Controlled Growth Of Carbon Nanotubes Based On Field-Effect Catalysis

Single-walled carbon nanotubes(SWCNTs)are anticipated as the most promising material to construct high-performance chip and flexible devices in the future for their nanostructures and extraordinary electronic properties.A prerequisite of practical applications of SWCNTs in electronics is directly synthesizing pure s-CNT arrays with high quality.However,as-grown SWCNTs,usually,are the admixture of metallic SWCNTs(m-CNTs)and semiconducting SWCNTs(s-CNTs),which has been one of the biggest hurdle for their industrial applications.Although the difference in electronic density of state between s-CNTs and m-CNTs is dramatic,the difference in structures between them is really small.The growth of highly-pure s-CNTs is impeded by the structural similarity of SWCNTs.To eliminate this roadblock,growth mechanism of SWCNTs should be further studied.In this paper,we started the discussion of growth mechanism and controlled growth of CNTs with an unexpected discovery that growing ultralong CNTs are negatively charged during the growth.By ex-situ measuring the charge distribution of as-grown ultralong CNTs and in-situ monitoring the charge generation during the growth of CNTs,we confirmed that the charge generation and transfer generally happened in a catalytic growth of CNTs and proposed an electrochemical mechanism to illustrate this process.With more detailed and quantitative analysis,the different growth behavior of m-CNTs and s-CNTs under applied electric field is predicted.Based on the electrochemical mechanism of the growth of CNTs,we further reported a concept of field-effect catalysis(FEC).Using the dramatic difference in the electronic density of state between m-CNTs and s-CNTs,the free energy difference between the growth of m-CNTs and s-CNTs is amplified by negatively charging the catalyst.As a result,m-CNTs tend to be twisted to s-CNTs with an energy barrier through the formation of a chiral junction.To selective grow s-CNTs,carefully designed electric field pulse is used to negatively charge the catalyst and to introduce a perturbation to overcome aforementioned energy barrier.Based on this method,high quality s-CNT arrays are obtained stably with the purity up to 99.9%.Moreover,with the control of nucleation time of CNTs in inch-size area,stable growth of wafer-scale highly-pure s-CNT arrays is realized.Calculation of FEC shows that direct synthesis of s-CNTs with any purity to meet the requirement of semiconductor industry can be achieved if the diameter of CNTs is further confined.The study of electrochemical mechanism and FEC in CVD system not only realize the growth of highly-pure s-CNT arrays that paves the way for the industrial application of CNT electronics,but also provides a new way of thinking about selective synthesis of other nanomaterials and promotes the development of semiconductor industry.