Metal halide perovskites (MHPs) are gaining increasing interest because of their extraordinary performance in optoelectronic devices and solar cells. However, developing an effective strategy for achieving the band-gap engineering of MHPs that will satisfy the practical applications remains a great challenge. In this study, high pressure is introduced to tailor the optical and structural properties of MHP-based cesium lead bromide nanocrystals (CsPbBr NCs), which exhibit excellent thermodynamic stability. Both the pressure-dependent steady-state photoluminescence and absorption spectra experience a stark discontinuity at ∼1.2 GPa, where an isostructural phase transformation regarding the Pbnm space group occurs. The physical origin points to the repulsive force impact due to the overlap between the valence electron charge clouds of neighboring layers. Simultaneous band-gap narrowing and carrier-lifetime prolongation of CsPbBr trihalide perovskite NCs were also achieved as expected, which facilitates the broader solar spectrum absorption for photovoltaic applications. Note that the values of the phase change interval and band-gap red-shift of CsPbBr nanowires are between those for CsPbBr nanocubes and the corresponding bulk counterparts, which results from the unique geometrical morphology effect. First-principles calculations unravel that the band-gap engineering is governed by orbital interactions within the inorganic Pb-Br frame through structural modification. Changes of band structures are attributed to the synergistic effect of pressure-induced modulations of the Br-Pb bond length and Pb-Br-Pb bond angle for the PbBr octahedral framework. Furthermore, the significant distortion of the lead-bromide octahedron to accommodate the Jahn-Teller effect at much higher pressure would eventually lead to a direct to indirect band-gap electronic transition. This study enables high pressure as a robust tool to control the structure and band gap of CsPbBr NCs, thus providing insight into the microscopic physiochemical mechanism of these compressed MHP nanosystems.

Metal halide perovskites (MHPs) have attracted increasing research attention given the ease of solution processability with excellent optical absorption and emission qualities. However, effective strategies for engineering the band gap of MHPs to satisfy the requirements of practical applications are difficult to develop. Cubic cesium lead iodide (α-CsPbI 3 ), a typical MHP with an ideal band gap of 1.73 eV, is an intriguing optoelectric material owing to the approaching Shockley−Queisser limit. Here, we carried out a combination of in situ photoluminescence, absorption, and angle-dispersive synchrotron X-ray diffraction spectra to investigate the pressure-induced optical and structural changes of α-CsPbI 3 nanocrystals (NCs). The α-CsPbI 3 NCs underwent a phase transition from cubic (α) to orthorhombic phase and subsequent amorphization upon further compression. The structural changes with octahedron distortion to accommodate the Jahn−Teller effect were strongly responsible for the optical variation with the increase of pressure. First-principles calculations reveal that the band-gap engineering is governed by orbital interactions within the inorganic Pb−I frame through the structural modification. Our high-pressure studies not only established structure− property relationships at the atomic scale of α-CsPbI 3 NCs, but also provided significant clues in optimizing photovoltaic performance, thus facilitating the design of novel MHPs with increased stimulus-resistant capability.

The structural stability and visible light response of two-dimensional layered perovskite-like CsPb2Br5 microplates were investigated by in situ high-pressure technology.

Metal-halide perovskites (MHPs) have attracted tremendous attention because of their excellent performance in photovoltaic devices, such as solar cells. However, because of the crucial relationship between emission intensity and performance, pressure-quenching of optical emission greatly restrict the potential application of MHPs. Here, we reported the unique pressure-induced emission enhancement (PIEE) of CsPb x Mn 1−x Cl 3 NCs. Different from other PIEE phenomena, the PIEE of CsPb x Mn 1−x Cl 3 NCs was caused by the enhancement of energy release from 4 T 1 to 6 A 1 of the Mn, attributed to the pressure-induced isostructural phase transition. Meanwhile, the photoluminescence (PL) can exist until almost 20 GPa, suggesting that CsPb x Mn 1−x Cl 3 NCs exhibited better environmental suitability and worked under high pressure. Our studies explored the relationship between bandgap microstructure and optical properties of CsPb x Mn 1−x Cl 3 NCs at high pressure and also gave insights into the optimization of photovoltaic performance, which promoting the design of functional MHPs.

The isomerization of spiropyrans in crystals was realized under high pressure, and the corresponding mechanochromic response could be observed by the naked eye. In situ UV-Vis spectroscopy study demonstrated that the equilibrium constant increases with the increasing pressure, from which we proposed that the negative volume of reaction determined the isomerization under high pressure.

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The high-pressure response of few-layer black phosphorus (BP) nanosheets remains elusive, despite the special interest in it particularly after the achievement of an exotic few-layer BP based field effect transistor. Here, we identified a pressure-induced reversible phase transition on few-layer BP nanosheets by performing in situ ADXRD and Raman spectroscopy with the assistance of DAC apparatus. The few-layer BP nanosheets transformed from orthorhombic semiconductors to simple cubic metal with increasing pressure, which is well interpreted using the pressure-induced inverse Peierls distortion. The obtained simple cubic BP nanosheets exhibited an enhanced isothermal bulk modulus of 147.0(2) GPa, and negative Grüneisen parameters that were attributed to the pressure-driven softening of phonon energies. Note that the simple cubic BP nanosheets adopted the highest symmetry which is in stark contrast to the general phase transformation under high pressure. First-principles calculations indicated that the metallic BP was significantly related to the band overlapped metallization, resulting from the traversing of density of states across the Fermi level at high pressure. Such findings paved a potential pathway to design targeted BP nanostructures with functional properties at extremes, and opened up possibilities for conceptually new devices.

The multi-colored mechanochromic materials based on a spiropyran and pyrene derivative were realized through a combination of supramolecular and chemical structural changes.