Excited-State Properties of Defected Halide Perovskite Quantum Dots: Insights from Computation

Nuclear Quantum Effects Accelerate Charge Recombination but Boost the Stability of Inorganic Perovskites in Mild Humidity

Experiments have demonstrated that mild humidity can enhance the stability of the CsPbBr3 perovskite, though the underlying mechanism remains unclear. Utilizing ab initio molecular dynamics, ring polymer molecular dynamics, and non-adiabatic molecular dynamics, our study reveals that nuclear quantum effects (NQEs) play a crucial role in stabilizing the lattice rigidity of the perovskite while simultaneously shortening the charge carrier lifetime. NQEs reduce the extent of geometric disorder and the number of atomic fluctuations, diminish the extent of hole localization, and thereby improve the electron-hole overlap and non-adiabatic coupling. Concurrently, these effects significantly suppress phonon modes and slow decoherence. As a result, these factors collectively accelerate charge recombination by a factor of 1.42 compared to that in scenarios excluding NQEs. The resulting sub-10 ns recombination time scales align remarkably well with experimental findings. This research offers novel insight into how moisture resistance impacts the stability and charge carrier lifetime in all-inorganic perovskites.

Point defect-mediated hot carrier relaxation dynamics of lead-free FASnI3 perovskites

In search of a promising optoelectronic performance, we herein investigated the hot carrier relaxation dynamics of a lead-free cubic phased bulk formamidinium tin triiodide (FASnI3) perovskite. To gain detailed theoretical insights, we should estimate the carrier relaxation dynamics of this pristine perovskite. To control the dynamics, point defects like central tin (Sn), iodine(I) anions, and formamidinium (FA) cations were introduced. With the iodine vacancy in the FASnI3 perovskite, the system seems to be unstable at room temperature, whereas the other three types of FASnI3 perovskites (pristine, Sn vacancy, and FA vacancy) are significantly stable at 300 K having semiconducting nature and excellent optical absorption in the UV-visible range. The computed electron-hole recombination time for the pristine system is 3.9 nanoseconds, which is in good agreement with the experimental investigation. The exciton relaxation processes in Sn and FA vacancy perovskites require 2.8 and 4.8 nanoseconds, respectively. These variations in the hot carrier relaxation dynamics processes are caused by the generation of significant changes in non-adiabatic coupling between energy levels, electron-phonon coupling, and quantum decoherence in different point defect analogous systems. The results presented here offer deeper insight into the temperature-dependent carrier relaxation dynamics of FASnI3 perovskites and thus open up opportunities for future exploration of their optoelectronic properties.

Defect-Passivated Photosensor Based on Cesium Lead Bromide (CsPbBr3) Perovskite Quantum Dots for Microbial Detection

A defect-passivated photosensor based on cesium lead bromide (CsPbBr3) perovskite quantum dots (QD) was fabricated using parylene films, and the photosensor was applied for the microbial detection. The CsPbBr3 perovskite QDs were synthesized to be homogeneous in size under thermodynamic control, and the perovskite QD-based photosensor was fabricated using MoS2 flakes as the electron transfer layer. In this work, a parylene film with functional groups was deposited on a photosensor for physical protection (waterproof) and defect (halide vacancy) passivation of the perovskite QD. As the first effect of the parylene film, the physical protection of the perovskite QD from water was estimated by comparing the photosensor performance after incubation in water. As the second effect of the parylene, the interaction between the functional groups of the parylene film and the halide vacancies of the perovskite QDs was investigated through the bandgap, crystal structure, and trap-state density analysis. Additionally, density functional theory analysis on Mulliken charges, lattice parameters, and Gibbs free energy demonstrated the effect of the defect passivation by parylene films. Finally, the parylene-passivated QD-based photosensor was applied to the detection of two kinds of food-poisoning and gastroduodenal disease bacteria (Listeria monocytogenes and Helicobacter pylori).

Utilizing the Undesirable Oxidation of Lead-Free Hybrid Halide Perovskite Nanosheets for Solar-Driven Photocatalytic C(sp3)─H Activation: Unraveling the Serendipity

Hybrid halide perovskites (HHPs), whose every branch generates intrusiveness, have been utilized in solar cells from a broader perspective. However, the inclusiveness of employing HHP as a photocatalyst is in its initial stage. This study mainly focuses on the unexpected utilization of, so far, undesirable material vacancy-ordered MA2SnBr6 quantum dots synthesized from MASnBr3 nanosheets. Here, the quantum confinement grounded a large blue shift in ultraviolet (UV) and photoluminescence (PL) spectra with a Stokes shift of 420 meV, where the band gap increase is observed as size decreases in MA2SnBr6. Remarkably, MA2SnBr6 exhibits air and moisture stability, better charge transfer, and high oxidation potential compared to MASnBr3. The first-principles-based atomistic computations reveal the strain relaxation in the Sn-Br framework that structurally stabilizes the MA2SnBr6 lattice. Furthermore, the direct band gap and strongly localized valence band edge give rise to a new potential photocatalyst MA2SnBr6 for efficient solar-driven C(sp3)─H activation of cyclohexane and toluene under ambient conditions.

Stable and Ultrafast Blue Cavity-Enhanced Superfluorescence in Mixed Halide Perovskites

Cavity-enhanced superfluorescence (CESF) in quantum dot (QD) system is an ultrafast and intense lasing generated by combination of quantum coupling effect and optically stimulated amplification effect, which can provide a new idea for realizing high quality blue light sources and address the limitation of conventional inefficient blue light sources. Modifying halide composition is a straightforward method to achieve blue emission in perovskite QD system. However, the spectral instability introduced by photoinduced halide phase segregation and low coupling efficiency between QDs and optical cavities make it challenging to achieve stable blue CESF in such halide-doped QD system. Herein, long-range-ordered, densely packed CsPbBr₂ Cl QD-assembled superlattice microcavities in which the two core issues can be appropriately addressed are developed. The QD superlattice structure facilitates excitonic delocalization to decrease exciton-phonon coupling, thus alleviating photoinduced phase segregation. By combination of theoretical analysis and temperature-dependent photoluminescence (PL) measurements, the underlying photoinduced phase segregation mitigation mechanism in mixed halide superlattices is clarified. Based on the CsPbBr₂ Cl QD superlattices with regularly geometrical structures, in which the gain medium can be strongly coupled to the naturally formed microcavity, stable and ultrafast (3 ps) blue CESF with excellent optical performance (threshold ≈33 µJ cm^-2 , quality factor ≈1900) is realized.

Point Defects in Two-Dimensional Ruddlesden-Popper Perovskites Explored with Ab Initio Calculations

Two-dimensional Ruddlesden-Popper (RP) halide perovskites stand out as excellent layered materials with favorable optoelectronic properties for efficient light-emitting, spintronic, and other spin-related applications. However, properties often determined by defects are not well understood in these perovskite systems. This work investigates the ground state electronic structure of commonly formed defects in a typical RP perovskite structure by density functional theory. Our study reveals that these 2D perovskites generally retain their defect tolerance with limited perturbation of the electronic structure in the case of neutral-type point defects. In contrast, donor/acceptor defects induce deep midgap states, potentially causing harm to the material's electronic performance. To retain positive intrinsic properties, the halide vacancies and interstitial defects should be avoided. The observed strong electron localization results in trap states and consequently leads to reduced device performance. This understanding can guide experimental efforts that aim for improved 2D halide perovskite-based device performance.

Induced Chirality in Halide Perovskite Clusters through Surface Chemistry

Chiroptical properties are of interest for various applications, including structure determination, polarized photodetectors, and spintronics. Inducing chiroptical activity into semiconductors is challenging because of difficulties in creating asymmetric crystal structures. One promising method is to use chirality transfer by deploying chiral organic molecules as capping ligands for nanocrystals. Experimentally, chiral-capped nanocrystals show emergent chiroptical signatures, but the mechanisms for chirality transfer remain unclear. Here we utilize atomistic modeling using time-dependent density functional theory calculations to explore chirality transfer in CsPbX₃ (X = Cl, I) clusters capped with chiral diaminocyclohexane (DACH) enantiomers. When DACH enantiomers are bound to the cluster surface, the perovskite optical transitions gain chiral signatures. This observed chirality transfer is best rationalized by chiral molecular dipole-cluster transition dipole coupling. With multiple DACH molecules bound to the cluster surface, anisotropy factors are found to increase proportionally to the surface ligand density, providing mechanistic insight toward improving chiroptical functionality in semiconductor nanomaterials.

Defect Tolerance Mechanism Revealed! Influence of Polaron Occupied Surface Trap States on CsPbBr3 Nanocrystal Photoluminescence: Ab Initio Excited-State Dynamics

Lead halide perovskite (LHP) nanocrystals (NCs) show exceptional defect tolerance which has been attributed to their unique electronic structure, where defect energy levels are not introduced inside the fundamental bandgap, and the role of polarons in screening charge carriers from defects. Here, we use ab initio atomistic simulations to explore the interplay between various surface chemistries (A = Cs⁺, R'NH₃⁺; X = Br^-, RCOO^-) used to passivate a CsPbBr₃ NC surface and their impact on the ground-state (GS) and excited-state (ES) photophysical properties. We investigate pristine fully passivated surfaces and A-X vacancy defects that reflect chemical reactions A⁺ + X^- → AX on the surface, which result in ligand desorption. For each surface configuration, calculations are performed in the GS and lowest ES (L-ES) electronic configurations, approximating polaron formation after photoexcitation. For models with A-X surface vacancies, we find that localized electron surface trap (ST) states emerge ∼100-400 meV below the pristine S_e band in the L-ES configuration due to polaronic nuclear reorganization. Surprisingly, these trap states contribute relatively bright S_h → ST spectral features. To test if these surface trap states remain bright in a dynamic (thermal) situation we implement excited-state molecular dynamics simulations. It is found that the surface defected model shows an enhanced nonradiative recombination rate which reduces the photoluminescence quantum yield (PLQY) from 95% for the pristine surface to 75%. This is accompanied by an order of magnitude reduction in PL intensity and a red shift of the transition energy. This study provides more evidence of the defect tolerance of LHP NCs along with evidence of surface trap states contributing to efficient photoluminescence. The observation of relatively bright surface trap states could provide insight into photophysical phenomena, such as size-dependent stretched-exponential photoluminescence decay and Stokes shifts.