Evolution of the Single-Nanocrystal Photoluminescence Linewidth with Size and Shell: Implications for Exciton–Phonon Coupling and the Optimization of Spectral Linewidths

Ligand Controls Excited Charge Carrier Dynamics in Metal-Rich CdSe Quantum Dots: Computational Insights

Small metal-rich semiconducting quantum dots (QDs) are promising for solid-state lighting and single-photon emission due to their highly tunable yet narrow emission line widths. Nonetheless, the anionic ligands commonly employed to passivate these QDs exert a substantial influence on the optoelectronic characteristics, primarily owing to strong electron-phonon interactions. In this work, we combine time-domain density functional theory and nonadiabatic molecular dynamics to investigate the excited charge carrier dynamics of Cd28Se17X22 QDs (X = HCOO-, OH-, Cl-, and SH-) at ambient conditions. These chemically distinct but regularly used molecular groups influence the dynamic surface-ligand interfacial interactions in Cd-rich QDs, drastically modifying their vibrational characteristics. The strong electron-phonon coupling leads to substantial transient variations at the band edge states. The strength of these interactions closely depends on the physicochemical characteristics of passivating ligands. Consequently, the ligands largely control the nonradiative recombination rates and emission characteristics in these QDs. Our simulations indicate that Cd28Se17(OH)22 has the fastest nonradiative recombination rate due to the strongest electron-phonon interactions. Conversely, QDs passivated with thiolate or chloride exhibit considerably longer carrier lifetimes and suppressed nonradiative processes. The ligand-controlled electron-phonon interactions further give rise to the broadest and narrowest intrinsic optical line widths for OH and Cl-passivated single QDs, respectively. Obtained computational insights lay the groundwork for designing appropriate passivating ligands on metal-rich QDs, making them suitable for a wide range of applications, from blue LEDs to quantum emitters.

Bidirectional Inhibiting Interfacial Ion Migration in the Inorganic Hole Transport Layer for Perovskite Light-Emitting Diodes

Cu2ZnSnS4 (CZTS) is strong candidate for hole transport in perovskite light emitting diodes (PeLEDs) due to their cost-effectiveness, deep highest occupied molecular orbital (HOMO), and high hole mobility. However, its inherent polymetallic ions usually deteriorate the quality of the perovskite emission layer (EML) affecting device performance. In this study, a bidirectional anchoring strategy is proposed by adding 15-crown-5 ether (15C5) into CZTS hole transport layer (HTL) to suppress the reaction between HTL and EML. The 15C5 molecule interacts with Cu+, Zn2+ and Sn2+ cations forming host-guest complexes to impede their migration, which is elucidated by density functional theory calculations. Additionally, 15C5 can neutralize lead (Pb) defects by the abundant oxygen (O) and high electronegative cavities to reduce the nonradiative recombination of FAPbBr3 film. This bidirectional anchoring strategy effectively improves hole charge transport efficiency and suppresses nonradiative recombination at the HTL/EML interface. As a result, the optimized PeLEDs present a 3.5 times peak external quantum efficiency (EQE) from 3.12% to 11.08% and the maximum luminance (Lmax) increased from 24495 to 50584 cd m-2. These findings offer innovative insights into addressing the metal ion migration issue commonly observed in inorganic HTLs.

Multihierarchical Regulation To Achieve Quantum Dot Nanospheres with a Photoluminescence Quantum Yield Close to 100

Quantum dots (QDs) exhibit superior brightness and photochemical stability, making them the preferred option for highly sensitive single-molecule detection compared with fluorescent dyes or proteins. Nevertheless, their high surface energy leads to nonspecific adsorption and poor colloidal stability. In the past decades, we have found that QD-based fluorescent nanoparticles (FNs) can not only address these limitations but also enhance detection sensitivity. However, the photoluminescence quantum yield (PLQY) of FNs is significantly lower compared with that of original QDs. It is urgent to develop a strategy to solve the issue, aiming to further enhance detection sensitivity. In this study, we found that the decrease of PLQY of FNs prepared by free radical polymerization was attributed to two factors: (1) generation of defects that can cause nonradiative transitions resulting from QD-ligands desorption and QD-shell oxidation induced by free radicals; (2) self-absorption resulting from aggregation caused by incompatibility of QDs with polymers. Based on these, we proposed a multihierarchical regulation strategy that includes: (1) regulating QD-ligands; (2) precisely controlling free radical concentration; and (3) constructing cross-linked structures of polymer to improve compatibility and to reduce the formation of surface defects. It is crucial to emphasize that the simultaneous coordination of multiple factors is essential. Consequently, a world-record PLQY of 97.6% for FNs was achieved, breaking through the current bottleneck at 65%. The flexible application of this regulatory concept paves the way for the large-scale production of high-brightness QD-polymer complexes, enhancing their potential applications in sensitive biomedical detection.

Band-Edge Energy Levels of Dynamic Excitons in Cube-Shaped CdSe/CdS Core/Shell Nanocrystals

An electron-hole pair in a cube-shaped CdSe/CdS core/shell nanocrystal exists in the form of dynamic excitons across the strongly and weakly confined regimes under ambient temperatures. Photochemical doping is applied to distinguish the band-edge electron and hole levels, confirming an effective mass model with universal constants. Reduction of the optical bandgap upon epitaxy of the CdS shells is caused by lowering the band-edge electron level and barely affecting the band-edge hole level. Similar shifts of the electron levels, yet retaining the hole levels, can switch the order in energy of the three lowest-energy transitions. Thermal distribution of 1-4 electrons among the two thermally accessible electron levels follows number-counting statistics, instead of Fermi-Dirac distribution.

Reducing Emission Linewidth of Pure-Blue ZnSeTe Quantum Dots through Shell Engineering toward High Color Purity Light-Emitting Diodes

High color purity blue quantum dot light-emitting diodes (QLEDs) have great potential applications in the field of ultra-high-definition display. However, the realization of eco-friendly pure-blue QLEDs with a narrow emission linewidth for high color purity remains a significant challenge. Herein, a strategy for fabricating high color purity and efficient pure-blue QLEDs based on ZnSeTe/ZnSe/ZnS quantum dots (QDs) is presented. It is found that by finely controlling the internal ZnSe shell thickness of the QDs, the emission linewidth can be narrowed by reducing the exciton-longitudinal optical phonon coupling and trap states in the QDs. Additionally, the regulation of the QD shell thickness can suppress the Förster energy transfer between QDs in the QLED emission layer, which will help to reduce the emission linewidth of the device. As a result, the fabricated pure-blue (452 nm) ZnSeTe QLED with ultra-narrow electroluminescence linewidth (22 nm) exhibit high color purity with the Commission Internationale de l'Eclairage chromatic coordinates of (0.148, 0.042) and considerable external quantum efficiency (18%). This work provides a demonstration of the preparation of pure-blue eco-friendly QLEDs with both high color purity and efficiency, and it is believed that it will accelerate the application process of eco-friendly QLEDs in ultra-high-definition displays.

Size-Dependent Optical Properties of InP Colloidal Quantum Dots

Indium phosphide colloidal quantum dots (CQDs) are the main alternative for toxic and restricted Cd based CQDs for lighting and display applications. Here we systematically report on the size-dependent optical absorption, ensemble, and single particle photoluminescence (PL) and biexciton lifetimes of core-only InP CQDs. This systematic study is enabled by improvements in the synthesis of InP CQDs to yield a broad size series of monodisperse core-only InP CQDs with narrow absorption and PL line width and significant PL quantum yield.

Theory of Photoluminescence Spectral Line Shapes of Semiconductor Nanocrystals

Single-molecule photoluminescence (PL) spectroscopy of semiconductor nanocrystals (NCs) reveals the nature of exciton-phonon interactions in NCs. Understanding the homogeneous spectral line shapes and their temperature dependence remains an open problem. Here, we develop an atomistic model to describe the PL spectrum of NCs, accounting for excitonic effects, phonon dispersion relations, and exciton-phonon couplings. We validate our model using single-NC measurements on CdSe/CdS NCs from T = 4 to 290 K, and we find that the slightly asymmetric main peak at low temperatures is comprised of a narrow zero-phonon line (ZPL) and acoustic phonon sidebands. Furthermore, we identify the specific phonon modes that give rise to the optical phonon sidebands. At temperatures above 200 K, the spectral line width shows a stronger dependence upon the temperature, which we demonstrate to be correlated with higher order exciton-phonon couplings. We also identify the line width dependence upon reorganization energy, NC core sizes, and shell thicknesses.

Toward Surface Chemistry of Semiconductor Nanocrystals at an Atomic-Molecular Level

ConspectusProperties of colloidal semiconductor nanocrystals with a single-crystalline structure are largely dominated by their surface structure at an atomic-molecular level, which is not well understood and controlled, due to a lack of experimental tools. However, if viewing the nanocrystal surface as three relatively independent spatial zones (i.e., crystal facets, inorganic-ligands interface, and ligands monolayer), we may approach an atomic-molecular level by coupling advanced experimental techniques and theoretical calculations.Semiconductor nanocrystals of interest are mainly based on compound semiconductors and mostly in two (or related) crystal structures, namely zinc-blende and wurtzite, which results in a small group of common low-index crystal facets. These low-index facets, from a surface-chemistry perspective, can be further classified into polar and nonpolar ones. Albeit far from being successful, the controlled formation of either polar or nonpolar facets is available for cadmium chalcogenide nanocrystals. Such facet-controlled systems offer a reliable basis for studying the inorganic-ligands interface. For convenience, here facet-controlled nanocrystals refer to a special class of shape-controlled ones, in which shape control is at an atomic level, instead of those with poorly defined facets (e.g., typical spheroids, nanorods, etc).Experimental and theoretical results reveal that type and bonding mode of surface ligands on nanocrystals is facet-specific and often beyond Green's classification (X-type, Z-type, and L-type). For instance, alkylamines bond strongly to the anion-terminated (0001) wurtzite facet in the form of ammonium ions, with three hydrogens of an ammonium ion bonding to three adjacent surface anion sites. With theoretically assessable experimental data, facet-ligands pairing can be identified using density functional theory (DFT) calculations. To make the pairing meaningful, possible forms of all potential ligands in the system need to be examined systematically, revealing the advantage of simple solution systems.Unlike the other two spatial zones, the ligands monolayer is disordered and dynamic at an atomic level. Thus, an understanding of the ligands monolayer on a molecular scale is sufficient for many cases. For colloidal nanocrystals stably coordinated with surface ligands, their solution properties are dictated by the ligands monolayer. Experimental and theoretical results reveal that solubility of a nanocrystal-ligands complex is an interplay between the intramolecular entropy of the ligands monolayer and intermolecular interactions of the ligands/nanocrystals. By introducing entropic ligands, solubility of nanocrystal-ligands complexes can be universally boosted by several orders of magnitude, i.e., up to >1 g/mL in typical organic solvents. Molecular environment in the pseudophase surrounding each nanocrystal plays a critical role in its chemical, photochemical, and photophysical properties.For some cases, such as the synthesis of high-quality nanocrystals, all three spatial zones of the nanocrystal surface must be taken into account. By optimizing nanocrystal surface at an atomic-molecular level, semiconductor nanocrystals with monodisperse size and facet structure become available recently through either direct synthesis or afterward facet reconstruction, implying full realization of their size-dependent properties.

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.

Elastic Phonon Scattering Dominates Dephasing in Weakly Confined Cesium Lead Bromide Nanocrystals at Cryogenic Temperatures

Cesium lead halide perovskite nanocrystals (PNCs) have emerged as a potential next-generation single quantum emitter (QE) material for quantum optics and quantum information science. Optical dephasing processes at cryogenic temperatures are critical to the quality of a QE, making a mechanistic understanding of coherence losses of fundamental interest. We use photon-correlation Fourier spectroscopy (PCFS) to obtain a lower bound to the optical coherence times of single PNCs as a function of temperature. We find that 20 nm CsPbBr₃ PNCs emit nearly exclusively into a narrow zero-phonon line from 4 to 13 K. Remarkably, no spectral diffusion is observed at time scales of 10 μs to 5 ms. Our results suggest that exciton dephasing in this temperature range is dominated by elastic scattering from phonon modes with characteristic frequencies of 1-3 meV, while inelastic scattering is minimal due to weak exciton-phonon coupling.

Charge Injection and Energy Transfer of Surface-Engineered InP/ZnSe/ZnS Quantum Dots

Surface passivation is a critical aspect of preventing surface oxidation and improving the emission properties of nanocrystal quantum dots (QDs). Recent studies have demonstrated the critical role of surface ligands in determining the performance of QD-based light-emitting diodes (QD-LEDs). Herein, the underlying mechanism by which the capping ligands of InP/ZnSe/ZnS QDs influence the brightness and lifetime of the QD-LEDs is investigated. The electrochemical results demonstrate that highly luminescent InP/ZnSe/ZnS QDs exhibit modulated charge injection depending on the length of the surface ligand chains: short alkyl chains on the ligands are favorable for charge transport to the QDs. In addition, the correlation between the spectroscopic and XRD analyses suggests that the length of the ligand chain tunes the ligand-ligand coupling strength, thereby controlling the inter-QD energy transfer dynamics. The present findings shed new light on the crucial role of surface ligands for InP/ZnSe/ZnS QD-LED applications.

Reversible Facet Reconstruction of CdSe/CdS Core/Shell Nanocrystals by Facet-Ligand Pairing

Synthesis of colloidal semiconductor nanocrystals with defined facet structures is challenging, though such nanocrystals are essential for fully realizing their size-dependent optical and optoelectronic properties. Here, for the mostly developed colloidal wurtzite CdSe/CdS core/shell nanocrystals, facet reconstruction is investigated under typical synthetic conditions, excluding nucleation, growth, and interparticle ripening. Within the reaction time window, two reproducible sets of facets─each with a specific group of low-index facets─can be reversibly reconstructed by switching the ligand system, indicating thermodynamic stability of each set. With a unique <0001> axis, atomic structures of the low-index facets of wurtzite nanocrystals are diverse. Experimental and theoretical studies reveal that each facet in a given set is paired with a common ligand in the solution, namely, either fatty amine and/or cadmium alkanoate. The robust bonding modes of ligands are found to be strongly facet-dependent and often unconventional, instead of following Green's classification. Results suggest that facet-controlled nanocrystals can be synthesized by optimal facet-ligand pairing either in synthesis or after-synthesis reconstruction, implying semiconductor nanocrystal formation with size-dependent properties down to an atomic level.

Sequential Carrier Transfer Can Accelerate Triplet Energy Transfer from Functionalized CdSe Nanocrystals

Nanocrystal (NC)-sensitized triplet-fusion upconversion is a rising strategy to convert long-wavelength, incoherent light into higher-energy output photons. Here, we chart the photophysics of tailor-functionalized CdSe NCs to understand energy transfer to surface-anchored transmitter ligands, which can proceed via correlated exciton transfer or sequential carrier hops. Varying NC size, we observe a pronounced acceleration of energy transfer (from k_quench = 0.0096 ns^-1 ligand^-1 to 0.064 ns^-1 ligand^-1) when the barrier to hole-first sequential transfer is lowered from 100 ± 25 meV to 50 ± 25 meV. This acceleration is 5.1× the expected effect of increased carrier wave function leakage, so we conclude that sequential transfer becomes kinetically dominant under the latter conditions. Last, transient photoluminescence shows that NC band-edge and trap states are comparably quenched by functionalization (up to ∼98% for sequential transfer) and exhibit matched dynamics for t > 300 ns, consistent with a dynamic quasi-equilibrium where photoexcitations can ultimately be extracted even when a carrier is initially trapped.

Impact of Morphological Inhomogeneity on Excitonic States in Highly Mismatched Alloy ZnSe1-XTeX Nanocrystals

ZnSe_1-XTe_X nanocrystals (NCs) are promising photon emitters with tunable emission across the violet to orange range and near-unity quantum yields. However, these NCs suffer from broad emission line widths and multiple exciton decay dynamics, which discourage their practicable use. Here, we explore the excitonic states in ZnSe_1-XTe_X NCs and their photophysical characteristics in relation to the morphological inhomogeneity of highly mismatched alloys. Ensemble and single-dot spectroscopic analysis of a series of ZnSe_1-XTe_X NC samples with varying Te ratios coupled with computational calculations shows that, due to the distinct electronegativity between Se and Te, nearest-neighbor Te pairs in ZnSe_1-XTe_X alloys create localized hole states spectrally distributed approximately 130 meV above the 1S_h level of homogeneous ZnSe_1-XTe_X NCs. This forms spatially separated excitons (delocalized electron and localized hole in trap), accounting for both inhomogeneous and homogeneous line width broadening with delayed recombination dynamics. Our results identify photophysical characteristics of excitonic states in NCs made of highly mismatched alloys and provide future research directions with potential implications for photonic applications.

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.

Uniaxial Strain Engineering via Core Position Control in CdSe/CdS Core/Shell Nanorods and Their Optical Response

Anisotropic strain engineering has emerged as a powerful strategy for enhancing the optoelectronic performance of semiconductor nanocrystals. Here, we show that CdSe/CdS dot-in-rod structures offer a platform for fine-tuning the optical response of CdSe quantum dots through anisotropic strain. By controlling the spatial position of the CdSe core within a growing CdS nanorod shell, varying degrees of uniaxial strain can be introduced. Placing CdSe cores at the end of the CdS nanorod induces strong asymmetric compression along the c-axis of the wurtzite CdSe core, dramatically altering its absorption and emission characteristics, whereas CdSe cores located near the middle of the nanorod experience a comparatively weak uniaxial strain field. The change in absorption and emission spectra and dynamics for highly strained end-position CdSe/CdS nanorods is explained by (1) relative shifting of the valence band light hole and heavy hole levels and (2) introduction of a strong piezoelectric potential, which spatially separates the electron and hole wave functions. The ability to tune the degree of uniaxial strain through core position control in a nanorod structure creates opportunities for precisely modulating the electronic properties of CdSe nanocrystals while simultaneously taking advantage of dielectric and optical anisotropies intrinsic to 1D nanostructures.

Synthesis of Weakly Confined, Cube-Shaped, and Monodisperse Cadmium Chalcogenide Nanocrystals with Unexpected Photophysical Properties

Zinc-blende CdSe, CdS, and CdSe/CdS core/shell nanocrystals with a structure-matched shape (cube-shaped, edge length ≤30 nm) are synthesized via a universal scheme. With the edge length up to five times larger than exciton diameter of the bulk semiconductors, the nanocrystals exhibit novel properties in the weakly confined size regime, such as near-unity single exciton and biexciton photoluminescence (PL) quantum yields, single-nanocrystal PL nonblinking, mixed PL decay dynamics of exciton and free carriers with sub-microsecond monoexponential decay lifetime, and stable yet extremely narrow PL full width at half maximum (FWHM < 0.1 meV) at 1.8 K. Their monodisperse edge length, shape, and facet structure enable demonstration of unexpected yet size-dependent PL properties at room temperature, including unusually broad and abnormally size-dependent PL FWHM (∼100 meV), nonmonotonic size dependence of PL peak energy, and dual-peak single-exciton PL. Calculations suggest that these unusual properties should be originated from the band-edge electron/hole states of the dynamic-exciton, whose exciton binding energy is too small to hold the photogenerated electron-hole pair as a bonded Wannier exciton in a weakly confined nanocrystal.

Simulations of nonradiative processes in semiconductor nanocrystals

The description of carrier dynamics in spatially confined semiconductor nanocrystals (NCs), which have enhanced electron-hole and exciton-phonon interactions, is a great challenge for modern computational science. These NCs typically contain thousands of atoms and tens of thousands of valence electrons with discrete spectra at low excitation energies, similar to atoms and molecules, that converge to the continuum bulk limit at higher energies. Computational methods developed for molecules are limited to very small nanoclusters, and methods for bulk systems with periodic boundary conditions are not suitable due to the lack of translational symmetry in NCs. This perspective focuses on our recent efforts in developing a unified atomistic model based on the semiempirical pseudopotential approach, which is parameterized by first-principle calculations and validated against experimental measurements, to describe two of the main nonradiative relaxation processes of quantum confined excitons: exciton cooling and Auger recombination. We focus on the description of both electron-hole and exciton-phonon interactions in our approach and discuss the role of size, shape, and interfacing on the electronic properties and dynamics for II-VI and III-V semiconductor NCs.

Coherent InP/ZnS core@shell quantum dots with narrow-band green emissions

We report, for the first time, that the coherent growth of zinc sulfide (ZnS) on a colloidal indium phosphide (InP) quantum dot (QD) yields a InP/ZnS core/shell structure with a single lattice constant of 0.563 nm. Compared to the bulk crystal of zinc-blend (cubic) InP, the lattice of the core QD is compressed by 4.1%. In contrast, the lattice of the shell expands by 4.1% relative to the bulky ZnS crystal throughout the core/shell QD if the shell is thinner than or equal to 0.81 nm and the diameter of the core QD is smaller than 2.64 nm. Under these conditions, the bandgap of the core QD increases, resulting in a blueshift of absorption and photoluminescence (PL) spectra. The PL peak is centered at 523 nm. Furthermore, the PL quantum yield is enhanced up to 70% and the PL bandwidth narrows to 36 nm based on the strengthened quantum confinement effect. The temperature dependence of the PL properties is investigated to discuss the effect of the core/shell lattice coherency on the improved PL performances.

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.

Narrow emission is desired for light-emitting devices. Here, Kovalenko et al. demonstrate that the emission line-broadening in perovskite quantum dots is dominated by the coupling between excitons and surface phonon modes which can be controlled by minimal surface modifications.

Room-Temperature, Highly Pure Single-Photon Sources from All-Inorganic Lead Halide Perovskite Quantum Dots

Attaining pure single-photon emission is key for many quantum technologies, from optical quantum computing to quantum key distribution and quantum imaging. The past 20 years have seen the development of several solid-state quantum emitters, but most of them require highly sophisticated techniques (e.g., ultrahigh vacuum growth methods and cryostats for low-temperature operation). The system complexity may be significantly reduced by employing quantum emitters capable of working at room temperature. Here, we present a systematic study across ∼170 photostable single CsPbX3 (X: Br and I) colloidal quantum dots (QDs) of different sizes and compositions, unveiling that increasing quantum confinement is an effective strategy for maximizing single-photon purity due to the suppressed biexciton quantum yield. Leveraging the latter, we achieve 98% single-photon purity (g(2)(0) as low as 2%) from a cavity-free, nonresonantly excited single 6.6 nm CsPbI3 QDs, showcasing the great potential of CsPbX3 QDs as room-temperature highly pure single-photon sources for quantum technologies.

Epitaxial Integration of Multiple CdSe Quantum Dots in a Colloidal CdS Nanoplatelet

Presynthesized CdSe/CdS core/shell quantum dots (QDs) are two-dimensionally (2D) and epitaxially fused in solution to form a CdS nanoplatelet with multiple epitaxially embedded CdSe QDs (CdSe@CdS coupled-dots@platelet). In addition to providing spatial confinement for the excitonic states of multiple CdSe QDs in a CdS nanoplatelet, the continuous and single-crystalline nanoplatelet with controlled thickness enables quantum coupling between the CdSe QDs, resulting in inhomogeneous-free optical properties for the colloidal CdSe@CdS coupled-dots@nanoplatelets with bright photoluminescence. The results here suggest that solution synthesis can offer a simple means to obtain semiconductor nanocrystals for realizing unique yet complex excitonic properties that are otherwise difficult to achieve.

Persistent nucleation and size dependent attachment kinetics produce monodisperse PbS nanocrystals

Modern syntheses of colloidal nanocrystals yield extraordinarily narrow size distributions that are believed to result from a rapid "burst of nucleation" (La Mer, JACS, 1950, 72(11), 4847-4854) followed by diffusion limited growth and size distribution focusing (Reiss, J. Chem. Phys., 1951, 19, 482). Using a combination of in situ X-ray scattering, optical absorption, and 13C nuclear magnetic resonance (NMR) spectroscopy, we monitor the kinetics of PbS solute generation, nucleation, and crystal growth from three thiourea precursors whose conversion reactivity spans a 2-fold range. In all three cases, nucleation is found to be slow and continues during >50% of the precipitation. A population balance model based on a size dependent growth law (1/r) fits the data with a single growth rate constant (k G) across all three precursors. However, the magnitude of the k G and the lack of solvent viscosity dependence indicates that the rate limiting step is not diffusion from solution to the nanoparticle surface. Several surface reaction limited mechanisms and a ligand penetration model that fits data our experiments using a single fit parameter are proposed to explain the results.

Growth kinetics determine the polydispersity and size of PbS and PbSe nanocrystals

A library of thio- and selenourea derivatives is used to adjust the kinetics of PbE (E = S, Se) nanocrystal formation across a 1000-fold range (k r = 10-1 to 10-4 s-1), at several temperatures (80-120 °C), under a standard set of conditions (Pb : E = 1.2 : 1, [Pb(oleate)2] = 10.8 mM, [chalcogenourea] = 9.0 mM). An induction delay (t ind) is observed prior to the onset of nanocrystal absorption during which PbE solute is observed using in situ X-ray total scattering. Density functional theory models fit to the X-ray pair distribution function (PDF) support a Pb2(μ2-S)2(Pb(O2CR)2)2 structure. Absorption spectra of aliquots reveal a continuous increase in the number of nanocrystals over more than half of the total reaction time at low temperatures. A strong correlation between the width of the nucleation phase and reaction temperature is observed that does not correlate with the polydispersity. These findings are antithetical to the critical concentration dependence of nucleation that underpins the La Mer hypothesis and demonstrates that the duration of the nucleation period has a minor influence on the size distribution. The results can be explained by growth kinetics that are size dependent, more rapid at high temperature, and self limiting at low temperatures.

Effects of Structural and Microstructural Features on the Total Scattering Pattern of Nanocrystalline Materials

Atomic- and nanometer-scale features of nanomaterials have a strong influence on their chemical and physical properties and a detailed description of these elements is a crucial step in their characterization. Total scattering methods, in real and reciprocal spaces, have been established as fundamental techniques to retrieve this information. Although the impact of microstructural features, such as defectiveness of different kinds, has been extensively studied in reciprocal space, disentangling these effects from size- and morphology-induced properties, upon downsizing, is not a trivial task. Additionally, once the experimental pattern is Fourier transformed to calculate the pair distribution function, the direct fingerprint of structural and microstructural features is severely lost and no modification of the histogram of interatomic distances derived therefrom is clearly discussed nor considered in the currently available protocols. Hereby, starting from atomistic models of a prototypical system (cadmium selenide), we simulate multiple effects on the atomic pair distribution function, obtained from reciprocal space patterns computed through the Debye scattering equation. Size and size dispersion effects, as well as different structures, morphologies, and their interplay with several kinds of planar defects, are explored, aiming at identifying the main (measurable and informative) fingerprints of these features on the total scattering pattern in real and reciprocal spaces, highlighting how, and how much, they become evident when comparing different cases. The results shown herein have general validity and, as such, can be further extended to other classes of nanomaterials.

Insight into the role of reduced graphene oxide for enhancing photocatalytic hydrogen evolution in disordered carbon nitride

Compared to crystalline carbon nitride, the performance of disordered carbon nitride (d-CN) as a hydrogen production photocatalyst is extremely poor. Owing to disordered atomic orientation, it is prone to numerous...

Ultra-small PbS nanocrystals as sensitizers for red-to-blue triplet-fusion upconversion

Photon upconversion is a strategy to generate high-energy excitations from low-energy photon input, enabling advanced architectures for imaging and photochemistry. Here, we show that ultra-small PbS nanocrystals can sensitize red-to-blue triplet-fusion upconversion with a large anti-Stokes shift (ΔE = 1.04 eV), and achieve max-efficiency upconversion at near-solar fluences (I th = 220 mW cm-2) despite endothermic triplet sensitization. This system facilitates the photo-initiated polymerization of methyl methacrylate using only long-wavelength light (λ exc: 637 nm); a demonstration of nanocrystal-sensitized upconversion photochemistry. Time-resolved spectroscopy and kinetic modelling clarify key loss channels, highlighting the benefit of long-lifetime nanocrystal sensitizers, but revealing that many (48%) excitons that reach triplet-extracting carboxyphenylanthracene ligands decay before they can transfer to free-floating acceptors-emphasizing the need to address the reduced lifetimes that we determine for molecular triplets near the nanocrystal surface. Finally, we find that the inferred thermodynamics of triplet sensitization from these ultra-small PbS quantum dots are surprisingly favourable-completing an advantageous suite of properties for upconversion photochemistry-and do not vary significantly across the ensemble, which indicates minimal effects from nanocrystal heterogeneity. Together, our demonstration and study of red-to-blue upconversion using ultra-small PbS nanocrystals in a quasi-equilibrium, mildly endothermic sensitization scheme offer design rules to advance implementations of triplet fusion, especially where large anti-Stokes wavelength shifts are sought.

Interplay of Surface and Interior Modes in Exciton-Phonon Coupling at the Nanoscale

Exciton-phonon coupling (EXPC) plays a key role in the optoelectronic properties of semiconductor nanocrystals (NCs), but a microscopic picture of EXPC is still lacking, particularly regarding the magnitude and scaling with NC size, the dependence on phonon frequency, and the role of the NC surface. The computational complexity associated with accurately describing excitons and phonons has limited previous theoretical studies of EXPC to small NCs, noninteracting electron-hole models, and/or a small number of phonon modes. Here, we develop an atomistic approach for describing EXPC in NCs of experimentally relevant sizes. We validate our approach by calculating the reorganization energies, a measure of EXPC, for CdSe and CdSe-CdS core-shell NCs, finding good agreement with experimental measurements. We demonstrate that exciton formation distorts the NC lattice primarily along the coordinates of low-frequency acoustic modes. Modes at the NC surface play a significant role in smaller NCs while interior modes dominate for larger systems.

First-Principles Calculations of Luminescence Spectra of Real-Scale Quantum Dots

The luminescence line shape is an important feature of semiconductor quantum dots (QDs) and affects performance in various optical applications. Here, we report a first-principles method to predict the luminescence spectrum of thousands of atom QDs. In our approach, neural network potential calculations are combined with density functional theory calculations to describe exciton-phonon coupling (EPC). Using the calculated EPC, the luminescence spectrum is evaluated within the Franck-Condon approximation. Our approach results in the luminescence line shape for an InP/ZnSe core/shell QD (3406 atoms) that exhibits excellent agreement with the experiments. From a detailed analysis of EPC, we reveal that the coupling of both acoustic and optical phonons to an exciton are important in determining the spectral line shapes of core/shell QDs, which is in contrast with previous studies. On the basis of the present simulation results, we provide guidelines for designing high-performance core/shell QDs with ultrasharp emission spectra.

Photoluminescent Nanoparticles for Chemical and Biological Analysis and Imaging

Research related to the development and application of luminescent nanoparticles (LNPs) for chemical and biological analysis and imaging is flourishing. Novel materials and new applications continue to be reported after two decades of research. This review provides a comprehensive and heuristic overview of this field. It is targeted to both newcomers and experts who are interested in a critical assessment of LNP materials, their properties, strengths and weaknesses, and prospective applications. Numerous LNP materials are cataloged by fundamental descriptions of their chemical identities and physical morphology, quantitative photoluminescence (PL) properties, PL mechanisms, and surface chemistry. These materials include various semiconductor quantum dots, carbon nanotubes, graphene derivatives, carbon dots, nanodiamonds, luminescent metal nanoclusters, lanthanide-doped upconversion nanoparticles and downshifting nanoparticles, triplet-triplet annihilation nanoparticles, persistent-luminescence nanoparticles, conjugated polymer nanoparticles and semiconducting polymer dots, multi-nanoparticle assemblies, and doped and labeled nanoparticles, including but not limited to those based on polymers and silica. As an exercise in the critical assessment of LNP properties, these materials are ranked by several application-related functional criteria. Additional sections highlight recent examples of advances in chemical and biological analysis, point-of-care diagnostics, and cellular, tissue, and in vivo imaging and theranostics. These examples are drawn from the recent literature and organized by both LNP material and the particular properties that are leveraged to an advantage. Finally, a perspective on what comes next for the field is offered.

Biexciton Binding Energy and Line width of Single Quantum Dots at Room Temperature

Broadening of multiexciton emission from colloidal quantum dots (QDs) at room temperature is important for their use in high-power applications, but an in-depth characterization has not been possible until now. We present and apply a novel spectroscopic method to quantify the biexciton line width and biexciton binding energy of single CdSe/CdS/ZnS colloidal QDs at room temperature. In our method, which we term "cascade spectroscopy", we select emission events from the biexciton cascade and reconstruct their spectrum. The biexciton has an average emission line width of 86 meV on the single-QD scale, similar to that of the exciton. Variations in the biexciton repulsion (E_b = 4.0 ± 3.1 meV; mean ± standard deviation of 15 QDs) are correlated with but are more narrowly distributed than variations in the exciton energy (10.0 meV standard deviation). Using a simple quantum-mechanical model, we conclude that inhomogeneous broadening in our sample is primarily due to variations in the CdS shell thickness.

Phonon-assisted up-conversion photoluminescence of quantum dots

Phonon-assisted up-conversion photoluminescence can boost energy of an emission photon to be higher than that of the excitation photon by absorbing vibration energy (or phonons) of the emitter. Here, up-conversion photoluminescence power-conversion efficiency (power ratio between the emission and excitation photons) for CdSe/CdS core/shell quantum dots is observed to be beyond unity. Instead of commonly known defect-assisted up-conversion photoluminescence for colloidal quantum dots, temperature-dependent measurements and single-dot spectroscopy reveal the up-conversion photoluminescence and conventional down-conversion photoluminescence share the same electron-phonon coupled electronic states. Ultrafast spectroscopy results imply the thermalized excitons for up-conversion photoluminescence form within 200 fs, which is 100,000 times faster than the radiative recombination rate of the exciton. Results suggest that colloidal quantum dots can be exploited as efficient, stable, and cost-effective emitters for up-conversion photoluminescence in various applications.

Up-conversion photoluminescence in colloidal quantum dots is generally believed to be mediated by thermal activation from defect states. Here, the authors reveal that highly-efficient up-conversion photoluminescence instead is related to electron-phonon coupling.

Relations between absorption, emission, and excited state chemical potentials from nanocrystal 2D spectra

For quantum-confined nanomaterials, size dispersion causes a static broadening of spectra that has been difficult to measure and invalidates all-optical methods for determining the maximum photovoltage that an excited state can generate. Using femtosecond two-dimensional (2D) spectroscopy to separate size dispersion broadening of absorption and emission spectra allows a test of single-molecule generalized Einstein relations between such spectra for colloidal PbS quantum dots. We show that 2D spectra and these relations determine the thermodynamic standard chemical potential difference between the lowest excited and ground electronic states, which gives the maximum photovoltage. Further, we find that the static line broadening from many slightly different quantum dot structures allows single-molecule generalized Einstein relations to determine the average single-molecule linewidth from Stokes' frequency shift between ensemble absorption and emission spectra.

PbS/CdS Quantum Dot Room-Temperature Single-Emitter Spectroscopy Reaches the Telecom O and S Bands via an Engineered Stability

We synthesized PbS/CdS core/shell quantum dots (QDs) to have functional single-emitter properties for room-temperature, solid-state operation in the telecom O and S bands. Two shell-growth methods-cation exchange and successive ionic layer adsorption and reaction (SILAR)-were employed to prepare QD heterostructures with shells of 2-16 monolayers. PbS/CdS QDs were sufficiently bright and stable to resolve photoluminescence (PL) spectra representing both bands from single nanocrystals using standard detection methods, and for a QD emitting in the O-band a second-order correlation function showed strong photon antibunching, important steps toward demonstrating the utility of lead chalcogenide QDs as single-photon emitters (SPEs). Irrespective of type, few telecom-SPEs exist that are capable of such room-temperature operation. Access to single-QD spectra enabled a direct assessment of spectral line width, which was ∼70-90 meV compared to much broader ensemble spectra (∼300 meV). We show inhomogeneous broadening results from dispersity in PbS core sizes that increases dramatically with extended cation exchange. Quantum yields (QYs) are negatively impacted at thick shells (>6 monolayers) and, especially, by SILAR-growth conditions. Time-resolved PL measurements revealed that, with SILAR, initially single-exponential PL-decays transition to biexponential, with opening of nonradiative carrier-recombination channels. Radiative decay times are, overall, longer for core/shell QDs compared to PbS cores, which we demonstrate can be partially attributed to some core/shell sizes occupying a quasi-type II electron-hole localization regime. Finally, we demonstrate that shell engineering and the use of lower laser-excitation powers can afford significantly suppressed blinking and photobleaching. However, dependence on shell thickness comes at a cost of less-than-optimal brightness, with implications for both materials and experimental design.

Monodisperse CdSe Quantum Dots Encased in Six (100) Facets via Ligand-Controlled Nucleation and Growth

Zinc-blende CdSe quantum dots (QDs) encased in six equal (100) facets are synthesized in a noncoordinating solvent. Their monodispersed size, unique facet structure, and single crystallinity render the narrowest ensemble photoluminescence for CdSe QDs (full width at half-maximum being 52 meV). The nucleation stage can selectively form small-size CdSe QDs (≤3 nm) as seeds suited for the growth of cube-shaped QDs by reducing the concentration of cadmium carboxylates (Cd(RCOO)₂) as the sole source of ligands. While resulting in poorly controlled nucleation, chloride-ion ligands introduced in the form of soluble CdCl_x(RCOO)_1-x (x = 0.1∼0.2) would thermodynamically stabilize the cadmium-terminated (100) facets yet kinetically accelerate the deposition of selenium ions onto the (100) facets. Results suggest that it is fully feasible to synthesize QDs simultaneously with monodisperse size and surface structure through ligand-controlled nucleation and growth.

Charge Transfer in the Heterostructure of CsPbBr3 Nanocrystals with Nitrogen-Doped Carbon Dots

Heterostructures of inorganic halide perovskites with mixed-dimensional inorganic nanomaterials have shown great potential not only in the field of optoelectronic energy devices and photocatalysis but also for improving our fundamental understanding of the charge transfer across the heterostructure interface. Herein, we present for the first time the heterostructure integration of the CsPbBr₃ nanocrystal with an N-doped carbon dot. We explore the photoluminescence (PL) and photoconductivity of the heterostructure of CsPbBr₃ nanocrystals and N-doped carbon dots. PL quenching of CsPbBr₃ nanocrystals with the addition of N-doped carbon dots was observed. The photoexcited electrons from the conduction band of CsPbBr₃ are trapped in the N-acceptor state of N-doped carbon dots, and the charge transfer occurs via quasi type II-like electronic band alignment. The charge transfer in the halide perovskite-based heterostructure should motivate further research into the new heterostructure synthesis with perovskites and the fundamental understanding of the mechanism of charge/energy transfer across the heterostructure interface.

Quantum Dots for Display Applications

This article offers a materials-chemistry perspective for colloidal quantum dots (QDs) in the field of display, including QD-enhanced liquid-crystal-display (QD-LCD) and QD-based light-emitting-diodes (QLEDs) display. The rapid successes of QDs for display in the past five years are not accidental but have a deep root in both maturity of their synthetic chemistry and their unique chemical, optical, and optoelectronic properties. This article intends to discuss the natural match of QD emitters for display and chemical means to eventually bring about their full potential.

Material and device engineering for high-performance blue quantum dot light-emitting diodes

Colloidal quantum dots (QDs) have attracted extensive attention due to their excellent optoelectronic properties, such as high quantum efficiency, narrow emission peaks, high color saturation, high stability and solution processability. Compared with the traditional display technology, QD based light-emitting diodes (QLEDs) show broad application prospects in the field of flat-panel displays and solid-state lighting. However, for full-color displays, the efficiency and lifetime of blue QLEDs are inferior to those of their green and red counterparts. Therefore, it is urgent for us to deeply understand the device physics and improve the performance of blue QLEDs through material and device engineering. An in-depth understanding of the optoelectronic properties (such as the structure of electronic states, electron-phonon interactions, Auger processes, etc.) and material engineering (such as size distribution control, composition control, and surface engineering) of blue emission QDs is greatly helpful for their applications in other fields. Herein, we review the key progress in the area of blue QLEDs, including the compositions and nanostructures of blue quantum dots, advances in the device architectures and the improvement of the device lifetime of blue QLEDs. The key factors that influence the blue device performance, including the nanostructure design and surface modification of QDs, interface engineering and architecture design of devices are discussed, aiming to propose possible solutions for these challenges, which will help to promote the commercialization process of QLEDs.

Nanocrystal synthesis, μfluidic sample dilution and direct extraction of single emission linewidths in continuous flow

The rational design of semiconductor nanocrystal populations requires control of their emission linewidths, which are dictated by interparticle inhomogeneities and single-nanocrystal spectral linewidths. To date, research efforts have concentrated on minimizing the ensemble emission linewidths, however there is little knowledge about the synthetic parameters dictating single-nanocrystal linewidths. In this direction, we present a flow-based system coupled with an optical interferometry setup for the extraction of single nanocrystal properties. The platform has the ability to synthesize nanocrystals at high temperature <300 °C, adjust the particle concentration after synthesis and extract ensemble-averaged single nanocrystal emission linewidths using flow photon-correlation Fourier spectroscopy.

Fundamental Limit of the Emission Linewidths of Quantum Dots: An Ab Initio Study of CdSe Nanocrystals

The emission linewidth of a semiconducting nanocrystal (NC) significantly affects its performance in light-emitting applications, but its fundamental limit is still elusive. Herein, we analyze the exciton-phonon coupling (EPC) from Huang-Rhys (HR) factors using ab initio calculations and compute emission line shapes of CdSe NCs. When surface traps are absent, acoustic modes are found to dominate EPC. The computed linewidths are mainly determined by the size of NCs, being largely insensitive to the shape and crystal structure. Linewidths obtained in this work are much smaller than most measurements on homogeneous linewidths, but they are consistent with a CdSe/Cd_xZn_1-xSe (core/shell) NC [Park, Y.-S.; Lim, J.; Klimov, V. I. Nat. Mater. 2019 18, 249-255]. Based on this comparison, it is concluded that the large linewidths in most experiments originated from internal fields by surface (or interface) traps or quasi-type II band alignment that amplifies EPC. Thus, the present results on NCs with ideal passivation provide the fundamental minimum of homogeneous linewidths, indicating that only the CdSe/Cd_xZn_1-xSe NC has achieved this limit through well-controlled synthesis of shell structures. To further verify the role of internal fields, we model NCs with charged surface defects. We find that the internal field significantly increases HR factors and linewidths, in reasonable agreement with experiments on single cores. By revealing the fundamental limit of the emission linewidths of quantum dots, this work will pave the way for engineering quantum dots with an ultrasharp spectrum.

Quantized Reaction Pathways for Solution Synthesis of Colloidal ZnSe Nanostructures: A Connection between Clusters, Nanowires, and Two-Dimensional Nanoplatelets

The morphology of nanocrystals serves as a powerful handle to modulate their functional properties. For semiconducting nanostructures, the shape is no less important than the size and composition, in terms of determining the electronic structure. For example, in the case of nanoplatelets (NPLs), their two-dimensional (2D) electronic structure and atomic precision along the axis of quantum confinement makes them well-suited as pure color emitters and optical gain media. In this study, we describe synthetic efforts to develop ZnSe NPLs emitting in the ultraviolet part of the spectrum. We focus on two populations of NPLs, the first having a sharp absorption onset at 345 nm and a previously unreported species with an absorption onset at 380 nm. Interestingly, we observe that the nanoplatelets are one step in a quantized reaction pathway that starts with (zero-dimensional (0D)) magic-sized clusters, then proceeds through the formation of (one-dimensional (1D)) nanowires toward the (2D) "345 nm" species of NPLs, which finally interconvert into the "380 nm" NPL species. We seek to rationalize this evolution of the morphology, in terms of a general free-energy landscape, which, under reaction control, allows for the isolation of well-defined structures, while thermodynamic control leads to the formation of three-dimensional (3D) nanocrystals.

Phonon-Mediated and Weakly Size-Dependent Electron and Hole Cooling in CsPbBr3 Nanocrystals Revealed by Atomistic Simulations and Ultrafast Spectroscopy

We combine state-of-the-art ultrafast photoluminescence and absorption spectroscopy and nonadiabatic molecular dynamics simulations to investigate charge-carrier cooling in CsPbBr3 nanocrystals over a very broad size regime, from 0.8 to 12 nm. Contrary to the prevailing notion that polaron formation slows down charge-carrier cooling in lead-halide perovskites, no suppression of carrier cooling is observed in CsPbBr3 nanocrystals except for a slow cooling (over ∼10 ps) of "warm" electrons in the vicinity (within ∼0.1 eV) of the conduction band edge. At higher excess energies, electrons and holes cool with similar rates, on the order of 1 eV ps-1 carrier-1, increasing weakly with size. Our ab initio simulations suggest that cooling proceeds via fast phonon-mediated intraband transitions driven by strong and size-dependent electron-phonon coupling. The presented experimental and computational methods yield the spectrum of involved phonons and may guide the development of devices utilizing hot charge carriers.

The effects of discrete and gradient mid-shell structures on the photoluminescence of single InP quantum dots

We investigated the dependence of the spectral diffusion and blinking behaviors of indium phosphide (InP) based core/shell/shell quantum dots (QDs) on their mid-shell compositions. We synthesized two types of core/shell/shell QDs having different mid-shell structures by controlling the shell thickness, the total sizes, and the selenium to sulfur ratios. The QDs with a discrete mid-shell (DS-QDs) exhibited a higher photoluminescence (PL) quantum yield (QY) and a narrower PL linewidth than the QDs with a gradient mid-shell (GS-QDs). By analyzing X-ray diffraction (XRD) patterns, and Raman spectra, we found that GS-QDs showed a larger lattice mismatch between the core and the shell than DS-QDs. Also, the spectral diffusion, PL blinking, Auger ionization efficiencies, and the lifetime blinking behavior on single QDs revealed that DS-QDs were nearly unaffected by the defect traps.

Intrinsic Exciton Photophysics of PbS Quantum Dots Revealed by Low-Temperature Single Nanocrystal Spectroscopy

With a tunable size-dependent photoluminescence (PL) over a wide infrared wavelength range, lead chalcogenide quantum dots (QDs) have attracted significant scientific and technological interest. Nevertheless, the investigation of intrinsic exciton photophysics at the single-QD level has remained a challenge. Herein, we present a comprehensive study of PL properties for the individual core/shell PbS/CdS QDs emissive near 1.0 eV. In contrast to the sub-meV spectral line widths observed for II/VI QDs, PbS/CdS QDs are predicted to possess broad homogeneous line widths. Performing spectroscopy at cryogenic (4 K) temperatures, we provide direct evidence confirming theoretical predictions, showing that intrinsic line widths for PbS/CdS QDs are in the range of 8-25 meV, with an average of 16.4 meV. In addition, low-temperature, single-QD spectroscopy reveals a broad low-energy side emission attributable to optical as well as localized acoustic phonon-assisted transitions. By tracking single QDs from 4 to 250 K, we were able to probe temperature-dependent evolutions of emission energy, line width, and line shape. Finally, polarization-resolved PL imaging showed that PbS/CdS QDs are characterized by a 3D emission dipole, in contrast with the 2D dipole observed for CdSe QDs.

Machine Learning Accelerates Discovery of Optimal Colloidal Quantum Dot Synthesis

Colloidal quantum dots (CQDs) allow broad tuning of the bandgap across the visible and near-infrared spectral regions. Recent advances in applying CQDs in light sensing, photovoltaics, and light emission have heightened interest in achieving further synthetic improvements. In particular, improving monodispersity remains a key priority in order to improve solar cells' open-circuit voltage, decrease lasing thresholds, and improve photodetectors' noise-equivalent power. Here we utilize machine-learning-in-the-loop to learn from available experimental data, propose experimental parameters to try, and, ultimately, point to regions of synthetic parameter space that will enable record-monodispersity PbS quantum dots. The resultant studies reveal that adding a growth-slowing precursor (oleylamine) allows nucleation to prevail over growth, a strategy that enables record-large-bandgap (611 nm exciton) PbS nanoparticles with a well-defined excitonic absorption peak (half-width at half-maximum (hwhm) of 145 meV). At longer wavelengths, we also achieve improved monodispersity, with an hwhm of 55 meV at 950 nm and 24 meV at 1500 nm, compared to the best published to date values of 75 and 26 meV, respectively.