Fluorescence Lifetime Of Quantum Dots Research Articles

Study on ZnO quantum dot ligands for inkjet printing of light-emitting diodes

Inkjet printing as a contact-free, high material utilization, low cost, patternable solution processing technology, may become the future color quantum-dot light-emitting diodes (QLED) production of the key technology. However, owing to the existence of the pixel structure of the substrate array, the film prepared by inkjet printing technology is not so smooth and uniform as that by spin coating technology, which affects the luminescence performance of the device. Therefore, it is necessary to study the film forming mechanism and process of inkjet printing. In order to solve the above problems, this paper focuses on the material and process problems of preparing high-quality electronic transport layer ZnO thin films by inkjet printing. From the perspective of ligand, the ZnO quantum dot material, which is an indispensable electron transport layer, is optimized to prepare the thin films with good morphology and stability in air, and the methods to improve the performances of inkjet printing QLED devices are explored. Firstly, the drying process and influence mechanism of ZnO quantum dot ink in air are investigated. As the thickness of ZnO quantum dot film prepared by inkjet printing increases, the film becomes loose and the surface becomes rough with the continuous extension of the time in the air. These changes make the efficiency of the QLED device decrease and the luminous region uneven. The reason for this phenomenon is that after the ZnO quantum dots are printed into the pixel pit, the random Brownian motion of quantum dots will continue in the process of solvent evaporation. In this process, the ligand ethanolamine connected to the surface of ZnO quantum dots will fall off with the movement and cannot play the role in dispersing the quantum dots. Besides, the external water oxygen and carbon dioxide attach to the film. The combination between the two effects makes the ZnO quantum dots aggregate into large particles and the film becomes loose and porous. In this paper, we use ethylenediamine tetraacetic acid (EDTA) as ligand to synthesize ZnO quantum dots, which can effectively improve this phenomenon. The carboxyl group at the end of EDTA is more stable in connection with ZnO quantum dots. Moreover, the EDTA has multiple branch chains, which makes it have steric hindrance effect and can disperse ZnO effectively. A smooth and compact film with a roughness of 1.97 nm is prepared by inkjet printing. After 40 min exposure to air, the film is still able to maintain a stable morphology. Then, based on the EDTA-ZnO quantum dots synthesized above, red QLED devices with optimized performance are prepared. The current efficiency of the spin-coating device is 16.8 cd/A, which is better than that of the ZnO quantum dots before modification. The EDTA ligand passivates the surface defects of ZnO, alleviates the quenching phenomenon when contacting red quantum dots, and improves the fluorescence quantum efficiency and transient fluorescence lifetime of quantum dots on the electron transport layer. In addition, the EDTA regulates the carrier transport performance, making the carrier recombination of the device more balanced. In the inkjet printing device, the improvement of device morphology plays a leading role in improving the performance. The inkjet printing QLED based on EDTA-ZnO achieves a current efficiency of 9.24 cd/A, and maintains a current efficiency of 6.82 cd/A after 40 min in air, which are 23% and 89% higher than those of the control device. This work provides a reference for solving the problems of film thickness increase and morphology variation caused by particle agglomeration in the preparation of large area devices in inkjet printing.

Study on fluorescence properties and stability of Cu2+-Substituted CsPbBr3 perovskite quantum dots

The toxicity and stability of all-inorganic perovskite quantum dots are major challenge to be addressed in its commercialization process. Here, the toxicity of lead is reduced while improving the optical properties of quantum dots via replacing Pb2+ ions in CsPbBr3 perovskites quantum dots with low-toxicity and cost-effectiveness Cu2+ cations. The PL emission and absorption peaks of quantum dots appear blue shift due to lattice shrinkage caused by copper ion doping. At the same time, we also observed that with the increase of Cu2+ doping concentration, the fluorescence lifetime of quantum dots decreased from 40.66 ns to 8.23 ns. Next, the slim CsPbBr3:Cu@SiO2 core-shell QDs were synthesized via a simply one-step synthesis method. Compared with the original CsPbBr3:Cu QDs, the stability of the quantum dots after coated has been significantly improved. Benefiting from this effective cladding strategy, these CsPbBr3:Cu@SiO2 can be well used for manufacturing high-performance perovskite LEDs.

Reduction of the fluorescence lifetime of quantum dots in presence of plasmonic nanostructures

Developing a single photon source in the near infrared spectrum for optical fibre communications implies the control of the spontaneous emission of quantum dots. In this paper we show that the fluorescence lifetime of lead sulphide quantum dots can be drastically decreased due to the presence of plasmonic nanostructures specially designed to confine the electromagnetic field to which the emitted photons could decay at λ = 1550nm. To demonstrate this, we first measured the fluorescence lifetime of colloidal quantum dots in toluene solution and took this as a reference measurement. Then, we compared this value with the fluorescence lifetime of quantum dots in two different plasmonic nanostructures. The measurement of lifetime was made using the Time-Correlated Single Photon Counting technique. We obtained for the quantum dots in solution a fluorescence lifetime of 0.98084μs and for the quantum dots in the nanostructures, fluorescence lifetimes of 0.06227μs and 0.11789μs. We also numerically calculated the distribution of the electromagnetic field near the plasmonic nanostructures. These results demonstrate that the plasmonic nanostructures modified the fluorescence lifetime of the quantum dots, and that these plasmonic nanostructures with the lead sulphide quantum dots could work as a single photon source that could be integrated in a photonic circuit for infrared optical fibre communications.

Study on Photoluminescence Quenching of CdSe Core/Shell Quantum Dots with Organic Charge Transferring Material

The effect of organic charge transferring material (CTM) on fluorescence of CdSe/ZnS and CdSe/CdS/ZnS core/shell quantum dots (QDs) are investigated by spectral methods. With the increase of organic molecular concentration, CTM can greatly quench the fluorescence of QDs and shorten the fluorescence lifetime of QDs. In the process of interacting with CTM, the efficiency of fluorescence quenching for CdSe/ZnS is significantly higher than that for CdSe/CdS/ZnS. The results of experiment show that the shell structure of QDs plays the major role in photoluminescence (PL) quenching. The mechanism of PL quenching of QDs is also analyzed.

Background-Free In vivo Time Domain Optical Molecular Imaging Using Colloidal Quantum Dots

The interest in optical molecular imaging of small animals in vivo has been steadily increased in the last two decades as it is being adopted by not only academic laboratories but also the biotechnical and pharmaceutical industries. In this Spotlight paper, the elements for in vivo optical molecular imaging are briefly reviewed, including contrast agents, i.e., various fluorescent reporters, and the most commonly used technologies to detect the reporters. The challenges particularly for in vivo fluorescence imaging are discussed and solutions to overcome the said-challenges are presented. An advanced imaging technique, in vivo fluorescence lifetime imaging, is introduced together with a few application examples. Taking advantage of the long fluorescence lifetime of quantum dots, a method to achieve background-free in vivo fluorescence small animal imaging is demonstrated.

A Novel Homogeneous Time-Resolved Fluoroimmunoassay for Carcinoembryonic Antigen Based on Water-Soluble Quantum Dots

Quantum dots are not widely used in clinical diagnosis. However, the homogeneous time-resolved fluorescence assay possesses many advantages over current methods for the detection of carcinoembryonic antigen (CEA), a primary marker for many cancers and diseases. Therefore, a novel luminescent terbium chelates- (LTCs) and quantum dots-based homogeneous time-resolved fluorescence assay was developed to detect CEA. Glutathione-capped quantum dots (QDs) were prepared from oil-soluble QDs with a 565nm emission peak. Conjugates (QDs-6F11) were prepared with QDs and anti-CEA monoclonal antibody. LTCs were prepared and conjugates (LTCs-S001) were prepared with another anti-CEA monoclonal antibody. The fluorescence lifetime of QDs was optimized for sequential analysis. The Förster distance (R0) was calculated as 61.9Å based on the overlap of the spectra of QDs-6F11 and LTCs-S001. Using a double-antibody sandwich approach, the above antibody conjugates were used as energy acceptor and donor, respectively. The signals from QDs were collected in time-resolved mode and analyzed for the detection of CEA. The results show that the QDs were suitable for time-resolved fluoroassays. The spatial distance of the donor-acceptor pair was calculated to be 61.9Å. The signals from QDs were proportional to CEA concentration. The standard curve was LogY = 2.75566 + 0.94457 LogX (R = 0.998) using the fluorescence counts (Y) of QDs and the concentrations of CEA (X). The calculated sensitivity was 0.4ng/mL. The results indicate that water-soluble QDs are suitable for the homogenous immunoassay. This work has expanded future applications of QDs in homogeneous clinical bioassays. Furthermore, a QDs-based homogeneous multiplex immunoassay will be investigated as a biomarker for infectious diseases in future research.

Induction of pH sensitivity on the fluorescence lifetime of quantum dots by NIR fluorescent dyes.

Modulation of the fluorescence lifetime (FLT) of CdTeSe/ZnS quantum dots (QDs) by near-IR (NIR) organic chromophores represents a new strategy for generating reproducible pH-sensing nanomaterials. The hybrid construct transfers the pH sensitivity of photolabile NIR cyanine dyes to highly emissive and long-lifetime pH-insensitive QDs, thereby inducing a reproducible FLT change from 29 ns at pH >7 to 12 ns at pH <5. This approach provides an unparalleled large dynamic FLT range for pH sensing at NIR wavelengths.