As-synthesized Quantum Dots Research Articles

Ion-Intercalation Assisted Solvothermal Synthesis and Optical Characterization of MoS2 Quantum Dots

Many of the previously reported preparation methods for MoS2 quantum dots (QDs) are limited by production rate, time consumption, tedious processes or the final quality of the as-prepared QDs. Therefore, a simple and productive method for large-scale production of high-quality MoS2 QDs is still a challenge. We report a facile, low cost and environmentally friendly ion-intercalation assisted solvothermal route for the preparation of MoS2 QDs. In the reported method, NaOH is used as the Na+ ion source to intercalate and exfoliate the commercial MoS2 powder into nanosheets and then QDs. The reaction is carried out at a certain temperature in a Teflon-lined autoclave reactor. The UV-Vis absorption spectra of the as synthesized QDs show a peak in the near UV region (λ < 300 nm) instead of the characteristic peaks for the nanosheets. Characterization by X-ray diffraction, atomic force microscopy and photoluminescence spectroscopy also confirmed that the as-synthesized QDs had a uniform size distribution in the range of a few nanometers with mostly a monolayer structure and showed good photoluminescence (PL) properties. The proposed method has much potential for further enhancing the yield of MoS2 QDs by taking advantage of the nature of solution-based processes.

Ligand Exchange Functionalization of CIS Quantum Dots for CIS/ZnO Film Heterojunctions

Colloidal quantum dots (QD) are rapidly making their way into several optoelectronic applications. While Cd- and Pb-based QDs are promising for high-performance devices, toxicity remains a concern. In this regard, Cu-In-S (CIS) QDs provide an alternative option for scalable, commercial production. A low-temperature, high-throughput process was used to synthesize CIS QDs with 1-dodecanethiol (DDT) ligands. While QD synthesis with a DDT ligand is facile, there are drawbacks when it comes to device implementation and interfacing with electron transport films such as ZnO. A ligand exchange process was employed to replace the DDT ligands in the as-synthesized QDs with 3-mercaptopropionic acid (MPA); owing to the short and bifunctional nature of the MPA molecules, this process improved the surface adhesion and carrier transport to ZnO. I-V measurements on planar structures showed the significance of ligand exchange in CIS QDs for heterojunction device implementation.

Facile Synthesis and Characterization of CdSe/ZnSe Core/Shell and ZnxCd1−xSe Alloy Quantum Dots via Non-organometallic Route

Hexadecylamine (HDA) capped CdSe/ZnSe core/shell and ZnxCd1−xSe alloy quantum dots (QDs) have been synthesized successfully via a non-organometallic, hot injection method. The optical, structural and morphological properties of the as-synthesized QDs were determined by using, UV–visible (UV–Vis) and photoluminescence spectroscopy, X-ray diffraction and transmission electron microscopy (TEM). The effect of reaction time and temperature on the core/shell and alloy formation were investigated in detail. The addition of ZnSe precursor to the pre-synthesized HDA capped CdSe QDs at 230 °C produced core/shell CdSe/ZnSe QDs whereas at 250 °C an alloy QDs were obtained initially which later transformed to core/shell structure with time. On the other hand, the immediate addition of ZnSe precursor to the hot CdSe-precursor solution in HDA at 230 °C resulted in HDA capped ZnxCd1−xSe alloy QDs. The optical and structural characterisation shows that the as-synthesised core–shell and alloy QDs are of high quality. This method is simple, and fast for the preparation of core/shell and alloy binary QDs and will encourage their synthesis for various industrial applications.

Zener diode behavior of nitrogen-doped graphene quantum dots

Nitrogen-doped highly fluorescent graphene quantum dots (N-GQDs) were synthesized, using ammonia as nitrogen and d-glucose as carbon source, using a facile microwave-assisted protocol, where the N/C ratio could be varied from 0.19 to 0.25 (% w/w, determined from EDAX). The as-synthesized quantum dots consisting of one to three graphene monolayers exhibited high crystalline morphology with an average size of 1.8 ± 0.2 nm. HRTEM data showed the presence of both pyridinic-N and pyrrolic-N structures. Semiconductor profile of the N-GQDs was extensively probed, and it was noticed that the optical bandgap, knee-voltage and break-down voltage varied linearly with the N/C ratio. The doped samples showed with an optical bandgap ≈5.3 eV at the maximum nitrogen doping yielding n-type semiconductor property. Clear Zener diode attributes with a large forward bias current (100–200 mA), and a smaller reverse bias current of typically half that value was found.

Total Count of Salmonella typhimurium Coupled on Water Soluble CdSe Quantum Dots by Fluorescence Detection

Health diseases due to the ingestion of water or food contaminated with pathogenic microorganisms are a main health problem around the world. The traditional methods for detecting foodborne pathogens are time-consuming (on the order of days). The development of methods that can help to detect and identify foodborne pathogens with high sensitivity and specificity have been proposed to overcome the limitations of traditional methods. Accordingly, this research is focused on the development of an experimental protocol for a high-sensitivity detection and quantification of bacterial pathogens with reduced detection times. This will lead to the development of a portable and low-cost technology with the opportunity to make onsite detection of pathogenic species. The proposed approach has modified the route reported in the literature; the method proposed is expected to be sensitive enough to detect a low limit of 102 CFU/mL counts of bacteria. The fluorescence-based method was tested in presence of Salmonella typhimurium (ATCC 14020) and Escherichia coli (ATCC 25922). CdSe water-soluble quantum dots (QDs) were synthesized in aqueous phase in presence of thioglycolic acid (TGA) as a capping agent. As-synthesized QDs were characterized by x-ray diffraction, near infrared and Fourier transform infrared spectroscopy, UV–Vis and photoluminescence techniques. Results of the CdSe/TGA-bacteria coupling and the determination of the corresponding quantification profiles (calibration curves) will be presented and discussed.

Green synthesis of SnO2 quantum dots using Parkia speciosa Hassk pods extract for the evaluation of anti-oxidant and photocatalytic properties

In the present study, microwave heating method was established for the biosynthesis of SnO2 Quantum dots (QDs) using Parkia speciosa Hassk pods extract. The as-synthesized quantum dots have been characterized by various techniques such as UV, XRD, EDX, TEM, HRTEM, SAED and FTIR spectroscopy. The biosynthesized SnO2 QDs was employed for the first time as an efficient photocatalyst for the degradation of a food dye, acid yellow 23 dye from aqueous phase under the UV254 light. Various parameters, such as the effect of catalyst dose, the initial concentration of acid yellow 23 dye (AY23), pH of the solution and irradiation time on the photodegradation process are also studied for efficient and better use of the synthesized SnO2 QDs as a catalyst. The biosynthesized SnO2 QDs exhibited excellent photocatalytic performances with degradation efficiency 98% on the degradation of an aqueous solution of AY23 of concentration 5 mg/L with a catalyst dose of 20 mg under UV254 light within 24 min. The synthesized SnO2 QDs can be reused up to 5 cycles of photodegradation experiment without losing its stability and efficiency. The biosynthesized SnO2 QDs also shows a fair activity in the scavenging of 2,2-diphenyl-1-picrylhydrazyl free radical with the IC50 value of 312.6 ± 0.025 μg/mL.

A New Passivation Route Leading to Over 8% Efficient PbSe Quantum-Dot Solar Cells via Direct Ion Exchange with Perovskite Nanocrystals.

Colloidal quantum dots (QDs) are promising candidate materials for photovoltaics (PV) owing to the tunable bandgap and low-cost solution processability. Lead selenide (PbSe) QDs are particularly attractive to PV applications due to the efficient multiple-exciton generation and carrier transportation. However, surface defects arising from the oxidation of the PbSe QDs have been the major limitation for their development in PV. Here, a new passivation method for chlorinated PbSe QDs via ion exchange with cesium lead halide (Br, I) perovskite nanocrystals is reported. The surface chloride ions on the as-synthesized QDs can be partially exchanged with bromide or iodide ions from the perovskite nanocrystals, hence forming a hybrid halide passivation. Consistent with the improved photoluminescence quantum yield, the champion PV device fabricated with these PbSe QDs achieves a PCE of 8.2%, compared to 7.3% of that fabricated with the untreated QDs. This new method also leads to devices with excellent air-stability, retaining at least 93% of their initial PCEs after being stored in ambient conditions for 57 d. This is considered as the first reported PbSe QD solar cell with a PCE of over 8% to date.

Gelatin stabilization of quantum dots for improved stability and biocompatibility

We herein report an aqueous synthesis of gelatin stabilized CdTe/CdS/ZnS (CSSG) core/double shell quantum dots (QDs) with improved biocompatibility. The as-synthesized QDs were characterized by ultraviolet-visible (UV–vis) and photoluminescence (PL) spectroscopic techniques, x-ray diffraction technique (XRD), x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The CSSG QDs revealed high photoluminescence quantum yield (PLQY) with excellent stability over a period of one year and retained 90% of its initial PLQY without any aggregation or precipitation under ambient condition. The cell viability study conducted on HeLa, cervical cancer cell lines indicated that the gelatin stabilization effectively decreased the QDs cytotoxicity by about 50%. The CSSG QDs were conjugated with transferrin (Tf) for the efficient delivery to the cancer cells followed by fluorescence imaging. The results showed that the CSSG QDs illuminates the entire cell which renders the QDs as cell labeling markers. The gelatin stabilized core/double shell QDs are potential candidates for long time fluorescent bio-imaging.

Functionalization of Cadmium Selenide Quantum Dots with Poly(ethylene glycol): Ligand Exchange, Surface Coverage, and Dispersion Stability.

Semiconductor quantum dots synthesized using rapid mixing of precursors by injection into a hot solution of solvents and surfactants have surface ligands that sterically stabilize the dispersions in nonpolar solvents. Often, these ligands are exchanged to disperse the quantum dots in polar solvents, but quantitative studies of quantum dot surfaces before and after ligand exchange are scarce. We studied exchanging trioctylphosphine (TOP) and trioctylphosphine oxide (TOPO) ligands on as-synthesized CdSe quantum dots dispersed in hexane with a 2000 g/mol thiolated poly(ethylene glycol) (PEG) polymer. Using infrared spectroscopy we quantify the absolute surface concentration of TOP/TOPO and PEG ligands per unit area before and after ligand exchange. While 50-85% of the TOP/TOPO ligands are removed upon ligand exchange, only a few are replaced with PEG. Surprisingly, the remaining TOP/TOPO ligands outnumber the PEG ligands, but these few PEG ligands are sufficient to disperse the quantum dots in polar solvents such as chloroform, tetrahydrofuran, and water. Moreover, as-synthesized quantum dots once easily dispersed in hexane are no longer dispersible in nonpolar solvents after ligand exchange. A subtle coverage-dependent balance between attractive PEG-solvent interactions and repulsive TOP/TOPO-solvent interactions determines the dispersion stability.

Generation of singlet oxygen by water-stable CdSe(S) and ZnSe(S) quantum dots

Quantum dots (QDs) are particularly valuable nanostructures for photodynamic therapy (PDT) applications due to their capacity of generating cytotoxic species, e.g. singlet oxygen (SO), when they are activated by light. We synthesized water-stable QDs with different composition (not doped and doped Cd- and Zn-based QDs) and surface chemistry (using different thiols compounds) under microwave irradiation heating conditions. As-synthesized QDs were optically characterized using UV–vis and photoluminescence spectroscopy techniques. The generation of SO species as a function of illumination time in the presence of the QDs dots was indirectly determined using a singlet oxygen sensor green (SOSG) kit. The corresponding constant rate “k”, associated to the generation of SO, was estimated for the different kind of materials and synthesis conditions. Results evidenced that the generation of SO was strongly dependent on the intrinsic composition, surface chemistry, type of doping species, and concentration of the QDs. The k values determined at 45μg/mL of Thioglycolic acid (TGA)-capped Cd-based QDs and TGA-capped Zn-based QDs were, respectively, 1.15×10−2±3.14×10−4 and 3.28×10−3±8.75×10−5. Obtained results evidenced the capability of thiol-functionalized QDs to produce SO directly in aqueous phase, which increases their attractiveness as direct photo-sensitizers for 2-photon PDT applications.

Tunable emission of Cu (Mn)-doped ZnInS quantum dots via dopant interaction

In this work, transition metal ion- doped zinc-based quantum dots (QDs) are synthesized via a greener controllable method to avoid the toxicity of the traditional cadmium-based QDs and broaden the tunable emission. Herein, the tunable emission of Cu-doped ZnInS/ZnS core-shell QDs (Cu:ZnInS/ZnS) can cover from 500 to 620nm by varying the Cu dopant concentration from 1 to 20% and the maximum quantum yield can reach 49.8%. Based on the single-doped QDs, Cu,Mn co-doped ZnInS/ZnS core-shell QDs (Cu,Mn:ZnInS/ZnS) with a photoluminance (PL) quantum yield of 30.4% are obtained. All the as-synthesized QDs have the zinc blende structure and the average size is about 3.55nm. Besides, the interaction mechanism between the Cu and Mn dopant luminescence centers is proposed in this work, which is rarely investigated in the previous report. Diffusion priority and energy transfer between these two dopants are supposed to play an important role in the co-doped QDs and Cu ions could affect the splitting of Mn d states. Color coordinates of these doped QDs show the line-tunability from (0.200, 0.397) to (0.408, 0.508) on the Commission Internationale de L'Eclairage (CIE) chromatic diagram, showing a promising potential in high-quality white light output by integration of these QDs with blue chips.

Thiolated selenium as a new precursor for the aqueous synthesis of CdSe/CdS core/shell quantum dots

A new hydrothermal method for the synthesis of thiol-protected CdSe/CdS core/shell quantum dots (QDs) starting from thiolated cadmium and selenium is reported. Four different thiols namely; 3-mercaptopropane-1-sulfonic acid (MPSA), 2-mercaptosuccinic acid, 3-mercaptopropanoic acid (MPA) and 2-mercaptoacetic acid were used to prepare the precursors which also served as capping agent and sulphur source. The as-synthesized QDs were characterized by ultraviolet–visible (UV–Vis) and photoluminescence (PL) spectroscopy, fourier transform infrared spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy, Raman spectroscopy and transmission electron microscopy. The as-prepared QDs exhibited broad trap emission with longer lifetime (>100 ns) due to thiolate trap sites. The particle growth kinetics and the photoluminescent quantum yield (PLQY) of the resulting QDs were found to be depended on the type of thiol and pH. The MPA capped QDs synthesized at pH 11.0 exhibited the high PLQY among other thiol capped QDs. The size of the QDs was calculated to be ~3 nm by XRD analysis, which is consistent with TEM results. The possible mechanism of the core/shell formation is also presented.

Phosphine-Free Synthesis of Metal Chalcogenide Quantum Dots by Directly Dissolving Chalcogen Dioxides in Alkylthiol as the Precursor.

Semiconductor quantum dots (QDs) are competitive emitting materials in developing new-generation light-emitting diodes (LEDs) with high color rendering and broad color gamut. However, the use of highly toxic alkylphosphines cannot be fully avoided in the synthesis of metal selenide and telluride QDs because they are requisite reducing agents and solvents for preparing chalcogen precursors. In this work, we demonstrate the phosphine-free preparation of selenium (Se) and tellurium (Te) precursors by directly dissolving chalcogen dioxides in the alkylthiol under the mild condition. The chalcogen dioxides are reduced to elemental chalcogen clusters, while the alkylthiol is oxidized to disulfides. The chalcogen clusters further combine with the disulfides, generating dispersible chalcogen precursors. The resulting chalcogen precursors are suitable for synthesizing various metal chalcogenide QDs, including CdSe, CdTe, Cu2Te, Ag2Te, PbTe, HgTe, and so forth. In addition, the precursors are of high reactivity, which permits a shorter QD synthesis process at lower temperature. Owing to the high quantum yield (QYs) and easy tunability of the photoluminescence (PL), the as-synthesized QDs are further employed as down-conversion materials to fabricate monochrome and white LEDs.

Continuous Purification of Colloidal Quantum Dots in Large-Scale Using Porous Electrodes in Flow Channel

Colloidal quantum dots (QDs) afford huge potential in numerous applications owing to their excellent optical and electronic properties. After the synthesis of QDs, separating QDs from unreacted impurities in large scale is one of the biggest issues to achieve scalable and high performance optoelectronic applications. Thus far, however, continuous purification method, which is essential for mass production, has rarely been reported. In this study, we developed a new continuous purification process that is suitable to the mass production of high-quality QDs. As-synthesized QDs are driven by electrophoresis in a flow channel and captured by porous electrodes and finally separated from the unreacted impurities. Nuclear magnetic resonance and ultraviolet/visible/near-infrared absorption spectroscopic data clearly showed that the impurities were efficiently removed from QDs with the purification yield, defined as the ratio of the mass of purified QDs to that of QDs in the crude solution, up to 87%. Also, we could successfully predict the purification yield depending on purification conditions with a simple theoretical model. The proposed large-scale purification process could be an important cornerstone for the mass production and industrial use of high-quality QDs.

Effect of surfactant-free addition and γ-irradiation on the synthesis of CdO quantum dots by thermal decomposition of γ-irradiated anhydrous cadmium acetate

AbstractPure phase of cubic (FCC) CdO quantum dots were successfully synthesized by thermal oxidation of γ-irradiated anhydrous cadmium acetate at 400°C for three hours in the presence and absence of benzyl alcohol as surfactant-free. Morphological and structural characteristic of the as-synthesized quantum dots were performed with X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FT-IR). SEM image of CdO quantum dots synthesized using γ-irradiated anhydrous cadmium acetate with 102 kGy absorbed dose shows the formation of the mesoporous nanostructure. In the presence of benzyl alcohol surfactant, the calcination process afforded cauliflower-like structure. TEM image of CdO quantum dots synthesized using γ-irradiated anhydrous cadmium acetate in presence of benzyl alcohol surfactant shows formation of nanoribbon of CdO quantum dots.

Crystal symmetry breaking and vacancies in colloidal lead chalcogenide quantum dots.

Size and shape tunability and low-cost solution processability make colloidal lead chalcogenide quantum dots (QDs) an emerging class of building blocks for innovative photovoltaic, thermoelectric and optoelectronic devices. Lead chalcogenide QDs are known to crystallize in the rock-salt structure, although with very different atomic order and stoichiometry in the core and surface regions; however, there exists no convincing prior identification of how extreme downsizing and surface-induced ligand effects influence structural distortion. Using forefront X-ray scattering techniques and density functional theory calculations, here we have identified that, at sizes below 8 nm, PbS and PbSe QDs undergo a lattice distortion with displacement of the Pb sublattice, driven by ligand-induced tensile strain. The resulting permanent electric dipoles may have implications on the oriented attachment of these QDs. Evidence is found for a Pb-deficient core and, in the as-synthesized QDs, for a rhombic dodecahedral shape with nonpolar {110} facets. On varying the nature of the surface ligands, differences in lattice strains are found.

One-Pot Aqueous Phase Synthesis of CdTe and CdTe/ZnS Core/Shell Quantum Dots.

A facile and economical one-pot strategy has been developed for the synthesis of water-solute CdTe and CdTe/ZnS core/shell quantum dots (QDs) using tellurium dioxide (TeO2) as a tellurium precursor and thioglycolic acid (TGA) as stabilizer without any pre-treatment and inert atmosphere protection. As-synthesized QDs were characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), electron diffraction spectroscopy (EDS), X-ray powder diffraction (XRD), UV-vis and photoluminescence (PL). The spherical particles were uniformly distributed with the average diameters of 3.2 nm (CdTe QDs) and -5 nm (CdTe/ZnS QDs). By altering the reaction conditions, the emission wavelengths of the CdTe core QDs and CdTe/ZnS core/shell QDs could be tuned from 508 to 574 nm and 526 to 600 nm with narrow full widths at half maximum (FWHM) of 33 to 58 nm, respectively. Meanwhile, on the optimum condition, the luminescence efficiency of CdTe/ZnS QDs can achieve to 74%, which was higher than that of CdTe core QDs (24%).

Irradiation route to aqueous synthesis of highly luminescent ZnSe quantum dots and its function as a copper ion fluorescence sensor

Size-controlled synthesis of stable ZnSe QDs with narrow distribution in aqueous environment through conventional soft chemical method still poses a challenge. The proposed radiation assisted strategy demonstrates aqueous synthesis of stable, monodisperse and luminescent ZnSe QDs capped with chelating ethylene diamine under ambient conditions and at room temperature. Radiation chemical method facilitates in slow and in-situ release of selenium ion from sodium selenosulfate. The concentrations of precursors, such as zinc salt, selenium source, ethylene diamine and absorbed radiation (7–90kGy) dose were optimized for obtaining good quality particles. Selective quenching of luminescence of as-synthesized quantum dots (QDs) by Cu2+ ions vis-à-vis other physiologically important cations provide evidence for use of ZnSe quantum dots as alternative to toxic Cd-based quantum dots to probe Cu2+ ions. The linear relation of ratio of loss in emission intensity as a function of concentration of Cu(II) indicates detection limit in nano-molar range.

Aqueous Synthesis of Tunable Highly Photoluminescent CdTe Quantum Dots Using Rongalite and Bioimaging Application

Abstract Rongalite is a cheap, strong and stable reducing agent with broad industrial applications. Herein we report novel method for the preparation of water-soluble and small-size CdTe quantum dots (QDs) using rongalite to produce Te 2− species in reaction medium for the first time. The synthesized QDs were characterized with transmission electron microscopy, spectroscopy, and confocal laser scanning microscopy. The influence of parameters such as the pH value of the precursor solution and the molar ratio of Cd 2+ to TGA and Cd 2+ to Te 2− on the quantum yields of CdTe QDs was systematically investigated. Under the optimal conditions at pH 11.6 and selecting thioglycolic acid as capping agent, the as-synthesized CdTe QDs possessed 66% quantum yield with narrow bandwidth of 24 nm. Moreover, methyl thiazolyl tetrazolium (MTT) assays were employed to evaluate the cytotoxicity of the as-prepared semiconductor nanocrystals on HeLa cell lines. Bioimaging studies showed that the as-synthesized QDs were quickly taken up by HeLa cells and did not have obvious damage to the cells within an incubation period of 24 h. The results show that Rongalite is a very promising agent for the synthesis of a variety of quantum dots and Rongalite-based QDs may find broad applications.

Nonlinear Optical Properties of PbS Colloidal Quantum Dots Fabricated via Solvothermal Method

Lead sulfide (PbS) quantum dots were prepared by a solvothermal method. The as-synthesized quantum dots were dispersed by toluene, n-hexane, and tetrachloromethane separately and characterized by high-resolution transmission electron microscopy. Femtosecond laser pulse-based single-beam Z-scan analysis was employed to investigate the third-order nonlinear optical response of the quantum dots. Dispersants used in the fabrication process play the essential role not only in the nucleation and crystallization of the nanoparticles but also in modifying the nonlinear optical responses of PbS colloidal quantum dots. Additionally, laser power of the detecting beam has a major influence on the accuracy of single-beam Z-scan analysis.