Vol.4 No.3 2012

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Research paper : Innovative electron microscope for light-element atom visualization (Y.Sato et al.)−178−Synthesiology - English edition Vol.4 No.3 (2012) 5 Application of low-voltage microscopes and their problemsWe have already completed the fundamental performance examination of our first microscope, which was developed prior to the second microscope, and have started using it to make scientific observations. In this chapter, we describe some typical results obtained by using the first microscope along with the problems associated with its application.5.1 Identifying single metal atoms inside fullerenesWe carried out the STEM-EELS analysis of fullerene-incorporated SWCNTs (so-called nano-peapods) using our first microscope to detect individual metal ions encapsulated inside the fullerene cages and to identify their elements. Due to the serious irradiation-induced damage of fullerenes, which causes rapid coalescence and/or opening of their carbon cages[20][21], it was impossible to directly observe the original isolated state of these metal ions in previous STEM observations made at 100 kV or higher[22]. Our microscope enables the STEM-EELS analysis of nano-peapods with drastically reduced damage at 60 kV[23].Figure 11 shows the results of the STEM-EELS analysis for a nano-peapod sample containing calcium-encapsulated fullerenes Ca@C82. In the bright-field (BF) STEM image (a), the structures of seven fullerene molecules appear to be maintained without undergoing significant beam damage. Seven calcium ions (Ca2+) inside these carbon cages are not clearly seen in this image due to their low contrast, but they are successfully detected by EELS elemental mapping for calcium (b), as indicated by the arrows. We thus found that low-voltage STEM-EELS enables the direct detection and elemental identification of individual Ca2+ ions with drastically reduced beam damage to the samples. Such a technique will greatly contribute to the analysis of biological specimens, especially in the elucidation of the mechanisms of ion channels, which play important roles in neurotransmission. Although the observation of ion channels has often been attempted by using electron microscopes in the past, identifying individual ions inside and obtaining high-resolution images of the channel structures has never been achieved because of the serious beam damage that occurs at higher acceleration voltages. The high-performance, low-voltage electron microscopy that we detailed here should be indispensable for the atomic-level characterization of structures and mechanisms inherent in biological specimens.5.2 Mapping the electronic state on a graphene edgeGraphene is a single layer of carbon atoms that forms a honeycomb network. Owing to its unique and excellent properties such as its theoretically predicted and/or experimentally proven electronic characteristics, graphene is now regarded as a strong candidate for functional materials to be used in constructing next-generation nano-electronics. Because electronic properties of graphene significantly depend on the atom arrangement on its edge, it is quite important to understand precisely the correlation between local structures and electronic states in this material. Through the Triple-C project, we succeeded in STEM-EELS mapping of the electronic states of graphene edges without introducing serious beam damage[24].Figure 12 shows the results of STEM-EELS analysis for a peripheral area of graphene at 60 kV. Possible local structures of three carbon atoms indicated by the colored arrows in the ADF-STEM image (a) are illustrated in (b), and the EELS spectra observed at these atoms are shown in (c). Note that two carbon atoms on the edge (colored blue and red) give characteristic EELS peaks at different energies (indicated by the black arrows), which were not observed for an inner carbon atom (colored green). We ascribed these EELS peaks to the electronic states peculiar to the carbon atoms on the graphene edge. Our atomic-level analysis thus provided the first direct proof that carbon atoms on a graphene edge possess different electronic states from those of inner atoms on the same layer. Fig. 11 Elemental analysis with low-voltage STEM-EELS (a) ADF-STEM image, (b) elemental maps for calcium and carbon (left and right, respectively) Nano-peapods of calcium-encapsulated fullerenes Ca@C82 were analyzed at 60 kV.Fig. 12 Observation of electronic states with low-voltage STEM-EELS (a) ADF-STEM image, (b) possible local structures, (c) EELS spectra of individual carbon atoms Graphene was analyzed at 60 kV. (b)(a)CaC1 nm(a)(b)(c)Intensity (a.u.)0.5 nmEnergy loss (eV)280290300310

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