Quantum Tunneling Induced Optical Rectification and Plasmon-Enhanced Photocurrent in Nanocavity Molecular Junctions

Abstract

Molecular junctions offer the opportunity for downscaling optoelectronic devices. Separating two electrodes with a single layer of molecules accesses the quantum-tunneling regime at low voltages (<1 V), where tunneling currents become highly sensitive to local nanometer-scale geometric features of the electrodes. These features generate asymmetries in the electrical response of the junction which combine with the incident oscillating optical fields to produce optical rectification and photocurrents. Maximizing photocurrents requires accurate control of the overall junction geometry and a large confined optical field in the optimal location. Plasmonic nanostructures such as metallic nanoparticles are prime candidates for this application, because their size and shape dictate a consistent junction geometry while strongly enhancing the optical field from incident light. Here we demonstrate a robust lithography-free molecular optoelectronic device geometry, where a metallic nanoparticle on a self-assembled molecular monolayer is sandwiched between planar bottom and semitransparent top electrodes, to create molecular junctions with reproducible morphology and electrical response. The well-defined geometry enables predictable and intense plasmonic localization, which we show creates optical-frequency voltages ∼ 30 mV in the molecular junction from 100 μW incident light, generating photocurrent by optical rectification (>10 μA/W) from only a few hundred molecules. Quantitative agreement is thus obtained between DC-and optical-frequency quantum-tunneling currents, predicted by a simple analytic equation. By measuring the degree of junction asymmetry for different molecular monolayers, we find that molecules with a large DC rectification ratio also boost zero-bias electrical asymmetry, making them good candidates for sensing and energy harvesting applications in combination with plasmonic nanomaterials.

Photo‐Responsive Molecular Junctions Activated by Perovskite/Graphene Heterostructure Electrode

“…However, this current enhancement of BPT molecular junctions is somewhat smaller than the previous results using photo-stimulated molecules such as diarylethene or azobenzene which have shown increases of about an order of magnitude. [30,[33][34][35] ] In such context, for further improvement in current enhancement at molecular junctions, we chose to vary the molecular dipole moment. Effect of dipoles on the transport characteristics had often considered to be elusive, due to depolarizations by disorders or interactions that occur among the molecules in SAM layer.…”

“…Conventionally, optical control of the charge transport in a molecular junction was implemented by employing intrinsically photo-active molecules whose molecular conformation changes upon photo-excitation. [30][31][32] However, SAM-based molecular junctions with photo-switching molecules suffer from either long switching times or irreversible switching properties. [33][34][35] Therefore, rather than relying on the intrinsically photoactive molecules, functionalizing other device components (e.g., electrodes) can be a suitable engineering option to explore the photo-modulated conduction properties in molecular junctions.…”

1,4-Benzenedithiol-Bridged Nanogap-Based Individual Particle Surface-Enhanced Raman Spectroscopy Mechanical Probe for Revealing the Endocytic Force

“…External forces can destroy or connect the dithiol molecules between the nanogaps, thus selectively changing the vibration mode. This principle can be explained by the chemical enhancement mechanism of SERS, in which metal-molecule charge transfer occurs. , Moreover, density functional theory (DFT) calculations, based on a simple adsorption model consisting of a ligand bound to the metal cluster, have been proven to be an effective tool to support the charge transfer between adsorbed molecules and the metal surface at the molecular level. , Such selective enhancement features make mechanical signals transform into spectral signals, which provides a promising pathway for investigating changes in mechanical forces.…”

A planar plasmonic nano-gap and its array for enhancing light-matter interactions at the nanoscale

“…In the past few decades, great efforts have been made to investigate GSP in different types of metallic structures, including flat metal films separated by a dielectric layer, [2][3][4] coupled metal nanoparticles, 5 metal nanoparticles on flat metal film structures (MNOF), [6][7][8] and their derivatives. 9,10 Various novel plasmonic phenomena, such as nonlocality, 11,12 huge electromagnetic enhancements, 13 ultra-strong coupling, 14,15 quantum tunneling, 16,17 and charge-transfer plasmons, 18 have been revealed, leading to a few interesting applications like surface-enhanced Raman scattering (SERS) with sensitivity down to the single-molecule level, 19,20 photothermal cancer therapy, [21][22][23] super-continuum generation, 24,25 and plasmon-enhanced electron emission. [26][27][28][29][30] In comparison with GSP modes from free-standing coupled metallic nanoparticles or MNOF, planar coupled nanostructures supported on a flat substrate are more favorable for device applications for the following reasons: (i) planar architectures can be facilely integrated with other functional components; (ii) the gap formed by planar structures is more easily accessed.…”

“…In the past few decades, great efforts have been made to investigate GSP in different types of metallic structures, including flat metal films separated by a dielectric layer, 2–4 coupled metal nanoparticles, 5 metal nanoparticles on flat metal film structures (MNOF), 6–8 and their derivatives. 9,10 Various novel plasmonic phenomena, such as non-locality, 11,12 huge electromagnetic enhancements, 13 ultra-strong coupling, 14,15 quantum tunneling, 16,17 and charge-transfer plasmons, 18 have been revealed, leading to a few interesting applications like surface-enhanced Raman scattering (SERS) with sensitivity down to the single-molecule level, 19,20 photothermal cancer therapy, 21–23 super-continuum generation, 24,25 and plasmon-enhanced electron emission. 26–30…”