Wetting Property of the Edges of Monoatomic Step on Graphite: Frictional-Force Microscopy and ab Initio Quantum Chemical Studies

Abstract

In this work, we present the wetting property of the edges of a monatomic step on graphite. We have used frictional force microscope to experimentally investigate the wetting property of these edges. We have carried out quantum chemical calculations on a model system of nanographene to characterize hydrogen-bonding interactions between water and two different edges of the graphene, namely, the zigzag and arm-chair edges, to explain our experimental results. We have clearly observed two distinct frictional properties along the monatomic steps on graphite surface under varying conditions of humidity. The distinct frictional properties of the edges have been attributed to the different wetting properties associated with two different edges. Our subsequent quantum chemical calculations complement experimental findings with edge-specific graphene–water interaction.

Measuring nanoscale friction at graphene step edges

“…The origin of this resistive force during descending remains controversial [19,21,22,27]. Moreover, the shape of the friction signal during descending reported in the literature also varied drastically-in some papers, the friction profile was a single peak [19,20,22,23,26,27,30], but in some others, the friction profile seemed to be composed of two peaks with a valley between them [20,21,28,29]. For better understanding the origin of the resistance during descending, it is critical to resolve this discrepancy.…”

“…This finding implies that the previous Refs. [19,20,22,23,26,27,30], in which the scan resolution was not high enough, might have missed the fine features in the friction response of the single-layer graphene step edge during the step-down scan.…”

“…When the AFM tip ascends a step edge, the friction always increases to a level more than an order of magnitude larger than that of the basal plane [17][18][19][20][21][22][23][24][25][26][27][28][29][30]. In the descending case, a majority of previous works showed that there is a resistive force acting against the tip sliding, increasing friction [17-21, 23, 26-30], while some other researchers reported that the friction force decreases and even becomes negative [21,24].…”

“…However, flat graphene basal planes cannot represent entire graphene coatings at macroscales; coatings over a large area will contain defects such as wrinkles, folds, step edges, humps, and so on. Among the structures mentioned above, the graphene step edge plays significant roles in friction because of its relatively high abundance and extremely large friction compared to the basal plane [17][18][19][20][21][22][23][24][25][26][27][28][29][30].…”

Oxidative Tearing of Graphene Sheets: Insights into the Probable Situations by Computational and Experimental Studies

“…26 The trans-edge is more hydrophilic as compared to the overall cis-edge, which has been established both experimentally and theoretically. 27−29 It was theoretically shown that chain of epoxy groups are responsible for oxidative unzipping of GO sheets 22,23,30,31 but none of the studies indicated any preference in generation of cis-edge or trans-edge after reduction A nanoscale graphene sheet has highly hydrophobic faces and slightly polar edges 25,29,32 with trans-edge being more polar.…”