Pressure-induced robust emission in a zero-dimensional hybrid metal halide (C₉NH₂₀)₆Pb₃Br₁₂

Organic–inorganic hybrid metal halides are promising materials for use in advanced optoelectronics applications owing to their unique structural and physical properties, such as soft lattices, adjustable bandgap, and high absorption coefficient.^1–3 Dimensionality engineering at molecular levels has been demonstrated to be an effective route for increasing the diversity of their architectures and improving their stability as the optically active inorganic clusters are tailored by hydrophobic cations.^4–6 Recently, low-dimensional (low-D) metal halides have drawn great interest for their outstanding properties, including broadband emission from self-trapped excitons (STEs), potential high emission efficiency, and large Stokes shift.^7,8 Especially for the zero-dimensional (0D) metal halides, near-unity photoluminescence quantum yield (PLQY) has been achieved by optimizing compositions and local structures.^9,10 Current research focuses mainly on chemical tailoring, such as cation engineering, which is limited by tunability and may introduce a variety of influencing factors.^11–13 A systematic understanding of how structure tuning affects optical properties is still lacking, and there is therefore an urgent need for the development of advanced methods for regulation and in situ characterization.

It is well known that exciton emission properties depend strongly on lattice and electronic characteristics, such as structural distortions of polyhedral frameworks and the exciton binding energy.^14–16 Thus, the exciton emission can be effectively modulated by controllably regulating the lattice and electronic features using chemical methods or external stimuli like temperature and pressure.^17–19 The application of pressure is an effective and clean way to tune the structures and properties of functional materials.^20–23 Using diamond anvil cells to apply high pressure, the bond distances and structural distortions of soft low-D metal halides can be effectively adjusted and their photophysical process and optical properties can be monitored in situ. There has recently been an increase in the number of studies focusing on pressure-enhanced and/or induced emission in low-D metal halides, although it is known that emission will be quenched at higher pressures. So far, no system has been found that can exhibit photoluminescence (PL) at pressures above 30 GPa.^24,25 The microscopic origin of the enhancement also remains unclear.

In this work, we report pressure-induced, enhanced, and robust emission up to 80 GPa in a 0D hybrid metal halide (C₉NH₂₀)₆Pb₃Br₁₂, hereinafter referred to as (bmpy)₆[Pb₃Br₁₂] (where bmpy = 1-butyl-1-methylpyrrolidinium), which possesses linear [Pb₃Br₁₂]⁶⁻ trimer clusters (face-sharing octahedra) isolated by organic cations.^13,26 Using pressure regulation in combination with in situ structural and optical characterization, we have systematically investigated pressure-dependent photophysical properties. The pressure-induced emission (PIE) is attributed to the largely suppressed phonon-assisted nonradiative pathway. The exciton–phonon interaction is considerably reduced owing to the phonon hardening that occurs upon compression. Impressively, the emission of (bmpy)₆[Pb₃Br₁₂] is still observable even at 80 GPa, which is the highest-recorded emission pressure for any kind of hybrid metal halide material. Such a robust emission is mainly due to the stiffness of the spring-like [Pb₃Br₁₂]⁶⁻ trimers.

To investigate the evolution of the emission properties of 0D (bmpy)₆[Pb₃Br₁₂] under high pressure, in situ PL measurements were performed, as shown in Fig. 1. The emission is undetectable at ambient pressure, and a pressure-induced emission (PIE) centered at around 650 nm with a full width at half maximum (FWHM) of 175 nm can be observed at 1.6 GPa [inset in the left panel of Fig. 1(a)]. Such a broadband emission in this material originates from the excitons resulting from the strong electron–phonon coupling in the completely isolated individual [Pb₃Br₁₂]⁶⁻ trimer clusters. As the pressure increases, the PL shows a remarkable enhancement and reaches a maximum at around 26 GPa. Upon further compression, the PL intensity gradually decreases with further compression [Fig. 1(a), right panel]. Figure 1(b) shows the integrated PL intensity of (bmpy)₆[Pb₃Br₁₂] as a function of pressure. Strikingly, the emission can still be detected even at 80 GPa [inset in the right panel of Fig. 1(a)], a record pressure for emission in hybrid metal halides, indicating the extraordinary structural stability of this hybrid system. Such an unusually robust emission is likely due to the unique trimer clusters of this 0D metal halide, which we will elaborate on later. Besides, the fluorescence micrographs in Fig. 1(c) and in Fig. S1 in the supplementary material clearly reveal pressure-induced PL variations.

Ultraviolet–visible (UV-vis) absorption spectroscopy was utilized to elucidate the bandgap evolution of (bmpy)₆[Pb₃Br₁₂] under high pressure [Fig. 2(a) and Fig. S2 in the supplementary material]. At ambient pressure, a sharp absorption edge can be found at around 350 nm upon UV irradiation. During compression, the absorption edge exhibits a continuous red shift up to 80 GPa. The bandgap evolution of (bmpy)₆[Pb₃Br₁₂] is presented in Fig. 2(b), from which it can be seen that the bandgap narrows from its initial value of 3.55–2.56 eV at 80 GPa. Density functional theory (DFT) calculations of electronic structures confirm the highly localized electronic states of isolated [Pb₃Br₁₂]⁶⁻ trimer clusters and the pressure-induced bandgap narrowing (Fig. S3 in the supplementary material) due to the increased overlapping of Pb and Br electron clouds with the shortening of Pb–Br bonds.²⁷ The highest occupied crystal orbital is contributed by the Pb-6s and Br-4p states, while the lowest unoccupied crystal orbital consists mainly of Pb-6p and Br-4p states.²⁸ By linearly fitting the pressure-dependent bandgaps, the bandgap can be tuned by less than 12.1 meV/GPa, which is lower than for the reported 0D metal halides as well as typical 3D, 2D, and 1D compounds [Fig. 2(c)], indicating a less compressible nature of the optically active inorganic unit. Optical micrographs show the color changes of the sample during compression [inset in Fig. 2(b) and Fig. S4 in the supplementary material].

To better understand the pressure-induced, enhanced, and robust emission of this material, in situ x-ray diffraction (XRD) measurements were performed up to 80 GPa. Under ambient conditions, (bmpy)₆[Pb₃Br₁₂] crystallizes in a trigonal structure with space group R$3¯$ and unit cell parameters a = 17.31 Å and c = 22.51 Å.²⁶ The [Pb₃Br₁₂]⁶⁻ oligomers are composed of three face-sharing [PbBr₆]⁴⁻ octahedra, and the adjacent inorganic clusters are completely isolated from each other by the surrounding large hydrophobic N-alkylpyrrolium cations [Fig. 3(a)]. XRD patterns at selected pressures are shown in Fig. 3(b) and in Fig. S5 in the supplementary material, where all Bragg diffraction peaks shift to higher two-theta angle with increasing pressure owing to the lattice contraction. Impressively, two main diffraction peaks, which belong to the (−1,1,1) and (−1,2,0) planes, can still be observed even at 80 GPa. No amorphization is found at such high pressures, indicating an extremely tough structure with low compressibility of the trimer clusters.

The crystal structures under high pressure were refined using the Rietveld method (Fig. S6 in the supplementary material).³⁷ The variations of the lattice constants are shown in Fig. 3(c) and in Table S1 in the supplementary material, and they indicate anisotropic compressibility and three-step variation during compression. With increasing pressure, the a/c ratio increases rapidly below 2 GPa, which is caused by the highly compressible organic parts and the relatively low packing factor along the c axis. Further increases in pressure induce an identical compression owing to the similar compressibility of the organic cations and the Pb–Br oligomers between 2 and 12 GPa. Above that, the a/c ratio continues to increase because of the different compressibilities of the octahedra along the stacking direction and those perpendicular to it. Using the Birch–Murnaghan equation of state (EoS), the pressure-dependent unit-cell volume yields bulk moduli K₀ of 13.8 and 80.8 GPa for the low- and high-pressure ranges, respectively [Fig. 3(d)].³⁸ Such a high bulk modulus is unusual compared with the reported values for high-pressure phases of other metal halides, such as the 0D hybrid (bmpy)₉[ZnBr₄]₂[Pb₃Br₁₁] (25 GPa) and the 1D inorganic CsCu₂I₃ (63 GPa).^35,39 The relatively high stiffness of (bmpy)₆[Pb₃Br₁₂] at high pressure is presumably due to the mechanical spring-like behavior of the [Pb₃Br₁₂]⁶⁻ trimers, which is similar to what occurs for nanohelices and the pTPE-AN dimer with antiparallel π–π stacking.^40,41 The claw-shaped spring frames play a role of buffering and damping, suppressing excessive deformation of the [Pb₃Br₁₂]⁶⁻ clusters and stabilize the inorganic units.

To the best of our knowledge, almost all the hybrid metal halides possess soft lattices, which impart a highly compressible structure whether they are 3D, 2D, 1D, or 0D in nature. When the pressure exceeds a certain threshold, the structure can no longer hold, resulting in amorphization. The emission will quench during crystalline structural damage. To date, no emission from these materials can survive above 30 GPa. As is summarized in Fig. 4(a) and Table I, the PL quenching pressure value of (bmpy)₆[Pb₃Br₁₂] is as high as 80 GPa, far higher than that of any other low-D metal halide.^{14,24,35,39,42,43} Such a robust PL of (bmpy)₆[Pb₃Br₁₂] could be attributable to its relatively high structural stability discussed above.

To obtain the local bonding information under high pressure, in situ Raman spectra were collected, as shown in Fig. S7 in the supplementary material. Under ambient conditions, a Raman-active mode at around 129.4 cm⁻¹ can be observed, which is associated with the symmetric stretching vibration of Pb–Br bonds. With increasing pressure, the right shift of the Raman peak indicates hardening of the phonon modes. We calculated the corresponding phonon energy as a function of pressure [Fig. 4(b)], which shows a continuous increase upon compression, suggesting reduced exciton–phonon coupling.^46,55

The strong exciton–phonon coupling gives rise to localized excitons and leads to the broadband emission in low-D metal halides. However, an overly strong exciton–phonon coupling would lead to overlap between the excited-state (ES) curve and the ground-state (GS) one, resulting in phonon-assisted nonradiative recombination of the excited electrons and holes.¹⁵ As has been reported, a relatively large Stokes shift exists in the low-D metal halides with face-sharing octahedra compared with the edge-sharing and corner-sharing compounds.^{12,13,56–58} As shown in Fig. 4(b), (bmpy)₆[Pb₃Br₁₂] exhibits a large Stokes shift of 1.7 eV at 1.6 GPa, which decreases rapidly during compression. DFT calculations on electronic structures of (bmpy)₆[Pb₃Br₁₂] reveal higher dispersive frontier orbitals under high pressures, which indicates reduced exciton localization (Fig. S3 in the supplementary material). The mechanism of pressure-induced emission is illustrated in Fig. 4(c). The severe structural distortion of the face-sharing frameworks brings strong exciton–phonon interaction in (bmpy)₆[Pb₃Br₁₂] at ambient conditions, raises the GS energy, and thus results in crossover between the ES and the GS. Consequently, the excitons can easily annihilate by the phonon-assisted pathway, leading to a highly nonradiative energy transfer and undetectable PL under ambient conditions. Upon compression, the lattice contraction leads to phonon hardening, which considerably weakens the exciton–phonon interaction and thus reduces the wavefunction overlap between the ES and the GS. Therefore, the phonon-assisted nonradiative loss is greatly suppressed, giving rise to exciton emission under certain high pressures.

In summary, pressure-induced emission has been revealed in the 0D hybrid metal halide (bmpy)₆[Pb₃Br₁₂], with PL emerging at around 1.6 GPa and surviving to a record high pressure of 80 GPa among all kinds of hybrid metal halides. Through comprehensive in situ high-pressure characterization and first-principles calculations, the pressure-induced emission has been demonstrated to be caused by suppression of the phonon-assisted nonradiative pathway. Lattice compression hardens the phonons and reduces the exciton–phonon interaction. The robust emission is attributed to the tough sublattice of optically active spring-like trimers [Pb₃Br₁₂]⁶⁻, indicated by the low compressibility with a relatively high bulk modulus K₀ = 80.8 GPa and the slow bandgap decrease (12.1 meV/GPa). Our findings provide new insight into the design and optimization of trimers and oligomers in low-D metal halides with the aim of achieving high performance in optoelectronics applications.

See the supplementary material for the experimental section and supplementary figures and tables.

This work was supported by the National Nature Science Foundation of China (NSFC) (Grant Nos. U1930401 and 51527801). S.L. and B.M. are grateful for support from the National Science Foundation (Grant No. DMR-1709116). Portions of this work were performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation – Earth Sciences (Grant No. EAR-1634415) and the Department of Energy – GeoSciences (Grant No. DE-FG02-94ER14466), and partially by COMPRES under NSF Cooperative Agreement No. EAR-1606856.