Theoretical studies of light emission from molecular junctions
Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
Final Report Abstract
In summary, based on the nonequilibrium Keldysh Green’s functions technique and with the help of density functional theory we studied the inelastic effects on current noise in systems of multiple electronic levels and phonon modes. As compared to the SLHM that predicts values of G ≈ 0.15G0 and G ≈ 0.85G0, Sign crossover thresholds for inelastic noise are observed at conductance of G ≈ 0.2G0 and G ≈ 0.90G0-0.95G0 for Au-BDT-Au single-molecule and pure Au single-atom contacts, respectively. This increase can be understood by the presence of several partially open transmission eigenchannels that contribute to the total transmission in addition to a dominant one and couple differently to various vibrations. We have also reported shot noise measurements for Au contacts, using the mechanically controllable breakjunction technique and applying a custom-made, versatile setup with simplified measurement electronics. The measurements show nonlinearities in the shot noise power for bias voltages around corresponding characteristic vibrational mode energies. The observed crossover from positive to negative for a relative Fano factor δF/F1 occurs in a range between 0.93G0 and 0.97G0. Our results confirm the previous experimental results by Kumar et al. who observed the crossover at 0.95 G0. We conclude that the deviation between the theoretically predicted crossover at 0.86 for a single channel and the experimental observation for Au contacts may be explained by the occurrence of multichannel contacts in the experiment. Multiple transmissive eigenchannels also provide a natural explanation for why the transition is not sharp. We further discuss the influence of the nonequilibrium phonon fluctuations and the effects of phonon heating on the current-noise properties. A quadratic increase in the noise as a function of voltage at large bias is observed theoretically for Au-benzenedithiol-Au junctions. The inelastic sign crossover in the noise at low conductance values could not be measured yet and thus remains to be verified. The challenging experiments employing high bias voltages are expected to reveal important insights into charge transport through nanosystems beyond elastic theories. The theoretical model can be extended to investigate shot noise for various frequency ranges and to clarify the role of the molecule in the emission rate.
Publications
- Inelastic electron tunneling spectroscopy of difurylethene-based photochromic single-molecule junctions. Beilstein Journal of Nanotechnology 8, 2606-2614 (2017)
Youngsang Kim, Safa G Bahoosh, Dmytro Sysoiev, Thomas Huhn, Fabian Pauly und Elke Scheer
(See online at https://doi.org/10.3762/bjnano.8.261) - Influence of electron-vibration interactions on electronic current noise of atomic and molecular junctions
S. G. Bahoosh, M. A. Karimi, W. Belzig, E. Scheer, and F. Pauly
(See online at https://doi.org/10.48550/arXiv.1912.13496)
