Abstract
Predicting the electronic framework of an organic molecule under practical conditions is essential if the molecules are to be wired in a realistic circuit. This demands a clear description of the molecular energy levels and dynamics as it adapts to the feedback from its evolving chemical environment and the surface topology. Here, we address this issue by monitoring in real-time the structural stability and intrinsic molecular resonance states of fullerene (C60)-based hybrid molecules in the presence of the solvent. Energetic levels of C60 hybrids are resolved by in situ scanning tunnelling spectroscopy with an energy resolution in the order of 0.1 eV at room-temperature. An ultra-thin organic spacer layer serves to limit contact metal-molecule energy overlap. The measured molecular conductance gap spread is statistically benchmarked against first principles electronic structure calculations and used to quantify the diversity in electronic species within a standard population of molecules. These findings provide important progress towards understanding conduction mechanisms at a single-molecular level and in serving as useful guidelines for rational design of robust nanoscale devices based on functional organic molecules.
Highlights
Predicting the electronic framework of an organic molecule under practical conditions is essential if the molecules are to be wired in a realistic circuit. This demands a clear description of the molecular energy levels and dynamics as it adapts to the feedback from its evolving chemical environment and the surface topology
The calculated conductance gaps for single molecules are (0.8 ± 0.3) eV are in good agreement with the structure-and time-averaged STS measured conductance gap values of (0.8 ± 0.2) eV
We derive a mean conductance gap value (0.8 ± 0.2) eV for the individual molecules which is lower than the mean conductance gap of (1.1 ± 0.1) eV for the regular C60 dimers, and in agreement with the density functional theory (DFT) computed highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps
Summary
Peter Nirmalraj[1], Andrea La Rosa[2], Damien Thompson[3,4], Marilyne Sousa[1], Nazario Martin[2], Bernd Gotsmann1 & Heike Riel[1]. Predicting the electronic framework of an organic molecule under practical conditions is essential if the molecules are to be wired in a realistic circuit This demands a clear description of the molecular energy levels and dynamics as it adapts to the feedback from its evolving chemical environment and the surface topology. The structural stability of the contact metal[10], local chemical potential of the molecular environment[11,12], hydration effects[13], trapped charges at the metal-organic interface[6,14], temperature[15], intermolecular interactions[16] and chemical functionality[17] are other factors that can contribute to the spread in values of molecular quantum conductance (G0) and tunneling attenuation factor (β ) values These arguments are valid in the case of relatively simple and short-length molecules[18] wired between metal electrodes. The experimental and theoretical studies on C60 dimers (C60—linker—bridge—linker—C60) report on structurally stable molecules and do not take into account the possibility of mixed electronic species, which has limited the interpretation of measured conductance values and charge propagation modes
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