-
[show abstract]
[hide abstract]
ABSTRACT: We present evidence for a partly chemisorptive bonding between single monolayers of copper-II-phthalocyanine (CuPc) and 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) that are stacked on Ag(111). A commensurate registry between the two molecular layers and the substrate, i.e., a common crystallographic lattice for CuPc and PTCDA films as well as for the Ag(111) surface, indicates that the growth of the upper layer is dominated by the structure of the lower. Photoemission spectroscopy clearly reveals a gradual filling of the lowest unoccupied molecular orbital of PTCDA due to CuPc adsorption, which proves the chemisorptive character.
Physical Review Letters 03/2012; 108(10):106103. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The understanding and control of epitaxial growth of organic thin films is of crucial importance in order to optimize the performance of future electronic devices. In particular, the start of the submonolayer growth plays an important role since it often determines the structure of the first layer and subsequently of the entire molecular film. We have investigated the structure formation of 3,4,9,10-perylene-tetracarboxylic dianhydride and copper-phthalocyanine molecules on Au(111) using pair-potential calculations based on van der Waals and electrostatic intermolecular interactions. The results are compared with the fundamental lateral structures known from experiment and an excellent agreement was found for these weakly interacting systems. Furthermore, the calculations are even suitable for chemisorptive adsorption as demonstrated for copper-phthalocyanine/Cu(111), if the influence of charge transfer between substrate and molecules is known and the corresponding charge redistribution in the molecules can be estimated. The calculations are of general applicability for molecular adsorbate systems which are dominated by electrostatic and van der Waals interaction.
The Journal of chemical physics 12/2011; 135(23):234703. · 3.09 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Individual Xe atoms as well as single CO and CH(4) molecules adsorbed at the tip apex of a scanning tunneling microscope (STM) function as microscopic force sensors that change the tunneling current in response to the forces acting from the surface. An STM equipped with any of these sensors is able to image the short-range Pauli repulsion and thus resolve the inner structure of large organic adsorbate molecules. Differences in the performance of the three studied sensors suggest that the sensor functionality can be tailored by tuning the interaction between the sensor particle and the STM tip.
Journal of the American Chemical Society 10/2011; 133(42):16847-51. · 9.91 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Die Erfindung des Rastertunnelmikroskops im Jahre 1981 hat die Oberflächenforschung revolutioniert und darüber hinaus einen wichtigen Beitrag zur Entstehung der Nanotechnologie geleistet. Erstmals war es möglich, atomare Strukturen direkt sichtbar zu machen. Durch eine einfache Modifikation des Tunnelkontakts kann die Auflösung eines Rastertunnelmikroskops noch dramatisch verbessert werden, wie unsere Gruppe im Forschungszentrum Jülich kürzlich zeigen konnte.
Physik in unserer Zeit 10/2010; 41(6):266 - 267.
-
Thomas Brumme,
Olga Neucheva,
Cormac Toher,
Rafael Gutiérrez,
Christian Weiss, Ruslan Temirov,
Andreas Greuling,
Marcin Kaczmarski,
Michael Rohlfing,
Stefan Tautz,
Gianaurelio Cuniberti
[show abstract]
[hide abstract]
ABSTRACT: The dynamics of a molecular junction consisting of a PTCDA molecule between
the tip of a scanning tunneling microscope and a Ag(111) surface have been
investigated experimentally and theoretically. Repeated switching of a PTCDA
molecule between two conductance states is studied by low-temperature scanning
tunneling microscopy for the first time, and is found to be dependent on the
tip-substrate distance and the applied bias. Using a minimal model Hamiltonian
approach combined with density-functional calculations, the switching is shown
to be related to the scattering of electrons tunneling through the junction,
which progressively excite the relevant chemical bond. Depending on the
direction in which the molecule switches, different molecular orbitals are
shown to dominate the transport and thus the vibrational heating process. This
in turn can dramatically affect the switching rate, leading to non-monotonic
behavior with respect to bias under certain conditions. In this work, rather
than simply assuming a constant density of states as in previous works, it was
modeled by Lorentzians. This allows for the successful description of this
non-monotonic behavior of the switching rate, thus demonstrating the importance
of modeling the density of states realistically.
09/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: Local, noncovalent intermolecular interactions in organic monolayers have been directly imaged using scanning tunneling hydrogen microscopy (STHM). Unprecedented spatial resolution directly reveals the relation between the intermolecular interactions, the molecular chemical structure, and the ordering in the film.
Journal of the American Chemical Society 09/2010; 132(34):11864-5. · 9.91 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Using a scanning tunneling microscope we have measured the quantum conductance through a PTCDA molecule for different configurations of the tip-molecule-surface junction. A peculiar conductance resonance arises at the Fermi level for certain tip to surface distances. We have relaxed the molecular junction coordinates and calculated transport by means of the Landauer/Keldysh approach. The zero bias transmission calculated for fixed tip positions in lateral dimensions but different tip substrate distances show a clear shift and sharpening of the molecular chemisorption level on increasing the STM-surface distance, in agreement with experiment.
09/2008;
-
[show abstract]
[hide abstract]
ABSTRACT: We present density-functional calculations of a monolayer of 3,4,9,10-perylene-tetracarboxylic-dianhydride adsorbed on the Ag(111) surface, yielding detailed insight into the structural and electronic properties of this prototypical adsorption system. Using the local-density approximation as the best choice for the exchange-correlation functional, we discuss the bond lengths inside the molecules, the distortion of the molecules due to adsorption, and their position and orientation relative to the substrate and to each other. Based on the calculated geometric and electronic structures, we calculate scanning tunneling microscopy and spectroscopy data within the Tersoff-Hamann framework [ Phys. Rev. B. 31 805 (1985)]. To this end, two-dimensional Fourier transform methods and spatial extrapolation techniques are employed to evaluate the sample wave functions at the tip position. We obtain constant-current images and spectral data that reveal detailed information about the electronic structure of the system. In addition, we have measured the same data by low-temperature scanning tunneling microscopy and scanning tunneling spectroscopy. Our measured and calculated data are in good agreement with one another.
Phys. Rev. B. 09/2007; 76(11).
-
[show abstract]
[hide abstract]
ABSTRACT: The effect of molecular flexibility on the surface ordering of complex organic adsorbates is explored, using alpha,omega-dihexylquaterthiophene (DH4T) and mixed DH4T|tetracene phases on Ag(111) as model systems. The structure of DH4T/Ag(111) interfaces is determined by the flexibility of the hexyl chains at either end of the quaterthiophene backbone: Above 273 K, DH4T forms a nematic liquid crystalline phase with a director close to the [112] direction of the silver substrate. At 273 K, a reversible phase transition to a long-range ordered, point-on-line coincident phase is observed. However, this ordered state is still affected substantially by the flexible nature of DH4T, which materializes in a large number of local structural defects. If traces of DH4T are coevaporated with tetracene, inclusions of a 1:1 stoichiometric DH4T|tetracene phase are found in a tetracene/Ag(111) matrix (alpha-phase). In this mixed phase, the two surface enantiomers of pro-chiral DH4T on one hand and tetracene on the other form a complex stripe structure. The mixed phase shows a higher degree of order than present at the pure DH4T/Ag(111) interface, which also lacks chiral organization. The addition of tetracene molecules as structural templates stabilizes certain conformations of DH4T and thus, by balancing its structural flexibility, allows the surface-induced chirality of DH4T to become a decisive factor in determining the structure of the mixed phase.
Langmuir 12/2006; 22(23):9572-9. · 4.19 Impact Factor
