Shiming Huang

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.

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.

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.

Improving image quality of x-ray in-line phase contrast imaging using an image restoration method.

CsI(Tl) scintillation films with columnar structure are now widely used for X-ray imaging. In this work, CsI(Tl) films were manufactured on glass substrates by the thermal deposition method. The influence of deposition conditions on the microstructure and crystalline property of the films was studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The films showed only (200) plane preferred orientation when deposited at low vacuum. The light outputs were evaluated. The films with only one preferred orientation showed better luminescence performance than that with several crystal planes. In addition, the spatial resolution of samples was given in terms of modulation transfer function (MTF).

Polychromatic X-ray in-line phase-contrast tomography for soft tissue

Transparent, Crack-free and dense M′-type LuTaO4:Ln3+ (Ln = Eu, Tb) scintillator films have been successfully prepared by Pechini sol-gel technique. The structure, morphology, photoluminescence and X-ray excited luminescence (XEL) of the films were investigated in detail. The optical band gap of the LuTaO4 thin film was experimentally determined for the first time. Eu3+ or Tb3+ activated M′-type LuTaO4 thin films exhibited intensive 5D0→7F2 or 5D4→7F5 emission, corresponding to the decay time of 1.08 ms or 1.10 ms, respectively. It was demonstrated that the sol-gel derived M′-type LuTaO4:Ln3+ (Ln = Eu, Tb) scintillator films have a superior XEL performance, which indicates that this new type of scintillator film is expected to be a promising candidate for applications in high-spatial-resolution X-ray imaging.

Fabrication and Performance of Columnar CsI(Tl) Scintillation Films With Single Preferred Orientation

Ga- and In-doped ZnO nanorods were prepared by low temperature hydrothermal method, and their structure, morphology and luminescence properties were investigated through X-ray diffraction, scanning electron microscopy, photoluminescence and X-ray excited fluorescence spectrometers. The results indicated that the nanorods had a hexagonal wurtzite structure, and the average diameter and length of the nanorod were about 1 μm and 10 μm, respectively. Ultraviolet and X-ray excited luminescence spectra of the nanorods show a relatively strong visible emission peak located around 450-750 nm. The In-doped ZnO nanorods exhibited a superior X-ray excited luminescence performance, which implies that it is expected to be a promising candidate for applications in high-spatial-resolution and fast X-ray imaging detector.