Phosphorene Flakes Research Articles

Breathing Mode's Temperature Coefficient Estimation and Interlayer Phonon Scattering Model of Few-Layer Phosphorene.

The breathing mode's Raman characteristic is a key parameter that estimates the number of layers and helps to determine interlayer thermal coupling in multilayer phosphorene. However, its temperature coefficient is not investigated yet, probably due to phosphorene's ambient instability, difficulties in capturing its Raman modes, and relatively weak temperature sensitivity than the corresponding primary intralayer Raman modes. Here, we captured the breathing modes' Raman scattering in multiple phosphorene flakes at different temperatures and estimated the corresponding first-order temperature coefficient. The captured modes show a negative temperature coefficient of around -0.0025 cm-1/K. Besides, we have explored a unique feature of the breathing mode phonon scattering with temperature. The modes closely follow the dominant three-phonon process and four-phonon process scattering phenomena at low- and high-temperature ranges. The three-phonon process scattering is dominant below ∼100 K, shifting to the dominant four-phonon process scattering beyond ∼150 K. Moreover, the phonon modes show anomalous behavior of blue shift with temperature during 100-150 K, probably due to transition in the scattering process. Our study shows the significant dependency of the breathing modes over temperature, which helps to understand and model phosphorene's interlayer thermal and mechanical properties. The study also reflects that phosphorene has significant interlayer heat transport capability due to three- and four-phonon scattering features.

Exfoliation of black phosphorus in isopropanol-water cosolvents

Black phosphorous (BP) has been gaining in popularity in the past decade in the 2D materials community because of the interesting properties of phosphorene, the graphene-type monolayered product obtained by the exfoliation of BP. Not all exfoliations are created equal: the properties – and hence, the usability – of phosphorene flakes can be fine-tuned by the optimization of the process parameters. Moreover, phosphorene processing is inherently sensitive to oxidation issues. Here we report on the liquid phase exfoliation of BP in isopropanol–water cosolvents. The efficiency of the exfoliation was monitored by AFM, DLS, and Raman spectroscopy. Our main findings are that (i) the solvent composition affects the size of the flakes, and that (ii) the extent of oxidation suffered by phosphorene during exfoliation is independent of the isopropanol:water ratio. We achieved optimal results using 30% isopropanol solvent that yielded mostly monolayers and bilayers measuring over 110 nm in diameter.

A Novel Laser‐Stimulated Technique for Direct Formation of Few‐Layer Phosphorene on Silicon Substrates

Direct formation of few‐layered phosphorene flakes through a laser stimulated crystallization on silicon substrates is reported. The formation of such layers is initiated by depositing red phosphorus (RP) on silicon followed by laser‐induced crystallization. After deposition, a glass plate is used to encapsulate the amorphous layer on the substrate. Being capable of a programmable scanning, a 1064‐nm laser is used as the source to impart energy onto the RP layer and to achieve the desired allotrope transformation. Exposure to laser beam at the presence of temperature and local pressure exerted by argon ambient leads to the conversion of amorphous material into crystalline flakes. Furthermore, interesting core‐shell, flower‐like and vertical structures are achieved. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman spectroscopy and energy dispersive X‐ray spectroscopy (EDS) are used to examine the conversion of red phosphorus into few‐layer phosphorene flakes. These flakes are used to fabricate a photodetector with a reasonable response to green laser and a field‐effect transistor with a mobility of 12 cm2 V−1 s−1, further indicating the evolution of well‐crystalline layers. The fabricated transistors demonstrate triode and saturation regimes in their electrical response, evidencing their high quality.

Electrochemical Stability of Few-Layered Phosphorene Flakes on Boron-Doped Diamond: A Wide Potential Range of Studies in Aqueous Solutions

Two-dimensional phosphorene has attracted great interest since its discovery as a result of its extraordinary properties. Two-dimensional single crystals of phosphorene can be useful for electroche...

Phosphorene and graphene flakes under the effect of external electric field as an anode material for high-performance lithium-ion batteries: A first-principles study

A new phenomenon, i.e. the improvement of the adsorption energy and the diffusion of Li ion in a phosphorene monolayer under the effect of perpendicular external electric field (EEF) was studied using M06-2X/6-31G(d,p) density functional theory (DFT) framework, and also outcomes were compared with the results obtained by using of graphene. Potential energy surface (PES) scanning and analysis of origin-dependency of charged molecular systems’ energy, were used to obtain intrinsic binding energy of the charged molecular compounds within the external electric field. Our calculations revealed that, increasing the strength of field from −0.02 to 0.032 a u, caused the adsorption energies of the Li ion in the phosphorene monolayer to be increased from −64.34 to −524.60 kJmol−1 and additionally, the qualitative diffusion constant of most preferred diffusion path for Li ion on phosphorene was about 1.3 × 103 times faster than that of graphene, indicating its ultrafast and anisotropic diffusivity. The corresponding diffusion barriers were synchronously decreased from 5.57 to 2.29 kJmol−1 for the Li ion under the EEF and similar results were found for graphene. The theoretical Li capacity for a monolayer phosphorene is predicted to be 865 mAh g−1 and 973.6 mAh g−1 in the absence and presence of EEF, respectively. We have concluded that the EEF can enhance the Li theoretical capacity for a monolayer phosphorene and can increase the adsorption of Li ion onto the phosphorene and graphene monolayers, and simultaneously decrease the diffusion barriers, which in turn can improve the performance of phosphorene or graphene anode-based rechargeable ion batteries.

Microphotoluminescence (μPL) measurements of bidimensional materials in a custom-made setup

The continuously growing interest in the so called bidimensional (2D) materials has been accompanied by a parallel development of techniques specifically devoted to manipulating and measuring micro-sized objects. Here a flexible microphotoluminescence (μPL) custom-made setup has been built on purpose: every setup component has been optimized for 2D materials; an automatized system able to perform measurements in different positions of the sample and execute mapping have been made; the dependence of the μPL intensity, peak center and other properties such as the Full Width Half Maximum (FWHM) on the position has been then extracted. The attention has been focused on WS2 and phosphorene. Optical response has been compared between Chemical Vapor Deposition (CVD) grown WS2 transferred on graphene and sapphire, especially focusing on the changes in the trion peak. Signatures have been found that can be attributed to photoluminescence from thick (more than five layers) phosphorene flakes.

Electron Microscopy and Electron Energy-Loss Spectroscopy (EELS) of Few-Layer Chemically-Exfoliated Phosphorene Flakes

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Phosphorene flakes may find use on the farm

Conservative gamblers may not want to bet the farm just yet, but two-dimensional phosphorus may soon be coming to bovine health care. Satish K. Tuteja and Suresh Neethirajan of the University of Guelph have shown that 2-D sheets of phosphorus, dubbed phosphorene, can be functionalized and used to quickly and inexpensively measure bovine haptoglobin levels (Nanotechnology 2018, DOI: 10.1088/1361-6528/aaab15). In beef and dairy cows, elevated levels of this protein are indicative of inflammation, infection, and other ailments associated with a common and costly pneumonia-like condition known as bovine respiratory disease. Currently, clinicians measure haptoglobin via time-consuming and costly immunoassay and spectroscopy methods. The Guelph researchers sought to capitalize on phosphorene’s high charge mobility and reactive surface to make fast-acting, sensitive detectors. They exfoliated phosphorus into nearly atomically thin sheets, treated them with poly-l-lysine, and deposited the functionalized flakes on disposable electrodes. Then they attached haptoglobin antibodies to the electrodes and

Exfoliation of Few-Layer Black Phosphorus in Low-Boiling-Point Solvents and Its Application in Li-Ion Batteries

The liquid-phase exfoliation (LPE) of black phosphorus (BP) is a strategic route for the large-scale production of phosphorene and few-layer BP (FL-BP) flakes. The exploitation of this exfoliated material in cutting-edge technologies, e.g., in flexible electronics and energy storage, is however limited by the fact that the LPE of BP is usually carried out at a high boiling point and in toxic solvents. In fact, the solvent residual is detrimental to device performance in real applications; thus, complete solvent removal is critical. Here, we tackle these issues by exfoliating BP in different low boiling-point solvents. Among these solvents, we find that acetone also provides a high concentration of exfoliated BP, leading to the production of FL-BP flakes with an average lateral size and thickness of c.a. 30 and 7 nm, respectively. The use of acetone to produce less defective few-layer BP flakes (FL-BPacetone) from bulk crystals is a straightforward process which enables the rapid preparation of homogeneous thin films thanks to the fast solvent evaporation. The ratio of edge to bulk atoms for the BP flakes here produced, combined with the use of low-boiling-point solvents for the exfoliation process suggests that these thin films are promising anodes for lithium-ion batteries. To this end, we tested Li-ion half cells with FL-BPacetone anodes achieving a reversible specific capacity of 480 mAh/g at a current density of 100 mA/g, over 100 charge/discharge cycles. Moreover, a reversible specific capacity of 345 mAh/g is achieved for the FL-BPacetone-based anode at a high current density (i.e., 1 A/g). These findings indicate that the FL-BPacetone-based battery is promising with regards to the design of fast charge/discharge devices. Overall, the presented process is a step forward toward the fabrication of phosphorene-based devices.

Chemically exfoliating large sheets of phosphorene via choline chloride urea viscosity-tuning

Exfoliation of two-dimensional phosphorene from bulk black phosphorous through chemical means is demonstrated where the solvent system of choice (choline chloride urea diluted with ethanol) has the ability to successfully exfoliate large-area multi-layer phosphorene sheets and further protect the flakes from ambient degradation. The intercalant solvent molecules, aided by low-powered sonication, diffuse between the layers of the bulk black phosphorus, allowing for the exfoliation of the multi-layer phosphorene through breaking of the interlayer van der Waals bonds. Through viscosity tuning, the optimal parameters (1:1 ratio between the intercalant and the diluting solvent) at which the exfoliation takes place is determined. Our exfoliation technique is shown to produce multi-layer phosphorene flakes with surface areas greater than 3 μm2 (a factor of three larger than what has previously been reported for a similar exfoliation method) while limiting exposure to the ambient environment, thereby protecting the flakes from degradation. Characterization techniques such as optical microscopy, Raman spectroscopy, ultraviolet–visible spectroscopy, and (scanning) transmission electron microscopy are used to investigate the quality, quantity, and thickness of the exfoliated flakes.

Fracture mechanisms in multilayer phosphorene assemblies: from brittle to ductile.

The outstanding mechanical performance of nacre has stimulated numerous studies on the design of artificial nacres. Phosphorene, a new two-dimensional (2D) material, has a crystalline in-plane structure and non-bonded interaction between adjacent flakes. Therefore, multi-layer phosphorene assemblies (MLPs), in which phosphorene flakes are piled up in a staggered manner, may exhibit outstanding mechanical performance, especially exceptional toughness. Therefore, molecular dynamics simulations are performed to study the dependence of the mechanical properties on the overlap distance between adjacent phosphorene layers and the number of phosphorene flakes per layer. The results indicate that when the flake number is equal to 1, a transition of fracture patterns is observed by increasing the overlap distance, from a ductile failure controlled by interfacial friction to a brittle failure dominated by the breakage of covalent bonds inside phosphorene flakes. Moreover, the failure pattern can be tuned by changing the number of flakes in each phosphorene layer. The results imply that the ultimate strength follows a power law with the exponent -0.5 in terms of the flake number, which is in good agreement with our analytical model. Furthermore, the flake number in each phosphorene layer is optimized as 2 when the temperature is 1 K in order to potentially achieve both high toughness and strength. Moreover, our results regarding the relations between mechanical performance and overlap distance can be explained well using a shear-lag model. However, it should be pointed out that increasing the temperature of MLPs could cause the transition of fracture patterns from ductile to brittle. Therefore, the optimal flake number depends heavily on temperature to achieve both its outstanding strength and toughness. Overall, our findings unveil the fundamental mechanism at the nanoscale for MLPs as well as provide a method to design phosphorene-based structures with targeted properties via tunable overlap distance and flake number in phosphorene layers.

Stable aqueous dispersions of optically and electronically active phosphorene

Understanding and exploiting the remarkable optical and electronic properties of phosphorene require mass production methods that avoid chemical degradation. Although solution-based strategies have been developed for scalable exfoliation of black phosphorus, these techniques have thus far used anhydrous organic solvents in an effort to minimize exposure to known oxidants, but at the cost of limited exfoliation yield and flake size distribution. Here, we present an alternative phosphorene production method based on surfactant-assisted exfoliation and postprocessing of black phosphorus in deoxygenated water. From comprehensive microscopic and spectroscopic analysis, this approach is shown to yield phosphorene dispersions that are stable, highly concentrated, and comparable to micromechanically exfoliated phosphorene in structure and chemistry. Due to the high exfoliation efficiency of this process, the resulting phosphorene flakes are thinner than anhydrous organic solvent dispersions, thus allowing the observation of layer-dependent photoluminescence down to the monolayer limit. Furthermore, to demonstrate preservation of electronic properties following solution processing, the aqueous-exfoliated phosphorene flakes are used in field-effect transistors with high drive currents and current modulation ratios. Overall, this method enables the isolation and mass production of few-layer phosphorene, which will accelerate ongoing efforts to realize a diverse range of phosphorene-based applications.

Producing air-stable monolayers of phosphorene and their defect engineering.

It has been a long-standing challenge to produce air-stable few- or monolayer samples of phosphorene because thin phosphorene films degrade rapidly in ambient conditions. Here we demonstrate a new highly controllable method for fabricating high quality, air-stable phosphorene films with a designated number of layers ranging from a few down to monolayer. Our approach involves the use of oxygen plasma dry etching to thin down thick-exfoliated phosphorene flakes, layer by layer with atomic precision. Moreover, in a stabilized phosphorene monolayer, we were able to precisely engineer defects for the first time, which led to efficient emission of photons at new frequencies in the near infrared at room temperature. In addition, we demonstrate the use of an electrostatic gate to tune the photon emission from the defects in a monolayer phosphorene. This could lead to new electronic and optoelectronic devices, such as electrically tunable, broadband near infrared lighting devices operating at room temperature.

The Role of Water in the Preparation and Stabilization of High-Quality Phosphorene Flakes.

The stability of phosphorene is shown to be degraded by water. However, the presence of a small amount of water allows the synthesis of high-quality material in liquid exfoliation of black phosphorus using dimethylsulfoxide as solvent. A phosphorus/water molar ratio between 1.5 and 14 maximizes the quality of the phosphorene flakes and their stability.