Reduction in Step Height Variation and Correcting Contrast Inversion in Dynamic AFM of WS2 Monolayers

Kyle Godin,Christian Cupo,Eui-Hyeok Yang

doi:10.1038/s41598-017-18077-4

Abstract

A model has been developed to account for and prevent the anomalies encountered in topographic images of transition metal dichalcogenide monolayers using dynamic atomic force microscopy (dAFM). The height of WS2 monolayers measured using dAFM appeared to be increased or decreased, resulting from the interactions between the tip and the surface. The hydrophilic SiO2 substrate appeared higher than the weakly hydrophilic WS2 when the tip amplitude was low or at a high set point (high force). Large amplitudes and low set points corrected the step height inversion, but did not recover the true step height. Removing water from the sample resulted in an order of magnitude reduced variation in step height, but the WS2 appeared inverted except at low amplitudes and high set points. Our model explains the varying step heights in dAFM of TMDs as a result of varying tip-sample interactions between the sample and substrate, in the presence or absence of capillaries. To eliminate contrast inversion, high amplitudes can be used to reduce the effect of capillary forces. However, when capillaries are not present, low amplitudes and high set points produce images with proper contrast due to tool operation in the repulsive regime on both materials.

Highlights

Transition metal dichalcogenides (TMDs) are a class of layered materials from which a single layer can be isolated with a thickness less than 1 nm
For the same experiment with exfoliated materials, large steps (>6 nm) were recorded and no contrast inversion was observed over a wide range of set points and amplitudes which we attribute to contaminants trapped between the TMD and the substrate
We have shown that varying tip-surface interactions are the cause of step height measurement errors on TMDs, and contrast inversion in topographic images can be corrected by appropriate set point and tip amplitude: namely high amplitudes when capillaries are present and low amplitudes with high set points on annealed samples

Summary

Introduction

Transition metal dichalcogenides (TMDs) are a class of layered materials from which a single layer can be isolated with a thickness less than 1 nm. Thin film and nanoparticle topographies from many sample-substrate systems have shown varying height and contrast inversion, including block copolymers, nanoparticles, and self-assembled monolayers on Au and mica[24,25,26,29,30,31,32,33] For these other material systems, the measured layer thicknesses and nanoparticle heights have been found to be dependent on tip amplitude and force[34]. Thickness, and topography measurements in AFM of TMD monolayers exhibit many artifacts and features which have not been systematically studied

Methods

Results

Discussion

Conclusion