Infoscience

Thesis

Plasmon enhanced luminescence from fullerene molecules excited by local electron tunneling

This thesis reports on the optical properties of fullerene molecules forming nanoscale crystals grown on ultrathin NaCl films on Au(111) investigated by Scanning Tunneling Microscope (STM)-induced light emission spectroscopy. In this regard, an existing optical setup has been improved. The new instrument facilitates the alignment of the detection lens with the tunnel junction. STM images of the investigated samples show that, upon adsorption on a NaCl covered Au(111) surface, C60 and C70 crystallize into multilayer islands with a hexagonally arranged close-packed top layer in which electron-poor regions on one molecule face electron-rich regions on the adjacent molecule. Luminescence spectra from these nanocrystals induced by electrons tunneling through a double barrier STM junction are presented. Their bias dependence as well as their similarity with laser-induced photoluminescence spectra indicate a hot electron injection mechanism followed by a radiative decay associated with the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO–LUMO) gap of the molecules. Local fluorescence and phosphorescence of C70 molecules are identified, leading to unambiguous chemical recognition on the nanoscale. Moreover, the molecular luminescence is demonstrated to be selectively enhanced by localized surface plasmons in the STM tip-sample gap, the tunnel junction acting as an optical antenna. Finally, the results of a scanning tunneling microscopy and scanning tunneling spectroscopy investigations of self-assembled islands of Azure A molecules on a Au(111) substrate are presented. Although no final conclusion can be drawn on the molecular arrangement owing to the impossibility to recognize specific parts of the molecule, the presence of these structures is found to modify the Au(111) herringbone reconstruction.

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