Developing Composite Insulating Cross-Arms for 400 kV Lattice Towers

\u3cp\u3ePolymorphism of organic semiconducting materials exerts critical effects on their physical properties such as optical absorption, emission and electrical conductivity, and provides an excellent platform for investigating structure–property relations. It is, however, challenging to efficiently tune the polymorphism of conjugated polymers in aggregated, semi-crystalline phases due to their conformational freedom and anisotropic nature. Here, two distinctly different semi-crystalline polymorphs (β\u3csub\u3e1\u3c/sub\u3e and β\u3csub\u3e2\u3c/sub\u3e) of a low-bandgap diketopyrrolopyrrole polymer are formed through controlling the solvent quality, as evidenced by spectroscopic, structural, thermal and charge transport studies. Compared to β\u3csub\u3e1\u3c/sub\u3e, the β\u3csub\u3e2\u3c/sub\u3e polymorph exhibits a lower optical band gap, an enhanced photoluminescence, a reduced π-stacking distance, a higher hole mobility in field-effect transistors and improved photocurrent generation in polymer solar cells. The β\u3csub\u3e1\u3c/sub\u3e and β\u3csub\u3e2\u3c/sub\u3e polymorphs provide insights into the control of polymer self-organization for plastic electronics and hold potential for developing programmable ink formulations for next-generation electronic devices.\u3c/p\u3

An efficient zero-order description of the fine structure in the infrared reflection band of cubic ionic crystals and the phonon-polariton dispersion using Lorentz gauge

\u3cp\u3eThe reflection of infrared light by ionic crystals with cubic symmetry such as lithium fluoride, LiF, is analyzed in terms of phonon-polaritons. In contrast to the conventional view on phonon-polaritons that uses the Coulomb gauge and assumes a purely local dielectric response of the material, we here develop an alternative description making use of the Lorentz gauge. This involves retarded interactions between charges, implying a non-local response of the material to electromagnetic radiation. The resulting new phonon-polariton dispersion relation features polaritons with negative group velocity in the frequency range in between the transverse (ω\u3csub\u3eT\u3c/sub\u3e) and longitudinal frequency (ω\u3csub\u3eL\u3c/sub\u3e). By contrast, the conventional description predicts, in zero order, the absence of any propagating polaritons in the frequency interval between ω\u3csub\u3eT\u3c/sub\u3e and ω\u3csub\u3eL\u3c/sub\u3e. The new dispersion relation provides an efficient, zero-order description of the fine structure within the reststrahlen band of LiF. The local minimum near the middle of the reflectance band is due to excitation of a phonon-polariton whose energy and momentum matches that of the incoming photon. The Lorentz gauge description can also describe off-normal reflection and accounts for the experimentally observed widening of the reflection band with increasing angle of incidence.\u3c/p\u3

Reflection of light by anisotropic molecular crystals including exciton-polaritons and spatial dispersion

A theory for the reflection of light by molecular crystals is described, which reproduces the minimum within the reflection band that is observed experimentally. The minimum in reflection is related to the excitation of polaritons in the crystal. The theory involves reformulation of the boundary conditions for electromagnetic waves at the interface between vacuum and material. The material is modeled by a cubic lattice of oriented Lorentz oscillators. By requiring uniformity of gauge of the electromagnetic potential across the interface between vacuum and the dipole lattice, the need for additional boundary conditions is obviated. The frequency separation between the maxima in reflectance on both sides of the minimum allows for the extraction of a plasma frequency. The plasma frequencies extracted from reflection spectra are compared to the plasma frequencies calculated directly from structural data on the crystals and the oscillator strengths of the constituent molecules. A good agreement between extracted and calculated plasma frequency is obtained for a set of 11 dye molecules

Circular selective reflection of light proving cholesteric ordering in thin layers of chiral fluorene polymers

Thermally annealed and aligned thin films of polyfluorene and two other chiral fluorene copolymers preferentially reflect left-circular-polarized light with wavelengths just below the onset of the lowest-allowed p–p* optical transition. This provides evidence for a left-handed cholesteric arrangement of the polymer chains in the film

Probing a conjugated polymer's transfer of organization-dependent properties from solutions to films

The dynamic transfer of a conjugated polymer's organization-dependent properties from the solution state to the solid film state was probed by circularly polarized luminescence (CPL) and circular dichroism (CD) spectroscopy. Different supramolecular organizations within films and aggregate solutions of a chiral poly(p-phenylenevinylene) derivative led to opposite CPL and CD spectra. These dramatic property differences were controlled by regulating the polymer's self-assembly through solvent selection and film annealing. Therefore, different processing conditions can greatly affect the functional properties of conjugated polymer films employed in various optoelectronic application

Electronic memory effects in a sexithiophene - poly(ethylene oxide) block copolymer doped with NaCl. combined diode and resistive switching behavior

Electrical transport in devices consisting of an electrode of a conducting polymer (poly(ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS); a layer of a sexithiophene-poly(ethylene oxide) (6T-PEO) block copolymer with an Al top electrode is investigated. These devices show diode behavior with the larger current density flowing when the PEDOT:PSS electrode is biased positive with respect to the Al electrode (forward bias). Introduction of inorganic salt (NaCl) in the PEDOT layer results in resistive switching behavior under forward bias while retaining the diode character. The switching allows for storage of information and rewritable memory operation is demonstrated for the diodes although the retention time of the information is still very short (~10 s). The reported combination of switching and diode behavior is an important requirement for passive matrix addressing of resistive switching memory cells in an array and shows that materials with combined ion and charge transport properties are interesting for information storage

Anisotropic dielectric tensor for chiral polyfluorene at optical frequencies

The anisotropic dielectric tensor of chiral poly[9,9-bis((3s)-3,7- dimethyloctyl)-2,7-fluorene] is determined via variable angle spectroscopic ellipsometry and circular dichroism spectroscopy. The uniaxial anisotropy indicates a high in-plane alignment of polymer chains in the thin film. Chirality of the polymer results in small but nonzero off-diagonal matrix elements in the dielectric tensor, indicating a helical interchain organization in the vertical direction. This method for determining the dielectric tensor appears to be selfconsistent when checked against various theoretical models for the optical activity in the reflection of light. The dielectric constants for the chiral polyfluorene are compared to those for an achiral polyfluorene

Optical modulation of nano-gap tunnelling junctions comprising self-assembled monolayers of hemicyanine dyes

Light-driven conductance switching in molecular tunnelling junctions that relies on photoisomerization is constrained by the limitations of kinetic traps and either by the sterics of rearranging atoms in a densely packed monolayer or the small absorbance of individual molecules. Here we demonstrate light-driven conductance gating; devices comprising monolayers of hemicyanine dyes trapped between two metallic nanowires exhibit higher conductance under irradiation than in the dark. The modulation of the tunnelling current occurs faster than the timescale of the measurement (∼1 min). We propose a mechanism in which a fraction of molecules enters an excited state that brings the conjugated portion of the monolayer into resonance with the electrodes. This mechanism is supported by calculations showing the delocalization of molecular orbitals near the Fermi energy in the excited and cationic states, but not the ground state and a reasonable change in conductance with respect to the effective barrier width

Surface modification of zinc oxide nanoparticles influences the electronic memory effects in ZnO-polystyrene diodes

Resistive switching effects in diode structures with a spin-coated active layer containing zinc oxide (ZnO) nanoparticles in a polystyrene matrix are studied. The switching effect can be influenced by modification of the surface of the nanoparticles with coordinating ligands (amines and thiols). Using n-propylamine as a ligand, memory effects are observed without the diodes having undergone the forming step that is usually required before switching effects can be observed in bulk metal oxides. Memory effects are characterized by impedance spectroscopy and temperature-dependent current-voltage measurements and involve a spontaneous, thermally activated gradual transition from a state with high frequency independent conduction to a state with lower conductivity

Time delayed collection field experiments on polymer: fullerene bulk-heterojunction solar cells

The recombination of photogenerated charge carriers in poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene vinylene]:1-(3-methoxycarbonyl)-propyl-1-phenyl-[6,6]C61 bulk-heterojunction solar cells is investigated using the time delayed collection field technique. Here the lifetime of photogenerated electrons and holes that have escaped charge recombination can be determined from current measurements using a pulsed collection voltage that is delayed with respect to the excitation pulse. At 80 K, the number of long lived charge carriers decays in time according to t– with =0.2, practically independent of laser fluence in the range of 1–1000 µJ/cm2. For excitation densit