Abstract:In this work, the friction characteristics of single-layer MoS 2 prepared with chemical vapor deposition (CVD) at three different temperatures were quantitatively investigated and compared to those of single-layer MoS 2 prepared using mechanical exfoliation. The surface and crystalline qualities of the MoS 2 specimens were characterized using an optical microscope, atomic force microscope (AFM), and Raman spectroscopy. The surfaces of the MoS 2 specimens were generally flat and smooth. However, the Raman data showed that the crystalline qualities of CVD-grown single-layer MoS 2 at 800 °C and 850 °C were relatively similar to those of mechanically exfoliated MoS 2 whereas the crystalline quality of the CVD-grown single-layer MoS 2 at 900 °C was lower. The CVD-grown single-layer MoS 2 exhibited higher friction than mechanically exfoliated single-layer MoS 2 , which might be related to the crystalline imperfections in the CVD-grown MoS 2 . In addition, the friction of CVD-grown single-layer MoS 2 increased as the CVD growth temperature increased. In terms of tribological properties, 800 °C was the optimal temperature for the CVD process used in this work. Furthermore, it was observed that the friction at the grain boundary was significantly larger than that at the grain, potentially due to defects at the grain boundary. This result indicates that the temperature used during CVD should be optimized considering the grain size to achieve low friction characteristics. The outcomes of this work will be useful for understanding the intrinsic friction characteristics of single-layer MoS 2 and elucidating the feasibility of single-layer MoS 2 as protective or lubricant layers for micro-and nano-devices.

Atomically thin layered materials such as MoS2 and graphene have attracted a lot of interest as protective coating layers for micro- and nano-electromechanical devices based on their superior mechanical properties and chemical inertness. In this work, the frictional characteristics of single layer MoS2 and graphene prepared by the mechanical exfoliation method were quantitatively investigated using atomic force microscopy. The results showed that both MoS2 and graphene exhibited relatively low friction forces of 1-3 nN under normal forces ranging from 1 to 30 nN. However, a higher increase in the friction force as the normal force increased was observed in the case of MoS2. The differences in the adhesion characteristics and mechanical properties of atomically thin layered materials may influence the puckering of the layer, which in turn influences the frictional behavior.