Study On The Mechanical-electronic Coupling Effects Of Two-dimensional Transition Metal Dichalcogenides Based On Atomic-bond-relaxation Method

With the continuous development of miniaturization of micro/nano devices,the traditional silicon-based electronic devices have encountered bottlenecks and are facing more and more difficulties and challenges.The emergence of two-dimensional layered materials provides opportunities and challenges for the design of efficient and stable electronic and optoelectronic devices,and also brings hope for the continuation of Moore's law.Moreover,two-dimensional transition metal dichalcogenides(TMDs)possess numerous remarkable properties with their bandgaps in the range of 1?2.5 e V,strong light-matter interaction,and more flexible,supplying a fertile soil for practical applications in electronic and optoelectronic devices.In addition,the mechanical-electronic coupling effects can effectively modulate the electronic properties of TMDs,and also provides an effective way for the optimization of device performance and the design of new types of electronic and optoelectronic devices.The electronic and optoelectronic properties of a material have a great correlation with its electronic band structure.Moreover,the electronic properties of TMDs can be effectively modulated by doping,pressure,and geometry effect.Meanwhile,the piezoelectric and flexoelectric properties have a significant impact on the electronic band structure and electronic transport properties of material.Although some progresses have been made in the experiments and theoretical calculations,there are still some fundamental problems need to be further clarified.For example,how does the alloying and pressure affect electronic properties of TMDs?What is the relationship between the piezoelectric properties and the size and boundary type of monolayer TMDs?Moreover,how does the flexoelectricity affect the electronic and optoelectronic properties of TMD nanotubes?Therefore,a systematic study with regard to the electronic and optoelectronic properties of TMDs under the external perturbation and geometry effect is necessary.In our work,we investigate the electronic and optoelectronic properties of TMDs via doping,pressure,and geometry effect based on the atomic-bond-relaxation consideration and first-principles calculations.First,we establish a relationship among semiconductor-to-metal transition,composition,and hydrostatic pressure in the ternary alloys of TMDs.Then,taking monolayer MSe₂(M=Cr,Mo,W)as an example,we explore the influence of size and boundary type on the piezoelectric properties.We construct an optoelectronic device model based on the spontaneous flexoelectric effect of MoS₂ and WS₂ nanotubes.Also,we develop a theoretical model to clarify the collection and recombination of photogenerated electron-hole pairs in MoS₂ and WS₂ nanotubes.Finally,we study the diameter-dependent power conversion efficiency(PCE)in MoS₂-based solar cells.The achievements are shown as follows:1.We develop a theoretical relationship between the bond identities and the band shift based on the atomic-bond-relaxation correlation mechanism.We find that the interaction parameter in TMD alloys can be obtained from the distortion energy and further reveal the bowing mechanism in monolayer TMDs.Moreover,we find that the bandgap displays a slightly red-shift and then blue-shift as the composition increases for cation doping,while the bandgap of anion doping shows a monotonically red-shift with increasing composition.In addition,we establish a relationship among bond identities,band offset,and related semiconductor-to-metal transition in Mo_(1-x)W_xS₂ with different thicknesses under hydrostatic pressure.Moreover,the bandgaps of monolayer and bilayer show a blue-shift and then red-shift with increasing pressure,whereas the bandgap of bulk Mo_(1-x)W_xS₂ displays a red-shift with increasing pressure owing to the strong coupling of interlayer.2.We investigate the piezoelectric properties of monolayer MSe₂and reveal how piezoelectric properties depend on the size and boundary type.We find that the edge effect can result in lattice distortion and induce lattice periodic potential deviating from the infinite cases.In addition,the monolayer MSe₂ exhibits anisotropic piezoelectric properties that are dependent on the boundary type.Moreover,the piezoelectric power output is strongly dependent on the stress direction.Our results show that the size and boundary type can significantly affect the piezoelectric properties of 2D systems,which provide a guide for future experiments to optimize the piezoelectric and related properties.3.We address the influence of spontaneous flexoelectric effect on the band engineering in single-and double-wall MoS₂ and WS₂nanotubes with different diameters based on first-principles calculations and atomic-bond-relaxation mechanism.We find that decreasing diameter of single-wall MoS₂ and WS₂ nanotubes can lead to a decrease of bandgap and an increase of polarization as well as flexoelectric voltage in the radial direction.Polarization charges promoted by flexoelectric polarization will take place,resulting in a straddling-to-staggered bandgap transition in the double-wall MoS₂ and WS₂ nanotubes.The critical diameters for bandgap transition are,respectively,about 3.1 and3.6 nm for double-wall MoS₂ and WS₂ nanotubes,which is autonomous for chirality.4.We further explore the influence of spontaneous flexoelectric effect on the collection efficiency of carriers and PCE of double-wall MoS₂ nanotubes based on the atomic-bond-relaxation method and detailed balance principle.Especially,the optimal PCE of double-wall MoS₂ nanotubes can reach 5.25%at fixed diameter of 5.2 nm.Our results show that the optimal PCE of double-wall MoS₂ nanotubes is seven times larger than that of bilayer MoS₂,suggesting that the flexoelectricity in TMD nanotubes can be as an effective way to develop new types of photovoltaic devices.