Zhiwei Ma, Guanjun Xiao, , and Liming Ding

1State Key Laboratory of SuperhardMaterials, College of Physics, Jilin University, Changchun130012, China

2Center for Excellencein Nanoscience (CAS), Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS), National Center for

Nanoscience and Technology,Beijing100190, China

White light emitting diodes from a single emitter is of significance in application of illumination and image display because of avoiding the color instability in devices with multiple emitters[1,2]. Recently, as a single emitter, low-dimensional halide perovskites have drawn great attention for their broadband solid-state lighting based on the radiative recombination of self-trapped excitons (STEs) and excellent device stability[3−8]. The dimensionality reduction could effectively enhance the deformability of octahedral framework and thus electron–phonon coupling strength, facilitating exciton selftrapping. However, low-dimensional perovskites cannot satisfy the requirement of device commercialization due to the rare achievement of high photoluminescence quantum yield[9]. Developing an effective strategy to improve the emission and elucidating the underlying mechanism have become an urgent need.

Pressure,produced bydiamondanvilcell (DAC, Fig. 1(a)),could effectively shorten interatomic distances and profoundlymodify electronic orbitals andbondingpatterns, generating new structures with novel properties[10]. Recently,high pressure has been regarded astheone of the “Top-Ten Emerging Technologies in Chemistry”, and no longer a small field,which accelerates the development ofscience and technology and enhances luminescence and super- conductivity[11]. Therefore, pressureisexpected to greatly improve the luminescent performance without altering the componentof the material. In 1992,Drickameret al.observed pressure-induced emission enhancement (PIEE), i.e., a solid-state luminogen gave enhanced emissionuponcompression[12].Zouet al. first reported pressure-induced emission (PIE) from zero-dimensionalall-inorganic halide perovskite, Cs4PbBr6nanocrystals (NCs), i.e., an unexpected broad emission was generatedby pressingan non-emissive sample (Figs. 1(b) and 1(c))[13]. The exotic PIE occurred at 3.01 GPa, and Cs4PbBr6NCs experienced areversible phasetransition from a rhombohedral phase to a monoclinic phase, accompanied by a significantdistortionof[PbBr6]4–octahedra. Thischange enhances the wavefunction overlap between the excited state and the groundstate,promotingthe dipole momentcompared to the ambient-pressure state (Fig. 1(d)). Meanwhile,the electron–phononcouplingstrengthassociatedwith the Huang–Rhys factor is enhanced under high pressure, which effectively strengthensthe binding energyofSTEs and inhibits the transformation from STEs to bound excitons. Finally, under stimulus of external pressure, the radiative recombination of STEs generates the exotic broad emission. Here, the concept of PIE refers to a novel phenomenon, i.e., a non-luminescent material gives emission upon compression. In fact,PIE could completely cover the description of PIEE. Such PIE phenomenonwas also observed in one-dimensionalSnbased perovskites (Figs.1(e)and 1(f)), reported by Shiet al.[14].Maet al.provedthatthe distortion determined PIE, and the greater the zigzagdistortion, the more remarkable the PIE(Fig. 1(g))[15].

ThequenchedPIEperovskitesgenerally recover totheinitialnon-emissive state, which significantly inhibits their practical applications. In this regard, Fuet al. recently proposed a new strategy to keep inorganic octahedral distortion through the introduction of steric hindrance of appropriate organic cations. The high-efficiency emission of quenched two-dimensional perovskite, (PEA)2PbCl4NCs (PEA+= C6H5CH2CH2NH3+),wasachievedas anticipated (Figs.2(a)–2(c)). This worknot merelyprovidedapathway to yieldstable perovskitewith high emissionviaa“clean” pressure treatment,butalso enabled an obvious color change from “warm” (4403 K) to“cold” white light (14 295 K) (Fig. 2(d))[16]. Furthermore, retention of PIE was also realized in microtubules (MTs) of zero-dimensional hybrid halide (4-AMP)2ZnBr4(4-AMP+= C5H7N2+)(Fig. 2(e)). Jointexperimental and theoretical analysesindicate irreversible electronictransitioninduced by thesteric hindrance isresponsiblefor theirreversible opticalproperties (Figs. 2(f) and 2(g))[17].

Fig. 1. (Color online) (a) Schematic diagram of a symmetric DAC apparatus used in the high-pressure experiments for 0-, 1- and 2D perovskites.(b) and (c) High-pressure emission from Cs4PbBr6 NCs. (d) Calculated absorption oscillator strength by using excited-state structure associated with STE at 1 atm and 4 GPa, respectively. Reproduced with permission[13], Copyright 2018, Springer Nature. (e) Pressure-dependent PL spectra for 1D perovskite C4N2H14SnBr4. (f) Transformation of C4N2H14SnBr4 under high pressure. Stage I and stage II show octahedral chains before and after the phase transition, respectively. Reproduced with permission[14], Copyright 2019, American Chemical Society. (g) Schematic illustrations of Pb–Br–Pb bond angle and Pb–Br bond length within octahedral framework before and after phase transition. Pb 6s orbital and Br 4p orbital are represented by light green sphere and red spindle, respectively. Reproduced with permission[15], Copyright 2020, Wiley Publishing Group.

Fig. 2. (Color online) (a) Evolution of PL spectra and microphotographs under pressures from 1 atm to 0.4 GPa. (b) PL spectra for (PEA)2PbCl4 NCs at 1 atm and after pressure release; inset shows the corresponding photographs. (c) Illustrated mechanism of PIE retention affected by steric hindrance of organic cation. ASS: atmospheric stable phase; HPMS: high-pressure metastable state; PB: potential barrier; PB’: increased potential barrier. (d) Chromaticity coordinate diagram at 1 atm and after pressure release. Reproduced with permission[16], Copyright 2021, Wiley Publishing Group. (e) PL spectra and photographs for (4-AMP)2ZnBr4 MTs (blue: atmospheric pressure; red: pressure removed). (f) Calculated partial charge density located on CBM and VBM at 1 atm and 5 GPa. (g) UV/Vis absorption spectra at 1 atm and after removal of 18.01 GPa pressure. Reproduced with permission[17], Copyright 2021, Wiley Publishing Group.

In conclusion, the pressure causes distorted octahedra within low-dimensionalperovskites toaffectSTE behavior and generates PIE. The PIEcan be modulated by introducing steric hindrance from appropriate organic cations. There is stillalargeroom for adjusting the stericeffectviatuning the structure and size of organic cations.

Acknowledgements

This work was financially supported by the National Key Research and DevelopmentProgramof China(2019YFE0120300 and 2017YFA0206600), the National Natural Science Foundation of China (21725304, 11774125,51773045,21772030, 51922032and 21961160720), and the Fundamental Research Funds for the Central Universities.