Yosuke Maehara

Low voltage electron diffractive imaging of atomic structure in single-wall carbon nanotubes

The demand for atomic-scale analysis without serious damage to the specimen has been increasing due to the spread of applications with light-element three-dimensional (3D) materials. Low voltage electron diffractive imaging has the potential possibility to clarify the atomic-scale structure of 3D materials without causing serious damage to specimens. We demonstrate low-voltage (30 kV) electron diffractive imaging of single-wall carbon nanotube at a resolution of 0.12 nm. In the reconstructed pattern, the intensity difference between single carbon atom and two overlapping atoms can be clearly distinguished. The present method can generally be applied to other materials including biologically important ones.

Spherical shell structure of distribution of images reconstructed by diffractive imaging.

Image reconstruction from Fourier intensity through phase retrieval was investigated when the intensity was contaminated with Poisson noise. Although different initial conditions and/or the instability of the iterative phase retrieval process led to different reconstructed images, we found that the distribution of the resulting images in both the object and Fourier spaces formed spherical shell structures. Averaging of the images over the distribution corresponds to the position of the image at the sphere center.

K-means clustering for support construction in diffractive imaging

A method for constructing an object support based on K-means clustering of the object-intensity distribution is newly presented in diffractive imaging. This releases the adjustment of unknown parameters in the support construction, and it is well incorporated with the Gerchberg and Saxton diagram. A simple numerical simulation reveals that the proposed method is effective for dynamically constructing the support without an initial prior support.

Sequential on-line construction for ensemble phase-retrieved images

The reconstruction of a target image from its diffraction pattern is referred to as “diffractive imaging,” and this has been focused as a new microscopy. Fourier phase retrieval is required during the imaging process by means of numerical computations using computers. However, it is difficult to establish a complete convergence using an incomplete diffraction pattern such as experimental data in the retrieving process. A simple averaging procedure by weak phase-retrieved results using different initials is often used. Recently, the spherical shell structure of the retrieved images has been investigated and the center of the shell is to be a superior retrieved result. Therefore, it is important to construct such a structure by the retrieved images in order to reconstruct the target image. In this paper, a novel sequential on-line algorithm for effectively constructing the spherical shell structure of the images is introduced. A numerical demonstration is also presented using the Fourier intensities contaminated with Poisson noise.

Atomic Arrangement of Contamination on Graphene

Graphene is an ideal sample support since it has the thickness of a single atom and is stable under lowvoltage irradiation that is below the knock-on damage threshold of graphene. However, the surface of free-standing graphene frequently becomes contaminated during long periods of electron microscopic observation, and this contamination interferes with the observation of the target. Such contamination is thought to consist of amorphous carbon, but its atomic structure is not clear. On the other hand, crystalline graphene grows at the step-edge of bilayer graphene at high temperature [1]. In this paper, we report the atomic-resolution observation of contamination grown on monolayer graphene at room temperature and the contamination’s atomic arrangement.

20-kV Diffractive Imaging of Graphene by using an SEM-based Dedicated Microscope

The optical microscope has reached a resolution finer than the wavelength of light; however, in the electron-microscopy field, decrease aberrations of lenses remains a challenge. Even with the help of aberration correctors and monochrometers, the resolution is limited to more than multiples of ten times the wavelength of the electron beam [1]. On the other hand, diffractive imaging, which is an imaging method using iterative phase retrieval from a diffraction pattern [2], can obtain high-resolution images without suffering any aberrations of the imaging lens. This method (using low acceleration voltage) have been applied by the authors to reach atomic resolution with a dedicated microscope based on a conventional scanning electron microscope (SEM) [3, 4], and this microscope resolved the atomic arrangement of a single-wall carbon nanotube at 30 kV [5]. In the present study, the atomic arrangement of multi-layer graphene in the case of an acceleration voltage of 20 kV was reconstructed, and the possibility of reconstruction of a non-periodic structure was investigated.

Invited Paper: An Information-Theoretic Approach to Phase Retrieval

Phase problems arise from the lost Fourier phase in measuring the diffraction waves. Re-constructing the phase information using the diffraction pattern of a target object yields the target image, and it is called phase retrieval. This paper introduces an information-theoretic approach to phase retrieval based on information measures, and a refined derivation of the generalized phase retrieval algorithm based on the density power divergence is presented with a simple numerical example using the Poisson-noise-contaminated Fourier intensity.

10-kV Electron-Diffractive Imaging of Multiwall Carbon Nanotube

Electron microscopes are often used to observe the atomic scale structure of materials. However, for the light element materials, e.g., carbon nanomaterials and organic semiconductors, specimen damage due to beam irradiation is a serious problem. Knock-on damage, which is significantly increased when a high-energy electron beam is used, is suppressed in the observation below the threshold energy. The threshold energy is specific to the specimen, depending on its component elements and the strength of their binding energy, e.g., about 27 keV for carbon and 5 keV for hydrogen [1, 2]. Using a low-energy (low acceleration voltage) electron beam decreases the amount of knock-on damage. However, even at a low acceleration voltage, the ionization damage becomes significant. Moreover, low acceleration voltage causes various experimental difficulties, such as increasing of inelastic scattering and decreasing of the brightness of electron gun. Relating to these factors, we need to choice a suitable acceleration voltage for each specimen. Furthermore, lens aberrations mean that obtaining the atomic-resolution image at a low acceleration voltage is difficult. Meanwhile, diffractive imaging with iterative phase retrieval [3, 4] is one of the most promising techniques for high-resolution imaging. The object image is reconstructed from diffraction intensities by retrieving phases obtained from iteration procedures. This method is mainly used with x-rays [5-8] and published works on imaging done with the electron beam [9-13] have recently increased. To achieve high-resolution imaging without serious damage to the specimen, we verified the diffractive imaging with a relatively low-energy (20 kV) electron beam [11], and developed an electron microscope that can be used for this method [14]. This microscope was based on the conventional scanning electron microscope, and a film loader system for the transmission electron microscope and a CCD camera system were installed to record the diffraction pattern without using a post-specimen lens (without projecting the back-focal plane of the objective lens). An additional function was the use of projection lenses to control the size of the diffraction pattern, i.e., camera length. To enable this imaging to be used in a wider range of applications for radiation sensitive materials, we need to verify the diffractive imaging at a lower energy. We tried to record the diffraction pattern of a multiwall carbon nanotube (MWCNT) below an acceleration voltage of 20 kV. However, the sensitivity of the imaging plate (IP) of the electron microscope (Fujifilm FDL-UR-V) decreased rapidly below the 20 kV, and at 10 kV it became almost unable to detect. We saw that the problem might be caused by the protective layer and selected the IP used for tritium detection (Fujifilm BAS-TR), which does not have such a layer, to record the lower voltage electron beam. The diffraction pattern of the MWCNT at 10 kV with 2048 x 2048 pixels is shown in Figure 1(a). The exposure time was 30 sec and the geometrical convergence angle of illumination beam was estimated to be 0.15 mrad. The camera length was 453 mm by using a projection lens. The intensity distribution of the equatorial line (indicated by the arrows) in the diffraction pattern was clearly resolved. The beam size on the specimen was estimated to be about 100 nm mainly because of the diffraction aberration. The large illumination beam meant that some additional intensities from Microsc Microanal 15(Suppl 2), 2009 Copyright 2009 Microscopy Society of America doi: 10.1017/S1431927609093842 746