A key display characteristic is its efficiency (emitted light power divided by input power). Although display efficiencies are being improved through emissive (for example, quantum dot and organic light-emitting) display designs, which remove the highly inefficient colour filters found in traditional liquid crystal displays, polarization filters, which block about 50% light, remain necessary to inhibit ambient light reflection. We introduce a luminescent concentrator design to replace both colour and polarization filters. Narrow-band, large-Stokes-shift, CdSe/CdS quantum dot emitters are embedded in a luminescent concentrator pixel element with a small top aperture. The remainder of the top surface is coated black, reducing ambient light reflection. A single pixel demonstrates an extraction efficiency of 40.9% from a pixel with an aperture opening of 11.0%. A simple proof-of-concept multipixel array is demonstrated. Inefficient filters and overall efficiency are issues for display technology. Luminescent concentrator pixels have been used with CdSe/CdS quantum dot emitters, which enable both colour and polarization filtering, as well as nearly 41% extraction efficiency.

A key display characteristic is its efficiency (emitted light power divided by input power). While display efficiencies are being improved through emissive (e.g., quantum dot and organic light emitting display (OLED) designs1,2, which remove the highly inefficient color filters found in traditional liquid crystal displays (LCDs)3,4, polarization filters, which block about 50% of the light, remain required to inhibit ambient light reflection. We introduce a luminescent cavity design to replace both the color and polarization filters. Narrow-band, large Stokes shift, CdSe/CdS quantum dot emitters are embedded in a reflective cavity pixel element with a small top aperture. The remainder of the top surface is coated black reducing ambient light reflection. A single pixel demonstrates an extraction efficiency of 40.9% from a cavity with an 11% aperture opening. A simple proof-of-concept multi-pixel array is demonstrated.

Increasing the quantum yields of InP quantum dots is important for their applications, particularly for use in consumer displays. While several methods exist to improve quantum yield, the addition of inorganic metal halide salts has proven promising. To further investigate this phenomenon, InP quantum dots dispersed in tetrahydrofuran were titrated with ZnCl2, ZnBr2, and InCl3. The optical properties were observed, and the reactions were studied by using quantitative 1H NMR and thermodynamic measurements from isothermal titration calorimetry. These measurements contradict the previously hypothesized reaction mechanism in which metal halide salts, acting as Z-type ligands, passivate undercoordinated anions on the surface of the quantum dots. This work provides evidence for a newly proposed mechanism wherein the metal halide salts undergo a ligand exchange with indium myristate. Thermodynamic measurements prove key to supporting this new mechanism, particularly in describing the organic ligand interactions on the surface. An Ising model was used to simulate the quantum dot surface and was fit by using thermodynamic and 1H NMR data. Together, these data and the proposed exchange mechanism provide greater insight into the surface chemistry of quantum dots.

Lead toxicity has sparked interest into alternative halide nanomaterials with properties similar to CsPbX3 perovskites. A promising alternative suggested from bulk studies is the family of double perovskites of the form Cs2AgMX6. Here, we report the synthesis of colloidal Cs2AgInCl6 and Cs2AgSbCl6 nanocrystals via injection of acyl halides into a metal acetate solution under atmospheric conditions and relatively mild temperatures. We demonstrate the synthesis of single-crystalline cubic nanocrystals of ca. 10 nm side length and their morphological similarities to other double perovskite nanostructures in terms of their [200] facet termination and decoration with Ag smaller nanocrystallites. To compare the stabilities of the synthesized materials, we develop a titration assay based on the degradation of nanocrystals with amines as a proxy for degradation by humidity, which provides a quantifiable stability metric. This measurement shows that Cs2AgSbCl6 releases more than twice the decomposition energy compared to Cs2AgInCl6 or CsPbCl3 and degrades in the presence of approximately one molar equivalent of amine, whereas the other two materials require more than a 100-fold excess. Using facile chemical titration to quantitatively determine chemical stability provides an additional tool to aid in the basic understanding of what makes some of these materials more environmentally stable than others. ■ INTRODUCTION Metal halide nanocrystals are a class of material for which it is possible to create and explore a wide variety of compositions. The prototypical lead system is well studied because of the attractive optoelectronic properties, which include high photoluminescence (PL) quantum yield and high tolerance to defects. Recent synthetic advances have demonstrated the ability to precisely control the size, shape, cation and anion compositions of the nanocrystals to fine tune the respective band gaps and optical properties. Of particular interest from the point of view of solid-state chemistry and nanochemistry is isoelectronic nanocrystal systems in which the doubly charged Pb ions are replaced by alternating singly and triply charged cations, a crystal structure referred to as double perovskite or elpasolite. Exploration of these systems, and their relative stabilities, offers rich opportunities for learning more about the chemical and physical principles that influence material properties and stability in metal halides. The investigation of these questions is all of the more timely, since the practical application of lead halide perovskite nanocrystals is severely limited by their long-term instability and regulations restricting the use of lead in devices. Identifying environmentally stable, synthetically accessible metal halide nanocrystals that avoid the use of lead while retaining the physical properties of lead halide perovskite archetypes might allow the realization of the many proof-ofconcept solar and device applications in development. Several groups of related materials that substitute lead with other metals have been explored in the recent literature. In principle, 3rdor 4th-row p-block metal halides should have similar electronic structures, due to a potential defect tolerant band structure arising from the Fermi level lying between two antibonding orbitals (mostly composed of metal ns and np) and strong spin-orbit coupling. Tin halide and germanium halide perovskites are the closest structural and electronic analogs to lead halide perovskites and are predicted to Received: October 3, 2018 Revised: April 12, 2019 Published: April 24, 2019 Article pubs.acs.org/cm Cite This: Chem. Mater. 2019, 31, 3134−3143 © 2019 American Chemical Society 3134 DOI: 10.1021/acs.chemmater.8b04202 Chem. Mater. 2019, 31, 3134−3143 D ow nl oa de d by U N IV O F C A L IF O R N IA B E R K E L E Y a t 1 5: 35 :0 9: 12 3 on J un e 28 , 2 01 9 fr om h ttp s: //p ub s. ac s. or g/ do i/1 0. 10 21 /a cs .c he m m at er .8 b0 42 02 . demonstrate efficient absorption and emission as well as defect tolerance; Unfortunately, these materials are even less stable to light, heat, and moisture than lead halide perovskites. Cs3M2X9 (M = Sb, Bi) and Cs2MX6 27 (M = Sn, Pb, Te) structures are ternary metal halide structures that can accommodate p-block metals, and several interesting examples, including Cs2SnI6, 28−30 Cs3Bi2I9, 31,32 and Cs3Sb2I9, 32,33 have been synthesized successfully both in bulk and at the nanoscale. The lower level of structural connectivity of the metal halide octahedra in these structure types reduces conductivity and exciton radii, which may limit their utility in electrical devices. Cs2AgMX6 (M = Bi, Sb, In) is a promising structure for lead-free halide perovskites since the metal halide octahedra are connected across all three dimensions. This 3D connectivity may explain why photovoltaic devices made from these materials have exhibited some of the highest power conversion efficiencies reported for lead-free halide materials despite the indirect nature of their band gaps. Recently, several groups have reported syntheses of Cs2AgBiX6 (X = Br, Cl) nanocrystals. 14,15,34,35 There are discrepancies in reports of optical spectra between nanocrystal and bulk measurements such as the peak at 430 nm for Cs2AgBiBr6 which could lead to the interpretation that there is significant quantum confinement in these systems. However, we proposed that these discrepancies arise from differences in measurement technique (transmission vs reflection) and concentration of the sample instead of from the underlying optical response. This hypothesis received further support from thin film transmission measurements of single-crystalline Cs2AgBiBr6, 36 which shows very similar optical signals to the nanocrystal solutions. In this work, we further elaborate on how these discrepancies may arise and how to eliminate them to ensure comparability between optical absorption measurements from the nanocrystal and solid-state chemistry fields. Besides Cs2AgBiX6, two other silver-containing double perovskite crystal structures have recently been reported in bulk: Cs2AgSbCl6 and Cs2AgInCl6. 37−39 Although neither of these materials is well suited for photovoltaic applications due to their large band gaps, they may have applications in optoelectronic technologies such as the low-noise UV photodetectors recently demonstrated for Cs2AgInCl6. 40 Here, we report the successful synthesis of Cs2AgSbCl6 and Cs2AgInCl6 nanocrystals and investigate their structure, absorption, and emission properties. In contrast to a recent report on Cs2AgInCl6 nanocrystals, 41 we demonstrate that the optical properties of these nanomaterials are mostly unchanged from bulk. Concerning the understanding of stability, most work has been predicated on the idea that the Goldschmidt tolerance factor can be used as a first guide to predict not only whether a material will form but also what the stability will be. In recent publications, both thermodynamic measurements and more extensive theoretical calculations or literature comparisons support the idea that the intrinsic stability of these halide systems does indeed correlate well with the Goldschmidt tolerance factor. However, this does not take into account the relative propensity to decompose according to specific pathways. When a perovskite decomposition product consists of metal−ligand complexes or other molecular species with varied bonding arrangements, the stability of those products can also vary widely across a series of metals, and it is the net reaction thermodynamics that is most relevant. The instability of many members of this class of compounds in the presence of water is of particular practical concern. Even slight exposure to humidity is sufficient to decompose materials such as CsPbI3. 46 Many reports include discussions of stability by documenting X-ray diffraction (XRD) data after a period of storage or immersion of the material in water, ethanol, or another solvent. In this work, we develop an assay based on an amine degradation reaction observed previously in CsPbBr3 nanocrystals 49 to quantitatively evaluate the stability of Cs2AgInCl6 and Cs2AgSbCl6 nanocrystals relative to Cs2AgBiCl6 and CsPbCl3. We demonstrate that Cs2AgSbCl6 decomposes with a 1000-fold lower amine concentration than Cs2AgInCl6 and extract both equilibrium constants and free reaction energies from an equilibrium model of the data. ■ EXPERIMENTAL METHODS Materials. Acetone (99.9%, Fischer Scientific), antimony(III) acetate (99.99%, Aldrich), benzoyl chloride (99%, VWR), bismuth(III) acetate (99.99%, Aldrich), cesium Acetate (99.9% Aldrich), cesium standard for ICP (1000 ppm in 2% aqueous nitric acid, Aldrich, TraceCert Lot: BCBK9448V), 2% aqueous nitric acid (Aldrich, 99.999%), 2-ethylhexanoyl chloride (98%, Aldrich), hexanes (mixture of isomers 99.99%, Fischer Scientific), Indium(III) acetate (99.99%, Aldrich), lead(II) acetate trihydrate (99.99%, Aldrich), octylamine (99%, Aldrich), oleic acid (90%, Aldrich), oleylamine (70%, Aldrich), silver acetate (99.99%, Aldrich), and m-xylene (99.9%, Fisher Scientific). All chemicals were used as purchased without further purification. Synthesis of Cs2AgInCl6 Nanocrystals. In a typical synthesis, 5 mg (0.025 mmol) of cesium acetate, 8 mg (0.05 mmol) of silver acetate, and 16 mg (0.05 mmol) of indium(III) acetate were placed into a 4 mL glass vial with a stir bar. Xylene (1 mL), oleic acid (0.25 mL, 0.8 mmol), and oleylamine (0.075 mL, 0.2 mmol) were added, and the vial heated to 100 °C for 10 min in an oil bath. Benzoyl chloride (0.040 mL, 0.3 mmol) was injected quickly. Then, the vials were taken out of the oil bath and left to cool to room temperature. The cooling rate and precise time of removal after injection did not appear to have any major effects on the result of this synthesis. For routine measurements, the nanocrystal reaction mixture was precipitated by centrifugation at 14,000 rpm for 5 min

A variety of optical applications rely on the absorption and reemission of light. The quantum yield of this process often plays an essential role. When the quantum yield deviates from unity by significantly less than 1%, applications such as luminescent concentrators and optical refrigerators become possible. To evaluate such high performance, we develop a measurement technique for luminescence efficiency with sufficient accuracy below one part per thousand. Photothermal threshold quantum yield is based on the quantization of light to minimize overall measurement uncertainty. This technique is used to guide a procedure capable of making ensembles of near-unity emitting cadmium selenide/cadmium sulfide (CdSe/CdS) core-shell quantum dots. We obtain a photothermal threshold quantum yield luminescence efficiency of 99.6 ± 0.2%, indicating nearly complete suppression of nonradiative decay channels.

Lead toxicity has sparked interest into alternative halide nanomaterials with properties similar to CsPbX3 perovskites. A promising alternative suggested from bulk studies is the family of double perovskites of the form Cs2AgMX6. Here, we report the synthesis of colloidal Cs2AgInCl6 and Cs2AgSbCl6 nanocrystals via injection of acyl halides into a metal acetate solution under atmospheric conditions and relatively mild temperatures. We demonstrate the synthesis of single-crystalline cubic nanocrystals of ca. 10 nm side length and their morphological similarities to other double perovskite nanostructures in terms of their [200] facet termination and decoration with Ag-(0) smaller nanocrystallites. To compare the stabilities of the synthesized materials, we develop a titration assay based on the degradation of nanocrystals with amines as a proxy for degradation by humidity, which provides a quantifiable stability metric. This measurement shows that Cs2AgSbCl6 releases more than twice the decomposition energy compared to Cs2AgInCl6 or CsPbCl3 and degrades in the presence of approximately one molar equivalent of amine, whereas the other two materials require more than a 100-fold excess. Using facile chemical titration to quantitatively determine chemical stability provides an additional tool to aid in the basic understanding of what makes some of these materials more environmentally stable than others.

Replacing lead in halide perovskites is of great interest due to concerns about stability and toxicity. Recently, lead free double perovskites in which the unit cell is doubled and two divalent lead cations are substituted by a combination of mono- and trivalent cations have been synthesized as bulk single crystals and as thin films. Here, we study stability and optical properties of all-inorganic cesium silver(I) bismuth(III) chloride and bromide nanocrystals with the double perovskite crystal structure. The cube-shaped nanocrystals are monodisperse in size with typical side lengths of 8 to 15 nm. The absorption spectrum of the nanocrystals presents a sharp peak, which we assign to a direct bismuth s-p transition and not to a quantum confined excitonic transition. Using this spectroscopic handle combined with high-resolution transmission electron microscopy (TEM) based elemental analysis, we conduct stoichiometric studies at the single nanocrystal level as well as decomposition assays in solution and observe that Ag+ diffusion and coalescence is one of the pathways by which this material degrades. Drying the nanocrystals leads to self-assembly into ordered nanocrystal solids, and these exhibit less degradation than nanocrystals in solution. Our results demonstrate that Cs2AgBiX6 (X = Cl, Br) nanocrystals are a useful model system to study structure-function relationships in the search for stable nontoxic halide perovskites.

: Semiconductor quantum dot sensitized lanthanide ions hold great promise in producing a broadly absorbing and sharply emitting luminophore, but their synthesis has proven to be difficult. We report the first synthesis of core/shell/shell InP/Ln x Y 1-x F 3 /ShF 3 (Ln = Yb, Nd, Sh = Lu, Y) nanocrystals that exhibit a broad visible absorption coupled to a sharp near-infrared emission. Additionally, this is the first report of Nd being coupled to a quantum dot absorber. We characterize the system with a variety of electron microscopy and x-ray techniques that prove this unique structure. Optical measurements confirm the correlation of the Ln 3+ emission to the quantum dot ab-sorption, while the presence of a trap state emission gives a clue as to the mechanism of energy transfer between the dot and the lanthanide.

Semiconductor quantum dot (QD)-sensitized lanthanide ions hold great promise in producing a broadly absorbing and sharply emitting luminophore, but their synthesis has proven to be difficult. We report the first synthesis of core/shell/shell InP/Ln xY1- xF3/ShF3 (Ln = Yb, Nd; Sh = Lu, Y) nanocrystals that exhibit a broad visible absorption coupled to a sharp near-infrared emission. Additionally, this is the first report of Nd being coupled to a QD absorber. We characterize the system with a variety of electron microscopy and X-ray techniques that prove this unique structure. Optical measurements confirm the correlation of the Ln3+ emission to the QD absorption, while the presence of a trap-state emission gives a clue as to the mechanism of energy transfer between the dot and the lanthanide.

Lead halide perovskite nanocrystals (NCs) have emerged as attractive nanomaterials owing to their excellent optical and optoelectronic properties. Their intrinsic instability and soft nature enable a post-synthetic controlled chemical transformation. We studied a ligand mediated transformation of presynthesized CsPbBr3 NCs to a new type of lead-halide depleted perovskite derivative nanocrystal, namely Cs4PbBr6. The transformation is initiated by amine addition, and the use of alkyl-thiol ligands greatly improves the size uniformity and chemical stability of the derived NCs. The thermodynamically driven transformation is governed by a two-step dissolution-recrystallization mechanism, which is monitored optically. Our results not only shed light on a decomposition pathway of CsPbBr3 NCs but also present a method to synthesize uniform colloidal Cs4PbBr6 NCs, which may actually be a common product of perovskite NCs degradation.

Precise morphology and composition control is vital for designing multifunctional lanthanide-doped core/shell nanocrystals. Herein, we report controlled isotropic and anisotropic shell growth techniques in hexagonal sodium rare-earth tetrafluoride (β-NaLnF4) nanocrystals by exploiting the kinetics of the shell growth. A drastic change of the shell morphology was observed by changing the injection rate of the shell precursors while keeping all other reaction conditions constant. We obtained isotropic shell growth for fast sequential injection and a preferred growth of the shell layers along the crystal's c-axis [001] for slow dropwise injection. Using this slow shell growth technique, we have grown rod-like shells around different almost spherical core nanocrystals. Bright and efficient upconversion was measured for both isotropic and rod-like shells around β-NaYF4 nanocrystals doped with Yb3+/Er3+ and Yb3+/Tm3+. Photoluminescence upconversion quantum yield and lifetime measurements reveal the high quality of the core/shell nanocrystal. Furthermore, multishell rod-like nanostructures have been prepared with optically active cores and tips separated by an inert intermediate shell layer. The controlled anisotropic shell growth allows the design of new core/multishell nanostructures and enables independent investigations of the chemistry and physics of different nanocrystal facets.

The reabsorption of photoluminescence within a medium, an effect known as the inner filter effect (IFE), has been well studied in solutions, but has garnered less attention in regards to solid-state nanocomposites. Photoluminescence from a quantum dot (QD) can selectively excite larger QDs around it resulting in a net red-shift in the reemitted photon. In CdSe/CdS core/shell QD-polymer nanocomposites, we observe a large spectral red-shift of over a third of the line width of the photoluminescence of the nanocomposites over a distance of 100 μm resulting from the IFE. Unlike fluorescent dyes, which do not show a large IFE red-shift, QDs have a component of inhomogeneous broadening that originates from their size distribution and quantum confinement. By controlling the photoluminescence broadening as well as the sample dispersion and concentration, we show that the magnitude of the IFE within the nanocomposite can be tuned. We further demonstrate that this shift can be exploited in order to spectroscopically monitor the vertical displacement of a nanocomposite in a fluorescence microscope. Large energetic shifts in the measured emission with displacement can be maximized, resulting in a displacement sensor with submicrometer resolution. We further show that the composite can be easily attached to biological samples and is able to measure deformations with high temporal and spatial precision.

We demonstrate postsynthetic modification of CsPbBr3 nanocrystals by a thiocyanate salt treatment. This treatment improves the quantum yield of both freshly synthesized (PLQY ≈ 90%) and aged nanocrystals (PLQY ≈ 70%) to within measurement error (2-3%) of unity, while simultaneously maintaining the shape, size, and colloidal stability. Additionally, the luminescence decay kinetics transform from multiexponential decays typical of nanocrystalline semiconductors with a distribution of trap sites, to a monoexponential decay, typical of single energy level emitters. Thiocyanate only needs to access a limited number of CsPbBr3 nanocrystal surface sites, likely representing under-coordinated lead atoms on the surface, in order to have this effect.

Lanthanide-doped nanocrystals are of particular interest for the research community not only due to their ability to shape light by downshifting, quantum cutting, and upconversion but also because novel optical properties can be found by the precise engineering of core-shell nanocrystals. Because of the large surface area-to-volume ratio of nanocrystals, the luminescence is typically suppressed by surface quenching. Here, we demonstrate a mechanism that exploits surface quenching processes to improve the luminescence of our core-shell lanthanide-doped nanocrystals. By carefully tuning the shell thickness of inert β-NaLuF4 around β-NaYF4 nanocrystals doped with Yb3+ and Er3+, we unravel the relationship between quantum yield and shell thickness, and quantify surface quenching rates for the relevant Er3+ and Yb3+ energy levels. This enhanced understanding of the system's dynamics allowed us to design nanocrystals with a surface quenching-assisted mechanism for bright NIR to NIR downshifting with a distinctive efficiency peak for an optimized shell thickness.

AbstractCa‐mediated treatment of Bu3SnH with TmsCF3 allows an efficient access to the title stannane (III).