New Eu2+-activated alkaline-earth aluminate luminescent nanoribbons and nanowires
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Developing novel one-dimensional (1-D) luminescent nanostructures (e.g., nanowires and nanoribbons) is highly desired for continuous progress in nanophotonics and other emerging optical technologies. Previous studies on 1-D luminescent nanostructures were mostly focused on semiconductor materials whose light emissions are originated from the radiative recombination of electrons and holes via either intrinsic states or extrinsic defect states. In this work, we have developed two series of Eu2+-activated alkaline-earth aluminate luminescent 1-D nanostructures (including nanoribbons and nanowires) that have localized Eu2+ luminescent centers and exhibit new compositions, new crystal structures, and new luminescence properties and mechanisms. We first synthesized three kinds of quaternary Eu2+-activated strontium europium aluminate nanoribbons and nanowires/whiskers through a carbothermal reaction and vapor-phase deposition method. The three strontium europium aluminates are: blue luminescent Sr1-xEuxAl6O10 (0<x<1) with new compositions and a new tetragonal crystal structure, green luminescent Sr1-xEuxAl2O4 (0<x<1) with a known monoclinic crystal structure, and yellow luminescent Sr1-xEuxAl2O4 (0<x<1) with a new hexagonal crystal structure and extremely large band width and Stokes shift of emission. In each of the three strontium europium aluminates, Sr can be substituted with Eu over the full composition range without changing the crystal structures, and when Eu completely replaces Sr, three isostructural, fully concentrated europium aluminates, i.e., new tetragonal, blue luminescent EuAl6O10, monoclinic, green luminescent EuAl2O4, and new hexagonal, orange luminescent EuAl2O4, are obtained. In the tetragonal Sr1-xEuxAl6O10 (including the EuAl6O10) and monoclinic Sr1-xEuxAl2O4 (including the EuAl2O4), the blue and green emissions are not significantly changed with the variation of Eu concentration, however, the hexagonal Sr1-xEuxAl2O4 shows tunable emission color from green-yellow to orange-yellow with the increase of Eu concentration, and finally reaching orange with full Eu concentration, i.e., the hexagonal EuAl2O4. By using the similar carbothermal reaction and vapor-phase deposition, we also synthesized four kinds of Eu2+-activated barium europium aluminate nanoribbons and nanowires: blue luminescent Ba0.8Eu0.2Al6O10 nanoribbons with a new composition and a new tetragonal crystal structure (isostructural to the blue luminescent tetragonal Sr1-xEuxAl6O10 and EuAl6O10), green luminescent Ba0.95Eu0.05Al2O4 nanowires with a known hexagonal crystal structure, yellow luminescent Ba0.93Eu0.07Al2O4 nanowires with a new orthorhombic crystal structure, and red luminescent Ba0.6Eu0.4Al2O4 nanowires with a new hexagonal crystal structure (isostructural to the yellow luminescent hexagonal Sr1-xEuxAl2O4 but with different a, b, and c lengths). The strontium europium aluminate and barium europium aluminate nanoribbons and nanowires luminescent nanoribbons and nanowires have promising applications as nanometer-scale light generators and waveguides in nanophotonics. In addition, the yellow luminescent Ba0.93Eu0.07Al2O4 shows a very broad emission band with sufficient red component under blue light excitation, making it a very promising material for achieving warm-white light-emitting diodes (LEDs) with high color rendition through a single-phosphor, single-emitting-center-conversion model.