TEM: FEI Tecnai Transmission Electron Microscope
The transmission electron microscope is used to characterize the microstructure of materials with very high spatial resolution. Information about the morphology, crystal structure and defects, crystal phases and composition, and magnetic microstructure can be obtained by a combination of electron-optical imaging, electron diffraction, and small probe capabilities. Further, in situ analysis can be performed using a variety of specimen holders, including heating, cooling, and strain experiments to name a few. The trade-off for this diverse range of structural information and high resolution is the challenge of producing very thin samples for electron transmission.
The FEI Tecnai G2 F20 X-TWIN Transmission Electron Microscope has a field-emission gun (FEG), scanning unit (STEM) with bright field/dark field detector, x-ray detector (EDS) for compositional analysis and CCD camera.
The Tecnai's specifications are as follows:
- 2.5 Å point-to-point resolution, 1.02 Å line resolution, 1.4 Å information limit
- 200kV operating voltage
- ±30° tilt with double-tilt holder, ±80° tilt with tomography holder (expected mid-2010)
- EDAX SUTW (super ultra thin window) and analyzer, 0.3 srad EDS solid angle
The instrument is run using the TIA interface, and additional software includes: Gatan Digital Micrograph (TIA-embedded version), Xplore3D (tomography package with data acquisition, Inspect3D, ResolveRT (FEI edition)), TrueImage (focal series reconstruction), Gatan HoloWorks, Adobe Photoshop, MacTempas (image calculation), Desktop Microscopist (crystallographic analysis), and ImageJ (image processing). Further, the Tecnai is integrated into our Collaboratory remote access system.
Principles of operation: The transmission electron microscope uses a high energy electron beam transmitted through a very thin sample to image and analyze the microstructure of materials with atomic scale resolution. The electrons are focused with electromagnetic lenses and the image is observed on a fluorescent screen, or recorded on film or digital camera. The electrons are accelerated at several hundred kV, giving wavelengths much smaller than that of light: 200kV electrons have a wavelength of 0.025Å. However, whereas the resolution of the optical microscope is limited by the wavelength of light, that of the electron microscope is limited by aberrations inherent in electromagnetic lenses, to about 1-2 Å.
Because even for very thin samples one is looking through many atoms, one does not usually see individual atoms. Rather the high resolution imaging mode of the microscope images the crystal lattice of a material as an interference pattern between the transmitted and diffracted beams. This allows one to observe planar and line defects, grain boundaries, interfaces, etc. with atomic scale resolution. The brightfield/darkfield imaging modes of the microscope, which operate at intermediate magnification, combined with electron diffraction, are also invaluable for giving information about the morphology, crystal phases, and defects in a material. Finally the microscope is equipped with a special imaging lens allowing for the observation of micromagnetic domain structures in a field-free environment.
The TEM is also capable of forming a focused electron probe, as small as 20 Å, which can be positioned on very fine features in the sample for microdiffraction information or analysis of x-rays for compositional information. The latter is the same signal as that used for EMPA and SEM composition analysis (see EMPA facility), where the resolution is on the order of one micron due to beam spreading in the bulk sample. The spatial resolution for this compositional analysis in TEM is much higher, on the order of the probe size, because the sample is so thin. Conversely the signal is much smaller and therefore less quantitative. The high brightness field-emission gun improves the sensitivity and resolution of x-ray compositional analysis over that available with more traditional thermionic sources.
Restrictions on Samples: Sample preparation for TEM generally requires more time and experience than for most other characterization techniques. A TEM specimen must be approximately 1000 Å or less in thickness in the area of interest. The entire specimen must fit into a 3mm diameter cup and be less than about 100 microns in thickness. A thin, disc shaped sample with a hole in the middle, the edges of the hole being thin enough for TEM viewing, is typical. The initial disk is usually formed by cutting and grinding from bulk or thin film/substrate material, and the final thinning done by ion milling. Other specimen preparation possibilities include direct deposition onto a TEM-thin substrate (Si3N4, carbon); direct dispersion of powders on such a substrate; grinding and polishing using special devices (t-tool, tripod); chemical etching and electropolishing; lithographic patterning of walls and pillars for cross-section viewing; and focused ion beam (FIB) sectioning for site specific samples.
Artifacts are common in TEM samples, due both to the thinning process and to changing the form of the original material. For example surface oxide films may be introduced during ion milling and the strain state of a thin film may change if the substrate is removed. Most artifacts can either be minimized by appropriate preparation techniques or be systematically identified and separated from real information.
In order to become a qualified user on the FEI Tecnai TEM, you need to follow each of these steps in the order as listed here:
- Complete the process to become a lab member of SNSF and follow the instructions to activate a Badger account.
- Complete the SNSF COVID-19 Authorization protocol and receive notification of authorization.
- Contact firstname.lastname@example.org to coordinate subsequent training. Basic Tecnai training consists of two 3-hr sessions and a 2-hr final, plus observation during start-up in your first session with your own samples. Further training on specific techniques can be requested following basic training. TEM users with prior experience will be trained at the level required. An accredited TEM laboratory course is periodically offered through the Materials Science and Engineering Department.
- Training in TEM specimen preparation is also available. Specimen prep for TEM requires more time and effort than for most other characterization techniques since a TEM specimen must be approximately 1000 Å or less in thickness in the area of interest and the entire specimen must fit into a shallow 3 mm diameter cup. There are a variety of techniques and instruments available for thinning materials to these specifications. Please refer to the TEM specimen preparation page for more details.
TEM service is available for projects where extensive instrument training is not practical. However, users must generally make or provide their own TEM-ready samples.