| Since the discovery of two-dimensional(2D)materials,their excellent photoelectric properties have attracted wide attention and in-depth research in various fields.Due to its unique electronic structure and high specific surface area,two-dimensional semiconductor materials have significant advantages in the realization of high-performance optoelectronic devices,which makes them promising to become the ideal candidate materials for various optoelectronic devices,and have broad application prospects.The physical properties of a material are usually determined by its crystal structure and type.As a clean and undoped means,high pressure affects the structural stability of the material itself by changing the atomic spacing and bonding mode,and then regulates the properties of the material,and leads to a series of novel physical phenomena,such as structural phase transformation,electronic phase transformation,metallization,etc.,which promotes the possibility of application of materials in many fields.In-depth study of the behavior of two-dimensional semiconductor materials under high pressure can provide key information and important reference value for understanding their physical mechanism,which is crucial for the development of new materials.For example,high pressure treatment of PbI2,g-C3N4 and graphene,which have great application potential in photoelectric detection,photocatalysis and other fields,may further optimize their crystal and electronic structures,breaking through the current limitations of such materials in their application,so as to enhance the application value of these materials.This paper systematically studies the structural characteristics and physical properties of two-dimensional semiconductor materials represented by PbI2,g-C3N4 and graphene under high pressure.Certain progress has been made in the structural understanding of materials under high pressure,the performance improvement and response range extension of photodetectors,the design of semi-metallic materials,band gap regulation and other aspects,and some new understandings have been obtained.It has a certain guiding significance for the application and development of two-dimensional semiconductor materials in the direction of photoelectric detection and photocatalysis.The research content of the full paper is as follows:(1)The structural phase transition of 2H-PbI2 under high pressure was systematically studied by in-situ Raman spectroscopy and XRD pattern.The results of Raman spectroscopy show that two phase transitions occur at 0.58 GPa and 2.6 GPa.XRD spectrum further illustrates the PbI2 under 20 GPa into two-dimensional hexagonal 2H-P3m1→polytype 4H-P3m1→three-dimensional(3D)Pnma.The results of electrical transport at high voltage reveal the semiconductor-metal phase transition of PbI2.At approximately 24.8 GPa and 37.6 GPa,there are two discontinuous kinks in the pressure-dependent relationship of the ultrafast spectrum.Raman spectroscopy and XRD confirm the structural phase transition of PbI2 from orthogonal Pnma to tetragonal I4/mmm symmetry at the first discontinuity.The second discontinuity arises from the closing of the bandgap and the enhancement of the electron-phonon interaction in the semiconductor-metal transition,which is further demonstrated by the temperature-dependent resistance of PbI2 under pressure.Measurements of the photocurrent of the pressure-induced PbI2 at high voltages showed that the photocurrent under visible light suddenly increased at about 27 GPa.At the same time,the response range extends from the visible to the infrared region with a wavelength greater than at least 1550 nm.High pressure absorption spectra show that the bandgap of PbI2 closes at the phase transition point,while the charge transport results show that the sample is still non-metallic.The results of theoretical calculation show that the photocurrent surge and infrared band response are due to the semiconductor-semi-metal phase transition under high pressure.(2)The pressure-induced amorphous behavior of g-C3N4 with nitrogen vacancies was studied under high pressure.The introduction of nitrogen vacancy leads to the initial bandgap of 2.40 eV,which is further reduced to 1.70 eV under high pressure.The bandgap value of 1.87 eV is retained when the pressure is discharged back to the ambient pressure.At the same time,the greater the maximum pressure,the smaller the band gap value back to normal pressure,realizing the adjustable bandgap.In situ synchronous X-ray diffraction and Raman spectroscopy evidence shows that tunable bandgap of g-C3N4 results from irreversible pressure-induced amortization and achieves a nearly 50%enhancement in photocurrent performance at high pressures.(3)Structure,charge transport and photocurrent response behavior of multilayer graphene materials under high pressure were investigated.Preliminary experimental results show that when the pressure increases to 39.5 GPa,the multilayer graphene exhibits photocurrent response under the incident laser in both visible and infrared wavebands.Meanwhile,the photocurrent value decreases with the increase of the pressure until it becomes undetectable at 52.9 GPa.The preliminary experimental results show that the formation of sp3 bonds between graphene layers is induced by high pressure,leading to the opening of the bandgap. |
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