Scanning Force Microscopy of Graphene on Water: From Nanoscale Imaging to Interfacial Forces and Phenomena

Abstract

Graphene’s atomic thinness and remarkable properties make it an ideal ultrathin, mechanically robust and functional layer on liquid surface. This enables a wide range of applications, including cellular and molecular sensing, spectro-microscopy of wet specimens, tribological engineering, and fundamental studies of water at the nanoscale. Techniques that can simultaneously probe fundamental forces and visualize nanoscale phenomena are powerful tools for studying such interfaces. In this context, this thesis employs dynamic atomic force microscopy (AFM) with high-frequency, high-Q oscillating probes in ambient conditions. Out-of-plane and in-plane tip-surface interactions captured through cantilever dynamics were used for nanomechanical characterization of three systems: (i) suspended graphene on water in a microfluidic device, (ii) water confined beneath mono- and few-layer graphene on hydrophilic substrates, and (iii) water encapsulated within graphene liquid cells (GLCs). The first part of this thesis explores suspended graphene on water within a microfluidic platform. Flexural-mode dynamic AFM enabled stable imaging. It showed how graphene stabilizes the water surface with a ten-fold increase in stiffness. Combined with van der Waals forces on graphene, this results in complete elimination of tip–water capillary instabilities. Phase imaging revealed nanoscale defects that can trap water. These defects form a self-seal that persists for hours after the water dries from the channels. High-resolution force measurements further captured drying-induced membrane dynamics under surface tension and capillary pressure, providing insights into the robustness of the device. This work opens up avenues for bio-nanomechanical imaging and sensing with graphene-microfluidic platforms by monitoring cantilever dynamics. The second part investigates ultrathin water confined beneath mono- and few-layer graphene on hydrophilic SiO₂, where trapped water significantly impacts surface properties. Direct visualization of trapped water has been difficult. This is because the substrate roughness and graphene’s topological features are comparable to the water layer thickness. Using torsional-resonance AFM with small-amplitude in-plane oscillations, we have imaged water layers less than 2 nm thick. They were resolved through 0.5°–1.5° phase shifts that were independent of stiffness, friction, or topography. This approach also identified water inside graphene nanobubbles and interlayers, and mapped its distribution under varying substrate temperatures. The results establish TR-AFM as a sensitive method for nanoscale visualization of trapped ultrathin water, and for understanding fundamental behaviour of ultrathin water beneath graphene. The final part presents direct experimental evidence that water confined within GLCs can exhibit solid-like characteristics. Such behaviour was previously observed only for a few molecular layers. Using in-plane TR-AFM combined with out-of-plane Peak Force Tapping, we visualized water pockets less than 12 nm thick and about 100 nm in lateral size. These pockets showed higher vertical stiffness than graphene along with indications of ordered molecular structuring. These findings highlight the role of confinement pressure, interfacial hydrogen bonding, and geometry in suppressing viscous behaviour, while pointing to how confinement volume, probing frequency, and coupled vertical–lateral measurements can reveal the transition between liquid-like and solid-like phases of nanoscale water.