| Two-dimensional tungsten disulfide(WS2)possesses high excitonic binding energy,tunable band gap in visible spectrum,high carrier mobility and strong spin-orbit coupling,offering great potential for optoelectronic devices and logic devices,and will play an essential role in valleytronics.In order to construct new-type functional devices or obtain better device performance,on the one hand,the crystal quality of WS2 need to be improved,on the other hand,it is usually necessary to tune the properties.In this thesis,the effect of the substrate on valley properties of WS2 is studied based on the first-principles calculations.Meanwhile,the growth of monolayer WS2 by chemical vapor deposition method is explored experimentally,and group-III nitrides(GaN and AlN),which are lattice-matched to WS2,are selected as substrates.The effects of different substrates on optical properties of WS2 are analyzed.The main contents are as follows:(1)The first-principles calculation is employed to calculated the electronic structure of WS2/GaN heterostructure.The mechanism and effect of GaN substrate on tuning valley properties of WS2 is studied detailedly.The results show that the Ga polar surface of GaN is nonmagnetic,while the N polar surface is ferromagnetic.When WS2 and N-polar GaN are constructed to heterostructure,WS2 shows a certain valley splitting,which is related to the interfacial stacking configuration.The most stable stacking configuration owns the largest valley splitting of 251.2 meV.By further studying of the mechanism,the emergence of sizable valley splitting is attributed to the hybridization between the pz orbit of surface N atom by N-GaN substrate and the electronic orbit of WS2 in valley states.Based on the k·p model,it is found that the orbital hybridization is equivalent to a giant magnetic field up to 1569 T.Moreover,it is found that the valley splitting can be enhanced effectively by reducing the interlayer distance and applying compressive strain.Under keeping the valley characteristic of WS2,when decreasing the interlayer distance to 2.3 A or applying a compressive strain of-3%,giant valley splitting of 426.3 meV and 353.2 meV can be achieved,respectively.(2)The growth of monolayer WS2 on group-Ⅲ nitrides substrate by chemical vapor deposition method is explored,and the effects of different substrates on its optical properties are investigated.The results show that the growth temperature plays a crucial role in the growth of WS2.Monolayer triangular WS2 with good quality can be grown under the growth temperature of 850℃.When the growth temperature is greater than 900℃,slight decomposition of the substrate surface occurs,which is not conducive to the growth.The nucleation density can be regulated by controlling the flow rate of H2 carrier gas.When the H2 flow rate is 9 sccm,monolayer WS2 with full coverage can be grown on the substrate.The as-grown triangular WS2 on GaN substrate shows a 60° rotation symmetry owing to the tiny lattice mismatch between GaN and WS2.The most stable structure of WS2/GaN heterostructure is predicted by combining the growth of WS2 on GaN nanorod with the first-principles calculations,where W and S atoms directly locate on the N and Ga atoms respectively.The optimized growth factors are applied to grow WS2 on AlN substrate,which show a similar result.It is found by Raman characterization that GaN substrate would exert certain tensile stress on WS2,resulting in the redshift of the Raman peak E2g1.While the WS2 on AlN substrate is free of strain due to the wave-shape surface of AlN substrate,resulting in nearly no shift of Raman peak and better quality by the smaller FWHM.Based on photoluminescence(PL)characterization,both exciton peak and trion peak in WS2/GaN show a redshift due to the compressive strain,and the emission is quenched due to the formation of type-Ⅱ band alignment.While the emission of WS2/AIN show a strong emission intensity due to the formation of type-Ⅰ band alignment. |
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