Tunable Symmetry-Breaking-Induced Dual Functions in Stable and Photoswitched Single-Molecule Junctions

The aim of molecular electronics is to miniaturize active electronic devices and ultimately construct single-molecule nanocircuits using molecules with diverse structures featuring various functions, which is extremely challenging. Here, we realize a gate-controlled rectifying function (the on/off ratio reaches ∼60) and a high-performance field effect (maximum on/off ratio >100) simultaneously in an initially symmetric single-molecule photoswitch comprising a dinuclear ruthenium-diarylethene (Ru-DAE) complex sandwiched covalently between graphene electrodes. Both experimental and theoretical results consistently demonstrate that the initially degenerated frontier molecular orbitals localized at each Ru fragment in the open-ring Ru-DAE molecule can be tuned separately and shift asymmetrically under gate electric fields. This symmetric orbital shifting (AOS) lifts the degeneracy and breaks the molecular symmetry, which is not only essential to achieve a diode-like behavior with tunable rectification ratio and controlled polarity, but also enhances the field-effect on/off ratio at the rectification direction. In addition, this gate-controlled symmetry-breaking effect can be switched on/off by isomerizing the DAE unit between its open-ring and closed-ring forms with light stimulus. This new scheme offers a general and efficient strategy to build high-performance multifunctional molecular nanocircuits.

Second Linear Response Theory and the Analytic Calculation of Excited-State Properties

We present a method based on second linear response time-dependent density functional theory (TDDFT) to calculate permanent and transition multipoles of excited states, which are required to compute excited-state absorption/emission spectra and multiphoton optical processes, among others. In previous work, we examined computations based on second linear response theory in which linear response TDDFT was employed twice. In contrast, the present methodology requires information from only a single linear response calculation to compute the excited-state properties. These are evaluated analytically through various algebraic operations involving electron repulsion integrals and excitation vectors. The present derivation focuses on full many-body wave functions instead of single orbitals, as in our previous approach. We test the proposed method by applying it to several diatomic and triatomic molecules. This shows that the computed excited-state dipoles are consistent with respect to reference equation-of-motion coupled-cluster calculations.

Influence of Vibronic Coupling on Ultrafast Singlet Fission in a Linear Terrylenediimide Dimer

Singlet fission (SF) is a photophysical process capable of boosting the efficiency of solar cells. Recent experimental investigations into the mechanism of SF provide evidence for coherent mixing between the singlet, triplet, and charge transfer basis states. Up until now, this interpretation has largely focused on electronic interactions; however, nuclear motions resulting in vibronic coupling have been suggested to support rapid and efficient SF in organic chromophore assemblies. Further information about the complex interactions between vibronic excited states is needed to understand the potential role of this coupling in SF. Here, we report mixed singlet and correlated triplet pair states giving rise to sub-50 fs SF in a terrylene-3,4:11,12-bis(dicarboximide) (TDI) dimer in which the two TDI molecules are covalently linked by a direct N-N connection at one of their imide positions, leading to a linear dimer with perpendicular TDI π systems. We observe the transfer of low-frequency coherent wavepackets between the initial predominantly singlet states to the product triplet-dominated states. This implies a non-negligible dependence of SF on nonadiabatic coupling in this dimer. We interpret our experimental results in the framework of a modified Holstein Hamiltonian, which predicts that vibronic interactions between low-frequency singlet modes and high-frequency correlated triplet pair motions lead to mixing of the pure basis states. These results highlight how nonadiabatic mixing can shape the complex potential energy landscape underlying ultrafast SF.

Two-photon excited deep-red and near-infrared emissive organic co-crystals

Two-photon excited near-infrared fluorescence materials have garnered considerable attention because of their superior optical penetration, higher spatial resolution, and lower optical scattering compared with other optical materials. Herein, a convenient and efficient supramolecular approach is used to synthesize a two-photon excited near-infrared emissive co-crystalline material. A naphthalenediimide-based triangular macrocycle and coronene form selectively two co-crystals. The triangle-shaped co-crystal emits deep-red fluorescence, while the quadrangle-shaped co-crystal displays deep-red and near-infrared emission centered on 668 nm, which represents a 162 nm red-shift compared with its precursors. Benefiting from intermolecular charge transfer interactions, the two co-crystals possess higher calculated two-photon absorption cross-sections than those of their individual constituents. Their two-photon absorption bands reach into the NIR-II region of the electromagnetic spectrum. The quadrangle-shaped co-crystal constitutes a unique material that exhibits two-photon absorption and near-infrared emission simultaneously. This co-crystallization strategy holds considerable promise for the future design and synthesis of more advanced optical materials.

Embedding Methods for Quantum Chemistry: Applications from Materials to Life Sciences

Quantum mechanical embedding methods hold the promise to transform not just the way calculations are performed, but to significantly reduce computational costs and improve scaling for macro-molecular systems containing hundreds if not thousands of atoms. The field of embedding has grown increasingly broad with many approaches of different intersecting flavors. In this perspective, we lay out the methods into two streams: QM:MM and QM:QM, showcasing the advantages and disadvantages of both. We provide a review of the literature, the underpinning theories including our contributions, and we highlight current applications with select examples spanning both materials and life sciences. We conclude with prospects and future outlook on embedding, and our view on the use of universal test case scenarios for cross-comparisons of the many available (and future) embedding theories.

Control of Charge Carriers and Band Structure in 2D Monolayer Molybdenum Disulfide via Covalent Functionalization

The fine-tuning of electro-optic properties is critical for high-performing technologies. This is now obtainable with advanced nanostructures, particularly two-dimensional (2D) monolayer materials such as molybdenum disulfide (MoS₂). Using spin-polarized periodic density functional theory (DFT), we find that the direct band gap (K → K') can be chemically tuned with covalently bound functional groups. With an electron-withdrawing group such as fluorine, we observe one occupied α and one unoccupied β band, which correspond to the addition of an α electron and a β hole, confirmed with the spin difference (Q_α - Q_β) being 1. By increasing the electron-donating behavior with the replacement of F by H and then by Me, the occupied (valence) α band shifts upward in energy relative to the Fermi energy, and the unoccupied β shifts down until they are in contact with the Fermi energy. In addition, both α and β unoccupied (conduction) bands of the MoS₂ shift down, relative to the Fermi energy, until they are in contact with the Fermi and the system can be described as metallic. The MoS₂ + F system is thus a small gap semiconductor (0.96 eV), and the MoS₂ + H and MoS₂ + Me gaps are 0.21 and 0 eV (metallic), respectively. Spin density calculations illustrate the semilocalized nature of the α spin; however, this is not formed from the radical of the functionalizing group, but rather the resulting unpaired electron is on the sulfur atom after radical abstraction to form a covalent bond with the group. Five- and six-membered heterocycles were studied and further confirm these observations. Distinct from typical functional groups such as phenyl, we find evidence for the covalent bonding of pyrrole, cyclopentadiene, and pyridine to a sulfur atom of the MoS₂ surface, from the new α and β bands in the band structure. The charge carrier nature of the 2D monolayers of functionalized MoS₂ can be further tuned with charge doping (hole or electron), such that even the metallic systems can be returned to semiconducting states, but importantly as p-type conductors. Semilocalization of the spin states and control of the band gap can be generalized to other covalently functionalized 2D materials and appears suitable for electronic applications, such as photoluminescence devices, contact-free transistors, and quantum communication.

Quantum Interference and Substantial Property Tuning in Conjugated Z-ortho-Regio-Resistive Organic (ZORRO) Junctions

Coherence is a significant factor in nanoscale electronic insulator technology and necessitates an understanding of the structure-property relationship between constructive and destructive quantum interference. This is particularly important in organic dielectric circuitry, which is the subject of this work. It is known that molecular wires composed of (i) meta-substituted phenylene rings, (ii) cross-conjugated double bonds (orthogonal to the molecular long axis), and (iii) single bonds can dramatically reduce electrical transmission. Here we add to these tools the use of an unexplored molecular shape to create strong and counterintuitive interference: a fully conjugated molecular wire with a structure that is forced back on itself in a Z shape, thereby exhibiting remarkably low conductance (G = 0.43 × 10^-9 S) even though the phenylene arrangements are ortho- rather than meta-disposed. We call these Z-shaped molecules having ultralow conduction Z-ortho-regio-resistive organics (ZORROs). Here we analyze a series of ZORRO molecules and find them to have significant insulating properties in the coherent electron-transport regime due to interfering transmission pathways in the phenylene rings. Importantly, we find that both electron-withdrawing (fluorine) and electron-donating (methoxy) substituents enhance the transmission, which is not desirable. The former is due to the suppression of the destructive quantum interference at the F site, thereby enhancing the overall transmission, much like a Büttiker probe. The latter is due to a methoxy unit resonance additive effect, akin to oxygen doping, and positively contributes to the transmission. We then examine the effects of replacing the phenylene rings with 4,5- and 3,4-disubstituted thiophenes and how this ZORRO modification further reduces the transmission. An ultralow conductance of 0.13 × 10^-9 S and a relatively high dielectric constant (ε_r) of ∼5 are predicted for the 3,4-thiophene ZORRO derivative, which closely resembles two cross-conjugated units, making it an intriguing candidate for a gate dielectric material.

Steric Interactions Impact Vibronic and Vibrational Coherences in Perylenediimide Cyclophanes

Designing molecular systems that exploit vibronic coherence to improve light harvesting efficiencies relies on understanding how interchromophoric interactions, such as van der Waals forces and dipolar coupling, influence these coherences in multichromophoric arrays. However, disentangling these interactions requires studies of molecular systems with tunable structural relationships. Here, we use a combination of two-dimensional electronic spectroscopy and femtosecond stimulated Raman spectroscopy to investigate the role of steric hindrance between chromophores in driving changes to vibronic and vibrational coherences in a series of substituted perylenediimide (PDI) cyclophane dimers. We report significant differences in the frequency power spectra from the cyclophane dimers versus the corresponding monomer reference. We attribute these differences to distortion of the PDI cores from steric interactions between the substituents. These results highlight the importance of considering structural changes when rationalizing vibronic coupling in multichromophoric systems.

Molecular Junctions Inspired by Nature: Electrical Conduction through Noncovalent Nanobelts

Charge transport occurs in a range of biomolecular systems, whose structures have covalent and noncovalent bonds. Understanding from these systems have yet to translate into molecular junction devices. We design junctions which have hydrogen-bonds between the edges of a series of prototype noncovalent nanobelts (NCNs) and vary the number of donor-acceptors to study their electrical properties. From frontier molecular orbitals (FMOs) and projected density of state (DOS) calculations, we found these NCN dimer junctions to have low HOMO-LUMO gaps and states at the Fermi level, suggesting these are metallic-like systems. Their conductance properties were studied with nonequilibrium Green's functions density functional theory (NEGF-DFT) and was found to decrease with cooperative H-bonding, that is, the conductance decreased as the alternating donor-acceptors around the nanobelts attenuates to a uniform distribution in the H-bonding arrays. The latter gave the highest conductance of 51.3 × 10^-6 S and the Seebeck coefficient showed n-type (-36 to -39 μV K^-1) behavior, while the lower conductors with alternating H-bonds are p-type (49.7 to 204 μV K^-1). In addition, the NCNs have appreciable binding energies (19.8 to 46.1 kcal mol^-1), implying they could form self-assembled monolayer (SAM) heterojunctions leading to a polymeric network for long-range charge transport.

Germanium Fluoride Nanocages as Optically Transparent n-Type Materials and Their Endohedral Metallofullerene Derivatives

Carbon- and silicon-based n-type materials tend to suffer from instability of the corresponding radical anions. With DFT calculations, we explore a promising route to overcome such challenges with molecular nanocages which utilize the heavier element Ge. The addition of fluorine substituents creates large electron affinities in the range 2.5-5.5 eV and HOMO-LUMO gaps between 1.6 and 3.2 eV. The LUMOs envelop the surfaces of these structures, suggesting extensive delocalization of injected electrons, analogous to fullerene acceptors. Moreover, these Ge _nF _n inorganic cages are found to be transparent in the UV-visible region as probed with their excited states. Their capacitance, linear polarizabilities, and dielectric constants are computed and found to be on the same order of magnitude as saturated oligomers and some extended π-organics (azobenzenes). Furthermore, we explore fullerene-type endohedral isomers, i.e., cages with internal substituents or guest atoms, and find them to be more stable than the parent exohedral isomers by up to -206.45 kcal mol^-1. We also consider the addition of Li, He, Cs, and Bi, to probe the utility of the exo/ endo cages as host-guest systems. The endohedral He/Li@F₈@Ge₆₀F₅₂ cages are significantly more stable than their parent exohedral isomers He/Li@Ge₆₀F₅₂ by -182.46 and -49.22 kcal mol^-1, respectively. The energy of formation of endohedral He@F₈@Ge₆₀F₅₂ is exothermic by -10.4 kcal mol^-1, while Cs and Bi guests are too large to be accommodated but are stable in the exohedral parent cages. Conceivable applications of these materials include n-type semiconductors and transparent electrodes, with potential for novel energy storage modalities.

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.

Designing Principles of Molecular Quantum Interference Effect Transistors

To explore the designing principles for the quantum interference effect transistors, a series of simulations are carried out on a 2,5-linked perylene molecular junction composed of two subsystems connected via destructive quantum interference. Simulation results suggest that the overall conductance of a large π-conjugated system is determined by its subsystem connected directly to the electrodes. A Büttiker probe can be treated as a resistor, and to first-order approximation, its effect is found equivalent to severing its surrounding bonds. These findings greatly simplify the design of molecular quantum interference effect transistors.

Measuring Dipole Inversion in Self-Assembled Nano-Dielectric Molecular Layers

A self-assembled nanodielectric (SAND) is an ultrathin film, typically with periodic layer pairs of high-k oxide and phosphonic-acid-based π-electron (PAE) molecular layers. IPAE, having a molecular structure similar to that of PAE but with an inverted dipole direction, has recently been developed for use in thin-film transistors. Here we report that replacing PAE with IPAE in SAND-based thin-film transistors induces sizable threshold and turn-on voltage shifts, indicating the flipping of the built-in SAND polarity. The bromide counteranion (Br^-) associated with the cationic stilbazolium portion of PAE or IPAE is of great importance, because its relative position strongly affects the electric dipole moment of the organic layer. Hence, a set of X-ray synchrotron measurements were designed and performed to directly measure and compare the Br^- distributions within the PAE and IPAE SANDs. Two trilayer SANDs, consisting of a PAE or IPAE layer sandwiched between an HfO_x and a ZrO_x layer, were deposited on the SiO_x surface of Si substrates or periodic Si/Mo multilayer substrates for X-ray reflectivity and X-ray standing wave measurements, respectively. Along with complementary DFT simulations, the spacings, elemental (Hf, Br, and Zr) distributions, molecular orientations, and Mulliken charge distributions of the PAE and IPAE molecules within each of the SAND trilayers were determined and correlated with the dipole inversion.

Probing Intermolecular Vibrational Symmetry Breaking in Self-Assembled Monolayers with Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy

Ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS) combines the atomic-scale imaging capability of scanning probe microscopy with the single-molecule chemical sensitivity and structural specificity of surface-enhanced Raman spectroscopy. Here, we use these techniques in combination with theory to reveal insights into the influence of intermolecular interactions on the vibrational spectra of a N-N'-bis(2,6-diisopropylphenyl)-perylene-3,4:9,10-bis(dicarboximide) (PDI) self-assembled monolayer adsorbed on single-crystal Ag substrates at room temperature. In particular, we have revealed the lifting of a vibrational degeneracy of a mode of PDI on Ag(111) and Ag(100) surfaces, with the most strongly perturbed mode being that associated with the largest vibrational amplitude on the periphery of the molecule. This work demonstrates that UHV-TERS enables direct measurement of molecule-molecule interaction at nanoscale. We anticipate that this information will advance the fundamental understanding of the most important effect of intermolecular interactions on the vibrational modes of surface-bound molecules.

Can Molecular Quantum Interference Effect Transistors Survive Vibration?

Quantum interference in cross-conjugated molecules can be utilized to construct molecular quantum interference effect transistors. However, whether its application can be achieved depends on the survivability of the quantum interference under real conditions such as nuclear vibration. We use two simulation methods to investigate the effects of nuclear vibration on quantum interference in a meta-linked benzene system. The simulation results suggest that the quantum interference is robust against nuclear vibration not only in the steady state but also in its transient dynamics, and thus the molecular quantum interference effect transistors can be realized.

Redfield Treatment of Multipathway Electron Transfer in Artificial Photosynthetic Systems

Coherence effects on electron transfer in a series of symmetric and asymmetric two-, three-, four-, and five-site molecular model systems for photosystem I in cyanobacteria and green plants were studied. The total site energies of the electronic Hamiltonian were calculated using the density functional theory (DFT) formalism and included the zero point vibrational energies of the electron donors and acceptors. Site energies and couplings were calculated using a polarizable continuum model to represent various solvent environments, and the site-to-site couplings were calculated using fragment charge difference methods at the DFT level of theory. The Redfield formalism was used to propagate the electron density from the donors to the acceptors, incorporating relaxation and dephasing effects to describe the electron transfer processes. Changing the relative energies of the donor, intermediate acceptor, and final acceptor molecules in these assemblies has profound effects on the electron transfer rates as well as on the amplitude of the quantum oscillations observed. Increasing the ratio of a particular energy gap to the electronic coupling for a given pair of states leads to weaker quantum oscillations between sites. Biasing the intermediate acceptor energies to slightly favor one pathway leads to a general decrease in electron transfer yield.

Chain Length Dependence of the Dielectric Constant and Polarizability in Conjugated Organic Thin Films

Dielectric materials are ubiquitous in optics, electronics, and materials science. Recently, there have been new efforts to characterize the dielectric performance of thin films composed of molecular assemblies. In this context, we investigate here the relationship between the polarizability of the constituent molecules and the film dielectric constant, using periodic density functional theory (DFT) calculations, for polyyne and saturated alkane chains. In particular, we explore the implication of the superlinear chain length dependence of the polarizability, a specific feature of conjugated molecules. We show and explain from DFT calculations and a simple depolarization model that this superlinearity is attenuated by the collective polarization. However, it is not completely suppressed. This confers a very high sensitivity of the dielectric constant to the thin film thickness. This latter can increase by a factor of 3-4 at reasonable coverages, by extending the molecular length. This significantly limits the decline of the thin film capacitance with the film thickness. Therefore, the conventional fit of the capacitance versus thickness is not appropriate to determine the dielectric constant of the film. Finally, we show that the failures of semilocal approximations of the exchange-correlation functional lead to a very significant overestimation of this effect.

Improved Scaling of Molecular Network Calculations: The Emergence of Molecular Domains

The design of materials needed for the storage, delivery, and conversion of (re)useable energy is still hindered by the lack of new, hierarchical molecular screening methodologies that encode information on more than one length scale. Using a molecular network theory as a foundation, we show that to describe charge transport in disordered materials the network methodology must be scaled-up. We detail the scale-up through the use of adjacency lists and depth first search algorithms for during operations on the adjacency matrix. We consider two types of electronic acceptors, perylenediimide (PDI) and the fullerene derivative phenyl-C61-butyric acid methyl ester (PCBM), and we demonstrate that the method is scalable to length scales relevant to grain boundary and trap formations. Such boundaries lead to a decrease in the percolation ratio of PDI with system size, while the ratio for PCBM remains constant, further quantifying the stable, diverse transport pathways of PCBM and its success as a charge-accepting material.

Theoretical modeling of voltage effects and the chemical mechanism in surface-enhanced Raman scattering

Theoretical approaches can provide insight into the mechanisms and magnitudes of electromagnetic and chemical effects in surface-enhanced Raman scattering (SERS), properties that are not readily available experimentally. Here, we model the SERS spectra of two geometries of the prototypical Ag₂₀–pyridine cluster using a semiempirical INDO/SCI approach that allows a straightforward decomposition of the enhancement factors at each wavelength into electromagnetic and chemical terms, with proper treatment of resonant charge-transfer contributions to the enhancement. The method also enables us to determine the dependence of the enhancement on the electrochemical potential. We show that the electromagnetic enhancements for the Ag₂₀ cluster are <10 far from resonance but can increase to 10² to 10³ on resonance with plasmon excitation in the cluster. The decomposition also shows that for the systems studied here, the chemical enhancements are primarily due to resonance with excited states with significant charge-transfer character. This term is typically <10 but can be >10² at electrochemical potentials where the charge-transfer excited states are resonant with the incoming light, leading to total enhancements of >10⁴.

Semiempirical modeling of electrochemical charge transfer

Nanoelectrochemical experiments using detection based on tip enhanced Raman spectroscopy (TERS) show a broad distribution of single-molecule formal potentials E°′ for large π-conjugated molecules; theoretical studies are needed to understand the origins of this distribution. In this paper, we present a theoretical approach to determine E°′ for electrochemical reactions involving a single molecule interacting with an electrode represented as a metal nanocluster and apply this method to the Ag₂₀–pyridine system. The theory is based on the semiempirical INDO electronic structure approach, together with the COSMO solvation model and an approach for tuning the Fermi energy, in which the silver atomic orbital energies are varied until the ground singlet state of Ag₂₀–pyridine matches the lowest triplet energy, corresponding to electron transfer from the metal cluster to pyridine. Based on this theory, we find that the variation of E°′ with the structure of the Ag₂₀–pyridine system is only weakly correlated with changes in either the ground-state interaction energy or the charge-transfer excited-state energies at zero applied potential, which shows the importance of calculations that include an applied potential in determining the variation of formal potential with geometry. Factors which determine E°′ include wavefunction overlap for geometries when pyridine is close to the surface, and electrostatics when the molecule-cluster separation is large.

Computation of Dielectric Response in Molecular Solids for High Capacitance Organic Dielectrics

The dielectric response of a material is central to numerous processes spanning the fields of chemistry, materials science, biology, and physics. Despite this broad importance across these disciplines, describing the dielectric environment of a molecular system at the level of first-principles theory and computation remains a great challenge and is of importance to understand the behavior of existing systems as well as to guide the design and synthetic realization of new ones. Furthermore, with recent advances in molecular electronics, nanotechnology, and molecular biology, it has become necessary to predict the dielectric properties of molecular systems that are often difficult or impossible to measure experimentally. In these scenarios, it is would be highly desirable to be able to determine dielectric response through efficient, accurate, and chemically informative calculations. A good example of where theoretical modeling of dielectric response would be valuable is in the development of high-capacitance organic gate dielectrics for unconventional electronics such as those that could be fabricated by high-throughput printing techniques. Gate dielectrics are fundamental components of all transistor-based logic circuitry, and the combination high dielectric constant and nanoscopic thickness (i.e., high capacitance) is essential to achieving high switching speeds and low power consumption. Molecule-based dielectrics offer the promise of cheap, flexible, and mass producible electronics when used in conjunction with unconventional organic or inorganic semiconducting materials to fabricate organic field effect transistors (OFETs). The molecular dielectrics developed to date typically have limited dielectric response, which results in low capacitances, translating into poor performance of the resulting OFETs. Furthermore, the development of better performing dielectric materials has been hindered by the current highly empirical and labor-intensive pace of synthetic progress. An accurate and efficient theoretical computational approach could drastically decrease this time by screening potential dielectric materials and providing reliable design rules for future molecular dielectrics. Until recently, accurate calculation of dielectric responses in molecular materials was difficult and highly approximate. Most previous modeling efforts relied on classical formalisms to relate molecular polarizability to macroscopic dielectric properties. These efforts often vastly overestimated polarizability in the subject materials and ignored crucial material properties that can affect dielectric response. Recent advances in first-principles calculations via density functional theory (DFT) with periodic boundary conditions have allowed accurate computation of dielectric properties in molecular materials. In this Account, we outline the methodology used to calculate dielectric properties of molecular materials. We demonstrate the validity of this approach on model systems, capturing the frequency dependence of the dielectric response and achieving quantitative accuracy compared with experiment. This method is then used as a guide to new high-capacitance molecular dielectrics by determining what materials and chemical properties are important in maximizing dielectric response in self-assembled monolayers (SAMs). It will be seen that this technique is a powerful tool for understanding and designing new molecular dielectric systems, the properties of which are fundamental to many scientific areas.

Covalently bonded single-molecule junctions with stable and reversible photoswitched conductivity

Through molecular engineering, single diarylethenes were covalently sandwiched between graphene electrodes to form stable molecular conduction junctions. Our experimental and theoretical studies of these junctions consistently show and interpret reversible conductance photoswitching at room temperature and stochastic switching between different conductive states at low temperature at a single-molecule level. We demonstrate a fully reversible, two-mode, single-molecule electrical switch with unprecedented levels of accuracy (on/off ratio of ~100), stability (over a year), and reproducibility (46 devices with more than 100 cycles for photoswitching and ~10(5) to 10(6) cycles for stochastic switching).

Nanoscale Chemical Imaging of a Dynamic Molecular Phase Boundary with Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy

Nanoscale chemical imaging of a dynamic molecular phase boundary has broad implications for a range of problems in catalysis, surface science, and molecular electronics. While scanning probe microscopy (SPM) is commonly used to study molecular phase boundaries, its information content can be severely compromised by surface diffusion, irregular packing, or three-dimensional adsorbate geometry. Here, we demonstrate the simultaneous chemical and structural analysis of N-N'-bis(2,6-diisopropylphenyl)-1,7-(4'-t-butylphenoxy)perylene-3,4:9,10-bis(dicarboximide) (PPDI) molecules by UHV tip-enhanced Raman spectroscopy. Both condensed and diffusing domains of PPDI coexist on Ag(100) at room temperature. Through comparison with time-dependent density functional theory simulations, we unravel the orientation of PPDI molecules at the dynamic molecular domain boundary with unprecedented ∼4 nm spatial resolution.

Identification of two mechanisms for current production in a biharmonic flashing electron ratchet

Ratchets rectify the motion of randomly moving particles, which are driven by isotropic sources of energy such as thermal and chemical energy, without applying a net, time-averaged force between source and drain. This paper describes the behavior of a damped electron, modeled by a quantum Lindblad master equation, within a flashing ratchet (a one-dimensional potential that oscillates between a flat surface and a periodic asymmetric surface). By examining the complete space of all biharmonic potential shapes and a large range of oscillation frequencies, two modes of ratchet operation, differentiated by their oscillation frequencies (relative to the rate of electron relaxation), are identified. Slow-oscillating, strong friction ratchets operate by a classical, overdamped mechanism. In fast-oscillating, weak friction ratchets, current is primarily produced when the frequency of the oscillating potential is resonant with the beating of the electron wave function in the potential well. The shape of the ratchet potential determines the direction of the current (and, in some cases, straightforwardly accounts for current reversals), but the maximum achievable current at any shape is controlled by the degree of friction applied to the electron.

Enhancement of Resonant Energy Transfer Due to an Evanescent Wave from the Metal

The high density of evanescent modes in the vicinity of a metal leads to enhancement of the near-field Förster resonant energy transfer (FRET) rate. We present a classical approach to calculate the FRET rate based on the dyadic Green's function of an arbitrary dielectric environment and consider the nonlocal limit of material permittivity in the case of the metallic half-space and thin film. In a dimer system, we find that the FRET rate is enhanced due to shared evanescent photon modes bridging a donor and an acceptor. Furthermore, a general expression for the FRET rate for multimer systems is derived. The presence of a dielectric environment and the path interference effect enhance the transfer rate, depending on the combination of distance and geometry.