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-fluorescence1-5. The first discovery of an effective UC host material was NaYF4 by 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 applications7-9, photon-conversion devices10,11, super-resolution nanoscopy12-14, and information storage and security etc15-17. 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 luminescence18-23.
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 etc24-27. 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 systems28-31. 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 themselves32-40. 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-NaYS2 presents 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 NaLnS2 prepared 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, MLnS2 phosphors 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 MLnS2 phosphors 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 4S3/2/2H11/2→4I15/2 and 4F9/2→4I15/2 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-MLnS2 are 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-MLnS2 are randomly disordered on an identical lattice site, inevitably leading to the intensified negative energy exchange42,43. 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 4I9/2, 4I11/2→4I15/2 transitions of Er3+, respectively. A schematic illustration of UC 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 on the ground state (4I15/2) of Er3+ ions are pumped to 4I13/2, and further excited to 4I9/2 via resonantly illumination, producing the 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 processes, realizing the green emissions. Moreover, a part of electrons on 4I9/2 is non-radiative to 4I11/2, then populates to 4F9/2, generating 4I11/2→4I15/2 and red (4F9/2→4I15/2) 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+ presents both 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 4S3/2→4I15/2 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 4I13/2 and 4I9/2 play 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 4I13/2 and 4I9/2 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 for 4I9/2; 4.9-8.0 ms for 4I13/2) is significantly shorter than that of β-MLnS2:Er3+ (7.4-9.2 ms for 4I9/2; 13.1-30.2 ms for 4I13/2). 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 4I13/2→4I15/2 and 4I9/2→4I15/2 transitions, respectively. Such ultralong decay time constants could be stemmed from the low non-radiative transition and weak electron-phonon coupling in NaYS2 host. 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 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. It is in line with the results in Fig. 2c,d, where the stronger of UC emissions, the longer lifetimes of 4I13/2→4I15/2 and 4I9/2→4I15/2 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):
![](https://myfiles.space/user_files/58854_b38fc7f3db2c487f/58854_custom_files/img1625854836.png)
where R3 presents the radiative rates of 4I9/2 level of Er3+, and R'ij is 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. σij denotes the absorption cross-section between level i and j of Er3+. It can be found from Equation (1) that the Igreen 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 4I9/2 level, especially for 4I11/2 level can lead to the strong green emission. The Igreen is inversely proportional to the R3 and R'32, that is, it is inversely proportional to the radiative lifetime (τ) of 4I9/2 level. The ultralong lifetime of 4I9/2 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 4I9/2→4I11/2, 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 folds than others under around 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 4I9/2 and 4I13/2 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 4I9/2 and 4I13/2 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:Er3+. 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 etc49,50. As a proof of concept, we designed and fabricated narrowband responsive NIR photodetectors (PDs) using a simple NaYS2:Er3+/MAPbI3 hybrids. 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+/MAPbI3 PDs 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-NaYS2 phosphors 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.