Significant enhancement of visible up-conversion emissions of Y₂O₂S:Er³⁺ phosphors by Mn²⁺ sensitizing under 1550 nm excitation

Abstract

A novel up-conversion (UC) emission route of Er³⁺ by Mn²⁺ sensitizing in a Y₂O₂S:Er³⁺ phosphor prepared via a high-temperature solid-state reaction method is reported here. This methodology resulted in a significant enhancement of visible up-conversion emission by 1550 nm laser diode excitation. By co-doping Mn²⁺, the visible (red and green) UC emission intensity, which is assigned to the transitions of ⁴F_9/2 → ⁴I_15/2 (670 nm), ⁴S_3/2 → ⁴I_15/2 (547 nm), ²H_11/2 → ⁴I_15/2 (528 nm) of Er³⁺, was 2 to 3 times higher than that of the Mn²⁺-free one. As the concentration of Mn²⁺ was increased, the visible UC emission intensity was enhanced and then reduced. The phosphor powders were well crystallized in a hexagonal shape with an average size of ca. 2 μm. Based on the results of the luminescence spectra, energy matching conditions and three-photon process dependence on excitation power, a possible UC mechanism is proposed. The proposed sensitizing route may lead to a promising step towards high-intensity UC emissions of rare-earth ion doped phosphors.

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Introduction

The lanthanide infrared (IR)-to-visible up-conversion (UC) phenomenon has attracted great attention for its potential applications in IR pumped imaging, bio-imaging, displays, lasers, and solar cells.^1–3 With the advent of low cost, readily available and powerful IR laser diodes, there has been a renewed interest in UC. Notably, the trivalent Er³⁺ ions are considered the potential up-convertors for red, green, and blue emissions because of their rich energy levels, which well fit the excitation of commercial IR laser diodes. However, Er³⁺ ions doped samples usually exhibit low UC emission intensity when excited by the lights in the range of 1000–1600 nm, which strongly limits the application and development of these UC phosphors in the infrared wavelength region. Yb³⁺ ions have been virtually exclusively used as a trivalent rare earth sensitizer at the 980 nm excitation, but they are not effective in the 1000–1600 nm excitation range.^4–6 In particular, Yb³⁺ ions perform poorly when excited by a commercial 1550 nm laser diode, which is one of the most widely used laser diodes on market currently. In order to improve the UC emission intensity at the 1550 nm excitation, we must seek alternative efficient sensitizers.

Non-rare-earth ions, which exhibit excellent performance, have been frequently used as co-dopants to improve the phosphor optical properties.^7–9 As reported in the literature,^10–14 UC of rare earth (RE)-transition metal (TM) ions co-doping system benefits from the combined advantages of specific energy levels of RE ions and tunable energy levels of TM ions, which are very attractive for a wide range of applications.^15–17 The choice of different hosts and RE/TM dopants may result in novel and desirable UC properties.¹⁸

TM ions have been widely used in luminescent materials. Among them, Mn²⁺, as a typical luminescent center, has attracted high interest for various applications because of its unique optical properties.^19–24 Mn²⁺ has been studied in various inorganic hosts covering a wide emission range from blue to red.²⁵ Recently, green UC emission of Mn²⁺ at room temperature was observed in non-halide material LaMgAl₁₁O₁₉:Yb³⁺, Mn²⁺.^15,17 The UC luminescence of the Er³⁺/Mn²⁺ pair was extensively investigated in Yb₃Al₅O₁₂ at low temperatures under IR laser excitation.²⁶ Compared to other host matrices (e.g., chalcogenides, fluorides, oxides), the commercial phosphor host Y₂O₂S exhibits favorable chemical stability as well as higher luminescent efficiency due to its moderate phonon energies (about 520 cm⁻¹), wide band gap, and high energy transfer efficiency.²⁷ Thus, Y₂O₂S is expected to exhibit UC when co-doped with Er³⁺ and Mn²⁺ due to the crystal local field.

In this work, Er³⁺/Mn²⁺ co-doped Y₂O₂S phosphors were prepared, with an aim to develop a novel route that can significantly enhance the visible UC emission intensity by the 1550 nm excitation. To the best of our knowledge, this is the first report on the 1550 nm excited UC spectral studies in Er³⁺/Mn²⁺ co-doped Y₂O₂S phosphors.

Experimental

Sample preparation

The phosphors were prepared by the high-temperature solid-state reaction method. Stoichiometric amounts of starting materials Y₂O₃ (99.99%), Er₂O₃ (99.99%) and S (99.999%), MnCO₃ (analytical grade) and Na₂CO₃ (analytical grade) were thoroughly mixed in an agate mortar. The concentrations of these rare earth oxides and MnCO₃ were formulated according to the chemical formula of (Y_0.95−xEr_0.05Mn_x)₂O₂S (x = 0, 0.01, 0.03, 0.05, 0.07, 0.10) and (Y_0.97Mn_0.03)O₂S (denoted as YM1). The Na₂CO₃ acted as a flux for the sintering process. The pre-determined amount of the starting materials were mixed, and sintered at 1100 °C for 3 h using an alumina crucible with an alumina cover in carbon reducing atmosphere,²⁸ and then cooled down to room temperature in the furnace. The sintered samples were further treated by washing with dilute hydrochloric acid (pH ≈ 1) and distilled water several times. The washed powders were subsequently filtered and dried. The resultant fine powders were collected for characterization.

Characterization

The phosphor samples were characterized by X-ray diffraction (XRD) (Rigaku, D/Max-RB, Cu K_α radiation, λ = 0.15406 nm). Electron paramagnetic resonance (EPR) spectra of the samples were recorded on a Bruker EMX-8/2.7 EPR spectrometer operating in the X-band frequency (9.877 GHz). The morphology of the phosphor samples was examined by a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi High-Technologies). Absorption spectra of the samples were acquired on a UV-Vis-NIR spectrophotometer (CARY 500, Varian Company). The UC luminescence spectra were recorded at room temperature on an Edinburgh FLS920P spectrometer, excited by a tunable 1550 nm semiconductor laser diode (0–500 mW).

Results and discussion

The XRD patterns of the as-synthesized samples with various Mn²⁺ ions concentrations are shown in Fig. 1. According to the XRD patterns, the phases of all samples are hexagonal Y₂O₂S (JCPDS # 24-1424) without any detectable impurity, which suggests that the crystalline hexagonal phase of Y₂O₂S samples were obtained by the high-temperature solid-state reaction method and their structural morphology was not influenced by the addition of Mn²⁺ ions. Fig. 1 also shows that the Mn²⁺-doping approach does not result in phase transformation, but the 2θ values of the XRD peaks for all Er/Mn co-doped samples are slightly lower than those of typical hexagonal Y₂O₂S. Y₂O₂S crystallizes in space group P3̄m1 (164) with the metal ions at the Wyckoff site 2d with 3m (C_3v) point group symmetry, surrounded by four oxygen and three sulfur ions.²⁹ The calculated cell parameters (a, b, c) (listed in Table 1) of the samples become slightly larger, indicating that the Mn²⁺-doped ions have a weak influence on the host crystal lattice.

Fig. 1 XRD patterns of (Y_0.95−x)₂O₂S:Er_0.1,Mn_x (x = 0, 0.01, 0.03, 0.05, 0.07, 0.10) samples and standard Y₂O₂S crystal.

Table 1 The unit cell parameters of the samples (Y_0.95−xEr_0.05Mn_x)₂O₂S (x = 0, 0.01, 0.03, 0.05, 0.07, 0.10) compared with the standard trigonal Y₂O₂S

Sample no.	Formula	Unit cell parameters
		a/Å	b/Å	c/Å
Reference	Y2O2S	3.785	3.785	6.571
YEM0	(Y0.95Er0.05)2O2S	3.789	3.789	6.609
YEM1	(Y0.95−xEr0.05Mn0.01)2O2S	3.799	3.799	6.616
YEM2	(Y0.95−xEr0.05Mn0.03)2O2S	3.806	3.806	6.621
YEM3	(Y0.95−xEr0.05Mn0.05)2O2S	3.812	3.812	6.625
YEM4	(Y0.95−xEr0.05Mn0.07)2O2S	3.817	3.817	6.627
YEM5	(Y0.95−xEr0.05Mn0.10)2O2S	3.824	3.824	6.628

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Generally, there are two possible sites for the doped Mn²⁺: the interstices or the sites of Y³⁺ (the radius difference is less than 30%).³⁰ From the increase of the cell parameters (a, b, c) as shown in Table 1, one can conclude that the occupy sites should be the interstices because the ionic size of Mn²⁺ is significantly smaller than that of Y³⁺. This is also consistent with a recent report by Yang et al.,³¹ which suggested that Mn²⁺ could partially enter interstitial sites when Mn²⁺ cations were doped into SrTiO₃ due to the relatively large difference in ionic radius between Sr²⁺ and Mn²⁺. To compensate the charge of Mn²⁺ in the interstitial sites, some O²⁻ or S²⁻ should also exist there. Hence, luminescence defects will be produced during the interstitial process of Mn²⁺ (assigned as I_(Mn)).^32,33 These defects would introduce some new energy levels in the energy band of Y₂O₂S host matrix, which has significant influence on the photoluminescence properties of the phosphors.^34,35

The prepared samples with various Mn²⁺ concentrations exhibit similar particle size and size distribution. Fig. 2 shows a representative SEM image of the as-fabricated (Y_0.92Er_0.05Mn_0.03)₂O₂S sample. The sample is of hexagonal morphology with smooth surface and an average grain size is ca. 2 μm.

Fig. 2 SEM image of (Y_0.92Er_0.05Mn_0.03)₂O₂S sample.

EPR was further utilized to determine the existence of Mn²⁺. Fig. 3 presents the EPR spectra of (Y_0.95−xEr_0.05Mn_0.03)₂O₂S and (Y_0.94Mn_0.03)₂O₂S samples. The resonance signals at a g value of ca. 2.0 were observed, in good agreement with the characteristic electron spin S and nuclear spin I of Mn²⁺ (S = I = 5/2).³⁶ Therefore, the EPR results support that Mn²⁺ ions have been successfully doped into the Y₂O₂S host lattice.

Fig. 3 EPR spectra of (Y_0.92Er_0.05Mn_0.03)₂O₂S and (Y_0.97Mn_0.03)₂O₂S samples.

The absorption spectra of the samples with various Mn²⁺ concentrations are shown in Fig. 4, from which several absorption bands of Er³⁺ in all Er or Er/Mn co-doped samples can be observed. These absorption bands can be attributed to the transitions of ⁴I_15/2 → ⁴G_11/4 (381 nm), ⁴I_15/2 → ⁴F_7/2 (495 nm), ⁴I_15/2 → ²H_11/2 (524 nm), ⁴I_15/2 → ⁴F_9/2 (661 nm), ⁴I_15/2 → ⁴I_9/2 (808 nm), ⁴I_15/2 → ⁴I_11/2 (980 nm), ⁴I_15/2 → ⁴I_13/2 (1550 nm) of Er³⁺.³⁷ By increasing Mn²⁺ concentration from 0 to 10.0 mol%, there was little influence on the entire absorption in the VIS-IR range. The zoomed absorption spectrum of the Mn²⁺ single-doped sample YM1 is shown in the inset of Fig. 4, from which one can observe an obvious absorption band in the range of 800 to 1000 nm wavelength. This absorption band does not match any Mn²⁺ energy level according to Tanabe–Sugano diagram for d⁵ ions,³⁸ which suggests that the band probably originates from I_(Mn). Moreover, such a band overlaps with the Er³⁺ doped samples.

Fig. 4 Absorption spectra of (Y_0.95−xEr_0.05Mn_x)₂O₂S (x = 0, 1.0, 3.0, 5.0, 7.0, 10.0 mol%).

Room temperature UC luminescence spectra of (Y_0.97Mn_0.03)₂O₂S, (Y_0.95Er_0.05)₂O₂S and (Y_0.92Er_0.05Mn_0.03)₂O₂S samples when excited at 980 nm are shown in Fig. 5. It is interesting to note that the sample doped with Mn²⁺ only exhibited a broad non-symmetric emission band peaking at ca. 830 nm accompanied with a weak hump centered at ca. 771 nm. This is in accordance with the absorption spectrum of (Y_0.97Mn_0.03)₂O₂S. Such broad emission band only exists in Mn²⁺ single-doped samples, but is not shown in the Er³⁺/Mn²⁺ co-doped samples. Therefore, it is concluded that doping of Mn²⁺ would introduce a new energy level (assigned as E_[Mn]) at ca. 830 nm. This energy level overlaps with I_9/2 level of Er³⁺, which can potentially facilitate the energy transfer from E_[Mn] to Er³⁺ in Er³⁺/Mn²⁺ co-doped samples.

Fig. 5 UC luminescence spectra of samples excited at 980 nm.

Room temperature UC luminescence spectra of the samples with various Mn²⁺ concentrations when excited at 1550 nm are shown in Fig. 6. The inset shows a photograph of the UC emission (Y_0.92Er_0.05Mn_0.03)₂O₂S upon 50 mW excitation at 1550 nm. In Fig. 6(a), the emission spectra in the 520–680 nm range are characterized by an intense green band at ca. 547 nm followed by an intense red band at ca. 670 nm. These emissions are strong to the naked eyes. The emission bands are observed in three regions that are composed of several overlapping peaks due to the crystal-field splitting, and can be attributed to the transitions ⁴F_9/2 → ⁴I_15/2 (670 nm), ⁴S_3/2 → ⁴I_15/2 (547 nm), ²H_11/2 → ⁴I_15/2 (528 nm) of Er³⁺. The sample without Mn²⁺ exhibits the lowest emission intensity among all samples. During the measurement, we also found that doping of Mn²⁺ did not result in peak shape or position evolution. And, Mn²⁺ single-doped sample (Y_0.97Mn_0.03)₂O₂S has no UC emission, which means that the introduced new energy level E_[Mn] by Mn²⁺ does not match the energy of 1550 nm. In Fig. 6(b), as the concentration of Mn²⁺ ions increases from 1.0 to 3.0 mol%, the emission intensity increases gradually. It is noticeable that further increasing the concentration of Mn²⁺ to 10.0 mol% remarkably decreases the intensity of both green and red emission bands. The involved mechanism will be further discussed in the following paragraphs. It should be noted that the UC emission intensity of (Y_0.92Er_0.05Mn_0.03)₂O₂S phosphor was 2–3 times as intense as (Y_0.95Er_0.05)₂O₂S.

Fig. 6 (a) UC luminescence spectra of (Y_0.95−xEr_0.05Mn_x)₂O₂S (x = 0, 1.0, 3.0, 5.0, 7.0, 10.0 mol%) samples excited at 1550 nm. The inset is a photograph of the UC emission of (Y_0.92Er_0.05Mn_0.03)₂O₂S under 50 mW excitation power. (b) Variations of UC emission (528, 548 and 670 nm) intensity as a function of Mn²⁺ concentration.

The dependence of the pumping power and luminescence intensity of (Y_0.92Er_0.05Mn_0.03)₂O₂S sample is shown in Fig. 7, in which the number of photons n determined from the slope coefficient of the linear fitted lines is 2.21, 2.83, 2.97 monitored at the emission peak wavelength 670, 548 and 524 nm, respectively. This result indicates that the UC luminescence is attributed to a three-photon absorption (TPA) mechanism, which is consistent with the literature.²

Fig. 7 Log–log plot of the UC emission intensities as a function of the 1550 nm excitation power for (Y_0.92Er_0.05Mn_0.03)₂O₂S sample.

Based on the results of the luminescence spectra, the energy matching conditions, and the TPA processes, the following possible UC mechanism is proposed as shown in Fig. 8. Initially, Er³⁺ can be directly excited from ground state to ⁴I_13/2 level via the absorption of a 1550 nm photon, and it can be further excited to ⁴I_9/2 level by absorbing another 1550 nm photon. The electron of Er³⁺ at ⁴I_9/2 level can be further excited to a higher energy level (²H_11/2), generating the emission bands at 528, 548, and 670 nm (²H_11/2 → ⁴I_15/2, ⁴S_3/2 → 4I_15/2, and ⁴F_9/2 → 4I_15/2 transitions, respectively) through non-radiation relaxation. For this reason, the Er³⁺ doped sample can exhibit UC luminescence. When Mn²⁺ was co-doped in the matrix, it turned to be different because I_(Mn) could act as an efficient sensitizing center. Since the energy of ⁴I_9/2 level of Er³⁺ is slightly larger than that of the E_[Mn] level originated from I_(Mn), the energy transfer (ET) process from Er³⁺ to I_(Mn) dominates in the excitation of E_[Mn] level (ca. 830 nm, which can be reflected from Fig. 5). As the E_[Mn] matches well with ⁴I_13/2–²H_11/2, part of the populated ⁴I_13/2 level is excited to the ²H_11/2 state from the I_(Mn) emission channel. Similar energy transfer between Er³⁺ and Mn²⁺ was also reported in a recent research.³⁹ After these excited state processes, de-excitation accumulates electrons in different excited states and the intensity of emission depends on the electron density at that particular level as well as the non-radiative contribution to the emission band. The level ²H_11/2 acts as a meta-stable level and the emission from ²H_11/2 to ⁴I_15/2 results in 528 nm emission. Since the ²H_11/2 level couples to the ⁴S_3/2 level by lattice phonon relaxation between closely spaced energy levels, the level ⁴S_3/2 act as a meta-stable level and the emission from ⁴S_3/2 to ⁴I_15/2 results in 547 nm emission. Fractional populations accumulated in level ⁴F_9/2 decays to ⁴I_15/2, giving rise to emissions at 670 nm. In oxysulfide hosts, because of the weak lattice phonon relaxation, the non-radiative contribution to 547, 670 nm emission bands forms the energy gap between ⁴S_3/2 ↔ ⁴F_9/2 (Er³⁺).

Fig. 8 Proposed UC luminescence mechanism of Er³⁺, Mn²⁺ co-doped Y₂O₂S excited at 1550 nm.

The optimal doping concentration of Mn²⁺ is found to be 3.0 mol%, and the corresponding emission intensity is 2–3 times higher than that of (Y_0.95Er_0.05)₂O₂S. By Mn²⁺ sensitizing in Y₂O₂S:Er³⁺ phosphor, the visible UC emission intensity enhances significantly due to the increase of the photon numbers of the ²H_11/2 (Er³⁺) level. With the further increase of Mn²⁺ ions, however, the intensity of luminescence grows less intense. Similar phenomena have been widely reported for the Mn²⁺ ions doped glasses,^40,41 and are ascribed to the concentration quenching effect that is due to the cross-relaxation caused by the interactions of Mn²⁺ ions.

Conclusions

The 1550 nm excited UC processes of Er³⁺/Mn²⁺ co-doped Y₂O₂S phosphors prepared by a high-temperature solid-state reaction method is reported for the first time. Through Mn²⁺ ions doping, hexagonal phase of the samples remains intact, and their average size is ca. 2 μm. By co-doping Mn²⁺, Y₂O₂S:Er³⁺ samples exhibit strong green and red UC emission under the excitation of 1550 nm laser. The UC emission intensity of (Y_0.92Er_0.05Mn_0.03)₂O₂S phosphor is 2–3 times higher than that of (Y_0.95Er_0.05)₂O₂S. It is hoped that this report will stimulate further discoveries of other UC materials with the designed structure and desirable optical functionalities.

Acknowledgements

This work is financially supported by the Natural Science Foundation of Shanghai (12ZR1407600, 11ZR1409300), the Fundamental Research Funds for the Central Universities, the Shanghai Leading Academic Discipline Project (B502), and the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology). L.S. acknowledges the National Science Foundation (Partnerships for Research and Education in Materials, DMR-1205670), Air Force Office of Scientific Research (no. FA9550-12-1-0159), and a Faculty Large Grant from the University of Connecticut for partial support of this project.

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