Conductance spectroscopy of confined water in Montmorillonite clay nanoparticles

Montmorillonite (MNT) clay confines water and ions to few-monolayer sheets. We use an SPM-based nano-dielectric-spectroscopy (NDS) technique to study frequency-dependent (10Hz-10 kHz) electrical properties of individual MNT nanoparticles (NP) with variable water content. We study ionic conductivity in this 2D nano-confined geometry, and compare with bulk material with inter-particle contributions. We focus on the high-frequency (HF) power-law regime of the conductivity peak in the electric modulus, M(f) ~ f^1-α, related to a hopping-conductivity, σ ~ σ_dc + σ_acf^α . We apply a sinusoidal voltage to the conducting tip during non-contact portion of a double pass scan, using high-harmonic for feedback, and the fundamental mode to measure the phase and amplitude of the force-derivative (FD). For a range of relative humidity (RH), the FD phase, φ(f) exhibits a peak, similar to the peak in bulk M(f), but shifted to lower frequencies. We use finite-element-modelling to extract NP conductivity spectra from data (poster by A. Wells). Simulated φ(f) for anisotropically conducting NP have HF power-laws matching M(f). In bulk, for various RH we find exponents, α = 0.3 ±0.05. NP exhibited exponents of α = 0.6 ±0.1. This suggests a different conductivity mechanism in bulk and NP.