Flexible black phosphorus ambipolar transistors, circuits and AM demodulator

Layer-by-layer thinning of two-dimensional materials

Etching technology - one of the representative modern semiconductor device makers - serves as a broad descriptor for the process of removing material from the surfaces of various materials, whether partially or entirely. Meanwhile, thinning technology represents a novel and highly specialized approach within the realm of etching technology. It indicates the importance of achieving an exceptionally sophisticated and precise removal of material, layer-by-layer, at the nanoscale. Notably, thinning technology has gained substantial momentum, particularly in top-down strategies aimed at pushing the frontiers of nano-worlds. This rapid development in thinning technology has generated substantial interest among researchers from diverse backgrounds, including those in the fields of chemistry, physics, and engineering. Precisely and expertly controlling the layer numbers of 2D materials through the thinning procedure has been considered as a crucial step. This is because the thinning processes lead to variations in the electrical and optical characteristics. In this comprehensive review, the strategies for top-down thinning of representative 2D materials (e.g., graphene, black phosphorus, MoS2, h-BN, WS2, MoSe2, and WSe2) based on conventional plasma-assisted thinning, integrated cyclic plasma-assisted thinning, laser-assisted thinning, metal-assisted splitting, and layer-resolved splitting are covered in detail, along with their mechanisms and benefits. Additionally, this review further explores the latest advancements in terms of the potential advantages of semiconductor devices achieved by top-down 2D material thinning procedures.

Strong mechanical anisotropy and an anisotropic Dirac state in 2D C5N3

Two-dimensional (2D) carbon nitride materials have emerged as a versatile platform for the design of high-performance nanoelectronics, but strong anisotropy in 2D carbon nitrides has rarely been reported. In this work, a 2D carbon nitride with strong anisotropy composed of tetra-, penta-, and hexa-rings (named as TPH-C5N3) is proposed. This TPH-C5N3 exhibits both dynamical and mechanical stability. Furthermore, it also showcases remarkable thermal stability, reaching up to 2300 K, as evidenced by AIMD simulations conducted in an NVT environment utilizing the Nosé-Hoover thermostat. Significantly, TPH-C5N3 demonstrates high anisotropic ratios in its mechanical properties, positioning it as the frontrunner in the current carbon nitride systems. In addition, a Dirac cone with an anisotropic ratio of 55.8% and Fermi velocity of 7.26 × 105 m s-1 is revealed in TPH-C5N3. The nontrivial topological properties of TPH-C5N3 are demonstrated by a non-zero Z2 invariant and topologically protected edge states. Our study offers theoretical insights into an anisotropic 2D carbon nitride material, laying the groundwork for its design and synthesis.

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

Controllable Modulation of the Electronic Properties of a Two-Dimensional Ambipolar Semiconductor by Interface Ferroelectric Polarization

Precise control of charge carrier type and density of two-dimensional (2D) ambipolar semiconductors is the prerequisite for their applications in next-generation integrated circuits and electronic devices. Here, by fabricating a heterointerface between a 2D ambipolar semiconductor (hydrogenated germanene, GeH) and a ferroelectric substrate (PbMg1/3Nb2/3O3-PbTiO3, PMN-PT), fine-tuning of charge carrier type and density of GeH is achieved. Due to ambipolar properties, proper band gap, and high carrier mobility of GeH, by applying the opposite local bias (±8 V), a lateral polarization in GeH is constructed with a change of work function by 0.6 eV. Besides, the built-in polarization in GeH nanoflake could promote the separation of photoexcited electron-hole pairs, which lead to 4 times enhancement of the photoconductivity after poling by 200 V. In addition, a gradient regulation of the work function of GeH from 4.94 to 5.21 eV by adjusting the local substrate polarization is demonstrated, which could be used for data storage at the micrometer size by forming p-n homojunctions. This work of constructing such heterointerfaces provides a pathway for applying 2D ambipolar semiconductors in nonvolatile memory devices, photoelectronic devices, and next-generation integrated circuit.

Recent Advances in Phosphorene: Structure, Synthesis, and Properties

Phosphorene is a 2D phosphorus atomic layer arranged in a honeycomb lattice like graphene but with a buckled structure. Since its exfoliation from black phosphorus in 2014, phosphorene has attracted tremendous research interest both in terms of synthesis and fundamental research, as well as in potential applications. Recently, significant attention in phosphorene is motivated not only by research on its fundamental physical properties as a novel 2D semiconductor material, such as tunable bandgap, strong in-plane anisotropy, and high carrier mobility, but also by the study of its wide range of potential applications, such as electronic, optoelectronic, and spintronic devices, energy conversion and storage devices. However, a lot of avenues remain to be explored including the fundamental properties of phosphorene and its device applications. This review recalls the current state of the art of phosphorene and its derivatives, touching upon topics on structure, synthesis, characterization, properties, stability, and applications. The current needs and future opportunities for phosphorene are also discussed.

Cascaded Logic Gates Based on High-Performance Ambipolar Dual-Gate WSe2 Thin Film Transistors

Ambipolar dual-gate transistors based on low-dimensional materials, such as graphene, carbon nanotubes, black phosphorus, and certain transition metal dichalcogenides (TMDs), enable reconfigurable logic circuits with a suppressed off-state current. These circuits achieve the same logical output as complementary metal-oxide semiconductor (CMOS) with fewer transistors and offer greater flexibility in design. The primary challenge lies in the cascadability and power consumption of these logic gates with static CMOS-like connections. In this article, high-performance ambipolar dual-gate transistors based on tungsten diselenide (WSe₂) are fabricated. A high on-off ratio of 10⁸ and 10⁶, a low off-state current of 100 to 300 fA, a negligible hysteresis, and an ideal subthreshold swing of 62 and 63 mV/dec are measured in the p- and n-type transport, respectively. We demonstrate cascadable and cascaded logic gates using ambipolar TMD transistors with minimal static power consumption, including inverters, XOR, NAND, NOR, and buffers made by cascaded inverters. A thorough study of both the control gate and the polarity gate behavior is conducted. The noise margin of the logic gates is measured and analyzed. The large noise margin enables the implementation of V_T-drop circuits, a type of logic with reduced transistor number and simplified circuit design. Finally, the speed performance of the V_T-drop and other circuits built by dual-gate devices is qualitatively analyzed. This work makes advancements in the field of ambipolar dual-gate TMD transistors, showing their potential for low-power, high-speed, and more flexible logic circuits.

Recent Advances in Surface Modifications of Elemental Two-Dimensional Materials: Structures, Properties, and Applications

The advent of graphene opens up the research into two-dimensional (2D) materials, which are considered revolutionary materials. Due to its unique geometric structure, graphene exhibits a series of exotic physical and chemical properties. In addition, single-element-based 2D materials (Xenes) have garnered tremendous interest. At present, 16 kinds of Xenes (silicene, borophene, germanene, phosphorene, tellurene, etc.) have been explored, mainly distributed in the third, fourth, fifth, and sixth main groups. The current methods to prepare monolayers or few-layer 2D materials include epitaxy growth, mechanical exfoliation, and liquid phase exfoliation. Although two Xenes (aluminene and indiene) have not been synthesized due to the limitations of synthetic methods and the stability of Xenes, other Xenes have been successfully created via elaborate artificial design and synthesis. Focusing on elemental 2D materials, this review mainly summarizes the recently reported work about tuning the electronic, optical, mechanical, and chemical properties of Xenes via surface modifications, achieved using controllable approaches (doping, adsorption, strain, intercalation, phase transition, etc.) to broaden their applications in various fields, including spintronics, electronics, optoelectronics, superconducting, photovoltaics, sensors, catalysis, and biomedicines. These advances in the surface modification of Xenes have laid a theoretical and experimental foundation for the development of 2D materials and their practical applications in diverse fields.

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.

Non-equilibrium spin-transport properties of Co/phosphorene/Co MTJ with non-collinear electrodes under mechanical bending

Monolayer phosphorene has outstanding mechanical flexibility, making it rather attractive in flexible spintronics that are based on 2D materials. Here, we report a first-principles study on non-equilibrium electronic-transport properties of the Co/phosphorene/Co magnetic tunnel junction (MTJ) with two α-Co electrodes. The magnetic moments of the two electrodes are considered in the parallel configuration (PC) and the anti-parallel configuration (APC). The tunneling current through the MTJ is investigated at a small bias from 0 to 40 mV when mechanical bending is applied on the MTJ with different central angle (θ) values. For both the PC and APC, the tunneling current increases evidently and monotonously with increasing mechanical bending for 25° < θ < 40°, as compared to that without bending, which is mainly due to the reduced tunnel barrier. In the PC, the spin-injection efficiency (SIE) of the current is largely increased at a small bias from 0 to 40 mV for 25° ≤ θ ≤ 30° with a maximum of 90%, while the SIE is overall increased under all mechanical bending angles for the APC. The tunnel magnetoresistance is decreased with an increasing bias voltage, which can be largely enhanced for θ ≥ 25°, especially at small bias. Our results indicate that the Co/phosphorene/Co MTJ has promising applications in flexible low-power spintronic devices.

Two-Dimensional Black Phosphorus: Preparation, Passivation and Lithium-Ion Battery Applications

As a new type of single element direct-bandgap semiconductor, black phosphorus (BP) shows many excellent characteristics due to its unique two-dimensional (2D) structure, which has great potential in the fields of optoelectronics, biology, sensing, information, and so on. In recent years, a series of physical and chemical methods have been developed to modify the surface of 2D BP to inhibit its contact with water and oxygen and improve the stability and physical properties of 2D BP. By doping and coating other materials, the stability of BP applied in the anode of a lithium-ion battery was improved. In this work, the preparation, passivation, and lithium-ion battery applications of two-dimensional black phosphorus are summarized and reviewed. Firstly, a variety of BP preparation methods are summarized. Secondly, starting from the environmental instability of BP, different passivation technologies are compared. Thirdly, the applications of BP in energy storage are introduced, especially the application of BP-based materials in lithium-ion batteries. Finally, based on preparation, surface functionalization, and lithium-ion battery of 2D BP, the current research status and possible future development direction are put forward.

Basal Plane Functionalization of Niobium Disulfide Nanosheets with Cyclopentadienyl Manganese(I) Dicarbonyl

Basal plane-functionalized NbS₂ nanosheets were obtained using in situ photolysis to generate the coordinatively unsaturated organometallic fragment cyclopentadienyl manganese(I) dicarbonyl (CpMn(CO)₂). Under UV irradiation, a labile carbonyl ligand dissociates from the tricarbonyl complex, creating an open coordination site for bonding between the Mn atom and the electron-rich sulfur atoms on the surface of the NbS₂ nanosheets. In contrast, no reaction is observed with 2H-MoS₂ nanosheets under the same reaction conditions. This difference in reactivity is consistent with the electronic structure calculations, which indicate stronger bonding of the organometallic fragment to electron-poor, metallic NbS₂ than to semiconducting, electron-rich MoS₂. X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared (FTIR) spectroscopy, and powder X-ray diffraction (PXRD) were used to characterize the bonding between Mn and S atoms on the surface-functionalized nanosheets.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Room-Temperature Infrared Photodetectors with Zero-Dimensional and New Two-Dimensional Materials

Infrared photodetectors have received much attention for several decades due to their broad applications in the military, science, and daily life. However, for achieving an ideal signal-to-noise ratio and a very fast response, cooling is necessary in those devices, which makes them bulky and costly. Thus, room-temperature infrared photodetectors have emerged as a hot research direction. Novel low-dimensional materials with their easy fabrication and excellent photoelectronic properties provide a possible solution for room-temperature infrared photodetectors. This review aims to summarize the preparation methods and characterization of several low-dimensional materials (PbS, PbSe and HgTe, new two-dimensional materials) with great concern and the room-temperature infrared photodetectors based on them.

Highly Sensitive Photodetectors Based on Monolayer MoS2 Field-Effect Transistors

Molybdenum disulfide (MoS₂) is a promising candidate for the development of high-performance photodetectors, due to its excellent electric and optoelectronic properties. However, most of the reported MoS₂ phototransistors have adopted a back-gate field-effect transistor (FET) structure, requiring applied gate bias voltages as high as 70 V, which made it impossible to modulate each detecting device in the fabricated array. In this paper, buried-gate FETs based on CVD-grown monolayer MoS₂ were fabricated and their electric and photoelectric properties were also systematically investigated. A photoresponsivity of around 6.86 A/W was obtained at 395 nm, under the conditions of zero gate bias voltage and a light power intensity of 2.57 mW/cm². By application of a buried-gate voltage of 8 V, the photoresponsivity increased by nearly 10 times. Furthermore, the response speed of the buried-gate MoS₂ FET phototransistors is measured to be around 350 ms. These results pave the way for MoS₂ photodetectors in practical applications.

Optothermal Raman Spectroscopy of Black Phosphorus on a Gold Substrate

With black phosphorus being a promising two-dimensional layered semiconductor for application to electronics and optoelectronics, an issue remains as to how heat diffusion is managed when black phosphorus is interfaced with metals, namely in a typical device heterojunction. We use Raman spectroscopy to investigate how the laser-induced heat affects the phonon modes at the interface by comparing the experimental data with a finite element simulation based on a localized heat diffusion. The best convergence is found taking into account an effective interface thermal conductance, thus indicating that heat dissipation at the Au-supported black phosphorus nanosheets is limited by interface effect.

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.

High Photoresponse Black Phosphorus TFTs Capping with Transparent Hexagonal Boron Nitride

Black phosphorus (BP), a single elemental two-dimensional (2D) material with a sizable band gap, meets several critical material requirements in the development of future nanoelectronic applications. This work reports the ambipolar characteristics of few-layer BP, induced using 2D transparent hexagonal boron nitride (h-BN) capping. The 2D h-BN capping have several advantages over conventional Al₂O₃ capping in flexible and transparent 2D device applications. The h-BN capping technique was used to achieve an electron mobility in the BP devices of 73 cm²V⁻¹s⁻¹, thereby demonstrating n-type behavior. The ambipolar BP devices exhibited ultrafast photodetector behavior with a very high photoresponsivity of 1980 mA/W over the ultraviolet (UV), visible, and infrared (IR) spectral ranges. The h-BN capping process offers a feasible approach to fabricating n-type behavior BP semiconductors and high photoresponse BP photodetectors.

Ultrafast and Highly Sensitive Dual-Channel FET Photodetector Based on a Two-Dimensional MoS2 Homojunction

Two-dimensional transition metal dichalcogenide-based phototransistors have been intensively studied in recent years due to their high detection rate and flexibility. However, the photogating effect, usually appearing in the devices, leads to a poor transient photoresponse, which slows down the imaging rate of the camera based on the devices. Here, we demonstrate a dual-channel two-dimensional field-effect phototransistor composed of a vertical molybdenum disulfide (MoS₂) p-n homojunction as the sensitizing channel layer. Owing to the effective separation by the vertical built-in electric field and rapid migration of photoexcited electrons and holes in the separated channels, the fabricated dual-channel FET device simultaneously exhibits prominent responsivity and greatly improved time response in comparison to the pristine MoS₂ FET detectors. Excellent device performance has been achieved, with a responsivity of 3.4 × 10⁴ A/W at a source-drain voltage (V_DS) of 1 V, corresponding to a detectivity (D*) of 1.9 × 10¹³ Jones@532 nm and a gain of more than 10⁵ electrons per photon, an external quantum efficiency of 9.6%, and a response time of tens of milliseconds. Especially, the response time of the dual-channel FET device is 3 orders of magnitude faster than that of the pristine device. Our results provide a new way to overcome the inherent photogating drawback of two-dimensional FET optoelectronic devices and to develop a related high frame rate imaging system.

A Roadmap for Disruptive Applications and Heterogeneous Integration Using Two-Dimensional Materials: State-of-the-Art and Technological Challenges

This Mini Review attempts to establish a roadmap for two-dimensional (2D) material-based microelectronic technologies for future/disruptive applications with a vision for the semiconductor industry to enable a universal technology platform for heterogeneous integration. The heterogeneous integration would involve integrating orthogonal capabilities, such as different forms of computing (classical, neuromorphic, and quantum), all forms of sensing, digital and analog memories, energy harvesting, and so forth, all in a single chip using a universal technology platform. We have reviewed the state-of-the-art 2D materials such as graphene, transition metal dichalcogenides, phosphorene and hexagonal boron nitride, and so forth, and how they offer unique possibilities for a range of futuristic/disruptive applications. Besides, we have discussed the technological and fundamental challenges in enabling such a universal technology platform, where the world stands today, and what gaps are required to be filled.

Outstanding spin-transport properties of a flexible phosphorene photodetector driven by the photogalvanic effect under mechanical strains

Monolayer phosphorene has outstanding spintronic properties including a nanosecond spin lifetime, and micrometer spin relaxation length, combined with excellent mechanical flexibility, making it rather attractive in low-dimensional flexible spintronic devices. However, knowledge on the spin-transport properties of phosphorene under mechanical strain is currently very limited. Here, we study the transport properties of the spin-polarized photocurrent in the flexible Ni-phosphorene-Ni photodetector, which is driven by the photogalvanic effect (PGE) under mechanical tension stress and bending. Broadband PGE photocurrent is generated at zero bias under the illumination of linearly polarized light due to the broken inversion symmetry of the photodetector. Remarkable spin-transport performances including the fully spin-polarized photocurrent, perfect spin-valve effect, and enhanced pure spin current are generated in a broad visible range by applying appropriate mechanical tension stress or bending. Our results indicate that the PGE-driven phosphorene-based photodetector has promising applications in flexible and low-power spintronic devices.

Probing the Laser Ablation of Black Phosphorus by Raman Spectroscopy

Laser ablation in conjunction with Raman spectroscopy can be used to attain a controllable reduction of the thickness of exfoliated black phosphorus flakes and simultaneous measurement of the local temperature. However, this approach can be affected by several parameters, such as the thickness-dependent heat dissipation. Optical, thermal, and mechanical effects in the flakes and the substrate can influence the laser ablation and may become a source of artifacts on the measurement of the local temperature. In this work, we carry out a systematic investigation of the laser thinning of black phosphorus flakes on SiO₂/Si substrates. The counterintuitive results from Raman thermometry are analyzed and elucidated with the help of numerical solutions of the problem, laying the groundwork for a controlled thinning process of this material.

Recent advances in black phosphorus-based electrochemical sensors: A review

Since the discovery of liquid-phase-exfoliated black phosphorus (BP) as a field-effect transistor in 2014, BP, with its 2D layered structure, has attracted significant attention, owing to its anisotropic electroconductivity, tunable direct bandgap, extraordinary surface activity, moderate switching ratio, high hole mobility, good biocompatibility, and biodegradability. Several pioneering research efforts have explored the application of BP in different types of electrochemical sensors. This review summarizes the latest synthesis methods, protection strategies, and electrochemical sensing applications of BP and its derivatives. The typical synthesis methods for BP-based crystals, nanosheets, and quantum dots are discussed in detail; the degradation of BP under ambient conditions is introduced; and state-of-the-art protection methodologies for enhancing BP stability are explored. Various electrochemical sensing applications, including chemically modified electrodes, electrochemiluminescence sensors, enzyme electrodes, electrochemical aptasensors, electrochemical immunosensors, and ion-selective electrodes are discussed in detail, along with the mechanisms of BP functionalization, sensing strategies, and sensing properties. Finally, the major challenges in this field are outlined and future research avenues for BP-based electrochemical sensors are highlighted.

Formation Mechanisms for Phosphorene and SnIP

Phosphorene-the monolayered material of the element allotrope black phosphorus (Pblack )-and SnIP are 2D and 1D semiconductors with intriguing physical properties. Pblack and SnIP have in common that they can be synthesized via short way transport or mineralization using tin, tin(IV) iodide and amorphous red phosphorus. This top-down approach is the most important access route to phosphorene. The two preparation routes are closely connected and differ mainly in reaction temperature and molar ratios of starting materials. Many speculative intermediates or activator side phases have been postulated especially for top-down Pblack /phosphorene synthesis, such as Hittorf's phosphorus or Sn24 P19.3 I8 clathrate. The importance of phosphorus-based 2D and 1D materials for energy conversion, storage, and catalysis inspired us to elucidate the formation mechanisms of these two compounds. Herein, we report on the reaction mechanisms of Pblack /phosphorene and SnIP from P4 and SnI2 via direct gas phase formation.

Frequency Doubler and Universal Logic Gate Based on Two-Dimensional Transition Metal Dichalcogenide Transistors with Low Power Consumption

Two-dimensional transition metal dichalcogenide semiconductors are very promising candidates for future electronic applications with low power consumption due to a low leakage current and high on-off current ratio. In this study, we suggest a complementary circuit consisting of ambipolar WSe₂ and n-MoS₂ field-effect transistors (FETs), which demonstrate dual functions of a frequency doubler and single inversion AND (SAND) logic gate. In order to reduce the power consumption, a high-quality thin h-BN single crystal is used as a gate dielectric that leads to a low operating voltage of less than 5 V. By combining the low operating voltage with a low operating current in the complementary circuit, a low power consumption of 300 nW (a minimum of 10 pW) has been achieved, which is a significant improvement compared to the tens of μW consumed by a graphene channel. The complementary circuit shows the effective frequency doubling of the input with a dynamic range from 20 to 100 Hz. Furthermore, this circuit satisfies all the truth tables of a SAND logic gate that can be used as a universal logic gate like NAND. Considering that the NAND logic gate generally consists of four transistors, it is significantly advantageous to implement the equivalent circuit SAND logic gate with only two FETs. Our results open up possibilities for analog- and logic-circuit applications based on low-dimensional semiconductors.

Electrochemical Delamination of Ultralarge Few-Layer Black Phosphorus with a Hydrogen-Free Intercalation Mechanism

Due to strong interlayer interaction and ease of oxidation issues of black phosphorus (BP), the domain size of artificial synthesized few-layer black phosphorus (FL-BP) crystals is often below 10 µm, which extremely limits its further applications in large-area thin-film devices and integrated circuits. Herein, a hydrogen-free electrochemical delamination strategy through weak Lewis acid intercalation enabled exfoliation is developed to produce ultralarge FL-BP single-crystalline domains with high quality. The interaction between the weak Lewis acid tetra-n-butylammonium acetate (CH₃ COOTBA) and P atoms promotes the average domain size of FL-BP crystal up to 77.6 ± 15.0 µm and the largest domain size is found to be as large as 119 µm. The presence of H⁺ and H₂ O is found to sharply decrease the size of as-exfoliated FL-BP flakes. The electronic transport measurements show that the delaminated FL-BP crystals exhibit a high hole mobility of 76 cm² V^-1 s^-1 and an on/off ratio of 10³ at 298 K. A broadband photoresponse from 532 to 1850 nm with ultrahigh responsivity is achieved. This work provides a scalable, simple, and low-cost approach for large-area BP films that meet industrial requirements for nanodevices applications.

Ambipolar 2D Semiconductors and Emerging Device Applications

With the rise of 2D materials, new physics and new processing techniques have emerged, triggering possibilities for the innovation of electronic and optoelectronic devices. Among them, ambipolar 2D semiconductors are of excellent gate-controlled capability and distinctive physical characteristic that the major charge carriers can be dynamically, reversibly and rapidly tuned between holes and electrons by electrostatic field. Based on such properties, novel devices, like ambipolar field-effect transistors, light-emitting transistors, electrostatic-field-charging PN diodes, are developed and show great advantages in logic and reconfigurable circuits, integrated optoelectronic circuits, and artificial neural network image sensors, enriching the functions of conventional devices and bringing breakthroughs to build new architectures. This review first focuses on the basic knowledge including fundamental principle of ambipolar semiconductors, basic material preparation techniques, and how to obtain the ambipolar behavior through electrical contact engineering. Then, the current ambipolar 2D semiconductors and their preparation approaches and main properties are summarized. Finally, the emerging new device structures are overviewed in detail, along with their novel electronic and optoelectronic applications. It is expected to shed light on the future development of ambipolar 2D semiconductors, exploring more new devices with novel functions and promoting the applications of 2D materials.

Strain-Controllable High Curie Temperature, Large Valley Polarization, and Magnetic Crystal Anisotropy in a 2D Ferromagnetic Janus VSeTe Monolayer

Two-dimensional (2D) intrinsic ferromagnetic semiconductors are important for spintronics. A highly stable ML (monolayer) Janus 2H-VSeTe with intrinsic ferromagnetism is investigated by density functional theory. The biaxial strain could effectively tune the magnetic and electronic properties of Janus VSeTe. Specifically, the magnetic moment, band gap, Curie temperature (T_c), and valley splitting (Δ) could be modulated, as the states near the Fermi level are mainly contributed by the in-plane atomic orbitals. The VSeTe could be switched from ferromagnetic (FM) order to antiferromagnetic (AFM) ground state, under biaxial strains. And the corresponding T_c is tuned from 360 K (4%) to 0 K (-10.7%). However, VSeTe can be modulated from bipolar magnetic semiconductor (BMS) to half-semiconductor (HSC), spin gapless semiconductor (SGS), half-metal (HM), and even normal metal as the biaxial strain varies from -13 to 10%. Moreover, the easy and hard axes could be switched from each other, and the magnetocrystalline anisotropy (MCA) energy is also controlled by the strains. The Δ is also increased from 158 to 169 meV as the strain varies from 3.3 to -3.0%. The magnetic and electronic phase transitions in the strained VSeTe are observed, which could help researchers to investigate the controllable electronic and magnetic properties in electronics, spintronics, and valleytronics.

Realization of a Buckled Antimonene Monolayer on Ag(111) via Surface Engineering

The degree of buckling of two-dimensional (2D) materials can have a dramatic impact on their corresponding electronic structures. Antimonene (β-phase), a new 2D material with air stability and promising electronic properties, has been engineered to adopt flat or two-heights-buckling geometries by employing different supporting substrates for epitaxial growth. However, studies of the antimonene monolayer with a more buckled configuration are still lacking. Here, we report the synthesis of an antimonene monolayer with a three-heights-buckling configuration overlaid on SbAg₂ surface alloy-covered Ag(111) by molecular beam epitaxy, in which the underlying surface alloy provides interfacial interactions to modulate the structure of the antimonene monolayer. The atomic structure of the synthesized antimonene has been precisely identified through a combination of low-temperature scanning tunneling microscopy and density functional theory calculations. The successful fabrication of a buckled antimonene monolayer could provide a promising way to modulate the structures of 2D materials for future electronic and optoelectronic applications.

Recent advances on TMDCs for medical diagnosis

Transition metal dichalcogenides (TMDCs), such as MoS₂ and WS₂, have attracted much attention in biosensing and bioimaging due to its excellent stability, biocompatibility, high specific surface area, and wide varieties. In this review, we overviewed the application of TMDCs in biosensing and bioimaging. Firstly, the synthesis methods and surface functionalization methods of TMDCs were summarized. Secondly, according to the working mechanism, we classified and gave a detailed account of the latest research progress of TMDC-based biosensing for the detection of the enzyme, DNA, and other biological molecules. Then, we outlined the recent progress of applying TMDCs in bio-imaging, including fluorescence, X-ray computed tomographic, magnetic response imaging, photographic and multimodal imaging, respectively. Finally, we discussed the future challenges and development direction of the application of TMDCs in medical diagnosis. Also, we put forward our view on the opportunity of TMDCs in the big data of modern medical diagnosis.

Two-Dimensional Black Phosphorus Nanomaterials: Emerging Advances in Electrochemical Energy Storage Science

Two-dimensional black phosphorus (2D BP), well known as phosphorene, has triggered tremendous attention since the first discovery in 2014. The unique puckered monolayer structure endows 2D BP intriguing properties, which facilitate its potential applications in various fields, such as catalyst, energy storage, sensor, etc. Owing to the large surface area, good electric conductivity, and high theoretical specific capacity, 2D BP has been widely studied as electrode materials and significantly enhanced the performance of energy storage devices. With the rapid development of energy storage devices based on 2D BP, a timely review on this topic is in demand to further extend the application of 2D BP in energy storage. In this review, recent advances in experimental and theoretical development of 2D BP are presented along with its structures, properties, and synthetic methods. Particularly, their emerging applications in electrochemical energy storage, including Li-/K-/Mg-/Na-ion, Li-S batteries, and supercapacitors, are systematically summarized with milestones as well as the challenges. Benefited from the fast-growing dynamic investigation of 2D BP, some possible improvements and constructive perspectives are provided to guide the design of 2D BP-based energy storage devices with high performance.

Nanohybrid Membrane Synthesis with Phosphorene Nanoparticles: A Study of the Addition, Stability and Toxicity

Phosphorene is a promising candidate as a membrane material additive because of its inherent photocatalytic properties and electrical conductance which can help reduce fouling and improve membrane properties. The main objective of this study was to characterize structural and morphologic changes arising from the addition of phosphorene to polymeric membranes. Here, phosphorene was physically incorporated into a blend of polysulfone (PSf) and sulfonated poly ether ether ketone (SPEEK) doping solution. Protein and dye rejection studies were carried out to determine the permeability and selectivity of the membranes. Since loss of material additives during filtration processes is a challenge, the stability of phosphorene nanoparticles in different environments was also examined. Furthermore, given that phosphorene is a new material, toxicity studies with a model nematode, Caenorhabditis elegans, were carried out to provide insight into the biocompatibility and safety of phosphorene. Results showed that membranes modified with phosphorene displayed a higher protein rejection, but lower flux values. Phosphorene also led to a 70% reduction in dye fouling after filtration. Additionally, data showed that phosphorene loss was negligible within the membrane matrix irrespective of the pH environment. Phosphorene caused toxicity to nematodes in a free form, while no toxicity was observed for membrane permeates.

Strain-Engineered Anisotropic Optical and Electrical Properties in 2D Chiral-Chain Tellurium

Atomically thin materials, leveraging their low-dimensional geometries and superior mechanical properties, are amenable to exquisite strain manipulation with a broad tunability inaccessible to bulk or thin-film materials. Such capability offers unexplored possibilities for probing intriguing physics and materials science in the 2D limit as well as enabling unprecedented device applications. Here, the strain-engineered anisotropic optical and electrical properties in solution-grown, sub-millimeter-size 2D Te are systematically investigated through designing and introducing a controlled buckled geometry in its intriguing chiral-chain lattice. The observed Raman spectra reveal anisotropic lattice vibrations under the corresponding straining conditions. The feasibility of using buckled 2D Te for ultrastretchable strain sensors with a high gauge factor (≈380) is further explored. 2D Te is an emerging material boasting attractive characteristics for electronics, sensors, quantum devices, and optoelectronics. The results suggest the potential of 2D Te as a promising candidate for designing and implementing flexible and stretchable devices with strain-engineered functionalities.

Homogeneous 2D MoTe2 CMOS Inverters and p-n Junctions Formed by Laser-Irradiation-Induced p-Type Doping

Among all typical transition-metal dichalcogenides (TMDs), the bandgap of α-MoTe₂ is smallest and is close to that of conventional 3D Si. The properties of α-MoTe₂ make it a favorable candidate for future electronic devices. Even though there are a few reports regarding fabrication of complementary metal-oxide-semiconductor (CMOS) inverters or p-n junction by controlling the charge-carrier polarity of TMDs, the fabrication process is complicated. Here, a straightforward selective doping technique is demonstrated to fabricate a 2D p-n junction diode and CMOS inverter on a single α-MoTe₂ nanoflake. The n-doped channel of a single α-MoTe₂ nanoflake is selectively converted to a p-doped region via laser-irradiation-induced MoO_x doping. The homogeneous 2D MoTe₂ CMOS inverter has a high DC voltage gain of 28, desirable noise margin (NM_H = 0.52 V_DD , NM_L = 0.40 V_DD ), and an AC gain of 4 at 10 kHz. The results show that the doping technique by laser scan can be potentially used for future larger-scale MoTe₂ CMOS circuits.

Emerging pnictogen-based 2D semiconductors: sensing and electronic devices

Pnictogens are an intensively studied group of monoelemental two-dimensional materials. This group of elements consists of phosphorus, arsenic, antimony, and bismuth. In this group, the elements adopt two different layered structural allotropes, orthorhombic structure with true van der Waals layered interactions and rhombohedral structure, where covalent interactions between layers are also present. The orthorhombic structure is well known for phosphorus and arsenic, and the rhombohedral structure is the most thermodynamically stable allotropic modification of arsenic, antimony, and bismuth. Due to the electronic structure of pnictogen layers and their semiconducting character, these materials have huge application potential for electronic devices such as transistors and sensors including photosensitive devices as well as gas and electrochemical sensors. While photodetection and gas sensing applications are often related to lithography processed materials, chemical sensing proceeds in a liquid environment (either aqueous or non-aqueous) and can be influenced by surface oxidation of these materials. In this review, we explore the current state of pnictogen applications in sensing and electronic devices including transistors, photodetectors, gas sensors, and chemical/electrochemical sensors.

Giant Thickness-Tunable Bandgap and Robust Air Stability of 2D Palladium Diselenide

Uncovering the thickness-dependent electronic property and environmental stability for 2D materials are crucial issues for promoting their applications in high-performance electronic and optoelectronic devices. Herein, the extrahigh air stability and giant tunable electronic bandgap of chemical vapor deposition (CVD)-derived few-layer PdSe₂ on Au foils, by using scanning tunneling microscope/spectroscopy (STM/STS), are reported. The robust stability of 2D PdSe₂ is uncovered by the observation of nearly defect/adsorption-free atomic lattices on long-time air-exposed samples. A one-to-one correspondence between the electronic bandgap (from ≈1.15 to ≈0 eV) and thickness of PdSe₂ /Au (from bilayer to bulk) is established. It is also revealed that few-layer semiconducting PdSe₂ flakes present zero-gap edges, induced by hybridization of Pd 4d and Se 4p orbitals. This work hereby provides straightforward evidence for the thickness-tunable electronic property and air stability of 2D semiconductors, thus shedding light on their applications in next-generation electronic devices.

Natural antioxidant functionalization for fabricating ambient-stable black phosphorus nanosheets toward enhancing flame retardancy and toxic gases suppression of polyurethane

Herein, as a natural antioxidant, tannin (TA) is firstly used to functionalize black phosphorous (BP) nanosheets to improve the ambient stability and toxic suppression, thus decreasing the fire hazards of polymer materials. Compared to pure BP nanosheets, higher temperature for thermal oxidation decomposition is achieved for TA-BP nanosheets, directly confirming the ambient stability of TA-BP nanosheets. Meanwhile, from high resolution TEM and XPS results, TA-BP nanosheets after being exposed at air for 10 days present well-organized crystal structure and low PO_x bonds content. Cone calorimeter results illustrate that the incorporation of 2.0 wt% TA-BP nanosheets significantly decreases the peak value of heat release rate (-56.5 %), total heat release (-43.0 %), CO₂ concentration (-57.3 %) of TPU composite. Meanwhile, with addition of low to 1.5 wt%, the release of highly-toxic CO gas is significantly suppressed, confirmed by lower peak value (0.52 mg/m³) and decreased total release amount (-55.1 %). The obviously enlarged tensile strength (36.7 MPa) and desirable elongation at break (622 %) are also observed. This strategy not only firstly adopts bio-based antioxidant to impart excellent environmental stability for BP nanosheets, but also promotes the promising potentials of BP nanosheets in the fire safety application of polymer composites.

Ultrafast 27 GHz cutoff frequency in vertical WSe2 Schottky diodes with extremely low contact resistance

Ultra-thin two-dimensional semiconducting crystals in their monolayer and few-layer forms show promising aspects in nanoelectronic applications. However, the ultra-thin nature of two-dimensional crystals inevitably results in high contact resistance from limited channel/contact volume as well as device-to-device variability, which seriously limit reliable applications using two-dimensional semiconductors. Here, we incorporate rather thick two-dimensional layered semiconducting crystals for reliable vertical diodes showing excellent Ohmic and Schottky contacts. Using the vertical transport of WSe₂, we demonstrate devices which are functional at various frequency ranges from megahertz AM demodulation of audio signals, to gigahertz rectification for fifth-generation wireless electronics, to ultraviolet-visible photodetection. The WSe₂ exhibits an excellent Ohmic contact to bottom platinum electrode with record-low contact resistance (~50 Ω) and an exemplary Schottky junction to top transparent conducting oxide electrode. Our semitransparent vertical WSe₂ Schottky diodes could be a key component of future high frequency electronics in the era of fifth-generation wireless communication.

An overview of the optical properties and applications of black phosphorus

Since the year 2014, when scientists first obtained black phosphorus using a sticky tape to peel the layers off, it has attracted tremendous interest as a novel two-dimensional material. After it was successfully produced, its outstanding optical properties have been unveiled. Various applications based on these properties have been reported. This study mainly reviews the unique optical properties and potential applications of black phosphorus. The optical performances of black phosphorus mainly include linear optical properties and nonlinear optical properties. Some examples include the anisotropic optical response, saturable absorption effect and Kerr effect. The researchers found that the nonlinear saturable absorption coefficients of black phosphorus are better than that of MoS₂ and WS₂ from the visible region to the near-infrared region. Compared with graphene, black phosphorus has a better nonlinear saturable absorption performance. After passivation or surface modification, black phosphorus is stable when exposed to oxygen and water. Herein, black phosphorus has the potential to be used in detector/sensors, solar energy harvesting, photocatalysts, optical saturable absorbers in ultrafast lasers, all optical switches, optical modulation, nanomedicine and some others in the near future.

In-plane anisotropic electronics based on low-symmetry 2D materials: progress and prospects

Low-symmetry layered materials such as black phosphorus (BP) have been revived recently due to their high intrinsic mobility and in-plane anisotropic properties, which can be used in anisotropic electronic and optoelectronic devices. Since the anisotropic properties have a close relationship with their anisotropic structural characters, especially for materials with low-symmetry, exploring new low-symmetry layered materials and investigating their anisotropic properties have inspired numerous research efforts. In this paper, we review the recent experimental progresses on low-symmetry layered materials and their corresponding anisotropic electrical transport, magneto-transport, optoelectronic, thermoelectric, ferroelectric, and piezoelectric properties. The boom of new low-symmetry layered materials with high anisotropy could open up considerable possibilities for next-generation anisotropic multifunctional electronic devices.

Two-Dimensional Pnictogen for Field-Effect Transistors

Two-dimensional (2D) layered materials hold great promise for various future electronic and optoelectronic devices that traditional semiconductors cannot afford. 2D pnictogen, group-VA atomic sheet (including phosphorene, arsenene, antimonene, and bismuthene) is believed to be a competitive candidate for next-generation logic devices. This is due to their intriguing physical and chemical properties, such as tunable midrange bandgap and controllable stability. Since the first black phosphorus field-effect transistor (FET) demo in 2014, there has been abundant exciting research advancement on the fundamental properties, preparation methods, and related electronic applications of 2D pnictogen. Herein, we review the recent progress in both material and device aspects of 2D pnictogen FETs. This includes a brief survey on the crystal structure, electronic properties and synthesis, or growth experiments. With more device orientation, this review emphasizes experimental fabrication, performance enhancing approaches, and configuration engineering of 2D pnictogen FETs. At the end, this review outlines current challenges and prospects for 2D pnictogen FETs as a potential platform for novel nanoelectronics.

Chemical vapor transport growth of bulk black phosphorus single crystals

Recent progress in growth of bulk black phosphorus single crystal by CVT method has been briefly reviewed with the emphasis on reaction system, nucleation and growth mechanism as well as advancement in growth of doped BP bulk single crystal.

Passivation of Layered Gallium Telluride by Double Encapsulation with Graphene

Layered semiconductor gallium telluride (GaTe) undergoes a rapid structural transition to a degraded phase in ambient conditions, limiting its utility in devices such as optical switches. In this work, we demonstrate that the degradation process in GaTe flakes can be slowed down dramatically via encapsulation with graphene. Through examining Raman signatures of degradation, we show that the choice of substrate significantly impacts the degradation rate and that the process is accelerated by the transfer of GaTe to hydrophilic substrates such as SiO₂/Si. We find that double encapsulation with both top and bottom graphene layers can extend the lifetime of the material for several weeks. The photoresponse of flakes encapsulated in this way is only reduced by 17.6 ± 0.4% after 2 weeks, whereas unencapsulated flakes display no response after this time. Our results demonstrate the potential for alternative, van der Waals material-based passivation strategies in unstable layered materials and highlight the need for careful selection of substrates for 2D electronic devices.

Raman and electrical transport properties of few-layered arsenic-doped black phosphorus

Black phosphorus (b-P) is an allotrope of phosphorus whose properties have attracted great attention. In contrast to other 2D compounds, or pristine b-P, the properties of b-P alloys have yet to be explored. In this report, we present a detailed study on the Raman spectra and on the temperature dependence of the electrical transport properties of As-doped black phosphorus (b-AsP) for an As fraction x = 0.25. The observed complex Raman spectra were interpreted with the support of Density Functional Theory (DFT) calculations since each original mode splits in three due to P-P, P-As, and As-As bonds. Field-effect transistors (FET) fabricated from few-layered b-AsP exfoliated onto Si/SiO2 substrates exhibit hole-doped like conduction with a room temperature ON/OFF current ratio of ∼103 and an intrinsic field-effect mobility approaching ∼300 cm2 V-1 s-1 at 300 K which increases up to 600 cm2 V-1 s-1 at 100 K when measured via a 4-terminal method. Remarkably, these values are comparable to, or higher, than those initially reported for pristine b-P, indicating that this level of As doping is not detrimental to its transport properties. The ON to OFF current ratio is observed to increase up to 105 at 4 K. At high gate voltages b-AsP displays metallic behavior with the resistivity decreasing with decreasing temperature and saturating below T ∼100 K, indicating a gate-induced insulator to metal transition. Similarly to pristine b-P, its transport properties reveal a high anisotropy between armchair (AC) and zig-zag (ZZ) directions. Electronic band structure computed through periodic dispersion-corrected hybrid Density Functional Theory (DFT) indicate close proximity between the Fermi level and the top of the valence band(s) thus explaining its hole doped character. Our study shows that b-AsP has potential for optoelectronics applications that benefit from its anisotropic character and the ability to tune its band gap as a function of the number of layers and As content.