The Structural And Electrical Transport Properties Of Compressed SnSe Nanosheets

Semiconductor materials and topologically insulated materials are popular materials in the field of scientific research.Due to their new properties and new structures under high pressure,IV-VI compounds have attracted great attention.SnSe is one of the most representative narrow-bandgap P-type semiconductor materials in IV-VI compounds.Under ambient temperature and pressure,SnSe crystals are symmetric layered orthorhombic structures with a space group of Pnma.Because of its more unique physicochemical properties,it is commonly used in memory switching devices,infrared detectors and solar cell anode materials.In recent years,it has been found that most researches focus on the bulk SnSe,and study the physical properties and structures of SnSe under different conditions.Although there are also a few reports on nanomaterial SnSe,it is limited to the researches of synthetic method,structure stability,and spectral properties.However,the behaves of SnSe nanomaterials are similar with bulk SnSe,and will it transform from semiconducting phase to metallic phase under high pressure is still unknown.Therefore,both the phase transition and electrical transport properties of SnSe nanosheets have been studied with the methods of high pressure synchrotron radiation X-ray diffraction technique,high pressure in-situ resistivity and variable-temperature resistivity measurements on diamond anvil?DAC?.The results are listed as follows:1.With the synchrotron radiation X-ray diffraction and GSAS refinement,the structures of SnSe nanosheets have been studied under high pressure.With the pressure increasing from ambient to 7.2 GPa,SnSe nanosheets undergo a second-order structural phase transition from Pnma to Cmcm.The pressure continues to increase until 36.4 GPa.Due to the appearance of a new peak,we find the crystal structure have changed again.We have refined the spectrum at 34.66 GPa.The crystal structure of the SnSe nanosheets has been transformed from the orthogonal structure Cmcm into a monoclinic structure P2₁.2.With the in situ high pressure resistivity measurement,we have found that the resistivity of SnSe nanosheets decrease with the increasing pressure.In the pressure range of 0-7.3 GPa,the resistivity decreases by four orders of magnitude,during the pressure range of 7.3-14.0 GPa,the SnSe nanosheets undergoes a isostructural continuous phase transition from the initial phase?Pnma?to a higher-symmetry isomorphic structure?Cmcm?,which in accordance with the results of high-pressure synchrotron X-ray diffraction experiments.Whereas in the range of 14.0-27.6 GPa,the resistivity decreases significantly,showing a big discontinuous change at 27.6GPa.We conclude that this is related to pressure-induced structural phase transitions,after a sharp drop in resistivity in the 27.6-40.2 GPa interval,tends to be flat.3.Through high pressure in situ variable-temperature resistivity experiments,the experimental temperature is controlled between 100 K-250 K.At the pressure below12.5 GPa,the resistivity decreases with the increase of temperature,and it can be judged that the SnSe nanosheets show obvious semiconductor characteristics in this interval.Between 13.6-19.8 GPa,due to the appearance of the partial broken state of the SnSe nanosheets,the electrical signal become unstable.SnSe nanosheets exhibites obvious semi-metallic properties in the pressure range of 22.5-40.2 GPa.4.From the relationship of resistivity with temperature under different pressures,the pressure-dependent activation energy is obtained.The activation energy decreases from 1.9 GPa to 7.0 GPa,and suddenly rises at 7.9 GPa,then keep the downward trend.The decrease trend in pressure-dependent activation energy over the pressure range of 1.9 GPa to 12.5 GPa is due to the pressure-induced band gap narrowing in the SnSe nanosheets.In this process,carriers at the impurity and defect levels are more easily to transition to Conductor bottom.The sudden increase of activation energy value at 7.9 GPa is attributed to the Pnma?Cmcm structural phase transition at this pressure.