Rainbow Emission from an Atomic Transition in Doped Quantum Dots

Although semiconductor quantum dots are promising materials for displays and lighting due to their tunable emissions, these materials also suffer from the serious disadvantage of self-absorption of emitted light. The reabsorption of emitted light is a serious loss mechanism in practical situations because most phosphors exhibit subunity quantum yields. Manganese-based phosphors that also exhibit high stability and quantum efficiency do not suffer from this problem but in turn lack emission tunability, seriously affecting their practical utility. Here, we present a class of manganese-doped quantum dot materials, where strain is used to tune the wavelength of the dopant emission, extending the otherwise limited emission tunability over the yellow–orange range for manganese ions to almost the entire visible spectrum covering all colors from blue to red. These new materials thus combine the advantages of both quantum dots and conventional doped phosphors, thereby opening new possibilities for a wide range of applications in the future

Substrate Integrated Nickel–Iron Ultrabattery with Extraordinarily Enhanced Performances

A substrate-integrated nickel–iron ultrabattery is realized using nickel oxide (NiO) nanoflakes and hematite (α-Fe₂O₃) nanorods as electroactive materials for its positive and negative electrodes, respectively. Direct growth of electroactive materials on a highly conductive stainless steel substrate enhances the mechanical stability of the system together with reducing its internal resistance. The proposed nanoarchitectural design of the electroactive materials provides a large number of interaction sites for the electrolytic ions with the electrode materials in conjunction with short ion-diffusion paths, which significantly improve the capacitive performance of individual electrodes and hence of the fabricated ultrabattery. As a consequence, the as-assembled ultrabattery exhibits a specific capacity value of ∼36 mAh/g (94 F/g and volumetric capacitance ∼0.77 F/cm³) at a current density of 0.5 A/g in a potential window between 0 and 1.4 V, with capacitance retention of ∼60% of its original value even when the load current density is increased 20 times

Rainbow Emission from an Atomic Transition in Doped Quantum Dots

Although semiconductor quantum dots are promising materials for displays and lighting due to their tunable emissions, these materials also suffer from the serious disadvantage of self-absorption of emitted light. The reabsorption of emitted light is a serious loss mechanism in practical situations because most phosphors exhibit subunity quantum yields. Manganese-based phosphors that also exhibit high stability and quantum efficiency do not suffer from this problem but in turn lack emission tunability, seriously affecting their practical utility. Here, we present a class of manganese-doped quantum dot materials, where strain is used to tune the wavelength of the dopant emission, extending the otherwise limited emission tunability over the yellow–orange range for manganese ions to almost the entire visible spectrum covering all colors from blue to red. These new materials thus combine the advantages of both quantum dots and conventional doped phosphors, thereby opening new possibilities for a wide range of applications in the future

Suppression of the Coffee-Ring Effect and Evaporation-Driven Disorder to Order Transition in Colloidal Droplets

The formation of a ring-like deposit at the periphery of a drying colloidal droplet is a vexing problem in many applications. We show a complete suppression of such deposits when a droplet of aqueous colloidal suspension, deposited on a glass substrate coated with a thin layer of silicone oil, is evaporated. This coating prevents the periphery of the aqueous droplet from getting pinned to the substrate and helps in suppressing the ring formation. It also decreases the surface area of the droplet, thereby decreasing the evaporation rate. These two factors together, driving the colloidal particles slowly to the center of the droplet, contribute to form an ordered crystallite at the end of the evaporation process. Brownian dynamics simulations performed to study ordering in the aggregate show that the spherical colloidal particles form face-centered cubic structures. Experiments and simulations show that slow rates of droplet evaporation and smaller-sized colloidal particles further lead to high-quality ordered colloidal crystallites

An Alternate Route to High-Quality ZnSe and Mn-Doped ZnSe Nanocrystals

We report here a synthetic route for high-quality Mn-doped ZnSe nanocrystals using selenourea as a selenium source, avoiding the more conventional route using tributylphosphine (TBP) that restricts the growth of spherical ZnSe nanocrystals below 5 nm in size, besides being highly toxic and pyrophoric. Spherical ZnSe nanocrystals with unprecedented sizes (up to 12 nm) are synthesized, the large size of the host helps to keep dopant ions well inside the nanocrystal, leading to intense and stable dopant emission. Mn-doped ZnSe nanocrystals with more than 50% quantum yield (QY) are synthesized in this method and found to be stable both in aqueous and nonaqueous dispersions for months

Doping Transition Metal (Mn or Cu) Ions in Semiconductor Nanocrystals

Following growth doping strategy and using dopant oxides nanocrystals as dopant sources, we report here two different transition-metal ions doped in a variety of group II−VI semiconductor nanocrystals. Using manganese oxide and copper oxide nanocrystals as corresponding dopant sources, intense photoluminescence emission over a wide range of wavelength has been observed for different host nanocrystals. Interestingly, this single doping strategy is successful in providing such highly emissive nanocrystals considered here, in contrast with the literature reports that would suggest synthesis strategies to be highly specific to the particular dopant, host, or both. We investigate and discuss the possible mechanism of the doping process, supporting the migration of dopant ions from dopant oxide nanocrystals to host nanocrystals as the most likely scenario

Beyond the “Coffee Ring”: Re-entrant Ordering in an Evaporation-Driven Self-Assembly in a Colloidal Suspension on a Substrate

We study the phenomenon of evaporation-driven self-assembly of a colloid suspension of silica microspheres in the interior region and away from the rim of the droplet on a glass plate. In view of the importance of achieving a large-area, monolayer assembly, we first realize a suitable choice of experimental conditions, minimizing the influence of many other competing phenomena that usually complicate the understanding of fundamental concepts of such self-assembly processes in the interior region of a drying droplet. Under these simplifying conditions to bring out essential aspects, our experiments unveil an interesting competition between ordering and compaction in such drying systems in analogy to an impending glass transition. We establish a re-entrant behavior in the order–disorder phase diagram as a function of the particle density, such that there is an optimal range of the particle density to realize the long-range ordering. The results are explained with the help of simulations and phenomenological theory

Competing Roles of Substrate Composition, Microstructure, and Sustained Strontium Release in Directing Osteogenic Differentiation of hMSCs

Strontium releasing bioactive ceramics constitute an important class of biomaterials for osteoporosis treatment. In the present study, we evaluated the synthesis, phase assemblage, and magnetic properties of strontium hexaferrite, SrFe₁₂O₁₉, (SrFe) nanoparticles. On the biocompatibility front, the size- and dose-dependent cytotoxicity of SrFe against human mesenchymal stem cells (hMSCs) were investigated. After establishing their non-toxic nature, we used the strontium hexaferrite nanoparticles (SrFeNPs) in varying amount (<i>x</i> = 0, 10, and 20 wt %) to consolidate bioactive composites with hydroxyapatite (HA) by multi-stage spark plasma sintering (SPS). Rietveld refinement of these spark plasma sintered composites revealed a near complete decomposition of SrFe₁₂O₁₉ to magnetite (Fe₃O₄) along with a marked increase in the unit cell volume of HA, commensurate with strontium-doped HA. The cytocompatibility of SrHA-Fe composites with hMSCs was assessed using qualitative and quantitative morphological analysis along with phenotypic and genotypic expression for stem cell differentiation. A marked decrease in the stemness of hMSCs, indicated by reduced vimentin expression and acquisition of osteogenic phenotype, evinced by alkaline phosphatase (ALP) and collagen deposition was recorded on SrHA-Fe composites in osteoinductive culture. A significant upregulation of osteogenic marker genes (Runx2, ALP and OPN) was detected in case of the SrHA-Fe composites, whereas OCN and Col IA expression were similarly high for baseline HA. However, matrix mineralization was elevated on SrHA-Fe composites in commensurate with the release of Sr²⁺ and Fe²⁺. Summarizing, the current work is the first report of strontium hexaferrite as a non-toxic nanobiomaterial. Also, SrHA-based iron oxide composites can potentially better facilitate bone formation, when compared to pristine HA

Magnetic Properties of Fe/Cu Codoped ZnO Nanocrystals

Free-standing ZnO nanocrystals simultaneously doped with Fe and Cu with varying Fe/Cu compositions have been synthesized using colloidal methods with a mean size of ∼7.7 nm. Interestingly, while the Cu-doped ZnO nanocrystal remains diamagnetic and Fe-doped samples show antiferromagnetic interactions between Fe sites without any magnetic ordering down to the lowest temperature investigated, samples doped simultaneously with Fe and Cu show a qualitative departure in exhibiting ferromagnetic interactions, with suggestions of ferromagnetic order at low temperature. XAS measurements establish the presence of Fe²⁺ and Fe³⁺ ions, with the concentration of the trivalent species increasing in the presence of Cu doping, providing direct evidence of the Fe²⁺ + Cu²⁺ ⇌ Fe³⁺ + Cu⁺ redox couple being correlated with the ferromagnetic property. Using DFT, the unexpected ferromagnetic nature of these systems is explained in terms of a double exchange between Fe atoms, mediated by the Cu atom, in agreement with experimental observations

Solution-Processed Free-Standing Ultrathin Two-Dimensional PbS Nanocrystals with Efficient and Highly Stable Dielectric Properties

Two-dimensional (2D) materials with downscaled thicknesses are the quest of the electronics industry because of their immense potential in modern microelectronics. Despite the discovery of several novel 2D materials, the flexible design of high-performance free-standing ultrathin 2D dielectric nanocrystals (NCs) with a large planar morphology remains the most challenging task. We develop a method for synthesizing high-quality free-standing ultrathin 2D NCs of PbS with a well-defined large rectangular morphology with a thickness of ∼2 nm. The lateral size can be tuned up to a few hundred nanometers by changing only the reaction annealing time. Microscopic and spectroscopic analyses at different stages of the reaction reveal formation of 2D NCs by a continuous growth mechanism. The 2D NCs exhibit a nearly temperature and frequency independent high dielectric constant (>13.4) with a small dielectric loss (0.0006 at 20 K and <0.06 at 350 K for 100 kHz) over broad temperature and frequency ranges. Low-frequency dispersion from 125 Hz to 1 MHz, frequency stability with a small dielectric loss (<0.03 at 100 kHz), and a stable temperature coefficient of the dielectric constant outline the merits of 2D NCs as a potential dielectric material. Complex impedance analyses demonstrate dominant intrinsic effects contributed by polarons in covalent NCs. Equal activation energies for the conduction and relaxation processes offer uniform energy barriers for the charges in NCs leading to high-performance dielectric behavior. This work opens up promising features of non-oxide binary semiconductors as dielectric alternatives for miniaturized electronics using flexible solution processing routes