Transmission Electron Microscope (TEM)
Transmission Electron Microscopy (TEM) is a well-known technique for imaging solid materials at atomic resolution. The design of a transmission electron microscope (TEM) is analogous to that of an optical microscope. In a TEM high-energy (>60 kV) electrons are used instead of photons and electromagnetic lenses instead of glass lenses. The electron beam passes an electron-transparent sample (typically 10-200nm) and a magnified image is formed using a set of lenses. This image is projected onto a CCD camera. Structural information can be acquired both by (high resolution) imaging as well as by electron diffraction.
A TEM can be operated in two modes:
- Conventional TEM (CTEM) using a parallel, broad beam, allowing for High Resolution TEM and Bright Field TEM imaging, and
- Scanning TEM (STEM). In the latter mode, a focused probe is scanned over the sample, while recording the transmitted of scattered electron beam intensity. The resulting image displays contrasts that depend on the atomic number of the elements present. Our current TEM, a JEOL ARM 200F, is equipped with a so-called probe-corrector, implying that imperfections in the optical path are corrected, yielding sub-Angstrom resolution in STEM.
The combination of STEM with an elemental analysis technique allows for mapping of the lateral distribution of elements at the sub-nm scale: As a result of the interaction of the incident electron beam with the sample, X-rays are emitted. Their energies are characteristic for the atoms present in the volume that is probed. The detection and analysis of these X-rays is called Energy Dispersive X-Ray analysis (EDX or EDS). The X-ray spectrum that is obtained allows for quantification of the elemental composition of the irradiated area. The JEOL ARM is equipped with a Silicon Drift Detector (SDD), allowing for high count rates, and has an exceptionally large detector area of 100 mm2, yielding a high sensitivity for elemental analysis at the (sub-) nanometer scale.
One of the additional merits of the ARM is the possibility to go to lower acceleration voltages (60 / 80 kV), while maintaining atomic resolution in both TEM and STEM mode, thus enabling high resolution studies of beam sensitive materials, such as graphene.