Jing Zhang

Vertical-structure devices offer distinct advantages in terms of integration feasibility, efficiency and stability. However, poor out-of-plane carrier transport in layered 2D perovskite hampers vertical-structure X-ray detection performance. Layered CsPb₂Br₅ single crystals, grown with ligand-assisted methods, achieve excellent out-of-plane µτ product. The resulting vertical-structure X-ray detectors demonstrate a sensitivity of 8865.6 µC Gyair⁻¹ cm⁻² and good imaging performance.

Abstract

The emerging 2D layered perovskites have promising optoelectronic properties, good intrinsic stability and reduced ion migration, making them effective for detecting X-ray radiation. However, their application is constrained by poor out-of-plane carrier transport. In this study, inch-sized high-quality CsPb₂Br₅ layered single crystals (SCs) are developed using an organic ligand-assisted solution process. By modifying the surface energy, the anisotropy of crystal growth is conquered, resulting in CsPb₂Br₅ SCs with sufficient thickness for X-ray detection. Importantly, this modification significantly enhanced the crystal quality as the grown CsPb₂Br₅ SCs exhibited longer photoluminescence lifetime and smaller trap density. Notably, the CsPb₂Br₅ SCs demonstrate unprecedented out-of-plane carrier transport, achieving a high carrier mobility-lifetime product of 2.53 × 10⁻² cm²V⁻¹. This can be attributed to the small interlayer distance and the strong interlayer force of Cs─Br bonding. Furthermore, CsPb₂Br₅ SCs possess other intriguing attributes for X-ray detection, including high bulk resistivity and outstanding thermal stability. These advantageous properties enable high-performance vertical-structure X-ray detection with a superior sensitivity of up to 8865.6 µC Gy_air ⁻¹cm⁻² and a low detectable dose rate of 12.7 nGy_airs⁻¹. Additionally, CsPb₂Br₅ SCs exhibit high spatial resolution in X-ray imaging and exceptional thermal stability, making them promising candidates for nondestructive determination.

This work reviews the recent progress of 2D ferroelectric devices for in-sensing and in-memory neuromorphic computing. Experimental and theoretical progresses on 2D ferroelectric devices, including passive ferroelectrics-integrated 2D devices and active ferroelectrics-integrated 2D devices are reviewed followed by the integration of perception, memory, and computing application. Notably, the 2D ferroelectric devices have been used to simulate synaptic weights, neuronal model functions, and neural networks for image processing.

Abstract

The quantity of sensor nodes within current computing systems is rapidly increasing in tandem with the sensing data. The presence of a bottleneck in data transmission between the sensors, computing, and memory units obstructs the system's efficiency and speed. To minimize the latency of data transmission between units, novel in-memory and in-sensor computing architectures are proposed as alternatives to the conventional von Neumann architecture, aiming for data-intensive sensing and computing applications. The integration of 2D materials and 2D ferroelectric materials has been expected to build these novel sensing and computing architectures due to the dangling-bond-free surface, ultra-fast polarization flipping, and ultra-low power consumption of the 2D ferroelectrics. Here, the recent progress of 2D ferroelectric devices for in-sensing and in-memory neuromorphic computing is reviewed. Experimental and theoretical progresses on 2D ferroelectric devices, including passive ferroelectrics-integrated 2D devices and active ferroelectrics-integrated 2D devices, are reviewed followed by the integration of perception, memory, and computing application. Notably, 2D ferroelectric devices have been used to simulate synaptic weights, neuronal model functions, and neural networks for image processing. As an emerging device configuration, 2D ferroelectric devices have the potential to expand into the sensor-memory and computing integration application field, leading to new possibilities for modern electronics.

Thermochromic afterglow (TCAG) in carbon dots-inked paper (CDs@Paper) is achieved by tuning the temperature-dependent dual-mode afterglow of room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF). The TCAG properties of CDs@Paper are leveraged to design a multidimensional color code containing color and temperature information for advanced dynamic information encryption. And the two-photon afterglow is first reported in CDs materials.

Abstract

Achieving thermochromic afterglow (TCAG) in a single material for advanced information encryption remains a significant challenge. Herein, TCAG in carbon dots (CDs)-inked paper (CDs@Paper) is achieved by tuning the temperature-dependent dual-mode afterglow of room temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF). The CDs are synthesized through thermal treatment of levofloxacin in melting boric acid with postpurification via dialysis. CDs@Paper exhibit both TCAG and excitation-dependent afterglow color properties. The TCAG of CDs@Paper exhibits dynamic color changes from blue at high temperatures to yellow at low temperatures by adjusting the proportion of the temperature-dependent TADF and phosphorescence. Notably, two-photon afterglow in CDs-based afterglow materials and time-dependent two-photon afterglow colors are achieved for the first time. Moreover, leveraging the opposite emission responses of phosphorescence and TADF to temperature, CDs@Paper demonstrate TCAG with temperature-sensing capabilities across a wide temperature range. Furthermore, a CDs@Paper-based 3D code containing color and temperature information is successfully developed for advanced dynamic information encryption.

A graphene and MoS₂-based implantable multi-modal sensor array is developed, presenting a minimally invasive implantation process for neural monitoring apparatus. The sensor array is injected via syringe and air pressure through a small hole in the skull and spreads out to conformally cover the cortical surface. The sensors detect epileptic discharges and monitor intracranial temperature and pressure.

Abstract

Intracranial implants for diagnosis and treatment of brain diseases have been developed over the past few decades. However, the platform of conventional implantable devices still relies on invasive probes and bulky sensors in conjunction with large-area craniotomy and provides only limited biometric information. Here, an implantable multi-modal sensor array that can be injected through a small hole in the skull and inherently spread out for conformal contact with the cortical surface is reported. The injectable sensor array, composed of graphene multi-channel electrodes for neural recording and electrical stimulation and MoS₂-based sensors for monitoring intracranial temperature and pressure, is designed based on a mesh structure whose elastic restoring force enables the contracted device to spread out. It is demonstrated that the sensor array injected into a rabbit's head can detect epileptic discharges on the surface of the cortex and mitigate it by electrical stimulation while monitoring both intracranial temperature and pressure. This method provides good potential for implanting a variety of functional devices via minimally invasive surgery.

High-quality transition metal dichalcogenide (TMD) thin films can be synthesized using a two-step approach where a solid transition metal precursor layer is converted in a chalcogen-containing atmosphere to a TMD. Herein, a critical review of this method, demonstrating its versatility and outlining key features, applications, and outlook on this method's impact in the TMD synthesis community is given.

Abstract

The widely studied class of two-dimensional (2D) materials known as transition metal dichalcogenides (TMDs) are now well-poised to be employed in real-world applications ranging from electronic logic and memory devices to gas and biological sensors. Several scalable thin film synthesis techniques have demonstrated nanoscale control of TMD material thickness, morphology, structure, and chemistry and correlated these properties with high-performing, application-specific device metrics. In this review, the particularly versatile two-step conversion (2SC) method of TMD film synthesis is highlighted. The 2SC technique relies on deposition of a solid metal or metal oxide precursor material, followed by a reaction with a chalcogen vapor at an elevated temperature, converting the precursor film to a crystalline TMD. Herein, the variables at each step of the 2SC process including the impact of the precursor film material and deposition technique, the influence of gas composition and temperature during conversion, as well as other factors controlling high-quality 2D TMD synthesis are considered. The specific advantages of the 2SC approach including deposition on diverse substrates, low-temperature processing, orientation control, and heterostructure synthesis, among others, are featured. Finally, emergent opportunities that take advantage of the 2SC approach are discussed to include next-generation electronics, sensing, and optoelectronic devices, as well as catalysis for energy-related applications.

Ferroelectricity with out-of-plane polarization has so far been found in several two-dimensional (2D) materials, including monolayers comprising three to five planes of atoms, e.g. α-In2Se3 and MoTe2. Here, we explore the generation of out-of-plane polarization within a one-atom-thick monolayer material, namely hexagonal boron nitride. We performed density-functional-theory calculations to explore inducing ferroelectric-like distortions through incorporation of isovalent substitutional impurities that are larger than the host atoms. This disparity in bond lengths causes a buckling of the h-BN, either up or down, which amounts to a dipole with two equivalent energies and opposing orientations. We tested several impurities to explore the magnitude of the induced dipole and the switching energy barrier for dipole inversion. The effects of strain, dipole–dipole interactions, and vertical heterostructures with graphene are further explored. Our results suggest a highly-tunable system with ground state antiferroelectricity and metastable ferroelectricity. We expect that this work will help foster new ways to include functionality in layered 2D-material-based applications.

As shown, macroscale superlubricity with high pressure on engineering steel is enabled by partially oxidized violet phosphorus nanosheets (oVP) in poly alpha olefin (PAO) oil. The outstanding tribological performance (coefficient of friction: 0.0064) and load-bearing capacity (interfacial maximal stress: 810 MPa) are mainly attributed to a reliable chemical tribofilm formed on on steel surface with the catalyzation effect of oVP.

Abstract

As a novel cross-structured 2D material, violet phosphorus (VP) promises to further develop the dynamic performance, energy efficiency, and service lifetime of mechanical components owing to its high strength and toughness, large carrier mobility, wide bandgap, and good friction-reducing properties. In this work, the partially oxidized violet phosphorus nanosheets (oVP) are synthesized and employed as the lubricating additives in the poly alpha olefin (PAO) oil environment with less oleic acid (OA) improver, which can trigger the macroscale superlubricity on the diamond-like carbon (DLC) film deposited steel surface at high loading pressures (>800 MPa) and sliding speeds (>0.1 m s⁻¹). The friction coefficient (COF) of the steel-DLC tribopair lubricated by the oVP-PAO oil can reduce down to 0.0064 with little wear. At the high-pressure sliding interface, the force-thermal coupling action during the running-in process promotes the cleavage and recombination of oVP nanosheets and OA molecules to form a reliable chemical adsorption tribofilm mainly composed of phosphorus oxides and amorphous carbon, which prevents the original asperities from direct contact and provides an ultralow shear strength. These findings suggest a new method to achieve macroscale oil-based high-pressure superlubricity for engineering steel through the lubrication and catalyzation effects of oVP.

Through the self-leveling and self-evaporative properties of gelatin solution, nano-scale flexible transparent films are obtained. Leveraging its adhesive properties with a mixture of liquid metal, wet-adhesive flexible electronic devices are fabricated, enabling rapid adhesion in moist environments in vivo and improving the accuracy and stability of implanted monitoring.

Abstract

Implantable flexible electronic has attracted significant research interest in various fields. However, it still faces the challenge of simultaneously achieving tight adhesion to tissues in a mildly wet environment and possessing excellent biocompatibility to reduce immune rejection reactions after implantation. Here, a degradable wet-adhesive flexible electronic device based on liquid metal and ultrathin gelatin film is developed. The ultrathin gelatin film forms numerous hydrogen bonds with tissue in a slightly humid environment, rapidly constructing a wet-adhesive interface without damaging tissue structure. Inkjet printing is utilized to pattern the mixture of liquid metal and PVP on the surface of the ultrathin gelatin to create flexible patch. With the excellent conductivity of liquid metal, low toxicity, and similarity to natural tissue components of gelatin, flexible patch exhibits outstanding biocompatibility and fatigue resistance. It can be implanted in the body for up to 6 weeks, retaining monitoring capabilities and resisting 1 000 000 cycles of bending fatigue. This study provides a novel strategy for the future development of implantable flexible electronics.

Metal halide perovskite single crystals (MHP SCs) have attracted much attention due to their dramatically enhanced optoelectronic properties and improved stability compared with their polycrystalline counterparts. The recent advancements in MHP SC-based light-emitting diodes, potential strategies for further device performance improvement and future application prospects in electrically pumped lasers are discussed in this review.

Abstract

Metal halide perovskite single crystals (MHP SCs) have attracted extensive attention due to their superior properties, such as higher carrier mobility, longer carrier diffusion length, and better stability than their polycrystalline counterparts. In particular, the suppression of ion migration and Auger recombination endows MHP SCs with excellent electroluminescence (EL) properties, thus holding great potential for highly efficient and stable light-emitting devices. In this review, general overview of MHP crystal structures are begin, and highlight the merits of MHP SCs in terms of outstanding optoelectronic properties and high stability. Then, appropriate growth methods of high-quality, thickness-controlled MHP SCs for EL device applications are systematically summarized. Subsequently, recent advancements in developing MHP SC-based perovskite light-emitting diodes (PeLEDs) are discussed, and the effective strategies to further enhance the device performance are reviewed. Moreover, the potential application of MHP SCs for electrically pumped lasers is highlighted. Finally, the review is concluded with a detailed account of the current challenges and a perspective on the key approaches and opportunities on the optimization of SC growth, the improvement of device performance and the integration of SC-based optoelectronic devices.

This study assesses the potential of layered 2D materials for gas sensing using second harmonic generation (SHG). It focuses on the adsorption behaviors of oxygen, ammonia, and water vapor on WS₂ surfaces. By applying the simplified bond hyperpolarizability model, it confirms physical adsorption and explores competitive interactions between gases, aligning with Langmuir's model and theoretical predictions from density functional theory.

Abstract

While understanding the competitive adsorption behavior of gas sensor is important, it is yet to be unraveled. Especially for the influence of water molecules to the gas adsorbed on 2D materials. This study explores the potential of layered 2D materials as a candidate material for gas sensing, employing non-destructive measurement, and second harmonic generation (SHG). The investigation focuses on analyzing oxygen, ammonia, and water vapor adsorbed on a WS₂ surface by studying the evolutions in electric dipole and electric field. Leveraging the simplified bond hyperpolarizability model (SBHM), a foundation is established for gas sensors utilizing high-quality 2D materials. This approach facilitates the detection of material modifications in response to environmental influences, including the inevitable water molecules. The obtained hyperpolarizability from SBHM exhibits remarkable consistency with Langmuir's adsorption model, confirming the physical adsorption in the system. In addition, the competitive effects between gases are explored by comparing experimental results with theoretical predictions based on Boltzmann distribution and density functional theory (DFT) calculations. This highlights the effectiveness of SHG and SBHM in studying gas adsorption on layered van der Waals materials.

2D flexible magnets hold great promise in flexible spintronics. By employing an aged precursor, 2D chromium selenide with internal voids can be synthesized. The unique structure induces ferrimagnetism and a small Young's modulus. This work offers avenues for obtaining 2D magnets with desired mechanical properties, paving the way for future flexible spintronics.

Abstract

2D magnetic materials with distinct mechanical properties are of great importance for flexible spintronics. However, synthesizing 2D magnets with atomic thickness is challenging and their mechanical properties remain largely unexplored. Here, the growth of a ferrimagnetic 2D Cr₉Se₁₃ with anomalous elasticity is reported by an aged-precursor-assisted method. The obtained 2D Cr₉Se₁₃ exhibits an out-of-plane ferrimagnetic order with a coercivity larger than those of conventional magnetic materials. Noteworthy, it presents decent breaking strength and a Young's modulus of 52 ± 8 GPa that is among the smallest of the 2D family. This exceptional elasticity is attributed to the unique internal voids in Cr₉Se₁₃, as evidenced by the formed edge dislocations under strain. This work not only offers a facile method to synthesize 2D magnets but also develops avenues for obtaining 2D materials with desired mechanical properties, paving the way for future flexible spintronics.

Recent advances in poly(N-isopropylacrylamide) and its copolymers, with a specific focus on their structural and compositional features in the area of sensor and biosensor applications from 2016 until now are comprehensively summarized here.

Abstract

Stimuli-responsive polymers have received increasing attention for various applications due to their ability to adapt physical and chemical properties in response to external environmental stimuli. In this regard, poly(N-isopropylacrylamide) (PNIPAM) is the most extensively studied stimuli-responsive polymer and, consequently has been prominently featured in (bio)-sensor development, adaptive coating technology, drug delivery, wound healing, tissue regeneration, artificial actuator design, sensor technology, responsive coatings, and soft robotics. This success can be mainly attributed to the accessible and versatile nature of the PNIPAM platform, thus allowing the synthesis of a wide variety of copolymer architectures, topologies and compositions. Within this review, the structural and compositional features of PNIPAM-based materials in sensor and biosensor applications are discussed with a focus on the literature from 2016 until now. The reader is provided with the current state of the art regarding PNIPAM-based sensor development and their molecular design. Finally, the challenges ahead in the successful implementation of PNIPAM-based sensors are highlighted, as well as the opportunities in the rational design of improved PNIPAM-based sensors. Altogether, this review provides comprehensive insights into the exciting and rapidly expanding field of PNIPAM-based sensing systems, which will benefit the chemical, pharmaceutical, textile, and biotech industries is believed.

Black phosphorus (BP)/graphdiyne oxide (GDYO) lateral/vertical heterostructures are prepared via solid-state mechanochemistry. The sp²-hybridized mode P-C and out-of-plane P-O-C bonds realize the lateral and vertical connection, respectively. GDYO regulates the volume expansion of BP, provides active sites, and restrains the shuttle effect of Li_xP_y. The heterostructures combine interface and structural engineering, demonstrating high-rate performance and long-term stability in lithium storage.

Abstract

Artificial heterostructures with structural advancements and customizable electronic interfaces are fundamental for achieving high-performance lithium-ion batteries (LIBs). Here, a design idea for a covalently bonded lateral/vertical black phosphorus (BP)-graphdiyne oxide (GDYO) heterostructure achieved through a facile ball-milling approach, is designed. Lateral heterogeneity is realized by the sp²-hybridized mode P-C bonds, which connect the phosphorus atoms at the edges of BP with the carbon atoms of the terminal acetylene in GDYO. The vertical connection of the heterojunction of BP and GDYO is connected by P-O-C bond. Experimental and theoretical studies demonstrate that BP-GDYO incorporates interfacial and structural engineering features, including built-in electric fields, chemical bond interactions, and maximized nanospace confinement effects. Therefore, BP-GDYO exhibits improved electrochemical kinetics and enhanced structural stability. Moreover, through ex- and in-situ studies, the lithiation mechanism of BP-GDYO, highlighting that the introduction of GDYO inhibits the shuttle/dissolution effect of phosphorus intermediates, hinders volume expansion, provides more reactive sites, and ultimately promotes reversible lithium storage, is clarified. The BP-GDYO anode exhibits lithium storage performance with high-rate capacity and long-cycle stability (602.6 mAh g⁻¹ after 1 000 cycles at 2.0 A g⁻¹). The proposed interfacial and structural engineering is universal and represents a conceptual advance in building high-performance LIBs electrode.

Various synthesis techniques on how to subtly incorporate rare earth/transition metals into various matrices in the NIR are reviewed, followed by a discussion of strategies to improve the excitation absorption and emission efficiency of NIR materials. Finally, functionalization strategies and their applications are presented. As such it provides a valuable overview of the field.

Abstract

The spotlight has shifted to near-infrared (NIR) luminescent materials emitting beyond 1000 nm, with growing interest due to their unique characteristics. The ability of NIR-II emission (1000–1700 nm) to penetrate deeply and transmit independently positions these NIR luminescent materials for applications in optical-communication devices, bioimaging, and photodetectors. The combination of rare earth metals/transition metals with a variety of matrix materials provides a new platform for creating new chemical and physical properties for materials science and device applications. In this review, the recent advancements in NIR emission activated by rare earth and transition metal ions are summarized and their role in applications spanning bioimaging, sensing, and optoelectronics is illustrated. It started with various synthesis techniques and explored how rare earths/transition metals can be skillfully incorporated into various matrixes, thereby endowing them with unique characteristics. The discussion to strategies of enhancing excitation absorption and emission efficiency, spotlighting innovations like dye sensitization and surface plasmon resonance effects is then extended. Subsequently, a significant focus is placed on functionalization strategies and their applications. Finally, a comprehensive analysis of the challenges and proposed strategies for rare earth/transition metal ion-doped near-infrared luminescent materials, summarizing the insights of each section is provided.

Zinc-Air Batteries

In article number 2308443, Soorathep Kheawhom, and co-workers delve into the critical role of lanthanum and strontium perovskite oxides in augmenting the efficiency of zinc-air batteries, highlighting their cost-effectiveness and adaptability as electrocatalysts for oxygen-related reactions. They examine their structures, synthesis, and how they function in oxygen reduction and evolution, pivotal for energy conversion and storage technologies.

Nature Physics, Published online: 09 May 2024; doi:10.1038/s41567-024-02498-w

Some magnetic phase transitions can be understood as Bose–Einstein condensation of magnons. Close to a quantum critical point, YbCl3 now provides a realization of a Bose–Einstein condensate that is dominated by two-dimensional physical behaviour.

Ligand engineering contributes to the precise control of the photochromic behavior of CdS quantum dots. This characteristic is harnessed to craft complex and exquisite patterns intricately through UV lithography. Simultaneously, within a hydrogel matrix, the variation in fluorescence induced by light-induced aggregated QDs enables dual-mode information encryption. Leveraging these capabilities, a dual-mode information encryption technology is developed, incorporating interactive, temporal, multilevel, and spatial features.

Abstract

The explosive growth of information and its widespread availability underscores the need for robust encryption and anticounterfeiting measures. In this study, CdS quantum dots are engineered (QDs) to manifest multiple visual responses to a single trigger through strategic ligand design. The surface engineering method allows QDs to transition from yellow to black upon photoexcitation-induced electron transfer from Cd(II) to Cd(0). Surface ligands desorption under hole injection, leading to an increase in QDs size and resulting in a redshift in photoluminescence. This photoexcitation-induced redox reaction reveals unprecedented photochromism and photoluminescence phenomena, establishing a foundation for advanced information protection measures. Utilizing these QDs, excellent writing performance under UV irradiation is achieved in solid-state substrates, while a dual-mode encryption system is realized in gel matrices, opening up new avenues for information encryption as well as cumulative and interactive information protection. Furthermore, the redox reaction of CdS QDs is employed as ink for 3D printing, enabling for the creation of digitally programmable materials with distinct temporally evolving appearances by controlling the oxygen content in the ink to regulate the rate of photochromism. This advancement also sheds light on the progress in 3D printing technology.

Nature Communications, Published online: 08 May 2024; doi:10.1038/s41467-024-47674-x

Room temperature phosphorescent materials have many desirable properties, but fine control of these can be challenging. Here, the authors report a strategy for this control by dynamic lanthanide coordination in combination with organic phosphors.

Nature, Published online: 08 May 2024; doi:10.1038/s41586-024-07369-1

Electro-optical photonic integrated circuits based on lithium tantalate perform as well as current state-of-the-art ones using lithium niobate but the material has the advantage of existing commercial uses in consumer electronics, easing the problem of scalability.

Nature, Published online: 08 May 2024; doi:10.1038/s41586-024-07505-x

Elastic films of single-crystal two-dimensional covalent organic frameworks

A triboelectric potential powered vertical field-effect transistor (tribotronic vertical field-effect transistor or graphene barristor) is demonstrated, offering an effective way to modulate the Schottky barrier of vertical van der Waals heterostructure by mechanical displacement.

Abstract

Graphene has attracted considerable interest for next-generation electronics. However, the absence of natural bandgap has limited the current on/off ratio of graphene-based transistors. Vertical integration of 2D heterostructures offers a promising approach to address this challenge, enabling high-current-density vertical field-effect transistor (VFET) with large on/off ratio. Here, a triboelectric potential-powered VFET with a vertical stacked graphene/MoS₂ heterostructure and a sliding-mode triboelectric nanogenerator (TENG) coupled with gate dielectrics are proposed. The tribotronic VFET has an ultrashort channel length in vertical direction, exhibiting excellent current driving capability with an ultrahigh on-state current density of 950 A cm⁻² and a good current on/off ratio of 630. It also demonstrates reconfigurable diode behavior with a rectification ratio over 10². Temperature-dependent studies are applied to tribotronic devices for the first time, indicating an effective modulation on the Schottky barrier height of 150 meV by the triboelectric potential. A green LED pixel is driven by the tribotronic VFET as a demonstration to work as a tactile interactive light-emitting device. The demonstrated tribotronic vertical device offers a promising strategy for integrating various 2D layered materials with TENG in vertical direction, enabling 3D integration of low-power and interactive devices for next-generation electronics.

The influence of the structural nonequivalence of AlN–AlGaN interfaces on the interface vibrational modes is probed through off-axis monochromated electron energy-loss spectroscopy. By carefully aligning off-axis deflection with the polar bonds, phonon softening at one interface (but not the other) are isolated and the nonequivalence is measured. This opens the way for polarization-selective vibrational spectroscopy with ultrahigh spatial resolution.

Abstract

In heterostructures made from polar materials, e.g., AlN–GaN–AlN, the nonequivalence of the two interfaces is long recognized as a critical aspect of their electronic properties; in that, they host different 2D carrier gases. Interfaces play an important role in the vibrational properties of materials, where interface states enhance thermal conductivity and can generate unique infrared-optical activity. The nonequivalence of the corresponding interface atomic vibrations, however, is not investigated so far due to a lack of experimental techniques with both high spatial and high spectral resolution. Herein, the nonequivalence of AlN–(Al_0.65Ga_0.35)N and (Al_0.65Ga_0.35)N–AlN interface vibrations is experimentally demonstrated using monochromated electron energy-loss spectroscopy in the scanning transmission electron microscope (STEM-EELS) and density-functional-theory (DFT) calculations are employed to gain insights in the physical origins of observations. It is demonstrated that STEM-EELS possesses sensitivity to the displacement vector of the vibrational modes as well as the frequency, which is as critical to understanding vibrations as polarization in optical spectroscopies. The combination enables direct mapping of the nonequivalent interface phonons between materials with different phonon polarizations. The results demonstrate the capacity to carefully assess the vibrational properties of complex heterostructures where interface states dominate the functional properties.

“Graphene's Wetting Transparency: A Liquid Interface Study – This compelling digital visualization showcases a single-layer graphene sheet's interaction with a submerged organic droplet. The graphene, almost optically transparent, conforms to the liquid surface, offering a vivid display of surface tension and intermolecular forces at play between the droplet, graphene, and the liquid substrate.”

Abstract

Graphene's wetting transparency offers promising avenues for creating multifunctional devices by allowing real-time wettability control on liquid substrates via the flow of different liquids beneath graphene. Despite its potential, direct measurement of floating graphene's wettability remains a challenge, hindering the exploration of these applications. The current study develops an experimental methodology to assess the wetting transparency of single-layer graphene (SLG) on liquid substrates. By employing contact angle measurements and Neumann's Triangle model, the challenge of evaluating the wettability of floating free-suspended single-layer graphene is addressed. The research reveals that for successful contact angle measurements, the testing and substrate liquids must be immiscible. Using diiodomethane as the testing liquid and ammonium persulfate solution as liquid substrate, the study demonstrates the near-complete wetting transparency of graphene. Furthermore, it successfully showcases the feasibility of real-time wettability control using graphene on liquid substrates. This work not only advances the understanding of graphene's interaction with liquid interfaces but also suggests a new avenue for the development of multifunctional materials and devices by exploiting the unique wetting transparency of graphene.

Nature, Published online: 08 May 2024; doi:10.1038/s41586-024-07369-1

Electro-optical photonic integrated circuits based on lithium tantalate perform as well as current state-of-the-art ones using lithium niobate but the material has the advantage of existing commercial uses in consumer electronics, easing the problem of scalability.

Non-metallic substrates offer a promising platform for direct growth of 2D materials, reducing transfer complexities and enabling wafer-scale fabrication. The substrates and growth methods play critical roles in controlling crystal nucleation and growth, with potential for preserving material properties and scaling up production.

Abstract

The advent of 2D materials has ushered in the exploration of their synthesis, characterization and application. While plenty of 2D materials have been synthesized on various metallic substrates, interfacial interaction significantly affects their intrinsic electronic properties. Additionally, the complex transfer process presents further challenges. In this context, experimental efforts are devoted to the direct growth on technologically important semiconductor/insulator substrates. This review aims to uncover the effects of substrate on the growth of 2D materials. The focus is on non-metallic substrate used for epitaxial growth and how this highlights the necessity for phase engineering and advanced characterization at atomic scale. Special attention is paid to monoelemental 2D structures with topological properties. The conclusion is drawn through a discussion of the requirements for integrating 2D materials with current semiconductor-based technology and the unique properties of heterostructures based on 2D materials. Overall, this review describes how 2D materials can be fabricated directly on non-metallic substrates and the exploration of growth mechanism at atomic scale.

Black phosphorus dual-gate devices are shown to form electrostatically induced pn-homojunctions when asymmetric gate biases are applied. This allows the demonstration of photodiodes with specific detectivities of 8.5 × 10⁸ cm Hz^1/2 W⁻¹ and open circuit photovoltages of 175 mV, under infrared illumination. This development extends the application of dual-gate van der Waals materials photodetectors into the short-wave infrared range.

Abstract

Homojunctions are key elements in many mainstream electronic devices. However, conventional dopant-based “pn” homojunctions are not easily achievable in new material families, such as the 2D materials. Several recent 2D material studies have shown that lateral pn homojunctions can instead be electrostatically induced using back gates localized to either the source or drain contacts. Here, a hBN-encapsulated black phosphorus dual-gate device containing a lateral pn homojunction, whose orientation can be switched via application of back gate voltages, is demonstrated. Importantly, this study extends the state-of-the-art for this architecture by characterizing the photoresponse under infrared (λ = 2.2 µm) illumination. It is shown that when biased to form a homojunction, the device exhibits the photovoltaic effect, resulting in a specific detectivity of 8.5 × 10⁸ cm Hz^1/2 W⁻¹ at 77 K under short-circuit conditions, and an open circuit photovoltage up to 175 mV at 77 K. Further, it is shown that the device can be operated in photoconductive mode, allowing a high responsivity of 0.55 A W⁻¹. This device is thus highly reconfigurable as it can be switched between photovoltaic and photoconductive modes of operation to prioritize low noise and fast response or high responsivity.