Study On The Strain Modulation And Photoelectric Properties Of InSe

In recent years,two-dimensional semiconductor materials,have attracted much attention from researchers,because of their excellent properties,such as atomic-level thickness,high mobility and tunable band gap.However,the zero bandgap of graphene limits its application in photodetection.InSe is favored by researchers due to its tunable bandgap,high carrier mobility,and excellent optical polarization properties.InSe presents high work function,low carrier concentration,low hole mobility,and weak fluorescence for few-layer InSe,which limit its wide application in photoelectric devices.To further improve the performance of InSe,strain,doping,temperature,external electromagnetic field and heterojunction have been used to further tune and optimize its performance.InSe has a small Young’s modulus of about 23.5GPa,and can withstand tensile forces of up to 27%.Therefore,strain has a unique advantage in regulating the structure and properties of InSe.In this work,multilayer InSe flakes were mechanically exfoliated from a bulk InSe crystal.It was confirmed that its layer number can be effectively identified by Raman and PL spectra.Four different type strains,including in-plane uniaxial tensile strain,in-plane biaxial tensile strain,hydrostatic compressive strain and out-ofplane axial compressive strain are used to modulate the structure and performance of InSe.Especailly,high pressure not only provides an all-isotropic strain but also provides one extreme condition,which can be used to explore special structure and special property that can not present at ambient condition.Thus,we have systematically investigated the structural stability,electronic structure and polarization property changes of InSe under high-pressure conditions.In addition,InSe photodetectors are constructed,and their photovoltaic properties are studied in detail.The specific research work is as follows:(1)In this work,multilayer γ-InSe flakes were mechanically exfoliated from a bulk crystal InSe,and their thicknesses were characterized by optical microscopy,Raman spectroscopy and fluorescence spectroscopy.The fluorescence peak of InSe gradually blue shifts with decreasing thickness,and its band gap increases from 1.25 e V for bulk material to1.46 e V for 5-layers.When the thickness less than five layers,its emission can not be detected because of the direct-to-indirect band transition.The results indicate that photoluminescence is one effective method to characterize its thickness.(2)High pressure study of InSe.The structural evolution and phase transition of InSe under pressure were systematically investigated by Raman,PL,FT-IR and First-principles calculations.Around 11 GPa,the sample color became completely black Raman and infrared spectroscopy confirmed that it underwent an irreversible metallization phase transition.Below11 GPa,InSe will finally irreversibly transformed into the high-temperature high-pressure phase of InSe-II after several pressure up-down cycles.In addition,as the pressure increased,the fluorescence peak of InSe presented blue-shift,and a transition from direct-to-indirect band was detected near 4 GPa.Simultaneously,its fluorescence intensity first increased and then decreased during compression,which reached a maximum at 3.8 GPa.Pressure also caused a change in polarization properties,with a 15° shift in the polarization angle at 3.3 GPa and another 30° shift when reaching 6.5 GPa.Therefore,pressure can effectively modify the crystal structure,luminescent property and anisotropic property of InSe.(3)Strain modulation study of InSe.Different devices were designed to generate four different types of strains,including in-plane uniaxial tensile strain,in-plane biaxial tensile strain,hydrostatic compressive strain and out-of-plane axial compressive strain,and the effects of different types of strains on its phonon mode and fluorescence performance were systematically investigated.The results showed that four types of strains would lead to its fluorescence redshift and band gap reduction.For the same phonon mode,the modulation effect of in-plane biaxial tensile strain and hydrostatic compressive strain is better than that of uniaxial tensile strain and out-of-plane axial compressive strain.Both Raman and fluorescence results show that the modulation effect of biaxial tensile strain is much better for InSe.(4)Construction of InSe photoelectric device.The photoconductive InSe photodetectors were constructed.The effect of annealing,temperature and laser power on the photoelectric performance were investigated.The results showed that annealing can greatly improve the electrode contact and thus improve its photoelectric performance.Under the light of 532 nm laser,the photovoltage and photocurrent of the sample increased with the enhancement of laser power.Temperature can effectively improve its contact potential and photocurrent.In addition,its photovoltaic response was stable,and its photocurrent did not diminish after several test cycles.