Direct HRTEM observation of ultrathin freestanding ionic liquid film on carbon nanotube grid

Direct imaging of liquids by electron microscopy has been extremely difficult due to their high volatility. Ionic liquids are a unique liquid material with almost zero vapor pressure, which allows us to characterize them under high-vacuum conditions. Here we report the first direct observation of the microstructure and phase behavior of an imidazolium-based room-temperature ionic liquid by high-resolution transmission electron microscopy with the aid of a special carbon nanotube network, which is able to support a freestanding ultrathin ionic liquid film on its nanosized holes. It was found that the existence of cluster structures is one of the intrinsic properties of the ionic liquid in its whole liquid phase range. Furthermore, the size and mobility of the clusters play an important role during phase transition of the ionic liquid. We show that the direct HRTEM imaging on freestanding liquid film is a powerful technique to obtain insight into the structure of ionic liquids and their phase behavior. The present study can provide an important starting point for more sophisticated structural studies on the microstructure of liquidus materials.