Doping and ion substitution in colloidal metal halide perovskite nanocrystals.

The past decade has witnessed tremendous advances in synthesis of metal halide perovskites and their use for a rich variety of optoelectronics applications. Metal halide perovskite has the general formula ABX3, where A is a monovalent cation (which can be either organic (e.g., CH3NH3+ (MA), CH(NH2)2+ (FA)) or inorganic (e.g., Cs+)), B is a divalent metal cation (usually Pb2+), and X is a halogen anion (Cl-, Br-, I-). Particularly, the photoluminescence (PL) properties of metal halide perovskites have garnered much attention due to the recent rapid development of perovskite nanocrystals. The introduction of capping ligands enables the synthesis of colloidal perovskite nanocrystals which offer new insight into dimension-dependent physical properties compared to their bulk counterparts. It is notable that doping and ion substitution represent effective strategies for tailoring the optoelectronic properties (e.g., absorption band gap, PL emission, and quantum yield (QY)) and stabilities of perovskite nanocrystals. The doping and ion substitution processes can be performed during or after the synthesis of colloidal nanocrystals by incorporating new A', B', or X' site ions into the A, B, or X sites of ABX3 perovskites. Interestingly, both isovalent and heterovalent doping and ion substitution can be conducted on colloidal perovskite nanocrystals. In this review, the general background of perovskite nanocrystals synthesis is first introduced. The effects of A-site, B-site, and X-site ionic doping and substitution on the optoelectronic properties and stabilities of colloidal metal halide perovskite nanocrystals are then detailed. Finally, possible applications and future research directions of doped and ion-substituted colloidal perovskite nanocrystals are also discussed.

Tailoring Optical Properties of Luminescent Semiconducting Nanocrystals through Hydrostatic, Anisotropic Static, and Dynamic Pressures.

Luminescent semiconductor nanocrystals are a fascinating class of materials because of their size-dependent emissions. Numerous past studies have demonstrated that semiconductor nanoparticles with radii smaller than their Bohr radius experience quantum confinement and thus size-dependent emissions. Exerting pressure on these nanoparticles represents an additional, more dynamic, strategy to alter their size and shift their emission. The application of pressure results in the lattices becoming strained and the electronic structure altered. In this Minireview, colloidal semiconductor nanocrystals are first introduced. The effects of uniform hydrostatic pressure on the optical properties of metal halide perovskite (ABX3 ), II-VI, III-V, and IV-VI semiconductor nanocrystals are then examined. The optical properties of semiconductor nanocrystals under static and dynamic anisotropic pressure are then summarized. Finally, future research directions and applications utilizing the pressure-dependent optical properties of semiconductor nanocrystals are discussed.

Enhancement of fluorescent emission in photonic crystal film and application in photocatalysis.

Fluorescent photonic crystal films composed of monodisperse NaYF4:15Yb,0.5Tm@SiO2 (where 15 and 0.5 represent the mole percentage of reactants) core-shell spheres were successfully fabricated and applied in photocatalysis. The core-shell spheres were prepared using a modified Stober method, and fluorescent photonic crystal films were fabricated via a simple self-assembly method. The morphologies, structures and upconversion fluorescent properties of the fluorescent photonic crystal films with different photonic band gaps were characterized. Moreover, their photocatalytic capability in decomposing rhodamine B using near-infrared light was studied. Results indicate that the band edge effect plays a critical role in the enhancement of short wave emission intensity of fluorescent photonic crystal films. Specifically, in comparison to the reference sample without a band edge effect, the 363 nm emission intensity was enhanced by 5.97 times, while the percentage of UV upconversion emission was improved by 6.23%. In addition, the 451 nm emission intensity was enhanced by 5.81 times, and the percentage of visible upconversion emission was improved by 8.88%. Furthermore, fluorescent photonic crystal films with enhanced short wave emission exhibited great photocatalytic performance in the degradation of rhodamine B aqueous solutions under near-infrared light.

Design and fabrication of distributed Bragg reflector multilayers for dynamic pressure sensing.

A novel 2D-surface shock pressure sensor is designed and tested based on 1D-Photonic Crystal, i.e., Distributed Bragg Reflector Multilayer (DBR/ML) structures. The fast opto-mechanical response of these structures to changes in layer thicknesses and refractive indices are ideally suited for dynamic pressure sensing. They offer the potential to minimize acoustic impedance mismatch between the material layers, and most importantly, the potential to monitor both temporal and spatial (lateral) variations during shock compression. In this feasibility study, different materials and device designs are investigated to identify material/device design combinations with optimum response to dynamic loading. Structural and material effects are studied in terms of spectral and mechanical properties, structure stability, and the ease of fabrication process. Structures comprising of different numbers of SiO1.5/SiO1.7 bilayer stacks are modeled, and fabricated. A 10-bilayer structure placed under a dynamic compressive load of ~7.2 GPa, exhibits a blueshift of 29 nm with a response time of ~5 ns which is well within the shock pressure rise time measured with PDV velocimetry. This promising result successfully demonstrates the feasibility of the specifically designed DBR/ML structure as a dynamic pressure sensor.

Unconventional Route to Uniform Hollow Semiconducting Nanoparticles with Tailorable Dimensions, Compositions, Surface Chemistry, and Near-Infrared Absorption.

Despite impressive recent advances in the synthesis of lead chalcogenide solid nanoparticles, there are no examples of lead chalcogenide hollow nanoparticles (HNPs) with controlled diameter and shell thickness as current synthetic approaches for HNPs have inherent limitations associated with their complexity, inability to precisely control the dimensions, and limited possibilities with regard to applicable materials. Herein, we report on an unconventional strategy for crafting uniform lead chalcogenide (PbS and PbTe) HNPs with tailorable size, surface chemistry, and near-IR absorption. Amphiphilic star-like triblock copolymers [polystyrene-block-poly(acrylic acid)-block-polystyrene and polystyrene-block-poly(acrylic acid)-block-poly(3,4-ethylenedioxythiophene)] were rationally synthesized and exploited as nanoreactors for the formation of uniform PbS and PbTe HNPs. Compared to their solid counterparts, the near-IR absorption of the HNPs is blue-shifted owing to the hollow interior. This strategy can be readily extended to other types of intriguing low-band-gap HNPs for diverse applications.

Asymmetrical optical microcavity structures for dynamic pressure sensing: design, fabrication, validation.

Optical microcavity (OMC) structures have spectral properties that are directly related to their physical dimensions and material refractive indices. Their intrinsically fast optical response to mechanically-induced changes in these parameters makes OMCs uniquely suited for dynamic sensing when paired with a suitably fast streak camera and spectrograph. Various designs and processes of fabrication for asymmetrical OMC (AOMC) structures were investigated to optimize and assess their feasibility for dynamic sensing. Structural and material effects were studied in terms of spectral properties, structure stabilities and fabrication process. From this study, it was shown that an AOMC structure with a SiO2 cavity layer and Ag mirror layers, fabricated with thin adhesion Al2O3 layers exhibited the best structural stability and spectral properties. Under dynamic compressive loading of ~4 GPa, the structure exhibited a blueshift of 22 nm and a temporal response time of < 3.3 ns, thus demonstrating the potential of AOMC based dynamic pressure sensing.

Synthesis and characterization of a BaGdF5:Tb glass ceramic as a nanocomposite scintillator for x-ray imaging.

Transparent glass ceramics with embedded light-emitting nanocrystals show great potential as low-cost nanocomposite scintillators in comparison to single crystal and transparent ceramic scintillators. In this study, cubic structure BaGdF5:Tb nanocrystals embedded in an aluminosilicate glass matrix are reported for potential high performance MeV imaging applications. Scintillator samples with systematically varied compositions were prepared by a simple conventional melt-quenching method followed by annealing. Optical, structural and scintillation properties were characterized to guide the design and optimization of selected material systems, aiming at the development of a system with higher crystal volume and larger crystal size for improved luminosity. It is observed that enhanced scintillation performance was achieved by tuning the glass matrix composition and using GdF3 in the raw materials, which served as a nucleation agent. A 26% improvement in light output was observed from a BaGdF5:Tb glass ceramic with addition of GdF3.

Synthesis and luminescence properties of Tb doped LaBGeO5 and GdBGeO5 glass scintillators.

Borogermanate glasses show promise as scintillators due to their ability to incorporate high levels of heavy metal oxides without excessive loss of luminescent output intensity. Heavy metal oxide scintillating glasses of 50GeO2-25B2O3-(25-x)La2O3/Gd2O3-xTb2O3 (x=1,2,3,4) with the same stoichiometric composition of crystalline Tb doped LaBGeO5/GdBGeO5 were synthesized via the melt-quench method. Three times of higher light output under gamma ray excitation was observed from Tb doped GdBGeO5 based glass compared to LaBGeO5 glass due to efficient energy transfer between Gd-Tb pairs and higher luminescence efficiency in the GdBGeO5 glasses. The potential to form LaBGeO5/GdBGeO5 glass ceramic scintillators was also discussed and preliminarily investigated.

Composition optimization of scintillating rare-earth nanocrystals in oxide glass-ceramics for radiation spectroscopy.

Glass-ceramic nanocomposites comprising GdBr3/CeBr3 loaded sodium-aluminosilicate glasses in which scintillating crystallites are precipitated in situ from a host glass matrix were studied. This materials system shows promise as an alternative to single-crystal scintillators, with potential to be fabricated into a wide variety of sizes, shapes, and compositions. Batch compositions containing 15-18 mol. % GdBr3 and 3-4 mol. % CeBr3 were prepared and analyzed for photoluminescent light yield. Light yield peaked with rare-earth content of 15 mol. % GdBr3 and 4 mol. % CeBr3. Preliminary ceramization studies on this composition found that the precipitated phase more closely matched a Gd2O3-CeO2 mixture rather than the GdBr3(Ce) that was targeted.

Synthesis and Luminescence Properties of Transparent Nanocrystalline GdF3:Tb Glass-Ceramic Scintillator.

Transparent glass-ceramic containing rare-earth doped halide nanocrystals exhibits enhanced luminescence performance. In this study, a glass-ceramic with Tb doped gadolinium fluoride nanocrystals embedded in an aluminosilicate glass matrix is investigated for X-ray imaging applications. The nanocrystalline glass-ceramic scintillator was prepared by a melt-quench method followed by an anneal. The GdF3:Tb nanocrystals precipitated within the oxide glass matrix during the processing and their luminescence and scintillation properties were investigated. In this nanocomposite scintillator system, the incorporation of high atomic number Gd compound into the glass matrix increases the X-ray stopping power of the glass scintillator, and effective energy transfer between Gd3+ and Tb3+ ions in the nanocrystals enhances the scintillation efficiency.

Chemical reduction of three-dimensional silica micro-assemblies into microporous silicon replicas.

The carbothermal reduction of silica into silicon requires the use of temperatures well above the silicon melting point (> or =2,000 degrees C). Solid silicon has recently been generated directly from silica at much lower temperatures (< or =850 degrees C) via electrochemical reduction in molten salts. However, the silicon products of such electrochemical reduction did not retain the microscale morphology of the starting silica reactants. Here we demonstrate a low-temperature (650 degrees C) magnesiothermic reduction process for converting three-dimensional nanostructured silica micro-assemblies into microporous nanocrystalline silicon replicas. The intricate nanostructured silica microshells (frustules) of diatoms (unicellular algae) were converted into co-continuous, nanocrystalline mixtures of silicon and magnesia by reaction with magnesium gas. Selective magnesia dissolution then yielded an interconnected network of silicon nanocrystals that retained the starting three-dimensional frustule morphology. The silicon replicas possessed a high specific surface area (>500 m(2) g(-1)), and contained a significant population of micropores (< or =20 A). The silicon replicas were photoluminescent, and exhibited rapid changes in impedance upon exposure to gaseous nitric oxide (suggesting a possible application in microscale gas sensing). This process enables the syntheses of microporous nanocrystalline silicon micro-assemblies with multifarious three-dimensional shapes inherited from biological or synthetic silica templates for sensor, electronic, optical or biomedical applications.

Continuous manufacturing of highly stable lead halide perovskite nanocrystals via a dual-reactor strategy.

Lead halide perovskite nanocrystals possess incredible potential as next generation emitters due to their stellar set of optoelectronic properties. Unfortunately, their instability towards many ambient conditions and reliance on batch processing hinder their widespread utilities. Herein, we address both challenges by continuously synthesizing highly stable perovskite nanocrystals via integrating star-like block copolymer nanoreactors into a house-built flow reactor. Perovskite nanocrystals manufactured in this strategy display significantly enhanced colloidal, UV, and thermal stabilities over those synthesized with conventional ligands. Such scaling up of highly stable perovskite nanocrystals represents an important step towards their eventual use in many practical applications in optoelectronic materials and devices.

Continuous production of ultrathin organic-inorganic Ruddlesden-Popper perovskite nanoplatelets via a flow reactor.

Because of their enhanced quantum confinement, colloidal two-dimensional Ruddlesden-Popper (RP) perovskite nanosheets with a general formula L2[ABX3]n-1BX4 stand as a promising narrow-wavelength blue-emitting nanomaterial. Despite ample studies on batch synthesis, for RP perovskites to be broadly applied, continuous synthetic routes are needed. Herein, we design and optimize a flow reactor to continuously produce high-quality n = 1 RP perovskite nanoplatelets. The effects of antisolvent composition, reactor tube length, precursor solution injection rate, and antisolvent injection rate on the morphology and optical properties of the nanoplatelets are systematically examined. Our investigation suggests that flow reactors can be employed to synthesize high-quality L2PbX4 perovskite nanoplatelets (i.e., n = 1) at rates greater than 8 times that of batch synthesis. Mass-produced perovskite nanoplatelets promise a variety of potential applications in optoelectronics, including light emitting diodes, photodetectors, and solar cells.

Unconventional route to dual-shelled organolead halide perovskite nanocrystals with controlled dimensions, surface chemistry, and stabilities.

The past few years have witnessed rapid advances in the synthesis of high-quality perovskite nanocrystals (PNCs). However, despite the impressive developments, the stability of PNCs remains a substantial challenge. The ability to reliably improve stability of PNCs while retaining their individual nanometer size represents a critical step that underpins future advances in optoelectronic applications. Here, we report an unconventional strategy for crafting dual-shelled PNCs (i.e., polymer-ligated perovskite/SiO2 core/shell NCs) with exquisite control over dimensions, surface chemistry, and stabilities. In stark contrast to conventional methods, our strategy relies on capitalizing on judiciously designed star-like copolymers as nanoreactors to render the growth of core/shell NCs with controlled yet tunable perovskite core diameter, SiO2 shell thickness, and surface chemistry. Consequently, the resulting polymer-tethered perovskite/SiO2 core/shell NCs display concurrently a stellar set of substantially improved stabilities (i.e., colloidal stability, chemical composition stability, photostability, water stability), while having appealing solution processability, which are unattainable by conventional methods.

Enabling Tailorable Optical Properties and Markedly Enhanced Stability of Perovskite Quantum Dots by Permanently Ligating with Polymer Hairs.

Instability of perovskite quantum dots (QDs) toward humidity remains one of the major obstacles for their long-term use in optoelectronic devices. Herein, a general amphiphilic star-like block copolymer nanoreactor strategy for in situ crafting a set of hairy perovskite QDs with precisely tunable size and exceptionally high water and colloidal stabilities is presented. The selective partition of precursors within the compartment occupied by inner hydrophilic blocks of star-like diblock copolymers imparts in situ formation of robust hairy perovskite QDs permanently ligated by outer hydrophobic blocks via coprecipitation in nonpolar solvent. These size- and composition-tunable perovskite QDs reveal impressive water and colloidal stabilities as the surface of QDs is intimately and permanently ligated by a layer of outer hydrophobic polymer hairs. More intriguingly, the readily alterable length of outer hydrophobic polymers renders the remarkable control over the stability enhancement of hairy perovskite QDs.

All-Inorganic Perovskite Nanocrystals with a Stellar Set of Stabilities and Their Use in White Light-Emitting Diodes.

We report a simple, robust, and inexpensive strategy to enable all-inorganic CsPbX3 perovskite nanocrystals (NCs) with a set of markedly improved stabilities, that is, water stability, compositional stability, phase stability, and phase segregation stability via impregnating them in solid organic salt matrices (i.e., metal stearate; MSt). In addition to acting as matrices, MSt also functions as the ligand bound to the surface of CsPbX3 NCs, thereby eliminating the potential damage of NCs commonly encountered during purification as in copious past work. Quite intriguingly, the resulting CsPbX3-MSt nanocomposites display an outstanding suite of stabilities. First, they retain high emission in the presence of water because of the insolubility of MSt in water, signifying their excellent water stability. Second, anion exchange between CsPbBr3-MSt and CsPbI3-MSt nanocomposites is greatly suppressed. This can be ascribed to the efficient coating of MSt, thus effectively isolating the contact between CsPbBr3 and CsPbI3 NCs, reflecting notable compositional stability. Third, remarkably, after being impregnated by MSt, the resulting CsPbI3-MSt nanocomposites sustain the cubic phase of CsPbI3 and high emission, manifesting the strikingly improved phase stability. Finally, phase segregation of CsPbBr1.5I1.5 NCs is arrested via the MSt encapsulation (i.e., no formation of the respective CsPbBr3 and CsPbI3), thus rendering pure and stable photoluminescence (i.e., demonstration of phase segregation stability). Notably, when assembled into typical white light-emitting diode architecture, CsPbBr1.5I1.5-MSt nanocomposites exhibit appealing performance, including a high color rendering index ( Ra) and a low color temperature ( Tc). As such, the judicious encapsulation of perovskite NCs into organic salts represents a facile and robust strategy for creating high-quality solid-state luminophores for use in optoelectronic devices.

Analyses of factors affecting nickel ferrite nanoparticles synthesis in ultrasound-assisted aqueous solution ball milling.

Ball milling experiments were conducted with and without ultrasound wave assistance in deionized water using NiCO3·2Ni(OH)2·4H2O as raw materials. In the reaction process of NiFe2O4 prepared by ultrasound-assisted aqueous solution ball milling, some influencing factors including raw materials, ultrasonic frequency, ball to powder ratio and liquid level were changed. Samples were characterized by X-ray diffraction, fluorescence measurements and electroconductivity detections. The results indicate that more hydroxyl radicals and ions can be generated under the coupling effect of ultrasonic and ball milling. The fluorescence measurements and electroconductivity detections also reflect the reaction speed, allowing for optimal parameters to be determined.

Preparation of magnesium ferrite nanoparticles by ultrasonic wave-assisted aqueous solution ball milling.

Magnesium ferrite, MgFe2O4 nanoparticles with high saturation magnetization were successfully synthesized using ultrasonic wave-assisted ball milling. In this study, the raw materials were 4MgCO3·Mg(OH)2·5H2O and Fe2O3 powders and the grinding media was stainless steel ball. The average particle diameter of the product MgFe2O4 powders was 20 nm and the saturation magnetization of them reached 54.8 emu/g. The different results of aqueous solution ball milling with and without ultrasonic wave revealed that it was the coupling effect of ultrasonic wave and mechanical force that played an important role during the synthesis of MgFe2O4. In addition, the effect of the frequency of the ultrasonic wave on the ball milling process was investigated.

GdBr3: CE in a glass wafer as a nuclear radiation monitor.

A glass wafer that contains cerium-activated gadolinium-based scintillator has been tested as a nuclear radiation monitor. The detector is prepared by mixing powdered gadolinium and cerium (3+) bromides with alumina, silica, and lithium fluoride, melting the mixture at 1,400°C, and then quenching and annealing the glass. The resulting clear glass matrix emits stimulated blue light that can be collected by a conventional photomultiplier tube. Spectral analysis of radionuclides with this detector shows the energy peaks for alpha particles, the energy continuum for beta particles, the Compton continuum and full-energy peaks for gamma rays, and an energy continuum with specific reaction-product peaks for neutrons. Energy resolution for the 5.5-MeV alpha particle and 0.662-MeV gamma-ray peaks is about 20%. This resolution, although threefold poorer than for single-crystal NaI(Tl) scintillators, contributes to radionuclide identification and quantification. Application of this detector to radiation monitoring is proposed, as well as approaches for improving light collection and energy resolution that will facilitate radionuclide identification and monitoring, especially for alpha particles, beta particles, and low-energy gamma rays.

CdTe quantum dots and polymer nanocomposites for x-ray scintillation and imaging.

Investigations are reported on the x-ray scintillation and imaging application of CdTe quantum dots (QDs) and their polymer nanocomposites. Aqueous CdTe QDs with emissions ranging between 510 and 680 nm were prepared and incorporated into polyvinyl alcohol or polymethyl methacrylate polymer matrices. The x-ray luminescent properties were evaluated and a resolution of 5 lines∕mm was obtained from the nanocomposite films. Additionally, the fast decay time, nonafterglow, and superior spectral match to conventional charge coupled devices, show that CdTe QD nanocomposites have high promise for x-ray imaging applications.