First Principles Study Of Novel Two-Dimensional Van Der Waals Heterostructures

For a long time,those "fascinating" but at the same time "unusual" physical predictions were sometimes just models for thought experiments for novel theory in order to consider the origin of the universe or even to be shown in courses for students.It's can be called the pre-graphene era.However,since the discovery of graphene just over a decade ago,two-dimensional materials are attracting great interest since they are suitable model systems to play with theoretically predicted anomalous physical and chemical phenomena,and have been a central focus of materials research.Although this material required "one small step" to prepare,exfoliating using Scotch tape,it is actually "one giant leap" for general science.Each layer in two-dimensional materials consists of a covalently bonded,dangling-bond-free lattice and is weakly bound to neighbouring layers by van der Waals interactions.This makes it feasible to isolate,mix and match highly disparate atomic layers to create a wide range of van der Waals heterostructures without the constraints of lattice matching and processing compatibility.Exploiting the novel properties in these vdWHs with diverse layering of metals,semiconductors or insulators,new designs of electronic devices emerge,including tunnelling transistors,barristors and flexible electronics,as well as optoelectronic devices,including photodetectors,photovoltaics and light-emitting devices with unprecedented characteristics or unique functionalities.However,there are still many problem and mechanism need to be clarified.In this thesis,the electronic properties,including carrier mobility and mechanical properties of van der Waals heterotructures are theoretically investigated by using first principles calculation?The mainly results are as follows:1)Using a state-of-the-art first principles method,we have calculated phonon limited carrier mobility of hBN encapsulated two-dimensional InSe(ISBN)with different thicknesses,the largest system containing 2212 atoms.The hBN capping layer significantly alters the elastic stiffness coefficient as compared with pure InSe,thus the acoustic phonons in the ISBN composite,giving rise to the observed large mobility of ISBN films.We conclude that the mechanical properties of the composite 2D ISBN material play the crucial role for inducing the large carrier mobility,a principle that could be applied to many other 2D heterostructures.2)In this work,we report a theoretical investigation of Moiré patterns on twisted bilayer black phosphorus(tbBP).It is found that the Moiré pattern has extraordinary effects and leads to significant asymmetry with respect to transport direction and carrier type.The high-symmetry local stacking configurations in the Moiré pattern act as impurities with sizes at the Moiré length scale,and these "Moiré impurities" induce flatbands and localized states in tbBPs.Because both the conduction band minimum and valence band maximum are dominated by these localized states,the deformation potential limited carrier mobility is significantly affected.Furthermore,the variation of twisted angles could impact the size of Moiré impurities,suggesting that it significantly changes the overlap status of localized wavefunctions,leading to the effective regulation of carrier mobility.3)By performing state-of-the-art first-principles methods,we report a theoretical investigation of mechanical and electronic properties of small-angle-twisted bilayer black phosphorus,as Moiré is constructed,under uniaxial tensile strain.It is found that considerable hole carrier mobility and the property of direct band gaps are demonstrated to retain under armchair tensile strain(as large as 25%)before fracture.The Moiré impurities in tbBP induce the real-space localization of wave functions of carrier mobility,which leads to the electronic properties less sensitive to the strain field compared to that in untwisted bilayer black phosphorus.Furthermore,the fracture mechanism of tbBPs is investigated by molecular dynamics method and the boundaries of high-symmetry local stacking configurations in tbBPs are proven as regime of initiation of crack.This in-plane region-selected behavior is demonstrated to be related to the interaction between each constituent layer.