xS Quantum Dots Research Articles

Bottom‐Up Synthesis of CoxSn1−xS Nanosheets: A Ferromagnetic and Photoconductive Semiconductor

Abstract2D ferromagnetic semiconductors are key to next‐generation spintronic devices in the post‐Moore era. The combination of ferromagnetic and optoelectronic properties offers exciting opportunities for advanced multifunctional devices in spin‐optoelectronic applications. Herein, the authors synthesize 2D van der Waals (vdW) CoxSn1‐xS with ferromagnetism and photoresponse through a bottom‐up reaction, which has a high yield compared to typical mechanical exfoliation. Ferromagnetic ordering is realized in 2D vdW semiconductor SnS by Co doping at the Sn sites. Magnetic properties are thoroughly studied at different doping concentrations, and first‐principles calculations are further performed to reveal the magnetism origin and spin interactions. In particular, a low Gilbert damping of 1.69 × 10−3 is obtained in vdW CoxSn1−xS through ferromagnetic resonance. In addition, photodetectors based on CoxSn1−xS quantum dots are demonstrated. These studies establish a promising semiconductor with both ferromagnetic ordering and photoelectric response, which provides unprecedented opportunities in spintronic‐photonic integrated applications.

Rare-earth-incorporated ternary Ce x Cd1-x S quantum dot-sensitized solar cells.

This work presents a new absorber material - rare-earth-doped ternary Ce x Cd1-x S quantum dots (QDs) - for solar cells. Ce x Cd1-x S QDs were synthesized by partially replacing the cation Cd in the binary sulfide CdS with Ce using a two-step solution processing process. First, Ce-S QDs were grown on a mesoporous TiO2 electrode. Second, Cd-S QDs were grown on top of the Ce-S QDs. Post annealing transformed the Ce-S/Cd-S double layers into the ternary Ce x Cd1-x S structure. The synthesized Ce x Cd1-x S QDs have the same hexagonal structure as the host CdS, with an average particle size of 11.8 nm. X-ray diffraction reveals a slight lattice expansion in Ce x Cd1-x S relative to CdS. The band gap E g of Ce x Cd1-x S exhibits a monotonic decrease from 2.40 to 2.24 eV with increasing Ce content x from 0 to 0.20, indicating an E g tunable by controlling the dopant content. Ce x Cd1-x S QDSCs were fabricated with a polysulfide electrolyte and CuS counter electrode. The best Ce x Cd1-x S cell yields a J sc of 8.16 mA cm-2, a V oc of 0.73 V, a fill factor of 62.5%, and an efficiency of 3.72% under 1 sun. The efficiency increases to 4.24% under the reduced light intensity of 0.25 sun. The efficiency of the Ce x Cd1-x S cell is 25% higher than that of the host CdS cell. The improved performance is attributed to the broader absorption range resulting from Ce doping. These results suggest the potential of using Ce as a dopant in CdS to tune the E g and improve the photovoltaic performance.

Non-stoichiometric CuxIn1−xS quantum dots for robust photodegradation of gemifloxacin: influencing parameters, intermediates, and insights into the mechanism

Promising 0D non-stoichiometric CuxIn1−xS quantum dots show robust photodegradation towards gemifloxacin under visible light.

Double-Phase Heterostructure within Fe-Doped Cu2–xS Quantum Dots with Boosted Electrocatalytic Nitrogen Reduction

Electrochemical nitrogen reduction reaction (NRR) is considered as one of the most promising methods for NH₃ synthesis under room temperature and ambient pressure. A grand challenge of NRR is the development of efficient electrocatalysts, for which the delicate nanostructuring of catalysts plays an important role. Herein, a series of Fe-doped Cu₂–ₓS quantum dots (QDs) are synthesized with multiple active sites and interface engineering, in which the double-phase heterostructure plays a key role for boosting NRR activity. The yield of NH₃ was obviously improved with the increase of Fe content from 0 to 3% but started to decrease with Fe from 3 to 9%. The optimized Fe₃%–Cu₂–ₓS QDs show an outstanding NH₃ yield of 26.4 μg h–¹ mg–¹cₐₜ at −0.7 V (vs the reversible hydrogen electrode), which is 5 times higher than that of Cu₂–ₓS QDs. More importantly, we observed that the highest NRR activity in Fe₃%–Cu₂–ₓS QDs was ascribed to the formation of an inherent double-phase heterostructure of Cu₂–ₓS/Cu₅FeS₄, whereas the complete conversion to single-phase Cu₅FeS₄ with increased Fe doping (9%) resulted in the activity decrease. Further, N₂ temperature-programmed desorption and electrochemical impedance spectra characterizations confirm the stronger chemical adsorption of N₂ and faster charge transfer in the Cu₂–ₓS/Cu₅FeS₄ QDs. A plausible mechanism was proposed for the double-phase Cu₂–ₓS/Cu₅FeS₄ heterostructure, where the interface provides efficient charge transfer and more active sites of Cu, Fe, and S for the synergetic adsorption and activation of N₂. Our work provides a simple strategy for the design of NRR electrocatalysts, which may also bring new inspiration for the preparation of the inherent double-phase heterostructure within other doped QDs.

The binding energy of biexcitons in alloy ZnxCd1−xS quantum dots detected by femtosecond laser spectroscopy

Biexcitons localized at ZnxCd1−xS quantum dots (x = 0.37 or 0.45) with a diameter of ~ 45 A synthesized by two different methods were studied by femtosecond laser spectroscopy. The spectral features of ultrafast transient absorption spectra at the short-time delay of 70 fs are associated with the three lowest energy transitions of quantum dots. The shapes of the transient absorption bands were modeled by fitting to linear absorption. The spectral positions of the absorption components of the excited state in the transient spectra take into account the energy of the biexciton coupling. By fitting the experimental transient absorption spectra of ZnxCd1−xS QDs, the binding energies of biexcitons were determined. The biexciton binding energies vary from 16.6 to 37 meV depending on the biexciton transition excited at the ZnxCd1−xS quantum dot.

A General In Situ Growth Strategy of Designing Theranostic NaLnF4@Cu2−xS Nanoplatform for In Vivo NIR‐II Optical Imaging Beyond 1500 nm and Photothermal Therapy

AbstractTheranostic nanoprobes with a combination of highly sensitive optical bioimaging and photothermal therapy (PTT) are considered advanced tools for improving the detection precision and the imaging‐guided hyperthermal therapy efficacy against tumor in the biomedical area. Compared with the traditional visible/first near‐infrared (NIR‐I, 650–900 nm) light‐emitting optical probe, a nanoprobe capable of generating the second near‐infrared (NIR‐II, 1000–1700 nm) emission is emerging as the next‐generation optical imaging technique with high‐sensitivity, and high spatial/time resolution owing to its remarkably reduced photon scattering losses. However, a multifunctional theranostic nanoplatform incorporated with the new advanced NIR‐II optical imaging and PTT has not yet been explored. Herein, a general strategy for designing theranostic nanoplatforms by integrating NIR‐II optical bioimaging with photothermal functions via in situ growth of Cu2−xS quantum dots on the lanthanide nanorods is demonstrated. The as‐prepared NaLnF4:Yb/Er@Cu2−xS hybrid nanoprobes with a core‐satellite structure present excellent NIR‐II emission centered at 1525 nm, highly stable photothermal effects and good biocompatibility. These designed theranostic nanoprobes are utilized for NIR‐II optical imaging, small tumor detection (5 mm in diameter), and PTT. More importantly, non‐invasive brain vessel visualization with high spatial resolution (44.2 µm) through scalp and skull without craniotomy is demonstrated. Therefore, these results pave the way to designing new multifunction theranostic nanoplatforms for highly sensitive NIR‐II optical‐guided tumor detection, non‐invasive blood vessel imaging, and PTT.

Multiply-Binding Polymeric Imidazole Ligands: Influence of Molecular Weight and Monomer Sequence on Colloidal Quantum Dot Stability

This study addresses the colloidal stability of polymer-coated core/shell CdSe/CdxZn1–xS quantum dots (QDs) in the presence of l-glutathione (GSH) under neutral conditions by means of photoluminesc...

Influence of manganese doping on the luminescence characteristics of colloidal ZnxCd1−xS quantum dots in gelatin

Colloidal pure and Mn-doped mixed ZnxCd1−xS quantum dots were successfully prepared by aqueous synthesis in photographic inert gelatin. Quantum dots are formed in a cubic crystal lattice with the average size of about 2nm. Two emission bands in photoluminescence spectra of Mn-doped quantum dots are observed: trap state luminescence and Mn characteristic emission. The possibility of Mn-doped quantum dots spectral characteristics tuning by changing the synthesis conditions is shown. It is shown that value of trap state – Mn ion excitation transfer efficiency in mixed ZnxCd1−xS:Mn quantum dots is about 24–30%.

Identification of trapping and recombination levels, structure, morphology, photoluminescence and optical absorption behavior of alloyed ZnxCd1−xS quantum dots

Cubic ZnxCd1−xS nanoparticles (NPs) synthesized by chemical precipitation method with average crystallite size of 2.9 ± 0.2 nm were characterized using X-ray diffraction (XRD), X-ray fluorescence (XRF), high-resolution transmission electron microscope (HRTEM), UV–vis absorption, Fourier transform infrared red (FTIR), photoluminescence (PL) emission and Raman spectroscopy. XRD analysis demonstrated systematic shift of diffraction peaks to higher diffraction angles accompanied by a decrease in the lattice parameters with increasing Zn content (x); this confirmed the formation and homogeneity of ZnxCd1−xS nanoalloys. In addition, the dependence of lattice parameters and Raman shift on x showed linear behavior and good agreement with Vegard's law. TEM images of ZnxCd1−xS NPs revealed nearly spherical shape NPs, relatively narrow particle size distribution and standard deviation in the range 1.8–3.4%.; as well as HRTEM images showed well resolved diffraction crystalline planes of zinc-blende cubic type structure. Analysis of optical absorption spectra showed blue shift of both the direct optical band gap from 3.25 to 4.05 eV and excitonic absorption shoulder from 2.56 to 3.88 eV with increasing x, confirming the homogeneity of ZnxCd1−xS alloyed semiconductor NPs. At excitation wavelength 325 nm, the deconvoluted structural defects related PL emission bands are broadened and revealed stronger PL intensity than that at 370 nm. Furthermore, increasing x resulted in PL enhancement accompanied by blue shift of green emission band centered at 515 nm–445 nm. To explain composition dependent PL emission process; trapping and recombination localized levels in ZnxCd1−xS alloyed NPs were identified quantitatively and an energy band diagram was suggested.

Improving the Photocurrent in Quantum-Dot-Sensitized Solar Cells by Employing Alloy PbxCd1-xS Quantum Dots as Photosensitizers.

Ternary alloy PbxCd1−xS quantum dots (QDs) were explored as photosensitizers for quantum-dot-sensitized solar cells (QDSCs). Alloy PbxCd1−xS QDs (Pb0.54Cd0.46S, Pb0.31Cd0.69S, and Pb0.24Cd0.76S) were found to substantially improve the photocurrent of the solar cells compared to the single CdS or PbS QDs. Moreover, it was found that the photocurrent increases and the photovoltage decreases when the ratio of Pb in PbxCd1−xS is increased. Without surface protecting layer deposition, the highest short-circuit current density reaches 20 mA/cm2 under simulated AM 1.5 illumination (100 mW/cm2). After an additional CdS coating layer was deposited onto the PbxCd1−xS electrode, the photovoltaic performance further improved, with a photocurrent of 22.6 mA/cm2 and an efficiency of 3.2%.

Relationship between structural and optical properties of colloidal Cd x Zn 1 − x S quantum dots in gelatin

Aqueous synthesis of mixed cadmium and zinc sulfides colloidal quantum dots (QDs) has been successfully realized. Colloidal CdxZn1−xS QDs are formed in a cubic crystal lattice with particle size of ∼2 nm. The blueshift of optical absorption spectra from 420 to 295 nm and recombination photoluminescence from 646 to 483 nm with increasing zinc content in QDs was observed. Optimum photoluminescence intensity occurs for QDs with Cd0.3Zn0.7S composition. With increasing zinc content up to Cd0.3Zn0.7S, luminescence intensity increases and decreases when zinc content is larger than 0.7. The increase in photoluminescence intensity is explained by the increase in the number of point defects, such as complexes of interstitial metal atoms–metal vacancies [Mei−VMe]. Such complexes occur due to displacement of the metal atom at the center of the elementary tetrahedron due to substitution of one of the four sulfur atoms by an impurity atom, such as an oxygen atom.

Low toxic and highly luminescent CdSe/Cd x Zn1−x S quantum dots with thin organic SiO2 coating for application in cell imaging

A silanization process was employed to transfer hydrophobic quantum dots (QDs) prepared via an organic route at high temperature into water phase. The QDs were further coated with a thin organic SiO2 shell to form QDs@SiO2 composite nanoparticles by ligand exchange or remaining initial organic ligands on the surface. In this study, QDs with different ligands, either trioctylphosphine oxide (TOPO) or oleic acid (OA), were employed to investigate the effects of ligands on the reverse micelles in preparing QDs@SiO2 nanoparticles. In the preparing process, hydrophobic QDs were silanized by partially hydrolyzed tetraethyl orthosilicate (TEOS). For TOPO-capped CdSe QDs, surface TOPO ligands were completely replaced by partially hydrolyzed TEOS. As for OA-capped CdSe/Cd x Zn1−x S QDs, surface OA ligands were partially replaced. It was found that the ligand exchange drastically reduced the photoluminescence (PL) efficiency of CdSe QDs. Furthermore, the cytotoxicity studies of QDs@SiO2 have been carried out in detail. The results indicate that CdSe/Cd x Zn1−x S QDs@SiO2 composite nanoparticles exhibit lower cytotoxicity compared with CdSe QDs@SiO2, because the SiO2 shell and remained OA ligand layer can effectively prevent the leakage of toxic Cd2+ ions. Meanwhile, it was found that these CdSe/Cd x Zn1−x S QDs@SiO2 nanocomposites could keep excellent PL properties even for 24 h incubating with Siha cells, which indicating that our prepared composite nanoparticles are potentially applicable for cell imaging in biological systems.

Preparation and properties of highly stable quantum dot-based flexible silica films

Highly luminescent hydrophobic CdSe and CdSe/CdxZn1−xS quantum dots (QDs) were synthesized via an organic route. The phase transfer of the QDs was carried out through a ligand exchange from 3-aminopropyltrimethoxysilane (APS) instead of an organic capping agent to get aqueous QDs. A functional sol–gel SiO2 sol with a high QD concentration was obtained from the aqueous QD colloidal solution with APS through the hydrolysis and condensation which subsequently occurred. Flexible inorganic SiO2 films with QDs were fabricated via various methods. The photodegradation experiments of the resulting films were completed. It is surprising that the QDs in films were revealed to be highly stable. Especially, the PL intensity of the films increased dramatically after irradiation by 365 nm UV light. By integrating a thin CdSe QD–silica film on a solar cell, the enhanced current demonstrated that a thin film can facilitate the continuous development of solar cells. Because of their high PL brightness, multicolor emission, flexibility and stability, these films will have great potential applications.

Surface complexation reaction for phase transfer of hydrophobic quantum dot from nonpolar to polar medium.

Chemical reaction between oleate-capped Zn(x)Cd(1-x)S quantum dots (Qdots) and 8-hydroxyquinoline (HQ) led to formation of a surface complex, which was accompanied by transfer of hydrophobic Qdots from nonpolar (hexane) to polar (water) medium with high efficiency. The stability of the complex on the surface was achieved via involvement of dangling sulfide bonds. Moreover, the transferred hydrophilic Qdots--herein called as quantum dot complex (QDC)--exhibited new and superior optical properties in comparison to bare inorganic complexes with retention of the dimension and core structure of the Qdots. Finally, the new and superior optical properties of water-soluble QDC make them potentially useful for biological--in addition to light emitting device (LED)--applications.

One-pot synthesis of graphene–ZnxCd1−xS QDs composites with improved photoelectrochemical performance for selective determination of Cu2+

Graphene–ZnxCd1−xS quantum dots (QDs) hybrid materials were successfully prepared via one-step hydrothermal method. TEM and HRTEM results proved that Zn0.8Cd0.2S QDs with an average size of ∼6nm were homogeneously dispersed on graphene sheets, which favorited the charge separation and improvement of the photocurrent intensity. The controlled experiments proved that graphene–Zn0.8Cd0.2S/ITO electrode with 2mg graphene dosage exhibited the highest photocurrent at an applied voltage of 0.2V. Therefore, a photoelectrochemical sensor was developed to detect Cu2+ using graphene–Zn0.8Cd0.2S/ITO electrode. The sensor exhibited good selectivity and linear relationship from 1μmoldm−3 to 120μmoldm−3 with detection limit of 6.5nmoldm−3 at a signal-to-noise (S/N) ratio of 3.

Novel Au+-doped ZnxCd1−xS/ZnS core/shell quantum dots

Au+ ions have been successfully incorporated into the lattice of ZnxCd1−xS quantum dots as luminescent centers via an aqueous method. The dopant emission can be tuned in the range of 445 to 570 nm, and the highest quantum yield can reach as high as 30%. Our results showed that Au+ ions can be used as an excellent luminescent dopant in doped quantum dots. Finally, these water-soluble quantum dots can be used directly in PL imaging of HepG2 cells without conjugating any biological targets, indicating a high potential application in bio-labeling.

Water-soluble CdSe/CdS and CdSe/Cd x Zn1−x S quantum dots with tunable and narrow luminescent spectra and high photoluminescence efficiency

Nearly monodisperse CdSe/CdS and CdSe/Cd x Zn1−x S core–shell quantum dots (QDs) were synthesized by the epitaxial growth of CdS or Cd x Zn1–x S shells on pre-synthesized CdSe cores through a facile organic route. The thickness effect of CdS and Cd x Zn1−x S shells on the properties of resulting core–shell QDs, such as the photoluminescence (PL) efficiency, PL full width at half-maximum (fwhm), and PL decay lifetime, were investigated. The PL efficiency is greatly enhanced from 9 % of CdSe cores to 64 % of CdSe/Cd x Zn1−x S core–shell QDs. The PL spectra are narrowed after coating (fwhm ~28–32 nm). The alloy Cd x Zn1−x S shell is especially effective to enhance the PL efficiency, and it leads to a smaller red-shift in PL spectra compared with that of the CdS shell. The water-soluble CdSe/Cd x Zn1−x S core–shell QDs were obtained by the encapsulation of amphiphilic poly(styrene-co-maleic anhydride) (PSMA)–ethanolamine (EA) polymers, which was initially dispersed in oil phase. It was found that the CdSe/Cd x Zn1−x S QDs in water still retained a high PL efficiency as well as a small value in fwhm after encapsulating the CdSe/Cd x Zn1−x S QDs with PSMA–EA polymers, expanding the applications of these core–shell QDs in aqueous environment.

Aqueous synthesis of glutathione-capped Cu+and Ag+-doped ZnxCd1−xS quantum dots with full color emission

Water-soluble Cu+- and Ag+-doped ZnxCd1−xS quantum dots have been successfully synthesized via a facile and green method. All synthetic procedures were conducted in the open air at 95 °C. Hydrophilic glutathione was used as the capping agent. The influence of the dopant concentration in alloyed ZnxCd1−xS QDs has been investigated. The as-prepared Cu+- and Ag+-doped ZnxCd1−xS/ZnS core–shell quantum dots exhibit tunable emission covering the whole visible light region and the photoluminescence quantum yield (PL QY) can reach as high as 20% and 31%, respectively.

Surface modification of SiO2 beads with multiple hydrophobic quantum dots for bioapplications

Multi hydrophobic CdSe/ZnS or CdSe/CdxZn1−xS quantum dots were encapsulated into SiO2 beads through a three-step sol–gel procedure. First, the quantum dots were transferred from toluene to water phase via silanization using tetraethyl orthosilicate. The control of ligand exchange (partially hydrolyzed tetraethyl orthosilicate instead of organic amine) resulted in the quantum dots retaining their initial photoluminescence efficiencies after the phase transfer. Second, the assembly of the quantum dots occurs by the hydrolysis and condensation of tetraethyl orthosilicate to form seeds. The amount of 3-mercaptopropyltrimethoxysilane during incorporation plays an important role in controlling the quantum dot number per seed. Third, the seeds are coated with a SiO2 shell by a subsequent Stöber process. The resulting SiO2 beads with a controlled number of hydrophobic quantum dots revealed high photoluminescence efficiency. The SiO2 beads were functionalized with amine, carboxyl, and thiol-terminated biolinkers for surface modification. To confirm the surface modification by carboxyl groups, the SiO2 beads were conjugated with amino functional polystyrene beads. The silica beads introduced here represent a new platform for nanoparticulate multimodality bioanalysis.

Multiple hydrophobic QDs assembled in SiO2 particles using silane coupling agent

Hydrophobic oleic acid-capped CdSe/CdxZn1−xS core/shell quantum dots (QDs) with high photoluminescence (PL) efficiency of 62% and a PL peak wavelength of 602nm were assembled in SiO2 particles through a novel silanization technique. Ligand exchange (3-mercaptopropyltrimethoxysilane (MPS) instead of oleic acid) occurred via the connection of mercapto groups and QDs when MPS was added in the toluene solution of the QDs. After being silanized using MPS, partially hydrolyzed tetraethyl orthosilicate (TEOS) was attached to the surface of the silanized QDs. The hydrophobic CdSe/CdxZn1−xS QDs were then transferred from toluene phase to water phase by the hydrolysis of TEOS through the addition of ethanol, ammonia, and water. Further hydrolysis and condensation of the TEOS and MPS causes them to form assemblies in the water phase. MPS is crucial for proper assembly of the QDs in SiO2 particles and for controlling the size of resulting SiO2 particles. The resulting SiO2 particles exhibited tunable diameters from several tens to 100nm. After being encapsulated in SiO2 particles, the QDs revealed a PL efficiency of 25%. Because of the resulting SiO2 particles encapsulating multiple hydrophobic CdSe/CdxZn1−xS QDs and having a functional surface (mercapto group), they are applicable to bio-probes.