Atomic and nanoscale imaging of a cellulose nanofiber and Pd nanoparticles composite using lower-voltage high-resolution TEM.

We have examined the advanced application of transmission electron microscopy (TEM) for the structural characterization of a composite of cellulose nanofiber (CNF) and palladium (Pd) nanoparticles. In the present study, we focused on electron-irradiation damage and optimization of high-resolution TEM imaging of the composite. The investigation indicates that the CNF breaks even under low-electron-dose conditions at an acceleration voltage of 200 kV. We then applied lower-voltage TEM at 60 kV using a spherical aberration corrector and a monochromator, in order to reduce electron-irradiation damage and improve the spatial resolution. The TEM observation achieved high-resolution imaging and revealed the existence of small Pd nanoparticles, around 2 nm in diameter, supported on the CNF. It is considered that the use of a monochromator in combination with spherical aberration correction contributed to the atomic and nanoscale imaging of the composite, owing to the improvement of the information limit under a lower-acceleration voltage.

EELS study of Fe- or Co-doped titania nanosheets.

Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets are Fe- and Co-doped titanium oxides, respectively, and they are synthesized by the exfoliation of lepidocrocite-type layered titanates. We have investigated these nanosheets by electron energy-loss spectroscopy (EELS) using a monochromated transmission electron microscope. The energy-loss near-edge structures (ELNESs) of Fe-L and Co-L indicate that Fe(3+) and Co(2+) ions are substituted in the octahedral sites in each nanosheet. The Ti-L edges of Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets correspond to the octahedral coordination of Ti(4+) and oxygen atoms as well as an undoped titania nanosheet (Ti0.87O2). On the other hand, the electron transitions from 2p3/2 to 3d eg in Ti-L3 regions are different in each nanosheet. We have also investigated the electron-beam-induced damage of Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets. The results indicated that Fe(3+) ions in the Ti0.6Fe0.4O2 nanosheets were selectively reduced to Fe(2+) ions in the reduction process by electron irradiation. In contrast, the chemical shift of the Ti-L edge of the Ti0.8Co0.2O2 nanosheets indicated that Ti(4+) ions were reduced. These results suggest that the substitution of 3d metals in titania nanosheets affects their crystal and electronic structures and material properties such as their long-range atomic configuration and reduction mechanism.

Atomic structure of titania nanosheet with vacancies.

Titania nanosheets are two-dimensional single crystallites of titanium oxide with a thickness of one titanium or two oxygen atoms, and they show attractive material properties, such as photocatalytic reactions. Since a titania (Ti0.87O2) nanosheet is synthesized by the delamination of a parent layered K0.8Ti1.73Li0.27O4 crystal using a soft chemical procedure, substantial Ti vacancies are expected to be included and affect the material properties. The atomic arrangement of a titania nanosheet with vacancies has not been revealed owing to the difficulties of direct observation. Here, we have directly visualized the atomic arrangement and Ti vacancies of a titania nanosheet using advanced lower-voltage transmission electron microscopy (TEM). Analyses of the results of first-principles calculations and TEM image simulations for various Ti vacancy structure models indicate that two particular oxygen atoms around each Ti vacancy are desorbed, suggesting the sites where atomic reduction first occurs.

Quantitative evaluation of temporal partial coherence using 3D Fourier transforms of through-focus TEM images.

We evaluate the temporal partial coherence of transmission electron microscopy (TEM) using the three-dimensional (3D) Fourier transform (FT) of through-focus images. Young's fringe method often indicates the unexpected high-frequency information due to non-linear imaging terms. We have already used the 3D FT of axial (non-tilted) through-focus images to reduce the effect of non-linear terms on the linear imaging term, and demonstrated the improvement of monochromated lower-voltage TEM performance [Kimoto et al., Ultramicroscopy 121 (2012) 31-39]. Here we apply the 3D FT method with intentionally tilted incidence to normalize various factors associated with a TEM specimen and an imaging device. The temporal partial coherence of two microscopes operated at 30, 60 and 80 kV is evaluated. Our method is applicable to such cases where the non-linear terms become more significant in lower acceleration voltage or aberration-corrected high spatial resolution TEM.

Assessment of lower-voltage TEM performance using 3D Fourier transform of through-focus series.

We assess the imaging performance of a transmission electron microscopy (TEM) system operated at a relatively low acceleration voltage using the three-dimensional (3D) Fourier transform of through-focus images. Although a single diffractogram and the Thon diagram cannot distinguish between the linear and non-linear TEM imaging terms, the 3D Fourier transform allows us to evaluate linear imaging terms, resulting in a conclusive assessment of TEM performance. Using this method, information transfer up to 98 pm is demonstrated for an 80 kV TEM system equipped with a spherical aberration corrector and a monochromator. We also revisit the Young fringe method in the light of the 3D Fourier transform, and have found a considerable amount of non-linear terms in Young fringes at 80 kV even from a typical standard specimen, such as an amorphous Ge thin film.