Quantum Dot Structures Research Articles

Ice template induced assembly strategy for preparation of 3D porous carbon frameworks from low-cost carbon quantum dots for high-performance lithium-ion batteries

Consider the natural similarities between the structures of coal tar pitch and carbon quantum dots (CQDs), the mass-production of low-cost zero-dimensional (0D) CQDs was achieved by using the mild oxidation with hydrogen peroxide (H2O2). The as-obtained CQDs possesses a uniformly distributed size with a diameter of 4.16 ± 0.46 nm, many oxygen-containing functional groups, distinct crystallinity nature and a production yield of ∼18 wt%. Subsequently, ice-template induced assembly coupled with carbonization strategy reveals a transition from 0D CQDs to 3D porous carbon frameworks. Differing from the well-defined porous carbon, the obtained porous carbon frameworks consist of cross-linked and twisted nanosheets, achieving a well-developed hierarchical porous structure with a reasonable specific surface area and a gradient distribution of meso/micropores. LIBs based on porous carbon frameworks as anode materials show excellent long-cycle performance, as evidenced by reversible capacity of up to 273 mAh·g−1 after 1000 charge-discharge cycles at 2000 mA·g−1. The research emphasizes a green method of preparing CQDs from representative low-value coal tar pitch and the directional transition from 0D CQDs to 3D porous carbon frameworks, providing a promising idea to prepare functional carbon materials and endow them opportunities for LIBs anode.

High-Throughput Characterization of Single-Quantum-Dot Emission Spectra and Spectral Diffusion by Multiparticle Spectroscopy

In recent years, quantum dots (QDs) have emerged as bright, color-tunable light sources for various applications such as light-emitting devices, lasing, and bioimaging. One important next step to advance their applicability is to reduce particle-to-particle variations of the emission properties as well as fluctuations of a single QD’s emission spectrum, also known as spectral diffusion (SD). Characterizing SD is typically inefficient as it requires time-consuming measurements at the single-particle level. Here, however, we demonstrate multiparticle spectroscopy (MPS) as a high-throughput method to acquire statistically relevant information about both fluctuations at the single-particle level and variations at the level of a synthesis batch. In MPS, we simultaneously measure emission spectra of many (20–100) QDs with a high time resolution. We obtain statistics on single-particle emission line broadening for a batch of traditional CdSe-based core–shell QDs and a batch of the less toxic InP-based core–shell QDs. The CdSe-based QDs show significantly narrower homogeneous line widths, less SD, and less inhomogeneous broadening than the InP-based QDs. The time scales of SD are longer in the InP-based QDs than in the CdSe-based QDs. Based on the distributions and correlations in single-particle properties, we discuss the possible origins of line-width broadening of the two types of QDs. Our experiments pave the way to large-scale, high-throughput characterization of single-QD emission properties and will ultimately contribute to facilitating rational design of future QD structures.

Effects of applied external fields on the nonlinear optical second harmonic generation in a GaAs/Ga0.7Al0.3As quantum dots

This work has studied some nonlinear optical properties of Al0.3Ga0.7As/GaAs quantum dot system with tangent-square potential (TSP), with special emphasis on the role of second harmonic generation (SHG) coefficient under hydrostatic pressure and temperature. Due to the strain of external application conditions, the changes of the energy level and wave function of the neutron band in the system structure provide valuable insights into the optical properties of the quantum dot structure. In this paper, we report the effects of structural parameters, temperature and hydrostatic pressure on the SHG coefficient. According to the results obtained in the work, it can be reported that the nonlinear optical properties of the quantum dot system can be improved by adjusting the hydrostatic pressure and temperature. At the same time, the interaction of various parameters has produced some interesting observations, which are relevant in relevant research fields.

Facile Synthesis of Multicolored Stacked Quantum Dot Films for Efficient White Light Emission

Quantum dot (QD)-based white light-emitting diodes (LEDs) or white QD-LEDs were successfully fabricated by stacking QD films of primary colors (i.e., red, green, and blue). An ultrathin ZnO film was deposited between each QD layer to facilitate the formation of a tricolor-stacked QD structure. Such stacked QD films effectively suppress the Förster resonance energy transfer between QDs and enable efficient blue light emission, unlike randomly mixed QD films of different colors. The photoluminescence (PL) spectra of various QD films were obtained to study the change in the corresponding color gamut with applied voltage. In addition, QD-LEDs were fabricated based on the PL behavior and electronic band diagrams of QDs of different colors. Maximum luminance and peak EQE of white QD-LED shows 5700 cd/m2 and 1.1% compared to 2200 cd/m2 and 0.3% of QD-LED with mixed QD layer. Finally, a smooth color transition and white light emission were achieved using a blue/green/red stacking sequence for the QDs, which successfully suppressed the energy transfer effect. The overall performance and brightness of the white QD-LEDs synthesized in this study can be further enhanced by developing high-performance blue QDs.

Relativistic correction of biexciton fine structure in ellipsoidal quantum dots

In the framework of the effective mass approximation the biexciton states have been investigated in the oblate and prolate ellipsoidal quantum dots. To calculate the ground state energy of the biexciton, three-parameters’ variational trial function was utilized. Based on the non-relativistic energy and wave function the lowest-order relativistic correction to the non-relativistic Hamiltonian have been calculated. The relativistic correction energy of the biexciton fine structure have been found to be the order 10−6meV. The relativistic correction approaches zero with increase of the quantum dot sizes. It has been also revealed that the relativistic correction of the biexciton fine structure increases with the increase of size quantization. As a result, the mentioned effect is more pronounced for the case of prolate ellipsoidal quantum dot in the comparison with oblate one.

Stability of self-encapsulated CsPbX3@PbX(OH)(X=Cl,Br) perovskite quantum dots in polar solvents

Inorganic halide perovskite quantum dots has excellent optical and photoelectric properties, so it has great potential in photovoltaic, photoelectric devices, display lighting, medical detection and other fields. However, its commercial application is hampered by its instability. In this paper, the perovskite quantum dots encapsulated by PbX (OH)(X = Cl,Br) can obtain hexahedral CsPbBr3@PbBr(OH), cube CsPbCl3@PbCl(OH) and spindle CsPbCl3@PbBr(OH) between hexahedron and cube. The core-shell encapsulation and interchange combination are realized. Before and after encapsulation, the color coordinates of CsPbX3 (X = Cl,Br) perovskite quantum dots basically did not change, no other crystal phases were found in the XRD lines, which indicatied that the PbX (OH) (X = Cl,Br) encapsulation layer did not change the crystal structure of perovskite quantum dots. PbX (OH) (X = Cl,Br) encapsulation layer plays the role of water-blocking layer and protective layer, which greatly improves the stability of CsPbX3 (X = Cl,Br) perovskite quantum dots in polar solvents. After encapsulation, the photoluminescence quantum yield (PLQY) and time-resolved photoluminescence decay performance of CsPbX3 (X = Cl,Br) perovskite quantum dots are greatly improved. The surface passivation and trap density of perovskite nanocrystals can be reduced by PbX (OH) (X = Cl,Br) wrapping, which extends the fluorescence life of CsPbX3 (X = Cl,Br) and improving the PL stability of perovskite structure. Room temperature (RT) crystallization method does not require inert gas protection, high temperature and high pressure conditions. It also has good repeatability, which provides a better alternative to hot injection. Meanwhile, the improvement of package stability provides a broader prospect for the application and promotion of lead halide perovskite.

Luminescence behavior of colloid quantum dots in the presence anthracycline antibiotic mitoxantrone: Surface interaction and luminescence quenching, size and composition dependence, potential for clinical study

The luminescence of colloidal quantum dots (QDs) is often affected by the passivating layer, the stabilizing ligand, and the interactions in solution. The anthracycline antibiotic mitoxantrone (MTX) effectively quenches QDs luminescence while attaching to their surface in the aqua colloid. Careful selection of the architecture of QDs with significant luminescence quenching in the presence of MTX will make future for developing a simple and fast luminescence turn-off method to determine the concentration of MTX. To find the most sensitive QDs architecture to the presence of MTX, two types of nanoparticles were examined as detector systems: (i) a series of size-selected fractions of non-cadmium AgInS/ZnS QDs and (ii) alloyed CdZnSeS and CdZnSeS/ZnS QDs. All tested QDs in aqueous colloids were coated with thioglycolic acid as a surface stabilizing ligand. An examination of luminescence intensity and lifetime decreasing in the presence of MTX allowed us to evaluate the effect of the structure and composition of QDs on the efficiency of luminescence quenching by MTX. A detailed study demonstrates a predominantly static quenching mechanism, via the formation of complexes with high binding constants (∼106-107 M−1). The wide variability in the composition and size of QDs made it possible to achieve a low MTX detection limits (3 and 5 nM for the third fraction of AgInS/ZnS and CdZnSeS/ZnS QDs, respectively). Thus, the use of luminescent QDs as a direct detection system is promising for monitoring MTX concentrations

Majorana tunneling in a non-Hermitian double-quantum-dot structure

In this paper, we investigate a non-Hermitian quantum transport system, which is formed by two quantum dots possessing PT-symmetric complex potentials and side coupled to Majorana fermions. We study the transmission coefficient first. It is found that the conductance is reduced by a factor of 1/4, i.e., G=e2/4h, which can be seen as a signature of Majorana fermion in this double quantum dots structure. It is also found that the joint effects of Majorana tunneling, PT-symmetric complex potentials and dot energy level are mainly as follows: merging process, asymmetry and exceptional point. Then we study the current and conductance driven by a voltage or temperature gradient. Due to the merging process and asymmetry, the thermal current and conductance change sign drastically near the exceptional point. With large fixed temperature difference, the relationship between thermal current and dot level becomes linear, which is a manifestation of Majorana fermion’s robustness. These results can be useful in understanding the quantum transport behaviors modified by the interplay between PT symmetry and Majorana fermions.

Structural and Optical Characterisation of Size-Selected Glutathione-Capped Colloidal Cu–In–S Quantum Dots

Size-selected series of copper-deficient colloidal Cu–In–S quantum dots (QDs) stabilized with glutathione are obtained by the exchange reaction in aqueous solutions under mild synthesis conditions. The optical bandgap and photoluminescence maximum position shift toward higher energies with decreasing QD size. Based on X-ray diffraction data, the QDs are assigned to a tetragonal chalcopyrite-type structure. The average size of QDs, estimated from the Scherrer formula and from the comparison with the absorption edge-based sizing curves, exhibits a fair agreement, being in the interval of 1.2–2.9 nm. The Raman spectra of Cu–In–S QDs are analyzed with the account for the QD structure, confinement-related effects, non-stoichiometry, and possible existence of secondary phases.

Bi-functional photocatalytic heterostructures combining titania thin films with carbon quantum dots (C-QDs/TiO2) for effective elimination of water pollutants

In this study, carbon quantum dots (C-QDs), prepared via hydrothermal-microwave procedures, were successfully combined with nanostructured titania (TiO2). The photocatalytic oxidation/reduction activity of the C-QDs/TiO2 composite films was evaluated in the decomposition of organic-inorganic contaminants from aqueous solutions under UV illumination. Physicochemical characterizations were applied to investigate the crystal structure of the carbon quantum dots and the composites. It was found that the prepared C-QDs/TiO2 composites had great contribution to the photocatalytic reduction of hexavalent chromium (Cr+6) species and 4-Nitrophenol (PNP) as well as to the photocatalytic oxidation of methylene blue (MB) and Rhodamine B (RhB) dyes. The mechanism of the photocatalytic reaction was studied with trapping experiments, revealing that the electron (e−) radical species were powerfully supported for the photocatalytic reduction of Cr+6 and PNP and the holes (h+) are the main active species for the photocatalytic oxidation reactions.

Regulated the ferromagnetism of In[formula omitted]O[formula omitted] quantum dots by non-stoichiometry compositions

Due to different versions of ferromagnetic origin, the magnetic properties of oxide diluted magnetic semiconductor (ODMS) cannot be accurately determined. We have systematically investigate the magnetic properties and electronic structure of In2O3 quantum dots (Qds) with non-stoichiometry ratio by first-principle method with band-structure calculations. For the sake of contrastive analysis, some models are proposed to passivate the surface dangling bonds by pseudo-hydrogen atoms. We find that: (i), dangling bonds of oxygen atoms on the surface of Qds can induce 6 μB magnetic moments due to imbalanced chemical proportion. Spin polarization energy of the structure is as low as −763 eV. (ii), internal cation vacancy can induce magnetic moment, while internal anionic vacancy or surface hanging bond passivated by pseudo-hydrogen atoms cannot produce magnetic moment; and (iii), it has broken the chemical balance ratio of doping cobalt atoms where the magnetic moment mainly local area in the cobalt atoms and a little are distributed on some special O atoms which binding with the Co atoms. All three kinds of above results indicate that the non-stoichiometry compositions play an special roles to effect the magnetic properties and electronic structure in In2O3 Qds.

The surface chemistry of ionic liquid-treated CsPbBr3 quantum dots.

The power conversion efficiencies of lead halide perovskite thin film solar cells have surged in the short time since their inception. Compounds, such as ionic liquids (ILs), have been explored as chemical additives and interface modifiers in perovskite solar cells, contributing to the rapid increase in cell efficiencies. However, due to the small surface area-to-volume ratio of the large grained polycrystalline halide perovskite films, an atomistic understanding of the interaction between ILs and perovskite surfaces is limited. Here, we use quantum dots (QDs) to study the coordinative surface interaction between phosphonium-based ILs and CsPbBr3. When native oleylammonium oleate ligands are exchanged off the QD surface with the phosphonium cation as well as the IL anion, a threefold increase in photoluminescent quantum yield of as-synthesized QDs is observed. The CsPbBr3 QD structure, shape, and size remain unchanged after ligand exchange, indicating only a surface ligand interaction at approximately equimolar additions of the IL. Increased concentrations of the IL lead to a disadvantageous phase change and a concomitant decrease in photoluminescent quantum yields. Valuable information regarding the coordinative interaction between certain ILs and lead halide perovskites has been elucidated and can be used for informed pairing of beneficial combinations of IL cations and anions.

Identifying Pauli blockade regimes in bilayer graphene double quantum dots

Recent experimental observations of current blockades in 2D material quantum-dot platforms have opened new avenues for spin and valley-qubit processing. Motivated by experimental results, we construct a model capturing the delicate interplay of Coulomb interactions, inter-dot tunneling, Zeeman splittings, and intrinsic spin–orbit coupling in a double quantum dot (DQD) structure to simulate the Pauli blockades. Analyzing the relevant Fock-subspaces of the generalized Hamiltonian, coupled with the density matrix master equation technique for transport across the setup, we identify the generic class of blockade mechanisms. Most importantly, and contrary to what is widely recognized, we show that conducting and blocking states responsible for the Pauli-blockades are a result of the coupled effect of all degrees of freedom and cannot be explained using the spin or the valley pseudo-spin only. We then numerically predict the regimes where Pauli blockades might occur, and, to this end, we verify our model against actual experimental data and propose that our model can be used to generate data sets for different values of parameters with the ultimate goal of training on a machine learning algorithm. Our work provides an enabling platform for a predictable theory-aided experimental realization of single-shot readout of the spin and valley states on DQDs based on 2D-material platforms.

Modeling and Simulation of Output Spectrum of Quantum Dot Doped Broadband Fiber Light Source in 1200-1700 nm Range

In the face of the demand for increased communication capacity of fiber optic communication and the requirement for accuracy and stability of various fiber optic devices, the light source, as one of the key devices, is also required to have higher bandwidth and output power. In this paper, this study model and simulate the output spectrum of a quantum-doped optical fiber light source in the range of 1200nm to 1700nm. The rate equation and power propagation equation are established based on the energy level structure of quantum dots. MATLAB solves the numerical model to obtain the output spectra of quantum dot-doped optical fiber light source under different variables. The fiber length, doping concentration, and pumping optical power was further varied to study the effects of several variables on the peak output spectrum. The results show that the fiber length and doping concentration increase and then decrease the peak value of the output spectrum while the pump power increases the peak value of the output spectrum. A pump power of 200 mW was chosen to obtain good experimental results at a fiber length of 3 m and a doping concentration of . The maximum output power is 4.9 mW, and the spectral line width is about 80 nm. The maximum output power is 69 mW, and the spectral line width is about 100 nm at a fiber length of 4 m and a doping concentration of with a pump power of 400 mW.

Facile synthesis of strongly coupled platinum-molybdenum/silicon carbide quantum-dots heterostructures as electrocatalyst for hydrogen evolution reaction

Strongly coupled platinum-based transition-metal oxide and the development of quantum-dot structures of hybrid catalysts are cost-effective and maximize accessible active sites. However, a significant obstacle still exists for the rational proposal and simple synthesis of hybrid quantum-dot material catalysts. Herein, novel PtxMo1-xSiC quantum dots (PtxMo1-xSiC QDs) for catalyzing the hydrogen evolution reaction (HER) were fabricated through a simple solution-induced impregnation method. The optimized Pt0.05Mo0.95SiC QDs catalyst only require overpotentials of 28 mV to deliver a current density of 10 mA cm−2 in acidic media, respectively. The synergistic effects of the PtxMo1-xSiC QDs networks that promote electron transfer are responsible for the outstanding HER performance. This work presents a novel method for producing an extremely effective HER catalyst for applications on large-scale.

Adjusting Microscale to Atomic-Scale Structural Order in PbS Nanocrystal Superlattice for Enhanced Photodetector Performance.

An investigation is presented into the effect of the long-range order on the optoelectronic properties of PbS quantum dot (QD) superlattices, which form mesocrystals, for potential use in photodetector applications. By self-assembly of QD nanocrystals on an Si/SiOx substrate, a highly ordered and densely packed PbS QD superlattice with a microscale size is obtained. The results demonstrate that annealing treatment induces mesocrystalline superlattices with preferred growth orientation, achieved by dislodging ligands. The improved orientation and electronic coupling of the mesocrystalline superlattices exhibit superior photodetector performance compared to disordered QD structures and closely packed superlattices. This improved performance is attributed to atomic alignment between QDs, leading to enhanced electronic coupling. The findings suggest that these mesocrystalline superlattices have promising potential for the next generation of QD optoelectronic devices.

Circuit quantum electrodynamics with a quadruple quantum dot

In this theoretical work, we describe a mechanism for the coupling between a plane structure consisting of four quantum dots and a resonator. We systematically study the dependence of the quadruple coupling strength and the qubit decoherence rate and point out the optimized operating position of the hybrid system. According to the transmission given by the input–output theory, the signatures in the resonator spectrum are predicted. Furthermore, based on the parameters already achieved in previous works, we prove that the device described in this paper can achieve the strong coupling limit, i.e., this approach can be used for system extension under the existing technical conditions. Our results show an effective and promotable approach to couple quantum dot structures in plane with the resonator and propose a meaningful extension method.

Physics-informed reduced-order learning from the first principles for simulation of quantum nanostructures

Multi-dimensional direct numerical simulation (DNS) of the Schrödinger equation is needed for design and analysis of quantum nanostructures that offer numerous applications in biology, medicine, materials, electronic/photonic devices, etc. In large-scale nanostructures, extensive computational effort needed in DNS may become prohibitive due to the high degrees of freedom (DoF). This study employs a physics-based reduced-order learning algorithm, enabled by the first principles, for simulation of the Schrödinger equation to achieve high accuracy and efficiency. The proposed simulation methodology is applied to investigate two quantum-dot structures; one operates under external electric field, and the other is influenced by internal potential variation with periodic boundary conditions. The former is similar to typical operations of nanoelectronic devices, and the latter is of interest to simulation and design of nanostructures and materials, such as applications of density functional theory. In each structure, cases within and beyond training conditions are examined. Using the proposed methodology, a very accurate prediction can be realized with a reduction in the DoF by more than 3 orders of magnitude and in the computational time by 2 orders, compared to DNS. An accurate prediction beyond the training conditions, including higher external field and larger internal potential in untrained quantum states, is also achieved. Comparison is also carried out between the physics-based learning and Fourier-based plane-wave approaches for a periodic case.

Impact of Local Composition on the Emission Spectra of InGaN Quantum-Dot LEDs.

A possible solution for the realization of high-efficiency visible light-emitting diodes (LEDs) exploits InGaN-quantum-dot-based active regions. However, the role of local composition fluctuations inside the quantum dots and their effect of the device characteristics have not yet been examined in sufficient detail. Here, we present numerical simulations of a quantum-dot structure restored from an experimental high-resolution transmission electron microscopy image. A single InGaN island with the size of ten nanometers and nonuniform indium content distribution is analyzed. A number of two- and three-dimensional models of the quantum dot are derived from the experimental image by a special numerical algorithm, which enables electromechanical, continuum k→·p→, and empirical tight-binding calculations, including emission spectra prediction. Effectiveness of continuous and atomistic approaches are compared, and the impact of InGaN composition fluctuations on the ground-state electron and hole wave functions and quantum dot emission spectrum is analyzed in detail. Finally, comparison of the predicted spectrum with the experimental one is performed to assess the applicability of various simulation approaches.

Near infrared photoluminescence of Si1–xGex quantum dots fabricated by double hot Ge+/Si+ implantation into SiO2 layer

In this study, we developed very simple and ULSI (ultra large scale integration) compatible fabrication processes for group-IV (Si1–xGex and Si) semiconductor quantum dots (QDs) to apply hybrid ULSIs with photonic and electron devices, using double Ge+/Si+ hot-ion implantation into a SiO2 layer with larger bandgap EG and the post-furnace annealing. We successfully demonstrated the near-infrared (IR) photoluminescence (PL) from Si1–xGex-QDs. Transmission electron microscopy observations of single-crystallized Si1–xGex-QDs revealed that the diameter and the QD density were 3.6 ± 0.9 nm and (2.6 ± 0.4) × 1012 cm−2, respectively. In addition, Ge atoms were detected in the Si1–xGex-QDs by the energy dispersive x-ray spectroscopy analysis, and the Ge fraction of Si1–xGex-QDs was varied from 0.06 to 0.26 by changing the Ge ion dose. The increase in the PL intensity by forming gas annealing was attributed to the dangling-bond reduction by the H-atom termination method. The PL spectrum of Si1–xGex-QDs was fitted by PL components of two QD structures containing Si1–xGex and Si materials. The PL intensity and PL-peak photon energy of Si1–xGex-QDs strongly depended on the Ge fraction. The Si1–xGex-QDs achieved the maximum PL intensity at x ≈ 0.13. High PL-peak photon energy (∼1.31 eV) of Si1–xGex-QDs is attributed to the quantum confinement effect of carriers in QDs. Consequently, group-IV semiconductor QDs including Si1–xGex, Si, SiC, and C, through the simple hot-ion implantation into the SiO2 layer, exhibited a wide range of PL emissions from the near-IR to ultraviolet regions.