Control Over The Electrical Type And Array Density Of Single-wall Carbon Nanotubes By Catalyst Design

Single-wall carbon nanotube(SWCNT)is a hollow cylinder made up of rolled up single-layer graphene.SWCNTs can be either metallic or semiconducting,depending on how the graphene is rolled up,i.e.the chirality.Metallic SWCNTs can be used as interconnecting wires,semiconducting SWCNTs are ideal channel material of transistors.So,SWCNTs are considered as one of the most promising alternatives of silicon in the next generation high-performance chips.However,to realize the application of SWCNTs in nano-devices,it is essential to obtain high-density horizontally aligned high-purity semiconducting SWCNT arrays.At present,the mechanism of controllable growth of SWCNTs is not clear,and efficient methods that permit the synthesis of SWCNTs with desired structures and properties need developed.Catalyst is one of the key factors determining the structure of SWCNTs.Therefore,in this dissertation,we explore controllable growth of high-density horizontally aligned SWCNT arrays and semiconducting SWCNTs by catalyst design,including continuous formation of catalyst nanoparticles and control over the structure and composition of catalysts.The main results obtained are as follows:A continuous vaporization and nucleation method is developed to realize the continuous supply of high activity catalyst nanoparticles during the growth of SWCNTs,so that the growth of high density horizontally aligned SWCNT arrays is achieved.A layer of Fe catalyst was deposited on a SiO2/Si substrate put at upstream of the reactor,Fe atoms were evaporated at elevated temperatures,and deposited and nucleated to nanoparticles that catalyze the growth of SWCNTs on a quartz substrate put downstream.The effect of growth temperature on the nanoparticle size and SWCNT density was studied.High density horizontally aligned SWCNT arrays were synthesized with an average density of 30/μm under the optimum condition.A method of etching the caps of metallic SWCNTs at the nucleation stage and permitting the growth of high purity semiconducting SWCNTs is developed.The key to achieving this goal is to find a catalyst that can separate the nucleation and growth stages of SWCNTs.We selected non-metal SiC as a catalyst to study the depletion of Si atoms and the formation carbon caps.The selective etching mechanism of mild etchant H2 on the metallic carbon caps was investigated.By optimizing the thermodynamic and dynamic conditions for the etching of metallic carbon caps and the growth of semiconducting SWCNTs,high quality and high purity(～97%)semiconducting SWCNTs were obtained.Thin film transistors fabricated using the as-grown SWCNTs show excellent performance with current on/off ratios higher than 105 and carrier mobilities as high as 57 cm2V-1s-1.Monodispersed cobalt-platinum alloy nanoparticles with high crystallinity,uniform shape and size,adjustable composition and face-centered cubic structure were prepared by a hydrothermal method using oleylamine as a medium.Semiconducting SWCNTs were synthesized by using these nanoparticles as a catalyst.The influence of CoPt particle composition on the structure of the prepared SWCNTs was studied.Under an optimum condition,SWCNT samples containing～94%semiconducting tubes were obtained.