Stabilization of Fluorescent [Agm] n+ Quantum Clusters in Multi-phase

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Stabilization of Fluorescent [Ag ] Quantum Clusters in Multi-phase Inorganic Glass-ceramics for White LEDs Xiuxia Xu, Junjie Zhao, Xue Luo, Ronghua Ma, Jiangyun Qian, Xvsheng Qiao, Jincheng Du, Guodong Qian, Xiang-Hua Zhang, and Xianping Fan ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00312 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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ACS Applied Nano Materials

Stabilization of Fluorescent [Agm]n+ Quantum Clusters in Multi-phase Inorganic Glass-ceramics for White LEDs

Xiuxia Xu, a Junjie Zhao,a Xue Luo, a Ronghua Ma,a Jiangyun Qian,a Xvsheng Qiao,a,* Jincheng Du, b Guodong Qian, a Xianghua Zhang c and Xianping Fan a,*

a

State Key Laboratory of Silicon Materials & School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

b

Department of Materials Science and Engineering, University of North Texas, Denton, Texas 76203-5017, U.S.

c Laboratory

of Glasses and Ceramics, Institute of Chemical Science UMR CNRS 6226, University of Rennes 1, 35042 Rennes, France

ABSTRACT Silver quantum clusters ([Agm]n+ QCs) is a type of efficient broadband fluorescence centers with m and n related quantum size effects, but usually lack of chemical and thermal stability. In order to solve such problem and exploit [Agm]n+ QCs potential application in white LED lighting, [Agm]n+ QCs and rare earth ions (RE3+) were designed to selectively enriched into B2O3-rich spinodal nano-phase separation and SrF2 nanocrystals in a fluoroborosilicate multi-phase glass-ceramics. In this work, Ag/RE3+ co-doped glasses and glass-ceramics with designed composition were prepared through melt-quenching method and subsequent heat treatment. The B2O3-

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rich spinodal nano-phase separation and SrF2 nanocrystals were stepwise formed in these glass-ceramics. Taking Ag/Er3+ co-doped glass-ceramics for the example, the special microstructures were clearly revealed by transmission electron microscope (TEM), high resolution transmission electron microscope (HRTEM), scanning transmission electron microscope (STEM) and energy dispersive X-ray spectroscopy (EDX) mappings. By the strategies of controlling [Ag+] solubility and charge compensating to [AlO4]−, [BO4]− and [ZnO4]2− tetrahedra, a large quantity of [Agm]n+ QCs were stabilized in the glass networks with regulated m and n. By the strategies of RE3+/Al3+ and Ag+/Na+ competitive distribution, the solubility of Ag+ in B2O3-rich glassy phases was effectively increased and [Agm]n+ QCs were eventually stabilized to standby the post-crystallization procedures at high temperatures up to 650 °C. By the multi-phase strategy, Ag and RE3+ were selectively partitioned into the B2O3-rich nanosize glassy phases and SrF2 nanocrystals, respectively. So energy transfers (ETs) between [Agm]n+ and RE3+ can be well suppressed to enhance the photoluminescence quantum yields (PL QYs) of the multi-phase glass-ceramics. Those engaged [Agm]n+ with largely improved White Light Emitting Diode (WLED) performances, e.g. QY and color rendering index (CRI), in the glass-ceramics. It suggests that Ag/RE3+ codoped glass-ceramics can be ideal candidate phosphors for high power WLED lighting devices.

Keywords: [Agm]n+ quantum clusters; fluorescence; chemical and thermal stability; selective enrichment; spinodal-phase; nanocrystal; multi-phase glass-ceramics


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1. INTRODUCTION Silver quantum clusters

1-3

([Agm]n+ QCs) are rather a new type of molecule like

fluorescence centers, consisting of only several to tens of atoms and/or cations, with extraordinary optical, spectroscopic and catalytic properties. The continuous density of the [Agm]n+ energy states split into discrete energy levels. It produces strong quantum effects and “molecule-like” fluorescence4-5 of [Agm]n+ QCs and demonstrates their great potential in biomarkers6 and detection of metal ions7, uric acids8 and protein9. But [Agm]n+ QCs, formed by Ag0 atoms and Ag+ cations, are always chemically active and are thermally unstable, so how to stabilize [Agm]n+ QCs is still kept as a challenge10. To solve such problem, organic ligands (e.g. polymers and proteins) and inorganic scaffolds (e.g. zeolite) were employed to stabilized [Agm]n+ QCs11. Alternatively, inorganic glass matrices were recently revealed to have capacities to chemically stabilize [Agm]n+ QCs12-14. For example, [Agm]n+ QCs can be chemically stabilized by oxyfluoride glass matrix to achieve high PL quantum yields (QYs) up to 96.75 %15. The strategies13,

14

via solubility control and charge compensation have also been

reported to chemically stabilize [Agm]n+ QCs so as to prevent them from being reduced into luminescently inactive Ag0 nanoparticles (NPs). Those further extend the application of [Agm]n+ QCs to solid state lighting, data storage and display16-17. With super-broad visible emission bands and producing near white light with high CRI, [Agm]n+-doped inorganic glass or glass-ceramics 17-19 can further act as a superior



candidate for commercial yellow Y3Al5O12: Ce3+ phosphor so as to overcome the low CRI and severe thermal deterioration problems of the latter. However, [Agm]n+ QCs can also be easily reduced and polymerized into luminescently inactive Ag0 NPs at a high LED work temperature. It thus seriously diminishes the quantity of luminescently active [Agm]n+ QCs. To solve such problem18,

20-24,

we propose the strategies with

introducing RE3+/Al3+ and Ag+/Na+ competitive distribution to effectively hold a high [Agm]n+ concentration at temperatures as high as the first crystallization peak temperature (about 650

°C).

Under such strategy, by introducing RE3+ in

fluoroborosilicate glass system and inducing separation of F-rich, B2O3-rich and SiO2rich phases, RE3+ and Al3+ will compete for the coordinating F- in F-rich phase, as well as Ag+ and Na+ compete for the charge compensating of the [BO4] − tetrahedron in B2O3-rich phase. As a practice in this study, Al3+ can be extruded from F-rich phase into SiO2-rich phase to form [AlO4]– tetrahedron, [AlO4]− tetrahedron further extracts Na+ from B2O3-rich phase into SiO2-rich phase for charge compensation, and eventually the solubility of Ag+ in B2O3-rich phase could be increased adequately to prevent the over-polymerization of [Agm]n+, i.e. to prevent the formation of luminescently inactive Ag0 NPs. However, [Agm]n+/RE3+-codoping in glass usually brings about a serious PL quenching due to energy transfers (ETs) between RE3+ and [Agm]n+. For the aim to suppress the ET processes, selective enrichment strategy in multi-phase glass-ceramics would provide an ideal solution. In some previously reported multi-phase glassceramics, RE and TM (transition metal) ions could be enriched into different crystalline


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phases, respectively. The ET routes were accordingly baffled to extremely lower the ET probability. For example, in the multi-phase glass-ceramics25-27 containing Ga2O3: Ni2+/LaF3: Er3+ or Ga2O3: Mn2+/YF3: Tm3+ or ZnAl2O4: Co2+/YF3:Er3+, RE3+ and TM2+ were enriched in the oxide and fluoride crystals, respectively. Similarly, our previous study

13-14

had revealed Ag could be enriched into B2O3-rich glass phase separation

domain in the fluorosilicate glasses. Thereby, the selective enrichment strategy can be applied to form the [Agm]n+/RE3+ co-doped multi-phase glass-ceramics. Hence, this work is focused on developing a new type of multi-phase glass-ceramics, in which stable [Agm]n+ QCs are partitioned into B2O3-rich spinodal phase separation and RE3+ ions are enriched into the precipitated SrF2 nano-crystals. Such special microstructure firstly introduces RE3+/Al3+ and Ag+/Na+ competitive distribution in different sub-phases. It improves the solubility of Ag+ in B2O3-rich phase and prevent [Agm]n+ from being reduced and aggregated into inactive Ag NPs. Simultaneously, it effectively suppresses non-radiative ETs between [Agm]n+ and RE3+ so as to improve QY and CRI performances of Ag/RE3+ co-doped glass-ceramics. The newly developed multi-phase glass-ceramics exhibit a bright prospect for application in future high power white light emitting diodes (WLEDs).

2. EXPERIMENT AND SIMULATION DETAILS Reagent grade anhydrous powders of SiO2, Al2O3, H3BO3, Na2CO3, ZnF2, SrF2, AgNO3 and REF3 (RE = Er; Sm; Tb; Nd; Ho) were used to prepare 3 mol % Ag doped, 1 mol % REF3 doped and 1 mol % ErF3/3 mol % Ag co-doped 47.0 SiO2 − 15.0 Al2O3 − 6.4



B2O3 − 3.2 Na2O − 11.3 ZnF2 − 13.1 SrF2 (in mol %) glass samples (named as GAg, GEr and GAgRE, respectively). Minor B2O3-Na2O component was found to be very important for the precipitation and the homogenous distribution of silver NPs. Batches of 40 g were fully mixed and melted at 1450 °C for 45 minutes in partially covered alumina crucibles in air. Then the melts were cast onto a copper plate and pressed to form a glass disk with another copper plate. Glass ceramics samples, GCAg, GCRE and GCAgRE, were obtained by annealing GAg, GEr and GAgRE at 650 °C for 4 hours. Finally, all samples were polished to 1 mm thick for spectroscopic measurements. Powder X-ray diffraction (XRD) was performed on a Shimadzu XRD-6000 X-ray diffractometer from 10 ° to 80 ° with a 4 °/minute scanning speed. Transmission electron microscope (TEM), selected area electron diffraction (SAED) and scanning transmission electron microscope (STEM) images were obtained on a 200 kV FEI Tecnai G2 F20 S-TWIN transmission electron microscope. The ultrathin samples for electron microscope observation were prepared using focused ion beam (FIB) combined with a scanning electron microscope (SEM) on a FEI Quanta 3D FEG FIB/SEM system. The optical absorption spectra were measured on a Hitachi U-4100 spectrophotometer. The photo-luminescence (PL) and decay curves were recorded on an Edinburgh Instruments (EI) FLSP920 spectrophotometer with excitation from a Xe lamp, a picosecond laser diode (PLD), or a microsecond Xe flash lamp. The PL quantum yields (QYs) were also measured with an integrated sphere on the Edinburgh Instruments (EI) FLSP920 spectrophotometer.


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Taking La3+ to represent RE3+, the effect of introducing La3+ on the distribution of Al3+ and Na+ in different sub-phases of the glasses were comparatively studied with a molecular dynamic (MD) simulation on the glasses with the compositions (in mol %) of 45 SiO2 – 15 Al2O3 – 12 Na2O – 28 SrF2 and the 45 SiO2 – 15Al2O3 – 12 Na2O – 20 SrF2 – 8 LaF3. The MD simulation details are the same with those in Ref. 28.

3. RESULTS AND DISCUSSION 3.1 Multi-phase glass-ceramics containing SrF2: Er3+ and B2O3: Ag sub-phases Fluoride nano-crystal containing oxyfluoride glass-ceramics combine the merits of glass and crystals as well as fluorides and oxides. Such type of glass-ceramics possesses good mechanical and chemical stabilities, exhibit highly efficient luminescence and is easy to be heavily doped with lanthanides. In our previous studies, the fluorosilicate glass-ceramics 29-32 containing MF2 or M2RF7 (M = Ca; Sr; Ba; R= Y; La; Gd), display potential applications in lasers, optical communication, solar spectrum modification and 3D displays. As a starting point, SiO2 – Al2O3 – ZnF2 – SrF2 – ErF3 glass (GEr) was chosen as the basic system to develop a type of glass-ceramics simultaneously containing lanthanide enriched fluoride nano-crystalline phases and silver enriched nano-glassy phases. By annealing the precursor glass at the temperature between the glass transition point and the first crystallization peak, the glass-ceramics, GCEr, was obtained with SrF2 nano-crystals homogenously precipitated. A typical glass-ceramic microstructure was then revealed for GCEr by TEM image (Figure 1a), where two phases with different contrasts corresponded to SrF2 nano-crystalline phase (dark nano-



spheres sized at 20 ~ 30 nm) and remnant glassy phase (grey contrast), respectively. The insets of Figure 1(a) is selected area electron diffraction (SAED) patterns, where the polycrystalline diffraction ring can be assigned to the (111) lattice planes of cubic SrF2. The inset SAED patterns in Figure 1b reveal discrete bright spots corresponding to different lattice planes of SrF2. HRTEM image further exhibits the (111) lattice fringes of an isolated SrF2 nano-crystalline sphere with about 25 nm size. EDXS analysis (Figure 1d) evidences that the concentration of Er3+ of SrF2 nano-crystalline region was much higher than that of the residual glassy region. It indicates that Er3+ ions were selectively enriched in SrF2 nano-crystals. SrF2 lattice can provide Er3+ with relatively low maximum phonon vibration energy favored by de-excitation transitions with low non-radiative probabilities, thus can further promote the glass-ceramics to achieve high luminescence efficiency. Therefore, fluorosilicate glass-ceramics have been considered as ideal hosts for luminescent lanthanide ions to be selectively enriched into the precipitated fluoride crystalline phases. The glass system with heavily doped and homogenously dispersed silver could be designed by introducing the B2O3–Na2O–Ag components into the above SiO2 – Al2O3 – ZnF2 – SrF2 – ErF3 glass system. The B2O3–Na2O–Ag2O glass33-34 is considered as ideal host to heavily and uniformly dissolve silver ions, but suffers from its bad chemical stability and water resistance. By contrast, silicate glass is inert in chemical reactivity and water sensitivity, but is not suited as silver doped host because of its low silver solubility. Accordingly, to well refrain from the agglomeration of silver in the glass, the microstructure of SiO2 – Al2O3 – ZnF2 – SrF2 – ErF3 and B2O3 – Na2O – Ag


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mixing glass (GAgEr) was mapped out with B2O3 – Na2O – Ag sub-phases separated from the major glass network. With reference to the glass phase diagram35, droplet binodal or interpenetrating spinodal B2O3-rich phase can be separated from the Na2O – B2O3 – SiO2 glass at certain composition region and thermodynamic condition. Here is the type of spinodal interpenetrating spinodal B2O3-rich phase separated from the silica rich glass phase. TEM images (Figure 2) show double phases without clear interface in the glass: the dark gray region is assigned to the B2O3-rich phase, while the light gray region corresponds to the SiO2-rich phase. As a matter of fact, the darkness of B2O3rich phase is deriving from quantities of dark spots involved, which are eventually identified as silver nano-particles by the high resolved TEM (HRTEM, Figure 2c) and EDX spectra (Figure 2d). The distribution of Ag in dark contrast indeed draws the outline of the interpenetrating spinodal B2O3-rich phases. The (103) crystallographic plane is also clearly distinguished by the consistent interplanar spacing with the standard JCPDS card (#87-0598) of Ag-4H. Silver is also selectively enriched in the B2O3-rich phase, because not any X-ray bands of Ag are present in the EDX spectra of the SiO2-rich phase (domain 1in Figure 2b), but intense feature X-ray bands assigned to Ag: Lα1/ Lα2/ Lβ1/ Lβ2 emerged from 2.9 to 3.5 keV in the EDX spectra of the B2O3rich phase (domain 1in Figure 2c). Due to the small size of silver nano-particles, it is difficult to be detected by electron diffraction (only central bright diffraction spot in the insets of Figure 2a-b), but easy to calculate the interplanar spacing by fast Fourier transform (FFT in the inset of Figure 2c). Interpenetrating spinodal B2O3-rich phases homogenously disperse in the glass and Ag selectively disperse in the B2O3-rich phases,



so it ingeniously develops one way to heavily introducing Ag in the glass without any large size agglomeration. By remixing the above two types of glass, Er3+ selectively enriched SrF2 nanocrystalline sub-phases and Ag selectively enriched B2O3-rich glassy sub-phases were simultaneously precipitated and separated in the SiO2 – Al2O3 – ZnF2 – SrF2 – ErF3 – B2O3 – Na2O – Ag glass-ceramics (GCAgEr). The similar morphology in Figure 1 is observed for GCAgEr, where 30 ~ 50 nm SrF2 nano-crystals (dark contrast spheres) homogenously dispersed in the glass-ceramic host. However, the residual glass phase does not possess uniform contrast, revealing its multi-phase constitution. It actually includes quantities of deep dark contrast fold structures assigned to B2O3-rich glassy sub-phases. Such fold structures are clearly observed as the exactly same morphology as that in Figure 2a, where quantities of dark spots dispersed inside are silver nanoparticles less than 5 nm. The lattice fringes of cubic SrF2 (111) (Figure 3c) and Ag-4H (100) (Figure 3d) are identified simultaneously in the HRTEM image. Only SAED patterns of cubic SrF2 could be observed in the inset of Figure 3a-c, while electron diffractions assigned to cubic SrF2 and Ag-4H could be simultaneously observed in the inset of Figure 3c. That is because silver nano-lattices have ultra-small size and are imperfect in lattice structures. Therefore, the SiO2 – Al2O3 – ZnF2 – SrF2 – ErF3 – B2O3 – Na2O – Ag glass-ceramics (GCAgEr) exhibit both structural features of GCEr with fluoride nano-crystals and GAgEr with borate rich glassy sub-phases.

3.2 Selective enrichment behaviors of Er3+ and Ag in SrF2 and B2O3 sub-phases


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The micro-structural feature of such newly invented fluoroborosilicate glass-ceramics is abstractly illustrated by inheriting the glass-ceramics dispersed with Er3+ selectively enriched SrF2 nano-crystals (Figure 4a) and the phase separated glass with Ag selectively enriched B2O3-rich phase (Figure 4b) into a triple-phase glass-ceramics simultaneously containing SrF2: Er3+ nano-crystals and B2O3: Ag sub-phases (Figure 4c). That is further evidenced by STEM and EDX mapping (Figure 3e-k). Under highangle annular dark field (HAADF) STEM observation (Figure 3e), the heavier the element is, the brighter the STEM contrast will be. Accordingly, the brightest nanospheres are SrF2: Er3+ nano-crystals (30 ~ 50 nm), while the tiny points with lower brightness are Ag nano-particles, which are divided into two groups in size. One is several nano-meter size easily observed under TEM (Figure 2-3), while the other group is sub-nano-meter size abundantly in the glass. The former is subsequently found to behave as surface plasmon resonant Ag nano-particles (Ag NPs) without any luminescence behaviors. The latter Ag species is sub-nanometer size [Agm]n+ quantum clusters ([Agm]n+ QCs), which are believed to be responsible for the broad band luminescence of the glass. EDX mapping (Figure 5f-k) evidenced that elements are inhomogenously distributed in the glass-ceramics. Sr and Er appear at the same position, indicating that Er has enriched into SrF2 nano-crystals. For Ag, it emerges not only at several nano-meter sizes but also at sub-nano-meter size. Those agree well with the TEM observation. The selective enrichment behaviors of Er and Ag are further evidenced by the STEM observation and EDX mapping (Figure 3), where the 30 ~ 50 nm sized bright nano-



spheres could be identified as SrF2 nano-crystals and the small bright points ought to be Ag nano-particles. In high-angle annular dark field (HAADF) STEM images (Figure 3e), heavy elements appear much brighter than light elements. Atomic numbers have the following sequence: Ag > Sr > Si > Al > Na > F > O > B. So the brightness sequence of different phases should be SrF2: Er3+ > B2O3-Na2O: Ag > SiO2-Al2O3-Na2O with Er and Ag inhomogenously dispersed. In Figure 3(e), large SrF2 nano-crystals is much brighter than small silver nano-particles, indicating that Er3+ has selectively enriched into the SrF2 nano-crystalline phases, while spinodal B2O3-rich glassy phase is brighter than other dark glassy phase (SiO2-rich residual glassy phase), inferring that there should be extra undetectable sub-nanometer Ag clusters, besides Ag nanoparticles, to light up the B2O3-rich glassy phase.

3.3 Spectroscopic behaviors of quantum cluster [Agm]n+ It has been believed that there are four types of silver species13 in the glass and glassceramics: (i) Silver ions (Ag+). Ionic Ag+ acts as the charge compensators of [BO4] − tetrahedral, so could be isolated and stabilized by [BO4] − tetrahedral and homogenously dispersed in the B2O3-rich glassy phase. The feature absorption of Ag+ locates at 200 ~ 300 nm (GAg and GCAg in Figure 5(a-b)), and its PL excitation and emission bands (GAg and GCAg in Figure 5(c)) are identified at UV and visible spectral region with peaks centered at 270 nm and 390 nm, respectively. Those can be assigned to the electronic transition of isolated Ag+ ions: 4d10 (1S0) ↔ 4d95s1 (3D1) 24.


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(ii) Silver atoms (Ag0). Atomic Ag0 is generated by the reduction of Ag+ + e- → Ag0 when silver content exceeds its solubility in the host. Similar with those in the silicate glasses

36-37,

the electrons required for the reduction might be extracted from non-

bridging oxygen originally in the glass matrix via the following reactions: 2[≡B−O− Ag+] ↔ ≡B−O−B≡ + O− +[Ag2]+

(1)

2[＝B−O− Ag+] ↔ ＝B−O−B＝ + O− +[Ag2]+

(2)

[≡B−O− Ag+] + [＝B−O− Ag+] ↔ ≡B−O−B＝ + O− +[Ag2]+

(3)

O− + [Ag2]+ →O + [Ag2]0

(4)

Such reduction is similar with the decomposition of Ag2O into metallic Ag and O2, which was experimentally observed at the temperature range of 100 ~ 490 °C36, 38. But here the oxygen product in Eqs. (4) should be physically dissolved as dissociative “O” in the glass. Due to the homoatomic inner-shell d10-d10 interactions39, two neighbored Ag0 atoms tend to form [Ag2]0aggregate. These neutral [Ag2]0 aggregators cannot exist in isolation, but contact to other Ag+ ions, locating around [BO4]– tetrahedral as charge compensators, to form large size sub-nanometer [Agm]n+ in the glass-ceramics. Those [Agm]n+ aggregate occupy the central cavities in chelate-like anion groups composed of boroxol rings, such as [B3O7]5− and [B12O22]8−, which will be subsequently discussed in details. (iii) Sub-nanometer silver quantum clusters ([Agm]n+ QCs) with photoluminescence (PL) behaviors. Sub-nanometer Ag QCs have been expressed as [Agm]n+ in many literatures12, 17-18, 24, 40, which was indeed the aggregation of Ag+ and Ag0 according to the following process: n Ag+ + (m − n) Ag0 → [Agm]n+. The feature absorption of



[Agm]n+ locates at 300 ~ 400 nm (GAg in Figure 5(a-b)), and its PL excitation and emission bands (GAg in Figure 5(c)) are identified at UV and visible spectral region with peaks centered at 330 nm and 490 nm, respectively. Those can be assigned to singlet-singlet transition (S0 ↔ S1) of molecule-like [Agm]n+ QCs. (iv) Silver nano-particles (Ag NPs) with surface plasma resonance (SPR) behaviors. Heavily introduction of silver or annealing the glass, such as sample GCAg, at high temperature (above 490 °C) leads to a massive reduction of Ag+36. It then deduces a further growth of Ag QCs (sub-nanometer aggregators) into nanoscale Ag NPs with a feature surface plasmon resonance (SPR) absorption band41. In Figure 5(b), GCAg and GCAgEr show such SPR absorption at about 410 nm. But the emerging of Ag NPs is related with the decreasing of [Agm]n+ QCs. And SPR causes serious self-absorption phenomena around 410 nm. Especially, GCAg shows a lightless valley around 410 nm on Figure 5(c) due to the SPR self-absorption. Therefore, the PL emission of GCAg assigned to [Agm]n+ QCs (Figure 5(c)) appears weaker than GAg. Among the above four silver species, only ionic Ag+ and molecule like [Agm]n+ QCs were observed with PL behaviors in Figure 5. Spectroscopic properties of Ag+ has been intensively reported in the previous literature 18, 42-43, so here we mainly focus on discussing the PL nature of molecular [Agm]n+ QCs. From DFT calculation, Velazquez et al.45 proposed that the PL nature of [Agm]n+ QCs, such as [Ag4]2+ in an oxyfluoride glass, is a type of molecular luminescence, including singlet-singlet fluorescence and triplet-singlet phosphorescence. Similarly, [Agm]n+ QCs in the present glasses and glass-ceramics behavior also like molecules. As shown in Figure 5, under the 328nm


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excitation, the emission of glass is broad spectral luminescence, covering the whole visible and even near infrared spectral regions. These emission transitions, as illustrated in Figure 6, could be decomposed into the sub-bands (Figure 6(a-c)) locating at ultraviolet (UV)-violet (S1→S0), blue-green (T2→S0), yellow-red (T1→S0) and near infrared (S1→T1 and T2→T1) spectral regions. Theoretically, emissions due to singletsinglet or triplet-triplet transitions (S1→S0; T2→T1) are parity allowed fluorescence with ultra-short lifetimes, while emissions due to inter-system transitions between singlet and triplet (S1→T1; T2→S0; T1→S0) are parity forbidden phosphorescence with relatively long lifetimes. The S1→S0 transition at room temperature can be further quenched by the inter-system cross of S1→T2, and the near-infrared transitions of S1→T1 and T2→T1 can also be quenched by cross-relaxation processes of (S1,S0→T1,T1) and (T2,S0→T1,T1), respectively. Those eventually largely enhance blue-green (T2→S0) and yellow-red (T1→S0) transitions and bring them as principle PL sub-bands of [Agm]n+ QCs. The quenched S1→S0 emission can be experimentally evidenced by the low temperature PL spectra (Figure S1(d)) with nanosecond lifetimes (Figure S1(a) and the inset of Figure 6(a)). The nature of (T2→S0) and (T1→S0) transitions can also be reflected with their microsecond lifetimes (Figure S1(b-c) and the inset of Figure 6(b-c)).

3.4 To chemically stabilize [Agm]n+ by solubility and charge compensation strategies



Silver QCs ([Agm]n+) show further potentials to achieve highly efficient and color tunable broadband PL performance by regulating polymerization degree (m) and charge quantity (n) in the fluoroborosilicate glass matrix. From DFT simulation, as shown in Figure S2, energy states and absorption spectra were computed for [Agm]n+ QCs with m = 0; 1; 2; 3; 4; 5 and n = 0; 1. An obvious quantum effect is observed with small m and n values. Energy states of Ag10 and Ag11+ are completely discrete, but those of [Agm]n+ (m ≥ 2) become more quasi-continuous. The extreme case should be bulk Ag with the energy states overlapping to each other (metallic energy band). Such evolution makes it possible to design an energy gap by changing m value (polymerization degree). As shown in Figure S3, by forming Ag2+, Ag3+, Ag4+ and Ag6+, the absorption band of [Agm]n+ move from deep UV to visible blue spectral region. The experimental method to increase polymerization degree (m) could rely on a solubility strategy. 15 Beyond the solubility, Ag+ in a host tends to be reduced into Ag0 and polymerized into [Agm]n+. In practice, effective routes include increasing [Ag+] concentration, introducing competitors of Ag+, such as Na+, annealing below 490 °C and replacing hosts with low Ag+ solubility. Similarly, the charge quantity, n, also relates with the regulation of energy gaps. As shown in Figure S4, Ag32+ tends to accept electrons to transform into Ag31+ and Ag30, thus their absorption band also move from UV to visible spectral region. It has been previously reported that the introduction of negatively charged tetrahedral, such as [AlO4]−, [BO4]− and [ZnO4]2−, could help to the formation of [Agm]n+ with certain negative charge (n). 15


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3.5 To thermally stabilize [Agm]n+ by codoping with RE3+ Co-doping with trivalent rare earth cations (RE3+) can thermally stabilize the chemical state as well as hold the efficient PL of [Agm]n+ QCs, since it can prevent [Agm]n+ from being reduced and aggregated into Ag NPs at a high temperature. The introduction of RE3+ leads to redistribution of cations (RE3+/Al3+ and Ag+/Na+) in different sub-phases of the multi-phase glass-ceramics. RE3+ and Al3+ will compete for the coordinated F- in F-rich phase, and Ag+ and Na+ compete for the charge compensator of the [BO4] − tetrahedron in B2O3-rich phase. Accordingly, the introduction of RE3+ will exclude Al3+ from fluoride phase and result in an increasing amount of Al3+ in the residual silicate phase. The newly increased Al3+ will further extract Na+ from fluoride phase and B2O3rich phase. Thus, the concentration of [Ag+] is increased to be a much high level in B2O3-rich phase. That means the effective inhibition of reduction of [Agm]n+ into inactive Ag0 NPs. The above phenomenon can be clearly illustrated with the structural molecular dynamic (MD) simulation of the 45 SiO2 – 15 Al2O3 – 12 Na2O – 28 SrF2 (in mol %) glass (Figure S5(a-b)) and the 45 SiO2 –15 Al2O3 – 12 Na2O – 20 SrF2 – 8 LaF3 glass (in mol %) (Figure S5(c-d)). Here La3+ is taken as a representative cation for other trivalent RE3+. Without La doping (Figure S5(a-b)), a minority of Al3+ (blue colored) are found in the fluoride phase, and Na+ (black colored) are almost equally distributed in fluoride and silicate phase. In contrast, with the introduction of La3+ (Figure S5(c-d)), almost no Al3+ can be found inside the fluoride phase, a majority of Al3+ can be found on the interface between fluoride and silicate phases. Then, the newly formed [AlO4]–



tetrahedra could extract most of Na+ from fluoride phase (and similarly from the B2O3: Ag rich phase) into the silicate phase, so Na+ are seldom found in the fluoride phase. Such elemental distribution evolution can also be revealed by the coordination statistics (Table S1) on the cations in 45 SiO2 – 15 Al2O3 – 12 Na2O – (28-x) SrF2 – x LaF3 (in mol %) glasses with different LaF3 concentration. On the one hand, in the B2O3-rich

phase, Ag+ and Na+ are competitors for compensating the negative charge of [BO4]– tetrahedra. Less Na+ cations is favorable to large solubility of Ag+ in the B2O3-rich phase. Then, a low concentration of [Na+] results in a low polymerization degree of [Agm]n+ in a large quantity and is beneficial to stabilize [Agm]n+. Therefore, co-doping with RE3+ in such multi-phase glass can extract most of Na+ from the fluoride and B2O3rich phase, and the low [Na+] concentration is favorite to the stabilization of [Agm]n+ in a large quantity with low polymerization degree (m). It eventually prevents [Agm]n+ in the glasses (GAgRE) and especially in the glass-ceramics (GCAgRE) after annealing at high temperature from being grown into large Ag0 NPs with intense SPR absorption and diminished PL.

3.6 Optimization of QY and CRI performances for WLEDs application By introducing Ag/RE-codoping, the PL quantum yields (QYs) of the multi-phase glass-ceramics can also be well improved largely. This is due to the selective enrichment of two luminescent centers in two different phases (SrF2: RE3+ nanocrystals and B2O3: Ag sub-phases, see Figure 3-4 in section 3.2). Such special multiphase micro-structure can effectively suppress the energy transfers (ETs) between


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[Agm]n+ and RE3+, then refrain [Agm]n+ from serious PL quenching. For a pair of sensitizer (S) and activator (A), two essential factors to the ET process are spectral overlap and distance between them. According to the energy transfer theories, the distance between S and A should be shorter than a critical distance Rc whether in Dextor45 or Förster46 energy transfer cases. If the optical transitions of S and A are allowed electric-dipole transitions with a considerable spectral overlap, Rc may be some 30 Å. If these transitions are forbidden, Rc should be 5~8 Å for the occurrence of exchange interaction. It is easy to meet the spectral overlap prerequisite between broadband emission of [Agm]n+ and narrow absorption of RE ions, thus the distance between [Agm]n+ and RE ions become much crucial to the ET possibility. Take Ag/Er3+ co-doping as an example, in GAgEr, although [Agm]n+ is mainly enriched in the B2O3rich phase, Er3+ ions still distribute throughout the whole glass. So the distance is short enough for [Agm]n+ and Er3+ to transfer energy resonantly, especially for [Agm]n+ and Er3+ pairs near phase interfaces, as Figure 7(a; c) showed. For this reason, PL of GAgEr is much weaker that of GAg (Figure 5(c; d)), and its PL quantum efficiency is as low as 26.26 % (Table 1). In GCAgEr, [Agm]n+ and Er3+ in the glass-ceramics (Figure 1-4) are proved to be partitioned into B2O3-rich glassy phase and SrF2 nano-crystals, respectively. Thereby, [Agm]n+ and Er3+ are completely separated and the distance between them is not short enough for an ET process, as Figure 7 (b; d) showed. Therefore, PL of GCAgEr is strongly enhanced to be 4 times as intense as that of GAgEr (Figure 5(c; d)); PL lifetime of [Agm]n+ in GCAgEr is much longer than in GAgEr (Figure 8c); QY of GCAgEr (57.03 %) is strongly improved as compared to that of



GAgEr (26.26 %), as listed in Table 1. These ET-quenching PL of the other [Agm]n+/RE3+-codoped glasses are also easily observed in the case of co-doping [Agm]n+/Nd3+, [Agm]n+/Ho3+, [Agm]n+/Tb3+, [Agm]n+/Sm3+ (Figure 8a-c; Figure S14; Table S3-S5), where the QYs of the glasses (Table 1) are as low as 25.18 %, 28.97 %, 56.97 % and 46.06 %, respectively. In contrast, the QYs of the corresponding glassceramics are 46.10 %, 54.39 %, 61.19 % and 58.06 %, respectively. At the same time, practical CIE color coordinates and color rendering index (CRI) performances can also be well optimized for white LED (WLED) lighting application. As shown in Figure 7(d), Table 1 and Table S2, under the excitation of ultraviolet (328nm), quantum efficiency, color coordinates and color rendering index of the GAg emission are 58.46 %, (0.27, 0.32) and 85, respectively. But the problem is, at a high LED work temperature, the [Agm]n+ are easily reduced and aggregated into inactive Ag0 nano-particles (NPs) resulting in a sharp PL decrease. To display such deterioration, reference sample GCAg was obtained by thermal preserving GAg for 4 hours at 650 °C. As shown in Figure 5 (a-c), an intense SPR feature absorption emerges on the absorption of GCAg, and the PL of GCAg is significantly weakened after experiencing the SrF2 nano-crystallization procedure. The strong SPR absorption of Ag NPs even leads to one large sunken PL valley centered at 410 nm for GCAg. This is due to the existed d10-d10 attraction40 between Ag atoms and/or Ag+ ions, which drives Ag and/or Ag+ to be aggregated into larger Ag NPs with strong SPR absorption centered at 410 nm. In a result, as shown in Figure 7(d), Table 1 and Table S2, QY of GCAg sharply drops to 8.60 %，PL (Figure 5(c)) of GCAg is only ~ 35 % as intense as that of GAg,


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CIE coordinates of GCAg emission seriously off track to a green-white light (0.34, 0.44) with a color rendering index as low as 72. By [Agm]n+/RE3+-codoping, PL of the GCAgRE (co-doping [Agm]n+/Er3+, [Agm]n+/Nd3+, [Agm]n+/Ho3+, [Agm]n+/Tb3+, [Agm]n+/Sm3+, respectively) are remedied back as similar as GAg. The CRIs of GCAgRE are mostly kept as > 80, and the QY of GCAgRE are also largely improved, as shown in Table 1 and Table S2. Therefore, the strategies with multi-phase glassceramics and elemental selective enrichment are effective strategies to optimize QY and CRI performances of [Agm]n+/RE3+ co-doped glass-ceramics. Those demonstrate GCAgRE are potential candidate glass-ceramic phosphors for WLED applications. In addition, ETs from [Agm]n+ and RE3+ ions are usually utilized to broaden PL excitation bands and enhance PL of RE3+ [47-49]. For comparison, a similar investigation has also been performed to confirm possible PL broadening or enhancement effect to RE3+ in the multiphase glass-ceramics. Here two forms of ETs, namely non-radiative ETs and radiative ETs, are observed for the Ag/RE3+ co-doped glasses (GAgRE) and glass-ceramics (GCAgRE), respectively. Figures S6-S12 clearly reveal feature excitations of [Agm]n+ in the PL excitation spectra of the Ag/RE-codoped samples when monitoring at emission of RE3+ and feature emission of RE3+ when exciting [Agm]n+ at 328 nm (out of [Agm]n+/RE3+ resonant region). Those sufficiently verify the ET assignment from [Agm]n+ to RE3+. (GAgHo and GHo are exceptions, where no NIR emission could be detected under 328 nm excitation. The PL of them may be had been seriously quenched by large phonon vibration of the glass matrix. On the contrary, the PL of GCAgHo and GCHo can be easily observed since Ho3+ has been enriched into



the precipitated SrF2 crystalline phase with much smaller phonon vibration energy.) Both ETs require a spectral overlap between the emission of S ([Agm]n+ in this case) and the absorption of A (RE3+ in this case), but they happen at different distance (R). Usually non-radiative ET (i.e. resonant ET) in a solid takes place as R

Stabilization of Fluorescent [Agm] n+ Quantum Clusters in Multi-phase

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