Performances of aberration‐corrected monochromatic low‐voltage analytical electron microscope

Abstract

To study the detailed electronic structures of carbon‐related materials at an energy resolution better than 25 meV, we have developed a monochromatic low‐voltage analytical electron microscope under a project “Triple‐C phase‐2”. The developed microscope is equipped with a double Wien‐filter monochromator system [1] and delta‐type aberration correctors [2]. It works at an accelerating voltage range from 15 kV to 60 kV. The two Wien‐filters for the monochromator, which is located just below the extraction anode of Schottky source, enables us to obtain an achromatic probe at an exit of the monochromator, since the second filter cancels out the energy dispersion generated by the first filter. The energy spread of electrons (ΔE) is controllable by choosing the width of the slits, which are located between two filters, independently on the probe size at the specimen. The delta‐type aberration corrector consists of three dodeca‐poles to correct geometrical aberrations up to the fifth‐order including six‐fold astigmatism. In addition, so as to obtain high energy resolution spectra in electron energy‐loss spectroscopy (EELS), the microscope is equipped with a high energy resolution spectrometer (Quantum‐ERS from Gatan Inc.), which incorporates with the highly sensitive detection system at lower accelerating voltage and the highly stabilized power supplies for the prism and the lens system. The ultimate energy resolution was acquired to be 14 meV at an accelerating voltage of 30 kV with an acquisition time of 2 ms, as shown in Fig. 1. And at the longer acquisition time of 0.5 seconds, the energy resolution was measured to be 20 meV. These results exceeded the original target of 25 meV. Figures 2 (a) and (b) show raw ADF‐STEM images of a single‐layered graphene at 60 kV and 30 kV. These images were obtained with a monochromatic electron probe, whose ΔE was 228 meV (using a 4 μm slit). By using a monochromatic electron probe at 60 kV, C‐C dumbbells of single‐layered graphene were clearly resolved. And a single‐carbon atom on a graphene was successfully imaged at 30 kV. These results indicate that the developed microscope enables us to analyze materials with high energy and spatial resolutions at lower accelerating voltage. We tested a low‐loss EELS map using a hexagonal boron nitride (h‐BN) specimen with a monochromatic probe at 30 kV using a 0.1 μm slit. The experimental conditions are listed as probe size = 1 nm, probe current = 10 pA and acquisition time for each pixel = 0.3 seconds. Figure 2 (a) shows the ADF‐STEM image from the EELS mapping area. Fig. 2 (b) shows the extracted low‐loss spectrum from the edge of the specimen indicated with the framed yellow square in Fig. 2 (a). This spectrum, which was measured with an energy resolution of 22 meV, showed a sharp peak corresponding to an optical phonon at about 170 meV. Figure 2 (c) shows the EELS map at the phonon excitation energy, whose intensity is normalized with the zero‐loss peak intensity in each pixel. The phonon intensity was found to be strongly delocalized at the vacuum area beyond the edge of the specimen of more than 100 nm.