Enhanced Blue Emission in Er³⁺/Yb3³⁺ Doped Glass-ceramics Containing Ag Nanoparticles and ZnO Nanocrystals

Abstract

We report the precipitation of ZnO nanocrystals, Ag-clusters, and Ag nanoparticles in Ag/Er/Yb doped borate glasses by furnace annealing and $CO_2$ laser annealing. The XRD analysis revealed the precipitation of ZnO and Ag phases. The absorption spectra, the TEM and energy dispersive spectroscopy (EDS) revealed the incorporation of Er and Yb ions into ZnO nanocrystals formed by a laser technique and showed the surface plasmon band of Ag nanoparticles. The down-converted blue emission intensity of $Er^{3+}$ ions obtained under 365 nm excitation was enhanced by more than a hundred times in the glass treated by furnace annealing, mainly due to the energy transfer from Ag-clusters. Moreover, we discussed the contribution of Ag nanoparticles and defects to emission characteristics in the glasses treated by two annealing techniques. Up-conversion emissions of the $Er^{3+}$ ions under 980 nm excitation were enhanced due to the incorporation of $Er^{3+}$ and $Yb^{3+}$ ions into ZnO nanocrystals after thermal treatments.

I. INTRODUCTION

Glass-ceramics containing nanocrystals doped with rare-earth ions have been widely investigated as an economical alternative to existing materials and have substantial performance improvements [1]. ZnO materials have been recently reported as good doping hosts for rare-earth ions based on the energy transfer from ZnO to rare-earth ions [2]. ZnO is a non-toxic optical material with a wide transparent spectral region and high solubility of rare-earth ions [3]. It also has the advantage of not only high chemical and mechanical stabilities [4], but also much-reduced phonon energy (436 cm-1) [5], yielding large quantum efficiency. Er-doped ZnO is known as a promising material and has potential applications in optoelectronic devices [6-8]. However, ZnO and Er-doped ZnO nanocrystals showed only defect-related emissions under ultraviolet excitation due to efficient energy transfer from excitons to defect centers [7, 8].

Another approach for improving the emission efficiency of rare-earth ions is coupling the ions with Ag species [9]. Emission enhancement of glasses embedded with silver species of Ag+ ions, Ag-clusters, and Ag nanoparticles(NPs) [10, 11] have attracted a great deal of attention due to their wide potential applications, including the improvement of Si-based solar cells conversion efficiency [12].The aggregation lasts and forms Ag NPs during the heat treatment. Emission enhancement requires the energy transfer from Ag+ ions or Ag-clusters to rare-earth ions or the local field enhancement due to Ag NPs. The local-field surface plasmon resonance (SPR) effect of Ag NPs were reported when the spectral range of rare-earth ions is overlapped with the SPR band [13, 14]. The enhanced infrared emissions of Er3+ ions in Ag and Er co-doped glasses have been investigated and explained by energy transfer from Agclusters to Er3+ ions [10]. However, only a few observations of the visible emissions from Er ions and Ag-clusters in Ag and Er co-doped glasses and ZnO nanocrystals have been recently reported [13, 15]. Meanwhile, the laser irradiation glass has been reported to be an alternative method to the traditional furnace annealing to form glass-ceramics or NPs with an advantage of spatially selected structural modification and reduction process inside glass [16].

In this work, we report the precipitation of ZnO nanocrystals co-doped with Er and Yb and Ag NPs inside the glass by employing not only the traditional furnace treatment but also the CO2-laser-induced crystallization technique for thermal annealing. We compared the glass-ceramic samples prepared using two kinds of thermal annealing techniques for their surprising enhancements of visible emission of Er3+ions and the emission of defects in ZnO under ultraviolet excitation at 365 nm. Recently, the enhanced emission of Tm ions and broad emission from Ag species have been reported [17]. However, this is the first report for strong visible emissions of Er ions in Ag and Er codoped glasses containing nanoparticles and ZnO nanocrystals as far as we know. We report also the enhanced upconversion emission under the excitation at 980 nm, and discussed the origins of these enhancements, including energy transfer processes.

II. EXPERIMENT

The conventional melt quenching technique was used to prepare Ag⁺ and Er³⁺ codoped glass ceramics. The starting materials used in the present work were fine grained powders from high purity commercial chemicals of 37ZnO,37B₂O_3, 10K₂O, 10CaO, 1Al₂O₃, 3.5Yb₂O3, 1Er₂O₃, and 0.5Ag₂O. Starting batches were thoroughly mixed and melted at 1350°C for 1 h in a covered alumina crucible under normal atmosphere. Then, the melt was cast into an iron mold and annealed at 530°C for 10 h in order to release inner stress and then allowed to cool slowly to room temperature. Finally, the glass was cut and polished into glass samples with thickness of 1 mm. For thermal treatment to precipitate Ag NPs and ZnO nanocrystals, we separately used both furnace heating and CO2 laser heating.

The glass samples were annealed at 600~650°C for 5 h in order to induce crystallization. We confirmed the formation of ZnO crystals and Ag NP phases by x-ray diffraction(XRD, RIGAKU, SmartLab) analysis. Additional information of Er³⁺, Ag clusters and Ag NPs were obtained from the absorption spectra for each thermal treatment by employing a UV-VIS spectrophotometer. For laser heating,we scanned a CO₂ laser beam using different laser powers and scan speeds so as to control the exposure. The CO2laser beam width was ~100 µm on the surface. Furthermore,in order to clarify the crystallization of nanocrystals, we performed transmission electron microscopy (TEM, Titan G260-300) and EDS (Energy Dispersive Spectroscopy) analysis for the laser-treated surface. We employed micro-X-ray diffraction (D/MAX RAPID-S) analysis for the irradiated and un-irradiated regions in order to confirm the nanocrystal and NP formation. The up-conversion and down-conversion emissions for the glasses treated by a furnace and a laser were measured using a monochromator (DK240) and photomultiplier tube (PMT) under 980 nm LD and 365 nm LED excitations respectively.

III. RESULTS AND DISCUSSION

Figure 1(a) presents the XRD patterns of the precursor and the glasses treated by a furnace at 600~650°C. No diffraction peak was found for the precursor glass, whereas several diffraction peaks found in the treated glasses at600~625°C for 5 h correspond to ZnO (JCPDS No. 36-1451)crystals. The sharp diffraction peaks at 31.8°, 34.5°, 36.3°,47.7°, 56.7°, 63.0°, and 68.1° are easily assigned to the diffraction from the (100), (002), (101), (102), (110), (103), and (112) planes of the ZnO phase, respectively. The glass treated at 650°C did not show any clear peaks from Ag particles. Instead, the peaks from ZnAl2O4 (JCPDS No.05-0669) and YbBO3 (JCPDS No. 74-1937) phases appeared indicated by arrows [18, 19]. Micro- x-ray diffraction patterns of the glass unexposed and exposed to a 1.8 WCO2 laser with 0.1~0.2 mm/s scan speeds and a 4.5 Walser with 0.1 mm/s speed were shown in Fig. 1(b).

FIG. 1. XRD patterns of nanocrystals precipitated in Er,Yb, Ag triply-doped glass and glass-ceramics treated by electric furnace with 600~650°C for 5 h (a), and CO2 laser with 1.8 W at 0.1~0.3 mm/s and 4.5 W at 0.1 mm/s scan speed (b).

The x-ray beam has a smaller size than the laser-beam size on the surface, and the laser thermal treatment is effective only on the surface with an effective depth of~100 µm [20] because of the low thermal conductivity of glass. Thus, most of the sample volume exposed to the x-ray beam produces amorphous features in the XRD pattern. The exposed glass with a slow scan speed of 0.1 mm/shows several narrow peaks, especially in the sample treated with 4.5 W laser power, indicating the clear diffraction patterns of both ZnO and Ag crystalline phases. The peaks at 38.32°, 44.49°, 64.61°, and 77.53° (JCPDS 04-0783)indicate the (111), (200), (220), and (311) planes of the Ag structure in addition to the ZnO structure.

The narrower and taller peaks compared to those obtained for the furnace treatment indicate the larger sizes of ZnO and Ag nanoparticles. The average diameter size of the nanocrystals or nanoparticles has been calculated using the Scherrer formula [21] for XRD data of the lattice plane(hkl), using the full width at half maximum β, diffraction angle θ, and x-ray wavelength λ,

\(D=\frac{0.9 \lambda}{\beta \cos \theta}\)       (1)

The sizes of ZnO nanocrystals formed in the glass treated by an electric furnace at 650°C for 5 h and by a CO2laser with 4.5 W and 0.1 mm/s scan speed were estimated to be 13 and 30 nm, respectively. The size of Ag NPsformed in the glass treated by the laser with 4.5 W was estimated to be 15 nm.

Figure 2 shows the TEM images for nanocrystals precipitated in glass-ceramics treated by furnace treatment at 650°C for 5 h. It also includes the EDS image of Zn,Ag, Er, Yb, and Al elements. Both ZnO and ZnAl2O4nanocrystals are formed because we observed their phases in Fig. 1(a). We assume that the TEM image shows two types of nanocrystals doped with Er and Yb ions. The Alelements belong to ZnAl2O4 nanocrystals only, and the Znelements belong to both ZnO and ZnAl2O4 nanocrystals.YbBO3 nanocrystals have a low density because starting materials have only 3.5 mol%Yb2O3. The density of Agreements are very low and they seem to be distributed randomly. However, Fig. 3 shows the TEM image for nanocrystals precipitated in glass-ceramics treated by CO2laser with 4.5 W power and 0.1 mm/s scan speed. It also includes the EDS images of Zn, Ag, Er, Yb, and Al. The images of Er, Yb, and Zn elements are distributed in the same area, indicating the formation of ZnO nanocrystals doped with Er and Yb ions. The size of the ZnOnanocrystal is larger than the estimated average value of 30 nm from the XRD data. The Ag NPs were not observed near the ZnO nanocrystal and the Ag and Al elements are distributed randomly regardless of the ZnO. Another TEMmicrograph showed the existence of a nanoparticle at a different area as shown in Fig. 3(g). The EDS element maps of the other elements except for the Ag elements do not clearly show their localized distributions. We assume that some of the elements are distributed even on the surface of Ag nanoparticles. All were observed in glass-ceramics created by CO2 laser with 4.5 W and 0.1 mm/s scan speed.The size of the silver nanoparticle is close to the estimated value of 15 nm from the XRD data in Fig. 1(b).

FIG. 2. TEM micrograph of nanocrystals (a), EDS element maps of Zn (b), Ag (c), Er (d), Yb (e), and Al (f) in glass-ceramics treated by furnace treatment at 650°C for 5 h.​​​​​​​

FIG. 3. TEM micrograph of ZnO nanocrystals measured at one area (a), EDS element maps of Zn (b), Ag (c), Er (d), Yb (e), and Al (f). TEM micrograph of Ag nanoparticles measured at another area (g), EDS element maps of Zn (h), Ag (i), Er (j), Yb (k), and Al (l). All were observed in glass-ceramics treated by CO2 laser with 4.5 W and 0.1 mm/s scan speed.​​​​​​​

Figure 4(a) presents the absorption spectra of the precursor and the glasses treated at 600~650°C by an electric furnace. The treated glasses at 600~650°C showed the enhancement of the Ag-cluster absorption in the 320~380 nm range [15].Thermal treatment produced the reduction of Ag⁺ ⟶Ag0and Ag⁺ - Ag⁺ ⟶Ag⁺ - Ag⁰ [22], followed by aggregation of molecular-like Ag-clusters. The Er3+ absorption was little changed in the overlapped region with Ag-cluster absorption in the visible. However, the Yb³⁺ absorption at980 nm increased in the sample treated at 650°C. The enhancement of Yb³⁺ ion absorption of the ²F_7/2 ^{- 2}F_5/2 transition might be explained by the change of ligand field around Yb³⁺ ions due to the environment change from glass to ZnO nanocrystal [23]. Furthermore, the many broadbands in the sample treated at 650°C might be due to the scattering by several kinds of unknown oxide crystals.

FIG. 4. Absorption spectra of glass-ceramics treated by electric furnace with 600~650°C for 5 h (a), and CO₂ laser with 1.8 and 4.5 W at 0.1 mm/s scan speed (b).​​​​​​​

Figure 4(b) shows the absorption spectra of the glasses treated by the CO2 laser with different laser powers and the scan speed of 0.1 mm/s. The laser treatment exhibits relatively small enhancement of the Ag-cluster absorption in the 320~380 nm region compared to the furnace treatment case. The high power treatment with 4.5 W increased Erand Yb absorption and produced the enhanced SPR band which is expected from the existence of Ag NPs in XRD data.

Figure 5(a) shows the emission spectra obtained under365 nm excitation for the precursor glass and the samples treated at 600~650°C by a furnace. The untreated glass shows no blue emission but rather typical green emissions of the 2H11/2 and 4S3/2 to 4I15/2 transitions of Er3+ ions. The thermally treated samples exhibit several blue emission bands of the 4FJ (J = 3/2, 5/2, 7/2) ⟶⁴I_15/2 and the ⁴G_9/2 ⟶ ⁴I_13/2 transitions of Er₃₊ ions. The excitation at 365 nm reaches not only the ²K_15/2 and ⁴G_9/2 levels of Er_3⁺ ions but also the singlet state S1 of Ag-clusters in the treated samples. The excited Ag-clusters exhibit a broadband emission centered at 460 nm due to a large Stokes shift [24]. The well-overlapping emission band of Ag^-clusters and the Er³⁺ excited states enable the efficient energy transfer from Ag^-clusters to Er3+ ions. The emission intensity from Ag-clusters was also enhanced with annealing temperature because the density of the clusters was increased. The enhancement of blue emission of Er³⁺ ions is remarkable.Four times enhancement of blue emission in Er: ZnO -AgNPs hybrid structure compared to Er: ZnO was reported under excitation at 375 nm and explained by energy transfer from Ag NPs to Er3⁺ ions [25]. Our results showed more than 100 times enhancement of blue emission of the furnace annealed Ag, Er-doped glass at 650°C for 5 h compared to the untreated Ag, Er-doped glass and disappearance of green emission of Er³⁺ ions. The disappearance of green emission from the lower states of the ²H_11/2 and 4S3/2 to the ⁴I_15/2 transitions can be explained by no feeding from the upper states because of the efficient blue radiative transitions to the ground state. The visible emissions at 460 and 566 nm was reported in Er:ZnO nanocrystals and attributed to the surface defects caused by Er³⁺ ions without any Er emission [6]. Our results showed the defect emissions at 470 and 570 nm increased with annealing temperature. The band edge of ZnO is around 3.37 eV nm and the 365 nm excitation reaches the tail of the host absorption. Efficient energy transfer to the defects produces green emissions. Our defect emissions showed a little shift to the red. Moreover, our results showed the strong blue emission of Er³⁺ ions. The 365 nm excitation reaches the absorption band of Ag-clusters and only the tail of ZnO. Thus, we assume that energy transfer from defects to Er³⁺ions is negligible, while energy transfer from Ag-clusters to Er³⁺ ions are efficient as in other systems.

Similarly, the CO₂ laser-treated glasses treated with1.8 W and 4.5 W with 0.1 mm/s scan speed showed also enormous enhancement in Er³⁺ emissions compared to the precursor glass, as shown in Fig. 5(b). The broad emission in 440~530 nm is relatively weaker than that of the glass treated by a furnace because of the smaller density of the Ag-clusters as shown in Fig. 4(b). Most of the CO₂ laser energy is absorbed on the surface. The defect emissions at 470 and 570 nm observed in the sample treated by anelectric furnace were much increased, indicating the effectof non-uniform temperature distribution caused by the gaussian beam, because the defect formation is sensitive to the temperature. When Zn2+ ions are substituted with trivalent Er³⁺ ions, the overall emission line shape is modified due to the change of the local symmetries of Er³⁺ions and the defect creation due to a large difference in the ionic radii between Zn and rare-earth ions.

FIG. 5. Down-conversion emission spectra of glass-ceramics treated by electric furnace with 600~650°C for 5 h (a), and CO₂ laser with 1.8 and 4.5 W at 0.1 mm/s scan speed (b) obtained under 365 nm excitation.

In addition to energy transfer from Ag-clusters, we can not exclude the contribution of Ag NPs to the emission enhancement. The Ag NPs provide plasmonic coupling with nearby Er ions in ZnO nanocrystals because the SPRband and the Er emission band are spectrally overlapped, resulting in the increase in the radiative emission rate [14]. The effects of Ag NPs are very limited in the furnace treated samples due to their low density. However, the effects cannot be neglected in the laser-treated sample because the 365 nm excitation reaches the tail of the SPRband. In addition, the ZnO nanocrystals doped with Er ions formed during thermal annealing process may contribute to the increased radiative transition rates of dopants in the crystalline environment.

The up-conversion spectra under the 980-nm excitation in Figs. 6(a) and 6(b) show the narrow green and red emissions from the glass-ceramics treated by a furnace and a CO₂ laser respectively. The up-conversion process has been reported to originate from the excited state absorption in Er³⁺ ions and the energy transfer processes from the 2F7/2 - 2F5/2 transition of Yb³⁺ ions to the ⁴I_15/2 ⟶ 4I11/2, ⁴I_13/2 ⟶⁴F_9/2, and 4I11/2 ⟶⁴F_7/2 transitions of Er³⁺ ions [26]. The strong emission at 545 nm is due to the fast relaxation from the ⁴F_7/2 to ⁴S_3/2 levels. The excited state absorption of ⁴I_13/2 ⟶⁴F_9/2 transition feeds the population of the 4F7/2 level. Moreover, the cooperative energy transfer of ⁴S_3/2 ⟶⁴F_9/2 and 4I9/2 ⟶4F9/2 transitions [27]also increases the population of the ⁴F_9/2, resulting in thered emission at 660 nm. The emission intensities increase with the annealing temperature or the laser exposure, because the effective energy transfer of closely located Erand Yb ions occurs in the environment of ZnO nanocrystals created more in the higher temperature annealing as shown in Fig. 1. However, the ⁴F_7/2 level energy overlaps with the tail of the SPR band, resulting in reducing the emissions.The Er³⁺ emission bands of up-conversion spectra in all of the glass-ceramics are much narrower than down-conversion emission bands, implying that only the selected sites ofEr3+ ions in various crystal environments are involved in the up-conversion process. Figure 7 represents schematically the energetic diagram and main mechanisms for downconversion and up-conversion processes including energy transfer and possible transition routes expected to affect the emission process of Er³⁺ and Ag species.

FIG. 6. Up-conversion emission spectra of glass-ceramics treated by electric furnace with 600~650°C for 5 h (a), and CO₂ laser with 1.8 W at 0.1~0.3 mm/s scan speeds (b) obtained under 980 nm excitation.​​​​​​​

FIG. 7. Energy level diagram of Er³⁺, Yb³⁺, Ag-clusters, and Ag nanoparticles explaining down-conversion emission under 365 nm excitation (left), and upconversion emission under 980 nm excitation (right), incuding energy transfer (ET) and cooperative energy transfer.​​​​​​​

The furnace treatment produces several phases and takes a long time to find optimized conditions. In contrast, the laser treatment provides different thermal energy distribution on the surface of the glass, and it is more useful for precipitating nanoparticles or nanocrystals than the furnace technique. However, the CO₂ laser treatment is effective only on the surface, and most of the sample volume is in the glass phase which has low emission efficiency as shown in Figs. 4 and 5.

IV. CONCLUSION

We precipitated Er and Yb co-doped ZnO nanocrystals, Ag-clusters, and Ag NPs in borate glasses by both furnace and CO₂ laser annealing. Two annealing processes produced remarkably enhanced blue emissions of Er3+ ions due to the formation of Ag-clusters mainly consisting of Ag⁺ - Ag0pair species. We confirmed that the Ag-clusters produce their own broad blue emission and perform the energy transfer to the ⁴F_3/2 and ⁴F_5/2 levels of Er³⁺ ions to enhance the blue emission of Er³⁺ ions enormously. The overlap of the SPR band of Ag NPs with the blue emission band ofEr3+ ions in the laser-treated sample also contributed to enhance the emissions. Upconversion emissions of Er³⁺ions in green and red are enhanced because Er³⁺ and Yb³⁺ions were well incorporated into ZnO nanocrystals by thermal treatment. We also observed the enhancements of the emissions at 470 and 570 nm from the host defects more created by replacing Zn ions by rare earth ions.