Size-dependent amorphization of nanoscale Y2O3 at high pressure.

Y2O3 with particle sizes ranging from 5 nm to 1 µm were studied at high pressure using x-ray diffraction and Raman spectroscopy techniques. Nanometer-sized Y2O3 particles are shown to be more stable than their bulk counterparts, and a grain size-dependent crystalline-amorphous transition was discovered in these materials. High-energy atomic pair distribution function measurements reveal that the amorphization is associated with the breakdown of the long-rang order of the YO6 octahedra, while the nearest-neighbor edge-shared octahedral linkages are preserved.

Novel red-emitting phosphor Li2MgZrO4:Mn4+,Ga3+ for warm white LEDs based on blue-emitting chip.

Li2MgZrO4:Mn4+,Ga3+ phosphors are synthesized via a conventional high-temperature solid-state reaction. The red emission of Mn4+ is enhanced 6.68 times and 13.0 times under 352 nm and 468 nm excitation when Ga3+ ions with a concentration of 40% are introduced into the phosphor. The thermal stability of the phosphor is also slightly improved by Ga3+ doping. It is believed that the GaZr dopants in the Li2MgZrO4 host not only stabilize Mn ions in the 4+ state, but also lower the symmetry of Mn4+ sites, resulting in the significant improvement of the luminescence performance, especially under blue excitation. The results might provide meaningful reference to seek high-performance Mn4+ doped oxide phosphors for W-LED applications based on a blue-emitting InGaN chip.

[The upconversion mechanisms for the red and green emissions in Er3+ doped and Er3+ -Yb3+ codopoed fluoride materials based on experiment].

The rare earth Er3+ doped and Er3+ -Yb3+ co-doped fluoride samples were prepared. Under 980 nm laser excitation, the intense red (650 nm) and green (545 nm) upconversion luminescence was obtained corresponding to the transitions 4S3/2, 2H11/2--4I15/2 and 4F9/2--4I15/2, respectively. For the Er3+ -Yb3+ co-doped fluoride samples, it was found that the ratio of the red upconversion luminescence to the green one decreases as the Er3+ concentration increases. The dependence of upconversion intensities on excitation power indicates that two-photon absorption processes are responsible for the upconverted luminescence. The possible upconversion mechanisms of the red and green emissions are discussed based on the energy-matching conditions and the quadratic dependence on excitation power.

[Direct sensitization up-conversion mechanism from Yb3+ to Ho3+ and Tm3+ in fluoride].

The direct sensitization up-conversion processes from Yb3+ to Ho3+ and Tm3+ in fluoride were studied. The properties of the up-converted spectra of Yb3+ /Ho3+ and Ho3+ /Yb3+ /Tm3+ co-doped fluorides pumped by a 980 nm laser were compared. Their up-conversion mechanism has been analyzed and discussed. Two strong up-conversion luminescence bands have been observed, which were the transitions of 5 F4 5 S2 (Ho3+) -->18 (Ho3+) in the Yb3+ /Ho3+ co-doped fluoride system and 3 H4 (Tm3+) --> 3 H6 (Tm3+) in the Ho+ /Yb3+ /Tm3+ co-doped fluoride system. The addition of Tm3+ into Yb3+ /Ho3+ co-doped fluoride decreases the up-converted emission intensities of Ho3+ in the visible region.

[Multiply upconversion emission in oxyfluoride ceramics].

Oxyfluoride ceramics with the host composition of SiO2 and PbF2 have been prepared. X-ray diffraction analysis of the ceramics revealed that fluoride type beta-PbF2 solid solution regions are precipitated in the glass matrix. Rare earth ions in the beta-PbF2 solid solution show highly efficient upconversion performance due to the very small multi-phonon relaxation rates. Eight upconversion emission bands whose central wavelength are 846, 803, 665, 549, 523, 487, 456 and 411 nm have been observed when the sample was excited with 930 nm diode light. Four possible energy transfer processes between Er3+ and Yb3+ cause the electronic population of high energy level of Er3+ and realize the abound upconversion luminescence bands.

[Upconversion luminescence in Ho3+ doped fluoride thin film with excitation red laser].

Ho3+ doped fluoride thin film was deposited on common glass matrix by low energy pulse laser (532 nm, quadratic harmonics of Nd: YAG laser). Red and green upconversion luminescences with central wavelength of 770 and 540 nm have been observed through Japan R-500 spectrometer with excitation of 632.8 nm He-Ne laser. These upconverted emissions are ascribed to transitions of 5S2-->5I7, 5F4-->5I7 and 5S2-->5I8 and they are realized by two step absorption of Ho3+ ions and energy transfer among Ho3+ ions.

[Luminescence properties and up-conversion mechanism of Er3+ -Yb3+ -Tm3+ co-doped CdF2: PbF2-based glass].

Under 980 nm excitation, we obtained five intense up-conversion luminescence bands in Er3+ -Yb3+ -Tm3+ co-doped CdF2: PbF2-based glass as follows: infrared (800 nm), red (645 nm), green (545 nm and 525 nm), blue (480 nm ) and violet (407 nm) up-conversion luminescence. With the addition of Tm3+ , the intensity of 480 nm blue luminescence increased notably compared with that of co-doped Yb3+ -Er3+ sample, which attributed to Tm3+ special energy level structure; logI-logP plot of the luminescence intensity versus pump power shows that 480 nm luminescence emission is a two-photon excitation process induced by the cooperative up-conversion between two Yb3+ ions and the slope of logI-logP plot decreases gradually with increasing pump power and shows a tendency to bend down. The up-conversion mechanism is analyzed in detail under 980 nm semiconductor laser excitation; the dependence of 480 nm up-conversion luminescence intensity on the pump power under the steady states is discussed by using rate equation and the results agree well with the experiments.

[The study of up-conversion fluorescence in Er3+:Yb3+ co-doped oxy-fluoride glasses].

Er3+:Yb3+ co-doped oxy-fluoride glasses with composition of (50-x) GeO2.PbF2.WO3.(6 + x) CdF2.1.4Yb2O3.0.6Er2O3 (x = 10,20,30) were prepared. Their up-conversion fluorescence characteristics were investigated under excitation of 930 nm diode laser. Three strong fluorescence bands of Er3+ whose central wavelength lie in 543, 550 and 655 nm have been observed. The maximum phonon energy is obtained by measuring anti-stokes Raman shifts of the samples, and the up-conversion mechanism is briefly discussed. The influences of host materials composition on up-conversion efficiency are also discussed adopting the average electronegative difference and the cationic field strength of the host. It is found that relative small average electronegative difference and large cationic field strength of host benefit to energy up-conversion.

Visible photon-avalanche upconversion in Ho3+ singly doped beta-Na(Y1.5Na0.5)F6 under 980 nm excitation.

Visible IR-to-green photon-avalanche upconversion is reported in an Ho3+ singly doped beta-Na(Y1.5Na0.5)F6 crystal under 980 nm excitation. Upconverted green, red, and IR emissions are observed at 540, 645, and 751 nm, respectively. Temporal evolution and excitation power dependent upconversion intensity are measured, suggesting that a photon-avalanche mechanism is responsible for the upconversion process. It is believed that an efficient cross relaxation (5S2,5I8)-->(5I6,5I6) mainly performs the population of 5I6 excited state, resulting in the intense photon-avalanche upconversion emission in the synthesized samples.