Microscopy Advance Access published July 11, 2014
Microscopy, 2014, 1–6 doi: 10.1093/jmicro/dfu023
Article
Downloaded from http://jmicro.oxfordjournals.org/ at Queen's University on August 18, 2014
Measurement of vibrational spectrum of liquid using monochromated scanning transmission electron microscopy–electron energy loss spectroscopy Tomohiro Miyata1, Mao Fukuyama2, Akihide Hibara2, Eiji Okunishi3, Masaki Mukai3, and Teruyasu Mizoguchi1,* 1
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan, 2Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan, and 3JEOL Ltd, 3-1-2 Musashino, Akishima-shi, Tokyo 196-8558, Japan *To whom correspondence should be addressed. E-mail: [email protected] Received 21 April 2014; Accepted 11 June 2014
Abstract Investigations on the dynamic behavior of molecules in liquids at high spatial resolution are greatly desired because localized regions, such as solid–liquid interfaces or sites of reacting molecules, have assumed increasing importance with respect to improving material performance. In application to liquids, electron energy loss spectroscopy (EELS) observed with transmission electron microscopy (TEM) is a promising analytical technique with the appropriate resolutions. In this study, we obtained EELS spectra from an ionic liquid, 1-ethyl-3-methylimidazolium bis (trifluoromethyl-sulfonyl) imide (C2mimTFSI), chosen as the sampled liquid, using monochromated scanning TEM (STEM). The molecular vibrational spectrum and the highest occupied molecular orbital (HOMO)– lowest unoccupied molecular orbital (LUMO) gap of the liquid were investigated. The HOMO–LUMO gap measurement coincided with that obtained from the ultraviolet–visible spectrum. A shoulder in the spectrum observed ∼0.4 eV is believed to originate from the molecular vibration. From a separately performed infrared observation and first-principles calculations, we found that this shoulder coincided with the vibrational peak attributed to the C–H stretching vibration of the [C2mim+] cation. This study demonstrates that a vibrational peak for a liquid can be observed using monochromated STEM–EELS, and leads one to expect observations of chemical reactions or aids in the analysis of the dynamic behavior of molecules in liquid. Key words: EELS, molecular vibration, liquid, low loss, ionic liquid, first-principles calculation
© The Author 2014. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: [email protected]
1
Microscopy, 2014, Vol. 0, No. 0
2
Introduction
Methods The EEL spectrum was obtained using an EEL spectrometer (Tridiem ERS, Gatan, Inc.) attached to a monochromated aberration-corrected STEM (JEM-2400FCS, JEOL Ltd), which was operated at 60 keV. The monochromator system, a two-stage Wien filter with an energy selection slit between the two filters, achieves an energy resolution of 0.026 eV. The drop of zero-loss intensity at 0.35 eV is 1/10 000 of its maximum at an accelerating voltage of 80 kV [16]. This monochromator system was used in measuring the band gap of Ge, which is 0.7 eV, indicating that this system has potential to identify spectral profiles under 1 eV [17]. The energy dispersion of the EEL spectrometer was set to 0.002 eV pix−1 in the zero-loss and vibrational spectrum measurements, and 0.005 eV pix−1 in the low-loss measurement. A generic ionic liquid, 1-ethyl-3-methylimidazolium bis (trifluoromethyl-sulfonyl) imide (C2mim-TFSI), was chosen as a model liquid sample. Ionic liquids are salts (comprising of cations and anions) that form liquid states at room temperature, and have properties suitable for electron microscopy, that is, electrically conducting and near-zero vapor pressure. As depicted in Fig. 1, C2mim-TFSI is composed of [TFSI−] anions and [C2mim+] cations; both are organic ions. As an electrolyte, C2mim-TFSI has been considered in
Fig. 1. Structure of ionic liquid C2mim-TFSI. C2mim-TFSI is composed of [C2mim+] cation and [TFSI−] anion.
Downloaded from http://jmicro.oxfordjournals.org/ at Queen's University on August 18, 2014
Liquids exhibit dynamics that lie between that of gases and solids, that is, their molecules move freely while interacting with each other. Because of this unique property, liquids have been found useful as transport and reaction media, and also reactants. For structural analyses of liquids, measurements of vibrational spectrum, such as infrared (IR) and Raman spectra, have been widely performed because spectra reflect the dynamics and interactions of molecules in the liquid. However, in general, these spectroscopic techniques enable averaged information of the entire sample to be obtained. With recent developments in the performance of devices and materials, understanding mechanisms at submicron and nanometer scales becomes indispensable. For liquids, there has been an increasing need to study localized areas, such as solid/liquid interfaces in batteries for structural analyses and sites of chemical reactions of molecules in liquids for direct observations of pathways. Transmission electron microscopy (TEM) and scanningTEM (STEM) are well-known experimental techniques which excel in localized measurements at the nanometer scale, and by combining electron energy loss spectroscopy (EELS) with TEM/STEM, it is possible to acquire the state of chemical bonding or other elemental information [1–3]. The application of TEM/STEM to liquids has already been performed using a liquid cell [4–6], and it was shown that the electron energy loss near-edge structure (ELNES) or the gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of a liquid can be measured using TEM/STEM– EELS [7,8]. Additionally, the experimental ELNES of a liquid can be reproduced by averaging the calculated electron energy loss (EEL) spectra derived for each molecule in the liquid structure; the ELNES thereby reflects the dynamic behavior of liquid molecules [9]. Furthermore, the peaks
Microscopy, 2014, 1–6 doi: 10.1093/jmicro/dfu023
Article
Downloaded from http://jmicro.oxfordjournals.org/ at Queen's University on August 18, 2014
Measurement of vibrational spectrum of liquid using monochromated scanning transmission electron microscopy–electron energy loss spectroscopy Tomohiro Miyata1, Mao Fukuyama2, Akihide Hibara2, Eiji Okunishi3, Masaki Mukai3, and Teruyasu Mizoguchi1,* 1
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan, 2Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan, and 3JEOL Ltd, 3-1-2 Musashino, Akishima-shi, Tokyo 196-8558, Japan *To whom correspondence should be addressed. E-mail: [email protected] Received 21 April 2014; Accepted 11 June 2014
Abstract Investigations on the dynamic behavior of molecules in liquids at high spatial resolution are greatly desired because localized regions, such as solid–liquid interfaces or sites of reacting molecules, have assumed increasing importance with respect to improving material performance. In application to liquids, electron energy loss spectroscopy (EELS) observed with transmission electron microscopy (TEM) is a promising analytical technique with the appropriate resolutions. In this study, we obtained EELS spectra from an ionic liquid, 1-ethyl-3-methylimidazolium bis (trifluoromethyl-sulfonyl) imide (C2mimTFSI), chosen as the sampled liquid, using monochromated scanning TEM (STEM). The molecular vibrational spectrum and the highest occupied molecular orbital (HOMO)– lowest unoccupied molecular orbital (LUMO) gap of the liquid were investigated. The HOMO–LUMO gap measurement coincided with that obtained from the ultraviolet–visible spectrum. A shoulder in the spectrum observed ∼0.4 eV is believed to originate from the molecular vibration. From a separately performed infrared observation and first-principles calculations, we found that this shoulder coincided with the vibrational peak attributed to the C–H stretching vibration of the [C2mim+] cation. This study demonstrates that a vibrational peak for a liquid can be observed using monochromated STEM–EELS, and leads one to expect observations of chemical reactions or aids in the analysis of the dynamic behavior of molecules in liquid. Key words: EELS, molecular vibration, liquid, low loss, ionic liquid, first-principles calculation
© The Author 2014. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: [email protected]
1
Microscopy, 2014, Vol. 0, No. 0
2
Introduction
Methods The EEL spectrum was obtained using an EEL spectrometer (Tridiem ERS, Gatan, Inc.) attached to a monochromated aberration-corrected STEM (JEM-2400FCS, JEOL Ltd), which was operated at 60 keV. The monochromator system, a two-stage Wien filter with an energy selection slit between the two filters, achieves an energy resolution of 0.026 eV. The drop of zero-loss intensity at 0.35 eV is 1/10 000 of its maximum at an accelerating voltage of 80 kV [16]. This monochromator system was used in measuring the band gap of Ge, which is 0.7 eV, indicating that this system has potential to identify spectral profiles under 1 eV [17]. The energy dispersion of the EEL spectrometer was set to 0.002 eV pix−1 in the zero-loss and vibrational spectrum measurements, and 0.005 eV pix−1 in the low-loss measurement. A generic ionic liquid, 1-ethyl-3-methylimidazolium bis (trifluoromethyl-sulfonyl) imide (C2mim-TFSI), was chosen as a model liquid sample. Ionic liquids are salts (comprising of cations and anions) that form liquid states at room temperature, and have properties suitable for electron microscopy, that is, electrically conducting and near-zero vapor pressure. As depicted in Fig. 1, C2mim-TFSI is composed of [TFSI−] anions and [C2mim+] cations; both are organic ions. As an electrolyte, C2mim-TFSI has been considered in
Fig. 1. Structure of ionic liquid C2mim-TFSI. C2mim-TFSI is composed of [C2mim+] cation and [TFSI−] anion.
Downloaded from http://jmicro.oxfordjournals.org/ at Queen's University on August 18, 2014
Liquids exhibit dynamics that lie between that of gases and solids, that is, their molecules move freely while interacting with each other. Because of this unique property, liquids have been found useful as transport and reaction media, and also reactants. For structural analyses of liquids, measurements of vibrational spectrum, such as infrared (IR) and Raman spectra, have been widely performed because spectra reflect the dynamics and interactions of molecules in the liquid. However, in general, these spectroscopic techniques enable averaged information of the entire sample to be obtained. With recent developments in the performance of devices and materials, understanding mechanisms at submicron and nanometer scales becomes indispensable. For liquids, there has been an increasing need to study localized areas, such as solid/liquid interfaces in batteries for structural analyses and sites of chemical reactions of molecules in liquids for direct observations of pathways. Transmission electron microscopy (TEM) and scanningTEM (STEM) are well-known experimental techniques which excel in localized measurements at the nanometer scale, and by combining electron energy loss spectroscopy (EELS) with TEM/STEM, it is possible to acquire the state of chemical bonding or other elemental information [1–3]. The application of TEM/STEM to liquids has already been performed using a liquid cell [4–6], and it was shown that the electron energy loss near-edge structure (ELNES) or the gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of a liquid can be measured using TEM/STEM– EELS [7,8]. Additionally, the experimental ELNES of a liquid can be reproduced by averaging the calculated electron energy loss (EEL) spectra derived for each molecule in the liquid structure; the ELNES thereby reflects the dynamic behavior of liquid molecules [9]. Furthermore, the peaks
