Up-conversion routines of Er3+–Yb3+ doped Y6O5F8 and YOF phosphors
Graphical abstract
Introduction
Quantum counter action or excited-state absorption stimulated by rare-earth ions in host materials, which generates visible light, was proposed in 1959 [1]. These luminescence compounds emit visible light based on the principles of anti-Stokes (up-conversion) processes wherein the excitation photons produce higher energies from weaker input energies. In 1966, Auzel suggested the function of the energy transfer, which takes place between two ions such as Er3+ and Yb3+ ions in host lattices, via the up-conversion process [2]. The Er3+ ions in host lattices emit blue (2H9/2 → 4I15/2), green (2H11/2, 4S3/2 → 4I15/2), and red (4F9/2 → 4I15/2) light as an activator under infrared radiation. Yb3+ ions act as a sensitizer via energy transfer (ET) from the activator to enhance the up-converting efficiency. Infrared-pumped visible laser light, stimulated by the ET, was developed with up-conversion luminescent materials in the early 1970s. Since then, solid-state 3D displays, oxygenic photosynthesis, optical sensing and imaging, photodynamic therapy, and solar cells were extensively developed from these materials [3], [4], [5], [6], [7], [8]. Strümpel et al. suggested that Stark splitting of energy levels caused by an electric field and the creation of phonon energy due to non-radiative relaxation can play important roles in producing highly efficient up-conversion emission in rare-ion doped host-materials [7]. Furthermore, they reported that smaller absorption spectrum by Stark splitting occurs with increasing ionic radius and that relatively low phonon energy arises in chloride or bromide host-materials compared to fluoride or oxide compounds. In comparison with pure chloride and fluoride compounds, oxide-chloride or oxide-fluoride host-materials have several merits, including chemical stability, low toxicity, and convenient methods of synthesis [9], [10], [11], [12], [13], [14].
In a previous study, the spectral up-converting properties of Er3+ and Er3+–Yb3+ doped Y(La)OCl phosphors, exhibiting multi-photon processes, were elucidated under 980 and 1550 nm diode laser excitation [13], [14]. Subsequently in this paper, Er3+–Yb3+ substituted yttrium-oxyfluoride compounds, which contain stoichiometric YOF and nonstoichiometric Y6O5F8 structures, were prepared as spectral-converting luminescent host materials using a simple process with NH4F flux in air. Fig. 1 shows YOF stoichiometric and Y6O5F8 nonstoichiometric oxyfluorides, which are comprised of alternating layers of cations (YO)n+ and Fn− ions with a rhombohedral crystal system (R-3m space group) and a single (YO)n+ layer sandwiched by Fn− layers with orthorhombic centrosymmetric symmetry (Pcmb), respectively [15], [16]. The host structure of YOF contains a single Y3+ site coordinated with four oxygen and four fluorine atoms; on the other hand, in the Y6O5F8 host lattice, which is called the vernier phase, the Y3+ sites are coordinated by four O2− and three or four F− anions. The up-conversion luminescence properties of Er3+ and Er3+–Yb3+ doped yttrium-oxyfluoride phosphors were investigated under excitation with a 980-nm-wavelength diode laser. From these results, the up-conversion mechanisms are proposed on the basis of an energy-level diagram of the Er3+ and Yb3+ ions. Using these up-conversion luminescence materials, the desired emission of green, orange, and red light was achieved. Intense up-conversion emission spectra was attained and the dependence of the emission intensity of the Y6(0.995−q)Er0.03Yb6qO5F8 (q = 0.01, 0.05, and 0.1) phosphors on the excitation power was investigated.
Section snippets
Experimental
Samples of Y6(1−p−q)Er6pYb6qO5F8 (p = 0.001–0.1, q = 0.005–0.1) and Y0.99Er0.01OF were prepared by heating the appropriate stoichiometric amounts of Y2O3 (Alfa 99.9%), Er2O3 (Alfa 99.9%), Yb2O3 (Alfa 99.9%) and NH4F (Alfa 99%) at 950 °C for 30 min in air [13], [14]. Phase identification was established using a Shimadzu XRD-6000 powder diffractometer (Cu-Kα radiation) and the unit cell parameters were determined by using the Rietveld refinement program Rietica. UV spectroscopy to measure the
Results and discussion
Phase identification of YOF:Er0.01 and Y6(1−p−q)Er6pYb6qO5F8 (p = 0.001–0.1, q = 0.005–0.1) solid solutions was performed based on the powder X-ray diffraction (XRD) patterns obtained after Er3+ and Yb3+ ions had substituted for Y3+ ions in the YOF and Y6O5F8 host lattice. The synthesis scheme for single-phase Y6(1−p−q)Er6pYb6qO5F8 phosphors was optimized by heating the material at 950 °C for 30 min and using Y2O3, Er2O3, Yb2O3, and NH4F. XRD patterns of the calculated Y2O3 (ICSD 27772), YF3 (ICSD
Conclusions
Promising single-phase, spectral converting phosphors composed of Y6(1−p−q)Er6pYb6qO5F8 (p = 0.001–0.1, q = 0.005–0.1) and Y0.99Er0.01OF were efficiently prepared via a simple solid-state reaction using NH4F flux at 950 °C for 30 min. The up-conversion spectra of Er3+ and/or Er3+–Yb3+ in Y6O5F8 and YOF phosphors were analyzed under excitation with a 980-nm diode-laser. The effective up-converted photon processes in Er3+-doped orthorhombic Y6O5F8 phosphors were eventually monitored. The up-conversion
Acknowledgment
We acknowledge financial support from KIST institutional funding (project No. 2E23891).
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Cited by (9)
Multicolor-tunable emissions of YOF: Ln<sup>3+</sup>/Yb<sup>3+</sup> (Ln<sup>3+</sup> = Ho<sup>3+</sup>, Er<sup>3+</sup>, Tm<sup>3+</sup>) nanophosphors
2018, Dyes and PigmentsCitation Excerpt :The cubic β-YOF structure occurs as the result of the phase transition from the rhombohedral YOF at temperature of 560–570 °C [1,21]. Nonstoichiometric compounds usually have a tetragonal crystal form, but additional studies have shown that these materials are more complex than it was previously assumed; one example are compound with general formula REnOn-1Fn+2, 5 ≤ n ≤ 9 such as Y5O4F7, Y6O5F8 and Y7O6F9, which crystallize in an orto-rhombohedral form [10,19,22–25]. To prepare oxyfluorides, several methods can be employed: thermolysis method, sol-gel method, fluorolytic sol-gel, hydrothermal method, co-precipitation method, solid state synthesis, combustion synthesis, stearic acid method, urea based homogenous precipitation [4,16,23,26,27].
Ionic liquid-assisted synthesis of Yb<sup>3+</sup>-Tm<sup>3+</sup> codoped Y<inf>7</inf>O<inf>6</inf>F<inf>9</inf> petal shaped microcrystals with enhanced upconversion emission
2018, Materials Research BulletinCitation Excerpt :Oxyfluorides, combining the advantages of both fluoride and oxide crystals, not only have better chemical and thermal stability than fluorides but also have effective lower phonon energy than oxides [20]. Recently, a number of oxyfluorides compounds have been prepared and studied, such as YOF: Er3+, Yb3+; GdOF: Er3+, Yb3+; LaOF: Er3+, Yb3+; Y6O5F8: Er3+, Yb3+; Y7O6F9: Eu2+, Mn2+ and Y7O6F9: Yb3+, Tm3+ [21–25]. Among these works, the Y7O6F9 powders codoped with Yb3+ and Tm3+ ions was first successfully achieved by co-precipitation and subsequent calcining route.
Synthesis and distinct up-converting behaviors of Er<sup>3+</sup>, Yb<sup>3+</sup> doped LaOF and LaO<inf>0.65</inf>F<inf>1.7</inf> phosphors
2017, Optical MaterialsCitation Excerpt :In a previous study, the spectral up-converting properties of Er3+-Yb3+ doped stoichiometric YOF and non-stoichiometric Y6O5F8 vernier structures, as spectral-converting luminescent host materials, were elucidated under 980 nm diode laser excitation [13]. When rare-earth ions such as Er3+ and Yb3+ were substituted in a non-stoichiometric Y6O5F8 host lattice, improved spectral converting properties were observed owing to the low non-radiative relaxation due to the strong formation of a metastable energy level [13,14]. Fig. 1 shows the structures of tetragonal stoichiometric LaOF and non-stoichiometric LaO0.65F1.7 oxyfluorides.