Atomic-scale characterization of interfacial electric polarization in van der Waals heterostructures and two-dimensionally confined water

Abstract

This thesis focuses on the measurement of electric polarization at two-dimensional (2D) interfaces using scanning probe microscopy (SPM). The atomically thin interfacial layer at the boundary of a material often has properties profoundly different from the same substance in bulk. Interfacial properties are typically poorly characterized due to the difficulty of probing them at molecular resolution. Here, atomic force microscopy (AFM), scanning dielectric microscopy (SDM), electrostatic force microscopy (EFM), and Kelvin probe force microscopy (KPFM) were applied to 2D interfaces to measure their interfacial polarization properties. Using SDM, we measured the dielectric and electrodynamic response of deionized (DI) water confined inside nanoslits of different thicknesses, fabricated from graphene (Gr), hexagonal boron nitride (hBN), or micas. We found that the water confined inside hydrophobic hBN/hBN nanochannels displayed an anomalously high dielectric response in the in-plane direction, due to the high in-plane dielectric constant and high in-plane conductivity of interfacial water. Preliminary results for water inside hydrophilic nanochannels of muscovite mica showed a suppressed out-of-plane dielectric constant, consistent with previous findings for water in Gr/hBN channels. Water confined in graphite/mica nanoslits, obtained from direct exfoliation of mica flakes onto a graphite substrate, formed hexagonally-symmetric plateaus, which also showed a suppressed out-of-plane dielectric constant, in agreement with previous results. Furthermore, we unexpectedly observed ferroelectric-like charge-polarized triangular domains at the marginally twisted hBN/hBN interface. Electrostatic imaging of these domains using AFM, EFM and KPFM, combined with theoretical calculations of the atomic lattice reconstruction, showed that they originated from interfacial dipoles created by reconstruction of the hBN lattice into regions of different stacking order. These findings provide much-needed experimental data on interfacial water and constitute an advance in our understanding of electric polarization at 2D interfaces. They open up possibilities for designing novel van der Waals heterostructures, and demonstrate new ways to probe interfacial polarization properties directly at the atomic scale.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Andre Geim (Supervisor) & Laura Fumagalli (Supervisor)