E-beams get a new twist

A new method to expand the capabilities of conventional transmission electron microscopes (TEMs) has been discovered by researchers at the National Institute of Standards and Technology.

Passing electrons through a nanometer-scale grating imparted the resulting electron waves with so much orbital momentum that they were able to maintain a corkscrew shape in free space, the scientists found.

Adapting transmission electron microscopy with the new development could lead to quick, inexpensive imaging of a larger set of magnetic and biological materials with atom-scale resolutions. The spiral shapes and angular momentum created within the electrons will allow scientists to look at a greater variety of materials in ways not previously possible with TEMs. Using a nanograting can expand the microscope’s capabilities.

The researchers were not the first to manipulate electron beams in this way, but their smaller device enabled them to separate the fanned-out beams 10 times wider than in previous experiments. They also spun the electrons with 100 times the orbital momentum, enabling them to determine that the electron corkscrew separates over time, despite its relative stability. Their work appears in Science, Vol. 331, No. 6014, pp. 155-156 (2011).

NIST researchers twisted the flat electron wavefronts into a fan of helices using a very thin film with a 5-μm-diameter pattern of nanoscale slits, combining the wavefronts to create spiral forms similar to a pasta maker extruding rotini. The method produces several electron beams fanning out in different directions, with each beam made of electrons that orbit around the direction of the beam. Courtesy of Ben McMorran, NIST.

The pattern amplifies some of the wave peaks and eliminates some of the wave valleys in the electron wavefronts passing through it, creating a spiral form similar to rotini coming out of a pasta maker. The team determined the experiment’s success when it determined that the electrons had formed doughnutlike, or spiral, patterns, indicating a helical shape.

The technique could help provide more information from magnetic materials as well as help improve TEM images of transparent objects such as biological specimens. Although biological material can be difficult to image in ordinary TEMs because electrons pass through it without deflecting, using corkscrew electron beams could produce high-contrast, high-resolution images of biological samples by looking at how the spiral wavefronts get distorted as they pass through such transparent objects.

The imaging applications have not yet been demonstrated, but by producing the corkscrew electrons with nanogratings in a TEM, scientists hope that it will provide a significant step toward expanding the capabilities of existing microscopes.