Takashi Dobashi

Diffraction microscopy using 20 kV electron beam for multiwall carbon nanotubes

Diffraction microscopy with iterative phase retrieval using a 20kV electron beam was carried out to explore the possibility of high-resolution imaging for radiation-sensitive materials. Fine, homogeneous, and isolated multiwall carbon nanotubes (MWCNTs) were used as specimens. To avoid lens aberrations, the diffraction patterns were recorded without a postspecimen lens. One- and two-dimensional iterative phase retrievals were executed. Images reconstructed from the diffraction pattern alone showed a characteristic structure of MWCNTs with the finest feature corresponding to a carbon wall spacing of 0.34nm.

10-kV diffractive imaging using newly developed electron diffraction microscope.

A new electron diffraction microscope based on a conventional scanning electron microscope (SEM), for obtaining atomic-level resolution images without causing serious damage to the specimen, has been developed. This microscope in the relatively low-voltage region makes it possible to observe specimens at suitable resolution and record diffraction patterns. Using the microscope we accomplished 10-kV diffractive imaging with the iterative phase retrieval and reconstructed the structure of a multi-wall carbon nanotube with its finest feature corresponding to 0.34-nm carbon wall spacing. These results demonstrate the possibility of seamless connection between observing specimens by SEM and obtaining their images at high resolution by diffractive imaging.

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.

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.

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

Diffraction Microscopy using 20-kV Electron Beam for Multi-Wall Carbon Nanotubes

Title Diffraction microscopy using 20 kV electron beam for multiwall carbon nanotubes Author(s) Kamimura, Osamu; Kawahara, Kota; Doi, Takahisa; Dobashi, Takashi; Abe, Takashi; Gohara, Kazutoshi Citation Applied Physics Letters, 92(2): 024106-1-024106-3 Issue Date 2008-01 Doc URL http://hdl.handle.net/2115/45432 Rights Copyright 2008 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. Type article File Information 20080115_20kVDiffractionMicroscopy_kamimura_APL.pdf