Covalent Functionalization of Black Phosphorus from First-Principles

Polyelectrolyte modified black phosphorus/titania nanosheet heterojunction enhanced photocatalysis: Synergistic enhancement effect of interface affinity and electron transport channel

The instability and high electron-hole recombination have limited the application of black phosphorus (BP) as an excellent photocatalyst. To address these challenges, poly dimethyl diallyl ammonium chloride (PDDA), poly (allylamine hydrochloride) (PAH), and polyethyleneimine (PEI) are introduced to the functionalization of BP (F-BP), which can not only enhance its stability, but also boost the carrier transfer. Furthermore, a high-performance heterojunction photocatalyst is fabricated using F-BP and titania nanosheets (TNs) via a layer-by-layer self-assembly approach. The experimental outcomes unequivocally indicate that F-BP exhibits fast charge migration compared to BP. The density functional theory (DFT), in situ Kelvin-probe force microscopy (KPFM) and other advanced characterization techniques collectively unfold that PDDA modified BP can notably boost separation and propagation of charges, along with an enhanced carrier abundance. In summary, this novel strategy of using polyelectrolytes to enhance the electron transfer and the stability of BP permits immense potential in building next-generation BP-based high efficiency photocatalysts.

Bimetallic Alloys b-AsxP1-x at High Concentration Differences: Ideal for Photonic Devices

Black arsenic phosphorus (b-As_xP_1-x) is expected to be one of the primary materials for future photonic devices. However, the x-factor is randomly estimated and applied in photonic devices in current studies, rather than systematically analyzing it for a comprehensive understanding. Herein, As_xP_1-x switches from a direct band gap semiconductor to an indirect band gap one at x = 0.75. As_xP_1-x at x ≤ 0.25 is capable of broadband absorption, while b-As_xP_1-x at x ≥ 0.75 can only absorb at specific wavelengths in the perspective of the electron energy transition. Additionally, the optoelectronic response of the integral field-effect transistor configurations constructed with b-As_xP_1-x is investigated systematically as a photodetector device. The photonic response characteristics show high polarization sensitivity at x ≥ 0.75, but a typical circuit system signal at x ≤ 0.25. These results suggest that b-As_xP_1-x with high concentration differences is a perfect candidate for photonic material.

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.

Black phosphorus for near-infrared ultrafast lasers in the spatial/temporal domain

Two-dimensional (2D) materials have attracted extensive interests due to their wide range of electronic and optical properties. After continuous and extensive research, black phosphorus (BP), a novel member of 2D layered semiconductor material, benefit for the unique in-plane anisotropic structure, controllable direct bandgap characteristic, and high charge carrier mobility, has attracted tremendous attention and successfully applied in ultrafast pulse generation. This article, which focuses on near-infrared ultrafast laser demonstration of BP, present discussion of preparation methods for high quality BP nanosheet, various BP based ultrafast lasers in the spatial/temporal domain, and the future research needs.

Black Phosphorus, an Emerging Versatile Nanoplatform for Cancer Immunotherapy

Black phosphorus (BP) is one of the emerging versatile nanomaterials with outstanding biocompatibility and biodegradability, exhibiting great potential as a promising inorganic nanomaterial in the biomedical field. BP nanomaterials possess excellent ability for valid bio-conjugation and molecular loading in anticancer therapy. Generally, BP nanomaterials can be classified into BP nanosheets (BPNSs) and BP quantum dots (BPQDs), both of which can be synthesized through various preparation routes. In addition, BP nanomaterials can be applied as photothermal agents (PTA) for the photothermal therapy (PTT) due to their high photothermal conversion efficiency and larger extinction coefficients. The generated local hyperpyrexia leads to thermal elimination of tumor. Besides, BP nanomaterials are capable of producing singlet oxygen, which enable its application as a photosensitizer for photodynamic therapy (PDT). Moreover, BP nanomaterials can be oxidized and degraded to nontoxic phosphonates and phosphate under physiological conditions, improving their safety as a nano drug carrier in cancer therapy. Recently, it has been reported that BP-based PTT is capable of activating immune responses and alleviating the immunosuppressive tumor microenvironment by detection of T lymphocytes and various immunocytokines, indicating that BP-based nanocomposites not only serve as effective PTAs to ablate large solid tumors but also function as an immunomodulation agent to eliminate discrete tumorlets. Therefore, BP-mediated immunotherapy would provide more possibilities for synergistic cancer treatment.

Tunable phase transitions and high photovoltaic performance of two-dimensional In2Ge2Te6 semiconductors

Ultrathin semiconductors with great electrical and photovoltaic performance hold tremendous promise for fundamental research and applications in next-generation electronic devices. Here, we report new 2D direct-bandgap semiconductors, namely mono- and few-layer In₂Ge₂Te₆, with a range of desired properties from ab initio simulations. We suggest that 2D In₂Ge₂Te₆ samples should be highly stable and can be experimentally fabricated by mechanical exfoliation. They are predicted to exhibit extraordinary optical absorption and high photovoltaic conversion efficiency (≥31.8%), comparable to the most efficient single-junction GaAs solar cell. We reveal that, thanks to the presence of van Hove singularities in the band structure, unusual quantum-phase transitions could be induced in monolayers via electrostatic doping. Furthermore, taking bilayer In₂Ge₂Te₆ as a prototypical system, we demonstrate the application of van der Waals pressure as a promising strategy to tune the electronic and stacking property of 2D crystals. Our work creates exciting opportunities to explore various quantum phases and atomic stacking, as well as potential applications of 2D In₂Ge₂Te₆ in future nanoelectronics.

Structural, electronic and optical properties of pristine and functionalized MgO monolayers: a first principles study

In this paper, we present a detailed investigation of the structural, electronic, and optical properties of pristine, nitrogenated, and fluorinated MgO monolayers using ab initio calculations. The two dimensional (2D) material stability is confirmed by the phonon dispersion curves and binding energies. Full functionalization causes notable changes in the monolayer structure and slightly reduces the chemical stability. The simulations predict that the MgO single layer is an indirect semiconductor with an energy gap of 3.481 (4.693) eV as determined by the GGA-PBE (HSE06) functional. The electronic structure of the MgO monolayer exhibits high sensitivity to chemical functionalization. Specifically, nitrogenation induces metallization of the MgO monolayer, while an indirect-direct band gap transition and band gap reduction of 81.34 (59.96)% are achieved by means of fluorination. Consequently, the functionalized single layers display strong optical absorption in the infrared and visible regimes. The results suggest that full nitrogenation and fluorination may be a quite effective approach to enhance the optoelectronic properties of the MgO monolayer for application in nano-devices.

Recent advances in long-term stable black phosphorus transistors

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

Black phosphorus for fighting antibiotic-resistant bacteria: What is known and what is missing

Recently, two-dimensional black phosphorus (BP) nanomaterial has captured much attention due to its superb physiochemical and electronic properties and various promising biomedical applications. However, relatively few studies have explored its antimicrobial properties, particularly for targeting antibiotic-resistant pathogens. A comprehensive understanding of the bactericidal mechanisms of BP is essential for application of this material as an antimicrobial. This review discusses the physicochemical and electronic properties of BP that are relevant for antimicrobial applications, especially the unique characteristics that may play a role in overcoming drug resistance. The literature is discussed in the context of what is known and what information is missing. We also highlight the differences and advantages of BP over other two-dimensional nanomaterials (i.e., graphene oxide and molybdenum disulfide) for bactericidal activity. Finally, we analyze existing challenges and note topics that require future investigation to overcome current inadequacies, aiming to assist the safe development of BP-based nanotechnology for pathogen control.

Covalent Functionalized Black Phosphorus Greatly Inhibits Nonradiative Charge Recombination: A Time Domain Ab Initio Study

Mono- or few-layer black phosphorus (BP) has emerged as a promising photovoltaic and optoelectronic material with realistic applications subjected to instability and short charge carrier lifetime. Experiments show that covalent functionalization can improve the stability, but the underlying mechanism for the prolonged lifetime remains elusive. By performing spin-polarized time domain density functional theory combined with nonadiabatic (NA) molecular dynamics simulations, we demonstrate that BP passivated with both phenyl and nitrophenyl can suppress the nonradiative electron-hole recombination by a factor of 2 and 3, respectively, relative to the pristine system. The slow recombination is due to the interplay between energy gap, NA coupling, and decoherence time, which happens either through a hole-trap-assisted process or in a direct way between a free electron and hole in the spin-up channel. The observations hold in the spin-down channel. The study suggests that the passivating strategy should work for BP and other two-dimensional materials.

Black phosphorus electronics

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

Unveiling chemical reactivity and oxidation of 1T-phased group VI disulfides

Transition metal dichalcogenides (TMDs) are of particular interest because of their unique electrical and optical properties that evolve from the quantum confinement and surface effects. However, their long-term stability in air is proved to be a main concern for practical applications of the ultrathin materials, especially for TMDs with 1T phased structures. Here, we provide an in-depth understanding of the oxidation and degradation mechanisms of monolayers of group VIB disulfides, including TiS₂, ZrS₂, and HfS₂. As the atomic radius of the transitional metals increases, their air stability significantly decreases and the oxidation mechanisms are quite different from one another. In particular, the oxygen induced oxidations initiated at both the surface vacancy sites and edges of ZrS₂ and HfS₂ are studied, while the oxidation of TiS₂ starts at the edges and water plays a crucial role in the continuous oxidation process. Moreover, the defective sites expose the metals for activation and dissociation of either oxygen or water, causing the breakdown of the systems eventually. Meanwhile, these sites can be used as active centers for specific applications in catalysts and surface functionalized materials.

Role of Structural Distortion in Stabilizing Electrosynthesized Blue-Emitting Phosphorene Quantum Dots

Luminescent phosphorene quantum dots (PQDs) have emerged as fascinating nanomaterials for potential applications in optoelectronics, catalysis, and sensing. Herein, we investigate the structural distortion of black phosphorus (BP) under an applied electric field to yield blue luminescent PQDs [average diameter 8 ± 1.5 nm ( N = 60)]. The electrosynthesized PQDs exhibit photoluminescence emission independent of excitation wavelength with 84% quantum efficiency. Structural distortion that occurred during the transformation of BP to PQDs is confirmed by results obtained during transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Further, using first-principles-based density functional theory, calculations on oxygenated and nonoxygenated PQDs augment the experimental observations that an optimum oxygen content maintains the structural integrity of PQDs, above which the structural robustness of PQDs is drastically diminished.

Transport and photogalvanic properties of covalent functionalized monolayer black phosphorus

Opposite responses of the transport and photogalvanic properties of the two-probe devices based on covalent functionalized black phosphorus.

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.

Tunable transport and optoelectronic properties of monolayer black phosphorus by grafting PdCl2 quantum dots

The electronic, transport, and optoelectronic properties of monolayer black phosphorus (MLBP) are much influenced by grafting PdCl2 groups, demonstrated here by using density functional theory (DFT) and non-equilibrium Green's function (NEGF) as well as the Keldysh Nonequilibrium Green's Functions (KNEGF) methods. We find that the PdCl2 groups prefer to locate over the furrow site of MLBP and form a planar quadridentate structure of . The PdCl2 groups serve as quantum dots by introducing discrete flat levels between the MLBP valence band and the Fermi level (E f). The conductivity is much lowered after attaching PdCl2 quantum dots, due to the fact that the scattering effect of PdCl2 plays a major role in the process of electron transporting. A threshold voltage is found for the functionalized system with a large density of PdCl2 quantum dots, a valuable clue for exploring current switches. However, no evident threshold voltage is found for the pure MLBP. Electrons permeate easier through the armchair direction compared with the zigzag either in the pure MLBP or in the functionalized composites. More importantly, grafting PdCl2 quantum dots is very beneficial for enhancing photoresponse. The values of photoresponse for the modified species are about 20 times higher than the free MLBP. A significant photoresponse anisotropy is observed for both MLBP and nPdCl2-BP (n = 1, 2, and 4), contrary to the conductivity, the zigzag direction shows much stronger photoresponse than the armchair. All of the aforementioned unique properties make these new two-dimensional (2D) MLBP based materials especially attractive for both electronic and optoelectronic devices.

Photo-oxidative Degradation and Protection Mechanism of Black Phosphorus: Insights from Ultrafast Dynamics

The environmental instability and protection of black phosphorus (BP) is one of the most attractive hotspots in two-dimensional materials. The generation of superoxide is believed to be the key culprit, while the photogenerated electron dynamics is yet to be known. In this work, we carry out time domain ab initio nonadiabatic molecular dynamics to understand the photogenerated electron dynamics at the molecule/BP interface. It is found that oxygen can trap the photogenerated electrons of BP rapidly owing to strong electron-phonon (e-p) coupling and becomes an active superoxide under light illumination. A good protection layer, such as perylene diimide (PDI), has comparable capabilities of trapping photogenerated electrons to oxygen, which can efficiently prevent the formation of superoxide and thus suppress the degradation of BP. Moreover, PDI can enhance the separation of photogenerated electron-hole pairs of BP by prolonging the time of holding photogenerated electrons of BP. In contrast, 7,7,8,8-tetracyano- p-quinodimethane (TCNQ) has weak e-p coupling and a large band edge offset between trapping and donor states and thereby poor trapping ability for photogenerated electrons, leading to poor protection efficiency. This study provides the first in-depth understanding of the BP degradation and protection mechanism from excited-state dynamics and can be applicable to other 2D photo-oxidative degradation.

Defects and oxidation of group-III monochalcogenide monolayers

Among various two-dimensional (2D) materials, monolayer group-III monochalcogenides (GaS, GaSe, InS, and InSe) stand out owing to their potential applications in microelectronics and optoelectronics. Devices made of these novel 2D materials are sensitive to environmental gases, especially O₂ molecules. To address this critical issue, here we systematically investigate the oxidization behaviors of perfect and defective group-III monochalcogenide monolayers by first-principles calculations. The perfect monolayers show superior oxidation resistance with large barriers of 3.02-3.20 eV for the dissociation and chemisorption of O₂ molecules. In contrast, the defective monolayers with single chalcogen vacancy are vulnerable to O₂, showing small barriers of only 0.26-0.36 eV for the chemisorption of an O₂ molecule. Interestingly, filling an O₂ molecule to the chalcogen vacancy of group-III monochalcogenide monolayers could preserve the electronic band structure of the perfect system-the bandgaps are almost intact and the carrier effective masses are only moderately disturbed. On the other hand, the defective monolayers with single vacancies of group-III atoms carry local magnetic moments of 1-2 μ_B. These results help experimental design and synthesis of group-III monochalcogenides based 2D devices with high performance and stability.

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.

Covalent functionalization of black phosphorus nanoflakes by carbon free radicals for durable air and water stability

This work developed a simple and efficient method to covalently functionalize black phosphorus nanoflakes (BPNFs) with carbon free radicals from azodiisobutyronitrile (AIBN) molecules. BPNFs after successful modification (BPNFs-AIBN) not only had good stability in air and aqueous solution, but also still maintained good optical properties.

Polymer-Based Black Phosphorus (bP) Hybrid Materials by in Situ Radical Polymerization: An Effective Tool To Exfoliate bP and Stabilize bP Nanoflakes

Black phosphorus (bP) has been recently investigated for next generation nanoelectronic multifunctional devices. However, the intrinsic instability of exfoliated bP (the bP nanoflakes) toward both moisture and air has so far overshadowed its practical implementation. In order to contribute to fill this gap, we report here the preparation of new hybrid polymer-based materials where bP nanoflakes (bPn) exhibit a significantly improved stability. The new materials have been prepared by different synthetic paths including: (i) the mixing of conventionally liquid-phase exfoliated bP (in dimethyl sulfoxide, DMSO) with poly(methyl methacrylate) (PMMA) solution; (ii) the direct exfoliation of bP in a polymeric solution; (iii) the in situ radical polymerization after exfoliating bP in the liquid monomer (methyl methacrylate, MMA). This last methodology concerns the preparation of stable suspensions of bPn-MMA by sonication-assisted liquid-phase exfoliation (LPE) of bP in the presence of MMA followed by radical polymerization. The hybrids characteristics have been compared in order to evaluate the bP dispersion and the effectiveness of the bPn interfacial interactions with polymer chains aimed at their long-term environmental stabilization. The passivation of the bPn is particularly effective when the hybrid material is prepared by in situ polymerization. By using this synthetic methodology, the nanoflakes, even if with a gradient of dispersion (size of aggregates), preserve their chemical structure from oxidation (as proved by both Raman and 31P-solid state NMR studies) and are particularly stable to air and UV light exposure. The feasibility of this approach, capable of efficiently exfoliating bP while protecting the bPn, has been then verified by using different vinyl monomers (styrene and N-vinylpyrrolidone), thus obtaining hybrids where the nanoflakes are embedded in polymer matrices with a variety of intriguing thermal, mechanical, and solubility characteristics.

Electron Pair Repulsion Responsible for the Peculiar Edge Effects and Surface Chemistry of Black Phosphorus

The electronic and optical properties of black phosphorus (black-P) are significantly modulated by fabricating the edges of this two-dimensional material. Electron lone pairs (ELPs) are ubiquitous in black-P, but their role in creating the edge effects of black-P is poorly understood. Using first-principle calculations, we report ELPs of black-P experience severe Coulomb repulsion and play a central role in creating the edge effects of black-P. We discover the outermost P atoms of the zigzag edges of black-PQDs are free of the Coulomb repulsion, but the P atoms of the armchair edges do experience the Coulomb repulsion. The Coulomb repulsion serves as a new chemical driving force to make electron donor-acceptor bonds with chemical groups bearing vacant orbitals. Our results provide insights into the mechanism responsible for the peculiar edge effects of black-P and highlight the opportunity to use the ELPs of black-P for their damage-free surface functionalization.

Rational Surface Modification of Two-Dimensional Layered Black Phosphorus: Insights from First-Principles Calculations

Surface modification of atomically thin semiconductors enables their electronic, optical, chemical, and mechanical properties to be tailored and allows these nanosheets to be processed in solutions. Here, we report first-principles density functional theory calculations, through which we show chemical functionalization of black phosphorus using phenyl, phenolate, and nitrene species, which were widely investigated for carbon-based materials. We find that covalent functionalization using nitrene-derived species introduces a strong P-N dative bond at the interface without perturbing its intrinsic electronic structure. The Lewis basic and nucleophilic P atom attacks, through a free pair of electrons, the Lewis acidic nitrene species. These results are further compared to other nitrene-derived functional groups on black phosphorus, including N-methylbenzene, N-aminobenzene, and N-nitrobenzene. We find that by tuning the charge redistribution at the interface, the work function of black phosphorus can be tuned by more than 2 eV. These results suggest valuable tunability of the electronic properties of two-dimensional layered black phosphorus by covalent functionalization for future device applications.

On-surface synthesis: a promising strategy toward the encapsulation of air unstable ultra-thin 2D materials

2D black phosphorus (BP) and transition metal chalcogenides (TMCs) have beneficial electronic, optical, and physical properties at the few-layer limit. However, irreversible degradation of exfoliated or chemical vapor deposition-grown ultrathin BP and TMCs like GaSe via oxidation under ambient conditions limits their applications. Herein, the on-surface growth of an oxidation-resistant 2D thin film of a metal coordination polymer is demonstrated by multiscale simulations. We show that the preparation of such heterostructures can be conducted in solution, in which pristine BP and GaSe present better stability than in an air environment. Our calculations reveal that the interaction between the polymer layer and 2D materials is dominated by van der Waals forces; thus, the electronic properties of pristine BP and GaSe are well preserved. Meanwhile, the isolation from oxygen and water can be achieved by monolayer polymers, due to the nature of their close-packed layers. Our facile strategy for enhancing the environmental stability of ultrathin materials is expected to accelerate efforts to implement 2D materials in electronic and optoelectronic applications.

Quasi-Monolayer Black Phosphorus with High Mobility and Air Stability

Black phosphorus (BP) exhibits thickness-dependent band gap and high electronic mobility. The chemical intercalation of BP with alkali metal has attracted attention recently due to the generation of universal superconductivity regardless of the type of alkali metals. However, both ultrathin BP, as well as alkali metal-intercalated BP, are highly unstable and corrode rapidly under ambient conditions. This study demonstrates that alkali metal hydride intercalation decouples monolayer to few layers BP from the bulk BP, allowing an optical gap of ≈1.7 eV and an electronic gap of 1.98 eV to be measured by photoluminescence and electron energy loss spectroscopy at the intercalated regions. Raman and transport measurements confirm that chemically intercalated BP exhibits enhanced stability, while maintaining a high hole mobility of up to ≈800 cm² V^-1 s^-1 and on/off ratio exceeding 10³ . The use of alkali metal hydrides as intercalants should be applicable to a wide range of layered 2D materials and pave the way for generating highly stable, quasi-monolayer 2D materials.

Emerging novel electronic structure in hydrogen-Arsenene-halogen nanosheets: A computational study

Based on first-principles calculations including spin-orbit coupling, we investigated the stability and electronic structure of unexplored double-side decorated arsenenes. It has been found that these new double-side decorated arsenenes, which we call "hydrogen-arsenene-halogen (H-As-X, X is halogen)", are dynamically stable via the phonon dispersion calculations except H-As-F sheets. In particular, all of H-As-X nanosheets are direct band gap semiconductors with a strong dispersion near the Fermi level, which is substantially different from the previous works of double-side decorated arsenenes with zero band gaps. Our results reveal a new route to change the band gap of arsenene from indirect to direct. Furthermore, we also studied bilayer, trilayer, and multilayer H-As-Cl sheets to explore the effects of the layer number. The results indicate that bilayer, trilayer, and multilayer H-As-Cl sheets display novel electronic structure, namely multi-Dirac cones character, and the Dirac character depends sensitively on the layer number. It is noted that the frontier states near the Fermi level are dominantly controlled by the top and bottom layers in trilayer and multilayer H-As-Cl sheets. Our findings may provide the valuable information about the new double-side decorated arsenene sheets in various practical applications in the future.

Reliable passivation of black phosphorus by thin hybrid coating

Black phosphorus (BP) possesses several extraordinary physical properties, which include in-plane anisotropy, thickness dependent direct bandgap and high carrier mobility. These physical properties make BP highly desirable from the point of view of fundamental science and modern optoelectronics applications. The excitement about this material has always been accompanied by unreserved skepticism due to its extraordinary degradation under ambient conditions. Here we show ambient degradation of exfoliated BP can be effectively suppressed using thin layer of hybrid metal organic chemical vapor deposition coating of boron nitride (BN) followed by atomic layer deposition coating of Al₂O₃. We have extensively studied the time dependent surface, optical and electrical properties of BP encapsulated by BN and/or Al₂O₃ using nanoscale infrared imaging and I-V characterizations. Our results show hybrid thin layer (∼5 nm) BN/Al₂O₃ coated BP exfoliated on SiO₂ substrate is protected from degradation in ambient for over 6 months, much longer than those coated only by BN or Al₂O₃ layers. Our theoretical modeling of the experimental degradation growth pattern shows that the influence of neighboring elements on the degradation of a given element is minimal for BP flakes with hybrid coating. Electrical characterization further confirms the effectiveness of BN/Al₂O₃ as encapsulation layer and gate dielectrics with minor changes after several weeks.

Oxidation Resistance of Monolayer Group-IV Monochalcogenides

Ridged, orthorhombic two-dimensional (2D) group-V elemental and group IV-VI compound analogues of phosphorene provide a versatile platform for nanoelectronics, optoelectronics, and clean energy. However, phosphorene is vulnerable to oxygen in ambient air, which is a major obstacle for its applications. Regarding this issue, here we explore the oxidation behavior of monolayer group-IV monochalcogenides (GeS, GeSe, SnS, and SnSe), in comparison to that of phosphorene and arsenene by first-principles calculations. We find superior oxidation resistance of the monolayer group-IV monochalcogenides, with activation energies for the chemisorption of O₂ on the 2D sheets in the range of 1.26-1.60 eV, about twice of the values of phosphorene and arsenene. The distinct oxidation behaviors of monolayer group-IV monochalcogenides and group-V phosphorene analogues originate from their different bond natures. Moreover, the chemisorption of a moderate amount of oxygen atoms does not severely deteriorate the electronic band structures of the monolayer group-IV monochalcogenides. These results shine light on the utilization of the monolayer group-IV monochalcogenides for next-generation 2D electronics and optoelectronics with high performance and stability.

Band-edge engineering via molecule intercalation: a new strategy to improve stability of few-layer black phosphorus

Through embedding small molecules into few-layer black phosphorous, the production of photo-generated superoxide on its surface can be suppressed.