Metals and organic adsorbates on GaAs(110) : a scanning tunneling microscopy, spectroscopy and luminescence study
Surface plasmon polaritons of the tip of a scanning tunneling microscope are used to induce fluorescence from tetraphenylporphine molecules on GaAs(110) surfaces at very low coverages. Light spectra exhibiting vibrationally resolved Q-band transitions reveal that the luminescence is of molecular origin. The emission is not directly induced by the tunneling current. Rather, radiationless energy transfer from plasmon modes of the tip leads to molecular excitation. The spin transfer efficiency from a remanently magnetized ferromagnet to p-type GaAs across a tunneling barrier is studied. A strong energy dependence of the circular polarization of the recombination luminescence and unexpectedly high maximum values exceeding 50 % are found. A model of a two-parts light generation process with a plasmon induced excitation dominating at low energies, and a superimposed electron injection luminescence becoming dominant at energies of ≈ 100 meV above the conduction band edge is proposed. The degree of circular polarization above 50 %, seemingly contradictory to the well-known selection rules, is tentatively explained in terms of a dopant induced broadening of the acceptor level and its merging with the valence band. Evidence is presented that individual phthalocyanine molecules on unpassivated GaAs(110) largely preserve their electronic characteristics. The tip of an STM can be used to manipulate the band structure of the substrate, leading to a reversible modification of the charge state of the molecule. The room temperature growth of the planar FePc and the non-planar AgPc on GaAs(110) is discussed. For both species, unidirectionally growing islands with very high long-range order are found, and their electronic as well as structural properties are examined. For AgPc, irreversible switching between different molecular appearances is observed and attributed to a transition between a non-planar and a planar conformation.