In this work, a host which interacts and enhanced energy transfer to the luminescent center such that it facilitates the infrared emission while avoiding undesired emissions was found. An intense emission at ~1530 nm with no other visible emissions was observed in Er- and Yb-Er- doped CeF3 nanoparticles upon excitation at ~975 nm. The average measured luminescence lifetimes of the ~1530 nm emission for heat-treated CeF3:Er and CeF3:Yb,Er nanoparticles was ~4.5−6.5 ms, with internal quantum efficiencies up to ~52−75%. These nanoparticles offer a vast range of potential applications, which include optical amplifiers, waveguides, laser materials and infrared imaging probes.
© 2009 OSA
Infrared-emitting rare-earth doped materials have been extensively used in fiber amplifiers, solid-state lasers, telecommunications, optoelectronics and remote sensing applications [1–5]. In addition, the recent advent of infrared optical imaging systems has expanded the biomedical applications for infrared-emitting rare-earth doped nanomaterials for diagnostics and deep tissue imaging [6–9]. Optical transitions for rare-earth doped materials are governed mainly by radiative transitions between energy levels of 4f electrons that are shielded by 5s and 5p electrons . The absorption and emission properties of rare-earth doped materials can be further tailored by controlling the local environment, such as site symmetry, crystal field strength and electron-phonon interaction strength of rare-earth dopants.
Er-doped materials with a broad ~1530 nm emission are commonly used in optical communications, eye-safe measurements and spectroscopy [11,12]. Together with the infrared emission at ~1530 nm, upconversion upon excitation at ~975 nm results in visible emissions (~550 nm and ~670 nm) from Er-doped materials (Fig. 1(a) ) [8,13–19]. This near infrared-to-visible upconversion phenomenon observed in Er-doped materials reduces the intensity of infrared emission at ~1530 nm. Subsequently, the low branching ratio of ~0.1-0.2 for the ~1530 nm emission in Er fluoride glasses is a factor that limits its performance in commercial fiber amplifiers. Efficiency improvements for the ~1530 nm emission in Er-doped fluoride glasses are realized with Ce3+ and Yb3+ co-dopants [20–23]. The branching ratio improves from ~0.1-0.2 to ~0.8-0.9 with Ce3+ co-doping , due to the phonon-assisted energy transfer between Er3+ and Ce3+ which facilitates the population of the 4I13/2 level and simultaneously decreases upconversion losses (Fig. 1(b)). Furthermore, the 2F5/2 level of Yb3+ and 4I11/2 level of Er3+ are nearly similar in energy so that the high absorption cross-section of Yb3+ further enhances energy transfer to Er3+ upon excitation at ~975 nm (Fig. 1(c)). Nonetheless, Fig. 1(b) implies that co-doping of Ce and Er can eliminate the near infrared-to-visible upconversion, which has not been reported in the literature. The structural and optical characteristics of Er- and Yb-Er- doped CeF3 nanoparticles synthesized by hydrothermal methods are discussed in this work.
2. Results and discussion
Er- and Yb-Er- doped CeF3 nanoparticles were synthesized using a hydrothermal process . The as-synthesized nanoparticles were heat-treated in a controlled environment using the double crucible method to prevent CeF3 oxidation. Further details on particle processing can be found in the experimental section. Figure 2 is a TEM micrograph that shows Er-doped CeF3 nanoparticles with particle sizes of ~14 ± 6 nm were synthesized. After heat treatment, the particle size distribution increased to ~42 ± 15 nm as a result of particle aggregation. X-ray powder diffraction patterns of batches of both as-synthesized and heat-treated powders confirmed the formation of hexagonal CeF3 (Fig. 3 ). Further evaluation of the diffraction peak width for (1 1 1) using Scherrer’s equation , showed that the average crystallite sizes of as-synthesized and heat treated particles were ~13 − 17 nm and ~16 − 19 nm, respectively. Since particle and crystallite size were similar for as-synthesized CeF3:Er powders, these data indicate that single crystallite nanoparticles were synthesized. In contrast, the difference in particle and crystallite size for heat treated CeF3:Er showed that polycrystalline nanoparticle aggregates were obtained after heat treatment. Since there was significant CeF3 (1 1 1) and YbF3 (1 1 1) peaks overlap, the crystallite size for CeF3:Yb-Er where Yb ≥ 7.5 mol% could not be accurately determined.
The emission spectra of as-synthesized and heat-treated CeF3:Er and CeF3:Yb-Er upon excitation at ~975 nm are shown in Fig. 4(a) and 4(b), respectively. A broad near-infrared emission at ~1530 nm with no visible emissions (i.e. both green and red emissions) was observed for all batches of powder. The absence of visible emissions was consistent with our proposed scheme in Fig. 1(b), where phonon-assisted energy transfer between Er3+ and Ce3+ had eliminated upconversion losses by increasing population density of electrons to the Er3+ 4I13/2 level . Considering that the cutoff phonon energy (ħω) for CeF3 was ~320 cm-1 and energy difference of between the 4I11/2 → 4I13/2 transition of Er3+ () and 2F7/2 → 2F5/2 transition of Ce3+ () [27,28], the estimated number of phonons (N) emitted in the non-radiative decay was ~5 using , assuming that the phonons involved in the energy transfer are of equal energy. The addition of Yb3+ co-dopant in CeF3:Er further increased the emission intensity at ~1530 nm. Comparing the maximum intensities of CeF3:Er and CeF3:Yb-Er in Fig. 4(a) and 4(c), respectively, an intensity enhancement of ~25 – 30 times was achieved with the addition of Yb as a co-dopant. The improved emission intensity with addition of Yb3+ suggests Yb3+-Er3+ interactions were strong, because energy transfer from Yb → Er further increased population density of the Er3+ 4I13/2 level. Further details on the electronic transitions and optical properties of CeF3:Yb-Er will be discussed in a future publication.
Comparing the maximum intensities in Figs. 4(a) with 4(b) and Figs. 4(c) with 4(d), it was observed that heat-treatment further enhanced the emission intensities for both CeF3:Er and CeF3:Yb-Er nanopowders by ~1.5 – 2 times. Amongst the differently doped samples, the maximum emission intensity of ~870 mV (see Fig. 4(d)) and maximum measured luminescence lifetime of ~6.5 ms with a quantum efficiency of ~ 75% (see Fig. 5(b) ), was observed for heat treated CeF3:Yb-Er (7.5, 0.5 mol%) nanoparticles. This measured intensity and luminescence decay time for heat-treated CeF3:Yb-Er (7.5, 0.5 mol%) nanoparticles was comparable to that measured from a standard sample of Er-doped phosphate laser glass (Kigre Inc., Hilton Head Island, South Carolina) of ~900 mV and ~8 ms, respectively. The enhanced emission after heat treatment could be attributed to either enhanced Yb3+-Er3+-Ce3+ interactions due to improved rare earth uniformity, or reduced emission quenching from reduction in the concentration of lattice and surface defects. Possible defects include surface and bulk hydroxyl groups and cation-anion vacancies. Heat treatment also decreased the surface-to-volume ratio as particle size increases, thereby decreasing the contribution of surface defects to observed optical phenomena.
The luminescence decay time and quantum efficiency of the ~1530 nm emission from heat-treated CeF3:Er and CeF3:Yb-Er nanoparticles were measured, as shown in Fig. 5. The quantum efficiency was determined by taking the ratio of measured decay time with that of theoretically calculated decay time. The average measured luminescence lifetimes of the ~1530 nm emission for heat-treated CeF3:Er and CeF3:Yb-Er nanoparticles was ~4.5 − 6.5 ms, with quantum efficiencies ~52 − 75%. The low quantum efficiency of ~52 − 75% for these materials could be attributed to non-radiative recombination losses from the presence of lattice and surface defects that remained after heat treatment. Figure 5(a) shows that for heat-treated CeF3:Er, the luminescence decay time and quantum efficiency decreased linearly as Er concentration increased due to reducing Er-Er interatomic distance. Figure 5(b) shows that for heat-treated CeF3:Yb-Er, the luminescence decay time and quantum efficiency increased with increasing Yb concentration up to ~ 7.5 mol%, as energy transfer efficiency from Yb to Er improved.
The decrease in emission intensity and quantum efficiency for heat-treated CeF3:Yb-Er (10, 0.5 mol%) could be attributed to the phase separation of YbF3. YbF3 phase separation was observed for Yb ≥ 7.5 mol%, as shown by the presence of orthorhombic YbF3 peaks (see Fig. 3(b) and 3(c)). As a consequence of YbF3 phase separation, fewer Yb3+-Er3+ pairs exist for energy transfer leading to lower emission intensities and quantum efficiencies in CeF3:Yb-Er (10, 0.5 mol%). Considering that YbF3 (orthorhombic) and CeF3 (hexagonal) are non-isostructural, Yb solubility in CeF3 is limited to ~7.5 mol%. The limited solubility leads to Yb3+ ion clustering as the solubility limit approaches, which induces concentration quenching. In addition, high concentration Yb doping could induce lattice distortion and alter the rare-earth ion separation distances. This leads to non-radiative losses that would affect the phosphor’s quantum efficiency.
In summary, near-infrared emitting Er- and Yb-Er- doped CeF3 nanoparticles were synthesized using hydrothermal methods. A broad and intense emission at ~1530 nm with no other visible emissions was observed in the as-synthesized and heat-treated CeF3 nanoparticles upon excitation at ~975 nm. Prior to our work, hosts either non-radiatively quench electronic transitions or they play a passive non-interactive role. The novel aspect in this work is the discovery of an active host, which interacts and enhances energy transfer to the rare-earth luminescent center in a manner that facilitates infrared emission while avoiding parasitic emissions that are not of interest. In addition, it was also observed that intensity of the ~1530 nm emission was significantly improved by ~25 times with the addition of Yb in Er-doped CeF3. The average measured luminescence lifetimes of the ~1530 nm emission for heat-treated CeF3:Er and CeF3:Yb,Er nanoparticles was ~4.5 − 6.5 ms, with quantum efficiencies up to ~52 − 75%. These nanoparticles offer a vast range of potential applications, which include optical amplifiers, waveguides and laser materials.
4. Experimental methods
Stoichiometric amounts of 99.5% cerium (III) nitrate, 99.9% erbium (III) nitrate, 99.9% ytterbium (III) nitrate and 98% ammonium fluoride (Sigma Aldrich, St. Louis, Missouri) were mixed in ~75 mL of water for 30 min. This mixture was next transferred to a 125 mL Teflon liner and heated to ~200°C for 2 h in a Parr pressure vessel (Parr Instrument Company, Moline, Illinois). The as-synthesized nanoparticles were washed three times in deionized water by centrifuging and dried at 70°C in a oven (Thermo Scientific Thermolyne, Waltham, Massachusetts) for further powder characterization. Heat-treatment of as-synthesized particles was completed in a controlled environment using the double crucible method to prevent CeF3 oxidation. 10 mL and 50 mL alumina crucibles (CoorsTek, Golden, Colardo) were used for the heat treatment. ~0.9 g of as-synthesized nanoparticles (inner 10 mL crucible) was heated with ~3.0 g of 95% ammonium bifluoride (outer 50 mL crucible) at ~400°C for 1 h in a box furnace.
Transmission electron microscopy (TEM) images of samples on 400-mesh carbon-coated copper grids (Electron Microscopy Sciences, Hatfield, Pennsylvania) were taken using the JEOL 100CX transmission electron microscope (JEOL, Tokyo, Japan) equipped with a LaB6 gun operating at an accelerating voltage of 80 kV. Powder x-ray diffraction (XRD) patterns were obtained with a resolution of 0.04º/step and 2 sec/step with the Siemens D500 (Bruker AXS Inc., Madison, Wisconsin) powder diffractometer (40 kV, 30 mA), using Cu Kα radiation (λ = 1.54 Å). Powder diffraction files (PDF) from International Centre for Diffraction Data (ICDD, Newtown Square, Pennsylvania) for CeF3 PDF#97-000-0004 and YbF3 PDF#97-000-9844 were used as references.
The emission spectra of nanoparticles excited at ~976 nm with a 0.7 W laser (BW976, BW Tek, Newark, New Jersey), was collected, focused and dispersed using a 0.55 m Triax 550 monochromator (Jobin Yvon, Edison, New Jersey). The signals were detected with a thermoelectrically cooled InxGa1-xAs detector (Electro-Optical Systems, Phoenixville, Pennsylvania). A lock-in amplifier (SR850 DSP, Stanford Research System, Sunnyvale, California) amplified the output signal from the detector. The spectrometer and detection systems were interfaced using a data acquisition system that was controlled with Synerjy commercial software (Jobin Yvon and Origin Lab Corporation). Radiative decay time of CeF3:Er nanoparticles excited at 976 nm with a 0.7 W laser modulated at ~40 Hz, was measured using a digital storage oscilloscope (TDS 220, Tektronix, Richardson, Texas).
We would like to thank V. Starovoytovy for his assistance with TEM. The authors acknowledge the Defense Advanced Research Projects Agency (ONR-N00014-08-1-0131) for funding this research.
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