Reversible Tailoring Of Structures And Electrical Properties Of Carbon Nanotubes Via Defect Engineering

Sp2 carbon has gained tremendous research interests in the fields of environment,new energy and information technology,etc.owning to its extremely high technological importance.From the structure point of view,it is also very appealing for its fascinating structural variety of these C systems.Like no other material,sp2 carbon systems have the unique ability to evolve into various structures by diverse C-C bond reconstruction under certain circumstances.Such structural evolution can be readily driven by,e.g.the irradiation with energetic particles(such as electrons and ions),which has been recently demonstrated as an effective means to tailor the structures of a variety of materials with high precision.This technique,however,usually causes irreversible deterioration of the material structures and related properties due to defect introduction.In this thesis,we present a highly controllable defect handling technique,i.e.defect creation followed by their elimination,as demonstrated in carbon nanotubes(CNTs)tailoring by in-situ transmission electron microscopy.Such defect handling can be achieved via the alternate use of room-temperature electron irradiation and a separated heat annealing treatment,which enables reversible tuning of the structure and various properties of CNTs.The research contents and main results of this thesis are summarized as follows:A chip-based TEM heating holder was utilized to study the microstructuralevolution of CNTs under irradiation/annealing processes,as well as the factors that may influence the structural changes.The regarded CNTs thus undergo the crystal/amorphous structural transition,which can be repeated up to at least 10 cycles with the CNT integrity largely maintained.Reversible conversion of defects at the atomic scale(interstitial atoms and vacancies)was in parallel documented under high-resolution imaging.A temperatre-dependent annealing experiment shows that the CNT recrystallization can be initiated at a surprisingly low temperature of 300?,and a high temperature of over 1000? is required for the irradiated CNTs to primarily regain their structural perfectness.Besides,the annealed CNTs can exhibit a somewhat memory-like effect if a limited irradiation dose is used,thus regaining certain structural details at atomic scale.More importantly,by using a TEM-STM system,we demonstrated that this technique allows for the reversible and repeatable tuning of a series of important CNT electrical properties.The property tuning range and precision can be well controlled by choosing deferent irradiation/annealing conditions,as demonstrated in the tuning of the CNT resistance.The reversible tailoring of CNT field emission(FE)performance was accomplished for the first time,with the direct linkage of FE onset voltage to the microstructural evolution.In addition,the present technique exhibits the great potential as a general route for reversible tuning of the defect-dependent properties of CNTs.Furthermore,this single-CNT based approach can be practically extended to the property tuning of other sp2 carbon nanomaterials(like graphene),on a large scale via powerful broad-beam irradiation and batch heating.