Study Of The Electromagnetic Properties Of CVD-prepared Graphene Films And Carbon Nanofibers In Sub-THz Domain

Terahertz(THz)technology is one of the important emerging technologies in the 21 st century.THz wave generally refers to the electromagnetic(EM)wave with frequency between 0.1-10 THz,which is in the transition stage from macroscopic classical theory to microscopic quantum theory and electronics to photonics.Terahertz waves have frequencies higher than microwaves and lower than infrared.In recent years,the technology has been gradually applied to semiconductor materials,high temperature superconducting materials,tomographic imaging technology,unlabeled gene detection,cell microscopic imaging,chemical and biological detection,broadband communication,microwave positioning and other fields.With the development of technology,carbon-based materials such as graphite,graphene,carbon nanofibers and their composites have been widely used in the research of Terahertz devices due to their excellent electrical properties and atomic structure.In the development of terahertz technology,Vector Network Analyzer(VNA)and Terahertz time-domain Spectroscopy(THZ-TDS)take up two different domains,respectively.Among them,VNA is often used to characterize the electromagnetic response of materials or devices,while TDS can be used to identify and explore the wideband dielectric properties of materials.Through literature search,we found that the graphene membrane research in the field of terahertz devices and electromagnetic characterization is still mainly in the simulation and experimental validation stage,about the chemical vapor deposition of carbon nanofibers in the electromagnetic characterization research papers in the field of terahertz are limited,especially to study the electromagnetic response of carbon nanofibers in more than 300 GHz frequency bands.Although a large number of studies have shown that single-layer graphene films have excellent electromagnetic modulation functions due to their band tunable properties.However,there are still bottlenecks in the preparation of single-layer graphene films with high uniformity and high crystalline quality,which hinder their application in terahertz devices.In addition,the preparation of graphene films with controllable number of layers is also a relatively complex technology,and studies on the electromagnetic response of graphene films with different thickness still need to be filled in.Based on the above background,we used vector network analyzer to carry out simulation modeling and experimental test of the electromagnetic response of graphene and carbon nanofibers in millimeter to sub-terahertz frequency band,and further explored its potential in electromagnetic devices,electromagnetic characterization and other application fields.The research work of this topic is divided into three parts: the preparation of graphene film samples and carbon nanofiber samples;Test out this simulation using CST Microwave Studio 2017 while studying the design parameters of this sample.Through the combination of experiment and simulation,the electromagnetic properties of the material were characterized by using VNA system.Finally,we found that in the 500-750 GHz band test,the randomly oriented graphene nanofibers had obvious polarized light absorption behavior,and this effect would also cause the polarized reflection.This result provides an important reference for the research and development of carbon nanofiber terahertz devices in the future.The obvious anisotropy of absorption lays a foundation for the development of variable reflector or attenuator,terahertz absorption modulator and other devices.We also found that the terahertz transverse waves transmitted by the rectangular waveguide were very sensitive to the surface roughness of the graphene film and the defects caused by folds and cracks.The surface defects of the graphene film,such as suspension bonds,would cause the interfacial polarization effect,while the surface roughness would cause the surface scattering effect of the graphene film on the electromagnetic wave.The surface defects of graphene films also affect the local conductivity at the surface defects,thus reducing their ability to reflect electromagnetic waves.On the other hand,the effect of this interfacial polarization can be characterized by testing its anisotropy.As graphene film preparation technology matures,we need to shift the focus from "are we making graphene?" to "What is the uniformity and quality of graphene films?" Therefore,based on the electromagnetic response principle of terahertz transverse radio waves to graphene films of different thickness,a potential macroscopic quality control method for the homogeneity and crystal mass of graphene films was proposed in this study,and two new quality detection parameters were proposed to characterize the homogeneous degree and crystal mass of graphene films,respectively.