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Energy Material Advances / 2021 / Article

Research Article | Open Access

Volume 2021 |Article ID 2585274 | https://doi.org/10.34133/2021/2585274

Ying Liu, Maxim S. Molokeev, Zhiguo Xia, "Lattice Doping of Lanthanide Ions in Cs2AgInCl6 Nanocrystals Enabling Tunable Photoluminescence", Energy Material Advances, vol. 2021, Article ID 2585274, 9 pages, 2021. https://doi.org/10.34133/2021/2585274

Lattice Doping of Lanthanide Ions in Cs2AgInCl6 Nanocrystals Enabling Tunable Photoluminescence

Received11 Dec 2020
Accepted22 Jan 2021
Published24 Feb 2021

Abstract

Lead-free halide double perovskite Cs2AgInCl6 has become the research hotspot in the optoelectronic fields. It is a challenge to utilize the lattice doping by different lanthanide ions with rich and unique photoluminescence (PL) emissions for emerging photonic applications. Here, we successfully incorporated Dy3+, Sm3+, and Tb3+ ions into Cs2AgInCl6 nanocrystals (NCs) by the hot-injection method, bringing diverse PL emissions of yellowish, orange, and green light in Cs2AgInCl6:Ln3+ (Ln3+ = Dy3+, Sm3+, Tb3+). Moreover, benefiting from the energy transfer process, Sm3+ and Tb3+ ion-codoped Cs2AgInCl6 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 indicates that lanthanide ions could endow Cs2AgInCl6 NCs the unique and tunable PL properties and stimulate the development of lead-free halide perovskite materials for new optoelectronic applications.

1. 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 [14]. Nevertheless, considering their lead toxicity and low stability, it is urgent to seek environmentally friendly semiconductor materials in this database. At this time, lead-free halide perovskites were discovered with lower toxicity and higher stability and have attracted great interests [59]. There are many choices for the replacement of Pb2+ by other benign metal ions, including the incorporation of isovalent Sn2+ ions [10] and substitution of trivalent Bi3+ or Sb3+ ions forming the similar composition as Cs3Bi2Cl9 [1113]. 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 Pb2+ ions with one monovalent cation (B+ ions) and one trivalent cation (B3+ 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 A2B+B3+X6 (; ; ; ) crystallize in a cubic unit cell with the space group Fmm [18]. Among them, Cs2AgBiX6 and Cs2NaBiCl6 possess an indirect band gap leading to a low absorption coefficient and a weak photoluminescence (PL) emission [19, 20]. In contrast, Cs2AgInCl6, 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]. Cs2AgInCl6 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 Cs2AgInCl6 has been summarized recently for the details [23]. The poor PLQY has been improved by different doping and alloying strategies [7, 2426]. Nevertheless, the PL of Cs2AgInCl6 nanocrystals (NCs) contains a broadband spectral profile owing to the origin of self-trapped excitons (STEs) [27]. Therefore, to explore doped Cs2AgInCl6 NCs with improved PLQY and tunable emission is a main challenge. Generally, lanthanide (Ln3+) 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 Cs2AgInCl6 NCs [3335].

In this work, different lanthanide ions () were successfully incorporated into Cs2AgInCl6 perovskite NCs through the hot-injection method developed by our group [26]. Dy3+, Tb3+, and Sm3+ ions were verified to occupy the In3+ site in the Cs2AgInCl6 lattice. The introduction of these rare earth ions endowed Cs2AgInCl6 with diverse PL emissions in the visible region. Benefiting from the energy transfer process, Sm3+/Tb3+-codoped Cs2AgInCl6 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.

2. Materials and Methods

2.1. Materials

Cesium carbonate (Cs2CO3, 99.9%), indium chloride (InCl3, 99.99%), dysprosium chloride hexahydrate (DyCl3∙6H2O, 99.9%), terbium (III) nitrate pentahydrate (Tb(NO3)3·5H2O, 99.9%), samarium (III) chloride (SmCl3, 99.9%), octadecene (ODE, >90%), oleylamine (OLA, 80-90%), oleic acid (OA, analytical pure), hexane (C6H14, ≥98%), and ethyl acetate (C4H8O2, analytical pure) were purchased from Aldrich. Silver nitrate (AgNO3, analytical pure) and hydrochloric acid (HCl, analytical pure) were purchased from Beijing Chemical Works, China. All the chemicals were used directly without further purification.

2.2. Synthesis of Cs-Oleate

0.814 g of Cs2CO3 was loaded into a mixture of ODE (10 mL) and OA (2.5 mL), heated to 120°C and degassed by alternating vacuum and N2 for 1 h. Then, the reaction mixture was filled with N2 and heated to 150°C.

2.3. Synthesis of Ln3+ ()-Doped Cs2AgInCl6 NCs

Adequate amount of lanthanide raw materials (DyCl3·6H2O: 0.036 mmol, Tb(NO3)3·5H2O: 0.108 mmol, SmCl3: 0.072 mmol) was added into the mixture of AgNO3 (0.36 mmol), InCl3 (0.36 mmol), ODE (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 N2 for 1 h. Then, the mixture was heated to 260°C under N2. 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 Sm3+- and Tb3+-codoped samples, different doping concentrations (5 mol%, 10 mol%, 20 mol%, and 40 mol%) of Sm3+ were added at the fixed concentration of Tb3+ (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. X-ray 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.

3. Results

3.1. Structural Analysis of Ln3+ ()-Doped Cs2AgInCl6 NCs

Ln3+ ion (Dy3+, Sm3+, Tb3+)-doped Cs2AgInCl6 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 (Fmm) with the parameters close to Cs2AgInCl6 (Figures 1(b)–1(e)) [21]. This indicated that the incorporation of Ln3+ ions into Cs2AgInCl6 does not change the phase structure. To verify the location of Ln3+ ions, Rietveld refinement was performed using TOPAS 4.2 software. The refinements were stable and showed low 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 Ln3+ ions doped (Figure 1(f)). All the ion radii of Ln3+ dopants with 6-coordination (IR ; IR ; IR ) were smaller than those of Ag+ (IR ) and Cs+ (IR ) ions, inconsistent with the increasing trend of cell volumes. Therefore, it cannot be explained by the model of Ln3+ ↔ Cs+ or Ln3+ ↔ Ag+ ion replacements. On the other hand, the ion radii of Ln3+ dopants were larger than those of In3+ ion (IR ), which was in a good agreement with the increasing trend of cell volumes. Hence, Ln3+ ions are proposed to occupy the sites of In3+ ions, as shown in the inset of Figure 1(f). The actual doping concentrations detected by inductively coupled plasma (ICP) measurement were 5% for Dy3+ ions, 12% for Sm3+ ions, and 17% for Tb3+ ions. To see the micromorphology of the NCs, transmission electron microscopy (TEM) images of Ln3+ ion-doped NCs were exhibited in Figures 1(g)–1(i). As revealed by TEM, all the Ln3+ ion (Dy3+, Sm3+, Tb3+)-doped Cs2AgInCl6 NCs demonstrated the similar uniform cubic shape with the mean size of 9.68, 10.26, and 10.46 nm, respectively (Figure S2). The selected area electron diffraction (SAED) signals for the three Ln3+ ion-doped NCs all showed the presence of (022) and (004) planes of cubic phase, further verifying the formation of the same perovskite structure as Cs2AgInCl6. The existence of doped Dy3+, Sm3+, and Tb3+ ions in Cs2AgInCl6 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 Ln3+ 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 Dy3+, Sm3+, and Tb3+ ions doped, respectively, corresponded to the (022) interplane distance (3.7 Å) of Cs2AgInCl6. The increased interplane distances further indicated the successful incorporation of Dy3+, Sm3+, and Tb3+ ions.

To further characterize the chemical compositions of Ln3+-doped Cs2AgInCl6 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(e). As for the Cs 3d and In 3d XPS spectra, there was a slight shift to higher binding energy as Dy3+, Sm3+, and Tb3+ ions were introduced, attributed to changed chemical environments of In3+ and Cs+ in terms of the samples doped with Ln3+ ions, while for the Ag 3d the spectra showed almost the same peak position for the undoped and three Ln3+ ion-doped Cs2AgInCl6 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 Ln3+ ions were successfully doped into the perovskite host lattice and located in the site of In3+ to alter the local coordination structures.

3.2. Optical Properties of Ln3+ ()-Doped Cs2AgInCl6 NCs

The optical features of the as-prepared Ln3+-doped Cs2AgInCl6 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 Ln3+ 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 Dy3+-doped, Sm3+-doped, and Tb3+-doped NCs were quantified from the plots of , which were calculated from the corresponding absorption spectra (Figure 3(b)). The decrease in optical band gaps compared with ~4 eV of undoped Cs2AgInCl6 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)). Under 310 nm excitation, Dy3+-doped, Sm3+-doped, and Tb3+-doped NCs exhibited the characteristic emissions of Dy3+, Sm3+, and Tb3+ ions with the PLQY values of 2.8%, 3.1%, and 9.2%, respectively, and the irradiated NC solutions upon UV light were demonstrated in the insets of Figure 3(c). The sharp peaks therein were corresponding to the intrinsic transitions of 4F5/2-6HJ (, 13/2, 11/2) for Dy3+ ions, 4G5/2-6HJ (, 2/7, 2/9, 2/11) for Sm3+ ions, and 5D4-7FJ (, 5, 4, 3) for Tb3+ ions, respectively. All the PLE spectra monitored at the respective peak positions of three Ln3+ ions were almost the same, which matches closely with the PLE spectrum of Cs2AgInCl6 NC host seen in the previous work by Alivisatos et al. [39] and in our group [26]. That indicated that the emissions of Ln3+-doped NCs were most likely to originate from an efficient energy transfer from Cs2AgInCl6 NC host to the energy levels of Dy3+, Sm3+, and Tb3+ 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 Dy3+-doped, Sm3+-doped, and Tb3+-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].

3.3. Tunable Luminescence of Sm3+- and Tb3+-Codoped Cs2AgInCl6 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 Tb3+ ions in Cs2AgInCl6 NCs with different amounts of Sm3+ ions (Figure 4). The general amount of Sm3+ and Tb3+ 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 Cs2AgIn(0.89-x)Cl6:0.11Tb,xSm NCs were almost the same when monitored at 548 and 605 nm, further suggesting that the emissions of Sm3+ and Tb3+ ions were also derived from the efficient energy transfer from Cs2AgInCl6 NC host to lanthanide ions (Figure 4(b)). Figure 4(c) reveals the PL emission for different amounts of Sm3+-doped Cs2AgIn(0.89-x)Cl6:0.11Tb NCs under the excitation of 311 nm. The PLQYs were measured to be 5.9%, 5.5%, and 5.0%, respectively, corresponding to the Sm3+ concentrations of 3%, 5%, and 11%. With the increase in the amount of Sm3+ dopants, the PL intensity of Tb3+ emission decreases and the PL intensity of Sm3+ emission increases first and then decreases. Thus, the emission colors could be tuned from green to yellow orange. The weakening of Sm3+ 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 Tb3+ ions decreased and the luminescent intensity of Sm3+ ions increased gradually. Those results indicated the possible occurrence of Tb3+ → Sm3+ energy transfer in Cs2AgInCl6 NCs. Moreover, the decay curves of 11%Tb3+/xSm3+ (, 2%, 3%, 5%, and 11%)-codoped Cs2AgInCl6 NCs by recording Tb3+ 548 nm emission at 311 nm excitation are shown in Figure 4(d) to investigate the energy transfer process from Tb3+ to Sm3+ ions. The lifetimes calculated from Figure 4(d) and Table S5 for xSm3+ (, 2%, 3%, 5%, and 11%)-doped Cs2AgIn(0.89-x)Tb0.11Cl6 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 Sm3+ ion dopants, the fluorescence lifetime of Tb3+ ion emission decreased gradually. That evidence further confirmed the existence of the energy transfer channel from Tb3+ to Sm3+ ions in Cs2AgInCl6 NCs. Sm3+ 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 Sm3+ ions, the fluorescence decays became faster, attributed to the concentration quenching effect of Sm3+ ion dopants. In addition, we used Bi3+-doped Cs2AgIn(0.89-x)Tb0.11Cl6:xSm NCs to make fluorescent signs by spray coating. Bi3+ 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 Cs2AgInCl6 NCs in the field of anticounterfeiting technology and fluorescent signs.

4. Discussion

In conclusion, we demonstrated the successful lattice doping of various lanthanide ions, including Dy3+, Tb3+, and Sm3+, into lead-free perovskite Cs2AgInCl6 NCs through the hot-injection method. It was confirmed by structural refinements that Dy3+, Tb3+, and Sm3+ ions occupied the site of In3+ ions, and the TEM images and XPS analysis further verified this result. The introduction of Ln3+ doping endowed Cs2AgInCl6 with diverse PL emissions in the visible region. Benefiting from the energy transfer process, Sm3+/Tb3+-codoped Cs2AgInCl6 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 application in fluorescent signs and anticounterfeiting technology. This work extends the study on lanthanide ion doping into lead-free halide perovskite Cs2AgInCl6 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.

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this article.

Authors’ Contributions

Z.G.X. initiated and guided the research. Y.L. and Z.G.X. discussed and wrote the manuscript. Y.L. performed the experiments. M.S.M. performed Rietveld refinement of the power X-ray diffraction results.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (grant numbers 51961145101 and 51972118), the Fundamental Research Funds for the Central Universities (grant number FRFTP-18-002C1), the Guangzhou Science & Technology Project (202007020005), and the Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (grant number 2017BT01X137). This work was also funded by RFBR according to the research project no. 19-52-80003.

Supplementary Materials

Figures S1–S4 and Tables S1–S5 show the synthesis scheme, size distribution of NCs, TEM-EDS spectra and mapping images, energy-level diagram, structural refinement data, and calculated PL lifetimes. (Supplementary Materials)

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