Highly Luminous N3--Substituted Li2MSiO4-δN2/3δ:Eu2+ (M = Ca, Sr, and Ba) for White NUV Light-Emitting Diodes

The N^3--substituted Li₂MSiO₄:Eu²⁺ (M = Ca, Sr, and Ba) phosphors were systematically prepared and analyzed. Secondary-ion mass spectroscopy measurements revealed that the average N^3- contents are 0.003 for Ca, 0.009 for Sr, and 0.032 for Ba. Furthermore, the N^3- incorporation in the host lattices was corroborated by infrared and X-ray photoelectron spectroscopies. From the photoluminescence spectra of Li₂MSiO₄:Eu²⁺ (M = Ca, Sr, and Ba) phosphors before and after N^3- doping, it was verified that the enhanced emission intensity of the phosphors is most likely due to the N^3- doping. In Li₂MSiO₄:Eu²⁺ (M = Ca, Sr, and Ba) phosphors, the maximum wavelengths of the emission band were red-shifted in the order Ca < Ba < Sr, which is not consistent with the trend of crystal field splitting: Ba < Sr < Ca. This discrepancy was clearly explained by electron-electron repulsions among polyhedra, LiO₄-MO _n , SiO₄-MO _n , and MO _n -M'O _n associated with structural difference in the host lattices. Therefore, the energy levels associated with the 4f⁶5d energy levels of Eu²⁺ are definitely established in the following order: Li₂CaSiO₄:Eu²⁺ > Li₂BaSiO₄:Eu²⁺ > Li₂SrSiO₄:Eu²⁺. Furthermore, using the Williamson-Hall (W-H) method, the determined structural strains of Li₂MSiO₄:Eu²⁺ (M = Ca, Sr, and Ba) phosphors revealed that the increased compressive strain after N^3- doping induces the enhanced emission intensity of these phosphors. White light-emitting diodes made by three N^3--doped phosphors and a 365 nm emitting InGaN chip showed the (0.333, 0.373) color coordinate and high color-rendering index (R _a = 83). These phosphor materials may provide a platform for development of new efficient phosphors in solid-state lighting field.

Self-emitting blue and red EuOX (X = F, Cl, Br, I) materials: band structure, charge transfer energy, and emission energy

Self-emitting blue and red EuOX (X = F, Cl, Br, and I) were successfully synthesized and characterized. Far-infrared and Raman measurements revealed that the vibration modes prominently reflected the Eu-O and Eu-X bond characters of these materials. X-ray photoelectron spectroscopy (XPS) of the red-emitting EuOX compounds showed that Eu exclusively existed as Eu3+, while in the blue-emitting EuOX, a mixed Eu3+/Eu2+ state was observed. For the red-emitting EuOX (X = F, Cl, and Br), the maximum wavelengths of the charge-transfer (CT) bands were red-shifted: F → Cl → Br (282, 320, and 330 nm for F, Cl, and Br, respectively). Using one-electron spin-polarized band structure calculations, it was verified that the red-shift of the CT energy from F to Br in EuOX was mainly due to the relative positions of the halogen orbital energies being gradually increased, following the trend in their electronegativity. For the blue-emitting EuOX (X = Cl, Br, and I), the emission band maxima were red-shifted from Cl to I (409, 414, and 432 nm for Cl, Br, and I, respectively), which was quite opposite to the trend predicted based on the spectrochemical series in crystal field theory, which was in good agreement with the previous results of the calculated 5d → 4f transition energies of the Eu2+ activator based on the crystal field theory. Through photoluminescence, UV-visible absorbance, and XPS, it was elucidated that the red emission due to Eu3+ was strongly masked by the intensified blue emission associated with the small amount of Eu2+ in the blue-emitting EuOX (X = Cl, Br, and I). These materials may provide a platform for modeling new phosphors for application in solid-state lighting.

High-Performance Quantum Dot Thin-Film Transistors with Environmentally Benign Surface Functionalization and Robust Defect Passivation

The recent development of high-performance colloidal quantum dot (QD) thin-film transistors (TFTs) has been achieved with removal of surface ligand, defect passivation, and facile electronic doping. Here, we report on high-performance solution-processed CdSe QD-TFTs with an optimized surface functionalization and robust defect passivation via hydrazine-free metal chalcogenide (MCC) ligands. The underlying mechanism of the ligand effects on CdSe QDs has been studied with hydrazine-free ex situ reaction derived MCC ligands, such as Sn₂S₆^4-, Sn₂Se₆^4-, and In₂Se₄^2-, to allow benign solution-process available. Furthermore, the defect passivation and remote n-type doping effects have been investigated by incorporating indium nanoparticles over the QD layer. Strong electronic coupling and solid defect passivation of QDs could be achieved by introducing electronically active MCC capping and thermal diffusion of the indium nanoparticles, respectively. It is also noteworthy that the diffused indium nanoparticles facilitate charge injection not only inter-QDs but also between source/drain electrodes and the QD semiconductors, significantly reducing contact resistance. With benign organic solvents, the Sn₂S₆^4-, Sn₂Se₆^4-, and In₂Se₄^2- ligand based QD-TFTs exhibited field-effect mobilities exceeding 4.8, 12.0, and 44.2 cm²/(V s), respectively. The results reported here imply that the incorporation of MCC ligands and appropriate dopants provide a general route to high-performance, extremely stable solution-processed QD-based electronic devices with marginal toxicity, offering compatibility with standard complementary metal oxide semiconductor processing and large-scale on-chip device applications.

ZnTe Alloying Effect on Enhanced Thermoelectric Properties of p-Type PbTe

We investigate the effect of ZnTe incorporation on PbTe to enhance thermoelectric performance. We report structural, microscopic, and spectroscopic characterizations, ab initio theoretical calculations, and thermoelectric transport properties of Pb_0.985Na_0.015Te-x% ZnTe (x = 0, 1, 2, 4). We find that the solid solubility limit of ZnTe in PbTe is less than 1 mol %. The introduction of 2% ZnTe in p-type Pb_0.985Na_0.015Te reduces the lattice thermal conductivity through the ZnTe precipitates at the microscale. Consequently, a maximum thermoelectric figure of merit (ZT) of 1.73 at 700 K is achieved for the spark plasma-sintered Pb_0.985Na_0.015Te-2% ZnTe, which arises from a decreased lattice thermal conductivity of ∼0.69 W m^-1 K^-1 at ∼700 K in comparison with Pb_0.985Na_0.015Te.

Extraordinary Off-Stoichiometric Bismuth Telluride for Enhanced n-Type Thermoelectric Power Factor

Thermoelectrics directly converts waste heat into electricity and is considered a promising means of sustainable energy generation. While most of the recent advances in the enhancement of the thermoelectric figure of merit (ZT) resulted from a decrease in lattice thermal conductivity by nanostructuring, there have been very few attempts to enhance electrical transport properties, i.e., the power factor. Here we use nanochemistry to stabilize bulk bismuth telluride (Bi₂Te₃) that violates phase equilibrium, namely, phase-pure n-type K_0.06Bi₂Te_3.18. Incorporated potassium and tellurium in Bi₂Te₃ far exceed their solubility limit, inducing simultaneous increase in the electrical conductivity and the Seebeck coefficient along with decrease in the thermal conductivity. Consequently, a high power factor of ∼43 μW cm^-1 K^-2 and a high ZT > 1.1 at 323 K are achieved. Our current synthetic method can be used to produce a new family of materials with novel physical and chemical characteristics for various applications.

K4GeP4Se12: a case for phase-change nonlinear optical chalcogenide

We report on broadband nonlinear optical (NLO) responses from a phase-change chalcogenide compound K(4)GeP(4)Se(12). Its glassy phase exhibits unusual second-harmonic generation (SHG) due to the preservation of local crystallographic order. The SHG efficiency of the glassy form can be boosted by more than 2 orders of magnitude by simple heat treatment. Strong SHG and third-harmonic generation from both glassy and crystalline compounds were characterized over a wide wavelength range of 1.2-4.0 μm. Our results imply that K(4)GeP(4)Se(12) can be utilized for various NLO applications in the mid-infrared spectrum.

Effect of amiloride to retinal toxicity induced by tissue plasminogen activator

Purpose

The effects of amiloride on cellular toxicity caused by tissue plasminogen activator (tPA) in mouse primary retinal cells were investigated.

Methods

Primary retinal cell cultures were maintained using glial conditioned medium. Commercial tPA and L-arginine were added, and the level of cyclic guanosine monophosphate (cyclic-GMP) in the culture supernatant was assessed using an ELISA assay. We measured the cell viability of cultured retinal cells pretreated with three different concentrations of amiloride (1, 10, and 100 µm) in addition to commercial tPA or L-arginine treatment.

Results

After exposing the cultured mouse retinal cells to tPA plus L-arginine or L-arginine alone, cyclic-GMP concentrations were 61.9 ± 5.1 pmole/mL and 63.1 ± 6.1 pmole/mL, respectively. However, the control group had a significantly lower concentration of cyclic-GMP (37.2 ± 3.4 pmole/mL, p < 0.01). The cyclic GMP-dissolved solution did not cause retinal cell death. In the control group and the group treated with 1 µm amiloride and tPA containing L-arginine, the cell viability was 43.7% and 44.5%, respectively. However, cell viability increased to 70.6% with 10 µm amiloride and 78.4% with 100 µm amiloride (p = 0.015).

Conclusions

L-arginine increases intracellular cyclic-GMP and may give rise to retinal cells through this mechanism. In addition, amiloride in concentrations greater than 10 µm protects against L-arginine-induced retinal cell death.

All-solid-state dye-sensitized solar cells with high efficiency

Dye-sensitized solar cells based on titanium dioxide (TiO(2)) are promising low-cost alternatives to conventional solid-state photovoltaic devices based on materials such as Si, CdTe and CuIn(1-x)Ga(x)Se(2) (refs 1, 2). Despite offering relatively high conversion efficiencies for solar energy, typical dye-sensitized solar cells suffer from durability problems that result from their use of organic liquid electrolytes containing the iodide/tri-iodide redox couple, which causes serious problems such as electrode corrosion and electrolyte leakage. Replacements for iodine-based liquid electrolytes have been extensively studied, but the efficiencies of the resulting devices remain low. Here we show that the solution-processable p-type direct bandgap semiconductor CsSnI(3) can be used for hole conduction in lieu of a liquid electrolyte. The resulting solid-state dye-sensitized solar cells consist of CsSnI(2.95)F(0.05) doped with SnF(2), nanoporous TiO(2) and the dye N719, and show conversion efficiencies of up to 10.2 per cent (8.51 per cent with a mask). With a bandgap of 1.3 electronvolts, CsSnI(3) enhances visible light absorption on the red side of the spectrum to outperform the typical dye-sensitized solar cells in this spectral region.

CsSnI3: Semiconductor or metal? High electrical conductivity and strong near-infrared photoluminescence from a single material. High hole mobility and phase-transitions

CsSnI(3) is an unusual perovskite that undergoes complex displacive and reconstructive phase transitions and exhibits near-infrared emission at room temperature. Experimental and theoretical studies of CsSnI(3) have been limited by the lack of detailed crystal structure characterization and chemical instability. Here we describe the synthesis of pure polymorphic crystals, the preparation of large crack-/bubble-free ingots, the refined single-crystal structures, and temperature-dependent charge transport and optical properties of CsSnI(3), coupled with ab initio first-principles density functional theory (DFT) calculations. In situ temperature-dependent single-crystal and synchrotron powder X-ray diffraction studies reveal the origin of polymorphous phase transitions of CsSnI(3). The black orthorhombic form of CsSnI(3) demonstrates one of the largest volumetric thermal expansion coefficients for inorganic solids. Electrical conductivity, Hall effect, and thermopower measurements on it show p-type metallic behavior with low carrier density, despite the optical band gap of 1.3 eV. Hall effect measurements of the black orthorhombic perovskite phase of CsSnI(3) indicate that it is a p-type direct band gap semiconductor with carrier concentration at room temperature of ∼ 10(17) cm(-3) and a hole mobility of ∼585 cm(2) V(-1) s(-1). The hole mobility is one of the highest observed among p-type semiconductors with comparable band gaps. Its powders exhibit a strong room-temperature near-IR emission spectrum at 950 nm. Remarkably, the values of the electrical conductivity and photoluminescence intensity increase with heat treatment. The DFT calculations show that the screened-exchange local density approximation-derived band gap agrees well with the experimentally measured band gap. Calculations of the formation energy of defects strongly suggest that the electrical and light emission properties possibly result from Sn defects in the crystal structure, which arise intrinsically. Thus, although stoichiometric CsSnI(3) is a semiconductor, the material is prone to intrinsic defects associated with Sn vacancies. This creates highly mobile holes which cause the materials to appear metallic.