Near‐Complete Suppression of NIR‐II Luminescence Quenching in Halide Double Perovskites for Surface Functionalization Through Facet Engineering

Abstract Lanthanide‐based NIR‐II‐emitting materials (1000–1700 nm) show promise for optoelectronic devices, phototherapy, and bioimaging. However, one major bottleneck to prevent their widespread use lies in low quantum efficiencies, which are significantly constrained by various quenching effects. Here, a highly oriented (222) facet is achieved via facet engineering for Cs2NaErCl6 double perovskites, enabling near‐complete suppression of NIR‐II luminescence quenching. The optimally (222)‐oriented Cs2Ag0.10Na0.90ErCl6 microcrystals emit Er3+ 1540 nm light with unprecedented high quantum efficiencies of 90 ± 6% under 379 nm UV excitation (ultralarge Stokes shift >1000 nm), and a record near‐unity quantum yield of 98.6% is also obtained for (222)‐based Cs2NaYb0.40Er0.60Cl6 microcrystallites under 980 nm excitation. With combined experimental and theoretical studies, the underlying mechanism of facet‐dependent Er3+ 1540 nm emissions is revealed, which can contribute to surface asymmetry‐induced breakdown of parity‐forbidden transition and suppression of undesired non‐radiative processes. Further, the role of surface quenching is reexamined by molecular dynamics based on two facets, highlighting the drastic two‐phonon coupling effect of a hydroxyl group to 4I13/2 level of Er3+. Surface‐functionalized facets will provide new insights for tunable luminescence in double perovskites, and open up a new avenue for developing highly efficient NIR‐II emitters toward broad applications.


Introduction
[3] Among them, lead-free halide perovskites are considered promising luminescent candidates for greatly overcoming the instability and toxicity of lead halide perovskites. [4][7] What is more, the octahedral coordination environment in DPs provides abundant opportunities for doping with lanthanide (Ln 3+ ), transition metal ions (Mn 2+ and Cr 3+ ), and ns 2 electrons of Bi 3+ and Sb 3+ .
In particular, trivalent Ln 3+ ions with unique 4f electronic configuration and rich energy levels can produce NIR light, covering from NIR-I (700-1000 nm) to NIR-II (1000-1700 nm) and beyond.6a] Specially, an NIR-II light at 1540 nm is needed because of its great promise in NIR photography, deep-tissue, optical communication, and high-resolution imaging. [8]Although 1540 nm emission can be supplied by the 4 I 13/2 → 4 I 15/2 transition of Er 3+ , Er 3+ -doped DPs inevitably suffer from poor PLQY due to low absorption coefficient, parity-forbidden optical transition, and inefficient energy transfer.It has thus remained a challenge to obtain bright Er 3+ 1540 nm emission.Doping ions can be an effective method for improving NIR efficiency, yet the concentration of lanthanide ions should be carefully controlled because of concentration quenching.In particular, cross-relaxation between lanthanide ions at high doping levels is historically regarded as the major course of concentration quenching, [9] leading to luminescence quenching in Ln 3+ -doped DP single crystals.222) and ( 220) facets from cubic unit cell.b) Atomic arrangements of ( 220) and ( 222) facets on a perovskite cubic crystal.
[12] However, it is quite difficult to tailor the facet orientation in DPs due to the uncontrolled crystal growth process.Currently, nearly all the DPs have the same (220)-dominated facet, [13] and thus it is impossible to investigate the impact of crystal orientation on their optical properties.However, from the viewpoint of facet orientation, it may be a new way to manipulate visible and NIR luminescence and provides an important insight into the photophysical pathways in DPs.
Here, we report that the preferred high-index (222) facet is thoroughly favorable for Er 3+ 1540 nm emission in Cs 2 NaErCl 6 DPs under multiwavelength-excitation, exhibiting a record quantum yield up to 98.6% under 980 nm excitation.In particular, we demonstrate the major deactivation pathway at high Er 3+ concentrations is surface quenching via high-energy vibrations of the surface OH − group, providing a fundamental mechanistic understanding of various quenching pathways, which are commonly misunderstood in Er-based microcrystallites.

Results and Discussion
The synthetic method of Cs 2 NaErCl 6 DPs can be seen in the Supporting Information.Figure 1a shows the evolution of two different facets from cubic unit cells.The growth rate is exponentially proportional to the surface energy of each crystal facet, [14] and therefore the conventional DP crystals will be enclosed by (220) facets with minimum surface energy.Meanwhile, the crystallographic planes can also be controlled by the growth rate ratio along the (100) to that of (111), [15] indicating the possible controllable growth of (111) facet.It should be noted that (111) and highindex (222) facets are equally crystallographic orientations with similar truncated octahedral or octahedral shapes.It is expected that the surface properties of DP crystals would be different due to diverse atomic arrangement and coordination.Figure 1b exhibits atomic arrangements of ( 220) and ( 222) facets on a perovskite cubic crystal.The (222) facet can be terminated 3/8 Cs cation and 3/2 chloride anion, which are negatively charged.In contrast, (220) facet shows positively charged characters due to less chloride anions.Thus, this big difference is an essential prerequisite for tailoring the luminescent properties of DPs.
X-ray diffraction (XRD) was used to evaluate the orientation of Cs 2 NaErCl 6 DP.The Cs 2 NaErCl 6 samples with favorable (220) plane, labeled as CNEC (220), show the same XRD patterns with the standard card (PDF#89-0053) and as-reported Cs 2 NaErCl 6 DPs (Figure 2a). [16]Most impressively, (222) facet orientation is dramatically promoted after altering the growth rate, marked as CNEC ( 222), indicating that the preferred orientation turns to (222) from conventional (220).Similarly, Ag-alloyed Cs 2 NaErCl 6 DPs simplified as CNEC: Ag (222), don't change the oriented (222) crystal plane.The XRD Rietveld refinement results demonstrate that both pristine Cs 2 NaErCl 6 and Ag-alloyed Cs 2 NaErCl 6 (222) samples still belong to the Fm-3m space group and exhibit a highly symmetrical face-centered cubic crystal structure (Figure 2b; Table S1, Supporting Information).The decreased lattice parameters after Ag doping are a result of stronger covalency between Ag + and Cl − .XRD pole-figure was further used to unveil the preferred facet distribution.The (222) peak intensity of pristine CNEC ( 222) is mainly located at the center, suggesting that (222) facet is one dominant plane (Figure 2c).Meanwhile, pole figures of ( 444) and ( 220) facets are also obtained (Figure 2d; Figure S2, Supporting Information) for roughly analyzing the crystal planes.As seen in Figure 2e, the percentage of ( 222) is calculated to be 60%, and the total percentage of ( 111) and ( 444) is estimated to be 30%.Thus, (222) facet and its equal orientations are dominate in controlled CNEC samples.
The modeling (222) crystal structure agrees well with those determined by electron microscopy (Figure 2h; Figure S3, Supporting Information).The scanning electron microscopy (SEM) images show that the products are micro-sized crystals with uniform distribution of all the elements using energy-dispersive Xray spectroscopy (EDS).The actual doping contents were determined from both EDS and the induced coupled plasma optical emission spectrometry in Table S2 and Figure S4 (Supporting Information).A truncated octahedral shape is observed based on CNEC:10% Ag (222), a typical shape of ( 111) or ( 222) facets in perovskites (Figure 2g).The high-resolution transmission electron microscopy (HRTEM) images confirm a high crystallinity and the lattice spacing values are 3-3.1 Å, corresponding to the (222) lattice planes (Figure 2f-i).Meanwhile, all the selected area electron diffraction patterns exhibit polycrystalline rings, which can be attributed to the polycrystal characteristics of as-prepared samples (Figure S5, Supporting Information).
The optical properties of CNEC ( 222) and Ag-alloyed CNEC (222) with different concentrations were systematically studied to check the facet effect.Figure 3a shows UV-vis absorption spectra of undoped and 10% Ag + -doped CNEC (222) and CNEC (220).All the absorption curves are similar with intense absorption peaks at 379 and 524 nm, arising from characteristic f-f transitions of Er 3+ .PL excitation (PLE) spectra are consistent with absorption spectra at 1540 nm for Ag-alloyed CNEC (222) with different concentrations (Figure 3b).Although self-trapped excitons (STE) typically exist in Ag-based materials due to Jahn-Teller distortion, low-temperature PL data present that narrow 554 nm emission corresponding to 4 S 3/2 → 4 I 15/2 transition of Er 3+ is observed instead of broad STE emission for CNEC:10% Ag (222), indicating very weak electron-phonon coupling (Figure 3c).Meanwhile, both green and red emissions are largely suppressed with increasing temperature based on (222) facets compared with (220) counterparts (Figure S6, Supporting Information), which can also be considered as another kind of "non-radiative" emissions compared with desired 1540 luminescence.The short-wave infrared emission shows main non-asymmetrical peaks at 1540, 1547, and 1561 nm under 365 or 379 nm excitation, corresponding to Stark splitting 4 I 13/2 sublevels of Er 3+ (Figure 3d; Figure S7, Supporting Information).Besides, only a weak 808 nm emission can be found in PL spectra, which can be assigned to the 4 I 9/2 → 4 I 15/2 transition of Er 3+ .The PL decay curves of Ag-alloyed CNEC (222) with different concentrations were then measured (Figure 3e; Table S3, Supporting Information).The lifetimes of all the samples exhibit a single-exponential function, with a maximum value of 21.05 ms for CNEC:10% Ag (222), in consistent with PLQY results (Figure S8, Supporting Information).Further increased Ag + content would reduce lifetime, as an evidence of enhanced non-radiative recombination rate.
Meanwhile, obvious Er 3+ 1540 nm luminescence can also be achieved for Yb 3+ -doped CNEC (222) under 980 nm excitation.The corresponding characterizations can be seen in Figure S14 (Supporting Information).( 222) facet enables highconcentration doping of Yb 3+ , which is beneficial for bright emission (Figure 4a).The fitting lifetimes of all the samples exhibit the single-exponential function, with a maximum value of 36.74 ms for CNEC:40% Yb (222), as shown in the PL decay curves (Figure 4b; Figure S15 and Table S4, Supporting Information).PL decay curves at different emission wavelengths were also measured to better understand the excited-state dynamics in Cs 2 NaErCl 6 DPs (Figure S16 and Table S5, Supporting Information).The NIR-II emission is originated from an energy transfer from Yb 3+ to Er 3+ because the emission intensity shows a linear relationship with the pump power of the 980 nm laser (Figure 4c).Furthermore, both pristine CNEC ( 222) and Yb 3+ -doped CNEC (222) samples exhibit high quantum efficiencies under 980 nm excitation (Figure 4d; Figures S17 and S18, Supporting Information).Specially, CNEC:40% Yb (222) shows a near-unity quantum yield of 98.6% under 980 nm excitation, which is the highest value for Er-based halide perovskites, to the best of our knowledge (Table S6, Supporting Information).Moreover, the operational stability of DP emitters under continuous illumination with 379 and 980 nm has been measured as shown in Figure S19 (Supporting Information).The samples show good emission stability over 100 h with negligible degradation of PLQY, indicating their potential for broad applications.
The PLQY is defined as the ratio of the radiative recombination rate (k r ) to the sum of the radiative and non-radiative (k nr ) recombination rates.Therefore, increasing k r and reducing k nr are two strategies to enhance the PLQY.As presented in Table 1, k r of pristine CNEC ( 222) is much larger than that of CNEC (220), meaning the effective breakdown of parity-forbidden transition by ( 222) facet.Considering the bulk centrosymmetric structure of all the Cs 2 NaErCl 6 DPs, the origin of breaking symmetry must originate from surface asymmetry due to an imbalance charged surface.The built-in electric field at (222) surface is beneficial for alleviating the 4 I 13/2 → 4 I 15/2 transition selection rule, promoting the structure asymmetry, and enhancing the luminescence efficiency.The lower symmetry has also been verified by the larger Ω 2 from Judd-Ofelt (J-O) analysis (Table 1). [17]The higher value of Ω 2 /Ω 4 in CNEC:10% Ag (222) confirms stronger covalency after Ag + doping, in accord with the Rietveld refinement results (Table S7, Supporting Information).In addition, CNEC:10% Ag (222) has a slightly larger k r compared with CNEC ( 222), indicating the same effect of ( 222) facet on the symmetry or the radiative recombination rate.The calculated transition probabilities by J-O analysis are in accordance with the k r values calculated by the fitted data of the time-resolved PL decay, providing theoretical evidence that facet engineering can manipulate the symmetry for increasing the oscillator strength to improve the PLQY.On the other hand, the efficiency is usually limited by undesired competing non-radiative processes, including multiphonon relaxation (MPR), surface quenching (SQ), crossrelaxation (CR) and energy migration to defects (EM), [18][19][20][21][22][23] i.e., the total k nr = k MPR +k SQ +k CR +k EM .Specifically concerning 1540 nm emission (≈6500 cm −1 of energy gap), k MPR is closely related to phonon-mediated relaxation, and its contribution can be omitted due to the low phonon frequency of Cs 2 NaErCl 6 DPs according to the "energy gap law" (Figure S20, Supporting Information). [19]k EM is dependent on the density of defects.Regarding the lattice defects, they are very few in high-crystallinity CNEC DPs as evidenced by ultra-narrow diffraction peaks.Besides, antisite defects can also be quantitatively evaluated by the relative intensity ratio of ( 111) and ( 200) peak (I 111 /I 200 ). [5]As shown in Figure S21 (Supporting Information), CNEC:10% Ag (222) has the largest I 111 /I 200 , even better than that of the standard pattern of Cs 2 NaErCl 6 , suggesting a high degree of Na(I) and Er(III) site ordering and negligible lattice defects.Although cross-relaxation is usually recognized as the main origin of concentration quenching, its effect (k CR ) is less at high Er 3+ concentrations in our case, only 1.5% for 4 I 9/2 → 4 I 15/2 transition under 379 nm excitation.Consequently, surface quenching (k SQ ) should be responsible for the facet-dependent quantum efficiency.As shown in Table 1, the calculated k nr of CNEC:10% Ag ( 222) is one order of magnitude slower than that of CNEC ( 220), proving that non-radiative processes can be strongly suppressed by ( 222) facet.
It is known that 4 I 13/2 emission of Er 3+ is very sensitive to the presence of OH − vibrations, due to energy resonance for strong quenching (energy gap of ≈6500 cm -1 vs 3200-3700 cm −1  220) and ( 222) facets, respectively.d) The distribution of water-number-density along the Z axis for ( 220) and ( 222) facets.e) The calculated interaction energy for ( 220) and ( 222) facets.f) Photophysical processes for Er 3+ 1540 nm emission and proposed mechanism for bright NIR-II luminescence in (222)-dominated Cs 2 NaErCl 6 DPs.
of O-H vibrations). [24]However, compared to nanocrystals, surface quenching is commonly not considered the major quenching pathway in microcrystalline materials.In our case, we can infer that the predominant deactivation pathway is indeed a surface effect via the two-phonon coupling of OH − group.To validate this conjecture, Fourier transform infrared spectroscopy (FTIR) was used to investigate the surface information for two facets.Molecular dynamics simulations were further used to examine the interaction between H 2 O molecules and two facets (details in the Supporting Information).( 220) and ( 222) facets were simulated as surfaces comparatively to interact with amounts of water molecules from the atomic level (Figure 5b,c).Figure 5d displays the simulation results of the distribution of water-number densities along the Z axis.CNEC (220) surface has a higher density of water molecules, demonstrating its stronger interaction with H 2 O and OH − species.The interaction energy per nm 2 was also calculated based on the two systems.The resulting values are −196.94and −185.32 kcal mol −1 for ( 220) and ( 222) facets, respectively (Figure 5e).From the calculation, it is clear that ( 220) is more energetically favorable for H 2 O molecules than (222) facet in terms of molecular dynamics.Both the experimental and theoretical evidences support that (222) facet can overcome the surface quenching from the coordinated water molecules.
The underlying mechanism is then proposed to explain the facet effects in Cs 2 NaErCl 6 DPs.The energy level diagram is shown in Figure 5f.The excitation energy, for example, 379 or 980 nm, is well adsorbed by the 4 G 11/2 or 2 F 5/2 levels and then relaxed to 4 I 13/2 level for 1540 nm emissions.At high Er 3+ concentrations, the CR process occurs inevitably, 4 I 13/2 + 4 I 13/2 → 4 I 15/2 + 4 I 9/2 , although its contribution is low.Energy can diffuse or hop rapidly between Er 3+ via energy migration, which may be released by the OH − group from the surrounding environment.For conventional (220)-dominated facet, the surface can absorb H 2 O molecules easily due to electrostatic interaction, leading to surface quenching for huge efficiency loss.In comparison, two significant benefits from ( 222) facet are electrostatic repulsion to OH − and protection by the outside ion layer like a "core-shell" structure.Therefore, the excitation energy can be preserved well and then converted efficiently to 1540 nm luminescence.Therefore, the studies elucidate the essence of surface quenching in Er 3+ -based microcrystalline materials and offer an alternative strategy for near-complete suppression of NIR luminescence quenching.
As proof, the negatively charged ( 222) surface shows surface functionalization via electrostatic interaction.It can be easily passivated by self-assembled long-chain organic amine for stable Er 3+ 1540 luminescence or can be merged with amino polymer for NIR-II emission in aqueous solution (Figure 6a).The cetyltrimethylammonium chloride (CTAC)-modified (222) facets show a little luminescence degradation (Figure 6b).Besides, CNEC:50% Yb ( 222) is mixed with the protonated form of polyethyleneimine (PEI•HCl) to produce weak Er 3+ 1540 emission under 980 nm irradiation (Figure 6c).The transparent solution indicates that DP samples are protected well by the outside polymer in the acidic aqueous environment (inset of Figure 6c).What is more, the CTAC-modified CNEC:50% Yb (222) samples exhibit good stability over 5 months, with degradation of PLQYs slowly (Figure 6d).In contrast, PLQYs of unmodified CNEC:50% Yb (222) samples drop to only 3% in the fifth month due to high hygroscopicity of Cs 2 NaErCl 6 , as evidenced by the contact angle tests (Figure S22, Supporting Information).The effective passivation of CTAC makes sure that (222)-dominated CNEC can be further applied for broader in-laboratory use and potential practical applications.

Conclusion
In summary, we have discovered anisotropic NIR-II luminescence in lead-free Cs 2 NaErCl 6 double perovskites based on two different facets.The underlying mechanisms of facet-dependent Er 3+ 1540 nm luminescence are breaking the parity-forbidden transition by the built-in electric field at (222) surface for enhancing radiative recombination rates and suppressing various quenching processes for reducing non-radiative recombination rates.The exact role of various quenching pathways in Er 3+ -based perovskites can also been elucidated on (222) facet, including multi-phonon relaxation, surface quenching, cross-relaxation, and energy migration to defects.Moreover, surface quenching effects, mainly the two-phonon coupling effect of hydroxyl group to 4 I 13/2 level of Er 3+ , show a direct impact on Er 3+ 1540 nm emissions in microcrystalline materials, as evidenced by both experimental and theoretical studies.Our findings suggest facet en-gineering is a new way out of the existing technologies for realizing highly efficient Er 3+ -based NIR-II emitters, toward broad applications in high-resolution fluorescence imaging, NIR LEDs, short-wave infrared lasers, and beyond.

Figure 1 .
Figure 1.a) Schematic diagram of the evolution of (222) and (220) facets from cubic unit cell.b) Atomic arrangements of (220) and (222) facets on a perovskite cubic crystal.
Figure 5a presents two characteristic vibrations at 1634 and 3430 cm -1 , which can be attributed to bending vibrations of absorbed H 2 O molecules and stretching vibrations of O-H group.CNEC (222) exhibits much weaker vibration strengths of both H 2 O and OH − than those of CNEC (220), indicating that (222) facet can effectively suppress the absorption of H 2 O and OH − .Thus, much lower PLQYs of CNEC (220) are expected via an intense two-phonon relaxation process due to strong coordination with surface species.

Table 1 .
Fitted average lifetimes, calculated k r and k nr , J-O parameters, and transition probabilities from J-O analysis.