ARTICLE pubs.acs.org/JPCC
Optical Efficiency of Short Wave Infrared Emitting Phosphors Mei Chee Tan,† John Connolly,‡ and Richard E. Riman*,† †
Department of Materials Science and Engineering, Rutgers, The State University of New Jersey, 607 Taylor Road, Piscataway, New Jersey 08854, United States ‡ Innovative Photonic Solutions, 4250 U.S. Route 1, Suite 1, Monmouth Junction, New Jersey 08852, United States ABSTRACT: Several methods and corresponding parameters have been used to measure the emission intensities and efficiencies of luminescent materials (e.g., organic dyes, quantum dots, rare-earth-doped materials). The important performance parameters characterizing the emission intensities or brightness are overall quantum yield, intrinsic quantum efficiency, and radiant efficiency. Currently, the measurement parameter used to evaluate the brightness varies with the type of material (e.g., organic dyes vs rare-earth-doped phosphors) that is studied. Since different performance parameters are used, the relative performance among the different material classes cannot be compared easily. This work demonstrates that optical efficiency can be use as a single performance parameter to measure the short wave infrared (SWIR) emission performance of rare-earth-doped materials with different emission mechanisms and branching. NaYF4:YbEr (with visible upconversion emissions), CeF3:YbEr (no upconversion), ErGdVO4 (quantum cutting phosphor), and commercial Kigre phosphate glass were tested. Phosphors that emit on the basis of a quantum cutting mechanism were compared to those that emit through the single-photon downconversion mechanism. It was found that the optical efficiency of ErGdVO4 as a three-photon quantum cutting phosphor was poorer than any single-photon downconversion phosphor tested. NaYF4:YbEr, a single-photon downconversion phosphor, was found to emit most brightly. Its emission was ∼2.5 times more efficient than the Kigre phosphate glass.
1. INTRODUCTION Short wave infrared (SWIR) emitting phosphors have been extensively used in fiber amplifiers, solid-state lasers, telecommunications, and optoelectronics.110 The recent emergence of SWIR imaging systems has expanded the applications of SWIR emitting phosphors in surveillance applications, biomedical imaging, and diagnosis.1114 In most cases, brightness (i.e., emission intensities) and energy efficiency are used as measures of the phosphors’ performance. The performance of phosphors is crucial to the successful implementation of the above-mentioned technological applications. For example, brighter and more efficient phosphors improve the diagnostic sensitivity of biomedical phosphor probes and enhance the energy efficiency of phosphor-based illuminators. Therefore, it is essential to set benchmarks that accurately reflect the brightness and energy efficiency of these phosphors. Several methods and corresponding parameters have been used to measure the emission intensities and efficiency of various luminescent materials (e.g., organic dyes, quantum dots, rare-earth-doped materials). The important parameters characterizing the emission intensities or brightness are (1) overall quantum yield (QY), (2) intrinsic quantum efficiency (IQE), and (3) radiant efficiency (RE) as shown in eqs 14.1517 Z Nem dλ QY ¼ Z em ð1Þ Nabs dλ abs
r 2011 American Chemical Society
R R where emNemdλ and absNabsdλ are the number of emitted and absorbed photons, respectively, and λ is wavelength. τlum IQE ¼ ð2Þ τrad where τlum and τrad are the luminescence and radiative lifetimes of a specific excited state, respectively. QY ¼ ηeff IQE
ð3Þ
where ηeff is the efficacy with which energy is transferred to the excited state. Z Pem dλ em Z RE ¼ ð4Þ Pabs dλ abs
R where emPemdλ and absPabsdλ are the emitted and absorbed powers, respectively. Comprehensive reviews covering different aspects of the QY measurements were reported by Demas and Crosby and by Rohwer and Martin,18,19 and the various methods for IQE measurements were discussed by Henderson and Imbusch.20 In brief, calibration using standard light sources and accurately R
Received: April 21, 2011 Revised: July 9, 2011 Published: July 13, 2011 17952
dx.doi.org/10.1021/jp203735n | J. Phys. Chem. C 2011, 115, 17952–17957
The Journal of Physical Chemistry C
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known QY standard samples is required to make the overall quantum yield measurements. An ideal QY standard should have no overlap between absorption and emission bands and should not be susceptible to oxygen or concentration quenching. The absorption and emission spectra should also be similar to those of the tested compounds so that any source or detectorcalibration errors will not greatly affect the results. Secondary processes that follow optical excitation like polarization and reabsorption reemission are some of the problems related to design of the setup for QY measurements that affect accuracy. To minimize reabsorption losses, dilute samples or low power excitation sources are used. The setup for RE measurements is similar to that for overall quantum yield measurements, where optical power is measured instead of the number of photons. The IQE is determined from the luminescence (τlum) and radiative (τrad) lifetimes. Judd-Ofelt parameters extracted from the absorption spectrum are used for the calculation of τrad, while τlum is measured using transient spectroscopic techniques (i.e., collecting the emission signal on an oscilloscope from a modulated excitation source). The merits and inadequacies associated with the various performance parameters shown in eqs 1 4 will be discussed in greater detail later in this work. To date, the measurement parameter that is used to evaluate performance depends on the type of material (e.g., organic dyes vs rare-earth doped phosphors) that is studied. For example, QY is more commonly used as a performance metric for organic dyes, while the IQE is more commonly used for rare-earth-doped phosphors. Since different performance parameters are used, the relative performance of different material classes cannot be compared. In addition, separate efficiency parameters are currently used to measure the performance of phosphors that emit using different mechanisms. Most phosphors emit using the single-photon downconversion mechanism where one photon is emitted for each absorbed photon. Phosphors that emit using the quantum cutting mechanism are a unique class of downconversion materials with the potential of emitting two to three photons for each absorbed photon of light.2123 Subsequently, these quantum cutting phosphors can potentially have efficiencies of more than 100% which suggests that these phosphors could be brighter and more efficient than single-photon downconversion phosphors. However, the efficiencies of quantum cutting phosphors are determined using a different method which is based on the ratios of integrated emission areas obtained from the steady-state emission spectra obtained at different excitation wavelengths.2123 Thus, with all the above approaches, there is a need for a single approach that is relevant for evaluating the performance of all types of phosphors so that a direct comparison of brightness can be made. This work proposes the use of optical efficiency to accomplish a direct comparison of brightness among different phosphors. We will demonstrate this approach by measuring the SWIR emission performance of rare-earth-doped materials. The optical efficiency (OE) is defined as Z Pem dλ ð5Þ OE ¼ Zem Pinc dλ where
R
inc incPincdλ
is the incident power.
OE ¼ fabs RE
ð6Þ
where fabs is the absorbed fraction of incident excitation light.
The following systems were chosen to demonstrate the utility of optical efficiency as a measure of brightness: (1) NaYF4: YbEr, (2) CeF3:YbEr, (3) ErGdVO4 (quantum cutting phosphor), and (4) commercial Kigre phosphate glass. Besides the infrared emission, NaYF4:YbEr exhibits intense visible upconversion emissions while CeF3:YbEr does not show any visible emissions.8,9 Therefore, the infrared emission performance of NaYF4:YbEr and CeF3:YbEr was compared to evaluate the effects of different emission branching ratios. The performance of phosphors that emit on the basis of a multiphoton quantum cutting mechanism was compared to those that emit through the single-photon downconversion mechanism. Thus, ErGdVO4 was chosen because it had been earlier reported to be a three-photon infrared quantum cutting phosphor having an efficiency of ∼179% when excited with green light at 522 nm.21 Commercial Kigre phosphate glass, wellknown to emit brightly at 1530 nm, was used as the benchmark comparison.
2. EXPERIMENTAL METHODS 2.1. Synthesis. All chemicals used for the hydrothermal synthesis of NaY0.78Yb0.20Er0.02F4 (NaYF4:YbEr), Ce0.68Yb0.30Er0.02F3 (CeF3:YbEr), and Er0.3Gd0.7VO4 (ErGdVO4) powders were acquired from Sigma Aldrich (Sigma Aldrich, St. Louis, MO). Details on the synthesis and characterization for NaYF4:YbEr and CeF3:YbEr were discussed in our earlier work.8,9 In brief, NaYF4:YbEr powders were prepared by mixing stoichiometric amounts of rare-earth nitrates with 1.5 times excess sodium fluoride in ∼70 mL of a water:ethanol mixture (80:20 v/v) and 24 g of polyvinylpyrrolidinone for 30 min. This mixture was then transferred to a 125 mL Teflon liner and was heated to ∼240 °C for 4 h in a Parr pressure vessel (Parr Instrument Company, Moline, IL). For the synthesis of CeF3:YbEr, stoichiometric amounts of rare-earth nitrates and ammonium fluoride were mixed in ∼75 mL of water for 30 min. Subsequently, this mixture was transferred to a 125 mL Teflon liner and was heated to ∼200 °C for 2 h in a Parr pressure vessel. ErGdVO4 was synthesized by mixing stoichiometric amounts of rare-earth nitrates and ammonium vanadate in ∼75 mL of water for 30 min. The pH of the solution was adjusted to ∼pH 4 using nitric acid or ammonia. This mixture was next transferred to a 125 mL Teflon lined Parr pressure vessel and was heated to ∼240 °C for 2 h. The above as-synthesized powders were washed three times in deionized water by centrifuging (Beckman-Coulter Avanti J-26 XP, Fullerton, CA) and were dried at 70 °C in air in a mechanical convection oven (Thermo Scientific Thermolyne, Waltham, MA) for further powder characterization. The as-synthesized ErGdVO4 was heat-treated in 10 mL alumina crucibles (CoorsTek, Golden, CO) using an FD1535 M box furnace (Thermo Scientific Thermolyne, Waltham, MA) at 800 °C for 1 h in air. Heat-treatment of as-synthesized CeF3:YbEr powders was completed in a controlled environment using the double crucible method to prevent CeF3 oxidation. Ten and fifty milliliter alumina crucibles were used for the heat treatment. The ∼0.9 g of as-synthesized powders in the inner 10 mL crucible was heated in a box furnace with ∼3.0 g of ammonium bifluoride (outer 50 mL crucible) at ∼400 °C for 1 h. The structure and phase purity for each of these powders were characterized using X-ray diffraction and were verified to be phase-pure. Powder X-ray diffraction (XRD) patterns were 17953
dx.doi.org/10.1021/jp203735n |J. Phys. Chem. C 2011, 115, 17952–17957
The Journal of Physical Chemistry C obtained with a resolution of 0.04°/step and 2 s/step with the Siemens D500 (Bruker AXS Inc., Madison, WI) powder diffractometer (40 kV, 30 mA) using Cu KR radiation (λ = 1.54 Å). The collected XRD profiles were interpreted using reference powder diffraction files (PDF).24 2.2. Steady-State and Time-Resolved Fluorescence Spectroscopy. For the optical measurements, powder samples of asprepared phosphors were pressed into pellets of 1 cm diameter and 2 mm thickness which had the same thickness as the Kigre phosphate glass. This ensured that the light interaction volume was the same for all samples given that the incident beam diameter (∼5 mm) was smaller than the sample size. The emission spectra were collected using the FSP920 Edinburgh Instruments spectrometer (Edinburgh Instruments, Livingston, United Kingdom) that was equipped with a Hamamatsu G585223 thermoelectrically cooled analog InGaAs photodiode (Edinburgh Instruments). Emission spectra were collected using a 975 nm laser excitation source (BW976, BW Tek, Newark, NJ) at 0.5 W, which was focused on a spot size with ∼5 mm diameter. For the emission spectra collected using 522 nm excitation, a 450 W Xe lamp was used as an excitation source, where the excitation wavelength was selected using a TMS300-X monochromator (Edinburgh Instruments). The ∼1 mW of 522 nm light focused on an area of ∼5 mm 5 mm was found to be incident on the sample. The emission slit width, step size, and integration time were fixed at 0.5 nm, 0.5 nm, and 0.5 s, respectively, for emission spectra collected using both excitations. The decay time was measured by modulating the continuous wave 975 nm laser beam with a chopper, and the signal from the infrared emission was collected using the Hamamatsu G5852-23 thermoelectrically cooled analog InGaAs photodiode that was coupled to a digital oscilloscope (TDS 220, 200 MHz, Tektronix, Beaverton, OR). 2.3. Optical Efficiency Measurements. A modification of the C9220-03 quantum yield measurement system from Hamamatsu (Hamamatsu, Bridgewater, NJ) was used to make the optical efficiency measurements. In brief, the measurement principle is based on direct illumination and indirect reflection. Light enters the integrating sphere through the sample port, goes through multiple reflections, and is scattered uniformly around the interior of the sphere. For our measurements, the integrating sphere was set up in the reflectance mode to measure total integrated reflectance of a surface (see Figure 1). The PD300-IR power detector (Ophir-Spiricon, Logan, UT), which measures the power of emitted light, was used in place of the photomultiplier tube that was originally on the C9220-03 quantum yield measurement system. It was positioned at the port at the side of the sphere where the emitted beam is independent of the angular properties of light at the sample port. Further assumptions made during measurements are that all light emanating from the different samples is isotropic and that internal reflection effects and reflection losses are similar for different samples. Considering the similarity in excitation and emission wavelengths with the as-prepared phosphors (CeF3:YbEr, NaYF4:YbEr, and ErGdVO4), SWIR-emitting commercial Kigre phosphate QE-7S glass (3.8 2.9 0.2 cm, Kigre Inc., Hilton Head, SC) was used as the reference standard to verify the reliability and reproducibility of the optical efficiency measurement setup. Technical data for τrad and average optical efficiency of 7.9 ms and 0.4%, respectively, (from an output energy/input energy at an input energy of 20 J) was obtained from published Kigre product specifications.25
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Figure 1. Optical efficiency measurement setup.
3. RESULTS AND DISCUSSIONS Several different methods and parameters have been used to measure the emission intensities and efficiency of various luminescence materials (e.g., organic dyes, quantum dots, rare-earthdoped materials).1518 Some of the parameters commonly used to evaluate the brightness and efficiency of luminescent materials are QY, IQE, and RE (see eqs 14). Each of these methods and parameters has their merits and inadequacies. Measurements of near-infrared (NIR) QY values have been made for Nd3+ and Yb3+ solid-state samples and are reported to be 0.10.4 and 0.61.4%, respectively.15,16 Since these numbers are often very low, the precision and accuracy of the measurement method will be critical. There are few studies that report the details about the QY measurements especially in the SWIR ranges. Part of the reason for the absence in QY measurements is that few laboratories are equipped with instruments to make measurements in the NIR-SWIR primarily because of issues related to lack of photon calibration standards in the SWIR region. The overlap of absorption and emission bands or low material absorption coefficients are additional problems associated with making accurate QY measurements. For example, for quantum dots and organic dyes, the overlap of absorption and emission bands presents a challenge in accurately determining the QY values. An accurate and precise QY measurement often requires the samples to have absorption cross sections greater than ∼0.06 cm2.19 The low absorption cross sections of rare-earth-doped phosphors (typically of the order from 1 1021 to 5 1020 cm2) because of low dopant concentrations (
