Low-Energy Charge-Transfer Excitons in Organic Solids from First-Principles: The Case of Pentacene

The nature of low energy optical excitations, or excitons, in organic solids is of central relevance to many optoelectronic applications, including solar energy conversion. Excitons in solid pentacene, a prototypical organic semiconductor, have been the subject of many experimental and theoretical studies, with differing conclusions as to the degree of their charge-transfer character. Using first-principles calculations based on density functional theory and many-body perturbation theory, we compute the average electron–hole distance and quantify the degree of charge-transfer character within optical excitations in solid-state pentacene. We show that several low-energy singlet excitations are characterized by a weak overlap between electron and hole and an average electron–hole distance greater than 6 Å. Additionally, we show that the character of the lowest-lying singlet and triplet excitons is well-described with a simple analytic envelope function of the electron–hole distance

First-Principles Investigation of Borophene as a Monolayer Transparent Conductor

Two-dimensional boron is promising as a tunable monolayer metal for nano-optoelectronics. We study the optoelectronic properties of two likely allotropes of two-dimensional boron, β₁₂ and δ₆, using first-principles density functional theory and many-body perturbation theory. We find that both systems are anisotropic metals, with strong energy- and thickness-dependent optical transparency and a weak (<1%) absorbance in the visible range. Additionally, using state-of-the-art methods for the description of the electron–phonon and electron–electron interactions, we show that the electrical conductivity is limited by electron–phonon interactions. Our results indicate that both structures are suitable as a transparent electrode

Simultaneous Determination of Conductance and Thermopower of Single Molecule Junctions

We report the first concurrent determination of conductance (<i>G</i>) and thermopower (<i>S</i>) of single-molecule junctions via direct measurement of electrical and thermoelectric currents using a scanning tunneling microscope-based break-junction technique. We explore several amine-Au and pyridine-Au linked molecules that are predicted to conduct through either the highest occupied molecular orbital (HOMO) or the lowest unoccupied molecular orbital (LUMO), respectively. We find that the Seebeck coefficient is negative for pyridine-Au linked LUMO-conducting junctions and positive for amine-Au linked HOMO-conducting junctions. Within the accessible temperature gradients (<30 K), we do not observe a strong dependence of the junction Seebeck coefficient on temperature. From histograms of thousands of junctions, we use the most probable Seebeck coefficient to determine a power factor, <i>GS</i>², for each junction studied, and find that <i>GS</i>² increases with <i>G</i>. Finally, we find that conductance and Seebeck coefficient values are in good quantitative agreement with our self-energy corrected density functional theory calculations

Surface-Area-Dependent Electron Transfer Between Isoenergetic 2D Quantum Wells and a Molecular Acceptor

We report measurements of electron transfer rates for four isoenergetic donor–acceptor pairs comprising a molecular electron acceptor, methylviologen (MV), and morphology-controlled colloidal semiconductor nanoparticles of CdSe. The four nanoparticles include a spherical quantum dot (QD) and three differing lateral areas of 4-monolayer-thick nanoplatelets (NPLs), each with a 2.42 eV energy gap. As such, the measurements, performed via ultrafast photoluminescence, relate the dependence of charge transfer rate on the spatial extent of the initial electron–hole pair wave function explicitly, which we show for the first time to be related to surface area in this regime that is intermediate between homogeneous and heterogeneous charge transfer as well as 2D to 0D electron transfer. The observed nonlinear dependence of rate with surface area is attributed to exciton delocalization within each structure, which we show via temperature-dependent absorption measurements remains constant

Quantitative Current–Voltage Characteristics in Molecular Junctions from First Principles

Using self-energy-corrected density functional theory (DFT) and a coherent scattering-state approach, we explain current–voltage (IV) measurements of four pyridine-Au and amine-Au linked molecular junctions with quantitative accuracy. Parameter-free many-electron self-energy corrections to DFT Kohn–Sham eigenvalues are demonstrated to lead to excellent agreement with experiments at finite bias, improving upon order-of-magnitude errors in currents obtained with standard DFT approaches. We further propose an approximate route for prediction of quantitative IV characteristics for both symmetric and asymmetric molecular junctions based on linear response theory and knowledge of the Stark shifts of junction resonance energies. Our work demonstrates that a quantitative, computationally inexpensive description of coherent transport in molecular junctions is readily achievable, enabling new understanding and control of charge transport properties of molecular-scale interfaces at large bias voltages

The impact of physical performance and cognitive status on subsequent ADL disability in low-functioning older adults

We demonstrate that rectification ratios (RR) of ≳250 (≳1000) at biases of 0.5 V (1.2 V) are achievable at the two-molecule limit for donor–acceptor bilayers of pentacene on C₆₀ on Cu using scanning tunneling spectroscopy and microscopy. Using first-principles calculations, we show that the system behaves as a molecular Schottky diode with a tunneling transport mechanism from semiconducting pentacene to Cu-hybridized metallic C₆₀. Low-bias RRs vary by two orders-of-magnitude at the edge of these molecular heterojunctions due to increased Stark shifts and confinement effects

Determination of Energy Level Alignment and Coupling Strength in 4,4′-Bipyridine Single-Molecule Junctions

We measure conductance and thermopower of single Au–4,4′-bipyridine–Au junctions in distinct low and high conductance binding geometries accessed by modulating the electrode separation. We use these data to determine the electronic energy level alignment and coupling strength for these junctions, which are known to conduct through the lowest unoccupied molecular orbital (LUMO). Contrary to intuition, we find that, in the high-conductance junction, the LUMO resonance energy is further away from the Au Fermi energy than in the low-conductance junction. However, the LUMO of the high-conducting junction is better coupled to the electrode. These results are in good quantitative agreement with self-energy corrected zero-bias density functional theory calculations. Our calculations show further that measurements of conductance and thermopower in amine-terminated oligophenyl–Au junctions, where conduction occurs through the highest occupied molecular orbitals, cannot be used to extract electronic parameters as their transmission functions do not follow a simple Lorentzian form

Tuning Rectification in Single-Molecular Diodes

We demonstrate a new method of achieving rectification in single molecule devices using the high-bias properties of gold–carbon bonds. Our design for molecular rectifiers uses a symmetric, conjugated molecular backbone with a single methylsulfide group linking one end to a gold electrode and a covalent gold–carbon bond at the other end. The gold–carbon bond results in a hybrid gold-molecule “gateway” state pinned close to the Fermi level of one electrode. Through nonequilibrium transport calculations, we show that the energy of this state shifts drastically with applied bias, resulting in rectification at surprisingly low voltages. We use this concept to design and synthesize a family of diodes and demonstrate through single-molecule current–voltage measurements that the rectification ratio can be predictably and efficiently tuned. This result constitutes the first experimental demonstration of a rationally tunable system of single-molecule rectifiers. More generally, the results demonstrate that the high-bias properties of “gateway” states can be used to provide additional functionality to molecular electronic systems

Inverse Rectification in Donor–Acceptor Molecular Heterojunctions

The transport properties of a junction consisting of small donor–acceptor molecules bound to Au electrodes are studied and understood in terms of its hybrid donor–acceptor–electrode interfaces. A newly synthesized donor–acceptor molecule consisting of a bithiophene donor and a naphthalenediimide acceptor separated by a conjugated phenylacetylene bridge and a nonconjugated end group shows rectification in the reverse polarization, behavior opposite to that observed in mesoscopic p–n junctions. Solution-based spectroscopic measurements demonstrate that the molecule retains many of its original constituent properties, suggesting a weak hybridization between the wave functions of the donor and acceptor moieties, even in the presence of a conjugated bridge. Differential conductance measurements for biases as high as 1.5 V are reported and indicate a large asymmetry in the orbital contributions to transport arising from disproportionate electronic coupling at anode–donor and acceptor–cathode interfaces. A semi-empirical single Lorentzian coherent transport model, developed from experimental data and density functional theory based calculations, is found to explain the inverse rectification

Tunable Charge Transport in Single-Molecule Junctions via Electrolytic Gating

We modulate the conductance of electrochemically inactive molecules in single-molecule junctions using an electrolytic gate to controllably tune the energy level alignment of the system. Molecular junctions that conduct through their highest occupied molecular orbital show a decrease in conductance when applying a positive electrochemical potential, and those that conduct though their lowest unoccupied molecular orbital show the opposite trend. We fit the experimentally measured conductance data as a function of gate voltage with a Lorentzian function and find the fitting parameters to be in quantitative agreement with self-energy corrected density functional theory calculations of transmission probability across single-molecule junctions. This work shows that electrochemical gating can directly modulate the alignment of the conducting orbital relative to the metal Fermi energy, thereby changing the junction transport properties