Influences of copper–potassium ion exchange process on the optical bandgaps and spectroscopic properties of Cr³⁺/Yb³⁺ co-doped in lanthanum aluminosilicate glasses

Abstract

In this study, lanthanum aluminosilicate glasses with compositions of 45SiO₂–20Al₂O₃–12.5LaF₃–10BaF₂–9K₂O–1Cr₂O₃–2.5Yb₂O₃ (SALBK) were prepared using the conventional melting method and copper–potassium ion exchange process. Influences of the ion exchange process between copper and potassium on the visible, upconversion, and near-infrared luminescence spectra of Cr³⁺/Yb³⁺ co-doped under excitations of 343, 490, and 980 nm LD were investigated. The EDS analysis of SALBK glasses was measured to confirm the presence of atoms in the glasses. The values of direct and indirect bandgaps of Cr³⁺/Yb³⁺ co-doped SALBK glasses were calculated and analyzed. Besides, the energy exchange processes between Cu⁺, Cu²⁺ ions, and Cr³⁺, Yb³⁺ ions were also proposed and discussed.

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1. Introduction

In recent years, the spectroscopy and optical properties of chromium single-doped and chromium/rare-earth (RE) co-doped have been extensively studied^1–5 due to their advantages.^6,7 Chromium is a transition metal (TM) with many different valence states.^8–10 In the glass networks, it often exists in the trivalent state Cr³⁺,^1–5,8 which can emit radiation in the visible (VIS), near-infrared (NIR) regions under different excitation wavelengths.^5,11,12 Also, the ⁴T_1g(F) → ⁴A_2g, ⁴T_2g(F) → ⁴A_2g, ²T_1g → ⁴A_2g, ²T_2g → ⁴A_2g and ²E → ⁴A_2g transitions of Cr³⁺ can be combined with Yb³⁺ to generate the VIS, NIR emission spectra.^5,13,14 In 2001, H. U. Güdel et al.¹⁵ confirmed that Cr³⁺, in association with Yb³⁺, creates VIS emission in the wavelength range from 400 to 700 nm.¹⁶ Moreover, our recent study¹⁷ showed that the Cr³⁺/Yb³⁺ co-doped in the glasses generates emission spectra in the wavelength regions of 420–700 nm, 660–860 nm, and 970–1150 nm corresponding to the excitations 358, 488, and 690 nm LD. Since then, we have been interested in enhancing emissions and optical properties of Cr³⁺/Yb³⁺ co-doped in the glasses.^17,18 To this aim, embedding the coinage ions (such as, Ag⁺, Cu⁺ ions) into the glass through the ion exchange process^20–22 is one of the different solutions which brought about positive results.^3,12,19 In fact, the ion exchange process between coinage ions and alkali ions has many advantages compared with other traditional methods.²¹ For example, in many previous researches^22–24 as well as in our recent work,²⁵ it was shown that the coinage ions introduced by the ion exchange processes could significantly affect the optical properties of the glasses such as the refractive index, the optical density, the near-infrared emission spectrum, or even chemically strengthen the glasses^26,27 as well as modify glass structure.²⁸

Through the ion exchange process, the Cu⁺ and Cu²⁺ ions, as well as Ag⁺ ions, could be ejected into the surfaces of the glasses,^{22,24,29–32} then become neutral copper or/and silver atoms and grow into copper or/and silver nanoparticles (CuNPs or/and AgNPs).^24,29,32 Therefore, the ion exchange process between copper or/and silver cations and alkali cations to enhance the luminescence of Cr³⁺/RE³⁺ co-doped has been studied in recent times.^21,29,33 In this paper, we study the influences of the ion exchange process between copper and potassium on the optical bandgaps and spectroscopic properties of Cr³⁺/Yb³⁺ co-doped in 45SiO₂–20Al₂O₃–12.5LaF₃–10BaF₂–9K₂O–1Cr₂O₃–2.5Yb₂O₃ lanthanum aluminosilicate glasses. We calculated the values of both the direct and indirect optical bandgaps and figured out its manner of dependence on the salt concentration ratios between CuSO₄:K₂SO₄. Besides, the energy transfer mechanism between Cu⁺ and Cu²⁺ ions with Cr³⁺, Yb³⁺ ions was also proposed and discussed.

2. Experimental details

In this study, we used the highly pure (99.99%) reagents of SiO₂, Al₂O₃, LaF₃, BaF₂, Cr₂O₃, K₂O, Yb₂O₃, K₂SO₄, and CuSO₄ to prepare compositions of 45SiO₂–20Al₂O₃–12.5LaF₃–10BaF₂–9K₂O–1Cr₂O₃–2.5Yb₂O₃ (SALBK) by the conventional melting method. After being condensed in a platinum crucible, 12 g of the mixture of these materials were put into an electric furnace to be heated at 1600 °C for 1 h under air atmosphere. After that, we poured the molten mixture into a mold placed on a polished plate made from stainless steel to form glass samples, then annealed all of them at 450 °C for 6 h to remove thermal strains.³⁴ We next cut them into 10 mm × 10 mm × 2 mm size and finally polished their surface for the sake of measurements.

In order to perform the ion exchange process between copper and potassium, we prepared salt mixtures with different concentration ratios between xCuSO₄:(100 − x)K₂SO₄ where x varies as particularly given in Table 1.

Table 1 The concentration ratio of salts mixture for the ion exchange process

Name of glass samples	CuSO4x mol%	K2SO4(100 − x) mol%	Salt concentration ratio p between CuSO4 and K2SO4
SALBK-1Cr2.5Yb-0Cu	0	100	0
SALBK-1Cr2.5Yb-15Cu	15	85	0.18
SALBK-1Cr2.5Yb-20Cu	20	80	0.25
SALBK-1Cr2.5Yb-25Cu	25	75	0.33
SALBK-1Cr2.5Yb-30Cu	30	70	0.43
SALBK-1Cr2.5Yb-35Cu	35	65	0.54

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All the experimental measurements with the glass samples, which were carried out at the ambient air temperature, and their corresponding instruments are listed in Table 2.

Table 2 The measurements and their corresponding instruments

Measurement	Instrument
Differential thermal analysis (DTA)	DTA-60AH Shimadzu
X-ray photoelectron spectroscopy (XPS) spectra	PHI5500 ESCA spectrometer
XRD analysis	Powder diffractometer (BRUKER AXS GMBH) using CuKα radiation
Absorption spectra	U-4100 Hitachi spectrophotometer
NIR emission spectra	SBP300 Zolix spectrophotometer with an InGaAs detector
Visible emission spectra	F-7000 Hitachi fluorescence spectrophotometer
Energy-dispersive X-ray spectroscopy (EDS)	Field emission scanning electron microscopy (FESEM)
Decay lifetimes	Edinburgh instruments FLS-1000 fluorescence spectrometer

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To find the suitable temperature ranges for the ion exchange process between copper and potassium, we conducted DTA analysis for 45SiO₂–20Al₂O₃–12.5LaF₃–10BaF₂–9K₂O–1Cr₂O₃–2.5Yb₂O₃ (SALBK) glass sample. The results are shown in Fig. 1, where T_g, T_x, and T_p are the glass transition temperature, the crystallization onset temperature and crystallization peak temperature, respectively. According to the results, we chose 587 °C as the temperature of the salt mixtures into which we submerged the glass samples for 24 h.³⁴

Fig. 1 DTA analysis of 45SiO₂–20Al₂O₃–12.5LaF₃–10BaF₂–9K₂O–1Cr₂O₃–2.5Yb₂O₃ (SALBK) glass sample.

Next, they were taken out and washed with alcohol and distilled water to eliminate all the residual salt on their surfaces. After all, the glass samples were further heat-treated at 686 °C for 8 h to promote the copper nanoparticles (CuNPs) formation.

3. Results and discussion

The EDS analysis of the SALBK-1Cr2.5Yb-35Cu glass sample is shown in Fig. 2. In addition to the EDS peaks of the Si, O, Al, La, F, Ba, K, Cr, and Yb atoms in SALBK glass,³⁵ the ones of copper were strongly obtained corresponding to the energy values about 0.95, 8.04, and 8.99 keV,³⁶ which means that the copper and potassium ions have been added into the glass network through the ion exchange process.³⁷

Fig. 2 EDS analysis of SALBK-1Cr2.5Yb-35Cu glass sample.

To investigate the influence of the ion exchange process between copper and potassium on the structure of glass materials, we performed the XRD analysis of all the glass samples together with using X'Pert HighScore Plus software,³⁸ showing the results in Fig. 3. Conspicuously, the XRD pattern of the SALBK-1Cr2.5Yb-0Cu glass sample has no diffraction peak,³⁹ while all of the others show three main peaks at 2θ = 30.6 degree (110), 2θ = 44.2 degree (111) and 2θ = 52.1 degree (200)⁴⁰ due to the formation of copper nanoparticles (CuNPs).⁴¹ Besides, no diffraction peak of other nanocrystals is observed for all the samples.

Fig. 3 XRD analysis of SALBK-1Cr2.5Yb-0Cu, SALBK-1Cr2.5Yb-15Cu, SALBK-1Cr2.5Yb-20Cu, SALBK-1Cr2.5Yb-25Cu, SALBK-1Cr2.5Yb-30Cu, and SALBK-1Cr2.5Yb-35Cu glass samples.

Next, we exhibit the absorption spectra of SALBK-1Cr2.5Yb-0Cu, SALBK-1Cr2.5Yb-15Cu, SALBK-1Cr2.5Yb-20Cu, SALBK-1Cr2.5Yb-25Cu, SALBK-1Cr2.5Yb-30Cu, and SALBK-1Cr2.5Yb-35Cu glass samples in Fig. 4. For the SALBK-1Cr2.5Yb-0Cu glass sample, we can observe three absorption peaks bands centered at 470, 650, and 878 nm, which is evidence that they are from the ⁴A_2g → ⁴T_1g(F), ⁴A_2g → ⁴T_2g(F) and ⁴A_2g → ²E transitions of Cr³⁺.^17,42,43 In addition, with the increase in salt concentration ratio p from 0.18 to 0.54, the emission intensities of Cr³⁺/Yb³⁺ co-doped bands centered at 470, 650 nm, and 878 nm strongly increased. These increments may be ascribed to the local surface plasmon resonance (LSPR) of copper ions⁴⁴ and the crystal field variation caused by copper ions.^17,43,45 Moreover, the intensities of two absorption spectra bands centered at 306 nm and 878 nm also increased significantly, which confirms the existence and the role of Cu⁺ and Cu²⁺, respectively,⁴⁶ in the absorption spectra of Cr³⁺/Yb³⁺ co-doped. The Cu⁺ formed and exists in silicate glass due to reducing of Cu²⁺.^47,48 To further demonstrate the existence of both Cu⁺ and Cu²⁺ ions in the glass after ion exchange. We analyzed XPS spectra of the SABLK-15Cu glass sample, the results are shown in Fig. 5 (the blue and green curves are the curve-fitting Gaussian of XPS spectra at 934.91 eV and 932.39 eV, respectively). Based on these results, we can confirm the existence of both Cu⁺ and Cu²⁺ ions in the SALBK glasses, corresponding to the XPS major peaks at 932.39 eV and 934.91 eV.^49–52

Fig. 4 Absorption spectra of SALBK-1Cr2.5Yb-0Cu, SALBK-1Cr2.5Yb-15Cu, SALBK-1Cr2.5Yb-20Cu, SALBK-1Cr2.5Yb-25Cu, SALBK-1Cr2.5Yb-30Cu, and SALBK-1Cr2.5Yb-35Cu glass samples.

Fig. 5 XPS spectra of SABLK-15Cu glass sample.

In Fig. 6, we present the visible emission spectra under excitation 343 nm of all these glass samples with two peaks at about 488 and 653 nm, which are assigned to the ²T₂ → ⁴A_2g and ⁴T_2g(⁴F) → ⁴A_2g transitions of Cr³⁺.^17,43,45 As shown, the visible emission intensities of Cr³⁺/Yb³⁺ co-doped bands centered at 488 and 653 nm significantly increased with the increase in salt concentration ratio p from 0.18 to 0.54.⁴⁷ Interestingly, the peak at 488 nm of Cr³⁺ under 343 nm excitation has a slight blue-shift of about 8 nm. The emission peak shift at 488 nm of Cr³⁺ is assigned to the role of Cu⁺ cations. This result is also consistent with the suggestions discussed in the paper⁵³ of Tian-Shuai Lv et al.

Fig. 6 Visible emission spectra of SALBK-1Cr2.5Yb-0Cu, SALBK-1Cr2.5Yb-15Cu, SALBK-1Cr2.5Yb-20Cu, SALBK-1Cr2.5Yb-25Cu, SALBK-1Cr2.5Yb-30Cu, and SALBK-1Cr2.5Yb-35Cu glass samples under excitation 343 nm.

The increases in visible emission intensity of Cr³⁺/Yb³⁺ co-doped band centered at 488 nm and 653 nm were assigned to the energy transfer (ET₁) process from ³E_g(T_2g) → ¹A_g transition of Cu⁺ to ²T₂ → ⁴A_2g transition of Cr³⁺,^47,48 and to the ET₂ process from ³E_g(T_1g) → ¹A_g transition of Cu⁺ to ⁴T_2g(⁴F) → ⁴A_2g transition of Cr³⁺,^47,48 respectively. These two ET₁, ET₂ processes are proposed as follows:

ET₁: ³E_g(T_2g)(Cu⁺) + ⁴A_2g(Cr³⁺) → ²T₂(Cr³⁺) + ¹A_g(Cu⁺).

ET₂: ³E_g(T_1g)(Cu⁺) + ⁴A_2g(Cr³⁺) → ⁴T_2g(⁴F)(Cr³⁺) + ¹A_g(Cu⁺).

Furthermore, the calculated results of the CIE 1931(x; y) chromaticity coordinates for luminescence of Cr³⁺/Yb³⁺ co-doped in SALBK-1Cr2.5Yb-0Cu, SALBK-1Cr2.5Yb-15Cu, SALBK-1Cr2.5Yb-20Cu, SALBK-1Cr2.5Yb-25Cu, SALBK-1Cr2.5Yb-30Cu, and SALBK-1Cr2.5Yb-35Cu glass samples in correspondence to the P0Cu, P15Cu, P20Cu, P25Cu, P30Cu, and P35Cu points on the CIE 1931(x, y) chromaticity coordinates are given in Table 3, while Fig. 7 is for the sake of illustration. It can be seen that except that the CIE 1931(x; y) chromaticity coordinates for luminescence of Cr³⁺/Yb³⁺ co-doped in SALBK-1Cr2.5Yb-35Cu glass sample were shifted to the yellowish-green color region, the remaining were originally in the green color region.

Table 3 CIE 1931 (x; y) chromaticity coordinates for luminescence of Cr³⁺/Yb³⁺ co-doped in SALBK-1Cr2.5Yb-0Cu, SALBK-1Cr2.5Yb-15Cu, SALBK-1Cr2.5Yb-20Cu, SALBK-1Cr2.5Yb-25Cu, SALBK-1Cr2.5Yb-30Cu, and SALBK-1Cr2.5Yb-35Cu glass samples

Name of glass samples	CIEx	CIEy	Position on the CIE 1931(x, y) chromaticity coordinates	Color region
SALBK-1Cr2.5Yb-0Cu	0.2511	0.4241	P0Cu	Green
SALBK-1Cr2.5Yb-15Cu	0.2648	0.4356	P15Cu	Green
SALBK-1Cr2.5Yb-20Cu	0.2562	0.4294	P20Cu	Green
SALBK-1Cr2.5Yb-25Cu	0.2722	0.4363	P25Cu	Green
SALBK-1Cr2.5Yb-30Cu	0.2779	0.4402	P30Cu	Green
SALBK-1Cr2.5Yb-35Cu	0.2851	0.4501	P35Cu	Yellowish-green

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Fig. 7 CIE 1931(x; y) chromaticity coordinates for luminescence of Cr³⁺/Yb³⁺ co-doped in SALBK-1Cr2.5Yb-0Cu, SALBK-1Cr2.5Yb-15Cu, SALBK-1Cr2.5Yb-20Cu, SALBK-1Cr2.5Yb-25Cu, SALBK-1Cr2.5Yb-30Cu, and SALBK-1Cr2.5Yb-35Cu glass samples.

Fig. 8 shows the upconversion (UC) emission spectra of Cr³⁺/Yb³⁺ co-doped in all the glass samples under excitation 980 nm LD with two bands centered at ∼537 and 853 nm corresponding to ²T₂ → ⁴A_2g, and ²E → ⁴A_2g transitions of Cr³⁺, respectively.^{17,45,54–56} With the increase in salt concentration ratio p from 0.18 to 0.54, the emission intensities of these bands markedly increased. These increments were assigned to the aforementioned ET₁ process (for the band centered at 537 nm) and to the ET₃ process from ²B_2g → ²B_1g transition of Cu²⁺ to ²E → ⁴A_2g transition of Cr³⁺ (for the one at 853 nm),^5,46 which is proposed as follows:

Fig. 8 UC spectra of Cr³⁺/Yb³⁺ co-doped in SALBK-1Cr2.5Yb-0Cu, SALBK-1Cr2.5Yb-15Cu, SALBK-1Cr2.5Yb-20Cu, SALBK-1Cr2.5Yb-25Cu, SALBK-1Cr2.5Yb-30Cu, and SALBK-1Cr2.5Yb-35Cu glass samples under excitation 980 nm LD.

ET₃: ²E_g(Cu²⁺) + ⁴A_2g(Cr³⁺) → ²E(Cr³⁺) + ²B_1g(Cu²⁺).

Likewise, Fig. 9 shows the NIR emission spectra under excitation 490 nm. There was only one NIR emission peak at 1016 nm, and it is assigned to the ²F_5/2 → ²F_7/2 transition of Yb³⁺.^3,5 Similarly, the NIR emission intensity of the band centered at 1016 nm also increased with the increase in salt concentration ratio p, which is a manifestation of the ET₄ process from ²B_2g → ²B_1g transition of Cu²⁺ to ²F_5/2 → ²F_7/2 transition of Yb³⁺:⁵⁷

Fig. 9 NIR emission spectra of Cr³⁺/Yb³⁺ co-doped in SALBK-1Cr2.5Yb-0Cu, SALBK-1Cr2.5Yb-15Cu, SALBK-1Cr2.5Yb-20Cu, SALBK-1Cr2.5Yb-25Cu, SALBK-1Cr2.5Yb-30Cu, and SALBK-1Cr2.5Yb-35Cu glass samples, excited by 490 nm.

ET₄: ²E_g(Cu²⁺) + ²F_7/2(Yb³⁺) → ²F_5/2(Yb³⁺) + ²B_1g(Cu²⁺).

The mechanism for the visible and NIR luminescence of Cr³⁺/Yb³⁺ co-doped under excitations of 343, 490, and 980 nm LD, and the details of all the above-mentioned ET₁, ET₂, ET₃, and ET₄ processes are described in Fig. 10.

Fig. 10 Mechanism ET processes for the visible, UC, and NIR luminescence of Cr³⁺/Yb³⁺ co-doped under excitations 343, 490, and 980 nm LD.

To further validate evidence for the ET₁, ET₂, ET₃, ET₄ processes from Cu⁺/Cu²⁺ ions to Cr³⁺ and Yb³⁺ ions, we carried out the decay lifetimes measurement for several glass samples. Namely, the decay lifetimes curves of Cr³⁺ at 537, 653, and 853 nm to demonstrations for the ET₁, ET₂, ET₃ processes are shown in Fig. 11A–C, respectively while the decay lifetimes curve of Yb³⁺ at 1016 nm to demonstrations for the ET₄ is in Fig. 11D.

Fig. 11 (A) Decay lifetimes curves of Cr³⁺ at 537 nm in SALBK-0Cu, SALBK-15Cu, SALBK-20Cu, SALBK-25Cu, SALBK-30Cu, and SALBK-35Cu glass samples, under excitation 980 nm. (B) Decay lifetimes curves of Cr³⁺ at 653 nm in SALBK-0Cu, SALBK-15Cu, SALBK-20Cu, SALBK-25Cu, SALBK-30Cu, and SALBK-35Cu glass samples, under excitation 343 nm. (C) Decay lifetimes curves of Cr³⁺ at 853 nm in SALBK-0Cu, SALBK-15Cu, SALBK-20Cu, SALBK-25Cu, SALBK-30Cu, and SALBK-35Cu glass samples, under excitation 980 nm LD. (D) Decay lifetimes curves of Yb³⁺ at 1016 nm in SALBK-0Cu, SALBK-15Cu, SALBK-20Cu, SALBK-25Cu, SALBK-30Cu, and SALBK-35Cu glass samples, under excitation 490 nm.

We recall that the decay lifetimes can be calculated by the following equation:^58,59

where A₁, A₂ are constants; τ₁, τ₂ are rapid and slow lifetimes for exponential components, respectively. Using the formula (1) for the data obtained in Fig. 11A–D, we calculated the decay lifetime values of Cr³⁺ and Yb³⁺, corresponding to the ET₁, ET₂, ET₃ and ET₄ processes, details are presented in Table 4.

Table 4 Decay lifetimes values of Cr³⁺ and Yb³⁺ demonstrating for the ET₁, ET₂, ET₃, and ET₄ processes

Name of glass samples	Decay lifetimes of Cr^3+ at 537 nm, λex = 980 nm LD (ET1 process)	Decay lifetimes of Cr^3+ at 653 nm, λex = 343 nm (ET2 process)	Decay lifetimes of Cr^3+ at 853 nm, λex = 980 nm LD (ET3 process)	Decay lifetimes of Yb^3+ at 1016 nm, λex = 490 nm (ET4 process)
SALBK-1Cr2.5Yb-35Cu	98.32 μs	21.68 μs	62.78 μs	28.91 μs
SALBK-1Cr2.5Yb-30Cu	94.12 μs	20.52 μs	61.63 μs	26.12 μs
SALBK-1Cr2.5Yb-25Cu	91.04 μs	19.46 μs	60.54 μs	24.01 μs
SALBK-1Cr2.5Yb-20Cu	88.16 μs	18.34 μs	59.56 μs	22.89 μs
SALBK-1Cr2.5Yb-15Cu	86.21 μs	17.59 μs	58.66 μs	21.28 μs
SALBK-1Cr2.5Yb-0Cu	84.56 μs	16.78 μs	57.82 μs	20.57 μs

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Finally, we calculated and analyzed the optical bandgap (E_g) of Cr³⁺/Yb³⁺ co-doped in SALBK glasses to evaluate whether and how it is affected by the ion exchange process between copper and potassium. The optical bandgap E_g can be calculated using Tauc's formula:⁶⁰

αhν = A(hν − E_g)^m (2)

where α is the absorption coefficient, A is a constant, m is the power depending on the nature of transition (m = 1/2 for direct transition, and m = 2 for indirect transition).^61–63 From formula (2), the values of both types (direct and indirect) of the optical bandgap in SALBK glasses are calculated and detailed in Table 5. For the sake of demonstration, we also present the plot of (hν) versus both (αhν)² and (αhν)^1/2 for estimating E_g of all the considered samples in Fig. 12 and 13.

Table 5 The values of both the direct and indirect optical bandgaps in SALBK glasses

Glass samples	Direct bandgap hν [(eV)]	Indirect bandgap hν [(eV)]
SALBK-1Cr2.5Yb-0Cu	3.33	3.02
SALBK-1Cr2.5Yb-15Cu	3.32	2.98
SALBK-1Cr2.5Yb-20Cu	3.30	2.91
SALBK-1Cr2.5Yb-25Cu	3.28	2.88
SALBK-1Cr2.5Yb-30Cu	3.25	2.87
SALBK-1Cr2.5Yb-35Cu	3.23	2.83

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Fig. 12 Plot of (hν) versus (αhν)² for estimating the E_g of SALBK-1Cr2.5Yb-0Cu, SALBK-1Cr2.5Yb-15Cu, SALBK-1Cr2.5Yb-20Cu, SALBK-1Cr2.5Yb-25Cu, SALBK-1Cr2.5Yb-30Cu, and SALBK-1Cr2.5Yb-35Cu glass samples.

Fig. 13 Plot of (hν) versus (αhν)^1/2 for estimating the E_g of SALBK-1Cr2.5Yb-0Cu, SALBK-1Cr2.5Yb-15Cu, SALBK-1Cr2.5Yb-20Cu, SALBK-1Cr2.5Yb-25Cu, SALBK-1Cr2.5Yb-30Cu, and SALBK-1Cr2.5Yb-35Cu glass samples.

The calculation gave us the estimated results of 3.23–3.33 eV for direct bandgap and 2.83–3.02 eV for indirect bandgap. On the other hand, both bandgap types decrease when the salt concentration ratio p increases from 0.18 to 0.54. Thus, we can conclude that the ion exchange process between copper and potassium had a diminishing effect on the bandgap. The reasons for this effect can be: (i) Because there are non-bridging oxygens (NBOs) in the silicate glasses network,⁶⁴ the Al³⁺ ions have some options to link with SiO₄ tetrahedra to form (Al, Si)–O–Si, (Al, Si)–O–Al bonds,⁶⁵ or with others groups to form (Al, Si)–O bonds.⁶⁵ After the copper ions were introduced into the silica network by the copper-potassium ion exchange process, these bonds may be broken and then Al–O⁻ and Si–O⁻ groups can combine with copper ions to create the Si–O–Cu, and Al–O–Cu bonds. (ii) With the increase of salt concentration ratio, more negative sites appeared due to the local structure of the NBOs in the silicate glasses network.^64,66

4. Conclusions

The CuNPs were formed in 45SiO₂–20Al₂O₃–12.5LaF₃–10BaF₂–9K₂O–1Cr₂O₃–2.5Yb₂O₃ lanthanum aluminosilicate glasses through the ion exchange process between copper and potassium process. The intensities of all three visible, UC, and NIR emissions of Cr³⁺/Yb³⁺ co-doped bands at 480, 537, 653, 853, and 1016 nm increased with the increase in the ratio of salt concentrations CuSO₄:K₂SO₄ from 0.18 to 0.54. When the salt concentration ratio of the ion exchange process achieved 35 mol%CuSO₄:65 mol%K₂SO₄, the CIE 1931(x; y) chromaticity coordinates for the luminescence of Cr³⁺/Yb³⁺ co-doped in SALBK-1Cr2.5Yb-35Cu glass sample shifted to the yellowish-green color region. At the same time, the estimated results of the optical bandgap (E_g) confirmed that with the increase in the ratio of salt concentrations CuSO₄:K₂SO₄ from 0.18 to 0.54, the value of both direct and indirect bandgaps decrease. Besides, the possible energy transfer processes from Cu⁺, Cu²⁺ ions to Cr³⁺, Yb³⁺ ions were determined through the experimental results of the spectroscopic emissions and the decay lifetimes.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.03-2020.30.

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