Research On Low Pressure Sensing Mechanism Of Carbon Nanotubes Based On Work Function Measurement

Carbon nanotubes can be widely used in the fields of field emission(FE)and gas adsorption due to their excellent mechanical structure,unique electronic properties and good thermal conductivity.Earlier studies had shown that gas adsorption of carbon nanotube films would lead to FE sensing effects,but there were no reports on the intrinsic FE sensing effects of a single carbon nanotube.In this paper,a single carbon nanotube is assembled on a tungsten tip by dielectrophoresis,and the work function and band gap change of the single carbon nanotube after adsorbing nitrogen and hydrogen ware studied by field-emission electron energy distribution(FEED).This paper mainly includes the following three aspects:(1)A dielectrophoresis method for solvent evaporation was developed to assemble a single carbon nanotube on the tungsten tip.The assembly method solves the difficulty of assembling carbon nanotubes with sensing effect,which is low concentration of carbon nanotubes with sensing effect and difficult to achieve small continuous displacement of the tungsten tip.The experimental results suggested that the solvent evaporated naturally under an alternating electric field of 12 V and 2 MHz,then a single carbon nanotube with a length of 1-10 μm could be prepared.(2)The FEED was used to investigate the work function changes of nitrogen and hydrogen adsorption for carbon nanotubes under different gas pressures.The research results showed that when the vacuum pressure increases from 10-7 Pa to 10-4 Pa,hydrogen molecules dissociate and chemically adsorb on the surface of the carbon nanotubes,causing the work function to drop by 0.7 eV.Nitrogen molecules will physically adsorb on the carbon nanotubes,reducing their work function by 0.3 eV.(3)The band gap of carbon nanotubes before and after adsorption was studied by using the relationship of FEED with voltage.The result of research shows that the FE of a single carbon nanotube eigenstate under a pressure of 3.6×10-7 Pa exhibits the field emission characteristics of a metal.However,under a pressure of 4.9×10-5 Pa,the field emission of a single carbon nanotube in the hydrogen-adsorbed state exhibits the field emission characteristics of a semiconductor.Compared with the eigenstate,the band gap of carbon nanotubes became wider after hydrogen adsorption,that is,the semiconductor properties of carbon nanotubes were enhanced.