Photoelectronic Properties Of Graphene/Si Nanowires、graphene/ultrathin Ge And 3D Graphene

Graphene has wide spectrum absorption characteristic from ultraviolet to infrared as well as ultra-high carrier mobility at room temperature.Combining graphene with semiconductors to form heterojunction optoelectronic devices can be applied in fields such as ultra-wide spectrum and ultra-fast photodetectors.However,due to the low light absorption rate of graphene,the low light response rate severely affects the performance of the photodetector.In this work,Chemical Vapor Deposition(CVD)and Pulsed Laser assisted Deposition(PLD)are used to grew Si nanowires and ultra-thin Ge epitaxial layers on graphene substrate to enhance the light response.Several type of flexible,highly responsive visible near-infrared graphene based photodetector are fabricated by direct transfer method.On this basis,a 3D graphene form is constructed by template method and 3D graphene structure is prepared by origami self-folding method which response to visible near-infrared light and vital sign.The main research as follows:1.Graphene/Si nanowire heterojunction photodetector with high VIS response were prepared based on Si nanowires.Si nanowires were directly grown on the graphene substrate by using the metal-catalyzed CVD method.Furthermore,a high quality and transferable 3D graphene heterojunction was prepared by wet transfer process on PMMA/PDMS bilayer film.Experiment result shows the diameter of single crystal structure Si nano wires is 20 nm uniformly,and the growth orientation is[21-1]direction.Photoelectric results show good rectification characteristics and promising optical response under VIS light.the photocurrent Jsc and opencircuit voltage VOC are 4.85μA and 265mV,respectively.2.Graphene/ultra-thin Ge flexible heterojunction photodetector is fabricated.Ge thin films with thickness of 50 nm were successfully deposited on graphene by PLD method.The process parameters such as substrate temperature and pulse laser frequency are optimized.Photoelectric results indicate that compare with single-layer graphene,the light absorption of graphene/ultrathin Ge heterojunction is significantly improved and shows promising rectification characteristics,the leakage current 1.42×10-5 A under-3V bias,the calculated schottky barrier height is 0.81eV.Graphene/ultra-thin Ge heterojunction exhibits significant light response of 0.08mA·W-1 and 6.25×104Jones of detection rate under VIS-NIR irradiation.Under 650nm and 850nm laser irradiation,the responsivity was 0.38 mA·W-1 and 0.25 mA·W-1,with detection rate of 9.5×104Jones and 6.25×104Jones,respectively.Switching characteristics indicate that the response and recovery time are 600ms and 400ms,respectively.3.3D graphene photodetector was prepared by template method and the photoelectric characteristic and piezoelectric characteristics is studied.SEM morphology shows that graphite oxide(GO)uniformly covered PDMS skeleton surface.3D graphene is a porous structure with 57%porosity and covered with 5μm GO layer.Photoelectric response of 3D graphene is 0.9μA under 3V bias and VIS-NIR irradiation.Furthermore,3D graphene exhibits pressure-sensitive properties.The test results indicate that 3D graphene exhibit highly responsive state with a current of 35μA when under pressure.On the contrary,3D graphene exhibit a current of 30μA as graphene in relaxed state。Cyclic test shows a good stability in 20 circles.4.In order to explore the mechanism of self-folding,a surface tension model was established and internal mechanism of origami self-folding was experimentally and theoretically studied.Simulation results show that the surface tension direction is parallel to sample at the initial stage and as the hinge melts,the surface tension direction gradually changes from horizontal to vertical direction.The maximum surface tension is 16.6kPa,which is greater than the weight of the panel and the adhesion force between SU-8 panel and substrate.The folding speed decrease from 22.5°/s to 7°/s as surface tension reduced,indicate that glass transition temperature caused solidliquid transition of the SPR-220 hinge and the surface tension is the source of self-folding.5.Based on simulation results,a 2D graphene frame is constructed through etching process and photolithography,which use SU-8 as panel and SPR-220 as hinge.3D graphene cubic structure was successfully prepared by stress guided origami self-folding method and the process mechanism is studied.Experiment results explicit that cubic structure will self-folding together with 130℃ of heating temperature,and the self-folding process is completed under 8s.Raman spectra indicate that 3D graphene cubic structure has good structural integrity without obvious damage and has a low defect state during self-folding process.