Introduction

Self-assembled growth of 2D layered microstructure and its evolution to 3D microstructure is unique in nature in the realm of crystal growth and design of low dimensional solids. Self-assembled molecular structures give rise to unique macromolecular structure often found in nature giving interesting physical properties and functionalities of molecules1,2,3,4,5,6. However, controlled crystallization has paramount importance in exploiting specific functionalities of molecular material and designing devices to work. The process of crystallization is often very complex and is limited by the growth kinetics controlled by various thermodynamical parameters leading to an equilibrium state. In a number of physical (MBE, CVD etc.) as well as chemical process, controlled molecular growth has been obtained and specific devices are fabricated in various dimensions (1D, 2D or 3D)7,8,9,10,11,12. Now, understanding the self-assembled molecular growth mechanism in 2D and its subsequent evolution in 3D giving rise to unique molecular architectures is fundamentally a very important aspect in materials science research.

How does a crystal grow? The generic question is often encountered in various studies in condensed matter physics, materials science and surface science. Crystallization of molecules and their growth kinetics eventually determine the macromolecular structure and properties. Numerous studies have been reported on the growth self-assembled or artificially grown 1D, 2D or 3D crystals along with their physical properties1,2,3,4,5,6,7,8,9,10,11,13,14,15,16,17,18,19,20,21. Nevertheless, finding the growth mechanism of crystals and their equilibrium surface structures is a pertinent problem to look out given the various growth parameters, preparation methodologies, type of materials and many more22. A seminal paper published in 1951 by Burton, Cabrera and Frank (referred as BCF theory)23, brings out first an extensive discussion on the growth of crystals, particularly on the role of dislocations for the growth of crystals from a metastable phase and some reviews thereafter will be of worth mentioning24,25,26. Now, nucleation is the first step of any growth of crystals either from solutions or vapors followed by its expansion in 2D or 3D and finally the termination. As suggested by BCF theory, for a perfect crystal to grow in a supersaturated environment, the formation of 2D critical nuclei is a must which further grows freely to form a molecular layer upon addition of new molecules to the adjacent sites of the critical nucleus. Statistically, the rate of formation of such critical nucleus at a temperature T is proportional to exp(-E0/kBT) where E0 is the free energy of the nucleus or the activation energy for 2D nucleation and kB is the Boltzmann constant23. On the other hand, if the crystal is not perfect, contains lattice imperfections, more specifically dislocations, then each dislocation becomes the origin of the formation of a step. The growth takes place by the formation of such steps and attaching molecules thereon the crystal surface and may require certain activation energy for the growth to occur23,24,25,26,27. Interestingly, in addition to steps, the surface may contain kinks (steps on steps) and can facilitate growth of a monolayer with addition of molecules at the kinks without any cost of extra energy. The concentration of kinks and the rate of advancement of steps of course depends on the thermodynamics and crystal growth environments.

We describe here a unique growth of 2D layered surfaces on quartz substrate for a hybrid halide perovskite material ((CH3NH3)3Bi2Cl9) named as MABiCl, MA = CH3NH3+) with each molecular layer having the shape of ‘raspberry surface’ stacked one above the other presumably bonded by some interlayer adhesion or weak van der Walls forces. A comprehensive morphological analysis obtained from scanning and transmission electron microscopy revealed growth of ‘raspberry surface’ driven by a combined edge and screw dislocations23,24,25,28. All together the molecular layers form a disc-shaped 2D crystal (or microcrystal) with rough or uneven surface of visible steps or height and boundary as well lying flat on the substrate. However, the 2D crystal lacks stability with time and suffers spontaneous recrystallization. Interestingly, upon morphological evolution with time, the 2D layered structure is recrystallized into a self-assembled 3D flower-like structure as clearly revealed in the electron microscopic images. The possible growth mechanism is presented here based on information inferred from microscopic images and molecular dynamics simulations. For the growth of 2D layered raspberry surface structure, we propose a dislocation driven growth following a ‘terrace-ledge-kink (TLK)’ model. On the other hand, the conversion of 2d layered sheets into 3D flower-like crystal is plausibly caused by elastic strains within the sheets29,30. Interestingly, in a mixed halide ((CH3NH3)3Bi2(BrxCl1−x)9, x = 0.44 named as MABiBrCl) obtained upon bromine doping to MABiCl, the 2D growth and its further evolution to 3D structure is significantly influenced suggesting an interesting aspect of dislocation driven growth of perovskites. To be noted, the mixed halide system is found to exhibit the lowest bandgap of 2.557 eV corresponding to bromine content x = 0.44 in our experimental study31. In this case, the growth is dominated by rounded flat one or two layered structures without formation of any multi-layered raspberry surface structure. The rounded facets structure upon evolution in time becomes curved and is reshaped into a ‘lotus leaf-like’ structure instead of a ‘flower-like’ structure as that of observed before. Definitely, therefore the growth kinetics changes after insertion of Br into the halide structure which can be examined considering a dislocation induced growth as discussed here. Beside experiments, we also performed molecular dynamics simulations to reconstruct the 3D flower structure from 2D layered sheets of MABiCl and computed the values of bending stiffness of the folded/curved surfaces (petals) which is a very crucial parameter to explain the stability of such folded 3D microstructures.

Worthwhile to mention that halide perovskites have emerged as the potential material for highly efficient solar cell and optoelectronic device applications, particularly light emitting diodes (LEDs) in recent times32,33,34,35. Use of hybrid lead-halide based polycrystalline perovskite film as the light-absorbing layer has rapidly changed the photovoltaic research to develop highly efficient perovskite based solar cells (PSCs) and devices in the last decade. An impressive certified efficiency of more than 25% has been reported and PSCs appear to be the most promising one for the next generation solar cells36,37,38. However, a number of factors have limited a transformation of lab-scale devices to commercial devices. A key bottleneck lies in long term stability of PSCs and is of utmost importance in the field of PSC technology31,39,40. It is further worthwhile to mention that, although replacement of toxic lead by bismuth is found to increase the operational stability of halide perovskites, the high band gap above 2 eV of MABiCl is still a bottleneck in its use in photovoltaic applications and necessitates strategy to tune the band gap of Bi-based halide perovskites to make it useful in device applications41,42.

Results and discussion

Morphological analysis

MABiCl film

First, we fabricate MABiCl film on quartz substrate by drop-casting method, details of which are provided in Experimental section. To understand the microstructural evolution of the perovskite film/sample, scanning electron microscope (SEM) images were captured over a period of one week and analyzed further. Since its first drop-casting to quartz substrate, we allow the samples to age over a period of one week and SEM images were recorded accordingly on each day. The data eventually show the formation of 2D flake and its transformation to 3D flower like microstructures with time. The distinct formation of 2D flake and 3D hierarchical flower-like compact microstructure is shown in Fig. 1a and b respectively. A closer look of the 2D flake depicts a unique layered structure of molecular layers having the shape of ‘raspberry surface’ stacked one above the other as shown in an enlarged view in Fig. 1c, which is further presented with a schematic diagram in Fig. 1d for convenience. Now from the analysis of microscopic images, it is understood that the process of 2D to 3D transformation is not an abrupt one, rather gradual and involves a series of steps from 2D nucleation to formation of 2D flakes/sheets with a finite size and finally to 3D flower-like structures. In Figs. 2(a-f), we presented SEM images which are quite revealing for the steps involved for the formation of 2D layer and the final configuration of a 3D microstructure. The steps listed below sequentially inferred from Fig. 2 are:

Fig. 1
figure 1

2D and 3D growth of MABiCl. SEM images of the perovskite MABiCl sample indicating growth of (a) 2D layered structure and its conversion into a (b) 3D flower-like structure. (c) An enlarged view of the surface of the SEM image presented in (a) resembling that of a ‘raspberry’ surface and (d) schematic diagram of the raspberry surface.

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Fig. 2
figure 2

Nucleation and growth of MABiCl microstructures. SEM images obtained for a period of one week for the step-by-step growth obtained for the evolution of 3D flower structure: (a) day 1: initial nucleation of molecular clusters, (b) day 2: 2D nucleation of the molecular layer, (c) day 3: growth of 2D larger flakes and formation of steps, (d) day 5: top layer of the 2D surface, (e–f) day 6: lift-off of the layers to grow flower petals and deformation of the 2D layers into sectors to form the petals for the flower structure. A fully grown flower structure can be observed on day 7 as shown in Fig. 1(b).

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  • i. Initial nucleation: A number of small molecular clusters or simply small particles (size ~ 20 ± 5 nm) randomly distributed on the scan area indicating initial nucleation for the growth of crystal as shown in Fig. 2a. Any smaller size of particles is probably beyond the resolution of image taken here and precludes their true size or shape.

  • ii. 2D nucleation: As revealed in Fig. 2b, a number of relatively larger size (~ 100 nm) particles plausibly suggesting the critical size of 2D nucleation formed by the unstable tiny particles. The size estimation is done after observing a number of images recorded for this purpose. A very early stage of 2D growth is initiated by aggregation of 2D islands. The microscopic image further reveals the growth of one layer piling over another layer and plausibly forming the steps for further growth. In this process of growth, it is observed that a 2D nucleus is stuck to the step edge and its subsequent growth over time results in a top layer. Sticking of 2D nucleus takes place in various step edges and becomes the source material for growth of larger flakes under dissolution and recrystallization process eventually forming consecutive layers over the top of other as can be clearly seen in the latter stage Fig. 2c.

  • iii. Formation of 2D flake: Combined action as stated in the steps above, when proceeds over time culminates into a larger 2D flake with a truncated round shaped growth as shown in Fig. 1a. On the surface of the larger flake, a large number of steps (on the top of one into other) can be observed (Fig. 2d). The growth of each layer takes place by adsorption of small particles (a number of adatoms) to the step edges giving rise to a complete 2D flat layered structure. The microstructure of each layer resembles that of a ‘raspberry surface’ uniquely assembled on the substrate.

  • iv. Sector formation on 2D flake: In this process, on the top layer (s) of the thicker 2D sheet, cracks start to develop and parts of the flake start to break apart as can be seen Fig. 2e. The broken part resembled to that of a sector of circle. The sectors are neither concentric nor having same radii. The process is a continuous one suggesting formation of sectors on the other parts (top as well as bottom) of the flake too.

  • v. Formation of petals: Interestingly, after formation, sectors are found to be partially delaminated or lifted off the substrate/surface (Fig. 2e) and start to be inclined to the vertical partly adhering to the flat or unfolded segment of the layers. Each sector has different surface area as well as inclination to one another. These sectors resemble ‘petals’ of a flower (of thickness ~ 40 nm) and initiates the process of formation of 3D flower like structures as can be viewed in Fig. 2f.

  • vi. 3D flower-like structure: The sectors or petals become inter-connected with time across the 2D sheet and the morphology starts to grow like a flower. The orientations of the delaminated sheets are not quite regular rather random within the evolved 3D structures. In its final configuration, the 2D sheets eventually evolves to a compact, well-defined 3D flower-like structure with a network of interconnected petals as can be clearly seen from Fig. 1b. The flowers have a distribution in size as can be viewed in supplementary Figs. S1a and S1b.

MABiBrCl film

Interestingly, in a mixed halide system MABiBrCl (Experimental, Methods for details of fabrication) growth of the perovskite on quartz becomes significantly different than that of observed for MABiCl. We repeat the same time dependent experiment on Br doped mixed halide perovskite film and the morphological images obtained through SEM are presented in Fig. 3. There is marked differences in both 2D and 3D structure of Br doped sample in comparison to the pristine single halide system. The initial morphology (Fig. 3a) retains the 2D flake-like structure without any formation of ‘raspberry surface’ which in time transformed into a 2D ‘lotus leaf-like’ structure (Fig. 3b). The 2D sheet is possibly grown out of addition of smaller particles as indicated in Figs. 4 (a-c). It does not possess any multiple petal structures, instead grows in the shape of single leaf-like microstructure (Fig. 4c). The growth of 2D layer does not seem to have many layers as that of pristine single halide system, instead grows as a single sheet of thickness ~ 20 nm (Fig. 4d). The lotus leaf like structure is formed from the 2D flakes upon self-folding of outer edges with concave curvatures (Fig. 4e) with a final shape as shown in Fig. 4f. The results are indicative of the influence of Br on the halide perovskite structure and provides a better stability of the perovskite layer31.

Fig. 3
figure 3

2D and 3D growth of MABiBrCl. SEM image of the perovskite MABiBrCl sample showing (a) initial morphology of the 2D flake-like structure and (b) its evolution into a 2D ‘lotus leaf-like’ structure.

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Fig. 4
figure 4

Nucleation and growth of MABiBrCl microstructures. SEM images for the step-by-step growth obtained for the evolution of 2D lotus leaf-like structure: (a) day 1: initial nucleation of molecular clusters, (b) day 2: initial stage of 2D nucleation of the molecular layer, (c) day 3: growth of 2D larger flake, (d) larger view of top layer of the 2D flake surface (e) day 5: lift-off of the 2D sheet from the substrate to form curvature and (f) day 6: deformation of the 2D layers into a lotus-leaf like structure. A complete ‘lotus leaf-like’ structure was observed on day 7 as shown in Fig. 3(b).

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Fig. 5
figure 5

TEM and AFM images for the growth of steps and surfaces for MABiCl. (a) TEM image obtained for MABiCl sample indicating layered growth of 2D layers, (b) AFM image of the layered 2D structure, (c) HRTEM lattice image indicating steps between terraces for the growth 2D layered raspberry surface and (d) a schematic view of the atomic arrangements indicating formation of layers and steps. Both TEM and AFM images were obtained on day 3 since the sample was drop-casted.

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To elucidate further on the growth and plausible growth mechanism, we presented in Fig. 5, high resolution transmission electron microscope (HRTEM) and atomic force microscope (AFM) images obtained for the pristine MABiCl film. The layered 2D growth of the microstructures is clearly visible in the TEM as shown in Fig. 5a. The boundary of each layer is terminated with the crystallized small particles driving the growth of the surface/layer. The HRTEM image dominantly supports the growth of ‘raspberry surface’ as observed by SEM. The 2D layered structures are also clearly visible in the AFM image (Fig. 5b) as well in commensurate with that observed in SEM and TEM. The typical thickness of each layer obtained from AFM is ~ 20 ± 5 nm which is nearly the same as that of the size of the small particles (Fig. 2a) composed of few molecules.

Now for a typical layered crystal growth to occur, nucleation of new steps is essential for the addition of atoms/molecules to crystal facets. Formation of such new steps edges and growth of new facets in a repetitive sequence may lead to the observed fashion of raspberry like crystal surface. Quite interestingly, the plausible model of growth can be directly inferred from the high-resolution lattice images obtained through TEM image as shown in Fig. 5c. The image clearly shows the formation of steps (ledges) as well as kinks in between two surfaces (terraces). The image is quite revealing to suggest the ‘terrace-ledge-kink’ (TLK) model to be appropriate one to describe the observed 2D growth of the perovskite films. To be mentioned, terraces are described by a small atomically smooth flat surfaces, while steps, more specifically atomic steps, are arranged vertically above the surfaces with possible formation of kinks within it. These steps and kinks are the active reacting sites for the incoming atoms and molecules from vapor or gases to facilitate the crystal growth. Now the steps here are not straight and smooth, rather exhibit curvatures to generate raspberry surfaces. The geometry of steps and the number of kinks created at a finite temperature is determined of course by statistical mechanics. For clarity, the atomic arrangements indicating formation of layers and steps as observed in the HRTEM lattice image is presented schematically in Fig. 5d.

Role of dislocations

HRTEM images provide a great deal of insight into the atomic arrangements of the lattice to elucidate and understand the possible growth mechanism of 2D structures for MABiCl and MABiBrCl perovskites. Lattice images are quite revealing to the fact that dislocations play the pivotal role in both cases. Interestingly, MABiCl exhibits mixed type of dislocations, namely edge and screw dislocations as can be clearly seen from supplementary Figs. S2a and S2b respectively. The edge dislocations give rise to the lift-off of the atoms for formation of steps, while the screw dislocations drive the growth at steps edges with addition of adatoms and becomes the active site of nucleation giving rise to the raspberry layer/surface as shown in Fig. 1a. In case of MABiBrCl, the lattice exhibits multiple screw dislocations (spiral-type) without any visible edge dislocations (supplementary Fig. S3). The Br doped lattice is found to be more compact with a reduction in lattice spacing (supplementary Table-1) possibly impeding formation of edge dislocations. Each screw dislocation sources result in piling up adatoms at atomic step edges for the growth of a layer as shown in Fig. 2a and further dissolution of the resulting structure led to formation of single layered 2D leaf-like structures.

Now for MABiCl, the transformation from 2D layered sheets to 3D flower-like structure can be understood by considering the self-folding of the 2D sheets of the layered structures. The self-folding is initiated by partial delamination of the flake (sector) from the substrate and then subsequent folding to stick to its unfolded (flat) segment of the layer. The process is found to be repetitive on the folded segment giving rise to a number of folded segments (petals) adhered to each other eventually giving rise to the formation of a 3D flower-like structure. In the final structure, plausibly all the 2D layers are lifted off from the substrate and folded segments are arranged one over the other at different orientations with adhesive interactions between the self-folded segments. The stability of the 3D flower-like structure could arise out of a balance between the bending rigidity of the folded segments and inter-layer interactions between the 2D segments of the adhered flakes. The interactions between the substrate and the 2D layer becomes less relevant once all the segments are lifted off and self-folded on the out-of-plane of the substrate. The bending rigidity of the 2D flakes could be different from bulk solid due to the strong in-plane bonding amongst atoms or molecules and plays the pivotal role in self-folding of the flakes43.

Fig. 6
figure 6

Formation of cracks in MABiCl layer. (a) TEM image of a portion of a 2D flake indicating formation and propagation of cracks and (b) schematic view of the development of cracks within the 2D layered surface. The TEM image was recorded on day 5.

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The initial stage of delamination (Fig. 2e) starts with the development of cracks on the 2D layered sheets. The sheet is subjected to enormous compressive stress as can been viewed in the lattice image (supplementary Fig. S2b). Direct visualization of the development of cracks can be seen in TEM image as shown in Fig. 6a. The cracks developed at the edges of the surface subsequently propagates to the core which in later stage gives to the formation of sectors. The origin for the development of cracks can be ascribed to the dislocation pile up at the edges, possibly at the kinks and subsequent propagation of cracks to the interior as schematically shown in Fig. 6b. For reasons, mainly topological in nature, the dislocation lines could not start or end in the interior of the crystal lattice. They may end up with the formation of a closed loop or with a fracture in the crystal. The mechanism of crack development and its propagation across the grain boundaries leading to plastic deformation is an important subject of investigation, particularly in the field of metallurgy44,45,46. Our inference lies in direct observation of the development of cracks from microscopic investigations instead of any specific modelling or simulations. We find lattice relaxation due to strains and piling up of dislocations at the boundary as can clearly visible in the TEM images (supplementary Fig. S4). The molecular layers are oriented in different directions (as indicated by different lattice spacing) and dislocations preferentially glide in these different compliant regions and gets trapped into the stiffer regions47. The MABiBrCl film suffers less compressive stress than MABiCl film and does not develop any crack, instead leads to concave curvature of the 2D layer and hence the leaf-like structure.

Calculation of bending stiffness for the flower petals

To elucidate further on the growth mechanism from 2D sheets to 3D flower structure, we simulate the self-folding of the sheets using a nonlinear continuum mechanics model which is largely considered to calculate the bending stiffness (D) of layered sheets of graphene, MoS2 or h-BN under self-folded configurations48,49,50. The details of simulations are provided in the supplementary information, Simulations. The simulation has been done to reconstruct the 3D flower structure from 2D sheets as shown in Fig. 7 (a-d). Interestingly, calculations show that flower petals have a high bending stiffness ~ 200-25000 eV depending on curvatures (κ) (Fig. 7e) of the flower petals indicating how relaxation of the strained 2D layers through bending giving rise to the formation of flower structure as observed experimentally. The bending stiffness, D comes out to be an important parameter in governing 3D deformations from 2D structures of soft lattices. The value of bending stiffness of course sometimes brings discrepancy between experiments and theory predicting a wide range of acceptable values. For example, literature values of bending stiffness for few layered graphene range from ~ 0.8 to 10,000 eV49. Consensus also lacks on how the bending stiffness varies with the number of layers of the soft materials. This is probably dependent on the methods/techniques employed for experiments, models and boundary conditions imposed for theoretical simulations51,52,53,54,55,56. Although no reported value of bending stiffness for the layered hybrid perovskites is available to us, we believe that the observed high bending stiffness is quite reasonable to explain the experimentally observed 3D flower structure and the simulation process adopted here57 corroborate well with the observation from experiments.

Fig. 7
figure 7

Molecular dynamics simulations for the growth MABiCl layers and their bending mechanisms. (a) Unit cell for MABiCl perovskite crystal structure used for simulation of the layered 2D structure, (b) simulated 2D microstructure of MABiCl layers on substrate, (c) bending of 2D microstructure, (d) simulated 3D flower structure and (e) plot of bending stiffness vs. curvature κ of the 2D flakes (flower petals). The details of simulation software developed by us is available at https://doi.org/10.5281/zenodo.1798864957.

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Conclusions

In this report, we conduct experiments on the morphological evolution of the perovskite microstructures for a hybrid perovskite (CH3NH3)3Bi2Cl9 thin film on quartz substrate. We made a comprehensive analysis from microscopic images on the self-assembled growth mechanism of a 2D sheet/flake like structures resembling ‘raspberry surface’ and its subsequent evolution to 3D flower-like structures reflecting an interesting growth kinetics with time. Interestingly, in a mixed halide perovskite system obtained with Br doping, the growth of 2D surface is found to be markedly different eventually leading to a lotus-leaf structures. In both systems, dislocations played the crucial role for the observed crystal growth and can be explained in the framework of BCF theory23. We further reconstruct the 3D flower structure from 2D flat sheets through molecular dynamics simulations and obtained values of the bending stiffness of the curved sheets (petals) of hybrid perovskites which bears paramount significance is such folded structure for practical applications. Further, the results might be quite useful to consider the morphological stability of perovskite layers, a topic of large interest in perovskite based photovoltaic research today.

Experimental

Details of materials and fabrication process

Materials

Methylamine, hydrochloric acid (HCl, Merck chemicals), hydrobromic acid (HBr, Merck chemicals), and anhydrous N, N dimethylformamide (DMF) (C3H7NO, Merck chemicals,99.5%) were used as raw materials for the synthesis of perovskite materials. All chemicals were of analytical grade purity and used with no additional purification carried out prior to use.

Methods

The microstructures of MABiCl and MABiBrCl (MA = CH3NH3+) films were observed by scanning electron microscopy (SEM ZEISS GEMINI SEM 450) and high-resolution transmission electron microscopy (HRTEM, Talos F200X G2). The samples for HRTEM were prepared by drop casting method using 400 mesh carbon coated Cu grids. The data for crystal structure of those samples were performed by synchrotron X-ray diffractometer (INDUS-II Grazing incidence X-ray scattering (GIXS) Beam line no-13, RRCAT Indore). The surface topography and nanoscale features were measured with atomic force microscopy (AFM, AIST NT). Energy dispersive X-ray spectroscopy (EDS) was carried out with EDAX Inc., USA. Optical absorption properties were measured with Beckman Coulter DU720 UV–Visible spectrophotometer. X-ray Photoelectron spectroscopy (XPS) data were collected using a ThermoFisher Scientific instrument. The particle size distribution of samples in solution was obtained by dynamic light scattering technique using ZETA SIZER, Nano ZS90, Photoluminescence (PL) and quantum yield measurements were done using PicoQuant instrument. Raman data were recorded using a HORIBA XPLORA PLUS instrument.

Details for the synthesis process of MABiCl and MABiBrCl films

CH3NH3Cl (MACl) was prepared by mixing 1:1 ratio of methylamine and hydrochloric acid (35%) in absolute ethanol in ice bath under constant stirring for 2 h. The clear transparent solution thereafter was allowed to evaporate slowly maintaining the temperature of the bath at 80 °C. Under continuous stirring, the ethanol was removed slowly which produced a light yellow solid MACl powder. Now, lead-free Bi-based perovskite (CH3NH3)3Bi2Cl9 (MABiCl) was obtained by mixing CH3NH3Cl and BiCl3 in 3:2 molar ratio in DMF. The solution was further stirred at 50⸰C for 30 min, resulting in a foggy white solution. The solution further diluted with the addition of ethyl alcohol and sonicated for 15 min resulting the formation of MABiCl solution. The solution was then drop-casted on clean quartz substrate to obtain the MABiCl film. For synthesis of (CH3NH3)3Bi2(BrxCl1−x)9 (x = 0.44) (MABiBrCl), desired amount of CH3NH3Br (MABr) and MACl was mixed properly under stirring condition before reacting with BiCl3. CH3NH3Br (MABr) was also synthesized by reacting 1:1 ratio of methylamine (33 wt% in ethanol) and hydrobromic acid (35%) in absolute ethanol in ice bath for 2 h under constant stirring. MABiBrCl solution was then further drop-casted to obtain films for further measurements.

Material characterizations

The details of material characterization by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), compositional analysis by energy dispersive x-ray spectroscopy (EDS), Raman spectroscopy, optical absorption and photoluminescence (PL) are provided in the supplementary information (Methods: Experimental: Material Characterizations).