11:00-12:00
Highly coherent electron sources:noble-metal covered W(111)single-atom tips
Ing-Shouh Hwang (Institute of Physics, Academia Sinica, Nankang, Taipei, Taiwan, R.O.C.)
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Spatial coherence and brightness of electron sources are two key factors for their application to electron interferometry and holography, electron diffraction, and electron and microscopies. It has been long considered that a smaller source size would yield a higher brightness and a larger transverse coherence width. We have used noblemetal covered W(111)single-atom tips as the point sources in an electron point projection microscope. The interference fringes of a single-walled carbon nanotube(SWNT) bundle exhibit a very high contrast. These results, along with recent investigations(Ref.1-3), indicate that this type of single-atom tips can produce very bright and highly coherent electron beams. In addition, the long lifetime and the reliable preparation method of these single-atom tips maybring the electron-beam-based techniques into a new era.
We have systematically measured the interference patterns of isolated SWNT at different tip-sample separations d. Wide and semi-transparent patterns are seen in the projection image when d is within 5-15 μm. The electron wave passing through a SWNT might experience certain phase shifts and interferes with the primary beam on the screen, resulting in the observed patterns. This is the in-line holography originally proposed by Gabor in 1948 (Ref. 4). The biprism effect becomes more significant when d is reduced below 5 μm. Simulation results for the biprism effect and the relation of the effective refractive index versus electron energy will be presented.
Reference
1) H.-S.Kuo, I.-S.Hwang, T.-Y.Fu, J.-Y.Wu, C.-C.Chang, T.T.Tsong, NANO Lett. 4(12), 2379(2004).
2) C.Oshima, E.Rokuta, T.Itagaki, T.Ishikawa, B.Cho, H.-S.Kuo, I.-S.Hwang, T.T.Tsong, e-J. Surf. Sci. Nanotechnol. 3, 412(2005).
3) T.Ishikawa, T.Urata,B.Cho, E.Rokuta, C.Oshima, Y.Terui, H.Saito, A.Yonezawa, T.T.Tsong, Appl.Phys. Lett. 90, 143120(2007).
4) D.Gabor, Nature 161, 777(1948).
