Exciton radiative lifetime in CdSe quantum dots

Colloidal CdSe quantum dots(QDs)are promising materials for solar cells because of their simple preparation pro-cess and compatibility with flexible substrates.The QD radiative recombination lifetime has attracted enormous attention as it affects the probability of photogenerated charges leaving the QDs and being collected at the battery electrodes.However,the scaling law for the exciton radiative lifetime in CdSe QDs is still a puzzle.This article presents a novel explanation that recon-ciles this controversy.Our calculations agree with the experimental measurements of all three divergent trends in a broadened energy window.Further,we proved that the exciton radiative lifetime is a consequence of the thermal average of decays for all thermally accessible exciton states.Each of the contradictory size-dependent patterns reflects this trend in a specific size range.As the optical band gap increases,the radiative lifetime decreases in larger QDs,increases in smaller QDs,and is weakly depend-ent on size in the intermediate energy region.This study addresses the inconsistencies in the scaling law of the exciton life-time and gives a unified interpretation over a widened framework.Moreover,it provides valuable guidance for carrier separa-tion in the thin film solar cell of CdSe QDs.

Photocatalytic degradation of Brilliant Green dye using CdSe quantum dots hybridized with graphene oxide under sunlight irradiation

CdSe quantum dots(QDs)hybridized with graphene oxide(GO)are synthesized by a facile chemical precipitation method.The absorption of the CdSe/GO nanocomposite is increased with a significantblue shift with respect to CdSe QDs.The specific surface area of the CdSe/GO nanocomposite is10.4m2/g,which is higher than that of CdSe QDs(5m2/g).The PL intensity of the CdSe/GO nanocomposite is lower than that of the CdSe QDs owing to the inhibition of the recombination of electron‐hole pairs in the composite.In Raman analysis,the two bands of the CdSe/GO nanocomposite are shifted to higher wavenumbers with respect to graphene oxide,which is attributed to electron injection that is induced by CdSe QDs into graphene oxide.Using the Brilliant Green dye,the photocatalytic reduction efficiency of CdSe QDs and the CdSe/GO nanocomposite under sunlight irradiation for90min are approximately81.9%and95.5%,respectively.The calculated photodegradation rate constants for CdSe QDs and the CdSe/GO nanocomposite are0.0190min–1and0.0345min–1,respectively.The enhanced photocatalytic activity of the CdSe/GO nanocomposite can be attributed to the high specific surface area and the reduction of electron‐hole pair recombination because of the introduction of graphene oxide.

Room-Temperature Organic Negative Differential Resistance Device Using CdSe Quantum Dots as the ITO Modification Layer

Room-temperature negative differential resistance （NDR） has been observed in different types of organic materials. However, detailed study on the influence of the organic material on NDR performance is still scarce. In this work, room-temperature NDR ＆ observed when CdSe quantum dot （QD） modified ITO is used as the electrode. Furthermore, material dependence of the NDR performance is observed by selecting materials with different charge transporting properties as the active layer, respectively. A peak-to-valley current ratio up to 9 is observed. It is demonstrated that the injection barrier between ITO and the organic active layer plays a decisive role for the device NDR performance. The influence of the aggregation state of CdSe QDs on the NDR performance is also studied, which indicates that the NDR is caused by the resonant tunneling process in the ITO/CdSe QD/organic active layer structure.

Synthesis of CdSe quantum dots

Water-soluble CdSe quantum dots(QDs) were synthesized in aqueous solution with thioglycollic acid as stabilizer.CdSe QDswere characterized using Transmission electron microscopy(TEM),UV-Vis absorption spectrum(UV-Vis) and fluorescence spectra(FL).The characterization results indicate that as-synthesized CdSe QD was uniform at about 3.5 nm.

Graphene quantum dots assisted photovoltage and efficiency enhancement in CdSe quantum dot sensitized solar cells

CdSe quantum dot sensitized solar cells （QDSCs） modified with graphene quantum dots （GQDs） have been successfully achieved in this work for the first time. Satisfactorily, the optimized photovoltage （Voc） of the modified QDSCs was approximately 0.04 V higher than that of plain CdSe QDSCs, consequently improving the photovoltaic performance of the resulting QDSCs. Served as a novel coating on the CdSe QD sensitized photoanode, GQDs played a vital role in improving Voc due to the suppressed charge recombination which has been confirmed by electron impedance spectroscopy as well as transient photovoltage decay measure- ments. Moreover, different adsorption sequences, concentration and deposition time of GQDs have also been systematically investigated to boost the power conversion efficiency （PCE） of CdSe QDSCs. After the coating of CdSe with GQDs, the resulting champion CdSe QDSCs exhibited an improved PCE of 6.59% under AM 1.5G full one sun illumination.

Study on the Interaction between CdSe Quantum Dots and Bovine Serum Albumin with Ultraviolet Visible Absorption Spectroscopy

The interaction of CdSe quantum dots （QDs） with bovine serum albumin （BSA） has been investigated with ultraviolet visible absorption spectroscopy （UVAS）. It was found that the absorption intensity of CdSe QDs significantly decreased after adding BSA solution, showing that CdSe QDs were bonded to BSA. The binding molar ratio was 1：1 and the binding constant was 9.7 × 10^6 L mol^-1.

Preparation and Optical Properties Assessment of CdSe Quantum Dots

CdSe quantum dots (QDs) (2 - 3 nm) were synthesized by chemical precipitation method. Optical and structure properties of the products were investigated by scanning tunneling microscope (STM), X-ray diffraction (XRD), and ultra violet-visible (UV-Vis) spectrophotometer. The results show that high-quality cubic CdSe QDs were obtained. It is also obtained that temperature is one of the most important factors the affect on the particle size and optical properties of the prepared QDs samples.

Ultrafast Carrier Dynamics in CdSe/CdS/ZnS Quantum Dots

The intra- and inter-band relaxation dynamics of CdSe/CdS/ZnS core/shell/shell quantum dots are investigated with the aid of time-resolved nonlinear transmission spectra which are obtained using femtosecond pump-probe technique. By selectively exciting the core and shell carrier, the dynamics are studied in detail. Carrier relaxation is found faster in the conduction band of the CdS shell （about 130 fs） than that in the conduction band of the CdSe core （about 400 fs）. From the experiments it is distinctly demonstrated the existence of the defect states in the interface between the CdSe core and the CdS shell, indicating that ultrafast spectroscopy might be a suitable tool in studying interface and surface morphology properties in nanosystems.

Ultrathin zinc selenide nanosheet-based intercalation hybrid coupled with CdSe quantum dots showing enhanced photocatalytic CO_(2) reduction

Fabrication of well-designed heterojunctions is an extraordinarily attractive pathway for boosting the photocatalytic activity toward CO_(2) photoreduction.Herein,a novel kind of na nosheet-based intercalation hybrid coupled with CdSe quantum dots(QDs) was successfully fabricated by a facile solvothermal method and served as photocatalyst for full-spectrum-light-driven CO_(2) reduction.Ultra-small CdSe QDs were rationally in-situ introduced and coupled with lamellar ZnSe-intercalation hybrid nanosheet,resulting in the formation of CdSe Q.Ds/ZnSe hybrid heterojunction.Significantly,the concentration of Cd^(2+) could change directly the crystallinity and micromorphology of ZnSe intercalation hybrid,which in turn would impact on the photocatalysis activity.The optimized CdSe QDs/ZnSe hybrid-5 composite demonstrated a considerable CO yield rate of the 25.6 μmol g^(-1) h^(-1) without any additional cocatalysts or sacrificial agents assisting,making it one of the best reported performance toward CO_(2) photoreduction under full-spectrum light.The elevated CO_(2) photoreduction activity could be attributed to the special surface heterojunction,leading to improving the ability of light absorption and promoting the separation/transfer of photogenerated carriers.This present study developed a new strategy for designing inorganic-organic heterojunctions with enhanced photocatalyst for CO_(2) photoreduction and provided an available way to simultaneously mitigate the greenhouse effect and alleviate energy shortage pressure.

The effect of differential temperatures on the latent heat in the nucleation of CdSe quantum dots

This work studied the effect of differential temperatures on the latent heat in the nucleation of CdSe quantum dots（QDs）.The result showed that,by the formula of phase change,with increasing the reaction temperature,the latent heat in the nucleation of QDs reduced.CdSe QDs with the size-dispersion from 2.7 to 3.6 nm were synthesized via oleic acid-paraffin liquid system by controlling the reaction temperature from 180 to 220℃.Synthesized QDs were characterized by UV-vis absorption spectra and X-ray diffraction（XRD）.The result of UV-vis absorption spectra showed that with increasing of reaction temperature,the first absorption peak was red-shifted and the size of QD increased.The result of XRD showed that the synthesized QDs were zinc-blende structure.

Roles of glutathione and L-cysteine in the biomimetic green synthesis of CdSe quantum dots

Biological synthesis of quantum dots （QDs） as an environmental-friendly and facile preparation method has attracted increasing interests. However, it is difficult to distinguish the roles ofbio-thiols in QDs synthesis process because of the complex nature in organisms. In this work, the CdSe QDs synthesis conditions in organisms were reconstructed by using a simplified in vitro approach to uncover the roles of two small bio-thiols in the QDs formation. CdSe QDs were synthesized with glutathione （GSH） and L-cysteine （Cys） respectively. Compared with Cys at the same molar concentration, the CdSe QDs synthesized by GSH had a larger and broader particle size distribution with improved optical properties and crystal structure. Furthermore, quantum chemical calculations indicate that the stronger Cd^2＋ binding capacity ofGSH contributed a lot to the CdSe QDs formation despite ofthe greater capability Cys for selenite reduction. This work clearly demonstrates the different roles of small thiols in the Cd2^＋- stabilization in the environment and biomimetic QDs synthesis process.

Optical properties and dynamic process in metal ions doped on CdSe quantum dots sensitized solar cells

In recent years, the nanostructure for solar cells have attracted considerable attention from scientists as a result of a promising candidate for low cost devices. In this work, quantum dots sensitized solar cells with effective performance based on a co-sensitized Cd S∕Cd Se：Mn2＋（or Cu2＋） nanocrystal, which was made by successive ionic layer absorption and reaction, are discussed. The optical, physical, chemical, and photovoltaic properties of quantum dots sensitized solar cells were sensitized to Mn2＋and Cu2＋dopants. Therefore, the short current（JSC）of the quantum dot sensitized solar cells is boosted dramatically from 12.351 mA∕cm2 for pure Cd Se nanoparticles to 18.990 mA∕cm2 for Mn2＋ions and 19.915 mA∕cm2 for Cu2＋ions. Actually, metal dopant extended the band gap of pure Cd Se nanoparticles, reduced recombination, enhanced the efficiency of devices, and improved the charge transfer and collection. In addition, Mn2＋and Cu2＋dopants rose to the level of the conduction band of pure Cd Se nanoparticles, which leads to the reduction of the charge recombination, enhances the lightharvesting efficiency, and improves the charge diffusion and collection. The results also were confirmed by the obtained experimental data of photoluminescence decay and electrochemical impedance spectroscopy.

Optimization of CdSe Quantum Dots Sensitized TiO_2 Photo Anodes in Fabrication of Quantum Dots Sensitized Solar Cells

As one of the most promising candidates for the third generation solar cells,quantum dots sensitized solar cells(QDSCs) have been comprehensively studied.In this work,we synthesize the CdSe QDs with the absorption range from 450-550 nm,which are suitable to be applied in the QDSCs.Then,we found that the self-assembly(SA) deposition method is superior to the successive ionic layer adsorption and reaction(SILAR) deposition method in the fabrication of the photo anodes.Furthermore,the influence of TiO_2's thickness of the photo anodes to the QDSCs' efficiency has been studied.With the optimized CdSe QDs sensitized photo anodes,the efficiency of the QDSCs can reach 3.38%in this work.

Living cell synthesis of CdSe quantum dots： Manipulation based on the transformation mechanism of intracellular Se-precursors

Currently, the biosynthesis of nanomaterials by organisms is attracting considerable attention because of the sustainable and environmentally friendly nature of the reactions involved in this process compared with those in the conventional nanomaterial synthesis. However, the manipulation and control of nanomaterial biosynthesis remain difficult because of the lack of knowledge about the biosynthetic mechanisms. In the present study, we elucidated the selenium （Se）-precursor and Se metabolic flux in the biosynthesis of cadmium-selenium quantum dots （CdSe QDs） in Saccharomyces cerevisiae and improved the cells＇ ability to biosynthesize CdSe QDs through gene modification based on the regulation mechanism. By deleting the genes involved in Se metabolism and measuring seleno-amino acids, we identified selenocysteine （SeCys） as the primary Se-precursor in the intracellular biosynthesis of CdSe QDs. Further studies demonstrated that the selenomethionine （SeMet）-to-SeCys pathway regulates CdSe QD biosynthesis. Knowledge of the regulatory pathway allowed us to enhance SeMet synthesis by overexpression of the MET6 gene, and an increased CdSe QD yield was realized in the engineered cells. Understanding the mechanism of CdSe QD biosynthesis helped to determine the relationship between nanocrystal formation and biological processes, and offers a new perspective to manipulation of nanomaterial biosynthesis.

Biomimetic Synthesis of CdSe Quantum Dots through Emulsion Liquid Membrane System of Gas-Liquid Transport

The cadmium selenide quantum dots (QD) have been synthesized by template-control in an emulsion liquid membrane system. The system consisted of kerosene as solvent, L152 (dialkylene succinimide) as surfactant, N7301 (trialiphatic amine, Ra, R=C8-C10) as carrier, 0.1 mol/L CdCl2 solution as internal-aqueous phase and H2Se gas as external phase. Additive organic template agent in internal-aqueous phase was necessary to fom CdSe QD. The influence of the nature of template and its concentration on sizes of the formed CdSe QD has also been studied. Transmission electron microscopy showed that the sizes of the products could be controlled down to 3-4 nm. X-ray diffraction analysis revealed that the crystals had cubic structure. The formation process and the optical properties of CdSe QD have also been presented.

Citrate-stabilized CdSe/CdS quantum dots as fluorescence probe for protein determination

A rapid, ultrasensitive and convenient fluorescence measurement technology based on the enhancement of the fluorescence intensity resulting from the interaction of functionalized CdSe/CdS quantum dots （QDs） with bov/ne serum albumin （BSA） was proposed. The citrate-stabilized CdSe/CdS （QDs） were synthesized by using Se powder and Na2S as precursors instead of any pyrophoric organometallic precursors. The modified CdSe/CdS QDs are brighter and more stable against photobleaching in comparison with organic fluorophores. At pH 7.0, the fluorescence signal of CdSe/CdS is enhanced by increasing the concentration of BSA in the range of 0.1-10 μg/mL, and the low detection limit is 0.06 μg/mL. A linear relationship between the enhanced fluorescence peak intensity （△F） and BSA concentration （c） is established using equation △F=50.7c＋16.4 （R=0.996 36）. Results of determination for BSA in three synthetic samples are identical with the true values, and the recovery （98.9%-102.4%） and relative standard deviation （RSD, 1.8%-2.5%） are satisfactory.

Hierarchical ZnO nanorod-on-nanosheet arrays electrodes for efficient CdSe quantum dot-sensitized solar cells

Two-dimensional(2D) ZnO nanosheet arrays were prepared via vanadium(V)-doping assisted hydrothermal method, and then the nanosheet was successfully converted to a nanorod-on-nanosheet ZnO hierarchical structure by treating with Na_2S solution and subsequent hydrothermal reaction. Hierarchical films with different nanorod growth time(1–8h) were prepared and their photovoltaic properties were also investigated after electrodeposition of CdSe quantum dots. For the hierarchical nanorod-on-nanosheet ZnO films, increasing the ZnO nanorod growth time can enormously enlarge the length of branched nanorods and light-scattering ability, resulting in better light-harvesting efficiency and higher photo-generated electron concentration, which leads to higher short-circuit current density(J_(sc)) and open-circuit voltage(V_(oc)). However,further increasing nanorod growth time to 8h leads to the over-dense coverage of nanorods, which is harmful for light-harvesting efficiency and leads to severe electron recombination, eventually diminishes the power conversion efficiency(PCE). With the optimized nanorod modification and Cu_2S counter electrode, the PCE reaches a maximum value of 4.26%, which to the best of our knowledge, is among the highest PCE record for CdSe sensitized solar cells based on ZnO photoanodes.

Preparation of Electrospun CdSe-Quantum-Dots-Doped Porous Films

Electrospun porous films doped with the green-synthesized CdSe quantum dots were synthesized. Glycerol was chosen to prepare the quantum dots （ QDs）, with the highest quantum yield of 78.28%. Polycaprolactone （PCL） was electrospun with CdSe QDs to avoid the QDs＇ toxicity and improve the QDs＇ cytocompatibility. The electrospun QDs-doped films preserve the original QDs＇ fluorescence. Pores can be detected from the SEM of the films, predicting the possibility of loading drugs in the cancer therapy. The cell proliferation assay shows excellent cytocompatibility of the eletrospun CdSe-QDs-doped films. The present eletrospun CdSe- QDs-doped porous films are cytocompatibale, highly-fluorescent and ootential to load drugs in cancer therapy.

胶体CdSe量子点敏化TiO_(2)太阳能电池中光电转换行为研究

以NaSeH_(4)水溶液、Cd^(2+)水溶液及巯基乙酸为原料采用水热法制备了水溶性CdSe量子点(QDs),并直接敏化在TiO_(2)光阳极上用于量子点敏化太阳能电池(QDSSC)。通过NaSeH_(4)的用量实现了CdSe QDs的尺寸调节和光谱响应范围调控。光伏性能分析表明尺寸约6nm的CdSe QDSSC效率达到2.26%,明显高于尺寸约4nm和10nm的CdSe QDSSC效率(分别为0.62%和1.39%)。光谱和阻抗分析表明合适的CdSe QDs尺寸能扩大光捕获效率并减少光阳极-电解液界面间的电荷复合,从而提高了光电转换效率。

Water-soluble CdSe/ZnS quantum dots passivated by poly(glycino amino acid) phosphazenes

Poly(organophosphazenes) have potential applications in making water-soluble and biocompatible quantum dots (QDs) due to their wide variety of properties. The CdSe QDs of green emission and the core-shell CdSe/ZnS QDs of red emission were prepared. Subsequently the trioctylphosphine oxide-stabilized CdSe/ZnS QDs were transferred from chloroform into water through a ligand exchange process with poly(glycino amino acid)phosphazenes, which can be obtained from the saponification of poly (glycino amino ester)phosphazenes at room temperature. The resulting QDs-polymer nanocomposite particles can form colloidally stable suspensions in water and exhibit good photostability.