Vol.4 No.3 2012

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Research paper : Innovative electron microscope for light-element atom visualization (Y. Sato et al.)−173−Synthesiology - English edition Vol.4 No.3 (2012) to decrease the irradiation-induced damage to specimens and improve both spatial and time resolution. No existing electron microscope is specially designed for characterizing such light-element materials. It is therefore essential to develop innovative microscopes based on concepts that are completely different from those used for previous ultra-high voltage microscopes, in order to realize the direct imaging and identification of individual atoms and molecules of soft matter. Based on these requirements, we attempt to overcome two difficult parameters that have been considered incompatible with each other with regard to conventional electron microscopes: extremely low electron acceleration voltage and atomic resolution. In 2006, we launched a full-scale project supported by JST-CREST[8] to develop new electron microscopes suitable for the high-resolution observation of soft matter with less damage to the specimens and increased sensitivity. This is the first project aimed at constructing completely new electron microscopes that are specially designed to work at low acceleration voltages. 2 Scenario for achieving our research goalOur project to develop new low-voltage electron microscopes is supported by JST-CREST[8]. It was started in October 2006 and will end in March 2012. The originally planned design for a low-voltage microscope and its presumed application are schematically shown in Fig. 2, in which Cs and Cc denote spherical and chromatic aberrations, respectively. Based on the currently available TEM/STEM systems and the problems associated with them, we have decided to intensively develop the following three components as the most important elemental technologies for the first stage of this project.•Low acceleration voltage electron gun: highly stable at 30–60 kV with excellent monochromaticity and luminance.•A spherical aberration corrector having higher correction performance than existing products to fully compensate for the disadvantages affecting spatial resolution due to the use of low acceleration voltages.•A chromatic aberration corrector having a new optical system to realize chromatic aberration correction that has been almost unprecedented so far[9]. During the second stage of the project, low-voltage electron microscopes were constructed by integrating the components including those picked up above, and their performance was examined. We originally planned to employ a TEM/STEM dual-use model equipped with both spherical and chromatic aberration correctors, as shown in Fig. 2. We later revised the plan to be more practicable and more efficient based on the progress made in the first stage, and we finally constructed two microscopes with different setups optimized for their respective applications. The performance of these microscopes was examined with the main consideration of how closely the spatial resolution d approaches its critical value, as determined by the acceleration voltage E. Therefore, instead of d, we focused on the ratio d/, where denotes the wavelength of electrons accelerated at E. Taking account that the highest spatial resolution of d = 0.05 nm previously recorded at 300 kV[10] corresponds to d/ = 25, we aimed to obtain a d/ smaller than 25 at a much lower E by using our new microscope.After the completion of the performance examination of the microscopes, this project has now entered the third stage, where the designed microscopes are practically used for the observation and analysis of various nano- and light-element materials. Here, we intend to demonstrate the advantage of our low-voltage microscopes by providing scientifically valuable results that can appeal to a wide audience. We primarily use carbon nano-materials as specimens to verify the effects of using low acceleration voltages in TEM, STEM, and EELS observations, while referring to our extensive data on the same materials previously obtained by using conventional microscopes with higher voltages. At the same time, we assign this stage for detecting and solving possible latent problems in our microscopes to make them more practicable.Fig. 1 Comparison of TEM images of SWCNTs(a) Recorded at an acceleration voltage of 120 kV without spherical aberration (Cs) correction, (b) recorded at 120 kV with Cs correction, (c) recorded at 80 kV with Cs correction Scale bar = 1 nm Fig. 2 Outline of low-voltage TEM/STEM and future prospects(b)(a)(c)Damage-free observation(long exposure imaging)Low-voltage TEM/STEM technologiesHigh-sensitivity detection systemCs and Cc correction system for TEMCs and Cc correction system for STEMCold-filament field emission gun for low voltages (5-60 kV)Polymer materialsBio-materials(e.g. nucleotide sequence)Observation of liquids and gases(gas adsorption, phase transition, etc.)In-situ observation of chemical reactions(e.g. catalytic reactions)Application to inorganic materials(observation of point defects, ionic conduction, etc.)Application to soft matters and nano-materials(this project)Higher spatial resolution(approaching the limits at lower voltages)

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