Ultrafast electronic relaxation and charge-carrier localization in CdS/CdSe/CdS quantum-dot quantum-well heterostructures

The relaxation and localization times of excited electrons in CdS/CdSe/CdS colloidal quantum wells were measured using subpicosecond spectroscopy. HRTEM analysis and steady-state PL demonstrate a narrow size distribution of 5-6 nm epitaxial crystallites. By monitoring the rise time of the stimulated emission as a function of pump intensity, the relaxation times of the electron from the CdS core into the CdSe well are determined and assigned. Two-component rise times in the stimulated emission are attributed to intraband relaxation of carriers generated directly within the CdSe well (fast component) and charge transfer of core-localized carriers across the CdS/CdSe interface (slow component). This is the first reported observation of simultaneous photon absorption in the core and well of a quantum-dot heterostructure. With increasing pump intensity, the charge-transfer channel between the CdS core CdSe well contributes less to the stimulated emission signal because of filling and saturation of the CdSe well state, making the interfacial charge-transfer component less efficient. The interfacial charge-transfer time of the excited electron was determined from the slow component of the stimulated emission build-up time and is found to have a value of 1.2 ps.     

Seeded growth of highly luminescent CdSe/CdS nanoheterostructures with rod and tetrapod morphologies

We have demonstrated that seeded growth of nanocrystals offers a convenient way to design nanoheterostructures with complex shapes and morphologies by changing the crystalline structure of the seed. By using CdSe nanocrystals with wurtzite and zinc blende structure as seeds for growth of CdS nanorods, we synthesized CdSe/CdS heterostructure nanorods and nanotetrapods, respectively. Both of these structures showed excellent luminescent properties, combining high photoluminescence efficiency (approximately 80 and approximately 50% for nanorods and nanotetrapods, correspondingly), giant extinction coefficients (approximately 2 x 10(7) and approximately 1.5 x 10(8) M(-1) cm(-1) at 350 nm for nanorods and nanotetrapods, correspondingly), and efficient energy transfer from the CdS arms into the emitting CdSe core.     

CdSe／CdS核壳量子点拉曼光谱的表面模分析

研究了不同壳层厚度(0～5.5ML)的CdSe/CdS核壳量子点的一次和高次拉曼散射,具体分析了CdSe和CdS的表面模随着壳层厚度的变化情况.结果表明,随着壳层厚度的增加,CdSe表面模(SO1)从198cm-1频移到185cm-1,CdS的表面模(SO2)从275cm-1频移到267cm-1,并且SO1和SO2试验结果与由介电连续模型得到的理论值很接近.此外,根据CdSe表面模的频移,对随着CdS壳层厚度的增加而引起的核层(CdSe)的介电常数随环境的变化做出了修正.     

TiO2 inverse opal based CdS/CdSe quantum dot co-sensitized solar cells

CdS and CdSe quantum dots were introduced as co-sensitizers into TiO₂ inverse opal quantum dot sensitized solar cells. Herein, the three-dimensionally ordered porous TiO₂ inverse opal film leads to a better infiltration of both sensitizers and hole transporting material, and the smaller surface area of TiO₂ inverse opal film is effectively offset by the incorporating of co-sensitization. It was found that the presence of CdS/CdSe co-sensitizers provides enhanced light absorption, and leads to a lower recombination rate of the electrons due to the stepwise structure of band edge in TiO₂/CdS/CdSe, which resulted in the observed enhanced photocurrent and energy conversion efficiency of the solar cells. A cell efficiency of 1.01 % has been attained.     

Size tuning of MAA capped CdSe and CdSe/CdS quantum dots and their stability in different pH environments

The present work reports synthesis of mercaptoacetic acid capped CdSe nanoparticles soluble in water at different temperatures and with different precursor ratios. This enabled to tune the particle size of QDs from 2.7 to 5.8 nm. The particles consist of nanocrystals; with mixed phase, hexagonal wurtzite as well as sphalerite cubic and are luminescent with quantum yield 10%. The quantum yield up to 20% has been obtained by growing a shell of CdS over the CdSe. HR-TEM images, XRD patterns and the photoluminescence excitation spectra shows epitaxial growth of CdS shell over CdSe and with average size 3.2 ± 1.2 and 4.7 ± 1.2 nm for CdSe and CdSe/CdS quantum dots respectively. FT-IR spectrum and the negative zeta potential value together confirms the attachment of mercaptoacetic acid to the QD surface, where the carboxylic acid group is facing towards solvent and provides stability due to electrostatic hindrance. Further, the QDs are checked for their stability and the luminescence in environments of different pH (4–11 pH). Both CdSe and CdSe/CdS agglomerate with total elimination of fluorescence for 4 pH medium, and no shift in the fluorescence emission peak observed for the 6–9 pH, therefore QDs can be applicable as the fluorescence tags in this specific range of pH.     

Aqueous synthesis of CdSe and CdSe/CdS quantum dots with controllable introduction of Se and S sources

A new and convenient route is developed to synthesize CdSe and core–shell CdSe/CdS quantum dots (QDs) in aqueous solution. CdSe QDs are prepared by introducing H₂Se gas into the aqueous medium containing Cd²⁺ ions. The synthesized CdSe QDs are further capped with CdS to form core–shell CdSe/CdS QDs by reacting with H₂S gas. The gaseous precursors, H₂Se and H₂S, are generated on-line by reducing SeO₃ ^2? with NaBH₄ and the reaction between Na₂S and H₂SO₄, and introduced sequentially into the solution to form CdSe and CdSe/CdS QDs, respectively. The synthesized water-soluble CdSe and CdSe/CdS QDs possess high quantum yield (3 and 20 %) and narrow full-width-at-half-maximum (43 and 38 nm). The synthesis process is easily reproducible with simple apparatus and low-toxic chemicals. The relatively standard deviation of maxima fluorescence intensity is only 2.1 % (n = 7) for CdSe and 3.6 % (n = 7) for CdSe/CdS QDs. This developed route is simple, environmentally friendly and can be readily extended to the large-scale aqueous synthesis of QDs.     

CdSe/CdS core/shell quantum dots as sensitizer of a photorefractive polymer composite

Abstract

CdSe/CdS core/shell, tri-n-octylphosphine oxide passivated, quantum dots are used to sensitize a photorefractive polymer composite. The composite also consists of poly(N-vinylcarbazole) as the nominally charge transporting matrix and an electro-optic chromophore. The efficacy of sensitization and consequent photorefractive performance is investigated using transmission spectroscopy and ellipsometry, two-beam coupling and degenerate four-wave mixing experiments. The photorefractive nature of the photo-induced grating is confirmed by the observation of asymmetric two-beam coupling. Four-wave mixing reveals record diffraction efficiencies for a nano-particle-sensitized photorefractive polymer at the field levels applied (1.3% at 70 V.μm^?1). A recently developed analytical technique is used to extract space-charge field rise time values from degenerate four-wave mixing transients. In turn, analysis of the dependence of the rise time on applied field is used to determine the zero-field charge dissociation efficiency to be 3.6 × 10^?5 ± 0.5 × 10^?5. It is further shown that the magnitude of this parameter accounts for most of the difference in photorefractive response rate between the present material and a similar C₆₀ sensitized composite.     

CdSe/CdS semiconductor quantum rods as robust fluorescent probes for paraffin-embedded tissue imaging

Immunofluorescence techniques on formalin fixed paraffin-embedded sections allow for the evaluation of the expression and spatial distribution of specific markers in patient tissue specimens or for monitoring the fate of labeled cells after in vivo injection. This technique suffers however from the auto-fluorescence background signal of the embedded tissue that eventually confounds the analysis. Here we show that rod-like semiconductor nanocrystals (QRs), intramuscularly injected in living mice, could be clearly detected by confocal microscopy in formalin fixed paraffin-embedded tissue sections. Despite the low amount of QRs amount injected (25 picomoles), these were clearly visible after 24 h in the muscle sections and their fluorescence signal was stronger than that of CdSe/ZnS quantum dots (QDs) similarly functionalized and in the case of QRs only, the signal lasted even after 21 days after the injection.     

长余辉荧光结构层增强CdS/CdSe量子点敏化太阳能电池（英文）

光吸收在提高量子点敏化太阳能电池(QDSSCs)的功率转换效率(PCE)方面起着至关重要的作用.本研究采用简单的刮涂法将多功能长余辉荧光层(LPP)引入到CdS/CdSe QDSSCs中.LPP层不仅可以增强光的捕获,还可以加速CdS/CdSe QDSSCs的电荷转移.因此,LPP层的引入有效地提高了CdS/CdSe QDSSCs的短路电流密度和相应的PCE.当采用橄榄绿荧光层时,PCE高达5.07%,与常规CdS/CdSe QDSSCs(4.08%)的功率转换效率相比,PCE提高了24%.此外,经过一分钟的太阳光照射(AM 1.5G,100 mW cm^-2),由于LPPs的储能特性,太阳能电池可在黑暗中继续工作.本研究不仅为QDSSCs提供了提高PCE的有效方法,而且为全天候QDSSCs的制备提供了可能.     

Efficient CdSe/CdS quantum dot light-emitting diodes using a thermally polymerized hole transport layer

We report multilayer nanocrystal quantum dot light-emitting diodes (QD-LEDs) fabricated by spin-coating a monolayer of colloidal CdSe/CdS nanocrystals on top of thermally polymerized solvent-resistant hole-transport layers (HTLs). We obtain high-quality QD layers of controlled thickness (down to submonolayer) simply by spin-coating QD solutions directly onto the HTL. The resulting QD-LEDs exhibit narrow ( approximately 30 nm, fwhm) electroluminescence from the QDs with virtually no emission from the organic matrix at any voltage. Using multiple spin-on HTLs improves the external quantum efficiency of the QD-LEDs to approximately 0.8% at a brightness of 100 cd/m(2) (with a maximum brightness over 1,000 cd/m(2)). We conclude that QD-LEDs could be made more efficient by further optimization of the organic semiconductors.     

Ligand-mediated modification of the electronic structure of CdSe quantum dots

X-ray absorption spectroscopy and ab initio modeling of the experimental spectra have been used to investigate the effects of surface passivation on the unoccupied electronic states of CdSe quantum dots (QDs). Significant differences are observed in the unoccupied electronic structure of the CdSe QDs, which are shown to arise from variations in specific ligand-surface bonding interactions.     

石墨烯量子点/CdS/CdSe共敏化太阳能电池

以三维锐钛矿TiO₂微球为上层光散射层材料, 以商业纳米TiO₂为下层连接材料, 采用刮刀法制备了一种新颖的双层TiO₂薄膜, 并应用于量子点敏化太阳能电池(QDSSC)。其中, 石墨烯量子点(GQDs)采用滴液法引入, CdS/CdSe量子点采用连续离子层吸附法(SILAR)制备。采用场发射扫描电镜、透射电镜、X射线衍射、紫外-可见漫反射光谱及荧光光谱对样品进行表征。实验还制备了CdS/CdSe量子点敏化及石墨烯量子点/CdS/CdSe共敏化太阳能电池, 并研究了石墨烯量子点及CdS不同敏化周期及对电池性能影响。研究结果表明, 石墨烯量子点及CdS不同敏化周期对薄膜的光学性质、电子传输及载流子复合均有较大影响。优选条件下, TiO₂/QGDs/CdS(4)/CdSe电池的光电转换效率为1.24%, 光电流密度为9.47 mA/cm², 显著高于TiO₂/CdS(4)/CdSe电池的这些参数(0.59%与6.22 mA/cm²)。这主要是由于TiO₂表层吸附石墨烯量子点后增强了电子的传输, 减少了载流子的复合。     

Multiexcitonic dual emission in CdSe/CdS tetrapods and nanorods

CdSe/CdS semiconductor nanocrystal heterostructures are currently of high interest for the peculiar electronic structure offering unique optical properties. Here, we show that nanorods and tetrapods made of such material combination enable efficient multiexcitonic emission, when the volume of the nanoparticle is maximized. This condition is fulfilled by tetrapods with an arm length of 55 nm and results in a dual emission with comparable intensities from the CdS arms and CdSe core. The relative intensities of the dual emission, originating from exciton phase-space filling and reduced Auger recombination, can be effectively modulated by the photon fluence of the pump laser. The results, obtained under steady-state detection conditions, highlight the properties of tetrapods as multiexciton dual-color emitters.     

Optical memory based on photo-activated fluorescence of core/shell CdSe/CdS quantum dots embedded in poly(butylmethacrylate)

This work presents the observation of a photo-activated fluorescence from core/shell quantum dots of CdSe/CdS incorporated in a poly(butylmethacrylate) matrix. Upon illumination with UV-light, the intensity of fluorescence from the quantum dots increases as seen by naked eyes at ambient conditions. This allows its utilization in optical memory media based on thin films of CdSe/CdS polymer nanocomposites suitable for practical application. The quantum dots are synthesized by the hot-injection method and embedded in poly(butylmethacrylate) matrix by radical polymerization with 1,1′-azobis-(cyclohexanecarbonitrile). The fluorescence of quantum dots quenched during the polymerization process, but appeared again after illumination of the nanocomposite material with UV-light. The fluorescence properties of quantum dots are governed by the presence of trioctylphosphine oxide in the matrix, which allows control of the optical memory effect.     

巯基包覆CdSe和CdSe/CdS核壳纳米晶的水相合成与表征

利用水相合成的方法制备了巯基包覆的具有较高荧光量子产率的CdSe和CdSe／CdS纳米晶．水相合成方法的优点是原料低廉、安全可靠和重复性高，缺点是纳米晶的尺寸分布较宽，发光效率不是很高．采用X-射线粉末衍射、吸收和荧光等光谱手段对纳米晶的平均尺度、粒径分布、晶体结构及发光特性进行了表征。在77K到300K的温度范围内，随着温度降低，CdSe纳米晶的发光峰逐渐蓝移，而CdSe／Cds纳米晶发光峰位基本不随温度变化而变化．此外，在325nm激光辐照下，CdSe／CdS纳米晶的荧光寿命比CdSe纳米晶延长了6倍左右，稳定性大幅度提高．以上结果表明，核壳结构的CdSe／CdS纳米晶具有较高的发光效率和良好的稳定性，具有广阔的应用前景．     

CdSe/CdS/SiO2 core/shell/shell nanoparticles

Quantum dots (QD) of a CdSe-ZnS core-shell structure are coated with silica spheres to improve their stability in biological buffers and biocompatibility in fluorescence imaging. We found that it was critical to transfer quantum dots from organic phase to aqueous phase before the silica shell growth process. As a result, high quality CdSe-ZnS-SiO2 core-shell-shell nanoparticles were prepared in high yields and their size and distribution are characterized with transmission electron microscopy and dynamic light scattering, which yielded uniform sizes and narrow polydispersity. Single particle fluorescence spectroscopy on the silica-protected quantum dots showed they were stronger emitters with consistent fluorescence intensity and "on-off" behaviors than bare CdSe-ZnS nanocrystals.     

水相中CdSe与核/壳CdSe/CdS量子点的制备与发光特性研究

以巯基乙酸为稳定剂在水相中制备了CdSe与核/壳型CdSe/CdS量子点水溶胶, 用紫外-可见吸收光谱和发射光谱研究了它们的发光特性, 并且用X射线粉末衍射(XRD)、透射电镜(TEM)和X射线光电子能谱(XPS)表征了它们的结构、形貌和化学组成, 结果表明使用该方法制备的量子点分散性良好, 而且用CdS对CdSe进行表面修饰以后的发光强度明显提高, 发射光谱和吸收光谱都有红移现象, 不同粒径颗粒的吸收峰的位置也有所不同.     

Enhanced performance of Al2O3 coated ZnO nanorods in CdS/CdSe quantum dot-sensitized solar cell

CdS/CdSe quantum dot-sensitized solar cells (QDSCs) based on ZnO nanorods, 4.55 μm in length, were studied. Many studies have shown that the performance of QDSCs is limited by a recombination process. Therefore, the interface layer was fabricated on the surface of the ZnO nanorods to retard recombination at the interface between the semiconductor and electrolyte. Overall, the performance of the QDSCs was improved by a surface coating of aluminum isopropoxide (Al₂O₃) on the ZnO nanorod, which facilitates a decrease in electron recombination and increased adsorption of CdS/CdSe QDs on the ZnO nanorods.