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Accueil > Actualités > Imaging the rotation of a single molecule adsorbed on a surface

Imaging the rotation of a single molecule adsorbed on a surface

Collaboration between J.Schaffert, M.C.Cottin, A.Sonntag, H.Karacuban, C.A.Bobisch and R.Möller from CeNIDE in Duisburg-Essen, N.Lorente from CIN2 in Barcelona and J.P.Gauyacq from ISMO

Nature Materials 12 (2013) 223

Topographical images of an adsorbed molecule can be obtained by Scanning Tunnelling Microscopy (STM) : the spatial variation of the tunnelling current provides an ‘image’ of the molecule, or at least of some of its electronic properties. However, electrons travelling through the molecule can also modify it ; the electron can transfer some energy to the molecule resulting in a molecular excitation or a molecular displacement on the surface. Numerous examples of atomic and molecular manipulations by an STM tip have been reported in the literature. A recent experimental break-through by R.Möller group at Duisburg-Essen University allows to analyse in detail such a manipulation and to provide an image of the manipulation efficiency, simultaneously with the topographic image. For that purpose, they designed a very efficient analytical tool for the fluctuations in the tunnelling current (telegraphic noise), thus allowing to reveal the manipulation induced by the tunnelling electrons (SNM : Scanning Noise Microscopy).

A Cu-phthalocynanine molecule exhibits four lobes that are equivalent in the free molecule. It can adsorb individually and flat on a Cu(111) surface, however the surface symmetry is different from that of the molecule and the four lobes cannot adsorb on equivalent regions of the surface. As a consequence, the lobe degeneracy is partly lifted. This effect is barely visible on a topographic image of the adsorbed molecule (see below, left panel, ‘Topo’) ; two opposite lobes exhibits a more grainy image than the other two.

The fuzzy character of the image is actually due to fluctuations in the tunnelling current when the electron travels through these two lobes. The fluctuations have been interpreted as resulting from the rotation of the molecule induced by the tunnelling electrons. A DFT study revealed a metastable configuration of the adsorbed molecule, rotated by 7 degrees from the equilibrium position. The adsorbed molecule is then in a quasi-bistable situation and it fluctuates between the two positions under the action of the tunnelling electrons. The right picture above (‘Rate’) displays the spatial distribution of the rotation rate determined experimentally. It stresses the striking difference between the two sets of lobes : only two of the molecular lobes are active in the electron-induced rotation process.

The electron-induced rotation process has been studied theoretically, from a DFT description of the tunnelling current. The electron travel time through the molecule is very brief, much shorter that the typical rotation time of the molecule so that a sudden approximation, well suited for such an angular momentum transfer process, can be used. The spatial distribution of the electron-induced rotation rate obtained theoretically is presented below, superimposed on a schematic picture of the adsorbed Cu-phthalocyanine molecule (circles). One recognizes the four lobes in the schematic molecule and the ab initio study well reproduces the high spatial selectivity of the rotational excitation process, with only two active lobes.

These results illustrate all the capabilities of the SNM (Scanning Noise Microscopy) for quantitative studies at the atomic level of the action of tunnelling electrons on adsorbed molecules, while stressing the availability of ab initio-based theoretical approaches able to describe manipulations of sizeable adsorbed molecules.