Instruments


 
       
 

Transmission Electron Microscopy

A beam of electrons is transmitted through an ultra-thin specimen where the electrons interact with the material. The sample can be either illuminated with a focused beam that is scanned over a region (STEM) or with a broader beam of parallel electrons (TEM). Samples have thicknesses ranging from a couple of hundred all the way down to a few nanometers. The latter allow to visualize the atomic structure of the material. The typical energy range of the primary electrons lies between 80 and 300 keV. Due to the high energies and corresponding small wavelengths of the electron wave the achievable spatial resolution is in the Angstrom range.

The primary mode of interaction is elastic scattering of electrons which leads to material-specific diffraction. The microscope's imaging objective lens is able to visualize the diffracted signal directly in its focal plane or construct an image in its image plane. The corresponding signal is further magnified by projector lenses and then detected by electron sensitive detectors, in the case of STEM typically by an annular ring detector with the microscope set in diffraction mode, in the case of TEM the image is recorded with help of a CCD camera.

Additionally the electron can transfer characteristic energies to the sample by exciting inner degrees of freedom. For TEM relevant excitations are phonons, plasmons and characterisic X-rays. Those energy losses can be analyzed via electron energy loss spectroscopy (EELS). Characteristic X-rays are detected with an energy dispersive X-ray spectrometer (EDX). Overall TEM and STEM provide a versatile and powerfull set of tools that allow exploration of materials morphology, chemistry and crystallography on the nanometer scall all the way down to the atomic level.

Traditional TEM analysis is based on the premise that the sample is thin enough to capture a quasi two-dimensional representation of the material. Nevertheless many matarial system contain complex three-dimensional morphologies with characteristic lengths of a few nanometer. The microscopy facility is equipped with dedicated sample holders and computational capabilities to record tilt series and reconstruct corresponding 3D electron tomograms.

A first crucial step for a successful TEM analysis is the preparation of a high-quality sample with the right thickness whereas the procedure is typically material specific. The microscopy facility provides all necessary tools in form of FIB, ion mills and dedicated polishing equipment.

 

FEI Tecnai G2 Sphera 200kV Cryo
(for Life Science Studies)

This microscope is equipped with a Twin objecitve lens which provides at a beam energy of 200keV a moderate resolution of 3A. It is characterized by a large pole piece gap which leaves enough space for larger sample holders such as a cryo holder. It also allows a larger tilt range and is therefore particularly suitable for nanoscale defect analysis in crystalline samples.

Features:
LaB6 emitter; CompuStage with Smart-Tilt software; Low-dose exposure modes; CCD camera (Gatan Ultrascan 1000 2k x 2k).

Holders:
(1) FEI single tilt, tilt range α=±70° (2) FEI double tilt, α=±60°, β=±20° (3) cryo station for vitrified aqueous solutions (4) Fischione 2020 dual axis for tomography, α=±70° (5) Hummingbird combi TEM-APT holder for tomography, α=±80° depending on sample geometry.

T2\0C
 

FEI Tecnai G2 Sphera 200kV EDX
(for Materials Science Studies)

This microscope is equipped with a Super-Twin objective lens which provides at 200keV and the given LaB6 emitter a point resolution of 2.2A. This is suitable for basic atomic resolution TEM analysis of for example nano particles. An Oxford electron dispersive X-ray spectrosometer (EDX) allows point-to-point chemical analysis with a chemical sensitivity down to 0.2%

Holders:
(1) FEI single tilt, tilt range α=±45° (2) FEI double tilt, α=±40°, β=±18°

T20C
 

FEI Titan 80-300kV FEG TEM-STEM with EDS/EELS
(for Analytical Studies)

The Titan is the most versatile microscope which is currently the only one that can be operated both in STEM and TEM mode. Working in an energy range between 80 and 300keV and equipped with a field emission gun it can reach a spatial resolution of 1.3A in STEM, and an information limit of 0.9A in high resolution TEM mode. An energy dispersive X-ray (EDX) detector and an electron energy loss spectrometer (EELS) allow analytical work on the nanometer scale.

Features:
(1) A scanning unit with a high-angle annular dark-field (HAADF) detector for STEM (Z-contrast imaging), an extremely useful imaging mode if crystallographic effects need to be suppressed and chemical variations emphasized; and for atomic resolution HAADF STEM imaging of FIB samples (2) Gatan Enfina 1000 system for electron energy-loss spectroscopy (EELS); (3) Electron dispersive X-ray analysis system (EDAX) (4) Gatan wide-angle CCD (2kx2k) for image recording.

Holders:
(1) FEI double tilt, α=±40°, β=±18° (2) Gatan analytical double tilt with lN2 cooling (3) Fischione 2020 dual axis for tomography, α=±70° (4) Hummingbird combi TEM-APT holder for tomography, α=±70°

T20C