Direct imaging, three-dimensional interaction spectroscopy, and friction anisotropy of atomic-scale ripples on MoS2

Omur E Dagdeviren,Mehmet Z Baykara,Ogulcan Acikgoz,Peter Grütter

doi:10.1038/s41699-020-00164-2

Abstract

Theory predicts that two-dimensional (2D) materials may only exist in the presence of out-of-plane deformations on atomic length scales, frequently referred to as ripples. While such ripples can be detected via electron microscopy, their direct observation via surface-based techniques and characterization in terms of interaction forces and energies remain limited, preventing an unambiguous study of their effect on mechanical characteristics, including but not limited to friction anisotropy. Here, we employ high-resolution atomic force microscopy to demonstrate the presence of atomic-scale ripples on supported samples of few-layer molybdenum disulfide (MoS2). Three-dimensional force/energy spectroscopy is utilized to study the effect of ripples on the interaction landscape. Friction force microscopy reveals multiple symmetries for friction anisotropy, explained by studying rippled sample areas as a function of scan size. Our experiments contribute to the continuing development of a rigorous understanding of the nanoscale mechanics of 2D materials.

Highlights

The discovery that atomically thin sheets can be isolated from bulk crystals and the exciting physical properties exhibited by them, initiated the thriving field of two-dimensional (2D) materials[1]
The presence of ripples was confirmed by transmission electron microscopy (TEM) imaging performed on suspended single-layer graphene[7], as well as singlelayer MoS29, in an indirect fashion, i.e., by studying the broadening of diffraction spots in reciprocal space
The results reported here, which constitute the first direct atomic force microscopy (AFM)-based imaging of defining role that ripples are thought to play in the friction anisotropy of such materials[16,17], we initially conducted FFM experiments on few-layer MoS2 samples exfoliated onto SiO2, whereby topographical and friction force maps are recorded atomic-scale ripples on a 2D material such as MoS2, at the same time open up the way for their detailed characterization in the form of interaction forces and energies

Summary

INTRODUCTION

The discovery that atomically thin sheets can be isolated from bulk crystals and the exciting physical properties exhibited by them, initiated the thriving field of two-dimensional (2D) materials[1]. We employ AFM-based high-resolution imaging, threedimensional (3D) force/energy spectroscopy, and nanoscale friction measurements for a detailed physical characterization of atomic-scale ripples on few-layer MoS2 samples supported on SiO2. In order to overcome the limitations of FFM and tapping-mode the interaction energies experienced by the probe tip in close AFM in terms of spatial resolution, we imaged the topography of proximity to the sample surface, revealing modulations of the potential energy landscape down to a few meV. A Fourier- contact is avoided during the measurements, resulted in transform-based analysis of lateral force data derived from 3D potential energy maps as a function of scan size provides clues the direct imaging of ripples on the MoS2 surface (on a flake of ~65 Å height, corresponding to ~10 layers) in the form of linearly regarding the observed variety in friction anisotropy. Aligned, minute undulations in the surface topography, with outof-plane corrugations of 1–5 Å and lateral spacings on the order of

RESULTS

Findings

DISCUSSION