Mono- and Bilayer WS2 Light-Emitting Transistors

Abstract: We have realized ambipolar ionic liquid gated field-effect transistors based on WS2 mono- and bilayers, and investigated their opto-electronic response. A thorough characterization of the transport properties demonstrates the high quality of these devices for both electron and hole accumulation, which enables the quantitative determination of the band gap ({\Delta}1L = 2.14 eV for monolayers and {\Delta}2L = 1.82 eV for bilayers). It also enables the operation of the transistors in the ambipolar injection regime with electrons and holes injected simultaneously at the two opposite contacts of the devices in which we observe light emission from the FET channel. A quantitative analysis of the spectral properties of the emitted light, together with a comparison with the band gap values obtained from transport, show the internal consistency of our results and allow a quantitative estimate of the excitonic binding energies to be made. Our results demonstrate the power of ionic liquid gating in combination with nanoelectronic systems, as well as the compatibility of this technique with optical measurements on semiconducting transition metal dichalcogenides. These findings further open the way to the investigation of the optical properties of these systems in a carrier density range much broader than that explored until now.

• The paper demonstrates the fabrication of high-performance WS2 transistors with ionic liquid gating, achieving room-temperature mobility of ~50 cm²/V·s.
• The paper quantifies the band gaps as 2.14 eV for monolayers and 1.82 eV for bilayers, confirming reliable light emission in the ambipolar regime.
• The paper correlates optical emission with excitonic binding energies of 160 meV (monolayer) and 80 meV (bilayer), highlighting thickness-dependent optoelectronic properties.

Analysis of Mono- and Bilayer WS$_2$ Light-Emitting Transistors

This paper presents an in-depth exploration into the ambipolar ionic liquid gated field-effect transistors (FETs) utilizing monolayer and bilayer WS$_2$ as the active material. Transition metal dichalcogenides (TMDs) are a focal area of interest in nanoelectronic research due to their unique properties derived from their two-dimensional structure. The study broadens this scope by focusing on the opto-electronic properties of TMDs, specifically in the context of light-emitting transistors.

Key Findings

The authors have successfully demonstrated the realization of light-emitting transistors by employing WS$_2$ mono- and bilayers. The significant contributions and findings of this research are summarized as follows:

• Device Fabrication and Characterization: The fabrication of the ionic liquid gated FETs involved using WS$_2$ monolayer and bilayer flakes. High-quality ambipolar transport behavior was demonstrated, which is critical for applications such as solar cells and light-emitting devices. The observed room-temperature mobility values of approximately 50 cm$^2$/Vs indicate substantial carrier transport efficiency.
• Opto-Electronic Properties and Band Gap Estimation: Through comprehensive transport measurements, the researchers quantitatively determined the band gaps: $\Delta_{1L}=2.14$ eV for monolayers and $\Delta_{2L}=1.82$ eV for bilayers. This determination is vital for evaluating the potential of TMDs in nanoelectronic applications. Additionally, the light emission from the FET channel was achieved in the ambipolar injection regime, validating the device's operational capabilities.
• Optical and Transport Correlation: The study conducted a comparative analysis of the spectral properties of the emitted light with the band gap values. This comparison allowed for a quantitative estimation of the excitonic binding energies, which are significant for understanding the optical responses in TMDs.
• Exciton Binding Energy: The binding energy of excitons was estimated as 160 meV for monolayers and 80 meV for bilayers, indicating how TMD layer thickness affects excitonic characteristics. The observed trend aligns with theoretical expectations of reduced Coulombic interactions with increasing thickness.

Implications and Future Directions

The findings from this research emphasize the utility of ionic liquid gating in enhancing the functional capabilities of TMD-based devices. These developments could have strong implications for future nanoelectronic and optoelectronic applications, enabling devices that operate efficiently at much broader carrier density ranges than previously accessible.

Further exploration into the opto-electronic applications of TMDs could lead to the development of innovative device concepts and potential commercial applications. Continued research might also extend to various other TMD compositions and configurations, particularly exploring heterostructures or mixed-dimensionality systems. Understanding and refining carrier modulation techniques, along with reducing contact resistance, could pave the way towards high-performance, integrated circuits based on TMDs.

The interdisciplinary relevance of this work stretches across materials science and electrical engineering, presenting avenues for advanced light-emitting, sensing, and quantum information technologies by taking advantage of the unique properties of monolayer and bilayer systems.