Molecular Dynamics Study On Stability Of Black Phosphorus Nanotubes

Since the monolayer black phosphorus(MLBP)was synthesized in 2014,it has become the most popular 2-D material due to its excellent electronic and photovoltaic properties.Considering the important application of nanotubes(1-D material)in nano-electro-mechanical systems,we believe that prediction on the dynamic response of nanotube from MLBP is significant for its potential applications.In present dissertation,the stability of two types of black phosphorus nanotubes(BPNTs)are investigated using molecular dynamics simulations and following results are obtained:(1)The thermal stability of the two types of BPNTs is analyzed.MLBP shows puckered sheet.Hence,the armchair type BPNT(A-BPNT)formed by curling a square MLBP along pucker direction will have lower initial in-shell stress than the zigzag type BPNT(Z-BPNT)formed by curling the square sheet along the in-plane direction normal to pucker direction.The difference of stability between the two types of BPNTs is obvious.On the other hand,the phosphorus atoms on a BPNT have great thermal vibration,which enhances the variation of the bond lengths and bond angles in the tube.At higher temperature,higher variation of bond lengths and bond angles leads to breakage of BPNT randomly.Results demonstrate that the breakage on an A-BPNT is caused by broken bonds on the outer surface and strong attraction among the atoms on the inner surface of tube.The major reason for the damage of Z-BPNT is that the buckling of bond angle at the center of outer surface of tube.At the same temperature,the critical value of radius of A-BPNT is far less than that of Z-BPNT;(2)At canonical NVT ensemble,strength and stability of an A-BPNT under uniaxial compression are investigated.The effects of four major factors,e.g.,diameter of tube,tube length,loading speed and temperature,on the strength and stability of tube are considered.During loading,buckling of tube happens earlier than damage on tube.For the tubes with the same ratio between length and diameter,the maximal axial stress is lower for a longer tube.For the tubes with the same diameter,the maximal axial stress decreases with the increase of tube length and the critical axial strain decreases,simultaneously.At higher temperature,A-BPNT has weaker strength and stability;(3)Strength and stability of composite double-walled nanotubes(DWNTs)from a carbon nanotube(CNT)and a BPNT are investigated by putting the DWNTs under uniaxial load.The influence of temperature of environment,ratio between effective length and diameter and configuration of DWNTs on their dynamic response is researched.Results show that the two co-axial tubes are deformed coordinately due to van der Waals interaction between two shells.When one tube buckles,the other is driven to buckle immediately.Some phosphorus-phosphorus bonds on BPNT break after the maximal axial stress which happens after buckling.Whilst the deformation of CNT becomes higher and CNT has no broken bonds as the axial loading stops.The axial stresses of two tubes jumps down soon after buckling.Comparing with the single BPNT under compression,the BPNT in composite DWNTs has higher compressed deformation.The strength of composite DWNTs is lower at higher temperature.Stability of DWNTs is not obviously sensitive to temperature as BPNT is covered with a CNT.