Crystal Growth And Characterization Of A Novel Ⅳ-Ⅴ Group Two-Dimensional Semiconductor Of SiP₂

Since the discovery of graphene in 2004,many excellent physical and chemical properties,such as ultra-high carrier mobility,high thermal conductivity have aroused a research upsurge of two-dimensional materials due to its unique two-dimensional layered structure.However,the "zero band gap" of graphene has also seriously hindered its application in the field of optoelectronics.The low carrier mobility of transition metal disulfides(TMDs)at room temperature is still a problem that researchers are eager to improve.Black phosphorus(BP),as a typical representative of the fifth main group of two-dimensional materials,has not only continuous adjustable band gap,but also high mobility and obvious in-plane anisotropy.This in-plane anisotropy makes black phosphorus have more abundant physical properties,which provides a new way to explore the regulation of electrical,thermal and optical properties of two-dimensional materials.However,the stability of black phosphorus is very poor.Although scientists have improved the stability of black phosphorus in various ways,the effect is still unsatisfactory.Therefore,there is an urgent need to find a new type of two-dimensional materials which take into account the advantages of graphene,TMDs,black phosphorus and other materials(high carrier mobility,adjustable bandgap,good stability,in-plane anisotropy),and realize their practical application in the field of optoelectronics.Here,we have made a preliminary study on a new type of in-plane anisotropic SiP2 two-dimensional material of group Ⅳ-Ⅴ.The specific work is as follows:(1)Crystal growth and characterization of properties of orthorhombic SiP2Orthorhombic SiP2 crystals were successfully grown by Sn flux using Gd as catalyst,and the maximum planar size of the crystals was 1 × 8mm2.The single crystal structure analysis confirmed that the crystals were orthorhombic and layered in Pnma space group,in which the c-axis direction was layered stacking direction.XRD results confirmed that the natural growth plane of the material was {002} plane group;The results of EDS and EPMA showed that Gd did not enter the lattice,but only played a catalytic role;the obvious layered structure of SiP2 could be seen by scanning electron microscopy;the bandgap of SiP2 was 1.63 eV by UV-visible diffuse reflectance spectroscopy.(2)Exfoliation and band calculation of orthorhombic SiP2SiP2 nanosheets were successfully prepared by mechanical exfoliation method and liquid ultrasound exfoliation method.The samples obtained by the two methods can observe obvious lattice fringes under TEM high resolution electron microscopy,and the diffraction spots of selected area electron diffraction patterns are clear and bright.Mechanical exfoliation method can obtain a few or even two layers of SIP2 nanosheets(about 1.5 nm),liquid-phase ultrasound method can obtain a large area of uniform thickness(about 4 nm)of SiP2 nanosheets,and the stability of the few layers of SiP2 nanosheets in air is good.The band gaps of SiP2 with different layers are calculated by first principles.The results show that the band gaps of SiP2 increase from 1.25 eV to 1.58 eV with the decrease of layers,which indicates that the band gaps of SiP2 can be adjusted by changing the layers.(3)Raman anisotropy of orthorhombic Sii2The in-plane Raman anisotropy of bulk SiP2 was characterized by angular resolved polarization Raman spectroscopy.The polarization Raman spectra of bulk SiP2 were measured in parallel and vertical configurations.Under parallel configurations,the variation periods of the intensity of Raman peak position with polarization angle are two types:π and π/2.Under vertical configurations,the variation periods of the intensity of all peaks with polarization angle are π/2.By fitting the test data points with the formula derived from Raman selection law,the different peaks can be attributed to Ag and Big Raman vibration modes.Among them,the change of Raman shift peak intensity of Ag mode in parallel configuration can be used as a basis for judging the orientation of bulk SiP2.(4)Study on light response of orthorhombic SiP2The optical response performance of the photodetectors fabricated by orthogonal phase SiP2 is tested.The results show that the photodetectors based on orthogonal phase SiP2 have optical response under the excitation of multiple wavelengths in the visible range,and the switching current ratio can reach 104.The optical response is the highest when the incident wavelength is 633 nm and the laser power is 5 mW.At the same time,the switching ratio,optical responsivity,detectivity and response time of the device under different excitation wavelength and laser power are compared.