Synthesis of Cs3Cu2I5 Nanocrystals in a Continuous Flow System.

Achieving the goal of generating all of the world's energy via renewable sources and significantly reducing the energy usage will require the development of novel, abundant, nontoxic energy conversion materials. Here, a cost-efficient and scalable continuous flow synthesis of Cs3Cu2I5 nanocrystals is developed as a basis for the rapid advancement of novel nanomaterials. Ideal precursor solutions are obtained through a novel batch synthesis, whose product served as a benchmark for the subsequent flow synthesis. Realizing this setup enabled a reproducible fabrication of Cs3Cu2I5 nanocrystals. The effect of volumetric flow rate and temperature on the final product's morphology and optical properties are determined, obtaining 21% quantum yield with the optimal configuration. Consequently, the size and morphology of the nanocrystals can be tuned with far more precision and in a much broader range than previously achievable. The flow setup is readily applicable to other relevant nanomaterials. It should enable a rapid determination of a material's potential and subsequently optimize its desired properties for renewable energy generation or efficient optoelectronics.

Lead-Free, Luminescent Perovskite Nanocrystals Obtained through Ambient Condition Synthesis.

Heterovalently substituting toxic lead is an increasingly popular design strategy to obtain environmentally sustainable variants of the exciting material class of halide perovskites. Perovskite nanocrystals (NCs) obtained through solution-based methods exhibit exceedingly high optical quality. Unfortunately, most of these synthesis routes still require reaction under inert gas and at very high temperatures. Herein a novel synthesis routine for lead-free double perovskite (LFDP) NCs is presented. An approach based upon the hot injection and ligand-assisted reprecipitation (LARP) methods to achieve a low-temperature and ambient atmosphere-based synthesis for manganese-doped Cs2 NaBiCl6 NCs is presented. Mn incorporation is critical for the otherwise non-emissive material, with a 9:1 Bi:Mn precursor ratio maximizing the bright orange photoluminescence (PL) and quantum yield (QY). Higher synthesis temperatures slightly increase the material's performance, yet NCs synthesized at room temperature are still emissive, highlighting the versatility of the synthetic approach. While the material's indirect bandgap limits its appeal for optoelectronics, this feature could benefit photocatalysis due to longer carrier lifetimes. Moreover, the developed synthesis is facile and can rapidly be adapted to other more viable material compositions and up-scaled to realize applications directly.

Energy Transfer in Stability-Optimized Perovskite Nanocrystals.

Outstanding optoelectronic properties and a facile synthesis render halide perovskite nanocrystals (NCs) a promising material for nanostructure-based devices. However, the commercialization is hindered mainly by the lack of NC stability under ambient conditions and inefficient charge carrier injection. Here, we investigate solutions to both problems, employing methylammonium lead bromide (MAPbBr3) NCs encapsulated in diblock copolymer core-shell micelles of tunable size. We confirm that the shell does not prohibit energy transfer, as FRET efficiencies between these NCs and 2D CsPbBr3 nanoplatelets (NPLs) reach 73.6%. This value strongly correlates to the micelle size, with thicker shells displaying significantly reduced FRET efficiencies. Those high efficiencies come with a price, as the thinnest shells protect the encapsulated NCs less from environmentally induced degradation. Finding the sweet spot between efficiency and protection could lead to the realization of tailored energy funnels with enhanced carrier densities for high-power perovskite NC-based optoelectronic applications.

Thickness-Dependent Dark-Bright Exciton Splitting and Phonon Bottleneck in CsPbBr3-Based Nanoplatelets Revealed via Magneto-Optical Spectroscopy.

The optimized exploitation of perovskite nanocrystals and nanoplatelets as highly efficient light sources requires a detailed understanding of the energy spacing within the exciton manifold. Dark exciton states are particularly relevant because they represent a channel that reduces radiative efficiency. Here, we apply large in-plane magnetic fields to brighten optically inactive states of CsPbBr3-based nanoplatelets for the first time. This approach allows us to access the dark states and directly determine the dark-bright splitting, which reaches 22 meV for the thinnest nanoplatelets. The splitting is significantly less for thicker nanoplatelets due to reduced exciton confinement. Additionally, the form of the magneto-PL spectrum suggests that dark and bright state populations are nonthermalized, which is indicative of a phonon bottleneck in the exciton relaxation process.

Metal Halide Perovskite Nanocrystals: Synthesis, Post-Synthesis Modifications, and Their Optical Properties.

Metal halide perovskites represent a flourishing area of research, which is driven by both their potential application in photovoltaics and optoelectronics and by the fundamental science behind their unique optoelectronic properties. The emergence of new colloidal methods for the synthesis of halide perovskite nanocrystals, as well as the interesting characteristics of this new type of material, has attracted the attention of many researchers. This review aims to provide an up-to-date survey of this fast-moving field and will mainly focus on the different colloidal synthesis approaches that have been developed. We will examine the chemistry and the capability of different colloidal synthetic routes with regard to controlling the shape, size, and optical properties of the resulting nanocrystals. We will also provide an up-to-date overview of their postsynthesis transformations, and summarize the various solution processes that are aimed at fabricating halide perovskite-based nanocomposites. Furthermore, we will review the fundamental optical properties of halide perovskite nanocrystals by focusing on their linear optical properties, on the effects of quantum confinement, and on the current knowledge of their exciton binding energies. We will also discuss the emergence of nonlinear phenomena such as multiphoton absorption, biexcitons, and carrier multiplication. Finally, we will discuss open questions and possible future directions.

Real-Time Electron and Hole Transport Dynamics in Halide Perovskite Nanowires.

For optoelectronic devices, high transport mobilities of electrons and holes are desirable, which, moreover, should be close to identical. Acousto-optoelectric spectroscopy is employed to probe the spatiotemporal dynamics of both electrons and holes inside CsPbI3 nanowires. These dynamics are induced without the need for electrical contacts simply by the piezoelectric field of a surface acoustic wave. Its radio frequency of fSAW = 324 MHz natively avoids spurious contributions from ion migration typically occurring in these materials. The observed dynamic modulation of the photoluminescence is faithfully reproduced by solving the drift and diffusion currents of electrons and holes induced by the surface acoustic wave. These calculations confirm that the mobilities of electrons and holes are equal and quantify them to be µe = µh = 3 ± 1 cm2 V-1 s-1. Additionally, carrier loss due to surface recombination is shown to be largely suppressed in CsPbI3 nanowires. Both findings mark significant advantages over traditional compound semiconductors, in particular, GaAs, for applications in future optoelectronic and photovoltaic devices. The demonstrated sublifetime modulation of the optical emission may find direct application in switchable perovskite light-emitting devices employing mature surface acoustic wave technology.

Polymer Nanoreactors Shield Perovskite Nanocrystals from Degradation.

Halide perovskite nanocrystals (NCs) have shown impressive advances, exhibiting optical properties that outpace conventional semiconductor NCs, such as near-unity quantum yields and ultrafast radiative decay rates. Nevertheless, the NCs suffer even more from stability problems at ambient conditions and due to moisture than their bulk counterparts. Herein, we report a strategy of employing polymer micelles as nanoreactors for the synthesis of methylammonium lead trihalide perovskite NCs. Encapsulated by this polymer shell, the NCs display strong stability against water degradation and halide ion migration. Thin films comprising these NCs exhibit a more than 15-fold increase in lifespan in comparison to unprotected NCs in ambient conditions and even survive over 75 days of complete immersion in water. Furthermore, the NCs, which exhibit quantum yields of up to 63% and tunability of the emission wavelength throughout the visible range, show no signs of halide ion exchange. Additionally, heterostructures of MAPI and MAPBr NC layers exhibit efficient Förster resonance energy transfer (FRET), revealing a strategy for optoelectronic integration.

Boosting Tunable Blue Luminescence of Halide Perovskite Nanoplatelets through Postsynthetic Surface Trap Repair.

The easily tunable emission of halide perovskite nanocrystals throughout the visible spectrum makes them an extremely promising material for light-emitting applications. Whereas high quantum yields and long-term colloidal stability have already been achieved for nanocrystals emitting in the red and green spectral range, the blue region currently lags behind with low quantum yields, broad emission profiles, and insufficient colloidal stability. In this work, we present a facile synthetic approach for obtaining two-dimensional CsPbBr3 nanoplatelets with monolayer-precise control over their thickness, resulting in sharp photoluminescence and electroluminescence peaks with a tunable emission wavelength between 432 and 497 nm due to quantum confinement. Subsequent addition of a PbBr2-ligand solution repairs surface defects likely stemming from bromide and lead vacancies in a subensemble of weakly emissive nanoplatelets. The overall photoluminescence quantum yield of the blue-emissive colloidal dispersions is consequently enhanced up to a value of 73 ± 2%. Transient optical spectroscopy measurements focusing on the excitonic resonances further confirm the proposed repair process. Additionally, the high stability of these nanoplatelets in films and to prolonged ultraviolet light exposure is shown.

Tracking the Source of Carbon Dot Photoluminescence: Aromatic Domains versus Molecular Fluorophores.

Carbon dots (CDs) are an intriguing fluorescent material; however, due to a plethora of synthesis techniques and precursor materials, there is still significant debate on their structure and the origin of their optical properties. The two most prevalent mechanisms to explain them are based on polycyclic aromatic hydrocarbon domains and small molecular fluorophores, for instance, citrazinic acid. Yet, how these form and whether they can exist simultaneously is still under study. To address this, we vary the hydrothermal synthesis time of CDs obtained from citric acid and ethylenediamine and show that in the initial phase molecular fluorophores, likely 2-pyridone derivatives, account for the blue luminescence of the dots. However, over time, while the overall size of the CDs does not change, aromatic domains form and grow, resulting in a second, faster decay channel at similar wavelengths and also creating additional lower energetic states. Electrophoresis provides further evidence that the ensemble of CDs consists of several subsets with different internal structure and surface charge. The understanding of the formation mechanism enables a control of the chemical origin of these emitters and the ensuing optical properties of the CDs through synthetic means.

From Precursor Powders to CsPbX3 Perovskite Nanowires: One-Pot Synthesis, Growth Mechanism, and Oriented Self-Assembly.

The colloidal synthesis and assembly of semiconductor nanowires continues to attract a great deal of interest. Herein, we describe the single-step ligand-mediated synthesis of single-crystalline CsPbBr3 perovskite nanowires (NWs) directly from the precursor powders. Studies of the reaction process and the morphological evolution revealed that the initially formed CsPbBr3 nanocubes are transformed into NWs through an oriented-attachment mechanism. The optical properties of the NWs can be tuned across the entire visible range by varying the halide (Cl, Br, and I) composition through subsequent halide ion exchange. Single-particle studies showed that these NWs exhibit strongly polarized emission with a polarization anisotropy of 0.36. More importantly, the NWs can self-assemble in a quasi-oriented fashion at an air/liquid interface. This process should also be easily applicable to perovskite nanocrystals of different morphologies for their integration into nanoscale optoelectronic devices.

Carbon Dots: A Unique Fluorescent Cocktail of Polycyclic Aromatic Hydrocarbons.

Carbon dots (CDs) have attracted rapidly growing interest in recent years due to their unique and tunable optical properties, the low cost of fabrication, and their widespread uses. However, due to the complex structure of CDs, both the molecular ingredients and the intrinsic mechanisms governing photoluminescence of CDs are poorly understood. Among other features, a large Stokes shift of over 100 nm and a photoluminescence spectrally dependent on the excitation wavelength have so far not been adequately explained. In this Letter we investigate CDs and develop a model system to mimic their optical properties. This system comprised three types of polycyclic aromatic hydrocarbon (PAH) molecules with fine-tuned concentrations embedded in a polymer matrix. The model suggests that the Stokes shift in CDs is due to the self-trapping of an exciton in the PAH network. The width and the excitation dependence of the emission comes from a selective excitation of PAHs with slightly different energy gaps and from energy transfer between them. These insights will help to tailor the optical properties of CDs and help their implementation into applications, e.g., light-emitting devices and biomarkers. This could also lead to "artificial" tunable carbon dots by locally modifying the composition and consequently the optical properties of composite PAH films.

Quantum Size Effect in Organometal Halide Perovskite Nanoplatelets.

Organometal halide perovskites have recently emerged displaying a huge potential for not only photovoltaic, but also light emitting applications. Exploiting the optical properties of specifically tailored perovskite nanocrystals could greatly enhance the efficiency and functionality of applications based on this material. In this study, we investigate the quantum size effect in colloidal organometal halide perovskite nanoplatelets. By tuning the ratio of the organic cations used, we can control the thickness and consequently the photoluminescence emission of the platelets. Quantum mechanical calculations match well with the experimental values. We find that not only do the properties of the perovskite, but also those of the organic ligands play an important role. Stacking of nanoplatelets leads to the formation of minibands, further shifting the bandgap energies. In addition, we find a large exciton binding energy of up to several hundreds of meV for nanoplatelets thinner than three unit cells, partially counteracting the blueshift induced by quantum confinement. Understanding of the quantum size effects in perovskite nanoplatelets and the ability to tune them provide an additional method with which to manipulate the optical properties of organometal halide perovskites.

Highly Luminescent Cesium Lead Halide Perovskite Nanocrystals with Tunable Composition and Thickness by Ultrasonication.

We describe the simple, scalable, single-step, and polar-solvent-free synthesis of high-quality colloidal CsPbX3 (X=Cl, Br, and I) perovskite nanocrystals (NCs) with tunable halide ion composition and thickness by direct ultrasonication of the corresponding precursor solutions in the presence of organic capping molecules. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) revealed the cubic crystal structure and surface termination of the NCs with atomic resolution. The NCs exhibit high photoluminescence quantum yields, narrow emission line widths, and considerable air stability. Furthermore, we investigated the quantum size effects in CsPbBr3 and CsPbI3 nanoplatelets by tuning their thickness down to only three to six monolayers. The high quality of the prepared NCs (CsPbBr3 ) was confirmed by amplified spontaneous emission with low thresholds. The versatility of this synthesis approach was demonstrated by synthesizing different perovskite NCs.

Nanoparticles heat through light localization.

Aqueous solutions containing light-absorbing nanoparticles have recently been shown to produce steam at high efficiencies upon solar illumination, even when the temperature of the bulk fluid volume remains far below its boiling point. Here we show that this phenomenon is due to a collective effect mediated by multiple light scattering from the dispersed nanoparticles. Randomly positioned nanoparticles that both scatter and absorb light are able to concentrate light energy into mesoscale volumes near the illuminated surface of the liquid. The resulting light absorption creates intense localized heating and efficient vaporization of the surrounding liquid. Light trapping-induced localized heating provides the mechanism for low-temperature light-induced steam generation and is consistent with classical heat transfer.

Near-field mediated plexcitonic coupling and giant Rabi splitting in individual metallic dimers.

Strong coupling between resonantly matched localized surface plasmons and molecular excitons results in the formation of new hybridized energy states called plexcitons. Understanding the nature and tunability of these hybrid nanostructures is important for both fundamental studies and the development of new applications. We investigate the interactions between J-aggregate excitons and single plasmonic dimers and report for the first time a unique strong coupling regime in individual plexcitonic nanostructures. Dark-field scattering measurements and finite-difference time-domain simulations of the hybrid nanostructures show strong plexcitonic coupling mediated by the near-field inside each dimer gap, which can be actively controlled by rotating the polarization of the optical excitation. The plexciton dispersion curves, obtained from coupled harmonic oscillator models, show anticrossing behavior at the exciton transition energy and giant Rabi splitting ranging between 230 and 400 meV. These energies are, to the best of our knowledge, the largest obtained on individual hybrid nanostructures.

Embedding plasmonic nanostructure diodes enhances hot electron emission.

When plasmonic nanostructures serve as the metallic counterpart of a metal-semiconductor Schottky interface, hot electrons due to plasmon decay are emitted across the Schottky barrier, generating measurable photocurrents in the semiconductor. When the plasmonic nanostructure is atop the semiconductor, only a small percentage of hot electrons are excited with a wavevector permitting transport across the Schottky barrier. Here we show that embedding plasmonic structures into the semiconductor substantially increases hot electron emission. Responsivities increase by 25× over planar diodes for embedding depths as small as 5 nm. The vertical Schottky barriers created by this geometry make the plasmon-induced hot electron process the dominant contributor to photocurrent in plasmonic nanostructure-diode-based devices.

Three-dimensional plasmonic nanoclusters.

Assembling nanoparticles into well-defined structures is an important way to create and tailor the optical properties of materials. Most advances in metamaterials research to date have been based on structures fabricated in two-dimensional planar geometries. Here, we show an efficient method for assembling noble metal nanoparticles into stable, three-dimensional (3-D) clusters, whose optical properties can be highly sensitive or remarkably independent of cluster orientation, depending on particle number and cluster geometry. Some of the clusters, such as tetrahedra and icosahedra, could serve as the optical kernels for metafluids, imparting metamaterial optical properties into disordered media such as liquids, glasses, or plastics, free from the requirement of nanostructure orientation.

Rapid Data-Efficient Optimization of Perovskite Nanocrystal Syntheses through Machine Learning Algorithm Fusion.

With the demand for renewable energy and efficient devices rapidly increasing, a need arises to find and optimize novel (nano)materials. With sheer limitless possibilities for material combinations and synthetic procedures, obtaining novel, highly functional materials has been a tedious trial and error process. Recently, machine learning has emerged as a powerful tool to help optimize syntheses; however, most approaches require a substantial amount of input data, limiting their pertinence. Here, three well-known machine-learning models are merged with Bayesian optimization into one to optimize the synthesis of CsPbBr3 nanoplatelets with limited data demand. The algorithm can accurately predict the photoluminescence emission maxima of nanoplatelet dispersions using only the three precursor ratios as input parameters. This allows us to fabricate previously unobtainable seven and eight monolayer-thick nanoplatelets. Moreover, the algorithm dramatically improves the homogeneity of 2-6-monolayer-thick nanoplatelet dispersions, as evidenced by narrower and more symmetric photoluminescence spectra. Decisively, only 200 total syntheses are required to achieve this vast improvement, highlighting how rapidly material properties can be optimized. The algorithm is highly versatile and can incorporate additional synthetic parameters. Accordingly, it is readily applicable to other less-explored nanocrystal syntheses and can help rapidly identify and improve exciting compositions' quality.

Optical force stamping lithography.

Here we introduce a new paradigm of far-field optical lithography, optical force stamping lithography. The approach employs optical forces exerted by a spatially modulated light field on colloidal nanoparticles to rapidly stamp large arbitrary patterns comprised of single nanoparticles onto a substrate with a single-nanoparticle positioning accuracy well beyond the diffraction limit. Because the process is all-optical, the stamping pattern can be changed almost instantly and there is no constraint on the type of nanoparticle or substrates used.

Dark and Bright Excitons in Halide Perovskite Nanoplatelets.

Semiconductor nanoplatelets (NPLs), with their large exciton binding energy, narrow photoluminescence (PL), and absence of dielectric screening for photons emitted normal to the NPL surface, could be expected to become the fastest luminophores amongst all colloidal nanostructures. However, super-fast emission is suppressed by a dark (optically passive) exciton ground state, substantially split from a higher-lying bright (optically active) state. Here, the exciton fine structure in 2-8 monolayer (ML) thick Csn - 1 Pbn Br3n + 1 NPLs is revealed by merging temperature-resolved PL spectra and time-resolved PL decay with an effective mass model taking quantum confinement and dielectric confinement anisotropy into account. This approach exposes a thickness-dependent bright-dark exciton splitting reaching 32.3 meV for the 2 ML NPLs. The model also reveals a 5-16 meV splitting of the bright exciton states with transition dipoles polarized parallel and perpendicular to the NPL surfaces, the order of which is reversed for the thinnest NPLs, as confirmed by TR-PL measurements. Accordingly, the individual bright states must be taken into account, while the dark exciton state strongly affects the optical properties of the thinnest NPLs even at room temperature. Significantly, the derived model can be generalized for any isotropically or anisotropically confined nanostructure.