English

• 0.   Introduction

  Tellurite glasses have many advantages such as wide range of transparency, large optical nonlinearity, refractive index and density, high solubility of rare-earth (RE) ions, as well as low phonon energy compared to other oxide glasses^[1]. These characteristics make RE doped tellurite glasses promising candidates for the optical fiber amplifier, laser and new band optical communication^[2]. Er³⁺-doped tellurite glass has generated considerable interest due to their favorable emissions of blue, red, green and near-infrared light^[3]. Yb³⁺ is known as a resourceful dopant can enhance the luminescence emissions of Er³⁺ due to its large absorption cross-section. What′s more, the resonance energy transfer between Yb³⁺ and Er³⁺ can significantly improve the pumping efficiency of Er³⁺, and improve the up-conversion (UC) properties of glasses^[4]. However, the luminous intensity of RE doped tellurite glasses is still weaker than some luminescent crystals^[5].

  Over the last few years, glass-ceramics have attracted great interest because of their prospective applications as promising hosts for RE ions. The formation of micro-crystals in the glass can greatly change the coupled environment around RE ions, and reduce the covalence of crystal field as well as the vibration energy of lattice^[6]. Gao et al.^[7] obtain NaYF₄:Tb³⁺, Yb³⁺, Li⁺ crystallites in silicate oxide system. RE ions are gathering around the micro-crystals, which increases the probability of the energy transfer between RE ions, and improves the luminous performance of the glasses. In addition, the simple process of preparation and synthesis makes RE ions doped glass-ceramics become important UC luminescence materials. Ansari et al.^[8] report the synthesis of YbF₃/ErF₃-codoped lithium tungsten tellurite oxyfluoride glass-ceramics contain LiYbErF₄ nanocrystals. An intense visible emission originated from Er³⁺ can be observed due to the cooperative UC processed 980 nm excitation.

  The localized surface plasmon resonance (LSPR) can increase the local field on the RE ions which near the metal nanoparticles and transferring energy to the RE ions, When the incident light wavelength or photoluminescence (PL) wavelength of the glass is close to the localized surface plasmon resonance wavelength, due to the local field enhancement (LFE) and energy transfer (ET) from Ag to RE ions, the luminescence intensity of glasses can be enhanced^[9]. Therefore, inorganic glasses doped with silver nanocrystals (Ag NPs) have received considerable attention^[10] due to their unique optical properties^[11]. Amjad et al.^[12] report significant luminescence enhancement of Er³⁺ ions as well as raman intensity of Ag NPs embedded zinctellurite glasses. The radiative transition of Er³⁺ ions and Ag NPs induce the electric dipoles, leading to the enhancement of PL^[2-13]. Ma et al.^[14] successfully introduce Ag NPs into SiO₂-Al₂O₃-CaF₂ system and obtain the glass-ceramics containing CaF₂ crystallites with significant enhanced luminous intensity. To our best knowledge, the study of tellurite glass-ceramics containing Ag NPs is rare.

  In this paper, we report the optical properties of Er³⁺/Yb³⁺ co-doped tellurite glass-ceramics containing Ag NPs. The effect of different introducing ways of Ag NPs on the UC luminescence properties of Er³⁺/Yb³⁺ co-doped tellurite glass-ceramics were systematically investigated. Our study further demonstrates the luminescent intensity of the sample co-doped with AgCl and AgNO₃ has the better UC luminescence properties than the sample which was single doped with AgCl or AgNO₃. Tellurite glass-ceramics doped with RE ions and Ag NPs are promising candidates for the development of lasers and optical amplifiers for PL based devices.

  1.   Experimental

  1.1   Material preparation

  Tellurite glasses with compositions 69TeO₂-23WO₃ -8La₂O₃ (TWL) containing fixed concentration of Er³⁺ (0.5% (n/n)), Yb³⁺ (1.0% (n/n)), AgCl (0~2.0% (w/w)) and AgNO₃ (0~3.0% (w/w)) were prepared by conventional melt-quenching method, melting anhydrous mixtures of TeO₂ (99.99%), La₂O₃ (99.99%), Er₂O₃ (99.99%), Yb₂O₃ (99.99%), WO₃ (99.99%), AgCl (99.9%) and AgNO₃ (99.9%). A gold crucible containing the glass constituents was placed in an electric furnace at (790±10) ℃ for 15 min and the melt was poured onto a preheated stainless steel plates. Subsequently, the samples were annealed at 380 ℃ for 2 h to remove the thermal and mechanical strains completely. The samples were then cooled down to the room temperature. 69TeO₂-23WO₃-8La₂O₃-0.5%Er₂O₃-1.0%Yb₂O₃ (TWL-ErYb base glass) and 69TeO₂-23WO₃-8La₂O₃-0.5%Er₂O₃-1.0%Yb₂O₃-1.0%AgCl (TWL-ErYb-1AgCl) glasses were heat treated at different heat treatment conditions and other samples were heat treated at appropriate heat treatment condition (at 390 ℃ for 15 min) to form glass ceramics. The samples heat treated at different temperatures (T) and times (t) are denoted as T-t. Finally, all the samples were cut and polished for the structural and optical measurements.

  1.2   Characterizations

  Differential thermal analysis (DTA) measurement (NETZSCH STA 449C) was carried out by heating about 0.02 g of glass powder in alumina crucible at the heating rate of 10 K·min^-1 from 200 to 800 ℃. The used atmosphere in DTA was N₂, and the gas flow was set as 40 mL·min^-1. The DTA results of samples were referenced to that of alumina powder. Phase identification of the samples was performed by X-ray diffraction (XRD) analysis with Cu Kα (λ=0.154 06 nm, 35 kV, 30 mA) radiation (D8 Advance, Bruker Inc., Germany) at room temperature in the 2θ range of 10°~70° with a step size of 0.02°and a step scanning time of 1 s. The UV-NIR absorption spectra of glasses were recorded by a UV-4100 UV/VIS/NIR spectrophotometer in the range of 400~1 100 nm. The fluorescence spectrometer (Model Omni-λ300 Zolix), together with a photomultiplier detector (PMTH-SI-CR131) were used to measure the luminescence spectra under 980 nm diode laser within the range of 500~700 nm. Laser power is 194 mW and focusing methodology is lens focusing. Standard sample was used to compare the intensity between different spectra. Luminescence decay times in the microsecond time scale was measured on an Edinburgh FLSP920 spectrophotometer. All spectroscopic measurements were performed at room temperature. The characterizations of nanocrystal or microcrystal in glasses were carried out by a Transmission electron microscopy (TEM) (JEM-2100) with an accelerating voltage of 200 kV. The samples were grinded into fine powders in an agate mortar. Subsequently, the powders were dissolved in ethanol and then dispersed by supersonic before the solution was dropped on the copper grid.

  2.   Results and discussion

  2.1   Effect of micro-crystallization on luminescence of the TWL-ErYb glasses

  Comparing to the base glass, appropriate micro-crystallization can improve the strength and thermal stability of glass. For example, oxyfluoride glass-ceramics doped with RE ions show higher chemical and mechanical stability and lower phonon energy than fluoride glasses^[7].

  The transparent TWL-ErYb glass-ceramics were prepared by appropriate heat treating. The absorption spectra of TWL-ErYb glasses before and after heat treatment at 390 ℃ for 15 min are presented in Fig. 1. After heat treatment, the absorption edge shows no obvious shift. The spectra exhibit a number of distinct absorption bands around 974, 800, 654, 544, 522 and 499 nm, which can be well assigned as the electronic transitions of Er³⁺ from its ground ⁴I_15/2 state to the ⁴I_11/2, ⁴I_9/2, ⁴F_9/2, ⁴S_3/2, ²H_11/2 and ⁴F_7/2 excited states, the absorption bands around 974 nm also include the electronic transitions of Yb³⁺ from its ground ²F_7/2 state to the ²F_5/2 besides^[15]. This result clearly shows that Er³⁺ ions exists in the glass. In order to further check whether micro-crystals were formed, the samples were characterized by TEM, instead of the XRD which is found to be hard to detective small amount of nanometer size micro-crystals in the glass.

  图 1

  

  图 1  Absorption spectra of TWL-ErYb glasses before and after heat treatment (390 ℃-15 min)

  Figure 1.  Absorption spectra of TWL-ErYb glasses before and after heat treatment (390 ℃-15 min)

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  Fig. 2 shows representative HR-TEM images of TWL-ErYb base glass after heat treatment at 390 ℃ for 15 min. There were many micro-crystals precipitated in the glass, and the size of micro-crystals varies from 40 to 150 nm. The measurement results of crystalline inter-planar space are 0.357 14 and 0.329 8 nm, corresponding to the (311) plane of Er₂WO₆ (PDF No.38-0102) and La₂(WO₄)₃ (PDF No.19-0669). For this reason, it can be inferred that Er³⁺ ions have been incorporated into micro-crystals, which has been demonstrated to improve the efficiency of the UC luminescence to a certain extent^[16].

  图 2

  

  图 2  HR-TEM images of TWL-ErYb base glass with heat treatment (390 ℃-15 min) and (b, c) are the enlarged view of (a)

  Figure 2.  HR-TEM images of TWL-ErYb base glass with heat treatment (390 ℃-15 min) and (b, c) are the enlarged view of (a)

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  Fig. 3 shows the UC emission spectra of TWL-ErYb base glasses with different heat treatment temperatures under the excitation of 980 nm. Three UC emission bands that located at 538, 557 and 674 nm can be clearly observed, which can be assigned to ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions of Er³⁺ ions, respectively^[17]. With the increase of heat treatment temperature, luminous intensity of each band gradually enhanced. As shown in Fig. 1, after heat treatment, the transmittance of glass declined, to the contrary, the intensity of RE absorption peaks accordingly increased and the non-radiative-relaxation influences which caused by RE ions co-doping can be eliminated due to the lower phonon energy of glass ceramics^[18].

  图 3

  

  图 3  Up-conversion emission spectra of TWL-ErYb base glasses for different heat treatment temperatures with the same treatment time of 15 min

  Figure 3.  Up-conversion emission spectra of TWL-ErYb base glasses for different heat treatment temperatures with the same treatment time of 15 min

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  What′s more, comparing to the red emission (674 nm), the enhancement of the green emission (557 nm) is more obvious (Fig. 3). However, the enhancement of luminescence caused by micro-crystals is not significant because of the incomplete structure of micro-crystals. Even when the heat treatment temperature was raised to 410 ℃, due to the overgrowth of micro-crystals, the luminous intensity of the glass was decreased. In order to investigate the luminescence mechanism of RE ions in glasses, the power dependence of UC emission intensity for TWL-ErYb base glasses without and with heat treatment at 390 ℃ for 15 min were studied, as shown in the inset of Fig. 3. The slope (n) for 557 nm wavelength of the sample without heat treatment was 1.84, and the sample with heat treatment at 390 ℃ for 15 min was 1.71. The result confirms that the green emission (557 nm) originates from the two-photon process absorption of Er³⁺ ions, and micro-crystallization has little effect on the UC mechanism of Er³⁺ and Yb³⁺ ions^[19].

  2.2   Effect of AgCl on luminescence of the TWL-ErYb glasses

  Introducing Ag NPs into RE ions doped glasses can effectively alter the free space spectral properties of RE ions and enhance the yield of their weak optical transitions^[20-21]. It is common to introduce AgCl as source of silver in glasses. The introduction of AgCl can reduce the glass transition temperature (T_g) of glass and open the network structure of the glass well, leading to the precipitation of Ag NPs easily despite of the small solubility of AgCl in glass^[22].

  In order to study the effect of Ag NPs on the micro-crystallization process, XRD patterns of TWL-ErYb and TWL-ErYb-1AgCl glasses with different heat treatment conditions (Fig. 4(a~c)) and DTA curves of TWL-ErYb and TWL-ErYb-1AgCl glasses (Fig. 4(d)) at the heating rate of 10 K·min^-1 from 200 to 800 ℃ were measured. T_g and the first crystallization temperature (T_c1), the second crystallization temperature (T_c2) and the third crystallization temperature (T_c3) are pointed by the arrow in the Fig. 4(d). When the glasses were been heat treated at 420 ℃ for 24 h, TWL-ErYb base glass was still transparent and TWL-ErYb-1AgCl glass became purple. There is no obvious crystallization peak on the XRD patterns, which may be caused by the small content of microcrystal and Ag NPs. When the glasses were been heat treated at 500 ℃ for 24 h and 640 ℃ for 24 h, the glasses were all opaque. The crystalline peaks of lanthanum tellurium oxides compounds were observed in Fig. 4(b, c). In addition, crystallization peaks of TWL-ErYb-1AgCl glasses are more obvious than TWL-ErYb base glass. Therefore, the Ag NPs are expected to be the nucleation agent and promote the precipitation of microcrystals. In general, this kind of glass is very stable and Ag NPs will not destroy the structure of the glass at a moderate heat treatment temperature.

  图 4

  

  图 4  XRD patterns of TWL-ErYb and TWL-ErYb-1AgCl glasses with different heat treatment conditions of (a) 420 ℃-24 h, (b) 500 ℃-24 h and (c) 640 ℃-24 h; (d) DTA curve of TWL-ErYb and TWL-ErYb-1AgCl glasses

  Figure 4.  XRD patterns of TWL-ErYb and TWL-ErYb-1AgCl glasses with different heat treatment conditions of (a) 420 ℃-24 h, (b) 500 ℃-24 h and (c) 640 ℃-24 h; (d) DTA curve of TWL-ErYb and TWL-ErYb-1AgCl glasses

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  The absorption spectra of TWL-ErYb-1AgCl glasses with different heat treatment conditions are shown in Fig. 5. After introducing Ag NPs into the glass, the absorption peak positions didn′t change. However, with the growth of Ag NPs caused by the increase of heat treatment temperature, the transmittance of TWL-ErYb glass with Ag NPs decreased. When the heat treatment temperature was raised to 460 ℃, in addition to the intrinsic absorption peaks of RE ions, the LSPR peak of Ag NPs was detected at the range of 500~650 nm^[23]. The emergence of the LSPR peak means that the content of Ag NPs increased significantly in the glass.

  图 5

  

  图 5  Absorption spectra of TWL-ErYb-1AgCl with different heat treatment

  Figure 5.  Absorption spectra of TWL-ErYb-1AgCl with different heat treatment

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  Fig. 6(a) presents the HR-TEM image of TWL-ErYb-1AgCl glass with heat treatment at 390 ℃ for 15 min. The shapes of Ag NPs are mainly spherical and ellipsoidal. Moreover, the size of Ag NPs is about 4~6 nm, while the size of micro-crystals is about 6~8 nm. The lattice fringes can be clearly observed in an enlarge image Fig. 6(b). The interplanar spacing of NPs is 0.238 1 nm, corresponding to the (111) plane of silver crystal (PDF No.65-8424). These results prove the precipitation of Ag NPs in TWL-ErYb-1AgCl glass. It is important to note that, in the Fig. 6(c), we can find the precipitation of La₂WO₆ micro-crystals around the Ag NPs. However, the number of Ag NPs is overall dominant.

  图 6

  

  图 6  TEM images of the sample TWL-ErYb-1AgCl heat treated at 390 ℃ for 15 min

  Figure 6.  TEM images of the sample TWL-ErYb-1AgCl heat treated at 390 ℃ for 15 min

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  Ag NPs are formed from AgCl or AgNO₃ throughout the melting procedure and grown during the annealing. The reduction of the Ag NPs can be discussed by the reduction potentials (E⁰) of redox system elements, as^[24]:

  $ {\rm{T}}{{\rm{e}}^{{\rm{6 + }}}}{\rm{/T}}{{\rm{e}}^{{\rm{4 + }}}}{\rm{ = 1}}{\rm{.02}}\;{\rm{V}} $

  $ {{\rm{W}}^{{\rm{6 + }}}}{\rm{/}}{{\rm{W}}^{{\rm{4 + }}}}{\rm{ = 0}}{\rm{.036}}\;{\rm{V}} $

  $ {{\rm{W}}^{{\rm{5 + }}}}{\rm{/ }}{{\rm{W}}^{{\rm{4 + }}}}{\rm{ = - 0}}{\rm{.031}}\;{\rm{V}} $

  $ {\rm{A}}{{\rm{g}}^{\rm{ + }}}{\rm{/Ag = 0}}{\rm{.799}}\;{\rm{6}}\;{\rm{V}} $

  Following reduction processes are likely to ensue:

  $ {\rm{T}}{{\rm{e}}^{{\rm{4 + }}}}{\rm{ + 2A}}{{\rm{g}}^{\rm{ + }}} \to {\rm{T}}{{\rm{e}}^{{\rm{6 + }}}}{\rm{ + 2Ag}}\;\;\;\;\;\;\;\;\Delta {\mathit{E}^{\rm{0}}}{\rm{ = 0}}{\rm{.579}}\;{\rm{2}}\;{\rm{V}} $

  $ {{\rm{W}}^{{\rm{4 + }}}}{\rm{ + 2A}}{{\rm{g}}^{\rm{ + }}} \to {{\rm{W}}^{{\rm{6 + }}}}{\rm{ + 2Ag}}\;\;\;\;\;\;\;\;\;\Delta {\mathit{E}^{\rm{0}}}{\rm{ = 1}}{\rm{.563}}\;{\rm{2}}\;{\rm{V}} $

  $ {{\rm{W}}^{{\rm{4 + }}}}{\rm{ + A}}{{\rm{g}}^{\rm{ + }}} \to {{\rm{W}}^{{\rm{5 + }}}}{\rm{ + Ag}}\;\;\;\;\;\;\;\;\;\;\;\;\Delta {\mathit{E}^{\rm{0}}}{\rm{ = 0}}{\rm{.830}}\;{\rm{6}}\;{\rm{V}} $

  where ΔE⁰ is the total potential of reduction process. The equation (5~7) are all feasible reactions (with ΔE⁰ > 0). Therefore, these reactions guarantee the presence of Ag NPs in the system in addition to the absorption spectra results and TEM images.

  The luminescence spectra of TWL-ErYb-1AgCl glasses with different heat treatment conditions were studied, the results are shown in Fig. 7. After heat treatment, all of the samples obtained the stronger emissions. The sample with heat treatment at 390 ℃ for 15 min acquired the best luminescence property. In this heat treatment condition, Ag NPs precipitated a lot and the average size of the micro-crystals was small. A lot of precipitations of Ag NPs, leading to short distance between Ag NPs and Er³⁺ ions, made the energy transfer from Ag NPs to Er³⁺ ions become a possible explanation for the enhanced luminescence^[25]. The non-resonance excitation light excites the d-band electron to unoccupied sp-conduction band^[1]. Subsequently, electron and hole recombine and moves to Fermi level through a phonon-electron interaction. Therefore, luminescence is mainly in visible region^[26]. The local electric field change cause the enhancement of photoluminescence and the effective electric field ($ {{\overrightarrow{E}}_{\rm{eff}}} $) can be written as^[17]:

  $ {{\vec{E}}_{\rm{eff}}}=\frac{{{\varepsilon }_{\rm{0}}}+2}{3}\left\{ 1+\frac{q\omega _{\rm{p}}^{2}}{3{{\varepsilon }_{\rm{0}}}\left[ \left( 1-q \right)\omega _{\rm{p}}^{2}/\left( 3{{\varepsilon }_{\rm{0}}} \right)-{{\omega }^{\rm{2}}}+i\gamma \omega \right]} \right\}{{\overrightarrow{E}}_{\rm{0}}} $

  图 7

  

  图 7  Up-conversion emission spectra of TWL-ErYb-1AgCl glasses at different heat treatment conditions

  Figure 7.  Up-conversion emission spectra of TWL-ErYb-1AgCl glasses at different heat treatment conditions

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  here ε₀ is the dielectric constant in the presence of an external electromagnetic field of amplitude $ {{\overrightarrow{E}} _{{\rm{0}}}}$, q is the nanocrystal specific volume, ω_p is the plasma frequency, i is the ground level, and γ is the damping of the resonance. The damping γ is the contribution of the conduction electrons and it can be described as^[2]:

  $ \mathit{\gamma }{\rm{ = }}\frac{1}{{{\mathit{\tau }_{{\rm{NP}}}}}} = \frac{1}{{{\mathit{\tau }_{\rm{0}}}}} + 2\frac{{{\mathit{g}_{\rm{s}}}{\mathit{V}_{\rm{F}}}}}{\mathit{D}} $

  The first term 1/τ₀ is related to the bulk electron scattering process in the nanoparticle. The interaction between quasi-electron-free and the surface of a sphere causes the second term, where V_F is the Fermi velocity, D is the mean core particle size, and g_s is the surface factor^[26]. The shape of the NPs is related to surface plasmon resonance (SPR) and the appropriate size of the NPs can enhance local field^[27]. Therefore, the glass with more Ag NPs, fewer micro-crystals, as well as high transmittance can obtain the enhancement of the luminescence property^[28]. The inset of Fig. 7 shows the dependence of the UC luminescence intensity on the 980 nm pump laser power for 557 nm. The slope (n) for 557 nm wavelength of the sample without heat treatment was found to be 1.87, and the sample with heat treatment at 390 ℃ for 15 min was found to be 2.05. This experimental result confirms that the green emission (557 nm) is due to the two-photon process absorption of Er³⁺ ions. It is worth noting that the slope (n) of the glass doped with Ag NPs is larger than the tellurite base glass (Fig. 4), which implies that Ag NPs can promote the two-photon absorption of Er³⁺ ions in the process of the energy transfer and promote particles jump to a high level^[29].

  Under the appropriate heat treatment condition at 390 ℃ for 15 min, the up-conversion emission spectra of the glasses with different contents of AgCl are shown in Fig. 8. The co-doped AgCl samples are denoted as TWL-ErYb-xAgCl (x=0~2% (w/w)). The reduction and growth of Ag nanocrystals generate an efficient localized electric field around the Ag NPs. The local electric field can increase the rate of excitations of Er³⁺ ions in vicinity of Ag NPs. As a result, the rates of transitions from emitting levels are enhanced^[29]. According to our testing results, the glass sample containing 0.75% (w/w) AgCl shows the maximum intensity enhancement. Furthermore, we find that the glass will be opaque after introducing 2%(w/w) AgCl into the glass. It means that doping content of AgCl is limited.

  图 8

  

  图 8  Up-conversion emission spectra of TWL-ErYb-xAgCl with heat treatment temperature (390 ℃-15 min)

  Figure 8.  Up-conversion emission spectra of TWL-ErYb-xAgCl with heat treatment temperature (390 ℃-15 min)

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  2.3   Effect of co-doped AgCl and AgNO₃ on luminescence of the TWL-ErYb glass-ceramics

  Ag NPs can be produced by the introduction of AgCl, however, it will cause the overgrowth of microcrystals at the same time. In order to find a better way to increase the precipitation of Ag NPs, the effect of different contents of AgNO₃ on luminescence of TWL-ErYb glass-ceramics has been studied, and the luminescence spectra are shown in Fig. 9. The single doped AgNO₃ samples are denoted as TWL-ErYb-yAgNO₃ (y=0~3%, (w/w)). Under the heat treatment condition at 390 ℃ for 15 min, the glass sample containing 2.5% (w/w) AgNO₃ shows the maximum intensity enhancement. Under the condition of large content, the luminescent property of the glass single doped AgNO₃ is better than that with single doped AgCl. However, the UC emission intensity enhancement of the glasses doped with a small content of AgNO₃ is not obvious. Besides, it was difficult to find Ag NPs in the TEM images of TWL-ErYb-2.5AgNO₃. It is not easy for AgNO₃ to generate Ag NPs in the glasses. Therefore, we consider co-doping AgCl and AgNO₃ to inhibit the precipitation of AgCl crystals and introduce more Ag⁺ ions into the glasses.

  图 9

  

  图 9  Up-conversion emission spectra of TWL-ErYb-yAgNO₃ with heat treatment temperature (390 ℃-15 min)

  Figure 9.  Up-conversion emission spectra of TWL-ErYb-yAgNO₃ with heat treatment temperature (390 ℃-15 min)

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  We studied the effect of the proportion of AgCl and AgNO₃ on luminescent properties, the results are shown in Fig. 10. The single doped AgCl and AgNO₃ samples are denoted as TWL-ErYb-xAgCl-yAgNO₃ (x=0~1%, y=0~1%, (w/w)) and the proportion of AgCl and AgNO₃ is x:y. The luminescent intensity of the glass co-doped with AgCl and AgNO₃ was stronger than the glass which was single doped with AgCl or AgNO₃. The sample with x:y=0.5:0.5 displayed the highest luminescence intensity. The glass with x:y=0.7:0.3 show the better luminescence intensity than the glass with x:y=0.3:0.7, which proves that AgCl can have greater effect on fluorescence enhancement of glasses when the content of silver source is limited. However, when x:y=1:1, the glass didn′t obtain further enhanced luminescence intensity. The luminescence microsecond time resolution were performed on the luminescence of the Er³⁺ ions in TWL-ErYb base glass and TWL-ErYb-0.5AgCl-0.5AgNO₃ glass. The samples were excited at 823 nm, and the decay curves were detected at 557 nm, as described in Fig. 11. Both the luminescence decays are well fitted to single exponential decay function, the calculated lifetime of the Er³⁺ ions in TWL-ErYb base glass (τ_Er) and TWL-ErYb-0.5AgCl-0.5AgNO₃ glass (τ_ErAg) is 81.001 and 95.081 μs, respectively. The energy transfer between Ag NPs and Er³⁺ ions and the effect of LSPR enhanced emission are might responsible for the longer lifetime of the glass with Ag NPs^[30]. Zhang et al^[31]. also find a longer lifetime of Er³⁺ ions (⁴I_13/2) in TeO₂-WO₃-La₂O₃-AgNO₃ glass than TeO₂-WO₃-La₂O₃ glass.

  图 10

  

  图 10  Up-conversion emission spectra of TWL-ErYb-xAgCl-yAgNO₃ glasses with heat treatment temperature (390 ℃-15 min)

  Figure 10.  Up-conversion emission spectra of TWL-ErYb-xAgCl-yAgNO₃ glasses with heat treatment temperature (390 ℃-15 min)

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  图 11

  

  图 11  Luminescence decay curves of the Er³⁺ ions inTWL-ErYb base glass and TWL-ErYb-0.5AgCl-0.5AgNO₃ glass

  Figure 11.  Luminescence decay curves of the Er³⁺ ions inTWL-ErYb base glass and TWL-ErYb-0.5AgCl-0.5AgNO₃ glass

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  LSPR and plasma coupling effect between particles make the effective enhancement of local electric field near the nanoparticles, leading to the increase of the radiative transition probability of each energy level of Er³⁺ ions, which eventually makes luminescence emission enhanced. Comparing to LSPR and plasma coupling effect, the energy transfer between Ag NPs and Er³⁺ ions is however the secondary factor lead to the enhancement of luminescence^[30]. Besides, micro-crystallization can decrease the phonon energy of glasses, and boost the energy level transition probability of RE ions^[32].

  TWL-ErYb-0.5AgCl-0.5AgNO₃ glass with heat treatment at 390 ℃ for 15 min shows the maximum enhancement and the HR-TEM images of the glass are shown in Fig. 12. A large number of Ag NPs in uniform distribution were observed. The shapes of Ag NPs are mainly spherical and ellipsoidal. It can be clearly seen the regular arrangement of silver atoms, and the size is about 4~6 nm. However, we didn′t find micro-crystals in this TEM image. It is probably that the amount of Cl^- ions of TWL-ErYb-0.5AgCl-0.5AgNO₃ glass is less than TWL-ErYb-1AgCl glass. The effect of AgCl on the precipitation of micro-crystals becomes weak, and the glass becomes more stable. Therefore, the precipitation of micro-crystals was not obvious in TWL-ErYb-0.5AgCl-0.5AgNO₃ glass. Besides, TWL-ErYb-0.5AgCl-0.5AgNO₃ glass can provide the same amount of Ag⁺ ions comparing to the TWL-ErYb-1AgCl glass. According to the above results, we conclude the follow results: (1) The introduction of AgCl can be helpful to produce Ag NPs, but it will cause the overgrowth of microcrystals; (2) It is not easy for AgNO₃ to produce Ag NPs, but introducing AgNO₃ can increase the content of Ag⁺ ions. Therefore, co-doping AgCl and AgNO₃ can combine the characteristics of AgCl and AgNO₃. Comparing to single doped AgCl or AgNO₃, co-doping AgCl and AgNO₃ can bring more Ag NPs into the glass, and keep the glass transparent. Based on the above reasons, the glass with appropriate co-doping proportion of AgCl and AgNO₃ show an enhancement of up-conversion emission intensity due to the formation of a lot of Ag NPs and a small amount of micro-crystals.

  图 12

  

  图 12  TEM images of the sample TWL-ErYb-0.5AgCl-0.5AgNO₃ annealed at 380 ℃ for 2 h: (a) is the enlarged view of (c) and (b) is the enlarged view of (a)

  Figure 12.  TEM images of the sample TWL-ErYb-0.5AgCl-0.5AgNO₃ annealed at 380 ℃ for 2 h: (a) is the enlarged view of (c) and (b) is the enlarged view of (a)

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  3.   Conclusions

  The role of micro-crystals and Ag NPs on the thermal, structural and spectroscopic properties of TWL-ErYb glasses have been studied in this paper. Micro-crystallization can improve the efficiency of luminescence emission. In the meantime, Ag NPs can also increase the strength of luminescence emission. The luminous efficiency of glasses can be further increased by introducing micro-crystals and Ag NPs at the same time. However, the excessive growth of micro-crystals will decrease the strength of luminescence emission. We find that co-doping AgCl and AgNO₃ can increase the precipitation of Ag NPs and reduce the overgrowth of micro-crystals. Besides, appropriate heat treatment temperature can also promote the precipitation of Ag NPs, and help to control the precipitation of micro-crystals. TWL-ErYb glasses with a lot of Ag NPs and a small amount of micro-crystals can further improve the up-conversion luminescence intensity due to the enhanced LSPR effect and a low phonon energy environment.

• 1. [1]

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  Figure 1  Absorption spectra of TWL-ErYb glasses before and after heat treatment (390 ℃-15 min)

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  Figure 2  HR-TEM images of TWL-ErYb base glass with heat treatment (390 ℃-15 min) and (b, c) are the enlarged view of (a)

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  Figure 3  Up-conversion emission spectra of TWL-ErYb base glasses for different heat treatment temperatures with the same treatment time of 15 min

  Inset is the power dependence spectra of up-conversion emission intensity for TWL-ErYb base glasses before and after heat treatment (390 ℃-15 min)

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  Figure 4  XRD patterns of TWL-ErYb and TWL-ErYb-1AgCl glasses with different heat treatment conditions of (a) 420 ℃-24 h, (b) 500 ℃-24 h and (c) 640 ℃-24 h; (d) DTA curve of TWL-ErYb and TWL-ErYb-1AgCl glasses

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  Figure 5  Absorption spectra of TWL-ErYb-1AgCl with different heat treatment

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  Figure 6  TEM images of the sample TWL-ErYb-1AgCl heat treated at 390 ℃ for 15 min

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  Figure 7  Up-conversion emission spectra of TWL-ErYb-1AgCl glasses at different heat treatment conditions

  Inset is power dependence spectra of up-conversion emission intensity for TWL-ErYb-1AgCl glasses before and after heat treatment (390 ℃-15 min)

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  Figure 8  Up-conversion emission spectra of TWL-ErYb-xAgCl with heat treatment temperature (390 ℃-15 min)

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  Figure 9  Up-conversion emission spectra of TWL-ErYb-yAgNO₃ with heat treatment temperature (390 ℃-15 min)

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  Figure 10  Up-conversion emission spectra of TWL-ErYb-xAgCl-yAgNO₃ glasses with heat treatment temperature (390 ℃-15 min)

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  Figure 11  Luminescence decay curves of the Er³⁺ ions inTWL-ErYb base glass and TWL-ErYb-0.5AgCl-0.5AgNO₃ glass

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  Figure 12  TEM images of the sample TWL-ErYb-0.5AgCl-0.5AgNO₃ annealed at 380 ℃ for 2 h: (a) is the enlarged view of (c) and (b) is the enlarged view of (a)

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