Quantum Dot Structures Research Articles

Droplet free self-assembling of high density nanoholes on GaAs(100) via thermal drilling

Dense arrays of self-assembled nanoholes are fabricated in GaAs(100) surfaces by As-free oxide cleaning thermal process. The formation of pit-like structures above 500 °C occurs by congruent evaporation of GaAs in the areas where the oxides have already desorbed. Thermally etched nanoholes exhibit inverted symmetric conical shape with an average depth/base diameter ratio of 0.21. Shallow and deep nanoholes are formed with this method, their depth ranging from 2 nm to 15 nm. Metallic Indium deposited onto the etched surfaces accumulates at the bottom of the nanoholes, thus making the etched pits a convenient template for the fabrication of high density quantum dot structures.

Statistical model of electronic structure in InAs, InP, GaSb, and Si quantum dots with surface roughness

Simulations based on the sp 3 d 5 s * empirical tight-binding method were performed to provide a statistical understanding of the electronic structures and bandgap distributions of III–V (InAs, InP, GaSb) and IV (Si) semiconductor quantum dots (QDs) with surface roughness. The electronic states and wavefunctions of QDs with surface roughness of different sizes, shapes, and materials were computed. The effects of surface roughness on the electronic structures and the bandgap distributions of QDs were investigated. The results show that the bandgaps of QDs of considered materials/sizes/shapes increase on average when introducing surface roughness. It is shown that the simulated bandgap distributions of QDs with surface roughness can be reproduced by a simple model formula, which can be applied to different materials, sizes, and shapes. The model formula was derived by assuming that removing and adding of one atom procedures are independent random processes.

Spatial-Arrangement-Assisted Emission Energy Fine Tuning of CdSe Quantum Dots (QDs) in QD–Block Copolymer Complexes

Quantum dots (QDs) exhibit size-dependent optical properties, where both the absorption and fluorescence energy levels vary with QD size. However, this dependence results in a discontinuity of intrinsically accessible energy levels for the bandgap, posing challenges in achieving precise energy tuning within a specific range. Herein, we demonstrate emission energy control of QDs with identical absorption energy levels by manipulating the spatial arrangement of QDs within QD–polymer complexes through hydrophobic interactions. The phase behavior of the QD–polymer complexes was modulated by adjusting the mass fraction of hydrophilic and hydrophobic blocks in the block copolymer, utilizing two types of amphiphilic block copolymers and varying temperatures. The QDs were spontaneously trapped within the hydrophobic region of the polymer template in water, resulting in spherical, cylindrical, and vesicle structures of QD–polymer complexes, corresponding to spherical, cylindrical, and layered assemblies of QDs, respectively. Depending on the QDs’ location within the QD–polymer complex, the surface area in contact with water varied, leading to different degrees of oxidation and, consequently, a change in the fluorescence energy level of QDs. This study introduces a novel method to fine tune the emission energy (<15 eV) of QDs by adjusting the polymer form factor without complicated procedures.

A kinetic model of a quantum-dot microlaser

The results of a qualitative analysis of a semiclassical model of light generation in low-dimensional solidstate lasers, including quantum-dot microlasers and, on its basis, a numerical modelling of the regular pulsation regime that occurs under conditions of nonlinear shift and broadening of the resonant spectral gain line due to the influence of dipoledipoles interaction and absorption in quasi-resonant transitions on the dielectric susceptibility of the active medium are herein presented. The modelling of lasing was carried out for the parameters of semiconductor quantum-dot structures.

Effect of interfacial electronic structure on conductivity and space charge characteristics of core-shell quantum dots/polyethylene nanocomposite insulation

To investigate the effect of the interface electronic structure of core-shell quantum dots on the conductivity and space charge characteristics of polyethylene insulation, nanocomposite insulations, namely CdSe@ZnS/LDPE and ZnSe@ZnS/LDPE, are synthesized. The study focuses on elucidating the evolution patterns of DC conductivity and space charge in the nanocomposite insulation, and analyzing the effect of the interfacial electronic structure of core-shell quantum dots on the distribution of charge traps. Comparative analysis reveals that in contrast to LDPE insulation, ZnSe@ZnS/LDPE nanocomposite insulation demonstrates a substantial reduction in DC conductivity by 47.2% and a decrease in space charge accumulation by 40.3% under the conditions of elevated temperature and strong electric field. The increase of trap energy level means an enhanced trap effect on charger carriers. According to density functional theory, the band structure characteristics of core-shell quantum dots integrated with polyethylene are computationally assessed. The findings underscore that the band misalignment at the core-shell interface and the shell-insulation interface induces shifts in the conduction band bottom and at the valence band top, respectively. These shifts impose a confinement effect on electrons and holes, with the extent of this effect escalating with the augment of the difference in band gap between the core layer and the shell layer. Consequently, this phenomenon curtails carrier migration, thereby inhibiting space charge accumulation under the conditions of elevated temperature and strong electric fields.

Ligand effect on surface reconstruction in CdSe quantum dots driven by electron injection in electroluminescence processes.

The short lifetime of blue quantum dots (QDs) in the electroluminescence process is indeed one of the main obstacles that hinder their applications in new display technologies. One of the speculations about the short lifespan is believed to be the reduction reactions at the interface between the QD and the ligand caused by electron injection, but little is known about how the reactions proceed. The evolution of geometrical and electronic structures of ligated (CdSe)6 is simulated with the real-time time-dependent density functional theory (rt-TDDFT) method. Two distinct reactions are characterized in the QDs with different ligand types. One involves the localization of an electron at one specified surface atom, making the ligand separated from the QD, as well as large changes in the QD structures. The other involves the delocalization of an electron across the QD and the ligand, leading to only small changes. In the first case, the destroyed structure becomes irreversible once the ligand fails to re-bond with the QD after the electron-hole recombination. Our simulations provide direct evidence that the reduction reactions caused by electron injection are responsible for the performance loss of blue QDs in the electroluminescence process, and suggest that the delocalization of injected electrons is an interesting strategy for future studies.

Mapping the Ge/InAl(Ga)As interfacial electronic structure and strain relief mechanism in germanium quantum dots

Germanium quantum dots (QDs) with defect-free regions and clusters of stacking faults (SFs) relieved the strain from Ge QDs.

Quantum rate as a spectroscopic methodology for measuring the electronic structure of quantum dots

The quantum mechanical rate concept allows to access the density-of-state of quantum dots assemblies over an electrode interface, allowing for establishing a method of measuring the electronic structure of nanoscale assemblies at room temperature.

Repercussions of the Inner Shell Layer on the Performance of Cd-Free Quantum Dots and Their Light-Emitting Diodes.

Indium phosphide (InP) and zinc selenium tellurium (ZnSeTe) quantum dots (QDs) as less toxic alternatives have received substantial attention. The structure of QDs generally consists of a QD core, inner shell layer, and outer shell layer. We reckon that the inner shell layer, especially its components and thickness, have a significant influence on the optical and electronic performances of QDs. In this Perspective, we compare optical properties of these QDs with different inner shells and summarize how typical inner shell components and thickness influence their optical properties. The impact of the inner shell on the performance of QD light-emitting diodes (QLEDs) has also been discussed. The appropriate components and thickness of the inner shell both contribute to alleviate valence or lattice mismatch, thereby enhancing the performance of QDs. We expect that this Perspective could heighten awareness of the significance and impact of the inner shell layer in QDs and facilitate further development of QDs and QLEDs.

Structure and Defect Engineering of V3S4-xSex Quantum Dots Confined in a Nitrogen-Doped Carbon Framework for High-Performance Sodium-Ion Storage.

Constructing quantum dot-scale metal sulfides with defects and strongly coupled with carbon is significant for advanced sodium-ion batteries (SIBs). Herein, Se substituted V3S4 quantum dots with anionic defects confined in nitrogen-doped carbon matrix (V3S4-xSex/NC) are fabricated. Introducing element Se into V3S4 crystal expands the interlayer distance of V3S4, and triggers anionic defects, which can facilitate Na+ diffusions and act as active sites for Na+ storage. Meanwhile, the quantum dots tightly encapsulated by conductive carbon framework improve the stability and conductivity of the electrode. Theoretical calculations also unveil that the presence of Se enhances the conductivity and Na+ adsorption ability of V3S4-xSex. These properties contribute to the V3S4-xSex/NC with high specific capacity of 447 mAh g-1 at 0.2 A g-1, and prominent rate and cyclic performance with 504 mAh g-1 after 1000 cycles at 10 A g-1. The sodium-ion hybrid capacitors (SIHCs) with V3S4-xSex/NC anode and activated carbon cathode can achieve high energy/power density (maximum 144 Wh kg-1/5960 W kg-1), capacity retention ratio of 71% after 4000 cycles at 2 A g-1. This work not only synthesizes V3S4-xSex/NC, but also provides a promising opportunity for designing quantum dots and utilizing defects to improve the electrochemical properties.

Cross gain modulation in quantum dot semiconductor optical amplifiers under the influence of the Probe

The Pulse effect on the cross-gain modulation (XGM) in the quantum dot (QD) semiconductor optical amplifiers (SOAs) is investigated using the combining of the SOA power with the QD rate equations system. The QDs structure includes three regions: ground state (GS), excited state (ES) and wetting layer (WL). Thus, a set of rate equations for pump and probe signals is introduced for both steady-state and small-signal power values, which are solved numerically. The pulse shape was included in the analysis, for pump and probe signals. The theoretical results showed a good agreement with the experimental results. It was found that decreasing pulse width of pump/probe ratio is efficient to increase XGM efficiency and bandwidth.

Growth of Medium-Sized InP Quantum Dots with High Photoluminescence Efficiency and Enhanced Stability

This study introduces medium-sized core/shell InP quantum dots (QDs) with high photoluminescence (PL) efficiency and remarkable material stability. The incorporation of a ZnSe interlayer between the core and the thicker outer ZnS shell enhances both the optical performance and material stability. When further increasing the S/Se ratio to synthesize larger QDs with a thicker ZnS layer, optical performance significantly declines despite the improvement in stability. To achieve QDs with superior optical performance and stability, we controlled the S/Se ratio variation during shell growth and examined its impact on the structure, morphology, and size of InP QDs. Our approach resulted in medium-sized InP/ZnSe/ZnS QDs with tunable wavelengths (606-630 nm), PL quantum yields (PLQY) > 90%, full width at half maximum (FWHM) values < 40 nm, and enhanced material stability.Keywords: indium phosphide (InP), quantum dots (QDs), high efficiency and stability, S/Se ratio of shell, medium-size quantum dots, stainless steel reactor Figure 1

Modeling of light absorption in self-assembled truncated conical quantum dot structures

Quantum Dots have shown a significant potential as a top candidate for infrared photodetection at higher temperatures. In the presented work, a theoretical model for estimating the coefficient of optical absorption of self-assembled truncated conical quantum dot is developed. This model considers both bound-to-continuum and bound-to-bound absorption mechanisms that increase the accuracy of the absorption coefficient estimation. The developed model is based on estimating the bound states by diagonalizing the Hamiltonian matrix, where the density of states is computed using the Non-Equilibrium Greens function and the effective mass theory to obtain the unbound states. The kinetic equation of Green’s function is solved numerically by finite difference method. Besides, the effects of quantum dot size, height, aspect ratio, and density on the coefficient of the optical absorption are investigated. The results of the developed model are contrasted with those of other alternative QD structures where the truncated conical QD structure results in a higher absorption coefficient in infrared range than semispherical and conical QD structures.

RuOx Quantum Dots Loaded on Graphdiyne for High-Performance Lithium-Sulfur Batteries.

Here, a strategy to strengthen d-p orbital hybridization by fabricating π backbonding in the catalyst for efficient lithium polysulfides (LiPSs) conversion is reported. A special interface structure of RuOx quantum dots (QDs) anchored on graphdiyne (GDY) nanoboxes (RuOx QDs/GDY) is prepared to enable strong Ru-to-alkyne π backdonation, which effectively regulates the d-electron structures of Ru centers to promote the d-p orbital hybridization between the catalyst and LiPSs and significantly boosts the catalytic performance of RuOx QDs/GDY. The strong affinity with Li ions and fast Li-ion diffusion of RuOx QDs/GDY also enable ultrastable Li metal anodes. Thus, S@RuOx QDs/GDY cathodes exhibit excellent cycling performance under harsh conditions, and Li@RuOx QDs/GDY anodes show an ultralong cycling life over 8800h without Li dendrite growth. Lithium-sulfur (Li-S) full cells with S@RuOx QDs/GDY cathodes and Li@RuOx QDs/GDY anodes can deliver an impressive areal capacity of 17.8mA h cm-2 and good cycling stability under the practical conditions of low negative-to-positive electrode capacity (N/P) ratio (N/P = 1.4), lean electrolyte (E/S = 3µL mg-1 ), and high S mass loading (15.4mgcm-2 ).

Shape and quantum size effects on the electronic structure, oscillator strengths and state lifetimes of helium-like quantum dots

Transition energies, electric static polarizability, oscillator strengths and state lifetimes of helium-like quantum dots are determined as a function of their shape and size. A configuration interaction approach based on B-spline functions is used. We found that, the oscillator strengths present an extremum around a low value of the dot radius whatever the shape of the confining potential. This extremum is due to the pressure on the energy levels in such a way that for a particular value of the dot radius, the second electron in the excited one-electron state reaches the borders of the confinement potential. The extremum position depends on the impurity charge and the oscillator strength value at this point changes with the potential well shape. As well, the increase of the effective mass involves the shift of the position of the oscillator strength extremum towards weak radius values. The first excited state lifetime globally increases with the dot radius and the reduction of the nuclear charge but presents a shoulder for a certain low value of the dot radius mostly marked for a triangular form of the confining potential.

Study of the Optical Properties of 3MPA CdTe and 3MPA CdTe/CdSe Quantum Dots at PH 12 in Different Periods of Time

This research aims to study the optical characteristics of semiconductor quantum dots (QDs) composed of CdTe and CdTe/CdSe core-shell structures. It utilizes the refluxed method to synthesize these nanoscale particles and aims to comprehend the growth process by monitoring their optical properties over varied periods of time and pH 12. Specifically, the optical evolution of these QDs is evaluated using photoluminescence (PL) and ultraviolet (UV) spectroscopy. For CdTe QDs, a consistent absorbance and peak intensity increase were observed across the spectrum over time. Conversely, CdTe/CdSe QDs displayed distinctive absorbance and peak intensity variations. These disparities might stem from irregularities in forming selenium (Se) layers around CdTe QDs during growth stages, which could potentially induce quenching in the emission spectrum. The optical examinations unveiled a discernible redshift towards higher wavelength values as the reaction progressed. This spectral shift was coupled with an enlargement in QDs size and a decrease in the energy gap. Using PL and UV analysis techniques enabled a comprehensive study of the optical attributes of the CdTe and CdTe/CdSe QD systems. Our findings underscored the influence of growth conditions and shell materials on the optical properties of QDs. The observed changes in absorbance, peak intensity, wavelength values, QDs size, and energy gap with increasing reaction time provided valuable insights into the growth dynamics of these QD structures.

Panning ideal In(Ga)As(P)/InGaP quantum dot structures for intermediate band solar cells

The In(Ga)As(P)/InGaP quantum dot (QD)system has been investigated for QD intermediate band solar cells. In order to obtain optical transition energies compatible with the ideal ones required for the device record performance, namely: 1.95 eV, 1.24 eV and 0.7 eV, first disordered InGaP with a bandgap of 1.91 eV as the barrier material has been obtained. Then InAs QDs nucleated at 490 °C, 1.32 MLs thick, covered by a 3–4 nm GaAs capping layer and In-flushed provided radiation emission in the energy interval between 1.15 eV and 1.35 eV, fully compatible with the ideal 1.24 eV. The 4 nm capped structures have been proven to exhibit a stronger PL emission at 1.24 eV. Nominal InAs QDs suffer a pronounced incorporation of Ga, a consequence of In/Ga intermixing at their capping layer and barrier interfaces. An As/P intermixing also takes place at the quantum dot—barrier interface. The encouraging results herald further improvement of QD intermediate band solar cell’s performance.

Theoretical Investigation of Organic Ligands on the Surface Structure of InP Quantum Dots: Implications for Display Materials with Enhanced Surface Stability

Theoretical Investigation of Organic Ligands on the Surface Structure of InP Quantum Dots: Implications for Display Materials with Enhanced Surface Stability

Synthesis of diethylenetriamine-modified carbon quantum dots for dual sensing Fe3+ and Co2+and its application

Surface modification is an important method for regulating the carbon quantum dots structure and enhancing the performance. Herein, green fluorescent carbon quantum dots (G-CQDs) were synthesized by the solvothermal method using p-phenylenediamine as carbon source, diethylenetriamine as modifier and anhydrous ethanol as solvent. G-CQDs were characterized by TEM, FTIR, XPS, UV–Vis absorption spectroscopy, fluorescence spectroscopy and Zeta potential. The particle size of G-CQDs was about 2.11 nm and the fluorescence quantum yield of G-CQDs was increased from 1.07 % to 28.90 % after diethylenetriamine modification. The increase of nitrogen and oxygen contents after modification enriches the oxygen-containing and nitrogen-containing groups on the surface, providing more reaction sites for metal ions. It was further found that G-CQDs can detect Fe3+ and Co2+ based on two different fluorescence “on-off” mechanisms with detection limits of 0.88 μM and 0.95 μM, respectively. In practical applications, the excellent fluorescence performance of G-CQDs for information anti-counterfeiting has a good effect. It was used for the detection of Fe3+ and Co2+ in tap water, the spike recovery were in the range of 98.11 % ∼ 106.80 % and 95.87 % ∼ 104.49 %, respectively, showing good accuracy and reproducibility.

Density-functional study of electronic structure of quantum dots in high magnetic fields

Density-functional theory is used to study the electronic structure of quantum dots in magnetic field. New series of magic numbers are found for the total angular momentum of electrons. The empirical formula for the plateau width is obtained. The effect of a charged impurity on the electronic structure of a quantum dot has been studied.