Few-layer Black Phosphorus Research Articles

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.

Modulation of Phosphorene for Optimal Hydrogen Evolution Reaction.

Economical and highly effective catalysts are crucial to the electrocatalytic hydrogen evolution reaction (HER), and few-layer black phosphorus (phosphorene) is a promising candidate because of the high carrier mobility, large specific surface area, and tunable physicochemical characteristics. However, the HER activity of phosphorene is limited by the weak hydrogen adsorption ability on the basal plane. In this work, optimal active sites are created to modulate the electronic structure of phosphorene to improve the HER activity and the effectiveness is investigated theoretically by density-functional theory calculation and verified experimentally. The edges and defects affect the electronic density of states, and a linear relationship between the HER activity and lowest unoccupied states (εLUS) is discovered. The medium εLUS value corresponds to the suitable hydrogen adsorption strength. Experiments are designed and performed to verify the prediction, and our results show that a smaller phosphorene moiety with more edges and defects exhibits better HER activity and surface doping with metal adatoms improves the catalytic performance. The results suggest that modified phosphorene has large potential in efficient HER and provides a convenient standard to explore ideal electrocatalysts.

High-quality in situ fabricated Nb Josephson junctions with black phosphorus barriers

Owing to appealing physical properties such as broad tunability in bandgaps and structural anisotropy, black phosphorus (BP) holds great potential in exploring novel electronic devices. However, it is extremely challenging to use BP to fabricate electronic devices, since it is prone to deteriorate in air. To address this challenge, we demonstrate an in situ fabrication technique which enables us to minimize interfacial degradation and to fabricate vertical Josephson junctions by employing few-layer BP as a barrier between two closely spaced Nb electrodes. The current–voltage characteristics of the junctions are hysteretic at low temperatures and become nonhysteretic when approaching the junction critical temperature. In the resistive state the differential conductance increases with decreasing voltage. Microwave-induced Shapiro steps were observed, confirming the presence of the ac Josephson effect. We present different models to analyze the current–voltage characteristics and conclude that resistive state of the current–voltage characteristics points to a zero bias anomaly, which is presumably caused by Andreev reflections. Our in situ fabrication technique represents a viable way to incorporate air-unstable materials for electronics and offer a chance to explore their unique functionalities.

Tuning the Optical and Electrical Properties of Few-Layer Black Phosphorus via Physisorption of Small Solvent Molecules.

Black phosphorus (BP) is recently becoming more and more popular among semiconducting 2D materials for (opto)electronic applications. The controlled physisorption of molecules on the BP surface is a viable approach to modulate its optical and electronic properties. Solvents consisting of small molecules are often used for washing 2D materials or as liquid media for their chemical functionalization with larger molecules, disregarding their ability to change the opto-electronic properties of BP. Herein, it is shown that the opto-electronic properties of mechanically exfoliated few-layer BP are altered when physically interacting with common solvents. Significantly, charge transport analysis in field-effect transistors reveals that physisorbed solvent molecules induce a modulation of the charge carrier density which can be as high as 1012 cm-2 in BP, i.e., comparable to common dopants such as F4 -TCNQ and MoO3 . By combining experimental evidences with density functional theory calculations, it is confirmed that BP doping by solvent molecules not only depends on charge transfer, but is also influenced by molecular dipole. The results clearly demonstrate how an exquisite tuning of the opto-electronic properties of few-layer BP can be achieved through physisorption of small solvent molecules. Such findings are of interest both for fundamental studies and more technological applications in opto-electronics.

Optical anisotropy of black phosphorus by total internal reflection

Although abundant research on the anisotropy of van der Waals (vdW) materials has been published, we undertake an in-depth study of their optical properties as they have an important guiding role for light control in two-dimensional (2D) nanospace. As an example, we study the reflectance of few-layered black phosphorus (BP) in the total internal reflection (TIR) mode in detail. We demonstrate that its optical anisotropy can be changed on a large scale by varying the incident angles, polarization states, and the in-plane rotation angles of the BP samples. Theoretical analysis indicates that the phenomena observed are common to all the atom-thick biaxial crystals, so these conclusions can be widely applied to other anisotropic 2D materials. This research furthers the current understanding of the properties of BP more comprehensively, and provides guidance for developing new optoelectronic applications, especially when BP and other atom-thick biaxial crystals are integrated with TIR devices.

Review on Applications of Black Phosphorus in Catalysis.

As emerging non-metallic semiconductors, black phosphorus (BP) and few-layered BP (phosphorene) have exhibited unique physicochemical properties and promising application prospect for catalysis related fields. In this review, we first introduced the intriguing properties of BP, including high carrier mobility, a wide optical absorption range and tunable direct band gap. We then summarized the recent research progress about the applications of BP as catalysts, mainly focus on photocatalytic water splitting, photocatalytic degradation of organic pollutants and electrocatalytic H₂ and O₂ evolution. The recent works about other catalytic applications were also introduced.

Single-step exfoliation and functionalization of few-layers black phosphorus and its application for polymer composites

We demonstrate the large scale exfoliation of bulk black phosphorus into few-layers black phosphorus. We use wet-jet mill (WJM) as an industrially scalable exfoliation technique. In order to avoid the degradation by oxidation of the exfoliated flakes, the few-layers black phosphorus is protected by functionalisation (with 4-bromobenzene-diazonium tetrafluoroborate) during the exfoliation process. We used the exfoliated and functionalised flakes as fillers in a polymethyl methacrylate matrix to test the mechanical endurance, obtaining an improvement in the Young’s modulus of 106% compared to the bare polymer. These data support the development of black phosphorus materials as 2D-polymer fillers and performance-enhancers.

Phonon Anharmonicity in Few-Layer Black Phosphorus.

We report a temperature-dependent Raman spectroscopy study of few-layer black phosphorus (BP) with varied incident polarization and sample thickness. The Raman-active modes Ag1, B2g, and Ag2 exhibit a frequency downshift, while their line width tends to increase with increasing temperature. To understand the details of these phenomena, we perform first-principles density functional theory calculations on freestanding monolayer BP. The effect of thermal expansion is included by constraining the temperature-dependent lattice constant. The study of the temperature-induced shift of the phonon frequencies is carried out using ab initio molecular dynamics simulations. The normal-mode frequencies are calculated by identifying the peak positions from the magnitude of the Fourier transform of the total velocity autocorrelation. Anharmonicity induces a frequency shift for each individual mode, and the three- and four-phonon process coefficients are extracted. These results are compared with those obtained from many-body perturbation theory, giving access to phonon lifetimes and lattice thermal conductivity. We establish that the frequency downshift is primarily due to phonon-phonon scattering while thermal expansion only contributes indirectly by renormalizing the phonon-phonon scattering. Overall, the theoretical results are in excellent agreement with experiment, thus showing that controlling phonon scattering in BP could result in better thermoelectric devices or transistors that dissipate heat more effectively when confined to the nanoscale.

Few-layer black phosphorus-on-MAPbI3 for superb visible-light photocatalytic hydrogen evolution from HI splitting

Organic-inorganic perovskites like MAPbI3 with excellent optoelectric properties have recently gained concerns in photocatalytic HI splitting. However, their achieved hydrogen evolution reaction (HER) rates are still insufficient. Herein, a 2D few-layer black phosphorus (BP) as cocatalyst was anchored on MAPbI3 via electrostatic coupling. The resultant BP/MAPbI3 is rather stable in HI solution during the whole photoreaction, yielding a superb HER rate of 3742 μmol h−1 g−1 under visible irradiation, which is two orders of magnitude higher than that of MAPbI3-only, far higher than that of MAPbI3/Pt and also shows the advanced HER performance among MAPbI3 based systems. The remarkably boosted HER activity is thoroughly explored by optical/optoelectrochemical measurements, showing that BP can act as an electron promoter to trap electrons derived from MAPbI3 through a type I heterojunction in the interface. This contributes to a new paradigm for high-efficiency photocatalysts by anchoring non-metal cocatalyst onto MAPbI3 for solar energy conversion.

Black phosphorus quantum dot based all-optical signal processing: ultrafast optical switching and wavelength converting

As a unique two-dimensional material, few-layered black phosphorus (BP) nanosheets have shown promising applications in electronics and optoelectronics. Black phosphorus quantum dots (BPQDs) have attracted attention due to the unique properties of BP combined with the edge effects. In this paper, we report on the all-optical application of BPQD based on its nonlinear Kerr effect. BPQDs were synthesized using a liquid exfoliation method combined with probe sonication and bath sonication. BPQD deposited on the microfiber as an optical device was demonstrated as the Kerr switcher with extinction ratio of 20 dB and four-wave mixing based wavelength converter with −40 dB conversion efficiency. These findings suggest that BPQD-based novel nonlinear photonics devices could be further developed in the applications of next generation high-speed optical communication.

Black phosphorus-based polyvinylidene fluoride nanocomposites: Synthesis, processing and characterization

In this study, the polyvinylidene fluoride (PVDF) nanocomposites were produced based on the few-layers Black phosphorus nanosheets (BP) via the liquid exfoliation method, and at different BP nanofiller contents (1, 2.5 and 5 wt%). A masterbatch with 5 wt% of BP was prepared by solution blending followed by melt compounding process to elaborate the different nanocomposites and compression molding to molding all samples. The characterization of the nanocomposites in terms of X-ray diffraction, infrared spectroscopy, and scanning electron microscopy confirmed that BP nanosheets were successfully prepared. The nanocomposites were characterized in terms of structural, thermal, electrical, and mechanical performances. The thermal stability of neat PVDF was found increased with 5 wt% of BP nanofillers addition. For the same concentration, the electrical conductivity was sharply increased from 3.3 e−14 to 5 e−11 S/cm. Similarly, Young's modulus value was found increased by 56% as compared to the neat matrix. BP nanofillers appear to be a good alternative to elaborate new polymer nanocomposites with improved performances.

Reversible Oxidation of Blue Phosphorus Monolayer on Au(111)

Practical applications of two-dimensional (2D) black phosphorus (BP) are limited by its fast degradation under ambient conditions, for which many different mechanisms have been proposed; however, an atomic level understanding of the degradation process is still hindered by the absence of bottom-up methods for the growth of large-scale few-layer black phosphorus. Recent experimental success in the fabrication of single-layer blue phosphorus provides a model system to probe the oxidation mechanism of two-dimensional (2D) phosphorene down to single-layer thicknesses. Here, we report an atomic-scale investigation of the interaction between molecular oxygen and blue phosphorus. The atomic structure of blue phosphorus and the local binding sites of oxygen have been precisely identified using qPlus-based noncontact atomic force microscopy. A combination of low-temperature scanning tunneling microscopy and X-ray photoelectron spectroscopy measurements reveal a thermally reversible oxidation process of blue phosphorus in a pure oxygen atmosphere. Our study clearly demonstrates the essential role of oxygen in the initial oxidation process, and it sheds further light on the fundamental pathways of the degradation mechanism.

Accurate Threshold Voltage Reliability Evaluation of Thin Al2O3 Top-Gated Dielectric Black Phosphorous FETs Using Ultrafast Measurement Pulses.

Few-layer black phosphorus (BP) has attracted significant interest in recent years due to electrical and photonic properties that are far superior to those of other two-dimensional layered semiconductors. The study of long term electrical stability and reliability of black phosphorus field effect transistors (BP-FETs) with technologically relevant thin, and device-selective, gate dielectrics, stressed under realistic (closer to operation) bias and measured using state-of-the-art ultrafast reliability characterization techniques, is essential for their qualification and use in different applications. In this work, air-stable BP-FETs with a thin top-gated dielectric (15 nm Al2O3, SiO2 equivalent thickness of 5 nm) were fabricated and comprehensively characterized for threshold voltage ( Vth) instability under negative gate bias stress at various measurement delays ( tm), stress biases ( VGSTR), temperatures ( T), and stress times ( tstr) for the first time. Thin top-gated oxide enables low VGSTR that is closer to the operating condition and ultrafast Vth measurements with low delay ( tm = 10 μs, due to high drain current) that ensure minimal recovery. The resultant time kinetics of Vth degradation (Δ Vth) shows fast saturation at longer stress times and low-temperature activation energy. Vth instability in these top-gated devices is suggested to be dominated by hole trapping, which is modeled using first-order equations at different VGSTR and T. It is shown that measurements using larger tm show lower degradation magnitude that do not saturate due to recovery artifacts and give inaccurate estimation of hole trap densities. Conventional, thick, and global back-gated oxide BP-FETs were also fabricated and characterized for varying tm (1 ms being the lowest due to a low drain current level for thick oxide), VGSTR, and T to benchmark our top-gated results. Nonsaturating Δ Vth in the back-gated devices is shown to result from recovery artifacts due to the large tm (1 ms and greater) values. Finally, using a VGSTR and T-dependent first-order model, we show that the top-gated Al2O3 BP-FETs with scaled gate oxide thickness can match state-of-the-art Si reliability specifications at operating voltage and room/elevated temperature.

Strain-tunable van der Waals interactions in few-layer black phosphorus

Interlayer interactions in 2D materials, also known as van der Waals (vdWs) interactions, play a critical role in the physical properties of layered materials. It is fascinating to manipulate the vdWs interaction, and hence to “redefine” the material properties. Here, we demonstrate that in-plane biaxial strain can effectively tune the vdWs interaction of few-layer black phosphorus with thickness of 2-10 layers, using infrared spectroscopy. Surprisingly, our results reveal that in-plane tensile strain efficiently weakens the interlayer coupling, even though the sample shrinks in the vertical direction due to the Poisson effect, in sharp contrast to one’s intuition. Moreover, density functional theory (DFT) calculations further confirm our observations and indicate a dominant role of the puckered lattice structure. Our study highlights the important role played by vdWs interactions in 2D materials during external physical perturbations.

Physical Fingerprints of the 2O-tαP Phase in Phosphorene Stacking.

The 2O-tαP phase is a bilayer phosphorene stacking twisted by ∼70.5° standing out from all the potential candidates predicted by our previous work. Here, by linear response theory, we directly verified that the 2O-tαP phase preserves the intrinsic features of phonon spectrum of the existing AB phase, reflecting a stable thermodynamic behavior. Then we provided three distinct fingerprints to help finding this new phase: upon comparison to the existing shifting bilayer phosphorene, the in-plane elastic constants showed a much weaker anisotropic response, providing a characteristic mechanical criterion; the calculated Raman spectrum revealed forthe low frequency rang thelayer-breathing modeand theout-of-plane twisted mode, L-A1 and L-A2, both of which together stabilize the twisted structure; in particular, thesimulated scanning tunneling microscope image presented recognizable cross stripes, which should withstand an examination of exfoliated bilayer and few-layer black phosphorus.

Surface coordination modification and electrical properties of few-layer black phosphorus exfoliated by the liquid-phase method

Black phosphorus (BP) is a novel multifunctional two-dimensional semiconductor material, which has wide application prospects in nano-electronic devices and optoelectronic devices. However, its poor environmental stability hinders its industrial applications. In this work, liquid exfoliation method was employed to isolate few-layer BP in N-methyl-2-pyrrolidone (NMP), and a titanium sulfonate ligand (TiL4) was synthesized to coordinate with BP to prevent oxidation in air and water. We have shown that TiL4-modified BP nanoplates present the excellent stability against water and oxygen due to the passivation of lone pair electrons in P atoms. The current-voltage curves, which do not change with temperature, indicates that surface coordination modification may passivate the defects or impurities on surfaces of few-layer BP and inhibits the production of carriers from the ionization of these defects or impurities, and the carrier migration mobility is also greatly affected. These results indicated a possible method to tailor the electrical properties of few-layer BP by the surface coordination coverage of titanium sulfonate ligand.

Ultrafast Cathodic Exfoliation of Few-Layer Black Phosphorus in Aqueous Solution

Preparing few-layer black phosphorus flakes, in a scalable, fast, and benign way, is a key step to promote both the fundamental research and practical applications of black phosphorus. Although the electrochemically assisted liquid exfoliation of bulk black phosphorus is a promising strategy, it is still challenging to obtain few-layer black phosphorus flakes in polar protic solvent through cathodic exfoliation. In this work, we demonstrate an ultrafast approach to exfoliate bulk black phosphorus, under a cathodic potential, in an aqueous solution of hexadecyltrimethylammonium chloride. Particularly, the exfoliation can be trigged within ∼20 s, and the obtained flakes exhibit satisfactory quality. Further detailed mechanical explorations reveal that the cathodic intercalation of hexadecyltrimethylammonium cations and their subsequent decomposition contribute to the ultrafast expansion and exfoliation of bulk black phosphorus crystals. In addition, the as-exfoliated flakes show improved stability due to the protection of hexadecyltrimethylammonium chloride. This exfoliation method, together with the findings, should shed light on the development of new exfoliation methods for preparing few-layer black phosphorus flakes in various liquid systems, thus facilitating the future applications of black phosphorus.

Synergy of Black Phosphorus-Graphite-Polyaniline-Based Ternary Composites for Stable High Reversible Capacity Na-Ion Battery Anodes.

In recent times, few-layer black phosphorus (BP) has attracted tremendous attention as a promising anode material for sodium-ion batteries due to its particular two-dimensional structure, good electron conductivity, and high theoretical capacity. The main disadvantages of BP-based materials are the lower practical specific capacity of the BP-based composite than expectation because of the low P atom utilization and the structural fracture due to the large volume expansion that occurs during sodiation/desodiation cycles. In this work, we report a ternary composite comprising BP, graphite, and polyaniline (BP-G/PANI) with a BP mass content of ∼65 wt %. The ternary composite provides an optimized ion pathway (electrolyte → PANI → BP-G → BP), which reduces the charge transfer resistance of the electrode. Also, further ex situ X-ray absorption spectroscopy measurements demonstrate that the presence of graphite in the BP-G composite allows a deep sodiation of BP and also leads to a higher sodiation/desodiation reversibility. In addition, the uniformly coated PANI also restricts the huge volume expansion of the BP electrode through discharge/charge processes, which promise the stable cycling performance of BP-G/PANI. Thus, our composite shows a high reversible gravimetric capacity of 1530 mAh gcompo.-1 at 0.25 A g-1 and a capacity retention of 520 mAh gcompo.-1 after 1000 cycles at a high current density of 4 A g-1.

Electrochemically Exfoliated Functionalized Black Phosphorene and Its Polyurethane Acrylate Nanocomposites: Synthesis and Applications.

Owing to its mechanical performance, thermal stability, and size effects, single or few-layer black phosphorus (BP) has the potential to prepare the polymer nanocomposites as a candidate of nanoadditives, similar to graphene. The step to realize the scalable exfoliation of single or few-layer BP nanosheets is crucial to BP applications. Herein, we utilized a facile, green, and scalable electrochemical strategy for generating cobaltous phytate-functionalized BP nanosheets (BP-EC-Exf) wherein the BP crystal served as the cathode and phytic acid served as a modifier and an electrolyte simultaneously. Moreover, high-performance polyurethane acrylate/BP-EC-Exf (PUA/BP-EC) nanocomposites are easily prepared by a convenient UV-curable strategy for the first time. Significantly, the conclusion of introducing BP-EC-Exf into the PUA matrix resulted in enhancement in mechanical properties of PUA in terms of the tensile strength (increased by 59.8%) and tensile fracture strain (increased by 88.1%), in the distinct improvement in flame retardancy of PUA in terms of the decreased peak heat release rate (reduced by 44.5%) and total heat release (decreased by 34.5%), and in lower intensities of pyrolysis products including toxic CO. Moreover, it was confirmed by X-ray diffraction and Raman spectra that the air stability of PUA/BP-EC nanocomposites was maintained after exposure to environmental conditions for 4 months. The air-stable BP nanosheets, which were wrapped and embedded in the PUA matrix, can achieve the isolation and protection effect. This modified electrochemical method toward the simultaneous exfoliation and functionalization of BP nanosheets provides an efficient approach for fabricating BP-polymer-based nanocomposites.

Optical conductivity of black phosphorus with a tunable electronic structure

Black phosphorus (BP) is a two-dimensional layered material composed of phosphorus atoms. Recently, it was demonstrated that external perturbations such as an electric field close the band gap in few-layer BP, and can even induce a band inversion, resulting in an insulator phase with a finite energy gap or a Dirac semimetal phase characterized by two separate Dirac nodes. At the transition between the two phases, a semi-Dirac state appears in which energy disperses linearly along one direction and quadratically along the other. In this work, we study the optical conductivity of few-layer BP using a lattice model and the corresponding continuum model, incorporating the effects of an external electric field and finite temperature. We find that the low-frequency optical conductivity scales a power law that differs depending on the phase, which can be utilized as an experimental signature of few-layer BP in different phases. We also systematically analyze the evolution of the material parameters as the electric field increases, and the consequence on the power-law behavior of the optical conductivity.