In this work, the structures, stabilities and electronic properties of 25 possible two‐dimensional (2D) binary P0.5As0.5 alloys were systematically investigated. We found that α‐ and β‐phase are the two most favourable lattice configurations for atomic positional fluctuation. Moreover, we show that the predicted structure IV is slightly more stable than the previously proposed structure III for elementary distribution. Our results indicate that atomic positional fluctuation and elementary distribution are two effective routes to modulating the electronic properties of the 2D P0.5As0.5 alloy with direct or indirect band‐gaps. New ground states for 2D binary P0.5As0.5 alloy.

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The upconversion luminescences of YAlO3:Er 3+ phosphor co-doping with different Gd 3+ concentrations are investigated under the excitation of 980-and 532-nm diode lasers. A near ultraviolet upconversion emission at 410 nm is observed in YAlO3 under 532-nm excitation. Moreover, the inactive Gd 3+ ions can improve the upconversion intensity efficiently in a certain range of concentration. Under 980-nm excitation, the visible upconversion emissions at 546 and 646 nm are enhanced by about 10 and 8 times at the Gd 3+ concentration of 40%, respectively. The upconversion emission at 410 nm under 532-nm excitation is also enhanced by 7 times. The substitution of Gd 3+ ions for Y 3+ sites changes the local symmetry of Er 3+ , leading to the improvement of upconversion efficiency.OCIS codes: 160.5690, 160.4670, 160.4760. doi: 10.3788/COL201210.081602.In the last several decades, the upconversion luminescence of rare-earth-ion-doped materials has been investigated extensively due to their wide applications in solid-state lasers, flat panel displays, biological labeling, and so on [1−3] . Er 3+ has been reported as one of the most popular and efficient ions for upconversion [4−8] . It can provide several intermediate levels with long life time, which can be directly pumped by several diode lights [9−14] . Recently, upconversion phenomena in Er 3+ -doped YAlO 3 crystals have been widely studied because of their high mechanical hardness and considerable thermal conductivity, among others. Several authors have reported upconversion luminescence in Er 3+ -doped YAlO 3 under 518-, 542.4-, 548.9-, 652.2-, and 980-nm excitations [10,15−17] . Therefore, it is very important to improve their upconversion efficiency. There are many factors affecting the upconversion efficiency, such as the local environment, the dopant concentration, and the distribution of active ions in host materials [18] . A recent study has reported that the luminescence intensity can be enhanced by changing the local environment of luminescent centers, which can be performed by doping some suitable inactive ions in the host. , xGd 3+ (x=0, 0.15, 0.25, 0.4, 0.5, and 0.7) was prepared through a solution combustion synthesis procedure. An aqueous solution containing citric acid, Y(NO 3 ) 3 , Er(NO 3 ) 3 , and Gd(NO 3 ) 3 was used to synthesize the Er 3+ and Gd 3+ co-doped YAlO 3 powders. A citric acid-to-metal nitrate molar ratio of 1.5:1 was employed to prepare the precursor solution. After the combustion, the precursor was calcined at 1 100• C for 2 h. The powders were characterized by X-ray diffraction (XRD) on a Bruker D8 advanced equipment (x) using Cu tube with K α radiation of 0.15406 nm in the 2θ range of 20• -60• . The morphology of the prepared powders was observed by scanning electron microscopy (SEM) using JSM-6610. Upconversion luminescence spectra excited by 980-and 532-nm lasers were obtained in the spectrophotometer (R-500, Japan Spectroscopic Co., Japan). All the measurements were performed at room temperature. Figure 1 shows the representat...

The influence of annealing temperature on the photo-luminescence (PL) of high co ncentration Er/Yb co-doped Al２O３ films was studied. The relationship betwe en PL spectra and microstructure of the films at various annealing temperatures was revealed by analyzing the dependence of the intensity and the full width at half-maximum of the PL spectra on the annealing temperature. The PL measurement shows that the annealing behavior may be split into three different regimes. Bel ow 750℃, the intensity increases with the increase of annealing temperature wit h a small slope, which corresponds to the amorphous Al２O３ films. Between 800℃ and 900℃, the PL intensity increases considerably, where the microstructure of the films was identified to be composed of γ-Al２O３ grains in nano-meter scale; At the temperature of 1000℃, the intensity of PL spectrum decre ases to a very low level, γ-Al２O３ grains and the segreg ation of Er2O ３ and Yb2O３ phases were observed by using transmis sion electron microscopy . Furthermore, the dependence of PL spectrum shape on the PL intensity of sub-le vel transition was analyzed and discussed by fitting the PL spectra at various t emperatures．

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