Few-layer Black Phosphorus Research Articles

Optical Anisotropy of Few-Layer Black Phosphorus Visualized by Scanning Polarization Modulation Microscopy

In-plane anisotropy, as one of the most intriguing properties of newly rediscovered 2D black phosphorus (BP), supplies another degree of freedom to design novel optical and optoelectronic devices. Here, the optical in-plane anisotropy of few-layer BP was directly visualized and studied by scanning polarization modulation microscopy (SPMM). The polarization analysis and experiment showed that the SPMM signal was dependent on the polarization angle of the laser beam in a sinusoidal way, by which the crystallographic orientation of BP was determined. The differential reflectance (ΔR = Rzz – Rac) showed the excitation wavelength and thickness dependence, which was well explained by considering the anisotropic multireflection effect. The intrinsic anisotropic complex refractive indices (n + iκ) were also derived on the basis of the Fresnel equation.

Anomalous Acoustoelectric Currents in Few-Layer Black Phosphorus Nanocrystals

We report. on the anomalous acoustoelectric (AE) currents in few-layer black phosphorus (BP) nanocrystals as the result of strong Bragg reflections. The device consists of a surface acoustic wave (SAW) delay line and a BP field-effect transistor, which uses a graphene film as its gate electrode with an atomically thin boron nitride gate dielectric layer. It was found that the Bragg reflection could give rise to large anomalous AE currents when the hole concentration was sufficiently large. Our experimental results showed that the magnitude and the direction of the anomalous AE currents were related to the applied bias and the position of the device with respect to the nodes/antinodes of the standing waves. The observations have been explained based on some assumption. Our work provides more insights into the interaction between SAW and 2D materials and highlights the particularity of anomalous AE effect in such systems.

Layer-Dependent Ultrafast Carrier and Coherent Phonon Dynamics in Black Phosphorus.

Black phosphorus is a layered semiconducting material, demonstrating strong layer-dependent optical and electronic properties. Probing the photophysical properties on ultrafast time scales is of central importance in understanding many-body interactions and nonequilibrium quasiparticle dynamics. Here, we applied temporally, spectrally, and spatially resolved pump-probe microscopy to study the transient optical responses of mechanically exfoliated few-layer black phosphorus, with layer numbers ranging from 2 to 9. We have observed layer-dependent resonant transient absorption spectra with both photobleaching and red-shifted photoinduced absorption features, which could be attributed to band gap renormalization of higher subband transitions. Surprisingly, coherent phonon oscillations with unprecedented intensities were observed when the probe photons were in resonance with the optical transitions, which correspond to the low-frequency layer-breathing mode. Our results reveal strong Coulomb interactions and electron-phonon couplings in photoexcited black phosphorus, providing important insights into the ultrafast optical, nanomechanical, and optoelectronic properties of this novel two-dimensional material.

Simulation and Analysis of Phosphorene Nanoribbon Field Effect Transistors Using Non-Equilibrium Green's Function Formalism

Few-layer black phosphorus is a semiconductor material which was discovered in 2014. Phosphorus has both high mobility (1100-1140 cm2/V.s in armchair direction) and finite bandgap (~0.35 eV for bulk) compared to the other two-dimensional materials such as MoS2 and graphene. A single layer of black phosphorus is named as phosphorene. Multilayer (ML) phosphorene has an adjustable direct bandgap that can be controlled either by changing the width of the ribbon or the number of layers. In this work, first we calculate the band structure of ML phosphorene nanoribbons (PNRs) using the tight-binding method. Then, we analyzed the performance characteristics of SL- and ML-PNR field effect transistors (PNRFETs) with abrupt doping between the Source/Drain (S/D) region and the channel region by employing the non-equilibrium Green’s function (NEGF) formalism in real space. This type of FET with abrupt doping near S/D and channel regions is known as conventional FET. We found that the bandgap of armchair PNRs with 16 atoms across the width of PNR for single-layer (SL) and bilayer (BL) structures is 1.899 and 1.224 eV, respectively. We showed that increasing the number of layers reduces both the bandgap of the material and the turn on voltage of device, however, it increases the ON-current of device. Simultaneously, the off-current also increases due to a higher probability of tunneling in low bandgap devices. In addition, increasing the number of atoms across the width of PNR leads to an increase in the Ids due to the decrease of the bandgap. Moreover, both on- and off-currents increase by decreasing the channel length in multilayer PNRFETs. Next, we compared the performance characteristics of conventional and junction-less (JL) PNRFETs. Recently, JL-FET with high doping concentration in S/D and channel regions was proposed to replace the conventional FET. The JL silicon-on insulator FETs fabricated by Tyndall group show outstanding output characteristics with low Drain Induced Barrier Lowering (DIBL), ideal subthreshold swing, etc. There is no p-n junction between S/D and channel regions in a JL transistor. Instead, all regions are highly doped with the same amount of dopant to increase the conductivity of the channel material. In this case, the barrier in front of the electrons coming from source, is formed by applying suitable gate voltage. From experimental point of view, it should be much easier to dope all S/D and channel materials with the same doping concentration compared to the conventional FETs which needs abrupt doping. Thus, it should be a point of interest to have a performance comparison between conventional and JL phosphorene FETs. We investigated the performance of phosphorene FETs in terms of Id-Vgs, Id-Vds characteristics, energy levels, subthreshold swing, DIBL, and ION/IOFF.

Anisotropic visible photoluminescence from thermally annealed few-layer black phosphorus

Black phosphorus, a two-dimensional material, with high carrier mobility, tunable direct bandgap and anisotropic electronic properties has attracted enormous research interest towards potential application in electronic, optoelectronic and optomechanical devices. The bandgap of BP is thickness dependent, ranging from 0.3 eV for bulk to 1.3 eV for monolayer, while lacking in the visible region, a widely used optical regime for practical optoelectronic applications. In this work, photoluminescence (PL) centered at 605 nm is observed from the thermally annealed BP with thickness ≤20 nm. This higher energy PL is most likely the consequence of the formation of higher bandgap phosphorene oxides and suboxides on the surface BP layers as a result of the enhanced rate of oxidation. Moreover, the polarization-resolved PL measurements show that the emitted light is anisotropic when the excitation polarization is along the armchair direction. However, if excited along zigzag direction, the PL is nearly isotropic. Our findings suggest that the thermal annealing of BP can be used as a convenient route to fill the visible gap of the BP-based optoelectronic and optomechanical devices.

Absorption and temperature effects on the tensile strength of a black phosphorus ribbon in argon environment

Due to its excellent electrical properties, a few-layer black phosphorus (BP) ribbon has wide potential application in nano-devices. Its strength is essential for fabrication of the nano-devices. However, BP suffers from severe performance degradation in ambient conditions. In the present study, using argon as shielding gas, we investigate the tensile strength of a few-layer ribbon with consideration of both temperature and gas absorption via molecular dynamics simulations. Results demonstrate that the tensile strength of a BP ribbon is weakened at high temperature. If the ribbon has full relaxation before stretching, its tensile strength can be improved slightly by the argon membrane absorbed onto its surfaces. At 300 K, the strength is improved obviously due to gas absorption. The conclusions can be used for potential application in fabrication of few-layer BP ribbons.

Degradation Chemistry and Stabilization of Exfoliated Few-Layer Black Phosphorus in Water.

Exfoliated black phosphorus (BP), as a monolayer or few-layer material, has attracted tremendous attention owing to its unique physical properties for applications ranging from optoelectronics to photocatalytic hydrogen production. Approaching intrinsic properties has been, however, challenged by chemical reactions and structure degradation of BP under ambient conditions. Surface passivation by capping agents has been proposed to extend the processing time window, yet contamination or structure damage rise challenges for BP applications. Here, we report experiments combined with first-principle calculations that address the degradation chemistry of BP. Our results show that BP reacts with oxygen in water even without light illumination. The reaction follows a pseudo-first-order parallel reaction kinetics, produces PO23-, PO33-, and PO43- with reaction rate constants of 0.019, 0.034, and 0.023 per day, respectively, and occurs preferentially from the P atoms locating at BP edges, which yields structural decay from the nanoflake edges in water. In addition, a negligible decay ratio (0.9 ± 0.3 mol %) and preserved photocatalytic activity of BP are observed after storage in deoxygenated water for 15 days without surface passivation under ambient light. Our results reveal the chemistry of BP degradation and provide a practical approach for exfoliation, delivery, and application of BP.

Facile bottom-up synthesis of partially oxidized black phosphorus nanosheets as metal-free photocatalyst for hydrogen evolution

Few-layer black phosphorus (BP) nanosheets were first reported as a 2D material for the application of field-effect transistors in 2014 and have stimulated intense activity among physicists, chemists, and material and biomedical scientists, driving research into novel synthetic techniques to produce BP nanosheets. At present, exfoliation is the main route toward few-layer BP nanosheets via employing bulk BP as raw material. However, this is a complicated and time-consuming process, which is difficult for the large-scale synthesis of BP nanosheets. Moreover, BP degrades rapidly when exfoliated to nanoscale dimensions, resulting in the rapid loss of semiconducting properties. Here, we report the direct wet-chemical synthesis of few-layer BP nanosheets in gram-scale quantities in a bottom-up approach based on common laboratory reagents at low temperature, showing excellent stability due to partial oxidation of surface. Solvent and temperature are two critical factors, controlling not only the formation of BP nanosheets but also the thickness. The as-prepared BP nanosheets can extract hydrogen from pure water (pH = 6.8), exhibiting more than 24-fold higher activity than the well-known C3N4 nanosheets. Our results reporting the ability to prepare few-layer BP nanosheets with a facile, scalable, low-cost approach take us a step closer to real-world applications of phosphorene including next-generation metal-free photocatalysts for photosynthesis.

Uniform Tellurium Doping in Black Phosphorus Single Crystals by Chemical Vapor Transport.

Doping has been a reliable way to improve the properties of black phosphorus (BP). However, a uniform and large amount of doping in BP remains a challenge. Herein, the synthesis of tellurium-doped black phosphorus (Te-doped BP) single crystals with high crystalline quality is achieved by the chemical vapor transport reaction method. The synthetic route enables a uniform and relatively large amount (up to 0.5% atomic ratio) of Te-doping in BP single crystals. The electrocatalytic oxygen evolution reaction (OER) properties of few-layer Te-doped BP nanosheets prepared by liquid exfoliation were also investigated for the first time. Electrochemical tests demonstrated that the OER onset-potential of undoped and Te-doped BP nanosheets was 1.63 and 1.49 V, respectively. The result implies that doping provides an effective route to enhance the electrochemical OER performance of BP.

Determination of layer-dependent exciton binding energies in few-layer black phosphorus.

The attraction between electrons and holes in semiconductors forms excitons, which largely determine the optical properties of the hosting material, and hence the device performance, especially for low-dimensional systems. Mono- and few-layer black phosphorus (BP) are emerging two-dimensional (2D) semiconductors. Despite its fundamental importance and technological interest, experimental investigation of exciton physics has been rather limited. We report the first systematic measurement of exciton binding energies in ultrahigh-quality few-layer BP by infrared absorption spectroscopy, with layer (L) thickness ranging from 2 to 6 layers. Our experiments allow us to determine the exciton binding energy, decreasing from 213 meV (2L) to 106 meV (6L). The scaling behavior with layer numbers can be well described by an analytical model, which takes into account the nonlocal screening effect. Extrapolation to free-standing monolayer yields a large binding energy of ~800 meV. Our study provides insights into 2D excitons and their crossover from 2D to 3D, and demonstrates that few-layer BP is a promising high-quality optoelectronic material for potential infrared applications.

Electrostatic Functionalization and Passivation of Water-Exfoliated Few-Layer Black Phosphorus by Poly Dimethyldiallyl Ammonium Chloride and Its Ultrafast Laser Application.

Few-layer black phosphorus (BP) which exhibits excellent optical and electronic properties, has great potential applications in nanodevices. However, BP inevitably suffers from the rapid degradation in ambient air because of the high reactivity of P atoms with oxygen and water, which greatly hinders its wide applications. Herein, we demonstrate the electrostatic functionalization as an effective way to simultaneously enhance the stability and dispersity of aqueous phase exfoliated few-layer BP. The poly dimethyldiallyl ammonium chloride (PDDA) is selected to spontaneously and uniformly adsorb on the surface of few-layer BP via electrostatic interaction. The positive charge-center of the N atom of PDDA, which passivates the lone-pair electrons of P, plays a critical role in stabilizing the BP. Meanwhile, the PDDA could serve as hydrophilic ligands to improve the dispersity of exfoliated BP in water. The thinner PDDA-BP nanosheets can stabilize in both air and water even after 15 days of exposure. Finally, the uniform PDDA-BP-polymer film was used as a saturable absorber to realize passive mode-locking operations in a fiber laser, delivering a train of ultrafast pulses with the duration of 1.2 ps at 1557.8 nm. This work provides a new way to obtain highly stable few-layer BP, which shows great promise in ultrafast optics application.

Biaxial hyperbolic metamaterials using anisotropic few-layer black phosphorus.

Most of hyperbolic metamaterials (HMMs) investigated to date are based on isotropic materials resulting in uniaxial HMMs in which dielectric permittivities perpendicular to the propagation direction are the same. Using an anisotropic material constituent to form a HMM is a promising research direction providing opportunities to control the dielectric permittivity in all three directions independently. Herein, we propose and theoretically demonstrate novel biaxial HMMs composed of multilayer stacks of few-layer black phosphorus (BP) and Au thin films. Black phosphorus is an anisotropic material exhibiting crystal axis-dependent dielectric permittivity due to its puckered crystal structure. The proposed HMM provides previously unattained hyperbolic dispersion relations in which the dielectric permittivity in Z-direction of the structure shows opposite sign from that in X- and Y-directions in the most wavelengths from 400~900nm. Furthermore, we calculated the Purcell factor of the proposed biaxial HMMs using full-field electromagnetic simulations.

On mechanical behaviors of few-layer black phosphorus

This paper investigates the mechanical behaviors of few-layer black phosphorus (FLBP) by using molecular dynamics simulations. Results show that both tensile and compressive behaviors are strongly anisotropic in the armchair and zigzag directions due to the unidirectional puckers in each atomic layer, and that the compressive behavior is dependent on the number of atomic layers. In particular, the compressive and buckling strengths of FLBP can be significantly enhanced by stacking more atomic layers together, while this has little influence on both Young’s modulus and tensile strength. It is interesting to found that increasing the number of atomic layers in FLBP or the dimension ratio can lead to a drastically reduced flexibility in armchair direction, showing that both compressive and buckling strengths become higher than those in zigzag direction. It is also demonstrated that the reorientation of FLBP’s atomic configuration occurs under certain conditions. The mechanism of deformation underlying the mechanical behaviors of FLBP is also discussed, suggesting that changing the number of atomic layers is an effective way to engineer two-dimensional materials for desired material properties.

A New Effective Approach to Prevent the Degradation of Black Phosphorus: The Scandium Transition Metal Doping

Density functional calculations are used to investigate the effective approach to prevent the degradation of black phosphorus (BP). For the few-layer BP, it is found that the Sc atom monodoped the 3 × 3 unit cell with a doping ratio of 1.4%, which can retain the semiconductor character with a moderate band gap (Egap) of 0.84 eV. Obviously, the two layer BP(2MLBP) with one Sc atom binding to three P atoms on the upmost layer has the lowest formation energy (−4.57 eV), exploring its greatest stability. Moreover, the calculated binding energy of Sc above the three P atoms is the largest (4.24 eV) energy, which is larger than the experimental cohesive energy of bulk Sc (3.90 eV), indicating that the clustering of Sc atoms on the surface of BP can be avoided. Importantly, the Sc atom adsorbed 2MLBP structure (2MLBP-Sc) has a conduction band minimum (CBM) value of −4.18 eV, which is below the redox potential of O2/O2– (−4.11 eV). When the O2 molecule is adsorbed on the 2MLBP-Sc, although the Sc–O and O–P bonds ...

Electrochemical cathode exfoliation of bulky black phosphorus into few-layer phosphorene nanosheets

Black phosphorus (BP) has gained great interest due to its tunable band-gap structure which strongly depends on the number of layers. The key step therefore towards application of BP is the preparation of few-layer or multilayer BP materials. However, because of the high chemical reactivity of BP, the preparation process must be handled with special care to avoid oxidation-induced degradation. Here we introduced a facile and scalable electrochemical method to generate BP nanosheets where BP served as the cathode. Subsequently, the product was characterized by TEM, Raman, UV–Vis, AFM, XPS and FTIR techniques. The relevant characterization data indicated that the thickness of the obtained BP nanosheets was mainly in the range of 2–7 nm. Furthermore, oxidation can be completely avoided during this electrochemical cathode exfoliation process, which provided high-quality BP nanosheets.

Liquid phase mass production of air-stable black phosphorus/phospholipids nanocomposite with ultralow tunneling barrier

Few-layer black phosphorus (FLBP), a recently discovered two-dimensional semiconductor, has attracted substantial attention in the scientific and technical communities due to its great potential in electronic and optoelectronic applications. However, reactivity of FLBP flakes with ambient species limits its direct applications. Among various methods to passivate FLBP in ambient environment, nanocomposites mixing FLBP flakes with stable matrix may be one of the most promising approaches for industry applications. Here, we report a simple one-step procedure to mass produce air-stable FLBP/phospholipids nanocomposite in liquid phase. The resultant nanocomposite is found to have ultralow tunneling barrier for charge carriers which can be described by an Efros–Shklovskii variable range hopping mechanism. Devices made from such mass-produced FLBP/phospholipids nanocomposite show highly stable electrical conductivity and opto-electrical response in ambient conditions, indicating its promising applications in both electronic and optoelectronic applications. This method could also be generalized to the mass production of nanocomposites consisting of other air-sensitive 2D materials, such as FeSe, NbSe2, WTe2, etc.

Electrocatalytic Activity of a 2D Phosphorene‐Based Heteroelectrocatalyst for Photoelectrochemical Cells

AbstractResearch into efficient synthesis, fundamental properties, and potential applications of phosphorene is currently the subject of intense investigation. Herein, solution‐processed phosphorene or few‐layer black phosphorus (FL‐BP) sheets are prepared using a microwave exfoliation method and used in photoelectrochemical cells. Based on experimental and theoretical (DFT) studies, the FL‐BP sheets are found to act as catalytically active sites and show excellent electrocatalytic activity for triiodide reduction in dye‐sensitized solar cells. Importantly, the device fabricated based on the newly designed cobalt sulfide (CoSx) decorated nitrogen and sulfur co‐doped carbon nanotube heteroelectrocatalyst coated with FL‐BP (FL‐BP@N,S‐doped CNTs‐CoSx) displayed an impressive photovoltaic efficiency of 8.31 %, outperforming expensive platinum based cells. This work paves the way for using phosphorene‐based electrocatalysts for next‐generation energy‐storage systems.

Electrocatalytic Activity of a 2D Phosphorene-Based Heteroelectrocatalyst for Photoelectrochemical Cells.

Research into efficient synthesis, fundamental properties, and potential applications of phosphorene is currently the subject of intense investigation. Herein, solution-processed phosphorene or few-layer black phosphorus (FL-BP) sheets are prepared using a microwave exfoliation method and used in photoelectrochemical cells. Based on experimental and theoretical (DFT) studies, the FL-BP sheets are found to act as catalytically active sites and show excellent electrocatalytic activity for triiodide reduction in dye-sensitized solar cells. Importantly, the device fabricated based on the newly designed cobalt sulfide (CoSx ) decorated nitrogen and sulfur co-doped carbon nanotube heteroelectrocatalyst coated with FL-BP (FL-BP@N,S-doped CNTs-CoSx ) displayed an impressive photovoltaic efficiency of 8.31 %, outperforming expensive platinum based cells. This work paves the way for using phosphorene-based electrocatalysts for next-generation energy-storage systems.

Density functional theory study of inter-layer coupling in bulk tin selenide

We study the inter-layer coupling in bulk tin selenide (SnSe) through density functional theory based calculations. Different approximations for the exchange-correlation functionals and the van der Waals interaction are employed. By performing comparison with graphite, MoS2 and black phosphorus, we analyze the inter-layer coupling from different points of view, including the binding energy, the low frequency inter-layer optical phonons, and the inter-layer charge transfer. We find that, there is a strong charge transfer between layers of SnSe, resulting in the strongest inter-layer coupling. Moreover, the charge transfer renders the inter-layer coupling in SnSe not of van der Waals type. Mechanical exfoliation has been used to fabricate mono- or few-layer graphene, MoS2 and black phosphorus. But, our results show that it may be difficult to apply similar technique to SnSe.

Repeatable Photodynamic Therapy with Triggered Signaling Pathways of Fibroblast Cell Proliferation and Differentiation To Promote Bacteria-Accompanied Wound Healing.

Despite the development of advanced antibacterial materials, bacterial infection is still a serious problem for wound healing because it usually induces severe complications and cannot be eradicated completely. Most current materials cannot simultaneously provide antibacterial activity, reusability, and biocompatibility as well as participate in stimulating cell behaviors to promote bacteria-accompanied wound healing. This work fabricated a hybrid hydrogel embedded with two-dimensional (2D) few-layer black phosphorus nanosheets (BPs) via simple electrostatic interaction. Within 10 min, 98.90% Escherichia coli and 99.51% Staphylococcus aureus can be killed rapidly by this hybrid, due to its powerful ability to produce singlet oxygen (1O2) under simulated visible light. In addition, this hydrogel also shows a high repeatability; that is, the antibacterial efficacy can still reach up to 95.6 and 94.58% against E. coli and S. aureus, respectively, even after challenging bacteria up to four times repeatedly. In vitro and in vivo results reveal that BPs in this hybrid hydrogel can promote the formation of the fibrinogen at the early stages during the tissue reconstruction process for accelerated incrustation. In addition, BPs can also trigger phosphoinositide 3-kinase (PI3K), phosphorylation of protein kinase B (Akt), and extracellular signal-regulated kinase (ERK1/2) signaling pathways for enhanced cellular proliferation and differentiation. Moreover, the hydrogel causes no appreciable abnormalities or damage to major organs (heart, liver, spleen, lung, and kidney) in rats during the wound healing process. Therefore, this BP-based hydrogel will have great potential as a safe multimodal therapeutic system for active wound healing and sterilization.