II-VI Quantum Dots Research Articles

Comparison of various core electron treatments for studying the properties of II-VI quantum dots and their bulk counterparts: a DFT study

Comparison of various core electron treatments for studying the properties of II-VI quantum dots and their bulk counterparts: a DFT study

Metal Fluorides Passivate II-VI and III-V Quantum Dots.

Quantum dots (QDs) with metal fluoride surface ligands were prepared via reaction with anhydrous oleylammonium fluoride. Carboxylate terminated II-VI QDs underwent carboxylate for fluoride exchange, while InP QDs underwent photochemical acidolysis yielding oleylamine, PH3, and InF3. The final photoluminescence quantum yield (PLQY) reached 83% for InP and near unity for core-shell QDs. Core-only CdS QDs showed dramatic improvements in PLQY, but only after exposure to air. Following etching, the InP QDs were bound by oleylamine ligands that were characterized by the frequency and breadth of the corresponding ν(N-H) bands in the infrared absorption spectrum. The fluoride content (1.6-9.2 nm-2) was measured by titration with chlorotrimethylsilane and compared with the oleylamine content (2.3-5.1 nm-2) supporting the formation of densely covered surfaces. The influence of metal fluoride adsorption on the air stability of QDs is discussed.

Recent Progress in the Composites of Perovskite Nanocrystals and II-VI Quantum Dots: Their Synthesis, Applications, and Prospects

Abstract: The remarkable photoelectric characteristics of perovskite nanocrystals (NCs), including high fault tolerance, tunable photoluminescence (PL) emission, and high carrier mobility, contribute to making them especially attractive for photonic and optoelectronic applications. Unfortunately, the poor environmental thermal and light stability set obstacles to their industrial applications. Over the past 40 years, II-VI semiconductor quantum dots (QDs) have achieved many important photophysics findings and optoelectronic applications. Compared with perovskite NCs, II-VI semiconductor QDs still have a relatively weaker molar absorbance coefficient. Whereas, significant enhancement of both the stability and the optical performance of the composites of perovskite NCs and II-VI QDs are of interest for photovoltaic and optoelectronic devices. The composites of perovskite NCs and II-VI QDs come in two primary types: core/shell structures and heterojunction structures. To better understand the composites of perovskite NCs and II-VI QDs, the approaches of synthesis methods, their optoelectronic properties, carrier dynamics and potential applications in solar cells, light emitting diodes (LEDs) and photodetectors are summarized. Furthermore, the unmet problems and the potential applications are also presented.

Enhanced Spin Polarization from Biaxially Strained Colloidal Quantum Dots.

Electron and hole spin polarization is crucial for quantum dots to be used in spin lasers and quantum information processing. However, the degree of spin polarization in II-VI and III-V semiconductor quantum dots is low because of the degenerated valence band. Here, we increase the light and heavy hole degeneracy by introducing biaxial strain into CdSe-based quantum dots, enabling the degree of spin polarization to be increased from 20% to 50% under photoexcitation. The optical gain threshold measurement further reveals that the increase in polarization helps to reduce the gain threshold.

Surface Reconstructions in II-VI Quantum Dots.

Although density functional theory (DFT) calculations have been crucial in our understanding of colloidal quantum dots (QDs), simulations are commonly carried out on QD models that are significantly smaller than those generally found experimentally. While smaller models allow for efficient study of local surface configurations, increasing the size of the QD model will increase the size or number of facets, which can in turn influence the energetics and characteristics of trap formation. Moreover, core-shell structures can only be studied with QD models that are large enough to accommodate the different layers with the correct thickness. Here, we use DFT calculations to study the electronic properties of QDs as a function of size, up to a diameter of ∼4.5 nm. We show that increasing the size of QD models traditionally used in DFT studies leads to a disappearance of the band gap and localization of the HOMO and LUMO levels on facet-specific regions of the QD surface. We attribute this to the lateral coupling of surface orbitals and the formation of surface bands. The introduction of surface vacancies and their a posteriori refilling with Z-type ligands leads to surface reconstructions that widen the band gap and delocalize both the HOMO and LUMO. These results show that the surface geometry of the facets plays a pivotal role in defining the electronic properties of the QD.

Design Rules for Obtaining Narrow Luminescence from Semiconductors Made in Solution.

Solution-processed semiconductors are in demand for present and next-generation optoelectronic technologies ranging from displays to quantum light sources because of their scalability and ease of integration into devices with diverse form factors. One of the central requirements for semiconductors used in these applications is a narrow photoluminescence (PL) line width. Narrow emission line widths are needed to ensure both color and single-photon purity, raising the question of what design rules are needed to obtain narrow emission from semiconductors made in solution. In this review, we first examine the requirements for colloidal emitters for a variety of applications including light-emitting diodes, photodetectors, lasers, and quantum information science. Next, we will delve into the sources of spectral broadening, including "homogeneous" broadening from dynamical broadening mechanisms in single-particle spectra, heterogeneous broadening from static structural differences in ensemble spectra, and spectral diffusion. Then, we compare the current state of the art in terms of emission line width for a variety of colloidal materials including II-VI quantum dots (QDs) and nanoplatelets, III-V QDs, alloyed QDs, metal-halide perovskites including nanocrystals and 2D structures, doped nanocrystals, and, finally, as a point of comparison, organic molecules. We end with some conclusions and connections, including an outline of promising paths forward.

Nanoscale LiZnN - Luminescent Half-Heusler Quantum Dots.

Colloidal semiconductor quantum dots are a well-established technology, with numerous materials available either commercially or through the vast body of literature. The prevalent materials are cadmium-based and are unlikely to find general acceptance in most applications. While the III-V family of materials is a likely substitute, issues remain about its long-term suitability, and other earth-abundant materials are being explored. In this report, we highlight a nanoscale half-Heusler semiconductor, LiZnN, composed of readily available elements as a potential alternative system to luminescent II-VI and III-V nanoparticle quantum dots.

Sequential Co-Passivation in InAs Colloidal Quantum Dot Solids Enables Efficient Near-Infrared Photodetectors.

III-V colloidal quantum dots (CQDs) are promising materials for optoelectronic applications, for they avoid heavy metals while achieving absorption spanning the visible to the infrared (IR). However, the covalent nature of III-V CQDs requires the development of new passivation strategies to fabricate conductive CQD solids for optoelectronics: this work shows herein that ligand exchanges, previously developed in II-VI and IV-VI quantum dots and employing a single ligand, do not fully passivate CQDs, and that this curtails device efficiency. Guided by density functional theory (DFT) simulations, this work develops a co-passivation strategy to fabricate indium arsenide CQD photodetectors, an approach that employs the combination of X-type methyl ammonium acetate (MaAc) and Z-type ligands InBr3 . This approach maintains charge carrier mobility and improves passivation, seen in a 25% decrease in Stokes shift, a fourfold reduction in the rate of first-exciton absorption linewidth broadening over time-under-stress, and leads to a doubling in photoluminescence (PL) lifetime. The resulting devices show 37% external quantum efficiency (EQE) at 950nm, the highest value reported for InAs CQD photodetectors.

Advances in Solution-Processed Blue Quantum Dot Light-Emitting Diodes.

Quantum dot light-emitting diodes (QLEDs) have been identified as a next-generation display technology owing to their low-cost manufacturing, wide color gamut, and electrically driven self-emission properties. However, the efficiency and stability of blue QLEDs still pose a significant challenge, limiting their production and potential application. This review aims to analyse the factors leading to the failure of blue QLEDs and presents a roadmap to accelerate their development based on the progress made in the synthesis of II-VI (CdSe, ZnSe) quantum dots (QDs), III-V (InP) QDs, carbon dots, and perovskite QDs. The proposed analysis will include discussions on material synthesis, core-shell structures, ligand interactions, and device fabrication, providing a comprehensive overview of these materials and their development.

An Ultrafast and Room-Temperature Strategy for Kilogram-Scale Synthesis of Sub-5nm Eu3+ -doped CaMO4 Nanocrystals with a Photoluminescence Quantum Yield Exceeding 85.

Rare earth-doped metal oxide nanocrystals have a high potential in display, lighting, and bio-imaging, owing to their excellent emission efficiency, superior chemical, and thermal stability. However, the photoluminescence quantum yields (PLQYs) of rare earth-doped metal oxide nanocrystals have been reported to be much lower than those of the corresponding bulk phosphors, group II-VI, and halide-based perovskite quantum dots because of their poor crystallinity and high-concentration surface defects. Here, an ultrafast and room-temperature strategy for the kilogram-scale synthesis of sub-5nm Eu3+ -doped CaMoO4 nanocrystals is presented, and this reaction can be finished in 1 min underambientconditions. The absolute PLQYs for sub-5nm Eu3+ -doped CaMoO4 nanocrystals can reach over 85%, which are comparable to those of the corresponding bulk phosphors prepared by the high-temperature solid state reaction. Moreover, the as-produced nanocrystals exhibit a superiorthermalstability and their emission intensity unexpectedly increases after sintering at 600°C for 2h in air. 1.9 kg of Eu3+ -doped CaMoO4 nanocrystals with a PLQY of 85.1% can be obtained in single reaction.

Amplified Spontaneous Emission and Lasing from Zn-Processed AgIn5S8 Core/Shell Quantum Dots

I-III-VI ternary quantum dots (QDs) have emerged as favorable alternatives to the toxic II-VI QDs for optoelectronic and biological applications. However, their use as optical gain media for microlasers is still limited by a low fluorescence efficiency. Here, we demonstrate amplified spontaneous emission (ASE) and lasing from colloidal QDs of Zn-processed AgIn5S8 (AIS) for the first time. The passivation treatment on the AIS QDs yields a 3.4-fold enhancement of fluorescence quantum efficiency and a 30% increase in the two-photon absorption cross section. ASE is achieved from the AIS/ZnS core/shell QD films under both one- and two-photon pumping with a threshold fluence of ∼84.5 μJ/cm2 and 3.1 mJ/cm2, respectively. These thresholds are comparable to the best optical gain performance of Cd based-QDs reported in the literature. Moreover, we demonstrate a facile whispering-gallery-mode microlaser of the core/shell QDs with a lasing threshold of ∼233 μJ/cm2. The passivated AIS QDs can be promising optical gain media for photonic applications.

Surface Stoichiometry Control of Colloidal Heterostructured Quantum Dots for High-Performance Photoelectrochemical Hydrogen Generation.

Manipulating the separation and transfer behaviors of charges has long been pursued for promoting the photoelectrochemical (PEC) hydrogen generation based on II-VI quantum dot (QDs), but remains challenging due to the lack of effective strategies. Herein, a facile strategy is reported to regulate the recombination and transfer of interfacial charges through tuning the surface stoichiometry of heterostructured QDs. Using this method, it is demonstrated that the PEC cells based on CdSe-(Sex S1- x )4 -(CdS)2 core/shell QDs with a proper Ssurface /Cdsurface ratio exhibits a remarkably improved photocurrent density (≈18.4mA cm-2 under one sun illumination), superior to the PEC cells based on QDs with Cd-rich or excessive S-rich surface. In-depth electrochemical and spectroscopic characterizations reveal the critical role (hole traps) of surface S atoms in suppressing the recombination of photogenerated charges, and further attribute the inferior performance of excessive S-rich QDs to the impeded charge transfer from QDs to TiO2 and electrolyte. This work puts forward a simple surface engineering strategy for improving the performance of QDs PEC cells, providing an efficient method to guide the surface design of QDs for their applications in other optoelectronic devices.

Slow Hole Localization and Fast Electron Cooling in Cu-Doped InP/ZnSe Quantum Dots.

Impurity doping of low-dimensional semiconductors is an interesting route towards achieving control over carrier dynamics and energetics, e.g., to improve hot carrier extraction, or to obtain strongly Stokes shifted luminescence. Such studies remain, however, underexplored for the emerging family of III-V colloidal quantum dots (QDs). Here, we show through a detailed global analysis of multiresonant pump-probe spectroscopy that electron cooling in copper-doped InP quantum dot (QDs) proceeds on subpicosecond time scales. Conversely, hole localization on Cu dopants is remarkably slow (1.8 ps), yet still leads to very efficient subgap emission. Due to this slow hole localization, common Auger assisted pathways in electron cooling cannot be blocked by Cu doping III-V systems, in contrast with the case of II-VI QDs. Finally, we argue that the structural relaxation around the Cu dopants, estimated to impart a reorganization energy of 220 meV, most likely proceeds simultaneously with the localization itself leading to efficient luminescence.

Confined Excitons in Spherical-Like Halide Perovskite Quantum Dots.

Quantum dots (QDs) offer unique physical properties and novel application possibilities like single-photon emitters for quantum technologies. While strongly confined III-V and II-VI QDs have been studied extensively, their complex valence band structure often limits clear observations of individual transitions. In recently emerged lead-halide perovskites, band degeneracies are absent around the bandgap reducing the complexity of optical spectra. We show that for spherical-like CsPbBr3 QDs with diameters >6 nm, excitons confine with respect to their center-of-mass motion leading to well-pronounced resonances in their absorption spectra. Optical pumping of the lowest-confined exciton with femtosecond laser pulses not only bleaches all excitons but also reveals a series of distinct induced absorption resonances which we attribute to exciton-to-biexciton transitions and are red-shifted by the biexciton binding energy (∼40 meV). The temporal dynamics of the bleached excitons further support our exciton confinement model. Our study provides the first insight into confined excitons in CsPbBr3 QDs and gives a detailed understanding of their linear and nonlinear optical spectra.

Photoluminescence Quenching in CsPbCl3 upon Fe Doping: Colloidal Synthesis, Structural and Optical Properties.

Doped perovskite lead halide nanocrystals (PHNCs) are promising materials for various optoelectronic applications, but the major challenge faced by the researchers is the inability to dope foreign elements into perovskite lattice because of the strong lead-halide bond energies. In this work, we have used Fe as a dopant in CsPbCl3 to explore different doping techniques based on the colloidal synthesis of PHNCs to investigate the advantages and disadvantages of different techniques. We are able to dope a relatively higher amount of Fe (∼10%) than reported and observe clear optical signatures when the precursor does not have pre-existing Pb-Cl bonds. We prove that there are two competing processes inside a doped PHNC - one is the effect of dopant energy levels, and the other is surface passivation by halide ions. Using the most optimal synthesis strategy, we show that although Fe does act as a luminescence quencher in perovskite similar to II-VI quantum dots (QDs), the quenching requires much more Fe compared to trace amounts of Fe required in traditional QDs. Our work will assist in giving an overall comparative idea of doping and finding the most optimized strategy and help identify the underlying physical processes in perovskite based QDs.

Ultra-narrow room-temperature emission from single CsPbBr3 perovskite quantum dots

Semiconductor quantum dots have long been considered artificial atoms, but despite the overarching analogies in the strong energy-level quantization and the single-photon emission capability, their emission spectrum is far broader than typical atomic emission lines. Here, by using ab-initio molecular dynamics for simulating exciton-surface-phonon interactions in structurally dynamic CsPbBr3 quantum dots, followed by single quantum dot optical spectroscopy, we demonstrate that emission line-broadening in these quantum dots is primarily governed by the coupling of excitons to low-energy surface phonons. Mild adjustments of the surface chemical composition allow for attaining much smaller emission linewidths of 35−65 meV (vs. initial values of 70–120 meV), which are on par with the best values known for structurally rigid, colloidal II-VI quantum dots (20−60 meV). Ultra-narrow emission at room-temperature is desired for conventional light-emitting devices and paramount for emerging quantum light sources.

Recent advances in II-VI quantum dots based-signal strategy of electrochemiluminescence sensor

Electrochemiluminescence technology is widely used in the highly sensitive detection of environmental pollutants, biologically active molecules, nucleic acids and tumor markers based on its high sensitivity, good selectivity and excellent stability. Quantum dots have become an ideal electrochemiluminescence luminescent material due to their amazing optical properties, good water solubility and good electrochemiluminescence monochromaticity. The further research on signal transduction strategies of electrochemiluminescence sensors based on II-VI quantum dots on will contribute to the construction of advanced sensors. Therefore, this review summarizes the electrochemiluminescence mechanism and electrochemiluminescence sensing signal transduction strategies based on II-VI quantum dots. Three electrochemiluminescence sensing mode (signal-off transduction strategy, signal-on transduction strategy and ratiometric transduction strategy) are introduced emphatically. Finally, the challenges and future development directions faced by II-VI quantum dots based- electrochemiluminescence sensors are discussed according to the development status of the II-VI quantum dot electrochemiluminescence sensing signal transduction strategy and its application in electrochemiluminescence sensors.

Direct Patterning of CsPbBr3 Nanocrystals via Electron-Beam Lithography.

Lead-halide perovskite (LHP) nanocrystals have proven themselves as an interesting material platform due to their easy synthesis and compositional versatility, allowing for a tunable band gap, strong absorption, and high photoluminescence quantum yield (PLQY). This tunability and performance make LHP nanocrystals interesting for optoelectronic applications. Patterning active materials like these is a useful way to expand their tunability and applicability as it may allow more intricate designs that can improve efficiencies or increase functionality. Based on a technique for II–VI quantum dots, here we pattern colloidal LHP nanocrystals using electron-beam lithography (EBL). We create patterns of LHP nanocrystals on the order of 100s of nanometers to several microns and use these patterns to form intricate designs. The patterning mechanism is induced by ligand cross-linking, which binds adjacent nanocrystals together. We find that the luminescent properties are somewhat diminished after exposure, but that the structures are nonetheless still emissive. We believe that this is an interesting step toward patterning LHP nanocrystals at the nanoscale for device fabrication.

II-VI core/shell quantum dots and doping with transition metal ions as a means of tuning the magnetoelectronic properties of CdS/ZnS core/shell QDs: A DFT study

This paper examines the alterations in the properties of II-VI Quantum Dots (QDs) when these are coated with a shell made of another material of the same family and investigates the structural, electronic and magnetic properties of doped CdS/ZnS core/shell QDs. The core/shell QDs have been constructed by building the shell over the bare core QD and it is found that this construction of a shell over the bare QD can bring about dramatic changes in its optical properties. On changing the shell by varying either the cation or the anion, substantial variations are brought about in the band gap and electrophilicity. The trend of Fermi energies is more negative for core/shell QDs than for the QDs without a shell, and the value is almost the same for core/shell QDs with the same core. Swapping of the core and the shell materials brings greater stability in the case of shells of the wider band gap materials. Binding energy data demonstrates that the CdS/ZnS, CdSe/ZnSe, CdSe/CdS core/shell systems are more stable than ZnS/CdS, ZnSe/CdSe, CdS/CdSe core/shell systems, respectively. An augmentation in the properties is found on doping the QD with transition metal ions. The binding energies are found to be functions of the kind of dopant as well as the spin multiplicity and account for the stability of one spin state over the other at a specific site of the QD. The most fascinating property that plays a decisive role in the extant work is the introduction of magnetism in core/shell QDs as a result of the entry of unpaired electrons within the CdS/ZnS QDs on doping with transition metal ions. The deviation of the observed magnetic moments from the expected values increases as the dopant is varied from Mn2+ to Fe2+ to Co2+ to Ni2+ to Cu2+. Hirshfeld charge analysis shows that the doped ion accepts negative charge from the sulfide ions in the core, with the smallest charge transfer seen in the case of Hg2+ ions. As we move from Mn2+ to Hg2+, the trend followed for the Hirshfeld charges indicates that the overall charge on the core is lower and that on the shell is higher for all the doped cases in comparison to the undoped CdS/ZnS core/shell QD. The band gap values reveal that the Fe2+ doped CdS/ZnS core/shell structures have the smallest band gaps. Hence, we expect that this paper will help researchers to develop a strategy to produce QDs of the anticipated properties for various applications, and transition metal ions can be successfully employed for modification of various magnetoelectronic properties of the host semiconductor for future applications in nanotechnology.

Light hole states in a strained quantum dot: Numerical calculation and phenomenological models

Starting from the numerical solution of the 6-band \textbf{k.p} description of a lattice-mismatched ellipsoidal quantum dot situated inside a nanowire, including a spin Zeeman effect with values appropriate to a dilute magnetic semiconductor, we propose and test phenomenological models of the effect of the built-in strain on the heavy hole, light hole and exciton states. We test the validity and the limits of a description restricted to a ($\Gamma_8$) quadruplet of ground states and we demonstrate the role of the interactions of the light-hole state with light-hole excited states. We show that the built-in axial strain not only defines the character, heavy-hole or light-hole, of the ground state, but also mixes significantly the light-hole state with the split-off band's states: Even for a spin-orbit energy as large as 1 eV, that mixing induces first-order modifications of properties such as the spin value and anisotropy, the oscillator strength, and the electron-hole exchange, for which we extend the description to the light-hole exciton. CdTe/ZnTe quantum dots are mainly used as a test case but the concepts we discuss apply to many heterostructures, from mismatched II-VI and III-V quantum dots and nanowires, to III-V nanostructures submitted to an applied stress and to silicon nanodevices with even smaller residual strains.