Fluorinated Phosphorene: Electrochemical Synthesis, Atomistic Fluorination, and Enhanced Stability

Difluorophosphorane-Flattened Phosphorene through Difluorination

We computationally showed that by difluorination of phosphorene we can make a new material difluorphosphorane (DFP) with perfectly planar honeycomb structure out of phosphorus atoms with fluorine atoms attached to every phosphorus atom from above and below. The structure is dynamically stable. It is a semiconductor with a direct band gap of 4.51 eV and an indirect band gap of 3.88 eV. We hope that with the passivation this new DFP material if made could find many applications in nanoelectronics.

Functionalized Phosphorene Quantum Dots as Efficient Electrocatalyst for Oxygen Evolution Reaction

Phosphorene has attracted great interest in the rapidly emerging field of two-dimensional layered nanomaterials. Recent studies show promising electrocatalytic activity of few-layered phosphorene sheets toward the oxygen evolution reaction (OER). However, controllable synthesis of mono/few-layered phosphorene nanostructures with a large number of electrocatalytically active sites and exposed surface area is important to achieve significant enhancement in OER activity. Here, a novel strategy for controlled synthesis and in situ surface functionalization of phosphorene quantum dots (PQDs) using a single-step electrochemical exfoliation process is demonstrated. Phosphorene quantum dots functionalized with nitrogen-containing groups (FPQDs) exhibit efficient and stable electrocatalytic activity for OER with an overpotential of 1.66 V @ 10 mA cm^-2, a low Tafel slope of 48 mV dec^-1, and excellent stability. Further, we observe enhanced electron transfer kinetics for FPQDs toward the Fe²⁺/Fe³⁺ redox probe in comparison with pristine PQDs. The results demonstrate the promising potential of phosphorene as technologically viable OER electrodes for water-splitting devices.

Two-dimensional beta-lead oxide quantum dots

In recent years, black-phosphorus-analogue (BPA) two-dimensional (2D) materials have been explored to demonstrate promising optoelectronic performances and distinguished ambient stabilities, holding great promise in practical applications. Here, one new kind of BPA material, orthorhombic β-PbO quantum dots (QDs), is successfully fabricated by a facile liquid phase exfoliation (LPE) technique. The as-prepared β-PbO QDs show a homogeneous distribution of the lateral size (3.2 ± 0.9 nm) and thickness (2.5 ± 0.5 nm), corresponding to 4 ± 1 layers. The carrier dynamics of β-PbO QDs was systematically investigated via a femtosecond resolution transient absorption approach in the visible wavelength regime and it was clarified that two decay components were resolved with a decay time of τ1 = 2.3 ± 0.3 ps and τ2 = 87.9 ± 6.0 ps, respectively, providing important insights into their potential applications in the field of ultrafast optics, nanomechanics and optoelectronics. As a proof-of-concept, β-PbO QDs were, for the first time to our knowledge, fabricated as a working electrode in a photoelectrochemical (PEC)-typed photodetector that exhibits significantly high photocurrent density and excellent stability under ambient conditions.

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.

Fluorination-Enhanced Ambient Stability and Electronic Tolerance of Black Phosphorus Quantum Dots

The environmental instability and uneliminable electronic trap states in black phosphorus quantum dots (BPQDs) limit the optoelectronics and related applications of BPQDs. Here, fluorinated BPQDs (F-BPQDs) are successfully synthesized by using a facile electrochemical exfoliation and synchronous fluorination method. The F-BPQDs exhibit robust ambient stability and limited fluorination capability, showing a nonstoichiometric fluorination degree (DF) maximum of ≈0.68. Density functional theory calculations confirm that due to the edge etching effect of fluorine adatoms, the simulated F-BPQDs become structurally unstable when DF surpasses the limit. Furthermore, the trap states of BPQDs can be effectively eliminated via fluorination to obtain a coordination number of 3 or 5 for fluorinated and unfluorinated phosphorus atoms. The results reveal that the air-stable F-BPQDs exhibit fluorine defect-enhanced electronic tolerance, which is crucial for nanophotonics and nanoelectronics applications.

Products of Degradation of Black Phosphorus in Protic Solvents

Deterioration of the surface of black phosphorus (BP) caused by ambient atmosphere is an undesired process, limiting broader use of BP in many areas. The mechanism of BP degradation was explained theoretically, and the oxidized materials were thoroughly characterized experimentally. However, the surface analysis techniques introduce only a limited insight into the real state of the material. Here, we report a thorough analysis of the composition of mixtures obtained after a prolonged exposure of suspensions of BP to atmospheric oxygen with the aim to further disclosure the processes involved in the decomposition process. The results are compared with the predicted structures of the oxidized material and confirm the results of the theoretical calculations. The comparison of reactivity of BP with reactivity of white phosphorus under similar conditions concludes a similar distribution of the products in both cases.

New solvent-stabilized few-layer black phosphorus for antibacterial applications

Discovering highly efficient, environmentally friendly, and low-cost exfoliating media that can both disperse and protect black phosphorus (BP) remains a challenge. Herein, we demonstrate such a new molecule, N,N'-dimethylpropyleneurea (DMPU), for effective exfoliation and dispersion of two-dimensional BP nanosheets. A very high exfoliation efficiency of up to 16% was achieved in DMPU, significantly surpassing other good solvents. Exfoliated flakes are free from structural disorder or oxidation. Nanosheets retain high stability in DMPU even after addition of 25 vol% of common solvents. The solvation shell appears to protect the nanosheets from reacting with water and air, more remarkably than the best solvent N-cyclohexyl-2-pyrrolidone reported so far. Molecular dynamics simulations of the exfoliation process show that DMPU is among the effective solvents, although energetically it does not appear as favorable as some other amides. We also demonstrate that our exfoliated BP nanosheets exhibit excellent antimicrobial activities against both Escherichia coli and Staphylococcus aureus, outperforming other common two-dimensional materials of graphene and MoS2, suggesting promise in biomedical applications.

Realizing Long-Term Stability and Thickness Control of Black Phosphorus by Ambient Thermal Treatment

Few-layer black phosphorus (BP) has shown great potential for next-generation electronics with tunable band gap and high carrier mobility. For the electronic applications, the thickness modulation of a BP flake is essential due to its thickness-dependent electronic properties. However, controlling the precise thickness of few-layer BP is a challenge for the high-performance device applications. In this study, we demonstrate that thermal treatment under ambient condition precisely controls the thickness of BP flake. The thermal etching method utilizes the chemical reactivity of BP surface with oxygen and water molecules by the repeated formation and evaporation of phosphoric acid during thermal annealing. Field-effect transistor of the thickness-modulated BP sheet by thermal etching method shows a high hole mobility of ∼576 cm² V^-1 s^-1 and a high on-off ratio of ∼10⁵. The stability of the BP devices remained for 1 month under ambient condition without an additional protecting layer, resulting from the preservation of active BP layers below native surface phosphorus oxide.