Raman studies of black phosphorus from 0.25 to 7.7 GPa at 15 K

Imaging Ultrafast Dynamics of Pressure-Driven Phase Transitions in Black Phosphorus and Anomalous Coherent Phonon Softening

Applying high pressure to effectively modulate the electronic and lattice structures of materials could unravel various physical properties associated with phase transitions. In this work, high-pressure-compatible femtosecond pump-probe microscopy was constructed to study the pressure-dependent ultrafast dynamics in black phosphorus (BP) thin films. We observed pressure-driven evolution of the electronic topological transition and three structural phases as the pressure reached ∼22 GPa, which could be clearly differentiated in the transient absorption images containing spatially resolved ultrafast carrier and coherent phonon dynamics. Surprisingly, an anomalous coherent acoustic phonon mode with pressure softening behavior was observed within the range of ∼3-8 GPa, showing distinct laser power and time dependences. Density functional theory calculations show that this mode, identified as the shear mode along the armchair orientation, gains significant electron-phonon coupling strength from out-of-plane compression that leads to decreased phonon frequency. Our results provide insights into the structure evolution of BP with pressure and hold potential for applications in microelectromechanical devices.

Polarized Raman spectroscopy in low-symmetry 2D materials: angle-resolved experiments and complex number tensor elements

In this perspective review, we discuss the power of polarized Raman spectroscopy to study optically anisotropic 2D materials, belonging to the orthorhombic, monoclinic and triclinic crystal families. We start by showing that the polarization dependence of the peak intensities is described by the Raman tensor that is unique for each phonon mode, and then we discuss how to determine the tensor elements from the angle-resolved polarized measurements by analyzing the intensities in both the parallel- and cross-polarized scattering configurations. We present specific examples of orthorhombic black phosphorus and monoclinic 1T'-MoTe₂, where the Raman tensors have null elements and their principal axes coincide with the crystallographic ones, followed by a discussion on the results for triclinic ReS₂ and ReSe₂, where the axes of the Raman tensor do not coincide with the crystallographic axes and all elements are non-zero. We show that the Raman tensor elements are, in general, given by complex numbers and that phase differences between tensor elements are needed to describe the experimental results. We discuss the dependence of the Raman tensors on the excitation laser energy and thickness of the sample within the framework of the quantum model for the Raman intensities. We show that the wavevector dependence of the electron-phonon interaction is essential for explaining the distinct Raman tensor for each phonon mode. Finally, we close with our concluding remarks and perspectives to be explored using angle-resolved polarized Raman spectroscopy in optically anisotropic 2D materials.

Reversible Pressure-Induced Partial Phase Transition in Few-Layer Black Phosphorus

The experimental identification of structural transitions in layered black phosphorus (BP) under mechanical stress is essential to extend its application in microelectromechanical (MEMS) devices under harsh conditions. High-pressure Raman spectroscopic analysis of BP flakes suggests a transition pressure at ∼4.2 GPa, where the BP's crystal structure progressively transforms from an orthorhombic to a rhombohedral symmetry (blue phosphorus, bP). The phase transition has been identified by observing a transition from blueshift to redshift of the in-plane characteristic Raman modes (B_2g and A_g²) with increasing pressure. Recovery of the vibrational frequencies for all three characteristic Raman modes confirms the reversibility of the structural phase transition. First-principles calculations provide insight into the behavior of the Raman modes of BP under high pressure and reveal the mechanism responsible for the partial phase transition from BP to bP, corresponding to a metastable equilibrium state where both phases coexist.

Non-aqueous solution-processed phosphorene by controlled low-potential electrochemical exfoliation and thin film preparation

Black phosphorus (BP) in its monolayer form called phosphorene is thought of as a successor of graphene and is of great interest for (opto)electronic applications. A quantitative and scalable method for the synthesis of (mono-)few-layer phosphorene has been an outstanding challenge due to the process irreproducibility and environmental degradation capability of the BP. Here, we report a facile controlled electrochemical exfoliation method for the preparation of a few-layer phosphorene (FP) with nearly 100% yield. Our approach relies on the low-potential influence in anhydrous and oxygen-free low-boiling acetonitrile (AN) and N,N-dimethylformamide (DMF) using alkylammonium ions. Herein, intercalation of positive ions into BP interlayers occurred with a minimum potential of -2.95 V in DMF and -2.85 V in AN and the non-damaging and highly accurate electrochemical exfoliation lasted at -3.8 V. A variety of analytical methods have revealed that in particular DMF-based exfoliation results in high-quality phosphorene of 1-5 layers with good crystallinity and lateral sizes up to tens of micrometers. Moreover, assurance of the oxygen- and water-free environment allowed us to minimize the surface oxidation of BP and, consequently, exfoliated phosphorene. We pioneer an effective and reproducible printing transfer of electrochemically exfoliated phosphorene films onto various flexible and rigid substrates. The surfactant-free process of exfoliation allowed assembly and transfer of thin films based on FP. The phosphorene-based films characterized as direct gap semiconductors have a layer-number-dependent bandgap with a tuning range larger than that of other 2D materials. We show that on varying the films' thickness, it is possible to modify their optical properties, which is a significant advantage for compact and switchable optoelectronic components.

Nanomaterial-based SERS sensing technology for biomedical application

Over the past few years, nanomaterial-based surface-enhanced Raman scattering (SERS) detection has emerged as a new exciting field in which theoretical and experimental studies of the structure and function of nanomaterials have become a focus.

Precise and label-free tumour cell recognition based on a black phosphorus nanoquenching platform

Duplexed recognition of label-free breast cancer cells: a duplexed assay platform based on a BP nanoquencher allows simultaneous detection of two tumor markers within one run.

Electrochemical Na-Insertion/Extraction Property of Ni-Coated Black Phosphorus Prepared by an Electroless Deposition Method

Electrical conductivity is one of the properties required for an active material, and it is extremely essential to exert its potential. In the present study, the strategy of coating a metal at a single particle level by an electroless deposition method was applied to enhance the cycling performance of phosphorus-based negative electrodes for Na-ion batteries. The deposition morphology and composition of the Ni coating layer were characterized by field-emission scanning electron microscopy, scanning transmission electron microscopy, and X-ray diffraction. In the 5 wt % Ni coating, an amorphous Ni layer of several nanometers thickness was homogeneously formed on the phosphorus surface, whereas a shell layer having a 200 nm thickness was formed in the order of Ni₁₂P₅, NiP₂, NiP₃, and metallic Ni from the surface toward the center in the 30 wt % Ni coating. Electrochemical impedance spectroscopic measurements clarified that the good electron transport proceeded throughout the developed conduction pathway to promote the phase transition to trisodium phosphide (Na₃P), leading to a high reversible capacity for phosphorus; the as-prepared black phosphorus showed only a reversible capacity of 140 mA h g^-1 at the 60th cycle, whereas the 30 wt % Ni-coated composite delivered a relatively high capacity of 780 mA h g^(P)-1. In addition, the expansion ratio of the electrode after the 30th desodiation was the lowest among the three kinds of electrodes. By contrast, cracks and exfoliation of the active material layer from the current collector were confirmed in the as-prepared black phosphorus. These results demonstrate that the upgraded performance accomplished using the 30 wt % Ni-coated composite with the Ni/Ni-P layer is due to the synergetic effects of the electron conduction channel and a buffer matrix against a large volumetric change (∼400%) in phosphorus during the charge-discharge reactions.

A phosphorene-graphene hybrid material as a high-capacity anode for sodium-ion batteries

Sodium-ion batteries have recently attracted significant attention as an alternative to lithium-ion batteries because sodium sources do not present the geopolitical issues that lithium sources might. Although recent reports on cathode materials for sodium-ion batteries have demonstrated performances comparable to their lithium-ion counterparts, the major scientific challenge for a competitive sodium-ion battery technology is to develop viable anode materials. Here we show that a hybrid material made out of a few phosphorene layers sandwiched between graphene layers shows a specific capacity of 2,440 mA h g(-1) (calculated using the mass of phosphorus only) at a current density of 0.05 A g(-1) and an 83% capacity retention after 100 cycles while operating between 0 and 1.5 V. Using in situ transmission electron microscopy and ex situ X-ray diffraction techniques, we explain the large capacity of our anode through a dual mechanism of intercalation of sodium ions along the x axis of the phosphorene layers followed by the formation of a Na3P alloy. The presence of graphene layers in the hybrid material works as a mechanical backbone and an electrical highway, ensuring that a suitable elastic buffer space accommodates the anisotropic expansion of phosphorene layers along the y and z axial directions for stable cycling operation.

Photooxidation and quantum confinement effects in exfoliated black phosphorus

Thin layers of black phosphorus have recently raised interest owing to their two-dimensional (2D) semiconducting properties, such as tunable direct bandgap and high carrier mobilities. This lamellar crystal of phosphorus atoms can be exfoliated down to monolayer 2D-phosphane (also called phosphorene) using procedures similar to those used for graphene. Probing the properties has, however, been challenged by a fast degradation of the thinnest layers on exposure to ambient conditions. Herein, we investigate this chemistry using in situ Raman and transmission electron spectroscopies. The results highlight a thickness-dependent photoassisted oxidation reaction with oxygen dissolved in adsorbed water. The oxidation kinetics is consistent with a phenomenological model involving electron transfer and quantum confinement as key parameters. A procedure carried out in a glove box is used to prepare mono-, bi- and multilayer 2D-phosphane in their pristine states for further studies on the effect of layer thickness on the Raman modes. Controlled experiments in ambient conditions are shown to lower the A(g)(1)/A(g)(2) intensity ratio for ultrathin layers, a signature of oxidation.

Structures, stabilities, and electronic properties of defects in monolayer black phosphorus

The structures, stabilities, and electronic properties of monolayer black phosphorus (M-BP) with different kinds of defects are investigated within the frame of density-functional theory. All the possible configurations of defects in M-BP are explored, and the calculated results suggest that the stabilities of the configurations with different kinds of defects are greatly related to broken bonds, structural deformation and the character of the bonding. The configurations with two or three vacancies are energetically more favorable than the ones with a single vacancy. Meanwhile, the doping of two foreign atoms, such as sulfur, silicon or aluminum, is more stable than that of the corresponding single dopant. The electronic properties of M-BP are greatly affected by the types of defects. The single S-doped M-BP not only retains the character of a direct semiconductor, but it also can enlarge the band gap by 0.24 eV relative to the perfect one. Such results reveal that the defects not only greatly affect the electronic properties, but they also can be used as an effective way to modulate the band gap for the different applications of M-BP in electronic devices.

Unusual angular dependence of the Raman response in black phosphorus

Anisotropic materials are characterized by a unique optical response, which is highly polarization-dependent. Of particular interest are layered materials formed by the stacking of two-dimensional (2D) crystals that are naturally anisotropic in the direction perpendicular to the 2D planes. Black phosphorus (BP) is a stack of 2D phosphorene crystals and a highly anisotropic semiconductor with a direct band gap. We show that the angular dependence of polarized Raman spectra of BP is rather unusual and can be explained only by considering complex values for the Raman tensor elements. This result can be traced back to the electron-photon and electron-phonon interactions in this material.

Ambipolar insulator-to-metal transition in black phosphorus by ionic-liquid gating

We report ambipolar transport properties in black phosphorus using an electric-double-layer transistor configuration. The transfer curve clearly exhibits ambipolar transistor behavior with an ON-OFF ratio of ∼5 × 10(3). The band gap was determined as ≅0.35 eV from the transfer curve, and Hall-effect measurements revealed that the hole mobility was ∼190 cm(2)/(V s) at 170 K, which is 1 order of magnitude larger than the electron mobility. By inducing an ultrahigh carrier density of ∼10(14) cm(-2), an electric-field-induced transition from the insulating state to the metallic state was realized, due to both electron and hole doping. Our results suggest that black phosphorus will be a good candidate for the fabrication of functional devices, such as lateral p-n junctions and tunnel diodes, due to the intrinsic narrow band gap.

Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

Formation of stable phosphorus-carbon bond for enhanced performance in black phosphorus nanoparticle-graphite composite battery anodes

High specific capacity battery electrode materials have attracted great research attention. Phosphorus as a low-cost abundant material has a high theoretical specific capacity of 2596 mAh/g with most of its capacity at the discharge potential range of 0.4-1.2 V, suitable as anodes. Although numerous research progress have shown other high capacity anodes such as Si, Ge, Sn, and SnO2, there are only a few studies on phosphorus anodes despite its high theoretical capacity. Successful applications of phosphorus anodes have been impeded by rapid capacity fading, mainly caused by large volume change (around 300%) upon lithiation and thus loss of electrical contact. Using the conducting allotrope of phosphorus, "black phosphorus" as starting materials, here we fabricated composites of black phosphorus nanoparticle-graphite by mechanochemical reaction in a high energy mechanical milling process. This process produces phosphorus-carbon bonds, which are stable during lithium insertion/extraction, maintaining excellent electrical connection between phosphorus and carbon. We demonstrated high initial discharge capacity of 2786 mAh·g(-1) at 0.2 C and an excellent cycle life of 100 cycles with 80% capacity retention. High specific discharge capacities are maintained at fast C rates (2270, 1750, 1500, and 1240 mAh·g(-1) at C/5, 1, 2, and 4.5 C, respectively).

Black phosphorus field-effect transistors

Two-dimensional crystals have emerged as a class of materials that may impact future electronic technologies. Experimentally identifying and characterizing new functional two-dimensional materials is challenging, but also potentially rewarding. Here, we fabricate field-effect transistors based on few-layer black phosphorus crystals with thickness down to a few nanometres. Reliable transistor performance is achieved at room temperature in samples thinner than 7.5 nm, with drain current modulation on the order of 10(5) and well-developed current saturation in the I-V characteristics. The charge-carrier mobility is found to be thickness-dependent, with the highest values up to ∼ 1,000 cm(2) V(-1) s(-1) obtained for a thickness of ∼ 10 nm. Our results demonstrate the potential of black phosphorus thin crystals as a new two-dimensional material for applications in nanoelectronic devices.

Phosphorene: an unexplored 2D semiconductor with a high hole mobility

We introduce the 2D counterpart of layered black phosphorus, which we call phosphorene, as an unexplored p-type semiconducting material. Same as graphene and MoS2, single-layer phosphorene is flexible and can be mechanically exfoliated. We find phosphorene to be stable and, unlike graphene, to have an inherent, direct, and appreciable band gap. Our ab initio calculations indicate that the band gap is direct, depends on the number of layers and the in-layer strain, and is significantly larger than the bulk value of 0.31-0.36 eV. The observed photoluminescence peak of single-layer phosphorene in the visible optical range confirms that the band gap is larger than that of the bulk system. Our transport studies indicate a hole mobility that reflects the structural anisotropy of phosphorene and complements n-type MoS2. At room temperature, our few-layer phosphorene field-effect transistors with 1.0 μm channel length display a high on-current of 194 mA/mm, a high hole field-effect mobility of 286 cm(2)/V·s, and an on/off ratio of up to 10(4). We demonstrate the possibility of phosphorene integration by constructing a 2D CMOS inverter consisting of phosphorene PMOS and MoS2 NMOS transistors.