Interaction of radiation-induced defects of glasses with CdSe and CdS nanocrystals

The influence of X-ray irradiation on the absorption spectra of commercial light filters based on the glasses containing selenium, cadmium, and sulfur has been investigated. The light filters were preliminary subjected to the heat treatment, which resulted in the precipitation of nanocrystals in the glass matrix. The irradiation leads to stationary partial bleaching of glasses. Analysis demonstrates that the difference spectra contain the induced absorption bands of conventional radiation-induced color centers of alkali silicate glasses and a fine structure. The fine structure closely resembles the known difference spectra obtained under the action of the external constant or low-frequency alternating electric field on glasses containing CdSe nanocrystals. The conclusion is drawn that the stable radiation-induced centers of the glass at the interface with nanocrystals serve as a source of the Coulomb field affecting the energy levels of the size quantization of the nanocrystals. It is suggested that nanocrystals of the type of the CdS_xS_1−x solid solutions in these glasses are either lacking or do not manifest themselves in the spectra after irradiation.     

Specific features of the growth of CdSe nanocrystals in fluorophosphate glasses

The choice of the composition of the fluorophosphate vitreous matrix used for optimizing the growth of CdSe nanocrystals with a narrow size distribution is justified. Investigation of the X-ray diffraction parameters has revealed that nanocrystals with sizes varying over a wide range have a hexagonal structure. The evolution of the absorption spectra of fluorophosphate glasses containing CdSe semiconductor nanocrystals is studied. It is shown that the growth mechanism of CdSe nanocrystals differs radically from the nucleation mechanism. The time dependences of the nanocrystal size exhibit an intricate three-stage behavior. The first stage is accompanied by a shift in the absorption spectrum toward the long-wavelength range. After the first stage, the time dependences are characterized by “fast” and “slow” portions. In the fast portion, the exponent of the time dependence of the nanocrystal size increases from 0.4 to 1.0 with an increase in the annealing temperature. The assumption is made that the spinodal decomposition of phases under hyperbolic diffusion occurs in the system under investigation.     

Spatially resolved photoconductivity of thin films formed by colloidal octapod-shaped CdSe/CdS nanocrystals

We studied the optical absorption and photoconductive properties of thin films consisting of core-shell octapod-shaped nanocrystals, which consisted of CdS pods that branch out from a CdSe core. The current-voltage characteristics were measured at room and cryogenic temperatures and agreed well with a phenomenological exponential fitting model, from which we could extract the sheet resistance and the average voltage barrier for the charge tunneling between the octapods. The temperature dependence of the photocurrent showed temperature activated behavior above 220 K and a non-Arrhenius exponential (T/T(0))(n) dispersion below 220 K. Furthermore, we mapped the photocurrent generation within the octapod film via scanning photocurrent microscopy, which revealed photocurrent enhancement near micron-size voids and spatial shifts of the photocurrent maxima with bias voltage.     

Morphology of silicate glasses with lead sulfide nanocrystals

The specific features in the structure of nanostructured glass-ceramic materials based on sodium zinc silicate glasses doped with lead sulfide are investigated using small-angle X-ray scattering and X-ray powder diffraction. It is established that a close-to-monodisperse size distribution of PbS nanocrystals is an important structural feature of the system under investigation. An analysis of the X-ray powder diffraction data demonstrates that the shape of the lead sulfide nanocrystals precipitated depends on the heat treatment conditions and can differ from a spherical shape.     

Photoelectrochemical cells with mixed polycrystalline n-type CdSPbS and CdSCdSe electrodes

The photoelectrochemical behaviour of n-type CdS (polycrystalline) containing small amount of PbS or CdSe in S^2? /S^2?_n redox system has been studied. Mixed polycrystalline n-type CdSPbS electrodes were prepared by electrodeposition and the n-type CdSCdSe electrodes were made by partial replacement of sulphide ions of CdS electrode with selenide ions from a solution of sodium selenosulphate. It has been observed that both the mixed chalcogenide electrodes exhibit better photoresponse than the simple CdS electrode.     

Photoelectrochemical solar cells fabricated from porous CdSe and CdS layers

Porous CdSe layers were prepared by spray pyrolysis deposition using sodium selenosulfate as a selenium source and its surface area and porosity were increased by the dissolution of sodium sulfate formed as by-product. The porous CdSe as both photoanode and absorber could efficiently transport electrons to fluorine-doped tin oxide electrode and extract holes to the electrolyte. The cells were optimized by controlling the number of spray pyrolysis deposition cycles and then etching with sodium sulfate. An efficient solar cell having a power conversion efficiency of 2.6% at 1 sun illumination (100 mW cm⁻¹) was fabricated. Further, we extend this approach to fabricate an efficient porous CdS-sensitized solar cell with power conversion efficiency greater than 1.0% at 1 sun illumination.     

A ceramic microreactor for the synthesis of water soluble CdS and CdS/ZnS nanocrystals with on-line optical characterization

In this paper, a computer controlled microreactor to synthesize water soluble CdS and CdS/ZnS nanocrystals with in situ monitoring of the reaction progress is developed. It is based on ceramic tapes and the Low-Temperature Co-fired Ceramics technology (LTCC). As well the microsystem set-up, the microreactor fluidic design has also been thoroughly optimized. The final device is based on a hydrodynamic focusing of the reagents followed by a three-dimensional micromixer. This generates monodispersed and stable CdS and core-shell CdS/ZnS nanocrystals of 4.5 and 4.2 nm, respectively, with reproducible optical properties in terms of fluorescence emission wavelengths, bandwidth, and quantum yields, which is a key requirement for their future analytical applications. The synthetic process is also controlled in real time with the integration of an optical detection system for absorbance and fluorescence measurements based on commercial miniaturized optical components. This makes possible the efficient managing of the hydrodynamic variables to obtain the desired colloidal suspension. As a result, a simple, economic, robust and portable microsystem for the well controlled synthesis of CdS and CdS/ZnS nanocrystals is presented. Moreover, the reaction takes place in aqueous medium, thus allowing the direct modular integration of this microreactor in specific analytical microsystems, which require the use of such quantum dots as labels.     

CdSe/CdS核壳型量子点的合成及性质研究

在三相体系中合成了核壳型CdSe/CdS量子点。在三相体系中,油酸和乙醇的混和物组成液相(L),油酸钠为固相(S),Cd2+溶液和乙醇组成溶液相(S)。量子点在两相的界面形成。所制备的量子点用TEM、XRD和FL进行表征。结果表明,量子点的粒径分布均匀,形貌规则,为立方型晶体。在核表面包覆上宽带隙的半导体后,其光量子产率极大提高,表明核的表面缺陷被消除。     

High catalytic performance of CuO nanocrystals with largest defects

To improve the calalytic performance of CuO, nanometer-sized CuO particles were prepared with ultrafiltration surface contact method (UMSCM). X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the crystal structure and morphology of the particles made with UMSCM, hydrothermal method, grounding method as well as a commercial one. SEM results illustrate that UMSCM prepared CuO particles mainly show the form of orthorhombic sliced crystals with the largest amounts of defects compared to CuO particles made by the other three methods. In the oxidation of cumene, CuO prepared with UMSCM also shows better catalytic performance than the other three particles, which can be ascribed to its smaller particle sizes and larger defects so as to accelerate the surface adsorption rate of O2 molecules and the diffusion of ions and atoms, thus improving the catalytic activity.     

Synthesis of Tapered CdS Nanobelts and CdSe Nanowires with Good Optical Property by Hydrogen-Assisted Thermal Evaporation

The tapered CdS nanobelts and CdSe nanowires were prepared by hydrogen-assisted thermal evaporation method. Different supersaturation leads to two different kinds of 1D nanostructures. The PL measurements recorded from the as-prepared tapered CdS nanobelts and CdSe nanowires show only a bandgap emission with relatively narrow full-width half maximum, which means that they possess good optical property. The as-synthesized high-quality tapered CdS nanobelts and CdSe nanowires may be excellent building blocks for photonic devices.     

The kinetics of the formation of CdSe nanocrystals in sodium-zinc-silica glass

The kinetics of precipitation of CdSe nanocrystals in sodium-zinc-silica glass subjected to a two-stage heat treatment at different temperatures of the primary and secondary heat treatments has been studied by small-angle X-ray scattering (SAXS). The primary low-temperature heat treatment is accompanied by the nucleation of crystallization centers, i.e., supercritical nucleation centers. During the secondary heat treatment, crystals largely grow on the formed centers. In this case, the crystals are distributed in the bulk of the glass in a certain order, which apparently results from the regular pattern of distribution of the crystallization centers formed during the primary heat treatment. It has been found that the precipitated nanocrystals have a narrow size distribution at all stages of heat treatment. After the completion of precipitation of the crystalline phase, a stable structure is formed. The crystal sizes and the volume fraction of the phase do not change within the measurement accuracy (±1–2%) for the heating times that are at least an order of magnitude longer than the duration of the precipitation stage.     

In situ growth of well-dispersed CdS nanocrystals in semiconducting polymers

A straight synthetic route to fabricate hybrid nanocomposite films of well-dispersed CdS nanocrystals (NCs) in poly[2-methoxy-5-(2''-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) is reported. A soluble cadmium complex [Cd(SBz)₂]₂·MI, obtained by incorporating a Lewis base (1-methylimidazole, MI) on the cadmium bis(benzyl)thiol, is used as starting reagent in an in situ thermolytic process. CdS NCs with spherical shape nucleate and grow well below 200°C in a relatively short time (30 min). Photoluminescence spectroscopy measurements performed on CdS/MEH-PPV nanocomposites show that CdS photoluminescence peaks are totally quenched inside MEH-PPV, if compared to CdS/PMMA nanocomposites, as expected due to overlapping of the polymer absorption and CdS emission spectra. The CdS NCs are well-dispersed in size and homogeneously distributed within MEH-PPV matrix as proved by transmission electron microscopy. Nanocomposites with different precursor/polymer weight ratios were prepared in the range from 1:4 to 4:1. Highly dense materials, without NCs clustering, were obtained for a weight/weight ratio of 2:3 between precursor and polymer, making these nanocomposites particularly suitable for optoelectronic and solar energy conversion applications.     

Shape and phase control of CdS nanocrystals using cationic surfactant in noninjection synthesis

Monodispersed CdS nanocrystals with controllable shape and phase have been successfully synthesized in this study by adding cationic surfactant in noninjection synthesis system. With the increase of the amount of cetyltrimethylammonium chloride (CTAC) added, the shape of the CdS nanocrystals changed from spherical to multi-armed, and the phase changed from zinc-blende to wurtzite. It was found that halide ion Cl- plays a key role in the transformation, and other halide ions such as Br- can also induce similar transformation. We proposed that the strong binding between Cd2+ and halide ions reduced the reactivity of the precursors, decreased the nuclei formed in the nucleation stage, and led to the high concentration of precursor in the growth stage, resulting in the increase of size and phase transformation of CdS nanocrystals. In addition, it was found that the multi-armed CdS nanocrystals lost quantum confinement effect because of the increase of the size with the increase of the concentration of CTAC.     

Ways of eliminating defects in borosilicate glasses

Translated from Steklo i Keramika, No. 5, pp. 26–27, May, 1989.     

Facile synthesis of monodisperse ZnS capped CdS nanocrystals exhibiting efficient blue emission

A new method for the capping of colloidal CdS nanocrystals with ZnS shells is presented. A combination of the monomolecular precursor zinc ethylxanthate (Zn(ex)₂) and zinc stearate was used to replace hazardous organometallic reagents usually applied in this procedure, i.e. bis(trimethylsilyl) sulfide and diethylzinc. Its simple preparation, air-stability and low decomposition temperature of 150 °C make Zn(ex)₂ a very suitable source for the ZnS shell growth. With this precursor, highly luminescent CdS/ZnS core/shell nanocrystals (Q.Y. 35–45%), exhibiting narrow emission linewidths of 15–18 nm (FWHM) in the blue spectral region, can reproducibly be obtained.     

Synthesis and characterization of CdSe nanocrystals in the presence of butylamine as a capping agent

TOPO-capped cadmium selenide (CdSe) nanocrystals of sizes between 3 and 8 nm have been synthesized, and the surface-capping molecule, trioctylphosphine oxide, was replaced by butylamine. The effects of changing the surface ligands of the synthesized CdSe nanocrystals on the structural, optical, and electrical properties were investigated. The shift toward shorter wavelength (higher energy) in the visible range of the optical absorption band edge was observed by UV-Vis spectroscopy, and a blue-shift of the photoluminescence peaks was observed with luminescent quenching. Surface modification was found to cause an increase in the surface energy of nanocrystals, resulting in the improvement in charge carrier separation and cell performance in applications towards bulk hetero-junction solar cells.     

Performances of some low-cost counter electrode materials in CdS and CdSe quantum dot-sensitized solar cells

Different counter electrode (CE) materials based on carbon and Cu₂S were prepared for the application in CdS and CdSe quantum dot-sensitized solar cells (QDSSCs). The CEs were prepared using low-cost and facile methods. Platinum was used as the reference CE material to compare the performances of the other materials. While carbon-based materials produced the best solar cell performance in CdS QDSSCs, platinum and Cu₂S were superior in CdSe QDSSCs. Different CE materials have different performance in the two types of QDSSCs employed due to the different type of sensitizers and composition of polysulfide electrolytes used. The poor performance of QDSSCs with some CE materials is largely due to the lower photocurrent density and open-circuit voltage. The electrochemical impedance spectroscopy performed on the cells showed that the poor-performing QDSSCs had higher charge-transfer resistances and CPE values at their CE/electrolyte interfaces.     

Facile solution growth of vertically aligned ZnO nanorods sensitized with aqueous CdS and CdSe quantum dots for photovoltaic applications

Vertically aligned single crystalline ZnO nanorod arrays, approximately 3 μm in length and 50-450 nm in diameter are grown by a simple solution approach on a Zn foil substrate. CdS and CdSe colloidal quantum dots are assembled onto ZnO nanorods array using water-soluble nanocrystals capped as-synthesized with a short-chain bifuncional linker thioglycolic acid. The solar cells co-sensitized with both CdS and CdSe quantum dots demonstrate superior efficiency compared with the cells using only one type of quantum dots. A thin Al2O3 layer deposited prior to quantum dot anchoring successfully acts as a barrier inhibiting electron recombination at the Zn/ZnO/electrolyte interface, resulting in power conversion efficiency of approximately 1% with an improved fill factor of 0.55. The in situ growth of ZnO nanorod arrays in a solution containing CdSe quantum dots provides better contact between two materials resulting in enhanced open circuit voltage.     

One-Pot Synthesis of Biocompatible CdSe/CdS Quantum Dots and Their Applications as Fluorescent Biological Labels

We developed a novel one-pot polyol approach for the synthesis of biocompatible CdSe quantum dots (QDs) using poly(acrylic acid) (PAA) as a capping ligand at 240°C. The morphological and structural characterization confirmed the formation of biocompatible and monodisperse CdSe QDs with several nanometers in size. The encapsulation of CdS thin layers on the surface of CdSe QDs (CdSe/CdS core–shell QDs) was used for passivating the defect emission (650 nm) and enhancing the fluorescent quantum yields up to 30% of band-to-band emission (530–600 nm). Moreover, the PL emission peak of CdSe/CdS core–shell QDs could be tuned from 530 to 600 nm by the size of CdSe core. The as-prepared CdSe/CdS core–shell QDs with small size, well water solubility, good monodispersity, and bright PL emission showed high performance as fluorescent cell labels in vitro. The viability of QDs-labeled 293T cells was evaluated using a 3-(4,5-dimethylthiazol)-2-diphenyltertrazolium bromide (MTT) assay. The results showed the satisfactory (>80%) biocompatibility of as-synthesized PAA-capped QDs at the Cd concentration of 15 μg/ml.     

硅壳包裹核/壳型CdSe/CdS纳米晶的制备与性能研究

采用低温水热技术,分别以柠檬酸、聚乙二醇(PEG400)和甲硫氨酸为稳定剂,在水相中合成了核壳型CdSe/CdS量子点,研究了稳定剂、CdSe与CdS物质的量比对量子点发光性能和结构的影响。XRD结果表明,当CdSe∶CdS在1∶3~4时,CdS主要在CdSe的外延生长,形成核壳型纳米粒子,当比例达到1∶5时,CdS单独成晶现象严重。CdSe∶CdS=1∶4时,核壳型量子点具有较高的荧光发射效率。TEM研究表明CdS在CdSe外表面生长形成较为完整的壳层,有效钝化CdSe表面,减少表面缺陷,从而显著提高CdSe量子点的发光效率。CdSe核尺寸为2~3nm的核壳型纳米粒子外包裹一层SiO2壳后,荧光发射效率没有显著提高,发射峰位置无明显红移。量子点包壳后能有效提高该量子点的光化学稳定性,提高量子点的生物相容性。