Spray-lithography of hybrid graphene-perovskite paper-based photodetectors for sustainable electronics

Paper is an ideal substrate for the development of flexible and environmentally sustainable ubiquitous electronic systems. When combined with nanomaterial-based devices, it can be harnessed for various Internet-of-Things applications, ranging from wearable electronics to smart packaging. However, paper remains a challenging substrate for electronics due to its rough and porous nature. In addition, the absence of established fabrication methods is impeding its utilization in wearable applications. Unlike other paper-based electronics with added layers, in this study, we present a scalable spray-lithography on a commercial paper substrate. We present a non-vacuum spray-lithography of chemical vapor deposition (CVD) single-layer graphene (SLG), carbon nanotubes (CNTs) and perovskite quantum dots (QDs) on a paper substrate. This approach combines the advantages of two large-area techniques: CVD and spray-coating. The first technique allows for the growth of SLG, while the second enables the spray coating of a mask to pattern CVD SLG, electrodes (CNTs), and photoactive (QDs) layers. We harness the advantages of perovskite QDs in photodetection, leveraging their strong absorption coefficients. Integrating them with the graphene enhances the photoconductive gain mechanism, leading to high external responsivity. The presented device shows high external responsivity of ∼520 A W-1at 405 nm at <1 V bias due to the photoconductive gain mechanism. The prepared paper-based photodetectors (PDs) achieve an external responsivity of 520 A W-1under 405 nm illumination at <1 V operating voltage. To the best of our knowledge, our devices have the highest external responsivity among paper-based PDs. By fabricating arrays of PDs on a paper substrate in the air, this work highlights the potential of this scalable approach for enabling ubiquitous electronics on paper.

High-Quality Solution-Processed Quasi-2D Perovskite for Low-Threshold Lasers

Spin-coated quasi-two-dimensional halide perovskite films, which exhibit superior optoelectronic properties and environmental stability, have recently been extensively studied for lasers. Crystallinity is of great importance for the laser performance. Although some parameters related to the spin-coating process have been studied, the in-depth understanding and effective control of the acceleration rate on two-dimensional perovskite crystallization during spin-coating are still unknown. Here we investigate the effect of solvent evaporation on the microstructure of the final perovskite films during the spin-coating process. The crystallization quality of the film can be significantly improved by controlling solvent evaporation. As a result, the prepared quasi-2D perovskite film exhibits a stimulated emission threshold (pump: 343 nm, 6 kHz, 290 fs) of 550 nm as low as 16.2 μJ/cm2. Transient absorption characterization shows that the radiative biexciton recombination time is reduced from 738.5 to 438.3 ps, benefiting from the improved crystallinity. The faster biexciton recombination significantly enhanced the photoluminescence efficiency, which is critical for population inversion. This work could contribute to the development of low-threshold lasers.

Research Progress on Rashba Effect in Two-Dimensional Organic-Inorganic Hybrid Lead Halide Perovskites

The Rashba effect appears in the semiconductors with an inversion-asymmetric structure and strong spin-orbit coupling, which splits the spin-degenerated band into two sub-bands with opposite spin states. The Rashba effect can not only be used to regulate carrier relaxations, thereby improving the performance of photoelectric devices, but also used to expand the applications of semiconductors in spintronics. In this mini-review, recent research progress on the Rashba effect of two-dimensional (2D) organic-inorganic hybrid perovskites is summarized. The origin and magnitude of Rashba spin splitting, layer-dependent Rashba band splitting of 2D perovskites, the Rashba effect in 2D perovskite quantum dots, a 2D/3D perovskite composite, and 2D-perovskites-based van der Waals heterostructures are discussed. Moreover, applications of the 2D Rashba effect in circularly polarized light detection are reviewed. Finally, future research to modulate the Rashba strength in 2D perovskites is prospected, which is conceived to promote the optoelectronic and spintronic applications of 2D perovskites.

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.

Optical Second Harmonic Generation of Low-Dimensional Semiconductor Materials

In recent years, the phenomenon of optical second harmonic generation (SHG) has attracted significant attention as a pivotal nonlinear optical effect in research. Notably, in low-dimensional materials (LDMs), SHG detection has become an instrumental tool for elucidating nonlinear optical properties due to their pronounced second-order susceptibility and distinct electronic structure. This review offers an exhaustive overview of the generation process and experimental configurations for SHG in such materials. It underscores the latest advancements in harnessing SHG as a sensitive probe for investigating the nonlinear optical attributes of these materials, with a particular focus on its pivotal role in unveiling electronic structures, bandgap characteristics, and crystal symmetry. By analyzing SHG signals, researchers can glean invaluable insights into the microscopic properties of these materials. Furthermore, this paper delves into the applications of optical SHG in imaging and time-resolved experiments. Finally, future directions and challenges toward the improvement in the NLO in LDMs are discussed to provide an outlook in this rapidly developing field, offering crucial perspectives for the design and optimization of pertinent devices.

Molecular Electronics: From Nanostructure Assembly to Device Integration

Integrated electronics and optoelectronics based on organic semiconductors have attracted considerable interest in displays, photovoltaics, and biosensing owing to their designable electronic properties, solution processability, and flexibility. Miniaturization and integration of devices are growing trends in molecular electronics and optoelectronics for practical applications, which requires large-scale and versatile assembly strategies for patterning organic micro/nano-structures with simultaneously long-range order, pure orientation, and high resolution. Although various integration methods have been developed in past decades, molecular electronics still needs a versatile platform to avoid defects and disorders due to weak intermolecular interactions in organic materials. In this perspective, a roadmap of organic integration technologies in recent three decades is provided to review the history of molecular electronics. First, we highlight the importance of long-range-ordered molecular packing for achieving exotic electronic and photophysical properties. Second, we classify the strategies for large-scale integration of molecular electronics through the control of nucleation and crystallographic orientation, and evaluate them based on factors of resolution, crystallinity, orientation, scalability, and versatility. Third, we discuss the multifunctional devices and integrated circuits based on organic field-effect transistors (OFETs) and photodetectors. Finally, we explore future research directions and outlines the need for further development of molecular electronics, including assembly of doped organic semiconductors and heterostructures, biological interfaces in molecular electronics and integrated organic logics based on complementary FETs.

Superior Phonon-Limited Exciton Mobility in Lead-Free Two-Dimensional Perovskites

Tin-based two-dimensional (2D) perovskites are emerging as lead-free alternatives in halide perovskite materials, yet their exciton dynamics and transport remain less understood due to defect scattering. Addressing this, we employed temperature-dependent transient photoluminescence (PL) microscopy to investigate intrinsic exciton transport in three structurally analogous Sn- and Pb-based 2D perovskites. Employing conjugated ligands, we synthesized high-quality crystals with enhanced phase stability at various temperatures. Our results revealed phonon-limited exciton transport in Sn perovskites, with diffusion constants increasing from 0.2 cm2 s-1 at room temperature to 0.6 cm2 s-1 at 40 K, and a narrowing PL line width. Notably, Sn-based perovskites exhibited greater exciton mobility than their Pb-based equivalents, which is attributed to lighter effective masses. Thermally activated optical phonon scattering was observed in Sn-based compounds but was absent in Pb-based materials. These findings, supported by molecular dynamics simulations, demonstrate that the phonon scattering mechanism in Sn-based halide perovskites can be distinct from their Pb counterparts.

Spacer Conformation Induced Multiple Hydrogen Bonds in 2D Perovskite toward Highly Efficient Optoelectronic Devices

Two-dimensional (2D) Dion-Jacobson (DJ) perovskites typically outperform Ruddlesden-Popper (RP) analogs in terms of photodetection (PD). However, the mechanism behind this enhanced performance remains elusive. Theoretical calculations for elucidating interlayer spacer conformation-induced multiple hydrogen bonds in 2D perovskite are presented, along with the synthesis of DPAPbBr4 (DPB) single crystals (SCs) and their PD properties under X-ray/ultraviolet (UV) excitation. The high-quality DPB SC enhances PD with exceptional photoresponse attributes, including a high on/off ratio (4.89 × 104), high responsivity (2.44 A W⁻1), along with large dynamic linear range (154 dB) and low detection limit (7.1 nW cm⁻2), which are currently the best results among 2D perovskite SC detectors, respectively. Importantly, high-resolution images are obtained under UV illumination with weak light levels. The SC X-ray detector exhibits a high sensitivity of 663 µC Gyair⁻1 cm-2 at 10 V and a detection limit of 1.44 µGyair s⁻1. This study explores 2D DJ perovskites for efficient and innovative optoelectronic applications.

Directed Assembly of Ordered Mixed-Spacer Quasi-2D Halide Perovskites through Homomeric Chains of Intermolecular Bonds

Two-dimensional (2D) halide perovskites (HPs) are of significant interest to researchers because of their natural structural frameworks and intriguing optoelectronic properties. However, the direct fabrication of ordered mixed-spacer quasi-2D HPs remains challenging. Herein, a synthetic strategy inspired by the principle of supramolecular synthons is employed for the self-assembly of a series of ordered mixed-spacer bilayered HPs. The key innovation involves the introduction of intermolecular hydrogen bonds using a bifunctional 3-aminopropionitrile cation. Three homogeneous n = 2 structures are obtained, with a subtly ordered perovskite connected by two distinct types of organic cation layers, resulting in a recurrent ABAB' stacking sequence. These three compounds exhibit attractive semiconducting properties. Moderate bandgaps in the range of 2.70 to 2.76 eV with an absorption wavelength range of 448-459 nm exhibit excellent photoelectric response. Moreover, the ordered structures facilitate excellent polarization-sensitive photodetection, with an impressive on/off ratio of 103. The response speed ranged from 298 to 381 µs, and the out-of-plane polarization-related dichroism ratio is determined to be 1.19. Such ordered mixed-spacer bilayered perovskites have not been reported. These results enrich the HPs system and play a significant role in the direct assembly of novel perovskites with ordered structures.

Superalkali halide perovskites with suitable direct band gaps for photovoltaic applications

The construction of superalkali halide perovskites has attracted attention for the development of new photovoltaic materials, but stable superalkalis have not been found until now. Herein, to construct new three-dimensional superalkali halide perovskites with a MI3 frame (M = Sn and Pb), a new Li(H2O)3+ superalkali cation is designed and selected based on low vertical ionization potential, suitable tolerance factor, small ionic radius and large dissociation energy. High-throughput first-principles calculations show that superalkalis with lower vertical ionization potentials exhibit stronger interactions with the MI3 frame. The normal and cubic Li(H2O)3MI3 perovskites and cubic Li(H2O)4PbI3 perovskites have direct band gaps, s-p and p-p electron transitions, effective carrier masses of less than 0.45me and exciton binding energies of less than 291 meV. Moreover, the cubic Li(H2O)3PbI3 perovskite with a direct band gap of 1.40 eV can in theory show a power conversion efficiency of 33.49%. These results strongly suggest that superalkali cations with large dissociation energy can be used to develop stable superalkali perovskites for photovoltaic applications.

Large-n quasi-phase-pure two-dimensional halide perovskite: A toolbox from materials to devices

Despite their excellent environmental stability, low defect density, and high carrier mobility, large-n quasi-two-dimensional halide perovskites (quasi-2DHPs) feature a limited application scope because of the formation of self-assembled multiple quantum wells (QWs) due to the similar thermal stabilities of large-n phases. However, large-n quasi-phase-pure 2DHPs (quasi-PP-2DHPs) can solve this problem perfectly. This review discusses the structures, formation mechanisms, and photoelectronic and physical properties of quasi-PP-2DHPs, summarises the corresponding single crystals, thin films, and heterojunction preparation methods, and presents the related advances. Moreover, we focus on applications of large-n quasi-PP-2DHPs in solar cells, photodetectors, lasers, light-emitting diodes, and field-effect transistors, discuss the challenges and prospects of these emerging photoelectronic materials, and review the potential technological developments in this area.

Electron-Phonon Coupling-Mediated Ultralong Carrier Lifetime in an All-Inorganic Two-Dimensional Cs2PbI2Cl2 Perovskite: Explanation for the High Antisite Defect Tolerance

Two-dimensional (2D) halide perovskite are appealing candidates for applications in optoelectronics and photovoltaics, but their energy conversion efficiency is severely limited by nonradiative electron-hole recombination. In most investigations, point defects with deep defect levels and deep charge-state transition levels in the band gap are treated as the carrier recombination centers. For the all-inorganic 2D Css 2PbI2Cl2, the IPb antisite defect is the most likely to form and cause nonradiative electron-hole recombination. By using density functional theory and ab initio nonradiative molecular dynamics calculations, we found that the IPb defect can introduce the deep acceptor and donor levels into the band gap. Because electron-phonon coupling gives rise to weak nonadiabatic coupling and rapid loss of electronic coherence, those levels lead to a reduction of the carrier loss and the prolongation of the excited-state carrier lifetime, thereby enhancing the photoelectric and defect tolerance properties of the Cs2PbI2Cl2 material. These results could deepen the understanding of the chemistry of defects and carrier dynamics in perovskite materials.

Optical readout of charge carriers stored in a 2D memory cell of monolayer WSe2

Owing to their superior charge retaining and transport characteristics, 2D transition metal dichalcogenides are investigated for practical applications in various memory-cell structures. Herein, we fabricated a quasi-one-terminal 2D memory cell by partially depositing a WSe2 monolayer on an Au electrode, which can be manipulated to achieve efficient charge injection upon the application or removal of external bias. Furthermore, the amount of charge carriers stored in the memory cell could be optically probed because of its close correlation with the fluorescence efficiency of WSe2, allowing us to achieve an electron retention time of ∼300 s at the cryogenic temperature of 4 K. Accordingly, the simplified device structure and the non-contact optical readout of the stored charge carriers present new research opportunities for 2D memory cells in terms of both fundamental mechanism studies and practical development for integrated nanophotonic devices.

Phase Engineering Reinforced Energy Transfer for High-Performance Blue Perovskite Light-Emitting Diodes

Layered metal-halide perovskites, a category of self-assembled quantum wells, are of paramount importance in emerging photonic sources, such as lasers and light-emitting diodes (LEDs). Despite high trap density in two-dimensional (2D) perovskites, efficient non-radiative energy funneling from wide- to narrow-bandgap components, sustained by the Förster resonance energy transfer (FRET) mechanism, contributes to efficient luminescence by light or electrical injection. Herein, it is demonstrated that bandgap extension of layered perovskites to the blue-emitting regime will cause sluggish and inefficient FRET, stemming from the tiny spectral overlap between different phases. Motivated by the importance of blue LEDs and inefficient energy transfer in materials with phase polydispersity, wide-bandgap quasi-2D perovskites with narrow phase distribution, improved crystallinity, and the pure crystal orientation perpendicular to the charge transport layer are developed. Based on this emitter, high-performance blue perovskite LEDs with improved electroluminescence (EL) external quantum efficiency (EQE) of 7.9% at 478 nm, a narrow full width at half-maximum (FWHM) of 22 nm and a more stable EL spectra are achieved. These results provide an important insight into spectrally stable and efficient blue emitters and EL devices based on perovskites.

Dual-Hyperspectral Optical Pump-Probe Microscopy with Single-Nanosecond Time Resolution

In recent years, optical pump-probe microscopy (PPM) has become a vital technique for spatiotemporally imaging electronic excitations and charge-carrier transport in metals and semiconductors. However, existing methods are limited by mechanical delay lines with a probe time window up to several nanoseconds (ns) or monochromatic pump and probe sources with restricted spectral coverage and temporal resolution, hindering their amenability in studying relatively slow processes. To bridge these gaps, we introduce a dual-hyperspectral PPM setup with a time window spanning from nanoseconds to milliseconds and single-nanosecond resolution. Our method features a wide-field probe tunable from 370 to 1000 nm and a pump spanning from 330 nm to 16 μm. We apply this PPM technique to study various two-dimensional metal-halide perovskites (2D-MHPs) as representative semiconductors by imaging their transient responses near the exciton resonances under both above-band gap electronic pump excitation and below-band gap vibrational pump excitation. The resulting spatially and temporally resolved images reveal insights into heat dissipation, film uniformity, distribution of impurity phases, and film-substrate interfaces. In addition, the single-nanosecond temporal resolution enables the imaging of in-plane strain wave propagation in 2D-MHP single crystals. Our method, which offers extensive spectral tunability and significantly improved time resolution, opens new possibilities for the imaging of charge carriers, heat, and transient phase transformation processes, particularly in materials with spatially varying composition, strain, crystalline structure, and interfaces.

The Nanoplasmonic Purcell Effect in Ultrafast and High-Light-Yield Perovskite Scintillators

The development of X-ray scintillators with ultrahigh light yields and ultrafast response times is a long sought-after goal. In this work, a fundamental mechanism that pushes the frontiers of ultrafast X-ray scintillator performance is theoretically predicted and experimentally demonstrated: the use of nanoscale-confined surface plasmon polariton modes to tailor the scintillator response time via the Purcell effect. By incorporating nanoplasmonic materials in scintillator devices, this work predicts over tenfold enhancement in decay rate and 38% reduction in time resolution even with only a simple planar design. The nanoplasmonic Purcell effect is experimentally demonstrated using perovskite scintillators, enhancing the light yield by over 120% to 88 ± 11 ph/keV, and the decay rate by over 60% to 2.0 ± 0.2 ns for the average decay time, and 0.7 ± 0.1 ns for the ultrafast decay component, in good agreement with the predictions of our theoretical framework. Proof-of-concept X-ray imaging experiments are performed using nanoplasmonic scintillators, demonstrating 182% enhancement in the modulation transfer function at four line pairs per millimeter spatial frequency. This work highlights the enormous potential of nanoplasmonics in optimizing ultrafast scintillator devices for applications including time-of-flight X-ray imaging and photon-counting computed tomography.

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.

Gain enhancement of perovskite nanosheets by a patterned waveguide: excitation and temperature dependence of gain saturation

Optical gain enhancement of two-dimensional CsPbBr3 nanosheets was studied when the amplified spontaneous emission is guided by a patterned structure of polyurethane-acrylate. Given the uncertainties and pitfalls in retrieving a gain coefficient from the variable stripe length method, a gain contour [Formula: see text] was obtained in the plane of spectrum energy (ℏω) and stripe length (x), whereby an average gain was obtained, and gain saturation was analysed. Excitation and temperature dependence of the gain contour show that the waveguide enhances both gain and thermal stability due to the increased optical confinement and heat dissipation, and the gain origins were attributed to the two-dimensional excitons and the localized states.

The Scale Effects of Organometal Halide Perovskites

Organometal halide perovskites have achieved great success in solution-processed photovoltaics. The explorations quickly expanded into other optoelectronic applications, including light-emitting diodes, lasers, and photodetectors. An in-depth analysis of the special scale effects is essential to understand the working mechanisms of devices and optimize the materials towards an enhanced performance. Generally speaking, organometal halide perovskites can be classified in two ways. By controlling the morphological dimensionality, 2D perovskite nanoplatelets, 1D perovskite nanowires, and 0D perovskite quantum dots have been studied. Using appropriate organic and inorganic components, low-dimensional organic-inorganic metal halide hybrids with 2D, quasi-2D, 1D, and 0D structures at the molecular level have been developed and studied. This provides opportunities to investigate the scale-dependent properties. Here, we present the progress on the characteristics of scale effects in organometal halide perovskites in these two classifications, with a focus on carrier diffusion, excitonic features, and defect properties.

Flexible Crystal Heterojunctions of Low-Dimensional Organic Metal Halides Enabling Color-Tunable Space-Resolved Optical Waveguides

Molecular luminescent materials with optical waveguide have wide application prospects in light-emitting diodes, sensors, and logic gates. However, the majority of traditional optical waveguide systems are based on brittle molecular crystals, which limited the fabrication, transportation, storage, and adaptation of flexible devices under diverse application situations. To date, the design and synthesis of photofunctional materials with high flexibility, novel optical waveguide, and multi-port color-tunable emission in the same solid-state system remain an open challenge. Here, we have constructed new types of zero-dimensional organic metal halides (Au-4-dimethylaminopyridine [DMAP] and In-DMAP) with a rarely high elasticity and rather low loss coefficients for optical waveguide. Theoretical calculations on the intermolecular interactions showed that the high elasticity of 2 molecular crystalline materials was original from their herringbone structure and slip plane. Based on one-dimensional flexible microrods of 2 crystals and the 2-dimensional microplate of the Mn-DMAP, heterojunctions with multi-color and space-resolved optical waveguides have been fabricated. The formation mechanism of heterojunctions is based on the surface selective growth on account of the low lattice mismatch ratio between contacting crystal planes. Therefore, this work describes the first attempt to the design of metal-halide-based crystal heterojunctions with high flexibility and optical waveguide, expanding the prospects of traditional luminescent materials for smart optical devices, such as logic gates and multiplexers.

Exceptional ultrafast nonlinear optical response of functionalized silicon nanosheets

The present work reports on the ultrafast saturable absorption (SA), optical limiting (OL), and the nonlinear refractive response of hydride-terminated silicon nanosheets (SiNS-H) differently functionalized with styrene and tert-butyl methacrylate (tBuMA), namely, SiNS-styrene and SiNS-tBuMA, using 50 fs, 400 nm and 70 fs, 800 nm laser pulses. SiNS-styrene and SiNS-tBuMA exhibit dramatically enhanced nonlinear optical (NLO) responses compared to SiNS-H, with their absorptive nonlinearity strongly dependent on the laser excitation wavelength. More specifically, the studied functionalized SiNSs reveal strong SA behavior under 400 nm laser excitation, with NLO absorption coefficients, saturable intensities, and modulation depths comparable to various two-dimensional (2D) materials, known to exhibit strong SA, such as graphene, black phosphorous (BP), some transition metal dichalcogenides (TMDs), and some MXenes. On the other hand, under 800 nm laser excitation, SiNS-styrene and SiNS-tBuMA show highly efficient OL performance with OL onset values of about 0.0045 and 0.0065 J cm-2, respectively, which are significantly lower than those of other 2D nanostructures. In addition, it is shown that both SiNS samples have great potential in already existing Si-based optoelectronic devices for optical-switching applications since they exhibit very strong NLO refraction comparable to that of bulk Si. The results of the present work demonstrate that the chemical functionalization of SiNSs provides a highly efficient strategy for the preparation of 2D Si-based nanostructures with enhanced NLO response in view of several optoelectronic and photonic applications, such as OL, SA, and all-optical switching.

Scientific Machine Learning of 2D Perovskite Nanosheet Formation

We apply a scientific machine learning (ML) framework to aid the prediction and understanding of nanomaterial formation processes via a joint spectral-kinetic model. We apply this framework to study the nucleation and growth of two-dimensional (2D) perovskite nanosheets. Colloidal nanomaterials have size-dependent optical properties and can be observed in situ, all of which make them a good model for understanding the complex processes of nucleation, growth, and phase transformation of 2D perovskites. Our results demonstrate that this model nanomaterial can form through two processes at the nanoscale: either via a layer-by-layer chemical exfoliation process from lead bromide nanocrystals or via direct nucleation from precursors. We utilize a phenomenological kinetic analysis to study the exfoliation process and scientific machine learning to study the direct nucleation and growth and discuss the circumstances under which it is more appropriate to use phenomenological or more complex machine learning models. Data for both analysis techniques are collected through in situ spectroscopy in a stopped flow chamber, incorporating over 500,000 spectra taken under more than 100 different conditions. More broadly, our research shows that the ability to utilize and integrate traditional kinetics and machine learning methods will greatly assist in the understanding of complex chemical systems.

Atomically thin two-dimensional hybrid perovskites using hydrophobic superalkali cations with tunable electron transition type

The direct band gaps of two-dimensional (2D) metal halide perovskites can be tuned via component engineering, but their electron transition type hardly changes. Herein, atomically thin (C5NH6)2MX4 (M = Ge, Sn, Pb; X = Cl, Br, I) hybrid perovskites with hydrophobic superalkali cations were systematically explored using high-throughput hybrid density functional calculations and ab initio molecular dynamics simulations. We found that the electron transition between the M and X atoms was converted into that between the C5NH6 parts and X atoms via X change in the 2D (C5NH6)2MX4 perovskites. Negative formation energy, stable thermodynamic and kinetic properties, sharp valence bands, and tunable direct band gaps were obtained for the 2D perovskites. A power conversion efficiency (PCE) of 32.54% was obtained in theory for the passivated cubic NH2CHNH2PbI3 (FAPbI3) perovskite containing the 2D (C5NH6)2PbI4 perovskite. The hybrid Pb-free (C5NH6)2SnI4 perovskite with a direct bandgap of 1.56 eV may be viewed as a potential passivation material for perovskite devices. Moreover, the C5NH6 cations and X atoms show different hydrogen bonding interactions, which can be extended to other atomically thin organic-inorganic hybrid perovskites.

Fluorination and Conjugation Engineering Synergistically Enhance the Optoelectronic Properties of Two-Dimensional Hybrid Organic-Inorganic Perovskites

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) are expected to be a viable alternative to three-dimensional (3D) analogs in solar cells (SCs) and optoelectronic devices due to their high stability, diverse composition, and physical properties. However, unsuitable band alignment and large bandgaps limit the power conversion efficiency (PCE) improvement of SCs based on 2D HOIPs. Here, we report a molecular design strategy that combines fluorination and conjugation engineering to tune the electronic structure and optimize the PCE of 2D HOIPs. Our results show that type IIa band alignment and tunable bandgaps can be achieved in 2D Dion-Jacobson (DJ) HOIPs by H/F substitution of organic cations with different degrees of conjugation. In general, the bandgap of 2D DJ-HOIPs decreases monotonously with the increase of the number of F atoms, which is due to the gradual decrease of the lowest unoccupied molecular orbitals (LUMO) of organic cations. In addition, the enhanced interlayer charge transfer and higher dielectric constant suggest that the fluorination-induced dielectric limitations are weakened. The estimated PCE of 2D DJ-HOIPs is exponentially increased and positively correlated with the degree of conjugation and fluorination of organic cations, with a PCE approaching 29% under their synergistic effect. Our results not only provide promising candidates for photovoltaic device applications but also provide an effective method for PCE optimization.

Direct Fast-Neutron Detection by 2D Perovskite Semiconductor

Fast-neutrons play a critical role in a range of applications, including medical imaging, therapy, and nondestructive inspection. However, direct detecting fast-neutrons by semiconductors has proven to be challenging due to their weak interaction with most matter and the requirement of high carrier mobility-lifetime (µτ) product for efficient charge collection. Herein, a novel approach is presented to direct fast-neutron detection using 2D Dion-Jacobson perovskite semiconductor BDAPbBr4 . This material features a high fast-neutron caption cross-section, good electrical stability, high resistivity, and, most importantly, a record-high µτ product of 3.3 × 10-4 cm2 V-1 , outperforming most reported fast-neutron detection semiconductors. As a result, BDAPbBr4 detector exhibited good response to fast-neutrons, not only achieving fast-neutron energy spectra in counting mode, but also obtaining linear and fast response in integration mode. This work provides a paradigm-shifting strategy for designing materials that efficiently detect fast-neutrons and paves the way toward exciting applications in fast-neutron imaging and therapy.

Achieving Ultralong Room-Temperature Phosphorescence in Two-Dimensional Metal Halide Perovskites by Alkyl Chain Engineering

Two-dimensional (2D) metal halide perovskites with highly efficient ultralong room-temperature phosphorescence (URTP) are rare due to their uncertain structures and complicated intermolecular interactions. Herein, by varying the alkyl length of organic units, we synthesized two single-component 2D metal hybrid perovskites, i.e., B-MACC and B-EACC, with obvious URTP emission. In particular, B-EACC exhibits a green-yellow URTP emission with an ultralong lifetime (579 ms) and a high efficiency (14.86%). It is found that the molecular packing of B-EA+ cations because of the presence one more carbon in the alkyl chain affords strong hydrogen bonding and π-π stacking interactions, which immobilizes and reduces the triplet exciton quenching. Moreover, B-MACC and B-EACC with space-time dual-resolved characteristics can be utilized for dynamic information encryption and optical logic gate applications. This study is the first to disclose the relation between the characteristics of molecular packing and the resultant URTP of 2D metal hybrid perovskites, significantly advancing the development of next-generation URTP materials for versatile applications.

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.

Unraveling the Effect of Stacking Configurations on Charge Transfer in WS2 and Organic Semiconductor Heterojunctions

Photoinduced interfacial charge transfer plays a critical role in energy conversion involving van der Waals (vdW) heterostructures constructed of inorganic nanostructures and organic materials. However, the effect of molecular stacking configurations on charge transfer dynamics is less understood. In this study, we demonstrated the tunability of interfacial charge separation in a type-II heterojunction between monolayer (ML) WS2 and an organic semiconducting molecule [2-(3″',4'-dimethyl-[2,2':5',2':5″,2″'-quaterthiophen]-5-yl)ethan-1-ammonium halide (4Tm)] by rational design of relative stacking configurations. The assembly between ML-WS2 and the 4Tm molecule forms a face-to-face stacking when 4Tm molecules are in a self-aggregation state. In contrast, a face-to-edge stacking is observed when 4Tm molecule is incorporated into a 2D organic-inorganic hybrid perovskite lattice. The face-to-face stacking was proved to be more favorable for hole transfer from WS2 to 4Tm and led to interlayer excitons (IEs) emission. Transient absorption measurements show that the hole transfer occurs on a time scale of 150 fs. On the other hand, the face-to-edge stacking resulted in much slower hole transfer without formation of IEs. This inefficient hole transfer occurs on a similar time scale as A exciton recombination in WS2, leading to the formation of negative trions. These investigations offer important fundamental insights into the charge transfer processes at organic-inorganic interfaces.

High Emission Efficiency and Thermal Stability in Zero-Dimensional Hybrid Zinc Halide as a Blue Light Emitter

Exploring highly efficient blue-emissive lead-free halide materials is a significant and challenging objective in the study of luminescent materials. This study reports the synthesis of a new zero-dimensional (0D) hybrid zinc halide of [CYP]ZnBr4 (CYP = 1-cyclohexylpiperazine) containing an isolated [ZnBr4]2- tetrahedron. [CYP]ZnBr4 exhibits strong blue light emission with a high photoluminescence quantum yield (PLQY) of 79.22%, surpassing all previously reported 0D zinc halide counterparts. According to the theoretical and experimental studies, the blue light emission is attributed to intrinsic self-trapped excitons resulting from strong electron-phonon coupling and structural deformation. Importantly, [CYP]ZnBr4 demonstrates excellent structural and luminescence stability toward high temperatures (180 °C) over at least half a month. High luminescence efficiency and stability enable [CYP]ZnBr4 to be an efficient blue phosphor to fabricate white light-emitting diodes (LEDs), which produces high-quality white light with a color rendering index (CRI) of 93.1 and a correlated color temperature (CCT) of 5304 K, closely resembling natural sunlight. This white LED also exhibits consistent performance and stability across different drive currents, suggesting the potential for high-power optoelectronic applications. Overall, this study paves the way for the utilization of 0D hybrid halides in advanced solid-state lighting applications.

Hydrophobic Chain-Induced Conversion of Three-Dimensional Perovskite Nanocrystals to Gold Nanocluster-Grafted Two-Dimensional Platelets for Photoinduced Electron Transfer Substrate Formulation

Considering the augmentation of new generation energy harvesting devices and applications of electron-hole separation therein, conversion of 3D cubic CsPbBr3 perovskite nanocrystals into 2D-platelets through ligand-ligand hydrophobic interactions has been conceived here. Cationic surfactants with various chain length coated the gold nanoclusters (AuNCs) that interact with oleic acid (OA) and oleylamine (OAm) coated 3D CsPbBr3 nanocrystals to disintegrate the crystallinity of the perovskites and reformation of AuNC-grafted 2D-platelets of unusually large size. The planar perovskite-derivatives act as an exciton donor to the embedded AuNCs through photoinduced electron transfer (PET). This process is controlled by the optimum surfactant chain length. Transient absorption spectroscopy shows that the fastest radical growth time (4 ps) was with the 14-carbon containing tail of the surfactant, followed by the 16-carbon (45 ps) and the 12-carbon (290 ps) ones. PET is administered by the energy gaps of the participating candidates that control the transition dynamics. Our findings can be a potential tool to develop metal nanocluster-based hybrid 2D perovskite-derived platelets for optoelectronic applications.

Two-Dimensional Halide Pb-Perovskite-Double Perovskite Epitaxial Heterostructures

Epitaxial heterostructures of two-dimensional (2D) halide perovskites offer a new platform for studying intriguing structural, optical, and electronic properties. However, difficulties with the stability of Pb- and Sn-based heterostructures have repeatedly slowed the progress. Recently, Pb-free halide double perovskites are gaining a lot of attention due to their superior stability and greater chemical diversity, but they have not been successfully incorporated into epitaxial heterostructures for further investigation. Here, we report epitaxial core-shell heterostructures via growing Pb-free double perovskites (involving combinations of Ag(I)-Bi(III), Ag-Sb, Ag-In, Na-Bi, Na-Sb, and Na-In) around Pb perovskite 2D crystals. Distinct from Pb-Pb and Pb-Sn perovskite heterostructures, growths of the Pb-free shell at 45° on the (100) surface of the lead perovskite core are observed in all Pb-free cases. The in-depth structural analysis carried out with electron diffraction unequivocally demonstrates the growth of the Pb-free shell along the [110] direction of the Pb perovskite, which is likely due to the relatively lower surface energy of the (110) surface. Furthermore, an investigation of anionic interdiffusion across heterostructure interfaces under the influence of heat was carried out. Interestingly, halide anion diffusion in the Pb-free 2D perovskites is found to be significantly suppressed as compared to Pb-based 2D perovskites. The great structural tunability and excellent stability of Pb-free perovskite heterostructures may find uses in electronic and optoelectronic devices in the near future.

Optical Sensing Capability Evaluation for Methylammonium Based Perovskites for Explosive

Here, we have synthesized methylammonium based two metal halide perovskites (MHP) such as MAPbBr3, and MAPbI3 using methylammonium bromide, methylammonium iodide, lead bromide, respective at room temperature under certain experimental conditions. All synthesized MHPs have been confirmed through X-ray diffraction technique (XRD), scanning electron microscope (SEM), Fourier transform infra-red (FTIR) and photoluminescence (PL) analysis. Afterward, comparative evaluation on optical sensing capability has been made for both MHPs using PL in different solvents. Importantly, we find out that MAPbBr3 exhibit an excellent optical feature over MAPbI3 in hexane only. Afterward, MAPbBr3 has also been explored to know the sensing capability for nitrobenzene sensing. Our model study confirms that MAPbBr3 is an excellent sensing material with R square (0.87), selectivity (16.9%) and Stern Volmer constant (Ksv=10- 2 × 0.464) for nitrobenzene in hexane.

Enhanced photocatalytic performance of a stable type-II PtSe2/GaSe van der Waals heterostructure

In this investigation, the structural, electronic, and optical properties of two-dimensional van der Waals heterostructure (vdwHS) PtSe2/GaSe with three different configurations have been studied using density functional theory with the generalized gradient approximation. All three optimized vdwHSs PtSe2/GaSe have positive phonon frequencies and hexagonal unit cells. The hybrid exchange-correlation functional has been employed to study the electronic properties of vdwHSs PtSe2/GaSe. The vdwHSs PtSe2/GaSe shows semiconducting behavior with indirect Type-II bandgaps, which have been confirmed by the charge density difference, electrostatic potential, work function, and band edge calculations. Additionally, from the band edge positions, the vdwHSs PtSe2/GaSe are analyzed for photocatalytic activities. The optical properties such as extinction coefficient, refractive index, reflectivity, energy loss spectrum, and absorption coefficient have been studied using norm-conserving pseudo-potentials. The vdwHSs PtSe2/GaSe exhibit consistent absorption from the visible to the ultraviolet region of the electromagnetic spectrum. From the obtained results, we conclude that vdwHSs PtSe2/GaSe could be utilized for H2 production through photocatalytic activity as well as for optoelectronic devices and their application.

Discovery of enhanced lattice dynamics in a single-layered hybrid perovskite

Layered hybrid perovskites exhibit emergent physical properties and exceptional functional performances, but the coexistence of lattice order and structural disorder severely hinders our understanding of these materials. One unsolved problem regards how the lattice dynamics are affected by the dimensional engineering of the inorganic frameworks and their interaction with the molecular moieties. Here, we address this question by using a combination of spontaneous Raman scattering, terahertz spectroscopy, and molecular dynamics simulations. This approach reveals the structural dynamics in and out of equilibrium and provides unexpected observables that differentiate single- and double-layered perovskites. While no distinct vibrational coherence is observed in double-layered perovskites, an off-resonant terahertz pulse can drive a long-lived coherent phonon mode in the single-layered system. This difference highlights the dramatic change in the lattice environment as the dimension is reduced, and the findings pave the way for ultrafast structural engineering and high-speed optical modulators based on layered perovskites.

Accurate Dimension Prediction for Low-Dimensional Organic-Inorganic Halide Perovskites via a Self-Established Machine Learning Strategy

Low-dimensional perovskites (LDPs) have enormous potential for the development of advanced optoelectronic devices and tackling the stability issue for the commercial application of perovskites. However, quantified structural dimensionality prediction for LDPs is still an intractable issue. Herein, we develop a self-established machine learning (ML)-assisted approach to predict the dimensionality of LDPs based on 195 reported amines that are classified as two-dimensional, one-dimensional, and zero-dimensional. The optimal K-nearest neighbor model allows us to realize an accuracy rate of 92.3% for the test data set containing 39 reported amines. Two features, i.e., ATSC1pe and SlogP_VSA2, associated with polarity and electrostatic potential on the van der Waals surface of an organic spacer, are identified from >1800 descriptors as key controlling factors determining the structure dimensionality. This work develops a typical paradigm for the application of a multiple-classification strategy of ML with an extremely high accuracy rate, which would thereby motivate the development of new types of LDPs.

CsPbX3 (X = Cl, Br, and I) Nanocrystals in Substrates toward Stable Photoluminescence: Nanoarchitectonics, Properties, and Applications

Cesium lead halide (CsPbX3, X = Cl, Br, and I) perovskite nanocrystals (NCs) possess great potential in light-emitting diode applications because of their high brightness, low cost, tunable luminescence, and facile synthesis nature. However, these NCs are often disadvantaged by their instability in nonsolvent environment that hinders the practical applications of the material. In order to solve these issues, cesium lead halide NCs prepared using a solvent environment can be placed on substrates to retain the high stability and expand the applicability of the material. This Review focuses on the transfer of the all-inorganic cesium lead halide NCs (synthesized in solutions) onto matrix materials and their direct synthesis on these bases, including the inert shell growth (inorganic and organic shell), embedment in matrixes (e.g., metal organic frameworks, porous SiO2, glass, ZrO2, Al2O3, and AlOOH), and direct synthesis in substrates. In particular, the strategies for stability and PL property improvement of the materials are also summarized. The purpose of this Review is to provide inspiration for the encapsulation of cesium lead halide NCs with high brightness and stability in matrixes to expand the applicability of these materials in wide color gamut backlighting (e.g., white-light-emitting devices).

Enhanced optical absorption in two-dimensional Ruddlesden-Popper (C6H5CH2NH3)2PbI4 perovskites via biaxial strain and surface doping

Two-dimensional Ruddlesden-Popper (2D RP) perovskites can form layered protective materials using long organic cations as "barrier" caps, which is expected to solve the problem of instability of perovskites in the working environment. In this work, we systematically studied the 2D Ruddlesden-Popper (C6H5CH2NH3)2PbI4 hybrid perovskites using density functional theory. The results reveal that the 2D (C6H5CH2NH3)2PbI4 perovskites are semiconductors with band gaps of 2.22 eV. The optical absorption peak of the 2D (C6H5CH2NH3)2PbI4 perovskite structure is located at 532 nm in the visible region. Interestingly, the optical absorption spectrum of the 2D (C6H5CH2NH3)2PbI4 perovskite structure enhanced under suitable strains. The highest optical absorption peak appears in 2D (C6H5CH2NH3)2PbI4 under a -2% strain, and its theoretical photoelectric conversion efficiency is 28.5%. More interestingly, the replacement of surface I atoms with Br is another ways to enhance the optical absorption spectrum of the 2D (C6H5CH2NH3)2PbI4 perovskite structure. The optical absorption peak blue-shifts to the high energy region, which has higher solar energy flux density than the low energy region. The good stability, tuneable band gap and excellent theoretical photoelectric conversion efficiency of the 2D (C6H5CH2NH3)2PbI4 perovskite structure make it a promising candidate for novel 2D hybrid perovskite based photoelectronic devices and solar cells.

Giant Polarization Sensitivity via the Anomalous Photovoltaic Effect in a Two-Dimensional Perovskite Ferroelectric

Polarization sensitivity, which shows great potential in photoelectric detection, is expected to be significantly improved by the ferroelectric anomalous photovoltaic (APV) effect. However, it is challenging to explore new APV-active ferroelectrics due to severe polarization fatigue induced by the leakage current of photoexcited carriers. For the first time, we report a strong APV effect in a 2D hybrid perovskite ferroelectric assembled by alloying mixed organic cations, (HA)₂(EA)₂Pb₃Br₁₀ (1, where HA⁺ is n-hexylammonium and EA⁺ is ethylammonium), which has a large spontaneous polarization ∼3.8 μC/cm² and high a Curie temperature ∼378 K. Its ferroelectricity allows a strong APV effect with an above-bandgap photovoltage up to 7.4 V, which exceeds its bandgap (∼2.7 eV). Most strikingly, based on the dependence on polarized-light angle, this strong APV effect renders the highest level of polarization sensitivity with a giant current ratio of ∼25, far beyond other 2D single-phase materials. This study sheds light on the exploration of APV-active ferroelectrics and inspires their future high-performance optoelectronic device applications.

Chemical Behavior and Local Structure of the Ruddlesden-Popper and Dion-Jacobson Alloyed Pb/Sn Bromide 2D Perovskites

The alloyed lead/tin (Pb/Sn) halide perovskites have gained significant attention in the development of tandem solar cells and other optoelectronic devices due to their widely tunable absorption edge. To gain a better understanding of the intriguing properties of Pb/Sn perovskites, such as their anomalous bandgap's dependence on stoichiometry, it is important to deepen the understanding of their chemical behavior and local structure. Herein, we investigate a series of two-dimensional Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) phase alloyed Pb/Sn bromide perovskites using butylammonium (BA) and 3-(aminomethyl)pyridinium (3AMPY) as the spacer cations: (BA)₂(MA)_n-1Pb_xSn_n-xBr_3n+1 (n = 1-3) and (3AMPY)(MA)_n-1Pb_xSn_n-xBr_3n+1 (n = 1-3) through a solution-based approach. Our results show that the ratio and site preference of Pb/Sn atoms are influenced by the layer thickness (n) and spacer cations (A'), as determined by single-crystal X-ray diffraction. Solid-state ¹H, ¹¹⁹Sn, and ²⁰⁷Pb NMR spectroscopy analysis shows that the Pb atoms prefer the outer layers in n = 3 members: (BA)₂(MA)Pb_xSn_n-xBr₁₀ and (3AMPY)(MA)Pb_xSn_n-xBr₁₀. Layered 2D DJ alloyed Pb/Sn bromide perovskites (3AMPY)(MA)_n-1Pb_xSn_n-xBr_3n+1 (n = 1-3) demonstrate much narrower optical band gaps, lower energy PL emission peaks, and longer carrier lifetimes compared to those of RP analogs. Density functional theory calculations suggest that Pb-rich alloys (Pb:Sn ∼4:1) for n = 1 compounds are thermodynamically favored over 50:50 (Pb:Sn ∼1:1) compositions. From grazing-incidence wide-angle X-ray scattering (GIWAXS), we see that films in the RP phase orient parallel to the substrate, whereas for DJ cases, random orientations are observed relative to the substrate.

Real-Time In Situ Observation of CsPbBr3 Perovskite Nanoplatelets Transforming into Nanosheets

The manipulation of nano-objects through heating is an effective strategy for inducing structural modifications and therefore changing the optoelectronic properties of semiconducting materials. Despite its potential, the underlying mechanism of the structural transformations remains elusive, largely due to the challenges associated with their in situ observations. To address these issues, we synthesize temperature-sensitive CsPbBr₃ perovskite nanoplatelets and investigate their structural evolution at the nanoscale using in situ heating transmission electron microscopy. We observe the morphological changes that start from the self-assembly of the nanoplatelets into ribbons on a substrate. We identify several paths of merging nanoplates within ribbons that ultimately lead to the formation of nanosheets dispersed randomly on the substrate. These observations are supported by molecular dynamics simulations. We correlate the various paths for merging to the random orientation of the initial ribbons along with the ligand mobility (especially from the edges of the nanoplatelets). This leads to the preferential growth of individual nanosheets and the merging of neighboring ones. These processes enable the creation of structures with tunable emission, ranging from blue to green, all from a single material. Our real-time observations of the transformation of perovskite 2D nanocrystals reveal a route to achieve large-area nanosheets by controlling the initial orientation of the self-assembled objects with potential for large-scale applications.

Dual Emission Bands of a 2D Perovskite Single Crystal with Charge Transfer State Characteristics

Several hybrid halide 2D-perovskite species emit light with an emergent and controversial broadband emission Stokes-shifted down from the narrow band emission. This paper uncovers the sub- and above-bandgap emission and absorption characteristics of PEA₂PbI₄ prepared with gap states introduced during single crystal growth. Here, gap states led to coexistent intrinsic and heterostructured electronic frameworks that are selectively accessible with ultraviolet (UV) and infrared (IR) light, respectively, resulting in the phenomenon of photoluminescence (PL) switching from narrowband green to broadband red. Electron-energy dependent cathodoluminescence shows a relative increase in the broadband red PL intensity as the electron penetration depth increases from 30 nm to 2 μm, confirming the heterostructured framework is formed in the bulk of the crystal. Excitation-emission power slope of 2.5 and up-conversion pump transient absorption (TA) spectra suggest that the IR up-conversion excitation with red photoluminescence, peaked at 655 nm, is a multiphoton process occurring in the heterostructured framework through a nonlinear optical response. The energetic pathways toward the dual emission bands are revealed by pump-probe transient absorption spectroscopy, showing energetically broad gap states with high sensitivity to an IR pump are upconverted and subsequently quickly relax from high to low energy levels within 4 ps. Furthermore, the up-conversion red PL demonstrates a linear polarization with magnetic field effects, thus affirming that the band-like heterostructured framework is crystallographically aligned with characteristics of spatially extended charge-transfer states.

Colloid driven low supersaturation crystallization for atomically thin Bismuth halide perovskite

It is challenging to grow atomically thin non-van der Waals perovskite due to the strong electronic coupling between adjacent layers. Here, we present a colloid-driven low supersaturation crystallization strategy to grow atomically thin Cs₃Bi₂Br₉. The colloid solution drives low-concentration solute in a supersaturation state, contributing to initial heterogeneous nucleation. Simultaneously, the colloids provide a stable precursor source in the low-concentration solute. The surfactant is absorbed in specific crystal nucleation facet resulting in the anisotropic growth of planar dominance. Ionic perovskite Cs₃Bi₂Br₉ is readily grown from monolayered to six-layered Cs₃Bi₂Br₉ corresponding to thicknesses of 0.7, 1.6, 2.7, 3.6, 4.6 and 5.7 nm. The atomically thin Cs₃Bi₂Br₉ presents layer-dependent nonlinear optical performance and stacking-induced second harmonic generation. This work provides a concept for growing atomically thin halide perovskite with non-van der Waal structures and demonstrates potential application for atomically thin single crystals' growth with strong electronic coupling between adjacent layers.

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

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

One-Dimensional Organic-Inorganic Lead Bromide Hybrids with Excitation-Dependent White-Light Emission Templated by Pyridinium Derivatives

Organic-inorganic hybrid metal halides have attracted widespread attention due to their excellent tunability and versatility. Here, we have selected pyridinium derivatives with different substituent groups or substitution positions as the organic templating cations and obtained six 1D chain-like structures. They are divided into three types: type I (single chain), type II (double chain), and type III (triple chain), with tunable optical band gaps and emission properties. Among them, only (2,4-LD)PbBr₃ (2,4-LD = 2,4-lutidine) shows an exciton-dependent emission phenomenon, ranging from strong yellow-white to weak red-white light. By comparing its photoluminescence spectrum with that of its bromate (2,4-LD)Br, it is found that the strong yellow-white emission at 534 nm mainly came from the organic component. Furthermore, through a comparison of the fluorescence spectra and lifetimes of (2,4-LD)PbBr₃ and (2-MP)PbBr₃ (2-MP = 2-methylpyridine) with similar structures at different temperatures, we confirm that the tunable emission of (2,4-LD)PbBr₃ comes from different photoluminescent sources corresponding to organic cations and self-trapped excitons. Density functional theory calculations further reveal that (2,4-LD)PbBr₃ has a stronger interaction between organic and inorganic components compared to (2-MP)PbBr₃. This work highlights the importance of organic templating cations in hybrid metal halides and the new functionalities associated with them.

Modulating Resonance Energy Transfer with Supramolecular Control in a Layered Hybrid Perovskite and Chromium Photosensitizer Assembly

Recently, the low-dimensional organic-inorganic halide perovskites (OIHP) have been exploited heavily for their favorable exciton dynamics, broad-band emission, remarkable stability, and tunable band-edge excited-state energy compared to their 3D counterparts for potential optoelectronic applications. Low-dimensional perovskites are generally good candidates for utilization as room-temperature photoluminescence (PL) materials. Further, doping divalent transition metals like Mn²⁺ into OIHP is expected to introduce a ⁴T₁-⁶A₁-based low-energy luminescence emission around 600 nm; an optical property that is favorable for biomedical optoelectronics. Doping Mn²⁺ in the perovskite lattice is also expected to induce the generation of cytotoxic singlet oxygen species (¹O₂), a ROS that is being exploited for various therapeutic applications. To integrate these optical and therapeutic properties of a 2D (PEA)₂PbBr₄ (Pb PeV; PEA = phenylethylammonium cation) perovskite alloyed with Mn²⁺ ions (Mn:PbPeV) and the option for a photoinduced energy transfer process involving a Cr(III)-based ¹O₂ generating photosensitizer (CrPS), we designed a unique purpose-built nanoassembly (Mn:PbPeV@PCD) using the encapsulation properties of a water-soluble polymer derived from β-cyclodextrin (PCD). Here the PCD is observed to modulate the classical internal energy transfer of Pb²⁺ exciton to alloyed Mn²⁺ orange emission, resulting in the emergence of a new blue emission. The addition of CrPS into the Mn:PbPeV@PCD to generate the CrPS@Mn:PbPeV@PCD assembly results in restoring perovskite luminescence followed by the external energy transfer to CrPS. We have elucidated the mechanism of these cascade energy transfer processes between multiple components using steady-state and time-resolved luminescence techniques. Efficient ROS generation and its potential to induce an oxidation reaction of a biomolecule are realized using guanine as the target molecule. Further photoinduced cleavage studies with biomolecules confirmed the efficacy of the nanoassembly in inducing the cleavage of guanine-rich DNA. The study opens up a new direction in the field of perovskite for biomedical applications.

Lattice Expansion in Rb-Doped Hybrid Organic-Inorganic Perovskite Crystals Resulting in Smaller Band-Gap and Higher Light-Yield Scintillators

Two-dimensional hybrid-organic-inorganic perovskite (2D-HOIP) lead bromide perovskite crystals have demonstrated great potential as scintillators with high light yields and fast decay times while also being low cost with solution-processable materials for wide energy radiation detection. Ion doping has been also shown to be a very promising avenue for improvements of the scintillation properties of 2D-HOIP crystals. In this paper, we discuss the effect of rubidium (Rb) doping on two previously reported 2D-HOIP single crystals, BA₂PbBr₄ and PEA₂PbBr₄. We observe that doping the perovskite crystals with Rb ions leads to an expansion of the crystal lattices of the materials, which also leads to narrowing of band gaps down to 84% of the pure compounds. Rb doping of BA₂PbBr₄ and PEA₂PbBr₄ shows a broadening in the photoluminescence and scintillation emissions of both perovskite crystals. Rb doping also leads to faster γ-ray scintillation decay times, as fast as 4.4 ns, with average decay time decreases of 15% and 8% for Rb-doped BA₂PbBr₄ and PEA₂PbBr₄, respectively, compared to those of undoped crystals. The inclusion of Rb ions also leads to a slightly longer afterglow, with residual scintillation still being below 1% after 5 s at 10 K, for both undoped and Rb-doped perovskite crystals. The light yield of both perovskites is significantly increased by Rb doping with improvements of 58% and 25% for BA₂PbBr₄ and PEA₂PbBr₄, respectively. This work shows that Rb doping leads to a significant enhancement of the 2D-HOIP crystal performance, which is of particular significance for high light yield and fast timing applications, such as photon counting or positron emission tomography.

Structural, vibrational, photoelectrochemical, and optical properties of two-dimensional Ruddlesden-Popper perovskite BA2PbI4 crystals

Two-dimensional organic-inorganic hybrid Ruddlesden-Popper perovskites have attracted a lot of attention due to their unique photochemical properties and enhanced stability towards photoluminescence devices. Compared with three-dimensional materials, two-dimensional perovskites show great potential for photoelectric applications due to their tunable band gap, great excitation binding energy, and large crystal anisotropy. Although the synthesis and optical properties of BA₂PbI₄ crystals have been extensively studied, the role of their microstructure in photoelectric applications, their electronic structure, and their electron-phonon interaction are still poorly understood. In this paper, based on the preparation of BA₂PbI₄ crystals, the electronic structure, phonon dispersion, and vibrational properties of BA₂PbI₄ crystals were revealed in detail with the help of density functional theory. The BA₂PbI₄ stability diagram of formation enthalpy was calculated. The crystal structure of the BA₂PbI₄ crystals was characterized and calculated with the aid of Rietveld refinement. A contactless fixed-point lighting device was designed based on the principle of an electromagnetic induction coil, and the points with different thicknesses of BA₂PbI₄ crystal were tested. It is proved that the excitation peak of the bulk is 564 nm, and the surface luminescence peak is 520 nm. Phonon dispersion curves and the total and partial phonon densities of states have been calculated for the BA₂PbI₄ crystals. The calculated results are in good agreement with the experimental Fourier infrared spectra. Besides the basic characterization of the BA₂PbI₄ crystals, the photoelectrochemical properties of the materials were also studied, which further proves the excellent photoelectric properties of the BA₂PbI₄ crystals and the broad application prospect.

Intersubband Transitions in Lead Halide Perovskite-Based Quantum Wells for Mid-Infrared Detectors

Due to their excellent optical and electrical properties as well as versatile growth and fabrication processes, lead halide perovskites have been widely considered as promising candidates for green energy and applications related to optoelectronics. Here, we investigate their potential applications at infrared wavelengths by modeling the intersubband transitions in perovskite-based quantum well systems. Both single-well and double-well structures are studied, and their energy levels as well as the corresponding wave functions and intersubband transition energies are calculated by solving the one-dimensional Schrödinger equations. Via adjustment of the quantum well and barrier thicknesses, the intersubband transition energies can be tuned to cover a broad infrared wavelength range. We also find that the lead halide perovskite-based quantum wells possess high absorption coefficients. The widely tunable transition energies and high absorption coefficients of the perovskite-based quantum well systems, combined with their unique material and electrical properties, may enable an alternative material system for infrared photodetector applications.

Localized Bound Multiexcitons in Engineered Quasi-2D Perovskites Grains at Room Temperature for Efficient Lasers

Reducing the excitation threshold to minimize the Joule heating is critical for the realization of perovskite laser diodes. Although bound excitons are promising for low threshold laser, how to generate them at room temperature for laser applications is still unclear in quasi-2D perovskite-based devices. In this work, via engineering quasi-2D perovskite PEA₂ (CH₃ NH₃ )_{n -1} Pb_n Br_{3 n +1} microscopic grains by the anti-solvent method, room-temperature multiexciton radiative recombination is successfully demonstrated at a remarkably low pump density of 0.97 µJ cm^-2 , which is only one-fourth of that required in 2D CdSe nanosheets. In addition, the well-defined translational momentum in quasi-2D perovskite grains can restrict the Auger recombination which is detrimental to radiative emission. Furthermore, the quasi-2D perovskite grains are favorable for increasing binding energies of excitons and biexcitons and so as the related radiative recombination. Consequently, the prepared <n = 8> phase quasi-2D perovskite film renders a threshold of room-temperature stimulated emission as low as 13.7 µJ cm^-2 , reduced by 58.6% relative to the amorphous counterpart with larger grains. The findings in this work are expected to facilitate the development of solution-processable perovskite multiexcitonic laser diodes.

Localised strain and doping of 2D materials

There is a growing interest in 2D materials-based devices as the replacement for established materials, such as silicon and metal oxides in microelectronics and sensing, respectively. However, the atomically thin nature of 2D materials makes them susceptible to slight variations caused by their immediate environment, inducing doping and strain, which can vary between, and even microscopically within, devices. One of the misapprehensions for using 2D materials is the consideration of unanimous intrinsic properties over different support surfaces. The interfacial interaction, intrinsic structural disorder and external strain modulate the properties of 2D materials and govern the device performance. The understanding, measurement and control of these factors are thus one of the significant challenges for the adoption of 2D materials in industrial electronics, sensing, and polymer composites. This topical review provides a comprehensive overview of the effect of strain-induced lattice deformation and its relationship with physical and electronic properties. Using the example of graphene and MoS₂ (as the prototypical 2D semiconductor), we rationalise the importance of scanning probe techniques and Raman spectroscopy to elucidate strain and doping in 2D materials. These effects can be directly and accurately characterised through Raman shifts in a non-destructive manner. A generalised model has been presented that deconvolutes the intertwined relationship between strain and doping in graphene and MoS₂ that could apply to other members of the 2D materials family. The emerging field of straintronics is presented, where the controlled application of strain over 2D materials induces tuneable physical and electronic properties. These perspectives highlight practical considerations for strain engineering and related microelectromechanical applications.