Hexagonal Boron Nitride for Next-Generation Photonics and Electronics

Ion-Electron Interactions in 2D Nanomaterials-Based Artificial Synapses for Neuromorphic Applications

With the increasing limitations of conventional computing techniques, particularly the von Neumann bottleneck, the brain's seamless integration of memory and processing through synapses offers a valuable model for technological innovation. Inspired by biological synapse facilitating adaptive, low-power computation by modulating signal transmission via ionic conduction, iontronic synaptic devices have emerged as one of the most promising candidates for neuromorphic computing. Meanwhile, the atomic-scale thickness and tunable electronic properties of van der Waals two-dimensional (2D) materials enable the possibility of designing highly integrated, energy-efficient devices that closely replicate synaptic plasticity. This review comprehensively analyzes advancements in iontronic synaptic devices based on 2D materials, focusing on electron-ion interactions in both iontronic transistors and memristors. The challenges of material stability, scalability, and device integration are evaluated, along with potential solutions and future research directions. By highlighting these developments, this review offers insights into the potential of 2D materials in advancing neuromorphic systems.

Amorphous boron nitride: synthesis, properties and device application

Amorphous boron nitride (a-BN) exhibits remarkable electrical, optical, and chemical properties, alongside robust mechanical stability, making it a compelling material for advanced applications in nanoelectronics and photonics. This review comprehensively examines the unique characteristics of a-BN, emphasizing its electrical and optical attributes, state-of-the-art synthesis techniques, and device applications. Key advancements in low-temperature growth methods for a-BN are highlighted, offering insights into their potential for integration into scalable, CMOS-compatible platforms. Additionally, the review discusses the emerging role of a-BN as a dielectric material in electronic and photonic devices, serving as substrates, encapsulation layers, and gate insulators. Finally, perspectives on future challenges, including defect control, interface engineering, and scalability, are presented, providing a roadmap for realizing the full potential of a-BN in next-generation device technologies.

Research progress on the epitaxial growth of hexagonal boron nitride on different substrates by the CVD method

Hexagonal boron nitride (h-BN) has a hexagonal structure similar to graphene, comprising alternating boron and nitrogen atoms. This unique structure endows h-BN with a plethora of excellent properties, including a low dielectric constant, elevated thermal and chemical stability, substantial mechanical rigidity, and an exceptionally low friction coefficient, rendering it versatile across a spectrum of applications ranging from semiconductors to aerospace. Moreover, its smooth surface, absence of dangling bonds, and wide band gap make h-BN an optimal substrate and gate dielectric material for two-dimensional electronic devices. This article details the synthesis methodologies and research progress of h-BN epitaxial growth on solid transition metal, liquid metal, alloy, sapphire/metal and semiconductor substrates. In particular, progress in improving the quality and functionality of h-BN films by adapting processes and substrates has been rigorously reviewed. Finally, the characteristics of different substrates are summarized and the challenges faced by h-BN in future applications are discussed.

Wafer-scale AA-stacked hexagonal boron nitride grown on a GaN substrate

The stacking sequence of two-dimensional hexagonal boron nitride (hBN) is a critical factor that determines its polytypes and its distinct physical properties. Although most hBN layers adopt the thermodynamically stable AA' stacking sequence, achieving alternative stacking configurations has remained a long-standing challenge. Here we demonstrate the scalable synthesis of hBN featuring unprecedented AA stacking, where atomic monolayers align along the c axis without any translation or rotation. This previously considered thermodynamically unfavourable hBN polytype is achieved through epitaxial growth on a two-inch single-crystalline gallium nitride wafer, using a metal-organic chemical vapour deposition technique. Comprehensive structural and optical characterizations, complemented by theoretical modelling, evidence the formation of AA-stacked multilayer hBN and reveal that hBN nucleation on the vicinal gallium nitride surface drives the unidirectional alignment of layers. Here electron doping plays a central role in stabilizing the AA stacking configuration. Our findings provide further insights into the scalable synthesis of engineered hBN polytypes, characterized by unique properties such as large optical nonlinearity.

Wearable Biodevices Based on Two-Dimensional Materials: From Flexible Sensors to Smart Integrated Systems

The proliferation of wearable biodevices has boosted the development of soft, innovative, and multifunctional materials for human health monitoring. The integration of wearable sensors with intelligent systems is an overwhelming tendency, providing powerful tools for remote health monitoring and personal health management. Among many candidates, two-dimensional (2D) materials stand out due to several exotic mechanical, electrical, optical, and chemical properties that can be efficiently integrated into atomic-thin films. While previous reviews on 2D materials for biodevices primarily focus on conventional configurations and materials like graphene, the rapid development of new 2D materials with exotic properties has opened up novel applications, particularly in smart interaction and integrated functionalities. This review aims to consolidate recent progress, highlight the unique advantages of 2D materials, and guide future research by discussing existing challenges and opportunities in applying 2D materials for smart wearable biodevices. We begin with an in-depth analysis of the advantages, sensing mechanisms, and potential applications of 2D materials in wearable biodevice fabrication. Following this, we systematically discuss state-of-the-art biodevices based on 2D materials for monitoring various physiological signals within the human body. Special attention is given to showcasing the integration of multi-functionality in 2D smart devices, mainly including self-power supply, integrated diagnosis/treatment, and human-machine interaction. Finally, the review concludes with a concise summary of existing challenges and prospective solutions concerning the utilization of 2D materials for advanced biodevices.

Electron Correlation in 2D Periodic Systems from Periodic Bootstrap Embedding

Given the growing significance of 2D materials in various optoelectronic applications, it is imperative to have simulation tools that can accurately and efficiently describe electron correlation effects in these systems. Here, we show that the recently developed bootstrap embedding (BE) accurately predicts electron correlation energies and structural properties for 2D systems. Without explicit dependence on the reciprocal space sum (k-points) in the correlation calculation, our proof-of-concept calculations shows that BE can typically recover ∼99.5% of the total minimal basis electron correlation energy in 2D semimetal, insulator, and semiconductors. We demonstrate that BE can predict lattice constants and bulk moduli for 2D systems with high precision. Furthermore, we highlight the capability of BE to treat electron correlation in twisted bilayer graphene superlattices with large unit cells containing hundreds of carbon atoms. We find that as the twist angle decreases toward the magic angle, the correlation energy initially decreases in magnitude, followed by a subsequent increase. We conclude that BE is a promising electronic structure method for future applications to 2D materials.

Nanomaterials for Flexible Neuromorphics

The quest to imbue machines with intelligence akin to that of humans, through the development of adaptable neuromorphic devices and the creation of artificial neural systems, has long stood as a pivotal goal in both scientific inquiry and industrial advancement. Recent advancements in flexible neuromorphic electronics primarily rely on nanomaterials and polymers owing to their inherent uniformity, superior mechanical and electrical capabilities, and versatile functionalities. However, this field is still in its nascent stage, necessitating continuous efforts in materials innovation and device/system design. Therefore, it is imperative to conduct an extensive and comprehensive analysis to summarize current progress. This review highlights the advancements and applications of flexible neuromorphics, involving inorganic nanomaterials (zero-/one-/two-dimensional, and heterostructure), carbon-based nanomaterials such as carbon nanotubes (CNTs) and graphene, and polymers. Additionally, a comprehensive comparison and summary of the structural compositions, design strategies, key performance, and significant applications of these devices are provided. Furthermore, the challenges and future directions pertaining to materials/devices/systems associated with flexible neuromorphics are also addressed. The aim of this review is to shed light on the rapidly growing field of flexible neuromorphics, attract experts from diverse disciplines (e.g., electronics, materials science, neurobiology), and foster further innovation for its accelerated development.

Attojoule Hexagonal Boron Nitride-Based Memristor for High-Performance Neuromorphic Computing

In next-generation neuromorphic computing applications, the primary challenge lies in achieving energy-efficient and reliable memristors while minimizing their energy consumption to a level comparable to that of biological synapses. In this work, hexagonal boron nitride (h-BN)-based metal-insulator-semiconductor (MIS) memristors operating is presented at the attojoule-level tailored for high-performance artificial neural networks. The memristors benefit from a wafer-scale uniform h-BN resistive switching medium grown directly on a highly doped Si wafer using metal-organic chemical vapor deposition (MOCVD), resulting in outstanding reliability and low variability. Notably, the h-BN-based memristors exhibit exceptionally low energy consumption of attojoule levels, coupled with fast switching speed. The switching mechanisms are systematically substantiated by electrical and nano-structural analysis, confirming that the h-BN layer facilitates the resistive switching with extremely low high resistance states (HRS) and the native SiOx on Si contributes to suppressing excessive current, enabling attojoule-level energy consumption. Furthermore, the formation of atomic-scale conductive filaments leads to remarkably fast response times within the nanosecond range, and allows for the attainment of multi-resistance states, making these memristors well-suited for next-generation neuromorphic applications. The h-BN-based MIS memristors hold the potential to revolutionize energy consumption limitations in neuromorphic devices, bridging the gap between artificial and biological synapses.

Understanding vapor phase growth of hexagonal boron nitride

Hexagonal boron nitride (hBN), with its atomically flat structure, excellent chemical stability, and large band gap energy (∼6 eV), serves as an exemplary 2D insulator in electronics. Additionally, it offers exceptional attributes for the growth and encapsulation of semiconductor transition metal dichalcogenides (TMDCs). Current methodologies for producing hBN thin films primarily involve exfoliating multi-layer or bulk crystals and thin film growth via chemical vapor deposition (CVD), which entails the thermal decomposition and surface reaction of molecular precursors like ammonia boranes (NH3BH3) and borazine (B3N3H6). These molecular precursors contain pre-existing B-N bonds, thus promoting the nucleation of BN. However, the quality and phase purity of resulting BN films are greatly influenced by the film preparation and deposition process conditions that remain a substantial concern. This study aims to comprehensively investigate the impact of varied CVD systems, parameters, and precursor chemistry on the synthesis of high-quality, large scale hBN on both catalytic and non-catalytic substrates. The comparative analysis provided new insights into most effective approaches concerning both quality and scalability of vapor phase grown hBN films.

Enhanced Energy Transfer in Cavity QED Based Phototransistors

We leveraged strong light-matter coupling, a quantum process generating hybridized states, to prepare phototransistors using donor-acceptor pairs that transfer energy via Rabi oscillations. In a prototype experiment, we used a cyanine J-aggregate (TDBC; donor) and MoS2 monolayer (acceptor) in a field effect transistor cavity to study photoresponsivity. Energy migrates through the newly formed polaritonic ladder, with enhanced device efficiency when the cavity is resonant with donors. A theoretical model based on the time-dependent Schrödinger equation helped interpret results, with polaritonic states acting as a strong energy funnel to the MoS2 monolayer. These findings suggest novel applications of strong light-matter coupling in quantum materials.

A new highly stable multifunctional two-dimensional Si2BN monolayer quantum material with a direct bandgap predicted by density functional theory

In this work, we present a novel two-dimensional (2D) Si2BN structure (2D δ-Si2BN) predicted using density functional theory (DFT). The proposed structure exhibits a unique double quasi-planar layer interconnected by covalent bonds, demonstrating lower energy compared to the previously reported planar Si2BN nanosheet. Our calculations, conducted at the HSE06 level of theory, reveal its semiconductor nature with a direct band gap of 1.24 eV at the gamma point. The 2D material exhibits exceptional light absorption in the visible region, prompting an exploration of its potential in photovoltaic applications. Remarkably, our findings indicate a maximum theoretical efficiency of 27.6%, underscoring its promise for renewable energy technologies. Furthermore, employing modern polarization theory, we unveil the ferroelectric properties of the Si2BN monolayer. Notably, a large out-of-plane polarization is observed. It was found that the unstrained 2D δ-Si2BN monolayer demonstrates an impressive out-of-plane spontaneous electric polarization of 28.98 × 10-10 C m-1, a value six times greater than previously referenced Janus materials. This remarkable enhancement in ferroelectric capabilities positions the Si2BN monolayer as a promising candidate for applications in next generation novel information storage, nano-electronic, and optoelectronic devices. These findings not only contribute to the understanding of the structural and electronic properties of the 2D δ-Si2BN monolayer but also highlight its potential for various technological applications, marking a significant advancement in the field of nanomaterials.

Flexible nanoimprint lithography enables high-throughput manufacturing of bioinspired microstructures on warped substrates for efficient III-nitride optoelectronic devices

III-nitride materials are of great importance in the development of modern optoelectronics, but they have been limited over years by low light utilization rate and high dislocation densities in heteroepitaxial films grown on foreign substrate with limited refractive index contrast and large lattice mismatches. Here, we demonstrate a paradigm of high-throughput manufacturing bioinspired microstructures on warped substrates by flexible nanoimprint lithography for promoting the light extraction capability. We design a flexible nanoimprinting mold of copolymer and a two-step etching process that enable high-efficiency fabrication of nanoimprinted compound-eye-like Al2O3 microstructure (NCAM) and nanoimprinted compound-eye-like SiO2 microstructure (NCSM) template, achieving a 6.4-fold increase in throughput and 25% savings in economic costs over stepper projection lithography. Compared to NCAM template, we find that the NCSM template can not only improve the light extraction capability, but also modulate the morphology of AlN nucleation layer and reduce the formation of misoriented GaN grains on the inclined sidewall of microstructures, which suppresses the dislocations generated during coalescence, resulting in 40% reduction in dislocation density. This study provides a low-cost, high-quality, and high-throughput solution for manufacturing microstructures on warped surfaces of III-nitride optoelectronic devices.

Electroluminescence from pure resonant states in hBN-based vertical tunneling junctions

Defect centers in wide-band-gap crystals have garnered interest for their potential in applications among optoelectronic and sensor technologies. However, defects embedded in highly insulating crystals, like diamond, silicon carbide, or aluminum oxide, have been notoriously difficult to excite electrically due to their large internal resistance. To address this challenge, we realized a new paradigm of exciting defects in vertical tunneling junctions based on carbon centers in hexagonal boron nitride (hBN). The rational design of the devices via van der Waals technology enabled us to raise and control optical processes related to defect-to-band and intradefect electroluminescence. The fundamental understanding of the tunneling events was based on the transfer of the electronic wave function amplitude between resonant defect states in hBN to the metallic state in graphene, which leads to dramatic changes in the characteristics of electrons due to different band structures of constituent materials. In our devices, the decay of electrons via tunneling pathways competed with radiative recombination, resulting in an unprecedented degree of tuneability of carrier dynamics due to the significant sensitivity of the characteristic tunneling times on the thickness and structure of the barrier. This enabled us to achieve a high-efficiency electrical excitation of intradefect transitions, exceeding by several orders of magnitude the efficiency of optical excitation in the sub-band-gap regime. This work represents a significant advancement towards a universal and scalable platform for electrically driven devices utilizing defect centers in wide-band-gap crystals with properties modulated via activation of different tunneling mechanisms at a level of device engineering.

Synthesis of Few-Layer Hexagonal Boron Nitride for Magnetic Tunnel Junction Application

Hexagonal boron nitride (hBN), a wide-gap two-dimensional (2D) insulator, is an ideal tunneling barrier for many applications because of the atomically flat surface, high crystalline quality, and high stability. Few-layer hBN with a thickness of 1-2 nm is an effective barrier for electron tunneling, but the preparation of few-layer hBN relies on mechanical exfoliation from bulk hBN crystals. Here, we report the large-area growth of few-layer hBN by chemical vapor deposition on ferromagnetic Ni-Fe thin films and its application to tunnel barriers of magnetic tunnel junction (MTJ) devices. Few-layer hBN sheets mainly consisting of two to three layers have been successfully synthesized on a Ni-Fe catalyst at a high growth temperature of 1200 °C. The MTJ devices were fabricated on as-grown hBN by using the Ni-Fe film as the bottom ferromagnetic electrode to avoid contamination and surface oxidation. We found that trilayer hBN gives a higher tunneling magnetoresistance (TMR) ratio than bilayer hBN, resulting in a high TMR ratio up to 10% at ∼10 K.

Sequential Dual Alignments Introduce Synergistic Effect on Hexagonal Boron Nitride Platelets for Superior Thermal Performance

Planarly aligning 2D platelets is challenging due to their additional orientational freedom compared to 1D materials. This study reports a sequential dual-alignment approach, employing an extrusion-printing-induced shear force and rotating-magnetic-field-induced force couple for platelet planarly alignment in a yield-stress support bath. It is hypothesized that the partial alignment induced by a directional shear force facilitates subsequent axial rotation of the platelets for planar alignment under an external force couple, resulting in a synergistic alignment effect. This sequential dual-alignment approach achieves better planar alignment of 2D modified hexagonal boron nitride (mhBN). Specifically, the thermal conductivity of the 40 wt% mhBN/epoxy composite is significantly higher (692%) than that of unaligned composites, surpassing the cumulative effect of individual methods (only 133%) with a 5 times more synergistic effect. For 30, 40, and 50 wt% mhBN composites, the thermal conductivity values (5.9, 9.5, and 13.8 W m-1 K-1) show considerable improvement compared to the previously reported highest values (5.3, 6.6, and 8.6 W m-1 K-1). Additionally, a 3D mhBN/epoxy heat sink is printed and evaluated to demonstrate the feasibility of device fabrication. The approach enables the planar alignment of electrically or thermally conducting 2D fillers during 3D fabrication.

Miracle in "White":Hexagonal Boron Nitride

The exploration of 2D materials has captured significant attention due to their unique performances, notably focusing on graphene and hexagonal boron nitride (h-BN). Characterized by closely resembling atomic structures arranged in a honeycomb lattice, both graphene and h-BN share comparable traits, including exceptional thermal conductivity, impressive carrier mobility, and robust pi-pi interactions with organic molecules. Notably, h-BN has been extensively examined for its exceptional electrical insulating properties, inert passivation capabilities, and provision of an ideal ultraflat surface devoid of dangling bonds. These distinct attributes, contrasting with those of h-BN, such as its conductive versus insulating behavior, active versus inert nature, and absence of dangling surface bonds versus absorbent tendencies, render it a compelling material with broad application potential. Moreover, the unity of such contradictions endows h-BN with intriguing possibilities for unique applications in specific contexts. This review aims to underscore these key attributes and elucidate the intriguing contradictions inherent in current investigations of h-BN, fostering significant insights into the understanding of material properties.

Near-Unity Light-Matter Interaction in Mid-Infrared van der Waals Metasurfaces

Accessing mid-infrared radiation is of great importance for a range of applications, including thermal imaging, sensing, and radiative cooling. Here, we study light interaction with hexagonal boron nitride (hBN) nanocavities and reveal strong and tunable resonances across its hyperbolic transition. In addition to conventional phonon-polariton excitations, we demonstrate that the high refractive index of hexagonal boron nitride outside the Reststrahlen band allows enhanced light-matter interactions in deep subwavelength (<λ/15) nanostructures across a broad 7-8 μm range. Emergence and interplay of Fabry-Perot and Mie-like resonances are examined experimentally and theoretically. Near-unity absorption and high quality (Q ≥ 80) resonance interaction in the vicinity of the hBN transverse optical phonon is further observed. Our study provides avenues to design highly efficient and ultracompact structures for controlling mid-infrared radiation and accessing strong light-matter interactions with hBN.

Quantum Conductance in Vertical Hexagonal Boron Nitride Memristors with Graphene-Edge Contacts

Two-dimensional materials (2DMs) have gained significant interest for resistive-switching memory toward neuromorphic and in-memory computing (IMC). To achieve atomic-level miniaturization, we introduce vertical hexagonal boron nitride (h-BN) memristors with graphene edge contacts. In addition to enabling three-dimensional (3D) integration (i.e., vertical stacking) for ultimate scalability, the proposed structure delivers ultralow power by isolating single conductive nanofilaments (CNFs) in ultrasmall active areas with negligible leakage thanks to atomically thin (∼0.3 nm) graphene edge contacts. Moreover, it facilitates studying fundamental resistive-switching behavior of single CNFs in CVD-grown 2DMs that was previously unattainable with planar devices. This way, we studied their programming characteristics and observed a consistent single quantum step in conductance attributed to unique atomically constrained nanofilament behavior in CVD-grown 2DMs. This resistive-switching property was previously suggested for h-BN memristors and linked to potential improvements in stability (robustness of CNFs), and now we show experimental evidence including superior retention of quantized conductance.

Advanced materials for smart protective coatings: Unleashing the potential of metal/covalent organic frameworks, 2D nanomaterials and carbonaceous structures

The detrimental impact of corrosion on metallic materials remains a pressing concern across industries. Recently, intelligent anti-corrosive coatings for safeguarding metal infrastructures have garnered significant interest. These coatings are equipped with micro/nano carriers that store corrosion inhibitors and release them when triggered by external stimuli. These advanced coatings have the capability to elevate the electrochemical impedance values of steel by 2-3 orders of magnitude compared to the blank coating. However, achieving intelligent, durable, and reliable anti-corrosive coatings requires careful consideration in the design of these micro/nano carriers. This review paper primarily focuses on investigating the corrosion inhibition mechanism of various nano/micro carriers/barriers and identifying the challenges associated with using them for achieving desired properties in anti-corrosive coatings. Furthermore, the fundamental aspects required for nano/micro carriers, including compatibility with the coating matrix, high specific surface area, stability in different environments, stimuli-responsive behavior, and facile synthesis were investigated. To achieve this aim, we explored the properties of micro/nanocarriers based on oxide nanoparticles, carbonaceous and two-dimensional (2D) nanomaterials. Finally, we reviewed recent literature on the application of state-of the art nanocarriers based on metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs). We believe that the outcomes of this review paper offer valuable insights for researchers in selecting appropriate materials that can effectively enhance the corrosion resistance of coatings.

Coaxial Wet Spinning of Boron Nitride Nanosheet-Based Composite Fibers with Enhanced Thermal Conductivity and Mechanical Strength

Hexagonal boron nitride nanosheets (BNNSs) exhibit remarkable thermal and dielectric properties. However, their self-assembly and alignment in macroscopic forms remain challenging due to the chemical inertness of boron nitride, thereby limiting their performance in applications such as thermal management. In this study, we present a coaxial wet spinning approach for the fabrication of BNNSs/polymer composite fibers with high nanosheet orientation. The composite fibers were prepared using a superacid-based solvent system and showed a layered structure comprising an aramid core and an aramid/BNNSs sheath. Notably, the coaxial fibers exhibited significantly higher BNNSs alignment compared to uniaxial aramid/BNNSs fibers, primarily due to the additional compressive forces exerted at the core-sheath interface during the hot drawing process. With a BNNSs loading of 60 wt%, the resulting coaxial fibers showed exceptional properties, including an ultrahigh Herman orientation parameter of 0.81, thermal conductivity of 17.2 W m-1 K-1, and tensile strength of 192.5 MPa. These results surpassed those of uniaxial fibers and previously reported BNNSs composite fibers, making them highly suitable for applications such as wearable thermal management textiles. Our findings present a promising strategy for fabricating high-performance composite fibers based on BNNSs.

On the CO 2 adsorption in a boron nitride analog for the recently synthesized biphenylene network: a DFT study

CONTEXT

Recent advances in nanomaterial synthesis and characterization have led to exploring novel 2D materials. The biphenylene network (BPN) is a notable achievement in current fabrication efforts. Numerical studies have indicated the stability of its boron nitride counterpart, known as BN-BPN. In this study, we employ computational simulations to investigate the electronic and structural properties of pristine and doped BN-BPN monolayers upon CO2 adsorption. Our findings demonstrate that pristine BN-BPN layers exhibit moderate adsorption energies for CO2 molecules, approximately− 0.16 eV, indicating physisorption. However, introducing one-atom doping with silver, germanium, nickel, palladium, platinum, or silicon significantly enhances CO2 adsorption, leading to adsorption energies ranging from− 0.13 to− 0.65 eV. This enhancement indicates the presence of both physisorption and chemisorption mechanisms. BN-BPN does not show precise CO2 sensing and selectivity. Furthermore, our investigation of the recovery time for adsorbed CO2 molecules suggests that the interaction between BN-BPN and CO2 cannot modify the electronic properties of BN-BPN before the CO2 molecules escape.

METHODS

We performed density functional theory (DFT) simulations using the DMol3 code in the Biovia Materials Studio software. We incorporated Van der Waals corrections (DFT-D) within the Grimme scheme for an accurate representation. The exchange and correlation functions were treated using the Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA). We used a double-zeta plus polarization (DZP) basis set to describe the electronic structure. Additionally, we accounted for the basis set superposition error (BSSE) through the counterpoise method. We included semicore DFT pseudopotentials to accurately model the interactions between the nuclei and valence electrons.

Metal Chalcogenide-Hydroxide Hybrids as an Emerging Family of Two-Dimensional Heterolayered Materials: An Early Review

Two-dimensional (2D) materials and phenomena attract huge attention in modern science. Herein, we introduce a family of layered materials inspired by the minerals valleriite and tochilinite, which are composed of alternating "incompatible", and often incommensurate, quasi-atomic sheets of transition metal chalcogenide (sulfides and selenides of Fe, Fe-Cu and other metals) and hydroxide of Mg, Al, Fe, Li, etc., stacked via electrostatic interaction rather than van der Waals forces. We survey the data available on the composition and structure of the layered minerals, laboratory syntheses of such materials and the effect of reaction conditions on the phase purity, morphology and composition of the products. The spectroscopic results (Mössbauer, X-ray photoelectron, X-ray absorption, Raman, UV-vis, etc.), physical (electron, magnetic, optical and some others) characteristics, a specificity of thermal behavior of the materials are discussed. The family of superconductors (FeSe)·(Li,Fe)(OH) having a similar layered structure is briefly considered too. Finally, promising research directions and applications of the valleriite-type substances as a new class of prospective multifunctional 2D materials are outlined.

Electromechanical behaviour of violet phosphorene nanoflakes

Violet phosphorene (vP), a two-dimensional structure with various unique properties, has attracted much attention recently. The electromechanical behaviour, a mechanical-electrical-thermal coupling failure behaviour, of supported and suspended vP nanoflakes has been investigated by the c-AFM nanoindentation method under different loads and bias voltages in this work. A bump due to the oxidation caused by electric current-induced local heating was observed for the supported vP on substrates under mild load and bias voltage. A concave morphology was observed when a load of 1500 nN and a bias voltage of 10 V were applied. No bump was observed for the suspended vP due to the larger contact area between the probe and the sample, resulting in a lower temperature. Fatigue damage was observed for the vP nanoflakes under constant load and a cycled bias voltage from 10 V to -10 V. The electromechanical behaviour of vP was also analyzed through its specific heat capacity and conductivity by density functional theory.

Stacking-Controlled Growth of rBN Crystalline Films with High Nonlinear Optical Conversion Efficiency up to 1

Nonlinear optical crystals lie at the core of ultrafast laser science and quantum communication technology. The emergence of 2D materials provides a revolutionary potential for nonlinear optical crystals due to their exceptionally high nonlinear coefficients. However, uncontrolled stacking orders generally induce the destructive nonlinear response due to the optical phase deviation in different 2D layers. Therefore, conversion efficiency of 2D nonlinear crystals is typically limited to less than 0.01% (far below the practical criterion of >1%). Here, crystalline films of rhombohedral boron nitride (rBN) with parallel stacked layers are controllably synthesized. This success is realized by the utilization of vicinal FeNi (111) single crystal, where both the unidirectional arrangement of BN grains into a single-crystal monolayer and the continuous precipitation of (B,N) source for thick layers are guaranteed. The preserved in-plane inversion asymmetry in rBN films keeps the in-phase second-harmonic generation field in every layer and leads to a record-high conversion efficiency of 1% in the whole family of 2D materials within the coherence thickness of only 1.6 µm. The work provides a route for designing ultrathin nonlinear optical crystals from 2D materials, and will promote the on-demand fabrication of integrated photonic and compact quantum optical devices.

Pentagonal CmXnY6-m-n (m = 2, 3; n = 1, 2; X, Y = B, N, Al, Si, P) Monolayers: Janus Ternaries Combine Omnidirectional Negative Poisson Ratios with Giant Piezoelectric Effects

Two-dimensional (2D) materials composed of pentagon and Janus motifs usually exhibit unique mechanical and electronic properties. In this work, a class of ternary carbon-based 2D materials, C_mX_nY_6-m-n (m = 2, 3; n = 1, 2; X, Y = B, N, Al, Si, P), are systematically studied by first-principles calculations. Six of 21 Janus penta-C_mX_nY_6-m-n monolayers are dynamically and thermally stable. The Janus penta-C₂B₂Al₂ and Janus penta-Si₂C₂N₂ exhibit auxeticity. More strikingly, Janus penta-Si₂C₂N₂ exhibits an omnidirectional negative Poisson ratio (NPR) with values ranging from -0.13 to -0.15; in other words, it is auxetic under stretch in any direction. The calculations of piezoelectricity reveal that the out-of-plane piezoelectric strain coefficient (d₃₂) of Janus panta-C₂B₂Al₂ is up to 0.63 pm/V and increases to 1 pm/V after a strain engineering. These omnidirectional NPR, giant piezoelectric coefficients endow the Janus pentagonal ternary carbon-based monolayers as potential candidates in the future nanoelectronics, especially in the electromechanical devices.

Charged Boron Nitride Nanosheet Membranes for Improved Organic Solvent Nanofiltration

Two-dimensional nanomaterial-based membranes have earned broad attention because of their excellent capability of separation performance in a mixture that can challenge the conventional membrane materials utilized in the organic solvent nanofiltration (OSN) field. Boron nitride (BN) nanosheet membranes have displayed superb stability and separation ability in aqueous and organic solutions compared to the widely researched analogous graphene-based membranes; nevertheless, the concentration polarization of organic dye pollutants fades their separation performance and eclipses their potential adoption as a feasible technology. Herein, PDDA-modified BN (PBN) and sodium alginate-modified BN (SBN) nanosheet membranes with a thinner laminar structure are facially fabricated to improve the molecule separation performance compared to that of the pristine BN membrane. In aqueous separation application, the SBN membranes (2 μm) can reject positively charged dyes up to 100% and the PBN membrane (2 μm) could reject negatively charged dyes up to 100%. Impressively, the PBN membranes (3 μm) and SBN membranes (3 μm) demonstrate record high performances in OSN, with a permeance of 809 L m^-2 h^-1 bar^-1 and 97.71% rejection to acid fuchsin in acetonitrile and 290 L m^-2 h^-1 bar^-1 and 94.94% rejection to Azure B in dimethyl sulfoxide, respectively. The charged PBN and SBN nanosheet membranes demonstrate stable separation capability, exhibiting their potential for practical water and organic solvent purification processes.

Epitaxial Hexagonal Boron Nitride for Hydrogen Generation by Radiolysis of Interfacial Water

Hydrogen is an important building block in global strategies toward a future green energy system. To make this transition possible, intense scientific efforts are needed, also in the field of materials science. Two-dimensional crystals, such as hexagonal boron nitride (hBN), are very promising in this regard, as it has been demonstrated that micrometer-sized flakes are excellent barriers to molecular hydrogen. However, it remains an open question whether large-area layers fabricated by industrially relevant methods preserve such promising properties. In this work, we show that electron-beam-induced splitting of water creates hBN bubbles that effectively store molecular hydrogen for weeks and under extreme mechanical deformation. We demonstrate that epitaxial hBN allows direct visualization and monitoring of the process of hydrogen generation by radiolysis of interfacial water. Our findings show that hBN is not only a potential candidate for hydrogen storage but also holds promise for the development of unconventional hydrogen production schemes.