Introduction

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 [10]. 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 [20], due to the phonon-assisted energy transfer between Er³⁺ and Ce³⁺ which facilitates the population of the ⁴I_13/2 level and simultaneously decreases upconversion losses (Fig. 1(b)). Furthermore, the ²F_5/2 level of Yb³⁺ and ⁴I_11/2 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.

 

Fig. 1 Schematic of electronic transitions in (a) Er-doped fluorides (e.g. NaYF₄) where near infrared-to-visible upconversion process dominates, (b) CeF₃:Er and (c) CeF₃:Yb-Er. Dashed lines indicate non-radiative transitions.

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2. Results and discussion

Er- and Yb-Er- doped CeF₃ nanoparticles were synthesized using a hydrothermal process [24]. 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 [25], 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.

 

Fig. 2 TEM micrographs of CeF₃:Er (0.5 mol%), (a) as-synthesized and (b) heat-treated at 400°C, 1h.

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Fig. 3 XRD profile of (a) as-synthesized and heat-treated CeF₃:Er, (b) as-synthesized CeF₃:Yb,Er and (c) heat treated CeF₃:Yb,Er. YbF₃ diffraction peak positions indicated by *. All other diffraction peaks were identified as CeF₃.

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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 Er^{3+ 4}I_13/2 level [26]. Considering that the cutoff phonon energy (ħω) for CeF₃ was ~320 cm^-1 and energy difference of $\Delta E~1486c{m}^{-1}$ between the ⁴I_11/2 → ⁴I_13/2 transition of Er³⁺ ($\Delta E_{Er}~3656c{m}^{-1}$) and ²F_7/2 → ²F_5/2 transition of Ce³⁺ ($\Delta E_{Ce}~2170c{m}^{-1}$) [27,28], the estimated number of phonons (N) emitted in the non-radiative decay was ~5 using $N=\Delta E/\hslash \omega$, 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 Er^{3+ 4}I_13/2 level. Further details on the electronic transitions and optical properties of CeF₃:Yb-Er will be discussed in a future publication.

 

Fig. 4 Measured emission from (a) as-synthesized and (b) heat-treated CeF₃:Er, and (c) as-synthesized and (d) heat treated CeF₃:Yb-Er upon excitation at ~975 nm.

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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.

 

Fig. 5 Measured decay time and quantum efficiency for ~1530 nm emission from heat treated (a) CeF₃:Er and (b) CeF₃:Yb-Er upon excitation at ~975 nm.

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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.

3. Conclusions

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 K_α 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 In_xGa_1-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).