Transmission Electron Microscopy (TEM)¶
Transmission electron microscopy (TEM) is a characterization technique in which a beam of electrons is transmitted through a specimen thinner than roughly 100 nm. Electrons that pass through the sample are focused into an image or a diffraction pattern, revealing crystal structure, defects, interfaces, and chemical composition at resolutions below 0.2 nm - small enough to resolve individual atomic columns.
How transmission electron microscopy works¶
An electron gun accelerates electrons to high energy, 20 to 200 kV on a typical analytical instrument. Because an accelerated electron has a wavelength thousands of times shorter than visible light, the resolution limit that constrains optical microscopes does not apply.
Electromagnetic lenses focus the beam onto a specimen that has been thinned until it is electron-transparent. Three things then happen to the electrons, and each is a different measurement:
- Transmitted electrons form the conventional bright-field image, where contrast comes from differences in thickness, density, and crystal orientation.
- Scattered electrons are collected at high angle to form a STEM-HAADF image, in which brightness scales with atomic number. Heavy elements appear bright, so composition can be read directly from the image.
- Energy lost by the beam is measured by electron energy loss spectroscopy (EELS), which reports bonding state and electronic structure. Separately, X-rays emitted by the excited sample are measured by energy-dispersive spectroscopy (EDS) to give elemental composition.
When to use TEM¶
TEM is the correct choice when the question is about internal structure at or near the atomic scale:
- Identifying crystal phase and orientation from electron diffraction
- Imaging dislocations, stacking faults, and grain boundaries
- Measuring layer thickness and abruptness in a semiconductor stack
- Locating dopants or precipitates and identifying them by composition
- Distinguishing an amorphous region from a crystalline one
If the question concerns surface topography, particle counts, or features larger than about 100 nm, scanning electron microscopy answers it faster and with far less sample preparation. If the question concerns bulk crystal structure averaged over a large volume, use X-ray diffraction.
What TEM cannot do¶
Being clear about the limits saves everyone a wasted session:
- The sample must be destroyed. Thinning to electron transparency is irreversible.
- The field of view is tiny. A TEM image covers a few micrometres at most. It cannot tell you whether what you are looking at is representative; that requires complementary bulk measurement.
- Beam damage is real. Polymers, biological material, and some oxides degrade under a 200 kV beam within seconds.
- Preparation dominates the schedule. Producing a good specimen routinely takes longer than the microscope session itself.
TEM at MPaCT Lab, Flagstaff, Arizona¶
The MPaCT Lab at Northern Arizona University operates a JEOL JEM-F200 transmission electron microscope in Flagstaff, Arizona. It is a 200 kV analytical TEM/STEM with a field-emission gun, configured for both high-resolution imaging and chemical analysis.
The instrument is available to NAU researchers, external academic users, and industry partners, on a fee-for-service basis or as a trained hands-on user. NAU is the only university in northern Arizona offering shared-use access to an analytical TEM of this class.
| Specification | Value |
|---|---|
| Accelerating voltage | 20 to 200 kV |
| TEM point resolution | 0.19 nm |
| STEM-HAADF resolution | 0.14 nm |
| Electron gun | Schottky FEG / Cold FEG |
| Magnification range (TEM) | 20x to 2,000,000x |
| Magnification range (STEM) | 200x to 150,000,000x |
| Analytical options | EDS, EELS, tomography |
Available accessories include a backscattered electron detector for enhanced Z-contrast and an electron biprism for electron holography and phase imaging.
Full JEOL JEM-F200 specifications and booking →
Sample requirements¶
Specimens must be electron-transparent: below roughly 100 nm, and below 50 nm for high-resolution imaging. The lab operates a full preparation suite:
- Dimple grinder - thins the centre of a disk while leaving a supporting rim
- Disk grinder - produces flat, parallel-sided disks of uniform thickness
- Ion beam mill - final thinning to electron transparency by argon ion sputtering
- Site-specific lift-out is the listed DualBeam
If you are unsure whether your material can be prepared, contact the lab before submitting a request. Staff will advise on preparation route and realistic turnaround.
Frequently asked questions¶
How thin does a TEM sample need to be?¶
Below roughly 100 nm, and often below 50 nm for high-resolution work. The MPaCT Lab operates a dimple grinder, a disk grinder, and an ion beam mill for preparing electron-transparent specimens. See TEM sample preparation.
What is the difference between TEM and SEM?¶
A TEM transmits electrons through a thin specimen to reveal internal structure at atomic resolution. An SEM scans a beam across a bulk surface and collects scattered or secondary electrons, giving surface topography at lower resolution with far simpler sample preparation.
Can external companies use the TEM at NAU?¶
Yes. The MPaCT Lab is a shared-use facility in Flagstaff, Arizona, open to NAU researchers, external academic users, and industry partners on either a fee-for-service or hands-on trained-user basis.
What can EELS detect that EDS cannot?¶
Light elements. EDS sensitivity falls away below boron, whereas EELS detects from lithium upward and is the method of choice for elements with atomic number of 10 or below. For sodium the detection limit is roughly an order of magnitude better with EELS than with EDS.
Can TEM determine oxidation state?¶
Yes, through EELS. The fine structure of an EELS edge depends on bonding environment, so it distinguishes oxidation states of transition metals and separates allotropes of carbon. This is information no imaging mode and no EDS spectrum provides.
Request time on this instrument¶
MPaCT Lab - Building 98E, South Engineering Lab
561 E Pine Knoll Dr, Flagstaff, AZ 86001
Phone: 928-523-2343 · Email: mpct.nano@nau.edu