Shigeyuki Morishita

Resolution enhancement in transmission electron microscopy with 60-kV monochromated electron source

Transmission electron microscopy (TEM) at low accelerating voltages is useful to obtain images with low irradiation damage. For a low accelerating voltage, linear information transfer, which determines the resolution for observation of single-layered materials, is largely limited by defocus spread, which improves when a narrow energy spread is used in the electron source. In this study, we have evaluated the resolution of images obtained at 60 kV by TEM performed with a monochromated electron source. The defocus spread has been evaluated by comparing diffractogram tableaux from TEM images obtained under nonmonochromated and monochromated illumination. The information limits for different energy spreads were precisely measured by using diffractograms with a large beam tilt. The result shows that the information limit reaches 0.1 nm with an energy width of 0.10 eV. With this monochromated source and a higher-order aberration corrector, we have obtained images of single carbon atoms in a graphene sheet by TE...

Unexpected Huge Dimerization Ratio in One-Dimensional Carbon Atomic Chains

Peierls theory predicted atomic distortion in one-dimensional (1D) crystal due to its intrinsic instability in 1930. Free-standing carbon atomic chains created in situ in transmission electron microscope (TEM)1-3 are an ideal example to experimentally observe the dimerization behavior of carbon atomic chain within a finite length. We report here a surprisingly huge distortion found in the free-standing carbon atomic chains at 773 K, which is 10 times larger than the value expected in the system. Such an abnormally distorted phase only dominates at the elevated temperatures, while two distinct phases, distorted and undistorted, coexist at lower or ambient temperatures. Atom-by-atom spectroscopy indeed shows considerable variations in the carbon 1s spectra at each atomic site but commonly observes a slightly downshifted π* peak, which proves its sp1 bonding feature. These results suggest that the simple model, relaxed and straight, is not fully adequate to describe the realistic 1D structure, which is extremely sensitive to perturbations such as external force or boundary conditions.

Attainment of 40.5 pm spatial resolution using 300 kV scanning transmission electron microscope equipped with fifth-order aberration corrector

The achievement of a fine electron probe for high-resolution imaging in scanning transmission electron microscopy requires technological developments, especially in electron optics. For this purpose, we developed a microscope with a fifth-order aberration corrector that operates at 300 kV. The contrast flat region in an experimental Ronchigram, which indicates the aberration-free angle, was expanded to 70 mrad. By using a probe with convergence angle of 40 mrad in the scanning transmission electron microscope at 300 kV, we attained the spatial resolution of 40.5 pm, which is the projected interatomic distance between Ga-Ga atomic columns of GaN observed along [212] direction.

Evaluation of residual aberration in fifth-order geometrical aberration correctors

Higher order geometrical aberration correctors for transmission electron microscopes are essential for atomic-resolution imaging, especially at low-accelerating voltages. We quantitatively calculated the residual aberrations of fifth-order aberration correctors to determine the dominant aberrations. The calculations showed that the sixth-order three-lobe aberration was dominant when fifth-order aberrations were corrected by using the double-hexapole or delta types of aberration correctors. It was also deduced that the sixth-order three-lobe aberration was generally smaller in the delta corrector than in the double-hexapole corrector. The sixth-order three-lobe aberration was counterbalanced with a finite amount of the fourth-order three-lobe aberration and 3-fold astigmatism. In the experiments, we used a low-voltage microscope equipped with delta correctors for probe- and image-forming systems. Residual aberrations in each system were evaluated using Ronchigrams and diffractogram tableaux, respectively. The counterbalanced aberration correction was applied to obtain high-resolution transmission electron microscopy images of graphene and WS2 samples at 60 and 15 kV, respectively.

Performance of Low-kV Aberration-corrected STEM with Delta-corrector and CFEG in Ultrahigh Vacuum Environment

Single-layered materials, such as graphene or BN, are susceptive to knock-on damage. To reduce this effect, observation of these materials by (scanning) transmission electron microscopy (STEM/TEM) at low acceleration voltage is well-known to be effective. We have developed low-kV STEM/TEM instruments such as TripleC #1 (CFEG and Delta correctors, 15-60 kV) [1] and TripleC #2 (Monochromator FEG, and Delta correctors, 15-60 kV) [2] to observe the layered materials with less damage. Even using with lower acceleration voltages from 15 to 60 kV, it was found that layered materials were etched with residual gas atoms in a microscope column under electron irradiation. To evaluate the etching of the single-layered materials, a base pressure in a microscope column had better to be changeable from a high vacuum to an ultrahigh vacuum (UHV) range. This paper reports the evacuation system and results of resolution test in the developed UHV low-kV electron microscope.

New STEM/TEM Objective Lens for Atomic Resolution Lorentz Imaging

During high resolution STEM/TEM imaging, a specimen has been always placed in strong perpendicular magnetic field using ordinary magnetic objective lens. If the specimen is magnetic material, such strong field can strongly affect its magnetic structures or even break the whole specimen. The simplest way to avoid such effect is turning off the objective lens and using the next nearest lens from the objective lens. This method can be used for almost all STEM/TEM, but the focal length of the lens should inevitably become much longer. The long focal length results in large chromatic aberrations, and being susceptible to noises. These drawbacks make it difficult to observe specimen at very high resolution with objective lens off mode, i.e. in Lorentz mode.

Ultra High Energy Resolution EELS Mapping using Aberration-corrected Low-voltage STEM Equipped with Monochromator

We have been developing a low-voltage analytical electron microscope working at an accelerating voltage of from 60 kV to 15 kV, which enables us to observe and analyze carbon-related materials with less sample damage by irradiation of electrons. This microscope, which employs a double Wien-filter monochromator system [1], is equipped with delta-type aberration correctors for probeand imageforming lens system [2]. The double Wien-filter monochromator, which is located between the extraction anode of Schottky source and the accelerator, enables us to obtain an achromatic monochromatic probe. Therefore the energy spread of electrons after the monochromation is controllable by choosing the width of the slit which is located between two filters, independently on the probe size at the specimen. The delta-type aberration corrector consists of triple dodeca-poles to correct the fifth-order aberration including six-fold astigmatism as well as the spherical aberration. In addition, so as to detect the high energy resolution spectra in electron energy-loss spectroscopy (EELS) by the monochromated electron source, 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.

Improvement of TEM Spatial Resolution at Low Accelerating Voltages (15-30 kV) with Monochromator

Development of transmission electron microscope at accelerating voltage lower than 60 kV is important for observing carbon related materials with low irradiation damage. The geometrical aberration corrector is a mandatory tool, especially for microscopy at low accelerating voltage, because the scattered electrons up to higher angle must be included in the formation of a high spatial resolution image. We have already developed the higher-order geometrical aberration corrector (Delta corrector) which can correct up to fifth-order aberrations including six-fold astigmatism. In a STEM ADF image at 30 kV, CC dumbbells of graphene with a separation of 0.14 nm were resolved by the corrector with cold-field emission gun (CFEG) [1]. However, the C-C dumbbells cannot be resolved in TEM images with the Delta corrector and a CFEG. This is due to chromatic aberration; the resolution reduction by chromatic aberration in TEM is severer than that in STEM.

Development of a Monochromated and Aberration-Corrected Low-Voltage (S)TEM

Low-voltage scanning/transmission electron microscope (STEM/TEM) equipped with delta-type aberration correctors [1] was developed under a project “Triple-C phase-1” to observe the atomic structure of carbon materials with less knock-on damage. This microscope enables us to observe and analyze defected structures of graphene edge by using electron energy-loss spectroscopy (EELS) at an atomic scale [2]. However this microscope was equipped with a cold field emission gun to obtain high brightness therefore its energy resolution remains to be approximately 0.3 eV. To study the electronic structures of materials in detail, we have developed a monochromator working at 15 60 kV for a lowvoltage aberration-corrected microscope under a project “Triple-C phase-2”, whose targeted energy resolution is better than 25 meV. The developed microscope, which is based on JEM-ARM200F, is equipped with a high resolution EELS (Quantum-ERS optimized in low-voltage, GATAN Inc.) and delta-type aberration correctors for STEM and TEM. The optical system for the monochromator is chosen to be a double Wien-filter. The Wienfilter arranged between the extraction anode of Schottky source and the accelerator, as we developed in the previous design [3]. After the monochromator, the electron probe is achromatic and the energy spread is controlled by the width of the slit, independently on the probe size at the specimen. In addition, the setting of the monochromator and the electron trajectories inside the monochromator are independent of the change of the accelerating voltage, since the monochromator is located before the accelerator and the potential along the optical axis inside the monochromator is kept constant. The ultimate energy resolution with acquisition time of 2 ms was obtained to be 14 meV with a 0.1μmwidth slit at 30 kV as shown in Fig.1 (a). Figure 1(b) shows the low loss spectrum from Hexagonal BN obtained with an energy resolution of 22 meV and an acquisition time of 300 ms at 30 kV. This spectrum, which was measured with a probe size of about 1 nm, a beam current of about 10 pA, a convergence semi-angle of 15 mrad and a collection semi-angle of 30 mrad, showed a sharp peak corresponding to an optical phonon at about 170 meV. These results suggest that the microscope enables us to analyze materials with very high energy resolution ≤ 25 meV in nanometer scale, owing to a large scattering cross-section and small specimen damage by using lower voltage electrons. Figure 2 shows a monochromated and aberration-corrected TEM image of single-layered graphene obtained with an energy spread of 172 meV at 60 kV. This image implies that the monochromated electron source in aberration-corrected TEM provides clearly resolved C-C bonds and the enhancement of spatial resolution arising from a small chromatic aberration in TEM at low accelerating voltage. This work is supported by Japan Science and Technology agency, Research Acceleration Program.

Resolution Improvement in Aberration-Corrected Low- Voltage TEM with Monochromator at 60 kV

We have developed a low-voltage electron microscope equipped with a monochromator and Delta-type Cs correctors, which shows atomic resolution at accelerating voltages of 60, 30 and 15 kV. In theory, resolution of TEM images at 60 kV is severely affected by chromatic aberration, which is proven by our calculations of contrast transfer functions and multi-slice image simulation taking chromatic aberration into account with experimental conditions. Experimentally, TEM images of gold nano-particles were observed with non-monochromated and monochromated electron sources at 60 kV. Detectable spatial frequency in the image with the monochromated source was higher than that with non- monochromated source. We have demonstrated that the TEM image resolution at the low- voltage is improved by using a monochromated electron source, which reduce the energy spread of the electron source.