Multishell Quantum Dots Research Articles

Investigation of band parameters and electrochemical analysis of multi core-shell CdSe/CdS/ZnS quantum dots

Highly emissive core−multi-shell (CdSe/CdS/ZnS) quantum dots (QDs) within size range of ∼ 2 nm–5 nm were prepared via hot injection method. Capping agent (polymers) were used to increase the lifetime of QDs, without a protective atmosphere. Red shift was observed in an absorption spectrum of QDs peaks at 362 nm, 365 nm, 558 nm, and 556 nm for CdSe/CdS and CdSe/CdS/ZnS with respect to CdSe core. Voltammetry measurements (Eg ∼ 2. 09 eV) for band gaps were in good agreement with the UV–visible spectroscopic data (Eg ∼ 2.65 eV). QDs exhibited three distinguished peaks comparing to (111), (220), and (311) planes of cubic structures of CdSe, CdSe/CdS and CdSe/CdS/ZnS within crystallite size of 20–27 nm as observed by XRD spectra. A larger active surface area 0.06 cm−2, computed by CV increased the catalytic activity of sensors by exposing the electroactive sites. Linear, nonlinear optical susceptibility and absorption coefficients were computed by an effective mass model. Quantum confinement was confirmed by taking electron probability density as function of radial distance of quantum dots.

Effect of number of well and core radius on the binding energy and optical properties of multishell spherical quantum dot with parabolic potential

In the current study, the probability densities and binding energy of the ground state and first excited sate in multishell quantum dot with parabolic confinement potential have been examined. Also, the effects of core radius, the number of well in structure and the presence of impurity on the total optical absorption coefficient have been investigated. The finite difference method has been used to obtain the energy levels and related wave functions of the system within framework effective mass approximation. The results obtained reveal that the number of well, especially the first five values, have a considerable impact on the critical core values of binding energy. It is found that the presence of impurity and the number of wells cause a blue shift in the peak positions of total absorption coefficients. Moreover, our results show that these factors have an important effect on the magnitude of optical properties.

One-Pot Synthesis of InP Multishell Quantum Dots for Narrow-Bandwidth Light-Emitting Devices

With tunable emission in the full visible region, the ecofriendly InP quantum dots (QDs) show unique application prospects in light-emitting devices. At present, InP QDs suffer from wide-bandwidth emission, especially for electroluminescence (EL), which hinders their applications in high-performance display devices. Here, we report a facile one-pot synthesis of narrow-bandwidth InP/ZnSeS/ZnS QDs using a safe phosphorus source of tris(dimethylamino)phosphine, in which the ZnSeS inner-shell layer is formed via temperature-gradient solution growth from 240 to 280 °C. The synthesized green QDs have a high photoluminescence quantum yield (PLQY) of 91% and full width at half-maximum (fwhm) of 36 nm. Moreover, the resultant quantum dot light-emitting devices (QLEDs) also show a narrow fwhm of 42 nm, which is the narrowest emission of green InP QLEDs based on a safe phosphorus source reported so far. Further modulation of the electron injection into the device by inserting a thin layer of lithium fluoride results in a peak external quantum efficiency of 5.56%.

Advanced tunability of optical properties of CdS/ZnSe/ZnTe/CdSe multi-shell quantum dot by the band edge engineering

In this study, the advanced manipulability of wavefunctions in a type-II multi-shell hetero-nanostructure (MS-HNS) and the tunability of radiative exciton lifetime over a wide range with and/or without changing in transition energies has been demonstrated by the band edge engineering. For this purpose, the electronic and optical properties of exciton (X) and biexciton (XX) in a spherical CdS/ZnSe/ZnTe/CdSe HNS have been explored in detail. In the calculations, effects of all Coulombic interactions between the charges have been taken into account on the wavefunctions. Moreover, in the case of XX, the exchange–correlation potential between the same charged particles has also been considered. The results have been presented as a function of CdS core radius and ZnSe shell thickness and the probable physical reasons have been discussed in detail. • The band edge engineering allows an advanced manipulability of wavefunctions in type-II CdS/ZnSe/ZnTe/CdSe multi-shell quantum dots. • The transition energies and exciton lifetime can be tuned separately and simultaneously by band edge engineering. • These simultaneous tunability properties allow an important degree of freedom for device applications.

Physico-Chemical Transformation and Toxicity of Multi-Shell InP Quantum Dots under Simulated Sunlight Irradiation, in an Environmentally Realistic Scenario.

Quantum dots (QDs) are widely used in optoelectronics, lighting, and photovoltaics leading to their potential release into the environment. The most promising alternative to the highly toxic cadmium selenide (CdSe) QDs are indium phosphide (InP) QDs, which show reduced toxicity and comparable optical and electronic properties. QD degradation leads to the release of toxic metal ions into the environment. Coating the QD core with robust shell(s) composed of another semi-conductor material enhances their properties and protects the QD from degradation. We recently developed double-shelled InP QDs, which proved to be less toxic than single-shell QDs. In the present study, we confirm their reduced cytotoxicity, with an LC50 at 77 nM for pristine gradient shell QDs and >100 nM for pristine thin and thick shell QDs. We also confirm that these three QDs, when exposed to simulated sunlight, show greater cytotoxicity compared to pristine ones, with LC50 ranging from 15 to 23 nM. Using a combination of spectroscopic and microscopic techniques, we characterize the degradation kinetics and transformation products of single- and double-shell QDs, when exposed to solar light at high temperature, simulating environmental conditions. Non-toxic pristine QDs degrade to form toxic In–phosphate, In–carboxylate, Zn–phosphate, and oxidized Se, all of which precipitate as heterogeneous deposits. Comparison of their degradation kinetics highlights that the QDs bearing the thickest ZnS outer shell are, as expected, the most resistant to photodegradation among the three tested QDs, as gradient shell, thin shell, and thick shell QDs lose their optical properties in less than 15 min, 60 min, and more than 90 min, respectively. They exhibit the highest photoluminescence efficiency, i.e., the best functionality, with a photoluminescence quantum yield in aqueous solution of 24%, as compared to 18% for the gradient shell and thin shell QDs. Therefore, they can be considered as safer-by-design QDs.

Efficient InP/ZnSe/ZnS quantum dot shelling and the effect of a bi-layered organic-inorganic electron-transport layer on the performance of quantum dot light-emitting diode devices

We report the fabrication of red-emitting multi-shell InP quantum dots (QDs) via a two-pot synthesis method and their application to high-efficiency QD light-emitting diodes (QD-LEDs). We synthesized highly efficient and monodisperse deep-red-emitting InP/ZnSe/ZnS QDs with a photoluminescence quantum efficiency of 90% and a full width at half maximum of 49 nm using an eco-friendly phosphine precursor, tris(dimethylamino)phosphine. Furthermore, conventional QD-LED device structures in which metal oxide nanoparticles are used as the electron-transport layer (ETL) suffer from charge imbalance problems related to the charges injected from the cathode and anode. Therefore, in this study, we explored a new approach for efficiently balancing the electrons and holes, which involved the insertion of an additional organic layer of 2,2′,2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) between the inorganic zinc magnesium oxide (ZnMgO) as ETL and LiF as electron injection layer. The bi-layered organic-inorganic ETL reduced the electron mobility and modulated charge balance, thereby enhancing the properties of the conventional QD-LED device. The QD-LED device with the bi-layered ETL was 1.35 times and 1.36 times higher than those of the QD-LED device employing a mono-layered ZnMgO ETL, respectively.

Heterostructured core/gradient multi-shell quantum dots for high-performance and durable photoelectrochemical hydrogen generation

Colloidal Quantum dots (QDs) are considered promising light harvesters for photoelectrochemical (PEC) hydrogen generation devices due to their size tunable optoelectronic properties. However, the solar-to-hydrogen (STH) efficiency and long-term stability of devices based on QDs are still relatively low, thus limiting the commercial development. These limitations are attributed to the limited absorption range of QDs, unfavorable band energy alignment and photo-oxidation. Here we propose and realize two core/multiple-shell architecture based on CdSe/CdS/ZnS QDs. The shell composition is optimized with gradient layers, forming CdSe/CdSexS1−x/CdS/ZnyCd1−yS/ZnS core/multiple-shell structures, which reduces the surface traps and defects of QDs and simultaneously suppresses exciton recombination by providing intermediated alloyed interlayers. The PEC device based on a mesoporous TiO2 sensitized with two types of core/multiple-shell QDs exhibited an outstanding saturated photocurrent density of 20.5 mA/cm2 for alloyed core/multiple-shell QDs, under one sun illumination (AM 1.5 G, 100 mW/cm2). To our knowledge, this is comparable to the highest value reported so far for the PEC devices based on colloidal QDs. In addition, the as-prepared PEC devices exhibited excellent stability, maintaining ~93.4 % of the initial photocurrent density after 2-hour continuous illumination (100 mW/cm2). This work provides an efficient approach for improving the performance of PEC devices through QDs structure engineering.

The Effect of Cation Doped ZnO Electron Transport Layer for Pure Blue Emitting Zntese/ZnSe/ZnS Quantum Dot Light Emitting Diodes

Colloidal quantum dot light emitting diodes (QLED) has considered as next generation display technology due to their accurate color information, high brightness, with low process cost, and potential flexible substrates. Red and green QLED achieved nearly the efficiency of theoretical limitation by optimizing quantum dot materials and device architecture with the tremendous efforts of many fellow researchers [1,2]. However, the authentic blue color QLED with high performance and high stability is yet remained as a obstacle for the commercialization. Herein, we report the fabrication of ZnTeSe based blue QLED device with multi shell quantum dots with very high quantum yield over >90% at 443nm with narrow FWHM of 15 nm. The peak wavelength of quantum dot could be tuned with the different Te/Se ratio of the core and the thickness of intermediate ZnSe shell. With the combination of zinc magnesium oxide (ZMO), which is the most common electron transport layer (ETL) for blue QLED, and our highly luminescent blue quantum dot, bright blue QLEDs with the max luminance of 700 cd/m2 in the inverted structure are demonstrated. Moreover, to obtain the better-performed and stable electron transport layer, different cations such as lithium, nickel, aluminum, cerium, erbium and indium-doped zinc oxides were successfully synthesized and studied. we have analyzed the band gap of ZnO can be fine tuned by doping of different cations and the effect of different zinc oxide in the device will be tested in the further study. According to the recent researches of the device stability of blue QLED [3,4], inferior stability of blue QLED is originated from the interface of ETL and quantum dots. I believe that the study of various ETL materials can give us a key to solve the poor lifetime problem of blue QLED.Reference1. Li, Y., Hou, X., Dai, X., Yao, Z., Lv, L., Jin, Y., & Peng, X. (2019). Stoichiometry-Controlled InP-Based Quantum Dots: Synthesis, Photoluminescence, and Electroluminescence. Journal of the American Chemical Society, 141(16), 6448–6452.2. Won, Y.-H., Cho, O., Kim, T., Chung, D.-Y., Kim, T., Chung, H., ... Jang, E. (2019). Highly efficient and stable InP/ZnSe/ZnS quantum dot light-emitting diodes. Nature, 575(7784), 634–638.3. Chen, S., Cao, W., Liu, T., Tsang, S.-W., Yang, Y., Yan, X., & Qian, L. (2019). On the degradation mechanisms of quantum-dot light-emitting diodes. Nature Communications, 10(1), 765.4. Kim, T., Kim, K.-H., Kim, S., Choi, S.-M., Jang, H., Seo, H.-K., ... Jang, E. (2020). Efficient and stable blue quantum dot light-emitting diode. Nature, 586(7829), 385–389.

Importance of Surface Functionalization and Purification for Narrow FWHM and Bright Green-Emitting InP Core–Multishell Quantum Dots via a Two-Step Growth Process

Importance of Surface Functionalization and Purification for Narrow FWHM and Bright Green-Emitting InP Core–Multishell Quantum Dots via a Two-Step Growth Process

Dispersion of InPZnS/ZnSe/ZnS multishell quantum dots (QDs) in water: extension to QDs with different core sizes and identical shell thickness

We have previously shown (Nanomaterials, 2020, 10 (9), 1858) that InPZnS/ZnSe/ZnS multishell quantum dots (QDs) with a variety of shell thicknesses can be dispersed in water using a ligand exchange reaction with mercaptocarboxylic acids. Here, we demonstrate that the concept can further be extended to a larger variety of QD core sizes while keeping the shell thickness constant. The photoluminescence quantum yield of the QDs in the aqueous phase depends on the QD size and the QDs only show a slight red-shift upon dispersion in water. This provides access to a larger group of particles with different emission colors, which is interesting for application in e. g. biological or medical diagnostics.

Efficient multi-shell CuInS2/ZnS/ZnS quantum-dots based light-emitting diodes: Time-controlled synthesis of quantum-dots and carrier balance effects of PEI

CuInS2-based quantum-dots (QDs) have been highlighted as promising photoelectric materials for their low toxicity and color adjustable emissions as well as stability. Here, multi-shell CuInS2/ZnS/ZnS QDs with high PLQY of 76% were synthesized by employing a high temperature and long-time growth strategy while highly bright and efficient quantum-dot-light-emitting diodes (QLEDs) were fabricated by using these multi-shell QDs as an emission layer and polyethylenimine (PEI) as an insulating layer, respectively. By introducing PEI between the QDs and ZnO EML layers of the QLEDs, the balance of the carriers within the QDs layer injected from HTL and ETL was greatly improved. As a result, comparing to the QLEDs without PEI, the greatly improved efficiency (1.9 times as high as that of the QLEDs without PEI) has been achieved in the optimized QLEDs with PEI. These QLEDs exhibits an outstanding performance with the maximum EQE of 1.56% and the maximum brightness of 5825 cd/m2.

Effect of Polymer Capping on Photonic Multi-Core–Shell Quantum Dots CdSe/CdS/ZnS: Impact of Sunlight and Antibacterial Activity

The highly luminescent CdSe/CdS/ZnS core–multi-shell quantum dots (QDs) were prepared without a protective atmosphere through the precursor injection method (phosphine free) in paraffin liquid and oleic acid. Polymers (PEG, PVA, PVP, and PAA) were coated to CdSe/CdS/ZnS core–multi-shell quantum dots to increase stability. However, core–multi-shell structured QDs reveal enhanced emission in the range 355–410 nm by suppressing the defect sensitive cores and nonradioactive recombination in PL spectra. The cubic zinc blended quantum dots with crystallite size in the range 22–44 nm, as confirmed by XRD, were obtained. The resultant absorption spectra of all the samples showed that the samples were absorbent in the UV region over the 302–380 nm range. In the FT-IR spectrum 712, 731, and 400–700 cm–1 band values were assigned to CdSe, CdS, and ZnS band stretching, respectively. Images of CdSe, CdSe/CdS, and CdSe/CdS/ZnS quantum dots obtained from the SEM were spherical whereas QDs capped with different polymers (PEG, PVA, PVP, and PAA) showed nanofibers that were linear and homogeneous size ranged between 12 and 38 nm. These as prepared QDs were placed under visible light for 48 h. After absorbing UV light, the range of UV–vis intensity was enhanced until 389–464 nm and emission intensity enhanced until 492–509 nm, which was confirmed by UV and PL spectra. CdSe/CdS/ZnS QDs with organic ligands revealed antibacterial activity over a broad range against Klebsiella Pneumoniae and Pseudomonas aeruginosa.

The electronic and optical properties of an exciton, biexciton and charged excitons in CdSe/CdTe-based multi-shell type-II quantum dot nanocrystals

It has been recently reported that multi-shell type-II quantum dot nanocrystals (QDNCs) have higher quantum yields. Besides these higher quantum yields of multi-shell type-II QDNCs, additional second layer has been a critical influence on the formation mechanisms of the excitonic structures. Understanding of bound and unbound cases of the excitonic structures in multi-shell type-II QDNCs gives some important information for applications. In this study, we have investigated the electronic and optical properties of a single exciton (X), biexciton (XX), and positively and negatively charged excitons ( $${\mathsf {X^+}}$$ and $${\mathsf {X^-}}$$ ) in CdSe/CdTe-based multi-shell type-II QDNCs. In the study, three different structure compositions, i.e., CdSe/CdTe, CdSe/CdTe/CdS, and CdSe/CdTe/ZnTe, have been considered. We have observed that CdS and ZnTe materials have drastically changed the electronic and optical properties of the bare CdSe/CdTe type-II QDNCs.

Endocytic pathway of vascular cell adhesion molecule 1 in human umbilical vein endothelial cell identified in vitro by using functionalized nontoxic fluorescent quantum dots

Studies about vascular cell adhesion molecule 1 (VCAM1) in tumor growth, metastasis, and angiogenesis suggest that targeting VCAM1 expression is an attractive strategy for diagnosis and anti-tumor therapy. However, the endocytic pathway of VCAM1 in vascular cells has not been well characterized. In this study we visualize the endocytic pathway of tumor necrosis factor α (TNFα) induced VCAM1 in human umbilical vein endothelial cell (HUVEC) in vitro using 5-carboxyfluorescein labeled VCAM1 binding peptides and fluorescent water-dispersible 3-mercaptopropionic acid (3MPA)-coated CdSe-CdS/Cd0.5Zn0.5S/ZnS core–multishell nontoxic quantum dots (3MPA-QDs) functionalized with VCAM1 binding peptides. Clear key in vitro observations are as follows: (a) 3MPA-QDs functionalized with VCAM1 binding peptides, denoted as VQDs, adhered and aggregated cumulatively to cell membrane around 2 h after VQD deposition to cell culture medium and were found in lysosomes in TNFα-treated HUVECs approximately 24 h after VQD deposition; (b) VQDs remained in TNFα-treated HUVECs for the whole 16 days of the experimental observation period; (c) quite differently, 3MPA-QDs were endocytosed then exocytosed by HUVECs via endosomes in about 24–48 h after 3MPA-QD deposition. Our study suggests that VCAM1 molecules, initially expressed on cell membrane induced by TNFα treatment, are internalized into lysosomes. This provides a novel means to deliver materials to lysosomes such as enzyme replacement therapy. Moreover, our meticulous sensing methodology of devising fluorescent nontoxic QDs advances biosensing technique for studying cellular activities in vitro and in vivo.

Structural dependent, dielectric and conduction analysis of CdSe based quantum dots

Nowadays, the effect of temperature on nanocrystal’s dielectric has become a routine study while their structural effects on dielectric properties, lacks the attention of researchers. The correlation of different shell arrangement on dielectric enhancement in CdSe based quantum dots, is addressed in this letter. Detrimental impact of surface defects on dielectric properties of quantum dots is investigated and shown that surface defects of bare core increases dielectric loss, dissipation factor and decreases conductivity of the quantum dots. Dielectric properties of quantum dots is evaluated after deposition of different shells and multishells. It is observed that loss factor is declined in core/shell(multishell) quantum dots due to passivation of surface defects and consequently ease in charge carrier tunneling. Higher conductivity of the core/shell and multishell structures has also been demonstrated here. Structural and optical investigation of the quantum dots after shell deposition is also considered.

Luminescence properties and exciton dynamics of core-multi-shell semiconductor quantum dots leading to QLEDs.

In this study, we grew CdSe core quantum dots (QDs) along with CdSe/ZnS/ZnS and CdSe/CdS/CdS core-multi-shell semiconductor QDs via successive ionic layer adsorption and reaction. The effects of the shell layers on the optical properties and exciton dynamics of the QDs were examined using steady-state and time-resolved measurements together with transient absorption (TA). The photoluminescence (PL) intensity and quantum yield (QY) of the synthesized QDs were enhanced by shell growth. The QY of the QDs increased from 23% to 41% with shell growth due to the decrease in nonradiative recombination sites, which was attributed to surface passivation by the shell layer. The average PL lifetime decreased with shell growth. This effect might be related to the reduction of electron and hole densities on the QD surfaces. The TA spectra show bleaching and photoinduced absorption, corresponding to the state-filling effect and trap states, respectively. The findings are essential for understanding the growth and applications of core-multi-shell QDs.

Optical Properties of Quantum Dots with a Core–Multishell Structure

In the last decade, colloidal semiconductor nanocrystals (quantum dots) have been not only studied fundamentally but also applied in photovoltaics, optoelectronics, and biomedicine. Beginning with simple approaches to the deposition of protective shells, e.g., ZnS on CdSe cores, searches for ways to increase the quantum yield of photoluminescence of quantum dots have resulted now in the development of new types of quantum dots characterized not only by record high extinction coefficients but also by high photoluminescence quantum yields. In this work, the optical properties of core–multishell quantum dots have been analyzed. These quantum dots have been specially designed to reach the maximum possible localization of excited charge carriers inside luminescent cores, which makes it possible to reach a photoluminescence quantum yield close to 100%. Core–multishell quantum dot samples with a shell thickness of 3–7 monolayers have been fabricated. Changes in the characteristics of optical transitions in such quantum dots with an increase in the number of layers of the shell have been studied. The effect of the thickness of the shell on the optical properties of prepared quantum dots has been analyzed. In particular, analysis of photoluminescence lifetimes of such quantum dots has revealed a possible alternative mechanism of radiation of core–multishell quantum dots based on the slow charge carrier transfer from the excited outer layer of the CdS shell to the CdSe core.

A detailed investigation of electronic and optical properties of single exciton in GaAs/AlxGa1-xAs/GaAs/AlyGa1-yAs multi-shell quantum dot

ABSTRACTThe fundamental quantum mechanical properties of quantum dots (QDs), such as energy levels and corresponding wave functions, have important effects on characteristics of hi-tech electro-optical nano devices. The quantum mechanical properties of QDs are determined via the material content, shape and size of the structures. In this study, the electronic and interband optical properties of multi-shell quantum dot (MSQD) heterostructures as a function of the Al concentrations both in the interior and in the exterior barrier regions have been investigated theoretically for different sizes of core, barrier and well regions. For this purpose, Poisson and Schrödinger equations have been solved numerically by a self-consistent approach. Results of our calculations show that the electronic and optical properties of MSQDs can be changed effectively and this controllability makes them excellent candidates for new generation devices.

Intersubband optical properties of three electrons confined in multishell quantum dots: comparison of two semiconducting compounds

We investigated the effect of the number of wells and quantum dot radius on the dipole matrix elements and optical properties of three electrons confined in concentric multishell quantum dots, comparing two different semiconducting compound nanostructures: $$\hbox {GaAs/Ga}_{{x}}\hbox {Al}_{{(1-x)}}\hbox {As}$$ and InGaAs/InAlAs. We used a high-accuracy interpolation-based 14-point finite difference method to solve the corresponding Schrodinger equation. The results showed that, although InGaAs/InAlAs multishell quantum dots have lower absorption peak heights than the $$\hbox {GaAs/Ga}_{{x}}\hbox {Al}_{{(1-x)}}\hbox {As}$$ ones, the effects of the quantum dot radius and number of wells on the optical properties are more intense in the studied InGaAs/InAlAs structures. Therefore, since a wider absorption peak height range can be spanned when using the InGaAs/InAlAs system, these structures are more tunable, which could facilitate selection of desired systems by experimentalists.

Interface optical phonons and their electron-phonon interactions in ZnS/CdS multi-shell spherical quantum dots

The properties of interface optical phonons and their electron-phonon interactions in ZnS/CdS multi-shell spherical quantum dots are studied by adopting dielectric continuum model. The dispersion curves and electron-phonon coupling strengths for interface optical phonons in ZnS/CdS multi-shell dots are calculated and analyzed in detail. It is shown that the number of interface optical phonon branches increase with the increase of layers of multi-shell quantum dots. The number of interface optical phonons increase two branches for increasing one layer material. The dispersions of interface optical phonon with low quantum number l are weak. The electron-interface optical phonon interactions are mainly localized at hetero-interfaces. The interface optical phonons with low quantum number l have important contribution to electron-interface optical phonon interactions.