J. Phys. Chem. C 2007, 111, 15427-15432
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Luminescence of Nanostructured Eu3+/ZnO Mixed Films Prepared by Electrodeposition T. Pauporte´ ,*,† 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, EÄ 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 Eu3+-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 Eu3+ 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 5D0 emitting level. Eu3+ 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 vacancies6,7 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 (Ln3+) 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 Ln3+. Among lanthanides, Eu3+ 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 powders19,20 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 method19,20), 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 * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Laboratoire d’Electrochimie et de Chimie Analytique. ‡ Laboratoire de Chimie de la Matie ` re Condense´e de Paris.
red-emitting ZnO/Eu thin films has attracted much less attention.13,20 For advanced display and lighting applications, an efficient energy transfer between the ZnO matrix and Eu3+ light emission center is an important requirement. In the present paper, we report an original method for the direct preparation of nanostructured Eu3+/ZnO mixed thin films. The effects of film composition and thermal annealing at different temperatures and in different atmospheres on ZnO and Eu3+ luminescences are thoroughly investigated. After a thermal treatment at 400 or 800 °C in air or argon, Eu3+ red emission is markedly activated. Eu3+ 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:SnO2-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 homogeneous 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 ZnCl2 (Merck, reagent grade), 0.1 M KCl (Merck, reagent grade), and 0.6 or 1.2 µM EuCl3 (EuCl3·4H2O, Alfa Aesar, 99.9%) and was prepared with MilliQ quality water. It was saturated with molecular oxygen, and a slight O2 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% H2). It was observed that, after the
10.1021/jp0747860 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/03/2007
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annealing treatment at 800 °C in forming gas, ZnO was almost fully evaporated and no significant luminescence was measured. The highest luminescence intensities were measured after annealing in air; therefore, hereinafter we mainly discuss the effect of this treatment. The films were observed with a scanning electron microscope (SEM) (Leica stereoscan 440). Their compositions were determined by energy dispersive X-ray spectroscopy (EDS) (Voyager 1 model from Tracor-Noran). The zinc deposit content was determined at the K line and the europium content at the L line. The global film compositions were also checked by inductively coupled plasma-atomic emission spectrometry (ICP-AES) titration after film dissolution in a 10% HCl solution. It was found that the films deposited from 0.6 mM EuCl3 contained about 4.3 atom % of Eu (defined as Eu atom %/(Eu atom % + Zn atom %)), whereas those deposited from 1.2 mM EuCl3 contained about 50 atom % of Eu and 50 atom % of Zn. X-ray diffraction (XRD) experiments were carried out with a Siemens D5000 type diffractometer, using the Co KR1 radiation (λ ) 1.7889 Å) and a back graphite monochromator. The diffraction pattern was scanned by steps of 0.02° (2θ) between 32° and 60°. The PL measurements were carried out at room temperature (RT). The excitation at 464 nm was provided by an O.P.O. pumped with the third-harmonic of an yttrium-aluminumgarnet (YAG:Nd) laser (10 Hz, 10 ns, Thales). The excitation source at 266 nm was provided by the YAG:Nd quadrupled frequency. The emission was analyzed using an HR250 monochromator (Jobin-Yvon) coupled with a UV-enhanced intensified charge coupled device (ICCD) (Princeton Instrument). Under pulsed laser excitation, luminescence spectra were recorded either in a gate mode with a delay of 50 ns and a measurement width of 100 µs or in pseudo-CW mode with a continuous integration of the intensity during 300 ms corresponding to three full illumination pulses. The excitation power and the geometrical arrangement of the experiment was the same for the various samples investigated; therefore, for a given gate delay and excitation wavelength the spectra of different samples before and after annealing in various atmospheres could be compared for shape and intensity. 3. Results and Discussion 3.1. Structure and Composition of the Eu/ZnO Films. The film deposition process is based on the local interfacial pH change in the vicinity of the electrode by means of the cathodic reduction of molecular oxygen.23 In the presence of Zn2+ in solution, ZnO is precipitated at the electrode according to the reaction
Zn2+ + 2OH- f ZnO + H2O Eu(OH)3 is more stable thermodynamically than Eu2O324 and can also be obtained by precipitation:25
Eu3+ + 3OH- f Eu(OH)3 Films were prepared from solutions containing 5 mM ZnCl2 and various concentrations of EuCl3. They were made of pure ZnO for EuCl3 concentrations lower than 0.55 mM. Above this threshold, Eu was detected in the layer. Figure 1 shows SEM views of films prepared by this coprecipitation process and annealed at 800 °C in air. The film morphology was identical before annealing, and no morphological evolution was observed with the heating treatment. Figure 1a shows that the films
Figure 1. (a) SEM view of a Eu3+/ZnO film prepared with 0.6 mM EuCl3 and annealed at 800 °C (A-film). (b) Schematic cross-sectional view of film in (a). (c) Same as (a); film prepared with 1.2 mM EuCl3 (B-film).
prepared from 0.6 mM EuCl3 (noted as A-films) present two distinct phases. The electrode is covered by grains with a cylindrical rodlike shape. The ZnO hexagonal facets which are observed on electrodeposited pure zinc oxide26 are not found here. The mean rod size is about 500 nm in width and 1.2-1.4 µm in length. The grains are surrounded at their bottom by a covering thin basal layer (b-layer). The rods were analyzed by EDS as made of ZnO with a low Eu content (1-2 atom %). On the contrary, the b-layer was found to be europium rich with a composition of about 50 atom % Zn and 50 atom % Eu. In these films, europium ions are not homogeneously distributed since they are mainly in the basal layer. The global film europium content was measured at about 4.3 atom %, the basal layer representing only a small fraction of the electrodeposited material. XRD patterns of the films, before (not shown) and after annealing at 800 °C under air (Figure 2), are indexed with the wu¨rtzite hexagonal crystallographic structure of ZnO. A
Luminescence of Nanostructured Eu3+/ZnO Films
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Figure 2. XRD diagram of a ZnO/4.3 atom % Eu film deposited on F:SnO2/Si and annealed at 800 °C (/ substrate).
scheme of the film cross-sectional structure is presented in Figure 1b. At 0.55 mM EuCl3, the presence of the basal layer was not easy to observe, but they could not be described as only made of rodlike material. Films were also prepared with using 1.2 mM EuCl3 in the deposition bath (noted as B-films). SEM views (Figure 1c) reveal that they are smoother. The substrate is covered by a dense layer, and the dispersed small white spots are identified by EDS analysis as ZnO rods doped with europium ions. ZnO is also detected by XRD before and after the thermal treatments. EDS global analysis gives a film composition almost similar to that of the b-layer described above with about 50 atom % Eu and 50 atom % Zn. No XRD peaks specific of this layer could be detected before the annealing treatments. 3.2. Luminescence Properties of As-Deposited Films and Annealing Effects. 3.2.1. Luminescence under Direct Europium Excitation. The luminescence of europium included in the film was first probed by exciting in the 5D2 multiplet at 464 nm. The as-deposited films present a weak PL signal. The spectra are similar in shape and intensity for A- and B-films (Figure 3a) and are characteristic for Eu3+ ions in a nonsymmetric environment. Emission peaks observed at 570, 590, 612, 646, and 700 nm are assigned to the transitions from the 5D0 (Eu3+) excited state to the 7FJ (with J ) 0, 1, 2, 3, 4) multiplets, respectively. The 5D0 f 7F2 is the most intense transition as usually observed for Eu3+ embedded in materials.27 Under UV illumination, no spectral features due to Eu2+ ions were recorded from as-deposited films and from films after annealing (see below). These results reveal that most of the europium ions are incorporated in the film in the trivalent oxidation state. The electrochemical synthesis is performed at rather negative potential (-1 V vs SCE), but we have shown elsewhere that Eu(OH)2 is not a thermodynamically stable compound and it cannot be formed in the present case.24 The films were subsequently annealed in different atmospheres at 400 and 800 °C. The best luminescence activation was found after annealing in air. The variation of the A-film PL emission spectrum with annealing temperature is reported in Figure 3b. The most intense luminescence was obtained after the treatment at 800 °C during 5 h. The luminescence improvement can be assigned to the recrystallization of ZnO around the europium emission centers which leads to a dramatic decrease in defect concentration (killing centers). In order to understand which of the b-layer or the rod components is at the origin of the intense luminescence, PL spectra were recorded under the same experimental conditions (wavelength and
Figure 3. (a) Comparison of PL emission spectra under 464 nm excitation of as-deposited A- and B-films. (b) Effect of annealing in air on the emission spectra of A-films. (c) Comparison of PL emission spectra of A- and B-films annealed at 800 °C in air.
excitation power, geometrical arrangement of the experiment, etc.) for A- and B-films annealed at 800 °C (Figure 3c). The A-film signal is much more intense and dominated by the rod signal which is the main film component. We can reasonably suppose that the PL emission in annealed A-film mainly arises from the europium-doped rods. The B-film gives a similar spectrum but much less intense; therefore, the signal most likely results from a combination of the small rods and the basal layer emissions. In Figure 3, a high value of the intensity ratio between the 5D f 7F and 5D f 7F transitions is observed for the two 0 2 0 1 different films and the different annealing treatments. It reflects
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Figure 4. 5D0 f 7F0 transition of A- and B-films at 10 K under excitation at 464 nm.
Figure 5. Decay curves of the 612 nm emission intensity (in a logarithmic scale) of an A-film annealed in air at 800 °C (λexc ) 464 nm): (a) room temperature, (b) 10 K. Inset : same curves reported as a function of xt.
a low-symmetry site for the coordination shell of europium ions. Low site symmetry for Eu3+ ions is expected in these films, for instance, in the crystallographic lattice of the ZnO rods, in which a charge compensation is needed (for instance, by means of zinc vacancies) to substitute Zn2+ for Eu3+. In order to get further information on the europium sites, the 5D0 f 7F0 transition was studied at low temperature (10 K) for the A- and B-films under an excitation at 464 nm (Figure 4). Since the involved states are nondegenerate, it provides the best information concerning the distribution of Eu3+ ions among different sites. A single emission was observed at 580.8 nm with the A-film. From SEM observation, the A-film is mainly composed of nanorods; therefore, the emission can be assigned to europium included in the nanorods. In the case of the B-films, two emission peaks at 579.5 and 581 nm are observed for the 5D0 f 7F0 transition, then two different environments are expected in that films. The former at 579.5 nm can be assigned to the weak emission of europium in the basal layer. The wavelength of the latter is about the same as the emission wavelength of the A-film and then can be assigned to the emission of Eu3+ in the nanorods. The two emission peaks present about the same intensity, whereas most of the europium is located in the b-layer. This can be explained if we suppose that the nanorod phase yields a much more intense emission than the basal layer. 5D time-resolved emission was measured by recording at 0 RT and 10 K a set of luminescence spectra at increasing gate delay up to 1.5 ms (Figure 5). The decay curve recorded at RT exhibits two distinct regimes. At short times (
