Towards highly efficient NIR II response up-conversion phosphor enabled by long lifetimes of Er3+

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 develop a series of Er3+ doped ternary sulfides phosphors with highly efficient UC emissions under 1532 nm irradiation. β-NaYS2:Er3+ achieves a visible UC efficiency as high as 2.6%, along with high brightness, spectral stability of lights illumination and temperature. Such efficient UC is dominated by excited state absorption, accompanied by the advantage of long lifetimes (4I9/2, 9.24 ms; 4I13/2, 30.27 ms) 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 narrowband NIR photodetectors. Our findings suggest a novel approach for developing NIR II response UC materials, and simulate new applications, eg., simultaneous NIR and visible optical communication.

sensitizer owing to the lack of matchable activators. A number of reports has announced Er 3+ doped UC materials pumping around 1530 nm realized via the excited state absorption process of Er 3+ themselves [33][34][35][36][37][38][39] . Disappointedly, their up-conversion quantum yields (UCQYs) are low, less than 0.5%, limited by the deleterious quenching interactions, and strong electron-phonon coupling etc. To date, because of the lack of appropriate host materials, it still remains a challenge to achieve efficient NIR II 1530 nm response UC.
Ternary rare rarth sulfide (MLnS 2 ; where M is an alkali metal ion, and Ln is a rare earth ion) materials process the layer structure, low unit cell symmetry, strong covalence and low phonon energy 40 , which are promising materials for efficient NIR II response UC. It was discovered in 1970s 41 , and Güdel et al. reported the UC emissions with UCQY of 21% in NaYS 2 :Er 3+ crystals under the excitation of 980 nm, but this was obtained at a high incident power of 0.79 kW cm −2 at low temperature of 12 K 42 . Since then, little studies have paid attention to these materials.
In this work, we fabricated a series of efficient NIR II 1532 nm response UC materials: MLnS 2 :Er 3+ (M = Li, Na, K; Ln = Y, Lu, La, Gd). They exhibit cubic (α-) and trigonal (β-) phases depending on the radius ratios between the trivalent lanthanide ions and the monovalent alkali metal ions (RLn 3+ /RM + ). It should be highlighted that β-NaYS 2 presents a particularly high efficiency UC emissions with the UCQY as high as 2.6% under 1532 nm excitation (~4.5 W cm −2 ), originating from the exceptionally long lifetimes ( 4 I 9/2 , 9.24 ms; 4 I 13/2 , 30.27 ms) of excited state levels of Er 3+ . Compared to the commercial NaYF 4 :Yb 3+ , Er 3+ phosphor, the NaYS 2 :Er 3+ owns much higher UCQY, brightness, and spectral stability of illumination and temperature under the same conditions. Then, the NaYS 2 :Er 3+ phosphors were employed to achieve multiband responsive NIR photodetectors (800 nm, 980 nm, and 1532 nm), and green light underwater communication.

Results
Synthesis and structure characterization of the MLnS 2 :Er 3+ phosphors A series of MLnS 2 :Er 3+ UC phosphors were synthesized by the gas-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 Methods). Figures 1a and 1b display the X-ray diffraction (XRD) patterns of MYS 2 and NaLnS 2 prepared at 1173 K, which coincide well with the corresponding standard cards. The XRD patterns and UC luminescence spectra further reveal that the optimized temperature for synthesis is 1173 K, otherwise the impurity phase of Y 2 O 2 S is formed at 1273 and 1373 K (Supplementary Figs. 3,4), which leads to the decrease of UC luminescence. Interestingly, MLnS 2 phosphors have two structural types: NaLaS 2 and LiYS 2 possess cubic structure (α-), while KYS 2 , NaGdS 2 , NaLuS 2 , and NaYS 2 are indexed as trigonal phase (β-) under the same conditions ( Fig. 1a, b). As represented in Fig. 1c, the phase structure of MLnS 2 phosphors mainly depends on the radius ratios between the trivalent lanthanide ions and the monovalent alkali metal ions (RLn 3+ /RM + ) 43 . When the RLn 3+ /RM + ratio is larger than 1.0, MLnS 2 tends to form cubic phase (NaCl or Th 3 P 4 type). Conversely, the β-phase structure is dominant. The α-NaLaS 2 and β-NaYS 2 were taken as examples to Er 3+ (M = Li, Na, K) and NaLnS 2 :Er 3+ (Ln = La, Y, Lu, Gd) prepared at 1173 K. c Radius ratios between the trivalent lanthanide ion and the monovalent alkali metal ion (RLn 3+ /RM + ) of MLnS 2 (M = Li, Na, K; Ln = La, Y, Lu, Gd), the cubic and trigonal phases are divided by the green dotted line. d, e Crystal structure models of cubic NaLaS 2 (α-NaLaS 2 ) and trigonal NaYS 2 (β-NaYS 2 ). f XRD patterns of β-NaYS 2 doped with 2, 5, 10, 15 mol% Er 3+ . TEM, HRTEM images, and corresponding SAED of g-i α-NaLaS 2 and j-l β-NaYS 2 . Source data is provided in this paper.
illustrate the representative structure of MLnS 2 family (Fig. 1d, e). In α-NaLaS 2 with cubic structure, Na + and La 3+ are coordinated with six S 2− to form (Na/La-S)6 octahedrons 44 . In β-NaYS 2 , both Na + and Y 3+ are sixfold coordinated by S 2− in form of a regular octahedron and contain empty voids on the special 3a and 3b sites, respectively. The S 2− are surrounded by three Na + and three Y 3+ . Both MS6 and LnS6 distorted octahedra are more precisely described as trigonal anti-prisms with centro-symmetric D3d symmetry, where all six bonds M-S in the octahedron possess identical length (as in Oh symmetry) 45,46 . Therefore, Na + and Y 3+ are orderly situated in alternating NaS6 and YS6 octahedral layers.
In addition, the XRD patterns of NaYS 2 with various doping concentration of Er 3+ are recorded in Fig. 1f. β-NaYS 2 is obtained according to the standard cards (JCPDS No. 46-1051), and two characteristic peaks located at 26.34°and 31.64°correspond to the (101) and (104) crystal planes, respectively. The diffraction peaks of NaYS 2 :Er 3+ shift to large angle on increasing the doping concentration of Er 3+ owing to the replacement of Y 3+ (89.3 pm) by Er 3+ (88.1 pm) with a smaller radius. The similar morphologies of MLnS 2 :Er 3+ are obtained as displayed in Supplementary Fig. 5. The transmission electron microscopic (TEM) and high resolution TEM (HRTEM) images of the NaLaS 2 :Er 3+ and NaYS 2 :Er 3+ phosphors are presented in Fig. 1g, h and Fig. 1j, k, which are irregular morphology with micron size (Supplementary Fig. 5). The lattice fringes of 0.34 nm and 0.28 nm are associated with (111) and (104) planes of NaLaS 2 (d-spacing of 0.339 nm; JCPDS No. 38-1391) and NaYS 2 (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 MLnS 2 :Er 3+ phosphors with two phases are successfully fabricated.
The UC intensity ratios of green-to-red emissions in β-MLnS 2 :Er 3+ are larger compared to the cubic phase ones. These suggest that the β-MLnS 2 :Er 3+ phosphors are benefit to the UC emissions. The Raman spectra of the MLnS 2 hosts in Supplementary Fig. 6 demonstrate that the phonon energies of all the samples are lower than 300 cm −1 . Based on the theory of multi-phonon relaxation, the low-frequency phonon should rarely take contributions of nonradiative relaxation 47 . The phonon energies of all the MLnS 2 hosts locate within 260-280 cm −1 , consistently with the reported results 48 . Considering the similar phonon energies of MLnS 2 hosts, such phenomenon can be explained by e UC luminescence spectrum of NaYS 2 :Er 3+ in visible and NIR regions. f UC luminescence populating mechanism of NaYS 2 :Er 3+ under 1532 nm excitation, the 1-5 present the excited state absorption (ESA) process, 6-8 present the cross-relaxation process. g UC luminescence spectra of NaYS 2 doped with 2, 5, 10, 15 mol% Er 3+ (inset: integral intensity varies with concentration of Er 3+ ). h-i, Lifetimes of 4 I 9/2 → 4 I 15/2 and 4 I 13/2 → 4 I 15/2 transitions of Er 3+ in MLnS 2 :Er 3+ . Source data is provided in this paper.
Article https://doi.org/10.1038/s41467-022-34350-1 the following reason: the Er 3+ in β-MLnS 2 are ordered separated by NaS6 octahedral layers along the c axis direction, which blocks the cross relaxation between each other to a certain extent. However, the Er 3+ in α-MLnS 2 are randomly disordered on an identical lattice site, inevitably leading to the intensified negative energy exchange 49,50 . The comparison of UC emission intensity between α-NaLaS 2 :0.1-15%Er 3+ and β-NaYS 2 :0.1-15%Er 3+ (Supplementary Fig. 7) further proves the above deduction. In addition, the distance of the nearest Y 3+ (dY-Y) in NaYS 2 is determined to be 3.9808 Å. Since the probability of energy transfer between luminous centers (concentration quenching) is proportional to 1/d6, the large adjacent distance d often causes a high doping level and intense luminescence. Crystal structural data for NaYS 2 and NaYF 4 are summarized in Supplementary Table 1. The excellent characteristics of NaYS 2 compared to NaYF 4 are mainly ascribed to the layer structure, lower unit cell symmetry (rhombohedral vs hexagonal), longer minimum Y 3+ separation distance, stronger covalence, and lower phonon energy. These advantages of NaYS 2 all contribute to the efficient UC of NaYS 2 :Er 3+ .
The visible-NIR UC luminescence of NaYS 2 :Er 3+ illuminated at 1532 nm was further investigated to clarify the populating process. Five UC emission bands spanning from 400-1100 nm are given in Fig. 2e. The NIR emission bands in the range of 790 to~840 nm and 950 tõ 1050 nm are attributed to the 4 I 9/2 , 4 I 11/2 → 4 I 15/2 transitions of Er 3+ , respectively. A schematic illustration of populating mechanism for NaYS 2 :Er 3+ pumping under 1532 nm is provided in Fig. 2f, originating from the multi-photon excited state absorption (ESA) process. Firstly, the electrons in the ground state ( 4 I 15/2 ) of Er 3+ are pumped to 4 I 13/2, and further excited to 4 I 9/2 via resonant illumination, generating 4 I 9/2 → 4 I 15/2 transition. The electrons on 4 I 9/2 jump to the higher excited levels ( 4 S 3/ 2 / 2 H 11/2 ) through continuous three-photon absorption, realizing the green emissions. A fraction of electrons on 4 I 9/2 decays nonradiatively to 4 I 11/2 , then populates the 4 F 9/2 state and thereby generating 4 I 11/2 → 4 I 15/2 and red ( 4 F 9/2 → 4 I 15/2 ) emissions, respectively. ET-sensitized UC is always considered as the most efficient strategy to achieve UC emissions, while the excited state absorption is basically limited by the weak absorption cross-section. However, the efficient UC emissions in MLnS 2 :Er 3+ are realized through an ESA process. It is different from the UC from typical energy transfer systems containing Yb 3+ , Er 3+ , where Yb 3+ serves as a sensitizer and Er 3+ as an activator. Thus, the visible and NIR emissions are ascribed to three-and twophoton UCs, as evidenced by the power dependence of the integral UC emission intensity of NaYS 2 :Er 3+ ( Supplementary Fig. 8).
As revealed in Fig. 2g, the energy levels of 4 I 13/2 and 4 I 9/2 play a crucial role in UC emissions for MLnS 2 :Er 3+ . To deeply understand the essence of highly efficient UC emissions for NaYS 2 :Er 3+ , the decay time curves of MLnS 2 :Er 3+ were measured ( Supplementary Figs. 14, 15) and the lifetimes histogram of 4 I 13/2 and 4 I 9/2 are performed as Fig. 2h, i, corresponding lifetimes constants are listed in Supplementary  Table 3. It is very interesting that the decay time constants for α-MLnS 2 :Er 3+ (2.4-2.84 ms for 4 I 9/2 ; 4.69-7.08 ms for 4 I 13/2 ) are significantly shorter than that of β-MLnS 2 :Er 3+ (7.45-9.24 ms for 4 I 9/2 ; 13.12-30.27 ms for 4 I 13/2 ). M + and Ln 3+ are randomly disordered on an identical lattice site in α-MLnS 2 with cubic structure, inevitably leading to the intensified negative energy exchange. While the Er 3+ in β-MLnS 2 are ordered separated by NaS6 octahedral layers along the c-axis direction, which blocks the cross-relaxation between each other to a certain extent. Therefore, in virtue of different cation distribution, the long decay time of the 4 I 13/2 level in the β compared to the α polymorph is observed. Excitingly, the NaYS 2 :Er 3+ phosphors have the longest lifetimes in all the samples, reaching 30.27 ms and 9.24 ms for 4 I 13/2 → 4 I 15/2 and 4 I 9/2 → 4 I 15/2 transitions, respectively. As mentioned in Fig. 2f, the UC emissions origin in the electrons populating process via ESA from low to high energy levels step by step. Significantly, the delayed lifetimes in NaYS 2 :Er 3+ mean that the electrons on 4 I 13/2 and 4 I 9/2 levels stay longer, which is in favor of being re-simulated to higher green energy levels ( 4 S 3/2 / 2 H 11/2 ), bringing about efficient green UC emissions. This is in line with the results in Fig. 2c, d, that show the stronger UC emissions and the longer lifetimes of 4 I 13/2 → 4 I 15/2 and 4 I 9/2 → 4 I 15/2 transitions in MLnS 2 :Er 3+ phosphors. From the UC emission spectra and luminescent decay curves as a function of temperature ranging from 10 -300 K in Supplementary Figs. 16-21, the radiative and non-radiative rates of Er 3+ in NaYS 2 are significantly shorter than that of the other UC materials 52 . It can be concluded that the long lifetimes of NaY-S 2 :Er 3+ phosphors originate from the long radiative and non-radiative lifetimes of Er 3+ (See Supplementary Fig. 2

and Supplementary
Note 3). The long lifetimes are caused by the crystal structure and low phonon energy of NaYS 2 ( Supplementary Fig. 6) that lead to less non-radiative transition and weak electron-phonon coupling in NaYS 2 host.
In order to understand qualitatively the UC luminescence mechanism in Er 3+ , a set of rate equations were established based on the UC process in MLnS 2 :Er 3+ phosphors (in Fig. 2f, See Supplementary Note 4): where R 1 and R 3 represent the radiative rates of 4 I 13/2 and 4 I 9/2 level of Er 3+ , and R' ij is non-radiative rate from level i to level j. ρ is the laser photon number density. σ ij denotes the absorption cross-section between level i and j of Er 3+ . C 8 represents cross-relaxion process of 4 I 13/ 2 + 4 I 9/2 → 4 I 15/2 + 4 S 3/2 . It can be concluded from Eq. (1) that the I green value shows a cubic dependence on the laser photon number density, which means that the green emission results from a three-photon process, coinciding with the results in Supplementary Fig. 8. The green emission intensity is proportional to σ 35 and σ 2 13 , and the larger absorption cross-section of 4 I 9/2 level, especially for 4 I 11/2 level can lead to the strong green emission. The I green is inversely proportional to the R 1 , R 3 and R' 32 , that is, it is proportional to the radiative lifetime (τ) of 4 I 13/2 , 4 I 9/2 levels. From the UC spectra and dynamics at low temperature ( Supplementary Figs. 16-21), the radiative lifetimes of Er 3+ is much longer than that of commercial NaYF 4 :Yb 3+ , Er 3+52 , leading to efficient green UC emissions in NaYS 2 :Er 3+ . In addition, the long non-radiative lifetimes of Er 3+ lead to weak C 8 and induce strong green UC luminescence. Therefore, such efficient UC is dominated by the advantage of the exceptionally long lifetimes of the excited-state levels of Er 3+ .
Rare earth-doped β-NaYF 4 (such as β-NaYF 4 :Yb 3+ , Er 3+ ) has been recognized as the most efficient UC material. Therefore, the commercial NaYF 4 :Yb 3+ , Er 3+ phosphor of similar size ( Supplementary  Fig. 22) is selected as a comparison with the NaYS 2 :Er 3+ phosphor. Figure 3a shows the normalized UC luminescence spectra of NaYS 2 :Er 3+ under 1532 nm excitation and NaYF 4 :Yb 3+ ,Er 3+ under 980 nm excitation with the same power density (0.45 W cm -2 ). It is amazing to observe that the green emission intensity of NaYS 2 :Er 3+ is 3.4 times stronger than that of NaYF 4 :Yb 3+ , Er 3+ (Fig. 3a). The ratio of green to red emissions is much higher in NaYS 2 :Er 3+ , suggesting the much weaker nonradiative transition from 4 I 9/2 → 4 I 11/2 and further confirming the weak electron-phonon coupling. Supplementary Figs. 23 demonstrates that NaYS 2 :Er 3+ displays more efficient UC emission than NaYF 4 :Yb 3+ , Er 3+ as increasing the illuminating power density. The luminescence brightness of NaYS 2 :Er 3+ and NaYF 4 :Yb 3+ , Er 3+ at different power densities are measured, as displayed in Supplementary Figs. 24, 25. The brightness of NaYS 2 :Er 3+ is as high as 8224 cd/m 2 , while it is 3985 cd/m 2 for NaY-F 4 :Yb 3+ , Er 3+ at the excitation power density of 1532 and 980 nm of 1.45 W cm −2 . To maintain the same photon flux, the UC brightness of NaYS 2 :Er 3+ excited with 1532 nm (0.27-0.95 W cm −2 ) and NaYF 4 :Yb 3+ , Er 3+ excited with 980 nm (0.45-1.45 W cm −2 ) are compared, as shown in Supplementary Fig. 25 and Supplementary movie 1. It can be seen that the brightness of NaYS 2 :Er 3+ is also higher than that of NaYF 4 :Yb 3+ , Er 3+ . The UCQYs of MLnS 2 :Er 3+ (M = Li, Na, K; Ln = La, Y, Lu, Gd) are measured using the typical method with two detectors in an integrating sphere (See Supplementary Fig. 1 and Supplementary Note 1) 53 , under the illumination of 1532 nm as a function of excitation power density of 0-4.5 W cm −2 , as displayed in Fig. 3b and Supplementary Fig. 26. The MLnS 2 :Er 3+ show high UCQYs under 1532 nm excitation with the power density of 4.5 W cm −2 ranging of 0.5-2.6% ( Supplementary Fig. 26 and Supplementary Data 1). The UCQY of NaYS 2 :Er 3+ is best, reaching 2.6%, which is much higher than that of NaYF 4 :Yb 3+ , Er 3+ (1.3%) under the illumination of 980 nm with the same power density ( Supplementary  Fig. 27). We estimated the theoretical UCQY of NaYS 2 :Er 3+ under the weak excitation from the rate equations (See Supplementary Note 2) combining with the decay times at low temperature (10 K) and room temperature (Supplementary Table 4). The estimated green UCQY in NaYS 2 :Er 3+ is 7.6% and 3.0% at low temperature and room temperature, respectively. Compared to the other representative UC materials, the UCQY of NaYS 2 :Er 3+ significantly improves under around 1532 nm excitation, whereas the excitation power density in our system is much smaller than that of the literatures [53][54][55][56] . For example, the reported UCQYs of LiYF 4 :Er 3+ and SrF 2 :5%Yb 3+ , 1%Er 3+ are <0.2% with the power density of 150-850 W cm −2 excited at 1532 nm and 1490 nm, respectively, which are markedly higher than that of 4.5 W cm −2 57 . Furthermore, NaYS 2 :Er 3+ show bright UC luminescence under 980 nm excitation. The UCQYs of NaYS 2 :Er 3+ were recorded with varying power density at 980 nm ( Supplementary Fig. 28). It was measured to be 0.4% with the power density of 4.5 W cm −2 . Since the absorption crosssection of Er 3+ at 980 nm is much smaller than that of at 1532 nm 58 , the UC intensity/efficiency of NaYS 2 :Er 3+ excited at 980 is lower than that of 1532 nm excitation ( Supplementary Fig. 29). The UC emission intensity of NaYS 2 :Er 3+ compared with commercial NaYF 4 :Yb 3+ , Er 3+ is shown in Supplementary Fig. 30. The UC emission intensity of NaY-S 2 :Er 3+ is just lower 8.1 fold than that of NaYF 4 :Yb 3+ , Er 3+ excited at the same power density of 980 nm (0.45 W cm −2 ; Supplementary Fig. 30). Considering the higher concentration (18%) and absorption crosssection (1.2 × 10 −20 cm 2 ) at 980 nm of Yb 3+ in NaYF 4 :Yb 3+ , Er 3+ than that of Er 3+ (5%, 1.7 × 10 −21 cm 2 ), NaYS 2 :Er 3+ is indeed efficient and a promising UC material for 980 nm pumping. It should be noted that the NaYS 2 :Er 3+ -based ESA process not only obtains the more efficient UC emissions, but also expands its pumping wavelength to NIR II region. It should be highlighted that green and red UC emissions in NaYS 2 :Er 3+ are from a three-photon process under 1532 nm excitation, while they are a two-photon process in the NaYF 4 :Yb 3+ , Er 3+ phosphor under 980 nm excitation. It is difficult for the former one to obtain highly efficient UC. In fact, the multi-photons UC is also possible to be more effective than two-photon UC, owing to the enriched energy levels and diversity of transitions in UC 59 . Such efficient UC emissions-induced ESA may be ascribed to the significantly longer lifetimes of 4 I 9/2 and 4 I 13/2 levels of Er 3+ in NaYS 2 :Er 3+ , which are 9.4 fold and 3.05 fold longer than that of NaYF 4 :Yb 3+ , Er 3+ (0.98 ms and 9.92 ms for 4 I 9/2 and 4 I 13/2 levels, as shown in Fig. 3c Table 5), both the radiative and nonradiative rates in NaYS 2 :Er 3+ are much smaller than that of NaYF 4 :Yb 3+ , Er 3+ 52 . These can be explained by the ordered layered structures and low phonon energy of NaYS 2 host. Such long lifetimes of rare earth ions in MLnS 2 hosts are in favor of multi-photon UC, providing novel approach to develop efficient UC materials.
In addition, the UC spectra of NaYS 2 :Er 3+ remain unchanged with varying the excitation power density under 1532 nm excitation (Fig. 3d). The CIE chromaticity coordinates and digital camera images in Fig. 3e demonstrate that the emission color of NaYF 4 :Yb 3+ , Er 3+ gradually shifts from green (0.3671, 0.6057) to red (0.4290, 0.5596) region, while NaYS 2 :Er 3+ 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 NaYF 4 :Yb 3+ , Er 3+ significantly increases, whereas it changes little for NaYS 2 :Er 3+ (Fig. 3f). These advantages can be attributed to the lower non-radiation in NaYS 2 :Er 3+ . These indicate that NaYS 2 :Er 3+ phosphors have high stability towards light and temperature.

NIR photodetection and underwater optical communication applications
Narrowband NIR photodetection has been attracting substantial attention in diverse areas, including biological analysis, bio-imaging/ sensing, and encrypted communications etc 61,62 . As a proof of concept, we designed and fabricated narrowband responsive NIR photodetectors (PDs) using a simple NaYS 2 :Er 3+ /MAPbI 3 hybrid. As illustrated in Fig. 4a, a high-quality MAPbI 3 film acting as the photon-to-current material was spin-coated on the top of a NaYS 2 :Er 3+ film (the top view SEM image of the NaYS 2 :Er 3+ /MAPbI 3 hybrids is shown in Supplementary Fig. 31), Then the silver electrodes were deposited on the MAPbI 3 film. The working mechanism of the NaYS 2 :Er 3+ /MAPbI 3 PD can be explained in Supplementary Fig. 32. Briefly, the NaYS 2 :Er 3+ phosphor can absorb the incident NIR photons peaking around 808, 980, and 1532 nm, respectively, and convert them to visible light in the spectral range of 400-700 nm through photon UC. The up-converted light can be efficiently absorbed by the perovskite MAPbI 3 with a narrow band gap (~800 nm), thereby producing photocurrents. Figure 4b displays the typical on-off photocurrent-time (I-t) response curves of the NaY-S 2 :Er 3+ /MAPbI 3 device separately under 808, 980, and 1532 nm illumination with an incident light intensity of 5 mW cm −2 at the bias voltage of 1 V. The photocurrents are 1.26, 2.19, and 3.78 µA in NaYS 2 :Er 3+ / MAPbI 3 PDs for 808, 980, and 1532 nm. This is in well agreement with the UC luminescence intensities for the NaYS 2 :Er 3+ phosphor under corresponding excitations, respectively, and shown in Supplementary  Fig. 32. Three representative parameters (See Supplementary Note 5) can be used to characterize the performance of the PDs, namely photoresponsivity (R), detectivity (D*) and external quantum efficiency (EQE). As exhibited in the insets of Fig. 4b and Supplementary Fig. 33, R, D* and EQE of the NaYS 2 :Er 3+ /MAPbI 3 PDs are determined to be 0.26 A/W, 0.44 A/W, and 0.73 A/W; 0.46 × 10 10 Jones, 0.56 × 10 10 Jones, and 0.84 × 10 10 Jones; 39%, 55%, and 59% for the 808, 980, and 1532 nm light, respectively. For the photodetector, EQE is equal to the number of electron-hole pairs per second collected to produce the photo-currentI ph , divided by the number of incident photons per second. In a detector with 100% EQE, R = 1 A/W for photon energy E ph = 1 eV 63 . The photoconductive gain leads to the EQE > 50% when applied a bias in PDs, similar to previous reports [64][65][66] . In general, UC visible emissions of NaYS 2 :Er 3+ under 1532 nm excitation excite the semiconductor (MAPbI 3 ) to produce electron-hole (e-h) pairs. Since holes generally move more slowly than electrons in semiconductors, N electrons have already been collected by the electrode when the slower-moving hole c On-off switching currents at lowest detectable excitation power density of NaYS 2 :Er 3+ /MAPbI 3 device under 808, 980, and 1532 nm, respectively. d Schematic illustration of the NaYS 2 :Er 3+ underwater information transmission in seawater. e Input and output signal information of transmitter and receiver at different frequencies (Hz) with the seawater depth of 0.6 m. f Received signal strength of NaYS 2 :Er 3+ in air and water at different distance between transmitter and receiver. Source data is provided in this paper.
Article https://doi.org/10.1038/s41467-022-34350-1 is searched by the electrode under an applied electric field. 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 photonresponse times, which were extracted from the dynamic response curves of photocurrents of the device under 808, 980, or 1532 nm light illumination. The NaYS 2 :Er 3+ /MAPbI 3 PD exhibits response times in the range of 310-570 ms. As shown in Supplementary Fig. 34, the photodetection thresholds for the NaYS 2 :Er 3+ /MAPbI 3 PDs reached below 2 mW cm −2 for 808 and 980 nm light, particularly below 1 mW cm −2 for 1532 nm light. Compared to other representative NIR PDs, our PDs based on NaYS 2 :Er 3+ phosphors demonstrate the excellent capability for multi-wavelength detection and good performance (Supplementary Table 6).
Visible light communication is environmentally friendly, and can realize low energy consumption communication, effectively avoid the weaknesses of radio communication, such as electromagnetic signal leakage, and will also interact and integrate with WiFi, cellular network technologies, bringing innovative applications in the fields of Internet of Things, navigation, and high-speed rail etc 67 . For example, the development and collection of marine resources are closely related to underwater optical communication. In contrast to high absorption of the UV and NIR wavelength, the low absorption at the visible wavelength (green light) of seawater is suitable for under seawater communication, which usually requires visible light source with high power for long-distance seawater transmission 68 . As known, optical fiber communication based on 1532 nm is mature technology, which can be easily coupled into optical fiber with low coupling and transmission losses. Based on the above results, NaY-S 2 :Er 3+ phosphors can efficiently convert 1532 nm to green emissions, which simultaneously combines the advantages of NIR optical fiber communication and visible optical communication. As a proof of concept demonstration, the NIR-II responsive NaYS 2 :Er 3+ 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 NaYS 2 :Er 3+ phosphors mixed with epoxy resin were packaged on the optical fiber, and 1532 nm laser diode (LD) was coupled into the optical fiber which further simulated the NaYS 2 :Er 3+ 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 NaYS 2 :Er 3+ excited by the 1532 nm. The green UC emission lights remain the frequency after passing seawater over a distance of 0.6 m (See Supplementary Movie 2). 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 NaYS 2 :Er 3+ are basically the same in these two media. These suggest that such NIR II-responsive NaYS 2 :Er 3+ phosphors can effectively convert 1532 nm light into green light to realize underwater optical communication. Because of the large loss of visible light in optical fiber, the transmission distance of visible light cannot be matched with that of 1532 nm light in an optical fiber. Therefore, the visible light communication based on the UC green emission of NaYS 2 :Er 3+ phosphor using 1532 nm as light source possesses great value in underwater optical communication.

Discussion
In summary, the Er 3+ doped MLnS 2 (M = Li, Na, K; Ln = La, Y, Gd, Lu; phases: cubic, or trigonal) phosphors were successfully prepared through the gas-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-NaYS 2 phosphor is recognized as the most UC material in MLnS 2 :Er 3+ 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 exceptionally long lifetimes of excited state levels of NaYS 2 :Er 3+ . β-NaYS 2 :Er 3+ displays remarkably higher UCQY and brightness, and much better spectral stability of lights illumination and temperature than those of commercial NaYF 4 :Yb 3+ , Er 3+ phosphor. β-NaYS 2 :Er 3+ realizes a breakthrough UC efficiency as high as 2.6% under 1532 nm excitation, and the brightness of NaYS 2 :Er 3+ is 8224 cd/m 2 under 1532 nm irradiation. Such high optical performances can be assigned to low non-radiative transition and electronphonon coupling in NaYS 2 :Er 3+ phosphor. 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 strategy for constructing NIR II responsive UC materials and expanding the scope of applications.

Synthesis of micron-sized MLnS 2 :Er 3+ phosphors
MLnS 2 :Er 3+ (M = Li, Na, or K; Ln = Y, Lu, La, or Gd) phosphors were fabricated by the gas-solid reaction method. In the typical synthesis process, the Ln 2 O 3 (15 mmol) and M 2 CO 3 (16.5 mmol) were mixed and thoroughly ground in an agate mortar for 30 min according to the 1:1.1 ratio, and the doping concentrations of Er 3+ were 2, 5, 10, and 15 mol%, respectively. The mixture was transferred to the quartz boat and placed into the tube furnace. The samples were heated to 773 K under argon (Ar) atmosphere, and then saturated vapor of CS 2 (40 kPa) was flowed in Ar at a flow rate of 50 mL/min. The asprepared samples were obtained after calcining the mixture at 1073, 1173, 1273, or 1373 K for two hours and washing by water and ethanol for several times.

Structure and morphology characterization
The X-ray diffraction (XRD) patterns of samples were characterized by the Shimadzu XRD-6000 X-ray diffractometer with a Cu Kα (λ = 0.1541 nm). Scanning electron microscopic (SEM) images of samples were performed by the Hitachi S-4800 scanning electron microscope. Transmission electron microscopic (TEM) and high-resolution transmission electron microscopic (HRTEM) images of samples were measured by using a JEM-2100F transmission electron microscope.

Optical properties measurement
All samples were pressed into the plate shape with the thickness of 2 mm for optical characterization. The absorption spectra of the samples were recorded on a Lambda 750 UV-Vis-NIR spectrophotometer (Perkin-Elmer, USA). The Raman spectra were measured by the Renishaw InVia Raman Microscope (maximum power: 150 W) equipped 532 nm laser as a source of excitation, and the absolute accuracy is 0.55 cm −1 for λ = 532 nm. The UC luminescence spectra and decay curves were measured by a Jobin Yvon iHR550 monochromator equipped with R928 and H10220B-75 photomultiplier tubes from Hamamatsu Photonics using the light sources of laser diodes (808, 980, and 1532 nm; maximum power of 2.0 W), and a pulsed work Horizon OPO laser, for signal collection from 400 nm to 1700 nm with a step length of 0.2 nm. The decay profiles were collected using a Tektronix DPO 5104 digital oscilloscope. A closed-cycle helium gas cryostat (Advanced Research Systems, Model CSW-202) equipped with a Lakeshore Model 331 temperature controller was used to control the sample temperature in the 10-300 K range. The underwater optical communication test was performed by a self-designed system, the signal transmitter was composed of a signal generator and 1532 nm LD, Jobin Yvon iHR550 monochromator and oscilloscope constitute the signal receiver. The UCQYs of MLnS 2 :Er 3+ were directly measured using a commercial setup (XPQY-EQE-SolTM 1.7, Guangzhou Xi Pu Optoelectronics Technology Co., Ltd.) equipped with an integrated sphere (GPS-4P-SL, Labsphere).

Data availability
All data generated or analysed during this study are included in this published article and its supplementary information files. Source data are provided with this paper.
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