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choline chloride, ChCl) and a neutral hydrogen-bond donor such as glycerol (Gly), urea and natural carboxylic acids. Common mixtures derive from a quaternary ammonium salt (e.g. DES components, often solid compounds at room temperature, are able to interact with each other mainly through hydrogen-bonding interactions, thereby forming eutectic mixtures whose eutectic point temperature is far below those of the individual components. DESs, which share several physical-chemical properties with traditional ionic liquids (e.g., low vapour pressure, thermal stability, non-flammability, easy of recycling), are biodegradable fluids usually made by mixing and gently heating two or three safe and inexpensive components. In particular, we first propose DESs as electrolytic media for tuning sheet resistance and Fermi level of graphene in order to fully control its microwave response (reflection, transmission and absorption). We propose an optically transparent, flexible and reconfigurable microwave absorber using two main strategies/approaches: (i) an electrolyte-gated graphene capacitor exploiting a Deep Eutectic Solvents (DES) as electrolytic medium and (ii) a “quasi-metal” graphene microwave mirror. In this paper, we take a step forward on the path towards innovative and reconfigurable “optically transparent” graphene-based devices, overcoming both limitations. This strongly limits their potential applications in commercial devices.
Moreover, electrolytic gating of graphene typically makes use of ionic liquids (especially of imidazolium-derived ionic liquids) 18, 19 which have several drawbacks in terms of cost and toxicity 20. Thus, they are not transparent in the visible range. However, the reported examples of microwave absorbers typically have configurations based on metallic and opaque surfaces. This opens the possibility (for an optimized configuration) to switch from a microwave transparent window to an absorbing or reflecting one. Electrolytic gating allows the controlled switching from pristine (high Rs) to doped graphene (low Rs). Up to now, few reconfigurable examples of microwave absorbers based on electrolyte-gated graphene have been proposed 15, 16, 17. These devices offer new functionalities, although they share a common limitation: they are “static” and their properties mainly relate to their geometry.
Recently, the possibility to integrate an optically transparent absorber with an acoustic absorber has also been reported 14, leading to a multi-physical regulation system. In the literature, there are recent examples where attempts are made to achieve microwave absorption in optically transparent structures by means of several configurations based on metamaterials 3, 4, 5, 6, 7, 8, 9, metasurfaces 10, metallic meshes 11, 12, and Salisbury screens 13. In this framework, one important aspect is related to the ability to manipulate and control reflection, transmission and absorption of microwave radiation in devices that are also optically transparent. This growing research interest is driven by the opportunity to combine photonic and microwave technologies, thus creating new applications 1, 2. The quest for optically transparent microwave devices has increased over the last years. These results could find applications in several technological fields, ranging from electromagnetic pollution to integrated multi-physical regulation systems, thereby helping the advance of the performance of microwave cloaking systems, stealth windows, frequency selective surfaces, modulators and polarizers. The numerical simulations and experimental measurements also show the ability of the absorber, in the Salisbury screen configuration, to achieve near perfect absorption with a modulation of about 20%. We consider the tunability of the reconfigurable surfaces in terms of transmittance, absorption and reflectance, respectively, over the X and Ku bands when the gate voltage is varied in the −1.4/+1.4 V range. DESs have been first explored as electrolytic and environmentally friendly media for tuning sheet resistance and Fermi level of graphene together with its microwave response (reflection, transmission and absorption).
Electrolytically tunable graphene “building blocks” for reconfigurable and optically transparent microwave surfaces and absorbers have been designed and fabricated by exploiting Deep Eutectic Solvents (DESs).