Internal Electric Field Modulation in Molecular Electronic Devices by Atmosphere and Mobile Ions

The internal potential profile and electric field are major factors controlling the electronic behavior of molecular electronic junctions consisting of ∼1–10 nm thick layers of molecules oriented in parallel between conducting contacts. The potential profile is assumed linear in the simplest cases, but can be affected by internal dipoles, charge polarization, and electronic coupling between the contacts and the molecular layer. Electrochemical processes in solutions or the solid state are entirely dependent on modification of the electric field by electrolyte ions, which screen the electrodes and form the ionic double layers that are fundamental to electrode kinetics and widespread applications. The current report investigates the effects of mobile ions on nominally solid-state molecular junctions containing aromatic molecules covalently bonded between flat, conducting carbon surfaces, focusing on changes in device conductance when ions are introduced into an otherwise conventional junction design. Small changes in conductance were observed when a polar molecule, acetonitrile, was present in the junction, and a large decrease of conductance was observed when both acetonitrile (ACN) and lithium ions (Li⁺) were present. Transient experiments revealed that conductance changes occur on a microsecond–millisecond time scale, and are accompanied by significant alteration of device impedance and temperature dependence. A single molecular junction containing lithium benzoate could be reversibly transformed from symmetric current–voltage behavior to a rectifier by repetitive bias scans. The results are consistent with field-induced reorientation of acetonitrile molecules and Li⁺ ion motion, which screen the electrodes and modify the internal potential profile and provide a potentially useful means to dynamically alter junction electronic behavior

Unipolar Injection and Bipolar Transport in Electroluminescent Ru-Centered Molecular Electronic Junctions

peer reviewedBias-induced light emission and light-induced photocurrents were used as independent probes of charge transport in carbon-based molecular junctions containing Ru(bpy)3. The thickness, bias, and temperature dependence of both the total device current and photoemission were compared, as well as their response to bias pulses lasting from a few milliseconds to several seconds. The device current was exponentially dependent on the square root of the applied electric field, with weak dependence on thickness when compared at a constant field. In contrast, light emission was strongly dependent on thickness at a given electric field, with a thickness-independent onset for light emission and a large intensity increase when the bias exceeded the 2.7 V HOMO-LUMO gap of Ru(bpy)3. The apparent activation energies for light emission and current were similar but much smaller than those expected for thermionic emission or redox exchange. Light emission lagged current by several milliseconds but reached maximum emission in 5-10 ms and then decreased slowly for 1 s, in contrast to previously reported solid-state Ru(bpy)3 light-emitting devices that relied on electrochemical charge injection. We conclude that at least two transport mechanisms are present, that is, "unipolar injection" initiated by electron transfer from a Ru(bpy)3 HOMO to the positive electrode and "bipolar injection" involving hole and electron injection followed by migration, recombination, and light emission. The unipolar mechanism is field-driven and the majority of the device is current, while the bipolar mechanism is bias-driven and involves electrode screening by PF6 ions or mobile charges. In addition, significant changes in thickness and temperature dependence for thicknesses exceeding 15 nm imply a change from injection-limited transport to bulk-limited transport. The current results establish unequivocally that electrons and holes reside in the molecular layer during transport once the transport distance exceeds the ∼5 nm limit for coherent tunneling and that redox events involving nuclear reorganization accompany transport. In addition, they demonstrate luminescence in a single organometallic layer without hole or electron transport layers, thicknesses below 30 nm, and symmetric electrodes with similar work functions

Robust All-Carbon Molecular Junctions on Flexible or Semi-Transparent Substrates Using “Process-Friendly” Fabrication

Large area molecular junctions were fabricated on electron-beam deposited carbon (eC) surfaces with molecular layers in the range of 2–5.5 nm between conducting, amorphous carbon contacts. Incorporating eC as an interconnect between Au and the molecular layer improves substrate roughness, prevents electromigration and uses well-known electrochemistry to form a covalent C–C bond to the molecular layer. Au/eC/anthraquinone/eC/Au junctions were fabricated on Si/SiO<i>_x</i> with high yield and reproducibility and were unchanged by 10⁷ current–voltage cycles and temperatures between 80 and 450 K. Au/eC/AQ/eC/Au devices fabricated on plastic films were unchanged by 10⁷ current density vs bias voltage (<i>J</i>–<i>V</i>) cycles and repeated bending of the entire assembled junction. The low sheet resistance of Au/eC substrates permitted junctions with sufficiently transparent electrodes to conduct Raman or UV–vis absorption spectroscopy in either reflection or transmission geometries. Lithographic patterning of Au/eC substrates permitted wafer-scale integration yielding 500 devices on 20 chips on a 100 mm diameter wafer. Collectively, eC on Au provides a platform for fabrication and operation of chemically stable, optically and electrically functional molecules on rigid or flexible materials. The relative ease of processing and the robustness of molecular junctions incorporating eC layers should help address the challenge of economic fabrication of practical, flexible molecular junctions for a potentially wide range of applications

Electrogenerated Chemiluminescence of Iridium-Containing ROMP Block Copolymer and Self-Assembled Micelles

The electrochemical properties and electrogenerated chemiluminescence (ECL) of an Ir­(ppy)₂(bpy)⁺-containing ROMP monomer, block copolymer (containing Ir­(ppy)₂(bpy)⁺ complexes, PEG chains, and butyl moieties), and self-assembled micelles were investigated. Following polymerization of the iridium complex, we observed multiple oxidation peaks for the block copolymer in cyclic voltammograms (CV) and differential pulse voltammograms (DPV), suggesting the presence of multiple environments for the iridium complexes along the polymer backbone. The ECL signals from monomer <b>1</b> and polymer <b>2</b> were reproducible over continuous CV cycles and stable over prolonged potential biases, demonstrating their robustness toward ECL-based detection. Comparison of the ECL signal of the block copolymer, containing multiple iridium complexes attached to the backbone, and the monomeric complex showed enhanced signals for the polymer. In fact, formation and reopening of the self-assembled micelles allowed recovery of the polymer and near complete retention of its original ECL intensity

Cyclometalated Iridium(III) Imidazole Phenanthroline Complexes as Luminescent and Electrochemiluminescent G-Quadruplex DNA Binders

Six cyclometalated iridium(III) phenanthroimidazole complexes with different modifications to the imidazole phenanthroline ligand exhibit enhanced luminescence when bound to guanine (G-) quadruplex DNA sequences. The complexes bind with low micromolar affinity to human telomeric and c-myc sequences in a 1:1 complex:quadruplex stoichiometry. Due to the luminescence enhancement upon binding to G-quadruplex DNA, the complexes can be used as selective quadruplex indicators. In addition, the electrogenerated chemiluminescence of all complexes increases in the presence of specific G-quadruplex sequences, demonstrating potential for the development of an ECL-based G-quadruplex assay

Reducing the corrosion rate of magnesium alloys using ethylene glycol for advanced electrochemical imaging

a b s t r a c t The corrosion of an AM50 Mg alloy was studied in ethylene glycol using electrochemical and electron microscopy techniques. Switching from H 2 O to ethylene glycol, it was shown that the corrosion of the AM50 alloy was significantly suppressed thereby slowing H 2 evolution. The corrosion of the AM50 alloy was mapped using scanning electrochemical microscopy in the feedback mode. Ferrocenemethanol can be used to expose the reactive anodic areas on the Mg alloy. These studies confirmed that studies in ethylene glycol can be used to elucidate reaction features obscured by rapid corrosion in H 2 O without significantly altering the mechanism and damage morphology. Crow

Robust Bipolar Light Emission and Charge Transport in Symmetric Molecular Junctions

Molecular junctions consisting of a Ru­(bpy)₃ oligomer between conducting carbon contacts exhibit an exponential dependence of junction current on molecular layer thickness (<i>d</i>) similar to that observed for other aromatic devices when <i>d</i> < 4 nm. However, when <i>d</i> > 4 nm, a change in transport mechanism occurs which coincides with light emission in the range of 600–900 nm. Unlike light emission from electrochemical cells or solid-state films containing Ru­(bpy)₃, emission is bipolar, occurs in vacuum, has rapid rise time (<5 ms), and persists for >10 h. Light emission directly indicates simultaneous hole and electron injection and transport, possibly resonant due to the high electric field present (>3 MV/cm). Transport differs fundamentally from previous tunneling and hopping mechanisms and is a clear “molecular signature” relating molecular structure to electronic behavior