Improvement of a 40–120 kV analytical TEM system for electron beam irradiation sensitive nano materials

Abstract

The performance of advanced nanomaterials such as the electrode catalysts of fuel cells is closely related to their composition, morphology and crystal structure. At the nanoscale, high resolution TEM is essential to understand the relationship between structure and electrochemical properties. In case of TEM characterization of chemically synthesized nanomaterials, highest attention should be paid to electron irradiation damage. If the observation conditions, especially accelerating voltage and electron beam density, are not optimized, an initial structure will be changed. For example, different from general inorganic materials, some materials such as chemically synthesized amorphous metal nano particles are quickly crystallized by the electron irradiation and it often mislead the characterization. Therefore, careful control of illumination conditions and irradiation time are essential for the analysis of those composites. The selection of an optimal accelerating voltage based on composition is one option to reduce irradiation damage. In addition, sample observation with lower accelerating voltages is advantageous to generate higher image contrast. Since standard 120 kV TEMs are optimized for applications at lower magnification with higher contrast requirements, we have developed an ultra high resolution objective lens (UHRLENS) to extend the application in nanomaterial field of a 40–120 kV HT7700 analytical TEM [1,2]. It mounted with the UHRLENS which provides lattice resolution of 0.2 nm with on‐axis illumination at 120 kV and accommodates a high solid angle silicon drift detector of an energy dispersive X‐ray (EDX) analyzer. Figure 1 shows an example of high resolution and high contrast observation of the fuel cell electrode catalyst by the newly developed TEM. Lattice images of platinum nanoparticles (lattice spacing: 0.23 nm) and the carbon support (lattice spacing: 0.34 nm) were observed clearly at an acceleration voltage of 120 kV. The binder is clearly observed with sufficient contrast. To analyze the crystal structure of nanomaterials, we have used a selected‐area electron beam diffraction (SAED) technique with a micro‐fabricated SA aperture hole rather than nano‐probe diffraction [3]. Because the damage caused by the electron beam irradiation was much less than that of the nano‐probe electron diffraction technique. An FIB fabricated apertures with the diameter of 1mm are equipped for structural analysis of individual nanomaterial. The smallest selected area diameter on the specimen is calculated to be 18nm. In case of characterization of nano materials, the spatial error in SAED pattern due to a spherical aberration is not serious because; No high‐order diffraction spots available from nano‐materials such as nano particles High order diffraction spots are not used in practice To improve operability for acquirements of the SAED, a new automatic operation function, called “nano analysis function”, is produced. This function enables automatic acquirements of SAED at plural analysis positions pre‐designated by a user. The analysis position of the SAED is precisely controlled by an image shift coil mounted just below the objective lens. The minimum diameter of SA aperture is 1 um, which corresponds to the diameter of the selected area of 18 nm on the specimen. Figure 2 shows a TEM image (a) of an asbestos specimen with the corresponding SAED patterns (b) acquired by the nano analysis function. The selected area of each analysis position is displayed by a circle on the TEM image. The diffraction patterns of the acquired SAED images can be analyzed with an optional function of Hitachi EMIP software called “diffraction analysis function”. This function enables automatic measurement of diffraction spot intervals and assumption of elements contained in the selected area of the specimen from the database. The conventional 40–120kV analytical TEM has been improved for characterization of electron beam sensitive nanomaterial by high‐resolution TEM with high contrast. The improved functions made it possible to identify the crystal structure by selected nano area electron diffraction technique.