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Séminaire général

Using text book Quantum mechanics to control metallic nano-islands height and liquid like diffusion in Pb/Si at low temperatures

M. C. Tringides

As nanostructures become smaller in size the confined electrons occupy discrete energy levels because of Quantum Size Effects (QSE). It has been standard to search with spectroscopic probes (ARPES, STS etc) to correlate the energy spectra with the dimensions of epitaxially grown metallic islands. However it was an unexpected surprise when it was found [1] that for Pb/Si(111) the reverse is true as seen in STM and SPA-LEED experiments, i.e., islands of single 7-layer height are grown with unprecedented -function distribution because of the modulation of the QSE energy with height (fig.1). This is an intriguing growth mode [2] and opens up new ways to control the dimensions of nanostructures.

Over the last 10 years Pb/Si(111) have been a prototype system to observe how other island properties (kinetic barriers, reactivity, superconductivity, interfaces etc) can be tuned with island dimensions and QSE. Different experimental methods by different groups have been used with robust and mutually consistent results : STM, STS, High resolution LEED, angle resolved photoemission and surface X-ray scattering. The experimental results have been also confirmed with theoretical calculations. This successful activity will be briefly reviewed and open questions listed.

However the QSE energetics provides only part of the reason, since kinetics must be also unusually fast for the islands to assemble within a few minutes at the relatively low temperatures T 150K. Coarsening experiments in a mixture of unstable and stable islands (generated at high flux rates) have revealed a novel type of non-classical coarsening. The unstable islands transform to stable islands very quickly, because the dense wetting layer between the islands moves collectively transferring atoms to the stable islands at rate much higher than normal diffusion [3]. This unusual mass transport has been confirmed in real time experiments with Low Energy Electron Microscopy [4] where the wetting layer displays a “liquid-like”super-diffusive behavior. It is also an open question to discover other systems where such unusual QSE-driven self organization is present.

In collaboration with M. Hupalo, S. Binz, C. Z. Wang, K.M.Ho, Z. Chvoj, Z. Kuntova, P.Miceli, E. Conrad, M. Altman, M.Loy.

References

1. K. Budde, E. Abram, V. Yeh, M. C. Tringides, Phys. Rev. B (R) 61, 10602 (2000)
2. M. C. Tringides, M. Jalochowski and E. Bauer in Physics Today 60, No. 4, 50 (2007)
3. M. Hupalo and M. C. Tringides Phys. Rev. B 75, 235443 (2007)
4. M.C. Tringides, M. Hupalo K.L. Man, M.M.T. Loy, M.S. Altman Nanophenomena at Surfaces : Fundamentals of Exotic Condensed Matter Properties Springer ed. M.Michailov (2010) ; K.L. Man, M.C. Tringides, M.M.T. Loy, M.S. Altman Phys. Rev. Lett. 101, 226102 (2008).