Investigation of e-h Trapping Efficiency in Eu3+ Doped YPO4 Using VUV Spectroscopy

Absorption of Vacuum Ultraviolet (VUV) radiation creates excited e – h pairs in rare – earth doped phosphors that can either be trapped by a defect state (killer) or Ln³⁺ activator. The e – h trapping efficiency by Ln³⁺ is proportional to the rate of transfer to the activators. Recent work on YBO₃:Eu³⁺ nano-sized particles indicate that with decreasing particle size, there is an increase in loss of energy to surface states [1]. Micron-sized Ln³⁺ doped YBO₃ particles seem to have no surface loss effects. Our ultimate goal is to study particle size effects on surface loss and trapping efficiency in Ln³⁺ doped phosphor compounds. To develop a point of reference for surface loss and trapping efficiency for other nano-sized Ln³⁺ doped phosphor compounds, the corresponding micron-sized phosphors must be analyzed first. Thus, the e - h trapping efficiencies under VUV excitation have been determined from absorbance and excitation spectra for micron-sized Ln³⁺ doped YBO₃, YPO₄, and LaPO₄. Several of these phosphor compounds contain multiple d(Ln³⁺) and or a Ln²⁺ - charge transfer (CT) state in the band gap for e - h trapping to occur. For a single CT strap state, a unique method for estimating the transfer efficiency under VUV excitation has been developed [2], and we have recently proposed a method to determine transfer efficiencies under VUV excitation for each of the individual trap states for Ln³⁺ doped phosphor compounds that contain multiple trap states [3].  Treating these data using accepted kinetic models allows us to calculate relative trapping efficiencies as well as the degree of surface loss.  To understand the trapping mechanism, the Ln³⁺ energy levels relative to host states must be taken into consideration. We have developed an energy level scheme of Ln²⁺ and Ln³⁺ ground states relative to the valence band maximum (VBM) of Ln³⁺ doped compounds using techniques proposed by Dorenbos and Thiel [4,5,6]. Results indicate that if the Ln³⁺ ground state is higher in energy than the VBM, and d – orbitals are in the band gap, e - h trapping by the activator will form a 5d(Ln³⁺) state.  The e - h trapping efficiency shows a positive linear correlation with the energy difference of the Ln³⁺ ground state and VBM. That is, the observed trapping efficiency appears to be dictated by hole trapping efficiency of the Ln³⁺ ground state for the corresponding activators.  For the activators with the Ln³⁺ ground state approximately equivalent to or lower in energy relative to the VBM, and a Ln²⁺ ground state CT that is in the band gap, a linear correlation is observed between the trapping efficiency and the difference in energy between the CT state and conduction band edge (CBE). In this case, a CT state is formed and the trapping efficiency must be dictated by the electron trapping efficiency for the corresponding activator. Having completed a preliminary study of the micron – sized materials, data will be presented on our studies of surface loss effects in the nano – sized materials.

References

[1]     Olsen, K; Lawler, A; Diaz, A. L; J. Phys. Chem. C (2011), 115, 17136-17146

[2]     Watrous-Kelly,T.; Diaz, A. L.; Dang, T. A. Chem. Mater. (2006), 18, 3130.

[3]     Wallace, Maxwell K.; Diaz, Anthony L. P.58: Quantitative Assessment of Host — to — Activator Energy — Transfer Efficiency of Multiple d — Orbital Trap States for Microcrystalline YBO₃:Tb³⁺. SID Symposium Digest of Technical Papers. (2013), 44, 1

[4]     Dorenbos, P. Phys Rev B. 85, 165107 (2012)

[5]     Dorenbos, P. Journal of Luminescence 135 93-104 (2013)

[6]     C.W. Thiel; Y. Sun; R.L. Cone; J. Modern Optics 49, 2399 – 2411 (2002)