First Principles Investigations On The Application Of Two-dimensional Materials In Nanodevices

Nanomaterials are in the spotlight of scientific research in many fields.Compared with traditional materials,nanomaterials exhibit better electronic,mechanical,optical,thermal,and magnetic properties,which allow them to be widely used in electronic devices,magnetic storage,energy transition and storage,surface catalysts,sensors,and even bioapplications.Two-dimensional(2D)materials are a category of novel nanomaterials,with the most famous being graphene,which consists of a 2D hexagonal network of carbon atoms.Graphene has attracted much research interest in academia and industry because of its ultrahigh carrier mobility,extremely strong mechanical performance,high optical transmittance,superb thermal conductivity,large specific surface area,and many remarkable physical effects such as the quantum Hall effect.In recent years,many new 2D materials have been predicted or experimentally synthesized,such as molybdenum disulfide,tungsten diselenide,black phosphorene,blue phosphorene,arsenene,and graphene-like gallium nitride.In addition to their excellent physical and chemical properties,these materials also have great potential for application in many important fields.Meanwhile,along with the rapid development of computer hardware and the great advances in numerical methods,various accurate,reliable,and highly efficient quantum mechanical methods have been developed by researchers.In particular,first-principles calculations based on density functional theory have been widely adopted to investigate various properties of nanomaterials.The results predicted by this method are rather reliable and generally close to those obtained in experiments.This type of theoretical investigation can provide useful guidelines for experiments to significantly enhance their efficiency,reduce unnecessary expenses,and pinpoint potential applications.Therefore,in this thesis,first-principles calculations were performed to investigate and explore various properties and potential applications of a serious nanosystems based on 2D materials.In the first chapter,the most recent and widely studied 2D materials are introduced.The structure,properties,applications,and fabrication methods of graphene are firstly described.Then the properties and features of the other popular 2D materials,such as molybdenum disulfide,tungsten diselenide,black phosphorene,blue phosphorene,arsenene,and graphene-like gallium nitride are described.The second chapter begins with a brief description of the first-principles method based on density functional theory,which is the approach used in this thesis,including the Born–Oppenheimer approximation,the Hartree–Fock approximation,the Hohenberg–Kohn theorem,the Kohn–Sham equations,and various exchange-correlation functionals.The simulation package used for the density functional theory calculations in this thesis is then described.The structural,interfacial,and electronic properties of graphene-based van der Waals heterostructures,including graphene/graphene-like gallium nitride,graphene/blue phosphorene,and graphene/tungsten diselenide,are discussed based on first-principles calculations in the third chapter.The potential applications of these heterostructures in nanoelectronic devices are then examined.In addition,the effects of interlayer coupling,external electric field,and defects on the electronic properties of these heterostructures are investigated.Finally,the ionic barristor,a novel nanoelectronic device based on a heterostructure consisting of 2D materials,is presented.In the fourth chapter,the electronic and magnetic properties of 2D material systems,including 4d-transition-metal-doped graphene,non-metal-doped blue phosphorene,and non-metal-adsorbed graphene-like gallium nitride,are explored using first-principles calculations.Their potential application in spintronics is also discussed.In addition,for 4d-transition-metal-doped graphene,the defect states are analysed by hybridisation theory and group theory;a hybridization model based on the orbital symmetry matching rule is then proposed.In addition,the Curie temperature of some magnetic systems in 4d-transition-metal-doped graphene and non-metal-adsorbed graphene-like gallium nitride systems is estimated by mean-field approximation.As discussed in the fifth chapter,first-principles calculations were used to determine whether molecular doping of arsenene and blue phosphorene can enhance their efficiency of solar-energy utilization.The potential application of doped arsenene and blue phosphorene in photovoltaics is also discussed.