J. Phys. Chem. C 2009, 113, 10773–10779
10773
Spectral Band Shifts in the Electronic Spectra of Rare Earth Sesquioxide Nanomaterials Doped with Europium Peter A. Tanner* and Lianshe Fu Department of Biology and Chemistry, City UniVersity of Hong Kong, Tat Chee AVenue, Kowloon, Hong Kong S.A.R., People’s Republic of China
Bing-Ming Cheng National Synchrotron Radiation Research Center, Hsinchu, Taiwan ReceiVed: February 23, 2009; ReVised Manuscript ReceiVed: April 14, 2009
The UV-visible and vacuum ultraviolet emission and excitation spectra of Y2O3:Eu3+ (1 and 0.1 atom %) nanomaterials, prepared by combustion syntheses with use of glycine or hydrazine, have been recorded in order to investigate spectral band shifts as a function of particle size. FT-IR spectra and site-selective emission spectra at low temperature show the presence of impurities for all of these samples, with the least impurities for the samples prepared using stoichiometric ratios of reactants, where (i) the relative intensity of the free exciton (FE) and the charge transfer (CT) bands changes significantly with Eu3+ concentration, (ii) there is a small red-shift of the CT band position with decreasing particle size, and (iii) the shifts of the FE and band-to-band (CB) transitions are within a separation of 116 cm-1 (14 meV) at room temperature for samples of size 20-40 nm. The band positions change for samples synthesized when not using stoichiometric ratios of reactants. 1. Introduction Several recent reviews have highlighted differences in the properties of bulk and nanocrystalline rare earth materials.1-3 Concerning their spectral properties, phenomena such as thermalization, phonon confinement, band shifts, band broadening, radiative lifetime, quantum efficiency, energy transfer, and upconversion have received attention. The present study investigates one of these topics, with Eu3+ as the major focus. The spectra of this ion are particularly intense, simple, and instructive since, for example, nondegenerate initial and final states are involved in the 5D0-7F0 transition, and the intensity mechanisms differ for 5D0-7F1 and D50-7F2. Thus, the number of luminescent Eu3+ sites and the relative contributions of electric dipole and magnetic dipole intensity have been investigated in many previous studies. The rare earth sesquioxide host has been employed in the present work. In Ln2O3 (Ln ) Y, Gd), the Ln3+ ion is situated at two different sites of symmetry C2 and S6, with the nominal occupation ratio of 3:1.4 The coordination of each Ln3+ is 6-fold to oxygen atoms at the corners of a cube, with body diagonal (for S6) or face diagonal (for C2) oxygen atoms missing. The emission and excitation spectra of Eu3+ in these hosts have been thoroughly investigated4-10 and nearly all of the spectral intensity is due to the Eu3+ ions situated at the C2 site. Energy transfer occurs between ions at the different sites,7,11,12 and at higher Eu3+ concentrations, the S6 site emission from 5D0 decreases in relative intensity compared with the C2 site emission.13 The interest herein concerns the spectral band shifts which have been stated to occur in the excitation spectra of Ln2O3: Eu3+. Figure 1 shows some relevant spectral processes: namely, charge transfer (CT), free exciton (FE) formation, and the valence band to conduction band (CB) transition.14-16 With * To whom correspondence should be addressed.
Figure 1. Relevant energy levels and optical transitions for Ln2O3: Eu3+ (not to scale). E(CT) represents the charge transfer transition from the valence band to Eu3+ to form the ground state of Eu2+. E(CB) and E(FE) represent the band to band and free exciton transitions, respectively. STE represents the self-trapped exciton. The literature value of the Y2O3 band gap energy is 5.6 eV (221 nm)14,15 and the Eu3+ charge transfer energy is 5.06 eV (245 nm).16
decreasing size of the nanocrystals, the charge transfer band has shifted to the red according to many studies,17-21 although a blue shift22,23 or no shift24-26 have also been observed. CT band shifts have been reported to occur for different dopant ion concentrations.27 The host lattice absorption has been reported to blue shift24,28-31 with decreasing particle size. Several explanations have been, or could be, put forward to account for such band shifts. Chemical reasons include that the formation of longer Eu-O bonds (due to coordination number increase,19,21 surface sites,32 or other phases or impurities33) would lead to a red shift of the CT band. Certainly, the differences between the luminescence spectra of very small (
