Theoretical Investigations Of The Electronic And Related Properties In Novel Two-dimensional Materials

Advances in nanotechnology have created the condition for the development of two-dimensional materials,while the development of two-dimensional systems have provided the potential candidates to promote the progress of nanotechnology in turn,and neither can exist effectively without each other.It is well known that two-dimensional materials have caught lots of attention from researchers since graphene is exploited from graphite.Subsequently,h-BN,germanene,silicene,black phosphorus,antimonene and transition metal dichalcogenides(TMDs)have been successfully synthesized in experiment.It is shocking that two-dimensional ferromagnetic semiconductors,such as Cr13,Cr2Ge2Te6 and VSe2 have been fabricated as well.Due to the excellent physicochemical properties,they have been widely applied in energy,environmental and semiconductor science.Two-dimensional systems have served as potential materials in generation of clean energy and energy storage,which are attributed to their particular advantages in photovoltaic,photocatalytic and electrochemical fields.The appearance of two-dimensional materials also meet the requirement of device miniaturization in semiconductor science.Especially,the freedoms of charge and spin of electrons in two-dimensional ferromagnetic semiconductors make it possible for the production of miniaturized electronic devices with low power consumption and high efficiency.Furthermore,valley,as a new freedom of electrons,not only enriches properties of electrons,but also develops into a new discipline,namely,valleytronics.The valley polarization in ferromagnetic systems could give rise to the anomalous Hall effect of carriers,which provide a new platform for develop nonvolatile devices.In this thesis,we systematically investigate the geometry and electronic structures of two-dimensional materials.We show that the two-dimensional materials have great potential in photocatalytic,photovoltaic and electronic field.And the underlying physical and chemical mechanisms are unveiled,which provides the guidance for further development of two-dimensional systems.The thesis consists of six chapters:in the first chapter,we briefly describe the progress and background of two-dimensional materials in energy,environment and semiconductor science.In the second chapter,we introduce the theoretical basis,method and computational code in first-principles calculations.In the third chapter,we study the two-dimensional materials with intrinsic electric fields and its applications in photocatalysis.In the fourth chapter,we investigate the photovoltaic effect and its applications in solar cell in two-dimensional systems.In the fifth chapter,we explore the quantum anomalous Hall effect and valley Hall effect in two-dimensional systems,and unveil the underlying physical mechanism.In the sixth chapter,we summarize the conclusion and innovations and look ahead to the future development of two-dimensional materials.The main content and results are listed as follow:(1)We systematically investigate the electronic properties of two-dimensional In2X3(X=S,Se,Te)and demonstrate that they hold great potential and advantages in overall water splitting under infrared spectrum.The ferroelectrict properties induced by broken symmetry of their structures lead to a spontaneous intrinsic electric field,which can break the limitation of 1.23 eV band gaps in overall water splitting.In addition,with the help of intrinsic electric field,the photogenerated electrons and holes move towards opposite directions,thus promoting the effective separation of carriers.Furthermore,these systems hold suitable band edge positions,meeting the requirement of overall water splitting.For In2X3 bilayers,the photogenerated electrons and holes are located on top and bottom layers,respectively,which further promotes the separation of carriers.Remarkably,these systems exhibit good optical absorption.(2)We propose a novel class of two-dimensional Janus chromium dichalcogenide(CrXY,X/Y=S,Se and Te)serving as high efficient photocatalysts for overall water-splitting under infrared light irradiation.We reveal that these Janus systems harbor an intrinsic dipole,which promotes the spatial separation of photogenerated carriers.It is noted that these systems exhibit suitable band gaps and band edge positions,preeminent infrared optical absorption and high carrier mobility.Furthermore,the nonradiative recombination of photo-induced charge carriers in two-dimensional CrXY is evaluated by time domain density functional theory.The lifetime of excited carriers can reach 2 ns,which is even comparable with that in TMDs heterostructures,benefiting for the high-efficiency photocatalytic reaction.Our results provide a new guidance for designing brand new photocatalytic systems with broad optical absorption and low carrier recombination.(3)We find three stable phases of two-dimensional AsSb and investigate their electronic and optoelectronic properties.Due to their direct band gaps,the transition probability of photogenerated electrons increase,thus improving the efficiency of photoelectrict conversion.In addition,the broken inversion symmetry makes AsSb monolayer harboring excellent linear photogalvanic effect along both the zigzag and armchair directions.And they exhibit excellent photoresponse in a broad spectrum ranging from ultraviolet to infrared regions.These indicate that they can serve as potential photovoltaic materials.(4)We systematically investigate the optoelectronic properties of two-dimensional MX(M=Ge,Sn;X=S,Se)based on density functional theory combined with Keldysh nonequilibrium Green's function.It is found that the proposed two-dimensional MX possess many attractive characteristics,such as high carrier mobility,suitable band gaps and small exciton binding energies,which make two-dimensional MX promising candidates for the advanced application in photovoltaic devices.The photoresponse and the photovoltaic performance of two-dimensional MX monoalyers are evaluated by means of quantum transport simulations.Under illumination,two-dimensional MX exhibit high photoresponsivity and external quantum efficiency in the visible region.Our results can provide an insight in the design of next-generation solar cell devices.(5)We predict a series of novel two-dimensional ferromagnets MP2S6(M=Co,Mn,Cr)and demonstrate their thermal and dynamical stability.The results indicate that two-dimensional CoP2S6 is a ferromagnetic half-metal,two-dimensional MnP2S6 is a ferromagnetic semiconductor,while two-dimensional CrP2S6 is considered as a quantum anomalous Hall insulator.When the spin orbital coupling is included,CrP2S6 holds a nontrivial band gap of 53 meV.And it is identified with a nonzero Chern number(C=-1).Meanwhile,the obtained Curie temperature is about 350 K based on Monte Carlo simulations.All these excellent properties demonstrate two-dimensional CrP2S6 is a quantum anomalous Hall insulator which can survive at room temperature Our findings thus present a feasible platform for achieving the QAH effect at room temperature.(6)We propose a series of novel valleytronic materials in two-dimensional 2H-MNX(M=Ti,Zr,Hf,X=Cl,Br)and systematically investigate their valleytronic properties.The underlying valley-contrasting physics including valley spin splitting and valley-dependent optical selection rules in two-dimensional 2H-MNX are unveiled.Moreover,we find that the intriguing valley polarization in two-dimensional 2H-MNX can be achieved through both circularized polarized light and ferromagnetic substrates.And the substrate-induced valley polarization in two-dimensional 2H-MNX is stacking-dependent and can be enhanced by decreasing the interlayer distance.Our findings thus provide a tantalizing platform for operating valley index in two-dimensional materials.(7)We reveal that two-dimensional LaBr2 is found to be a spin and valley polarization ferromagnetic semiconductor.And it is dynamically and thermally stable,which could be exfoliated from its bulk material.More interestingly,we show that two-dimensional LaBr2 harbors the extremely rare intrinsic valley polarization,owing to the coexistence of broken inversion symmetry and time-reversal symmetry.Its spontaneous valley polarization reaches 33 meV,sizable enough for operating room-temperature valleytronic physics.Our work thus provides a promising material for experimental studies and practical applications of two-dimensional spintronics and valleytronics.(8)We show that two-dimensional FeCl2 exhibits a large spontaneous valley polarization(?101 meV)arising from the broken time-reversal symmetry and strong spin-orbital coupling,which can be continuously tuned by varying the directions of magnetic crystalline.By employing the perturbation theory,the underlying physical mechanisms are unveiled.Moreover,the coupling between valley degree of freedom and ferromagnetic orders could generate a spin-and valley-polarized anomalous Hall current in the presence of the in-plane electric field,facilitating its experimental exploration and practical applications.