huangpanqi

In this research, high-quality epitaxial films of VO₂ (020) are directly grown on fluorophlogopite substrates, and the relationship between phase-transition temperature and external strains is revealed. Based on experimental results and density functional theory calculations, it is speculated that lattice constant and bandgap between d_// and π * are strongly affected by external strains, which allow for more effective dynamic modulation of phase-transition process.

Abstract

Unique metal–insulator transition behaviors of strongly correlated electronic materials, vanadium dioxide (VO₂), and their wide potential applications have gained much attention for investigation. In this research, high-quality epitaxial films of VO₂ (020) are directly grown on fluorophlogopite (001) substrates, and the relationship between phase-transition temperature and external strains is revealed. After verifying the like-freestanding property and low intrinsic resistance changing of VO₂/fluorophlogopite, variable phase-transition temperatures under different external strains with a tuning rate of 5.37 K per 0.1% strain are obtained. Based on experimental results and theoretical calculation, it is speculated that lattice constant and bandgap between d_// and π * are strongly affected by the external strains, which allow for more effective dynamic modulation of phase-transition process. This research provides a comprehensive understanding of strain engineering on phase-transition properties and also broadens the possibility of potential applications of certain optoelectronic devices for strain modulation.

By utilizing the monolayer thickness of graphene in Nanoelectromechanical system (NEMS) switches, sub-1 V switching characteristics are achieved. The irreversible static friction at the switch contact is overcome by employing the weak van der Waals (vdW) bonding of graphene-hexagonal boron nitride. These Graphene NEMS vdW switches show sub-0.5 V switching voltage, 10⁵ ON/OFF ratio, and nearly zero hysteretic window characteristics.

Abstract

The Nanoelectromechanical (NEM) switches are a promising candidate to overcome the physical limitations of the complementary metal-oxide-semiconductor (CMOS) switches due to their quasi-zero leakage behavior, sub-thermal switching, and suitability to operate in harsh environments. The main obstacles affecting NEM switches are their irreversible switch-contact stiction, the large switching voltage, and its hysteretic loop. In this study, the irreversible static friction is overcome by employing the weak van der Waals (vdW) bonding of graphene-hexagonal boron nitride (hBN) contact in the Graphene NEM (GNEM) switches. These vdW switches show sub-0.5 V switching voltage with an ON/OFF ratio higher than 10⁵ and nearly zero hysteretic window characteristics with a high endurance of over 50 000 switching cycles. These remarkable performances are achieved by exploiting graphene's monolayer thickness, high Young's modulus, cubic mechanical restoring force, and low vdW binding energy characteristics. As chemical vapor deposition graphene and hBN are used in these GNEM switches, it exhibits the prospect for large-scale graphene NEM system applications. These GNEM switches can be potentially used in ultralow-power CMOS integrated circuits, hybrid NEM-CMOS systems, logic devices, NEM resonator mass sensing, and single-molecule sensors.

Nature Photonics, Published online: 13 October 2022; doi:10.1038/s41566-022-01085-w

Indirect-bandgap transition lasing, even under continuous-wave excitation at room temperature, is demonstrated in an ultra-thin WS2 disk.

Nature Communications, Published online: 17 October 2022; doi:10.1038/s41467-022-33699-7

Designing efficient Bayesian neural networks remains a challenge. Here, the authors use the cycle variation in the programming of the 2D memtransistors to achieve Gaussian random number generator-based synapses, and combine it with the complementary 2D memtransistors-based tanh function to implement a Bayesian neural network.

Compared to the other types of gas sensors, this 1T′/2H/1T′ device possesses distinctive advantages. First, the polycrystalline 2H MoTe₂ provides numerous defects, such as vacancies, edges, and grain boundaries, which play crucial roles in enhancing the response rate and sensitivity of a gas sensor. Second, the low-resistance planar heterostructure ensures a microamp scale current output even at larger channel aspect ratios.

Abstract

2D materials, with their extraordinary physical and chemical properties, have gained extensive interest for physical, chemical and biological sensing applications. However, 2D material-based devices, such as field effect transistors (FETs) often show high contact resistance and low output signals, which severely affect their sensing performance. In this study, a new strategy is developed to combine metallic and semiconducting polymorphs of transition-metal dichalcogenides (TMDCs) to solve this critical issue. Such a phase engineering methodology to integrate large-scale and spatially assembled multilayers of 2H MoTe₂ FETs with coplanar metallic 1T′ MoTe₂ contacts is applied. Such in-plane heterophase-based FETs exhibit an ohmic contact behavior with an extremely low contact resistance due to the coplanar and seamless connections between 2H and 1T′ phases of MoTe₂. These 1T′/2H/1T′ based FETs are successfully demonstrated for detecting NH₃ with high current outputs increased up to microamp levels without using any conventional interdigital electrodes, which is compatible with the current CMOS circuits for practical applications. Furthermore, the as-fabricated sensors can detect NH₃ gas concentrations down to 5 ppm at room temperature. This study demonstrates a new strategy of applying the heterophase MoTe₂-based nanoelectronics for high-performance sensing applications.

The latest advances and developments in the field of chemical sensors based on van der Waals heterostructures of 2D materials, with specific insights into sensing mechanisms, are reviewed and future directions, challenges, and opportunities for the next generation of (bio)chemical sensors with potential impact in environmental sciences and biomedical applications are discussed.

Abstract

During the last 15 years, 2D materials have revolutionized the field of materials science. Moreover, because of their highest surface-to-volume ratio and properties extremely susceptible to their interaction with the local environment they became powerful active components for the development the high-performance chemical sensors. By combining different 2D materials to form van der Waals heterostructures (VDWHs) it is possible to overcome the drawback of individual materials (such as inertness and zero-bandgap of pristine graphene and less environmental stability of transition metal dichalcogenides). Meanwhile, VDWHs possess unprecedented and fascinating properties arising from the intimate interaction between the components, which can yield superior sensitivities, higher selectivity, and stability when employed to detect gases, biomolecules, and other organic/inorganic molecules. Herein, the latest developments and advances in the field of chemical sensors based on VDWH of 2D materials, with specific insight into the sensing mechanisms, are reviewed and future directions, challenges, and opportunities for the development of the next generation of (bio)chemical sensors with potential impact in environmental sciences and biomedical applications, and more specifically in (bio)chemical defense, industrial safety, food, and environmental surveillance, and medical (early) diagnostics, are discussed.

The optical and optoelectronic properties of 2D Xenes and their applications in photonic devices including all-optical modulators, wavelength converters, ultrafast lasers, and photodetectors are comprehensively reviewed. The article mainly focuses on five most extensively studied Xenes: graphene, black phosphorus, antimonene, bismuthene, and tellurenene. The properties and applications of other Xenes are also briefly introduced.

Abstract

In recent years, tremendous attention has been paid to the investigation of single-element 2D materials. These 2D materials mainly consist of elements from group IV and group V such as silicene, phosphorene, and antimonene. Together with other four elements from groups III and VI, they are classified as 2D Xenes and exhibit rich optical and optoelectronic properties such as broadband optical response, strong nonlinearity, ultrafast recovery time, and layer-dependent bandgap. 2D Xenes can be easily integrated with microfibers and other optical platforms. On the basis of their attracting characteristics, 2D Xenes have been utilized in various functional devices. In this review, the optical and optoelectronic properties of the most intensively studied 2D Xenes are introduced. Their applications in photonic devices including all-optical modulators, wavelength converters, ultrafast lasers, and photodetectors are explicitly explored. Finally, the challenges and future perspectives of photonic devices based on 2D Xenes are discussed.

The interface carrier transfer process is detected in MoS₂/WS₂ heterostructures via transient absorption microscopy. Owing to the weakened interlayer coupling, an energy transfer process with a rate of ≈240 fs is observed in chemical vapor deposition heterostructures. It elucidates the role of the energy transfer process in interfacial carrier dynamics and provides guidance for engineering interfaces for optoelectronic and quantum applications of transition metal dichalcogenide heterostructures.

Abstract

Van der Waals semiconducting heterostructures, known as stacks of atomically thin transition-metal dichalcogenide (TMD) layers, have recently been reported as new quantum materials with fascinating optoelectronic properties and novel functionalities. These discoveries are significantly related to the interfacial carrier dynamics of the excited states. Carrier dynamics have been reported to be predominantly driven by the ultrafast charge transfer (CT) process; however, the energy transfer (ET) process remains elusive. Herein, the ET process in MoS₂/WS₂ heterostructures via transient absorption microscopy is reported. By analyzing the ultrafast dynamics using various MoS₂/WS₂ interfaces, an ET rate of ≈240 fs is obtain, which is not trivial to the CT process. This study elucidates the role of the ET process in interfacial carrier dynamics and provides guidance for engineering interfaces for optoelectronic and quantum applications of TMD heterostructures.

Nature Synthesis, Published online: 29 September 2022; doi:10.1038/s44160-022-00165-7

Two-dimensional materials have many desirable properties but controllable synthesis is difficult. Now, a flux-assisted growth approach has been designed to reproducibly prepare high-quality, atomically thin materials. Eighty atomically thin composite flakes have been prepared by this approach.

Polycrystalline ErMnO₃ displays an anomalous domain size/grain size scaling behavior in comparison to classical ferroelectric materials, such as BaTiO₃ or Pb(Zr,Ti)O₃. The fundamental difference is due to the formation of topologically protected vortex/anti-vortex pairs in polycrystalline ErMnO₃ and their interaction with elastic strain fields.

Abstract

The research on topological phenomena in ferroelectric materials has revolutionized the way people understand polar order. Intriguing examples are polar skyrmions, vortex/anti-vortex structures, and ferroelectric incommensurabilties, which promote emergent physical properties ranging from electric-field-controllable chirality to negative capacitance effects. Here, the impact of topologically protected vortices on the domain formation in improper ferroelectric ErMnO₃ polycrystals is studied, demonstrating inverted domain scaling behavior compared to classical ferroelectrics. It is observed that as the grain size increases, smaller domains are formed. Phase field simulations reveal that elastic strain fields drive the annihilation of vortex/anti-vortex pairs within the grains and individual vortices at the grain boundaries. The inversion of the domain scaling behavior has far-reaching implications, providing fundamentally new opportunities for topology-based domain engineering and the tuning of the electromechanical and dielectric performance of ferroelectrics in general.

This review summarizes the theoretical and experimental progress of Ge-based monoelemental and binary two-dimensional (2D) materials, with an emphasis on their crystal structures and electronic, mechanical, thermal, optical, and photoelectric properties. The application prospects of these materials in field effect transistors, photodetectors, optical devices, catalysts, energy storage devices, solar cells, thermoelectric devices, sensors, biomedical materials, and spintronic devices are discussed in detail.

Abstract

Two-dimensional (2D) materials based on group IVA elements have attracted extensive attention owing to their rich chemical structures and novel properties. This comprehensive review focuses on the phases of Ge monoelemental and binary 2D materials including germanene and its derivatives, Ge-IVA binary compounds, Ge-VA binary compounds, and Ge-VIA binary compounds. The latest progress in predictive modeling, fabrication, and fundamental and physical property modulation of their stable 2D configurations are presented. Accordingly, various interesting applications of these Ge-based 2D materials are discussed, particularly field effect transistors, photodetectors, optical devices, catalysts, energy storage devices, solar cells, thermoelectric devices, sensors, biomedical materials, and spintronic devices. Finally, this review concludes with a few perspectives and an outlook for quickly expanding the application scope Ge-based 2D materials based on recent developments.

Here, the authors discover a universal Ag-assisted exfoliation method, which can be used to directly integrate large-scale 2D crystals with plasmonic nanostructures. The long propagation of surface plasmonic polariton in the hybrid structures results in extraordinarily photoluminescence enhancement for monolayer MoS₂ and MoSe₂. This technique allows to direct preparation and exploration of optical properties of emergent 2D crystals, which paves a new way for preparing large-scale 2D materials and their future applications.

Abstract

Advanced exfoliation techniques are crucial for exploring the intrinsic properties and applications of 2D materials. Though the recently discovered Au-enhanced exfoliation technique provides an effective strategy for the preparation of large-scale 2D crystals, the high cost of gold hinders this method from being widely adopted in industrial applications. In addition, direct Au contact could significantly quench photoluminescence (PL) emission in 2D semiconductors. It is therefore crucial to find alternative metals that can replace gold to achieve efficient exfoliation of 2D materials. Here, the authors present a one-step Ag-assisted method that can efficiently exfoliate many large-area 2D monolayers, where the yield ratio is comparable to Au-enhanced exfoliation method. Differing from Au film, however, the surface roughness of as-prepared Ag films on SiO₂/Si substrate is much higher, which facilitates the generation of surface plasmons resulting from the nanostructures formed on the rough Ag surface. More interestingly, the strong coupling between 2D semiconductor crystals (e.g., MoS₂, MoSe₂) and Ag film leads to a unique PL enhancement that has not been observed in other mechanical exfoliation techniques, which can be mainly attributed to enhanced light-matter interaction as a result of extended propagation of surface plasmonic polariton (SPP). This work provides a lower-cost and universal Ag-assisted exfoliation method, while at the same time offering enhanced SPP-matter interactions.

Recent advances in etching of 2D materials are comprehensively presented and typical etching modes of chemical vapor deposition (CVD) etching are shown. Linear etching usually results in etched lines along specific crystal lattice. Anisotropic etching tends to form etched holes with Euclidean geometric patterns. Fractal etching typically leads to self-similar symmetric patterns, such as snow-like patterns.

Abstract

Etching, considered as the reverse process of growth, has drawn intensive interests in the past decades. Rather from offering building blocks for formation of materials, etching is served as removing basic units from the matrix. Generally, etching plays a critical role in three aspects: first, it can serve as direct top-down method to precisely make designed patterns for electronic devices. Second, it can offer an indirect way to probe the detailed growth mechanism of 2D materials, enhancing understanding of growth process. Finally, it can be an efficient and facile way to visualize grain boundaries. Herein, several commonly used etching techniques for 2D materials are presented of which chemical vapor deposition etching has attracted the most intensive attentions. Thereafter, the typical etching modes of 2D materials are demonstrated, wherein linear etching, anisotropic etching, and fractal etching are comprehensively exhibited, respectively. Furthermore, the etching mechanism of 2D materials is elucidated, thereby offering a guideline for probing their in-depth etching dynamics and kinetics. Finally, relevant concerns regarding uniformity and reproducibility within etching process are discussed and the expected future is envisaged.

Wafer-scale continuous hexagonal boron nitride thick films are prepared by magnetron sputtering, and the two-inch complete films are peeled and transferred. The integrity and continuity of the transferred films are verified by measuring the resistance switching behavior. This work systematically studies the stripping process, characterizes the transferred films, and explores the application in the field of resistance switching.

Abstract

A film stripping method that allows for liquid phase exfoliation assisted by spin coating polymethyl methacrylate has been investigated, resulting in a two-inch hexagonal boron nitride (hBN) film to be fully stripped and then transferred. A number of key factors that can influence the stripping and the transferring process of the films grown by sputtering have been systematically analyzed, including different solutions, different concentration of solution and different thickness of films. The morphology and properties of the hBN films before and after stripping have been characterized. The band edge absorption peak of the transferred film is 229 nm and the corresponding optical band gap is 5.50 eV. Such transferred hBN films have been fabricated into transparent resistive switching devices on indium-tin-oxide glass, demonstrating a constant resistance window of ≈10² even under different applied voltages. This work systematically studies the stripping process, characterizes the transferred films, and explores the application in the field of resistance switching, which lay a foundation for the further application of hBN materials in optoelectronic devices.

Epitaxially grown vertical layered heterostructures (VLHs) of mixed 2D layered materials are often thought to align without angular misorientation. However, it is shown that high-mismatch VLHs show discrete and sometimes non-zero twist angles dependent on their natural lattice mismatch value. This opens a pathway for scalable Moire VLH systems.

Abstract

Artificially introduced small twist angles at the interfaces of vertical layered heterostructures (VLHs) have allowed deterministic tuning of electronic and optical properties such as strongly correlated electronic phases and Moiré excitons. But creating a Moiré twist in van der Waals (vdWs) systems by manual stacking is challenging in reproducibility, uniformity, and accuracy of the twist angle, which hinders future studies. Here, it is demonstrated that contrary to the commonly believed 0°-orientation in vdWs epitaxy, these VLHs show small twist angles controlled by the low-order commensurate phase with low energy and local atomic relaxation. A commensurate multilevel map is proposed to predict possible orientations. Remarkably, high-mismatch VLHs show discrete and sometimes non-zero twist angles dependent on their natural mismatch value. Such framework is experimentally confirmed in five epitaxially grown VLHs under high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and can provide significant insights for large-scale engineering of twist angle in VLHs.

The modulating of energy migration is achieved by the well-designed lanthanide dumbbell-like nanocrystals, resulting in excitation wavelength-dependent dual-mode emission. The dual-mode emission of nanocrystals is used for deep learning fluorescence imaging, where the blurred visible light information disturbed by phantom tissue can be established to a high resolution comparable to that of second near infrared window (NIR-II).

Abstract

Fluorescence bioimaging has always been a research hotspot in the field of life sciences and medicine. Although many studies focus on the promising second near infrared window (NIR-II) imaging, the NIR-II imaging with deep tissue penetration is limited by the broad emission band widths. Herein, a well-designed lanthanide doped nanocrystal is presented that can modulate the energy migration processes by controlling energy migration pathway and cerium-assisted energy transfer processes, resulting in switchable emission modes of visible and NIR-II that dependent by the excitation wavelengths. Subsequently, the multimode emissions of dumbbell-like nanocrystals are cooperated with deep learning, where the advantages of narrow emission peak of visible fluorescence and deep tissue penetration of NIR-II fluorescence are combined to offer a unique deep learning fluorescence bioimaging. By this new imaging method, fluorescence signals can be obtained with narrow emission peak and high signal-to-noise ratio after penetrating phantom tissue. This study brings a powerful idea for cutting-edge applications of intelligent optical materials, such as in vivo information security.