Theoretical Studies Of The Boron Nitride Nanotubes By The Density Functional Methods

Ever since the discovery of carbon nanotubes,the nanotubular materials,as new type nanostructure systems,have attracted considerable research interests.With the development of nanoscience and nanotechnology,nanotubes have been the subject of extensive study for years.Boron nitride nanotubes(BNNTs) were successfully synthesized soon after the discovery of carbon nanotubes.BNNTs characterize the unique atomic structures,physical properties and potential applications.In the present dissertation,by using DFT calculations,we have investigated the structural defects, chemical modifications and doping impurities in BNNT systems.Our work will be helpful for a better understanding of the BNNTs and will also provide the theoretical support for their potential applications.The dissertation contains five chapters.In chapter 1,we introduce the basic concept of DFT method and review its recent progress.The basis of DFT is the charge density of a system at ground state,which determines any properties of a many body system.With the help of Kohn-Sham equation,many body interaction is included in the exchange-correlation functional energy and the many body problem becomes an effective single particle problem.Searching for a good approximation of the exchange-correlation functional is one of the main targets in DFT.In addition, combining DFT with other theories can also lead to its progress and extend its application.At the end of this chapter,we briefly introduce some DFT based simulation packages used in this dissertation.In chapter 2,we give a brief review of the research progress made on the inorganic nanotubular materials.We pay a close attention to BNNTs,including their research progress and the last achievements.It is found that BNNTs are very important functional nanomaterials.But compared with carbon nanotubes,their research progress are far lingering behind.This is primarily as a results of the difficulties involved in the experimental preparations and characterizations of BNNT samples.Especially,the pioneering and systematic research works performed in this field are still lacking.In this context,our research works are focused on BNNT systems.Starting from chapter 3,we start to introduce our research works performed on BNNTs. In chapter 3,we systematically study the structural defects on BNNTs with DFT methods.We perform the calculations on the structural configurations and energetic stabilities of various defects,including single vacancy,divacancy,and Stone-Wales defects,where the size(tube diameter) dependent properties of the considered defects are found.In order to understand the defects healing mechanism on the BNNTs,the migration of the structural defects in a finite-length BNNT are under investigation. Based on our findings,we discuss the potential experimental consequences and provide the theoretical support to one experimental method for the synthesis of BN tubes.In chapter 4,we investigate the chemical modification of BNNTs.The chemical modification of BNNTs include the covalent and non-covalent modifications.Using DFT calculations,we study the covalent modification of BNNTs with transition metal Fe and non-covalent modification of BNNTs by the perylene derivative molecules (PTAS).We focus on the electronic and magnetic properties of Fe functionalized BNNTs.For non-covalent modification of BNNTs with PTAS,we provide the direct theoretical understanding to the experimental observation on the red shift of infrared adsorption peaks for PTAS-BNNTs.In chapter 5,we review the experimental and theoretical research progress made on the optical properties of BNNTs.In order to understand the low-energy luminescence spectra resulting from radiative transitions of the O-doped BNNT samples,we predict a novel stable O-doped BNNT model.On the basis of our calculations,the low-energy luminescence peaks experimentally observed can be identified as electronic interband transitions induced by the O substitutional impurities. Moreover,O-doped impurity of BNNT can result in the characteristic B-O vibrational modes,which can serve as a fingerprint for experimental identification.Our theoretical simulations can account for major experimental findings,and can therefore illustrate the origins for these results.We also expect that our work can be helpful for reveling the nature of the radiative transitions in BNNT systems and provide theoretical support for the future experimental works..