Imaging of isotope diffusion using atomic-scale vibrational spectroscopy.

The spatial resolutions of even the most sensitive isotope analysis techniques based on light or ion probes are limited to a few hundred nanometres. Although vibrational spectroscopy using electron probes has achieved higher spatial resolution1-3, the detection of isotopes at the atomic level4 has been challenging so far. Here we show the unambiguous isotopic imaging of 12C carbon atoms embedded in 13C graphene and the monitoring of their self-diffusion via atomic-level vibrational spectroscopy. We first grow a domain of 12C carbon atoms in a pre-existing crack of 13C graphene, which is then annealed at 600 degrees Celsius for several hours. Using scanning transmission electron microscopy-electron energy loss spectroscopy, we obtain an isotope map that confirms the segregation of 12C atoms that diffused rapidly. The map also indicates that the graphene layer becomes isotopically homogeneous over 100-nanometre regions after 2 hours. Our results demonstrate the high mobility of carbon atoms during growth and annealing via self-diffusion. This imaging technique can provide a fundamental methodology for nanoisotope engineering and monitoring, which will aid in the creation of isotope labels and tracing at the nanoscale.

Position and momentum mapping of vibrations in graphene nanostructures.

Propagating atomic vibrational waves-phonons-determine important thermal, mechanical, optoelectronic and transport characteristics of materials. Thus a knowledge of phonon dispersion (that is, the dependence of vibrational energy on momentum) is a key part of our understanding and optimization of a material's behaviour. However, the phonon dispersion of a free-standing monolayer of a two-dimensional material such as graphene, and its local variations, have remained elusive for the past decade because of the experimental limitations of vibrational spectroscopy. Even though electron energy loss spectroscopy (EELS) in transmission has recently been shown to probe local vibrational charge responses1-4, such studies are still limited by momentum space integration due to the focused beam geometry; they are also restricted to polar materials such as boron nitride or oxides1-4, in which huge signals induced by strong dipole moments are present. On the other hand, measurements on graphene performed by inelastic X-ray (neutron) scattering spectroscopy5-7 or EELS in reflection8,9 do not have any spatial resolution and require large microcrystals. Here we provide a new pathway to determine phonon dispersions down to the scale of an individual free-standing graphene monolayer by mapping the distinct vibrational modes for a large momentum transfer. The measured scattering intensities are accurately reproduced and interpreted with density functional perturbation theory10. Additionally, a nanometre-scale mapping of selected momentum-resolved vibrational modes using graphene nanoribbon structures has enabled us to spatially disentangle bulk, edge and surface vibrations. Our results are a proof-of-principle demonstration of the feasibility of studying local vibrational modes in two-dimensional monolayer materials at the nanometre scale.

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.

Atomic Resolution Imaging at an Ultralow Accelerating Voltage by a Monochromatic Transmission Electron Microscope.

Transmission electron microscopy using low-energy electrons would be very useful for atomic resolution imaging of specimens that would be damaged at higher energies. However, the resolution at low voltages is degraded because of geometrical and chromatic aberrations. In the present study, we diminish the effect of these aberrations by using a delta-type corrector and a monochromator. The dominant residual aberration in a delta-type corrector, which is the sixth-order three-lobe aberration, is counterbalanced by other threefold aberrations. Defocus spread caused by chromatic aberration is reduced by using a monochromated beam with an energy spread of 0.05 eV. We obtain images of graphene and demonstrate atomic resolution at an ultralow accelerating voltage of 15 kV.

Spatial and phase resolution in electron microscopy.

With the invention of the aberration corrector in electron optics, the spatial resolution in electron microscopy has progressively improved and has now reached the sub-50-pm regime, and atomic-resolution electron microscopy has become a versatile tool for investigating the atomic structures in materials and devices. Furthermore, the phase resolution in electron microscopy also exhibits outstanding progress, and it has become possible to visualize electromagnetic fields at atomic dimensions, which strongly contributes to understanding the physical and chemical properties of materials. The electron microscopy society has grown with the improvements in spatial and phase resolutions, and hence, we must continuously develop new hardware, software and methodologies to boost these resolutions. Here, we review the historical progress of spatial and phase resolutions in electron microscopy, where we clarify the definition of these resolutions. We also discuss the future targets in electron microscopy.

Theoretical study on sixth-order geometrical aberration correction.

Spherical aberration correctors using hexapole fields are widely used and are pivotal in atomic-resolution imaging. Although hexapole-field correctors increase the aberration-free angular range, the angular range is limited by higher-order aberrations, such as six-fold astigmatism or sixth-order three-lobe aberration. Here, we propose two types of spherical aberration correctors to compensate for geometrical aberrations up to the sixth order. The first is a four-hexapole corrector, while the second is a two-hexapole corrector, where each hexapole has a nonuniform magnetic field. The four-hexapole corrector can increase the aberration-free angle up to almost 100 mrad. The two-hexapole corrector with a nonuniform magnetic field has a smaller aberration-free angle than that of the four-hexapole corrector, but it is more compact. The dominant residual aberration in these correctors is seventh-order spherical aberration or chaplet aberration, which is seventh-order geometrical aberration.

Development of tilt-scan system for differential phase contrast scanning transmission electron microscopy.

Differential phase contrast (DPC) scanning transmission electron microscopy can directly visualize electromagnetic fields inside a specimen. However, their image contrast is not only sensitive to the electromagnetic fields in the sample, but also the changes in diffraction conditions such as sample bends or thickness changes. These additional contrasts are called diffraction contrasts, and sometimes make it difficult to extract pure electromagnetic field information from the experimental DPC images. In this study, we developed a beam scan system that can acquire many DPC images from the same sample region with arbitrarily varying incident beam tilt angles to the sample. Then, these images are precisely averaged to form tilt-scan averaged DPC images. It is shown that the diffraction contrast can be effectively reduced in the tilt-scan averaged DPC images.

Estimation of wave fields of incident beams in a transmission electron microscope by using a small selected-area aperture.

The direction of an electron beam in a nanometer-sized area is measured directly by utilizing a selected-area aperture. By the measurements at several areas in a beam, the wavefront curvature and thus the defocus value of the beam are detected. From the defocus value, the wave field at the specimen plane is also reproduced in consideration of the influences of the condenser aperture and spherical aberration of the illumination lens. The result shows that phase deviation of 2π is caused only at about 10 nm apart from the beam center in a beam with a typical diameter for high-resolution transmission electron microscopy. Based on the defocus value, the convergence angle of the beam is also estimated to be about 6 mrad without being influenced by the partial coherence, that is, independently of the type of the electron gun. Measuring the defocus values for only two beam diameters enables us to determine geometrical parameters peculiar to the illumination system, based on which wave fields of any beam diameters by any condenser aperture sizes can be estimated. The technique proposed in this paper is effective in evaluating the influence of wavefront curvature of incident beams on various kinds of precise measurements conducted in transmission electron microscopes.

Reprint of: Automated geometric aberration correction for large-angle illumination STEM.

Depth resolution in scanning transmission electron microscopy (STEM) is physically limited by the illumination angle. In recent notable progress on aberration correction technology, the illumination angle is significantly improved to be larger than 60 milliradians, which is 2 or 3 times larger than those in the previous generation. However, for three-dimensional depth sectioning with the large illumination angles, it is prerequisite to ultimately minimize lower orders of aberrations such as 2- and 3-fold astigmatisms and axial coma. Here, we demonstrate a live aberration correction using atomic-resolution STEM images rather than Ronchigram images. The present method could save the required time for aberration correction, and moreover, it is possible to build up a fully automated program. We demonstrate the method should be useful not only for axial depth sectioning but also phase imaging in STEM including differential phase-contrast imaging.

Automated geometric aberration correction for large-angle illumination STEM.

Depth resolution in scanning transmission electron microscopy (STEM) is physically limited by the illumination angle. In recent notable progress on aberration correction technology, the illumination angle is significantly improved to be larger than 60 milliradians, which is 2 or 3 times larger than those in the previous generation. However, for three-dimensional depth sectioning with the large illumination angles, it is prerequisite to ultimately minimize lower orders of aberrations such as 2- and 3-fold astigmatisms and axial coma. Here, we demonstrate a live aberration correction using atomic-resolution STEM images rather than Ronchigram images. The present method could save the required time for aberration correction, and moreover, it is possible to build up a fully automated program. We demonstrate the method should be useful not only for axial depth sectioning but also phase imaging in STEM including differential phase-contrast imaging.

High spatiotemporal-resolution imaging in the scanning transmission electron microscope.

The temporal resolution in scanning transmission electron microscopy (STEM) is limited by the scanning system of an electron probe, leading to only a few frames per second (fps) at most in the current microscopes. To push the boundary of atomic-resolution STEM imaging into dynamic observations, an unprecedentedly faster scanning system combined with fast electron detection systems should be a prerequisite. Here we develop a new scanning probe system with the acquisition time of 83 nanoseconds per pixel and the fly-back time of 35 microseconds, leading to 25 fps STEM imaging with the image size of 512 × 512 pixels that is faster than a human perception speed. Using such high-speed probe scanning system, we have demonstrated the observations of shape-transformation of Pt nanoparticles and Pt single atomic motions on TiO2 (110) surface at atomic-resolution with the temporal resolution of 40 milliseconds. The present probe scanning system opens the door to use atomic-resolution STEM imaging for in situ observations of material dynamics under the temperatures of cooling or heating, the atmosphere of liquid or gas, electric-basing or mechanical test.

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.

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.

Measurement of spatial coherence of electron beams by using a small selected-area aperture.

A new method for measuring the spatial coherence of an electron beam in a transmission electron microscope is proposed. In this method, an Airy pattern produced by a circular selected-area (SA) aperture with an effective diameter of several nanometers is analyzed to obtain the degree of coherence as a function of separation in the specimen plane. Using typical TEM illumination conditions, demonstrative measurements were carried out to determine the spatial coherence length, angular size of the electron source and shape of the coherence function. Based on the results, it was shown that the ratio of the spatial coherence length to the beam radius is about 5% for a condenser aperture with a diameter of 100 µm. This means that perfectly coherent illumination exists within the small SA aperture for beam diameters larger than 560 nm. As an example application of these results, the advantage of SA diffraction over nano-beam diffraction in electron diffractive imaging is discussed. The proposed method is unaffected by temporal coherence or geometric aberrations of the lenses. The possibility of carrying out future measurements using SA apertures with conventional sizes is also discussed.

Characterization of 2,4-dichlorophenoxyacetic acid and 2,4,5-trichlorophenoxyacetic acid-degrading fungi in Vietnamese soils.

Sixty-nine fungal strains were isolated countrywide from 10 Vietnamese soils, in areas both with and without a history of exposure to Agent Orange, and their degrading activities on the phenoxy acid herbicides 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), as well as related compounds, were examined. Among taxonomically various fungi, 45, 12 and 4% of the isolates degraded phenoxyacetic acid (PA), 2,4-D and 2,4,5-T, respectively. While the PA-degrading fungi were distributed to all sites and among many genera, the 2,4-D-degraders were found only in order Eurotiales in class Eurotiomycetes. All of the 2,4,5-T-degrading fungal strains were phylogenetically close to Eupenicillium spp. and were isolated from southern Vietnam. As a degradation intermediate, the corresponding phenol compounds were detected in some strains. The degradation substrate spectrum for 26 compounds of Eupenicillium spp. strains including 2,4,5-T-degraders and -non-degraders seemed to be related to phylogenetic similarity and soil sampling location of the isolates. These results suggest that the heavily contaminated environments enhanced the adaptation of the phylogenetic group of Eupenicillium spp. toward to obtain the ability to degrade 2,4,5-T.