M. C. Cottin

Imaging the dynamics of individually adsorbed molecules

Although noise is observed in many experiments, it is rarely used as a source of information. However, valuable information can be extracted from noisy signals. The motion of particles on a surface induced, for example, by thermal activation or by the interaction with the tip of a scanning tunnelling microscope may lead to fluctuations or switching of the tunnelling current. The analysis of these processes gives insight into dynamics on a single atomic or molecular level. Unfortunately, scanning tunnelling microscopy (STM) is not a useful tool to study dynamics in detail, as it is an intrinsically slow technique. Here, we show that this problem can be solved by providing a full real-time characterization of random telegraph noise in the current signal. The hopping rate, the noise amplitude and the relative occupation of the involved states are measured as a function of the tunnelling parameters, providing spatially resolved maps. In contrast to standard STM, our technique gives access to transiently populated states revealing an electron-driven hindered rotation between the equilibrium and two metastable positions of an individually adsorbed molecule. The new approach yields a complete characterization of copper phthalocyanine molecules on Cu(111), ranging from dynamical processes on surfaces to the underlying electronic structure on the single-molecule level.

Anisotropic scattering of surface state electrons at a point defect on Bi(111)

Scanning tunneling microscopy was applied to study the lateral variation of the local density of electronic states on the Bi(111) surface in the vicinity of a point defect. At an energy close to the Fermi level a characteristic pattern with a threefold symmetry is found. The pattern can be attributed to the scattering between two electronic surface states which are split by spin orbit coupling. The observation is well described by the superposition of three monochromatic waves. The phase of the waves relative to the center of the defect leads to a reduction to a threefold symmetry.

Interplay between forward and backward scattering of spin-orbit split surface states of Bi(111).

The electronic structure at the surface of Bi(111) enables us to study the effect of defects scattering into multiple channels. By performing scanning tunneling spectroscopy near step edges, we analyze the resulting oscillations in the local density of electronic states (LDOS) as function of position. At a given energy, forward and backward scattering not only occur simultaneously but may contribute to the same scattering vector Δk. If the scattering phase of both processes differs by π and the amplitudes are almost equal, the oscillations cancel out. A sharp dip in the magnitude of the Fourier transform of the LDOS marks the crossover between forward and backward scattering channels.

Scanning noise microscopy

The paper describes a simple scheme enabling the real-time characterization of fluctuations, e.g., of the conductance in scanning tunneling microscopy. The technique can be used in parallel to other data acquisition, evaluating the rate, the amplitude, and the duty cycle of telegraphic noise in the tunneling current. This kind of scanning probe microscopy allows to evaluate the noise parameters as a function of the average tunneling current, the electron energy, and the lateral position. Images of the noise with Ångstrom spatial resolution are acquired simultaneously to the topographic information providing a direct correlation between the structural information and the noise. The method can be applied to a large variety of systems to monitor dynamics on the nanoscale, e.g., the localization of tunneling current induced switching within a single molecule. Noise spectroscopy may reveal the involved molecular orbitals, even if they cannot be resolved in standard scanning tunneling spectroscopy. As an example we present experimental data of the organic molecule copper phthalocyanine on a Cu(111) surface [J. Schaffert, M. C. Cottin, A. Sonntag, H. Karacuban, C. A. Bobisch, N. Lorente, J.-P. Gauyacq, and R. Möller, Nature Mater. 12, 223-227 (2013)].

Tunneling electron induced rotation of a copper phthalocyanine molecule on Cu(111)

J.S., A.S., C.A.B., and R.M. gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft through the SFB616 ‘Energy Dissipation at Surfaces.’ N.L. is supported by the ICT-FET Integrated Project AtMol (http://www.atmol.eu). M.C.C. thanks the Studienstiftung desdeutschen Volkes.

Communication: Substrate induced dehydrogenation: Transformation of octa-ethyl-porphyrin into tetra-benzo-porphyrin

Individual molecules of octa-ethyl-porhphyrin-iron(III)-chloride adsorbed on a Cu(111) surface are studied by scanning tunneling microscopy. Upon moderate heating the molecules are found to transform into Fe-tetra-benzo-porphyrin at a surprisingly low temperature of 380 K. If the annealing is interrupted, the different steps of the transformation can be imaged. By evaluating the ratio of transformed molecules as function of annealing temperature, an approximate activation energy of 1.2 eV ± 0.1 eV could be determined.

A chemically inert Rashba split interface electronic structure of C60, FeOEP and PTCDA on BiAg2/Ag(111) substrates

The fields of organic electronics and spintronics have the potential to revolutionize the electronics industry. Finding the right materials that can retain their electrical and spin properties when combined is a technological and fundamental challenge. We carry out the study of three archetypal organic molecules in intimate contact with the BiAg2 surface alloy. We show that the BiAg2 alloy is an especially suited substrate due to its inertness as support for molecular films, exhibiting an almost complete absence of substrate–molecular interactions. This is inferred from the persistence of a completely unaltered giant spin-orbit split surface state of the BiAg2 substrate, and from the absence of significant metallic screening of charged molecular levels in the organic layer. Spin-orbit split states in BiAg2 turn out to be far more robust to organic overlayers than previously thought.

Surface-Induced Dechlorination of FeOEPCl on Cu(111)†

To be or not to be chlorinated: When octaethylporphyrin iron(III) chloride (FeOEP-Cl) molecules are sublimated onto Cu(111) surfaces, two different molecular species are observed through scanning tunneling microscopy, showing either a protrusion or a depression at the center. In combination with van der Waals-corrected density functional calculations, our experiments reveal that one species corresponds to FeOEP-Cl molecules with the chlorine atom pointing away from the surface, whereas the other species has been dechlorinated.

Electronic Transport on the Nanoscale

A scanning tunneling microscope with several tips is ideally suited to analyze the electronic transport through objects on the nanoscale. Two different configurations will be discussed. The lateral transport of electrons may be studied by using two tips to drive a current parallel to the surface. A third tip enables to map the corresponding electrochemical potential μ ec. Measurements for a 2D conducting layer will be discussed. To analyze the transport perpendicular to the surface, a thin metallic layer is placed on a semiconducting surface. At the interface a Schottky barrier is formed, which can only be overcome by electrons of sufficient energy. This may be used to split the tunneling current coming from the tip of the microscope, into the ballistic electrons and the electrons which underwent inelastic scattering processes. This technique has been applied to study the ballistic transport of electrons through a thin epitaxial Bi(111) layer as well as through individual molecules.

STM-induced light emission enhanced by weakly coupled organic ad-layers

We analyze the light emission induced by the tunneling current flowing in a scanning tunneling microscopy experiment. In particular, we study the influence of organic ad-layers on the light emission on the initial monolayer of bismuth (Bi) on Cu(111) in comparison to the well-known case of organic ad-layers on Ag(111). On the Bi/Cu(111)-surface, we find that the scanning tunneling microscopy-induced light emission is considerably enhanced if an organic layer, e.g., the fullerene C60 or the perylene derivate perylene-tetracarboxylic-dianhydride, is introduced into the tip-sample junction. The enhancement can be correlated with a peculiarly weak interaction between the adsorbed molecules and the underlying Bi/Cu(111) substrate as compared to the Ag(111) substrate. This allows us to efficiently enhance and tune the coupling of the tunneling current to localized excitations of the tip-sample junction, which in turn couple to radiative decay channels.