Study On The Structure And Electrical Transport Properties Of SnS Nanosheets Under High Pressure

After graphene has been reported to contain a unique layered structure,the IV-VI two-dimensional layered semiconductor material with similar structure has attracted wide research interest.As one of the most representative semiconductor materials in its family,stannous sulfide?SnS?has aroused great interest among scientists.In this thesis,the few layer SnS nanosheets synthesized by direct current?DC?arc plasma discharge method are selected as the research object,the microstructure and morphology of the samples at ambient conditions were characterized by X-ray diffraction?XRD?,scanning electron microscopy?SEM?and transmission electron microscopy?TEM?,respectively.The structural properties of SnS nanosheets under high pressure were investigated by high pressure in situ Raman spectroscopy and synchrotron radiation XRD spectroscopy.The electrical transport properties under high pressure were systematically studied by high-pressure in situ room temperature resistivity,high-pressure in situ temperature-dependence resistivity and high-pressure in situ Hall effect measurements.Our goal is to explore how pressure tune the structure and electrical transport properties of SnS nanosheets.The specific research results are as follows:1.We successfully synthesized SnS nanosheets by DC arc plasma discharge method.The morphology of SnS nanosheets was characterized by SEM and TEM.The average length and width of SnS nanosheets were 400-500 nm,and the average thickness was 20-30 nm.The structure of SnS nanosheets under ambient conditions was characterized by XRD technique.The results show that the SnS nanosheets are single crystal powder samples with orthorhombic structure?Pnma?.By high-pressure in situ Raman spectroscopy and high-pressure in situ synchrotron radiation XRD spectroscopy measurements,we find that the SnS nanosheets undergo a second-order isostructural continuous phase transition from orthorhombic structure?Pnma?to orthogonal structure?Cmcm?at 3.0 GPa,and then another structural phase transition occurred from orthorhombic structure?Cmcm?to monoclinic structure?P2₁/C?at 13.0GPa.2.Through high-pressure in situ DC resistivity experiments,we find that the electrical resistivity of SnS nanosheets decreases with increasing pressure over the entire pressure range.The resistance values have two discontinuous changes at 3.0GPa and 12.0 GPa,which are attributed to the structural phase transitions.It is worth noting that both the two phase transition pressures of the SnS nanosheets are considerably lower than the bulk material SnS.The first phase transition pressure reduces about 9.6 GPa,and the second phase transition pressure reduces about 6.2GPa,respectively.These differences can be attributed to the microscopic size effect of the SnS nanosheets and their special surface and interface structure of the layered morphology.After the pressure release,the electrical resistivity value of the SnS nanosheets recovered to its original order of magnitude,indicating that the phase transition process is reversible.3.The effect of pressure on the electrical transport properties of SnS nanosheets was investigated by high-pressure in-situ Hall effect measurements.It is found that the value of Hall coefficient?R_H?was always positive in the whole compression process,which indicates that the holes are the dominant carrier under compression.The electrical transport parameters include electrical resistivity,Hall coefficient,carrier concentration and carrier mobility change obviously at around 3.0 GPa and12.4 GPa,mainly due to the structural phase transition.4.Through the high-pressure in situ temperature dependent electrical resistivity experiment,we find that the electrical resistivity of the sample decrease with increasing temperature from ambient pressure to 7.0 GPa,which indicates that the SnS nanosheets exhibited obvious semiconductor characteristics.At the pressure above 8.0 GPa,the electrical resistivity of SnS increased with increasing temperature,and the SnS nanosheets exhibit metallic behaviors.Compared with the hall effect measurement results,we find that the metal or semi-metal phase is a“bad metal”,which is dominated by hole conduction.The dependence of activation energy of semiconductor SnS nanosheets with pressure is given to reflect the change of impurity level with pressure.In addition,compared with the data of SnS bulk materials,we also find that the metallization phase transition pressure of SnS nanosheets was about2.3 GPa lower than that in bulk materials,showing a strong size-dependent behavior under high compression.