Friction characteristics of mechanically exfoliated and CVD-grown single-layer MoS2

Dinh Le Cao Ky,Bien-Cuong Tran Khac,Chinh Tam Le,Koo-Hyun Chung,Yong Soo Kim

doi:10.1007/s40544-017-0172-8

Abstract

In this work, the friction characteristics of single-layer MoS2 prepared with chemical vapor deposition (CVD) at three different temperatures were quantitatively investigated and compared to those of single-layer MoS2 prepared using mechanical exfoliation. The surface and crystalline qualities of the MoS2 specimens were characterized using an optical microscope, atomic force microscope (AFM), and Raman spectroscopy. The surfaces of the MoS2 specimens were generally flat and smooth. However, the Raman data showed that the crystalline qualities of CVD-grown single-layer MoS2 at 800 °C and 850 °C were relatively similar to those of mechanically exfoliated MoS2 whereas the crystalline quality of the CVD-grown single-layer MoS2 at 900 °C was lower. The CVD-grown single-layer MoS2 exhibited higher friction than mechanically exfoliated single-layer MoS2, which might be related to the crystalline imperfections in the CVD-grown MoS2. In addition, the friction of CVD-grown single-layer MoS2 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 MoS2 and elucidating the feasibility of single-layer MoS2 as protective or lubricant layers for micro- and nano-devices.

Highlights

Thin molybdenum disulfide (MoS2) exhibits remarkable thermal [1], electrical [2, 3], and optical [4] properties that differ from those of bulk MoS2
The superior mechanical properties [11, 12] and low frictional characteristics [13, 14] of atomically thin MoS2 suggest that it has potential as an effective protective or lubricant layer [15, 16] for micro- and nano-devices that experience contact sliding during operation
The chemical vapor deposition (CVD)-grown MoS2 exhibited greater friction than the mechanically exfoliated single-layer MoS2, which might have been associated with the crystalline imperfections in CVD-grown MoS2

Summary

Introduction

Thin molybdenum disulfide (MoS2) exhibits remarkable thermal [1], electrical [2, 3], and optical [4] properties that differ from those of bulk MoS2. To properly implement atomically thin MoS2 as a protective or a lubricant layer, more data should be gathered over a wide variety of experimental conditions Various methods, such as mechanical exfoliation [9, 26], lithium intercalation-assisted exfoliation [24], liquid exfoliation [27], and physical vapor deposition [28] have been used to prepare atomically thin MoS2. The friction characteristics of single-layer MoS2 prepared using CVD at various temperatures were systematically investigated using AFM. The outcomes of this work are expected to aid in evaluating the feasibility of single-layer MoS2 as protective or lubricant layers for micro- and nano-devices, and optimize CVD growth conditions from a tribological point of view

Experimental details

Results and discussion

Conclusions