Formation of visible single-mode light sources using quantum dots

A review devoted to the theory of excitonic quasimolecules (Biexcitons) (formed of spatially separated electrons and holes) in a nanosystems that consists of semiconductor and dielectric quantum dots synthesized in a dielectric matrix.It is shown that exciton quasimolecules formation is of the threshold character and possible in nanosystems, in with the spacing between the quantum dots surfaces is larger than a certain critical spacing.It was found that the binding energy of singlet ground state of exciton quasimolecules, consisting of two semiconductor quantum dots is a significant large values, larger than the binding energy of the biexciton in a semiconductor single crystal almost two orders of magnitude.

Colloidal semiconductor quantum dots (QDs) have strong light absorption, continuously adjustable narrowband emission, and high photoluminescence quantum yields, thereby making them promising materials for light-emitting diodes, solar cells, detectors, and lasers. Single-QD photoluminescence spectroscopy can remove the ensemble average to reveal the structure information and exciton dynamics of QD materials at a single-particle level. The study of single-QD spectroscopy can provide guidelines for rationally designing the QDs and giving the mechanism basis for QD-based applications. We can also carry out the research of the interaction between light and single QDs on a nanoscale, and prepare QD-based single-photon sources and entangled photon sources. Here, we review the recent research progress of single-QD photoluminescence spectroscopy and exciton dynamics, mainly including photoluminescence blinking dynamics, and exciton and multi-exciton dynamics of single colloidal CdSe-based QDs and perovskite QDs. Finally, we briefly discuss the possible future development trends of single-QD spectroscopy and exciton dynamics.

Over the past decade, semiconducting halide perovskite lasers have emerged as a transformative platform in optoelectronics, owing to unique properties such as high photoluminescence quantum yields, tunable bandgaps, and low-cost fabrication processes. This review systematically examines the advancements in halide perovskite lasers, covering diverse laser architectures, such as whispering gallery mode, Fabry-Pérot, plasmonic, bound states in the continuum (BIC), quantum dot, and polariton lasers. The mechanisms of optical gain, the role of material engineering in optimizing lasing performance, and the challenges associated with continuous-wave (CW) pumping and electrically driven lasing are discussed. Furthermore, recent progress in improving the stability and scalability of perovskite lasers, essential for their integration into practical applications in displays, optical communications, sensing, and integrated photonics is highlighted. Finally, future research directions are discussed, emphasizing the potential of perovskite lasers to revolutionize various technological domains by enabling the development of next-generation photonic devices.

Semiconductor quantum dots possess attractive optical properties useful in the photonic field. Thanks to the evolution of core-shell colloidal synthesis, quantum dots exhibit almost ideal emitting properties. Their employment in optoelectronic devices is encouraged by their size-tunable emission as well as by their high absorption cross-section and high photoluminescence quantum yield. In this work, starting from the archetypal CdSe based system, the influence of shell thickness and composition on exciton dynamics in CdSe/Cd_xZn1_-xScore-shell QDs is investigated. By means of steadystate and transient optical spectroscopy, single- and multi-exciton recombination processes are characterized and correlated with different shell materials. The present study clarifies why graded CdSe-CdS-Cd0.5Zn0.5-ZnS core-shell represents the best performing heterostructure for optical gain applications, since such quantum dots combine the high radiative rates of CdS with the superior confinement potential of ZnS in a single dot.

Inorganic nanoparticles have been introduced into biological systems as useful probes for in vitro diagnosis and in vivo imaging, due to their relatively small size and exceptional physical and chemical properties. A new kind of color tunable Gd-Zn-Cu-In-S/ZnS (GZCIS/ZnS) quantum dots (QDs) with stable crystal structure was successfully synthesized and utilized for magnetic resonance (MR) and fluorescence dual modality imaging. This strategy allows successful fabrication of GZCIS/ZnS QDs by incorporating Gd into ZCIS/ZnS QDs to achieve great MR enhancement without compromising the fluorescence properties of the initial ZCIS/ZnS QDs. The as-prepared GZCIS/ZnS QDs show high T1 MR contrast as well as "color-tunable" photoluminescence (PL) in the range of 550-725 nm by adjusting the Zn/Cu feeding ratio with high PL quantum yield (QY). The GZCIS/ZnS QDs were transferred into water via a bovine serum albumin (BSA) coating strategy. The resulting Cd-free GZCIS/ZnS QDs reveal negligible cytotoxicity on both HeLa and A549 cells. Both fluorescence and MR imaging studies were successfully performed in vitro and in vivo. The results demonstrated that GZCIS/ZnS QDs could be a dual-modal contrast agent to simultaneously produce strong MR contrast enhancement as well as fluorescence emission for in vivo imaging.

Traditional methods for fabricating nanoscale arrays are usually based on lithographic techniques. Alternative new approaches rely on the use of nanoscale templates made of synthetic or biological materials. Some proteins, for example, have been used to form ordered two-dimensional arrays. Here, we fabricated nanoscale ordered arrays of metal and semiconductor quantum dots by binding preformed nanoparticles onto crystalline protein templates made from genetically engineered hollow double-ring structures called chaperonins. Using structural information as a guide, a thermostable recombinant chaperonin subunit was modified to assemble into chaperonins with either 3 nm or 9 nm apical pores surrounded by chemically reactive thiols. These engineered chaperonins were crystallized into two-dimensional templates up to 20 microm in diameter. The periodic solvent-exposed thiols within these crystalline templates were used to size-selectively bind and organize either gold (1.4, 5 or 10nm) or CdSe-ZnS semiconductor (4.5 nm) quantum dots into arrays. The order within the arrays was defined by the lattice of the underlying protein crystal. By combining the self-assembling properties of chaperonins with mutations guided by structural modelling, we demonstrate that quantum dots can be manipulated using modified chaperonins and organized into arrays for use in next-generation electronic and photonic devices.

Systems of confined electrons are found everywhere in nature in the form of atoms where the orbiting electrons are confined by the Coulomb attraction of the nucleus. Advancement of nanotechnology has, however, provided us with an alternative way to confine electrons by using artificial confining potentials. A typical structure of this nature is the quantum dot, a nanoscale system which consists of few confined electrons. There are many types of quantum dots ranging from self-assembled to miniaturized semiconductor quantum dots. In this work we are interested in electrostatically confined semiconductor quantum dot systems where the electrostatic confining potential that traps the electrons is generated by external electrodes, doping, strain or other factors. A large number of semiconductor quantum dots of this type are fabricated by applying lithographically patterned gate electrodes or by etching on two-dimensional electron gases in semiconductor heterostructures. Because of this, the whole structure can be treated as a confined two-dimensional electron system. Quantum confinement profoundly affects the way in which electrons interact with each other, and external parameters such as a magnetic field. Since a magnetic field affects both the orbital and the spin motion of the electrons, the interplay between quantum confinement, electron–electron correlation effects and the magnetic field gives rise to very interesting physical phenomena. Thus, confined systems of electrons in a semiconductor quantum dot represent a unique opportunity to study fundamental quantum theories in a controllable atomic-like setup. In this work, we describe some common theoretical models which are used to study confined systems of electrons in a two-dimensional semiconductor quantum dot. The main emphasis of the work is to draw attention to important physical phenomena that arise in confined two-dimensional electron systems under various quantum regimes.

Large-scale synthesis of semiconductor nanocrystals or quantum dots (QDs) with high concentration and high yield through simultaneously increasing the precursor concentration was introduced. This synthetic route conducted in diesel has produced gram-scale CdSe semiconductor quantum dots (In optimal scale-up synthetic condition, the one-pot yield of QDs is up to 9.6g). The reaction has been conducted in open air and at relatively low temperature at 190-230 degrees C in the absence of expensive organic phosphine ligands, aliphatic amine and octadecene, which is really green chemistry without high energy cost for high temperature reaction and unessential toxic chemicals except for Cd, which is the essential building block for QDs.

We have designed a series of short RGD peptide ligands and developed one-pot aqueous synthesis of integrin-binding CdTe and CdZnTe quantum dots (QDs). We first examined the effects of different RGD peptides, including RGDS, CRGDS, Ac-CRGDS, CRGDS-CONH₂, Ac-CRGDS-CONH₂, RGDSC, CCRGDS, and CCCRGDS, on the synthesis of CdTe QDs. CRGDS were found to be the optimal ligand, providing the CdTe QDs with well-defined wavelength ranges (500-650 nm) and relatively high photoluminescence quantum yields (up to 15%). The key synthesis parameters (the pH value of the Cd²⁺-RGD precursors and the molar ratio of RGD/Cd²⁺) were assessed. In order to further improve the optical properties of the RGD-capped QDs, zinc was then incorporated by the simultaneous reaction of Cd²⁺ and Zn²⁺ with NaHTe. By using a mixture of CRGDS and cysteine as the stabilizer, the quantum yields of CdZnTe alloy QDs reached as high as 60% without any post-treatment, and they also showed excellent stability against time, pH, and salinity. Note that these properties could not be obtained with CRGDS or cysteine alone as the stabilizer. Finally, we demonstrated that the RGD-capped QDs preferentially bind to cell surfaces because of the specific recognition of the RGD sequence to cell surface integrin receptors. Our synthesis strategy based on RGD peptides thus represents a convenient route for opening up QD technologies for cell-specific tagging and labeling applicable to a wide range of diagnostics and therapy.

A facile route for highly efficient color-tunable Cu-Ga-Se/ZnSe quantum dots

Synthesis of PbS quantum dots (QDs) with uniform and controllable size is of great importance in realizing functionality manipulation, as well as building advanced devices, and these QDs have been normally synthesized via "bottom-up" colloidal chemistry. However, the problems of complicated techniques and the relative high cost of the "bottom-up" methods still need to be overcome. Herein, we present a facile and cost-effective "top-down" strategy for the production of PbS QDs with controllable sizes and narrow dispersions (4.8% < σ < 7%) based on the sulfuration reaction of highly active lead oxide and oxychloride intermediates. We investigated the two-step reaction mechanism of the QD synthesis. Initially, Pb(OH)Cl undergoes a reaction with oleic acid in the presence of oleylamine as an activator, leading to the formation of active lead oxide/oxychloride intermediates. Notably, this distinctive reaction induces the creation of numerous cracks within the intermediates, thereby augmenting the active sites available for the subsequent sulfuration reactions. In that, the sulfur precursor reacts with the intermediates, resulting in the rapid generation of a substantial number of PbS fragments. Over time, these small fragments undergo "ripening" until reaching the "critical" size threshold. Different from the ones obtained by the traditional "bottom-up" method, our synthesized colloidal QDs exhibit a S-rich surface and are confirmed to be N-type. In addition, size-tunable near-infrared photoluminescence renders these QDs a promising material for various applications.

Perovskite quantum dots (QDs) are emerging as one of the most promising candidates for the monochromatic light‐emitting diodes (LEDs) approaching the Rec. 2020 color gamut due to their extremely narrow emission bandwidth. Another important aspect are the high photoluminescence quantum yield (PLQY) values that can be obtained either in solution or thin films making these materials as promising candidates for optoelectronic applications such as LEDs or solar cells. Considerable research efforts in chemistry, chemical engineering, solid‐state physics, and material sciences have been made in the past years. The new opportunity in the field of semiconductor quantum dots is still in the beginning phase and is expected to remain active in the following years. Here, in this invited commentary, we briefly discuss and summarize the opportunities and challenges in both fundamental and technological aspects, based on our work and the recent work in this field.

Exploring the synthesis of new biocompatible quantum dots (QDs) helps in overcoming the intrinsic toxicity of the existing QDs composed of highly toxic heavy metals (e.g., Cd, Hg, Pb, etc.) and is particularly interesting for the future practical application of QDs in biomedical imaging. Hence, in this report, a new one-pot approach to oil-soluble (highly toxic heavy metal-free) highly luminescent quaternary Zn-Ag-In-Se (ZAISe) QDs was designed. Their photoluminescence (PL) emission could be systematically tuned from 660 to 800 nm by controlling the Ag/Zn feed ratio, and their highest PL quantum yield is close to 50% after detailed optimization. Next, by using biodegradable RGD peptide (arginine-glycine-aspartic acid)-modified N-succinyl-N'-octyl-chitosan (RGD-SOC) micelles as a water transfer agent, the versatility of these quaternary ZAISe QDs for multiscale bioimaging of micelles (namely, in vitro and in vivo evaluating the tumor targeting of drug carriers) was further explored, as a promising alternative for Cd- and Pb-based QDs.

: This overall objective of this research task was to push forward the state-of-the-art in utilizing semiconductor quantum dots in optical and optoelectronic devices of interest to the Air Force. The full potential of semiconductor quantum dots cannot be achieved by random formation and assembly processes. For narrow optical linewidths, suppressed thermal excitation, and tailored dot-to-dot interactions, the quantum dot size, composition and location needs to be precisely controlled to engineer the desired properties. Two complementary but distinct methods for fabricating uniform quantum dots with controlled dot/nanoparticle size, composition and location are proposed. 1) Nanopatterning and annealing of MBE grown planar InAs. 2) Synthesis of semiconductor nanocrystals by Supercritical Fluid CO2 process. In addition to the fabrication process development, a key objective is to model and characterize the fundamental physics and chemistry of these nanoscale heterostructures to provide the firm scientific basis for these materials to transition into advanced devices, such as detectors, lasers, optical communication and quantum computing.

A key challenge in quantum photonics today is the efficient and on-demand generation of high-quality single photons and entangled photon pairs. In this regard, one of the most promising types of emitters are semiconductor quantum dots, fluorescent nanostructures also described as artificial atoms. The main technological challenge in upscaling to an industrial level is the typically random spatial and spectral distribution in their growth. Furthermore, depending on the intended application, different requirements are imposed on a quantum dot, which are reflected in its spectral properties. Given that an in-depth suitability analysis is lengthy and costly, it is common practice to pre-select promising candidate quantum dots using their emission spectrum. Currently, this is done by hand. Therefore, to automate and expedite this process, in this paper, we propose a data-driven machine-learning-based method of evaluating the applicability of a semiconductor quantum dot as single photon source. For this, first, a minimally redundant, but maximally relevant feature representation for quantum dot emission spectra is derived by combining conventional spectral analysis with an autoencoding convolutional neural network. The obtained feature vector is subsequently used as input to a neural network regression model, which is specifically designed to not only return a rating score, gauging the technical suitability of a quantum dot, but also a measure of confidence for its evaluation. For training and testing, a large dataset of self-assembled InAs/GaAs semiconductor quantum dot emission spectra is used, partially labelled by a team of experts in the field. Overall, highly convincing results are achieved, as quantum dots are reliably evaluated correctly. Note, that the presented methodology can account for different spectral requirements and is applicable regardless of the underlying photonic structure, fabrication method and material composition. We therefore consider it the first step towards a fully integrated evaluation framework for quantum dots, proving the use of machine learning beneficial in the advancement of future quantum technologies.