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Research paper : Innovative electron microscope for light-element atom visualization (Y. Sato et al.)−179−Synthesiology - English edition Vol.4 No.3 (2012) 5.3 Known practical problems with low-voltage microscopyIn the experiments described above, we found some practical problems associated with our low-voltage microscope that would not occur for conventional observations made at higher acceleration voltages without aberration correction. For example, the etching effect of residual gaseous species in the microscope column, which causes the gradual loss of atoms in a sample during observation, is more obvious at lower voltages, while the knock-on effect of electrons itself is drastically reduced. In addition, even a slight change in condition of the CFEG emitter can significantly affect the spatial resolution and image quality of the microscope, where spherical aberration and various astigmatisms are finely corrected by the Delta corrector. We are currently taking measures to solve these problems by reinforcing the vacuum system of the microscope, and by optimizing the emitter shape and its operating conditions. Since we have proven the potential of our low-voltage microscopes as described above, our final goal is to improve them to the level that they can stably and easily exhibit their highest performance even for practical uses such as for material research and development.6 Future prospectsIn this paper, we reviewed our Triple-C project, through which we have developed completely new low-voltage electron microscopes for the first time. The advantage of using low acceleration voltages was not recognized in the field of electron microscopy when we started the concrete planning of this project in 2006. However, the situation completely changed within a few years of launching this project. A foreign group has also started developing low-voltage microscopes based on a similar concept[25], and some existing projects originally planned to improve the performance of middle-voltage microscopes for 80–300 kV have been modified for low-voltage systems[26][27]. Low-voltage TEM and STEM are now regarded as the main advanced forms of electron microscopy, and the competition between full-scale projects to develop such microscopes is intensifying worldwide. Each project is aimed at achieving a spatial resolution close to 0.1 nm at 50 kV or lower by using newly developed chromatic aberration correctors such as those employed in our project or by introducing conventional monochrometers. Low-voltage microscopes can be considered an innovation in electron microscopy that follows spherical aberration correctors developed in 1990s. These innovations are expected to satisfy the growing requirement for microscopes to be able to observe new targets such as nano-materials and single organic molecules.As low-voltage microscopes become widespread and are put into practical use, they are expected to contribute significantly in the characterization of various kinds of materials, especially the materials used in the fields of chemistry and biology. If these microscopes can enable us to more easily observe the dynamic behavior of molecules and atoms, we can immediately apply them to important investigations, such as the structure analysis of ion channels and the in-situ observation of catalytic reactions. For example, direct observation of the reconstruction of individual molecules in the presence of metal clusters should help us further understand the mechanisms responsible for catalytic reactions at an atomic level, thereby making a huge impact on society. If we can observe in real time how a particular functional group in a molecule is excited by light or heat and how the electronic state of a particular atom is changed, we will be able to obtain fundamental data through which we can elucidate the mechanisms responsible for various chemical reactions at the atomic level.Low-voltage electron microscopy and spectroscopy, which cause less irradiation damage to samples, are beneficial for analyzing not only soft matter but also inorganic materials. When characterizing crystalline materials, for example, they can provide us with new viewpoints on the physical properties of samples such as the formation and annihilation processes associated with point defects. We may also be able to gain a better understanding of the correlation between the atomic-level structures and the electronic states of individual quantum objects such as CNTs and fullerenes based on high-precision EELS analyses. AcknowledgementsWe thank Koji Kimoto (NIMS) for his cooperation throughout the Triple-C project; Eiji Okunishi (JEOL) for his cooperation during the low-voltage EELS experiments; and Hiromichi Kataura, Toshiya Okazaki, Yoko Iizumi, and Haruka Kobayashi (AIST) for their help with sample preparation. A portion of the experiments performed with the low-voltage microscope was financially supported by KAKENHI (19054017 and 23750250). S. Horiguchi: Ko-bunkaino Denshi Kenbikyo (High-Resolution Electron Microscopy), Kyoritsu Shuppan, Tokyo (1988) (in Japanese).D. B. Williams and C. B. Carter: Transmission Electron Microscopy (2nd Ed.), Springer (2009).A. Hashimoto, H. Yorimitsu, K. Ajima, K. Suenaga, H. Isobe, J. Miyawaki, M. Yudasaka, S. Iijima and E. Nakamura: Selective deposition of a gadolinium (III) cluster in a hole opening of single-wall carbon nanohorn, Proc. Natl. Acad. Sci. USA, 101, 8527-8530 (2004).A. Hashimoto, K. Suenaga, A. Gloter, K. Urita and S. Iijima: Direct evidence for atomic defects in grapheme layers, Nature, 430, 870-873 (2004).K. Suenaga, H. Wakabayashi, M. Koshino, Y. Sato, K. Urita and S. Iijima: Imaging active topological defects in carbon nanotubes, Nat. Nanotechnol., 2, 358-360 (2007).[1][2][3][4][5]References

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