Passivation of Black Phosphorus via Self-Assembled Organic Monolayers by van der Waals Epitaxy

Self-assembled inorganic nanomaterials for biomedical applications

Controlled self-assembly of inorganic nanoparticles has the potential to generate complex nanostructures with distinctive properties. The advancement of more precise techniques empowers researchers in constructing and assembling diverse building blocks, marking a pivotal evolution in nanotechnology and biomedicine. This progress enables the creation of customizable biomaterials with unique characteristics and functions. This comprehensive review takes an innovative approach to explore the current state-of-the-art self-assembly methods and the key interactions driving the self-assembly processes and provides a range of examples of biomedical and therapeutic applications involving inorganic or hybrid nanoparticles and structures. Self-assembly methods applied to bionanomaterials are presented, ranging from commonly used methods in cancer phototherapy and drug delivery to emerging techniques in bioimaging and tissue engineering. The most promising in vitro and in vivo experimental results achieved thus far are presented. Additionally, the review engages in a discourse on safety and biocompatibility concerns related to inorganic self-assembled nanomaterials. Finally, opinions on future challenges and prospects anticipated in this evolving field are provided.

Ultrahigh carrier mobility and multidirectional piezoelectricity in 2D Janus copper-containing chalcogenide monolayers

Two-dimensional (2D) materials have attracted enormous research attention due to their remarkable properties and potential applications in electronic and optoelectronic devices. In this work, Janus 2D copper-containing chalcogenides, CuP2Se0.5S0.5 and CuP2Te0.5Se0.5 monolayers, are proposed and studied systematically based on first-principles calculations. These two Janus-structured materials possess the same thermal and dynamic stability as the perfect CuP2Se structure. Remarkably, we observe multiple VBM and CBM points with negligible energy differences in the band structures of perfect CuP2Se and Janus CuP2Se0.5S0.5 and CuP2Te0.5Se0.5 monolayers. This will significantly impact the electronic and transport properties of the material. The calculated anisotropic carrier mobilities can reach 104-105 cm2 V-1 s-1 orders of magnitude, which are higher than those of most reported materials. Meanwhile, the two Janus derivatives, CuP2Se0.5S0.5 and CuP2Te0.5Se0.5 monolayers, exhibit outstanding multidirectional piezoelectricity, which are comparable with those of traditional piezoelectric materials. The combination of ultrahigh carrier mobility and multidirectional piezoelectricity indicates that these novel 2D Janus materials could be promising for applications in electronic and piezoelectric devices under special conditions.

Modification Strategies and Prospects for Enhancing the Stability of Black Phosphorus

Black phosphorus is a two-dimensional layer material with promising applications due to its many excellent physicochemical properties, including high carrier mobility, ambipolar field effect and unusual in-plane anisotropy. Currently, BP has been widely used in biomedical engineering, photocatalysis, semiconductor devices, and energy storage electrode materials. However, the unique structure of BP makes it highly chemically active, leading to its easy oxidation and degradation in air, which limits its practical applications. Recently, researchers have proposed a number of initiatives that can address the environmental instability of BP, and the application of these physical and chemical passivation techniques can effectively enhance the environmental stability of BP, including four modification methods: covalent functionalization, non-covalent functionalization, surface coordination, physical encapsulation and edge passivation. This review highlights the mechanisms of the above modification techniques in addressing the severe instability of BP in different application scenarios, as well as the advantages and disadvantages of each method. This review can provide guidance for more researchers in studying the marvellous properties of BP and accelerate the practical application of BP in different fields.

Investigations of Crucial Factors for the Non-Covalent Functionalization of Black Phosphorus (BP) using Perylene Diimide Derivatives for the Passivation of BP Nanosheets

The non-covalent functionalization of black phosphorus (BP) was studied with a scope of ten tailor-made perylene diimides (PDIs). A combination of UV/Vis-, fluorescence-, as well as Raman spectroscopy and atomic force microscopy was used to investigate the structural factors, which contribute to a pronounced PDI-BP interaction and thus support the protection of BP nanosheets against oxidative degradation. We were able to show, that water-soluble, amphiphilic PDIs with highly charged head groups can be used for the non-covalent functionalization of BP in aqueous media. Here, based on the hydrophobic effect, an efficient adsorption of the respective PDI molecules takes place and leads to the formation of a passivating film, yielding a considerable stabilization of the BP flakes under ambient conditions exceeding 30 days.

Surface Engineering of Two-Dimensional Black Phosphorus for Advanced Nanophotonics

Everything in the world has two sides. We should correctly understand its two sides to pursue the positive side and get rid of the negative side. Recently, two-dimensional (2D) black phosphorus (BP) has received a tremendous amount of attention and has been applied for broad applications in optoelectronics, transistors, logic devices, and biomedicines due to its intrinsic properties, e.g., thickness-dependent bandgap, high mobility, highly anisotropic charge transport, and excellent biodegradability and biocompatibility. On one hand, rapid degradation of 2D BP under ambient conditions becomes a vital bottleneck that largely hampers its practical applications in optical and optoelectronic devices and photocatalysis. On the other hand, just because of its ambient instability, 2D BP as a novel kind of nanomedicine in a cancer drug delivery system can not only satisfy effective cancer therapy but also enable its safe biodegradation in vivo. Until now, a variety of surface functionality types and approaches have been employed to rationally modify 2D BP to meet the growing requirements of advanced nanophotonics. In this Account, we describe our research on surface engineering of 2D BP in two opposite ways: (i) stabilizing 2D BP by various approaches for advanced nanophotonic devices with both remarkable photoresponse behavior and environmentally structural stability and (ii) making full use of biodegradation, biocompatibility, and biological activity (e.g., photothermal therapy, photodynamic therapy, and bioimaging) of 2D BP for the construction of high-performance delivery nanoplatforms for biophotonic applications. We highlight the targeted aims of the surface-engineered 2D BP for advanced nanophotonics, including photonic devices (optics, optoelectronics, and photocatalysis) and photoinduced cancer therapy, by means of various surface functionalities, such as heteroatom incorporation, polymer functionalization, coating, heterostructure design, etc. From the viewpoint of potential applications, the fundamental properties of surface-engineered 2D BP and recent advances in surface-engineered 2D BP-based nanophotonic devices are briefly discussed. For the photonic devices, surface-engineered 2D BP can not only effectively improve environmentally structural stability but also simultaneously maintain photoresponse performance, enabling 2D BP-based devices for a wide range of practical applications. In terms of the photoinduced cancer therapy, surface-engineered 2D BP is more appropriate for the treatment of cancer due to negligible toxicity and excellent biodegradation and biocompatibility. We also provide our perspectives on future opportunities and challenges in this important and fast-growing field. It is envisioned that this Account can attract more attention in this area and inspire more scientists in a variety of research communities to accelerate the development of 2D BP for more widespread high-performance nanophotonic applications.

Asymmetric Laser Field Interaction with MXene Coated on the Side Surface of Optical Fibers for Ultrafast Nonlinear Switches

In recent years, there has been significant interest in researching ultrafast nonlinear optical phenomena involving light-matter interactions in two-dimensional (2D) materials, owing to their potential applications in optics and photonics. MXene, a recently developed 2D material, has garnered considerable attention due to its graphene-like properties and highly tunable electronic/optical characteristics. Herein, we demonstrate ultrafast all-optical switches based on four-wave-mixing (FWM) utilizing the nonlinear optical property of MXene Ti3C2Tx. In order to realize the device, we deposited multilayered Ti3C2Tx in the form of a supernatant solution onto the polished surface of a side-polished optical fiber, enabling the interaction of Ti3C2Tx with the asymmetric evanescent field of the incident input. We systematically characterized the nonlinear optical responses derived from the Ti3C2Tx layers. The fabricated device exhibits notable performance metrics, an enhancement of the extinction ratio, and a conversion efficiency of the newly generated signal, displaying 5.3 and 5.2 dB, respectively. Additionally, the device operates at high modulation frequencies, reaching up to 20 GHz, and demonstrates high-resolution detuning with channel distances of up to 15 nm. Our findings highlight the potential of MXene-based materials for ultrafast optical data management systems.

A 2D-0D-2D Sandwich Heterostructure toward High-Performance Room-Temperature Gas Sensing

The construction of two-dimensional (2D) van der Waals (vdW) heterostructures over black phosphorus (BP) has been attracting significant attention to better utilize its inherent properties. The sandwich of zero-dimensional (0D) noble metals within BP-based vdW heterostructures can provide efficient catalytic channels, modulating their surface redox potentials and therefore inducing versatile functionalities. Herein, we realize a 2D WS2-Au-BP heterostructure, in which Au nanoparticles are connected between BP and WS2 via ionic bonds. The ultralow conduction band minimum position, the reduced adsorption energies of O2, and the increased dissociation barrier energy of O2- into 2O contribute greatly to improving the long-term stability of BP in the air. The formation of heterostructures can reduce the potential barrier energy in target gas molecules, thus enhancing the absorption energy and charge transfer. Taking the paramagnetic NO2 gas molecules as a representative, a stable response magnitude of 2.11 to 100 ppb NO2 is achieved for 80 days, which is far larger than the initial responses of most BP-based materials. A practical gas sensing system is also developed to demonstrate its real-world implementation. This work provides a promising demonstration of 0D noble metal within 2D BP-based vdW heterostructure for simultaneously improving the long-term stability and room-temperature reversible gas sensing.

Catalysis-Mediated Male Contraception through Black Phosphorus Nanosheets

Nanocontraception has been proposed and received extensive attention in recent years for population control. However, currently developed methods for nanocontraception still face problems in efficacy and safety. Here, we propose catalysis-mediated oxidation as a new strategy for nanocontraception. With the catalytic production of highly oxidative species, male contraception was successfully achieved after the administration of black phosphorus nanosheets into the testes of male mice. Further mechanistic studies revealed that contraception was induced by oxidative stress and apoptosis of spermatogenesis cells. Meanwhile, the apoptosis of germ cells released testis antigen and induced immune cell infiltration, which enhanced reproductive damage. Notably, the introduced black phosphorus nanosheets naturally degraded during the catalytic oxidation process and ultimately converted to harmless phosphates, indicating the safety of the strategy. Furthermore, the catalysis-mediated strategy avoids utilizing additional inducers, such as near-infrared irradiation, magnetic fields, or ultrasound, which may cause severe pain. In summary, the proposed catalysis-mediated contraception can be a self-cleared, convenient, and safe strategy for controlling male fertility.

Black phosphorus nanosheets/poly(allylamine hydrochloride) based electrochemical immunosensor for the selective detection of human epididymis protein 4

In this work, an electrochemical immunosensor based on black phosphorus nanosheets (BPNS)/poly(allylamine hydrochloride) (PAH) nanocomposite modified glassy carbon electrode was developed for the detection of ovarian cancer biomarker HE4. PAH has been applied to retain BPNS in its original honeycomb structure and to anchor biomolecules electrostatically on the transducer surface. The as synthesized nanocomposite was characterized by zeta potential analysis, scanning electron microscopy, x-ray photoelectron spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy. Subsequently, the performance of the electrochemical immunosensor was evaluated through cyclic voltammetry, differential pulse voltammetry and electrochemical impedance spectroscopy. Under the optimal condition, the developed electrochemical immunosensor permitted to detect HE4 with a linear range of 0.1-300 ng ml^-1and a detection limit of 0.01 ng ml^-1. The developed sensor exhibited good selectivity and specificity to HE4 with negligible interference effect from common biomolecules like bovine serum albumin, lysozyme, protamine, glucose, fructose, hemoglobin and fetal bovine serum. Further, practical application of developed electrochemical immunosensor was demonstrated in spiked human serum which showed satisfactory recovery percentages.

Effects of oppositely charged moieties on the self-assembly and biophysicochemical properties of polyurethane micelles

Gemini quaternary ammonium (GQA), a type of cationic surfactant, exhibits excellent micellization ability and acts as a cell internalization promoter to increase the permeability of the cell membrane. GQA is sensitive to ionic solutions, which disturb its stabilization and leads to the rapid degradation of its polymer micelles due to its unique hydrophilic N⁺ structure. However, the effect of negatively charged moieties in the polymer chains of GQA on its action in polymer micelles, typically with regard to its micellization and biological performance, remains unclear. In this work, a series of polyurethane micelles containing various ratios of oppositely charged moieties was prepared. We found that the interchain electrostatic interaction severely undermines the function of the GQA surfactant and hinders the self-assembly and stabilization of polyurethane micelles. Specifically, a hydrophilic corona with a longer length cannot completely overcome this effect. By regulating the ratio of oppositely charged moieties, micelles exhibited tunable biological properties, such as biocompatibility, cytotoxicity, cell internalization, and phagocytosis by macrophages. Based on our results, a moderate molecular weight of mPEG (Mn = 1900) and a slight positive surface potential (∼10 mV) are the best surface parameters for the comprehensive performance of the studied nanoplatforms. This study provides a further understanding of the electrostatic interaction effect on the properties of the cationic GQA, offering rational guidance for the design and fabrication of GQA polymer micelles.

Emerging Phases of Layered Metal Chalcogenides

Layered metal chalcogenides, as a "rich" family of 2D materials, have attracted increasing research interest due to the abundant choices of materials with diverse structures and rich electronic characteristics. Although the common metal chalcogenide phases such as 2H and 1T have been intensively studied, many other unusual phases are rarely explored, and some of these show fascinating behaviors including superconductivity, ferroelectrics, ferromagnetism, etc. From this perspective, the unusual phases of metal chalcogenides and their characteristics, as well as potential applications are introduced. First, the unusual phases of metal chalcogenides from different classes, including transition metal dichalcogenides, magnetic element-based chalcogenides, and metal phosphorus chalcogenides, are discussed, respectively. Meanwhile, their excellent properties of different unusual phases are introduced. Then, the methods for producing the unusual phases are discussed, specifically, the stabilization strategies during the chemical vapor deposition process for the unusual phase growth are discussed, followed by an outlook and discussions on how to prepare the unusual phase metal dichalcogenides in terms of synthetic methodology and potential applications.

Emerging Two-Dimensional Inorganic Molecular Crystals: The Concept and Beyond

The concept of two-dimensional (2D) inorganic molecular crystals (IMCs) was first introduced by Zhai and coauthors in 2019. In contrast to the layered structures of graphene-like 2D materials, 2D IMCs consist of tiny inorganic molecules bonded together through all-around van der Waals (vdW) interactions. Their structural peculiarities lead to some special behaviors and appealing properties in their synthesis and applications. In this Perspective, we first introduce the concept of 2D IMCs and present the very first synthesis of 2D IMCs using a surface-passivated growth approach. The special intermolecular effects between the inorganic molecules are also summarized. In addition, because of its molecular structure, a vdW film of IMCs can be facilely fabricated, which exhibits appealing potential in integrated 2D devices. More importantly, we give a general outlook for the further development of 2D IMCs with the goal of attracting more attention to this emerging research frontier.

Commensurate Assembly of C60 on Black Phosphorus for Mixed-Dimensional van der Waals Transistors

2D crystals can serve as templates for the realization of new van der Waals (vdW) heterostructures via controlled assembly of low-dimensional functional components. Among available 2D crystals, black phosphorus (BP) is unique due to its puckered atomic surface topography, which may lead to strong epitaxial phenomena through guided vdW assembly. Here, it is demonstrated that a BP template can induce highly oriented assembly of C₆₀ molecular crystals. Transmission electron microscopy and theoretical analysis of the C₆₀ /BP vdW heterostructure clearly confirm that the BP template results in oriented C₆₀ assembly with higher-order commensurism. Lateral and vertical devices with C₆₀ /BP junctions are fabricated via a lithography-free clean process, which allows one to investigate the ideal electrical properties of pristine C₆₀ /BP junctions. Effective tuning of the C₆₀ /BP junction barrier from 0.2 to 0.5 eV and maximum on-current density higher than 10⁴  mA cm^-2 are achieved with graphite/C₆₀ /BP vertical vdW transistors. Due to the formation of high-quality C₆₀ film and the semitransparent graphite top-electrode, the vertical transistors show high photoresponsivities up to ≈100 A W^-1 as well as a fast response time under visible light illumination.

Scalable Van der Waals Encapsulation by Inorganic Molecular Crystals

Encapsulation is critical for devices to guarantee their stability and reliability. It becomes an even more essential requirement for devices based on 2D materials with atomic thinness and far inferior stability compared to their bulk counterparts. Here a general van der Waals (vdW) encapsulation method for 2D materials using Sb₂ O₃ layer of inorganic molecular crystal fabricated via thermal evaporation deposition is reported. It is demonstrated that such a scalable encapsulation method not only maintains the intrinsic properties of typical air-susceptible 2D materials due to their vdW interactions but also remarkably improves their environmental stability. Specifically, the encapsulated black phosphorus (BP) exhibits greatly enhanced structural stability of over 80 days and more sustaining-electrical properties of 19 days, while the bare BP undergoes degradation within hours. Moreover, the encapsulation layer can be facilely removed by sublimation in vacuum without damaging the underlying materials. This scalable encapsulation method shows a promising pathway to effectively enhance the environmental stability of 2D materials, which may further boost their practical application in novel (opto)electronic devices.

SiCP4 Monolayer with a Direct Band Gap and High Carrier Mobility for Photocatalytic Water Splitting

Photocatalytic water splitting is a promising method that uses sunlight to generate hydrogen from water to provide clean and renewable energy resources. Two-dimensional materials with abundant active sites are ideal candidates for achieving this goal; however, few of the known ones can meet the rigorous requirement of photocatalytic water splitting. By using first-principles swarm-intelligence search calculations, we have successfully identified two new semiconducting SiCP₂ and SiCP₄ monolayers. Their band-edge heights evidently straddle the redox potentials of water. For the more prominent SiCP₄ monolayer, additional external biases of 0.32 V for water oxidation and 0.03 V for the hydrogen reduction half-reaction would be enough to drive its reaction sequences at pH 0, and it can spontaneously proceed to the water oxidation half-reaction in a neutral solution. Interestingly, the excellent optical absorbance ability (∼10⁴ cm^-1) and high carrier mobility (∼10⁵ cm² V^-1 s^-1) of SiCP₂ and SiCP₄ facilitate the utilization of sunlight and the fast transportation of photogenerated carriers. All of these properties make SiCP₂ and SiCP₄ monolayers promising candidates for applications in photocatalytic water splitting.

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.

Electron Matters: Recent Advances in Passivation and Applications of Black Phosphorus

2D materials have experienced rapid and explosive development in the past decades. Among them, black phosphorus (BP) is one of the most promising materials on account of its thickness-dependent bandgap, high charge-carrier mobility, in-plane anisotropic structure, and excellent biocompatibility, as well as the broad applications brought by the properties. In view of the electron configuration, the most unique feature of BP is the lone-pair electrons on each P atom. The lone-pair electrons inevitably cause high reactivity of BP, particularly toward water/oxygen, which greatly limits the practical application of BP under ambient conditions. The other side of the coin is that BP can serve as an electron donor to promote the construction of BP-based hybrid materials and/or to boost the performance of BP or BP-based hybrid materials in applications. Here, recent advances in passivation and application of BP by addressing the interaction between the lone-pair electrons of BP and the other materials are discussed, and prospects for future research on BP are also proposed.

Synthesis and stabilization of black phosphorus and phosphorene: recent progress and perspectives

Two-dimensional black phosphorus (BP) has triggered tremendous research interest owing to its unique crystal structure, high carrier mobility, and tunable direct bandgap. Preparation of few-layer BP with high quality and stability is very important for its related research and applications in biomedicine, electronics, and optoelectronics. In this review, the synthesis methods of BP, including the preparation of bulk BP crystal which is an important raw material for preparing few-layer BP, the popular top-down methods, and some direct growth strategies of few-layer BP are comprehensively overviewed. Then chemical ways to enhance the stability of few-layer BP are concretely introduced. Finally, we propose a selection rule of preparation methods of few-layer BP according to the requirement of specific BP properties for different applications. We hope this review would bring some insight for future researches on BP and contributes to the acceleration of BP's commercial progress.

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.

Covalent and non-covalent chemistry of 2D black phosphorus

The post-graphene era is undoubtedly marked by two-dimensional (2D) sheet polymers, such as black phosphorus (BP). This emerging material has a fascinating structure and outstanding electronic properties and has been postulated for a plethora of applications. The need to circumvent the pronounced oxophilicity of P atoms has dominated the research on this material in recent years, with the objective of finding the most effective method to improve its environmental stability. When it comes to chemical functionalization, the few approaches reported so far involve some drawbacks such as low degree of addition and low production ability. This review presents the concepts and strategies of our studies on the chemical functionalization of BP, both non-covalent and covalent, emphazising the current synthetic challenges. Moreover, we also provide some effective pathways for the chemical activation of the unreactive basal plane, the identification of the effective binding strategies, and the concept to overcome hurdles associated with characterization tools. This work will provide fundamental insights into the controlled chemical functionalization and characterization of BP, fostering the research on this appealing 2D material.

Interface Amorphization of Two-Dimensional Black Phosphorus upon Treatment with Diazonium Salts

Two-dimensional (2D) black phosphorus (BP) represents one of the most appealing 2D materials due to its electronic, optical, and chemical properties. Many strategies have been pursued to face its environmental instability, covalent functionalization being one of the most promising. However, the extremely low functionalization degrees and the limitations in proving the nature of the covalent functionalization still represent challenges in many of these sheet architectures reported to date. Here we shine light on the structural evolution of 2D-BP upon the addition of electrophilic diazonium salts. We demonstrated the absence of covalent functionalization in both the neutral and the reductive routes, observing in the latter case an unexpected interface conversion of BP to red phosphorus (RP), as characterized by Raman, 31 P-MAS NMR, and X-ray photoelectron spectroscopies (XPS). Furthermore, thermogravimetric analysis coupled to gas chromatography and mass spectrometry (TG-GC-MS), as well as electron paramagnetic resonance (EPR) gave insights into the potential underlying radical mechanism, suggesting a Sandmeyer-like reaction.

Enhancing the photodynamic therapy efficacy of black phosphorus nanosheets by covalently grafting fullerene C60

Few-layer black phosphorus (BP) nanosheets show potential application in biomedicine such as photodynamic therapy (PDT), and are therefore commonly used in anticancer therapy and nanomedicine due to being relatively less invasive. However, they suffer from low ambient stability and poor therapeutic efficacy. Herein, C60 was covalently grafted onto the edges of BP nanosheets, and the resultant BP-C60 hybrid was applied as a novel endocytosing photosensitizer, resulting in not only significantly enhanced PDT efficacy relative to that of the pristine BP nanosheets, but also drastically improved stability in a physiological environment, as confirmed by both in vitro and in vivo studies. Such improved stability was due to shielding effect of the stable hydrophobic C60 molecules. The enhanced PDT efficacy is interpreted from the photoinduced electron transfer from BP to C60, leading to the promoted generation of ˙OH radicals, acting as a reactive oxygen species (ROS) that is effective in killing tumor cells. Furthermore, the BP-C60 hybrid exhibited low systemic toxicity in the major organs of mice. The BP-C60 hybrid represents the first BP-fullerene hybrid nanomaterial fulfilling promoted ROS generation and consequently enhanced PDT efficacy.

Aggregation-Induced Emission Luminogens Married to 2D Black Phosphorus Nanosheets for Highly Efficient Multimodal Theranostics

Inspired by the respective advantages of aggregation-induced emission (AIE)-active photosensitizers and black phosphorus nanomaterials in cancer treatment, the facile construction of novel AIE photosensitizers married to 2D black phosphorus nanosheets and their application for multimodal theranostics are demonstrated. The developed nanomaterial simultaneously possesses distinctive properties and multiple functions including excellent stability, good biocompatibility, intensive fluorescence emission in the NIR region, high-performance reactive oxygen species generation, good photothermal conversion efficiency, outstanding cellular uptake, and effective accumulation at the tumor site. Both in vitro and in vivo evaluation show that the presented nanotheranostic system is an excellent candidate for NIR fluorescence-photothermal dual imaging-guided synergistic photodynamic-photothermal therapies. This study thus not only extends the applications scope of AIE and black phosphorus materials, but also offers useful insights into designing a new generation of cancer theranostic protocol for potential clinical applications.

Stable C2N/h-BN van der Waals heterostructure: flexibly tunable electronic and optic properties

Monolayer C2N has been successfully synthesized. To explore the enhancement of its stability and the expansion of its potential applications in electronics and optoelectronics, a vertical van der Waals heterostructure is constructed by C2N monolayer integrated withh-BN monolayer. The first-principles calculations based on the density functional theory show that this structure possesses a type-II band alignment with a smaller direct band gap and lager band offsets, suggesting that photo-generated electron-hole pairs can be well spatially separated, and thus an excellent photoelectric and photovoltaic material can be obtained. Also shown is that the electronic properties of such a heterostructure can be effectively regulated by a vertical strain and external electric field. For example, under compressive strain or forward electric field, its band gap can be significantly reduced to enhance light-excitation electron transition further, meanwhile the feature of direct band-gap and large band offsets is always well-preserved. Furthermore, it is found that the intrinsic heterostructure holds a wide optic adsorption range and large adsorption coefficient, and the applied compressive strain or a positive electric field can lead to a wide and high main absorption peak across the near-infrared, visible light, and ultraviolet region, implying that a tuned heterostructure has more promising applications in optoelectronics.

Wireless Hand-Held Device Based on Polylactic Acid-Protected, Highly Stable, CTAB-Functionalized Phosphorene for CO2 Gas Sensing

Phosphorene is a novel two-dimensional (2D) material with exceptional properties and is connecting the gaps between graphene and transition-metal chalcogenides but having environmental instability. In this study, we present effective liquid exfoliation of few-layer phosphorene (FLP) from bulk black phosphorous (BP) in the presence of cetyltrimethylammonium bromide (CTAB), a cationic surfactant that is highly stable. It successfully stabilizes FLP in deionized water, which is consistent with obtained characterization and gas-sensing studies. Our investigation shows that the dynamic sensing response of the CTAB-grafted phosphorene (P-CTAB) sensor increases by ∼1.5 times as the relative humidity (RH) varies from 33 to 75%, which is the first published result for CO₂ gas detection. The sensitivity values of the P-CTAB and P-CTAB/polylactic acid (PLA) are found to be 0.0356 and 0.0329 ppm^-1, respectively, toward CO₂ gas. It is notable that when a polylactic acid (PLA) membrane is introduced as a barrier layer in our fabricated Arduino-based Bluetooth-enabled hand-held device, it obstructs the environmental effect with a trace-level detection capability and negligible change over time (up to 30 days). Herein, for the first time, we discover the gas-sensing characteristics of CTAB-grafted phosphorene and witness an ultrasensitive and selective response toward CO₂ gas detection.

Two-dimensional materials for next-generation computing technologies

Rapid digital technology advancement has resulted in a tremendous increase in computing tasks imposing stringent energy efficiency and area efficiency requirements on next-generation computing. To meet the growing data-driven demand, in-memory computing and transistor-based computing have emerged as potent technologies for the implementation of matrix and logic computing. However, to fulfil the future computing requirements new materials are urgently needed to complement the existing Si complementary metal-oxide-semiconductor technology and new technologies must be developed to enable further diversification of electronics and their applications. The abundance and rich variety of electronic properties of two-dimensional materials have endowed them with the potential to enhance computing energy efficiency while enabling continued device downscaling to a feature size below 5 nm. In this Review, from the perspective of matrix and logic computing, we discuss the opportunities, progress and challenges of integrating two-dimensional materials with in-memory computing and transistor-based computing technologies.

Molecular-Scale Investigation of the Thermal and Chemical Stability of Monolayer PTCDA on Cu(111) and Cu(110)

Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) has been intensively investigated for decades because of its unique electronic and optical properties and its applications in organic electronics and surface engineering and passivation of 2D materials. Recently, the high demand for achieving selective area deposition in device fabrications drives the research of utilizing organic molecules as a passivation layer on metals in the semiconductor industry. PTCDA molecules show promising potential to be used as a passivation layer on a metal surface because of their ability to form self-assembled compact lying-down layers with the well-exposed inert conjugated molecular π-plane. However, the thermal and chemical stabilities of monolayer PTCDA on metal surfaces have not been thoroughly studied. In this paper, we demonstrate that monolayer PTCDA on Cu(110) and Cu(111) surfaces exhibit good thermal and chemical stabilities, as revealed through the combination of in situ X-ray photoelectron spectroscopy and in situ low-temperature scanning tunneling microscopy measurements. We show that monolayer PTCDA on copper is stable up to 220 °C and decomposes to perylene at higher temperature. Monolayer PTCDA also shows good chemical stability when exposed to O₂ and water, demonstrating good potential for its future applications as passivation layers in selective area deposition.

The Rise of 2D Photothermal Materials beyond Graphene for Clean Water Production

Water shortage is one of the most concerning global challenges in the 21st century. Solar-inspired vaporization employing photothermal nanomaterials is considered to be a feasible and green technology for addressing the water challenge by virtue of abundant and clean solar energy. 2D nanomaterials aroused considerable attention in photothermal evaporation-induced water production owing to their large absorption surface, strong absorption in broadband solar spectrum, and efficient photothermal conversion. Herein, the recent progress of 2D nanomaterials-based photothermal evaporation, mainly including emerging Xenes (phosphorene, antimonene, tellurene, and borophene) and binary-enes (MXenes and transition metal dichalcogenides), is reviewed. Then, the optimization strategies for higher evaporation performance are summarized in terms of modulation of the intrinsic photothermal performance of 2D nanomaterials and design of the complete evaporation system. Finally, the challenges and prospective of various kinds of 2D photothermal nanomaterials are discussed in terms of the photothermal performance, stability, environmental influence, and cost. One important principle is that solutions for water challenges should not introduce new environmental and social problems. This Review aims to highlight the role of 2D photothermal nanomaterials in solving water challenges and provides a viable scheme toward the practical use in photothermal materials selection, design, and evaporation systems building.

Recent Advances in Chemical Functionalization of 2D Black Phosphorous Nanosheets

Owing to their tunable direct bandgap, high charge carrier mobility, and unique in-plane anisotropic structure, black phosphorus nanosheets (BPNSs) have emerged as one of the most important candidates among the 2D materials beyond graphene. However, the poor ambient stability of black phosphorus limits its practical application, due to the chemical degradation of phosphorus atoms to phosphorus oxides in the presence of oxygen and/or water. Chemical functionalization is demonstrated as an efficient approach to enhance the ambient stability of BPNSs. Herein, various covalent strategies including radical addition, nitrene addition, nucleophilic substitution, and metal coordination are summarized. In addition, efficient noncovalent functionalization methods such as van der Waals interactions, electrostatic interactions, and cation-π interactions are described in detail. Furthermore, the preparations, characterization, and diverse applications of functionalized BPNSs in various fields are recapped. The challenges faced and future directions for the chemical functionalization of BPNSs are also highlighted.

Carbon Dot-Passivated Black Phosphorus Nanosheet Hybrids for Synergistic Cancer Therapy in the NIR-II Window

Metal-free layered black phosphorus (BP) nanosheets with an excellent photothermal effect and large surface areas have been widely applied in biomedicine but are easily oxidized in ambient conditions yielding insulating phosphorus oxides adsorbed on its surface. Several chemical-functionalized strategies have been explored to protect thin layers of BP; however, the performance of passivated BP often decreases significantly, falling behind the single BP due to the strong structure perturbation. Herein, we designed and constructed 0D/2D hybrid photothermal agents by assembling NIR-II-responsive carbon dots (NIR-II-CDs) on BP nanosheets. NIR-II-CDs improve the ambient stability of BP by isolating them from water and oxygen and enhance the photothermal properties of BP nanosheets. Such NIR-II-CD/BP hybrids strengthen the light-harvesting ability, achieving high photothermal conversion efficiencies in the NIR-I (77.3%) and NIR-II (61.4%) windows, which is significantly higher than that of pristine BP (49.5 and 28.4% at 808 and 1064 nm). Owing to the intrinsic advantage of 1064 nm laser and the excellent PTT effect of our NIR-II-CD/BP hybrids, complete tumor eradication was realized in a deep-tissue tumor model.

Epitaxial multilayers of alkanes on two-dimensional black phosphorus as passivating and electrically insulating nanostructures

Mechanically exfoliated two-dimensional (2D) black phosphorus (bP) is epitaxially terminated by monolayers and multilayers of tetracosane, a linear alkane, to form a weakly interacting van der Waals heterostructure. Atomic force microscopy (AFM) and computational modelling show that epitaxial domains of alkane chains are ordered in parallel lamellae along the principal crystalline axis of bP, and this order is extended over a few layers above the interface. Epitaxial alkane multilayers delay the oxidation of 2D bP in air by 18 hours, in comparison to 1 hour for bare 2D bP, and act as an electrical insulator, as demonstrated using electrostatic force microscopy. The presented heterostructure is a technologically relevant insulator-semiconductor model system that can open the way to the use of 2D bP in micro- and nanoelectronic, optoelectronic and photonic applications.

Waterproof molecular monolayers stabilize 2D materials

Two-dimensional van der Waals materials have rich and unique functional properties, but many are susceptible to corrosion under ambient conditions. Here we show that linear alkylamines n-C _m H_2m+1NH₂, with m = 4 through 11, are highly effective in protecting the optoelectronic properties of these materials, such as black phosphorus (BP) and transition-metal dichalcogenides (TMDs: WS₂, 1T'-MoTe₂, WTe₂, WSe₂, TaS₂, and NbSe₂). As a representative example, n-hexylamine (m = 6) can be applied in the form of thin molecular monolayers on BP flakes with less than 2-nm thickness and can prolong BP's lifetime from a few hours to several weeks and even months in ambient environments. Characterizations combined with our theoretical analysis show that the thin monolayers selectively sift out water molecules, forming a drying layer to achieve the passivation of the protected 2D materials. The monolayer coating is also stable in air, H₂ annealing, and organic solvents, but can be removed by certain organic acids.

HfO2-passivated black phosphorus field effect transistor with long-termed stability and enhanced current on/off ratio

Enhanced on/off ratio, obvious threshold voltage left shift, newly emerging bipolar field effect performance and most importantly, excellent stability in ambient condition have been reported for the HfO₂-passivated black phosphorus field effect transistors . Both Raman spectra and x-ray photoelectron spectroscopy (XPS) show a thickness reduction effect after HfO₂ passivation, XPS further demonstrates that the formation of P-Hf and P-O chemical bonds contributes to the thinning of layered black phosphorus (BP), in which P-Hf bonds also provide chemical protection for BP flakes from degradation. Atomic force microscopy measures the thickness of the passivation layer and also verifies the stability of the passivated BP flakes.

2D-Organic Hybrid Heterostructures for Optoelectronic Applications

The unique properties of hybrid heterostructures have motivated the integration of two or more different types of nanomaterials into a single optoelectronic device structure. Despite the promising features of organic semiconductors, such as their acceptable optoelectronic properties, availability of low-cost processes for their fabrication, and flexibility, further optimization of both material properties and device performances remains to be achieved. With the emergence of atomically thin 2D materials, they have been integrated with conventional organic semiconductors to form multidimensional heterostructures that overcome the present limitations and provide further opportunities in the field of optoelectronics. Herein, a comprehensive review of emerging 2D-organic heterostructures-from their synthesis and fabrication to their state-of-the-art optoelectronic applications-is presented. Future challenges and opportunities associated with these heterostructures are highlighted.

Lattice Opening upon Bulk Reductive Covalent Functionalization of Black Phosphorus

The chemical bulk reductive covalent functionalization of thin-layer black phosphorus (BP) using BP intercalation compounds has been developed. Through effective reductive activation, covalent functionalization of the charged BP by reaction with organic alkyl halides is achieved. Functionalization was extensively demonstrated by means of several spectroscopic techniques and DFT calculations; the products showed higher functionalization degrees than those obtained by neutral routes.

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.

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.

Two-dimensional black phosphorus: its fabrication, functionalization and applications

Phosphorus, one of the most abundant elements in the Earth (∼0.1%), has attracted much attention in the last five years since the rediscovery of two-dimensional (2D) black phosphorus (BP) in 2014. The successful scaling down of BP endows this 'old material' with new vitality, resulting from the intriguing semiconducting properties in the atomic scale limit, i.e. layer-dependent bandgap that covers from the visible light to mid-infrared light spectrum as well as hole-dominated ambipolar transport characteristics. Intensive research effort has been devoted to the fabrication, characterization, functionalization and application of BP and other phosphorus allotropes. In this review article, we summarize the fundamental properties and fabrication techniques of BP, with particular emphasis on the recent progress in molecular beam epitaxy growth of 2D phosphorus. Subsequently, we highlight recent progress in BP (opto)electronic device applications achieved via customized manipulation methods, such as interface, defect and bandgap engineering as well as forming Lego-like stacked heterostructures.

P3Cl2: A Unique Post-Phosphorene 2D Material with Superior Properties against Oxidation

Herein, a unique class of post-phosphorene materials, namely, phosphorene halogenides (e.g., α-P₃Cl₂) with superior oxidation resistance and desirable bandgap characteristics, are proposed. Our first-principles computations show that monolayer α-P₃Cl₂ is a direct semiconductor with a wide bandgap of 2.41 eV (HSE06) or 4.02 eV (G₀W₀), while the bandgap exhibits only slight reduction with increasing number of layers. The monolayer α-P₃Cl₂ also possesses highly anisotropic carrier mobility, with both ultrahigh electron mobility (56 890 cm² V^-1 s^-1) and hole mobility (26 450 cm² V^-1 s^-1). Meanwhile, the outstanding optical properties and favorable band alignment of 2D P₃Cl₂ suggest its potential as a photocatalyst for visible-light water splitting. 2D α-P₃X₂ (X = F, Br, I) also exhibit good oxidation resistance and possess wide direct bandgaps ranging from 2.16 to 2.43 eV (HSE06). These unique electronic and optical properties render 2D phosphorene halogenide as promising functional materials for broad applications in electronic and optoelectronic devices.

Stabilizing black phosphorus nanosheets via edge-selective bonding of sacrificial C60 molecules

Few-layer black phosphorus (BP) with an anisotropic two-dimensional (2D)-layered structure shows potential applications in photoelectric conversion and photocatalysis, but is easily oxidized under ambient condition preferentially at its edge sites. Improving the ambient stability of BP nanosheets has been fulfilled by chemical functionalization, however this functionalization is typically non-selective. Here we show that edge-selective functionalization of BP nanosheets by covalently bonding stable C60 molecules leads to its significant stability improvement. Owing to the high stability of the hydrophobic C60 molecule, C60 functions as a sacrificial shield and effectively protects BP nanosheets from oxidation under ambient condition. C60 bonding leads to a rapid photoinduced electron transfer from BP to C60, affording enhanced photoelectrochemical and photocatalytic activities. The selective passivation of the reactive edge sites of BP nanosheets by sacrificial C60 molecules paves the way toward ambient processing and applications of BP.

Self-assembly of an oligo(p-phenylenevinylene)-based molecule on an HOPG surface: insights from multi-scale simulation and STM observation

To gain knowledge of the most important weak interactions for supramolecular self-assembly and observe molecular structure for self-assembled architectures, the two-dimensional self-assembly of an oligo(p-phenylenevinylene)-based molecule (AS-OPV) on highly oriented pyrolytic graphite has been investigated. Accurate atomic configuration for the AS-OPV self-assembled pattern has been identified by means of multi-scale simulation combined with scanning tunneling microscopy (STM) experiments. The weak interactions which contribute to the formation of AS-OPV self-assembly are studied by analysis of non-covalent interactions existing in the system and theoretical calculation of their energy values. Investigation of the molecular structure of self-assembly and STM images at a certain temperature range is performed by molecular dynamics and density functional theory simulations. This work paves the way to explore the contribution of weak interactions for the self-assembly system, as well as providing a reference to observe the possible self-assembled structure at temperatures not convenient for direct experimental observation.