Effect of energy back transfer from Er³⁺ to Yb³⁺ ions on the upconversion luminescence of Er:NaYb(MoO₄)₂ and Yb,Er:NaBi(MoO₄)₂

Abstract

Under the excitation of a 980 nm laser, the visible upconversion (UC) luminescence of Er³⁺ ions doped Yb³⁺ ions self-activated NaYb(MoO₄)₂ phosphor and crystal, as well as the Yb³⁺/Er³⁺ ions codoped NaBi(MoO₄)₂ crystal were investigated comprehensively. The results indicate that all three samples exhibit two significant green emission bands and a weak red emission band in the visible band corresponding to the transitions of ²H_11/2/⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 of Er³⁺ ions, respectively. Through the variable power density spectra of three different samples, the relationship between the energy back transfer (EBT) process of Yb³⁺-Er³⁺ ions and the power density point and Yb³⁺ ion concentration was investigated. The EBT process was observed in both the Er³⁺ ions doped Yb³⁺ ions self-activated NaYb(MoO₄)₂ phosphor and crystal, as confirmed by the luminescence image of the sample. At high power density, the Yb³⁺ ions self-activated sample exhibited yellow luminescence, with the crystal appearing later than the phosphor. In contrast, the NaBi(MoO₄)₂ crystal displayed bright green emission within the measured power density range. In addition, by monitoring the relative intensity change of Yb³⁺ emission in 5 at% Er³⁺:NaYb(MoO₄)₂ crystal, the generation of EBT process in self-activated samples at high power density is more directly explained. These experimental results provide a reliable basis for our comprehensive understanding of the EBT mechanism, and also provide a reliable direction for the final determination of the optimal excitation power density for optical temperature measurement.

Graphical abstract

Keywords

Upconversion luminescence / Energy back transfer / Yb³⁺/Er³⁺ cooping / Phosphor / Crystal

1 Introduction

Upconversion (UC) luminescence is the process of achieving higher energy visible emission photons by continuously absorbing at least two low-energy photons [1]. Rare-earth ions doped UC luminescent materials have attracted the attention of many researchers, including potential applications in biological imaging, solid-state lasers, fluorescent anti-counterfeiting and temperature sensors [1–5]. Of particular interest is the optical temperature sensors based on fluorescence intensity ratio (FIR), which enable non-contact, high-precision and fast temperature measurement [6–8]. The FIR technology is based on the intensity ratio of emission bands, corresponding to the two thermally coupled energy levels of rare earth ions, to realize the detection of temperature-dependent UC luminescence intensity [8]. At present, numerous Yb³⁺ and Er³⁺ ions codoped temperature sensing luminescent materials based on FIR technology have been effectively synthesized, including NaGd(MoO₄)₂ [9], La₄GeO₈ [10], Y₂O₃ [11], Cs₂NaGdCl₆ [12], NaYF₄ [13], among others. Unfortunately, we found that these researchers were not very comprehensive in their discourse on the mechanism of UC luminescence, and even some of the literature neglected to describe the energy back-transfer (EBT) mechanism.

Numerous scholars have extensively investigated the UC luminescent materials codoped with Yb³⁺ and Er³⁺ ions, with a discussion on the UC luminescence mechanism [14–16]. Among them, some literatures also mentioned the EBT mechanism. It is noteworthy that EBT is intricately linked to excitation power, but the excitation power used in the existing optical temperature measurement experiments is not consistent. For example, Chen et al. [10] focused on the optical temperature measurement of Y₂GeO₅:Er³⁺, Yb³⁺ phosphors, revealing a peak absolute sensitivity (S_A) of 1.85 × 10³ K⁻¹ at 473 K (excitation power: 150 mW). Similarly, Liao et al. [17] explored the optical temperature measurement of Gd₂TiO₅: 2 at% Yb³⁺/2 at% Er³⁺ phosphors, identifying a maximum S_A of 40.76 × 10⁴ K⁻¹ at 565 K (excitation power density: 3 W/cm²). Additionally, Li et al. [18] observed that KBaY(MoO₄)₃:Yb³⁺, Er³⁺ phosphors exhibited a peak S_A of 0.01206 K⁻¹ at 420 K (excitation power density: 4.6 W/cm²). EBT represents a crucial mechanism state affecting the layout of Yb³⁺/Er³⁺ ion intermediate states [3, 19]. Regrettably, current literature often only mentions the existence of this mechanism, yet fails to provide an in-depth analysis encompassing aspects such as sample conditions, doped Yb³⁺ ion concentrations, and the correlation between excitation power and EBT.

Consequently, to comprehensively investigate the process of EBT in the UC luminescence mechanism, various samples were synthesized with differing states (phosphors and crystals) and different doping conditions to conduct experiments. The experiment aimed to enhance the understanding of the EBT mechanism, and establish a pathway for the optimum excitation power point for optical temperature measurements. The characteristics of the EBT process between the phosphor (polycrystalline) and the crystal (monocrystalline), that is, the local order and long-range order systems, were analyzed and compared. The impact of Yb³⁺ ion concentration on the EBT process was scrutinized under various conditions including Er³⁺ ions doped Yb³⁺ ions self-activation and Yb³⁺/Er³⁺ ions codoping configurations.

2 Experimental section

The NaYb(MoO₄)₂: 5 at% Er³⁺ phosphor was synthesized by high temperature solid state method, and the crystals of NaYb(MoO₄)₂: 5 at% Er³⁺ and NaBi(MoO₄)₂: 10 at% Yb³⁺, 1 at% Er³⁺ were successfully grown by Czochralski method using a JGD-400 top-mounted single crystal growth furnace (produced by CETC26 th) [20]. The UC emission spectra of the samples were measured using an Omni-λ5028i and a charge coupler (Andor DU401 BVF). The excitation source for the UC emission was a 980 nm semiconductor laser with a maximum output power of 10 W (BWT Beijing Ltd.).

The phase structural analysis was performed using a Bruker D8 Advance X-ray diffractometer equipped with Cu-Kα radiation (λ = 1.5406 Å). The diffraction angle (2θ) ranged from 10° to 80° with a step size of 0.02°. The diffuse reflectance UV–Vis–IR absorption spectra were measured on a UV-3600 spectrophotometer.

3 Results and discussion

Figure 1a displays the X-ray diffraction (XRD) patterns for the relevant phosphor and single crystals. All patterns correspond to the standard reference compounds NaYb(MoO₄)₂ (PDF#57–0839) and NaBi(MoO₄)₂ (PDF#51–1508), demonstrating the pure phase state of the prepared samples and that the crystal structure of the main lattice is not significantly altered by the addition of dopant ions. The photograph of the as-prepared samples is given in Fig. 1b.

Figure 1c shows the measured diffuse reflection spectra of Er³⁺ doped NaYb(MoO₄)₂ samples and Er³⁺/Yb³⁺ co-doped NaBi(MoO₄)₂ single crystal in the range of 200–1000 nm. In the Er³⁺ doped NaYb(MoO₄)₂ samples, nine absorption bands with considerable intensity were displayed, corresponding to the transition from the ⁴I_15/2 ground state of Er³⁺ to different excited states. These absorption peaks are located at 379, 407, 451, 489, 523, 545, 655, 802, and 970 nm, corresponding to the transitions of the excited states ⁴G_11/2, ²H_9/2, ⁴F_5/2, ⁴F_7/2, ²H_11/2, ⁴S_3/2, ⁴F_9/2, ⁴I_9/2, and ⁴I_11/2, respectively [21]. Among them, the absorption band appears wider near 970 nm, which may be due to the superposition of two absorption peaks, attributed to the ⁴I_15/2 → ⁴I_11/2 transition of Er³⁺ and the ²F_7/2 → ²F_5/2 transition of high concentration of Yb³⁺ ions in the host. In addition, an absorption band corresponding to the transition from the ground state ⁴I_15/2 to the excited state ²H_11/2 of Er³⁺ ions was observed in the Er³⁺/Yb³⁺ co-doped NaBi(MoO₄)₂ single crystal, and its absorption peak was at 523 nm.

The optical band gap energy (E_g) of the samples is estimated from the diffuse reflectance spectra using the Tauc relation given in [22]

\begin{array}{}\text{(1)}& {\left(\alpha h\nu \right)}^{\frac{1}{n}}=A\left(h\nu -E_{\text{g}}\right)\mathrm{,}\end{array}

where α is the absorption constant, h is Planck's constant, ν is the frequency of the incident photon, A is the proportionality constant, E_g is the material band gap, the index n represents the transition properties that occur in the sample, and their values are n = 1/2 and 2 for direct and indirect transitions, respectively. Here, we consider the direct band gap, so the value of n remains 1/2. The Tauc plots of the prepared samples are shown in Fig. 1d. The band gap of Er³⁺/Yb³⁺ co-doped NaBi(MoO₄)₂ single crystal is observed to be 3.20 eV, while the band gap of Er³⁺ doped NaYb(MoO₄)₂ phosphor is calculated to be 3.68 eV. This is attributed to the larger ionic radius of Bi³⁺ ions, leading to a smaller band gap in the former [23]. In addition, the band gap of Er³⁺ doped NaYb(MoO₄)₂ single crystal is slightly larger than that of the phosphor. This is due to the segregation coefficient of Er³⁺ within the NaYb(MoO₄)₂ host, resulting in incomplete substitution of Yb³⁺ by doped Er³⁺ during crystal growth.

To investigate the correlation between the sensitizer Yb³⁺ ions and the UC luminescence of the materials, the UC spectra of the three synthesized samples were measured under 980 nm laser excitation, as illustrated in Fig. 2a. The samples exhibited two prominent green emission bands and a weak red emission band within the visible light band. Among them, the green emission bands are a result of the transitions ²H_11/2 → ⁴I_15/2 (centered at 530 nm) and ⁴S_3/2 → ⁴I_15/2 (centered at 554 nm) involving the Er³⁺ ions. The weak red emission band originates from the transition ⁴F_9/2 → ⁴I_15/2 (centered at 660 nm) of the Er³⁺ ions. It is worth noting that the concentration of Yb³⁺ ions in the matrix does not exert a significant influence on the emission peak position of the UC emission spectra.

The contour plots for different power densities of three different samples were examined to learn more about the UC luminescence mechanism. The corresponding contour plots obtained after normalizing the data were shown in Fig. 2b–d. Over a broad power density range, the concentration of Yb³⁺ ions and the pump power density were examined in relation to the energy back transfer (EBT) process of Yb³⁺-Er³⁺ ions. A comparison of the three spectra reveals that the spectrum of the NaBi(MoO₄)₂ crystal remains constant throughout the power density range, dominated by the green emission peak. Conversely, the spectra of the self-activated NaYb(MoO₄)₂ phosphor and NaYb(MoO₄)₂ crystal were distorted at higher power density, potentially attributed to the emergence of the EBT process between Yb³⁺-Er³⁺ ions under high-power density conditions.

In addition, to illustrate the phenomenon more intuitively, the luminescence images of the three samples were captured at different power densities, as shown in Fig. 3. The NaYb(MoO₄)₂ phosphor and crystal exhibit a yellow luminescence trend at higher power densities, and the phosphor has demonstrated yellow luminescence at lower power densities than the crystal. This may be due to the better thermal stability of the crystal compared to the phosphor, so that the power point at which its EBT occurs appears to be shifted back. The thermal conductivity of a material is intricately linked to its internal structure. In general, the long-range ordered structure will have better thermal conductivity. Thus, in contrast to phosphors exhibiting a locally ordered structure, crystals possessing a long-range ordered structure demonstrate better thermal stability, potentially accounting for the weaker EBT process within crystals.

Notably, the luminescence image of the Yb³⁺/Er³⁺ ions codoped NaBi(MoO₄)₂ crystal in Fig. 3c shows that the sample exhibits bright green emission throughout the power density range. This observation is consistent with the findings depicted in the contour plot of Fig. 2d, indicating the absence of an EBT process between Yb³⁺ and Er³⁺ ions. The results of the experiment revealed a direct correlation between the concentration of Yb³⁺ ions and the EBT process of Yb³⁺-Er³⁺ ions, with the excitation power density level also playing a role in this process. A higher concentration of Yb³⁺ ions was found to facilitate the EBT process between Yb³⁺ and Er³⁺ ions, leading to the suppression of green light emission and the enhancement of red light emission from Er³⁺ ions. Furthermore, it was observed that the phosphor initiated the EBT process at a lower power threshold compared to the crystal, which may be related to the poor thermal stability of the phosphor.

Figure 4a summarizes the qualitative relationship between pump power density and luminescence color for the three samples. The NaYb(MoO₄)₂ phosphor exhibits the earliest transition to yellow emission (lower power threshold), followed by the NaYb(MoO₄)₂ crystal, while the NaBi(MoO₄)₂ crystal retains green emission throughout. This trend aligns with the thermal stability and Yb³⁺ concentration differences between samples. The color shift is attributed to EBT-driven suppression of green emission and enhancement of red emission, as confirmed by spectral distortion analysis (Fig. 2b–d) and luminescence imaging (Fig. 3). Furthermore, a schematic diagram of the energy transfer relationship between Yb³⁺ and Er³⁺ ions is provided to enhance the understanding, as depicted in Fig. 4b.

The simplified energy level diagrams of Er³⁺ and Yb³⁺ ions and possible luminescence processes at low pump power densities are illustrated in Fig. 5. The large absorption cross-section of Yb³⁺ ions in the near-infrared region and the amazing energy level overlap between Yb³⁺ and Er³⁺ ions result in the energy transfer (ET) process of Yb³⁺ to Er³⁺ ions, which primarily contributes to the generation of visible green and red light emission in the current work. Under the irradiation of a 980 nm laser, a large number of Yb³⁺ ions absorb near infrared photons and are excited from the ground state of ²F_7/2 to the excited energy level of ²F_5/2. Subsequently, the excited Yb³⁺ ions excite the neighboring Er³⁺ ions from the ⁴I_15/2 ground state to the ⁴I_11/2 energy level through an effective energy transfer process (ET1: ²F_5/2 + ⁴I_15/2 → ²F_7/2 + ⁴I_11/2). Additionally, Er³⁺ ions can directly absorb near-infrared photons through the ground state absorption process (GSA) and undergo a transition from the ⁴I_15/2 ground state energy level to the ⁴I_11/2 energy level. The adjacent Yb³⁺ ions excite a portion of the Er³⁺ ions located at the ⁴I_11/2 level to the higher ⁴F_7/2 level through the secondary energy transfer process (ET3: ²F_5/2 + ⁴I_11/2 → ²F_7/2 + ⁴F_7/2). Similarly, Er³⁺ ions can absorb a near-infrared photon through the excited state absorption process (ESA1), achieving the transition from the ⁴I_11/2 to the ⁴F_7/2 level. The ⁴F_7/2 energy level undergoes nonradiative relaxation (NR) filling into the ²H_11/2 and ⁴S_3/2 energy levels. Ultimately, the transitions ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 result in green UC emission centered at 530 nm and 554 nm. The electrons in the ²H_11/2/⁴S_3/2 (Er³⁺) state can be partially non-radiatively relaxed to the ⁴F_9/2 state (Er³⁺), and the ⁴F_9/2 level can be filled directly from the ⁴S_3/2 level. In addition, the electrons in the ⁴I_11/2 can be depopulate by ET3, ESA1 and nonradiative to ⁴I_13/2. Obviously, the electrons of the ⁴I_13/2 energy level can be further excited to the ⁴F_9/2 energy level through the excited state absorption process ESA2: ⁴I_13/2 (Er³⁺) + a photon → ⁴F_9/2 (Er³⁺), or through the energy transfer process (ET2) of the adjacent Yb³⁺ ions. Eventually, the radiative transition ⁴F_9/2 → ⁴I_15/2 occurs, resulting in the generation of red UC emission.

To directly explain the EBT process in the sample, we monitored the change of the emission intensity of Yb³⁺ ions under different power density excitation of 5 at% Er³⁺:NaYb(MoO₄)₂ crystal. Figure 6a gives the contour plot of the sample at low power density. The spectrum shows that the emission intensity of Yb³⁺ gradually decreases with the increase of power density, and the emission process of Yb³⁺ becomes weaker. As described in the energy transfer process of Fig. 5, this realizes the positive energy transfer from Yb³⁺ to Er³⁺ ions. In the case of high power density, as shown in Fig. 6b, it can be found that the very wide emission band of Yb³⁺ around 1um centered at 1048 nm is gradually weakened. In contrast, it shows an enhanced signal around 1010 nm. This is most likely due to the energy transfer of Er³⁺ to Yb³⁺ ions, that is, the possible EBT process occurs, resulting in the enhancement of Yb³⁺ signal. Finally, the yellow luminescence shown in Fig. 3b is realized.

As shown in the contour plot of Fig. 5b, when the concentration of Yb³⁺ ions in the sample reaches a significant level and the pump power is high enough, it cannot be explained by the traditional UC mechanism of Fig. 5. The influence of other mechanisms on the luminescence of the sample should be considered, as shown in Fig. 7. At sufficient concentrations of Yb³⁺ ions, the red luminescence can be accounted for by the following two processes: the energy back transfer (EBT) process from Er³⁺ to Yb³⁺ (⁴S_3/2 + ²F_7/2 → ⁴I_13/2 + ²F_5/2) [5, 24, 25] and the cross-relaxation (CR) process between the Er³⁺ ions (⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2 + ⁴F_9/2) [26, 27]. The EBT process can layout ²F_5/2 energy levels and lowering the layout of the ⁴S_3/2 energy level. Since the energy level difference between ⁴F_7/2 and ⁴F_9/2 (5200 cm⁻¹) is comparable to the energy level difference between ⁴I_11/2 and ⁴F_9/2 (5100 cm⁻¹), the CR process diminishes the intensity of green light emission [17, 28]. More importantly, the CR process becomes the dominant process only when the Er³⁺ ions concentration is high. Therefore, the EBT process is more efficient and dominates among the two processes [29], leading to the enhancement of red UC emission at high Yb³⁺ ion concentrations. This EBT-dominated mechanism leads to suppression of green UC luminescence and enhancement of red UC luminescence at higher Yb³⁺ ion concentrations, which is consistent with experimental observations. Notably, under high pump power excitation, it may be necessary to consider the energy exchange between two Er³⁺ ions. The highly filled ⁴F_7/2 energy level reaches saturation, and any further energy contribution is transferred through the virtual energy level of the Yb–Yb cluster to the nearest ⁴F_7/2 energy level of the other Er³⁺ ion, leading to a simultaneous transfer of energy through energy bridging between the two Er³⁺ ions [30].

4 Conclusions

Overall, rare-earth ions doped NaYb(MoO₄)₂ phosphor and crystal, along with NaBi(MoO₄)₂ crystal, were successfully synthesized. The three samples exhibited two prominent green emission bands and a weak red emission band in the visible band. The luminescence images clearly show that the Er³⁺ ions doped Yb³⁺ ions self-activated NaYb(MoO₄)₂ phosphor and crystal exhibit yellow luminescence at high power density levels, attributed to the efficient EBT process (⁴S_3/2 + ²F_7/2 → ⁴I_13/2 + ²F_5/2). In contrast, the Yb³⁺/Er³⁺ ions codoped NaBi(MoO₄)₂ crystal displays bright green emission in the whole power density range. The findings indicate that the high Yb³⁺ ion concentration promotes an enhanced EBT process between Yb³⁺ and Er³⁺ ions, leading to the suppression of green emission and enhancement of red emission from Er³⁺ ions. In addition, crystals with long-range ordered structure exhibit weaker EBT process than phosphors with local ordered structure, a phenomenon possibly ascribed to their superior thermal stability. The relative intensity change of Yb³⁺ emission in 5 at%Er³⁺:NaYb(MoO₄)₂ crystal was further monitored, and the EBT process in self-activated samples at high power density was visually displayed. Based on the experimental results and comprehensive understanding of EBT mechanism, we provide a reliable direction for determining the optimal excitation power for optical temperature measurement.

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