materials
Article Upconversion Luminescence of Silica–Calcia Nanoparticles Co-doped with Tm3+ and Yb3+ Ions
Katarzyna Halubek-Gluchowska 1,*, Damian Szyma ´nski 1 , Thi Ngoc Lam Tran 2 , Maurizio Ferrari 2 and Anna Lukowiak 1,*
1 Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Okolna 2, 50-422 Wroclaw, Poland; [email protected] 2 IFN-CNR CSMFO Lab. and FBK Photonics Unit, Via alla Cascata 56/C, Povo, 38100 Trento, Italy; [email protected] (T.N.L.T.); [email protected] (M.F.) * Correspondence: [email protected] (K.H.-G.); [email protected] (A.L.)
Abstract: Looking for upconverting biocompatible nanoparticles, we have prepared by the sol–gel method, silica–calcia glass nanopowders doped with different concentration of Tm3+ and Yb3+ ions (Tm3+ from 0.15 mol% up to 0.5 mol% and Yb3+ from 1 mol% up to 4 mol%) and characterized their structure, morphology, and optical properties. X-ray diffraction patterns indicated an amorphous phase of the silica-based glass with partial crystallization of samples with a higher content of lan- thanides ions. Transmission electron microscopy images showed that the average size of particles decreased with increasing lanthanides content. The upconversion (UC) emission spectra and fluores- cence lifetimes were registered under near infrared excitation (980 nm) at room temperature to study the energy transfer between Yb3+ and Tm3+ at various active ions concentrations. Characteristic emission bands of Tm3+ ions in the range of 350 nm to 850 nm were observed. To understand the 3+ 3+ mechanism of Yb –Tm UC energy transfer in the SiO2–CaO powders, the kinetics of luminescence decays were studied. Citation: Halubek-Gluchowska, K.; Szyma´nski,D.; Tran, T.N.L.; Ferrari, Keywords: bioactive glass; upconversion; ytterbium; thulium M.; Lukowiak, A. Upconversion Luminescence of Silica–Calcia Nanoparticles Co-doped with Tm3+ 3+ and Yb Ions. Materials 2021, 14, 937. 1. Introduction http://doi.org/10.3390/ma14040937 Rare-earth-activated systems have been demonstrated to be the pillar of photonic
Academic Editor: Efrat Lifshitz technologies enabling a broad spectrum of crucial applications in strategic social and Received: 30 December 2020 economic priorities [1–5]. Systems based on rare-earth-doped upconverters are largely Accepted: 7 February 2021 employed in bioimaging, drug delivery, laser, lighting, photon management, environmental Published: 16 February 2021 sensing, and nanothermometry [6–15]. Upconversion (UC) mechanism is a process of energy transfer from a sensitizer in a proper host matrix, which is excited under low- Publisher’s Note: MDPI stays neutral energy radiation (usually near infrared, NIR), to an emitter that emits higher energy with regard to jurisdictional claims in photons than the excitation ones. This is a multi-photon process in which two or more published maps and institutional affil- low-energy photons are needed to generate in consequence shorter wavelength photon and iations. the emission with higher photon energy (ultraviolet (UV), visible (VIS), or NIR radiation) than the energy of the pumping source [16,17]. Yb3+ ions are excellent luminescence sensitizers for other rare earth ions due to their effective absorption cross section at 980 nm [16]. The Yb3+ ion has been demonstrated to be Copyright: © 2021 by the authors. a crucial element for upconverters [16], high power lasers [18], photovoltaic systems [19], 3+ 3+ Licensee MDPI, Basel, Switzerland. and integrated optics [5]. The couple Tm /Yb ions have been largely investigated be- 3+ This article is an open access article cause of the need to efficiently pump the Tm ion to obtain upconversion-based photonic 3+ 3+ distributed under the terms and devices on a broad spectrum of wavelengths. There are reports on Tm /Yb upcon- conditions of the Creative Commons version luminescence in single crystals [9], nanocrystals [17], glasses [20], ceramics [21], Attribution (CC BY) license (https:// and glass-ceramics [19]. Interesting research was performed on Tm3+/Yb3+ upconversion creativecommons.org/licenses/by/ luminescence in silicate glasses [22,23] to enhance the optical amplification spectroscopic 4.0/).
Materials 2021, 14, 937. https://doi.org/10.3390/ma14040937 https://www.mdpi.com/journal/materials Materials 2021, 14, 937 2 of 19
properties of thulium in silica. Moreover, studies were also dedicated to the downconver- sion mechanism between Tm3+/Yb3+ in glass or nanocrystals [17,24]. Properly synthesized silica-based glasses can be described as bioactive materials able to form a hydroxyapatite-like surface layer when immersed in a simulated body fluid similar to the blood plasma. Non-toxic and biocompatible bioactive glass is used in medicine to replace defects in bones or to stimulate growing new bones. Recently, lanthanides co-doped with ytterbium ions were introduced into bioactive glass matrices to study their optical properties and upconversion luminescence. For example, Li et al. 3+ 3+ described a couple of Er /Yb ions in CaSiO3 [25] and Kalaivani et al. doped sol–gel- 3+ 3+ derived glasses of SiO2–Na2O–CaO–P2O5 with Tb and Yb [26]. The systems were tested to estimate their biological properties and applications for regenerative medicine. In 3+ 3+ addition to conducting a bioactivity test of Gd /Yb -co-doped SiO2–Na2O–CaO–P2O5 glass [27], the group of Borges had also a second goal to study the role of rare earth ions in the modification of glass structure [28]. Within this work, we successfully used the sol–gel method to prepare powders of silica–calcia glass doped with different concentrations of Tm3+ and Yb3+ ions (not studied so far) and characterized their structure and morphology. The spectroscopic properties of the samples, such as absorption, UC emission, and photoluminescence decay times, were also investigated to propose the energy transfer mechanism in the studied system.
2. Materials and Methods 2.1. Synthesis of SiO2–CaO Particles Doped with Rare Earth Ions The analytical reagents comprising tetraethyl orthosilicate (TEOS, Sigma-Aldrich, Darmstadt, Germany), HNO3, NH4OH, Ca(NO3)2·4H2O (Avantor Performance Mate- rials, Gliwice, Poland), and high purity Yb2O3 and Tm2O3 (≥99.99%, Stanford Mate- rials Co., Lake Forest, CA, USA) were used as starting materials to obtain seven sam- ples of different compositions varied through the incorporation of rare earth elements: 3+ 3+ (xTm /yYb ):63SiO2–37CaO, where x = 0.15, 0.3 or 0.5 and y = 0, 1, 2, 3 or 4 (mol%). Additionally, one sample without rare earth ions was synthesized as well. The silica–calcia glasses were prepared by the sol–gel method as described previ- ously [29]. The starting materials were mixed in requisite proportions and order. The derived powders were centrifuged and washed three times with distilled water and dried at 70 ◦C overnight. Then, the samples were calcined at 800 ◦C for 2 h.
2.2. Characterization The structure of the samples was identified by X-ray diffractometer (XRD, X’Pert PRO, PANalytical, Bruker, Germany) with Cu Kα radiation (λ = 1.5406 Å) scanning the diffraction angles (2θ) between 10◦ and 70◦ at room temperature. The morphology and microstructure were examined by a Transmission Electron Microscope (TEM) (Philips CM-20 SuperTwin, Eindhoven, The Netherlands, operating at 160 kV) equipped with a selected area electron diffraction (SAED). TEM measurements have been performed on a copper grid coated with carbon. The grain size distribution has been determined for about 200 particles. The chemical composition of the samples was determined using a Field Emission Scanning Electron Microscope (FE-SEM) (FEI Nova NanoSEM 230, Fremont, CA, USA) equipped with an energy dispersive X-ray spectrometer (EDAX Genesis XM4). The EDS analyses were performed at 18.0 kV from the large area (250 µm × 200 µm) of the samples. The powder samples were included in the carbon resin and then pressed to obtain a large and flat area. Signals from three randomly selected areas were collected to ensure satisfactory statistical averaging. The absorption spectra of the powders were collected in the reflection mode using a UV-VIS-NIR spectrophotometer (Agilent Cary 5000, Santa Clara, CA, USA) in the wave- length range from 200 to 1300 nm. Photoluminescence spectra were measured using an FLS980 fluorescence spectrometer (Edinburgh Instruments, Livingstone, UK) equipped with a 980 nm laser diode as an excitation source and a photomultiplier as a detector. For Materials 2021, 14, x FOR PEER REVIEW 3 of 20
The absorption spectra of the powders were collected in the reflection mode using a UV-VIS-NIR spectrophotometer (Agilent Cary 5000, Santa Clara, CA, USA) in the wave- length range from 200 to 1300 nm. Photoluminescence spectra were measured using an FLS980 fluorescence spectrometer (Edinburgh Instruments, Livingstone, UK) equipped with a 980 nm laser diode as an excitation source and a photomultiplier as a detector. For decay time measurements, triggering was used with a 150 W xenon pulsed flash lamp (pulse duration 1–2 μs, 20 Hz) and the same photomultiplier as a detector. Spectra were corrected with respect to the detector sensitivity and lamp characteristic. All the measure- ments were done at room temperature. From the measured photoluminescence decay curves, 1/e decay times (τ1/e) were determined for all samples [30].
Materials 2021, 14, 937 3 of 19 3. Results and Discussion
3.1. Structuraldecay time and measurements, Morphological triggering Characterization was used with a 150 W xenon pulsed flash lamp The(pulse chosen duration method 1–2 µs, allowed 20 Hz) and the the samepreparation photomultiplier of two as aseries detector. of Spectra samples were with various corrected with respect to the detector sensitivity and lamp characteristic. All the measure- 3+ 3+ Tm andments Yb were concentrations done at room temperature. (Table 1). FromThe theXRD measured patterns photoluminescence of all compositions decay are shown in Figurecurves, 1. All 1/e patterns decay times were (τ1/e )characterized were determined forby allsignals samples displaying [30]. the amorphous nature
of the glass3. Results with and typical Discussion broad reflections. For three samples with the lowest thulium con- centration3.1. of Structural 0.15% and (Figure Morphological 1a), a Characterization low reflection signal at 2θ = 30° was recorded indicating partial crystallizationThe chosen method of the allowed glass theprobably preparation in ofthe two wollastonite series of samples phase with variousof calcium silicate Tm3+ and Yb3+ concentrations (Table1). The XRD patterns of all compositions are shown (CaSiO3, JCPDS No. 96-900-5779). Due to the low samples’ crystallinity, this phase cannot in Figure1. All patterns were characterized by signals displaying the amorphous nature be fully ofconfirmed the glass with but typical it is pointed broad reflections. out here For to three be the samples most with probable. the lowest In thuliumthe case of samples doped withconcentration 4% of Yb of3+ 0.15% and (Figure a higher1a), a concentration low reflection signal of Tm at 2 3+θ =ions 30◦ was(Figure recorded 1b), indi- a higher degree of crystallizationcating partial occurred crystallization and of thethe glass second probably phase in the wollastoniteof calcium phase silicate of calcium was detected— silicate (CaSiO3, JCPDS No. 96-900-5779). Due to the low samples’ crystallinity, this phase pseudowollastonitecannot be fully confirmed(JCPDS No. but it 96-900-2180). is pointed out here Thus, to be it the can most be probable. concluded In the that case the introduc- tion of selectedof samples rare doped earth with ions 4% of in Yb 3+theand sy astem higher favors concentration the formation of Tm3+ ions of (Figure glass-ceramic.1b), a higher degree of crystallization occurred and the second phase of calcium silicate was detected—pseudowollastonite (JCPDS No. 96-900-2180). Thus, it can be concluded that the introduction of selected rare earth ions in the system favors the formation of glass-ceramic.
(a) 0.15Tm0Yb 0.15Tm1Yb ▼ 0.15Tm2Yb 0.15Tm3Yb 0.15Tm4Yb ▼
▼ counts(a.u.)
10 20 30 40 50 60 70 2ϴ (deg.)
Figure 1. Cont.
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(b) 0.3Tm4Yb * 0.5Tm4Yb ▼ * * wollastonite pseudowollastonite * * ▼* * * *
* * counts (a.u.) *
10 20 30 40 50 60 70 2ϴ (deg.)
Figure 1. XRD patterns of the samples with various rare earth ions concentrations: (a) different Figure 1.Yb XRD3+ concentrations patterns of (0, the 1, 2,samples 3, and 4 mol%)with various and the same rare Tm earth3+ concentration ions concentrations: (0.15 mol%); ((ba)) different 3+ 3+ Yb3+ concentrationsdifferent Tm (0,(0.3 1, and 2, 3, 0.5 and mol%) 4 mol%) and the sameand the Yb sameconcentration Tm3+ concentration (4 mol%) (H—wollastonite (0.15 mol%); (b) dif- and —pseudowollastonite phase of CaSiO assigned to JCPDS Nos. 96-900-5779 and 96-900-2180 ferent Tm3+ (0.3* and 0.5 mol%) and the same3 Yb3+ concentration (4 mol%) (▼—wollastonite and ⁎— patterns, respectively). pseudowollastonite phase of CaSiO3 assigned to JCPDS Nos. 96-900-5779 and 96-900-2180 pat- terns, respectively).In order to determine the morphology of the silica–calcia powders, a transmission electron microscope was used. Figure2 shows representative TEM images of the un-doped 3+ 3+ and Tm /Yb -co-doped samples, which reveal that examined SiO2–CaO particles were In ordernanosized to and determine exhibited two the different morphology morphologies. of the TEM silica–calcia image of the un-doped powders, glass and a transmission electronthe microscope particle size distribution was used. (Figure Figure2a) indicated 2 shows that SiO representative2–CaO had a regular TEM morphology images of the un- and relatively good monodispersity with diameters of particles from 80 to 120 nm. It can doped and Tm3+/Yb3+-co-doped samples, which reveal that examined SiO2–CaO particles also be seen that spherical particles were non-aggregated and uniformly distributed. On were nanosizedthe other hand, and the exhibited particles of Tmtwo3+ /Ybdifferent3+-doped morphologies. samples exhibited undefinedTEM image shapes of and the un-doped glass andformed the largeparticle groups size of agglomeratesdistribution (see (Figure Figure2b–d). 2a) indicated The size distribution that SiO histograms2–CaO had a regular morphologyshowed and that therelatively maximum good size of monodispersity particles decreased from with 30–80 diameters nm to 15–30 of nm particles for the from 80 to 0.15Tm2Yb and 0.5Tm4Yb samples, respectively. As is well known, the sol–gel synthesis 120 nm.conditions It can also strongly be seen influence that the spherical final morphology particles of the were samples. non-aggregated This effect is easily and uniformly distributed.observed On in the the other case of synthesishand, the of silica–calciaparticles systemof Tm where3+/Yb3+ both-doped particles samples of undefined exhibited unde- fined shapesshape asand well formed as spheres large with different groups diameters of agglomerates can be obtained (see [29 ].Figure From these 2b–d). studies, The size distri- it is evident that even a low concentration of introduced rare earth ions modifies synthesis bution histogramsconditions and showed effects in differentthat the shapes maximum of the particles. size of particles decreased from 30–80 nm to 15–30 nm for the 0.15Tm2Yb and 0.5Tm4Yb samples, respectively. As is well known, the sol–gel synthesis conditions strongly influence the final morphology of the samples. This effect is easily observed in the case of synthesis of silica–calcia system where both particles of undefined shape as well as spheres with different diameters can be obtained [29]. From these studies, it is evident that even a low concentration of introduced rare earth ions modifies synthesis conditions and effects in different shapes of the particles.
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FigureFigure 2. 2. TEMTEM images images (on (on the left) withwith selectedselected area area electron electron diffraction diffraction (SAED) (SAED) (as (as insets), insets), histograms histograms of size of size particle particle distribution (in the middle), and EDS spectra (on the right) of (a) un-doped (SiO2-CaO), (b) 0.15Tm2Yb, (c) 0.3Tm4Yb, and distribution (in the middle), and EDS spectra (on the right) of (a) un-doped (SiO2-CaO), (b) 0.15Tm2Yb, (c) 0.3Tm4Yb, and (d) 0.5Tm4Yb samples. (d) 0.5Tm4Yb samples.
To confirmconfirm thethe structure structure of of SiO SiO2–CaO2–CaO samples, samples, SAED SAED images images were were also also employed. employed. The selectedselected area area electron electron diffraction diffraction pattern pattern displayed displayed an amorphous an amorphous pattern pattern of SiO2–CaO of SiO2– CaOglass glass (see insets(see insets in Figure in Figure2). It 2). is clearlyIt is clearly visible visible that athat diffraction a diffraction pattern pattern of examined of examined samplessamples exhibited aa ringring pattern pattern as as a a result result of of the the absence absence of long-rangeof long-range order, order, which which is is consistentconsistent with thethe XRDXRD resultsresults showing showing broad broad reflections reflections from from the the amorphous amorphous phase. phase. SAEDSAED technique hashas notnot shown shown the the minor minor crystalline crystalline phases, phases, most most probably probably due due to the to the analysisanalysis performed on on a a selected selected area area of of the the samples. samples. The elemental compositions of the SiO –CaO glasses were analyzed by Energy Disper- The elemental compositions of the SiO2 2–CaO glasses were analyzed by Energy Dis- persivesive X-ray X-ray Spectroscopy. Spectroscopy. The EDS The spectra EDS spectra presented presented in Figure 2in show Figure three 2 strongshow bandsthree ofstrong oxygen (O), silicon (Si), and calcium (Ca). Other recorded bands with maxima at 7.15 and bands of oxygen (O), silicon (Si), and calcium (Ca). Other recorded bands with maxima at 7.42 keV have been attributed to thulium (Tm) and ytterbium (Yb) elements, respectively. 7.15 and 7.42 keV have been attributed to thulium (Tm) and ytterbium (Yb) elements, re- Moreover, additional bands from other elements were not detected which indicates that spectively. Moreover, additional bands from other elements were not detected which in- the samples were free from contaminants. Table1 shows the weight percentage of the SiO 2, dicates that the samples were free from contaminants. Table 1 shows the weight percent- CaO, Tm2O3, and Yb2O3 obtained from the EDS analysis. age of the SiO2, CaO, Tm2O3, and Yb2O3 obtained from the EDS analysis.
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Table 1. Samples names, nominal lanthanides concentration (mol%), and chemical composition (wt.%) of SiO2–CaO powders calculated on the basis of a quantitative EDS analysis.
Lanthanides Concentration Chemical Composition (wt.%) (a) Sample Name (mol%) Tm3+ Yb3+ SiO2 CaO Tm2O3 (b) Yb2O3 Materials 2021, 14, 937 0Tm0Yb 0 0 86.8 13.2 - 6 of 19 - 0.15Tm0Yb 0.15 0 86.4 13.6 - - 0.15Tm1YbTable 1. Samples names,0.15 nominal lanthanides 1 concentration86.5 (mol%),9.6 and chemical- composition 3.9 0.15Tm2Yb(wt.%) of SiO 2–CaO powders0.15 calculated on the 2 basis of a quantitative76.5 16.3 EDS analysis. - 7.2 0.15Tm3Yb Lanthanides0.15 Concentration 3 73.4 15.8 - 10.8 Chemical Composition (wt.%) (a) 0.15Tm4YbSample Name 0.15 (mol%) 4 70,7 16.9 - 12.4 3+ 3+ (b) 0.3Tm4Yb Tm0.3 Yb 4 SiO269.4 CaO18.0 Tm2O3 - Yb2O3 12.6 0.5Tm4Yb0Tm0Yb 00.5 0 4 86.869.5 13.216.7 -1.2 - 12.6 0.15Tm0Yb 0.15 0 86.4 13.6 - - (a) The relative errors of EDS method are less than 2% and 4% for main (above 20%) and major (5– 0.15Tm1Yb 0.15 1 86.5 9.6 - 3.9 20%) elements,0.15Tm2Yb respectively. 0.15 (b) The concentratio 2n of 76.5the Tm element 16.3 was below - the detection 7.2 limit. 0.15Tm3Yb 0.15 3 73.4 15.8 - 10.8 0.15Tm4Yb 0.15 4 70,7 16.9 - 12.4 3.2. Spectroscopic0.3Tm4Yb Properties 0.3 4 69.4 18.0 - 12.6 0.5Tm4Yb 0.5 4 69.5 16.7 1.2 12.6 3.2.1. (a)AbsorptionThe relative errors Spectra of EDS method are less than 2% and 4% for main (above 20%) and major (5–20%) elements, (b) Therespectively. absorptionThe concentration spectrum of theof Tm 0.5Tm4Yb element was belowpowder the detection in the limit. NIR-Vis spectral range is shown3.2. in Spectroscopic Figure 3. In Properties the 300–1300 nm range, there were observed relatively intense and well separated3.2.1. Absorption bands Spectra of intraconfigurational 4fN → 4fN transitions from 3H6 and 2F5/2 3+ 3+ ground stateThe absorptionto the high-lying spectrum levels of 0.5Tm4Yb of Tm powder and inYb the ions, NIR-Vis respectively. spectral range The is shown absorption bandsin with Figure maxima3. In the at 300–1300 356, 463, nm 683, range, and there 1210 were nm observed have been relatively assigned intense to andthe transitions well N N 3 2 from separatedthe 3H6 ground bands of state intraconfigurational to the 3H5, 3H4f4, <→3F4f3, 3Ftransitions2>, 1G4, and from 1DH2 6excitedand F5/2 statesground of Tm3+, 3+ 3+ respectively.state to the The high-lying absorption levels bands of Tm relatedand Ybto theions, higher respectively. energy Thelevel absorption of trivalent bands thulium with maxima at 356, 463, 683, and 1210 nm have been assigned to the transitions from ion were 3not observed due to the3 strong3 3absorption3 1 of the 1SiO2–CaO host and scattering3+ the H6 ground state to the H5, H4, < F3, F2>, G4, and D2 excited states of Tm , of therespectively. light on the The sample. absorption bands related to the higher energy level of trivalent thulium ion were not observed due to the strong absorption of the SiO2–CaO host and scattering of the light on the sample.
2 3+ F7/2 (Yb )
) 3+ ) ) ) ) (Tm Absorbance (a.u.) Absorbance 3+ 3+ 3+ 3+ 2 F 3 (Tm (Tm (Tm (Tm + 4 2 5 4 3 D G F H H 1 1 3 3 3
300 400 500 600 700 800 900 1000 1100 1200 1300 Wavelength (nm)
Figure 3. Absorption spectra of the 0.5Tm4Yb sample. Figure 3. Absorption spectra of the 0.5Tm4Yb sample. 2 2 The absorption band located at 840–1135 nm, assigned to the spin-allowed F7/2 → F5/2 3+ 3+ Thetransition absorption of Yb (presentband located in the sample at 840–1135 at a higher nm, concentration assigned thanto the Tm spin-allowedions), exhibits 2F7/2 → the maximum value of absorbance. The observed shape of the Yb3+ absorption band 2F5/2 transition of Yb3+ (present in the sample at a higher concentration than Tm3+ ions), showed inhomogeneous broadening features due to the short-range order in the amorphous
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exhibits the maximum value of absorbance. The observed shape of the Yb3+ absorption band showed inhomogeneous broadening features due to the short-range order in the amorphous matrix. This band consisted of broad weakly resolved bands located between 840 and 1110 nm, which corresponded to the Stark components of the 2F7/2 → 2F5/2 transi- tions. The strongest absorption line is the 0-phonon line at around 976 nm. This is the transition from the lowest Stark level of the 2F7/2(1) ground state to the lowest Stark level of the 2F5/2(5) excited state. The others, much less intensive and less separated components, are more difficult to spectrally resolve. Materials 2021, 14, 937 7 of 19 3.2.2. Emission Spectra Thematrix. upconversion This band consistedemission of spectra broad weakly shown resolved in Figure bands 4 locatedwere registered between 840 under and near 2 2 infrared1110 excitation nm, which (980 corresponded nm) to study to thethe Stark energy components transfer of between the F7/2 Yb→ 3+F and5/2 transitions. Tm3+ at various The strongest absorption line is the 0-phonon line at around 976 nm. This is the transition active ions concentrations. In these spectra,2 four emission bands in the ultraviolet (cen- from the lowest Stark level of the F7/2(1) ground state to the lowest Stark level of the tered at 2366 nm), blue (475 nm), red (651 nm), and near infrared (805 nm) were observed F5/2(5) excited state. The others, much less intensive and less separated components, are and all themore emission difficult to bands spectrally corresponded resolve. to the Tm3+ intraconfigurational f-f transitions. The ultraviolet emission band is assigned to the 1D2 → 3H6 transition while the blue and 3.2.2. Emission Spectra the red emission bands are corresponding to the 1G4 → 3H6 and the 1G4 → 3F4 transitions, The upconversion emission spectra shown in Figure4 were registered under near respectively.infrared In excitation particular, (980 nm)the toNIR study band the energycan be transfer contributed between by Yb3+ twoand different Tm3+ at various transitions: 1G4 → 3Hactive5 and ions 3H concentrations.4 → 3H6, of Inwhich these spectra,the luminescence four emission bandsbands in overlap the ultraviolet at around (centered 800 nm region asat 366reported nm), blue in (475the previous nm), red (651 works nm), and[23,31,32]. near infrared From (805 both nm) sets were of observed spectra and shown in Figure 4,all one the emissioncan see bandsthat the corresponded most intense to the upconversion Tm3+ intraconfigurational emission f-fwastransitions. the NIR The emission. 1 3 ultraviolet emission band is assigned to the3+ D2 → H6 transition while3+ the blue and the At the same constant concentration of Tm (0.15%),1 3 increasing1 Yb concentration3 up to red emission bands are corresponding to the G4 → H6 and the G4 → F4 transitions, 4% led torespectively. an enhancement In particular, of all the the NIR observed band can beUC contributed emission by bands two different (Figure transitions: 4a). Most prob- 1 3 3 3 3+ 3+ ably, thisG effect4 → H arose5 and dueH4 → toH a6 ,more of which efficient the luminescence energy transfer bands overlap between at around Yb and 800 nmTm ions at a higherregion concentration as reported in theof previousthe donor. works At [ 23the,31 ,same,32]. From Yb both3+ concentration sets of spectra shownof 4%, in the UC emissionFigure bands4, one were can seeenhanced that the most when intense the upconversionTm3+ concentration emission was increased the NIR emission.from 0.15% to At the same constant concentration of Tm3+ (0.15%), increasing Yb3+ concentration up 3+ 0.3%, butto 4%they led diminished to an enhancement when of the all theTm observed concentration UC emission reached bands (Figure 0.5%4 a).(Figure Most 4b). A higher concentrationprobably, this effect of aroseacceptor due to(Tm a more3+) might efficient also energy favor transfer energy between migration Yb3+ and observed Tm3+ as more intenseions at luminescence, a higher concentration but above of the donor. some At level the same, of Tm Yb3+3+, concentrationconcentration of 4%, quenching the UC may 3+ occur. emission bands were enhanced when the Tm concentration increased from 0.15% to 0.3%, but they diminished when the Tm3+ concentration reached 0.5% (Figure4b). A higher concentration of acceptor (Tm3+) might also favor energy migration observed as more intense luminescence, but above some level of Tm3+, concentration quenching may occur.
0.15Tm1Yb 3 → 3 H4 H6 0.15Tm2Yb 1 → 3 G4 H5 0.15Tm3Yb (a) 0.15Tm4Yb λ exc: 980 nm
1 → 3 1 → 3 G4 F4 D2 H6
340 345 350 355 360 365 370 375 380 Emission Intensity (a.u.)
620 630 640 650 660 670 680 690 700 1 → 3 G4 H6
300 400 500 600 700 800 Wavelength (nm)
Figure 4. Cont.
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3 → 3 0.15Tm4Yb H4 H6 0.3Tm4Yb 1 → 3 G4 H5 0.5Tm4Yb (b)
λ exc: 980 nm
1D → 3H 2 6 1 → 3 G4 F4 Emission Intensity (a.u.)
340 350 360 370 380
1 → 3 620 630 640 650 660 670 680 G4 H6
300 400 500 600 700 800 Wavelength (nm)
Figure 4. Upconversion emission spectra of silica–calcia matrix doped with different (a) Yb3+ (1, 2, 3, 3+ Figure 4.and Upconversion 4 mol%) and (b emission) Tm (0.15, spectra 0.3, and of 0.5 silica–calcia mol%) concentrations. matrix doped with different (a) Yb3+ (1, 2, 3, and 4 mol%)3.2.3. Time-Resolved and (b) Tm3+ UC(0.15, Photoluminescence 0.3, and 0.5 mol%) concentrations. In this section, the time-resolved UC photoluminescence in the visible and NIR ranges 3.2.3. Time-Resolvedare analyzed separately. UC Photoluminescence The emission intensity in the UV, i.e., at 366 nm, had a very low intensity and therefore, these bands were not taken into consideration in the studies. In this section, the time-resolved UC photoluminescence1 in 3+the visible and NIR To study the luminescence decay times of the G4 manifolds of Tm in silica–calcia ranges areco-doped analyzed with differentseparately. Tm3+ Theand emission Yb3+ concentrations, intensity thein time-resolvedthe UV, i.e., UCat photolu-366 nm, had a 1 3 1 3 very lowminescence intensity spectra and therefore, of the branching these Gband4 → sH were6 transition not taken (at 475 into nm) consideration and G4 → F4 in the studies.transition (at 650 nm) were measured as shown in Figures5 and6, respectively. In the par- ticular case of the SiO –CaO powder co-doped with 0.3% Tm3+ and 4% Yb3+ (0.3Tm4Yb), To study the luminescence2 decay times of the 1G4 manifolds of Tm3+ in silica–calcia 1 → 3 1 → 3 the time-resolved luminescence3+ spectra3+ of both branching G4 H6 and G4 F4 tran- co-doped with different Tm and Yb concentrations, the time-resolved1 UC photolumi- sitions showed a single exponential decay with comparable values of G4 decay times of 1 3 1 3 nescencearound spectra 100 ofµs the (see branching Table2). This singleG4 → exponentialH6 transition decay (at (indicating 475 nm) that and only G4 one → siteF4 transition of (at 650 Tmnm)3+ wereions in measured the 0.3Tm4Yb as powder shown is involvedin Figures in these 5 and UC processes)6, respectively. is in nature In withthe theparticular 1 3+ 3+ case of thecharacteristic SiO2–CaO lifetime powder of the co-dopedG4 manifolds with of 0.3% Tm reportedTm3+ and in cases4% Yb of3+ Tm (0.3Tm4Yb),-single-doped the time- 1 silica fibers [23,33,34] and thus, it is suggested to be the spontaneous decay time of the G4 resolved luminescence spectra of both branching 1G4 → 3H6 and 1G4 → 3F4 transitions manifolds of Tm3+ in the SiO –CaO matrix. In comparison with the 1G decay times of 2 1 4 showedTm a single3+ in fluorides exponential being typically decay with in ms comparable range, these ions values in silica-based of G4 decay matrices times exhibit of around 100 μs (seea shorter Table luminescence 2). This single lifetime exponential [35] indicating decay a strong (indicating contribution that from only non-radiative one site of Tm3+ ions in pathways.the 0.3Tm4Yb This was powder expected is involved due to the chemicalin these compositionUC processes) of the is network in nature where with the 4− −1 [SiO4] groups give rise1 to4 the vibrational energies3+ in the order of 1100 cm (relatively3+ characteristic lifetime of the G manifolds of Tm reported −in1 cases of Tm -single-doped−1 high compared to other host materials, such as fluorides (350 cm ) or tellurites (700 cm )). 1 silica fibersAdditionally, [23,33,34] possible and thus, defective it is structuresuggested on ato microscopic be the spontaneous level of the decay sol–gel-derived time of the G4 3+ 2 1 4 3+ 3+ manifoldsSiO 2of–CaO Tm glassceramic in the SiO can–CaO also inducematrix. the In shortening comparison of the with decay the times G of decay Tm .times of Tm in fluoridesThe being powders typically other than in 0.3Tm4Ybms range, exhibited these non-singleions in silica-based exponential decays matrices for both exhibit a 1 3 1 3 3+ shorter branchingluminescenceG4 → lifetimeH6 and [35]G4 → indicatingF4 transitions a strong of Tm contributionwith shorter 1/e from decay non-radiative times. pathways.As discussedThis was in expected Section 3.1 ,due the structureto the ch ofemical the SiO 2composition–CaO changed of with the the network dopants where concentrations. These changes most probably give rise to the presence of multiple, crys- 4− −1 [SiO4] tallographicallygroups give rise inequivalent to the vibrational sites due to theenergies short-range in the order order in the of glassceramics 1100 cm (relatively [2]. high comparedConsequently, to other this host can induce materials, the complex such as features fluorides and (350 shortening cm−1) ofor thetellurites decay times (700 cm−1)). 1 Additionally,in the investigated possible defective samples. Moreover, structure the on shortening a microscopic of the 1/e level decay of times the ofsol–gel-derived the G4 SiO2–CaO glassceramic can also induce the shortening of the decay times of Tm3+. The powders other than 0.3Tm4Yb exhibited non-single exponential decays for both branching 1G4 → 3H6 and 1G4 → 3F4 transitions of Tm3+ with shorter 1/e decay times. As discussed in Section 3.1, the structure of the SiO2–CaO changed with the dopants concen- trations. These changes most probably give rise to the presence of multiple, crystallo- graphically inequivalent sites due to the short-range order in the glassceramics [2]. Con- sequently, this can induce the complex features and shortening of the decay times in the
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investigated samples. Moreover, the shortening of the 1/e decay times of the 1G4 manifolds can be contributed as well by several processes such as cross-relaxation between Tm3+ ions manifolds can be contributed as well by several processes such as cross-relaxation between and back energy transfer from Tm3+ to Yb3+ [23,36]. In other words, there are several decay Tm3+ ions and back energy transfer from Tm3+ to Yb3+ [23,36]. In other words, there are 1 processes possibly occurring at the depletion of the G4 populated1 ions but further inves- several decay processes possibly occurring at the depletion of the G4 populated ions but tigations would be needed to describe them in detail. further investigations would be needed to describe them in detail.
λ : 980 nm 1Yb0.15Tm 1 exc λ 2Yb0.15Tm em: 475 nm 3Yb0.15Tm (a) 4Yb0.15Tm
0.1 Normalized intensity 0.01
0 200 400 600 800 1000 time (μs)
λ 4Yb0.15Tm exc: 980 nm 1 4Yb0.3Tm λ : 475 nm em (b) 4Yb0.5Tm
0.1
0.01 Normalized intensity
0.001
0 200 400 600 800 1000 time (μs)
1 3 Figure 5. The time-resolved upconversion (UC) photoluminescence of the branching of G4– H6 Figure 5. The time-resolved1 upconversion3+ (UC) photoluminescence of the branching of 1G4–3+3H6 transition from G4 manifolds of Tm in silica–calcia matrix doped with: (a) different Yb con- transition from 1G4 manifolds of Tm3+ in silica–calcia matrix doped with: (a) different Yb3+ concen- centrations (1, 2, 3, and 4 mol%) and the same Tm3+ concentration (0.15 mol%); (b) different Tm3+ trations (1, 2, 3, and 4 mol%) and the same Tm3+ concentration (0.15 mol%); (b) different Tm3+ con- concentrations (0.15, 0.3, and 0.5 mol%) and the same Yb3+ concentration (4 mol%). centrations (0.15, 0.3, and 0.5 mol%) and the same Yb3+ concentration (4 mol%).
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λ : 980 nm 1Yb0.15Tm 1 exc λ 2Yb0.15Tm em: 651 nm 3Yb0.15Tm (a) 4Yb0.15Tm
0.1 Normalized intensity
0.01
0 200 400 600 800 1000 time (μs)
λ : 980 nm 4Yb0.15Tm 1 exc λ : 651 nm 4Yb0.3Tm em 4Yb0.5 Tm (b)
0.1 Normalized intensity
0.01
0 200 400 600 800 1000 time (μs)
1 3 1 Figure 6. The time-resolved UC photoluminescence of the branching of G4– F4 transition from G4 1 3 1 manifoldsFigure 6. The of Tm time-resolved3+ in silica–calcia UC photoluminescence matrix doped with: of (a the) different branching Yb3+ ofconcentrations G4– F4 transition (1, 2, from 3, and G4 manifolds of Tm3+ in silica–calcia matrix doped with: (a) different Yb3+ concentrations (1, 2, 3, and 4 mol%) and the same Tm3+ concentration (0.15 mol%); (b) different Tm3+ concentrations (0.15, 0.3, mol%) and the same Tm3+ concentration (0.15 mol%); (b) different Tm3+ concentrations (0.15, 0.3, and 0.5 mol%) and the same Yb3+ concentration (4 mol%). and 0.5 mol%) and the same Yb3+ concentration (4 mol%). To investigate more deeply the effect of Tm3+ and Yb3+ concentrations on the UC To investigate more deeply the effect of Tm3+ and Yb3+ concentrations on the UC pho- photoluminescence, the 1/e decay times of the 1G manifolds for all the samples were 1 4 toluminescence, the 1/e decay times of the G4 manifolds1 for all the samples were collected collected in Table2 and analyzed. The decay times of G4 manifolds derived from both in Table 2 and analyzed.1 The3 decay times1 of 31G4 manifolds derived from both branching branching transitions G4 → H6 and G4 → F4 of the investigated powders showed the 1 → 3 1 → 3 sametransitions behavior G4 concerning H6 and G the4 concentrationF4 of the investigated of the dopants, powders as showed well as the their same Yb behavior3+:Tm3+ concerning the concentration of the dopants, as well as their Yb3+:Tm3+ concentration ratios concentration ratios varying from 7 to 27 (Table2). In the SiO 2–CaO samples doped 3+ withvarying the from same 7 Tm to 3+27 concentration(Table 2). In the of SiO 0.15%,2–CaO the samples powders doped doped with with the the same smallest Tm Ybcon-3+ 3+ concentration,centration of 0.15%, i.e., 1%, the withpowders Yb3+ doped:Tm3+ withratio the equal smallest to 7, exhibited Yb concentration, a short 1/e i.e., lifetime 1%, with of
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1 the G4 manifolds in the range of 13–15 µs. The 0.5Tm4Yb sample having almost the same Yb3+:Tm3+ ions ratio (equal to 8) showed a similar 1/e lifetime. Considering the powders doped with 0.15% of Tm3+ and higher Yb3+ concentrations (2, 3, and 4%), the 1/e decay times showed comparable longer values in the range from 23 to 63 µs. On the other hand, 3+ concerning the SiO2–CaO powders doped with a constant Yb concentration of 4%, they have not shown a clear tendency of the variation of the 1/e decay times with Tm3+ content. 1 Among them, the 0.3Tm4Yb sample exhibited the longest G4 decay time of about 100 µs.
1 3 3+ Table 2. The 1/e luminescence decay times of the G4 and H4 manifolds of Tm of the silica–calcia powders doped with different Tm3+ and Yb3+ concentrations.
1/e Decay Time (µs) 3+ 3+ 3+ 3+ Yb :Tm 1 3 1 3 3 3 1 3 Tm Yb G4– H6 G4– F4 H4– H6, G4– H5 (mol%) (mol%) Concentration Ratio Transition Transition Transitions (@475 nm) (@651 nm) (@805 nm) 0.15 1 7 13 ± 7 15 ± 8 14 ± 2 0.15 2 13 63 ± 7 27 ± 14 36 ± 5 0.15 3 20 41 ± 14 23 ± 6 31 ± 2 0.15 4 27 63 ± 14 34 ± 4 31 ± 2 0.3 4 13 104 ± 28 109 ± 9 39 ± 5 0.5 4 8 19 ± 2 15 ± 4 13 ± 1
The above observations may rise to the following conclusions. The 0.3Tm4Yb powder 1 showed a single exponential G4 decay times indicating that the processes of spontaneous 1 3+ decays from the G4 manifolds to other lower energy electronic states of the Tm ions 1 dominated the depletion of such ions at the excited G4 state. Moreover, the possible impact of the multiple, crystallographically inequivalent sites, back energy transfer from Tm3+ to Yb3+, and non-radiative processes were negligible in the de-population of Tm3+ 1 3+ 3+ ions at G4 manifolds for this sample. For other Yb :Tm concentration ratios, especially at low Yb3+:Tm3+ ratios of 7 or 8, fast decay processes may take place evidenced by the non- 1 single-exponential behaviors and shortening of the G4 decay times (see Figures5 and6 ). The aforementioned processes absent in the case of 0.3Tm4Yb may derive the depletion 1 of the G4 populated ions in the other samples. Eventually, the SiO2–CaO powder co- doped with 0.3% Tm3+ and 4% Yb3+ with their dominant spontaneous emission is the most optimal composition for obtaining upconversion photoluminescence in the blue and red spectral ranges. Looking at the time-resolved UC photoluminescence at 805 nm emission band of the 3 investigated samples (Figure7), one may note that the decay curves of the H4 manifolds of Tm3+ in all the samples exhibited non-single-exponential decays profiles. The 1/e decay 3 3+ time τ1/e of the H4 manifolds of Tm are also tabulated in Table2. For powders with a Yb3+:Tm3+ molar ratio ranging from 13 to 27, the decay times had comparative values 3+ 3+ ranging from 31 µs to 39 µs. On the other hand, for the SiO2–CaO powders with a Yb :Tm molar ratio of 7 and 8, the same parameter exhibited shorter time of about 13 µs. Thus, 3 3+ the 1/e decay times of the H4 of Tm ions concerning dopants concentrations showed 1 similar behavior with the 1/e G4 decay times. The 0.3Tm4Yb sample also exhibited the 3 1 longest 1/e decay time of the H4 manifolds as in the case of the G4. However, unlike 1 the cases of the blue and red emission bands from the G4 manifold, the time-resolved photoluminescence NIR band of Tm3+ ions of this sample did not show single-exponential behavior. These observations are not surprising since, at the 805 nm emission band, there 3 3 1 3 exists an overlap of the two transitions: H4 → H6 and G4 → H5. Materials 2021, 14, x FOR PEER REVIEW 12 of 20
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λ : 980 nm 1Yb0.15Tm 1 exc 2Yb0.15Tm λ em: 805 nm 3Yb0.15Tm 4Yb0.15Tm (a) 0.1
0.01 Normalized intensity
0.001 0 200 400 600 800 1000 time (μs)
λ : 980 nm 1 exc 4Yb0.15Tm λ em: 805 nm 4Yb0.3Tm 4Yb0.5Tm (b) 0.1
0.01 Normalized intensity
0.001 0 200 400 600 800 1000 Time (μs)
Figure 7. The time-resolved UC photoluminescence at 805 nm emission band of Tm3+ in silica–calcia 3+ matrixFigure doped 7. The with:time-resolved (a) different UC photoluminescence Yb3+ concentrations at (1,805 2, nm 3, emission and 4 mol%) band andof Tm the in same silica– Tm3+ calcia matrix doped with: (a) different Yb3+ concentrations (1, 2, 3, and 4 mol%) and the same Tm3+ concentration (0.15 mol%); (b) different Tm3+ concentrations (0.15, 0.3, and 0.5 mol%) and the same concentration (0.15 mol%); (b) different Tm3+ concentrations (0.15, 0.3, and 0.5 mol%) and the same Yb3+ concentration (4 mol%). Yb3+ concentration (4 mol%). To define the decay time of the 3H manifolds of Tm3+ ions as well as reveal the 3 4 3+ To define the decay3 time3 of the1 H4 manifolds3 of Tm ions as well as reveal the con- contribution of both H4 → H6 and G4 → H5 transitions to the emission band centered tribution of both 3H4 → 3H6 and 1G4 → 3H5 transitions to the emission band centered at 805 at 805 nm, the time-resolved UC photoluminescence at 805 nm emission band of the 0.3Tm4Ybnm, the time-resolved sample was fittedUC photoluminescence into a double exponential at 805 nm function emission as band shown of the in 0.3Tm4Yb Figure8 sample was fitted into a double exponential1 function as shown in Figure 8 together with together with the decay curves from G4 manifolds, i.e., blue and red emission band. The 1 fittingthe decay result curves is listed from in TableG4 manifolds,3. i.e., blue and red emission band. The fitting result is listed in Table 3.
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λ : 980 nm λ : 651 nm 1 exc em λ em: 475 nm λ em: 805 nm Double exponential fitting
0.1
0.01 Normalized intensity
0.001 0 200 400 600 Time (μs)
Figure 8. The time-resolved UC photoluminescence at 805 nm emission band of Tm3+ of the 0.3Tm4Yb 3+ sampleFigure and8. The its time-resolved fitting based onUC a photoluminescence double exponential at function. 805 nm emission The time-resolved band of Tm UC of photolumi- the 0.3Tm4Yb sample and its fitting based on a double exponential1 function. The time-resolved UC nescence at the 651 nm and 475 nm emission bands from the G4 manifolds are also recalled for a photoluminescence at the 651 nm and 475 nm emission bands from the 1G4 manifolds are also re- comparison. called for a comparison. As one can see from Figure8, the time-resolved UC photoluminescence at 805 nm emissionAs one band can of see Tm from3+ of Figure the 0.3Tm4Yb 8, the time-r powderesolved is well-fitted UC photoluminescence to the double exponen-at 805 nm 3+ tialemission function. band Thus, of Tm a sum of the function 0.3Tm4Yb of two powder exponentials is well-fitted was employed to the double to describe exponential the luminescencefunction. Thus, decay a sum function function as suggestedof two expone by Righinintials was et al. employed [30]: to describe the lumi- nescence decay function as suggested by Righini et al. [30]: t t t t φ(t)ɸ=(t)A=A×exp −× exp − + +(1−A)exp −(1 − A) exp − (1) τ1 τ2 (1) τ τ
wherewhereφ ϕ(t)(t) is is the the decay decay function, function,τ1 τand1 andτ2 τare2 are the the two two decay decay time time components, components, and and A is theA is fittingthe fitting parameter parameter describing describing the amplitude the amplitude of the of contribution the contribution of the decayof the timedecay component time com- τponent1 (Table τ31). (Table 3).
TableTable 3. 3.Values Values of ofτ 1τ,1,τ τ22,, andand AA ofof thethe doubledouble exponentialexponential func functiontion fitting fitting of of the the time-resolved time-resolved UC UC 3+3+ 11 photoluminescencephotoluminescence atat 805805 nmnm emissionemission bandband ofof TmTm ofof the the 0.3Tm4Yb 0.3Tm4Yb powder. powder. The The values values of of GG4 4 decaydecay times times are are also also listed listed for for comparison. comparison.
DoubleDouble Exponential Exponential Function Function Fitting Fitting of 805 of nm 805 Decay nm CurveDecay 1 1 3 G4 Decay Time (µs) ( G4Curve+ H4 ) 1G4 Decay Time (µs) Long decay component Short(1G4 + decay 3H4) component Derived from Derived from 1G –3H 1G –3F Long decay component Short decay component R-square Derived4 6from Derived4 4from A τ1 (µs) 1-A τ2 (µs) Transition Transition (@4751G4 – nm) 3H6 (@6511G4 – nm) 3F4 0.55 ± 0.01 88 ± 2 0.45 ± 0.01 9 ± 1R-square 0.998 104 ± 28 109 ± 9 A τ1 (μs) 1-A τ2 (μs) transition transition (@475 nm) (@651 nm) 1 3 0.55The ± 0.01 double 88 exponential ± 2 0.45 fitting ± 0.01 results 9 ± unravel 1 the 0.998 contribution 104 of ± both28 the G 1094 → ± 9 H5 3 3 and H4 → H6 transitions. The short component of around 9 µs is assigned to the 3 decayThe time double of the exponentialH4 manifolds, fitting which results is slightlyunravel lowerthe contribution than the value of both of around the 1G4 20→ µ3Hs 5 3 reportedand 3H4 → in 3 theH6 transitions. literature [ 23The]. Thisshort reduction component of of the aroundH4 manifold 9 μs is assigned decay time to the can decay be duetime to of the the multiphonon 3H4 manifolds, processes which is or slightly cross-relaxation lower than in the the value SiO2 of–CaO around matrix. 20 μs The reported long 1 decayin thecomponent literature [23]. of around This reduction 88 µs which of the is 3 comparableH4 manifold to decay the Gtime4 decay can be time due (see to Tablethe mul-3 ) 1 3 indicatestiphonon the processes contribution or cross-relaxation of the G4 → inH the5 transition SiO2–CaO to matrix. the UC The photoluminescence long decay compo- at 1 3 805nent nm. of around Moreover, 88 μ thes which obtained is comparable fitting parameter to the 1 AG4 of decay 0.55 indicatestime (see thatTable both 3) indicatesG4 → H the5
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3 3 and H4 → H6 transitions contributed comparably to the UC photoluminescence at 805 nm. It must be also mentioned that the existence of the crystalline phases in selected samples may influence the kinetics of the photoluminescence processes because the decay times of thulium ions depend also on the matrix [37,38].
3.2.4. UC Excitation Power Dependence and Yb3+-Tm3+ UC Energy Transfer The upconversion emission intensity for the observed UV, blue, red, and NIR signals was examined as a function of the laser diode (980 nm) excitation power density (Figure9). In general, the number of photons which are required to populate the upper emitting state under unsaturated conditions can be obtained by the relation [39,40]:
N IL ∝ P (2)
−2 where IL is the photoluminescence intensity, P is the pump laser power density (W·cm ), and N is the number of the laser photons required to observe UC emission. At low incident power at which the unsaturated condition is satisfied, the UC photoluminescence intensity is proportional to the incident pump power following the relation (2). Therefore, the slope values N in Figure9 can provide information on the number of photons absorbed to provide upconversion photoluminescence. The values are determined from the linear dependencies range as marked in Figure9 (points at high incident powers are not taken into account due to the saturation effect [41]). The summary of N numbers for different UC processes observed as emission at 366 nm, 475 nm, 651 nm, and 805 nm for the investigated samples is listed in Table4. The slope value N of all transitions varies slightly upon the concentrations of the 3 3 active ions in the investigated samples. It was concluded that in the H4 → H6 transitions, absorption of one photon is required whereas, for other transitions, two or more photons 1 are involved in the upconversion process. The unexpected low value of N = 0.32 for D2 3 → H6 transition in the case of 0.15Tm1Yb sample was not taken into account because of the difficulties to correctly estimate the power dependence due to the low intensity of this luminescence. Materials 2021, 14, x FOR PEER REVIEW 15 of 20
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0.15Tm1Yb 0.15Tm4Yb 0.15Tm2Yb N: 1.65 0.3Tm4Yb N: 2.12 0.15Tm3Yb 0.5Tm4Yb 0.15Tm4Yb fit fit N: 1.73
N: 1.67 N: 1.62 N: 0.32 N: 1.65 1 →3 1 →3 D2 H6 D2 H6
0.15Tm1Yb 0.15Tm4Yb 0.15Tm2Yb 0.3Tm4Yb N: 1.57 N: 1.50 0.15Tm3Yb 0.5Tm4Yb 0.15Tm4Yb fit fit N: 1.83 N: 1.41 N: 1.95
N: 1.57 N: 2.07 1 →3 1 →3 G4 H6 G4 H6
ln(I) 0.15Tm1Yb 0.15Tm4Yb 0.15Tm2Yb N: 1.51 0.3Tm4Yb N: 1.43 0.15Tm3Yb 0.5Tm4Yb 0.15Tm4Yb fit fit N: 1.76
N: 1.82 N: 1.43
N: 1.56 N: 1.51 1 →3 1 →3 G4 F4 G4 F4
0.15Tm1Yb 0.15Tm4Yb 0.15Tm2Yb N: 1.04 0.3Tm4Yb N: 0.70 0.15Tm3Yb 0.5Tm4Yb 0.15Tm4Yb fit fit N: 1.08 N: 0.75 N: 1.11 N: 1.04 N: 1.26
3 →3 1 →3 3 →3 1 →3 H4 H6 , G4 H5 H4 H6 , G4 H5 3.4 3.6 3.8 4.0 4.2 4.4 3.4 3.6 3.8 4.0 4.2 4.4 ln(ρP) / Wcm-2
FigureFigure 9. A log-log 9. A log-log plot plotof the of thedependence dependence of ofluminescence luminescence inte intensitiesnsities (bands(bands at at 366 366 nm, nm, 475 475 nm, nm, 651 651 nm, nm, and and 805 nm)805 onnm) on 3+ 3+ the laserthe laserpower power density density of excitation of excitation source source for for sa samplesmples co-doped co-doped withwith different different concentration concentration of Tm of Tmand3+ and Yb Ybions.3+ ions.
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Table 4. The summary of N numbers of different UC photoluminescence at 366 nm, 475 nm, 651 nm, and 805 nm for different concentrations of Tm3+ and Yb3+ in silica–calcia powders.
Tm3+ N Number Yb3+ (mol%) (mol%) 366 nm 475 nm 651 nm 805 nm 0.15 1 (0.32) 2.07 1.56 1.26 0.15 2 1.62 1.95 1.82 1.08 0.15 3 1.67 1.83 1.76 1.04 0.15 4 1.65 1.57 1.51 1.11 0.3 4 2.12 1.50 1.43 0.70 0.5 4 1.73 1.41 1.43 0.75
To understand the mechanism of Yb3+-Tm3+ UC energy transfer, the energy level diagrams of the Yb3+ and Tm3+ ions were prepared as illustrated in Figure 10. Taking into account the NIR upconversion at 805 nm, as discussed above, two main transitions are 1 3 3 3 contributing to this emission band: the G4 → H5 and H4 → H6 transitions. Concerning 3 3+ the latter one, under 980 nm excitation, the population of H4 manifolds of Tm can be 3+ 2 derived by one photon energy transfer from Yb ion at the excited state F5/2 (1 photon ETU; mentioned here possible energy transfer processes are marked in Figure 10) and the 2 3+ 3 missing energy gap between F5/2 (Yb ) and H4 states in this process can be fulfilled by phonons, i.e., phonon assistance upconversion (PAET). However, there is another 3+ 3 possibility that several Tm ions at a ground state can be excited into the H4 by the cross- relaxation processes (CR) between the excited Tm3+ ions and the Tm3+ ions at the ground state, i.e., ground state absorption (GSA). This can be an explanation for the complex decay times behavior of the investigated powders in the observed UC photoluminescence. With this UC mechanism, the number of 980 nm excitation photons needed is 1. On the other 1 3 3+ hand, referring to the G4 → H5 transition, the UC mechanism to populate the Tm into 1 3+ 2 the G4 is assigned to a two-photon energy transfer from Yb ion at the excited state F5/2 to the Tm3+ ions and the necessary number of 980 nm excitation photon is 2 (2 photon ETU). If there is only the combination of these two UC energy transfers according to the 1 3 excitation to G4 and H4 manifolds, the slope number of this transition should be in the range from 1 to 2 as in the cases of the samples with the lowest Tm3+ concentration (0.15%). However, since there can be the occurrence of the GSA and CR processes and contribution of phonon energy, the slope number of the powder can be lower than 1 as also observed in the work of Simpson et al. [23]. Furthermore, this effect probably occurs in the cases of the powders with Tm3+ concentration higher than 0.15%, which can be due to the higher 3 contribution of the CR and GSA processes to the excitation of the ions to the H4 excited state occurring when there are higher Tm3+ concentration. Materials 2021, 14, x FOR PEER REVIEW 17 of 20
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30 3 photon ETU 1 D2 25 PAET
2 photon ETU CR 366 ) 1G –1 20 4 (cm 475 3 651 805 3 15 F2,3
3 1 photon ETU PAET H4
Energy/10 10 2 CR F5/2 3 H5 GSA 3 5 F4 980 805 2 3 0 F7/2 H6 Yb3+ Tm3+
Figure 10. Simplified energy-level diagram of the Yb3+ and Tm3+ ions and the proposed UC mecha- nism from different energy states of Tm3+ ions. ETU—energy transfer upconversion; GSA—ground state absorption; PAET—phonon assistance upconversion; CR—cross-relaxation.
Figure 10.In Simplified the case ofenergy-level the UC photoluminescence diagram of the Yb3+ atand both Tm3+ the ions blue and and the redproposed emission UC mecha- bands 1 3 1 3+ 3 nismaccording from different to the energyG4 → statesH6 andof TmG4 ions.→ FETU—energy4 transitions, transfer the observed upconversion; slope values GSA— are rang- grounding state from absorption; 1.41 to 2.07 PAET—phonon among the investigated assistance powders,upconversion; which CR—cross-relaxation. is sufficiently in agreement 3+ 2 with the proposed two-photon energy transfer from Yb ion at the excited state F5/2 to theIn Tmthe3+ caseions of (2 the photon UC ETU)photoluminescence as shown in Figure at both 10. As the mentioned, blue and thered cases emission of the bands slope accordingvalues to below the 1 2G can4 → be3H due6 and to 1G the4 → contribution 3F4 transitions, of the the CR observed processes slope and thevalues contribution are ranging of frommultiphonon 1.41 to 2.07 among processes. the investigated powders, which is sufficiently in agreement with the proposedLast but two-photon not least, concerningenergy transfer the UC from photoluminescence Yb3+ ion at the excited at 366 state nm, in 2F principle,5/2 to the Tm the3+ 3+ 2 3+ ionsthree-photon (2 photon ETU) energy as shown transfer in from Figure Yb 10.ion As at mentioned, the excited the state casesF5/2 ofto the the slope Tm valuesions below(3 photon2 can be ETU) dueis to proposed the contribution to be the mechanism of the CR drivingprocesses this and UV the emission contribution band. However, of mul- tiphononthe slope processes. values of the powders are generally close to 2 or even below 2. As discussed, the effectLast ofbut cross-relaxation not least, concerning and multiphonon the UC phot processesoluminescence can account at 366 for nm, this in as principle, in the other the emission bands. three-photon energy transfer from Yb3+ ion at the excited state 2F5/2 to the Tm3+ ions (3 photon4. Conclusions ETU) is proposed to be the mechanism driving this UV emission band. However, the slope values of the powders are generally close to 2 or even below 2. As discussed, the In the present work, we have successfully synthesized new Tm3+/Yb3+-co-doped effect of cross-relaxation and multiphonon processes can account for this as in the other SiO2–CaO powders by the sol–gel method. XRD analysis and transmission electron micro- emissionscope bands. images showed the influence of thulium and ytterbium ions on the structure and morphology of derived nanoparticles. For a smaller amount of dopants, the amorphous 4. Conclusionsstructure of the material was registered. With an increased amount of the dopants, a smaller sizeIn the of thepresent particles work, and we partial have successfully crystallization synthesized of calcium new silicate Tm phases3+/Yb3+-co-doped were observed. SiO2– CaOFurthermore, powders by opticalthe sol–gel measurements method. XRD were analys performed.is and Upconversiontransmission electron emission microscope was regis- imagestered showed upon 980 the nm influence excitation of atthulium room temperature and ytterbium and ions four mainon the emission structure bands and weremor- 1 3 phologyobserved of derived from 300 nanoparticles. nm to 850 nm For due a smal to theler transitions amount of from dopants,D2 excitation the amorphous level to struc-H6 1 3 3 3 3 (ultraviolet), from G4 level to H6 (blue), F4 (red) and H5 (NIR), and from H4 level to ture3 of the material was registered. With an increased amount of the dopants,3+ a smaller size ofH6 the(NIR). particles The intensities and partial of photoluminescence crystallization of andcalcium decay silicate times ofphases Tm werewere dependent observed. on the concentration of both Yb3+ (changing from 1 to 4 mol%) and Tm3+ (changing from Furthermore, optical measurements were performed. Upconversion emission was regis- 0.15 to 0.5 mol%) ions. The highest emission intensity and single exponential decay time tered upon 980 nm excitation at room temperature and four main emission bands were were registered for the concentration of 0.3% Tm3+ and 4% Yb3+, which would indicate this 1 3 observedsample from as the 300 most nm promising to 850 nm one due in further to the studies. transitions The opticalfrom measurementD2 excitation datalevel includ- to H6 (ultraviolet), from 1G4 level to 3H6 (blue), 3F4 (red) and 3H5 (NIR), and from 3H4 level to 3H6
Materials 2021, 14, 937 18 of 19
ing decay times were used to propose the most likely energy transfer diagram between ytterbium and thulium ions. The experimental results indicate that these new up-converting nanoparticles are promising materials for biological testing and applications; although, further investi- gation is mandatory to optimize the properties of the system. Compared to the com- monly used nanoparticles like fluorides, bioactive glasses have many advantages, such as non-toxicity, biocompatibility, the possibility of surface functionalization, and more environment-friendly synthesis methods [25,26,42]. The luminescent ions in the amor- phous glass are weaker coordinated, so usually, the emission from these ions is less intense and has a lower efficiency compared to fluorides and thus, the way these two materials might be used is different. In fluorides, high emission efficiency is helpful for biomarkers applications. In the case of glasses, that are designed mainly as bioactive materials in regenerative medicine or drug delivery, the UC luminescence can be helpful in biosensing and controlling the compositional and structural changes of the glass.
Author Contributions: Conceptualization, K.H.-G., A.L.; formal analysis, K.H.-G., D.S., T.N.L.T.; investigation, K.H.-G.; data curation, K.H.-G., D.S.; writing—original draft preparation, K.H.-G., A.L.; writing—review and editing, K.H.-G., D.S., T.N.L.T., M.F., A.L.; visualization, K.H.-G., D.S.; supervision, A.L.; funding acquisition, A.L. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Science Centre, Poland under grant SONATA BIS no. 2016/22/E/ST5/00530. Data Availability Statement: Data sharing is not applicable to this article. Conflicts of Interest: The authors declare no conflict of interest.
References 1. Lukowiak, A.; Zur, L.; Tomala, R.; LamTran, T.N.; Bouajaj, A.; Strek, W.; Righini, G.C.; Wickleder, M.; Ferrari, M. Rare earth elements and urban mines: Critical strategies for sustainable development. Ceram. Int. 2020, 46, 26247–26250. [CrossRef] 2. Lukowiak, A.; Chiasera, A.; Chiappini, A.; Righini, G.C.; Ferrari, M. Active sol-gel materials, fluorescence spectra, and lifetimes. In Handbook of Sol-Gel Science and Technology; Klein, L., Aparicio, M., Jitianu, A., Eds.; Springer International Publishing: Berlin/Heidelberg, Germany, 2018; pp. 1–43. ISBN 978-3-319-32099-1. 3. Quandt, A.; Ferrari, M.; Righini, G.C. Advancement of glass-ceramic materials for photonic applications. In Sol-Gel Based Nanoce- ramic Materials: Preparation, Properties and Applications; Mishra, A.K., Ed.; Springer International Publishing: Berlin/Heidelberg, Germany, 2016; pp. 133–155. ISBN 978-3-319-49512-5. 4. Ferrari, M.; Righini, G.C. Glass-ceramic materials for guided-wave optics. Int. J. Appl. Glass Sci. 2015, 6, 240–248. [CrossRef] 5. Ferrari, M.; Righini, G.C. Physics and Chemistry of Rare-Earth Ions Doped Glasses; Trans Tech Publications: Bäch, Switzerland, 2008; pp. 71–120. 6. Chen, Y.; Chen, G.; Liu, X.; Xu, J.; Zhou, X.; Yang, T.; Yuan, C.; Zhou, C. Upconversion luminescence, optical thermometric properties and energy transfer in Yb3+/Tm3+ co-doped phosphate glass. Opt. Mater. (Amst.) 2018, 81, 78–83. [CrossRef] 7. Xia, H.; Lei, L.; Xia, J.; Hua, Y.; Deng, D.; Xu, S. Yb/Er/Tm tri-doped Na3ZrF7 upconversion nanocrystals for high performance temperature sensing. J. Lumin. 2019, 209, 8–13. [CrossRef] 8. Deng, Y.; Niu, C. Up-conversion luminescence properties of Er3+/Yb3+ co-doped oxyfluoride glass ceramic. J. Lumin. 2019, 209, 39–44. [CrossRef] 9. Kasprowicz, D.; Brik, M.G.; Majchrowski, A.; Michalski, E.; Głuchowski, P. Up-conversion emission in triply-doped 3+ 3+ 3+ Ho /Yb /Tm KGd(WO4)2 single crystals. Opt. Commun. 2011, 284, 2895–2899. [CrossRef] 10. Wang, Z.; Wang, C.; Han, Q.; Wang, G.; Zhang, M.; Zhang, J.; Gao, W.; Zheng, H. Metal-enhanced upconversion luminescence of NaYF4:Yb/Er with Ag nanoparticles. Mater. Res. Bull. 2017, 88, 182–187. [CrossRef] 3+ 3+ 11. Pokhrel, M.; Valdes, C.; Mao, Y. Ultraviolet upconversion enhancement in triply doped NaYF4:Tm ,Yb particles: The role of Nd3+ or Gd3+ Co-doping. Opt. Mater. (Amst.) 2016, 58, 67–75. [CrossRef] 12. Hassairi, M.A.; Dammak, M.; Zambon, D.; Chadeyron, G.; Mahiou, R. Red–green–blue upconversion luminescence and energy 3+ 3+ 3+ transfer in Yb /Er /Tm doped YP5O14 ultraphosphates. J. Lumin. 2017, 181, 393–399. [CrossRef] 13. Xu, S.; Huang, S.; He, Q.; Wang, L. Upconversion nanophosphores for bioimaging. TrAC Trends Anal. Chem. 2015, 66, 72–79. [CrossRef] 14. Zmojda, J.; Kochanowicz, M.; Miluski, P.; Righini, G.C.; Ferrari, M.; Dorosz, D. Investigation of upconversion luminescence in Yb3+/Tm3+/Ho3+ triply doped antimony-germanate glass and double-clad optical fiber. Opt. Mater. (Amst.) 2016, 58, 279–284. [CrossRef] Materials 2021, 14, 937 19 of 19
15. Rafique, R.; Kailasa, S.K.; Park, T.J. Recent advances of upconversion nanoparticles in theranostics and bioimaging applications. TrAC Trends Anal. Chem. 2019, 120, 115646. [CrossRef] 16. Auzel, F. Upconversion and anti-Stokes processes with f and d ions in solids. Chem. Rev. 2004, 104, 139–173. [CrossRef] 17. Lukowiak, A.; Stefanski, M.; Ferrari, M.; Strek, W. Nanocrystalline lanthanide tetraphosphates: Energy transfer processes in samples co-doped with Pr3+/Yb3+ and Tm3+/Yb3+. Opt. Mater. (Amst.) 2017, 74, 159–165. [CrossRef] 18. Sudarshanam, V.; Abedin, K.S.; Nicholson, J.W.; Headley, C.E.; DiGiovanni, D.J. Kilo-Watt high-power Yb fiber laser at 1117 nm. In Proceedings of the Fiber Lasers XV: Technology and Systems, San Francisco, CA, USA, 29 January–1 February 2018; Carter, A.L., Hartl, I., Eds.; SPIE: Bellingham, WA, USA, 2018; Volume 10512, p. 12. 19. Chiappini, A.; Zur, L.; Enrichi, F.; Boulard, B.; Lukowiak, A.; Righini, G.C.; Ferrari, M. Glass ceramics for frequency conversion. In Solar Cells and Light Management: Materials, Strategies and Sustainability; Enrichi, F., Righini, G., Eds.; Elsevier: Amsterdam, The Netherlands, 2019; pp. 391–414. ISBN 9780081027622. 20. Boulard, B.; Dieudonné, B.; Gao, Y.; Chiasera, A.; Ferrari, M. Up-conversion visible emission in rare-earth doped fluoride glass waveguides. Opt. Eng. 2014, 53, 071814. [CrossRef] 21. Lahoz, F.; Martín, I.R.; Méndez-Ramos, J.; Núñez, P. Dopant distribution in a Tm3+-Yb3+ codoped silica based glass ceramic: An infrared-laser induced upconversion study. J. Chem. Phys. 2004, 120, 6180–6190. [CrossRef][PubMed] 22. Watekar, P.R.; Ju, S.; Boo, S.; Han, W.T. Linear and non-linear optical properties of Yb3+/Tm3+ co-doped alumino-silicate glass prepared by sol-gel method. J. Non-Cryst. Solids 2005, 351, 2446–2452. [CrossRef] 23. Simpson, D.A.; Gibbs, W.E.; Collins, S.F.; Blanc, W.; Dussardier, B.; Monnom, G.; Peterka, P.; Baxter, G.W. Visible and near infra-red up-conversion in Tm3+/Yb3+ co-doped silica fibers under 980 nm excitation. Opt. Express 2008, 16, 13781. [CrossRef] 24. Maalej, O.; Lukowiak, A.; Bouajaj, A.; Chiasera, A.; Righini, G.C.; Ferrari, M.; Boulard, B. Blue to NIR down-conversion in Tm3+/Yb3+-codoped fluorozirconate glasses compared to Pr3+/Yb3+ ion-pair. J. Lumin. 2018, 193, 22–28. [CrossRef] 25. Li, Q.; Xing, M.; Chen, Z.; Wang, X.; Zhao, C.; Qiu, J.; Yu, J.; Chang, J. Er3+/Yb3+ co-doped bioactive glasses with up-conversion luminescence prepared by containerless processing. Ceram. Int. 2016, 42, 13168–13175. [CrossRef] 26. Kalaivani, S.; Srividiya, S.; Vijayalakshmi, U.; Kannan, S. Bioactivity and up-conversion luminescence characteristics of Yb3+/Tb3+ co-doped bioglass system. Ceram. Int. 2019, 45, 18640–18647. [CrossRef] 27. Zambanini, T.; Borges, R.; Faria, P.C.; Delpino, G.P.; Pereira, I.S.; Marques, M.M.; Marchi, J. Dissolution, bioactivity behavior, and cytotoxicity of rare earth-containing bioactive glasses (RE = Gd, Yb). Int. J. Appl. Ceram. Technol. 2019, 16, 2028–2039. [CrossRef] 28. Borges, R.; Schneider, J.F.; Marchi, J. Structural characterization of bioactive glasses containing rare earth elements (Gd and/or Yb). J. Mater. Sci. 2019, 54, 11390–11399. [CrossRef] 29. Lukowiak, A.; Lao, J.; Lacroix, J.; Marie Nedelec, J. Bioactive glass nanoparticles obtained through sol–gel chemistry. Chem. Commun. 2013, 49, 6620–6622. [CrossRef][PubMed] 30. Righini, G.C.; Ferrari, M. Photoluminescence of rare-earth-doped glasses. La Rivista del Nuovo Cimento 2005, 28, 1–53. [CrossRef] 3+ 31. Tsuboi, T. Luminescence of Tm Ion in LiNbO3 Crystal. J. Electrochem. Soc. 2000, 147, 1997. [CrossRef] 32. Serra, O.A.; Nassar, E.J.; Calefi, P.S.; Rosa, I.L.V. Luminescence of a new Tm3+ β-diketonate compound. J. Alloys Compd. 1998, 275–277, 838–840. [CrossRef] 33. Gomes, A.S.L.; Carvalho, M.T.; Sundheimer, M.L.; Bastos-Filho, C.J.A.; Martins-Filho, J.F.; Von der Weid, J.P.; Margulis, W. Low-pump-power, short-fiber copropagating dual-pumped (800 and 1050 nm) thulium-doped fiber amplifier. Opt. Lett. 2003, 28, 334. [CrossRef] 34. Davydov, A.S. The Radiationless transfer of energy of electronic excitation between impurity molecules in crystals. Phys. Status Solidi 1968, 30, 357–366. [CrossRef] 35. Yu, D.; Yu, T.; Bunningen, A.J.; Zhang, Q.; Meijerink, A.; Rabouw, F.T. Understanding and tuning blue-to-near-infrared photon cutting by the Tm3+/Yb3+ couple. Light Sci. Appl. 2020, 9, 107. [CrossRef] 36. Marconi da Silva MD Linhares, H.; Felipe Henriques Librantz, A.; Gomes, L.; Coronato Courrol, L.; Lícia Baldochi, S.; Marcia 1 1 3+ Ranieri, I. Energy transfer rates and population inversion investigation of G4 and D2 excited states of Tm in Yb:Tm:Nd:KY3F10 crystals. J. Appl. Phys. 2011, 109, 083533. [CrossRef] 37. Dwaraka Viswanath, C.S.; Babu, P.; Martín, I.R.; Venkatramu, V.; Lavín, V.; Jayasankar, C.K. Near-infrared and upconversion luminescence of Tm3+ and Tm3+/Yb3+-doped oxyfluorosilicate glasses. J. Non-Cryst. Solids 2019, 507, 1–10. [CrossRef] 38. Grzyb, T.; Balabhadra, S.; Przybylska, D.; W˛ecławiak,M. Upconversion luminescence in BaYF5, BaGdF5 and BaLuF5 nanocrystals doped with Yb3+/Ho3+, Yb3+/Er3+ or Yb3+/Tm3+ ions. J. Alloys Compd. 2015, 649, 606–616. [CrossRef] 3+ 3+ 39. Chen, G.; Ohulchanskyy, T.Y.; Kumar, R.; Ågren, H.; Prasad, P.N. Ultrasmall monodisperse NaYF4:Yb /Tm nanocrystals with enhanced near-infrared to near-infrared upconversion photoluminescence. ACS Nano 2010, 4, 3163–3168. [CrossRef] 40. Qiu, H.; Yang, C.; Shao, W.; Damasco, J.; Wang, X.; Ågren, H.; Prasad, P.; Chen, G. Enhanced Upconversion Luminescence in Yb3+/Tm3+-Codoped Fluoride Active Core/Active Shell/Inert Shell Nanoparticles through Directed Energy Migration. Nanomaterials 2014, 4, 55–68. [CrossRef][PubMed] 41. Pollnau, M.; Gamelin, D.R.; Lüthi, S.R.; Güdel, H.U. Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems. Phys. Rev. B 2000, 61, 3337–3346. [CrossRef] 42. Chen, Q.-Z.; Rezwan, K.; Françon, V.; Armitage, D.; Nazhat, S.N.; Jones, F.H.; Boccaccini, A.R. Surface functionalization of Bioglass®-derived porous scaffolds. Acta Biomater. 2007, 3, 551–562. [CrossRef][PubMed]