Syntheses,Performance And Optoelectronic Devices Of Two-Dimentional ReS₂ And MoS₂

Two-dimentional(2D)transition metal dichalcogenides(TMDs)have attached great attention in recent years,among which 2D ReS₂ owns the promising application in the area of electronic devices,optoelectronic devices,energy storage and so on,due to its distinctive in-plane anisotropic properties and interlayer decoupling.Though,caused by the interlayer decoupling of 2D ReS₂,the growth of 2D ReS₂ large-area film is extremely difficult.In the other hand,2D MoS₂,a member of TMDs family,not only boasts the ultrathin thickness and mechanical flexibility like graphene,but also exhibits layer-dependent bandgap,which leads to a bright future in electronic applications,such as field effect transistors,and applications of wearable devices,and so on.Besides,the edge sites of base plane of MoS₂ provide tremendous chemically active sites for catalytic reactions,making MoS₂ auspicious as a catalyst.However,the various applications respectively require the specific growth direction of MoS₂—horizontal or vertical.Herein,2D ReS₂ flakes were synthesised by chemical vapour deposition(CVD),in which the effects of process conditions on structure,morphology,thickness,optoelectronic performance were studied.Moreover,the CVD system was successfully improved to grow 2D ReS₂ large-area film on mica and control its thickness,where Re source and S source(sulphur)were separated by an inner tube.Then,the optoelectronic devices based on ReS₂ film were fabricated and their performance characterization was also carried out.In addition to ReS₂,MoS₂ was also grown by CVD with controllable growth direction,horizontal or vertical.The results are shown as follow:First,the 2D ReS₂ flakes were synthesised on mica substrate by CVD where sulphur powder and Re-Te binary eutectic,used as reactants,were heated separately,using argon as carrier gas.The research focused on the effects of CVD process conditions on morphology,thickness,structure,composition,optical properties,and the conclusions are listed as follow:1.The experiments for three control variables,the inclinationqof substrate,the distance d between substrate and reactants,and the evapourating timepoint of sulphur(measured by the temperature T_start of furnace),were conducted,showing that the inclinationqof substrate can change the velocity of carrier gas on substrate surface as well as concentration of reactants vapour;Re-S group exhibits different concentration and transport speed with different d,leading to corresponding growth modes of ReS₂flakes;the nucleation density is highly dependent on T_start that changes the initial reaction temperature.2.Monolayer 30-?m ReS₂ flakes were successfully grown withq=45°,d=1cm,T_start=600?.The characterisation of Raman spectrum,transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS),optical transmittance spectrum were conducted,representing the good crystallinity of synthesised ReS₂ flakes,optical bandgap of 1.42e V,and optical absorption coefficient of 10⁵-10⁶ cm^-1,refractive index of1.3-1.5,extinction coefficient of 2.5-4,for visible light.3.The dependence of the morphology and growth mechanism of ReS₂ flakes on process conditions was analysed.If the in-plane growth dominates,ReS₂ flakes with horizontal growth direction would be obtained.Otherwise,out-of-plane growth would lead to the product of ReS₂ nano-flowers composed of many ReS₂ nanosheets with verical growth direction.Second,the CVD system for the synthesis of 2D ReS₂ flakes was improved,where the S source(sulphur powder)and Re source(Re-Te Re-Te binary eutectic)were separated by an inner tube for the continuous,steady transport and reaction of evapourated reactants,to grow thickness-controllable 2D ReS₂ film.Here are the conclusions:1.Atomic force microscopy(AFM)results indicated the good thickness uniformity of as-synthesised 2D ReS₂ film,and its good crystallinity was also confirmed by TEM and Raman spectrum.XPS spectra suggested the atom ratio of S and Re is approximately2:1.2.2D ReS₂ film exhibited high optical absorption coefficient of 10⁵-10⁶ cm^-1 for visible light,and bandgap of 1.59 e V for monolayer ReS₂,1.50 e V for 10 layers,which decreases with the increase of layer number.3.The field effect transistors based on 2D ReS₂ film were fabricated,and the influences of channel length and layer number of ReS₂ film on carrier mobility were tested.It's concluded that mobility declines with the increase of channel length,but rises,then level off with the increase of layer number.The photocurrent curve indicated the high photoresponsivity of ReS₂ film as large as 278 m A/W under 405 nm laser,almost two orders larger than those reported.Third,2D MoS₂ was grown on Si O₂/Si substrate by CVD,using sulphur powder and Mo O₃ powder as reactants,argon as carrier gas.The controllable growth of MoS₂ with vertical or horizontal growth direction was achieved by modulating the location of sulphur and distance between substrate and Mo O₃ on which the Mo:S atom ratio is dependent.The detail is listed as follow:1.The vertical growth of 2D MoS₂ relates to the formation of freestanding Mo O₂.The concentration of S and initial reaction temperature are modulated by the location of sulphur powder,as the concentration of Mo on substrate surface is highly sensitive to the distance between substrate and Mo O₃ powder.The Mo O₂ would form in the condition of low initial reaction temperature,or high Mo concentration and low S concentration,and be continuously sulphurized to finally become MoS₂ with vertical growth direction.The characterisation of Raman spectrum,TEM,scanning electron microscope(SEM)confirmed that the highly crystalline 2D MoS₂ nanosheets with vertical growth direction were obtained.Besides,the structure of MoS₂/Mo O₂ caused by partially sulphurising was also found.2.Horizontal growth of 2D MoS₂ flakes were achieved on the zone of substrate far away from Mo O₃ powder,under 650? for 30 min,while the sulphur was 25 cm far from the inlet of furnace.SEM,AFM,Raman spectrum,TEM,photoluminescence(PL)spectrum showed the as-synthesised 2D MoS₂ flakes with a maximum size of 16?m are single crystal,having a strong PL peak located at 673 nm.