Broadband white-light emission from a novel two-dimensional metal halide assembled by Pb-Cl hendecahedrons

Broadband white-light emission from a novel two-dimensional metal halide assembled by Pb-Cl hendecahedrons

Item Type Article

Authors Chen, Runan;Gu, Hao;Han, Ying;Yin, Jun;Xing, Guichuan;Cui, Bin- Bin

Citation Chen, R., Gu, H., Han, Y., Yin, J., Xing, G., & Cui, B.-B. (2022).

Broadband white-light emission from a novel two-dimensional metal halide assembled by Pb–Cl hendecahedrons. Journal of Materials Chemistry C, 10(25), 9465–9470. https://doi.org/10.1039/

d2tc01472f Eprint version Post-print

DOI 10.1039/d2tc01472f

Publisher Royal Society of Chemistry (RSC)

Journal JOURNAL OF MATERIALS CHEMISTRY C

Rights Archived with thanks to JOURNAL OF MATERIALS CHEMISTRY C Download date 2024-01-26 17:08:59

Link to Item http://hdl.handle.net/10754/679601

Broadband white-light emission from a novel two-dimensional metal halide assembled by Pb-Cl hendecahedrons

Runan Chen,¹ Hao Gu,² Ying Han,¹ Jun Yin,³ Guichuan Xing,² and Bin-Bin Cui^1,*

Abstract

Abundant combination of organic molecules and inorganic units has allowed organic-inorganic hybrid metal halides (OIMHs) the development of various morphologies and crystal structures with controllable molecular dimensions and unique photoelectric functions. This work reports a unique two- dimensional (2D) OIMH C6H10N2Pb2Cl6 assembled by distinctive lead chloride hendecahedrons (Pb2Cl6)^2- and p-phenylenediamium cations. The extremely large lattice distortion brings this 2D OIMH rich self-trapped excitons (STEs) and strong electron-phonon coupling which are contributed to its broadband “cold” white-light emission with a large Stokes shift (~200 nm) and a remarkable photoluminescence quantum efficiency (PLQE) over 17% in 2D OIMHs.

TOC

1Advanced Research Institute of Multidisciplinary Science, School of Materials Science and Engineering, School of Chemistry and Chemical Engineering, Beijing Institute of Technology (BIT), Beijing 100081, P. R. China. ²Joint Key Laboratory of the Ministry of Education, Institute of Applied Physics and Materials Engineering, University of Macau, Avenida da Universidade, Taipa, Macau 999078, P. R. China. ³Advanced Membranes & Porous Materials Center, KAUST Catalysis Center, Division of Physical Science and Engineering, King Abdullah University of Science and Technology, Thuwal 23955- 6900, Kingdom of Saudi Arabia. Correspondence and requests for materials should be addressed to B.B.C. (email: cui- chem@bit.edu.cn).

Introduction

Recently, new organic-inorganic metal halide (OIMH) crystal materials are constantly emerging and have attracted a lot of attentions,^1-4 as variety organic and inorganic components can be chosen to assemble zero-dimensional (0D),^{5, 6} one-dimensional (1D),^{7, 8} two-dimensional (2D)⁹ and three- dimensional (3D)¹⁰ structures with respectively unique photoelectric applications, such as perovskite solar cells (PSCs),¹¹ light-emitting diodes (LEDs),^{12, 13} photodetectors,^{14, 15} and optically pumped lasers, etc.¹⁶ Furthermore, it is an important advantage that the structure and luminescence properties of the hybrid crystal materials can be tailored at the molecular level.In addition to the color-tunable narrow emission achieved by OIMH perovskite quantum dots, the development of low-dimensional (0~2D) OIMH hybrid materials, especially in single-component white-light emitters¹⁷ and mixed fluorescence (blue,¹⁸ green and cyan) phosphors,^19-22 are essential for creating novelsolid-state light-emitting and display devices.

In general, 2D OIMH crystal materials have high deformation tolerance in the inorganic layer and can be synthesized by simple solution preparation method. Hu et al.²³ induced structural distortion of the inorganic layer by adjusting the substituted positions of methoxy group in organic component, and found that the lattice distortion caused by the coupling of organic cations and inorganic layer would produce broadband emissions. Thus, by controlling the lattice distortion, the emission could shift from blue to white. Karunadasa et al.^{24, 25} reported a family of (110)-oriented corrugated 2D OIMH perovskites, emitting broadband white-light with moderate photoluminescence quantum efficiencies (PLQEs) of 0.5-9%. Fei et al.²⁶ synthesized a class of 2D lead halide layered materials which achieved large Stokes shift and broadband emission with an external PLQE as high as 11.8%. These luminous characters are attributed to short-range electron-phonon coupling in the strongly deformable lattices and thus generated useful self-trapped excitons (STEs). These novel single-component, broadband white-light emitters are promising solid lighting materials due to their tunable optical properties with high PLQEs.

It has been proved that there is an intense relationship between the lattice distortion and electron- phonon coupling in OIMHs, and this relationship determines the difficulties of producing STE states.^27,

28 In general, excluding defect luminescence, the broadband white-light emission of 2D OIMH

perovskites originate from STE states due to lattice deformation and the strong electron-phonon coupling leading to a large Stokes shift.²⁹ Smith et al.³⁰ demonstrated the correlation between lattice distortion and broadband emission in (100)-oriented perovskites, thus high quality broadband white emissions can be achieved by optimizing the crystal structure. The band gap, optical and electronic properties of 2D perovskites, can be altered by tuning the distortion degree of metal halide octahedrons.^31-33 In addition, the degree of octahedral distortion is positively correlated with the average lifetime of time-resolved photoluminescence (TRPL), which can be attributed to the fact that larger distorted structures generate more STE states needing to be filled by excitons.³⁴ Therefore, in order to further expand the application of OIMH materials in optoelectronic devices, it is of great significance to synthesize new OIMH crystals with larger lattice distortions.

Herein, we design and synthesize a new OIMH C6H10N2Pb2Cl6 (PPD)Pb2Cl6 (C6H10N2 (PPD)= p- phenylenediamium), with [Pb2Cl6]^2- hendecahedrons sharing ten edges and three faces to form undulate inorganic layers and the p-phenylenediamium cations are evenly arranged on both sides of inorganic layers to form a stable 2D structure. This unique structure formed large lattice distortions and self-trapped exciton states with strong quantum confinement. Its Huang-Rhys factor (S) is as high as 168, indicating the presence of strong electron-phonon coupling. At room temperature (R.T.), the broadband “cold” white-light emission of its bulk crystal peaks at ~500 nm and has a large Stokes shift (~200 nm) with a competitive PLQE over 17%.In addition, with high color rendition and long-term emission stability in atmospheric conditions (~65% relative humidity), this 2D organic-inorganic hybrid lead chloride OIMH has great application potential in solid-state lighting and displays.

Results and discussion

Single crystal X-ray diffraction (SCXRD) well defines the crystal structure of (PPD)Pb2Cl6 (Figure 1a) and the specific crystal cell parameters are summarized in Supplementary information Table S1.

Powder X-ray diffraction (PXRD) pattern of the ball-milled sample is consistent with the simulated PXRD pattern from the single crystals (Figure S1 of the Supporting Information). In the infrared (IR) spectrum, a stronger interaction between N-H and Cl^- in (PPD)Pb2Cl6 can be found, leading to a broaden emission peak at around 3465 cm^-1 of N-H bonds (Figure S2). A [Pb2Cl6]^2- hendecahedron shares ten edges and three faces with other surrounding hendecahedrons to form the inorganic

frameworks, and the p-phenylenediamium cations are evenly arranged on both sides of an inorganic layer to form a 2D hilly-like structure. The unit cell volume of (PPD)Pb2Cl6 is ~768.56 ų. A parallelogram and ten irregular triangles form a hendecahedron [Pb2Cl6]^2-, and four hendecahedrons form a unit cell, which separates organic cations out of dense inorganic layers. Thermal gravimetric analysis (TGA) test indicates its considerable thermal stability (Figure S3). Meanwhile, it was found that this unique single crystal material is difficult to dissolve in polar solvents (e.g water, ethanol), showing strong environmental stability.

Figure 1 ǀ (a) SCXRD structure of C6H10N2Pb2Cl6 and representation of the lead halide post-perovskite type chain and hydrogen bond between ammonium ion (brown spheres: lead atoms; green spheres: chlorine atoms;

blue spheres: nitrogen atoms; gray spheres: carbon atoms; cyan hendecahedron: [Pb2X62-]; white spheres:

hydrogen atoms; (b) Dry single crystals of C6H10N2Pb2Cl6 under ambient and UV light (300 nm); (c) CIE chromaticity coordinate of the bulk crystal C6H10N2Pb2Cl6.

This OIMH crystal material has a wide spectral emission range and presents an excellent bluish- white emitter. (PPD)Pb2Cl6 has a correlated color temperature (CCT) of 9241K and the Commission International de I’ Eclairage (CIE) chromaticity coordinate of its broadband emission is (0.26, 0.35).

Under the excitation of near ultraviolet 300 nm shows high color reproducibility and a high PLQE of 17.17% (Figure 1b, c) at room-temperature, which is a significant white emission compared to the reported 2D perovskites (Table S2).

In the low-dimensional OIMH single crystal, the inorganic layers and organic cations are alternately

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

y-chromaticity coordinate

x-chromaticity coordinate PPD-Cl

b

c

PLQE=17%

hendecahedron

a

arranged to form a natural quantum well structure, and the dielectric constant of the two is different, forming a dielectric confinement effect.^{35, 36} Owing to dielectric confinement, sharp excitonic peaks⁶ can be observed in absorption spectra for (PPD)Pb2Cl6. According to this data, we can estimate its band gap of 3.81 eV by extrapolating the slope of the high-energy absorption edge to the imaginary axis parallel to the X-axis (Figure S4).³⁷ The distortion of the lattice structure affects the luminescent properties of OIMH materials to some extent. Distortion of the hendecahedron [Pb2Cl6]^2- building blocks that comprise the inorganic layers were evaluated using the following parameters.³⁸

Bond length distortion: 𝛥_𝑜𝑐𝑡 = ¹

8∑ [^{𝑑𝑖−𝑑𝑚}

𝑑_𝑚 ]²

8 𝑖=1

(1) Hendecahedral angle variance: 𝜎_𝑜𝑐𝑡² = ¹

19𝛴_𝑖̇=1¹⁹ (𝛼_𝑙̇− 90)² (2)

where dm is the average bond length of Pb-Cl and di are the lengths of individual Pb-Cl bonds in an individual hendecahedron [Pb2Cl6]^2-. Δoct is calculated to be 4.07310^-2 and the calculated hendecahedral angle variance (σ²oct) is 252.6 for (PPD)Pb2Cl6, suggesting that the crystal has distortions with a tremendous degree.The corresponding bond length and angle data are indicated in support information Table S3 and Figure S10.

Figure 2 ǀ (a) Excitation spectra at room temperature and emission spectra at room temperature and 77K for (PPD)Pb2Cl6; (b) Temperature dependent PL spectra of 2D (PPD)Pb2Cl6 crystals; (c) Transient absorption

spectrum upon photoexcitation at 325 nm; (d) Time-resolved PL experimental decay (circle) and fitting curve of (PPD)Pb2Cl6 (blue line) probed at 500 nm; (e) FWHM of broadband emission vs temperature with a solid line fit (FWHM at each temperature is represented by a circle); (f) Fitting of Raman data 50-300 cm^-1 for thin films under excitation at 532 nm.

Strong coupling of excited electrons/holes to lattice deformations leads (PPD)Pb2Cl6 to exhibits a broadband emission with a significant Stokes shift of ~200 nm,²⁷ and the distortion of hendecahedron [Pb2Cl6]^2- is conducive to the formation of STEs, determining the production of broadband emissions.

As shown in Figure 2a, the broadband emission presents regular changes with temperatures, and this is related to the capture degree of excitons. With the temperature decreasing from 300 to 77K, the emission in temperature dependent photoluminescence (PL) spectra becomes narrower and narrower, and the peak position has a significant red shift, which can be attribute to the reduction of non-radiative recombination centers (Figure 2b).²⁹ To better investigate the principle of its luminescence and establish that the broadband emission is not derived from defects, we grinded the single crystal into powder for testing their PLspectrum. In result, the emission of lamellar crystals and ball-milled power are nearly identical (Figure S5). This well rules out the possibility that the emission originate from surface defects. Moreover, we recrystallized the (PPD)Pb2Cl6 crystals to passivate defects, avoiding that the broadband emission is caused by missing elements induced intrinsic defects. Comparation of PL spectrum (Figure S6a) and the PXRD result (Figure S6b) of the recrystallized crystal with that of the reference sample indicates that crystals after defects passivation have same structures and emission properties with the untreated samples. This experimentally excludes the intrinsic defects luminescence.^{39, 40} In addition, the more direct evidence of that excitons are self-trapping in STE states rather than defect states comes from transient absorption data (TA).^{41, 42} The TA spectrum of (PPD)Pb2Cl6 single crystal wasmeasured as shown in Figure 2c. A broad pump-induced absorption with lower energy than that of free exciton (FE) states was observed in (PPD)Pb2Cl6 crystals under the excitation of 325 nm light with low pulse energy of 14 μJ·cm^-2. (PPD)Pb2Cl6 exhibits extensive absorption below the exciton energy extending to the visible range. This is the feature signal of STE states, further confirming the existence of the STE states of (PPD)Pb2Cl6. The luminescent decay of this white-light emitter at RT is displayed in Figure 2d. By the tri-exponential decays fitting, (PPD)Pb2Cl6 shows a relatively long lifetime of approximately 1.6 μs. It is of interest to note that the

average lifetimes increase to 9.6 μs when the temperature decreased to 77K (Figure S7).These data suggest that the luminescence of crystal originates from STEs is reasonable.

One of the most important factors affecting lattice vibration is temperature. The higher the temperature, the stronger the lattice vibration and the more frequent the energy exchange with electrons-holes.⁴³ By studying the photoluminescence spectra at different temperatures, the competitive relationship between the STE states and FE states can be explored. This is particularly true for the present material (Figure S8), as the broadband emission is almost exclusively reflected during the temperature dependent PL spectra collection,(PPD)Pb2Cl6 happens lattice distortion witha large degree and so most FEs turn into STEs, resulting in the disappearance of the FE emission.The full width at half maximum (FWHM) narrows along with decreasing temperature, and the intensity of the broadband emission greatly enhanced at the same time. Typically, nonradiative recombination is suppressed at low temperatures and therefore the PL intensity increases with the decreasing temperatures. Conversely, as the temperature increases, nonradiative recombination is enhanced and the coupling of phonons becomes stronger and the number of de-trapped excitons from STE states to FE states increased, leading to a decreased PL intensity.^{30, 44} The FWHM is linearly related to temperatures and the FWHM is about 177 nm (0.85 eV) at RT (Figure S9), which is in accordance with existing research.²⁹ Additionally, the Huang-Rhys factor (S) reflects the strength of electron- phonon coupling. It can be obtained by fitting the temperature-dependent FWHM of photoluminescence peaks using the following equation:

FWHM = 2.36√Sћw_phonon√cot⁡(ћw_phonon 2K_BT )

Here, ћ𝑤_{𝑝ℎ𝑜𝑛𝑜𝑛} is the phonon frequency, and K_B is the Boltzmann constant. The Huang-Rhys factor for (PPD)Pb2Cl6 is calculated to be 168 (Figure 2e), which represents the stronger electron coupling to phonons compared with other common emitters, such as Cs2AgInCl645 and (C7H7N2)2PbBr4,⁴⁶ indicating the easy formation of STEs in (PPD)Pb2Cl6.

As shown in Figure 2f, Raman spectrum in the range of 50~300 cm^-1 which was obtained by Lorentz fitting could correspond to the character of the inorganic frameworks and some features due to the influence of organic molecules. For examples, the mode at 61 cm^-1 represents the bending vibration of

Cl-Pb-Cl and the characteristic mode at 92 cm^-1 represents the stretching vibration of Pb-Cl.

Additionally, the mode at 169 cm^-1 represents the vibration of organic molecules induced by the electron coupling from inorganic layers. The lattice distortion in (PPD)Pb2Cl6 mainly originates from the irregularity of the hendecahedrons, which most likely corresponds to the strong STE emission phenomenon.⁴⁷ A comparison of the FWHM and intensity at different bands of the Raman spectrum is summarized in the Supporting Information Table S4.

Figure 3 ǀ (a) Calculated band structure and projected density of states (PDOS) of (PPD)Pb2Cl6 calculated at HSE06+SOC level of theory; (b) Schematic of the photophysical processes in a metal halide single crystal.

To understand the electronic structure and emission mechanism of (PPD)Pb2Cl6, we further performed density functional theory (DFT) calculations by combining the hybrid functional (HSE06) and spin-orbit coupling (SOC). As shown in Figure 3a, the calculated indirect band gap is 3.82 eV, matching well with the experimental value (3.81 eV). From the projected density of states (PDOS), we find that the valence band maximum (VBM) is composed of both Cl-3p and Pb-6s states, while the conduction band minimum (CBM) is dominated by the Pb-6p state. In addition, the band dispersions along the tube direct for top VB and bottom CB are small because of weak interactions between Pb- Cl hendecahedrons separated by organic spacers.^{48, 49} Therefore, we can propose a hypothesis in photophysical process for this 2D crystal (Figure 3b): Upon optical excitation, FEs are excited from the ground states to the excited states, and then due to the large degree of lattice distortion and strong electron-phonon coupling, FEs are trapped and stabilized at the multiple intermediate energy levels, namely, the STE states which are prone to form broadband emissions.

Conclusions

In conclusion, a novel organic-inorganic lead chloride hybrid (PPD)Pb2Cl6 with bluish-white emission was prepared via a simple solution phase synthesis method. Lead chloride hendecahedrons are closely connected in face- and edge- sharing modes to form 2D inorganic layers. The p- phenylenediamine cations are uniformly distributed on both sides of the inorganic layers. This unique 2D structure has abundant self-trapped states with strong quantum confinement, and the inside lead- chloride hendecahedrons have distortions with a large degree in crystal lattices, where excitons are strongly coupled with crystal lattices, producing a wide emission with a large Stokes shift of ~200 nm.

At R.T., the broadband emission of the bulk crystal peaks at 500 nm with the highest PLQE of approximately 17%, representing a potential alternative to commercial bluish-white phosphors.The discovery further enriches the OIHM family and expands the structure-function relationship of new white light-emitting OIHMs with high emitting efficiencies.

ASSOCIATED CONTENT Supporting Information

Experimental methods, crystallographic data, optical spectra, and thermal analysis. This material is available free of charge via the Internet at http://XXXXXX. CIF for (PPD)Pb2Cl6 has been deposited in the Cambridge Crystallographic Data Centre under deposition numbers CCDC 2131056.

AUTHOR INFORMATION Corresponding Author cui-chem@bit.edu.cn

Author contributions

B.B.C. and R.C. conceived the two-dimensional OIMHs and summarized their measurement data. R.C.

synthesized and produced the bulk crystal (PPD)Pb2Cl6 and collected Powder XRD,IR spectrum, Raman spectrum and TGA data; R.C. measured the photophysical and photoluminescence (PL) properties with the help of Y.H.; H.G. performed TA measurement; R.C. analyzed the SCXRD, TA

data with the help of Y.H. and H.G.; J.Y. and G.X. were in charge of the part of density functional theory (DFT) calculations. The manuscript was mainly written and revised by B.B.C. and R.C. This project was directed and supervised by B.B.C. All authors discussed the results and commented on the manuscript.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was supported by funding from the National Natural Science Foundation (22075022 and 21703008) and the “Cultivate Creative Talents Project” of Beijing Institute of Technology (BIT).

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