low-energy-cathodoluminescence-for-oxy-nitride-phosphors

Low-energy Cathodoluminescence for (Oxy)Nitride Phosphors

Nitride and oxynitride (Sialon) phosphors are good candidates for the ultraviolet and visible emission applications. High performance, good stability and ...

university of tsukuba

National Institute for Materials Science (NIMS)

Zinc oxide (ZnO) nanoparticles (NPs) are used in diverse applications ranging from paints and cosmetics to biomedicine and food. Although micron-sized ZnO is a traditional food supplement, ZnO NPs are an unknown public health risk because of their unique physicochemical properties. Herein, we studied the 13-week subchronic toxicity of ZnO NPs administered via the oral route according to Organization for Economic Cooperation and Development (OECD) test guideline 408. Well-dispersed ZnO NPs were administered to Sprague-Dawley (SD) rats (11/sex/group) at doses of 67.1, 134.2, 268.4 or 536.8 mg kg(-1) per body weight over a 13-week period. The mean body weight gain in males given 536.8 mg kg(-1) ZnO NPs was significantly lower than that of control male rats, whereas no significant differences were observed between the other treatment groups and the controls. Male and female rats dosed at 536.8 mg kg(-1) ZnO NPs had significant changes in anemia-related hematologic parameters. Mild to moderate pancreatitis also developed in both sexes dosed at 536.8 mg kg(-1) , whereas no histological changes were observed in the other treatment groups. To evaluate the mechanism of toxicity, we performed a bio-persistence study and evaluated the effects of the ZnO NPs on cell proliferation. The treatment of a human gastric adenocarcinoma cell line with ZnO NPs resulted in a significant inhibition of cellular proliferation. The anti-proliferative effect of ZnO NPs or Zn(2+) was effectively blocked by treatment with chelators. These results indicate that the bio-persistence of ZnO NPs after ingestion is key to their toxicity; the no-observed-adverse effect level (NOAEL) of ZnO NPs was found to be 268.4 mg kg(-1) per day for both sexes.

Rat pancreatitis produced by 13-week administration of zinc oxide nanoparticles: biopersistence of nanoparticles and possible solutions.

Major challenge in nanotechnology is to improve the solar cells efficiency. This can be achieved by controlling the silicon solar cell wafer surface structure. Herein, we report a KOH wet etching process along with an ultrasonic cleaning process to improve the surface texture of silicon solar cell wafers. We evaluated the KOH temperature, concentration, and ultra-sonication time. It was observed that the surface texture of the silicon solar wafer changed from a pyramid shape to a rectangular shape under edge cutting as the concentration of the KOH solution was increased. We controlled the etching time to avoid pattern damage and any further increase of the reflectance. The present study will be helpful for the mass processing of silicon solar cell wafers with improved reflectance.

Nanoetching process on silicon solar cell wafers during mass production for surface texture improvement.

A series of Eu(2+)- and Mn(2+)-codoped γ-AlON (Al1.7O2.1N0.3) phosphors was synthesized at 1800 °C under 0.5 MPa N2 by using the gas-pressure sintering method (GPS). Eu(2+) and Mn(2+) ions were proved to enter into γ-AlON host lattice by means of XRD, CL, and EDS measurements. Under 365 nm excitation, two emission peaks located at 472 and 517 nm, resulting from 4f(6)5d(1) → 4f(7) and (4)T1(4G) → (6)A1 electron transitions of Eu(2+) and Mn(2+), respectively, can be observed. Energy transfer from Eu(2+) to Mn(2+) was evidenced by directly observing appreciable overlap between the excitation spectrum of Mn(2+) and the emission spectrum of Eu(2+) as well as by the decreased decay time of Eu(2+) with increasing Mn(2+) concentration. The critical energy-transfer distance between Eu(2+) and Mn(2+) and the energy-transfer efficiency were also calculated. The mechanism of energy transfer was identified as a resonant type via a dipole-dipole mechanism. The external quantum efficiency was increased 7 times (from 7% for γ-AlON:Mn(2+) to 49% for γ-AlON:Mn(2+),Eu(2+) under 365 nm excitation), and color-tunable emissions from blue-green to green-yellow were also realized with the Eu(2+) → Mn(2+) energy transfer in γ-AlON.

Strong Energy-Transfer-Induced Enhancement of Luminescence Efficiency of Eu(2+)- and Mn(2+)-Codoped Gamma-AlON for Near-UV-LED-Pumped Solid State Lighting.

Graphene and related two-dimensional materials are promising candidates for atomically thin, flexible and transparent optoelectronics. In particular, the strong light-matter interaction in graphene has allowed for the development of state-of-the-art photodetectors, optical modulators and plasmonic devices. In addition, electrically biased graphene on SiO2 substrates can be used as a low-efficiency emitter in the mid-infrared range. However, emission in the visible range has remained elusive. Here, we report the observation of bright visible light emission from electrically biased suspended graphene devices. In these devices, heat transport is greatly reduced. Hot electrons (∼2,800 K) therefore become spatially localized at the centre of the graphene layer, resulting in a 1,000-fold enhancement in thermal radiation efficiency. Moreover, strong optical interference between the suspended graphene and substrate can be used to tune the emission spectrum. We also demonstrate the scalability of this technique by realizing arrays of chemical-vapour-deposited graphene light emitters. These results pave the way towards the realization of commercially viable large-scale, atomically thin, flexible and transparent light emitters and displays with low operation voltage and graphene-based on-chip ultrafast optical communications.

Bright visible light emission from graphene.

(GaN)1-x(ZnO)x solid-solution nanostructures with superior crystallinity, large surface areas and visible light absorption have been regarded as promising photocatalysts for overall water splitting to produce H2. In this work, we report the preparation of (GaN)1-x(ZnO)x solid-solution nanorods with a high ZnO solubility up to 95% via a two-step synthetic route, which starts from a sol-gel reaction and follows with a nitridation process. Moreover, we clearly demonstrated that the crystallographic facets of (GaN)1-x(ZnO)x solid-solution nanorods can be finely tailored from non-polar {10̄10} to semipolar {10̄11} and then finally to mixed {10̄1} and polar {000̄1} by carefully controlling the growth temperature and nitridation time. Correspondingly, the ZnO content in the GaN lattice can be achieved in the range of ∼25%-95%. Room-temperature cathodoluminescence (CL) measurements on the three types of (GaN)1-x(ZnO)x solid-solution nanorods indicate that the minimum band-gap of 2.46 eV of the solid-solution nanorods is achieved under a ZnO solubility of 25%. The efficiency and versatility of our strategy in the band-gap and facet engineering of (GaN)1-x(ZnO)x solid-solution nanorods will enhance their promising photocatalytic utilizations like an overall water splitting for H2 production under visible-light irradiation.

Solubility and crystallographic facet tailoring of (GaN)(1-x)(ZnO)(x) pseudobinary solid-solution nanostructures as promising photocatalysts.

A red phosphor of Sr2Si5N8:Eu(2+) powder was synthesized by a solid state reaction. The synthesized phosphor was thermally post-treated in an inert and reductive N2-H2 mixed-gas atmosphere at 300-1200 °C. The main phase of the resultant phosphor was identified as Sr2Si5N8. A passivation layer of ∼0.2 μm thickness was formed around the phosphor surface via thermal treatment. Moreover, two different luminescence centers of Eu(SrI) and Eu(SrII) in the synthesized Sr2Si5N8:Eu(2+) phosphor were proposed to be responsible for 620 nm and 670 nm emissions, respectively. More interestingly, thermal- and moisture-induced degradation of PL intensity was effectively reduced by the formation of a passivation layer around the phosphor surface, that is, the relative PL intensity recovered 99.8% of the initial intensity even after encountering thermal degradation; both moisture-induced degraded external and internal QEs were merely 1% of the initial QEs. The formed surface layer was concluded to primarily prevent the Eu(2+) activator from being oxidized, based on the systemic analysis of the mechanisms of thermal- and moisture-induced degradation.

Prevention of thermal- and moisture-induced degradation of the photoluminescence properties of the Sr2Si5N8:Eu(2+) red phosphor by thermal post-treatment in N2-H2.

Graphene was chemically aminated via cycloaddition. Aziridine-ring linkages were formed by covalently modifying the C-C double bonds in graphene. The aminated graphene presents an enhanced hydrophilicity, the contact angle with water decreases from 80.5 degrees to 58.5 degrees. And the conductivity of aminated graphene exhibits exponential decay as the reaction time increase. If the reaction time is 90 min, the resistance of aminated graphene was increased from -32 Ω to -2744 Ω. Because the amino group has good biocompatibility, the aminated graphene is designed for use as an enzyme sensor platform, such as glucose sensor based on glucose oxidase. The aminated graphene exhibited a good detection response for glucose. The increase in device current is about 12% in 1.2 mg/mL glucose solution.

Chemical Amination via Cycloaddition of Graphene for Use in a Glucose Sensor.

Silicon dioxide (SiO2) is one of the key materials in many modern technological applications such as in metal oxide semiconductor transistors, photovoltaic solar cells, pollution removal, and biomedicine. We report the accidental discovery of free-standing grassy silica nanoribbons directly grown on SiO2/Si platform which is commonly used for field-effect transistors fabrication without other precursor. We investigate the formation mechanism of this novel silica nanostructure that has not been previously documented. The silica nanoribbons are flexible and can be manipulated by electron-beam. The silica nanoribbons exhibit strong blue emission at about 467 nm, together with UV and red emissions as investigated by cathodoluminescence technique. The origins of the luminescence are attributed to various defects in the silica nanoribbons; and the intensity change of the blue emission and green emission at about 550 nm is discussed in the frame of the defect density. Our study may lead to rational design of the new silica-based materials for a wide range of applications.

Yujin Cho

Graduate School of Pure and Applied Science

university of tsukuba

Low-energy Cathodoluminescence for (Oxy)Nitride Phosphors