Electronic Structure And Its Modulation Of Spin Polarization,Quantum Spin Hall Effect And Semiconducting Features For Novel Two Dimensional Materials

Two dimensional(2D)nanomaterials are attracting tremendous attentions currently since they are abundant and with high performance.Among them,graphene,considered as a single atom-layer carbon based honeycomb lattice,has a dominating position on two dimensional(2D)nanomaterials because of its excellent mechanic,physical and chemical properties.The birth of graphene has stimulated increasing interests of graphene-like 2D nanomaterials.Up to now,many two dimensional nanomaterials have been successfully predicted and even more have been experimentally achieved,which are highly expected in various areas.For instance,in the spintronic applications,2D magnetic nanomaterials possess steady magnetic moments and ideal structure-property stability,which can fulfill the demands of both high storage capabilities and minimized device size.In the field of quantum spin Hall(QSH)applications,2D topological insulator(TI)owns intriguing quantum spin Hall effect(QSHE),which means that the existence of edge states is topologically protected and the spins in the edge states are with dissipationless transport features.It provides a new route to improve performance and reduce energy consumption in spintronic devices.In the field of band engineering and relevant applications,2D semiconducting nanomaterials always have external field controllable band structures,high carrier mobility as well as appropriate band edge positions,which will shed a light on applications such as new generation electronic devices and novel photocatalysts.In this dissertation,we systematically investigated the electronic structures,correlated properties and underlying physical mechanisms of a various kinds of 2D nanomaterials by using first principles calculations based on density functional theory.We have uncovered the potential applications of 2D dimensional nanomaterials in the field of spintronic devices,quantum spin Hall effects and semiconducting band engineering,which provide some systematical instructions especially for energy applications.The dissertation is divided into six chapters.In the first chapter,we introduce the background,current progress and applications of 2D nanomaterials.In the second chapter,we give a brief introduction of the theoretical fundamentals of first-principles calculations and the relevant software packages.In the third chapter,we discuss the electronic properties and spin polarized features of some typical 2D transition metal(TM)based monolayers and nanowires,and further explore the modulation mechanism of spin polarization.In the fourth chapter,we investigate the quantum spin Hall effect in 2D TIs and design a series of novel 2D TIs with larger band gaps which can be used as room temperature devices.In the fifth chapter,we discuss the band engineering and relevant applications in 2D semiconducting nanomaterials.In the sixth chapter,we summarize the research contents in this dissertation and further provide an outlook for future exploration of low dimensional nanomaterials.The main work and results are listed as follows:(1)We explore a kind of TM based nanomaterials including TM monolayers(TM = Ti,Zr,Hf,V,N and Ta)and[(Ge5)TM]?(TM = Ti,V,Cr,Mn and Fe)nanowires.TM monolayers(TM = Ti,Zr,and Hf)and[(Ge5)TM]?(TM = Ti and Fe)have ferromagnetic ground states,which are derived from d electrons of TM atoms.Moreover,the Curie temperature of TM monolayers are larger than 580 K indicating promising spin polarization at room temperature.These results may pave the way for further experimental studies on novel spintronic devices.(2)We investigate some novel 2D TIs with QSHE including honeycomb like g-TIA(A = N,P,As and Sb)monolayers and TMC6(TM = Mo and W)kagome lattice systems.We identify that band inversions and edge states occur due to the strong spin orbit coupling(SOC)in these systems.The nontrivial topological features are confirmed by direct calculation of topological invariants.These 2D TIs have larger band gaps which can be used into room temperature devices.The novel 2D TIs can be used to further improve our spintronic devices.(3)Studies show that MoTe2 is a good match for 2D TI Gel monolayer by preserving QSHE.The thermal stability of the van der Waals(vdW)GeI/MoTe2 heterosheet is examined via molecular dynamics simulations.The Gel on MoTe2 is confirmed to maintain its topological feature with a sizable indirect bulk bandgap of 0.24 eV by directly calculating the spin Chern number to be-1.Our results pave way for further realization and utilization of 2D TIs at room temperature.(4)Other 2D vdW combined systems with semiconducting features are also investigated such as graphene/g-C3N4 and MoS2/MXene(MXene = Ti2CO2,Zr2CO2 and Hf2CO2)systems.The electronic structures and band modulations are studied in detail.We demonstrate that weak vdW interactions can lead to a charge redistribution which make combined systems possess electronic features derived from both two primitive 2D monolayers.Results show that graphene/g-C3N4 system is a semiconductor with the band gap of 106 meV,while the electrons near the Fermi level can still maintain the features of small effective mass and large Fermi velocity.On the other hand,the MoS2/MXene systems can form Type-? band alignments which will make a spatial separation of electrons and holes.Moreover,the band structures of these vdW systems can be modulated regularly by exerting electric field or strains.(5)We explore a new family of metal-free semiconducting nanomaterials including Si-based pentagonal monolayers,p-SiX(X = B,C,N and Si)and inorganic DNA-like helical nanowires,XYP(X = Si,Ge and Sn;Y = Cl,Br and I).Band structures show that p-SiC and XYP systems are all semiconductors with the band gap ranging from 2.21 to 2.96 eV,which corresponds to the visible bands of solar spectrum.Further investigations show that the carrier mobility is estimated to be high,and the band gaps as well as the band edge positions of these systems can meet the requirement of the reduction and oxidation levels in photocatalytic water splitting.Our results offer theoretical guidance for the experimental feasibility of novel photocatalysts.