Rapid Degradation of the Electrical Properties of 2D MoS Thin Films under Long-Term Ambient Exposure.

The MoS thin film has attracted a lot of attention due to its potential applications in flexible electronics, sensors, catalysis, and heterostructures. Understanding the effect of long-term ambient exposure on the electrical properties of the thin film is important for achieving many overreaching goals of this material. Here, we report for the first time a systematic study of electrical property variation and stability of MoS thin films under ambient exposure of up to a year. The MoS thin films were grown via the sulfurization of 6 nm thick molybdenum films. We found that the resistance of the samples increases by 114% just in 4 weeks and 430% in 4 months and they become fully insulated in a year of ambient exposure. The dual-sweep current-voltage (-) characteristic shows hysteretic behavior for a 4-month-old sample which further exhibits pronounced nonlinear - curves and hysteretic behavior after 8 months. The X-ray photoelectron spectroscopy measurements show that the MoS thin film gradually oxidizes and 13.1% of MoO and 11.8% oxide of sulfur were formed in 4 months, which further increased to 23.1 and 12.7% in a year, respectively. The oxide of the sulfur peak was not reported in any previous stability studies of exfoliated and chemical vapor deposition-grown MoS, suggesting that the origin of this peak is related to the distinct crystallinity of the MoS thin film due to its smaller grain sizes, abundant grain boundaries, and exposed edges. Raman studies show the broadening of E and A peaks with increasing exposure time, suggesting an increase in the disorder in MoS. It is also found that coating the MoS thin film with polymethylmethacrylate can effectively prevent the electrical property degradation, showing only a 6% increase in resistance in 4 months and 40% over a year of ambient exposure.