Plasma-Treated Thickness-Controlled Two-Dimensional Black Phosphorus and Its Electronic Transport Properties

An updated review of few-layer black phosphorus serving as a promising photocatalyst: synthesis, modification and applications

Semiconductor photocatalysts represent a potential strategy to simultaneously solve the global energy shortage and environmental pollution, and black phosphorus (BP) has gained widespread applications in photocatalysis due to its high hole mobility, strong light trapping capabilities, and adjustable band gap. Nevertheless, the original material exhibits unsatisfactory photocatalytic activity in terms of low carrier separation efficiency, weak environmental stability, and difficult to control layer thickness. The following review briefly presents the fundamental characteristics and extensively discusses the synthesis methods and modification strategies for few-layer black phosphorus (FL-BP). Furthermore, various applications of composite photocatalysts derived from FL-BP such as water splitting, pollutant degradation, the carbon dioxide reduction reaction (CO2RR), phototherapy, bacterial disinfection, N2 fixation, and hydrogenation reactions are reviewed. Finally, the opportunities and challenges for the development and further investigation of advanced FL-BP-based photocatalysts are also presented.

Current advances in black phosphorus-based antibacterial nanoplatform for infection therpy

Black phosphorus (BP) has attracted much attention due to its excellent physiochemical properties. However, due to its biodegradability and simple antibacterial mechanism, using only BP nanomaterials to combat bacterial infections caused by drug-resistant pathogens remains a significant challenge. In order to improve the antibacterial efficiency and avoid the emergence of drug resistance, BP nanomaterials have been combined with other functional materials to form black phosphorus-based antibacterial nanoplatform (BPANP), which provides unprecedented opportunities for the treatment of drug-resistant infections. This article reviews the performance of BPANP and its multiple antibacterial mechanisms while emphatically introducing its design direction and latest application progress in antibacterial fields. Moreover, this paper additionally summarizes and discusses the current challenges and inadequacies of BPANP that need to be improved in future research. We believe that this review will provide researchers with an up-to-date and multifaceted reference, and provide new ideas for designing effective strategies against drug-resistant bacteria.

Polymer Dielectric-Based Emerging Devices: Advancements in Memory, Field-Effect Transistor, and Nanogenerator Technologies

Polymer dielectric materials have recently attracted attention for their versatile applications in emerging electronic devices such as memory, field-effect transistors (FETs), and triboelectric nanogenerators (TENGs). This review highlights the advances in polymer dielectric materials and their integration into these devices, emphasizing their unique electrical, mechanical, and thermal properties that enable high performance and flexibility. By exploring their roles in self-sustaining technologies (e.g., artificial intelligence (AI) and Internet of Everything (IoE)), this review emphasizes the importance of polymer dielectric materials in enabling low-power, flexible, and sustainable electronic devices. The discussion covers design strategies to improve the dielectric constant, charge trapping, and overall device stability. Specific challenges, such as optimizing electrical properties, ensuring process scalability, and enhancing environmental stability, are also addressed. In addition, the review explores the synergistic integration of memory devices, FETs, and TENGs, focusing on their potential in flexible and wearable electronics, self-powered systems, and sustainable technologies. This review provides a comprehensive overview of the current state and prospects of polymer dielectric-based devices in advanced electronic applications by examining recent research breakthroughs and identifying future opportunities.

Full Two-Dimensional Ambipolar Field-Effect Transistors for Transparent and Flexible Electronics

The unique features of two-dimensional (2D) materials provide significant opportunities for the development of transparent and flexible electronics. Recently, ambipolar 2D semiconductors have advanced innovative applications such as CMOS-like circuits, reconfigurable circuits, and ultrafast neuromorphic image sensors. Here, we report on the fabrication of full 2D ambipolar field-effect transistors (FETs), in which graphene serves as the source/drain/gate electrodes, WSe2 is for the channel, and h-BN is for the dielectric. The produced ambipolar FETs exhibit comparable on-currents in the n-branch and p-branch with on/off ratios up to 108. By using two ambipolar FETs in series, a CMOS-like inverter is demonstrated with a maximum gain of up to 147, which can work in both the first and third quadrants by controlling the supply voltages and input voltages. The full 2D ambipolar FETs yield a transmittance of over 70% for visible light on transparent glass and achieve a curvature radius of less than 0.5 cm for bending on polyethylene terephthalate (PET) substrate. The work is helpful for the application of ambipolar 2D materials-based devices in transparent and flexible electronics.

Emerging 2D Nanomaterials-Integrated Hydrogels: Advancements in Designing Theragenerative Materials for Bone Regeneration and Disease Therapy

This review highlights recent advancements in the synthesis, processing, properties, and applications of 2D-material integrated hydrogels, with a focus on their performance in bone-related applications. Various synthesis methods and types of 2D nanomaterials, including graphene, graphene oxide, transition metal dichalcogenides, black phosphorus, and MXene are discussed, along with strategies for their incorporation into hydrogel matrices. These composite hydrogels exhibit tunable mechanical properties, high surface area, strong near-infrared (NIR) photon absorption and controlled release capabilities, making them suitable for a range of regeneration and therapeutic applications. In cancer therapy, 2D-material-based hydrogels show promise for photothermal and photodynamic therapies, and drug delivery (chemotherapy). The photothermal properties of these materials enable selective tumor ablation upon NIR irradiation, while their high drug-loading capacity facilitates targeted and controlled release of chemotherapeutic agents. Additionally, 2D-materials -infused hydrogels exhibit potent antibacterial activity, making them effective against multidrug-resistant infections and disruption of biofilm generated on implant surface. Moreover, their synergistic therapy approach combines multiple treatment modalities such as photothermal, chemo, and immunotherapy to enhance therapeutic outcomes. In bio-imaging, these materials serve as versatile contrast agents and imaging probes, enabling their real-time monitoring during tumor imaging. Furthermore, in bone regeneration, most 2D-materials incorporated hydrogels promote osteogenesis and tissue regeneration, offering potential solutions for bone defects repair. Overall, the integration of 2D materials into hydrogels presents a promising platform for developing multifunctional theragenerative biomaterials.

Flexible and Ultra Low Weight Energy Harvesters Based on 2D Phosphorene or Black phosphorus (BP): Current and Futuristic Prospects

Phosphorene, or two-dimensional (2D) black phosphorus, has recently emerged as a competitor of graphene as it offers several advantages, including a tunable band gap, higher on/off current ratio, piezoelectric nature, and biocompatibility. Researchers have succeeded in obtaining several forms of phosphorene, such as nanosheets, nanorods, nanoribbons, and quantum dots, with satisfactory yields. Nanostructures with various controlled properties have been fabricated in multiple devices for energy production. These phosphorene-based devices are lightweight, flexible, and efficient, demonstrating great potential for energy-harvesting applications in sensors and nanogenerators. While ongoing exploration and advancements continue for these lightweight energy harvesters, it is essential to review the current progress in order to develop a future roadmap for the potential use of these phosphorene-based energy harvesters in space programs. They could be employed in applications such as wearable devices for astronauts, where ultralow weight is a vital component of any integrated device. This review also anticipates the growing significance of phosphorene in various emerging applications such as robots, information storage devices, and artificial intelligence.

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.

Wet-Chemical Synthesis of Elemental 2D Materials

Wet-chemical synthesis refers to the bottom-up chemical synthesis in solution, which is among the most popular synthetic approaches towards functional two-dimensional (2D) materials. It offers several advantages, including cost-effectiveness, high yields,, precious control over the production process. As an emerging family of 2D materials, elemental 2D materials (Xenes) have shown great potential in various applications such as electronics, catalysts, biochemistry,, sensing technologies due to their exceptional/exotic properties such as large surface area, tunable band gap,, high carrier mobility. In this review, we provide a comprehensive overview of the current state-of-the-art in wet-chemical synthesis of Xenes including tellurene, bismuthene, antimonene, phosphorene,, arsenene. The current solvent compositions, process parameters utilized in wet-chemical synthesis, their effects on the thickness, stability of the resulting Xenes are also presented. Key factors considered involves ligands, precursors, surfactants, reaction time, temperature. Finally, we highlight recent advances, existing challenges in the current application of wet-chemical synthesis for Xenes production, provide perspectives on future improvement.

Black phosphorus, an advanced versatile nanoparticles of antitumor, antibacterial and bone regeneration for OS therapy

Osteosarcoma (OS) is the most common primary malignant bone tumor. In the clinic, usual strategies for OS treatment include surgery, chemotherapy, and radiation. However, all of these therapies have complications that cannot be ignored. Therefore, the search for better OS treatments is urgent. Black phosphorus (BP), a rising star of 2D inorganic nanoparticles, has shown excellent results in OS therapy due to its outstanding photothermal, photodynamic, biodegradable and biocompatible properties. This review aims to present current advances in the use of BP nanoparticles in OS therapy, including the synthesis of BP nanoparticles, properties of BP nanoparticles, types of BP nanoparticles, and modification strategies for BP nanoparticles. In addition, we have discussed comprehensively the application of BP in OS therapy, including single, dual, and multimodal synergistic OS therapies, as well as studies about bone regeneration and antibacterial properties. Finally, we have summarized the conclusions, limitations and perspectives of BP nanoparticles for OS therapy.

The prediction of two-dimensional PbN: opened bandgap in heterostructure with CdO

The development of two-dimensional (2D) materials has received wide attention as a generation of optoelectronics, thermoelectric, and other applications. In this study, a novel 2D material, PbN, is proposed as an elemental method using the prototype of a recent reported nitride (J. Phys. Chem. C 2023, 127, 43, 21,006-21014). Based on first-principle calculations, the PbN monolayer is investigated as stable at 900 K, and the isotropic mechanical behavior is addressed by the Young's modulus and Poisson's ratio at 67.4 N m-1 and 0.15, respectively. The PbN monolayer also presents excellent catalytic performance with Gibbs free energy of 0.41 eV. Zero bandgap is found for the PbN monolayer, and it can be opened at about 0.128 eV by forming a heterostructure with CdO. Furthermore, the PbN/CdO is constructed by Van der Waals interaction, while the apparent potential drop and charge transfer are investigated at the interface. The PbN/CdO heterostructure also possesses excellent light absorption properties. The results provide theoretical guidance for the design of layered functional materials.

Photodegradation and van der Waals Passivation of Violet Phosphorus

Violet phosphorus (VP), a novel two-dimensional (2D) nanomaterial, boasts structural anisotropy, a tunable optical bandgap, and superior thermal stability compared with its allotropes. Its multifunctionality has sparked widespread interest in the community. Yet, the VP's air susceptibility impedes both probing its intrinsic features and device integration, thus making it of urgent significance to unveil the degradation mechanism. Herein, we conduct a comprehensive study of photoactivated degradation effects on VP. A nitrogen annealing method is presented for the effective elimination of surface adsorbates from VP, as evidenced by a giant surface-roughness improvement from 65.639 nm to 7.09 nm, enabling direct observation of the intrinsic morphology changes induced by photodegradation. Laser illumination demonstrates a significant thickness-thinning effect on VP, manifested in the remarkable morphological changes and the 73% quenching of PL intensity within 160 s, implying its great potential for the efficient selected-area etching of VP at high resolution. Furthermore, van der Waals passivation of VP using 2D hexagonal boron nitride (hBN) was achieved. The hBN-passivated channel exhibited improved surface roughness (0.512 nm), reduced photocurrent hysteresis, and lower responsivity (0.11 A/W @ 450 nm; 2 μW), effectively excluding adsorbate-induced electrical and optoelectrical effects while disabling photodegradation. Based on our experimental results, we conclude that three possible factors contribute to the photodegradation of VP: illumination with photon energy higher than the bandgap, adsorbed H2O, and adsorbed O2.

The Roadmap of 2D Materials and Devices Toward Chips

Due to the constraints imposed by physical effects and performance degradation, silicon-based chip technology is facing certain limitations in sustaining the advancement of Moore's law. Two-dimensional (2D) materials have emerged as highly promising candidates for the post-Moore era, offering significant potential in domains such as integrated circuits and next-generation computing. Here, in this review, the progress of 2D semiconductors in process engineering and various electronic applications are summarized. A careful introduction of material synthesis, transistor engineering focused on device configuration, dielectric engineering, contact engineering, and material integration are given first. Then 2D transistors for certain electronic applications including digital and analog circuits, heterogeneous integration chips, and sensing circuits are discussed. Moreover, several promising applications (artificial intelligence chips and quantum chips) based on specific mechanism devices are introduced. Finally, the challenges for 2D materials encountered in achieving circuit-level or system-level applications are analyzed, and potential development pathways or roadmaps are further speculated and outlooked.

Reversible Stacking of 2D ZnIn2 S4 Atomic Layers for Enhanced Photocatalytic Hydrogen Evolution

It is technically challenging to reversibly tune the layer number of 2D materials in the solution. Herein, a facile concentration modulation strategy is demonstrated to reversibly tailor the aggregation state of 2D ZnIn2 S4 (ZIS) atomic layers, and they are implemented for effective photocatalytic hydrogen (H2 ) evolution. By adjusting the colloidal concentration of ZIS (ZIS-X, X = 0.09, 0.25, or 3.0 mg mL-1 ), ZIS atomic layers exhibit the significant aggregation of (006) facet stacking in the solution, leading to the bandgap shift from 3.21 to 2.66 eV. The colloidal stacked layers are further assembled into hollow microsphere after freeze-drying the solution into solid powders, which can be redispersed into colloidal solution with reversibility. The photocatalytic hydrogen evolution of ZIS-X colloids is evaluated, and the slightly aggregated ZIS-0.25 displays the enhanced photocatalytic H2 evolution rates (1.11 µmol m-2 h-1 ). The charge-transfer/recombination dynamics are characterized by time-resolved photoluminescence (TRPL) spectroscopy, and ZIS-0.25 displays the longest lifetime (5.55 µs), consistent with the best photocatalytic performance. This work provides a facile, consecutive, and reversible strategy for regulating the photo-electrochemical properties of 2D ZIS, which is beneficial for efficient solar energy conversion.

Combination of Polymer Gate Dielectric and Two-Dimensional Semiconductor for Emerging Field-Effect Transistors

Two-dimensional (2D) materials are considered attractive semiconducting layers for emerging field-effect transistors owing to their unique electronic and optoelectronic properties. Polymers have been utilized in combination with 2D semiconductors as gate dielectric layers in field-effect transistors (FETs). Despite their distinctive advantages, the applicability of polymer gate dielectric materials for 2D semiconductor FETs has rarely been discussed in a comprehensive manner. Therefore, this paper reviews recent progress relating to 2D semiconductor FETs based on a wide range of polymeric gate dielectric materials, including (1) solution-based polymer dielectrics, (2) vacuum-deposited polymer dielectrics, (3) ferroelectric polymers, and (4) ion gels. Exploiting appropriate materials and corresponding processes, polymer gate dielectrics have enhanced the performance of 2D semiconductor FETs and enabled the development of versatile device structures in energy-efficient ways. Furthermore, FET-based functional electronic devices, such as flash memory devices, photodetectors, ferroelectric memory devices, and flexible electronics, are highlighted in this review. This paper also outlines challenges and opportunities in order to help develop high-performance FETs based on 2D semiconductors and polymer gate dielectrics and realize their practical applications.

Alkene-Catalyzed Rapid Layer-by-Layer Thinning of Black Phosphorus for Precise Nanomanufacturing

Black phosphorus (BP) is a promising material for electronic and optoelectronic applications. However, it is still challenging to obtain geometrically well-defined BP with desirable thickness. The method involving rapid BP surface reaction via alkene-catalyzed oxidation and easy removal of reactants by a mechanical effect was proposed to achieve the precise layer-by-layer thinning and real-time thickness monitoring of BP for nanopatterning with high spatial resolution based on mechanical scanning probe nanolithography. The enhanced electron affinity of oxygen with the assistance of a carbon-carbon double bond (C═C) in the alkene was demonstrated by density functional theory calculations, shortening the BP surface oxidation period by 99%, which provides access for the rapid thinning. The few-layer BP nanoflake with nested structure and arbitrary thickness on various substrates and the nanopatterned heterojunctions (BP/graphene and BP/hexagonal boron nitride) can be precisely fabricated by the adjustment of scanning number under a small load. This thinning technology was efficient and universal, which could be used to fabricate a BP field-effect transistor with a thinned channel to enhance the capability for current modulation, showing great potential applications for designing high-performance nanodevices.

Black Phosphorus: Fundamental Properties and Influence of Impurities Induced by Its Synthesis

Black phosphorus (BP) has been among the most widely explored materials in recent years because of its exceptional properties. A vapor transport method using tin and iodide as mineralizers was used to synthesize large crystals which can be used for fundamental physical characterization including electrical and heat transport and heat capacity. This method is compared to other reported procedures (high-pressure crystal growth and mercury catalysis) which are broadly used and the most dominant procedures for the obtainment of bulk layered BP. In addition, we have investigated any possible impurities which could have been introduced by synthesis and their possible incorporation into BP and their influence on the physical properties of BP.

Defect Engineering in Thickness-Controlled Bi2O2Se-Based Transistors by Argon Plasma Treatment

We present a simple, effective, and controllable method to uniformly thin down the thickness of as-exfoliated two-dimensional Bi₂O₂Se nanoflakes using Ar⁺ plasma treatment. Atomic force microscopy (AFM) images and Raman spectra indicate that the surface morphology and crystalline quality of etched Bi₂O₂Se nanoflakes remain almost unaffected. X-ray photoelectron spectra (XPS) indicate that the O and Se vacancies created during Ar⁺ plasma etching on the top surface of Bi₂O₂Se nanoflakes are passivated by forming an ultrathin oxide layer with UV O₃ treatment. Moreover, a bottom-gate Bi₂O₂Se-based field-effect transistor (FET) was constructed to research the effect of thicknesses and defects on electronic properties. The on-current/off-current (I_on/I_off) ratio of the Bi₂O₂Se FET increases with decreasing Bi₂O₂Se thickness and is further improved by UV O₃ treatment. Eventually, the thickness-controlled Bi₂O₂Se FET achieves a high I_on/I_off ratio of 6.0 × 10⁴ and a high field-effect mobility of 5.7 cm² V^-1 s^-1. Specifically, the variation trend of the I_on/I_off ratio and the electronic transport properties for the bottom-gate Bi₂O₂Se-based FET are well described by a parallel resistor model (including bulk, channel, and defect resistance). Furthermore, the I_ds-V_gs hysteresis and its inversion with UV irradiation were observed. The pulsed gate and drain voltage measurements were used to extract trap time constants and analyze the formation mechanism of different hysteresis. Before UV irradiation, the origin of clockwise hysteresis is attributed to the charge trapping/detrapping of defects at the Bi₂O₂Se/SiO₂ interface and in the Bi₂O₂Se bulk. After UV irradiation, the large anticlockwise hysteresis is mainly due to the tunneling between deep-level oxygen defects in SiO₂ and p⁺⁺-Si gate, which implies the potential in nonvolatile memory.

Study on Black Phosphorus Characteristics Using a Two-Step Thinning Method

A mild two-step method of black phosphorus (BP) flake thinning was demonstrated in this article. Slight ultraviolet-ozone (UVO) radiation followed by an argon plasma treatment was employed to oxidize mechanically exfoliated BP flakes and remove the surface remains of previous ozone treatment. The annealing process introduced aims to reduce impurities and defects. Low damage and efficient electronic devices were fabricated in terms of controlling the thickness of BP flakes through this method. These results lead to an important step toward the fabrication of high-performance devices based on two-dimensioned materials.

Perspective on Micro-Supercapacitors

The rapid development of portable, wearable, and implantable electronic devices greatly stimulated the urgent demand for modern society for multifunctional and miniaturized electrochemical energy storage devices and their integrated microsystems. This article reviews material design and manufacturing technology in different micro-supercapacitors (MSCs) along with devices integrate to achieve the targets of their various applications in recent years. Finally, We also critically prospect the future development directions and challenges of MSCs.

P=O Functionalized Black Phosphorus/1T-WS2 Nanocomposite High Efficiency Hybrid Photocatalyst for Air/Water Pollutant Degradation

Research on layered two-dimensional (2D) materials is at the forefront of material science. Because 2D materialshave variousplate shapes, there is a great deal of research on the layer-by-layer-type junction structure. In this study, we designed a composite catalyst with a dimension lower than two dimensions and with catalysts that canbe combined so that the band structures can be designed to suit various applications and cover for each other’s disadvantages. Among transition metal dichalcogenides, 1T-WS₂ can be a promising catalytic material because of its unique electrical properties. Black phosphorus with properly controlled surface oxidation can act as a redox functional group. We synthesized black phosphorus that was properly surface oxidized by oxygen plasma treatment and made a catalyst for water quality improvement through composite with 1T-WS₂. This photocatalytic activity was highly efficient such that the reaction rate constant k was 10.31 × 10⁻² min⁻¹. In addition, a high-concentration methylene blue solution (20 ppm) was rapidly decomposed after more than 10 cycles and showed photo stability. Designing and fabricating bandgap energy-matching nanocomposite photocatalysts could provide a fundamental direction in solving the future’s clean energy problem.

A multifunctional 2D black phosphorene-based platform for improved photovoltaics

As one of the latest additions to the 2D nanomaterials family, black phosphorene (BP, monolayer or few-layer black phosphorus) has gained much attention in various forms of solar cells. This is due largely to its intriguing semiconducting properties such as tunable direct bandgap (from 0.3 eV in the bulk to 2.0 eV in the monolayer), extremely high ambipolar carrier mobility, broad visible to infrared light absorption, etc. These appealing optoelectronic attributes make BP a multifunctional nanomaterial for use in solar cells via tailoring carrier dynamics, band energy alignment, and light harvesting, thereby promoting the rapid development of third-generation solar cells. Notably, in sharp contrast to the copious work on revealing the fundamental properties of BP, investigation into the utility of BP is comparatively less, particularly in the area of photovoltaics. Herein, we first identify and summarize an array of unique characteristics of BP that underpin its application in photovoltaics, aiming at providing inspiration to develop new designs and device architectures of photovoltaics. Subsequently, state-of-the-art synthetic routes (i.e., top-down and bottom-up) to scalable BP production that facilitates its applications in optoelectronic materials and devices are outlined. Afterward, recent advances in a diverse set of BP-incorporated solar cells, where BP may impart electron and/or hole extraction and transport, function as a light absorber, provide dielectric screening for enhancing exciton dissociation, and modify the morphology of photoabsorbers, are discussed, including organic solar cells, dye-sensitized solar cells, heterojunction solar cells and perovskite solar cells. Finally, the challenges and opportunities in this rapidly evolving field are presented.

Physical insights on transistors based on lateral heterostructures of monolayer and multilayer PtSe2 via Ab initio modelling of interfaces

Lateral heterostructures (LH) of monolayer-multilayer regions of the same noble transition metal dichalcogenide, such as platinum diselenide (PtSe₂), are promising options for the fabrication of efficient two-dimensional field-effect transistors (FETs), by exploiting the dependence of the energy gap on the number of layers and the intrinsically high quality of the heterojunctions. Key for future progress in this direction is understanding the effects of the physics of the lateral interfaces on far-from-equilibrium transport properties. In this work, a multi-scale approach to device simulation, capable to include ab-initio modelling of the interfaces in a computationally efficient way, is presented. As an application, p- and n-type monolayer-multilayer PtSe₂ LH-FETs are investigated, considering design parameters such as channel length, number of layers and junction quality. The simulations suggest that such transistors can provide high performance in terms of subthreshold characteristics and switching behavior, and that a single channel device is not capable, even in the ballistic defectless limit, to satisfy the requirements of the semiconductor roadmap for the next decade, and that stacked channel devices would be required. It is shown how ab-initio modelling of interfaces provides a reliable physical description of charge displacements in their proximity, which can be crucial to correctly predict device transport properties, especially in presence of strong dipoles, mixed stoichiometries or imperfections.

Recent Progress of Two-Dimensional Materials for Ultrafast Photonics

Owing to their extraordinary physical and chemical properties, two-dimensional (2D) materials have aroused extensive attention and have been widely used in photonic and optoelectronic devices, catalytic reactions, and biomedicine. In particular, 2D materials possess a unique bandgap structure and nonlinear optical properties, which can be used as saturable absorbers in ultrafast lasers. Here, we mainly review the top-down and bottom-up methods for preparing 2D materials, such as graphene, topological insulators, transition metal dichalcogenides, black phosphorus, and MXenes. Then, we focus on the ultrafast applications of 2D materials at the typical operating wavelengths of 1, 1.5, 2, and 3 μm. The key parameters and output performance of ultrafast pulsed lasers based on 2D materials are discussed. Furthermore, an outlook regarding the fabrication methods and the development of 2D materials in ultrafast photonics is also presented.

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 of monoelemental 2D materials for photocatalytic applications

As a sustainable environmental governance strategy and energy conversion method, photocatalysis has considered to have great potential in this field due to its excellent optical properties and has become one of the most attractive technologies today. Among 2D materials, the emerging two-dimensional (2D) monoelemental materials mainly distributed in the -IIIA, -IVA, -VA and -VIA groups and show excellent performance in solar energy conversion due to their graphene-like 2D atomic structure and unique properties, thereby drawing increasing attention. This review briefly summarizes the preparation processes and fundamental properties of 2D single-element nanomaterials, as well as various modification strategies and adjustment mechanisms to enhance their photocatalytic properties. In particular, this article comprehensively discusses the related practical applications of 2D single-element materials in the field of photocatalysis, including photocatalytic degradation for contaminants removal, photocatalytic pathogen inactivation, photocatalytic fouling control and photocatalytic energy conversion. This review will provide some new opportunities for the rational design of other excellent photocatalysts based on 2D monoelemental materials, as well as present tremendous novel ideas for 2D monoelemental materials in other environmental conservation and energy-related applications, such as supercapacitors, electrocatalysis, solar cells, and so on.

Extrinsic Photoconduction Induced Short-Wavelength Infrared Photodetectors Based on Ge-Based Chalcogenides

2D layered photodetectors have been widely researched for intriguing optoelectronic properties but their application fields are limited by the bandgap. Extending the detection waveband can significantly enrich functionalities and applications of photodetectors. For example, after breaking through bandgap limitation, extrinsic Si photodetectors are used for short-wavelength infrared or even long-wavelength infrared detection. Utilizing extrinsic photoconduction to extend the detection waveband of 2D layered photodetectors is attractive and desirable. However, extrinsic photoconduction has yet not been observed in 2D layered materials. Here, extrinsic photoconduction-induced short-wavelength infrared photodetectors based on Ge-based chalcogenides are reported for the first time and the effectiveness of intrinsic point defects are demonstrated. The detection waveband of room-temperature extrinsic GeSe photodetectors with the assistance of Ge vacancies is broadened to 1.6 µm. Extrinsic GeSe photodetectors have an excellent external quantum efficiency (0.5%) at the communication band of 1.31 µm and polarization-resolved capability to subwaveband radiation. Moreover, room-temperature extrinsic GeS photodetectors with a detection waveband to the communication band of 1.55 µm further verify the versatility of intrinsic point defects. This approach provides design strategies to enrich the functionalities of 2D layered photodetectors.

Recent advances in long-term stable black phosphorus transistors

Two-dimensional black phosphorus (BP) presents extensive exciting properties attributed to the high mobility and non-dangling bonds uniform surface with simultaneously obtained atomically ultrathin body and offer opportunities beyond the traditional materials. BP has thus emerged as a unique material in the post-silicon era for low-power electronics and photo-electronics. Tremendous efforts have been invested in fully developing the extreme potentiality of BP for future nanoelectronics. However, the accompanying challenges, especially the poor stability that originates from the active surface, in fabricating large-area BP transistors with comparable electrical performance to silicon electronics prevent their practical application. Herein, we review the progress of recent works that demonstrated the feasibility of enhancing the stability of BP electronics, and identify the opportunities and challenges in developing BP as atomically thin semiconductors for next-generation nanoelectronics.

Thickness determination of anisotropic van der Waals crystals by raman spectroscopy: the case of black phosphorus

The large foreseeable use two-dimensional materials in nanotechnology consequently demands precise methods for their thickness measurements. Usually, having a quick and easy methodology is a key requisite for the inspection of the large number of flakes produced by exfoliation methods. An effective option in this respect relies on the measurement of the intensity of Raman spectra, which can be used even when the flakes are encapsulated by a transparent protective layer. However, when using this methodology, special attention should be paid to the crystalline anisotropy of the examined material. Specifically, for the case of black phosphorus flakes, the absolute experimental determination of the thickness is rather difficult because the material is characterized by a low symmetry and also because the Raman tensors are complex quantities. In this work, we exploited Raman spectroscopy to measure the thickness of black phosphorous flakes using silicon as reference material for intensity calibrations. We found out that we can determine the thickness of a flake above 5 nm with an accuracy of about 20%. We tested the reproducibility of the method on two different setups, finding similar results. The method can be applied also to other van der Waals materials with a Raman band characterized by the same Raman tensor.

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.

Intercalation of Nb2O5 nano-flowers into the walls of few-layer black phosphorus creating a heterostructure of FL-BP@Nb2O5 with the potential for environmental application

Herein we report the successful exfoliation of few-layer BP (FL-BP) from bulk BP via ultrasonication in N-methylpyrrolidone (NMP). FL-BP exhibited an orthorhombic phase structure similar to that of bulk BP with weak electrostatic out-of-plane interactions and strong ionic in-plane bonds. The weakened out-of-plane bonds allowed the intercalation of Nb2O5 nano-flowers that were hydrothermally synthesized, forming an intimate contact with the exfoliated BP. The successful formation of the heterointerface was confirmed by the co-existence of crystal phases of both compounds as per the XRD results. The formation of the new intrinsic Nb-P bond was confirmed by the presence of Raman shoulders of both compounds, further substantiated by the XPS analysis. The heterointerface enhanced Nb2O5 light-harvesting capacity as per the UV-vis measurements. The FL-BP's properties of higher carrier effective mass and density were successfully incorporated in the composite, implying an increased flow of electrons in the composite's lattice structure. This was displayed by the great suppression of the fast recombination rate of charge carriers in the composites. The 3% BP@Nb2O5 composite exhibited excellent optoelectrical properties, compared to the other composites, as suggested by the microstrain calculations, PL, and the EIS data. Mott-Schottky plots verified the p-n type heterojunction formed in the composites, and further verified the increased electron density/concentration in the composites, with respect to Nb2O5. Noteworthy, the incorporation of FL-BP in the lattice of Nb2O5 increased the surface area and the pore size and volume, which is a character beneficial for photocatalysis as it presents active sites and diffusion pathways.

A Scalable Method for Thickness and Lateral Engineering of 2D Materials

The physical properties of two-dimensional (2D) materials depend strongly on the number of layers. Hence, methods for controlling their thickness with atomic layer precision are highly desirable, yet still too rare, and demonstrated for only a limited number of 2D materials. Here, we present a simple and scalable method for the continuous layer-by-layer thinning that works for a large class of 2D materials, notably layered germanium pnictides and chalcogenides. It is based on a simple oxidation/etching process, which selectively occurs on the topmost layers. Through a combination of atomic force microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray diffraction experiments we demonstrate the thinning method on germanium arsenide (GeAs), germanium sulfide (GeS), and germanium disulfide (GeS₂). We use first-principles simulation to provide insights into the oxidation mechanism. Our strategy, which could be applied to other classes of 2D materials upon proper choice of the oxidation/etching reagent, supports 2D material-based device applications, e.g., in electronics or optoelectronics, where a precise control over the number of layers (hence over the material's physical properties) is needed. Finally, we also show that when used in combination with lithography, our method can be used to make precise patterns in the 2D materials.

Investigation of the growth of few-layer SnS2 thin films via atomic layer deposition on an O2 plasma-treated substrate

Despite increasing interest in tin disulfide (SnS₂) as a two-dimensional (2D) material due to its promising electrical and optical properties, the surface treatment of silicon dioxide (SiO₂) substrates prior to the atomic layer deposition (ALD) deposition of SnS₂ has not been thoroughly studied. In this paper, we prepared two types of SiO₂ substrates with and without using an O₂ plasma surface treatment and compared the ALD growth behavior of SnS₂ on the SiO₂ substrates. The hydrophilic properties of the two SiO₂ substrates were investigated by x-ray photoelectron spectroscopy and contact angle measurements, which showed that using an O₂ plasma surface treatment tuned the surface to be more hydrophilic. ALD-grown SnS₂ thin films on the two different SiO₂ substrates were characterized by x-ray diffraction, Raman spectroscopy, atomic force microscopy, and x-ray photoelectron spectroscopy. To estimate the exact thickness of the ALD-grown SnS₂ thin films, transmission electron microscopy was used. Our data revealed that using O₂ plasma surface treatment increased the growth rate of the initial ALD stage. Thus, the ALD-grown SnS₂ thin film on the SiO₂ substrate treated with O₂ plasma was thicker than the film grown on the non-treated SiO₂ substrate.

Advanced Black Phosphorus Nanomaterials for Bone Regeneration

Bone regeneration remains a great clinical challenge. Two-dimensional materials, especially graphene and its derivative graphene oxide, have been widely used for bone regeneration. Since its discovery in 2014, black phosphorus (BP) nanomaterials including BP nanosheets and BP quantum dots have attracted considerable scientific attention and are considered as prospective graphene substitutes. BP nanomaterials exhibit numerous advantages such as excellent optical and mechanical properties, electrical conductivity, excellent biocompatibility, and good biodegradation, all of which make them particularly attractive in biomedicine. In this review, we comprehensively summarize recent advances of BP-based nanomaterials in bone regeneration. The advantages are reviewed, the different synthesis methods of BP are summarized, and the applications to promote bone regeneration are highlighted. Finally, the existing challenges and perspectives of BP in bone regeneration are briefly discussed.

Property-Activity Relationship of Black Phosphorus at the Nano-Bio Interface: From Molecules to Organisms

As a novel member of the two-dimensional nanomaterial family, mono- or few-layer black phosphorus (BP) with direct bandgap and high charge carrier mobility is promising in many applications such as microelectronic devices, photoelectronic devices, energy technologies, and catalysis agents. Due to its benign elemental composition (phosphorus), large surface area, electronic/photonic performances, and chemical/biological activities, BP has also demonstrated a great potential in biomedical applications including biosensing, photothermal/photodynamic therapies, controlled drug releases, and antibacterial uses. The nature of the BP-bio interface is comprised of dynamic contacts between nanomaterials (NMs) and biological systems, where BP and the biological system interact. The physicochemical interactions at the nano-bio interface play a critical role in the biological effects of NMs. In this review, we discuss the interface in the context of BP as a nanomaterial and its unique physicochemical properties that may affect its biological effects. Herein, we comprehensively reviewed the recent studies on the interactions between BP and biomolecules, cells, and animals and summarized various cellular responses, inflammatory/immunological effects, as well as other biological outcomes of BP depending on its own physical properties, exposure routes, and biodistribution. In addition, we also discussed the environmental behaviors and potential risks on environmental organisms of BP. Based on accumulating knowledge on the BP-bio interfaces, this review also summarizes various safer-by-design strategies to change the physicochemical properties including chemical stability and nano-bio interactions, which are critical in tuning the biological behaviors of BP. The better understanding of the biological activity of BP at BP-bio interfaces and corresponding methods to overcome the challenges would promote its future exploration in terms of bringing this new nanomaterial to practical applications.

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.

Fabrication and Imaging of Monolayer Phosphorene with Preferred Edge Configurations via Graphene-Assisted Layer-by-Layer Thinning

Phosphorene, a monolayer of black phosphorus (BP), is an elemental two-dimensional material with interesting physical properties, such as high charge carrier mobility and exotic anisotropic in-plane properties. To fundamentally understand these various physical properties, it is critically important to conduct an atomic-scale structural investigation of phosphorene, particularly regarding various defects and preferred edge configurations. However, it has been challenging to investigate mono- and few-layer phosphorene because of technical difficulties arising in the preparation of a high-quality sample and damages induced during the characterization process. Here, we successfully fabricate high-quality monolayer phosphorene using a controlled thinning process with transmission electron microscopy and subsequently perform atomic-resolution imaging. Graphene protection suppresses the e-beam-induced damage to multilayer BP and one-side graphene protection facilitates the layer-by-layer thinning of the samples, rendering high-quality monolayer and bilayer regions. We also observe the formation of atomic-scale crystalline edges predominantly aligned along the zigzag and (101) terminations, which is originated from edge kinetics under e-beam-induced sputtering process. Our study demonstrates a new method to image and precisely manipulate the thickness and edge configurations of air-sensitive two-dimensional materials.

Avalanche Carrier Multiplication in Multilayer Black Phosphorus and Avalanche Photodetector

A highly sensitive avalanche photodetector (APD) is fabricated by utilizing the avalanche multiplication mechanism in black phosphorus (BP), where a strong avalanche multiplication of electron-hole pairs is observed. Owing to the small bandgap (0.33 eV) of the multilayer BP, the carrier multiplication occurs at a significantly lower electric field than those of other 2D semiconductor materials. In order to further enhance the quantum efficiency and increase the signal-to-noise (S/N) ratio, Au nanoparticles (NPs) are integrated on the BP surface, which improves the light absorption by plasmonic effects. The BP-Au-NPs structure effectively reduces both dark current (≈10 times lower) and onset of avalanche electric field, leading to higher carrier multiplication, photogain, quantum efficiency, and S/N ratio. For the BP-Au-NPs APD, it is obtained that the external quantum efficiency (EQE) is 382 and the responsivity is 160 A W^-1 at an electric field of 5 kV cm^-1 (V_d ≈ 3.5 V, note that for the BP APD, EQE = 4.77 and responsivity = 2 A W^-1 obtained at the same electric field). The significantly increased performance of the BP APD is promising for low-power-consumption, high-sensitivity, and low-noise photodevice applications, which can enable high-performance optical communication and imaging systems.

Black phosphorus electronics

As the scaling of silicon-based field-effect transistors has approached its physical limits, the search for alternative channel materials for future logic devices has attracted much attention. The discovery of graphene has unveiled another material family with layered structures called two-dimensional (2D) materials. Black phosphorus (BP), the most stable allotrope of phosphorus, was introduced as a new type of 2D material in 2014. Thanks to its high mobility, in-plane anisotropy and direct band gap, BP is considered to be a promising candidate for next-generation electronic and optoelectronic devices. Numerous studies have demonstrated the beneficial effects of introducing BP for device architectures. Herein, we present a review outlining recent progress towards high performance BP-based transistors. This review starts with the fundamental properties of BP, including its crystal structure, bandgap, and direct current (DC) and radio-frequency (RF) characteristics, followed by a detailed description of the modulation and application of those properties, involving anisotropy, functionalization and superlattices. Furthermore, we also discuss device design for high-performance transistors, with particular emphasis on interface engineering and device stability. Finally, we offer our perspective on the future of BP electronics, aiming to benefit colleagues who are interested in this exciting research field.

Electrochemically Exfoliated Functionalized Black Phosphorene and Its Polyurethane Acrylate Nanocomposites: Synthesis and Applications

Owing to its mechanical performance, thermal stability, and size effects, single or few-layer black phosphorus (BP) has the potential to prepare the polymer nanocomposites as a candidate of nanoadditives, similar to graphene. The step to realize the scalable exfoliation of single or few-layer BP nanosheets is crucial to BP applications. Herein, we utilized a facile, green, and scalable electrochemical strategy for generating cobaltous phytate-functionalized BP nanosheets (BP-EC-Exf) wherein the BP crystal served as the cathode and phytic acid served as a modifier and an electrolyte simultaneously. Moreover, high-performance polyurethane acrylate/BP-EC-Exf (PUA/BP-EC) nanocomposites are easily prepared by a convenient UV-curable strategy for the first time. Significantly, the conclusion of introducing BP-EC-Exf into the PUA matrix resulted in enhancement in mechanical properties of PUA in terms of the tensile strength (increased by 59.8%) and tensile fracture strain (increased by 88.1%), in the distinct improvement in flame retardancy of PUA in terms of the decreased peak heat release rate (reduced by 44.5%) and total heat release (decreased by 34.5%), and in lower intensities of pyrolysis products including toxic CO. Moreover, it was confirmed by X-ray diffraction and Raman spectra that the air stability of PUA/BP-EC nanocomposites was maintained after exposure to environmental conditions for 4 months. The air-stable BP nanosheets, which were wrapped and embedded in the PUA matrix, can achieve the isolation and protection effect. This modified electrochemical method toward the simultaneous exfoliation and functionalization of BP nanosheets provides an efficient approach for fabricating BP-polymer-based nanocomposites.

Mono-Elemental Properties of 2D Black Phosphorus Ensure Extended Charge Carrier Lifetimes under Oxidation: Time-Domain Ab Initio Analysis

An attractive two-dimensional semiconductor with tunable direct bandgap and high carrier mobility, black phosphorus (BP), is used in batteries, solar cells, photocatalysis, plasmonics, and optoelectronics. BP is sensitive to ambient conditions, with oxygen playing a critical role in structure degradation. Our simulations show that BP oxidation slows down charge recombination. This is unexpected, since typically charges are trapped and lost on defects. First, BP has no ionic character. It interacts with oxygen and water weakly, experiencing little perturbation to electronic structure. Second, phosphorus supports different oxidation states and binds extraneous atoms avoiding deep defect levels. Third, soft BP structure can accommodate foreign species without disrupting periodic geometry. Finally, BP phonon scattering on defects shortens quantum coherence and suppresses recombination. Thus, oxidation can be regarded as production of a self-protective layer that improves BP properties. These BP features should be common to other monoelemental 2D materials, stimulating energy and electronics applications.

Wet Chemical Method for Black Phosphorus Thinning and Passivation

Layered black phosphorus (BP) has been expected to be a promising material for future electronic and optoelectronic applications since its discovery. However, the difficulty in mass fabricating layered air-stable BP severely obstructs its potential industry applications. Here, we report a new BP chemical modification method to implement all-solution-based mass production of layered air-stable BP. This method uses the combination of two electron-deficient reagents 2,2,6,6-tetramethylpiperidinyl- N-oxyl (TEMPO) and triphenylcarbenium tetrafluorobor ([Ph₃C]BF₄) to accomplish thinning and/or passivation of BP in organic solvent. The field-effect transistor and photodetection devices constructed from the chemically modified BP flakes exhibit enhanced performances with environmental stability up to 4 months. A proof-of-concept BP thin-film transistor fabricated through the all-solution-based exfoliation and modification displays an air-stable and a typical p-type transistor behavior. This all-solution-based method improves the prospects of BP for industry applications.

Black Phosphorus-New Nanostructured Material for Humidity Sensors: Achievements and Limitations

The prospects of using nanostructured black phosphorus for the development of humidity sensors are considered. It was shown that black phosphorus has a set of parameters that distinguish it from other two-dimensional (2D) materials such as graphene, silicone, and dichalcogenides. At the same time, an analysis of shortcomings, limiting the use of black phosphorus as a humidity sensitive material in devices aimed for market of humidity sensors, was also conducted.

Monolayer black phosphorus by sequential wet-chemical surface oxidation

We report a straightforward chemical methodology for controlling the thickness of black phosphorus flakes down to the monolayer limit by layer-by-layer oxidation and thinning, using water as solubilizing agent. Moreover, the oxidation process can be stopped at will by two different passivation procedures, namely the non-covalent functionalization with perylene diimide chromophores, which prevents the photooxidation, or by using a protective ionic liquid layer. The obtained flakes preserve their electronic properties as demonstrated by fabricating a BP field-effect transistor (FET). This work paves the way for the preparation of BP devices with controlled thickness.