Layered number and stacking order of graphene are strongly related to their electrical and optical properties. Evaluations of these atomic-level characteristics are paid much attention in terms of both scientific and technology interests. Raman spectrophotometry is general method for evaluation of graphene, however, this technique obtains not the atomic-level local structure but an large-area information which is limited by wavelength of light. High-resolution transmission electron microscopy (TEM) is usually applied for observing the atomic-level structures. The graphene is, however, composed of carbon which is easily received electron-irradiation damage, especially the knock-on damage . The knock-on damage shows characteristic energy of threshold and is reduced by applying the relatively low-voltage-electron beam. The atomic-level observation, however, is difficult at the low-voltage-electron beam because of some additional aberrations such as chromatic and high-order aberration. To realize the low-voltage atomic-level characterizing technique is needed.
Diffractive imaging is one of the high-resolution imaging techniques. A specimen image is calculated by a computer processing of a diffraction pattern [2, 3]. We developed the dedicated microscope which was based on a conventional scanning electron microscope (S-5500, Hitachi High-Technologies Corp.) for the diffractive imaging and demonstrated the atomic-level observation of carbon nanotube . To obtain the quantitative diffraction pattern for the diffractive imaging, the diffraction pattern is recorded on the imaging plate by applying a film-loader system for the TEM below the objective lens.
The small-aperture stage (about 75 nm in diameter) is prepared for recording limited-area information of the graphene. By comparing the diffraction pattern to simulation quantitatively, approximately layered-number (2 or 3 layers) and tilt angle to an electron beam (3 to 4 degrees) become clear. By means of the diffractive imaging, the atomic-level structure including stacking order of graphene is obtained.
We conclude that combining diffraction pattern with imaging based on dedicated microscope characterize the graphene at the atomic level. We will present out recent analyzing results obtained using this dedicated microscope.
Part of this work was supported by the Japan Science and Technology Agency.
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