Nonvolatile flexible organic bistable devices fabricated utilizing CdSe/ZnS nanoparticles embedded in a conducting poly N-vinylcarbazole polymer layer

The bistable effects of CdSe/ZnS nanoparticles embedded in a conducting poly N-vinylcarbazole (PVK) polymer layer by using flexible poly-vinylidene difluoride (PVDF) and polyethylene terephthalate (PET) substrates were investigated. Transmission electron microscopy (TEM) images revealed that CdSe/ZnS nanoparticles were formed inside the PVK polymer layer. Current-voltage (I-V) measurement on the Al/[CdSe/ZnS?nanoparticles+?PVK]/ITO/PVDF and Al/[CdSe/ZnS nanoparticles+?PVK ]/ITO/PET structures at 300?K showed a nonvolatile electrical bistability behavior with a flat-band voltage shift due to the existence of the CdSe/ZnS nanoparticles, indicative of trapping, storing and emission of charges in the electronic states of the CdSe nanoparticles. A bistable behavior for the fabricated organic bistable device (OBD) structures is described on the basis of the I-V results. These results indicate that OBDs fabricated by embedding inorganic CdSe/ZnS nanoparticles in a conducting polymer matrix on flexible substrates are prospects for potential applications in flexible nonvolatile flash memory devices.     

Synthesis and blue electroluminescent properties of zinc (II) [2-(2-hydroxyphenyl)benzoxazole]

This study reports on the properties of organic light-emitting diodes (OLEDs) with zinc (II) [2-(2-hydroxyphenyl)benzoxazole] as a hole-blocking layer. OLEDs devices are prepared in a conventional OLEDs structure (i.e., anode/HTL/EL/HBL/cathode and anode/HTL/HBL/EL/cathode). The luminescence efficiencies and the turn-on voltage are significantly affected by the existence of the hole-blocking layer. This is attributed to an excellent hole-blocking property, which is in turn due to the high HOMO energy level (6.5 eV). The device showed luminous efficiency 2.46 lm/W at 5 V. The maximum luminance of about 10,000 cd/m² is obtained, and the turn-on voltage (2.5 V) is affected by the existence of the hole-blocking layer.     

CdSe胶质量子点的电致发光特性研究

采用胶体化学法合成硒化镉(CdSe)胶质量子点, 在此基础上制成了以CdSe胶质量子点为有源层, 结构为ITO/ZnS/CdSe/ZnS/Al的电致发光(EL)器件. 透射电镜测量表明量子点的尺寸为4.3 nm, 扫描电子显微镜测量ZnS薄膜和Al薄膜结果显示表面均较为平整, 由器件结构的X射线衍射分析观察到了CdSe(111)、ZnS(111)等晶面的衍射, 表明器件中包含了CdSe量子点和ZnS绝缘层材料. 光致发光谱表征胶质量子点的室温发光峰位于614 nm, 电致发光测量得到器件在室温下的发光波长位于450 ~ 850 nm, 峰值在800 nm附近. 本文对电致发光机制及其与光致发光谱的区别进行了讨论.     

Single active-layer structured dual-function devices using hybrid polymer-quantum dots

We demonstrate hybrid polymer-quantum dot dual-function devices with a single active-layer structure consisting of CdSe/ZnS semiconductor quantum dots dispersed with poly N-vinylcarbazole (PVK) and 1,3,5-tirs-(N-phenylbenzimidazol-2-yl) benzene (TPBi) fabricated on an indium-tin-oxide (ITO)/glass substrate by using a simple spin-coating technique. The dual-function devices are composed of light-emitting diodes (LED) on the top side and nonvolatile organic bistable memory devices (OBD) on the bottom side and can show electroluminescence (EL) along with electrical bistability concurrently. Both the functionality of LEDs and OBDs can be successfully achieved by adding an electron transport layer (ETL) TPBi to the OBD to attain an LED in which the lowest unoccupied molecular orbital (LUMO) level of TPBi is positioned at the energy level between the conduction band of CdSe/ZnS and the LiF/Al electrode. Through transmission electron microscopy (TEM) study, it is revealed that CdSe/ZnS QDs distributed on the interface of the hole transport layer (HTL) and ETL significantly take part in the electroluminescence process rather than those existing at the outer surface of the ETL.     

Traps and performance of MEH-PPV/CdSe(ZnS) nanocomposite-based organic light-emitting diodes

We report the fabrication and investigations of organic light-emitting diodes (OLEDs) using a composite made by mixing poly[2-methoxy-5(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) with CdSe/ZnS core/shell quantum dots (QDs). The electroluminescence efficiency of the studied devices was found to be improved as compared to devices using MEH-PPV. Moreover, the current density decreased with increasing QD concentration. We checked the effects of QDs on the electrical transport by determining the trap states, making use of the charge-based deep level transient spectroscopy (Q-DLTS) technique. The most striking result obtained is the decrease in trap density when adding QDs to the MEH-PPV polymer film. These results suggest that QDs would heal defects in nanocomposite-based devices and that CdSe/ZnS QDs prevent the trap center formation.     

Synthesis and photoluminescence of bright water-soluble CdSe/ZnS quantum dots overcoated by hybrid organic shell

Photoluminescence properties from water soluble CdSe/ZnS QDs encapsulated with hybrid trioctylphosphine-poly(acrylamide-co-acrylic acid)-ethanolamine (TOPO-PSMA-EA) shell have been investigated. It was found that PL efficiency of CdSe/ZnS QDs in water was increased 5–30% after introducing PSMA-EA polymers to encapsulate CdSe/ZnS-TOPO QDs. Higher PSMA concentrations were found to enhance the PL efficiency of QDs up to 1.8 folds, which is ascribed to a better packing and passivation of the TOPO-PSMA-EA shell over the QDs. Time-resolved photoluminescence suggested that the mean lifetime of photoexcited carriers in the water-soluble CdSe/ZnS-TOPO-PSMA-EA QDs elongated 2–17 ns compared with that of uncoated samples, indicating that PL quenching defects were effectively removed for CdSe/ZnS QDs with hybrid TOPO-PSMA-EA shell.     

Radiative interface state study in CdSe/ZnS quantum dots covered by polymer

The paper presents the transformation of photoluminescence (PL) spectra of nonconjugated and bioconjugated core/shell CdSe/ZnS QDs covered by PEG polymer at the aging in ambient air. Studied QDs are characterized by the sizes: (i) 3.6-4.0 nm and color emission with the maxima at 560-565 nm (2.19-2.25 eV) and (ii) 5.2-5.3 nm and with emission at 605-610 nm (2.02-2.08 eV). The part of 565 nm CdSe/ZnS QDs has been bioconjugated to the mouse anti PSA (Prostate-Specific Antigen) antibodies and the part of 605 nm QDs has been bioconjugated to the antihuman IL10 (Interleukin 10) antibodies using the commercially available 565 nm and 605 nm QD conjugation kits. It is revealed that the aging process in ambient air has the very strong impact on PL spectra of nonconjugated core/shell CdSe/ZnS QDs covered with PEG polymer. The aging process relates to the polymer modification in ambient air that is accompanied by the three effects: (i) polymer transparency increasing for the emission of CdSe cores (2.03 or 2.20 eV), (ii) the intensity stimulation of high energy PL bands (2.37, 2.73 and 3.06 eV) related to the interface states at the ZnS/PEG polymer interface and (iii) the elastic strain modification in QD systems. The concentration of interface states at the ZnS/polymer interface increases at the aging of PEG polymer in ambient air.     

Nanocrystalline Y3Al5O12:Ce phosphor-based white light-emitting diodes embedded with CdS:Mn/ZnS core/shell quantum dots

120 nm-sized yellow emitting Y₃Al₅O₁₂:Ce (YAG:Ce) nanocrystalline phosphors and 4.5 nm-sized orange emitting CdS:Mn/ZnS core/shell structured quantum dots (QDs) were prepared by a modified polyol and a reverse micelle chemistry, respectively. To compensate a poor color rendering index (CRI) of YAG:Ce nanophosphor-based white light-emitting diodes (LEDs) due to the lack of red spectral component, CdS:Mn/ZnS QDs were blended into YAG:Ce nanophosphors. Blends of nanophosphors and QDs exhibited the prominent spectral evolution with an increasing content of QDs. A hybrid white LED, which combines a blue LED with the blend of nanophosphor and QDs with a weight ratio of 1:1, was demonstrated with an improved CRI value of 85.     

High color rendering index white LED based on nano-YAG:Ce3+ phosphor hybrid with CdSe/CdS/ZnS core/shell/shell quantum dots

To improve the poor color rendering index (CRI) of YAG:Ce-based white light-emitting diode (LED) due to the lack of red spectral component, core/shell/shell CdSe/CdS/ZnS quantum dots (QDs) were synthesized and blended into nano-YAG:Ce³⁺ phosphors. Prominent spectral evolution has been achieved by increasing the content of QDs. A white LED combining a blue LED with the blends of nano-YAG phosphors and orange- and red-emission QDs with a weight ratio of 1:1:1 was obtained. This kind of white LED showed excellent white light with luminescent efficiency, color coordinates, CRI and correlated color temperature (CCT) of 82.5?lm/W, (0.3264, 0.3255), 91 and 4580?K, respectively.     

In vitro degradation of porous poly(lactic acid)/quantum dots scaffolds

In this study, cadmium selenide/zinc sulfide (CdSe/ZnS) quantum dots (QDs) were introduced into poly(lactic acid) (PLA) for fabrication of photoluminescent PLA/QDs scaffolds. TEM images revealed that the QDs were uniformly dispersed in the PLA. Compressive modulus and thermal stability of the PLA/QDs scaffolds are higher than those of the unfilled PLA scaffold. Cytotoxicity test results confirmed the non-cytotoxicity of the PLA/QDs scaffolds. During the process of in vitro degradation, the degradation rate of the PLA was accelerated by the presence of the QDs, and the molecular weight distributions of the PLA/QDs scaffolds were much broader when compared with the unfilled PLA ones. During the first 84 weeks of the degradation process, the photoluminescence (PL) intensity of the PLA/QDs scaffolds decreased with almost the same degradation ratio. The results suggested that the CdSe/ZnS QDs have potential applications for monitoring in vivo degradation of tissue engineering scaffolds.     

Fabrication of quantum dot-sensitized solar cells with multilayer TiO2/PbS(X)/CdS/CdSe/ZnS/SiO2 photoanode and optimization of the PbS nanocrystalline layer

In this work, two multilayer photoanode structures of TiO₂/PbS(X)/CdS/ZnS/SiO₂ and TiO₂/PbS(X)/CdS/CdSe/ZnS/SiO₂ were fabricated and applied in quantum dot-sensitized solar cells (QDSCs). Then, the effect of PbS QDs layer on the photovoltaic performance of corresponding cells was investigated. The sensitization was carried out by PbS and CdS QDs layers deposited on TiO₂ scaffold through successive ionic layer adsorption and reaction (SILAR) method. The CdSe QDs film was also formed by a fast, modified chemical bath deposition (CBD) approach. Two passivating ZnS and SiO₂ layers were finally deposited by SILAR and CBD methods, respectively. It was shown that the reference cell with TiO₂/CdS/ZnS/SiO₂ photoanode demonstrated a power conversion efficiency (PCE) of 3.0%. This efficiency was increased to 4.0% for the QDSC with TiO₂/PbS(2)/CdS/ZnS/SiO₂ photoelectrode. This was due to the co-absorption of incident light by low-bandgap PbS nanocrystalline film and also the CdS QDs layer and well transport of the charge carriers. For the CdSe included QDSCs, the PbS-free reference cell represented a PCE of 4.1%. This efficiency was improved to 5.1% for the optimized cell with TiO₂/PbS(2)/CdS/CdSe/ZnS/SiO₂ photoelectrode. The maximized efficiency was enhanced about 25% and 70% compared to the PbS-free reference cells with and without the CdSe QDs layer.

     

Preparation,optical and thermal properties of CdSe–ZnS/poly(lactic acid) (PLA) nanocomposites

In this study, CdSe–ZnS/poly(lactic acid) (PLA) nanocomposite films, containing different concentrations of surface-modified CdSe–ZnS quantum dots (QDs), were prepared via a solution casting method. The optical microstructural and thermal properties of the as-prepared QDs/PLA films were investigated. The QDs/PLA films exhibited strong and stable photoluminescence (PL) intensity with concentration dependent amplitudes. The transmission electron microscopy (TEM) pictures revealed that QDs of ∼5 nm diameter were uniformly dispersed in the PLA matrix. According to the results of thermogravimetric analysis, the weight-loss onset temperature of PLA clearly decreased with the QD content. A combination of Fourier transform infrared (FT-IR) spectroscopy, X-ray diffractometry (XRD) and differential scanning calorimetry (DSC) results suggested that the QDs exhibit obvious nucleation activity on the crystallization behavior of the PLA matrix. This research provides useful information to the foundations of practical applications of QDs/PLA nanocomposites as fluorescent and biodegradable functionalized materials.     

Indium phthalocyanine–CdSe/ZnS quantum dots nanocomposites showing size dependent and near ideal optical limiting behaviour

Indium phthalocyanine–CdSe/ZnS quantum dots (QDs) nanocomposites (InPc–CdSe/ZnS) of three sizes (5.57, 8.12 and 8.75 nm) were synthesized according to known procedures. The particle size of the CdSe/ZnS QDs alone are 3.95, 6.02, and 6.66 nm, and are denoted as QD1, QD2 and QD3 respectively. The nonlinear absorption (NLA) properties of the nanoconjugates (InPc–CdSe/ZnS) were investigated with nanosecond laser radiation at 532 nm wavelength. Enhanced NLA properties compared to the InPc alone were observed in the conjugates. The NLA was found to increase with the size of the CdSe/ZnS particles attached to the phthalocyanine. The observed increase was due to the availability of more free-carrier ions in the larger QDs, thus giving rise to the enhanced free-carrier absorption. The measured free-carrier absorption cross-sections (σ_FCA) are 1.10, 1.65 and 1.95 (×10⁻¹⁹ cm²) for InPc-QD1, InPc-QD2 and InPc-QD3 respectively. The nanoconjugates (InPc–CdSe/ZnS) showed a much lower threshold for optical limiting together with a much lower transmission at high fluences, than the previously reported nanocomposite limiters.     

Hybrid white-light emitting-LED based on luminescent polyfluorene polymer and quantum dots

We report the fabrication and electroluminescence (EL) characterization of a white-emitting hybrid organic/inorganic light emitting device (LED) by integrating core-shell CdSe/ZnS quantum dots (QDs) acting as a yellow emitter and polyfluorenes as the blue emitter in a multilayered structure. The hybrid white-emitting device was fabricated by spin-coating QDs in varied concentrations onto the layer of polyfluorenes with different functional groups (i.e., sulfur-containing PF1 and PFO). Depending on the QDs concentrations and the design of the hybrid LED, our device is able to exhibit either electroluminescence from QDs or a combination of both PF and QD. The EL spectra at different driving voltages, I-V characteristics, and EL chromaticity of the LEDs based on PF1/QD and PFO/QD LEDs have been measured, investigated, and compared, respectively.     

核壳型量子点-纳米金颗粒组装体高效检测神经性毒剂模拟剂

实验设计制备了一种由12层硫化锌包覆硒化镉的核壳型量子点(CdSe/12ZnS QDs)和纳米金颗粒(Au NPs)自组装形成的CdSe/12ZnS QDs/Au NPs复合结构, 并将其应用于神经性毒剂模拟剂氰基磷酸二乙酯(Diethyl Cyanophosphonate, DCNP)的高效检测。QDs由于与Au NPs存在荧光共振能量转移作用(Fluorescence Resonance Energy Transfer, FRET)而发生荧光猝灭, 乙酰胆碱酯酶(AChE)水解氯化硫代乙酰胆碱(ATC)生成的硫胆碱能够将量子点取代而使量子点荧光恢复。当QDs与Au NPs的摩尔浓度比为20 : 1时, QDs荧光猝灭效果最佳, AChE浓度为1.0×10 ^-3 U/L时, QDs荧光恢复效果最好。DCNP的存在会抑制AChE的活性, 减少硫胆碱的生成并降低QDs的荧光恢复效率, 通过对QDs荧光恢复效率测定能够检测DCNP。在最优条件下对DCNP的检测结果表明, 量子点的荧光恢复效率与DCNP浓度的对数在5.0×10 ^-9~5.0×10 ^-4mol/L的范围内存在良好的线性关系, 检出限达5.0×10 ^-9mol/L。     

Novel quantum dots: Water-based CdTeSe/ZnS and YAG hybrid phosphor for white light-emitting diodes

Novel water-based core/shell CdTeSe/ZnS quantum dots (QDs) were synthesized by aqueous method. The CdTeSe/ZnS QDs were investigated by high resolution transmission electron microscopy, energy dispersive spectrometry, UV–vis absorption spectra, and photoluminescence spectrum. The as-prepared QDs capped with ZnS shell were spherical in shape with an excellent quantum yield of 16% and emitted bright yellow light. In addition, the CdTeSe/ZnS QDs can be excited by blue or near-UV region, which is an advantage over wavelength converters for white light-emitting diodes (LEDs). White LEDs based on CdTeSe/ZnS QDs, commercially known as Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce), and hybrid phosphor of CdTeSe/ZnS QDs and YAG:Ce, were fabricated. The luminescent properties of the resultant white LEDs were evaluated. The higher red-component in the emission spectrum from CdTeSe/ZnS QDs increased the color rendering index (CRI) value of the commercial YAG:Ce-based white LEDs, and the hybrid phosphor-based white LED had CIE-1993 color coordinate, color temperature, and CRI values of (0.3125, 0.2806), 7108 K and 83.3, respectively.     

ZnS:Cu polymer nanocomposites for thin film electroluminescent devices

Copper-doped zinc sulphide nanoparticles with varying copper content were synthesized via a coprecepitation method and embedded in polymer thin films by phase transfer technique to examine direct current electroluminescence (DC-EL) properties. A single layer structure (ITO/ZnS:Cu@ polymer/Al) was chosen to examine the influence of film thickness, copper content and polymeric matrix. Two types of devices were investigated based either on an insulating polymethylmethacrylate (PMMA) matrix or a semiconducting poly(9-vinylcarbazole) (PVK) matrix. The resulting DC-EL differs in spectral characteristics showing a broad EL emission over the whole visible range for PMMA based devices and EL with narrow full width at half-maxima values (FWHM) and maxima positions close to those observed in photoluminescence (PL) spectra of particle dispersions.     

水溶胶CdSe纳米复合器件电致发光特性的研究

以巯基乙酸为稳定剂在水相中制备了水溶胶CdSe纳米晶,透射电子显微镜表明了纳米晶的形态和尺寸大小.用表面活性剂将CdSe纳米晶从水相中转移到有机相中,将其与具有空穴传输性能的聚合物PVK互溶在一起作为电致发光器件的发光层,以Alq3作为电子传输层,在发光层与Alq3之间加入了空穴阻挡层BCP制备了多层电致发光器件,研究了不同CdSe/PVK配比下水溶胶CdSe纳米复合器件的电致发光特性,结果发现随着水溶胶CdSe纳米晶在纳米复合物中所占比例的降低,电致发光器件的发光强度有所提高,起亮电压有所降低.     

Fabrication of white light-emitting diodes based on UV light-emitting diodes with conjugated polymers-(CdSe/ZnS) quantum dots as hybrid phosphors

White light-emitting diodes (LEDs) were fabricated using GaN-based 380-nm UV LEDs precoated with the composite of blue-emitting polymer (poly[(9,9-dihexylfluorenyl-2,7-diyl)-alt-co-(2-methoxy-5-{2-ethylhexyloxy)-1 ,4-phenylene)]), yellow green-emitting polymer (poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1',3}-thiadiazole)]), and 605-nm red-emitting quantum dots (QDs). CdSe cores were obtained by solvothermal route using CdO, Se precursors and ZnS shells were synthesized by using diethylzinc, and hexamethyldisilathiane precursors. The optical properties of CdSe/ZnS QDs were characterized by UV-visible and photoluminescence (PL) spectra. The structural data and composition of the QDs were transmission electron microscopy (TEM), and EDX technique. The quantum yield and size of the QDs were 58.7% and about 6.7 nm, respectively. Three-band white light was generated by hybridizing blue (430 nm), green (535 nm), and red (605 nm) emission. The color-rendering index (CRI) of the device was extremely improved by introducing the QDs. The CIE-1931 chromaticity coordinate, color temperature, and CRI of a white LED at 20 mA were (0.379, 0.368), 3969 K, and 90, respectively.     

Enhancement of the efficiency and the color stabilization in green organic light-emitting devices with multiple heterostructures acting as a hole transport layer

The electrical and the optical properties of organic light-emitting devices (OLEDs), with and without multiple heterostructures consisting of N, N-bis-(1-naphthyl)-N, N-diphenyl-1,1-biphenyl-4,4-diamine (NPB)/5,6,11,12-tetraphenylnaphthacene (rubrene) acting as a hole transport layer (HTL), were investigated. The OLEDs with 3 periods of NPB/mixed rubrene:NPB multiple heterostructures acting as an HTL showed the highest luminances and efficiencies. While the electroluminescence (EL) peak corresponding to the rubrene layer did not appear for the OLEDs with 3 periods of NPB/mixed rubrene:NPB multiple heterostructures, only the EL peak related to the tris (8-hydroxyquinoline) aluminum layer was observed. The Commission Internationale de l’Eclairage chromaticity coordinates of the OLEDs with 3 periods of NPB/mixed rubrene:NPB multiple heterostructures at 14 V were (0.321, 0.531), indicative of a deep stabilized green color.