Modulation of Remote Epitaxial Heterointerface by Graphene-Assisted Attenuative Charge Transfer.

Remote epitaxy (RE), substrate polarity can "penetrate" two-dimensional materials (2DMs) and act on the epi-layer, showing a prospective universal growth strategy. However, essentially, the role that 2DMs plays in RE has not been deeply investigated so far. Here, the RE of single-crystal films on the weakest polarity/iconicity substrate is realized to reveal its essence physical properties. Graphene facilitates attenuative charge transfer (ACT) from a substrate to epi-layer to construct remote interactions. Interfacial atoms are assembled into "incommensurate" epitaxial relationships through graphene to reduce misfit dislocations in the epi-layer. Moreover, graphene reduces the atomic migration barrier, leading to a tendency toward a "layer-by-layer" growth mode. Such film growth mode is different with the conventional epitaxy (CE), and it is beneficial for the fast growth of epi-layers and the reduction of dislocations at coalescence boundaries. The insightful revelation of the role of graphene reveals the interface physics of RE and provides a more valuable guide to using 2DMs to expand three-dimensional materials (3DMs) for application in devices.

Ca-α-SiAlON:Eu Phosphors: Oxidation States, Energy Transfer, and Emission Enhancement by Incorporation-Aimed Surface Engineering.

Ca-α-SiAlON:Eu is one of the most promising phosphors exhibiting potential applications in high power LEDs owing to its excellent thermal stability and relatively high luminescence efficiency comparable to YAG:Ce commercial phosphors. The oxidation states of Eu ions in Ca-α-SiAlON:Eu are still questionable, and furthermore it exhibits lower luminescence intensity than commercial yellow phosphors. Therefore, in the present work, the valences of Eu ions in Ca-α-SiAlON have been examined, from which a mixture of divalence and trivalence is observed. Further improvement in emission intensity involves finding a way to increase the incorporation of larger Eu ions into the Ca-α-SiAlON. Here, we observed preferred doping of Eu around phosphor grain surface and then propose a surface engineering strategy involving HF pickling of glassy surface layer, formation of Eu-rich precursors, and finally post-annealing, aiming to increase surface incorporation of Eu ions. The surface engineered samples exhibit great enhancement in emission intensity with a maximum increment by 80%.

Dark Matter Results from First 98.7 Days of Data from the PandaX-II Experiment.

We report the weakly interacting massive particle (WIMP) dark matter search results using the first physics-run data of the PandaX-II 500 kg liquid xenon dual-phase time-projection chamber, operating at the China JinPing underground laboratory. No dark matter candidate is identified above background. In combination with the data set during the commissioning run, with a total exposure of 3.3×10^{4} kg day, the most stringent limit to the spin-independent interaction between the ordinary and WIMP dark matter is set for a range of dark matter mass between 5 and 1000 GeV/c^{2}. The best upper limit on the scattering cross section is found 2.5×10^{-46} cm^{2} for the WIMP mass 40 GeV/c^{2} at 90% confidence level.

Multiple doping structures of the rare-earth atoms in ß-SiAlON:Ce phosphors and their effects on luminescence properties.

The critical doping structures of rare-earth atoms in the promising ß-SiAlON phosphors have long been argued owing to the lack of direct evidence. Here, the exact locations and coordination of the Ce rare-earth atoms in the ß-SiAlON structure have been examined using an atom-resolved Cs-corrected scanning transmission electron microscope. Three different occupation sites for the Ce atoms have been directly observed: two of them are in the structural channel coordinated with six and nine N(O) atoms, respectively; the other one is the unexpected substitution site for Si(Al). The chemical valences and stabilities of the doping Ce ions at the different occupation sites have been evaluated using density functional calculations. Correlation of the different doping structures with the luminescence properties has been investigated by the aid of cathodoluminescence (CL) microanalysis, which verifies the different contribution of the interstitial trivalent Ce ions to the light emission while no luminescence is observed for the substitutional doping of quadrivalent Ce.

Effects of gamma-irradiation and air annealing on Yb-doped Y3Al5O12 single crystal.

The effects of gamma-irradiation on the air-annealed 10at.% Yb:Y(3)Al(5)O(12) (YAG) and air annealing on the gamma-irradiated 10at.% Yb:YAG have been studied by the difference absorption spectra before and after treatment. The gamma-irradiation and air annealing led to opposite changes of the absorption properties of the Yb:YAG crystal. After air annealing, the gamma-irradiation induced centers were totally removed and the concentration of Fe(3+) and Yb(3+) were lightly increased. For the first time, the gamma-irradiation induced valence changes between Yb(3+) and Yb(2+) ions in Yb:YAG crystals have been observed.

Effects of Yb concentration on the fluorescence spectra of Yb-doped YAlO3 single crystals.

0.5 at.% Yb:YAlO(3)(YAP), 5 at.% Yb:YAP and 15 at.% Yb:YAP were grown using the Czochralski method. Their absorption and fluorescence spectra were measured at room temperature and their emission line shape was calculated using the method of reciprocity. It was observed that the fluorescence spectra changed appreciably with the increasing of Yb concentration. For 0.5 at.% Yb:YAP, the line shape of fluorescence is very similar with the calculated emission line shape; with the increasing of Yb doping concentration, the line shape of fluorescence is very different from the calculated emission line shape. These phenomena are caused by the strong self-absorption at 979 and 999 nm for Yb:YAP.

Effects of Yb concentration on the spectroscopic properties of Yb: Y3Al5O12.

Yb:YAG single crystals with Yb doping concentration 5.4, 16.3, 27.1, 53.6, and 100 at.% were grown by the Czochralski process. The effects of Yb concentration on the absorption spectra (190-1100 nm), fluorescence spectra under 940 nm and X-ray excitation were studied. The concentration quenching of fluorescence was observed when the Yb doping concentration reaches to as high as 27.1at.% for Yb:YAG. Under 940 nm excitation, the influence of the self-absorption at 969 and 1029 nm on the fluorescence spectra is not evident when the Yb doping concentration is as high as 27.1at.%. However, it can greatly change the shape of fluorescence spectra of Yb:YAG when the Yb doping concentration reaches to above 53.6 at.%.