Nano-Crystallization of Ln-Fluoride Crystals in Glass-Ceramics via Inducing of Yb3+ for Efficient Near-Infrared Upconversion Luminescence of Tm3+

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1 nanomaterials Article Nano-Crystallization Ln-Fluoride Crystals in Glass-Ceramics via Inducing Yb 3+ for Efficient Near-Infrared Upconversion Luminescence Tm 3+ Jianfeng Li, Yi Long, Qichao Zhao, Shupei Zheng *, Zaijin Fang * Bai-Ou Guan Guangdong Provincial Key Laboratory Optical Fiber Sensing Communications, Institute Photonics Technology, Jinan University, Guangzhou , China; ljf151@stu2019.jnu.edu.cn (J.L.); longyi9604@stu2019.jnu.edu.cn (Y.L.); a @stu2020.jnu.edu.cn (Q.Z.); tguanbo@jnu.edu.cn (B.-O.G.) * Correspondence: spzheng@jnu.edu.cn (S.Z.); fzj909@jnu.edu.cn (Z.F.) Citation: Li, J.; Long, Y.; Zhao, Q.; Zheng, S.; Fang, Z.; Guan, B.-O. Nano-Crystallization Ln-Fluoride Crystals in Glass-Ceramics via Inducing Yb 3+ for Efficient Near-Infrared Upconversion Luminescence Tm 3+. Nanomaterials 2021, 11, /nano Abstract: Transparent glass-ceramic composites embedded with Ln-fluoride nanocrystals are prepared in this work to enhance upconversion luminescence Tm 3+. crystalline phases, microstructures, photoluminescence properties samples are carefully investigated. KYb 3 F 10 nanocrystals are proved to controllably precipitate in glass-ceramics via inducing Yb 3+ when doping concentration varies 0.5 to 1.5 mol%. Pure near-infrared upconversion emissions are observed emission intensities are enhanced in glass-ceramics as compared to in precursor glass due to incorporation Tm 3+ into KYb 3 F 10 crystal structures via substitutions for Yb 3+. Furrmore, KYb 2 F 7 crystals are also nano-crystallized in glass-ceramics when Yb 3+ concentration exceeds 2.0 mol%. upconversion emission intensity Tm 3+ is furr enhanced by seven times as Tm 3+ enters lattice sites pure KYb 2 F 7 nanocrystals. designed glass ceramics provide efficient gain materials for optical applications in biological transmission window. Moreover, controllable nano-crystallization strategy induced by Yb 3+ opens a new way for engineering a wide range functional nanomaterials with effective incorporation Ln 3+ ions into fluoride crystal structures. Keywords: nano-crystallized glass ceramic; nano-crystallization; luminescence; Tm 3+ doped; upconversion Academic Editor: Federico Rosei Received: 31 March 2021 Accepted: 16 April 2021 Published: 18 April 2021 Publisher s Note: MDPI stays neutral with regard to jurisdictional claims in published maps institutional affiliations. Copyright: 2021 by authors. Licensee MDPI, Basel, Switzerl. This article is an open access article distributed under terms conditions Creative Commons Attribution (CC BY) license ( creativecommons.org/licenses/by/ 4.0/). 1. Introduction Trivalent lanthanide (Ln 3+ ) ions doped upconversion (UC) luminescent materials have been extensively investigated for applications in lighting, solar cells, solid-state laser, biological imaging, temperature sensing due to multiple luminescence wavelength ultraviolet to near-infrared (NIR) regions [1 9]. Among se, Tm 3+ doped materials possess UC luminescence in NIR region at around 800 nm ( 3 H 4 3 H 6 ), which is located in biological transmission window, light in that region can easily penetrate biological tissues [10 13]. Moreover, UC emissions Tm 3+ are dramatically enhanced via energy transfer (ET) Yb 3+, which possesses a large absorption section for commercial 980 nm lasers. Thus, Yb 3+ -Tm 3+ co-doped materials were more efficient cidates for NIR-to-NIR UC luminescence were significant light sources for biologically non-destructive detection. In past decades, a large number investigations about Yb 3+ -Tm 3+ co-doped UC materials have been widely reported for achieving high-efficiency NIR UC luminescence [14 20]. Glass exhibits high transmittance, easy fabrication, excellent processability due to its amorphous stening characteristics. se properties make glass an appealing matrix for design fabrication special optics devices, particularly optical fiber lasers [21,22], which can hardly be achieved by using crystals. Furrmore, nano-crystallized glass ceramic (GC) composites can be obtained via precipitation Nanomaterials 2021, 11,

2 Nanomaterials 2021, 11, functional nanocrystals in a glass matrix via heat treatments. nano-crystallized GCs still possess high optical transmittance UC luminescence efficiency in GCs can be greatly enhanced, as compared to glass, when Ln 3+ ions are successfully incorporated into crystal structures featuring lower probability non-radiative transition ascribed to lower phonon energy. So far, Yb 3+ -Tm 3+ co-doped GCs have been considered as significantly optical gain materials for UC NIR fiber lasers, noncontact optical rmometers, bio-imaging [23 26]. Generally, Ln 3+ ions were expected to incorporate into crystal structures via ionic substitution for Y 3+, La 3+, Lu 3+, Sc 3+, Sr 2+ in GCs embedded with Na(Y/La/Lu)F 4, LaF 3, YF 3, KSc 2 F 7, SrF 2 crystals, etc [27 31]. However, ionic substitution process was uncontrollable amount Ln 3+ incorporated into crystal structures was usually small due to large mismatch between Ln 3+ replaceable sites. enhancements luminescence in GCs were limited. Moreover, a lot remaining crystals not occupied by Ln 3+ ions made no contribution to enhancement luminescence but will trigger severe issues optical scattering low optical transmittance in glass. se made traditional GCs difficult to be used in practical applications. Accordingly, it is highly desirable for design fabrication novel GCs for controllably incorporating a large number Ln 3+ into crystal structures to achieve high-efficiency NIR UC luminescence Tm 3+. Ln-fluoride crystal, intrinsically containing Ln 3+ (for example, Yb 3+ or Er 3+ ) in crystal lattices, is a significant host for incorporation or active Ln 3+ ions because mismatches in ionic radius between different Ln 3+ ions are smaller active Ln 3+ ions enter Ln-fluoride crystals more easily [32]. In this work, GCs containing Lnfluoride nanocrystals were designed to greatly enhance UC luminescence Tm 3+ achieve pure, high-efficiency NIR-to-NIR emissions. Two kinds Ln-fluoride nanocrystals (KYb 3 F 10 KYb 2 F 7 ) were controllably precipitated a glass matrix via heat treatments dependent on doping Yb 3+. Tm 3+ ions entered fluoride crystal lattices easily by replacing Yb 3+ sites. As a result, a large number Ln 3+ were incorporated into fluoride crystals featuring extremely low phonon energy UC emission Tm 3+ in GCs were dramatically enhanced. Moreover, pure NIR UC emissions were also obtained by adjusting doping concentration Tm 3+. Thus, designed GCs provide an efficient material for pure NIR UC luminescence fer highly promising developments for UC fiber lasers, non-contact optical rmometers, bio-imaging. 2. Materials Methods 2.1. Materials Preparation Samples with a molar composition 70SiO 2-15KF-15ZnF 2 -xybf 3 -ytm 2 O 3 (x = 0~3.5, y = 0~0.3) were prepared using a melt quenching method. First, 30 g reagent grade stoichiometric mixtures were mixed thoroughly in an agate mortar melted in covered quartz crucibles in an electric furnace at 1550 C for 30 min; melts were poured onto a brass plate n pressed using anor brass plate to obtain precursor glasses (PGs). PG samples were heated at 540 C for 5 10 h to obtain GC samples according to differential scanning calorimetry (DSC) results in ref. [33]. PG GC samples were cut polished to 2 mm thick for measurements Characterizations To identify crystalline phase in GCs, X-ray diffraction (XRD) patterns were performed on a X-ray diffractometer (Bruker, Fällen, Switzerl) with Cu/Ka (λ = nm) radiation. morphology size distribution nanocrystals in GCs were measured via high-resolution transmission electron microscopy (HRTEM) (FEI, Hillsboro, OR, USA). UC emission spectra samples were recorded using an Edinburgh FLS980 fluorescence spectrometer (Edinburgh Instruments, Edinburgh, UK). A 980 nm laser diode (LD) was used as exciting source for measurement UC emission spectra. emission decay curves were measured using same spectrometer with a microsecond lamp as excitation source. All measurements were performed at room temperature.

3 spectra. emission decay curves were measured using same spectrometer with a microsecond lamp as excitation source. All measurements were performed at room temperature. Nanomaterials 2021, 11, Results Discussion 3. Results Figure 1a shows Discussion XRD patterns Yb 3+ -Tm 3+ co doped PG GCs. A broad b is observed in XRD pattern PG due to Figure 1a shows XRD patterns Yb 3+ amorphous -Tm 3+ characteristic glass. Sharp co-doped PG GCs. A broad b peaks is observed are observed in XRD in pattern XRD patterns PG due to two amorphous GCs. characteristic main peaks at 26.99, glass. Sharp 31.27, 44.81, peaks are 53.1 observed are ascribed in XRDto patterns (222), (400), two (440), GCs. (622) maincrystal peaks at facets KYb3F10, 31.27, (No: ),, 53.1 respectively, are ascribed indicating to (222), that (400), KYb3F10 (440), crystals (622) have crystal been facets precipitated KYb 3 in F 10 GCs. (No: Moreover, ), respectively, weak peaks indicating 18.86, that 31.04, KYb 3 F crystalsattributed have beento precipitated diffraction in peaks GCs. K2SiF6 Moreover, (No: ) weak peaks crystals at are also, observed, in attributed XRD patterns to diffraction GC heated at peaks 540 C or K 2 5 SiF h. 6 It (No: is also ) found crystals that are intensities also observed diffraction in XRDpeaks patterns for KYb3F10 GC heated crystal are atall 540 increased C for 5when h. It is also heating foundtime thatincreases intensities 5 to diffraction 10 h. However, peaks for diffraction KYb 3 F 10 peaks crystal K2SiF6 are allcrystal increased all when weaken due heating to time furr increases heat treatment 5 to 10 to 10 h. h. However, se results indicate diffraction that KYb3F10 peaks crystals K 2 SiF 6 crystal prefer all to precipitate weaken duein to 1.0Yb furr Tm heat treatment 3+ co doped to 10GCs h. via se furr results heat indicate treatment. that KYbAdditionally, 3 F 10 crystals prefer average to precipitate size in crystals 1.0Yb can 3+ be Tm calculated 3+ by co-doped Scherrer s GCs via equation. furr heat diffraction treatment. peak, Additionally, around 2θ = average size 18.42, crystals was can be calculated by Scherrer s equation. diffraction peak, around 2θ = selected for calculation, average size KYb3F10 K2SiF6 nanocrystals in, was selected for calculation, average size KYb 3 F 10 K 2 SiF GC heated at 540 C for 5 h was calculated 6 nanocrystals in GC heated at 540 to be approximately nm, C for 5 h was calculated to be approximately respectively nm, respectively. Figure Figure 1. (a) 1. XRD (a) XRD patterns 1.0Yb Tm Tm co-dopedpg PG GCs. (b) TEM (c) HRTEM images 1.5Yb 1.5Yb Tm Tm co doped co-doped GC GC heat heat treated treated at C Cfor for h. h. HAADF TEM HAADF-TEM image shown in in Figure 1b 1b reveals reveals that that nanocrystals nanocrystals are in-situ, are insitu, precipitated precipitated among among glass glass matrix. matrix. measured measured size se size nanoparticles se nanoparticles is 10 is to nm. to 30 nm. HR-TEM HR TEM image is shown image in is Figure shown 1c. in Figure crystal 1c. lattice crystal fringeslattice are obvious, fringes are which obvious, is different which is different that amorphous that glass amorphous matrix. glass interval matrix. crystal interval lattice fringes d can be measured directly, its value is about nm, which corresponds to crystal lattice fringes d can be measured directly, its value is about nm, (400) crystal facet cubic KYb 3 F 10. se also prove that KYb 3 F 10 nanocrystals are precipitated in GCs.

4 Nanomaterials 2021, 11, x FOR PEER REVIEW 4 10 Nanomaterials 2021, 11, which corresponds to (400) crystal facet cubic KYb3F10. se also prove that KYb3F10 nanocrystals are precipitated in GCs. optical transmission spectra 1.0Yb optical transmission spectra 1.0Yb Tm -0.1Tm co doped samples are shown co-doped samples are shown in Figure 2a. PG sample possesses high transmittance (~90%) 300 to 800 nm in Figure 2a. PG sample possesses high transmittance (~90%) 300 to 800 nm with witha athickness thickness 2 mm. mm. Though Though nanocrystals nanocrystals are are precipitated precipitated in in GCs, GCs, optical optical transmittances GCs GCsare arestill as high as as 85%. Interestingly, transmittance transmittance GC GC heated heatedfor for10 10h his ishigher than that heated for 5 h. h. As As calculated XRD XRD patterns, patterns, average size KYb3F10 3 F 10 crystals is smaller than that KK2SiF6 2 6 crystals. When When heat heat treatment time increases 5 to 10 h, h, more KYb KYb3F10 3 F crystals less less K 2 K2SiF6 6 crystals are are precipitated in in GC, resulting in increase transmittance. Figure Figure 2. (a) 2. (a) Transmission spectra 1.0Yb Tm -0.1Tmco doped co-doped PG GC. (b) (b) UC UC emission spectra 1.0Yb 1.0Yb Tm codoped PG PG GCs. GCs. (c) (c) Emission decay curves 1.0Yb Tm Tm 3+ co doped co-doped PG GCs monitored at at nm nm emission. (d) (d) Emission decay decay curves 1.0Yb Tm Tm co doped co-dopedpg PG GCs monitored at 802 nm emission. Figure2b 2b shows emission spectra 1.0Yb Tm 0.005Tm co-doped co doped PG PG GCs GCs recordedat at room temperature upon upon excitation excitation a 980 nm a 980 laser nm diode laser (LD). diode (LD). spectra 3+ codoped spectra all include all include a NIR emission a NIR emission peak atpeak 802 nm at 802 attributed nm attributed to 3 Hto 4 3 H 3 6 transition H4 3 H6 transition Tm 3+. Tm Meanwhile, 3+. Meanwhile, blue (450 blue ( nm), 480 red nm), (650 red nm), (650 nm), deep red emission deep red peaks emission (680 peaks 700 nm) attributed to ( nm) attributed 1 D 2 3 F to 4, 1 G 1 D2 4 3 H 3 F4, 6, 1 G 1 G4 4 3 H6, 3 F 4, 3 F 1 G4 2 3 H 3 F4, 4, 3 F2 3 F 3 H4, 3 3 H 4 transition 3 F3 3 H4 transition 3+ [34,35], respectively, are all observed in emission spectra PG GC samples. Tm Tm 3+ [34,35], respectively, are all observed in emission spectra PG GC Since Tm 3+ exhibits no absorption for 980 nm light, se emissions are all obtained via samples. Since Tm 3+ exhibits no absorption for 980 nm light, se emissions are all obtained ET processes Yb 3+ to Tm 3+. In addition, emission peaks GCs are narrower than that via PG ET processes splitting spectrum Yb 3+ to by Tm crystal 3+. In fields addition, are observed emission in peaks spectra GCs GCs, are narrower indicatingthan incorporation that PG Tm 3+ splitting into crystal spectrum coordinated by sites. crystal It isfields also found are observed in spectra spectra that GCs, emission indicating intensity incorporation nm Tm increases 3+ into crystal 15 6coordinated times after sites. heat It is also treatment found for 10 h, respectively. spectra that Figure emission 2c,d showintensity emission at 450 decay curves 802 nm monitored increases at 15 6 times after heat treatment for 10 h, respectively. Figure 2c Figure 2d show

5 Nanomaterials 2021, 11, x FOR PEER REVIEW 5 10 Nanomaterials 2021, 11, 1033 emission decay curves monitored at nm, respectively. emission 5 10 lifetime at 450 nm increases 441 to μs, that at 802 nm increases 393 to μs when sample is heated at 540 C for 5 10 h, respectively. se results are attributed to fact that low phonon energy environment reduces nm, respectively. emission lifetime at 450 nm increases 441 to 601 possibility 719 µs, that non-radiative at 802 nm increases transition 393 enhances to emission µs when intensity sample is lifetime heated in at 540 sample, C forwhich 5 10 in h, turn respectively. prove se incorporation results are attributed Tm 3+ into fact fluoride that nanocrystals low phonon in GCs. energy Actually, environment it is difficult reduces for Tmpossibility 3+ to enter K2SiF6 non-radiative crystal structures transition due to enhances lack appropriate emission sites intensity for substitution lifetime in Tmsample, 3+. However, which in Tmturn 3+ can prove easily in- corporate incorporation into KYb3F10 Tm 3+ into crystal structures fluoride nanocrystals by occupying GCs. Actually, sites Yb it is 3+ difficult because for Tmionic 3+ radiuses to enter K 2 Tm SiF 3+ 6 crystal (R = structures nm) dueyb to 3+ (R lack = appropriate nm) are very sites similar. for Accordingly, substitution incorporation Tm 3+. However, Tm into can easily KYb3F10 incorporate nanocrystals into KYb is responsible 3 F 10 crystal structures for enhancements by occupying emission sitesintensities Yb 3+ because lifetimes ionic radiuses in GCs. Tm 3+ (R = nm) Yb 3+ (R = nm) are verydouble logarithmic similar. Accordingly, plots incorporation excitation power Tm 3+ into dependency KYb 3 F 10 nanocrystals on emission intensities responsible are for presented enhancements in Figure 3a. emission fitted intensities slope (n) lifetimes plot in Ln GCs. (IUC) versus Ln (Power) is double-logarithmic used to determine plots absorbed excitation photon numbers power dependency per UC emitted on [36,37]. emission intensities are presented in Figure 3a. fitted slope (n) plot Ln (I UC ) versus Ln obtained value n was 1.67, 1.73, 2.67, 3.40 corresponding to 802,700, 650, (Power) is used to determine absorbed photon numbers per UC emitted [36,37]. 450 nm emission 1.0Yb obtained value n was 1.67, Tm 1.73, 2.67, 3+ co-doped GC, respectively. se prove that 3.40 corresponding to 802,700, 650, emissions 450 nm emission are all attributed 1.0Yb 3+ to Tm UC emission 3+ co-doped Tm GC, 3+ respectively. obtained se via prove ET that Yb 3+. Two, emissions two, three, are all attributed four pump to UCphotons emissionare needed Tm 3+ to obtained pump via electrons ET to 3 H4, Yb 3+ 3 F2,3,. 1 G4, Two, two, 1 D2 three, energy levels four pump Tmphotons 3+ achieve are needed to corresponding pump electrons UC emissions, to 3 H 4, 3 F 2,3 respectively. 1 D 2 energy levels Tm 3+ to achieve corresponding UC emissions,, 1 G 4, respectively. Figure Figure 3. (a) 3. (a) Ln-Ln Ln-Ln plots plots UC UC emission intensity versus excited power density nm nm laser. laser. (b) (b) Schematic Schematic diagrams diagrams energy levels relative transitions in in KYb KYb3F10:Tm 3+ 3 F GCs. Basedon on se, energy levels Tm Tm Yb Yb relative transitions corresponding corresponding to toabove above UC UC emissions emissions are are illuminated in in Figure Figure 3b 3b [38 40]. Under 980 nm 980 LD nmexcitation, LD excitation, electrons electrons Yb 3+ are Yb 3+ excited are excited ground ground state state 2 F7/2 to 2 F 7/2 to 2 F5/2 excited 2 Fstate, 5/2 excited n state, energy n is transferred energy is transferred to Tm 3+ through to Tm 3+ through ET1 process: ET1 [ 2 process: F5/2 (Yb 3+ ) + 3 [ 2 H6 (Tm F 5/2 3+ (Yb ) 3+ 2 ) + 3 F7/2 (Yb H 6 (Tm 3+ ) ) 2 H5 (Tm F 7/2 3+ (Yb ) + 3+ phonons]. ) + 3 H 5 (Tm 3+ ) electrons + phonons]. Tm 3+ electrons are excited Tm 3+ are excited ground ground state to 3 state to H5 excited state 3 H 5 excited state n undergo a non-radiative n undergo a non radiative relaxation relaxation transition to transition to 3 F4 energy level. 3 F 4 energy level. Via ET2 process, [ Via ET2 process, [ 2 F5/2 (Yb 3+ ) + 2 F 3 F4 (Tm 5/2 (Yb 3+ ) 3+ ) + 2 F7/2 (Yb 3 F 4 3+ ) (Tm 3+ ) 2 F + 3 F2,3 (Tm 3+ ) 7/2 (Yb 3+ ) + 3 F + phonons], 2,3 (Tm 3+ ) + phonons], electrons are populated to 3 F electrons are populated to 3 F2, 3 energy level. A part 2, 3 energy level. A part electrons undergo a non-radiative relaxation transition to 3 electrons undergo a non-radiative relaxation transition to 3 H4 H energy level. In this 4 energy level. In this process, 3 H 4 3 H 6 (802 nm) 3 F 2,3 3 H 6 ( nm) process, 3 H4 3 H6 (802 nm) 3 F2,3 3 H6 transitions are obtained. Next, part electrons (700 at680 3 H nm) transitions are obtained. 4 energy levels are excited to Next, 1 G part electrons at 3 H4 energy levels are excited to 1 G4 4 energy through process ET3: [ 2 F 5/2 (Yb 3+ ) + 3 H 4 (Tm 3+ ) energy 2 F 7/2 (Ybthrough 3+ ) + 1 G 4 process (Tm 3+ ) + phonons]. ET3: [ 2 F5/2 This (Yb 3+ produces ) + 3 H4 (Tm 1 G 3+ ) 2 F7/2 (Yb 3+ ) + 1 G4 (Tm H 6 (480 nm) 1 G 4 3 F) 4 + (650 phonons]. nm) transitions. This produces Finally, 1 G4 3 electrons H6 (480 nm) 1 G 1 G4 3 4 energyf4 level (650 are nm) populated transitions. to Finally, 1 D 2 energy electrons level through 1 G4 energy level are populated to 1 D2 energy level through process ET4, [ 2 F5/2

6 Nanomaterials 2021, 11, x FOR PEER REVIEW 6 10 Nanomaterials 2021, 11, 1033 (Yb 3+ ) + 1 G4 (Tm 3+ ) 2 F7/2 (Yb 3+ ) + 1 D2 (Tm 3+ ) + phonons], producing a 1 D2 3 F4 6( nm) transition. Furrmore, transitions Tm 3+ are also modulated via interaction between neighboring process Tm 3+ ET4, ions, [ 2 which F 5/2 (Ybis 3+ directly ) + 1 G 4 (Tm determined 3+ ) 2 F 7/2 by (Yb 3+ ) doping + 1 D 2 (Tm concentration 3+ ) + phonons], Tm 3+ ions producing [41,42]. a 1 Ddependence 2 3 F 4 (450 nm) transition. UC emission spectrum on doping concentration Tm 3+ in Furrmore, co-doped PG transitions GC is presented Tm 3+ are also in Figure modulated 4a,b. viait can interaction be observed between spectra neighboring that Tm blue 3+ ions, emission whichintensities is directly determined at 450 nm by 480 doping nm concentration both decrease monoton- ically with increase Tm 3+ Tm 3+ ions [41,42]. dependence UC emission spectrum on doping concentration Tm in co-doped PG GC is 3+ concentration, while NIR emission intensity at 802 nm presented in Figure 4a,b. It can be observed spectra experiences first an enhancement n a decrease due to furr increase Tm that blue emission intensities at 450 nm 480 nm both decrease monotonically with 3+ concentration. increase Actually, Tm 3+ concentration, as a result while increase NIR emission Tmintensity 3+ content, at 802 nm distance experiences between Tm 3+ first becomes an enhancement shorter. n interactions a decreasebetween due to neighboring furr increase Tm 3+ Tm ions 3+ get concentration. larger, which benefits Actually, as cross-relaxation a result increase (CR) process, Tm 3+ 1 content, G4 + 3 H6 distance 3 F2,3 + 3 F4, between as shown Tm 3+ in becomes Figure 3b. shorter. CR process interactions depopulates between electrons neighboring in 1 G4 Tmlevel, 3+ ions while get larger, electrons which benefits in 3 F2,3 3 F4 levels cross-relaxation are populated (CR) via process, CR 1 process. G H 6 Thus, 3 F 2,3 blue + 3 Femission 4, as shown intensities Figuredecrease 3b. CR quickly with process increase depopulates Tmelectrons 3+ even though in 1 G 4 level, doping while concentration electronsis inlow. 3 F 2,3 NIR 3 F 4 levels emission are populated via CR process. Thus, blue emission intensities decrease quickly with increases firstly increase Tm 3+ n decreases when doping concentration is furr increased even though doping concentration is low. NIR emission due to concentration quenching effect. Compared to a glass matrix, crystal exhibits a increases firstly n decreases when doping concentration is furr increased more due compact to concentration structure. quenching distance effect. between Compared Tm 3+ toin agc glass is matrix, shorter crystal than exhibits that in PG a at more same compact doping structure. concentration, distance resulting between in Tm 3+ lower in GCluminescence quenching is shorter than that PG atconcen- tration samein doping GC as concentration, compared to resulting PG. inemission lowerintensity luminescence-quenching at 802 nm reaches concentration a maximum with in GCdoping as compared to PG. 0.01Tm emission 3+ in intensity PG at GC, 802respectively. nm reaches aas maximum shown with in Figure 4b doping inset, blue 0.01Tm emissions 3+ in PG around GC, 450 respectively. 480 nm Asin shown PG in Figure GC both 4b disappear inset, UC spectra blue emissions almost around exhibit 450 pure NIR 480 nm emissions in PG when GC both disappear contents Tm 3+ UC exceed spectra almost exhibit pure NIR emissions when contents Tm 3+ exceed 0.01 mol% mol%. A blue UC emission only being obtained in GC at a low doping concentration A blue UC emission only being obtained in GC at a low doping concentration indicates indicates most most Tm 3+ Tm ions are 3+ ions are incorporated into KYb3F10 crystal structure with short incorporated into KYb 3 F 10 crystal structure with short interionic interionic distance. distance. pure NIRpure emissions NIR emissions that are obtained that are atobtained a very lowat doping a very concentration low doping concentration Tm 3+ indicates Tm 3+ indicates that GC that is an excellent GC is an cidate excellent for cidate pure NIRfor UCpure emission NIR UC isemis- sion a significant is a significant UC luminescent UC luminescent material formaterial promising for applications promising in applications NIR fiber lasers in NIR fiber lasers high-resolution high-resolution biological biological imaging. imaging. Figure 4. UC emission spectra 1.0Yb Figure 4. UC emission spectra 1.0Yb 3+ -ytm 3+ -ytm 3+ co-doped (a) PG (b) GC excited by a 980 nm LD. (y = 0.005~0.3). 3+ co doped (a) PG (b) GC excited by a 980 nm LD. (y = 0.005~0.3). As mentioned above, Yb 3+ works as sensitizer ion for UC emissions Tm 3+. More As mentioned importantly, above, Yb 3+ also Ybparticipates 3+ works as insensitizer construction for fluoride UC nanocrystals emissions Tm 3+. More provides importantly, appropriate Yb 3+ crystal also participates sites for incorporation construction Tm 3+. fluoride dopingnanocrystals Yb 3+ plays provides a vital appropriate role in precipitation crystal sites nanocrystals for incorporation enhancement Tm 3+. UC doping emissions. Yb 3+ plays a vital Yb 3+ role -concentration-dependent in precipitation XRDnanocrystals patterns Ybenhancement 3+ -Tm 3+ co-doped UC no-doped emissions. GCs Yb 3+ are concentration dependent presented in Figure 5. It is found XRD that patterns only K 2 SiF 6 crystals Yb 3+ Tm are 3+ precipitated co doped in nodoped GCs are presented in Figure 5. It is found that only K2SiF6 crystals are precipitated in no-doped GC se make no contribution to enhancement UC emission.

7 Nanomaterials 2021, 11, Nanomaterials 2021, 11, x FOR PEER REVIEW 7 10 no-doped Via doping GC se make no contribution to enhancement UC emission. Via doping Yb 3+ Yb 3+, KYb3F10 are precipitated in GCs apart K2SiF6 crystals, KYb 3 F 10 are precipitated in GCs apart K 2 SiF 6 crystals as Yb 3+ as Yb content 3+ content increases 0.5 to 1.0 mol%. increases 0.5 to 1.0 mol%. When Yb 3+ When Yb concentration 3+ concentration is set to 1.5 is set to 1.5 mol%, only mol%, only diffraction peaks KYb3F10 crystals are observed in XRD pattern diffraction peaks KYb 3 F 10 crystals are observed in XRD pattern GC. se GC. se results indicate that precipitation KYb3F10 nanocrystals in GCs is results indicate that precipitation KYb 3 F 10 nanocrystals in GCs is governed by governed by doping concentration doping concentration Yb 3+ Yb induced by Yb induced by Yb. More interestingly, 3+. More interestingly, KYb 2 F 7 crystals are KYb2F7 crystals are also precipitated in also precipitated in GCs when Yb 3+ GCs when Yb concentration is increased 3+ concentration is increased 2.0 to 3.5 mol%. 2.0 to 3.5 mol%. In XRD In XRD pattern 3.5Yb 3+ pattern -0.1Tm Yb co-doped sample, Tm only 3+ co doped sample, only diffraction peaks diffraction peaks KYb2F7 crystals are observed. refore, nano crystallization in KYb 2 F 7 crystals are observed. refore, nano-crystallization in GC is induced by GC is induced doped Yb 3+ by doped Yb crystalline phase 3+ crystalline phase in GC can be controllably in GC can be controllably regulated K 2 SiF 6 regulated K2SiF6 to KYb3F10 furr changes to KYb2F7 crystals by adjusting to KYb 3 F 10 furr changes to KYb 2 F 7 crystals by adjusting concentration Yb 3+ concentration 0 to 3.5 mol%. Yb 3+ 0 to 3.5 mol%. Figure 5. XRD patterns xyb Tm -0.1Tm 3+ co doped co-doped no doped no-doped GCs heat treated at 540 C C for 10 h. (x = 0~3.5%). Via nano-crystallizations nano crystallizations KYb KYb3F10 3 F KYb2F7, 2 F 7, Yb ions are spontaneously confined within fluoride crystal structures. More More importantly, Tm Tm 3+ can 3+ can easily easily incorporate incorporate se two two crystal crystal structures structures by by substitution substitution Yb 3+ Yb. 3+ Owing. to to extremely low phonon energies in fluoride crystals, probabilities non-radiative non radiative transitions are low, which is is beneficial for for efficient ET ET Yb Yb to to Tm Tm enhanced UC emission Tm UC emission spectra Yb Yb Tm -Tm co doped co-doped GC with various concentrations Yb Yb are shown in Figure 6a. Excited by a 980 nm LD, pure NIR UC UC emissions around 802 nm nm Tm Tm are observed in in spectra GCs. GCs. As As Yb Yb concentration increases to 1.5 mol%, quantity quantity KYb KYb3F10 3 F 10 nanocrystals nanocrystals precipitated precipitated in in GCs GCs is in- is se increased, creased, amount amount Tm Tm Yb Yb ions ions located located in in fluoride fluoride crystal crystal structures structures are are increased. increased. emission emission intensity intensity enhances enhances firstly, firstly, reaching reaching a maximum maximum at at mol% mol% Yb Yb 3+ 3+, n n decreases decreases when when Yb Yb concentration concentration is is increased increased to to mol% mol% due due to to concentration concentration quenching quenching effect effect caused caused by by severe severe interactions interactions between between Ln 3+ ions. Ln When 3+ ions. Yb When 3+ concentration is furr increased to 2.0 mol%, KYb Yb 3+ concentration is furr increased to 2.0 mol%, 2 F 7 crystals start to precipitate KYb2F7 crystals start to

8 Nanomaterials 2021, 11, x FOR PEER REVIEW 8 10 Nanomaterials 2021, 11, precipitate in GCs. A part Tm 3+ enter KYb2F7 nanocrystals in 2.0Yb Tm 3+ co-doped in GCs. GC, A part which disperses Tm 3+ enter KYb distribution 2 F 7 nanocrystals Tm 3+ in increases 2.0Yb Tm NIR 3+ co-doped UC emission GC, intensity which disperses again. When distribution Yb 3+ concentration Tm 3+ increases increases NIR UC 2.0 emission to 3.5 mol%, intensity more again. When Yb 3+ KYb2F7 crystals are precipitated concentration in increases GCs, more Tm toions 3.5 mol%, are incorporated more KYb 2 F 7 into crystals are precipitated in GCs, more Tm 3+ KYb2F7 crystals, UC emission intensity ions at 800 are is incorporated increased monotonically. into KYb 2 F 7 crystals, It is also found that UC emission prile intensity emission at 800 is increased spectrum monotonically. It is also found that prile emission spectrum for 3.5 Yb 3+ for -0.1Tm Yb Tm 3+ co doped GCs is different co-doped GCs is different that 1.0Yb 3+ that 0.1Tm Yb co-doped Tm samples 3+ co doped samples (inset Figure 6a), which (inset Figure 6a), which in turn proves that Tm 3+ in turn proves that Tm are incorporated in two 3+ are incorporated in two different crystal environments in two different crystal environments in two GCs containing different GCs nanocrystals. containing Compared different to nanocrystals. KYb 2 F 7 crystal, Compared more Yb to 3+ KYb2F7 ions are crystal, distributed more in Yb 3+ crystal ions are distributed structure in in KYb crystal 3 F 10. structure luminescent-quenching in KYb3F10. concentration luminescent quenching UC emission concentration in KYb 2 UC F 7 crystal emission higher in KYb2F7 thancrystal that in is KYb higher 3 F 10. than emission that in KYb3F10. intensity 3.5Yb emission Tm intensity 3+ in co-doped 3.5Yb Tm GC is 3+ 7 co doped times higher GC than is 7 that times inhigher 1.0Yb than Tm that in 3+ sample, 1.0Ybwhich Tmindicates 3+ sample, which that indicates nano-crystallization that nano crystallization KYb 2 F 7 provides more KYb2F7 excellent provides crystal more environments excellent crystal for environments achieving more for efficient achieving NIRmore UC emission efficient NIR TmUC 3+. emission Tm 3+. Figure (a) (a) UC UC emission spectra xyb Tm co-doped GCs heat treated at 540 C C for for 5 5 h. h. (x (x = 0.0~3.5). = (b) (b) Emission decay decay curves curves xyb xyb Tm Tm co-doped PG PG GCs GCs heat heat treated treated 540 at 540 C for C for 5 h 5 h monitored at at nm nm emission. (x = 0.5~3.5). (x = 0.5~3.5). UC emission decay curves Yb Yb Tm Tm co-doped GCs GCs monitored at at nm nm are are shownin in Figure 6b. lifetime NIR NIR emission decreases decreases as Yb Yb 3+ concentration 3+ increases increases 0.5 to 1.5 mol%. mol%. Especially, Especially, lifetime lifetime 1.5Yb 1.5Yb Tm Tm 3+ co-doped 3+ co doped GC decreases dramatically to 373 µs due to severe interaction between neighboring GC decreases dramatically to 373 μs due to severe interaction between neighboring 3+ in KYb Tm Tm 3+ in 3 F 10 crystals. However, lifetime increases 16.5 to 39.8 µs when KYb3F10 crystals. However, lifetime increases 16.5 to 39.8 μs when Yb Yb 3+ concentration is increased 2.5 to 3.5 mol%. se results also prove that 3+ concentration is increased 2.5 to 3.5 mol%. se results also prove that coordinated environments Tm 3+ in high-doping GCs ( mol% Yb Tm 3+ ) coordinated is distinct to environments that in low-doping Tm 3+ GCs in high doping (0.5, 1.0, GCs 1.5 mol% (2.5 Yb Tm mol% 3+ Yb ). 3+ refore, 0.1Tm 3+ ) is distinct precipitation to that in low doping KYb GCs (0.5, 1.0, 1.5 mol% Yb Tm 3+ ). refore, 2 F 7 nanocrystals in GCs furr enhances NIR UC emission precipitation Tm 3+ provides KYb2F7 a significant nanocrystals matrixin for GCs applications furr enhances in photonic devices, NIR UC in particular emission Tm 3+ high-efficiency provides UC a significant fiber lasers. matrix for applications in photonic devices, in particular high efficiency UC fiber lasers. 4. Conclusions 4. Conclusions In summary, transparent Yb 3+ -Tm 3+ co-doped GCs were prepared in this work for greatly In summary, enhancingtransparent UC luminescence Yb 3+ Tm 3+ co doped Tm 3+. GCs GCswere possessed prepared high in transmittance this work for greatly (>85.0%) enhancing though fluoride UC nanocrystals luminescence were precipitated Tm 3+. GCs among possessed glass high matrices. transmittance K 2 SiF 6 crystals are precipitated in no-doped GC. However, KYb (>85.0%) though fluoride nanocrystals were precipitated among 3 F 10 KYb glass 2 F 7 nanocrystals matrices. K2SiF6 were controllably successively precipitated in GCs via inducing Yb crystals are precipitated in no doped GC. However, KYb3F10 KYb2F7 nanocrystals 3+ when were controllably successively precipitated in GCs via inducing Yb 3+ when doping concentration Yb 3+ changed 0.5 to 3.5 mol%. UC emissions Tm 3+

9 Nanomaterials 2021, 11, References doping concentration Yb 3+ changed 0.5 to 3.5 mol%. UC emissions Tm 3+ were dramatically enhanced when Ln 3+ ions were incorporated into KYb 3 F 10 crystal structures as Yb 3+ concentration changed 0.5 to 1.5 mol%. Pure NIR UC emissions were obtained by adjusting concentration Tm 3+. KYb 2 F 7 nanocrystals were precipitated in GCs when Yb 3+ concentration exceeded 2.0 mol%. More efficient UC emissions were achieved via precipitation KYb 2 F 7 than that KYb 3 F 10 in GCs. designed GCs fer potential optical gain materials for highly-efficient NIR UC photoluminescence. More importantly, Yb 3+ -induced nano-crystallization strategy paves a new way for design fabrication emerging GCs to provide excellent crystal environments for Ln 3+. Author Contributions: Conceptualization, Z.F. J.L.; methodology, J.L.; Y.L. Q.Z.; formal analysis, J.L.; Y.L.; Q.Z. S.Z.; investigation, J.L.; Y.L.; Q.Z. Z.F.; resources, S.Z.; Z.F. B.-O.G.; data curation, J.L.; Y.L.; Q.Z. Z.F.; writing-original draft, J.L.; writing-review editing, S.Z.; Z.F. B.-O.G.; supervision, Z.F. B.-O.G.; project administration, Z.F. B.-O.G.; funding acquisition, Z.F.; S.Z. B.-O.G. All authors have read agreed to published version manuscript. Funding: authors gratefully acknowledge financial support National Natural Science Foundation China (No , ). This work was also supported by Open Fund Guangdong Provincial Key Laboratory Fiber Laser Materials Applied Techniques (South China University Technology) Fundamental Research Funds for Central Universities ( ). Conflicts Interest: authors declare no conflict interest. 1. Zhou, J.; Gu, F.; Liu, X.; Qiu, J. Enhanced multiphoton upconversion in single nanowires by waveguiding excitation. Adv. Opt. Mater. 2016, 4, [CrossRef] 2. Zhou, J.; Leano, J.L., Jr.; Liu, Z.; Jin, D.; Wong, K.L.; Liu, R.S.; Bunzli, J.G. Impact lanthanide nanomaterials on photonic devices smart applications. Small 2018, 14, e [CrossRef] 3. Wen, S.; Zhou, J.; Zheng, K.; Bednarkiewicz, A.; Liu, X.; Jin, D. Advances in highly doped upconversion nanoparticles. Nat. Commun. 2018, 9, [CrossRef] [PubMed] 4. Wang, F.; Han, Y.; Lim, C.S.; Lu, Y.; Wang, J.; Xu, J.; Chen, H.; Zhang, C.; Hong, M.; Liu, X. Simultaneous phase size control upconversion nanocrystals through lanthanide doping. Nature 2010, 463, [CrossRef] 5. Wang, F.; Wang, J.; Liu, X. Direct evidence a surface quenching effect on size-dependent luminescence upconversion nanoparticles. Angew. Chem. Int. Ed. Engl. 2010, 49, [CrossRef] [PubMed] 6. Wang, J.; Deng, R.; MacDonald, M.A.; Chen, B.; Yuan, J.; Wang, F.; Chi, D.; Hor, T.S.; Zhang, P.; Liu, G.; et al. Enhancing multiphoton upconversion through energy clustering at sublattice level. Nat. Mater. 2014, 13, [CrossRef] 7. Wang, J.; Wang, F.; Wang, C.; Liu, Z.; Liu, X. Single-b upconversion emission in lanthanide-doped KMnF 3 nanocrystals. Angew. Chem. Int. Ed. Engl. 2011, 50, [CrossRef] 8. Zhou, B.; Shi, B.; Jin, D.; Liu, X. Controlling upconversion nanocrystals for emerging applications. Nat. Nanotechnol. 2015, 10, [CrossRef] 9. Auzel, F. Upconversion Anti-Stokes Processes with f d Ions in Solids. Chem. Rev. 2004, 104, [CrossRef] 10. Chen, G.Y.; Ohulchanskyy, T.Y.; Kumar, R.; Agren, H.; Prasad, P.N. Ultrasmall monodisperse NaYF 4 :Yb 3+ /Tm 3+ nanocrystals with enhanced Near-Infrared to Near-Infrared upconversion photoluminescence. Acs Nano 2010, 4, [CrossRef] 11. Fernez-Bravo, A.; Yao, K.; Barnard, E.S.; Borys, N.J.; Levy, E.S.; Tian, B.; Tajon, C.A.; Moretti, L.; Altoe, M.V.; Aloni, S.; et al. Continuous-wave upconverting nanoparticle microlasers. Nat. Nanotechnol. 2018, 13, [CrossRef] 12. Liu, Q.; Sun, Y.; Yang, T.; Feng, W.; Li, C.; Li, F. Sub-10 nm hexagonal lanthanide-doped NaLuF 4 upconversion nanocrystals for sensitive bioimaging in vivo. J. Am. Chem. Soc. 2011, 133, [CrossRef] [PubMed] 13. Zhang, W.J.; Zhang, J.P.; Wang, Z.; Wang, W.C.; Zhang, Q.Y. Spectroscopic structural characterization transparent fluorogermanate glass ceramics with LaF 3 :Tm 3+ nanocrystals for optical amplifications. J. Alloys Compd. 2015, 634, [CrossRef] 14. Chen, G.; Lei, R.; Huang, F.; Wang, H.; Zhao, S.; Xu, S. Effects Tm 3+ concentration on upconversion luminescence temperature-sensing behavior in Tm 3+ /Yb 3+ :Y 2 O 3 nanocrystals. J. Biolumin. Chemilumin. 2018, 33, Chen, S.; Song, W.; Cao, J.; Hu, F.; Guo, H. Highly sensitive optical rmometer based on FIR technique transparent NaY 2 F 7 :Tm 3+ /Yb 3+ glass ceramic. J. Alloys Compd. 2020, 825, [CrossRef] 16. Li, X.; Yang, C.; Yu, Y.; Li, Z.; Lin, J.; Guan, X.; Zheng, Z.; Chen, D. Dual-modal photon upconverting downshifting emissions ultra-stable CsPbBr 3 perovskite nanocrystals triggered by co-growth Tm:NaYbF 4 nanocrystals in glass. ACS Appl Mater. Interfaces 2020, 12, [CrossRef] [PubMed]

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