Stabilization of the Cubic Crystalline Phase in Organometal Halide Perovskite Quantum Dots via Surface Energy Manipulation

Octahedral Distortions Generate a Thermally Activated Phonon-Assisted Radiative Recombination Pathway in Cubic CsPbBr3 Perovskite Quantum Dots

Exciton-phonon interactions elucidate structure-function relationships that aid in the control of color purity and carrier diffusion, which is necessary for the performance-driven design of solid-state optical emitters. Temperature-dependent steady-state photoluminescence (PL) and time-resolved PL (TRPL) reveal that thermally activated exciton-phonon interactions originate from structural distortions related to vibrations in cubic CsPbBr3 perovskite quantum dots (PQDs) at room temperature. Exciton-phonon interactions cause performance-degrading PL line width broadening and slower electron-hole recombination. Structural distortions in cubic PQDs at room temperature exist as the bending and stretching of the PbBr6 octahedra subunit. The PbBr6 octahedral distortions cause symmetry breaking, resulting in thermally activated longitudinal optical (LO) phonon coupling to the photoexcited electron-hole pair that manifests as inhomogeneous PL line width broadening. At cryogenic temperatures, the line width broadening is minimized due to a decrease in phonon-assisted recombination through shallow traps. A fundamental understanding of these intrinsic exciton-phonon interactions gives insight into the polymorphic nature of the cubic phase and the origins of performance degradation in PQD optical emitters.

Velocity field distribution control in antisolvent flow realizing highly stable and efficient perovskite nanocrystals

Achieving highly stable and efficient perovskite nanocrystals (NCs) without applying functional additives or encapsulation, particularly sustaining the stability in ultra-dilute solution, is still a formidable challenge. Here, we show the FAPbI₃ perovskite NCs with achieved ∼100 % photoluminescence quantum yield (PLQY) and low defect density (∼0.2 cm^-3 per NCs), which is obtained by controlling the velocity field distribution of antisolvent flow in ligand-assisted reprecipitation process. The NCs show incredible reproducibility with narrow deviation of PLQY and linewidth between batch by batch, as well as remarkable stability of maintaining over 80 % PLQY, either in an ultra-diluted solution (9.3 × 10^-6 mg/mL), or storing in ambient condition after 90 days with concentration of 0.09 mg/mL. The results in this work demonstrate the interplay of fluid mechanics and crystallization kinetics of perovskite, which pioneers a novel and unprecedent understanding for improving the stability of perovskite NCs for efficient quantum light source.

Optical Properties of a CsMnBr3 Single Crystal

Lead-free perovskite materials with good stability are promising for various applications. In order to explore their application in optoelectronic devices, it is essential to investigate their fundamental optical properties. In this work, we have synthesized a CsMnBr₃ single crystal (SC) with red emission at ∼621 nm and studied their optical properties. Through the measurement of temperature-dependent photoluminescence (PL) spectra, it is found that a phase transition occurs at approximately 100 K in the SC, which is absent in the CsMnBr₃ nanocrystals (NCs). Furthermore, the SC exhibits stronger electron and longitudinal optical phonon coupling strength than that of the NCs at low temperatures. In addition, under the resonant excitation at 600 nm, the SC possesses strong saturable absorption property, with a modulation depth of ∼27%. Interestingly, the SC also exhibits a large two-photon absorption coefficient of ∼0.035 cm GW^-1 at 800 nm and an excellent optical limiting behavior. The experimental results indicate that the CsMnBr₃ SC is a class of excellent environmentally friendly optoelectronic materials.

Low-Temperature Energy Transfer via Self-Trapped Excitons in Mn2+-Doped 2D Organometal Halide Perovskites

We investigate the mechanisms of energy transfer in Mn²⁺-doped ethylammonium lead bromide (EA₂PbBr₄:Mn²⁺), a two-dimensional layered perovskite (2DLP), using cryogenic optical spectroscopy. At temperature T > 120 K, photoluminescence (PL) is dominated by emission from Mn²⁺, with complete suppression of band edge (BE) emission and self-trapped exciton (STE) emission. However, for T < 120 K, in addition to Mn²⁺ emission, PL is observed from BE and STEs. Data further reveal that for 20 K < T < 120 K, STEs form the most dominant routes in assisting energy transfer (ET) from 2DLP to Mn²⁺ dopants. However, at higher Mn²⁺ concentration, higher activation energies indicate defect states come into play, successfully competing with STEs for ET both from BE to STE states and from STE to Mn²⁺. Finally, using polarization-resolved spectroscopy, we demonstrate optical spin orientation of the Mn²⁺ ions via ET from 2DLP excitons at zero magnetic field. Our results reveal fundamental insights on the interactions between quantum confined charge carriers and dopants in organometal halide perovskites.

Modulating Charge Carrier Dynamics and Transfer via Surface Modifications in Organometallic Halide Perovskite Quantum Dots

We investigate the effect of functionalization by acid/amine combinations of four aromatic capping ligands on the optoelectronic properties of CH₃NH₃PbBr₃ perovskite quantum dots (PQDs). These include benzoic acid (BA), phenylacetic acid (PAA), benzylamine, and isopropyl benzylamine. We observe that charge transfer efficiency in PQD films comprising BA-ligated samples varies between 12% and 95% as the dot density is tuned from 10² to 10⁵ dots/μm² but is consistently ∼92% over that entire range for PAA-ligated PQDs. As temperature T decreases, initially, recombination is dominated by bound or trapped excitons, but below 80 K, spectral broadening, accompanied by free excitonic behavior, is observed. Our results indicate enhanced charge delocalization at lower values of T, which reduces the level of exciton confinement and recombination decay rates and underlines the importance of investigating PQD-ligand interactions at a fundamental level given the significant effect minute changes in ligand structures have on the optoelectronic properties of quantum dots.

Transition from Doublet to Triplet Excitons in Single Perovskite Nanocrystals

Lead halide perovskite nanocrystals (NCs) have emerged as novel semiconductor nanostructures possessing great potential for optoelectronic, photovoltaic, and quantum information processing applications. Success in these applications requires a comprehensive understanding of the perovskite NCs' electronic structures, which mysteriously exhibit either doublet or triplet peaks of exciton luminescence at the single-particle level. Here we show that the transition from doublet- to triplet-exciton peaks can be triggered in single CsPbI₃ NCs from the same batch of samples when they are stored in the ambient environment. We propose theoretically that the doublet-exciton peaks originate from two in-plane dipole moments, while the optical transition arising from the out-of-plane dipole moment becomes prominent only after the crystal-field splitting is strongly reduced by the structural transformation in the deterioration process. Furthermore, the quantum-confinement effect is strongly reinforced in the single CsPbI₃ NCs with a triplet-exciton configuration, leading to enhanced Auger recombination and allowing us to extract the emission-energy dependence of the exciton-energy-level fine structure.

Size-Dependent Phase Transition in Perovskite Nanocrystals

The complex structure of halide and oxide perovskites strongly affects their physical properties. Here, the effect of dimensions reduced to the nanoscale has been investigated by a combination of single-dot optical experiments with a phase transition theory. Methylammonium lead bromide (CH₃NH₃PbBr₃) nanocrystals with two average particle sizes of ∼2 and ∼4 nm with blue and green photoluminescence, respectively, were spectrally and temporally probed on a single-particle level from 5 to 295 K. The results show that the abrupt blue shift of the photoluminescence spectra and lifetimes at ∼150 K can be attributed to the cubic-to-tetragonal phase transition in the large 4 nm nanocrystals, while this phase transition is completely absent for the small 2 nm particles in the investigated temperature range. Theoretical calculations based on Landau theory reveal a strong size-dependent effect on temperature-induced phase transitions in individual CH₃NH₃PbBr₃ nanocrystals, corroborating experimental observations. This effect should be considered in structure-property analysis of ultrasmall perovskite crystals.

A "Cocktail" Approach to Effective Surface Passivation of Multiple Surface Defects of Metal Halide Perovskites Using a Combination of Ligands

Surface passivation of metal halide perovskites (MHPs) is essential for their stability and various properties as well as functionalities, including optical and electronic. Passivation is important for both stabilizing intrinsic defects and preventing extrinsic damaging species from reaching the perovskite (PVK), such as water and oxygen. Because of the ternary nature of their chemical composition, multiple surface defects exist for both bulk and nanostructured PVKs, with the latter particularly prominent because of their extremely large surface-to-volume ratio. To effectively passivate the different surface defects, a multitude of different ligands are necessary because each type of defect likely requires a different ligand for optimal passivation, as has been successfully demonstrated in a number of systems in essentially a "cocktail" approach. Characteristics of the ligands that affect effectiveness of passivation include size, shape, charge and charge distribution, orientation, conductivity, and interligand interaction. Examples of ligands for MHPs include both cationic and anionic or zwitterionic species with varied valences. The challenge is to identify the most effective ligand for each type of defect, and addressing this will require further experimental and theoretical study.

Spin-optotronic Properties of Organometal Halide Perovskites

Spin is an intrinsic quantum mechanical property of fundamental particles including the electron. The spin property is intimately related to electronic and optical properties of molecules and materials. The combination of spin (magnetic), electronic, and optical properties of materials, such as organometal halide perovskites (OMHP), has attracted increasing attention, which has led to a new field termed spin-optotronics based on all three key properties. This growing field has implications in emerging technological applications across disciplines, including photonics, electronics, spintronics, quantum computation, and information storage. This Perspective provides a brief introduction to this field from both experimental and computational aspects, with a focus on the effect of spin on charge carrier dynamics in OMHP, a class of materials with novel properties and promising applications in a number of fields. For instance, recent studies have demonstrated the use of ultrafast laser techniques in probing the fundamental charge carrier dynamics in relation to spin properties. Because of strong spin-orbit coupling (SOC) and broken inversion symmetry that result in Rashba and Dresselhaus effects, OMHP are considered ideal for manipulating spin states for spin-optotronics applications. In the meantime, on the basis of first-principles calculations and effective model Hamiltonians, the Rashba splitting in locally polarized domains can result in spin-forbidden recombination with significantly slow transition rate due to the mismatch of spin and momentum. We summarize the state-of-the-art first-principles methods and their current limitations for ultrafast charge and spin dynamics for realistic solid-state systems in general. To conclude, we note some promising future research and development directions for both experimental and theoretical ultrafast spin dynamics studies of OMHP.

Unique Optical Properties of Methylammonium Lead Iodide Nanocrystals Below the Bulk Tetragonal-Orthorhombic Phase Transition

Methylammonium (MA) and formamidinium (FA) lead halides are widely studied for their potential as low-cost, high-performance optoelectronic materials. Here, we present measurements of visible and IR absorption, steady state, and time-resolved photoluminescence from 300 K to cryogenic temperatures. Whereas FAPbI₃ nanocrystals (NCs) are found to behave in a very similar manner to reported bulk behavior, colloidal nanocrystals of MAPbI₃ show a departure from the low-temperature optical behavior of the bulk material. Using photoluminescence, visible, and infrared absorption measurements, we demonstrate that unlike single crystals and polycrystalline films NCs of MAPbI₃ do not undergo optical changes associated with the bulk tetragonal-to-orthorhombic phase transition, which occurs near 160 K. We find no evidence of frozen organic cation rotation to as low as 80 K or altered exciton binding energy to as low as 3 K in MAPbI₃ NCs. Similar results are obtained in MAPbI₃ NCs ranging from 20 to over 100 nm and in morphologies including cubes and plates. Colloidal MAPbI₃ NCs therefore offer a window into the properties of the solar-relevant, room-temperature phase of MAPbI₃ at temperatures inaccessible with single crystals or polycrystalline samples. Exploiting this phenomenon, these measurements reveal the existence of an optically passive photoexcited state close to the band edge and persistent slow Auger recombination at low temperature.

Acetone vapour-assisted growth of 2D single-crystalline organic lead halide perovskite microplates and their temperature-enhanced photoluminescence

We adopt an acetone vapour-assisted method to grow high quality single-crystalline microplates of two-dimensional (2D) perovskite, 2-phenylethylammonium lead bromide [(C₆H₅C₂H₄NH₃)₂PbBr₄]. The microplates, converted from the spin-coated films, are well-defined rectangles. Temperature dependent photoluminescence (PL) spectroscopy shows that the band gap PL is enhanced markedly with increasing temperature up to 218 K, accompanied by the quenching of the PL related to the trap states, which perhaps results from the exciton–phonon couplings. The optical phonon energy around 50 meV and the exciton binding energy around 120 meV are derived by fitting the band gap PL linewidths and intensities at different temperatures, respectively.

We report an acetone vapour-assisted method to grow single-crystalline 2D perovskite microplates and find their temperature-enhanced photoluminescence.