Structural Tuning of Layered Perovskites for Enhanced Broadband Emission at Room Temperature

Structural Tuning of Layered Perovskites for Enhanced Broadband Emission at Room Temperature Aarya Prabhakaran, Quentin Evrard, Danilo Marchetti, Sirous Khabbaz Abkenar, Luiz Gustavo Bonato, Balaji Dhanabalan, Iryna Andrusenko, Simone Lauciello, Pavlo Solokha, Serena De Negri, Luca Goldoni, Moussa D. Faye Diouf, Liberato Manna, Giorgio Divitini, Mauro Gemmi, and Milena P. Arciniegas* 1. Introduction Low-dimensional organic–inorganic metal halide perovskites have emerged as a cost-effective alternative for broadband whitelight emitters from a single-component material. [1,2] These structures have the potential to overcome the need to use multiple layers that emit different colors to obtain white light, simplifying device engineering while mitigating detrimental factors such as interfacial defects, selfabsorption, and layer degradation at different rates. [3,4] Ruddlesden-popper layered perovskites are among the most common low-dimensional structures, and they are formed by alternating bilayers of bulky monovalent organic cations with inorganic layers formed by corner-sharing metal halide octahedra, where the emission originates. White light emission from this class of metal halide perovskites has been enabled The versatile structure of organic–inorganic layered perovskites has enabled the fabrication of strongly luminescent material sources with narrow emission. However, although demonstrated to be beneficial for many applications, their structural tunability to achieve broadband emission and thus white light is often limited to changes in the organic cation. Herein, the role of solvent–organic cation interactions on the structural arrangement of the organic and inorganic building blocks in a room-temperature synthesis is investigated. It is shown that changes in the solvent properties coupled with the molecular configuration of the organic cation can disrupt the conventional platelet-like shape of layered perovskites, leading to disconnected corner-sharing ribbons of octahedra, from which micrometer-elongated crystals with nanoscale grooves are formed. With an 18% white light photoluminescence quantum efficiency, the crystal shape enables localized Mn doping at the groove interfaces. These findings highlight the crucial role of the interaction among solvent molecules and organic cations in shaping the assembly of the structural framework and their optical properties. A. Prabhakaran, Q. Evrard, L. G. Bonato, B. Dhanabalan, M. P. Arciniegas Automated Nanomaterials Engineering Center for Convergent Technologies Istituto Italiano di Tecnologia Via Morego 30, 16163 Genova, Italy E-mail: [email protected] A. Prabhakaran, P. Solokha, S. De Negri Dipartimento di Chimica e Chimica Industriale Università degli Studi di Genova Via Dodecaneso, 31, 16146 Genova, Italy The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/sstr.202500311. [+] Present address: Center for Instrument Sharing of the University of Pisa (CISUP), University of Pisa, Lungarno Antonio Pacinotti, 43, 56126 Pisa, Italy © 2025 The Author(s). Small Structures published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. DOI: 10.1002/sstr.202500311 D. Marchetti, I. Andrusenko, [+] M. D. Faye Diouf, M. Gemmi Electron Crystallography Center for Materials Interfaces Istituto Italiano di Tecnologia Viale Rinaldo Piaggio 34, 56025 Pontedera, Italy D. Marchetti, M. D. Faye Diouf Dipartimento di Scienze Chimiche, della Vita e della Sostenibilità Ambientale University of Parma Parco Area delle Scienze 17/A, 43123 Parma, Italy S. Khabbaz Abkenar, G. Divitini Electron Spectroscopy and Nanoscopy Center for Convergent Technologies Istituto Italiano di Tecnologia Via Morego 30, Genova 16163, Italy S. Lauciello Electron Microscopy Facility Center for Convergent Technologies Istituto Italiano di Tecnologia Via Morego 30, 16163 Genova, Italy RESEARCH ARTICLE www.small-structures.com Small Struct. 2025, 2500311 2500311 (1 of 13) © 2025 The Author(s). Small Structures published by Wiley-VCH GmbH in substantial part by the random tuning of the organic molecules used in the synthesis, as they have a significant impact on the metal halide octahedra layer where they sit. The accommodation of the organic cations can induce strong lattice distortions, such as octahedral tilting, contraction, and elongation of metal halide bonds. [5,6] In return, lattice distortions may lead to strongly coupled excitons and phonons and thus layered structures with intrinsic broadband white light emission. [3,5,7] Inducing distortions relies on templating hydrogen bonding interactions between the organic cation and the metal halide inorganic framework. Such interactions are difficult to predict due to the extended library of available organic cations and the fact that slight changes in their molecular configuration strongly influence the inorganic layers in the structure. [5,6,8] The assembly of the organic cations depends on the chemical environment where the intercalation with inorganic layers occurs, especially solvents, which influence the solubility and miscibility of perovskite precursors. Thus, solvents affect the crystallization kinetics and thereby the material’s crystal structure and optoelectronic properties. [9–11] Hydrohalic acids [12] and their combination with polar solvents, such as water, [13] methanol, [14] γ-butyrolactone (GBL), [15] dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF), [16] are often used in the preparation of organic–inorganic layered perovskites. They are also used with solvents such as chloroform, chlorobenzene, and ethyl acetate in the solvent/antisolvent approach for the preparation of single crystals and films for efficient transistors [16] and blue emitters. [15] Although solvents are an important parameter in material synthesis, their role in the structural tunability of low-dimensional organic–inorganic perovskites toward the fabrication of efficient white light emitters remains largely unexplored. This motivated us to approach the preparation of these materials via a room-temperature injection synthesis and generate different solvent–organic cation interactions that may alter the precursor diffusion (by creating solvent interfaces in some conditions) and thus the arrangement of metal halide octahedra responsible for their emission. In this work, we investigate the structural tuning of organic– inorganic single-layered structures (n=1, with nindicating the number of inorganic layers in the crystallographic motif) as a function of the solvent by developing a simple room-temperature injection synthesis strategy which led to a new set of broadband-emitting organic–inorganic materials. We demonstrate through a comprehensive study involving 3D electron diffraction (3D ED) and optical spectroscopy (including absorption,photoluminescence(PL),and time-resolved PL) that switching from acetone to toluene in the synthesis breaks the traditional platelet morphology of organic–inorganic layered perovskites prepared with benzylamine and induces the formation of elongated crystals with grooves of a few hundred nanometers deep. Such structures display an intrinsic broadband emission with an emission peak centered at 505 nm, an emission bandwidth of 224 nm (full width at half maximum [FWHM]), and a color rendering index (CRI) of 81—relevant to indoor lighting applications. Moreover, the groovy crystals have an average PL quantum yield (PLQY) of 18% 3%, a notable improvement for materials of this class prepared through a few steps and at room temperature. Using these structures as templates in a Mn doping strategy, we observe an anisotropic localization of the foreign atoms, with Mn-enriched groove’sedges.Theincorporation of Mn into the grooves brings the broadband emission center to 607 nm, with 30% 2% PLQY. The spontaneous spatial localization of foreign atoms is a desired feature for nanoscale patterning, which is traditionally achieved through nanoimprint lithography or self-assembly of nanostructures to control the spatial distribution of, for example, plasmonic and semiconducting quantum dots to produce optical waveguiding and lasing behavior. [17,18] In our case, the structural grooves may serve as nanoscale scaffolds for the localized incorporation of dopants or deposition of co-catalysts. Their intrinsic spatial arrangement could facilitate applications in directional energy transfer or site-specific catalysis to enable the formation of energetically favorable metal– support bonds. [19,20] 2. Results and Discussion 2.1. Design, Synthesis, and Structural Characterization The interactions among organic cations are a key driving force in the assembly of the organic–inorganic components in layered perovskites. [6,14,21–24] Such interactions are maximized by π–π stacking when adding, for example, aromatic rings to the tail of the organic cations. [25,26] Because of their molecular flexibility, organic cations can adopt various configurations. [25,27,28] However, commonly used organic cations, such as aromatic and aliphatic amines, can behave differently depending on the choice of solvent, dictating the way the organic and inorganic components intercalate and accommodate within the structure. We thought, therefore, to explore primary amines, with and without an aromatic ring in their tail, in the synthesis of layered perovskites by using different solvents. Following the same reasoning, we selected benzene, toluene, p-xylene, and diethyl ether as nonpolar solvents and acetonitrileandacetoneaspolarsolvents. We chose benzylamine as it is among the smallest primary amines that incorporate a benzyl ring as the N-substituent and form cornersharing layered perovskites [22] and butylamine as the aliphatic alternative with a comparable molecular length (4.90 Å, see Figure S1, Supporting Information). Table S1, Supporting Information, compares the two organic molecules in terms of their molecular descriptors. We anticipate lower molecular flexibility in benzylamine, given its lower number of rotatable bonds than butylamine, which may result in lower molecular motion. [15] These reduced molecular flexibility may then impact the packing of the organic layer in different solvents. From now on, we refer to the corresponding organic cations, benzylammonium and butylammonium, as BzA and BA, respectively. Our material preparation relies on an injection synthesis protocol performed at room temperature and in a few steps L. Goldoni Materials Characterization Facility Center for Convergent Technologies Istituto Italiano di Tecnologia Via Morego 30, 16163 Genova, Italy L. Manna Nanochemistry Center for Convergent Technologies Istituto Italiano di Tecnologia Via Morego 30, 16163 Genova, Italy www.advancedsciencenews.com www.small-structures.com Small Struct. 2025, 2500311 2500311 (2 of 13) © 2025 The Author(s). Small Structures published by Wiley-VCH GmbH 26884062, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/sstr.202500311 by CochraneItalia, Wiley Online Library on [15/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License (see Experimental Section), resulting in reduced costs and short processing time (see detailed comparison in Table S2, Supporting Information). Figure 1a illustrates the chemical structure of the solvents and amines used in the experiments and the steps of the synthesis protocol. We started by dissolving PbBr 2 powder in a mixture of the aqueous solution of HBr and the selected solvent. Then, we injected the corresponding amine into the mixture, as summarized in Table S3, Supporting Information. We found that the addition of the amine to the mixture of perovskite precursors and the selected solvent resulted in the fast formation of precipitateinallthemixtures(FigureS2,SupportingInformation). Scanning electron microscopy (SEM) reveals a strong dependence of the crystal morphology and size of the benzylaminebased samples on the type of solvent used in their synthesis (Figure 1b and S3, Supporting Information). While acetonitrile, acetone, and diethyl ether induce a conventional round-edged platelet morphology (left panel in Figure 1b for a sample prepared in acetone) with a lateral size spanning up to 180 μm, the use of benzene, toluene, or p-xylene results in elongated structures of 10–30 μm with lateral sizes smaller than 5 μm (right panel in Figure 1b). Such anisotropic structures exhibit natural grooves and tend to form well-aligned domains as observed in Figure 1c. Confirming the growth mechanism of these crystals is not trivial, as their size/thickness and electron beam sensitivity did not facilitate transmission electron imaging at high resolution. Thus, we can only provide sensible pathways: 1) oriented aggregation of small 2D structures over time, as it occurs with self-assembled arrays from nanocrystals; [29,30] 2) the facet-selective nucleation led by controlling facet ligand densities, [31] in our case, surface distribution of the organic cations; and 3) the formation of twin boundaries to reduce crystal strain, which results in so-called re-entrants, [32,33] similar to what has been observed in Ag and AuGe crystals. [32,33] The drastic changes in crystal morphology, from lamella to elongated groovy crystals, observed through the experiments, suggest that the crystal formation is governed by both the reactivity and solubility of the organic cation in the selected solvent. These factors are among those identified by Kanatzidis and coworkers, along with the organic cation charge, shape, size, and hydrogen bonding, to affect the conformation of 2D layered perovskites in the presence of different precursors. [34] It is, however, worth noting that the reaction kinetics can change when using the same organic cation. Whether a solvent can alter the rate of perovskite crystal formation has been extensively studied in CsPbBr 3 nanocrystals in solution. [11,35] Figure 1. Screening solvent–amine combinations. a) Chemical structure of the solvents and amines [84] screened through a room temperature (RT) injection synthesis. Gray, white, blue, and red represent C, H, N, and O atoms, respectively. b) Top-view SEM images of representative benzylamine-based crystals synthesized in the polar (acetone) and aromatic nonpolar (toluene) solvents, showing two different morphologies, from platelets to elongated crystals. Scale bars: 10 μm. c) Close SEM image of the nanometer-sized grooves observed in the crystals prepared in toluene. Scale bar: 2 μm. d) Collection of p-XRD patterns of the benzylamine-based crystals prepared using different solvents. The XRD patterns from the samples prepared in acetone and acetonitrile are in good agreement with (BzA) 2 PbBr 4 platelet-like crystals, [36] while the crystals prepared in nonpolar solvents show additional peaks at a low 2θangle, which is highlighted with an asterisk. The periodic reflections in the (h0 0) planes are highlighted with dotted lines, and they are present in all the crystals. www.advancedsciencenews.com www.small-structures.com Small Struct. 2025, 2500311 2500311 (3 of 13) © 2025 The Author(s). Small Structures published by Wiley-VCH GmbH 26884062, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/sstr.202500311 by CochraneItalia, Wiley Online Library on [15/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License These works suggest that the arrangement of the network of octahedra is governed by the rates at which Pb is released from the PbBr 2 precursor across different solvents. In our synthesis, when using polar solvents that are miscible with HBr, all the PbBr 2 is immediately available to react with the organic ammonium cations. In contrast, in the nonpolar/HBr mixtures, the reaction is slowed down due to the diffusion barrier, leading to slow release of PbBr 2 for the formation of [PbBr 6 ] 2 octahedra. In this scenario, benzene, toluene, and p-xylene reduce the Pb release rate because of their immiscibility with HBr, which is used to dissolve PbBr 2 . As a result, in the presence of an excess of organic molecules, a smaller amount of PbBr 2 is available to form [PbBr 6 ] 2 octahedra compared to the larger amount immediately available when polar solvents are used. X-ray photoelectron spectroscopy (XPS) analysis indicates that the Pb, Br, and N elements preserve the same chemical state when changing from acetone to toluene, with no significant difference in the binding energy peak positions (Figure S4, Supporting Information). Elemental analysis performed by XPS (Table S4, Supporting Information) and energy-dispersive X-ray spectroscopy (EDX) in the SEM (Table S5 and Figure S5 and S6, Supporting Information) shows that the samples prepared in polar solvents follow the expected Br:Pb ratio of 4:1, in line with conventional Ruddlesden–Popper layered perovskites. [5] To gain insight into the crystallographic details of the synthesized structures, we performed powder X-ray diffraction (p-XRD) on all the samples. The p-XRD results are displayed in Figure 1d. We observe periodic reflections (highlighted with dotted lines) from all the samples with an interplanar distance of 16.7 0.01 Å, which match well with the distance between Pb and Br inorganic layers reported for Ruddlesden–Popper crystals prepared with benzylamine (CCDC number 1542460). [36] Compared to the samples prepared with polar solvents (acetone and acetonitrile), additional diffraction peaks are observed in the samples prepared with nonpolar solvents (diethyl ether, benzene, toluene, and p-xylene). Some of these peaks are located at low 2θ values (e.g., a prominent diffraction peak at 2θ6º, highlighted with asterisksdare in Figure 1d), suggesting the presence of a crystalline phase with moderately low symmetry and large unit cell parameters in the samples prepared with aromatic nonpolar solvents. Note that only trace amounts of benzylammonium bromide salt remain in the samples prepared with toluene as a by-product of the reaction process (Figure S7, Supporting Information). To understand whether aromatic nonpolar or aliphatic polar solvents promote different octahedra connectivity, we selected the benzylamine-based samples prepared in acetone and toluene as examples of each solvent type for further analysis. The crystallographic information extracted from single-crystal XRD (sc-XRD) data through a fast scan procedure (Figure S8 and S9, Supporting Information) allowed us to confirm the orthorhombic unit cell of the benzylamine-based samples prepared in acetone with lattice parameters a=33.37 Å, b=8.14 Å, and c=8.13 Å in agreement with previous works on Ruddlesden– Popper (BzA) 2 PbBr 4 crystals (see comparison in Table S6, Supporting Information). [36,37] This phase is also observed in the butylamine-based samples prepared in the same solvent from which a complete sc-XRD dataset was collected (Figure S10, Table S7 and S8, and CIF file 1, Supporting Information). While polar solvents such as GBL, DMF, and DMSO are often used for the preparation of halide layered perovskites, acetone is excluded due to its potential reactivity with amines. [3,5,38] Our results indicate that acetone can be used as a solvent for the preparation of 2D Pb-based layered perovskites with benzylamine and butylamine through room-temperature injection synthesis. Crystallographic data from the BzA-based samples prepared in toluene were collected via 3D ED (see Experimental Section), as their small crystal size, along with the associated difficulties in isolating a single crystal, hindered sc-XRD analysis. Note that our attempts to prepare single crystals or larger groovy structures by using a slow crystallization strategy [39,40] or longer crystallization times led to either platelets that do not preserve the grooves or groovy crystals with similar sizes (see details in Figure S11 and S12, Supporting Information). In 3D ED, the strong elastic scattering typical of electrons is reduced. This ability, together with the possibility to analyse isolate submicrometric crystals, enables 3D ED to collect single-crystal diffraction data from microcrystalline samples. This technique is particularly useful for materials, such as organic–inorganic compounds, that often are only analyzed by p-XRD. [41,42] Figure 2a shows a scanning transmission electron microscopy (STEM) image of a representative benzylamine-based crystal prepared in toluene used for 3D ED analysis. While benzylamine and butylamine both template a Ruddlesden–Popper phase in acetone, we found that benzylamine in toluene leads to a C-centered monoclinic unit cell with parameters of a=31.89 Å, b=8.10 Å, c=28.19 Å, and β=113.45° as indicated by the reciprocal space reconstruction in Figure 2b and sections in Figure S13, Supporting Information. The inorganic backbone is formed by segments of corner-sharing octahedra arranged along the crystallographic b-axis, leading to ribbon-like inorganic structures (Figure 2c). The distance between parallel ribbons is around 17 Å, retaining a similar organic interlayer space to the (BzA) 2 PbBr 4 crystals. Each [Pb 3 Br 13 ] n ribbon unit consists of three independent Pb atoms (inset in Figure 2c), each surrounded by 6 Br atoms in an octahedral coordination environment. The Pb1 and Pb2 atoms use all their equatorial Br atoms as bridging μ 2 -ligands, leading to octahedra connected by sharing four vertices (Figure 2d). Instead, the Pb3 atoms, located at the edges of the ribbons, are coordinated by only two Br atoms acting as bridging ligands toward the neighboring Pb2 atoms, breaking what would be a continuous layer into separate ribbons. The ribbons run parallel along the b-axis and are faced side by side along c, forming a kind of disconnected stepping layer (Figure 2d,e). Figure S14, Supporting Information, shows additional crystallographic views of the inorganic component in these structures extracted from the 3D ED data (CIF file 2, Supporting Information). A similar disruption of the corner-sharing connectivity in nonemitting crystals was observed by Hoffman et al. when investigating whether or not a 2D Ruddlesden–Popper phase could be formed by using propylamine, as the shortest alkylamine cation able to stabilize a perovskite. [43] This was observed only on crystals with n=2 when changing the propylamine/methylamine ratio, as structures with n=1 could not be synthesized through this approach. However, as a derivative of the Ruddlesden–Popper phase, those structures have a mixed connectivity as the ribbons are www.advancedsciencenews.com www.small-structures.com Small Struct. 2025, 2500311 2500311 (4 of 13) © 2025 The Author(s). Small Structures published by Wiley-VCH GmbH 26884062, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/sstr.202500311 by CochraneItalia, Wiley Online Library on [15/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License connected via a face-sharing octahedron, explained by two consecutive cleaved directions, one along the (100) plane, followed by one along the (110) plane. The identification of the new homology with n=1 in our work tells us that the crystal structure is not only dependent on the steric hindrance of the organic cation, but it can also be modulated by the solvent mixture. The structural model obtained from 3D ED analysis was then refined against p-XRD data through the Rietveld method, demonstrating the consistency of the structural model in both crystallographic techniques (Figure S15 and Table S9, Supporting Information). The high scattering power of Pb and Br, combined with the low scattering coefficients of the organic cations and beam sensitivity of the samples (Figure S16, Supporting Information), led to a limited localization of the organic cations. However, a Fourier difference map (Figure 2f) derived from the Rietveld refinement of the p-XRD data based on the 3D structural model allows us to distinguish residual electron density maps between the inorganic ribbons, indicating that the organic cations occupy the interlayer space. To assess the presence of potential residual solvent molecules in these structures, we performed nuclear magnetic resonance (NMR) analysis. The crystals were dissolved in DMSO-d6, and aliquots of toluene dissolved in the same solvent were gradually added (see Experimental Section for further details). The collected 1 H NMR quantitative spectra in Figure S17, Supporting Information, reveal that toluene is present in the samples in trace amounts, with a calculated benzylammonium/toluene ratio below 0.03%. Such a very low concentration is likely due to physiosorbed toluene molecules on the surface of the crystals. From the NMR analysis, we also observe a slight difference in the content of benzylammonium between the samples prepared in acetone and those prepared in toluene, from 708 mM to 684 mM (Table S10, Supporting Information), despite the excess of benzylammonium bromide in the samples synthesized in toluene, as discussed in the XRD analysis. EDX-SEM and XPS show similar trends (Table S4 and S5, Supporting Information). These results suggest an overall lower content of the organic cation needed to stabilize the disconnected ribbons versus the full inorganic Pb–Br slabs in the mother Ruddlesden–Popper (BzA) 2 PbBr 4 crystals obtained when using acetone in the synthesis. Figure 2. Crystallographic details of the benzylamine-based samples synthesized in toluene. a) STEM image of a representative benzylamine-based crystal prepared in toluene. Scale bar: 500 nm. b) Crystal structure of the inorganic layers retrieved from 3D ED data. Blue and red represent Pb and Br atoms, respectively. The highlighted view on the dotted box shows the three different Pb atoms in terms of connectivity. c) Projections of the 3D ED reconstruction oriented along its reciprocal axes a*, b*, and c*. d,e) Crystallographic views of an expanded [Pb 3 Br 13 ] n ribbon oriented along the a-axis (d) and b-axis (e). f) Difference Fourier map obtained from p-XRD Rietveld refinement, highlighting the presence of electron density between the inorganic regions due to the presence of the organic cation. Isosurface levels: 2σand 2.5σas light blue and blue surfaces, respectively. www.advancedsciencenews.com www.small-structures.com Small Struct. 2025, 2500311 2500311 (5 of 13) © 2025 The Author(s). Small Structures published by Wiley-VCH GmbH 26884062, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/sstr.202500311 by CochraneItalia, Wiley Online Library on [15/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 2.2. Implications for Light-Emitting Properties We have investigated the photophysical properties of both the platelets prepared in acetone and the elongated crystals with grooves obtained in toluene when using benzylamine to investigate the impact of the different structural arrangements. The analysis confirms that there are two types of optical behaviour depending on the choice of the solvent used in the synthesis and thus structural morphology and related crystallographic phase (Figure S18, Supporting Information). In the case of the (BzA) 2 PbBr 4 crystals prepared in acetone and in general for all the samples prepared in polar solvents that show a platelet-like morphology, we observe an excitonic absorption peak at around 395 nm (3.14 eV) and a narrow PL peak centered at around 418 nm (2.97 eV) with an FWHM of 25 nm, followed by a long tail up to 600 nm, which are typical characteristics of 2D layered perovskites that are associated to free exciton recombination from the inorganic layer. [44] On the other hand, the benzylamine-based samples prepared in aromatic solvents with their unusual crystallographic structure and related grooves feature a double absorption peak at high (around 3.29 eV, 377 nm) and low energy (3.14 eV, 395 nm), with sharper profiles, compared to the samples prepared in polar solvents, denoting a more quantum-confined structure (dotted lines in Figure S18, Supporting Information). Interestingly, under excitation of 375 nm, this set of samples shows broad PL peaks (solid lines in Figure S18, Supporting Information) centered around 500 nm along with a narrower emission peak at around 400 nm that resonates with the absorption peak at low energy (3.14 eV), as observed on the samples prepared in polar solvents. Figure 3a shows a representative absorbance and PL spectra of the samples prepared in toluene, indicating the peaks at low (P 1 ) and high energy (P 2 ) in the absorption profile. The PL spectra can be deconvoluted into two major components, an emission peak located at 403 nm (3.07 eV; FWHM 21 nm) and a peak at 505 nm (2.48 eV; FWHM 224 nm). The PL excitation spectrum collected at 505 nm matches the P 2 absorption peak with minor contributions from P 1 , resulting in a Stokes shift of around 0.8 eV, 128 nm, while the PL emission at 403 nm predominantly stems from P 1 (green and pink dotted lines in Figure 3a). The inset in Figure 3a shows a photograph of the vial containing the corresponding crystals emitting white light under ultraviolet light illumination. The corresponding color coordinates for this sample are (0.25, 0.30) with a CRI of 81, which is close to the standards for indoor illumination. [45–47] The average PLQY value of these white-emitting crystals is 18% 3%, a significant enhancement compared with values reported for organic– inorganic layered structures that require more complex preparation steps, as those summarized in Table S2, Supporting Information, compared to our simple room-temperature injection synthesis. We attribute the improved PLQY of such low-dimensional white light-emitting microcrystals to the disconnected ribbons, which may favor a reduction in electron–phonon interactions, combined with the rigidity of benzylamine, as it occurs in bright blue emitters. [15] Importantly, after 2 years of storage under ambient conditions, the samples remain emissive under UV illumination with a PLQY of 7%, still preserving their structure and characteristic broadband emission (Figure S19, Supporting Information), albeit showing signals of degradation. We observe a slight decrease in the intensity of the low-angle diffraction peaks associated with the perovskite phase, accompanied Figure 3. Optical features of the benzylamine-based crystals synthesized in toluene. a) Absorbance (gray solid line), PL (blue solid line), and PLE spectra (pink and green profiles) collected from the benzylamine-based sample prepared in toluene. The gray dotted line indicates the position of the low (P 1 ) and high (P 2 ) energy absorbance peaks. The inset displays a photograph of the vials containing the crystals under UV illumination (λ=366 nm). b) PL decay profiles acquired at the maxima of the emission peaks (403 nm and 500 nm). The label indicates the corresponding biexponential fitting components τ 1 and τ 2 . c) Excitation power-dependent PL spectra (light excitation at 372 nm). d) PL intensity as a function of the excitation power at the two different spectral positions, 403 and 500 nm, fitted by using the power law equation y¼y0þaxk. www.advancedsciencenews.com www.small-structures.com Small Struct. 2025, 2500311 2500311 (6 of 13) © 2025 The Author(s). Small Structures published by Wiley-VCH GmbH 26884062, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/sstr.202500311 by CochraneItalia, Wiley Online Library on [15/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License by an increase in the reflections from the benzylammonium bromide salt, without reflections associated with crystalline PbBr 2 degradation products. This observation is in agreement with the formation of amorphous degradation products caused by either ion migration [48,49] or upon illumination. [50] This degradation pathway is particularly relevant in the grooved structures, which show large exposed surface areas and edge sites. The degradation of the inorganic network likely increases the number of undercoordinated surface atoms, which act as trap sites for charge carriers, facilitating nonradiative recombination pathways and thereby reducing the overall PLQY over time. Although there is a reduction of around 60% in the PLQY over this extended period, these results highlight the structural robustness of the crystals, without the need for post-treatments. Moreover, the crystals retain their broadband emission profile (Figure S20, Supporting Information) up to 120 °C (see Experimental Section for details), with a slight decrease in their emission intensity after 90 °C, further confirming the robustness of the structures under this range of temperatures. To understand the excited charge carrier recombination process of the sample prepared in toluene, we performed timeresolved PL measurements at room temperature. Figure 3b shows the PL decay curves collected at the maxima of the observed emission peaks at 403 nm and 500 nm. The results of the fitting by a biexponential decay function are summarized in Table S11, Supporting Information. The emission peak at 403 nm has a τ avg of 27.49 ns with fast and slow contributions τ 1 and τ 2 of 8.35 ns (37%) and 30.63 ns (63%), respectively; the emission peak at 500 nm has a faster τ avg of 19.22 ns, with similar τ 1 and τ 2 components to those of the higher energy peak, with a high weight from the slow component (Table S11, Supporting Information). The similar decay profile for both emission peaks, which suggests a strong interaction between the corresponding recombination processes, along with the high structural distortions observed in these samples of around 50° on the in and out of plane Pb-Br-Pb angles (Figure S21, Supporting Information), points to the presence of self-trapped excitons, as observed by our group and others in white light-emitting organic–inorganic layered perovskites. [5,6,51] Typically, the broadband emission of low-dimensional metal halide perovskites is attributed to the recombination of selftrapped excitons, which are excitons trapped transiently by the lattice distortion (mainly out-of-plane metal halide–metal angle) induced by the same excitons through exciton-lattice coupling. [5–7,52,5–7] However, recent studies have challenged this explanation and suggest that broadband emissions in some lowdimensional layered perovskites may have an extrinsic origin, attributed to defect-related luminescence centers in both single layers [53,54] and multiple-layer systems. [55] To assess potential contributions from halide and/or organic vacancies as permanent defects on the broadband emission of these samples, we carried out excitation power-dependent PL measurements at room temperature by applying an excited power in the range of 10–400 μW, corresponding to a low fluence range of 0.3–10 μJm 2 (Experimental Section) to avoid the formation of surface defects and damage the samples. Figure 3c displays the collected emission profiles with no signs of saturation and preserving the band profile. We also observe a consistent increment of the PL intensity for both the narrow and broad PL peaks when increasing the excitation power (Figure 3d). We fitted the PL intensity as a function of the excitation power by using the power law equation, which is defined as y¼y0þaxk where kdenotes the recombination mechanism from free or bound excitons when it is above 1 (up to 2) and from defects when it is below 1. [6,7,56] We found from our analysis that the dependence of PL intensity with excitation power at these two spectral positions, 403 nm and 500 nm, has a linear dependency with a kvalue close to 1. Such a linear increment in PL intensity with increasing excitation power excludes the participation of permanent defects in the observed broadband emission. To further confirm this behavior, we treated the crystals with an excess of organic cations and halides in the form of benzylammonium bromide salt, a strategy known to reduce vacancy-related nonradiative recombination pathways. [53] In our samples, the broadband emission is retained, showing an enhancement in intensity (Figure S22, Supporting Information), while retaining the overall PLQY. This observation indicates that the treatment effectively reduced defect states without suppressing the broadband emission, evidencing that the emission is not driven by defects. Instead, the enhancement upon passivation supports the hypothesis that the origin of the observed broadband emission is intrinsic to the structure, likely due to self-trapped excitons promoted by local lattice distortions. Note that given the complex morphology of the benzylaminebased crystals prepared in toluene with numerous ridges and edges, to establish the real origin of the observed broadband emission (intrinsic versus extrinsic) requires a dedicated optical spectroscopy study. Recent results from Nag and coworkers [51,57] indicate that even minor structural variations between the surface and interior, such as the presence of multiple edges, can result in two different band gaps, each producing a unique excitonic broadband emission. On the other hand, replacing the benzylammonium cation with butylammonium does not induce significant changes in the morphology and related optical properties of the crystals when switching from acetone to toluene (Figure S23, Supporting Information), regardless of changes in the precursor concentrations (Figure S24, Supporting Information), which is in strong contrast with the results observed when using benzylamine. Although we observe a reduction of the crystal size from 100–250 μm when preparing the samples in acetone to 50 μm in toluene, the platelet-like morphology of the crystals is preserved. The collected p-XRD patterns from samples prepared in different solvents (Figure S25a, Supporting Information) evidence that the structures consist of parallel inorganic layers (periodic peaks all in the same position for all the samples) with an interlayer spacing of 13.81 0.06 Å, which agrees with the thickness of the butylammonium bilayer in Ruddlesden– Popper layered perovskites. [6,58] sc-XRD further confirms that the samples prepared in acetone correspond to (BA) 2 PbBr 4 layered perovskites (Figure S10, Table S7 and S8, and Supporting CIF file 1, Supporting Information). Table S12, Supporting Information, summarizes the structural and optical characteristics of all the synthesized crystals. These results highlight the importance of solvent–organic cation interactions in inducing different layered configurations. www.advancedsciencenews.com www.small-structures.com Small Struct. 2025, 2500311 2500311 (7 of 13) © 2025 The Author(s). Small Structures published by Wiley-VCH GmbH 26884062, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/sstr.202500311 by CochraneItalia, Wiley Online Library on [15/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License The increased rigidity of benzylamine compared to butylamine, combined with different solvent–amine π–πinteractions, may enhance lattice distortions in this system. For butylamine, an alkylamine of similar length, the lack of such interactions, in addition to the higher flexibility of the amine compared to benzylamine, may result in a less strained system, preserving the traditional organic–inorganic layer intercalation. This is supported by the similar optical properties observed from all the samples prepared with butylamine, regardless of the solvent used in the synthesis (Figure S25b and S26, Supporting Information). This behavior can also be explained by comparing steric parameters, such as molecular volume and Sterimol parameters, from butylamine and benzylamine (Table S13, Supporting Information), [59–61] highlighting the significant differences in the steric profile between butylamine and benzylamine. Butylamine has a smaller molecular volume and B 5 value, indicating lower steric hindrance and greater conformational flexibility. In contrast, the benzylamine’s aromatic moiety contributes to increased rigidity and a larger steric footprint, which can lead to greater structural strain and the observed solventdependent behavior. Previous studies also support this argument, showing that steric hindrance in organic cations can restrict molecular mobility within the perovskite lattice. [5,45,62] On the other hand, solvent–amine interactions can be strongly influenced by further changes in the organic cation configuration, such as the addition of relatively longer alkyl chains compared to benzylamine and the incorporation of heteroatoms, which might modulate the organic cation flexibility and motion and induce strong steric hindrance effects, as no grooves and broadband emission are observed in the case of organic cations such as phenethylamine, 4-fluorobenzylamine, and 4-methoxyphenethylamine-based compounds synthesized following the same protocol in toluene. These compounds preserve similar morphology and optical features of the BA crystals. In contrast, 3-phenyl-1-propylamine replicates the behavior of benzylamine (details in the Figures S27 and S28, Supporting Information). Further studies are required to fully understand the parameters mentioned above and guide the preparation of broadband emitters from a single perovskite material. These include the potential role of the packing arrangement adopted by the organic cation within the structures, depending on the length of the alkyl chain (so-called odd-even effects), [63,64] as it occurs in conventional 2D layered perovskites. Inspired by the crystallographic tuning induced in the benzylamine-based crystals, we sought to use the grooves present on the surface of the crystals as potential sites to enrich the structures with another element. Localized doping (for example, at grain boundaries and edges) has been explored to reduce the nonradiative recombination channels in perovskite thin films, [65] to improve the mechanical properties of graphene, [66] and to reduce resistivity in copper nanowires. [67] In metal halide perovskites, one of the most explored doping elements so far is manganese, which typically results in a strong broadband emission in the red region coming from the Mn 2þ pair d–dtransition. [68–70] In our approach, we used fresh and washed structures as hosts and carried out a postdoping strategy also performed at ambient conditions by adding MnBr 2 dissolved in HBr to the crystals in toluene (Experimental Section). We compare the impact of the Mn intake on the optical properties by examining two different Pb:Mn loadings, 1:1 and 1:3, relative to the content of Pb used in the synthesis of the initial crystals acting as hosts. From the SEM analysis, we observe that the crystals used as hosts for Mn doping retain their characteristic groove shape and size (Figure 4a and S29, Supporting Information), indicating no severe structural reorganization during Mn incorporation. Although the Mn-dopant concentration is relatively low (<5%), as assessed via inductively coupled plasma optical emission spectroscopy (Table S14, Supporting Information), p-XRD data (Figure 4b) show that the primary diffraction peaks shift to higher 2θangles, indicating a decrease in the lattice parameter. This observation suggests a potential incorporation of Mn 2þ , [71] which has a relatively small atomic ratio compared to Pb, into the lattice, rather than a dissolution–recrystallization process that would likely lead to significant changes in crystal habit/shape. Note that there were no traces of Mn when using lower Pb:Mn loading (1:0.3). We then used STEM coupled with EDX (STEM-EDX) to assess the location of Mn in the doped structures. Elemental mapping of different crystals reveals an anisotropic distribution of manganese in the structures, with Mn-rich regions preferentially located at the groove’s boundaries and in areas of their facets (Figure 4c). Additional STEM-EDX analysis is presented in Figure S30, Supporting Information. A high-resolution STEM image of a groove region (Figure S30, Supporting Information), accompanied by a line scan EDX composite elemental map for Pb, Mn, and Br, reveals that the Mn enrichment is spatially correlated with the groovy feature. This observation suggests that the groove boundaries have higher surface energy compared to other crystallographic facets, and thus, they are more prone to allocate the manganese atoms favorably, driven by surface energy minimization. To further support the Mn incorporation into the grooved regions, we performed highresolution STEM coupled with EDX mapping on the Mn-doped samples (Figure S31 and S32, Supporting Information). This provided spatially resolved elemental distributions, allowing a direct comparison between Mn-rich grooves and Mn-poor bulk areas. At higher magnification, the elemental maps reveal a strong spatial correlation between Mn and Br signals at the grooves, while the Pb signal appears significantly depleted in these regions (Figure S31 and Table S15, Supporting Information). While there is a vast literature on 3D perovskites, very few studies report the controlled introduction of metal dopants into disrupted inorganic networks. [68,71,72] A previous report on 2D layered perovskites (with their anisotropic nature) indicates the existence of internal edge-doping sites due to the breakage of the lattice within inorganic layers. [72] We reason that the disconnected ribbons in the structure (Figure 3) offer inherently many available internal edge-doping sites, which may modulate the dopant incorporation energy, affecting both the dopant localization and its electronic contribution. By studying the effect of the Mn doping, we found that the Mn intake strongly conditions the color of the emission of the host, from white to pink/orange, as a result of the appearance of a red emission band in the PL spectra (Figure 4d), which is centered at 607 nm with an FWHM of 80 nm. This observation aligns with prior reports on similar systems, which demonstrate that Mn 2þ ions substitute Pb 2þ ones, leading to a distinct Mn d–demission www.advancedsciencenews.com www.small-structures.com Small Struct. 2025, 2500311 2500311 (8 of 13) © 2025 The Author(s). Small Structures published by Wiley-VCH GmbH 26884062, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/sstr.202500311 by CochraneItalia, Wiley Online Library on [15/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License peak around 600 nm, without significantly altering the host crystal structure or band-edge absorption. [68,72] These works suggest that energy transfer from host excitons to localized Mn 2þ d-states is responsible for the dopant emission, rather than the creation of mid-gap states or significant band realignment. Similarly, in our system, the absorbance spectra of the doped samples show no additional bands, indicating that the absorption contributing to this peak overlaps with the band of the host material, while a red emission band appears in the PL spectra of the doped samples, suggesting that the Mn 2þ incorporation does not introduce mid-gap trap states but rather acts as a luminescent center following exciton-to-dopant energy transfer mechanisms. Moreover, increasing the content of Mn in the samples, the blue emission peak diminishes in intensity, and the samples emit mostly from the Mn channel (red solid profile in Figure 4d), shifting the CIE coordinates from (0.49, 0.36) to (0.55, 0.38) (Figure 4e). The Mn-doped samples also show a more efficient energy transfer, reaching a PLQY of 30% 2%. 3. Conclusions Our findings highlight the importance of interactions between the organic cation and the solvent in shaping the structure of low-dimensional organic–inorganic perovskites. We hypothesize that the choice of the solvent–organic cation combination also influences the controlled release of metal halide precursors in solution, which can lead to white light-emitting crystals (18% 3% PLQY). Further studies need to address the complex interplay between the origin of white light and the formation of grooves in such crystals. However, these insights offer valuable guidance for the engineering of low-dimensional organic– inorganic perovskites, including solvent-assisted interactions. Moreover, as suggested by our experiments, the grooves in the structures can template an anisotropic doping distribution. Such channels might be used to selectively locate foreign atoms to achieve mechanically reinforced nanostructures or to modulate the electronic, optical, and/or catalytic properties of the host material. 4. Experimental Section Materials: Lead (II) bromide (98%), manganese (II) bromide (98), hydrobromic acid (48% aqueous solution), butylamine (99.5%), benzylamine, acetone (99.5%), acetonitrile (99.8%), diethyl ether (99.8%), benzene (99.8%), toluene (99.7%), and p-xylene (99%) were purchased from Sigma–Aldrich and used without any further purification. Synthesis: In a 4 mL glass vial, PbBr 2 powders (110 mg, 0.3 mmol) were dissolved in HBr (0.14 mL, 1.2 mmol), and 2 mL of the selected solvent was added to the mixture. Next, the solution was shaken well in a vortex for 5 min. Then, the organic cation (0.126 mL of benzylamine or 0.118 mL of butylamine, 1.2 mmol) was quickly injected into the previous mixture. The vials were kept under shaking (1700 rpm) for 24 h. The produced crystals were washed by centrifugation at 5000 rpm for 5 min, followed by the addition of 2 mL of the corresponding solvent. This step was repeated 3 times to remove the excess of precursors. The crystals were then collected and dried on filter paper using a vacuum pump before analysis. All the Figure 4. Localized Mn doping. a) SEM image of a Mn-doped crystal confirming that the grooves are preserved in the structures. The sketch illustrates the Mn doping strategy performed at room temperature (RT). Scale bar: 10 μm. b) p-XRD patterns of the samples compared to the host structure, indicating a right shift of the reflections highlighted by the dotted lines. c) HAADF image of a selected crystal and the corresponding STEM-EDX composite elemental map for Pb and Mn. The arrows indicate the Mn-rich regions. d) Absorbance (dotted lines) and PL (solid lines) spectra of the Mn-doped crystals at different Pb:Mn ratio. The orange arrows highlight the new emission peak at 630 nm in the doped samples, while the blue arrows indicate the decrease in the intensity of the blue peak when increasing the Mn content. e) CIE diagram showing the changes in the color of the emission depending on the solvent and further doping. The insets show photographs of the host and Mn-doped crystals (Pb:Mn ratio of 1:0 and 1:3, respectively). www.advancedsciencenews.com www.small-structures.com Small Struct. 2025, 2500311 2500311 (9 of 13) © 2025 The Author(s). Small Structures published by Wiley-VCH GmbH 26884062, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/sstr.202500311 by CochraneItalia, Wiley Online Library on [15/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License