Shell Quantum Dots Research Articles

Electronic and magnetic properties of many-electron complexes in charged InAsxP1−x quantum dots in InP nanowires

We present here a microscopic theory of electronic complexes in charged $\mathrm{In}{\mathrm{As}}_{x}{\mathrm{P}}_{1\ensuremath{-}x}$ quantum dots in InP nanowires with a hexagonal cross section and determine the potential use of an array of such quantum dots as a synthetic spin chain for the possible construction of a topological qubit. The single-particle energies and wave functions are obtained by diagonalizing a microscopic atomistic tight-binding Hamiltonian of multiple quantum dots in the basis of $s{p}^{3}{d}^{5}{s}^{*}$ local atomic orbitals for a given random distribution of arsenic (As) vs phosphorus (P) atoms. The conduction band electronic states are found grouped into $s, p$, and $d$ quantum dot shells. For a double dot, the electronic shells can be understood in terms of interdot tunneling despite the random distribution of As atoms in each quantum dot. The single- and double-dot structures were charged with a finite number of electrons. The many-body Hamiltonian including Coulomb electron-electron interactions was constructed using single atomistic particle states and then diagonalized in the space of many-electron configurations. For a single dot filled with ${N}_{e}=1--7$ electrons, the ground state of a half-filled $p$-shell configuration with ${N}_{e}=4$ was found with total electronic spin $S=1$. The low-energy spectrum obtained using exact diagonalization of a Hamiltonian of a charged double dot filled with ${N}_{e}=8$ electrons, i.e., half-filled $p$ shells in each dot, was successfully fitted to the Hubbard-Kanamori and antiferromagnetic Heisenberg spin-1 Hamiltonians. The atomistic simulation confirmed the potential of InAsP/InP quantum dots in a nanowire for the design of synthetic spin chains.

Controllable synthesis of CdSe/ZnS core–shell quantum dots by one-step thermal injection and application in light-emitting diodes

Controllable synthesis of CdSe/ZnS core–shell quantum dots by one-step thermal injection and application in light-emitting diodes

A First Wide-Open LDH Structure Hosting InP/ZnS QDs: A New Route Toward Efficient and Photostable Red-Emitting Phosphor.

The architecture of Zn-Al layered double hydroxides (LDHs), organo-modified with bola-amphiphiles molecules, is matching its interlayer space to the size of narrow-band red-emitting InP/ZnS core-shell quantum dots (QDs) to form original high-performance functional organic-inorganic QD-bola-LDH hybrids. The success of size-matching interlayer space (SMIS) approach is confirmed by X-ray diffraction, small angle X-ray scattering (SAXS), TEM, STEM-HAADF, and photoluminescence investigations. The QD-Bola-LDH hybrid exhibits a photoluminescence quantum yield three times higher than that of pristine InP/ZnS QDs and provides an easy dispersion into silicone-based resins, what makes the SMIS approach a change of paradigm compared to intercalation chemistry using common host structures. Moreover, this novel hybrid presents low QD-QD energy transfer comparable to that obtained for QDs in suspension. Composite silicone films incorporating InP/ZnS (0.27wt%) QD-bola-LDH hybrids further show remarkable improved photostability relative to pristine QDs. An LED overlay consisting of a blue LED chip and silicone films loaded with QD-bola-LDH hybrids and YAG:Ce phosphors exhibits a color rendering index close to 94.

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.

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.

Electrosynthesised CdS@ZnS quantum dots decorated multi walled carbon nanotubes for analysis of propranolol in biological fluids and pharmaceutical samples

Quantum dots (QDs) based electrochemical assays have received a great deal of attention. They preserve the outstanding characteristics of electrochemical methodologies with respect to simplicity, ease-of-use, and cost-effective instrumentation. In this work, a new challenge was opened up to explore the electrochemical features of CdS@ZnS as two well-known semiconductors for analytical utilizations. Here, CdS@ZnS core–shell QDs have been prepared electrochemically and applied for electrochemical sensor development in this approach. An activated glassy carbon electrode modified with a thin film of multi walled carbon nanotube was coated with CdS@ZnS QDs (activated GCE/MWCNTs/CdS@ZnS) as a thin uniform layer for analysis and monitoring of propranolol (PRO), a non-selective β-blocker drug. Field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectroscopy (EDX) were used to characterize the surface morphology of the developed electrodes. The results demonstrated that CdS@ZnS QDs were well-dispersed on the activated GCE/MWCNTs surface and were able to enhance the number of adsorbed analyte molecules. The electrochemical behavior of the PRO was studied at the modified electrode surface. The kinetic parameters of electron transfer coefficient (α) and the catalytic rate constant (kcat) for the electron transfer between PRO and the modified electrode were calculated. The electrochemical oxidation mechanism of PRO was also investigated by square wave voltammetry (SWV). Under optimal conditions, the proposed sensor demonstrated a linear response towards propranolol in the range of 0.06–27 μmol L−1 and a detection limit of 12 nmol L−1. Furthermore, the analytical performance of the developed sensor was evaluated for measuring propranolol in biological fluids and pharmaceutical samples. The observed promising results confirm the suitability of the developed sensor for applications in measuring complex materials.

Design and operation of a portable micro-photoluminescence spectrometer for education on semiconductor quantum structures and graphene sheets.

The design and operation of a portable micro-photoluminescence spectrometer for applications in education is described. Guidelines are a compact, robust, portable, and flexible design; operation without cryogenic media for sample cooling; and a limited budget. Targeted samples are semiconductor quantum structures emitting in a wavelength range of 600-1000nm and graphene sheets. The portable spectrometer includes a reflected-light microscope with a motorized sample stage of 156nm step size, a thermoelectric sample cooler allowing temperatures down to 196K, a green and a blue laser for focused excitation, a monochromator with 0.18nm spectral resolution, and a cooled camera as the image sensor. For demonstration of the capabilities of the spectrometer, measurements of the quantized energy levels of molecular beam epitaxy grown GaAs quantum dots (QDs) are shown. Here, different sample designs are used, the sample temperature as well as the laser excitation power and energy is varied, and the respective influence on the measurements is discussed. A clear QD shell structure with four states is shown for a sample, where approximately four QDs are directly excited by a focused laser. Limitations of the spectrometer for QD characterization mainly due to the waiver of cryogenic media for sample cooling are discussed. As a further example, which does not require sample cooling, local Raman spectroscopy of a graphene sheet is demonstrated where clear Raman signatures allow the identification of a single-layer thickness.

CdSe-ZnO Core–Shell Quantum Dots for Protein Detection: A Potential Sensing Platform

A highly sensitive biosensing platform comprised of CdSe-ZnO core–shell nanostructures for targeted applications in protein detection is demonstrated. This innovative technique uses a microwave-assisted thermal decomposition method to produce a rapid, less hazardous, and user-friendly procedure to synthesize a semiconductor core surrounded by nanometer-thick metal oxide shells. The benefit of using a metal oxide shell includes mitigating the toxicity of the CdSe core, thus increasing its biocompatibility and minimizing its photochemical corrosion and oxidation. We present a simple one-pot microwave-assisted protocol for the formation of CdSe-ZnO core–shell quantum dots (QDs). These QDs optimize the recognition limit of bovine serum albumin (BSA) protein through a spectral signal at a considerably low concentration (2.5 × 10−6 M), thus demonstrating its potential to become a highly effective surface-plasmon-enhanced Raman spectroscopy (SERS)-like sensing platform. We report a QD material that can mimic a strong SERS-like behavior due to charge transfer affecting the local electric field.

Quantum dot sensitized solar cells using type-II CdSe-Cu2Se core-shell QDs

Oleylamine capped CdSe-Cu2Se type-II core–shell quantum dots were synthesised by organometallic high temperature route. The nanostructures were characterized using XRD, XPS, HRTEM, optical absorption and EDS and their suitability to serve as an effective sensitizer for TiO2 deposited fluorine doped tin oxide coated glass slides was explored. Employing aqueous sulfide/polysulfide redox couple as the electrolyte, Cu2S as the counter electrode, ZnS as the surface passivation layer and TiO2 as the anode material, quantum dot sensitized solar cells were constructed. A power conversion efficiency of 4.3% was observed for the best performing device. Sintering of the photoanode, before embedding in the cell structure, resulted in an efficiency enhancement to 4.83%.

Upconversion photoluminescence in colloidal CdSe/CdS nanocrystal-based solid films

Upconversion is the process based on the transformation of incoming low energy photons to higher energy emitted radiation. Upconverting materials are exploited in several novel applications like biolabeling and radiation harvesting, among others. Thanks to their intrinsic potential, it is very important to be able to fabricate functional devices based on such materials, and to do so in a controllable and affordable way. We investigated the below-bandgap stimulated radiation of heterostructures, obtained from core–shell semiconductor quantum dots, for the development of a large area upconverting system. The upconversion photoluminescence, after sintering of the nanoparticles, has been measured and the conversion mechanism investigated. The presence of intermediate states within the bandgap of the structure, derived from the specific quasi type-II configuration of the core–shell quantum dots, activated a uniform single photon upconversion mechanism on a wide surface area of the developed films, resulting in an anti-Stokes shift larger than 0.4 eV.

Organometallic synthesis, structural and optical properties of CdSe quantum dots passivated with ternary AgZnS alloyed shell

We report on the surface passivation of a new alloyed ternary AgZnS shell layer on luminescent CdSe quantum dots (QDs) via the organometallic hot-injection pyrolysis of metal precursors, surfactant precursors and organic ligands in a non-coordinating solvent. Transmission electron microscopy, X-ray diffraction, Raman spectroscopy and UV/vis absorption and fluorescence emission spectrophotometry were used to characterize the QD nanocrystals. Fixed AgZnS shell alloying was used as the main fabrication strategy to engineer the band gap of CdSe/AgZnS core/alloyed shell QDs which in turn led to a blended mixture of homogenous and heterogenous particle size growth with spherical, quasi-spherical and trigonal-shaped particles. The crystal structure of CdSe/AgZnS QDs was mainly zinc blende but as the QDs size increased with time, a superimposition of zinc blende and wurtzite hexagonal structure was observed with the former being predominant in the diffraction pattern. The photoluminescence (PL) emission spectra of CdSe/AgZnS QDs were tuned across the visible region while the full width at half maximum was as low as 33 nm, representing an 8 nm decrease compared to 41 nm FWHM exhibited by CdSe QDs. As CdSe/AgZnS core/alloyed shell QDs grew with time, the PL quantum yield (QY) increased from 67% for CdSe to a maximum of ~100% for CdSe/AgZnS QDs and thereafter decreased steadily to a minimum of 36% upon further growth. By achieving ~100% PL QY, it appears that interfacial surface defect states in the core/alloyed shell QDs were completely eliminated via the AgZnS shell alloying process on the CdSe core. The unique optical properties exhibited by CdSe/AgZnS core/alloyed shell QDs as reported in this work shows great promise for light emitting QD applications.

Exciton radiative recombination time in a group III-V/II-VI core/shell quantum dot: Simultaneous effects of pressure and temperature

Excitonic properties of a compound semiconductor from group III-V and group II-VI materials are studied applying external perturbations. The simultaneous effects of pressure and temperature on exciton energies in an InP/ZnS quantum dot (core/shell) are found taking into account the ratio of radius of core/shell dot. The results bring out that the radiative lifetime enhances monotonically when the ratio of radius of core to shell quantum dot increases and it reaches a saturation value when Rc/Rs → 1 and the radiative recombination lifetime enhances upto 9 ns. The results can be applied for tailoring the excitonic optical transitions in nano-optical devices.

Theoretical modeling of a “giant” colloidal core–shell quantum dot with an alloyed interfacial layer for solar cell applications

We proposed a theoretical modeling of a “giant” colloidal core–shell quantum dot with a strain-adapting alloyed interfacial layer between the core and shell materials for solar cell applications. The recently modified detailed balance model [J. Appl. Phys. 125, 174302 (2019)JAPIAU0021-897910.1063/1.5086015] is further modified in this paper to obtain a more realistic conversion efficiency (CE) of the C d S e / C d S e x S 1 − x / C d S quantum dot solar cell. This proposed model computes the CE, considering the significant impact of the energy gap and oscillator strength simultaneously. The CE is investigated for different alloying factors “ x ” by considering the strain effect between the heterostructure. The results obtained in terms of CE and photoluminescence peak wavelength nearly approximate the experimental studies of similar dot structure and dimensions.

Deep learning STEM-EDX tomography of nanocrystals

Energy-dispersive X-ray spectroscopy (EDX) is often performed simultaneously with high-angle annular dark-field scanning transmission electron microscopy (STEM) for nanoscale physico-chemical analysis. However, high-quality STEM-EDX tomographic imaging is still challenging due to fundamental limitations such as sample degradation with prolonged scan time and the low probability of X-ray generation. To address this, we propose an unsupervised deep learning method for high-quality 3D EDX tomography of core–shell nanocrystals, which can be usually permanently dammaged by prolonged electron beam. The proposed deep learning STEM-EDX tomography method was used to accurately reconstruct Au nanoparticles and InP/ZnSe/ZnS core–shell quantum dots, used in commercial display devices. Furthermore, the shape and thickness uniformity of the reconstructed ZnSe/ZnS shell closely correlates with optical properties of the quantum dots, such as quantum efficiency and chemical stability. Advanced electron microscopy and spectroscopy techniques can reveal useful structural and chemical details at the nanoscale. An unsupervised deep learning approach helps to reconstruct 3D images and observe the relationship between optical and structural properties of semiconductor nanocrystals, of interest in optoelectronic applications.

Organic polymer and perovskite CdSe–CdS QDs hybrid thin film: a new model for the direct detection of light elements

A window-less X-ray detector fabrication is a challenging job for physicists and technologists. Using a high atomic number (Z) nano-particles (NPs), such as CdSe–CdS quantum dots (QDs) in nano-form, are the most versatile nano-particles and the most suitable choice for energy-dispersive X-ray spectroscopy analysis of light elements. There is no report on the fabrication of a windowless X-ray detector by using core–shell CdSe–CdS QDs, so far as we know. We have observed the luminescence properties of CdSe/CdS core–shell QDs–cellulose hybrid thin films and characterized them. The blaze union of high photoluminescence (PL) of CdSe/CdS core–shell QDs was synthesized by a high-temperature short-time handling approach with a quantum yield of 70%. The surface, configurationally characterizations, and optical characteristics of the above thin films were examined utilizing X-ray diffraction, transmission electron microscopy, PL, scanning electron microscopy, and I–V trademark strategies. This experimental work confirmed thin films coated with QDs of core–shell CdSe–CdS which improve the PL capability of nano-crystalline cellulose. The distribution of a quantum size possesses a rigid structure in the total growth process of the entire shell during the experiment, which is indicated in the absorption spectra followed by a sharp PL peak at 630 nm. It is expected that the present study will have a good promise for X-ray-sensing applications.

Exciton and Bi-exciton Binding Energy Calculation in a Core Shell Quantum Dot

We report the effect of shell thickness on exciton and biexciton binding energy calculation for a CdSe/ZnSe core shell quantum dot. Wentzel Kramers Brillouin approximation help us to create proper wave function for the system. It incorporates core shell boundary effect correctly. For excitons, the binding energy is affected by the Columbic confinement potentials and during biexciton binding energy calculation, the exchange and correlation effects were taken into account. It is observed that the excitonic binding energy shows an initial increase with increasing shell thickness but saturates for thick shell while it always gives a negative value for bi-excitonic binding energy. This confirms the existence of antibonding states for biexcitons.

Luminescent Quantum Dots Stabilized by N-Heterocyclic Carbene Polymer Ligands

We have tested the ability of N-heterocyclic carbene (NHC)-modified ligands to coordinate and stabilize luminescent CdSe-ZnS core-shell quantum dot (QD) dispersions in hydrophilic media. In particular, we probed the effects of ligand structure and coordination number on the coating affinity to the nanocrystals. We find that such NHC-based ligands rapidly coordinate onto the QDs (requiring ∼5-10 min of reaction time), which reflects the soft Lewis base nature of the NHC groups, with its two electrons sharing capacity. Removal of the hydrophobic cap and promotion of carbene-driven coordination on the nanocrystals have been verified by 1H NMR spectroscopy, while 13C NMR was used to identify the formation of carbene-Zn complexes. The newly coated QD dispersions exhibit great long-term colloidal stability over a wide range of conditions. Additionally, we find that coordination onto the QD surfaces affects the optical and spectroscopic properties of the nanocrystals. These include a size-dependent red-shift of the absorption and fluorescence spectra and a pronounced increase in the measured fluorescence intensity when the samples are stored under white light exposure compared to those stored in the dark.

Tunable Circular Dichroism by Photoluminescent Moiré Gratings

AbstractIn nanophotonics, there is a current demand for ultrathin, flexible nanostructures that are simultaneously easily tunable, demonstrate a high contrast, and have a strong response in photoluminescent polarization. In this work, the template‐assisted self‐assembly of water‐dispersed colloidal core–shell quantum dots into 1D light‐emitting sub‐micrometer gratings on a flexible substrate is demonstrated. Combining such structures with a light‐absorbing metallic counterpart by simple stacking at various angles results in a tunable Moiré pattern with strong lateral contrast. Furthermore, a combination with an identical emitter‐based grating leads to a chiroptical effect with a remarkably high degree of polarization of 0.72. Such a structure demonstrates direct circular polarized photoluminescence, for the first time, without a need for an additional chiral template as an intermediary. The suggested approach allows for reproducible, large‐area manufacturing at reasonable costs and is of potential use for chiroptical sensors, photonic circuit applications, or preventing counterfeit.

Reversible Photoinduced Luminescence Modulation from Nanospheres Containing CdSe/ZnS Quantum Dots and Photochromic Diarylethene

We prepared for the first time photoactive colloidal nanospheres consisting of hydrophobic CdSe/ZnS core–shell quantum dots encapsulated in an amphiphilic PMAT polymeric shell containing photochrom...

Relaxation Dynamics of Excited States in Mn2+-Doped ZnCdS (Core)/ZnS (Shell) Quantum Dots Ions in Propylene Carbonate

It has been shown that luminescence in Mn@Zn0.49Cd0.51S/ZnS quantum dots (QDs) in propylene carbonate is due to a single Mn2+(4T1 → 6A1) band and has a quantum yield of ~24%. The Mn2+(4T1) level is excited through the light absorption in the exciton absorption bands of Zn0.49Cd0.51S/ZnS. The femtosecond dynamics of the difference spectra of Mn@Zn0.49Cd0.51S/ZnS QDs show fast relaxation of the exciton on the subpicosecond–picosecond timescale. At delay times in picosecond range, the band of stimulated luminescence has been detected in difference absorption spectra in the Mn2+(4T1 → 6A1) transition region. A qualitative hypothetical mechanism of nonradiative energy transfer from the exciton to Mn2+(4T1) involving the Auger transition in the trap states has been proposed. The assumption solves the problems of the energy excess of 0.98 eV in the energy transfer to the 3d5 level of Mn and of the conservation of the total spin of the system.