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We have used surface plasmon resonant metal gratings to induce and probe the dielectric response (i.e., electro-optic modulation) of ionic liquids (ILs) at electrode interfaces. Here, the cross-plane electric field at the electrode surface modulates the refractive index of the IL due to the Pockels effect. This is observed as a shift in the resonant angle of the grating (i.e., Δϕ), which can be related to the change in the local index of refraction of the electrolyte (i.e., Δnlocal). The reflection modulation of the IL is compared against a polar (D2O) and a non-polar solvent (benzene) to confirm the electro-optic origin of resonance shift. The electrostatic accumulation of ions from the IL induces local index changes to the gratings over the extent of electrical double layer (EDL) thickness. Finite difference time domain simulations are used to relate the observed shifts in the plasmon resonance and change in reflection to the change in the local index of refraction of the electrolyte and the thickness of the EDL. Simultaneously using the wavelength and intensity shift of the resonance enables us to determine both the effective thickness and Δn of the double layer. We believe that this technique can be used more broadly, allowing the dynamics associated with the potential-induced ordering and rearrangement of ionic species in electrode–solution interfaces. [ABSTRACT FROM AUTHOR]

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 (Δn = 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 ΔEF = 1.1 eV. [ABSTRACT FROM AUTHOR]

We report electrochemical measurements using in situ Raman spectroscopy at graphene/D 2 O interfaces under extremely low applied potentials. Here, the hydrophobic and catalytically inert nature of graphene and the insulating nature of the deionized (DI) water enables potentials as low as V applied = -7 V vs Ag/AgCl to be applied without exceeding 200 μA/cm 2 of current density. At higher currents, bubble formation (i.e., hydrogen evolution reaction) prohibits reliable spectra from being obtained from the electrode surface. Using CaF 2 as the supporting substrate enables significantly lower reducing potentials to be reached compared to glass substrates, likely due to trapped charge and impurities in the glass substrate. G band Raman spectra taken under various applied electrochemical potentials exhibit a linear relationship between the G band shift (Δω G ) and the applied potential, with blueshifts as high as Δω G = 18 cm -1 . These large Raman shifts indicate a large change in the Fermi level of Δ E F = -0.43 eV for graphene electrodes in contact with water, favoring reduction half-reactions. Based on the solution resistance measurement, there is a V IR = 3.1 V voltage drop across the solution for D 2 O (when the applied potential was V applied = -7 V vs Ag/AgCl) and the effective reducing potential on the working electrode is V effective = -3.9 V vs Ag/AgCl. We have also tested these graphene electrodes in ionic liquids [DEME][TFSI], which are limited to applied potentials above V applied = -2.7 V vs Ag/AgCl and a corresponding shift in the Fermi level Δ E F = -0.32 eV, indicating that pure water can provide a more robust electrolyte for reaching low reducing potentials than ionic liquids.

In situ surface-enhanced Raman scattering (SERS) spectroscopy is used to identify the key reaction intermediates during the plasma-based removal of NO and SO 2 under dry and wet conditions on Ag nanoparticles. Density functional theory (DFT) calculations are used to confirm the experimental observations by calculating the vibrational modes of the surface-bound intermediate species. Here, we provide spectroscopic evidence that the wet plasma increases the SO 2 and the NO x removal through the formation of highly reactive OH radicals, driving the reactions to H 2 SO 4 and HNO 3 , respectively. We observed the formation of SO 3 and SO 4 species in the SO 2 wet-plasma-driven remediation, while in the dry plasma, we only identified SO 3 adsorbed on the Ag surface. During the removal of NO in the dry and wet plasma, both NO 2 and NO 3 species were observed on the Ag surface; however, the concentration of NO 3 species was enhanced under wet-plasma conditions. By closing the loop between the experimental and DFT-calculated spectra, we identified not only the adsorbed species associated with each peak in the SERS spectra but also their orientation and adsorption site, providing a detailed atomistic picture of the chemical reaction pathway and surface interaction chemistry.

We explore the effect of charge density wave (CDW) on the in-plane thermoelectric transport properties of (PbSe) 1+δ (VSe 2 ) 1 and (PbSe) 1+δ (VSe 2 ) 2 heterostructures. In (PbSe) 1+δ (VSe 2 ) 1 we observe an abrupt 86% increase in the Seebeck coefficient, 245% increase in the power factor, and a slight decrease in resistivity over the CDW transition. This behavior is not observed in (PbSe) 1+δ (VSe 2 ) 2 and is rather unusual compared to the general trend observed in other materials. The abrupt transition causes a deviation from the Mott relationship through correlated electron states. Raman spectra of the (PbSe) 1+δ (VSe 2 ) 1 material show the emergence of additional peaks below the CDW transition temperature associated with VSe 2 material. Temperature-dependent in-plane X-ray diffraction (XRD) spectra show a change in the in-plane thermal expansion of VSe 2 in (PbSe) 1+δ (VSe 2 ) 1 due to lattice distortion. The increase in the power factor and decrease in the resistivity due to CDW suggest a potential mechanism for enhancing the thermoelectric performance at the low temperature region.