Probing Electrocatalytic Synergy in Graphene/MoS2/Nickel Networks for Water Splitting through a Combined Experimental and Theoretical Lens.

The development of low-cost and active electrocatalysts signifies an important effort toward accelerating economical water electrolysis and overcoming the sluggish hydrogen or oxygen evolution reaction (HER or OER) kinetics. Herein, we report a scalable and rapid synthesis of inexpensive Ni and MoS2 electrocatalysts on N-doped graphene/carbon cloth substrate to address these challenges. Mesoporous N-doped graphene is synthesized by using electrochemical polymerization of polyaniline (PANI), followed by a rapid one-step photothermal pyrolysis process. The N-doped graphene/carbon cloth substrate improves the interconnection between the electrocatalyst and substrate. Consequently, Ni species deposited on an N-doped graphene OER electrocatalyst shows a low Tafel slope value of 35 mV/decade at an overpotential of 130 mV at 10 mA/cm2 current density in 1 M KOH electrolytes. In addition, Ni-doped MoS2 on N-doped graphene HER electrocatalyst shows Tafel slopes of 37 and 42 mV/decade and overpotentials of 159 and 175 mV, respectively, in acidic and alkaline electrolytes at 10 mA/cm2 current density. Both these values are lower than recently reported nonplatinum-group-metal-based OER and HER electrocatalysts. These excellent electrochemical performances are due to the high electrochemical surface area, a porous structure that improves the charge transfer between electrode and electrolytes, and the synergistic effect between the substrate and electrocatalyst. Raman spectroscopy, X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations demonstrate that the Ni hydroxide species and Ni-doped MoS2 edge sites serve as active sites for OER and HER, respectively. Finally, we also evaluate the performance of the HER electrocatalyst in commercial alkaline electrolyzers.

Alice (and Bob) in Flatland.

2D quantum materials have opened infinite doors, hosting intriguing phenomena and featuring incredible engineering potential. Whether these qualities can boost the use of 2D crystals for quantum applications remains an open field with yet unexplored paths.

Graphene Terahertz Devices for Sensing and Communication.

Graphene-based terahertz (THz) devices have emerged as promising platforms for a variety of applications, leveraging graphene's unique optoelectronic properties. This review explores recent advancements in utilizing graphene in THz technology, focusing on two main aspects: THz molecular sensing and THz wave modulation. In molecular sensing, the environment-sensitive THz transmission and emission properties of graphene are utilized for enabling molecular adsorption detection and biomolecular sensing. This capability holds significant potential, from the detection of pesticides to DNA at high sensitivity and selectivity. In THz wave modulation, crucial for next-generation wireless communication systems, graphene demonstrates remarkable potential in absorption modulation when gated. Novel device structures, spectroscopic systems, and metasurface architectures have enabled enhanced absorption and wave modulation. Furthermore, techniques such as spatial phase modulation and polarization manipulation have been explored. From sensing to communication, graphene-based THz devices present a wide array of opportunities for future research and development. Finally, advancements in sensing techniques not only enhance biomolecular analysis but also contribute to optimizing graphene's properties for communication by enabling efficient modulation of electromagnetic waves. Conversely, developments in communication strategies inform and enhance sensing capabilities, establishing a mutually beneficial relationship.

Cascade Förster resonance energy transfer between layered silicate edge-linked chromophores.

Förster resonance energy transfer (FRET) serves as a critical mechanism to study intermolecular interactions and the formation of macromolecular assemblies. Cascade FRET is a multi-step FRET process which can overcome limitations associated with traditional single-step FRET. Herein, a novel organic-inorganic hybrid composed of a nile red derivative attached to the edge of the layered silicate clay Laponite (Lap-NR) was used to facilitate cascade FRET between Laponite sheets. Utilizing naphthalene-diimide edge-modified Laponite (Lap-NDI) as the initial donor, Rhodamine 6G on the basal surface of Laponite as the first acceptor, and Lap-NR as the second acceptor, cascade FRET was achieved. The influence of solvent composition in a DMF/water mixture on cascade FRET was investigated, revealing that a higher water content led to an enhancement of the cascade FRET process, which is attributed to increased aggregation-induced emission of Lap-NDI and the enhanced quantum yield of R6G in water. This study provides a unique approach to achieve cascade FRET by taking advantage of the anisotropic surface chemistry of a two-dimensional nanomaterial, providing a colloidally-driven alternative with improved tunability compared to macromolecular routes. The flexibility and simplicity of this approach will advance the state of the art of organic-inorganic hybrids for applications in optoelectronics, sensors, and hybrid photovoltaics.

Modeling of Nanomaterials for Supercapacitors: Beyond Carbon Electrodes.

Capacitive storage devices allow for fast charge and discharge cycles, making them the perfect complements to batteries for high power applications. Many materials display interesting capacitive properties when they are put in contact with ionic solutions despite their very different structures and (surface) reactivity. Among them, nanocarbons are the most important for practical applications, but many nanomaterials have recently emerged, such as conductive metal-organic frameworks, 2D materials, and a wide variety of metal oxides. These heterogeneous and complex electrode materials are difficult to model with conventional approaches. However, the development of computational methods, the incorporation of machine learning techniques, and the increasing power in high performance computing now allow us to tackle these types of systems. In this Review, we summarize the current efforts in this direction. We show that depending on the nature of the materials and of the charging mechanisms, different methods, or combinations of them, can provide desirable atomic-scale insight on the interactions at play. We mainly focus on two important aspects: (i) the study of ion adsorption in complex nanoporous materials, which require the extension of constant potential molecular dynamics to multicomponent systems, and (ii) the characterization of Faradaic processes in pseudocapacitors, that involves the use of electronic structure-based methods. We also discuss how recently developed simulation methods will allow bridges to be made between double-layer capacitors and pseudocapacitors for future high power electricity storage devices.

Direct Determination of Torsion in Twisted Graphite and MoS2 Interfaces.

The design space of two-dimensional materials is undergoing significant expansion through the stacking of layers in non-equilibrium configurations. However, the lack of quantitative insights into twist dynamics impedes the development of such heterostructures. Herein, we utilize the lateral force sensitivity of an atomic force microscope cantilever and specially designed rotational bearing structures to measure the torque in graphite and MoS2 interfaces. While the extracted torsional energies are virtually zero across all angular misfit configurations, commensurate interfaces of graphite and MoS2 are characterized by values of 0.1533 and 0.6384 N-m/m2, respectively. Furthermore, we measured the adhesion energies of graphite and MoS2 to elucidate the interplay between twist and slide. The adhesion energy dominates over the torsional energy for the graphitic interface, suggesting a tendency to twist prior to superlubric sliding. Conversely, MoS2 displays an increased torsional energy exceeding its adhesion energy. Consequently, our findings demonstrate a fundamental disparity between the sliding-to-twisting dynamics at MoS2 and graphite interfaces.

Enhanced Sensitivity in Photovoltaic 2D MoS2/Te Heterojunction VOC Sensors.

Volatile organic compound (VOC) sensors have a broad range of applications including healthcare monitoring, product quality control, and air quality management. However, many such applications are demanding, requiring sensors with high sensitivity and selectivity. 2D materials are extensively used in many VOC sensing devices due to their large surface-to-volume ratio and fascinating electronic properties. These properties, along with their exceptional flexibility, low power consumption, room-temperature operation, chemical functionalization potential, and defect engineering capabilities, make 2D materials ideal for high-performance VOC sensing. Here, a 2D MoS2/Te heterojunction is reported that significantly improves the VOC detection compared to MoS2 and Te sensors on their own. Density functional theory (DFT) analysis shows that the MoS2/Te heterojunction significantly enhances the adsorption energy and therefore sensing sensitivity of the sensor. The sensor response, which denotes the percentage change in the sensor's conductance upon VOC exposure, is further enhanced under photo-illumination and zero-bias conditions to values up to ≈7000% when exposed to butanone. The MoS2/Te heterojunction is therefore a promising device architecture for portable and wearable sensing applications.

Superatomic Layer of Cubic Mo4S4 Clusters Connected by Cl Cross-Linking.

Superatomic clusters - assemblies of atoms with various sizes, shapes, and compositions - can form hierarchical architectures that exhibit emergent electronic properties not found in their individual units. In particular, cubic M4X4 clusters of chalcogenides (M = transition metal; X = chalcogen) are recognized as versatile building blocks for 3D structures with tunable morphologies and electronic properties. However, tetrahedral M4X4 clusters rarely assemble into 2D architectures, which could offer a distinct class of functional materials from their 3D analogues. Here, this work reports the preparation of 2D Mo8S8Cl11, a superatomic layer with a sandwich structure consisting of Mo4S4 clusters interconnected through Cl cross-linking. The vapor-phase reaction inside nanotubes promotes the selective growth of Mo8S8Cl11 nanoribbons, allowing detailed characterization via transmission electron microscopy. This methodology can be applied to the growth of layered structures containing Mo8S8Cl11 at the micrometer scale. This work has demonstrated that mono- and few-layer Mo8S8Cl11 can be prepared by exfoliation of parent solids. Electronic structure calculations indicate that the 2D monolayer has quasi-flat bands, giving rise to an indirect-to-direct bandgap transition under mechanical strain. Furthermore, scanning electrochemical microscopy reveals the potential of the layered structures as highly efficient catalysts for the hydrogen-evolution reaction.

Exploring surface charge dynamics: implications for AFM height measurements in 2D materials.

An often observed artifact in atomic force microscopy investigations of individual monolayer flakes of 2D materials is the inaccurate height derived from topography images, often attributed to capillary or electrostatic forces. Here, we show the existence of a Joule dissipative mechanism related to charge dynamics and supplementing the dissipation due to capillary forces. This particular mechanism arises from the surface conductivity and assumes significance specially in the context of 2D materials on insulating supports. In such scenarios, the oscillating tip induces in-plane charge currents that in many circumstances constitute the main dissipative contribution to amplitude reduction and, consequently, affect the measured height. To investigate this phenomenon, we conduct measurements on monolayer flakes of co-deposited graphene oxide and reduced graphene oxide. Subsequently, we introduce a general model that elucidates our observations. This approach offers valuable insights into the dynamics of surface charges and their intricate interaction with the tip.

2D-Bio-FETs for sensitive detection of cardiovascular diseases.

The biosensing industry has seen exponential growth in the past decade. Impact of biosensors in the current scenario cannot be overlooked. Cardiovascular diseases (CvDs) have been recognized as one of the major causes for millions of deaths globally. This mortality can be minimized by early and accurate detection/diagnosis of CvDs with the help of biosensing devices. This also presents a global market opportunity for the development of biosensors for CvDs. A vast variety of biosensing methods and devices have been developed for this problem. Most of commercially available platforms for CvD detection rely on optical (fluorometric and colorimetric analysis) techniques using serum biomarkers since optical testing is the gold standard in medical diagnosis. Field effect transistors-based biosensors, termed as Bio-FETs, are the upcoming devices for blood or serum analyte detection due to excellent sensitivity, low operational voltage, handheld device structure and simple chip-based operation. Further, the discovery of two dimensional (2D) materials and their integration with conventional FETs has improved the overvoltage problem, sensitivity and strict operating conditions as compared to conventional FETs. Graphene-FETs based biosensing devices have been proven as promising candidates due to their attractive properties. Despite the severe threat of CvDs which has further increased in post-covid era, the Bio-FET sensor studies in literature are still rare. In this review, we aim to provide a comprehensive view of all the multidisciplinary concepts related to 2D-BioFETs for CvDs. A critical review of the different platforms has been covered with detailed discussions of related studies to provide a clear concept and present status of 2D-BioFETs based CvD biosensors.

Beyond Conventional Catalysts: Monoelemental Tellurium as a Game Changer for Piezo-Driven Hydrogen Evolution.

The increasing demand for clean hydrogen production over fossil fuels necessitates the development of sustainable piezoelectrochemical methods that can overcome the limitations of conventional electrocatalytic and photocatalytic approaches. In this regard, existing piezocatalysts face challenges related to their low piezoelectricity or active site coverage for hydrogen evolution reaction (HER). Driven by global environmental concerns, there is a compelling push to engineer practical materials for highly efficient HER. Herein, monoelemental 2D tellurium (Te) is presented as a class of layered chalcogenide with a non-centrosymmetric crystal structure (P3121 space group). The refined Te nanosheets demonstrate an unprecedented highly efficient H2 production rate ≈9000 µmol g-1 h-1 under ultrasonic mechanical vibration due to built-in piezo-potential in the system. The remarkable piezocatalytic performance of Te nanosheets arises from a synergistic interplay between their semi-metallic nature, favorable free energy landscape, enhanced electrical conductivity and outstanding piezoelectricity. As a proof of concept, the theoretical approach based on Density Functional Theory (DFT) validates the findings due to the gradual exposure of active sites on the Te nanosheets leading to a self-optimized catalytic performance for hydrogen generation. Therefore, mechanically driven Te emerges as a promising piezocatalyst with the potential to revolutionize highly efficient and sustainable technology for futuristic applications.

Robust Ferrimagnetism and Ferroelectricity in 2D ɛ-Fe2O3 Semiconductor with Ultrahigh Ordering Temperature.

2D single-phase multiferroic materials with the coexistence of electric and spin polarization offer a tantalizing potential for high-density multilevel data storage. One of the current limitations for application is the scarcity of the materials, especially those combine ferromagnetism and ferroelectricity at high temperatures. Here, robust ferrimagnetism and ferroelectricity in 2D ɛ-Fe2O3 samples with both single-crystalline and polycrystalline form are demonstrated. Interestingly, the polycrystalline nanosheets also exhibit easily switchable ferroelectric polarizations comparable to that of single crystals. The existence of grain boundary does not hinder the switching and retention of ferroelectric polarization. Furthermore, the ɛ-Fe2O3 nanosheets show ferrimagnetic and ferroelectric Curie temperatures up to 800 K, which reaches record highs in 2D single-phase multiferroic materials. This work provides important progress in the exploration of 2D high-temperature single-phase multiferroics for potentially compact high-temperature information nanodevices.

Infrared Interlayer Excitons in Twist-Free MoTe2/MoS2 Heterobilayers.

Excitonic devices based on interlayer excitons in van der Waals heterobilayers are a promising platform for advancing photoelectric interconnection telecommunications. However, the absence of exciton emission in the crucial telecom C-band has constrained their practical applications. Here, this limitation is addressed by reporting exciton emission at 0.8 eV (1550 nm) in a chemically vapor-deposited, strictly aligned MoTe2/MoS2 heterobilayer, resulting from the direct bandgap transitions of interlayer excitons as identified by momentum-space imaging of their electrons and holes. The decay mechanisms dominated by direct radiative recombination ensure constant emission quantum yields, a basic demand for efficient excitonic devices. The atomically sharp interface enables the resolution of two narrowly-splitter transitions induced by spin-orbit coupling, further distinguished through the distinct Landé g-factors as the fingerprint of spin configurations. By electrical control, the double transitions coupling into opposite circularly-polarized photon modes, preserve or reverse the helicities of the incident light with a degree of polarization up to 90%. The Stark effect tuning extends the emission energy range by over 150 meV (270 nm), covering the telecom C-band. The findings provide a material platform for studying the excitonic complexes and significantly boost the application prospects of excitonic devices in silicon photonics and all-optical telecommunications.

Toward Scalable Electrochemical Exfoliation of Molybdenum Disulfide Powder through an Accessible Electrode Design.

Cathodic electrochemical intercalation/exfoliation of transition metal dichalcogenides (TMDs) with bulky tetraalkylammonium-based cations is gaining popularity as it avoids the semiconducting (2H) to metallic (1T) phase transformation in TMDs like molybdenum disulfide (MoS2) and, generally, produces sheets with a larger aspect ratio - important for achieving conformal sheet-to-sheet contact in optoelectronic devices. Large single crystals are typically used as the precursor, but these are expensive, often inaccessible, and result in limited quantities of material. In this paper, a 3D-printable electrochemical cell capable of intercalating gram-scale quantities of commercially available TMD powders is presented. By incorporating a reference electrode in the cell and physically restraining the powder with a spring-loaded mechanism, the system can probe the intercalation electrochemistry, for example, determining the onset of intercalation to be near -2.5 V versus the ferrocene redox couple. While the extent of intercalation depends on precursor quantity and reaction time, a high yield of exfoliated product can be obtained exhibiting average aspect ratios as high as 49 ± 44 similar to values obtained by crystal intercalation. The intercalation and exfoliation of a wide variety of pelletized commercial powders including molybdenum diselenide (MoSe2), tungsten diselenide (WSe2), tungsten disulfide (WS2), and graphitic carbon nitride (gCN) are also demonstrated.

Innovative AI-Enhanced Ice Detection System Using Graphene-Based Sensors for Enhanced Aviation Safety and Efficiency.

Ice formation on aircraft surfaces poses significant safety risks, and current detection systems often struggle to provide accurate, real-time predictions. This paper presents the development and comprehensive evaluation of a smart ice control system using a suite of machine learning models. The system utilizes various sensors to detect temperature anomalies and signal potential ice formation. We trained and tested supervised learning models (Logistic Regression, Support Vector Machine, and Random Forest), unsupervised learning models (K-Means Clustering), and neural networks (Multilayer Perceptron) to predict and identify ice formation patterns. The experimental results demonstrate that our smart system, driven by machine learning, accurately predicts ice formation in real time, optimizes deicing processes, and enhances safety while reducing power consumption. This solution holds the potential for improving ice detection accuracy in aviation and other critical industries requiring robust predictive maintenance.

Spin-Glass States Generated in a van der Waals Magnet by Alkali-Ion Intercalation.

Tuning magnetic properties in layered van der Waals (vdW) materials has captured significant attention due to the efficient control of ground states by heterostructuring and external stimuli. Electron doping by electrostatic gating, interfacial charge transfer, and intercalation is particularly effective in manipulating the exchange and spin-orbit properties, resulting in a control of Curie temperature (TC) and magnetic anisotropy. Here, an uncharted role of intercalation is discovered to generate magnetic frustration. As a model study, Na atoms are intercalated into the vdW gaps of pristine Cr2Ge2Te6 (CGT) where generated magnetic frustration leads to emerging spin-glass states coexisting with a ferromagnetic order. A series of dynamic magnetic susceptibility measurements/analysis confirms the formation of magnetic clusters representing slow dynamics with a distribution of relaxation times. The intercalation also modifies other macroscopic physical parameters including the significant enhancement of TC from 66 to 240 K and the switching of magnetic easy-hard axis direction. This study identifies intercalation as a unique route to generate emerging frustrated spin states in simple vdW crystals.

In-Sublattice Carrier Transition Enabled Polarimetric Photodetectors with Reconfigurable Polarity Transition.

Miniaturized polarimetric photodetectors based on anisotropic two-dimensional materials attract potential applications in ultra-compact polarimeters. However, these photodetectors are hindered by the small polarization ratio values and complicated artificial structures. Here, a novel polarization photodetector based on in-sublattice carrier transition in the CdSb2Se3Br2/WSe2 heterostructure, with a giant and reconfigurable PR value, is demonstrated. The unique periodic sublattice structure of CdSb2Se3Br2 features an in-sublattice carrier transition preferred along Sb2Se3 chains. Leveraging on the in-sublattice carrier transition in the CdSb2Se3Br2/WSe2 heterostructure, gate voltage has an anisotropic modulation effect on the band alignment of heterostructure along sublattice. Consequently, the heterostructure exhibits a polarization-tunable photo-induced threshold voltage shift, which provides reconfigurable PR values from positive (unipolar regime) to negative (bipolar regime), covering all possible numbers (1→+∞/-∞→-1). Using this anisotropic photovoltaic effect, gate-tunable polarimetric imaging is successfully implemented. This work provides a new platform for developing next-generation highly polarimetric optoelectronics.

Ionic Photovoltaics-in-Memory in van der Waals Material.

The photovoltaic effect is gaining growing attention in the optoelectronics field due to its low power consumption, sustainable nature, and high efficiency. However, the photovoltaic effects hitherto reported are hindered by the stringent band-alignment requirement or inversion symmetry-breaking, and are challenging for achieving multifunctional photovoltaic properties (such as reconfiguration, nonvolatility, and so on). Here, a novel ionic photovoltaic effect in centrosymmetric CdSb2Se3Br2 that can overcome these limitations is demonstrated. The photovoltaic effect displays significant anisotropy, with the photocurrent being most apparent along the CdBr2 chains while absent perpendicular to them. Additionally, the device shows electrically-induced nonvolatile photocurrent switching characteristics. The photovoltaic effect is attributed to the modulation of the built-in electric field through the migration of Br ions. Using these unique photovoltaic properties, a highly secure circuit with electrical and optical keys is successfully implemented. The findings not only broaden the understanding of the photovoltaic mechanism, but also provide a new material platform for the development of in-memory sensing and computing devices.

Biomimetic Porous MXene Antibacterial Adsorbents with Enhanced Toxins Trapping Ability for Hemoperfusion.

2D transition metal carbides and nitrides, i.e., MXene, are recently attracting wide attentions and presenting competitive performances as adsorbents used in hemoperfusion. Nonetheless, the nonporous texture and easily restacking feature limit the efficient adsorption of toxin molecules inside MXene and between layers. To circumvent this concern, here a plerogyra sinuosa biomimetic porous titanium carbide MXene (P-Ti3C2) is reported. The hollow and hierarchically porous structure with large surface area benefits the maximum access of toxins as well as trapping them inside the spherical cavity. The cambered surface of P-Ti3C2 prevents layers restacking, thus affording better interlaminar adsorption. In addition to enhanced toxin removal ability, the P-Ti3C2 is found to selectively adsorb more middle and large toxin molecules than small toxin molecules. It possibly originates from the rich Ti-deficient vacancies in the P-MXene lattice that increases the affinity with middle/large toxin molecules. Also, the vacancies as active sites facilitate the production of reactive oxygen under NIR irradiation to promote the photodynamic antibacterial performance. Then, the versatility of P-MXene is validated by extension to niobium carbide (P-Nb2C). And the simulated hemoperfusion proves the practicability of the P-MXene as polymeric adhesives-free adsorbents to eliminate the broad-spectrum toxins.

Electrically Controlled High Sensitivity Strain Modulation in MoS2 Field-Effect Transistors via a Piezoelectric Thin Film on Silicon Substrates.

Strain can modulate bandgap and carrier mobilities in two-dimensional (2D) materials. Conventional strain-application methodologies relying on flexible/patterned/nanoindented substrates are limited by low thermal tolerance, poor tunability, and/or scalability. Here, we leverage the converse piezoelectric effect to electrically generate and control strain transfer from a piezoelectric thin film to electromechanically coupled 2D MoS2. Electrical bias polarity change across the piezo film tunes the nature of strain transferred to MoS2 from compressive (∼0.23%) to tensile (∼0.14%) as verified through Raman and photoluminescence spectroscopies and substantiated by density functional theory calculations. The device architecture, on silicon substrate, integrates an MoS2 field-effect transistor on a metal-piezoelectric-metal stack enabling strain modulation of transistor drain current (130×), on/off ratio (150×), and mobility (1.19×) with high precision, reversibility, and resolution. Large, tunable tensile (1056) and compressive (-1498) strain gauge factors, electrical strain modulation, and high thermal tolerance promise facile integration with silicon-based CMOS and micro-electromechanical systems.