Abstract-Broadband Terahertz Spectroscopy of Imidazolium-Based Ionic Liquids

Sen Mou, Andrea Rubano, Domenico Paparo

https://cdn-pubs.acs.org/doi/abs/10.1021/acs.jpcb.7b10886

Ionic liquids are liquid salts at ambient temperature composed of organic cations and organic/inorganic anions. Outstanding physical and chemical properties of ionic liquids lead to increasing application in scientific and industrial field. Ionic liquids have been already investigated by different spectroscopic techniques, including terahertz (THz) time-domain spectroscopy. The usual THz frequency range extends up to 2-3 THz, a relatively narrow band, which can only show the intermolecular vibrational modes. Here we report about broadband THz spectroscopy of ILs up to 13 THz. Bandwidth of intermolecular absorption band presents unexpected behavior and strong sharp intramolecular absorptions are shown. In addition, we found violation of the approximation of harmonic oscillator used to predict the peak shift of intermolecular absorption band.

Abstract-Complex Permittivity of Ionic Liquid Mixtures Investigated by Terahertz Time-Domain Spectroscopy

Sen Mou, Andrea Rubano, and Domenico Paparo

http://pubs.acs.org/doi/abs/10.1021/acs.jpcb.7b04706

Ionic liquids are salts found in their liquid state at ambient temperature. The physicochemical properties of ionic liquids can be tailored by selecting constituent cation and anion from numerous available ions. The physicochemical properties can be further tuned by mixing different neat ionic liquids. Reported data of ionic liquid mixtures reveal that frequently investigated properties such as density, viscosity and thermal stability follow corresponding mixing laws. Complex permittivity in the interval of terahertz frequencies is of great importance to understand the molecular interactions and the solvation dynamics which drive the macroscopic properties of ionic liquids, however, to the best of our knowledge, there are scarce reports about the mixing behavior of complex permittivity in ionic liquid mixtures. In this contribution, binary mixtures of 1-butyl-3-methylimidazoulium iodide ([C4C1im]I) and 1-butyl-3- methylimidazoulium bis(trifluoromethylsulfonyl)imide ([C4C1im][NTf2]) are investigated in the terahertz spectral range, and the resulting low-energy spectra are analyzed in order to clarify the mixing laws at play. The results show that the complex permittivity of mixtures of [C4C1im]I and ([C4C1im][NTf2] obeys a linear mixing law.

Abstract-Tunable terahertz reflection of graphene via ionic liquid gating

Yang Wu1,2,3, Xuepeng Qiu1, Hongwei Liu3, Jingbo Liu4, Yuanfu Chen4, Lin Ke3 and 
Hyunsoo Yang1,2

http://iopscience.iop.org/article/10.1088/1361-6528/aa57ad

We report a highly efficient tunable THz reflector in graphene. By applying a small gate voltage (up to ±3 V), the reflectance of graphene is modulated from a minimum of 0.79% to a maximum of 33.4% using graphene/ionic liquid structures at room temperature, and the reflection tuning is uniform within a wide spectral range (0.1–1.5 THz). Our observation is explained by the Drude model, which describes the THz wave-induced intraband transition in graphene. This tunable reflectance of graphene may contribute to broadband THz mirrors, deformable THz mirrors, variable THz beam splitters and other optical components.

Abstract-Terahertz and Infrared Spectroscopy of Room Temperature Imidazolium-Based Ionic Liquids

Yamada T, Tominari Y, Tanaka S, Mizuno M.
http://www.ncbi.nlm.nih.gov/pubmed/26625193

The terahertz and infrared frequency vibrational modes of various room-temperature imidazolium-based ionic liquids with molecular anions were examined extensively. We found that the molar concentration normalized absorption coefficient spectra in the low-wavenumber region for imidazolium cations with different alkyl-chain lengths are nearly identical for the same anion. In regards to the overall view of a wide range of imidazolium-based ionic liquids, we found that the reduced mass, consisting of an imidazolium ring cation and the anion, as well as the force constant have a significant role in determining the central frequency of the broad absorption band. In addition to these findings, we also discuss the correlation between the +C-H stretching vibrational modes in the 3000 to 3300 cm-1 region of infrared spectroscopy and the intermolecular vibrational band in the low-wavenumber region. Finally, we describe some interesting characteristics of the intermolecular vibrational band observed in a wide range of imidazolium-based ionic liquids.

Abstract-Graphene Terahertz Modulators by Ionic Liquid Gating

1. Yang Wu^1,2, 
2. Chan La-o-vorakiat^3,4,
3. Xuepeng Qiu⁵, 
4. Jingbo Liu⁶, 
5. Praveen Deorani⁵, 
6. Karan Banerjee⁵, 
7. Jaesung Son⁵, 
8. Yuanfu Chen⁶, 
9. Elbert E. M. Chia^4,*and
10. Hyunsoo Yang^1,2,*

Article first published online: 3 FEB 2015

http://onlinelibrary.wiley.com/doi/10.1002/adma.201405251/abstract;jsessionid=4D3B95BDF171BDBF815E1504D6336819.f03t03?deniedAccessCustomisedMessage=&userIsAuthenticated=false

DOI: 10.1002/adma.201405251

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Excellent-performance terahertz (THz) modulators based on graphene/ionic liquid/graphene sandwich structures are demonstrated. The modulation covers a broadband frequency range from 0.1 to 2.5 THz with a modulation depth of up to 99% by applying a small gate voltage of 3 V. The outstanding performance of the proposed devices is due to the conical band structure of the graphene and the powerful gating effect of the ionic liquid in proximity to the graphene.

Abstract-Vibration Modes at Terahertz and Infrared Frequencies of Ionic Liquids Consisting of an Imidazolium Cation and a Halogen Anion

Toshiki Yamada ^1,* , Yukihiro Tominari ¹

, Shukichi Tanaka ¹

, Maya Mizuno ²

 and Kaori Fukunaga ²

http://www.mdpi.com/1996-1944/7/11/7409

¹ Advanced ICT Research Institute, National Institute of Information and Communications Technology, 588-2 Iwaoka, Kobe 651-2492, Japan² Applied Electromagnetic Research Institute, National Institute of Information and Communications Technology, 4-2-1 Nukuikitamachi, Koganei, Tokyo 184-8795, Japan

Abstract: The terahertz and infrared frequency vibration modes of room-temperature ionic liquids with imidazolium cations and halogen anions were extensively investigated. There is an intermolecular vibrational mode between the imidazolium ring of an imidazolium cation, a halogen atomic anion with a large absorption coefficient and a broad bandwidth in the low THz frequency region (13–130 cm−1), the intramolecular vibrational modes of the alkyl-chain part of an imidazolium cation with a relatively small absorption coefficient in the mid THz frequency region (130–500 cm−1), the intramolecular skeletal vibrational modes of an imidazolium ring affected by the interaction between the imidazolium ring, and a halogen anion with a relatively large absorption coefficient in a high THz frequency region (500–670 cm−1). Interesting spectroscopic features on the interaction between imidazolium cations and halogen anions was also obtained from spectroscopic studies at IR frequencies (550–3300 cm−1). As far as the frequency of the intermolecular vibrational mode is concerned, we found the significance of the reduced mass in determining the intermolecular vibration frequency.