Enhanced Size Selection in Two-Photon Excitation for CsPbBr3 Perovskite Nanocrystals

Data-Driven Controlled Synthesis of Oriented Quasi-Spherical CsPbBr3 Perovskite Materials

Controlled synthesis of lead-halide perovskite crystals is challenging yet attractive because of the pivotal role played by the crystal structure and growth conditions in regulating their properties. This study introduces data-driven strategies for the controlled synthesis of oriented quasi-spherical CsPbBr3, alongside an investigation into the synthesis mechanism. High-throughput rapid characterization of absorption spectra and color under ultraviolet illumination was conducted using 23 possible ligands for the synthesis of CsPbBr3 crystals. The links between the absorption spectra slope (difference in the absorbance at 400 nm and 450 nm divided by a wavelength interval of 50 nm) and crystal size were determined through statistical analysis of more than 100 related publications. Big data analysis and machine learning were employed to investigate a total of 688 absorption spectra and 652 color values, revealing correlations between synthesis parameters and properties. Ex situ characterization confirmed successful synthesis of oriented quasi-spherical CsPbBr3 perovskites using polyvinylpyrrolidone and Acacia. Density functional theory calculations highlighted strong adsorption of Acacia on the (110) facet of CsPbBr3. Optical properties of the oriented quasi-spherical perovskites prepared with these data-driven strategies were significantly improved. This study demonstrates that data-driven controlled synthesis facilitates morphology-controlled perovskites with excellent optical properties.

Photophysics and its application in photon upconversion

Photoluminescence (PL) upconversion is a phenomenon involving light-matter interaction, where the energy of the emitted photons is higher than that of the incident photons. PL upconversion has promising applications in optoelectronic devices, displays, photovoltaics, imaging, diagnosis and treatment. In this review, we summarize the mechanism of PL upconversion and ultrafast PL physical processes. In particular, we highlight the advances in laser cooling, biological imaging, volumetric displays and photonics.

Surface Versus Bulk State Transitions in Inkjet-Printed All-Inorganic Perovskite Quantum Dot Films

The anion exchange of the halides, Br and I, is demonstrated through the direct mixing of two pure perovskite quantum dot solutions, CsPbBr3 and CsPbI3, and is shown to be both facile and result in a completely alloyed single phase mixed halide perovskite. Anion exchange is also observed in an interlayer printing method utilizing the pure, unalloyed perovskite solutions and a commercial inkjet printer. The halide exchange was confirmed by optical absorption spectroscopy, photoluminescent spectroscopy, X-ray diffraction, and X-ray photoemission spectroscopy characterization and indicates that alloying is thermodynamically favorable, while the formation of a clustered alloy is not favored. Additionally, a surface-to-bulk photoemission core level transition is observed for the Cs 4d photoemission feature, which indicates that the electronic structure of the surface is different from the bulk. Time resolved photoluminescence spectroscopy indicates the presence of multiple excitonic decay features, which is argued to originate from states residing at surface and bulk environments.

Direct Observation of Size-Dependent Phase Transition in Methylammonium Lead Bromide Perovskite Microcrystals and Nanocrystals

Methylammonium (MA) lead halide perovskites have been widely studied as active materials for advanced optoelectronics. As crystalline semiconductor materials, their properties are strongly affected by their crystal structure. Depending on their applications, the size of MA lead halide perovskite crystals varies by several orders of magnitude. The particle size can lead to different structural phase transitions and optoelectronic properties. Herein, we investigate the size effect for phase transition of MA lead bromide (MAPbBr₃) by comparing the temperature-dependent neutron powder diffraction patterns of microcrystals and nanocrystals. The orthorhombic-to-tetragonal phase transition occurs in MAPbBr₃ microcrystals within the temperature range from 100 to 310 K. However, the phase transition is absent in nanocrystals in this temperature range. In this work, we offer a persuasive and direct evidence of the relationship between the particle size and the phase transition in perovskite crystals.

Two-photon absorption in halide perovskites and their applications

Active research on halide perovskites has given us a deep understanding of this family of materials and their potential for applications in advanced optoelectronic devices. One of the prominent outcomes is the use of perovskite materials for nonlinear optical applications. Two-photon absorption in perovskites, in particular their nanostructures, has been extensively studied and shows huge promise for many applications. However, we are still far from a thorough understanding of two-photon absorption in halide perovskites from a micro to macro perspective. Here we summarize different techniques for studying the two-photon absorption in nonlinear optical materials. We discuss the in-depth photophysics in two-photon absorption in halide perovskites. A comprehensive summary about the factors which influence two-photon absorption provides the direction to improve the two-photon absorption properties of halide perovskites. A summary of the recent applications of two-photon absorption in halide perovskites provides inspirations for engineers to utilize halide perovskites in two-photon absorption device development. This review will help readers to have a comprehensive and in-depth understanding of the research field of two-photon absorption of halide perovskites from microscopic mechanisms to applications. The article can serve as a manual and give inspiration for future researchers.

Morphology-Dependent One- and Two-Photon Absorption Properties in Blue Emitting CsPbBr3 Nanocrystals

The linear and nonlinear optical parameters and morphologic dependence of CsPbBr₃ nanocrystals (NCs) are crucial for device engineering. In particular, such information in asymmetric nanocrystals is still insufficient. We characterized the OPLA (σ¹) and TPA cross sections (σ²) of a series CsPbBr₃ nanocrystals with various aspect ratios (AR) using femtosecond transient absorption spectroscopy (TAS). The σ¹ presents a linear volume dependence of all the samples, which agrees with the previous behavior in CsPbBr₃ QDs. However, the σ² values do not exhibit conventional power dependency of the crystal volume but are also modulated by the shape-dependent local field factors. In addition, the local field effect in CsPbBr₃ NCs is contributed by their asymmetric morphologies and polar ionic lattices, which is more pronounced than in conventional semiconductor NCs. Finally, we revealed that the lifetimes of photogenerated multiexcitonic species of those nanocrystals feature identical morphology independence in both OPLA and TPA.

Bright Triplet Self-Trapped Excitons to Dopant Energy Transfer in Halide Double-Perovskite Nanocrystals

For inorganic semiconductor nanostructure, excitons in the triplet states are known as the "dark exciton" with poor emitting properties, because of the spin-forbidden transition. Herein, we report a design principle to boost triplet excitons photoluminescence (PL) in all-inorganic lead-free double-perovskite nanocrystals (NCs). Our experimental data reveal that singlet self-trapped excitons (STEs) experience fast intersystem crossing (80 ps) to triplet states. These triplet STEs give bright green color emission with unity PL quantum yield (PLQY). Furthermore, efficient energy transfer from triplet STEs to dopants (Mn²⁺) can be achieved, which leads to white-light emitting with 87% PLQY in both colloidal and solid thin film NCs. These findings illustrate a fundamental principle to design efficient white-light emitting inorganic phosphors, propelling the development of illumination-related applications.

Controlling Photoluminescence and Photocatalysis Activities in Lead-Free Cs2 Ptx Sn1-x Cl6 Perovskites via Ion Substitution

Lead-free halide perovskites have triggered interest in the field of optoelectronics and photocatalysis because of their low toxicity, and tunable optical and charge-carrier properties. From an application point of view, it is desirable to develop stable multifunctional lead-free halide perovskites. We have developed a series of Cs₂ Pt_x Sn_1-x Cl₆ perovskites (0≤x≤1) with high stability, which show switchable photoluminescence and photocatalytic functions by varying the amount of Pt⁴⁺ substitution. A Cs₂ Pt_x Sn_1-x Cl₆ solid solution with a dominant proportion of Pt⁴⁺ shows broadband photoluminescence with a lifetime on the microsecond timescale. A Cs₂ Pt_x Sn_1-x Cl₆ solid solution with a small amount of Pt⁴⁺ substitution exhibits photocatalytic hydrogen evolution activity. An optical spectroscopy study reveals that the switch between photoluminescence and photocatalysis functions is controlled by sub-band gap states. Our finding provides a new way to develop lead-free multifunctional halide perovskites with high stability.

Advances of Nonlinear Photonics in Low-Dimensional Halide Perovskites

Hybrid halide perovskites emerging as a highly promising class of functional materials for semiconductor optoelectronic applications have drawn great attention from worldwide researchers. In the past few years, prominent nonlinear optical properties have been demonstrated in perovskite bulk structures indicating their bright prospect in the field of nonlinear optics (NLO). Following the surge of 3D perovskites, more recently, the low-dimensional perovskites (LDPs) materials ranging from two-, one-, to zero-dimension such as quantum-wells or colloidal nanostructures have displayed unexpectedly attractive NLO response due to the strong quantum confinement, remarkable exciton effect, and structural diversity. In this perspective, the current state of the art is reviewed in the field of NLO for LDP materials. The relationship between confinement effect and NLO is analyzed systematically to give a comprehensive understanding of the function of dimension reduction. Furthermore, future directions and challenges toward the improvement of the NLO in LDP materials are discussed to provide an outlook in this rapidly developing field.

Phase Engineering of Cesium Manganese Bromides Nanocrystals with Color-Tunable Emission

For display applications, it is highly desirable to obtain tunable red/green/blue emission. However, lead-free perovskite nanocrystals (NCs) generally exhibit broadband emission with poor color purity. Herein, we developed a unique phase transition strategy to engineer the emission color of lead-free cesium manganese bromides NCs and we can achieve a tunable red/green/blue emission with high color purity in these NCs. Such phase transition can be triggered by isopropanol: from one dimensional (1D) CsMnBr₃ NCs (red-color emission) to zero dimensional (0D) Cs₃ MnBr₅ NCs (green-color emission). Furthermore, in a humid environment both 1D CsMnBr₃ NCs and 0D Cs₃ MnBr₅ NCs can be transformed into 0D Cs₂ MnBr₄ ⋅2 H₂ O NCs (blue-color emission). Cs₂ MnBr₄ ⋅2 H₂ O NCs could inversely transform into the mixture of CsMnBr₃ and Cs₃ MnBr₅ phase during the thermal annealing dehydration step. Our work highlights the tunable optical properties in single component NCs via phase engineering and provides a new avenue for future endeavors in light-emitting devices.

Nonlinear Photonics Using Low-Dimensional Metal-Halide Perovskites: Recent Advances and Future Challenges

Low-dimensional metal-halide perovskites have exhibited significantly superior nonlinear optical properties compared to traditional semiconductor counterparts, thanks to their peculiar physical and electronic structures. Their exceptional nonlinear optical characteristics make them excellent candidates for revolutionizing widespread applications. However, the research of nonlinear photonics based on low-dimensional metal-halide perovskites is in its infancy. There is a lack of comprehensive and in-depth summary of this research realm. Here, the state-of-the-art research progress related to third-and higher-order nonlinear optical properties of low-dimensional metal-halide perovskites with diverse crystal structures from 3D down to 0D, together with their practical applications, is summarized comprehensively. Critical discussions are offered on the fundamental mechanisms beneath their exceptional nonlinear optical performance from the physics viewpoint, attempting to disclose the role of intrinsic attributes (e.g., composition, bandgap, size, shape, and structure) and external modulation strategies (e.g., developing core-shell structures, transition metal ion doping, and hybridization with dielectric microspheres) in tuning the response. Additionally, their potential applications in nonlinear photonics, nonlinear optoelectronics, and biophotonics are systematically and thoroughly summed up and categorized. Lastly, insights into the current technical challenges and future research opportunities of nonlinear photonics based on low-dimensional metal-halide perovskites are provided.

Spectral Dynamics and Multiphoton Absorption Properties of All-Inorganic Perovskite Nanorods

All-inorganic perovskite (CsPbX₃, X = Cl, Br, I) nanorods (NRs) not only retain their inherent advantages such as a high photoluminescence quantum yield and broad wavelength tunability but also exhibit superior photophysical properties including their extremely strong multiphoton absorption (MPA). However, the spectral dynamics and MPA properties of CsPbX₃ NRs have not been fully investigated. Here, we report comprehensive comparison studies on the femtosecond spectral dynamical properties of CsPb(Br_0.8Cl_0.2)₃, CsPbBr₃, and CsPb(Br_0.85I_0.15)₃ NRs, including their influences on their hot-carrier cooling, biexciton lifetime, and biexciton binding energy. Interestingly, although the three kinds of perovskite NRs have similar diameters and lengths, they differ significantly in their nonlinear optical properties, among which the CsPb(Br_0.85I_0.15)₃ displayed the greatest MPA cross sections. Furthermore, the multiphoton-excited stimulated emission of CsPb(Br_0.8Cl_0.2)₃ and CsPbBr₃ NRs is demonstrated. This work indicates that CsPbX₃ (X = Cl, Br, I) NRs are excellent candidates for exploring their applications in different optoelectronic devices.

Amplified spontaneous emission and random lasing using CsPbBr3 quantum dot glass through controlling crystallization

CsPbBr₃ QD glasses heat-treated at different temperatures have different sizes of QDs and lasing properties.

The preparation and up-conversion properties of full spectrum CsPbX3 (X = Cl, Br, I) quantum dot glasses

All-inorganic perovskite (CsPbX3) quantum dots (QDs) have achieved unprecedented success in various applications due to their outstanding performance. Nevertheless, the inherent instability of these QDs severely limits their practical applications. Here, ultra-stable full visible spectrum CsPbX3 (X = Cl, Br, I) QD glasses are prepared successfully. For the first time, the full spectrum tunable up-conversion (UC) emission of CsPbX3 QD glasses ranging from 420 nm to 711 nm under excitation with an 800 nm femtosecond (fs) laser was achieved. Importantly, the two-photon absorption properties, the exciton binding energy and the UC full width at half maximum (FWHM) of CsPbCl1.5Br1.5 (blue), CsPbBr3 (green), and CsPbBr1.5I1.5 (red) QD glasses were studied in depth. Furthermore, CsPbCl1.5Br1.5 showed the highest exciton binding energy (∼87.5 meV), allowing the CsPbCl1.5Br1.5 QD glass to act as a good optical gain material that could more easily achieve amplified spontaneous emission (ASE).

Giant two- to five-photon absorption in CsPbBr2.7I0.3 two-dimensional nanoplatelets

CsPbBr_2.7I_0.3 two-dimensional (2D) nanoplatelets (NPs) with emission wavelengths of 469 nm and 527 nm were synthesized and characterized. Femtosecond transient absorption spectra revealed hot carrier cooling times of ∼368  fs and ∼438  fs for 469 nm and 527 nm 2D NPs, respectively. Importantly, the 2D NPs exhibit giant two-, three-, four-, and five-photon absorption cross-sections, reaching ∼4.1×10⁶ GM at 830 nm, ∼2.3×10^-74  cm⁶ s² photon^-2 at 1300 nm, 2.06×10^-104  cm⁸ s³ photon^-3 at 1600 nm, and 1.50×10^-136  cm¹⁰ s⁴ photon^-4 at 2200 nm, respectively, which are 3-8 orders of magnitude larger, compared to specially designed organic molecules.

Stable and Strong Emission CsPbBr3 Quantum Dots by Surface Engineering for High-Performance Optoelectronic Films

We demonstrated complete surface passivation of CsPbBr₃ quantum dots (QDs) by treatment with di-dodecyldimethylammonium bromide (DDAB) and sodium thiocyanate (NaSCN), resulting in dispersions with photostable photoluminescence of near-unity quantum yield (∼100%) as well as high carrier mobility of QDs' film. Br^- from DDAB and SCN^- from NaSCN passivated the bromine vacancies of the QDs to reduce the surface defect density and increase the stability. The QDs-passivated maintained the original photoluminescence intensity under ultraviolet irradiation from a 150 W xenon lamp for 1 h, whereas the PL intensity of QDs-control dropped quickly to 20% of its initial value. The shorter DDA⁺ ligands also improved carrier transport in the QDs-passivated film, which was verified by conductivity and space charge limited current measurements. When used as the photoemitting species in a solution-processed light-emitting diode structure, the surface treatment increased the maximum luminance from 550 to 1200 cd·m^-2 and reduced the turn-on voltage from 3.1 to 2.8 V.

Ultrafast carrier dynamics of metal halide perovskite nanocrystals and perovskite-composites

Perovskite nanocrystals (NCs), especially those based on cesium lead halides, have emerged in recent years as highly promising materials for efficient solar cells and photonic applications. The key to realization of full potential of these materials lies however in the molecular level understanding of the processes triggered by light. Herein we highlight the knowledge gained from photophysical investigations on these NCs of various sizes and compositions employing primarily the femtosecond pump-probe technique. We show how spectral and temporal characterization of the photo-induced transients provide insight into the mechanism and dynamics of relaxation of hot and thermalized charge carriers through their recombination and trapping. We discuss how the multiple excitons including the charged ones (trions), generated using high pump fluence or photon energy, recombine through the Auger-assisted process. We discussed the harvesting of hot carriers prior to their cooling and band-edge carriers from these perovskite NCs to wide band-gap metal oxides, metal chalcogenide NCs and molecular acceptors. How perovskites can influence the charge carrier dynamics in composites of organic and inorganic semiconductors is also discussed.