Two-dimensional Lead Halide Perovskites Research Articles

Optical Studies of Doped Two-Dimensional Lead Halide Perovskites: Evidence for Rashba-Split Branches in the Conduction Band.

Two-dimensional (2D) hybrid organic/inorganic perovskites are an emerging materials class for optoelectronic and spintronic applications due to strong excitonic absorption and emission, large spin-orbit coupling, and Rashba spin-splitting effects. For many of the envisioned applications, tuning the majority charge carrier (electron or hole) concentration is desirable, but electronic doping of metal-halide perovskites has proven to be challenging. Here, we demonstrate electron injection into the lower-energy branch of the Rashba-split conduction band of 2D phenethylammonium lead iodide by means of n-type molecular doping at room temperature. The molecular dopant, benzyl viologen (BV), is shown to compensate adventitious p-type impurities and can lead to a tunable Fermi level above the conduction band minimum and increased conductivity in intrinsic samples. The doping-induced carrier concentration is monitored by the observation of free-carrier absorption and intraband optical transitions in the infrared spectral range. These optical measurements allow for an estimation of the Rashba splitting energy ER ≈38 ± 4 meV. Photoinduced quantum beating measurements demonstrate that the excess electron density reduces the electron spin g-factor by ca. 6%. This work demonstrates controllable carrier concentrations in hybrid organic/inorganic perovskites and yields potential for room temperature spin control through the Rashba effect.

Heterostructures via a Solution-Based Anion Exchange in Microcrystalline 2D Layered Metal-Halide Perovskites.

Layered perovskites consist of stacks of inorganic semiconducting metal-halide octahedra lattices sandwiched between organic layers with typically dielectric behavior. The in-plane confinement of electrical carriers in such two-dimensional metal halide perovskites drives a large range of appealing electronic properties, such as strong exciton binding, anisotropic charge diffusion, and polarization-directionality. Heterostructures provide additional control on carrier diffusion and localization, and in-plane heterojunctions are interesting because of the associated high charge mobility. Here, this work demonstrates a versatile solution-based approach to fabricate in-plane heterostructures with different halide composition in two-dimensional lead-halide perovskite microcrystals. This leads to spatially separated halide phases with different band gap and light emission. Interestingly, the composition of the exchanged phase and the morphology of the phase boundary depends on the exchange route, which can be related to the preferred localization of the halides at the equatorial or axial octahedra positions that either leads to dissolution and recrystallization of the octahedra lattice (for bromide to iodide), or allows for ion diffusion within the lattice (for iodide to bromide). These detailed insights on the ion exchange processes in layered perovskites will stimulate the development of heterostructures that can be tailored for different applications such as photocatalysis, energy storage, and light emission.

Two-Dimensional Lead Halide Perovskites with Spirocyclic Intercalating Cations

Two-Dimensional Lead Halide Perovskites with Spirocyclic Intercalating Cations

Organic and inorganic sublattice coupling in two-dimensional lead halide perovskites

Two-dimensional layered organic-inorganic halide perovskites have successfully spread to diverse optoelectronic applications. Nevertheless, there remain gaps in our understanding of the interactions between organic and inorganic sublattices that form the foundation of their remarkable properties. Here, we examine these interactions using pump-probe spectroscopy and ab initio molecular dynamics simulations. Unlike off-resonant pumping, resonant excitation of the organic sublattice alters both the electronic and lattice degrees of freedom within the inorganic sublattice, indicating the existence of electronic coupling. Theoretical simulations verify that the reduced bandgap is likely due to the enhanced distortion index of the inorganic octahedra. Further evidence of the mechanical coupling between these two sublattices is revealed through the slow heat transfer process, where the resultant lattice tensile strain launches coherent longitudinal acoustic phonons. Our findings explicate the intimate electronic and mechanical couplings between the organic and inorganic sublattices, crucial for tailoring the optoelectronic properties of two-dimensional halide perovskites.

Two-dimensional lead halide perovskite lateral homojunctions enabled by phase pinning

Two-dimensional organic-inorganic hybrid halide perovskites possess diverse structural polymorphs with versatile physical properties, which can be controlled by order-disorder transition of the spacer cation, making them attractive for constructing semiconductor homojunctions. Here, we demonstrate a space-cation-dopant-induced phase stabilization approach to creating a lateral homojunction composed of ordered and disordered phases within a two-dimensional perovskite. By doping a small quantity of pentylammonium into (butylammonium)2PbI4 or vice versa, we effectively suppress the ordering transition of the spacer cation and the associated out-of-plane octahedral tilting in the inorganic framework, resulting in phase pining of the disordered phase when decreasing temperature or increasing pressure. This enables epitaxial growth of a two-dimensional perovskite homojunction with tunable optical properties under temperature and pressure stimuli, as well as directional exciton diffusion across the interface. Our results demonstrate a previously unexplored strategy for constructing two-dimensional perovskite heterostructures by thermodynamic tuning and spacer cation doping.

Origin of the low-energy tail in the photoluminescence spectrum of CsPbBr3 nanoplatelets: a femtosecond transient absorption spectroscopic study.

CsPbBr3 nanoplatelets (NPLs), as some of the two-dimensional lead halide perovskites, have been intensively investigated due to their outstanding photophysical and photoelectric properties. However, there remain unclear fundamental issues on their carrier kinetics and the low-energy tail in their photoluminescence (PL) spectrum. In this paper, we synthesized CsPbBr3 NPLs with five [PbBr6]4- monolayers and performed comprehensive studies by using steady-state absorption, PL, and femtosecond transient absorption (fs-TA) spectroscopic measurements. We determined both the biexciton Auger recombination time (7 ± 2 ps) and trapped exciton lifetime (110 ± 15 ps) of the five monolayer CsPbBr3 NPLs. We also investigated the origin of the low-energy tail emission in their PL spectrum. More importantly, we found that a negative ΔA feature in the energy range of 2.45-2.55 eV appears in their fs-TA spectrum at 2, 4 and 10 ps delay times, which could help them act as a laser gain medium. The low-energy tail emission in their PL spectrum overlaps well with the negative ΔA feature in the energy range of 2.45-2.55 eV in their fs-TA spectrum at 2, 4 and 10 ps delay times.

Ultrafast and Nonlinear Optical Properties of Two-Dimensional Mo-Doped Dual Phase Inorganic Lead Halide Perovskite

Ultrafast and Nonlinear Optical Properties of Two-Dimensional Mo-Doped Dual Phase Inorganic Lead Halide Perovskite

Correlating Photophysical Properties with Stereochemical Expression of 6s2 Lone Pairs in Two-dimensional Lead Halide Perovskites.

Two-dimensional (2D) lead halide perovskites (LHPs) have shown great promises for light-emitting applications and excitonic devices. Fulfilling these promises demands an in-depth understanding on the relationships between the structural dynamics and exciton-phonon interactions that govern the optical properties. Here, we unveil the structural dynamics of 2D lead iodide perovskites with different spacer cations. Loose packing of an undersized spacer cation leads to out-of-plane octahedral tilting, whereas compact packing of an oversized spacer cation stretches Pb-I bond length, resulting in Pb2+ off-center displacement driven by stereochemical expression of the Pb2+ 6s2 lone pair electrons. Density functional theory calculations indicate that the Pb2+ cation is off-center displaced mainly along the direction where the octahedra are stretched the most by the spacer cation. We find dynamic structural distortions associated with either octahedral tilting or Pb2+ off-centering lead to a broad Raman central peak background and phonon softening, which increase the non-radiative recombination loss via exciton-phonon interactions and quench the photoluminescence intensity. The correlations between the structural, phonon, and optical properties are further confirmed by the pressure tuning of the 2D LHPs. Our results demonstrate that minimizing the dynamic structural distortions via a judicious selection of the spacer cations is essential to realize high luminescence properties in 2D LHPs.

Cyclic versus Linear Alkylammonium Cations: Preventing Phase Transitions at Operational Temperatures in 2D Perovskites.

Two-dimensional lead halide perovskites offer numerous attractive features for optoelectronics owing to their soft, deformable lattices and high degree of chemical tunability. While alteration of the metal and halide ions gives rise to significant modification of the bandgap energy, the organic spacer cations offer in-roads to tuning phase behavior and more subtle functionalities in ways that remain to be understood. Here, we study six variations of 2D perovskites changing only the organic spacer cations and demonstrate that these components intrinsically impact material response in important ways such as altering crystallographic structure, temperature-induced phase transitions, and photoluminescence emission. Two-dimensional perovskites containing commonly utilized aliphatic linear spacers, such as butylammonium, undergo phase transitions near room temperature. These transitions and temperature changes induce spacer-dependent variations in the emission spectra. Conversely, 2D perovskites comprising cyclic aliphatic spacers, such as cyclobutylammonium, are found to lack first-order phase transitions. These cyclic molecules are more sterically hindered within the crystal lattice, leading to temperature-induced contraction or expansion along certain crystallographic planes but no other significant thermal effects; additionally, they undergo changes in their emission spectra that cannot be explained by simple thermal expansion. Given the similarities in the dielectric and chemical makeup of this set of six alkylammonium molecules, these results are unexpected and suggest a large structural and thermal phase space via spacer manipulation that could lead to improved 2D perovskite functionalization.

Crystal structure prediction of quasi-two-dimensional lead halide perovskites

Two-dimensional lead halide perovskites are promising materials for optoelectronics due to the tunability of their properties with the number of lead halide layers and the choice of an organic spacer. Physical understanding for the rational design of materials primarily requires knowledge of crystal structure. Two-dimensional lead halide perovskites are usually prepared in the form of films, complicating the experimental determination of structure. To enable theoretical studies of experimentally unresolvable structures as well as high-throughput virtual screening, we present an algorithm for crystal structure prediction of lead halide perovskites. Using an automatically prepared classical potential we show that our algorithm enables fast access to a structure that can be used for further first-principles studies.

Enhanced Two-Dimensional Exciton Propagation via Strong Light–Matter Coupling with Surface Lattice Plasmons

Exciton–polaritons─hybrid exciton–photon quasi-particles─have enabled exciting long-range coherent transport by taking advantage of the properties of their photonic component. However, most experimental demonstrations of strong coupling have been based on semiconductor Fabry–Pérot microcavities. Here, we report an open and versatile exciton–polariton platform by integrating two-dimensional lead halide perovskites with plasmonic nanoparticle arrays, which support hybridization between excitons and surface lattice resonances in the strong coupling regime. Benefitting from the open architecture, we directly visualized polariton-enhanced exciton transport via angle-dependent transient absorption microscopy measurements. The observed transport enhancement extends into the picosecond timescale, leading to more than 1 order of magnitude improvement of exciton migration, from ∼10 nm to 100s of nanometers. We proposed a polariton-mediated exciton transport model and obtained an average polariton velocity of ∼1 × 106 m/s for states with an ∼40% photonic fraction. These results demonstrate plasmonic particle arrays as a versatile and adaptable cavity to enhance exciton transport.

Fluorescence Emission Is Highly Structure-Dependent in Hybrid Lead Halides.

Hybrid lead halide perovskites have received extensive scientific attention owing to their great potential in the field of fluorescent displays and light-emitting diodes. Currently, most luminescent materials contain functional molecular and rare-earth metal ion parts. However, the mechanism of photoluminescence property in two-dimensional hybrid lead halide perovskites with different layered inorganic skeletons has been reported rarely. To better understand the effect of an inorganic skeleton on the fluorescence property, here, we report three organic-inorganic hybrid materials with different layered inorganic frameworks: (MACH)2·PbBr4 (Prv-1, MACH = cyclohexylmethylammonium), (2-MPQ)·PbBr4 (Prv-2, 2-MPQ = 2-methylpiperazinium), and (TMBA)4·Pb3Br10 (Prv-3, TMBA = N'N'N'-trimethylbenzylammonium). Among them, Prv-1 is a (100)-oriented perovskite, Prv-2 belongs to the (110)-oriented perovskite, and the inorganic framework of Prv-3 possesses [Pb3Br10] units. Interestingly, Prv-1 has a strong blue-violet fluorescence emission, while the luminescence effect of Prv-2 is very weak; notably, Prv-3 emits a charming bright-orange light. Meanwhile, results of theoretical computational studies also reveal that the electronic structure of all three compounds is highly dependent on structurally distorted [PbBr6] octahedra, and the frontier molecular orbital (FMO) analysis further suggests that HOMO and LUMO of Prv-3 are contributed by inorganic and organic components, respectively. In addition, all three materials belong to direct band gap semiconductors, and the band gaps are 2.79, 2.97, and 2.76 eV, respectively. Significantly, there are obvious differences in conduction bands. Based on the above analysis, the photoluminescence mechanism of three hybrid materials is explained from the electronic levels. Consequentially, this work might provide practical strategies and perspectives for exploring novel structure-related properties.

Fabrication of two-dimensional hybrid organic–inorganic lead halide perovskites with controlled multilayer structures by liquid-phase laser ablation

We proposed a novel fabrication method of layered two-dimensional lead halide perovskites by liquid-phase laser ablation. Spacings between lead halide monolayer sheets can be controlled by choosing species of organic amine additives.

Machine learning assisted synthetic acceleration of Ruddlesden-Popper and Dion-Jacobson 2D lead halide perovskites

Two-dimensional lead halide perovskites (2D LHPs) can be generally categorized into the Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) phases, with unbiased and biased structural packing manners, respectively. To distinguish the two different phases during materials discovery, the synthesis has heavily relied on the traditional trial-and-error methods which severely delay the rapid development of these emerging 2D materials. In this work, we applied machine learning (ML) to accelerate the synthetic development of (100)-oriented 2D A2BX4 and ABX4 LHPs (A = organic amine cation, B = Pb, X = halide). The experimentally available 264 crystal structures were used as training data for our ML model to identify the descriptors influencing the formation of the RP- and DJ-phases. The number of nitrogen atoms able to be protonated, the heteroatom type, and the nitrogen content of the organic amine are identified as the most influential factors. To validate our model, three organic amines were tested by growing single crystals and two of the three new structures are consistent with our ML results. This study takes one step further toward the rational synthesis of RP- and DJ-phase hybrid 2D LHPs via an approach of ML.

Effects of Ammonium and Alkali Metal Additives on Anisotropic Photoconductivities and Solar Cell Efficiencies of Two-Dimensional Lead Halide Perovskites

Effects of Ammonium and Alkali Metal Additives on Anisotropic Photoconductivities and Solar Cell Efficiencies of Two-Dimensional Lead Halide Perovskites

Terahertz Modulation and Ultrafast Characteristic of Two-Dimensional Lead Halide Perovskites.

In recent years, two-dimensional (2D) halide perovskites have been widely used in solar cells and photoelectric devices due to their excellent photoelectric properties and high environmental stability. However, the terahertz (THz) and ultrafast responses of the 2D halide perovskites are seldom studied, limiting the developments and applications of tunable terahertz devices based on 2D perovskites. Here, 2D R-P type (PEA)2(MA)2Pb3I10 perovskite films are fabricated on quartz substrates by a one-step spin-coating process to study their THz and ultrafast characteristics. Based on our homemade ultrafast optical pump–THz probe (OPTP) system, the 2D perovskite film shows an intensity modulation depth of about 10% and an ultrafast relaxation time of about 3 ps at a pump power of 100 mW due to the quantum confinement effect. To further analyze the recombination mechanisms of the photogenerated carriers, a three-exponential function is used to fit the carrier decay processes, obtaining three different decay channels, originating from free carrier recombination, exciton recombination, and trap-assisted recombination, respectively. In addition, the photoconductor changes (∆σ) at different pump–probe delay times are also investigated using the Drude-Smith model, and a maximum difference of 600 S/m is obtained at τp = 0 ps for a pump power of 100 mW. Therefore, these results show that the 2D (PEA)2(MA)2Pb3I10 film has potential applications in high-performance tunable and ultrafast THz devices.

Mie Exciton-Polariton in a Perovskite Metasurface

Exciton-polariton arising from strong light-matter interaction between exciton and optical cavity has attracted considerable attention due to its potential applications in Bose-Einstein condensation, low-threshold lasing, and slow light. In recent years, two-dimensional lead halide perovskite has emerged as an ideal candidate for realizing exciton polariton at room temperature because it has large exciton binding energy and quantum yield. Here, we demonstrate that strong coupling could be enabled with a perovskite metasurface that supports multipolar Mie resonance, including magnetic quadrupole dominant, anapole, and toroidal resonances. For an array of perovskite nanodisks, the strong coupling behavior between these resonances and exciton is confirmed by the anticrossing features in absorption spectra mapping, while the Rabi splitting is increased from 230.7 meV in magnetic quadrupole-exciton strong coupling to 253 meV in both anapole-exciton and toroidal-exciton strong coupling. The enhanced Rabi splitting is attributed to the stronger field localization within the perovskite instead of within the air gap. In addition, we find that the Rabi splitting depends on the oscillatory strength of the exciton mode and can then be boosted to 362 meV in anapole-exciton strong coupling. Our results provide promising ways to improve the performance of optoelectronic devices such as low-threshold lasers and slow-light devices.

Solvothermal synthesis of perovskite CsPbCl3 nanoplates and improved photoluminescence performance through postsynthetic treatment

The controllable synthesis of two-dimensional all-inorganic lead halide perovskite (CsPbX3, X = Cl, Br and I) nanoplates (NPLs) has attracted wide attention of researchers. However, the direct synthesis of high-quality CsPbCl3 NPLs with controllable morphology still faces a challenge. Herein we report a solvothermal method to directly synthesize CsPbCl3 NPLs. By varying solvothermal reaction time, the thickness of the NPLs has be successfully adjusted from 2.8 to 3.6 nm, and the red shift of the photoluminescence (PL) emission from 391 to 402 nm has been realized. Subsequently, effective post-treatment on the as-prepared CsPbCl3 NPLs using CoCl2 results in the greatly enhancement of PL quantum yield (PLQY) from original 14.09 to final 50.4%, showing the best PLQY among all CsPbCl3 NPLs samples to date. This work not only provides a reliable process for the controllable preparation of CsPbCl3 NPLs, but also supplies more selectivity for the post-treatment of CsPbCl3 nanocrystals using chloride compounds.

Boosted luminescence efficiency and stability of Mn-doped perovskite nanoplatelets via incorporating Cd2+ ions

Mn-doped two-dimensional lead halide perovskites nanoplatelets (NPLs) possess promising application in solid state lighting. The Mn:L2[CsPbBr3]PbBr4 NPLs with corporation of Cd2+ were prepared via a hot injection method. The photoluminescence (PL) properties the NPLs with different Mn/Pb/Cd molar ratio from 0.5/1/0 to 5/1/3 were investigated by steady-state and time-resolved PL spectroscopy, indicating that the doping efficiency of Mn2+ can be boosted by adding Cd2+ to the NPLs. After thermal annealing, the PL quantum yield of Mn2+ ions reached up to 75%, which is Cd2+ content dependent. The temperature-dependent PL spectra demonstrated that the thermal activation energies of the NPLs could be increased by incorporating Cd2+, meaning that the nonradiative defect/trap states were effectively suppressed. These experimental results suggested the PL properties of Mn:L2[CsPbBr3]PbBr4 NPLs were optimized by adding Cd2+ ions.

Long carrier diffusion length in two-dimensional lead halide perovskite single crystals

<h2>Summary</h2> Ruddlesden-Popper (RP) perovskites are two-dimensional semiconductors for high-performance optoelectronic devices. In this work, we report a long in-plane carrier diffusion length in 2D RP perovskite single crystals probed by scanning photocurrent microscopy. Carrier diffusion lengths of 7–14 μm are observed when the number of PbI₆⁻² octahedra between organic spacers increases from 1 to 3. Using detailed light intensity and electric-field-dependent photocurrent measurements, we attribute the observed long diffusion length to the dominating dissociated free carrier transport. This is further validated by time-resolved photoluminescence measurements, where the decay lifetime increases in the presence of an electric field. From our experiments, we conclude that the in-plane transport in RP perovskites is efficient because of the partial free carrier generation, which overcomes strong excitonic effects. Our results suggest that semiconducting devices fabricated from RP perovskite single crystals can be as efficient as their 3D counterparts.