Impact of Ionic Liquids on Effectiveness of Tuning the Emissivity of Multilayer Graphene

Multilayer graphene has been employed as a functional material for tuning the emissivity in mid- and long-infrared range, which shows great potential for various applications, such as radiative cooling and thermal camouflage. However, the stability of the multilayer graphene is not sufficient for practical applications yet. Even though it is reported that the integrity of the multilayer graphene is compromised by ion intercalation, the detailed mechanism is rather unclear. Here, a set of ionic liquids is deployed as sources of electronic charges for tuning the emissivity of multilayer graphene. It is found that the emissivity modulator using 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([EMIm]NTf2) as the ionic liquid provides a modulation depth of about 0.52 (i.e., about 21% larger than the best-reported value) while maintaining a reasonable device lifetime. The microscopic structures of the multilayer graphene in an operational and failure modulator are investigated by scanning electron microscopy, Raman spectroscopy, X-ray diffraction. The results indicate that the modulation depth of emissivity is negatively correlated with the initial voltage, which represents the reaction potential between the ionic liquid and graphene. Furthermore, not only the chemical reactivity but also the size of both anion and cation in the ionic liquids play important roles in maintaining stability of the modulator. Therefore, a set of criteria (e.g., low initial voltage and small size of anion and cation) is proposed to select proper ionic liquids for emissivity modulation. This not only sheds light on the underlying physics of the modulator but also promotes its practical applications.