Cystamine-configured lead halide based 2 D hybrid molecular crystals : Synthesis and photoluminescence systematics

We have synthesized and studied a specific family of disulfide bridge based 2D organic inorganic hybrid perovskites using the cation, cystamine [2,2′-dithiobis(ethylammonium), abbreviated as SS] in the three lead halide (X = I, Br, and Cl) based systems, and explored their unique photo-physical properties. Green, blue, and white emissions are noted in I, Br, and Cl based systems, respectively. The experimental observations are compared with the results of first principles DFT calculations. The role of the halide ion (X) in the [Pb-X4] cages on the luminescence of the disulfide bridge based hybrid system is elucidated, and the corresponding systematics are analyzed.

state lighting (SSL) is currently dominated by 2D materials for fabrication of LEDs and lasers. 14,235][26] The sublime luminescent properties are known to originate in the inorganic sheet consisting of Pb-X cages.A recent development is the emergence of a new sub-class of 2D materials known to be broadband emitting phosphors. 27,28he genesis for observing white light luminescence from 2D hybrid perovskites is suggested to be due to self-trapping of excitons mediated by lattice distortions in inorganic Pb-X cages or their assemblies. 29,302][43] The organic moieties explored in this subclass largely belong to long chain amines. 28,37he incorporation of organic moieties having disulfide bridges in 2D hybrid perovskites is a relatively unexplored class especially in the context of their photo-physical properties.Unlike the simple ammonium cations such as alkyl diammonium, the disulfide bridge offers uniqueness due to greater intermolecular interactions.Disulfide molecules and allied derivatives belonging to the family of R-SS-R (where R and R are the organic groups) exhibit skewed structures in their crystalline phase.5][46][47][48] The use of these molecules in hybrid perovskite systems leads to a rich and structurally versatile group of low dimensional hybrid perovskites which can be exploited for various applications such as second harmonic generation (SHG) switches and non-linear optics. 15,49,500][51][52][53] A study has also been performed by the same group on the bromide based system using this molecule, but it is focused on the synthesis and a structural comparison with the iodide system. 54None of these studies have however explored the interesting photo-physical properties of these materials, which forms the goal of the present work.Thus, we report on the family of three lead halide (X = I, Br, Cl) based 2D perovskites using 2,2 -dithiobis(ethylammonium) (abbreviated as SS) as the structure defining cation.The single crystals of the three systems, [SS]PbI 4 (I-1), [SS]PbBr 4 (Br-2), and [SS]PbCl 4 (Cl-3), are prepared under similar experimental conditions using a solution processed growth technique.The photo-physical properties of these systems are studied on crushed single crystals (powder).The experimental observations have been compared with the first principles DFT calculations for better understanding of the observed results.The details for the synthesis and characterizations are given in Secs.S1 and S2 of the supplementary material.
We first discuss the three 2D hybrid perovskite systems, namely, (SS)[PbI 4 ] (I-1), (SS)[PbBr 4 ] (Br-2), and (SS)[PbCl 4 ] (Cl-3), which were synthesized using a method that is different from that employed by Louvain et al. 54 The relavant crystallographic data are given in Sec.S3 and Table SI of the supplementary material.The 2D structures shown in Figs.1(a)-1(c) belong to centrosymmetric monoclinic space groups of P21/n, C2/c, and P2/c for the I, Br, and Cl cases, respectively.The crystal structures reflect periodic arrangements of inorganic [PbX 4 ] −2 layers stacked along the crystallographic a-direction.The overall packing configuration is seen to be in the form of a linear chain of perfectly ordered diprotonated 2,2 -dithiobis(ethylammonium) (SS) cations in the ab-plane, running along the crystallographic b-direction and connected through N-H• • • X hydrogen bonds (X = I, Br, Cl).
The difference in the orientation of the S-S bond and NH 3 + with the halide in the inorganic cage causes subtle changes in the three systems under study.In system 1, a disulfide cation is present in two alternating conformations, the stable P and M chiral helical conformations in equal proportion, as seen in Fig. 1(a). 52Since our studies are limited to room temperature, we do not observe any transformations within the two conformations.The optical images of the three crystalline powders in day light and their luminescence in dark under excitation at specific wavelengths (as stated) are shown in Figs.1(d)-1(f).The crystal system 1 shows highly shiny crystals of orange color under visible light illumination and exhibits a strong green luminescence under 450 nm excitation.System 2 exhibits bright blue luminescence and system 3 exhibits white luminescence, under excitation at 350 nm, while they appear greyish white in visible light.The PXRDs for crushed crystals (powder) and simulated pattern obtained from single crystal XRD analysis for all three systems show good correspondence, as seen in Fig. S1.The high resolution transmission electron microscopy (HRTEM) images of the crystal are shown in Fig. S2 for system 2. The lattice image shows the lattice parameter of 1.2 nm that corresponds to the 2D peak in powder x-ray diffraction (PXRD) at 2θ = 7.7 • , as observed from the single crystal data submitted to the Cambridge Crystallographic Data Centre (CCDC).
The vibrational spectroscopy studies using FTIR measurements on the perovskite systems as well as the respective organic salts were conducted on crystals crushed into powder which were mixed with high purity KBr to make a pellet; the details are presented in Fig. S3 of the supplementary material.The identification of all the modes is presented in Table SII of the supplementary material.
The vibrational spectra of the halide perovskites and the respective salts do not show any discernible differences, as seen from Fig. S3 of the supplementary material.On comparing the spectra for the three different perovskite systems, almost all the peaks in the series are found to shift to higher frequency in going from I to Br to Cl.The change in the ionic radius and the electronegativity of the halide ion render the vibration induced change in the dipole moment that is reflected in the FTIR shift.The S-S stretching mode at 505 cm −1 and stretching modes of C-N at 799 and 868 cm −1 remain mostly unchanged.The rocking modes of CH 3 -NH 3 at 933, 1014, and 1075 cm −1 as well as the bending modes for CH 3 and NH 3 in the range 1400-1500 cm −1 also do not show any drastic changes.This is in accordance with the observation by Glaser et al. in their report on IR studies in 3D methylammonium lead halides.The absorption and photoluminescence properties of the powdered crystals were measured at room temperature.The corresponding results are shown in Figs.3(a)-3(c).The absorption spectra for all three systems follow a broadly similar trend as that of the other 2D lead halide based materials. 31he absorption bands span the region from 300 to 520 nm, 200 to 410 nm, and 200 to 350 nm for the three systems 1, 2, and 3, respectively.The sharp excitonic absorption bands are positioned at (484 nm, 520 nm), (378 nm, 408 nm), and (316 nm, 339 nm) for systems 1, 2, and 3, respectively.
The optical bandgap is estimated from the Kubelka-Munk function fitted to the direct bandgap material, and the values are obtained as 2.3 eV, 2.9 eV, and 3.5 eV for the 1, 2, and 3 systems, respectively. 56In order to record the photoluminescence (PL), we used different excitation wavelengths representing photon energy higher than the corresponding bandgaps as estimated and stated above for the three systems.Some key data providing comparative insights into the PL behavior [Figs.3(a The nature of photoluminescence for system 1 under excitation at 450 nm is seen to have two prominently sharp peaks at 522 nm and 554 nm in the green region, which can be attributed to free and bound excitons, respectively; the latter being a broader feature as expected.The 522 nm free exciton contribution overlaps with the absorption feature.In the case of 2, one witnesses an asymmetric emission band peaked at 434 nm with a tail extending to 600 nm, and a just discernible tiny feature at 404 nm which lies at the absorption band edge, as shown the in inset of Fig. 2(b).The asymmetric line shape of emission in this case 2 can in fact be fitted with two contributions at about 430 nm and 500 nm.The 500 nm contribution with significant Stoke shift can be attributed to self-trapped exciton.Interestingly, in the case of system 3, a broad dual peak structure is noted under the excitation at 300 nm with contributions peaking at about 380 nm and 510 nm (the latter with a large Stoke shift and again attributed to the self-trapped exciton state), while only the broad emission at 510 nm is observed under the excitation at 350 nm.Interestingly, a progressive blue shift is observed in going from the (I-1) to (Br-2) to (Cl-3) system.This must originate from the subtle structural changes caused by the decreasing size of halogen ions.
The broadband (white) luminescence, which has invited significant attention lately in the case of hybrid perovskite systems, occurs in the present case for the Cl-based 2D system. 33Such broadband luminescence has been generally attributed in the recent literature to self-trapped excitonic emission and is suggested to result from distortions or corrugations of the octahedral networks in the crystals. 51,57Interestingly, in the case of our disulfide based materials, the white luminescence is seen in the least-distorted Cl-based material, while the other two cases exhibit blue and green emissions.
The time decay of photoluminescence in powder (crushed crystals) samples is shown in Figs.3(d)-3(f).These data were collected using the laser wavelengths close to the excitation FIG. 3. [(a)-(c)] The absorption spectra for the powder (crushed crystals) obtained from diffused reflectance spectroscopy (DRS) along with the PL spectra for the three perovskite systems, 1 excited at 450 nm, 2 excited at 350 nm (the inset shows the region between 380 nm and 420 nm), and 3 excited at 350 nm.The time-resolved PL studies for (d) system 1, excitation at 405 nm, emission recorded at 520 nm and 550 nm; (e) system 2, excitation at 340 nm, emission recorded at 405 nm and 428 nm; (f) system 3, excitation 340 nm, and emission at 500 nm.
wavelengths used to record the steady state PL data in Figs.3(a)-3(c).The time decay was seen to follow a mono-exponential decay for all three systems.For system 1, for excitation at 405 nm, the emissions at 520 nm and 550 nm were noted to have time constants of 0.81 ns and 0.55 ns, respectively.On the other hand, for system 2, for excitation at 340 nm, the emissions at 405 nm and 428 nm had almost the same time constant of about 1 ns.Interestingly, for the broadband emission in system 3, peaking around 500 nm, a single component contribution of 7.04 ns was noted.This can be understood as the relatively slower contribution emanating from self-trapping of excitons (STE). 58ince the low dimensional luminescent hybrid materials comprise of the inorganic units (e.g., Pb-X 4 octahedra) configured in the crystalline forms with participating organic moieties, it is of interest to examine the possible contributions of the organic moieties to the electronic states and optical absorption spectra of the hybrid materials. 42Figures S4(a)-S4(c) in the supplementary material show the absorbance (as obtained by DRS) and PL data for the three organic salts.The disulfide organic salts for I, Br, and Cl are seen to have PL maxima at 520 nm (λex = 450 nm), 435 nm (λex = 350 nm), and 435 nm (λex = 350 nm), respectively.In Figs.S5(a)-S5(c), we have compared the absorbance spectra for the perovskite and respective salts for the three cases individually.In case 2 (Br), the absorbance of the salt is shown in the inset on the log scale to highlight the long wavelength tail in the absorbance.In all the cases, sharp excitonic absorbance features are observed in the case of perovskite systems and these are seen to ride over the absorbance features corresponding to the respective salts.The nature of excitonic absorption features is similar in the three cases, though shifted on the wavelength scale for the I, Br, and Cl cases.This suggests that the inorganic PbX 6 octahedra define the excitonic energy scale, while the organic component broadly defines the photoexcitation.
We have systematically performed Density Functional Theory (DFT) based electronic structure calculations to determine the optical absorption spectra of three synthesized hybrid perovskites (SS)[PbI 4 ] (I-1), (SS)[PbBr 4 ] (Br-2), and (SS)[PbCl 4 ] (Cl-3).The unit cell that represents the crystal structure of systems 1 and 2 consists of total 216 atoms as 8 lead (Pb), 16 sulfur (S), 16 nitrogen (N), 112 hydrogen (H), 32 carbon (C), 32 iodine (I), and 32 bromine (Br), respectively.In case of 3, the total number of atoms in the unit cell is 108 and they are distributed as 4 lead (Pb), 8 sulfur (S), 8 nitrogen (N), 56 hydrogen (H), 16 carbon (C), and 16 chlorine (Cl).The calculated optical absorption spectra (Fig. 4) give an estimation of the optical bandgap of the three systems. 59,60The calculated absorption spectrum for 1 is observed to have a prominent feature at 3.5 eV and subtle shoulders at 2.6 eV and 2.2 eV with a tailing down to 1.8 eV.The shoulder at 2.2 eV corresponds impeccably to the experimental value of 2.3 eV [Fig.4(b)].Similarly, for the case 2, the calculated absorption maximum is seen to be at 4.1 eV along with broader features at 3.5 eV and 2.8 eV.The 2.9 eV feature seen in experimental data [Fig.4(c)] corroborates well with the 2.8 eV calculated signature.In case 3, in the calculated absorption [Fig.4(a)], a prominent feature is seen at 5.8 eV followed by humps at 5.3 eV, 4.9 eV, 4.0 eV, 3.6 eV, and 3.2 eV.The experimental value for this case shows a feature 3.5 eV [Fig.4(d)].Thus the DFT calculated spectra matches reasonably well with the experimentally observed features.
The main observations from the present study pertaining to absorbance data can be summarized as follows: (a) In all three crystal systems 1, 2, and 3, the primary absorption edges as reflected by theory match quite well with the edges for the molecular crystals.(b) The appearance of sharp and clear excitonic absorbance contributions at the respective band edges implies high purity of the crystals studied herein, and that the vibronic states are thereby well defined.(c) In all three cases, sharp excitonic absorbance features are seen to ride over the absorbance features corresponding to the respective salts, though blue shifted on the wavelength scale for the 1, 2, and 3 cases.This suggests that the inorganic PbX 4 octahedra define the excitonic energy scale, while the organic component broadly defines the photoexcitation.
The primary observations pertaining to emission data can be summarized as follows: (a) The excitonic emission structures in the crystalline powders show contributions from free excitons, bound excitons, and self-trapped excitons.
(b) In the case of iodide (I-1), the self-trapped excitonic contribution is not discernible, while it is weak in the case of the bromide case (Br-2).In the chloride case (Cl-3), the self-trapped excitonic contribution is significantly broad and represents the major emission component.It is also significantly Stoke shifted and exhibits white luminescence.(c) The emission features evolve with the evolution of the respective structures, as controlled by the halide ionic size and electronegativity.(d) Interestingly, system 3 shows least structural distortion amongst the three halide cases and, yet, it is the one which shows white luminescence.This brings into question the need of octahedral structural distortions or corrugations for generating white luminescence.
The 2D perovskite family of materials formed under the same experimental conditions using a disulfide organic moiety and lead halides (X = I, Br, Cl) is found to reflect subtle but interesting crystal structure changes due to steric and electronic effects in the organic moiety and the influence of the electronegativity and the ionic radius of the halide ion.Interesting systematics are observed in the excitonic absorption and emission features, which are traced to the possible organic and inorganic contributions.Importantly, the chloride based system (3) with least structural distortion is the one which exhibits broadband (white) luminescence, bringing into question the need of octahedral structural distortions or corrugations for generating white luminescence, as suggested in some recent papers.
The supplementary material contains the synthesis details of single crystals and method of measurement, Secs.S1 and S2.The single crystal XRD, the refinement method (S3), and crystalographic Table SI with related figures.The CIF files can be found at CCDC-724584, CCDC-1841476, and CCDC-1841478.Table SII contains vibrational modes and comparison with the literature.S.K. would like to thank CSIR for fellowship.Funding support from the DST-CERI, DST Nanomission (thematic unit) DST-UKEIRI, and SUNRISE projects is gratefully acknowledged.We thank Dr. S. K. Asha (CSIR-NCL), Dr. Parmod Pillai, and Dr. Anshuman Nag (IISER Pune) for help with the photophysical measurements and Ms. Archana Patil for single crystal XRD studies.For the computing time, we acknowledge PRACE facility.