Long-Lived Hot Carriers in Formamidinium Lead Iodide Nanocrystals

The efficient harvesting of hot carrier energy in semiconductors is typically inhibited by their ultrafast thermalization process. Recently, highly promising experiments reported on the slowdown of the intraband relaxation in hybrid metal halide perovskites. In this work, we report on the presence of long-lived hot carriers in weakly confined colloidal nanocrystals (NCs) of formamidinium lead iodide perovskite (FAPbI₃). The effect is apparent from the excitation-dependent lengthening of the rise time and broadening of the high-energy tail of the transient absorption bleaching signal, yielding a retardation of the carrier relaxation by 2 orders of magnitude compared to typical time scales in colloidal semiconductor NCs. Three distinct cooling stages are observed, occurring at sub-picosecond, ∼5 ps, and ∼40 ps time scales, which we attribute to scattering from LO-phonons, contribution from a hot phonon bottleneck effect and Auger heating, respectively. Thermalization appears also influenced by the FAPbI₃ NCs purity, with trapping at unreacted precursor impurities further reducing the carrier energy loss rate

A General Synthesis Strategy for Monodisperse Metallic and Metalloid Nanoparticles (In, Ga, Bi, Sb, Zn, Cu, Sn, and Their Alloys) via in Situ Formed Metal Long-Chain Amides

We report a facile one-pot synthesis of highly monodisperse nanoparticles (5–30 nm in diameter, 5–10% in standard size distribution) of various metals and metalloids such as In, Sn, Bi, Sb, Ga, Cu, Zn, and their alloys (Cu₆Sn₅, Cu₂Sb, Bi_<i>x</i>Sb_1–<i>x</i>, etc.) using inexpensive commercial precursors. Several of these metals and alloys had not been previously obtained in the form of uniform nanoparticles. The proposed reaction mechanism has been elucidated with multinuclear (¹H, ⁷Li, ¹¹⁹Sn) NMR spectroscopy combined with DFT and molecular dynamics simulations. Metal chloride is reacted with long-chain primary or secondary amine such as oleylamine and dioctylamine in the presence of a strong Brønsted base that deprotonates the amine and thus promotes the formation of metal long-chain amide. The in situ formed amide is then reduced or thermally decomposed into corresponding metal nanoparticles. This simple methodology eliminates elaborate preparation, storage, and handling of highly reactive, moisture and oxygen sensitive molecular precursors of these metals, while providing a compelling quality of nanomaterials

Low-Cost Synthesis of Highly Luminescent Colloidal Lead Halide Perovskite Nanocrystals by Wet Ball Milling

Lead halide perovskites of APbX₃ type [A = Cs, formamidinium (FA), methylammonium; X = Br, I] in the form of ligand-capped colloidal nanocrystals (NCs) are widely studied as versatile photonic sources. FAPbBr₃ and CsPbBr₃ NCs have become promising as spectrally narrow green primary emitters in backlighting of liquid-crystal displays (peak at 520–530 nm, full width at half-maximum of 22–30 nm). Herein, we report that wet ball milling of bulk APbBr₃ (A = Cs, FA) mixed with solvents and capping ligands yields green luminescent colloidal NCs with a high overall reaction yield and optoelectronic quality on par with that of NCs of the same composition obtained by hot-injection method. We emphasize the superiority of oleylammonium bromide as a capping ligand used for this procedure over the standard oleic acid and oleylamine. We also show a mechanically induced anion-exchange reaction for the formation of orange-emissive CsPb­(Br/I)₃ NCs

Fast Anion-Exchange in Highly Luminescent Nanocrystals of Cesium Lead Halide Perovskites (CsPbX₃, X = Cl, Br, I)

Postsynthetic chemical transformations of colloidal nanocrystals, such as ion-exchange reactions, provide an avenue to compositional fine-tuning or to otherwise inaccessible materials and morphologies. While cation-exchange is facile and commonplace, anion-exchange reactions have not received substantial deployment. Here we report fast, low-temperature, deliberately partial, or complete anion-exchange in highly luminescent semiconductor nanocrystals of cesium lead halide perovskites (CsPbX₃, X = Cl, Br, I). By adjusting the halide ratios in the colloidal nanocrystal solution, the bright photoluminescence can be tuned over the entire visible spectral region (410–700 nm) while maintaining high quantum yields of 20–80% and narrow emission line widths of 10–40 nm (from blue to red). Furthermore, fast internanocrystal anion-exchange is demonstrated, leading to uniform CsPb­(Cl/Br)₃ or CsPb­(Br/I)₃ compositions simply by mixing CsPbCl₃, CsPbBr₃, and CsPbI₃ nanocrystals in appropriate ratios

Synthesis of Cesium Lead Halide Perovskite Nanocrystals in a Droplet-Based Microfluidic Platform: Fast Parametric Space Mapping

Prior to this work, fully inorganic nanocrystals of cesium lead halide perovskite (CsPbX₃, X = Br, I, Cl and Cl/Br and Br/I mixed halide systems), exhibiting bright and tunable photoluminescence, have been synthesized using conventional batch (flask-based) reactions. Unfortunately, our understanding of the parameters governing the formation of these nanocrystals is still very limited due to extremely fast reaction kinetics and multiple variables involved in ion-metathesis-based synthesis of such multinary halide systems. Herein, we report the use of a droplet-based microfluidic platform for the synthesis of CsPbX₃ nanocrystals. The combination of online photoluminescence and absorption measurements and the fast mixing of reagents within such a platform allows the rigorous and rapid mapping of the reaction parameters, including molar ratios of Cs, Pb, and halide precursors, reaction temperatures, and reaction times. This translates into enormous savings in reagent usage and screening times when compared to analogous batch synthetic approaches. The early-stage insight into the mechanism of nucleation of metal halide nanocrystals suggests similarities with multinary metal chalcogenide systems, albeit with much faster reaction kinetics in the case of halides. Furthermore, we show that microfluidics-optimized synthesis parameters are also directly transferrable to the conventional flask-based reaction

Single Cesium Lead Halide Perovskite Nanocrystals at Low Temperature: Fast Single-Photon Emission, Reduced Blinking, and Exciton Fine Structure

Metal-halide semiconductors with perovskite crystal structure are attractive due to their facile solution processability, and have recently been harnessed very successfully for high-efficiency photovoltaics and bright light sources. Here, we show that at low temperature single colloidal cesium lead halide (CsPbX₃, where X = Cl/Br) nanocrystals exhibit stable, narrow-band emission with suppressed blinking and small spectral diffusion. Photon antibunching demonstrates unambiguously nonclassical single-photon emission with radiative decay on the order of 250 ps, representing a significant acceleration compared to other common quantum emitters. High-resolution spectroscopy provides insight into the complex nature of the emission process such as the fine structure and charged exciton dynamics

Microfluidic Reactors Provide Preparative and Mechanistic Insights into the Synthesis of Formamidinium Lead Halide Perovskite Nanocrystals

Formamidinium lead bromide and iodide (FAPbX₃, X = Br, I) in the form of colloidal nanocrystals (NCs) exhibit outstanding photoluminescence properties in the green and infrared regions of the electromagnetic spectrum, characterized by narrow emission line widths (below 90 meV) and high quantum yields (above 90%). The controlled formation of Br-I mixed halide NCs is a facile strategy for tuning band-gap energies, in particular between 700 and 800 nm, not accessible with CsPbX3 NCs. Herein, we report a mechanistic and high-throughput parametric screening study of the synthesis of such NCs using droplet-based microfluidic platforms, equipped with in situ optical characterization. We establish the growth conditions that fully suppress the formation of nanoplatelet impurities in the final colloid and demonstrate that the formation mechanism of FAPbBr₃ NCs proceeds via the formation of nanoplatelets as transient species, whereas FAPbI₃ forms directly as cubic-shaped NCs. In contrast to CsPb­(Br/I)₃ NCs, the stability of FAPb­(Br/I)₃ NCs increases with iodine content. Such NCs form by first nucleating pure FAPbI₃ NCs, followed by incorporation of bromide ions

Monodisperse and Inorganically Capped Sn and Sn/SnO₂ Nanocrystals for High-Performance Li-Ion Battery Anodes

We report a facile synthesis of highly monodisperse colloidal Sn and Sn/SnO₂ nanocrystals with mean sizes tunable over the range 9–23 nm and size distributions below 10%. For testing the utility of Sn/SnO₂ nanocrystals as an active anode material in Li-ion batteries, a simple ligand-exchange procedure using inorganic capping ligands was applied to facilitate electronic connectivity within the components of the nanocrystalline electrode. Electrochemical measurements demonstrated that 10 nm Sn/SnO₂ nanocrystals enable high Li insertion/removal cycling stability, in striking contrast to commercial 100–150 nm powders of Sn and SnO₂. In particular, reversible Li-storage capacities above 700 mA h g^–1 were obtained after 100 cycles of deep charging (0.005–2 V) at a relatively high current of 1000 mA h g^–1

Harnessing Defect-Tolerance at the Nanoscale: Highly Luminescent Lead Halide Perovskite Nanocrystals in Mesoporous Silica Matrixes

Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as a novel class of bright emitters with pure colors spanning the entire visible spectral range. Contrary to conventional quantum dots, such as CdSe and InP NCs, perovskite NCs feature unusual, defect-tolerant photophysics. Specifically, surface dangling bonds and intrinsic point defects such as vacancies do not form midgap states, known to trap carriers and thereby quench photoluminescence (PL). Accordingly, perovskite NCs need not be electronically surface-passivated (with, for instance, ligands and wider-gap materials) and do not noticeably suffer from photo-oxidation. Novel opportunities for their preparation therefore can be envisaged. Herein, we show that the infiltration of perovskite precursor solutions into the pores of mesoporous silica, followed by drying, leads to the template-assisted formation of perovskite NCs. The most striking outcome of this simple methodology is very bright PL with quantum efficiencies exceeding 50%. This facile strategy can be applied to a large variety of perovskite compounds, hybrid and fully inorganic, with the general formula APbX₃, where A is cesium (Cs), methylammonium (MA), or formamidinium (FA), and X is Cl, Br, I or a mixture thereof. The luminescent properties of the resulting templated NCs can be tuned by both quantum size effects as well as composition. Also exhibiting intrinsic haze due to scattering within the composite, such materials may find applications as replacements for conventional phosphors in liquid-crystal television display technologies and in related luminescence down-conversion-based devices

Facile Droplet-based Microfluidic Synthesis of Monodisperse IV–VI Semiconductor Nanocrystals with Coupled In-Line NIR Fluorescence Detection

We describe the realization of a droplet-based microfluidic platform for the controlled and reproducible synthesis of lead chalcogenide (PbS, PbSe) nanocrystal quantum dots (QDs). Monodisperse nanocrystals were synthesized over a wide range of experimental conditions, with real-time assessment and fine-tuning of material properties being achieved using NIR fluorescence spectroscopy. Importantly, we show for the first time that real-time monitoring of the synthetic process allows for rapid optimization of reaction conditions and the synthesis of high quality PbS nanocrystals, emitting in the range of 765–1600 nm, without any post-synthetic processing. The segmented-flow capillary reactor exhibits stable droplet generation and reproducible synthesis of PbS nanocrystals with high photoluminescence quantum yields (28%) over extended periods of time (3–6 h). Furthermore, the produced NIR-emitting nanoparticles were successfully used in the fabrication of Schottky solar cells, exhibiting a power conversion efficiency of 3.4% under simulated AM 1.5 illumination. Finally, the droplet-based microfluidic platform was used to synthesize PbSe nanocrystals having photoluminescence peaks in the range of 860–1600 nm, showing the exceptional control and stability of the reactor