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1. https://doi.org/10.1017/S1431927619014697

   https://doi.org/https://doi.org/10.1017/S1431927619014697  

   Chen, Zhe; Luo, Jiawei; Doudevski, Ivo; Erten, Sema; Kim, Seong H. (January 2019, Microscopy and microanalysis)

   Atomic force microscopy (AFM) is typically used for analysis of relatively flat surfaces with topographic features smaller than the height of the AFM tip. On flat surfaces, it is relatively easy to find the object of interest and deconvolute imaging artifacts resulting from the finite size of the AFM tip. In contrast, AFM imaging of three-dimensional objects much larger than the AFM tip height is rarely attempted although it could provide topographic information that is not readily available from two-dimensional imaging, such as scanning electron microscopy. In this paper, we report AFM measurements of a vertically-mounted razor blade, which is taller and sharper than the AFM tip. In this case, the AFM height data, except for the data collected around the cutting edge of the blade, reflect the shape of the AFM tip. The height data around the apex area are effectively the convolution of the AFM tip and the blade cutting edge. Based on computer simulations mimicking an AFM tip scanning across a round sample, a simple algorithm is proposed to deconvolute the AFM height data of an object taller and sharper than the AFM tip and estimate its effective curvature. 

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2. Measuring nanoscale friction at graphene step edges

   https://doi.org/10.1007/s40544-019-0334-y  

   Chen, Zhe Kim (January 2020, Friction)

   Although graphene is well known for super-lubricity on its basal plane, friction at its step edge is not well understood and contradictory friction behaviors have been reported. In this study, friction of mono-layer thick graphene step edges was studied using atomic force microscopy (AFM) with a Si tip in dry nitrogen atmosphere. It is found that, when the tip slides over a ‘buried’ graphene step edge, there is a resistive force during the step-up motion and an assistive force during the step-down motion due to the topographic height change. The magnitude of these two forces is small and the same in both step-up and step-down motions. As for the ‘exposed’ graphene step edge, friction increases in magnitude and exhibits more complicated behaviors. During the step-down motion of the tip over the exposed step edge, both resistive and assistive components can be detected in the lateral force signal of AFM if the scan resolution is sufficiently high. The resistive component is attributed to chemical interactions between the functional groups at the tip and step-edge surfaces, and the assistive component is due to the topographic effect, same as the case of buried step edge. If a blunt tip is used, the distinct effects of these two components become more prominent. In the step-up scan direction, the blunt tip appears to have two separate topographic effects elastic deformation of the contact region at the bottom of the tip due to the substrate height change at the step edge and tilting of the tip while the vertical position of the cantilever (the end of the tip) ascends from the lower terrace to the upper terrace. The high-resolution measurement of friction behaviors at graphene step edges will further enrich understanding of interfacial friction behaviors on graphene-covered surfaces. 

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3. Electric-Field-Assisted Contact Mode AFM-Based Nanolithography with Low Stiffness Conductive Probes

   https://doi.org/10.1115/1.4054316  

   Zhou, Huimin; Jiang, Yingchun; Dmuchowski, Christopher M; Ke, Changhong; Deng, Jia (April 2022, Journal of Micro and Nano-Manufacturing)

   Abstract Electric-field-assisted atomic force microscope (E-AFM) nanolithography is a novel polymer-patterning technique that has diverse applications. E-AFM uses a biased AFM tip with conductive coatings to make patterns with little probe-sample interaction, which thereby avoids the tip wear that is a major issue for contact-mode AFM-based lithography, which usually requires a high probe-sample contact force to fabricate nanopatterns; however, the relatively large tip radius and large tip-sample separation limit its capacity to fabricate high-resolution nanopatterns. In this paper, we developed a contact mode E-AFM nanolithography approach to achieve high-resolution nanolithography of poly (methyl methacrylate) (PMMA) using a conductive AFM probe with a low stiffness (~0.16 N/m). The nanolithography process generates features by biasing the AFM probe across a thin polymer film on a metal substrate. A small constant force (0.5-1 nN) applied on the AFM tip helps engage the tip-film contact, which enhances nanomachining resolution. This E-AFM nanolithography approach enables high-resolution nanopatterning with feature width down to ~16 nm, which is less than one half of the nominal tip radius of the employed conductive AFM probes. 

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4. Identifying Physical and Chemical Contributions to Friction: A Comparative Study of Chemically Inert and Active Graphene Step Edges

   https://doi.org/10.1021/acsami.0c08121  

   Chen, Zhe; Khajeh, Arash; Martini, Ashlie; Kim, Seong H. (January 2020, ACS applied materials interfaces)

   Friction has both physical and chemical origins. To differentiate these origins and understand their combined effects, we study friction at graphene step edges with the same height and different terminating chemical moieties using atomic force microscopy (AFM) and reactive molecular dynamics (MD) simulations. A step edge produced by physical exfoliation of graphite layers in ambient air is terminated with hydroxyl (OH) groups. Measurements with a silica countersurface at this exposed step edge in dry nitrogen provide a reference where both physical topography effects and chemical hydrogen-bonding (H-bonding) interactions are significant. H-bonding is then suppressed in AFM experiments performed in alcohol vapor environments, where the OH groups at the step edge are covered with physisorbed alcohol molecules. Finally, a step edge buried under another graphene layer provides a chemically inert topographic feature with the same height. These systems are modeled by reactive MD simulations of sliding on an OH-terminated step edge, a step edge with alkoxide group termination, or a buried step edge. Results from AFM experiments and MD simulations demonstrate hysteresis in friction measured during the step-up versus step-down processes in all cases except the buried step edge. The origin of this hysteresis is shown to be the anisotropic deflection of terminal groups at the exposed step edge, which varies depending on their chemical functionality. The findings explain why friction is high on atomically corrugated and chemically active surfaces, which provides the insight needed to achieve superlubricity more broadly. 

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5. Effect of Atomic Corrugation on Adhesion and Friction: A Model Study with Graphene Step Edges

   https://doi.org/10.1021/acs.jpclett.9b02501  

   Chen, Zhe Vazirisereshk (January 2020, The journal of physical chemistry letters)

   This Letter reports that the atomic corrugation of the surface can affect nanoscale interfacial adhesion and friction differently. Both atomic force microscopy (AFM) and molecular dynamics (MD) simulations showed that the adhesion force needed to separate a silica tip from a graphene step edge increases as the side wall of the tip approaches the step edge when the tip is on the lower terrace and decreases as the tip ascends or descends the step edge. However, the friction force measured with the same AFM tip moving across the step edge does not positively correlate with the measured adhesion, which implies that the conventional contact mechanics approach of correlating interfacial adhesion and friction could be invalid for surfaces with atomic-scale features. The chemical and physical origins for the observed discrepancy between adhesion and friction at the atomic step edge are discussed. 

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