Tunable photoluminescence and an enhanced photoelectric response of Mn2+-doped CsPbCl3 perovskite nanocrystals via pressure-induced structure evolution

Ultrafast Dynamics Across Pressure-Induced Electronic State Transitions, Fluorescence Quenching, and Bandgap Evolution in CsPbBr3 Quantum Dots

This work investigates the impact of pressure on the structural, optical properties, and electronic structure of CsPbBr3 quantum dots (QDs) using steady-state photoluminescence, steady-state absorption, and femtosecond transient absorption spectroscopy, reaching a maximum pressure of 3.38 GPa. The experimental results indicate that CsPbBr3 QDs undergo electronic state (ES) transitions from ES-I to ES-II and ES-II to ES-III at 0.38 and 1.08 GPa, respectively. Intriguingly, a mixed state of ES-II and ES-III is observed within the pressure range of 1.08-1.68 GPa. The pressure-induced fluorescence quenching in ES-II is attributed to enhanced defect trapping and reduced radiative recombination. Above 1.68 GPa, fluorescence vanishes entirely, attributed to the complete phase transformation from ES-II to ES-III in which radiative recombination becomes non-existent. Notably, owing to stronger quantum confinement effects, CsPbBr3 QDs exhibit an impressive bandgap tuning range of 0.497 eV from 0 to 2.08 GPa, outperforming nanocrystals by 1.4 times and bulk counterparts by 11.3 times. Furthermore, this work analyzes various carrier dynamics processes in the pressure-induced bandgap evolution and electron state transitions, and systematically studies the microphysical mechanisms of optical properties in CsPbBr3 QDs under pressure, offering insights for optimizing optical properties and designing novel materials.

Theoretical and Experimental Advances in High-Pressure Behaviors of Nanoparticles

Using compressive mechanical forces, such as pressure, to induce crystallographic phase transitions and mesostructural changes while modulating material properties in nanoparticles (NPs) is a unique way to discover new phase behaviors, create novel nanostructures, and study emerging properties that are difficult to achieve under conventional conditions. In recent decades, NPs of a plethora of chemical compositions, sizes, shapes, surface ligands, and self-assembled mesostructures have been studied under pressure by in-situ scattering and/or spectroscopy techniques. As a result, the fundamental knowledge of pressure-structure-property relationships has been significantly improved, leading to a better understanding of the design guidelines for nanomaterial synthesis. In the present review, we discuss experimental progress in NP high-pressure research conducted primarily over roughly the past four years on semiconductor NPs, metal and metal oxide NPs, and perovskite NPs. We focus on the pressure-induced behaviors of NPs at both the atomic- and mesoscales, inorganic NP property changes upon compression, and the structural and property transitions of perovskite NPs under pressure. We further discuss in depth progress on molecular modeling, including simulations of ligand behavior, phase-change chalcogenides, layered transition metal dichalcogenides, boron nitride, and inorganic and hybrid organic-inorganic perovskites NPs. These models now provide both mechanistic explanations of experimental observations and predictive guidelines for future experimental design. We conclude with a summary and our insights on future directions for exploration of nanomaterial phase transition, coupling, growth, and nanoelectronic and photonic properties.

Fluorescence-based monitoring of the pressure-induced aggregation microenvironment evolution for an AIEgen under multiple excitation channels

The development of organic solid-state luminescent materials, especially those sensitive to aggregation microenvironment, is critical for their applications in devices such as pressure-sensitive elements, sensors, and photoelectric devices. However, it still faces certain challenges and a deep understanding of the corresponding internal mechanisms is required. Here, we put forward an unconventional strategy to explore the pressure-induced evolution of the aggregation microenvironment, involving changes in molecular conformation, stacking mode, and intermolecular interaction, by monitoring the emission under multiple excitation channels based on a luminogen with aggregation-induced emission characteristics of di(p-methoxylphenyl)dibenzofulvene. Under three excitation wavelengths, the distinct emission behaviors have been interestingly observed to reveal the pressure-induced structural evolution, well consistent with the results from ultraviolet-visible absorption, high-pressure angle-dispersive X-ray diffraction, and infrared studies, which have rarely been reported before. This finding provides important insights into the design of organic solid luminescent materials and greatly promotes the development of stimulus-responsive luminescent materials.

Precise incorporation of transition metals into organolead oxyhalide crystalline materials for photocatalysis

Organolead halide crystalline materials are an emerging class of high-performance photocatalysts. However, limited studies have been performed to tune their photoactive properties by precise introduction of transition metals. Herein, we report the successful incorporation of four different transition metal centers (Mn²⁺, Co²⁺, Ni²⁺ and Zn²⁺) into a lead oxyhalide crystalline matrix via isoreticular synthesis. Importantly, the precise control of the incoming transition metal positions has been achieved by its octahedral coordination with three organic ligands. Among them, the Zn²⁺-incorporated material exhibits the highest catalytic activity and recyclable activity in benzylamine oxidation under UV light, which is probably ascribed to the long carrier lifetime and efficient carrier transfer.

Tailoring Optical Properties of Luminescent Semiconducting Nanocrystals through Hydrostatic, Anisotropic Static, and Dynamic Pressures

Luminescent semiconductor nanocrystals are a fascinating class of materials because of their size-dependent emissions. Numerous past studies have demonstrated that semiconductor nanoparticles with radii smaller than their Bohr radius experience quantum confinement and thus size-dependent emissions. Exerting pressure on these nanoparticles represents an additional, more dynamic, strategy to alter their size and shift their emission. The application of pressure results in the lattices becoming strained and the electronic structure altered. In this Minireview, colloidal semiconductor nanocrystals are first introduced. The effects of uniform hydrostatic pressure on the optical properties of metal halide perovskite (ABX₃ ), II-VI, III-V, and IV-VI semiconductor nanocrystals are then examined. The optical properties of semiconductor nanocrystals under static and dynamic anisotropic pressure are then summarized. Finally, future research directions and applications utilizing the pressure-dependent optical properties of semiconductor nanocrystals are discussed.

Pressure-Engineered Optical and Charge Transport Properties of Mn2+/Cu2+ Codoped CsPbCl3 Perovskite Nanocrystals via Structural Progression

In this work, compared with the corresponding pure CsPbCl₃ nanocrystals (NCs) and Mn²⁺-doped CsPbCl₃ NCs, Mn²⁺/Cu²⁺-codoped CsPbCl₃ NCs exhibited improved photoluminescence (PL) and photoluminescence quantum yields (PL QYs) (57.6%), prolonged PL lifetimes (1.78 ms), and enhanced thermal endurance (523 K) as a result of efficient Mn²⁺ doping (3.66%) induced by the addition of CuCl₂. Furthermore, we applied pressure on Mn²⁺/Cu²⁺-codoped CsPbCl₃ NCs to reveal that a red shift of photoluminescence followed by a blue shift was caused by band gap evolution and related to the structural phase transition from cubic to orthorhombic. Moreover, we also found that under the preheating condition of 523 K, such phase transition exhibited obvious morphological invariance, accompanied by significantly enhanced conductivity. The pressure applied to the products treated with high temperature enlarged the electrical difference and easily intensified the interface by closer packaging. Interestingly, defect-triggered mixed ionic and electronic conducting (MIEC) was observed in annealed NCs when the applied pressure was 2.9 GPa. The pressure-dependent ionic conduction was closely related to local nanocrystal amorphization and increased deviatoric stress, as clearly described by in situ impedance spectra. Finally, retrieved products exhibited better conductivity (improved by 5-6 times) and enhanced photoelectric response than those when pressure was not applied. Our findings not only reveal the pressure-tuned optical and electrical properties via structural progression but also open up the promising exploration of more amorphous all-inorganic CsPbX₃-based photoelectric applications.

Fluorescence Lifetime-Tunable Water-Resistant Perovskite Quantum Dots for Multidimensional Encryption

Using temporal dimension in optical multiplexing is a promising method to increase the security of data encryption. However, adjusting the fluorescence lifetime of light-emitting material often results in inevitable changes in their fluorescence spectra, which is unfavorable for confidential information protection. Here, we report the preparation of various perovskite quantum dot/polymer nanospheres (PQD/polymer) with tunable and long fluorescence lifetimes but identical fluorescence spectra, which are ideal multidimensional data encryption materials. This new data encryption strategy utilizes the water sensitivity of perovskite and achieves spatial dimension encryption of information using different water stabilities between uncoated perovskite quantum dots and PQD/polymer. The fluorescence lifetime of PQD/polymer is used as the coding element to achieve temporal dimension data encryption, and the data are decrypted by fluorescence lifetime imaging microscopy and time-gated luminescence imaging techniques. This study shows the potential of PQD/polymer as a new class of materials for advanced data encryption.

Doping and ion substitution in colloidal metal halide perovskite nanocrystals

The past decade has witnessed tremendous advances in synthesis of metal halide perovskites and their use for a rich variety of optoelectronics applications. Metal halide perovskite has the general formula ABX3, where A is a monovalent cation (which can be either organic (e.g., CH3NH3+ (MA), CH(NH2)2+ (FA)) or inorganic (e.g., Cs+)), B is a divalent metal cation (usually Pb2+), and X is a halogen anion (Cl-, Br-, I-). Particularly, the photoluminescence (PL) properties of metal halide perovskites have garnered much attention due to the recent rapid development of perovskite nanocrystals. The introduction of capping ligands enables the synthesis of colloidal perovskite nanocrystals which offer new insight into dimension-dependent physical properties compared to their bulk counterparts. It is notable that doping and ion substitution represent effective strategies for tailoring the optoelectronic properties (e.g., absorption band gap, PL emission, and quantum yield (QY)) and stabilities of perovskite nanocrystals. The doping and ion substitution processes can be performed during or after the synthesis of colloidal nanocrystals by incorporating new A', B', or X' site ions into the A, B, or X sites of ABX3 perovskites. Interestingly, both isovalent and heterovalent doping and ion substitution can be conducted on colloidal perovskite nanocrystals. In this review, the general background of perovskite nanocrystals synthesis is first introduced. The effects of A-site, B-site, and X-site ionic doping and substitution on the optoelectronic properties and stabilities of colloidal metal halide perovskite nanocrystals are then detailed. Finally, possible applications and future research directions of doped and ion-substituted colloidal perovskite nanocrystals are also discussed.