The discovery of graphene played a key role in creating two-dimensional (2D) electronic materials with potential applications in optoelectronic and electronic devices due to their remarkable physical properties. Lately, many types of 2D electronic materials systems have been developed. Among them, monolayer and few-layer transition metal dichalcogenides (TMDs) with similar characteristics as semiconductors with direct band gap have been extensively studied. Besides, 2D TMDs exhibit unusual valleytronic properties making them potential candidates for fabricating information storage and processing devices. More particularly, molybdenum disulfide (MoS2), a kind of 2D semiconductor material, have demonstrated the distinct differences between MoS2 and ML MoS2 from different perspectives. Besides the notable structural difference, MoS2 has an indirect band gap while ML MoS2 has a direct band gap. These, together with other features, show that ML MoS2 can also be used to observe various quantum effects such as valley optical polarization and Hall effects.
Like conventional semiconductor-based devices, ML MoS2 should be placed on a substrate during the fabrication of optoelectronic and electronic devices. Because the substrate can affect the device performance due to the scattering centers, extra charges and proximity effects, it is imperative to examine and understand the physical properties of ML MoS2 on different substrates to ensure high-performance electronic and optoelectronic devices. Recently, terahertz (THz) time-domain spectroscopy (TDS) has emerged as a robust and effective optical technique for studying electronic materials. It allows users to obtain imaginary and real parts of the optical conductivity directly without Kramas-Kronig transformation. Consequently, THz TDS can accurately measure the imaginary and real parts of transverse and longitudinal magneto-optical conductivities of electronic materials in the presence of external magnetic fields, providing more insights into the physical properties of electronic materials.
Herein, PhD candidate Hua Wen, Professor Wen Xu, Dr. Chao Wang, Dr. Dan Song from the Chinese Academy of Sciences, in collaboration with Dr. Jie Zhang and Professor Lan Ding from the Yunnan University and Dr. Hongying Mei from Huanghuai University studied the terahertz magneto-optical (MO) properties of ML MoS2 fabricated on SiO2/Si substrate using THz TDS technique. The experiment was conducted in a magnetic field in a Faraday geometry at a liquid nitrogen temperature of 80 K. The complex longitudinal MO conductivity for ML MoS2 was measured at different magnetic fields. The authors also determined the localization and electronic backscattering effects on the THz MO conductivity. The aim was to provide detailed experimental insights into THz magneto-optical properties of ML MoS2 on a dielectric substrate. The work is currently published in the journal, Nano Select.
The authors successfully obtained the real and imaginary parts of the longitudinal MO conductivity for ML MoS2, which agreed well with the magneto-optical Drude Smith formula developed theoretically by the same group (J. Appl. Phys. 119, 245706 (2016)). Moreover, the magnetic fields significantly weakened the effects of localization and electronic backscattering in the ML MoS2. Additionally, by fitting the experimental data and the Drude-Smith formula, the sample and material parameters for ML MoS2, such as electron density, were obtained magneto-optically. Whereas the electronic relaxation time exhibited a linear relationship with the magnetic field, the localization factor decreased with an increase in the magnetic field.
In summary, the study reported the fabrication of ML MoS2 on SiO2/Si substrate and the subsequent investigation of its THz magneto-optical properties via THz TDS technique. It was experimentally established that the electrons in the ML MoS2 could strongly respond to the magnetic field in the THz regime. The results showcased the superiority of the THz TDS technique in studying the optoelectronic properties of electronic systems. This provided more information on the electronic systems required to effectively investigate atomically thin materials like ML MoS2 on a substrate. In a statement to Advances in Engineering, first authors Hua Wen explained that their findings would provide a scientific path for the advanced application of ML MoS2 as high-performance magneto-optical devices.
Wen, H., Xu, W., Wang, C., Song, D., Mei, H., Zhang, J., & Ding, L. (2020). Magneto‐optical properties of monolayer MoS 2 ‐SiO 2 /Si structure measured via terahertz time‐domain spectroscopy. Nano Select, 2(1), 90-98.