S. Jugade, P. Sen, A. Naik
Indian Institute of Science,
India
Keywords: graphene, intercalated water, torsional resonance
Summary:
Exfoliation of graphene in an ambient environment leads to the trapping of moisture between graphene and a hydrophilic substrate [1]. This interfacial ice-like water layer significantly modifies graphene's tribological [2] and electronic properties [3]. The confined water also governs several other properties like adhesion, surface chemistry, conductance, and local strain in graphene. Visualizing micron-scale islands of trapped liquid is possible through topography in Atomic Force Microscopy (AFM). However, localized mapping of the sub-surface liquid regions is not easy. In this work, we show Torsional Resonance AFM phase-contrast as a simple tool to enable nanoscale mapping of intercalated water in graphene. TR phase remains nearly the same on SiO2 and graphene with intercalated water, while it is ∼1°-3° less on graphene without intercalated water. This technique probes the graphene-liquid interface in a non-invasive way, nonetheless extremely sensitive to near-field lateral forces. The TR phase contrast most likely arises due to the screening ability of graphene. Variation in phase contrast with tip-sample distance shows the opacity of graphene to tip and SiO2 van der Waals interaction, whereas its transparency to tip and hydrogen-bonded water interaction. Furthermore, we utilize this TR phase mapping technique to study the effect of moisture intrusion and thermal processing on the distribution of intercalated water in graphene. References: [1] Ochedowski, O. et al. Sci Rep (2014) [2] Lee et al. J. Phys. Chem. Lett. (2017) [3] Shim J. et al. Nano Lett. (2012)