In Situ High-Resolution Transmission Electron Microscopy in the Study of Nanomaterials and Properties
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Resolution Transmission Electron Microscopy in the Study of Nanomaterials and Properties James M. Howe, Hirotaro Mori, and Zhong Lin Wang
Abstract This article introduces the use of in situ high-resolution transmission electron microscopy (HRTEM) techniques for the study and development of nanomaterials and their properties. Specifically, it shows how in situ HRTEM (and TEM) can be used to understand diverse phenomena at the nanoscale, such as the behavior of alloy phase formation in isolated nanometer-sized particles, the mechanical and transport properties of carbon nanotubes and nanowires, and the dynamic behavior of interphase boundaries at the atomic level. Current limitations and future potential advances in in situ HRTEM of nanomaterials are also discussed.
Introduction It is often necessary to study nanoscale materials under the actual conditions in which they exist in order to understand their functionality, or the fundamental phenomena governing their behavior in realistic environments. As materials systems continue to decrease in size, the capability to study these materials at the highest possible levels of spatial resolution is becoming increasingly important. In situ high-resolution transmission electron microscopy (HRTEM) enables such studies and can be applied to a variety of nanoscale materials phenomena. HRTEM imaging is a phase-contrast dominated imaging technique where two or more beams are allowed to interfere to
form an image. Because the lens system of the TEM must preserve the coherence of the image-forming beams, the spacing being resolved in the image must be within the resolution limits of the microscope. This requirement generally limits HRTEM to observations of projected specimen structures along relatively low-index zone axes with wide interplanar spacings that are within the resolution limits of the microscope. Under optimum conditions, an HRTEM image can be directly interpreted as a map of the projected crystal structure (crystal potential) along the electron-beam direction. Such interpretation is usually possible only for a narrow range of specimen and microscope parameters, but this
MRS BULLETIN • VOLUME 33 • FEBRUARY 2008 • www.mrs.org/bulletin
situation and its variations are well understood. In any case, the resulting HRTEM image is a two-dimensional projection of the three-dimensional structure, with the columns (or planes) of atoms being either dark or bright, depending on the exact specimen and imaging conditions. The specimen and microscope requirements for in situ HRTEM imaging are not generally different from those of usual HRTEM, except that one must have specialized holders capable of inducing changes in the specimen and some method of recording and analyzing the resulting dynamic responses. Many different types of holders capable of heating or cooling, straining, lasing, evaporating/depositing, applying a current/voltage, etc., are available commercially or can be fabricated without great difficulty. Some of these holders are mentioned in the following examples. At
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