Molten-Salt-Assisted Annealing for Making Colloidal ZnGa2O4:Cr Nanocrystals with High Persistent Luminescence

Persistent luminescent nanocrystals (PLNCs) in the sub-10 nm domain are considered to be the most fascinating inventions in lighting technology owing to their excellent performance in anti-counterfeiting, luminous paints, bioimaging, security applications, etc. Further improvement of persistent luminescence (PersL) intensity and lifetime is needed to achieve the desired success of PLNCs while keeping the uniform sub-10 nm size. In this work, the concept of molten salt confinement to thermally anneal as-synthesized ZnGa2O4:Cr3+ (ZGOC) colloidal NCs (CNCs) in a molten salt medium at 650 °C is introduced. This method led to significantly monodispersed and few agglomerated NCs with a much improved photoluminescence (PL) and PersL intensity without much growth in the size of the pristine CNCs. Other strategies such as i) thermal annealing, ii) overcoating, and iii) the core–shell strategy have also been tried to improve PL and PersL but did not improve them simultaneously. Moreover, directly annealing the CNCs in air without the assistance of molten salt could significantly improve both PL and PersL but led to particle heterogeneity and aggregation, which are highly unsuitable for in vivo imaging. We believe this work provides a novel strategy to design PLNCs with high PL intensity and long PersL duration without losing their nanostructural characteristics, water dispersibility and biocompatibility

Piezoelectric Properties of PVDF-Zn2GeO4 Fine Fiber Mats

The current paper presents the development and characterization of polyvinylidene fluoride (PVDF)-Zn2GeO4 (ZGO) fine fiber mats. ZGO nanorods (NRs) were synthesized using a hydrothermal method and incorporated in a PVDF solution to produce fine fiber mats. The fiber mats were prepared by varying the concentration of ZGO NRs (1.25–10 wt %) using the Forcespinning® method. The developed mats showed long, continuous, and homogeneous fibers, with average fiber diameters varying from 0.7 to 1 µm, depending on the ZGO concentration. X-ray diffraction spectra depicted a positive correlation among concentration of ZGO NRs and strengthening of the beta phase within the PVDF fibers. The composite system containing 1.25 wt % of ZGO displayed the highest piezoelectric response of 172 V. This fine fiber composite system has promising potential applications for energy harvesting and the powering of wearable and portable electronics. View Full-Tex

Tunable CsPb(Br/Cl)3 perovskite nanocrystals and further advancement in designing light emitting fiber membranes

Cesium lead halide perovskite nanocrystals (NCs) have drawn a great deal of interest in optoelectronic and photonic applications due to their intrinsic and attractive photoluminescence properties. Though, their commercially viability is of concern due to their intrinsic instability. In this study, blue and green luminous PMMA-CsPbX3(X=Cl/Br) fibers were fabricated via forcespinning technique, where the polymer matrix encapsulated the NCs. Blue CsPbX3 NCs (b- CPX NCs) were synthesized at ambient conditions while blue to green CsPbX3 NCs (g-CPX NCs) fine color tuning was obtained after heat treatment at 150°C.Field emission scanning electron microscopy (FESEM) shows fibers with diameters in the single digit microscale. Efficient encapsulation of NCs in the PMMA fiber was confirmed using FTIR spectroscopy. UV visible spectra of the NCs suggested a quantum confinement effect. Pristine NCs shows bright blue and green emission from b-CPX and g-CPX NCs under UV irradiation (365 nm) which was successfully reproduced even upon encapsulation in the PMMA matrix. In both cases, the PMMA besides promoting QD encapsulation also enhanced the photoluminescent quantum yield (PLQY) from 25.5% to 31.1% (blue PMMA fibers) and 42.6% to 51.4% (green PMMA fibers) compare to bare NCs PLQY. The PMMA-CsPbX3(X=Cl/Br) also possessed narrow half-peak width compared to pristine NCs suggesting high color purity. This work provides a novel polymer fiber-based encapsulation approach to solve the intrinsic instability issues of CsPbX3 NCs, therefore prompting promising practical applications

Rare earth free bright and persistent white light emitting zinc gallo-germanate nanosheets: technological advancement to fibers with enhanced quantum efficiency

Recent materials science and engineering research focused on defects, dopants, hosts, and morphological structures has resulted in novel cost efficient and sustainable phases with extraordinary properties and performance. Contributing in this direction we have designed dopant and rare earth free white lightemitting zincgallogermanate (ZGG) phosphors in nanosheet (NS) morphology. These ZGG NSs with interlayer and interfacial defects display bright white photoluminescence (PL) with significant quantum yield (QY). Thermal treatment of the as-synthesized ZGG NSs at 750 1C does not degrade their sheet-like morphology while resulting in long persistent luminescence (PerL) with a duration of approximately one hour. Furthermore, to improve the commercial viability of the as-synthesized ZGG NSs, we have assembled them as fine fibers of polyvinyl alcohol (PVA) using Forcespinnings technology. The ZGG–PVA fibers displayed efficient white PL with increased quantum yield compared to the as-synthesized ZGG NSs. We believe this technological evolution, transitioning from bulk ZGG to ZGG nanosheets, will lead to dopant-free/rare-earth-free persistent white light emission and an enhancement in QY. This technology will be a boon to the optoelectronic and lighting industries, and will benefit commercial applications in smart textiles, energy efficient lighting, night vision, anti-counterfeiting, traffic signals, and security, among other potential uses

Color tunable aerogels/sponge-like structures developed from fine fiber membranes

The development of macroscopic aerogels from 1D systems, such as nanofibers, has resulted in a novel pathway to obtain porous and lightweight architectures. In this work, bright green, red, and tunable color emitting aerogels were obtained with luminescent nanofibers as the precursor system. A simple, low cost, and environmentally friendly process is followed where luminescent fillers are encapsulated within fibers which were subsequently freeze-dried to form 3D aerogels and sponge-like structures. Moreover, the aerogels/sponge-like structures show higher photoluminescence intensity than the fiber mats due to an increase of porosity which provides higher and direct interaction with the fillers and therefore an efficient light absorption resulting in higher luminescence. Manganese doped zinc germanate (Mn: Zn2GeO4) nanorods and chromium doped zinc gallate (Cr: ZnGa2O4) nanoparticles were used as the source of green and red emissions respectively. By precisely adjusting the stoichiometric ratios of nanorods and nanoparticles within the nanofibers, a broad spectrum output is obtained from the final aerogels. We foresee that these types of photoluminescent aerogels have promising potential applications in a variety of fields such as display devices, solid-state lighting, sensors, etc

Disorder driven asymmetry and singular red emission in doped Lu2Hf2O7 nanocrystals with no charge compensating defects

High performance luminescent materials possess low symmetry, high color purity, no charge compensating defects, and high quantum yield. In this work, we have synthesized Lu2Hf2O7 (LuHO) and Lu2Hf2O7:Eu3+ (LuHOE) nanocrystals (NCs) using a molten salt synthesis and confirmed that both are stabilized in defect fluorite structure with a high degree of structural disordering. The LuHO NCs depicted green emission under ultraviolet irradiation, which decreases and increases after being treated in oxidizing and reducing environments, respectively, confirming the role of oxygen vacancies in the emission process. The LuHOE NCs (i) show excitation wavelength-dependent host to dopant energy transfer efficiency, (ii) give a singular red emission with high color purity of ~95%, (iii) have a dominant occupation of the Lu3+ sites by Eu3+ ions without the formation of charge compensating defects (CCDs), and (iv) possess low non-radiative channels with a quantum yield of ~88%. We believe these LuHOE NCs with singular red emission, high quantum yield, and color purity acquired through structural disordering and the absence of CCDs warrant further investigation as efficient phosphors

Computational and Experimental Study on Undoped and Er-Doped Lithium Tantalate Nanofluorescent Probes

We present a combined density functional theory (DFT) and experimental work on lithium tantalate LiTaO3 (LT) and its Er-doped counterparts. We calculate the electronic and optical properties for both LT and LT:Er+3, with Er occupying either Li or Ta sites, at 4.167 mol%. The generalized gradient approximation (GGA) calculations show that the Er-4 f bands appear closer to the conduction band bottom and to the valance band top, for the first and second doped configurations, respectively. This agrees with changes in the imaginary part of the frequency dependent dielectric function between the doped configurations. There are striking differences between the GGA and the hybrid functional HSE06 calculations for the band structures of the doped configurations. HSE06 accurately predicts the location in energy for all Er-4 f orbitals: These are now spread in energy and appear above and below the Fermi energy. We synthesized LT:Er+3 nanoparticles, validated through X-ray diffraction and Scanning Electron Microscopy. Differential scanning calorimetry and thermogravimetric analysis confirmed increases in the activation energy and lowering of the reaction temperature due to Er+3 doping. The LT:Er+3photoluminescence showed strong f–f emission in the visible and near-infrared regions, in an excellent agreement with the HSE06 electronic information

Molten Salt Assisted Annealing for Making Colloidal ZnGa2O4:Cr Nanocrystals with High Persistent Luminescence

Persistent luminescent nanocrystals (PLNCs) in the sub-10 nm domain are considered to be the most fascinating inventions in lighting technology owing to their excellent performance in anti-counterfeiting, luminous paints, bioimaging, security applications, etc. Further improvement of persistent luminescence (PersL) intensity and lifetime is needed to achieve the desired success of PLNCs while keeping the uniform sub-10 nm size. In this work, the concept of molten salt confinement to thermally anneal as-synthesized ZnGa2O4:Cr3+ (ZGOC) colloidal NCs (CNCs) in a molten salt medium at 650 °C is introduced. This method led to significantly monodispersed and few agglomerated NCs with a much improved photoluminescence (PL) and PersL intensity without much growth in the size of the pristine CNCs. Other strategies such as i) thermal annealing, ii) overcoating, and iii) the core–shell strategy have also been tried to improve PL and PersL but did not improve them simultaneously. Moreover, directly annealing the CNCs in air without the assistance of molten salt could significantly improve both PL and PersL but led to particle heterogeneity and aggregation, which are highly unsuitable for in vivo imaging. We believe this work provides a novel strategy to design PLNCs with high PL intensity and long PersL duration without losing their nanostructural characteristics, water dispersibility and biocompatibility

Surface Ligand Population Controlled Oriented Attachment: A Case of CdS Nanowires

using two types of organic ligands having similar chemical structure but different physical properties and varying their dynamic population at the surface of zinc blende seed nanocrystals, self-assembled zinc blende semicircular-shaped bent nanowires of CdS are synthesized via a colloidal synthetic approach. It is found that the hydrophobic tail interaction of long-chain ligands puts strain on these thin nanowires (< 2 nm diameter) and bend them to some extent, forming strained nanowires

A Controlled Growth Process To Design Relatively Larger Size Semiconductor Nanocrystals

The growth of semiconductor nanocrystals in solution is mostly governed by the kinetic and thermal modes of control of the reaction process. In most of the cases, the size of the particles is limited within 5–6 nm, and further annealing mostly defocuses the particles size distribution. But, herein, we report a self-driven growth protocol which supplies the monomer continuously to significant extent and delays the thermal diffusion-controlled ripening process. This has been achieved by choosing appropriate sulfur precursor in the synthesis of metal sulfide nanocrystals which controls the sulfide ion concentration in the reaction medium via establishing an appropriate chemical equilibrium. As a consequence, the monomer concentration retains above their critical limit and it delays the ripening process. Finally, the nanocrystals can grow even larger than 10 nm, which are difficult to obtain from different established synthetic approaches. This has been observed for several semiconductor nanocrystals such as ZnS, CdS, CdZnS, and also in ZnSe nanocrystals. Further, this growth process has been adopted to dope Mn in larger sized ZnS and CdZnS nanocrystals, and efficient dopant emission has been obtained