DOI: 10.1007/s00339-007-3941-9 Appl. Phys. A 88, 129–133 (2007) Materials Science & Processing Applied Physics A m. peres 1 a. cruz 1 s. pereira 1,3 m.r. correia 1 m.j. soares 1 a. neves 1 m.c. carmo 1 t. monteiro 1, ✉ a.s. pereira 2 m.a. martins 2 t. trindade 2 e. alves 3 s.s. nobre 4 r.a. s ´ a ferreira 4 Optical studies of ZnO nanocrystals doped with Eu 3+ ions 1 Departamento de F´ ısica e I3N, Universidade de Aveiro, Aveiro, Portugal 2 Departamento de Qu´ ımica e CICECO, Universidade de Aveiro, Aveiro, Portugal 3 Instituto Tecnol´ ogico e Nuclear (ITN), Lisboa, Portugal 4 Departamento de F´ ısica e CICECO, Universidade de Aveiro, Aveiro, Portugal Received: 31 October 2005/Accepted: 13 January 2007 Published online: 28 March 2007 • © Springer-Verlag 2007 ABSTRACT Synthetic ZnO nanocrystals have been intention- ally doped with Eu 3+ ions. Structural analysis performed on the nanocrystals showed wurtzite-ZnO as the only phase present in the samples. Photoluminescence in emission and excitation modes allows the assignment of the intra-4 f 6 transitions for the Eu 3+ ions. From the analysis of the optical data we are able to demonstrate that multiple Eu-related optical centres are present in the studied samples. Oxygen vacancies are likely candidates to be responsible for the ion accommodation in the ZnO lattice and from the photoluminescence excitation data we tentatively assign a trap level at ∼ 200 meV below the conduction band to this intrinsic defect. PACS 78.66.Hf; 78.67.-n; 82.80.Yc 1 Introduction Doping of wide band gap materials with trivalent rare earth (RE) ions to obtain optical activity is well es- tablished. For instance, the RE doping of GaN, a semicon- ductor with a band gap of ∼ 3.5 eV and wurtzite structure, is currently under intensive investigation by several groups in order to obtain RE-based efficient light emitters in red, green and blue spectral regions [1–7]. Both thin films and quantum dots (QDs) of GaN-based materials have been in- tentionally doped with RE ions by using in situ doping and ion-implantation techniques. The RE ion is preferentially lo- cated in Ga sites and multiple RE-related optical centres can be observed [1–7]. ZnO is a wide band gap (∼ 3.4 eV) semiconductor with the same wurtzite structure as GaN. In this material the incorporation of RE ions using the ion-implantation tech- nique is not as straightforward as for GaN [8]. For bulk samples, ion-implanted Tm and and Er ions are mostly sit- ting in cation sites (Zn sites). Air thermal annealing of the ✉ Fax: +351-234-424-965, E-mail: tita@fis.ua.pt ion-implanted samples promotes a simultaneous lattice dam- age recovery and out-diffusion of the RE ions [8]. While Tm 3+ - and Er 3+ -related optical activation was observed in ion-implanted ZnO samples, no intra-4 f n related transitions were detected for the Tb- and Eu-doped samples [8]. Previ- ous studies suggest that mobile native defects and residual contaminants have an important role in the RE ion lattice accommodation and consequently in the observed optical properties [8]. In order to obtain a better understanding of the photo- luminescence behaviour of the RE ions incorporated in the ZnO host, we have extended our work to ZnO nanocrys- tals (nc). ZnO nanostructures have been obtained by adding tetramethylammonium hydroxide to dimethylsulfoxide solu- tions containing Zn 2+ cations [9]. The addition of transition- metal and RE ions to the zinc salt solution allows the incor- poration of dopants in the nanosized host [9, 10]. On the other hand, particle-size control can be achieved by varying the amount of base added to the zinc salt solution [10]. In this work, we focus on the optical properties of Eu- doped ZnO-nc prepared at mild temperatures, synthesized as described elsewhere [10]. Emission (PL) and excitation (PLE) spectra were recorded and a phenomenological description and interpretation of the observed data is given. 2 Experimental details Undoped and Eu-doped ZnO colloids were syn- thesized as described elsewhere [10]. In brief, ZnO col- loids were synthesized at room temperature (RT) by the drop-wise addition of tetramethylammonium hydroxide in ethanol (0.552 mol dm −3 ) to a Zn(CH 3 COO) 2 · 2H 2 O solu- tion in dimethylsulfoxide (0.101 mol dm −3 ). To dope the ZnO colloids, the synthesis was performed similarly but adding Eu(CH 3 COO) 3 · H 2 O to the precursor solution. Dis- tinct ZnO colloids have been prepared by varying the amount of Eu(CH 3 COO) 3 · H 2 O present in the precursor solution. To obtain the doped ZnO-nc as powders, ethyl acetate was added to the colloids and then centrifuged. The powders were then washed with ethyl acetate.