Gate-tunable modulation of the optical properties of multilayer graphene by the reversible intercalation of ionic liquid anions

Abstract

We demonstrate a substantial modulation of the optical properties of multilayer graphene (∼100 layers) using a simple device consisting of a multilayer graphene/polymer electrolyte membrane/gold film stack. Applying a voltage of 3–4 V drives the intercalation of anion [TFSI]− [ion liquid diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide [DEME][TFSI]] 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 (Δ n = 1020 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 Δ EF = 1.1 eV.

Harvesting Planck radiation for free-space optical communications in the long-wave infrared band

“…The design, fabrication, and material characterization of the Planck modulator devices we have developed follow work from [10] and is described in detail in [13]. The experimental setup for the electrothermo-optical characterization of the Planck modulator MLG devices is shown in Fig.…”

“…Our communications transmitter design was inspired by electro-chemical engineering of multi-layer graphene (MLG) devices, where the MLG acts as a cathode sandwiching a layer ionic liquid between itself and a conducting cathode [10,11,12]. Voltage modulation across the devices enables the rapid electronically controlled switching of the device's surface emissivity over a broad wavelength range covering the LWIR band [13]. These devices have been shown able to modulate the emissivity of a surface enough to change the apparent surface temperature by more than 10 Kelvin.…”

Dynamic Study of Intercalation/Deintercalation of Ionic Liquids in Multilayer Graphene Using an Alternating Current Raman Spectroscopy Technique

“…Raman spectroscopy is well-suited for studying the intercalation of MLG because its features contain important information about the material, including Fermi energy Δ E F , free carrier density N , and the formation of defects . Previously, infrared (IR) and Raman spectroscopy was carried out on these types of MLG devices under applied direct current (DC) voltages and collected on the time scale of several minutes . In the work presented here, we present a new strategy for collecting Raman spectra of this intercalation–deintercalation process on relatively short time scales.…”

“…Here, instead of using a copper bottom electrode, indium tin oxide (ITO)-coated glass is used as the bottom electrode and a glass correction (Olympus LUCPlanFL N 40×/0.60) lens is used to measure the Raman spectra. Here, the alumina membrane is crucial to obtaining a clear optical path to the MLG compared to previously used polymer-based membranes. , Figure b shows a series of Raman spectra taken from the bottom surface of the MLG electrode, which is in direct contact with the ionic liquid solution. We observe a G int / G ratio of 1:2 in the intercalated state, corresponding to N = 2 × 10 20 cm –3 .…”

“… 12 Previously, infrared (IR) and Raman spectroscopy was carried out on these types of MLG devices under applied direct current (DC) voltages and collected on the time scale of several minutes. 13 In the work presented here, we present a new strategy for collecting Raman spectra of this intercalation–deintercalation process on relatively short time scales.…”