The First-principles Studies On The Boron-based And Related Nano-structures

The research on nano—structures have attract lots of interests in the fields of con-densed matter physics and nano—technology．Low-dimensional nano-structure,due toits small size,has different phvssical and chemical properties from the bulk structure．Through controlling its shape,width,composition and other parameters,physical prop—erties of the nano—structure can be changed and tuned．In this thesis,we perform first—principles studies on the nano—structures composed of pure boron,binary boron com—pounds,and other related nano—materialS．We find that the boron—based nano—structureshave rich electronic and magnetic properties,which can be applied in future nano—electronic and spintronics devices．Firstly,we investigate three types of one—dimensional baron—based nanoribbons:i．e．boron nanoribbons,BC₃ nanoribbons,and boron nitride nanoribbons．Two types ofboron nanoribbons,lined and zigzag ones,are constructed from the stable boron sheet．All the bare boron nanoribbons are metalS．wmle the boron nanoribbons with two-hydrogen passivated on the zigzag edges become semiconductors．For BC₃ systems,the armchair BC₃ nanoribbons are all semiconductors,while zigzag BC₃ nanoribbonsare semiconductors or metals depending on the edge atoms．The metallicity of zigzagBC₃ nanoribbons is attributed to theÏ€bond formation between boron and carbon atomsin the edge zigzag line．The stabilities of boron nitride nanoribbons with differenthydrogen—terminated edges are determined．Five kinds of the stable edges are deter-mined for the zigzag boron nitride nanoribbons．While for the armchair boron nitridenanoribbons,there are three kinds of the stable edges．Then,we investigate a two—dimensional nano—structure of BC₃ sheet．The hydro—BC₃ sheets with only carbon atoms bonding with hydrogen atoms are all semicon—ductors．When both boron and carbon atoms bond with hydrogen atoms,the hydro—BC₃ sheets become metals．During step by step hydrogenation,semiconductor—semiconductor-metal transitions appear in the hydro—BC₃ sheets．By inducing defects in the BC₃ sheet,the BC₃ sheet can become a stable magnetic metal,magnetic semi-conductor,or nonmagnetic semiconductor．The metallic BC₃ sheets can be obtainedthrough replacing boron atoms by vacancies or carbon atoms．Also replacing carbonatoms by vacancies or boron atoms can rune the band gaps of semiconduⁱng BC₃sheets．This demonstrates that the properties of the BC₃ sheet can be tuned by a defectengineering,which leads to potential applications as functional nano-structures．Finally,we investigate two related nano—structures of silicon nanoribbons and thegraphene nanoribbons embedded in boron nitride sheets．For silicon nanoribbons,theyhave similar structures and properties as the graphene nanoribbons．For the armchairgraphene nanoribbons embedded in boron nitride sheets,we have found they are semi—conductors．While for the Ngzag graphene nanoribbons embedded in boron nitridesheets,thev become half-metals owing to the different interfaces in boron nitride sheets．...