Scanning transmission electron microscopy

What is STEM used for?

STEM is used to analyze and characterize thin film materials down to nanoscopic and even atomic levels. Its extremely high resolution makes STEM fundamental to nanotechnology and similar material science fields that require powerful nanoscopic imaging techniques. When combined with EELS (electron energy loss spectroscopy) in STEM-EELS analysis, information on the elemental composition of the sample can also be obtained.

STEM finds applications in the development of new technologies through semiconductors, solar cells, catalysts, and battery materials, where it is not only helpful for imaging but also for investigating electronic properties. STEM can even be used to analyze biological samples, providing insight into the mechanics of molecular biology.

How does scanning transmission electron microscopy work?

STEM works by combining the principles behind two well-established analytical techniques: transmission electron microscopy and scanning electron microscopy. A probe pointed at an ultrathin sample emits a beam of electrons that interact with a very fine point on the surface of the sample. These electrons are transmitted through the sample and are picked up by a screen on the other side. The way that the electrons interact with the sample will cause patterns to display on the screen which represent the individual atoms present. This process is repeated as the probe scans across the surface of the sample until a spatial image has been produced. 

Suitable samples and sample preparation

STEM can be carried out on a wide range of materials, provided that electrons are able to pass through them. This means that samples must be ultrathin, generally less than 100 nanometres in thickness. Thinning should be done with care to reduce the risk of contamination or damage to the sample surface, as this can affect the quality of the final results. New methods, such as a focused ion beam (FIB), can help to speed up the thinning process, which has historically been the most time-consuming part of gathering STEM data.

STEM vs. TEM

Fundamentally transmission electron microscopy (TEM) and STEM are the same technique, with the key difference being that a TEM probe does not scan. Therefore, TEM provides the same benefits of being able to image materials on a nanoscopic scale with a high resolution. However, as the probe does not move across the surface, it is limited to individual scanning areas and does not provide an overall image of the material in the way that STEM does. The trade-off for this is that a TEM-only setup is generally less expensive and therefore can be viewed as a faster, more accessible option to STEM analysis.

Advantages and limitations of STEM analysis

STEM is able to provide ultra-high resolution images of areas that are too small to be viewed using a scanning electron microscope, let alone an optical one. STEM is, for the most part, a non-destructive technique and therefore is suitable for analyzing a wide variety of sample types without running the risk of destroying the sample. However, in some cases, materials can be sensitive to electron exposure and it may accelerate their decomposition.

One key disadvantage is that in order to remove interactions with air, STEM must be conducted in a vacuum. This means it will not be suitable for certain materials, particularly some biological samples, which are not vacuum-compatible. Furthermore, the images produced by STEM are inherently monochrome and thus tend to require a substantial level of interpretation.