Contents I. Introduction
Due to their non-volatility under room temperature, ionic liquids (ILs) have become the subject of extensive studies in the field of chemistry and material sciences as one type of important green chemical organic solvents [1], [2], [3]. The two major components of ILs are inorganic or organic anions with smaller molecular mass and organic cations with larger molecular mass, which result in a liquid molten phase under room temperature. Therefore ionic liquids are often referred to room-temperature ionic liquids (RTILs) [4]. In contrast to ordinary molecular liquids, ionic liquids exhibit intriguing dipole interactions due to the significant difference in geometric size between anions and cations, resulting in high molecular polarizability, which provides an ideal material for research on intermolecular forces, dispersion forces [5] and Raman-spectra-related phenomenon [6]. In addition, their distinct physical as well as chemical properties, including no-vapor pressure [7], high conductivity [8], and optical nonlinearity [9], make ILs indispensable functional materials in various scientific and engineering applications such as gas separation [10], energy storage [11], [12] sensing arrays [13], and life science [14], [15], etc.
