Dimensionality and Photophysical

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Synthetic Control on Structure/Dimensionality and Photophysical Properties of Low Dimensional Organic Lead Bromide Perovskite Muhammed P. U. Haris,∥,† Rangarajan Bakthavatsalam,∥,† Samir Shaikh,† Bhushan P. Kore,‡ Dhanashree Moghe,§ Rajesh G. Gonnade,† D. D. Sarma,‡ Dinesh Kabra,§ and Janardan Kundu*,†

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†

Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune, Dr. Homi Bhabha Road, Pashan Pune, Maharashtra-411008, India ‡ Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, CV Raman Rd, Bengaluru, Karnataka-560012, India § Department of Physics, Indian Institute of Technology Bombay, Main Gate Road, Powai, Mumbai, Maharashtra-400076, India S Supporting Information *

ABSTRACT: Low dimensional lead halide perovskites have attracted huge research interest due to their structural diversity and remarkable photophysical properties. The ability to controllably change dimensionality/structure of perovskites remains highly challenging. Here, we report synthetic control on structure/ dimensionality of ethylenediammonium (ED) lead bromide perovskite from a two dimensionally networked (2DN) sheet to a one dimensionally networked (1DN) chain structure. Intercalation of solvent molecules into the perovskite plays a crucial role in directing the final dimensionality/structure. This change in dimensionality reflects strongly in the observed differences in photophysical properties. Upon UV excitation, the 1DN structure emits white light due to easily formed “self-trapped” excitons. 2DN perovskites show band edge blue emission (∼410 nm). Interestingly, Mn2+ incorporated 2DN perovskites show a highly red-shif ted Mn2+ emission peak at ∼670 nm. Such a long wavelength Mn2+ emission peak is unprecedented in the perovskite family. This report highlights the synthetic ability to control the dimensionality/structure of perovskite and consequently its photophysical properties.

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INTRODUCTION Organic−inorganic hybrid metal halide perovskites continue to be a fascinating research frontier due to their amazing photophysical properties and myriads of applications.1−3 Depending upon the connectivity of the constituting metal halide octahedra, these perovskites can be classified as three dimensionally networked (3DN) or two dimensionally networked (2DN) or one dimensionally networked (1DN) or zero dimensionally networked (0DN, isolated octahedra) structures.4−6 3DN perovskites with the general formula APbX3 are characterized by corner shared metal halide octahedra with “A” type cation fitting the void created by the interconnected (in three dimensions) network of the octahedra. Recently, 2DN perovskites, which can be thought of as derived from 3DN perovskites by slicing along particular crystal directions, with the general formula of L2PbX4, have seen a resurgence due to their intriguing fundamental properties and applications.7 The 2DN perovskites have sheet like layered structures where the corner shared metal halide octahedra are partitioned by the long organic ligand (L) layer. Depending upon the direction of the slicing, the 2DN perovskites could be further classified as flat (001) or corrugated (110) inorganic sheet structures.8 The structure © XXXX American Chemical Society

and photophysical properties of these perovskites with different dimensionalities are widely different. (001) 2DN perovskite shows narrow and blue emission at room temperature when excited with near UV light. In comparison, few (110) perovskites with corrugated sheet structure show dramatically different emission profile with very broad emission covering the entire visible spectrum.9 Such materials emit white light under near UV illumination and are potential candidates for single phase phosphors for solid state lighting applications. It is noteworthy that the observed broad emission in these corrugated systems arises due to self-trapped excitons (light induced transient “excited defect states”).9 The whole family of organic−inorganic metal halide perovskites (3DN, 2DN, 1DN, 0DN) has metal halide octahedra and organic ligands that are central to the chemistry of the system. Manipulating the metal halides, organic ions, and their reaction chemistry are the crucial handles that act as powerful synthetic tools to create new systems with different band structures and photophysical properties.10 Control on the degree of confinement (dimensionality) in the perovskite based system is likely Received: July 20, 2018

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DOI: 10.1021/acs.inorgchem.8b02042 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

attained dimensionality, which largely shapes their photophysical properties. The Result and Discussion of this article is arranged as follows: a brief synthetic methodology is followed by optical and physical characterization of the products. Such characterizations, indicating different dimensionality of the products, necessitates solving the single crystal structure. This is further followed by gaining insight into the observed optical properties of the products. Finally, rationale for the observed change in dimensionality of the products is discussed.

to be achieved by deeper understanding and control of the solution chemistry of their synthesis. Hence, it is of paramount interest to control reaction conditions to tailor the preparation of systems of different structure/dimensionality and photophysical properties. The impact of the synthetic route utilized in the fabrication of perovskites has been previously reported in the literature.11−14 Changing the “A” type cation from small inorganic/organic moiety (Cs, methylammonium, formamidinium) to long chain organic ligands (alkyl/aryl ammoniums) can lead to stabilization of 2DN perovskites over the 3DN perovskites.8 Interestingly, utilizing a mixture of “A” type cation and long chain ammonium ligands can stabilize quasi2DN structures that bridge the 2DN and 3DN family.14 This already depicts the versatility of the adopted solution chemistry in dictating structure/dimensionality of the final product perovskites. Stoumpos et al. demonstrated12 that the physical and chemical properties of the 3DN perovskite (MAPbI3) largely depends on the synthetic route utilized for the synthesis. The effect of solvent engineering has also been demonstrated for enhancing the efficiency of solar cells fabricated using 3D perovskites as the absorber layer.15,16 Gardner and co-workers have demonstrated that changing the organic cation can change the dimensionality of the metal halide perovskites.17,18 Moreover, they also reported a change in dimensionality of the products by changing reaction conditions for synthesis of butyl-1,4-diammonium lead(II) iodide materials.19 Along similar lines, Ma and co-workers have demonstrated the change in dimensionality of tin(II) bromide based perovskites by altering the reaction route and reactant precursors.11 This accessibility to reaction route tailored diversity of final product dimensionality/structures and associated photophysical properties provides a considerable opportunity for scientific research on material design and their properties. Here, we demonstrate synthetic control in directing changes in the dimensionality/structure that remarkably shapes the photophysical properties of the lead(II) bromide based perovskite utilizing ethylenediammonium (ED) as the ligand. Contorted (110) 2DN (n = 1) perovskites result when DMSO−DCM (dimethyl sulfoxide-dichloromethane) is used as the solvent−antisolvent pair while 1DN perovskites are formed when HBr−acetone acts as the solvent−antisolvent pair. For both cases, the ratio of the ligand to lead precursor is kept constant at 2:1. Single crystal structure analysis clearly depicts the structure of the system highlighting the role of the solvent incorporation in determining the final dimensionality of the perovskites. This change in dimensionality of products reflects strongly in the observed remarkable differences in their photophysical properties. The 1DN system shows a broad emission band which emits white light under UV illumination. Such white light emission in the 1DN perovskites arises due to “self-trapped” excitons. The 2DN perovskites show their characteristic strong excitonic blue emission across the band edge. Further, the 2DN systems are amenable to Mn2+ incorporation, showing strong energy transfer from host semiconductor to Mn2+ ions. Intriguingly, the Mn2+ emission is highly red-shifted (∼670 nm) which is unprecedented for any reported metal halide based perovskite system. This report highlights that the nature of the synthetic route, utilized solvents, and its incorporation into the final perovskite structure may be playing a key role that governs the final

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EXPERIMENTAL SECTION

Materials. Lead(II) bromide (99%) was purchased from Loba Chemie. Dichloromethane (DCM, anhydrous) and hydrobromic acid (47%) were purchased from TCI Chemicals. Manganese(II) bromide, acetone, and dimethyl sulfoxide (DMSO) were purchased from Sigma-Aldrich. Ethane 1,2- diammoniumdibromide (EDBr2) was purchased from Greatcell Solar. All chemicals were used as purchased without further purification. Synthesis of 1D ED HBr Perovskite. A total of 0.444 g of EDBr2 salt (2 mmol) and 0.367 g of PbBr2 salt (1 mmol) was dissolved in 17 mL of concentrated HBr and separated into two parts. A total of 30 mL of acetone was added to one part of the solution, which resulted in precipitation of white powders. The precipitate was filtered and dried under a vacuum at 50 °C. Another part was kept overnight in an acetone vapor chamber to obtain single crystals that were handpicked for analysis. Synthesis of 2D ED DMSO Perovskite. A total of 6 mL of DMSO solution containing 0.266 g of EDBr2 salt (1.2 mmol) and 0.221 g of PbBr2 salt (0.6 mmol) was kept overnight in a DCM vapor chamber for crystallization. Samples of single crystals thus obtained were handpicked for further analysis. Solvent was removed by filtration, and the crystals were dried under a vacuum at 60 °C for 12 h for other characterizations. Synthesis of Mn2+ Incorporated 2D ED DMSO Perovskites. The as prepared 2D ED DMSO perovskite is ground with MnBr2 (at different mole percentages with respect to Pb) in an agate mortar and pestle. The homogeneous powder was annealed at 100 °C for 1 h and used for further optical characterization. Material Characterization. Samples are finely powdered for all the analysis unless stated otherwise. Room temperature diffuse reflectance spectra (DRS) of powder samples were measured with a Shimadzu UV−vis−NIR spectrophotometer (Model UV-3600 plus), and reflectance data were converted to absorbance data via Kubelka− Munk transformation. BaSO4 powder was used as a reflectance reference sample. All steady state photoluminescence (PL) measurements were performed using an Edinburg FLS 980 spectrometer. PL decay dynamics of Mn2+ emissions were measured using a microsecond flash lamp of 100 W power excited at 340 nm. PL decay measurements for shorter lifetimes were carried out on a TCSPC setup from Horiba Jobin Yvon using a 375 nm diode laser (IBH, UK, NanoLED-375L) and analysis done with DAS v6.5 software. For steady-state power dependent measurement, the sample was excited with the 355 nm line of an Innolas Picolo AOT Nd:YAG laser using a variable neutral optical density filter, where the power was varied over 0.38 mW cm−2 to 0.55 W cm−2 (3 orders of magnitude). The emission was collected in the reflection mode using an Andor-iCCD spectrometer. Low temperature steady state PL measurements were performed using an Edinburgh FLS-900 spectrofluorometer with a closed cycle He cryostat (Janis). A 405 nm pulsed diode laser was used for time-resolved measured measurement. For performing temperature dependent measurements, a pellet of 10 mm diameter and 0.6 mm thickness was prepared by cold pressing the sample in a cylindrical die. The excitation beam was guided toward the sample through an optical fiber, which has a wavelength limitation below 400 nm. The entire measurement was carried out with approximately 25 min of temperature stabilization time at each temperature. Powder Xray diffraction measurements B

DOI: 10.1021/acs.inorgchem.8b02042 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Reaction scheme highlighting the synthetic conditions adopted for the synthesis of ethylenediammonium (ED) based lead bromide perovskites with remarkably different photophysical properties. (b) PL and PLE spectra of the as prepared ED HBr perovskite; c) Absorbance, PL, and PLE spectra of ED DMSO perovskite. (d) White light emission characteristics on chromaticity plot for ED HBr system with inset photograph of the material under UV illumination showing white light emission. were carried with a PANalytic X’Pert Pro using Cu Kα radiation (λ = 1.5406 A0). Thermogravimetric (TGA) analyses were performed using a Mettler-Toledo TGA/SDTA851e system. All the samples were heated from 25 to 800 °C at a rate of 10 °C/min under a nitrogen atmosphere. Details of single crystal X-ray diffraction including data collection, structure solution and refinement, and crystallographic table of crystals are provided in the SI.

crystal structure elucidation. For brevity and clarity, products formed using HBr as the solvent are termed as ED HBr, while the products formed using DMSO as the solvent are termed as ED DMSO in the rest of the report. The as prepared, bulk phase powder of ED HBr crystals shows white light emission under UV excitation inside a UV chamber. Figure 1b shows the photoluminescence (PL) emission profile of the ED-HBr product when excited at 350 nm. The emission has a shoulder at ∼400 nm and a dominant broad band peaked at 550 nm that spans from 450 to 750 nm covering the entire visible range. The large Stokes shift (∼200 nm) of the 550 nm broad emission (absorption onset: ∼410 nm, Figure S1, SI) leads to low self-reabsorption. The color coordinates of such an emission profile are calculated to be (0.37, 0.41) as presented in Figure 1d, in the CIE 1931 chromaticity plot. The broad emission profile of the 1DN perovskites is consistent with the observed white light emission from the samples (inset to Figure 1d) when illuminated inside a UV chamber. Although the quantum yield of the broad emission band is low (3.3%), the physical origin of the observed broad emission is worth probing further (as discussed later in the report). The photoluminescence excitation (PLE) spectra for the narrow and broad emission peaks are shown in Figure 1b. The nature of the PLE curve for the band edge excitonic emission peak (narrow shoulder peak) and the broad emission peak is similar and closely resembles the absorption spectra (see Figure S1, SI). This indicates that the excitation of the band edge charge carriers is responsible for the broad emission in the ED HBr system. The optical absorbance, PL, and PLE spectra of the as prepared ED DMSO perovskites are shown in Figure 1c.

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RESULTS AND DISCUSSION Typical wet chemical synthetic method of preparation of organic inorganic hybrid perovskites involves dissolution of the organic halide salt and lead halide salt in a suitable solvent by heating. To obtain the self-assembled products, the influence of the solvent is reduced by solvent evaporation, or gradual cooling, or by addition of an antisolvent. This results in the precipitation of products.8,10,19 The solvent activity and its incorporation can play a significant role in directing the final structure/dimensionality of the products.20−23 Here, ethylenediammoniumdibromide (ED) and lead(II) bromide salts are dissolved in a 2:1 molar ratio to prepare a clear precursor solution in a chosen polar solvent. When HBr is used as the solvent, the dissolution is carried out by stirring for 10 h at 45 °C. Antisolvent acetone is added to this clear solution to afford crystals (ED HBr) that emit white light under UV illumination. Interestingly, when dimethyl sulfoxide (DMSO) is utilized as a solvent to prepare the precursor solution followed by diffusion of dichloromethane (DCM) as the antisolvent, crystals with blue emission are formed. The reaction scheme is shown in Figure 1a. Both types of crystals were washed, dried under a vacuum, and utilized further for characterizations. Hand-picked single crystals for both the products were utilized for single C

DOI: 10.1021/acs.inorgchem.8b02042 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Ultraviolet−visible light (UV−vis) absorption measurement of the ground powder of ED DMSO perovskite, performed through diffuse reflectance, shows an absorption band at 390 and 350 nm. Steady-state PL measurement of ED DMSO shows a strong blue emission peak at 410 nm when excited at 360 nm above the band edge with a small Stokes shift. The PLE spectra collected at 430 nm show excitonic absorption features at 397 and 350 nm that compare well with the absorbance profile. The excitation spectra are characterized by a sharply rising threshold region occurring near the absorption edge of the ED DMSO perovskite. The resemblance of the absorption band with the PLE spectra of ED DMSO perovskite for the blue emission signifies that the blue emission is due to the formation of electron−hole pairs in the ED DMSO perovskite semiconductor material. These observations, in general, are consistent with reports on the optical characterizations of 2DN layered lead bromide based perovskites suggesting two dimensionally networked structures of the ED DMSO product.7,8,10,24 In order to rationalize the observed remarkably different photophysical properties of the two as prepared perovskites (ED HBr and ED DMSO), structural characterizations were performed for both the products. Thermogravimetric analysis (TGA) of the ED HBr crystals (Figure S2, SI) shows a loss of organic ligands at 344 °C, while the inorganic component loss is at 584 °C. Given the reactant mixture composition (absence of small “A” site cation), the obtained ED HBr crystals are anticipated to be lead halide perovskite of dimensionality lower than 3DN. The collected powder XRD pattern of the ED HBr crystals (Figure S3, SI) also suggests the low dimensional nature of the formed ED HBr perovskites (absence of a series of (00l) diffraction peaks with a given periodicity suggests lower than 2DN structure). On the other hand, the collected powder XRD pattern of the ED DMSO crystals, as shown in Figure 2a, shows a series of (00l) diffraction peaks corresponding to interlayer spacing, indicating the formation of a two-dimensional layered structure for the ED DMSO sample. The interlayer spacing for the ED DMSO perovskite material, calculated from the XRD pattern using the Bragg equation, is 1.3 nm. This calculated interlayer spacing seems higher (interlayer spacing of n = 1 2D perovskite (butylammonium)2PbBr4 is ∼1.4 nm).25−27 Further, we have performed a TGA of ED DMSO perovskites (Figure 2b). TGA data show three weight loss peaks at 200, 344, and 669 °C. The weight loss peaks at 344 and 669 °C can be assigned to a loss of organic ligands and inorganic framework, respectively. Given that the boiling point of the utilized solvent (DMSO) is ∼190 °C, the weight loss peak at 200 °C could be attributed to a loss of intercalated DMSO molecules from the layered 2DN ED DMSO perovskite. The DMSO solvent molecules, if intercalated into the layered perovskite structure, can lead to increased interlayer spacing. In order to confirmatively assign the structure/dimensionality of the prepared perovskites, single crystal structure analysis was performed for both the products. Single crystal structure elucidation of ED HBr crystals [CCDC 1850343] reveals the 1DN nature of the formed perovskite that crystallizes in monoclinic space group P21/n (Figure 3a; Tables S1, S2, and S3, Supporting Information). Here, the corner shared PbBr6 octahedra form a 1D ribbon like structure along the a axis with each ribbon having a width of four corner-sharing lead bromide octahedra. They are surrounded by ethylenediammonium ligands and isolated bromide atoms. The organic molecules follow a similar wave

Figure 2. (a) PXRD pattern of powdered crystals of ED DMSO 2DN perovskites. (b) Thermogravimetric data for weight loss of ED DMSO 2DN perovskites.

like pattern to that of the inorganic component, as can be seen in Figure 2b. The asymmetric unit and the atomic numbering scheme are depicted in Figure S4, SI. Importantly, the angles Pb(1)−Br(4)−Pb(2) and Pb(1)−Br(6)−Pb(2) are 172.83(2)° and 160.57(2)°, respectively, and are severely distorted from the ideal 180° value. Such distortions play an important role in conferring the 1D ED HBr system with broad white light emission properties as discussed later in this report.28 Single crystal structure analysis of ED DMSO crystals reveal 2D nature of the formed perovskite crystallizing in monoclinic space group P21/c (Figure 3b; Table S4, S5, and S6, Supporting Information; [CCDC 1850344]). The corner shared PbBr6 octahedra form a zigzag 2D sheet like structure showing contortion along the b axis with the ethylenediammonium ligand localizing in these contortion “pockets” on either side of the sheet. The crystal structure also shows the presence of DMSO solvent molecules intercalated into these metal halide octahedra layers. This structural analysis clearly provides the rationale for the observed increased interlayer spacing distance and the TGA weight loss peak at ∼200 °C (boiling point of DMSO ∼ 190 °C). Notably, the ammonium ions form hydrogen bonds with the oxygen atom of the intercalated DMSO molecule (see Figure S5, SI). This favorable hydrogen D

DOI: 10.1021/acs.inorgchem.8b02042 Inorg. Chem. XXXX, XXX, XXX−XXX

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not due to defects. The PLE spectra of the annealed samples are akin to as prepared samples, which again indicates that the broad emission is a bulk property (see Figure S6, SI). There are no observable differences in the peak positions in the PXRD pattern of the as prepared and annealed samples (see Figure S3, SI). Time resolved PL measurements reveal that the lifetime of the emission (∼11 ns) at different wavelengths across the broad emission peak remains constant (see Figure S7, SI), indicating an origin of the broad emission from similar excited states. Further, we studied the dependence of emission on excitation intensity. If the emission arises from permanent defects, PL saturation is expected, as the defect/traps are filled at higher pump fluences.34,35 Figure 4d shows that the PL intensity increases linearly with excitation power density from 0.3 to 100 mW/cm2 at room temperature with no signs of PL saturation. Although material degradation precludes the use of higher excitation intensities on these samples, this linear behavior suggests that emission does not arise from permanent defects/trap states. We also see no change in emission band shape throughout this experiment (see Figure S8, SI), indicating that different emissive defect sites are not accessed at different excitation intensities. In addition, we have performed low temperature steady state PL measurements for ED HBr perovskite. Figure 5a compares the steady state PL at room temperature and at 60 K. The PL profile at low temperature is broad (fwhm ∼160 nm @ 60 K compared to ∼200 nm @ 298 K), strongly Stokes shifted, with the same features as those of self-trapped excitons observed in many metal halide crystals at low temperature, corrugated 2D halide perovskites9,29,36−42 and low dimensional perovskites.43,44 With decreasing temperature, the broad luminescence becomes more dominant compared to free exciton (FE) luminescence (see Figure S9, SI). Figure 5b shows an Arrhenius plot of the ratio of the peak area of the broad (STE) and narrow (FE) emission as a function of inverse temperature. This plot clearly shows that cooling increases the ratio until 60 K and then decreases as temperature is lowered. This trend has been observed in other materials and is due to the thermal equilibrium between the conduction band state of the band edge excitonic emission and the self-trapped emission states.9 The self-trapping depth (or difference in activation energies for carrier transfer between the FE and STE states),9 obtained by fitting the measured PL intensity ratios as a function of temperature through Arrhenius relation as shown by the blue dotted line in Figure 5b, is estimated to be ∼14 meV. The observed increase in the intensity of the broad STE emission (with relation to FE emission) as the temperatures are lowered can be understood as arising from a thermally driven back transfer from the STE to the FE state, which becomes more difficult at lower temperatures.9 Then, at very low temperatures (below 60 K), the carriers are less able to surmount the activation barrier from the FE to the STE states leading to a relative increase in narrow FE PL.9 At sufficiently low temperatures, this equilibrium between the FE and STE states becomes more complicated9 due to the tunneling between the FE and STE states,45 singlet−triplet exciton interconversion, FEs bound to defects,46,47 or biexcitons.48 As an alternative to permanent defects giving rise to broadband emission, photogenerated excitons can couple to lattice distortions of the soft inorganic/organic framework and be stabilized as “self-trapped” carriers.49 Strong vibronic coupling between excitons and the lattice can cause broadband emission due to the formation of a transient excited state that

Figure 3. (a) The wave-like arrangement of the 1D ribbons of corner shared PbBr6 octahedra in ED HBr perovskite. Br = green sphere, C = black sphere, N = blue sphere, H = gray sphere; Pb is at the center of octahedra. (b) 2D ED DMSO perovskite showing the 2D contorted nature of the formed perovskite. The sulfur and oxygen atoms of the DMSO molecule are shown as yellow and red spheres, respectively. Hydrogen bonding interactions between the terminal ammonium groups and DMSO oxygen atoms are highlighted through dashed red lines.

bonding interaction of the ethylenediammonium ligand with the DMSO molecule orients the ligand in such a way that the amine group faces the oxygen atom. This largely drives the arrangement of the inorganic metal octahedra into a contorted 2 × 2 (110)-oriented 2D sheet structure. The structural contortion in 2DN perovskite is known to broaden the emission profile of the perovskites.29−31 In consonance with this, the contorted 2DN ED DMSO perovskite shows broadened emission profile (see Figure 1c, black curve) here. Notably, the ED DMSO has a 2D (n = 1) perovskite structure with a network of corner shared octahedra in two dimensions while the ED HBr has a 1D wire like structure wherein the ligands are wrapped around the metal octahedra chain networked in one dimension. Further, the origin of the broad white emission of 1DN ED HBr perovskite is probed here. Defect states are well-known to create such broad emission profiles in materials.32,33 Hence it is imperative to find the origin of the observed broad emission: whether it is a inherent bulk property or it is due to defects. In order to gain insight, we collected emission spectra of the samples when excited at different wavelengths (Figure 4a). The emission profile is seen to be insensitive to the excitation wavelength, which indicates that the broad emissions are very likely not due to defects. The nature of the PLE spectra collected across the emission peak, as shown in Figure 4b, remains the same as that of the band edge shoulder PLE. This again indicates that the broad emission and the narrow emission share similar emissive excited states and are inherent bulk properties. In order to the eliminate defect state based broad emission hypothesis, we have performed thermal annealing of the prepared crystals at 125 °C. The observed similarity of the emission profile of the as prepared and annealed samples (see Figure 4c) suggests that the emission is E

DOI: 10.1021/acs.inorgchem.8b02042 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) PL emission profile of 1D ED HBr perovskite under different excitation wavelengths. (b) Photoluminescence excitation (PLE) spectra collected at different wavelengths for 1D ED HBr perovskites. (c) PL emission profile of as prepared and annealed 1D ED HBr perovskites. (d) Dependence of emission intensity (550 nm) of 1D ED HBr perovskites on excitation (350 nm) intensity and the linear fit to the data.

generated transient self-trapped excited states55 in our ED HBr 1DN system. The ED DMSO 2DN (n = 1) layered perovskite, like many other conventional 2DN layered perovskites, is expected to be highly deformable/malleable due to the inherent mechanical and electronic “softness” of constituent atoms of the perovskites.7,8,10 This has been previously exploited by Kundu and co-workers to demonstrate Mn2+ doping into (butylammonium)2PbBr4 based 2DN layered perovskites utilizing a mechanochemical synthetic strategy.26 In lieu of this, we demonstrate successful Mn2+ incorporation into the 2DN ED DMSO perovskites utilizing a solid state grinding and annealing methodology. The PL and PLE spectra of the Mn2+ incorporated ED DMSO perovskites are shown in Figure 6. The emission spectra of the Mn2+ incorporated ED DMSO system show a characteristic band edge blue emission along with a broad emission peak for the Mn2+ emission. The lifetime of the Mn2+ broad emission is measured to be ∼1 ms (see Figure S10, SI) in accordance to the forbidden nature of the 4 T1g → 6A1g internal transition. By changing the amount of the input MnBr2 salt, the intensity of the Mn2+ emission can also be altered as shown in Figure S11, SI. Importantly, the PLE spectra collected at Mn2+ emission (Figure 6, black curve) shows features that very closely resemble those of the PLE spectra of the band edge emission of the Mn2+ incorporated ED DMSO system (Figure 6, red curve). Moreover, the excitation spectra here are characterized by a sharply rising threshold region occurring near the absorption band edge of the host ED DMSO perovskite. This clearly signifies that the

is highly distorted with respect to the ground state. Such selftrapped excitons are reported to be present in corrugated 2DN layered hybrid perovskites and low dimensional perovskites.7,9 Karunadasa and co-workers have studied a variety of 2D perovskites that emit white light due to self-trapped excitons.9,28 The authors reported a linear correlation between the largest measured out-of-plane distortion Dout (180° − θout) and ISTE/IFE at a given temperature (where θout is the out-ofplane projection of θtilt onto the plane of Pb atoms and θtilt = metal−(μ-halide)−metal angle).9,28 The single crystal structure data analysis of our 1D ED HBr system reveals a high value of Dout (20°), which is in agreement with reports by Karunadasa et al. relating structural deformations to selftrapping/white light emission.28 It is also well-known that exciton self-trapping depends critically on the dimensionality of the system.50,51 Lowering the dimensionality to 1DN makes exciton self-trapping easier at any exciton−phonon interaction strength.52,53 Exciton self-trapping has been observed in 1D metal halides that exhibit broad PL with large Stokes shifts.43,54 In lieu of these, it is very likely that self-trapped excitons are supported in our 1DN perovskite systems that cause broadband emission. More sophisticated experiments such as transient absorption (TA) studies are required to confirm the nature of the self-trapping mechanism in these systems. However, the observed linear power dependence, results of the annealing experiments, time-resolved PL experiments, low temperature PL studies, and the observed similarity to reported cases of self-trapping in corrugated 2D and 1D chain perovskites are consistent with emission from such photoF

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color with peak position in a narrow range (600−630 nm).56−60 Even for the heavily investigated Mn2+ doped II− VI quantum dot systems, the tunability of the Mn2+ emission is minimal.57,61−64 This is very likely due to the fact that states involved in the emission are essentially localized at the Mn site.61,62 However, strain engineered ZnSe/CdSe/ZnSe based quantum dots, when doped at select radial positions, does show tunability of the Mn emission over the entire visible range, highlighting the effect of changes in the local geometry at the Mn center.62 Here, the observed long wavelength Mn2+ emission peak at ∼670 nm in Mn2+ incorporated ED DMSO perovskite is unprecedented in the lead halide based perovskite family. We suspect that the presence of the intercalated DMSO solvent molecules in the 2DN ED DMSO system might play a role in red-shifting the Mn2+ emission peak. In order to gain insight into the plausible cause of the observed red-shift, we have performed Mn2+ incorporation into 2DN perovskite prepared by dissolving 1,4-butanediammoniumdibromide and PbBr2 in HBr following a reported protocol.13 The resulting perovskite [(H3NC4H8NH3)PbBr4] is known to form a 2DN layered structure with no insertion of solvent molecules (refer to single crystal structure data in ref 13). Mn2+ incorporation into this diammonium based 2D perovskite shows Mn2+ emission at ∼610 nm (see Figure S12, SI), which is in stark contrast to Mn2+ incorporated in ED DMSO diammonium based 2DN perovskite. This indicates that intercalated DMSO molecules do affect the Mn2+ emission peak, thereby suggesting interaction between the incorporated Mn2+ ions and the DMSO molecules within the first coordination sphere of the Mn2+ ions. This can cause heterogeneity in the local site geometry around Mn2+ ions that can lead to red-shifted emission. We do note that these experiments are incapable of providing direct rationale for the observed red-shifted Mn2+ emission, and experiments are being designed to gain further insight into the observed red-shift. However, the underlying observation made here is sound and represents the first example of highly red-shifted Mn2+ emission in the entire lead halide based perovskite family. Notably, preparation of the perovskites under different synthetic conditions (albeit under the same precursor ratio) leads to structurally diverse 1DN and 2DN type perovskite with widely different photophysical properties (see Figure 1 and Figure 3). This clearly signifies the impact of the chosen synthetic route in controlling the final dimensionality/structure of the products. Single crystal structure analysis presented above already highlights the effect of the incorporated solvent molecules (DMSO/HBr) in stabilizing final structure/ dimensionality (2DN/1DN). When butylammonium instead of ethylenediammonium is utilized as the ligand, both the above synthetic procedures produced 2DN sheet structures. These observations indicate that the presence of the solvent molecules along with the short chain diammonium ligand plays an important role in directing the final structure/dimensionality. The single crystal structure of the 2D ED DMSO product reveals the operative interactions between the ligand, solvent, and the metal octahedra that largely dictates the final structutre/dimensionality. The small length of the chosen ED ligand allows the terminal ammonium groups to be on the cis side to maximize hydrogen bonding interaction with the oxygen atom of the DMSO solvent molecule. Here, one DMSO molecule interacts with the two terminal ammonium groups of one ED ligand. The footprint of the ED ligand is such that it fits into the contortion pockets of the metal halide

Figure 5. (a) Steady state PL emission profile of 1D ED HBr perovskites at room temperature and at 60 K. (b) Temperaturedependent ratio of integrated intensity of broad STE emission and narrow FE, ln(ISTE/IFE), for a pellet of 1D ED HBr perovskites when photoexcited at 350 nm. The blue dotted line is a linear fit to the curve.

Figure 6. Optical characterization of Mn2+ incorporated ED DMSO 2DN perovskite showing highly red-shifted Mn2+ emission.

Mn2+ emission is sensitized by band-edge absorption of the host ED DMSO perovskite. Interestingly, the Mn2+ emission is observed to be red in color centered at ∼670 nm, which is highly red-shifted. The majority of the reports on Mn2+ doped perovskite demonstrate Mn2+ emission to be yellow-orange in G

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for any reported metal halide based doped perovskite system. This report highlights that nature of the synthetic route, utilized solvents, and its incorporation into the final perovskite structure may be playing a key role that governs the final dimensionality, which largely shape their remarkable photophysical properties.

octahedra. The length of the contortion is likely governed by the length/footprint of the ED ligand. Here, the DMSO molecule acts as the spacer between the two layers of the metal octahedra. The ED ligands, that charge balance the metal octahedra in a layer, undergo hydrogen bonding with DMSO molecules that bind two separate metal octahedra layers together. In the case of ED HBr, since the ED ligand is not oriented toward a particular direction, it forms 1D ribbons as opposed to 2D layers. Further, we have performed control experiment that highlights the role of intercalated solvent in directing the final dimensionality/structure (1DN, 2DN). Room temperature solid state grinding of PbBr2, MnBr2 and ethylenediammoniumdibromide salt inside a UV chamber shows a highly dynamic range of emission colors. Within the first few minutes of grinding, a clear red emission is observed that gradually changes into a yellow-orange emission and finally to a white emission. This change in the color can be rationalized by realizing that the 2DN structures are formed momentarily during the initial grinding phase. Mn2+ ions can incorporate into this initially formed 2DN perovskite giving rise to the red emission (∼670 nm). With further grinding and in the presence of bromide anions (with no DMSO solvent molecules present), the 2DN perovskite converts to 1DN perovskite that emits white light, possibly still incorporating Mn2+ ions. This does provide indirect evidence of the effect exerted by the solvent molecules (DMSO/bromide) in stabilizing the 2DN/1DN perovskite structure. Here, in this simple solid state grinding control experiment, we have utilized Mn2+ emission as a reporter for 2DN structure, while the white light emission was the reporter for 1DN perovskite structure. The video showing the changes in the color during this control experiment can be found in the SI. A detailed understanding of the interactions of the solvent molecules with the metal octahedra can help in rationalizing the attained structure/ dimensionality of the final perovskite end products.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02042. Video file of solid state grinding reaction of MnBr2, PbBr2, and EDBr2 salts showing dynamic change of colors (AVI) Single crystal structure data analysis results for 1DN and 2DN perovskites, CIF and CheckCIF files for the crystals, hydrogen bonding interaction in 2DN ED DMSO perovskite, PLE comparison for 1DN, comparison of PLE and PXRD for as prepared and annealed 1DN, lifetime analysis of 1DN, fluence dependent PL of 1DN, low temperature PL of 1DN, lifetime analysis of Mn2+:2DN, PL of Mn2+:2DN at different Mn concentrations, PL of host and Mn2+ incorporated 2DN perovskite [(H3NC4H8NH3)PbBr4] (PDF) Accession Codes

CCDC 1850343−1850344 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

CONCLUSIONS Here, we report synthetic control in directing dimensionality/ structure and photophysical properties of low dimensional organic lead bromide perovskites. Utilizing organic (ethylenediammoniumdibromide) and inorganic (PbBr2) components in a given ratio (2:1) under different reaction conditions (solvent-antisolvent pairs) can give rise to one dimensionally networked (1DN) and two dimensionally networked (2DN) hybrid perovskites in high yields. Contorted (110) 2DN perovskites are formed when dimethyl sulfoxide-dichloromethane is used while 1DN perovskites are formed when HBr-acetone is used as a solvent−antisolvent pair. Importantly, the incorporation of the solvent molecules into the final structure of the perovskites, as observed here for both the 2DN and 1DN cases, stabilizes the final structure/dimensionality of the resulting perovskites. This change in dimensionality of products reflects strongly in the observed remarkable differences in their photophysical properties. The 1DN system shows a broad emission band spanning the visible range which emits white light under UV illumination. Such white light emission in the 1DN perovskites arises likely due to selftrapped excitons. The 2DN perovskite shows its characteristic strong excitonic blue emission at room temperature. Further, the 2DN systems are amenable to Mn2+ incorporation, showing energy transfer from 2DN host to Mn2+ ions. Intriguingly, this long-lived Mn2+ emission is red in color and is highly red-shifted (∼670 nm), which is unprecedented

*E-mail: [email protected]. ORCID

Bhushan P. Kore: 0000-0003-3921-6194 Rajesh G. Gonnade: 0000-0002-2841-0197 D. D. Sarma: 0000-0001-6433-1069 Janardan Kundu: 0000-0003-4879-0235 Author Contributions ∥

Equal contribution

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.H.P.U. and R.B. contributed equally to this work. The authors would like to thank Dr. S. B. Sukumaran, Dr. P. Hazra, Dr. P. Poddar, and Dr. A. Nag for insightful discussions. This work was supported by DST (Grant Nos. SB/S2/RJN-61/ 2013 and EMR/2014/000478) and CSIR-NCL start-up (Grant No. MLP030326).

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REFERENCES

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