Research On The Applications Of Graphene In Novel Terahertz And Photoelectric Devices

Graphene is widely used in millimeter-wave-terahertz devices and photoelectric devices due to its excellent physical properties such as high light transmittance,high conductivity,and high stability.Particularly,the chemical potential(Fermi level)of graphene can be changed within a certain range through applied voltage or doping,providing a convenient way for tuning and optimizing device performance.The present thesis designs and fabricates several novel terahertz and photoelectric devices based on the graphene or its composite structure,and the device performance can be tuned or optimized by adjusting the Fermi level of graphene.The main contributions are as follows:1.Graphene-based tunable terahertz devices.(1)A novel two-beam antenna is designed,which consists of graphene dipole and parasitic graphene strips on both sides.The chemical potential of dipole graphene mainly affects the working frequency of the antenna,which increases with the increase of the chemical potential of dipole graphene in a certain range.While the chemical potential of parasitic graphene mainly affects the beam direction of the antenna.Among the two parasitic graphenes,the graphene strip with high chemical potential can reflect the radiation of the dipole,while the graphene strip with 0 eV chemical potential has no effect on the radiation of the dipole.This structure realizes a flexible switching of the antenna beam between the two directions of±90°.Moreover,through simple structural transformation of the above antenna,a six-beam antenna can be obtained.The beam direction of the new structure can be flexibly switched between±30°,±90°,±150°.In addition,a two-beam antenna based on the classic Yagi antenna is also designed.The antenna's beam can also switch flexibly between±90°,but it is difficult to expand it to a six-beam antenna through simple structure transformation.(2)A graphene-based dual-band antenna is fabricated to verify the feasibility of graphene in tunable antenna applications.The antenna consists of graphene radiator and metallic coplanar waveguide feed structure,working at 28.1 GHz and 37.4 GHz.By applying DC bias on the garphene,the S11 parameters can be regulated.Moreover,when the DC bias changes from-70 V to+70 V,the S11 parameter of the antenna at 28.1 GHz can vary from-27.79 dB to-23.59 dB,while the S11 parameter at 37.4 GHz can vary from-13.65 dB to-16.10 dB.(3)A terahertz negative refractive index metamaterial based on graphene composite structure is designed.The proposed metamaterial unit consists of a metallic resonant structure and embedded graphene.When the chemical potential of graphene is 0 eV,the metamaterial exhibits negative refraction index in the 4.26 THz band.When the chemical potential of the graphene increases to 0.5 eV,the negative refractive index band of the metamaterial is switched to 4.28 THz.After loading the single layer metamaterial above a microstrip antenna,when the chemical potential of the graphene is 0 eV,the beam tilt angle of the microstrip antenna changes from 0° to 8°.While when the chemical potential of the graphene is adjusted to 0.5 eV,the beam tilt angle of the microstrip antenna changes to 11°.In addition,by increasing the number of layers of the proposed metamaterial,the tilt angle can be further expanded.(4)A terahertz frequency selective surface(FSS)based on graphene composite structure is designed.The proposed FSS unit consists of convoluted metallic resonance strips and graphene embedded between adjacent metallic strips,and its dimension is only 11.7%of the working wavelength.Moreover,the working frequency of the proposed structure can be regulated within the range of 1.76 THz?2.56 THz,showing an excellent band-stop property.Loading the FSS into the dual-transmission-line structure and two-antenna array structure,the isolation between the ports of transmission lines and the antenna elements can be improved by 6.9 dB and 16.5 dB,respectively.2.Graphene/semiconductor nanowire Schottky junction photoelectric conversion device.(1)A graphene/single GaAs nano wire Schottky junction photoelectric conversion device is fabricated for the first time.There is a strong built-in electric field in the junction area due to the difference in work functions between graphene and GaAs nanowire,which promotes the effective and rapid separation of the photogenerated carriers,and the device presents an excellent photoelectric characteristic.Under 532 nm laser excitation,the device exhibits a high responsivity of 231 mA/W,a large detectivity of 1.60×109 cm Hz1/2/W and a short response/recover time of 85/118 ?s at zero bias.Under AM 1.5G solar illumination,the device has an open-circuit voltage of 75.0 mV and a short-circuit current density of 425 mA/cm2,yielding a remarkable conversion efficiency of 8.8%,which is at the leading level in the similar devices(2)Tuning and optimizing the performance of graphene/nanowire schottky junction devices are realized by changing the chemical potential of graphene.Using nitric acid to achieve chemical doping of graphene,the Schottky barrier height of the device increases from 0.29 eV to 0.35 eV.Under 532 nm laser excitation,the detectivity of the device is increased from 2.10×109 cm Hz1/2/W to 1.0×1010 cm Hz1/2W,and the response/recover time is improved from 42/140 ?s to 20/128 ?s.Under AM 1.5G solar illumination,the photovoltaic efficiency of the device is improved to 15.88%.In addition,applying an appropriate gate bias voltage on the graphene,the barrier height of the device also increases.Meanwhile,the performance of the device has also been improved greatly.