Observation of giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor

Abstract: Two-dimensional (2D) transition metal dichalcogenides (TMDs) exhibit novel electrical and optical properties and are emerging as a new platform for exploring 2D semiconductor physics. Reduced screening in 2D results in dramatically enhanced electron-electron interactions, which have been predicted to generate giant bandgap renormalization and excitonic effects. Currently, however, there is little direct experimental confirmation of such many-body effects in these materials. Here we present an experimental observation of extraordinarily large exciton binding energy in a 2D semiconducting TMD. We accomplished this by determining the single-particle electronic bandgap of single-layer MoSe2 via scanning tunneling spectroscopy (STS), as well as the two-particle exciton transition energy via photoluminescence spectroscopy (PL). These quantities yield an exciton binding energy of 0.55 eV for monolayer MoSe2, a value that is orders of magnitude larger than what is seen in conventional 3D semiconductors. This finding is corroborated by our ab initio GW and Bethe Salpeter equation calculations, which include electron correlation effects. The renormalized bandgap and large exciton binding observed here will have a profound impact on electronic and optoelectronic device technologies based on single-layer semiconducting TMDs.

• The paper experimentally confirms giant bandgap renormalization and a 0.55 eV exciton binding energy in monolayer MoSe2.
• It employs complementary STS and PL spectroscopy alongside GW-BSE calculations to robustly determine electronic and optical gaps.
• The findings have significant implications for designing advanced optoelectronic devices based on 2D TMD semiconductors.

Overview of Giant Bandgap Renormalization and Excitonic Effects in Monolayer TMDs

The study presented in the paper focuses on the empirical verification of many-body interactions in monolayer transition metal dichalcogenides (TMDs), specifically MoSe$_2$. Two-dimensional TMDs exhibit unique optical and electronic properties due to reduced screening, which enhances electron-electron interactions significantly. Previous theoretical works have pointed to giant bandgap renormalization and excitonic effects as a consequence of these interactions, but direct experimental confirmation has been limited.

This paper claims a substantial advancement by presenting experimental evidence of remarkably large exciton binding energies in monolayer MoSe$_2$. The authors achieved this by utilizing the combination of scanning tunneling spectroscopy (STS) and photoluminescence (PL) spectroscopy to determine the single-particle electronic bandgap and the two-particle exciton transition energy, respectively. Their findings disclosed an exciton binding energy of 0.55 eV, which is substantially higher than values observed in conventional 3D semiconductors.

The experimental results were further substantiated by ab initio GW and Bethe-Salpeter equation (GW-BSE) calculations. These computational methods incorporate electron correlation effects, corroborating the observed renormalized bandgap and large exciton binding energy.

Methodology and Results

The researchers employed complementary experimental and theoretical approaches to explore the electronic structure and optical transitions in monolayer MoSe$_2$ deposited on bilayer graphene (BLG) on a SiC substrate. With STS, they determined an electronic bandgap of 2.18 eV ± 0.04 eV, while PL measurements indicated an optical bandgap of 1.63 eV ± 0.01 eV. This difference directly yields the calculated exciton binding energy.

The theoretical calculations used a GW approach and GW-BSE method, optimized for substrate effects to better mirror experimental conditions. The results underscored GW's capability to replace initial mean-field approximations with more precise quasiparticle energies. The agreement between the experimental and theoretical values for electronic and optical gaps, and exciton binding energies underscores the robustness of these methodologies.

Implications and Future Directions

The findings have vital implications for the development of electronic and optoelectronic devices based on single-layer TMDs. The strong excitonic effects in such materials are poised to play a critical role in the performance of photodetectors, photovoltaic cells, and other nanodevices, particularly at room temperature where many-body interactions can notably influence device characteristics.

The work paves the way for further research into the role of substrate effects and many-body interactions in other 2D materials. Understanding these interactions could further bridge the gap between experimental results and theoretical predictions, enabling more precise manipulation of electronic properties for targeted applications.

Future developments could explore the modulation of electronic properties through doping or by using different substrates, potentially leading to tunable exciton binding energies in multilayered or hybrid material systems. This could facilitate the creation of bespoke optoelectronic devices with enhanced performance and functionality.

In summary, the paper delivers significant empirical contributions to the understanding of many-body effects in TMDs, offering a methodology that blends empirical and theoretical techniques to explore the exciting complexities of 2D semiconductors.