Ultra-high resolution STM with hydrogen as a contrast enhancing agent
Christian Weiss, Ruslan Temirov, Stefan Tautz
Scanning tunnelling microscopy (STM) is a powerful tool for studying the structural, electronic, vibrational and transport properties of complex adsorbates on a single- molecule scale. A major disadvantage of the method is its lack of chemical sensitivity, which would allow the direct identification of molecular species. Recently we have demonstrated that low-temperature STM in the presence of condensed molecular hydrogen or deuterium resolves the geometric structure of planar organic adsorbate molecules on metal surfaces (see Fig1). Because the images that are obtained with this technique closely resemble chemists' structure formulae, the geometric imaging mode of scanning tunnelling hydrogen microscopy (STHM) can be considered as a new type of 'chemical' resolution for the scanning tunnelling microscope.
Figure 1:- STM images of PTCDA/Ag(111) in conventional and deuterium-sensitized modes. (a) 5×5 nm2 conventional constant current image of the electronic local density of states, recorded with 1 nA, −0.340 V. In the centre of the image a point defect (impurity molecule) is visible. (b) 5×5 nm2 constant height dI/dV image of the area shown in (a), recorded in the geometric imaging mode. Prior to recording the image, the tip was stabilized at 1 nA, −0.010V and then the bias voltage was set to 0. In the upper left part of the image, the calculated atomic structure of a PTCDA layer on Ag(111), is superimposed. (c) Enlarged section of (b), 1×1.5 nm2. (d) Structural formula of PTCDA.
Remarkably, the imaging is not the only intriguing feature of transport in the presence of condensed H2 and D2. Our studies as well as the experiments done by other groups have revealed a complicated evolution of the STM junction conductance upon condensation of the gas in the junction (see Fig2). The observed strongly non-linear conductance behaviour as well as the highly resolved images obtained in the presence of H2 and D2 suggest that the transport across the junction involves a complex dynamics. Notably, there have been attempts to attribute similar conductance behaviour to the manifestation of two channel Kondo effect (2CK). Despite the seeming simplicity of hydrogen molecule, there appear more and more observations of intricate transport effects induced by its adsorption which means that the consistent picture of transport in the presence of hydrogen has yet to be established.
Figure 2 - The colour sequence black-red-blue-green-magenta shows the evolution of the junction conductance during molecular hydrogen exposure. (a) Evolution of the I(V) characteristics of the junction. Black - conductance spectrum in the absence of hydrogen. Red - noise in the low bias region appears as a first sign of hydrogen in the junction. After measuring the red spectrum, hydrogen exposure was stopped. Blue - spectrum was measured 22 minutes after the exposure was stopped. The switching frequency has increased. Green - spectrum was measured 80 minutes after the end of hydrogen exposure. Magenta - spectrum was measured 14 hours after the end of hydrogen exposure. Spectra are off-set vertically (by 200 pA) for clarity. (b) dI/dV spectra corresponding to the I(V) curves shown in a. All spectra in (a) and (b) were measured above the centre of one PTCDA molecule adsorbed on Ag(111), with the tip which after sensitization yielded the contrast shown in Fig. 1. The stabilization point of all spectra was 0.1 nA, -0.340 V.
R. Temirov, S. Soubatch, O. Neucheva, A. Lassise, F.S. Tautz, A Novel Method Achieving Ultra-High Geometrical Resolution in Scanning Tunnelling Microcopy, New J. Phys. 10, 053012 (2008)
J. Kroha, Univ. Bonn,
F. Anders, Univ. Bremen,
J.M. van Ruitenbeek, Univ. Leiden, The Netherlands,
T. Michely, Univ. Köln.