Michigan Tech recently commissioned a FEI 200kV Titan Themis Scanning Transmission Electron Microscope (S-TEM). This microscope positions Michigan Tech faculty on the leading edge of new imaging capability for structural and chemical analysis at the nano-scale. The microscope is housed in a building specially constructed for such an instrument capable of atomic resolution. This instrument represents one of only two Titans found in higher education in the state of Michigan. The Themis has a full complement of state-of-the-art accessories, including six specialized specimen holders that extend the S-TEM utility.
CTEM is the TEM operation that we are most familiar where a static beam is imposed on an ultra-thin specimen (50 – 75 nm thick). The beam electrons are scattered in the specimen and form an image where the contrast in the image is a complicated function of Bragg diffraction (for crystalline materials) and mass-thickness. At very high magnifications, the periodic arrays of atoms in crystals can be identified with spacing less than 0.15 nm. With the correct interpretation, such images of the atomic lattice can be used to identify local crystal structure and defects within that structure. Electron diffraction is also a CTEM capability that provides information on atomic order (or disorder) from 10 um to nano-scale diameter areas in the specimen. Electron diffraction can allow identification of local crystal structure and defects within a crystal structure and can be very powerful when used in tandem with the lattice imaging capability of the Titan.
STEM mode provides an image of the ultra-thin specimen produced by scanning the electron beam over the specimen. The resolution in STEM mode is enhanced by an aberration-correcting lens that produces an electron probe with a diameter less than that of an atom, which then permits atomic-level resolution—80 picometers as seen in the image of Silicon atoms (Figure 2). The STEM mode is also utilized in EFTEM and HAADF modes described below.
EELS and EFTEM result from inelastic scattering in a thin specimen that results in electron energy loss that is characteristic of the element in the ultra-thin specimen. The electron beam emerging from the specimen is filtered through a magnetic prism that separates the signal based on its kinetic energy. This hardware produces an EELS spectrum but selectively passing electrons of defined energy can be used to reconstruct a live STEM-mode energy filtered electron image. Since different elements produce different amounts of energy loss (different kinetic energy), the Titan sub-angstrom probe interaction with the specimen can be correlated with the specimen chemistry at each probe position. Thus, not only can the Titan indicate the positions of atomic arrays, it can also provide the element (chemical) identity of the positions within the array.
HAADF is a STEM mode technique where a dedicated detector senses atomic number (Z) contrast changes in the specimen. The detector is positioned in a unique location below the specimen where it can collect more scattered electrons than is possible in conventional dark field imaging. Based on the intensity of scattering at each position of the sub-atomic electron probe, a qualitative indication of the chemistry variation within an atomic array can be developed. Thus the HAADF images are complimentary to the results obtained using the EFTEM function.
ChemiSTEM and Super-X Energy Dispersive X-ray nano-analysis system consists of the high beam current field emission electron gun and four high X-ray throughput silicon drift X-ray detectors radially positioned and directed towards the specimen. In conjunction with a beryllium specimen holder the system is capable of collecting a high quality X-ray map at atomic resolution in seconds. The x-ray maps are a 2D map describing the spatial distribution of chemical phases and structures from an ultra small-scale region of the specimen.
Unique specimen holders include the standard single tilt axis holder but also the NanoEX single tilt heating and biasing holder for in-situ STEM imaging and elemental analysis at elevated temperatures. The low background (beryllium) holder is used with the Super-X X-ray nanoanalysis to minimize any contribution to x-ray analysis from a specimen holder. The tomography holder and reconstruction software facilitate electron tomography at high specimen tilt angles that produce 3-dimensional imagery of nano structures in ultrathin specimens. The nano-indenter holder permits mechanical testing studies of atomic scale materials. In-situ liquid electrochemistry holder permits testing in an aqueous cell while observing the experiment under the electron beam.
Free online eTraining is available for this instrument. This self-paced tutorial and reference content does not replace course requirements for authorized usage.
Available topics related to this instrument:
Michigan Tech offers many undergraduate and graduate courses related to materials characterization.
One of these courses offers direct, hands-on training in transmission electron microscopy:
MY 5250 – Transmission Electron Microscopy
Practical aspects of materials characterization by transmission electron microscopy.
Semesters Offered: On Demand
Restrictions: Must be enrolled in one of the following Level(s): Graduate
Last updated November 14, 2017