NaYS2:Er3+ - a highly efficient NIR II response multi-photon up-conversion phosphor

doi:10.21203/rs.3.rs-506182/v1

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NaYS₂:Er³⁺ - a highly efficient NIR II response multi-photon up-conversion phosphor

https://doi.org/10.21203/rs.3.rs-506182/v1

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The second near-infrared (NIR II) response photon up-conversion (UC) materials show great application prospects in the fields of biology and optical communication. However, it is still an enormous challenge to obtain efficient NIR II response materials. Herein, we developed a series of Er3+ doping ternary sulfides phosphors with highly efficient UC emissions under 1532 nm irradiation. Excitingly, β-NaYS2:Er3+ achieves a breakthrough visible UC efficiency as high as 6.13%, along with outstanding brightness, spectral stability of lights illumination and temperature. Such efficient UC dominates by excited state absorption process accompanied by the advantage of super long lifetimes of excited state levels of Er3+, instead of the well-recognized energy transfer UC between sensitizer and activator. NaYS2:Er3+ phosphors are further developed for high-performance underwater communication and a narrowband NIR photodetectors. Our findings breakthrough the traditional thinking of realizing efficient UC, and open up a novel frontier for developing NIR II response UC materials.

Optical Materials and Devices

Photonics/optics

NIR II response

UC materials

β-NaYS2:Er3+

Lanthanide doped photon up-conversion (UC) materials can absorb two or more low-energy photons, and emit high-energy photons, which have attracted extensive attention due to the superior optical performance, such as large anti-Stokes shift, excellent stability against photo-bleaching, and no auto-fluorescence^1-5. The first discovery of an effective UC host material was NaYF₄by Pierce in 1972. Until 2004, the hexagonal phase NaYF₄ (β-NaYF₄) with high UC efficiency was demonstrated⁶. With the development of nanotechnology, lanthanide doped UC materials experienced the explosive growth, especially in biological applications^7-9, photon-conversion devices^10,11, super-resolution nanoscopy^12-14, and information storage and security etc^15-17. Much efforts have been devoted to manipulate UC, and Yb³⁺, Nd^3+_sensitized energy transfer (ET) and interfacial energy transfer (IET) are recognized as the promising ways to achieve efficient UC luminescence^18-23.

Recently, the near-infrared second (NIR II) spectral region, especially for 1530 nm, shows great potential in the large penetration depth for tissue, and high-resolution imaging etc^24-27. Particularly, 1530 nm is also the wavelength of optical communication, exhibits a low loss in transmission, and is widely used in metro, long-distance, ultra-long-distance and submarine optical cable systems^28-31. However, the development of the highly efficient NIR II 1530 nm response photon UC materials still faces great challenges. Up to date, the recognized efficient UC materials (eg., NaYF₄:Yb,Er/Tm ) are based on the sensitized ET, owing to the efficient ET process from sensitizers (eg., Yb³⁺, Nd³⁺) to activators (eg., Er³⁺, Tm³⁺, Ho³⁺). Despite the great progress made in UC, the existing UC materials strongly depend on the excitation of short near-infrared wavelengths (as 808 and 980 nm). Er³⁺has a well spectral response around 1530 nm with a relative large absorption cross-section, but it is not suitable for sensitizer owing to the lacking of the matchable activator. Actually, a number of the literatures has announced Er³⁺ doped UC materials pumping around 1530 nm realized via the excited state absorption process of Er³⁺ ions themselves^32-40. Disappointedly, their up-conversion quantum yields (UCQY) are extremely low, less than 0.5%, limited by the deleterious quenching interactions, and strong electron-phonon coupling etc. To date, because of lacking of appropriate host materials, it still remains a challenge to achieve efficient NIR II 1530 nm response UC materials.

In this work, we fabricated a series of efficient NIR II 1532 nm response UC materials: Er³⁺ sensitized MLnS₂ (M=Li, Na, K; Ln= Y, Lu, La, Gd) materials. They exhibit cubic (α-) and hexagonal (β-) phase depending on the radius ratios between the trivalent lanthanide metal ions and the monovalent alkali metal ions (RLn³⁺/RM⁺). It should be highlighted that β-NaYS₂presents the most efficient UC emissions with the UCQY as high as 6.13% under 1532 nm excitation (~ 4.8 W/cm²), originating from the super long lifetimes of excited state levels of Er³⁺. Compared to the commercial NaYF₄:Yb³⁺, Er³⁺ phosphors, the NaYS₂: Er³⁺ owns much higher UCQY, brightness, and spectral stability of lights illumination and temperature at the same conditions. Finally, the NaYS₂:Er³⁺ phosphors were employed to obtain a sensitive narrowband responsive NIR photodetectors (800 nm, 980 nm, and 1532 nm), and green light underwater communication.

A series of Er³⁺-doped MLnS₂ UC phosphors were synthesized by the high temperature solid reaction method, in which the IA alkali metal elements (M=Li, Na, or K) and trivalent rare earth elements (Ln=Y, Lu, La, or Gd) were selected to occupy the M and Ln sites, respectively (See the experimental methods). Fig. 1a and 1b display the X-ray diffraction (XRD) patterns of MYS₂ and NaLnS₂prepared at 1173 K, which coincide well with the corresponding standard cards. The synthesis temperature dependent XRD patterns and UC luminescence spectra further reveal that the optimized temperature is 1173 K, otherwise the impurity phase of Y₂O₂S is formed at 1273 and 1373 K (Fig. S1, 2), which leads to the decrease of UC luminescence. Interestingly, MLnS₂phosphors have two structural types: NaLaS₂ and LiYS₂ present a cubic structure (α-), while KYS₂, NaGdS₂, NaLuS₂, and NaYS₂ are indexed as hexagonal phase (β-) under the same conditions (Fig. 1a, b). As represented in Fig. 1c, the phase structure of MLnS₂phosphors mainly depends on the radius ratios between the trivalent lanthanide metal ions and the monovalent alkali metal ions (RLn³⁺/RM⁺)⁴¹. When the RLn³⁺/RM⁺ ratio is larger than 1.0, MLnS₂ tends to form cubic phase (NaCl or Th3P4 type). Conversely, the hexagonal phase structure is dominant. The α-NaLaS₂ and β-NaYS₂ were taken as examples to illustrate the representative structure of MLnS₂ family (Fig. 1d, e). In α-NaLaS₂ with cubic structure, Na and La are randomly disordered on an identical lattice site and are coordinated with six S atoms to form (NaLa-S)6 octahedrons. In hexagonal β-NaYS₂, Na and Y ions are orderly situated in alternating NaS6 and YS6 octahedral layers.

In addition, the XRD patterns of NaYS₂ with various doping concentration Er³⁺ are recorded in Fig. 1f. β-NaYS₂ is obtained according to the standard cards (PDF No. 46-1051), and two characteristic peaks located at 26.34^° and 31.64^° correspond to the (101) and (104) crystal planes, respectively. Meanwhile, the diffraction peaks of NaYS₂:Er³⁺ shift to large angle with increasing the doping concentration of Er³⁺ owing to the replacement of Y³⁺ (89.3 pm) by Er³⁺ (88.1 pm) with a smaller radius. The similar morphologies of MLnS₂:Er³⁺ are obtained as displayed in Fig. S3. The transmission electron microscopic (TEM) and high resolution TEM (HRTEM) images of the as-prepared NaLaS₂ and NaYS₂ phosphors are presented in Fig. 1g, h and Fig. 1j, k. The lattice fringes of 0.34 nm and 0.28 nm are associated with (111) and (104) planes of NaLaS₂ (d-spacing of 0.339 nm; JCPDS No. 38-1391) and NaYS₂ (d-spacing of 0.282 nm; JCPDS No. 46-1051), respectively. The selected area electron diffraction (SAED) patterns (Fig. 1i, l) further demonstrate the good crystallinity of samples. All of the above results indicate that the Er³⁺-doped MLnS₂ phosphors with two phases are successfully fabricated.

The NIR II response UC luminescence characteristics of MLnS₂:Er³⁺ under 1532 nm excitation are systematically evaluated. As presented in Fig. 2a, b, two emission bands in green and red luminescence region are identified in the normalized visible UC spectra of MLnS₂:Er³⁺, assigned to the ⁴S_3/2/²H_11/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions of Er³⁺. Note that, β-MLnS₂:Er³⁺ shows clearer and sharper splitting of emission lines than those of α-MLnS₂:Er³⁺, implying the better optical performance of β-phosphors. As expected in Fig. 2c, d, the emission intensities of β-MYS₂:Er³⁺and β-NaLnS₂:Er³⁺are significantly higher than those of α- phase phosphors, and NaYS₂:Er³⁺ is found to be the most efficient UC phosphors in MLnS₂:Er³⁺ excited by 1532 nm. Meanwhile, the UC intensity ratios of green to red emissions in β-MLnS₂:Er³⁺are larger compared to the cubic phase ones. These suggest that the β-MLnS₂:Er³⁺phosphors are benefit to the UC emissions. Such phenomenon can be explained by the following reason: the Er³⁺ in β-MLnS₂are ordered separated by NaS6 octahedral layers along the c axis direction, which blocks the energy transfer and/or cross relaxation between each other to a certain extent. However, the Er³⁺ in α-MLnS₂are randomly disordered on an identical lattice site, inevitably leading to the intensified negative energy exchange^42,43. Furthermore, the distance of the nearest Y³⁺ ions (dY-Y) in NaYS₂ is determined to be 3.9808 Å. Since the probability of energy transfer between luminous centers (concentration quenching) is proportional to 1/d6, therefore, the large adjacent distance d often causes high doping level and intense luminescence.

The visible-NIR UC luminescence spectra of NaYS₂:Er³⁺ illuminated by 1532 nm are further performed to clarify the UC populating process. Five UC emission bands spanning from 400-1100 nm are recorded in Fig. 2e. There are two additional NIR emission bands in the range of 790~840 nm and 950~1050 nm expect the visible ones, attributed to the ⁴I_9/2, ⁴I_11/2→⁴I_15/2 transitions of Er³⁺, respectively. A schematic illustration of UC populating mechanism for NaYS₂:Er³⁺pumpingunder 1532 nmis provided in Fig. 2f, originating from the multi-photon excited state absorption (ESA) process. Firstly, the electrons on the ground state (⁴I_15/2) of Er³⁺ ions are pumped to ⁴I_13/2, and further excited to ⁴I_9/2 via resonantly illumination, producing the ⁴I_9/2→⁴I_15/2 transition. The electrons on ⁴I_9/2 jump to the higher excited levels (⁴S_3/2/²H_11/2) through continuous three-photon absorption processes, realizing the green emissions. Moreover, a part of electrons on ⁴I_9/2isnon-radiative to ⁴I_11/2, then populates to ⁴F_9/2, generating ⁴I_11/2→⁴I_15/2 and red (⁴F_9/2→⁴I_15/2) emissions, respectively. ET- sensitized UC is always being considered as the most efficient strategy to achieve UC emissions, while the excited state absorption is basically denied limited by the weak absorption. However, the efficient UC emissions in MLnS₂:Er³⁺are realized through ESA process, where Er³⁺presentsboth sensitizer and activator in such system. It is different from the UC from typical energy transfer (eg., Yb³⁺, Er³⁺ system: Yb³⁺ serves as sensitizer and Er³⁺ ions are as activators.). Thus, the visible and NIR emissions are ascribed to the three and two photons UC, as evidenced by the power dependence of the integrated UC emission intensity of NaYS₂:Er³⁺ (Fig. S4).

As displayed in Fig. 2g, the visible UC luminescence intensity of NaYS₂:Er³⁺ initially increases, then decreases with increasing the Er³⁺ doping concentration due to the concentration quenching, in which the optimal doping concentration of Er³⁺ ion is 5 mol%. Similarly, the corresponding decay time curves and constants of Er³⁺ in Fig. S5-S9 and Table S1 present that the lifetime first increases, and then decreases as the Er³⁺ concentration continuously increases to 15%. Such increase of decay time constants may be attributed to the re-absorbed process induced by high doping concentration of Er³⁺ ions, and further decrease comes from the concentration quenching. It should be highlighted that the decay profiles of ⁴S_3/2→⁴I_15/2 transition (green emissions) change from single exponential to double exponential, suggesting that the energy transfer UC mechanism among Er³⁺ ions involve into the UC populating process (Fig. 2f).

As revealed in Fig. 2g, the energy levels of ⁴I_13/2and⁴I_9/2play a crucial in UC emissions for MLnS₂:Er³⁺_. To deeply understand the essence of highly efficient UC emissions for NaYS₂:Er³⁺, the decay time curves of MLnS₂:Er³⁺ were measured (Fig. S10,11) and the lifetimes histogram of ⁴I_13/2 and⁴I_9/2 are performed as Fig. 2h,i, the corresponding lifetimes constants were listed in Table S2. It is very interesting to observe that the decay time constants for α-MLnS₂:Er³⁺(2.4-2.8 ms for⁴I_9/2; 4.9-8.0 ms for ⁴I_13/2) is significantly shorter than that of β-MLnS₂:Er³⁺(7.4-9.2 ms for⁴I_9/2; 13.1-30.2 ms for ⁴I_13/2). Excitingly, the NaYS₂:Er³⁺phosphors have super long and the longest lifetimes in all the samples, reaching 30.27 ms and 9.24 ms for ⁴I_13/2→⁴I_15/2 and ⁴I_9/2→⁴I_15/2 transitions, respectively. Such ultralong decay time constants could be stemmed from the low non-radiative transition and weak electron-phonon coupling in NaYS₂host. As mentioned in Fig. 2f, the UC emissions origins in the electrons populating process via ESA from low to high energy levels step by step. Significantly, the delayed lifetimes in NaYS₂:Er³⁺ mean that the electrons on ⁴I_13/2and⁴I_9/2levels stay longer, which is in favor of being re-simulated to higher green energy levels (⁴S_3/2/²H_11/2), bringing about efficient green UC emissions. It is in line with the results in Fig. 2c,d, where the stronger of UC emissions, the longer lifetimes of ⁴I_13/2→⁴I_15/2 and ⁴I_9/2→⁴I_15/2 transitions in MLnS₂:Er³⁺ phosphors.

In order to understand qualitatively the UC luminescence mechanism in Er³⁺, a set of rate equations were established based on the well-known UC process in MLnS₂:Er³⁺ phosphors (See Supplementary Note 1):

where R₃ presents the radiative rates of ⁴I_9/2 level of Er³⁺, and R'_ijis non-radiative rate from level i to level j. W is the rate of the energy transfer process between Er³⁺ ions. ρ is the laser photon number density. σ_ij denotes the absorption cross-section between level i and j of Er³⁺. It can be found from Equation (1) that the I_green value shows a cubic dependence on the laser photon number density, which means the green emission is a three-photon process, coinciding with the results in Fig. S6. The green emission intensity is proportional to and , and the lager absorption cross-section of ⁴I_9/2 level, especially for ⁴I_11/2 level can lead to the strong green emission. The I_green is inversely proportional to the R₃ and R'₃₂, that is, it is inversely proportional to the radiative lifetime (τ) of ⁴I_9/2 level. The ultralong lifetime of ⁴I_9/2 level for 9.24 ms in NaYS₂:Er³⁺ should generates the strong green UC luminescence. Meanwhile, because of small nonradiatve rate (R'₃₂), it leads to weak red UC emission.

Table 1 Comparison of reported visible UCQYs with NaYS₂:Er³⁺ by using 980 or 1500 nm as the excitation source.

Sample	UCQY	λex	Power density	Ref.
LiLuF₄:20%Yb³⁺,1%Er³⁺	5.00 %	980 nm	127 W/cm²	44
NaYF₄:20%Yb³⁺,2%Er³⁺	1.66 %	980 nm	22 W/cm²	45
Gd₂O₂S:3%Yb³⁺,7%Er³⁺	0.46 %	980 nm	3.8W/cm²	45
NaYF₄:20%Yb³⁺,2%Er³⁺	3.00 %	980 nm	150 W/cm²	46
SrF₂:5%Yb³⁺,1%Er³⁺	0.20 %	1532 nm	850 W/cm²	47
LiYF₄:10%Er³⁺	0.20 %	1490 nm	150 W/cm²	48
NaYS₂:5%Er³⁺	6.13 %	1532 nm	4.8 W/cm²	This work

As is well known, the rare earth doped β-NaYF₄(eg., β-NaYF₄:Yb³⁺,Er³⁺) has been recognized as the most efficient UC luminescent materials.Therefore, the commercial NaYF₄:Yb³⁺,Er³⁺ phosphor is selected as a comparison with the as-prepared NaYS₂:Er³⁺ phosphors. Fig. 3a shows the normalized UC luminescence spectra of NaYS₂:Er³⁺ excited by 1532 nm and NaYF₄:Yb³⁺,Er³⁺ pumped by 980 nm with the same power density (0.57 W/cm²). It is amazing to observe that the green emission intensity of NaYS₂:Er³⁺ is 3.4 times stronger than that of NaYF₄:Yb³⁺,Er³⁺ (Fig. 3a). The ratio of green to red emissions is much higher in NaYS₂:Er³⁺, suggesting the much weaker non-radiative transition from ⁴I_9/2→⁴I_11/2, further confirming the weak electron-phonon coupling. Fig. S12 demonstrates that NaYS₂:Er³⁺ has higher UC emission intensity than NaYF₄:Yb³⁺,Er³⁺ as increasing the illuminating power density. The luminescence brightness of NaYS₂:Er³⁺ and NaYF₄:Yb³⁺,Er³⁺ at different power densities are measured, as displayed in Fig. 3b. The brightness of NaYS₂:Er³⁺ is as high as 8224 cd/m³, while 3985 cd/m³ for NaYF₄:Yb³⁺,Er³⁺ at the excitation power density of 1532 and 980 nm of 1.42 W/cm³. Up-conversion quantum yield (UCQY) of the NaYS₂:Er³⁺ sample is estimated to be 6.13% for the green and red emissions under 1532 nm excitation of 4.8 W/cm² (See Supplementary Note 2-3 and Fig. S13,14 in supporting information). At the same excitation power density, the UCQY of commercial NaYF₄:Yb³⁺,Er³⁺ excited by 980 nm is measured to be 2.61%. Compared to the other representative UC materials (Table 1), the UCQY of NaYS₂:Er³⁺improves 30 foldsthan others underaround 1532 nm excitation, and is also much higher than that of 980 nm excitation, whereas the excitation power density in our system is much smaller than that of the literatures44-48. For example, the reported UCQYs of LiYF₄: Er³⁺and NaLuF₄:Yb³⁺, Er³⁺ are 0.2% and 5.0% with the power density of 127-150 W/cm² at 1532 nm and 980 nm, respectively, which are markedly higher than that of 4.8 W/cm². It should be noted that the NaYS₂:Er³⁺ based ESA process not only obtains the more efficient UC emissions, but also expands its pumping wavelength to NIR II region. Such efficient UC emissions induced ESA may be ascribed to the significantly longer lifetimes of ⁴I_9/2 and ⁴I_13/2 levels of Er³⁺ ions in NaYS₂:Er³⁺, which are 9.4 folds and 3.05 folds longer than that of NaYF₄:Yb³⁺,Er³⁺ (0.98 ms and 9.92 ms for ⁴I_9/2 and ⁴I_13/2 levels, as shown in Fig. 3c and Table S2.

More importantly, the UC spectra of NaYS₂:Er³⁺remains unchanged with varying the excitation power density under 1532 nm excitation, nevertheless, the red emission ratio obviously enhances in NaYF₄:Yb³⁺,Er³⁺illuminated by 980 nm(Fig. 3d). The CIE chromaticity coordinates and digital camera images in Fig. 3e demonstrate that the emission color of NaYF₄:Yb³⁺,Er³⁺ gradually shifts from green (0.3671, 0.6057) to red (0.4290, 0.5596) region, while NaYS₂:Er³⁺ shows higher brightness and excellent color stability with concentrating in the green emission region (0.3131, 0.6737) by varying the pumping power. In addition, the UC luminescence ratio of red to green emissions in NaYF₄:Yb³⁺,Er³⁺ significantly increases, whereas it changes little for NaYS₂:Er³⁺ (Fig. 3f). These advantages can be attributed to the lower non-radiation in NaYS₂:Er^3+. These indicate that NaYS₂:Er³⁺phosphors have outstanding lighting and temperature spectral stability.

Narrowband NIR photodetection has been attracting substantial attention in diverse areas, including biological analysis, bio-imaging/sensing, and encrypted communications etc^49,50. As a proof of concept, we designed and fabricated narrowband responsive NIR photodetectors (PDs) using a simple NaYS₂:Er³⁺/MAPbI₃hybrids. As illustrated in Fig. 4a, a high quality MAPbI₃ film acting as the photon-to-current material was spin-coated on the top of NaYS₂:Er³⁺ film (the top view SEM image of the NaYS₂:Er³⁺/MAPbI₃ hybrids was shown in Fig. S15), then the silver electrodes were deposited on the MAPbI₃ film. The working mechanism of the NaYS₂:Er³⁺/MAPbI₃ PD can be explained in Fig. S16. Briefly, the NaYS₂:Er³⁺ phosphor can absorb the incident NIR photons peaking around 808, 980, and 1532 nm, respectively, and convert them to visible lights in the spectral range of 400-700 nm through photon UC processes. The up-converted light can be efficiently absorbed by the perovskite MAPbI₃ with a narrow band gap (~800 nm), thereby producing photocurrents. Fig. 4b displays the typical on-off photocurrent-time (I-t) response curves of the NaYS₂:Er³⁺/MAPbI₃ device separately under 808, 980, and 1532 nm illumination with an incident light intensity of 5 mW/cm² at the bias voltage of 1V. The photocurrents are 1.26 µA, 2.19 µA, and 3.78 µA in NaYS₂:Er³⁺/MAPbI₃ PDs for 808, 980, and 1532 nm, while it is in well agreement with the UC luminescence intensities for the NaYS₂:Er³⁺ phosphor under corresponding excitations, respectively, and shown in Fig. S16. Importantly, three representative parameters (See Supplementary Note 4) to characterize the performance of the PDs including photo-responsivity (R), detectivity (D*) and external quantum efficiency (EQE). As exhibited in the inset of the Fig. 4b and Fig. S17, R, D* and EQE of the NaYS₂:Er³⁺/MAPbI₃PDs are determined to be 0.26 A/W, 0.44 A/W, and 0.73 A/W; 0.4610¹⁰ Jones, 0.5610¹⁰ Jones, and 0.8410¹⁰ Jones; 39%, 55%, and 59% for the 808, 980, and 1532 nm light, respectively. The device at 1532 nm shows the best performance owing to the most effective UC at this excitation wavelength. The Fig. 4c shows that the photon-response times, which were extracted from the dynamic response curves of photocurrents of the device under 808, 980, or 1532 nm light illumination. The NaYS₂:Er³⁺/MAPbI₃ PD exhibits response times in the range of 310-570 ms. As shown in Fig. S18, the photodetection thresholds for the NaYS₂:Er³⁺/MAPbI₃ PDs reached below 2 mW/cm² for 808 nm and 980 nm light, particularly below 1 mW/cm² for 1532 nm light. Compared to other representative NIR PDs, our PDs based on NaYS₂: Er³⁺ phosphors demonstrate the excellent ability to multi wavelength detection and good performance (Table S3).

The development and collection of marine resources are closely related to underwater optical communication. Because 1532 nm is an optical communication wavelength, which can be easily coupled into optical fiber with low coupling and transmission losses. As a proof of concept demonstration, the NIR II responsive NaYS₂:Er³⁺ phosphors are used to construct the UC underwater information transmission system, as illustrated in Fig. 4d. The system is composed of three parts: transmitter, channel, and receiver. Firstly, the NaYS₂:Er³⁺phosphors mixed with epoxy resin were packaged on the optical fiber, and 1532 LDs was coupled into the optical fiber which further simulated the NaYS₂:Er³⁺phosphors, then generating UC green emission lights. The input signal of 1532 nm with different frequencies was adjusted by the signal generator, Then, the emitted UC lights carrying information passed through the seawater, and collected by receivers. As shown in Fig. 4e, the input signals are the pulsed 1532 nm lights at different working frequency, and the output signals present the 564 nm green emissions of NaYS₂:Er³⁺ excited by the 1532 nm. The green UC emission lights remain the frequency after passing through 0.6 m seawater (See the video in supporting information). In addition, the received signal strengths of UC green emissions are compared in air and seawater medium at the fixed distance of 1 m. It is clearly observed from Fig. 4f that the reduced rates of green emission of NaYS₂:Er³⁺ are basically the same in these two mediums. These suggest that such NIR II responsive NaYS₂:Er³⁺ phosphors can effectively convert 1532 nm into green light signals to realize underwater optical communication.

In summary, the Er³⁺ doped MLnS₂ (M=Li, Na, K; Ln=La, Y, Gd, Lu; phases: cubic, or hexagonal) phosphors were successfully prepared through the high temperature solid reaction method. All of them demonstrate the efficient UC emissions under the illumination of NIR II 1532 nm based on the ESA process. By comparison, β-NaYS₂phosphors are recognized as the most UC materials in MLnS₂:Er³⁺ under 1532 nm excitation. It can be explained by ordered layer structure of β phase instead of disordered structure of α phase, as well as super long lifetimes of excited state levels of NaYS₂:Er³⁺. More encouragingly, β-NaYS₂:Er³⁺displays remarkably higher UCQY and brightness, and much better spectral stability of lights illumination and temperature than those of commercial NaYF₄:Yb³⁺, Er³⁺ phosphors. β-NaYS₂:Er³⁺realizes a breakthrough UC efficiency as high as 6.13% under 1532 nm excitation, and the brightness of NaYS₂:Er³⁺ is 8224 cd/m³ under 1532 nm irradiation. Such high optical performances can be assigned to low non-radiative transition and electron-phonon coupling in NaYS₂:Er³⁺phosphors. Furthermore, we designed and fabricated the sensitive narrowband responsive NIR PDs at wavelength of 808, 980, and 1532 nm, and the UC green light underwater communication application. Our work provides a new strategy for constructing NIR II responsive UC materials and expanding the scope of applications.

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All experimental details including samples synthesis, characterization and optical measurements are provided in the Supplementary Information.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.

Acknowledgement

This work is supported by the National Natural Science Foundation of China (Grant No. 11974069), Liao Ning Revitalization Talents Program (XLYC1902113), Science and Technology Project of Liaoning Province (2020JH2/10100012).

Author contributions

B.D., X.L., W.X. and L.G. conceived and supervised the project. X.Y. designed the experiment. X.Y. synthesized the samples with the help of Q.X., X.D, and N.Z. G.Z. Y.J., M.H., B.C., Q.X. and M.Y. performed the characterization and optical measurements. W.X., M. Y. and B.D. wrote the manuscript, with input from all authors.

Competing interests

The authors declare no competing interests.

There is NO Competing Interest.

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NaYS₂:Er³⁺ - a highly efficient NIR II response multi-photon up-conversion phosphor

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