Atomically thin molybdenum disulfide prepared via chemical vapor deposition and mechanical exfoliation: aging studies and reliability testing

Abstract

Transition metal dichalcogenides (TMDs) are a class of two-dimensional materials that have attracted significant attention due to their unique electronic, optical, and mechanical properties, which makes them promising candidates for next-generation electronic, optoelectronic, and sensing devices. Among TMDs, molybdenum disulfide (MoS₂) stands out for its semiconducting nature, large band gap (1.9 eV for monolayer MoS₂), and high career mobility (50-200 cm²V⁻¹s⁻¹), which positions it as a key material for various applications. However, like most TMDs, MoS₂ suffers stability issues when subjected to extreme conditions, such as elevated temperatures and atmospheric environments, limiting its applications in next-generation electronics, optoelectronics, and sensing devices. This study reports on the synthesis of atomically thin two-dimensional MoS₂ using both chemical vapor deposition (CVD) and mechanical exfoliation methods, the aging studies, and the electrical performance of the synthesized MoS₂ flakes and MoS₂ devices, respectively. The study is divided into two parts. In the first part, we aimed to modify an existing CVD furnace in the lab to produce large-area, atomically thin MoS₂ flakes following procedures described in published works. In the initial CVD experiments, we replicated the same conditions as reported in literature, including parameters such as growth temperature, argon gas flow rate, pressure, amounts of molybdenum and sulfur precursors, heating rate, and growth time. Large area MoS₂ flakes could not be observed, instead, the synthesized material consisted of particles and thick films. In order to achieve thin, large-area flakes, we varied the growth parameters such as the growth temperature, pressure, amounts of precursor, and argon gas flow rate. The optimized growth conditions (Growth temperature=850 °C, Agon flow rate=100 sccm, Sulfur=1000 mg, MoO₃=50 mg, Growth time=10 mins, Heating rate = 13℃/min) successfully yielded large area, atomically thin MoS₂ flakes. In addition, the distance of the growth substrates (Si/SiO₂ wafers) from precursor source were studied. Two substrates were arranged faced down and close together , covering one side of the ceramic vessel or boat supporting the wafers, with no gap between the wafers and one side of the end of the boat, resulting in the growth of large MoS₂ flakes across the entire wafer surface. Repeating the experiment under the same conditions yielded consistent results, confirming that the chosen growth parameters and wafer arrangement reliably produce MoS₂ flakes. Later, we explored the effects of argon gas flow rate on MoS₂ thin flakes by varying the argon gas flow rates during the synthesis of MoS₂ thin films while maintaining the other conditions. The results showed that the argon gas flow rate had an influence on the size of the MoS₂ flakes. In the second part of the study, we conducted aging studies on the MoS₂ films fabricated by CVD as well as those produced via mechanical exfoliation. The thin MoS₂ films on SiO₂/Si wafers were heated on a hot plate at 200 ℃ for one month in ambient air. The degradation of MoS₂ over time was characterized using optical microscopy, Raman spectroscopy, and Image J software. It was observed that the CVD-grown MoS₂ developed some surface cracks, experienced a reduction in flake size or oxidative etching at the edges, and thinning or reduction in its thickness, whereas the mechanically exfoliated MoS₂ flakes showed a reduction in size and thickness but no visible cracks throughout the heating period. These results indicate that mechanically exfoliated MoS₂ flakes show relatively greater thermal stability, while CVD-grown MoS₂ flakes experienced more pronounced degradation under thermal stress. Additionally, we investigated the effects of heating on CVD-grown MoS₂ two-terminal electrical devices. The chips were fabricated using photolithography and subject to 150 °C on a hot plate for approximately 1100 hours. Current-voltage (I-V) measurements were carried out regularly using a two-point probe station. We measured the current at 5V on different days and observed that the current increased at the same voltage in the MoS₂-based device. These findings provide insights into the influence of heating MoS₂-based devices in the air. They contribute to the broader understanding of MoS₂ reliability and pave the way for its application in nanoelectronics, optoelectronics, and sensors.