Substrate-induced electronic localization in monolayer MoS2 measured via terahertz spectroscopy

We study terahertz (THz) optoelectronic properties of monolayer (ML) MoS₂ placed on different substrates such as SiO₂/Si, sapphire, and quartz. Through the measurements of THz Fourier transform spectroscopy (2.5-6.5 THz) and THz time-domain spectroscopy (TDS, 0.2-1.2 THz), we find that the real part of optical conductivity increases for ML MoS₂ on SiO₂/Si and sapphire substrates and decreases for it on quartz with increasing radiation frequency. It is shown that the complex optical conductivity for ML MoS₂, obtained from THz TDS measurements, can fit very well to the Drude-Smith formula. Thus, the dependence of optical conductivity of ML MoS₂ on different substrates can be understood via a mechanism of electronic localization, and the electron density, relaxation time, and localization factor of the sample can be determined optically. Furthermore, we examine the influence of temperature on these key parameters in ML MoS₂ on different substrates. The results obtained from this Letter indicate that THz spectroscopy is a very powerful tool in studying and characterizing ML MoS₂-based electronic systems, especially in examining the electronic localization effect which cannot be directly measured in conventional electrical transport experiment. This Letter is relevant to an in-depth understanding of the optoelectronic properties of ML MoS₂ and of the proximity effect induced by different substrates.