Effects of Heat Treatment and Yb ³⁺ Concentration on the Downconversion Emission of Er ³⁺ /Yb ³⁺ Co-Doped Transparent Silicate Glass-Ceramics

Abstract

The SiO₂-Al₂O₃-BaF₂-TiO₂-CaF₂ transparent silicate glass-ceramics containing BaF₂ nanocrystals were successfully prepared by heat treatment process through conventional melting method. Effects of heat treatment processes and Yb³⁺ concentration on the downconversion (DC) emission of the co-doped Er³⁺/Yb³⁺ transparent silicate glass-ceramics were investigated. With the increase of temperatures and times of heat treatment process, the DC emission intensity of the co-doped Er³⁺/Yb³⁺ glass-ceramics was significantly enhanced. At the same time, with the increase of Yb³⁺ concentration, the value of DC intensity of Er³⁺/Yb³⁺ co-doped bands centered at 849, 883 and 1533 nm is maximized when the concentration of Yb³⁺ reaches 2.5 mol.%. When the concentration exceed 2.5 mol. %, the DC emission intensity of Er³⁺/Yb³⁺ co-doped bands centered at 849, 883 and 1533 nm was decreased, owing to the self-quenching effect. It’s interesting that the DC emission intensity of Er³⁺/Yb³⁺ co-doped band centered at 978 nm didn’t quench when the Yb³⁺ concentration exceed 2.5 mol. %. At the same time, the DC mechanism and ET processes between Yb³⁺ and Er³⁺ ions were discussed.

Keyword:
Downconversion; BaF₂; heat treatment; glass-ceramics; Er³⁺/Yb³⁺.

1. Introduction

In recent years, the silicon solar cells (Si-SC) are widely used to produce electric energy, it is considered a green and inexhaustible source of energy. Therefore, many studies have developed to enhance the emission spectrum of Si-SC energy ^1-4. Usually, there are two processes that contribute to the increase in emission solar cells (SC) spectrum, which is the downconversion (DC) and the upconversion (UC) of rare earth (RE³⁺) ions. Among them, the DC emission of the single doped Er³⁺ and the co-doped Er³⁺ with others RE³⁺ ions is a promising way to increase the efficiency spectrum of SC ^5-7.

In reality, the solar spectrum is within the wavelength range of 300-2500 nm ⁸, whereas the band-gap of the Si-SC converts only a small band around (1.000 nm at full efficiency into the electricity. The spectrum below the band-gap is not absorbed at all, and the spectrum above the band-gap is fully absorbed but is converted into electricity with high thermal losses. This spectral mismatch are caused a major loss of energy. Therefore, the researchers have interested in improve DC luminescence intensity of the co-doped RE³⁺ ions to deliver the highest spectrum efficiency for the SC energy ^2,9-11.

Among the existing trivalent RE³⁺ ions, the Yb³⁺ has a relatively simple electronic structure of two energy-level manifolds: the ²F_7/2 ground state and ²F_5/2 excited state around (1000 nm in the near-infrared (NIR) region, which located just above the band-gap of Si-SC ^1,12. Similar to Yb³⁺, the Er³⁺ also is one of most efficient ions combining to enhance SC spectrum because it has a favorable energy level structure with ⁴I_15/2 → ⁴I_11/2 transition corresponding to NIR emission of about 980 nm. Therefore, enhancement on the DC emission can be achieved by combine of the co-doped Er³⁺/Yb³⁺, through energy transfer (ET) process between between Er³⁺ and Yb³⁺ ions. There upon the energy is transferred to two Yb³⁺ ions via a resonant ET process. Finally, the Yb³⁺ ions will emit the two required photons with the band-gap energy of Si-SC ¹³.

In 2009, L. Aarts et al., ¹⁴ have investigated the DC emission for SC in NaYF₄: Er³⁺, Yb³⁺. This result indicated that the desired DC process from the ⁴F_7/2 level has very low efficiency due to fast multi-phonon relaxation from the ⁴F_7/2 to ⁴S_3/2 level via the intermediate ²H_11/2 level. Recently, In the paper of M.B. de la Mora et al., ¹⁵ mentioned the materials for DC in SC: Perspectives and challenges. Results of this paper affirmed among different options, downconversion is an appealing way to harvest the efficiency in solar cells because it permits to optimize the solar spectrum usage ¹⁵. With the purposed to improved efficiency photoluminescence for the solar cells application. In previous studies, we have investigated enhancement of upconversion emission of Er³⁺/Yb³⁺ co-doped transparent silicate glass-ceramics containing BaF₂ nanocrystals by effects of Mn²⁺ concentrations ¹⁶ and heat treatment processes ¹⁷. In this work, we continues to investigation the effects of the heat treatment processes and Yb³⁺ concentration on the DC emission intensity of the co-doped Er³⁺/Yb³⁺ transparent silicate glass-ceramics containing BaF₂ nanocrystals. At the same time, the mechanism of DC and ET processes between Yb³⁺ and Er³⁺ ions are also proposed and discussed.

2. Experimental Details

The glasses were prepared according to a conventional melt-quenching method. High-purity SiO₂, Al₂O₃, BaF₂, TiO₂, CaF₂, Er₂O_3, and Yb₂O₃ (99.99%) were used as the starting materials. All glass components (SiO₂, Al₂O₃, BaF₂, TiO₂, CaF_2, Er₂O₃ and Yb₂O₃) were purchased from the Aladdin Industrial Corporation, China. The compositions chosen in the present study are shown in Table 1. Mixtures with a sufficient weight of approximately 10 g, compacted into a platinum crucible, were set in an electric furnace. The electric furnace in this study manufactured by Nabertherm, Germany. After holding at 1500 ºC for 45 min under air atmosphere in an electric furnace, the melts were quenched by putting them onto a polished plate of stainless steel. According to the glass transition temperature (T_g) of differential thermal analysis which was determined by differential scanning calorimeter (DTA-60AH SHIMADZU) with a heating rate of 10 ºC/min under a nitrogen atmosphere.

Table 1
Chemical composition of SiO₂–Al₂ O₃ –BaF₂–TiO₂– C a F₂–Er₂O₃–Yb₂O₃ glasses (in mol. %)

Glasses name		Composition ratios of reagents (in mol. %)
		SiO2	Al2O3	BaF2	TiO2	CaF2	Er2O3	Yb2O3
SEY-1	SEY-0.2E0Y	45	19.8	20	10	5	0.2	0
	SEY-0.2E1Y	45	18.8	20	10	5	0.2	1
	SEY-0.2E1.5Y	45	18.3	20	10	5	0.2	1.5
	SEY-0.2E2Y	45	17.8	20	10	5	0.2	2
	SEY-0.2E2.5Y	45	17.3	20	10	5	0.2	2.5
	SEY-0.2E3Y	45	16.8	20	10	5	0.2	3
SEY-2	SEY-0.1E2.5Y	45	17.4	20	10	5	0.1	2.5
	SEY-0.3E2.5Y	45	17.2	20	10	5	0.3	2.5
	SEY-0.5E2.5Y	45	17	20	10	5	0.5	2.5
	SEY-0.8E2.5Y	45	16.7	20	10	5	0.8	2.5
	SEY-1E2.5Y	45	16.5	20	10	5	1	2.5

The samples were cut into the size of 10×10×2 mm³ and polished for optical measurements. To identify the crystallization phase, XRD (X-ray diffraction) analysis was carried out with a powder diffractometer (BRUKER AXS GMBH) using CuKα radiation. The sizes, shape, structure and component compositions of the asprepared nanocrystals were characterized by transmission electron microscopy (TEM, JEM-2100) at 200 kV. The reflectance spectra in the wavelength range of 350-1800 nm were measured on a Hitachi U-4100 spectrophotometer. The DC spectra in the wavelength range of 800-1650 nm and lifetime curves were measured on an Edinburgh Instruments FLS980 fluorescence spectrometer using a µF920 microsecond flash lamp as the excitation source and detected using a liquid-nitrogen-cooled PbS detector upon excitation at 410 nm. All spectral, DTA, XRD, TEM measurements were conducted at ambient temperatures.

3. Results and Discussion

To characterize the thermal stability of the prepared SiO₂-Al₂O₃-BaF₂-TiO₂-CaF₂ glass system, a DTA curve of SEY-1 glass sample was measured and showed in Fig.1.

Figure 1
The DTA curves of SEY-0.2E2.5Y glasses.

As can be seen in this figure, three temperature parameters: the glass transition temperature (T_g) was located around 554 ºC, the crystallization onset temperature (T_x1 = 675 ºC), two crystallization peaks temperatures (T_p1, T_p2) are located around 685 ^oC and 773 ^oC, respectively. Therefore, the transparent silicate glass-ceramics can be prepared by heat-treat in the first crystallization peak near 665 ºC, by controlling the appropriate crystallization temperature and process. Besides, between ~710ºC and 773 ºC, an endothermic reaction occurs. It’s also the crystallization onset temperature (T_x2) and the T_x2 is determined value around 753ºC. The difference ΔT between the crystallization onset temperature T_x1 and the glass transition temperature T_g (ΔT = T_x1 - T_g) is used as a rough indicator of glass thermal stability, and the ΔT = 675 ºC - 554 ºC = 121ºC > 100 ºC indicating the prepared glass is stable and suitable for applications such as fiber amplifiers and solar cells, etc. Based on the analysis results of the DTA curve, all the prepared glasses were heat-treated within the range of 665^oC to 773 ^oC. However, when glass-ceramics samples heat-treated up to 695 ^oC, the glass-ceramics sample is no longer transparent glass-ceramics. The optical images of glass-ceramics samples heat treatment at ~600, 685, 695 and 773 ^oC as shown in inset of Fig. 1. Therefore, in this study, we had chosen heat treatment temperatures for transparent silicate glass-ceramics samples within the range of 600-685 ^oC. Polished SEY-0.2E2.5Y glass sample was then heat treated at four different temperatures: 600, 630, 660 and 685 ºC, which were selected to carry out heat treatment for 5 h to form transparent silicate glass-ceramics and the fabricated samples were named as SEY-0.2E2.5Y-600, SEY-0.2E2.5Y-630, SEY-0.2E2.5Y-660, and SEY-0.2E2.5Y-685, respectively. At the same time, the polished SEY-0.2E2.5Y glass samples were selected to carry out heat treatment at 685 ºC for different times 10, 15, 20, 25 and 30 h to form transparent silicate glass-ceramics and the fabricated samples were named as SEY-0.2E2.5Y-10h, SEY-0.2E2.5Y-15h, SEY-0.2E2.5Y-20h, SEY-0.2E2.5Y-25h, and SEY-0.2E2.5Y-30h, respectively.

The transparent silicate glass-ceramics was prepared and the nanocrystals structures in the glass-ceramics were monitored by XRD. The XRD patterns of glass-ceramics after heat treatment at different temperatures are shown in Fig. 2 (a). From the results of Fig. 2(a) shows when the increase of processing temperature from 600 up to 685 ºC, crystal size of BaF₂ nanocrystals was increased from 10.7 up to 17.9 nm. Relationship between crystal size with the heat treatment temperatures are shown in the Fig. 2(b).

Figure 2
(a) XRD patterns of SEY-0.2E2.5Y glass sample and SEY-0.2E2.5Y-600, SEY-0.2E2.5Y-630, SEY-0.2E2.5Y-660 and SEY-0.2E2.5Y-685 transparent glass-ceramics samples; (b) Relationship between crystal size with the heat treatment temperatures; (c) XRD patterns of the SEY-0.2E2.5Y-10h, SEY-0.2E2.5Y-15h, SEY-0.2E2.5Y-20h, SEY-0.2E2.5Y-25h and SEY-0.2E2.5Y-30h transparent glass-ceramics samples; (d) Relationship between crystal size with the heat treatment times.

Fig. 2 (a) XRD patterns of SEY-0.2E2.5Y glass sample and SEY-0.2E2.5Y-600, SEY-0.2E2.5Y-630, SEY-0.2E2.5Y-660 and SEY-0.2E2.5Y-685 transparent glass-ceramics samples; (b) Relationship between crystal size with the heat treatment temperatures; (c) XRD patterns of the SEY-0.2E2.5Y-10h, SEY-0.2E2.5Y-15h, SEY-0.2E2.5Y-20h, SEY-0.2E2.5Y-25h and SEY-0.2E2.5Y-30h transparent glass-ceramics samples; (d) Relationship between crystal size with the heat treatment times.

Also from the result of the Fig. 2(a), the precursor glass sample presents a broad diffraction curve characteristic of the amorphous state, while in the patterns of transparent silicate glass-ceramics, the intense diffraction peaks are clearly observed, indicating that microcrystallites are successfully precipitated during thermal treatment. The diffraction pattern of the crystalline element is typical of a face-centered-cubic and these diffraction peaks around 2θ(degree) = 26º, 30º, 43º, 50º and 53º can be assigned respectively to the (111), (200), (220), (311) and (222) planes of the BaF₂ cubic phase.

The XRD patterns of glass-ceramics after heat treatment at different times are shown in Fig. 2 (c). From the results of Fig. 2(c) shows when the increase of processing times from 10 up to 30h, crystal size of BaF₂ nanocrystals was increased from 17.6 up to 19.9 nm. Relationship between crystal size with the heat treatment times are shown in the Fig. 2(d).

The crystallites size D for a given (hkl) plane was estimated from the XRD patterns following the Scherrer equation:

(1) $D=\frac{\lambda \times K}{\beta \times \cos \theta }$

Where K = 0.9, λ is the wavelength of the incident XRD [CuK_α (λ = 0.154056 nm)], β is the FWHM in radians and θ is the diffraction angle for the (hkl) plane. By using Debye-Scherrer equation, the average of BaF₂ crystallites size of SEY-0.2E2.5Y-600, SEY-0.2E2.5Y-630, SEY-0.2E2.5Y-660, SEY-0.2E2.5Y-685, SEY-0.2E2.5Y-10h, SEY-0.2E2.5Y-15h, SEY-0.2E2.5Y-20h, SEY-0.2E2.5Y-25h and SEY-0.2E2.5Y-30h transparent glass-ceramics samples has been calculated and displayed in the Figs. 2 (c&d). The results calculation of BaF₂ crystallites size and the relationship between the crystal size with the heat-treated different temperatures and times in the glass-ceramics are shown in Figs. 2 (c & d). Clearly, in this figure, the increase of the heat treatment temperatures and times were led to the crystal size increased, similar to the result of our previous works ^17-19.

The TEM image of SEY-0.2E2.5Y-685 transparent silicate glass-ceramics sample is shown in Fig. 3. From result of Fig. 3, it demonstrates that the BaF₂ nanocrystals were distributed homogeneously among the glass matrix and the mean sizes of nanocrystals were about 18-19 nm, which was similar to those calculated by Debye-Scherrer equation. The HRTEM image of the SEY-0.2E2.5Y-685 transparent silicate glass-ceramics sample is shown in inset of Fig. 3. As from this figure, the lattice spacing of (111) was estimated about 0.334 nm.

Figure 3
(a) TEM image of SEY-0.2E2.5Y-685 transparent silicate glass-ceramics sample; (b) HRTEM image of SEY-0.2E2.5Y-685 transparent silicate glass-ceramics sample.

The reflectance spectra of the Er³⁺/Yb³⁺ co-doped SEY-0.2E2.5Y glass and SEY-0.2E2.5Y-600, SEY-0.2E2.5Y-630, SEY-0.2E2.5Y-660 and SEY-0.2E2.5Y-685 transparent glass-ceramics samples within the range of 350 to 1800 nm are exhibited in Fig. 4. The reflectance bands corresponding to transitions from the ground-state (⁴I_15/2) to excited states: ⁴G_11/2, ²H_11/2, (⁴F_3/2, ⁴F_5/2), ⁴F_7/2, ²H_11/2, ⁴S_3/2, ⁴F_9/2, ⁴I_9/2, ⁴I_11/2 and ⁴I_13/2 transitions of the Er³⁺ and ²F_7/2 → ²F_5/2 of the Yb³⁺ ions, respectively, were observed. Furthermore, the reflectance intensity at ⁴F_5/2, ⁴F_7/2, ²H_11/2, ⁴S_3/2, ⁴F_9/2, ⁴I_9/2, ⁴I_11/2, ⁴I_13/2 states of the Er³⁺ and ²F_5/2 of the Yb³⁺ ions were increased with the increase of heat treatment temperatures from 600 to 685 ºC.

Figure 4
Reflectance spectra of Er³⁺/Yb³⁺ co-doped in SEY-0.2E2.5Y-glass, SEY-0.2E2.5Y-600, SEY-0.2E2.5Y-630, SEY-0.2E2.5Y-660, and SEY-0.2E2.5Y-685 transparent glass-ceramics samples.

Figure 5 shows DC emission spectra of the SEY-0.2E2.5Y-glass, SEY-0.2E2.5Y-600, SEY-0.2E2.5Y-630, SEY-0.2E2.5Y-660 and SEY-0.2E2.5Y-685 transparent glass-ceramics samples under excitation 410 nm in resonance with the ²H_9/2 (Er³⁺) level. In contrast to the slight DC luminescence observed through the precursor glass, the strong DC emission intensity of the Er³⁺/Yb³⁺ co-doped bands centered at 824 nm (²H_9/2 → ⁴I_11/2), 849 nm (²H_11/2 → ⁴I_13/2), 883 nm (⁴S_3/2 → ⁴I_13/2), 918 nm (⁴F_7/2 → ⁴I_11/2), 978 nm (⁴I_11/2 → ⁴I_15/2 of the Er³⁺ and ²F_5/2 → ²F_7/2 of the Yb³⁺), 1265 nm (⁴S_3/2 → ⁴I_11/2), 1533 nm (⁴I_13/2 → ⁴I_15/2) were observed in the glass-ceramics after heat treatment process changing temperatures ¹⁸. The reasons contributing to the increase of the DC emission intensity: in the case of the specimens after heat treatment, the Er³⁺ and Yb³⁺ ions practically dispersive into precipitated nanocrystal, the Er³⁺ and Yb³⁺ ions are condensed in the glass-ceramics, so that the distances between the Er³⁺ and Yb³⁺ become closer and consequently result in increasing of the DC emission intensity ²⁰.

Figure 5
DC emission spectra of SEY-0.2E2.5Y-glass, SEY-0.2E2.5Y-600, SEY-0.2E2.5Y-630, SEY-0.2E2.5Y-660 and SEY-0.2E2.5Y-685 transparent glass-ceramics samples.

The DC emission spectra of the SEY-0.2E2.5Y-10h, SEY-0.2E2.5Y-15h, SEY-0.2E2.5Y-20h, SEY-0.2E2.5Y-25h, and SEY-0.2E2.5Y-30h transparent glass-ceramics samples, under excitation 410 nm are shown in Fig. 6. Similar in the case of changing heat treatment temperatures, the DC emission intensity of the Er³⁺/Yb³⁺ co-doped bands centered at 824, 849, 883, 918, 978, 1265 and 1533 nm were strongly increased with the increase of heat treatment times from 10 to 30 h. These results confirms that the heat treatment processes greatly affects the DC emission intensity of Er³⁺/Yb³⁺ co-doped transparent silicate glass-ceramics.

Figure 6
DC emission spectra of SEY-0.2E2.5Y-10h, SEY-0.2E2.5Y-15h, SEY-0.2E2.5Y-20h, SEY-0.2E2.5Y-25h, and SEY-0.2E2.5Y-30h transparent glass-ceramics samples.

Furthermore, the effect of Yb³⁺ concentration on the DC emission intensity of Er³⁺/Yb³⁺ co-doped transparent silicate glass-ceramics were also presented follows. The DC emission spectra of SEY-0.2E0Y, SEY-0.2E1.0Y, SEY-0.2E1.5Y, SEY-0.2E2.0Y, SEY-0.2E2.5Y and SEY-0.2E3.0Y transparent glass-ceramics samples, under 410 nm excitation are shown in Fig. 7. As shown in the Fig. 7, in the DC process, the Yb³⁺ ions act as an efficient sensitizer. While Er³⁺ fixed concentration, with the increase of Yb³⁺ concentration, the DC emission intensity of Er³⁺/Yb³⁺ co-doped bands centered at 849, 883 and 1533 nm were strongly increased and reaches its maximum value when the content of Yb₂O₃ is 2.5 mol. %. When the concentration exceed 2.5 mol. %, the DC emission intensity of Er³⁺/Yb³⁺ co-doped bands centered at 849, 883 and 1533 nm was decreased. This result may be owing to the reasons mainly of the self-quenching effect can be attributed to the cluster or the ions pair between the Yb³⁺ ions is possibly formed in high the Yb³⁺ concentration ²¹. Further, the increase of Yb³⁺ concentration has enhanced the probability of interaction between the Yb³⁺ ions and some impurity, such as OH⁻ impurities was born from atmospheric moisture during melting ²². Therefore, the Yb³⁺ could not effectively absorb the pumping energy leading to the quenching of the DC emission intensities.

Figure 7
DC emission spectra of SEY-0.2E0Y, SEY-0.2E1.0Y, SEY-0.2E1.5Y, SEY-0.2E2.0Y, SEY-0.2E2.5Y and SEY-0.2E3.0Y transparent glass-ceramics samples.

It is interesting that the DC emission intensity band centered 978 nm, corresponding to the transitions: ⁴I_11/2 → ⁴I_15/2 of Er³⁺ and ²F_5/2 → ²F_7/2 of Yb³⁺ didn’t quench when the Yb³⁺ concentration excess 2.5 mol. % (see inset of Fig. 7). The strong DC emission intensity band around 978 nm consists in two contributions: (i) the ⁴I_11/2 → ⁴I_15/2 transition of Er³⁺ ions and (ii) the ²F_5/2 → ²F_7/2 transition of Yb³⁺ ions. On the other hand, the increase of the DC emission intensity bands at 849, 883 and 1533 nm can be explained by the following reasons: Firstly, we deem that the ET from ²F_5/2 → ²F_7/2 transition of Yb³⁺ to ⁴I_11/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_13/2 transitions of the Er³⁺ might be occurred. Secondly, the cross-relaxation (CR) may be occur between two neighboring Er³⁺ ions [⁴F_9/2-⁴I_13/2]; [²H_9/2-⁴F_9/2], the efficiency of the ET strongly depends on the distance of two Er³⁺ ions. The mechanism of the ET from Yb³⁺ to Er³⁺ ions and CR from Er³⁺ to Er³⁺ ions was suggested as follows:

²F_5/2 (Yb³⁺) + ⁴I_11/2 (Er³⁺) → ²F_7/2 (Yb³⁺) + ⁴I_15/2 (Er³⁺) (ET1)

²F_5/2 (Yb³⁺) + ⁴I_13/2 (Er³⁺) → ²F_7/2 (Yb³⁺) + ⁴F_9/2 (Er³⁺). (ET2)

⁴F_9/2 (Er³⁺) + ²H_9/2 (Er³⁺) → ⁴I_13/2 (Er³⁺) + ⁴F_9/2 (Er³⁺). (CR).

In addition, a variation of the molar concentration of Er³⁺ ions while keeping the concentration of Yb³⁺ ions in transparent silicate glass-ceramics composition was also given for comparison in the second component of SEY-2 transparent glass-ceramics sample. The DC emission of SEY-0.1E2.5Y, SEY-0.3E2.5Y, SEY-0.5E2.5Y, SEY-0.8E2.5Y, and SEY-1.0E2.5Y glass-ceramics samples under excitation 410 nm are shown in Fig. 8. From results in Figure 8, the DC emission intensity bands centered at 824 nm (²H_9/2 → ⁴I_11/2), 849 nm (²H_11/2 → ⁴I_13/2), 883 nm (⁴S_3/2 → ⁴I_13/2), 918 nm (⁴F_7/2 → ⁴I_11/2), 1265 nm (⁴S_3/2 → ⁴I_11/2) and 1533 nm (⁴I_13/2 → ⁴I_15/2) originated from Er³⁺ ions were significantly increased with the increase of Er³⁺ ions. In contrast, the DC emission intensity band centered around 978 nm, corresponding to the transitions: ⁴I_11/2 → ⁴I_15/2 transition of Er³⁺ and ²F_5/2→²F_7/2 transition of Yb³⁺ decreased with increasing molar concentration of Er³⁺ ions. This phenomenon can be explained by these reasons: Firstly, as the molarity of Er³⁺ ions increased, the increased luminescent centers lead the emission intensity bands centered at 824, 849, 883, 918, 1265 and 1533 nm significantly increased. Secondly, the possible ET from Yb³⁺ to Er³⁺ ions, contribute to the emission intensity bands centered at 824, 849, 883, 918, 1265 and 1533 nm improved while emission intensity bands centered at 978 nm decreased. The mechanism of the ET from Yb³⁺ to Er³⁺ ions was proposed as above section.

Figure 8
DC emission spectra of SEY-0.1E2.5Y, SEY-0.3E2.5Y, SEY-0.5E2.5Y, SEY-0.8E2.5Y and SEY-1.0E2.5Y transparent glass-ceramics samples.

The DC emission mechanism of Er³⁺/Yb³⁺ co-doped glass-ceramics are depicted schematically in Fig. 9.

Figure 9
Mechanism for DC processes of the Er³⁺/Yb³⁺ co-doped in SEY-1 glass-ceramics, under excitation 410 nm.

Fig. 9 Mechanism for DC processes of the Er³⁺/Yb³⁺ co-doped in SEY-1 glass-ceramics, under excitation 410 nm.

First of all, the Er³⁺ ions were the excited to the ²H_9/2 level under excitation at 410 nm. From the ²H_9/2 level, the Er³⁺ ions decay radiative to the ⁴I_11/2 state generating the DC emission around at 824 nm, and then quickly relaxes to the ⁴F_7/2 level with multi-phonon relaxing process ²³. The next step, from the ⁴F_7/2 level, the Er³⁺ ions decay radiative to the ⁴I_11/2 state generating the DC emission around 918 nm, and the ⁴F_7/2 level relaxes to the ²H_{11/ 2} and further relaxation to the ⁴S_3/2 levels. At the same time, the cooperative energy transfer (CET) process from one Er³⁺ ion to two neighboring Yb³⁺ ions occurs via cooperative dipole-dipole interaction. Subsequently, the ET from ²F_5/2 → ²F_7/2 transition of Yb³⁺ to ⁴I_11/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_13/2 transitions of Er³⁺ were occurred. And after the ET process, the Yb³⁺ emission is observed around 978 nm corresponding to the ²F_5/2 → ²F_7/2 transition. From the ²H_11/2 level, the Er³⁺ ions decay radiative to the ⁴I_11/2 state generating the DC emission band at 849 nm. Similarly, from the ⁴S_3/2 level, the Er³⁺ ions decay radiative to ⁴I_13/2 and ⁴I_11/2 states generating the DC emissions bands around 883 and 1265 nm, respectively ²⁴. The major contribution to the DC emission at 1533 nm is attributed to the ⁴I_13/2 → ⁴I_15/2 transition as shown in Fig. 9.

Furthermore, the fluorescence lifetimes of Yb³⁺ have been measured in SEY-0.1E2.5Y, SEY-0.3E2.5Y, SEY-0.5E2.5Y, SEY-0.8E2.5Y, and SEY-1.0E2.5Y transparent glass-ceramics samples to have a further evidence of energy transfer from Yb³⁺ to Er³⁺. The fluorescence lifetimes τ of Yb³⁺ at ²F_5/2 → ²F_7/2 under 410 nm excitation were measured and were presented in Fig. 10. The fluorescence lifetime was monitored at 1000 nm to avoid the luminescence owing to the ⁴I_11/2 → ⁴I_15/2 (Er³⁺) emission. Decay curves were well calculated as a double-exponential luminescence decay by using equation ²⁵:

(2) $I\left(t\right)=A_1.\exp \left(\frac{-t}{t_1}\right)+A_2.\exp \left(\frac{-t}{t_2}\right)$

Figure 10
Decay lifetimes τ of Yb³⁺ at ²F_5/2 → ²F_7/2 under excitation 410 nm

Where I(t) is the luminescence intensity; A₁ and A₂ are fitting constants; t is the time; τ₁ and τ₂ are short and long decay lifetimes for exponential components, respectively. By using these parameters, average photoluminescence lifetimes (τ*) for SEY-0.1E2.5Y, SEY-0.3E2.5Y, SEY-0.5E2.5Y, SEY-0.8E2.5Y and SEY-1.0E2.5Y transparent glass-ceramics samples can be resolved and calculated by using equation ²⁶:

(3) $t^{*}=\frac{A_1{t}_1^2+A_2{t}_2^2}{A_1{t}_1+A_2{t}_2}$

Average decay lifetimes of SEY-0.1E2.5Y, SEY-0.3E2.5Y, SEY-0.5E2.5Y, SEY-0.8E2.5Y, and SEY-1.0E2.5Y transparent glass-ceramics samples has been calculated to be about 32.5, 30.9, 29.8, 27.6 and 25.4 µs, respectively. Decay lifetimes for Yb³⁺ at (²F_5/2 → ²F_7/2) transition were found to decrease with increase of Er³⁺ concentration, this result confirms that the strong evidence for the ET from Yb³⁺ to Er³⁺ ions ^14,27.

4. Conclusions

In study of this article, the effects of heat treatment and Yb³⁺ concentration on the DC emission of Er³⁺/Yb³⁺ co-doped in transparent silicate glass-ceramics containing BaF₂ nanocrystals were successfully investigated. Comparison with the precursor glass, the DC luminescence of Er³⁺/Yb³⁺ co-doped transparent glass-ceramics has significantly enhanced after heat treatment process changing temperatures and times. With the increase of Yb³⁺ concentration, the DC emission intensity of Er³⁺/Yb³⁺ co-doped bands centered at 849, 883 and 1533 nm were strongly increased and reaches its maximum at 2.5 mol. % Yb³⁺ concentration.

When the concentration exceed 2.5 mol. %, the DC emission intensity of Er³⁺/Yb³⁺ co-doped bands centered at 849, 883 and 1533 nm was decreased, owing to the self-quenching effect. Whereas the DC emission intensity band centered 978 nm, corresponding to the transitions: ⁴I_11/2 → ⁴I_15/2 of Er³⁺ and ²F_5/2 → ²F_7/2 of Yb³⁺ didn’t quench when the Yb³⁺ concentration exceed 2.5 mol. %. At the same time, we deem that there was possibly an energy transition process from the ²F_5/2 → ²F_7/2 transition of Yb³⁺ to the ⁴I_11/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_13/2 transitions of Er³⁺ ions. In addition, the data presented for this study might provide useful information for further development of the DC in transparent silicate glass-ceramics associated with the ET between Yb³⁺ and Er³⁺ ions. These materials are promising for applications in enhancing conversion efficiency of SC.

5. Acknowledgments

This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.03-2019.56

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Publication Dates

• Publication in this collection
  28 Oct 2019
• Date of issue
  2019

History

• Received
  07 Feb 2019
• Reviewed
  28 June 2019
• Accepted
  05 Aug 2019