A Novel Red-Emitting NaYS2 Phosphor Under 1550 nm Excitation

1. Introduction

Lanthanide ions (Ln³⁺)-doped up-conversion luminescence (UCL) materials have attracted significant interests in the areas of display, sensing, anti-counterfeiting, bioimaging and diagnostics owing to their superior optical properties of sharp and tunable 4f → 4f emissions, resistance to photobleaching and no autofluorescence background [1,2,3,4,5,6]. However, the utilization of UCL material as a luminescence probe for bioimaging is currently restricted by the strong absorption of short-wavelength light (below 600 nm) by the tissues, resulting in limited efficacy [7]. Therefore, the development of red emissions in the “tissue transparent window” (600-1200 nm) is of great significance in achieving deep tissue penetration depth and high-resolution bioimaging [8,9,10,11,12]. For Ln³⁺-doped UCL, it is known that a host crystal lattice with low symmetry is essential to relax the Laporte selection rule and significantly enhance f-f emissions [13]. Additionally, the ideal host material should possess low lattice phonon energy to minimize non-radiative multi-phonon relaxation, enhance electron population of excited states, and prolong the excited state lifetime [14,15]. Currently, research into Ln³⁺-doped UCL materials has predominantly focused on fluoride hosts, particularly β-NaYF₄, which is renowned for its low phonon energy (418 cm^-1) and asymmetric hexagonal crystal structure [16,17,18,19,20,21,22,23,24]. However, the NaYF₄ is easy to deliquescence in humidity environment due to their poor chemical stability, which finally quench the UCL of Ln³⁺ [25]. Recently, Luo et al have found a new host of ternary rare earth sulfide with phonon energy of 279 cm^-1, which is even lower than that of β-NaYF₄ and have proven its high quantum yield of UCL under excitation of ~1550 nm [26,27]. However, both Er³⁺ single-doped and Yb³⁺/Er³⁺ co-doped NaYS₂ samples exhibit strong green emission due to the long fluorescent lifetime of Er³⁺ at ⁴I_9/2 energy level which results in the favorable population of the green-emitting level (²H_11/2/⁴S_3/2). The red UCL has not been achieved in this promising ternary sulfide matrix material.

In this work, Tm³⁺ and Ho³⁺ ions are introduced into Er³⁺ doped NaYS₂ material to achieve pure and efficient red UCL under 1550 nm excitation. The results show that the I_R/I_G significantly promotes by 31 and 80 times, respectively. This is due to the newly generated energy transfer pathways after cooperating with Tm³⁺ and Ho³⁺ ions. The mechanisms of UCL and energy transfer processes are investigated in detail by steady-state fluorescence spectra and dynamics luminescence decay measurements.

2. Results and Discussions

The XRD patterns of NaYS₂:6%Er³⁺,0.5%Tm³⁺ and NaYS₂:6%Er³⁺,0.5%Ho³⁺ samples are shown in Figure 1a. As shown in the figure, all the peaks can be well indexed to trigonal crystal structure NaYS₂ with space group of R$\overline{3}$m (PDF#46-1051). This indicates that the pure trigonal NaYS₂ are successfully synthesized for the NaYS₂:6%Er³⁺,0.5%Tm³⁺ and NaYS₂:6%Er³⁺,0.5%Ho³⁺ phosphors. The crystal structure of NaYS₂ belongs to the α-NaFeO₂-type structure. The [NaS₆] and [YS₆] octahedrons are arranged into alternating layers through edge sharing, which are perpendicular to the c axis of the crystal (Figure 1b) [28,29]. The SEM images (Figure 1c and d) show that the sample presents irregular shapes and are agglomerate, which may due to the high synthesis temperature.

The optical properties of Er³⁺/Tm³⁺ and Er³⁺/Ho³⁺ co-doped NaYS₂ samples are investigated and presented in Figure 2. The reflection spectra of Er³⁺/Tm³⁺ and Er³⁺/Ho³⁺ co-doped NaYS₂ samples exhibit same reflection peaks at 365, 379, 405, 452, 489, 525, 555, 655, 799, 974, 1473 and 1537 nm, which correspond to the electronic transitions of Er³⁺ ions. It is clearly shown that in both samples, the absorption at 1550 nm is much stronger than that at 980 nm. This is due to the large absorption cross-section of Er³⁺ ion at ⁴I_13/2 energy level. The reflection peaks at 463, 686, 777, 1133 and 1207 nm correspond to the transitions of ⁵I₈ → ⁵F₂, ³H₆ → ³F₂, ³H₆ → ³H₄, ⁵I₈ → ⁵I₆ and ³H₆ → ³H₅ of Ho³⁺ or Tm³⁺ ions, respectively.

As shown in Figure 2b, the NaYS₂:6%Er³⁺,0.5%Ho³⁺ and NaYS₂:6%Er³⁺,1.5%Tm³⁺ samples exhibit relatively weak green emissions at 515 ~ 565 nm and intense red emission bands at 640 ~ 680 nm, which are assigned to the ²H_11/2/⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ions, respectively. The UCL of NaYS₂:6%Er³⁺,1.5%Tm³⁺ and NaYS₂:6%Er³⁺,0.5%Ho³⁺ samples are compared under 980 and 1550 nm excitations with the same laser power. It is observed that the UCL intensities of the both samples under 1550 nm excitation is significantly higher than that excited by 980 nm laser. The integrated UCL intensity excited by 1550 nm is 11 and 10 times higher than that under 980 nm excitation for the NaYS₂:6%Er³⁺,1.5%Tm³⁺ and NaYS₂:6%Er³⁺,0.5%Ho³⁺ samples, respectively. This is due to the high efficiency absorbance at 1550 nm arising from the large absorption cross-section of the ⁴I_13/2 state of Er³⁺ ions. This is consistent with the results of reflectance spectra, i.e., the reflection peak at 1550 nm is much stronger than that at 980 nm as observed in Figure 2a.

The efficient red UCL is obtained after introducing Tm³⁺ or Ho³⁺ ions to NaYS₂:Er³⁺ phosphors. To investigate the impact of Tm³⁺ or Ho³⁺ ions on the UCL property of Er³⁺ self-sensitized NaYS₂, the UCL spectra and red to green emission intensity ratio (I_R/I_G) of NaYS₂:Er³⁺,Tm³⁺ and NaYS₂:Er³⁺,Ho³⁺ samples are shown in Figure 3. The NaYS₂:Er³⁺ sample without doping Tm³⁺ or Ho³⁺ exhibit a dominant green emission with a minor red emission. After the incorporation of Tm³⁺ or Ho³⁺ ions, the red UCL of the samples is significantly enhanced and gradually replaces the green UCL as the principal peak. The brightest red luminescence is achieved with a doping concentration of 1.5 mol%Tm³⁺ or 0.5 mol%Ho³⁺. The insets of Figure 3a and c show the photo images of the phosphors under 1550 nm excitation. It can be observed that the color of the samples varies significantly from green to red after introducing the Tm³⁺ or Ho³⁺ ions. The I_R/I_G values increase from 0.17 to 4.73 and 4.90 with low concentration of Tm³⁺% = 1.5 mol% and Ho³⁺% = 0.5 mol%, respectively. The maximum I_R/I_G of NaYS₂:Er³⁺,Tm³⁺ and NaYS₂:Er³⁺,Ho³⁺ samples increase by 31 and 80 times compared to the NaYS₂:Er³⁺ sample, respectively (Figure 3b and d). These findings suggest that after doping Tm³⁺ or Ho³⁺ ions into NaYS₂:Er³⁺ sample, the luminescence color effectively changes from green to red.

To understand the UCL mechanism of samples, the fluorescence decay dynamics of NaYS₂:6%Er³⁺, NaYS₂:6%Er³⁺,1.5%Tm³⁺ and NaYS₂:6%Er³⁺,0.5%Ho³⁺ samples were measured under 1550 nm excitation (Figure 4a,c and Figure S1). According to the formula of

$$\tau =\frac{{\int }_0^{\mathrm{\infty }}I\left(t\right)tdt}{{\int }_0^{\mathrm{\infty }}I\left(t\right)dt}$$

(1)

where I(t) is the fluorescence luminescence intensity at time t. The calculated lifetime τ of the ⁴S_3/2 (555 nm), ⁴F_9/2 (655 nm) and ⁴I_9/2 (800 nm) levels are listed in Table 1. As shown in Figure 4b, the green-emitting state of ⁴S_3/2/²H_11/2(Er³⁺) is pumped through a simple continuous three-photon absorption processes from the ground state of ⁴I_15/2. Alternatively, the population of the red-emitting level of ⁴F_9/2 is more complicated. The critical step is to populate the ⁴I_11/2 state. Then after absorbing a 1550 nm photon, the Er³⁺ ions can be excited to the ⁴F_9/2 level for red UCL [23]. However, in the single-doped NaYS₂:Er³⁺ sample, the UCL spectra present highly dominant green with faint red emission at 1550 nm excitation (Figure 3a). This may due to the large energy gap between the ⁴I_9/2 and ⁴I_11/2 levels, making this multi-phonon relaxation (MPR) process (⁴I_9/2 → ⁴I_11/2) difficult to occur [23]. When Tm³⁺ ions are incorporated, due to the characteristics of the energy structure of Tm³⁺ ions, the energy level of the ⁴I_9/2(Er³⁺) matches with that of ³H₄(Tm³⁺), resulting in energy transfer (ET) between Er³⁺ and Tm³⁺ ions, as indicated ET1 in Figure 4b. The Tm³⁺ ions act as an energy trapping center that effectively stored the transferred energy from Er³⁺ ions. This ET from Er³⁺ to Tm³⁺ ions can result in the lifetime (τ) shortening of the ⁴I_9/2(Er³⁺) level, which is consistent with the τ measurements. As shown in Table 1, the τ of the ⁴I_9/2 level decrease significantly from 3432 μs (NaYS₂:Er³⁺) down to 636 (NaYS₂:Er³⁺,Tm³⁺). After that, the population of the ⁴I_11/2 level is achieved via cross-relaxation (CR) process of ³H₄(Tm³⁺) + ⁴I_13/2(Er³⁺) → ³H₅(Tm³⁺) + ⁴I_11/2(Er³⁺) (step 5 in Figure 4b). Then, the red-emitting state of ⁴F_9/2 are populated by further absorbing a 1550 nm photon or via energy transfer up-conversion (ETU) process of ³F₄(Tm³⁺) + ⁴I_11/2(Er³⁺) → ³H₆(Tm³⁺) + ⁴F_9/2(Er³⁺) [6] (step 6 in Figure 4b). These energy transfer pathways after introducing Tm³⁺ ions effectively restrain the population of ⁴I_9/2(Er³⁺) level, thereby achieving efficient red UCL.

When co-dope with Ho³⁺ ions, similar energy trapping process occurs in the NaYS₂:Er³⁺,Ho³⁺ sample. As discussed above, the large energy gap between the ⁴I_9/2 and ⁴I_11/2 levels makes the MPR of ⁴I_9/2 → ⁴I_11/2 difficult to occur (MPR1). The energy state of ⁵I₅(Ho³⁺) locates between ⁴I_9/2 and ⁴I_11/2 states and can store the energy transferred from the ⁴I_9/2 level of Er³⁺ via ET1, as illustrated in Figure 4d. Further, the ⁴I_11/2 state is populated via ET from Ho³⁺ to Er³⁺ ions (ET2). This ET processes are confirmed by the fluorescence decay dynamics which presents a reduction of the τ of ⁴I_9/2(Er³⁺) level after doping the Ho³⁺ ions, i.e., the τ decreases from 3432 to 423 μs. After the population of the ⁴I_11/2 state, the red-emitting ⁴F_9/2 level are populated by further absorbing a 1550 nm photon or via ETU process of ⁵I₇(Ho³⁺) + ⁴I_11/2(Er³⁺) → ⁵I₈(Ho³⁺) + ⁴F_9/2(Er³⁺) (step 5 in Figure 4d). Therefore, the dominant red UCL is achieved in the NaYS₂:Er³⁺,Ho³⁺ sample by altering the energy transfer pathways via Ho³⁺ ions.

3. Materials and Methods

The raw material used in this study were Y₂O₃ (99.99%), Er₂O₃ (99.99%), Tm₂O₃ (99.99%), Ho₂O₃ (99.99%), Na₂CO₃ (99.99%), and CS₂ (99.99%). The raw material was accurately weighed according to the chemical formula NaY_1-x-yEr_xTm_yS₂ (x = 6 mol%, y = 0.5, 1.0, 1.5, 2.0, 2.5 mol%) and NaY_1-x-yEr_xHo_yS₂ (x = 6 mol%, y = 0.5, 1.0, 1.5, 2.0, 2.5 mol%). The weighed powders were ground for 30 mins and placed into a corundum boat. Then samples were annealed at 1050 °C for 2 h in the atmosphere of argon and vapor of CS₂. After annealing, the NaYS₂:Er³⁺,Tm³⁺ and NaYS₂:Er³⁺,Ho³⁺ samples were cooled to room temperature under a flow of Ar.

X-ray diffraction (XRD) analysis was measured by using a SHIMADZU-6000 X-ray diffractometer (Cu-Kα radiation, λ = 0.15406 nm, 40 kV, 40 mA, 2θ = 10 ~ 80°). Scanning electron microscope (SEM) analysis was carried out with SUPRA 55 SAPPHIRE electron microscope. Diffuse reflectance spectra were measured with a UV-3600 Shimadzu UV-Vis-NIR spectrophotometer. Photoluminescence spectra and luminescence decay curves were recorded by using the Edinburgh FS5 spectrometer. Excitation sources were provided by a 1550 nm laser diode (CNI laser MDL-III-1550 nm) and a 980 nm laser diode (CNI laser MDL-III-980 nm) with tunable output power. The digital photos were taken using a Canon EOS 5D Mark III camera.

4. Conclusions

In summary, Er³⁺/Tm³⁺ and Er³⁺/Ho³⁺ co-doped NaYS₂ are prepared to realize red UCL under 1550 nm excitation. The ⁴I_9/2(Er³⁺) level exhibits shorter lifetime after the addition of Tm³⁺ or Ho³⁺ ions due to the newly generated energy transfer pathways, which is beneficial for the population of ⁴F_9/2 level of Er³⁺ ions, realizing efficient red UCL. Therefore, the main emission peak is tuned from green to red successfully after the addition of Tm³⁺ or Ho³⁺ ions. The I_R/I_G of NaYS₂:Er³⁺,Tm³⁺ and NaYS₂:Er³⁺,Ho³⁺ samples increase by 31 and 80 times, respectively. The optimum doping concentration of Ho³⁺ and Tm³⁺ ions are 1.5 mol% and 0.5 mol%, respectively. The mechanisms of the red UCL and upconversion pathways are discussed in detail by steady-state fluorescence spectra and dynamics luminescence decay measurements. The findings demonstrate the achievement of red-emitting UCL within the ternary sulfide matrix, introducing a novel category of red UCL phosphors.