II-VI Quantum Dots Research Articles

Stable and highly efficient blue-emitting CsPbBr3 perovskite nanomaterials via kinetic-controlled growth

Compared to the classical II-VI quantum dots (QDs), the reaction kinetics of cesium lead halide perovskite nanocrystals (NCs) are not clear yet because of the fast reaction rate. Here, an Ag+-assisted slow-release strategy is proposed to synthesize CsPbBr3 nanomaterials, where the reaction rate is decreased and the reaction time is prolonged to days. It is found that the Ag+ ions can bind with the PbBr2 precursor, which makes the formation of PbBr64- as the rate-limiting step. Shape conversion from dots to nanowires and then to platelets has been observed for the CsPbBr3. Pure blue-emitting nanoplatelets (NPLs) with near unity photoluminescence quantum yield (QY) are acquired. In addition, the blue-emitting CsPbBr3 solution is very stable, which has been stored for over 1.5 years without performance degradation. A liquid light-emitting diode (LED) prototype is further fabricated and exhibits excellent stability towards driving currents.

Femtosecond Transfer and Manipulation of Persistent Hot-Trion Coherence in a Single CdSe/ZnSe Quantum Dot.

Ultrafast transmission changes around the fundamental trion resonance are studied after exciting a p-shell exciton in a negatively charged II-VI quantum dot. The biexcitonic induced absorption reveals quantum beats between hot-trion states at 133GHz. While interband dephasing is dominated by relaxation of the P-shell hole within 390fs, trionic coherence remains stored in the spin system for 85ps due to Pauli blocking of the triplet electron. The complex spectrotemporal evolution of transmission is explained analytically by solving the Maxwell-Liouville equations. Pump and probe polarizations provide full control over amplitude and phase of the quantum beats.

Site- and Spatial-Selective Integration of Non-noble Metal Ions into Quantum Dots for Robust Hydrogen Photogeneration

Summary Artificial photosystems consisting of semiconductor quantum dots (QDs) and non-noble metal ions (e.g., Fe2+, Co2+, Ni2+) show intrinsic advantages in efficient and cost-effective hydrogen (H2) evolution. However, well-controlled integration of these metal ions into ultra-small QDs is a major challenge. Here, we present a two-step strategy to realize site- and spatial-selective integration of earth-abundant Fe2+ ions in CdSe QDs; i.e., CdSe QDs are modified with a thin and anisotropic ZnS shell, and partial Zn2+ ions are selectively substituted by Fe2+ via cation exchange. The anisotropic ZnS layer not only passivates CdSe core but also works as an ideal medium to intimately anchor Fe2+. The multifunctional CdSe/Zn1−xFexS QD exhibits extraordinary activity and stability toward H2 photogeneration. Specifically, more than 880 mL (∼39,300 μmol) H2 gas is produced from 6 mL of aqueous solution during a consecutive 172 h (>1 week) experiment, thus achieving a turnover number of >600,000 per QD.

Highly Efficient Near-Infrared Light-Emitting Diodes Based on Chloride Treated CdTe/CdSe Type-II Quantum Dots.

Quantum dot light-emitting diodes (QLEDs) have been considered as the most promising candidate of light sources for the new generation display and solid-state lighting applications. Especially, the performance of visible QLEDs based on II-VI quantum dots (QDs) has satisfied the requirements of the above applications. However, the optoelectronic properties of the corresponding near-infrared (NIR) QLEDs still lag far behind the visible ones. Here, we demonstrated the highly efficient NIR QLEDs based on chloride treated CdTe/CdSe type-II QDs. The maximum radiant emittance and peak external quantum efficiency (EQE) increased by 24.5 and 26.3%, up to 66 mW/cm2 and 7.2% for the corresponding devices based on the chloride treated CdTe/CdSe QDs with the PL peak located at 788 nm, respectively, compared with those of devices before chloride treatment. Remarkably, the EQE of > 5% can be sustained at the current density of 0.3–250 mA/cm2 after the chloride treatment. Compared with NIR LEDs based on transition metal complex, the efficiency roll-off has been suppressed to some extent for chloride treated CdTe/CdSe based NIR QLEDs. Based on the optimized conditions, the peak EQE of 7.4, 5.0, and 1.8% can be obtained for other devices based on chloride treated CdTe/CdSe with PL peak of 744, 852, and 910 nm, respectively. This improved performance can be mainly attributed to the chloride surface ligand that not only increases the carrier mobility and reduces the carrier accumulation, but also increases the probability of electron-hole radiative efficiency within QD layers.

Heterometal-Doped M23 (M = Au/Ag/Cd) Nanoclusters with Large Dipole Moments.

Dipole moment (μ) is a critical parameter for molecules and nanomaterials as it affects many properties. In metal-thiolate (SR) nanoclusters (NCs), μ is commonly low (0-5 D) compared to quantum dots. Herein, we report a doping strategy to give giant dipoles (∼18 D) in M23 (M = Au/Ag/Cd) NCs, falling in the experimental trend for II-VI quantum dots. In M23 NCs, high μ is caused by the Cd-Br bond and the arrangement of heteroatoms along the C3 axis. Strong dipole-dipole interactions are observed in crystalline state, with energy exceeding 5 kJ/mol, directing a "head-to-tail" alignment of Au22-nAgnCd1(SR)15X (SR = adamantanethiolate) dipoles. The alignment can be controlled by μ via doping. The optical absorption peaks of M23 show solvent polarity-dependent shifts (∼25 meV) with negative solvatochromism. Detailed electronic structures of M23 are revealed by density functional theory and time-dependent DFT calculations. Overall, the doping strategy for obtaining large dipole moments demonstrates an atomic-level design of clusters with useful properties.

Silicon Quantum Dots: Synthesis, Encapsulation, and Application in Light-Emitting Diodes.

Silicon quantum dots (SiQDs) are semiconductor Si nanoparticles ranging from 1 to 10 nm that hold great applicative potential as optoelectronic devices and fluorescent bio-marking agents due to their ability to fluoresce blue and red light. Their biocompatibility compared to conventional toxic Group II-VI and III-V metal-based quantum dots makes their practical utilization even more attractive to prevent environmental pollution and harm to living organisms. This work focuses on their possible use for light-emitting diode (LED) manufacturing. Summarizing the main achievements over the past few years concerning different Si quantum dot synthetic methods, LED formation and characteristics, and strategies for their stabilization by microencapsulation and modification of their surface by specific ligands, this work aims to provide guidance en route to the development of the first stable Si-based light-emitting diodes.

Influence of the cationic composition on the optical properties and photostability of AgInS2 and AgInS2/ZnS quantum dots

Semiconductor I-III-VI nanocrystals are highly luminescent low toxic materials with direct band gap and tunable emission in visible and near infrared region which can be synthesized in aqueous media via simple procedure making them competitive object in compare with well developed and investigated II-VI quantum dots. However the nature of radiative recombination pathways definition and effect of synthesis conditions on spectral characteristics of the nanocrystals investigation remains an important task. In the present work influence of the cation precursors ratio as well as the heat treatment duration on the composition, optical properties and photostability of AgInS2 and AgInS2/ZnS nanocrystals synthesized in aqueous media have been investigated. Due to the low reactivity of indium salts in aqueous solution coused by formation of a stable complexes with stabilizer or hydroxycomplexes the nearest stoichiometric Ag1,1InSx quantum dots are formed at the initial [In]:[Ag] ratio 7:1. Under high excess of Ag formation of Ag2S/AgInS2 core/shell quantum dots confirming by presence of large 12-15 nm nanocrystals and red shift of the photoluminescence maximum with increasing [In]:[Ag] ratio from 1 to 3 (shell thickness should increase proportionally) is possible. With a further increase of the [In]:[Ag] ratio the absorption edge and the photoluminescence maximum are green shifted indicating increase of the quantum dots band energy. That can be explained by lowering of the valence band ceiling energy and rising of the conduction band bottom energy due to decrease of density of states of Ag 4d orbitals and increase of density of states of In 5s and 5p orbitals involved in the AgInS2 band gap formation. Increase of Indium content leads to significant increase of the photoluminescence intensity of AgInS2 nanocrystals eliminating nonradiative defects such as Agi. It have been shown that the ZnS shell epitaxial grow occurs due to the cation exchange between Zn2+ and In3+ and takes place only at low concentration of Zinc precursor ([Zn]:[Ag] ≤ 4). With further enhancement of the ratio the solid solution of AgInS2-ZnS is forming. To achieve the highest possible photoluminescence intensity and energy the AgInS2-ZnS nanocrystals should be heated at 950C at least for 120 minutes. It have been shown that the nearly stoichiometric Ag1,1InSx nanocrystals posess the highest photostability under UV light irradiation.

Radiative and non-radiative exciton recombination rate constants in ZnSe clusters

Understanding the origin of photoluminescence intermittency and its correlation with microstructure is crucial for the design and preparation of quantum dots (QDs) with high fluorescence quantum yield. ZnSe clusters provide a typical model for studying the effect of their size, geometrical and electronic structures on their radiative and non-radiative process of II-VI QDs. The rate constants of radiative and non-radiative processes, kr and knr, of the (ZnSe)n clusters were computed by using first-principles calculations, Einstein spontaneous radiation theory and Fermi’s golden rule. The kr and knr variations with cluster size were analyzed in term of a number of quantities. Emission energy and reorganization energy were identified to play dominant roles in the determination of kr and knr for the studied clusters. Furthermore, a correlation between these two quantities and the geometric rigidity of the ZnSe clusters was revealed. The clusters with greater geometric rigidity tend to possess larger emission energy, and smaller reorganization energy. While radiative and non-radiative electron–hole recombination rates of the ZnSe clusters vary in a complicated way because of their diverse structures and prominent quantum size effect, our study highlights the correlation between recombination rate and cluster structure, which would be helpful for the design of QD materials with high fluorescence quantum yields.

Hybrid Light Emitters and UV Solar-Blind Avalanche Photodiodes based on III-Nitride Semiconductors.

In the last two decades, remarkable progress has been achieved in the field of optoelectronic devices based on III-nitride semiconductors. In terms of photonics applications in the visible-UV spectral range, III-nitrides are one of the most promising materials. For instance, emerging gallium nitride (GaN)-based micro-light-emitting diode (LED) technology for high-resolution display, and UV photo-detection for environmental monitoring, health, and medical applications. In this work, hybrid micro/nano-LEDs with integration of II-VI quantum dots by means of lithography and nano-imprinting patterning techniques are demonstrated, and high-performance red/green/blue and white emissions are achieved. Consequently, plasmonic nanolasers are designed and fabricated using a metal-oxide-semiconductor structure, where strong surface plasmon polariton coupling leads to the efficient lasing with a low excitation threshold from the visible to UV tunable spectral range. Furthermore, performance-improved AlGaN UV solar-blind avalanche photodiodes (APDs) with a separate absorption and multiplication structure by polarization engineering are reported. These APDs deliver a record-high avalanche gain of up to 1.6 × 105 . These newest advances in nano/micro-LEDs, nanolasers, and APDs can shed light on the emerging capabilities of III-nitride in cutting-edge applications.

Synergistic Path for Dual Anisotropic and Electrically Switchable Emission From a Nanocomposite of CsPbBr3 Quantum Cuboids and Nematic Liquid Crystal

We report photophysical properties of a nanocomposite consisting of perovskite quantum cuboids (QCs) formed by CsPbBr3 and a wide temperature range nematic liquid crystal. Contrary to observations made with conventional II-VI quantum dots dispersed in a liquid crystal, the used QCs form, under the influence of the nematic orientation, linear assemblies over macroscopic length scales evidenced by polarizing optical microscopy. Interestingly, the linear assembly is actually caused by such an anisotropic arrangement at the nm scale, as seen in TEM images. Thin films of the nanocomposite exhibiting this unique and fascinating character exhibit absorption and emission features, which are quite appealing. These include retention of the sharp bandwidth of emission characteristic of the native QCs and establishment of dual anisotropies, arising from the values being different along the director as well in the two directions orthogonal to it. We also present data on voltage-driven switching between one of the anisotropic limits.

Engineering Auger recombination in colloidal quantum dots via dielectric screening

Auger recombination is the main non-radiative decay pathway for multi-carrier states of colloidal quantum dots, which affects performance of most of their optical and optoelectronic applications. Outstanding single-exciton properties of CdSe/CdS core/shell quantum dots enable us to simultaneously study the two basic types of Auger recombination channels—negative trion and positive trion channels. Though Auger rates of positive trion are regarded to be much faster than that of negative trion for II-VI quantum dots in literature, our experiments find the two rates can be inverted for certain core/shell geometries. This is confirmed by theoretical calculations as a result of geometry-dependent dielectric screening. By varying the core/shell geometry, both types of Auger rates can be independently tuned for ~ 1 order of magnitude. Experimental and theoretical findings shed new light on designing quantum dots with necessary Auger recombination characteristics for high-power light-emitting-diodes, lasers, single-molecular tracking, super-resolution microscope, and advanced quantum light sources.

Defect-induced magnetism in II-VI quantum dots

The identification of physical parameters that lead to magnetism in apparent nonmagnetic semiconductor systems has continuously challenged our community. In particular, for quantum-dot systems, they are key factors that contribute to their magneto-optical properties. We report, experimentally, optical evidences of induced nanomagnetism in nonmagnetic CdSe quantum dots and assess theoretically the role played by charged and uncharged vacancies. The analysis of these effects rests upon both the chemical and strain environments where the quantum dots are embedded. The interplay of spin-orbit interaction with built-in axial strains has been demonstrated to be a key factor for the magnetic moment alignment. This has been achieved in this paper by emulating the electronic structure at atomistic levels, considering various defect configurations and taking into account both the quantum dot composition and the influence of the host lattice.

Some radiation effects in II-VI quantum dots

The effect of electron irradiation (E=0.6-1.8 MeV) on the optical characteristics (photoluminescence, PL) of CdTe/ZnTe structures with quantum dots, QDs, was investigated in the temperature range from 4.2 to 250 K. The data on the influence of irradiation on the temperature dependence of PL intensity, energy position and PL line width, W, from QDs were obtained. The narrowing of PL band and the blue shift of the QD peak position are explained by quenching of the low energy component connected with larger QDs. A slight decrease in PL intensity for both QDs and the buffer ZnTe layer as well as a sharp drop in PL intensity for 1 ML CdTe quantum wells, QWs, and fragments accompanied by a change in the activation energy of the PL quenching is explained by the radiation defects localized near the QD interface. The obtained results show that under e-beam irradiation QDs are more stable than QWs, which is in agreement with our previous investigations.

Spectroscopy of Surface-State p-Doped CdSe/CdS Quantum Dots.

Transient absorption (TA) and time-resolved photoluminescence (PL) spectroscopies have been used to provide direct spectroscopic evidence for the recently reported phenomenon of thermal "surface charging" in II-VI quantum dots (QDs). In these studies, zincblende CdSe cores are synthesized by standard methods, and a thin CdS shell deposited by the decomposition of Cd(DDTC)2, resulting in core/shell QDs with chalcogenide-rich surfaces. Following ligand exchange with oleylamine, these QDs have empty low-lying surface states that can be thermally populated from the valence band. At room temperature, the surface charging equilibrium results in some fraction of the particles having a hole in the valence band, i.e., the surface acceptor states make the particle p-type. Photoexcitation of the surface charged state results in what is essentially a positive trion, which can undergo a fast Auger recombination. Both PL and TA (bleach recovery) kinetics of the CdSe/CdS QDs show a 70 ps decay component, which is assigned to Auger recombination. The empty nonbonding surface orbitals are passivated by ligation with a trialkylphosphine, and the fast decay component is absent when tributylphosphine is present. The comparison of the TA and PL kinetics shows that the relative amplitude of the 70 ps component is a factor of about 1.5 greater in the TA than in the PL. They also show that the fast component in the PL spectrum is shifted about 6 nm to the blue of the exciton luminescence. The above observations can be understood in terms of the trion versus exciton spectroscopy and strongly support the assignment of the 70 ps transient to the decay of a trion formed from the surface charged state.

Enhancement of Two-Photon Fluorescence and Low Threshold Amplification of Spontaneous Emission of Zn-processed CuInS2 Quantum Dots

Heavy-metal-free I-III-VI2 quantum dots (QDs) have recently emerged as favorable alternatives to toxic II-VI QDs for optoelectronic and biological applications, but low fluorescence efficiency is a key issue related to the promising nanocrystals. In this work, we demonstrate big enhancement of two-photon fluorescence efficiency and low threshold two-photon pumped amplification of spontaneous emission (ASE) of Zn-processed CuInS2 (CIS) QDs for the first time. The change of two-photon process caused by Zn2+ cation exchange in CIS QDs is systematically investigated. Two-photon ASE in Zn-processed CIS QDs films is achieved with a record of low threshold fluence of 5.7μJ/cm^2. With high two-photon fluorescence quantum yields and low threshold ASE, the Zn-processed CIS QDs could become a promising candidate for two-photon bio-imaging, nonlinear optical and novel QDs laser devices.

Uncovering active precursors in colloidal quantum dot synthesis

Studies of the fundamental physics and chemistry of colloidal semiconductor nanocrystal quantum dots (QDs) have been central to the field for over 30 years. Although the photophysics of QDs has been intensely studied, much less is understood about the underlying chemical reaction mechanism leading to monomer formation and subsequent QD growth. Here we investigate the reaction mechanism behind CdSe QD synthesis, the most widely studied QD system. Remarkably, we find that it is not necessary for chemical precursors used in the most common synthetic methods to directly react to form QD monomers, but rather they can generate in situ the same highly reactive Cd and Se precursors that were used in some of the original II-VI QD syntheses decades ago, i.e., hydrogen chalcogenide gas and alkyl cadmium. Appreciating this surprising finding may allow for directed manipulation of these reactive intermediates, leading to more controlled syntheses with improved reproducibility.

Suppression of slow decaying emission in II-VI quantum dots with Förster resonance energy transfer

We report on time-resolved photoluminescence studies of Förster resonance energy transfer (FRET) in structures with two arrays of epitaxial Cd(Zn)Se quantum dots (QDs) of different sizes separated by the ZnSe barrier of a variable width. The acceleration of recombination rate of both fast and slowly decaying components of emission from the energy-donating small QDs with the decrease of the barrier width is well consistent with the FRET mechanism. The found Förster radii turn out to be different for the fast and slow components. The rate acceleration is accompanied by the strong suppression of the slow emission component related, presumably, to the dark excitons. These findings open a way to control the characteristic of QD-based devices.

Aqueous Synthesis of Protein-Encapsulated ZnSe Quantum Dots and Physical Significance of Semiconductor-Induced CuII Ion Sensing.

In view of their promising bio-applicability, we have synthesized water-soluble bovine serum albumin (BSA)-encapsulated ZnSe quantum dots (QDs) with visible emission with longer average luminescence lifetimes of approximately 125 ns at ambient conditions. BSA-ZnSe QDs are shown to be efficient selective copper ion probes in the presence of physiologically important metal ions through luminescence quenching with a high Stern-Volmer constant (3.3×105 m-1 ). The mechanism of sensing has been explained in terms of electron transfer processes and the apparent rate of electron transfer (Ket ) from ZnSe QDs to Cu2+ has been calculated to be 2.8×108 s-1 . It is demonstrated that the negative conduction band potential plays a major role in the feasibility of the electron transfer process, which is reflected in the higher efficacy of ZnSe QDs in sensing copper(II) ions over other group II-VI quantum dots, namely, CdSe, ZnS, or CdS. The results observed with cysteine-capped QDs are almost identical to those with BSA-encapsulated QDs and this presumably negates the possible reason of CuII ion induced quenching ascribed to its binding with surface groups or replacement of metal sites as proposed by several groups previously.

Lead Halide Perovskite Nanocrystals in the Research Spotlight: Stability and Defect Tolerance.

This Perspective outlines basic structural and optical properties of lead halide perovskite colloidal nanocrystals, highlighting differences and similarities between them and conventional II–VI and III–V semiconductor quantum dots. A detailed insight into two important issues inherent to lead halide perovskite nanocrystals then follows, namely, the advantages of defect tolerance and the necessity to improve their stability in environmental conditions. The defect tolerance of lead halide perovskites offers an impetus to search for similar attributes in other related heavy metal-free compounds. We discuss the origins of the significantly blue-shifted emission from CsPbBr3 nanocrystals and the synthetic strategies toward fabrication of stable perovskite nanocrystal materials with emission in the red and infrared parts of the optical spectrum, which are related to fabrication of mixed cation compounds guided by Goldschmidt tolerance factor considerations. We conclude with the view on perspectives of use of the colloidal perovskite nanocrystals for applications in backlighting of liquid-crystal TV displays.

Solution‐Grown CsPbBr3/Cs4PbBr6 Perovskite Nanocomposites: Toward Temperature‐Insensitive Optical Gain

With regards to developing miniaturized coherent light sources, the temperature-insensitivity in gain spectrum and threshold is highly desirable. Quantum dots (QDs) are predicted to possess a temperature-insensitive threshold by virtue of the separated electronic states; however, it is never observed in colloidal QDs due to the poor thermal stability. Besides, for the classical II-VI QDs, the gain profile generally redshifts with increasing temperature, plaguing the device chromaticity. Herein, this paper addresses the above two issues simultaneously by embedding ligands-free CsPbBr3 nanocrystals in a wider band gap Cs4 PbBr6 matrix by solution-phase synthesis. The unique electronic structures of CsPbBr3 nanocrystals enable temperature-insensitive gain spectrum while the lack of ligands and protection from Cs4 PbBr6 matrix ensure the thermal stability and high temperature operation. Specifically, a color drift-free stimulated emission irrespective of temperature change (20-150 °C) upon two-photon pumping is presented and the characteristic temperature is determined to be as high as ≈260 K. The superior gain properties of the CsPbBr3 /Cs4 PbBr6 perovskite nanocomposites are directly validated by a vertical cavity surface emitting laser operating at temperature as high as 100 °C. The results shed light on manipulating optical gain from the advantageous CsPbBr3 nanocrystals and represent a significant step toward the temperature-insensitive frequency-upconverted lasers.