Emerging perovskite monolayers

Deterministic Fabrication and Quantum-Well Modulation of Phase-Pure 2D Perovskite Heterostructures for Encrypted Light Communication

Deterministic integration of phase-pure Ruddlesden-Popper (RP) perovskites has great significance for realizing functional optoelectronic devices. However, precise fabrications of artificial perovskite heterostructures with pristine interfaces and rational design over electronic structure configurations remain a challenge. Here, the controllable synthesis of large-area ultrathin single-crystalline RP perovskite nanosheets and the deterministic fabrication of arbitrary 2D vertical perovskite heterostructures are reported. The 2D heterostructures exhibit intriguing dual-peak emission phenomenon and dual-band photoresponse characteristic. Importantly, the interlayer energy transfer behaviors from wide-bandgap component to narrow-bandgap component modulated by comprising quantum wells are thoroughly revealed. Functional nanoscale photodetectors are further constructed based on the 2D heterostructures. Moreover, by combining the modulated dual-band photoresponse characteristic with double-beam irradiation modes, and introducing an encryption algorithm mechanism, a light communication system with high security and reliability is achieved. This work can greatly promote the development of heterogeneous integration technologies of 2D perovskites, and could provide a competitive candidate for advanced integrated optoelectronics.

Growth of millimeter-sized 2D metal iodide crystals induced by ion-specific preference at water-air interfaces

Conventional liquid-phase methods lack precise control in synthesizing and processing materials with macroscopic sizes and atomic thicknesses. Water interfaces are ubiquitous and unique in catalyzing many chemical reactions. However, investigations on two-dimensional (2D) materials related to water interfaces remain limited. Here we report the growth of millimeter-sized 2D PbI2 single crystals at the water-air interface. The growth mechanism is based on an inherent ion-specific preference, i.e. iodine and lead ions tend to remain at the water-air interface and in bulk water, respectively. The spontaneous accumulation and in-plane arrangement within the 2D crystal of iodide ions at the water-air interface leads to the unique crystallization of PbI2 as well as other metal iodides. In particular, PbI2 crystals can be customized to specific thicknesses and further transformed into millimeter-sized mono- to few-layer perovskites. Additionally, we have developed water-based techniques, including water-soaking, spin-coating, water-etching, and water-flow-assisted transfer to recycle, thin, pattern, and position PbI2, and subsequently, perovskites. Our water-interface mediated synthesis and processing methods represents a significant advancement in achieving simple, cost-effective, and energy-efficient production of functional materials and their integrated devices.

Inorganic-Organic Hybrid Layered Semiconductor AgSePh: Quasi-Solution Synthesis, Optical Properties, and Thermolysis Behavior

Two-dimensional inorganic-organic hybrid layered semiconductors are actively studied because of their naturally formed multiquantum well (MQW) structures and associated optical, photoelectric, and quantum optics characteristics. Silver benzeneselenolate (AgSePh, Ph = C6H5) is a new member of such hybrid layered materials, but has not fully been exploited. Herein, we present a quasi-solution method to prepare high quality free-standing AgSePh flake-like microcrystals by reacting diphenyl diselenide (Ph2Se2) with silver nanoparticles. The resultant AgSePh microflakes exhibit room-temperature (RT) resolvable MQW-induced quasi-particle quantization and interesting optical properties, such as three distinct excitonic resonance absorptions X1 (2.67 eV), X2 (2.71 eV), and X3 (2.83 eV) in the visible region, strong narrow-line width blue photoluminescence at ∼2.64 eV (470 nm) from the radiative recombination of the X1 exciton state, and a large exciton binding energy (∼0.35 eV). Furthermore, AgSePh microcrystals show high stability under water, oxygen, and heat environments, while above 220 °C, they will thermally decompose to silver and Ph2Se2 as evidenced by a combination of thermogravimetry and differential scanning calorimetry and pyrolysis-coupled gas chromatography-mass spectrometry studies. Finally, a comparison is extended between AgSePh and other metal benzeneselenolates, benzenethiolates, and alkanethiolates to clarify differences in their solubility, decomposition/melting temperature, and pyrolytic products.

Crystal structural effects on up/down-conversion luminescence properties of GdInO3:Tm,Yb perovskite phosphors for effective dual-mode anti-counterfeit applications

Developing advanced luminescent materials that are recognizable under specified conditions provides better opportunity for reliable optical anti-counterfeiting techniques. In this work, to the best of our knowledge, novel GdInO3:Tm,Yb perovskite phosphors with ultrafine sizes and rounded morphologies were successfully synthesized by a facile chemical precipitation route. Two-type perovskites with orthorhombic and hexagonal structures could be obtained by calcining the precursor at 850 and 1100 °C, respectively. Under 980 nm excitation, the two phosphors exhibited cyan-bluish emission at ∼460-565 nm, red emission at 645-680 nm, and near-infrared emission at 770-825 nm arising from 1G4 + 1D2→3H5,6, 3F2,3→3H6, and 3H4→3H6 transitions of Tm3+, respectively, where the hexagonal perovskite phosphor had relatively strong and sharp red emission as well as red-shifted cyan-bluish emission via successive cross relaxations. The Yb3+ sensitizer enhanced the upconversion luminescence via effective Yb3+→Tm3+ energy transfer and the optimal Yb3+ concentrations were 10 at.% for orthorhombic perovskite and 5 at.% for hexagonal one. The upconversion mechanism mainly ascribed to two-photon processes while three-photon was also present. Upon excitation at 254 nm, their down-conversion spectra exhibited broad multibands in the wavelength range of 400-500 nm deriving from combined effects of the defect-induced emission of GdInO3 and the 1D2→3F4 + 4G4→3H6 emissions of Tm3+. The energy transfer from GdInO3 defect level to Tm3+ excitation state was observed for the first time. The unclonable security codes prepared by screen printing from those dual-mode emitting perovskite phosphors were almost invisible under natural light, which had promising potential for anti-counterfeiting application.

Interfacial Rivet to Fill Structural Defects: A Spacer Engineering Gift for 3D Solar Cells

The combination of 2D and 3D perovskites to passivate surfaces or interfaces with a high concentration of defects shows great promise for improving the efficiency of perovskite solar cells (PSCs). Constructing high-quality perovskite film systems by precisely modulating 2D perovskites with good morphologies and growth sites on 3D perovskite films remains a formidable challenge due to the complexity of spacer-engineered surface reactions. In this study, phase-pure 2D (HA)2(MA)n-1PbnI3n+1 perovskites with a controlled number of layers (n) are separated on a large scale and exploited as interface rivets to optimize 3D perovskite films, resulting in tunable film structural defects and grain boundaries. The optimized PSCs system benefits from a reduction in non-radiative recombination, resulting in improved optical performance, higher mobility, and lower trap density. The corresponding device achieves a champion power conversion efficiency (PCE) of more than 25%, especially for voltage (VOC) and fill factor (FF). The quality and uniformity of the perovskite films are further confirmed using large-area devices with an active area of 14 cm2, which exhibits a PCE of more than 21.24%. The high-quality thin-film system based on the 2D perovskites presented herein provides a new perspective for improving the efficiency and stability of PSCs.

Recent advance of high-quality perovskite nanostructure and its application in flexible photodetectors

Flexible photodetectors (PDs) have garnered increasing attention for their potential applications in diverse fields, including weather monitoring, smart robotics, smart textiles, electronic eyes, wearable biomedical monitoring devices, and so on. Notably, perovskite nanostructures have emerged as a promising material for flexible PDs due to their distinctive features, such as a large optical absorption coefficient, tunable band gap, extended photoluminescence decay time, high carrier mobility, low defect density, long exciton diffusion lengths, strong self-trapped effect, good mechanical flexibility, and facile synthesis methods. In this review, we first introduce various synthesis methods for perovskite nanostructures and elucidate their corresponding optical and electrical properties, encompassing quantum dots, nanocrystals, nanowires, nanobelts, nanosheets, single-crystal thin films, polycrystalline thin films, and nanostructured arrays. Furthermore, the working mechanism and key performance parameters of optoelectronic devices are summarized. The review also systematically compiles recent advancements in flexible PDs based on various nanostructured perovskites. Finally, we present the current challenges and prospects for the development of perovskite nanostructures-based flexible PDs.

Strong Sliding Ferroelectricity and Interlayer Sliding Controllable Spintronic Effect in Two-Dimensional HgI2 Layers

Exploration of two-dimensional (2D) sliding ferroelectric (FE) materials with experimentally detectable ferroelectricity and value-added novel functionalities is highly sought for the development of 2D "slidetronics". Herein, based on first-principles calculations, we identify the synthesizable van der Waals (vdW) layered crystals HgX2 (X = Br and I) as a new class of 2D sliding ferroelectrics. Both HgBr2 and HgI2 in 2D multilayered forms adopt the preferential stacking sequence, leading to room temperature stable out-of-plane (vertical) ferroelectricity that can be reversed via the sliding of adjacent monolayers. Owing to strong interlayer coupling and interfacial charge rearrangement, 2D HgI2 layers possess strong sliding ferroelectricity up to 0.16 μC/cm2, readily detectable in experiment. Moreover, robust sliding ferroelectricity and interlayer sliding controllable Rashba spin texture of FE-HgI2 layers enable potential applications as 2D spintronic devices such that the electric control of electron spin detection can be realized at the 2D regime.

Topological Nodal-Point Superconductivity in Two-Dimensional Ferroelectric Hybrid Perovskites

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) with enhanced stability, high tunability, and strong spin-orbit coupling have shown great potential in vast applications. Here, we extend the already rich functionality of 2D HOIPs to a new territory, realizing topological superconductivity and Majorana modes for fault-tolerant quantum computation. Especially, we predict that room-temperature ferroelectric BA2PbCl4 (BA for benzylammonium) exhibits topological nodal-point superconductivity (NSC) and gapless Majorana modes on selected edges and ferroelectric domain walls when proximity-coupled to an s-wave superconductor and an in-plane Zeeman field, attractive for experimental verification and application. Since NSC is protected by spatial symmetry of 2D HOIPs, we envision more exotic topological superconducting states to be found in this class of materials due to their diverse noncentrosymmetric space groups, which may open a new avenue in the fields of HOIPs and topological superconductivity.

Atomic Imaging of Multi-Dimensional Ruddlesden-Popper Interfaces in Lead-Halide Perovskites

Ruddlesden-Popper (RP) interface with defined stacking structure will fundamentally influence the optoelectronic performances of lead-halide perovskite (LHP) materials and devices. However, it remains challenging to observe the atomic local structures in LHPs, especially for multi-dimensional RP interface hidden inside the nanocrystal. In this work, the advantages of two imaging modes in scanning transmission electron microscopy (STEM), including high-angle annular dark field (HAADF) and integrated differential phase contrast (iDPC) STEM, are successfully combined to study the bulk and local structures of inorganic and organic/inorganic hybrid LHP nanocrystals. Then, the multi-dimensional RP interfaces in these LHPs are atomically resolved with clear gap and blurred transition region, respectively. In particular, the complex interface by the RP stacking in 3D directions can be analyzed in 2D projected image. Finally, the phase transition, ion missing, and electronic structures related to this interface are investigated. These results provide real-space evidence for observing and analyzing atomic multi-dimensional RP interfaces, which may help to better understand the structure-property relation of LHPs, especially their complex local structures.

Highly efficient and fast modulation of magnetic coupling interaction in the SrCoO2.5/La0.7Ca0.3MnO3 heterostructure

Effective manipulation of magnetic properties in transition-metal oxides is one of the crucial issues for the application of materials. Up to now, most investigations have focused on electrolyte-based ionic control, which is limited by the slow speed. In this work, the interfacial coupling of the SrCoO2.5/La0.7Ca0.3MnO3 (LCMO) bilayer is effectively modulated with fast response time. After being treated with diluted acetic acid, the bilayer changes from antiferromagnetic/ferromagnetic (AFM/FM) coupling to FM/FM coupling and the Curie temperature is also effectively increased. Meanwhile, the corresponding electric transport properties are modulated within a very short time. Combined with the structure characterization and X-ray absorption measurements, we find that the top SrCoO2.5 layer is changed from the antiferromagnetic insulator to the ferromagnetic metal phase, which is attributed to the formation of the active oxygen species due to the reaction between the protons in the acid and the SrCoO2.5 layer. The bottom LCMO layer remains unchanged during this process. The response time of the bilayer with the acid treatment method is more than an order of magnitude faster than other methods. It is expected that this acid treatment method may open more possibilities for manipulating the magnetic and electric properties in oxide-based devices.

High-efficiency reinforcement learning with hybrid architecture photonic integrated circuit

Reinforcement learning (RL) stands as one of the three fundamental paradigms within machine learning and has made a substantial leap to build general-purpose learning systems. However, using traditional electrical computers to simulate agent-environment interactions in RL models consumes tremendous computing resources, posing a significant challenge to the efficiency of RL. Here, we propose a universal framework that utilizes a photonic integrated circuit (PIC) to simulate the interactions in RL for improving the algorithm efficiency. High parallelism and precision on-chip optical interaction calculations are implemented with the assistance of link calibration in the hybrid architecture PIC. By introducing similarity information into the reward function of the RL model, PIC-RL successfully accomplishes perovskite materials synthesis task within a 3472-dimensional state space, resulting in a notable 56% improvement in efficiency. Our results validate the effectiveness of simulating RL algorithm interactions on the PIC platform, highlighting its potential to boost computing power in large-scale and sophisticated RL tasks.

Thickness control of organic semiconductor-incorporated perovskites

Two-dimensional organic semiconductor-incorporated perovskites are a promising family of hybrid materials for optoelectronic applications, owing in part to their inherent quantum well architecture. Tuning their structures and properties for specific properties, however, has remained challenging. Here we report a general method to tune the dimensionality of phase-pure organic semiconductor-incorporated perovskite single crystals during their synthesis, by judicious choice of solvent. The length of the conjugated semiconducting organic cations and the dimensionality (n value) of the inorganic layers can be manipulated at the same time. The energy band offsets and exciton dynamics at the organic-inorganic interfaces can therefore be precisely controlled. Furthermore, we show that longer and more planar π-conjugated organic cations induce a more rigid inorganic crystal lattice, which leads to suppressed exciton-phonon interactions and better optoelectronic properties as compared to conventional two-dimensional perovskites. As a demonstration, optically driven lasing behaviour with substantially lower lasing thresholds was realized.

A promising perovskite primary explosive

A primary explosive is an ideal chemical substance for performing ignition in military and commercial applications. For over 150 years, nearly all of the developed primary explosives have suffered from various issues, such as troublesome syntheses, high toxicity, poor stability or/and weak ignition performance. Now we report an interesting example of a primary explosive with double perovskite framework, {(C6H14N2)2[Na(NH4)(IO4)6]}n (DPPE-1), which was synthesized using a simple green one-pot method in an aqueous solution at room temperature. DPPE-1 is free of heavy metals, toxic organic components, and doesn't involve any explosive precursors. It exhibits good stability towards air, moisture, sunlight, and heat and has acceptable mechanical sensitivities. It affords ignition performance on par with the most powerful primary explosives reported to date. DPPE-1 promises to meet the challenges existing with current primary explosives, and this work could trigger more extensive applications of perovskite.

Ambient-Stable 2D Dion-Jacobson Phase Tin Halide Perovskite Field-Effect Transistors with Mobility over 1.6 Cm2 V-1 s-1

Solution-processed metal halide perovskites hold immense potential for the advancement of next-generation field-effect transistors (FETs). However, the instability of perovskite-based transistors has impeded their progress and practical applications. Here, ambient-stable high-performance FETs based on 2D Dion-Jacobson phase tin halide perovskite BDASnI4 , which has high film quality and excellent electrical properties, are reported. The perovskite channels are established by engineering the film crystallization process via the employment of ammonium salt interlayers and the incorporation of NH4 SCN additives within the precursor solution. The refined FETs demonstrate field-effect hole mobilities up to 1.61 cm2 V-1 s-1 and an on/off ratio surpassing 106 . Moreover, the devices show impressive operational and environmental stability and retain their functional performance even after being exposed to ambient conditions with a temperature of 45 °C and humidity of 45% for over 150 h.

A Self-Powered UV Photodetector With Ultrahigh Responsivity Based on 2D Perovskite Ferroelectric Films With Mixed Spacer Cations

Self-powered photodetectors (PDs) have the advantages of no external power requirement, wireless operation, and long life. Spontaneous ferroelectric polarizations can significantly increase built-in electric field intensity, showing great potential in self-powered photodetection. Moreover, ferroelectrics possess pyroelectric and piezoelectric properties, beneficial for enhancing self-powered PDs. 2D metal halide perovskites (MHPs), which have ferroelectric properties, are suitable for fabricating high-performance self-powered PDs. However, the research on 2D metal halide perovskites ferroelectrics focuses on growing bulk crystals. Herein, 2D ferroelectric perovskite films with mixed spacer cations for self-powered PDs are demonstrated by mixing Ruddlesden-Popper (RP)-type and Dion-Jacobson (DJ)-type perovskite. The (BDA0.7 (BA2 )0.3 )(EA)2 Pb3 Br10 film possesses, overall, the best film qualities with the best crystalline quality, lowest trap density, good phase purity, and obvious ferroelectricity. Based on the ferro-pyro-phototronic effect, the PD at 360 nm exhibits excellent photoelectric properties, with an ultrahigh peak responsivity greater than 93 A W-1 and a detectivity of 2.5 × 1015 Jones, together with excellent reproducibility and stability. The maximum responsivities can be modulated by piezo-phototronic effect with an effective enhancement ratio of 480%. This work will open up a new route of designing MHP ferroelectric films for high-performance PDs and offers the opportunity to utilize it for various optoelectronics applications.

Theoretical study of electrocatalytic properties of low-dimensional freestanding PbTiO3 for hydrogen evolution reactions

The discovery of novel materials for catalytic purposes that are highly stable is one of the main challenges nowadays for reducing our dependence on fossil fuels. Here, low-dimensional PbTiO3 is introduced as an electrocatalyst using first-principles calculations. Density-functional theory calculations indicate that 2D-PbTiO3 is dynamically and thermodynamically stable. Our results show that a single oxygen defect vacancy in 2D-PbTiO3 can play a key role in enhancing the hydrogen evolution reaction (HER), together with the Ti atoms. Our study concludes that the Volmer-Heyrovsky mechanism is a more favorable route to achieve HER than the Volmer-Tafel mechanism, including solvation and vacuum conditions.

2D Materials in the Display Industry: Status and Prospects

With advances in flexible electronics, innovative foldable, rollable, and stretchable displays have been developed to maintain their performance under various deformations. These flexible devices can develop more innovative designs than conventional devices due to their light weight, high space efficiency, and practical convenience. However, developing flexible devices requires material innovation because the devices must be flexible and exhibit desirable electrical insulating/semiconducting/metallic properties. Recently, emerging 2D materials such as graphene, hexagonal boron nitride, and transition metal dichalcogenides have attracted considerable research attention because of their outstanding electrical, optical, and mechanical properties, which are ideal for flexible electronics. The recent progress and challenges of 2D material growth and display applications are reviewed and perspectives for exploring 2D materials for display applications are discussed.

Functionalizing nanophotonic structures with 2D van der Waals materials

The integration of two-dimensional (2D) van der Waals materials with nanostructures has triggered a wide spectrum of optical and optoelectronic applications. Photonic structures of conventional materials typically lack efficient reconfigurability or multifunctionality. Atomically thin 2D materials can thus generate new functionality and reconfigurability for a well-established library of photonic structures such as integrated waveguides, optical fibers, photonic crystals, and metasurfaces, to name a few. Meanwhile, the interaction between light and van der Waals materials can be drastically enhanced as well by leveraging micro-cavities or resonators with high optical confinement. The unique van der Waals surfaces of the 2D materials enable handiness in transfer and mixing with various prefabricated photonic templates with high degrees of freedom, functionalizing as the optical gain, modulation, sensing, or plasmonic media for diverse applications. Here, we review recent advances in synergizing 2D materials to nanophotonic structures for prototyping novel functionality or performance enhancements. Challenges in scalable 2D materials preparations and transfer, as well as emerging opportunities in integrating van der Waals building blocks beyond 2D materials are also discussed.

Recent Advances in Moiré Superlattice Systems by Angle-Resolved Photoemission Spectroscopy

The last decade has witnessed a flourish in 2D materials including graphene and transition metal dichalcogenides (TMDs) as atomic-scale Legos. Artificial moiré superlattices via stacking 2D materials with a twist angle and/or a lattice mismatch have recently become a fertile playground exhibiting a plethora of emergent properties beyond their building blocks. These rich quantum phenomena stem from their nontrivial electronic structures that are effectively tuned by the moiré periodicity. Modern angle-resolved photoemission spectroscopy (ARPES) can directly visualize electronic structures with decent momentum, energy, and spatial resolution, thus can provide enlightening insights into fundamental physics in moiré superlattice systems and guides for designing novel devices. In this review, first, a brief introduction is given on advanced ARPES techniques and basic ideas of band structures in a moiré superlattice system. Then ARPES research results of various moiré superlattice systems are highlighted, including graphene on substrates with small lattice mismatches, twisted graphene/TMD moiré systems, and high-order moiré superlattice systems. Finally, it discusses important questions that remain open, challenges in current experimental investigations, and presents an outlook on this field of research.

Multiple-ion Management of Perovskites by Regulating Spatial Distribution of Hydroxyls in Oligosaccharides

Suppressing migrations of intrinsic and extrinsic ions (e.g., Pb2+ , I- , FA+ /MA+ , and Li+ ) in organic-inorganic hybrid perovskites is critical for alleviating the hysteresis and degradation of perovskite solar cells (PSCs). However, various additives reported for that purpose usually interact with one or two types of those ions, not inhibiting multiple-ion migrations simultaneously. Two oligosaccharides (β-cyclodextrin (β-CD) and maltotetraose (G4)), containing 14 hydroxyls (-OH) with different spatial distributions, for the suppression of multiple-ion migrations in PSCs is herein employed. Compared to linear arrangement of -OH in G4, annular distribution of -OH around wide and narrow rims of β-CD can form supramolecular multi-site interactions in a focal manner with various ions, more effectively capturing and immobilizing these migrated ions. With this multiple-ion management strategy, β-CD-based PSCs exhibit an impressive efficiency of 24.22% with negligible hysteresis and excellent device stability. This work highlights the significances of multi-site interactions and molecular configuration of the additive for inhibiting multi-ion migrations in PSCs.

High-Temperature Ferroelasticity and Photoluminescence in a 2D Monolayer Perovskite Compound: (C5NH8Br)2PbBr4

Hybrid organic-inorganic perovskites (HOIPs) have attracted much attention due to their excellent properties and easy synthesis. As far as we know, most documented ferroelastics mainly focus on the 3D (three-dimensional) perovskites, the 2D monolayer perovskite ferroelastics are rarely reported before. In this work, we synthesized a 2D lead-based perovskite (C₅NH₁₃Br)₂PbBr₄ (1) (C₅NH₁₃Br = 5-bromoamylamine cation) by introducing flexible chain organic cations. The evolution of ferroelastic domains observed by a polarized light microscope confirms that compound 1 undergoes a ferroelastic phase transition at 392/384 K. In addition, its direct band gap is 2.877 eV. Interestingly, the material emits an attractive blue light (quantum yield 5.06%) under UV light. Three structural descriptors are introduced to quantitatively analyze the relationship between structural distortion and the shape of emission peak. This work provides a way to design multifunctional perovskite-type materials.

Moisture-Tailored 2D Dion-Jacobson Perovskites for Reconfigurable Optoelectronics

Humidity- and moisture-induced degradation has been a longstanding problem in perovskite materials, affecting their long-term stability during applications. Counterintuitively, the moisture is leveraged to tailor the reversible hydrochromic behaviors of a new series of 2D Dion-Jacobson (DJ) perovskites for reconfigurable optoelectronics. In particular, the hydrogen bonds between organic cations and water molecules can be dynamically modulated via moisture removal/exposure. Remarkably, such modulation confines the movement of the organic cations close to the original position, preventing their escape from crystal lattices. Furthermore, this mechanism is elucidated by theoretical analysis using first-principles calculations and confirmed with the experimental characterizations. The reversible fluorescent transition 2D DJ perovskites show excellent cyclical properties, presenting untapped opportunities for reconfigurable optoelectronic applications. As a proof-of-concept demonstration, an anti-counterfeiting display is shown based on patterned reversible 2D DJ perovskites. The results represent a new avenue of reconfigurable optoelectronic application with 2D DJ perovskites for humidity detection, anti-counterfeiting, sensing, and other emerging photoelectric intelligent technologies.

Strong Ferromagnetic Manipulation of SrTiO3 from CO2 -Straining Effect on Electronic Structure Modulation

2D magnetic materials are ideal to fabricate magneto-optical, magneto-electric, and data storage devices, which are proposed to be critical to the next generation of information technologies. Benefited from their labile structures, 2D perovskites are amenable for magnetic manipulation through structural optimization. In this work, 2D room-temperature ferromagnetic SrTiO₃ is achieved through straining effect induced by supercritical carbon dioxide (SC CO₂ ). According to experimental results, the cubic phase of SrTiO₃ is converted to tetragonal with exposure of (110), (200), (111), and (211) planes over the SC CO₂ treatment, leading to significant ferromagnetic enhancement. Theoretical calculations illustrate that over the conversion from cubic to tetragonal, the electronic structure of SrTiO₃ is significantly modulated. Specifically, the spin density of planes of (200), (111), and (211) is enhanced, presumably due to the stabilization of the highest occupied molecular orbital over straining by SC CO₂ , leading to magnetic optimizations. This work suggests that magnetic optimization can be achieved from SC CO₂ -induced electronic structure modulation.

Single-Photon Emitting Arrays by Capillary Assembly of Colloidal Semiconductor CdSe/CdS/SiO2 Nanocrystals

The controlled placement of colloidal semiconductor nanocrystals (NCs) onto planar surfaces is crucial for scalable fabrication of single-photon emitters on-chip, which are critical elements of optical quantum computing, communication, and encryption. The positioning of colloidal semiconductor NCs such as metal chalcogenides or perovskites is still challenging, as it requires a nonaggressive fabrication process to preserve the optical properties of the NCs. In this work, periodic arrays of 2500 nanoholes are patterned by electron beam lithography in a poly(methyl methacrylate) (PMMA) thin film on indium tin oxide/glass substrates. Colloidal core/shell CdSe/CdS NCs, functionalized with a SiO₂ capping layer to increase their size and facilitate deposition into 100 nm holes, are trapped with a close to optimal Poisson distribution into the PMMA nanoholes via a capillary assembly method. The resulting arrays of NCs contain hundreds of single-photon emitters each. We believe this work paves the way to an affordable, fast, and practical method for the fabrication of nanodevices, such as single-photon-emitting light-emitting diodes based on colloidal semiconductor NCs.

Emerging 2D Metal Oxides: From Synthesis to Device Integration

2D metal oxides have aroused increasing attention in the field of electronics and optoelectronics due to their intriguing physical properties. In this review, an overview of recent advances on synthesis of 2D metal oxides and their electronic applications is presented. First, the tunable physical properties of 2D metal oxides that relate to the structure (various oxidation-state forms, polymorphism, etc.), crystallinity and defects (anisotropy, point defects, and grain boundary), and thickness (quantum confinement effect, interfacial effect, etc.) are discussed. Then, advanced synthesis methods for 2D metal oxides besides mechanical exfoliation are introduced and classified into solution process, vapor-phase deposition, and native oxidation on a metal source. Later, the various roles of 2D metal oxides in widespread applications, i.e., transistors, inverters, photodetectors, piezotronics, memristors, and potential applications (solar cell, spintronics, and superconducting devices) are discussed. Finally, an outlook of existing challenges and future opportunities in 2D metal oxides is proposed.

Performance Enhancement of Lead-Free 2D Tin Halide Perovskite Transistors by Surface Passivation and Its Impact on Non-Volatile Photomemory Characteristics

Two-dimensional (2D) tin (Sn)-based perovskites have recently received increasing research attention for perovskite transistor application. Although some progress is made, Sn-based perovskites have long suffered from easy oxidation from Sn²⁺ to Sn⁴⁺ , leading to undesirable p-doping and instability. In this study, it is demonstrated that surface passivation by phenethylammonium iodide (PEAI) and 4-fluorophenethylammonium iodide (FPEAI) effectively passivates surface defects in 2D phenethylammonium tin iodide (PEA₂ SnI₄ ) films, increases the grain size by surface recrystallization, and p-dopes the PEA₂ SnI₄ film to form a better energy-level alignment with the electrodes and promote charge transport properties. As a result, the passivated devices exhibit better ambient and gate bias stability, improved photo-response, and higher mobility, for example, 2.96 cm² V^-1 s^-1 for the FPEAI-passivated films-four times higher than the control film (0.76 cm² V^-1 s^-1 ). In addition, these perovskite transistors display non-volatile photomemory characteristics and are used as perovskite-transistor-based memories. Although the reduction of surface defects in perovskite films results in reduced charge retention time due to lower trap density, these passivated devices with better photoresponse and air stability show promise for future photomemory applications.

Stereopsis-Inspired 3D Visual Imaging System Based on 2D Ruddlesden-Popper Perovskite

Stereopsis is of great important functions for humans to perceive and interact with the world. To realize the function of stereoscopic imaging, optoelectronic sensors shall possess good photoresponsive performance, multidirectional sensing, and 3D building capabilities. However, the current imaging sensors are mainly focused on 2D imaging, limiting their practical application scenarios. In this study, a stereopsis-inspired flexible 3D visual imaging system (VIS) based on 2D Ruddlesden-Popper perovskite is demonstrated. The 3D-VIS consists of 800 device units, each of which demonstrates excellent photoresponse performance, mechanical characteristics, and environmental stability. In addition to the capability of detecting 2D reflective images, the 3D-VIS realizes the function of detecting the depth of field and fusing object projections of two directions to invert the 3D image by utilizing voxels to rebuild the spatial structure of the object. In the future, the 3D-VIS will have broad application prospects in medical imaging, virtual reality, industrial automation, and other fields.

A mini review of the recent progress of electrode materials for low-temperature solid oxide fuel cells

Lowering the operating temperature (450-650 °C) of solid oxide fuel cells (SOFCs) faces the intrinsic challenge of sluggish electrode reaction kinetics in the low temperature (LT) range. To accelerate the electrode reaction rate, many efforts have been put into the optimization of electrode composition and morphology. In this review, we have summarized recent developments of LT-SOFC electrodes, including anode and cathode materials. For anode performance improvement, the internal structure design, fine anode structure, reforming layer addition, and in situ exsolution techniques are introduced and their related functionalities are also explained, respectively. While for the cathode, we focus on the perovskite-type materials because of their superior catalytic performance and relatively good stability. The optimization of perovskite composition, including A site alkali or alkali-earth metal doping and B site variable-valence transition metal doping, is discussed in detail based on their effects on oxygen reduction reaction (ORR). Besides, nanostructure assembly and 3D morphology design are also recent hotspots for cathode research. Finally, we also propose several research directions in this field, hoping to provide guidelines for future research.

Hybrid mixed-dimensional WTe2/CsPbI3perovskite heterojunction for high-performance photodetectors

Perovskites have showed significant potential for the application in photodetectors due to their outstanding electrical and optical properties. Integrating two-dimensional (2D) materials with perovskites can make full use of the high carrier mobility of 2D materials and strong light absorption of perovskite to realize excellent optoelectrical properties. Here, we demonstrate a photodetector based on the WTe₂/CsPbI₃heterostructure. The quenching and the shortened lifetime of photoluminescence (PL) for CsPbI₃perovskite confirms the efficient charge transfer at the WTe₂/CsPbI₃heterojunction. After coupled with WTe₂, the photoresponsivity of the CsPbI₃photodetector is improved by almost two orders of magnitude due to the high-gain photogating effect. The WTe₂/CsPbI₃heterojunction photodetector reveals a large responsivity of 1157 A W^-1and a high detectivity of 2.1 × 10¹³Jones. The results pave the way for the development of high-performance optoelectronic devices based on 2D materials/perovskite heterojunctions.

Nanomaterial-Based Synaptic Optoelectronic Devices for In-Sensor Preprocessing of Image Data

With the advance in information technologies involving machine vision applications, the demand for energy- and time-efficient acquisition, transfer, and processing of a large amount of image data has rapidly increased. However, current architectures of the machine vision system have inherent limitations in terms of power consumption and data latency owing to the physical isolation of image sensors and processors. Meanwhile, synaptic optoelectronic devices that exhibit photoresponse similar to the behaviors of the human synapse enable in-sensor preprocessing, which makes the front-end part of the image recognition process more efficient. Herein, we review recent progress in the development of synaptic optoelectronic devices using functional nanomaterials and their unique interfacial characteristics. First, we provide an overview of representative functional nanomaterials and device configurations for the synaptic optoelectronic devices. Then, we discuss the underlying physics of each nanomaterial in the synaptic optoelectronic device and explain related device characteristics that allow for the in-sensor preprocessing. We also discuss advantages achieved by the application of the synaptic optoelectronic devices to image preprocessing, such as contrast enhancement and image filtering. Finally, we conclude this review and present a short prospect.

Static and Dynamic Disorder in Formamidinium Lead Bromide Single Crystals

We show that formamidinium-based crystals are distinct from methylammonium-based halide perovskite crystals because their inorganic sublattice exhibits intrinsic local static disorder that coexists with a well-defined average crystal structure. Our study combines terahertz-range Raman scattering with single-crystal X-ray diffraction and first-principles calculations to probe the evolution of inorganic sublattice dynamics with temperature in the range of 10-300 K. The temperature evolution of the Raman spectra shows that low-temperature, local static disorder strongly affects the crystal structural dynamics and phase transitions at higher temperatures.

Coherent Spin Dynamics of Electrons in Two-Dimensional (PEA)2PbI4 Perovskites

The versatile potential of lead halide perovskites and two-dimensional materials is merged in the Ruddlesden-Popper perovskites having outstanding optical properties. Here, the coherent spin dynamics in Ruddlesden-Popper (PEA)₂PbI₄ perovskites is investigated by picosecond pump-probe Kerr rotation in an external magnetic field. The Larmor spin precession of resident electrons with a spin dephasing time of 190 ps is identified. The longitudinal spin relaxation time in weak magnetic fields measured by the spin inertia method is as long as 25 μs. A significant anisotropy of the electron g-factor with the in-plane value of +2.45 and out-of-plane value of +2.05 is found. The exciton out-of-plane g-factor of +1.6 is measured by magneto-reflectivity. This work contributes to the understanding of the spin-dependent properties of two-dimensional perovskites and their spin dynamics.

Recent Advances in Ultrathin Chiral Metasurfaces by Twisted Stacking

Artificial chiral nanostructures have been subjected to extensive research for their unique chiroptical activities. Planarized chiral films of ultrathin thicknesses are in particular demand for easy on-chip integration and improved energy efficiency as polarization-sensitive metadevices. Recently, controlled twisted stacking of two or more layers of nanomaterials, such as 2D van der Waals materials, ultrathin films, or traditional metasurfaces, at an angle has emerged as a general strategy to introduce optical chirality into achiral solid-state systems. This method endows new degrees of freedom, e.g., the interlayer twist angle, to flexibly engineer and tune the chiroptical responses without having to change the material or the design, thus greatly facilitating the development of multifunctional metamaterials. In this review, recent exciting progress in planar chiral metasurfaces are summarized and discussed from the viewpoints of building blocks, fabrication methods, as well as circular dichroism and modulation thereof in twisted stacked nanostructures. The review further highlights the ever-growing portfolio of applications of these chiral metasurfaces, including polarization conversion, information encryption, chiral sensing, and as an engineering platform for hybrid metadevices. Finally, forward-looking prospects are provided.

Quasi-One-Dimensional Metallicity in Compressed CsSnI3

Low-dimensional metal halides exhibit strong structural and electronic anisotropies, making them candidates for accessing unusual electronic properties. Here, we demonstrate pressure-induced quasi-one-dimensional (quasi-1D) metallicity in δ-CsSnI₃. With the application of pressure up to 40 GPa, the initially insulating δ-CsSnI₃ transforms to a metallic state. Synchrotron X-ray diffraction and Raman spectroscopy indicate that the starting 1D chain structure of edge-sharing Sn-I octahedra in δ-CsSnI₃ is maintained in the high-pressure metallic phase while the SnI₆ octahedral chains are distorted. Our experiments combined with first-principles density functional theory calculations reveal that pressure induces Sn-Sn hybridization and enhances Sn-I coupling within the chain, leading to band gap closure and formation of conductive SnI₆ distorted octahedral chains. In contrast, the interchain I...I interactions remain minimal, resulting in a highly anisotropic electronic structure and quasi-1D metallicity. Our study offers a high-pressure approach for achieving diverse electronic platforms in the broad family of low-dimensional metal halides.

Mechanisms of metal-insulator transitions in ultrathin SrMoO3films

As the thickness of a transition metal oxide thin film is reduced to several unit cells, dimensional and interfacial effects modulate its structure and properties, and initiate low-dimension quantum phase transitions different from its bulk counterparts. To check if a metal-insulator transition (MIT) occurs to a low-dimensional 4d2electron systems, we investigated SrMoO₃thin films by characterizing and analyzing their lattice structures, electric transport properties and electronic states. Among various dimensional effects and interfacial effects, quantum confinement effect (QCE) was discerned as the dominating mechanism of the thickness-driven MIT. Surface/interface scattering contributes to the residual resistivity while the competition of several interactions modulated by QCE governs the temperature dependence of the resistivity of SrMoO₃ultrathin films.

Hot-carrier tunable abnormal nonlinear absorption conversion in quasi-2D perovskite

Controlling the high-power laser transmittance is built on the diverse manipulation of multiple nonlinear absorption (NLA) processes in the nonlinear optical (NLO) materials. According to standard saturable absorption (SA) and reverse saturable absorption (RSA) model adapted for traditional semiconductor materials, the coexistence of SA and RSA will result in SA induced transparency at low laser intensity, yet switch to RSA with pump fluence increasing. Here, we observed, in contrast, an unusual RSA to SA conversion in quasi-two-dimensional (2D) perovskite film with a low threshold around 2.6 GW cm−2. With ultrafast transient absorption (TA) spectra measurement, such abnormal NLA is attributed to the competition between excitonic absorption enhancement and non-thermalized carrier induced bleaching. TA singularity from non-thermalized “Fermi Sea” is observed in quasi-2D perovskite film, indicating an ultrafast carrier thermalization within 100 fs. Moreover, the comparative study between the 2D and 3D perovskites uncovers the crucial role of hot-carrier effect to tune the NLA response. The ultrafast carrier cooling of quasi-2D perovskite is pointed out as an important factor to realize such abnormal NLA conversion process. These results provide fresh insights into the NLA mechanisms in low-dimensional perovskites, which may pave a promising way to diversify the NLO material applications.

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

Revealing Giant Exciton Fine-Structure Splitting in Two-Dimensional Perovskites Using van der Waals Passivation

Organic-inorganic layered perovskites are currently some of the most promising 2D van der Waals materials. Low crystal quality usually broadens the exciton line width, obscuring the fine structure of the exciton in conventional photoluminescence experiments. Here, we propose a mechanical approach to reducing the effect of spectral diffusion by means of hBN capping on layered perovskites, revealing the exciton fine structure. We used a stochastic model to link the reduction of the spectral line width with the population of charge fluctuation centers present in the organic spacer. van der Waals forces between both lattices cause the partial clamping of the perovskite organic spacer molecules, and hence the amplitude of the overall spectral diffusion effect is reduced. Our work provides a low-cost solution to the problem of accessing important fine-structure excitonic state information, along with an explanation of the important carrier dynamics present in the organic spacer that affect the quality of the optical emission.

A Review of Performance Prediction Based on Machine Learning in Materials Science

With increasing demand in many areas, materials are constantly evolving. However, they still have numerous practical constraints. The rational design and discovery of new materials can create a huge technological and social impact. However, such rational design and discovery require a holistic, multi-stage design process, including the design of the material composition, material structure, material properties as well as process design and engineering. Such a complex exploration using traditional scientific methods is not only blind but also a huge waste of time and resources. Machine learning (ML), which is used across data to find correlations in material properties and understand the chemical properties of materials, is being considered a new way to explore the materials field. This paper reviews some of the major recent advances and applications of ML in the field of properties prediction of materials and discusses the key challenges and opportunities in this cross-cutting area.

Simultaneous Interfacial Modification and Defect Passivation for Wide-Bandgap Semitransparent Perovskite Solar Cells with 14.4% Power Conversion Efficiency and 38% Average Visible Transmittance

Perovskite materials offer a great potential in the application of semitransparent solar cells, owing to the tunable bandgap, ease of preparation and excellent photovoltaic property. A majority of works exhibit high average visible-light transmittance (AVT) for semitransparent perovskite solar cells (ST-PSCs) through decreasing perovskite thickness, leading to sacrificing the power conversion efficiency (PCE) of the device. Herein, a wide-bandgap (WBG) perovskite of Cs_0.2 FA_0.8 Pb(I_0.6 Br_0.4 )₃ is applied as absorber in ST-PSCs, which is a tremendous progress to balance both large PCE and high AVT. Moreover, a strategy of simultaneous interfacial modification and defect passivation is provided to enhance the performance of WBG ST-PSCs. Consequently, an inverted planar structure WBG perovskite solar cell (PSC) achieves 15.06% of PCE with excellent stability by restraining the interfacial energy loss and suppressing the nonradiative recombination. Furthermore, the ST-PSC obtains high PCE of 14.40% with an AVT of 38% by means of optimizing the transparent electrode. This work provides an efficient and simple method to improve the performance and AVT of ST-PSCs for the application in building-integrated photovoltaics.

Epitaxial Growth of Lead-Free 2D Cs3 Cu2 I5 Perovskites for High-Performance UV Photodetectors

The all-inorganic lead-free Cu-based halide perovskites represented by the Cs-Cu-I system, have sparked extensive interest recently due to their impressive photophysical characteristics. However, successive works on their potential application in light emission diodes and photodetectors rely on tiny polycrystals, in which the grain boundaries and defects may lead to the performance degradation of their embodied devices. Here, 2D all-inorganic perovskite Cs₃ Cu₂ I₅ single crystals are epitaxially grown on mica substrates, with a thickness down to 10 nm. The strong blue emission of the Cs₃ Cu₂ I₅ flakes may originate from the radiative transition of self-trapped excitons associated with a large Stocks shift and long (microsecond) decay time. Ultravioelt (UV) photodetectors based on individual Cs₃ Cu₂ I₅ nanosheets are fabricated via a swift and etching-free dry transfer approach, which reveal a high responsivity of 3.78 A W^-1 (270 nm, 5 V bias), as well as a fast response speed (τ_rise  ≈163 ms, τ_decay  ≈203 ms), outperforming congeneric UV sensors based on other 2D metal halide perovskites. This work therefore sheds light on the fabrication of green optoelectronic devices based on lead-free 2D perovskites, vital for the sustainable development of photoelectric technology.

Tailoring Photoluminescence by Strain-Engineering in Layered Perovskite Flakes

Strain is an effective strategy to modulate the optoelectronic properties of 2D materials, but it has been almost unexplored in layered hybrid organic-inorganic metal halide perovskites (HOIPs) due to their complex band structure and mechanical properties. Here, we investigate the temperature-dependent microphotoluminescence (PL) of 2D (C₆H₅CH₂CH₂NH₃)₂Cs₃Pb₄Br₁₃ HOIP subject to biaxial strain induced by a SiO₂ ring platform on which flakes are placed by viscoelastic stamping. At 80 K, we found that a strain of <1% can change the PL emission from a single peak (unstrained) to three well-resolved peaks. Supported by micro-Raman spectroscopy, we show that the thermomechanically generated strain modulates the bandgap due to changes in the octahedral tilting and lattice expansion. Mechanical simulations demonstrate the coexistence of tensile and compressive strain along the flake. The observed PL peaks add an interesting feature to the rich phenomenology of photoluminescence in 2D HOIPs, which can be exploited in tailored sensing and optoelectronic devices.

Single Crystals of Mixed-Cation Copper-Based Perovskite with Trimodal Bandgap Behavior

Two-dimensional (2D) hybrid perovskites with novel functionalities and structural diversity are a perfect platform for emerging optoelectronic devices such as photodetectors, light-emitting diodes, and solar cells. Here, we demonstrate that excess concentration of Cesium bromide (CsBr) is key to the formation of easily exfoliated 2D Cs₂ Cu(Cl/Br)₄ perovskite crystal. Furthermore, by employing this trick to 2D perovskite MA₂ Cu(Cl/Br)₄ (MA=methylammonium), we achieve a phase-pure easily exfoliated 2D mixed-cation (MA/Cs)₂ Cu(Cl/Br)₄ perovskite crystal, which exhibits reduced bandgap (1.53 eV) with ferromagnetic behavior and photovoltaic property. The resultant mixed-cation structured device reveals enhanced efficiency compared to all MA and all Cs counterparts. These findings demonstrate the importance of cation-engineering in developing innovative materials with novel properties.

The Universal Growth of Ultrathin Perovskite Single Crystals

Perovskites have engaged significant attention owing to rich species and remarkable physical properties as well as optoelectronic applications. Compared to bulk counterparts, ultrathin perovskites exhibit more available compositions due to the breaking of bulk lattice limitation. Coupled with crystal lattice relaxation and quantum confinement, infinite intriguing properties of ultrathin perovskites deserve to be explored. Developing ultrathin perovskites with alterable composition and structure is a necessity to fully explore this versatile family. Herein, a universal strategy is conceived via constructing oriented solvent microenvironment induced by the interfacial electric field originated from the charge separation between solid and liquid phases, which is conducive to controlling the precursor distribution and makes crystals preferentially nucleate and grow in the preferentially lateral mode. From layered to nonlayered, organic to inorganic, and toxic to low-toxic lead-free perovskite, a full-range synthesis is achieved of ultrathin perovskites. This work opens up opportunities both for ultrathin perovskite exploration through compositional engineering and for device miniaturization in energy conversion applications.

Progress and challenges in layered two-dimensional hybrid perovskites

Dimensionality is the game-changer property of a material. The optical and electronic properties of a compound get dramatically influenced by confining dimensions from 3D to 2D. The bulk 3D perovskite materials have shown remarkable up-gradation in the power conversion efficiency, hence grabbing worldwide attention. But instability against moisture, temperature, and ion migration are the factors constantly back-stabbing and hindering from full-scale commercialization. 2D perovskite material has emerged as an excellent bridging entity between structural-chemical stability, and viable commercialization. Organic-inorganic 2D perovskite materials come with a layered structure in which a large organic cation layer as a spacer is sandwiched between two inorganic metal halide octahedra layers. Moreover, hydrophobic spacer cations are employed which isolate inorganic octahedral layers from water molecules. Hydrophobic spacer cations protect the authentic structure from being degraded. These layered structures occur in two phases namely the Ruddlesden-Popper phase and Dion-Jacobson phase, depending on the spacer cation types. Alternating inorganic and organic layers form multiple quantum wells naturally, along with spin-orbit-coupling gives Rashba splitting. 2D perovskite materials are coming up with interesting chemical, physical properties like exciton dynamics, charge carrier transport, and electron-phonon coupling as a result of the quantum confinement effect. Despite appreciable stability, limited charge transport and large bandgap are limiting the application of 2D perovskite materials in solar cells. These limitations can be overcome by using the concept of 2D/3D multidimensional hybrid perovskites, which includes the long-term stability of 2D perovskite and the high performance of 3D perovskite at the same time. Here in this perspective, we have given brief insight on structural versatility, synthesis techniques, some of the unique photophysical properties, potential device fabrication, and recent advancements in the 2D structure to stand against degradation. Certain shortcomings and future outlooks are also discussed to make the perspective more informative.

2D Ultrawide Bandgap Semiconductors: Odyssey and Challenges

2D ultrawide bandgap (UWBG) semiconductors have aroused increasing interest in the field of high-power transparent electronic devices, deep-ultraviolet photodetectors, flexible electronic skins, and energy-efficient displays, owing to their intriguing physical properties. Compared with dominant narrow bandgap semiconductor material families, 2D UWBG semiconductors are less investigated but stand out because of their propensity for high optical transparency, tunable electrical conductivity, high mobility, and ultrahigh gate dielectrics. At the current stage of research, the most intensively investigated 2D UWBG semiconductors are metal oxides, metal chalcogenides, metal halides, and metal nitrides. This paper provides an up-to-date review of recent research progress on new 2D UWBG semiconductor materials and novel physical properties. The widespread applications, i.e., transistors, photodetector, touch screen, and inverter are summarized, which employ 2D UWBG semiconductors as either a passive or active layer. Finally, the existing challenges and opportunities of the enticing class of 2D UWBG semiconductors are highlighted.

Two-Dimensional Violet Phosphorus: A p-Type Semiconductor for (Opto)electronics

The synthesis of novel two-dimensional (2D) materials displaying an unprecedented composition and structure via the exfoliation of layered systems provides access to uncharted properties. For application in optoelectronics, a vast majority of exfoliated 2D semiconductors possess n-type or more seldom ambipolar characteristics. The shortage of p-type 2D semiconductors enormously hinders the extensive engineering of 2D devices for complementary metal oxide semiconductors (CMOSs) and beyond CMOS applications. However, despite the recent progress in the development of 2D materials endowed with p-type behaviors by direct synthesis or p-doping strategies, finding new structures is still of primary importance. Here, we report the sonication-assisted liquid-phase exfoliation of violet phosphorus (VP) crystals into few-layer-thick flakes and the first exploration of their electrical and optical properties. Field-effect transistors based on exfoliated VP thin films exhibit a p-type transport feature with an I_on/I_off ratio of 10⁴ and a hole mobility of 2.25 cm² V^-1 s^-1 at room temperature. In addition, the VP film-based photodetectors display a photoresponsivity (R) of 10 mA W^-1 and a response time down to 0.16 s. Finally, VP embedded into CMOS inverter arrays displays a voltage gain of ∼17. This scalable production method and high quality of the exfoliated material combined with the excellent optoelectronic performances make VP an enticing and versatile p-type candidate for next-generation more-than-Moore (opto)electronics.

Perovskite-Type 2D Materials for High-Performance Photodetectors

Photodetectors are light sensors in widespread use in image sensing, optical communication, and consumer electronics. In current smart optoelectronic technology, conventional semiconductors have encountered a bottleneck caused by inflexibility and opacity. With the ever-increasing demands for versatile optoelectronic applications, perovskite-type 2D materials demonstrate great potential for advanced photodetectors inspired by molecularly thin 2D materials. Through the reduction of thickness to thin or molecularly thin levels, single-crystalline 2D perovskites can exhibit superior optoelectronic performance characteristics, such as tunable absorption property by chemical design, enhanced carrier separation by remarkable photosensing capability, and improved carrier extraction by versatile band engineering. More importantly, perovskite-type 2D materials exhibit great potential for large-scale monolithic integration to achieve all-in-one sensing-memory-computing optoelectronic devices. In this Perspective, recent progress in 2D perovskite-based photodetectors is presented in detail. The focus is on growth strategies for reducing thickness, thickness-dependent optical and electrical properties, device engineering, heterojunction fabrication, and device performance. Finally, the current challenges and future prospects in this field are presented.

Tin-based halide perovskite materials: properties and applications

Organic–inorganic hybrid halide perovskite materials have attracted considerable research interest, especially for photovoltaics. In addition, their scope has been extended towards light-emitting devices, photodetectors, or detectors. However, the toxicity of lead (Pb) element in perovskite compositions limits their applications. Therefore, a tremendous research effort on replacing is underway. More specifically, tin-based perovskites have shown the highest potential for this purpose. However, many challenges remain before these materials reach the goals of stability, safety, and eventually commercial application. This perspective considers many aspects and the critical development possibilities of tin-based perovskites, including drawbacks and challenges based on their physical properties. Additionally, it provides insights for future device applications that go beyond solar cells. Finally, the existing challenges and opportunities in tin-based perovskites are discussed.

This perspective presents the current status and prospects of tin-perovskites and the relevant optoelectronic device applications.