Encapsulation of Cadmium-Free InP-based Quantum Dots in Cross-Linked Core-Shell Microparticles via Coaxial Electrospraying.

Quantum dots (QDs) are inorganic semiconductor nanocrystals capable of emitting light. The current major challenge lies in the use of heavy metals, which are known to be highly toxic to humans and pose significant environmental risks. Researchers have turned to indium (In) as a promising option for more environmentally benign QDs, specifically indium phosphide (InP). A significant obstacle remains in sustaining the long-term photostability of InP-based QDs when exposed to the environment. To tackle this, electrospraying is used in this work to protect indium phosphide/zinc selenide/zinc sulfide (InP/ZnSe/ZnS) QDs by embedding them within polymer core-shell microparticles of poly[(lauryl methacrylate)-co-(ethylene glycol dimethacrylate)]/poly(methyl methacrylate) (poly(LMA-co-EGDMA)/PMMA). During the flight of droplets, the liquid monomer core of LMA and EGDMA with QDs is encapsulated by the solid shell of PMMA formed due to solvent evaporation, resulting in a liquid-core/solid-shell particle structure. After that, the captured core of monomers is polymerized into a cross-linked polymer with the embedded QDs via a thermal initiation. They demonstrate how a successful core-shell particle formation is achieved to produce structures for initially liquid monomer systems via coaxial electrospraying that are used for cross-linked polymers, which are of major interest for the encapsulation of InP-based QDs for generally improved photostability over pristine QDs.

Ligands on Nanocrystal Surfaces, the 1H Nuclear Magnetic Resonance Fingerprint.

ConspectusSurfaces are an integral part of colloidal nanocrystals (NCs). Hence, understanding the binding and packing to NC surfaces of organic ligands, which are often used to stabilize NC colloids, is an essential aspect of the formation of NCs with desired chemical or physical properties. Since NCs lack a unique structure, not a single analytical technique can provide a complete description of the chemistry of NC surfaces. Even so, solution 1H nuclear magnetic resonance spectroscopy stands out as a unique method to study the organic ligand shell for its capability to distinguish between surface bound species and surface inactive residues from NC synthesis and purification.In this account, we first set the stage by highlighting the fingerprints of ligands bound to NCs in solution 1H NMR, which are broadened and shifted resonances, slow diffusion, and pronounced transfer of spin polarization between nearby protons. These characteristics enable bound ligands to be identified and quantified by 1D 1H NMR spectroscopy, diffusion-ordered spectroscopy (DOSY), and nuclear Overhauser effect spectroscopy (NOESY). Even so, we argue in a second part that much more insight in surface chemistry can be obtained from the in situ monitoring of ligand exchange processes. The chemical analysis of released compounds and the thermodynamic study of exchange equilibria provide a surprisingly detailed picture of the chemistry of the NC-ligand bond, the heterogeneity of binding sites, and the bunching of ligands on the NC surface. Multiple case studies are discussed to illustrate these different aspects of NC surface chemistry, where work on CdSe NCs in particular indicates that binding sites at facet edges are most vulnerable for ligand loss. While such weak binding sites are a liability for optoelectronic applications, they could offer an opportunity for catalysis. Moreover, the general character of the methodology introduced calls for realizing a broad, quantitative survey of NC-ligand interactions, well beyond the extensively studied case of CdSe NCs.In a third part, we address in more detail the line broadening that characterizes ligands bound to NCs, which results from a combination of reduced mobility and a diversity of chemical environments. Hence, chemical shift and line shape, or rates of transversal relaxation and interligand cross-relaxation, can all convey information on the ligand environment, especially when solvents are used that are chemically distinct from the ligand chain, such as aromatic versus aliphatic. Two examples that illustrate this point are the relation between line width and ligand solvation, where better solvated ligands yield more narrow resonances, and the possibility to identify different parts of the inhomogeneously broadened resonance with ligands bound on different locations at the NC surface. Interestingly, such results question the limits of NC size and ligand packing density at which the current bound-ligand paradigm, modest inhomogeneous broadening, will break down. Building on this question, we summarize in a final part the current status of NC ligand analysis by solution 1H NMR and outline directions for further research.

Colloidal CdSe/CdS Core/Crown Nanoplatelets for Efficient Blue Light Emission and Optical Amplification.

The application of CdSe nanoplatelets (NPLs) in the ultraviolet/blue region remains an open challenge due to charge trapping typically leading to limited photoluminescence quantum efficiency (PL QE) and sub-bandgap emission in core-only NPLs. Here, we synthesized 3.5 monolayer core/crown CdSe/CdS NPLs with various crown dimensions, exhibiting saturated blue emission and PL QE up to 55%. Compared to core-only NPLs, the PL intensity decays monoexponentially over two decades due to suppressed deep trapping and delayed emission. In both core-only and core/crown NPLs we observe biexciton-mediated optical gain between 470 and 510 nm, with material gain coefficients up to 7900 cm-1 and consistently lower gain thresholds in crowned NPLs. Gain lifetimes are limited to 40 ps, due to residual ultrafast trapping and higher exciton densities at threshold. Our results provide guidelines for rational optimization of thin CdSe NPLs toward lighting and light-amplification applications.

Bridging the Green Gap: Monochromatic InP-Based Quantum-Dot-on-Chip LEDs with over 50% Color Conversion Efficiency.

Solid-state light-emitting diodes (LEDs) emit nearly monochromatic light, yet seamless tuning of emission color throughout the visible region remains elusive. Color-converting powder phosphors are therefore used for making LEDs with a bespoke emission spectrum, yet broad emission lines and low absorption coefficients compromise the formation of small-footprint monochromatic LEDs. Color conversion by quantum dots (QDs) can address these issues, but high-performance monochromatic LEDs made using QDs free of restricted, hazardous elements remain to be demonstrated. Here, we show green, amber, and red LEDs formed using InP-based QDs as on-chip color convertor for blue LEDs. Implementing QDs with near-unity photoluminescence efficiency yields a color conversion efficiency over 50% with little intensity roll-off and nearly complete blue light rejection. Moreover, as the conversion efficiency is mostly limited by package losses, we conclude that on-chip color conversion using InP-based QDs can provide spectrum-on-demand LEDs, including monochromatic LEDs that bridge the green gap.

The Dynamic Interaction of Surfactants with Colloidal Molybdenum Disulfide Nanosheets Calls for Thermodynamic Stabilization by Solvents.

Top-down liquid-phase exfoliation (LPE) and bottom-up hot-injection synthesis are scalable methods to produce colloids of two-dimensional (2D) van der Waals (vdW) solids. Generally thought off as two entirely different fields, we show that similar stabilization mechanisms apply to colloids of molybdenum disulfide (MoS2) produced by both methods. By screening the colloidal stability of MoS2 produced in a hot-injection synthesis in a wide range of solvents, we observe that colloidal stability can be understood based on solution thermodynamics, wherein matching the solubility parameter of solvent and nanomaterial maximizes colloidal stability. Identical to MoS2 produced through LPE, optimal solvents to disperse MoS2 produced from the bottom-up have similar solubility parameters of ≈22 MPa1/2 and include aromatic solvents with polar functionalities, such as o-dichlorobenzene, and polar aprotic solvents, such as N,N-dimethylformamide. We further complemented our findings by nuclear magnetic resonance (NMR) spectrscopy, highlighting that organic surfactants, such as oleylamine and oleic acid, have a minimal affinity toward the nanocrystal surface and engage in a highly dynamic adsorption/desorption equilibrium. We thus conclude that hot injection yields MoS2 colloids with comparable surfaces as those produced by LPE. These similarities might offer the prospect of using established procedures developed for LPE nanomaterials to postprocess colloidally synthesized dispersions of 2D colloids as processable inks.

Classical Force Field Parameters for InP and InAs Quantum Dots with Various Surface Passivations.

Classical molecular dynamics (MD) simulations on realistic colloidal quantum dot (QD) systems are often hampered by missing force field (FF) parameters for an accurate description of the QD-ligand interface. However, such calculations are of major interest, specifically for studying the surface chemistry of colloidal nanocrystals. In this work, we have utilized a previously published stochastic optimization algorithm to obtain FF parameters for InP and InAs QDs capped by Cl, amine, carboxylate, and thiolate ligands. Our FF parameters are interfaced with well-established FFs for organic molecules, allowing for the simulation of InP and InAs QDs with a broad range of organic ligands in explicit apolar solvents. The quality of our FF parameters was assessed by comparing properties of the classical MD simulations with ab initio MD simulations and experimental and theoretical values from the literature.

A roadmap to decipher ultrafast photophysics in two-dimensional nanomaterials.

Atomically thin two-dimensional (2D) semiconductors are extensively investigated for optoelectronic applications that require strong light-matter interactions. In view of such applications, it is essential to understand how (photo)excitation alters the non-linear optical response of these materials under high carrier density conditions. Broadband transient absorption (TA) spectroscopy is by now a widely used tool to study the semiconductor physics in such highly excited systems. However, the complex interplay between different many-body interactions in 2D materials produces highly congested spectral information and an ensuing non-trivial non-linear photo-response, thereby masking the desired intrinsic photophysics. Herein, we outline a concise roadmap for analyzing such congested datasets based on examples of TA analysis of various 2D materials. In particular, we emphasize the synergy between an initial qualitative understanding of the transient photo-response based on line shapes and their derivatives and a consequent quantitative spectral deconvolution backed by such insights.

Mechanistic study of ZnSe nanocrystal formation from zinc halides.

We studied the formation of zinc selenide (ZnSe) from zinc chloride (ZnCl2) and trioctylphosphine selenide (TOP=Se) in oleylamine, a chemistry originally proposed to grow ZnSe shells around InP core quantum dots. By monitoring the formation of ZnSe in reactions with and without InP seeds by quantitative absorbance and nuclear magnetic resonance (NMR) spectroscopy, we observe that the ZnSe formation rate is independent of the presence of InP cores. Similar to the seeded growth of CdSe and CdS, this observation supports a ZnSe growth mechanism through the inclusion of reactive ZnSe monomers that form homogeneously in the solution. Furthermore, by combining NMR and mass spectrometry, we identified the dominant reaction products of the ZnSe formation reaction as oleylammonium chloride and amino-substitutions of TOP, i.e., iminophosphoranes (TOP=NR), aminophosphonium chloride salts [TOP(NHR)Cl], and bis(amino)phosphoranes [TOP(NHR)2]. Based on the acquired results, we outline a reaction scheme that involves the complexation of TOP=Se by ZnCl2, followed by the nucleophilic addition of oleylamine onto the Lewis acid activated P-Se bond, thereby eliminating ZnSe monomers and forming amino-substitutions of TOP. Our work highlights the central role of oleylamine, acting as both the nucleophile and Brønsted base, in the transformation of metal halides and alkylphosphine chalcogenides into metal chalcogenides.

Narrow homogeneous linewidths and slow cooling dynamics across infrared intra-band transitions in n-doped HgSe colloidal quantum dots.

Intra-band transitions in colloidal quantum dots (QDs) are promising for opto-electronic applications in the mid-IR spectral region. However, such intra-band transitions are typically very broad and spectrally overlapping, making the study of individual excited states and their ultrafast dynamics very challenging. Here, we present the first full spectrum two-dimensional continuum infrared (2D CIR) spectroscopy study of intrinsically n-doped HgSe QDs, which exhibit mid-infrared intra-band transitions in their ground state. The obtained 2D CIR spectra reveal that underneath the broad absorption line shape of ∼500 cm-1, the transitions exhibit surprisingly narrow intrinsic linewidths with a homogeneous broadening of 175-250 cm-1. Furthermore, the 2D IR spectra are remarkably invariant, with no sign of spectral diffusion dynamics at waiting times up to 50 ps. Accordingly, we attribute the large static inhomogeneous broadening to the distribution of size and doping level of the QDs. In addition, the two higher-lying P-states of the QDs can be clearly identified in the 2D IR spectra along the diagonal with a cross-peak. However, there is no indication of cross-peak dynamics indicating that, despite the strong spin-orbit coupling in HgSe, transitions between the P-states must be longer than our maximum waiting time of 50 ps. This study illustrates a new frontier of 2D IR spectroscopy enabling the study of intra-band carrier dynamics in nanocrystalline materials across the entire mid-infrared spectrum.

Area-Independence of the Biexciton Oscillator Strength in CdSe Colloidal Nanoplatelets.

Colloidal CdSe nanoplatelets (NPLs) are unique systems to study two-dimensional excitons and excitonic complexes. However, while absorption and emission of photons through exciton formation and recombination have been extensively quantified, few studies have addressed the exciton-biexciton transition. Here, we use cross-polarized pump-probe spectroscopy to measure the absorption coefficient spectrum of this transition and determine the biexciton oscillator strength (fBX). We show that fBX is independent of the NPL area and that the concomitant biexciton area (SBX) agrees with predictions of a short-range interaction model. Moreover, we show that fBX is comparable to the oscillator strength of forming localized excitons at room temperature while being unaffected itself by center-of-mass localization. These results confirm the relevance of biexcitons for light-matter interaction in NPLs. Moreover, the quantification of the exciton-biexciton transition introduced here will enable researchers to rank 2D materials by the strength of this transition and to compare experimental results with theoretical predictions.

General Expression for the Size-Dependent Optical Properties of Quantum Dots.

While initial theories on quantum confinement in colloidal quantum dots (QDs) led to analytical band gap/size relations or sizing functions, numerical methods describe size quantization more accurately. However, because of the lack of reliable sizing functions, researchers fit experimental band gap/size data sets using models with redundant, physically meaningless parameters that break down upon extrapolation. Here, we propose a new sizing function based on a proportional correction for nonparabolic bands. Using known bulk parameters, we predict size quantization for groups IV, III-V, II-VI, and IV-VI and metal-halide perovskite semiconductors, including straightforward adaptations for negative-gap semiconductors and nonspherical QDs. Refinement with respect to experimental data is possible using the Bohr diameter as a fitting parameter, by which we show a statistically relevant difference in the band gap/size relation for wurtzite and zinc blende CdSe. The general sizing function proposed here unifies the QD size calibration and enables researchers to assess bulk semiconductor parameters and predict the size quantization in unexplored materials.

Comparison of different RGB InP-quantum-dot-on-chip LED configurations.

InP/ZnSe/ZnS quantum dots (QDs) offer a cadmium-free solution to make white LEDs with a narrow blue, green and red emission peak. Such LEDs are required for display and lighting applications with high color gamut. An important phenomenon that hampers the efficiency of such quantum-dot-on-chip LEDs is re-absorption of already converted light by the QDs. Proposed solutions to remedy this effect often rely on complex or cost-ineffective manufacturing methods. In this work, four different RGB QD-on-chip LED package configurations are investigated that can be fabricated with a simple cavity encapsulation method. Using accurate optical simulations, the impact of QD re-absorption on the overall luminous efficacy of the light source is analyzed for these four configurations as a function of the photo-luminescent quantum yield (PLQY) of the QDs. The simulation results are validated by implementing these configurations in QD-on-chip LEDs using a single set of red and green emitting InP/ZnSe/ZnS QDs. In this way, the benefits are demonstrated of adding volume scattering particles or a hemispherical extraction dome to the LED package. The best configuration in terms of luminous efficacy, however, is one where the red QDs are deposited in the recycling cavity, while the green QDs are incorporated in the extraction dome. Using this configuration with green and red InP/ZnSe/ZnS QDs with a PLQY of 75% and 65% respectively, luminous efficacy of 102 lm/W was realized for white light with a CCT of 3000 K.

Van Hove Singularities and Trap States in Two-Dimensional CdSe Nanoplatelets.

Semiconductor nanoplatelets, which offer a compelling combination of the flatness of two-dimensional semiconductors and the inherent richness brought about by colloidal nanostructure synthesis, form an ideal and general testbed to investigate fundamental physical effects related to the dimensionality of semiconductors. With low temperature scanning tunnelling spectroscopy and tight binding calculations, we investigate the conduction band density of states of individual CdSe nanoplatelets. We find an occurrence of peaks instead of the typical steplike function associated with a quantum well, that rule out a free in-plane electron motion, in agreement with the theoretical density of states. This finding, along with the detection of deep trap states located on the edge facets, which also restrict the electron motion, provides a detailed picture of the actual lateral confinement in quantum wells with finite length and width.

The Fine-Structure Constant as a Ruler for the Band-Edge Light Absorption Strength of Bulk and Quantum-Confined Semiconductors.

Low-dimensional semiconductors have found numerous applications in optoelectronics. However, a quantitative comparison of the absorption strength of low-dimensional versus bulk semiconductors has remained elusive. Here, we report generality in the band-edge light absorptance of semiconductors, independent of their dimensions. First, we provide atomistic tight-binding calculations that show that the absorptance of semiconductor quantum wells equals mπα (m = 1 or 2 with α as the fine-structure constant), in agreement with reported experimental results. Then, we show experimentally that a monolayer (superlattice) of quantum dots has similar absorptance, suggesting an absorptance quantum of mπα per (confined) exciton diameter. Extending this idea to bulk semiconductors, we experimentally demonstrate that an absorptance quantum equal to mπα per exciton Bohr diameter explains their widely varying absorption coefficients. We thus provided compelling evidence that the absorptance quantum πα per exciton diameter rules the band-edge absorption of all direct semiconductors, regardless of their dimension.

Extended Nucleation and Superfocusing in Colloidal Semiconductor Nanocrystal Synthesis.

Hot-injection synthesis is renowned for producing semiconductor nanocolloids with superb size dispersions. Burst nucleation and diffusion-controlled size focusing during growth have been invoked to rationalize this characteristic yet experimental evidence supporting the pertinence of these concepts is scant. By monitoring a CdSe synthesis in-situ with X-ray scattering, we find that nucleation is an extended event that coincides with growth during 15-20% of the reaction time. Moreover, we show that size focusing outpaces predictions of diffusion-limited growth. This observation indicates that nanocrystal growth is dictated by the surface reactivity, which drops sharply for larger nanocrystals. Kinetic reaction simulations confirm that this so-called superfocusing can lengthen the nucleation period and promote size focusing. The finding that narrow size dispersions can emerge from the counteracting effects of extended nucleation and reaction-limited size focusing ushers in an evidence-based perspective that turns hot injection into a rational scheme to produce monodisperse semiconductor nanocolloids.

Acid-Base Mediated Ligand Exchange on Near-Infrared Absorbing, Indium-Based III-V Colloidal Quantum Dots.

Colloidal quantum dots (QDs) made from In-based III-V semiconductors are emerging as a printable infrared material. However, the formulation of infrared inks and the formation of electrically conductive QD coatings is hampered by a limited understanding of the surface chemistry of In-based QDs. In this work, we present a case study on the surface termination of IR active III-V QDs absorbing at 1220 nm that were synthesized by reducing a mixture of indium halides and an aminoarsine by an aminophosphine in oleylamine. We find that this recently established synthesis method yields In(As,P) QDs with minor phosphorus admixing and a surface terminated by a mixture of oleylamine and chloride. Exposing these QDs to protic surface-active compounds RXH, such as fatty acids or alkanethiols, initiates a ligand exchange reaction involving the binding of the conjugate base RX- and the desorption of 1 equiv of alkylammonium chloride. Using density functional theory simulations, we confirm that the formation of the alkylammonium chloride salt can provide the energy needed to drive such acid/base mediated ligand exchange reactions, even for weak organic acids such as alkanethiols. We conclude that the unique surface termination of In(As,P) QDs, consisting of a mixture of L-type and X-type ligands and acid/base mediated ligand exchange, can form a general model for In-based III-V QDs synthesized using indium halides and aminopnictogens.

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.

Atomically Precise Nanocrystals.

Nanocrystals are a state-of-matter in the border area between molecules and bulk materials. Unlike bulk materials, nanocrystals have size-dependent properties, yet the question remains whether nanocrystal properties can be analyzed, understood, and controlled with atomic precision, a key characteristic of molecules. Acknowledging the inclination of nanocrystals to form defect structures, we first outline the prospects of atomically precise analysis. A broad spectrum of analytical methods has become available over the last five years, such that for heterogeneous nanocrystal ensembles, a single, atomically precise representative structure can be determined to explore structure-property relations. Atomically precise synthesis, on the other hand, remains an outstanding challenge that may well face fundamental limitations. However, to amplify properties and prepare nanocrystals for specific applications, full atomic precision may not be needed. Examples of an atomic precision light approach, focusing on exact thickness or facet control, exist and can inspire scientists to explore atomic precision in nanocrystal research further.

Liquid-Phase Exfoliation of Rhenium Disulfide by Solubility Parameter Matching.

In this work, we provide a detailed account of the liquid-phase exfoliation (LPE) of rhenium disulfide (ReS2), a promising new-generation two-dimensional material. By screening LPE in a wide range of solvents, we show that the most optimal solvents are characterized by similar Hildebrand or dispersive Hansen solubility parameters of 25 and 18 MPa1/2, respectively. Such values are attained by solvents such as N-methyl-2-pyrrolidone, N,N-dimethylformamide, and 1-butanol. In line with solution thermodynamics, we interpret the conditions for high-yield exfoliation as a matching of the solvent and ReS2 solubility parameters. Using N-methyl-2-pyrrolidone as an exemplary exfoliation solvent, we undertook a detailed analysis of the exfoliated ReS2. In-depth morphological, structural, and elemental characterization outlined that the LPE procedure presented here produces few-layer, anisotropically stacked, and chemically pure ReS2 platelets with long-term stability against oxidation. These results underscore the suitability of LPE to batch-produce few-layer and pristine ReS2 in solvents that have a solubility parameter close to 25 MPa1/2.

Integration of Colloidal PbS/CdS Quantum Dots with Plasmonic Antennas and Superconducting Detectors on a Silicon Nitride Photonic Platform.

Single-photon sources and detectors are indispensable building blocks for integrated quantum photonics, a research field that is seeing ever increasing interest for numerous applications. In this work, we implemented essential components for a quantum key distribution transceiver on a single photonic chip. Plasmonic antennas on top of silicon nitride waveguides provide Purcell enhancement with a concurrent increase of the count rate, speeding up the microsecond radiative lifetime of IR-emitting colloidal PbS/CdS quantum dots (QDs). The use of low-fluorescence silicon nitride, with a waveguide loss smaller than 1 dB/cm, made it possible to implement high extinction ratio optical filters and low insertion loss spectrometers. Waveguide-coupled superconducting nanowire single-photon detectors allow for low time-jitter single-photon detection. To showcase the performance of the components, we demonstrate on-chip lifetime spectroscopy of PbS/CdS QDs. The method developed in this paper is predicted to scale down to single QDs, and newly developed emitters can be readily integrated on the chip-based platform.