948
J. Phys. Chem. B 2001, 105, 948-953
The Effect of Particle Morphology and Crystallite Size on the Upconversion Luminescence Properties of Erbium and Ytterbium Co-doped Yttrium Oxide Phosphors J. Silver,* M. I. Martinez-Rubio, T. G. Ireland, G. R. Fern, and R. Withnall* Centre for Phosphors and Display Materials, Chemical and Life Sciences, UniVersity of Greenwich, London, SE18 6PF, United Kingdom ReceiVed: August 2, 2000; In Final Form: October 21, 2000
Phosphor particles (ca. 60 nm in size) of yttrium oxide co-doped with Er3+ and Yb3+ ions were prepared by a precipitation method in the presence of EDTA. Their physical properties were compared to much larger particles (ca. 600-800 nm in size) prepared in the absence of EDTA. All of the particles were shown to have crystallized in the cubic phase, and all exhibited blue, blue-green, and green-yellow upconversion emission when excited with laser light of wavelength equal to 632.8 nm. These upconversion emissions were shown to be excited by a two-photon process. The most intense yellow-green upconversion emission occurs when the crystallite size is between 75 and 200 nm and the particle size is 600 to 800 nm. Cross-relaxation processes between Er3+ ions are suggested to be responsible for the more efficient upconversion in the larger particles. There is evidence from spectra taken in the temperature range 30 to -190 °C that there are two different hot bands in the given region of the spectrum. These two emission manifolds are explained as arising from the two different Er3+ lattice sites in the cubic Y2O3:Er3+ structure. When using red excitation (rather than infrared excitation), the presence of Yb3+ was found to be detrimental, as it diminished the upconversion intensity.
1. Introduction There has been considerable research on upconverting phosphors since initial interest in the late 1950s.1-3 An upconverting phosphor is one which takes multiple photons of lower energy and converts them to one photon of higher energy (this is an anti-Stokes process). Much of the early work was aimed at producing upconverting phosphor lamps by coupling the phosphors with light emitting diodes (LED), though the research did not yield viable products.1 More recently, there has been a resurgence of interest in upconverting phosphors and an extremely successful and widely utilized photonic application of rare earth elements is in the area of optical fiberbased telecommunications.4 Fiber optics have been a revolutionary force in the telecommunications industry. The critical component in this application is the Er3+ ion doped fiber amplifier. This provides optical gain in the 1530-1560 nm lowloss window of glass fibers. Light emission from Er3+ doped silica glass has recently been reviewed.5 The principle behind anti-Stokes emission phosphors can be seen below for an Er3+ activator in NaYb(WO4)2:Er.3a,c In Figure 1, it can be seen that the first photon of infrared light (974 nm) elevates an electron to the 2F5/2 level of Yb3+ and the ion may then decay radiatively from this excited state back to the ground state (a forbidden transition, see below). Alternatively, it can transfer the energy to the Er3+ ion. This energy can promote an electron from the 4I15/2 to the 4I11/2 state, and if the latter is already populated a transition from the 4I11/2 to the 4F7/2 state can occur. The Er3+ ion, which is now in the 4F7/2 state, can now undergo phonon-assisted nonradiative decay to the 4S3/2 state. The 2H11/2 state can also be populated, as it is in thermal equilibrium with the 4S3/2 state.3c The ion can radiatively decay from the 2H11/2 state to the ground state, emitting green light of * Corresponding authors. E-mail:
[email protected];
[email protected]. Fax: 44 208 331 8405.
Figure 1. The relevant levels of Yb3+ and Er3+ ions when green light is emitted under infrared excitation.
wavelength approximately equal to 525 nm (see Figure 1). The Er3+ ion can also decay radiatively to the ground state from the 4S3/2 state, emitting green light of wavelength around 550 nm. Phonons are lattice vibrations in a material that can provide nonradiative decay pathways to suppress upconversion luminescence. The result is that when an electron is excited to the 4I 11/2 level (for instance) via an external pump source, phononassisted decay down to the 4I13/2 level (and then to the 4I15/2 ground state) can compete efficiently against other relaxation pathways. The energy lost in this nonradiative decay route is given up to the lattice phonons in the form of infrared energy (i.e., heat), and no light will be emitted. This is why phosphor glasses, having matrices with high phonon energies, are very
10.1021/jp002778c CCC: $20.00 © 2001 American Chemical Society Published on Web 01/12/2001
Upconversion Properties of Yttrium Oxide Phosphors inefficient emitters of long wavelength light. To overcome the phonon decay problem it is necessary to choose a lattice that has much lower phonon energies. If the phonon decay is less favored, the excited-state lifetime is extended so that it is possible to pump in a second photon of infrared light (λ ) 974 nm) to promote the electron to the 4F7/2 level (see Figure 1) so that visible emission can result. For instance, to enhance the initial population of the 4I11/2 level of Er3+, a Yb3+ sensitizer can be added to the host lattice, and this can be excited using 974 nm light (Figure 1). This sensitizer-absorber (S-A) energy transfer is the dominant mechanism since the transition from the 2F5/2 level back to the 2F7/2 ground level of isolated Yb3+ is parity forbidden for an electric dipole transition. However, in a crystal lattice, this selection rule can be relaxed by configuration interaction due to the odd components of the local crystal field acting on the rare earth ion. These relaxation effects depend very critically on the host lattice, which dictates the symmetry and magnitude of the crystal field. When the ion occupies a lattice site with inversion symmetry, there are no odd crystal field terms, hence in addition to phonon-assisted transitions,1 only magnetic dipole or higher order optical multipole transitions may occur. The crystal field splitting terms are also responsible for the small (ca. 100 cm-1) Stark splittings in the 4f states not shown in Figure 1. These terms cause the small host-dependent differences in the precise transition energies within the sensitizer absorption and activator emission. The parity selection rule has been found to be broken most easily in oxides and oxysulfides where configurational interaction between the 4f levels can occur. Usually, to ensure a good population of the 4I11/2 level of Er3+, up to eighteen times as many Yb3+ ions will be added to the lattice as Er3+ ions, when using infrared excitation.3c Possible applications of upconverting phosphors include (a) three-dimensional displays;6-8 (b) fiber optic amplifiers (referred to above) that operate at wavelengths of 1.55, 1.46 and 1.31 µm;4, 8-11 (c) upconversion lasers;7 (d) remote sensing (reading) thermometers for high-temperature applications (if the temperature dependence of the optical properties are studied).12 The efficiency of upconverting phosphors has been improved over the years with the use of different systems such as Y0.78: (Yb0.20 Er0.02)F3,13 NaY0.69:(Yb 0.30 Er0.01)F4,13 BaY1.34:(Yb0.60 Er0.06)F8 (green),14 Y0.04:(Yb0.74 Er0.01)OCl (red),13 Y0.65:(Yb0.35 Tm0.001)F3, (blue).13 The fluoride systems (particularly glasses) were used in many applications and were found to have some advantages over other materials.15 However, it has since been found that chloride systems, such as BaCl2(25ErCl3-75BaCl2),16-18 are more suitable hosts than fluorides for Er3+ ions, although moisture sensitivity of these compounds requires protection procedures. During the past decade the preparation of submicrometer phosphor particles with a defined morphology has been an active area of research.19-23 Spherical particles of cathodoluminescent europium-doped yttrium oxide (Y2O3:Eu)24-27 have recently been prepared with a range of particle sizes (
