Fluorescence lifetimes and correlated photon statistics from single CdSe/oligo(phenylene vinylene) composite nanostructures

We present measurements of fluorescence intensity trajectories and associated excited-state decay times from individual CdSe/oligo(phenylene vinylene) (CdSe-OPV) quantum dot nanostructures using time-tagged, time-resolved (TTTR) photon counting techniques. We find that fluorescence decay times for the quantum dot emitter in these composite systems are at least an order of magnitude shorter than ZnS-capped CdSe quantum dot systems. We show that both the blinking suppression and associated lifetime/count rate behavior can be described by a modified version of the diffusive reaction coordinate model which couples slow fluctuations in quantum dot electron (1Se, 1Pe) energies to Auger-assisted hole trapping processes, hence modifying both blinking statistics and excited-state decay rates.     

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。     

含吸电子基的PPV共轭聚合物的研究进展

聚苯撑乙烯类(PPVs)光电功能材料与器件因其广阔的应用前景一直受到广泛的关注。本文主要对含吸电子基团PPVs电致发光聚合物进行了综述。从聚合物吸电子基团种类的角度,介绍了PPVs的分子结构与光电性质及其在电致发光器件中的应用,并讨论了当前这些聚合物作为光电子材料存在的一些问题和发展方向。     

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.     

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.     

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.     

PPV类电致发光聚合物的合成及分子设计研究

PPV类电致发光聚合物以其优良的光电性能在微电子及显示领域中得到了广泛的应用,是当今材料科学研究的热点之一.综述了PPV类聚合物的主要合成方法,并对此类材料进行了分子结构设计,同时简要地介绍了相关器件的性能.     

Crystal morphology in pristine and doped films of poly (p-phenylene vinylene)

The crystal morphology of oriented films of poly (p-phenylene vinylene) (PPV) has been investigated using electron microscopy and X-ray diffraction. An X-ray diffraction rotation series confirmed the existence of fibre symmetry in bulk oriented films. Dark-field imaging by transmission electron microscopy (TEM) revealed small diffracting regions of the order of 7 nm in size with an aspect ratio near 1. These diffracting regions were shown by high resolution transmission electron microscopy (HREM) to be composed of small crystallites with an average size of 5 nm. Imaging of the lateral packing by HREM allowed the evaluation of local variations in crystallite orientation. This HREM method of orientation function determination compares well to bulk methods (e.g. wide-angle X-ray scattering, infrared dichroism) for PPV of similar draw ratio. A micellar model is presented to describe the crystalline morphology of oriented PPV. The model presents PPV as a highly connected network of small crystallites. The well-formed crystalline regions are thought to compose approximately 50% of the sample volume with the remainder of the volume being grain boundaries. Doping by AsF₅ led to the formation of an electron-dense overlayer, thought to be arsenic oxide, which prohibited darkfield imaging of the crystallites. After doping with H₂SO₄, crystallites of the electrically conductive phase were observed. The general morphological character is preserved in the conversion from insulating to conducting forms. For the conditions employed, the doped diffracting regions were 4 nm in size and retained the orientation initially present in the pristine film.     

KrF laser-induced photobleaching effects in O(2) planar laser-induced fluorescence signals: experiment and model

KrF excimer lasers are often employed as high-power excitation sources in planar laser-induced fluorescence (PLIF) imaging experiments to measure the distributions of O(2), OH, and H(2)O-all important species in combustion phenomena. However, due to the predissociative nature of these molecules, the high laser pumping rates typically required in such PLIF experiments may significantly deplete the ground-state population. The proper interpretation of the ground-state number density and/or the temperature from the fluorescence signals then requires the inclusion of photobleaching effects. We compare the results of a five-level rate-equation model incorporating photobleaching effects to the time-resolved PLIF signals from O(2) as obtained in the products of a fuel-lean CH(4) air flame. The results indicate that the fluorescence signals in a typical predissociated PLIF imaging experiment are subject to significant amounts of photobleaching. In an effort to provide a convenient way to account for photobleaching, a simple three-level model is developed. This model provides an analytic solution that describes satisfactorily the time-integrated fluorescence signal when compared with both the five-level model and the measurements. The results also indicate that at the low laser irradiances required to minimize the effects of photobleaching, the correspondingly low fluorescence signal levels make the acquisition of single-shot PLIF images a challenge to currently available camera systems.     

Biexciton Quantum Yield Heterogeneities in Single CdSe (CdS) Core (Shell) Nanocrystals and Its Correlation to Exciton Blinking

We explore biexciton (BX) nonradiative recombination processes in single semiconductor nanocrystals (NCs) using confocal fluorescence microscopy and second-order photon intensity correlation. More specifically, we measure the photoluminescence blinking and BX quantum yields (QYs) and study the correlation between these two measurements for single core (shell) CdSe (CdS) nanocrystals (NCs). We find that NCs with a high "on" time fraction are significantly more likely to have a high BX QY than NCs with a low "on" fraction, even though the BX QYs of NCs with a high "on" fraction vary dramatically. The BX QYs of single NCs are also weakly dependent upon excitation wavelength. The weak correlation between exciton "on" fractions and BX QYs suggests that multiple recombination processes are involved in the BX recombination. To explain our results, we propose a model that combines both trapping and an Auger mechanism for BX recombination.