Mixture Of Quantum Dots Research Articles

Formation of plasmon-exciton nanostructures based on quantum dots and metal nanoparticles with a nonlinear optical response

The establishment of the conditions for the formation of nanostructures with plasmon-exciton interaction based onquantum dots and plasmonic nanoparticles that provide unique nonlinear optical properties is an urgent task. The study demonstrates the formation of plasmon-exciton nanostructures based on hydrophilic colloidal Zn0.5Cd0.5S, Ag2S quantum dots and metal nanoparticles. Transmission electron microscopy and optical absorption and luminescence spectroscopy were used to substantiate the formation of plasmon-exciton hybrid nanostructures. The phase composition of the studied samples was determined by X-ray diffraction. The results obtained using ARLX’TRA diffractometer (Switzerland) indicated a cubic crystal structure (F43m) of synthesised Zn0.5Cd0.5S quantum dots and monoclinic (P21/C) crystal lattice of Ag2S. Transmission electron microscopy revealed that plasmonic nanoparticles are adsorption centres for quantum dots. The average sizes of the studied samples were determined: colloidal Ag2S quantum dots (2.6 nm), Zn0.5Cd0.5S(2.0 nm) and metal nanoparticles: silvernanospheres (10 nm) and gold nanorods (4x25 nm). The transformation of the extinction spectra of the light and the luminescence quenching of quantum dots have been established in mixtures of quantum dots and plasmonic nanoparticles. The nonlinear optical parameters of the studied samples were determined using the Z-scanning method at wavelengths of 355 and 532 nm in the field of nanosecond laser pulses. The conditions for the formation of hybrid nanostructures that provide an increase of the coefficient of nonlinear absorption of laser pulses (355 and 532 nm) up to 9 times with a duration of 10 ns due to the reverse saturable absorption occurring due to cascade two-quantum transitions in the intrinsic and local states of colloidal quantum dots and the suppression of nonlinear refraction, were determined. The observed changes were explained by the manifestation of the Purcell effect on the states of quantum dots in the presence of nanoresonators (gold nanorods and silver nanospheres). The results of these studies create new opportunities for the development of original systems for controlling the intensity of laser radiation, as well as quantum sensors of a new generation

An ultrathin fiber-based fluorescent imaging probe based on hyperspectral imaging

Most fluorescent imaging systems use pre-defined filter sets with limited flexibility in the choice of excitation and emission bands and suffer from crosstalk between different fluorophores for multiplexed imaging. When implemented in the context of optical fibers for biological imaging in a remote setting with space constraints, the size of the fiber probe is crucial. To overcome these challenges, we combined the merits of hyperspectral imaging and an ultrathin optical imaging fiber where fluorescent images at 311 spectral bands in the visible range were captured using a liquid crystal tunable filter with a fiber probe of 500 μm in diameter. Fluorescent experiments were performed on quantum dots mixtures and genetically modified E. coli bacteria mixtures to demonstrate the system’s capabilities for multiplexed imaging. Images were first processed using a Fourier transform filtering technique to remove the fiber core pattern artifact. Subsequently, the classification of different types of bacteria samples was calculated using two methods, namely, spectral unmixing with maximum abundance and spectral matching with minimum spectral angle distance. Finally, the spatial distributions of individual bacteria types were overlaid with the mixture image, and the two classification results matched well. A simple-to-use graphic user interface (GUI) platform for the hyperspectral imaging fiber probe system was developed which performs image processing and displays the classification results. The methods and results presented will appeal to the optical fiber-based imaging and hyperspectral imaging communities in general and have great potential for biological imaging applications.

Insights into the kinetics and self-assembly order of small-molecule organic semiconductor/quantum dot blends during blade coating.

Organic-inorganic nanocomposite films formed from blends of small-molecule organic semiconductors and colloidal quantum dots are attractive candidates for high efficiency, low-cost solar energy harvesting devices. Understanding and controlling the self-assembly of the resulting organic-inorganic nanocomposite films is crucial in optimising device performance, not only at a lab-scale but for large-scale, high-throughput printing and coating methods. Here, in situ grazing incidence X-ray scattering (GIXS) gives direct insights into how small-molecule organic semiconductors and colloidal quantum dots self-assemble during blade coating. Results show that for two blends separated only by a small difference in the structure of the small molecule forming the organic phase, crystallisation may proceed down two distinct routes. It either occurs spontaneously or is mediated by the formation of quantum dot aggregates. Irrespective of the initial crystallisation route, the small-molecule crystallisation acts to exclude the quantum dot inclusions from the growing crystalline matrix phase. These results provide important fundamental understanding of structure formation in nanocomposite films of organic small molecules and colloidal quantum dots prepared via solution processing routes. It highlights the fundamental difference to structural evolution which can be made by seemingly small changes in system composition. It provides routes for the structural design and optimisation of solution-processed nanocomposites that are compatible with the large-scale deposition manufacturing techniques that are crucial in driving their wider adoption in energy harvesting applications.

Size-dependent reactivity of highly photoluminescent CdZnTeS alloyed quantum dots to mercury and lead ions

Water-soluble and highly photoluminescent (PL) quantum dots (QDs) are attractive for metal ion sensing. Their size-dependent properties including UV stability and reactivity are important aspects to deploy QDs as PL probes. Herein, CdZnTeS alloyed QDs with a bandgap ranging from 2.6 eV to 2.1 eV were synthesized by a one-pot procedure and their size dependent reactivities to Pb 2+ or Hg 2+ ions were examined. Optical study on mixtures of QDs and Pb 2+ or Hg 2+ revealed that only Hg 2+ can exchange Cd 2+ of mother QDs while both Pb 2+ and Hg 2+ could act as Z-type ligands enhancing the PL. The cation exchanging ability and the PL enhancement increase as QDs size decreases. The results demonstrated new understandings on the interaction between CdZnTeS alloyed QDs and heavy metal ions which could be useful to deploy QDs in metal ion sensing.

Tuning energy transfer efficiency in quantum dots mixture by controling donor/acceptor ratio* *Project supported by the National Natural Science Foundation of China (Grant Nos. 52025023, 51991342, 52021006, 11888101, and 61922028), the Key R&D Program of GuangdongProvince, China (Grant Nos. 2020B010189001, 2019B010931001, and 2018B030327001), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB33000000), Beijing Natural Science Foundation,

Improving the emission performance of colloidal quantum dots (QDs) is of paramount importance for their applications on light-emitting diodes (LEDs), displays and lasers. A highly promising approach is to tune the carrier recombination channels and lifetime by exploiting the energy transfer process. However, to achieve this precise emission optimization, quantitative modulation on energy transfer efficiency is highly desirable but still challenging. Here, we demonstrate a convenient approach to realize tunable energy transfer efficiency by forming QDs mixture with controllable donor/acceptor (D/A) ratio. With the mixing ratio ranging from 16/1 to 1/16, the energy transfer efficiency could be effectively tuned from near zero to ∼ 70%. For the high mixing ratio of 16/1, acceptors obtain adequate energy supplied by closely surrounding donors, leading to ∼ 2.4-fold PL enhancement. While for the low mixing ratio, the ultrafast and efficient energy extraction process directly suppresses the multi-exciton and Auger recombination in the donor, bringing about a higher threshold. The facile modulation of emission performance by controllably designed mixing ratio and quantitatively tunable energy transfer efficiency will facilitate QD-based optoelectronic and photovoltaic applications.

Charge Carrier Diffusion Dynamics in Multisized Quaternary Alkylammonium-Capped CsPbBr3 Perovskite Nanocrystal Solids.

CsPbBr3 quantum dots (QDs) are promising candidates for optoelectronic devices. The substitution of oleic acid (OA) and oleylamine (OLA) capping agents with a quaternary alkylammonium such as di-dodecyl dimethyl ammonium bromide (DDAB) has shown an increase in external quantum efficiency (EQE) from 0.19% (OA/OLA) to 13.4% (DDAB) in LED devices. The device performance significantly depends on both the diffusion length and the mobility of photoexcited charge carriers in QD solids. Therefore, we investigated the charge carrier transport dynamics in DDAB-capped CsPbBr3 QD solids by constructing a bi-sized QD mixture film. Charge carrier diffusion can be monitored by quantitatively varying the ratio between two sizes of QDs, which varies the mean free path of the carriers in each QD cluster. Excited-state dynamics of the QD solids obtained from ultrafast transient absorption spectroscopy reveals that the photogenerated electrons and holes are difficult to diffuse among small-sized QDs (4 nm) due to the strong quantum confinement. On the other hand, both photoinduced electrons and holes in large-sized QDs (10 nm) would diffuse toward the interface with the small-sized QDs, followed by a recombination process. Combining the carrier diffusion study with a Monte Carlo simulation on the QD assembly in the mixture films, we can calculate the diffusion lengths of charge carriers to be ∼239 ± 16 nm in 10 nm CsPbBr3 QDs and the mobility values of electrons and holes to be 2.1 (± 0.1) and 0.69 (± 0.03) cm2/V s, respectively. Both parameters indicate an efficient charge carrier transport in DDAB-capped QD films, which rationalized the perfect performance of their LED device application.

Affinity Immobilization of Semiconductor Quantum Dots and Metal Nanoparticles on Cellulose Paper Substrates.

Colloidal semiconductor quantum dots (QDs), metal nanoparticles, and cellulose paper are materials with numerous applications in bioanalysis and beyond. The functional properties of QDs and metal NPs are substantially different than those of cellulose, such that their integration with cellulose paper is potentially enabling for many applications. Here, we characterize and evaluate multiple chemistries that modify cellulose paper substrates for the affinity-based immobilization of QDs, gold nanoparticles (Au NPs), and platinum nanoparticles (Pt NPs). These chemistries include grafting of cellulose fibers with imidazole and dithiol groups, as well as the aminosilanization of cellulose fibers (both with and without subsequent grafting with dithiol groups). Cellulose modifications and nanoparticle immobilization are characterized by multiple techniques, including, but not limited to, X-ray photoelectron spectroscopy, scanning electron microscopy, and optical imaging, extinction, and fluorescence measurements. We demonstrate the on-paper immobilization of color-tuned mixtures of QDs, on-paper patterning of QDs by microcontact printing, and post-immobilization enhancement of energy transfer and model assays of protease activity. The robustness of QD photoluminescence is also evaluated between immobilization chemistries. Paper-immobilized Au NPs and Pt NPs are evaluated as potential substrates for SERS and as supported catalysts for a model decolorization reaction. Our cumulative results indicate that there may not be a one-size-fits-all immobilization chemistry. Instead, the immobilization chemistry should be tailored and optimized for the downstream application.

Light-Induced Oxidase Activity of DNAzyme-Modified Quantum Dots.

Here, we report the synthesis of a quantum dot (QD)-DNA covalent conjugate to be used as an H2O2-free DNAzyme system with oxidase activity. Amino-coupling conjugation was carried out between amino-modified oligonucleotides (CatG4-NH2) and carboxylated quantum dots (CdTe@COOH QDs). The obtained products were characterized by spectroscopic methods (UV-Vis, fluorescence, circular dichroizm (CD), and IR) and the transmission electron microscopy (TEM) technique. A QD-DNA system with a low polydispersity and high stability in aqueous solutions was successfully obtained. The catalytic activity of the QD-DNA conjugate was examined with Amplex Red and ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate)) indicators using reactive oxygen species (ROS) generated by visible light irradiation. The synthesized QD-DNAzyme exhibited enhanced catalytic activity compared with the reference system (a mixture of QDs and DNAzyme). This proved the assumption that the covalent attachment of DNAzyme to the surface of QD resulted in a beneficial effect on its catalytic activity. The results proved that the QD-DNAzyme system can be used for generation of the signal by light irradiation. The light-induced oxidase activity of the conjugate was demonstrated, proving that the QD-DNAzyme system can be useful for the development of new cellular bioassays, e.g., for the determination of oxygen radical scavengers.

The study of the interaction of quantum dots with phosphatidylcholine to create hybrid functional materials

Quantum dots (QDs) are widely used as biomarkers that has both fundamental and applied importance. Since cell membranes are mainly composed of lipids such as phosphatidylcholine (PC) and its derivatives, it is important to investigate the interaction of QDs with PC in monolayers to understand the penetration mechanism. This work is devoted to the study of the interaction of QDs with PC by determining their surface-active properties at various interfaces by dynamic surface tension. The isotherms of the surface tension of 5 μg/L QDs solution or 0.17 mM (0.86 mM) PC solutions, as well as the solutions of the mixtures of QDs and PC: 0.17 mM (0.86 mM) PC and 5 μg/L QDs or 0.17 mM (0.86 mM) PC and 15 μg/L QDs were obtained. The mixed QDs/PC monolayers (at low content of QDs equal to 1:100) were characterized by the formation of a stable domain structures. An adsorption of PC at QDs allows to vary the degree of surface modification by changing the amount of the lipid. Such nanoparticles have dimensions not very different from the sizes of the initial QDs, due to the small size of lipids and are well suited for incorporation into biomembrane models.

Selective uptake and modulation of nanometal surface energy transfer from quantum dot to Au nanoparticle across lipid bilayer of liposomes

Understanding the initial adsorption and subsequent translocation of nanoparticles (NPs) across the phospholipid bilayer of liposome find importance in drug delivery and in vivo bio-imaging applications. Herein, we have investigated the time-dependent selective uptake of 2.0 ± 0.3 nm-sized CdTe quantum dots (QDs) into small unilamellar vesicles (SUV) of dipalmitoylphoshatidylcholine (DPPC) in its gel phase and subsequent alteration of nanometal surface energy transfer (NSET) in the presence of 18.0 ± 0.2 nm-sized citrate-capped gold NPs (Au NPs). The interactions between CdTe QD and Au NP across the lipid bilayer of DPPC liposome has been explored by monitoring the NSET process using fluorescence spectroscopy. The time-dependent selective partitioning of QDs into the liposomal phase has been demonstrated using confocal laser scanning microscopy (CLSM) and atomic force microscopy (AFM). It has been observed that instant mixing of QDs and Au NPs with liposomes does not alter the extent of NSET between QD-Au NP pair relative to that in the absence of liposome. However, equilibrating the binary mixture of liposome and QDs for 24 h at room temperature leads to the formation of liposome encapsulated QD (LipQD) which does not take part in NSET with aqueous phase Au NPs. In contrast, equilibrating the ternary mixture of QDs, liposomes, and Au NPs for 24 h results in the formation of Au NP adsorbed QD-encapsulated liposomes (AuLipQD) which exhibit moderate amount of NSET from encapsulated QDs to the adsorbed Au NPs at the surface of liposomes. It has been observed that the efficiency of NSET decreases from 62% in bulk aqueous medium to 24% in AuLipQD complex. Our present findings may be useful to understand the fundamental interaction of other metal and semiconductor NPs with liposomes and cell membrane for various drug-delivery and bio-imaging applications.

Functional DNA Structures and Their Biomedical Applications

Since the discovery of the double-helix structure in 1953, nucleic acids have been developed from natural genetic codes into functional building blocks in a wide range of biotechnology and material...

A Comparative Study of Optical Properties of Thin Films of MPS-encapsulated CdS Quantum Dots and SiO2/MPS-encapsulated CdS Quantum Dots Mixture

The aim of this study is to investigate the optical properties of thin films of MPS-encapsulated CdS quantum dots (QDs) and SiO2/MPS-encapsulated CdS QDs mixture. For this purpose, SiO2 sol and colloidal MPS-encapsulated CdS QDs were mixed and then deposited on a glass substrate by spin coating. Also, the spin coating method was used to deposit the colloid-produced MPS-encapsulated CdS QDs and in this way the QDs were self-assembled onto the glass substrate. The films were heat treated at 150°C under N2 atmosphere. The optical properties of the films were characterized by UV-Vis absorption spectrophotometer and NKD spectrophotometer measurements. The refractive index and thickness of the films were calculated by fitting the measured data in a Drude–Lorentz model. The refractive index values were found 1.90 and 1.51 for the thin films of the MPS-encapsulated CdS QDs and SiO2/QDs mixtures, respectively. According to this result, the refractive index value of the SiO2/CdS QDs mixture thin film is close to the glass substrate and such films may be a good candidate for planar luminous solar concentrators.

Synthesis of Si and CdTe quantum dots and their combined use as down-shifting photoluminescent centers in Si solar cells

This paper describes the synthesis and characterization of Si and CdTe quantum dots (QDs) and their use, either on their own or combined, as photoluminescent (PL) down-shifting nanostructured coatings aimed to enhance the photovoltaic efficiency of polycrystalline silicon solar cells. To this end, the front face of a set of silicon cells was coated with different volume ratios of the above-mentioned QDs, or some of its mixtures, dispersed in PMMA layers. Previously, the absorption and the PL (exc = 380 nm) response of the dispersions of the QDs were measured. It was observed that the PL response of the mixtures was strongly affected in location, spread, and intensity of the emission peak according to the volume ratio involved. As compared to the unmixed CdTe samples, a notorious red-shift of the main peak location was obtained for a couple of mixed QDs’ dispersions, which was one of the project objectives given that Si solar cells respond better to photons with wavelengths in the 650–700 nm range. This effect was confirmed in a set of polycrystalline Si solar cells covered with and without nanostructured PMMA/QDs layers tested under AM 1.5G solar simulator conditions. It was found that the use of the proposed mixtures of QDs gave an increase of 1.53% in solar cell power conversion efficiency.

Origin of the Below-Bandgap Turn-On Voltage in Light-Emitting Diodes and the High VOC in Solar Cells Comprising Colloidal Quantum Dots with an Engineered Density of States.

The turn-on voltage of a light-emitting diode (LED) is an important parameter as it determines the power consumption of the LED and influences the effective power conversion efficiency. LEDs based on nanoscale engineering of the blended PbS [mixture of quantum dots (QDs) with two different bandgaps] colloidal QDs have recently shown record performance in the infrared region. One of the most intriguing findings for these blended devices is the substantially lower-than-bandgap turn-on voltage and the achievement of an open circuit voltage ( VOC), approaching the radiative limit. In this work, we provide new insight into the origin of these phenomena. We posit that the decrease in the effective density of states in the conduction and valence band of the emitter QDs in the blended structures modifies the chemical potential, which controls the turn-on voltage for LEDs through the modification of the photon flux and VOC for photovoltaic devices with quasi-Fermi level modification.

Electric Freedericksz transition in nematic liquid crystals with graphene quantum dot mixture

A theoretical model to explain the Freedericksz transition for a nematic liquid crystal with graphene quantum dots adding in electric field below the instability area is proposed. A mathematical formula for the critical voltage was obtained by elastic continuum theory combined with Burilov - Zakhlevnykh anchoring model, classical electrostatic principles and temperature dependence of elastic constant. We compared the results with the experimental data recorded for three LC samples and a good agreement between them was found encouraging us to propose this model as a tool for theoretical analysis of graphene quantum dot's dispersions in liquid crystals.

A Highly Efficient Chemiluminescence System Based on an Enhancing Effect of Ag Nanoclusters/Graphene Quantum Dots Mixture for Ultrasensitive Detection of Rabeprazole.

Herein, an efficient chemiluminescence (CL) reaction with a high emission intensity is reported based on a synergistic improving effect of silver nanoclusters (AgNCs) and graphene quantum dots (GQDs). First, the syntheses of AgNCs and GQDs were simply performed by the chemical reducing of AgNO3 and a thermal treatment of glucose, respectively. After the characterization steps, the beneficial behavior of the prepared nanomaterial was investigated in CL systems. The oxidation reaction of KMnO4-rhodamine B produced weak CL emission. However, the presence of AgNCs and GQDs led to a synergetic enhancing effect, and thus higher emission was obtained. A possible mechanism was investigated for this effect using absorption and fluorescence experiments. Furthermore, rabeprazole showed a relatively selective enhancing impact on the CL emission. The CL intensity was linearly increased in the rabeprazole concentration range of 4 - 133 ng mL-1 with a detection limit (3Sb/m) of 1.1 ng mL-1. The developed CL method was utilized for the measurement of Rbp in biological samples with acceptable precision and accuracy.

The depuration fate of the mixtures of CdS/ZnS quantum dots (QDs) with different surface coatings on mangrove and wheat root epidermis: results from a novel method

BackgroundThe depuration fate of mixtures of metal-based quantum dots (QDs) in root epidermis is an important factor that determined their follow-up transportation and transformation processes in plant. However, the related mechanisms are poorly understood due to the lack of appropriate quantitative methods.ResultsIn our study, a novel method for in situ determination of mixtures of CdS/ZnS QDs with different surface coatings located on root epidermis was developed using an excitation–emission matrix combined with multi-way parallel factor analysis (PARAFAC) and n-way partial least squares (n-PLS). Then, the fates of CdS/ZnS QDs on mangrove and wheat root epidermis were investigated using the established method. During the rapid dissipation stage, intracellular and intercellular degradation of metal ions instead of transportation and rhizophytic bacteria degradation was the predominant reasons for the reduction in the CdS/ZnS QDs mixtures. However, due to vertical migration of CdS/ZnS QDs in the slow depuration stage, negative linear relationships were observed between the Ks values of oleic acid-CdS/ZnS QDs and the polarity index of the root epidermis (p < 0.05), which further indicated a difference between the single and mixture QDs.ConclusionsTo our knowledge, this study provided a new approach to investigate the fate of QDs located on root epidermis and revealed the related mechanisms.

Mixture of quantum dots and ZnS nanoparticles as emissive layer for improved quantum dots light emitting diodes.

Recently, quantum dots based light-emitting diodes (QLEDs) have received huge attention due to the properties of quantum dots (QDs), such as high photoluminescence quantum yield (PLQY) and narrow emission. To improve the performance of QLEDs, reducing non-radiative energy transfer is critical. So far, most conventional methods required additional chemical treatment like giant shell and/or ligands exchange. However that triggers unsought shifted emission or reduced PLQY of QDs. In this work, we have firstly suggested a novel approach to improve the efficiency of QLEDs by introducing inorganic nanoparticles (NPs) spacer between QDs, without additional chemical treatment. As ZnS NPs formed a mixture layer with QDs, the energy transfer was reduced and the distance between the QDs increased, leading to improved PLQY of mixture layer. As a result, current efficiency (CE) of the QLED device was improved by twice compared with one using only QDs layer. This is an early report on utilizing ZnS NPs as an efficient spacer, which can be utilized to other compositions of QDs.

Development of a Room Temperature-Operable PbS QD-Based Infrared Sensor by Using Bandgap Manipulation

A PbS quantum dot (QD)-based infrared sensor was developed for room temperature operability by using interlayer energy bandgap manipulation. The developed sensor constitutes various ratios of PbS QDs in a PCPDTBT matrix that are sandwiched by two metal electrodes. The offset barrier exerted a great effect on the sensitivity improvement. The PbS QD mixture ratio, layer stacking order, and sensitive layer thickness were controlled for optimization. The absorption characterization and the transmission electron microscopy analysis revealed the energy bandgap and the diameter of the PbS QDs.

CdSe quantum dots in chiral smectic C matrix: experimental evidence of smectic layer distortion by small and wide angle X-ray scattering and subsequent effect on electro-optical parameters

ABSTRACTCdSe quantum dots (QDs) dispersed ferroelectric liquid crystal (FLC) has been subjected to small and wide-angle X-ray scattering and atomic force microscopy to understand the molecular organization in chiral smectic C (SmC*) phase. SAXS indicates that the presence of QDs causes enhancement in the smectic layer separation. The smectic order parameter for neat FLC and FLC–QDs mixtures is obtained in the range of 0.6 to 0.85. Both smectic order parameter and structural tilt are found to be lesser for FLC–QDs mixtures as compared to neat FLC. The insertion of QDs in SmC* matrix causes localized smectic layer distortion in such a way that spontaneous polarization remains almost the same but the electro-optic switching of molecules becomes faster. We have outlined the superiority of FLC–QDs mixtures for electrical energy storage and their suitability in electronic devices.