Research On Preparation And Performance Of Low-Dimensional Materials Composite Photodetector

Graphene is a two-dimensional carbon nanomaterial with hexagonalc honeycomb lattice composed of carbon atoms in sp2 hybrid orbitals.It has excellent characteristics such as high mobility,tunable band gap,broadband absorption,high thermal conductivity,and easy large-area preparation which has attracted extensive attention of scholars.However,the electrical conductivity of graphene is very high,which leads to a large dark current of the graphene photoconductive photodetector,the absorption of light is limited by the nature of monoatomic layer,and the photoresponsivity of the detector is not high,which limits its application in the field of photoelectric detection.In this thesis,graphene is used as the basic material to design a metamaterial structure to enhance the modulation characteristics of terahertz light.Zero-dimensional quantum dot doping,one-dimensional nanotube doping,and compositing with other two-dimensional materials to form heterojunctions are used to improve the response performance of photodetectors.Then we expand the two-dimensional graphene material to“three-dimensional graphene”（cadmium arsenide）to enhance the absorption efficiency of light.In this thesis,the graphene-based high-performance photodetectors are studied from the aspects of simulation design,material composition and structure design.The main content of this thesis can be divided into the following four parts:1.Design of graphene metamaterials.The monolayer graphene maintains a constant absorption rate of 2.3%in the visible range,but as the wavelength increases,the light absorption of the monolayer graphene in the mid-far infrared and terahertz bands changes with the light frequency.In the terahertz range,the intra-band transition dominates the photoconductivity of graphene,while in the mid-to-far infrared range,the inter-band transition gradually plays a leading role.The static electron density of single-layer graphene is very low while the external electric field can adjust its Fermi level.This provides a theoretical basis for the modulation of the absorption and reflection of terahertz by graphene.By constructing artificial metasurface structures,the graphical graphene can further enhance the modulation of terahertz waves.In this thesis,by constructing the artificial complementary split ring resonators（CSSR）of graphene,the dynamic modulation of light transmittance in the range of 0.98 THz to 7.47 THz with a fixed pattern is achieved.A maximum extinction coefficient of 99%@E_F=0.9 eV is achieved.By changing the pattern parameters,the modulation capability in a wider frequency range can be obtained.2.Design of graphene doped structure.According to the basic photoelectric properties of graphene,we use different types of materials to dope graphene.Firstly,one-dimensional nanotubes are used for doping by region contacting,and a horizontal p-n-p junction is formed in the graphene region.A stable 55 nA photocurrent@405 nm 3.63mW/cm² was obtained with different bias voltages.Without external bias,the response time of the detector is about 30 ms@405 nm.Compared with the same batch of single-layer graphene transferred onto the Si/SiO₂ substrate,the photocurrent and response speed are improved.At the same time,it can work normally without bias voltage with a concerete potential of low-power photodetectors.Next,we studied the effect of zero-dimensional quantum dots doping on the photodetection performance of graphene.Doping the graphene prepared by mechanical exfoliation method with core-shell structure CuInS₂/ZnS quantum dots with can obtain a high responsivity of 2.12×10⁵ A/W@650 nm,and the corresponding response time is tens of seconds.Monolayer graphene prepared by chemical vapor deposition（CVD）is doped with lead sulfide quantum dots（PbS QDs）and black phosphorus quantum dots（BP QDs）.The former can obtain higher photocurrent,and the latter can obtain faster response time.Doping PbS QDs and BP QDs together into CVD graphene can obtain a responsivity of 569.2 A/W@650 nm,and the response time is several hundred milliseconds,and has better performance than single quantum dot doping.Finally,the three-dimensional pyroelectric material lithium tantalate is used for charge doping.The performance improvment is not obvious.The photocurrent can only be increased within a weak range,but the noise current intensity will be increased.We need to optimize the type of pyroelectric material and match the appropriate size.3.Design of graphene heterojunction devices.We use two-dimensional transition-metal dichalcogenides（TMDCs）and graphene to form a heterojunction to reduce the dark current and improve the light detection performance.First,a graphene/Molybdenum disulfide（MoS₂）heterojunction was prepared.At the bias voltage of 10 V,the dark current of the heterojunction was 7.12×10^-5 A,and a responsivity of 404 A/W@650 nm was obtained.The rise and fall times were 54 ms and 75 ms,respectively,and the spectral range of the response was from visible light 405 nm to short-wave infrared 1550 nm.Compared with the MoS₂,the photocurrent is increased by about 4 times and the photocurrent is increased by about 20 times compared with the graphene.Furthermore,the heterojunction device has lower noise current.Then a graphene/Tungsten disulfide（WS₂）heterojunction was prepared.The dark current was as low as 0.212 nA at 0 V bias voltage,and a switching ratio of more than 300 can be obtained under a 650 nm laser.The highest responsivity of 206 A/W@650 nm was obtained by applying a bias voltage of 8 V.The rise and fall times were 49.6 ms and 98.6 ms,respectively.The obvious light response can also be obtained in the wide wavelength range of 405 nm to 1550 nm,but the dark current of the graphene/WS₂ heterojunction is unstable,and it becomes weaker as the test time increases.Compared with the WS₂,the responsivity has increased by about 10.5 times.While compared with the graphene,the light responsivity has increased by about 61 times.The prepared graphene/Molybdenum diselenide（MoSe₂）heterojunction obtained a maximum response rate of 537.4 A/W@650 nm.The rise and fall times were 61 ms and 55 ms.There is a clear light response in the range from 405 nm to 1550 nm.Finally,a graphene/Tungsten diselenide（WSe₂）heterojunction was prepared.The maximum response rate was 365 A/W@650 nm,and the rise and fall times were149.1ms and 52.7ms,respectively.There is a clear light response in the range of 405-1310 nm.The heterojunction formed by graphene and four kinds of TMDCs can greatly improve the responsivity of the detector.The response time is generally around tens of milliseconds,and the light detection spectrum ranges from visible light to short-wave infrared.The devices provide important reference value for the development of broadband photodetectors.4.Expansion of three-dimensional graphene materials.On the basis of study of the graphene photodetector,we further explored the photodetector with three-dimensional graphene analogue Cadmium Arsenide（Cd₃As₂）based photodetector.First,the heterojunction of Cd₃As₂ and MoS₂ was prepared,and the maximum responsivity of2.74×10³ A/W@405 nm was obtained,which was about 4.6×10⁵ times higher than that of pure Cd₃As₂ photodetector,and 5.4×10⁴ times higher than pure MoS₂ photodetector.The rise and fall times of the photodetector are 43μs and 65μs respectively,which is 3orders of magnitude faster than the photodetector of graphene/TMDCs heterojunctions.The Cd₃As₂/MoS₂ heterojunction also has an ultra-wide detection spectrum,with a strong optical response from ultraviolet 365 nm to short-wave infrared 1550 nm.Then,we used organic perovskite FaPbBr₃ quantum dots to dope the Cd₃As₂ nanoplate.The photocurrent increased by about 6 times after using low concentration quantum dots doping.When the quantum dot concentration increased to form a thin film,the photocurrent was sharply increased by about 3.95×10⁴ times,while the switching ratio is as high as 9.6×10³.However,the detection wavelength of the Cd₃As₂ photodetector doped with FaPbBr₃quantum dot material is limited to a maximum of about 808 nm,though it cannot detect optical response at 980 nm and longer wavelength infrared light.Starting from the most typical two-dimensional material graphene,this thesis focuses on the exploration and research of high-performance photodetectors.Simulating of the modulation of graphene metamaterials in terahertz band,low dimensional material doping graphene photodetectors,graphene and other two-dimensional materials heterojunction photodetectors,“three-dimensional graphene”Cd₃As₂ heterojunction photodetectors are studied.It provides a reference for the next generation of novel materials photodetectors.