Editorial Feature

A Guide to Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM)

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Atomic Force Microscopy (AFM) was invented in 1986. It uses various forces that occur when two objects are brought within nanometres of each other. An AFM can work either when the probe is in contact with a surface, causing a repulsive force, or when it is a few nanometres away, where the force is attractive. Scanning works similarly to the Scanning Tunnelling Microscopy (STM), and also creates three-dimensional images. AFM is based on scanning a flexible, force-sensing cantilever across a specimen. Attractive and repulsive forces acting on the tiny diving-board-like arm cause deflections that can be measured with laser methods.

The Newer Proximal Probe

The newer proximal probe can be used in a number of modes of operation. Included is a contact mode, in which the tip touches the specimen surface and senses internuclear repulsive forces between nuclei in the tip and sample, and a non-contact mode that exploits electrostatic or van der Waals forces. As with STM, a feedback circuit can be used to adjust the tip-to-sample distance to maintain a constant force. The tip motions can be recorded and converted into relief maps. The AFM works on most materials. It can image down to atomic dimensions but is best for larger features. Recent advances have shown that individual molecules can be imaged with atomic-scale resolution using a non-contact method. This relies on functionalizing the tip appropriately. Since the tip is in direct contact with the surface, problems can occur if the material is soft, sticky, or has loose particles. Hence, such materials are usually imaged in the non-contact mode.

Solving Processing and Materials Problems

The AFM is being used to solve processing and materials problems in a wide range of technologies affecting the electronics, telecommunications, biological, chemical, automotive, aerospace, and energy industries. The materials being investigated include thin and thick film coatings, ceramics, composites, glasses, synthetic and biological membranes, metals, polymers, and semiconductors. The AFM is being applied to studies of phenomena such as abrasion, adhesion, cleaning, corrosion, etching, friction, lubrication, plating, and polishing.

New Uses for AFM

In addition to imaging surfaces, AFM is also used in nanomanufacturing such as nanoscale lithography and nanomachining. Nanopatterning of oxides on silicon using an electrical bias between the AFM tip and the surface has been reported. AFM has also been used to pattern organic self-assembled monolayers on surfaces. An AFM probe moving over a substrate removes the layer already present, which in turn is replaced by a different molecule by self-assembly from a solution. Such techniques of nanomanufacturing can be used to build two-dimensional and three-dimensional structures on both flat and curved surfaces.

The Background of Transmission Electron Microscopy (TEM)

Older techniques have also made their mark on science and technology at the small scale, and new instrumental methods continue to be developed. Transmission Electron Microscopy (TEM), for example, has been used for decades to examine tiny structures. Transmission electron microscopy and X-ray diffraction (XRD) are often used to determine the morphology and structure of nanomaterials. Electron microscopy can obtain nearly atomic resolution of a material’s atomic arrangement and chemical composition. This technique requires a clean sample that meets ultra high-vacuum standards in order to provide surface characterizations such as reconstruction and phase transitions.

Types of Transmission Electron Microscopy (TEM)

A standard TEM can be modified to perform various types of electron microscopy. By adding a scanning system and the appropriate detectors, the electron beam can raster across a surface to enable imaging a larger area.

Another type is cryogenic TEM. Although its development started in the 1970s, only recently has it become popular for studying biological molecules. It involves imaging samples at cryogenic temperatures and is useful for imaging materials that are volatile in the high vacuum used in standard TEM at room temperature. TEM can also be used to study sample responses to stimuli in situ, including biological materials. High-resolution TEM (HRTEM) increases the resolution limit and is used to directly image atomic structures.

Sources and Further Reading

  • Gross et al., Science, 325, (2009), 1110.
  • Lu et al., Journal of Nanoscience and Nanotechnology, 9(3), (2009), 1696.
  • Moores et al., Nanoscale Research Letters, 6, (2011), 185.

This article was updated on 25th January, 2019.

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