Luminescence from Sol-Gel-Derived Europium-Doped Films Confined in Mesoporous Anodic Alumina

Abstract

Photoluminescence ͑PL͒ properties of the structure porous anodic alumina ͑PAA͒/europium-doped xerogel film have been investigated with respect to the xerogel content (TiO 2 , Al 2 O 3 , In 2 O 3 ), its transmittance and PL excitation wavelength. Three mechanisms of lanthanide optical excitation in the structure are proposed: ͑i͒ excitation through the PAA matrix; (ii) excitation through the xerogel matrix that depends on its transparency and, in turn, on xerogel content and thickness within the porous morphology; and (iii) direct excitation of a lanthanide ion due to the relative transparency of both the PAA and xerogel films. It is speculated that exploiting PAA allows significant change in the excitation conditions of lanthanides, probably due to multiple scattering of the exciting light, which is the main reason for the reported PL enhancement of lanthanides from the structure compared with similar PAA/lanthanide-doped xerogel films fabricated on flat substrates.The sol-gel method attracts much attention as an alternative low cost technology for fabrication of a variety of materials. The applications of sol-gel synthesis vary from low temperature methods for fabrication of glasses to creation of optical waveguides and laser sources. The process allows fabrication of homogeneous inorganic oxide materials with desirable properties, such as optical transparence, porosity, hardness, and chemical and thermal resistance. The sol-gel reaction involves the evolution of a colloidal suspension ͑sol͒ to a network in a continuous liquid phase ͑gel͒ that is transformed into a glass-like solid ͑xerogel͒ after gel syneresis due to removal of solvents, drying and high temperature annealing. 1 One of the main advantages of sol-gel technology is the possibility of allowing most chemical elements to be introduced into the initial sol composition. Introduction of lanthanide salts into sols allows fabrication of efficient luminescent materials with high thermal stability and low temperature quenching. Particularly, PL of Eu, Er, and Tb from sol-gel-derived films has been reported over the last decade. 2-4 Further, it has been revealed that Er, Tb, and Eu-related PL from sol-gel films is strongly enhanced when the method is applied to mesoporous silicon 5,6 and mesoporous anodic alumina. 7-9 Among the mesoporous matrices utilized, including porous silicon, porous anodic alumina ͑PAA͒ and artificial opals, 10 PAA is the most promising template for fabrication of sol-gel derived films. This is due to its self-organized, reproducible morphology, low cost, and its ability to be patterned photolithographically using typical microelectronics processes. 11 The morphology of PAA comprises an array of hexagonal cells of anodic alumina, each containing a cylindrical pore. 12 Fabrication is through electrochemical anodizing of aluminum, generally in inorganic acid solutions. PAA is optically transparent in the visible to IR range, and it has high mechanical and thermal durability. By selecting the anodizing conditions, the appropriate geometr...

Energy Transfer Between Nanocrystalline Host and Eu[sup 3+] Ions in GaN:Eu[sup 3+] Powders

“…13,14 This value is similar to the position of the emission band of Eu 3+ ions in the oxide matrices at about 610 nm. 15 Thus, the obtained value is mostly due to the fact that in our case probably there are at least two different sites for the Eu 3+ ions, and the obtained signal is a superposition of the emission from the different emission centers, one related to Ga intersitials of Eu 3+ surrounded by N atoms ͑at ϳ622 nm͒ and the second related to oxygen surrounding the Eu 3+ ions. The later sites are rather related with the oxygen at the nanocrystalline surface, because the XRD results 9 do not show any additional Ga, N-oxide phase but only pure GaN crystalline phase.…”

Influence of Europium Concentration on Optical and Structural Properties of Nanocrystalline GaN:Eu[sup 3+] Powder

“…28 This suggests that some of the Eu ions in nanograins have oxygen neighbors and most probably are placed at the oxygen rich sites at the surface because the XRD results do not show any additional Ga, N-oxide phase but only pure GaN crystalline phase. A similar conclusion has been given based on the excitation properties of Eu ions and discussed in our previous paper.…”

Luminescence of Lanthanides from Xerogels Embedded in Mesoporous Matrices

“…High-transparency of anodic alumina oxide on the one hand and strong optical anisotropy on the other hand may be the reasons of growing interest to exploit this material as a cage for light-emitting species. Filling the channels of the pores of anodic alumina films with silica [36][37][38], titania [23,[39][40][41][42], alumina [43][44][45] xerogels doped with lanthanide ions Er [37][38][39], Tb [23,38,44] and Eu [35,[40][41][42]45], organic dyes [46], semiconductor nanocrystals [47,48], polymers [ 49,50], quantum dots [51,52], and biological samples [53] was investigated, and strong luminescence of optically active species incorporated in voids of anodic alumina or, probably, in its walls [54,55] was reported.…”

“…To investigate the spatial distribution of xerogel deposited onto PAA by spinning followed by thermal treatment, selected samples containing terbium-doped titania and alumina xerogels were examined by SIMS [37][38][39], transmission electron microscopy (TEM), SEM and energy distribution of X-ray (EDX) analyses [23,40,41]. After the first spin-on deposition of a sol followed by drying transmission electron micrographs revealed relatively dark, fine textured material of amorphous appearance at the base of the pore.…”

“…Because of the low efficiency of Er PL, fabrication of one spin-on layer is not sufficient for registration 1.54 µm light emission [8,22]. Generally, PL of lanthanides from the structure xerogel/PAA increases with the thickness of PAA and for some excitation wave- length increases with the number of xerogel layers inside the pores [41,56], and even with the chosen concentration of lanthanides in xerogels [23,42,59]. Con-centration quenching of lanthanides in xerogels embedded in mesoporous matrices has not been detected.…”