Self‐Assembled Au/CdSe Nanocrystal Clusters for Plasmon‐Mediated Photocatalytic Hydrogen Evolution

Plasmon‐mediated photocatalytic systems generally suffer from poor efficiency due to weak absorption overlap and thus limited energy transfer between the plasmonic metal and the semiconductor. Herein, a near‐ideal plasmon‐mediated photocatalyst system is developed. Au/CdSe nanocrystal clusters (NCs) are successfully fabricated through a facile emulsion‐based self‐assembly approach, containing Au nanoparticles (NPs) of size 2.8, 4.6, 7.2, or 9.0 nm and CdSe quantum dots (QDs) of size ≈3.3 nm. Under visible‐light irradiation, the Au/CdSe NCs with 7.2 nm Au NPs afford very stable operation and a remarkable H₂‐evolution rate of (10× higher than bare CdSe NCs). Plasmon resonance energy transfer from the Au NPs to the CdSe QDs, which enhances charge‐carrier generation in the semiconductor and suppresses bulk recombination, is responsible for the outstanding photocatalytic performance. The approach used here to fabricate the Au/CdSe NCs is suitable for the construction of other plasmon‐mediated photocatalysts.     

Endowing Perovskite Nanocrystals with Circularly Polarized Luminescence

Perovskite nanocrystals are attracting great interest due to their excellent photonic properties. Here, through a supramolecular self‐assembly approach, the perovskite nanocrystals (NCs) with a novel circularly polarized luminescence (CPL) are successfully endowed. It is found that the achiral perovskite NCs can coassemble with chiral gelator in nonpolar solvents, in which the gelator molecules modify the surface of the perovskite NCs. Through such cogelation, the molecular chirality can transfer to the NCs resulting in CPL signals with a dissymmetric factor (g_lum) up to 10^?3. Furthermore, depending on the molecular chirality of the gelator, the CPL sense can be selected and the mirror‐imaged CPL is obtained. Such gels can be further embedded into the polymer film to facilitate flexible CPL devices. It is envisaged that this approach will afford a new insight into the designing of the functional chiroptical materials.     

Rapid synthesis of CdSe nanocrystals in aqueous solution at room temperature

Water-soluble thioglycolic acid-capped CdSe nanocrystals (NCs) were prepared in aqueous solution at room temperature. We investigated the effects of pH values on the fluorescence intensity of the as-prepared CdSe NCs, and discussed the influence of the initial pH values on the fluorescence property. Their mean diameter was estimated to be 1.9 nm depending on the initial pH values in the preparation, the photoluminescence quantum yield could reach as high as 1.9%, almost comparable to the CdSe NCs prepared by an organometallic route. Finally, the products were characterized by Fourier transform infrared spectrometry (FTIR), atomic force microscope (AFM) and X-ray powder diffraction (XRD). AFM image showed that the NCs were ball-shaped with good dispersibility. XRD analysis disclosed that the CdSe NCs were of cubic zinc-blended structure.     

Fluorescence enhancement from Eu3+ ions in CdSe nanocrystal containing silica matrix hosts

Semiconductor cadmium selenide particles together with europium ions were incorporated into the silica matrix using sol–gel method. Here, the effect of nanocrystals on the absorption and fluorescence features of europium ions is discussed. The fluorescence spectra reveal that the intensity of characteristic emission of europium increases considerably in the presence of CdSe particles. This phenomenon can be explained as due to the energy transfer resulting from electron–hole recombination in the CdSe to the rare earth ion. These zero-dimensional materials, along with the effect of matrix incorporating europium ions, are found to have increased the optical gain.     

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．     

Phosphine-free synthesis of high-quality reverse type-I ZnSe/CdSe core with CdS/Cd(x)Zn(1 - x)S/ZnS multishell nanocrystals and their application for detection of human hepatitis B surface antigen

Highly photoluminescent (PL) reverse type-I ZnSe/CdSe nanocrystals (NCs) and ZnSe/CdSe/CdS/Cd(x)Zn(1 - x)S/ZnS core/multishell NCs were successfully synthesized by a phosphine-free method. By this low-cost, 'green' synthesis route, more than 10 g of high-quality ZnSe/CdSe/CdS/Cd(x)Zn(1 - x)S/ZnS NCs were synthesized in a large scale synthesis. After the overgrowth of a CdS/Cd(x)Zn(1 - x)S/ZnS multishell on ZnSe/CdSe cores, the PL quantum yields (QYs) increased from 28% to 75% along with the stability improvement. An amphiphilic oligomer was used as a surface coating agent to conduct a phase transfer experiment, core/multishell NCs were dissolved in water by such surface modification and the QYs were still kept above 70%. The as-prepared water dispersible ZnSe/CdSe/CdS/Cd(x)Zn(1 - x)S/ZnS core/multishell NCs not only have high fluorescence QYs but also are extremely stable in various physiological conditions. Furthermore, a biosensor system (lateral flow immunoassay system, LFIA) for the detection of human hepatitis B surface antigen (HBsAg) was developed by using this water-soluble core/multishell NCs as a fluorescent label and a nitrocellulose filter membrane for lateral flow. The result showed that such ZnSe/CdSe/CdS/Cd(x)Zn(1 - x)S/ZnS core/multishell NCs were excellent fluorescent labels to detect HBsAg. The sensitivity of HBsAg detection could reach as high as 0.05 ng ml( - 1).     

Electrochemiluminescence immunosensor based on CdSe nanocomposites

A novel strategy for the enhancement of electrochemiluminescence (ECL) was developed by combining CdSe nanocrystals (NCs), carbon nanotube-chitosan (CNT-CHIT), and 3-aminopropyl-triethoxysilane (APS). A label-free ECL immunosensor for the sensitive detection of human IgG (HIgG) was fabricated. The colloidal solution containing CdSe NCs/CNT-CHIT composite was first covered on the Au electrode surface to form a robust film, which showed high ECL intensity and good biocompatibility. After APS as a cross-linker was covalently conjugated to the CdSe NCs/CNT-CHIT film, the ECL intensity was greatly enhanced. And, an intensity about 20-fold higher than that of the CdSe NCs/CNT-CHIT film was observed. After antibody was bound to the functionalized film via glutaric dialdehyde (GLD), the modified electrode could be used as an ECL immunosensor for the detection of HIgG. The specific immunoreaction between HIgG and antibody resulted in the decrease in ECL intensity. The ECL intensity decreased linearly with HIgG concentration in the range of 0.02-200 ng mL(-1), and the detection limit was 0.001 ng mL(-1). The immunosensor has the advantages of high sensitivity, speed, specificity, and stability and could become a promising technique for protein detection.     

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.     

A mild phosphine-free synthesis of alkylamine-capped CdSe nanocrystals

A new phosphine-free approach has been developed to synthesize high-quality cadmium selenide (CdSe) nanocrystals with cubic zinc-blende structure, by using the highly reactive selenium (Se) precursor at milder temperature than that used in the traditional phosphine route. This Se precursor was obtained from the reduction of Se powder by sodium borohydride in N,N-dimetbylformamide, in the absence of phosphine. Without the addition of other long-chain coordinating substances in this approach, the alkylamines such as dodecylamine (DDA) and octylamine (OA) were used as reaction solvents, and they also acted as surface capping reagents to produce DDA-capped and OA-capped CdSe NCs, respectively. The rapid nucleation and slow growth were observed by ultraviolet-visible absorption spectrum. The resulting OA-capped CdSe NCs grew faster compared with DDA-capped CdSe NCs under the same other conditions. These as-synthesized CdSe nanocrystals showed relatively narrow size distribution and high photoluminescence quantum efficiency (up to 9.4% for OA-capped CdSe NCs). This mild approach is low cost, relatively low danger and high production yield (approximately 80%), indicating that it is very effective for the phosphine-free synthesis of alkylamine-capped CdSe nanocrystals.     

Efficient emission from core/(doped) shell nanoparticles: applications for chemical sensing

We have studied manganese doping of zinc selenide core/zinc sulfide shell nanocrystals (NCs) where the impurity phosphor resides primarily in the shell. We have found that a simple two-step synthesis can be used to create these nontoxic materials that display efficient energy transfer from the core to the Mn doped shell. These core/shell NCs retain ample quantum efficiency ( approximately 25%) when solubilized in water, which opens the possibility of using these materials as bioimaging agents. As recent work has shown that nanocrystals can be functionalized with organic dyes to operate as ratiometric chemical sensing agents, we have conjugated the doped NCs with an organic dye to showcase efficient F?rster resonant energy transfer from the shell-doped phosphor to the surface-bound dye. This result indicates that doped NCs can be used to develop nontoxic ratiometric sensing/biological imaging agents.     

Colloidal synthesis of ultrathin two-dimensional semiconductor nanocrystals

2D semiconductor quantum wells have been recognized as potential candidates for various quantum devices. In quantum wells, electrons and holes are spatially confined within a finite thickness and freely move in 2D space. Much effort has focused on shape control of colloidal semiconductor nanocrystals(NCs), and synthesis of 2D colloidal NCs has been achieved very recently. Here, recent advances in colloidal synthesis of uniform and ultrathin 2D CdSeNCs are highlighted. Structural and optical property characterization of these quantum-sized 2D CdSe NCs is discussed. Additionally, 2D CdSe NCs doped with Mn 2+ ions for dilute magnetic semiconductors (DMS) are presented.These 2D CdSe-based NCs can be used as model systems for studying quantum-well structures.     

Facile synthesis of CdSe nanocrystals for bulk-heterojunction solar cells

Oleic acid (OA)-capped cadmium selenide (CdSe) nanocrystals (NCs) have been synthesized via a new high-scale route. X-ray diffraction and transmission electron microscopy confirmed that rod-like hexagonal (wurtzite) CdSe NCs with an average size of 10 nm were obtained via this new route. The obtained CdSe NCs were treated individually with pyridine and tert-butylamine (t-BA) for ligand exchange. Fourier transform infrared spectra of the as-synthesized and treated CdSe NCs confirmed the removal of OA ligands from the surface of CdSe NCs after treatments with pyridine and t-BA. Bulk-heterojunction (BHJ) solar cell devices were prepared using untreated and surface treated CdSe NCs blended with poly (3-hexylthiophene-2,5-diyl) (P3HT) polymer. BHJ solar cell devices made from P3HT:(surface treated CdSe NCs) blends showed greater improvement in photovoltaic performances compared to P3HT:(untreated CdSe NCs) blend. The improvement in photovoltaic performances was due to the increase of electron mobility in P3HT:(CdSe NCs) blends after surface treatment of CdSe NCs.     

High‐Brightness Blue and White LEDs based on Inorganic Perovskite Nanocrystals and their Composites

Inorganic metal halide perovskite nanocrystals (NCs) have been employed universally in light‐emitting applications during the past two years. Here, blue‐emission (≈470 nm) Cs‐based perovskite NCs are derived by directly mixing synthesized bromide and chloride nanocrystals with a weight ratio of 2:1. High‐brightness blue perovskite light‐emitting diodes (PeLEDs) are obtained by controlling the grain size of the perovskite films. Moreover, a white PeLED is demonstrated for the first time by blending orange polymer materials with the blue perovskite nanocrystals as the active layer. Exciton transfer from the blue nanocrystals to the orange polymers via Förster or Dexter energy transfer is analyzed through time resolved photoluminescence. By tuning the ratio between the perovskite nanocrystals and polymers, pure white light is achieved with the a CIE coordinate at (0.33,0.34).     

Synthesis of high quality zinc-blende CdSe nanocrystals and their application in hybrid solar cells

Oleic acid (OA) modified zinc-blende cadmium selenium nanocrystals (NCs) with different diameters, 3-5?nm, have been prepared. We find that the morphology and fluorescent properties of the samples are related to the preparation conditions such as the chain-length and concentration of the cadmium precursor as well as the concentration of OA. The hybrid solar cells based on the obtained spherical CdSe NCs as an acceptor and Poly(2-methoxy-5-(2'-ethylhexoxy)-p-phenylenevinylene) (MEH-PPV) as a donor show an energy conversion efficiency (ECE) as high as 0.85%, three times higher than that reported before for spherical CdSe NCs/conjugated polymer hybrid solar cells. When poly(3-hexylthiophene) (P3HT) is used as the donor phase instead of MEH-PPV, the energy conversion efficiency increases up to 1.08%. The solar cell based on CdSe NCs/conjugated polymer has the potential to open up new production technologies for hybrid solar cells based on semiconductor NCs.     

High quality zinc-blende CdSe nanocrystals synthesized in a hexadecylamine–oleic acid–paraffin liquid mixture

Safe, common, and low-cost compounds were used as solvents for the non-tri-n-octylphosphine (TOP) synthesis of high quality CdSe nanocrystals (NCs) in open air. In oleic acid–paraffin liquid system, CdSe nanocrystals in the less common zinc-blende (ZB, cubic) crystal structure have been obtained. The effects of adding n-hexadecylamine (HDA) to different solutions were discussed. Stable, highly homogeneous and luminescent CdSe nanocrystals were formed by adding n-hexadecylamine to Cd solution rather than to Se solution. Without any size sorting, the size distribution of the nanocrystals can be readily controlled and the highest photoluminescence (PL) quantum efficiency (QE) of the nanocrystals was up to 20–30%.     

Layer-by-layer growth of CdSe-based nanocrystal light-emitting diodes

The partial exchange of surface-passivating trioctylphosphine oxide (TOPO) on CdSe and ZnS-clad CdSe (CdSe/ZnS) nanocrystals with primary amines was utilized to grow ultra-thin films of these nanocrystals under nonaqueous conditions. This growth was achieved using 1,12-diaminododecane in a layer-by-layer assembly format, where one of the amino groups binds with the nanocrystal surface and the other regenerates the interface for further binding of nanocrystals. The nature of the growth is dependent on the relative surface affinity between the TOPO and the primary amine toward the zinc or cadmium sites on the nanocrystals. Using this technique, high-quality luminescent films of these nanocrystals can be built with well-defined thicknesses. Electroluminescent devices have been fabricated using this methodology.     

MgZnO Nanocrystals: Mechanism for Dopant‐Stimulated Self‐Assembly

Understanding the growth behavior of nanocrystals (NCs), especially when heteroatoms are introduced, is very important for the optimization of doping (or alloying) and optoelectronic performances. Here, it is reported on the observation of alloying‐facilitated self‐assembly of MgZnO NCs and the underlying mechanism of alloying concentration‐dependent surface grafting. Using the developed one‐pot thermolysis of Zn and Mg organic salts with the help of oleylamine (OAm) and oleic acid (OA), the Mg ions can be introduced into the ZnO lattice without phase separation with concentrations as high as 20%. Interestingly, with the increase of Mg alloying concentration, the morphologies of the products transform from monodispersed NCs to nanoflowers, and then nanobouquet superstructures, which have quasi‐monocrystal features and obey the oriented attachment rules. According to the analyses of surface functional groups, a mechanism involving concentration‐dependent surface grafting is proposed for such alloying‐facilitated self‐assembly.     

Physical and chemical analyses on single source precursor growth CdSe semiconductor nanomaterials

CdSe nanocrystals (NCs) are synthesized by the single source precursor thermal method. The use of a temperature ramp allows to obtain faster elaboration and smaller nano-particles in size. A cross-disciplinary study between chemical analyses and physical techniques provides consistent data for these small size NCs. Joint mass spectroscopy, transmission electron microscopy and optical spectrometry techniques give a coherent picture about average size and size dispersion of the NCs, as well as their optical spectral response in correlation with their size via quantum confinement effects.     

Study on the photoluminescence character of water-soluble CdSe nanocrystals under ambient conditions

Water soluble, thioglycolic acid (TGA) modified CdSe nanocrystals (NCs) have been prepared in aqueous media by the reaction between Cd2+ and NaHSe. Although initially these quantum dots (QDs) display photoluminescence (PL) with very low quantum yields (QY), upon prolonged exposure to ambient light, a strong PL enhancement by illumination is observed which leads to water soluble QDs with high luminescence. This result may have important application potential in biological or other fields. The primary reason for the luminescence enhancement is concluded to be the incorporation of sulfide ions from TGA into the lattice of CdSe NCs and the subsequent formation of alloy structures. Moreover, the CdSe/CdS core-shell structured QDs synthesized in aqueous solutions also consolidate this conclusion.     

pH‐Dependent Galvanic Replacement of Supported and Colloidal Cu2O Nanocrystals with Gold and Palladium

Galvanic replacement reactions (GRRs) on nanoparticles (NPs) are typically performed between two metals, i.e., a solid metal NP and a replacing salt solution of a more noble metal. The solution pH in GRRs is commonly considered an irrelevant parameter. Yet, the solution pH plays a major role in GRRs involving metal oxide NPs. Here, Cu₂O nanocrystals (NCs) are studied as galvanic replacement (GR) precursors, undergoing replacement by gold and palladium, with the resulting nanostructures showing a strong dependence on the pH of the replacing metal salt solution. GRRs are reported for the first time on supported (chemically deposited) oxide NCs and the results are compared with those obtained with corresponding colloidal systems. Control of the pH enables production of different nanostructures, from metal‐decorated Cu₂O NCs to uniformly coated Cu₂O‐in‐metal (Cu₂O@Me) core–shell nanoarchitectures. Improved metal nucleation efficiencies at low pHs are attributed to changes in the Cu₂O surface charge resulting from protonation of the oxide surface. GR followed by etching of the Cu₂O cores provides metal nanocages that collapse upon drying; the latter is prevented using a sol–gel silica overlayer stabilizing the metal nanocages. Metal‐replaced Cu₂O NCs and their corresponding stabilized nanostructures may be useful as photocatalysts, electrocatalysts, and nanosensors.