H3 Acetylation-Induced Basal Progenitor Generation and Neocortex Expansion Depends on the Transcription Factor Pax6.

Enrichment of basal progenitors (BPs) in the developing neocortex is a central driver of cortical enlargement. The transcription factor Pax6 is known as an essential regulator in generation of BPs. H3 lysine 9 acetylation (H3K9ac) has emerged as a crucial epigenetic mechanism that activates the gene expression program required for BP pool amplification. In this current work, we applied immunohistochemistry, RNA sequencing, chromatin immunoprecipitation and sequencing, and the yeast two-hybrid assay to reveal that the BP-genic effect of H3 acetylation is dependent on Pax6 functionality in the developing mouse cortex. In the presence of Pax6, increased H3 acetylation caused BP pool expansion, leading to enhanced neurogenesis, which evoked expansion and quasi-convolution of the mouse neocortex. Interestingly, H3 acetylation activation exacerbates the BP depletion and corticogenesis reduction effect of Pax6 ablation in cortex-specific Pax6 mutants. Furthermore, we found that H3K9 acetyltransferase KAT2A/GCN5 interacts with Pax6 and potentiates Pax6-dependent transcriptional activity. This explains a genome-wide lack of H3K9ac, especially in the promoter regions of BP-genic genes, in the Pax6 mutant cortex. Together, these findings reveal a mechanistic coupling of H3 acetylation and Pax6 in orchestrating BP production and cortical expansion through the promotion of a BP gene expression program during cortical development.

Highly Stable White Light Emission from III-Nitride Nanowire LEDs Utilizing Nanostructured Alumina-Doped Mn4+ and Mg2.

In this report, red-emitting alumina nanophosphors doped with Mn4+ and Mg2+ (Al2O3:Mn4+, Mg2+) are synthesized by a hydrothermal method using a Pluronic surfactant. The prepared samples are ceramic-sintered at various temperatures. X-ray diffraction shows that Al2O3:Mn4+, Mg2+ annealed at 500 °C exhibits a cubic γ-Al2O3 phase with the space group Fd3m-227. The tetragonal δ-Al2O3 and rhombohedral α-Al2O3 phase is obtained at 1000 and 1300 °C, respectively. Cube-like nanoparticles in a size of ∼40 nm are observed for the alumina heated at 500-1000 °C. The size and red-emitting intensity of the phosphors remarkably increased with annealed temperature ∼1300 °C. Emission spectra of the phosphors show strong peaks at 678 and 692 nm due to 2 E g → 4 A 2 transitions of the Mn4+ ion, under a light excitation of 460 nm. A strong zero-phonon line (ZPL) emission is observed in the luminescence spectra of δ-Al2O3:Mn4+, Mg2+ at 298 K, whereas a weak one is observed in those of α- and γ-Al2O3:Mn4+, Mg2+. The alumina phosphors exhibited an excellent waterproof ability during 60 days in water and good thermal stability in the range of 77-573 K. A warm-white light-emitting diode (WLED) fabricated using In x Ga1-x N nanowire chips with Al2O3:Mn4+, Mg2+ red-emitting nanophosphors presents a high color rendering index of ∼95.1 and a low correlated color temperature of ∼4998 K. Moreover, the current-voltage characteristic of the nanowire LEDs could be improved using Al2O3:Mn4+, Mg2+ nanophosphors which is attributed to the increased heat dissipation in the nanowire LEDs.

High-efficiency InGaN blue LEDs with reduced positive sheet polarization.

The formation of positive sheet polarization charges at the interface of the last quantum barrier (QB) and the conventional p-type electron-blocking layer (EBL) creates significant band bending, leading to severe electron leakage and poor hole injection in III-nitride light-emitting diodes. We report that the positive sheet polarization charges are mitigated by employing a lattice matched AlGaN last QB. Electron leakage is dramatically reduced due to the increased effective conduction band height at the last QB and EBL. Furthermore, it favors hole injection into the active region due to the reduced effective valance band height for EBL.

Improving Color Quality of Nanowire White Light-Emitting Diodes with Mn4+ Doped Fluoride Nanosheets.

A two-dimensional nanostructured fluoride red-emitting phosphor with an excellent quantum yield of ~91% is studied for cost-effective and high-color quality nanowire white light-emitting diodes (WLEDs). K2TiF6:Mn4+ phosphors are synthesized via an emulsification method using surfactants as sodium dodecyl sulphonate and oleic acid. The K2TiF6:Mn4+ phosphors in ultra-thin and nanosheet crystals are observed via scanning electron microscopy and high-resolution transmission electron microscopy. The surfactants are found to play a key role in inhibition of KTFM crystal growth process and stabilization of Mn4+ ions doping into the K2TiF6 host. The prepared phosphors exhibited intensive red emission at approximately 632 nm and excellent thermal stability in the range of 300-500 K upon 460 nm light excitation. Moreover, the K2TiF6:Mn4+ nanosheets were integrated on InGaN/AlGaN nanowire WLEDs for color quality study. The results show that the nanowire WLEDs with red-emitting phosphor exhibit unprecedentedly high color rendering index ~96.4, and correlated color temperature ~4450 K.

Deep red fluoride dots-in-nanoparticles for high color quality micro white light-emitting diodes.

In this study, a novel nanostructure of fluoride red emitting phosphor is synthesized via soft templates. K2SiF6:Mn4+ nanocrystals in the range of 3-5 nm diameter are found inside the porous K2SiF6:Mn4+ nanoparticle hosts, forming unique dots-in-nanoparticles (d-NPs) structures with controlled optical properties. The porous K2SiF6:Mn4+ d-NPs exhibit a sharp and deep red emission with an excellent quantum yield of ∼95.9%, and ultra-high color purity with the corresponding x and y in the CIE chromaticity coordinates are 0.7102 and 0.2870, respectively. Moreover, this nanophosphor possesses good thermal stability in range of 300 K-500 K, under light excitation of 455 nm. The K2SiF6:Mn4+ d-NPs are covered onto a surface of 100×100 µm2 blue-yellow InxGa1-xN nanowire light-emitting diode (LED) to make warm white LEDs (WLEDs). The fabricated WLEDs present an excellent color rendering index of ∼95.4 and a low correlated color temperature of ∼3649 K. Porous K2SiF6:Mn4+ d-NPs are suggested as a potential red component for high color quality micro WLED applications.

High-performance electron-blocking-layer-free deep ultraviolet light-emitting diodes implementing a strip-in-a-barrier structure.

In this Letter, the electron-blocking-layer (EBL)-free AlGaN ultraviolet (UV) light-emitting diodes (LEDs) using a strip-in-a-barrier structure have been proposed. The quantum barrier (QB) structures are systematically engineered by integrating a 1 nm intrinsic AlxGa(1-x)N strip into the middle of QBs. The resulted structures exhibit significantly reduced electron leakage and improved hole injection into the active region, thus generating higher carrier radiative recombination. Our study shows that the proposed structure improves radiative recombination by ∼220%, reduces electron leakage by ∼11 times, and enhances optical power by ∼225% at 60 mA current injection compared to a conventional AlGaN EBL LED structure. Moreover, the EBL-free strip-in-a-barrier UV LED records the maximum internal quantum efficiency (IQE) of ∼61.5% which is ∼72% higher, and IQE droop is ∼12.4%, which is ∼333% less compared to the conventional AlGaN EBL LED structure at ∼284.5nm wavelength. Hence, the proposed EBL-free AlGaN LED is the potential solution to enhance the optical power and produce highly efficient UV emitters.

Improving carrier transport in AlGaN deep-ultraviolet light-emitting diodes using a strip-in-a-barrier structure.

This paper reports the illustration of electron blocking layer (EBL)-free AlGaN light-emitting diodes (LEDs) operating in the deep-ultraviolet (DUV) wavelength at ∼270nm. In this work, we demonstrated that the integration of an optimized thin undoped AlGaN strip layer in the middle of the last quantum barrier (LQB) could generate enough conduction band barrier height for the effectively reduced electron overflow into the p-GaN region. Moreover, the hole injection into the multi-quantum-well active region is significantly increased due to a large hole accumulation at the interface of the AlGaN strip and the LQB. As a result, the internal quantum efficiency and output power of the proposed LED structure has been enhanced tremendously compared to that of the conventional p-type EBL-based LED structure.

High performance electron blocking layer-free InGaN/GaN nanowire white-light-emitting diodes.

We investigated the effect of coupled quantum wells to reduce electron overflow in InGaN/GaN dot-in-a-wire phosphor-free white color light-emitting diodes (white LEDs) and to improve the device performance. The light output power and external quantum efficiency (EQE) of the white LEDs with coupled quantum wells were increased and indicated that the efficiency droop was reduced. The improved output power and EQE of LEDs with the coupled quantum wells were attributed to the significant reduction of electron overflow primarily responsible for efficiency degradation through the near-surface GaN region. Compared to the commonly used AlGaN electron blocking layer between the device active region and p-GaN, the incorporation of a suitable InGaN quantum well between the n-GaN and the active region does not adversely affect the hole injection process. Moreover, the electron transport to the device active region can be further controlled by optimizing the thickness and bandgap energy of this InGaN quantum well. In addition, a blue-emitting InGaN quantum well is incorporated between the quantum dot active region and the p-GaN, wherein electrons escaping from the device active region can recombine with holes and contribute to white-light emission. The resulting device exhibits high internal quantum efficiency of 58.5% with highly stable emission characteristics and virtually no efficiency droop.

Epitaxial Growth and Characterization of AlInN-Based Core-Shell Nanowire Light Emitting Diodes Operating in the Ultraviolet Spectrum.

We report the demonstration of the first axial AlInN ultraviolet core-shell nanowire light-emitting diodes with highly stable emission in the ultraviolet wavelength range. During epitaxial growth of the AlInN layer, an AlInN shell is spontaneously formed, resulting in reduced nonradiative recombination on the nanowire surface. The AlInN nanowires exhibit a high internal quantum efficiency of ~52% at room temperature for emission at 295 nm. The peak emission wavelength can be varied from 290 nm to 355 nm by changing the growth conditions. Moreover, significantly strong transverse magnetic (TM) polarized emission is recorded, which is ~4 times stronger than the transverse electric (TE) polarized light at 295 nm. This study provides an alternative approach for the fabrication of new types of high-performance ultraviolet light emitters.

Full-Color InGaN/AlGaN Nanowire Micro Light-Emitting Diodes Grown by Molecular Beam Epitaxy: A Promising Candidate for Next Generation Micro Displays.

We have demonstrated full-color and white-color micro light-emitting diodes (µLEDs) using InGaN/AlGaN core-shell nanowire heterostructures, grown on silicon substrate by molecular beam epitaxy. InGaN/AlGaN core-shell nanowire µLED arrays were fabricated with their wavelengths tunable from blue to red by controlling the indium composition in the device active regions. Moreover, our fabricated phosphor-free white-color µLEDs demonstrate strong and highly stable white-light emission with high color rendering index of ~ 94. The µLEDs are in circular shapes with the diameter varying from 30 to 100 µm. Such high-performance µLEDs are perfectly suitable for the next generation of high-resolution micro-display applications.

Fabrication of Phosphor-Free III-Nitride Nanowire Light-Emitting Diodes on Metal Substrates for Flexible Photonics.

In this paper, we report our study on high-performance III-nitride nanowire light-emitting diodes (LEDs) on copper (Cu) substrates via the substrate-transfer process. Nanowire LED structures were first grown on silicon-on-insulator (SOI) substrates by molecular beam epitaxy. Subsequently, the SOI substrate was removed by combining dry- and wet-etching processes. Compared to conventional nanowire LEDs on Si, the nanowire LEDs on Cu exhibit several advantages, including more efficient thermal management and enhanced light-extraction efficiency (LEE) because of the usage of metal reflectors and highly thermally conductive metal substrates. The LED on Cu, therefore, has stronger photoluminescence, electroluminescence intensities, and better current-voltage characteristics compared to the conventional nanowire LED on Si. Our simulation results further confirm the improved device performance of LEDs on Cu, compared to LEDs on Si. The LEE of the nanowire LED on Cu is nine times higher than that of the LED on Si at the same nanowire radius of 60 nm and spacing of 130 nm. Moreover, by engineering the device-active region, we achieved high-brightness phosphor-free LEDs on Cu with highly stable white-light emission and high color-rendering index of ∼95, showing their promising applications in general lighting, flexible displays, and wearable applications.

Improvement of the Water Resistance of a Narrow-Band Red-Emitting SrLiAl3 N4 :Eu(2+) Phosphor Synthesized under High Isostatic Pressure through Coating with an Organosilica Layer.

A SrLiAl3 N4 :Eu(2+) (SLA) red phosphor prepared through a high-pressure solid-state reaction was coated with an organosilica layer with a thickness of 400-600 nm to improve its water resistance. The observed 4f(6) 5d→4f(7) transition bands are thought to result from the existence of Eu(2+) at two different Sr(2+) sites. Luminescence spectra at 10 K revealed two zero-phonon lines at 15377 (for Eu(Sr1)) and 15780 cm(-1) (for Eu(Sr2)). The phosphor exhibited stable red emission under high pressure up to 312 kbar. The configurational coordinate diagram gave a theoretical explanation for the Eu(2+/3+) result. The coated samples showed excellent moisture resistance while retaining an external quantum efficiency (EQE) of 70 % of their initial EQE after aging for 5 days under harsh conditions. White-light-emitting diodes of the SLA red phosphor and a commercial Y3 Al5 O12 :Ce(3+) yellow phosphor on a blue InGaN chip showed high color rendition (CRI=89, R9=69) and a low correlated color temperature of 2406 K.

Waterproof Alkyl Phosphate Coated Fluoride Phosphors for Optoelectronic Materials.

A facile approach for coating red fluoride phosphors with a moisture-resistant alkyl phosphate layer with a thickness of 50-100 nm is reported. K2 SiF6 :Mn(4+) particles were prepared by co-precipitation and then coated by esterification of P2 O5 with alcohols (methanol, ethanol, and isopropanol). This route was adopted to encapsulate the prepared phosphors using transition-metal ions as cross-linkers between the alkyl phosphate moieties. The coated phosphor particles exhibited a high water tolerance and retained approximately 87 % of their initial external quantum efficiency after aging under high-humidity (85 %) and high-temperature (85 °C) conditions for one month. Warm white-light-emitting diodes that consisted of blue InGaN chips, the prepared K2 SiF6 :Mn(4+) phosphors, and either yellow Y3 Al5 O12 :Ce(3+) phosphors or green ß-SiAlON: Eu(2+) phosphors showed excellent color rendition.

Enhanced ultrafast optical nonlinearity of porous anodized aluminum oxide nanostructures.

Enhanced ultrafast optical nonlinearities of porous anodized aluminum oxide (AAO) nanostructures, well-known templates for quantum dots fabrication, have been investigated using the differential optical Kerr gate technique at 800 nm. The optical nonlinearity is strongly influenced by the pore number density, the pore size and the shape. Large values of the third-order nonlinear optical susceptibility (chi((3))) of the order of 10(-10)esu are measured. The nonlinear response time is faster than or comparable to the laser pulse width (90 fs) used. The origin and variation of such remarkable optical nonlinearities has been discussed by considering the nanoporous AAO as an effective medium and utilizing the extended Maxwell Garnet theory, and by considering the additional influence from pore diameter, pore shape and surface states.