New nanotechnology materials enable scientists to improve bulk physical properties for different applications and, with the resulting products, open new markets for the nanotechnology industry. The physical properties of nanotechnology materials are strongly correlated with their crystalline structure. It is, however, the real structure complete with interfaces and defects that is of importance and not just the ideal crystalline structure. Hence a thorough understanding of interfaces and defects is required to be able to correlate microscopic structures with macroscopic physical properties. High resolution transmission electron microscopy (HRTEM) is the key to understand these real structures down to the atomic level.
A transmission electron microscope is constituted of two or three condenser lenses to focus the electron beam on the sample, an objective lens to form the diffraction in the back focal plane and the image of the sample in the image plane, some intermediate lenses to magnify the image or the diffraction pattern on the screen. If the sample is thin (< 200 nm) and constituted of light chemical elements, the image presents a very low contrast when it is focused. To obtain an amplitude contrasted image, an objective diaphragm is inserted in the back focal plane to select the transmitted beam (and possibly few diffracted beam): the crystalline parts in Bragg orientation appear dark and the amorphous or not
Braggoriented parts appear bright. This imaging mode is called bright field mode BF.
If the diffraction is constituted by many diffracting phases, each of them can be differentiated by selecting one of its diffracted beams with the objective diaphragm. To do that, the incident beam must be tilted so that the diffracted beam is put on the objective lens axis to avoid off axis aberrations This mode is called dark field mode DF. The BF and DF modes are used for imaging materials to nanometer scale.
EFTEM: material analysis at nanometer resolution
Because of its high resolution, it is a valuable tool to study nanoscale properties of crystalline material such as semiconductors and metals. TEM imaging can be combined with several material analysis techniques like Electron Energy Loss Spectroscopy (EELS), Energy Filtered TEM (EFTEM) and Energy Dispersive X-ray (EDX). EELS is a technique that provides elemental information on a nanometer scale when coupled with Transmission Electron Microscopy. The energy of the incident electrons is altered as they pass through the sample. This Energy Loss can be characterised using EELS to provide elemental identification. Compared to EDX, EELS provides improved signal to noise, spatial resolution (down to 1 nm), energy resolution (<1 eV for EELS) and sensitivity to the lower atomic number elements.
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