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Submolecular Resolution Spectroscopic Imaging for Photoactive Molecules and Assemblies

  • Author(s): Wang, Shenkai
  • Advisor(s): Weiss, Paul S.
  • et al.

Characterizing photon absorption and excitation at the nanoscale is an important tool for optimizing and applying photoactive systems. Coupling laser irradiation and scanning tunneling microscopy (STM) enables the measurement of photo generated carriers with submolecular precision. Here, we introduce a custom built laser assisted STM that employs evanescent illumination and functions at ambient conditions and room temperature. Laser beams (405 nm, 633 nm, and 780 nm) are frequency modulated by a mechanical chopper wheel and incorporated into the tunneling junction through total internal reflection. Photo induced changes in tunneling current are detected phase sensitively by a lock in amplifier and in phase signal that is related to local photoelectron density is recorded at every pixel during the raster scans to generate maps of photo induced carriers. Non absorbing n dodecanethiolate (C12) self assembled monolayers (SAMs) are used to test thermal effects and as negative controls for photoactive molecules.

We measured photo-induced charge separation in isolated individual C60 tethered 2,5 dithienylpyrrole triad (C60 triad) molecules with submolecular resolution under 405 nm laser illumination. Photo-induced charge separation was not detected for all C60 triad molecules, indicating that the conformations of the molecules may affect their excitation probability, lifetime, and/or charge distribution. Photo induced signal was not observed for dodecanethiol molecules in the surrounding matrix nor for control molecules without C60 moieties, as neither absorbs incident photons at these energies. We also studied solution deposited titanyl phthalocyanine (TiOPc) SAMs on Au{111} using the laser assisted STM. We observed hexagonal and rectangular lattice structures in TiOPc SAMs that are different from previously reported self assembled structures in ultra high vacuum and more related to the crystal structures of TiOPc. The photo responses of TiOPc SAMs were characterized with 633 nm and 780 nm laser illumination. The distributions of photoelectrons in hexagonal lattice matches with theoretical calculated charge density changes in TiOPc molecules upon excitation. However, the photo responses of TiOPc molecules in rectangular lattices are different from those expected; TiOPc molecules in this arrangement may have low probabilities of activation by 633 nm and 780 nm light. Our results suggest that the photo carrier generation efficiency of TiOPc molecules is related to their packing arrangement in SAMs and local environment.

In a study aiming to fabricate molecular level mixed p n junctions in SAMs, we explored the reactivity of 1 decaboranethiolate SAMs and discovered that the reaction between 1 decaboranethiolate and 4 phenyl 1 butyne (4p1b) results in highly ordered 4p1b SAMs. The initially disordered 1 decaboranethiolate changed into ordered (√3�√3)R30� lattices on Au{111} typical of alkyne SAMs, indicating the complete substitution of 1-decaboranethiolate moieties, as determined by nanoscale imaging with scanning tunneling microscopy and X-ray photoelectron spectroscopy. Vibrational spectroscopy results indicate that the process happens gradually and that alkynyl groups are not totally oxidized in the ordered 4 phenyl-1-butyne monolayer.

Our spectroscopic imaging method can elucidate the change of electronic structures of photoactive molecules and assemblies upon activation and spatially resolve it. By analyzing the photo induced charge distributions in individual molecules and SAMs, we can elucidate the effects of molecular structure, intermolecular interactions, and local environment on the probabilities of photoactivation and energy conversion. Such information can guide the design of molecules and nanostructures for photovoltaics and the modification of processing route for devices for higher performance.

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