Microscopic origin of the effect of substrate metallicity on interfacial free energies

Microscopic Origin of Twist-Dependent Electron Transfer Rate in Bilayer Graphene

Using molecular simulation and continuum dielectric theory, we consider how electrochemical kinetics are modulated by the twist angle in bilayer graphene electrodes. By establishing a connection between the twist angle and the screening length of charge carriers within the electrode, we investigate how tunable metallicity modifies the statistics of the electron transfer energy gap. Constant potential molecular simulations show that the activation free energy for electron transfer increases with screening length, leading to a non-monotonic dependence on the twist angle. The twist angle alters the density of states, tuning the number of thermally accessible channels for electron transfer and the reorganization energy by affecting the stability of the vertically excited state through attenuated image charge interactions. Understanding these effects allows us to express the Marcus rate of interfacial electron transfer as a function of the twist angle in a manner consistent with experimental observations.

Ferroelectric field effect transistors based on two-dimensional CuInP2S6 (CIPS) and graphene heterostructures

Abstract

Heterostructures of two-dimensional (2D) materials comprise clean van der Waals (vdW) interfaces that can facilitate charge or energy transfer. Recently, the 2D ferroelectric CuInP2S6 (CIPS) has been integrated with graphene and other 2D materials to realize potentially novel low energy electronic devices. However, the influence of 2D CIPS on the properties of graphene and doping across the vdW interface has not been studied in detail. Here, we study graphene field effect transistors (FETs) with CIPS as the top gate. We find that CIPS leads to modulation of the graphene Fermi level due to local doping. We also find polarization-induced hysteresis in CIPS-gated graphene FETs. Electrical transport measurements from 50 to300 K show that above 200 K, the ferroelectric response decreases. As a result, the hysteresis voltage windows in the graphene ferroelectric FETs (FeFET) transfer curves decrease above 200 K. Our results show that interfacial remote doping affects the macroscopic polarization and performance of CIPS-based graphene FeFETs.

Graphical abstract

Highlights

This research studies the temperature-dependent local doping across a vdW ferroelectric/2D channel interface that affects the transport properties of ferroelectric field effect transistors (FeFETs).Experimental findings showed ferroelectric polarization switching-based hysteresis in CuInP2S6-gated graphene FeFETs.

Discussion

vdW ferroelectrics that can be scaled to atomic layer thicknesses are useful for miniaturised low energy electronics.Understanding the interface charge or energy transfer in vdW ferroelectrics is essential for their integration into current or future technologies.

Supplementary Information

The online version contains supplementary material available at 10.1557/s43581-024-00109-y.

Impedance of nanocapacitors from molecular simulations to understand the dynamics of confined electrolytes

Nanoelectrochemical devices have become a promising candidate technology across various applications, including sensing and energy storage, and provide new platforms for studying fundamental properties of electrode/electrolyte interfaces. In this work, we employ constant-potential molecular dynamics simulations to investigate the impedance of gold-aqueous electrolyte nanocapacitors, exploiting a recently introduced fluctuation-dissipation relation. In particular, we relate the frequency-dependent impedance of these nanocapacitors to the complex conductivity of the bulk electrolyte in different regimes, and use this connection to design simple but accurate equivalent circuit models. We show that the electrode/electrolyte interfacial contribution is essentially capacitive and that the electrolyte response is bulk-like even when the interelectrode distance is only a few nanometers, provided that the latter is sufficiently large compared to the Debye screening length. We extensively compare our simulation results with spectroscopy experiments and predictions from analytical theories. In contrast to experiments, direct access in simulations to the ionic and solvent contributions to the polarization allows us to highlight their significant and persistent anticorrelation and to investigate the microscopic origin of the timescales observed in the impedance spectrum. This work opens avenues for the molecular interpretation of impedance measurements, and offers valuable contributions for future developments of accurate coarse-grained representations of confined electrolytes.

Frequency-Dependent Impedance of Nanocapacitors from Electrode Charge Fluctuations as a Probe of Electrolyte Dynamics

The frequency-dependent impedance is a fundamental property of electrical components. We show that it can be determined from the equilibrium dynamical fluctuations of the electrode charge in constant-potential molecular simulations, extending in particular a fluctuation-dissipation relation for the capacitance recovered in the low-frequency limit and provide an illustration on water-gold nanocapacitors. This Letter opens the way to the interpretation of electrochemical impedance measurements in terms of microscopic mechanisms, directly from the dynamics of the electrolyte, or indirectly via equivalent circuit models as in experiments.

Coupled Interactions at the Ionic Graphene-Water Interface

We compute ionic free energy adsorption profiles at an aqueous graphene interface by developing a self-consistent approach. To do so, we design a microscopic model for water and put the liquid on an equal footing with the graphene described by its electronic band structure. By evaluating progressively the electronic and dipolar coupled electrostatic interactions, we show that the coupling level including mutual graphene and water screening permits one to recover remarkably the precision of extensive quantum simulations. We further derive the potential of mean force evolution of several alkali cations.

Microscopic origin of the effect of substrate metallicity on interfacial free energies

We investigate the effect of the metallic character of solid substrates on solid-liquid interfacial thermodynamics using molecular simulations. Building on the recent development of a semiclassical Thomas-Fermi model to tune the metallicity in classical molecular dynamics simulations, we introduce a thermodynamic integration framework to compute the evolution of the interfacial free energy as a function of the Thomas-Fermi screening length. We validate this approach against analytical results for empty capacitors and by comparing the predictions in the presence of an electrolyte with values determined from the contact angle of droplets on the surface. The general expression derived in this work highlights the role of the charge distribution within the metal. We further propose a simple model to interpret the evolution of the interfacial free energy with voltage and Thomas-Fermi length, which allows us to identify the charge correlations within the metal as the microscopic origin of the evolution of the interfacial free energy with the metallic character of the substrate. This methodology opens the door to the molecular-scale study of the effect of the metallic character of the substrate on confinement-induced transitions in ionic systems, as reported in recent atomic force microscopy and surface force apparatus experiments.