Broadband Emission in a New Two-Dimensional Cd-Based Hybrid

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Broadband Emission in a New Two-Dimensional Cd-based Hybrid Perovskite Aymen YANGUI, Sebastien Pillet, El-Eulmi Bendeif, Alain Lusson, Smail Triki, Younes Abid, and KAMEL BOUKHEDDADEN ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00052 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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Broadband Emission in a New Two-Dimensional Cd-based Hybrid Perovskite Aymen Yangui,*1,2 Sebastien Pillet,3 El-Eulmi Bendeif,3 Alain Lusson,2 Smail Triki,4 Younes Abid,5 and Kamel Boukheddaden**2 1

Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan Groupe d’Etudes de la Matière Condensée, UMR CNRS 8653-Université de Versailles Saint Quentin En Yvelines, Université Paris-Saclay, 45 Avenue des Etats-Unis, 78035 Versailles, France 3 Laboratoire de Cristallographie, Résonance Magnétique et Modélisations, UMR-CNRS 7036, Institut Jean Barriol, Université de Lorraine, BP 239, 54506 Vandœuvre-lès-Nancy, France 4 Laboratoire de Chimie, Electrochimie Moléculaires, Chimie Analytique, UMR CNRS 6521-Université de Brest (UBO), 6 Avenue V. Le Gorgeu, CS 93837, 29238 Brest, France 5 Laboratoire de Physique Appliquée, Faculté des Sciences de Sfax, Route de Soukra km 3.5 BP 1171, 3018 Sfax, Tunisia 2

ABSTRACT: Organic-inorganic hybrid perovskites (OIHP) are developing rapidly as high-performance semiconductors for solidstate solar cells and light emitting devices. Recently, lead-halide two-dimensional (2D) OIHP were found to present bright broadband visible emission, thus highlighting their potential as single-component white-light (WL) emitters. This contribution deals with the preparation of a new Cd-based 2D hybrid perovskite, of the chemical formula (C6H11NH3)2CdBr4 (abbreviated as compound 1), of which structural and optical properties have been studied and analyzed. Room temperature OA measurements, performed on spin-coated film of compound 1, revealed a sharp excitonic absorption peak at 3.24 eV, and a large exciton binding energy of 377 meV, estimated from low temperature OA spectrum. Upon 325 nm irradiation, compound 1 showed a very broadband WL emission consisting of one peak at 2.94 eV, attributed to exciton confined in the [CdBr4]2- inorganic layers, and a second peak at 2.53 eV resulting from the cyclohexylammonium cations emission. Temperature dependence of PL spectra evidenced anomalous behavior accompanied with singularities around 50 and 150K in the integrated intensity, the full width at half maximum and the PL peaks positions. These singularities have been traced back to structural phase transitions, from temperature dependence powder and single crystal X-ray diffraction investigations, from which strong correlations had emerged between the structural distortion of the CdBr6 pseudo-octahedron and the broadening characteristics of the WL emission band. These hitherto unrecognized properties turn this and similar OIHP into perspective candidates for potential applications as WL-emitting diodes.

KEYWORDS: Organic-inorganic hybrid perovskites, White light luminescence, Optical absorption, Excitons, X-ray diffraction, Phase transition

Among the crystalline solids that exhibit exceptional performance in optoelectronic and photovoltaic (PV) devices applications, organic-inorganic hybrid perovskites (OIHP) have drawn much attention.1-8 In particular, three dimensional (3D) OIHP have revolutionized the PV landscape, since 2009, with an extremely rapid evolution of their power conversion efficiency (PCE) from 3.8 %3 to more than 22 %8 in less than 10 years, and become the best performing solution-processed PV technology on record. In addition to the genuine 3D hybrid perovskites, their numerous two-dimensional (2D) counterparts have greater possibilities for property tuning than the 3D perovskites by changing widely the functional organic cations or adjusting the perovskite layer thickness.9 From the structure viewpoint, they can be interpreted as modular structures consisting of layer perovskite modules, which act as complex

anion, and interlayers of organic molecules, which act as cation. Different hybrid perovskites can be built by varying the thickness of either or both the perovskite and the organic layer. If the perovskite layer is kept fixed while the organic layer is variable, one obtains a merotype series; otherwise, the result is a polysomatic series.10 These 2D materials show promising optoelectronic properties for potential applications such as light-emitting diodes (LEDs),2, 11-12 light-emitting transistors (LETs),1, 13 laser gain media6, 14-15 and stable solar cells.16-18 Recently, white-light (WL) emitting 2D OIHP are attracting great interest for solid-state lighting and lasing applications.1925 Having the advantages of easy-processing, low cost, color stability, and high tenability, these WL emitting 2D OIHP are promising as single-component light emitters.26 The first WL emission based on hybrid perovskites were reported by Doh-

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ner et al.19-20 in 2D (110)-oriented (N-MEDA)[PbBr4−xClx] (NMEDA = N1-methylethane-1,2-diammonium) and (EDBE)[PbX4] (EDBE = 2,2′(ethylenedioxy)bis(ethylammonium)) with X = Cl or Br. The chromaticity of the emission was partially tuned with the choice of the halogen, and a stable photoluminescence quantum efficiency (PLQE) of 9% has been measured. Few months later, we reported the same phenomenon in a novel (100)oriented 2D OIHP namely (C6H11NH3)2[PbBr4].21 We presented in-depth experimental investigations of WL emission in thin films of (C6H11NH3)2[PbBr4], which showed that the broadband emission was strongly Stokes shifted and presented a maximum around 100 K. Our investigations suggested that the WL emission resulted from self-trapped excitons (STE) in a deformable lattice due to strong electron−phonon coupling.21-23 Since these pioneering works, several research groups have investigated the origin of this new phenomenon,27-44 which further confirms the great interest of these WL emitting OIHP in solid state lighting applications. In our previous works,22-23 we gradually substituted Bromine by Iodine in mixed (C6H11NH3)2[PbBr4-xIx] systems, and proved an impressive correlation between the WL emission and the structural spatial modulation of the inorganic lead halide PbX6 octahedra, which established the leading role of the structural distortions in the control of the optical properties. Here, we report a new Cd-based 2D OIHP WL emitter, namely (C6H11NH3)2[CdBr4] (compound 1). Single crystals have been synthesized by slow solvent evaporation at room temperature and the corresponding thin films were prepared by the spin coating technique. Crystal structures of compound 1 and the corresponding organic cyclohexylammonium salt C6H11NH2.HBr have been determined by X-ray diffraction (XRD) as a function of temperature and the temperature dependence of their optical properties have been investigated using optical absorption (OA) and photoluminescence (PL) spectroscopies.

EXPERIMENTAL SECTION Single crystals synthesis and thin films preparation. All the starting reagents were purchased from commercial sources (Sigma-Aldrich, Alfa Aesar and Merck) and used without further purification unless otherwise stated. Dried solvents were prepared by refluxing for one day under nitrogen over the appropriate drying agents, and then degassed before use. All manipulations were conducted in air. Single crystals of compound 1 were synthetized by slow solvent evaporation at room temperature.45-48 In a first step, cyclohexylammonium salt C6H11NH2.HBr was prepared using the reaction between cyclohexylamine C6H11NH2 and HBr (47 wt %) in the cold (20 °C), to remove the reaction heat, with stirring for 20 min. Water was evaporated by elevating the temperature followed by washing with diethylether. The colorless precipitate was well dried under vacuum. In a second step, stoichiometric amounts of CdBr2 (1 mmol, 272 mg) was added to the prepared cyclohexylammonium salt (2 mmol, 360 mg), then well-stirred in methanol solvent, and kept in the dark at room temperature. After five days, colorless millimeter-sized platelets were formed. Thin films of compound 1 were prepared by spin-coating technique:49 10 mg of single crystals of compound 1 were dissolved in 1 mL of N,N-dimethylformamide solvent (DMF). Then, 100 µL of the solution was deposited on a glass slide,

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which was first cleaned with acetone in an ultrasonic bath and immersed in KOH ethanol solution (1M) for 10 min in order to modify the surface charge.50 The solution was spin-coated on the substrate at 1500 rpm for 30 s, at ambient temperature. The obtained film was then annealed in air at 80°C for 5 min to remove the residual solvent. The average thickness of films was estimated to 200 nm with profilometry technique. Single crystals and powder X-ray diffraction measurements. For the structure determination, good quality single crystals of the C6H11NH2.HBr ammonium salt and compound 1 were mounted on a supernova 4-circle micro-source diffractometer (Oxford Diffraction) equipped with a two-dimensional ATLAS detector, and using graphite monochromatized MoKα radiation (λ = 0.71073 Å). For compound 1, data collection has been performed as a function of temperature in the range 10-300 K using a helijet helium gas flow (10K-70K) and a nitrogen gas flow cryosystem (90-300K). The unit-cell determination and data reduction were performed using the CRYSALIS program suite51 on the full set of data. Numerical absorption correction was performed according to the crystal faces for compound 1. Crystal structures were solved by direct method with the SHELXS software, and refined using SHELXL (within the WINGX package).52 Room temperature XRD spectrum of thin film of compound 1 were performed using a Panalytical X’Pert Pro diffractometer with monochromatized Cu Kα1 radiation (λ = 1.54060 Å). The spectrum shows well defined and equally spaced (h 0 0) (h = 2,4, 6,…) diffraction peaks (see Figure S1 in the supporting information), which reveals that the crystallites are highly oriented and indicates the good quality of the films with the presence of a single crystalline phase in the sample. Powder XRD patterns have further been collected on microcrystalline powder of compound 1 as a function of temperature [22-300K] using a phenix closed-cycle cryostat. The microcrystalline powder of compound 1 has been prepared by milling its single crystals. Optical measurements. Optical absorption measurements were performed on spin-coated films of the compound 1 and the cyclohexylammonium salt C6H11NH2.HBr, and were deduced from direct transmission spectra acquired using a Perkin Elmer (Lambda 950) spectrophotometer. The temperature of the samples was controlled using a cold finger of a helium closed cycle cryostat. Photoluminescence spectra were measured on single crystals of C6H11NH2.HBr and compound 1 using a double monochromator U1000 equipped with a photomultiplier. The excitation wavelength was the 325 nm (3.815 eV) line of a Spectra-Physics beamlock 2085 Argon laser. The sample was placed in a helium bath cryostat and the measurements were performed between 2 and 300 K. Calorimetric measurements. Differential scanning calorimetry (DSC) measurements were performed using a cryogenic DSC-1/LN2 Mettler Toledo calorimeter on 42.1 mg of powder sample, sealed in aluminum pans with a mechanical crimp. The experiments were carried out at constant scan rate of 4 K.min-1 in the temperature range 290K-125 K, which is minimum temperature reachable in the sample holder using the liquid nitrogen cooling system.

RESULTS AND DISCUSSION Structural analysis. We have already reported the crystal structures of the two parent compounds (C6H11NH3)2[PbBr4]21


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and (C6H11NH3)2[PbI4],22 as well as the mixed compounds (C6H11NH3)2[PbBr4-xIx] (0 ≤ x ≤ 4).23 Although the 2D structural topology is very similar, the iodine and bromine lead derivatives crystallize in the centrosymmetric Pbca and noncentrosymmetric Cmc21 space groups, respectively. We have further shown that these materials exhibit a very complex structural phase diagram with several commensurate to incommensurate structural phase transitions, which significantly affect their photo-physical properties, especially the OA and optical emission characteristics. Figures 1a and 1b show the projections of the crystal structure of the C6H11NH2.HBr salt at 100K along the crystallographic c- and b-axis. C6H11NH2.HBr crystallizes in the non-centrosymmetric and polar orthorhombic space group Pbc21, with a primitive unit cell of dimensions: a = 11.0825(5) Å, b = 9.4784(3) Å, c = 7.7630(3) Å, and four formula units (Z = 4). The relevant crystallographic data are summarized in Table S1 in the supporting information. The packing may be described as zig-zag chains of perfectly ordered cyclohexylammonium cations in the (ab) plane running along the crystallographic b direction and connected through N-H…Br hydrogen bonds (dH1A…Br = 2.3983(2) Å, dH1C…Br = 2.4031(4) Å) (Figure 1a). The chains are further stacked next to each other in the a direction without any significant short inter-chain contacts. In the third direction, the chains are packed on top of each other in a staggered configuration, assisted by N-H…Br hydrogen bonds (dH1B…Br = 2.3681(4) Å) (Figure 1b). Accordingly, the crystal packing of C6H11NH2.HBr does not evidence any direct interactions between the cyclohexylammonium cations, or formation of molecular dimers, which could have been the origin for excimer light emission.

N-H…Br hydrogen bonds are depicted in dotted lines. Br, N, and C atoms are depicted in orange, blue and grey respectively.

At room temperature, compound 1 crystallizes in the noncentrosymmetric and polar orthorhombic space group Cmc21, with a unit cell of dimensions: a = 26.6412(18) Å, b = 8.6728(6) Å, c = 8.6045(8) Å, and four formula units (Z = 4). The relevant crystallographic data are given in Table S1. The structure shows a periodic arrangement of inorganic [CdBr4]2layers stacked along the crystallographic a direction in a staggered manner (Figure 2a), similar to the parent compound (C6H11NH3)2[PbBr4].21 Cylohexylammonium cations C6H11NH3+ are located in the interlayer spacing, and link adjacent inorganic layers via charge-assisted weak hydrogen bonds between the ammonium NH3+ moieties and bromine Br- anions, one bridging, and two terminal bromine (the so-called terminal halogen configuration.53 The organic cation is furthermore disordered over two orientations, which are not related by crystallographic symmetry. By comparison with (C6H11NH3)2[PbBr4], the cadmium atom is located in a heavily distorted environment with Cd-Br distances ranging from 2.593(1) Å to 3.996(2) Å. Four of the Cd-Br distances correspond to real chemical bonds (2.593(1), 2.618(1), 2.618(1), 2.701(2) Å), below the sum of the corresponding ionic radii (rCd=1.03 Å, rBr=1.96 Å) while the two other Cd-Br distances are clearly out of the range for a real chemical bond (3.406(2) and 3.996(2) Å). These last two distances are depicted as thin blue lines in Figure 2. The Cd atoms are therefore formally coordinated by only four bromine atoms, neither in a square planar nor in a tetrahedral configuration, but rather in a vacant deformed octahedral geometry. It is astonishing that the two long Cd-Br distances are nevertheless oriented in the two vacant directions of the octahedron, and therefore in the same direction than Pb-Br bonds in the parent compound (C6H11NH3)2[PbBr4], leading to the same overall structural topology of the inorganic layer. In a way, the inorganic perovskite layer is therefore disrupted with respect to the ideal 3D perovskite structure. We must therefore admit that these two long Cd-Br distances correspond to indirect interactions in the crystal, most probably assisted by the N-H…Br hydrogen bonds which direct the formation of the inorganic layers (Figure 2b). An overlay of the inorganic layer structure of compound 1 and (C6H11NH3)2[PbBr4] is given in Figure S2. It is obvious that the two compounds exhibit the same structural topology of the inorganic layers, but with a drastically more pronounced distortion for compound 1. By comparison with the 3D hybrid perovskites (CH3NH3)CdBr3 and (N(CH3)4)CdBr3 for which the Cd-Br bond distances range from 2.770 to 2.788 Å,54 the two groups of Cd-Br distances in (C6H11NH3)2[CdBr4] are much shorter (2.593(1), 2.618(1), 2.618(1), 2.701(2) Å) and much larger (3.406(2) and 3.996(2) Å) respectively, which highlights the strong degree of octahedron distortion in this later. The distortion may be quantified by the quadratic elongation 〈λ〉 and bond angle variance parameters55 defined as 〈λ〉 ∑ ⁄ ⁄6

Figure 1. Crystal packing of the structure of the cyclohexylammonium salt C6H11NH2.HBr projected (a) along the crystallographic c-axis, and (b) along the crystallographic b-axis.

(Eq 1)

where l0 is the center-to-vertex distance for an octahedron with Oh symmetry whose volume is equal to that of the distorted octahedron with bond lengths li.


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∑ 90° ⁄11

(Eq 2)

In practice, these two parameters vary almost linearly with each other. For compound 1 at 300K, if we consider the CdBr6 “pseudo-octahedron”, the corresponding distortion is as high as 〈λ〉 1.0861, and 166.39°. The degree of structural distortion of the inorganic perovskite layer may be further estimated from the in plane and out of plane Cd-Br-Cd angles (tilt). Since in the compound 1, the inorganic layers are located on the mirror plane perpendicular to the crystallographic a direction, the out of plane Cd-Br-Cd tilt is exactly zero, while the in-plane values are 137.84(6)° and 167.12(5)°. It is interesting to note that a hybrid organic inorganic compound of formula (C6H11NH3)[CdBr3], built from cylohexylammonium cations C6H11NH3+ and CdBr6 octahedra, has been reported in the literature.56 In this compound the octahedra are much less distorted (from 2.746 to 2.841 Å) than in compound 1, and share faces to form infinite one-dimensional chains. Interchain contacts are assisted by N-H…Br hydrogen bonds. Accordingly, the two building blocks C6H11NH3+ and CdBr6 are able to form either a layer (in compound 1) or a chain inorganic topological structure.

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We have previously shown that for the parent compound (C6H11NH3)2[PbI4],22 the cylohexylammonium cations exhibit short repulsive C-H…H-C contacts between molecules attached to two neighboring perovskite layers. Such intermolecular contacts produce a structural instability, which may be at the origin of the observed disorder of the C6H11NH3+ cation. The same phenomenon (short contacts and disorder of the cations) occurs for the present compound 1 (Figure S3). This is at variance of the C6H11NH2.HBr salt for which no direct contacts between the cylohexylammonium cations are detected (see above). A comparison of the cations structures in their direct crystalline environment is given in Figure S4. It shows that the conformations of the cations, as well as the orientation of the N-H…Br hydrogen bonds in the crystal packing of the two structures are almost superimposable, at the exception of one N-H…Br systematically shorter by more than 0.2 Å, and therefore stronger, in the packing of the C6H11NH2.HBr salt. Optical study. Room temperature OA spectrum of compound 1 (Figure 3) shows a similar behavior to reported Cd-based OIHP,57-59 with a sharp absorption peak at 3.24 eV, assigned to the excitons confined into the [CdBr4]2- inorganic layers, and attributed to the electronic transition from the top of the valence band (VB) including Br(4p) orbital to the bottom of the conduction band (CB) made of the Cd(5s) level. The presence of an excitonic absorption even at room temperature suggests a large exciton binding energy.60 At 12K, the intensity of the excitonic absorption peak of compound 1 increases and the maximum absorption blue shifts (Figure 3), resulting from the high stability of excitons at low temperature.60 Moreover, a step-like absorption structure, typical of 2D OIHP, was clearly observed at 3.575 eV (see the inset of Figure 3). Based on previous works,61-63 we attribute the sharp absorption at E1 = 3.24 eV to the 1s exciton and the step-like absorption at E2 = 3.575 eV to the 2s exciton. The exciton binding energy in the frame of a 2D hydrogenic-like model is given by,64

Figure 2. (a) Projection of the compound 1 structure along caxis at room temperature. Only the major component (A) of the disorder of the organic moiety is depicted. (b) Fragment of the structure, showing the N-H…Br hydrogen bonds for one orientation of the disordered cylohexylammonium cation. The two longest Cd-Br1 and Cd-Br3 distances are shown as blue thin lines (see text for more explanations). Cd, Br, N, and C atoms are depicted in pink, orange, blue and grey respectively.

! # $

" $

(Eq 3)

where %& is the Rydberg constant and ' = 1, 2…are the quantum numbers associated with the high energy excitonic states, which relate to the band gap energy, ( , by ( .Using the above mentioned experimental values of ab sorptions bands, we could derive = 376.88 meV, %& = 94.22 meV, and ( = 3.617 eV. This value is more than ten times larger than those reported in inorganic semiconductors, in which electron-hole have large radii (typically, 30-100 Å) and low binding energies (10-30 meV) within the order of kBT at room temperature (∼ 26 meV);65 for example 30 meV on PbI2 crystals.66 In contrast, it is in excellent agreement with those measured for the homologous OIHP (C6H11NH3)2PbI4 (356 meV)22 and (C10H21NH3)2PbI4 (370 meV).60 This large exciton binding energy is attributed to the 2D confinement and the large contrast in dielectric constants between the organic cyclohexylammonium and the inorganic [CdBr4]2- layers.60, 67 This further confirms that by combining organic and inorganic components, exciton binding energies (60-545 meV) and radii (22.9-6.2 Å) can be tuned over a broad range in OIHP materials.68 Decay of stable excitons via recombination of the electron-hole pair results in spontaneous light emission, which can be used for lighting and high-light-yield scintillator applications.


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Figure 3. Optical absorption spectra of thin film of compound 1 measured at 300 K (blue dashed line) and at 12 K (green solid line). The inset shows the step-like absorption, characteristic of a 2D density of states. Dashed lines indicate the resonance energies of the 1s and 2s excitons.

Upon 3.815 eV (325 nm) laser excitation, the PL spectrum of single crystal of compound 1 (curve (b) in Figure 4) shows a very broad band WL emission covering the entire visible region (from 320 to 750 nm) consisting of two bands located at 2.94 eV and 2.53 eV, noted P1 and P2, respectively. In literature,69-71 extensive luminescence studies of pure CdX2 (X = I, Br, Cl) and their halide solid solutions have been carried out mostly at low temperature because the emission are much weaker at room temperature due to thermal quenching effects. According to these reports, CdX2 exhibit strong luminescence observed in the 2-3.8 eV range, with peak positions dependent on the nature of halogen, the temperature and the excitation wavelength. Emission spectra of hybrid Cd-based halides are very similar to those of CdX2; for example, the maximum emission was observed at 2.5 eV for (C2H5NH3)2CdCl4,72 2.99 eV for (CH3NH3)2CdBr4,40 2.79 eV for (C6H14N2)CdCl4,73 and 2.95 eV for (C5N2H9)CdCl3.59 Drawing parallels to these studies, we tentatively assign the high-energy PL peak P1 for compound 1 to photoinduced exciton confined in the CdBr6 inorganic pseudo-octahedra. On the other hand, P2 peak shows a broadband emission and a very large stokes-shift (710 meV) with respect to the excitonic absorption. One possibility is that P2 could result from bound exciton confined at an impurity center60, 74 or STE.75 To find the most plausible origin of the second PL band P2, we studied the room temperature optical properties of the corresponding cyclohexylammonium salt C6H11NH2.HBr. A broad OA band was observed at 3.44 eV (curve (c) in Figure 4), much less intense than that of compound 1 (curve (a) in Figure 4), while the PL spectrum of C6H11NH2.HBr organic salt (curve (d) in Figure 4) shows a very broadband, Gaussian shaped with the maximum intensity at 2.44 eV and a full width at half-maximum (FWHM) of ~ 1.10 eV. From a simple inspection of PL spectra of C6H11NH2.HBr and compound 1 (curves (b) and (d) in Figure 4), two preliminary observations can be outlined: first, under the same measurements conditions, the energy position of the PL peak of C6H11NH2.HBr and that of P2 in compound 1 are relatively close (2.44 eV and 2.53 eV, respectively) and the 90 meV blue shift of P2 lumi-

nescence of compound 1 compared to C6H11NH2.HBr emission, results from the formation of the hybrid perovskite compound 1 which leads to tune the band structure thus modifying the luminescence properties.76 Second, a remarkable enhancement of P2 PL intensity in compound 1 (~ 3 times), when it is self-assembled with CdBr4 wells, was observed. Similar behavior was already reported in some OIHP such as the (110)oriented OIHP (C6H13N3)[PbBr4]76 and recently in (C12H16N2O3S)[SnCl6]77 and (H2L)[CuCl4],78 where the authors proved the presence of a resonance energy transfer mechanism between the organic and the inorganic moieties using electronic band structure calculations, which lead to the enhancement of the PL within the perovskite layers.76, 77 According to these informations, the most plausible origin of P2 peak is an energy transfer mechanism between the perovskite inorganic sheets [CdBr4]2- and the cyclohexylamine counterpart after hybridization.76, 79 However, the exact mechanism and nature of emission in compound 1, warrant further scrutiny through electronic band structure calculations, which will be fully reported in a forthcoming paper. The WL emission on OIHP was first reported in some corrugated 2D hybrid perovskites, and later in the 1D, C4N2H14PbBr427 and the 0D (C4N2H14Br)4SnBr3I3.38 For 2D OIH perovskites, the best PLQE was very recently reported by Zhuang et al.43 of 11.8% in bulk form of single crystals of (-O2C(CH)2CO2-)[Pb2Cl2]2+. More importantly, their WL emitters are long-sought ultra-stable lead halide materials, which overcome the air/moisture-sensitivity problems of lead perovskites. In contrast to OIHP and other bulk emitters, the WL emission intensity of their material remains unchanged after continuous UV irradiation for 30 days under atmospheric conditions (~ 60% relative humidity). Another impressive result was reported by Yuan et al.27 on the 1D OIH material C4N2H14PbBr4, with PLQE of ~ 20% for bulk single crystals and 12% for micro-crystals. The origin of the WL emission was deeply discussed and explained as due to a short-range electron-phonon coupling in a strongly deformable lattice which generated self-trapped carriers.21-41 Moreover, it was evidenced that lowering the dimensionality of OIH materials from 2D to 1D enables a strong quantum confinement with the formation of self-trapped excited states that produce highly efficient below-gap broadband luminescence and give efficient bluish white-light emissions.27 In Table S2, we compare the maximum position and the FWHM of the WL emission of the title compound 1, with those already reported in the literature. The large FWHM of the overall WL emission (~ 1.1 eV) observed in compound 1 can be explained by the substantial overlap between P1 and P2 PL sub-bands which gives an overall broadband emission, at room temperature.


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the emission spectrum of compound 1 shows that the PL peak P2 is sharper than P1, and when the temperature decreases, the ratio between the intensities of P2 and P1 decreases until 170 K. Below this temperature, the PL excitonic peak P1 becomes sharper and sharper until it completely hides the PL peak P2 below 50 K (see inset of Figure 5). Such behavior is due to the high stability of excitons at low temperature.60 Therefore, at low temperature, the overall PL spectrum was dominated by the strong excitons emission P1. On the other hand, the abnormal disappearance of P2 PL peak might result from structural phase transitions often observed in OIHP. The presence of a structural transition is most often accompanied with an abrupt shift of the PL position46 as well as an appearance or disappearance60 of the PL peaks, as in the parent compound (C6H11NH3)2[PbI4].22 Figure 4. Room temperature spectra of (a) the OA, (b) PL of compound 1, (c) OA and (d) PL of C6H11NH2.HBr salt. To provide further arguments about the origin of the broadband emission showed in compound 1, we have compared the structure of the two cyclohexylammonium cations in the respective crystal packings. The molecular structure of the cations is identical, all bond distances and bond angles are equal within one standard deviation between the two structures. The N-H…Br hydrogen bond pattern is also quite similar. The major difference concerns the inter-cations interactions. In the hybrid perovskite 1, the cyclohexylammonium cations are confined by the inorganic layers to sit in specific locations which forces inter-cation short contacts below the sum of the Van der Waals radii (see Figure S3). On the contrary, the crystal packing of C6H11NH2.HBr does not evidence any direct interactions between the organic cations, or formation of molecular dimers, which could have been the origin for excimer light emission. Accordingly, the similarity in hydrogen bond pattern between the two compounds may suggest that an energy transfer mechanism is the most probable origin for the WL emission in the C6H11NH2.HBr salt. Thermal dependence of the optical properties. In our previous works, we reported a strong WL emission, even at ambient temperature, in OIHP based on the same cyclohexylamine organic units, namely (C6H11NH3)2[PbBr4] and (C6H11NH3)2[PbBr4-xIx], and we deeply studied the physical origin of this novel phenomenon. Our investigations have shown an excellent correlation between the WL emission efficiency and the structural changes. We concluded that the WL emission resulting from the self-trapped states was enhanced by the strong structural angular distortion of the inorganic framework.21, 23 Our hypothesis was recently supported by several investigations reported in the literature.21-41 Moreover, in our previous studies on WL emitter perovskites, the maximum emission efficiency was observed around 100K. It was also the case of (C6H11NH3)2[PbI4]22 which showed, at low temperature (below 130K), a broadband emission coupled with sharp free excitons PL peaks. In addition, the two OIHP namely (N-MEDA)[PbBr4]19 and (EDBE)[PbBr4],20 reported by Dohner et al. also exhibited a maximum WL emission around 100 K. To get more information’s about the PL behavior of the present 2D OIHP, we measured the temperature dependence of its PL spectra (Figure 5). Significant temperature dependent changes were revealed. At room temperature,

Figure 5. Temperature dependence of the PL spectra for compound 1, under 3.816 eV irradiation. The inset highlights the low-energy range showing the disappearance of P2 peak at low temperature.

From the PL spectra of Figure 5, we derived the temperature dependence of the integrated intensity (Figure 6a), the FWHM (Figure 6b) and the position (Figure 6c) of P1. The integrated intensity of the high energy PL peak of compound 1 decreases as a function of temperature and shows two clear anomalies around 50 and 140 K. Three different temperature ranges are therefore detected, noted "A", "B", and "C" in Figure 6a. In the region, "A", corresponding to the temperature range 2K< T < 40K, the integrated intensity decreases with temperature, it jumps between 40 K and 50 K and then deceases continuously in region "B" (50 < T < 130 K), before to jump again between 130 and 140 K, and finally dramatically quenches as a function of temperature in region "C" (T > 140 K). We described the quenching of this excitonic PL emission, in "B" and "C", as due to a competition with a nonradiative channel as described by the Arrhenius-type law,67 *+,

-.

,

456 78 9

/0123

(Eq 4)

where * is the low-temperature PL intensity, :; is the Boltzmann constant, < is the ratio between the radiative and the nonradiative decay rates, and 0 is the activation energy. The best fit gives * 2.5 ? 10@ A 350 , < 83 A 5, and 0 10 A 0.7 meV. This activation energy presents a good agreement with those measured in similar 2D OIHP such as


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(C6H11NH3)2[PbBr4]21 and (C6H5C2H4NH3)2[PbI4],80 and is far from the 377 meV exciton binding energy deduced from lowtemperature optical absorption spectroscopy (see Figure 3). Consequently, the decrease of the integrated PL intensity cannot be attributed to a partial ionization of the exciton. Moreover, the FWHM of the perovskite-excitonic peak slightly decreases in region "A", and then linearly increases as a function of temperature in both regions "B" and "C" by showing two different slopes (Figure 6b). The broadening of the P1 line as a function of temperature can be attributed to exciton-phonons interaction, described within the following law,81-82 C D C E CFG . D E

HIJ , 5 123K IJ L"

(Eq 5)

78 9

where, the first term is the natural linewidth at 0 K, the second term represents the broadening induced by acoustic phonons, and the third term corresponds to the contribution of optical phonons to the peak broadening. There, C,M is the excitonphonons coupling and ,M is the optical phonon energy. The best fitted parameters in the temperature region "C" give, C 123 A 1 meV, CFG 1.64 A 0.01 meV. K " , C,M 40 A 3 meV, and ,M 12 A 1 meV. These values are in good agreement with those reported for CdBr2 crystals83 and similar hybrid perovskites such as CsCdBr3.84 The presence of singularities in the thermal dependence of the integrated intensity and the FWHM of the PL peaks around 50 K and 150 K was also pointed out from the temperature dependence of the peak position plotted in Figure 6c. At least, three temperature regions were also identified, corresponding to the previous "A", "B" and "C" temperature regions, already found from the thermal dependence of the integrated intensity and the FWHM (Figures 6a and 6b). First, the PL peak red shifts until 50 K, then non-monotonous shifts were detected in region "B" and a continuous red shift as a function of temperature was observed in "C" temperature range. Usually, in semiconductor materials, the free exciton energy peak shifts to lower energy as a function of temperature, this shift is often described by Varshni’s model,85 12 ,12

TU $

Figure 6. Temperature dependence of (a) the integrated intensity, (b) FWHM, and (c) the position of P1 peak, deduced from the PL spectra of compound 1 showed in Figure 5. Three A, B, and C temperature regions clearly emerge from the data. The red lines (in Figures (a) and (b)) are the best fit of the experimental data, according to Eqs. (4) and (5), respectively.

The temperature dependence of the PL response was also investigated on single crystals of C6H11NH2.HBr cyclohexylammonium organic salt (Figure 7a). The intensity of the WL emission increases when lowering the temperature with the presence of a slight regime change around 100 K. Moreover, the position of the luminescence red shifts as a function of temperature and shows a clear "jump" around 150-200K (Figure 7b).

, where

V/U

,12 is the exciton energy at 0 K, W is responsible of the linear red-shift and X is close to Debye temperature. In compound 1, the red shift was observed only in "A" and "C" regions. The non-monotonous behavior of the peak energy in "B" region is attributed to a strong structure modulation of the inorganic framework in this temperature range (see below). Such structure modulations were observed in the parent perovskites (C6H11NH3)2[PbBr4]21 and (C6H11NH3)2[PbI4]22 based on the same organic cations and non-monotonous shifts of the PL peaks positions were also observed in the temperature ranges corresponding to distorted incommensurate structure. Moreover, the abnormal blue shift of the peak position of compound 1 between 50 and 80 K can be the result of the competition in the recombination process between the excitonic PL peak P1 on the one hand and the PL peak P2 on the other hand, resulting from the organic cation, since the later one appears clearly above 50 K.

Figure 7. (a) Temperature dependence of the PL spectra of C6H11NH2.HBr cyclohexylammonium organic salt. (b) Plot of the temperature dependence of the position (blue) and the integrated intensity (red) of the PL peak.

The PL behaviors of C6H11NH2.HBr salt and compound 1 suggest the presence of at least two critical structural changes as a function of temperature around 50 and 150 K. The presence of these singularities in the behavior of the organic salt indicates the existence of an important interaction between the inorganic sheets and the organic moieties. As a result, the total response is far from the sum of two moieties. The signatures of the presence of structural phase transitions in 2D OIHP can be usually detected by studying the tempera-


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ture dependence of the OA spectra.46, 60, 86-89 In previous works,22, 45-47 we have shown that the temperature dependence of the position of the excitonic absorption peak constitutes a sensitive probe of structural phase transitions. The possible presence of structural phase transitions in compound 1 around 50 and 150 K pushed us to investigate the temperature dependence of the optical absorption spectra. Results are plotted in Figure 8a. The intensity of the excitonic absorption line decreases while its width increases as a function of temperature. Moreover, a clear red-shift of the position of the maximum absorption was also found and confirmed by the plot of the temperature variation of the excitonic absorption peak (Figure 8b). The quasi-linear trend of the thermal red-shift is affected by two anomalies at 60 and 150K. Three different slopes are observed as a function of temperature in the "A", "B" and "C" ranges, already observed on the temperature dependence of the PL spectra of compound 1. Such results further strengthen the hypothesis of the presence of structural phase transitions around 60 and 150 K.

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for the conformational change of long-chain alkylammonium hybrid perovskite.87, 94 At this stage, we can conclude that the structural rearrangements occurring in the present phase transitions might be relatively subtle. We may further speculate that the weak value of ∆S is related to the existence of a residual short-range order of the cyclohexylammonium cations in the high-temperature disordered phase, due to the freezing of their motion.

Figure 9. Thermal variation of the heat flow of powder sample of compound 1 in heating (red line) and cooling (blue line) modes. Vertical dashed lines locate the phase transition temperatures.

Figure 8. (a) Thermal dependence of the optical absorption spectra of compound 1 measured between 12 and 300 K, and (b) temperature dependence of the excitonic absorption peak position.

Differential scanning calorimetry study. The critical behaviors of the optical properties of compound 1 as a function of temperature led us to suspect the presence of structural phase transition around 50 and 150 K. To check the consistency of our reasoning and to further derive the associated thermodynamic parameters of compound 1, we performed DSC measurements on microcrystalline powder of compound 1. The investigations were conducted in the heating and cooling modes, but they were limited to the temperature range [135295K] due to technical constraints. The DSC data are presented in Figure 9. Sharp endothermic (on heating) and exothermic (on cooling) peaks are detected at 162 K and 158K, respectively, denoting the existence of a first-order phase transition with a 4 K thermal hysteresis loop. Moreover, additional broader peaks were also detected at 154 K on heating and 149 K on cooling, in good agreement with the critical behaviors observed in optical measurements. The excess heat observed around 162 K is associated with a phase transition characterized by extremely small enthalpy and entropy changes of ∆H = 0.19 kJ mol-1 and ∆S = 1.2 J K-1mol-1, respectively. The relation ∆S = R LnΩ gives the value of Ω = 1.155 which is similar to those obtained for first-order and second-order transitions of some homologous OIHP, such as (C6H11NH3)2[PbI4]45 (4FC6H4C2H4NH3)2[PbI4]88 (C3H7NH3)2[PbCl4],90 91 (C2H5NH3)2[MnCl4], (CH3NH3)2[CdCl4].92-93 These enthalpy and entropy changes are much smaller than the values reported

Thermal dependence of X-ray diffraction spectra. In order to investigate further the structural phase diagram, and detect the presence of structural phase transitions as most probably revealed by the optical and calorimetric investigations (see above), we have undertaken single crystal and powder XRD experiments as a function of temperature from 10 to 300 K. The XRD pattern of microcrystalline powder of compound 1 as a function of temperature is given in Figure 10. In a whole, six different structural regimes have been identified which have been labelled S1-S6. As can be seen in Figure S5, the nice matching between the 300K experimental pattern and the pattern calculated from the 300K single crystal structure confirms the phase purity of the microcrystalline powder sample. In addition, the diffraction pattern recorded at the end of the complete temperature variation sequence is matching exactly the pattern recorded before the sequence; this indicates a complete reversibility of the different structural phase transitions, no sample degradation is detected (Figure S6). Increasing the temperature from 22 to 40 K exhibits a progressive modification of the diffraction pattern as shown in Figure S7, evidencing gradual structural modifications (from phase S1 to phase S2). At first, a diffuse feature appears in the 27-28° 2θ range (Figures S7b), which progressively condense to a sharp diffraction peak whose intensity maximum is observed at 34 K. This peak then decreases in intensity to 40 K. A similar feature appears also in the 23-24° 2θ range (Figures S7a). The diffraction pattern then does not show significant modifications up to 136K, at which a clear structural phase transition occurs (from phase S3 to phase S4), as highlighted in Figure S8. As will be shown below from single crystal structural diffraction, an additional structural phase transition happens at nearly 100 K (from phase S2 to phase S3). A further phase transition is detected from the powder XRD pattern at nearly 160 K (from phase S5 to phase S6), and no further modification occurs up to 300 K.


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Figure 10. Powder X-ray diffraction pattern of compound 1 as a function of temperature [22-300 K] in the 20-30° 2θ range. Four different structural regimes are evidenced. To confirm the different structural regimes, and structural phase transitions detected by powder XRD, we have undertaken a single crystal XRD analysis as a function of temperature from 10 K to room temperature, using a helium cryosystem (from 10 to 70 K) and a nitrogen cryosystem (from 90 to 300 K). The evolution of the corresponding unit cell parameters is given in Figure 11. The different structural regimes discussed above correspond to abrupt changes in the unit cell parameters. At the transition between regime S1 and S2, a sudden decrease of the unit cell volume is observed, resulting mainly from an abrupt decrease in the interlayer spacing (unit cell parameter a). Around 100 K, a sudden increase in interlayer spacing occurs, correlated with the appearance of satellite reflections on the diffraction pattern (Figures S9 and S10). This corresponds to phase transition S2→S3, which was not identified in the powder XRD study. The satellite reflections can be indexed using two modulation wavevectors q1 and q2 (depicted in blue in Figure S10). Upon further increase in temperature, a sudden modification of the position of the satellite reflections occurs between 130K and 140K, which corresponds to the phase transition (S3→S4) detected from the powder diffraction measurements. At this point, the inter-layer spacing undergoes a strong change of behavior, characterized by an abrupt contraction of the unit cell parameter a. This is almost compensated by a large increase in the c cell parameter, so that the unit cell volume continuously increases upon the phase transition (S3→S4). Phase S4 is stable on a very narrow temperature range, while an additional modification of modulation wavevector occurs at nearly 150K during phase transition (S4→S5). Finally, the incommensurate structural modulation disappears at nearly 160K upon the phase transition (S5→S6). The two series of peaks detected in the DSC measurements correspond to the (S4→S5) (broad DSC peak) and (S5→S6) (sharp DSC peak) phase transitions. This is in agreement with a subtle structural reorganization corresponding to the broad DSC peak (only a slight change of modulation wavector), and a larger structural reorganization corresponding to the sharp DSC peak (complete disappearance of the structural modulation and satellite reflections).

Figure 11. Evolution of the unit cell (a) volume, (b) a parameter, and (c) b and c parameters of compound 1 as a function of temperature, derived from single crystal XRD. S1-S6 denotes the sixth different identified structural regimes. Compound 1 exhibits therefore a very rich and complicated structural phase diagram with at least 6 different structural regimes detected by a combination of powder and single crystal XRD analysis. From 100K to 160K, the diffraction pattern exhibits additional reflections associated with wave vectors Q=H+mq1+nq2, where H is a reciprocal lattice vector, q1 and q2 are modulation vectors. It is noteworthy that q1 and q2 are lying in the (0kl) plane of reciprocal space. The structural modulation is therefore intrinsic to the 2D structural topology. The average crystal structure (without taking into account the structural modulation) has been determined as a function of temperature from the single crystal diffraction data. It does not show any change of the average space group Cmc21 upon passing the different structural phase transitions. It is commonly accepted that the broadness of the optical emission in such 2D HOIP is heavily influenced by the structural distortion, which can be evaluated through some structural parameters, such as the in-plane and out of plane Cd-Br-Cd tilt angles, octahedron quadratic elongation 〈λ〉 and bond angle variance distortion parameters.29, 31 In particular, PL broadening generally follows the increase in structural distortion parameters. To quantitatively measure the amplitude of distortion of the CdBr6 pseudo-octahedron, we have evaluated the quadratic elongation and angle variance distortion parameters as a function of temperature, the results are given in Figure 12. The obtained values are by far the largest obtained for similar white-light emission 2D HOIP.29 On the temperature range corresponding to the S1-S2 regime, the distortion parameters continuously increase, while the S2 to S3 phase transition corresponds to an abrupt decrease, which is most probably related to the appearance of the structural incommensurate modulation, releasing elastic strain in the crystals. From the S4 to S5 phase transition, the distortion of the pseudo-octahedron continuously increases with temperature up to 300K. It is very conclusive that the very large increase in elongation and angular distortion corresponds to the very large broadening of the PL spectrum in the region C (see increase in FWHM in Figure 6b) in the structural regime S6.


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efficiency to room temperature. Therefore, we believe that this work will enrich OIHP family with WL emission by arousing a renewed research on low cost, eco-friendly raw materials, with facile synthesis and excellent optical properties at room temperature. Within this context, this new perovskite phosphor represents a highly promising alternative to replace conventional inorganic rare-earth-based phosphors and quantum dotbased phosphors that currently dominate the field of optically pumped WL emitting diodes.

ASSOCIATED CONTENT Supporting Information

Figure 12. Quadratic elongation and angle variance distortion parameters of the CdBr6 pseudo-octahedron as a function of temperature. The parameters are defined as 〈λ〉 ∑ ⁄ ⁄6, where l0 is the center-to-vertex distance for an octahedron with Oh symmetry whose volume is equal to that of the distorted octahedron with bond lengths li, and ∑ 90° ⁄11.

CONCLUSION In summary, a new 2D OIHP with the general formula (C6H11NH3)2[CdBr4] has been synthesized by slow solvent evaporation method at room temperature and processed in thin films by spin coating. Structural, thermal and optical properties have been studied by means of X-ray diffraction, DSC, optical absorption, and photoluminescence measurements. The room temperature optical absorption spectrum of compound 1 perovskite revealed a sharp absorption peak at 3.24 eV, arising from the two-dimensional excitons confined into the [CdBr4]2layers and attributed to the electronic transition from the top of the valence band to the bottom of the conduction band. A strong exciton binding energy of 377 meV was estimated from the low-temperature optical absorption measurements; it is results from the large contrast in dielectric constants between organic cyclohexylammonium and inorganic [CdBr4]2- layers. Upon 325 nm irradiation, compound 1 exhibited a very broad band WL emission consisting of two peaks: the first one at 2.94 eV, attributed to exciton confined in the [CdBr4]2- inorganic layers, and the second one at 2.53 eV, attributed to the emission of the organic moiety, and a possible energy transfer mechanism between the inorganic and the organic parts, a result which is supported by the investigations of the optical absorption and PL spectra of the cyclohexylammonium bromine salt. Furthermore, we found the presence of abnormal PL behaviors around 50 and 150K at which singularities in the integrated intensity, the FWHM and the position of the PL peak were detected. These singularities were also revealed by studying the temperature dependence of the position of the excitonic absorption peak suggesting the presence of structural phase transitions often observed in this class of materials. All these singularities have been traced back to structural phase transitions from temperature dependent powder and single crystal XRD investigations. A strong correlation has been obtained between the structural distortion of the CdBr6 pseudo-octahedron and the broadening of the WL emission band. The observation of the large band emission, opens interesting perspectives in the design of WL emitters, after relevant chemical modifications leading to shift the maximum WL emission

Details of the crystals structure, thin film and powder X-ray diffraction data, and comparative table between the WL emission based on hybrid perovskites reported up to date. This material is available free of charge via the Internet at http://pubs.acs.org/

Accession Codes CCDC 1822417, 1823075 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge viawww.ccdc.cam.ac.uk/data_request/cif,orbyemailingdata_reque [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION Corresponding Authors *[email protected] **[email protected]

Author Contributions The manuscript was written through contributions of all authors. A.Y synthesized the samples, analyzed all the optical data and wrote the paper, A.L did the PL measurements, S.P and E-E.B did the XRD measurements and wrote the XRD section, S.T did the DSC measurements, Y.A contributed to the discussion of the PL data, and K.B supervised the experimental work and wrote the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by CNRS, the Universities of Versailles (member of University Paris-Saclay), Brest, Lorraine and Sfax, and CPER MatDS, Lorraine Université d’excellence (award No. ANR-15-IDEX-04-LUE). A.Y acknowledges the Japan Society for the Promotion of Science (JSPS) for the postdoctoral fellowship (16F16783). We also acknowledge Guillaume Bouchez and Catherine Charles for technical assistance with the optical absorption and DSC measurements, respectively.

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Table of Contents figure for submission A brief synopsis: this figure describes the crystal structure of the hybrid perovskite (C6H11NH3)2CdBr4 as well as its room temperature optical absorption and emission spectra.


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this figure describes the crystal structure of the hybrid perovskite (C6H11NH3)2CdBr4 as well as its room temperature optical absorption and emission spectra. 265x179mm (300 x 300 DPI)


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Figure 1. Crystal packing of the structure of the cyclohexylammonium salt C6H11NH2.HBr projected (a) along the crystallographic c-axis, and (b) along the crystallographic b-axis. N-H…Br hydrogen bonds are depicted in dotted lines. Br, N, and C atoms are depicted in orange, blue and grey respectively. 600x800mm (96 x 96 DPI)


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Figure 2. (a) Projection of the compound 1 structure along c-axis at room temperature. Only the major component (A) of the disorder of the organic moiety is depicted. (b) Fragment of the structure, showing the N-H…Br hydrogen bonds for one orientation of the disordered cylohexylammonium cation. The two longest Cd-Br1 and Cd-Br3 distances are shown as blue thin lines (see text for more explanations). Cd, Br, N, and C atoms are depicted in pink, orange, blue and grey respectively. 179x260mm (96 x 96 DPI)


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Figure 3. Optical absorption spectra of thin film of compound 1 measured at 300 K (blue dashed line) and at 12 K (green solid line). The inset shows the step-like absorption, characteristic of a 2D density of states. Dashed lines indicate the resonance energies of the 1s and 2s excitons. 241x198mm (300 x 300 DPI)


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Figure 4. Room temperature spectra of (a) the OA, (b) PL of compound 1, (c) OA and (d) PL of C6H11NH2.HBr salt. 263x205mm (300 x 300 DPI)


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Figure 5. Temperature dependence of the PL spectra for compound 1, under 3.816 eV irradiation. The inset highlights the low-energy range showing the disappearance of P2 peak at low temperature. 274x201mm (300 x 300 DPI)


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ACS Photonics

Figure 6. Temperature dependence of (a) the integrated intensity, (b) FWHM, and (c) the position of P1 peak, deduced from the PL spectra of compound 1 showed in Figure 5. Three A, B, and C temperature regions clearly emerge from the data. The red lines (in Figures (a) and (b)) are the best fit of the experimental data, according to Eqs. (4) and (5), respectively. 380x330mm (280 x 280 DPI)


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Figure 7. (a) Temperature dependence of the PL spectra of C6H11NH2.HBr cyclohexylammonium organic salt. (b) Plot of the temperature dependence of the position (blue) and the integrated intensity (red) of the PL peak. 406x162mm (300 x 300 DPI)


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ACS Photonics

Figure 8. (a) Thermal dependence of the optical absorption spectra of compound 1 measured between 12 and 300 K, and (b) temperature dependence of the excitonic absorption peak position. 315x153mm (300 x 300 DPI)


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Figure 9. Thermal variation of the heat flow of powder sample of compound 1 in heating (red line) and cooling (blue line) modes. Vertical dashed lines locate the phase transition temperatures. 247x192mm (300 x 300 DPI)


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Figure 10. Powder X-ray diffraction pattern of compound 1 as a function of temperature [22-300 K] in the 20-30° 2θ range. Four different structural regimes are evidenced. 254x179mm (96 x 96 DPI)


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Figure 11. Evolution of the unit cell (a) volume, (b) a parameter, and (c) b and c parameters of compound 1 as a function of temperature, derived from single crystal XRD. S1-S6 denotes the sixth different identified structural regimes. 378x341mm (300 x 300 DPI)


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ACS Photonics

Figure 12. Quadratic elongation and angle variance distortion parameters of the CdBr6 pseudo-octahedron as a function of temperature. The parameters are defined as 〈λ〉=∑_(i=1)^6▒(l_i⁄l_0 )^2⁄6, where l0 is the center-to-vertex distance for an octahedron with Oh symmetry whose volume is equal to that of the distorted octahedron with bond lengths li, and σ_θ^2=∑_(i=1)^12▒(θ_i-90°)^2⁄11. 295x199mm (300 x 300 DPI)


Broadband Emission in a New Two-Dimensional Cd-Based Hybrid

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