Ultrafast and large third-order nonlinear optical properties of CdS nanocrystals in polymeric film

We report the ultrafast and large third-order nonlinear optical properties of CdS nanocrystals (NCs) embedded in a polymeric film. The CdS NCs of 2 nm radius are synthesized by an ion-exchange method and highly concentrated in the two layers near the surfaces of the polymeric film. The two-photon absorption coefficient and the optical Kerr coefficient are measured with laser pulses of 250 fs duration at 800 nm wavelength. The one-photon and two-photon figures of merit are determined to be 3.1 and 1.3, respectively, at irradiance of 2 GW/cm(2). The observed nonlinearities have a recovery time of approximately 1 ps. The two-photon-generated free carrier effects have also been observed and discussed. These results demonstrate that CdS NCs embedded in polymeric film are a promising candidate for optical switching applications.     

Preparation of CdS nanocrystals within supramolecular self-assembled nanoreactors and their phase transfer behavior

A new strategy for the synthesis of CdS nanocrystals (NCs) within supramolecular self-assembly nanoreactors has been described. The self-assembly nanoreactors were readily constructed through the electrostatic interactions and ion pairs between palmitic acid and the terminal amine groups of hyperbranched polymer. In a chloroform/water two-phase system, aqueous Cd (2+) ions were spontaneously encapsulated into the cavities of self-assembly nanoreactors in chloroform. After reaction with S (2-) ions, the CdS NCs with high stability were obtained. By the addition of excess triethylamine, CdS NCs formed in the self-assembly nanoreactors were transferred from organic phase into aqueous phase. After dialysis and rotorary evaporation, aqueous CdS NCs could be redispersed into chloroform solution containing palmitic acid.     

Preparation and optical properties of blue-emitting colloidal CdS nanocrystallines by the solvothermal process using poly (ethylene oxide) as the stabilizer

Blue-emitting colloidal CdS nanocrystals have been synthesized through the solvothermal reaction of cadmium acetate and thiourea in N,N-dimethylformamide using poly(ethylene oxide; PEO) as the stabilizing polymer. The as-prepared CdS colloids were stable at ambient conditions for several weeks. The PEO-stabilized CdS colloids showed a narrow fluorescence band with the maximum at about 420 nm and thus emitting blue fluorescence under the ultraviolet (UV) lamp. A common red shift of fluorescence band is not detected for the prepared CdS colloids in the study, indicating that PEO-stabilized CdS NCs possess few crystalline defects on their surface. In addition, transmission electron microscope micrographs reveal that the sizes of CdS NCs are between 4.4 to 5.4 nm with small standard deviations from 0.5 to 0.7 nm. The particle growth kinetics was studied by monitoring UV-visible absorption onsets versus the reaction time and was found to nearly follow the Lifshitz–Slyozov–Wagner theory for the Ostwald ripening mechanism.     

CdS纳米晶@碳点复合物膜的电致化学发光

采用热解柠檬酸法制备碳点(CDs),并将之与表面无包裹剂的CdS纳米晶(CdS NCs)超声复合制备CdS纳米晶@碳点(CdS NCs@CDs)复合物。研究了复合物膜阴极电致化学发光(ECL),探讨了CDs对CdS纳米晶膜ECL增强的机理。CDs分散性良好、尺寸在1.5~4 nm之间;与粒径约为4 nm的CdS纳米晶按体积比2∶3复合后,在360 nm光激发下复合物具有最强的荧光发射且表现为CDs的荧光。同时,复合物膜产生归属于激发态CdS纳米晶的最强的ECL发射,且ECL发光峰起置电势正移至-1.05 V。复合物膜的ECL发射是pH依赖的,在pH值为6时,复合物膜具有最大的ECL强度,为CdS纳米晶膜ECL强度的19倍。这种ECL增强源于CDs能束缚大量电子产生局域电场从而促进近邻CdS纳米晶激发态的形成与弛豫。     

Phase transfer of CdS nanocrystals mediated by heptamine β-cyclodextrin

A fundamental and systematic study on the fabrication of a supramolecularly assembled nanostructure of an organic ligand-capped CdS nanocrystal (NC) and multiple heptamine β-cyclodextrin ((NH(2))(7)βCD) molecules in aqueous solution has been here reported. The functionalization process of presynthesized hydrophobic CdS NCs by means of (NH(2))(7)βCD has been extensively investigated by using different spectroscopic and structural techniques, as a function of different experimental parameters, such as the composition and the concentration of CD, the concentration of CdS NCs, the nature of the NC surface capping ligand (oleic acid and octylamine), and the organic solvent. The formation of a complex based on the direct coordination of the (NH(2))(7)βCD amine groups at the NC surface has been demonstrated and found responsible for the CdS NC phase transfer process. The amine functional group in (NH(2))(7)βCD and the appropriate combination of pristine capping agent coordinating the NC surface and a suitable solvent have been found decisive for the success of the CdS NC phase transfer process. Furthermore, a layer-by-layer assembly experiment has indicated that the obtained (NH(2))(7)βCD functionalized CdS NCs are still able to perform the host-guest chemistry. Thus, they offer a model of a nanoparticle-based material with molecular receptors, useful for bio applications.     

Synthesis of nanocrystalline CdS from cadmium(II) complex of S-benzyl dithiocarbazate as a precursor

The single X-ray crystal structure of the cadmium(II)–S-benzyl dithiocarbazate (SBDTC) complex, [Cd(SBDTC)Cl₂]₂, is reported. The compound has been found to be an effective single-source precursor for the preparation of CdS nanocrystals (NCs) via solvothermal method. CdS NCs including spheres and rods were prepared at a relatively low temperature by thermolysis of the precursor using chelating solvent like ethylene glycol (EG), ethylenediamine (EN), hydrazine hydrate (HH) or in a mixture of EG and EN. The influence of solvent, temperature and reaction time was investigated on the size and morphology of the NCs. Use of EG afforded spherical CdS NCs while EN uniquely yielded rod-shaped NCs, and mixture of spheres and rods are obtained from the mixture of EN and EG with a ratio 0.2 (v/v: EN/EG). UV–visible spectroscopy established pronounced quantum confinement with enhanced band gap and XRD analyses revealed hexagonal crystal phase for so obtained CdS NCs. The NCs were also characterized by transmission electron microscopy (TEM), photoluminescence spectroscopy (PL), energy-dispersive X-ray spectroscopy (EDS) and FTIR. The possible formation mechanism for the anisotropic growth of NCs was also discussed.     

Chemical synthesis and optical properties of CdS–poly(lactic acid) nanocomposites and their transparent fluorescent films

This paper describes the first synthesis of cadmium sulfide (CdS)-poly(lactic acid) (PLA) nanocomposites and their transparent fluorescent films by covalently grafting PLA onto the surfaces of CdS nanocrystals (NCs). Synthesis of the nanocomposites involved two steps. Lactic acid (LA)-capped CdS NCs were first prepared by reacting cadmium chloride (CdCl₂) with sodium sulfide (Na₂S) using LA as the organic ligand in H₂O/N,N-dimethylformamide (DMF) solution. CdS–PLA nanocomposites were then formed by in situ ring-opening polymerization of lactide on the surface of modified CdS NCs. We also demonstrated herein the fabrication of the transparent fluorescent films of CdS–PLA nanocomposites by blending as-prepared nanocomposites with high-molecular-weight PLA. The as-prepared CdS NCs and their nanocomposites were studied by transmission electron microscopic imaging, thermogravimetric analyses, and spectroscopic measurements (ultraviolet–visible absorption and photoluminescence). The results revealed that the CdS–polymer nanocomposites exhibited good optical properties in terms of their photoluminescence and transparency.     

Improving the Power-Conversion Efficiency through Alloying in Common Anion CdZnX (X=S,Se) Nanocrystal Sensitized Solar Cells

In this paper, we have investigated the possibility of utilizing CdZnS and CdZnSe alloy nanocrystals (NCs) as sensitizers in quantum-dot solar cells (QDSCs). The alloy NCs were synthesized by a high-temperature hot injection method and subsequently characterized through high photoluminescence quantum yield, along with larger size compared to binary NCs. Femtosecond transient absorption measurements revealed long-lived charge carriers in the alloy structure due to more structural rigidity and less defect states. Finally, the solar-cell efficiencies of the CdZnS (CdZnSe) NCs were found to be 3.05 % (3.69 %) as compared to 1.23 % (3.12 %) efficiencies for CdS (CdSe) NCs. Thus, common anion ternary NCs have been successfully utilized for solar-cell assembly and can be helpful for constructing tandem solar cells to harvest the high-energy portion of solar radiation.     

Alpha-cyclodextrin functionalized CdS nanocrystals for fabrication of 2/3 D assemblies

Different types of cyclodextrins (CDs) have been tested as mediators for the water phase transfer of organic-capped CdS nanocrystals (NCs), and alphaCD has been demonstrated to be the most effective system. The formation of a complex based on alphaCDs and colloidal NCs has been considered to be responsible for the phase transfer process and extensively investigated by optical, structural, and calorimetric measurements, as a function of the experimental parameters (pH and NC and CD concentration). A mechanism for the complexation phenomena has been suggested. The fabrication of 2/3 D supramolecular architectures has been proposed according to two different strategies. First, a layer-by-layer procedure has been used to obtain multilayered structures where polyelectrolyte layers have been intercalated with negatively charged alphaCD-CdS NC complexes by exploiting electrostatic interaction between polyelectrolyte and cyclodextrin OH groups. Second, a monolayer of CdS NCs has been deposited onto a self-assembled monolayer of sulfated CDs, thus combining the use of an electrostatic-force-based approach and host-guest chemistry. The important role played by host-guest interactions has then been revealed.     

Tuning the excitonic and plasmonic properties of copper chalcogenide nanocrystals

The optical properties of stoichiometric copper chalcogenide nanocrystals (NCs) are characterized by strong interband transitions in the blue part of the spectral range and a weaker absorption onset up to ~1000 nm, with negligible absorption in the near-infrared (NIR). Oxygen exposure leads to a gradual transformation of stoichiometric copper chalcogenide NCs (namely, Cu(2-x)S and Cu(2-x)Se, x = 0) into their nonstoichiometric counterparts (Cu(2-x)S and Cu(2-x)Se, x > 0), entailing the appearance and evolution of an intense localized surface plasmon (LSP) band in the NIR. We also show that well-defined copper telluride NCs (Cu(2-x)Te, x > 0) display a NIR LSP, in analogy to nonstoichiometric copper sulfide and selenide NCs. The LSP band in copper chalcogenide NCs can be tuned by actively controlling their degree of copper deficiency via oxidation and reduction experiments. We show that this controlled LSP tuning affects the excitonic transitions in the NCs, resulting in photoluminescence (PL) quenching upon oxidation and PL recovery upon subsequent reduction. Time-resolved PL spectroscopy reveals a decrease in exciton lifetime correlated to the PL quenching upon LSP evolution. Finally, we report on the dynamics of LSPs in nonstoichiometric copper chalcogenide NCs. Through pump-probe experiments, we determined the time constants for carrier-phonon scattering involved in LSP cooling. Our results demonstrate that copper chalcogenide NCs offer the unique property of holding excitons and highly tunable LSPs on demand, and hence they are envisaged as a unique platform for the evaluation of exciton/LSP interactions.     

Synthesis and properties of transparent luminescent nanocomposites with surface functionalized semiconductor nanocrystals

5-Amino-1,10-phenanthroline (Aphen) was used as an organic ligand to functionalize CdS nanocrystals (NCs) by a ligand-exchange process. The functional Aphen-CdS NCs have strong luminescent emission at 552 nm and good dispersibility in the polar organic monomers. The Aphen-CdS NCs were dispersed in polymeric monomers to prepare a series of transparent luminescent nanocomposites with excellent thermal stability via in-situ bulk polymerization. The fluorescent properties of the Aphen-CdS NCs were well retained in the polymer matrix. It was found that when the methacrylic acid (MAA) and glycidyl methacrylate (GMA) as the comonomers were introduced into the polymer matrix, the emission peaks of the resultant nanocomposites had a blue shift and the fluorescent intensities also increased due to the interaction between NCs and the polymer matrices. The transparent NCs/polymer nanocomposites with tunable fluorescent emission can be potentially used for the fabrication of optoelectronic devices.     

Controllable fabrication of nanocrystal-polymer hybrids via the catalytic chain transfer polymerization process

A facile catalytic chain transfer polymerization (CCTP) technique has been developed to synthesize covalently linked CdS nanocrystal-polymer hybrids with good optical properties. The in situ polymerization of methyl methacrylate (MMA) on the surface of modified CdS nanocrystals (NCs) with diameter of 5 nm via CCTP process yielded CdS-polymethylmethacrylate (PMMA) hybrid nanocomposites; while the incorporation of hydroxyl-coated CdS NCs into poly(methacryloxypropyltrimethoxysilane) (PMPS)-co-PMMA matrices prepared by CCTP afforded CdS-PMPS-co-PMMA hybrid nanocomposites, which were further cross-linked by free radical polymerization to form CdS NC-polymer network. The spectroscopic studies indicate that as-prepared CdS NC-polymer hybrids show good photoluminescence (PL) and the NC-polymer network exhibits highly enhanced PL property with respect to that before cross-linking. Also described are the probable mechanism for the catalytic chain transfer polymerization on the surface of modified nanocrystal and the measurement of chain transfer constants.     

改进溶胶凝胶过程制备CdS/SiO2量子点玻璃

The sol-gel process has been applied successfully to the preparation of small-sized CdS-doped silica glasses. Silica glasses containing 0.887.4wt% CdS were synthesized in a process in which ethylene diamine tetra-acetic acid (EDTA) or surfactant microenmulsion is added to obtain smaller size CdS crystal with narrower size distribution. These additives can be removed after the heat treatment during the densification procedure. The optical absorption edge of CdS crystals in doped glasses obtained by new methods exhibited larger blue shifts(comparing with the bulk absorption edge value of CdS crystal) than that without addition of EDTA or surfactant microemulsion in the preparing process. Thus the quantum size effect was found to be enhanced for glasses containing CdS prepared by the improved sol-gel process.     

Photophysical properties of spherical aggregations of CdS nanocrystals capped with a chromophoric surface agent

A novel sulfur-terminal Cd(II) complex, CdS(2)L (L = N-hexyl-3-{2-[4-(2,2':6',2'-terpyridin-4'-yl)phenyl]ethenyl}-carbazole), was successfully synthesized from CdS nanocrystals and the organic chromophores (L), which was confirmed by single-crystal X-ray diffraction analysis. Its photophysical properties have been investigated both experimentally and theoretically. The novel hybrid nanoparticles (CdS/L) were then obtained using the L as surface capped agent, which aggregate into large spheres, exhibiting novel luminescent properties, strong two photon absorption (TPA) and obvious prolonged fluorescence lifetime, which differ from those of the pure CdS nanocrystals and free L.     

Intraband spectroscopy and band offsets of colloidal II-VI core/shell structures

The interband and intraband spectra of colloidal II-VI CdS and CdSe quantum dot cores and CdSZnSe, CdSCdSe, CdSeCdS, and CdSeZnSe core/shell systems are reported. Infrared absorption peaks between 0.5 and 0.2 eV are observed. The slope of the intraband energy versus the first interband absorption feature is characteristic of the relative band alignments of the materials constituting the core and the shell and it is analyzed within an effective mass model. The analysis provides a new estimate of the band gap of zinc blende CdSe as well as the band offsets in zinc blende and wurtzite CdSe, CdS, and ZnSe.     

Polyelectrolyte multilayers as a platform for luminescent nanocrystal patterned assemblies

The fabrication of uniform and patterned nanocrystal (NC) assemblies has been investigated by exploiting the possibility of carefully tailoring colloidal NC surface chemistry and the ability of polyelectrolyte (PE) to functionalize substrates through an electrostatic layer-by-layer (LbL) strategy. Appropriate deposition conditions, substrate functionalization, and post-preparative treatments were selected to tailor the substrate surface chemistry to effectively direct the homogeneous electrostatic-induced assembly of NCs. Water-dispersible luminescent NCs, namely, (CdSe)ZnS and CdS, were differently functionalized by (1) ligand-exchange reaction, (2) growth of a hydrophilic silica shell, and (3) formation of a hydrophilic inclusion complex, thus providing functional NCs stable in a defined pH range. The electrostatically charged functional NCs represent a comprehensive selection of examples of surface-functionalized NCs, which enables the systematic investigation of experimental parameters in NC assembly processes carried out by combining LbL procedures with microcontact printing and also exploiting NC emission, relevant for potential applications, as a prompt and effective probe for evaluating assembly quality. Thus, an ample showcase of combinations has been investigated, and the spectroscopic and morphological features of the resulting NC-based structures have been discussed.     

Analysis of the local atomic structure of quantum dots of the CdS family

The microwave synthesis of quantum dots based on CdS has been performed, the temperature (T 180°C and 150°C) and synthesis duration (10 min and 5 min) have been varied. The analysis of the peak broadening in X-ray diffraction has shown that the average particle size in the synthesized samples is 10.02 nm for the CdS _{T = 180°C} sample and 5.22 nm for the CdS _{T = 150°C} sample. For both synthesis temperatures particles of sphalerite phase are formed but the sample CdS _{T = 180°C} contained some impurity of wurtzite phase too. CdK-XANES spectra in the standard samples and quantum dots have been recorded using a Rigaku R-XAS X-ray absorption laboratory spectrometer. The theoretical analysis of the CdK-XANES spectra of bulk samples of CdS and CdS nanoparticles has been performed. It has been shown that the theoretical difference spectra between bulk CdS and CdS with decreased lattice parameters demonstrate the same tendency as the experimental difference spectra between bulk CdS and the quantum dot samples under study. It has been shown that the theoretical CdK-edge HERFD-XANES spectrum in CdS demonstrates considerably more detailed structure pointed to the need of the analysis of experimental CdK-edge HERFDXANES spectra to pick out more precise information on local atomic structure parameters of small semiconducting quantum dots.     

溶胶凝胶法制备CdS/SiO2量子点玻璃的研究

通过溶胶凝胶转变过程制备了硫化镉／二氧化硅量子点复合玻璃材料（硫化镉在复合材料中的最大掺杂质量比可达到30％）；并针对两种催化剂对复合体系的不同影响对这一类体系的物理化学性质进行了较为系统的研究．复合体系的溶胶凝胶转变点可通过对复合溶胶粘度随时间变化的观测而确定．TEM法与低频Raman散射法的测定表明实验制备的样品中CdS颗粒基本为nm级的球形粒子。其CdS颗粒粒径随CdS含量增加而增大。随CdS／SiO2量子点玻璃材料中CdS颗粒粒径的减小，其吸收光谱中的吸收边界与常规尺寸颗粒的吸收光谱边界相比有明显的蓝移，体现出显著的量子效应．适宜的热处理过程对复合材料中CdS颗粒粒径的减小和量子效应的增强起着有利的作用．     

Greatly enhanced electrochemiluminescence of CdS nanocrystals upon heating in the presence of ammonia

CdS nanocrystals (NCs) usually exhibit very weak electrochemiluminescence (ECL) emission. It is showed that when CdS NCs were treated by heating in the presence of ammonia (heated-CdS–NH₃), greatly enhanced ECL was observed. The ECL of the heated-CdS–NH₃ modified glassy carbon electrode (heated-CdS–NH₃/GCE) in phosphate buffer solution (pH 7.0) containing 0.1 M K₂S₂O₈ was ca. 310 times higher than that of CdS/GCE. The treatment caused the changes in the morphology and surface electronic structure of CdS NCs, which facilitated the reduction process of CdS, consequently improved the quantity of the excited states (CdS*), leading to enormous enhancement in ECL.     

Ultrasound-Induced Formation of CdS Nanostructures in Oil-in-Water Microemulsions

In this study the formation of approximately spherical assemblies of nanocrystalline primary particles of CdS in a CS(2)-water-ethylenediamine (CWE) microemulsion induced by ultrasonic irradiation is described. CS(2) was employed as the sulfur source for CdS and also as the oil phase in the microemulsion. The particles were studied with X-ray powder diffraction, transmission electron microscopy, UV/Vis absorption spectroscopy, and photoluminescence spectroscopy. A blue shift compared to bulk CdS was observed in absorption and photoluminescence spectra. Copyright 2001 Academic Press.