Blinking suppression and intensity recurrences in single CdSe-oligo(phenylene vinylene) nanostructures: experiment and kinetic model

We report time-resolved single molecule fluorescence imaging of individual CdSe quantum dots that are functionalized with oligomeric conjugated organic ligands. The fluorescence intensity trajectories from these composite nanostructures display both a strong degree of blinking suppression and intensity fluctuations with characteristic recurrence times on the order of 10-60?s. In addition, fluorescence decay rate measurements of individual hybrid nanostructures indicate significantly modified non-radiative quantum dot decay rates relative to conventional ZnS-capped CdSe quantum dots. We show that a modified diffusive reaction coordinate model with slow fluctuations in quantum dot electron energies (1S(e), 1P(e)) can reproduce the experimentally observed behaviour.     

Dual amplified spontaneous emission and laser action from a model oligo(phenylene vinylene): comparison with the corresponding polymer

We report a comparative study of the spectroscopic and stimulated emission properties of a model oligo(phenylene vinylene) with those of the corresponding partially conjugated poly(phenylene vinylene). The model oligomer exhibits dual Amplified Spontaneous Emission (ASE) at 430 and 450 nm attributed to two independent vibrational modes in the excited state of the molecule. The corresponding polymer exhibits single ASE at 453 nm implying that in the polymer chains the amplification originates from a single vibrational mode. The efficiency of ASE and laser action is lower in the polymer than in the oligomer because of defects in the polymer chains.     

Modified spontaneous emission in oligo (p-phenylene vinylene) planar microcavities

This paper reports on the photoluminescence properties of phenylene vinylene oligomers films (OPV) the emission of which was modified by microcavity effect. Films with 4 and 6 vinyl groups (respectively denoted OPV-4 and OPV-6) were investigated. A strong blue luminescence at 470 nm is observed for OPV-4. OPV-6 shows an emission spectrum quite similar to that of poly phenylene vinylene (PPV). Photoluminescence excitation measurements (PLE) give similar behaviors for the two samples and do not show any change in the shape of the spectra. Ageing under UV irradiation (380 nm) of the unencapsulated OPV-4 layers was evidenced by an exponential decrease of the photoluminescence intensity of about 40% after 3 h a continuous illumination. OPV-6 samples, aged on shelf, for weeks does only show a small decrease (15%) of the PL level over more than 7 h under continuous UV irradiation. Microcavity related effects were observed by inserting the films between two mirrors. These are generally a TiO₂/SiO₂ distributed Bragg reflectors (DBR) as bottom mirror and a Ag film as top mirror. However best results were obtained with two dielectric DBR. As expected, a strong enhancement of the emission at the cavity resonance and pronounced angular effects (emission peaking in the direction normal to the surface of the device) were observed. The importance of the position of the emitting material within the microcavity is evidenced with a thin tris(8-hydroxy) quinoline aluminum (Alq) embedded in SiO₂ with various cavity configurations.     

共轭聚合物—聚对苯撑乙烯齐聚物的合成及发光性质的研究

以Ｗｉｔｔｉｇ反应的方法，分别合成了五种聚对苯撑乙烯（ＰＰＶ）齐聚物，并用红外光谱、核磁共振、紫外－可见光谱及荧光光谱对所得齐聚物进行了测试和表征，发现它们的溶解性能有所改善，紫外可见光谱均向红移。     

2D self-assembly of oligo(p-phenylene vinylene) derivatives: from dimers to chiral rosettes

Enantiomerically pure oligo(p-phenylene vinylene) diaminotriazine derivatives and a short structurally related achiral diaminotriazine derivative, all having a rigid backbone in common, are studied to self-assemble at the solution-graphite interface by scanning tunneling microscopy. As a function of the length of the backbone, different two-dimensional motifs are formed (dimers and rosettes) that are rationalized in terms of the balance between different intermolecular interactions, in this case, intermolecular hydrogen bonding and the packing requirements of the alkyl chains on a graphite surface. In addition, the effect of molecular chirality on monolayer chirality is investigated, revealing molecular size-dependent expressions of the monolayer chirality.     

Fullerene-containing C<Subscript>60</Subscript>-CdTe(CdSe) composite nanostructures

We have developed a technology for obtaining homogeneous films based on fullerene-containing C₆₀-CdTe and C₆₀-CdSe composites. The surface morphology of the initial films is characterized by an average lateral roughness size of about 150 nm. Annealing in a vacuum of 10^?5 Torr for 3 h showed that C₆₀-CdSe films are stable at temperatures up to T=180°C, while the same treatment of films of the (C₆₀)_1?x(CdTe)_x system with x<0.5 leads to the appearance of surface clusters of the semiconductor component with an average size of about 500 nm. The cluster density and size increase with the content of CdTe in the initial composite powder. The photoluminescence spectrum of a (C₆₀)_1?x(CdTe)_x film with x=0.5 upon annealing displays a dominating peak at 730 nm, which is indicative of a significant modification of the film structure as a result of this post-growth treatment. It is demonstrated that fullerene-containing composite network nanostructures with a lateral resolution up to 250 nm can be created by direct electron-beam lithography.     

Blue emitting diodes based on self-assembled multilayers of cationic oligo(p-phenylene vinylene) and sulfonated polyaniline

Boladication form oligo(p-phenylene vinylene) (OPV) was synthesized and mixed boladication–polyanion multilayer thin films have been self-assembled by a process based on electrostatic interaction. Electroluminescent spectral fine structures of these self-assembled thin films showed differences in vibronic coupling intensity and space. The poly(aniline-co-N-propane-sulfonic acid aniline) (PAPSAH) provided better charge transport ability than poly(sulfonated styrene) (PSS) and the turn-on voltage of the device based on boladication OPV/PAPSAH multilayer thin films is lower than that of the PSS counterpart.     

Theoretical investigations of oligo(phenylene ethylene) molecular wire: Effects from substituents and external electric field

Theoretical investigation of the substituent effect on the molecular wire, oligo(phenylene ethynylene) (OPE), has been performed with density functional calculations by considering the influence from the external electric field (EF). Compared to the electron-donating –NH₂ group, the electron-withdrawing –NO₂ group plays more important roles, such as increase of molecular dipole moment, decrease of LUMO–HOMO gap, and localization of LUMO. Both the geometric and electronic structures of the model molecular wires are sensitive to the external EF. In particularly, the –NO₂ substituted OPE yields obvious asymmetrical evolutions of both the frontier molecular orbital energies and their spatial distribution, which could be used to intuitively interpret the asymmetrical current–voltage behaviors of molecules.     

New semi-conducting poly(phenylene vinylene-alt-anthrylene vinylene)s: Synthesis,characterization and photophysical properties

An anthracene-based semi-conducting polymer (P1) and its cyano-analogue (P2) were synthesized via the Wittig and Knoevenagel polycondensations. The polymers were soluble in common organic solvents and have number-average molecular weights of 13,750 and 6430 g mol⁻¹ for P1 and P2, respectively. The DSC analyzes show a good thermal stability and an amorphous morphology in solid state for these organic materials. The optical properties of the polymers were investigated by UV–visible absorption and fluorescence spectroscopies. The HOMO and LUMO levels were estimated using cyclic voltammetry analysis. The effect of cyano group on the photophysical properties of poly(phenylene vinylene-alt-anthrylene vinylene)s was investigated. The results demonstrate an enhancement in the ionization potential and a significant improvement of the fluorescence yield due to introduction of such groups into the π-conjugated system. Single-layer diodes based on these organic semiconductors have been fabricated and showed relatively low turn-on voltages.     

Topography and transport properties of oligo(phenylene ethynylene) molecular wires studied by scanning tunneling microscopy

Conjugated phenylene(ethynylene) molecular wires are of interest as potential candidates for molecular electronic devices. Scanning tunneling microscopic study of the topography and current-voltage (I-V) characteristics of self-assembled monolayers of two types of molecular wires are presented here. The study shows that the topography and I-Vs, for small scan voltages, of the two wires are quite similar and that the electronic and structural changes introduced by the substitution of an electronegative N atom in the central phenyl ring of these wires does not significantly alter the self-assembly or the transport properties.     

Resonant infrared matrix-assisted pulsed laser evaporation of CdSe colloidal quantum dot/poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-(1-cyano vinylene)phenylene] hybrid nanocomposite thin films

CdSe colloidal quantum dot / poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-(1-cyano vinylene)phenylene] hybrid nanocomposite thin films were deposited using resonant infrared matrix-assisted pulsed laser evaporation. The distributions of CdSe colloidal quantum dots within the polymer matrices of as-grown films were evaluated using transmission electron microscopy, and the optical properties of these films were determined by photoluminescence spectroscopy. These measurements demonstrate that: i) depending upon the deposition parameters used, the CdSe colloidal quantum dot distribution can be tuned between two morphology extremes, i.e. clustering or homogenous dispersion; and ii) the constituent materials of the nanocomposite are not damaged in any way that affects structural or optical properties by the deposition process. The demonstrated ability to control nanoparticle distribution within organic films has not been achieved by other deposition techniques and could enhance the performance of optoelectronic devices based on these materials.     

Friction Spot Joining of aluminum AA2024/carbon-fiber reinforced poly(phenylene sulfide) composite single lap joints: Microstructure and mechanical performance

Friction Spot Joining is a promising alternative joining technology for polymer–metal hybrid structures. In this work, the feasibility of Friction Spot Joining of aluminum AA2024-T3 (bare and alclad)/carbon-fiber reinforced poly(phenylene sulfide) is reported. The process temperature and the microstructure of the joints were investigated. Lap shear tensile strength as high as 27 MPa was achieved by using aluminum bare specimens. Sand blasting was also performed as an effective mechanical surface pre-treatment on aluminum surfaces, which resulted in higher surface roughness and accordingly improved mechanical performance for the selected conditions. In addition, the alclad specimens exhibited promising mechanical performance (lap shear strength of up to 43 MPa) that justifies further investigations. Finally, the bonding and failure mechanisms of the joints are briefly discussed.     

聚乙二醇/磺化聚苯醚质子交换膜的制备与性能研究

采用溶液共混浇铸法制备了一系列的聚乙二醇/磺化聚苯醚(PEG/SPPO)共混膜SPPO的红外光谱图显示了磺酸基团的成功引入;X-射线衍射图表明,加入柔性的聚乙二醇,SPPO的有序程度降低;膜的离子交换容量(IEC值)、质子传导率和吸水率等理化性能结果表明:PEG/SPPO共混膜的质子传导率和离子交换容量虽然较纯SPPO膜有所下降,但是最大也分别达到了1.80mequiv.g-1和0.061S/cm(PEG/SPPO=10/90,90℃),仍然可以满足质子交换膜的使用要求,而复合膜的吸水率和溶胀率较SPPO膜大幅降低。整体来说,复合膜的各项综合性质良好,有望应用于直接甲醇燃料电池。     

Photocurrent generation in a CdS nanocrystals/poly[2-methoxy-5-(2′-ethyl-exyloxy)phenylene vinylene] electrochemical cell

In this paper the photoelectrochemical processes occurring in composites formed of organic-capped CdS nanocrystals and low molecular weight poly[2-methoxy-5-(2′-ethyl-exyloxy)phenylene vinylene] conjugated polymer were investigated. High quality colloidal CdS nanoparticles were synthesized by means of thermal decomposition of suitable precursors in non coordinating solvents, using oleic acid as surface capping agent.

The absorption and emission properties of the prepared heterojunctions were studied both in solutions and in composite films.

The dispersed hybrids were also investigated as photoactive materials, focusing on the photoinduced charge transfer and recombination processes at the interface between the two components. The composites have shown a fundamental role in photoelectrochemical applications due to the presence of a great number of interfaces able to enhance the charge transfer between mixture components.

Blend solutions prepared with octylamine capped CdS nanocrystals showed an improvement of the photoconductivity with respect to hybrids containing longer oleate surfactants.     

Understanding the charge transfer and defect states in ZnO/In2O3 composite nanostructures (CNs)

Journal of Materials Science: Materials in Electronics - This report presents the VLS synthesis and the study of the charge carrier transport through defect states across the interface in ZnO/In2O3...     

Relationship between structure and electroluminescence of oligo(p-phenylenevinylene)s

The preparation of LEDs with poly( p-phenylenevinylene) (PPV) as emitting material is well established. However, due to the presence of a distribution of conjugated chain lengths in the polymer, systematic investigations of the electroluminescence with polymeric materials are difficult, as far as the optical emission is concerned. We are studying the relationship between structural variation of substituted oligo(p-phenylenevinylene)s and their electroluminescent behaviour using a series of distyrylbenzenes with a variety of substituents in order to investigate their influence on the electroluminescence (EL). Furthermore, we synthesized a homologous series of monodisperse oligo(2,5-dipropoxy-1,4-phenylenevinylene)s with up to 11 repeating units. This series covers the spectrum from monomer to polymer. The influence on the EL can be investigated by preparing single layer LEDs using vapor deposition or spincoating of the oligomers in a polystyrene (PS) matrix. The comparison of photoluminescence (PL)- and EL-spectra shows that the photophysical properties of the oligomers are strongly altered by aggregation phenomena.