| ?A-?A group two-dimensional semiconducting materials including InSe,SnS,SnSe and SnSe2 exhibit relative narrow theoretical band gap in the range of 0.7-1.5 eV and unique physical optoelectrical properties,for instance,high carrier mobility,strong in-plane anisotropy,high photon absorption efficient,flexible,alloy engineering tunable band structure.The above materials can be applied in the field effect transistors and photodetectors with high performance.In recent years,van der Waal s heterostructures assembled with different two-dimensional materials likes graphene and transition metal dichalcogenides(TMDs)are developing,which shows excellent properties and device functions such as low temperature superconductivity,rectifying behavior,ultrahigh tunneling current,photovoltaic effect and negative differential resistance.Therefore,van der Waals heterostructure integrated with ?A-?A group materials show tremendous development space in the optical electronic device field.In this paper,we comprehensively and systematically studied the growth and physical characterization as well as device application of graphene,TMDs and ?A-?A group two-dimensional materials.The main contents and results are as follows.Frist,ultrathin SnS,SnSe and SnS1-xSex nanosheets have been synthesized successfully with narrow-gap physical vapor deposition(NGPVD)method.According to the analysis of the growth mechanism,the precursors vapor tended to grow along the horizontal direction rather than vertical direction in such a small space.Meanwhile,the hot etching effect of nitrogen gas decreased to a low level.Moreover,ultrathin SnS0.5Se0.5 nanosheets with orthorhombic structure delivered a high crystal quality and a deviated Raman shift under Se content due to the lattice disorder.Most importantly,ultrathin SnS0.5Se0.5 nanosheets exhibited an abnormal Raman anisotropy behavior.When the incident laser is parallel to scattered laser,the peak intensity of alloy like Ag(77 cm-1)showed a period of 180°.While under a parallel or cross configuration,alloy-Ag(77 cm-1)showed isotropy and the Raman intensities of SnSe-B3g(114 cm-1,177 cm-1)modes show a different variation period of 90°,respectively.In particular,the alloy-B3g(177 cm-1)is overlapped with SnS-Ag(192 cm-1)due to the disorder-induced effects and lattice distortion.We can determine the armchair or zigzag direction of ultrathin SnS0.5Se0.5 through Ag(77 cm-1)and SnSe-B3g(114 cm-1,177 cm-1)Raman modes.In future,we will pay more attention on the different proportion of SnS1-xSex in Raman,photoluminescence,optical and electrical transport.Second,ultrathin SnS0.sSe0.5 field effect transistors were fabricated and further studied for the electrical and optoelectrical properties.It was found that the maximum current on/off ratio reached 2.1 ×102,which is nearly two orders of magnitude larger than previous SnSe and SnS FETs devices.The SnS0.5Se0.5 photodetector can sensitively response to 405-808 nm.Under high power laser of 532 nm,the responsivity,external quantum efficient,detectivity and response speed were 1.69 A W-1,392%,3.96 × 104 Jones and 40 ms,respectively.When the sample thickness is less than 25 nm,the corresponding on/off function remain a normal level.By contrast,when the thickness exceeds 25 nm,the corresponding current on/off ratio suddenly decreases to about 1.10 and was seem to be semi-metal behavior.Furthermore,we have calculated the band structure of SnS1-xSex by the density function theory method.It is believed that the deep defects levels in the band structure can be shallowed by alloy engineering.The optoelectrical properties of alloy photodetectors will be improved,too.In result,we found that p-type SnS0.25Se0.75 FET delivered a highest hole mobility of 0.77 cm2·V-1s-1,moderate responsivity at 635 nm of 4.44 ×102 A W-1 and fast response speed of 32.1/57.5 ms by comparing other proportions.After stacking part of p-type SnS0.25Se0.75 onto the n-type Si substrate,a p-n mixed dimension junction can be generated and showed a good self-power photo-response.Under 635 nm illumination,the corresponding R635 nm,EQE,D*,response speed and light on/off ratio were 377 mA·W-1,73.8%,?1011 Jones,4.7/3.9 ms and 4.5 × 102,respectively.The above parameters are better than most of self-driven device.Besides,a type-? staggered band alignment between p-type SnS0.25Se0.75 and n-Si was confirmed by the calculated bandgap.A built-in electric field existed at the interface region,and then the photogenerated carriers can be diffused and separated fast.Last but not least,the photo-response in the range of 950-1100 nm are enhanced suddenly.Particularly,the R1050nm and response speed were 6.63 A·W-1 and 85.1/296.64 ms,respectively.It is indicated that the synergistic effect between suppressing deep defects level by SnS0.25Se0.75 and the narrow bandgap of n-Si,which accelerating the high absorption in spectral wavelength.Third,a series of vertical graphene/n-type SnSe2/graphene sandwiched device based on different SnSe2 thickness have been fabricated by the mechanical exfoliation and wet-transfer method.We have found the SnSe2 thickness of 96.5 nm showed good gate-tunable optoelectrical performance.When the gate voltage was+80 V,the R532 nm,EQE and D*were 1.3 × 103A·W-1,3×105%and 1.2 × 1012 Jones.Meanwhile,the selected device also displayed a stable and reproducible light on-off function with fast response of 30.2/27.2 ms.Besides,a lower surface potential difference for Gr/SnSe2 interface than that of Au/SnSe2 interface with the help of Kelvin Probe Force Microscopy measurement.The band alignment of graphene-SnSe2-graphene under various gate voltage were discussed.In result,when the gate voltage was+80 V,the band slope of SnSe2-Gr was downshifted and the number of photogenerated carriers was increased in SnSe2 layer.Last,the transport difference between planar Au-SnSe2 and vertical graphene-SnSe2 are discussed by the band alignment and structure diagrams.In conclusion,moderate multilayered SnSe2 integrated with vertical graphene structure emerges as a high optoelectronic device in future nano-electronic field.Last,a number of Au-multilayered InSe-graphene and vertical graphene-InSe/WSe2-graphene vertical p-n junction devices were prepared by the mechanical exfoliation and wet-transfer method.On the one hand,Au-multilayered InSe-graphene vertical FET showed a high tunneling surface current density of 1.64 × 103 A·cm-2 at Vds=1.0 V and Vg=-80 V In future,we can modify the thickness of InSe to study the vertical transport properties at low temperature and improve the on/off ratio.On the other hand,for vertical graphene-InSe/WSe2-graphene p-n junction,the interface between p-InSe and n-WSe2 revealed a photoluminescence quenching effect and a wide depletion region.The rectification ratio and current on/off ratio of those p-n junction was both 103.The corresponding photodetector can response to the range of 405-635 nm.Under a weak 532 nm illumination,the R532nm,EQE and D*at negative bias were 83 A·W-1,1.92 ×104%and 1.55 × 1012 Jones.The corelated response speed of 36/18 ms and augment of photocurrent were better than that of vertical graphene-InSe-graphene.At last,according to the DFT method,we found that a type-II staggered band alignment were generated between InSe and WSe2.Under illumination,the enhanced photogenerated electron-holes generated at each side of InSe and WSe2.Those carriers separated faster in reverse direction with a built-in field.In future,gate tunable,ultralow or zero bias tunneling behavior of vertical graphene sandwiched device can be studied.Such as ballistic avalanche,negative differential resistance,backward rectification and self-driven effect. |
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