Rastertunnelspektroskopische Untersuchungen von Terthiophen auf Au(111)

In the present work the adsorption behavior of (2,2")-(5",2")-terthiophene (3T) on Au(111) was analyzed. The gold surface was prepared by ex situ flame annealing and in situ treatment with oxygen plasma and annealing of commercially available polycrystalline gold films. The surface was investigated by means of low temperature scanning tunneling microscopy and scanning tunneling spectroscopy (STS). The spectroscopic measurements focused on z-V spectroscopy and inelastic electron tunneling spectroscopy (IETS) to analyze the electronic and vibronic structure of 3T, respectively. By STS the energy positions and the geometry of the HOMO-1, HOMO and the LUMO were analyzed. The HOMO-1 was found at (2.9 ± 0.15) eV, the HOMO at (1.25 ± 0.15) eV, and the LUMO at ( + 2.35 ± 0.10) eV. Thus, the result is a HOMO-LUMO gap of 3.6 eV, which is in agreement with DFT calculations and UV/vis measurements. The orientation of the 3T cannot be derived from topographic measurements, but from an asymmetry in dI/dV maps of the molecule. Using the differential tunneling barrier height, the imaging mechanism and the apparent molecular height within the HOMO-LUMO gap could be clarified. IETS measurements of isolated 3T molecules were considered at three different adsorption sites of the herringbone reconstructed Au(111) surface: the fcc area, the hcp area and the elbow site. In the first two sites the experimental uncertainty of the vibrational energies could be determined to be ±1 meV. The vibrational energies were compared to calculations of 3T in gas phase. The calculations included anharmonic contributions to the vibrational energies. All three measurements show a high level of accordance with the theory