Too Hot for Photon-Assisted Transport: Hot-Electrons Dominate Conductance Enhancement in Illuminated Single-Molecule Junctions

Abstract

We investigate light-induced conductance enhancement in single-molecule junctions via photon-assisted transport and hot-electron transport. Using 4,4'-bipyridine bound to Au electrodes as a prototypical single-molecule junction, we report a 20-40% enhancement in conductance under illumination with 980 nm wavelength radiation. We probe the effects of subtle changes in the transmission function on light-enhanced current and show that discrete variations in the binding geometry result in a 10% change in enhancement. Importantly, we prove theoretically that the steady-state behavior of photon-assisted transport and hot-electron transport is identical but that hot-electron transport is the dominant mechanism for optically induced conductance enhancement in single-molecule junctions when the wavelength used is absorbed by the electrodes and the hot-electron relaxation time is long. We confirm this experimentally by performing polarization-dependent conductance measurements of illuminated 4,4'-bipyridine junctions. Finally, we perform lock-in type measurements of optical current and conclude that currents due to laser-induced thermal expansion mask optical currents. This work provides a robust experimental framework for studying mechanisms of light-enhanced transport in single-molecule junctions and offers tools for tuning the performance of organic optoelectronic devices by analyzing detailed transport properties of the molecules involved.

Experimental Route to Scanning Probe Hot‐Electron Nanoscopy (HENs) Applied to 2D Material

“…In specific, from I-V measurement we infer a local variation of the Schottky barrier that would be responsible for this current fluctuation. This local variation can be caused by several different physical effects as the dependence of the band structure on the number of layers, band mixing at the interface, atomic orientational variation of the terminated surface species, such as sulfur atoms, [60,61] or defect and dangling bonds charge pinning.…”

Functional molecular electronic devices through environmental control

“…In addition to the intrinsic molecular structure, external manipulation of the molecular orbitals proves to be a direct and effective strategy to control the electron transport behavior. By taking advantage of the sensitivity of molecular orbitals to external stimuli such as an electric field [19,20] and light [21][22][23], the functionalities of molecular electronic devices can be largely enriched. For instance, in 2009, Song et al [9] fabricated a threeterminal field-effect transistor (FET)-like molecular device.…”

Photoinduced anomalous Coulomb blockade and the role of triplet states in electron transport through an irradiated molecular transistor. II. Effects of electron-phonon coupling and vibrational relaxation

“…Chemically, the high-and low-conductance states based on different molecular conformations have been able to be switched by a laser field at specific wavelengths 28,[30][31][32] . Physically, photoelectric current can be generated via the mechanisms of photon-assisted tunneling 18,29,[33][34][35][36][37] or electronic excitation 20,[38][39][40][41][42][43][44] in a molecular junction. From the theoretical aspects, the Floquet-based methods have successfully demonstrated several fascinating phenomena, including photon-assisted tunneling [45][46][47][48][49][50][51][52] , coherent destruction of tunneling 53,54 , coherent revival of tunneling 55 , and quantum ratchet effect 56 .…”