Efficient Near-Infrared Luminescence Based on Double Perovskite Cs2SnCl6

Cs2SnCl6 double perovskite has attracted wide attention as a promising optoelectronic material because of its better stability and lower toxicity than its lead counterparts. However, pure Cs2SnCl6 demonstrates quite poor optical properties, which usually calls for active element doping to realize efficient luminescence. Herein, a facile co-precipitation method was used to synthesize Te4+ and Er3+-co-doped Cs2SnCl6 microcrystals. The prepared microcrystals were polyhedral, with a size distribution around 1–3 μm. Highly efficient NIR emissions at 1540 nm and 1562 nm due to Er3+ were achieved in doped Cs2SnCl6 compounds for the first time. Moreover, the visible luminescence lifetimes of Te4+/Er3+-co-doped Cs2SnCl6 decreased with the increase in the Er3+ concentration due to the increasing energy transfer efficiency. The strong and multi-wavelength NIR luminescence of Te4+/Er3+-co-doped Cs2SnCl6 originates from the 4f→4f transition of Er3+, which was sensitized by the spin-orbital allowed 1S0→3P1 transition of Te4+ through a self-trapped exciton (STE) state. The findings suggest that ns2-metal and lanthanide ion co-doping is a promising method to extend the emission range of Cs2SnCl6 materials to the NIR region.


Introduction
The Pb-halide perovskite photovoltaics has been seen in the rapid rise of power conversion efficiency (PCE) in the past several years, expectedly contributing to sustainable development via green energy strategies [1,2]. In addition, Pb-halide perovskites have attracted increasing attention due to their outstanding photoelectric properties, such as their tunable band gap and high photoluminescence quantum yield (PLQY) [3][4][5]. However, there are growing concerns about the lead toxicity to human health and the environment [6,7]. Therefore, various efforts have been made to replace Pb with non-toxic metals, such as tin (Sn), germanium (Ge), bismuth (Bi), and indium (In) [8][9][10][11]. Among those alternative elements, Sn is deemed as a perfect choice because it has the most similar electronic properties in the same group of the periodic table with lead. As expected, tin halide perovskites (CsSnX 3 ) can also offer an outstanding optoelectronic performance including their narrow band gap, low exciton binding energy, and long carrier diffusion length [12][13][14]. Unfortunately, CsSnX 3 perovskites suffer from rather poor stability under ambient conditions due to the easy oxidation of Sn 2+ to Sn 4+ [15,16].
In such a context, Sn 4+ -based perovskite variants (Cs 2 SnX 6 ) are much more stable than CsSnX 3 perovskites [14]. However, Cs 2 SnX 6 exhibits significantly inferior optical properties in the visible and near-infrared (NIR) regions compared to Pb-halide perovskites [17,18]. It is reported that the 6s 2 electrons of Pb 2+ play a major role in avoiding the formation of deep trap defects, resulting in highly efficient optoelectronic processes [19,20]. Whereas, the ns 2 electronic configuration of Sn 4+ is lost in Cs 2 SnX 6 . Therefore, doping elements with ns 2 electrons is a very effective method to improve their optoelectronic performance [21].
For example, Bi 3+ -, Sb 3+ -, and Te 4+ -doped Cs 2 SnX 6 have produced intense blue, red, and yellow emissions, respectively [22][23][24]. In addition, white light emission was obtained from Cs 2 SnX 6 that was co-doped with Bi 3+ and Te 4+ ions [25,26]. Up until now, Cs 2 SnX 6 luminescence has almost covered the whole visible region through ion doping. However, to the best of our knowledge, there are few reports of achieving luminescence in the NIR region in Sn 4+ -based perovskites, especially for Cs 2 SnCl 6 with a wide band gap. While NIR is of significant importance in many applications, including night vision, thermal imaging, bioimaging, and wellness monitoring [27]. Therefore, it is critically demanding to achieve efficient NIR-emitting perovskite derivatives.
Herein, we realized the NIR emission in Te 4+ -and Er 3+ -co-doped Cs 2 SnCl 6 microcrystal following a simple co-precipitation method. Under the low energy excitation at 391 nm, Te 4+ /Er 3+ -co-doped Cs 2 SnCl 6 displays an efficient NIR emission peak at~1540 nm, in contrast to a negligible emission peak at this position in Er 3+ -singly doped Cs 2 SnCl 6 . The energy transfer processes from Te 4+ to Er 3+ f-electrons are proposed and discussed in detail based on the experimental findings.

Crystal Structure and Characterization
The Te 4+ /Er 3+ -co-doped Cs 2 SnCl 6 particles were synthesized through a facile coprecipitation method [33]. To be brief, the precursors SnCl 4 , TeO 2 , and ErCl 3 ·6H 2 O were mixed with HCl and ethanol and dissolved. Thereafter, Cs 2 CO 3 (dissolved in HCl) was added into the reaction mixture, and the perovskite MCs were immediately precipitated. More synthesis details are described in the Supporting Information (SI). As depicted in Figure 1a, the scanning electron microscopy (SEM) image showed that the size of the obtained Cs 2 SnCl 6 crystals with a nominal molar concentration of 1.4% Te 4+ and 10% Er 3+ was mainly in the range of about 1-3 µm ( Figure S1). The energy-dispersive spectroscopy (EDS) mapping in Figure 1b-g demonstrated that the constituent elements were uniformly distributed in the microcrystals, and the estimated Cs:(Sn+Te+Er):Cl composition ratio of microcrystals roughly agreed with the stoichiometric ratio of Cs 2 SnCl 6 , as given in Table S1. Moreover, the exact doping contents of Te 4+ and Er 3+ were determined to be 1.6% and 2.0% by inductively coupled plasma mass spectrometry (ICP-MS) (Table S2). It is noted that the Te 4+ actual concentration was a little higher than its feeding concentration of 1.4%, which is mainly due to the high solubility of Te 4+ in Cs 2 SnCl 6 and the lower formation energy of Cs 2 TeCl 6 than that of Cs 2 SnCl 6 [34,35].
Molecules 2023, 28, x FOR PEER REVIEW 3 of 9 eV), and Cl 2p (199.06 eV) peaks are consistent with the reported values [36] ( Figure S3), proving that the as-prepared Cs2SnCl6 crystals are composed of tetravalent Sn. The peaks located at 586.64 eV and 576.38 eV correspond to Te 4+ 3d, and 170.29 eV to Er 3+ 4d, respectively. The binding energy of Sn 3d was almost same in un-doped and Te 4+ -doped samples, while it shifted toward a high energy side in the Te 4+ /Er 3+ -co-doped Cs2SnCl6 ( Figure S4). After combining the above ICP-MS results, substitutional site of Sn rather than the interstitial site is likely occupied by Te in the doped sample [37].

Optical Properties
The optical properties of Te 4+ /Er 3+ -co-doped Cs2SnCl6 microcrystals were investigated via UV-Vis absorption and photoluminescence (PL) spectra. As shown in Figure 2a, Cs2SnCl6 microcrystals showed an optical absorption edge at around 315 nm, which is in agreement with the previous report [22]. While Er 3+ -singly doped Cs2SnCl6 has a similar result to the undoped one, interestingly, Te 4+ -singly doped and Te 4+ /Er 3+ -co-doped Cs2SnCl6 exhibited intense absorption peaks within the region of 280-450 nm. Compared to the pure white color of the sample without Te 4+ , these new absorption bands changed the hue of the Te 4+ -doped Cs2SnCl6 to a luminous yellow (see the photographs in the inset of Figure 2a). In accordance with the absorption spectra, the PL excitation (PLE) spectra also showed peaks between 280 and 450 nm ( Figure S5). Thereby, the absorption peaks located at 280-320 nm (A), 320-360 nm (B), and 360-450 nm (C) were derived from the Te 4+ -induced ion absorption and could be assigned to the inter-configurational 5s 2 →5s5p transitions of Te 4+ [36,38].
The PL spectra of undoped and doped Cs2SnCl6 upon excitation at 391 nm were given in Figure 2b,c. In the visible region, no emission peaks were observed for both the pristine and Er 3+ -doped Cs2SnCl6 microcrystals (Figure 2b). In contrast, an intense yellow emission at about 577 nm with a large Stokes shift of 127 nm occurs after the doping of Te 4+ in

Optical Properties
The optical properties of Te 4+ /Er 3+ -co-doped Cs 2 SnCl 6 microcrystals were investigated via UV-Vis absorption and photoluminescence (PL) spectra. As shown in Figure 2a, Cs 2 SnCl 6 microcrystals showed an optical absorption edge at around 315 nm, which is in agreement with the previous report [22]. While Er 3+ -singly doped Cs 2 SnCl 6 has a similar result to the undoped one, interestingly, Te 4+ -singly doped and Te 4+ /Er 3+ -co-doped Cs 2 SnCl 6 exhibited intense absorption peaks within the region of 280-450 nm. Compared to the pure white color of the sample without Te 4+ , these new absorption bands changed the hue of the Te 4+ -doped Cs 2 SnCl 6 to a luminous yellow (see the photographs in the inset of Figure 2a). In accordance with the absorption spectra, the PL excitation (PLE) spectra also showed peaks between 280 and 450 nm ( Figure S5). Thereby, the absorption peaks located at 280-320 nm (A), 320-360 nm (B), and 360-450 nm (C) were derived from the Te 4+ -induced ion absorption and could be assigned to the inter-configurational 5s 2 →5s5p transitions of Te 4+ [36,38].
contrast, Te 4+ /Er 3+ -co-doped Cs2SnCl6 displayed an intense NIR emission at 1540 nm and its PLQY was 0.8%. It is also noted that the PL spectrum has other peaks at 1562 nm along with shoulders at around 1480 nm and 1506 nm, which may be caused by the crystal field split manifold of 4 I13/2 and 4 I15/2 states [40]. The phenomenon that the decreased peak intensity at 577 nm accompanies the enhanced emission at 1540 nm in Te 4+ /Er 3+ -co-doped Cs2SnCl6 as compared with the Te 4+ singly doped sample suggests that the energy transfer and sensitization take place between the luminescent centers Te 4+ and Er 3+ in the former. To achieve the highest NIR emission intensity for Te 4+ /Er 3+ -co-doped Cs2SnCl6, the Er 3+ doping concentration was first optimized to 10% via monitoring the NIR emission intensity of Er 3+ -doped Cs2SnCl6 ( Figure S7). Subsequently, the Te 4+ precursor concentrations were varied while keeping a constant Er 3+ concentration of 10%. It was seen in Figure  S8 that the NIR emission intensity gradually increased to a maximum value at 1.4% Te 4+ content and then decreased upon increasing the Te 4+ doping amount (0.2-2.6%). The decrease in PL intensity is due to the concentration quenching effect arising from the energy migration among the ions [39]. As discerned in Figure 2c, Te 4+ -doped Cs2SnCl6 showed no luminescence at all in the NIR region of 1450-1600 nm. However, the luminescence intensity of Er 3+ was remarkably enhanced with increasing Te 4+ concentrations below 1.4%, which confirms the sensitization effect of Te 4+ on the NIR luminescence of Er 3+ .
To better understand the sensitization effect of Te 4+ on the Er 3+ NIR emission, PL and time-resolved PL (TRPL) measurements of Te 4+ /Er 3+ -co-doped Cs2SnCl6 were carried out at different Er 3+ concentrations. As expected, the NIR emission intensity gradually increased with an increasing Er 3+ concentration from 0 to 10% (Figure 3a), while the visible PL intensity continuously decreased (Figure 3b). The intensity of NIR emissions declines as the Er 3+ concentration exceeds 10%, which suggests the occurrence of the concentration quenching effect. For Er 3+ -singly doped Cs2SnCl6, however, different Er 3+ doping amounts all lead to a negligible NIR luminescence ( Figure S9). At the same time, the visible luminescence lifetimes of Te 4+ /Er 3+ -co-doped Cs2SnCl6 also decreased from 4.18 to 3.39 with the increase in the Er 3+ concentration (Figure 3c, Table S4), which corresponds to the continuously increasing energy transfer efficiency (1 − τx/τ0, where τ0 is the lifetime of visible luminescence with Er 3+ doping amount x = 0 [41].) of Te 4+ /Er 3+ from 0.96% to 18.9% and benefits intense NIR emissions. The PL spectra of undoped and doped Cs 2 SnCl 6 upon excitation at 391 nm were given in Figure 2b,c. In the visible region, no emission peaks were observed for both the pristine and Er 3+ -doped Cs 2 SnCl 6 microcrystals (Figure 2b). In contrast, an intense yellow emission at about 577 nm with a large Stokes shift of 127 nm occurs after the doping of Te 4+ in Cs 2 SnCl 6 , and the luminescence intensity of Te 4+ /Er 3+ -co-doped Cs 2 SnCl 6 is a little lower than that of the Te 4+ -singly doped one. Meanwhile, no emission was observed in the undoped and Te 4+ -doped Cs 2 SnCl 6 in the NIR region (Figure 2c). Quite weak NIR emissions of Er 3+ -doped Cs 2 SnCl 6 were observed in the spectra region from 1450 to 1600 nm, originating from the characteristic 4 I 13/2 → 4 I 15/2 transition of the Er 3+ ion ( Figure S6) [38,39]. The NIR emission intensity of Er 3+ -doped Cs 2 SnCl 6 is too weak to evaluate the PLQY. In sharp contrast, Te 4+ /Er 3+ -co-doped Cs 2 SnCl 6 displayed an intense NIR emission at 1540 nm and its PLQY was 0.8%. It is also noted that the PL spectrum has other peaks at 1562 nm along with shoulders at around 1480 nm and 1506 nm, which may be caused by the crystal field split manifold of 4 I 13/2 and 4 I 15/2 states [40]. The phenomenon that the decreased peak intensity at 577 nm accompanies the enhanced emission at 1540 nm in Te 4+ /Er 3+ -co-doped Cs 2 SnCl 6 as compared with the Te 4+ singly doped sample suggests that the energy transfer and sensitization take place between the luminescent centers Te 4+ and Er 3+ in the former.
To achieve the highest NIR emission intensity for Te 4+ /Er 3+ -co-doped Cs 2 SnCl 6 , the Er 3+ doping concentration was first optimized to 10% via monitoring the NIR emission intensity of Er 3+ -doped Cs 2 SnCl 6 ( Figure S7). Subsequently, the Te 4+ precursor concentrations were varied while keeping a constant Er 3+ concentration of 10%. It was seen in Figure S8 that the NIR emission intensity gradually increased to a maximum value at 1.4% Te 4+ content and then decreased upon increasing the Te 4+ doping amount (0.2-2.6%). The decrease in PL intensity is due to the concentration quenching effect arising from the energy migration among the ions [39]. As discerned in Figure 2c, Te 4+ -doped Cs 2 SnCl 6 showed no luminescence at all in the NIR region of 1450-1600 nm. However, the luminescence intensity of Er 3+ was remarkably enhanced with increasing Te 4+ concentrations below 1.4%, which confirms the sensitization effect of Te 4+ on the NIR luminescence of Er 3+ .
To better understand the sensitization effect of Te 4+ on the Er 3+ NIR emission, PL and time-resolved PL (TRPL) measurements of Te 4+ /Er 3+ -co-doped Cs 2 SnCl 6 were carried out at different Er 3+ concentrations. As expected, the NIR emission intensity gradually increased with an increasing Er 3+ concentration from 0 to 10% (Figure 3a), while the visible PL intensity continuously decreased (Figure 3b). The intensity of NIR emissions declines as the Er 3+ concentration exceeds 10%, which suggests the occurrence of the concentration quenching effect. For Er 3+ -singly doped Cs 2 SnCl 6 , however, different Er 3+ doping amounts all lead to a negligible NIR luminescence ( Figure S9). At the same time, the visible luminescence lifetimes of Te 4+ /Er 3+ -co-doped Cs 2 SnCl 6 also decreased from 4.18 to 3.39 with the increase in the Er 3+ concentration (Figure 3c, Table S4), which corresponds to the continuously increasing energy transfer efficiency (1 − τ x /τ 0 , where τ 0 is the lifetime Furthermore, temperature-dependent PL spectra were studied for the as-prepared Te 4+ /Er 3+ -co-doped Cs2SnCl6. Figure 4a showed that the PL intensities of Te 4+ declined monotonically upon increasing the temperature from 80 K to 300 K. This is attributed to the increased non-radiative transition probability of Te 4+ at higher temperatures and the thermal-enhanced energy transfer from Te 4+ to Er 3+ [33]. In addition, the full width at half maximum (FWHM) increased as the temperature increased ( Figure S10). A high Huang-Rhys factor (S) of 18 was obtained according to the temperature dependence of FWHM, which reveals the strong electron-phonon coupling effect in Cs2SnCl6:Te and facilitates the formation of STEs [24]. Nevertheless, the integrated PL intensities of Er 3+ increased slightly with rising temperature (Figures 4b and S11), which was accompanied by small variations in the PL lifetime at 1540 nm within this temperature range (Figure 4c, Table S5). The temperature-stable PL intensity of the NIR emission reflected a good protection of Er 3+ 4f electrons by the outer electrons in 5s 2 5p 6 shells [42]. It is worth noting that the intensity ratio of I1540/I1562 declined with an increasing temperature ( Figure S12). This variation is caused by the population redistribution among crystal field split manifolds of 4 I13/2 and 4 I15/2 states at different temperatures, which is consistent with the similar observations made in different Er 3+ -doped hosts [39,43].
According to the above optical results, the energy transfer mechanism was described in Figure 4d based on the energy level alignment of Te 4+ and Er 3+ . Thanks to the strong electron-phonon coupling in Cs2SnCl6 with soft lattice, transient elastic lattice deformation occurs upon photogeneration, where excitons tend to be self-trapped due to its lower energy and form self-trapped excitons (STEs) [43]. Therefore, carriers excited from 1 S0 to 3 P1 of Te 4+ ion under 391 nm excitation relax to form STEs, which then recombine to yield the broad band yellow emission at 577 nm in Te 4+ doped Cs2SnCl6. For Er 3+ singly-doped samples, the electrons in ground state 4 I15/2 transited to excited state 4 I13/2 under low energy excitation, and then generated very weak NIR emission (1540 nm) due to the parity-forbidden transitions within the 4f N configurations [24,40]. In Te 4+ /Er 3+ co-doped Cs2SnCl6, however, partial excitation energy was transferred from STEs to the well-matched 2 H11/2 energy level of Er 3+ ions in addition to the yellow emission [33,37]. The transferred carriers relaxed non-radiatively to the 4 I13/2 energy level, and finally return to the ground states of the 4 I15/2 energy level through radiative transition, resulting in the enhanced 1540 nm NIR emissions at the expense of the weakened yellow luminescence from STEs. Furthermore, temperature-dependent PL spectra were studied for the as-prepared Te 4+ /Er 3+ -co-doped Cs 2 SnCl 6 . Figure 4a showed that the PL intensities of Te 4+ declined monotonically upon increasing the temperature from 80 K to 300 K. This is attributed to the increased non-radiative transition probability of Te 4+ at higher temperatures and the thermal-enhanced energy transfer from Te 4+ to Er 3+ [33]. In addition, the full width at half maximum (FWHM) increased as the temperature increased ( Figure S10). A high Huang-Rhys factor (S) of 18 was obtained according to the temperature dependence of FWHM, which reveals the strong electron-phonon coupling effect in Cs 2 SnCl 6 :Te and facilitates the formation of STEs [24]. Nevertheless, the integrated PL intensities of Er 3+ increased slightly with rising temperature (Figure 4b and Figure S11), which was accompanied by small variations in the PL lifetime at 1540 nm within this temperature range (Figure 4c, Table S5). The temperature-stable PL intensity of the NIR emission reflected a good protection of Er 3+ 4f electrons by the outer electrons in 5s 2 5p 6 shells [42]. It is worth noting that the intensity ratio of I 1540 /I 1562 declined with an increasing temperature ( Figure S12). This variation is caused by the population redistribution among crystal field split manifolds of 4 I 13/2 and 4 I 15/2 states at different temperatures, which is consistent with the similar observations made in different Er 3+ -doped hosts [39,43].
According to the above optical results, the energy transfer mechanism was described in Figure 4d based on the energy level alignment of Te 4+ and Er 3+ . Thanks to the strong electron-phonon coupling in Cs 2 SnCl 6 with soft lattice, transient elastic lattice deformation occurs upon photogeneration, where excitons tend to be self-trapped due to its lower energy and form self-trapped excitons (STEs) [43]. Therefore, carriers excited from 1 S 0 to 3 P 1 of Te 4+ ion under 391 nm excitation relax to form STEs, which then recombine to yield the broad band yellow emission at 577 nm in Te 4+ doped Cs 2 SnCl 6 . For Er 3+ singlydoped samples, the electrons in ground state 4 I 15/2 transited to excited state 4 I 13/2 under low energy excitation, and then generated very weak NIR emission (1540 nm) due to the parity-forbidden transitions within the 4f N configurations [24,40]. In Te 4+ /Er 3+ co-doped Cs 2 SnCl 6 , however, partial excitation energy was transferred from STEs to the well-matched 2 H 11/2 energy level of Er 3+ ions in addition to the yellow emission [33,37]. The transferred carriers relaxed non-radiatively to the 4 I 13/2 energy level, and finally return to the ground states of the 4 I 15/2 energy level through radiative transition, resulting in the enhanced 1540 nm NIR emissions at the expense of the weakened yellow luminescence from STEs.

Moisture Stability
Moisture stability is essential for practical applications of perovskite materials. Impressively, the NIR emission was very stable when the samples were exposed to air and even immersed in water. As shown in Figure S13, the XRD pattern of Te 4+ /Er 3+ -co-doped Cs2SnCl6 microcrystals was basically unchanged after being left in ambient air for 100 days. The PL intensity decreased by only 13 % compared to the original data ( Figure S14). Moreover, a strong NIR emission of the microcrystals was maintained after being immersed in deionized water for 2 h ( Figure S15). Even after the samples were soaked in water for 8 h, the emissions still remained at 30 % of the initial level, while the shape and position of XRD peaks remained unchanged ( Figure S16). The superior stability in both the structure and NIR luminescence renders Cs2SnCl6 microcrystals more promising for practical applications relative to the common lead halide perovskites.

Conclusions
In conclusion, intense and multiple NIR emissions at 1540 nm and 1562 nm were achieved in a Sn-based double perovskite Cs2SnCl6 through Te 4+ /Er 3+ co-doping. Under a low energy excitation at 391 nm, the NIR luminescence originating from the 4f→4f transition of Er 3+ was significantly enhanced due to the effective energy transfer from the 1 S0→ 3 P1 transition of Te 4+ . Furthermore, the Te 4+ /Er 3+ Cs2SnCl6 microcrystals prepared via the simple co-precipitation method exhibited excellent emission and moisture stability. These findings bring novel emissive features to Cs2SnCl6 double perovskites, thus expanding their optoelectronic properties for future applications, such as NIR biosensors, anticounterfeit technologies, and optical fiber communication.
Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Figure S1: Particle size statistics of MCs; Figure S2: Enlarged shifts of (220)

Moisture Stability
Moisture stability is essential for practical applications of perovskite materials. Impressively, the NIR emission was very stable when the samples were exposed to air and even immersed in water. As shown in Figure S13, the XRD pattern of Te 4+ /Er 3+ -co-doped Cs 2 SnCl 6 microcrystals was basically unchanged after being left in ambient air for 100 days. The PL intensity decreased by only 13% compared to the original data ( Figure S14). Moreover, a strong NIR emission of the microcrystals was maintained after being immersed in deionized water for 2 h ( Figure S15). Even after the samples were soaked in water for 8 h, the emissions still remained at 30% of the initial level, while the shape and position of XRD peaks remained unchanged ( Figure S16). The superior stability in both the structure and NIR luminescence renders Cs 2 SnCl 6 microcrystals more promising for practical applications relative to the common lead halide perovskites.

Conclusions
In conclusion, intense and multiple NIR emissions at 1540 nm and 1562 nm were achieved in a Sn-based double perovskite Cs 2 SnCl 6 through Te 4+ /Er 3+ co-doping. Under a low energy excitation at 391 nm, the NIR luminescence originating from the 4f→4f transition of Er 3+ was significantly enhanced due to the effective energy transfer from the 1 S 0 → 3 P 1 transition of Te 4+ . Furthermore, the Te 4+ /Er 3+ Cs 2 SnCl 6 microcrystals prepared via the simple co-precipitation method exhibited excellent emission and moisture stability. These findings bring novel emissive features to Cs 2 SnCl 6 double perovskites, thus expanding their optoelectronic properties for future applications, such as NIR biosensors, anti-counterfeit technologies, and optical fiber communication.

Supplementary Materials:
The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules28083593/s1, Figure S1: Particle size statistics of MCs; Figure S2: Enlarged shifts of (220) peak of MCs; Figure S3: XPS spectra of MCs; Figure S4: Representative XPS spectra of Sn 3d in the MCs; Figure S5: PLE spectra of MCs in the visible region; Figure S6: PL spectra of MCs in the NIR region; Figure S7: PL emission spectra of MCs in the NIR region; Figure S8: PL emission spectra of x% Te 4+ -10% Er 3+ co-doped MCs; Figure S9: NIR intensity variations of MCs with different Er 3+ concentrations; Figure S10: Fitting results of the FWHM; Figure S11: Integral intensity of NIR peak at different temperatures ; Figure S12: Peak intensity ratio of 1540 nm (I 1540 )/1562 nm (I 1562 ) emissions at different temperatures; Figure S13: XRD patterns of MCs before and after their exposure to ambient air; Figure S14: PL emission spectra of MCs before and after their exposure to ambient air; Figure S15: PL emission spectra of MCs before and after their soak in water; Figure S16: XRD patterns of MCs before and after their soak in water; Table S1: Elemental analyses of the MCs by EDS;