Introduction

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-fluorescence1,2,3,4,5. The first discovery of an effective UC host material (NaYF4) was made by Pierce in 19726. Until 2004, the hexagonal phase NaYF4 (β-NaYF4) with high UC efficiency was demonstrated7. With the development of nanotechnology, lanthanide doped UC materials experienced the explosive growth, especially in biological applications8,9,10, photon-conversion devices11,12, super-resolution nanoscopy13,14,15, and information storage and security16,17,18. Much efforts have been devoted to manipulate UC, and Yb3+, Nd3+_sensitized energy transfer (ET) and interfacial energy transfer (IET) are recognized as the promising ways to achieve efficient UC luminescence19,20,21,22,23,24.

The near-infrared second spectral region (NIR II), especially for 1530 nm, shows great potential with respect to the information storage and security, 3D display, and high-resolution imaging, etc25,26,27,28. 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 systems29,30,31,32. 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:Yb3+, Er3+/Tm3+) 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 (such as 808 and 980 nm). Er3+ has a well spectral response around 1530 nm with a relatively large absorption cross-section, but it is not suitable for sensitizer owing to the lack of matchable activators. A number of reports has announced Er3+ doped UC materials pumping around 1530 nm realized via the excited state absorption process of Er3+ themselves33,34,35,36,37,38,39. 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 (MLnS2; 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 energy40, which are promising materials for efficient NIR II response UC. It was discovered in 1970s41, and Güdel et al. reported the UC emissions with UCQY of 21% in NaYS2:Er3+ 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 K42. 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: MLnS2:Er3+ (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 (RLn3+/RM+). It should be highlighted that β-NaYS2 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 (4I9/2, 9.24 ms; 4I13/2, 30.27 ms) of excited state levels of Er3+. Compared to the commercial NaYF4:Yb3+, Er3+ phosphor, the NaYS2:Er3+ owns much higher UCQY, brightness, and spectral stability of illumination and temperature under the same conditions. Then, the NaYS2:Er3+ 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 MLnS2:Er3+ phosphors

A series of MLnS2:Er3+ 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 MYS2 and NaLnS2 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 Y2O2S is formed at 1273 and 1373 K (Supplementary Figs. 3, 4), which leads to the decrease of UC luminescence. Interestingly, MLnS2 phosphors have two structural types: NaLaS2 and LiYS2 possess cubic structure (α-), while KYS2, NaGdS2, NaLuS2, and NaYS2 are indexed as trigonal phase (β-) under the same conditions (Fig. 1a, b). As represented in Fig. 1c, the phase structure of MLnS2 phosphors mainly depends on the radius ratios between the trivalent lanthanide ions and the monovalent alkali metal ions (RLn3+/RM+)43. When the RLn3+/RM+ ratio is larger than 1.0, MLnS2 tends to form cubic phase (NaCl or Th3P4 type). Conversely, the β-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 La3+ are coordinated with six S2− to form (Na/La-S)6 octahedrons44. In β-NaYS2, both Na+ and Y3+ are six-fold coordinated by S2− in form of a regular octahedron and contain empty voids on the special 3a and 3b sites, respectively. The S2− are surrounded by three Na+ and three Y3+. 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 Y3+ are orderly situated in alternating NaS6 and YS6 octahedral layers.

Fig. 1: Structure and morphology characterizations of MLnS2:Er3+.
figure 1

a, b XRD patterns of MYS2:Er3+ (M = Li, Na, K) and NaLnS2:Er3+ (Ln = La, Y, Lu, Gd) prepared at 1173 K. c Radius ratios between the trivalent lanthanide ion and the monovalent alkali metal ion (RLn3+/RM+) of MLnS2 (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 NaLaS2 (α-NaLaS2) and trigonal NaYS2 (β-NaYS2). f XRD patterns of β-NaYS2 doped with 2, 5, 10, 15 mol% Er3+. TEM, HRTEM images, and corresponding SAED of g–i α-NaLaS2 and j–l β-NaYS2. Source data is provided in this paper.

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In addition, the XRD patterns of NaYS2 with various doping concentration of Er3+ are recorded in Fig. 1f. β-NaYS2 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 NaYS2:Er3+ shift to large angle on 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 Supplementary Fig. 5. The transmission electron microscopic (TEM) and high resolution TEM (HRTEM) images of the NaLaS2:Er3+ and NaYS2:Er3+ 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 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 MLnS2:Er3+ phosphors with two phases are successfully fabricated.

UC luminescence properties of MLnS2:Er3+

The NIR II response UC luminescence characteristics of MLnS2:Er3+ under 1532 nm excitation are systematically evaluated. As presented in Fig. 2a, b, the blue, green, and red luminescence regions are identified in the normalized visible UC spectra of MLnS2:Er3+, assigned to the 2H9/2 → 4I15/2, 4S3/2/2H11/2 → 4I15/2, and 4F9/2 → 4I15/2 transitions of Er3+, respectively. Note that, β-MLnS2:Er3+ show 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. NaYS2:Er3+ is found to be the most efficient UC phosphor in the MLnS2:Er3+ series under 1532 nm excitation.

Fig. 2: UC luminescence characterization of MLnS2:Er3+.
figure 2

a, b Normalized visible UC luminescence spectra of MYS2:Er3+ (M = Li, Na, K) and NaLnS2:Er3+ (Ln = La, Y, Lu, Gd) under 1532 nm excitation. c, d Integral luminescence intensity and green to red emission ratio (Ig/Ir) of MYS2:Er3+ (M = Li, Na, K) and NaLnS2:Er3+ (Ln = La, Y, Lu, Gd). e UC luminescence spectrum of NaYS2:Er3+ in visible and NIR regions. f UC luminescence populating mechanism of NaYS2:Er3+ 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 NaYS2 doped with 2, 5, 10, 15 mol% Er3+ (inset: integral intensity varies with concentration of Er3+). h-i, Lifetimes of 4I9/2→4I15/2 and 4I13/2→4I15/2 transitions of Er3+ in MLnS2:Er3+. Source data is provided in this paper.

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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. The Raman spectra of the MLnS2 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 relaxation47. The phonon energies of all the MLnS2 hosts locate within 260–280 cm−1, consistently with the reported results48. Considering the similar phonon energies of MLnS2 hosts, such phenomenon can be explained by the following reason: the Er3+ in β-MLnS2 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 Er3+ in α-MLnS2 are randomly disordered on an identical lattice site, inevitably leading to the intensified negative energy exchange49,50. The comparison of UC emission intensity between α-NaLaS2:0.1–15%Er3+ and β-NaYS2:0.1–15%Er3+ (Supplementary Fig. 7) further proves the above deduction. In addition, the distance of the nearest Y3+ (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, the large adjacent distance d often causes a high doping level and intense luminescence. Crystal structural data for NaYS2 and NaYF4 are summarized in Supplementary Table 1. The excellent characteristics of NaYS2 compared to NaYF4 are mainly ascribed to the layer structure, lower unit cell symmetry (rhombohedral vs hexagonal), longer minimum Y3+ separation distance, stronger covalence, and lower phonon energy. These advantages of NaYS2 all contribute to the efficient UC of NaYS2:Er3+.

The visible-NIR UC luminescence of NaYS2:Er3+ 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 to ~1050 nm are attributed to the 4I9/2, 4I11/2 → 4I15/2 transitions of Er3+, respectively. A schematic illustration of populating mechanism for NaYS2:Er3+ 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 (4I15/2) of Er3+ are pumped to 4I13/2, and further excited to 4I9/2 via resonant illumination, generating 4I9/2 → 4I15/2 transition. The electrons on 4I9/2 jump to the higher excited levels (4S3/2/2H11/2) through continuous three-photon absorption, realizing the green emissions. A fraction of electrons on 4I9/2 decays non-radiatively to 4I11/2, then populates the 4F9/2 state and thereby generating 4I11/2 → 4I15/2 and red (4F9/2 → 4I15/2) 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 MLnS2:Er3+ are realized through an ESA process. It is different from the UC from typical energy transfer systems containing Yb3+, Er3+, where Yb3+ serves as a sensitizer and Er3+ as an activator. Thus, the visible and NIR emissions are ascribed to three- and two-photon UCs, as evidenced by the power dependence of the integral UC emission intensity of NaYS2:Er3+ (Supplementary Fig. 8).

As displayed in Fig. 2g, the visible UC luminescence intensity of NaYS2:Er3+ initially increases, then decreases with increasing the doping concentration of Er3+, in which the optimal doping concentration of Er3+ is 5 mol%. Similarly, the corresponding decay time curves and constants of Er3+ in Supplementary Figs. 9–13 and Supplementary Table 2 present that the lifetime first increases, and then decreases as the Er3+ concentration continuously increases to 15 mol%. Such increase of decay time constants may be attributed to the re-absorption induced by high doping concentration of Er3+. A further decrease results from concentration quenching. It should be highlighted that the decay profiles of 4S3/2 → 4I15/2 transition (green emissions) change from single exponential to double exponential, suggesting that cooperative UC among Er3+ is involved. While they decrease for 4I9/2 → 4I15/2 (800 nm), 4I11/2 → 4I15/2 (1000 nm), and 4I13/2 → 4I15/2 (1500 nm) on increasing the Er3+ doping concentration from 2 to 15 mol%. According to the previous reports51, the cross-relaxation should dominate the above process among Er3+. Three possible cross-relaxation processes (4I13/2 + 4I13/2 → 4I15/2 + 4I9/2; 4I13/2 + 4I11/2 → I15/2 + 4F9/2; 4I13/2 + 4I9/2 → 4I15/2 + 4S3/2) are added in Fig. 2f.

As revealed in Fig. 2g, the energy levels of 4I13/2 and 4I9/2 play a crucial role 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 (Supplementary Figs. 14, 15) and the lifetimes histogram of 4I13/2 and 4I9/2 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-MLnS2:Er3+ (2.4–2.84 ms for 4I9/2; 4.69–7.08 ms for 4I13/2) are significantly shorter than that of β-MLnS2:Er3+ (7.45-9.24 ms for 4I9/2; 13.12-30.27 ms for 4I13/2). M+ and Ln3+ are randomly disordered on an identical lattice site in α-MLnS2 with cubic structure, inevitably leading to the intensified negative energy exchange. While the Er3+ in β-MLnS2 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 4I13/2 level in the β compared to the α polymorph is observed. Excitingly, the NaYS2:Er3+ phosphors have the longest lifetimes in all the samples, reaching 30.27 ms and 9.24 ms for 4I13/2 → 4I15/2 and 4I9/2 → 4I15/2 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 NaYS2:Er3+ mean that the electrons on 4I13/2 and 4I9/2 levels stay longer, which is in favor of being re-simulated to higher green energy levels (4S3/2/2H11/2), 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 4I13/2 → 4I15/2 and 4I9/2 → 4I15/2 transitions in MLnS2:Er3+ 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 Er3+ in NaYS2 are significantly shorter than that of the other UC materials52. It can be concluded that the long lifetimes of NaYS2:Er3+ phosphors originate from the long radiative and non-radiative lifetimes of Er3+ (See Supplementary Fig. 2 and Supplementary Note 3). The long lifetimes are caused by the crystal structure and low phonon energy of NaYS2 (Supplementary Fig. 6) that lead to less non-radiative transition and weak electron-phonon coupling in NaYS2 host.

In order to understand qualitatively the UC luminescence mechanism in Er3+, a set of rate equations were established based on the UC process in MLnS2:Er3+ phosphors (in Fig. 2f, See Supplementary Note 4):

$${I}_{{green}}\propto \frac{{\sigma }_{13}^{2}{\sigma }_{35}{\rho }^{3}}{{C}_{8}\left({R}_{1}+{\delta }_{13}\rho \right)\left({R}_{3}+{{R}^{{\prime} }}_{32}+{\sigma }_{35}\rho \right)},$$
(1)

where R1 and R3 represent the radiative rates of 4I13/2 and 4I9/2 level of Er3+, and R’ij is non-radiative rate from level i to level j. ρ is the laser photon number density. σij denotes the absorption cross-section between level i and j of Er3+. C8 represents cross-relaxion process of 4I13/2+4I9/2→4I15/2+4S3/2. It can be concluded from Eq. (1) that the Igreen 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 \({\sigma }_{35}\) and \({\sigma }_{13}^{2}\), and the larger absorption cross-section of 4I9/2 level, especially for 4I11/2 level can lead to the strong green emission. The Igreen is inversely proportional to the R1, R3 and R'32, that is, it is proportional to the radiative lifetime (τ) of 4I13/2, 4I9/2 levels. From the UC spectra and dynamics at low temperature (Supplementary Figs. 16–21), the radiative lifetimes of Er3+ is much longer than that of commercial NaYF4:Yb3+, Er3+52, leading to efficient green UC emissions in NaYS2:Er3+. In addition, the long non-radiative lifetimes of Er3+ lead to weak C8 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 Er3+.

Rare earth-doped β-NaYF4 (such as β-NaYF4:Yb3+, Er3+) has been recognized as the most efficient UC material. Therefore, the commercial NaYF4:Yb3+, Er3+ phosphor of similar size (Supplementary Fig. 22) is selected as a comparison with the NaYS2:Er3+ phosphor. Figure 3a shows the normalized UC luminescence spectra of NaYS2:Er3+ under 1532 nm excitation and NaYF4:Yb3+,Er3+ 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 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 4I9/2→4I11/2 and further confirming the weak electron-phonon coupling. Supplementary Figs. 23 demonstrates that NaYS2:Er3+ displays more efficient UC emission 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 Supplementary Figs. 24, 25. The brightness of NaYS2:Er3+ is as high as 8224 cd/m2, while it is 3985 cd/m2 for NaYF4:Yb3+, Er3+ 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 NaYS2:Er3+ excited with 1532 nm (0.27–0.95 W cm−2) and NaYF4:Yb3+, Er3+ 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 NaYS2:Er3+ is also higher than that of NaYF4:Yb3+, Er3+. The UCQYs of MLnS2:Er3+ (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 MLnS2:Er3+ 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 NaYS2:Er3+ is best, reaching 2.6%, which is much higher than that of NaYF4:Yb3+, Er3+ (1.3%) under the illumination of 980 nm with the same power density (Supplementary Fig. 27). We estimated the theoretical UCQY of NaYS2:Er3+ 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 NaYS2:Er3+ is 7.6% and 3.0% at low temperature and room temperature, respectively. Compared to the other representative UC materials, the UCQY of NaYS2:Er3+ significantly improves under around 1532 nm excitation, whereas the excitation power density in our system is much smaller than that of the literatures53,54,55,56. For example, the reported UCQYs of LiYF4:Er3+ and SrF2:5%Yb3+, 1%Er3+ 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, NaYS2:Er3+ show bright UC luminescence under 980 nm excitation. The UCQYs of NaYS2:Er3+ 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 cross-section of Er3+ at 980 nm is much smaller than that of at 1532 nm58, the UC intensity/efficiency of NaYS2:Er3+ excited at 980 is lower than that of 1532 nm excitation (Supplementary Fig. 29). The UC emission intensity of NaYS2:Er3+ compared with commercial NaYF4:Yb3+, Er3+ is shown in Supplementary Fig. 30. The UC emission intensity of NaYS2:Er3+ is just lower 8.1 fold than that of NaYF4:Yb3+, Er3+ excited at the same power density of 980 nm (0.45 W cm−2; Supplementary Fig. 30). Considering the higher concentration (18%) and absorption cross-section (1.2 × 10−20 cm2) at 980 nm of Yb3+ in NaYF4:Yb3+, Er3+ than that of Er3+ (5%, 1.7 × 10−21 cm2), NaYS2:Er3+ is indeed efficient and a promising UC material for 980 nm pumping. 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. It should be highlighted that green and red UC emissions in NaYS2:Er3+ are from a three-photon process under 1532 nm excitation, while they are a two-photon process in the NaYF4:Yb3+, Er3+ 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 UC59. Such efficient UC emissions-induced ESA may be ascribed to the significantly longer lifetimes of 4I9/2 and 4I13/2 levels of Er3+ in NaYS2:Er3+, which are 9.4 fold and 3.05 fold longer than that of NaYF4:Yb3+, Er3+ (0.98 ms and 9.92 ms for 4I9/2 and 4I13/2 levels, as shown in Fig. 3c and Supplementary Table 3). UC emission as a multistep process not only relies on the absorption cross-section of rare earth ions and excitation light intensity, but also strongly depends on the electrons population on the intermediate energy levels. The long lifetime of the intermediate energy levels indicates more electrons resting on them for longer time, is of significance to realize efficient UC in NaYS2:Er3+, in accordance to the previous reports60. Based on the theory of multi-phonon relaxation (See Supplementary Note 3, Supplementary Fig. 21, and Supplementary Table 5), both the radiative and non-radiative rates in NaYS2:Er3+ are much smaller than that of NaYF4:Yb3+, Er3+ 52. These can be explained by the ordered layered structures and low phonon energy of NaYS2 host. Such long lifetimes of rare earth ions in MLnS2 hosts are in favor of multi-photon UC, providing novel approach to develop efficient UC materials.

Fig. 3: Luminescence properties comparison of NaYS2:Er3+ (1532 nm) and commercial NaYF4:Yb3+, Er3+ (980 nm).
figure 3

a Normalized UC luminescence spectra of NaYS2:Er3+ under 1532 nm excitation and NaYF4:Yb3+, Er3+ under 980 nm excitation with the same power density (0.45 W cm−2). b UCQYs of NaYS2:Er3+ under 1532 nm excitation at different power densities. c Decays curves of 4I9/2→4I15/2 and 4I13/2→4I15/2 transitions of Er3+ in NaYF4:Yb3+, Er3+ and NaYS2:Er3+. d Normalized UC luminescence spectra of NaYS2:Er3+ (1532 nm) and NaYF4:Yb3+, Er3+ (980 nm) at 0.45 and 1.45 W cm−2. e CIE chromaticity coordinates and luminescence detail images of NaYS2:Er3+ under 1532 nm excitation and NaYF4:Yb3+, Er3+ under 980 nm excitation as the power density increase from 0.45 to 1.45 W cm−2. f Temperature dependence of UC luminescence ratios between red and green emissions. Source data is provided in this paper.

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In addition, the UC spectra of NaYS2:Er3+ 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 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:Er3+. These indicate that NaYS2:Er3+ 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 etc61,62. As a proof of concept, we designed and fabricated narrowband responsive NIR photodetectors (PDs) using a simple NaYS2:Er3+/MAPbI3 hybrid. As illustrated in Fig. 4a, a high-quality MAPbI3 film acting as the photon-to-current material was spin-coated on the top of a NaYS2:Er3+ film (the top view SEM image of the NaYS2:Er3+/MAPbI3 hybrids is shown in Supplementary Fig. 31), Then the silver electrodes were deposited on the MAPbI3 film. The working mechanism of the NaYS2:Er3+/MAPbI3 PD can be explained in Supplementary Fig. 32. Briefly, the NaYS2:Er3+ 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 MAPbI3 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 NaYS2:Er3+/MAPbI3 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 NaYS2:Er3+/MAPbI3 PDs for 808, 980, and 1532 nm. This is in well agreement with the UC luminescence intensities for the NaYS2:Er3+ 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 photo-responsivity (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 NaYS2:Er3+/MAPbI3 PDs are determined to be 0.26 A/W, 0.44 A/W, and 0.73 A/W; 0.46\(\times\)1010 Jones, 0.56\(\times\)1010 Jones, and 0.84\(\times\)1010 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 photocurrentIph, divided by the number of incident photons per second. In a detector with 100% EQE, R = 1 A/W for photon energy Eph = 1 eV63. The photoconductive gain leads to the EQE > 50% when applied a bias in PDs, similar to previous reports64,65,66. In general, UC visible emissions of NaYS2:Er3+ under 1532 nm excitation excite the semiconductor (MAPbI3) 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 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 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 Supplementary Fig. 34, the photodetection thresholds for the NaYS2:Er3+/MAPbI3 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 NaYS2:Er3+ phosphors demonstrate the excellent capability for multi-wavelength detection and good performance (Supplementary Table 6).

Fig. 4: Multispectral narrowband NIR photodetection and underwater information transmission of NaYS2:Er3+ phosphor.
figure 4

a Schematic illustration of structure and working mechanism in the NaYS2:Er3+/MAPbI3 PDs. b Photocurrent response of NaYS2:Er3+/MAPbI3 PDs under 808, 980, and 1532 nm excitation at power density of 5 mW cm−2, respectively (inset: the responsivity and detectivity of NaYS2:Er3+/MAPbI3 device under separately 808, 980, and 1532 nm excitation). c On-off switching currents at lowest detectable excitation power density of NaYS2:Er3+/MAPbI3 device under 808, 980, and 1532 nm, respectively. d Schematic illustration of the NaYS2:Er3+ 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 NaYS2:Er3+ in air and water at different distance between transmitter and receiver. Source data is provided in this paper.

Full size image

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 etc67. 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 transmission68. 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, NaYS2:Er3+ 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 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 nm laser diode (LD) 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 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 NaYS2:Er3+ are basically the same in these two media. These suggest that such NIR II-responsive NaYS2:Er3+ 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 NaYS2:Er3+ phosphor using 1532 nm as light source possesses great value in underwater optical communication.

Discussion

In summary, the Er3+ doped MLnS2 (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-NaYS2 phosphor is recognized as the most UC material 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 exceptionally long lifetimes of excited state levels of NaYS2:Er3+. 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+ phosphor. β-NaYS2:Er3+ realizes a breakthrough UC efficiency as high as 2.6% under 1532 nm excitation, and the brightness of NaYS2:Er3+ is 8224 cd/m2 under 1532 nm irradiation. Such high optical performances can be assigned to low non-radiative transition and electron-phonon coupling in NaYS2:Er3+ 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.

Methods

Materials

Lanthanide oxides (Y2O3, Lu2O3, La2O3, Gd2O3, or Er2O3; 99.99%), and alkaline metal carbonates (Na2CO3, K2CO3, or Li2CO3; A.R.) were used as the primary materials. Carbon disulfide (CS2, Aladdin, ≥99%) was employed as the sulfur source.

Synthesis of micron-sized MLnS2:Er3+ phosphors

MLnS2:Er3+ (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 Ln2O3 (15 mmol) and M2CO3 (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 Er3+ 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 CS2 (40 kPa) was flowed in Ar at a flow rate of 50 mL/min. The as-prepared 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 MLnS2:Er3+ 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).