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Annular aperture boosts contrast in laser-driven electron microscope

June 2, 2010

A new imaging mode for dynamic transmission electron microscopes (DTEMs), which rely on a laser-driven photocathode to produce electron pulses, has been developed by a group of U.S. scientists.

John Wallace

Davis, CA--A new imaging mode for dynamic transmission electron microscopes (DTEMs), which rely on a laser-driven photocathode to produce electron pulses, has been developed by a group of U.S. scientists. The imaging technique enables studies on the dynamics of nanocatalysts at unprecedented spatial and temporal resolution.

"Our group has developed a dark-field imaging mode for DTEM that enables the highest combined spatial and temporal resolution imaging of nanoparticles achieved thus far," says Daniel Masiel of the University of California (Davis) and lead author of the work.¹ According to Masiel, annular dark-field DTEM (ADF-DTEM) could, for the first time, enable direct time-resolved observation of processes such as nanowire growth and other phenomena.

A DTEM is a transmission electron microscope that has been modified to include a laser-driven photocathode that can produce a single intense pulse of electrons with a duration of 15 ns. While the instrument has the potential to provide insight into nanoparticle catalyst dynamics by enabling direct imaging with high spatial and temporal resolution, the limited signal-to-background ratios attainable for dispersed nanoparticle samples have made such studies difficult to perform at optimal resolutions. To overcome these limitations, Masiel and co-workers have fabricated an annular electron-focusing objective-lens aperture that permits images to be obtained with a threefold increase in the signal-to-background ratio. This annular dark-field imaging mode improves the contrast attainable in 15 ns-pulsed electron images and allows particles as small as 30 nm in diameter to be observed.

Other techniques such as coherent diffractive imaging (using coherent X-rays) or in-situ TEM offer direct imaging data but at the cost of either spatial or temporal resolution. This is not the case for ADF-DTEM, the researchers say. While not a photonic device itself (other than the laser-driven photocathode), the ADF-DTEM could be of help to researchers working on novel nanomaterials for use in photonics.

REFERENCE:

1. Daniel Masiel and Ting Guo, ChemPhysChem 2010, 11, No. 10; Permalink to the article: http://dx.doi.org/10.1002/cphc.201000274

About the Author

John Wallace | Senior Technical Editor (1998-2022)

John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.