Lattice Doping of Lanthanide Ions in Cs2AgInCl6 Nanocrystals Enabling Tunable Photoluminescence

The Beijing Municipal Key Laboratory of New Energy Materials and Technologies, School of Materials Science and Engineering, University of Science and Technology, Beijing, China Laboratory of Crystal Physics, Kirensky Institute of Physics, Federal Research Center KSC SB RASs, Russia Department of Engineering Physics and Radioelectronics, Siberian Federal University, Russia Department of Physics, Far Eastern State Transport University, Russia The State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, School of Materials Science and Engineering, South China University of Technology, China


Introduction
Lead halide perovskites have become the legend in the history of material science for emerging optoelectronic application due to their tunable emissions, high photoluminescence quantum yield (PLQY), easy solution processability, and so on [1][2][3][4]. Nevertheless, considering their lead toxicity and low stability, it is urgent to seek environmentally friendly semiconductor materials in this database. At this time, leadfree halide perovskites were discovered with lower toxicity and higher stability and have attracted great interests [5][6][7][8][9]. There are many choices for the replacement of Pb 2+ by other benign metal ions, including the incorporation of isovalent Sn 2+ ions [10] and substitution of trivalent Bi 3+ or Sb 3+ ions forming the similar composition as Cs 3 Bi 2 Cl 9 [11][12][13]. However, those materials are either limited by stability challenges [14] or with lower electronic mobility because of the lower symmetry nonperovskite structure [15]. One different way to address the challenge is to replace two Pb 2+ ions with one monovalent cation (B + ions) and one trivalent cation (B 3+ ions), forming the three-dimensional (3D) double perovskite structure [16]. The possible combinations of various cations make the diversity of lead-free double perovskites and make them the most promising alternative for optoelectronic applications [17].
Lead-free halide double perovskites with the general formula A 2 B + B 3+ X 6 (A = Cs + ; B + = Cu + , Ag + , Na + ; B 3+ = Bi 3+ , Sb 3+ , In 3+ ; X = Cl − , Br − , I − ) crystallize in a cubic unit cell with the space group Fm 3 m [18]. Among them, Cs 2 AgBiX 6 and Cs 2 NaBiCl 6 possess an indirect band gap leading to a low absorption coefficient and a weak photoluminescence (PL) emission [19,20]. In contrast, Cs 2 AgInCl 6 , inheriting the relatively good performance of the lead halide perovskites mainly attributed to the nature of direct band gap, has drawn increasing attention after the discovery by Giustino et al. [21] and Zhou et al. [22] and the milestone work as white light emitters by Luo et al. [7]. Cs 2 AgInCl 6 is reported to have a long carrier lifetime, easy solution processability, and a direct band gap with a parity-forbidden transition that results in a low PLQY (<0.1%), and a full story on research history of Cs 2 AgInCl 6 has been summarized recently for the details [23]. The poor PLQY has been improved by different doping and alloying strategies [7,[24][25][26]. Nevertheless, the PL of Cs 2 AgInCl 6 nanocrystals (NCs) contains a broadband spectral profile owing to the origin of self-trapped excitons (STEs) [27]. Therefore, to explore doped Cs 2 AgInCl 6 NCs with improved PLQY and tunable emission is a main challenge. Generally, lanthanide (Ln 3+ ) ions would be the most suitable dopants for their rich and unique PL emissions in the visible to near-infrared range [28,29], which could be utilized to achieve tunable luminescence and increased PLQY [30]. Moreover, the successful incorporation of rare earth ions for the lead-based halide perovskites [31,32] and the structural similarity between lead-based and lead-free perovskites (both with the six octahedral coordination number) have provided the reference and opportunities to conduct the further lanthanide doping study on Cs 2 AgInCl 6 NCs [33][34][35].
In this work, different lanthanide ions (Ln 3+ = Dy 3+ , Sm 3+ , Tb 3+ ) were successfully incorporated into Cs 2 AgInCl 6 perovskite NCs through the hot-injection method developed by our group [26]. Dy 3+ , Tb 3+ , and Sm 3+ ions were verified to occupy the In 3+ site in the Cs 2 AgInCl 6 lattice. The introduction of these rare earth ions endowed Cs 2 AgInCl 6 with diverse PL emissions in the visible region. Benefiting from the energy transfer process, Sm 3+ /Tb 3+codoped Cs 2 AgInCl 6 NCs achieved tunable emission from green to yellow orange and a fluorescent pattern from the as-prepared NC-hexane inks by spray coating was made to show its potential application in fluorescent signs and anticounterfeiting technology. This work expands the PL emissions of lead-free perovskite NCs through lanthanide ion doping, making them more competitive and will promote a wider regulation for their optical properties and novel photonic applications in energy-related materials.  (14 mL), OA (1 mL), OLA (1 mL), and HCl (0.28 mL). The reaction solution was heated to 120°C and degassed by alternating vacuum and N 2 for 1 h. Then, the mixture was heated to 260°C under N 2 . The as-prepared hot (150°C) Cs-oleate solution (0.8 mL) was quickly injected into the solution. After~20 s, the system was transferred to an ice-water bath. The crude sample was centrifuged at 8000 rpm for 4 min, discarding the supernatant. Next, the precipitate was dispersed in hexane and centrifuged again at 5000 rpm for 4 min, leaving the supernatant. The final NCs were precipitated with ethyl acetate by centrifugating for 4 min at 10000 rpm. For Sm 3+and Tb 3+ -codoped samples, different doping concentrations (5 mol%, 10 mol%, 20 mol%, and 40 mol%) of Sm 3+ were added at the fixed concentration of Tb 3+ (0.108 mmol).

2.4.
Characterization. X-ray diffraction (XRD) measurements were carried out on an Aeris X-ray diffractometer (PANalytical Corporation, Netherlands) equipped with a 50000 mW Cu Kα radiation after dropping concentrated nanocrystal hexane solutions on the silicon substrates. Transmission electron microscopy (TEM) images and energy-dispersive X-ray spectroscopy (EDS) analysis were acquired on a JEM-2010 microscope transmission electron microscope at the voltage of 120 kV equipped with an energy-dispersive detector, for which the samples were prepared by dropping dilute nanocrystal hexane solutions on the ultrathin carbon film-mounted Cu grids. Steady-state photoluminescence (PL) spectra, photoluminescence excitation (PLE) spectra, and PL decay spectra were recorded using a FLS920 fluorescence spectrometer (Edinburgh Instruments Ltd., U.K.) which is equipped with the Xe900 lamp, nF920 flash lamp, and the PMT detector. UV-visible absorption spectra were collected using a Hitachi UH4150 UV-vis-near IR spectrophotometer. Elemental contents were determined by the inductively coupled plasma mass spectroscopy (ICP-MS) after treating samples with wet digestion method. Xray photoelectron spectroscopy (XPS) was carried out on the ESCALAB 250Xi instrument (Thermo Fisher). The PL quantum yields were obtained on the Hamamatsu absolute PL quantum yield spectrometer C11347 Quantaurus_QY.

Structural Analysis of Ln 3+ (Ln = Dy, Tb, Sm)-Doped
Cs 2 AgInCl 6 NCs. Ln 3+ ion (Dy 3+ , Sm 3+ , Tb 3+ )-doped Cs 2 A-gInCl 6 NCs were synthesized by a hot-injection method at 260°C as illustrated in Figure S1. The X-ray diffraction (XRD) patterns showed that all the doped samples possessed pure phase (Figure 1(a)) and all peaks of them were indexed by cubic cell (Fm 3 m) with the parameters close to Cs 2 AgInCl 6 (Figures 1(b)-1(e)) [21]. This indicated that the incorporation of Ln 3+ ions into Cs 2 AgInCl 6 does not change the phase structure. To verify the location of Ln 3+ ions, Rietveld refinement was performed using 2 Energy Material Advances TOPAS 4.2 software. The refinements were stable and showed low R factors (Table S1). The coordinates of atoms and main bond lengths are given in Tables S2 and S3, respectively. It was found that cell volumes of compounds increased with Ln 3+ ions doped (Figure 1(f)  3 Energy Material Advances same perovskite structure as Cs 2 AgInCl 6 . The existence of doped Dy 3+ , Sm 3+ , and Tb 3+ ions in Cs 2 AgInCl 6 NCs could be confirmed by energy-dispersive X-ray (EDS) analysis and corresponding elemental mapping images ( Figure S3). The high-resolution TEM (HRTEM) images in Figures 1(g)-1(i) revealed that the incorporation of Ln 3+ ions did not induce the formation of crystal defects and the clear lattice fringes with the increasing lattice constants of 3.75 Å, 3.8 Å, and 3.9 Å for Dy 3+ , Sm 3+ , and Tb 3+ ions doped, respectively, corresponded to the (022) interplane distance (3.7 Å) of Cs 2 AgInCl 6 . The increased interplane distances further indicated the successful incorporation of Dy 3+ , Sm 3+ , and Tb 3+ ions.
To further characterize the chemical compositions of Ln 3+ -doped Cs 2 AgInCl 6 NCs, X-ray photoelectron spectroscopy (XPS) measurements were carried out. As shown in the XPS survey spectra (Figure 2(a)), the signals of Cs, Ag, In, and Cl were clearly observed in every sample. The respective high-resolution XPS spectra are present in Figures 2(b)-2  Energy Material Advances environments of In 3+ and Cs + in terms of the samples doped with Ln 3+ ions, while for the Ag 3d the spectra showed almost the same peak position for the undoped and three Ln 3+ iondoped Cs 2 AgInCl 6 NCs. Moreover, the relatively weak signals peaked at 167.9 eV, 1085 and 1110 eV, and 167.3 eV are observed in Figure 2(f) corresponding to the binding energy of Dy 4d, Sm 3d, and Tb 4d, respectively [36,37]. The weak signals may be due to the small amount of lanthanide ions on the surface. Combined with the XRD analysis, those results further indicated that Ln 3+ ions were successfully doped into the perovskite host lattice and located in the site of In 3+ to alter the local coordination structures.

Optical Properties of Ln 3+ (Ln = Dy, Tb, Sm)-Doped
Cs 2 AgInCl 6 NCs. The optical features of the as-prepared Ln 3+ -doped Cs 2 AgInCl 6 NCs were investigated ( Figure 3). All samples showed a strong absorption starting at around 350 nm and peaked at~310 nm (Figure 3(a)). Additionally, it is clear that there was a red shift of the excitonic absorption peak with Ln 3+ ion doping, which could be ascribed to the size increase of NCs. The optical band gaps 3.83 eV, 3.85 eV, and 3.88 eV for Dy 3+ -doped, Sm 3+ -doped, and Tb 3+ -doped NCs were quantified from the T auc plots of ðαhνÞ 2 , which were calculated from the corresponding absorption spectra (Figure 3(b)). The decrease in optical band gaps compared with~4 eV of undoped Cs 2 AgInCl 6 NCs [26] could be attributed from the lattice expansion of doped NCs [38]. Doped with different lanthanide ions, the as-synthesized NCs present variable emission (Figure 3(c)).      Figure 3(c). The sharp peaks therein were corresponding to the intrinsic transitions of 4 F 5/2 -6 H J (J = 15/2, 13/2, 11/2) for Dy 3+ ions, 4 G 5/2 -6 H J (J = 2/5, 2/7, 2/9, 2/11) for Sm 3+ ions, and 5 D 4 -7 F J (J = 6, 5, 4, 3) for Tb 3+ ions, respectively. All the PLE spectra monitored at the respective peak positions of three Ln 3+ ions were almost the same, which matches closely with the PLE spectrum of Cs 2 AgInCl 6 NC host seen in the previous work by Alivisatos et al. [39] and in our group [26]. That indicated that the emissions of Ln 3+ -doped NCs were most likely to originate from an efficient energy transfer from Cs 2 AgInCl 6 NC host to the energy levels of Dy 3+ , Sm 3+ , and Tb 3+ ions [40], as illustrated in Figure S4. The PL decay curves of the three lanthanide ion-doped samples were measured ( Figure 3(d), Table S4) and fitted by The calculated lifetimes for Dy 3+ -doped, Sm 3+ -doped, and Tb 3+ -doped NCs were 3.29 ms, 8.1 ms, and 8.45 ms, respectively, consistent with the recent reports on these lanthanide ion-doped luminescent materials [41,42].

Tunable
Luminescence of Sm 3+ -and Tb 3+ -Codoped Cs 2 AgInCl 6 NCs. Energy transfer between the codoped lanthanide ions in one system is a general strategy to achieve tunable luminescence. We design the controlled experiments by doping Tb 3+ ions in Cs 2 AgInCl 6 NCs with different amounts of Sm 3+ ions ( Figure 4). The general amount of Sm 3+ and Tb 3+ dopants was determined by ICP-MS measurement. As shown in Figure 4(a), all samples showed a strong absorption starting at~350 nm and peaked at around 310 nm. The PLE spectra of Cs 2 AgIn (0.89-x) Cl 6 :0.11Tb,xSm NCs were almost the same when monitored at 548 and 605 nm, further suggesting that the emissions of Sm 3+ and Tb 3+ ions were also derived from the efficient energy transfer from Cs 2 AgInCl 6 NC host to lanthanide ions (Figure 4(b)). Figure 4(c) reveals the PL emission for different amounts of Sm 3+ -doped Cs 2 AgIn (0.89-x) Cl 6 :0.11Tb NCs under the excitation of 311 nm. The PLQYs were measured to be 5.9%, 5.5%, and 5.0%, respectively, corresponding to the Sm 3+ concentrations of 3%, 5%, and 11%. With the increase in the amount of Sm 3+ dopants, the PL intensity of Tb 3+ emission decreases and the PL intensity of Sm 3+ emission increases first and then decreases. Thus, the emission colors could be tuned from green to yellow orange. The weakening of Sm 3+ emission was attributed to the concentration quenching effect. To reveal the variation trend of PL intensity more directly, the PL spectra were normalized as shown in the inset of Figure 4(c). It was found that the normalized peak intensity of Tb 3+ ions decreased and the luminescent intensity of Sm 3+ ions increased gradually. Those results indicated the possible occurrence of Tb 3+ → Sm 3+ energy transfer in Cs 2 A-gInCl 6 NCs. Moreover, the decay curves of 11%Tb 3+ /xSm 3+ (x = 0, 2%, 3%, 5%, and 11%)-codoped Cs 2 AgInCl 6 NCs by recording Tb 3+ 548 nm emission at 311 nm excitation are shown in Figure 4(d) to investigate the energy transfer process from Tb 3+ to Sm 3+ ions. The lifetimes calculated from Figure 4(d) and Table S5 for xSm 3+ (x = 0, 2%, 3%, 5%, and 11%)-doped Cs 2 AgIn (0.89-x) Tb 0.11 Cl 6 NCs were 8.77, 8.39, 8.12, 7.70, and 7.35 ms, respectively, which showed that with the increase in the concentration of Sm 3+ ion dopants, the fluorescence lifetime of Tb 3+ ion emission decreased gradually. That evidence further confirmed the existence of the energy transfer channel from Tb 3+ to Sm 3+ ions in Cs 2 AgInCl 6 NCs. Sm 3+ emission decays monitored at 605 nm emission and 311 nm excitation were also revealed in Figure 4(e). It was found that with the increase in the doping amount of Sm 3+ ions, the fluorescence decays became faster, attributed to the concentration quenching effect of Sm 3+ ion dopants. In addition, we used Bi 3+ -doped Cs 2 AgIn (0.89-x) Tb 0.11 Cl 6 :xSm NCs to make fluorescent signs by spray coating. Bi 3+ ion incorporation could adjust the excitation to 365 nm for wider application from our previous work [34]. The scheme of spray coating process is demonstrated in Figure 4(f), in which different NC-hexane solutions were atomized into very small droplets from the nozzle with the high-pressurized nitrogen gas. Then, the droplets deposited onto the PMMA substrate, forming the desired uniform, stable, and high-resolution patterns. The fluorescence patterns with tunable emissions shown in the right side of Figure 4(f) could respond to the 365 nm UV excitation signal, revealing the potential application of lanthanide ion-doped Cs 2 AgInCl 6 NCs in the field of anticounterfeiting technology and fluorescent signs.

Discussion
In conclusion, we demonstrated the successful lattice doping of various lanthanide ions, including Dy 3+ , Tb 3+ , and Sm 3+ , into lead-free perovskite Cs 2 AgInCl 6 NCs through the hotinjection method. It was confirmed by structural refinements that Dy 3+ , Tb 3+ , and Sm 3+ ions occupied the site of In 3+ ions, and the TEM images and XPS analysis further verified this result. The introduction of Ln 3+ doping endowed Cs 2 AgInCl 6 with diverse PL emissions in the visible region. Benefiting from the energy transfer process, Sm 3+ /Tb 3+ -codoped Cs 2 A-gInCl 6 NCs achieved tunable emission from green to yellow orange and a fluorescent pattern from the as-prepared NChexane inks by spray coating was made to show its application in fluorescent signs and anticounterfeiting technology. This work extends the study on lanthanide ion doping into lead-free halide perovskite Cs 2 AgInCl 6 NCs and further enables a wider regulation for their optical properties and applications in energy-related materials.

Data Availability
All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.