Mn2+ co-doped dual-phase glass ceramics

Mn2+ co-doped dual-phase glass ceramics

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Ceramics International xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

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Tunable broadband upconversion luminescence from Yb3+/Mn2+ co-doped dual-phase glass ceramics Xueyun Liua,b,∗, Cuimei Chenga, Na Zenga, Xiaoman Lic, Qing Jiaoa, Changgui Lina, Shixun Daia,∗∗ a

Laboratory of Infrared Materials and Devices, The Research Institute of Advanced Technologies, Ningbo University, Ningbo, Zhengjiang, 315211, China State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, Guangdong, 510640, China c School of Physics and Physical Engineering, Qufu Normal University, Qufu, Shandong, 273165, China b



Keywords: Tunable broadband Upconversion Glass ceramics Yb3+-Mn2+ dimers

In comparison to narrow-band upconversion (UC) luminescence from lanthanide-doped systems, broadband UC emission enables a broad and tailorable emissive color, which is desirable for tunable lasers and multicolor display applications. Herein, dual continuous broadband UC emission covering the visible spectral region at 480–720 nm has been achieved for the first time in Yb3+/Mn2+ co-doped glass ceramics (GCs) functionalized with the precipitation of dual-phase γ-Ga2O3/β-YF3 nanocrystals. A tunable UC emissive color from yellowish to orange could be obtained under 980 nm excitation by varying the Mn2+ doping content and heat treatment temperature of GCs. We ascribe this phenomenon to the typical transition of exchange-coupled Yb3+-Mn2+ dimers in tetrahedral and octahedral ligand environments. The Stokes and power-dependent UC luminescence properties, fluorescence lifetimes, site occupation and the involved UC mechanism are discussed in detail. This research contributes to a better understanding of the UC process between Yb3+ and Mn2+ in transparent hosts and provides a novel perspective for designing broadband-emitting UC materials.

1. Introduction In recent decades, upconversion (UC) luminescent materials have attracted extensive interests due to their potential applications in functional lighting, multicolor display, bioimaging, solar energy harvesting and lasers [1–4]. Most research into UC materials has focused on rare earth ion (REI)-doped powders or transparent glasses/ceramics, particularly for Yb3+/Er3+ (Tm3+, Ho3+) co-doped systems due to their highly efficient UC performance that mainly stems from their unique 4f electron configuration [5–7]. Unfortunately, the relatively narrow emission band and fixed emission wavelength of REIs inevitably restricts their applications as broadband light sources in illuminations, photovoltaics and continuous wave-tunable lasers, etc. [8]. Compared with REIs, a transition metal (TM) ion, Mn2+, which is known as an important participant in inorganic phosphors, generally shows desirable wavelength-tunable broadband emission because of its typical 3d5 electronic configuration and susceptibility to the surrounding crystal field environment [9]. The UC luminescence behavior of Mn2+ was initially observed in Yb3+-doped manganese chlorides (CsMnCl3, Rb2MnCl4, MnCl2) and manganese bromides (MnBr2,

CsMnBr3) [10–13], befitting from the formation of Yb3+-Mn2+ pairs. As distinguished from the traditional sensitization of Yb3+ to REIs, the Yb3+-Mn2+ pairs can generate visible broadband luminescence by successively absorbing photon energy at 980 nm. For a long time, however, this unique UC emission occurred only in microtherms (12–100 K) and was quenched at room temperature (RT) because of the severe nonradiative transition of Mn2+. Such temperature limitation was overcome in 2009 by Rodríguez et al., who demonstrated UC green emission in a LaMgAl11O19:Yb3+/Mn2+ system at RT [14]. In 2016, Song et al. obtained bright RT broadband UC luminescence via selecting a series of Yb3+/Mn2+ co-doped fluoride perovskites ABF3 (A = Na+, K+, Rb+; B=Mg2+, Zn2+) nanocrystals (NCs), which has increased the attention on Mn2+-based UC materials [15]. In comparison to powder materials, glass ceramics (GCs) obtained by controlling the formation of NCs in a glass host possess excellent characteristics, such as higher optical transparency, easier manufacture procedure, better mechanical and chemical stability, and a good crystal environment for emitters [16–20]. It is thus highly meaningful to develop glass ceramics that combine the virtues of glass and crystalline materials for improving UC luminescence efficiency. Previous UC luminescence studies related to

Corresponding author. Laboratory of Infrared Materials and Devices, The Research Institute of Advanced Technologies, Ningbo University, Ningbo, Zhengjiang, 315211, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (X. Liu), [email protected] (S. Dai). ∗ Received 19 July 2019; Received in revised form 8 October 2019; Accepted 29 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Xueyun Liu, et al., Ceramics International,

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Mn2+ doped transparent hosts were mostly concerned with the influence of Mn2+ on UC enhancement for REIs (such as Tm3+ and Er3+), where Mn2+ served as an energy distributor to regulate the luminescence of REIs rather than acting as an emitter [21,22]. Most recently, an intense single-band UC emission associated with Mn2+ was realized from Yb3+/Mn2+ co-doped fluorosilicate GCs containing KZnF3 nanocrystals [23]. However, the evidence for the UC mechanism in such transparent matrix remains insufficient. In addition, there have been limited investigations on the dual broadband UC luminescence based on Mn2+ in tetrahedral and octahedral coordination sites. Here, we report the first realization of continuous dual-broadband UC emission from Yb3+/Mn2+ co-doped transparent GCs containing βYF3 and γ-Ga2O3 nanocrystals at RT. Fluoride phase β-YF3 was selected as one of the precipitated crystals due to its low phonon energy (~400 cm−1) environment–advantageous for efficient UC emission from the dopants. Moreover, oxide crystalline phase γ-Ga2O3 with multiple coordination environments has previously been demonstrated as a suitable host for Mn2+ incorporation [24]. In this case, the introduced Mn2+ and Yb3+ ions are expected to incorporate into the corresponding sites, thus giving rise to tunable dual-broadband UC luminescence. Under 980 nm pumping, tunable dual-emission bands peaking around 520 nm and 605 nm, associated with Yb3+-Mn2+ dimers in the tetrahedral and octahedral coordination sites, respectively, were obtained from the resultant GCs. The structural and UC emission properties with variable Mn2+ contents and heat treatment temperatures are thoroughly investigated. We also discuss the occupation of Yb3+ and Mn2+ and the involved UC mechanism in detail. This research provides an in-depth understanding of the unique Mn2+ UC emission in transparent glass ceramics, and may also create a novel pathway towards broadband UC light source.

Fig. 1. (a) XRD patterns of PG-YM1.5 and the corresponding GCs at diverse heat treatment temperatures (750, 760, and 770 °C); (b) XRD patterns of GC760-YMx samples (x = 0, 0.5, 0.75 and 1.0 mol %). The standard cards of γGa2O3 (No. 20–0426) and β-YF3 (No. 74–0911) are provided for reference.

3. Results and discussion 3.1. Microstructures and optical absorption Fig. 1(a) shows the XRD patterns of PG-YM1.5 and the corresponding GCs after a 2 h heat treatment at 750, 760 and 770 °C. The PG sample exhibited a typical amorphous structure characterized by only one broad hump [25]. The sharp diffraction peaks associated with spinel γ-Ga2O3 crystalline phase (JCPDS No. 20–0426) appeared on the diffuse humps first after heat treatment at 750 °C, revealing the dominant precipitation of γ-Ga2O3 nanocrystals among the glass matrix. As the heat treatment temperature further increased to 760 and 770 °C, additional characteristic sharp peaks corresponding to β-YF3 nanophase (JCPDS No. 74–0911) could also be observed, i.e., heat treatment above 760 °C induced the precipitation of these two NCs simultaneously. The diffraction peaks of γ-Ga2O3 and β-YF3 both became prominently sharper with elevated heat treatment temperatures, which indicated that dual-phase NCs grew gradually in a glass host. Employing the Scherrer equation [26], the sizes of γ-Ga2O3 and β-YF3 NCs were calculated in the range of 9–11 nm and 23–26 nm, respectively. On closer inspection, the diffraction peaks attributed to γ-Ga2O3 nanocrystals demonstrated a red shift compared with those of standard γ-Ga2O3 crystalline phase. Such a phenomenon may be explained in two respects. The growth of Ga2O3 nanocrystals is under compressive stress during the crystallization process, and because plenty of the fluorine available in glass becomes highly mobile at elevated temperature, replacement of O2− (r = 1.4 Å) by smaller ionic radius F- (r = 1.36 Å) is predicted to occur. This behavior would lead to the shrinkage of the crystal lattice, resulting in a red shift in the diffraction peaks [27,28]. Analogously, the Y3+ (r = 0.9 Å) may also be partially substituted by smaller Yb3+ (r = 0.868 Å) ions, as reflected by a slight red shift for the main diffraction peaks of YF3. The XRD patterns of the GCs doped with different Mn2+ concentrations (0.5, 0.75 and 1.0 mol%) and singly doped samples obtained by heat treatment at 760 °C were also obtained and shown in Fig. 1(b). All of the GC760 samples showed the precipitation of both γ-Ga2O3 and β-YF3 crystallites. Compared with those of Ga2O3, the diffraction peaks assigned to YF3 in GCs were more sensitive to the increased Mn2+ content. To get a deeper understanding of the microstructures of the fabricated dual-phase GC samples, TEM was employed to analyze the GC760-YM1.5, with the corresponding TEM images shown in Fig. 2. The TEM micrograph displays the existence of two types of crystalline particles in the amorphous glassy phase. Specifically, the bigger YF3 nanocrystals (marked by red circles) are closely surrounded by large amounts of smaller Ga2O3 nanocrystals (marked by yellow circles). The corresponding SAED patterns further confirmed the polycrystalline diffraction nature of the precipitated NCs. The lattice fringe intervals of crystallized dual-phase NCs were measured to be 0.174 nm and

2. Experimental Glass samples of 40SiO2–20Al2O3-10Ga2O3–15LiF–15YF3 doped with an additional 1 mol% YbF3 and x mol% MnCO3 (where x = 0, 0.5, 0.75, 1.0 and 1.5, labeled as Y and YMx, respectively) were prepared via a melt-quenching method in an ambient atmosphere. A sample doped with 1.5 mol% MnCO3 alone (M) was also synthesized for comparison. The starting materials used were SiO2, Al2O3 (A.R. purity, Sinopharm Chemical Reagent Co., Ltd) and Ga2O3, LiF, YF3, YbF3, MnCO3 (99.99%, Aladdin). Approximately 20 g well-mixed batches were placed into a corundum crucible and melted at 1500 °C for 1 h in a Nabertherm high-temperature furnace. SnO was introduced into the batches to prevent Mn2+ oxidation during the melting process. The melt was then poured onto a 350 °C preheated stainless-steel plate and quickly pressed by another stainless-steel plate to form precursor glass (PG), followed by annealing at 550 °C for 3 h to relinquish inner stress. Next, the PG samples were heat-treated at 750 °C, 760 °C and 770 °C for 2 h to form the GCs (labeled as GC750, GC760 and GC770, respectively). Eventually, all the resultant samples were sliced and doubleside polished with a thickness of 2.2 mm for further measurement. The crystalline phase of the GCs was identified using a Bruker Model D2 X-ray powder diffractometer (XRD) with Cu Kα irradiation (λ = 1.5406 Å). The microstructure of the GC was analyzed through a transmission electron microscope (TEM, FEI Talos F200X, USA) combined with selected area electron diffraction (SAED) and high-resolution TEM (HRTEM). Absorption spectra were measured by a PerkinElmer Lambda 950 spectrophotometer, with spectral range of 250–1100 nm. Photoluminescence (PL), photoexcitation (PLE) spectra and fluorescence decay curves were acquired on an Edinburgh Instruments FLS980 fluorescence spectrometer fitted with a 450 W Xe lamp and a μF900 flashlamp as excitation sources. The UC emission spectra and decay curves of the samples were obtained using the same spectrometer, equipped with both a continuous and a pulsed 980 nm laser diode. Signal light was detected by a Hamamatsu R928 photomultiplier tube (PMT). 2

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Fig. 2. (a) TEM micrograph and (b) the corresponding SEAD patterns of GC760-YM1.5; (c) HRTEM image of γ-Ga2O3 and β-YF3 nanocrystals; (d) Absorption spectra of PG-YM1.5, GCs and GC760-YMx samples with their corresponding photographs presented in (e), the inset of (d) is the transmission spectra of GC760-YM1.5.

0.208 nm using a HRTEM photo (Fig. 2(c)), corresponding to the (212) plane of β-YF3 and the (400) plane of γ-Ga2O3, respectively. Fig. 2(d) illustrates the optical absorption spectra of PG, GCs-YM1.5 and GC760YMx samples across the spectral range of 250–1100 nm. The obvious absorption observed at ~ 980 nm for all samples is assigned to the characteristic transition of Yb3+: 2F7/2 2F5/2. Moreover, one week absorption peak around 415 nm arising from Mn2+: 6A1(6S) [4A1(4G),4E(4G)] transition was detected in the PG sample, whereas it disappeared completely in GCs as a result of the obvious red shift of the absorption cut-off edge, which probably resulted from typical optical scattering of the precipitated NCs. Compared with PG, the transmittance of the GC samples decreased gradually with the elevated heat treatment temperature and (or) Mn2+ doping concentration, which is attributed to the increased grain size and crystallinity of the nanocrystals after further crystallization. Nevertheless, the fabricated GC samples still maintains a good transparency (> 60%), except for the GC770 sample, as reflected by the image shown in Fig. 2(e). Normally, further thermal treatment will promote the generation of nanocrystals in GC and sometimes favors the luminescence of the emitter. This process, however, often leads to several negative reactions such as further phase separation or devitrification due to the overgrowth of nanocrystals [29]. Considering the optical transparency, the representative GC760-YM1.5 sample thereby was selected and systematically studied to explore its possible application prospects in the field of photonic devices.

Fig. 3. Normalized excitation and emission spectra of (a) GC760-M, (b) GC760YM1.5 and (c) GC760-Y GC. 2+ (Mn2+ (Mn2+ (IV)) and octahedrally coordinated Mn (VI)), respectively [24,31,32]. This indicates that Mn2+ ions are associated with two different types of environments in the current matrix. Further identification of the locations of Mn2+ will be discussed later in Section 3.3. For the Yb3+/Mn2+ co-doped sample GC760-YM1.5, it shows an extra emission peak around 976 nm in addition to the aforementioned two broad bands from Mn2+ under 445 nm excitation (Fig. 3(b)). This NIR luminescence may originate from the 2F5/2 2F7/2 transition of Yb3+, being fully consistent with the emission profile of the Yb3+ singledoped GC (GC760-Y) sample (Fig. 3(c)). When monitoring the emission of Yb3+ at 976 nm in the GC760-YM1.5 sample, several excitation subbands ranging from 350 nm to 510 nm were identified. The shapes and positions of these excitation sub-bands are similar to those in GC760-M GC (Fig. 3(a)). In contrast, no excitation signals in this region were detected for the GC760-Y sample. These results indicate that the effective energy transfer (ET) from Mn2+ to Yb3+ occurred in the Yb3+/ Mn2+ co-doped GCs. Since the excitation of this sample effectively

3.2. Photoluminescence related to Yb3+ and Mn2+ single-doped GCs Normalized excitation and emission spectra of GC760-M, GC760YM1.5 and GC760-Y samples are presented in sequence in Fig. 3(a)–(c). It can be seen that the excitation spectrum of the Mn2+ single-doped sample (GC760-M) measured at 515 nm comprises several sharp excitation peaks, located at 356, 381, 445 and 488 nm, which can be ascribed to the typical transitions of octahedral Mn2+ from 6A1(6S) to 4 T2(4D), [4A1(4G), 4E(G)], 4T2(4G) and 4T1(4G) levels, respectively [30]. Upon optimal excitation at 445 nm, dual-mode continuous emitting bands centered around 515 and 605 nm appear, typically arising from the 4T1(4G) →6A1(6S) transition of tetrahedrally coordinated Mn2+ 3

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Fig. 4. UC emission spectra of (a) PG-YM1.5 and the corresponding GCs at different heat treatment temperatures (750, 760, and 770 °C); (b) UC emission spectra of GC760-YMx samples (x = 0, 0.5, 0.75, 1.0, 1.5); (c) CIE chromaticity diagram and (d) the corresponding PL photographs of GC samples. All samples were excited under 980 nm LD.

2+ Such red shift is probably due to the gradually enhanced Mn2+ (VI)–Mn(VI) 2+ 2+ interaction, i.e., when increasing the Mn content, the Mn(VI)–Mn2+ (VI) spatial distance becomes shorter, and thus the interaction between Mn2+ (VI) species is reinforced. The red shift of the orange emission with increasing degree of crystallization (Fig. 4(a)) might be caused by two effects: (1) the large change of the environment around Mn2+ (VI) after crystallization; and (2) the reduced volume fraction of the glassy phase, which consequently increases Mn2+ content in the residual glassy 2+ matrix and the Mn2+ (VI)–Mn(VI) interactions, to some extent [24,32]. Furthermore, it is clear that the UC luminescence was significantly intensified after crystallization. In particular, the PL-integrated intensity of PG was 30 times weaker than that of the GC760-YM1.5 sample, demonstrating that considerable UC emission intensity could be realized in this transparent GC system as expected. With elevated heat treatment temperature (Fig. 4(a)), both the green and orange emission components were increased continuously, but the increase of the latter was more prominent. The intense UC green luminescence further validated the incorporation of Mn2+ ions into a more ordered environment, i.e. tetrahedrally coordinated γ-Ga2O3 nanocrystals. As for the enhancement of the orange emission from Mn2+ (VI), it may result from the increased precipitation of β-YF3 nanocrystals in the glass host (Fig. 1(a)). That is, although the remaining Mn2+ ions do not enter into the precipitated β-YF3 crystalline phase, they are prone to accumulate around these fluoride nanocrystals. This provides a lower phonon energy environment compared to the amorphous glassy phase, which can somewhat reduce the non-radiative relaxation probability to boost the orange emission of Mn2+ in the residual glass host. In Fig. 4(b), the UC luminescence spectra of those GCs with various Mn2+ doping contents showed similar changes owing to the precipitated dual-phase NCs in the oxyfluoride glass matrix. The luminescence intensity of Mn2+ (IV) underwent an increase until the Mn2+ concentration reached at x = 1.0. Once the Mn2+ content was further increased, the green emission deriving from Mn2+ (IV) became weakened and it almost disappeared when x = 3.5 (See Fig. S1). However, the orange emission corresponding to Mn2+ (VI) reached a maximum at x = 2.5 (as shown in Fig. S1). With further increasing the Mn2+ content, the emission intensity of Mn2+ (VI) began to decrease. These results indicate that Mn2+ ions would

covers the UV and visible spectral regions and is accompanied by a NIR emission, the fabricated Yb3+/Mn2+ co-doped GCs may become potential candidates for solar spectrum conversion. 3.3. UC luminescence of Yb3+/Mn2+ co-doped GC samples Fig. 4(a)-(b) exhibit the UC luminescence spectra of all GCs excited by a 980 LD. Only a weak UC orange-emitting broad band centered at ~ 595 nm was observed for the PG sample. Interestingly, all co-doped GC samples produced a continuous dual-emission broadband covering 450–750 nm, the position of which resembles that of Mn2+ when directly excited by 445 nm. Such broadband is effectively deconvoluted into two Gaussian components, termed as P1 (green) and P2 (orange), respectively. Since Yb3+ ions showed no emission in the 450–750 nm region, the orange emission should be assigned to the 4T1(4G)→6A1(6S) transition of Mn2+ (VI), while the green emission in GCs originated from the 4T1(4G)→6A1(6S) transition of Mn2+ (IV). Based on previous research results, the transition metal Mn2+ will be selectively embedded into Ga2O3 crystallites rather than YF3 crystallites due to the coordination environment and the matched radius (Mn2+(r = 0.66 Å) and Ga3+(r = 0.47 Å)) [24,33]. In view of the fact that the γ-Ga2O3 unit cell contains two configurations—the [GaO6] octahedron and [GaO4] tetrahedron—it was deduced that some of the Mn2+ ions were incorporated into the tetrahedral coordination sites of γ-Ga2O3 after crystallization, thereby giving the green emission. As demonstrated theoretically and experimentally by Hayashi et al., Mn2+ ions were not embedded in the octahedral coordination sites of γ-Ga2O3 [34]. Therefore, the orange-emitting band in GCs definitely stems from the optical transitions of Mn2+ ions that remained in the residual glass matrix, which means that the glassy phase provides an octahedral coordination environment for the remaining Mn2+ ions. Notably, with increasing heat treatment temperature and/or Mn2+ doping concentration, the peak wavelength of green emission in GGs remained nearly constant (~520 nm), whereas the orange components shifted from 581 toward 609 nm, as listed in Table І (some extra peaks at 550 and 600 nm superimposed on the Mn2+-related UC broad band for the GC760-YM0.5 sample originated primarily from the impurity Er3+). 4

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Table 1 Peak positions of P1 and P2 and corresponding CIE chromaticity coordinates at 980 nm excitation. Serial number


P1(G) [nm]

P2(O) [nm]


1 2 3 4 5 6

GC760-YM0.5 GC760-YM0.75 GC760-YM1.0 GC750-YM1.5 GC760-YM1.5 GC770-YM1.5

519 520 519 520 520 520

– 581 584 596 603 609

(0.3648,0.5611) (0.3756,0.5548) (0.3948,0.5419) (0.4447,0.5109) (0.4643,0.4986) (0.4774,0.4875)



where index n represents the number of absorbed photons, which is determined by the slope of the log(UC integrated intensity)-log(pump power) diagram fitted linearly. Fig. 5(b) shows the logP-logI plots for both green and orange emissions. By fitting the logP-logI curve linearly, the n values obtained from the slope were 1.55 and 1.52 for the green and orange emitting components, respectively, suggesting a two-photon process involving in both UC emission components. To gain more detailed information in relation to the green and orange UC luminescence behavior, the relevant fluorescence decay curves upon a pulsed 980 nm excitation for all co-doped GCs were investigated, with a representative decay curve of the GC760-YM1.5 displayed in Fig. 6. All the decay curves fitted well according to the formula below [36].

preferentially occupy at tetrahedral site (tetrahedral Ga2O3 nanocrystals) as Mn2+ concentration x increased from 0.5 to 1.0, whereas they would prefer staying in glassy phase (octahedral site) at higher doping content (x ≥ 1.5) due to the formation of lower phonon energy YF3 nanocrystals. Based on this, the decreasing intensity for green emission might be mainly caused by the reduced Mn2+ (IV) ions. As for the weakening of orange emission, it was probably because of the typical concentration quenching between Mn2+ (VI) ions. The above results suggest that the green and orange UC emission components can be tuned in terms of their intensities and wavelength positions by controlling the crystallization temperature and/or Mn2+ concentration. This was eventually reflected in the luminescent color changes of the fabricated GC samples. Fig. 4(c) shows the luminescent colors of the Yb3+/Mn2+ co-doped GCs excited by 980 nm LD, depicted via the Commission Internationale de L'Eclairage (CIE) chromaticity plot. The luminescence color varies from yellowish green (point 1) to orange (point 6) upon increasing the heat treatment temperature and/ or the Mn2+ proportion (The corresponding CIE coordinates are given in Table 1), which is graphically reflected by the luminescent photos of the GCs (Fig. 4(d)). Noticeably, the GC760-YM1.5 sample emitted a pure green light when excited at 445 nm—distinctly different from its orange color upon 980 nm LD excitation. This manifests that the Yb3+/ Mn2+ co-doped glass ceramics could become an ideal multifunctional broadband illuminant for continuous wave-tunable lasers and lighting. Moving further into the emission spectra in Fig. 4(b), it was found that almost no emission was detected in the Mn2+-free (GC760-Y) or Yb3+-free (GC760-M) samples, indicating that the UC luminescence behavior in current case involves both Yb3+ and Mn2+ ions. The power-dependent UC luminescence spectra of the GC760-YM1.5 sample provide further insight into this UC process, as illustrated in Fig. 5(a). Evidently, both of the emission components were monotonically enhanced by increasing the excitation power. Normally, the relation between UC intensity I and excitation power P obeys the following equation [35].

I (t ) = A1 exp( t / 1) + A2 exp( t / 2 )


where I(t) denotes emission intensity at time t, A1 and A2 represent fitting constants, and 1 and 2 are the rapid- and slow-decay exponential parts, respectively. The mean lifetimes ( ) for the UC luminescence were determined eventually through the following equation:

= (A1



+ A2


2)/(A 1 1

+ A2 2)


The corresponding values are summarized in Table 2. Specifically, the luminescence lifetime τ(G) of Mn2+ (IV) (monitoring the green UC emission at 515 nm) was calculated to be about 3 m s, while the τ(O) related to Mn2+ (IV) (monitoring the orange UC emission at 605 nm) was approximately 5 m s. Increasing the Mn2+ doping concentration or crystallization temperature led to a slight change in the UC decay lifetime (that is, a decrease from 3.47 to 2.95 for τ(G) and 5.37 to 5.09 m s for τ(O), respectively). The above results reveal that the UC luminescence dynamic of Mn2+ in the current system occurs over a timescale of few milliseconds (ms), which is comparable to that in CaO:Yb3+/Mn2+ (7.2 m s) and LaMgAl11O19:Yb3+/Mn2+ (5.8 m s), but lower than that in KZnF3:Yb3+/Mn2+ (27 m s) and Rb2MnCl4:Yb3+ (9.7 m s) [10,14,37,38], (the UC decay lifetime of Mn2+ in transparent matrix has not yet been investigated). Moreover, the decay curves corresponding to Stokes emission upon 445 nm excitation were also measured for comparison (two insets of Fig. 6). The values of τ(G) and τ(O) were evaluated approximately 5.12 and 8.91 m s, respectively, which is longer than that excited by 980 nm LD. This means that the UC green and orange emissions should be related to other forms of Mn2+ that are distinguishable from the isolated Mn2+ ion. Cooperative sensitization and/or ground state/excited state absorption (GSA/ESA) based on Yb3+-Mn2+ pairs have been previously proposed by researchers to account for the UC luminescence of Mn2+ [13,14]. The former is a three-ion (Yb3+-Yb3+-Mn2+) process, the occurrence of which usually requires two excited Yb3+ ions (donors) to be close enough so as to concurrently transfer their energy to one adjacent Mn2+ ion (acceptors). If the UC luminescence of Mn2+ arises from cooperative sensitization, an additional peak around 490 nm, attributed to the cooperative Yb3+ pairs, could be observed in addition to the Mn2+-related UC emission. In the present work, however, the cooperative sensitization mechanism was excluded because of the absence of a cooperative luminescence band from Yb3+ pairs in the Yb3+ singledoped and Yb3+/Mn2+ co-doped GCs. Unlike most lanthanide ions, transition metal Mn2+ has no intermediate energy level in resonant with the Yb3+ ion. Hence, the GSA/ESA process in Yb3+/Mn2+ codoped systems must be induced by a unique emission center that is different from a single luminescent ion (such as Er3+, Tm3+ or Ho3+). In previous reports, an exchange-coupled Yb3+-Mn2+ pairs model involving a sequential GSA/ESA process was proposed to explain the UC luminescence behavior of Mn2+ in Yb3+/Mn2+-activated powders [39]. Such proposed Yb3+-Mn2+ pairs model may equally suit the UC luminescence of the resultant dual-phase GCs. As discussed above, both γ-Ga2O3 and β-YF3 NCs were precipitated in the present oxyfluoride

Fig. 5. (a) Power-dependent UC emission spectra and (b) the corresponding log (UC integrated intensity)-log(pump power) plots of GC760-YM1.5 under 980 nm LD excitation. 5

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Fig. 6. UC luminescence decay curves of GC760-YM1.5 for (a) green emission and (b) orange emission excited by a pulsed 980 nm LD. The inset shows the respective decay curves upon 445 nm excitation. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

intermediate state |2F5/2, 6A1(6S) > , and emitting state |2F7/2, 4 T1(4G) > . Upon absorption of a 980 nm photon, the Yb3+-Mn2+ dimer is firstly pumped from the ground state to the intermediate state through the GSA process, which will be further pumped to the |2F7/2, 4 T1(4G) > state by absorbing a second 980 nm photon (ESA process). After that, the electrons on the populated |2F7/2, 4T1(4G) > level radiatively relax to the |2F7/2, 6A1(6S) > ground state, releasing a visible photon in the form of green emission around 520 nm. The UC orange emission mechanism may be attributed to the Yb3+Mn2+ dimers formed in the residual glass matrix. Considering the mismatched valent state and coordination environment (Y3+ is in a nine-fold coordination site), the residual Mn2+ ions remain in a glassy phase instead of embedding into the precipitated β-YF3 NCs. Despite this, the remaining Yb3+ and Mn2+ ions in the glass phase preferentially gather around the YF3 nanocrystals due to the effects of low phonon energy and an intense crystal field environment. In this case, the distance between ions becomes sufficiently small, enabling the formation of the exchange-coupled Yb3+-Mn2+ dimers. Analogously, a visible photon approximately 605 nm is released via the GSA and ESA processes.

Fig. 7. Schematic diagram of the phonon UC process for Yb3+-Mn2+ dimer in a γ-Ga2O3&β-YF3 dual-phase based glass ceramic. Table 2 Average decay lifetimes for P1 and P2 at 980 nm excitation. Sample





GC760-YM0.75 GC760-YM1.0 GC750-YM1.5 GC760-YM1.5 GC770-YM1.5

3.47 3.32 2.97 3.01 2.95

99.89% 99.89% 99.85% 99.84% 99.84%

5.37 5.21 5.06 5.13 5.09

99.77% 99.84% 99.84% 99.77% 99.83%

4. Conclusions In summary, Yb3+/Mn2+ co-doped transparent oxyfluoride GCs comprising γ-Ga2O3/β-YF3 dual NCs were prepared through a meltquenching method. The effect of Mn2+ content and thermal treatment temperature on the microstructure and UC luminescent performance was systematically investigated through XRD, TEM, photoluminescence spectra and decay lifetime measurements. Under 980 nm LD pumping, two consecutive UC luminescent broad bands peaking at around 520 and 605 nm appeared in the co-doped GC samples, covering the visible spectral region across 480–720 nm. It was experimentally demonstrated that the intense green-emitting UC emission band could be assigned to the incorporations of Mn2+ and Yb3+ ions into the crystalized γ-Ga2O3 nanophase, where exchange-coupled Yb3+-Mn2+ dimers are easily formed and support a GSA/ESA UC process; while the UC mechanism for the orange-emitting band originates from the formation of Yb3+Mn2+ dimers in the residual glass phase and also involves a sequential GSA/ESA UC process. Both the position and intensity of the dual UC luminescence bands could be tailored by changing the Mn2+ ions concentration and/or the heat treatment temperature, giving rise to a tunable emissive color from yellowish green to orange for the GCs. The realization of such novel dual UC emission broad bands from Yb3+/ Mn2+-activated oxyfluoride GCs further enriches the existing UC spectral components and endows the resultant glass ceramics with potential applications in solar cells, multicolor display, and continuous

GCs. The cell structure of γ-Ga2O3 is constructed from [GaO4] tetrahedron and [GaO6] octahedron, and these polyhedrons are interconnected by sharing edges or sharing faces, resulting in a distance of 1.697 Å between the two nearest neighbor Ga3+ (IV) and 2.015 Å between the nearest Ga3+(IV) and Ga3+(VI). After crystallization, some of the Mn2+ ions would selectively incorporate into the γ-Ga2O3 nanocyrstals by substituting the tetrahedrally coordinated Ga3+ sites, meanwhile a part of Yb3+ ions may replace both of the Ga3+ sites with tetrahedrally coordination and octahedrally coordination. Thus, it could be inferred from this structure that the Yb3+(Ga3+(IV))–Mn2+(Ga3+(IV)) dimers and Yb3+(Ga3+(VI))–Mn2+(Ga3+(IV)) dimers are easily formed through an exchange interaction in the γ-Ga2O3-based GC samples. Fig. 6 depicts the GSA/ESA UC process involving exchange-coupled Yb3+-Mn2+ pairs to elucidate the UC green luminescence of Mn2+, in which the Yb3+Mn2+ pairs play the role of a chromophoric unit, and a series of newly hybrid states are established, i.e., the ground state |2F7/2, 6A1(6S) > , 6

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wave-tunable lasers.

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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Grant Nos. 51702172 and 61605093), Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology), China; Grant Nos. 2018skllmd-11, Natural Science Foundation of Ningbo (Grant Nos. 2018A610042), Science Research Fund Project of Ningbo University (Grant Nos. XYL18015), and is sponsored by the K. C. WongMagna Fund in Ningbo University. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// References [1] S. Wen, J. Zhou, K. Zheng, A. Bednarkiewicz, X. Liu, D. Jin, Advances in highly doped upconversion nanoparticles, Nat. Commun. 9 (1) (2018) 2415. [2] Y.Y. Liu, X.F. Meng, W.B. Bu, Upconversion-based photodynamic cancer therapy, Coord. Chem. Rev. 379 (2019) 82–98. [3] M. Alkahtani, Y.Y. Chen, J.J. Pedraza, J.M. González, D.Y. Parkinson, P.R. Hemmer, H. Liang, High resolution fluorescence bio-imaging upconversion nanoparticles in insects, Opt. Express 25 (2) (2017) 1030–1039. [4] J. Zhou, Y.Q. Chen, R.S. Lei, H.P. Wang, Q.G. Zhu, X.M. Wang, Y.Q. Wu, Q.H. Yang, S.Q. Xu, Excellent photoluminescence and temperature sensing properties in Ho3+/ Yb3+ codoped (Y0.88La0.09Zr0.03)2O3 transparent ceramics, Ceram. Int. 45 (6) (2019) 7696–7702. [5] D.Q. Chen, Y. Zhou, Z.Y. Wan, P. Huang, H. Yu, H.W. Lu, Z.G. Ji, Enhanced upconversion luminescence in phase-separation-controlled crystallization glass ceramics containing Yb/Er(Tm):NaLuF4 nanocrystals, J. Eur. Ceram. Soc. 35 (7) (2015) 2129–2137. [6] J. Dong, Y.M. Li, W. Zheng, R. Wang, Y.L. Xu, Lower power dependent upconversion multicolor tunable properties in TiO2:Yb3+/Er3+/ (Tm3+), Ceram. Int. 45 (1) (2019) 432–438. [7] S. Shi, L.D. Sun, Y.X. Xue, H. Dong, K. Wu, S.C. Guo, B.T. Wu, C.H. Yan, Scalable direct writing of lanthanide-doped KMnF3 perovskite nanowires into aligned arrays with polarized up-conversion emission, Nano Lett. 18 (5) (2018) 2964–2969. [8] S. Ye, E.H. Song, Q.Y. Zhang, Transition metal-involved photon upconversion, Adv. Sci. 3 (1) (2016) 1600302. [9] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, Heidelberg, 1994. [10] C. Reinhard, P. Gerner, R. Valiente, O.S. Wenger, H.U. Güdel, Upconversion phenomena in the Yb3+ doped transition metal compounds Rb2MnCl4 and CsMnBr3, J. Lumin. 94–95 (2001) 331–335. [11] R. Valiente, O. Wenger, H.U. Güdel, New photon upconversion processes in Yb3+ doped CsMnCl3 and RbMnCl3, Chem. Phys. Lett. 320 (5–6) (2000) 639–644. [12] P. Gerner, O.S. Wenger, R. Valiente, H.U. Güdel, Green and red light emission by upconversion from the near-IR in Yb3+ doped CsMnBr3, Inorg. Chem. 40 (18) (2001) 4534–4542. [13] P. Gerner, C. Reinhard, H.U. Güdel, Cooperative near-IR to visible photon upconversion in Yb3+-doped MnCl2 and MnBr2: comparison with a series of Yb3+-doped Mn2+ halides, Chem. Eur J. 10 (19) (2004) 4735–4741. [14] R. Martín-Rodríguez, R. Valiente, M. Bettinelli, Room-temperature green upconversion luminescence in LaMgAl11O19: Mn2+/Yb3+ upon infrared excitation, Appl. Phys. Lett. 95 (9) (2009) 091913. [15] E.H. Song, Z.T. Chen, M. Wu, S. Ding, S. Ye, S.F. Zhou, Q.Y. Zhang, Room-temperature wavelength-tunable single-band upconversion luminescence from Yb3+/