Exploring the limits of analytical electron microscopy

自从20世纪30年代发明电子显微镜以来，电子显微镜在揭示远低于光学显微镜可能的长度尺度的材料结构方面发挥了无价的作用。最近的发展（包括电子透镜像差的校正）使透射电子显微镜（TEM）能够将结构分解到原子尺度。此外，光谱仪可以连接到透射电镜，把它变成一个分析显微镜。能量色散x射线（EDX）光谱可以进行原子水平的化学分析，而电子能量损失光谱（EELS）可以以任何其他技术无法比拟的分辨率提供化学和结构信息。

Ever since their invention in the 1930's, electron microscopes have been invaluable for revealing the structure of materials on a length scale far below what is possible with the light microscope. Recent developments (including correction of the aberrations of electron lenses) enable the transmission electron microscope (TEM) to resolve structure down to the atomic scale. Also, spectrometers can be attached to the TEM, turning it into an analytical microscope. Energy-dispersive x-ray (EDX) spectroscopy allows chemical analysis down to the atomic level, while electron energy-loss spectroscopy (EELS) can provide both chemical and structural information at a resolution unmatched by any other technique. To achieve these goals, the analytical TEM must be very stable but there are also several physical factors that limit its spatial resolution. One of these is the delocalization of the inelastic scattering of electrons, upon which the spectroscopy techniques depend. This delocalization can be estimated from wave optics (treating the electrons as matter waves) but making precise allowance for delocalization requires an accurate knowledge of its point spread function (PSF). I propose to measure this PSF down to the atomic scale, using state-of-the art TEMs that are now available in Canada and elsewhere. In addition, I will collaborate with an experienced theoretician to calculate how the PSF may vary under various experimental conditions. This knowledge will improve the resolution obtainable from analytical TEM and help towards solving several important problems in materials science. For example, EELS can measure the local electronic properties of semiconductor devices but is severely limited by delocalization. Another limitation of analytical TEM is the damage that electrons create in the specimen, which forms the practical resolution limit in biological research and other fields, such as the development of improved catalysts required by the chemical industry. I plan to continue my study into how radiation damage can be minimized in the TEM. Improved analysis techniques will enhance the capability of Canada's premier electron microscope facilities, such as the CCEM at McMaster University, with potential benefits to Canadian research and industrial development, and will help to maintain Canada's reputation at the frontier of advanced electron microscopy.