First principles study of the electron transport through cis-polyacetylene based molecular wires

The electron transport properties of cis-polyacetylene and cis-polyacetylene based molecular wires (oligo(cyclopentadiene), oligo(pyrrole), and oligo(furan)) have been studied theoretically using a combination of density-functional theory and non-equilibrium Green′s functions method. The results demonstrate that the introduction of bridging group X (X=CH₂, NH, and O) in cis-polyacetylene has a profound effect on the electron transport behavior of the molecules. The conductance of the four molecular wires decreases in the order of polyacetylene>oligo(cyclopentadiene)>oligo(furan)>oligo(pyrrole). In particular, the conductances of oligo(furan) and oligo(pyrrole) are much lower than those of polyacetylene and oligo(cyclopentadiene). The mechanism of this difference of electron transport properties of these four molecular systems is analyzed in terms of their geometric structures, electronic structures, transmission spectra, and spatial distribution of frontier orbitals. It is found that the energy levels of frontier molecular orbitals and the evolution of spatial distribution of frontier molecular orbitals with the applied bias are the essential reason for generating this difference of electron transport behaviors of the four molecular systems.     

Femtosecond laser micromachining in the conjugated polymer MEH-PPV

Femtosecond-laser micromachining of poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] films is investigated using 130 fs pulses at 800 nm from a laser oscillator operating at 76 MHz repetition rate. We investigate the effect of pulse energy and translation speed on the depth and morphology of the micromachined regions. We quantified the MEH-PPV photobleaching induced by the fs-laser, and the conditions in which the emission of MEH-PPV is preserved after the micromaching.     

π-electron density dependence of third-order optical nonlinearities in poly(1,4-phenylene vinylene) derivatives

The third-order nonlinear optical properties of poly(1,4-phenylene vinylene) (PPV), poly(2-bromo-1,4-phenylene vinylene) (BrPPV) and poly(2-chloro-1,4-phenylene vinylene) (ClPPV) were studied by forward degenerate four-wave mixing (DFWM). They have large and ultrafast third-order optical nonlinear responses, which mainly arise from the distortion of the large π-conjugated electronic charge distribution in the backbone. It was found that there was a significant contribution to π-electron density and thus χ⁽³⁾ values from the substituents at the phenylene ring.     

Photovoltaic effects of p–n heterojunction device

Photoirradiation effects of poly(p-pyridyl vinylene), PPyV/poly(3-hexylthiophene), P3HT and polypyridine, PPy/poly(2-methoxy-5-dodecyloxy-p-phenylene vinylene), MDOPPV heterostructure photoelectric conversion devices have been investigated. The photovoltaic characteristics of the heterostructure photoelectric conversion device are considerably improved from those in a single-layer photoelectric conversion device. Quenching of photoluminescence both in the PPyV layer film and in the P3HT layer film has been observed in the PPyV/P3HT heterostructure film. The observed photoirradiation effects of the heterostructure device have been discussed in terms of interfacial photoinduced charge transfer between P3HT and PPyV.     

Broadband optical limiting performance of polymer-wrapped carbon nanotubes in the orange-NIR region

Soluble multi-walled carbon nanotubes (MWNTs) have been obtained by noncovalent modification with poly [2-methoxy,5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV). For the composite MWNT/MEH-PPV, there is π-π interaction between the MEH-PPV and MWNTs in addition to the wrapping of the polymer. The nonlinear optical transmittance was measured using a nanosecond optical parametric oscillator pumped with a Nd:YAG system. Excellent optical limiting performance of the composite MWNT/MEH-PPV was observed both in the visible region of 590-680 nm and at the wavelength of 1064 nm. By means of time-correlated single-photon counting fluorescence measurement, an explanation based on the nonlinear absorption of MWNT dominated by the intermolecular energy transfer was proposed.     

Electrical and optical characteristics of various PPV derivatives (MEH-PPV, CzEH-PPV, OxdEH-PPV)

To investigate the electrical characteristics of polymer based light emitting diode (LED) devices, we fabricated the hole transport device (HTD) and the electron transport device (ETD). The ITO and Au with high work function were used as electrodes for the HTD, and the Al and Li:Al with low work function were used for the ETD. The active layer materials were poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV), poly[2-(N-carbazolyl)-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (CzEH-PPV), and poly[2-(4-tert-butylphenyl)-5-phenyl-1,3,4-oxadiazole-5(2-ethylhexoxy)-1,4-phenylene vinylene] (OxdEH-PPV). We measured the current density–applied field (J–E) characteristics of the HTD and ETD with various thickness at different temperatures. The results of the J–E curves were analyzed by using tunneling model, space charge limited conduction (SCLC) model, etc. In the SCLC model, the mobility of the hole and the electron of MEH-PPV is 10⁻⁶ and 10⁻⁸ cm²/Vs, respectively. For CzEH-PPV and OxdEH-PPV, the hole mobility is similar to the value of the electron mobility with 10⁻¹⁰ cm²/Vs. The luminescent efficiency of CzEH-PPV or OxdEH-PPV is higher than that of MEH-PPV. The results of photoconductivity of the systems qualitatively agrees with the result of the electrical measurement. We analyze that the balance of the electron and the hole mobility plays an important role for the efficiency of the LEDs.     

Investigation of the electrical properties of a new PPV derivative-based on a sandwich structure for opto-electronic applications

The opto-electronic properties of native poly(p-phenylene vinylene) (PPV) were tuned by the confinement of the π-conjugation and the incorporation of a sulphur group as a spacer in order to obtain a blue-green emitting polymer (PPVS). The energy band gap of the PPVS thin film has been measured by UV-vis absorption spectroscopy and evaluated to 2.87 eV. Current-voltage characteristics and impedance spectroscopy measurements performed on sandwich structures [ITO/PPVS/Al] are used to elucidate the conduction mechanisms. The static electrical characterisations showed a space charge limited conduction (SCLC) and a conductivity with low power frequency behaviour characteristic of a hopping transport in disordered materials. The impedance spectra can be discussed in terms of an equivalent circuit model designed as a parallel resistor R_P and capacitor C_P network in series with a resistor R_S. We extract numerical values of these parameters by fitting experimental data. Their evolution with bias voltages has shown that the SCLC mechanism is characterised by an exponential trap distribution.     

Synthesis of conjugated polymers for application in light-emitting diodes (PLEDs)

In the field of electroluminescent organic materials, conjugated polymers have attracted much attention over recent years owing to their versatile synthesis, their relative ease of processing and the possibility of establishing predictive structure–function relationships between chemical structures and optical properties. This review article highlights the advances made in the synthesis of conjugated polymers for use in light-emitting devices (LEDs) covering the last two years. Research efforts were largely directed towards the improvement of the synthesis of monomers and polymers involving classical polymer structures such as poly( p -phenylene vinylene)s (PPVs), poly( p -phenylene)s (PPPs), poly(2,7-fluorene)s (PFs), or poly(2,5-thienylene)s (PTs). Control of the color of emission by modulation of the effective conjugation length, improving balanced charge injection and transport properties by introduction of electron-donating or -withdrawing moieties directly into the polymer backbone or enhancement of the emission efficiency by attempts to influence film morphology are some examples of recent research directions. All these investigations contributed to a significantly better understanding of the chemical and physical processes spanning topics from the manufacturing process to the operation of LEDs and leading to the announcement of the first commercial products. In addition to the classical π -conjugated polymers a few examples of rather unusual structures have also emerged.     

Emissive pattern formation by the photoreaction of poly(p-phenylene vinylene)

The vinylene unit of a p-phenylene polymer was photo-oxidized under atmospheric oxygen to provide emittive patterns. Thin films of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-p-phenylene vinylene] (MDMO-PPV) were directly photo-patterned by UV light through a photo-mask having 1 μm-wide lines. The absorption and fluorescence studies on the MDMO-PPV film by UV irradiation indicated that the photo-patterning was mainly originated from the oxidation of the vinylic bonds to give carbonyl (CO) containing polymers. The patterned MDMO-PPV film was applied to an electroluminescent cell, which showed 20 μm-wide luminescent line patterns at applied potential of 5 V.     

Comparative study of the effect of incorporation of vinylene linkages on the electronic structures of heterocyclic polymers

Using theab initioband structure results of double-zeta quality of polythiophene (PTP), polyfuran (PFU) and polyacetylene (PA), the effects of incorporation of varying percentages of vinylene units on the electronic structures and conduction properties of PTP and PFU have been investigated using a negative-factor counting method in the tight-binding approximation. Both periodic and aperiodic distributions of vinylene units have been studied. The results show that the incorporation of vinylene units makes the heterocyclic polymers better conductors of electricity both intrinsically as well as extrinsically and that the effects are more pronounced for the aperiodic distribution than for the periodic distribution.     

Control of mixed protonic and electronic conductivity by mixing rare-earth ortho-borates

In order to develop mixed protonic and electronic conductors, we proposed a novel concept for material design that enables to control partial conductivities by fabricating solid solutions of protonic and electronic conductors. In this work, Sr-doped LaBO₃ and Sr-doped CeBO₃ were chosen as model compounds conducting protons and electron holes, respectively. Solid solutions of the above borates, Sr-doped La_1 − xCe_xBO₃, were prepared, and their electrical conductivities were investigated in 8.5 × 10²-4.2 × 10³ Pa of p(H₂O) and 1.0 × 10-1.0 × 10⁵ Pa of p(H₂) at 1073 K. From the experimental results of the gas partial pressure dependences of the conductivities, major charge carrier species were identified as a function of x. It was found that proton was the major charge carrier when x < 0.2 while the contribution of the electron hole conduction became remarkable as x increased above 0.2. The contribution of the electron hole conduction can be interpreted by the percolation model.     

Interchain interactions effects on emission efficiency of poly(p-phenylene vinylene) films

In attempts to obtain poly(p-phenylene vinylene) (PPV) molecules without intermacromolecular aggregation two strategies were employed: the addition of a strong electrolyte, lithium chloride, to the solution of the precursor, or/and the preparation of solid solutions from co-dissolved precursor and poly(vinyl alcohol) (PVA). The effects on the thermal conversion process of PPV were studied. Complete isolation of the PPV chains was not attained, but a significant degree of chain separation was achieved, as demonstrated by spectroscopic data. The main observations were that strong intermacromolecular forces operate during the PPV elimination step, resulting in simultaneous aggregation with the formation of the final chains. The relative emission efficiency increases up to 115% for PPV and PPV:PVA films when 0.7% and 1.4% of LiCl, respectively, are added and lower conversion temperature (100 °C) procedure is employed. In the case of total annealing procedure (230 °C) the emission efficiency increases 65% for PPV:PVA film with 0.7% of LiCl. For PPV:LiCl films, this parameter decreases substantially (∼11%) when the salt concentration increases above 10%.     

Valence band and core level X-ray photoelectron spectroscopy of lead sulfide nanoparticle-polymer composites

Valence band and core level X-ray photoelectron spectroscopy (XPS) were used to probe lead sulfide (PbS) nanoparticle-polymer nanocomposites. Composite materials were prepared by trapping commercially available monodisperse 3 and 10 nm PbS nanoparticles in two polymers, the non-conducting polymer, polystyrene, and the conjugated polymer, poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene (referred to below as MEH-PPV). The nanocomposites prepared from commercial nanoparticles underwent oxidation, mainly to form lead sulfate. However, the narrow size distributions of the commercial nanoparticles allowed observation of distinct changes in the valence band from the 3 to 10 nm nanoparticles. Nanocomposites of 2-5 and 4-7 nm PbS nanoparticles were synthesized by growing the particles in poly(vinyl alcohol) (referred to below as PVA) and MEH-PPV, respectively. These composites both indicated the formation of lead sulfide nanoparticles. Furthermore, the XP spectra for the PVA/PbS composite displayed bonding between the PbS nanoparticles and the polymer while MEH-PPV showed no PbS-polymer bonding. The nanoparticles synthesized in MEH-PPV did not undergo oxidation. The particle size distributions of the synthesized nanoparticles were too broad to display size-dependent changes in the valence band.     

Photoluminescence quenching through resonant energy transfer in blends of conjugated polymer with low-molecular acceptor

A model is proposed for photoluminescence quenching due to resonant energy transfer in a blend of a conjugated polymer and a low-molecular energy acceptor. An analytical dependence of the normalized photoluminescence intensity on the acceptor concentration is derived for the case of a homogeneous blend. This dependence can be described by two fitting parameters related to the Förster radii for energy transfer between conjugated segments of the polymer and between the conjugated polymer segment and the energy acceptor. Asymptotic approximations are obtained for the model dependence that make it possible to estimate the contribution from the spatial migration of excitons to the photoluminescence quenching. The proposed model is used to analyze experimental data on the photoluminescence quenching in a blend of the soluble derivative of poly(p-phenylene vinylene) and trinitrofluorenone [13]. The Förster radius for resonant energy transfer between the characteristic conjugated segment of poly(p-phenylene vinylene) and the energy acceptor is determined to be r _F = 2.6 ± 0.3 nm.     

Hydration effect on the electronic transport properties of oligomeric phenylene ethynylene molecular junctions

A first-principles computational method based on the hybrid density functional theory is developed to simulate the electronic transport properties of oligomeric phenylene ethynylene molecular junctions with H₂O molecules accumulated in the vicinity as recently reported by Na {\it et al.} [\wx{Nanotechnology}{18} 424001 (2007)]. The numerical results show that the hydrogen bonds between the oxygen atoms of the oligomeric phenylene ethynylene molecule and H₂O molecules result in the localisation of the molecular orbitals and lead to the lower transition peaks. The H₂O molecular chains accumulated in the vicinity of the molecular junction can not only change the electronic structure of the molecular junctions, but also open additional electronic transport pathways. The obvious influence of H₂O molecules on the electronic structure of the molecular junction and its electronic transport properties is thus demonstrated.     

聚苯胺在聚合物发光二极管中的空穴传输作用

报道了聚苯胺中间氧化态(emeraldine base form PANI-EB)空穴传输层对以PPV衍生物(PTA-PPV,PTA-DMPPV和MEH-PPV)为发光层和Alq₃为电子传输层构成的多层结构的发光二极管的影响。实验结果表明,在器件中PANI-EB空穴层的引入能有效地降低器件的启亮电压和提高器件的发光亮度,其影响程度依赖于PPV衍生物的结构。     

Multipole surface plasmon resonance of an aluminium surface

The surface photoelectric effect and the surface plasmon resonances appear when a p/transverse magnetic polarized laser hits a gas-solid interface. We model this effect in the long wave length (LWL) domain (λ_vac > 10 nm, ?ω < 124 eV) by combining the Ampère-Maxwell equation, written in classical approximation, with the material equation for the susceptibility. The resulting model, called the vector potential from the electron density (VPED), calculates the susceptibility as a product of the bulk susceptibility and the electron density of the actual system. The bulk susceptibility is a sum of the bound electron scalar susceptibility taken from the experiment and of the conduction electron non-local isotropic susceptibility tensor in a jellium metal (Lindhard, 1954 [1]). The electron density is the square of the wave function solution of the Schrödinger equation. The analysis of observables, the reflectance R and the photoelectron yield Y as well as the induced charge density permits to identify and characterize the multipole surface plasmon resonance of Al(111) appearing at ω_m ∼ 0.8ω_p or 11-12 eV.     

Formation of vinylidene and vinylene by selective reactivity of Si(1 0 0)2 × 1 towards iso, cis and trans isomers of dichloroethylene

The room temperature (RT) chemisorption of three (iso, cis and trans) isomers of dichloroethylene (DCE) on Si(1 0 0)2 × 1 have been investigated by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). Unlike ethylene, the lack of molecular desorption features in the TPD data effectively rules out the cycloaddition adsorption mechanism for all three isomers. XPS spectra show that cis- and trans-DCE adsorb dissociatively on the 2 × 1 surface in equal proportion as mono-σ bonded 2-chlorovinyl and di-σ bonded vinylene adspecies, which could be produced by dechlorination mechanisms involving the proposed tri-atom π-complex and diradical intermediates, respectively. Acetylene (m/z 26) evolution from 2-chlorovinyl adspecies at 590 K and vinylene at 750 K are also observed for both cis- and trans-DCE, further confirming the common adsorption mechanisms for these geometrical isomers and the relative stabilities of the adspecies. In contrast, only vinylidene adspecies is found for iso-DCE, which indicates that the high ionicity of the CCl₂ group favours the diradical dechlorination mechanism. The single m/z 26 desorption peak for iso-DCE adspecies observed at a higher temperature (780 K) than cis and trans isomers is consistent with the higher adsorption energy of vinylidene than vinylene on Si(1 0 0) obtained in our ab initio calculations. The different relative locations of the Cl atoms in these isomers therefore play a crucial role in controlling the adsorption and thermal evolution on Si(1 0 0)2 × 1. The selective reactivity of the 2 × 1 surface towards these isomers can be used to generate vinylene or vinylidene templates from their corresponding adspecies.     

Oxidation of a cesium-covered Ni (1 1 0) surface studied by metastable-induced electron spectroscopy

An initial stage of oxidation of a cesium-covered Ni (1 1 0) surface has been studied by metastable-induced electron spectroscopy (MIES) and low-energy electron diffraction (LEED). The MIES brought spectra with Cs 6s induced peak (P_6s), Cs 5p (P_5p), O 2p induced peak (P_ox) and a structure related to the substrate Ni 3d states (P_3d). The work function change Δφ showed an oscillatory behavior in the progress of surface oxidation. The process is divided into three stages: (i) at low O₂ exposures, Δφ > 0 with unchanging P_5p and P_6s; (ii) at moderate exposures, Δφ < 0 with a drastic decrease in the P_6s intensity; (iii) at higher exposures, Δφ > 0 with shifts of peaks P_5p and P_ox to higher energies, together with an appearance of peak P_3d. A three-step model of initial oxidation of alkali-covered Ni (1 1 0) surfaces is presented.     

Temperature dependence of photoluminescence in MEH-PPV blend films

The temperature dependence of the optical properties in poly [2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) blending with 2-(4-biphenyl)-5-(4-tertbutylphenyl)-1,3,4-oxadiazole (PBD) has been investigated. The blending influence on the chain conformation and exciton energy migration in blend film is described. The PL emission from the pure and blend films are all blue shifted with increasing temperature. An addition of PBD in MEH-PPV films suppresses the relative intensity of 0-1 vibronic peaks at lower temperature. This phenomenon will not be promoted with increasing temperature, attributed to a temperature-dependent migration process of singlet excitons.