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Research paper : Innovative electron microscope for light-element atom visualization (Y.Sato et al.)−174−Synthesiology - English edition Vol.4 No.3 (2012) This project has been implemented, by following the scenario outlined above, to realize next-generation, high-performance low-voltage electron microscopes based on technologies that have all been developed within Japan. We named the project Triple-C because it focuses on the following three C’s: spherical aberration (Cs), chromatic aberration (Cc), and carbon (C) nano-materials. This project has been carried out through a collaboration between JEOL Ltd., The National Institute for Material Science (NIMS), and the National Institute of Advanced Industrial Science and Technology (AIST). This project allows us to combine the expertise of all these organizations, such as the theories on electron microscopy and spectroscopy, experience in developing instruments, knowledge of nano-material science and experimental skills. The JEOL team developed each component during the first stage, and constructed the microscopes and examined their performances during the second stage. The NIMS and AIST teams took charge of the preliminary planning of this project, the application of the low-voltage microscopes to nano-materials during the third stage, and reference experiments using conventional microscopes. To achieve the best results from this joint project, we fully cooperated with each other for each experiment and shared information through meetings that were held approximately every two months to check on the project’s progress.In this paper, we review the development and performance of new low-voltage microscopes as carried out by the JEOL team as the principal part of our Triple-C project. We also introduce some typical scientific results and problems associated with the microscopes with regard to the characterization of carbon nano-materials by the AIST team.3 Elemental technologies3.1 Low-voltage electron gunThe electron gun of a microscope has the important function of stably generating and accelerating an electron beam at a fixed voltage. It is required, in particular, for the electron guns in the Triple-C project to produce a high luminance (current density) and a small energy spread (E). High luminance is necessary for identifying light-element single atoms by STEM-EELS at a high signal-to-noise ratio (S/N). A small E contributes to a minimization of the blurring of images due to the chromatic aberration of the lens system. Assuming that the acceleration voltage E slightly changes to E+dE, the degree of electron-beam broadening due to chromatic aberration is proportional to dE/E. This implies that spatial resolution of a microscope is more seriously affected by chromatic aberration at a lower E. It is therefore essential to make the E of the low-voltage electron gun as small as possible. Note that E is generally evaluated as the full width at half maximum of the EELS zero-loss peak in electronvolts.We adopted cold filament field-emission guns (CFEG) for our low-voltage microscopes because they have great advantages both in luminance and energy spread. The electron emitter and construction of the accelerating tubes, which apply voltages to electrons, were optimized for use at 30–60 kV. We also implemented measures for reducing the noise in high-tension electric sources and circuits for higher CFEG stability. As a result, excellent energy spreads of 0.27 and 0.30 eV were obtained at acceleration voltages of 60 and 30 kV, respectively, as shown in the EELS zero-loss peaks in Fig. 3[11]. 3.2 Spherical aberration correctorIn electron microscope lenses, magnetic fields are applied to focus the electron beam by using the refracting effect of Lorentz forces in the fields. Although the electron beam passing through the lens system should ideally converge on a certain point along the optical axis, the focal point is actually spread out to some extent by various aberrations of the lenses, resulting in blurring and/or distortion of the projected images. In particular, spherical aberration of the objective lens of TEM is a major limiting factor of the spatial resolution. Recently, the correction of spherical aberration has been realized by using several stages of magnetic multipoles, which were incorporated in the rear of the objective lens in the microscope column[12]-[14] .This correction system itself generates negative spherical aberration to cancel the original positive aberration of the lens. A commercially available spherical aberration corrector developed by CEOS GmbH, which has become more popular both for TEM and STEM in recent years, consists of double hexapoles and transfer lenses. Each hexapole applies a three-fold magnetic field with a different azimuth, whereby the entire system corrects spherical and geometrical aberrations such as three-fold astigmatism at the same time[13].This correction system has largely contributed to the improved spatial resolution of conventional TEM and STEM that are generally designed for operation at 100 kV or higher. Fig. 3 Evaluation of energy spread of low-voltage electron gunRecorded at acceleration voltages of (a) 60 kV and (b) 30 kV Intensity (counts)Intensity (counts)Energy loss (eV)Energy loss (eV)0.30 eV30 kV0.27 eV60 kV(b)(a)0.80.40.0-0.4010002000300040005000600001000200030004000500060000.80.40.0-0.4

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