Evolution of defect formation during atomically precise desulfurization of monolayer MoS2

Surface/defect engineering of two-dimensional (2D) materials is crucial for modifying their properties due to their high surface-to-volume ratio. Plasma treatment offers excellent control and uniformity for this purpose, enabling chemical functionalization, heteroatom doping, defect generation, and etching in transition metal dichalcogenides (TMDs) like MoS2, MoTe2, and WSe2. Janus-type membranes of MoSSe and WSSe can be fabricated by selenizing the hydrogenated top surface. Defects in MoS2 can enhance catalytic performance for water splitting. However, the structural evolution of MoS2 during hydrogen plasma treatment has not been extensively studied due to the high reactivity and instability of plasma-treated MoS2. This research addresses this gap by systematically investigating the structural and property changes in MoS2 exposed to low-energy indirect hydrogen plasma, aiming for atomic precision etching and controllable defect generation.

Previous research has explored surface/defect engineering of 2D materials using various techniques like laser exposure, ion irradiation, and plasma treatment, demonstrating their potential in applications such as field-effect transistors, sensors, and catalysts. Plasma treatment, particularly, has shown promise in modifying TMDs, achieving chemical functionalization, heteroatom doping, and defect engineering. Studies have highlighted the role of defects in enhancing the catalytic performance of MoS2 for water splitting and hydrogen evolution. However, the precise structural changes during hydrogen plasma treatment of MoS2 have remained elusive due to the challenges in observing the process in real-time and the instability of the resulting material.

Monolayer MoS2 samples were synthesized using atmospheric pressure CVD (APCVD). A home-built remote hydrogen plasma system was employed for desulfurization, ensuring indirect exposure to avoid direct ion bombardment. The plasma was generated at the center of three ring-shaped electrodes, with the MoS2 samples positioned away from the ignition area. The structural changes were analyzed using Raman spectroscopy, transmission electron microscopy (TEM), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Chemical composition was characterized using Auger electron spectroscopy (AES) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). Optical properties were investigated through photoluminescence (PL) spectroscopy. Electrical properties were measured using MoS2 field-effect transistors (FETs). Mechanical properties, specifically friction, were characterized using atomic force microscopy (AFM). The CVD growth involved heating MoO3 and sulfur powders to generate MoS2 on a SiO2/Si substrate. For TEM/STEM, samples were transferred onto graphene-coated holey carbon TEM grids. AES and ToF-SIMS provided chemical composition information. FET fabrication involved e-beam lithography and metal deposition. High-resolution AFM was used to analyze frictional properties, measuring lateral stiffness.

Low-energy indirect hydrogen plasma treatment enabled atomically precise desulfurization of monolayer MoS2, initially removing only the top-layer sulfur atoms while preserving the molybdenum layer and bottom sulfur atoms. Raman spectroscopy revealed significant shifts and broadening of E' and A1' vibration modes, indicative of strain and sulfur vacancy formation. HAADF-STEM imaging confirmed a significant increase in the density of sulfur vacancies (V_S and V_S2) after plasma treatment, preferentially aligned along the zigzag orientation. AES showed a decrease in the S/Mo atomic ratio with increasing treatment time, corroborating the sulfur vacancy formation. ToF-SIMS depth profiling indicated that the plasma removed only the top sulfur layer. The formation of hexagonal nanocracks along the zigzag orientation was observed in samples with more than 50% sulfur atoms removed from the top layer. These cracks were shown to be a consequence of the relaxation of biaxial tensile strain caused by the vacancies. Photoluminescence (PL) intensity gradually decreased and was eventually quenched with increasing treatment time. This was attributed to an increase in non-radiative recombination centers from the vacancies. Conductivity decreased by two orders of magnitude due to increased scattering centers. AFM measurements showed a ~50% increase in friction, resulting from the defect-induced increase in contact stiffness and change from periodic sawtooth pattern indicating stick-slip friction to aperiodic behavior.

The results demonstrate the precise control offered by low-energy indirect hydrogen plasma for modifying the surface and defects in MoS2. The observed evolution from sulfur vacancies to nanocracks highlights the interplay between defect density and strain relaxation. The significant reduction in PL intensity and conductivity shows a strong correlation between defect density and electronic properties. The increased friction, contrasting with the expected behavior of a metallic MoSH surface, is attributed to the combined effects of defects and increased contact stiffness. The findings are significant because they illustrate atomic-scale control over the properties of MoS2, offering valuable insights for designing and tailoring TMD materials for specific applications. The observed behavior deviates from theoretical expectations of metallic behavior from sulfur vacancies and/or hydrogenation, emphasizing the importance of considering combined effects of strain and defects.

This study presents a detailed investigation of atomically precise desulfurization of monolayer MoS2 using low-energy indirect hydrogen plasma. The findings reveal a systematic evolution of defects, from the initial formation of sulfur vacancies to the generation of nanocracks upon strain relaxation. These structural changes significantly impact optical, electrical, and mechanical properties. The ability to controllably introduce defects provides a powerful tool for tailoring the properties of MoS2 and other TMDs for various applications. Future work could explore other plasma conditions to control defect types and distributions. Further investigations into the catalytic activity of the desulfurized MoS2 would also be valuable.

The study primarily focuses on the effects of hydrogen plasma on monolayer MoS2. The generalizability of the findings to other TMDs or multilayer structures requires further investigation. The use of indirect plasma may limit the applicability to certain types of defects or defect concentrations. The samples sizes and number are not explicitly stated in the paper, which may affect the statistical significance of findings.