Apr 7, 2009
Total spatial coherence of electron sources demonstrated
Researchers in Taiwan have come up with a special type of single-atom electron source that provides brightness and spatial coherence orders of magnitude better than those of the state-of-the-art electron sources used in current electron microscopes. The long lifetime and the reliable preparation method of these single-atom tips may bring electron-beam-based techniques into a new era. New techniques based on these electron sources will hopefully solve important scientific problems that are currently out of reach.
The phase of an electron wave is very sensitive to the electric and magnetic fields in a material down to the atomic dimension. Total coherence of an electron wave allows scientists to fully retrieve such atomic-scale information recorded in the phase images. This is the essence of electron holography and coherent electron diffraction. However, the electron sources used in the state-of-the-art electron beam devices have only partial coherence and also limited brightness. With the availability of totally coherent single-atom electron sources, the dream of retrieving full atomic scale information can finally be realized.
The coherence of an electron beam emitted from a conventional field emitter cannot be compared to the coherence of a laser beam. Nanotips or single-atom tips have been shown to provide very bright and coherent electron beams for electron microscopes. However, traditional ways of preparing nanotips or single-atom tips are tedious and unreliable. The most serious problem is their short lifetimes, which hinders real applications. Now, the discovery of thermally and chemically stable noble-metal covered W(111) single-atom tips and the development of simple and reliable preparation methods in our laboratory have made possible the practical application of single-atom electron sources.
In our lab, we have built a low-energy electron point projection microscope (PPM) to image single-walled carbon nanotubes (SWNTs). Under certain conditions, the SWNT bundle can act as a nanobiprism to split the wavefront of the incoming electron wave into two coherent partial waves, which are deflected by the electric fields around the nanoprism in opposite directions and form an interference pattern on a screen. The interference pattern reveals the spatial coherence of the electron wave.
When we use a noble-metal covered W(111) single-atom tip as the electron source, the interference pattern caused by the nanobiprism shows very good contrast and the fringes extend over the entire beam width. These results indicate very good phase correlation across the entire beam. In contrast, the electron beam emitted from the most coherent electron sources in current commercial electron microscopes can achieve good phase correlation across only a small part of the beam width. One way of improving the phase coherence is to include a small aperture that admits only the most coherent part of the beam. Unfortunately, the brightness or the beam current is greatly sacrificed, which often results in a poor signal-to-noise ratio.
Based on the high-performance single-atom sources we have developed, we are currently devising new coherent electron diffraction imaging techniques to obtain diffraction patterns of individual nanostructures. In the future, we would like to focus these electron beams to carry out more sophisticated experiments.
The researchers presented their work in Nanotechnology.
About the author
This work was performed in the Surface and Nanoscience Laboratory of the Institute of Physics, Academia Sinica (IPAS), Taiwan, ROC. Che-Cheng Chang is a PhD student studying at the Department of Materials Science and Engineering, National Tsing-Hua University (NTHU). Dr Hong-Shi Kuo is an assistant research fellow at IPAS. Dr Ing-Shouh Hwang is a research fellow at IPAS and an adjunct professor at NTHU, and Prof. Tien T Tsong is a distinguished research fellow at IPAS.