Modulation of phosphor luminescence performance by high concentration self-sensitization of Er and Ho–Yb ion co-doping under 1550 nm excitation

Abstract

In this paper, upconversion fluoride phosphors NaY_{1−x−y−z}F₄:Er_x³⁺, Ho_y³⁺, Yb_z³⁺ were synthesized by the low-temperature combustion method. And the optimal ratio of lanthanide ion doping in the matrix lattice was determined by the control variable method. First of all, the experimental results show that some Er³⁺-2 ions may be present in the samples doped with a high concentration of Er³⁺ ions, and the energy carried by the 1550 nm photons is absorbed by them and transferred to the remaining Er³⁺-1 ions in the form of sensitizers, which leads to saturation excitation of these Er³⁺-1 ions. Thus, the upconversion luminescence intensity of the Er³⁺ ion-doped samples was dramatically increased. Secondly, it was found that the singly doped Ho³⁺ ion samples also showed significant absorption of 1550 nm photons. In contrast, doping Yb³⁺ ions in samples singly doped with Ho³⁺ ions produces the opposite effect of Er³⁺ ions. The upconversion luminescence intensity of the Ho³⁺ ion-doped samples is significantly quenched. According to the above experimental phenomena, when a small amount of Ho³⁺ ions are doped into the matrix lattice of the sample doped with a high concentration of Er³⁺ ions, firstly, these Ho³⁺ ions can act as transient energy transition centers in the lattice. Secondly, they can also play the role of another self-absorption activation center in the matrix lattice. The upconversion luminescence performance of the Er³⁺–Ho³⁺ ion co-doped samples is significantly enhanced, so the characteristic emissions of Er³⁺ and Ho³⁺ ions are highly overlapped in the visible region. A small amount of Yb³⁺ ions continue to be doped into the Er³⁺–Ho³⁺ ion co-doped system, due to the significant quenching effect of the Yb³⁺ ions on the luminescence of the Ho³⁺ ions. The Yb³⁺ ions mainly play the role of reverse energy transfer centers between the Er³⁺–Yb³⁺ ions in the crystal lattice. This results in the upconversion luminescence intensity of the triple-doped samples being enhanced significantly by increasing the utilization of the system for the 1550 nm photons. In this paper, the phase composition and morphology of the phosphors were studied by an X-ray diffractometer and scanning electron microscope. The upconversion luminescence mechanism of Er³⁺–Ho³⁺–Yb³⁺ ion triple-doped samples under 1550 nm excitation and the sensitization interactions between the ions were systematically investigated by upconversion emission spectra and fluorescence lifetime. This work provides a new idea for the design of high-color purity upconversion luminescent phosphors under 1550 nm excitation, and the prepared phosphors can be applied in the field of display lighting.

---

1. Introduction

Upconversion luminescence is an anti-Stokes process by which low-energy infrared light can be converted into high-energy visible light.¹ Therefore, it has been widely studied in infrared detection, optical anti-counterfeiting, and bio-detection.^2–4 Currently, the research on upconversion luminescence is mainly concentrated in the 980 nm band,⁵ but the research on the 1550 nm band is relatively rare. This is mainly due to the lack of effective sensitizer ions in the 1550 nm band.⁶ However, compared with the 980 nm band, the 1550 nm band has strong anti-interference ability, strong anti-light scattering ability, and safety for living organisms.⁷ Hence, the 1550 nm band has better application prospects. Among lanthanide ions, Er³⁺ ions have the largest absorption cross-section for 1550 nm photons, so Er³⁺ ions have potential as sensitizer ions in this band.⁶ However, if only relying on the doping of Er³⁺ ions to achieve a good response of the matrix lattice to 1550 nm photons, it will inevitably lead to an increase in the doping concentration of Er³⁺ ions.⁸ In addition, the upconversion luminescence intensity of the samples under 1550 nm excitation is also limited by the maximum doping concentration of activator ions in the matrix lattice. Hence, the upconversion luminescence of samples in this band is generally weak. Therefore, it is crucial that the cross-relaxation and concentration quenching chances induced by high-concentration doping of Er³⁺ ions are attenuated. In this paper, it is found that high-concentration doping of Er³⁺ ions can be realized in the NaYF₄ matrix lattice. It is also found that at low-concentration doping, Er³⁺ ions mainly play the role of unsaturated luminescent centers in the lattice. As the doping concentration of Er³⁺ ions continued to increase, firstly, some of the Er³⁺-2 ions can act as sensitizers to absorb and transfer energy. This causes the remaining Er³⁺-1 ions to be saturated with excitation so that the upconversion luminescence of the sample is significantly enhanced. Secondly, 1550 nm photons can be well responded by the Ho³⁺ ions in lanthanide activator ions, so mutual sensitization between the Er³⁺ and Ho³⁺ ions can be present. Since the characteristic emissions of Er³⁺ and Ho³⁺ ions are highly overlapping in the visible region, the upconversion luminescence intensity of the sample is significantly enhanced by the additive effect of the two characteristic emissions. At this time, Ho³⁺ ions mainly play the roles of transient energy trapping centers and new luminescence centers in the matrix lattice. On this basis, we also find that the upconversion luminescence intensity of the samples is further enhanced by doping a small amount of Yb³⁺ ions into the above two co-doping systems. At this time, the Yb³⁺ ions mainly play the role of the reverse energy transfer center in the matrix lattice. The energy that should have been dissipated in the supersaturated excitation is stored in the excited state energy level of the Yb³⁺ ions through the energy transfer process. Then, the Er³⁺ ions are secondarily excited by the reverse energy transfer process. Due to this process, the utilization of photons by the sample is drastically increased. As a result, the upconversion luminescence intensity of the triple-doped system is significantly enhanced.

In this paper, upconversion fluoride phosphors NaY_{1−x−y−z}F₄:Er_x³⁺, Ho_y³⁺, Yb_z³⁺ were synthesized by the low temperature combustion method. The inter-sensitization interaction between Er³⁺–Ho³⁺–Yb³⁺ ions was investigated in detail. This study not only provides a new idea for the design of upconversion luminescent materials, but also further improves the energy transfer mechanism between the upconversion multi-doped lanthanide activator and sensitizer ions under 1550 nm excitation.

2. Experimental

2.1 Preparation

The experiments started with sodium fluoride (NaF), rare earth oxides Ln₂O₃ (Ln = Er, Y, Ho and Yb) as raw materials, ammonium hydrogen fluoride (NH₄HF₂), urea [CO(NH₂)₂], and ammonium nitrate (NH₄NO₃). The up-conversion fluoride phosphors NaY_{1−x−y−z}F₄:Er_x³⁺, Ho_y³⁺, Yb_z³⁺ were synthesized by the low-temperature synthesized combustion (LCS) method. First, the rare earth oxides Ln₂O₃ were dissolved in dilute nitric acid to make a solution of rare earth nitrates [Ln(NO₃)₃]. The co-precipitation method was adopted to convert rare-earth nitrates into solid rare-earth fluorides (LnF₃). Then the prepared fluoride raw materials were mixed with oxidizer [NH₄NO₃], reductant [CO(NH₂)₂] and NH₄HF₂ by adding alcohol to grind them well to obtain prefabricated combustion rods. Finally, they were ignited in a muffle furnace at 650 °C. 10 min later, the crucible was removed, and a fluffy pink powder was obtained. After the samples were cooled, they were gently ground and used for subsequent tests.

2.2 Measurements

The X-ray diffraction (XRD) patterns of the samples were tested with a Rigaku/Ultima IV X-ray diffractometer at 40 kV, 20 mA, with Cu K radiation (λ = 1.5405 Å) and a diffraction angle range of 10 to 80 (2θ). The morphology of the samples was tested using a JSM-6701F scanning electron microscope (SEM) at an accelerating voltage of 10 kV. The upconversion emission spectra of the samples were tested by a Shimadzu RF-5301PC spectrometer and a power-tunable 1550 nm laser at room temperature, the spectrometer's test slit was 1.5 μm and the excitation power was 1.57 mW cm⁻².

3. Results and discussion

3.1 Crystal structure analysis

Fig. 1 and 2 show the XRD patterns of the samples doped with different kinds of lanthanides, respectively. Table 1 shows the ionic radius of the lanthanide ions doped in this sample. It is well known that when other lanthanide ions are doped in the matrix lattice, the difference in ion radius will cause the contraction or expansion of the cell of the matrix lattice, thus influencing the sample's up-conversion luminescence characteristics.^9,10 The variation law of the matrix lattice can be described by the interplanar spacing of the hexagonal crystal system and Bragg's law.^11,12

2d sin θ = nλ (2)

where h, k and l are the crystal indices of the crystal cell, a and c are the prism lengths of the crystal cell, d is the interplanar spacing, θ is the half diffraction angle of the crystal, λ is the excitation wavelength, and n is an integer that indicates the number of diffraction levels of the crystal. Since the phosphor samples are based on NaYF₄ doping, the other lanthanide ions have replaced the Y³⁺ ion lattice locations in the matrix lattice. Therefore, when Er³⁺/Yb³⁺ ions are doped into the units of the matrix lattice, the ionic radii of all these ions are smaller than those of Y³⁺ ions.^13,14 Hence, a certain degree of contraction occurs in the matrix lattice structure. According to eqn (1), h, k and l is constant for a specific family of crystal planes. When the cell parameters become smaller owing to the doping of rare earth ions, the interplanar spacing d decreases accordingly, so the cell's crystal plane spacing decreases. And then, according to eqn (2), when the crystal plane spacing d decreases, the diffraction angle of the sample should be shifted to a large angle direction.⁹ This is consistent with the shift of the diffraction peak in the XRD local magnification of Fig. 2. Then, Ho³⁺ ions replace the lattice sites of Y³⁺ ions by doping the matrix lattice, as the radius of these ions is greater than that of Y³⁺ ions.¹⁵ Therefore, according to the above eqn (1) and (2), the XRD diffraction peak of the sample should be shifted in an appropriate angular direction.^10,16 However, due to the minimal doping of Ho³⁺ ions in the sample, the diffraction peak shift of the samples is minimized, which can be further explained by the XRD local magnification pattern in the above figure. Fig. 3 shows a schematic representation of the crystal structure doped with several lanthanide ions. Fig. 4 shows the SEM images of different lanthanide ions doped in the matrix lattice. It is evident from the graph that the majority of grains in the SEM of the sample are hexagonal phase crystals, which is consistent with the crystal phase shown in the XRD standard card of the sample, and the size of the grains mostly remains around 1 μm with a more uniform grain size.

Fig. 1 XRD patterns of doping with different concentrations of Er³⁺ ions.

Table 1 Ionic size of elements

Elements	Y	Er	Ho	Yb
Ionic size [Å]	0.9	0.89	0.901	0.868

---

Fig. 2 XRD patterns of samples with different kinds of ion doping.

Fig. 3 Schematic diagram of the crystal structure of the sample doped with different types of ions.

Fig. 4 SEM images of the matrix lattice doped with different kinds of ions: (a) doped Er³⁺ ions, (b) doped Ho³⁺ ions and (c) doped Yb³⁺ ions.

3.2 Modulating effect of the Er³⁺ ion dopant on the upconversion luminescence properties

Er³⁺ ions are often used as activator ions in the 1550 nm band due to their significant absorption of 1550 nm photons.^17,18 Still, they are limited by the cross-relaxation between ions when doped with a high concentration of Er³⁺ ions.^16,19 This has led to the fact that doping with high concentrations of Er³⁺ ions at 1550 nm excitation has been poorly studied.¹⁹ Therefore, the mechanism of the role played by Er³⁺ ions in the upconversion luminescence on the lattice at high doping concentrations is imperfectly studied. Hence, it is necessary to systematically study the mechanism of upconversion luminescence on Er³⁺ ions doped with high concentration.

In this paper, NaYF₄:Er³⁺ upconversion fluoride phosphors with different Er³⁺ ion doping concentrations were synthesized by the LCS method. The upconversion emission spectra were determined for different Er³⁺ ion doping concentrations, as shown in Fig. 5 and 6, and a schematic diagram of the energy transfer between Er³⁺ ion single-doped sample ions is also shown. As can be seen from the figure, the doping concentration of Er³⁺ ions gradually increases. The upconversion luminescence intensity of the sample increased first, then decreased, and then increased and then decreased. And thus, two inflection points of high and low upconversion luminescence intensity are generated, which correspond to the doping concentration of Er³⁺ ions of 12 mol% and 26 mol% respectively. The upconversion emission spectrum of the sample consists of 521 nm for green light, 544 nm for red light and 660 nm for red light. For the green emission of the sample, it corresponds to the ⁴I_15/2 → ²H_11/2 and ⁴I_15/2 → ⁴S_3/2 energy level transitions of Er³⁺ ions, and for the red emission of the sample, it corresponds to the ⁴I_15/2 → ⁴F_9/2 energy level transitions of Er³⁺ ions. For the red emission of the sample, it corresponds to the energy level transitions of the ⁴I_15/2 → ⁴F_9/2 energy levels of Er³⁺ ions. For Er³⁺ ions doped with a high concentration of 26 mol% and low concentration of 12 mol%, the total luminous intensity is increased by 1.62 times, and the red-green ratio is increased by 2.31 times. This is mainly due to the strong response of Er³⁺ ions to 1550 nm photons.⁶ The Er³⁺-1 ions are uniformly distributed in the NaYF₄ matrix lattice for the inflection point appearing at low-concentration doping. The probability of the ions undergoing cross-relaxation is small because of the long energy transfer distance between neighboring Er³⁺ ions.²⁰ In addition, due to the influence of the excitation power, the Er³⁺ ions are in the unsaturated excitation state at this time. With the increase of the Er³⁺ ion doping concentration, the upconversion luminescence intensity of the sample was increased steadily. Afterward, as the doping concentration of Er³⁺ ions continues to be increased, the lattice sites of Y³⁺ ions around the uniformly distributed Er³⁺-1 ions in the matrix lattice are gradually replaced by Er³⁺-2 ions. The energy transfer distance between the Er³⁺-1–Er³⁺-2 ions is reduced,²¹ which results in the probability of cross-relaxation between neighboring ions being dramatically increased. The upconversion luminescence intensity of the sample is gradually reduced because the probability of non-radiative transitions in the sample is increased, and the probability of radiative transitions is decreased. Afterward, the Y³⁺ ion lattice sites around the Er³⁺-1 ions are gradually and uniformly occupied by more Er³⁺-2 ions as the Er³⁺ ion doping concentration continues to increase. At this point, the upconversion luminescence intensity of the samples begins to be gradually increased because the energy transfer effect between the Er³⁺-1–Er³⁺-2 ions begins to outweigh the cross-relaxation between Er³⁺-2–Er³⁺-1 ions. This will make the sample upconversion luminescence intensity begin to increase gradually, so the Er³⁺-1 ions are gradually saturating excited. When the doping concentration of Er³⁺ ions reaches 26 mol%, the energy transfer between Er³⁺-2 ions to Er³⁺-1 ions is greater than the cross-relaxation effect between the ions, reaching the maximum value, and the Er³⁺-1 ions in the energy receptor exhibit completely saturated excitation. Afterward, as the doping concentration of Er³⁺ ions continues to be increased, the cross-relaxation effect between neighboring ions begins to dominate so that the upconversion luminescence intensity of the sample is gradually reduced.^22–24 This process is shown in Fig. 7.

Fig. 5 Upconversion emission spectra from the Er³⁺-doped samples under 1550 nm excitation.

Fig. 6 Emission spectra of Er ions doped with low and high concentration under 1550 nm excitation.

Fig. 7 Schematic diagram of Er³⁺ ion doping energy transfer.

As shown in Fig. 5, with the gradual increase in the doping concentration of Er³⁺ ions, the red light emission of the single-doped samples with Er³⁺ ions was substantially enhanced, and the green light emission was significantly reduced. Therefore, it is necessary to further analyze the upconversion of red emission and green emission of the samples from the perspective of energy level transition.

Fig. 8(a) shows the energy level transition of Er³⁺ ions under 1550 nm excitation, and Fig. 8(b) shows the IP curve under 1550 nm excitation, from which it can be seen that the energy level transition of Er³⁺ ions under 1550 nm excitation is a three-photon summation process. As for the self-absorption green light emission of Er³⁺ ions, the specific process is that the electrons of Er³⁺ ions in the ground state absorb three photons in succession to transit to the ²H_11/2 energy level, after which some of the electrons in the ²H_11/2 energy level directly relax to the ground state and produce green light emission at 525 nm. Secondly, some of the other electrons firstly relax to the ⁴S_3/2 energy level and then return to the ground state by the radiative transition process, producing green light emission of 540 nm.

Fig. 8 (a) Energy level transition of Er³⁺ ions under 1550 nm excitation; (b) IP curve of Er³⁺ ions under 1550 nm excitation.

The reason for the self-absorbed red light emission for Er³⁺ ions is specified as follows. The electrons in the ground state of Er³⁺ ions absorb two photons consecutively to transit to the ⁴I_9/2 energy level. Next, these electrons relax to the ⁴I_11/2 energy level and then they absorb another photon and transit to the ⁴F_9/2 energy level. Finally, these electrons relax to the ground state and produce red light emission at 660 nm. Meanwhile, the electrons of the Er³⁺ ions in the ground state absorb three photons directly to transit to the ²H_11/2 energy level. Then they first relax to the ⁴S_3/2 energy level and then relax to the ⁴F_9/2 energy level. They eventually return to the ground state by the radiation transition process and produce the red light emission of the Er³⁺ ions.^6,19,25,26

In summary, the Er³⁺ ions do not play a single role in the upconversion luminescence of the matrix lattice in the concentration-sequence doping experiments of the Er³⁺ ions. At low-concentration doping, the Er³⁺ ions mainly play the role of the energy acceptor of unsaturated excitation and the luminescence center of the matrix lattice, which leads to the first inflection point of concentration sequence doping. When the doping concentration of Er³⁺ ions is increased, some of the Er³⁺ ions gradually play the roles of energy carriers and energy transfer centers. However, the other part of the Er³⁺ ions play the role of an energy acceptor and saturation excitation center. This creates a second inflection point in the ion concentration doped series of samples. Therefore, we will use the saturation excitation of the Er³⁺ ion concentration as the basis for subsequent studies.

3.3 Modulating effect of the Er³⁺–Ho³⁺ ion dopant on the upconversion luminescence properties

Currently, studies on the 1550 nm band are mainly focused on the Er³⁺ ion single-doped samples, and there are fewer studies on the response of other ions to the 1550 nm band.^27–29 However, in recent years, it has been found that in the matrix lattice doped with Er³⁺ ions, the upconversion luminescence intensity of the samples can be greatly enhanced by doping with an appropriate amount of other ions.^29,30 In this section, based on the above Er³⁺ ion doping of 26 mol%, the effect of this ion doping on the upconversion luminescence performance of the samples was investigated. As shown in Fig. 9(a), the upconversion luminescence intensity of the samples increased and then decreased when Ho³⁺ ions were doped into the matrix lattice of single-doped Er³⁺ ions. When the doping concentration of Ho³⁺ ions is 0.2 mol%, the upconversion luminescence intensity of the Er³⁺–Ho³⁺ ion co-doped system reaches the optimum, which is enhanced 1.23 times compared with the undoped Ho³⁺ ion sample, as shown in Fig. 9(b). When the Ho³⁺ ion doping concentration is further increased, the red light emission of the samples gradually dominated, and the green light emission of the samples was significantly quenched. This phenomenon is mainly caused by the different roles played by Ho³⁺ ions in the Er³⁺–Ho³⁺ co-doped system.

Fig. 9 (a) Upconversion emission spectra of Er³⁺–Ho³⁺ co-doped samples under 1550 nm excitation. (b) Upconversion emission spectra of the optimally doped sample with Ho³⁺ ions and undoped sample with Ho³⁺ ions.

The intrinsic mechanism of upconversion luminescence enhancement and energy transfer is analyzed in more detail. It is first necessary to determine whether the Ho³⁺ ions can act as an independent 1550 nm response upconversion absorption and emission center in the Er³⁺–Ho³⁺ ion co-doped system. Fig. 10(a) is the upconversion emission spectrum of the Ho³⁺ ion single-doped and Ho³⁺–Yb³⁺ ion co-doped samples under 1550 nm excitation. Fig. 10(b) shows the absorption spectra of mono-doped with Ho³⁺ ions and mono-doped samples with different concentrations of Er³⁺ ions. As can be seen from the figure, 1550 nm photons are significantly absorbed by the Ho³⁺ ion single-doped samples. The upconversion luminescence of the samples is composed of red and green emission, and the total luminescence intensity is dominated by red emission. For the Ho³⁺–Yb³⁺ ion co-doped samples, the luminescence of the samples still consists of red and green emission, whereas the total luminescence intensity is still dominated by red emission. Both the red and green light emission of the sample is reduced by doping with Yb³⁺ ions compared to the single-doped samples with Ho³⁺ ions. However, since there are almost no studies on the 1550 nm band that can be well responded to by other lanthanide activator ions, the current mechanism of upconversion energy level transition of Ho³⁺ ions under 1550 nm excitation is not well developed. In this section, the energy of each spectral term of Ho³⁺ ions was determined by reviewing the relevant literature, as shown in Table 2. After that, the energy difference between each spectral term was calculated: ΔE₁ = 5049 cm⁻¹ (⁵I₈ → ⁵I₇), ΔE₂ = 6061 cm⁻¹ (⁵I₇ → ⁵I₅), ΔE₃ = 4264 cm⁻¹ (⁵F₅ → ⁵I₅), ΔE₄ = 5289 cm⁻¹ (⁵F₄ → ⁵I₄).³¹ By comparing with the energy carried by 1550 nm photons (ΔE_{1550 nm} = 6452 cm⁻¹) and combining with the IP curves of 1550 nm excitation of single-doped Ho³⁺ ions, as shown in Fig. 11(a), the upconversion energy level transition diagrams of the Ho³⁺ ions excited at 1550 nm were determined, as shown in Fig. 11(b). The absorption of 1550 nm photons by Ho³⁺ ions makes Ho³⁺ ions play more roles in the Er³⁺–Ho³⁺ ion co-doping system. Firstly Ho³⁺ ions can be self-absorbing emission centers in the matrix lattice. Since the characteristic emissions of Er³⁺ ions and Ho³⁺ ions are highly overlapped in the visible region,²⁹ the upconversion emission spectra of Er³⁺–Ho³⁺ ion co-doped samples are generated by the superposition of the characteristic emissions of both, which enhances the upconversion luminescence of the samples.²⁹ Secondly, Ho³⁺ ions can also act as a reverse energy transfer center in the matrix lattice. The photon energy absorbed by the Er³⁺ ions is first temporarily stored by the Ho³⁺ ions. Then, the energy is transferred back to the Er³⁺ ions again by the reverse energy transfer process. Through this process, some of the Er³⁺ ions are excited twice, and the upconversion luminescence of the sample is enhanced.^28,32 Finally, the Ho³⁺ ions can also act as an energy receptor for the Er³⁺ ions. The energy transferred from the Er³⁺ ions is absorbed by these Ho³⁺ ions and produces their characteristic emission, causing the upconversion luminescence of the Er³⁺–Ho³⁺ ion co-doped sample to be to enhanced.

Fig. 10 (a) Upconversion emission spectra of the single-doped sample with Ho³⁺ ions and co-doped sample with Ho³⁺–Yb³⁺ under 1550 nm excitation. (b) Absorption spectra of the samples singly doped with Ho³⁺ ions and singly doped with Er³⁺ ions.

Table 2 Energy difference of each spectral term for Ho³⁺ ions

Energy level	^5I7	^5I6	^5I5	^5I4	^5F5	^5S2	^5F4
ΔE (cm^−1)	5049	8550	11 110	13 155	15 374	18 325	18 444

---

Fig. 11 (a) IP curves of singly doped samples with Ho³⁺ ions under 1550 nm excitation. (b) Upconversion energy level transition diagram of Ho³⁺ ions under 1550 nm excitation.

In summary, combining the upconversion emission spectra of the samples of the Er³⁺–Ho³⁺ ion co-doped system, it can be seen that the addition of the green emission of Er³⁺ ions and Ho³⁺ ions in the visible region produces the upconversion green emission of the samples. Fig. 12(a) shows the energy level transition of the Er³⁺–Ho³⁺ ion co-doped sample under 1550 nm excitation, and Fig. 12(b) shows the IP curve of the Er³⁺–Ho³⁺ ion co-doped sample under 1550 nm excitation. It can be seen that the energy level transition of the Er³⁺–Ho³⁺ ion co-doping system under 1550 nm excitation is a three-photon summation process. For the green emission of Er³⁺ ions of the samples, due to the doping of Ho³⁺ ions, the green emission pathway of Er³⁺ ions is increased by the reverse energy transfer process between Er³⁺–Ho³⁺ ions. The process is that the electrons of the Er³⁺ ions in the ⁴S_3/2 energy level transfer to the neighboring Ho³⁺ ions by the energy transfer process. Then, these electrons are stored on the ⁵F₄ energy level of the Ho³⁺ ions, after which they return to the ⁴S_3/2 energy level of the Er³⁺ ions by the reverse energy transfer process. Finally, these electrons relax to the ground state and produce green light at 544 nm.^28,29,33

Fig. 12 (a) Energy level transition of Er³⁺–Ho³⁺ ions co-doped under 1550 nm excitation. (b) IP curve of Er³⁺–Ho³⁺ ions co-doped under 1550 nm excitation.

As for the upconversion green light emission of Ho³⁺ ions, it mainly consists of three parts, the first part is the self-absorption green light emission of Ho³⁺ ions, and the second part is the green light emission generated by the reverse energy transfer process between Ho³⁺–Er³⁺ ions. The third is the green light emission generated by the energy transfer process between Er³⁺–Ho³⁺.

For the self-absorbed green light emission of Ho³⁺ ions, the main process is that the electrons of the Ho³⁺ ions in the ground state absorb three photons consecutively to transit to the ⁵F₅ energy level, after which they relax to the ⁵I₄ energy level. Then these electrons absorb another photon to transit to the ⁵F₄ energy level. Finally, they return to the ground state by a radiative transition process and produce the green light emission of the Ho³⁺ ions at 544 nm, and this process is shown in Fig. 13(a).

Fig. 13 (a) Schematic diagram of the reverse energy transfer of Er³⁺–Ho³⁺ ions. (b) Schematic diagram of the energy transfer of Er³⁺–Ho³⁺ ions.

For the green light emission of the Ho³⁺ ions generated by the reverse energy transfer process between Ho³⁺–Er³⁺, the specific process is that the electrons in the ⁵F₅ energy level of the Ho³⁺ ions transfer to the ⁴F_9/2 energy level of the neighboring Er³⁺ ions by an energy transfer process. These electrons return to the ⁵F₅ energy level of the Ho³⁺ ions by the reverse energy transfer process and further relax to the ⁵I₄ energy level. Next these electrons absorb a photon to transit to the ⁵F₄ energy level. Finally, they return to the ground state by the radiative transition process and produce green light emission of the Ho³⁺ ions. On the other hand, the electrons in the ⁵F₄ energy level of the Ho³⁺ ions can also be transferred to the ⁴S_3/2 energy level of the neighboring Er³⁺ ions through an energy transfer process and return to the ⁵F₄ energy level of the Ho³⁺ ions by the reverse energy transfer process. They eventually relax to the ground state and produce green light emission of the Ho³⁺ ions, and this process is shown in Fig. 13(a).^28,29

The reason for the green light emission generated by the energy transfer process between Er³⁺–Ho³⁺ ions is specified as follows, the electrons in the ⁴I_13/2 energy level of Er³⁺ ions transfer to the neighboring Ho³⁺ ions by the energy transfer process, keeping these electrons in the ⁵I₇ energy level of Ho³⁺ ions. Meanwhile, the electrons at the ⁵I₇ energy level absorb two photons consecutively to transit to the ⁵F₅ energy level, and they relax to the ⁵I₄ energy level. And then, these electrons absorb a photon to transit to the ⁵F₄ energy level. Finally, they return to the ground state by the radiative transition process and produce green light emission of Ho³⁺ ions, and this process is shown in Fig. 13(b).^28,29,34

For the Er³⁺–Ho³⁺ ion co-doped system under 1550 nm excitation, the red light emission is also generated by summating the red light emission of Er³⁺ ions and the Ho³⁺ ions in the visible region. For the red emission of Er³⁺ ions of the samples, due to the doping of Ho³⁺ ions, the red emission pathway of Er³⁺ ions is increased by the reverse energy transfer process between Er³⁺–Ho³⁺ ions. The specific process is that the electrons of the Er³⁺ ions in the ⁴S_3/2 energy level transfer to the neighboring Ho³⁺ ions by the energy transfer process, and these electrons return to the ⁴S_3/2 energy level of the Er³⁺ ions by the reverse energy transfer process. Then these electrons relax to the ⁴F_9/2 energy level. Finally, they return to the ground state by the radiative transition process and produce red light emission of the Er³⁺ ions. On the other hand, the electrons in the ⁴F_9/2 energy level of the Er³⁺ ions can also transfer to the ⁵F₅ energy level of the neighboring Ho³⁺ ions by the energy transfer process, and these electrons return to the ⁴F_9/2 energy level by the reverse energy transfer process. Finally, they relax to the ground state and produce the red light emission of the Er³⁺ ions, and this process is shown in Fig. 13(a).^28,29,35

As for the upconversion red light emission of Ho³⁺ ions, it mainly consists of three parts, the first is the self-absorption red light emission of Ho³⁺ ions, and the second is the red light emission generated by the reverse energy transfer process between Ho³⁺–Er³⁺ ions. The third is the red light emission generated by the energy transfer process between Er³⁺–Ho³⁺ ions.

For the self-absorbed red light emission of Ho³⁺ ions, the main process is that the electrons of the Ho³⁺ ions in the ground state absorb three photons consecutively to transit to the ⁵F₅ energy level. Then they immediately relax to the ground state and produce the red light emission of the Ho³⁺ ions. On the other hand, the electrons of the Ho³⁺ ions in the ⁵F₅ energy level relax to the ⁵I₄ energy level, and absorb another photon to transit to the ⁵F₄ energy level. Finally, they return to the ground state by a radiative transition process and produce the red light emission of the Ho³⁺ ions.

For the red light emission of the Ho³⁺ ions generated by the reverse energy transfer process between Ho³⁺–Er³⁺, the specific process is that the electrons in the ⁵F₅ energy level of the Ho³⁺ ions can transfer to the ⁴F_9/2 energy level of the neighboring Er³⁺ ions by an energy transfer process. After that, these electrons return to the ⁵F₅ energy level of the Ho³⁺ ions by the reverse energy transfer process. Then, they return to the ground state by the radiative transition process and produce red light emission of the Ho³⁺ ions, and this process is shown in Fig. 13(a).^28,29,35

The reason for the red light emission generated by the energy transfer process between Er³⁺–Ho³⁺ ions is specified as follows. The electrons in the ⁴I_13/2 energy level of Er³⁺ ions transfer to the neighboring Ho³⁺ ions by the energy transfer process, keeping these electrons in the ⁵I₇ energy level of Ho³⁺ ions. Then, these electrons at the ⁵I₇ energy level absorb two photons consecutively to transit to the ⁵F₅ energy level. They relax to the ground state and produce red light emission of Ho³⁺ ions. On the other hand, the electrons of the Er³⁺ ions in the ⁴F_9/2 energy level transfer to the ⁵F₅ energy level of the neighboring Ho³⁺ ions by an energy transfer process, and then they return to the ground state by a radiative transition process and produce the red light emission of the Ho³⁺ ions; this process is shown Fig. 13(b).^28,29,35

Fig. 14 shows the fluorescence lifetime decay curves of the Er³⁺–Ho³⁺ ion co-doped samples under 980 nm excitation. The graph shows that the co-doped Er³⁺–Ho³⁺ will lead to a certain degree of enhancing the fluorescence lifetime of 540 nm. This is mainly due to the doping of Ho³⁺ ions, which makes the electrons on the ²H_11/2 and ⁴S_3/2 energy levels of the Er³⁺ ions sustain a longer period. This decreases the chance of electrons at this energy level returning to the ground state. Therefore, the fluorescence weakening of 540 nm will appear in the upconversion emission spectrum. On the contrary, doping the Er³⁺ ions with the appropriate amount of Ho³⁺ ions will result in a substantial enhancement of the red emission of the samples. And the fluorescence lifetime of 660 nm is reduced after being doped with Ho³⁺ ions. This is mainly due to many electrons preferentially returning to the ground state and producing the red light emission of 660 nm. Therefore, all these factors increase the red lifetime of Er³⁺–Ho³⁺ ion co-doped samples. The reduction of 660 nm lifetime also corroborates the significant enhancement of the red emission presented by the emission spectrum.³⁶

Fig. 14 Fluorescence lifetime of the sample when excited in the 980 nm band.

The fluorescence decay curves are experimental evidence of energy transfer between rare earths. In order to further clarify the energy transfer mechanism of Ho³⁺ ion doping, we calculated the energy transfer efficiency of Er³⁺ ions co-doped with Er–Ho samples as shown in eqn (3):

where τ_Er–Ho is the fluorescence lifetime of Er³⁺ ions in the Er³⁺–Ho³⁺ co-doped sample, and τ _Er is the fluorescence lifetime of Er³⁺ ions in the Er ion mono-doped sample. From the above equations and combining the fluorescence lifetimes of the Er³⁺ ion mono-doped with the Er³⁺–Ho³⁺ co-doped at 540 nm and 660 nm, it can be seen that the energy transfer efficiency of the sample is in the range of 5.99–8.53%. The above results indicate that there is a cross-relaxation process between Er³⁺ ions and Ho³⁺ ions, which leads to a positive trend in the population size. This leads to a positive trend in population accumulation at the ⁴F_9/2 energy level of the Er³⁺ ions, which promotes the emission of the sample at 660 nm.

Since in the Er³⁺–Ho³⁺ ion co-doped system, the doping amount of Ho³⁺ ions is extremely small compared to the optimal doping concentration of Er³⁺ ions, the contribution of the characteristic emission of Ho³⁺ ions to the upconversion luminescence of the sample is extremely weak. Therefore, the upconversion luminescence of the sample is still dominated by the luminescence of Er³⁺ ions. As compared to the increase in the green emission pathway of the Er³⁺–Ho³⁺ ion co-doped sample, the red light emission pathway is significantly more than the green light emission. Due to the combined effect of cross-relaxation and energy transfer between the ions, the chance of the electron population to the green light energy level is greatly reduced. So the green light emission of the Er³⁺–Ho³⁺ co-doped samples is significantly quenched, while the red light emission is substantially enhanced.

3.4 Modulating the effect of Er³⁺–Ho³⁺–Yb³⁺ ion dopants on the upconversion luminescence properties

In the above study, the role played by Ho³⁺ ions in the Er³⁺–Ho³⁺ co-doping system and the enhancement of upconversion luminescence was investigated. Afterward, it was found that the up-conversion luminescence of the samples doped with Er³⁺ ions was significantly enhanced after doping with Yb³⁺ ions. Combined with the above Ho³⁺–Yb³⁺ ion co-doping experiment, the doping of Yb³⁺ ions can make the Ho³⁺ ions continue to produce the characteristic red and green light emission, but the upconversion luminescence of Ho³⁺ ions is reduced under the excitation at 1550 nm. This may be due to the large energy difference between the ²F_5/2 energy level of the Yb³⁺ ion and the ⁵I₆ energy level of the Ho³⁺ ions.³⁷ The reverse energy transfer process between the Ho³⁺–Yb³⁺ ion was difficult to occur. Therefore, the doping of Yb³⁺ ions depletes the energy absorbed by Ho³⁺ ions, the upconversion luminescence intensity of the Ho³⁺–Yb³⁺ co-doped system was reduced. Consequently, in this section, the appropriate amount of Yb³⁺ ions based on the optimal doping concentration of Er³⁺–Ho³⁺ ions mentioned above was doped, and the role of Yb³⁺ ions in the multi-ion co-doped system and the effect on the upconversion luminescence performance of the samples was investigated.

Fig. 15 shows the upconversion emission spectra of the Er³⁺–Ho³⁺–Yb³⁺ ion triple-doped samples under 1550 nm excitation. As can be seen from the figure, as the doping concentration of Yb³⁺ ions was increased, the upconversion luminescence intensity of the samples increases firstly and then decreases. When the Yb³⁺ ion doping concentration is 1 mol%, the upconversion luminescence intensity of the sample reaches the best value. The red and green emission intensities of the samples are enhanced to a certain extent after the doping of Yb³⁺ ions in the matrix lattice. Since the photons of 1550 nm are not absorbed by Yb³⁺ ions, the photons in the Er³⁺–Ho³⁺–Yb³⁺ ion doped samples are mainly accomplished by Er³⁺ ions, and a small portion of the photons are absorbed by Ho³⁺ ions.^29,30 By reviewing the relevant literature and combining the experimental phenomena, it can be seen that Yb³⁺ ions for the Er³⁺–Ho³⁺–Yb³⁺ triple-doped system of upconversion luminescence mainly plays the role of a reverse energy transfer center of Er³⁺–Yb³⁺ ions. This process means that the utilization rate of the Er³⁺ ions for the photons is increased so that the intensity of the upconversion luminescence is significantly increased. The energy transfer between the Ho³⁺–Yb³⁺ ions leads to rapid energy depletion, so the upconversion luminescence intensity of the Ho³⁺–Yb³⁺ co-doped samples is significantly quenched.

Fig. 15 (a) Upconversion emission spectra of Er³⁺–Ho³⁺–Yb³⁺triple-doped samples under 1550 nm excitation. (b) Upconversion emission spectra of the optimally doped sample with Yb³⁺ ions and the undoped sample with Yb³⁺ ions.

Fig. 16(a) shows the energy level transition of the Er³⁺–Ho³⁺–Yb³⁺ion triple-doped sample under 1550 nm excitation, and Fig. 16(b) shows the IP curve of the Er³⁺–Ho³⁺–Yb³⁺ion triple-doped sample under 1550 nm excitation, from which it can be seen that the energy level transition of the Er³⁺–Ho³⁺–Yb³⁺ion triple-doped system under 1550 nm excitation is a three-photon summation process. For the Er³⁺–Ho³⁺–Yb³⁺ triple-doped system under 1550 nm excitation, the increased green emission pathway due to the doping of Yb³⁺ ions mainly consists of the green emission of Er³⁺ ions. The specific process is that the electrons of the Er³⁺ ions in the ground state absorb two photons in succession to transit to the ⁴I_9/2 energy level, after which they relax to the ⁴I_11/2 energy level. Then, these electrons transfer to the neighboring Yb³⁺ ions by the energy transfer process and keep the electrons in the ²F_5/2 energy level of the Yb³⁺ ions. And then, these electrons return to the neighboring Er³⁺ ions by a reverse energy transfer process, making the electrons in the ⁴I_11/2 energy level directly transit to the ⁴F_7/2 energy level, after which these electrons relax to the ²H_11/2 energy level and the ⁴S_3/2 energy level. Finally, they return to the ground state by the radiative transfer process and produce green light emission of the Er³⁺ ions.^38–40

Fig. 16 (a) Energy level transition of Er³⁺–Ho³⁺–Yb³⁺ion triple-doping under 1550 nm excitation. (b) IP curve of Er³⁺–Ho³⁺–Yb³⁺ion triple-doping under 1550 nm excitation.

For 1550 nm excitation in the Er³⁺–Ho³⁺–Yb³⁺ triple-doped system, the red light emission pathway is increased due to the doping of Yb³⁺ ions. This mainly originates from the reverse energy transfer process of Er³⁺–Yb³⁺ ions. The specific process is that the electrons of the Er³⁺ ions in the ground state continuously absorb two photons to transit to the ⁴I_9/2 energy level, and then they relax to the ⁴I_11/2 energy level. Then these electrons are transferred to the ²F_5/2 energy level of the neighboring Yb³⁺ ions by the energy transfer process, and afterward they return to the ⁴I_11/2 energy level of the Er³⁺ ions by the reverse energy transfer process. These electrons absorb a photon to transit to the ⁴F_9/2 energy level, and they eventually return to the ground state by the radiation transition process and produce red light emission of the Er³⁺ ions. Finally, the electrons at the ²H_11/2 energy level with the electrons on the ⁴I_11/2 energy level of the Er³⁺ ions can undergo a cross-relaxation process, making the electrons at the ⁴I_11/2 energy level transit to the ⁴F_9/2 energy level, after which these electrons return to the ground state by the radiation transition process and produce the red light emission of the Er³⁺ ions.^36,41–43Fig. 17 shows the schematic diagram of the reverse energy transfer between Yb³⁺ ions and Er³⁺ ions and Ho³⁺ ions. Through the above experimental results, we only observed the reverse energy transfer process between Er³⁺–Yb³⁺ ions; the reverse energy transfer phenomenon between Ho³⁺–Yb³⁺ ions did not occur. Therefore, for the Er³⁺–Yb³⁺ ion pair, the up-conversion luminescence intensity of the sample is significantly enhanced because the process can significantly increase the utilization rate of Er³⁺ ions for 1550 nm photons.^44–53 Since we do not observe the reverse energy transfer between Ho³⁺–Yb³⁺ ion pairs, the up-conversion luminescence intensity of the samples was significantly quenched due to the cross-relaxation between the Ho³⁺–Yb³⁺ ions. The schematic diagram of the reverse energy transfer between the ions in Fig. 17 can well summarize the process of the reverse energy transfer between the Er³⁺–Ho³⁺–Yb³⁺ ions.

Fig. 17 Schematic diagram of reverse energy transfer of Er³⁺–Yb³⁺ ions.

In summary, for the upconversion luminescence of the Er³⁺–Ho³⁺–Yb³⁺ multi-ion co-sensitized samples, the upconversion red and green emissions of the samples are generated by the summation of the characteristic emissions of the Er³⁺ ions and Ho³⁺ ions in the visible region. Since the doping concentration of Er³⁺ ions is much larger than that of Ho³⁺ ions, the contribution to the upconversion luminescence intensity of the triple-doped Er³⁺–Ho³⁺–Yb³⁺ system is still dominated by Er³⁺ ions. The Ho³⁺ ions play the energy transfer capture center role in the upconversion luminescence of the samples. Meanwhile, they may also be part of the self-absorption activation center and the energy receptor luminescence center of the Er³⁺ ions. The doping of Yb³⁺ ions mainly plays the role of the reverse energy transfer center between Er³⁺–Yb³⁺ ion pairs. Although the doping of Yb³⁺ ions in the Ho³⁺–Yb³⁺ co-doped samples significantly reduces the up-conversion luminescence intensity of the samples, the doping concentration of Ho³⁺ ions in the Er³⁺–Ho³⁺–Yb³⁺ triple-doped system is very small. Therefore, Yb³⁺ ions have little effect on the characteristic emission of Ho³⁺ ions in the triple-doped samples. Meanwhile, Ho³⁺ ions and Yb³⁺ ions can act as energy transient capture centers and reverse energy transfer centers in the matrix lattice structure, respectively. And the energy that should be dissipated in the energy transfer and electron transition process will be stored temporarily, which increases the ions’ utilization rate for photons. Absorption of 1550 nm photons by Ho³⁺ ions results in spectral addition. In summary, these factors can enhance the upconversion emission spectral intensity of the samples.

4. Conclusions

Er³⁺–Ho³⁺–Yb³⁺ triple-doped fluoride up-conversion phosphors were successfully prepared by the low-temperature combustion synthesis method. It is found that in the series of doping concentrations of Er³⁺ ions under 1550 nm excitation, when the doping concentration of Er³⁺ ions is 12 mol%, due to the limitation of excitation power, Er³⁺-1 ions mainly play the role of unsaturated excitation centers in the matrix lattice. When the doping concentration of Er³⁺ ions continues to increase until it reaches 26 mol%, Er³⁺-2 ions will be uniformly distributed around the Er³⁺-1 ions, playing the role of a sensitizer in the matrix lattice, and saturation excitation of the Er³⁺-1 ions will significantly enhance the upconversion of the luminescence intensity. The upconversion luminescence intensity of the Er³⁺–Ho³⁺ co-doped samples is enhanced when a small amount of Ho³⁺ ions are doped into the matrix lattice. The upconversion luminescence intensity of the samples reaches the optimal value when the doping concentration of Ho³⁺ ions is 0.2 mol%, and the doping of Ho³⁺ ions substantially enhances the red emission of the samples and bursts the green light emission. At this time, Ho³⁺ ions can firstly play the roles of transient energy capture centers and reverse energy transfer centers in the matrix lattice, and secondly also act as the self-absorption luminescence center in the matrix lattice. The upconversion luminescence intensity of the Er³⁺–Ho³⁺–Yb³⁺ triple-doped system reaches the optimal value when 1 mol% of Yb³⁺ ions is doped in the Er³⁺–Ho³⁺ co-doped sample. At this time, Yb³⁺ ions mainly play the role of the reverse energy transfer center between Er³⁺–Yb³⁺ ion pairs in the matrix lattice. This study has significant applications in enhancing the upconversion luminescence of the samples and displaying high color purity in the 1550 nm band.

Author contributions

Bohan Lei: conceptualization, data curation, formal analysis, investigation, writing original draft. Liping Lu: supervision, writing – review & editing. Haiying Sun: formal analysis, investigation.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the Application Innovation Program of the Equipment pre-research (627011103) and the Jilin Province Education Department Project (grant no. JJKH20200760KJ).

Notes and references

1. X. Zhu, Q. Su and W. Feng, et al., Anti-Stokes shift luminescent materials for bio-applications, Chem. Soc. Rev., 2017, 46(4), 1025–1039.
2. N. Li, N. Eedugurala and D. S. Leem, et al., Organic upconversion imager with dual electronic and optical readouts for shortwave infrared light detection, Adv. Funct. Mater., 2021, 31(16), 2100565.
3. L. Gao, X. Shan and X. Xu, et al., Correction: Video-rate upconversion display from optimized lanthanide ion doped upconversion nanoparticles, Nanoscale, 2020, 12(36), 18987.
4. F. Jia, G. Li and B. Yang, et al., Investigation of rare earth upconversion fluorescent nanoparticles in biomedical field, Nanotechnol. Rev., 2019, 8(1), 1–17.
5. W. Wang, J. Tian and J. Dong, et al., Growth, spectroscopic properties and up-conversion of Yb, Pr co-doped CaF₂ crystals, J. Lumin., 2021, 233, 117931.
6. H. N. Luitel, S. Mizuno and Y. Takeda, CaTiO₃:Er³⁺, Ni²⁺ broadband-sensitive upconverter: an effective way to harvest unused NIR solar irradiation for crystalline silicon solar cells, Phys. Status Solidi A, 2017, 214(8), 1600899.
7. Z. Li, H. Yuan and W. Yuan, et al., Upconversion nanoprobes for biodetections, Coord. Chem. Rev., 2018, 354, 155–168.
8. X. Yin, W. Hong and X. Mingming, et al., Upconversion luminescence of Y₂Ti₂O₇:Er³⁺ under 1550 and 980 nm excitation, J. Rare Earths, 2017, 35(3), 230–234.
9. N. Y. Mostafa, A. Badawi and S. I. Ahmed, Influence of Cu and Ag doping on structure and optical properties of In₂O₃ thin film prepared by spray pyrolysis, Results Phys., 2018, 10, 126–131.
10. H. Bian, Y. Liu and D. Yan, et al., Light-induced electrons suppressed by Eu³⁺ ions doped in Ca_11.94−xSr_xAl₁₄O₃₃ caged phosphors for LED and FEDs, J. Am. Ceram. Soc., 2017, 100(8), 3467–3477.
11. C. G. Pope, X-ray diffraction and the Bragg equation, J. Chem. Educ., 1997, 74(1), 129.
12. Y. Liu, Y. Liu and M. G. B. Drew, Comparison of calculations for interplanar distances in a crystal lattice, Crystallogr. Rev., 2017, 23(4), 252–301.
13. K. N. Nicholson and S. A. Wood, Aqueous Geochemistry of Rare Earth Elements and Yttrium. XII: Potentiometric Stability Constant Determination of Bis -Tris Complexes with La, Nd, Eu, Gd, Yb, Dy, Er, Lu, and Y, J. Solution Chem., 2002, 31(9), 703–717.
14. S.-O. Yoon, C.-B. Hong and S. Kim, Microwave dielectric properties of Na⁺ and M³⁺-doped Ba(Mg_0.5W_0.5)O₃ ceramics, J. Electroceram., 2018, 41, 16–22.
15. A. Ghafoor, M. A. Khan and M. U. Islam, et al., Structural and electromagnetic studies of Ni_0.7Zn_0.3Ho_2xFe_2−2xO₄ ferrites, Ceram. Int., 2016, 42(12), 14252–14256.
16. W. S. Silveira and A. J. S. Silva, Nascimento P a M D, et al. Improving the luminescence properties of YAG:Ce³⁺ phosphors by co-doping Sr²⁺ ions, Optik, 2021, 231, 166363.
17. Y. Chen, F. Peng and Q. Zhang, et al., Growth, structure and spectroscopic properties of 1 at% Er³⁺:GdTaO₄ laser crystal, J. Lumin., 2017, 192, 555–561.
18. M. Li, S. Sun and L. Zhang, et al., Growth and spectral properties of a promising laser crystal Yb³⁺/Er³⁺:Ca₉La(VO₄)₇, J. Cryst. Growth, 2016, 451, 52–56.
19. C. Xu, Q. Yang and G. Ren, et al., Pure red upconversion emission from Yb₃Al₅O₁₂ phase doped with high Er³⁺ concentration, J. Alloys Compd., 2010, 503(1), 82–85.
20. H. Krzyżanowska, Y. Fu and K. S. Ni, et al., Efficient Energy Transfer between Si Nanostructures and Er Located at a Controlled Distance, ACS Photonics, 2016, 3(4), 564–570.
21. N. V. Kononets, V. V. Seminko and P. O. Maksimchuk, et al., Processes of energy migration in mixed europium–lanthanum magnesium borate nanocrystals, Spectrosc. Lett., 2017, 50(7), 399–403.
22. H. Hanchang, Z. Yanyi and L. Mingchen, et al., The effect of Er³⁺ concentration on the kinetics of multiband upconversion in NaYF₄:Yb/Er microcrystals, Front. Chem., 2023, 11, 1097250.
23. L. Yan, B. Zhou and N. Song, et al., Self-sensitization induced upconversion of Er³⁺ in core–shell nanoparticles, Nanoscale, 2018, 10(37), 17949–17957.
24. C. Lee, et al., Origin of strong red emission in Er³⁺-based upconversion materials: role of intermediate states and cross relaxation, Phys. Chem. Chem. Phys., 2019, 21(43), 24026–24033.
25. A. R. Hong, J.-H. Kyhm and G. Kang, et al., Orthogonal R/G/B Upconversion Luminescence-based Full-Color Tunable Upconversion Nanophosphors for Transparent Displays, Nano Lett., 2021, 21(11), 4838–4844.
26. G. A. Kumar, M. Pokhrel and D. K. Sardar, Intense visible and near infrared upconversion in M₂O₂S:Er (M = Y, Gd, La) phosphor under 1550 nm excitation, Mater. Lett., 2012, 68, 395–398.
27. W. Wang, Z. Feng and B. Li, et al., Er³⁺ self-sensitized nanoprobes with enhanced 1525 nm downshifting emission for NIR-IIb in vivo bio-imaging, J. Mater. Chem. B, 2021, 9(12), 2899–2908.
28. Y. Bowen, G. Linna and L. Tiesheng, Nearly pure red up-conversion emission of Ba₄Bi₃F₁₇:Ln³⁺ with 1550 nm wavelength excitation by controlling the doping ions, Opt. Mater., 2022, 125, 112076.
29. X. Cheng, H. Ge and Y. Wei, et al., Design for Brighter Photon Upconversion Emissions via Energy Level Overlap of Lanthanide Ions, ACS Nano, 2018, 12(11), 10992–10999.
30. X. Cheng, Y. Pan and Z. Yuan, et al., Er³⁺ Sensitized Photon Upconversion Nanocrystals, Adv. Funct. Mater., 2018, 28(22), 1800208.
31. A. Y. Freidzon, I. A. Kurbatov and V. I. Vovna, Ab initio calculation of energy levels of trivalent lanthanide ions, Phys. Chem. Chem. Phys., 2018, 20(21), 14564–14577.
32. Y. Wei, C. Su and H. Zhang, et al., Color-tunable up-conversion emission from Yb³⁺/Er³⁺/Tm³⁺/Ho³⁺ codoped KY(MoO₄)₂ microcrystals based on energy transfer, Ceram. Int., 2016, 42(4), 4642–4647.
33. X. Zhou, G. Ju and T. Dai, et al., Investigation of new color-tunable up-conversion phosphors and their long-persistent luminescence properties for potential biomedical applications, Appl. Phys. A: Mater. Sci. Process., 2019, 125, 1–8.
34. K. Lemański, R. Pązik and P. J. Dereń, Efficient up-conversion emission and energy transfer in LaAlO₃ doped with Er³⁺, Ho³⁺, and Yb³⁺ ions, Opt. Mater., 2012, 34(12), 1990–1993.
35. S. Lu, S. Yao and Q. Chen, et al., Intense red upconversion emission and energy transfer in Yb³⁺/Ho³⁺/Er³⁺:CaYAlO₄, J. Lumin., 2018, 196, 36–39.
36. S. Balaji, D. Ghosh and K. Biswas, et al., Insights into Er³⁺ ↔ Yb³⁺ energy transfer dynamics upon infrared ∼1550 nm excitation in a low phonon fluoro-tellurite glass system, J. Lumin., 2017, 187, 441–448.
37. S. H. Jeong, Y. K. Kshetri and S. H. Kim, et al., Microstructure investigation and multicolor upconversion in Yb³⁺/Ln³⁺(Ln = Er/Tm/Ho) ions doped α-Sialon, Prog. Nat. Sci.: Mater. Int., 2019, 29(5), 549–555.
38. A. A. Lyapin, S. V. Gushchin and S. V. Kuznetsov, et al., Infrared-to-visible upconversion luminescence in SrF₂:Er powders upon excitation of the ⁴I_13/2 level, Opt. Mater. Express, 2018, 8(7), 1863–1869.
39. X. Yin, H. Wang and T. Jiang, et al., Up-conversion luminescence properties and thermal effects of LaVO₄:Er³⁺ under 1550 nm excitation, Mater. Res. Bull., 2017, 86, 228–233.
40. R. Lei, H. Wang and S. Xu, et al., Combustion synthesis and enhanced 1.5 μm emission in Y₂O₃:Er³⁺ powders codoped with La³⁺ ions, J. Rare Earths, 2016, 34(2), 125–129.
41. W. Yu, Y. Tian and M. Xing, et al., Up-conversion luminescence of NaY(WO₄)₂:Yb, Er under 1550 and 980 nm excitation, Mater. Res. Bull., 2016, 80, 223–229.
42. A. A. Lyapin, P. A. Ryabochkina and S. V. Gushchin, et al., Upconversion Luminescence of Fluoride Phosphors SrF₂:Er, Yb under Laser Excitation at 1.5 μm, Opt. Spectrosc., 2018, 125, 537–542.
43. H. Wang, M. Xing and X. Luo, et al., Upconversion emission colour modulation of Y₂O₂S:Yb, Er under 1.55 μm and 980 nm excitation, J. Alloys Compd., 2014, 587, 344–348.
44. Chiho Lee, et al., Origin of strong red emission in Er³⁺-based upconversion materials: role of intermediate states and cross relaxation, Phys. Chem. Chem. Phys., 2019, 21(43), 24026–24033.
45. Hong Wang, et al., Designing Er³⁺ Single-doped ternary sulfide for highly efficient upconversion luminescence under 1550 nm excitation, Chem. Eng. J., 2023, 468, 143558.
46. H. Wang, et al., Upconversion emission colour modulation of Y₂O₂S:Yb, Er under 1.55 μm and 980 nm excitation, J. Alloys Compd., 2014, 587, 344–348.
47. A. Tymiński, R. M. Inocencio and G. Tomasz, Upconversion in Detail: Multicolor Emission of Yb/Er/Tm-Doped Nanoparticles under 800, 975, 1208, and 1532 nm Excitation Wavelengths, Part. Part. Syst. Charact., 2020, 37(8), 2000068.
48. X. Yin, et al., Towards highly efficient NIR II response up-conversion phosphor enabled by long lifetimes of Er³⁺, Nat. Commun., 2022, 13(1), 6549.
49. Wen Xu, et al., Atomic-scale imaging of ytterbium ions in lead halide perovskites, Sci. Adv., 2023, 9(35), eadi7931.
50. Z. Li, G. Zhu and S. Li, et al., High-Performance NIR Emission in Chromium-Doped Garnet Phosphors Enabled by Structure and Excitation Regulation, Laser Photonics Rev., 2024, 18(1), 2300732.
51. Z. Li, G. Zhu and S. Li, et al., Ultra-small Stokes shift induced thermal robust efficient blue-emitting alkaline phosphate phosphors for LWUV WLEDs, Ceram. Int., 2023, 49(13), 21510–21520.
52. L. Yan, M. Xing and Y. Ma, et al., Promising lanthanide-doped double molybdates KYb(MoO₄)₂ phosphors for highly efficient upconversion luminescence and temperature sensing, Spectrochim. Acta, Part A, 2024, 308, 123751.
53. Y. Xu, M. Zou and H. Wang, et al., Upconversion nanoparticles@ single-walled carbon nanotubes composites as efficient self-monitored photo-thermal agents, Spectrochim. Acta, Part A, 2023, 303, 123173.

---