First-principle Study Of Mechanical,Thermal Transport Properties And Raman Spectroscopy Of Novel 2D Materials

Since the discovery of graphene in 2004,low-dimensional materials have attracted extensive attention in basic research and applications.Because of its unique physical and chemical structure,graphene has excellent electrical,optical,thermal and mechanical properties.These properties make graphene has great application value in many fields,such as quantum physics,nano electronics,energy storage and conversion research,nano composite materials research.After that,the research community have raised the research upsurge of other low dimensional thin film materials,such as black phosphorus,silicene,borophene,MoS2,ReS2,WTe2 and SnSe.The successful synthesization or exfoliation of these novel 2D materials greatly enriched the family of 2D materials.Because of excellent optoelectronic and mechanical properties,the novel 2D materials are rapidly used to design tiny electronic equipment and more powerful energy storage devices.The mechanical and thermal properties are essential and important to materials.Excellent thermal and mechanical properties are beneficial to the wide application of materials,and the characterization of the thermal and mechanical properties of the new materials is a major issue in the field of materials research.For two-dimensional materials,the identification of layer number perpendicular to the two dimensional plane and the determination of crystalline orientation in the two dimensional plane are the foundamental characterization.The typical layer number identification measured by atomic force microscopy(AFM)is always inaccurate because of the gap between 2D materials and substrate.The determination of crystalline orientation through transmission electron microscopy(TEM)needs complicated transfer operation.Then a fast and nondestructive method to identify the number layer and crystalline orientation deserves to be explored.In addition,the estimation and comparison of interlayer coupling are helpful for the mechincal,electronic and optical applications.Raman spectroscopy,which is a fast and nondestructive tool to characterize the crystal structure,is sensitive to the crystal symmetry and has important application in the identification of layer number and determination of crystalline orientation.The main conclusions are listed as follows:1.Strain effects on borophene:ideal strength,negative Possion's ratio and phonon instability.We performed first-principle calculations to investigate the mechanical properties of the monolayer borophene,including ideal tensile strength and critical strain.It was found that monolayer borophene can withstand stress up to 20.26 Nm-1 and 12.98 Nm-1 in a and b directions,respectively.However,its critical strain was found to be small.Our numerical results show that the tensile strain applied in the b direction enhances the bucking height of borophene resulting in an out-of-plane negative Poisson's ratio,which makes the boron sheet show superior mechanical flexibility along the b direction.The failure mechanism and phonon instability of monolayer borophene were also explored.2.Cleavage Tendency of Anisotropic Two Dimensional Materials:ReX2(X=S,Se)and WTe2.Based on experiment and first principle calculation,we investigate the fracture behavior of ReX2 and WTe2.In order to interpret the unique cleavege tendency of ReX2 flakes,we perform comprehensive investigations of uniaxial tensile stress-strain relations of monolayer and multi-layer ReX2 sheets.Our numerical calculation shows that the particular cleaved edges of ReX2 flakes are caused by the unique anisotropic ultimate tensile strengths and critical strains.We found the ultimate stresses of ReX2 flakes along the directions perpendicular to the a and b lattice vectors are much lower than the other directions,which is responsible for the observed phenomenon that cleavage edges preferentially lie along their crystal axes.Additionally,along the direction perpendicular to the b axis,our calculations show that the critical strains are significantly lower than in the other directions.This explains why the b axis usually forms the longer edge of the cleaved flakes.We also calculate the stress-strain relation of WTe2 and reveal the reason of why the edges of cleaved WTe2 thin flakes are not corresponding to their principle axes.Our proposal mechanism about the fracture angle has been supported by the calculated cleavage energies and surface energies for different edge surfaces.3.Anisotropic phonon transport and lattice thermal conductivities in tin dichalcogenides SnS2 and SnSe2.Using first-principles calculations combined with the Boltzmann transport theory,we carry out comprehensive investigations on the phonon transport properties and heat transfer phenomena in tin dichalcogenides,and compare them with the recent popular thermoelectric materials SnS(SnSe)and widely studied typical TMDs:MoS2(MoSe2).The obtained total thermal conductivities of SnS2 and SnSe2 agree well with the experimental measured values.The contributions of different phonon branches to the total lattice thermal conductivities are evaluated,and the results suggest that it would be difficult for alloying to reduce kL because the contribution of the optical branches is rather small.At last,we investigated the size dependence of the thermal conductivities and found that nanostructuring may be efficient to further reduce kL since the mean free paths of the dominant phonon modes are relatively long.4.Anomalous in-plane anisotropic Raman response of monoclinic semimetal 1T'-MoTe2.We systemically study the in-plane anisotropy of multilayer 1T'-MoTe2 by using the polarization and angle resolved Raman spectroscopy.Then we use the full-quantum model associated with the density functional calculated energy band as well as the transition selection rule to explain the anisotropic results,and revealed that both the anisotropic electron-photon interaction and the anisotropic electron-phonon interaction play an important role in the anisotropic Raman response.