Scanning transmission electron microscopy (STEM)

About this technique

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Scanning transmission electron microscopy (STEM) involves focusing an electron beam into a small probe and scanning it across a sample (similar to a SEM). The image is built up pixel-by-pixel by collecting the electrons transmitted through the sample at each point in the scan. The sample requirements are similar to those for conventional TEM analysis, as sufficient electrons must be transmitted through the sample to build up the image. The resolution of the image depends on the probe size, with atomic resolution possible on high-resolution instruments.

STEM imaging can be combined with spectroscopy techniques such as EDS or EELS to investigate the local composition or chemistry of a sample. A spectrum can be collected at each point in the scan making it possible to correlate features in the STEM image with spectral information. The resolution for microanalysis will be similar to the image resolution and, like the image resolution, will be dependent on the probe size.

Initially a STEM image of the sample is obtained. The region to be analysed by spectroscopy is then marked on the image. This could be the entire field of view, a sub-region of the image or even a line scan across a specific image feature, such as an interface. The marked area is then rescanned and spectra acquired at each point in the scan.

The resulting data, often referred to as a spectrum-image, can be post-processed to extract any of the information typically obtained from the spectroscopy technique, such as composition, valence, etc. as a function of position in the sample. Acquiring STEM spectrum-image data can be a lengthy process if high spectral signals are to be obtained by scanning over large fields of view.

A range of STEM detectors are available that provide different information about the sample by collecting different sub-sets of the transmitted electrons. Bright-field (BF) images are formed by collecting the unscattered electrons and those scattered through small angles. These images look similar to those obtained by conventional TEM imaging except that the image resolution is now determined by the probe size. Dark-field (DF) images are obtained by collecting only the scattered electrons while omitting the unscattered electrons.

High-angle annular dark-field (HAADF) STEM, also referred to as Z-contrast imaging, involves collecting only those electrons scattered through very large angles. The resulting image shows mass- (or Z- ) contrast with higher atomic number regions of the sample appearing brighter than light element regions.

STEM imaging can be conducted on both physical science and biological samples. In the case of biological samples, the constantly scanning beam often results in reduced sample damage providing an option for imaging beam-sensitive samples. STEM can often be applied to thicker samples than conventional TEM analysis with imaging possible on samples more than a micron thick depending on the microscope voltage and mass of the specimen.