Hexagonal metal oxide monolayers derived from the metal-gas interface

Flexoelectricity Modulated Electron Transport of 2D Indium Oxide

The phenomenon of flexoelectricity, wherein mechanical deformation induces alterations in the electron configuration of metal oxides, has emerged as a promising avenue for regulating electron transport. Leveraging this mechanism, stress sensing can be optimized through precise modulation of electron transport. In this study, the electron transport in 2D ultra-smooth In2O3 crystals is modulated via flexoelectricity. By subjecting cubic In2O3 (c-In2O3) crystals to significant strain gradients using an atomic force microscope (AFM) tip, the crystal symmetry is broken, resulting in the separation of positive and negative charge centers. Upon applying nano-scale stress up to 100 nN, the output voltage and power values reach their maximum, e.g. 2.2 mV and 0.2 pW, respectively. The flexoelectric coefficient and flexocoupling coefficient of c-In2O3 are determined as ≈0.49 nC m-1 and 0.4 V, respectively. More importantly, the sensitivity of the nano-stress sensor upon c-In2O3 flexoelectric effect reaches 20 nN, which is four to six orders smaller than that fabricated with other low dimensional materials based on the piezoresistive, capacitive, and piezoelectric effect. Such a deformation-induced polarization modulates the band structure of c-In2O3, significantly reducing the Schottky barrier height (SBH), thereby regulating its electron transport. This finding highlights the potential of flexoelectricity in enabling high-performance nano-stress sensing through precise control of electron transport.

Thin Ga2O3 Layers by Thermal Oxidation of van der Waals GaSe Nanostructures for Ultraviolet Photon Sensing

Two-dimensional semiconductors (2DSEM) based on van der Waals crystals offer important avenues for nanotechnologies beyond the constraints of Moore's law and traditional semiconductors, such as silicon (Si). However, their application necessitates precise engineering of material properties and scalable manufacturing processes. The ability to oxidize Si to form silicon dioxide (SiO2) was crucial for the adoption of Si in modern technologies. Here, we report on the thermal oxidation of the 2DSEM gallium selenide (GaSe). The nanometer-thick layers are grown by molecular beam epitaxy on transparent sapphire (Al2O3) and feature a centro-symmetric polymorph of GaSe. Thermal annealing of the layers in an oxygen-rich environment promotes the chemical transformation and full conversion of GaSe into a thin layer of crystalline Ga2O3, paralleled by the formation of coherent Ga2O3/Al2O3 interfaces. Versatile functionalities are demonstrated in photon sensors based on GaSe and Ga2O3, ranging from electrical insulation to unfiltered deep ultraviolet optoelectronics, unlocking the technological potential of GaSe nanostructures and their amorphous and crystalline oxides.

Single-crystalline metal-oxide dielectrics for top-gate 2D transistors

Two-dimensional (2D) structures composed of atomically thin materials with high carrier mobility have been studied as candidates for future transistors1-4. However, owing to the unavailability of suitable high-quality dielectrics, 2D field-effect transistors (FETs) cannot attain the full theoretical potential and advantages despite their superior physical and electrical properties3,5,6. Here we demonstrate the fabrication of atomically thin single-crystalline Al2O3 (c-Al2O3) as a high-quality top-gate dielectric in 2D FETs. By using intercalative oxidation techniques, a stable, stoichiometric and atomically thin c-Al2O3 layer with a thickness of 1.25 nm is formed on the single-crystalline Al surface at room temperature. Owing to the favourable crystalline structure and well-defined interfaces, the gate leakage current, interface state density and dielectric strength of c-Al2O3 meet the International Roadmap for Devices and Systems requirements3,5,7. Through a one-step transfer process consisting of the source, drain, dielectric materials and gate, we achieve top-gate MoS2 FETs characterized by a steep subthreshold swing of 61 mV dec-1, high on/off current ratio of 108 and very small hysteresis of 10 mV. This technique and material demonstrate the possibility of producing high-quality single-crystalline oxides suitable for integration into fully scalable advanced 2D FETs, including negative capacitance transistors and spin transistors.

Large-area grown ultrathin molybdenum oxides for label-free sensitive biomarker detection

The rise of two-dimensional (2D) materials has provided a confined geometry and yielded methods for guiding electrons at the nanoscale level. 2D material-enabled electronic devices can interact and transduce the subtle charge perturbation and permit significant advancement in molecule discrimination technology with high accuracy, sensitivity, and specificity, leaving a significant impact on disease diagnosis and health monitoring. However, high-performance biosensors with scalable fabrication ability and simple protocols have yet to be fully realized due to the challenges in wafer-scale 2D film synthesis and integration with electronics. Here, we propose a molybdenum oxide (MoOx)-interdigitated electrode (IDE)-based label-free biosensing chip, which stands out for its wafer-scale dimension, tunability, ease of integration and compatibility with the complementary metal-oxide-semiconductor (CMOS) fabrication. The device surface is biofunctionalized with monoclonal anti-carcinoembryonic antigen antibodies (anti-CEA) via the linkage agent (3-aminopropyl)triethoxysilane (APTES) for carcinoembryonic antigen (CEA) detection and is characterized step-by-step to reveal the working mechanism. A wide range and real-time response of the CEA concentration from 0.1 to 100 ng mL-1 and a low limit of detection (LOD) of 0.015 ng mL-1 were achieved, meeting the clinical requirements for cancer diagnosis and prognosis in serum. The MoOx-IDE biosensor also demonstrates strong surface affinity towards molecules and high selectivity using L-cysteine (L-Cys), glycine (Gly), glucose (Glu), bovine serum albumin (BSA), and immunoglobulin G (IgG). This study showcases a simple, scalable, and low-cost strategy to create a nanoelectronic biosensing platform to achieve high-performance cancer biomarker discrimination capabilities.

Scalable solid-state synthesis of 2D transition metal oxide/graphene hybrid materials and their utilization for microsupercapacitors

Two-dimensional metal oxide (MO) nanostructures have unique properties compared with their bulk or 0D and 1D (nanoparticle and nanowire) counterparts. Their abundant surface area and atomically thin 2D structure are advantageous for their applications in catalysis and energy, as well as integration with 2D layered materials such as graphene and reduced graphene oxide (rGO). However, fast and scalable synthesis of 2D MOs and their nanocomposites remains challenging. Here, we developed a microwave-assisted solid-state synthesis method for the scalable generation of 2D MOs and 2D MO/rGO nanocomposites with tunable structure and composition. The structures and properties of 2D Fe2O3 and 2D ZnO as well as their nanocomposites with rGO were systematically investigated. The excellent electrochemical properties of such 2D MO/rGO nanocomposites also enable us to use them as electrode materials to fabricate microsupercapacitors. This work provides new insights into the scalable and solid-state synthesis of 2D nanocomposites and their potential applications in catalysis, energy conversion and storage.

Magnetic State Control of Non-van der Waals 2D Materials by Hydrogenation

Controlling the magnetic state of two-dimensional (2D) materials is crucial for spintronics. By employing data-mining and autonomous density functional theory calculations, we demonstrate the switching of magnetic properties of 2D non-van der Waals materials upon hydrogen passivation. The magnetic configurations are tuned to states with flipped and enhanced moments. For 2D CdTiO3─a diamagnetic compound in the pristine case─we observe an onset of ferromagnetism upon hydrogenation. Further investigation of the magnetization density of the pristine and passivated systems provides a detailed analysis of modified local spin symmetries and the emergence of ferromagnetism. Our results indicate that selective surface passivation is a powerful tool for tailoring magnetic properties of nanomaterials, such as non-vdW 2D compounds.

Emerging two dimensional metastable-phase oxides: insights and prospects in synthesis and catalysis

Since the discovery of graphene, the development of new two-dimensional (2D) materials has received considerable interest. Recently, as a newly emerging member of the 2D family, 2D metastable-phase oxides that combine the unique advantages of metal oxides, 2D structures, and metastable-phase materials have shown enormous potential in various catalytic reactions. In this review, the potential of various 2D materials to form a metastable-phase is predicted. The advantages of 2D metastable-phase oxides for advanced applications, reliable methods of synthesizing 2D metastable-phase oxides, and the application of these oxides in different catalytic reactions are presented. Finally, the challenges associated with 2D metastable-phase oxides and future perspectives are discussed.

Realizing new 2D spintronic materials from the non-magnetic 1T-PdO2 monolayer through vacancy defects and doping

In this work, vacancy- and doping-based magnetism engineering in a non-magnetic 1T-PdO2 monolayer is explored in order to realize new two-dimensional (2D) spintronic materials. The pristine monolayer is an indirect gap semiconductor with a band gap of 1.45 (3.20) eV obtained using the PBE (HSE06) functional. Half-metallicity with a total magnetic moment of 3.95 μB is induced by creating a single Pd vacancy, where the magnetic properties are produced mainly by O atoms around the vacancy site. In contrast, the non-magnetic nature is preserved under the effects of a single O vacancy, however a band gap reduction in the order of 37.93% is achieved. Further doping with transition metals (TMs = V, Cr, Mn, and Fe) in the Pd sublattice and with non-metals (B, C, N, and F) in the O sublattice is investigated. TM impurities lead to the emergence of a diluted magnetic semiconductor nature, where total magnetic moments of 1.00, 2.00, and 3.00 μB are obtained in the V-, Cr(Fe)-, and Mn-doped systems, respectively. In these cases, the TMs' 3d electrons mainly originate the system's magnetism. Significant magnetization of the PdO2 monolayer is also achieved by doping with B, N, and F atoms, where either half-metallic or diluted magnetic semiconductor natures are induced. Herein, electronic and magnetic properties are regulated mainly by the interactions between the 2p orbital of the dopant, 4d orbital of the first neighbor Pd atoms, and 2p orbital of the second neighbor O atoms. Meanwhile, C impurity induces no magnetism in the PdO2 monolayer because of the strong electronic hybridization with their neighbor atoms. Results presented herein may introduce efficient approaches to engineer magnetism in a non-magnetic PdO2 monolayer, such that the functionalized systems are further recommended for prospective spintronic applications.

Two-Dimensional Oxide Crystals for Device Applications: Challenges and Opportunities

Atomically thin two-dimensional (2D) oxide crystals have garnered considerable attention because of their remarkable physical properties and potential for versatile applications. In recent years, significant advancements have been made in the design, preparation, and application of ultrathin 2D oxides, providing many opportunities for new-generation advanced technologies. This review focuses on the controllable preparation of 2D oxide crystals and their applications in electronic and optoelectronic devices. Based on their bonding nature, the various types of 2D oxide crystals are first summarized, including both layered and nonlayered crystals, as well as their current top-down and bottom-up synthetic approaches. Subsequently, in terms of the unique physical and electrical properties of 2D oxides, recent advances in device applications are emphasized, including photodetectors, field-effect transistors, dielectric layers, magnetic and ferroelectric devices, memories, and gas sensors. Finally, conclusions and future prospects of 2D oxide crystals are presented. It is hoped that this review will provide comprehensive and insightful guidance for the development of 2D oxide crystals and their device applications.

Thickness-Dependent Room-Temperature Optoelectronic Gas Sensing Performances of 2D Nonlayered Indium Oxide Crystals from a Liquid Metal Printing Process

Due to excellent gas sensing performances, such as high responsivity, good selectivity, and long-term stability, two-dimensional (2D) nonlayered metal oxide semiconductors have attracted wide attention. However, their thickness-dependent gas sensing behaviors are rarely investigated, which is critical in the development of practical 2D sensors. In this work, 2D In2O3 crystals with a range of thicknesses are realized by extracting the self-limited oxide layer from the liquid indium droplets in a controlled environment. A strong thickness-dependent optoelectronic NO2 sensing behavior at room temperature is observed. While full reversibility and excellent selectivity toward NO2 are shown despite the thicknesses of 2D In2O3, the 1.9 nm thick In2O3 exhibits a maximum response amplitude (ΔI/Ig = 1300) for 10 ppm of NO2 at room temperature with 365 nm light irradiation, which is about 18, 58, and 810 times larger than those of its 3.1 nm thick, 4.5 nm thick, and 6.2 nm thick counterparts, respectively. The shortest response and recovery times (i.e., 40 s/48 s) are demonstrated for the 1.88 nm thick In2O3 as well. We correlate such a phenomenon with the change in the In2O3 band structure, which is influenced by the thickness of 2D crystals. This work provides in-depth knowledge of the thickness-dependent gas-sensing performances of emerging 2D nonlayered metal oxide crystals, as well as the opportunities to develop next-generation high-performing room-temperature gas sensors.

Oriented lateral growth of two-dimensional materials on c-plane sapphire

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) represent the ultimate thickness for scaling down channel materials. They provide a tantalizing solution to push the limit of semiconductor technology nodes in the sub-1 nm range. One key challenge with 2D semiconducting TMD channel materials is to achieve large-scale batch growth on insulating substrates of single crystals with spatial homogeneity and compelling electrical properties. Recent studies have claimed the epitaxy growth of wafer-scale, single-crystal 2D TMDs on a c-plane sapphire substrate with deliberately engineered off-cut angles. It has been postulated that exposed step edges break the energy degeneracy of nucleation and thus drive the seamless stitching of mono-oriented flakes. Here we show that a more dominant factor should be considered: in particular, the interaction of 2D TMD grains with the exposed oxygen-aluminium atomic plane establishes an energy-minimized 2D TMD-sapphire configuration. Reconstructing the surfaces of c-plane sapphire substrates to only a single type of atomic plane (plane symmetry) already guarantees the single-crystal epitaxy of monolayer TMDs without the aid of step edges. Electrical results evidence the structural uniformity of the monolayers. Our findings elucidate a long-standing question that curbs the wafer-scale batch epitaxy of 2D TMD single crystals-an important step towards using 2D materials for future electronics. Experiments extended to perovskite materials also support the argument that the interaction with sapphire atomic surfaces is more dominant than step-edge docking.

p-Type Two-Dimensional Semiconductors: From Materials Preparation to Electronic Applications

Two-dimensional (2D) materials are regarded as promising candidates in many applications, including electronics and optoelectronics, because of their superior properties, including atomic-level thickness, tunable bandgaps, large specific surface area, and high carrier mobility. In order to bring 2D materials from the laboratory to industrialized applications, materials preparation is the first prerequisite. Compared to the n-type analogs, the family of p-type 2D semiconductors is relatively small, which limits the broad integration of 2D semiconductors in practical applications such as complementary logic circuits. So far, many efforts have been made in the preparation of p-type 2D semiconductors. In this review, we overview recent progresses achieved in the preparation of p-type 2D semiconductors and highlight some promising methods to realize their controllable preparation by following both the top-down and bottom-up strategies. Then, we summarize some significant application of p-type 2D semiconductors in electronic and optoelectronic devices and their superiorities. In end, we conclude the challenges existed in this field and propose the potential opportunities in aspects from the discovery of novel p-type 2D semiconductors, their controlled mass preparation, compatible engineering with silicon production line, high-κ dielectric materials, to integration and applications of p-type 2D semiconductors and their heterostructures in electronic and optoelectronic devices. Overall, we believe that this review will guide the design of preparation systems to fulfill the controllable growth of p-type 2D semiconductors with high quality and thus lay the foundations for their potential application in electronics and optoelectronics.

Microenvironment Restruction of Emerging 2D Materials and their Roles in Therapeutic and Diagnostic Nano-Bio-Platforms

Engineering advanced therapeutic and diagnostic nano-bio-platforms (NBPFs) have emerged as rapidly-developed pathways against a wide range of challenges in antitumor, antipathogen, tissue regeneration, bioimaging, and biosensing applications. Emerged 2D materials have attracted extensive scientific interest as fundamental building blocks or nanostructures among material scientists, chemists, biologists, and doctors due to their advantageous physicochemical and biological properties. This timely review provides a comprehensive summary of creating advanced NBPFs via emerging 2D materials (2D-NBPFs) with unique insights into the corresponding molecularly restructured microenvironments and biofunctionalities. First, it is focused on an up-to-date overview of the synthetic strategies for designing 2D-NBPFs with a cross-comparison of their advantages and disadvantages. After that, the recent key achievements are summarized in tuning the biofunctionalities of 2D-NBPFs via molecularly programmed microenvironments, including physiological stability, biocompatibility, bio-adhesiveness, specific binding to pathogens, broad-spectrum pathogen inhibitors, stimuli-responsive systems, and enzyme-mimetics. Moreover, the representative therapeutic and diagnostic applications of 2D-NBPFs are also discussed with detailed disclosure of their critical design principles and parameters. Finally, current challenges and future research directions are also discussed. Overall, this review will provide cutting-edge and multidisciplinary guidance for accelerating future developments and therapeutic/diagnostic applications of 2D-NBPFs.

Multiple-State Nonvolatile Memory Based on Ultrathin Indium Oxide Film via Liquid Metal Printing

In this work, the ultrathin two-dimensional (2D) indium oxide (InO_x) with a large area of more than 100 μm² and a high degree of uniformity was automatically peeled off from indium by the liquid-metal printing technique. Raman and optical measurements revealed that 2D-InO_x has a polycrystalline cubic structure. By altering the printing temperature which affects the crystallinity of 2D-InO_x, the mechanism of the existence and disappearance of memristive characteristics was established. The tunable characteristics of the 2D-InO_x memristor with reproducible one-order switching was manifest from the electrical measurements. Further adjustable multistate characteristics of the 2D-InO_x memristor and its resistance switching mechanism were evaluated. A detailed examination of the memristive process demonstrated the Ca²⁺ mimic dynamic in 2D-InO_x memristors as well as the fundamental principles underlying biological and artificial synapses. These surveys allow us to comprehend a 2D-InO_x memristor using the liquid-metal printing technique and could be applied to future neuromorphic applications and in the field of revolutionary 2D material exploration.

Emerging 2D Metal Oxides: From Synthesis to Device Integration

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

Gate Dielectrics Integration for 2D Electronics: Challenges, Advances, and Outlook

2D semiconductors have emerged both as an ideal platform for fundamental studies and as promising channel materials in beyond-silicon field-effect-transistors due to their outstanding electrical properties and exceptional tunability via external field. However, the lack of proper dielectrics for 2D semiconductors has become a major roadblock for their further development toward practical applications. The prominent issues between conventional 3D dielectrics and 2D semiconductors arise from the integration and interface quality, where defect states and imperfections lead to dramatic deterioration of device performance. In this review article, the root causes of such issues are briefly analyzed and recent advances on some possible solutions, including various approaches of adapting conventional dielectrics to 2D semiconductors, and the development of novel dielectrics with van der Waals surface toward high-performance 2D electronics are summarized. Then, in the perspective, the requirements of ideal dielectrics for state-of-the-art 2D devices are outlined and an outlook for their future development is provided.

Angstrom-Confined Electrochemical Synthesis of Sub-Unit-Cell Non-Van Der Waals 2D Metal Oxides

Bottom-up electrochemical synthesis of atomically thin materials is desirable yet challenging, especially for non-vanderWaals (non-vdW) materials. Thicknesses below a few nanometers have not been reported yet, posing the question how thin can non-vdW materials be electrochemically synthesized. This is important as materials with (sub-)unit-cell thickness often show remarkably different properties compared to their bulk form or thin films of several nanometers thickness. Here, a straightforward electrochemical method utilizing the angstrom-confinement of laminar reduced graphene oxide (rGO) nanochannels is introduced to obtain a centimeter-scale network of atomically thin (<4.3 Å) 2D-transition metal oxides (2D-TMO). The angstrom-confinement provides a thickness limitation, forcing sub-unit-cell growth of 2D-TMO with oxygen and metal vacancies. It is showcased that Cr₂ O₃ , a material without significant catalytic activity for the oxygen evolution reaction (OER) in bulk form, can be activated as a high-performing catalyst if synthesized in the 2D sub-unit-cell form. This method displays the high activity of sub-unit-cell form while retaining the stability of bulk form, promising to yield unexplored fundamental science and applications. It is shown that while retaining the advantages of bottom-up electrochemical synthesis, like simplicity, high yield, and mild conditions, the thickness of TMO can be limited to sub-unit-cell dimensions.

Graphene Metamaterial 3D Conformal Coating for Enhanced Light Harvesting

Silicon (Si) photovoltaic devices present possible avenues for overcoming global energy and environmental challenges. The high reflection and surface recombination losses caused by the Si interface and its nanofabrication process are the main hurdles for pursuing a high energy conversion efficiency. However, recent advances have demonstrated great success in improving device performance via proper Si interface modification with the optical and electrical features of two-dimensional (2D) materials. Firmly integrating large-area 2D materials with 3D Si nanostructures with no gap in between, which is essential for optimizing device performance, has rarely been achieved by any technique due to the complex 3D morphology of the nanostructures. Here we propose the concept of a 3D conformal coating of graphene metamaterials, in which the 2D graphene layers perfectly adapt to the 3D Si curvatures, leading to a universal 20% optical reflection decrease and a 60% surface passivation improvement. In a further application of this metamaterial 3D conformal coating methodology to standard Si solar cells, an overall 23% enhancement of the solar energy conversion efficiency is achieved. The 3D conformal coating strategy could be readily extended to various optoelectronic and semiconductor device systems with peculiar performance, offering a pathway for highly efficient energy-harvesting and storage solutions.

P-Type 2D Semiconductors for Future Electronics

2D semiconductors represent one of the best candidates to extend Moore's law for their superiorities, such as keeping high carrier mobility and remarkable gate-control capability at atomic thickness. Complementary transistors and van der Waals junctions are critical in realizing 2D semiconductors-based integrated circuits suitable for future electronics. N-type 2D semiconductors have been reported predominantly for the strong electron doping caused by interfacial charge impurities and internal structural defects. By contrast, superior and reliable p-type 2D semiconductors with holes as majority carriers are still scarce. Not only that, but some critical issues have not been adequately addressed, including their controlled synthesis in wafer size and high quality, defect and carrier modulation, optimization of interface and contact, and application in high-speed and low-power integrated devices. Here the material toolkit, synthesis strategies, device basics, and digital electronics closely related to p-type 2D semiconductors are reviewed. Their opportunities, challenges, and prospects for future electronic applications are also discussed, which would be promising or even shining in the post-Moore era.

Coupling of nanocrystal hexagonal array and two-dimensional metastable substrate boosts H2-production

Designing well-ordered nanocrystal arrays with subnanometre distances can provide promising materials for future nanoscale applications. However, the fabrication of aligned arrays with controllable accuracy in the subnanometre range with conventional lithography, template or self-assembly strategies faces many challenges. Here, we report a two-dimensional layered metastable oxide, trigonal phase rhodium oxide (space group, P-3m1 (164)), which provides a platform from which to construct well-ordered face-centred cubic rhodium nanocrystal arrays in a hexagonal pattern with an intersurface distance of only 0.5 nm. The coupling of the well-ordered rhodium array and metastable substrate in this catalyst triggers and improves hydrogen spillover, enhancing the acidic hydrogen evolution for H2 production, which is essential for various clean energy-related devices. The catalyst achieves a low overpotential of only 9.8 mV at a current density of -10 mA cm-2, a low Tafel slope of 24.0 mV dec-1, and high stability under a high potential (vs. RHE) of -0.4 V (current density of ~750 mA cm-2). This work highlights the important role of metastable materials in the design of advanced materials to achieve high-performance catalysis.

Ultrathin ferrite nanosheets for room-temperature two-dimensional magnetic semiconductors

The discovery of magnetism in ultrathin crystals opens up opportunities to explore new physics and to develop next-generation spintronic devices. Nevertheless, two-dimensional magnetic semiconductors with Curie temperatures higher than room temperature have rarely been reported. Ferrites with strongly correlated d-orbital electrons may be alternative candidates offering two-dimensional high-temperature magnetic ordering. This prospect is, however, hindered by their inherent three-dimensional bonded nature. Here, we develop a confined-van der Waals epitaxial approach to synthesizing air-stable semiconducting cobalt ferrite nanosheets with thickness down to one unit cell using a facile chemical vapor deposition process. The hard magnetic behavior and magnetic domain evolution are demonstrated by means of vibrating sample magnetometry, magnetic force microscopy and magneto-optical Kerr effect measurements, which shows high Curie temperature above 390 K and strong dimensionality effect. The addition of room-temperature magnetic semiconductors to two-dimensional material family provides possibilities for numerous novel applications in computing, sensing and information storage.

Plasmonic metal oxides and their biological applications

Metal oxides modified with dopants and defects are an emerging class of novel materials supporting the localized surface plasmon resonance across a wide range of optical wavelengths, which have attracted tremendous research interest particularly in biological applications in the past decade. Compared to conventional noble metal-based plasmonic materials, plasmonic metal oxides are particularly favored for their cost efficiency, flexible plasmonic properties, and improved biocompatibility, which can be important to accelerate their practical implementation. In this review, we first explicate the origin of plasmonics in dopant/defect-enabled metal oxides and their associated tunable localized surface plasmon resonance through the conventional Mie-Gans model. The research progress of dopant incorporation and defect generation in metal oxide hosts, including both in situ and ex situ approaches, is critically discussed. The implementation of plasmonic metal oxides in biological applications in terms of therapy, imaging, and sensing is summarized, in which the uniqueness of dopant/defect-driven plasmonics for inducing novel functionalities is particularly emphasized. This review may provide insightful guidance for developing next-generation plasmonic devices for human health monitoring, diagnosis and therapy.

Freestanding complex-oxide membranes

Complex oxides show a vast range of functional responses, unparalleled within the inorganic solids realm, making them promising materials for applications as varied as next-generation field-effect transistors, spintronic devices, electro-optic modulators, pyroelectric detectors, or oxygen reduction catalysts. Their stability in ambient conditions, chemical versatility, and large susceptibility to minute structural and electronic modifications make them ideal subjects of study to discover emergent phenomena and to generate novel functionalities for next-generation devices. Recent advances in the synthesis of single-crystal, freestanding complex oxide membranes provide an unprecedented opportunity to study these materials in a nearly-ideal system (e.g. free of mechanical/thermal interaction with substrates) as well as expanding the range of tools for tweaking their order parameters (i.e. (anti-)ferromagnetic, (anti-)ferroelectric, ferroelastic), and increasing the possibility of achieving novel heterointegration approaches (including interfacing dissimilar materials) by avoiding the chemical, structural, or thermal constraints in synthesis processes. Here, we review the recent developments in the fabrication and characterization of complex-oxide membranes and discuss their potential for unraveling novel physicochemical phenomena at the nanoscale and for further exploiting their functionalities in technologically relevant devices.

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

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

Morphotaxy of Layered van der Waals Materials

Layered van der Waals (vdW) materials have attracted significant attention due to their materials properties that can enhance diverse applications including next-generation computing, biomedical devices, and energy conversion and storage technologies. This class of materials is typically studied in the two-dimensional (2D) limit by growing them directly on bulk substrates or exfoliating them from parent layered crystals to obtain single or few layers that preserve the original bonding. However, these vdW materials can also function as a platform for obtaining additional phases of matter at the nanoscale. Here, we introduce and review a synthesis paradigm, morphotaxy, where low-dimensional materials are realized by using the shape of an initial nanoscale precursor to template growth or chemical conversion. Using morphotaxy, diverse non-vdW materials such as HfO₂ or InF₃ can be synthesized in ultrathin form by changing the composition but preserving the shape of the original 2D layered material. Morphotaxy can also enable diverse atomically precise heterojunctions and other exotic structures such as Janus materials. Using this morphotaxial approach, the family of low-dimensional materials can be substantially expanded, thus creating vast possibilities for future fundamental studies and applied technologies.

Two-dimensional ferroelectric MoS2/Ga2O3 heterogeneous bilayers with highly tunable photocatalytic and electrical properties

Two-dimensional van der Waals heterostructures with strong intrinsic ferroelectrics are highly promising for novel devices with designed electronic properties. The polarization reversal transition of the 2D ferroelectric Ga₂O₃ monolayer offers a new approach to tune the photocatalytic and electrical properties of MoS₂/Ga₂O₃ heterogeneous bilayers. In this work, we study MoS₂/Ga₂O₃ heterogeneous bilayers with different intrinsic polarization using hybrid-functional calculations. We closely investigate the structural, electronic and optical properties of two stable stacking configurations with opposite polarization. The results reveal a distinct switch from type-I to type-II heterostructures owing to polarization reversal transition of the 2D ferroelectric Ga₂O₃ monolayer. Biaxial strain engineering leads to type-I-to-II and type-II-to-III transitions in the two polarized models, respectively. Intriguingly, one of the MoS₂/Ga₂O₃ heterolayers has a larger spatial separation of the valence and conduction band edges and excellent optical absorption ranging from infrared to ultraviolet region under biaxial strain, thus ensuring promising novel applications such as flexible electrical and optical devices. Based on the highly tunable physical properties of the bilayer heterostructures, we further explore their potential applications, such as photocatalytic water splitting and field-controlled switch channel in MOSFET devices.

Flexible 2D Materials beyond Graphene: Synthesis, Properties, and Applications

2D materials are now at the forefront of state-of-the-art nanotechnologies due to their fascinating properties and unique structures. As expected, low-cost, high-volume, and high-quality 2D materials play an important role in the applications of flexible devices. Although considerable progress has been achieved in the integration of a series of novel 2D materials beyond graphene into flexible devices, a lot remains to be known. At this stage of their development, the key issues concern how to make further improvements to high-performance and scalable-production. Herein, recent progress in the quest to improve the current state of the art for 2D materials beyond graphene is reviewed. Namely, the properties and synthesis techniques of 2D materials are first introduced. Then, both the advantages and challenges of these 2D materials for flexible devices are also highlighted. Finally, important directions for future advancements toward efficient, low-cost, and stable flexible devices are outlined.

2D Ultrawide Bandgap Semiconductors: Odyssey and Challenges

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

Aluminum-based layered metal oxides activating peroxymonosulfate for bisphenol A degradation via surface-bound sulfate radicals and singlet oxygen

In this study, a series of aluminum-based layered metal oxide with various divalent metals (M²⁺Al-LMOs) were prepared and employed in activation of peroxymonosulfate (PMS) for bisphenol A (BPA) degradation. The BPA removal rates of M²⁺Al-LMOs were ordered as: CoAl(100%) > MnAl(75.6%) > CuAl(63.2%) > NiAl(9.0%) > MgAl = ZnAl-LMO(0%). CoAl-LMO showed the highest kinetic constant (k = 1.329 µmol^-1g_cat^-1s^-1), which was 3.95 times of MnAl-LMO, 5.36 times of CuAl-LMO, 88.6 times of NiAl-LMO and 443 times of MgAl-LMO and ZnAl-LMO, respectively, and also exhibited the highest TOC removal rate (83.3%). The surface-bound sulfate radical (SO₄^·-) and singlet oxygen (¹O₂) were elucidated as the dominant reactive oxygen species (ROS) for BPA degradation. The M²⁺Al-LMOs/PMS system not only displayed wide applicability in different pH and inorganic anions environments, but also had excellent stability and reusability. This work provides a novel family of M²⁺Al-LMOs to activate PMS for water treatment.

Highly accurate and label-free discrimination of single cancer cell using a plasmonic oxide-based nanoprobe

The detection of cancer cells at the single-cell level enables many novel functionalities such as next-generation cancer prognosis and accurate cellular analysis. While surface-enhanced Raman spectroscopy (SERS) has been widely considered as an effective tool in a low-cost and label-free manner, however, it is challenging to discriminate single cancer cells with an accuracy above 90% mainly due to the poor biocompatibility of the noble-metal-based SERS agents. Here, we report a dual-functional nanoprobe based on dopant-driven plasmonic oxides, demonstrating a maximum accuracy above 90% in distinguishing single THP-1 cell from peripheral blood mononuclear cell (PBMC) and human embryonic kidney (HEK) 293 from human macrophage cell line U937 based on their SERS patterns. Furthermore, this nanoprobe can be triggered by the bio-redox response from individual cells towards stimuli, empowering another complementary colorimetric cell detection, approximately achieving the unity discrimination accuracy at a single-cell level. Our strategy could potentially enable the future accurate and low-cost detection of cancer cells from mixed cell samples.

Data-Driven Quest for Two-Dimensional Non-van der Waals Materials

Two-dimensional (2D) materials are frequently associated with the sheets forming bulk layered compounds bonded by van der Waals (vdW) forces. The anisotropy and weak interaction between the sheets have also been the main criteria in the computational search for new 2D systems, predicting ∼2000 exfoliable compounds. However, some representatives of a new type of non-vdW 2D systems, without layered 3D analogues, were recently manufactured. For this novel materials class, data-driven design principles are still missing. Here, we outline a set of 8 binary and 20 ternary candidates by filtering the AFLOW-ICSD database according to structural prototypes. The oxidation state of the surface cations regulates the exfoliation energy with low oxidation numbers leading to weak bonding─a useful descriptor to obtain novel 2D materials also providing clear guidelines for experiments. A vast range of appealing electronic, optical, and magnetic properties make the candidates attractive for various applications and particularly spintronics.

2D Material-Based Optical Biosensor: Status and Prospect

The combination of 2D materials and optical biosensors has become a hot research topic in recent years. Graphene, transition metal dichalcogenides, black phosphorus, MXenes, and other 2D materials (metal oxides and degenerate semiconductors) have unique optical properties and play a unique role in the detection of different biomolecules. Through the modification of 2D materials, optical biosensor has the advantages that traditional sensors (such as electrical sensing) do not have, and the sensitivity and detection limit are greatly improved. Here, optical biosensors based on different 2D materials are reviewed. First, various detection methods of biomolecules, including surface plasmon resonance (SPR), fluorescence resonance energy transfer (FRET), and evanescent wave and properties, preparation and integration strategies of 2D material, are introduced in detail. Second, various biosensors based on 2D materials are summarized. Furthermore, the applications of these optical biosensors in biological imaging, food safety, pollution prevention/control, and biological medicine are discussed. Finally, the future development of optical biosensors is prospected. It is believed that with their in-depth research in the laboratory, optical biosensors will gradually become commercialized and improve people's quality of life in many aspects.

Recent advances in the fabrication of 2D metal oxides

Atomically thin two-dimensional (2D) metal oxides exhibit unique optical, electrical, magnetic, and chemical properties, rendering them a bright application prospect in high-performance smart devices. Given the large variety of both layered and non-layered 2D metal oxides, the controllable synthesis is the critical prerequisite for enabling the exploration of their great potentials. In this review, recent progress in the synthesis of 2D metal oxides is summarized and categorized. Particularly, a brief overview of categories and crystal structures of 2D metal oxides is firstly introduced, followed by a critical discussion of various synthesis methods regarding the growth mechanisms, advantages, and limitations. Finally, the existing challenges are presented to provide possible future research directions regarding the synthesis of 2D metal oxides. This work can provide useful guidance on developing innovative approaches for producing both 2D layered and non-layered nanostructures and assist with the acceleration of the research of 2D metal oxides.

Recent Progress on Asymmetric Carbon- and Silica-Based Nanomaterials: From Synthetic Strategies to Their Applications

HIGHLIGHTS

The synthetic strategies and fundamental mechanisms of various asymmetric carbon- and silica-based nanomaterials were systematically summarized. The advantages of asymmetric structure on their related applications were clarified by some representative applications of asymmetric carbon- and silica-based nanomaterials. The future development prospects and challenges of asymmetric carbon- and silica-based nanomaterials were proposed.

ABSTRACT

Carbon- and silica-based nanomaterials possess a set of merits including large surface area, good structural stability, diversified morphology, adjustable structure, and biocompatibility. These outstanding features make them widely applied in different fields. However, limited by the surface free energy effect, the current studies mainly focus on the symmetric structures, such as nanospheres, nanoflowers, nanowires, nanosheets, and core–shell structured composites. By comparison, the asymmetric structure with ingenious adjustability not only exhibits a larger effective surface area accompanied with more active sites, but also enables each component to work independently or corporately to harness their own merits, thus showing the unusual performances in some specific applications. The current review mainly focuses on the recent progress of design principles and synthesis methods of asymmetric carbon- and silica-based nanomaterials, and their applications in energy storage, catalysis, and biomedicine. Particularly, we provide some deep insights into their unique advantages in related fields from the perspective of materials’ structure–performance relationship. Furthermore, the challenges and development prospects on the synthesis and applications of asymmetric carbon- and silica-based nanomaterials are also presented and highlighted. [Image: see text]

High-Voltage, High-Current Electrical Switching Discharge Synthesis of ZnO Nanorods: A New Method toward Rapid and Highly Tunable Synthesis of Oxide Semiconductors in Open Air and Water for Optoelectronic Applications

A novel method of oxide semiconductor nanoparticle synthesis is proposed based on high-voltage, high-current electrical switching discharge (HVHC-ESD). Through a subsecond discharge in the HVHC-ESD method, we successfully synthesized zinc oxide (ZnO) nanorods. Crystallography and optical and electrical analyses approve the high crystal-quality and outstanding optoelectronic characteristics of our synthesized ZnO. The HVHC-ESD method enables the synthesis of ZnO nanorods with ultraviolet (UV) and visible emissions. To demonstrate the effectiveness of our prepared materials, we also fabricated two UV photodetectors based on the ZnO nanorods synthesized using the subsecond HVHC-ESD method. The UV-photodetector test under dark and UV light irradiation also had a promising result with a linear ohmic current-voltage output. In addition to the HVHC-ESD method's excellent tunability for ZnO properties, this method enables the rapid synthesis of ZnO nanorods in open air and water. The results demonstrate the preparation, highlight the synthesis of fine hexagonal-shaped nanorods under a second with controlled oxygen vacancies, and point defects for a wide range of applications in less than a second.

Topological Phase and Quantum Anomalous Hall Effect in Ferromagnetic Transition-Metal Dichalcogenides Monolayer 1T-VSe2

Magnetic two-dimensional (2D) van der Waals materials have attracted tremendous attention because of their high potential in spintronics. In particular, the quantum anomalous Hall (QAH) effect in magnetic 2D layers shows a very promising prospect for hosting Majorana zero modes at the topologically protected edge states in proximity to superconductors. However, the QAH effect has not yet been experimentally realized in monolayer systems to date. In this work, we study the electronic structures and topological properties of the 2D ferromagnetic transition-metal dichalcogenides (TMD) monolayer 1T−VSe2 by first-principles calculations with the Heyd–Scuseria–Ernzerhof (HSE) functional. We find that the spin-orbit coupling (SOC) opens a continuous band gap at the magnetic Weyl-like crossing point hosting the quantum anomalous Hall effect with Chern number C=2. Moreover, we demonstrate the topologically protected edge states and intrinsic (spin) Hall conductivity in this magnetic 2D TMD system. Our results indicate that 1T−VSe2 monolayer serves as a stoichiometric quantum anomalous Hall material.

Controlled growth of 2D ultrathin Ga2O3 crystals on liquid metal

2D metal oxides (2DMOs) have drawn intensive interest in the past few years owing to their rich surface chemistry and unique electronic structures. Striving for large-scale and high-quality novel 2DMOs is of great significance for developing future nano-enabled technologies. In this work, we demonstrate for the first time controllable growth of highly crystalline 2D ultrathin Ga2O3 single crystals on liquid Ga by the chemical vapor deposition approach. With the introduction of oxygen into the growth process, large-area hexagonal α-Ga2O3 crystals with a uniform size distribution have been produced. At high temperature, fast diffusion of oxygen atoms onto the liquid surface facilitates reaction with Ga and thus leads to in situ formation of 2D ultrathin crystals. By precisely controlling the amount of oxygen, the vertical growth of the Ga2O3 single crystal has been realized. Furthermore, phase engineering can be achieved and thus 2D β-Ga2O3 crystals were also prepared by precisely tuning the growth temperature. The controlled growth of 2D Ga2O3 crystals offers an applicable avenue for fabrication of other 2D metal oxides and can further open up possibilities for future electronics.