Luminescence of Nanostructured Eu 3+ /ZnO Mixed Films Prepared by Electrodeposition T. Pauporte ´ ,* ,² F. Pelle ´ , ‡ B. Viana, ‡ and P. Aschehoug ‡ Laboratoire d’Electrochimie et de Chimie Analytique, UMR 7575 CNRS, and Laboratoire de Chimie de la Matie ` re Condense ´ e de Paris, UMR 7574 CNRS, E Ä cole Nationale Supe ´ rieure de Chimie de Paris, UniVersite ´ Paris 6, 11 rue P. et M. Curie, 75231 Paris cedex 05, France ReceiVed: June 20, 2007; In Final Form: August 9, 2007 ZnO/Eu mixed films have been prepared by electrochemical precipitation at an electrode surface. The films present a composite nanostructure with rod-shaped columns of Eu 3+ -doped ZnO surrounded by a Eu/ZnO mixed basal layer. The fraction of basal layer increases rapidly with europium concentration in the deposition bath. The contribution of each component on layer photoluminescence has been distinguished by studying separately rod-rich and basal-layer-rich films. The films have been studied before and after annealing treatments in various atmospheres. Under UV light excitation, two photoluminescent ZnO emissions are observed: the exciton emission in the UV and a visible emission which is cancelled after a heating treatment at 400 °C in air. The sharp 4f-4f transition emissions of Eu 3+ can be directly excited at 464 nm and are intense after an annealing treatment at 400 or 800 °C in air. A lifetime of 450 μs is measured for the 5 D 0 emitting level. Eu 3+ emission is also observed under ZnO excitation below 380 nm, but after film annealing, when defect effects are minimized. 1. Introduction Zinc oxide has potential applications in many fields due to its interesting optical, electronic, and mechanical properties. ZnO is an n-type semiconductor with a wide band gap of 3.37 eV and a high exciton binding energy of 60 meV. 1 It is a promising candidate for high-stability, room-temperature UV luminescent or lasing devices. 2-4 ZnO also presents a broad green emission at around 520 nm which has been notably identified as interesting for field emission display (FED) phosphor because of its high efficiency at low-voltage operation. 5 The green photoluminescence (PL) under UV excitation below 380 nm is classically assigned to the presence of oxygen vacancies 6,7 and is then very sensitive to the preparation method of the oxide. For light-emitting device applications such as FED or plasma display panels (PDP), it would be greatly beneficial to control and enlarge the palette of the colors emitted by ZnO due to its outstanding stability. 8 One may take advantage of the energy transfer between ZnO and trivalent lanthanides (Ln 3+ ) which are well-known to present sharp and intense emission peaks involving 4f-4f transitions. The application aims require the doping of ZnO matrix with different Ln 3+ . Among lanthanides, Eu 3+ is an interesting candidate due to its strong red emission at about 610-620 nm. The preparation of zinc oxide/europium- (III) pellets, 9-14 (nano)particles, 15-18 or powders 19,20 has been reported in the literature. Solid-state chemistry was performed by pressing and sintering both components mixed together. 9-14 Another route for an intimate mixing of zinc oxide and europium was the use of solutions as done by spray pyrolysis, 15,16 by coprecipitation (microemulsions, 17,18 Pechini method 19,20 ), and by sol-gel synthesis. 13 Recently, europium implantation in ZnO was reported. 22 In some cases lithium ion was used as a codopant. 11,14 To our best knowledge, the direct preparation of red-emitting ZnO/Eu thin films has attracted much less atten- tion. 13,20 For advanced display and lighting applications, an efficient energy transfer between the ZnO matrix and Eu 3+ light emission center is an important requirement. In the present paper, we report an original method for the direct preparation of nanostructured Eu 3+ /ZnO mixed thin films. The effects of film composition and thermal annealing at different temperatures and in different atmospheres on ZnO and Eu 3+ luminescences are thoroughly investigated. After a thermal treatment at 400 or 800 °C in air or argon, Eu 3+ red emission is markedly activated. Eu 3+ can be indirectly excited after charge generation in ZnO under UV illumination of the film. 2. Experimental Section Electrodeposition was carried out in a three-electrode cell. The counter electrode was a zinc wire, and the reference electrode was a saturated calomel electrode (SCE) (with a potential at +0.25 V vs NHE) placed in a separate compartment maintained at room temperature. The deposits were prepared on F:SnO 2 -coated Si substrates. The substrates were cleaned under ultrasonics, 5 min in acetone, 5 min in ethanol, and 2 min in 45% nitric acid. To ensure a deposition as homoge- neous as possible, the substrate was fixed to a rotating electrode and the deposition was performed at a constant rotation speed of 300 rotations per min (rpm). The deposition bath contained 5 mM ZnCl 2 (Merck, reagent grade), 0.1 M KCl (Merck, reagent grade), and 0.6 or 1.2 μM EuCl 3 (EuCl 3 ·4H 2 O, Alfa Aesar, 99.9%) and was prepared with MilliQ quality water. It was saturated with molecular oxygen, and a slight O 2 bubbling was maintained during the deposition process. The bath temperature was kept constant at 70 °C. A constant potential of -1 V versus SCE was applied during 15 min. The films were annealed 1 h at 400 °C or 5 h at 800 °C. The samples were heated at a scan rate of 3 °C per min. Various annealing atmospheres were tested, namely, air, argon, and forming gas (90% Ar/10% H 2 ). It was observed that, after the * Author to whom correspondence should be addressed. E-mail: thierry- pauporte@enscp.fr. ² Laboratoire d’Electrochimie et de Chimie Analytique. ‡ Laboratoire de Chimie de la Matie `re Condense ´e de Paris. 15427 J. Phys. Chem. C 2007, 111, 15427-15432 10.1021/jp0747860 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/03/2007