Artifacts in AFM topography images of TMD monolayers
Initially motivated by work with graphene, the broad class of two dimensional (2D) materials has generated enormous interest. Monolayers and heterostructures of transition metal dichalcogenides (TMDs) have attracted intense investigation as the long awaited 2D semiconductor analogs of graphene. Due to their direct bandgap, TMD monolayers are extremely promising candidates for atomically thin electronic, optical, and photovoltaic applications.
Before the possibility of using TMD monolayers in various applications, a comprehensive characterization of these materials is required. Atomic force microscopy (AFM) is able to characterize topography including loosely bound contaminants and adlayers through amplitude-phase-distance and force-distance curves, and has the additional potential for probing local electronic and magnetic properties. However, unwanted interactions between the tip and sample result in forces on the same order as the force from the property under investigation. While existing reports contain a range of results, anomalies in AFM scans have not been fully explored. The interaction forces between TMD and substrate have been known to affect the measured step height (or thickness) of TMD monolayers in AFM. Decoupling various forces for analysis requires a full understanding of the tip-sample interactions. Such a description of tip-sample forces and step height anomalies relating to TMDs has not yet been reported.
In our work, we provide a detailed look into the analysis of artifacts in AFM topography images of WS2 monolayers including varied step height (WS2 showing different thicknesses depending on the AFM settings) and contrast inversion (WS2 appearing below the substrate). We describe our findings in terms of varying tip-sample interactions and capillary forces, and find that the step height depends strongly on the tip amplitude and set point. Furthermore, we prescribe tool settings to prevent contrast inversion and discuss “correct” TMD step height measurements, as we find our results applicable to other TMDs with similar wetting properties such as WSe2 and MoSe2.
Our key findings include 1) When present, capillary forces dominate measurement errors; 2) Removal of substrate water layers reduces the variation in height measurement from +/-6 nm to +/-0.5 nm; 3) When substrate water layers are present, contrast inversion is corrected by setting the AFM with high tip amplitudes and low set points; we show that high tip amplitudes reduce capillary forces; 4) When substrate water layers are absent, contrast inversion is corrected by setting the AFM with low amplitudes and high set points, which drive operation into the repulsive regime; and 5) Contaminants must be characterized through amplitude-phase-distance and force-distance curves, considering presence of surface contaminants and water layers.
This work represents a milestone in determining the “correct” height measurement in AFM, which cannot be determined from AFM topography images alone, but must be found through techniques such as force-distance characterization. Mr. Christian Cupo and Mr. Kyle Godin have experimentally led this project.
Kyle Godin, Christian Cupo, EH Yang
Stevens Institute of Technology, USA
Reduction in Step Height Variation and Correcting Contrast Inversion in Dynamic AFM of WS2 Monolayers.
Godin K, Cupo C, Yang EH
Sci Rep. 2017 Dec 19
|Ultra-wideband absorption based on… Due to high carrier mobility, the graphene has attracted enormous interests in developing high-speed photodetectors. In this study, for practical high-performance photodetectors, we focus on the remarkable enlargement of absorption…|
|Intense light boosts the circadian function of… Humans are daily exposed to intense light in the form of sunlight which is crucial for the circadian rhythms in our bodies to properly function. This is important as circadian…|
|Can nanographene cross placental barrier? Graphene is an upcoming revolutionized 2D material that has sketched its importance across a wide spectra of interdisciplinary arenas due to its distinguishing properties. Being a sp2 hybridized allotrope of…|
|Villi-like topography provides a native… The use of an organotypic culture system, as an alternative to animal models, for recapitulating physiological and biologically relevant intestinal tissue is constantly gaining success in various fields of science.…|
|Imaging metal nanoparticle adsorption on graphene aerogels The functionalization of porous carbon aerogels and sponge-like materials with metal nanoparticles (NPs) has shown significant promise for several applications, including energy storage and harvesting and environmental remediation. Methods for…|
|Electronic liquid crystal: A key ingredient for… The phases of matter are characterized by the symmetries inherent to each state. Imagine the freezing from water to ice. This phase transition is accompanied by the crystallization of H2O molecules,…|