結晶構造像

crystal structure image

高分解能電子顕微鏡法で薄い結晶試料(＜10nm)からの透過波と回折波を干渉させて得る像の内、対物レンズの球面収差と加速電圧によって決められる焦点はずしの条件(シェルツァー・フォーカス)で得られる結晶構造を表す像。通常、低次の晶帯軸と平行に電子線を入射し、対物絞りで透過波と多くの回折波を通して撮影する。シェルツァー・フォーカスに設定しないで得る像を格子像という。格子像は必ずしも結晶構造には直接対応しない。結晶構造に良く対応する像を撮るための3要素は、十分薄い試料を用意すること、精度の高い方位合わせ、シェルツァー・フォーカス条件合わせである。

HREM, which allows wave interference between transmitted and diffracted waves to be caused, enables us to obtain an image exhibiting the crystal structure of a thin crystalline specimen (thickness <10 nm). This image is obtained at the defocus condition, which is determined by the spherical aberration of the objective lens and the accelerating voltage of the incident beam (Scherzer focus). The image is taken by setting the electron beam parallel to a low-order zone axis and by passing a transmitted beam and many diffracted beams through the objective aperture. When the image is not taken at Scherzer focus, it is called "lattice image," which does not always correspond to the crystal structure. Three important factors to take an image corresponding to the crystal structure are: (1) Preparation of a sufficiently thin crystalline specimen, (2) High-accuracy adjustment of the crystal orientation, and (3) Adjustment of the Scherzer focus condition.