Liang-Yan Hsu

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 (βn in Rmeasured = R(n=0)e(βnN), where N 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 (n = 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.

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 (~10(5)) 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.

Energy‐Level Alignment for Single‐Molecule Conductance of Extended Metal‐Atom Chains

The use of single-molecule junctions for various functions constitutes a central goal of molecular electronics. The functional features and the efficiency of electron transport are dictated by the degree of energy-level alignment (ELA), that is, the offset potential between the electrode Fermi level and the frontier molecular orbitals. Examples manifesting ELA are rare owing to experimental challenges and the large energy barriers of typical model compounds. In this work, single-molecule junctions of organometallic compounds with five metal centers joined in a collinear fashion were analyzed. The single-molecule i-V scans could be conducted in a reliable manner, and the EFMO levels were electrochemically accessible. When the electrode Fermi level (EF ) is close to the frontier orbitals (EFMO ) of the bridging molecule, larger conductance was observed. The smaller |EF -EFMO | gap was also derived quantitatively, unambiguously confirming the ELA. The mechanism is described in terms of a two-level model involving co-tunneling and sequential tunneling processes.

Molecular Series-Tunneling Junctions

Charge transport through junctions consisting of insulating molecular units is a quantum phenomenon that cannot be described adequately by classical circuit laws. This paper explores tunneling current densities in self-assembled monolayer (SAM)-based junctions with the structure Ag(TS)/O2C-R1-R2-H//Ga2O3/EGaIn, where Ag(TS) is template-stripped silver and EGaIn is the eutectic alloy of gallium and indium; R1 and R2 refer to two classes of insulating molecular units-(CH2)n and (C6H4)m-that are connected in series and have different tunneling decay constants in the Simmons equation. These junctions can be analyzed as a form of series-tunneling junctions based on the observation that permuting the order of R1 and R2 in the junction does not alter the overall rate of charge transport. By using the Ag/O2C interface, this system decouples the highest occupied molecular orbital (HOMO, which is localized on the carboxylate group) from strong interactions with the R1 and R2 units. The differences in rates of tunneling are thus determined by the electronic structure of the groups R1 and R2; these differences are not influenced by the order of R1 and R2 in the SAM. In an electrical potential model that rationalizes this observation, R1 and R2 contribute independently to the height of the barrier. This model explicitly assumes that contributions to rates of tunneling from the Ag(TS)/O2C and H//Ga2O3 interfaces are constant across the series examined. The current density of these series-tunneling junctions can be described by J(V) = J0(V) exp(-β1d1 - β2d2), where J(V) is the current density (A/cm(2)) at applied voltage V and βi and di are the parameters describing the attenuation of the tunneling current through a rectangular tunneling barrier, with width d and a height related to the attenuation factor β.

Light-driven electron transport through a molecular junction based on cross- conjugated systems

This work explores light-driven electron transport through cross-conjugated molecules with different numbers of alkenyl groups. In the framework of coherent quantum transport, the analysis uses single-particle Greens functions together with non-Hermitian Floquet theory. With realistic parameters stemming from spectroscopy, the simulations show that measurable current (∼10(-11) A) caused by photon-assisted tunneling should be observed in a weak driving field (∼2 × 10(5) V/cm). Current-field intensity characteristics give one-photon and two-photon field amplitude power laws. The gap between the molecular orbital and the Fermi level of the electrodes is revealed by current-field frequency characteristics. Due to generalized parity symmetry, the cross-conjugated molecules with odd and even numbers of alkenyl groups exhibit completely different current-polarization characteristics, which may provide an advantageous feature in nanoelectronic applications.

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.

A new time-frequency method to reveal quantum dynamics of atomic hydrogen in intense laser pulses: Synchrosqueezing transform

This study introduces a new adaptive time-frequency (TF) analysis technique, the synchrosqueezing transform (SST), to explore the dynamics of a laser-driven hydrogen atom at an ab initio level, upon which we have demonstrated its versatility as a new viable venue for further exploring quantum dynamics. For a signal composed of oscillatory components which can be characterized by instantaneous frequency, the SST enables rendering the decomposed signal based on the phase information inherited in the linear TF representation with mathematical support. Compared with the classical type of TF methods, the SST clearly depicts several intrinsic quantum dynamical processes such as selection rules, AC Stark effects, and high harmonic generation.

Transport through a mixed-valence molecular transistor in the sequential-tunneling regime: Theoretical insight from the two-site Peierls–Hubbard model

Transport through a mixed-valence system in the sequential-tunneling region is investigated using the master equation method and a simple two-site Peierls-Hubbard model that includes electron-phonon (e-p) coupling, electron hopping, and electron-electron (e-e) repulsion. The characteristics of Coulomb diamonds in the conductance spectra under three regimes are discussed. In the regime of zero e-p coupling, we found that the widths of Coulomb diamonds are dominated by the competition of electron-hopping and Coulomb repulsion. In the regime of weak and intermediate e-p coupling, by virtue of the normal-mode transformation we found that coupling to the symmetric-mode decreases the widths of Coulomb diamonds. In the regime of strong e-p coupling, an analytical expression for the widths of Coulomb diamonds can be derived using the small polaron transformation. The derived formula provides a new way to estimate e-e interactions and e-p couplings experimentally.

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) J̃ (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 WET. Indeed, WET = (8.785 × 10-25 mol) ϕDτD-1J̃, where ϕD is donor fluorescence quantum yield and τD 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.

Plasmon-coupled resonance energy transfer: A real-time electrodynamics approach

This paper presents a new real-time electrodynamics approach for determining the rate of resonance energy transfer (RET) between two molecules in the presence of plasmonic or other nanostructures (inhomogeneous absorbing and dispersive media). In this approach to plasmon-coupled resonance energy transfer (PC-RET), we develop a classical electrodynamics expression for the energy transfer matrix element which is evaluated using the finite-difference time-domain (FDTD) method to solve Maxwells equations for the electric field generated by the molecular donor and evaluated at the position of the molecular acceptor. We demonstrate that this approach yields RET rates in homogeneous media that are in precise agreement with analytical theory based on quantum electrodynamics (QED). In the presence of gold nanoparticles, our theory shows that the long-range decay of the RET rates can be significantly modified by plasmon excitation, with rates increased by as much as a factor of 106 leading to energy transfer rates over hundreds of nm that are comparable to that over tens of nm in the absence of the nanoparticles. These promising results suggest important future applications of the PC-RET in areas involving light harvesting or sensing, where energy transfer processes involving inhomogeneous absorbing and dispersive media are commonplace.