First-principles Study Of Energy Band Regulation Of Carbon Nanotubes

In this essay,we use first principle calculation method based on density functional theory to study the carbon nanotube(cnt)'s electronic properties such as band structure and band gap.We use different methods to tune the band gap of the carbon nanotubes.We hope that our work can help develop the application of infrared detectors and solve the problem of impurity in the synthesis of carbon nanotubes.Our work is in the following.1.We utilize the interaction between two carbon nanotubes to modulate the band structure and tune band gap of carbon nanotubes.We calculate the metallic(3,3),(5,5),(7,7)carbon nanotubes and semiconducting(8,0),(10,0)carbon nanotubes.Our results show that within specific distance,two metallic carbon nanotubes can be semiconducting and two semiconducting carbon nanotubes can be metallic.Besides,we can tune the band gap of two carbon nanotubes by modulating the distance of two carbon nanotubes.2.We utilize external field to modulate the band structure and tune band gap of carbon nanotubes.We use three schemes.First is to add electric field only.Second is to add axial strain only.Third is to add both electric field and axial strain.The result of the first scheme shows that firstly,the band gap of semiconducting(8,0),(10,0)carbon nanotubes decreases slowly before the electric field intensity reaches the critcial point.However,after the electric field intensity reaches the critcial point,the band gap decreases very fast.Secondly,we find that two nanotubes have metal-semiconductor transition more easily than single nanotube.Thirdly,we find that smaller diameter nanotubes have metal-semiconductor transition more easily than larger diameter nanotubes.The result of the second scheme shows that firstly,when strain comes to-4%,the band gap of(8,0)carbon nanotube becomes largest.Secondly,when strain comes to-11%,cnt(8,0)has metal-semiconductor transition.Thirdly,different diameter carbon nanotube performs differently under axial strain.The result of the third scheme shows that firstly,the critical point of electric field and strain is smaller than adding only electric field or strain.Secondly,we find that strain has more influence on the band gap of carbon nanotubes than electric field.3.We utilize covalent functionalization to modulate the band structure and tune band gap of carbon nanotubes.Electronic band structure and other electronic properties,including Fermi energy,adsorption energy,preference on adsorption siteand optical properties of covalently functionalized carbon nanotubes are studied.We choose azidodichloro-triazine to functionalize carbon nanotubes because of the experimental studies by other researchers.We find that firstly,when azidodichloro-triazine functionalizes on semiconducting cnt(8,0),the band gap depends on adsorption site and adsorption concentration.Secondly,we find that Fermi energy only depends on adsorption concentration.Thirdly,we find that optical properties are enhanced when adsorption concentration increases.On the other hand,when azidodichloro-triazine functionalizes on metallic cnt(6,6),the entire system will transit into semiconductor or maintain its metallicity,depending on the adsorption site.We also find the preference of adsorption site.When azidodichloro-triazine functionalizes on semiconducting carbon nanotubes,azidodichloro-triazine prefers the first adsorption site.When azidodichloro-triazine functionalizes on metallic carbon nanotube,azidodichloro-triazine prefers the second adsorption site.Results are consistent with the related experiment and provide novel functionalization approach on tuning the band structure of carbon nanotubes.