H Inada

Imaging Single Atoms Using Secondary Electrons with an Aberration-Corrected Electron Microscope

Aberration correction has embarked on a new frontier in electron microscopy by overcoming the limitations of conventional round lenses, providing sub-angstrom-sized probes. However, improvement of spatial resolution using aberration correction so far has been limited to the use of transmitted electrons both in scanning and stationary mode, with an improvement of 20-40% (refs 3-8). In contrast, advances in the spatial resolution of scanning electron microscopes (SEMs), which are by far the most widely used instrument for surface imaging at the micrometre-nanometre scale, have been stagnant, despite several recent efforts. Here, we report a new SEM, with aberration correction, able to image single atoms by detecting electrons emerging from its surface as a result of interaction with the small probe. The spatial resolution achieved represents a fourfold improvement over the best-reported resolution in any SEM (refs 10-12). Furthermore, we can simultaneously probe the sample through its entire thickness with transmitted electrons. This ability is significant because it permits the selective visualization of bulk atoms and surface ones, beyond a traditional two-dimensional projection in transmission electron microscopy. It has the potential to revolutionize the field of microscopy and imaging, thereby opening the door to a wide range of applications, especially when combined with simultaneous nanoprobe spectroscopy.

Performance and image analysis of the aberration-corrected Hitachi HD-2700C STEM

We report the performance of the first aberration-corrected scanning transmission electron microscope (STEM) manufactured by Hitachi. We describe its unique features and versatile capabilities in atomic-scale characterization and its applications in materials research. We also discuss contrast variation of the STEM images obtained from different annular dark-field (ADF) detectors of the instrument, and the increased complexity in contrast interpretation and quantification due to the increased convergent angles of the electron probe associated with the aberration corrector. We demonstrate that the intensity of atomic columns in an ADF image depends strongly on a variety of imaging parameters, sample thickness, as well as the nuclear charge and the deviation from their periodic position of the atoms we are probing. Image simulations are often required to correctly interpret the atomic structure of an ADF-STEM image.

Atomic imaging using secondary electrons in a scanning transmission electron microscope: experimental observations and possible mechanisms.

We report detailed investigation of high-resolution imaging using secondary electrons (SE) with a sub-nanometer probe in an aberration-corrected transmission electron microscope, Hitachi HD2700C. This instrument also allows us to acquire the corresponding annular dark-field (ADF) images both simultaneously and separately. We demonstrate that atomic SE imaging is achievable for a wide range of elements, from uranium to carbon. Using the ADF images as a reference, we studied the SE image intensity and contrast as functions of applied bias, atomic number, crystal tilt, and thickness to shed light on the origin of the unexpected ultrahigh resolution in SE imaging. We have also demonstrated that the SE signal is sensitive to the terminating species at a crystal surface. A possible mechanism for atomic-scale SE imaging is proposed. The ability to image both the surface and bulk of a sample at atomic-scale is unprecedented, and can have important applications in the field of electron microscopy and materials characterization.

Application of 80-200 kV aberration corrected dedicated STEM with cold FEG

We have developed new STEM instrumentation with a cold field emission source (Hitachi HD-2700) in order to perform structural characterization and elemental mapping at the atomic level. The instrument utilises the CEOS GmbH (Germany, managing director: Dr. Max Haider) aberration corrector. The accelerating voltage range is between 80 kV and 200 kV. The cold field emission source proves to be the ideal emitter for analytical transmission electron microscopes due to its high brightness, high current density and small energy spread. In this study, we have examined low accelerating voltage conditions for obtaining high image contrast and high performance elemental analysis (in which FWHM of zero loss peaks are 0.3 eV for acquisition time of 1 second and 0.34 eV for acquisition time of 40 second by accelerating voltage of 80 kV, respectively). We have observed high contrast bright field STEM images of graphene carbon at an accelerating voltage of 80 kV, in which lattice fringes can be clearly seen.

Revealing the Origin of "Phonon Glass-Electron Crystal" Behavior in Thermoelectric Layered Cobaltate by Accurate Displacement Measurement

Measurement of local disorder and lattice vibrations is of great importance for understanding the mechanisms whereby thermoelectric materials efficiently convert heat to electricity. Calcium cobalt oxides (Ca2CoO3)0.62CoO2 is a model system in this regard with a figure of merit ZT above one. The compound has a complex misfit layered structure with significant lattice displacement (both static and dynamic) that is attributed to the reduced thermal conductivity. Its averaged structure consists of two interpenetrating subsystems of a CdI2-type CoO2 layer and a distorted tri-layered rock-salt-type Ca2CoO3 block (Fig.1,2), being incommensurately modulated along the b-axis. It is well known that both static displacement and thermal atomic vibration can effectively scatter phonons to reduce thermal conductivity, however, the exact scattering mechanisms are still unknown, largely because there is no reliable method available for such a measurement that can link the displacement to the phonon scattering.

High-resolution SEM observation at the atomic level using a dedicated STEM with aberration correction

We have developed a high-resolution secondary electron (SE) imaging technique using a dedicated scanning transmission electron microscope (STEM) instrument with a high efficiency SE detector (Hitachi HD-2700) in order to investigate the structural characterization at the atomic level without thin film processing. The HD-2700 is equipped with a CEOS GmbH spherical aberration (Cs) corrector and can routinely achieve 1A spatial resolution in SE and STEM imaging. We applied this technique for high accuracy critical dimension measurements of semiconductor materials using aberration corrected SE imaging. We have observed the cross sectional SE image of a semiconductor transistor along the Si[011] zone axis. The specimen thickness was 1μm and an accelerating voltage of 200 kV was used to observe the semiconductor structures, such as a metal gate. Under these conditions the high-k gate insulating layer, source, and drain structures can be clearly seen. The higher magnification SE images show that it is possible to resolve the Si111 plane, which has a spacing of 0.314 nm, and these images can be used for magnification calibration. These results indicate the ability to perform high precision measurements of crystal lattices for the structural characterization of semiconductor materials.

Imaging and Spectroscopy of Energy-Related Nanomaterials

energy technologies has become one of the most important missions of our nation. The scientific challenge is to discover new ways to efficiently generate, transport, store, and use energies. In the past decade, our group has devoted significant efforts on energy related materials, including superconductors [1], thermoelectrics [2], photovoltaics, fuel cells, and batteries to understand their structure and property relationship. In this presentation, we report our recent work on Li-ion batteries [3] and core-shell nanocatalysts for hydrogen fuel-cell applications [4].

Imaging and Spectroscopy of Interfaces and Surfaces with Advanced Electron Microscopy

Of prime importance in the study of materials’ interfaces is the use of Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM) to examine the interfacial crystal structure, strain distribution, dopant profile and bonding state to understand the interfacial behavior. Traditionally, TEM has been the method of choice. With the development and implementation of aberration correction nanoprobe high-angle-annular-dark field (HAADF) imaging and energy loss spectroscopy in STEM become increasing popular with extraordinary ability to quantitatively study interface structure at atomic scale.

Uranium single atom imaging and EELS mapping using aberration corrected STEM and LN2 cold stage

Single heavy atoms on a thin carbon substrate represent a nearly ideal test specimen to evaluate STEM performance [1,2]. The single atoms approximate point scatters when imaged with the STEM large angle annular detector. (This is not necessarily true when using small angle scattering in TEM to make a phase contrast image.) The high scattering power relative to the substrate gives a high signal-to-noise ratio, even with relatively low beam current. The thinness of the sample eliminates any issues regarding depth of focus or channelling effects. The specimen was prepared in a manner similar to negative staining, except with a much lower concentration of Uranyl Acetate. The sample shown consisted of tobacco mosaic virus (TMV) on a 2nm thick carbon film substrate supported by holey film. The sample was rinsed several times with 0.01% Uranyl Acetate (compare to 2% normally used for negative staining) and air dried.