Electrolyte-gated charge transport in molecularly linked gold nanoparticle films: The transition from a Mott insulator to an exotic metal with strong electron-electron interactions.

Abstract

Strong electron-electron interactions experienced by electrons as they delocalize are widely believed to play a key role in a range of remarkable phenomena such as high Tc superconductivity, colossal magnetoresistance, and others. Strongly correlated electrons are often described by the Hubbard model, which is the simplest description of a correlated system and captures important gross features of phase diagrams of strongly correlated materials. However, open challenges in this field include experimentally mapping correlated electron phenomena beyond those captured by the Hubbard model, and extending the model accordingly. Here we use electrolyte gating to study a metal-insulator transition (MIT) in a new class of strongly correlated material, namely, nanostructured materials, using 1,4-butanedithiol-linked Au nanoparticle films (NPFs) as an example. Electrolyte gating provides a means for tuning the chemical potential of the materials over a wide range, without significantly modifying film morphology. On the insulating side of the transition, we observe Efros-Shklovskii variable range hopping and a soft Coulomb gap, evidencing the importance of Coulomb barriers. On the metallic side of the transition, we observe signatures of strong disorder mediated electron-electron correlations. Gating films near MIT also reveal a zero-bias conductance peak, which we attribute to a resonance at the Fermi level predicted by the Hubbard and Anderson impurity models when electrons delocalize and experience strong Coulomb electron-electron interactions. This study shows that by enabling large changes in carrier density, electrolyte gating of Au NPFs is a powerful means for tuning through the Hubbard MIT in NPFs. By revealing the range of behaviours that strongly correlated electrons can exhibit, this platform can guide the development of an improved understanding of correlated materials.