Speaker : Huang Tzu-En  PhD Candidate 

Venue : Room 36567, Fifth Floor, Science Building, Cheng Kung Campus

Date & Time : 2026 / 09 / 16 15:00

Since the discovery of graphene, two-dimensional (2D) materials have attracted considerable attention because of their atomically thin structures and unique physical properties. Among the family of transition metal dichalcogenides (TMDs), molybdenum disulfide (MoS 2 ) has received particular interest. Owing to its direct bandgap of approximately 1.8 eV in the monolayer limit, high current on/off ratio, relatively mature fabrication processes, and excellent optical properties, MoS 2 has emerged as a promising candidate for next-generation optical and electronic sensors. However, its extremely large surface-to-volume ratio also makes its physical properties highly sensitive to the surrounding environment and to the surface conditions of contacting materials.

During the fabrication of materials and devices, interfacial carrier traps and surface atomic vacancies, such as sulfur vacancies, are often unavoidable. These imperfections can degrade the carrier-transport properties of the resulting devices. For example, in gas-sensing applications, surface atomic vacancies may promote chemical adsorption on the material surface, leading to incomplete recovery of the sensing signal and limiting quantitative analysis. In optical sensors, interfacial traps can not only increase the dark current but also induce an optically induced field (OIF), which further influences both the magnitude and direction of the photocurrent. In addition, previous studies have often overlooked the role of the gate material in photocurrent generation. This effect may become particularly important under weak illumination, where the contribution from the gate material can dominate the photoresponse. Beyond sensing applications, accurately evaluating the surface physical condition of MoS 2 remains an important challenge. Although surface molecular doping provides an effective approach for modulating surface properties, strain is often introduced simultaneously, which can interfere with and obscure the interpretation of photoluminescence (PL) signals.

To address these challenges, this work presents a series of fundamental studies aimed at decoupling complex interfacial phenomena in monolayer MoS 2 . For gas- sensing applications, we introduce a two-trap model and a surface modulation
method, thereby restoring signal reversibility and enabling repeatable, quantitative gas sensing. In optoelectronic devices, we characterize the OIF and reveal the active role of the gate material, which can serve as the dominant source of photocarriers under weak illumination. Finally, we demonstrate that angle-resolved second-harmonic generation (SHG) provides a powerful, non-destructive probe for distinguishing charge-transfer-induced doping from lattice strain, while also offering directional information on strain relaxation that cannot be resolved by conventional spectroscopy. Together, these findings establish a comprehensive framework for understanding, controlling, and engineering MoS 2 interfaces for next-generation sensing and optoelectronic applications.

Keywords: Two-dimensional materials, Transition metal dichalcogenides, Monolayer MoS 2 , Field effect transistors, Interface engineering, Charge trapping, Gas sensing, Surface modulation, Optoelectronic devices, Optically induced field, Negative photocurrent, Surface molecular doping, Angle-resolved SHG, DFT calculation