Phase transition and energy transfer of lead-free Cs2SnCl6 perovskite nanocrystals by controlling the precursors and doping manganese ions

Perovskite quantum dots (QDs), such as all-inorganic CsPbX3 (X = Cl, Br, and I), are novel fluorescent semiconductor nanocrystals (NCs) that have attracted tremendous attention due to their excellent optical properties and great applications (e.g. display backlights, light-emitting diodes, and photodetectors). The instability and toxicity of lead-based perovskite QDs, however, are intrinsic defects that obstruct their application and commercialization. Poison is released from the lead of the unstable CsPbX3 NCs, which are generally ascribed to the labile surface, ionic character, and metastable structure. In this work, lead-free Cs2SnCl6 perovskite NCs are successfully synthesized via hot injection. Particularly, by controlling the different precursor ratios, phase transition (CsCl to Cs2SnCl6) was clearly observed from X-ray diffraction (XRD) measurements. The Cs2SnCl6 NCs exhibited a highly efficient deep-blue emission at 425 nm, with a 55 nm Stokes shift and an 84 nm full width at half maximum (FWHM). After doping Mn ions, the preferred formation of CsSnCl3:Mn2+ with double-wavelength emission was demonstrated based on the XRD and photoluminescence spectra. The study showed that doping synthesis should be widely used in lead-free perovskite NCs as an important strategy for next-generation solid-state lighting.


Introduction
Perovskite quantum dots (QDs) were named such due to the calcium titanium oxide (CaTiO 3 ) mineral, which was found by L.A. Perovski (a Russian mineralogist), because the two materials possess the same perovskite structure [1]. Lead halide perovskites (LHPs) instead crystallize in the form of a colloidal APbX 3 -type perovskite lattice, such as hybrid organic-inorganic [CH 3 NH 3 PbX 3 or CH(NH 2 ) 2 PbX 3 ] LHPs and all-inorganic CsPbX 3 LHPs (where X = Cl, Br, or I), have been intensively investigated of late [2][3][4][5]. The cesium lead halide (CsPbX 3 ) perovskite QDs have become promising materials for light-emitting diodes (LEDs), solar cells, and laser due to their superior optical properties, including high photoluminescence quantum yield (PLQY; up to 90%), narrow full width at half-maximum (FWHM; down to 50 nm), small exciton binding energy, and tunable full visible spectrum (380-780 nm), achieved by controlling the halide ion composition and through size modification [5][6][7][8][9][10][11][12][13][14][15]. Unfortunately, the relatively low stability and serious toxicity of the Pb 2+ ions of lead-based colloidal perovskite nanocrystals (NCs) are major challenges for their practical application [16,17]. Many strategies have been developed to resolve and ameliorate these issues. First, surface ligand modification methods, such as the use of tightly bound ligands [poly(maleic anhydride-alt-1-octadecene); PMA], ligand replacement (alkyl phosphinic acid), and crosslinking intermolecular C = C bonding, can enhance the stability of CsPbX 3 NCs through various synthetic processes [18][19][20]. Second, the core-shell structure has contributed to the stabilization and application of CsPbX 3 NCs by encapsulating the SiO 2 or polystyrene matrix [21,22]. As a result, the core-shell nanoparticles (NPs) display excellent stability against long-term storage in air and dispersion in the aqueous solution. Third, during the chemical reaction, benzoyl or zinc halides can be efficiently used as halide sources to synthesize the stable CsPbX 3 NCs due to metal carboxylate formation and surface passivation [23,24].
The structural and optical characterizations indicate that the alternative halide-based CsPbX 3 NCs are highly stable, without distinct changes in the X-ray diffraction (XRD) results, photoluminescence (PL), particle size, and morphology under ambient air conditions. Fourth, an effective manganese substitution strategy can also significantly stabilize the crystal lattices of CsPbX 3 :Mn 2+ perovskite QDs for improving their optical performance and thermal stability. The lattice contraction and the increase in formation energy are major reasons for the formation of stable CsPbX 3 :Mn 2+ QDs [25].
Metal halide perovskite NCs predominantly rely on the adoption of lead halides (PbX 2 , X = Cl, Br, I) as the divalent metal precursors, but there are rising concerns about the hazards that lead poses to the environment and human health, and also about its being an obstruction in the application of Pb-based perovskite NCs. As such, much effort has been devoted to the exploration of lead substitutes with nontoxic elements that have an analogous electronic band structure, such as Sn, Ge, Bi, and Sb [26][27][28][29]. For instance, the structural and optical properties of ASnX 3 , A 3 Bi 2 X 9 , and A 3 Sb 2 X 9 (A = Cs or Rb; X = Cl, Br, I) lead-free perovskite QDs have been reported [30][31][32][33][34][35][36], but these lead-free perovskite QDs generally show low PLQY and poor stability, which give rise to their intrinsic shortcomings, including vulnerability to oxidation, undesirable ionic conductivity, large band gap, and many surface defects.
In this work, blue-emission Cs 2 SnCl 6 NCs were successfully synthesized through a modified precursorassisted hot-injection method. The specific phase transition from CsCl to Cs 2 SnCl 6 was proven based on the XRD results. The Mn-doped mechanism of Cs 2 SnCl 6 has not been reported to date. Especially, bright Mn 2+ red emission was discovered in the preferred blue-emission CsSnCl 3 host when Mn ions were gradually doped into the Cs 2 SnCl 6 matrix. The spectrum is ascribed to the energy transfer from the CsSnCl 3 host to the Mn 2+ activators with d-d orbital transition. These Sn-based perovskite NCs exhibited high air stability due to the strategy. Hence, highly efficient lead-free perovskite NCs will be the target of the next indispensable work.

Preparation of cesium oleate
A 0.4M Cs-oleate solution was prepared in a 50 mL three-neck flask by mixing cesium carbonate (0.8140 g) with ODE (10 mL) and OA (2.5 mL). The mixture was degassed under vacuum at 120°C for 1 h, and was subsequently heated to 150°C under N 2 flux until the solution appeared clear.

Synthesis of undoped and Mn-doped Cs 2 SnCl 6 NCs
In a representative synthesis, SnCl 2 (0.08-0.36 mmol), 7 mL ODE, 0.5 mL OA, and 0.5 mL OAm were loaded onto a 25 mL three-neck flask. The mixture was stirred and dried under vacuum for 3 h, at 100°C. The reaction vessel was heated to 230°C under N 2 flow, and then 0.4 mL Cs-oleate was swiftly injected into the flask. The reaction was quenched after 10 min by cooling through an ice water bath. The product solution was centrifuged at 4000 rpm for 10 min, and the supernatant was subsequently discarded. Then the precipitated NCs were redispersed in hexane. Furthermore, Cs 2 Sn 1−x Mn 2x Cl 6 (x = 2, 4, and 6%) samples were prepared following the same steps, except that the desired amount of MnCl 2 •4H 2 O was added to the starting reaction mixture.

Characterization methods
XRD analysis was performed on a Bruker D2 Phase X-ray diffractometer equipped with monochromatic Cu K α radiation (λ = 1.54056 Å) operating at 30 kV and 10 mA in transmission mode. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were collected by a JEM2100F electron microscope with a 200 kV accelerating voltage. These samples were prepared by dropping dilute suspensions of NCs onto the carbon-coated copper grids. The UV-visible absorption spectra were collected using a JASCO V-730 spectrophotometer in absorption mode. The PL spectra of the samples were recorded with a FluoroMax-4 HORIBA spectrophotometer equipped with an Xe lamp and a photomultiplier tube. The X-ray photoelectron spectroscopy (XPS) spectra were measured on a VG scientific ESCALAB 250 spectrometer equipped with twin aluminum-magnesium anodes, using monochromatic Al Kα X-ray radiation (hν = 1486.6 eV). The samples were gently spread over the sliding glass and were put inside a vacuum chamber for XPS measurements.

Results and discussion
The synthesis of lead-free perovskite NCs was performed using a modified precursor-assisted hot-injection method. The XRD patterns of the as-prepared samples with different Cs:Sn precursor ratios indicate that cubic perovskite structures were formed in the pure CsCl phase, the co-existing CsCl and Cs 2 SnCl 6 phases, and the pure Cs 2 SnCl 6 phase, as shown in Figure 1. Interestingly, distinct phase transition (CsCl → Cs 2 SnCl 6 ) was observed in the formation procedure of pure Cs 2 SnCl 6 NCs, which can be indexed to the planes of cubic space group Fm-3 m (ICSD #9023). The initial product CsCl with space group Pm-3 m (ICSD #53847) consisted of a stoichiometric (Cs:Sn = 1:0.5) molar ratio and was gradually transformed into the primary product Cs 2 SnCl 6 (Cs:Sn = 1:1.5-2.25) when the molar ratio of tin increased.    (Figure 2(c)), which consists of the face-centered cubic phase. In Figure 2(d), the selected-area electron diffraction (SAED) pattern of the Cs 2 SnCl 6 NCs reveals the presence of (-20-2), (02-2), and (220) planes of the cubic phase, further verifying the formation of a perovskite structure.
To understand the photophysical properties of Cs 2 Sn Cl 6 NCs, the UV-visible absorption and PL spectra were measured under ambient atmosphere, and are shown in Figure 3(a). The pristine Cs 2 SnCl 6 NPs show a precipitous optical absorption edge at 316 nm (3.92 eV), consistent with the previous studies for excitonic absorption at 3.9-3.93 eV (317-315 nm) [37,38]. The PL excitation (PLE) spectrum with a prominent peak at 370 nm (3.35 eV, red line) is very similar to the Cs 2 SnCl 6 :Bi compound, which was previously reported for excitation at 365 nm [38]. For the PL spectrum at 370 nm excitation, there is a strong luminescent center at 425 nm (2.92 eV, blue line) with a 55 nm Stokes shift and a wide FWHM (84 nm), attributed to the energy from the electron-hole pair recombination. The elemental analysis using high-resolution X-ray photoelectron spectra (XPS) analysis revealed a zoom-in scan highlighting the Sn element of Cs 2 SnCl 6 ( Figure 3(b)). The two peaks are from the spin-orbit splitting corresponding to electrons from the 3d 5/2 and 3d 3/2 states located at 488.2 and 496.5 eV, respectively. The XPS data show only tetravalent tin (Sn 4+ ) in the Cs 2 SnCl 6 sample. This means that the higher oxidation state of tin (Sn 4+ ) is more stable against For the nanomaterials, the appropriate dopants of the host lattices were extensively explored as an effective approach to modulating the optical and electronic performance of diverse NCs and to stabilizing specific crystallographic phases [39][40][41][42]. To stabilize Cs 2 SnCl 6 NCs, a desired amount of MnCl 2 ·4H 2 O was added in the synthesis of Cs 2 Sn 1−x Mn 2x Cl 6 (x = 0, 2, 4, and 6%) while maintaining all the other synthetic parameters. The preferred monoclinic-phase CsSnCl 3 (ICSD #14199) with a space group of P2 1 /n, however, gradually formed when the Mn doping concentration increased, as shown in Figure 4. Based on the results, it can be said that the derivative of perovskite NCs may be created by doping adequate dopants.
Irregular and aggregative NPs of the 6%Mn-doped Cs 2 SnCl 6 compound are clearly observed in Figure 5(a  and b). The changed morphology may be ascribed to the coexistence of the two Cs 2 SnCl 6 and CsSnCl 3 phases. Figure 5(c) shows that the interplanar spacing was 3.36 Å, which is smaller than that of the pure Cs 2 SnCl 6 NCs (3.76 Å, Figure 2(c)), suggesting phase transition from the initial Cs 2 SnCl 6 NCs to the final CsSnCl 3 NCs with different Mn doping concentrations. Furthermore, there were many extra dots (red circle) in the SAED pattern of the Cs 2 SnCl 6 NCs, as shown in Figure 5(d). Based on these results, it can be speculated that phase transformation occurs in the Mn doping reaction of lead-free perovskite NCs.
Except for the XRD results, the PL spectra can also be used as evidence of the phase transition. The excitation spectra show a small red shift and are slightly broader when the Mn concentration increases (Figure 6(a and b)). Upon 6%Mn doping, there is a strong excitation center at 333 nm under 628 nm emission, which suggests that a high doping concentration benefits the phase transition from Cs 2 SnCl 6 to CsSnCl 3 NCs. When a dopant is inside a NC, fast energy transfer from the NC exciton to the dopant is expected. The monitoring of the excitation spectra as well as PL revealed that the peak positions of the excitonic and dopant emissions provide clear signatures for the phase transition through the successful incorporation of a dopant. As shown in Figure 6(c), a bright Mn 2+ red emission was detected due to the d-d orbital transition emission ( 4 T 1 → 6 A 1 ), and energy transfer from the excitons to the Mn 2+ activators was proven. These results prove the existence of Mn 2+ in   the CsSnCl 3 matrix. Moreover, the stable CsSnCl 3 :Mn 2+ solution can maintain its red luminescence for over two weeks. Figure 7 illustrates the proposed reaction mechanism for the Cs 2 SnCl 6 NC formation and the incorporation of Mn 2+ in the CsSnCl 3 NCs. Utilizing a nonstoichiometric modified precursor-assisted hot-injection method, the pure Cs 2 SnCl 6 NCs can be easily synthesized without adding metal ions. Unexpectedly, phase transition from Cs 2 SnCl 6 to CsSnCl 3 was introduced by doping MnCl 2 •4H 2 O. The analogous precursor (SnCl 2 ) and the charge of Mn (MnCl 2 ) are possible reasons for the existence of bright-red-emission CsSnCl 3 :Mn 2+ NCs.
Stability is an important issue for the practical application of perovskite nanomaterials. Moisture and heat are serious problems that impede the development of the new class of materials in products for the lighting and solar cell market. The stability levels of Cs 2 SnCl 6 and CsSnCl 3 :6%Mn 2+ NCs were compared. As shown in Figure 8(a and b), the variation of the PL spectra demonstrated that Mn-doped Sn-based perovskite NCs are quite stable after storing for 4 days in air. On the contrary, a drastic change was found in the emission spectra of the Cs 2 SnCl 6 NCs. In Figure 8(c and d), the corresponding luminescent photographs of the Cs 2 SnCl 6 and CsSnCl 3 :6%Mn 2+ NCs also indicate that doping synthesis is an important approach to improving the stability of lead-free perovskite nanomaterials.

Conclusion
In conclusion, blue-emission Cs 2 SnCl 6 nanocrystals (NCs) were successfully synthesized using a modified precursor-assisted hot-injection approach. By doping Mn ions, phase transition of the CsSnCl 3 NCs appeared involuntarily in the process of Cs 2 SnCl 6 NC formation. The CsSnCl 3 :Mn 2+ NCs showed bright-red luminescence, and the compound displayed extraordinary stability towards oxygen in the ambient atmosphere. The strategy provides a general method of creating the derivative of free-lead perovskite NCs and stabilizing the compound's performance. These tin-based NCs will also create possibilities for application in the next-generation solid-state lighting and displays.

Disclosure statement
No potential conflict of interest was reported by the authors.

Notes on contributors
Tsai-Wei Lin, received her B.S. degree from Chinese Culture University in 2016. She is currently pursuing an M.S. degree at National Taipei University of Technology. Her main interests are the synthesis of lead-free perovskite quantum dots and the development of a doping technology for them.