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Monday, 06 July 2015

Prof Richard E. Palmer presents his atomic manipulation experiments in an ICN2 Seminar

Prof Richard E. Palmer, from the Nanoscale Physics Research Laboratory at the University of Birmingham (UK), visited ICN2 o July 3. He reported variable temperature atomic manipulation experiments on semiconductor surfaces, which reveal a rich variety of new phenomena.

Atomic manipulation is the extreme limit of nanotechnology, and is mainly performed at low temperatures on metal surfaces. By contrast, Prof Richard E. Palmer, from the Nanoscale Physics Research Laboratory at the University of Birmingham (UK), reported variable temperature atomic manipulation experiments on semiconductor surfaces during an ICN2 Seminar held on July 3. These results reveal a rich variety of new phenomena. The Seminar, presented by Prof Pablo Ordejón, ICN2 Director, was entitled “Atomic manipulation of polyatomic molecules and size-selected gold clusters”.

Prof Palmer focused on a specific system – chemisorbed chlorobenzene (C6H5Cl or PhCl) on the Si(111)-7x7 surface – and discussed the bond-selective manipulation of the molecule via electron or hole injection. Specifically, He reported non-local atomic manipulation (leading to molecular desorption) of PhCl, which is effective over a length scale of 100A or more and represents a kind of 'remote control' of atomic manipulation. New experiments explore the coherence of this charge-transfer process and the role of elastic electron scattering (diffusive transport).

This non-local electron attachment mechanism is also found to be thermally activated (barrier 0.4 eV), and suppressed by the proximity of the STM tip itself; both these results are explicable in terms of electron-driven excitation to an intermediate physisorbed state from which thermal desorption proceeds. Moreover C-Cl bond dissociation in the PhCl molecule is also thermally activated, with an energy barrier of 0.8 eV, which correlates with thermal excitation to the physisorbed (precursor) state of the molecule, where electron attachment occurs.

The STM atomic manipulation experiments performed by Prof Palmer’s Group have also stimulated a new approach to the question of metastability in the atomic structure of size-selected nanoclusters, assembled from atoms in the gas phase and deposited in vacuum. These clusters provide a new route to model catalysts, biochips etc, but there are fundamental questions over their equilibrium atomic structures, since direct gas phase structural studies have been limited and new techniques like aberration-corrected scanning transmission electron microscopy (ac-STEM) are only now being applied to soft-landed, size-selected clusters.

He also surveyed his latest ac-STEM experiments, which address the atomic structure of size-selected “magic number” gold clusters – Au20, Au55, Au309, Au561, and Au923, with emphasis on dynamical manipulation experiments, which probe the transformation of metastable isomers into more stable configurations under the electron beam. The results establish a hierarchy of isomer stability as a function of size and provide a reference for theoretical treatments of nanosystems.