2D Halide Research Articles

Phase-dependent materials design and characteristic properties of two-dimensional (2D) halide perovskite single crystals for optoelectronic applications

Two-dimensional organic-inorganic hybrid halide perovskites have garnered much attention owing to their outstanding stability alongside unique quantum-well structures and anisotropic properties, leading to improved charge dynamics. Two-dimensional perovskites can be divided into three phases including Ruddlesden-Popper, Dion-Jacobson, and Alternating cations in the interlayer space phase. Each phase of these perovskites shows distinguished phase-dependent structural and optoelectrical properties. Tuning their properties by designing the materials can be a key strategy to enhance the device performance in optoelectrical applications. Configuration of spacer cations and the control of octahedral layer numbers (n ) can be important parameters in material design, enabling the tuning of dielectric properties, exciton binding energy, and bandgaps, as well as materials structures, thereby influencing stability and charge transport behaviors. In this point, two-dimensional perovskite single crystals can play essential roles in not only understanding phase-dependent intrinsic natures but also enhancing performance of optoelectronic applications, specifically owing to their long carrier diffusion length and enhanced stability with little grain boundaries and low trap density. This review will deliver the strategy of phase-dependent materials design with an understanding of their anisotropic properties and charge dynamics for optoelectronic applications, including photodetectors and X-ray detectors.

Aryl-Acetylene Layered Hybrid Perovskites in Photovoltaics.

Metal halide perovskites have shown exceptional potential in converting solar energy to electric power in photovoltaics, yet their application is hampered by limited operational stability. This stimulated the development of hybrid layered (two-dimensional, 2D) halide perovskites based on hydrophobic organic spacers, templating perovskite slabs, as a more stable alternative. However, conventional organic spacer cations are electronically insulating, resulting in charge confinement within the inorganic slabs, thus limiting their functionality. This can be ameliorated by extending the π-conjugation of the spacer cations. Wedemonstrate the capacity to access Ruddlesden-Popper and Dion-Jacobson 2D perovskites incorporating for the first time aryl-acetylene-based (4-ethynylphenyl)methylammonium (BMAA) and buta-1,3-diyne-1,4-diylbis(4,1-phenylene)dimethylammonium (BDAA) spacers, respectively. We assess their unique opto(electro)ionic characteristics by a combination of techniques and apply them in mixed-dimensional perovskite solar cells that show superior device performances with a power conversion efficiency of up to 23% and higher operational stability, opening the way for multifunctionality in layered hybrid materials and their application.

Quotient Complex (QC)-Based Machine Learning for 2D Hybrid Perovskite Design.

With remarkable stability and exceptional optoelectronic properties, two-dimensional (2D) halide layered perovskites hold immense promise for revolutionizing photovoltaic technology. Effective data representations are key to the success of all learning models. Currently, the lack of comprehensive and accurate material representations has hindered AI-based design and discovery of 2D perovskites, limiting their potential for advanced photovoltaic applications. In this context, this work introduces a novel computational topology framework termed the quotient complex (QC), which serves as the foundation for the material representation. The proposed QC-based features are seamlessly integrated with learning models for the advancement of 2D perovskite design. At the heart of this framework lies the quotient complex descriptors (QCDs), representing a quotient variation of simplicial complexes derived from materials' unit cell and periodic boundary conditions. Differing from prior material representations, this approach encodes higher-order interactions and periodicity information simultaneously. Based on the well-established new materials for solar energetics (NMSE) databank, the proposed QC-based machine learning models exhibit superior performance against all existing counterparts. This underscores the paramount role of periodicity information in predicting material functionality, while also showcasing the remarkable efficiency of the QC-based model in characterizing materials' structural attributes.

New synthesis of 2D halide perovskites assisted by the Langmuir-Schaefer methodology

New synthesis of 2D halide perovskites assisted by the Langmuir-Schaefer methodology

Chiral 2D Halide Perovskites for Piezoelectric Energy Harvesting and Ultrasound Detection

AbstractHybrid organic‐inorganic perovskite (HOIPs) has emerged as a great promising class of piezoelectric materials due to their solution processability, ease of structural adjustment, and high piezoelectricity. However, to date, their energy application and ultrasound detection has been relatively unexplored. Here, a pair of chiral two‐dimensional (2D) HOIPs, R‐/S‐[3MeOPEA]2PbBr4 (3MeOPEA=1‐(3‐Methoxyphenyl) ethylamine), were synthesized, and the structure, photoluminescence and piezoelectric properties are reported. Our results demonstrate these materials exhibit consistent broad PL peaks under 360 nm excitation and density functional theory (DFT) calculations reveal their direct bandgap characteristic. Moreover, the incorporation of chirality into the structure of R‐ and S‐[3MeOPEA]2PbBr4 endows them with significant piezoelectric properties. 5 wt% R‐[3MeOPEA]2PbBr4/PDMS (polydimethylsiloxane) composite film can generate a maximum open‐circuit piezoelectric voltages (Voc) of 1.98 V and short circuit currents (Isc) of 0.50 μA upon a loading force of 2 N at 10 Hz. Additionally, the device can sensitively detect ultrasound waves underwater at varying distances of 3, 6, and 9 mm, with corresponding peak‐to‐peak voltage (Vp‐p) responses of 52.3 mV, 41.1 mV, and 31.7 mV, respectively. Our research broadens the potential applications of multifunctional chiral 2D perovskite materials in the fields of piezoelectric energy harvesting and underwater ultra‐sound detection.

Fast, Highly Stable, and Low-Bandgap 2D Halide Perovskite Photodetectors Based on Short-Chained Fluorinated Piperidinium as a Spacer.

Ruddlesden-Popper (RP) two-dimensional (2D) halide perovskite (HP), with attractive structural and optoelectronic properties, has shown great potential in optoelectrical devices. However, the relatively wide bandgap (Eg) and stability, which cause inferior efficiency, prevent its feasibility from further applications. To tackle these issues, for the first time, a novel fluorine-containing piperidinium spacer, (3-HCF2CF2CH2OCH2-PPH+), abbreviated as (4FH-PPH+), has been designed for the stable and efficient n = 1 2D HPs. Its fluorophobicity can significantly enhance the noncovalent interactions between cations and [PbI6]4- octahedra. The observed Eg of the fluorinated (4FH-PPH)2PbI4 perovskite is found to be 2.22 eV, which is the lowest value among all fluorinated perovskites reported so far. Interestingly, this fluorinated film-based 2D perovskite photodetector (PD) exhibits the outstanding responsivity of 502 mA W-1, photodetectivity of 5.73 × 1010 cm Hz1/2 W-1, and impressive response/recovery time of 42/46 ms under 450 nm at 20 V. To the best of our knowledge, it is found for the first time that the 2D HP, with the fluorinated short-chained segment included into the organic spacer, shows remarkable stability for up to 49 days. These results strongly demonstrate the potential of the 2D fluorinated short-chained (4FH-PPH)2PbI4 HP with a low Eg as a promising candidate for next-generation optoelectronic devices.

(Invited) 2-Dimensional Halide Perovskite Memristors for Data Storage and Neuromorphic Computing

Halide perovskites, fascinating memristive materials owing to mixed ionic-electronic conductivity, have been attracting great attention as nonvolatile data storage and artificial synapses recently. However, polycrystalline nature in thin film form and instability under ambient air hamper them to be implemented in demonstrating reliable electronic devices. We successfully fabricated vertically aligned 2D halide perovskite films (V-HPs) for active layers of ReRAM and artificial synapses, showing moisture stability. Unlike random-oriented HPs, which exhibit negligible current hysteresis, V-HPs with Ag top electrodes exhibited highly stable and reliable binary memory performance. We built a flexible crossbar array to verify data storage capability, achieving a high (~100 %) device yield, robust endurance (5 × 106 cycles), long retention (2 × 105 s), reliability to operate under bending conditions, and moisture stability over a year. V-HPs with Au top electrodes showed multilevel analog memristive characteristics, programmable potentiation and depression with distinguished multi-states, long-short-term plasticity, paired-pulse facilitation, and even spike-timing-dependent plasticity. Our findings provide material design strategy that can be extended to the development of new semiconductor materials for next-generation memory devices.

Ferroelectric Properties and Bulk Photovoltaic Effects of 2D Hybrid Halide Perovskites

Two-dimensional (2D) hybrid organic-inorganic halide perovskites have emerged as a new class of tunable 2D semiconductors with the potential to combine excellent optoelectronic properties with symmetry-enabled properties such as ferroelectricity. Although many lead-based ferroelectric 2D halide perovskites have been studied so far, there is strong motivation to develop non-toxic lead-free perovskites. In this work, we report the ferroelectricity of 2D hybrid perovskites enabled by the rational design of noncentrosymmetric hybrid structures, including examples of lead-free tin-based 2D perovskites. Polarization-electric field hysteresis and second harmonic generation measurements confirm they are ferroelectric at room temperature. The frequency-dependent ferroelectric properties of lead- and tin- based pervoskites are studied, providing switching speed of reversible change of ferroelectric domain. Furthermore, in the photodetector devices fabricated with these new 2D perovskites, we observe non-zero short-circuit photocurrents without an external voltage, essentially a bulk photovoltaic effect (BVPE). The short-circuit current is switchable and controllable with poling voltage, which shows potential multiple ferroelectric states. As the first direct experimental observation of the spontaneous ferroelectric polarization of tin-based 2D hybrid perovskites, this work opens up environmentally friendly 2D tin halide perovskites for ferroelectricity and other physical property studies and could enable potential spin-orbitronic and neuromphoric devices in the future.

(Invited) Designing Noncentrosymmetric and Chiral 2D Quantum Materials Via Screw Dislocations and Structural Tuning

Rationally controlling the structures of two-dimensional (2D) materials allows us to tune the eelectronic structures and quantum states of matter, discover new physical properties, thus enabling new applications. Noncentrymmetry of these MX2 materials and 2D halide perovskites could be enabled at the monolayer and bulk crystal structure level by structural tuning and screw dislocation growth to result in chiroptical properties of individual chiral spiral microplates. Moreover, we achieved systematic interlayer twisting of MX2 spiral layers grown via screw dislocations due to mismatched geometry between Euclidean crystal lattices and non-Euclidean (curved) surfaces to form moiré superlattices, which could lead to novel quantum phenomena. Beyond optoelectronic applications, the large spin-orbit coupling in highly tunable 2D hybrid halide perovskites can lead to Rashba and/or Dresselhaus spin splitting and make them promising for spin-orbitronic applications. We utilize the complex interplay between the organic spacer cations, the A-site cations, metal cations and dimensionality to produce non-centrosymmetric structures that exhibit ferroelectricity, Rashba splitting and persistent spin textures. These rational design strategies to unlock and control non-centrosymmetry, chirality, and twist of 2D materials open up new nonlinear and chiral optical properties, spin-orbitronic, twistronics, and neuromorphic applications.

Stabilizing Metal Halide Perovskite Films via Chemical Vapor Deposition and Cryogenic Electron Beam Patterning.

Halide perovskites are hailed as semiconductors of the 21st century. Chemical vapor deposition (CVD), a solvent-free method, allows versatility in the growth of thin films of 3- and 2D organic-inorganic halide perovskites. Using CVD grown methylammonium lead iodide (MAPbI3) films as a prototype, the impact of electron beam dosage under cryogenic conditions is evaluated. With 5 kV accelerating voltage, the dosage is varied between 50 and 50000 µC cm-2. An optimum dosage of 35 000 µC cm-2 results in a significant blue shift and enhancement of the photoluminescence peak. Concomitantly, a strong increase in the photocurrent is observed. A similar electron beam treatment on chlorine incorporated MAPbI3, where chlorine is known to passivate defects, shows a blue shift in the photoluminescence without improving the photocurrent properties. Low electron beam dosage under cryogenic conditions is found to damage CVD grown 2D phenylethlyammoinum lead iodide films. Monte Carlo simulations reveal differences in electron beam interaction with 3- and 2D halide perovskite films.

Releasing charge transport barriers at low bias in perpendicularly aligned 2D halide perovskite

2D halide perovskites have garnered significant interest due to the great potential in optoelectronic devices. The 2D Perovskites samples are structured as n layers of [MX6]4− octahedral sheets inducing strong quantum confinement effects. Here, we have synthesized a series of 2D (BA)2MAn-1PbnI3n+1 and its 3D counterpart by controlling the particles alignment to the substrate. We have performed a comprehensive study involving electrical transport and photoconductivity effect as a function of the particle's organization: parallel, perpendicular, and disordered orientation to the electrodes. We have observed that for intermediary value of n, the 2D layered particles are aligned perpendicular to the electrode and parallel to the transport electrical current. A distinct and complex S-shape behavior in V-i curves is disclosed, suggesting two electrical resistance regimes. The sharp increase in resistance suggests the presence of electronic barriers for all samples, which is suppressed for columnar arrays of n = 2 and 3 under illumination. We suggest that this linear behavior in the photoconductivity effect is close related to the absence of electrical barriers at the interface of the particles due to perpendicular alignment to the electrodes. The absence of interfaces in a more columnar array facilitates ionic conductivity enhancing photoelectric effect. Our findings reveal distinct differences in charge transport for perpendicularly aligned 2D samples, opening up possibilities for important applications at low bias.

Linearly programmable two-dimensional halide perovskite memristor arrays for neuromorphic computing.

The exotic properties of three-dimensional halide perovskites, such as mixed ionic-electronic conductivity and feasible ion migration, have enabled them to challenge traditional memristive materials. However, the poor moisture stability and difficulty in controlling ion transport due to their polycrystalline nature have hindered their use as a neuromorphic hardware. Recently, two-dimensional (2D) halide perovskites have emerged as promising artificial synapses owing to their phase versatility, microstructural anisotropy in electrical and optoelectronic properties, and excellent moisture resistance. However, their asymmetrical and nonlinear conductance changes still limit the efficiency of training and accuracy of inference. Here we achieve highly linear and symmetrical conductance changes in Dion-Jacobson 2D perovskites. We further build a 7 × 7 crossbar array based on analogue perovskite synapses, achieving a high device yield, low variation with synaptic weight storing capability, multi-level analogue states with long retention, and moisture stability over 7 months. We explore the potential of such devices in large-scale image inference via simulations and show an accuracy within 0.08% of the theoretical limit. The excellent device performance is attributed to the elimination of gaps between inorganic layers, allowing the halide vacancies to migrate homogeneously regardless of grain boundaries. This was confirmed by first-principles calculations and experimental analysis.

Deterministic Synthesis of a Two-Dimensional MAPbI3 Nanosheet and Twisted Structure with Moiré Superlattice.

The synthesis of extremely thin 2D halide perovskites and the exploration of their interlayer interactions have garnered significant attention in current research. A recent advancement we have made involves the development of a successful technique for generating ultrathin MAPbI3 nanosheets with controlled thickness and an exposed intrinsic surface. This innovative method relies on utilizing the Ruddlesden-Popper (RP) phase perovskite (BA2MAn-1PbnI3n+1) as a template. However, the precise reaction mechanism remains incompletely understood. In this work, we systematically examined the dynamic evolution of the phase conversion process, with a specific focus on the influence of inorganic slab (composed of [PbI6]4- octahedrons) numbers on regulating the thickness and quality of the resulting MAPbI3 nanosheets. Additionally, the atomic structure is directly visualized using the transmission electron microscopy (TEM) method, confirming its exceptional quality. To illustrate interfacial interactions in ultrathin structures, artificial moiré superlattices are constructed through a physical transfer approach, revealing multiple localized high-symmetry stacks within a distinctive square moiré pattern. These findings establish a novel framework for investigating the physics of interfacial interactions in ionic semiconducting crystals.

2D and Quasi-2D Halide Perovskite-Based Resistive Switching Memory Systems

Resistive switching (RS) memory devices are gaining recognition as data storage devices due to the significant interest in their switching material, Halide perovskite (HP). The electrical characteristics include hysteresis in its current–voltage (I–V) relationship. It can be attributed to the production and migration of defects. This property allows HPs to be used as RS materials in memory devices. However, 3D HPs are vulnerable to moisture and the surrounding environment, making their devices more susceptible to deterioration. The potential of two-dimensional (2D)/quasi-2D HPs for optoelectronic applications has been recognized, making them a viable alternative to address current restrictions. Two-dimensional/quasi-2D HPs are created by including extended organic cations into the ABX3 frameworks. By adjusting the number of HP layers, it is possible to control the optoelectronic properties to achieve specific features for certain applications. This article presents an overview of 2D/quasi-2D HPs, including their structures, binding energies, and charge transport, compared to 3D HPs. Next, we discuss the operational principles, RS modes (bipolar and unipolar switching), in RS memory devices. Finally, there have been notable and recent breakthroughs in developing RS memory systems using 2D/quasi-2D HPs.

Mixed cation ordering scaffold polar 2D halide perovskite semiconductor for self-powered polarization-sensitive photodetection

Mixed cation ordering scaffold polar 2D halide perovskite semiconductor for self-powered polarization-sensitive photodetection

Rational Strategies to Improve the Efficiency of 2D Perovskite Solar Cells.

In the quest for durable photovoltaic devices, 2D halide perovskites have emerged as a focus of extensive research. However, the reduced dimension in structure is accompanied by inferior optical-electrical properties, such as widened band gap, enhanced exciton binding energy, and obstructed charge transport. As a result, the efficiency of 2D perovskite solar cells (PSCs) lags significantly behind their 3D counterparts. To overcome these constraints, extensive investigations into materials and processing techniques are pursued rigorously to augment the efficiency of 2D PSCs. Herein, The cutting-edge delve into developments in 2D PSCs, with a focus on chemical and material engineering, as well as their structure and photovoltaic properties. The review starts with an introduction of the crystal structure, followed by the key evaluation criteria of 2D PSCs. Then, the strategies around solution chemical engineering, processing technique, and interface optimization, to simultaneously boost efficiency and stability are systematically discussed. Finally, the challenges and perspectives associated with 2D perovskites to provide insights into potential improvements in photovoltaic performance will be outlined.

Revisiting Sub-Band Gap Emission Mechanism in 2D Halide Perovskites: The Role of Defect States.

Understanding the sub-band gap luminescence in Ruddlesden-Popper 2D metal halide hybrid perovskites (2D HaPs) is essential for efficient charge injection and collection in optoelectronic devices. Still, its origins are still under debate with respect to the role of self-trapped excitons or radiative recombination via defect states. In this study, we characterized charge separation, recombination, and transport in single crystals, exfoliated layers, and polycrystalline thin films of butylammonium lead iodide (BA2PbI4), one of the most prominent 2D HaPs. We combined complementary defect- and exciton-sensitive methods such as photoluminescence (PL) spectroscopy, modulated and time-resolved surface photovoltage (SPV) spectroscopy, constant final state photoelectron yield spectroscopy (CFSYS), and constant light-induced magneto transport (CLIMAT), to demonstrate striking differences between charge separation induced by dissociation of excitons and by excitation of mobile charge carriers from defect states. Our results suggest that the broad sub-band gap emission in BA2PbI4 and other 2D HaPs is caused by radiative recombination via defect states (shallow as well as midgap states) rather than self-trapped excitons. Density functional theory (DFT) results show that common defects can readily occur and produce an energetic profile that agrees well with the experimental results. The DFT results suggest that the formation of iodine interstitials is the initial process leading to degradation, responsible for the emergence of midgap states, and that defect engineering will play a key role in enhancing the optoelectronic properties of 2D HaPs in the future.

Ultrahigh energy harvesting ability of PVDF incorporated with 2D halide perovskite nanosheets via interface effect

Two-dimensional (2D) perovskites have recently emerged as an new family of ferroelectrics and attracted extensive attention for their specific surface interface characteristics benefitting from the unique structure by inserting hydrophobic organic cations into conventional three-dimensional frameworks. In this study, 2D metal–organic perovskite nanosheets, (C4H9NH3)2CsPb2Br7, ((BA)2CsPb2Br7), were successfully synthesized by facile temperature-cooling method and introduced into poly (vinylidene fluoride) (PVDF) polymer fibers to construct flexible piezoelectric energy harvester (PEH). (BA)2CsPb2Br7/PVDF nanofibers reveal a high piezoelectric coefficient (d33) of 63.3 pm/V, which is 2.9 times of the PVDF polymer (21.7 pm/V). Principally, the PVDF composite with 5 wt% (BA)2CsPb2Br7 nanosheet filler assumes an superior electroactive phase amount of about ∼ 93 % profiting from the interfacial interaction between BA+ cations of (BA)2CsPb2Br7 and the dipoles of PVDF. The PEH based on (BA)2CsPb2Br7 nanosheets embedded in PVDF fibers showed a maximum output voltage, 135 V, which is almost 17 times of the PEH based on neat PVDF fibers. Furthermore, the interfacial interaction between (BA)2CsPb2Br7 and PVDF was analyzed by first principle DFT study which reveals electrons migrate and the interfacial polarization between the interface. Simultaneously, (BA)2CsPb2Br7/PVDF composite fibers based PEH reveals exceptional long-term stabilities and durability. More attractively, the exceptional performance of this fabricated PEH evinces a pressure sensitivity of ∼ 7.3 V/N and bio-mechanical harvesting behavior arising from varied human motions, such as beating, pressing, twisting, walking and running. This cost-effective and high performance of (BA)2CsPb2Br7/PVDF based PEH is favorable for the continuation of 2D halide perovskites for further application in new energy and biomechanical filed.

Resistive random-access memories using quasi-2D halide perovskites for wafer-scale reliable switching behaviors

In this study, we propose the (PEA)2MA3Pb4I13, a quasi-two-dimensional (2D) halide perovskite, in the realm of resistive switching memory devices. A quasi-2D perovskite film (PEA)2MA3Pb4I13, with a thickness measuring 450 nm, has been effectively fabricated on a silicon substrate coated with platinum through an all-solution process conducted at low temperatures. The constructed resistive switching memory devices, incorporating an Ag/(PEA)2MA3Pb4I13/Pt/Ti/SiO2/Si configuration, display dependable and consistent bipolar switching attributes. The device shows an ultra-low operating voltage of less than +0.1 V, an impressive high ON/OFF ratio exceeding 108, reversible resistive switching via pulse voltage operation, and the capability for multilevel data storage. In addition, there is little variation in device characteristics depending on the location on a wafer or device size.

Charge Transfer in 2D Halide Perovskites and 2D/3D Heterostructures

Charge Transfer in 2D Halide Perovskites and 2D/3D Heterostructures