Characterization And Optoelectronic Performance Of Ultrathin MoS₂ Thin Films

The discovery of graphene in 2004 has been ignited the enthusiasm for the study of two-dimensional materials.The Two-dimensional molybdenum disulfide（2D-MoS₂）,similar to graphene,also attracted great interest and extensive research.Monolayer MoS₂ has a direct band gap structure of 1.8eV and belongs to the semiconductor material.It has great potential application in the field of electronic materials and optoelectronics.In the paper,the ultra-thin MoS₂ film was prepared using the hot-filament chemical vapor deposition（CVD）in one-step method,and deposited on the Si/SiO₂ substrate.To confirm the thickness,a combination of AFM,XRD,HRTEM,PL,and XPS characterization methods is used to investigate the crystal structures,valence states,and composition of the ultrathin film.In galvanostatic mode,the electrical measurements of the four-terminal mesoscopic structure made from the atomic layer MoS₂ thin film were test under valley polarization.The work of this paper is mainly divided into two parts:film characterization and mesoscopic structure measurement.This paper is with five chapters:Chapter 1 presents a review of our research background,summarizes the research history,quo status and related applications of two-dimensional MoS₂ and The development of theory and experiments with valley electron based on 2D MoS₂ is introduced.Chapter 2 is the introduction of the experiment content,including experimental materials,instruments,experimental process.In Chapter 3,we apply different characterization methods to the two-dimensional ultrathin films,and further confirm the two-dimensional MoS₂ with graphene structure.Using XRD,we could get the massage the films grow with a preferred orientation（002）,that is,the graphene-like MoS₂ with a number of layers in the direction of the c axis perpendicular to theatomic layer.The HRTEM images show the graphene-like structures of stacked layers with low density of stacking faults,and the interlayer distance of the plane is measured to be about 0.63 nm.It shows 6-fold coordination symmetry.Both EDAX and XPA confirm that the major elements in the sample are molybdenum and sulfur.The XPS measurements indicate that our sample has a high purity.The stoichiometric mole ratio of S/Mo is about 2.0-2.1 for the atomic layer MoS₂.It seems that it is near the nominal value of MoS₂.In conclusion,the above results confirm that the sample we prepared is a graphene-like two-dimensional MoS₂ with an atomic layer thickness.It is found that the voltage U_14,234,23 oscillates and the corresponding period varies with applied current.The largest period appears around at The largest period appears around at 45μA.Chapter 4,a mesoscopic structure with four-terminals is used to test the voltage U_14,23.On atomic layer MoS₂ film grown on SiO₂/Si substrate,a mesoscopic structure with four-terminals is used to test the voltage U_14,234,23 by using a galvanostatic measurement under the conditions with or without light irradiation.Get the following result:1)The test result of the constant current setting shows that the effect of the elliptically polarized light on the voltage oscillation is very obvious when the constant current setting is relatively small.In the range of larger constant current setting,the influence of elliptically polarized light on voltage oscillation is obviously reduced.The largest period appears around at 45μA.The oscillation periods are also different under dark and under laser irradiation.Based on the relationship of the oscillation period and the applied current,the laser-irradiated curve is obtained and it shifts towards low applied current direction compared with the darkness curve.The drift velocity is estimated at 10⁷cm/s.2)The experimental results of elliptically polarized light intensity further verify the role of laser circularly polarized light.3)The experimental results of the polarization state of elliptically polarized light show that the period value of U_14,234,23 oscillates periodically with the change of angleɑand the period is 2π.4)When we use a sodium lamp light,rather than laser,the variation of the period induced by the U_14,23oscillation is not evident.In contrast,when the laser is periodically switched on and off,the voltage U_14,234,23 curve and the corresponding period curve jump up and down.The experimental results suggest that the change of the period is related to the circularly polarized light.In fact,currently,the specific mechanisms behind the oscillations have not been thoroughly and clearly proven.One possible explanation is the beating effect from the photoexcited electrons with energy differences due to the feature of valleys based on the coherence mechanism.