Zhitao Kang

Chemical reduction of three-dimensional silica micro-assemblies into microporous silicon replicas

The carbothermal reduction of silica into silicon requires the use of temperatures well above the silicon melting point (≥2,000 °C). Solid silicon has recently been generated directly from silica at much lower temperatures (≤850 °C) via electrochemical reduction in molten salts. However, the silicon products of such electrochemical reduction did not retain the microscale morphology of the starting silica reactants. Here we demonstrate a low-temperature (650 °C) magnesiothermic reduction process for converting three-dimensional nanostructured silica micro-assemblies into microporous nanocrystalline silicon replicas. The intricate nanostructured silica microshells (frustules) of diatoms (unicellular algae) were converted into co-continuous, nanocrystalline mixtures of silicon and magnesia by reaction with magnesium gas. Selective magnesia dissolution then yielded an interconnected network of silicon nanocrystals that retained the starting three-dimensional frustule morphology. The silicon replicas possessed a high specific surface area (>500 m2 g-1), and contained a significant population of micropores (≤20 Å). The silicon replicas were photoluminescent, and exhibited rapid changes in impedance upon exposure to gaseous nitric oxide (suggesting a possible application in microscale gas sensing). This process enables the syntheses of microporous nanocrystalline silicon micro-assemblies with multifarious three-dimensional shapes inherited from biological or synthetic silica templates for sensor, electronic, optical or biomedical applications.

CdTe quantum dots and polymer nanocomposites for x-ray scintillation and imaging.

Investigations are reported on the x-ray scintillation and imaging application of CdTe quantum dots (QDs) and their polymer nanocomposites. Aqueous CdTe QDs with emissions ranging between 510 and 680 nm were prepared and incorporated into polyvinyl alcohol or polymethyl methacrylate polymer matrices. The x-ray luminescent properties were evaluated and a resolution of 5 lines∕mm was obtained from the nanocomposite films. Additionally, the fast decay time, nonafterglow, and superior spectral match to conventional charge coupled devices, show that CdTe QD nanocomposites have high promise for x-ray imaging applications.

Synthesis of silicon quantum dot buried SiOx films with controlled luminescent properties for solid-state lighting

Highly luminescent Si quantum dot embedded SiOx films were studied as down-converting emitters for solid-state lighting applications. Strong red photoluminescence was observed from these Si nanocrystal embedded films prepared by thermal evaporation of SiO in vacuum or an O2 atmosphere followed by anneal at 1100 °C. The stoichiometry (1.0<x<1.9) and refractive indices (1.5–1.75) of these films could be well controlled by varying the oxygen flow rate and the deposition rate. The emission peak shifted from 840 to 745 nm with increasing O2 flow rate due to a decrease in the size of the Si QDs. Two excitation bands, peaked at 280 and 370 nm, were observed from these samples. The 370 nm band was much stronger than the 280 nm band, which is near the UV LED emission range required for solid-state lighting applications. Blue and green emissions were also observed from samples annealed at a lower temperature.

ZnTe:O phosphor development for x-ray imaging applications

An efficient ZnTe:O x-ray powder phosphor was prepared by a dry synthesis process using gaseous doping and etching medias. The x-ray luminescent properties were evaluated and compared to standard commercial phosphors exhibited an x-ray luminescent efficiency equivalent to 76% of Gd2O2S:Tb and an equal resolution of 2.5lines∕mm. In addition, the fast decay time, low afterglow, and superior spectral match to conventional charge-coupled devices-indicate that ZnTe:O is a very promising phosphor candidate for x-ray imaging applications.

Lanthanum halide nanoparticle scintillators for nuclear radiation detection

Nanoparticles with sizes <10 nm were fabricated and characterized for their nanocomposite radiation detector properties. This work investigated the properties of several nanostructured radiation scintillators, in order to determine the viability of using scintillators employing nanostructured lanthanum trifluoride. Preliminary results of this investigation are consistent with the idea that these materials have an intrinsic response to nuclear radiation that may be correlated to the energy of the incident radiation.

Analyses of factors affecting nickel ferrite nanoparticles synthesis in ultrasound-assisted aqueous solution ball milling.

Ball milling experiments were conducted with and without ultrasound wave assistance in deionized water using NiCO3·2Ni(OH)2·4H2O as raw materials. In the reaction process of NiFe2O4 prepared by ultrasound-assisted aqueous solution ball milling, some influencing factors including raw materials, ultrasonic frequency, ball to powder ratio and liquid level were changed. Samples were characterized by X-ray diffraction, fluorescence measurements and electroconductivity detections. The results indicate that more hydroxyl radicals and ions can be generated under the coupling effect of ultrasonic and ball milling. The fluorescence measurements and electroconductivity detections also reflect the reaction speed, allowing for optimal parameters to be determined.

Transparent oxyhalide glass and glass ceramics for gamma-ray detection

Nuclear radiation detection is continuously becoming more important for todays society. Conventional scintillator based gamma-ray detectors use single crystal materials such as NaI:Tl, LaBr3:Ce, which provide excellent radiation detection properties, but suffer from their environment-related fluctuation, high cost and size limitation. The incorporation of nanophosphors or quantum dots (QD) into a transparent host matrix has been studied recently as a cost-saving alternative that may solve the scalability and stability problems while still providing considerable optical performance. In this work, a new glass based detecting material with promising gamma-ray detection performance is reported. Transparent alumino-silicate glasses containing cerium-doped gadolinium halide nanocrystals were prepared by a melt-quench method followed by annealing to form nanocrystal precipitates. Samples were cast and polished for optical and radiation characterization. The preliminary results indicated a similar gamma-ray detection efficiency compared to a conventional NaI:Tl detector and a gamma-ray peak resolution of ~27% at 662 KeV from some of these samples. By controlling elemental composition and ratio of the in-situ precipitated nanoparticles, radiation detection performance is expected to be improved.

Enhancement of white luminescence from SiNx films by surface roughening

White photoluminescence was obtained from multi-layered silicon nitride thin films prepared by plasma-enhanced chemical vapor deposition. The emission colors from single-layered silicon nitride films could be adjusted over the range of 440–660 nm by controlling the SiH4/NH3 flow ratio during deposition. Surface roughening by anisotropic KOH etching of the Si(100) substrate significantly improved the emission extraction efficiency and changed the color-rendering properties from the silicon nitride thin films. This was attributed to the suppression of internal reflection and interference effects from the thin films.

Composition optimization of scintillating rare-earth nanocrystals in oxide glass-ceramics for radiation spectroscopy.

Glass-ceramic nanocomposites comprising GdBr₃/CeBr₃ loaded sodium-aluminosilicate glasses in which scintillating crystallites are precipitated in situ from a host glass matrix were studied. This materials system shows promise as an alternative to single-crystal scintillators, with potential to be fabricated into a wide variety of sizes, shapes, and compositions. Batch compositions containing 15-18 mol. % GdBr₃ and 3-4 mol. % CeBr₃ were prepared and analyzed for photoluminescent light yield. Light yield peaked with rare-earth content of 15 mol. % GdBr₃ and 4 mol. % CeBr₃. Preliminary ceramization studies on this composition found that the precipitated phase more closely matched a Gd₂O₃-CeO₂ mixture rather than the GdBr₃(Ce) that was targeted.

Nanocomposites for radiation sensing

The use of light emitting nanoparticles in polymer and glass matrices was studied for the detection of radiation. These nanocomposite scintillators were produced by various approaches including quantum dot/polymer, fluoride nanophosphor/epoxy and halide nanophosphor containing glass-ceramic composites. The synthesis and characterization of these nanoparticles as well as their incorporation into composites is discussed. Further, the application of these composites for radiation detection and spectroscopy is described.