The Relationship Of Morphological Structure,Electronic Structure And Electrical Transport Property Of One-Dimensional Nanomaterials By In-situ TEM Studies

Nanomaterials have attracted great research interests in chemical,physical,biological,environmental,and medical sciences.In recent years,various of nanomaterials with uniform structure and excellent performance have been synthesized and exhibited a broad application potential in energy,electronics,storage and other fields.The structure and properties of the functional nanomaterials directly affect the performance,stability and life of the nanodevices.Therefore,exploration of the structure and property relationship of the nanomaterials under the working conditions is very critical,which can help improving the device design and clarifying the device failure mechanism.In-situ transmission electron microscopy(TEM)combined with high-resolution transmission microscopy,composition analysis,electronic structure analysis and in-situ measurement system,is able to study the properties of nanomaterials in real-time and understand the relationship between the material structure and device performance.In this thesis,we use the in-situ STM-TEM and in-situ heating system to study the morphological effect on electronic structure and electrical transport property of one-dimensional carbon nanostructures,the phase change process of individual phase change nanowire and the sodiation process of the carbon coated α-Fe2O3 nanoparticles.Detailed are briefed as follows.(1)Based on in-situ TEM system and X-ray absorption near edge structure spectroscopy(XANES),the morphological effect on electronic structure and electrical transport property of one-dimensional carbon nanostructures was investigated.Aligned one-dimensional carbon nanostructures with different morphologies,carbon nanotubes(CNTs)and carbon nanofibers(CNFs),were synthesized by plasma-enhanced chemical vapor deposition(PECVD)method with different catalyst/underlayer combinations.The electronic structures of CNTs and CNFs were studied by XANES,which revealed that CNTs have much less oxidized groups than CNFs.Moreover,the electrical transport properties of single CNT or CNF were in-situ measured under the TEM observation and the results showed that the resistivity of CNTs is 2 orders lower than that of CNFs.Individual CNT can be applied with higher voltage and larger current before thermal breakdown compared to individual CNF,which can be related to the electronic structure as revealed by XANES.(2)The active ingredients in the carbon nanomaterials was identified on the atomic scale.Individual CNT and CNF were clearly imaged with a concurrent characterization of their electronic structure by nanoscale scanning transmission X-ray microscopy(STXM).Except for prominent catalyst nanoparticle at the tip,tiny catalyst clusters along the tube(fiber)were detected,indicating a migration of the catalysts with the growth of CNTs(CNFs).The observation provided the direct evidence on the atomic metal in CNT for oxygen reduction reported in the literature.Interaction between catalysts(Fe,Ni)and CNTs(CNFs)at the tip was also identified by comparing the X-ray absorption spectra.(3)By applying an electric signal,the phase change process and failure mechanism of individual In2Se3 nanowire was studied by in-situ TEM.Hexagonal single crystal In2Se3 nanowires were synthesized by chemical vapor deposition(CVD)under a vapor-liquid-solid(VLS)mechanism.With a direct voltage applied,the structure defects of the In2Se3 nanowire disappeared with an increasing crystallinity at lower voltage due to the joule-heating and the conductivity of the In2Se3 NW became better at the same time.The temperature of the nanowire increased when a higher voltage was applied,and finally the single crystal phase changed to the amorphous-like phase.The transform process experienced a poly-crystalline structure as the intermediate phase in the 10 seconds transformation time.The crystallinity of the In2Se3 NW increased more rapidly in axial.Moreover,the electric pulse signal loading system was designed based on the in-situ TEM holder and the nonuniformity phase change process was obseved in In2Se3 NW under pulse voltage signal,which is attibuted to the crystal defects in the nanowires.As the defect healed by the joule-heating,the phase changes occured in the whole nanowire.In addition,both the excessive direct voltage step and the pulse width can induce the void formation in In2Se3 NW,which results in the failure of the device.(4)The composition and size effects in electrical properties and thermal stability of the InxSey,ZnTe and GexSby nanowires were investigated.Among the InxSey nanowire materials,the In2Se3 structure exhibits the highest resistivity and the minimal corresponding power consumption of phase change.Due to the size effect,the melting points of In2Se3,ZnTe and GexSby nanowires decrease with the reduced diameters.Meanwhile,the melting point of GexSby nanowires increases with the larger Ge proportion.The sublimation process of single ZnTe nanowires was observed in real-time,which is attributed to a Langmuir-type gasification process.(5)The first sodiation process of the carbon coated α-Fe2O3 nanoparticles was studied by in-situ TEM system.A volume expansion with sodium insertion wasobserved in carbon coated a-Fe2O3 nanoparticles and the crystal structure was changed from the out layer to the core inside.The electronic structure of as-synthesized and sodiated a-Fe2O3 nanoparticles were investigated by electron energy loss spectroscopy(EELS),which proved that Na inserted successfully in carbon coated α-Fe2O3 nanoparticles with a decreased oxidation state of Fe element.