Mu Gu

First-principles study of fluorine-doped zinc oxide

We present first-principles calculations for fluorine-doped zinc oxide (ZnO:F) by using density-functional theory. Under O-poor condition, fluorine substitution for oxygen (FO) is energetically favorable in ZnO. FO can effectively diminish oxygen vacancies. With high fluorine concentration, fluorine interstitial (Fi) may appear. The high transition energies of FO and Fi suggest that FO and Fi could act as deep donor and acceptor which cannot provide free carriers in ZnO at room temperature. The increase of carriers and mobility in ZnO:F could not contribute from deep donor FO, but may be due to the surface passivation effect of fluorine.

X-ray excited luminescence of cuprous iodide single crystals: On the nature of red luminescence

The x-ray excited luminescence spectra of different quality CuI crystals were measured. The prevailing blue luminescence and the unusual red luminescence were found at the same time. The relative intensity of these two luminescence peaks was different because of the change of defect concentration in crystals. By comparing the spectra of CuI crystals before and after annealing in vacuum, air or iodine vapors, the origin of the red luminescence in the as-grown crystals was ascribed to the presence of iodine vacancy, as confirmed by the energy dispersive x-ray analysis.

Enhanced luminescence through ion-doping-induced higher energy phonons in GdTaO4:Eu3+ phosphor

The photoluminescence enhancement effect of Li-, Zn-, or Al-doped GdTaO4:Eu3+ is investigated at 20 and 295 K. The underlying reason of luminescence enhancement by ion doping is revealed, and a mechanism of the enhancement based on the temperature effect is proposed. It can be safely concluded that the photoluminescence enhancement effect originates from temperature enhancement effect, which can be further strengthened by ion-doping-induced higher energy phonons, which are demonstrated by infrared transmittance spectra. This can reasonably explain why the photoluminescence is remarkably enhanced by ion-doping in GdTaO4:Eu3+ phosphor at 295 K rather than at 20 K.

Improved light extraction efficiency of cerium-doped lutetium-yttrium oxyorthosilicate scintillator by monolayers of periodic arrays of polystyrene spheres

In this Letter, monolayers of arrays of periodic polystyrene (PS) spheres are designed to couple onto the surface of cerium-doped lutetium-yttrium oxyorthosilicate scintillator to improve the light extraction efficiency. The enhancement of extraction efficiency up to 38% relative to the reference case without polystyrene spheres is achieved. Combining with the simulation for the transmission as well as its dispersion relation, detailed analysis of the effect of whispering gallery modes and diffraction on the extraction mechanism are given. As a result, the optimal diameter of 414 nm is obtained based on a trade-off between the transmission loss and the diffraction enhancement.

Formation energies of antisite defects in Y3Al5O12: A first-principles study

We perform the first-principles calculations for the formation energies of cation antisite defects in Y3Al5O12. This method provides precise values of formation energy and thus allows us to estimate the defect concentration. The calculations show that YAl,16a is the most predominant antisite defects at high temperature for the single crystal growth and its concentration significantly decreases at low temperature for the single-crystalline film preparation. The calculated defect concentrations are quantitatively accord with the experimental estimation. AlY has high formation energy even with excess Al2O3, which indicates AlY is energetically unfavorable and the defect process is not intrinsic but nonstoichiometry.

Enhanced light extraction of scintillator using large-area photonic crystal structures fabricated by soft-X-ray interference lithography

Soft-X-ray interference lithography is utilized in combination with atomic layer deposition to prepare photonic crystal structures on the surface of Bi4Ge3O12 (BGO) scintillator in order to extract the light otherwise trapped in the internal of scintillator due to total internal reflection. An enhancement with wavelength- and emergence angle-integration by 95.1% has been achieved. This method is advantageous to fabricate photonic crystal structures with large-area and high-index-contrast which enable a high-efficient coupling of evanescent field and the photonic crystal structures. Generally, the method demonstrated in this work is also suitable for many other light emitting devices where a large-area is required in the practical applications.

Improvement of light extraction of LYSO scintillator by using a combination of self-assembly of nanospheres and atomic layer deposition

The self-assembled monolayer periodic array of polystyrene spheres conformally coated with TiO₂ layer using atomic layer deposition is designed to obtain a further enhancement of light extraction for LYSO scintillator. The maximum enhancement is 149% for the sample with polystyrene spheres conformally coated with TiO₂ layer, while the enhancement is only 76% for the sample with only polystyrene spheres. Such further enhancement could be contributed from the additional modes forming by TiO₂ layer due to its high refractive index, which can be approved by the simulation of electric field distribution. The experimental results are agreement with the simulated results. Furthermore, the prepared structured layer exhibits an excellent combination with the surface of scintillator, which is in favor of the practical application. Therefore, it is safely concluded that the combination of self-assembly method and atomic layer deposition is a promising approach to obtain a significant enhancement of light extraction for a large area. This method can be extended to many other luminescent materials and devices.

Luminescent Properties of

LSO:Ce_x, Li_y (x = 0.002-0.016 and y = 0.01-0.08) phosphors were prepared by Pechini sol-gel chemistry, and the influence of Li⁺ ions on luminescence properties of LSO:Ce phosphors was discussed. It was exhibited that the optimal doping concentrations were at x = 0.006 and y = 0.02, and the crystallization temperature of pure LSO phase was reduced distinctly by Li⁺ codoping. It was found that photoluminescence intensity of LSO:Ce_0.006 > Li_0.02 phosphors was increased by factors of 2.2 in comparison to that of LSO:Ce_0.006 phosphors. Enhanced luminescence was regarded as the result of alteration of crystal field surrounding Ce³⁺ activator. Moreover, the energy transfer from Ce2 to Cel centers was induced by Li⁺ codoping.

{\rm Lu}_{2}{\rm SiO}_{5}{\hbox{:}}{\rm Ce}

For practical application of x-ray in-line phase contrast imaging, a high-quality image is essential for object perceptibility and quantitative imaging. The existing approach to improve image quality is limited by high cost and physical limitations of the acquisition hardware. A useful image restoration algorithm based on fast wavelet transform is proposed. It takes advantage of degradation model and extends the modulation transform function (MTF) compensation algorithm from Fourier domain to wavelet domain. The modified algorithm is evaluated through comparison with the conventional MTF compensation algorithm. Its deblurring property is also characterized with the evaluation parameters of image quality. The results demonstrate that the modified algorithm is fast and robust, and it can effectively restore both the lost detail and edge information while ringing artifacts are reduced.

Phosphors Codoped With Li Ions

An in-line phase-contrast computed tomography algorithm is proposed for the reconstruction of the three-dimensional distribution of the electron density of a weak absorption sample using micro-focus X-ray source. It exploits the natural combination of Feldkamp-Davis-Kress (FDK) cone-beam reconstruction algorithm with a phase retrieval algorithm based on a modified phase-attenuation duality. The effects of the coherence of X-rays and the revolution of the detector are also incorporated into this algorithm. Then an experimental demonstration is given and the results show that the proposed method can reconstruct the electron density of soft tissue effectively.