| Low-dimensional materials have excellent electrical,magnetic,thermal and mechanical properties,and have great application prospects in the fields of electronic devices,flexible materials,new functional devices,and nano-electromechanical systems.Mechanical properties are key indicators to evaluate the service life of low-dimensional materials.Good plasticity,strength and electronic properties can ensure the practical application of low-dimensional materials in flexible devices.Therefore,it is of great significance to study the mechanical and electronic properties of low-dimensional.Many research works have focused on the experimental preparation of low-dimensional materials.Through reasonable control of experimental preparation parameters,low-dimensional materials with excellent performance and powerful functions have been prepared.However,it is difficult to explain the microscopic mechanism of the internal structurefrom the molecular or atomic point of viewwith experimental work,and only the morphology analysis can be tested through the experiment instruments and equipment.Compared with this,theoretical simulation work has shown great advantages,which can construct microscopic atomic level structure,tune the internal geometric structure and internal defects of low-dimensional materials at the atomic or molecular level,and then rationally modulate the mechanical and electronicproperties.In view of the above analysis,we study the influence mechanism of the internal structure of low-dimensional materials based on carbon and boron nitride on the mechanical and electrical properties from the perspective of theoretical simulation.First,we have constructed the pristine boron nitride nanotubes(BNNTs).By introducing Stone-Wales(SW),B or N vacancies and embedded graphene sheet structures,defective BNNTs was studied on the mechanical parameters(mechanical strength,stiffness,and critical strain)and electronic properties(band structure,charge population,etc.).Then,a new nanotube models(PGNTs)with five-membered rings was constructed to explore their internal special geometric structures,band structure changes,mechanical properties and fracture failure behavior under strain.Finally,we have studied the defective hexagonal boron nitride(h-BN)with applying tensile strain.These studies have found the mechanical enhancement effect by increasing the defect size.The main research contents and conclusions of this paper are as followsThe first chapter mainly expounded the research background and significance of this dissertation.First,it briefly introduces the basic geometrical structure,physical and chemical properties,preparation methods and related applications of low-dimensional materials.Then,we have introduced several typical internal defect structures of low-dimensional materials,and explained the influence mechanism of internal defects on the mechanical and electrical properties of macroscopic materials.Finally,the importance of defect engineering in the theoretical research and experimental work of low-dimensional materials is pointed outThe second chapter described the basic framework of density functional theory(DFT),the main targets of DFT is to find optimal approximation for exchange-correlation functional.From LDA,GGA,to the correction of self-consistent field interaction,a variety of functional have emerged to seek the best calculation results;then the DMol3calculation module was introduced;and finally described the basic principle of molecular dynamics,ensemble and the calculation process of molecular dynamics.In the third chapter,we performed density functional theory method to concentrate on the mechanical and electronic parameters of perfect and defective BNNTs.We have systematically carried out the discussion on the stiffness,intrinsic strength,and failure critical strain of different types of BNNTs.With the increased SW defect density around the tube axis,it has been foud from the calculating results that the intrinsic strength of BNNTs decreased linearly.The mechanical properties of defective BNNTs enhance with the increased SW defect density along axis,which is due to the special SW defect structure.The defective model with a B vacancy has higher intrinsic strength than that of the N vacancy system.Final fracture of the perfect BNNTs was owing to the break of the stress bearing bonds under the mechanical tensile strain.Defects including the SW or vacancy are assumed as the initial break sites of the defective BNNTs.In addition,applying strain or doping defects are effective method to decrease the band gap of tubes.In the fourth chapter,employed with density functional theory method,we further investigated the mechanical properties,electronic structure and fracture process under axial tensile stretch of hybrid boron nitride nanotubes(BN-CNTs)doped with hexagonal or triangular graphene structure.In comparison with the perfect BNNT,the tensile strength of the BN-CNTs increased due to the strengthening mechanism.Due to the disturbance of the carbon structure,the symmetry of BN-CNTs is destroyed that lead to the decrease of the stiffness.The newly formed C-B or C-N bond in the interface structure plays an important role on the fracture mechanism.In comparison with the C-N bond,the C-B bond in the interface would cause more damage on the symmetry structure of the BN-CNTs.Thus the C-B bond served as the initial break site during the tensile stretch.For the models with C-N bonds interface structure,the B-N bonds near the C-N bond would break first.It has been found that the band gap would decrease due to the doping of graphene domains.This special interface structure has a great effect on tuning the mechanical and electronic parameters of BN-CNTs.In the fifth chapter,we have employed the molecular dynamics and density functional theory method to investigate the mechanical behaviors and electronic properties of penta graphene nanotubes under axial compression and tension loadings.We have built several strucutures of PGNTs to devote important insights about charge population,band structure,atoms orbital and fracture behavior.With the size increasing,the band gap keeps decrease relatively.For the(9,9)PGNT,the band gap increases with the tensile strain while it decreases with the compressive strain.With the strain change,the charge transformation can be found and proved by the HOMO-LUMO orbitals.We have found the mechanical parameters fluctuate with the nanotubes diameter.C1-C1 and C1-C2 bonds serve as the main stress bearing bonds on the fracture procedure of(9,9)PGNT.An eight-member ring is formed at the critical condition and acts as a defect during the continuous tensile strain increase.This investigation has brought a deeper understanding of the PGNTs about their intrinsic mechanical and electronic properties.In the sixth chapter,investigation on the mechanical properties of defective h-BN sheets was performed using the molecular dynamics method.Here,the elliptical and rectangular defects in h-BN sheets were established,and the discussion on failure strength and critical strain was conducted systematically.The calculated results showed that the failure strength and critical strain enhanced as elongating the elliptical defects.This mechanical enhancement is due to the different stress distribution situation.The local stress concentration is formed in the h-BN model with small size elliptical defect that make the model easy to break.The stress is evenly distributed along the edge of elliptical defects for the h-BN model with large size defect.In comparison,the mechanical properties of h-BN models with elliptical defects are smaller than that of the models with rectangular defects,this due to the curved-edge structure for the elliptical defective h-BN sheet.The fracture mechanism found that the multiple ring structures were formed and propagated with increasing tensile loading strain,which results in the final break of h-BN sheetsFinally,in the seventh chapter,we have summarized the main conclusions and the innovation of this dissertation,and described the future research direction. |
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