Gate Control of the Conduction Mechanism Transition from Tunneling to Thermally Activated Hopping

We explore gate control of electron transport through molecules with different repeat units. In the framework of reduced density matrix theory, the computational results show (i) exponential decay in the tunneling regime and (ii) Arrhenius behavior and similar activation energies in the hopping regime, which are qualitatively consistent with experimental observations. Moreover, the gate enables tuning of the activation energy, indicating that the continuous transition from tunneling to hopping could be experimentally observed. The activation energy–gate voltage characteristics are introduced to investigate different conduction regimes

Single-Molecule Electric Revolving Door

This work proposes a new type of molecular machine, the single-molecule electric revolving door, which utilizes conductance dependence upon molecular conformation as well as destructive quantum interference. We perform electron transport simulations in the zero-bias limit using the Landauer formalism together with density functional theory. The simulations show that the open- and closed-door states, accompanied by significant conductance variation, can be operated by an external electric field. The large on–off conductance ratio (∼10⁵) implies that the molecular machine can also serve as an effective switching device. The simultaneous control and detection of the door states can function at the nanosecond scale, thereby offering a new capability for molecular-scale devices

Plasmon-Coupled Resonance Energy Transfer

In this study, we overview resonance energy transfer between molecules in the presence of plasmonic structures and derive an explicit Förster-type expression for the rate of plasmon-coupled resonance energy transfer (PC-RET). The proposed theory is general for energy transfer in the presence of materials with any space-dependent, frequency-dependent, or complex dielectric functions. Furthermore, the theory allows us to develop the concept of a generalized spectral overlap (GSO) <i>J̃</i> (the integral of the molecular absorption coefficient, normalized emission spectrum, and the plasmon coupling factor) for understanding the wavelength dependence of PC-RET and to estimate the rate of PC-RET <i>W</i>_ET. Indeed, <i>W</i>_ET = (8.785 × 10^–25 mol) ϕ_Dτ_D^–1<i>J̃</i>, where ϕ_D is donor fluorescence quantum yield and τ_D is the emission lifetime. Simulations of the GSO for PC-RET show that the most important spectral region for PC-RET is not necessarily near the maximum overlap of donor emission and acceptor absorption. Instead a significant plasmonic contribution can involve a different spectral region from the extinction maximum of the plasmonic structure. This study opens a promising direction for exploring exciton transport in plasmonic nanostructures, with possible applications in spectroscopy, photonics, biosensing, and energy devices

DataSheet1_Tavis-Cummings model revisited: A perspective from macroscopic quantum electrodynamics.PDF

The Tavis-Cummings (TC) model has been widely used to investigate the collective coupling effect in hybrid light-matter systems; however, the TC model neglects the effect of a dielectric environment (the spectral structure of photonic bath), and it remains unclear whether the TC model can adequately depict the light-matter interaction in a non-homogeneous, dispersive, and absorbing medium. To clarify the ambiguity, in this work, we first connect the macroscopic quantum electrodynamics and the TC model with dissipation. Based on the relationship between these two theoretical frameworks, we develop a guideline that allows us to examine the applicability of the TC model with dissipation. The guideline states that if 1) the generalized spectral densities are independent of the positions of molecules and 2) the generalized spectral densities resemble a Lorentzian function, then the hybrid light-matter system can be properly described by the TC model with dissipation. In order to demonstrate how to use the guideline, we examine the position dependence and the lineshape of the generalized spectral densities in three representative systems, including a silver Fabry-Pérot cavity, a silver surface, and an aluminum spherical cavity. We find that only the aluminum spherical cavity meets the two conditions, i.e., position independence and Lorentzian lineshape, required for the utilization of the dissipative TC model. Our results indicate that the use of the TC model with dissipation to study the collective coupling effect should be done with care, providing an important perspective on resonance energy transfer and polariton chemistry.</p

Photoinduced Anomalous Coulomb Blockade and the Role of Triplet States in Electron Transport through an Irradiated Molecular Transistor

In this study, we explore photoinduced electron transport through a molecule weakly coupled to two electrodes by combining first-principles quantum chemistry calculations with a Pauli master equation approach that accounts for many-electron states. In the incoherent limit, we demonstrate that energy-level alignment of triplet and charged states plays a crucial role, even when the rate of intersystem crossing is much smaller than the rate of fluorescence. Furthermore, the field intensity dependence and an upper bound to the photoinduced electric current can be analytically derived in our model. Under an optical field, the conductance spectra (charge stability diagrams) exhibit unusual Coulomb diamonds, which are associated with molecular excited states, and their widths can be expressed in terms of energies of the molecular electronic states. This study offers new directions for exploring optoelectronic response in nanoelectronics

Conductance of Tailored Molecular Segments: A Rudimentary Assessment by Landauer Formulation

One of the strengths of molecular electronics is the synthetic ability of tuning the electric properties by the derivatization and reshaping of the functional moieties. However, after the quantitative measurements of single-molecule resistance became available, it was soon apparent that the assumption of negligible influence of the headgroup–electrode contact on the molecular resistance was oversimplified. Due to the measurement scheme of the metal–-molecule–metal configuration, the contact resistance is always involved in the reported values. Consequently the electrical behavior of the tailored molecular moiety can only be conceptually inferred by the tunneling decay constant (β_<i>n</i> in <i>R</i>_measured = <i>R</i>_<i>n</i>=0<i>e</i>^β<i>nN</i>, where <i>N</i> is the number of repeated units), available only for compounds with a homologous series. This limitation hampers the exploration of novel structures for molecular devices. Based on the Landauer formula, we propose that the single-molecule resistance of the molecular backbones can be extracted. This simplified evaluation scheme is cross-examined by electrode materials of Au, Pd, and Pt and by anchoring groups of thiol (−SH), nitrile (−CN), and isothiocyanate (−NCS). The resistance values of molecular backbones for polymethylenes (<i>n</i> = 4, 6, 8, and 10) and phenyl (−C₆H₄−) moieties are found independent of the anchoring groups and electrode materials. The finding justifies the proposed approach that the resistance of functional moieties can be quantitatively evaluated from the measured values even for compounds without repeated units