Investigation of nanotribological properties of 2-dimensional transition metal dichalcogenides by atomic force microscopy

Abstract

With the continuous miniaturization and integration of devices, the influence of the properties of a single atomic layer has become increasingly significant, raising the importance of research on material surfaces. Atomic force microscopy (AFM) is an ideal tool for investigating phenomena occurring on material surfaces, especially for the precise evaluation of properties in two-dimensional materials, which consist of only a few atomic layers. Chapter 1 emphasizes the significance and historical context of the studies in surface science and 2D materials. Chapter 2 provides fundamental explanations about the principles and measurement methods of atomic force microscopy. In Chapter 3, the changes in nanotribological properties of mechanically exfoliated molybdenum disulfide (MoS2) were studied by AFM, when water layers are intercalated between MoS2 and hydrophilic substrate. 1 or 2 layers of intercalated water increased the frictional force of monolayer MoS2 by 2 and 3 times, respectively. This suggests that the physical or electrical properties of the 2D materials do not play an important role on the friction increase from water intercalation between exfoliated 2D materials and hydrophilic substrates. Chapter 4 compares the phase dependence in nanotribological properties of mechanically exfoliated transition metal dichalcogenides, molybdenum disulfide and molybdenum ditelluride (MoTe2). The results revealed that MoS2 and MoTe2 in the metallic 1T' phase with an octahedral structure exhibit friction in approximately 5 to 10 times higher than the trigonal prismatic semiconducting 2H phase, in nanoscale. Through density functional theory calculations, this phenomenon is explained by the difference in the overlap of phonon density of states between the material and the substrate, and the difference in potential energy barrier in the minimum energy scan path. The research precisely compares and elucidates the nanotribological phenomena occurring at the monolayer level in MoS2. These findings are expected to offer valuable insights and applicability not only for MoS2 but also for the broader realm of 2D materials.