Metallic impurities in black phosphorus nanoflakes prepared by different synthetic routes

Synthesis of High-Quality Bulk Single-Crystal Black Phosphorus by the Circulating Vapor Growth Approach

Black phosphorus (BP), a promising two-dimensional (2D) layered semiconductor material, has gained enormous attention due to its impressive properties over the past several years. Although plenty of methods have been developed to synthesize high-quality BP, most of the currently available BP materials still suffer from unsatisfactory crystallization, purity, and stability in air, hindering their practical application. A facile approach to synthesizing ultrahigh-quality single-crystal BP is of significance to shed light on the nature of 2D semiconductor materials and their massive application. In this work, we present the facile and efficient circulating vapor growth approach to growing bulk single-crystal BP. The as-grown BP material features high crystallinity and ultrahigh purity (higher than 99.999 at %), exceeding those of all the previously reported and some commercially available BP crystals. It also maintains excellent stability in air and water after 15 consecutive days of test. Moreover, the as-synthesized BP material features good thermal stability, oxidation resistance, and excellent electrical properties, as well. This study provides a new approach for the fabrication of ultrahigh-quality BP material and thus promotes its application.

Research Progress on the Preparation Methods for and Flame Retardant Mechanism of Black Phosphorus and Black Phosphorus Nanosheets

Black phosphorus and black phosphorus nanosheets are widely used in the flame retardant field because of their excellent properties, but the immature preparation methods have resulted in extremely high preparation cost, which greatly limits their development and application. In this paper, various preparation methods of black phosphorus and black phosphorus nanosheets are described in detail, the advantages and disadvantages of each method are analyzed in depth, the flame-retardant mechanism and application of black phosphorus and black phosphorus nanosheets in flame retardants are discussed, and the subsequent development direction of black phosphorus and black phosphorus nanosheets is proposed.

Characterization of emerging 2D materials after chemical functionalization

The chemical modification of 2D materials has proven a powerful tool to fine tune their properties. With this motivation, the development of new reactions has moved extremely fast. The need for speed, together with the intrinsic heterogeneity of the samples, has sometimes led to permissiveness in the purification and characterization protocols. In this review, we present the main tools available for the chemical characterization of functionalized 2D materials, and the information that can be derived from each of them. We then describe examples of chemical modification of 2D materials other than graphene, focusing on the chemical description of the products. We have intentionally selected examples where an above-average characterization effort has been carried out, yet we find some cases where further information would have been welcome. Our aim is to bring together the toolbox of techniques and practical examples on how to use them, to serve as guidelines for the full characterization of covalently modified 2D materials.

Significant differences in the electrochemical activity of black phosphorus anodes prepared under different atmospheres

The effects of atmosphere and temperature on the electrochemical reversibility of black phosphorus (BP) anodes were investigated. BP anodes prepared in ambient air exhibited much-enhanced electrochemical activity due to the newly formed Cu₃P phase. This work highlights the importance of maintaining intragranular electronic conduction for developing advanced BP-based anodes with high reversible capacities.

Black phosphorus conjugation of chemotherapeutic ginsenoside Rg3: enhancing targeted multimodal nanotheranostics against lung cancer metastasis

It is a significant challenge in lung cancer chemophotothermal (CPT) therapy to develop multifunctional theranostic nanoagent (MTN) for precise targeting and successful tumor treatments, especially for lung metastasis. To overcome this problem, we effectively design and construct multifunctional black phosphorus (BP) nanoagents, BPs/G-Rg3@PLGA. BPs quantum dots (BPsQDs) are co-loaded onto poly(lactic-co-glycolic acid) (PLGA) with subsequent conjugations of a cancer therapeutic compound, ginsenoside Rg3 (G-Rg3), in this composite nanoagent. The in vivo delivery findings suggest that BPs/G-Rg3@PLGA has an excellent affinity for primary tumors and metastatic lung tumors. Furthermore, when paired with near-light irradiation, BPs/G-Rg3@PLGA shows superior controllable CPT therapy synergetic therapeutics, significantly increasing photothermal tumor ablation effectiveness. Mechanistically, heating causes rapid G-Rg3 release from the non-complex, and thermal therapy induces apoptosis, culminating in the reduction of lung cancer metastasis. Additionally, in vivo and in vitro findings support the biocompatibility of BPs/G-Rg3@PLGA. This thesis identifies a versatile BPs-based MTN for lung cancer metastasis control.

Interlayer Coordination of Pd-Pd Units in Exfoliated Black Phosphorus

The chemical functionalization of 2D exfoliated black phosphorus (2D BP) continues to attract great interest, although a satisfactory structural characterization of the functionalized material has seldom been achieved. Herein, we provide the first complete structural characterization of 2D BP functionalized with rare discrete Pd₂ units, obtained through a mild decomposition of the organometallic dimeric precursor [Pd(η³-C₃H₅)Cl]₂. A multitechnique approach, including HAADF-STEM, solid-state NMR, XPS, and XAS, was used to study in detail the morphology of the palladated nanosheets (Pd₂/BP) and to unravel the coordination of Pd₂ units to phosphorus atoms of 2D BP. In particular, XAS, backed up by DFT modeling, revealed the existence of unprecedented interlayer Pd–Pd units, sandwiched between stacked BP layers. The preliminary application of Pd₂/BP as a catalyst for the hydrogen evolution reaction (HER) in acidic medium highlighted an activity increase due to the presence of Pd₂ units.

Two-dimensional materials in biomedical, biosensing and sensing applications

Two-dimensional (2D) materials are at the forefront of materials research. Here we overview their applications beyond graphene, such as transition metal dichalcogenides, monoelemental Xenes (including phosphorene and bismuthene), carbon nitrides, boron nitrides along with transition metal carbides and nitrides (MXenes). We discuss their usage in various biomedical and environmental monitoring applications, from biosensors to therapeutic treatment agents, their toxicity and their utility in chemical sensing. We highlight how a specific chemical, physical and optical property of 2D materials can influence the performance of bio/sensing, improve drug delivery and photo/thermal therapy as well as affect their toxicity. Such properties are determined by crystal phases electrical conductivity, degree of exfoliation, surface functionalization, strong photoluminescence, strong optical absorption in the near-infrared range and high photothermal conversion efficiency. This review conveys the great future of all the families of 2D materials, especially with the expanding 2D materials' landscape as new materials emerge such as germanene and silicene.

Binary Phosphorene Redox Behavior in Oxidoreductase Enzymatic Systems

Phosphorene is a two-dimensional material that has many advantageous electronic, electrochemical, and optical properties. However, phosphorene possesses a relatively poor stability in ambient atmosphere. This disadvantage limits its application in several systems and particularly in electrochemical biosensors. Here we evaluate phosphorene as an electrochemical biosensing platform in two different mediator-based oxidoreductase enzymatic systems (glucose oxidase (GOx) and peroxidase from horseradish (HRP)), in which their detection is based on the reduction or oxidation of a mediator. In both cases, the used mediator is the same, ferrocene methanol (FcMeOH). Enhanced electrochemical activity is observed only in the reductive system (HRP-based biosensor) when compared to the oxidative counterpart (GOx-based biosensor). This phenomenon is attributed to the fact that in a reductive environment the phosphorene structure remains intact, while in an oxidative potential, the phosphorene is readily oxidized. In this way, the electroactivity of phosphorene as a sensing platform is strongly dependent on the type of mediator-based enzymatic system. These findings of binary nature of phosphorene are of high importance for construction of phosphorene-sensing platforms and in the development of enzyme logic systems.

Improvement in the quality of black phosphorus by selecting a mineralizer

The low-cost synthesis of high-quality black phosphorus (BP) has always been a challenge. Herein, we selected different mineralizers to synthesize high-crystallinity BP by the chemical vapor transport (CVT) method and demonstrated that the use of Pb instead of Sn can lead to higher purity BP. Residual Sn in Sn-BP was confirmed by X-ray photoelectron spectroscopy (XPS), but no mineralizer impurity was observed in Pb-BP. The performance of FET devices showed that the hole mobility of Pb-BP was significantly higher than that of Sn-BP. On the other hand, the Pb-BP devices exhibited good bipolarity with the highest hole mobility of 523 cm² V^-1 s^-1 at room temperature and electron mobility of up to 28 cm² V^-1 s^-1.

Raman and electrical transport properties of few-layered arsenic-doped black phosphorus

Black phosphorus (b-P) is an allotrope of phosphorus whose properties have attracted great attention. In contrast to other 2D compounds, or pristine b-P, the properties of b-P alloys have yet to be explored. In this report, we present a detailed study on the Raman spectra and on the temperature dependence of the electrical transport properties of As-doped black phosphorus (b-AsP) for an As fraction x = 0.25. The observed complex Raman spectra were interpreted with the support of Density Functional Theory (DFT) calculations since each original mode splits in three due to P-P, P-As, and As-As bonds. Field-effect transistors (FET) fabricated from few-layered b-AsP exfoliated onto Si/SiO2 substrates exhibit hole-doped like conduction with a room temperature ON/OFF current ratio of ∼103 and an intrinsic field-effect mobility approaching ∼300 cm2 V-1 s-1 at 300 K which increases up to 600 cm2 V-1 s-1 at 100 K when measured via a 4-terminal method. Remarkably, these values are comparable to, or higher, than those initially reported for pristine b-P, indicating that this level of As doping is not detrimental to its transport properties. The ON to OFF current ratio is observed to increase up to 105 at 4 K. At high gate voltages b-AsP displays metallic behavior with the resistivity decreasing with decreasing temperature and saturating below T ∼100 K, indicating a gate-induced insulator to metal transition. Similarly to pristine b-P, its transport properties reveal a high anisotropy between armchair (AC) and zig-zag (ZZ) directions. Electronic band structure computed through periodic dispersion-corrected hybrid Density Functional Theory (DFT) indicate close proximity between the Fermi level and the top of the valence band(s) thus explaining its hole doped character. Our study shows that b-AsP has potential for optoelectronics applications that benefit from its anisotropic character and the ability to tune its band gap as a function of the number of layers and As content.

Silicon as a ubiquitous contaminant in graphene derivatives with significant impact on device performance

Silicon-based impurities are ubiquitous in natural graphite. However, their role as a contaminant in exfoliated graphene and their influence on devices have been overlooked. Herein atomic resolution microscopy is used to highlight the existence of silicon-based contamination on various solution-processed graphene. We found these impurities are extremely persistent and thus utilising high purity graphite as a precursor is the only route to produce silicon-free graphene. These impurities are found to hamper the effective utilisation of graphene in whereby surface area is of paramount importance. When non-contaminated graphene is used to fabricate supercapacitor microelectrodes, a capacitance value closest to the predicted theoretical capacitance for graphene is obtained. We also demonstrate a versatile humidity sensor made from pure graphene oxide which achieves the highest sensitivity and the lowest limit of detection ever reported. Our findings constitute a vital milestone to achieve commercially viable and high performance graphene-based devices.

Structure and properties of intrinsic and extrinsic defects in black phosphorus

The electronic and geometric structures of a range of intrinsic and extrinsic defects in black phosphorus (BP) are calculated using Density Functional Theory (DFT) and a hybrid density functional. The results demonstrate that energy barriers to form intrinsic defects, such as Frenkel pairs and Stone-Wales type defects, exceed 3.0 eV and their equilibrium concentrations are likely to be low. Therefore, growth conditions and sample preparation play a crucial role in defect chemistry of black phosphorus. Mono-vacancies (MV) are shown to introduce a shallow acceptor state in the bandgap of BP, but exhibit fast hopping rates at room temperature. Coalescence of MVs into di-vacancies (DV) is energetically favourable and eliminates the band gap states. Thus MVs are not likely to be the main contributor to p-doping in BP. Extrinsic defects are a plausible alternative, with Sn_P found to be the most promising candidate. Other defects considered include I, O, Fe, Cu, Zn and Ni in surface adsorbed, intercalated and substitutional geometries, respectively. Furthermore, BP was found to be magnetic for isolated MVs and Fe doping, motivating further research in the area of magnetic functionalisation.

Functional Protection of Exfoliated Black Phosphorus by Noncovalent Modification with Anthraquinone

Few and monolayer black phosphorus (phosphorene) is currently an intensively researched material. Shear exfoliated black phosphorus (BP_SE) nanosheets were functionalized with the redox active antraquinone (AQ) that can provide additional charge storage capacity. The noncovalent interaction of BP with AQ occurs due to van der Waals interactions. X-ray photoelectron spectroscopy results show that AQ coverage of BP_SE nanosheets led to a stabilization against BP_SE degradation. Electrochemistry of the BP_SE-AQ shows that AQ is stably anchored onto BP_SE and exhibits redox peaks stable for more than 100 cycles. The surface coverage by AQ on BP_SE is estimated to be 1.25 nmol AQ/mg BP and electron-transfer rate constant ( k_ET) of 33 s^-1. Furthermore, the proposed modification greatly increases the gravimetric capacitance of BP_SE-AQ with respect to the starting BP_bulk. Such coating of BP not only protects BP from degradation but also brings electroactive functionality to this two-dimensionally layered material.