Source: https://pubs.rsc.org/en/content/articlehtml/2018/sc/c7sc04118g
Timestamp: 2019-04-19 21:01:13+00:00

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Herein, we report the unusual broadband white-light emission as an intrinsic property from two cationic lead bromide frameworks. This is the first time that the metal halide materials adopting a purely inorganic positively-charged three-dimensional (3D) topology have been synthesized, thus affording highly distorted PbII centers. The single-component white-light emitters achieve an external quantum efficiency of up to 5.6% and a correlated color temperature of 5727 K, producing typical white-light close to that of fluorescent light sources. Unlike the air/moisture-sensitive 3D organolead halide perovskites, our cationic materials are chemically “inert” over a wide range of pH as well as aqueous boiling condition. Importantly, these long-sought ultrastable lead halide materials exhibit undiminished photoluminescence upon continuous UV-irradiation for 30 days under atmospheric condition (∼60% relative humidity, 1 bar). Our mechanistic studies indicate the broadband emission have contributions from the self-trapped excited states through electron-vibrational coupling in the highly deformable and anharmonic lattice, as demonstrated by variable-temperature photoluminescence/absorption spectra as well as X-ray crystallography studies. The chemical robustness and structural tunability of the 3D cationic bromoplumbates open new paths for the rational design of hybrid bulk emitters with high photostability.
Hybrid inorganic–organic lead halide perovskites containing vertex-sharing metal halide octahedra are an emerging class of photoactive materials, which have promising applications in photovoltaics and light-emitting devices.14,15 Among them, two-dimensional (2D) organolead halide perovskites usually exhibit narrow-band photoluminescence owing to their flat (100)-oriented layers and large exciton binding energy.16 Recently, a few instances of (110)-oriented 2D perovskites showed broadband photoemission as an intrinsic property, presumably ascribed to the formation of self-trapped excited states (e.g. Pb23+, Pb3+, X2−) (X = Cl, Br, I).17–24 Lowering the dimensionality of lead halide facilitates the self-trapping process and enhances the photoluminescence quantum efficiency (PLQE), albeit with a sacrifice in structural stability and photoluminescence tunability.25–29 The air/moisture-sensitive nature of organolead halide perovskites results in a gradual decrease of photoemission intensity upon UV-irradiation in air, thus hindering their industrial applications in LED technology.30–32 Therefore, the development of chemically ultrastable lead halide materials with efficient, tunable and broadband white-light emission is crucial to extend their photoactive applications.
Very recently, we reported a class of 2D (layered) cationic lead halide materials with high chemical resistance and high photoluminescence quantum efficiency.47 Herein, we report the synthesis, crystal structures and broadband photoluminescence of the first two 3D cationic metal halide frameworks, [Pb2Br2][O2C(CH2)4CO2] and [Pb3Br4][O2C(CH2)2CO2] (which we denote as TJU-6 for [Pb2Br2]2+ and TJU-7 for [Pb3Br4]2+, TJU = Tongji University). The unique positively charged 3D lead bromide networks define the arrays of unidimensional channels, in which reside the bridging organic anions. Intriguingly, both materials are intrinsically bulk white-light emitters spanning the entire visible-light spectrum. In contrast to lead halide perovskite and other hybrid bulk emitters, our cationic materials are essentially unaffected upon near-UV-irradiation (365 nm) for 30 days under ambient condition (∼60% relative humidity, 1 bar). The temperature-dependent photophysical studies (e.g. UV-vis absorption spectra and photoluminescence spectra) and X-ray crystallography studies attribute the broad emission to the self-trapped states from electron-vibrational coupling in the strongly deformable and anharmonic lattice.
where αi are the X–Pb–X (X = O/Br) angles. The PbBr3O3 units in TJU-6 have a large structural distortion (σoct2 = 735.0), which probably result from the inert pair of s2 electrons as well as the different electronegativity of oxygen and bromine atoms.
Fig. 1 Crystallographic view of TJU-6 and TJU-7: (a, b) ball-and-stick view of TJU-6 (a) and TJU-7 (b), with organic anions omitted for clarity. (c, d) Polyhedral view of TJU-6 (c) and TJU-7 (d). Cyan polyhedra represents PbBr3O3 in TJU-6 and PbBr4O2/PbBr2O2 in TJU-7, respectively. Carbon and hydrogen atoms are omitted for clarity.
The edge-sharing PbBr3O3 octahedra in TJU-6 are connected in both of the crystallographically (100) and (010) planes, defining the honeycomb arrays of 6-membered ring channels along the a- and b-axis, respectively (Fig. 1c and S3†). Adipates covalently bridge the lead centers within the hexagon-shaped channels, and the Pb–O bond lengths (2.505–2.664 Å) are well within the accepted Pb–O covalent range. The two crystallographically independent Br atoms are either vertex-bridging (μ2-Br) or quadruply bridging (μ4-Br) in a highly distorted tetrahedral geometry. Overall, the high-coordinated bridging Br atoms and the low-coordinated Pb centers collectively attribute to the cationic feature of a rare 3D metal halide topology.
Non-perovskite metal-halide hybrid materials usually have a low-dimensional inorganic extended structure (1D or 2D). In addition, all of the previously reported 3D lead halide examples occupy an anionic inorganic network hosting organoammonium cations.45,46,50,51 Overall, TJU-6 and TJU-7 are the first two lead halide materials with a cationic 3D M–X–M (M = metal, X = halogen) connectivity that have been unambiguously identified by single-crystal X-ray crystallography.
The high yield and phase purity of TJU-6 and TJU-7 was evidenced by Fourier-transform infrared spectroscopy (FTIR), elemental analysis and powder X-ray diffraction (PXRD), which matches well with the theoretical patterns simulated from single-crystal data (Fig. 2, S6 and S7†). Unlike organoammonium cations in perovskites, the anionic structure-directing agents (e.g. α,ω-alkanecarboxylate) covalently crosslink the metal centers within the pore channels of the cationic inorganic host. This structural feature plays a significant role to afford the chemical “inertness” of the resulting lead bromide materials. Stability tests were performed by treating the materials in water, ethanol, HCl solution (pH = 2), and NaOH solution (pH = 12) for 24 h. PXRD of the post-treated TJU-6 and TJU-7 remained intact, confirming the well-retained cationic topology (Fig. 2). In addition, no apparent loss in mass was observed during the chemical treatment, further proving the chemical inertness of TJU-6 and TJU-7. Moreover, thermogravimetric analysis (TGA) and ex situ thermodiffraction indicate that TJU-6 and TJU-7 are thermally stable up to 250 °C under air (Fig. 2, S8 and S9†). Overall, our cationic 3D bromoplumbate materials strikingly push forward the chemical-resistance and related photoactive applications of organolead halide hybrid materials.
Fig. 2 PXRD of TJU-6 (a) and TJU-7 (b) before and after heat or chemical treatment.
Given the high robustness and lattice deformation of TJU-6 and TJU-7, we sought to investigate their photoluminescent properties (Table 1). Both materials show a typical semiconductive properties with an optical bandgap of 3.70 eV (335 nm) for TJU-6 and 3.50 eV (354 nm) for TJU-7, respectively, evidenced by UV-vis absorption spectra (Fig. 3a and b). The large band-gaps probably result from the open inorganic framework of [PbBr]+ and [Pb3Br4]3+, which leads to an increase in the bandgap and the energy level of conduction band.52 In addition, excitonic features (the shoulder peak at 3.99 eV for TJU-6 and 3.90 eV for TJU-7) are observed in the optical absorption spectra. The less-defined excitonic peak is largely ascribed to the moderate exciton binding energy (290 meV) from the absorption spectra of TJU-6 at 103 K (Fig. S10†).
a λ abs is the wavelength at absorbance maximum; λex is the excitation wavelength; λem is the wavelength at the emission maxima; φ; is the external photoluminescence quantum efficiency; τav is the PL lifetime.
Fig. 3 (a, b) Emission spectra of the cm-sized bulk crystals (blue) and μm-sized microscopic crystals (red) of TJU-6 (excited at 360 nm) and TJU-7 (excited at 370 nm). (c) Photoimages of TJU-6, TJU-7, PbBr2 and adipate under 4 W 365 nm UV-light. (d) CIE chromaticity coordinates of TJU-6 and TJU-7.
Interestingly, colorless block-shaped single crystals of TJU-6 show a pronounced white-light emission upon irradiation with a 4 W, 365 nm UV-lamp (Fig. 3c). The photoemission spectra (under 360 nm excitation) of TJU-6 demonstrates the unusual broadband photoluminescence spanning the entire visible spectrum (400–700 nm). The strongly Stokes-shifted broadband emission of TJU-6 has a maximum at 530 nm with a large FWHM up to 124 nm (0.56 eV), significantly overcoming the self-absorption problems in the near-UV region. In addition, μm-sized particles of TJU-6 were prepared via manual grinding, and showed nearly identical photoemission spectra (Fig. 3 and S11a†). These results confirm that the broadband emission arises from the bulk materials instead of from surface defect sites (which often contribute to the photoluminescence of lead halide perovskite nanocrystals53,54). The high-energy shoulder at 420 nm is more evident in microscale crystals, likely due to the strong self-adsorption in cm-sized single crystals.25 Since the two-band emission profiles are similar to those of lead halide perovskites,17,18 it is reasonable to attribute the high-energy shoulder to free excitons and the low-energy broadband emission to self-trapped excitons. The detailed photoemission mechanism will be discussed later. Time-resolved photoluminescence decay experiments indicate the lifetime of the emission measured at 537 nm is 1.7 ns for TJU-6 (Fig. 4a). The ns-ranged lifetimes are characteristic of fluorescence emission, while the longer lifetime of TJU-6 over the previously reported 2-D lead-based perovskites (τav = 0.23–1.39 ns)17–20 is presumably attributed to the populated self-trapped excitons. Moreover, the cm-sized single crystals and μm-sized particles of TJU-6 show similar lifetimes, again suggesting the broadband emission as an intrinsic effect. The strongly Stokes-shifted broadband photoemission covering the entire visible-light spectrum has also been observed in TJU-7. A more evident high-energy photoemission shoulder at 415 nm blue-shifts the maximum emission wavelength to 480 nm with a large FWHM up to 166 nm (0.88 eV).
Fig. 4 (a) The photoluminescence decay of TJU-6 (measured at 537 nm) at room temperature. (b) Photoemission spectra of TJU-6 before and after continuous UV-irradiation (4 W, 365 nm) under atmospheric condition (∼60% relative intensity, 1 bar, room temperature). (c) Temperature-dependent UV-vis diffuse reflectance micro-spectrum of TJU-6 from 133 to 293 K. (d) Temperature-dependent photoemission spectra of TJU-6 from 77 K to 473 K. (e) Integrated photoluminescence intensity (red) and FWHM (blue) as a function of temperature. (f) Schematic summarizing the main mechanism responsible for broadband emissions in the cationic lead bromide frameworks.
Importantly, no apparent change to the color properties of the broadband emission is observed in TJU-6 when adjusting the excitation from 320 nm to 360 nm, confirming its nature as a broadband monochromatic emitter (Fig. S12†). The Commission International de l’Eclairage (CIE) chromaticity coordinates of the overall TJU-6 emission is determined to be (0.33, 0.48), corresponding to a color temperature of 5727 K (Fig. 3d). The emission is so-called “cold” white-light, and very close to that of conventional fluorescent light sources (∼5100 K). Meanwhile, the μm-sized crystals of TJU-6 exhibit a CIE coordinate of (0.30, 0.45) with a color temperature of 6464 K (Fig. 3d). The external quantum efficiency of the TJU-6 is measured to be 5.6%, which is significantly higher than most white-light emitters based on 2D organolead halide perovskites as well as a 3D anionic lead chloride framework (Table S3†). In addition, the photoemission band and intensity was monitored upon irradiation with a 4 W, 365 UV lamp under atmospheric condition (∼60% relative intensity, 1 bar, room temperature). Intriguingly, the broadband emission is remarkably stable after UV-irradiation in air for 30 days, achieving a substantial advance in hybrid lead halide white-light emitters (Fig. 4b).
The TJU-7 light emission was determined to have CIE chromaticity coordinates of (0.25, 0.32) for bulk cm-sized crystals and (0.25, 0.33) for μm-sized crystal particles, which are all closer to the coordinates of the sunlight source (0.33, 0.33) (Fig. 3d). The bluish white-light emission affords the correlated color temperature of 11 967 K for cm-sized single crystals and 10 640 K for μm-sized particles, respectively. The external PLQE of the TJU-7 was measured to be 1.8% with an expected lower efficiency from the dense [Pb3Br4]2+ framework.
where Eg(T) is the energy gap at T K; β is physically associated with the Debye temperature, and γ is the temperature coefficient of the band gap. The best fit is obtained with Eg(0) = 3.815 (5) eV, γ = 0.55(5) meV K−1, and β = 135(35) K (Fig. S13†). The value of β = 135(35) K for TJU-6 is within the reasonable range of Debye temperature for PbBr2 between 100 and 150 K.58–60 The temperature coefficient of the band gap (γ = 0.55(5) meV K−1) is higher than some other typical semiconductors (e.g. InN),61 suggesting the possible influence of thermal expansion and/or electron–phonon interaction on the band gap in our cationic bromoplumbate materials. A small thermal expansion of the unit cell volume from 1160.5(3) Å3 (150 K) to 1166.5(3) Å3 (298 K) was observed using single-crystal X-ray crystallography (Tables S1 and S4†). In addition, the anisotropic thermal expansion was noticed with stronger elongation (0.25%) along the c-axis than that of the other two directions (0.13% for the a- and b-axis). Despite the contribution of the anharmonic lattice, the high temperature coefficient of the band gap (γ = 0.55(5) meV K−1) is largely attributed to the strong electron–phonon coupling.
We have discovered two hybrid bromoplumbate bulk emitters with an unusual broadband white-light emission spanning the entire visible-light region. These are the first two 3D lead halide frameworks possessing an overall positive charge, thus exhibiting high distortion of PbII centers and generated self-trapped states. Upon 360 nm excitation, the [PbBr]+ exhibit a moderately high PLQE of 5.6%, exceeding most of the perovskite-type white-light emitters (<1%). Importantly, the ultrastable nature of our materials affords undiminished photoemission throughout UV-irradiation for 30 days under air, largely overcoming the chemically labile problems of 3D lead perovskites. Our mechanistic studies confirm that the broadband emission arises from electron–phonon coupling in a strongly deformable and anharmonic lattice, which contributes to the formation of self-trapped states. The long-sought “inert” lead halide materials are amenable to synthetic design for enhancing PLQEs, thus serving as potential alternatives to commercial white-light LED phosphors.
This work was supported by grants from the National Natural Science Foundation of China (21501136, 51772217), the Recruitment of Global Youth Experts by China, the Fundamental Research Funds for the Central Universities, and the Science & Technology Commission of Shanghai Municipality (14DZ2261100).
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