ARTICLE pubs.acs.org/JPCC
Transition from Long-Range to Short-Range Energy Transfer through Donor Migration in Garnet Hosts A.A. Setlur,* J.J. Shiang, and C.J. Vess GE Global Research Center, 1 Research Circle, Niskayuna, New York 12309, United States ABSTRACT: We demonstrate that the mechanism for nonradiative energy transfer, a critical aspect in many optical systems, can switch from a longer-range dipole-dipole mechanism to a short-range exchange mechanism through energy migration between donors. This transition significantly increases energy transfer rates and is analyzed using measurements of Tb3þ(5D4)fCe3þ(5d1) energy transfer as a function of Tb3þ concentration in (Lu,Tb)3Al5O12: Ce3þ garnet phosphors. The additional short-range energy transfer is assigned to exchange by the analysis of the migration and trapping rates as well as estimates of the relevant exchange integrals.
1. INTRODUCTION Nonradiative energy transfer (ET) plays an important role in a variety of fields, including inorganic phosphors,1,2 organic lightemitting diodes,3 biological assays,4 and photonic media.5 The fundamental mechanism behind ET is usually assigned to either a long-range dipole-dipole (F€orster) or a short-range (super) exchange (Dexter) mechanism whose rates are proportional to R-6 and exp(-βR),1,6 respectively, where R is the donoracceptor (DA) distance and β is a constant related to the microscopic nature of the (super)exchange interaction. The demarcation between these mechanisms is generally clear with dilute concentrations of donors and acceptors2,7,8 but is less clear for short DA distances when relevant acceptor transitions are dipole-allowed. In those cases, ET rates can be enhanced beyond dipole-dipole estimates due to (super)exchange and/or other through-bond components.9-11 This enhancement has been demonstrated in molecular systems that have defined bridges separating donors and acceptors9-11 or with significant molecular diffusion in solution.12 When there is a sufficiently high concentration of donors, donor-donor (DD) energy migration can mediate energy transfer between separated donors and acceptors. The enhancement of ET rates with DD energy transfer is usually studied experimentally and theoretically using a single mechanism for DA energy transfer.13-20 However, the presence of DD migration does not preclude multiple DA energy transfer mechanisms, and sufficient DD migration could reduce the distance distribution for DA energy transfer to enable fast, short-range ET. This will strongly affect ET rates and could modify luminescence efficiency, spectra, and/or response time. In this report, the effect of DD migration on the mechanism for DA energy transfer is studied for Tb3þ donors and Ce3þ acceptors in (Lu,Tb)3Al5O12: Ce3þ garnet powders. Dipole-dipole ET between the 5D4 level of Tb3þ and the allowed Ce3þ 4f1f5d1 blue absorption band has been previously demonstrated and analyzed in aluminate garnets at low Ce3þ and Tb3þ concentrations.21,22 However, at high r 2011 American Chemical Society
Tb3þ concentrations, the long Tb3þ 5D4 radiative lifetime combined with the absence of Tb3þ-Tb3þ cross-relaxation leads to Tb3þ(5D4)-Tb3þ(5D4) energy migration that reduces the effective distance for DA energy transfer. This reduction in the effective DA distance leads to additional short-range Tb3þfCe3þ ET that is assigned to exchange based upon the analysis of the Ce3þ decay profiles after 4f8f4f75d1 Tb3þ excitation as a function of Tb3þ concentration. In this analysis, the contribution of exchange to Tb3þfCe3þ ET becomes stronger than the dipole-dipole contribution when Tb3þ occupies ∼50% of the RE3þ sites in the garnet. Tb3þfCe3þ ET by exchange dominates at full Tb3þ replacement, increasing the rate of Tb3þ-Tb3þ migration and trapping by Ce3þ to greater than three times the fast migration limit when only dipole-dipole Tb3þfCe3þ ET is accounted for.
2. EXPERIMENTAL PROCEDURE Garnet powders are made by ceramic methods with high purity Lu2O3, Tb4O7, CeO2, and R-Al2O3 using an AlF3 flux; these powders are fired at 1400-1550 C in N2/H2 gas mixtures. Unless specifically mentioned, all reported compositions are nominal compositions with 1% Ce3þ on the RE3þ site and are referred to by the Tb3þ concentration [Tb3þ = 0.40 refers to (Lu0.59Tb0.40Ce0.01)3Al5O12]. Steady-state photoluminescence measurements used a SPEX Fluorolog 3 with a Xe lamp source. Relative quantum efficiency (QE) measurements are corrected for the sample absorption using a BaSO4 (Kodak) powder reflectance standard. Time-resolved photoluminescence measurements for Tb3þ excitation into the spin and parity-allowed Tb3þ 4f8f4f75d1 transition used a quadruple Nd:YAG laser at 266 nm (JDS Uniphase) coupled into an Edinburgh F900 spectrometer with a Peltier cooled photomultiplier tube Received: November 3, 2010 Revised: January 6, 2011 Published: February 07, 2011 3475
dx.doi.org/10.1021/jp110520j | J. Phys. Chem. C 2011, 115, 3475–3480
The Journal of Physical Chemistry C
ARTICLE
Table 1. Relative QE and Microsecond Decay Constant for (Lu0.99-xTbxCe0.01)3Al5O12 Phosphor Powders at 266 nm Excitation x
relative QE
0.40
100
68.0
0.50
97
35.0
0.60
93
16.5
0.70
92
7.2
0.80 0.99
91 85
4.2 2.7
microsecond decay time (μs)
Figure 1. Emission spectra of (Lu,Tb)3Al5O12:Ce (1%) powder samples under Tb3þ 4f8f4f75d1 excitation (λex = 266 nm) with varying Tb3þ concentration.
(PMT) detector. The effect of direct Ce3þ excitation under 266-nm excitation is minimized since the Ce3þ 4f1f5d1 absorption is weak at 266 nm23 and the Tb3þ concentration is >40 times that of Ce3þ. Time-resolved measurements for direct Ce3þ 4f1f5d1 excitation used a PicoQuant laser at 468 nm with same Edinburgh F900 spectrometer. All measurements were made at room temperature.
3. RESULTS The luminescence spectra (λex = 266 nm) of three (Lu, Tb)3Al5O12 Ce3þ garnets used in this study are shown in Figure 1. Tb3þ 4f8f4f75d1 excitation in aluminate garnets with Tb3þ and Ce3þ can lead to three potential radiative transitions: narrow-line Tb3þ 5D3f7FJ and 5D4f7FJ transitions and a broadband Ce3þ 5d1f4f1 transition. In previous reports of Tb3þ and Ce3þ codoped Y3Al5O12,21,22 Tb3þ 5D4f7FJ emission was not fully quenched at the relatively low Tb3þ ( 0.5 (Figure 1), the QE for these phosphors under Tb3þ 4f8f4f75d1 excitation (λex = 266 nm) is relatively constant with weak concentration quenching at high Tb3þ concentrations (Table 1). As a reference for the absolute QE for these Ce3þ-doped garnets, the QE of the Tb3þ = 0.40 sample (λex = 266 nm) matches the QE of a commercial Ca5(PO4)3(F,Cl):Sb3þ,Mn2þ fluorescent lamp phosphor that has an absolute QE greater than 80%.25 In these Ce3þ-doped samples, the luminescence decay measurements (λem = 560 nm) following Tb3þ 4f8f4f75d1 excitation have microsecond components that represent >50% of the total integrated intensity similar to previous reports in (Tb0.986Ce0.014)3Al5O12:Ce3þ thin films.26 These microsecond decays have a much smaller exponential time constant at high Tb3þ concentrations (Table 1 and Figure 2). While QE measurements show the presence of additional nonradiative transitions at high Tb3þ concentrations (Table 1), the ∼15% reduction
Figure 2. Decay profiles for (Lu,Tb)3Al5O12:Ce (1%) powder samples (λex = 266 nm, λem = 560 nm) with varying Tb3þ concentration. The initial fast component from any direct Ce3þ excitation is reduced in these decay profiles due to a delay after the excitation pulse.
in QE at high Tb3þ concentrations is much smaller than the ∼25x reduction in the microsecond decay time (Table 1). The reduction of the decay time therefore cannot be assigned to energy transfer to killer centers that lead to nonradiative transitions. Instead, we assign this behavior to Tb3þ-Tb3þ migration followed by energy transfer to Ce3þ(5d1) being the rate limiting process at long times after Tb3þ excitation.14-18 This assignment is supported by our studies of samples without any Ce3þ, where the absolute quantum efficiency of Tb3þ 5D4f7FJ emission is much lower. For example, (Lu0.1Tb0.9)3Al5O12 powders have a Tb3þ 5D4f7FJ decay time (λex = 266 nm; λem = 550 nm) that is greater than 60 μs (taken within a 50 μs time window) and Tb3þ 5D3f7FJ decay times (λex = 266 nm; λem = 415 nm) of ∼5 ns (Figure 3). Considering that the radiative lifetime of the 5D3 and 5D4 levels of Tb3þ in garnet hosts are in the millisecond range,21,22 these results directly show strong Tb3þ concentration quenching in the absence of Ce3þ. These time-resolved measurements are also supported by steady-state QE measurements that give a QE of
