On Some Key Mechanical Problems Of Blue Phosphorus And Related One-Dimensional Materials

Since the experimental exfoliation of the first two-dimensional material –graphene-by Geim and Novoselov from the University of Manchester in 2004,the unprecedented superior mechanical,electronic,optical and chemical properties observed has generated unparalleled cross-disciplinary scientific and technological interest.A long stand challenge in the field is the lack of bandgap in graphene,which limits its broader application in nanoelectronics,optical and electromechanical devices.Therefore,the search for alternative stable two-dimensional materials of semiconducting nature is of essential benefit.In this regard,2D blue phosphorus with a buckled hexagonal honeycomb structure has been successfully synthesized recently.This graphene-like material holds good semiconductor properties(an electronic energy gap of 2.0e V)and retains excellent carrier mobility making it an ideal candidate for replacement of graphene in functional devices.As material mechanics determines the integrity of the nanodevices formed,aside from clarifying the mechanical responses of the functional nanomaterials,strategies to enhance the stability of the functional materials in heterostructure formation and self-healing have also attracted significant attention from the scientific community.Relative to graphene and other more well-known two-dimensional materials,understanding of the behavior of blue phosphorus,its heterojunctions with other onedimensional materials and the self-healing phenomenon in nanostructures is still limited.The basic mechanical properties,behaviors and energetic evolutions require further systematic characterization in order to gain insights into the atomic scale mechanisms and provide theoretical guidelines for materials design or synthesis.Firstly,the influence of temperature,chirality,size and loading mode on the mechanical responses of blue phosphorus 2D monolayer and related one-dimensional ribbon structures are studied based on the molecular dynamic methods(MD).Secondly,as direct carbon nanotube(CNT)encapsulation of blue phosphorous nanoribbon may lead to improved structural stability and new physical properties useful for enhancing device functionality,the structural dynamics and energetics during formation of these Blue P/CNT heterostructure are studied in detail.Finally,the investigation of detailed mechanical response evolution during and the mechanism underlying the unique selfhealing behavior of nanostructures is also revealed utilizing Silicon nanowire(Si NW)as a representative material.The major issues discussed are:1)The mechanical behavior of monolayer blue phosphorus under uniaxial tension,biaxial tension and shear loading were studied by utilizing Stillinger-Weber potential based MD.Failure by brittle fracture is observed in blue phosphorous under all applied loading type with unique initial cracking and propagation direction induced.In addition,the mechanical properties of blue phosphorus are very sensitive to temperature.With the increase of temperature,its Young’s modulus,ultimate stress and ultimate strain all significantly decrease.2)With understanding of mechanical response to basic loading types established,the effect of asymmetric biaxial loads are then examined where various combination of loading direction and strain rate ratios are considered.The results demonstrate that when the blue phosphorus is subjected to tension in one direction and a compressive load in the other,both the fracture stress and fracture strain of the blue phosphorus are enhanced as compared with biaxial tensile with relatively uniform,smooth edge structures generated by cracking.3)Formation of one-dimensional van der Waals(vd W)heterostructures via selfassembly of blue phosphorene nanoribbons to CNT.Our results show that driven by the vd W interaction,1D Blue P nanoribbons(Blue PNR)can be spontaneously encapsulated into CNTs while the structural transition is determined by the interplay between vd W interaction and bending energy.4)Silicon nanowires failure under simulated uniaxial tensile test and the mechanism behind the subsequent self-healing of fractured surfaces is also extensively revealed by MD modelling.It is demonstrated that although the dangling bonds on the fractured surface are in a high-energy state upon silicon nanowire initial failure,the energy of the entire system can still be reduced by re-bonding within a relatively short time period.This behavior can facilitate partial re-integration of the fractured surface and allow a stable wire structure to be reformed where the nanowire self-healing ability is characterized by a max ultimate stress restoration of 49.8%.