Vol.4 No.3 2012

44/62

Research paper : Innovative electron microscope for light-element atom visualization (Y.Sato et al.)−176−Synthesiology - English edition Vol.4 No.3 (2012) their practical application as early as possible. The major components of these experimental microscopes are listed in Table 1. The first microscope is a spherical aberration-corrected TEM/STEM for 30–60 kV, while the second microscope is a spherical and chromatic aberration-corrected TEM for 30 kV. They were initially developed as specially designed low-voltages microscopes. We obtained valuable results regarding their performances as detailed below. 4.2 Performance of the first low-voltage microscope (TEM/STEM)We started the operation of our first experimental microscope (Fig. 6) equipped with two Delta spherical aberration correctors for TEM and STEM modes in 2008. Through our preliminary experiments using a conventional electron gun (Schottky-type FEG) for a 200 kV-class commercial microscope, we confirmed that spherical aberration and high-order geometrical astigmatisms up to six-fold were canceled by the Delta correctors both in TEM and STEM modes. We then installed the new low-voltage CFEG and an EELS spectrometer in the microscope.We evaluated the spatial resolution of this microscope in STEM mode by using it to observe silicon (Si) <110>, which is conventionally used as a standard sample for high-resolution STEM[11]. Both of the annular dark field (ADF-) STEM images observed at 60 and 30 kV (Fig. 7) clearly show the so-called dumbbell structures, where the projected atomic columns in each “dumbbell” are separated by 136 pm. Furthermore, the smallest lattice spacing observed here was 96 and 111 pm at 60 and 30 kV, respectively, as shown by fast Fourier transformation (FFT) of the ADF images. Regarding these distances as spatial resolution d, the ratios of d to the wavelengths of electrons , d/, were found to be 20 and 17 at 60 and 30 kV, respectively. Our microscope thus exhibited the highest spatial resolution in terms of d/, which exceeded the past record of d/ = 25, corresponding to d = 0.05 nm obtained at 300 kV[10].We then examined spatial resolution of the microscope in TEM mode by using it to observe gold (Au) nanoparticles[11]. Lattice fringes of Au<200> with a separation of 204 pm were unambiguously imaged both at 60 and 30 kV (Fig. 8), and smaller lattice spacing of 79 and 91 pm were also resolved at 60 and 30 kV, respectively, as shown by FFT of the TEM Fig. 6 External view of the first experimental microscope (spherical aberration-corrected low-voltage TEM/STEM)Table 1. Main functions and constitution of experimental low-voltage electron microscopesFig. 7 Performance examination of the first low-voltage microscope in STEM modeRecorded at acceleration voltages of (a) 60 kV and (b) 30 kV Si<110> was used as test specimen.○ : equipped, - : not equipped-○Spectrometer for EELS○-Chromatic aberration corrector (for TEM)○○Spherical aberration corrector (for TEM)-○Spherical aberration corrector (for STEM)○○Low-voltage electron gun3060, 30Acceleration voltage(s) (kV)TEMTEM/STEM/EELSMain function(s)Second systemFirst system(b)136 pm136 pm004, 136 pm004, 136 pm224, 111 pm115, 105 pm440, 96 pm(a)60 kV30 kV1 nm1 nm

※このページを正しく表示するにはFlashPlayer9以上が必要です