Studies On Surface/Interface Of Two-dimensional Transition Metal Dichalcogenides And The Optoelectronic Properties Of Related Van Der Waals Heterojunctions:An Atomic-bond-relaxtion Approach

Two-dimensional(2D)layered materials have attracted a great deal of attentions owing to their novel and intriguing electronic and optical properties,opening up new avenues for applications in field effect transistors,photodetectors,photovoltaics and so on.In the rich family of 2D layered materials,transition metal dichalcogenides(TMDCs)become the focus of fundamental researches and technological applications due to their unique crystal structures,variety of chemical compositions,intriguing physical and chemical properties.TMDCs exhibit great potentials in optoelectronic due to its intrinsic bandgaps in range of 1-2.5 eV,strong light-matter interaction,coupled spin,and so on.Furthermore,2D layered materials are weakly bonded by the van der Waals(vdW)forces,which results in the fabrication of high quality vdW homo-or heterostructures randomly despite the lattice mismatch.The electronic properties of 2D layered materials can be effectively tuned by the number of layers,external stress,chemical doping,interface twist and so on.With the decrease of the thickness,the proportion of surface atoms increases,and the boundary atoms with high surface-to-volume ratio and high coordination defects will lead to the spontaneous relaxation,resulting in the self-equilibrium state,thus the energy and electronic charge of the system will be redistributed.Furthermore,the lattice parameters and energy of the system will change under the perturbation of external environment,which will tailor the electronic properties of the system.Although several experiments and first principle calculations on the modulation of electronic properties in 2D materials have been made,some basic problems still need to be clarified.For instance,there is no analytical model for quantitative analysis of interlayer coupling.Also,the relationship between the evolution of interlayer spacing and interlayer coupling strength as well as bandgap with interface twist angle is ambiguous.The joint effect of size and composition on bandgap change in 2D-TMDC alloys still remains unclear.Otherwise,a systematic study with regard to the size dependence of electronic and optoelectronic properties of 2D-vdW structures at the atomic level is still lacking.Therefore,in order to explore the evolution of band enginerring in TMDCs under the external perturbation(including strain,interface twist,chemical doping,and size,etc.),we put forward an analytical model to explore the intrinsic mechanism of band engineering on 2D layered materials in terms of atomic-bond-relaxation considerarion.First,we study the modulation of external strain on the bandgap of single MoS2,and clarify the modulation mechanism of strain engineering on the band offset of single-layer MoS2 in terms of the relationship between bond identities and uniaxial strain.Second,we put forward an analytical method to address the size tunable band alignment and power conversion effciency(PCE)of MoS2/WSe2 vertical stacking van der Waals heterostructures,and realize the optimized PCE of the system.Furthermore,we explore the evolution of interlayer vdW coupling,interlayer spacing and electronic properties with random interface twist angle in MoS2 bilayers.Moreover,the joint effect from size and composition contributions on the band offset of 2D-TMDCs is also addressed in detail.The achievements are shown as follows,(1)We propose an analytical model to address the band offset in single-layer MoS2 under the approach of uniaxial tensile strain,and explore the evolution of bond identities(bond length,bond angles)and band offset with tensile.It was found that the bandgap of single-layer MoS2 shows an approximately linearly red shift with a rate of?53.4 meV/%strain under uniaxial tensile strain,and the tendency possesses virtually isotropic at small strain.The underlying mechanism can be attributed to the variation of crystal potential induced by the vatiations of bond identities such as bond length,bond angle,and bond strength.(2)We put forward an analytical model to address the band alignment of interface and optoelectronic properties of TMDC van der Waals heterostructures by extending the detailed balance principle approach to the nanoscale,and establish the ralationship between PCE and thickness.Furthermore,the effects of geometric parameters and intrinsic properties of materials on the photovoltaic properties of solar cells have been clarified.Importantly,we found that the PCE of bilayer MoS2/WSe2 unit can be up to 1.70%,and the PCE increases with thickness owing to the increasing of light absorption.(3)We address the evolution of interlayer spacing and interlayer vdW coupling with random stacking configurations in MoS2 homojunctions,and establish the relationship between the band offset and the interface twist angle in terms of atomic-bond-relaxation correlation mechanism.We found that the interlayer spacing changes substantially with respect to the orientations,and the energy bandgap increases from 1.53 eV(AB staking)to 1.68 eV(AA stacking).(4)We investigate the joint effect of size and composition on the band offset in 2D-TMDC alloys,and pursure the underlying mechanism of band engineering and bending factor owing to the variations of bond identities.We found that the composition-dependent bandgap of 2D-TMDC alloys presents obviously size tunable,and the bandgap shows blue-shift or red-shift due to different intermixing of alloy components.Both size and chemical composition can be performed as the useful methods to modulate the bandgap,suggesting some effective approaches to realize the desirable properties of 2D-TMDC alloys.