Kana ISHISONE1, Guido ORI1, Mauro BOERO1,2
1Institut de Physique et Chimie des Matériaux de Strasbourg, UMR 7504, Université de Strasbourg, CNRS, Strasbourg, France
2Institute of Materials and Systems for Sustainability, Nagoya University, Nagoya, Japan
Our computational study aims at providing an insight into the structure and dynamics properties of an ionic liquid (IL) and its use as a component forming an interface with two-dimensional (2D) nanomaterials. This class of compounds are pioneered for next-generation electronic devices. ILs consist of two molecular building blocks, cations and anions, in liquid phase at room temperature. Their high ion conductivity and chemical stability make ILs ideal electrolytes in three-terminal nanodevices. Transition metal dichalcogenides (TMDCs) are novel 2D layered materials where a sheet of metal atoms is sandwiched between two sheets of chalcogens forming covalent bonds while bounding among the layers is due to van der Waals interactions. By contacting ILs with 2D-TMDCs and a applying gate voltage, the ions in ILs move and electrons/holes accumulate at the interface, making high current ON/OFF ratio on gate voltage. This hybrid system leads to major experimental observations such as high-performance field effect transistors and superconductivity. Moreover, these systems represent a promising route to empower memory devices for neuromorphic computing. The performance and property of the system are determined by the interactions and charge distribution inside the IL and at the interface with the substrate. To date, many experimental measurements, such as X-ray diffraction or AFM, have provided useful information about the ionic structure at the interface. Yet, experimental probes fail in giving a detailed atomic-level information, essential for rational development of these materials. By resorting to first-principles molecular dynamic simulations empowered by free energy sampling of reactive paths, we provide a microscopic picture of 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI])/WSe2 composite systems. Long-lasting dynamical simulations of bulk [EMIM][TFSI] gives a clear picture of the intimate structure of the IL molecular component and an estimation of the diffusion constant of this specific liquid state. Furthermore, a thorough analysis of the electronic structure and partial charges distribution characterizing the two components, cation and anion, allow to rationalize the nature of the electrostatic interactions and hydrogen bonding properties of the two ionic counterparts. The combination of classical and first-principles molecular dynamics of [EMIM][TFSI]/WSe2 gives access to the atomic and electronic distribution change at the interface in the absence or presence of an electric field. Additional investigations by using ionic gels for a more practical use will be shown to overcome the fluidity-related instability problems of ILs resulting from its liquid phase.