Study of field ion emission from ionic liquids using molecular dynamics simulations

Abstract

Molecular dynamics simulations are employed to study the field ion emission from ionic liquids. Here, an all-atom polarizable force field is selected to model the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM BF4). This force field presents a superior ability to reproduce the transport properties of the liquid, and it is used for the first time in the analysis of field ion emission. Initially, the case of a suspended droplet of ionic liquid is employed to describe the characteristics of the liquid at equilibrium. Then, a simulation comprising a nano-droplet of ionic liquid attached to a wall and exposed to a uniform electric field is employed to study the emission of ions. The current emitted, the beam composition, the energy deficit, and the mean characteristics of the process are reported. An extensive analysis is then carried out based on the process characteristics, the theory of field evaporation, and previous numerical solutions. The results suggested that steady field evaporation is unlikely to take place in most of the cases simulated. Field evaporation appears to be limited to the use of low electric fields, which makes the rate of emission decrease significantly. Despite this limitation, insights are made regarding the role of droplet polarization in decreasing the vaporization energy of the ions. The emission process observed in the simulations seems to be related to the stability limit of the droplet and the tearing of its surface, resembling a periodic jet-type of emission.