(Invited) Tunable Infrared Optoelectronics Achieved by the Reversible Intercalation of Ionic Liquid Anions in Multilayer Graphene in the Mid-Infrared Wavelength Range

We report the use of multilayer graphene (MLG) grown by chemical vapor deposition (CVD) approximately 75 layers (26 nm) thick configured in a simple electrochemical device. Applying a voltage of 3–4 V drives the intercalation of anion TFSI - (ion liquid diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide DEMETFSI]) resulting in the reversible modulation of the properties of this optically dense material. Upon intercalation, we observe an abrupt shift of 35 cm −1 in the G band Raman mode, an abrupt increase in FTIR reflectance over the wavelength range from 1.67 to 5 μm (2000–6000 cm −1 ), and an abrupt increase in luminescent background observed in the Raman spectra of graphene. All of these abrupt changes in the optical properties of this material arise from the intercalation of the TFSI − ion and the associated change in the free carrier density (Δn = 10 21 cm −3 ). Suppression of the 2D band Raman mode observed around 3 V corresponds to Pauli blocking of the double resonance Raman process and indicates a modulation of the Fermi energy of ΔE F = 1.1 eV. The thermal emissivity of the MLG device is also substantially modulated over the range from 7 – 14 µm. Using this device, have demonstrated free-space optical communication that utilizes ambient Planck radiation from a warm body and modulates the emitted intensity. We present an experimental demonstration achieving error-free communications at 100 bits per second over laboratory scales. We have also developed an AC Raman technique for measuring the Raman spectra during the intercalation−deintercalation half cycles in a multilayer graphene (MLG) device operating at 0.2−10 Hz. Raman spectra taken just 200 ms apart show the emergence and disappearance of the intercalated G band mode at around 1610 cm −1 . By integration of over hundreds of cycles, a significant Raman signal can be obtained. The intercalation/ deintercalation is also monitored with thermal imaging via voltage-induced changes in the carrier density, complex dielectric function ε(ω), and thermal emissivity of the device. “Dynamic Study of Intercalation/Deintercalation of Ionic Liquids in Multilayer Graphene Using an Alternating Current Raman Spectroscopy Technique” Zhi Cai, Haley Weinstein, Indu Aravind, Ruoxi Li, Sizhe Weng, Boxin Zhang, Jonathan L. Habif, and Stephen B. Cronin. Journal of Physical Chemistry Letters , 14 , 7223 (2023). “Gate-tunable modulation of the optical properties of multilayer graphene by the reversible intercalation of ionic liquid anions” Zhi Cai, Indu Aravind, Haley Weinstein, Ruoxi Li, Jiangbin Wu, Han Wang, Jonathan Habif, and Stephen Cronin. Journal of Applied Physics . 132 , 095102 (2022). “Harvesting Planck radiation for free-space optical communications in the long-wave infrared band“ Haley A. Weinstein, Zhi Cai, Stephen B. Cronin, and Jonathan L. Habif, Optics Letters. 47 , 6225-6228 (2022). Figure 1