Synthesis,Optical And Frictional Properties Of Two-Dimensional Transition Metal Dichalcogenides

With the increase of transitors density in integrated circuits,the need for advanced materials with superior and tunable properties is becoming more and more urgent.In addition,as the decrease of device size in micro/nano electro-mechanical systems（M/NEMS）,the dramatically increase of specific surface area cause a serious size effects which make surface forces,like friction force,adhension force etc.a critical factor in limiting the performance and lifetime of micro/nano devices.In the past decades,two-dimensional（2D）transition metal dichalcogenides（TMDs）have attracted huge attentation due to their unique optical,electrical,thermal and mechanical properties.TMDs exhibit a combination of atomic-scale thickness with tunable optical and frictional properties,which makes them one of the most promising candiates for M/NEMS.In view of this,it is important to systematically study the controllable synthesis,optical and frictional properties of TMDs.In this paper,experimental methods such as chemical vapor deposition（CVD）,Raman and photoluminescence（PL）spectroscopy,Ozone treatment and atomic force microscopy（AFM）are used,combined with theoretical research methods such as molecular dynamics（MD）simulation and first-principle calculations,to investigate the precise synthesis,optical properties characterization,regulation and application,frictional properties and analysis of influencing factors.The main work of this dissertation includes:1.Precise control of the thickness,shape and lateral size of the grown samples is achieved by manipulating the precursor sulfur during the growth of TMDs by CVD.The importance of the precursor sulfur in the growth of Mo_xW_1-xS₂ alloys by two-step CVD was determined.The precise control of the thickness,shape and lateral size of Mo_xW_1-xS₂ alloy can be achieved directly by varying the parameters of the sulfur.By changing the feeding rate of sulfur vapor to promote the best mathch of sulfur and tungsten,molybdenum precursors,the number of layers of the as-grown alloy nanosheets can be regulated from monolayer to four layers,thus the thickness of samples can be controlled.Applying the amount ratio of sulfur and tungsten trioxide（WO₃）,the as-grown Mo_xW_1-xS₂ alloy can evolve from a hexagon to a perfect triangle,thus the shape of the sample is controlled.The lateral size tuning of the grown samples achieved by varying the exposure time of sulfur introduced into the CVD chamber to involve in the chamical reaction.This method enables the stable and controlled synthesis of large size,high quality TMDs by varying only a single parameter in the chemical vapor deposition process,paving the way for their wide application in micro/nano electo-mechanical systems.2.The optical characterization of the thermal stability of as-grown TMDs was investigated systematically,and the PL spectra were used to characterize the thermal stability of the grain boundaries of the samples.Firstly,large-area polycrystalline monolayer tungsten disulfide（WS₂）samples were prepared by CVD.And their optical properties were characterized using confocal Raman spectroscopy and the PL mapping images were measured.We found that there is a significant difference in the intensity of the PL spectra in the grain boundary and the non-grain boundary region.After the samples were annealed,the trends of the PL intensity in the grain boundary region and non-grain boundary region were also different.The PL intensity in the grain boundary region gradually decreases as the annealing temperature increases,while the PL intensity in the non-grain boundary first increases and then decreases.With the help of first-principle calculations and results analysis,it is concluded that the enhancement of the PL intensity in non-grain boundary region is due to the filling of oxygen into the sulfur vacancy defects,and the PL intensity in the grain boundary region can more truly reflect the thermal stability of the sample.Based on the experimental results and theoretical analysis,it is proposed to use the optical properties of TMDs to characterize their thermal stability.3.The optical properties of TMDs were regulated by developing an erasable method to directly write encrypted information onto TMDs monolayers,we demonstrate the thinnest light disk at atomic level.Monolayer WS₂ flakes were prepared by CVD.The write-in is realized by precisely control of PL emission by means of ozone functionalization and scanning focused laser beam.The visual decryption and reading-out of information are enable by fluorescence contrast.The high encryption level is ensured by the threshold power upon which the data deletion will be triggered.First-principle calculations suggest that the disk formatting is realized by ozone molecule absorption induced localized unoccupied states,while the read-in relies on the passivation of defects via substitution of the sulfur vacancies with oxygen atoms.We demonstrated TMDs monolayer work as the thinnst light disk with data storage and encryption by utilizating their optical properties.4.The frictional properties of TMDs were investigated and the factors affecting their tribological performance were analyzed.In addition,a new understanding of the energy dissipation during the friction process from the phonon perspective was proposed.We synthesized monolayer Mo_xW_1-xS₂ alloy by CVD method.By performing AFM measurements,the friction force is found to decrease monotonically from Mo S₂ region to WS₂ region with the reduced molybdenum concentration.MD simulation was used to explore the influence of different factors on the friction force,verifying that the interfacial interaction is the determining factor leading to the inconsistent frictional distribution of the monolayer Mo_xW_1-xS₂ in the experiment.Moreover,he phonon dissipation during the friction process were calculated quantitatively.The results show that the phonons in substrate are excited at both washboard frequency and its harmonics.Besides,the number and total energy of the excited phonons decrease with the weaking interfacial interaction,leading to the decreasing energy dissipated into the substrate.The reduced dissipated energy in turn leads to the decreasing friction force.Our work has significant implications for understanding the underlying mechanism of energy dissipation in nanotribology,which helps to control friction in TMDs and lays the foundation for wider application of TMDs in M/NEMS.