ZnS Core Shell Quantum Dots Research Articles

Study of FRET between Zn-Cu-In-S/ZnS core/shell quantum dots-Cresyl violet dye system

This study reports the fluorescence resonance energy transfer between Zn-Cu-In-S/ZnS core–shell quantum dots and Cresyl Violet dye. The quenching of Zn-Cu-In-S/ZnS core–shell QD (donor) emission and lifetime and simultaneous enhancement in Cresyl Violet (acceptor) emission validates and confirms the non-radiative resonance energy transfer from Zn-Cu-In-S/ZnS QDs to Cresyl violet. The values of spectral overlap, Förster distance, the distance between donor and acceptor pair and FRET efficiency were determined using the steady state as well as time-resolved fluorescence spectroscopic techniques. This study demonstrates that this systemexhibits efficient FRET and can act as a very sensitive chemical sensor. Since Zn-Cu-In-S/ZnS QDs are nontoxic compared to popular cadmium-based quantum dots, the application of FRET in biological systems will be advantageous. It can find vast applications in photonics and biomedical applications.

Three-Dimensional Photoluminescent Crypto-Images Doped with (CdSe)ZnS Quantum Dots by One-Photon and Two-Photon Polymerization

Three-dimensional (3D) photoluminescent crypto-images can be created by nanopatterning quantum dots (QDs) within a polymeric micro-object. A UV-sensitive photoresist, doped with (CdSe)ZnS core–shell QDs, is employed to fabricate fluorescent devices, by two-photon polymerization direct laser writing (TPP-DLW). In this work, we report on the expulsion of the majority of QDs from the irradiated area, caused by the cross-linking of the monomers, determining their accumulation on the photopolymer surface. At the same time, QDs get excited and trigger a surrounding thermal polymerization, leading to a variation of the spatial resolution of the fabricated structures. By opportunely combining these physical phenomena, it is possible to control the local density of QDs with nanometric resolution and, as a consequence, to tune the local photoemission of the fabricated polymeric structure. A photopolymer doped with five different concentrations of (CdSe)ZnS core–shell QDs is analyzed and tested. We define the best conditions to create nanostructures containing only few QDs to be integrated in optical devices, as well as to create 3D photoluminescent micro crypto-images. By TPP-DLW and one-photon lithography, a 3D fluorescent design can be concealed inside a polymeric microstructure that is swiftly fabricated using a photoresist. An example of a 3D crypto-image is shown to illustrate the technique.

Study of photoluminescent and photometric properties of ZnS:Mn2+ quantum dots for white light emission

Abstract In this paper, ZnS, Mn2+-doped ZnS, and ZnS:Mn2+/ZnS core-shell quantum dots (QDs) are successfully prepared by the co-precipitation route at 80 °C in basic aqueous solutions. X-ray diffraction (XRD), energy dispersive X-ray spectrometer (EDX), UV–Visible absorption (UV–Vis), photoluminescence (PL), photoluminescence excitation (PLE) spectroscopy and photometric studies were employed to characterize the samples. The QDs have the zinc blende structure, and their CIE (x, y) chromaticity coordinates can be tuned by the Mn2+ impurity and the TGA-Zn2+ concentrations. The overlap between these QDs spectral power and the human eye sensitivity spectra is significantly enhanced with the efficient energy transfer process. The scotopic/photopic (S/P) ratios, the luminous efficacy of optical radiation (LER), and the color correlated temperature (CCT) increase from 0.93 to 1.69, 389.84–706.53 l m/Wotp, and 2782–14,037 K, respectively. As a result, the ZnS:Mn2+ QDs are suitable for the development of tune color components in white thin-film electroluminescent (TFEL) devices as well as light-emitting diodes (LEDs).

Site- and Spatial-Selective Integration of Non-noble Metal Ions into Quantum Dots for Robust Hydrogen Photogeneration

Summary Artificial photosystems consisting of semiconductor quantum dots (QDs) and non-noble metal ions (e.g., Fe2+, Co2+, Ni2+) show intrinsic advantages in efficient and cost-effective hydrogen (H2) evolution. However, well-controlled integration of these metal ions into ultra-small QDs is a major challenge. Here, we present a two-step strategy to realize site- and spatial-selective integration of earth-abundant Fe2+ ions in CdSe QDs; i.e., CdSe QDs are modified with a thin and anisotropic ZnS shell, and partial Zn2+ ions are selectively substituted by Fe2+ via cation exchange. The anisotropic ZnS layer not only passivates CdSe core but also works as an ideal medium to intimately anchor Fe2+. The multifunctional CdSe/Zn1−xFexS QD exhibits extraordinary activity and stability toward H2 photogeneration. Specifically, more than 880 mL (∼39,300 μmol) H2 gas is produced from 6 mL of aqueous solution during a consecutive 172 h (>1 week) experiment, thus achieving a turnover number of >600,000 per QD.

ZCIS/ZnS QDs fluorescent aerogels with tunable emission prepared from porous 3D nanofibrillar bacterial cellulose

Bacterial cellulose (BC) features a nanofibrillar network structure that can provide a good template for quantum dots (QDs), to overcome the fluorescence quenching-effect of QDs in polymer composites. Here, we fabricated novel fluorescent aerogels with tunable emission by covalently binding environmentally-friendly ZnS(CuInS2)/ZnS core-shell quantum dots along the nanofibrillar BC. A new ligand of 3-(mercaptopropyl)trimethoxysilane allows QDs to transfer from toluene to alcohol solvent and stably bind to the BC. After supercritical CO2 drying, the resulting BC-QDs aerogels maintain the porous nanofibrillar morphology of BC with ultra-light-weight, the QDs are well-distributed along the BC fiber surfaces without aggregation. The emission wavelength can be tuned in a wide range from 470 to 750 nm by simply adjusting the QDs core component or shell layers. This work provides a new approach for fabricating QDs-polymer hydrogels and aerogels with well distributed QDs via chemical binding that potential as smart sensor, catalysis, and 3D display applications.

Au@SiO2@CuInS2–ZnS/Anti-AFP fluorescent probe improves HCC cell labeling

BackgroundClear tumor imaging is essential to the resection of hepatocellular carcinoma (HCC). This study aimed to create a novel biological probe to improve the HCC imaging. MethodsAu nano-flower particles and CuInS2–ZnS core-shell quantum dots were synthesized by hydrothermal method. Au was coated with porous SiO2 and combined with anti-AFP antibody. HCC cell line HepG2 was used to evaluate the targeting efficacy of the probe, while flow cytometry and MTT assay were used to detect the cytotoxicity and bio-compatibility of the probe. Probes were subcutaneously injected to nude mice to explore light intensity and tissue penetration. ResultsThe fluorescence stability of the probe was maintained 100% for 24 h, and the brightness value was 4 times stronger than that of the corresponding CuInS2–ZnS quantum dot. In the targeting experiment, the labeled HepG2 emitted yellow fluorescence. In the cytotoxicity experiments, MTT and flow cytometry results showed that the bio-compatibility of the probe was fine, the inhibition rate of HepG2 cell with 60% Cu-QDs/Anti-AFP probe and Au-QDs/Anti-AFP probe solution for 48 h were significantly different (86.3%±7.0% vs. 4.9%±1.3%, t = 19.745, P<0.05), and the apoptosis rates were 83.3%±5.1% vs. 4.4%±0.8% (P<0.001). In the animal experiment, the luminescence of the novel probe can penetrate the abdominal tissues of a mouse, stronger than that of CuInS2–ZnS quantum dot. ConclusionsThe Au@SiO2@CuInS2–ZnS/Anti-AFP probe can targetedly recognize and label HepG2 cells with good bio-compatibility and no toxicity, and the strong tissue penetrability of luminescence may be helpful to surgeons.

Optical Energy Transfer from Photonic Nanowire to Plasmonic Nanowire

Rational and versatile transfer of optical energy from photonic nanowire to plasmonic nanowire is highly desirable and extremely challengeable. Herein, we demonstrate that optical energy transfer from CdSe–ZnS core–shell quantum dots doped polymer photonic nanowire to silver plasmonic nanowire with an efficiency of 32 ± 1.5% via the Forster resonance energy transfer process. The quantum dots with an emission at 650 nm wavelength were embedded in polymer nanowire and were excited by a 532 nm green laser. In the crossing region of the photonic nanowire and plasmonic nanowire, propagated surface plasmons of silver nanowire can be effectively excited by the emitted light from quantum dots embedded in polymer nanowire. This energy transfer between photonic and plasmonic nanowires would provide potential applications in optical interconnection.

Preparation and Property of CuInS2/ZnS Core-shell Quantum Dots in Aqueous Phase

Preparation and Property of CuInS₂/ZnS Core-shell Quantum Dots in Aqueous Phase

Characterization of the Ligand Capping of Hydrophobic CdSe–ZnS Quantum Dots Using NMR Spectroscopy

We have combined a few advanced solution phase NMR spectroscopy techniques, namely, 1H, 31P, heteronuclear single quantum coherence (HSQC), and diffusion ordered spectroscopy (DOSY), to probe the composition of the organic capping layer on colloidal CdSe–ZnS core–shell quantum dots grown via the “hot injection” route. Combining solution phase 31P and 1H NMR with DOSY, we are able to distinguish between free ligands and those coordinated on the QD surfaces. Furthermore, when those NMR measurements are complemented with matrix-assisted laser desorption ionization (MALDI) and FTIR data, we find that the organic shell of the as-prepared QDs consists of a mixture of tri-n-octylphosphine oxide (TOPO), tri-n-octylphosphine (TOP), alkyl amine, and alkyl phosphonic acid (L- and X-type ligands); the latter molecules are usually added during growth at a rather small concentration to improve the quality of the prepared nanocrystals. However, NMR data collected from QD dispersions subjected to two or three rounds of p...

Photoresponse Enhancement in Monolayer ReS2 Phototransistor Decorated with CdSe-CdS-ZnS Quantum Dots.

ReS2 films are considered as a promising candidate for optoelectronic applications due to their direct band gap character and optical/electrical anisotropy. However, the direct band gap in a narrow spectrum and the low absorption of atomically thin flakes weaken the prospect for light-harvesting applications. Here, we developed an efficient approach to enhance the performance of a ReS2-based phototransistor by coupling CdSe-CdS-ZnS core-shell quantum dots. Under 589 nm laser irradiation, the responsivity of the ReS2 phototransistor decorated with quantum dots could be enhanced by more than 25 times (up to ∼654 A/W) and the rising and recovery time can be also reduced to 3.2 and 2.8 s, respectively. The excellent optoelectronic performance is originated from the coupling effect of quantum dots light absorber and cross-linker ligands 1,2-ethanedithiol. Photoexcited electron-hole pairs in quantum dots can separate and transfer efficiently due to the type-II band alignment and charge exchange process at the interface. Our work shows that the simple hybrid zero- and two-dimensional hybrid system can be employed for photodetection applications.

Temperature-dependent resonance energy transfer from CdSe–ZnS core–shell quantum dots to monolayer MoS2

We investigated the temperature-dependent resonance energy transfer (ET) from CdSe–ZnS core–shell quantum dots (QDs) to monolayer MoS2. QDs/MoS2 structures were fabricated using a spin-coating method. Photoluminescence (PL) spectra and decay curves of the QDs/MoS2 structures were measured in the temperature range of 80−400 K. The results indicate that the PL intensity of the QDs decreased approximately 81% with increasing temperature, whereas that of the MoS2 increased up to a maximum of 78% at 300 K because of the combined effect of thermal quenching and the ET in the QDs/MoS2 structures. The ET efficiency and ET rate also exhibited similar variation trends, both increased with increasing temperature from 80 to 260 K and then decreased until 400 K, resulting in a maximum ET efficiency of 22% and an ET rate of 1.17 ns–1 at ~260 K. These results are attributed to the varied distribution of the localized excitons and free excitons in the QDs/MoS2 structures with increasing temperature.

Temperature-sensitive photoluminescent CdSe-ZnS polymer composite film for lock-in photothermal characterization

The temperature dependence of the fluorescence spectrum of CdSe−ZnS core–shell quantum dots embedded in a polystyrene matrix is characterized between 30 °C and 60 °C. The spectrally integrated photoluminescence intensity is found to linearly decrease with −1.3%/ °C. This feature is exploited in a dual coating-substrate-configuration, consisting of a layer of this nanocomposite material, acting as a temperature sensor with optical readout, on top of an optically absorbing and opaque layer, acting as a photothermal excitation source, and covering a substrate material or structure of interest. From the frequency dependence of the optically detected photothermal signal in the frequency range between 5 Hz and 150 Hz, different thermal parameters of the constituent layers are determined. The fitted values of thermal properties of the different layers, determined in different scenarios in terms of the used a priori information about the layers, are found to be internally consistent, and consistent with literature values.

Acid Dissociation of 3-Mercaptopropionic Acid Coated CdSe–CdS/Cd0.5Zn0.5S/ZnS Core–Multishell Quantum Dot and Strong Ionic Interaction with Ca2+ Ion

By devising careful electrophoresis, it was shown that at pH below 7.0, the electrophoretic mobility of 3-mercaptopropionic acid (3MPA) coated CdSe–ZnS core–shell quantum dots (denoted as QD-3MPA) was very small. At pH above 7.0, QD-3MPA migrated toward the anode, implying acid dissociation, and the degree of which was proportional to the pH value. QD-3MPA’s electrophoretic mobility was impaired after adding sufficient Ca2+ ions to the QD solution and revived when a similar amount of Ca2+ chelators (ethylene glycol tetraacetic acid, EGTA) was added. This demonstrated that acid dissociation and its pH dependence of 3MPA on the QD surface are critical factors in understanding the electric and optical properties of QDs. The acid dissociated QD-3MPA interacted strongly with Ca2+, forming a charge neutral QD-3MPA–Ca2+ complex in the absence of EGTA. First-principles study confirmed the observed experimental evidence. The strong ionic interaction between acid dissociated QD-3MPA and Ca2+ is critical for develop...

Structural and optical behavior of hexa-propyl methyl cellulose (HPMC) capped ZnS core–shell quantum dots

HPMC capped ZnS core–shell quantum dots were synthesized by using simple and eco friendly chemical precipitation method in which HPMC acts as a surface passivator as well as a functionalization agent to tune the properties. Morphological and optical characterizations were done to see the effect of HPMC capping on ZnS quantum dots. Morphological studies was done by using XRD and TEM. XRD studies confirms the pure zinc blende phase for all the samples. TEM studies shows that HPMC is efficiently controlling the agglomeration. Optical studies were done by using UV–visible and FTIR. UV–visible studies shows that bandgap is tunable on both sides of the wavelength depending on the concentration of HPMC. FTIR studies confirms the coating of HPMC on the surface of ZnS quantum dots. Laser induced time resolved photoluminescence studies were also done to see the effect of HPMC on the intensity of excited state life time of quantum dots. It is observed that the structural and optical characteristics have strong dependence on capping concentration. As HPMC is a very important viscoelastic polymer and organic surfactant used in various types of cosmetic binding applications and drug formulations, these studies opens a new way in the area of novel cosmetic design, efficient drug delivery and targeting and also specific optoelectronic applications using chalcogenide quantum dots.

Tuning the Redox Coupling between Quantum Dots and Dopamine in Hybrid Nanoscale Assemblies

We explored the charge transfer interactions between CdSe–ZnS core–shell quantum dots (QDs) and the redox active neurotransmitter dopamine, using covalently assembled QD–dopamine conjugates. We combined steady-state fluorescence, time-resolved fluorescence, and transient absorption bleach measurements to probe the effects of changing the QD size (thus the QD energy levels) and the conjugate valence on the rate of QD photoluminescence quenching when the pH of the medium was adjusted from acidic to alkaline. We measured substantially larger quenching efficiencies, combined with more pronounced shortening of the carrier dynamics of these assemblies for smaller size QDs and in alkaline pH. Moreover, we found that changes in the QD size alter the electron and hole relaxation of photoexcited QDs but with different extents. For instance, a pronounced change in the hole relaxation was measured in alkaline buffers. Moreover, the hole relaxation was faster for conjugates of green-emitting QDs as compared to their r...

Bright white-light emission from Ag/SiO2/CdS-ZnS core/shell/shell plasmon couplers.

Well-defined plasmon couplers (PCs) that comprise a Ag core overcoated with a SiO(2) shell with controlled thickness, followed by a monolayer of CdS-ZnS core-shell quantum dots (QDs) were synthesized to modify the emission from trap-rich CdS-ZnS QDs by adjusting the distance between the QDs and Ag nanoparticles (NPs). When the thickness of the SiO(2) shell was 10 nm, because the shell could effectively suppress the non-radiative energy transfer from the semiconductor QDs to the metal NPs and the localized surface plasmon resonance (LSPR) of the Ag NPs spectrally matched the emission peak of the CdS-ZnS QDs to bring about strong plasmon coupling, optimum enhancements of the surface state emission (SSE) (17 times) and band-edge emission (BEE) (4 times) were simultaneously realized and the SSE to BEE intensity ratio was increased to 55%. As a result, a bright white-light source with 1931 Commission Internationale d'Eclairage (CIE) chromaticity coordinates of (0.32, 0.34) was realized by the superposition of the two emissions. The experimental results from Ag/SiO(2)/CdSe-ZnS and the Ag/SiO(2)/CdS:Mn-ZnS core/shell/shell PCs indicated that suppressing the non-radiative decay rate (k(nr)) was the underlying mechanism for plasmon coupling fluorescence enhancement.

Effect of ZnS shell on optical properties of CdSe–ZnS core–shell quantum dots

A modified microemulsion technique was used to synthesize CdSe–ZnS core–shell quantum dots (QDs) of ∼5nm diameter. Their nonlinear optical absorption was observed with nanosecond laser radiation of 532-nm wavelength. Linear optical studies show confinement effects. There is a new emission band around 745nm under excitation of 532nm which is an indication of the generation of deep trap states in CdSe QDs. As the ZnS shell thickness increases, there could be an increased electron–hole pair generation leading to large free-carrier concentration which in turn results in enhancement of nonlinear optical absorption. In addition, it was found that the optical limiting threshold gets decreased as the ZnS shell thickness increases.

Absorption spectra of CdSe–ZnS core–shell quantum dots at high photon energies: Experiment and modeling

Absorption spectra of CdSe–ZnS core–shell quantum dot (QD) ensembles, with average core diameters ranging from 2.6nm to 7.2nm have been obtained using both transmission and photoluminescence excitation measurements. In agreement with previous reports, the absorption coefficient at energies ≃1eV above the effective bandgap increases monotonically as in bulk solids. A simple effective-mass spherical core–shell potential model cannot explain the relatively high absorption at higher energies. The calculated electron and hole radial envelope wavefunctions show asymmetry due to the core–shell structure. It leads to normally symmetry-disallowed transitions acquiring a weak oscillator strength, with their number and strength increasing with energy. A phenomenological model that invokes normally disallowed transitions in general is shown to reproduce the absorption spectrum at higher energies quite well. The oscillator strength scaling factor for such transitions increases with decrease in QD size, consistent with expectations.

Influence of the solvent environments on the spectral features of CdSe quantum dots with and without ZnS shell

The spectral properties of 5nm size of bare CdSe and (CdSe)ZnS core–shell quantum dots (QDs) have been recorded and investigated under different solvent environments with different polarities and different concentrations. The results showed that the spectral profile of absorption did not change for both quantum dots in different solvents under a wide range of concentrations. On the other hand, the photoluminescence (PL) spectra of (CdSe)ZnS core–shell quantum dots in non-polar solvents showed two bands; the primary around 420nm and the secondary around 620nm. In contrast, the PL spectra of bare CdSe in non-polar solvents, showed a very strong band around 590nm, with a total absence of the primary wavelength band at 420nm. Under high polar solvent environments, bare CdSe showed a new peak around 420nm, which was totally absent in non-polar solvent. Therefore, the solvent plays an important role in the PL spectra of bare CdSe and (CdSe)ZnS core–shell quantum dots.

Optical and Morphological Studies of Doped Core Shell ZnS:Cu/ZnS Nanoparticles

The paper presents some results of study based on applications of ZnS core shell quantum dots (QDs) doped with Cu. Keeping the luminous properties in focus we synthesized the core shell QDs by chemical precipitation route, resulting in formation of core@shell QDs with ZnS core doped with copper and ZnS shell on it, i.e. [ZnS:Cu@Zn. We focus the application of these particles in field of OLEDs (AMOLED) to address the performance deficiencies like varying brightness of the different wavelength emitting LEDs, called Green Window problem. Efforts have been done to address the problems by synthesizing highly luminescent green emitting copper doped ZnS, core@shell QDs. Further a monolayer of core shell quantum dots was deposited on ITO by spin coating for analyzing the photometric properties of the QDs.