Anisotropic Raman Spectra Of SnS Flakes And Optoelectronic Properties Of SnS/SnS₂ Heterostructure

Two dimensional(2D)IV-VI materials(IV:Sn,Ge;VI:S,Se,Te),as new members of 2D family after transition metal dichalcogenide(TMD),have attracted more and more attention from researchers due to its diverse structures,strong light absorption,excellently optoelectronic properties,earth abundance,nonpoisonous,and environmentally friendly.Among these materials,the most representatives are SnS and SnS2.SnS is a p-type semiconductor,with an orthorhombic crystal and many advantages,including a large absorption coefficient(?105 cm-1),high carrier mobility and strongly anisotropic electronic,optical and thermal properties.On the other hand,SnS2 is a n-type semiconductor belongs to hexagonal crystal structure with high electron mobility of?230 cm2V-1s-1 and prominently optoelectronic properties.Although many works on SnS and SnS2 have been reported so far,the controllable preparation for high-quality SnS and SnS2 materials are still lacking.At the same time,the anisotropic property of SnS has yet been investigated thoroughly,especially its thickness dependence is under debated.More importantly,although SnS and SnS2 are p-and n-type semiconductors respectively,direct growth of p-n heterojunction composed of SnS and SnS2 remains a great challenge,owing to their different crystalline structures.In this context,we carried out the systematic studies in this dissertation on the controllable preparation of SnS and SnS2,the anisotropic Raman spectra of SnS,and the growth of SnS2/SnS heterojunction and its corresponding photoresponse.The contents of the dissertation are summarized briefly as below.In chapter one,we firstly introduce the general information and development process of 2D ?-? materials.Then we introduce the anisotropic electronic and Raman properties of SnS.Moreover,the crystal structure and properties,synthesis methods and applications of SnS and SnS2 are reviewed in detail.Finally,we present the motivation and main content of the dissertation.In chapter two,we systematically study the thickness dependence of anisotropic Raman properties of SnS.Firstly,the SnS flakes with different thickness were synthesized by physical vapor deposition method,and the crystallinity and morphologies of the flakes were characterized by TEM and AFM,respectively.Then angle-resolved Raman spectra were carried out at SnS flakes with different thickness.It was found that the anisotropic behavior of B3g mode does not change with thickness.However-the anisotropic feature of Ag(192.0 cm-1,||)is extraordinarily sensitive to the thickness.Specifically,the Raman intensity in zigzag direction is increasing with the decreasing of thickness for both 633 nm and 514 nm laser excitation.In addition,for 514 nm laser excitation,with the decrease of thickness,the intensity of Ag(192.0 cm-1,II)in zigzag direction becomes even larger than that in armchair direction.The results can be understood by the complex Raman tensor theory.In chapter three,using S and SnS as precursors,the SnS2/SnS heterostructure was successfully fabricated by one-step CVD method through the switch of SnS and SnS2 growth by the change of vapor concentrations of S and SnS during the growth.The Raman,AFM and TEM characterizations demonstrated that the heterostructure is composed of the pyramid-shaped SnS2 onto the orthorhombic SnS.The possible growth mechanism was proposed and discussed.After three-step EBL and sputter process,the photodetector based on the high quality SnS2/SnS heterostructure was successfully fabricated,which was demonstrated with high performance:photoresponsivity of 27.7 A/W,on off ratio of 2.2 × 103 and the specific detectivity of 2.1 × 1010 Jones.In chapter four,we regulated CVD growth of sulfur-tin compounds through different conditions.For SnS2 growth,we proposed a strategy that combined with high substrate temperature and pre-preparation of nucleation seeds on the substrate,successfully fabricated large size SnS2 flakes(?150 ?m);and we developed a new method to fabricate the device based on the as-grown SnS2 flake,which showed a high specific detectivity of 2.0×1010 Jones.Moreover,by increasing the growth temperature and adding CuPC assisted nucleation,we also realized the controllable preparation of p-type Sn2S3 nanowires with high crystalline quality.Finally,through two strategies with either adding H2 into the growth CVD system or using high substrate temperature,we successfully fabricated the SnS flakes which have the photoluminescence at infrared wavelength.In addition,it was also found that the photoluminescence is related with the red-shift of the Raman peak,and one possible interpretation was also proposed.In chapter five,we prospect some topics for future studies related to SnS and SnS2 and give corresponding suggestions.