exciton binding energies and Bohr radii via model fitting, which enables engineering of light-matter interactions for nanophotonic devices.

ABSTRACT

We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5-7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5-7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters.

Motivation & Objective

• To investigate the dielectric function of atomically thin MoS2 films across varying layer numbers.
• To determine the relative contributions of excitonic effects versus band structure to the dielectric response.
• To quantify layer-dependent exciton binding energy and Bohr radius in few-layer MoS2.
• To explain the anomalous non-monotonic trend in dielectric function with increasing layer count.
• To enable design principles for MoS2-based photonic devices through accurate dielectric function characterization.

Proposed method

• Systematic measurement of the dielectric function of MoS2 films with 1 to 10 layers using spectroscopic ellipsometry.
• Comparison of experimental dielectric functions with theoretical models incorporating both band structure and excitonic effects.
• Fitting experimental data to an intuitive model that accounts for layer-dependent excitonic contributions.
• Extraction of exciton binding energy and Bohr radius by fitting the optical response to a modified dielectric function model.
• Analysis of the transition from exciton-dominated to band-structure-dominated behavior as film thickness increases beyond 5–7 layers.
• Use of the model to explain the observed reversal in dielectric function trend at higher layer counts.

Experimental results

Research questions

• RQ1 How does the dielectric function of atomically thin MoS2 vary with layer number, and what causes its non-monotonic behavior?
• RQ2 To what extent do excitonic effects dominate the dielectric function in few-layer MoS2 compared to band-structure contributions?
• RQ3 What are the layer-dependent values of exciton binding energy and Bohr radius in MoS2 films?
• RQ4 Why does the dielectric function initially decrease and then increase with increasing layer count in MoS2?
• RQ5 How can the dielectric function be modeled to accurately capture excitonic effects in 2D transition metal dichalcogenides?

Key findings

• The dielectric function of MoS2 films decreases with increasing layer count when thickness is below 5–7 layers, indicating strong excitonic influence.
• For films thicker than 5–7 layers, the dielectric function begins to increase with layer count, signaling a shift to band-structure dominance.
• Excitonic effects dominate the dielectric function in monolayer and few-layer MoS2, contrary to conventional semiconductors where band structure prevails.
• The extracted exciton binding energy in monolayer MoS2 is approximately 0.6 eV, with a Bohr radius of about 1.2 nm, both decreasing with increasing layer count.
• The model fitting confirms that excitonic contributions exceed band-structure contributions in thin films, explaining the anomalous dielectric response.
• The results provide a foundation for engineering light-matter interactions in 2D materials for applications in metamaterials, waveguides, and optoelectronic devices.