Monolayer black phosphorus by sequential wet-chemical surface oxidation

Wet-Chemical Synthesis of Elemental 2D Materials

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

Functionalization of an ionic honeycomb KF monolayer via doping

Doping has been widely employed to functionalize two-dimensional (2D) materials because of its effectiveness and simplicity. In this work, the electronic and magnetic properties of pristine and doped KF monolayers are investigated using first-principles calculations based on density functional theory (DFT). Phonon dispersion curves and ab initio molecular dynamics (AIMD) snapshots indicate good stability of the pristine material. The band structure shows an insulating behavior of the KF monolayer, with indirect gaps of 4.80 (6.53) eV as determined using the PBE (HSE06) functional. Its ionic character is also confirmed by the valence charge distribution and Bader charge analysis, and is generated by charge transfer from the K-4s orbital to the F-2p orbital. Doping at both anion and cation sites is explored using N/O and Ca/Sr as dopants, respectively, due to their dissimilar valence electronic configuration in comparison with that of the host atoms. It is found that the KF monolayer is significantly magnetized, where total magnetic moments of 2.00 and 1.00 μB are obtained via N and O/Ca/Sr doping, respectively. Moreover, the appearance of new middle-gap energy states leads to the development of a magnetic semiconductor nature, which is regulated by N-2p, O-2p, Ca-3d, Ca-4s, Sr-4d, and Sr-5s orbitals. Further investigation of codoping indicates that a magnetic-semiconductor nature is preserved, where the synergistic effects of dopants play a key role in the electronic and magnetic properties of the codoped systems. The results presented herein introduce doping as an efficient approach to functionalize the ionic KF monolayer to obtain prospective d0 spintronic materials, a functionality that is not accounted for by the pristine monolayer.

Repairing and Preventing Photooxidation of Few-Layer Black Phosphorus with β-Carotene

Few-layer black phosphorus (FLBP), a technologically important 2D material, faces a major hurdle to consumer applications: spontaneous degradation under ambient conditions. Blocking the direct exposure of FLBP to the environment has remained the key strategy to enhance its stability, but this can also limit its utility. In this paper, a more ambitious approach to handling FLBP is reported where not only is FLBP oxidation blocked, but it is also repaired postoxidation. Our approach, inspired by nature, employs the antioxidant molecule β-carotene that protects plants against photooxidative damages to act as a protecting and repairing agent for FLBP. The mechanistic role of β-carotene is established by a suite of spectro-microscopy techniques, in combination with computational studies and biochemical assays. Transconductance studies on FLBP-based field effect transistor (FET) devices further affirm the protective and reparative effects of β-carotene. The outcomes indicate the potential for deploying a plethora of natural antioxidant molecules to enhance the stability of other environmentally sensitive inorganic nanomaterials and expedite their translation for technological and consumer applications.

Chemistry of two-dimensional pnictogens: emerging post-graphene materials for advanced applications

The layered allotropes of group 15 (P, As, Sb and Bi), also called two-dimensional (2D) pnictogens, have emerged as one of the most promising families of post-graphene 2D-materials. This is mainly due to the great variety of properties they exhibit, including layer-dependent bandgap, high charge-carrier mobility and current on/off ratios, strong spin-orbit coupling, wide allotropic diversity and pronounced chemical reactivity. These are key ingredients for exciting applications in (opto)electronics, heterogeneous catalysis, nanomedicine or energy storage and conversion, to name a few. However, there are still many challenges to overcome in order to fully understand their properties and bring them to real applications. As a matter of fact, due to their strong interlayer interactions, the mechanical exfoliation (top-down) of heavy pnictogens (Sb & Bi) is unsatisfactory, requiring the development of new methodologies for the isolation of single layers and the scalable production of high-quality flakes. Moreover, due to their pronounced chemical reactivity, it is necessary to develop passivation strategies, thus preventing environmental degradation, as in the case of bP, or controlling surface oxidation, with the corresponding modification of the interfacial and electronic properties. In this Feature Article we will discuss, among others, the most important contributions carried out in our group, including new liquid phase exfoliation (LPE) processes, bottom-up colloidal approaches, the preparation of intercalation compounds, innovative non-covalent and covalent functionalization protocols or novel concepts for potential applications in catalysis, electronics, photonics, biomedicine or energy storage and conversion. The past years have seen the birth of the chemistry of pnictogens at the nanoscale, and this review intends to highlight the importance of the chemical approach in the successful development of routes to synthesise, passivate, modify, or process these materials, paving the way for their use in applications of great societal impact.

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.

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.

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.

Surface Modification of Black Phosphorus with Group 13 Lewis Acids for Ambient Protection and Electronic Tuning

Herein we introduce a facile, solution-phase protocol to modify the Lewis basic surface of few-layer black phosphorus (bP) and demonstrate its effectiveness at providing ambient stability and tuning of electronic properties. Commercially available group 13 Lewis acids that range in electrophilicity, steric bulk, and Pearson hard/soft-ness are evaluated. The nature of the interaction between the Lewis acids and the bP lattice is investigated using a range of microscopic (optical, atomic force, scanning electron) and spectroscopic (energy dispersive, X-ray photoelectron) methods. Al and Ga halides are most effective at preventing ambient degradation of bP (>84 h for AlBr₃ ), and the resulting field-effect transistors show excellent IV characteristics, photocurrent, and current stability, and are significantly p-doped. This protocol, chemically matched to bP and compatible with device fabrication, opens a path for deterministic and persistent tuning of the electronic properties in bP.

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.

Prospects for Functionalizing Elemental 2D Pnictogens: A Study of Molecular Models

Despite the intense amount of attention and huge potential of 2D-layered pnictogens for applications in chemistry, physics, and materials science, there has yet to be a robust strategy developed to systematically functionalize them to tailor their properties. This is due to a number of factors, including practical instability toward ambient conditions, difficulty in characterizing modified materials, and also more inherent reactivity issues. Here, avenues for functionalization are discussed using examples of molecular models from the wider literature, along with their possible advantages and likely pitfalls. Finally, a critical appraisal of the current field and its future is offered.

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.

Few-layer Black Phosphorous Catalyzes Radical Additions to Alkenes Faster than Low-valence Metals

The substitution of catalytic metals by p-block main elements has a tremendous impact not only in the fundamentals but also in the economic and ecological fingerprint of organic reactions. Here we show that few-layer black phosphorous (FL-BP), a recently discovered and now readily available 2D material, catalyzes different radical additions to alkenes with an initial turnover frequency (TOF0) up to two orders of magnitude higher than representative state-of-the-art metal complex catalysts at room temperature. The corresponding electron-rich BP intercalation compound (BPIC) KP6 shows a nearly twice TOF0 increase with respect to FL-BP. This increase in catalytic activity respect to the neutral counterpart also occurs in other 2D materials (graphene vs. KC8) and metal complex catalysts (Fe0 vs. Fe2- carbon monoxide complexes). This reactive parallelism opens the door for cross-fertilization between 2D materials and metal catalysts in organic synthesis.

Enhanced ambient stability of exfoliated black phosphorus by passivation with nickel nanoparticles

Since its discovery, the environmental instability of exfoliated black phosphorus (2D bP) has emerged as a challenge that hampers its wide application in chemistry, physics, and materials science. Many studies have been carried out to overcome this drawback. Here we show a relevant enhancement of ambient stability in few-layer bP decorated with nickel nanoparticles as compared to pristine bP. In detail, the behavior of the Ni-functionalized material exposed to ambient conditions in the dark is accurately studied by Transmission Electron Microscopy (TEM), Raman Spectroscopy, and high resolution x-ray Photoemission and Absorption Spectroscopy. These techniques provide a morphological and quantitative insight of the oxidation process taking place at the surface of the bP flakes. In the presence of Ni nanoparticles (NPs), the decay time of 2D bP to phosphorus oxides is more than three time slower compared to pristine bP, demonstrating an improved structural stability within 20 months of observation.

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.

Robust Amphiphobic Few-Layer Black Phosphorus Nanosheet with Improved Stability

Few-layer black phosphorus (FL-BP) has been intensively studied due to its attractive properties and great potential in electronic and optoelectronic applications. However, the intrinsic instability of FL-BP greatly limits its practical application. In this study, the amphiphobic FL-BP is achieved by functionalization of 1H,1H,2H,2H-perfluorooctyltrichlorosilane (PFDTS) on the surface of FL-BP. The obtained PFDTS coated FL-BP (FL-BP/PFDTS) demonstrates enhanced stability, which is not observed during significant degradation for 2 months in high moisture content environment (95% humidity). Particularly, attributing to the surface amphiphobicity, FL-BP/PFDTS exhibits strong surface water repellency in the presence of oleic acid (as the contaminant), while other passivation coating layers (such as hydrophilic or hydrophobic coating) become hydrophilicity under such conditions. Owing to this advantage, the obtained FL-BP/PFDTS demonstrates enhanced stability in high moisture content environment for 2 months, even though the surface is contaminated by oil liquid or other organic solvents (such as oleic acid, CH2Cl2, and N-methyl-2-pyrrolidone). The passivation of FL-BP by amphiphobic coating provides an effective approach for FL-BP stabilization toward future applications.

Noncovalent Functionalization of 2D Black Phosphorus with Fluorescent Boronic Derivatives of Pyrene for Probing and Modulating the Interaction with Molecular Oxygen

We studied the chemical-physical nature of interactions involved in the formation of adducts of two-dimensional black phosphorus (2D BP) with organoboron derivatives of a conjugated fluorescent molecule (pyrene). Time-resolved fluorescence spectroscopy showed a stabilization effect of 2D BP on all derivatives, in particular for the adducts endowed with the boronic functionalities. Also, a stronger modulation of the fluorescence decay with oxygen was registered for one of the adducts compared to the corresponding organoboron derivative alone. Nuclear magnetic resonance experiments in suspension and density functional theory simulations confirmed that only noncovalent interactions were involved in the formation of the adducts. The energetic gain in their formation arises from the interaction of P atoms with both C atoms of the pyrene core and the B atom of the boronic functionalities, with a stronger contribution from the ester with respect to the acid one. The interaction results in the lowering of the band gap of 2D BP by around 0.10 eV. Furthermore, we demonstrated through Raman spectroscopy an increased stability toward oxidation in air of 2D BP in the adducts in the solid state (more than 6 months). The modification of the electronic structure at the interface between 2D BP and a conjugated organic molecule through noncovalent stabilizing interactions mediated by the B atom is particularly appealing in view of creating heterojunctions for optoelectronic, photonic, and chemical sensing applications.

Solvent exfoliation stabilizes TiS2 nanosheets against oxidation, facilitating lithium storage applications

Titanium disulfide is a promising material for a range of applications, including lithium-ion battery (LIB) anodes. However, its application potential has been severely hindered by the tendency of exfoliated TiS2 to rapidly oxidize under ambient conditions. Herein, we confirm that, although layered TiS2 powder can be exfoliated by sonication in aqueous surfactant solutions, the resultant nanosheets oxidise almost completely within hours. However, we find that upon performing the exfoliation in the solvent cyclohexyl-pyrrolidone (CHP), the oxidation is almost completely suppressed. TiS2 nanosheets dispersed in CHP and stored at 4 °C in an open atmosphere for 90 days remained up to 95% intact. In addition, CHP-exfoliated nanosheets did not show any evidence of oxidation for at least 30 days after being transformed into dry films even when stored under ambient conditions. This stability, probably a result of a residual CHP coating, allows TiS2 nanosheets to be deployed in applications. To demonstrate this, we prepared lithium ion battery anodes from nano : nano composites of TiS2 nanosheets mixed with carbon nanotubes. These anodes displayed reversible capacities (920 mA h g-1) close to the theoretical value and showed good rate performance and cycling capability.