Adhesion and Friction at Graphene/Self-Assembled Monolayer Interfaces Investigated by Atomic Force Microscopy

Abstract

The functional implementation of graphene as a solid boundary lubricant requires the ability to control its frictional response across a variety of interfaces. This is challenging, as being a single atomic layer thick, the nanotribological properties of graphene depend highly on the competing interaction strengths with the converse sides of the top and bottom contacts of the interfaces it is placed in between. One method to modulate these interactions is to tune the surface chemistry (of one or both counter-faces) with self-assembled monolayers (SAMs). To fully understand the effects on the graphene/SAM (G-SAM) composite interfaces formed, however, first necessitates a basic understanding of graphene–SAM interactions. To explore graphene–SAM adhesive and frictional interactions over a range of chemical functionalities, SAMs were used to functionalize atomic force microscopy (AFM) tips with varying terminal end-groups (−NH2, −CH3, and –phenyl, compared to unfunctionalized −OH terminated reference tips). AFM pull-off force measurements and thermal gravimetric analysis (TGA) were used to evaluate the work of adhesion (mJ/m2) and interaction energy (kcal/mol) of the functionalized tips with graphene. Friction force microscopy (FFM) measurements were performed with the same functionalized AFM tips to examine the graphene-molecule frictional response. Tip–graphene interaction strength was increased for hydrophobic and aromatic functional groups. The frictional response was found to depend on a balance of graphene-molecule adhesion and shear strain.

Quantification/mechanism of interfacial interaction modulated by electric potential in aqueous salt solution

“…Adhesion plays an important role in nanoscale friction, with friction generally increasing with adhesion forces [64,65]. Nanoscale friction is linearly correlated with load for weak adhesion between two surfaces [66,67]. However, the correlation changes from linear to nonlinear as the adhesion forces increase [66,67].…”

“…Nanoscale friction is linearly correlated with load for weak adhesion between two surfaces [66,67]. However, the correlation changes from linear to nonlinear as the adhesion forces increase [66,67]. Thus, it is necessary to explore the effect of adhesion on friction.…”

“…Accordingly, we analyzed the friction at positive potential using the classical binomial law of friction (Eq. (1)), which describes friction in terms of the mechanical resistance and molecular attraction [66,67]. Considering the load dependence, the friction force F f takes the form…”

Charge‐Induced Structural Changes of Confined Copolymer Hydrogels for Controlled Surface Morphology, Rheological Response, Adhesion, and Friction

“…Silanization of sharp tips was carried out using a procedure from Elinski et al. [ 46 ] Lateral force measurements were conducted with SiO 2 (silica) colloids of 20 µm nominal radius (Duke Scientific, Thermo Scientific, USA). Colloidal probes were created by mounting silica colloids of specific radii to tipless cantilevers (HQ:CSC37/No Al tipless, MikroMash, nominal spring constant 0.6–1.2 N m −1 ).…”

Robust Superlubric Interface across Nano- and Micro-Scales Enabled by Fluoroalkylsilane Self-Assembled Monolayers and Atomically Thin Graphene

“…Self-assembled monolayers (SAMs) have ordered molecular structure by adsorbing the active surfactant on a solid surface with a complicated shape and have been demonstrated as an effective lubricant across nano- and microscales because of good bonding strength, low surface energy, and shear strength. − The surface modification with SAMs provides a simple and batch chemical method for flexibly designing the selectable surface properties on arbitrary shapes, and thus are technologically attractive for building the needed lubricating coatings. Li et al achieved a liquid superlubric interface combining graphene with hydrophobic SAMs via water intercalation, where the maximum load studied was 250 nN.…”