First-Principles Studies On The Properties Of Two-dimensional Materials And Devices

Since the successful isolation of graphene in 2004,numerous 2D materials have been emerged in last decade.2D materials have attracted great interests with thier excel-lent electronic,optical,mechanical properties and chemical activity.Chemical workers need to spend a lot of reperties work to explore the properties of 2D materials.We can use first-principles calculations to efficiently predict the properties of 2D materials.Our work mainly involves the following three points:1.electronic and mechanical prop-erties of 2D materials;2.the stability and chemical reactivity of 2D materials;3.the application of 2D materials in electronic devices.The original complex Schrodinger equation becomes easy to solve after introduc-ing the concept of electron density functional theory.The density functional theory meets the accuracy requirements of chemical researchers under the premise of high calculation speed.Therefore,density functional theory is widely used to calculate the properties of the periodic system.A part of our work also involves the calculation of the transport properties of the devices.So we need to use the non-equilibrium Green's func-tion to solve the scattering information of the electron.Thus,in chapter 1,we mainly introduce the origin and development of density functional theory,and the theoretical framework of the non-equilibrium Green's function.Finally,we briefly introduce some first-principles calculations software packages.In chapter 2,we introduce the electronic and mechanical properties,chemical ac-tivity and application of 2D materials in electronic devices.Since the discovery of graphene,people have been focus on 2D materials,due to their excellent electronic,mechanical properties and chemical activity.Most transition metal dichalcogenides?TMDs?have both metallic phase and semiconducting phase.Metallic TMDs can be used as electrodes.Bandgaps of semiconducting TMDs cover the infrared and visi-ble regions.But carrier mobility is usually lower in TMDs-based electronics.Another popular 2D material,phosphene,with appropriate band gap,high carrier mobility and anisotropic mechanical properties,has attracted widespread attention.However,the ambient instability of few-layer black phosphorus?BP?hinders the efficient applications of BP-based devices.Metallic 2D MXenes are considered to be suitable electrode mate-rials,due to its wide range of work functions.Compared with graphene,nitrogen-doped graphene enhances the chemical activity and has attracted extensive researches in the field of electrocatalysts.Nitrogen-doped graphene has been successfully used in water cycle,carbon cycle and nitrogen cycle.But,it is difficult to synthesize a nitrogen-doped graphene catalyst with high selectivity and activity.Metal-semiconductor contacts are important parts in electronic devices.In order to obtain efficient 2D semiconductor-metal junctions,chemical workers have been working hard to investigate TMD?black phosphorus?contacts with traditional and 2D metals.Therefore,we need to explore the properties of the emerging 2D materials.In chapter 3,we focus on the stability of monolayer black arsenic?arsenene?in ambient?oxygen and water?,and mechanical properties of arsenene in-plane directions.Black arsenic has a similar geometry to black phosphorus.More importantly,black arsenic has shown better ambient stability than black phosphorus.In the first section of chapter 3,we explore the chemical stability of arsenene.Under light illumination,the oxidation of arsenene is almost a natural process,with a barrier of 0.012 eV.While the energy barrier for oxygen dissociation in the dark is about 0.56 eV.Two dissociated oxygen atoms occupy bridge sites with As-O-As bonds.Then the stable As-O-As bonds captures the As atoms on the surface to maintain the integrity of arsenens.Thus,we consider the case that one side of black arsenic is completely covered by oxygens.The oxidized arsenene reconstruct and form As402.In Ab initio molecular dynamics,As402 maintains good stability with or without water.In the second section of chapter 3,we study mechanical properties of arsenene in-plane directions.From the stress-strain curve of arsenene,we can know that the critical strain in the armchair direction is up to 41.5%,and critical strain in the zigzag direction is 21%.Because the puckered structure can reduce the required strain energy through compromised dihedral angle,arsenene has the excellent tensile properties in the arm-chair direction.In addition,we obtained the anisotropic Young's modulus and Poisson's ratio of arsenene.Moreover,the Young's modulus of arsenene is significantly smaller,compared with phosphorene.Through the exploration of arsenic in this chapter,we propose that As402 can be used to encapsulate black arsenic.As-O-As bond capture the As atoms on the surface to keep self-stability and protect the underlying black ar-senic.What's more,arsenene can be used in large-magnitude-strain engineering due to superior flexibility.In chapter 4,we mainly introduce the work of single-atomic Co catalytic sites on nitrogen-doped graphene catalysts for CO₂ electroreduction.With the development of industry,the balance between emission and consumption of CO₂ is broken,bringing greenhouse effect.Currently,many catalysts can convert CO₂ to effective products.Noble metal catalysts are usually efficient,but expensive.The activity of bimetallic catalysts with different compostition and content are difficult to control.The experi-mental group designs a series of single-atomic Co catalytic sites on the nitrogen-doped porous carbon.Single-atomic Co catalytic with different Co-N numbers can be selected by different temperatures,and designated as Co-N4,Co-N3,and Co-N2,respectively.Electrochemical measurements clearly demonstrate that Co-N2 catalyst has the highest activity and the best selectivity.Based on DFT theory,we choose single-atomic Co catalytic sites on nitrogen-doped graphene as catalysts,which is consistent with the ex-perimental EXAFS spectrum.Comparing the Gibbs free energy of CO₂ RR pathway with Co-N2,Co-N4 catalysts and Co surface,Co-N2 reveals lower required energy to from COOH*.And d electrons of Co atom in Co-N2 are more localized near the Fermi level,corresponding to the most stable adsorption of J2f4and lower onset potential.Our work is not only consistent with the experimental results,but also provides an un-derstanding of high activity of Co-N2 catalyst from the electronic structure.In chapter 5,we use the first-principles calculations to study the interface prop-erties of the layered semiconductor C₂N contact with traditional metals and 2D metal materials.In the first section of chapter 5,we choose traditional metals as the electrode materials,and these metals cover a wide range of work functions from Sc?3.42 eV?to Pt?5.75 eV?.We comprehensively analyze the configurations,electronic structures and FLP effect.There are two ways to achieve Ohmic contacts.The strongly hybridiza-tion between A1?Sc?and monlayer?ML?C₂N leads to ML C₂N metallization.Besides,due to the weak van der Waals?vdW?interactions at interfaces and low work functions of metal electrodes,Ohmic contacts can also be realized in ML/BL C₂N-Ag and BL C₂N-Sc systems.Moreover,the Schottky barrier heights will decrease in the bilayer C₂N-metal contacts.With the development of 2D metals,2D metal-semiconductor contacts have been extensively studied theoretically and experimentally with weak FLP effect.So in the second subsection of chapter 5,we choose a series of 2D metals with clean,saturated surfaces as the contact electrodes for ML C₂N.These 2D metal electrodes are two types,one is surface-engineered MXenes;the other is metallic TMDs.These weak vdW 2D metal-ML C₂N contacts,with small binding energies and large interlayer distances.We can tune the Schottky barrier height with 2D clean and saturated metal,owing to the weak FLP.Finally,Hf2C?OH?2 contact with C₂N are Ohmic in device,because of the low work function of Hf2C?OH?2 electrode.Our calculations provide a guidance for choosing electrode in C₂N devices.