Room Temperature Fabrication of Stable, Strongly Luminescent Dion–Jacobson Tin Bromide Perovskite Microcrystals Achieved through Use of Primary Alcohols

Lead-free two-dimensional metal halide perovskites have recently emerged as promising light-emitting materials due to their improved stability and attractive optical properties. Herein, a facile room temperature wet milling method has been developed to make Dion–Jacobson (DJ) phase ODASnBr4 perovskite microcrystals, whose crystallization was accomplished via the aid of introduced primary alcohols: ethanol, butanol, pentanol, and hexanol. Due to the strong intermolecular hydrogen bonding, the use of ethanol promoted the formation of non-doped ODASnBr4 microcrystals, with an emission peaked at 599 nm and a high photoluminescence quantum yield (PL QY) of 81%. By introducing other primary alcohols with weaker intermolecular hydrogen bonding such as butanol, pentanol, and hexanol, [SnBr6]4− octahedral slabs of the DJ perovskite microcrystals experienced various degrees of expansion while forming O–H…Br hydrogen bonds. This resulted in the emission spectra of these alcohol-doped microcrystals to be adjusted in the range from 572 to 601 nm, while keeping the PL QY high, at around 89%. Our synthetic strategy provides a viable pathway towards strongly emitting lead-free DJ perovskite microcrystals with an improved stability.


Introduction
Metal halide perovskites have recently emerged as a popular material owing to their attractive optical properties, such as tunable absorption and photoluminescence (PL), and high defect tolerance [1]. The so-called three-dimensional (3D) perovskites are represented by a general formula ABX 3 , where A is a monovalent cation such as CH 3 NH 3 + (methylammonium), HC(NH 2 ) 2 + (formamidinium) or Cs + ; B is a divalent metallic cation (typically Pb 2+ , but could be also Ge 2+ , Sn 2+ , etc.), and X is a monovalent halide anion (Cl − , Br − , I − , or their combination in mixed halide alloys). There have been already promising reports of their application for various kinds of devices, such as solar cells [2], light-emitting diodes [3], and photodetectors [4]. However, the toxicity of lead which still remains the main constituent of the large variety of the reported metal halide perovskites, and their poor stabilities towards polar solvents and light irradiation greatly limit the practical application of 3D perovskites.
Even more recently, two-dimensional (2D) perovskite structures, namely Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) perovskites, have ignited the interest of scientists owing to their higher structural stability and at the same time their rather promising PL performance [5][6][7]. These 2D structures retain the main structural feature of a 3D perovskite lattice, in which octahedral metal halide units are connected by shared corners. At the same time, the interlayers between the planes of interconnected octahedra of the 2D perovskites are populated with monovalent or divalent long-chain organic cations, which Nanomaterials 2021, 11, 2738 2 of 9 form either the RP or DJ phases, respectively. Unlike the RP perovskites, DJ perovskites do not experience van der Waals interactions within their interlayers, because their extended divalent organic cations are connected with inorganic perovskite layers by forming hydrogen bonds at both ends, which often renders them with comparatively higher structural stability [8][9][10][11]. These DJ perovskites have already found application in solar cells [10] and light-emitting diodes [12], but their external quantum efficiency still has a lot of room for improvements. Thus, synthetic efforts towards strongly luminescent DJ perovskites constitute an important research task. Our group has recently introduced small molecules acting as acidic proton donors, such as dichloromethane and chloroform, which served as molecular dopants to improve the crystallinity of DJ phase tin bromide ODASnBr 4 perovskite microcrystals (ODA stands for 1,8-octanediamine), achieving remarkable PL quantum yields (QY) approaching 90% [13]. Moreover, primary alcohols were also found to be able to form hydrogen bonds with DJ phase ODASnBr 4 microcrystals, and in addition helped to remove byproducts formed during the conventional saturation recrystallization process [14]. However, while applied as a post-synthetic treatment agent, ethanol (EtOH) required very long treatment times or higher temperatures to break intermolecular O-H . . . O hydrogen bonds and form O-H . . . Br hydrogen bonds with the perovskite lattice [14].
Herein, by using a mixture of perovskite precursors and EtOH, DJ phase non-doped ODASnBr 4 perovskite microcrystals denoted as ODASnBr 4 (EtOH) were prepared at room temperature and showed a high PL QY of 81% for emission peaked at 599 nm. By using longer-chain primary alcohols, 1-butanol (BuOH), 1-pentanol (PeOH), or 1-hexanol (HeOH), which were able to form O-H . . . Br hydrogen bonds with the perovskite lattice, doped ODASnBr 4 [alcohol] microcrystals with PL QYs reaching 89% and PL peaks adjustable between 572 nm and 601 nm have been observed. This study is a logical continuation of our work to develop the facile synthesis of strongly emitting lead-free DJ low-dimensional perovskites.

Synthesis of ODASnBr 4 (EtOH) and ODASnBr 4 [Alcohol] Perovskite Microcrystals
The 0.4 mmol SnBr 2 , 0.4 mmol ODA, 0.2 mL HBr, and 0.5 mL BuOH, PeOH, or HeOH were successively added into a mortar and ground together to prepare alcohol-doped ODASnBr 4 [BuOH], ODASnBr 4 [PeOH] and ODASnBr 4 [HeOH] perovskite microcrystals, respectively. Dopant-free ODASnBr 4 , denoted as ODASnBr 4 (EtOH) could also be made in a similar way using EtOH, but they would degrade within 10 min due to oxidation of Sn(II) in the air, because EtOH does not function as a molecular dopant under such a synthesis condition [14]. Therefore, in order to prepare ODASnBr 4 (EtOH) microcrystals, respective precursors were mixed together in a 2 mL glass vial, which was sealed and shook for 5 min. All samples were rinsed with EtOH at room temperature in order to remove any non-reacted precursors and undesired impurities [14], and dried in the vacuum box for further use.

Characterization
Powder X-Ray diffraction (XRD) patterns were collected on a Rigaku SmartLab Xray diffractometer, Tokyo, Japan. Optical diffuse-reflectance spectra were collected on a Shimadzu UV 3600 UV/visible/IR spectrophotometer, Kyoto, Japan with an integrating sphere accessory. PL and PL excitation (PLE) spectra, as well as time-resolved PL decays were measured on an FLS920P spectrometer (Edinburgh Instruments, Livingston, UK).
Absolute PL QYs were measured with the aid of an integrating sphere with its inner face coated with BENFLEC™ (Edinburgh Instruments, Livingston, UK). X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI model 5802 instrument (ULVAC-PHI, Inc., Kanagawa, Japan). Fourier-transform infrared (FTIR) spectra were collected on a Perkin Elmer FTIR spectrophotometer (Perkin Elmer, Waltham, MA, USA). Raman spectra were collected on a WITec Alpha300 R confocal Raman imaging system (WITec Wissenschaftliche Instrumente und Technologie GmbH, Ulm, Germany) equipped with a 532 nm laser. An FEI Quanta 250 e-scanning electron microscope (SEM) (Thermo Fisher Scientific Inc., Waltham, MA, USA) was used to study the morphology and elemental composition of the samples.

Results and Discussion
After mixing ODA, SnBr 2 , and HBr precursors with different primary alcohols in a mortar, a room temperature (25 • C) wet grinding process was conducted to prepare ODASnBr 4 perovskite microcrystals as shown in Figure 1a. Powder XRD patterns were collected to compare the structural characteristics of perovskites produced using different primary alcohols. As shown in Figure 1b 4 [PeOH], the diffraction peak of ODASnBr 4 (EtOH) at 6.28 • still experienced a shift of 0.08 • towards lower 2θ, even though EtOH possesses a shorter carbon chain than BuOH and PeOH. This slight lattice dilation was due to the presence of byproducts (ODA·2HBr), which were physically inserted in the perovskite lattices [15,16] which also showed up through the presence of a broad diffraction peak at 8.4 o (Figure 1b). Importantly, XRD patterns have also shown that the room temperature wet grinding method applied here could efficiently suppress the crystallization and growth of any byproducts while leading exclusively to the formation of thermodynamically favored DJ phase perovskites. As such, the once dominating diffraction peak of impurities at around 8.4 • (reported in our previous related study [14]) was drastically reduced in ODASnBr 4 (EtOH) and absent completely in all the other ODASnBr 4 [alcohol] microcrystals.
SEM images of the obtained perovskite microcrystals are shown in Figure 1c. ODASnBr 4 (EtOH) microcrystals possess a typical 2D sheet-like morphology with a lateral length of around 10 µm. The size of the sheets decreased while using long-chain alcohols BuOH, PeOH, and HeOH, probably due to the longer chain alcohols that may favor stronger hydrogen bonding through the electromeric effect of the alkyl portion of the molecule pushing more electron density back towards the OH group, unlike for smaller EtOH molecules dominated by intermolecular O-H . . . O hydrogen bonds [16]. Figure 1d provides energy-dispersive X-ray spectroscopy (EDS) elemental mapping images for the representative case of ODASnBr 4 [HeOH] microcrystals, showing the presence of N, O, Sn, and Br elements, with atomic ratios of 52%, 21%, 5%, and 22%, respectively. Excess oxygen and nitrogen may originate from the presence of alcohol dopants and eventually some residual amine precursors.  (Table 1). From the absorption spectra and the respective Tauc plots derived from these, optical bandgaps of the samples have been determined  , and ODASnBr 4 [HeOH] perovskite microcrystals were determined to be 81%, 89%, 88%, and 89%, outperforming most other low-dimensional perovskites (Table 1). From the absorption spectra and the respec-tive Tauc plots derived from these, optical bandgaps of the samples have been determined (Figure 2b). Three samples ODASnBr 4

[BuOH], ODASnBr 4 [PeOH], and ODASnBr 4 [HeOH]
microcrystals showed similar bandgaps of 2.96 eV, which were blue-shifted from the value of 2.81 eV for ODASnBr 4 (EtOH) due to the slight lattice expansion caused by these alcohol dopants and the ODA·2HBr impurities [14]. We notice that the XRD patterns in Figure  1b and absorption spectra in Figure 2b do not match fully with those of the post-treated ODASnBr 4 [alcohol] microcrystals reported in our previous study [14], which may be due to the different amount of crystalized/uncrystallized impurities that are mixed in with the perovskite lattices, considering the broad XRD peak at around 30 • .
As shown in Figure 2c, ODASnBr 4 (EtOH), ODASnBr 4 [BuOH], and ODASnBr 4 [PeOH] microcrystals excited at 334 nm exhibited PL peaks at 599, 596, 601 nm, respectively, while the PL maximum of ODASnBr 4 [HeOH] underwent a much larger blue shift to 572 nm. Such a strong blue shift in the latter case may be determined by a greatly increased distance between perovskite slabs in the ODASnBr 4 [HeOH] lattice, according to their XRD patterns, which influenced the radiative recombination channels. Time-resolved PL decays of the four samples are shown in Figure 2d. Average PL lifetimes (τ avg ) were calculated from these decays by using the following equation [17]: τ avg = ∞ 0 tI(t)dt  Furthermore, the Raman signal at around 64 cm −1 , corresponding to protonated amine impurities (ODA·2HBr) [14], was greatly suppressed in ODASnBr 4 [alcohol] samples, except for ODASnBr 4 (EtOH). It is noted that the broad Raman signal at around 120 cm −1 could arise from a combination of Br−Sn−Br asymmetric bending (~100 cm −1 ) and the Sn−Br asymmetric stretch (142 cm −1 ). Therefore, the different degrees of lattice expansion on the XRD patterns could primarily be attributed to the different lengths of alcohols and any long-chain byproducts (ODA·2HBr) for ODASnBr 4 [alcohol] and ODASnBr 4 (EtOH) samples, respectively. Nanomaterials 2021, 11, x FOR PEER REVIEW 6 of 10 Raman spectra (Figure 3a) 4− octahedral. Furthermore, the Raman signal at around 64 cm −1 , corresponding to protonated amine impurities (ODA·2HBr) [14], was greatly suppressed in ODASnBr4[alcohol] samples, except for ODASnBr4(EtOH). It is noted that the broad Raman signal at around 120 cm −1 could arise from a combination of Br−Sn−Br asymmetric bending (~100 cm −1 ) and the Sn−Br asymmetric stretch (142 cm −1 ). Therefore, the different degrees of lattice expansion on the XRD patterns could primarily be attributed to the different lengths of alcohols and any long-chain byproducts (ODA·2HBr) for ODASnBr4[alcohol] and ODASnBr4(EtOH) samples, respectively.  (Figure 2c). Thus, X-ray photoelectron spectroscopy (XPS) has been performed on two samples ODASnBr 4 [PeOH] and ODASnBr 4 [HeOH] in order to reveal more details of their coordination. As can be seen from the XPS O1s core-level spectra provided in Figure 3c (Figure 2c). Thus, X-ray photoelectron spectroscopy (XPS) has been performed on two samples ODASnBr4[PeOH] and ODASnBr4[HeOH] in order to reveal more details of their coordination. As can be seen from the XPS O1s corelevel spectra provided in Figure 3c, ODASnBr4[HeOH] microcrystals have a slightly lower C-O binding energy (533.18 eV) as compared with ODASnBr4[PeOH] (533.28eV), which may induce stronger O-H…Br hydrogen bonding between HeOH and [SnBr6] 4− octahedra; the peak at 534.7 eV which appears in ODASnBr4[HeOH] may be related to oxygen from water molecules [24,25]. According to the N1s spectra shown in Figure 3d  maintained around 80% of their initial intensity for up to 30 days. ODASnBr 4 (EtOH) microcrystals, in contrast, were more susceptible to oxidation, as their PL intensity dropped to only 20% of the initial value during the storage for 30. This can be contrasted with the reasonably high thermal stability of the, to date, more comprehensively explored ODASnBr 4 microcrystals probably due to the absence of molecular dopants [14]. XPS Sn3d core-level spectra measured on perovskite microcrystals after 30 days storage (Figure 4b) showed that the binding energy corresponding to Sn3d 5/2 changed from 487.53 eV for ODASnBr 4 [HeOH], indicating that perovskite microcrystals were better protected from oxidation after incorporating long-chain primary alcohol dopants.
reasonably high thermal stability of the, to date, more comprehensively explored ODASnBr4 microcrystals probably due to the absence of molecular dopants [14]. XPS Sn3d core-level spectra measured on perovskite microcrystals after 30 days storage (Figure 4b) showed that the binding energy corresponding to Sn3d5/2 changed from 487.53 eV for ODASnBr4(EtOH) to 487.43 eV for ODASnBr4[BuOH], 487.13 eV for ODASnBr4[PeOH], and 487.23 eV for ODASnBr4 [HeOH], indicating that perovskite microcrystals were better protected from oxidation after incorporating long-chain primary alcohol dopants.

Conclusions
Strongly luminescent lead-free DJ phase ODASnBr4 perovskite microcrystals were produced in this work using a room temperature, wet milling method with the aid of primary alcohols: ethanol, 1-butanol, 1-pentanol, and 1-hexanol. Dopant-free ODASnBr4(EtOH) perovskite microcrystals were formed because EtOH could not form O-H…Br hydrogen bonds with the perovskite lattices under this synthetic condition while providing a homogeneous reaction environment like octadecene in classic colloidal synthesis. When using BuOH, PeOH, and HeOH, doped ODASnBr4[alcohol] microcrystals were obtained due to the formation of O-H…Br hydrogen bonding between [SnBr6] 4− octahedra. Perovskite microcrystals synthesized by this method showed a strong PL emission (PL QY over 80%) tunable in the range of 572 nm to 601 nm. The PL intensity of ODASnBr4[alcohol] microcrystals could be maintained at around 80% of their initial PL intensity after being stored in a sealed glass bottle for a month, due to their improved crystallinity and remarkable stability against oxidation provided by the primary alcohol dopants.

Conclusions
Strongly luminescent lead-free DJ phase ODASnBr 4 perovskite microcrystals were produced in this work using a room temperature, wet milling method with the aid of primary alcohols: ethanol, 1-butanol, 1-pentanol, and 1-hexanol. Dopant-free ODASnBr 4 (EtOH) perovskite microcrystals were formed because EtOH could not form O-H . . . Br hydrogen bonds with the perovskite lattices under this synthetic condition while providing a homogeneous reaction environment like octadecene in classic colloidal synthesis. When using BuOH, PeOH, and HeOH, doped ODASnBr 4 [alcohol] microcrystals were obtained due to the formation of O-H . . . Br hydrogen bonding between [SnBr 6 ] 4− octahedra. Perovskite microcrystals synthesized by this method showed a strong PL emission (PL QY over 80%) tunable in the range of 572 nm to 601 nm. The PL intensity of ODASnBr 4 [alcohol] microcrystals could be maintained at around 80% of their initial PL intensity after being stored in a sealed glass bottle for a month, due to their improved crystallinity and remarkable stability against oxidation provided by the primary alcohol dopants.