Interfacial Interactions In Vertical And Lateral Heterostructures Of Two-Dimensional Materials

In recent years,two-dimensional(2D)materials have become one of the current research hotspots in physics,chemistry and materials science.Because of their unique structures and diverse properties,2D materials hold great promise in fundamental scientific research and new applications in electronics and optoelectronics.Up to now,2D materials experimentally synthesized or theoretically predicted include silicene,germanene,black phosphorus,transition metal dichalcogenides(TMDs),hexagonal boron nitride and so on.Among them,TMDs not only have a variety of structures such as 2H,1T and 1T’ phases,but also cover a wealth of physical properties such as semiconductors,metals,and superconductors.With the increasing requirements for the integration and functionality of devices,investigations on 2D materials are in the ascendant.In addition to the inherent properties,heterostructures constructed from 2D monolayers harbour new electronic properties due to the interfacial interactions.The interfacing between 2D materials provides new possibility for tunable electronic structures.In this sense,heterostructures of 2D materials lay the foundation for electronic and ptoelectronic devices in nanoscale.Therefore,study on the electronic structure,the interfacial interaction properties of heterostructures of 2D materials have been attracting extensive attention from theoretical and experimental researchers.In this thesis,on the basis of the first-principles calculations,we systematically investigate the interfacial interaction characteristics and the novel electronic properties of vertical(out-of-plane)and lateral(in-palne)heterostructures of Janus ZrSSe and HfSSe monolayers.We also study the interfacing effects in the vertical heterostructures of 2D MoS2 and Ca2N.The main research contents and conclusions of the thesis are summarized as following:(1)The interfacial interactions in heterostructures from different 2D monolayers can not only tune the electronic properties,but also lead to new physical phenomena,indicating great potential in applications in such as low-power tunable electronic and optoelectronic devices.In previous works,2H-phase TMDs have been extensively studied,while the study of stable T-phase TMDs is still missing.On the basis of the first-principles calculations,therefore,we investigate the modulation of the electronic structures through structural engineering to Janus T-ZrSSe and T-HfSSe monolayers.Our results show that the lateral heterostructures constructed from T-ZrSSe and T-HfSSe exhibit type-II band arrangement and there is an indirect-to-direct band gap transition.This is an indication of enhanced optical absorbance and facilitates the spatial separation of excitons,presenting great potential in applications in energy conversion.Interestingly,for the lateral electronic heterostructures consisting of T-ZrSSe and H-ZrSSe,a transition from semiconductor to metal occurs as the width increases.We also find that the bulk polarization of the H-phase ZrSSe is±2e/3a,and the bulk polarization of T-phase ZrSSe is zero.It is the polar discontinuity across the phase boundary of the T-H heterostructure that makes it behaves as a 1D metal quantum wire and gives rise to a one-dimensional electron/hole gas.This also indicates that the unstable or sub-stable states can be stabilized by constructing phase grain boundary.(2)In current stage,the practical applications of semiconductor/metal contacts are limited by the presence of Schottky barrier as well as the tunneling barrier.2D materials have attracted great interest due to their unique properties,therefore,searching for low-resistance metal contacts for 2D semiconductors has become one of the most important issues.In this respect,we propose a new strategy to reduce the contact potential by constructing donor-acceptor heterostructures between MoS2 with a new class of 2D electrene material,that is,Ca2N.It is noteworthy that Ca2N is featured by the rather small work function and ultrahigh carrier concentration.In the Ca2N/MoS2 vertical heterostructures,results show that superior n-type ohmic contacts are formed between metallic Ca2N and semiconducting MoS2 with 100%tunneling probability and perfect linear I-V curves.The ohmic contact of the Ca2N/MoS2 donor-acceptor heterostructure is robust against the vertical strain.In addition,we demonstrate that this strategy is applicable to other 2D TMDs materials,including WS2,MoSe2,WSe2,and MoTe2.In this sense,Ca2N is an ideal electrode for 2D semiconducting TMDs in electronic devices.Our results provide a theoretical reference for the design of high-performance 2D nanodevices.