Bandgap Engineering of Phosphorene by Laser Oxidation toward Functional 2D Materials

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.

Gap engineering effects on transport and tunneling magnetoresistance properties in phosphorene ferromagnetic/normal/ferromagnetic junction

Tuning the band gap is of utmost importance for the practicality of two-dimensional materials in the semiconductor industry. In this study, we investigate the ballistic transport and the tunneling magnetoresistance (TMR) properties within a modulated gap in a ferromagnetic/normal/ferromagnetic (F/N/F) phosphorene junction. The theoretical framework is established on a Dirac-like Hamiltonian, the transfer matrix method, and the Landauer-Büttiker formalism to characterize electron behavior and obtain transmittance, conductance and TMR. Our results reveal that a reduction in gap energy leads to an enhancement of conductance for both parallel and anti-parallel magnetization configurations. In contrast, a significant reduction and redshift in TMR have been observed. Notably, the application of an electrostatic field in a gapless phosphorene F/N/F junction induces a blueshift and a slight increase in TMR. Furthermore, we found that introducing an asymmetrically applied electrostatic field in this gapless junction results in a significant reduction and redshift in TMR. Additionally, intensifying the applied magnetic field leads to a substantial increase in TMR. These findings could be useful for designing and implementing practical applications that require precise control over the TMR properties of a phosphorene F/N/F junction with a modulated gap.

Laser irradiation induced structural transformation in layered transition metal trichalcogenide nanoflakes

Laser irradiation is a powerful tool in inducing changes in lattice structures and properties of two-dimensional (2D) materials through processes such as heating, bleaching, catalysis, etc. However, the underlying mechanisms of such transformations vary dramatically in different 2D materials. Here, we report the structural transformation of layered titanium trisulfide (TiS3) to titanium disulfide (TiS2) after irradiation. We systematically characterized the dependence of the transformation on laser power, flake thickness, irradiation time, and vacuum conditions using microscopic and spectroscopic methods. The underlying mechanism is confirmed as the heat-induced materials decomposition, a process that also occurs in many other transition metal trichalcogenide materials. Furthermore, we demonstrate that this spatial-resolved method also enables the creation of in-plane TiS3-TiS2 heterostructures. Our study identifies a new family of 2D materials that undergo a structural transformation after laser irradiation and enriches the methods available for developing new prototypes of low-dimensional devices in the future.

Phase Engineering of 2D Materials

Polymorphic 2D materials allow structural and electronic phase engineering, which can be used to realize energy-efficient, cost-effective, and scalable device applications. The phase engineering covers not only conventional structural and metal-insulator transitions but also magnetic states, strongly correlated band structures, and topological phases in rich 2D materials. The methods used for the local phase engineering of 2D materials include various optical, geometrical, and chemical processes as well as traditional thermodynamic approaches. In this Review, we survey the precise manipulation of local phases and phase patterning of 2D materials, particularly with ideal and versatile phase interfaces for electronic and energy device applications. Polymorphic 2D materials and diverse quantum materials with their layered, vertical, and lateral geometries are discussed with an emphasis on the role and use of their phase interfaces. Various phase interfaces have demonstrated superior and unique performance in electronic and energy devices. The phase patterning leads to novel homo- and heterojunction structures of 2D materials with low-dimensional phase boundaries, which highlights their potential for technological breakthroughs in future electronic, quantum, and energy devices. Accordingly, we encourage researchers to investigate and exploit phase patterning in emerging 2D materials.

DFT analysis of the adsorption of bisphenol A (BPA) on pristine and oxidized phosphorene

CONTEXT

Bisphenol A is an endocrine disruptor that may cause harmful effects on human health. Some residues of this compound have been found in water bodies, alerting for its possible risk as an environmental pollutant. Thus, this work proposes the use of pristine and oxidized phosphorene as removers of bisphenol A, through an adsorption mechanism. Our results indicate that the main interactions exhibited by the complexes are hydrogen bonds, van der Waals, and n-π stacking. All complexes show adsorption energies less than -1.08 eV for the gas phase, and -0.65 eV for the aqueous environment, suggesting that the models may be good capturers of this pollutant. According to the electronic properties, the systems are good donators/acceptors of charge; likewise, they are suitable to sense bisphenol A, because of their changes in |LUMO-HOMO| gap energy. The values obtained suggest that the number of oxygen atoms in the models is important for their adsorption capabilities; hence, the modulation in the oxidation is significant to enhance such properties.

METHODOLOGY

Density functional theory calculations were implemented at the PBE-D3/TZVP level of theory in the ORCA 5.0 program, to evaluate the adsorption of bisphenol A on pristine and oxidized phosphorene models and propose the last as removers of this molecule. The visualization of the structures was done in the VMD code.

Band Edges Engineering of 2D/2D Heterostructures: The C3 N4 /Phosphorene Interface

We investigate the interface between carbon nitride (C₃ N₄ ) and phosphorene nanosheets (P-ene) by means of Density Functional Theory (DFT) calculations. C₃ N₄ /P-ene composites have been recently obtained experimentally showing excellent photoactivity. Our results indicate that the formation of the interface is a favorable process driven by Van der Waals forces. The thickness of P-ene nanosheets determines the band edges offsets and the charge carriers' separation. The system is predicted to pass from a nearly type-II to a type-I junction when the thickness of P-ene increases, and the conduction band offset is particularly sensitive. Last, we apply the Transfer Matrix Method to estimate the efficiency for charge carriers' migration as a function of the P-ene thickness.

Direct Laser Irradiation and Modification of 2D Te for Development of Volatile Memristor

Laser irradiation, as a kind of post-fabrication method for two-dimensional (2D) materials, is a promising way to tune the properties of materials and the performance of corresponding nano-devices. As the memristor has been regarded as an excellent candidate for in-memory devices in next-generation computing system, the application of laser irradiation in developing excellent memristor based on 2D materials should be explored deeply. Here, tellurene (Te) flakes are exposed to a 532 nm laser in the air atmosphere to investigate the evolutions of the surface morphology and atom structures under different irradiation parameters. Laser is capable of thinning the flakes, inducing amorphous structures, oxides and defects, and forming nanostructures by controlling the irradiation power and time. Furthermore, the laser-induced oxides and defects promote the migration of metal ions in Te, resulting in the formation of the conductive filaments, which provides the switching behavers of volatile memristor, opening a route to the development of next-generation nano-devices.

Deep ultraviolet spontaneous emission enhanced by layer dependent black phosphorus plasmonics

Although graphene has been the primary material of interest recently for spontaneous emission engineering through the Purcell effect, it features isotropic and thickness-independent optical properties. In contrast, the optical properties of black Phosphorus (BP) are in-plane anisotropic; which supports plasmonic modes and are thickness-dependent, offering an additional degree of freedom for control. Here we investigate how the anisotropy and thickness of BP affect spontaneous emission from a Hydrogenic emitter. We find that the spontaneous emission enhancement rate i.e. Purcell factor (PF) depends on emitter orientation, and PF at a particular frequency and distance can be controlled by BP thickness. At lower frequencies, PF increases with increasing thickness due to infrared (IR) plasmons, which then enhances visible and UV far-field spectra, even at energies greater than 10 eV. By leveraging the thickness and distance-dependent PF, deep UV emission can be switched between 103 nm or 122 nm wavelength from a Hydrogenic emitter. Additionally, we find that doping can significantly tune the PF near BP and this alteration depends on the thickness of the BP. Our work shows that BP is a promising platform for studying strong plasmon-induced light-matter interactions tunable by varying doping levels, emitter orientation, and thickness.

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.

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.

Molding Broadband Dispersion in Twisted Trilayer Hyperbolic Polaritonic Surfaces

Recent advancement in twisted layered metasurfaces can be employed to control the nanoscale flow of light, including the exotic hyperbolic-to-elliptic topological transitions in twisted bilayers (tBL). Such topological transitions can only occur to limited frequency ranges, restricted by the intrinsic in-plane dispersion of individual hyperbolic surfaces. Here, we report that, by controlling interlayer evanescent coupling in twisted polaritonic trilayers, moldable topological transitions of light can be achieved in broadband. We reveal that the required minimum open angle of the individual hyperbolic polaritonic surface for open-to-close topological transitions can be significantly lowered compared to that of the twisted bilayer counterpart. This increases the degree of freedom to enhance and control near-field light-matter interactions and energy management. As an example, we demonstrate a knob to manipulate near-field radiative heat transfer (NFRHT). By rotating the relative angles of trilayers, exotic and tunable thermal conductance can be achieved. Our findings enrich the controllability of light at the nanoscale in broadband, bringing twisted optical materials one step closer to practical applications.

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.

Morphotaxy of Layered van der Waals Materials

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

Optical Modification of 2D Materials: Methods and Applications

2D materials are under extensive research due to their remarkable properties suitable for various optoelectronic, photonic, and biological applications, yet their conventional fabrication methods are typically harsh and cost-ineffective. Optical modification is demonstrated as an effective and scalable method for accurate and local in situ engineering and patterning of 2D materials in ambient conditions. This review focuses on the state of the art of optical modification of 2D materials and their applications. Perspectives for future developments in this field are also discussed, including novel laser tools, new optical modification strategies, and their emerging applications in quantum technologies and biotechnologies.

Visible Out-of-plane Polarized Luminescence and Electronic Resonance in Black Phosphorus

Black phosphorus (BP) is unique among layered materials because of its homonuclear lattice and strong structural anisotropy. While recent investigations on few-layer BP have extensively explored the in-plane (a, c) anisotropy, much less attention has been given to the out-of-plane direction (b). Here, the optical response from bulk BP is probed using polarization-resolved photoluminescence (PL), photoluminescence excitation (PLE), and resonant Raman scattering along the zigzag, out-of-plane, and armchair directions. An unexpected b-polarized luminescence emission is detected in the visible, far above the fundamental gap. PLE indicates that this emission is generated through b-polarized excitation at 2.3 eV. The same electronic resonance is observed in resonant Raman with the enhancement of the A_g phonon modes scattering efficiency. These experimental results are fully consistent with DFT calculations of the permittivity tensor elements and demonstrate the remarkable extent to which the anisotropy influences the optical properties and carrier dynamics in black phosphorus.

Fabrication Technologies for the On-Chip Integration of 2D Materials

With compact footprint, low energy consumption, high scalability, and mass producibility, chip-scale integrated devices are an indispensable part of modern technological change and development. Recent advances in 2D layered materials with their unique structures and distinctive properties have motivated their on-chip integration, yielding a variety of functional devices with superior performance and new features. To realize integrated devices incorporating 2D materials, it requires a diverse range of device fabrication techniques, which are of fundamental importance to achieve good performance and high reproducibility. This paper reviews the state-of-art fabrication techniques for the on-chip integration of 2D materials. First, an overview of the material properties and on-chip applications of 2D materials is provided. Second, different approaches used for integrating 2D materials on chips are comprehensively reviewed, which are categorized into material synthesis, on-chip transfer, film patterning, and property tuning/modification. Third, the methods for integrating 2D van der Waals heterostructures are also discussed and summarized. Finally, the current challenges and future perspectives are highlighted.

Multi-reward Reinforcement Learning Based Bond-Order Potential to Study Strain-Assisted Phase Transitions in Phosphorene

We introduce a multi-reward reinforcement learning (RL) approach to train a flexible bond-order potential (BOP) for 2D phosphorene based on ab initio training data sets. Our approach is based on a continuous action space Monte Carlo tree search algorithm that is general and scalable and presents an efficient multiobjective optimization scheme for high-dimensional materials design problems. As a proof-of-concept, we deploy this scheme to parametrize multiple structural and dynamical properties of 2D phosphorene polymorphs. Our RL-trained BOP model adequately captures the structure, energetics, transformation barriers, equation of state, elastic constants, and phonon dispersions of various 2D P polymorphs. We use this model to probe the impact of temperature and strain rate on the phase transition from black (α-P) to blue phosphorene (β-P) through molecular dynamics simulations. A decrease in critical strain for this phase transition with increase in temperature is observed, and the underlying atomistic mechanisms are discussed.

Two-Dimensional Nanomaterials for the Removal of Pharmaceuticals from Wastewater: A Critical Review

The removal of pharmaceuticals from wastewater is critical due to their considerable risk on ecosystems and human health. Additionally, they are resistant to conventional chemical and biological remediation methods. Two-dimensional nanomaterials are a promising approach to face this challenge due to their combination of high surface areas, high electrical conductivities, and partially optical transparency. This review discusses the state-of-the-art concerning their use as adsorbents, oxidation catalysts or photocatalysts, and electrochemical catalysts for water treatment purposes. The bibliographic search bases upon academic databases including articles published until August 2021. Regarding adsorption, high removal capacities (>200 mg g−1) and short equilibrium times (<30 min) are reported for molybdenum disulfide, metal-organic frameworks, MXenes, and graphene oxide/magnetite nanocomposites, attributed to a strong adsorbate-adsorbent chemical interaction. Concerning photocatalysis, MXenes and carbon nitride heterostructures show enhanced charge carriers separation, favoring the generation of reactive oxygen species to degrade most pharmaceuticals. Peroxymonosulfate activation via pure or photo-assisted catalytic oxidation is promising to completely degrade many compounds in less than 30 min. Future work should be focused on the exploration of greener synthesis methods, regeneration, and recycling at the end-of-life of two-dimensional materials towards their successful large-scale production and application.

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.

Photoluminescence and magnetism integrated multifunctional black phosphorus probes through controllable PO bond orbital hybridization

Biological probes with integrated photoluminescence and magnetism characteristics play a critical role in modern clinical diagnosis and surgical protocols combining fluorescence optical imaging (FOI) with magnetic resonance imaging (MRI) technology. However, traditional magnetic semiconductors can easily generate a spin splitting at the Fermi level and half-metallic electronic occupation, which will sharply reduce the radiation recombination efficiency of photogenerated carriers. To overcome this intrinsic contradiction, we propose a controllable oxidation strategy to introduce some particular PO bonds into black phosphorus nanosheets, in which the p orbital hybridization between P and O atoms not only provides some carrier recombination centers but also leads to a room-temperature spin polarization. As a result, the coexistence of photoluminescence and magnetism is realized in multifunctional black phosphorus probes with excellent biocompatibility. This work provides a new insight into integrating photoluminescence and magnetism together by intriguing atomic orbital hybridization.

Shear-Induced Interfacial Structural Conversion Triggers Macroscale Superlubricity: From Black Phosphorus Nanoflakes to Phosphorus Oxide

As a new two-dimensional (2D) material, black phosphorus (BP) exhibits great potential for friction reduction. However, achieving macroscale superlubricity with a BP coating remains a great challenge. In this study, we designed a new lubrication system to achieve superlubricity with a BP coating at the macroscale, involving the formation of a BP coating with deposited BP nanoflakes, followed by water lubrication. Robust superlubricity with a coefficient of friction of 0.001 can be achieved on the BP coating in a pure water environment. The superlubricity mechanism is mainly attributed to the shear-induced interfacial structural conversion of BP to phosphorus oxide, leading to the formation of tribofilms on the friction pairs with extremely low shear strength. This finding provides a new strategy for achieving superlubricity of 2D material coatings at the macroscale, which has important implications for the development of novel superlubrication systems for industrial applications.

The role of chalcogen vacancies for atomic defect emission in MoS2

For two-dimensional (2D) layered semiconductors, control over atomic defects and understanding of their electronic and optical functionality represent major challenges towards developing a mature semiconductor technology using such materials. Here, we correlate generation, optical spectroscopy, atomic resolution imaging, and ab initio theory of chalcogen vacancies in monolayer MoS2. Chalcogen vacancies are selectively generated by in-vacuo annealing, but also focused ion beam exposure. The defect generation rate, atomic imaging and the optical signatures support this claim. We discriminate the narrow linewidth photoluminescence signatures of vacancies, resulting predominantly from localized defect orbitals, from broad luminescence features in the same spectral range, resulting from adsorbates. Vacancies can be patterned with a precision below 10 nm by ion beams, show single photon emission, and open the possibility for advanced defect engineering of 2D semiconductors at the ultimate scale.

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.

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.

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.

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.

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.

Optical Patterning of Two-Dimensional Materials

Recent advances in the field of two-dimensional (2D) materials have led to new electronic and photonic devices enabled by their unique properties at atomic thickness. Structuring 2D materials into desired patterns on substrates is often an essential and foremost step for the optimum performance of the functional devices. In this regard, optical patterning of 2D materials has received enormous interest due to its advantages of high-throughput, site-specific, and on-demand fabrication. Recent years have witnessed scientific reports of a variety of optical techniques applicable to patterning 2D materials. In this minireview, we present the state-of-the-art optical patterning of 2D materials, including laser thinning, doping, phase transition, oxidation, and ablation. Several applications based on optically patterned 2D materials will be discussed as well. With further developments, optical patterning is expected to hold the key in pushing the frontiers of manufacturing and applications of 2D materials.

Atomic-Scale Fabrication of In-Plane Heterojunctions of Few-Layer MoS2 via In Situ Scanning Transmission Electron Microscopy

Layered MoS₂ is a prospective candidate for use in energy harvesting, valleytronics, and nanoelectronics. Its properties strongly related to its stacking configuration and the number of layers. Due to its atomically thin nature, understanding the atomic-level and structural modifications of 2D transition metal dichalcogenides is still underdeveloped, particularly the spatial control and selective precision. Therefore, the development of nanofabrication techniques is essential. Here, an atomic-scale approach used to sculpt 2D few-layer MoS₂ into lateral heterojunctions via in situ scanning/transmission electron microscopy (STEM/TEM) is developed. The dynamic evolution is tracked using ultrafast and high-resolution filming equipment. The assembly behaviors inherent to few-layer 2D-materials are observed during the process and included the following: scrolling, folding, etching, and restructuring. Atomic resolution STEM is employed to identify the layer variation and stacking sequence for this new 2D-architecture. Subsequent energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy analyses are performed to corroborate the elemental distribution. This sculpting technique that is established allows for the formation of sub-10 nm features, produces diverse nanostructures, and preserves the crystallinity of the material. The lateral heterointerfaces created in this study also pave the way for the design of quantum-relevant geometries, flexible optoelectronics, and energy storage devices.

Tuning the Catalytic Property of Phosphorene for Oxygen Evolution and Reduction Reactions by Changing Oxidation Degree

The development of inexpensive metal-free catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is highly desirable for fuel cells and rechargeable metal-air batteries. Black phosphorus (BP), as a new kind of two-dimensional (2D) layer material, has recently shown excellent OER electrocatalytic activity. However, atomistic understanding of the catalytic mechanism is lacking. Here, on the basis of ab initio calculations, we find that pristine phosphorene shows poor ORR/OER performances. However, oxidation can effectively tune the adsorption strength of reactive intermediates and thus change its OER/ORR electrocatalytic performance. For OER, the higher the local oxidation degree ( D_local^O) of phosphorene, the better the OER activity. Therefore, the oxidized phosphorene site with highest D_local^O shows the best OER catalytic property. In contrast, there exists an optimum D_local^O for ORR. These findings provide new insights for better understanding and improving the catalytic performances of BP-based electrocatalysts and could stimulate more theoretical and experimental studies in this area.

Few-Layer P4O2: A Promising Photocatalyst for Water Splitting

Photocatalytic water splitting by a two-dimensional material is a promising technology for producing clean and renewable energy. Development of this field requires candidate materials with desirable optoelectronic properties. Here, we present a detailed theoretical investigation of the atomic and electronic structure of few-layer P₄O₂ systems to predict their optoelectronic properties. We predict that the three-layer P₄O₂ with normal packing (α-3), ingeniously combining all desired optoelectronic features, is an ideal candidate for photocatalytic water splitting. It fascinatingly bears nearly a direct band gap (1.40 eV), appropriate band edge position, high solar-to-hydrogen efficiency (17.15%), high sunlight absorption efficiency, and ultrahigh carrier mobility (21 460 cm² V^-1 s^-1) at room temperature. These results make three-layer P₄O₂ a promising candidate for photocatalytic water splitting.

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.

Superbound Excitons in 2D Phosphorene Oxides

The optical excitations in layered phosphorene oxides are studied via ab initio calculation together with GW approximation for the self-energy and solving the Bethe-Salpeter equation (BSE) for the excitations. It is found that the electronic structure of phosphorene oxides closely depends on the oxygen concentration. For the high oxygen coverage structure P₄O₁₀, it shows a strong localized molecular-like electronic structure with exciton binding ( E_b) energy up to 3.0 eV, which is several times larger than the ordinary E_b value in various low-dimensional materials. This study may provide an alternative way to design functional layered materials with large exciton binding energies by controlling the oxidation level in phosphorene oxides.

Defect Engineering in 2D Materials: Precise Manipulation and Improved Functionalities

Two-dimensional (2D) materials have attracted increasing interests in the last decade. The ultrathin feature of 2D materials makes them promising building blocks for next-generation electronic and optoelectronic devices. With reducing dimensionality from 3D to 2D, the inevitable defects will play more important roles in determining the properties of materials. In order to maximize the functionality of 2D materials, deep understanding and precise manipulation of the defects are indispensable. In the recent years, increasing research efforts have been made on the observation, understanding, manipulation, and control of defects in 2D materials. Here, we summarize the recent research progress of defect engineering on 2D materials. The defect engineering triggered by electron beam (e-beam), plasma, chemical treatment, and so forth is comprehensively reviewed. Firstly, e-beam irradiation-induced defect evolution, structural transformation, and novel structure fabrication are introduced. With the assistance of a high-resolution electron microscope, the dynamics of defect engineering can be visualized in situ. Subsequently, defect engineering employed to improve the performance of 2D devices by means of other methods of plasma, chemical, and ozone treatments is reviewed. At last, the challenges and opportunities of defect engineering on promoting the development of 2D materials are discussed. Through this review, we aim to build a correlation between defects and properties of 2D materials to support the design and optimization of high-performance electronic and optoelectronic devices.

Disassembly of 2D Vertical Heterostructures

As one of the most widely discussed fields, the assembly of nanomaterials has always been extensively studied. However, its inverse process, namely disassembly, is still limited in the ambit of biomolecules. Specifically, in the emerging 2D research field, disassembly still remains unexplored. Inspired by the disassembly of DNA molecules via breaking intermolecular hydrogen bonds, the disassembly of 2D vertical heterostructures (2DVHs) is first achieved through the weakening of the interlayer van der Waals interactions. As a demonstration, ReS₂ /WS₂ VHs is successfully disassembled into individual building blocks. Density functional theory calculations are performed to study the disassembly of the 2DVHs, which simulate that 2DVHs are first activated by the disassembly promoters and then disassembled with weakened interlayer van der Waals interactions. Such a disassembly process demonstrates that it has great potential to be expanded as a general strategy to achieve the disassembly of other 2D superstructures.

Laser-Based Texturing of Graphene to Locally Tune Electrical Potential and Surface Chemistry

A simple procedure of producing three-dimensional blisters of graphene through irradiation of the visible range laser by Raman spectrometer has been presented. Fabrication of different volumes of the blisters and their characterization were carried out with Raman spectroscopy by tuning the irradiation dose. The produced blisters showed a consistency in altitude and a remarkable change in functionality, adhesion force map and local contact potential difference as compared to untreated monolayer graphene and naturally occurred graphene nanobubbles. Nevertheless, bilayer graphene is unaffected in the applied laser doses. The laser irradiation led to lattice expansion of carbon atoms and introduced oxygenic functional groups with the structural disorder. The internal pressure of the gaseous molecules was evaluated by monitoring the shape of the graphene blisters and nanobubbles. High-resolution Raman mapping showed the impact of laser-affected area and the defect density (n _d) is reported as a function of displacement. Our results reveal ease of applicability of the Raman laser for the imaging and texturing of graphene pointing toward the possibility of the desirable and cost-effective laser writing at the submicron scale by tuning photochemistry of graphene which is pivotal for numerous applications.

Interface Characterization and Control of 2D Materials and Heterostructures

2D materials and heterostructures have attracted significant attention for a variety of nanoelectronic and optoelectronic applications. At the atomically thin limit, the material characteristics and functionalities are dominated by surface chemistry and interface coupling. Therefore, methods for comprehensively characterizing and precisely controlling surfaces and interfaces are required to realize the full technological potential of 2D materials. Here, the surface and interface properties that govern the performance of 2D materials are introduced. Then the experimental approaches that resolve surface and interface phenomena down to the atomic scale, as well as strategies that allow tuning and optimization of interfacial interactions in van der Waals heterostructures, are systematically reviewed. Finally, a future outlook that delineates the remaining challenges and opportunities for 2D material interface characterization and control is presented.

Anisotropic visible photoluminescence from thermally annealed few-layer black phosphorus

Black phosphorus, a two-dimensional material, with high carrier mobility, tunable direct bandgap and anisotropic electronic properties has attracted enormous research interest towards potential application in electronic, optoelectronic and optomechanical devices. The bandgap of BP is thickness dependent, ranging from 0.3 eV for bulk to 1.3 eV for monolayer, while lacking in the visible region, a widely used optical regime for practical optoelectronic applications. In this work, photoluminescence (PL) centered at 605 nm is observed from the thermally annealed BP with thickness ≤20 nm. This higher energy PL is most likely the consequence of the formation of higher bandgap phosphorene oxides and suboxides on the surface BP layers as a result of the enhanced rate of oxidation. Moreover, the polarization-resolved PL measurements show that the emitted light is anisotropic when the excitation polarization is along the armchair direction. However, if excited along zigzag direction, the PL is nearly isotropic. Our findings suggest that the thermal annealing of BP can be used as a convenient route to fill the visible gap of the BP-based optoelectronic and optomechanical devices.

Field Effect Transistor Based on Layered NiPS 3

Layered metal phosphochalcogenides of molecular formula, MPX3 (M = Mn, Fe, Co, Ni, etc and X = S, Se) have been emerging as new class of semiconductors towards various catalytic and optoelectronic applications. The low cleavage energy associated with these layered chalcogenides may lead to devices with very thin semiconductor channels. Herein, we report the first successful fabrication of field effect transistor (FET) using layered NiPS3 that reveals n-type semiconducting behavior. Devices using bulk and few-layer NiPS3 with gold contacts show on/off ratios of ~103-105 at 25 °C. The device characteristics reveal an increase in on-state current with decrease in threshold voltage and the Schottky barrier height is extracted to be 112 meV. Density functional theory calculations reveal various parameters that affect electron/hole doping in the layered phosphochalcogenide material.

Recent Progress on Stability and Passivation of Black Phosphorus

From a fundamental science perspective, black phosphorus (BP) is a canonical example of a material that possesses fascinating surface and electronic properties. It has extraordinary in-plane anisotropic electrical, optical, and vibrational states, as well as a tunable band gap. However, instability of the surface due to chemical degradation in ambient conditions remains a major impediment to its prospective applications. Early studies were limited by the degradation of black phosphorous surfaces in air. Recently, several robust strategies have been developed to mitigate these issues, and these novel developments can potentially allow researchers to exploit the extraordinary properties of this material and devices made out of it. Here, the fundamental chemistry of BP degradation and the tremendous progress made to address this issue are extensively reviewed. Device performances of encapsulated BP are also compared with nonencapsulated BP. In addition, BP possesses sensitive anisotropic photophysical surface properties such as excitons, surface plasmons/phonons, and topologically protected and Dirac semi-metallic surface states. Ambient degradation as well as any passivation method used to protect the surface could affect the intrinsic surface properties of BP. These properties and the extent of their modifications by both the degradation and passivation are reviewed.

Tailoring electronic properties of multilayer phosphorene by siliconization

Controlling the thickness dependence of electronic properties for two-dimensional (2d) materials is among the primary goals for their large-scale applications. Herein, employing a first-principles computational approach, we predict that Si interaction with multilayer phosphorene (2d-P) can result in the formation of highly stable 2d-SiP and 2d-SiP₂ compounds with a weak interlayer interaction. Our analysis demonstrates that these systems are semiconductors with band gap energies that can be governed by varying the thicknesses and stacking arrangements. Specifically, the siliconization of phosphorene allows designing 2d-SiP_x materials with a significantly weaker thickness dependence of electronic properties than that in 2d-P and to develop ways for their tailoring. We also reveal the spatial dependence of electronic properties for 2d-SiP_x highlighting the difference in the effective band gaps for different layers. Particularly, our results show that the central layers in the multilayer 2d systems determine their overall electronic properties, while the role of the outermost layers is noticeably smaller.

Two-dimensional light-emitting materials: preparation, properties and applications

We review the recent development in two-dimensional (2D) light-emitting materials and describe their preparation methods, optical/optoelectronic properties and applications.

Tunable band offsets in the BP/P4O10 van der Waals heterostructure: first-principles calculations

The separation of photoexcited electron–hole pairs in the BP/P₄O₁₀ vdW heterostructure.

Enhanced spontaneous emission of quantum emitter in monolayer and double layer black phosphorus

We theoretically demonstrate that the spontaneous emission rate (SER) of quantum emitter is greatly enhanced in the proximity of single or double layers of black phosphorus (BP). The maximum enhancement factor is on the order of 10⁶, about 3 times larger than that of graphene, due to the large photonic density of states near BP. We also study how the transition frequency, the BP doping level, the distance between emitter and BP, the existence of substrate underneath BP, and the orientation of transition dipole moment influence the SER. With properly designed parameters, both high SER of emitter and low propagation losses of BP surface plasmon polaritons can be achieved in the quantum system, making it a promising candidate for mid-infrared single-photon generation.

Cyclical Thinning of Black Phosphorus with High Spatial Resolution for Heterostructure Devices

A high spatial resolution, cyclical thinning method for realizing black phosphorus (BP) heterostructures is reported. This process utilizes a cyclic technique involving BP surface oxidation and vacuum annealing to create BP flakes as thin as 1.6 nm. The process also utilizes a spatially patternable mask created by evaporating Al that oxidizes to form Al₂O₃, which stabilizes the unetched BP regions and enables the formation of lateral heterostructures with spatial resolution as small as 150 nm. This thinning/patterning technique has also been used to create the first-ever lateral heterostructure BP metal oxide semiconductor field-effect transistor (MOSFET), in which half of a BP flake was thinned in order to increase its band gap. This heterostructure MOSFET showed an ON/OFF current ratio improvement of 1000× compared to homojunction MOSFETs.

Control of Surface and Edge Oxidation on Phosphorene

Phosphorene is emerging as an important two-dimensional semiconductor, but controlling the surface chemistry of phosphorene remains a significant challenge. Here, we show that controlled oxidation of phosphorene determines the composition and spatial distribution of the resulting oxide. We used X-ray photoemission spectroscopy to measure the binding energy shifts that accompany oxidation. We interpreted these spectra by calculating the binding energy shift for 24 likely bonding configurations, including phosphorus oxides and hydroxides located on the basal surface or edges of flakes. After brief exposure to high-purity oxygen or high-purity water vapor at room temperature, we observed phosphorus in the +1 and +2 oxidation states; longer exposures led to a large population of phosphorus in the +3 oxidation state. To provide insight into the spatial distribution of the oxide, transmission electron microscopy was performed at several stages during the oxidation. We found crucial differences between oxygen and water oxidants: while pure oxygen produced an oxide layer on the van der Waals surface, water oxidized the material at pre-existing defects such as edges or steps. We propose a mechanism based on the thermodynamics of electron transfer to interpret these observations. This work opens a route to functionalize the basal surface or edges of two-dimensional (2D) black phosphorus through site-selective chemical reactions and presents the opportunity to explore the synthesis of 2D phosphorene oxide by oxidation.