Reactive molecular dynamics simulations of the mechanical properties of various phosphorene allotropes

Two-Dimensional Violet Phosphorus P11: A Large Band Gap Phosphorus Allotrope

The discovery of novel large band gap two-dimensional (2D) materials with good stability and high carrier mobility will innovate the next generation of electronics and optoelectronics. A new allotrope of 2D violet phosphorus P₁₁ was synthesized via a salt flux method in the presence of bismuth. Millimeter-sized crystals of violet-P₁₁ were collected after removing the salt flux with DI water. From single-crystal X-ray diffraction, the crystal structure of violet-P₁₁ was determined to be in the monoclinic space group C2/c (no. 15) with unit cell parameters of a = 9.166(6) Å, b = 9.121(6) Å, c = 21.803(14)Å, β = 97.638(17)°, and a unit cell volume of 1807(2) ų. The structure differences between violet-P₁₁, violet-P₂₁, and fibrous-P₂₁ are discussed. The violet-P₁₁ crystals can be mechanically exfoliated down to a few layers (∼6 nm). Photoluminescence and Raman measurements reveal the thickness-dependent nature of violet-P₁₁, and exfoliated violet-P₁₁ flakes were stable in ambient air for at least 1 h, exhibiting moderate ambient stability. The bulk violet-P₁₁ crystals exhibit excellent stability, being stable in ambient air for many days. The optical band gap of violet-P₁₁ bulk crystals is 2.0(1) eV measured by UV-Vis and electron energy-loss spectroscopy measurements, in agreement with density functional theory calculations which predict that violet-P₁₁ is a direct band gap semiconductor with band gaps of 1.8 and 1.9 eV for bulk and monolayer, respectively, and with a high carrier mobility. This band gap is the largest among the known single-element 2D layered bulk crystals and thus attractive for various optoelectronic devices.

Experimental and molecular dynamics studies of an ultra-fast sequential hydrogen plasma process for fabricating phosphorene-based sensors

Low concentration phosphorene-based sensors have been fabricated using a facile and ultra-fast process which is based on an exfoliation-free sequential hydrogen plasma treatment to convert the amorphous phosphorus thin film into mono- or few-layered phosphorene sheets. These sheets have been realized directly on silicon substrates followed by the fabrication of field-effect transistors showing the low leakage current and reasonable mobility for the nano-sensors. Being capable of covering the whole surface of the silicon substrate, red phosphorus (RP) coated substrate has been employed to achieve large area phosphorene sheets. Unlike the available techniques including mechanical exfoliation, there is no need for any exfoliation and/or transfer step which is significant progress in shortening the device fabrication procedure. These phosphorene sheets have been examined using transmission electron microscopy (TEM), Scanning electron microscopy (SEM), Raman spectroscopy and atomic-force microscopy (AFM). Electrical output in different states of the crystallization as well as its correlation with the test parameters have been also extensively used to examine the evolution of the phosphorene sheets. By utilizing the fabricated devices, the sensitivity of the phosphorene based-field effect transistors to the soluble L-Cysteine in low concentrations has been studied by measuring the FET response to the different concentrations. At a gate voltage of - 2.5 V, the range of 0.07 to 0.60 mg/ml of the L-Cysteine has been distinguishably detected presenting a gate-controlled sensor for a low-concentration solution. A reactive molecular dynamics simulation has been also performed to track the details of this plasma-based crystallization. The obtained results showed that the imparted energy from hydrogen plasma resulted in a phase transition from a system containing red phosphorus atoms to the crystal one. Interestingly and according to the simulation results, there is a directional preference of crystal growth as the crystalline domains are being formed and RP atoms are more likely to re-locate in armchair than in zigzag direction.

Critical fracture properties of puckered and buckled arsenenes by molecular dynamics simulations

The pioneering prediction and successful synthesis of monolayer arsenene in recent years have promoted intensive studies on this novel two-dimensional (2D) material. Strain-engineered arsenene monolayer can change its geometric structures with tuned charge distribution, which paves the way for achieving novel electronic properties. The practical applications of the strain-driven topological state in arsenene strongly depend on its critical strain value. In this work, mechanical properties such as fracture strain, fracture strength and Young's modulus of two arsenene structures, i.e. buckled arsenene (b-arsenene) and puckered arsenene (p-arsenene), are comprehensively investigated under different modulators such as system dimension, chirality, temperature, strain rate and random surface defect. A maximum fracture strain reduction of 41.7% from 0.24 to 0.14 is observed in armchair b-arsenene when the temperature increases from 100 to 500 K. The most significant impact factor on the mechanical properties of arseneneis found to be surface defects. A maximum fracture strength reduction of 85.7% is predicted in the armchair b-arsenene when the defect ratio increases from 0 to 5%. On the other hand, the strain rate has a negligible effect on the mechanical properties. Our results provide fundamental knowledge on the critical fracture properties of arsenene.

A coarse-grained model for mechanical behavior of phosphorene sheets

The popularity of phosphorene (known as monolayer black phosphorus) in electronic devices relies on not only its superior electrical properties, but also its mechanical stability beyond the nanoscale. However, the mechanical performance of phosphorene beyond the nanoscale remains poorly explored owing to the spatiotemporal limitation of experimental observations, first-principles calculations, and atomistic simulations. To overcome this limitation, here a coarse-grained molecular dynamics (CG-MD) model is developed via a strain energy conservation approach to offer a new computational tool for the investigation of the mechanical properties of phosphorene beyond the nanoscale. The mechanical properties of a single phosphorene sheet are first characterized by all-atom molecular dynamics (AA-MD) simulations, followed by a force-field parameter optimization of the CG-MD model by matching these mechanical properties from AA-MD simulations. The intrinsic out-of-plane puckered feature is conserved in our CG-MD model, rendering mechanical anisotropy and heterogeneity in both the in-plane and out-of-plane directions preserved. The results indicate that our coarse-grained model is able to accurately capture the anisotropic in-plane mechanical performance of phosphorene and quantitatively reproduce Young's modulus, ultimate strength, and fracture strain under various environmental temperatures. Our CG-MD model can also capture the anisotropic out-of-plane bending stiffness of phosphorene. We demonstrate the applicability of our model in capturing the fracture toughness of phosphorene in both the armchair and zigzag directions by comparison with the results from AA-MD simulations. This CG-MD model proposed here offers greater capability to perform mechanical mesoscale simulations for phosphorene-based systems, allowing for a deeper understanding of the mechanical properties of phosphorene beyond the nanoscale, and the potential transferability of the developed force-field can help design hybrid phosphorene devices and structures.